CH16 Cytoskeleton.pdf - finedrafts

uolelalsov() aql
palluc stue[ueg 3o ura1s,,(s elqDlrBruer e uo puadap daqt pue ,aarSap q8q ,,fuarr
E o] suorlJurg leorueqJeru pue prleds eseql 11e padolanap e^eq s[ec c4orfuecng
'secuslstunsrlf,
8u€ueqc o1 ldepe pup 'apr^rp rr,rorS rtaqt se sluauodruoJ lpuJelur
rreqt a8uerreal ol alqe aq o] eAEq sllet IIV.ace1d o1 aceld uor; arrou pue adeqs
rraqt a8ueqc o1 aneq,,(ueyq Lleura1u1 parnpnrls dlradord pue ,lsnqor.,typcrsdqd
'padeqs
,tllcarroc eq ot elpq ,taq1 .luaruuoJrlue
Jleql qfl^a ,,i.lpcrueqcaru 1ce
-ralur pue aceds ur sellesueql azrue8ro lsnu daql flradord uoncury ol sllec JoC
aql ur rfuaurqopru eql sapnord lI 'seJeJrns ssorJp I1v\eJJ ol .slleJ poolq etrqM pue
slselqoJqg se qons 'sJar{lo puu rurms ol rurads se qJns sllec salqeue 11 .sa8ueqc
pue sgrqs lueruuoJrlue eqt se gude paddp Suraq lnoqlpr suruJls pue sasseJls
reaq [ec aq] lal ]eql saSoprl leoruer{Jarrr aql sapprord pue eupJqrueru erusuld
ay8er; aq] suoddns lI 'reqlouu ol IIec aql go uud auo ruo+ slerJalpru Burdrra;
'sallaueBro
Jo lrJJerl relnllacprlur aql saprn8 pue selrJp 1I .ort,r] olur ilec Bupptp
aqt sqds ueql pue srsolru ]e uedB seruosoruoJqc aql sgnd uo1a1a1so1b aq1
'(1-91
arn81fl uolalarlsofc eqt
-ranrp Surzeure eql sloJtuoo pue IIB,r,r IIec 1ue1d eqlJo qlmor8 aql saprn8 tl .setrJp
-uep pu" uoxB ue pueua o1 IIaJ a^Jeu eql ur puE uorlf,BJluos JoJ IIaJ elssnru
'sluauelg
yo sadfi urcru eaJql ruJoJ ol alquesse qclqm ,selnJelour
uralord;o
sarlrrueJ eerqlJo rorler1eq eq} uo puedap suollJury pelrel spolaya4so{c eql
'sadeqs
1ec cqorfuecna;o .,i1rs
'saqradord pcrueqcaru tJurlslp seq luaruelggo adft qceg
'e^oru ol
-3o1orq pue 'scrureu.,(p
,irmqe str pup 'adeqs s1r 'q13uar1s slr IIef, e an€ ol d1a,r,11ca11oc uoucunJ dlerurou
lsmu srualsds lueurelg pteleTsol,{t aeJql IIe os :eqteSol {ro^a ot salJsnru puu
'sauoq 'sluarue8rl
rno eJrnbal arvr sE lsnf 'aluradooc slueuela tueJeJJrp aqt rvroq
pup s{rom uo1a1a4so{c eqt MoqJo Surpuelsrapun praua8 E JoJ srseq eql ep4ord
saldnurrd aseql'seldroulrd pluaruepury ureuac eJeqs eerql IIE lnq ,selor IeJr
'suorlef,ol
rvreu olJleslr eloru ol-eleudordde uaq^\-pue ,Jeqlouu
o1 aceld auo
ruo4 Jauupru pelJeJrp e ur salaue8Jo eloru o1 ,ecu;rns str laporueJ pue adeqs
o] tepJo lpuJelur urelureru pue qstlqelse o] IIaJ eql Sullqeue ,srualszts
luerxelg
ureru eeJql eql qllm lceJalu suralord Jaqlo ,lroq eqrJcsep uaq] eM.sequelncad
lenprlrpur Jreql pue 'z{.yquassesp pue rtlqruasse rraql Sur.rtlrapun saldrcuud crseq
eql 'sluarxplggo sed,ft ureru eeJql eql SulquJsep r(q ur8aq e,\,\ teldeqc srq] uI
slNtwvt tJ tvlltl)sol^) lo
]UNI)NUIS )IWVNAC CNV AIBW]SSV-JI]S 1Hl
go suorlrsod aql eurruJalep salnqruoJJrw 'qlauetls IpJrueqf,eru epnord vua1a
-a7{ a1a1paruta1u1 'selltadord
pcueqceu pue uoqezrue8ro Iupeds ,qlec aqt roJ
alqrsuodsar are ]eql slueruelg p1e1a>1so1,{c go sadr{t eeJql elpr{ sllac 1erurup lso6
Ilas-aloq r JoJ drussacau aJe puB eceJrns s,ilec eql Jo edEqs eql eulu]Jtelep sluau
-nI{ ullcv'Uodsuerl repllace$ur lJeJrp pue se[aup8Jo pasolJue-aueJqrueu
eq] JoJ Ispuesse s sunrcJd [tossacco Jo les aSJEI srql .raqlo qJpe o] se IIa^\ se
'sluauodruoo
IIaJ Jeqlo ol sluetuelu eqf {uI }eql suraloJd drossacce;o speJpunq
aql ]noqlyv\ e^IlceJJaur eq pFo^ slueluplg plelelsoil.c asaql tng .uouoruocol
roldeq) slql ul
lo lunl)nuts
fIWVNAC CNV
s96 lt8wlssv-Jtls tHl_
SIN]IAVIII
tvl_ltl)sol_A)
SIN]WVIII
tvl_il1)sol_Al uEHl_
266 trvtn9tu slll) MoH
0r.0r suol_ow uflnfilow
uotAVHlS Il) ONV
sejl Nolltl)sol^) tHI
r,uri 0l
fuossnol UaqlV
Jo Isalrnol)'anlg ut palaqel st snal)nu
aql u! vNc aql'par) sluaulelu ut]te
aql pue (uaal6) solnqnrol)tu aql ,sulatsls
lelela)soill lofer"u s1r 1o oMl Mor1s
ol palaqel pue paxu uaaq seq arnlln)
ur lla) V'uo1a1e1soy(r eql 1-91 arnbrl
996

uogala>1soy{1 aql:9 L raldeqf 996
'ace;,{eru
}I leq} ^lpn}uarra,(ue o1 .(lprder puodsar
ol alqe sr IIeJ aq] 'crureu,{p aq ol s}uau€1; p1a1a>1so1rtJ asnuJ }Pql suslueqJeru
alqelrerual eql Jo asnecaq ']Eql aes IIEqs aM'seJnlJnJls paztue8ro-Jlas pue pezl
-re1od ,(lecrsulJlul ruJoJ ol drIIqB Jlsql uo snJoJ eM 'uo1a1a>lsoytJ eql Jo sluarue
-ru aql dn aleur 1eq1 suralord aq];o sarlredord aql ssnJSIp e^ 'uollsas slql uI
'sanlesrueql slueruelu aql elotu Jo slueuellJ aqt Suop
sallaue8ro eloru Jeqlla uBJ ]Eql eJJoJ IeOlueqcau olul srsr(lorpdq dJVJo d8raua
aql Ue^uoc leql saulqJeu JEInJalou alqe{reIual 'sutalotd tolota eq} sapnlcul
tl pup 'suopeool relnf,Iued ur sluaurellJ lelalalsolb aqr;o .{lqruasse pellor}uoo
lelalelsot^)
elqerdepv
pue )lueu(6 ery sluauellJ
o1 dlaq sepqruorJlru;o s.{erre paztue8ro 's1iac 1ue1O .ti '*o* puornau 3uo1
umop slerreteur go podsuur] eql roJ $loeJ] se enJas leql sapunq pau81p fpq8p
ro ,lleJ eqt Jo aoeJrns aql uo ayaBnll pue DqlJ palpc sdtqrr elllotu IUJoJ osle
uec .{aq; 'uorsr1rp 1ac Sutrnp a Tputds ctlo4ta relodtq e IIJoJ o] sanlasuraql a8uur
-ruar r(11cmb upJ 'lle3 aseqdrelut uP Jo Jaluec aql 1IIo4 Surleueura Lurre cruseld
-o{c a1q-ruls e uI puno; r(1}uanbar; are qclq.t,\ 'selnqn}olJllN 'saJn}JnJ}s aseq}
Jo aruos pe^er I{I laued uI sqderSorctur eql 'srualsfs }uaruEIIJ clseq aaJql
eq] 11104 saJnlJnlls Jo aluer snoruroua ue p1nq o] sgac rtlodrecne Sa1ollg Slueul
-elrJ
IPleleIsol,tc aqt;o l.lqruasse puB rol^eqaq c[ueu.{p aql 3o uorlepSaS
.aSueqc
suoplpuoc uaqm dSraua BJua elllI sarrnbar IIef, E ur luauraSuerreal
IBrnlcnrls E ,lreD lue up Jo uorlEJal[B aql aI[ 'snql 'xnu Jo alB]s ]uBlsuoc e uI aJe
saJnlJnJls asaql dn e)e111 leql sluauodruoc lelnoeloluoJJeul pnpwpul eq] ]ng
.aullaJrT s,lleJ aq] o] dn atnunu e ueq] ssal uo4 Sur8uer erullJo sqfual ro3 3upse1
,paeu
ol Surprocce
,lsrslad ro a8ueqc uEf, saJnlsnJts ptalalso]r{.c apcs-a8rel ,{em
Jelrurs e uI .uorlenlrs ^ eu eq] qll { pep 01 firprder Surqsruolse qly!\Jlasll saSuer
-JeeJ arnlJnJls cnueu^,tp eql'e^Eal pue dn uealc sralcrucrd eqlJI Jo 'pooJJo eoJnos
rallaq pue ,t,rau p purJ stnoJs lup aql JI 'cllels 1nq Surq{ue ere IIEJ} aql uqryv\
slue lenptlrtpul aq] tnq 'a]ts ctuctd alqelJelep E ol lsau lue eql uror; Sutpuaga
,sJnoq dueru ro; lsrsrad .{eru s}ue Jo IreD a13urs y 's,,tulv\q8rq e}elsJetul uBq}
slreJ] lue aIII aroru pazruu8ro 'alqeldepe pue ctueu,{p are suralsds p1a1a1sof3
surpr'r'or d's'a r IE ruru' r o auer q."e ."
" T:r"jir#l ;lii #lr'J::Hi"tH fi1'J",,0
ilac eql qcurd ol elJSnIu,rtup e e411 slclJlsuoJ qclq/!\ Sutt aplcn4uot aql 'lleJ aI{}
Jo elpprur eq] punoJP lleq P ruJoJ uaql ulsor(ur utalord Jololu pe]€Icosse sll puB
upcy'adeqs pctraqds eJour P selunsse pue 'dn spunoJ 'Suttotu sdols 1ac aqt
lBql os elquesseslp qslp aql Jo esPJrns eq] ssoJlE I AeJc o1 lselqorqlJ aql alqeue
leql seJntcnJls urlJe pazlptcads eql 'aurp arues eql lV 'IalJnu ralq8nup aler
-edas oml olur eluosoluorqc paleclldeJ qJee Jo sardoc oml aq] Surlu8arfias .{1a1er
-nJJp
Jo uoDJunJ IerplrJ aq] senJes qJIqM 'a1putds J!\oL!u't relodrq aql olul peJn
-Squoca; sr urseldo/c aqt lnoq8norql sueds 1uql -{erre ePqntoJJIIu aseqdralut
aql 'palecqdal aAEq sauosoruoJqf, aql JaUV 'qslp eJnllnJ enssll e ut Suu,ror8
lselqoJqrJ u JoJ Z-gI arnt;g uI u^^oqs se (uoIsIAIp 1ac Surrnp sJncso uo1a1a4so1dr
eql Jo uorleztue8roer ptder ;o alduruxa slletueJp pue luelrodtul uV
'speura8ug pue
rreq se qcns sa8epuadde q8nol IuJoJ o] sn lltolle.{aqt pue 'suoxe }snqoJ puu 3uo1
pueua ot slleo anrau dlaq ro raqtaSol slaaqs 1ac plaqttda ploq uec ]eq] selqeJ
Suorts otur petslmt are daq] '1oso{c aq} uI :VNCI s,llar eql ro; a8ec anpcalord e
Sunuro; 'adolanua Jeelcnu eql Jo ef,PJ Jeuul eql euq stueuPIIJ elelperuJalul
'uoqdrosqe
]uelJlnu eoueque ol
EaJe aceJJns 1ac pcrde eql asuaJcur,(1lsen slac plaqltda pupsatulJo aoBJJns aql
uo tl7ltoJJrru pazrueflro z(1reluns pue 'punos o] asuodsar uI spoJ p€tr se lllf feql
slualuplrJ urlJe Jo salpunq alqels ulEluos JBe Jauul aqt uI sllal JIEq Jo esBJJns eql
uoDllnoanls Jo sepunq rep8ar eqJ'lJerluoJ o] alosntu alqeua pue unleJlsqns
Sur.Llrapun ue lsurefie se1lesuaql eteJq ot sllao ltlrollp srterre a1qe1s aJou :oml
ur sllac applp o1 .{puatsue4 salqurasseBuu allPaJiuor pesuq-ullJe aql'punore
se^lasueql 1nd pue ,rfuolrrrat aroldxa ol asn sflac leqt mpodo1{ pue atpodty
-pwal eql se qJns ,sernlcnJls crureudp aJe asaql Jo eluos 'suopcaford aoBJJns
-llar
Jo sad,& .{ueru ruroJ osle ,,taq1 ra,(egq prdq uqr s1l o1 aduqs pue qlSuarls

slNttiv]l lvllll)sol^) lo lunl)nuls )lt lvN^c cNV AlStJlSSV-llls lHl-
-urru eAU ueq] ssel ulqtyv\ se^lasueq] eSueJJeeJ zi.1a1a1duroc uE3 suolelelso].{3
alnqnloJcrru puB urtce eql 'o^Jqlua DITdOS1JO e uI suoIsIAIp JBalcnu ,{Fea aql
sE qJns 'sesec aruos uI :Jnoq uE tnoqe se{e} slua^e Jo asuenbas slq} '}sPlqoJqg
P uI 'lle3 Jeqlou 3u11,uerc
'pauaileu eql Jo suolsJa^ rellelus olul slles JelqSnsp
dn papunor oml el{l uanuoJ o} seJn}f,nJls aseqdJe}u 4eqt olu1 elquasseer s}sEI
-qorqlJ ralq8nep o^\l eql Jo suolale{so}(c aq} 'atelduroc sI uoIsIAIp ual{ \'o1v\l uI
'(g-91arnt1g) saln
u!ru 0
aseqolalul
ulur s I
aseqdoJd
u!r.u s t
aseqdeleur
e)eJJns lla) u.lol,|. a)uels!p
urri 9 urfl t urn z urn 0
196
'sr{ean
areredes
raqt lmpr) o] tlaql 6ur;qeua'llal raqlour
eql u! luasord aram leql asoql Jo suorsra^
ralleus olur suole;e>1soy(r u!])e pue
alnqnlor)rrrr aql qloq azrueOroer s;;ar
rolq6nep oMl aql'a1a;duo: sr uorsr^rp
lla) JaUV'paleredas aneq saluosor!olq)
aql raue oml ur lla) aq1 saqrurd leqt
lle) oql,o ralua) aql ]e 6uu olrl)eJluo)
e ulroJ sluaurel!, ult)e eqf'(urlolg)
sauiosouorq) pole)rldnp aql 6ulleredas
uaql pue 6uru6r;e lo, alqrsuodsej
sr q)rqM'a;purds rrlolrru le;odrq e ur.ro1
ol sa6uelrea: ,{ele a;nqnloorur pazuelod
aql'sapr^rp lla) eqr uaqM'snal)nu
aLll Jo luorJ ur pale)ol Jaluo) 6urzrue6ro
alnqnlor)rru a;6urs e uorJ aleueuJo lpql
salnqnlor)r,.u 6uo; ;o 6urlsrsu o)' (uaalb
ur uMoqs) uo1ala1so1{r alnqnlor)rur
aql r(q patsrsse sr uo1a1a1so1b urlre
aql Jo uorlezuelod aq1'1q6u aql preMol
e6pe 6urpeel s1r qsnd ol salqLuassp
reql (pal ur uMor.ls) uolalalsoy{> utlre
rrueu{p'pezuelod e seq oraq uMerp
]splqorqq 6ur;merr aql'uo!s!^!p llal qlr/vr
pale!)osse uollezlue6ro le1a1a4soy(>
ug sa6ueqr prdeg 7-91 ern6r3
fue^rllnS ue!lllM
lo Iseuno)) 'sloqq6rau
st! qrlM 6ulplllo)
Luo4 snel)nu q)ee 6urdaal alrq^
'srsolrur
lp saurosorrjorq) aql aleledas
ol parnbar are'o^rqura burnrg e ur
eraq uaes '(uaat6) sapqnlor)rur pue (pal)
sluaurelU u!l)e aLll Jo slueua6uelrea.r
prder aql'use;dolb uouruo> e
ur os Jo selnurur 61 ,tana Jn))o suorsr^rp
real)nu IUea aq]'lle) reel)nurllnur
tuer6 s;ql ul 'oftqure {Fee
olltldosotg e 1o guaurdo;anop aql 6uunp
pa^rasqo uo!lez!ueOro 1e1e1a1soy(>
u! se6ueq) prdeg g-91 ern6r1

sl_Ntytvll NUJV
auerqurau euse;d aq] qlpauaq lsnf'xayo: aql ut paleJlue)uo)
^lq6rq lsorx ere i(aql';;a: aq1 lnoq6noJql pesledstp ale sluaueltl
u11:e qbnoqlly s;a6 ;euorsuaulp-eetql pue,slJoMleu
'salpunq
leuotsuaultp-om]
reeurl 'Lo [1auen e olur pazrueElo e.re r{eq} pue ,u]u 6-s }o./a}eLuetp
e qllm 'sernDnJls
alqrxal+ se readde {aqt 'urpe uralo.rd eq1 1o slauf lod
le)llaq pepuerls-oMl ale (sluaile/ryoDtw se uMoul osle) sluauelt] utllv
abprrnE q]!e) :(ll) sso6rng U c pup erepnslpfl Id i(^r pue r) 6rer) ra6ou +o Isalrno) sqder60r)y\j
srrn8n10u)ry\
(^r) rpsac V i(ilr) eurqs pr^eo i(il) l)utl M U pue MorpooM 1 O j(r) apeM preq)U +o ^salnol sqdej6ol)ru\
'eu/oso4ua)
e pelle)
()OIW) ralua) 6urzrue6to-elnqnlotrrut a;6uls e ol peq)elle pua
euo a^eq [llerrdri1 pue 1q6re.r1s pue 6uol ale salnqn]oJ)tl4l s]ueutelU
urDe ueql pt6u alou q)nuJ aJe ,{aq1 'uu
SZ }o la}auletp lalno ue q}!A
ur;nqn1 uralo.rd aq) ]o epeul slepull[: molloq '6uo1
ale salnqn]ol)r!4
StN I lAVl ll l-LVl C :l tAU lIN I
sz
uru
uru 00[
uru 00 !
uru 00 I
(^r) rqav rlan i(ril) uroqso ^ren i(il) eqsrapa) I fuueN :(r) uelurnO ^oU ,o Issunor sqder60rlrW
'urnrlaqltde
altluo oql 6uruaql6uejls ^qaleql laqloue o1 uorpunl
lla)-lle) auo ulot1 useldop{t aq} ueds ^aq}'ensstl ;er;aqlrda ue u1 ql6uatp
le)rueq)aur s11a: 6ulnr6
'uiseldop{:
eq} ssorle puaya sad{1 laqlo auelquraur
Jeel)nu rauur aql qleauaq 1snI euruel reel)nu aq] palle) ]loMqsaul
e surroJ luauJelr+ alerpeuiJelur 1o ed[1 au6 ,{1tu.re1 snoauabo.ralaq pue a6le1
e alnlrlsuo) q)rq^/\'sutalord luauelt] aletpauJJalut lo apeu ale ^eql:ulu 0l
punore ]o Jelaurerp e LlltM sJeqt, a1r;adot aJe sluoL!elr+ alprpaurl;lul
(rrr)

slNtwvl|J tvl-ltl)souf Jo lunr)nuts )twvN o cNV AtSwlssv-ltls 3Ht
)os E )es z )os I )as 0 aulll
'raldeqe
slql Jo uol]f,es IeurJ aql ul
Fetep aJoru ur pessnJsrp aq IIIrlt sluarua8uerreeJ lelale{so1,b prdergo spury esaq]
Jo qlog '17-91 arn81J) spuoces ulqll^t sernl3nrls arrrsnrlord pazrelod slr azru
-e8roar o1 pasrod sl pqdorlnau aql
(uorlJeJrp
luaJeJJrp e ur seloru r.ard prrapeq
a^rsnla eql uar{liUslueuplrJ urlce pazrraur,,(1od,r{prrau qly!\ palllJ a8pa Burpeel aql
le aJnlJnJls aarsnrlord e Surpualxa dq acuunpe slqdorlnau ,s1ac
SurlaneJc ]soru
arrJ 'UDIS
eq] q slnJ qSnorql se fpoq eq]30 sped alrrals flerurou eql 0l ssaJcu
urc8 dleluaprcf,B tpqt s11ac p8urg pup lprretf,Eq s;p8ua pue sespqo ,llal poolq
allq \ Jo ad.{} e '71t1do4nau aql 'aldurBxa Joc 'lla1n\
se aseqdralur Buunp Buruorl
-JunJ
Ierurou neq] roJ sluarua8uerreeJ lelele{so{c prder arrnbar q1ac,,(uuytr
sernl)nlls alqels uroJ oslv ue) uolalalsot{) aql
asoql sp qJns sllal Ieneqlrde pezrrplod 'llef, er{l Jo eul}eJII eqt relo paurelurpu
aq ueuo osp ]snu uo1a1a1so1tc aq] Jo uorlEzruuSJo eql dq papoJue uorleuroJur
drlreyod apcs-a8re1 eqJ )req pup ]uoq ro (ruopoq pue do1 uaemteq aJuereJ
-JIp eqt ilet ot sllar Suqqeua ,ir1re1od Jelnllar apcs-a8re1 rog alqrsuodseJ osle sr
uo1a1a4so1dc eql 'suorsnJlord acey;ns IIaJ pazrlerJeds a1qels Sururrog saprsag
'sapef,ap
ro3 lsrsrad leql saml
-JnJls ecPJJns IIec alqels eseql ulqlll\{ ue^e 'sJnoq 97.{rana aSerane uo paceldar
pue palapouar dlsnonuquoc ere pue crureu(p dl8urnrrls urerual sluaruelg
ur]38 lEnpr^rpur eql 'ssaleqleuoN 'Jeno uJnl lou op silac aseql ecurs .lBrurup eql
Jo eurpaJll eJrlue eql JoJ uorlpzruESro a1qe1s Jraq] urplurpru lsnru Jea Jeuur eqlJo
sllef, Jrpq eq] uo prlrJoerats
Jo seroJ eql lp sepunq urlJu eqt 1ng .sdep .ue; e fpo
sI slq] slleo Ierleqllda puqsalur uo rllrlorJrru Jo seroJ aqt le sepunq urpe eql
Joc '[ec eq] Jo eurrleJrl eJr]ue eq] JeAo Jelauerp pue 'q13ua1 'uor1eco1
luu]suoc p
urElurBru ot elqe eJe prlrJ pue rllr^orsrru Surpnlcur suorsnrlord aceJrns IIa3 pespq
-p1a1a>1so1,b '3un1
aqr pue eurlselul eq] sB qcns senss! auI lpql s11ac prlaqlrda
pazqercads uO 'uorlezrue8ro re1n11ac JoJ seJnlonJls apcs-a8rel ,alqpls apnord
oslE ]snru uo1ala4so{c aql Jo slueuala cnueu,{p eql 'sllaf, plaqtrda ro suornsu
eJnlpu sE qcns .,{3o1oqd.rour pelpnueJeJJrp .elqpls e pelerr{Je a^eq }eq} sfiec uI
IecrlrJJ eql urElureu 01 sluetuplu alerpetuJalul pup (sluaruplu urpe ,seFqnl
-oJcrru
Jo sdeJJE pazue8ro esn 'eldurExe JoJ ,eurlselur aq] Jo SuruU aql uI punoJ
arcg,n'racoltns luJaruIosaq eql ol [q sessed pooJ aJeq ^ auuselul aq] Jo ueunl eql
ruo.t; sluerJlnu sqJosqe lar4l aJatJns 7n4da eql uaemleq sef,uaJeJJrp leuor}cunJ
3uo1) salqec uqce :sllec eseql ur serlquessp luauelrJ urlJe Jo sedil omt aJB eJer.{J
'uo1e1a>1so1,b
urlJe slleo er{} Jo uorle}uerro relod eql uro4 selrJap drlauudse
sILIJ 'llal Jeqlou a8rey e pue IIeo ralq8nep IIErus e eleerJ 01 Surppnq l{q apyrlp
daqt ,{em eql ur luaprla 'd-qaruurdse pe{Jeu aql sl sfiac eseq] Jo erntlnJ]s eql
Jo eJnleeJ elqelou lsotu eqJ ,ir1relod eIpss-e8JEI elqpls paau aq$uralas sat{LaoJ
-Dqnas lsea,rt Surppnq aqt se qons sllec aldurs dypcrSoloqdroru .llel.us
ua^g
'19-91
arnt;g) raureq pcrsdqd elrlcaJJa ue sp alJes ol slleJ Jo ra.,fu1 al8urs srql
elqeue 01 Jeqtoue auo qllm slJuluoc alrsaqpe Suorls urclureru lsmu osp daq;
'urBeJlspoolq
eql ol eupJquraru eurseld aq] qBnoJqt sluarJlnu raJsupJl slleJ eql
]e pelsrlueJuoJ dlqSq e{uoJeq seqJ}ed unce aq} 'ssacord sH} uI 'raSJBI
,rtluro;
-run SuluorB dldurrs ol pesoddo sp .eJEJJns
IIac eql uo elrs el8uts e ruoq pnq
e ,tlroJ8 ot IIeJ eql .{ ollp ot pazrJulod ^HBIq eq tsntu slleo }seed Surppnq Surlera;1
-or4 '(srsol,bopue
uelrJp-urlJe Jo salrs Suqreur ,xauoc IIef, eql ql!v\ peleloosse
slueuelrJ Jo sarlquesse IIBrus) ssqclEd ultf,E pue (slueuelrJ ullJp Jo selpunq
prp/v\ol Suqurod pup pau8qp selqBJ urlJp eq] ql!v\ ,pnq eqt Jo dl] Suriuor8 aql
('sla6ou pt^p6 Iq peplo)al
oopt^ e uotl) .spuolas
MaJ p utLlltM
luaLria^ouJ jo uorl)altp pup uotlpluauo
s1r a6ueqr ot lt salqeua lla) stqt
ur uola;a1sop{r ut})e aql ro I;quessea.l
pue {lquasseslp pldpU .eual)eq
oql Jo uorle)ol aLll preMol qsnd ol (pal)
a6pe 6urpeal aql le lJomlsu utl)p asuap
aql salqLUasseat I;1trnb ;rqdorlnau
aql'anout euat)eq aLll sV .llr.ldojlnau
e {q palnlder aq ol lnoqe sl (lvtoilD
attq/A) elepeq;o durnlr e ,poolq ueurnq
1o uorletedald stql ul .eual)eq
;o glnsrnd ur ;pqdorgnau y tr-91 arn6r1
696

uola;a1soy(3 aql :9 I raideql) oL6
sluauelu alelpaulalul
salnqnlol)ltl.J -
sluAuellloJllul utPe ->s----
snllrAor)rul
leseq
'ssaoord UoISIAIp IIec eql Jo pua aql l3 IIef,
ralqSnep eql olq peJe^rTep eq uE3 saruosoruorqJ palEJrldeJ Jo tes elaldluoJ P
lpql os ,alpurds sltolllu eql Jo uopetuerJo aql saJuenlJul uJn] uI salnlJnJls uI]lE
aqlJo uoBezruesJo paztrelod eql'(9{I aJntIC) Suppnq3o elIS eql o} sleIJe}EIu
Jaqlo pue IIE^ IIec ^\au Jo uollaJJas eql slserlp uollEZIueSJo ullJs slqJ
'ueq]
slrunqns ulaloJd
ralleus uolJ pal)nllsuo) sl lueuellJ lelelelsov() lo ed^I q)el
prdpJ o8Japun ueo sllal '[ert sq] uI ']ouueJ s]ueIlEIU pelqulesse aql seaJeqm
'ruseldopb eql ulqll^\ dprder asnJJIp uef, ^eq] 'ilerus eJe sllunqns esaql
asnBseg '$lJIJq Jo lno radBJJSEs e Sulplmq a)II (sllunqns ilelus eql Jo sreq
-unu a8relJo flqurasse e^l]padal eqi,(q saJn]3nJ]s a8rel aq1 splnq ilec eqJ'azIS
ur sJalaruoueu ,11aJ e .{po f.1prauafi eJp uolele{so1,b aql Jo selnf,elou uleloJd
pnptirlput aq]
'sraleuloJcllu
]a
Jo speJpunq uene Jo suel Suluueds taqto aqf
o1 IIaJ aq] Jo pua auo luo4 ,(em aql IIE qJeer dlluanbar; seJn]onJls lulelelsol'(f,
-
ulpe ]o
qa^^ leururJal
uorpunl sueraqpe
auosoulsep
euosor.!saprureq
eurulel
uri 0t
rado.rd rraql ot sluauodr.uor pezrsaqlu{s
I;mau lrarrp ol lla) aql salqeuo leql
ualsl(s aleurproor leqo;6 e aprno.rd pue
'uroDoq aql ol lla) aqr 1o dot oqt Luolj
Illerrlran unt (uaafi) salnqnlor)rW
'6
[
ratoeLlf ur passn)srp aq lll/v\ salnl)nlls
e^rsaqpe luelJodrur asoql :lla) aql
lo aprs lPseq aLll uo xuleuJ relnlla)Prlxa
6ur{;rapun aql ol uaq} q)e}}e
pue taaqs,(prnts e otur slla) 1e1laql1da
eql l)auuo) leql saulosoulsapluaq
pue sauJosotusap 6utpn;rut sornl)nlls
a^rsaqpe Jo spurl raqlo ol poloq)ue
de pnlq) sluaurelu alelpaulralul
'Ipoq aql olu! burleal u.lol; uaulnl
leurtsalur aql Jo sluatuol aql lua^ard leql
suorlrunf lla)-llo) 6urr-uro; ol satnquluo)
sluaurelu url)e Jo pueq lerluaraJun)ll)
e'rllr^orfil.l-r eqt Molaq lsn|poo] urorJ
sluaulnu 6urqrosqe ro, alqepene eaJe
a)e]lns lla) aql osear)ur leql !ll!Aor)!Lu
w)ol (Pu) stuauelu url)e palpunq
'uaulnl
leutlsalur aql 6ulre;'a)eJrns
paddn) lerrde eql lV'aultsolu! lleuls
aql eu!l lpqt sllar lerlaqr;da aql 6urpnl)ul
s;1e> pezrler:ads 1o sadeqs )!lsual)ereq)
aqt a)npold ol aleladoor uololalsot^r
aql;o sluauodtuo) aqt llV 'slla)
1e;leqr1de pazl.re;od u1 uolalelsoy{r
aqlJo uollezlue6rg 5-91 a'rnEr3
'suorle)ol
(ssar6 {lrsrenrul
JellaJal)ou aqf uorJ uorssrurao
qllM'866 t 'LLS\-LjSL|ZVL 'lo!8
ilD'f
"le la e^odle) 'S f ruoll) 'solqe) palueuo
Ipuopuer a^eq slle) qloq q6noqlle
'elrs
uorsr^rp aql rPau paleJlua)uo)
are q)rqM'soq)led Mau uJoJ
s;;a> rarq6nep pue raqlour
'uorsr^tp
llal
raue ^lalerpauLul (O) 'pezgelod
ureuar
lle) laqlour aql u! salqef 'pnq pazrs-llnJ e
Jo ale;ins oql ralo palnqulsrp {1tu.to;tun
lsor.!le are seq)led ()) 'rltntor6
lleM llal
Mau Jo alrs srql premol lurod ol anurtuol
llo) laqtoul aql u! salqef ']t uqllM
ureuar saq)led lsor-u'srvror6 pnq lleus
aql sV (g) 'a6raua lllM pnq aql araqM
alrs aql sl q)rq^^'saq)led Jo ralsnl) aql
premot lurod pue dn pourl are salqe) aqf
'pua auo le peielsnl) auro)aq soq)led
eqt Jo lsor! 'pnq eql Jo uolleuro]
aql arolaq
'lla) Jaqtour e ul (v) '(saurl
popualxa) salqe) urt)e pue (slods rq6Uq
punor) saq>ted ull)e apnl)u!
'u1p;olgeqd
tua)saronlj LlllM areq polaqel 'lla) tsea^
aql u! sarnl)nlls u!l)e snoluaLue;|1'a;r{>
11a> lsea{ aql lnoq6norql solqP) pue
saqrled ull)elo Ilrre1o6 9-91 arn6t1

Sl-Nlt JVlll lvl-ll3)SOIA) lO :lUnIfnUlS )IWVNAC CNV AlBl llSSV-JllS 3Hl-
'leu6rs
leuralxa ue o] asuodsat ut lla) e ut uola;a1soy{>
aql Jo uorlezrue6roel prdeg (B) 'ulalord
lleurs e ulo.U uolleuto1 lueuelu (V)
'uo1a;a1so/r aql adeqsa.r ol ,(;qr.uassesrp pue,{lqr,uasse tuaulelU pale;nbal
sMolle slrunqns uralojd ralleus qrnur uror] sluauelU uralold Jo uolleulloJ
aq1 'edeqs
;;a> u1 se6ueqr 6u;rnp uola;elsoyb eq1 4-9 1 arn6t1
-ruasspar pue elrs euo le slueuBIIJ Suqqruasseslp 'suollezlue8roar IeJnlonJls
'enoru
ol 'sasec duetu uI 'pup aJnlJnJls IEuJeluI
alqFeg 1nq pazrue8ro l.1q3rq e urelupru ol IIeo cportrecna e elqeue sutalord
frossacce eq] TaqtaSot Surlcv'a1c[c 1ac qrea Suunp rnoco teql uo1a1a>1so1dc
eql Jo suorleruJoJsupJl oIlBuEJp aql raSSrrl luql esoql Supnlrur 'spuBts repl
-leceJlur pue JelnllaJeJtxa Jo IoJluoJ aq] Japun aJntJnJls p1a1a1so1dc Suuq sutal
-ord rfuossacce aq] 'sassecord aseql q 'aueJqruaru eursuld eql pue sallaue8ro
se qsns saJnlJnJls IIac raqlo ol ro reqloue auo ol sluauPlrJ {uI 01 puP 'eJroJ
alereua8 ol ,{3raua sseuJeq o1 'l.lqurassesrp pue dlqruasse tueruelg Jo sorleuDl
aq] a8ueqc ol 'sruJoJ
llunqns pue luatuelrJ ueemleq suralord raurdlod 3o 8utuo1]
-4red aq] aleln8ar o1 'sluaurelg ^aau Jo ,{lqruasse Jo sells eql aulruJa}ap o} sllun
-qns Jleql Jo sluaruplrJ eql ol pulq sutalord,{rossacce eseqJ'uol}Je p1a1a1so1,b
olur s.{.ervrqled Surpu8rs q8norqt pallacer uol}eruroJul Surl.rarruoc 's}uelu
-elrJ aqt Jo ror^Bqeq crueu.{p aqt pup uopnqlJ}slp pqeds aqt atuln8ar suratord
zfuossacce pelprJosse-uo1a1a1so$.c lueJeJJrp Jo speJpunq 'llao eql ulqll111
'ue{oJq
Jo peuroJ bureq spuoq
lueluloc lnoqlr^ Llprder Jncoo uec .{lqurassesrp pue ,ri.lqruassp Jleql dpuanb
-asuo] ',,sraruz(1od,, p1a1a1so$c go sad,fi eeJql eql raqlaSol ploq suollsPrelul
luelp^oouou {ea^a 'lseJ}uoc uI 'sula}ord pue 'VNU 'VNC Sutpnlcut-sraurdlod
pcrSolorq Lueu raqtaSor ploq strunqns rraq] ueemleq sa8eluu tualplo3
'luatuelrJ
Jo addl qcea
go sa4radord pcrueqJaru pue ,bnqels aql ul se3ueJeJJlp IEJIIIJo acnpord ruaql
ueamlaq saJroJ elrlcprpe aql 3o sq13ua4s eql pue sllunqns eql Jo salnlonJls
aql ur seoueJaJJrO
'slceluoc
ulalord apIS-ol-epls pue pua-ol-pua Jo uolleulqruoJ
e Sursn 'alerf,osse-Jles
lpq] (gz-t arn8t4 eas) sllunqns Jo sellquasse IDJIIeq sP
ruroJ sluetrregl p1a1a>1so{c go sadft eeJql IIV 'salnqnlorolru toJvrunqns unnqnl
'sluaruelrJ
ullou JoJ sryunqns U4la-ll-lnqo13 pue lcedruoc eJe ]eql sllunqns
Jo aperu ere salnqnloJcru pue sluaruelu ullce seaJaq ^ 'snoJqIJ pue pale8uola
sallesueql eJE lpql slrunqns ra[eurs Jo dn apeur eJE slueuellJ elEIpaIuJa]uI
'14-gy
arnElg) ferte JEJ alrs raq]oue 1e ruaql 3u11q
se!uedotd snoo6elue^pv
e^eH stuorueluolold eldlrlnw uol, paulol sluaulel!l
Ierales ur spuoq leurpnlr8uol Jo sles Suqearq sarrnbar a1ppgr eql ut luerup
-g alrsodruoc eqlJo a8e4earq 'lseJluoJ ul 'spuoq IeJelBIJo sles oml ro auo reqlle
pup spuoq purpnlr8uol Jo les euo $lpaJq Jo se>leru lueruelrJotoJd auo ;o pua aqt
]B lrunqns E Jo ssol ro uourppe eqJ 'e3pl?l lPslleq P ur reqtoue euo punoJE ]slMl
stueruelrJolord aql l.1pcrddl ,(1era1u1 reqloup euo qllm alerJosse ]Eql-pue ol
pua paurof slrunqns Jo s8uuls ruau11 3uo1-slueurulgolo.rd aldrlpur Jo lno llnq
are.{aqt esneceq,ftqrqeldepe ql!v\ q}3ueJ}s eurqruoJ srarur(1od p1a1a>1so1,(3
'aJnlcnJls
InJesn ssel pue Jrlels B uolalalsot tc aqt
Suqeu 'ayqurassesrp plnoo slueuelrJ eq] qrq.tr te elpJ aqt lgrll plnom Surpurq
tq8p qcns 'sroqq8rau o^at sU ol .,(FqBp .{laruarga punoq sp^ lueupl.IJ eql ur
lpnqns qJEa sselun 'd8raua purJaq] luerqup ,(q a8aparq plo^e o] qlSuarls aql
>1ce1 ppo,,u. de^\ sq] q peruJoJ lueuElrJ B ]nq 'lla3 crlol.recna ilulus e Jo JeleuPrp
aql uuds plnom'pue ol pua dn pauq 'alduruxe JoJ 'sJeurouou u{nqn} puesnoql
y'a1g lq8re4s apurs e ur raqtaSol slrunqns Sut8uuls,{ldurrs dq t[nq tou erp sllac
SurnrT ur slueuelrJ plalalsogr ]snqor aq] 'JaAe^ oH 'purq UEO llunqns reqloue
lad qcrq,u o] pua ,vrau E saleaJc raudlod eql Jo pue eql o] lrunqns qcpe Jo uo1}
-lppe aqJ '1 + zr qfual Jo luaurelu e aleraua8 ol slrunqns u sureluoc lpqt ]uaru
-€lrJ p
Jo pue eql 01 spurq lpnqns aa4 V 'uouceeJ uorleroosse aldruts E se ]ueu
-elrJ E ruroJ o1 raqlaSol slrunqns uralord;o Suuul eql 1\^aI^ ue3 el 'praua8 u1
slr u nqns
alqnlos lleurs
(8)
+
raLuAlod
snoluaurelr; a6rel
a)i nos
lUarJln u
e se q)ns
';eu
6 rs
sltNnSns Jo
NOtSnTJC CtdVU
cNV SINil TV]H
ro A'rStAlssvsro
ll_ts A tN v
IV Sl_NltAV]H
ro 'rsWSSSVIU
LL6

uola;a4soy{3 aql:9l. raldeq3 ZL6
'sluetu
'/ l \r-
r'1€vlsNn A]lvhrulHr.lN3wv'lBoloud 319Nrs
3'18VtS A'l'lVtAUlHl :SlNltAV'lHOt_OUd l'ldt-L'tnW
IeruJaql lsrseJ ol slueuelrJ p1a1a1so1b lsoru s^ olp sassecord oml eseql ueemleq
acuaraJJrp ,{8raua a8rel aql'19-91 arn8gg) aurp erues aq} }€ IIe slueuegolord
IEf,rIaq-D uaamleq slJeluoJ IBJaIBI Suorls Sururo; Lq alqruasse 'alduexa .rog
'slueruElu
elBrperuJalul
'sluatuelu p1ala>1so1-ri: luereJJlp eq] roJ reJJIp slcPluor
trunqns-lrunqns Jo sadfi pue suorterol aqJ '(?-t arn8rg aas) raqlaSol tuetue
-lrJ
lEtela{solr{c e ur slrunqns aq} ploq spuoq }uale^oJuou {Ee^\ puE suorlJeJa}ul
crqoqdorpLq fueru 'suo1]oeJatur ulalord-u1a1ord cgtcads Jaq]o q]Irlt sV
^lproeJ Jncco uE3 slrunqns Jo ssol puE uorl
-lpp€ qJIq \ t€ sernlf,nrts cnueuf.p se spue ]ueuelrJ aqt Surneal apqa,t 'a8e4earq
-qns snorqrJ pale8uola qceeJo er8ual aqlJo lso{u ralo puetxe qJIqA\'s1toc paltoc
-elrJ elerpeuJalur op uBql luaq are ,{aq} uaqa,r,(lsea eJolu qcnu {PeJq sepqnl
-orJru 'uosEeJ srql JoC 2ea^\ ,{1ang}EJedruo3 ale JeqleSol sluetuBllJoloJd tI
aqt Burploq spuoq Ierelel aq] pup 'spuoq purpnlr8uol [q,{lrerurrd raqtaSot pyaq
slrunqns repqop luo4 llrnq erE 'tseJluoc.{q 'salnqntorsll tr
'(6-9I arntlg) sarnl
-JnJls alrl-ador Suo4s Sunuro; 'Surpuaq pue SulqJlaJls al€Jelol stueruellJ ale
-rpeuJalur 'luaruelrJ
aql ur paraSSets are slrunqns 1enpl^lpul aq] asnecag ']Un
6uo1 para66e1s
lerelel :slrunqns
slU_uldoud
t)t-l-ldou
aleururop s1reluo)
" tr.
pal ut uMoqs ele sluaLuelu aql
ur raqlabol slrunqns aqr 6urp;oq spuoq
aqf 'sluaueluololo a^u ulotj peulloJ sl
luauelU alqets aqt'a;durexa le)ltaqlodA.l
srql u1'a6e1earq leullaqi ol luelstsal ale
sa^lasLuaqt siuaurelu aql altq/\^'r[ueu,{p
aq ol spua aLll sMolle luaureluolold
auo ueql aloul LxolJ ]uauelu
;e1e1a1sol,{> eJo uottPulol .sPua
rrueu{p ql!tvr s}uaurelll 1e1e1e1so1fr
1o (1r;rqe1s
leureql ar,ll g-9 [ arn6rl
'luaq
^lrsPa ale ^aql qOnoqlle
/sa)roJ 6urq)lorls ol tuelstsal (;;er:adsa
,{;}uanbasuor are pue (em s;ql ur pauro}
are sluaurelu alprpaulelu l'sl)eluo) leJelel
6uor1s qlrrvr sl!unqns snorq!J pale6uo;a
rror; paruro, tuauellr 6uor1s y 5-91 ernbrl

THE SELF-ASSEMBLY AND DYNAMIC STRUCTURE OF CYTOSKELETAL FILAMENTS 973
c
o
E
o
c
c
l
! f
c
o
(A)
n ucleation
(lag phase)
elongation
(growth phase)
steady state
(equilibrium phase)
time after salt addition +
Nucleation ls the Rate-Limiting Step in the Formation of a
Cytoskeletal Polymer
c
o
E
o
c
c
f
.o f
c
o
0
elongation
(growth phase)
There is an important additional consequence of the multiple-protofilament
organization of cltoskeletal polymers. Short oligomers composed of a few subunits
can assemble spontaneously, but they are unstable and disassemble readily
because each monomer is bonded only to a few other monomers. For a new
large filament to form, subunits must assemble into an initial aggregate, or
nucleus, that is stabilized by many subunit-subunit contacts and can then elongate
rapidly by addition of more subunits. The initial process of nucleus assembly
is called filament nucleation, and it can take quite a long time, depending on
how many subunits must come together to form the nucleus.
The instability of smaller aggregates creates a kinetic barrier to nucleation,
which is easily observed in a solution of pure actin or tubulin-the subunits of
actin filaments and microtubules, respectively. When polymerization is initiated
in a test tube containing a solution of pure individual subunits (by raising the
temperature or raising the salt concentration), there is an initial lag phase, during
which no filaments are observed. During this lag phase, however, a few of the
small unstable aggregates succeed in making the transition to the more stable
filament form, so that the lag phase is followed by a phase of rapid filament elongation,
during which subunits add quickly onto the ends of the nucleated filaments
(Figure f 6-f 0A). Finally, the system approaches a steady state at which
the rate of addition of new subunits to the filament ends exactly balances the
rate of subunit dissociation from the ends. The concentration of free subunits
left in solution at this point is called the critical concentration, C.. As explained
in Panel f6-2 (pp. 978-979), the value ofthe critical concentration is equal to the
rate constant for subunit loss divided by the rate constant for subunit additionthat
is, Cc = koff I kon.
The lag phase in filament growth is eliminated if preexisting seeds (such as
filament fragments that have been chemically cross-linked) are added to the
solution at the beginning of the polymerizalion reaction (Figure l6-108). The
cell takes great advantage of this nucleation requirement: it uses special proteins
to catalyze filament nucleation at specific sites, thereby determining the location
at which new cytoskeletal filaments are assembled. Indeed, the regulation
of filament nucleation is a primary way for cells to control their shape and their
movement.
The Tubulin and Actin Subunits Assemble Head-to-Tail to Create
Polar Filaments
Microtubules are formed from protein subunits of tubulin. The tubulin subunit
is itself a heterodimer formed from two closely related globular proteins called
ct-tubulin and B-tubulin, tigll'tly bound together by noncovalent bonds (Figure
(B)
steady state
(equilibrium phase)
preformed filament seeds added here
coming on and off
time after salt addition +
Figure 16-1 0 The time course of actin
polymerization in a test tube.
(A) Polymerization is begun by raising the
salt concentration in a solution of pure
actin subunits. (B) Polymerization is
begun in the same way, but with
preformed fragments of actin filaments
present to act as nuclei for filament
growth. As indicated, the 0/o free subunits
reflects the critical concentration (Cc), the
point at which there is no net change in
porymer.

