Genetic interactors of the Cdc42 GTPase effectors Gic1 and Gic2: their identification and functions in budding yeast cell polarity
Gic1 and Gic2 are structurally and functionally related effectors of the evolutionarily conserved Cdc42 GTPase in Saccharomyces cerevisiae. Like many other effectors of Cdc42, Gic1 and Gic2 function in the process of polarized cell growth. In the absence of both Gic1 and Gic2, yeast cells exhibit depolarized actin cytoskeleton and polarized growth defects at elevated temperatures. To obtain further insight into the biological role of Gic1 and Gic2, genetic approaches were used to identify functionally interacting partners of these proteins. A screen for multi-copy suppressors of the temperature-sensitivity of gic1 gic2 cells identified many genes (including AXL2, BNI1, CLN2, MSB1, MSB2, RSR1 and STE20) that have known roles in polarized cell growth. In addition, two pairs of structurally related genes - VHS2 and MLF3, MGC1 and TOS2 - with no previously reported functions were also identified. Functional characterization of VHS2 and MLF3 revealed their role in a pathway that affects the actin cytoskeleton organization and cell wall integrity. This pathway is functionally redundant to that mediated by GIC1 and GIC2. Functional characterization of MGC1 and TOS2 indicated that these genes function in the process of polarized growth, particularly in the process of cytokinesis. A genome-wide Synthetic Genetic Analysis identified more than 30 nonessential genes as those whose function overlaps with that of GIC1 and GIC2. Mutation in each of these genes exacerbates the growth defect of gic1 gic2 cells. As expected, some of these genes are involved in polarity-related functions, such as actin cytoskeleton organization, bud-site selection and cell wall biosynthesis. Others participate in a variety of biological processes, including organelle biogenesis, secretion and vesicular transport. The latter finding suggests that GIC1 and GIC2 may have function outside the scope of actin cytoskeleton organization. Taken together, the work presented here has uncovered the function of four previously uncharacterized genes in polarized cell growth. It has also provided hints to additional potential functions of GIC1 and GIC2. Further exploration of these functions might provide important links between Cdc42 signaling and cellular processes such as organelle biogenesis, secretion and vesicular transport, all of which need to be executed coordinately during polarized cell growth.