tertiary structure of proteins involved in the bacillus subtilis cell ...

16 TERTIARY STRUCTURE OF PROTEINS...
role in ac ti va tion of the first cell-type spe cific fac tor - F
(Fig. 1B) [24, 25]. The crys tal struc ture of phosphorylated
and unphosphorylated form of SpoIIAA was solved from
Ba cil lus sphaericus [26]. This sin gle do main glob u lar pro -
tein con sists of a cen tral sheet of five strands, and four
he li ces (Fig. 2E). On one face, the -pleated sheet is em -
bed ded with a pair of -he li ces, while the other face is ex -
posed to the sol vent. The phosphorylation site,
de pho sphorylation of which is im por tant for ac ti va tion of
F , is lo cated at the N-ter mi nus of the he lix 2 so that the
pho spho ryl group points into the sol vent. Com par i sons be -
tween unphosphorylated and phosphorylated SpoIIAA had
shown, that struc tural changes ac com pa ny ing phospho -
rylation are only slight. Ex tended hy dro pho bic sur faces to -
gether with flex i bil ity in he lix 3 and the fol low ing loop
in di cate that this pro tein of ten un der goes a dis or der to or -
der tran si tion and there fore has a dis po si tion for form ing
sta ble pro tein-pro tein in ter ac tions [26]. Also the struc ture
of the anti- fac tor SpoIIAB in the com plex with F of Ba -
cil lus stearothermophilus was re solved [27]. SpoIIAB - F
com plex con sists of a SpoIIAB dimer bound to one F mol -
e cule so that F bind ing di rectly to the dimer in ter face
makes con tact with both SpoIIAB mono mers (Fig. 2F).
The whole struc ture of F is com prised of three com pactly
folded do mains con nected by flex i ble link ers [27]. F is
kept in ac tive in a com plex with SpoIIAB. In the pres ence
of unphosphorylated SpoIIAA mol e cules, SpoIIAB binds
to SpoIIAA and ac tive F is re leased. How ever, SpoIIAB
kinase phos phory lates SpoIIAA, and the SpoIIAB-
SpoIIAA com plex de cays, re leas ing so the anti- fac tor
ready to block an other F . SpoIIAB has the ATP bind ing
Bergerat fold [28], found in histidine kin ases and ATP-ases
of the GHKL superfamily. This con sists of an / - sand -
wich with a four - stranded antiparallel - sheet and three a
he li ces. In the SpoIIAB mono mer 1 ex tends the four -
stranded sheet. Dimerization interactions occur among
1 and 1 of each SpoIIAB mono mers. The ATP - bind ing
site is rep re sented by Asn50, which plays a key role in che -
lating Mg 2+ , crit i cal for ATP-ase or kinase ac tiv ity of the
GHKL superfamily mem bers [27]. The cat a lytic site is
formed by Glu46, which func tions as a cat a lytic base in the
kinase re ac tion [29]. SpoIIAA phosphorylated by SpoIIAB
re quires the phosphatase ac tiv ity of SpoIIE to be come ac -
tive again.
After activation of F , which fol lows only af ter com ple -
tion of the sporulation sep tum, other com part ment spe cific
fac tors are se quen tially ac ti vated. This leads to dif fer en -
ti a tion of gene ex pres sion. In the course of the sporulation
pro cess, the mother cell en gulfs the prespore and the pro -
cess of sporulation cul mi nates with pro grammed death of
the mother cell. The sur vi vor is the forespore, which ma -
tures into a very re sis tant spore able to out live hos tile en vi -
ron ments. The life cy cle is com pleted by the pro cess of
ger mi na tion, which is ini ti ated when the free spore re turns
to con di tions fa vor able for life.
Future perspectives
Es pe cially in ter est ing for our un der stand ing of this mech a -
nism of the gene ex pres sion asym me try would be the de -
tailed study of sporulation septa for ma tion by solv ing the
crys tal struc ture of key pro teins of this cell di vi sion pro -
cess. Al though the asym met ric cell di vi sion that oc curs
dur ing sporulation dif fers from the cell di vi sion dur ing
veg e ta tive growth, both pro cesses use fun da men tally the
same pro tein ma chin ery [30, 31, 32]. SpoIIE pro tein is one
of the most in ter est ing can di dates for crys tal lo graphic stud -
ies. This large (92 kDa) pro tein con sists of three do mains:
N-ter mi nal re gion con tain ing 10 mem brane-span ning he li -
ces [33, 34] fol lowed by cen tral do main, pos si bly in volved
in intermolecular in ter ac tion [22], and the C-ter mi nal do -
main, which is spe cific pro tein phosphatase in volved in ac -
ti va tion of the first forespore sigma fac tor [25]. An other
very in ter est ing and highly stud ied sporulation spe cific
pro tein from B. subtilis is DNA translocase SpoIIIE [35,
36]. Re cently it has been shown that this pro tein also co op -
er ates in mem brane fu sion dur ing the spore en gulf ment
[37]. SpoIIIE pro tein con sists of a N-ter mi nal mem brane
bound do main me di at ing its lo cal iza tion to the di vi sion
sep tum and cytosolic C-ter mi nal do main ca pa ble of track -
ing along DNA in the pres ence of ATP.
DivIVA pro tein is an im por tant reg u la tor of cell di vi -
sion dur ing veg e ta tive growth [38] and it also has been pro -
posed to have a cru cial role in the sporulation pro cess by
an chor ing the chro mo somes to the cell poles prior to asym -
met ric di vi sion [39]. It was shown that in its na tive state
DivIVA oligomerizes [40], and has a struc tural sim i lar ity
to my o sin and other pro teins hav ing -helical coiled-coil
struc ture [41].
This re view is an at tempt to sum ma rize what is known
from crys tal lo graphic stud ies about the struc ture of some
of the ap prox i mately 200 sporulation spe cific pro teins of B.
subtilis. The struc tures of only a small num ber of these pro -
teins are known due to prob lems as so ci ated with the crys -
tal li za tion of the pro teins. Struc tures for many of the
interesting candidates remain a challenge. Among them are
the mem brane bound pro teins and pro teins with very flex i -
ble do mains.
Acknowledgements
The work in IB's lab o ra tory is sup ported by grant
2/1004/21 from the Slo vak Acad emy of Sci ences and
Wellcome Trust Grant 066732/Z/01/Z.
Krystalografická spoleènost