974 Chapter 1 6: The Cytoskeleton
(A)
P-tubulin
d-tubulin
R
f,,uo tubulin heterodimer
(= microtubule subunit)
protofilament
prus
eno
mtnus
eno
1
5U nm
I
f 6-l f ). These two tubulin proteins are found only in this heterodimer. Each a or
B monomer has a binding site for one molecule of GTP The GTP that is bound
to the cr-tubulin monomer is physically trapped at the dimer interface and is
never hydrolyzed or exchanged; it can therefore be considered to be an integral
part of the tubulin heterodimer structure. The nucleotide on the p-tubulin, in
contrast, may be in either the GTP or the GDP form, and it is exchangeable. As
we shall see, the hydrolysis of GTP at this site to produce GDP has an important
effect on microtubule dynamics.
A microtubule is a hollow cylindrical structure built from l3 parallel protofilaments,
each composed of alternating s-tubulin and B-tubulin molecules.
\Mhen the tubulin heterodimers assemble to form the hollow cylindrical microtubule,
they generate two new types of protein-protein contacts. Along the longitudinal
axis of the microtubule, the "top" of one B-tubulin molecule forms an
interface with the "bottom" of the a-tubulin molecule in the adjacent heterodimer.
This interface is very similar to the interface holding the a and B
monomers together in the dimer subunit, and the binding energy is strong. perpendicular
to these interactions, neighboring protofilaments form lateral contacts.
In this dimension, the main lateral contacts are between monomers of the
same type (cr-cx and 0-0). Togetnel the longitudinal and lateral contacts are
repeated in the regular helical lattice of the microtubule. Because multiple contacts
within the lattice hold most of the subunits in a microtubule in place, the
addition and loss of subunits occurs almost exclusively at the microtubule ends
(see Figure 16-8). These multiple contacts among subunits make microtubules
stiff and difficult to bend. The stiffness of a filament can be characterized by its
persistence length, a property of the filament describing how long it must be
before random thermal fluctuations are likely to cause it to bend. The persistence
length of a microtubule is several millimeters, making microtubules the
stiffest and straightest structural elements found in most animal cells.
The subunits in each protofilament in a microtubule all point in the same
direction, and the protofilaments themselves are aligned in parallel (in Figure
16-11, for example, the cr-tubulin is dornm and the B-tubulin up in each hetero-
microtubule
Figure 1 6-1 1 The structure of a
microtubule and its subunit. (A) The
subunit of each protofilament is a tubulin
heterodimer, formed from a very tightly
linked pair of cr- and P-tubulin monomers.
The GTP molecule in the o-tubulin
monomer is so tightly bound that it can
be considered an integral part ofthe
protein. The GTP molecule in the B-tubulin
monomer, however, is less tightly bound
and has an important role in filament
dynamics. Both nucleotides are shown in
red. (B) One tubulin subunit
(cr-p heterodimer) and one protofilament
are shown schematically. Each
protofilament consists of many adjacent
subunits with the same orientation.
(C) The microtubule is a stiff hollow tube
formed from 13 protofilaments aligned in
parallel. (D) A short segment of a
microtubule viewed in an electron
microscope. (E) Electron micrograph of a
cross section of a microtubule showing a
ring of 13 distinct protofilaments.
(D, courtesy of Richard Wade; E, courtesy
of Richard Linck.)

ll8l llssv-llls 1Hl-
(v)
(luaulel!, u!
uaq/v\ dcv)
dfv
sINlwvlH lvllll)sou) lo lunl)nuls )lwvNAC cNV
pue snlo
PUA SnUrUr
'lueluellJ
ullcE IenpIAIpuI uE ueql JeSuoJls qsnlu seJnlcnJls
upJp elpJs-eBre1 esaql Suqeru 'raqtaSol ueq] alpunq pue luqssorJ (lrtolaq eas)
suralord ,trossacce '1ac Suurg € uI ]ng 'sJe]sIuoJOIlu Jo sue] ana; e dpo 3o t{}3ua1
aouelsrsJad B qrl \ 'salnqnloJJltu pcrrpulrtc ^ olloq eql qlIA\ pareduroc luaq
,i.1sea pue alqFeg '(lazrqelar ere stueruelg ultov'4laq papueq-rq31J e uI reqlo
qcea punoJe ]suvq leql slueuellJolord lalprud orvrl Jo lslsuoc ol peJeplsuoo eq
uBf, lueIuEIIJ ullJe aql firrelod leJnlcnJls ]JuIlsIp E qllm slueruelu aleraua8 ol
Irel-ol-peeq elquesse sllunqns uIl3E aq]'uqnqnl roJ sV'(ZI{1 arntlg) (dCC io)
dlg uBqt JeqtBI (dCVro) dJV sI eplloelsnu aql uDce JoJ ]nq 'epuoelcnu P JoJ alls
Surpurq E spq lpnqns ullJe qJee (ullnqnl e{I'I 'Jarulp e ueql Jeq}EJ Jeluouo(u
e snq] sr pue urcqc apqdaddlod re1nqo13 aputs e sI lrunqns ul]f,e aqJ
'pue
Jeqlo aqt te pasodxa surpqnl-fl pue pue auo le pasodxa suqnqnl-n
qlp\ ,quelod prnlcnrls ]Jullslp B seq JIeslI eFqnlorf,Ilu aql 'eJoJareq; '(rarup
seleu luereJJlo le /v\or9
leql spul l)u!ls!c o^ I e^eH sluaulelll ull)v pue selnqnlol)lw
d8raua eer; aq1 'srsfloJp^q dIC Jo dIVJo acuesqe eq} uI 'slpnqns I + u Jo }uau
-EIrJ e ur sllnseJ slrunqns il Jo ]ueluPlg P Jo pue reqlle ol lFnqns E Jo uoplppv
'saler
q1,r,ror8 lueuelrJ uo ]JeJJa puno;ord E eABq leql s,(E,vr ul ]uaJeJJIp raurf1od
qcea Jo spua oml aql seTetu uolleluelro srql Lhrrelod leJnlsnJls sapqruoJf,Iu
pue slueruelu ullce sanr8 slrunqns Jleql Jo uoneluelro lalered Sep8ar aql
'pua
snullu aql pellBc sI pue Jaqlo eql
pue 'pua snld aql pelleJ sI 'lsuJ aJE a3e>1uuqs pue qtMoJS qloq ereqM']uaruelJ
e Jo spue o^N eql Jo sIIuBu,{p eJolu aqJ '}selseJ sezlJsru^lodep oslB pua SuIMorS
-lseJ eql 'uollsJluaouoJ IBOIIIJJ aq] Moleq sdoJp uol]eJlueJuof, llunqns eeq eql
lPql os pefnlp dprder arB stuauleluJI'(tI-gI ernBIC) roqlo eql uEql ralsPJ qf,nlu
selp8uole luelu8uJJ qcEe Jo pua euo 'slueIuBIIJ peluJoJeJd Jo s}ueu8e4 pe{Ielu
oluo elquesse o] pe^ ollP sr sllunqns paulJnd Jo ssasxe ue JI '.snql 'Jeqlo eqt lE
ueql pue euo ]e JaleaJB qcnu uauo eJe-^lelllf,ad."t
'rrory pue u9l-uopelcosstp
puP uorlPrsossE JoJ sluelsuos alEJ lllaupl aql 'luauEIIJ JPIod [lleJn]JnJls B uI
'pue qJPe lE lueJeJJIp rfua,r aq z{eru s}uels
-uoc alEJ asaql Jo sanle^ alnlosqe aqr q8noql uele (spua oml aql lE lecnuapl
eq ]snu gJo4
luoq 'slueJsuoJ eleJ pJE^DIf,Eq puB pJEl!\loJ eql Jo olleJ eq] 'asPf, slq]
u1 'raur[1od eql Jo pua raq]Ie ]e s]runqns Jo uolllppe roJ eruBs eql eq lsnur '(uop
-eJluaJuoJ
lecrlrJc eql pue) ]uelsuoJ runuqiltnba eql eJoJeJeql pue 'eouaJeJJlp
(8)
aln)alour urpe
pua snlo
pua snu!ur
())
uru 0s
sL6
[6rer1 ra6og
1o,(salrnor'))'sluaLuelu url)e paurels
Ilanrle6au Jo sqder6orrrr.u uort)all ())
'uolleluauo
euies aql a^eq luaLuelu
aq] ulqtlM slrunqns eqr llv'ruu 19,(.rana
6ulleadel lsrMl e qtrM'xrlaq e Jo spueils
lallereo o/!{l se raqlo q)ea punore purM
q)rLlM'st)eluo) lerarel ,(q raqrabor plaq
'sluaue;ryolord orvrl Jo 6urlsrsuo) se Jo
lq6noql oq osle ue) ll 'siaurouour
Jo xrlaq
a;6urs e se paqu)sap uauo sr luauelr,
aqt qbnoqlly'luauielu e ur srououou.l jo
luaua6uerry (B) 'alnraloLu
aql,o lalua)
aql ul ual) daap e ur punoq (dcv lo dlv
raLll!a) aplloal)nu e seq raruouou] urt)e
aqf (v)'luauielu url)P puP Jaurouour
u!l)e ue ,o sarnl)nJls oql Z [-9 [ oJn6rJ

uola;a1soy(1 srll_:g I reldeq] 916
1 {srrog (re9,ro Isalr nof )'qder6ortrr.l
srql ug dol aql lp pua eqr'alpunq alnqnlot)Lll aqlJo pua sn;d aql
urorJ ]salspJ nnor6 sa;nqnlol)tW 'suotltpuo> 6ulzuer-uI;od rapun sltunqns
urlnqnl qlrM aLutl Uoqs e JoJ palpqn)ut seM (ralpl passn)stp) urntlt) p
Jo
aro) aq] urorJ pautelqo salnqnlot)il.I] Jo olpunq alqpls e ,ase) stql ul laLllo
aql le ueql pua auo le lalseJ moi6 sluer.ue;l; ut])e pue salnqnlollul qlog
'pue
sn;d aql tp solnqntortru 1o qynolb lpllusJalald aq1 g 1-9 1 arnbrg
('luauelrJ aql Suop punoq
uaq^ speaq urso,{ru go acuereadde peaqarrorre aql Jo asnef,eq ,spua ,,pa1urod,,
se spua snurru pue 'spua ,,paqreq,, sE ol parreJer ,{lensn JJE s}ueuel.rJ urlce
Jo spua snld aql 'suosear IuJrJolsrq JoC) 'pua snunu eq] pJpmol slurod Jaruouour
aqt uo galc Surpurq-dlv aq] 'slueuelrJ ul]Je uO 'pua snld aqt te pasodxa aJE
slrunqns fl pue 'pua snurur aql ]e pasodxa are slrunqns n ,selnqnlorJrur uO
ea4 slqt uaqa,r llsnoaueluods spaacord uorlezrraru,{1odap }uaruplrJ ,asu a{r.I
'uorleJluaJuoc
IeJrlrJJ aql spaeJxa uorlnlos ur slrunqns Jo uorleJluaJuoJ eql
ueq ^ aseJ aql sl srqJ 'oJaz ueq] ssal sr lrunqns elqnlos aq] Jo uoUrppB roJ (gV)
aBueqo d8raua ae4 aq] uaqm,{lsnoaue}uods spaacord uorle8uola tuauelrC
ol uorlezrraru,{1odap ro uor}ezrraudlod luarueg snoaueluods Surrnp peseelaJ
d8raua aa4 asn ueJ IIac aql 'snql lsassacord snoaueluods asar{} ol ssacord alqe
-roneJun ,{lpcqa8raua ue aldnoc uet 'orez
IIaf, V ueql ralear8 sr a8uuqc .{3raua
'JalEI ssnJsrp a^ sE 'slleJ
alrloru
go a8pa Surpeal aql apnrlord dlaq sluaurepy u4ce Suqe8uola ,{lrepurg 'aseqdeue
Surrnp sralsrs rraql ruor; de^ap seurosoruorqJ Jrlolnu pd dlaq ueJ selnqnloJJrru
Suqurrqs pue 'seueJqruaru lno qsnd dlaq uec sepqnlorcnu Suqe8uola ,a1d
-ruexa JoC 'peol peqJeue ue 1nd to qsnd o1 tepcrged ur-{Jo^ pcrueqJaru op
ull)v pue utlnqnl{q srs{;orp/(H ept}oal)nN Jo
saf uenbasuo) erv fil;lqelsu; l!ueu^C pue 6urllrutpeatl luaueltl
tpql .{la{II sI tr uaqt ,{lprder Suu,ror8 sr }uauEIrJ eq}JI sr rer{r 'q8rq sr uor}rppp }run
-qns
Jo eler aq] 'uorlrppe
JI ]1unqns Jo atpr aqt qllm paredruoc srsdlorp,{q srq} Jo
alpr eqt uo spuadap uroJ C eqt ro I aqt ur sr turuelrJ e;o pua.{ran aqt te tgnqns
aql raqIJqM 'epuoalJnu punoq rlaqt paz,{1orp,riq aneq o1 are ,{aqt ,{1a111 a.ror-u
aq] 'alrllel raru.r{1od aqt ur ueaq a^Pq strunqns }pql atup aq} raSuol eqJ aoc
pup dCVJo ]Eq] ueql raq8rq ploJ-ua] lnoqe sr dJC pup dJV qloqJo uorterluaJ
-uoJ aa4 aq] se 'uJoJ J aql ur aJE slrunqns ae4 eq] Jo lsotu 's11ac 3unr1 u1
.,r,ror8 sraur.,(1od ruro;-; a1q^a {urJqs aloJalaq} ilr^
sraru.,{1od ruJoJ-C 'strunqns aa4Jo suolterluaJuoJ ureual JoC
'(J)rJ ueq} ratuar8
sl (C)'J 'snq; 'raru.{1od uro;-1 aq} roJ oper Supuodsarroc aql ueqt ra8rel
sl 't(O)'Jl uolteJluaouoJ Iecrlrrc s1r o1 pnba ,r(lpcrraunu sr qcrqm 'raudlod ruro;
-C aql JoJ 'ory
/ rory Jo orler eqt -{ltuanbasuo3 'raru,(1od
urroy-1 eqt ruo4 }lunqns
e Jo uorlerJossrp roJ a8ueqc .{8raua aa4 aq} ueq} anr}e8au aJoru raru.,(1od uJoJ-O
aqt tuo4 llunqns e Jo uortprcossrp roJ a8ueqc .{3;aua aa4 aqt sa{Bru srqJ 'acp}el
raur.,(1od aqt ur parols sr puoq aleqdsoqd-aleqdsoqd d8raua-q8rq aq] Jo e8e^eelc
,{.q pasualar d8raua aa4 aq} Jo qlnu 'pazd1orp,4q sr aprloalJnu eqt uaql\
'(urFqn]
roJ
dcc 'urlre roJ dcIV) punoq ,,luroJ c,, aql qlylr euo pue '(uInqnl roJ dJc 'ultrE roJ
dJV) punoq eprtoalcnu aql Jo ,,ruroJ J,, eql qtl^\ auo ']sua uef, sernttnrls lueru
-EIIJ
Jo sad,ft luarag;lp omt 'snqJ ('ZI-91 arn8rg aes-trunqns oqt Jo raluar aql
reeu Ualo e ur daap sr eprloalJnu aql 'urlJe ur sEeJaqM '11-g1 arn8rg aas-slrun
-qns Surroqq8rau o,r,rl uaamlaq aceJrelur aq] te sarl alrs Surpurq-epqoalJnu aql
'ullnqnl uO) 'eJntJnJls
luauelrJ aql ur paddeJl surcurer aluqdsoqdrp aprsoalJnu
aql lnq 'llunqns qJea uorJ peseelal sr dnor8 aleqdsoqd eaq aql lsrncco srsf
-1orp,{q eprloalJnu 'tueruelrJ e olur trunqns uqnqnt Jo urlJe ue Jo uoueJodrorur
rage.{llroqg 'slueruelrJ olur palerodrof,ur aJE s}runqns aq} ueqm pe}pJaleJte sr ll
'ra,ra,r,roq :,r{1alo1s t(ra,n spaarord srs.{1orp,r(q slql 's]unqns aa4 aq} rog dlanrlcadsar
AJC ro 41y 'aleqdsoqdrrl aprsoalJnu e ;o srs[1orp,{q aqt az[1e1et uEJ teqt
saru,{zua qloq aJE s}runqns uqnqnl pue urlce aq} 'sraru.{1od
lualenocuou uroJ
ol dllflqe rraq] o] uourppe ul 'sapqnlorcrur pup s]uauelrJ ur]ce qloq o1 sarldde
teql IJEJ IeJrlrJJ e parou8r seq scruuu,{p tuaruelrJ Jo uorssnJsrp Jno 'rEJ snqJ
salnqnlol)t rx paujlo+ ^l^^au
rxr I

slNlwvlll lvllll)soIA) lo lunl)nuls )lt^vN^c ONV
AISWlSSV-llls
lHl
srour^lod
(v)
eJntIC) selnqntoroltu lBnpl^ryul roJ slla3 a^ll q pe^Jasqo aq uPf, Sugprupearl
a{ll s{ool }eq} uoueuouaqd P puP (uIl3B roloqln ?l? pa^resqo ueeq seq slueluellJ
a18urs;o Sulpupearl eq] 'uleUecun sI IIeo eq] apISuI Suqprupea4 Jo lualxe eq]
aln11,1:srs.,i.1oip.,tq aleqdsoqdr4 aprsoalsnu Jo {rrroJ aq} ut -{8raua;o uopdunsuoJ
]uplsuoJ B seJrnbal ,,8urruupearl alels-l.peats,, slqJ'peSueqJun suEIuaJ lueluP
-llJ eql Jo rp8ual lPlol aql ellq^a 'salels snolueluellJ puu ee4 eql ueeMleq '{lprder
alcrtc slrunqns eql '^\oN 'pue snullu eql le a3E{uuqs luelu€llJ aql sasuElPq
,(1]cexa pue snld aq] le q] ^or8 luaurug eq] 'uolleJluecuoJ llunqns alElpauJelul
repcqrud e lV'elqlssod SurqnupeaJl sa)Pu,rtqaraql pue eFqnloJOIIu ro ]uaru
-EIrJ urlce eq] Jo spua snullu pue snld aq] le suollJeeJ uollelcosslp/uollelsosse
aqilo,{8laua aarg aqt uI eouareJJlp aq} o} aslr sa,tr8 dea,t aqt Suop sJncso teql stsd
-lorpdq
dIC ro dIV eql'turoJ C aql ut pua snuru eql ruoq peqs pue urroJ I eq]
,rt rjrrXloa Jo
"qt pua snld aqt tu petrnr3al arp strunqns '3uq*upearl Surrnq
'(Z-gt
Iaued pue ?I-9I arn8lg) Bullprpuarl luarueg;;o dlra
-dord alqu4reual eql o] speal sIqJ 'pue snulru aql ruoq sllunqns sasol dlsnoauel
-lnurs pue ,pue snld aqt ]e strunqns sppP lueIxBIIJ eql-(pue snulul eql 'sl ]uql)
turoJ G aqlJo uorlEJtuaJuoJ IecllIJJ aql ueql Ja^ ol ]nq'(pua snld eql'sI lBq])
urJoJ J aql Jo uorleJluaJuoJ IuJrlrJJ aql uPql raqSrq-a8uuJ elulpaluJelul ue
ur sr uounlos uI sllunqns aa4 Jo uoneJlueJuoc eql JI '(Alquesse pre^\ol AIIpBaJ
eJolu supel uJoJ J eq] allqm .{lqruassestp pJe^ o}.{1puar eJoru sueel IUJoJ C eql
,spJo^\ Jeqto uI)
'tuJoJ
J aql ueql uolleJluaJuoJ IeJIIIJO raq8q € seq uJoJ C eI{}
,paureldxa
lsnf sy .uortutuJoJuot c eql sldope pua snulul aql elq ^ 'uolletuJoJ
-uoJ
J aql q sulpureJ lua1llelg aql Jo pua snld aqt 'asEl slq] uI 'pue snulu aql
1e srsr(1orp,{.q epuoalsnu ueq} re^/Iols lnq 'pua snld aql 1e srs'r{lorpdq eppoelf,nu
ueql JalseJ aq 01 uopppe llunqns Jo alEJ aql JoJ alqlssod aloJeJaql sl
'sllunqns
1I
ae4 Jo uorleJluecuof, elelpalurelul uu tV '(Z-9I leuPd aas) spue o^\l aql uaao\leq
asuareJJrp IernlJnJls p Jo asnPJaq pue snulu aql JoJ ueql luaruBlr; u;o pua snld
aql JoJ JelsBJ qf,nlu sl "oZ aqJ ''!
]uelsuoc eleJ aql pue uolleJluef,uoJ llunqns
aaJJ eql Jo lrnpord aq} sl lueluellJ e Jo pua aql ]e uoFIppE }Iunqns Jo e}Er eqJ
ruroJ c aql uI aq ueql ilI ^ ]ueIuEIIJ
aqf 1o dp eql puE 'pappB sI ]Iunqns lxeu eql aroJaq Jncco ^EIu sls'{lorpdq '/\,\ol sI
uoDlppe lpnqns Jo aleJ aql JI tanarvroll 'doc 419 rc dac dJV ue Suturo; 'urro; I
eql ur sureuar raudlod eqtJo dll aql leql os 'pazl.1orp,'i.q uaeq seq llunqns peppe
^,{.lsnonard aqt ul appoalcnu aq} eroJaq raru,{1od eq} o} uo pp€ IIIM llunqns lreu e
'(9r-9r
( C) ullo+ C pue (l) turolllo arnlxlu e a.re
( I ) ullo+ f ul ale sllunqns elqnlos
dn saqrle> srsf ;o.tp[q
-rltols sl uolllppe pue-snuru
a rl
+
(a)5 > (I))
sls^l0uo H
clMo]'lol N
TT
-_r
purqeq s6e1 srsr{lorp{q
-lsel sl uorlrppe pua-sn1d
a6uel 6ut1lttlPear1
uorlerlue)uo)
lr unq ns
srn)ro 6urlILupeaJl (g)
(a)5 >f > (f)) rol
(ssar6 Arsranlun rallaJa))oU aqf uot, uolsstt-uled qllM'/66 t 'vEt-LLtt6El
'lo!8
lP) T'uoules 'C'l pue lalols-ueulraleM'W') Luoll)
'I1t1tqe1sut
rrr"ueu{p s{e|ds1p osle pue snld aq1'sluuqs (ryal aq} uo) pua snulur aq} pue
snofi (lq6p aqt uo) pua sn;d aql elrqM'(peaqMoile pd aqlAq pate)lpul
allltel alnqntol)tur aqt ulqllm )reLupue; e i(q umoqs se) ^reuollels suleural
arlllel alnqnlor)ru aql '])ej ul ']nq 'U6!t ot Ualuol; 6urpr;s aq o1 sleadde
uMoqs alnqnlorllu aql'ejaule) )luoll)ala an lllsuas e qllM pa^losqo uaql
seM salnqnlol)lu lenpl^lpul Jo o)ue)salonlJ aqf'lua)salonu seM 0z ul
l!unqns urlnqnt I I;aler.urxotdde ]eqt os 'au!ulepoql a{p lualsarong aql o}
palur; ,{1}ua;eno) uoaq pPq leqr urlnqnl q}!M pa1>efur seM lla) y'11ar 6u;n;1
p u! pa^Josqo se 'alnqntol)llu e Jo ,o!^eqaq 6url1rutpear1 91-91 ern6t3
'6ur;;rupearl ut 6ut1;nset'slurlqs
pua snurur aLll olrqM srrnot6 pua sn;d aql
'sanleA oMl asaql uaaMlaq araqMatuos
s! uorlerlualuo) lrunqns lenl)e aql
,l r.uroj c aql u! pue luauelu e lo] ueq]
raMol sr uilo, I aLll ur pua luauelu e uo
uorlezuar.u{1od roJ uorlerlua)uo) lef r}tl)
aql'strunqns aaJJ Jo suollerluo)uo)
alerpau,ualur le sJn)lo 6ut;1tupeatl (g)
'turo,
c aLll ul aJe pue slql
lp slrunqns leurural os pue'pua snulLu
aql le uoltebuola ueql ratsej st sts{;olp{q
'ra^e^ oH
'rlroJ
I aql ur s^eMle orojalaql
are pua s!ql le sl!unqns leululol aql pue
'a;dr,uexa s!ql u! pua sn;d aqr re s1s{lotpIq
uelll ralsej sr uotlebuo;a'srrno.r6 1uaue1t1
aql sV'a)ruel luaueltj aqr u1 srs{lotplq
aprloal)nu obrapun uaqt pue'luauJelU
6urniror6 p
Jo spua qloq le azuaur{;od
(sl!unqns uuro;-1) aleqdsoqdtll
eptsoal)nu punoq qllM strunqns'spua
snuru pue snld aql te ()) suotlellua)uo)
le)rlrr) luaroJJrp oql loJ uorleueldxl (V)
'uotttppe
l!unqns s/'^ollo, leql s;s{;orp^q
aleqdsoqdrrl aplsoal)nu aql Iq olqlssod
apeu 'alnqnlor)lur ro luauellJ u!l)P
ue 1o 6ur1;rupeerl aql 71-91 a.rn6r1
)as 0 aur]
uld OI
LL6

S]IVU IJO CNV S].LVU NO
'(1_>as
1o slrun) #o) {q uanr6 sr
ssol 1|'o alel aLlI't_)os 1_u\| jo sllun seq q)tqIV\
''o)
]uelsuol aler oq] Iq uanrb st (slaulouoLu
palle)) sltunqns asoql +o uotltppe +o olej
eq1 'teur{;od aq} +o spue aq} }e s}tunqns +o
le^ourar pue uor]rppe eq1 Iq (sazuaufilodep)
solquassestp pue (sazuou^;od) sa;qu.resse
'olnqnloJ)tul
e lo luot!elt+ utpe ue se
q)ns 'saln)oloru
(slrunqns u qllM) Jeul^lod
urelo.rd ,ro teur{;od Jeautl V
I HI
NOtlVUtNl)NO) tV)U_tU)
aq] ol ppe leq) sraurouor.ll ,|'o Jaquinu aqf
utpe) loul^lod
.ulnuqtltnba
stql }v
'ssol
+runqns lo aleJ eq+ s;enbe uorllppe
l!unqns +o aler oql uorlerluo)uo) s!ql lv .())
uorlerlua)uo) le)tllJ) oql palle) 'onleA
luelsuo)
e saq)eal 1r ;r1un do.rp ol panlasqo st ) pue
'dn pasn oie sltunqns 'sanol6
letu{;od aq1 sy
')
uo puadap lou soop leql (so)) aler luelsuo)
p le pua Jou^lod oql a^eal lltnn s]tunqns aql
lnq 'Ouo)) ltunqns aol+ oq] +o uotlelluo)uo)
oql o] leuotyodotd aq lltlvr puo)os
'(Eh-€
ernbrl oas :uotlrppe ltunqns
io+ luelsuo) unUqtltnbe aql s! ) olor.lM)
cNV sntd
.', I l'",
(slrunqns !+ u qllM) reru^lod
red (a;nqnlotlul ro luaulelt+
1lo1 = 3 uo)
_J
*"\
L
soNl snNtn
eseqd 6e1 eql
tau,{;od aq} sla}ua }!
se ]runqns q)ea +o uotleulloluo) aql ur se6ueq> Iq e;qrssod
apeur sr spuo onnl eql le qUnotb +o salel aql ut a)uala#tp
aql puo snutu aql polle) st pue bulnnot6-anols ar-{} seaJoqM
'pua
snld oq] polle) sr pue 6uranol6-lset aqt.saleJ luara#tp le
azueur{;od alnqn}oJ)tLu Jo }uotuelt+ utpe ue }o spua oA } oql
lrunqns ror|' 9v oq] 'alolaraql 'le)tluapt st uotlet)osstp Jo+e +tunqns aq]
,o olels leur+ aq] pue 'pua raq]te }e lsol st ltunqns e uoqM ualolq oJe
suorperolut llunqns eules aq] I;Dexa asne)oq sr srql .(srsIlolpIq
319
ro dIV ou) uotpeor uorlezuatu{;od eldurs e Jo+ spua q}oq }e aules aq}
eq 1snur-) erueq pue-uo/ory orlel treql lauAlod aql lo spua snutLU
pue snld aq] JoI sanle^ luola#tp a^eq lltlvr,lo) pue uo) q6noq1 uan3
'spuo
oMl aql
o] ppe slrunqns q)tqM }e solel aq] spo#e a6ueLl) leuotleuJroluo) stql
'olqels
aJoul dnot6 elrlua
sa)eu loulll
_aql
e uJlo+ ol loulououl utpe pllql e lo uotltppe +nq loqlo q)eo o]
{;1eenn I;enr1e;e.r putq soln)aloul ut1re oM} ,utpe }o ase) oLl} ul .s}tunqns
lua>efpe
uaaMlaq speluo) ardrlrnLu {q pezr;rqe1s sr lau.rr;od re)rroq v
NOllvll)nN
sDe a.rolaraql q)tqM leuu] stLll oluo e>e;d a>1e1 ue) uotltppe raulouour JaLlunl
'sluauelu
utl)e Jo solnqnloj)tut
pezuau{;od {pea.r;e rto s1uaut6e.r,r se q)ns ,tol)nu apeuratd 6urppe Iq {;el1ua
poqstloqe Jo pa)npoJ eq ue: aseqd 6e1 eq1 .uotlezuautfi;od
6ur.rnp uees
aseqd 6e1 eql sureldxa qltqM lvrols {;enr1e;a.r st snal)nu e 1o I;qurasse aq_
'aures
oql sr e;dr:uud
NOtlVZtUltAAtOd lo lsuno) tmtl
:asJno) eur] 6urnnol;orr eq] sMoqs I11e:rd{1 tunl1e6elrt let:e})eq e lo }uauelt+
;e1a1e1soy{: e se q)ns.raur{;od 1e>r;eq 6uo; e olur urelo.rd e,to {;qLuesse eql
ro+ ua)el autl ol spuodsettot
wntu€tltnol
auJ!] uorleol)nu
'srollrouotu
o1 ))eq I;quassesrp o1 anp teullod
aql 1o e6eluuqs aql so)ueleq I;asl>atd uotltppe JaLuououl o1 anp leutfi;od
aqt 1o qynol6 oq] uoqM poq)eal st ,alels
Ipeals.ro ,aseqd
u.rnuqr;rnbe aq1
'uorlebuo;a
luoueltl 6ursner'luauielU 6urnnot6
oql lo spua pasodxa aq] ol ppe srauouorx se srnl)o aseqd qlnnot6 aq1
']ure.rlsuo)
stLll sa^oulaj uotlezuot!^lod utlnqnl pue utl)e
saruedLUo))e ]eql stsllolp^q aleqdsoqdul aptsoal)nu aql
')uuqs
spua qloq ') > ) rot 1nno.r6 spue qloq ,) < ) lo] ,snq1
rolsp+
saultl lno+ )uuqs osle lsnLU }t ,puo snutLu or.ll ueql latse+ saull
lnort snnotb pua snld aql+t :spua qloq 1e le)tluapt st ,puo oq] qltlvr
uorler)osse slt to+ luelsuo) utnlrqr;rnbe aql souturlalap q)tqlvr ,ssol

sts^touoAH lctlolt)nN
(dcD ro
raur{;od ur }runqns
66y 6urrtrer raurouorx = C)
(d19 ro ____.;s1*__-____
61y 6ur{rrer raurouotu = __
l-)
rau{;od oq} olur salquosse aln)alout orl} la4e uoos oln)olotu dOg
punoq I;1q611 e ol paua^uol sl leql dI9 punoq I;1q611 e satle) oln)olotu utlnqnl
qree '{;re;rur5
:rau{;od oq} o}ut ,{;qruesse s}t lal+e uoos aln)aloul 6qy punoq I;1q611
e o1 pez{;otp{q st }eq} eln)elor! dlv punoq {11q611 e saure) aln)olorr.l urpe q)el
:A1uo pua sn;d aq1 le sluana 6uueprsuo3
'sa^eal
leql urol ffi aql pue lueuelr+ aql o1 sppe ler{}
r.uro+ aqlIllensnst ]l (tustueq)orua;qrssodelo+9t-91,a.rnbr3eas)luauelt+
oLll+o puo q)ee [!oJ] o]pr)osstp o1 I;e1r1 aJoul ]t so)eu pue sltunqns 6urloqqbreu
rol lrunqns oLll +o Ilrur#e 6urpurq aql sa)npar optlool)nu punoq oql;o srsllotp{g
uaqm sosea) qlnnot6 teur{;od }eql os 'lleus fuen {1;ensn ate Iaql o)u!s sor) pue uo6)
arou6r uer enn'sasodtnd ontlellsnllt .ro1 ') =;r;r1un nno.rb 1;rnnraurr{;od oq},aro+aq sV
H;i =5 ro #oo1 =) uor)
'(
-ffi)
lrunqns aar+
aql lo uorpeal ebueqrxa aprlool)nu e Iq peqsrualdar aq lsnu pezl;orpIq
sl ]eql dI9 Jo dIV or.ll asneloq 'unuqr;rnbe anrl e lou pue alels {pea1s e sr srql
:leq] os lauI;od aql ,ro spuo qloq ]e oules aql oq lou paau uor)Poq)
orler raql 'suorl)eal luaJo#rp o] Ja+ar 'or) pr" #oo.>1 aruls :rar.u{1od
oql +o spuo onn+ aql le uorlelluo)uo) le)rlu) eq1 abueqr ol sr uotleullol
rau{;od sarueduorre 1eq1 srs{lotp{q apt}oal)nu aq1 go aruenbosuo) oug
'6ur;;rr.upeerg
se uMou) lauIlod aq1 q6nolql
slrunqns lo xnl,t ]au e sr oraql qbnoql uana'q16ua1 luelsuo) e suteluteul
laruI;od aql'aler le)rluopr ue le pua snurur oql 1e {;quassesrp 1au
e pue pua snld eq1 1e I;quesse 1eu e o6.rapun slrunqns 'e1e1s
fipee1s
stql }v'pua snutul aql ro; ) nno;eq lnq puo sn;d aqt lo,r )3 enoqe sr
leql anle^ e soq)eor rauouor.u aorl +o uorlerluo)uo) aql ltlun
spaaro.rd uorlezueu{;od 'pasodxe
ale leufi;od e lo spuo q}oq 'snqI
+t
Ar_nrsvlsNt )IAVNAO
(pua snld) ') < (pua snurtu) )
'sansua
uorlezuaurllodap uaq+ 'lsol
st lt,t lnq
'qporb
sro^el de) dI9 V 'utlnqn+
dl9 6urureluo) auo uol+ ueql ullnqnl
6q9 6urureluo) pua ue uloJl Jolse+ soulrl O0l lnoqe azuetuI;odep sa;nqnlotrryl
oe) dI9
uo,)
O
eNt)NtuHs O
{1r;rqelsur rrueulp palle) uouauroueqd e 'Ilquassesrp prder }o porJad e
pue qynorb Mols +o por.rad e uoaMloq alpurolle aro+araql ue) salnqnlor)il,! lenpr^rpul
sdv) dt9 cNV sdv) d-LV
'alnqnloJ)il.u
e uo de) dfg e lo ]ueuelt+
url)e ue uo der 31y ue-eleqdsoqdul
aprsoal)nu aql 6utuleluo) sltunqns
,,Cle)il e seq pua aql 'suotltpuo) q)ns
repup paz{;o.rp'{q sl eprloel)nu punoq
roql q)rq/V\ le aleJ aql ueq] lalse+ oq ue)
alnqnloJ)tLU Jo luaueltl urpe 6urnnorb
e ol slrunqns to uotltppe +o sleJ aql
lo
'sluauielr+
utDe ul
aleuruuopeld Ieu 6ur;;rrupeall sealaqM
'salnqnlor)rul
ut aleurulopatd o1 panar;aq sr
I1r; rqelsu r >r r.ueulq'srsI;orp{q eleqdsoqdul
aprsool)nu qltM polet)osse eJe
qlog 'sretur{;od
1e1a;a1soy(> ut pa^lasqo
sror^eqaq oMl ore gNl'l'lllAcvlul
pue Al-ntEvJ_sNt )ilAVNAC
t
-

086
uoga;e>1soy(1 aql:9 t laldeql
'slueluellJ uI]f,e
ur alpurruopard feru Sulprupuarl seeJeqlvr
'selnqn]oJlltu uI e]euluopard ot
rq8noqr s1 ,bgqelsur ctureufp 's11ar crlortreJna ]soru u1 'dlqrqelsul ctureuf.p 8ut
-o8rapun saFqnloJJIIu JoJ sJelaIuoJJIIuJo suel o] paredruoc sE 'selnulur IBJaAas
JeAo os Jo ralaruorcnu e ,,(po selerucnu {r8uay aql elels dpuals }e leql os 'eluJs
Jallerus qrnu E uo tnq suor]€nlJng qr8ual o8rapun osle slueruellJ UIIJV
'(l9I-9I
arn8tg)
salnqnlorJru 3uzrrau.,(1odap Jo spua aql JPau uaas eJe urnqn] Sutuleluoc-dCC
;o srauoBqo pe^rno pue s8uu pue flprder go laad 01 sluauelgolord palrnc aqt
sasner aoul€l apqnloJrllu eq] uI peJols srs.{lorpdq;o .{8taua eq} Jo esBaleJ eAI}e
-radooc srqg'lrede Suuds sluaruellJolord panmc aql pue 'pelotueJ sl ]uleJlsuoc
srql 'saprloalcnu Jraqt pazf1orpr(q aleq s]Iunqns pulluJal aql ueqm lng 'lq8lerls
readde spue eq] pue 'stuaruplgolord eql Jo aJnlEAJnJ aq] uleJlsuoo o1 lqBnoqt
sr dec dJC eql 'elnqnlorcllu Sumtor8 ,,(prder E uO '(ggl-91 arn8tg) pe^lnc
sluaruelrJotord aql saleur qJIq { 'utalotd aq1 ut a8ueqo Ieuol}eruJoJuof, al}qns B
qty!\ palerrosse sr dog o] dJC 1o srsf.lorprtq eqt tng 'raqloue auo qlp\ slJeluoc
pJelpl rep8ar pue 3uor1s a{BIu leql sluauelgo}ord 1q3rcr1s acnpord reurouour
-fl aqt or punoq dJC qt1,t,r stlunqns u1nqnJ'cIlEIuErp sI pua IuroJ-C e pue pue
ruJoJ-I e uaaMlaq eJueJeJJIp leJnllnJls aqJ 'uonequasuoJ llunqns ea4 aql pue
aler srsr(lorpdq aql uo Surpuadap olleJ eq] qllA\'urJoJ C eql uI eJe aruos pue uJoJ
J eql ur eJE spua eqlJo auos ]uelsul due le 'salnqnlorJlur;o uollepdod e u1
'anrsal
e pallec sg quvror8 o1 a8ueqc eqt ayqn'aqd
-or$atar E pelleo sr a8eluuqs prder o1 qwor8 ruor; a8ueqc eql'(V9I{I arn8lg)
,irilqursur cgrruw(p pallel sI 'uolleJlueJuoc llunqns aar; IuJoJIun e }e 'a}els 3ur
-Iurrqs pue Surrr,tor8 e ueamlaq uolsJaluof,Jalut ptder stql'ute8e.uo;8 o1 urBaq
pup pua uJoJ-J e ure8ar uaqr rq31u ll 'aurp Jalel etuos 1V ']uelsuoc pleq sI uop
-prluacuoc tpnqns ae{ aq} allqm ue^e dlprder {ulrqs o1 urBaq pue turoJ C eq}
o1 a8ueqc.{luappns uaqt }nq 'ruJoJ J P uI arul}Jo Q}3ua1 uleua3 e ro; rror8 }r{8gr
pua ue ']uaruelg a13urs e uO '{ulrqs IIIIv\ urroJ C aql ul aq o1 suaddeq leq} pua
.{ue searaq,vr'arror8 pan ruroJ J eq} ur aq o} suaddeq 1eq} pua,(ue 'rltoN '(pallesqo
sr Sulprupeal] eJaqm suorleJluaJuoc ;o e8uer aru€s aql ut 'q leql) pue IUJoJ
-c e JoJ uollEJluef,uoc IesIlIJc aql pue pue IUJOJ-J e JoJ uolleJlueJuoc Ief,IlIJs
aql uee^ leq alerpeluJalul sI sllunqns aaqJo uolleJlualuoo aq] ]eq] asoddng
'elurl
lrun
rad.,fitpqeqord urulrac e qlyv\ 'uropueJ pue uappns sI uolleluJoJsuen sIqJ'ruJoJ
cI E ot pue eq] turoJsuerl pue uonrppe eql qtp\ ,,dn qc1ec,, f.lpnluana p.r,r stsd
-1orp.,tq feqf lnq 'urroJ I e uI ]no Uels IIIM pua sql ]eqf Ilruqeqord elIuIJ E sI ereqt
'srs.{1orpr(q;o e}uJ eql o1 apnltu8uur uI JeIIIUIS sI pue euo lP uop1ppe lpnqns Jo
aleJ aql 'slueruellJ
JI Jo sJol^Eqaq aq] JoJ acuanbasuoc luulrodrur Jaqloue e^Eq
IuroJ c aq] pue urroJ J aq] Jo rol^Eqeq eq] uaaMlaq sacuereJJlp sllauH aql

THE SELF-ASSEMBLY AND DYNAMIC STRUCTURE OF CYTOSKELETAL FILAMENTs
triphosphate. As we explained with our ant-trail analogy at the beginning of the
chapter, the advantage to the cell seems to be the spatial and temporal flexibility
that is inherent in a structural system with constant turnover. Individual subunits
are small and can diffuse very rapidly; an actin or tubulin subunit can diffuse
across the diameter of a typical eucaryotic cell in several seconds. As noted
I
.*
(A) I ""
rapid growth with GTP-capped end
accidental loss of GTp cao
I rapid shrinkage
|
I
F
CATASTROPHE
l."ouin of GTP cap REscUE
t
rapid growth with GTP-capped end
rl
GTP-tubulin dimer
--;---l-
-
straight protofilament
I
GTP HYDROLYSIS CHANGES SUBUNIT CONFORMATION
AND WEAKENS BOND IN THE POLYMER
curved protofilament
i
F
GTP cap
less stable
region of
microtubule
containing
GDP-tubulin
dimers
GROWING
0'@
s os
o
o
ro
SHRINKING
0,.orn
(B) (c)
Figure I 6-16 Dynamic instability due to the structural differences between a growing and a shrinking microtubule end. (A) lf the free
tubulin concentration in solution is between the critical values indicated in Figure 16-148, a single microtubule end may undergo transitions
between a growing state and a shrinking state. A growing microtubule has GTP-containing subunits at its end, forming a GTP cap' lf nucleotide
hydrolysis proceeds more rapidly than subunit addition, ih".up is lost and the microtubule begins to shrink, an event called a "catastrophel'
But GTP-containing subunits may still add to the shrinking end, and if enough add to form a new cap, then microtubule growth resumes, an
event called "rescue." (B) Model for the structural consequences of GTP hydrolysis in the microtubule lattice. The addition of GTP-containing
tubulin subunits to the end of a protofilament causes the end to grow in a linear conformation that can readily pack into the cylindrical wall of
the microtubule. Hydrolysis of Gip after assembly changes the conformation of the subunits and tends to force the protofilament into a curved
shape that is less able to pack into the microtubule wall. (C) In an intact microtubule, protofilaments made from GDP-containing subunits are
forced into a linear conformation by the many lateral bonds within the microtubule wall, given a stable cap of GTP-containing subunits. Loss of
the GTP cap, however, allows the GDP-containing protofilaments to relax into their more curved conformation.This leads to a progressive
disruption of the microtubule. Above the drawings of a growing and a shrinking microtubule, electron micrographs show actual microtubules
in each of these two states, as observed in preparations in vitreous ice. Note particularly the curling, disintegrating GDP-containing
protofilaments at the end of the shrinking microtubule. (C, courtesy of E.M. MandelkoW E. Mandelkow and R.A. Milligan, J. Cell Biol.
114:977-991,1991. With permission from The Rockefeller University Press.)
c
981

982 Chapter 1 6: The Cytoskeleton
time 0 sec
1 25 sec 307 sec
previously, the rate-limiting step in the formation of a new filament is nucleation,
so these rapidly diffusing subunits tend to assemble either on the ends of
preexisting filaments or at particular sites where special proteins catalyze the
nucleation step. The new filaments in either case are highly dynamic, and unless
specifically stabilized, they have only a fleeting existence. By controlling where
filaments are nucleated and selectively stabilized, a cell can control the location
of its filament systems, and hence its structure. It seems that the cell is continually
testing a wide variety of internal structures and only preserving those that
are useful. \Arhen external conditions change, or when internal signals arise (as
during the transitions in the cell cycle), the cell is poised to change its structure
rapidly (see Figures 16-2 to 16-4).
Actin and tubulin have independently evolved their nucleoside triphosphate
hydrolysis to enable their filaments to depolymerize readily aftei they
have polymerized. These two proteins are completely unrelated in amino acid
sequence: actin is distantly related in structure to the glycolltic enzyme hexokinase,
whereas tubulin is distantly related to a large family of GTpases that
includes the heterotrimeric G proteins and monomeric GTpases such as Ras
tion you are reading now is transferred into long-term memory, even a cell as
stable as a neuron can grow new elongated processes to make new slmapses. To
do this, a neuron requires the dynamic, exploratory activities of its cytoikeletal
filaments.
Tubulin and Actin Have Been Highly conserved During Eucaryotic
Evolution
tube. However, they can have distinct locations in a cell and perform subtly different
functions. As a striking example, a specific form of 0ltubulin forms the
microtubules in six specialized touch-sensitive neurons in the nematode
Caenorhabditis elegans. Mutations that eliminate this protein result in the loss of
touch-sensitivity, with no apparent defect in other functions.
Figure 16-1 7 Direct observation of the
dynamic instability of microtubules in a
living cell. Microtubules in a
newt lung epithelial cell were observed
after the cell was injected with a small
amount of rhodamine labeled tubulin, as
in Figure 16-1 5. Notice the dynamic
instability of microtubules at the edge of
the cell. Four individual microtubules are
highlighted for clarity; each of these
shows alternating shrinkage and growth.
(Courtesy of Wendy C. Salmon and Clare
Waterman-Storer.)

cNV AtSt/\llssv-Jlts lHt
'peelsur
sJeuued Surpurq Jreql JellB ol pue 'auole ullnqn] pue uI13P
e^eal ol elqelrJoJd aJoru ]r punoJ e^eq susrueSJo Suvrlona 'arurl JeAo 'elrs eurBs
aql reau Jo ]E purq leql suraloJd Jeqlo qll^\ suollceJalul sll uI seSuBq3 elqeJls
-apun asnBs 1q3$r uralord auo qlur uorlJBJelul slr uI aSuPqJ elquJrsep P ur llnsaJ
plnoJ lpql ullJe ur uopulmu.{ue 'snql'(gt-gf arntlg) eJour uale qllm^llceJlpul
pup 'suraloJd raqlo Jo suazop q4rlt fllJeJrp slJeJalur urlcu lpqt peleJlsuouep
a^etl aarclnalal saJ/tutoJDtlJJas $ee1\ eql q sarpn]s lEcruaqcorq pue Jr]auac
'salntJnJls
Jraql Jo &g11qerre,r eql slluI elnqnloJJlru Jo tuatuellJ UIIJE uB Jo
aJeJJns alrlua aqt qll^\ ]cEJelur 01 suraloJd Jeqlo Jo sJaqrunu a3re1 ro3 peeu aql
leq] sl uorleueldxa,{1a4t aql;rou aJE (urlnqn} Jo ur}f,e ol pulq }eq1 suralord rtros
-seJJB
Jo serFruEJ a3re1 aql pue surelord ]ualuulrJ elErpaurelul SuIpnlJuI 'sulel
-ord pla1a1so1dc raqlo lsoru Jo sacuenbas eql seaJeq ^ 'uorlnlona JllodJef,na uI
pa^JasuoJ ,{1}cr.rls os uqnqn} puE urlJB Jo seJuenbas prse ourue eql aJP ^r{ru
,(g o1 alqeun sl lnq IBr.uJou $lool leq] ,(U e uI s]lnsal apqdosotg ut uI]cB ]sea^
;o uorssardxe eql le,{'leor}uapr oh61 an ur}oe alosnru apqdosotq pue urloe lsee1
'sllac
alcsnuuou Ip lsorule ur raqtaSol punoJ eJE l, pue fl alrqm 'slleJ el3snru ur
fpo passardxe sI upoe-n eql 'seJuanbas prce ourrue Jraq] ut.{11q8rts Jagrp }pq}
t pue '$ 'lr paruJel 'urlJp Jo sruJoJosr lueJeJJrp -{1fqns eerq} eJB aJaq} 'saleJqeuel
uI 'seouaJeJJrp
IeuorlJunJ luergru8rs asner ueJ eouenbes prop ourup urlop ur
suorlerJel 'urlnqn]
llerus a>1r1 urc3e ']ng 'pcrluapr
%06 ]noqp rtlensn a.re sareads
lueraJJrp ruorJ surlJe;o sacuanbas prJp ourrue aq; 'salo^,(rucna Suoure pe^Jes
-uoJ
lla-,vr dpeurproerlxa sr upcv 'xrs a^Eq sueunq luqce Surpocua saua8 aldp
-lnru aleq srusrue8ro lsohtr 's11ac crlol.recna ile ur punoJ sr urloe 'ugnqru a{r'I
slro) pallo),o burlsr/vrl puP
arnt)nrls lueueltl ele!pautolul
a{eru lEqt (sel{corpuepoS[o pallEr) slle3 IPI6 pezllBlJeds aqJ 'ed,4f IIeo ,{ra^e
Jo rusEldol,{J eq} ul peJrnbal lou eJE sluauPlrJ elErpeuJelu 'stuslue8Jo aseql ul
ue^g '$lsn[ou puE 'sapolerueu 'sa]eJqaue^ Sutpnlcut-suBozelelu eluos .{Iuo
rn lua[rrelu f,rursEldils e suJoJ '1uauru\yf arylpawJalu eq] 'ula]oJd IP]ale{soil.c
Jo ed,q ro[eru prlq] aqt
'ulnqnt
lng
pue urtJp urptuoJ sllec clto[rerne IIV
sutso{tu :1:l)
slNtt^Jv-ilr tvrttl)sol^) Jo tunDnuis )twvN^o
srso1,{:opue'uol}ar)as
{1ue1od
'6urppnq 'uorsr^rp
lle)
aqt uo spuedeq
li1l'
srsaqlu{s prdrl
srru-reur{p }uauelr}
laqlo
6u;;pung lerale'l
€8 6
('t66 t 'ssard
Aroteroqel
roqreH 6uuds plo):AN iroq.reH
6urrd5 p;o1 'fspa'sauof 'trtr'3 'a;6u116 'g'1'
'qreorB 'U'fl
salworDtp)Ds $e4\
aql Jo ^6010rg rplnllal pue reln)alory
eqf ur "le la uralslog 'C urotJ poldepv)
'^al
aq] ur pale)rpur sp lolo) aLues
aql u! uMoqs a.re ssa)oro relnlla)erlul
aures aql ur alelado leql suralo.rd
l.rossarry' a o t s Aat at sattrw d D tp) o S
lseol aqt ur 'senbruq:a}
le)rLlaq)orq
ro rrleua6.raqlla 6ursn'palerlsuotuap
uaoq e^eq teql suorl)eralul
aql Jo lsou.r sr*oqs uerberp srql
's1;a>
>r1ofue>na ;1e ur suralord ftossal:e
jo,{lalren a6.re; ,(ran e ol spurq ur})V
'spporssor) aql le u!l)V g t-9 t arn6rl

uolalelsoil) aql:9 [ raldeLlf v86
eq] Jo euEJqrueu Jeuur aql 3uru11 lromqsaru P urJoJ luql sureloJd lueIuBIIJ
elPrperuJelu arp sulluel Jealcnu eql'sulluol JOaIJnu luele^erd aJoru qcnru aqt
'sJolsecue
Jrar{l ot pelelal dlasolo eJE s}uaruellJ elelpeluJetul JlruseldoilJ
'sleruruE
Jerqsmbs eq] roJ sanssll 01 qlSueJls lecluPqf,eru 8uI
-uedurr ur aloJ ]uEtJodril uE,{eld sluauelrJ alerperuJelul teq} sueas }I 'sruJapo
-urqJa puB spodoJque se qJns suolelalsoxa prSu e^Pq lPql sIPIuIue uI punoJ lou
,(lerauaB aJp pue 'ssaJls lecrupqJaru o] tcafqns eJB leql sllao Jo ruseldoldc aq1 u1
luaurruoJd dlrepcqred aJB slueuellJ elelpalurelul 'slueluellJ alelpelurelul qJns
ureluof, ]ou op 'elduexa JoJ 'ruels^s sno^Jeu leJlues ateJqeue^ aq] uI ula,{.Iu
-edor eJnpord o1 panlona eAEq saleJrldnp aqt pue 'palecqdnp dpuaredde aneq
seue8 urtupl 'uoltnlo^a upozBlalu Surrnp selull pra^as '(ZI JaldPqJ uI pessnc
-s1p sl uorsrlrp IIac Suunp Jorlpqaq crrupu,{p JIeq}) salod JpalJnu puu seuos
-ouorqc JoJ selrs eBEJoqJue apr^ord daq] araqrvr 'adolanua JeelJnu Jlloftecne
'aleqdsoqdu
eprsoelcnu e JoJ ells Sutputq E ulsluoc tou op sllunqns
lueuBIU alErpauralul aq] 'uqnqnl Jo ur]rE eql e{Iun '(6I{I
eJntlc) Je(uouotu
ul]ce eql 01 ro teurp ullnqnl-dD eql 01 snoSolEue sl leq] ]Fnqns elqnlos
eqt sluesaJdar raruEJlal srqJ'JeluPJla] pala8Sets P tuJoJ ol uolqseJ IellEJEdIluE
ue ur salercosse uaq] sJarurp IelleJEd Jo Jred v 'Jaruouoru Jaqloue qlll!\ IIOJ
pelroc lallEJed e sruroJ lBq] ureuop lBsrleq-}] IeJ]uac pepueue ue q]l.4i\ selncelou
pale8uole erp sluaruBlu alBlpeuralul Jo seppded.{lod Ienpl1\Ipul aqJ
'slueuelrJ
elprperuJelur f,rrusEldoltf, 'eII
(a)
rrll
lqDre
sjaurejlel
e olur palsrM]
())
ZHN
ZHN
(v)
(s)
reqla6ol pa1:ed srauetlal om1
luauelr] a)rlaooJ
,TI
ZHN
ZHN
zHN
HOO)
J
f uelutnO IoU Jo
{salrno> qder6orrru uol})all)'Ual leddn
aql uo uMoqs st sluaulelu aletpauilalul
;o qder6ol>tr-u uoll)ala uV'uotl)alp
q>ea ur 6urlurod ale sraulrp asaql,o JleH
'uorl)as-ssor)
ut (slto) palo) Zt) sreLutp
9 [ spq q]tqM '^ellp
le)tlaq e ut laq]a60]
pel)ed are slaueltal'luoulelu altl-odol
r.I,u-o t leuu aLll ul (l) taulpllat lei.lloue
qllm ater)ossp o1 11 6uranol;e {qalaql
laqloue auo ol l)odsal q]!M lasJJo aie
sraurp oMl aql'iaruerla] q)ea ulr.lfn (C)
'sluaurplu
aletpaurlalut Jo ltunqns
alqnlos aql st leuretlal aqf'suteq)
aprldadllod lnoJ Jo raurellel lalleredrtue
ue ulo, ol apts Iq aprs dn eur; uaql
sraurp o/1/\t ()) 'lto) polro) e otur toqta6o]
punoM pue lalleled ut pau6tle ele
sureuop poi lellual pa^losuo) aql q)!q^/\
u! '(g) rauJrp e uiloJ ol laurououl le)ttuapl
uP q]!M srecl (v) ut un/\or{s leulouoru
eql 'uo!l)nrlsuo) tuaupl!J
olelpaurelu! ,o lapour V 6l-9 t arnFll
rauouorx ur uor6al le)tlaq-n
lro)-palro)
uu 8? ----------------f
.raurp
HOO)
sJaurp lro)-pepo) oml ]o rauella) pata66els
HOO)

s86 sl_Nlv\Jvtl tvlltl)sol^) lo tuntfnuls )twvN^c cNV AtSyllssv-llls lHr
;o sad{1lofew t-91 alqel
real)nN
elrl-urluaurn
lerlaL{}tdl
leuoxv
-rgrp.{laruarlxa aJe i{aqf fnq'(ur}Je JoJ sraleruoJJrru ue} }noqe pue sepqnloJorru
JoJ sreleuqlrur IeJaAas o1 paredruoc) JalauoJrru auo ueq] ssalgo qlSual eJuetsrs
-rad e qtyrt'1uaq,{lsea aq uec.{aq1 'JalreJeqf, a1q-ador e s}ueuelrJ elerperuJalur
sant8 'suralord goc-papoc ;o pcrddl suor]reJelur crqoqdorp.,tq IeJalEI Suorls
eq] q]!v\ 'raqtaSot dn paug 1e sap4daddlod;o raqunu a8rel srqJ 'slror Iec1aq
-n
lPnpl^Ipul zt Jo uorlcas ssoJJ e sBq eroJaJeql lualuElg elerperuJelul pnpl^
-rpur qJeg 'sJarueJlet
3o dn apeu stuarueluolord lalpred lq8la sapnl3ur qJrqm
']uaruellJ aql uroJ o1 [1era1e1 raqlaSo] >1ced sraruer]el aqJ
'saFqnloJJnu
pup
sluauelrJ ullJe roJ lef,rtrJJ sr ]pq] [1rre1od IEJn]JnJ]s IIeJaAo aq] s{cEI eJoJaJaq}
]uauelrJ alerpeuJalur palquassE aqJ 'atups aq] aJE spua o^1l slr 'suoucarrp
alrsoddo ur Suuurod sJarurp o^ l Jo dn apeu sl lpnqns JrJerueJlel aql aJurs
-cnrts crrueuLp l.U8tq wuo1 uuuarura Surpnlcur s]ueuelrJ e]ErpetuJelur;o sadiil
aluos lnq 'selnqnloJcru puE slueruEllJ ullJE Jo ueq] stualuulrJ alerpauJelur
;o .r(lquassesrp pue flquasse Jo rusrueqcau aql ]noqe poo]srap1;es;[lT'",
se qJns slleo ur saJnl
f pue'€ 'v suluel
ullueulA
rrpne,(.re;1uqr; 1er;6
urusap
(H-lN pue'w-lN ''t-lN)
suralord luauellrornau
()rseq) surlerel ;1 ed{}
()!pl)e) surleral 1 adll
urreqdtted
'sllJJ
eseqt ur sualsfs 1eta1a1so/.c
eeJql aql go uorler8alur leJruer{cau p}uauepunJ e sr aJaql tpq} Suuerls
-uouap 'seseJ aruos ur s{Jo^ }eu }ueurelrJ urlJe pue elnqnloJorru aqt 3o .{lqruas
-spsrp aql acnpur osle ueJ uorlcalur Jepel eql 'd18uqsara1ul '{Jo^ ]au }ueuelrJ
atprpaurelul eqt Jo flqruessesrp prder aqt seJnpur tpnqns aqt yo uo13er IEOrIeq
panresuoJ p ruo4 palrJap saprldad;o uorlcafur ue alrq ^ 'selnurur l aJ e ulq]lm
sluaruelrJ alerparuJalul Supsxa aq] oluo sallasuaq] ppe,{lprder sllal arn}lnJ ens
-sp otq palcalurorcrtu s]runqns peleqel telouJnl prder ro; eJueprne sV'@Z-ZI
arn8rg ees) srsolru ur surupl J€alcnu go ,tyqruassesrp eqt salep8ar uor1e1,{roqd
-soqd leql fem arues aq] qJnru ur '.{lqurassesrp Jraql salep8ar,r(lqeqord uor1e1
sllo)
llPdul sluauel!l ele!paulalul
slla) elerqauan ur suralo.rd lueuelrl elerpeulrelul
uralord
suoJnau
pue req "6'e) sa^lle^uap
rlaql pue slla) letlaqlldo
suolneu aLuos
(sller uueMq)S auos
pue say(rorlse) s;;ar 1e;16
etfsnur
urbuo
;eu{qruasetu ;o s;1er {ueu
(adolo^ua leal)nu Jo
6urur1 rauur) eu!ulel Jeal)nu
(slleu
-,{roqdsoqd uralord 'suoplpuoc IEruJou Jepun 'slselqoJqrJ
leuluV or ^t!llqers le)!uPqlaw
uroI uaql qJIqMJo o^a] 'sJarurpoJe]eq uJoJ eseq] lsuleqJ ulleJa{ (clseq/leJ}nau)
II ed,{t pue (JrprJe) I ad.{1 Jo arnlxru pnba ue 1o dn apelu sI tueruellJ
uqera4 zfuang
'surleJa{ lJullslp 09 lnoqe aq -,teru ereq} }eql palea^al seq acuanbas
auroua8 ueunq eq] Jo srsdpue isleu pue Jrpq o] cgrcads aJB leql aloru 0I
tnoqe pue 'sllec pleqtrda uerunq;o sadfi 1uara;;rp u punoJ 0Z rnoqe are araql
:,,(pure; tueruelrJ alprparuJelur esJelrp lsotu eql eJe suIlBJaX '(I-gI aIqBI) saddl
ilac tualaJJrp ur passardxa aJE sluetuelrJ alerperuJelul Jo seIlIuBJ ]ueJeJJIC
-{1}ear8
.[ren uec sureuop repqolS purruJa]-J pue -N aql lnq 's{uJoJosr
}uaJeJJrp aq}
ur Jplrurrs sr ureruop slqJ'(6-t arn8rg aes) aJnlonJls Iroo-pelloJ pepuelxe uE ruJoJ
leq] sJllour leadar peldaq os Jo 0t seq ureruop pcrTeq-n 'uqnqn]
IeJlueJ V Jo
ullJe Jo sruJoJosr aql ur sJncf,o ueql suJoJosl lrunqns aq] ur uorlerren acuanbas
aroru ,{1eque}sqns q}yrr 'sad.,i.1;o ,{.}arre,r. eplirr e ur aruoJ s}uetuellJ e}Brpaurelul

