THE RHO-GAP ENCODED BY BEM2 REGULATES CYTOSKELETAL STRUCTURE IN BUDDING YEAST

Microfilaments are required for polarized growth and morphogenesis in Saccharomyces cerevisine. To accomplish this, actin cables and patches are redistributed during the cell cycle to direct secretory components to appropriate sites for cell growth. A major component of actin cables is tropomyosin I, encoded by TPM1, that determines or stabilizes these structures. Disruption of TPM1 is not lethal but results in the loss of actin cables and confers a partial defect in polarized secretion. Using a synthetic lethal screen, we have identified seven mutations residing in six genes whose products are required in the absence of Tpm1p. Each mutant exhibited a morphological defect, suggesting a functional link to the actin cytoskeleton. Complementation cloning of one mutation revealed that it lies in BEM2, which encodes a GTPase-activating protein for the RHO1 product. bem2 mutations also show synthetic lethality with rho1 and mutations in certain other cytoskeletal genes (ACT1, MYO1, MYO2, and SAC6) but not with mutations in several noncytoskeletal genes. These data therefore provide a genetic link between the GAP encoded by BEM2 and the functional organization of microfilaments. In addition, we show that bem2 mutations confer benomyl sensitivity and have abnormal microtubule arrays, suggesting that the BEM2 product may also be involved directly or indirectly in regulating microtubule function.