Functions and Functional Domains of the GTPase Cdc42p

Cdc42p, a Rho family GTPase of the Ras superfamily, is a key regulator of cell polarity and morphogenesis in eukaryotes. Using 37 site-directed cdc42 mutants, we explored the functions and interactions of Cdc42p in the budding yeast Saccharomyces cerevisiae. Cytological and genetic analyses of these cdc42 mutants revealed novel and diverse phenotypes, showing that Cdc42p possesses at least two distinct essential functions and acts as a nodal point of cell polarity regulation in vivo. In addition, mapping the functional data for each cdc42 mutation onto a structural model of the protein revealed as functionally important a surface of Cdc42p that is distinct from the canonical protein-interacting domains (switch I, switch II, and the C terminus) identified previously in members of the Ras superfamily. This region overlaps with a region (α5-helix) recently predicted by structural models to be a specificity determinant for Cdc42p-protein interactions.