986
uolalalsolr() aql:9 l. raideqf
surleJeI Jo les JBInJrped aql se '(seurourcJef,) sJasuec 1etteqllde ;o stsouSetp
aql ur Iuesn dlecruqc sl suIlPJe{ q dllsJa lp eqJ 'selEf,s puP (sA\BIc 'sIeu 'JIPq uI
pue uDIS Jo Jedel Jalno aql ul sE 'slerurue JoJ s8uualoJ q8nol SururroJ 'slleo rleql
Jo qlPep eqt ua^e a^r^rns,{eur spuoq epulnslp ^q Jeq}e8o} plaq s{rolli\leu ul}eJel
pa{url-ssorJ '(6I-9I arnSrg eas) }runqns JIJaIuPrlel plueluBpury aql uroJ o}
luerulEarl Jo eJroqJ aql apmS or dlaq uEf, snqt puP pel€u
-r8uo Jaruec aq] qrq ^ ur enssll pllaqllda eq] Jo uonuclpul uE saAIS passeJdxe
elerparuralul eql Suuoqf,ue dq ued uI senssF lellaqllde 01 qlSueJls lecluEqcalu
uedur stueruelg urleJe) '(02-gI arnglC) {romleu elSuIS e olul azlJeudlodoJ
eseqt puB 'surtPre{ Jo sed.& aldnFlu acnpord ,(eu 1ac [BIIaq]Ide elSuIS V
JO spue aql JetlP sesPasrp asaq] esneJ ]Eq] suollelnru slJlseds eqt Jo ltuBw 'uo]
-eleIsol-,tc lualuelg urlEJa{ eql Jo Suldtunls Jo uoI}BZIuEBJosIp P puB PIuneJl [eO
-rupqJeru
Jo af,uenbesuoJ e sP eJnldnr ilec Iq peudr! eJE selpeplu aseql Jo IIV
'senssp
esoql ot f,rJrceds sr uorssardxe asoqM sulleJel lueJeJJIp eqt ul suollElnlu
Lq pasnec ale 'e.{e eq] Jo EauJoo eq] pue '8urur1 pe8eqdose 'qlnour eql Jo sJepJo
-srp Surpnlf,ur 'seseesrp Suualsqq Jo sed.,ir raqlo '(IZ{I
arntlC) sllec lesuq eqt
salnldnJ qolq \ 'sseJls Ief,rueqcau tq8Is r(ran uana ol esuodseJ uI sJalsllq UDIS
eql qorq ^ ur'xaldutts asonnq s8tlouJaptda pelleJ JapJoslp e ecnpoJd daql 'snu
-raprda aqt Jo JedBI IIaJ Ipseq eqt ul pesseJdxe eJe sullsre{ e^DceJap ueq ^ 'eld
-ruExe Joc'sespasrp JrleueS ueunq leJa^as asnBc saueS ul]eJel uI suollstnlN
'6I
Jaldeql ur lPlep eJolu uI seJnlf,nJls
e^rsaqpe ]uEuodr.ul eseq] ssnJSIp aM'(9-9I eJnSrc ees) sauoso|usaprulaq pelleJ
']f,EluoJ
xrJlpru-lleo ro'saruosoltlsap pal]:P-J 'lsBluo3
llac-lleJ Jo salls lB slueluBlg
ud O?
urd 0t
('o6reuy aurla6uen3 pue uaaJg
uaalqle) lo {selrno))'sauepunoq lla)
aLll ro uorle)ol aql lea^ar o] pautels uaoq
setl (aryq uralo.rd puolas y '(6 9 raldeql
ur possn)srp) saurosoursep {q sroqqbrau
str Jo asoql ol pal)auuo) {llrargpur ele
lla) q)ea ul sluaurelu aql'alnlln) ut slla)
lerlaqtlda jo taaqs e ur (uaar6,) stuauelU
ulleral ,o lroMlau aql 1o qder6orrru
a)uaf salonuounuiull .slla)
1e;laqr;da
ur slueuPlg urleray g7-91 arnbrl
fssal6 ftrslanrul
rallaJal)oU aql uJolj uorssil.ulad
qllM' t66 t'v L9L- t"99 L:S L L' lolB ilD',r
"le la aqLuolno)'vij Luoll) 'eutulel
leseq
6ur{pepun aqt o} slueulelU ullera) eql
l)auuo) q)!qM '(6 t raldeq) ut passn)srp)
saurosouisaptulaq aq] pue snal)nu
aql uaamlaq arnldnr slla) oql 'Moile pa)
eql ^q pole)rpur sy'Idorso.rrru uollfela
{q penresqo se'srr-ulaprda luelnul
eql;o ra,(e; lpseq oql ut slla) aarql,o
q)la)s v (f) '(s^ orP pal tioqs) srluroprda
luetnul oLll Jo rale; ;eseq aqt u! slle),o
6uunldnr aql uJorJ sllnsal 6uualstlq eql
leq] Moqs urls (8) luelnui pup (v) leullou
,o suorl)as ssol),o sqder6ooru tq611
'ulls aql Jo sller lespq aql ut lroMlau
luaurelU ullpJal aql sldn.rsrp,(qareql
pue surlera) leullou aLll qll/\/^ salquasse
urelord anrl:a;ap aql'asnour lrua6suetl
e ur passaJoxo seM (suteuop leutulal-)
pue -N aql qloq 6ull)pl) ulalold u!]elal
pale)unrl e 6utpo)ua aua6 1ue1nr.u y
'eue6 ullera>1 luelnu e Iq posne)
urls srlllo 6urrelsr;g 1g-91 arn6r3
lla) leseq
sr(xreproa J|'o

THE SELF-ASSEMBLY AND DYNAMIC STRUCTURE OF CYTOSKELETAL FILAMENTS
the central rod domain, demonstrating the importance of this particular part of
the protein for correct filament assembly.
A second family of intermediate filaments, called neurofilaments, is found
in high concentrations along the axons of vertebrate neurons (Figure 16-22).
Three types of neurofilament proteins (NF-L, NF-M, NF-H) coassemble in uiuo,
forming heteropolymers that contain NF-L plus one of the others. The NF-H and
NF-M proteins have lengthy C-terminal tail domains that bind to neighboring
filaments, generating aligned arrays with a uniform interfilament spacing. During
axonal growth, new neurofilament subunits are incorporated all along the
axon in a dgramic process that involves the addition of subunits along the filament
length, as well as the addition of subunits at the filament ends. After an
axon has gro\,&'n and connected with its target cell, the diameter of the axon may
increase as much as fivefold. The level of neurofilament gene expression seems
to directly control axonal diameter, which in turn influences how fast electrical
signals travel dor,tm the axon.
The neurodegenerative disease amyotrophic lateral sclerosis (ALS, or Lou
Gehrig's Disease) is associated with an accumulation and abnormal assembly of
neurofilaments in motor neuron cell bodies and in the axon, which may interfere
with normal axonal transport. The degeneration of the axons leads to muscle
weakness and atrophy, which is usually fatal. The over-expression of human
NF-L or NF-H in mice results in mice that have an ALS-like disease.
The vimentin-like filaments are a third family of intermediate filaments.
Desmin, a member of this family, is expressed in skeletal, cardiac, and smooth
muscle. Mice lacking desmin shownormal initial muscle development, but adults
have various muscle cell abnormalities, including misaligned muscle fibers.
Drugs Can Alter Filament Polymerization
Because the survival of eucaryotic cells depends on a balanced assembly and
disassembly of the highly conserved cytoskeletal filaments formed from actin
and tubulin, the two types of filaments are frequent targets for natural toxins.
These toxins are produced in self-defense by plants, fungi, or sponges that do
not wish to be eaten but cannot run away from predators, and they generally
disrupt the filament polymerization reaction. The toxin binds tightly to either
the filament form or the free subunit form of a polymer, driving the assembly
reaction in the direction that favors the form to which the toxin binds. For example,
the drug latrunculin, extracled from the sea sponge Latrunculia magnifica,
binds to actin monomers and prevents their assembly into filaments; it thereby
987
Figure 16-22 Two types of intermediate
filaments in cells of the nervous system.
(A) Freeze-etch electron microscopic
image of neurofilaments in a nerve cell
axon, showing the extensive cross-linking
through protein cross-bridges-an
arrangement believed to give this long
cell process great tensile strength. The
cross-bridges are formed by the long,
nonhelical extensions at the C-terminus
of the largest neurofilament protein
(NF-H). (B) Freeze-etch image of glial
filaments in glial cells, showing that these
intermediate filaments are smooth and
have few cross-bridges. (C) Conventional
electron micrograph of a cross section of
an axon showing the regular side-to-side
spacing of the neurofilaments, which
greatly outnumber the microtubules.
(A and B, courtesy of Nobutaka Hirokawa;
C, courtesy of John Hopkins.)

988 Chapter 1 6: The Cytoskeleton
Table 16-2 Drugs That Affect Actin Filaments and Microtubules
Phalloidin
Cytochalasin
Swinholide
Latrunculin
Taxol
Colchicine, colcemid
Vinblastine, vincristine
Nocodazole
binds and stabilizes filaments
caps filament plus ends
severs filaments
binds subunits and prevents their polymerization
binds and stabilizes microtubules
binds subunits and prevents their polymerization
binds subunits and prevents their polymerization
binds subunits and prevents their polymerization
stabilizes free tubulin, causing microtubule depolymerization. In contrast,
taxol, extracted from the bark of a rare species of yew tree, binds to and stabilizes
microtubules, causing a net increase in tubulin polyrnerization. These and some
other natural products that are commonly used by cell biologists to manipulate
the cyoskeleton are listed in Table 16-2.
(A)
o o
C -NH-CH-CH-C -O
15 pm
o
H3C-C -O
H,C
c-o
o
9oH
CH
o-c -
ll
o
CH:
Figure 1 6-23 Effect of the drug taxol on
microtubule organization. (A) Molecular
structure of taxol. Recently, organic
chemists have succeeded in synthesizing
this complex molecule, which is widely
used for cancer treatment.
(B) | mmunofluorescence micrograph
showing the microtubule organization in
a liver epithelial cell before the addition
of taxol. (C) Microtubule organization in
the same type of cell after taxol
treatment. Note the thick circumferential
bundles of microtubules around the
periphery of the cell. (D) A Pacific yew
tree, the natural source of taxol.
(8, C from N.A. Gloushankova et al., Proc.
Natl Acad. Sci. U.S.A. 91 :8597 -8601, 1 994.
With permission from National Academy
of Sciences; D courtesy of A.K. Mitchell
2001. o Her Majesty the Queen in Right
of Canada, Canadian Forest Service.)

THE SELF-ASSEMBLY AND DYNAMIC STRUCTURE OF CYTOSKELETAL FILAMENTS
Figure 16-24The bacterial FtsZ protein, a tubulin homolog in
procaryotes. (A) A band of FtsZ protein forms a ring in a dividing bacterial
cell. This ring has been labeled by fusing the FtsZ protein to the green
fluorescent protein (GFP), which allows it to be observed in living E coli
cells with a fluorescence microscope. Iop, side view shows the ring as a bar
in the middle of the dividing cell. Bottom, rotated view showing the ring
structure. (B) FtsZ filaments and rings, formed in vitro, as visualized using
electron microscopy. Compare this image with that of the microtubule
shown on the right in Figure 16-16C. (A, from X. Ma, D.W. Ehrhardt and
W. Margolin, Proc. Natl Acad. Sci. IJ.S.A.93:12998-13003, 1996; B, from
:i,:5:Tffi'-.'#l;illir'ifl ,':?y.',i.'"::i1e-s23'lee6Arrw*h
(A)
Bacterial Cell Organization and Cell Division Depend on
Homologs of the Eucaryotic Cytoskeleton
\Mhile eucaryotic cells are typically large and morphologically complex, bacterial
cells are usually only a few micrometers long and assume simple, modest shapes
such as spheres or rods. Bacteria also lack the elaborate networks of intracellular
membrane-enclosed organelles such as the endoplasmic reticulum and
Golgi apparatus. For many years, biologists assumed that the lack of a bacterial
cltoskeleton was one reason for these striking differences between cell organization
in the eucaryotic and bacterial kingdoms. This assumption was challenged
with the discovery in the early 1990s that nearly all bacteria and many
archaea contain a homolog of tubulin, FtsZ, that can polymerize into filaments
and assemble into a ring (called the Z-ring) at the site where the septum forms
during cell division (Figure 16-24).
The three-dimensional folded protein structure of FtsZ is remarkably simi-
Iar to the structure of o or B tubulin and, like tubulin, hydrolysis of GTP is triggered
by polymerization and causes a conformational change in the filament
structure. Although the Z-ring itself persists for many minutes, the individual filaments
within it are highly dynamic, with an average half-life of about thirty seconds.
As the bacterium divides, ttre Z-ring becomes smaller until it has completely
disassembled, and it is thought that the shrinkage of the Z-ring may contribute
to the membrane invagination necessary for the completion of cell division.
The Z-ringmay also serve as a site for localization of specialized cell wall
slnthesis enzymes required for building the septum between the two daughter
cells. The disassembled FtsZ subunits later reassemble at the new sites of septum
formation in the daughter cells (Figure f 6-25).
More recently, it has been found that many bacteria also contain homologs
of actin. TWo of these, MreB and Mbl, are found primarily in rod-shaped or spiral-shaped
cells, and mutations disrupting their expression cause extreme
abnormalities in cell shape and defects in chromosome segregation (Figure
f6-26).MreBandMblfilamentsassemble inuiuotoformlarge-scalespiralsthat
(A)
time 0
(min)
35
, trm
Figure l6-25 Rapid rearrangements of FtsZ through the bacterial cell cycle. (A) After chromosome segregation is complete,
thl ring formed by FtsZ at the middle of the cell becomes smaller as the cell pinches in two, much like the contractile ring
formed by actin and myosin filaments in eucaryotic cells. The FtsZ filaments that have disassembled as the cells have separated
then reassemble to form two new rings at the middle of the two daughter cells. (B) Dividing chloroplasts (red\ from a red alga
also make use of a protein ring made from FtsZ (yeltow)for cleavage. (A, from Q. Sun and W. Margolin, J. Bacteriol.
1 80:2050-2056, 1 998. With peimission from American Society for Microbiology; B, from S. Miyagishima et al., Plant Cell
13:2257-2268,2001. With permission from American Society of Plant Biologists')
37
(B)
(B)
1 rm
989
il;

990 Chapter 1 6: The Cytoskeleton
L-----.1 (B)
5um
span the length of the cell and apparently contribute to cell shape determination
by serving as a scaffold to direct the synthesis of the peptidoglycan cell wall,
in much the same way that microtubules help organize the synthesis of the cellulose
cell wall in higher plant cells (see Figure 19-82). As with Ftsz, the filaments
within the MreB and Mbl spirals are highly dynamic, with half-lives of a
few minutes; as for actin, ATP hydrolysis accompanies the polymerization pro-
CCSS.
Diverse relatives of MreB and Mbl have more specialized roles. A particularly
intriguing bacterial actin homolog is parM, which is encoded on certain bacterial
plasmids that also carry genes responsible for antibiotic resistance and frequently
cause the spread of multi-drug resistance in epidemics. Bacterial plasmids
typically encode all the gene products that are necessary for their owrsegregation,
presumably as a strategy to ensure their faithful inheritance and propagation
in their bacterial hosts. rn uiuo, parM assembles into a filamentous
structure that associates at each end with a copy of the plasmid that encodes it,
and growth of the ParM filament appears to push the replicated plasmid copies
apart, rather like a mitotic spindle operating in reverse (Figure 16-2z).Altholgh
ParM is a structural homolog of actin, its dynamic behavior differs significantiy.
ParM filaments undergo dramatic dynamic instability in uitro, more closeiy
resembling microtubules than actin filaments in the way that they grow and
shrink. The spindle-like structure is apparently built by the selective stabilization
of spontaneously nucleated filaments that bind to specialized proteins
recruited to the origins of replication on the plasmids.
The various bacterial actin homologs share similar molecular structures but
their amino acid sequence similarity to each other is quite low (- l0-15% identical
residues). They assemble into filaments with distinct helical packing patterns,
which may also have very different dynamic behaviors. Rather than uiing
the same well-conserved actin for many different purposes, as eucaryotic celli
do, bacteria have apparently opted to proliferate and specialize their actin
homologs for distinct purposes.
It is now clear that the general principle of organizing cell structure by the
self-association of nucleotide-binding proteins into dynamic helical filaments is
used in all cells, and that the two major families of actin and tubulin are verv
monomers replication f ilaments proteins
(A)
(B)
(c)
5[m
r
2ttm
Figure 16-26 Actin homologs in
bacteria determine cell shape. (A) The
common soil bacterium Bacillus subtilis
normally forms cells with a regular rodlike
shape. (B) B. subtilis cells lacking the
actin homolog Mbl grow into irregular
twisted tubes and eventually die. (C) The
Mbl protein forms long helices made of
up many short filaments that run the
length of the bacterial cell and help to
direct the sites of cell wall synthesis.
(From L.J. Jones, R. Carbadillo-Lopez and
J. Errington, Cell 104: 913-922, 2001. With
permission from Elsevier.)
Figure 16-27 Role of the actin homolog
ParM in plasmid segregation. (A) Some
bacterial drug-resistance plasmids
(yellow) encode an actin homolog, ParM,
that will spontaneously nucleate to form
small, dynamic filaments (green)
throughout the bacterial cytoplasm. A
second plasmid-encoded protein (b/ue)
binds to specific DNA sequences in the
plasmid, and also stabilizes the dynamic
ends of the ParM filaments. When the
plasmid has duplicated, so that the ParM
filaments can be stabilized at both ends,
the filaments grow and push the
duplicated plasmids to opposite ends of
the cell. (B) In these bacterial cells
harboring a drug-resistance plasmid, the
plasmids are labeled in red and the ParM
protein in green.Left, a short ParM
bundle connects the two daughter
plasmids shortly after their duplication.
Right, the fully assembled ParM filament
has pushed the duplicated plasmids to
the cell poles. (A, adapted from
E.C. Garner, C.S. Campbell ano
R.D. Mullins, Science 306:1 021 -1 025,
2004. With permission from AAAS; B, from
J. Moller-Jensen et al., Mol. Cell
1 2:1 47 7 -1 487, 2003. With permission
from Elsevier.)

THE SELF-ASSEMBLY AND DYNAMIC STRUCTURE OF CYTOSKELETAL FILAMENTS
L__ t (B)
zpm
ancient, probably predating the split between the eucaryotic and bacterial kingdoms.
However, the uses to which bacteria put their cltoskeletons appear somewhat
different from their eucaryotic homologs. For example, in bacteria it is the
tubulin (FtsZ) that is involved in cytokinesis (the pinching apart of a dividing cell
into two daughters), while actin drives this process in eucaryotic cells. Conversely,
eucaryotic microtubules are responsible for chromosome segregation,
while bacterial actins (ParM and possibly MreB) help to segregate replicated
DNA in bacteria.
At least one bacterial species with an unusual crescent shape, Caulobacter
crescentus, even appears to harbor a protein with significant structural similarity
to the third major class of cytoskeletal filaments found in animal cells, the
intermediate filaments. A protein called crescentin forms a filamentous structure
that seems to influence the cell shape; when the gene encoding crescentin
is deleted, the Caulobacter cells grow as straight rods (Figure f 6-28)'
Since we now know that bacteria do in fact have sophisticated dynamic
cltoskeletons, why then do they remain so small and morphologically simple?
As yet there have been no motor proleins identified that walk along the bacterial
filaments; perhaps the evolution of motor proteins was a critical step allowing
morphological elaboration in the eucaryotes.
Summary
(A)
I
2pm
The cytoplasm of eucaryotic cells is spatielly organized by a network of protein ftla'
ments known as the cytoskeleton. This network contains three principal types of filaments:
microtubules, actin filaments, and intermediate filaments. All three types of filaments
form as helical assemblies of subunits that self-associate using a combination
of end-to-end and side-to-side protein contacts. Dffirences in the structure of the subunits
and the manner of their self-assembly giue the fitaments dffirent mechanical
properties. Intermediate filaments are rope-like and easy to bend but hard to break.
Microtubules are strong, rigid hollow tubes. Actin filaments are the thinnest of the
three and are easy to break.
In liuing cells, the assembly and disassembly of their subunits constantly remodels
all three types of cytoskeletal filaments. Microtubules and actin filaments add and lose
subunits only at their ends, with one end (the plus end) growing faster than the other.
Tubulin and actin (the subunits of microtubules and actin filaments, respectiuely)
birul and hydrolyze nucleoside triphosphates (tubulin binds GTP and actin binds
ATP). Nucleotide hydrolysis underlies the characteristic dynamic behauior of these two
fiIaments. Actin filaments in cells seem to predominantly undergo treadmilling, where
a filament assembles at one end while simultaneously disassembling at the other end.
Microtubules in cells predominantly display dynamic instability, where a microtubule
end undergoes alternating bouts of growth and shrinkage.
tnAlhereas tubulin and actin haue been strongly conserued in euolution, the family
of intermediate filaments is uery diuerse. There are many tissue-speciftc forms found in
the cytoplasm of animal cells, including keratin filaments in epithelial cells, neurofilaments
in nerue cells, and desmin filaments in muscle cells. In all these cells, the primary
job of intermediate ftlaments is to prouide mechanical strength.
Bacterial cells also containhomologs of tubulin, actin and intermediate filaments
that form dynamic filamentous structures inuolued in determining cell shape and in
cell diuision.
991
Figure 1 6-28 Caulobacter and
crescentin. The sickle-shaped bacterium
Caulobacter crescenfus expresses a protein,
crescentin, with a series of coiled-coil
domains similar in size and organization to
the domains of eucaryotic intermediate
filaments. In cells, the crescentin protein
forms a fiber that runs down the inner side
of the curving bacterial cell wall. When the
gene is disrupted, the bacteria are viable
but grow in a straight rod-shaped form.
(From N. Ausmees, J.R. Kuhn and
C. Jacobs-Wagn er, Cell 115:705-713,2003'
With oermission from Elsevier.)

992 Chapter 16:The Cytoskeleton
FILAM ENTS
Microtubules, actin filaments, and intermediate filaments are much more
dynamic in cells than they are in the test tube. The cell regulates the length and
stability of its cytoskeletal filaments, as well as their number and geometry. It
does so largely by regulating their attachments to one another and to other components
of the cell, so that the filaments can form a wide variety of higher-order
structures. Direct covalent modification of the filament subunits regulates some
filament properties, but most of the regulation is performed by a large array of
accessory proteins that bind to either the filaments or their free subunits. Some
of the most important accessory proteins associated with microtubules and
actin filaments are outlined in Panel 16-3 (pp. gg4-gg5). This section describes
how these accessory proteins modify the dynamics and structure of cltoskeletal
filaments. we begin with a discussion of the way that microtubules and actin filaments
are nucleated in cells, because this plays a major part in determining the
overall organization of the cell's interior.
A Protein complex containing y-Tubulin Nucleates Microtubules
\.{/hile cr- and B-tubulins are the regular building blocks of microtubules, another
type of tubulin, carled y-tubulin, ]nas a more specialized role. present in much
smaller amounts than cr- and B-tubulin, this protein is involved in the nucleation
of microtubule growth in organisms ranging from yeasts to humans.
Microtubules are generally nucleated from a specific intracellular location
knor,rm as a microtubule-organizing center (MToc). Antibodies against y-tubulin
stain the MToc in virtually all species and cell types thus far examined.
Microtubules
Emanate
from the centrosome
in Animal cells
Most animal cells have a single, well-defined MToc called the centrosome,
located near the nucleus. From this focal point, the cytoplasmic microtubules
emanate in a star-like, "astral" conformation. Microtubules are nucleated at the
centrosome at their minus ends, so the plus ends point outward and grow
toward the cell periphery. Microtubules nucleated at ihe centrosome continuously
grow and shrink by dFramic instabiliry probing the entire three-dimensional
volume of the cell. A centrosome is composed of a fibrous centrosome
y-tu bul in
accessory proteins in
y-tubulin ring complex
Figure 16-29 Polymerization of tubulin
nucleated by ytubulin ring complexes.
(A) Structure of the y-tubulin ring
complex, reconstructed from averaging
electron micrographs of individual
purified complexes. (B) Model for the
nucleation of microtubule growth by the
lTuRC. The red outline indicates a pair of
proteins bound to two molecules of
y-tubulin; this group can be isolated as a
separate subcomplex of the larger ring.
Note the longitudinal discontinuity
between two protofilaments.
Microtubules generally have one such
"seam" breaking the otherwise uniform
helical packing of the protofilaments.
(C) Electron micrograph of a single
microtubule nucleated from the purified
y-tubulin ring complex. (A and C, from
M. Moritz et al., Ndf. Cell Biol.2:365-370,
2000. With permission from Macmillan
Publishers Ltd.)

srNtWVlll
lvrlll)soi^) ullHr llvln9lu slll) MoH
(v)
(saxaldLUo) 6ul.t ut 1nqnl-/"1
salts 6ut1ea;lnu
salorJlua)
IO l|eo
a
xlJleuJ
auosoJlua)
rqBIJ 1B pa8uerre seJnlsnJls
(8)
3 sE edolalue ruelrnu eq] uI peppaqua
.
+"..--d
auosortua) alll +o
saxalduor 6uu utlnqnl-l'
Luo.r1 6ur,uo:6 salnqnlol)tul i
pcrrpulrtc snorralsl.ru lEq^\eluos;o rred e sI aruosoJlueJ eql uI peppaqurg
'lunl-,| eql ale^pcE puP llnrseJ
,taqt lv\oq u^/\oDI ]a,{ }ou sI }I puB 'peJaAoJSIp aq o} UIEIIIaJ xlJlelu slql IIIJOJ
]eq} suraloJd aqt Jo lsontr .3gn1-1,1o sardoc d4q ueqr eJolu suleluos ]eq] x!&aut
uopern8guorpadeqs-1 ue uI Jeqlo qJea o] sa13uu
sr lEql coIN ue lB pelealJnu aJe selnqnloJcllu 'surolerp pue rSun; u1
'poolsJapun-ile^A lou SI UOI1
-ecqdnp slr JoJ srsEq Jplncelolu eqJ .suraloJd .{rossacce Jo Jeqlunu a3re1 u snld
'selnqnlorJlru palJlpotu Jo JepuII^o uoqs € Jo slslsuoc elolJluec V 'GL11-ZL1I
'dd 't-zt
Ieued aas) apurds qloilur aql;o salod o^u aql ruro; '{aqt pue 'sulSeq
srsolrru ueq A snalcnu--"qt 1o Sapls allsoddo ol a4olu Saluosolluac ralqfinep orul
a'saq1 :red alolJlue3 pafecqdnR e Suruteluoc Jpq qcBa 'aseqd:a1ur Surrnp slred
pnbe orntr o1u1 s111ds pue saleclldnp euosoJluef, eql '11 raldeq3 uI peqIJJSap
rV'(f g-gf arn8gg) alei{dnp sallesueql selolr}ueo aql se a1c'ic 1ac qoea Sulrnp
uopecldnp slr Sutrnsua '(pualuu JelolJluaclJed aqt palec oslu) xulutu etuos
-orf,r", aziuefiro selorrlueJ aql'(Jalel pequJsep) sllao alporu ur ella8eg pue
"qi
ellro Jo serpoq prnq "qi"-of,eq qJIqM 'selolrlual aql aru esaql'(0t-91 arn8lg)
a1od a1purds eq] peller enbeld IIEtus
seJnsolcua IprcrJrup ur ue^a slql saop ]I pue 'Ialuaf, sll le aulosoJlues aql uop
-rsod 01 pup ilel eqlJo suorSer Surrtpno eqt ^a^Jns ot eJIAep e se slcB eluosoJlueJ
aql tuo4 Superper salnqnlorclu Jo lualsl's aql 'snelJnu aqlfteu pelJallor spua
snurru aiqers pue draqdirad IIao eqt prumo] pJe,tirlno Supulod spue snld rlrueu^p
qll^\ ']snqoJ zirarr sr selnqnloJJlru Jo uonBJnBIJuoJ IPJISP eql 'sllao IeruIuP uI
'selnqnloJslru Jlaql aleelJnu o1 lI
asn o] ruaas pue urlnqn]-1, uteluoc slleJ aseqt ile 'secueJeJJlp aseq] eltdsaq 'sa1o
-rJluao uleluoJ s11ac tuuld ]soru Jou r8un; raqllag 'ado1a'Lua Jealcnu eql punoJe
IIE palnqlJlslp sells lB salnqnloJJllu elPelJnu o] luaes slleJ ]uEId-JaqBE1 'tpoq
866
('ssard Allslo^run ralleJol)ou aqf
u.ror, uolsslulad ql111n'€002' 99v- LSVitg L
'tolB
ilD'r "le ]a aloolo 'I'l tuolj '))
'euosorlua) aql Luot; 6utleuetue uaas
aq ue) salnqnlor)ur Jo la))lLll asuap
e '1a> suo0ala ) e uor, )OIW aql Jo
aber,ur palrnrlsuorer e ul (l) 'useldol^)
aq] u! aar, sl alnqnloJ)lru qlea Jo pua
snld aql searaqm'xa;duot 6utt utlnqnt-,t
e ulor; unror6 6utneq 'euosoltua) aqi
ur pappaque sl alnqnlol)ltu qfea Jo pua
snurur aLlI'salnqnlorlrLu paq)elle qllM
aurosortua) V (g)'lxal aql ul paqu)sap se
'seloulua)
Jo rred e r(q paztue6lo sl xllleur
srql'poq)elte ele qlmot6 alnqnlol)ltrr
aleal)nu leql saxalduot 6ut.t utlnqnl-/"
aLll q)lqM ot sulalold snolqu ro xlrleul
snoqdrou:e ue jo slstsuo) ]t 'snal)nu aql
ol txau r,useldoilt aql u; pale)o1 'slla)
lpurue Jo 191y11 tofetu aql st eulosorlual
aql (V) 'auosotluo) aql OE-91 arn6t1
('rarAasll ruorJ uorssrtIlJad
qlm'/66 t 'EtL-LLLijZL
'lotB'Dnrls,'r'le
la uall?rql 'c urorJ
poldepe'g 'suo5
I ,(allM uqof uorj
uolsslLurad q]!M 0002' 6 LV-60Vt6V
'
LPal'saa 2su)tw'reulreu'B'r
pue no I')|rew 'r'D uroll'v)
red aloulue) aql Jo alnl)nlls (8)
'{qreau uaes are selnqn}oJ)tul
snoror.rtnN'eloulua) ratq6nep
aql Jo uott)as ;eurPnttbuo;
e pue aloutue) laqlou-l eql Jo ^^al^
uo-pua uP 6urrvroqs auosorlua)
e Jo uorl)as urqt e Jo qdet6ottttu
uoll)ala uV (V) 'auosoJlua)
aql u! alolllusr y 1g-91 arn6t3

ACTIN FILAMENTS
./ a
{
A
cofilin
formin
nucleates assembly and remains
associated with the growing
plus end
thymosin
binds subunits,
prevents assembly
binds ADP-actin f ilaments,
accelerates disassembly
fimbrin
a-actinin
ar
rro
a't C
./
gelsolin
severs filaments and
binds to plus end
F
oa o -V
O2or
oo.
actin subunits
actin filament
ARP complex
nucleates assembly
to form a web and remains
associated with the minus end
'r t
-\
o\t.
a
capping protein
prevents assembly and
disassembly at plus end
filament bundling, cross-linking, and attachment to membranes
f ilamin
profilin
binds subunits,
speeds elongation
Some of the major accessory proteins of the actin cytoskeleton. Except for the myosrn motor proteins,
a later,section,
to be discussed
an
in
example oi each. major type is shown. Each of theie is discussed in the text. HoweveL
more than
most cells
a hundred
contain
different actin-bindin! proteins, and it is rit"rvihaiih*e are important
proteins
types of actin-associated
that are not yet recognized.

sllnSntou)t1A
-G
)llrl )\rl
E(5gr*"
E I ursaurl
utuiqlels
{lqurasse sluanard
'slrunqns
spurq
+- pua snld 1e {lqLrassesrp
:rqdorlsele: sa)uequa
o c -1,
\rr
,
u ruele)
salnqnlorlrr.lr sJa^os
)UnJ.-,t
pua snutu 3q]
r..llrM paler)osse sureuar
pue ^lqLUasse saleel)nu
\
\ elnqnloJ)rur
6ur1ur;-sso.o pue 6urlpunq luauelU
t i
t\|
saprs 6uo1e 6urpurq
{q salnqnl azrlrqe}s
u r1re1d
sluaurPlrj alerpauljalur ol s)u.,
'pazru6o)al
la^ lou oJe leq+ suralord paler)osse-alnqnlor)rur +o sodl+ lueuodur sre sraql leLll ^lolrl sr ]r-suroloJd
poler)osse-urpe aq] rol se-pue 'surelo.rd 6urpurq-elnqnloJ)rur luaJa#rp poJpunq e ueql aJour ureluo) slla) +sour
la^omoH '1xo+
aql ur possn)srp sr osaql +o q)el 'umoqs sr ed[1 .rofew q)eo ]o a;duexe ue 'uorpos ia]el e ur passn)srp
aq o1 'suralo.rd
rolouJ +o sassel) o,V\l Jo+ lda)xl 'uola;a1soy{r alnqn}oJ)il,1r aq1 ,ro suralord fuosse>>e roleur eq} +o aulos
il-
sdvy{
s t zdvhlx
i(lquasse saleralarle pue
spua snld saztltqels

uolelelsoil) aql :9 I raldeql) 966
'(9-91
arn8rg eas) eJeJrns Ipseq eql
se reJ sB Surpuaga spua snld qlIM 'llac eql Jo srxe 3uo1 aql Suop sruroJ selnqn]
-oJJrru
lalleJPd f.Feau Jo ^EJJE ue (uoueJol crJleruludsE slql luoJc 'eueJqluau
Eluspld pcrde aql reau uor8ar e ol e^otu spua snulru eFqnloJJIIu eql 'pazuElod
{UBU saruocaq pue suorlcunf 1ac-11ac sIuJoJ IIaJ Ielleqtlde ue uaq \ 'aldruexa
JoJ 'snql'surelsds eleurpJoo3 IEuJsluI elEJoqela aloru Jleql qslqetsa ol sruslu
-eq3eu Surrnsearu puoplppE asn lsnlu slleJ Ielleqllda pue 'salcsntu 'suoJneu
sE qrns serSoloqdrou xaldruoc qllm slleo pelellueraJJlp [tq8Uf '[ac aql ulqrp\
selleuB8Jo ^,(ueu uoltlsod o1 pasn uaql sl qJIqM 'uralsLs elEulpJooo IeJeuaB
B seqsqqelse IIes aqtJo ratuas aql pulJ o] uolele{sotdc alnqnlorJllu eq}Jo,irulqe
slql '(tt{I
arntgg) aruosoJtual pelBlofl eq} dq pesn usluErlcaru Sutqsnd a1d
-urs eq1 lsnf ueql slueuodruoJ eJolu aAIoAul ^Eur ssaf,oJd srqf q8noqlp teluec
eql ]e peJalsnlc spua snurru eFqnloJsllu eqr qll^\leJJe padeqs-re1s e olul se^Ies
-ueql aBuBJJu sellauB8ro snoeuuJqruau qlIMBullcEJalul seFqnloJcllu cnuuufp
'aruosoJlueo
eql SuDIJpl luaru8e4 IIaf, pelelosl ue uI uaAA '(ZS-9I arnS;g)
euerqurew eurseld aql le poteal)nN ueuo erv slueulellJ ulpv
uBc seJnlsnJls urlJe 'au€Jqrueru ErusBId eql ol puE Jaqloue euo o] slualuqceuP
Jraql uo Surpuadap 'aldurexa JoC 'aJeJJns IIec eql Jo luarua^olu puE edeqs
eq] surruJalep ]l ul slualuelrJ uI]38 aql puP 'xeuoJ IIaJ eql pall"3 sI euEJqlueu
euseld eq] qtBeueq rsnf rai(e1 aql draqduad ilec eql ]e sl sllal lsotu uI slueul
-PIrJ urlf,e Jo &rsuap tsaq8q eqt fpuenbesuo]
'euEJqrueru srusEld eq] reeu ro
]E sJnrso lilluanbe4 lsol'u uol}Palsnu ]ueluellJ ullcs 'snalJnu eql rueu ursBldoldc
eqf ulqlp{ daap l.Irzruud sJnJJo qJIq \ 'uouPalf,nu eFqn}oJf,Ilu ol }s€Jluoc uI
IIaJ Jo sadIl ]uaJeJJIp ^FuDIu]s ,(ueur urroJ
ur sdnr cll,tco8eqd eql pup 'salertsqns pqos re^o slleJ eloru dlaq teqf Dlpodll
-pwql palleJ slre^ e^rsnrlord pE'aryodol{ rc qunoJJlla sB qf,ns sapunq allds
pazflelP-J eq ue3 uor]palJnu srqJ'lueuuoJl^ua leuJalxa sll uI se8ueqJ o] asuodsaJ
ur dlprder sser4Jrts pue adeqs str e8uuqc 01 IIaJ eq] 3umo1p 'auerqtuaur etuseld
eql ]E sluatuelg urlJe Jo uorlEelsnu aq] elElnSar ^lluenba.{ spuSIS lBurexa
'se8eqdOJJEur
llo) oroLloouelour
lteo
oloulue) 6urureluor
euJosolluo)
1uau6e.r1
lla)
peraAos
uirl 0 t
apnlJur eseqJ 'suoucaford aJEJJns
6urzrue6ro
-alnqnloJ)rur /l/\eu
6urpel raluar
tuau6erl 1;ar /
salnq nlor)rrx
pezrue6JoaJ
qlliV\ saloulua)
fsaluar)s Jo
{urapery lpuor}eN utol; uotsstullod rlllM
'
166 L' Lt Zg-8229:V6',V',S', n', pS'pDV ltoN
iud "p 1a {;og '31 urorl) 'arnsol)ua
aql
Jo slleM lnoJ lle uo 6urqsnd solnqnlolltur
aqt Jo uorlnqulsrp aql Moqs ol
paurels pue pexu /auiosoJlua) paletua)
-Jlas V (€)'sle^ralur alnurul-aalql le ualel
araM sarnt)ro asaqf'llo/1/\ aql,o latua)
aql ol auosorluaf aql sa)roj uor]rsoo eql
azrlrqels ot suorl)arp ;;e ur 6urqsnd ;enbe
roJ luauarnDar aqf 'llali\ aqr Jo slleM
oql lsule6e qsnd {aql'auosorlue) aql
,(q paleapnu'azrratul;od salnqnlot)rur
oql sv'uortnlos ur slrunqns urlnqnl q]rM
6uo;e
's;;am asaql Jo auo otur pereld senn
aurosorlua) al6urs y'aletlsqns rrlse;d
e olur paurq)euror)rur araM sllaM aJenos
lleLus (v) 'oruosorlua) e ro lor^eqaq
6uglaas-reluar aq1 7g-9 1 aln611
aql ro raluar aql reau dn pua spua snutu
raqt leql os azruebtoer 1uau6er1 ;1ar
paq)elap eql u! salnqnlol)tu aq]'alpaau
e qllM #o tn) sr lla) lueubrd qsr; e
,o rlre aql rauv 'lla) p,o lalua) aql pulJ
uel {ere alnqnlor)lul V €€- 9 1 ern6!l
talua) 6utztue610
-alnqntoj)tul Mau e ul paunq 'luaur6elJ

HOW CELLS REGULATE THEIR CYTOSKELETAL FILAMENTS
by two different tlpes of regulated factors, the ARP complex and the formins
(discussed below). The first of these is a complex of proteins that includes two
actin-related proteins, or ARPs, each of which is about 45% identical to actin.
Analogous to the function of the y-TuRC, the ARP complex (also known as the
Arp 2/3 complex) nucleates actin filament growth from the minus end, allowing
rapid elongation at the plus end (Figure 16-34A and B). The complex can also
(A)
T..-l
".,in ll ]
activating
factor
ARP complex active ARP complex
(B)
t.\
\!/
nrpz [ ]
mrnus
eno
nucleated actin f ilament
(D)
arpr [ ]
Figure 16-34 Nucleation and actin web formation by the ARP comple;
actin. Afthough the face of the molecule equivalent to the plus end (toP.
differences on the sides and minus end (bottom) prevent these actin-reli
into filaments with actin. (B) A model for actin filament nucleation by th
are held by their accessory proteins in an orientation that prevents then
indicated by the blue triangle binds the complex, Arp2 and Arp3 are bro
an actin filament. Actin subunits can then assemble onto this structure,
1 6-10). (C) The ARP complex nucleates filaments most efficiently when i
filament branch that grows at a 70. angle relative to the original filament. Repeated rounds of branching nucleation result in a treelike web of
actin filaments. (D) Top, electron micrographs of branched u'.tin filur"ntt formed by mixing purified actin subunits with purified ARP
complexes. Bottom, reconstructed image of a branch where the crystal structures oi actin and the ARP complex have been fitted to the
electron density. The mother filament runs from top to bottom, and the daughter filament branches off to the right where the ARP complex
binds to three actin subunits in the mother filament (D, from R.D. Mullins et al., Proc. Natl Acad. Sci. IJ.S.A.95:6181-61 86, 1 998' With permission
from National Academy of Sciences, and from N. Volkmann et al., Science 293:2456-2459, 2001. With permission from Macmillan Publishers Ltd')
100 nm
10 nm
997

998 Cha pter 1 6: The Cytoskeleton
rR\
20 um 100 pm
attach to the side of another actin filament while remaining bound to the minus
end of the filament that it has nucleated, thereby building individual filaments
into a treelike web (Figure l6-34C and D).
near the plasma membrane in yeast, where it is required to form cortical actin
patches (see Figure 16-6), and in plant cells, where it directs the formation of
actin bundles at the surface that are required for the growth of complex cell
shapes in a variety ofdifferent tissues (Figure f6-3b).
The Mechanism of Nucleation Infruences Large-scale Filament
Organization
or microtubule and prevent both subunit addition and subunit loss at this end.
Figure 1 6-35 Function of the ARP
complex in plant cells. (A) Cells in the
maize leaf epidermis form small, actinrich
lobes that lock neighboring cells
together like pieces of a jigsaw puzzle.
(B) The regular pattern of interlocking
cells covers the leaf surface. (C) Epidermal
cells in a mutant plant lacking the ARp
complex do not form the interlocking
lobes. The brick-shaped cells are normal
in size and spacing, but form leaves that
appear too shiny to the naked eye. (From
M.J. Frank, H.N. Cartwright and
L.G. Smith, Development 1 30:7 53-7 62,
2003. With permission from the Company
of Biologists.)

lrilnDlu slll) A oH
pua snld
pua snulu.l
srNrwvl|l tvllrr)sou) ullHr
uotte6uoll
,llpow sllunqns aarJ aql ol pu!8 leql sulelord
lueurelll
qslJ-urlJP Jo uolsuaxe eql puB uollEzlJelu1(lod ullJe IBsol a^Isoldxa esnpord
o] ullrJord elelllcp uBJ sleu8rs JelnllaoeJlxa'uoltecol slql ]V'araql spldloqdsoqd
auerqruaru 3rpl3e o] spulq 1l esnecaq euBJqueu Bluseld eql Jo aJPJ slloso]]tc
aql l€ pezllesol sI uIIIJoJd 'euEJqueu Pluseld aql tE dlqruasse luauPIIJ JoJ
IEJIIIJJ sr sluetuellJ Jo spue Suntor8 eq] oluo sllunqns ullse palelsenbes aloul
ol dr$qp spggord 'aldruexa JoC 's]te ulllJoJd eJaqm salls aql euuep UBJ sluslu
-BqJeu aseqJ'spldlloqdsoqd IolISouI ol Supulq uIIIJord pue uorlel,fuoqdsoqd
uqgord Sutpnlcut drprpce ullgord eleln8ar stuslueqf,eru JelnllaceJlul IeJaAaS
'(gt-gl
arn8lg) u1goJd ot punoq aq ullJe rlraluouolu aqt leql eJlnbar
ueJ uorlB8uola ]uaruEIIJ ultJB 'sasec eseql q :(e^oqe pessnf,slp sutalord 3ut
-]eelJnu-ur13e;o dpuel eql) sultuJoJ uleueJ qll1v\ pelelf,osse sI pua snld asoqrr
slueruelg asoql JoJ uoll€Allf,E ullgord uo ,i.puor1s aJolu uela spuedep ql,lroJ8
tueruelu ullf,V 'spue snld ]ueIuEIIJ oluo Iood punoq-ursoudq] peralsenbes
eql uroq sllunqns ullJ€ Jo ]uatue^olu e sI selnJalour u1p;ord Jo uolle^Ilf,e
IBOOI
p Jo ]lnseJ lau aqt 'sJeluouoru ullcE IenpIAIpuI ol Supulq ur utsourr(ql qlyvr
saladruoJ ulluoJd asnpJag ra8uol llunqns euo luaIuBIIJ UIIJE eql Sulteal 3go
sllpJ urTgord eql os 'ulllJord rog trrluUJB slr saJnpeJ ]eqt ultJe eqt ur e8ueqJ leuoll
-BruJoJuoo e sacnpul uop1ppe sql'pue snld aar; E oluo ppe ,(llpear uec xald
-ruoJ ur]f,e-ulgord oql'Og-9I arntg) pua snld aq] ol spulq lBql reruouotu aq]
uo alrs aql pasodxa 3unea1 allqm rpua snulru luetuellJ aqr qUltlr aletcossu .{.1pru
-rou plno^ lEr{l Jaruouou aq} Jo epIS aql SuDlcolq 'rya1c Sutputq-dlv aq} a}IS
-oddo raurouoru ullJe eqtJo aceJ eql ol spulq uITgoJd
'u11{otd'uralord Surpurq
-Jaruouoru reqlouE uo spuadap ]ualullnJser 'pEslsul 'eseJ aq] aq ol punoJ uaaq
lou seq srql lnq 'ursourdql alep8ar plnoJ 9I raldeq3 uI passn3slp esoql se qJns
s,'i.ea,rqted uollJnpsueJl pu8ts JI sE ruees tq8pr lt ;uopeztrarudlod ro; ueql esn
pue lood a8erots pereJJnq qql ruoq sreruouoru ultoe llnJf,er sllao op ,vroH
'apuoalcnu punoq rgaq] a8ueqcxa rou azf.lorpr(q reqltau uec
pue luauEIU ulloe Jo spue snulru ro snld aql Jeqlle qllm alelcosse louuec ,(aq1
aJeqm'alels pe{ool e ur aJp ursourdql ol punoq sJeruouoru ultf,V'sulalord asaql
Jo luepunqp lsoru eq] st utsouttt4t palleJ ulaloJd leurs V '(ullnJunJlel Snrp eql
Jo teql ol TEIITuIS uollJe uE) elqEJo^EJ ssel qJnu uolluzuaurrtlod Suqeru Lqalaql
'sJaruouoru uuce eql ol pulq leql sutalord ptcads suleluoJ lood lrunqns aql
leq] sr uoseal aqJ isluatuullJ olul Surzrrarul.lod uuql JeqleJ 'elqnlos uPIuaJ ullce
eql Jo qcn{u os seop ,r(qq'aqn1 }sal e uI ul}ce arnd JoJ pa^-Iasqo INrl I ueq} ssel Jo
uorlerluacuoJ leor]rrr aqt uanr8 'q8rqdlSursrrdrns st sqt :(lru/3ru 8-Z) INt1 002-09
dlecrdIl sr uolleJluaJuoc Jatuouou elqnlos aq; 'spu8rs puJelxa ol asuodsar
ur.{lprder a8ueqc uer ollur slq} q8noqrle 'a1qn1os sl %09 pue s}ueuEIIJ uI sI uI}rP
aqf Jo %09 ,(laleurxordde 's11ac eleJqaue^ elJsnluuou lsoul ul 'sllunqns elqn
-los
Jo uoplppE eqt r(q a1e3uo1a dlerauaS slueluelu p1ala1so{c 'palEelsnu eJuo
('unqr5 'l a)uarel) pue r>1>,{zog laeq)rW }o (saynol) 'pua
snuru aql ot 6utputq uoll paluanatd {1;elttals sl ll }nq'luoulel!J u!1)e uelo
pua sn;d aql ale6uola pue ol pulq arolaraql ue) laulpolelaq ult)P-ullUoro
srrll'Ual) 6u;putq-61y aql allsoddo ull)P Jo a)eJ aql ol spulq u!llJord
'uaatb u7 umor.ls st dIV'par ut utl)e eql pue'an/q ul uMoqs sl aln)alour
uralord urlr;o.rd arll 'rauiouoiu u!l)e ue ol punoq u;1;1o16 1g-91 atn611
'a)p!ns auerquraur aql qleauaq All)eJrp
luaurelu u!t)e aqt lo uorlezuaullod
lPuorlrasur oql p!e pue'auerqureur
eurseld lla) aq] ot pal)auuo) Ill)arlpu!
ole sururroJ {ueyt1 '11ar aql ur sarnlfnrts
reln>l1ted o] ]l lull pue ^t!^!l)e st! aleln6ar
suorbar laqlg'eraq umoqs sr aln)elour
urr..tuo; abre; aqt;o tred ^luO 'alqLuasse ot
Iunqns Mau Ll)ea sMollP t! se pua snlo aql
le pasooxa slrunqns ur])e oMl aql jo auo
o1 6urpurq sll surelureur uralold urr-u.ro1
aq1 sa1e6uo1e lr sp pua sn;d 6urmor6
-{lprder aqt qllm poler)osse ureuar pue
(pal) ]uaLUPllJ ullf e Mau e Jo uorlPuuo]
aql aleel)nu ue) leql xaloulo) )llouJp
e w)o! (uad6) surato.rd uru.ro3 'sugtu,rol Iq
palelporu uollebuo;a ulr)v 9€-9[ a.tn6!l
]+al) 6urPurq-dIV
ur aln)elour dfv
666

