GTPase-Activating Proteins for Cdc42

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Abstract
The Rho-type GTPase, Cdc42, has been implicated in a variety of functions in the yeast life cycle, including septin organization for cytokinesis, pheromone response, and haploid invasive growth. A group of proteins called GTPase-activating proteins (GAPs) catalyze the hydrolysis of GTP to GDP, thereby inactivating Cdc42. At the time this study began, there was one known GAP, Bem3, and one putative GAP, Rga1, for Cdc42. We identified another putative GAP for Cdc42 and named it Rga2 (Rho GTPase-activating protein 2). We confirmed by genetic and biochemical criteria that Rga1, Rga2, and Bem3 act as GAPs for Cdc42. A detailed characterization of Rga1, Rga2, and Bem3 suggested that they regulate different subsets of Cdc42 function. In particular, deletion of the individual GAPs conferred different phenotypes. For example, deletion of RGA1, but not RGA2 or BEM3, caused hyperinvasive growth. Furthermore, overproduction or loss of Rga1 and Rga2, but not Bem3, affected the two-hybrid interaction of Cdc42 with Ste20, a p21-activated kinase (PAK) kinase required for haploid invasive growth. These results suggest Rga1, and possibly Rga2, facilitate the interaction of Cdc42 with Ste20 to mediate signaling in the haploid invasive growth pathway. Deletion of BEM3 resulted in cells with severe morphological defects not observed in rga1Δ or rga2Δ strains. These data suggest that Bem3 and, to a lesser extent, Rga1 and Rga2 facilitate the role of Cdc42 in septin organization. Thus, it appears that the GAPs play a role in modulating specific aspects of Cdc42 function. Alternatively, the different phenotypes could reflect quantitative rather than qualitative differences in GAP activity in the mutant strains.