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Mastoparan, which has been shown to active G piotems [I 1. mhlblts the ADP-rlbosylatlon of 20 kDa human platelet membrane protems catalyzed by Closrrrdww botu/nzurn exoenzyme C3 half-maximally and maximally (90%) at 20 and 100 yM concentrations. respectively Inhlbltlon of ADP-rlbosylatlon war enhanced by GYP+ Mastopardn mcreascd GTP hydrolysis by porcine brain rho protein and stimulated GTP binding m a concentration dependent manner The data suggest that mastopdrdn not only interacts wnh heterotltmerlc G protems but also with low molecular mdss GTP-bmdmg protems of the rholrac Famdy Mastopdran, Small GTP-bmdmg proteins, rho, rat, Closrrrdwrn botrt/orum exoenzyme C3. ADP-rlbosylatlon

ADP-ribosylation of recombinant rhoA and rhoB proteins by Closrrirliunr ljorulinurrt C3 cxoenzyme increased steady-state GTP hydrolysis by 50 to 80%. ADP-ribosylation and increase in GTP hydrolysis occurred at similar concentrations ofC3, depended on the presence of NAD and were prevented by anti-C3 antibody or heut inactivation ofC3. In contrast, GTP hydrolysis by Ile-41 rhoA or kkkrds. which are no substrates for the transfcrasc, were no1 affected by C3. ADP-ribosylation facilitated the ["H]CiDP release and subsequently, the binding of ['H]GTP to rhoA. The data indicate that the increase in the steady-state GTPasc activity by ADP-ribosylation is caused by increasing the rate of GDP release which is suggested to be the rate limiting step of the GTPase cycle of the small GTP-binding proteins.

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