1000 Chapter 16;The Cytoskeleton
Figure 16-38 Profilin and formins. Some members of the formin protein
family have unstructured domains or"whiskers"that contain several binding
sites for profilin or the profilin-actin complex. These flexible domains serve as
a staging area for addition of actin to the growing prus end of the actin
filament when formin is bound. Under some conditions, this can enhance the
rate of actin filament elongation so that filament growth is faster than that
expected for a diffusion-controlled reaction, and faster in the presence of
formin and profilin than the rate for pure actin alone (see also Figure 3-g0C).
motile structures such as filopodia and lamellipodia (see below). Besides binding
to actin and phospholipids, profilin also binds to various other intracellular
proteins that have domains rich in proline; these proteins can also help to localize
profilin to sites that require rapid actin asrembly.
As it does with actin monomers, the cell sequesters unpolymerized tubulin
subunits to maintain the subunit pool at a level substantially higher than the
critical concentration. one molecule of the small protein smnmtibinds to two
tubulin heterodimers and prevents their additlon onto the ends of microtubules.
Stathmin thus decreases the effective concentration of tubulin subunits
that are available for polymerization (an action analogous to that of the drug
colchicine). Furthermore, stathmin enhances the rk;hhood that a growing
microtubule will undergo the catastrophic transition to the shrinkin"g statel
Phosphorylation of stathmin inhibits iis binding ro tubulin, and signils that
c.ause stathmin phosphorylation can increase the rate of microtubui=e elongation
andsuppress dynamic instability. Cancer cells frequently overexpress stuihmin,
and the increased rate of microtubule turnover that results is thought to
contribute to the characteristic change in cell shape associated with mahlnant
transformation.
severing Proteins Regulate the Length and Kinetic Behavior of
Actin Filaments and Microtubules
In some situations, a cell may break an existing long filament into many smaller
filaments. This generates a large number of new filament ends: one iong filament
with just one plus end and one minus end might be broken into dozens of
short filaments, each with its own minus end and plus end. under some intracellular
conditions, these newly formed ends nucleite filament elongation, and
in-this case severing accelerates the assembly of new filament structures. under
other conditions, severing promotes the depolymerization of old filaments,
speeding up the depolymerization rate by tenfbldor more. In addition, severing
filaments changes the physical and mechanical properties of the cytoplasmi
stiff, large bundles and gels become more fluid when the filaments are severed.
To sever a microtubule, thirteen longitudinal bonds must be broken, one for
each protofilament. The protein katanin, named after the Japanese word for
"sword," accomplishes-this demanding task (Figure 16-39). Kaianin is made up
of two subunits, a smaller subunit thai hydrolyies Arp and performs the actual
severing, and a larger one that directs katanin to the centrosome. Katanin
releases microtubules from their attachment to a microtubule organizing center,
and it is thought to have an important role in the rapid microtub"ule dep"oll,rnerization
observed at the poles of spindles during mei,osis and mitosis. It may also
be involved in microtubule release and depolyrnerization in proliferating cells in
interphase and in postmitotic cells such ai nerr.ons.
In contrast to microtubule severing by katanin, which requires ArB the severing
of actin filaments does not requiie an extra energy input. Most actin-severing
proteins are members of the gekotin superfamity,'whbse severing activity
is activated by high levels of cytosolic ca2*. Gelsolin has subdomains that bind
to two different sites on the actin subunit, one exposed on the surface of the filament
and one that is normally hidden in the longitudinal bond to the next subunit
in the protofilament. According to one model for gelsolin severing, gelsolin
binds on the side of an actin filament and waits until a thermal fluctuation hanpens
to create a small gap between neighboring subunits in the protofilament;
gelsolin then insinuates its subdomain into the gap, breaking the filament.
't
tl
I nreear wrrH
I RELOADTNG OF
+ ACTTN ON WH|SKERS
continued rapid growth of actin
filament at plus end

t00 r slNl wvt I I tvl-ltilsor-A) u ll Hr lrfl n9lu slll) MoH
ueql azrlrqelsec ro az!l!qels
raqlll ue) sluauel!l Jo sapls eql 6uolv pu!8 reql sulalold
sdVIN eseqJ'(07-9I arn81g; .{poq 1ec aq} Ixoq puega }Eqt sallrpuap pue suoxe
aqt Jo aJoJ aql uroJ salpunq eFqnlorJlu paz[qe]s alaqM 'suoJneu ul ]ueu
-nuord sr lasqns srqJ 'sluauodruoc 1ac Jaqlo qlIM selnqnloJJllu Jo uollJeJelul
eql ele1patu os1e uec sdVIAtr Jo lasqns y ,{lqurassesp lsup8e salnqnloJclru azIIIq
-Els uEJ sdVIAl 'loxel Snrp aqt a{l'I 'sdVIAtr ro 'su1e1ord pelulcossu-eFqnloJoIru
palpc ,{.1anuf,alloc are salnqnlorrrru Jo sepls aqt Suop pulq }pqt sulatord
_
AIeAr]f,eosaJ
'raur^,(1od aqt azlrqelsep Jo azlpqpls reqtla dqaraql u(aql pue 'a1e1s rarul.lod
aql;o d8raua aa{ aq} asrer Jo Ja^ ol Jaq}Ie o} ^8Jaua Surputq JIaq} asn suta}ord
pelerlossE-tuaruplg 'sarlradord
lueJeJJIC
IBJIueqJau pue dlurqels s,]uauellJ aql
ralp,{.eru raru.{1od eqlJo sepls aq} Suole pulq }eql suralord;o 1as e '1ood }Fnqns
eql tuo4 pale8uola pup uonealf,nu,tq pauroJ sl lualuelg p1a1a1so{J e af,uo
aFqnloJJIIu aql 01 spulq leql ulelxop auo lsEel lP eABq
]€ql JaqlouB pue aceJJns
arn8lgl salnqnlorcrru pa>1ced [1aso1c eJoru Jo sapunq IuroJ 'uIEIxop Su1}caf
-ord ralroqs qonru E qll^ dVI tr e 'nal Eursserdxarano slleo allqm 'paceds flapyut
1da1 ere lpql sepqruorJllu elqpts Jo sepunq tuJoJ 'uleurop Supcaford 3uo1
e seq qorqm 'Z1VW Sutssardxara^o slla3 'sdVIAtr
]uaJaJJIp eJnpoJdJalo o] peJeeu
-r8ua qlac uI palertsuouap se 'raqtaSo] 4ced sapqnlorolru pateof,-dvlN.{.1aso1c
r oq eurruJalap ueJ ureuop Suqcaford eq] Jo r{}3ua1 eql
'pJe^
lno slcaford
ssarts l€3ruEq3eu srqJ
'ev$l
alntlc) dpqSn eroru aFlll e lslxu ol lueuEIIJ
aql SurJJoJ
'lueluelrJ ullce eql Jo l{}8uel eq} Suop spuq uITIJOS
'sluJoJ
}Iunqns
aeJJ puE ]uauelrJ eql qloq uI ullJe o] spulq ll ]eq] uI pnsnun sI UIIIJOJ
'tolcntSut
-nrautlodap uura palIeJ osIV 'slue(uellJ ullce sazulqBlsep qclq^\ 'ullltoJ sI slla3
JIlo^JeJne IIB ul ]uasaJd uleloJd Surpurq lueurBlg-upce luBuodlul Jeqlouv
'(gz-gI
aJnBrC ees) JaIBI ssnlslp e^\ se 'uollJeJluoJ
alJsnu ur dals ]uEuodrul ue sI SuIpuIq ulso^urodoJ] Jo uoqeln8al aql 'uoseal
slql JoJ lsuralord raqlo qlIM SutlcEralul luorJ luauel.IJ eql lue^aJd uec luaruep;
ur]re ue Suop urso.{tuodorl;o Surpurq eqJ '}uaurellJoloJd auo uI s}Iunqns ullce
luecEfpe ua^as ol .{lsnoauelpruls spulq lBq] ula}oJd pele8uole ue'urcotutodoJl
;o Surpurq aqt ^q pazlllqels eJe slla3 ]sotu uI stueuEIIJ UI1JB palcalas 'slueul
-ulrJ urlse s]f,aJJe osp sepls Jleql Suop suleloJd drossacce;o Surpurq aql
'eseaslp slq] qrL-v\ palelJossP uollB
-Jeue8apoJneu eqlJo eJuanbasuoJ e Jo esnE3 e aJP nulJo sa13uu1 eseql Jaqlaq^\
reelo ta^ lou sl ]I 'sa13ue1 ^,fuelprqgorneu pellec 'stuauely nelJo sale8erSSe a8rel
sureluoc eseaslp s,Jarulaqzlv qlp eldoed Jo suIBJq eql q useldol-,tJ IIaJ elJeu
eqJ 'suorleJluaouoJ q8rq .4.pua1clJJns tB luesard ueqm sluaruulg IeJIIeq umo
str suroJ ulaloJd ne] 'saFqnloJJIIu Jo sepls eql Suop Surpulq o] uoulppe uI
'(7-91
arn8rg aas) uoqe8
-ar8as aruosoruoJqJ ro; uoqeredard ur aypulds Jllolltu eq] uJoJ ot uolalalsopb
apqnloJf,rru slr se8ueJreal IIaJ eq] sE JnJJo ]Eql scnueufp apqnloJf,Ilx
ur sa8uuqc aql aleln8ar sarU^Itre dVIN telncFred u1 '(11 raldeq3 uI passnrslp)
ayc,{c lac aql q8norqt ssarSord sllar se JJo pup uo peurn} eJe teql esoql ale sdvtr^tr
aleln8ar ueo ler{l seseuDl uralord lueuodrur aq} Suoury'sllal aplsul uoIlEZIIBJol
pue d1r,lr1ce sll qloq Suqlorluoc uI eloJ d;eurtrd E eleq uP3 d\'trAl e;o uoqelrfu
-oqdsoqd Suqpsar aql pue 'saseur{ uralord IBJe^es;o sla8rel eq} aJE sdVIAtr
'JelBl ssnJSIp III,r,t JM qJtq,,t,t 'sJolotu relnJalotu ,{.q uantrp sallaue8ro pasolJua
-euerquau;o godsuu4 aql ateln8ar osp uef, sluaruelg o1 Surpurq neJ '(It-gI
(ralueB 'y,{re9 Pue llaPueW
'M
souref Jo {satlnol) 'sa}tlpuap ut }uasatd osle st ne} po}el^loqdsoqd
lnel palel{roqdsoqdun o1 I;uo spulq nel })alap o} araq pasn {poqtlue aq1
'salupuap str pue {poq lla) aqt ot peuUuol st 6ututels ZdVW'(uolnau slql
ur paq)uerq pue 6uo1) uoxP aql ol pauUuo) st 6ututels nel sealoqM'alnlln)
ur uornau ;edr.ueroddrq e u (a6uDro) bututels ZdVt pue (uaatb) 'uwels
npt,o uorlnqulstp aqt smoqs qde.r6o.r:tu a)ua)salonlJounuul slqf
'uornau e Jo sallrpuap pue uoxe ut sdVW ,o uollPzlle)o'l g9-9 1 arn6t1
('rar^asll urorj
uolssll..urad q]!M'966 t 'Lgz-Llzit6 ila)
^le
ta upurueH 'f'f
Luorl) salnqnlor)il.x
6uo; aqt;o suorle)ol snornard aql le
slueu6er; llpLus ro sauas e 6urnea;
'sare;d
,(ueu ur paralas uaaq aneq sluaurelU aqf
'uruelel
Jo uorlrppP aq] ra]re salnurur €
plau arues aql (€) uruelPl ,o uorlrppe
aq] raljP spuo)os 0€ solnqnlor)il.!
aql ur slearq MaJ e are araqf (V)
dfv qllm 6uole pappe seM uruelel
par;und pue 'aprls sse;6 e;o a)eJrns aqt
uo paqrospe ale^^ salnqnlol)rur palaqPl
-aulurepoqr'pazrl rqels-loxef 'u!uelel
Iq 6ulrenes alnqnlor)lw 69-91 arn6r1
Luri 0 !

200 t
uoFlelsoil) aql :9 [ ro]deql
'Surprerc
IIaJ Ipuor]JeJrprun JoJ elqrsuodsal
IJoMlau urtJe ar{} Jo qte\oJ8 patJalrp 'pazrJelod ar{} JoJ IeJrluJ sr stueuplrJ ur}Je
^\au lou lnq plo Jo .,{lquassesrp pa}erpau-urlrJoJ aql 'lJalel ssncsrp IIr,/\,\ a^\ sv
dprder JaAo uJnl stuauulrJ ulloe IIe leql Suunsue 'llal aq] ur sluatuelrJ Japlo eql
salluerusrp dlluarcrg;a aJoJaJaq] ulllJo:) 'urlgoJ .{q uor}uzrrarurtlodap ol tue}srser
ere pue dJV.{Fsour ure}uoJ III}s 11e] eql ur s]ueruelrJ urlJe lsamau aql flquresse
]uauBIrJ ueql remols f1ensn sr srsd1orp,,i.q dJV aJurs 's]ueruelrJ 3ururc1uoc-41y
ol ueqt Jaqler stuauelrJ urlce SurureluoJ-dCV o1 ,{lp4uaragard spurq ulllJo:)
'Surpurq
urso^uodoJl ,rtq urqoc ruor;
palcalord aq uec sluarxelrJ urlJv'aqnl lsal e ur urlJe aJnd ruo4 pauroJ slueure
-lIJ aJe uBq] pe^rl-Jauoqs qJnru aJE sllac aprsur sluauElrJ urlJE eqlJo ]solu'llnsal
e sy fyquassesrp ]uetuElrJ ur]JB elEJeleJ3e ,(pear8 serlr^rlcp aseqJ ']uaruBlrJ
aq] Jo pua snu[u eq] ruo4 alerf,ossrp ol lrunqns urlJe-dcJv ue JoJ Jarsea qJnu 1r
sa)Eru tl 'uollrppe ul 'suonor.u
[Er.r]Jeq] dq paranas .{.1rsea arour pue allluq luetu
-elrJ aql Suqeur 'luaruelrJ aql ut slrunqns ur]re uaamlaq slceluof, eql sue{eal
s)!ureu/(c luauelrJ e6ueq3
{;;e>rleruerg ue) spul }uauel!l qr!/v1 })eratul }eql sure}otd
,,fuana ro; urso,,(urodorl auo ]noqe 'aydruexa JoJ) seular.uorqcrols q8rq r(pre; le luas
-ard aq ot aleq [aq] suearu slq] pue i(1a1a1duroc ]uauelrJ aq] ]poJ ol peeu uago
sulalord esaql 'ra^eMoq 'lJaJJa runrurxEur JoC 'Jor^Eqaq Jrureu^p s,]uauelrJ eql
aSueqc uec lueuelrJ e Jo aprs aq] Suop purq leql suraloJd 'uaas 1sn[ aleq e^ sV
luerrrelr+
urpe (v)
'
.lnqnlortr-/
urlr+o) + luau,relr]|' utpe (g)
uru Ls
(a)
wu vL
(8)
\
ruu 00€
('prl sraqs!lqnd uellil.!)ew
nrorj uolssluilad qtlM 266t 'LVg-VLg:jgt
arnlDN ''P
la uaql'n;o,{selrno: ,c pup f)
'ule
6url>aford lroqs I;anr1e;a.r qnel
Jo asne)oq ()) ut arp,{aq1 ueqt taqtabol
{;aso;r arou pa>eds are salnqn}or)tut
aql alaH'ne1 6urssatdxarano
lla) e ut alpunq alnqnlol)tut
e q6norqt uotl)as ssol) reltults (C)
ZdVW aql Jo sr.ure 6urltaford aql
Jo rl]6uol luelsuo) aqt Luorj tlnsat alpunq
srqf ur (sfw) salnqnlol)tur aqlJo 6ut)eds
re;n6ar aq1 74yy1; 6urssardxareno
lla) e ut alpunq alnqnlortru e q6norql
uotl)as ssol) e 6urmoqs qder6or:rr.u
uort)all (1) dool 6ult>afotd troqs
e qllM'!ururral-) pue -N slt qloq le e)tuel
alnqnlol)il.! aq] ol spurq nef (B) 'pua
raqlo aq] ]e ureurop 6urpurq-a1nqn1ot>rr.u
puo)as e qtlm [!re 6ulttalord 6uol
e spualxe pue spua s]t jo auo lp e)tilel
alnqnlor)il.ll eql 6uole spulq zdvw (v)
'sdVW ^q salpunq alnqntol)!r,u
;o uollezgue6lg 1y-91 arn613
('ssord A]tsra^t un rallalal)ou aql
l..UorJ uolsslLulad q]!M l66t 'Lg/__Lll:g€L
'lolB 'f ''lP
ilD la q6no9)W'V
Luoll) 'xtlaLl
aq] jo ]stMl q)ea ^q pauusds
a)uetsrp aql 6ut)npal pue ^llq6ll
aloul lt 6utlstM]'a)tilpl luaulelu utl)e
oql uuoJep ot sanras burpulq utluo) Jo
I6raua aq1'ra1:1qr,(;iq61;s r(1uo s.readde
luauelU aql os pue '(suo]lepolr)
€t ) ult)e
ol paleduro) (suo]lepolr) tl) uletold
lleu,rs e sr u!lgol'luaurel!; aqt 6uo;e
lle slrunqns ur]>e ol {r}auotq)tols [:! e ul
spurq q)tqM'utluo) qtlM poteo) ]uaLuelu
utl)p ueJo uott)nrlsuo)au (B) .x!laq
u!])e aqlJo slstM] oMl Jo ueds aql sMoqs
lallerq aqf'utllp ornd Jo apeu stuauleltJ
1o sqder6ortr ut uorlta;a-o{lr ulor;
uorl)nJlsuo)at leuotsuaultp-aalqI (V)
'utltro)
^q pa)npu! luauelrJ
u!l)e uP ro 6ullsrrvrl 7tr-91atn61

s I NlWVlH 'lvl 'EvSOtI) )IEH-L tlvtnDtu Stttf MOH
d
a
@
d
3
0
I
TO
r3
'- :
UOIIeJIUa)UO) JOt!OUOU] e
'pazllrqels
tPq^\aruos
uaaq.,{pearp,{qaraqt aleq pue urso'(uodorl ^,{q paleoc ueeq a^EI{ leq} sluatupllJ
urtJp Jo puJ snuru aqr or .{1uo spurq urlnporuodorl
'pua snurtu eqr w utlnporu
-odot1 f,qpup pue snld aqt rc 7de3,{.q-spua qtoq }E paddec ,{.1prcads ere s}ueur
-EIrJ aq] 'panr1-3uo1 .{lpuoqdacxe ere sluauEIIJ ullJe aJeqm 'sllal alcsnu uI
'paddecun
aJe pue xaldruoc dUV aq] Iuoq pasealal erp sllec pcrddl
ur spua snurur luel'uBlrJ urlJE aqtJo,ri.ueu leql alqrssod sl ll qSnoqlP 'uorlEelJnu
slr JoJ alqrsuodsar sea,r leqt xalduror dUV aql ol punoq Suturetuar,{q paddec aq
.,(eu luauelg urlJe ue 'pua snunu aqi tV'(ulelor4 Surdde3 palleJ osle sl lr lmoleq
aJS 'pueq Z alJsnur Jql ur uortpJol stl roJ palueu) 7da3 su qtns sutalord ,{q
pua snld rraq] le paddec ere IIac p uI sluaurpllJ ultte eql 'paapul '(Sf-gI
Jo lsoru
arn8gg) alrJJeur pua snld aqt Suopru dq uorlezrraru,r{lodap lueruelrJ pue q}^r,ror8
tuauelrJ qloq Jo salBr aq] s,r,rols ,4.11ear3 TIJIqM 'pua snld sll ]e lualuellJ ullJe
ue sazrlrqels ulalotd Sutddac pua snld e;o Surpurq aqJ 'pua snld aqt lE rnJro
la^a^ oq 'sa8ueqc prder lsoru aqJ'ruJoJ C aqt ot ile^uoo ol dJVJraql paz[1orp,{q
aleq pua teql le saln3alou urlJe aql acuo 'pua snullu s1t ro snld sll reqlle uorJ
slrunqns asol uEJ 11 :r(lprder azrraruf1odap ueJ IIaJ aql ,(q pazrTrqE]s .{.1pcr;rcads
lou sr pue uorle8uola saseal leq] luaruellJ ullJp uE 'pessnJslp,r(lsnornard sy
'Tro,\i\]au
]uauelrJ urlce ue
Jo aJntcatrqJJe aq] ruJoJsueJl ol q8noua aq ueJ (sJaruouoru ulloe 009-002 lnoqu
rad auo ,{Wcrd,{r) }uauelrJ ur}ce rad uralord e qJns Jo alnJalotu auo 'spua }uarup
-g le .{prerurrd rncco ssol pue uourppe lrunqns aJuIS
'slalal
,r,to1 ,{ran }e luasard
are .,(aq] ueqm uela scrureufp ]uauelrJ uo s]JaJJa Jl]prueJp eleq upJ slueuelrJ
Jo spua aql o1 ,(1er1uara;ard purq ]Eqt suralord 'tsBJluoJ uI '(llunqns utlce frana
JoJ urlrJoJ auo Jo 'slrunqns urlnqnl rno;.{rarra roJ nB} auo 's}Iunqns ul}ce ualas
spul alnqnlor)!w 6u1nnor9
aql la}lv suletord lo spu!)l tuata}JI6
spua
snurLu pue snld le
qp.rol6:sluaurelr;
+o uorlelnood
paooe)un
{1uo pua snurur }e
qyiror6:sluaurelrl
+o uorlelnooo
padoe)
{lprdeg ro sa!}radord
ur ruJoJrun puE alqels qloq aJE salnqnlorJlru araq A '(Jalel passnJslpJ BITI3
ur selnqnlorJrru aqt Jo spua eql le punoJ xaldruoc uralord ptcads aqt sI sapqnl
-oJJrur ro; uralord Surddec anJl Jeqlouv 'spua snurur rlaql sdec pue JaluaJ
Surzrue8ro up te salnqnlorJrur ;o qrruror8 aql selealJnu qloq qJIqM '(lgnJ-,t)
xalduoc Surr urlnqnl-,t aql :raddef, elnqnlorcltu lupuodrul ue passnoslp.,{pearp
aleq a A'uorlJe uralord .,fuossacce JoJ serlilqrssod aJoru zlueu qlpu ']ueulellJ
urlce ue Jo pua aq] ueq] arnlJnrls xalduoc aloru pue ra3re1 qcnru P sI '(I I-9I eJn
-3rg aas) 8urr,r,ro11oq E ur stueurulgolord uaaulqt qll.t,t relnqnloJJlru e Jo pue aql
aleJ aqdoDsBlpJ eql sasEaJJuI .{puecgru8rs slopat aqdoJlsDlal se U^\Ou)I sule}
-oJd palplaJ-ursauDl Jo ,{lrueJ E 'aldrupxe JoC '(sencsal Jo ^cuenbarJ aq}) atels
Suurror8 E ol SuDluuqs e ruo4 Jo (saqdorlseleJ Jo ,,{.cuanbar; eq}) a}els Sur4urrqs
E o] Suu or8 e ruo4 saqclu s elnqnlorcltu e qJIqM lB eleJ aql aluanlJul uef,
,(aqt '(gi-g t arn8rg aas) sa1nqnlonru;o,rfiqrqe]sul JIIueu.{p aq} uo slcaJJa JI}eu
-EJp a^Eq uec r(aq] 'relnJlued uI 'ulelord Surddec aldruts e ro; palcadxa asoq]
puo.,(aq saloJ IETJnJJ a^Eq seFqnloJJrruJo spua aqt le IJE ]eq] sutalord aruog
'{}3ua1
€0 0 r
'pua
snutul luauPlu aql Jo ]eql
ol surqs uorlelndod aql loJ uotlerlua)uo)
le)rtu) aql'uor]rppe ul'uotlerlua)uol
lelrlu) eql Molaq suotlprlua)uo)
raLuouour lle le taMols aq llrM
a6eluuqs ]uaurelU pue /uotlellua)uo)
le)r]u) eql a^oqe suolle.tlua)uol
rauououj llp lP laMols aq lltM
L.ltMor6 luaupl!J Xliuanbasuo) 1s]runqns
asol ro ppe ol alqp sr pua snutLr.l aLl] ^luo
'(aul pil) pua snld aq] sdp) leql uralord p
Jo a)uasard aql ul (autl uail6) sJawouow
aaJJ alqelrPAe Jo uotlellua)uol
or.ll uo 6urpuadop 'a6e)urrrls
lo qtmorb prder ur 6ultlnsa, 'spua snurur
puP snlo sLli r.lloq te slrunqns sasol pue
sppe sluaurelU paddetun;o uorle;ndod y
'srrueu{p
luouel!} uo stlaJ}a
s1r pue 6ugdde) luatupllJ gtr-91 a.rn6r1

uola;a1soy(1 aql:9 [ laideq) too t
'pede
slueurelgolord drd o1 uees pue spue eFqntorJrru ol ,{lpcgtcads putq
,taqt '(gg-gt arn8rg aas ldgure; tl-ulsauq eqt Jo sJeqlueru erp sutalord esaql)
r{3raua uorlellloe leluJou aq] Suuemol
ol ]sead uo4 aBuer leq1 surque8Jo ur sSolouroq esolo seq leql gyT1yy1y snol
-mbrqn aq] sp qons sdVIAtr ere suoqcu 4eql SulsoddO '(Cgt-gf arn8rg ees) alels
Suopuqs eql Jo orlsrJeloeJeqo ]uaruelrJolord pal-rnc eql olul pude 8ul8urrds
eql pue 'uralord palerf,ossE-elnqnloJJlul sndouay roJ spuels dVIAIX) suElunq
-tgg) srolceg aqdorlselec qlpr uoppedruoJ str Jo acueluq aql sUHs puu dlvrnce
slr slrqrqu srsotnu Surrnp gIZdVI [XJo uor1efl-roqdsoqd aqJ'e]els Suquuqs u o1
Suurror8 E ruo4 qJllrv\s Jleqr ]lqg{ul pue spue aFqntoJJIIu aaq azlpquls ol Ir111qp
prcads e seq ueloJd slqt'(suolppoFn ur ssBru JEInJelou sll o] sJaJeJ Jeqlunu
eJn
alnqnloj)il.r.,| +o
pua snlo uo
oe) dI9
dVIA
(€ [ ursaur{)
rope] aqooJlselel
pa)ueque eleJ
qy'iror 6 roTpue passorddns
saqdo.rlsele: +o r{:uenbarl
paseerlu!
.1'lnslu
:f'lnstu
uro4 elnqnloJorru E sluanard ler{l JerJJeq
go ,{1gqe1sur Jrrupudp aql ur esealf,ur ploJuel p ur sllnsar UIqs eqJ '(?t{I
'(gt{I
ernt.rd {urrqs ol ur8aq sepqntorclru
aql ueqm spue eql {uo4 SurlelJosslp 'salnqnloJf,Iru Surirnor8 ,{lprdur Jo spua
eql le sJe8uassed se IIaJ aql punoJu taIJoJ ol JBaddB puE 'spue a^rlJe aseql ]u
elplnrunf,Je (sdIJ+) sunrcJd Sur4Jau pua-sryd pelleo sulelord peluroosse-alnqnt
-oJJrIN 'eJeds
IIeo eJrlua aql eqoJd puE eJoldxa dpuarcr;;a 'lsEJluoc u1 'spua snld
eqJ'se4s uorlezrraurdlodap alnqntoJcrru sB e^Jas esle Jo 'aruosoJluac aql qllllr
uorlproosse dq pazllrquls eJe salnqnloJorru Jo spue snu[x eql 'slleo dueur u1
'(gg-Zt
ern8rC ees) epurds JItolFu aqlJo uollonrlsuoc lualo
-rJJe aql JoJ IBJTITJJ sr ]Br{} uoprsusJl B 'srsolrru Suunp pe^Jasqo saFqruoJJrru
'(ratel passn)s!p) (;rue#adns
ureloro rolour utseutl aqllo lequ.taul
e'€ t-ursaurl se q)ns slo])el eqdotlselet
ale uotl)e s1r 6ursoddg'eraq1
6u;pulq lelluaralerd s1t rq a;nqnlor>rtu
6urmol6 e Jo pua oqt sazlltqels S tZdV6X
se q)ns dVW y'suralo.rd leoads {q s;1er ur
pelloJluo) sr 6urluuqs alnqnlol)tui pue
qlrvror6 e;nqnloJ)il,! uaoMlaq uotltsuell
oqf'sPua alnqnloJ)!ut or pu!q reql
sulalord 1o sl)e#a aql tt-91 arnbr3
ulrl s
('utMps uay Jo ^sauno) ,g
llatlesll ulo.lJ uotsstt.lJled
l.llf^'9002'vs- LV:IL'lo!8
p2'u1d6 zn1 'peelua6ooH .J.)
pue e^oueuqlv'v ruotj'v) 'slla)
padeqs-por aql lo sa;od oM] aLlt
re (pal) tgj;o bolouoq eql qIM
palef )ossp arc (uaa$) salnqnlot)tut
eq] jo spue snld aql'aqwod
sat(wot o t ltlosozlqr5 lsea{ uolssr;
pedeqs-por aqr ul (g) '@arl LAj
uroloro dlI+ aql qllM pelet)osse
sr q)rqM pua snld 6uuvrol6 e seq
(uaar6) olnqnlol)tLu q)ea'alnlln)
anssrl ur ur*or6 ;;a> ;e;leq1rda
ue ul (v) 'solnqnlor)lur
,o spue sn;d 6ugrvro"r6 aql le
punol sulelord d11+ St-91 ern6ll

HOW CELLS REGULATE THEIR CYTOSKELETAL FILAMENTS
Some of the +TIPs, such as the kinesin-related catastrophe factors and
XMAP215 mentioned above, modulate the growth and shrinkage of the microtubule
end to which they are attached. Others control microtubule positioning
by helping to capture and stabilize the growing microtubule end at the location
of specific target proteins in the cell cortex. EBl, a +TIP present in both yeasts
and humans, for example, is essential for yeast mitotic spindle positioning,
directing the growing plus ends of yeast spindle microtubules to a specific docking
region in the yeast bud and then helping to anchor them there.
Filaments Are Organized into Higher-Order Structures in Cells
So far, we have described how cells use accessory proteins to regulate the location
and dynamic behavior of cltoskeletal filaments. These proteins can nucleate
filament assembly, bind to the ends or sides of the filaments, or bind to the
free subunits of filaments. But in order for the cytoskeletal filaments to form a
useful intracellular scaffold that gives the cell mechanical integrity and determines
its shape, the individual filaments must be organized and attached to one
another in larger-scale structures. The centrosome is one example of such a
cytoskeletal organizer; in addition to nucleating the growth of microtubules, it
holds them together in a defined geometry, with all of the minus ends buried in
the centrosome and the plus ends pointing outward. In this way, the centrosome
creates the astral array of microtubules that is able to find the center of each cell
(see Figure 16-32).
Another mechanism that cells use to organize filaments into large structures
is filament cross-linking. As described earlier, some MAPs can bundle microtubules
together: they have two domains-one that binds along the microtubule
side (and thereby stabilizes the filament) and another that projects outward to
contact other MAP-coated microtubules. In the actin cytoskeleton, the stabilizing
and cross-linking functions are separated. Tropomyosin binds along the
sides of actin filaments, but it does not have an outward projecting domain. As
we shall see shortly, filament cross-linking is instead mediated by a second
group of actin-binding proteins that have only this function. Intermediate filaments
are different yet again; they are organized both by a lateral self-association
of the filaments themselves and by the cross-linking activity of accessory
proteins, as we describe next.
lntermediate Filaments Are Cross-Linked and Bundled Into
Strong Arrays
Each individual intermediate filament forms as a long bundle of tetrameric subunits
(see Figure 16-19). Many intermediate filaments further bundle themselves
by self-association; for example, the neurofilament proteins NF-M and
NF-H (see Table 16-I, p. 985) contain a C-terminal domain that extends outward
from the surface of the assembled intermediate filament and binds to a neighboring
filament. Thus groups of neurofilaments form robust parallel arrays that
are held together by multiple lateral contacts, giving strength and stability to the
long cell processes ofneurons (see Figure 16-22).
Other tlpes of intermediate filament bundles are held together by accessory
proteins, such as filaggrin, which bundles keratin filaments in differentiating
cells of the epidermis to give the outermost layers of the skin their special toughness.
Plectinis a particularly interesting cross-linking protein. Besides bundling
intermediate filaments, it also links the intermediate filaments to microtubules,
actin filament bundles, and filaments of the motor protein myosin II (discussed
below), as well as helping to attach intermediate filament bundles to adhesive
structures at the plasma membrane (Figure 16-46).
Mutations in the gene for plectin cause a devastating human disease that
combines epidermolysis bullosa (caused by disruption of skin keratin filaments),
muscular dystrophy (caused by disruption of desmin filaments), and
neurodegeneration (caused by disruption of neurofilaments). Mice lacking a
1 005

1006 Chapter 16:The Cytoskeleton
u.5 um
functional plectin gene die within a few days of birth, with blistered skin and
abnormal skeletal and heart muscles. Thus, although plectin may not be necessary
for the initial formation and assembly of intermediate filaments, its crosslinking
action is required to provide cells with the strength they need to withstand
the mechanical stresses
inherent to vertebrate life.
cross-linking Proteins with Distinct properties organize Different
Assemblies of Actin Filaments
Actin filaments in animal cells are organized into two types of arrays: bundles
and weblike (gel-like) networks (Figure lHz). As described earlier, these different
structures are initiated by the action of distinct nucleating proteins: the
long straight filaments produced by formins make bundles and the ARp complex
makes webs. The actin filament cross-linking proteins that help to stabilize
and maintain these distinct structures are divided into tvvo classes: bundling
proteins and gel-forming proteins. Bundling proteins cross-link actin filaments
into a parallel array, while gel-forming proteins hold two actin filaments
together at a large angle to each other, thereby creating a looser meshwork. Both
Each type of bundling protein also determines which other molecules can
interact with an actin filament. Myosin II (discussed later) is the motor protein
in stress fibers and other contractile arrays that enables them to contract. The
contractile bundle
gel-like network
100 nm
Figure 16-46 Plectin cross-linking of
diverse cytoskeletal elements. Plectin
(green) is seen here making cross-links
from intermediate filaments (blue) to
microtubules ( red). ln this electron
micrograph, the dots (yellow) are gold
particles linked to anti-plectin antibodies.
The entire actin filament network was
removed to reveal these proteins. (From
T.M. Svitkina and G.G.Borisy, J. Cell Biol.
1 35:991-1 007, 1 996. With permission
from The Rockefeller University Press.)
Figure 16-47 Actin arrays in a cell.
A fibroblast crawling in a tissue culture dish
is shown with three areas enlarged to show
the arrangement of actin filaments. The
actin fifaments are shown in red, with
arrowheads pointing toward the minus
end. Stress fibers are contractile and exert
tension. Filopodia are spike-like projections
of the plasma membrane that allow a cell
to explore its environment. The cortex
underlies the plasma membrane.

HOW CELLS REGULATE THEIRCYTOSKELETAL FI LAMENTS
spectrin (tetramer)
5U nm
very close packing of actin filaments caused by the small monomeric bundling
protein fimbrin apparently excludes myosin, and thus the parallel actin filaments
held together by fimbrin are not contractile; on the other hand, the looser
packing caused by the larger dimeric bundling protein a-actinin allows myosin
molecules to enter, making stress fibers contractile (Figure f6-49). Because of
the very different spacing between the actin filaments, bundling by fimbrin
automatically discourages bundling by o-actinin, and vice-versa, so that the two
types of bundling protein are themselves mutually exclusive.
Villinis another bundling protein that, like fimbrin, has two actin-filamentbinding
sites very close together in a single pollpeptide chain. Villin (together
with fimbrin) helps cross-link the 20 to 30 tightly bundled actin filaments found
in microvilli, the finger-like extensions of the plasma membrane on the surface
of many epithelial cells (Figure l$-50). A single absorptive epithelial cell in the
human small intestine, for example, has several thousand microvilli on its apical
surface. Each is about 0.08 pm wide and I pm long, making the cell's absorptive
surface area about 20 times greater than it would be without microvilli. \'Vhen
villin is introduced into cultured fibroblasts, which do not normally contain
villin and have only a few small microvilli, the existing microvilli become greatly
elongated and stabilized, and new ones are induced. The actin filament core of
the microvillus is attached to the plasma membrane along its sides by lateral
sidearms made of myosin I (discussed later), which has a binding site for filamentous
actin on one end and a domain that binds lipids on the other end'
These two types of cross-linkers, one binding actin filaments to each other and
the other binding these filaments to the membrane, seem to be sufficient to
form microvilli on cells. Interestingly, when the gene for villin is disrupted in a
mouse, the intestinal microvilli form with apparently normal morphology, indicating
that other bundling proteins provide sufficient redundant function for
this purpose. However, the remodeling of intestinal microvilli in response to certain
kinds of stress or starvation is impaired.
(A)
fimbrin
(monomer)
actin filaments and
d-actinin
contractile bundle
loose packing allows myosin-ll
to enter bundle
0-actinin
(dimer)
50 nm
actin filaments and
f imbrin
parallel bundle
tight packing prevents myosin-ll
from entering bundle
(B)
1 007
Figure l6-48 The modular structures of
four actin-cross-linking proteins. Each of
the proteins shown has two actinbinding
sites (red) that are related in
sequence. Fimbrin has two directly
adjacent actin-binding sites, so that it
holds its two actin filaments very close
together (14 nm apart), aligned with the
same polarity (see Figure 16-494). The
two actin-binding sites in cr-actinin are
separated by a spacer around 30 nm
long, so that it forms more looselY
packed actin bundles (see Figure
16-49A). Filamin has two actin-binding
sites with a V-shaped linkage between
them, so that it cross-links actin filaments
into a network with the filaments
oriented almost at right angles to one
another (see Figure 16-51). Spectrin is a
tetramer of two o and two 0 subunits,
and the tetramer has two actin-binding
sites spaced about 200-nm apart (see
Figure 10-41).
Figure 16-49 The formation of two
types of actin filament bundles.
(A) cr-actinin, which is a homodimet
cross-links actin filaments into loose
bundles, which allow the motor protein
myosin ll (not shown) to participate in
the assembly. Fimbrin cross-links actin
filaments into tight bundles, which
exclude myosin. Fimbrin and s-actinin
tend to exclude one another because of
the very different spacing of the actin
filament bundles that theY form.
(B) Electron micrograph of purified
c[-actinin molecules. (8, courtesy of
John Heuser.)
100 nm

1008 Chapter 1 6: The Cytoskeleton
(A)
amorpnous,
densely
staining region
plus end of
actin filament
lateral sidearm
(myosin-1,
calmodulin)
(B) (c) | |
1rt
microvillus
acItn
f ilament
bundle
Figure 16-50 A microvillus. (A) A bundle of parallel actin filaments cross-linked by the actin-bundling proteins villin and
fimbrin forms the core of a microvillus. Lateral sidearms (composed of myosin I and the Ca2+-binding protein calmodulin)
connect the sides of the actin filament bundle to the overlying plasma membrane. All the plus ends of the actin filaments
are at the tip of the microvillus, where they are embedded in an amorphous, densely staining substance of unKnown
composition. (B) Freeze-fracture electron micrograph of the apical surface of an intestinal epithelial cell, showing microvilli.
Actin bundles from the microvilli extend down into the cell and are rooted in the terminal web, where they are linked
together by a complex set of proteins that includes spectrin and myosin ll. Below the terminal web is a layer of
intermediate filaments. (C) Thin section electron micrograph of microvilli. (8, courtesy of John Heuser; C, from
P.T. Matsudaira and D.R. Burgess, Cold Spring Harb. Symp. Quont. Biol. 46:845-854,1 985. With permission from Cold Spring
Harbor Laboratory Press.)
Filamin and Spectrin Form Actin Filament Webs
The various bundling proteins that we have discussed so far have straight, stiff
connections between their two actin-filament-binding domains, and they tend
to align filaments in parallel bundles. In contrast, those actin cross-linking proteins
that have either a flexible or a stiff, bent connection between their two
binding domains form actin filament webs or gels, rather than actin bundles.
projections called lamellipodiathathelp them to crawl across solid surfaces. Fil-
A very different well-studied web-forming protein i s spectrin,which was first
identified in red blood cells. spectrin is a long, flexible protein made out of four
elongated pollpeptide chains (two cr subunits and two B subunits), arranged so
that the two actin-filament-binding sites are about 200 nm apart (compared

srNrwvrl lvl-lll)sol^) ull Ht l-Lfl n9lu sll3f MoH
'euerquau aJeJrns aql ueJJIls
pue adBr{s o1 dlaq oqe -,(aql alaq^r'sadd1 11ac elPJqaua^ raqlo lsoluJo xauoJ eq}
ur punoJ aJe urrlceds Jo sellt€lal aso13
'rfuepder e q8norql Sutzaanbs rage adeqs
purSrro slr ol {JEq Suuds o} IIaJ poolq pal aql Surruolp 'auuJqtualu eurseld 3utd1
-JeAo eq] ro; ]roddns IeJIueqJaIu saplloJd ]Pql xagoo IIal JJIIs e seleaJc {Iomleu
Suqpsar aqJ '(It-gl arnBIC aas) sutalord euerqrueu IBJBaIuI fq Ja,{e1q ptdl aqr
JBau peuoplsod se^lesruaq] aJe qJIqM 'suralold euEJqluelu praqduad IoJ salls
Surpurq elpJedes seq lI esnp3eq augIqlueu eusgld aql ol qe^ slql $lug ulJlceds
:slualuellJ ultce uoqs ,i.q raqtaSot plaq qe^ puolsuaulp-oMl B sluroJ ll araqM
'aueJqureru euseld eq] qleeuaq lsnf pa]er]uecuoc sI UIJ]ceds '{lleJ poolq peJ
eql uI .(87-gI eJnSrc aas 'ururlJe-D JoJ ruu 0t lnoqP pue UIJqIUIJ JoJ Iuu 7I qu^a
sauerqulew ol sluauq)ellv {uery elew slueuall lelelalso}/()
II3O Jo eJupualureru aql JoJ pennbeJ eJe teql sJaqlueu Suleluos '(ulseoul pup
'urrnpeJ 'uuza 'srequtaru aaJql lsrlJ sll JoJ pelueu) tt11't;l-1 y'1gg aql 'sulaloJd Jel
-nlleJeJtul patelar.{1aso1c Jo t{llrueJ peardsaprlu V'poolsrapun dlprued lluo are
eupJqueur eruseld aql ol uolela>1so$o ullJe Ieoluoc eql Jo suollJeuuoo aql
'auurqueu EursBId aql;o sprdll Jo sulelord puE {Joo\
-lau luauelrJ urlJg aq] ueegleq Slue111llJelle JIJIcedS SaJInbaJ saJnlsnJls esaql
Jo sseue^IlcaJJa aql'sllaf, lullaqllda;o eare aceJJns e^IldJosqe eql eBJEIua leql
rllr^oJcrru uI selpunq uI]3E-uIIIF eql pue seueJqrueu eruseld aql sallJepun lEql
qe^a uIIJE-uIJlJads aqt :selduExa o^i\l passncsrp ,{pearp e^eq eM'eueJqrxelu
eruseld aqt Jo adpqs eql a8ueqJ pue ueJJIls qloq sernpnrls Ielale{sol,{c ullsv
ur pe^Io^ur pue IlrJelod
Jo urEruop IpuruJa]-J aqJ .srsol{Jopue pue srsol-{3oxe
,r(q palerparu eupJqueu Eruseld eqt pue ullse uaamlaq slueuqcune eql
'uEuoJnlEAq
]uauoduos xrJlBur JelnllaJEJUa aql JoJ Joldacal eql 'v?cll su qJns 'sulal
-oJdoJ^18 aueJqrueursueJ] aJotu Jo auo Jo af,EJ Jlluseldoldl aql ot sputq uletuop
IPurrurel-N eqJ 'sluaruBllJ ullJe Jo sapls eq] o] fllJarlp spulq ulelord I trug ue
rBlnlleJPJlxa puB mlnllacPrlul qtoq,(q pelup8ar erc sulelord ntrUg
INUS 'slEuBIS
aq uec uorleturoJuoc e^rlcE eq] o] SulqJlr1v\S
'uollseJelul JulnlalorueJlul uP ^q
JeqlaSol plaq eJe IuIIuJel-3 puB -N eql qllq^\ uI 'uollEluJoJuoc peploJ a^Il3eul
ue pup 'utalord aueJqueusuBJl E puB uIIJP qloq ol spulq pue sazIJaIuoBIIo
lpql uoqeuloJuoc pepuelxa e1llce uE 'suollgluJoJuoc 06l ul ls1xe ugl sulelold
(ssar6 {lrsranrun lolloJal)ou eql uorsstutad qlm '/66
t 'Lsq-Svg:gtl
'to!B
ilD '1' ;e 1a r-ueq6uluunl 'f tuorl)
'Il!lllor.rl pue {6o;oqdtou lle) uo
aneq ue) uralord le.rnl>nr1s a;6urs e Jo a)uasqe lo a)uasaJd aql leql l)a;Ja
puno;ord aql 6urlPllsuotuep Iueu jo ouo sl a;druexe s!Lll')llelselau /(lq6lLl
are pue erpodr;;aue1 leiuJou alPtu Mou sllaf aql'palolsal I;1etlt;llte uaaq
seq uorssaldxa urutplt, q)tqM ut slle) euouPlaul aules aLll (8) 'azlselselaul
o] ]ou pual pue {uood ;nnetl r{aq} '}lnsor e sv ,;sqalq,, auelquaul
qllM pora^o) ole pealsul pue erpodt;;euel leulrou aleul ol alqe lou
are slla) asaql'ulJrelU jo la^al Mol {;;eutouqe ue a^eq leq} slla) eurouelaur
;o dnol6 V (V) '/ttltrou
lla) lerulouqe sasne) ulurellr 1o ssol 75-91 ern6r3
'la6
ro
qam buorls (;ge:rueq>au e su-uo1 urulelU
(q pa1ur1-ssorr sluaurelu url)e jo las V (B)
'sluaulelU
urlre luarelpe oml uaaMlaq
1ur1 al6ue-q61q
'alqrxag e sLuro; pue
papualxa I;;n; ueqan 6uo; uuu 09l lnoqe
sr raurpor.uoq ururelU tl)el 1y;
'1eb e
;o segl.radord ;errs{qd aql qllm ryoMtau
leuolsuour!p-aorql e olul sluauelu
u!l)e s{u!l-ssor) ulrrelll 19-91 arn6t1
(8)
0 I
600t
r.lr11

uole;a4soy(3 eql:9 [ raldeql 0 t0 t
'61
raldeq3 ur ssnf,srp IIu e^,lt lpqt suorlJunf xrr]eur
-llao pue suorlcunf Ilao-llal pazqercads aql armbar qJlr{lrt ,suor]JeuuoJ esaql
JoJ Ief,nrJJ are sluaruElrJ alerpauralu pue slueluelrJ uDJe qlog .eueJqrueu
]ueuasEq JElnllaf,EJlxe up o] Jo slles Jeqlo ol IIef, e Jo eJnlsnJls leuJelur aql lJau
-uoc o] SurdleqJo uorlJury leuoqrppe aqt a^eq suralord esaqlJo euros .(966-t66
'dd) g-gt lauud ur pemarlal aJE selnqnloJJrru pue sluetuplu urtre Jo uorl
-tsod pue ,,(lqruasse eql 'uorlJes
IoJluoJ leql srql ur pessnJsrp ,suralord
aq1
'(21
raldeq3 ees) rll aoJB IIarJo loJtuoo aq] ol stuaruele lern]JnJts IIar
slcauuoo leql ruels.{s )leqpaal
p
Jo suorleorpur dueur;o auo sr srqJ .uels^s snol
-reu eqlJo slred raqlo ureueJ pue sanrau dJolrpne aql ur doyarrap sJorunl uBruaq
aldrlpur qJIqM ur'stsolvutotqglotnau pellec aseesrp cqaua8 uerunq eq] Jo ruJoJ
e ur slFSeJ 'urlJeru palpc ,(prue; WUg eql Jo sJaqrueu aq] Jo euo Jo ssol aqJ
'(t9-9I
arn
-tgg)
1ac eql ^q pelratal spu8rs;o dleuen e ol elrlrsuas auooeq xeUoc [e] eql
yo saqradord eql 'tte.,tlr srql q '(sesEuDI
eursordl JoldaceJ IeJaAasJo slaBrel lcarrp
are suralord ^gg ar{}) spu8rs JEInlleJeJ1xe o1 asuodsar ur ,aldurexa JoJ tncao
uEf, q3rqm Jo raqtra 'zdld ol Surpurq ,(q ro uorle/.roqdsoqd uralord dq para88rrl
fteurun5
'auurqruaw
auso1d aqt o1 sluaua1g[Su1qcaLp [q s11acto sa4tadotd anlsaqpa puo adnqs
aqt auruuapp sularctd ftossacco Jaqjo pA's[am pau{ap [11au4auoaB ur JaqjouD auo
o1 waql But>1ut1-ssotc fq saJnpnJ$ parapto nBnT olur guauapfar4] salquasw sulapJd
[tossaccv lo ssDIJ raLflouv'sluauBatt nllaus olut s7uaua77[ aql Suuanas r(q n sruaru
-aN
lo spua aqj o1 3u1pu1q [q sauoutp ryatualg[ n77a sulaprd asaql lo awos .sa(4as
-uaqt sruatual{ aqt rc slrunqns aa{ arll raqtla oj pwq wt4t sutalotd [tossacca fq pala
-rapcJv Jo pamols Jaqtn aq uac [Tquassnnp pua [lquassD ]ualuuly[to sulaut4 aq1
'auoJqraau
autso1d aql Jaau pa7aalrnu aJo sl.uauollf upca rout
,]wJtuoc q.Ipc aql
to nluac aql toau palacol [Tqraassa xalduoc a .awosorluax aqJ w pawapnu aJD salnqru.
-orrrw '.sqac putuo jsota uI s1uaua1y[ mau to uorlaa1cnu aql aruulu! wq] sassacotd
aql to uotrulnBat av.ll s2 saJnpnus pppqsou[xto sa11s aql lo tunutuuapp tQotuttd y
'sutalotd
ftossacca qcns to Tasqns pawc
-pap aBnT a soq (sruauo1{ upca puD rstuaual{ arulpawtalm ,salnqnrcrrna) sassoll
ruaua14f rcfnw aarqt aql to tqcog
'sutalotd [tossacco lo auol.tadat aplaaan a uo puadap
s77ac nlofuacna ul saJnpnr$ ruawallt ln1a\at1so1rb lo suotlcunt pua suto! pauan ar,11
SUOIOW UilNf]IOW
pu€ 'lualuelg aqf Suop anour,,bq] qJlqm ur uorloalrp eql ,(seFqnloJJrru Jo urlJE
raqlta) of pulq ,taqt luarueg Jo eddf eqt ur raJJrp ,{aq1 '1ac rr}o.,fueone Lra.l.a ur 1sr
-xeoJ suraloJd rolour luereJrpJo suezoq 'lf Suop,rtppeals eloru ol srs^loJp^q dJV
Jo salJ,,b pelBeder ruo4 pe^rrap zt8raua eqt esn puE tueuEIU Ielala{sotft pezrrElod
E o] purq suraloJd alqDlJerueJ eseqJ 'suleloJd Jolour palleJ sJololu JBlnJelolu
aqt eJP uolalelsoil.c aql q]!v\ alercossE leql sulaloJd Sulleurcse; tsolu eql Suoury
'sleuots
lelnlla)eJlxa
ol asuodsei ut LUIoJ leql suotsnllold
a)e]lns-llo) aztltqets pup aleraua6
^qaralll ue) suratold {1;ue1-61g3
JO uorle^tl)v'utalold auejqurausuell
e loJ auo pue luauelu utt)e ue JoJ
auo'salrs 6urpurq oirll sasodxa ,26;6
o1
6urpurq ,(q to uortel,(roqdsoqd (q pasnet
'uralold
{1;ue1-6153 ue 10 6urp;o;un
paleln6aU .aueJquaut
euseld aql
ol slueulellJ uglre 6ugqre11e u; surelord
{lrue;-6193 ;o alor eq1 gg-91 arn613

