ASSOCIATIONS AMONG PH AND SH3 DOMAIN-CONTAINING PROTEINS AND RHO-TYPEGTPASES IN YEAST

The src homology region 3 (SH3) domain-bearing protein Bem1p and the R ho-type GTPase Cdc42p are important for bud emergence in Saccharomyces cerevisiae. Here, we present evidence that through its second SH3 dom ain, Bem1p binds to the structurally and functionally similar proteins Boi1p and Boi2p, each of which contain an SH3 and a pleckstrin homolo gy (PH) domain. Deletion of BOI1 and BOI2 together leads to impaired m orphogenesis and poor viability, A PH domain-bearing segment of Boi1p that lacks the Bem1p-binding site is necessary and sufficient for func tion. This segment of Boi1p displays a two-hybrid interaction with Cdc 42p, suggesting that Boi1p either binds directly to or is part of a la rger complex that contains Cdc42p. Consistent with these possibilities , overexpression of Boi1p inhibits bud emergence, but this inhibition is counteracted by cooverexpression of Cdc42p. Increased expression of the Rho-type GTPase Rho3p, which is implicated in bud growth, suppres ses the growth defects of boil boi2 mutants, suggesting that Boi1p and Boi2p may also play roles in the activation or function of Rho3p. The se findings provide an example of a tight coupling in function between PH domain-bearing proteins and both Rho-type GTPases and SH3 domain-c ontaining proteins, and they raise the possibility that Boi1p and Boi2 p play a role in linking the actions of Cdc42p and Rho3p.