Download (26Mb) - OAR@ICRISAT

have been exhaustively reviewd (Dai and Gill, 1993; Douek ct al, 1992; Wirth et al, 1997;
Guerchicoff et al, 1997; Purcell and Ellar, 1997). Two accessory proteins ~iarncly PI9 and
P20 that arc requ~red ill tile usse~nbly ol'a~i i~~clus~o~i body linvc also been cliaracter~zsd
(Wu and Federici, 1993; Tlliery et nl, 1997; M;~~i;isiierob et al. 1997).
Mode oJuctio~~: Early studlcs showed that the prunary target of Bti toxicity is tlie
niidgut epithelium, where e~izynialic systems tra~isfornis the protoxin tiito an active toxin
under alkaline coi~ditions [Al-yahyaee 2nd ElI;ir, 1995) Tliese toxins act coordinately and
synergistically to distupt tlie epithelial cells of tlie larval gut where niidgut cells vacuolisc
and lyse (Lahkim-Tsror et al., 1983). Tliese sym)~tonis are more or less snnie for toxins of'
all Bt strallis other than Bti. Though Cry and Cyt toxi~is are structurally dissi~n~lar, tiley
have the similal. ti~embrane-pem~eatirlg ab~lity, where Cry toxi~is bind to nicmbranal
protcilis and B~Cvtiilu binds to ulisatt~ralcd pliospholip~ds acting as "bind~ng sites"
(Feldriiann et ai., 1995; Gill ct al., 1992).
Mode of action takes place in two steps; binding to a cell reccplor and subseque~it
pore formation (Knowlcs and Ellar, 1987). Soon after conversio~i of proto-toxin into active
toxin by gul proteases, the toxin is distiliguished illto two doiliains (Donlain I and Donio~n
11) wherc Doiiia~~i I1 bt~tds to a brusli border membrane receptor, acts as an anchor, while
Domain I inserts ~tsclf illto the ~iiembra~ie for~ning ;I pore (Deal1 et al., 1996; Flores et al.,
1997). Binding of the toxin beco~iies irreversible alier llie i~isertion of Donlain 1 illto
mernbra~ie (Che~i et al., 1995). ~4 and =5 liel~ces 1nscl.t into the membrane and a7 serves
as sensor to initiate tlie structural rearrangelllent of the inserting dornaln (Gut and Shai,
1998). Many otlier fi~~ictio~is of seglnents of the toxin have been elucidated. It was found
tliat substitutio~i of glutamine at 149 by prohne in the center of helix 4 results in co~ilplcte