I t0t suorow uiln)ltot^t
'sJor^eqeq
1ac xalduroc saleJauaS,{lsnonard peqrJJsap s}uaruelu lelele{sol,{J
cnueu.{p aql pue suraloJd Jo}oru ueeMlaq uorleJoqElloc e n/t.oq euruexa ilu^
aan taldeqc slqlJo uoqcas puu eql uI 'stuaurelu le]a1a4soilc uro{ }lrnq seJnlonJ}s
;o adeqs aql a8ueqc o] Jo sVNUtu pue sallaue8Jo pesoltua-aueJqrueru podsuerl
o] {JoM uec.,taq1 ,rnoq aqrJrsap uaq} el 'sulaud.p pue 'sursauDl 'surso.{ru :suralo.rd
Jotoru p1a1a1so1b;o sdnor8 aarql aql Surqrrcsap,,i.q u€aq a,rn
'uorlJas
srql uI
'(uralord
rolour pnpp
-eur aqtJo uortoury pcrSolorq eq] eroJaraql pue) o8reJ eqlJo.,{1quapl eqt seuru
-Jatap uralord rotoru aql Jo IIBI eql sealaqm ']1 Suop luerueloru Jo uor]oeJrp aql
pue {JEr} eqlJo &euepr aql seurruJa}ap (peaq) uretuop Jo}oru aqJ'(sJelauouuu
,^i\eJ e Jo aJuelsrp e ,{1eerdfi) }ueurEIU aqt Suo1e eurr} e le dals euo aloru
o8rec palercosse slr pue uralord Joloru aq] 'Surputqal luaruplu pue ,uorlexelaJ
puorleuroJuoc raseeler
]uaruelrJ 'e8ueqc IeuorleuJoJuoJ ,Surpurq ]uauelrJ Jo
a1c.,b pcruaqJoupqJau e q8norq;'punoqun are daqt qlrqm ur salpls pup s{JEr}
]uaruelrJ Jreql o] ,{13uor1s punoq a.re [aq] qJIq \ ur sale]s ueemleq alcdc sural
-ord aq] 'sa8ueqc puorlpruroJuoc acnpord lpq] srs^loJp,{q aprloalcnu Jo selodc
Lq ua,rrrq 61y saz(1orp.r{.q puu spurq luq} 'umluop Joloru Jo 'uor8eJ ,,peeq,, e
qSnorql s{ceJl luaruelrJ Jraq} q}I,,t,r eler3osse sureloJd Jolou playalsolrb aq1
'(12-g
arn8rg aas) g raldeq3 ur paurcydxa se
'ura1o.rd e ur a8ueqc puorleuroJuoc apcs-a8rel e ot srs,{1orpz(q aleqdsoqdrrl aprs
-oelJnu Surldnoc ,{q uoqoru aleraua8 uaql 'IJuJ}
Jo IIV aq13o ,firrelod prnlcnrls
aq] uo luapuadap Surprts Jo uortcaJrp aql qtF '{oeJl
JeaurT e Suop sallasueql
ladord o] dSraua lulrueqo asn ol drruqe aqt a^eq esaqt Jo IIV 'seuosoqrr pue
'seseorlaq 'saseraru,{1od
VNU pue VNCI sp qcns '{ooq sn{} ul eraqmasla passncsp
saxaldruoc uralord pue suralord Jaqlo aruos Jo luacsrurruer eJe {oeJ} rarudlod
peluerJo ue Suop ,{ypuo4carrprun aloru teql suralord ro}otu p1a1a>1so{3
'uorsrnrp
IIac pue '3ur1eaq drerlrJ 'uorlJerluoJ alcsnru se eueurouaqd qcns sanrrp
leqt acJoJ aqt Suueraua8 taqlo qcea 1surc3e apus ol Jo uorsua] uexa o] slueu
-eru plala{so},tc asnec suralord Jo}oru Jaq}O 'ilec aq} ur suouecol alerrdordde
Jlaql ol-salcrsan drolarcas Jo 'sTcels €1og 'eupuoqJolru sp qcns-sal1aue3;o
pesopua-eupJqruau drrec suralord rotoru .,(ue141 frrec .{aql ,,o8rec,, aq}
urso^w aql ro sroquew alv sutalotd rolow paseg-utl)v
(;lureltedn5
IPurrural-N aq] ol asolJ purq surEqc ]q8ll o^\t eql'(?g-gl ernSID uollezrrarurp
urcqc fueaq selprpau ]Eql IloJ-pelroc papuaxa up sruJoJ ]eql ecuanbas prcE
ourure 3uo1 ,tran e dq pamollog 'draurqceur SuperauaS-a3JoJ eql surEluof, ]eql snu
-rruJe]-N slr ]E ureruop peaq JelnqolS e seq urpqJ dleaq qJeg 'sureqJ 1r{3I o^n1gs
qree Jo serdoJ oml pup sureqo .,i,,reaq om1 ruo4 peruJoJ sl leq] uraloJd pelp8uola
ue sr (l\^olaq ees) il u8o/tru palle3 'urso,'ilu slqJ 'uorlJeJluoJ elJsnu JoJ acJoJ eql
saleraue8 qJIq { 'ulsoriru al3snru Iptala{s sE^a perJr}uepr ureloJd roloru lsJg aql
I uru
snur urral-)
sa)rlaq n oMl +o lro)-palro)
uru 0s I
:o lrau
snuIullal-N
- sureq) lqDrl
uor6ar a6urq
00|' ('uoloqs pr^e6 Jo {salrnor'g) 'urnurleld qtlm pamopeqs sa;nre;ou urso{r-u
;o sqder6or>rr-u uorl)ala ur uaas,{;leaqr aq ue) lte} aq} pue speaq telnqo;6
oMr aql (B)
'lrel oql pelle) sr aln)alou aql,o lteo s!r.ll pue 'uotlnlos
ul por pepualxa up saleur luauabuelre ltol-peltol aq1 (6-9 arn613
aas) spr)e ourue rrqoqdorplq pareds Ilre;nbal jo uotlet)osse aql Iq ualup
'lror-papo) p uuoJ ol .raqlo q)ea punore derrn sureqr ,{reaq aqt;o sa>r;aq
D OMI Oql UaqM Sin))o uorlezuaurc 'peaq
ursor{ru qlea uo luasald sr
ad,{1 q>ee ;o ,{do> auo pue 'sad,(1
l)utlstp o^i\} Jo ale sureq> }qbrl aql'@ryq)
sureqr 1q6r; rnol pue (uaafi) 6uo1 spr)e oururp 0002 lnoqe qtea) sureq)
(neaq o,rn1 1o pasodr.uo> sr alnlalour ;; ursolu.r V (V) 'll ulsortw gg-91 alnbr3
\
\
r-uu

uolalelsol^) aq] :9 I raldPql) ZLOL
suorloeJrp alrsoddo ur patuerro 'speaq urso^u paJpunq IEJaAes e^eq leql ,,slualu
-elrJ
IJIql,, relodrq aBrel uroy suorlJpJelur Wl-lle_] asaqJ 'salnJelolu ulsortlu
raqlo Jo sllel eql qllmJIaslI sepunq IIE] Ilos-pa1oc Euol aq] ellqm 'uleruop peaq
dJV Jo ,48raua aqt Sursn i11y saz,r(1orp.{q pue spulq peeq utso,{ru qceg
'(99-9I
arn8;g) luatuellJ {rlq} eql Jo spua oml aql }e
'allsnru
1elala{s uI 'Jeqlo qcea lsed slueuelrJ ur}38 pa}uelro dlalrsoddo Jo sJled
Surprts 1e ]uerJIJJa luauEIU aqt se{eru luaruBIIJ {olql aq} uI speeq aql Jo uope}
-uauo Sulsoddo aq1 'luaruellJ ullJe uP Jo pua snld aq] plemol {lem o} sls,{loJp-{q
ulq],, ut pau81e eJB sluaIuEIIJ ullce pa8ueJre.'{.1p;aree qclqm uI
-snlx qlootus pue Jerpre] '(ralel passncsrp) uorlseJluos elJsnu uI stlnsal sluaur
-elrJ urlre ;o Surprts ua^Irp-dJv aq] 'stuaruellJ {3lql ulso,{ru eq} Sulpunorrns
leqt salnJaloru II ulso/tru ule]uoc el3
osle sB^ uraloJd uISo^IU pepeaq-oml TEIIIUIS e leq] punoJ sraqcJeesal 's0z6l
aql ur lnq 'alJsntu ur ,r(po luaseJd se^ urso^tu ]eql lq8noq] ^ileplul se^a U
'(99{I
arn8ld) pueq eqt uqll Adla}alduoo
peureluoJ sr dlrntlJe Jolour aql teql Sut.ord 'oJlln ut Surpls luaruellJ aleJaua8
upJ auop luaru8er; IS eqJ'(IS pallpJ) lueur8e4 lcelul ue sP peseelar sI uletuop
pPaq aql 'ulPded pue ulsd^Dour,r(qc.{q palse8lp sI ulso,{Iu elJSnIu E ueq \
'uaql apooue seua8
(8)
( ssard )ruape)V uor] uorssil.urad qil^ sg6 t '6€v-6zv:vgL'tolB
tow '1-'6tet1'U pue uolped 'U leqtMor:)
'V'U uo paseq 'f
:treMals ,{elrn61 1o {sauno> 'V)'(ZL-gL arn6rg aas) pazruebio (;re;rr-urs qbnoqlle 'ralletus q)nur arP slla) al)snLu-uou ul s}uauelU
;; urso,{r,u trLuse;do1{: aq1'uaatb ur palq6rlq6rq st aln)aloLu urso{Lu ;enprnrpur uy'sqdet6ot)rul uol})ela ulolj pal)nr}suo)al
se tuaulelt, ;1 urso{ru e }o uotl)as lleuls V ()) 's;re1
1; urso{tu ;o ,{;arr}ua s}srsuo) }uaurelU aql Jo lo}ua) aql ul auoz aleq
oq1 luaLu el!J aql Jo aprslno aqt ot 6u lltaford s peaq raql q]!M 'suor6ar
lret r laql jo su eau ,(q ale6ar66e saln)alou 1; u tso{tu
aql ale)s ol umprp 1ou
'r,uerberp )lteuraq)S (B) 'sureulop peaq lo aarJ sr r..l)rr1m'auoz arPq leltua) aql atoN 'al)snul 6or; r.uot1
palelosr luaurelu )i)rql ;; urso,{r,u e 1o qder6or>rr.u uor})all (v) 'al)sntrt u! luauel!J ))rq} lelodlq 11 ulsoltu aq1 95-91 atn6t1
]uereJJrp q8noqlp 'pa8uerre fyreyurs aJE
.-.---
sderre ,,]uauBIIJ
ssel6
aprls
tull S
;re1
ursoIr.u
('q)tpndS sauef
Jo Isalrno) 'v) ']uaLua^our
luaulplu
u!l)e jo uorl)arlp eql ale)!pu! sMoile paJ
a6rel aqt'luauuadxa aql Jo r.xprOetC (B)
')as/uJrl
t tnoqe Jo aler e le suotl)alp
a]rsoddo ur 6ur^our a.law (uaafi auo pue
pa) auo) uMor]s sluauJplu ut])e oMl aq]
iuede puo)as g'0 tnoqp paplo)ar areM
a)uanbas srql ur uMoqs sauerJ oapr^ aq1
'luauelu q)Pa ol punoq speaq urso^u
Jo suozop aql Jo q)ee {q ua)el sdats
lpnpr^rpur
(uer.u aql ol 6urmo 'a)eJrns
aqi 6uole aprl6 o1 ue6aq sluaLue;r;
url)e aqt'pappe seM d-LV uaqM (V)
'speaq
ursoA-u aql ol purq ol pamollp pup
poppp araM urpro;1eqd luarsaronl, q]rm
palaqel stuauelu url)P uaqt pue'eptls
sse;6 e o1 paq)eilp ararvt speaq ursoftu
15 parpnd'tuauuadxa srqt ul
'peaq ulsolur oql Jo ^l!^!l)e rolou.l
aql ro] o)uap!^a l)orlq a9-9 1 arnEr3

suol_ow ufln)ttow
ureurop rolou
spr)e ourure 0001
ad{t
urso{u.r
ursoz(u puorl
-uenuoc aq] 1(papeeq-euo JoJ) I ursotru porueu sellt pup Jeruouoru e se uoqcury
o] peuees alnf,aloru sIqJ 'llel tuaraJJrp Llalaldruoc e 1nq urso.{ur alJsnru Jo peaq
aql ol Jelrrurs ureruop Jolotu e Suneq ur puonueluof,un sem lertrl qualp$aJ
oqaouDqruaJy eqaouE Jate^ qserJ eql uI ursor(ru e punoJ sJeqJJeaseJ Jeqlo
'aurll
aures aql lnoqe lV'slleo ueozolord Surpnlcur 's11ac aycsmuuou ur luasard
sureqJ dleaq aq1 'paJaloJsrp ararr saddl ursortur raqlo dueru .{puanbesqng
'(pappeq-oml
to1) 11 utsotru perueueJ serr,r
arn8gg) surEr.uop lpl lpururel-O Jo dlarrezr p qlu dlapVtt a8ranrp ueql pue
'snururJal-N
eql ]B ureuop Jotoru ursodur alqezruSocar B q1.rm pels dleraua8
papEeq-euo Jo Jaqunu E epnlf,ur sursoziru ;o sadzil par;rluapr ,(prau aqJ '(zg-gl
pulq ot suralord aqt llurad ol uorlnlole Surrnp pegrsrelrp,tpuaredde aneq 1,(1p
-raua8 suralord Joloru Jo slre] eql pue) slrpl ursor(ur aq; dprue;radns aqt ur sell
-rue; ursodru lJurlqp Zg lspal ]e eJE aJaq] leq] aleJrpu salof.recna asJe^rp Suoure
suosrredruoc acuanbag 'gInX ursodu lspel le qBnorql 111 urso,,tru) ,,fuanoc
-slp
Jo JapJo alerurxordde rlaql sloegal lrlrou arnlelJueurou aql pup '11 ursortur
pue 1 urso,{ru o} palelal r(1enba,r(lalerurxordde aJE }pq} sar}erJe^ pepeeq-o^q puB
[p uI punoJ eJp 'JaAa^ oq ']sontr '(XD seleJqepel ur [1uo punoJ ueeq elpq aruos
pue 's1ue1d ur dpo punoJ uaeq a^pq (1y pue IIIA se qJns) surso,{.ru aurog
'sao8rec
]uaraJJrp qlyvl ]oeJalur ol pup slrunqns Jeqlo
'(09-gI
aJnBrC aes) Ilrlor3rruJo uorlonJlsuoc ar11 JoJ JarlJpa passnJsrp sE (aopJJns
IIeJ eqi lB seJnlJnJls qcrJ-ur]rp Jo uorsnJloJd aqr SupnlJur-uorlezrue8ro re1n1
-lesPrlur ur pan1o,rur,,(1eraua3 are,,(aq] pue 's[pl Jreql ul alrs Surpurq-aueJqrueu
E Jo elrs Surpurq-uqcp puocas p ureluor uauo suralord 1 ursor(ru aq;'uoqer8rur
1ac Surrnp ilac e Jo ,{poq aqf Jo uorleoolsupJl pJpMJoJ aql JoJ se IIe^ se '(sJel
-q8nep oml olur 1ac Surprnrp e;o gede Surqcurd aq]) slsauHofc .rog parrnbar
,(leraua8 sr 1I 'sllaf, alcsnruuou pue elf,snru qloq ut ,ir4lrce alrlJeJluoJ qtl \
pelproosse sr 1y urso,{141
'}rodsue.u
alaue8ro pup elcrsal ur pellolur sr n ursort141
'pauruJalap
aq ol urEruer sursor(ur aql Jo lsoru JoJ suorlsunJ tJexe aqJ
'pua
snulu eql pJP.^ao] sa^oru
qJIq ^ '1r1 ursodru sr uortdacxa aq; 'spaads lueJeJJrp ]e os op ,{aq1 q8noqrp
']uetuelrJ
ullJe u€ Jo pua snld aql pJemol anotu auo ]dacxa sursortru eql Jo IIV
'(tZ
ratdeql ur pessnrsrp)
spu8rs leJrJlJela olur sale^a punos ueluoJ pue punos ol asuodsar ul lU] eseql
1(99-6 arn8rg aas) sllal aseql Jo eJEJrns perde aql uo punoJ eJE leql prlrroeJels
qJrJ-urlop Jo sepunq InJllneeq pue xaldruoo eql Jo uoqcury pup uorlJnJlsuoo
aql JoJ luelrodrut are surso,{ru pazqercads ,{yaurarga asaql'ssauJeap drelrparaq
asnEc ol rr aou)I erE rueql Jo eArJ ur suorlelntu pup tea Jauur eql Jo sllac JrEq
aqt u1 dlarrrsnloxa Jo ,(preruud passardxa are sursortu upunq eql Jo aurN 'saua8
utsodru 0? ]noqe sepnlf,ur aruoua8 upunq eql :sasselJ IeJn]f,nJls uales lspal
1u Suquasardar 'saua8 ursortur gI lspal 1e seq 'aldruuxe JoJ 'sunBa1a
J apolpueu
aq;'sursrue8Jo JplnllaJulnru ur r(1;epcrlred 'suorlcunJ pazrleroads aroru ru.ro;rad
sutsodtu Jaqlo ]Eql pue eArAJns o1 1ac c4ol.r€cne e ro; dressarau are sursof.ur;o
sadft aarql aseql teql alelncads o] Suudrua] sl ]I 'sA ulso.{ru oml pue 'II ursodtu
auo 's1 ursortru oa,q :sursodru aAIJ suleluoo aursrnaJal sactruotaqccrg lseari.
eql 'uonnlona crlodrecna ur f.pea asore sursofru leql 3uusa83ns 'sa]o^,fuecna
I
tl
ill
ilA
iltn
XI
IX
Atx
€ t0t
'uoll)al!p
ut aoueLlt strll loJ
alqrsuodsar {lqeqotd st 'sutsoftu
laq}o ul
punoj lou 'uteLtop peaLl loloul sl! u!r'lll/v\
uoruasut lleuls eql 'luaulelu
utlle up Jo
(pua snld eqlJo peatsur) pua snutu aql
premol 6urnou ur anbrun st 'slaqulaLu
I;rue1 .raqlo o1 {1ue;ruts lelnl)nlls llela^o
slr alldsap'11 ulso{y1 'uteutop
lolout
auo lsnf qlrm'slaulouou_l se uotl)unJ
ot uaas (Alx pue'xl 'l sp qlns) /!{aJ e lnq
'oln)aloul
lad suteLuop lolor.u oMt qltM
'sleultp
uJloJ surso{lu ,(uey1 'sraquau
^lrueJ asaql loJ ainl)nlls leln)aloul
oql Jo suort)rdap are tqbrr aql uO 'asla^tp
Iran ere (aryq ry6fl suotsualxa leututJal-N
pue (uaarb trl6tl) sllel leulutal-f lraqt lnq
'(uaafi
4top ur uMoqs) sutpulop loloru
repurs areqs surso{r.u 1;y'sad(1 urso,(ru
auJos Jo sureq> Ineaq arllJo aln])nJls
ureurop aq] Jo uosuedLUo) .sloqtueu
{;rue;redns u;so(yy 4g-91 arn6;g

uolelalsol^) aql:9[ ra]deq) tt0t
su!au^c
,o sad^I oMI oJV ereql
N
N
(v)
rsaurl
[ -u
€- u
rseurl
? l-ursau
rl
uI punoJ.{.1qeqord are suleudp clurseldol,li.:) 'speeq
a
)
uretrop rolour
pue su!seu!)| :sulatord ,|olow alnqnlor)lw
,{,reaq ro; alqrsuodser IteI IroJ-palloc pe}E8uole uE pue suleruop Jololu peeq JEIn
-qo13 olu IuJoJ esaql lJolotu ell]f,e Jed suleqJ 1q311 omr pue suIBqJ Ileaq o^r1 3uI
-AEq ur II urso^u o1 ^,(lernlcnrls relIuIS sI uISauI) 'selnqnloJJrru Jo pue snld eql
prE,\i\ol SuUprt rtq pururral uoxB eql pJe,4,\o] fpoq 1ac Puorneu eq] luo4 Le]!\P
selleuE8ro pasol3ue-auEJqrueu seIrJPJ ll eJaqM 'plnbs eql Jo uoxP 1ue€ aql ut
palJlruepl IsJIJ sB^ ]I 'sapqntoJcru Suop saloru leql ulalord Jolotu E sI ulseury
J epolPueu JqI'sursaull ]Jullstp xls sPq aDrcInilaJ saJlkaoJOt4Jsrs lsEa^ eqJ
'(gg-gl
arn81g) lueuala uorutuoJ ^luo eqt sl uleruop Jololu eql qJIqM JoJ L11ure;
-Jedns uraloJd a8rel e Jo Jaqrueru P sI uIseuIT 'utsortur aIIT 'uoIlPZIJeruIp uleqc
IIe^\ puB ureqJ d^eeq aql Jo snuluJel-N eq] lE ulPluop Jololu eql e^EI{ ureql Jo
1so61 fprue;Jadns urseurl eq] ul sarlrueJ ]oullqp ueaunoJ lseel lE eJP eJaqJ
'9t
tnoqe e^eq suElunq puE 'sulsaul{ 9I sEq suDSaIa
II ulso/(lu e se qJn{u 'reqlouE euo ]s?d seFqn}oJJlru peluelJo,(lalrsoddo sap11s
leql Jolour JPIodrq P SuruJoJ 'urPruop IIEI eql qSnorqr elPlsosse-Jlas uec LilrxEJ
g-ursauDl aqlJo sJaqurelfisJelulporeleq aJe sJaqlo 1ad pue 'sretulpoluoq aru sJe
-qlo eluos 'I urso^ru o1 snoSopue'sJeurouolu sP uollJunJ ol luees pue acuenbas
IroJ-paFoc e {Jel sureqs .4leaq utsauDl euos 'eFqn}oJJlru eql Jo pua snulu
eql pJPo\ol 'uorlseJlp ellsoddo eql ul qpA\ puE snulruJel-3 eql ]P uleluop Jololu
aqt seq dgue;8uqsara1ur,(lrepcruud V'alnqruor3llu eqlJo pua snld aq] prPr!\ol
;o,{ue11 'eFqnloJJrru Jaqloue ro a[eue8Jo pesolJue-euEJqluau P Jeqlle JoJ IIel
eql uI elrs Surpurq B i{Jrec sursauDl lsow 'sluelllElu uDce JoJ seop }usurellJ {3lql
seq3uerq rofEtu o^\] seq,{.lrureJ uleu.{p eqJ'suIEqJ lqBI pue suIPqJ elElpeluJe}ul
pelprsosse Jo requnu elqerJE^ puB eSJel e pu? (ureluop rolou eql epnlJul leql)
surBqJ d^eaq eeJql Jo o^^lJo pesoduo3 erP ^eqJ dgure;radns ulseupl aql ol pelPl
-eJun sJolour eFqruoJJrru patseJlp-pue-snul[u Jo ^lIuPJ B aJE sulau^p eql
'uorsr^rp
1ac Sur.rnp uorlPJPdes aluosoruoJqc uI pue 'uoueluJoJ elpulds
cporalu puP 3r1o1[u ur saloJ srJrJads eAPq sreqtuetu .{FueJredns utsauol aql
sP suEruop rololu a8rel ortl qlF 'sJeulpouoq uluqs-,,ileaq f.1pcldfi eJe qJIqM
'sutaut[p
JllusaldoltJ eq] sureluo3 qJueJq lueIJuE lsolu eql '(69{I eJntld)
I
I
)N
)N
fN
)N
fN
spr)e ourue 00s
,taqt pue 's11ar cqodrcJne IIB
r.uu 0l
('rasnoH uqof Jo ^sauno) 'g) ual
eql uo sureuop peeq aql qll/!{ aln)eloul
ursaurl eJo qder6orrrr-u uoll)ala q)la
-azaarl (B) 'elnqnlor)rru
e,o pue snld aql
Jo ppalsur pua snurur eql pleMol'sutsaut),1
lo Iryofeur aq] urorJ uor])alrp a]lsoddo
aql ur lanerl 11;eraua6 sursaut) osaql
'grey
uralord tseal aqt pue prp uralotd
oltLloosqo aqt sapnl)ur leql ulsaur)
leurural-) e sr t[-utsaut)'(€00t 'd aas)
srol)ej aqoorlsele) palle) arojalaql ale
Ieql 1sa;nqnto.r>rru ;o,(lrlrqelsur rrueulp
aseaJ)ur ol spua alnqntolllr,.u ol putq
peolsur pup {1rnr1re rololu le:rdft lsol
a^eq leql sursaurl 1o ,{;ruel e Jo raquau
p sr ll 'ureq) Ineaq aqt;o alpprur aql u!
pale)ol ureuop rolour slr seLl € [-ursaur)
'1;
urso{r.u {q paurol s}uaurelU ))rql
re;odrq aqt 1o (tln!t>e aqt o1 sno6o;eue
'raqlo q)Pa tseo salnqnlor)il.u oM] eprls
ol alqe sr rauerlal S-ursour) relodtq eql
's;re1 rraqt Iq aler)ossp sraulp oMt araqM
srauerlal suilo, s-ursaur)'salnqnlol)rul
6uo;e sa;lauebro pasol)uo-auelquraur
a^ou pue sror.lrouoru se uotl)unj
ol uaas leql sulsaull Jo ssel) lensnun ue
slueserder €- u r s e u r ) ' a ; ; a u e 6pasol)ua r o
-auerqureur e se q)ns'o6rer ol saq)elte
reqr lle] e suroj ureuop leur[!Jal-] aql
'uortpzuauJrp
6u!te!pau lror-papor 6uo;
e suuoJ ureurop alpprJr aql'ureq: Ineaq
aqt Jo snuruual-N aql le ureuJop rolour
aql seq [-ursaur)'pa^Jasuo) are sureuop
rolou aq1 {;uo'{;rureyadns urso{r.u
aql u! sv'slequralu ,{;rule;redns ursaurl
a^U Jo sarnt)nrts (V)'sulalord palelar
-ulsaull pue u!sau!) gg-91 arn6r3

SUOIOW UflN]]IOIA
u!6UO fu euol1n ;o^f uol,utuo)
e sale)rpul ursaul) pue ulso^w,o /buelluls lelnl)nrls aql
eql esnEc eJoJ IeJlueJ eql tuo{ pJe^ }no Sutpuaga sdool a3re1 'aSuEqJ Ieuoq
-PruJoJuor crralsollP ue olur srs^loJpdl{ dIV elEIsuEJ} o} dJesseseu dJaulqceu
aql pue Surpurq dJV Jo elIS eq] sepnlf,ul uoluruoJ uI a^eq suleloJd Joloru
Jo sed& omt eql leq] tuauala SuuerauaS-aJJoJ Ierlueo eql'(09{I arn8lf,) seJoJ
IPsrluepr fFeau punoJE tllnq aJ€ sulEluop Jololu o^\l eseql lBq] palEe^eJ seq
urseuDl pue urso^ru Jo sureuop Joloru eql Jo eJnlcnrls IBuolsuaulp-eeJq] eql Jo
uorleurruJalap ta^a^^oH 'selllJelruls ef,uanbes pIJe oulluP elqelJltuepl ou eAPq
daql pue 'sattradord rllaul{ luaraJJlp e^Eq puu sluetuellJ }uereJJlp Suop >1cer1
suleloJd Joloru Jo sessEIJ o1!\l eqJ '09t lnoqB ql!v\ peJeduroc splcP oulluE 098
Jnoqe (sursauDl
Jo leq] upql JaBTPI ^llepuetsqns sI susodru Jo ulEluop rolou eqJ
u lsau r)
[.retp: urau(p rruse;dopb
se 'SuDIJIJJEJ] elclsel JoJ luulJodlul ale
uraui(p
-edde r31og eql Jo uoFezIIBJol JoJ sB IIa^
pessnJsrp) e11a3eg pue EIIIJJo Suqeaq aql e^Irp ]eql sa{nqnloJclruJo sluaua^oru
Surprts lueIJUJa pue plder eqt roJ pazllelJads r(tq8q are ri.aql ,tla,rpcadsar 'speeq
ureuop-Jolou eeJql Jo o^/tl qll^\ 'sJelulJloralaq pue sJarulpoJelaq apnlsul
'qcuerq
a8rel raqlo aql Sutaufp lazuauoxv 'lleJ aql Jo Je]uao eql JEau snlsJ
^ oleq lro.nn r(aqt ^ oq ssnJslp fieqs aAA'casTurrl t-z ]noqe lP sapquoJsl[u Jleql
e^our up3 surseuDl lsalseJ eqt 'uoslJ?druoJ uI 'f,es/uli tI Jo eluJ elqPIJEureJ eql
te aqru lse] e uI sepqnloJJllu e^o{u uec suleu,{p lPluauoxB :}sa}sPJ eql Suolue
osle arE ,(aqt pue 'sJoloru rElncelolu rr^\ou)I el{l Jo }se8Jel aql eJe sulau^C
'ellrcJo
Suqeaq 3ql ur pa^Io^ul aq 01 slueas lnq sulau^p plueuoxe qtyruEql slru
-se1do{c r{llrrr drpellunrs acuenbas relpeJ8 saleqs qJuEJq 'Joultu 'pJIq} V '(Ja1PI
sa]ls
rq-e;nq nlor:r ur
6u rpu
st0t
flasnoH ur.lof
;o Isetrnol) 'ursaurl
.ro urso{u raqlra
Jo peaq oql qllm pareduor a6re; fuan
sr peaq uraulp aql leql eloN'speaq oMl
seq sleurrue uorl urau,(p ^lerlr) :speaq
aarql seq pue ueozoloro e LUoJJ sr uMoqs
urau{p {rer1o aql'aln)alour papeaq
-oml e sr urau{p rrr-use;doy(r'ursaury pue
p; urso{tu a111 'urau,{p (leuouoxe) Iletltl
Jo aln)aloLu e pue urau{p rrr.use;do1,{r
Jo aln)alour e;o sqder6o.rtrur uoll)ala
q)la-azearl'sulau{q 69-9 1 arn6r1
(tat^asll ulotJ uotsstuled
qtlM'/66 t' 9VZ-6Ea: l' lotS' Dnlts
'utdo
'v'u pue souv
'v"l
uro4 paldepv) "iln)'ssol) 'ureurop
lotor.ll orlllo
suoruod uorl)npsueil-a)loJ pue 6ulpugq
-raLu^lod aqt u! se)uara#rp rofer.u o1 anp
ere sJolor.ll oMl aqlJo suotl)unJ pue sezts
luaraJ,rp {ren aql'r e1 r Lu rs (len {l I er n}rnr}s
arc (uo11at( ul papeqs) ursautl pue
ursolur ;o sureuop 6urpurq-aprloaltnu
lerlua) aLlI'speaq ulsaull pue urso{ur
,o sarnl)nrls 1e1sfn {er-;1 g9-91 ern6;1

t
uo1a1a1so{1 eql:9 t reldeql 9 tO
'$lrerl
tuaurelg Jraqt o] Surpurq,{q palenrlce
^,(13uor1s sr suralord Joloru;o ,!nr1ce asedJv aql eJuls 'suorloeJrp qloq ur {Joilr
ol srueas alrs srsflorpdq apl]oapnu aqt puu alrs Supurq-raurr(1od aql uaamleq
sa8ueqc prntJnrts ;o .,{.e1ar aqJ 'ef,eJJetur Surpurq-raru.4.1od aql o} pa,{e1ar
aq o1 alcitc srs,{lorpdq dJV eql Lq pasnec suoursuEJl lBrntJnrts aq} 3uu,ro11u
,{1a,r4cadsar 'Surpurq
ur}oe pue apqn}orJrru ur pellolur uralord aq};o suo€ar
asoqt qll,lr ,{Janrsualxa s}cerelur dool qcllvrs e 'ursol.ur pue ulseurl ul
'uralord
aql yo ped ]uaJaJJrp e Jo uot1elor
E asnpJ o1 pagqdruu ,ri.peprurs sr-aleqdsoqd putural E Jo aouasqe ro acuasard
aql-alls e^rlJp aq] ur a8ueqc IBJntJnJls lerus .,t1a,r,qe1ar e 'pazdlorp.{q s1 413
ueql JaqlEJ dJV puP 'suralord rolo(u oe\l eql JoJ JeJJIp luarue^oru eql Jo slrEiep
aqr q8noqrly'pasealar s1 lareqdsoqd purural eq]) eleqdsoqd-/" paz,{1orp,{q aqt
uaqm lno Suurs sdool aseql tnq 'elets punoq-dJg eql ur aleqdsoqd-/" aqr qrprr
]reluoc esolJ ur aJe alrs Supurq-appoalf,nu aql ur sdool ,,qctIMS,, alrqou :suroJ
(a^rlJpur) punoq-dcg puE (a^Ucp) punoq-dJg Jraql uI suorlBruJoJuof, l3urlsrp
tlq1qxe sutalord eseqr '(ZZ-t arn8rg aas) g JaldEqO ur passnrsrp sy ,{pue;radns
seg aq] Jo sesedJg llerus eql Jo elrs Surpurq appoelcnu eqt ol acuelqueser
IEJnlsnrls aruos sreaq oslp aJoc Joloru aql leql uorlp^Jasqo eq] ruo4 aruoJ
seq uorleJeua8 acro; ur pe^lolur sr aJoc IeJlueJ eql ,ttoq o] anlc luepodrur uy
'aroJ
leJlueo aq] uro4 ]no Sunuoc sdool aqt;o uorlnlo^e q8norql
uorleJrJrporu pue uorleorldnp aua8 erl esoJe suorlcunJ pazrTerJads snorJel
Jreql leq] pue losJncard uralord Joloru [pJlseJue uoruruoJ e ruoq papuecsep
a.re surso,{ru eq} pue sursauDl eq} qtoq leqt rq8noqt sI lI ,(larrpcadsar ,ursauDl
pue ursodur eql ur salrs Surpurq-elnqntoJcrru pue Surpurq-uuJe aq] epnlour
sdool asaql'{JeDJo ecroqJ aql JoJ alqrsuodsa; eJe pue azrs ureruop ur ecualaJJrp
sa6ueq) | euolteuiloruo)
a>.ro1 a}eJauaD surelold ro}o4
dJV ruo{ pa^rJap ,,{3raua eql esn lsnru urelord ro}oru p 'uouour
IBuorlJeJrprun
qrns JoC 'luaruulrJ eql uo alrs Surpurq ^ au p ol lueuelrJ e Suop uoqcarrp a18urs
e uI prpMJoJ ruaql ladord lsntu aseeleJ pue Surpurq 3o a1c.{c qope :luauarrnbar
Jequrg E alpr{ sura}ord ro}oru eq} 'ulaloJd raulred e q}lm suog}ceralur crycr{c
acnpord o] selrs Surpurq-aleqdsoqdrrl-aprsoalJnu neql q sa8ueqc IeJnlcnJls
asn qloq suralord Surpurq-41g pue suralord Jolotu plala>1so{c aqr q8noqrry
reJ aql Sursnec talal 3uo1 p a)rl xueq aql 3ur.us uec lurod srql le a8ueqc IIErus V
'(09-9I
arn8rg eas) ureuop rapeluoc pellpo-os aqtJo suo4ptor llerus ol peeq eql
ur Uap Supurq-41y aq] lE sluarualoru slcauuoJ ]eq] xtleq e{u-uolsrd e sr araql
'peeq
aqt o] txau rure relel srqt Jo eseq eqt lV 'sureqc tq8q 1o Surpurq aqt dq pazrl
-tqe1s ,{lernlonJls sr qJIq^{ 'raJU Janal Jo 'xtleq n Suoy-ruu-g'g up Jo Sur8uu,rs aqt
,i.q palerauaS sr urlce Suop luaruanotu aq];o dals qcea 'ursodru JoC 'elnJelotu
utalord eq] Jo lred ur ]uarualoru a8rel e eJJoJ ol srsdlorp,,(q pue Surpurq
IInJ aql 'raqtoue ]e ure8e ]l Jo ploq qcteus pue lurod euo tE lueuelrJ url3p
aql uo drr8 slr asealeJ ol peeq ursodru aql 8umo1e 'urlJe JoJ rtfpgJe Surpurq s1r
ur sa8ueqc o1 paldnoc are ursoLu aq] Jo uoueuJoJuoJ aql ur saSueql eseqJ
'ruu
0'9 lnoqe dq a^our o] x1leq eqt Jo pua
pcnuaqooueqcaru
go Suqcopun pue Suotcop aq1 alepBar alrs Surpurq-appoalcnu aqt ]e sdool qcl,vrs
Jo sluarualoru llerus eql 'ruJe Jelel p
;o Suqcor eql Jo ppalsur 'urseupl uI
'(fg-gf
arn8gg) luau
-e^oruJo dals ay8urs e sacnpord (,,a{oJ}s rarrod,, eq} sesneJ qJlr{llr) asealar eleqd
-soqd pue 'srs,{1orp,{q
3uo1 B ot urcrxop peaq rolour aql
ot srs^lorp{H aw 6u;1dno1{q
epnoalonu'Supurq epuoelonuJo aycrtc
tB ppeq rotoru slqt slf,euuoc 1eq1 uo6ar ra1u11
s/l\ollB Surycopun pue Suplcop ra{urTJo a1c[c slqt ]Bq] 0s 'pa]eurprooJ zi.laso1c aru
speaq o^ l aq1 ur salrdc srsdlorpdq eppoelcnu eqJ 'peeq ]sJU aqt JoJ etrs Surpurq
aql ueql pua snld eFqruoJonu eql 01 JesolJ ruu I 'lueruelgolord eql uo elrs ]uaru
-qJene ,t,reu p purq o] alqe eq ily\^ ll ereqm uoltlsod e ot pJEMroJ peeq puooas eql
s^\orq] srq] :peaq aqt Jo eprs aql Suop slcop uo€ar re{u[ slr 'peeq punoq srq] ol
41y;o Sutpulq eql uO 'peJnlcnJlsun [1azr4u1ar sr uor8ar ra4ull slr 'e{oJ]s ra,,vr.od aql
aroJaq epqruorcrru p 01 punoq $ peeq ursauDl (Surpeap tuo{ aq} uarlq'(Ig-gI
arn8rg ees) pua Jeqlo aq] ]e urpruop uorlezrJarurp 1oc-pa1or aq] o] pue euo

suolovu uflnfttoy\l
pua
sn u tLlt
ru pua
Snutul
F
luaure||+
urlle
r)ouls urmod
peaq urso^u
i"ff t ii:#'#i:
.{puaredde tlp uI acuaraJJlp slql dlluara;yrp patuarro are speaq paqre}teun
'puoJes
eql lnq 'ayqeqsrn8urlsrput ,{ller}uassa aJE alnqnloJJnu eq} o} paqcel}e
aJB ]eql speeq eql 'saFqnlorJrru o1 punoq flruesradns urseuDl 3q] Jo sJeqrueu
3ur4p.,r,r-premIJpq pup SuDIIBllr-prp^/r4.JoJ Jo sa8prur uoqnlosar-q8q ,rI
'peloau
-uoJ eJE speaq aq] qJIqA\ ur ,tem eql ur arl ol sruaes raMsuE aqJ isuouJeJrp elrs
-oddo ur anoru Laql uEJ ,lrroq 'pcrluapr f1er]uasse eJe sursaur)Jo saddl oml asaq]
JO SurEruop rolour eql af,urs 'pua snuru aql pJEl ol a^oru pue snurruJal-J aql le
sureuop Jolour Jraql aleq sreqrueru dprue;radns ,lvral E 'elnqnloJJrru eql Jo pua
snld aql pJeA\ol a^our 'snurruJet-N aq] ]e surctuop Jolotu Jreql qlrA\'.{puegradns
ursauDl aqlJo sJeqruaru lsoru searaq^ teql IIEsau 'luauanoru;o dlqeuor]3aJrp slr
auruJalap ol pue Jarurp ursauDl aql Jo (surptuop Joloru) speaq oml aql Jo salJ^J
IeJluaqsoueqJaru eql elEurpJooc o] qloq srueas uEruop Foc-palros eql
'da1s ruu-g eleJosrp e 8up4et aurl qtee '(Zgrgl
aJngIC) Jeuueru
asl,vrdals (ppaq-Ja^o-peaq Jo) pueq-Jalo-pueq e ur aloru ot Jolour papeeq-omt aq]
I
I
t
I
_)
luauJelr] )ilrq]
u I soAur
t
srs^roua^H
I
dav -1
pua
sn ld
pua
sn 1d
srqt 'apsnu 6urpe.rluot ^la^t])e ue ul (qleap +o Altpi6u
aq1'sr1tow to6rL to1 a;qlsuodsat st lr asne)aq poujeu
os) uolletnbtluot to6u e ut ]uaulelt] urlte ue oluo ,{;1q611
pa))ol sr aprloel)nu punoq e 6ur1:e; peaq urso{ur e
'aln6r;
srqt ut uMor]s ap{r aq1 ;o ue}s aq} }V O:ll{iVIfV
r u.,t:d"' r""'i
I p r d e
],{q p a 1e u r u r a1 A
( urpe aLll o1 asol: i{lan surewat r{lqeqotd
peaq aql Alrleet ul q6noqlle 'a6ueq) sl.{l sazlseqduta
urlre pue peaq aql uaaMlaq alaq uMelp aleds aql)
'tuaurelrj
aql 6uo1e e^ou ol lr snnolle pue utpe lol peaq
aql;o,{1rur;;e aq} sa)npal stLll a}ts 6urpurq-url>e aq}
dn a)eu leql suteulop aql ]o uotlputJoluo) aql ur a6ueqr
]q611s e sasne: (;alerpatuLlr pue (]uaueltl ut1re aql
uroJ] tsaqun] apts aql uo 'st
lpq]) peaq eql +o ,,))eq,, aql
uo gap a6re1 aql ot spurq d1V ]o aln)alour V OlSVll:lU
urelord aq1 o1 punoq {1gq6r1
uretuar parnpotd (d) aleqdsorld true6tour pue dCV
aq] lnq 'srn))o 6ty 1o srsflorp{H u]u S }noqp }o a)ue}stp
e,{q 1uau.relr; aq} 6uole pateldsrp eq o} peoq aq} sasne)
1eq1 a6ueqr adeqs a6le1 e 6uua66u1'aln)aloul dfv
aq] punore llaqs urel) e e)rl sasol) ]+al) aqf al)lf,of
a;t{> aaau e +o !e}s aq} o} 6uru.rnlat
^qaraql 'dcv punoq slr sasol peaq aq1 'a1or1s
.rer'aod
aql 'o osino) eql ul uorleullo;uor 1eur6r.ro s1r sule6al
peaq aql qrrqm 6uunp adeqs ur a6ueqt 6urletaua6-a:.ro;
aql-a)orls iaMod aql sta66u1 asealaJ stqt
ur])e ol peaq aql ]o 6utputq lq6!f aqf qynn {11ue1ru.ro>uo:
'srs{lorpAq
6ty {q parnpotd eleqdsoqd lue6lour aq1
lo asealal sasne) luaulelt] utDe aql uo alts Mau e ol peaq
urso{ur aq1 1o 6urpurq leam y gNlIvUlN!19-l)UOl
lueujelt+ utl)e aql uo uotltsod
n,rau e ol pa^our seq peaq aql leql aloN uollern6rluor
.ro6u e ur luau.rpltl utDe aql o1 ,i;1q6r1 pa1to1 ure6e
sr peaq urso(tu aq] ,al)b aqt +o pua aq] rV OlHlVltV
e q
LLOL
fsvvv LlorJ
uorssf LrJrad qllM'866 t lgs-}s:Lgz DuaDS
''lp
la tuauleu
'l
uo pase8) luaulplu
url)p ol6uts p a^ou ol Jeqteool ltoM ol
s u r solur,{ u e u 6 u r rrno ; ; e'a r.u r 1 a ; rr(> al rl u a
aq] Jo o/o5 lnoqe I;uo lo; luaulelU utt)e
aq] ol punoq surpr,.ual peeq aq1 'a;t{: 11
urso{ut aql ul 'tuaulel4 u!t)e ue
6uo;e 1;em o1 1g ulso(tu Iq pesn sa6upr{)
lern))ntls,o ol)^) aql [9-9 t o/n6tl

uolalalsolr() eql :9 L raldeqf 8 t0 t
-{ooq '(16-9
V arn8r4 aas) 9 Ja}dPqO uI pessncslp esPdIV slJe(uExeq eq} Jo
e^qeleJ xeldruoJ aJoru e eJoJaleql sl ]I luleluop pulluJal-J uIEI{J-,{^eeq aql snld
surBruop wvXIs :suleruop ue^as luo4 peruloJ 3uu ruueld e Jo slslsuos pBeq aql
'ppeq pedBqs-8uu 'eleJoqele ue uJoJ ot pasn sI uIPq3.{,reaq aqtSo uonJod Jo[Eur
aq] alrqM'alnseloru uleu,{p eq} uI suIEqJ lrreaq raqlo eq} o} speuuof, pu€ suleq:)
lq311o ]as E spuq teql IIB] e sruJoJ uoluod pupural-g sll 'luerue^olu aqt selEeJJ
lPql ernlsnDs JrsEq eq] surroJ suolpp 000'009 uPql alolu Jo uleqs I^eaq 1uer3 y
'eBuEqJ
IEuollBIUJoJuoo SulluJauaS-ac;o; e ol se ila^ sE Surpurqun pue SuIpuIq
apqnlorsrru 01 srs,ltlorp^q eppoelsnu eql SuIdnos Jo alnr [EJaueS eq] s,i\/\olloJ
ilps ]nq 'sursauDl pue sulso^lu ol pelEleJun dlprnpnrls sI rololu uleudp eql
' (tg-gl aJntIC) luelualoru Joloru Jo dllpuorlcarp
aq] saurruJelap ,,(qaraqt puE 'peeq puoces eql JoJ alts Sutputq ]xeu aql saselq
o
c
(V)
rolou pepalp-pua snld
l-ursaull
(ureurop rolour)
peaq
'urvroqs ssa>ord eql;o teadar l)exo ue ,{q spaalotd qrtqnn 'dals
lxau aql loJ
pasrod rvrou sr aln)alour ursaurl aql'(esPalal (!d) aleqdsoqd pue stsfilotp{q
dfv sal|nbal luaLuq)elap) punoq dCV tll!/vl alnqnlolllu, eql ulolJ
peq)elap seq lt a)uo 'preMlo, uteiriop loloLu leal aqr sllnd Ulqs slql'(llo)
palro) poutmualut aLll pue uteutop rolou aql uaamlaq autl 6utlleuuol
e se aroq umelp st lalutl lrau oql) uolleui.lo;uor 6urlutod-plervuo;
e o1 6urlurod-premrear e uol; Urqs ol,,le)utl ))au,, aql pa1;er aptldad ;1eus
e sasne) ureurop rolor.il srql ol dIV,o 6urpu;q aql'(6utarretp slql ul E pue z
s;eued uaerr,rlaq) ureuop rolot! luolJ aql u! dCV loj dIV lo abueqrxa ue ,{q
uan up sr ureuop rolou, rear aql,o luaula)eldsrp prennrol aql'al!s 6u1pu;q
sr! u! dov tlll/!{ alnqnlol)ll.! aql ol punoq ,(1asoo; sr peeq 6utpeel lo ]uol]
aql allqM ijIV ol pue alnqnlor)lul aql ol punoq ,(llqbtl sl '@aatb
4top) peaq
6ur66e1 ro rear aql'speoq utsautl oml aqlJo auo'dals q)ea,o uels aql lV
.l)el] sl! jo
o6 6ur11a1 {;a1alduor }noql!/v\ alnqntot)ttu aLll uo sa)ue}srp 6uo; to; anou
ue) raurp ursautl aql 'uoltotu,,pueq-la^o-pueq,, sltll 6urs61
'a1ts 6u;putq
ullnqnl elqelle^e lxau aql ol spulqal uaql puP'uleulop lolol'u laulleo
aqt sassed
'e1rs 6utpurq utlnqnl sll ulolJ saq)elap peaq Jeal aq],'dals,,
ursour) e 6uunp lrauueu paleutptoo) e ul llom suleulop lolou, ulseull
oml aql'(8S-91 arnbrl eas) lre] llo)-pallo) 6uo; e q6nolql pal)auuo)
ale leql (speoq) suleLuop lolotu 6u1pu1q-ap;toel)nu oMl Jo lauilp e
sr [-ursaur)
'ulsout>1 o a;r(r ;erluaqrouetl)au aql 79-91 etn613
peaq ursaurl
punoqun
)N
luaurel rloloro
- alnqnlor)rtri
rolor! paperp-pue snurur
(p)N)€ [-ursaurl
(ureurop
rolour)
(8)
peoq llel l!er
c
)N
a)eJJns alnqnlor)rur
uorlre;aoft > ^q peurLuralap)
salnqnlor)rut ol peq)ellP
sraurp aql,o a6eur uorln;osa.r
ra/vlol e oluo (,{qder6oq1e1s{rr {er
-x {q peururalap) sraLurp uralord
-Jolour aarj aqt Jo sarnl)nlls aql
6ulug,{q palerauab ararrn sa6eur
asaLll'salnqnlor)!ur ol punoq
sulalord I;ruepadns u!sou!I
6ur1;em-prem1)eq pue -prervrJo,
Jo uollelualrg 69-9 1 eln6;1
(y)'({do>sor>rru
Ipr] sreqs!lqnd
uPllru)ew uorJ uorssruJao
qllM'866 t '9 r8-€ [8:56€
atnloN ''P ]a ullqPs
ruoU) 'uorleluauo ailsoddo aq]
qlr/v\ sraurlp suloj'snururjal-J aql
le ureuop iolot! aql ql!/vl roloul
pal)ar rp-pua-snutw e' \D I t rloosot
u! p)N palle)) € [-ursaur) (B) 'dals
]xeu eql e)el ol pesrod 'pua snld
alnqntor)r.u eql prpmol 6urlurod
sr peaq punoqun'puo)os
aql'(ar!s 6urpurq aptloal)nu
aqr u! dlv qtlM) alels alorls-lsod
e ur alnqnlor)r[! aql ol punoq
sr rarurp aql Jo peeq auo uaq^
'olnqnlor)rLU aql,o pua snlo aql
preMol sa^ou pue snuruilal-N
s/uraloro aql le ureuop rolor.l_r
slr seq (ursaur) leuoltua^uol)
o
'l
[-ursour)
tr
dcv

MOLECULAR MOTORS I 019
(B)
+
attachment to cargo
or another microtubule
(c)
AAA domains AAA domains
ADP + Pr
power stroKe,
8nm
c
AAA domains
in motorhead
major ATPase
C-terminal domain
15 nm
shaped linker region connects the hear,y-chain tail to the AAA domain that is
most active as an ATPase. Between the fourth and fifth AAA domains is a heavy
chain domain that forms a long anti-parallel coiled-coil stalk. This stalk extends
from the top of the ring, with an ATP hydrolysis-regulated microtubule-binding
site at its tip. Dlmein's "power stroke" is driven by the release of ADP and inorganic
phosphate, and it causes the ring to rotate relative to the tail (Figure f 6-64).
Although kinesin, myosin, and dynein all undergo analogous mechanochemical
cycles, the exact nature of the coupling between the mechanical and
chemical cycles differs in the three cases. For example, myosin without any
nucleotide is tightly bound to its actin track, in a so-called "rigor" state, and it is
released from this track by the association of ATP In contrast, kinesin forms a
rigor-like tight association with a microtubule when ATP is bound to the kinesin,
and it is hydrolysis of AIP that promotes release of the motor from its track. The
mechanochemical cycle of dlmein is more similar to myosin than to kinesin, in
that nucleotide-free dynein is tightly bound to the microtubule and it is released
by binding AIP However, for dynein the inorganic phosphate and ADP appear
to be released at the same time, causing the conformational change driving the
power stroke, while for myosin the phosphate is released first and the power
stroke does not occur until the ADP subsequently dissociates from the motor
head.
Thus, cytoskeletal motor proteins work in a manner highly analogous to
GTP-binding proteins, except that in motor proteins the small protein conformational
changes (a few tenths of a nanometer) associated with nucleotide
hydrolysis are amplified by special protein domains-the lever arm in the case
of myosin, the linker in the case of kinesin, and the ring and stalk in the case of
dgrein-to generate large-scale (several nanometers) conformational changes
that move the motor proteins stepwise along their filament tracks. The analogy
Figure 1 6-64 The power stroke of dynein.
(A) The organization of the domains in each
dynein heavy chain. This is a huge
molecule, containing nearly 5000 amino
acids. The number of heavy chains in a
dynein is equal to its number of motor
heads. (B) Dynein c is a monomeric flagella
dynein found in unicellular green alga
Ch I omyd o m on a s rei n hardtii. rhe large
dynein motor head is a planar ring
containing a C-terminal domain (gray) and
six AAA domains, four of which retain ATPbinding
sequences, but only one of which
(dark red) has the major ATPase activity.
Extending from the head are a long, coiledcoil
stalk with the microtubule binding site
at the tip, and a tail with a cargoattachment
site. In the ATP-bound state,
the stalk is detached from the microtubule,
but ATP hydrolysis causes stalk-microtubule
attachment. Subsequent release of ADP
and Pi then leads to a large conformational
"power stroke" involving rotation of the
head and stalk relative to the tail. Each
cycle generates a step of about 8 nm along
the microtubule towards its minus end.
(C) Electron micrographs of purified
dyneins in two different conformations
representing different steps in the
mechanochemical cycle. (B, from
S.A. Burgess et al., Noture 421:715-718,
2003. With permission from Macmillan
Publishers Ltd.)

