A synthetic lethal screen identifies a role for the cortical actin patch/endocytosis complex in the response to nutrient deprivation in Saccharomyces cerevisiae

Saccharomyces cerevisiae whi2 cells are unable to halt cell division in response to nutrient limitation and are sensitive to a wide variety of stresses. A synthetic lethal screen resulted in the isolation of siw mutants that had a phenotype similar to that of whi2. Among these were mutations affecting SIW14, FEN2, SLT2, and THR4. Fluid-phase endocytosis is severely reduced or abolished in whi2, siw14, fen2, and thr4mutants. Furthermore, whi2 and siw14 mutants produce large actin clumps in stationary phase similar to those seen in prk1 ark1 mutants defective in protein kinases that regulate the actin cytoskeleton. Overexpression of SIW14 in a prk1 strain resulted in a loss of cortical actin patches and cables and was lethal. Overexpression of SIW14 also rescued the caffeine sensitivity of the slt2 mutant isolated in the screen, but this was not due to alteration of the phosphorylation state of Slt2. These observations suggest that endocytosis and the organization of the actin cytoskeleton are required for the proper response to nutrient limitation. This hypothesis is supported by the observation that rvs161, sla1, sla2, vrp1, ypt51, ypt52, and end3 mutations, which disrupt the organization of the actin cytoskeleton and/or reduce endocytosis, have a phenotype similar to that of whi2 mutants.