A GTP-BINDING PROTEIN REGULATES THE ACTIVITY OF (1-]3)-BETA-GLUCAN SYNTHASE, AN ENZYME DIRECTLY INVOLVED IN YEAST-CELL WALL MORPHOGENESIS

Synthesis of (1-->3)-beta-D-glucan, the major structural component of the yeast cell wall, is synchronized with the budding cycle. Membrane- bound, GTP-stimulated (1-->3)-beta-glucan synthase was dissociated by stepwise treatment with salt and detergents into two soluble fractions , A and B, both required for activity. Fraction A was purified about 8 00-fold by chromatography on Mono and and Sephacryl S-300 columns. Dur ing purification, GTP binding to protein correlated with synthase comp lementing activity. A 20-kDa GTP-binding protein was identified by pho tolabeling in the purified preparation. This preparation no longer req uired GTP for activity, but incubation with another fraction from the Mono Q column (A1) led to hydrolysis of bound GTP to GDP with a concom itant return of the GTP requirement. Thus, fraction A1 appears to cont ain a GTPase-activating protein. These results show that the GTP-bindi ng protein not only regulates glucan synthase activity but can be regu lated in turn, constituting a potential link between cell cycle contro ls and wall morphogenesis.