1020 Chapter 16:The Cytoskeleton
between the GTPases and the cytoskeletal motor proteins has recently been
extended by the observation that one of the GTP-binding proteins-the bacterial
elongation factor G-translates the chemical energy of GTP hydrolysis into
directional movement of the mRNA molecule on the ribosome.
Motor Protein Kinetics Are Adapted to Cell Functions
The motor proteins in the myosin and kinesin superfamilies exhibit a remarkable
diversity of motile properties, well beyond their choice of different polymer
tracks. Most strikingly, a single dimer of kinesin-1 moves in a highly processiue
fashion, traveling for hundreds of ATPase cycles along a microtubule without
dissociating. Skeietal muscle myosin II, in contrast, cannot move processively
and makes just one or a few steps along an actin filament before letting go. These
differences are critical for the motors' various biological roles. A small number
of kinesin- I molecules must be able to transport an organelle all the way down
a nerve cell axon, and therefore require a high level of processivity. Skeletal muscle
myosin, in contrast, never operates as a single molecule but rather as part of
a huge array of myosin II molecules in a thick filament. Here processivity would
actually inhibit biological function, since efficient muscle contraction requires
that each myosin head perform its power stroke and then quickly get out of the
way-in order to avoid interfering with the actions of the other heads attached
to the same actin filament.
There are two reasons for the high degree of processivity of kinesin-1 movement.
The first is that the mechanochemical cycles of the two motor heads in a
kinesin- I dimer are coordinated with each other, so that one kinesin head does
not let go until the other is poised to bind. This coordination allows the motor
protein to operate in a hand-over-hand fashion, never allowing the organelle
cargo to diffuse away from the microtubule track. In contrast, there is no apparent
coordination between the myosin heads in a myosin II dimer. The second
reason for the high processivity of kinesin- I movement is that kinesin- 1 spends
a relatively large fraction of its ATPase cycle tightly bound to the microtubule.
For both kinesin- I and myosin II, the conformational change that produces the
force-generating working stroke must occur while the motor protein is tightly
bound to its polymer, and the recovery stroke in preparation for the next step
must occur while the motor is unbound. But myosin II spends only about 5% of
its AIPase cycle in the tightly bound state, and it is unbound the rest of the time.
\.Vhat myosin loses in processivity it gains in speed; in an array in which
many motor heads are interacting with the same actin filament, a set of linked
myosins can move its filament a total distance equivalent to 20 steps during a
single cycle time, while kinesins can move only two. Thus, myosin II can typically
drive filament sliding much more rapidly than kinesin-1, even though the
two different motor proteins hydrolyze NIP at comparable rates and take molecular
steps of comparable length. This property is particularly important in the
rapid contraction of skeletal muscle, as we will discuss later.
Within each motor protein class, movement speeds vary widely, from about
0.2 to 60 pm/sec for myosins, and from about 0.02 to 2 pm/sec for kinesins.
These differences arise from a fine-tuning of the mechanochemical cycle. The
number of steps that an individual motor molecule can take in a given time, and
thereby the velocity, can be decreased by either decreasing the motor protein's
intrinsic ATPase rate or by increasing the proportion of cycle time spent bound
to the filament track. For example, myosin v (which acts as a processive vesicle
motor) spends up to 907o of its nucleotide cycle tightly bound to the actin filament,
in contrast to 5% for myosin IL Moreover, a motor protein can evolve to
change the size of each step by either changing the length of the lever arm (for
example, the lever arm of myosin V is about three times longer than the lever
arm of myosin II) or the angle through which the helix swings (Figure f 6-65).
Each of these parameters varies slightly among different members of the myosin
and kinesin families, corresponding to slightly different protein sequences and
structures.
mrnuS
end
Myosin ll
Myosin V
5 to 10 nm swing
of lever arm
30 to 40 nm swing
of lever arm
neao ---
pl us
end
Figure 1 6-65 The effect of lever arm
length on the step size for a motor
protein. The lever arm of myosin ll is
much shorter than the lever arm of
myosin V. The power stroke in the head
swings their lever arms through the same
angle, so myosin V is able to take a bigger
step than myosin ll.

LZOL suol_ow uflnfltow
aqt Jo spaau cr;rcads aql o] Surprocce ,{lrnrssacord pue paads ro3 ,ureurop Uel s}I
q8norql peqJelle o8rec aql;o zlquapr aqr rtq peuruJalap sr uor]JunJ esoq ^ ,ural
-ord rolour qJBa Jo ror eqaq aql paun]-aurJ seq uounlo^a leq] paunsse sr ]I
ro l\^oq realc 1a.,( lou sl ]I 'uraui{p cnuseldol,b Jo uJoJ auo -,(po ,{1ensn sr eJeql
'1ac
cqodrecna pcrddl B ur punoJ sJaq{uau,{1preg urseuDl pue ursodu }ueJeJJrp
dueru are alaq] seaJaq yJaqtuaru dgruey relnc4red lpq] Jo uorlounJ aq] roJ IIeJ
'llef,
eql Jo spaau 3uua;;rp o1 asuodsar
ur parJrpou aq uEf, uraudp cluseldolrtc ;o saqradord leJrueqJaru aq] Jeqleqm
tolow
sulolotd
eql alerpew
SAllAUe6ro pASOI)Ul
-auerquaw
,o ltodsuell lelnlla)erlul
alqpls 'aBJEI Surcnpord,{q aseesrp s,Jeruraqzlv o} esrr sa^r8 uralord srq}Jo Surssac
-ord prurouqp aql 'suoxB IIaJ-aAJau ur JoldaJal Jolotu eupJquorusuEJl p eq o]
pasodord sr pup I -ursauD{ Jo IrEl aqt uo ureqc lq8q p ol .,tl}JeJrp spurq r{Jrq^\ AdV
'ureloJd
JosJnJaJd pro1,{ure aq} aq o} surees sJoldaJer eseql Jo aug .sraqruaru
[1
-rupJ urseuDl aleudorddu aqt Jo slrel eql qluv\ dl]JeJrpur Jo dpcalp ]ceJelur slueu
-uedruoo pasoloue-aueJqrueru Jgrceds ol pepos eJp teql sroldeJeJ Jolour palerc
-osse-euEJqruehl 'o3JpJ
allaueSJo eleudoJddp Jreql o] qJEue 01 sJolotu eql lv\ollB
suraloJd Joloru cgrcads uo surpqc IqBII pelercosse Jreql pue slrp] ]ueJegrp ar{J
'eruosoJlueJ
aql lE spua snurru pJe.\i\o] $lJpJ] aFqnloJsrru Suop salcrsan
r31og Sur,rou taluao IIaJ eql JEau snleJedde r31og eql Suruoursod JoJ perrnbaJ
aJB surau^p 'r{yasra^rruo3 'q}llroJ8 elnqnloJJnu q}IA\ pe}ercosse sr draqdued IIaJ
eqt premol saFqn] UgJo luaue^ou pJemlno eql sllal Surnrl ur ,asrmalrT 'sllal ur
Ug aqt a41 qcnu dral qa^ snouprquau e Sururro; pup suorsnlord repqnl olur
lno seueJqrueur UA aqt 3ur33erp 'spua snld apqruoJJrru aq] pJemo] Ip^ pue
's{JeJ}
elnqn}ortru paruro;erd ol saueJqruau palrJep-Ug Jaq}a} uEJ surseur)
'oJiln
uI '(SS-gt
arnS;g) useldo{c aq} }noq8norq} sasredsrp pue slueru8er;
srueredde r31og aql e1tq ^ 'llal eql Jo Ja]ual aq] o1 sasdelloc Ug aql ,elozupotou
Jo aurJrq3loo sE qJns 'salnqnloJ3[u sazuaur.4.yodap lpq] Snrp e qlyv\ paleaJ]
aJE sller uar{ ,1'aruosor}ual aq} Jpau pelpcol sr snluredde €1og aqr seeJeqm
'lleo
aql Jo a8pa eql ol ]sorule spualxa pue selnqnloJf,rru qlrr!\ su8qu salnqnl
euerquau Ug Jo {ro^a}eu aql'snteredde r31o5 aqt pup (Ug) urnpcltar trurseld
-opue aql SurzrueBro ur eloJ Jraq] sl sauerqurau relnllaJeJlur Jo rorleqaq eql uo
sJolour alnqnlorJrru pup saFqntoJJtu Jo ]JaJJa arll Jo aldurexa lseJealJ eqJ
'sursauDl
se qJns sJolour palceJrp-pue-sn1d armbar draqdrrad aq] pJe^ao] sluaru
-alour p8n;uluac searaq A'urau,(p curseydolrb se qJns suralo.rd Jolotu palcaJrp
-pue-snurru
Jo uorlf,e aql armbar JelueJ IIaJ eql pJe^ ol selJrsel ro salaueBro;o
sluaruanoru pladrrluac 'snq;'l;aqdrrad
lac aq] ol Sueuexa spue snyd aq1 pue
'eruosoJlueJ
aql le IIaf, aq] Jo ralueJ aql JEeu spua snurur aql qllM paluerJo sr
1ac aseqdratur uB ur,{erre alnqnlorcru pcrdfl V .fuesseoau,,(1pnba
sr uodsupJ]
paztrelod Jleql (saJuBlsrp 3uo1 qcns ra^of, lou paau sllal lsolu ur sallaue8ro
q8noqry 's1eurruJet
alJau ]uplsrp aqt ot uoxe aql Jo s.{emq8rq apqnlorJrru aql
u^l,op slueuodruoc asdeu.{s snorJe^ pue (sJosJnJeJd
alJrsa^,,fuolercas ,erJpuor.lJ
-olnuJo luatueloru prder aqt 'uodsuerl Ipuoxp lspJ JoJ alqrsuodsar uralord aq1 se
peguuepr dleu€rro se^ urseur) 'sallaue8ro pesolJue-aueJqrueru Jo Buruorlrsod
pue lrodsue[ eq] sl s11ac aseqdralur ur sJo]our p1a1a>1so1dc Jo uorlJunJ rofeu y
urn 0[
('eurqS pr^e6 lo ^sallnof) 'urseldol^)
1;ar aqt tnoq6norql pas.redsrp sr
pue s1uau6e.r; snleredde 1610g aqi ,(7-g 1
alqel aas) azuau,{;odap o} salnqn}ol)il.u
sasne) q)tqM'alozepo)ou o1 atnsodxa
rel;V (g) 'snal)nu
eql,o saprs alrsoddo
ol pe^our a^eq saulosoltua) pale)tldnp
aql pue'aseqdord ur st tlal aql uo lla) aql
'aurosorlua)
a16urs e qlran'aseqdrelul u; s1
tqb7 a\t uo lla) aqf 'lla) aql jo lalua) aLll
lP snal)nu aql ol asol)'aulosollua) aql
rpau paztle)ol sr r6;o9 eql,l)plut suteulal
salnqnlor)l.ll 1o uals{s aqt se 6uo1
sy'1ura1o.rd r6;o9 e lsurebe trpoqltre ,e
6u6n1 uaatb ur palaqel sr snleredde r6;o9
aql pue pal ut palaqel ale salnqnlor)trrj
aql'lla) lerlaqropua srqr ul (V)
'snleredde
r6go9 aqr uo salnqnloJ)tul
6u1z;raur{;odap Jo l)a#l 99-91 arn611

uolelelsol^) aql:9l. ieldeq) zzol
e apnord deru sroldacal uIseuDI eseqJ 'SuIPuBts 1ac qllrr,r petelJosse sulaloJd
ploJJeJS ere ,(sureloJd Sunceralul-)Nl) sdl{ eqJ 'u0I}EZIIBJOI rleq} roJ Uodsuer}
paseq-aFqnloJJrru uo ^lar leq] sallaueSJo punoq-auEJqrueu Jaqlo snoIJuA uo sB
IIa^i\ se ,unlnsrlal crurseldopua aq] uo palJpuepl uaeq aAPq surseuDl cgtcads ro;
sJoldaJeJ Jeqlo '(96-9 arnflrg aas) uleJq aql Jo slleo a1Jeu uI selEBaJSSe uralord
o] lualuqJBl]e salelpatu uago r(lqruesse relnlaloluoJJBur a3rul e 'utau.{p ro4
'8u1pu8rs
IIaJ pue uodsuerl ueamlaq Tull
'sa11aue3ro Jaqlo Jo dlarrerr e pue sauosopua
qlllvr pelercosse aJe 'I ursol.u Sutpnput 'sutsoLtu Jeqlo '-{Fuatcg;a saltcouqe
-Je{ aql ot peJe^llap }ou aJB seuosouEleru eql esnEf,aq pepPJ $lool JoloJ
rnJ qcrqM ur 'adflouaqd ,,alnllp,, P ul ]FsaJ auaS nutsori.ru eq] ul suopelnlu (aclru
q 'aqnl
]sel e ur ]uarue^oru peseq-ullJe Jleql alBlperu ol alqP fl puu (89-9I
arnElg) saruosoueleur Jo aJpJJns aql qll^\ pelerJosse sr n utsoLl41
' (allul uI InJ eql
pup) uDIs aq] uJoJ lpql seldtoulleJa{ Surl.lJe^o aql o} pare^[ap eru ztaql eJaq^
,sa{couelaru eq} uI sassaJold Jl}upuap Jo spua aq} ol lno enolu seluosouelaru
esaqJ 'eJeJrns UDIS aql qleaueq salLJouapu pellec sllac uI pazlsaq]
-ur(s are ,sautosouDlaut pallpJ (salnueJfi
lueufiId pesolsua-aueJquau 'SuEIunq
pue eorru uI '(g9-gI aJnBrC ees) ezIS dals a8rel e qlp\ ulso.'tru papeaq-oml
e ^ ulso[ur se,rt rfiqtlour alleue8Jo e]elpeu 01 u ^oqs ulsolur lsJU aqJ 'slueruellJ
urlJe SuolE uodsuEJt allaue8ro uI eloJ IuPOIJIuBIS P a^eq oslB sulaloJd Jolohtr
'llal poJ er{} uI ulsdopoqJ Jo SuDIJgJEn IeIuJou ro; parmber sI uoIl3BJa}uI
sq] pue 'suleqr tq8l uraufp eql Jo euo ol fllcerlp spulq 'ada eq] Jo slleJ por eq] ul
punoJ uretord Surlcatap-]q3q aqr 'ulsdopoqr Jo IIet f,Iruseldot b aq;'o8rec naq]
qll-ry\ ^llJaJIp ]JpJaluI l.etu sJolotu utau.,(p crurseldoldc 'seseJ Jeqlo uI '(29-9I aln
-31fl allaue8ro aqt ot ulau,{p;o ruaruqJe}}e peluF8er eql alulperu,{eur }l-}uarue
-llJ
IdrV eql Prn-pue
'auerqruetu r31o3 eqr oi &nqets IeJnlcnrls sanr8 dlqeqord
derre utrlcads eqJ'(I?-01 arn8rg aes) uolala{so/.c auerqurau aptcorqldra aql
Jo lue3srurluar IEJJp le]ala{sol-,(J ruueld B IuJoJ o1 xalduroc ul}Jeu^p eql q luaru
-eg
ldry aq] qlp\ alelJosse ol pasodord uaaq e^eq qclq ^ 'utllcads pue utrllue
suraloJd aql qlrM paleoJ eJe snleJEdde r31og aqlJo sauEJquelN '(slueuEIIJ ullJe
IPuorlue^uof, Jo uollBelf,nu aql uI paAIoAuI xelduoc duv eql Jo sluauoduoc
aqt 'tdrv pue Tdry ruor; ]cunslp) l dry uralord petelar-ulloe aq] uroq sturoJ lPq]
]ualupllJ a{[-ul]Je uoqs e sepnlcul xalduroc upceu,{p aq; z(larrrlcaga salleuE8Jo
alpoolsuerl ol uuJzu[p palpc xaldruo] ue]oJd e8Jel puoJas e qlllvr uolle
-IJossE seJlnber 'xaldruoc uleloJd efinq e JIeslI 'uraufp crurseldof3 'SeueJqulatu
uolalelsol^) aql
saln)olow vNU )u!)od5 sazl;erol
JoJ IEJrlrJc JolcPJ uolldlJJsuEJl E 'ptoctg SutpoJua vNulu eq] 'aldtuExe Joc 'suoll
-nqrJlsrp
vNuru pezlerreds asaql qsllqelse 01 tolJalsod eql reeu spua snld pue
flac aqlJo ued JoIJe]uP aq] uI peJe]snlc eJe spua snullu alnqnloJclru ]soIU areq {
'uolele{so]Jb
aFqn}orclu pazlrPlod s1r ;o eBB}ueApB sa{Pl eldcoo aqJ
'e],,tJoo aql Jo uoISeJ
JorJalue aq] q pazrTPJol sr or^qua eql ul seJnlJnrls JoIJaluP Jo uoIlPJUI3eds
JoJ ,,fu€sseJeu sureloJd Sutpocua sVNUur Jo dnor8 tJullslp P pue 'al,{ooo eql ul
,{porralsod pazlTef,ol sI 'sllef, tura8 aq} Jo luaudole^ep Surpnlcur 'oIJqLua aql Jo
uor8al JorJalsod aql Jo luaudole^ap JadoJd JoJ dJesseJeu suleloJd apocue leql
sVNUru Jo dnor8 y '(77 nldeqa uI passnrslp) srsauaSo,{rqure ^lrEa uI slua^a uoD
-€JrJr3eds
IIeJ pldeJ aqlJo uoUPdIJIluu uI ilec aql uqlF salls clJlsads o] sVNUur
papocua 1tllEuJaleru Jo raqunu aBrPI B sazllPsol altlcoo apr4dosoJQ ]uBI8 eqJ
'suollunlls
Jo 53fld1 asaql uI selnJelolu
VNUru uorlrsod puP Uodsuerl sulaloJd Jololu Plala{so}.{3 puB uolala{sotdc
aq] 'rtlsulsudrns toN ,irlcpseld rlldeul's pue ^rorueru rure]-3uol Supep8ar ur
selor ]ueuodurl .{eld sa}ts ct}dPuds aql le uoFeln8al uoIlPISu"Jl puE uollezllEf,ol
vNutu leql acuapl^e sI aJaql pue 'suoJnau Aueur uI sesdEu^s o] asolJ pazITPJol
,(lecgrcads eJe uollcunJ asdeu,{s uI pe^lonul sule}oJd Sutpocua sVNUur IeJaAes
'aldtuexa Jaqloup sV'saleJ toullsp qtllt sralq8nep o t1 eleJaua8 ol sapIAIp IIec
]ueJed e ueq ^ ]ueuodlul LFelncrped sl slqJ'salJlelulu^s? Jelnlle3 seqsqqelse
1eq1 ssacord E 'selncelolu VNUtu s1r 3utzleco1 Lq utalord relncured e;o slsaqlu^s
eql lJulsel uauo slleJ 'uopcunJ Jo ells rtaql ]E suleloJd eleJluacuoc ol JepJo uI
alnqnloJ)ruj
rxu sz
'(tt-0t
arnbrl aas) llal
poolq par aq] Jo uolalalsoli{> paler>osse
-auprqurau aq] o] rplrlrs 'urrIlue pue
urrlreds 1o )romtau e qbnorqt sa1;auebro
pasol)ua-ouerquau ol xa;duo> abre;
srr-l] Jo luaLuq)elle alerpaur Ieu luaue;r;
ldry aqr reqt tq6noqi sr 11 1d.ry uralord
patPlar-ur])P aq] Jo apP|.U ]uaurPlu alrl
-url)P
lleurs e uJoJ leq] sluauoouro) pue
;;aslr ureulp ol purq teql sluauodruor
'salnqnloJ)rt! o1 ,(;leaarr purq iPqt
sluauoduol sapnl)ur ]€ql (par) xoldruo)
abre; e sr urlreu,(6'sa;;auebro pasol)ua
-auerquraur q]rM aler)osse ol suralord
fiossarre 1o raqu,rnu e6re; e;o aruasard
eqt salrnbar urau{6 'a;;auebro pasol)ua
-euerqulau e ol ulauIp 1o lustuq)eup
aql roJ lapotu y 49-9 1 arn611
(E) (v)
utti Ol
(sls;6o1org ;o,(uedu:o1 aql
urorj uolsslulad qll^ '1.66L '6sg-Lvg:}lL
'PS
ilD'f ''P la nM X uroll) uralord
rolour srql Jo sardor yo requrnu a6re1 e qlrm
paler)osse sr euosouelau {ran3 l urso{lu
tsure6e {poqrlue }ua)salonlJ e qllm palaqel
lla) aues aql (g) 'uluelaur
luaubrd urls aql
qllM pall1; sa;;aue6lo pasol)ua-auPlquiaur
are q)rq/1/\'saurosouplaur arp slods )i)elq
aql'asnour e uroiJ palelosr e1,{>ouelaur
e;o uorl.rod e;o a6eur lselluo)-aseqd (V)
'sauosouelau uo 1 ugso(61 B9-9 t ernbrJ
&
fl
t

tzo t suolow uvtnftlow
aJe uolala{so{c aql qtIM pa}Ercosse eJe teq} s}f,npoJd auaB Jaq}o xts lseel lE
lpql pelea^al suq aouaJaJJrp ralq8nepTJaqlotu aqt ]dnJsrp lpql suorlulnru Jaqlo
JoJ uaarJs crlauaB V 'VNUru tr4sv Jo uonnqrJlsrp crrlaruurr{se sq} roJ parrnbar sr
'dyo[141 ']sea[
ur puno; surso^,(ru nadfi omt aq]Jo auo 'llar ralq8nep aq] ur,(po
dn pua aJoJareql pue pnq Suunor8 aq] ol ^le^rsnlcxa pezrleJol are uralord pue
VNUIU f rysy qlog 'tllsv pe1eJ uralord ,trolep8ar aua8 e ,{q pasner aJp sacueJeJ
-Jrp asar{} Jo 'tue61 'alrs pnq uau reql Jo af,rorlJ eql uI pue (2 raldeq3 ur pessnl
-srp) Surqclrrr,rs addt-3upeu o8rapun o] drl[qe luanbasqns Jraql ur seouaJaJJrp
rofeur dq pelealal sE 'rsarlrluapl
]Jurlstp urulal sllal ralqBnup pue raqloru lsead
aq1 'luapuadap-urloe sr Surroqcue se IIaM se lrodsuerl VNUur ,sllal euros uI
dpueg WUg aql Jo Jaqtuau e 'ursaotu palleJ ureloJd Surpurq-urlJe uE o]
dpcarrp spurq 'aldurexa JoJ te>Iso Surpooua VNUtu aql .uolala{so1,{o
urlce 1ecr1
-Joc aql aAIoAur ol sreadde selnqnloJJnu er,r,{ra,u1ap raUB suor}pJo1 alerrdordde
Jlaql 01 sVNUru aq];o SurroqJue aqJ 'spua snld elnqnloJ3rru aql ol Uodsuerl
slr JoJ I-urseuu sarrnbar'odrqura eqlJo JorJalsod aql u1 ]uarudoya,rap 1ac urra8
ro; .,tressacau uralord e tE{sO Surpocua VNUru Jo }rodsuer} aql dlasranuo3 .JorJ
-alue
IIer aql lE spua snurru alnqnloJJnu aq] ol uodsuer] slr 8u[qeua dlqeruns
-ard 'ureqc
rq311 uraufp cruseldopb e ol spurq urn] ur qclqm ,^aollelrts pallpo
utalord P spulq lEql UIn ,t eql uHllllr eJnltnJls e spq ']uerudolanap JorJeluE
uralord 4 ursofur aqt pue VNUru tqsv aql ur acuanbas crycads e uaamlaq {uq
]Jarrp E ruJoJ ]eql suralord olrl Jo xalduroc e se llarlr se Tleslr urltu pue ,url3ord
'urso.,(ruodorl 'sunuJoJ
aql Jo auo apnlJur aseql i&lJelod purrou ro; permbar
'(69-9I arn31g1
slle)
a)uanbas uotleztle)ol VNU
,{
uo!t)unJ utetotd tolow eleln6au
IlaJ-aloq ^ ro salauu8ro pasolcua-auerquau s1r go Suruorlrsod aql ur a8ueqc
e Jarllle asnec dqaraqt pue surelord roloru 3o firnrlce aq1 alep8ar ueo IIaf, aql
leJalas ur uorleJoloc UDIS ur saSueqc prdur roy alqrsuodsar aJe qJrq A ,s11ac
lue€
asaql 'saldruexa Jrleruerp lsotu aql Jo auo apvrord saptcouularu qsrC 'sluarueloru
lno SuruurM ursauDl JaSuoJls eql qlrl\/\ 'suralord
Joloru elnqnlorJrru o^\l
aql uee^ taq JeM-Jo-8n] e ruoq llnseJ IlluaJedde sluaua^oru pJemlno dryaf aql
n urso,r(ru Joloru urlJe aq] sE IIa^a se 'sa1nuel3 ]uaru8rd aql qlll\i\ pelprcossE eJe
ursaur{ pue urau.,(p sJoloru apqruorJnu aql qlog 'sda1s preaqcuq luanbar; q}yrt
{1ra[ sr Juaruanour pJe^ ]no aql alq ^ 'qlootus pue prdur sr luarualoru premur
aql teqt slpanar (IZ-9I arn31g1 sapuer8 luaur8rd lenprlrpur Jo 8uqcu4 aq;
,{raqdrrad IIar aq} punore patnqrr}srp ere spua snld aqi a1rq ^ ,llar aqt Jo retuaJ
eq] ur spua snur{u rraql Surzrpcol 'salnqn}orf,rru eseq} salealtnu euosor}uel aql
'salnqnloJf,rruJo
{romleu alrsualxa ue Suop Surnour r(q asradsrp ro a1e8ar88e saln
-uer8 luaru8rd aqt'(OZ-91 arn81g; uorlelnrut1s puoruJoq Jo IpuoJnau o1 asuodsar
ur uorlpool Jraqt Jallp UEJ leq] salnuer8 luaur8rd a8rel urc1uoc ,qsg go sarcads
'llEJa^o
fale^ uou pue eMezrlpf
lalad Jo Isellnor ,B) .lla) rarq6nep
aql ol pauuuo) oslp st 'vNUut paztlp)ol
srql Luor] paqu)supl]'utaloJd lqsv.(pnq
a6te1 e 'araq)
;;r1s lla) tatq6nep eqt;o
dll re, aql o] pauUUo) sl VNUr! oql'lle)
1sea,{ 6urprarp srql ur (pal) VNUru I gsy aLlr
azrle)ol ol pesn seM (HSll) uollezlplrq,(q
nJls u/ ]ua)selonll (B) '^llstLuaLl)otq
pue
srlaua6 {q paururralep se ,uotlpztle)ol
vNUr.u I qsy lo u.tstupq)arrJ leln)aloLu
aqt (V) 'dtr pnq lsea{ eql u! uo!}ezlle)ol
VNUUI pazgelod 69-9t arn6rl

uola;a>1soy(1 aql:9 L laldeLl] vzoL
(v)
Jo ]uauoduoc tueuodrul uB osls sI uoneldJoqdsoqd utsor(ru ^ oleq ssnf,slp ilu
e^ sV'omt olut ilar cpollu aql saqcurd leq] 3u1r alpcertuoc paseq-urlce eql olul
alquasse o1 1y ursodru Sursnec 'slsolltu Surrnp pele FlP osp sI )3'IIAI '@L-91
arnS;g) uollJeJluoo IIaJ ol speel pue luauEIIJ relodrq e olur dlqurasse sll seloru
-ord qcrqrt 'elels papuelxa eq] arunsse ,tlpquara;ard ol II ursodru eql sasnuc
UJIW) asaury uruqt-ry?11 utsotru luapuadap-runlf,lec eql Lq ureqc tq31&ote1
-n8ar aq1;o uor1e1,{.roqdsoqd
'peaq
Jololu eq} q}yv\ slf,eJa}ultltuaredde uletuop
IrEt aql qf,rqm uI alels tueq e pue 'slueurelS relodtq ruJoJ ueo ]eql aluls papualxa
uE 'sllal qJns uI saluts leuoperuJoJuoJ luaJaJJIp oml uI lsrxa ueJ 11 utsodur
aq1 .{lqruasse }uetuelg {olq} pue &1nltce rolotu q}oq SulrreJJe
'suleqr tq311pue
dleaq qloq uo salts;o ,,!atJel e uo pa1e1,{roqdsoqd aq uuc 11 utsodur 'slleJ elJsnru
-uou uI &lrrpce ursortu alep8ar o1 uor1u1.{roqdsoqd asn osp ueJ IIac eql
dlntlce pue luauqJe]le utalord Jolotu qloq areln8al leql sleu
-8rs SurladuroJ Jo af,upleq xaldtuoc e t(q palonuoJ sI sula]oJd rolotu relnc[red
qlyv\ pelpoJ salaueflro aueJquau Jeqlo Jo ]uaruanolu aql fent relnuls P uI 'JoloJ
sqsrJ aqt Sur8ueqc 'ralual IIaf, eqt pru^ o],(prder sapuerB luau8rd aql Serp o1
aar; urauLp 8unea1 'palenqceut sI uISauII 'a8uuqr Jolos uDIs qeu8ts luql uopel
-nurrls
lBuoruroq P JeuE parelrfuoqdsoqd aruocaq suleqs 1q3u ulsauq eql uaqM
sJaqrueu Jeqlo Jo uoqelnSaH 'sllac alJsnu qloolus uI uollJPJluoc Jo IoJluoJ eql
pue snlo
alnqnlor)il,!
c3suldslc
pua snurur
tUfi OS
dWV)
aseal)ep
dIAV)
aseoJ)ul
clIv91u99V
('oureH qeal ;o {salrno)'B)'AL\6U)
lla) aqrJo lalua) aq] ur patebar66e
ro (ryal) useldov(r aqt tnoqbnorql
pasraosrp raq]ra saurosouelaur
s1r 6urrvroqs'qsq prpllr) ue)uJv
ue Jo ale)s e ur lla) a;6urs e;o sa6eur
plag-lq6;rg (8)'salnqnlor)rtu 6uole
rn))o sauJosouelaui Jo suorlnqulsrpar
qlog',(len$:adsar'(dwv)) 6yyy tr;tIt
Jelnlla)erlur ur asPar)ap ro asear)ur uP ol
asuodsar ur sauJosouelaur ;o uor1e6a.r66e
pue ;esradsrp aql 6ulrtoqs ';1ar
luaubrd e
,o Mar^ )lleuaq)s (v) 'snlnurrls
leuornau
ro leuouiloq e ol asuoosal ur lla) aq] u!
uorle)ol r;aql a6ueq> ue) saurosoueleur
aql' PMuq) saurosouelaur .ro'sa;nuerb
luar.u6rd a6re1 ureluor'qsr;;o saoads
leranas ur uorlerolo) ur>1s ur sa6ueqr
rol a;qrsuodsa.r are Ll)rqM's11ar tuerb
asaqf 'slla) luau6rd qsg u! sluouanour
ourosouplaur pa1eln6eg O/-9 I arn6tl
('sa)uar)s
1o r{uapery leuOrleN uo.r; uorssrr.urad
qrl^ '166
r '9ZLt-0ZLtit6'V'S'n 'pS
'pDV
lloN iud ''p 1a sla6og 'l'S Luorl)
'pua snlo aql preMol pua snurLrr aql urorj
sa^ouJ pue uort)arp saq)lrMs,{lldnrqe
Ir 'aruanbes osprn aql qbnorql Ierryleg
'pua snurur aql pre^ ol pua snld aql
LuorJ'aprls sse;6 e uo alnqnlor)rur e 6uo;e
sa^ow (/aoila() aurosouelaur palelosl
uv'alnqnlor)l[u e uo ouosouelaul e
,o luauralour leuorl)aJrprg 11-91 arn613
uiri 0 !

THE CYTOSKELETON AND CELL BEHAVIOR
myosin light chains
INACTIVE STATE:
(light chains not phosphorylated)
(A)
actin-binding site
myosin tail released
@
ACTIVE STATE:
(light chains phosphorylated)
bipolar filament
of 15-20 molecules
of the myosin superfamily is not as well understood, but the control of these
myosins is likewise thought to involve site-specific phosphorylations.
Summary
Motor proteins use the energy of ATP hydrolysis to moue along microtubules or actin
filaments. They mediate the sliding of ftlaments relatiue to one another and the transport
of cargo along filament tracks. All known motor proteins that moue on actin filaments
are members of the myosin superfamily. The motor proteins that moue on
microtubules are either members of the kinesin superfamily or the dynein family. The
myosin and kinesin superfamilies are diuerse, with about 40 genes encoding each type of
protein in humans. The only structural element shared among all members of each
superfamily is the motor "head" domain. These heads are fused to a wide uariery of different
"tails," which attach to different types of cargo and enable the uarious family members
to perform dffirent functions in the cell. These functions include the transportation
and localization of specific proteins, membrane-enclosed organelles, and mRNAs.
Although myosin and kinesin walk along different tracks and use different mech'
anisms to produce force and mouement by ATP hydrolysis, they share a common structural
core, suggesting that they are deriued from a common ancestor. The dynein motor
protein has independently euolued, and it has a distinct structure and mechanism of
action.
THE CYTOSKELETON AND CELL BEH IOR
A central challenge in all areas of cell biology is to understand how the functions
of many individual molecular components combine to produce complex cell
behaviors. The cell behaviors that we describe in this final section all rely on a
coordinated deployment of the components and processes that we have
explored in the first three sections of the chapter: the dynamic assembly and disassembly
of cltoskeletal polymers, the regulation and modification of their
structure by polymer-associated proteins, and the actions of motor proteins
moving along the polymers. How does the cell coordinate all these activities to
define its shape, to enable it to crawl, or to divide it neatly into two at mitosis?
These problems of cltoskeletal coordination will challenge scientists for many
years to come.
1025
Figure 16-72 Light-chain phosphorylation and the regulation of the assembly of myosin ll into thick filaments. (A) The
controlled phosphorylation by the enzyme myosin light-chain kinase (MLCK) of one of the two light chains (the so-called
regulatorylightchain,shownin lightblue)onnonmusclemyosinll inatesttubehasatleasttwoeffects: itcausesachange
in the conformation of the myosin head, exposing its actin-binding site, and it releases the myosin tail from a "sticky patch"
on the myosin head, thereby allowing the myosin molecules to assemble into short, bipolar, thick filaments. (B) Electron
micrograph of negatively stained short filaments of myosin ll that have been induced to assemble in a test tube by
phosphorylation of their light chains. These myosin ll filaments are much smaller than those found in skeletal muscle cells
(see Figure 1 6-55). (8, courtesy of John Kendrick-Jones.)

1026 Chapter 16:The Cytoskeleton
To provide a sense of our present understanding, we first discuss examples
where specialized cells build stable arrays of filaments and use highly ordered
arrays of motor proteins sliding them relative to each other to generate the
large-scale movements of muscle, cilia, and eucaryotic flagella. Next, we consider
two important instances where filament dynamics collude with motor
protein activity to generate complex, self-organized dynamic structures: the
microtubule-based mitotic spindle and the actin arrays involved in cell crawling.
Finally, we consider the extraordinary organization and behavior of the
neuronal c)'toskeleton.
Sliding of Myosin ll and Actin Filaments Causes Muscles to
Contract
Muscle contraction is the most familiar and the best understood form of movement
in animals. In vertebrates, running, walking, swimming, and flying all
depend on the rapid contraction of skeletal muscle on its scaffolding of bone,
while involuntary movements such as heart pumping and gut peristalsis depend
on the contraction of cardiac muscle and smooth muscle, respectively. All these
forms of muscle contraction depend on the ATP-driven sliding of highly organized
arrays of actin filaments against arrays of myosin II filaments.
Skeletal muscle was a relatively late evolutionary development, and muscle
cells are highly specialized for rapid and efficient contracrion. The long thin
muscle fibers of skeletal muscle are actually huge single cells that form during
development by the fusion of many separate cells, as discussed in Chapter 22.
The large muscle cell retains the many nuclei of the contributing cells. These
nuclei lie just beneath the plasma membrane (Figure f6-23). The bulk of the
cltoplasm inside is made up of myofibrils, which is the name given to the basic
contractile elements of the muscle cell. A myofibril is a cylindrical structure l-2
pm in diameter that is often as long as the giant muscle cell itself. It consists of a
long repeated chain of tiny contractile units-called snrcomeres, each about 2.2
pm long, which give the vertebrate myofibril its striated appearance (Figure
ro-7$.
Each sarcomere is formed from a miniature, precisely ordered array of parallel
and partly overlapping thin and thick filaments. Tlne thin fiIaments are composed
of actin and associated proteins, and they are attached at their plus ends
to a Z disc at each end of the sarcomere. The capped minus ends of the actin filaments
extend in toward the middle of the sarcomere, where they overlap with
thick fiIamenfs, the bipolar assemblies formed from specific muscle isoforms of
myosin II (see Figure 16-55). \Mhen this region of overlap is examined in cross
section by electron microscopy, the myosin filaments are seen to be arranged in
a regular hexagonal lattice, with the actin filaments evenly spaced between them
(Figure 16-75). cardiac muscle and smooth muscle also contain sarcomeres,
although the organization is not as regular as that in skeletal muscle.
Sarcomere shortening is caused by the myosin filaments sliding past the
actin thin filaments, with no change in the length of either type of filament (Figure
16-74 c and D). Bipolar thick filaments walk toward the plus ends of two sets
of thin filaments of opposite orientations, driven by dozens of independent
myosin heads that are positioned to interact with each thin filament. Because
there is no coordination among the movements of the myosin heads, it is critical
myofibril
Figure 16-73 Skeletal muscle cells (also
called muscle fibers). (A) These huge
multinucleated cells form by the fusion of
many muscle cell precursors, called
myoblasts. In an adult human, a muscle
cell is typically 50 pm in diameter and can
be up to several centimeters long.
(B) Fluorescence micrograph of rat muscle,
showing the peripherally located nuclei
(blue) in these giant cells. Myofibrils are
stained red; see also Figure 23-468.
(B, courtesy of Nancy L. Kedersha.)
50 pm

THE CYTOSKELETON AND CELL BEHAVIOR
Figure 16-74 Skeletal muscle myofibrils, (A) Low-magnification electron
micrograph of a longitudinal section through a skeletal muscle cell of a
rabbit, showing the regular pattern of cross-striations. The cell contains
many myofibrils aligned in parallel (see Figure 16-73). (B) Detail of the
skeletal muscle shown in (A), showing portions of two adjacent myofibrils
and the definition of a sarcomere (black arrow). (C) Schematic diagram of a
single sarcomere, showing the origin of the dark and light bands seen in
the electron micrographs. The Z discs, at each end of the sarcomere, are
attachment sites for the plus ends of actin filaments (thin filaments); the
M line, or midline, is the location of proteins that link adjacent myosin ll
filaments (thick filaments) to one another. The dark bands, which mark the
location of the thick filaments, are sometimes called A bands because they
appear anisotropic in polarized light (that is, their refractive index changes
with the plane of polarization). The light bands, which contain only thin
filaments and therefore have a lower density of protein, are relatively
isotropic in polarized light and are sometimes called I bands. (D) When the
sarcomere contracts, the actin and myosin filaments slide past one another
without shortening. (A and B, courtesy of Roger Craig.)
that they operate with a low processivity, remaining tightly bound to the actin
filament for only a small fraction of each AIPase cycle so that they do not hold
one another back. Each myosin thick filament has about 300 heads (294 in frog
muscle), and each head cycles about five times per second in the course of a
1um
e;' ' -.
2pm
(B)
Z disc
dark band light band
one sarcomere
thick filament (myosin)
thin filament (actinl -
light band dark band light band
1027
--l
I
I
)
_d
Figure 16-75 Electron micrographs of an
insect flight muscle viewed in cross
section. The myosin and actin filaments
are packed together with almost crystalline
regularity. Unlike their vertebrate
counterparts, these myosin filaments have
a hollow center, as seen in the
enlargement on the right. The geometry of
the hexagonal lattice is slightly different in
vertebrate muscle. (From J. Auber, J. de
Microsc. 8:197 -232, 1 969. With permission
from Societ6 frangaise de microscopie
6lectronique.)
E

1028 Chapter 16:The Cytoskeleton
4!g!'llLlD
.ra!i!rrtl,:)
Z disc
Cap Z tropomodulin
blus end
of actin
f ilament
mtnuseno
myosin (thick f ilament)
actin (thin filament)
rapid contraction-sliding the myosin and actin filaments past one another at
rates of up to 15 pm/sec and enabling the sarcomere to shorten by l0% of its
length in less than 1/50th ofa second. The rapid synchronized shortening ofthe
thousands of sarcomeres lying end-to-end in each myofibril enables skeletal
muscle to contract rapidly enough for running and flying, or for playing the
piano.
Accessory proteins produce the remarkable uniformity in filament organizalion,length,
and spacing in the sarcomere (Figure f 6-76). The actin filament
plus ends are anchored in the Z disc, which is built fromcapzand o-actinin; the
Z disc caps the filaments (preventing depolymerization), while holding them
together in a regularly spaced bundle. The precise length of each thin filament is
determined by a template protein of enormous size, called nebulin, which consists
almost entirely of a repeating 35-amino-acid actin-binding motif. Nebulin
stretches from the Z disc to the minus end of each thin filament and acts as a
"molecular ruler" to dictate the length of the filament. The minus ends of the
thin filaments are capped and stabilized by tropomodulin. Although there is
some slow exchange of actin subunits at both ends of the muscle thin filament,
such that the components of the thin filament turn over with a half-life of several
days, the actin filaments in sarcomeres are remarkably stable compared to
the dynamic actin filaments characteristic of most other cell t!?es that turn over
with half-lives of a few minutes or less.
Opposing pairs of an even longer template protein, called titin, position the
thick filaments midway between the Z discs. Titin acts as a molecular spring,
with a long series of immunoglobulin-like domains that can unfold one by one
as stress is applied to the protein. A springlike unfolding and refolding of these
domains keeps the thick filaments poised in the middle of the sarcomere and
allows the muscle fiber to recover after being overstretched. In c. elegans,whose
sarcomeres are longer than those in vertebrates, titin is also longer, suggesting
that it too serves as a molecular ruler, determining in this case the overall length
of each sarcomere (see Figure 3-33).
A Sudden Rise in Cytosolic Ca2+ Concentration Initiates Muscle
Contraction
The force-generating molecular interaction between myosin thick filaments and
actin thin filaments takes place only when a signal passes to the skeletal muscle
from its motor nerve. Immediately upon arrival of the signal, the muscle cell
act in rapid succession on the same thin filament without interfering with one
another. second, a specialized membrane system relays the incoming signal
rapidly throughout the entire cell. The signal from the nerve triggers an action
potential in the muscle cell plasma membrane (discussed in Chapter ll), and
' .:t
Figure I 6-76 Organization of accessory
proteins in a sarcomere. Each
giant titin molecule extends from the
Z disc to the M line-a distance of over
1 pm. Part of each titin molecule is
closely associated with a myosin thick
filament (which switches polarity at the
M line); the rest of the titin molecule is
elastic and changes length as the
sarcomere contracts and relaxes. Each
nebulin molecule is exactly the length of
a thin filament. The actin filaments are
also coated with tropomyosin and
troponin (not shown; see Figure i6-78)
and are capped at both ends.
Tropomodulin caps the minus end of the
actin filaments, and CapZ anchors the
plus end at the Z disc, which also
contains o(-actinin.

THE CYTOSKELETON AND CELL BEHAVIOR
polarized
T-tubule
t
membrane
CYTOSOL
05pm
depolarized T-tubule membrane
l.
:a
a
rl':rral
t.
t
!liir.:
arllili:!
'i:.:llf
a ai l
a a
a
a
olasma membrane
Ca2*-release
cha nnels
transverse (T)
tubules formed
from invaginations
of plasma membrane
sa rcoplasm ic
reticulum
:iji: 1 35 nm
I
l.! ri
i't'r..t:,
I
this electrical excitation spreads rapidly into a series of membraneous folds, the
transverse tubules, or T tubules, that extend inward from the plasma membrane
around each myofibril. The signal is then relayed across a small gap to tlre sarcoplasmic
reticulum, an adjacentweb-like sheath of modified endoplasmic reticulum
that surrounds each myofibril like a net stocking (Figure lg-77{and B).
rWhen the incoming action potential activates a Ca2+ channel in the T-tubule
membrane, a Ca2+ influx triggers the opening of Ca2+-release channels in the
sarcoplasmic reticulum (Figure l6-77C). CaZ* flooding into the cytosol then initiates
the contraction of each myofibril. Because the signal from the muscle-cell
plasma membrane is passed within milliseconds (via the T tubules and sarcoplasmic
reticulum) to every sarcomere in the cell, all of the myofibrils in the
cell contract at once. The increase in Ca2* concentration is transient because the
Ca2* is rapidly pumped back into the sarcoplasmic reticulum by an abundant,
AlP-dependent Ca2+-pump (also called a Caz*-ATPase) in its membrane (see
Figure 1l-I3). Typically, the cytoplasmic Ca2* concentration is restored to resting
levels within 30 msec, allowing the myofibrils to relax. Thus, muscle contraction
depends on two processes that consume enormous amounts ofATP: filament
sliding, driven by the ATPase of the myosin motor domain, and Ca2*
pumping, driven by the Caz*-pump.
The Ca2* dependence of vertebrate skeletal muscle contraction, and hence
its dependence on motor commands transmitted via nerves, is due entirely to a
set of specialized accessory proteins that are closely associated with the actin
thin filaments. One of these accessory proteins is a muscle form of tropomyosin,
an elongated molecule that binds along the groove of the actin helix. The other
is troponin, a complex of three polypeptides, troponins T I, and C (named for
their tropomyosin-binding, inhibitory, and Ca2*-binding activities, respectively).
Troponin I binds to actin as well as to troponin T. In a resting muscle, the
troponin I-T complex pulls the tropomyosin out of its normal binding groove
into a position along the actin filament that interferes with the binding of
(A)
1 029
Figure 16-77 T tubules and the
sarcoplasmic reticulum. (A) Drawing of
the two membrane systems that relay the
signal to contract from the muscle cell
plasma membrane to all of the myofibrils
in the cell. (B) Electron micrograph
showing two T tubules. Note the position
of the large Ca2+-release channels in the
sarcoplasmic reticulum membrane; they
look like square-shaped "feet" that
connect to the adjacent T-tubule
membrane. (C) Schematic diagram
showing how a Ca2+-release channel in
the sarcoolasmic reticulum membrane is
thought to be opened by the activation
of a voltage-gated Ca2+ channel.
(8, courtesy of Clara Franzini-Armstrong.)

1030 Chapter 16:The Cytoskeleton
,".
10
"t
tropon in
comPlex tropomvosin
rc T
+ ca2*
ca2*
myosin-binding site exposed
by Ca'*-mediated tropomyosin
movement
Figure 16-78 The control of skeletal muscle contraction by troponin. (A) A skeletal muscle cell thin filament, showing the
positions of tropomyosin and troponin along the actin filament. Each tropomyosin molecule has seven evenly spaced
regions with similar amino acid sequences, each of which is thought to bind to an actin subunit in the filament.
(B) A thin filament shown end-on, illustrating how Ca2+ (binding to troponin) is thought to relieve the tropomyosin
blockage of the interaction between actin and the myosin head. (A, adapted from G.N. Phillips, J.P. Fillers and C. Cohen,
J. Mol. Biol. 192:111-131,1986. With permission from Academic Press.)
myosin heads, thereby preventing any force-generating interaction. \Mhen the
level of Ca2* is raised, troponin C-which binds up to four molecules of Caz+causes
troponin I to release its hold on actin. This allows the tropomyosin
molecules to slip back into their normal position so that the myosin heads can
walk along the actin filaments (Figure f 6-78). Troponin C is closely related to
the ubiquitous Caz*-binding protein calmodulin (see Figure lS=44); it can be
thought of as a specialized form of calmodulin that has acquired binding sites
for troponin I and troponin ! thereby ensuring that the myofibril responds
extremely rapidly to an increase in Ca2+ concentration.
In smooth muscle cells, so-called because they lack the regular striations of
skeletal muscle, contraction is also triggered by an influx of calcium ions, but the
regulatory mechanism is different. Smooth muscle forms the contractile portion
of the stomach, intestine, and uterus, the walls of arteries, and many other structures
requiring slow and sustained contractions. smooth muscle is composed of
sheets of highly elongated spindle-shaped cells, each with a single nucleus.
Smooth muscle cells do not express the troponins. Instead, Caz* influx into the
cell regulates contraction by two mechanisms that depend on the ubiquitous
calcium binding protein calmodulin.
First, Ca2+-bound calmodulin binds to an actin-binding protein, caldesmon,
which blocks the actin sites where the myosin motor heads would normally
bind. This causes the caldesmon to fall off of the actin filaments, preparing the
filaments for contraction. Second, smooth muscle myosin is phosphorylated on
one of its two light chains by myosin light chain kinase (MLCK.), as described
previously for regulation of nonmuscle myosin II (see Figure 16-72).lVhen the
light chain is phosphorylated, the myosin head can interact with actin filaments
and cause contraction; when it is dephosphorylated, the myosin head tends to
dissociate from actin and becomes inactive (in contrast to nonmuscle myosin II,
light chain dephosphorylation does not cause thick filament disassembly in
smooth muscle cells). MLCK requires bound ca2*/calmodulin to be fully active.
External signaling molecules such as adrenaline (epinephrine) can also regulate
the contractile activity of smooth muscle. Adrenaline binding to its G-protein-coupled
cell surface receptor causes an increase in the intracellular level of
cyclic AMf; which in turn activates cyclic-AMP-dependent protein kinase (pKA)
(see Figure l5-35). PKA phosphorylates and inactivates MLCK, thereby causing
the smooth muscle cell to relax.
The phosphorylation events that regulate contraction in smooth muscle
cells occur realtively slowly, so that maximum contraction often requires nearly
a second (compared with the few milliseconds required for contraction of a
skeletal muscle cell). But rapid activation of contraction is not important in
smooth muscle: its myosin II hydrolyzes ArP about l0 times more slowly than
skeletal muscle myosin, producing a slow cycle of myosin conformational
changes that results in slow contraction.

THE CYTOSKELETON AND CELL BEHAVIOR
Heart Muscle ls a Precisely Engineered Machine
The heart is the most heavilyworked muscle in the body, contracting about 3 billion
(3 x 10s) times during the course of a human lifetime. This number is about
the same as the average number of revolutions in the lifetime of an automobile's
internal combustion engine. Heart cells express several specific isoforms of cardiac
muscle myosin and cardiac muscle actin. Even subtle changes in these contractile
proteins expressed in the heart-changes that would not cause any
noticeable consequences in other tissues-can cause serious heart disease (Figure
16-79).
The normal cardiac contractile apparatus is such a highly tuned machine
that a tiny abnormality anywhere in the works can be enough to gradually wear
it down over years of repetitive motion. Familial hypertrophic cardiomyopathy is
a frequent cause of sudden death in young athletes. It is a genetically dominant
inherited condition that affects about two out of every thousand people, and it
is associated with heart enlargement, abnormally small coronary vessels, and
disturbances in heart rhythm (cardiac arrhythmias). The cause of this condition
is either any one of over 40 subtle point mutations in the genes encoding cardiac
B myosin heavy chain (almost all causing changes in or near the motor domain),
or one of about a dozen mutations in other genes encoding contractile proteins-including
myosin light chains, cardiac troponin, and tropomyosin. Minor
missense mutations in the cardiac actin gene cause another type of heart condition,
called dilated cardiomyopathy, that also frequently results in early heart
failure.
Cilia and Flagella Are Motile Structures Built from Microtubules
and Dyneins
Just as myofibrils are highly specialized and efficient motility machines built
from actin and myosin filaments, cilia and flagella are highly specialized and
efficient motility structures built from microtubules and dynein. Both cilia and
flagella are hair-like cell appendages that have a bundle of microtubules at their
core. Flagella are found on sperm and many protozoa. By their undulating
motion, they enable the cells to which they are attached to swim through liquid
media (Figure f 6-80A). Cilia tend to be shorter than flagella and are organized
in a similar fashion, but they beat with a whip-like motion that resembles the
breast stroke in swimming (Figure f6-808). The cycles of adjacent cilia are
almost but not quite in synchrony, creating the wave-like patterns that can be
seen in fields of beating cilia under the microscope. Ciliary beating can either
propel single cells through a fluid (as in the swimming of the protozoan Paramecium)
or can move fluid over the surface of a group of cells in a tissue. In the
human body, huge numbers of cilia (10s/cmz or more) line our respiratory tract,
sweeping layers of mucus, trapped particles of dust, and bacteria up to the
mouth where they are swallowed and ultimately eliminated. Likewise, cilia along
the oviduct help to sweep eggs toward the uterus.
The movement of a cilium or a flagellum is produced by the bending of its
core, which is called the axoneme. The axoneme is composed of microtubules
and their associated proteins, arranged in a distinctive and regular pattern. Nine
special doublet microtubules (comprising one complete and one partial microtubule
fused together so that they share a common tubule wall) are arranged in
Figure 16-80 The contrasting motions of flagella and cilia. (A) The wave-like
motion of the flagellum of a sperm cell from a tunicate. The cell was
photographed with stroboscopic illumination at 400 flashes per second. Note that
waves of constant amplitude move continuously from the base to the tip of the
flagellum. (B) The beat of a cilium, which resembles the breast stroke in
swimming. A fast power stroke (red arrows), in which fluid is driven over the
surface of the cell, is followed by a slow recovery stroke. Each cycle typically
requires 0.1 -0.2 sec and generates a force perpendicular to the axis of the
axoneme (the ciliary core). (A, courtesy of C.J. Brokaw.)
1 031
Figure 16-79 Effect on the heart of a
subtle mutation in cardiac myosin. left,
normal heart from a 6-day old mouse
pup. Right, heart from a pup with a point
mutation in both copies of its cardiac
myosin gene, changing Arg 403 to Gln.
The arrows indicate the atria. In the heart
from the pup with the cardiac myosin
mutation, both atria are greatly enlarged
(hypertrophic), and the mice die within a
few weeks of birth. (From D. Fatkin et al.,
J. CIin. lnvest. '103:147
, 1999. With
permission from The Rockefeller
University Press.)

032 Chapter 1 6: The Cytoskeleton
100 nm
radial spoke
inner sheath
central singlet
microtubule
plasma membrane
a ring around a pair of single microtubules (Figure r6-sr). Almost all forms of
eucaryotic flagella and cilia (from protozoans to humans) have this characteristic
arrangement. The microtubules extend continuously for the length of the
axoneme, which can be f 0-200 pm. At regular positions along the length of the
microtubules, accessory proteins cross-link the microtubules together.
Molecules of ciliary dynein form bridges between the neighboring doublet
microtubules around the circumference of the axoneme (Figure f 6-S2). \fhen
the motor domain of this dynein is activated, the dynein molecules attached to
one microtubule doublet (see Figure 16-64) attempt to walk along the adjacent
microtubule doublet, tending to force the adjacent doublets to slide relative to
one another, much as actin thin filaments slide during muscle contraction.
However, the presence of other links between the microtubule doublets prevents
this sliding, and the dynein force is instead converted into a bending
motion (Figure 16-83).
The length of flagella is carefully regulared. If one of the two flagella on a
chlamydomonas cell is amputated, the remaining one will transiently shrink as
the stump regrows until they reach the same length, and then the two shortened
flagella will continue to elongate until both are as long as they were on the
unperturbed cell. New flagellar components including tubulin and dynein are
incorporated into the growing flagella at the distal tips. Thus, even in these
(A)
(B)
A microtubule B microtubule
outer doublet microtubule
outer dynein arm
inner dynein arm
Figure 16-81 The arrangement of
microtubules in a flagellum or cilium.
(A) Electron micrograph of the flagellum
of a green-alga cell (Chlamydomonas)
shown in cross section, illustrating the
distinctive "9 + 2" arrangement of
microtubules. (B) Diagram of the parts of
a flagellum or cilium. The various
projections from the microtubules link
the microtubules together and occur at
regular intervals along the length of the
axoneme. (A, courtesy of Lewis Tilney.)
50 nm 100 nm
Figure 16-82 Ciliary dynein. Ciliary (axonemal) dynein is a large protein assembly (nearly 2 million daltons) composed of
9-12 polypeptide chains, the largest of which is the heavy chain of more than 500,000 daltons. (A) The heavy chains form
the major portion of the globular head and stem domains, and many of the smaller chains are clustered around the base of
the stem. There are two heads in the outer dynein in metazoans, but three heads in protozoa, each formed from their own
heavy chain (see Figure 1 6-598 for a view of an isolated molecule). The tail of the molecule binds tightly to an A
microtubule in an ATP-|ndependent manner, while the large globular heads have an ATP-dependent binding site for a B
microtubule (see Figure 16-81). When the heads hydrolyze their bound ATP, they move toward the minus end of the B
microtubule, thereby producing a sliding force between the adjacent microtubule doublets in a cilium or flagellum. For
details, see Figure 16-64. (B) Freeze-etch electron micrograph of a cilium showing the dynein arms projecting at regutar
intervals from the doublet microtubules. (8, courtesy of John Heuser.)

IHI
(v)
uorAVHrS rrrf cNV Norrrrxsol^)
9NtCtls 3'lnSntou)r1
sl)ncoud
NENAC :S3'tnSnrOU)ttA
rl]gnoc c3rv]ost Nt
t
9NrCN3€
l'rn8n10u)rn srsnv)
NENAC :t/!nt]l9V'lJ
.]VIAUON
NI
'(IS-ZI
eJnBrC ees) usrueqJaru snorJels.^.ru ruls e ^q eJnl
-cnr1s pur8rro aql o] relncrpuedrad paurrog sr ralq8nep Jellelus p qJIq ^ uI 'ssaJ
-ord uoqecqdar snorrnc e ,tq asue sarpoq IESBq pup salorJtuac rvrall 'sa1od alputds
aql;o ged aruof,aq pue Jorralur IIaJ eql olul eloru selpoq IESeq aqt pue 'paqJosaJ
are ella8eg aq] 'aldurexa to!\auoraopfunpp e31e replaclun eqt uI slso]tru
qcea Surrnp :alqrualuoJJalur d1puo1tf,ury aJe salorJluef, pue sepoq Ipseq 'sursr
-ue8ro aruos ur 'paapul '(t8{I
arntlg; IaaqMIeJ e ur pa8uerre salnqnloJJrru
1a1drr1 pasnJ Jo sdnor8 auru qllm 'saurosoJluao
lprurue Jo Je]ual eq] lB pappequa
punoJ aJe leql salorJluar eql se urroJ eluEs eq] e^Eq sarpoq IesBq eqJ'43€JJns IIac
aql te ela8eg pue prln crtortrecna loor l.1urg salpoq psoq pelll.l; seJntf,nJls
]uaprJlE IeJrJolsrq aleunuoJun ue
sr snleredde Surunumrs 5o sadfi lueJaJJrp .{ran o,rnl eseql elouep ol arueu arups
aq] Jo asn aqJ '(IZ-9I arn8rg aas) IIe^i\ IIaf, IerrelJpq aql ur pappequa roloru
,fuelor prcads e dq uanrrp 'sralladord a{q a}etor ela8eg aq; 'urqa8eg ura}ord
aqt Jo strunqns Surleadar ;o dn aperu 'sluauelrJ IBcIIaq p€rr '3uo1 an a77aBa7l
'leeq
Iauapaq'pealsul ro aAeAA lou op pup urauLp Jo sepqruorJrur ureluoo lou
op eseql 1nq 'ela8eg pallpJ seJnlJnJls ecEJJns 1ac Sursn tur^ s osp EIJelf,Eg
Q7 :e1deq1
ur pessnJsrp) luaurdolanap cruo^,fuqua dlrea Suunp dpoq aqtJo srxe lq8u-ga1 aqr
Jo uoDEulr.uralep ur slJaJep pup 'puelJeq puE srJqep lualJ ol IreJ lpql ]ce.u fuole.r
-rdsar aql ur erlrc paz.{.pred aql o} 3uu,ro suorlcaJur Suny o1 ,fillqpdacsns q8rq e
'rurads elqorurur ot anp ,{}mrals apru,{q pazrreppJeqc sr aurorpuf.s eql'eruoJp
-u,r(s s,raua8eue) esneJ uraur{p fre1gc ur sloaJap ,fuelrparaq 'suerunq uI
'acuarradxa,{aq1 sa8ueqc aql
o1 ,{1pcrueu,{.p pue ,r(prder puodsar o} uo}ala{so}b aq} yo ftqrqeldepe pue ,Qr
-11qFaU JrsurJ]ur aq] asn s11ac 's.{erre Jotoru-tuaruelrJ alqpts pue palapro ,(UBU
(8)
tuu 001
'gg-91
ern613 ur uaas
sp'suorloru 6urleaq Jo saneM 6urleatt
'uollotu
6urpuaq e sasne) atoJataql
uort)e loloru aql'ralqnop aqr Jo
6utp1;s aql luanard slur; uralord alqrxap
'(ulrads
e ur se Ll)ns) auauoxp l)elut up
u1 (g) 'rred
reqlo aqt lsurp6e sa;nqnlorrru
]alqnop Jo rred auo eprls ol speaq
urau{p eqt;o uot})e Joloul aql smollp
dlvro uor]rppe aql'asp) stql ul'ualolq
are raqla6ol salnqnlol)tru ]elqnop
6uuoqq6reu 6urploq sa6e1ur1 eql'ursdftt
aLu^zuo tr1{;oelord eq} o} pasodxa
are sauauoxe uaLlM (v) 'atueuoxP
ue 1o 6urpuaq aq1 gg-9 1 arn6r3
('I)u!l'M'u pue Morpoo^ l'c
;o {salrnot'V)'(t€-9t eln6u
aas) aLues aql ,(;;erluessa st aloulua)
p ur salnqnlor)rlu;o lueua6uellp eq1
'raqlabol
salnqnlor)rur 1o,{ere lerupur;I>
eqt ploq lPq] s)iurl uilo] (€ urpal
ur uMoqs) suralord raqlo'(selnqnlol)tru
) pup B aLlt) salnqntoj)rLu alalduo)ul
oMl ol pasnj (alnqnlor)rur V oq])
alnqnlor)rnr elaldtuot auo 6urureluot
laldu] q)ea'salnqnlorrrlu galdr.r1
JO SlaS AU|U JO paSOOUIO) St pup aLuauoxe
fterp> e;o uorltod ramo; aql sultoJ
{poq ;eseq q)el'apts aq} LuoiJ paMor^
Ipoq ;eseq e;o r"uel6er6 (g) 'ueozoloid
P Jo xauo) aql ut satpoq leseq aelql
q6norqt uolt)as ssor) e;o qde.r6o.r>rr.u
uorl)all (V) 'salpoq
leseg trg-91 arn6r1
(8)
E€O I

uole;a1soy(1 eql:9 L raldeql t€0 t
-lep8ar 'erlc Lrerurrd aql spueq ^ oU pmg eql ueqm pauado eJE slauueqc runlc
-leJ enrlrsuesoueqralN
'sa1nqn1 faupH eq] q8norqt plngJo tuetuelolu eqt ]Jetep
l€ql sJosuas molJ se ]Je erlrc dmutrd '.slleJ
lellaqlrda ,i.aupq ug 'rualsrts snorr
-Jau
ler]uao eql ur suoJnau uo sJelllrusueJ]oJneu ro; sroldacal pue '(096 'd aasl
,{e,u.qled Sulpu8rs Soqa8pag eql uI pa lonug suralord Sutpnlcut 'runrpc dletu
-ud aq] ur pelpJluacuof, aJe sutalord Suueu8ts r(ue6 '(s1ac a8eggec) saldcorp
-uoqo pup 'sllal auoq'suoJnau 'slleJ Iellaqllda 'slselqorqg Surpnlcur sa&! 1ac
luareJJrp dueru;o acpJrns eqt uo punoJ eJE daql 'urau.{p {Jel pue r{r8ua1 uI sreta
-ruorcru,ua; e;tpo dlensn eJE erlro Lrerutrd 'unryn fnruud e peflec uoucalord
eceJrns qJrr-eFqnloJlrur 'elrlour-uou e yo qu,ror8 aql eleelJnu ,tlluanbaq salo
-rrtual 'e11a8eg ro Erlrc Surluaq padola,lep rtlnl 1ce1 letll slleJ IeIuIue uI uelg
Jo uorleJaJllordrano sasneJ leql JapJoslp cpaua8 uoluruol e 'eseeslp daupq cp
-s,{c[1od sesneo s11ac .{aupq aql q runlllf, ,{-rerupd eqt Jo s}ueuoduroc prntcnr}s
Jeqlo Jo IeuuEqJ runrJlpo eql Jo sso'I 'uoqeraylord puu qu,rorS lac ,(aupq 8ur
puDI pazrTerf,ads raqlouy'arnle;,taupo1 ur ztlunluala pue sue8ro aqt lnoq8norql
s1s,b palg-prng a8rel Jo uolteluroJ aql u1 Suppsar-slleo pqaqlrda daupr4 aql
'U7-ZZ
arn8rg aas) ofrqrua 3urdo1a,r.ap aql u1 drtaruur,{se lq8rr-ga1 8ur
-qs[qetse rog parmbar sr leeq o] alqu Sutaq ut pnsnun sI leql tunlllf, dreurud;o
sulelold rolow ^ue6l Jo suoll)PJatul aql pue s)lueu/(c
elnqnlor)!w selrnbau elpulds )!lol!w aql ro uoll)nrlsuo)
'(98{I
erntlg) a1purds rltol1ru relodrq parueleq rtpuu8ala eq} sl tFsar IEUIJ
aql'spua snulx epqnloJJnu asaql asrue8Jo pup atealrnu o1 dlaq leql seurosoJl
-ueo
Jo rred e ^,(q paprnB eJe sasseooJd eseql 'sllaf, IEurIuE lsoru uI 'sa1od alpurds
eqt ruroJ o1 raqlaSo] spua snunu eFqnloJorru eqt Jaqtp8 leql suralord ploJJpcs
qlyvr saxalduroc crrauro8qo urJoJ tl-ulseuq pue utaudp cturseldoldc sJoloru
pelJeJrp pue-snuru aql ,(1pu1g 'ssEru a(uosoruoJqc eq] ruo4 deme spua snul{u
rtaql saqsnd pue salnqnloJJru palproosse-eruosot[orqf, Jo spua snld eq] pJelv\o]
S)IEI\/\ 't-ursauDl 'suup eruosouroJq3 ol punoq sr ]Bql urseuDl raqloue 'lxag
'Jaqlo qJpe uror; de,,vre suorlJerrp alrsoddo ur poluarJo erc l€ql sepqnloJclu
seprTs pue Lern lalpred e olur saFqnlorouu eql sepunq (gg-gl arn8tg aas)
g-urseuDl relodrq eql ']sJIC 'sseru elqnloJcllu pezlue8rostp aq] uroq alpurds re1
-odrq luaraqoJ e plmq surelord Joloru IBJeAes Jo suottJe peluurprooc eql 'suorl
-oalrp ruopuBr ur Surlurod uqeurorqf, pasuepuoo uo elquesse selnqnloJf,Ilu
sV'sauosouoJqo pasuapuoo aql punoJp suor8ar aql ul paJuequa aJu .,(yqurasse
pue uoueelonu alnqntorcrl4l 'ctueudp aJou qcnru pue railoqs eJB salnqnl
-oJJnu aq] srsolru Suunp lnq 'seqdoqseler eJer o8rapun pue 3uo1 ,!1ecgd, 1
eJe sepqnlorcnu aq].{erre aseqdralur eq} uI 'sepqn}orJrru aqrlo qfual a8erane
pue Jorleqaq clueufp aq1 ug sa8ueqJ Jr]euerp eJe eJeql 'sIsoJILu,{1rea u1
'Suporlpred
euosoruoJqc alqBIIaJ eJnsue
o1 aceld ur srusrueqJau dnlceq aJe aJaq] 'uospal Lue ro; slleJ sluslueqf,elu
Jo ]es euo Jr leql os 'dcuepunpar;o aar8ap snopuerueJl E eJnleal alputds Jllot$r
aql yo SuruollJunJ pue uoucnJlsuoc aql 'lelJelpur cpaua8 aql Jo uolsslusueJl
ur ,!1gqe11ar ;o ecuegodur lErluar eql Jo esnEJeg 'sllar ralq8nep o^ l eql ol
,fi[apg 1ca;rad qlp\ seuosoruorqc paleclldar aqt Sulre8ar8as ro3 alqtsuodsar st
teq] ernlcnrts alpurds relodlq eqt uroJ o1 pazrue8roer pue pelquesseslp L1a1a1d
-ruoc sr sepqnloJJlu;o ,(erru aseqdralur eqt 's[ec IPuruP ul Jnoq uB uel{} ssel
se1e1 d1ecld.,! ]eql slue^e Jo acuanbas prdur e uI 'sa(uosoruoJqo eql Jo uoped1c11
-red anrlce eqt sarnteeJ osle 1I 'sluauplrJ playa>1so1.{c cnueurtp qll&\ SullJeJelul
suralord Joloru Jo srueel dq uoqezrue8ro-;1as yo raarrod aql;o aldruexa Surluurc
-seJ pue luelrodrur l.1re1nc4red e sr alpurds ollollu aq] Jo uollJnJlsuoo eqJ
alpurds Jllolru eql Jo uorlf,unJ IeJrupqoelu pue uollJnJls
-uoJ aql ol elnquluoJ teql suslupqJar.u 1e1a1a1soilc aqlJo ^ aJ e ssnlslp fgauq
a,vr 'eJeH 'uorsrlrp IIac Jo slua^a aql go 3unur1 eqt seulurelap lpql ualsd.s loJluoc
alcri.c 1ac aq] puE srsolrru;o ssacord eq] qloq lelap uI eqlJf,sep ily!\ a^\ 'Ll rcl
-duq3 uI
'uorsrlrp
1ac Surrnp uroJ leql 3uu aplcerluoo eql pue ayputds JIlolIu
aq] aJE eseql Jo JerlnupJ lsoru aqJ 'sqof rraql alaldruoc ,taql acuo readdesrp uaql
pue spu8ls luuJelxa ol asuodsar ur ro alcdc IIaJ al{l 3o sa8els cgtcads le readde
leql seJruJnrls elrqel uo puadap slueruenoru IIaf, ]soru ]ng 'luerualoru a,ulrladar
acnpord o1 pazqercads seJnlJnJls ]uauerurad [1anpe1ar aJE BIIIo pue sprqgor(7,q

UOIAVHIS lll) CNV Nol:lll)Sol-ll lHI
+_)
ourosolluel
salnqnlorlrur aJoq)olaur) salnqnloJ)rui lerlse
uri 0t
atrJosollue)
'slueuoduoc
SurzrueBro Jeqlo pue
sJoloru JElncelolu Jo suezop Jo suor]rE lEnpr^rpur aq] ql.rM selnqnloJsrru Jo saF
-radord crueur(p aq] Sururquroc uoJJa eArlBJoqBIIoJ e sluasardar alpurds JIlolIu
eq] IIeJa O 'er8ual slr seurruJalap osp alpurds eql uI senrlrlce uralord Joloru
30 saffi o1\^l esaql ueervuaq asuPlBq elEf,llep eq] 'zI reldeq3 ur passnssrp aq
IIIM sV'(98-gI arn8rg aas) pede ruaqt Surqsnd,tpuetsuoc ere teqt sapqnlorclru
relodralur eql uo sJoloru g-urseuDl patcalrp-pua snyd reyodrq aql pue 'selnqnl
-oJorru aql q Suqaar ,(ltuelsuoc ere leql alod alpurds aql lE suralord roloru
palcaJrp-pua snurru Jo uorlse aqr rtq ue^rJp sl xnu pJe^ elod 'a1od alpurds aql
Jeau spue snurru Jreql lE ssol leu e Surcuepq 'spua snld rraql lp slpnqns uqnqnl
Jo uonrppp tau e qlrm 'xnp[ ptnmalod palec Jor eqaq e tlqlqxa sapqruoJJrrx
alpurds aql Jo ,(ueur 'aldurexa JoJ 'asuqdeuu u1 alerudas o1 u€aq ,i.luappns
sauosoruoJqJ eq] ueq a ur8aq p^u. leql uorlf,p JoJ pasuel 'eJnlJnJls cnueufp
,tlaruarlxa ue ,tlenpe sr alpurds eq] esneJaq 'a,rpdacap sr acueruadde .{pea1s
qql '(IZ ralduq3 aas) a1c[c IIaJ eql Jo uorssarSord aq] sraS8rr] uoqezlrueJ Irtun
sqluoru ro sr(ep roJ slreM alpurds eql pue 'aseqdelaru cllorelu ur a1e,{.c IIaJ str
slseJJu 33a pazqrga;un aql 'slprurue Lueur u1 'eurl Jo sporrad 3uo1 rog luacsarnb
pue alqets ruadde upJ lr 'pelquesse seq alpurds JIlolFu relodrq eq] JaUV
990 r
(rar^asll r.uolJ
uolsslLurao qllM'0002 'g0su:0
t 'ptg'iln)
'resa6 'V
rrrorJ') lssald {lrslenrul
rollalallou aqf uot, uorssrulad q1111
'096t'S9€-S9E:99'tolg
ilD'1'11eba5'g'g
pue uourles'c'3 Luolr'B)'sauosoulolr{)
aql ol salnqntol)tur orll t)auuo) leql
soint)nrls pezr;er>ads /saloq)olautl
aql
lJeLu slods pal aqL'aryq ut sarrosoutolq)
aqt pue uaat6 ug a;purds eql
Jo salnqnlol)r.u aql smoqs qde.r6otrtr,u
a)ua)saronU slql ()) tofenba e;purds
aql le paubr;e saulosoruolq) aqt qllM
'eseqdelau
le elputds )tloltul palelosl
ue 1o qderbot>trll lseltuo)-aseqd V (g)
'se;od
a;purds aql pJemol xng snonutluo)
o6rapun qloq solnqnlol:rur le;odtalur
pue salnqnlot)tul aloq)olaut) aqt
apqm 'spua sn;d traql le 6urluuqs pue
6uranoi6'I1r;rqelsur :rueu,{p o6lapun
salnqnlolftur lellse aqt'smolre aql ,{q
palp)rput sy',(enne 6ulDa[ord spua sn;d
raql pue'aprsar sauosollua) aql aleqM
sa;od a;purds aqt le spua snutul ltoql
qllM polualro aJe salnqnlol)ilJl oql lo
llv'xauo) lla) aql qllM ])elatut ue) leqt
(uaaJ6) salnqnlol)tut letlse pue'raqta6ot
alpurds aqt Jo sa^leq oMl aql ploq
fpqf (pal) salnqnlol)tru le;odtalur 'a;od
elpurds aql ol aulosoulolq) q)ea q)eue
teql (anlg) solnqnlor)rur aroq)olaurl
:aseqdptau 1e a;putds tr1o1rr"u
aql u! salnqnlor)rru )tueu^p Jo sassel)
oerql are araqf (v) .slla)
leul|ue
ur e;purds )llorlu aql Sg-91 arnbrl

uola;a1soy(1 eql:9 [ raldeL]f 9€0 t
ue) slle) {ue6;
urnlertsqns
p!los e ssol)v l/v\er)
ruJoJ ot,{poq aql ur selrs
Jeqlo le a8rerua uaql pu€ slesse^ qdur,ri.1 Jo slesse^ poolq olul IMEJJ pue senssp
Surroqq8rau apelur Jounl ,{rerutrd e uI slleJ uaqm 'sJeJuEJ ,{ueru uI eloJ E seq
osp SurTMeJc lac ',{laleunuoJun 'snlll eql go dr1 aql 1" lsol slle3 e^ltdrosqe 8ur
-Jeldar 'rTIrA
Ipuusatul eqt Jo saprs eq] dn 1aner1 eunsalul aqt Jo Sulull lBlleqllda
eql ur sllal aql 'uorssaJord parapro ue uI 'ftnful Jo sells lE seJn]JnJls peSBluBp
plrnqal o1 Surdlaq pue LJessef,eu aJeqo\ ueq] SulaporueJ (senssll allpeuuoc
q8norql elEJBru uec slselqoJqrg KlrepurrS
'1e^ eueJ pue SuqepoueJ euoqJo ssaJ
-ord snonurluor e ur 'ureq] ragp ,vrolloJ teqt s]$lqoelso eql ,tq ut pa1pl ale teqt
slauuBqJ Sulurro; 'euoq olur Ieuunl slsBlJoelso 'esuodseJ aunlurul el?uul eql
Jo UBd leorlrJ3 e se sJepe^ur u8raro;;p8ua puP uollJeJulJo sells ol p\EJf, slqdorl
-neu pue se8eqdoJcelN 'q1ac Sugrr,reJJ qlIM seqlaes osp PIuluB llnpe aqJ
'ri.u,u aql Suop xuleru JelnllaouJxe pue seJBJ
-rns
IIeo ot punoq spu8rs pup slEuBIs elqnlosJo suolteulqruoc dq paprnB 'sla8rel
Jrldeu^s Ipnluale Jreql ol IeAeJl suoxe Surdolanap 3o sdp 8ulouu^pe eql l3 sauoo
q] \oJB q3rJ-url3E aql ]Bq] /(elv\ slql q sI 1I :Iualsds sno^Jeu eJnua aqlJo uollJnns
-uoJ aql ol plueuepun; st Suptero e3uPtslp-8uo'J'ecBJ eqlJo seJnpnJls snoIJEA
pue 'eq8 pue suorneu Jlteqledurds puu dJosuas 's1ac luaur8rd u11s Suturocaq
'saleJ
esJelrp a^Er{ sllal eseqJ
'orfuqrua
aq} }noq8noJql sa}IS ;o .Qeuel e o1
aqnl luJneu aql ur u13uo Jo alls JIaq] uro4 suoper8nu acuelstp-3uol Jlaql roJ alqe
-{JurueJ ere s11ai $aJJ IUJnau 'seleJqegal uI '(tZ Je}deq3 uI pessnf,slp) sleeqs pI
-aqllda aloqmJo slua(ualour paleulpJooJ aqt.{q puu suol}EJol IaBJE} JIJIsads ol
sflac lpnprlrpur Jo suor]eJ8rru aq] r{q palearc sI Ieruluu uE Jo eJnlonJls eq}
(slseu
-a3o,{rqura Surrng 'urrads Suruula,rs;o uotldacxa elqetou aq} qllrlt '3uq,r,rerc fq
sJnJf,o uoporuoJol IIaJ IIe lsotup 'sletulue uI 'ral€,!\ puodSo dorp e ut salella8eg
pue selerlrJ JallBr.rrs Jno^ap pue {JEllu ol pa^Jasqo aq ,tlsea uec ,t[aql pue 'pooJ
Jo qJJees ur z(lsnonulluoc I ^BJJ aeqeoluu drolepar4 'Ixu s ol elaBeg Jo PIIIs
3ursn,{q uuql Jeqlpr saJEJJns JaAo SulllvreJc r(q arrour slac duelX 'su1a1ord Jo}ou
pue 'sJoleln8ar 'sluaurpllJ
luaruelg
lelala{so}^c 3o uo4er8elur crueu{.p aql
alercardde uEJ al alaq \ eouelsur Jeqloue saprnord Supterc 1ac 3o ssacord aq1
eql uo luepuedep 'sseJotd peleJ8alul xaldruo3 {UBU e sI 8u!l ^eJO IIaD
'sasElsPle|.u
-lleJ] eqlJo {lnq aql qcrqM ur 'uolpaq pu? llurueJ}sqns eql 01 eueJqlualu PIusBId
aql ssorJe s]3euuo3 uolala{solb ur}Je eql qslq ^ ul 'luau1q30110 '.lleJ eql Jo
luo4 eq1 1B ]no paqsnd are sarnlrnJls qJIJ-uIt3e qJlqo\ uI 'uolsnJrcJd :pa,rlonut
aJe sarlr^r]rE ]Jurlslp aeJqJ 'euEJqruau Pluseld aql qleauaq xailoJ tIcIJ-uIlJE
sE qJns 's1ac SuqareJ3 eruos uI '(98{I aJntIJ) pJElv\JoJ rrx\eJp sI urseldoli.c 8uI
urnrpoorllourel xeuo) urpe
11
urso{tu
uorsual repun xeuol
ur nlerlsqns
urnlpoorllauJel
sapnJtoJd pua snld
1e uorlezuaur{1od urpe
NOtSnUtOUd ffi
urpe pezr:eu{;odun ;o luatuanout
ll
'pa)
ur uMoqs st utl)e le)tuo) paztjalri^lod
Ilnnau eq1'Ilqloorls premloJ lla) aql
6utnou 'peteulproo),{lfq6lf
aq uet sda}s
lle (lenrleurallv'uotLlseJ esrrndals e ur
preMroJ 11ar aql 6urnou.r 'paleadai
aq ue>
ol)I) eues aqf 'plemloj slMet) lla) aql
se l)eq aqt le pelqurassestp ale souo plo
pue'tuor] aqt te opeur are sl)eluo) le)oj
MAN
'(uorl)ejl)
uorsuel aqlJo outos xelal
ol ())pq lp Morre uaa;6) premro; ;;at aql
1o ,(poq eql s;adord lla) oqr Jo lear aql
le uorl)erluo]'xauo) utl)e aql saq)lalls
pue (luorJ le sttotrc uaatdlpleMtoJ
abpa aql sa^ou llar aqrro a6pe 6urpea;
aql le unlpodlllaLuel e,o luauiq)elle
uuU pue uorsnllo.rd guapuadap
-uollezuoullod-urt)e oql'pJeMroJ lla) e
elour xauo) r,l)!r-ull)p aqt u! poleJeuab
so)roJ Moq ,o lapout y 9g-9 1 arn6r1

LEOL uorAVHtS rtl) cNV NollllxsolD tHI
'reln8arr pue 61ra[
sr uorloruocol aql pue 'luapuadapul aJou ere sep.r llce eseql 'slselqorqrJ sB qJns
'sllec Jeqlo u1 'adeqs 3u€ueqc lnoqlnt r(lqtoorus pJeMJoJ apr13 ol ueas sllac aql
pue 'peteurprooc rtlasolc eJu sarlrlrlJe asaq] 'slurJaprda qsg eql tuoq saldcolera>l
uolsnrlord euerquaw eurseld se^uc uoltezueu{1o6 u;py
auuJqueu eruseld aql Suqsnd uorlezrraurdlod uure ltq paleraua8 sacro; uo dp
-rerurrd r(1ar o1 sruees 'a8pa Surpeal E Jo uorsnr1ord 'uorlourorol ur dals IsJIJ eql
,{aq; 'sarnlcnJ}s e{rl-leeqs 'leuorsueurp-oml aJu (suoJneu eruos ,tq sE IIel se
'slselqorqg pue slleJ plaqrlda dq paurro; 'ugpod;1arue1 'cnueu,(p
eroru sp lla,r,r
sE teuulql pue ra8uol ]nq illr oJJrru ur asoql Jo luacsrur{ueJ eru r{Jrq ^ 'slua{u
-BIU urlJe pepunq '3uo13o eJoJ E ureluoc r(aq1 'leuorsuaurp-auo rtlprluassa eJe
'sl$lqoJquJo sadrb auos pue sauoo qyroJ8 Surlur8rur ri.q peruJoJ 'u;podogg
'suralord pelErJosse -urlJB
tueJeJJrp;o acuasard eql uorJ sllnser slql moq passnJsrp ,(puarp eleq er\ pue
-rtlarrrpadsal 'suorsuaurp eaJq] Jo 'oml 'euo ur-pezlue8ro sr urlJp eql qolqm
ur ^,teivr aqt u1 ,tlguurrrd ragrp saJnlonJls eeJql eqJ 'sallaue8ro pesoloue-aueJq
-Iuelu sapnlsxa qsrqm 'ullJu snoluauelg Jo aloc asuep E qlyv\ pellrJ eJu 'elp
IIV
-odopnasd pue 'elpodq1aue1 '(salldsoJJlrx sB rr ^ou{ osp) epodolg Supnlcur
'saJnlcnJls
anrsnrlord go sad,fi lueJeJJrp aleraua8 sadr! 1ac lueJeJJIC 'prumlno
qseur pa{uI-ssorc dleuoSoquo uE ululuoo
(leeqs
lelleqlrda ue Sunurog dq purruu eql reloJ,tlprurou sllaf, esaql'slueIuBIIJ
up€Je{ luepunqe JraqlJo asnecaqsaT/-cownq sE rrmou{ eJE qJIqM's8or; pue qs.r;
;o srurraprda eqt Jo slleo pllaqlrda aqt q papnls larvr dlercadsa uaaq aleq daql
,ir111lour IIaJ roJ parmbar sr leql d:aurqreur eql Jo IIe uruluoJ utpodqlaurul
'erpodopnasd
ro erpodolrJ Jer{}le JoJ aABq e,rrr ueql
erpodllauelJo rusrueqcaru uorsnrlord pue uorlezrue8ro cnueudp eql lnoqe uon
-Er.uJoJur aJou e^eq etrlr
'adoJsoJcrul 1q311 aql qlyv\ uorleurluexe JoJ luerue^uoc
]soru sr drlauroa8 puorsuaurp-oml rlaql esnuJeq sduqra4 '1aB
luauug
-ultJe ue qllm pellg suopcaford leuorsuaurp-aarq1 ,tqqnls are 'qtqdonneu pue
epqaorup,(q paurrol 'ugpodopnasd 'runleJtsqns pilos aql 01 lallEr?d aueld e ur
dran spunon esoll ol pazrprcads are daql pue
ell qJrq ^ Jo tsoru 'slueruelu ullop Jo
gg o1 dn saleJ tE Surnour ,(1p1der
sIeeAeJ slql 'elEJ su Sururuexe pue urlsP Jo q3led IlEurs B SuolrBur ,{.q erpodq
-leurpl eil.Jol?re{ ur parpnls eq uBJ s}ueuBIU urlJeJo roFeqaq JIIueuIp eqJ
'saptcoleral
,tuq a41 3uryoo1
'r{.1purou
IAerJ ol enur}uoJ r(aq} 'salaue8ro pasolcua-eueJqluau pue salnqnt
-oJrrru
4cu1 ,i.lpraua8 sluaur8ur; eqf q8noql[V 'elladrdorcFu e qfl1v\ JJo peJIIS eq
ueJ unrpodqlauel slqf Jo sluau8erg '(Z8-9I arntlg) Iunlerlsqns aql o] peqJBltE
lou sr lEql ,i.poq 1ac Sumer] 'llElus e pue unrpodileuEl a8rel drarr e qlpvr adeqs
a^rlJurlslp 3 arunsss sa1,{co1eral 'sllal lenpr^rpur s3 peJnllnc ual{]vUuluTuri
('ael la!lnrlo {seunol
'B pue V) 'snal)nu
oql ot asol) suor6er
aql ol pat)ulse) de (anlq) sluaurelu
alerpauJalur pue (uail6) salnqnlol)tW
'luaura^our proer Ellar aql roj alqrsuodsor
are pue tunrpodrllarue; a6rel aqt lll,
(pa4 sluauelu u!l)v'lla) slql u! sluouelu
lelololsoil) Jo uotlnqulsto ()) lear
aql le unlerlsqns aql anoqe dn peu.rer
'snel)nu
aql 6u;pnpul [poq 1;er lleus
pue unrpodr;;aLuel teg'peolq s1r 6urrnoqs
'Ido>sooru uorl)ala 6uruuels Iq uees
ay(rolerey (g)')as/uli S I lnoqe le 6urnou
sr llo) srql'ueoe )as g I lnoqe ualel
'aJnlln)
ur a1r{ro1eta1 e;o sqdeloolrru
lq6!l (V) 'srureprde qsrl e
uro,r; selI>o1ere1 {ro1er6r1111 2g-9 1 arn6r1

uolalalsoy(1 aql:9 t teldeql 8€o t
arn8rg aas) saxalduoJ dUV ,(q sluauep; urtre reqlo Jo sepls eql ot peqf,elle
,(puanba4 aru spue snurru aqJ 'prumloJ 3utce; spua snyd rleqr q]1lvr patualro
,(psoru eJp {Jo^aqsau aql ut s}uatuplg ulloe aqJ 'aleJ}sqns aql ol lcadsar qlyvr
freuor]e]s urerual s]uaruPlrJ ur]Je eqt'pJe/vlroJ yrterc etpodqleul?l aql eIq { 'leqt
leuorsueurp-o^ l aq] IUJoJ o1 Surdlaq '(7t-gl
Ienpr^pu ur sJncJo leql SuUIrupBaD eql Jo lueJslurueJ 'IJEq eql le Suqquras
-seslp puB luo4 aql 1e Suqqurasse
'Suqpupeal Suro8rapun aq ol sueas eloq ^
IEJaAas Jo euo dpo sr xalduoc dUV eqr f,q l11nq qem cltlrpuep B Jo SuHIIIupEe{
'(tf-gt
arn8rg aas) flsnonard passncsp seFqnloJJIIu pue stuauPIIJ ullf,E
,(q paleurprooc f.yqrssod ,{lqruassesrp pue .,{yqruasse }uaIuEIIJ ullJp peleu
-rpJooc sarrnbar ilps ]nq 'xaydruoc dUV aql uo puadap lou seop ]eql uslueqJalu
p asn ot readde slselqorqrJ Surpnlcur slac Sunoru-.{.pro1s aurog 'a8pa Surpual
eql Jo uorsnrlo;d aq] alrJp ol sluaIuEIU ullJe Jltupur{.p asn UEJ slleJ leqt sderr
IeJaAes Jo uorleJ8elur lef,rueqoaru
eJInbaJ ol ]q8noqt s1 erpodrqaurul .{q uorlou lPuol]f,erlplunJo aJuPueluIeIN
'surruJoJ
puu uoqeradooo aqt
sluauelrJ ruJoJ-J ^aeu eql '(urro; c eql) ullcB-dclv Surureluoc slueuPlrJ urlJB
o1 ,(lequaragard pue ,{lanperadoof, spurq (ZV-qI ern8rC ees) urqtor asnecag
'a8pa
Surpeal aqr purqeq Ilalvr peluJol selrs te srnrro uouezlrerudlodap luaru
-PIrJ 'pJB^\JoJ
]sow eueJquau eurseld eql qsnd ol uoltEf,ol rEql ul zi.preurtrd
Surrrncco ewor8 ]uauEIrJ urtoe ^ eu qlyv\'e8pa Surpeal eql lE pazlpJol sI uollu
Iq uollEzlreurflodap o] luutsrseJ eq pFoqs a8pa Surpual eql ]e petEJeue8
urnrpodrllarupl eql ur ssecord Surlgurpearl puollJeJlplun 'luerorJJe uE surel
-uleru lBr{1 rusrueqJaru p JoJ slsBq aq} apnord o1 lqSnoql sr ullJe sno}ueIuEIIJ
dq srsu(lorpdq 41ypa,(e1ap eqt 'snql'stuaruelg replo eqt alqruasspslp r{pualcgga
upr urlrJof, 'spaacord srs,{loJpdq dIV pue e8p slueuelg eq} sV '(68-9I aJnBIC)
'(06{I arntlgl
(ssar6 r{lgsranrul
rallaJal)ou aqf urorl uotssil.lllact qrll '666
t '9201-600LiSVL'lolg
ila)'f
Xsrrog '9 pue eur)ll^S f tuo4) '(t€-9 t alnbH aas) tueLuelg 6utlstxaald
e Jo aprs aq] JJo luauelU url)e Mau e soleal)nu xaldulot dUV aql uaqm
paurol a;6ue.0l )rlsual)ejeq) aqr qllm'uaas aq ue) sluaulellJ paq)uelq
snoraunN 'g ur uA oqs a6pa 6utpea; aqt le qaM utlte aql Jo suot6al
paueur aqlJo sMar^ dn-aso;) ()) ') ul pa6le;ue seare alouap slaqel eqI
'lromqsaur
luaurelq url)p asuap eql 6ulnoqs 'eilroleral e;o abpe 6u;peel
aql lo er;;dar paMoppr.{s-ulnulteld e ;o qdel6o.t>tLu uoll)al] (8) 'anltle
tsou.t
sr uorleel)nu u!l)e araqM 'unrpodrllauel aql ro luolj aql leau paleJluo)uo)
Ilq6rq sr xa;dtror 6gy aq1'rolalleadde del.leno oml aql alaqm
suor6a.r aql'suralo.rd luauodtuor sl! Jo auo lsutebe pester lpoqlgue ue
t4tltrt uaafi ur palaqpl xalduor duv aqr pue 'urprol;eqd lua>saron;; {q pal
u! paloqel sluauielU u!l)e qllm au(toletal V (V) 'elPodlllauel u; xe;duro>
dUV eqr (q uorgeu.ro; qa/n pue uolleal)nu luoutellJ ulpy gg-91 atnbtl
eqr qlF aterJossE ot uaas sluaruslrJ II ulsodlx JEIodIq ,(1pu14
'(16-9I
erntl0 preMioJ
sa^oru ]l sE IIac aq] Jo saprs eql ur raqlBS o1 Surdlaq 'unrpodllaurEl SuuoruoJol
aqlJo seprs aq] ur saqJurd tr puu uorsnJloJd slua^aJd uorlceJluos sIqJ'a3pa 3ur
-pEel eq] o] IellEJed lsorulE uolleluelro ue ol aSpe Surpeal eqt ol rPlnJlpuedred
dlreau uo4-uorleluerJo ^ au E olur ueqt ilnd pup qem aql uI sluaruellJ ullJe
urri 0Z
B sp qam aql'(gg-9l arn8gg; qa^a
urlrlo) pue urpe
-elonu luauelrc 'sJolJEJ
urlrJoc
(ssald ^ltsla^!un ralloJal)oU
aql uor} uorssil,llad qlm.666 t
'9201-600LtSnL'p!8
ilD 'r Xsuo€ .D pue
eurll!^s f r.uoll) 'urluo) qllM paler)osse
lou ^llerauab alp a)tDel tuauelu aql ul
(url)e dCV ueql raqlei) utl)e dfv utpluo)
ol ^la)rl ]sot! ale leql pup,(;luatar lsotu
patilioJ a^eq leqt sauo aqt osle aJe q)tqM
'a6pa 6urpee; aqt ol tsasol) sluauelU
ult)e eql'v ur a;bueltal a1lq^4 aqt qll/!^
polreu uor6a.r aql;o Mar^ dn-asol) (B)
'a6pe
6urpeal &an aql te punoj lou
sr urlUo) 'unrpodr;;aue;
aqt q6norql ,(em
aql lle saq)ear )|lo/v\qsau utl)e asuap aql
qbnoqrlv'no11a( rcadde de;lano oml aql
araqivr suor6ar aql',(poqrrue ]ua)saJonlJ
e q4ur uaatb ur palaqel urlqor pue
ulplolleqd lua)sotonlJ {q pa; ur pa;aqe1
stuaurelu ull)e qllM ailrorera>l V (V)
'erpodrl;eure1
u! utltro) 69-9[ ern6rJ

THE CYTOSKELETON AND CELL BEHAVIOR
capprng 0
proTern
lr
a
€
po
tnC
q
net filament
assembly at
leading edge
dq
ql
t'
6
d
&
9q
?
&u
$
d
ts
!p
$ d.
The pushing force created by the polymerization of a branched web of actin
filaments plays an important role in many cell processes. The polymerization at
the plus end can push the plasma membrane outward, as in the example just
discussed (see Figure 16-90), or it can propel vesicles or particles through the
cell cltoplasm, as in the example of the bacterium Listeria monocytogenes discussed
in Chapter 24 (see Figure 24-37 ). Moreover, when anchored in a
more complex way to the membrane, the same t)?e of force drives plasma
membrane invaginations, as it does during the endocytotic and phagocl'totic
processes discussed in Chapter 13.
It is interesting to compare the organization of the actin-rich lamellipodium
to the organization of the microtubule-rich mitotic spindle. In both cases, the
cell harnesses and amplifies the intrinsic dynamic behavior of the cltoskeletal
filament systems to generate large-scale structures that determine the behavior
of the whole cell. Both structures feature rapid turnover of their constituent
cytoskeletal filaments, even though the structures themselves may remain intact
at steady state for long periods of time. The leading edge plasma membrane in
the lamellipodium fulfills an organizational role analogous to the condensed
chromosomes in organizing and stimulating the dynamics of the mitotic spindle.
In both cases, molecular motor proteins help to enhance cltoskeletal filament
flux and turnover in the large-scale arrays.
Figure 16-91 Contribution of myosin ll to polarized cell motility'
(A) Myosin ll bipolar filaments bind to actin filaments in the dendritic
lamellipodial meshwork and cause network contraction. The myosin-driven
reorientation of the actin filaments in the dendritic meshwork forms an
actin bundle that recruits more myosin ll and contributes to generating the
contractile forces required for retraction of the trailing edge of the moving
cell. (B) A fragment of the large lamellipodium of a keratocyte can be
separated from the main cell body either by surgery with a micropipette or
by treating the cell with certain drugs. Many of these fragments continue
to move rapidly, with the same overall cytoskeletal organization as the
intact keratocytes. Actin (btue) forms a protrusive meshwork at the front of
the fragment. Myosin ll (pink) is gathered into a band at the rear' (From
A. Verkovsky et al., Curr. Biol. 9:11-20,1 999. With permission from Elsevier,)
o
rd
1039
Figure 16-90 A model for protrusion of
the actin meshwork at the leading edge,
Two time points during advance of the
lamellipodium are illustrated, with newly
assembled structures at the later time
point shown in a lighter color. Nucleation
is mediated by the ARP complex at the
front. Newly nucleated actin filaments are
attached to the sides of preexisting
filaments, primarily at a 70'angle.
Filaments elongate, pushing the plasma
membrane forward because of some sort
of anchorage of the array behind. At a
steady rate, actin filament plus ends
become capped. After newly polymerized
actin subunits hydrolyze their bound ATP
in the filament lattice, the filaments
become susceptible to depolymerization
by cofilin. This cycle causes a spatial
separation between net filament
assembly at the front and net filament
disassembly at the reat so that the actin
filament network as a whole can move
forward, even though the individual
filaments within it remain stationary with
resoect to the substratum.

1040 Chapter 16:The Cytoskeleton
leading edge of cell
Cell Adhesion and Traction Allow Cells to PullThemselves
Forward
(c)
20 pm
Lamellipodia of all cells seem to share a basic, simple type of dynamic organization
where actin filament assembly occurs preferentially at the leading edge and
actin filament disassembly occurs preferentially at the rear. However, the interactions
between the cell and its normal physical environment usually make the
situation considerably more complex than for fish keratocFes crawling on a culture
dish. Particularly important in locomotion is the intimate crosstalk between
tightly to move forward. In these lamellipodia, the same cycle of localized nucleation
of new actin filaments, depolymerization of old filaments, and myosindependent
contraction continues to operate. But because the leading edge is
prevented physically from moving forward, the entire actin mesh moves backward
toward the cell body instead, pulled by myosins (Figure 16-92). The adhesion
of most cells lies somewhere between these two extremes, and most lamellipodia
exhibit some combination of forward actin filament protrusion (like keratocJ,tes)
and rearward actin flux (like Ihe Aplysia neurons).
As a lamellipodium, filopodium, or pseudopodium extends forward over a
substratum, it can form new attachment sites at the cell front that remain stationary
as the cell moves forward over them, persisting until the rear of the cell
catches up with them. \.A/hen an individual lamellipodium fails to adhere to the
(D)
Hrc-cH'-cg,
/- \
HrC-CH
CHOH
Hzc
HO
Hzc
HC
au
cytochalasin B
CH
il
CH
Figure 16-92 Rearward movement of
the actin network in a growth-cone
lamellipodium. (A) A growth cone from a
neuron of the sea slug Aplysiais cultured
on a highly adhesive substratum and
viewed by differential-interferencecontrast
microscopy. Microtubules and
membrane-enclosed organelles are
confined to the bright, rear area ofthe
growth cone (to the /eft), while a
meshwork of actin filaments fills the
lamellipodium (onthe right). (B) After
brief treatment with the drug
cytochalasin, which caps the plus ends of
actin filaments (see Table 16-2, p. 988),
the actin meshwork has detached from
the front edge of the lamellipodium and
has been pulled backward. (C) At the
time point shown in B, the cell was fixed
and labeled with fluorescent phalloidin
to show the distribution of the actin
filaments. Some actin filaments persist at
the leading edge, but the region behind
the leading edge is devoid of filaments.
Note the sharp boundary of the
rearward-moving actin meshwork.
(D) The complex cyclic structure of
cytochalasin B. (A-C, courtesy of Paul
Forscher.)
Figure 16-93 Lamellipodia and ruffles at
the leading edge of a human fibroblast
migrating in culture. The arow in this
scanning electron micrograph shows the
direction of cell movement. As the cell
moves forward, lamellipodia that fail to
attach to the substratum are sweot
backward over the dorsal surface of the
cell, a movement known as rufflinq.
(Courtesy of Julian Heath.)

THE CYTOSKELETON AND CELL BEHAVIOR
substratum, it is usually lifted up onto the dorsal surface of the cell and rapidly
carried backward as a "ruffle" (Figure 16-93).
The attachment sites established at the leading edge serve as anchorage
points, which allow the cell to generate traction on the substratum and pull its
body forward. Myosin motor proteins, especially myosin II, seem to generate
traction forces. In many locomoting cells, myosin II is highly concentrated at the
posterior of the cell where it may help to push the cell body forward like toothpaste
being squeezed out of a tube from the rear (Figure 16-94; see also Figure
16-91). Dictyostelium amoebae that are deficient in myosin II are able to protrude
pseudopodia at normal speeds, but the translocation of their cell body is
much slower than that of wild-type amoebae, indicating the importance of
myosin II contraction in this part of the cell locomotion cycle. In addition to
helping to push the cell body forward, contraction of the actin-rich cortex at the
rear of the cell may selectively weaken the older adhesive interactions that tend
to hold the cell back. Myosin II may also transport cell body components forward
over a polarized array of actin filaments.
The traction forces generated by locomoting cells exert a significant pull on
the substratum (Figure 16-95). In a living animal, most crawling cells move
across a semiflexible substratum made of extracellular matrix, which can be
deformed and rearranged by these cell forces. In culture, movement of fibroblasts
through a gel of collagen fibrils aligns the collagen, generating an organized
extracellular matrix that in turn affects the shape and direction of locomotion of
the fibroblasts within it (Figure 16-96). Conversely, mechanical tension or
stretching applied externally to a cell will cause it to assemble stress fibers and
focal adhesions, and become more contractile. Although poorly understood,
this two-way mechanical interaction between cells and their physical environment
is thought to be a primary way that vertebrate tissues organize themselves.
Members of the Rho Protein Family Cause Major Rearrangements
of the Actin Cytoskeleton
Cell migration is one example of a process that requires long-distance communication
and coordination between one end of a cell and the other. During
directed migration, it is important that the front end of the cell remain structurally
and functionally distinct from the back end. In addition to driving local
mechanical processes such as protrusion at the front and retraction at the rear,
the cltoskeleton is responsible for coordinating cell shape, organization, and
mechanical properties from one end of the cell to the other, a distance which is
typically several tens of micrometers for animal cells. In many cases, including
but not limited to cell migration, large-scale cyoskeletal coordination takes the
form of the establishment of cell polarity, where a cell builds different structures
with distinct molecular components at the front vs. the back, or at the top vs. the
100 $m
1 041
5[m
Figure 16-94 The localization of myosin
land myosin ll in a normal crawling
Dictyostelium amoeba. This cell was
crawling toward the upper right at the
time that it was fixed and labeled with
antibodies specific for two myosin
isoforms. Myosin | (green) is mainly
restricted to the leading edge of
pseudopodia at the front of the cell.
Myosin ll (red) is highest in the posterior,
actin-rich cortex. Contraction of the
cortex at the posterior of the cell by
myosin ll may help to push the cell body
forward. (Courtesy of Yoshio Fukui.)
Figure 16-95 Adhesive cells exert
traction forces on the substratum. These
fibroblasts have been cultured on a very
thin sheet of silicon rubber. Attachment of
the cells, followed by contraction of their
cytoskeleton, has caused the rubber
substratum to wrinkle. (From A.K' Harris,
P. Wild and D. Stopak, Science 208:177-179'
1980. With permission from AAAS.)

1042 Chapter 16:TheCytoskeleton
Figure 16-96 Shaping of the extracellular matrix by cell pulling. This
micrograph shows a region between two pieces of embryonic chick heart
(tissue explants rich in fibroblasts and heart muscle cells) that were grown
in culture on a collagen gel for 4 days. A dense tract of aligned collagen
fibers has formed between the two explants, apparently as a result of
fibroblasts tugging on the collagen. (From D. Stopak and A.K. Harris, Dey.
Blol. 90:383-398, 1982. With permission from Academic press.)
bottom. Cell locomotion requires an initial polarization of the cell to set it off in
a particular direction. Carefully controlled cell polarization processes are also
required for oriented cell divisions in tissues and for formation of a coherent,
organized multicellular structure. Genetic studies in yeast, flies, and worms have
provided most of our current understanding of the molecular basis of cell polarity.
The mechanisms that generate cell polarity in vertebrates are only beginning
to be explored. In all known cases, however, the c],toskeleton has a central role,
and many of the molecular components have been evolutionarily conserved.
For the actin cytoskeleton, diverse cell-surface receptors trigger global
structural rearrangements in response to external signals. But all of these signals
seem to converge inside the cell on a group of closely related monomeric
GTPases that are members of the Rho protein family-cdc42, Rac, andRho. The
same Rho family proteins are also involved in the establishment of many kinds
of cell polarity.
Like other members of the Ras superfamily, these Rho proteins act as molecular
switches to control cell processes by cycling between an active, GTp-bound
state and an inactive, GDP-bound state (see Figure 3-71). Activation of cdc42 on
the plasma membrane triggers actin polymerization and bundling to form
either filopodia or shorter cell protrusions called microspikes. Activation of Rac
promotes actin polyrnerization at the cell periphery leading to the formation of
sheet-like lamellipodial extensions and membrane ruffles, which are actin-rich
protrusions on the cell's dorsal surface (see Figure l6-93). Activation of Rho promotes
both the bundling of actin filaments with myosin II filaments into stress
fibers and the clustering of integrins and associated proteins to form focal contacts
(Figure 16-97). These dramatic and complex structural changes occur
because each of these three molecular switches has numerous downsueam rarget
proteins that affect actin organization and dynamics.
actin staining actin staining
(A) QUIE5CENT CELLS (B) Rho ACTIVATION
(C) Rac ACTIVATION (D) Cdc42 ACTIVATION
20 pm
Figure 16-97 The dramatic effects of
Rac, Rho, and Cdc42 on actin
organization in fibroblasts. In each case,
the actin filaments have been labeled
with fluorescent phalloidin.
(A) Serum-starved fibroblasts have actin
filaments primarily in the cortex, and
relatively few stress fibers.
(B) Microinjection of a constitutively
activated form of Rho causes the raoid
assembly of many prominent stress
fibers. (C) Microinjection of a
constitutively activated form of Rac, a
closely related monomeric GTPase,
causes the formation of an enormous
lamellipodium that extends from the
entire circumference of the cell.
(D) Microinjection of a constitutively
activated form of Cdc42, another Rho
family member, causes the protrusion of
many long filopodia at the cell periphery.
The distinct global effects of these three
GTPases on the organization of the actin
cytoskeleton are mediated by the actions
of dozens of other protein molecules that
are regulated by the GTPases. These
target proteins include some of the
various actin-associated proteins that we
have discussed in this chapter. (From
A. Hall, Science 279:509-514, 1998. With
permission from AAAS.)

THE CYTOSKELETON AND CELL BEHAVIOR
Some key targets of activated Cdc42 are members of the WASp protein family.
Human patients deficient in WASp suffer fromWiskott-Aldrich Syndrome, a
severe form of immunodeficiency where immune system cells have abnormal
actin-based motility and platelets do not form normally. AlthoughWASp itself is
expressed only in blood cells and immune system cells, other family members
are expressed ubiquitously that enable activated Cdc42 to enhance actin polymerization.
WASp proteins can exist in an inactive folded conformation and an
activated open conformation. Association with Cdc42-GTP stabilizes the open
form of WASp, enabling it to bind to the ARP complex and strongly enhancing
this complex's actin-nucleating activity (see Figure 16-34). In this way, activation
of Cdc42 increases actin nucleation.
Rac-GTP also activates WASp family members, as well as activating the
crosslinking activity of the gel-forming protein filamin, and inhibiting the contractile
activity of the motor protein myosin II, stabilizing the lamellipodia and
inhibiting the formation of contractile stress fibers (Figure l6-98A).
Rho-GTP has a very different set of targets. Instead of activating the ARP
complex to build actin networks, Rho-GTP turns on formin proteins to construct
parallel actin bundles. At the same time, Rho-GTP activates a protein kinase that
indirectly inhibits the activity of cofilin, leading to actin filament stabilization.
The same protein kinase inhibits a phosphatase acting on myosin light chains
(see Figure 16-72). The consequent increase in the net amount of myosin light
chain phosphorylation increases the amount of contractile myosin motor protein
activity in the cell, enhancing the formation of tension-dependent structures
such as stress fibers (Figure 16-988).
In some cell types, Rac-GTP activates Rho, usuallywith kinetics that are slow
compared to Rac's activation of the ARP complex. This enables cells to use the
Rac pathway to build a new actin structure while subsequently activating the
Rho pathway to induce a contractility that builds up tension in this structure.
This occurs, for example, during the formation and maturation of cell-cell contacts.
As we will explore in more detail below the communication between the
Rac and Rho pathways also facilitates maintenance of the large-scale differences
between the cell front and the cell rear during migration.
Extracellular Signals Can Activate the Three Rho Protein Family
Members
The activation of the monomeric GTPases Rho, Rac, and Cdc42 occurs through
an exchange of GTP for a tightly bound GDP molecule, catalyzed by guanine
nucleotide exchange factors (GEFs). Of the 85 GEFs that have been identified in
_.:

1044 Chapter 16:The Cytoskeleton
the human genome, some are specific for an individual Rho family GTpase,
whereas others seem to act on all three family members. The number of GEFs
exceeds the number of Rho GTPases that they regulate because different GEFs
are restricted to specific tissues and even specific subcellular locations, and they
are sensitive to distinct kinds of regulatory inputs. Various cell-surface receptors
activate GEFs. An example is the Eph receptor tyrosine kinase involved in neurite
growth cone guidance, which is discussed in detail in chapter 15. Interestingly,
several of the Rho family GEFs associate with the growing ends of microtubules
by binding to one of the +TIPs. This provides a connection between the
dlnamics of the microtubule cytoskeleton and the large-scale organization of
the actin cltoskeleton, which is important for the overall integration of cell
shape and movement.
The Rho family GTPases are also primary determinants of cell polarity in
budding yeast, where extensive genetic analyses have increased our understanding
of the general mechanisms involved. on starvation, yeasts, like many
other unicellular organisms, sporulate. But sporulation can occur only in diploid
budding yeast cells, whereas budding yeasts mainly proliferate as haploid cells.
A starving haploid individual must therefore locate a partner of the opposite
mating type, woo it, and mate with it before sporulating. yeast cells are unable to
swim and, instead, reach their mates by polarized growth. The haploid form of
signal molecule, which under normal circumstances would direct it toward an
amorous a cell located nearby.
This polarized cell growth requires alignment of the actin cytoskeleton in
response to the mating factor signal. \.A/hen the signal binds to its receptol the
receptor activates cdc42, which in turn induces assembly of actin fllaments at
the location closest to the source of the signal. Local activation of Cdc42 is further
enhanced by a positive feedback loop, requiring actin-dependent transport of
cdc42 itself as well as its GEF and other signaling components along the newly
assembled actin structures toward the site of the signal. subsequently, actin
cables are assembled pointing toward the site of cdc42 accumulation due to the
upstream and dor,rmstream pathways have been identified through genetic
Figure 16-99 Morphological polarization
of yeast cells in response to mating
factor. (A) Cells of Soccharomyces
cerevisiae are usually spherical. (B) They
become polarized when treated with
mating factor from cells of the opposite
mating type. The polarized cells are called
"shmoos." (C) Al Capp's famous cartoon
character, the original Shmoo. (A and B,
courtesy of Michael Snyder; C, @ 1948
Capp Enterprises, Inc. Used by permission.)

uor^VHrs trl) cNV NoIIT)solAf lHr
O
rope,r 6urleur
ureloro 9 pale^rpe
t
uorlPuro]
alqe) urpe
t urulJo,
,6urooulqs, pue
srsol^)oxa pazue;od
uorlezr:au{;od
u!pe
I I
I
,irpelod IIaJ Jo luauqslqelse eql ur pa^lo^ur esrma{rl
ueryo eJE daql araqir,r 'srusrue8lo Jar{}o ur sSolouroq aleq ol punoJ uaaq eAEq
suraloJd esaqlJo fueur 'lseef ur uortuogltuepr JIeq] ol ]uanbasqng
'sluaruuadxa
uollerotw lle) ro uoll)arlo aql eleplo ue) sleubts leurelxf
arnS;g) req}o eql snsrel IIer eql Jo epls auo uo
saprldad elqlsnJJlp eseqlJo uorlPrlueruor eql q 961 dpolo esuaraJJrp s lcelep 01
'ir4qe Jreql ^q sta8Jet IErJelJeq o1 paprn8 eJp slrqdoJtneu 'sJoldeJer esaql Sursn
'(auruorqlaurlrturrog-1"r qly!\ srsaqtuds ureloJd ur8aq salo,{-record fpo) suraloJd
lerJalJBq ruo4 pa^rJep aJe leqt seppdad pale/ruro;-1ur aqlJo suopeJluaJuos ,/v\ol
,i-rarr aql lJelep ol uaql alqpue spqdonnau Jo ef,eJJns aql uo sulalord roldacag
'uorlJeJur
lprra]Jeq Jo eJJnos p prBl!\ol 's\tt1do4nau pallm '.sllaf, poolq elrqM
Jo ssEIJ E Jo luarua^olu srlJuloruaqs aql s1 aldruexa parpnls-llaM euo 'e^oqP
peqlrosep ,ifIlloru [aJ roJ peJrnbeJ sn]eJeddB eq] Jo uopuzlus8ro eql SurJue
-ngu1 ,tq firrelod IIaJ eIBJS-eBJel dn tas ol sulalord ,!nue; oqg eqt Je88ul sleu
-3rs puraga araqm es€J SullseJalur dlJelnJrupd u sl slqJ 'leJlrueqo elqrsnJJrp e Jo
luarper8 e ,{q pallorluoJ uorlceJrp e ur }uerualou IIec sp peuuep q suulowaqJ
euqeotrru unna$otlJlg Jo srxelotuaqr relrurs eql q pu€ espJ slqt uI qtog
'(I0I{I
sv .rerlJEa passnssrp sesBdlc,i.guel-oqg slJeuouoru eqt uo spuedap esuodser
uopuzrretuf.lod urlJp srql 'spue8rl Jreqt purq sroldeceJ eril uaq ^ peteFurlls sl
sJotdef,eJ aql leeu upf,e Jo uorluzFaurdlod [EJol e [16Jy c11cfc go acrnos E preo\ot
rrMoDlun up qSnoJqJ'(96-9I ern8rJ ees) sllnsal uorsnJloJd lerpodl
-leluel puB xelduoJ duv aql sale^ll3E ueql f,Bu 'esPdJc J?u eql selu^rl3? urnl
uI teql (ed(9'7'g)Id) elnrelolu Suleu8rs paseq-prdl p selpraue8 qrlqM'(s)gld)
sesBur)l ,t applsouroqdsoqd salBAIlJE Joldaf,eJ paldnoJ uralord-c sll ol pue
-3q ]uelcerDuoruaqJ aqlJo Sulpurq '.slleJ JllJeloruaqo JoC '1eu8ls eqt pJemol uols
-nrlord e spuelxe IIaJ SurpuodseJ eq] '(66-9I arn8rg eas) lseef Butooruqs aq1 u1
'ursluuqraur
f prl slorlsllqnd uellrur)en urorJ uorssrurad 'lllM'666 [ 'lg-sLiL
'tolB
il4 'tDN "le le rauro A'c'o ruorj) '(t-9 [ ernbrl osle aas) ]! Ior]sep pue
urnuat)eq aqr Jlnbua plnoivr ;rqdo.rlnau aql 'altadld Erole6llsa^ut up ueql
raqler 'aprlded aqt Jo a)rnos aql aroM urnual)eq led e I'(wollog) eplldad
s!ql Jo a)rnos aql premol slmer) lla) aql (lleurl '@tpp!w) untpodt;;eue; aql
Jo uorlrsod aqt allsoddo lear aqt te ^lr.reuJud pate)ol st ;1 ursolur ellDerluol
]Pq] os uolalalsov(r slr sazr.re;od pue tuntpodt;1euel sltll spualxa uaql
ll '(dot) apltdad luet)eiueouraq) aql Jo a)rnos aql premol urntpodtllaulPl
Mau e spualxa r(|)|);nb |;r.ldortnou aql'(tz latdeq) ur passn)slp) ropP^ul
u6raro; e;o trnpord aql se eprlded slql sazru6o)ar l!qdorlneu ueunq eql os
'sanprsar auruo!qlaur pale;Iu.ro3 eneq sutelo.td leUal)eq {1u6'eq4-na1-1a61
-;{uro; aprgdad aq} Jo lunoure lleus e 6urleal sr 1q6u aq} }e drl eDadld eql
'slxelouraq) pue uollezyelod llqdorgnap 1g 1-91 arn6;1
\
5?0t
'(umoqs
lou) 6ulrpru loj lla) proldeq
aqt 6uuedard'(St raldeq) ur pessn)stp)
ape)spr aseurl dV4 e q6norql sesuodsar
raqlo sra66r.l1 uorlenrl:e roldata.r
'uolrlppe u1 'edeqs
ooLuqs e ]o uorlrslnbte
pue'qunor6 pezuelod'uolleulloJ
alqe) url)e o1 {;;eur; pue'uotlent})e
uruiloj puP oqu o^tsualxa ol spPal
sil.ll'po)uequa raqun, sr ,tlnlr>e zt>p)
{qalaqm dool lreqpaa; anrlrsod e aleal)
qv"ror6 luaue;U pue uorleal)nu url)e
;ero; eql'6urpurq rol)e1 6ulteu Jo alls
aql le uolleal)nu url)e le)ol ot 6urpeal
'xelduor
dUV aql sale^tl)e 1eq1 uralotd
,(1;ue1 d5yg e sete^tl)e Zt)p)'slla)
uerleuureuJ ur sV'Zt)pl'utalold 6u;pulq
-619,(1;ueJ oqu aql sate^ll)e urnl ul
slr.ll'(St roldeqf ur passnf,stp) uralo.rd-g
)rJauJuloleleq e uoi] ]tunqns Dg
punoq-dfg aql Jo uotle!tossrp s.re66u1
roloa)er aq] jo uolle^lt)v'auerquiaul
eurse;d eqt ur loldatel pa;dnor-urelord-g
e ol spulq rolre; 6ug1eu relnlla)eltxa
aq1'asuodsar to1rel Eulleu lsea{ aql
u; Iernqled 6ur;eu6rs aq1 96 1-9 1 arn6r3

uole;alsoy(3 aql:9 ! raldeql 9VOL
uorloruo)ol pue uollezuelod lla)-aloLl^ seleu!proo)
suolelelsorr() ulrrv pue elnqntor)lry aql uaa/nleg uolle)lunuuo)
-,{yod unce elrsnJ]oJdJo uorluool eq] dq peruengur sr aruosonuec eqlJo uoplsod
aql 'sllaf, Suqourocol ,tueru u1 'sluaruElg uIlf,B JIeq] r{fyvr Suop saFqnloJJlru
Jraql asn sllac uorlJaJrp JBInJrued e ur luarueloru luelslsJed ezrueSrc dlaq o1
aprs pJE^ JoJ aql uo puno; Sutaq 'uollezuelu
JopaJJa 'sapqnloJJlru
IeJa^aS slr uo Surlpd r(q auosorlual aql a^otu leq] sulal
-oJd Jolour alrT-ureudp alelrlJp,{.1pco1 osle lnq (uorsnr}ord IEJoI eJoJaJaql pue)
aJaq] uorlezrraru,{1od ullle elelnrups .{1uo lou rq8lu IIaf, p Jo e8pe euo uo sJol
-dacar;o uorlelrtre aq] ]eq] ]q8noql sI ]I 'pe^lo ur aq Leur ZwpJ ulalord d.11ue3
oqu eql ]eq] eJuapna sr aJaql qSnoqlp 'Jeelc lou sl uollBluelJoal aluosoJlual
ruealtsrrmop suraloJd
urso{ur-urpe
'saleururop ol-l
snld apqnloJf,ru Jrtueudp eql'ilac aqlJo uor8al elrsnJloJd aqt olul eruosoJlueJ
aq] ruor; Surpualxa spua snyd Jraql alpq seFqnloJJrru eseqlJo dueru lpql sueeur
Suruorlrsodar slr pue 'salnqnlorctu crueur(p Jo Jaqunu a8rel e salealJnu eruos
-oJluar eqJ'sluaua8uerruar urlJe eJuengur sJtueudp alnqnloJOrru 'uJnl uI
'salnqnloJJrru
azrTrqels o] sreadde uope^Ipe oqg pue 'salnqnlorcrur
Suzqrqelsap
'(966-t66 'dd 't-gl
laued aas) uruqtpls ulalord Surpurq ulnqnl aqt (tlqqul
eJoJeJaql pue) elelftoqdsoqd uec ceg dq pelelrlop esBuDI ulalord u 'elduexa JoC
atpFpour dllcarlpul ,{eur spua
:Od
- eu-l
/,uo,,n,ro,oi': uorperluo)
,rraro-......'.......--
'(Zg-9I
arn8rg aas) suoneupsap radord rraql ot sauoo qlMoJB ro s11ac
Suqoruocol eq] raets o1 spu8rs alqrsnsp-uou pue elqlsryJlp Jo uoueuqruoo B
uo puadap 'sauor qrrtor8 puoJneuJo slaAEJ] eql pue uollEJBIIu IIaJ-lsalJ-leJnau
Surpnlcur 'slerurue ur suorler8nu IIaJ acuptsrp-3uo1 lsolz{ 'uopeztrarur{1od uqce
p3]f,eJrp puu uorseqpe IIas peseaJJur asnBJ uec ,(aq] 'sroldacar elP^rlse slPu
-3ts asaq] uar111
'uoqur8rur IIaJ Jo uopcaJlp aqt eJuangu osle uEf, sllaf, Jo aoPJJns
aql o1 Jo xrJluru JelnllaseJlxe aql ol peqsBllE sanJ lecluaqJ elqlsnJJlpuoN
)Jeq aq] l3 uorlJPJl
-uoJ pue a8pa Surpeal aql tp uorsnrlord qlyv\ &IJpIod [EuoI]JurU sll UIE]uIpru
ot IIaf, eqt selqeue slqJ'(Z0I{1 arn81g) JEaJ aqt uI sel?ulruop uol}E^ltJe oqg
puE IIef, eql Jo luoJJ aql u! saleuruop uorlE^IlJp JEU 'llnsar e sE puB Teqlo qf,Ee
llqlqul ,tpcarrp sassacord o^(1 aqJ dl1ltcer1uoc paseq-utsofru seJuuqua puP
oqu uo surnl }eq1 ,(eMqlEd 3u1pu3rs JeqlouE salEArlJE roldacer s1t o1 pue3l ]uel
-cpJlleouaqJ eqlJo 3urpurq
'aruu
eures aql tV'esuluqdsoqd ptdq arrqce r(larrpnl
-llsuoJ e ,tq 2614 olur If,Eq paua^uoc .{lprder sI }I eJuls 'qsaqluds 3o alls sll ruo4
JeJ asnJJrp louuec JEu sale^rlJE teql td(9'7't)Id aqJ 'uorsrulord;o uollJnpul
aq1 SuruaqfueJls 'dool
{cpqpeeJ anr}rsod e ul II1,rI}JB )tld Jo }uarueoueque
Ierol raqury sasneo a8pa Surpeal aql le qa ^ ullte pazlreyod eqlJo uollelnrunJce
Jo rusrueqJeru eqJ 'snelJnu eq] Jo
,tpcarrp scrueu^,tp alnqntoJcrur elulnporu oqg pue JEU Jo
esedJD ceu aql alp^rlse oslE pue uorsaqpe IeJol
'spu8rs
TBa/\/\ o] asuodsal a^rlrsuos
E Sur aolle 'p1ro,vr eprslno eql ruo4 salracal uolala{soilJ ullJe eql leql uolleru
-roJur dlrrelod aq] acro;urar ol sruees eruosoJlueJ eq] Jo uoueluarJo eql 'ursruE
-q3eru lrExe eql Jo ssalpreSeu 'porJed paSuoloJd e roJ uorlcerrp a(uus aql ul lsls
-rad o1 z!ry1oru alrsnrloJd selqeua leql dool {Jeqpaal a^qrsod e 3ut1uarc 'uorsnJ]
-ord raqury e8uJnoJua [qeJer{l pFo^a salnqn}oJJnu Jo uorlEJluef,uof, paseeJJur
aqJ 'spua alnqnlorcltu Suur,ror8 uo Surlelert sdII+ aq] o] pulq teql sCHD-oEU
Suuanuap rtq uor8ar elrsnJloJd aql q uoDezueur,(lod urlJe eseaJJur Jequry ol
't-g
t arn6rl
aas 'uJatsls 6ur;eu6rs stql Jo ssaualtt)a#a
aqt Jo alourexa aJtl-leal e lol
'luauranolrl psl)altp 6urlsrsse pue leal
lla) aql Jo uor])ertuo) 6ultelnLulls 'ller
aql,o rear aLl] le Iluo palentl)e st oqu
'llo)
aql Jo luor1 aql le alrl)e sr uorsnllo.rd
paseq-)eU arur5'trlsruobelue {;;entnu
are s{ervrqled oml aql'oqu Jo uotlenll)p
aq1 sra66111 leqt' (E L /Z L9 palouap)
€ [g pue 719 suralord 9 )uotrtp]
eqt ern'{ervrqted 6u11eu6rs puoras
e salelnurls osle toloa)al aures eqf
'luelnurts
aql ot lsasol) 11at eqt 1o uorbar
aql ol palrurl sr uorsnrloJd os 'pa^ll-uoqs
are,{earqled srql u!q}!M sla6uasseu
puo)as'slJoMlau u!lte anrsnllold
;o quirol6 o1 '!9 uralold g fuaulu] aql
er^ 'speal {ealqred )eU aql Jo uollelnults
';eu6rs
leuat>eq aql Jo a)lnos oql ol esol)
'lla) aqr Jo luor, aqt lv 'uoltezueloo
s,lle)
aql ol alnquluor s{ervrqled 6ur;eubrs
l)urlsrp olvrl'tuorJ aql le puebr; leuol)eq
aq] o] punoq aq o1 l;a1r; arour ale
rnq 'lla) aqtJo a)eJrns aq] ro^o lle punoj
are slolda:er asaql'fi r;l1oru pat)altp
alplnurls sltqoollnau oql uo s.loloa)al
pa;dnor-ura1o.rd-9 ot se;ntaloul leus])eq
eqt;o 6urpurg's;rqdorlnau 6urpn;tur
'uels,{s
ounuult aql;o sller aql ,(q
ubrarol se pazru6orel aJe leql saln)alou
elarras {poq uetunq aql papelut oleLl
tei.lt euatreg'uollezgre;od ;;qdorlneu
6urrnp 6ur;eu6!5 ZO[-9t aln6!l

THE CYTOSKELETON AND CELL EEHAVIOR
(A)
t
localized
signal
Figure 1 6-1 03 The polarization of a cytotoxic T cell after target-cell
recognition. (A) Changes in the cytoskeleton of a cytotoxic T cell after it
has made contact with a target cell.The initial recognition event results in
signals that cause actin polymerization in both cells at the site of contact.
In theT cell, interactions between the actin-rich contact zone and
microtubules emanating from the centrosome result in reorientation of the
centrosome, so that the associated Golgi apparatus is directly apposed to
the target cell. (B) lmmuno-fluorescence micrograph in which both the
T cell (top)and its target cell (bottom) have been stained with an antibody
against microtubules. The centrosome and the microtubules radiating from
it in the T cell are oriented toward the point of cell-cell contact. In contrast,
the microtubule array in the target cell is not polarized. (B' from B. Geiger,
D. Rosen and G. Berke, J. Cett Biol.95137-143,1982. With permission from
The Rockefeller University Press.)
A similar cooperative feedback loop seems to operate in many other
instances of cell polarization. A particularly interesting example is the killing of
specific target cells byT lymphocytes. These cells are a critical component of the
vertebrate's adaptive immune response to infection by viruses. T cells, like neutrophils,
use actin-based motility to crawl through the body's tissue and find
infected target cells. \Mhen a T cell comes into contact with a virus-infected cell
and its receptors recognize foreign viral antigens on the surface of the target cell,
the same polarization machinery is engaged in a very different way to facilitate
kilting of the target cell. Rac is activated at the point of cell-cell contact and
causes actin polymerization at this site, creating a specialized region of the cortex.
This specialized site causes the centrosome to reorient, moving with its
microtubules to the zone of T-cell-target contact (Figure f6-f03). The microtubules,
in turn, position the Golgi apparatus right under the contact zone'
focusing the killing machinery onto the target cell. The mechanism of killing is
discussed in Chapter 25 (see Figure 25-47).
The Complex Morphological Specialization of Neurons Depends
on the Cytoskeleton
For our final case study of the ways that the intrinsic properties of the eucaryotic
cytoskeleton enable specific and enormously complicated large-scale cell behaviors,
we examine the neuron. Neurons begin life in the embryo as unremarkable
cells, which use actin-based motility to migrate to specific locations. Once there,
however, they send out a series of long specialized processes that will either
receive electrical signals (dendrites) or transmit electrical signals (axons) to their
target cells. The beautiful and elaborate branching morphology of axons
and dendri les neurons to form tremendously complex signaling networks,
interacting with many other cells simultaneously and making possible the
complicated and often unpredictable behavior of the higher animals. Both axons
and dendrites (collectively called. neurites) are filled with bundles of microtubules
that are critical to both their structure and their function.
In axons, all the microtubules are oriented in the same direction, with their
minus end pointing back toward the cell body and their plus end pointing forward
toward the axon terminals (Figure f 6-104). The microtubules do not reach
(8)
10 pm
1047

1048 Chapter 16:TheCytoskeleton
o vesicle with bound dynein
o vesicle with bound kinesin
r microtubule
(A) FIBRoBLAST (B) NEURoN
from the cell body all the way to the axon terminals; each is typically only a few
micrometers in length, but large numbers are staggered in an overlapping array.
This set of aligned microtubule tracks acts as a highway to transport many specific
proteins, protein-containing vesicles, and mRNAs to the axon terminals,
where synapses must be constructed and maintained. The longest axon in the
human body reaches from the base of the spinal cord to the foot, being up to a
meter in length.
Mitochondria, large numbers of specific proteins in transport vesicles, and
synaptic vesicle precursors make the long journey in the forward (anterograde)
direction. They are carried there by plus-end-directed kinesin-family moror proteins
that can move them a meter in as little as two or three days, which is a great
improvement over diffrrsion, which would take approximately several decades
to move a mitochondrion this distance. Many members of the kinesin superfamily
contribute to this anterograde axonal transport, most carrying sp"iific
subsets of membrane-enclosed organelles along the microtubuler. ih" g."ut
diversity of the kinesin family motor proteins used in axonal transport suggests
that they are involved in targeting their cargo to specific structures near the terminus
or along the way, as well as in cargo movement. old components from the
axon terminals are carried back to the cell body for degradation and recycling by
a retrograde axonal transport. This transport occurs along the same set of oriented
microtubules, but it relies on cytoplasmic dynein, t-tri.tr is a minus-enddirected
motor protein. Retrograde transport is also critical for communicating
the presence of growth and survival signals received by the nerve terminus back
to the nucleus, in order to influence gene expression.
one form of human peripheral neuropathy, charcot-Marie-Tooth disease, is
caused by a point mutation in a particular kinesin family member that transports
slmaptic vesicle precursors dovrrn the axon. other kinds of neurodegenerative
diseases such as Alzheimer's disease may also be caused in part by dlsruptions
in neuronal trafficking; as pointed out previously, the amyloid pr".r.r^o,
protein APP is part of a protein complex that serves as a receptor for kinesin-l
binding to other axonal transport vesicles.
Axonal structure depends on the axonal microtubules, as well as on the contributions
of the other two major cytoskeletal systems-actin filaments and
intermediate filaments. Actin filaments line the cortex of the axon, just beneath
the plasma membrane, and actin-based motor proteins such as myosin v are
also abundant in the rxon, presumably to help move materials. Neurofilaments,
the specialized intermediate filaments of nerve cells, provide the most important
structural support in the axon. A disruption in neurofilament structure, or
in the cross-linking proteins that attach the neurofilaments to the microtubules
and actin filaments distributed along the ixon, can result in axonal disorganization
and eventually axonal degeneration.
The construction ofthe elaborate branching architecture ofthe neuron during
embryonic development requires actin-based motility. As mentioned earlier,
the- tips of growing axons and dendrites extend by means of a growth cone, a specialized
motile structure rich in actin (Figure 16-105). Mosi neuronal growth
cones produce filopodia, and some make lamellipodia as well. The protrusion
Figure l6-104 Microtubule organization
in fibroblasts and neurons. (A) In a
fibroblast, microtubules emanate
outward from the centrosome in the
middle of the cell. Vesicles with Dlus-enddirected
kinesin attached move outward,
and vesicles with minus-end-directed
dynein attached move inward. (B) In a
neuron, microtubule organization is more
complex. In the axon, all microtubules
share the same polarity, with the plus
ends pointing outward toward the axon
terminus. No one microtubule stretches
the entire length ofthe axon; instead,
short overlapping segments of parallel
microtubules make the tracks for fast
axonal transport. In dendrites, the
microtubules are of mixed polarity, with
some plus ends pointing outward and
some pointing inward.

THE CYTOSKELETON AND CELL BEHAVIOR
tt(B)
10 pm 10 pm
and stabilization of growth-cone filopodia are exquisitely sensitive to environmental
cues. Some cells secrete soluble proteins such as netrin to attract or repel
growth cones. These modulate the structure and motility of the growth cone
cytoskeleton by altering the balance between Rac activity and Rho activity at the
leading edge (see Figure 15-62).In addition, there are fixed guidance markers
along the way, attached to the extracellular matrix or to the surfaces of cells.
\Mhen a filopodium encounters such a "guidepost" in its exploration, it quickly
forms adhesive contacts. It is thought that a myosin-dependent collapse of the
actin meshwork in the unstabilized part of the growth cone then causes the
developing ixon to turn toward the guidepost.
Thus, a complex combination of positive and negative signals, both soluble
and insoluble, accurately guide the growth cone to its final destination. Microtubules
then reinforce the directional decisions made by the actin-rich protrusive
structures at the leading edge of the growth cone. Microtubules from the
axonal parallel array just behind the growth cone are constantly growing into the
growth cone and shrinking back by dynamic instability. Adhesive guidance signals
are somehow relayed to the dynamic microtubule ends, so that microtubules
growing in the correct direction are stabilized against disassembly. In
this way, a microtubule-rich axon is left behind, marking the path that the
growth cone has traveled.
Dendrites are generally much shorter projections than axons' and they
receive synaptic inputs rather than being specialized for sending signals like
axons. The microtubules in dendrites all lie parallel to one another but their
polarities are mixed, with some pointing their plus ends toward the dendrite tip,
while others point back toward the cell body. Nevertheless, dendrites also form
as the result of growth-cone activity. Therefore, it is the growth cones at the tips
of axons and dendrites that create the intricate, highly individual morphology of
each mature neuronal cell (Figure f 6-f 06).
cell body
zlrm
axon (less than 1 mm to
more than 1 m in length)
dendrites receive
synaptic inputs
terminal branches of
axon make synapses on
target cells
104!l
Figure 1 6-1 05 Neuronal growth cones.
(A) Scanning electron micrograph
of two growth cones at the end of a
neurite, put out by a chick sympathetic
neuron in culture. Here, a previously single
growth cone has recently split into two'
Note the many filopodia and the large
lamellipodia. The taut appearance of the
neurite is due to tension generated by the
forward movement of the growth cones,
which are often the only firm points of
attachment of the axon to the substratum.
(B) Scanning electron micrograph of the
growth cone of a sensory neuron crawling
over the inner surface of the epidermis of a
Xenopus tadpole. (A, from D. Bray, in Cell
Behaviour [R. Bellairs, A. Curtis and
G. Dunn, eds.l. Cambridge, UK: Cambridge
University Press, 1982; B, from A. Roberts,
Brain Res. 1 1 8:526-530, 1 976. With
permission from Elsevier.)
Figure 1 6-106 The complex architecture
of a vertebrate neuron. The neuron
shown is from the retina of a monkey.
The arrows indicate the direction of travel
of the electrical signal along the axon.
The longest and largest neurons in the
human body extend for a distance of
about 1 m (1 million Um), from the base
of the spinal cord to the tip of the big
toe, and have an axon diameter of 15 pm.
(Adapted from B.B. Boycott, in Essays on
the Nervous System [R. Bellairs and
E.G. Gray, eds.l. Oxford, UK: Clarendon
Press, 1 974.)

1050 Chapter 16:The Cytoskeleton
(B) 20 40
time in mins
60 | lolr# 80
Although the neurons of the central nervous system are long-lived cells, they
are by no means static. synapses are constantly being created, strengthened,
weakened, and eliminated as the brain learns, evaluates, and forgets. uigh-r"rolution
imaging of the structure of neurons in the brains of adult mice has
revealed that neuronal morphology is undergoing constant rearrangement as
synapses are forged and broken (Figure l6-10z). These actin-dependent rearrangements
are rhought to be critical in learning and long-term memory. In this
way, the cltoskeleton provides the engine for construction of the entire nervous
system, as well as producing the supporting structures that strengthen, stabilize,
and maintain its parts.
Summary
Two distinct types of specialized structures in eucaryotic cells are formed from highty
ordered arrays of motor proteins that moue on stabilized filament tracks. The
system function in the adult animal-is another prime example of such complex,
coordinated cytoskeletal action. For a cell to crawl, it must generate and maintain an
ouerall structural polarity, which is influenced by external cues. In addition, the cell
must coordinate protrusion at the leading edge (by assembly of new actin filaments),
adhesion of the newly protruded part of the cell to the substratum, forces generated by
molecular motors to bring the cell body forward.
Complex cells, such as neurons, require the coordinated assembly of microtubules,
neurofilaments (neuronal intermediate filaments), and actin ftlaments, as well as the
actions of dozens of highly specialized molecular motors that transport subcellular
components to their appropriate destinations.
PROBLEMS
Which statements
are true? Explain why or why not.
16-1 The role of ATP hydrolysis in actin polymerization is
similar to the role of GTP hydrolysis in tubulin polyrneriza_
tion: both serve to weaken the bonds in the polymer and
thereby promote depolymerization.
Figure 16-107 Rapid changes in dendrite
structure within a living mouse brain.
(A) lmage of cortical neurons in a transgenic
mouse that has been engineered to express
green fluorescent protein in a small fraction
of its brain cells. Changes in these brain
neurons and their projections can be
followed for months using highly sensitive
fluorescence microscopy. To make this
possible, the mouse is subjected to an
operation that introduces a small
transparent window through its skull, and it
is anesthetized each time that an image is
recorded. (B) A single dendrite, imaged over
the period of 80 minutes, demonstrates that
dendrites are constantly sending out and
retracting tiny actin-dependent protrusions
to create the dendritic spines that receive
the vast majority of excitatory synapses
from axons in the brain. Those spines that
become stabilized and persist for months
are thought to be important for brain
function, and may be involved in long-term
memory. (Courtesy of Karel Svoboda.)
16-2 In most animal cells, minus end-directed micro_
tubule motors deliver their cargo to the periphery of the cell,
whereas plus end-directed microtubule motors deliver their
cargo to the interior ofthe cell.
16-3 Motor neurons trigger action potentials in muscle
cell membranes that open voltage-sensitive Ca2* channels
in T-tubules, allowing extracellular CaZ* to enter the cytosol,
bind to troponin C, and initiate rapid muscle contraction.

END.OF-CHAPTER PROBLEMS
Discuss the following problems.
16-4 At 1.4 mg/ml pure tubulin, microtubules grow at a
rate of about 2 pm/min. At this growth rate how many oBtubulin
dimers (B nm in length) are added to the ends of a
microtubule each second?
16*5 A solution of pure aB-tubulin dimers is thought to
nucleate microtubules by forming a linear protofilament
about seven dimers in length. At that point, the probabilities
that the next ap-dimer will bind laterally or to the end of the
protofilament are about equal. The critical event for microtubule
formation is thought to be the first lateral association
(Figure Qf6-f). How does lateral association promote the
subsequent rapid formation of a microtubule?
o -,
e
od
LINEAR GROWTH
I
I JaJ
o
od
o.cd
..ddd
'lj,t'l
oofo
dddd
dddd
.t.s,lJ
LATERAL ASSOCIATION
e
"tJ
e
.t
,f
Figure Q16-1 Model for microtubule nucleation by pure crB-tubulin
dimers (Problem 16 5).
16-6 How does a centrosome "know" when it has found
the center of the cell?
16-7 The concentration of actin in cells is 50-100 times
greater than the critical concentration observed for pure
actin in a test tube. How is this possible?'v\rhat prevents the
actin subunits in cells from polymerizing into filaments?
\A4ry is it advantageous to the cell to maintain such a large
pool of actin subunits?
16*8 The movements of single motor-protein molecules
can be analyzed directly. Using polarized laser light, it is possible
to create interference patterns that exert a centrally
directed force, ranging from zero aI the center to a few
piconewtons at the periphery (about 200 nm from the center).
Individual molecules that enter the interference pattern
are rapidly pushed to the center, allowing them to be
captured and moved at the experimenter's discretion.
Using such "optical tweezers," single kinesin molecules
can be positioned on a microtubule that is fixed to a coverslip.
Although a single kinesin molecule cannot be seen
optically, it can be tagged with a silica bead and tracked indirectly
by following the bead (Figure Qf 6-2,{). In the absence
of ATB the kinesin molecule remains at the center of the
interference pattern, but with AIP it moves toward the plus
end of the microtubule. As kinesin moves along the microtubule,
it encounters the force of the interference pattern,
which simulates the load kinesin carries during its actual
function in the cell. Moreover, the pressure against the silica
bead counters the effects of Brownian (thermal) motion, so
that the position of the bead more accurately reflects the
position of the kinesin molecule on the microtubule.
Traces of the movements of a kinesin molecule along a
microtubule are sho'nr,.n in Figure Q16-28.
(A) EXPERIMENTAL SETUP
Euo
a
Fqo
20
(B) POSITION OF KINESIN
1 051
microtubule
o z
.,.1 o".onlu
8
Figure Q16-2 Movement of kinesin along a microtubule (Problent
16 s). (A) Experimental setup with kinesin linked to a silica bead,
moving along a microtubule. (B) Position of kinesin (as visualized by
Dosition of silica bead) relative to center of interference pattern, as a
function of time of movement alonq the microtubule' The jagged
nature of the trace results from Brownian motion of the bead'
A. As shown in Figure Ql6-2B, all movement of kinesin is
in one direction (toward the plus end of the microtubule).
\A4rat supplies the free energy needed to ensure a unidirectional
movement along the microtubule?
B. \Altrat is the average rate of movement of kinesin along
the microtubule?
c. \A/hat is the length of each step that a kinesin takes as it
moves along a microtubule?
D. From other studies it is knor,rm that kinesin has two
globular domains that each can bind to B-tubulin, and that
kinesin moves along a single protofilament in a microtubule.
In each protofilament the B-tubulin subunit repeats
at B-nm intervals. Given the step length and the interval
between B-tubulin subunits, how do you suppose a kinesin
molecule moves along a microtubule?
E. Is there anything in the data in Figure Ql6-28 that tells
you how many AIP molecules are hydrolyzed per step?
16-9 How is the unidirectional motion of a lamellipodium
maintained?
1 6* 1 0 Detailed measurements of sarcomere length and tension
during isometric contraction in striated muscle provided
crucial early support for the sliding filament model of
muscle contraction. Based on your understanding of the
sliding filament model and the structure of a sarcomere,
propose a molecular explanation for the relationship of tension
to sarcomere length in the portions of Figure Qf 6-3
marked L II, III, and IV (In this muscle, the length of the
myosin filament is 1.6 pm and the lengths of the actin thin
filaments that project from the Z discs are 1.0 pm.)
E l
E i 100
o
E
b '-
-50
c
az)
q
0
23
sarcomere length (Pm)
Figure Ql6-3 Tension as a
function of sarcomere length
during isometric contraction
(Problem 16-10).

1052 Chapter 16:The Cytoskeleton
REFERENCES
General
Bray D (2001) Cell Movements: From Molecules to Motility, 2nd ed
New York: Garland Science
Howard .J (2001) Mechanics of Motor proterns and the Cytoskeleton
Sunderland, MA: Sinauer
The Self-Assembly and Dynamic Structure of Cytoskeletal
Filaments
Dogterom M & Yurke B (1997) Measurement of the force-velocity
relation for growing microtubuies Science 218856 860
Garner EC, Campbell CS & Mullins RD (2004) Dynamic instability in a
DNA-segregating prokaryotic actin homolog Science 306.1021 1O)5
Helfand BT, Chang L & Goidman RD (2003) The dynamic and motile
properties of intermediate filaments Annu Rev Cell Dev Biol
19.445,467
Hill TL & Kirschner MW (1982) Bioenergetics and kinetics of microtubule
and actin filament assembly-disassembly lnt Rev Cytol 7B:1 125
Hotani H & Horio T ('l 9BB) Dynamics of microtubules visualized by
darkfield microscopy: treadmilling and dynamic instability Cell Motil
Cytoskeleton 1 0:229-236
Jones LJ, Carballido-Lopez R & Errington J (2001) Conrrol ofcell shape
in bacteria: helical, actin like filaments in Bacillus subtilis Cell
104913-922
Luby Phelps K (2000) Cytoarchitecture and physical properties of
cytoplasm: volume, viscosity, diffusion, intracellular surface area /nt
Rev Cytol 192189-221
Mitchison T & Kirschner M (1984) Dynamic instability of microtubule
growth A/rture 312:237 242
Mitchison I I (1 995) Evolution of a dynamic cytoskeleton philos Trans R
Soc Lond B Biol Sci 349:299-304
Mukherjee A & Lutkenhaus J (1 994) Guanine nucleotide oepenoent
assembly of FtsZ into filaments J Bacteriol 116:2754 2/58
Oosawa F & Asakura S (1975) Thermodynamics of the polymerization of
protein New York: Academic press, pp 41 55, pp 9O_1OB
Pauling L (1953) Aggregation of Globular proteins Discuss Faraday Soc
13.170-176
Rodionov Vl & Bor sy GG (,1997) Microtubule treadmilling ln vlvo
Science 2/5.215-2lB
Shih Yt & Rothfreld t (2006) The bacterial cytoskeleton l,licrobiol Mol
Biol Rev 7a.729-/54
Theriot lA (2000) The polymerization mator Traffic 1:lg 28
How Cells Regulate Their Cytoskeletal Filaments
Aldaz H, Rice LM, Stearns T & Agard DA (2005) Insighrs into
microtubul nucleation from the crystal structure of hurnan gamma_
tubulin. A/d re 435523 52/
Bretscher A, Chambers D, Nguyen R & Reczek D (2000) ERM-Merlin and
EBP50 protein families in plasma membrane organization and
function Annu Rev Cell Dev Biol i6.113 143
Doxsey S, McCollum D & Theurkauf W (2005) Centrosomes in cellular
regulation Annu Rev Cell Dev Biol 21:411-434
Garcia ML & Cleveland DW (2OOl) Going new places using an old MAp:
tau, microtubules and human neurodegenerative disease Curr apin
Cell Biol 13:41-48.
Holy TE, Dogterom M, Yurke B & Leibler S (1997) Assembry anc
positioning of microtubule asters in microfabricated chambers proc
Narl Acad Sci USA 94:6228-6231
Mullins RD, Heuser JA & Pollard TD (1998) The interaction of Arp2/3
usA 95.618t 6186
Robinson RC,Turbedsky K, Kaiser DA et al (2001) Crystal structure of
Arp2l3 complex Science 294:1 67 9-1 684
Quarmby L (2000) Cellular Samurai: katanin and the severing of
microtubules .J Cell Sci 113:2821-2827
Stearns T & Kirschner M (1994) /n /ltro reconstitution of centrosome
assembly and function: the central role of gamma-tubulin Cel/
76:623-637
Wiese C & Zheng Y (2006) Microtubule nucleation: gamma tubulin and
beyond J Cel/ 5ct 1 19.41 43-41 53
Zheng Y, Wong ML, Alberts B & Mitchison T (1995) Nucleation of
microtubuie assembly by a gamma-tubulin-containing ring
complex N atu re 37 8.57 8-583
Zigmond 5H (2004) Formin-induced nucleation of actin filaments Curr
Opin Cell Biol 16.99-105
Molecular Motors
Burgess SA, Walker ML, Sakakibara H et al (2003) Dynein structure and
power stroke Nature 421:715 718
Hirokawa N (1998) Kinesin and dynein superfamily proteins and the
mechanism of organelle transport Science 219.519 526
Howard J, Hudspeth AJ & Vale RD ('1989) Movement of microtubules by
single kinesin molecules Nature 3421 54-1 58
Howard J (1997) Molecular motors: structural adaptations to cellular
functions, Nature 389:561 567
Rayment l, Rypniewski WR, Schmidt Base K et al (1993) Threedimensional
structure of myosin subfragment-1: a molecular motor
Science261:50 58
Reck-Peterson SL, Yildiz A, Carrer AP et al (2006) Single-molecule
analysis of dynein processivity and stepping behavior Cel/
126:335 348
Rice S, Lin AW, Safer D et al (1999) A structural change in the kinesin
motor protein that drives motility /Vdfure 402:178-784
Richards TA & Cavalier Smith T (2005) Myosin domain evolution and
the primary divergence of eukaryotes Nature 436:11 13-1 j j B
Svoboda K, Schmidt CF, Schnapp BJ & Block SM (1 993) Drrect
observation of kinesin stepping by optical trapping interferometry
Nature 365,721 727
Vikstrom KL & Leinwand LA (,l996) Contractile protein mutations and
heart disease Curr Opin Cell Biol B:97-105
Wells AL, Lin AW, Chen tQ et al (1999) Myosin Vl is an actin-based
motor that moves backwards lVorure 401:505-5OB
Yildiz A, Forkey JN, McKinney SA et al (2003) Myosin V walks hand overhand:
single fluorophore imaging with 1 5-nm localization sclence
300.2061-2065
Yildiz A & Selvin PR (2005) Kinesin: walking, crawling or sliding along?
Trends Cell Biol 15:112-120
The Cytoskeleton and Cell Behavior
Abercrombie M (1980) The crawling movement of metazoan cells proc
Roy Soc B 207.129-147
Cooke R (2004) The sliding filament model.1972-2004 1Gen physiol
123.643-656
Dent EW & Gertler FB (2003) Cytoskeletal dynamics and transport in
growth cone motility and axon guidance Neuron 40:209 227
Lauffenburger DA & Horwitz AF (1996) Cell migration: a phystcally
integrated molecular process Cel/ 84:359-369
Lo CM, Wang HB, Dembo M & Wang YL (2000) Cell movement rs
guided by the rigidity of the substrate Biophys J 79:144 152
Madden K & Snyder M (1998) Cell polarity and morphogenesis in
budding yea:r Annu Rev Microbiol 52:687.744
Ridley AJ, Schwartz MA, Burridge K et al (2003) Cell migration:
integrating signals from front to back Science 302J104-1709
Rafelski SM & Theriot .JA (2004) Crawling roward a unified model of cell
motility: spatial and temporal regulation of actin dynamics Annu Rev
BiochemT3:209 239
Parent CA & Devreotes PN (1 999) A cell's sense of direction Sclence
284:765-770
Pollard TD & Borisy cc (2003) Cellular motility driven by assembly and
disassembly of actin filamenrs Cell i12:453 465
Purcell EM (1977) ttfe at low Reynolds' number Am J phys
45:3-11
Wittmann I Hyman A & Desai A (200,l) The spindle: a dynamic
assembly of microtubules and motors Nature Cell Biol 3:E2B-E34