An investigation of Cdc42 and its effects on the Saccharomyces cerevisiae actin cytoskeleton

The development of polarity in yeast cells as they bud has become a paradigm for how cell polarity may be established. This single cell organism shares many of the same mechanisms for establishing polarity as multicellular organisms. Cdc42, a highly conserved Rho-like GTPase, has been implicated as an effector of the actin cytoskeleton in many organisms. In yeast Cdc42 is essential for the continuing rearrangements of the actin cytoskeleton through the cell cycle. This thesis will present evidence that Gic1, a downstream Cdc42 effector, affects cytokinesis; that Ynl240c/Foe1, an essential protein, interacts with Cdc42; and that Cdc42 functions in intracellular trafficking.; The homologous Cdc42 effectors Gic1 and Gic2 have been implicated as important in polarity establishment. Gic2 and perhaps Gic1 serve as molecular links between GTPCdc42 and several actin interacting proteins to contribute to the establishment of the polarity required for bud emergence. This thesis will provide evidence that Gic1 has an additional function as a molecular link between Cdc42 and cytokinesis. Several lines of evidence show that Gic1 interacts with Iqg1, a protein essential for cytokinesis. Genetic data suggests that Gic1 plays a previously uncharacterized role in cytokinesis.; Evidence that a novel protein Ynl240c/Foe1 interacts with Cdc42 and may play a role in actin polarity through Sla2, an actin interacting protein is provided in this thesis. Foe1 is an essential protein that has homology to anaerobic Fe-only hydrogenases and to several uncharacterized proteins in other organisms. Conditional alleles of Foe1 at restricted conditions exhibit a phenotype of large round cells with unpolarized actin, similar to the phenotype of arrested cdc42-1 cells. Biochemical and genetic evidence indicate that Foe1 interacts with Cdc42 and Sla2, a characterized actin effector, to direct actin dynamics.; Cdc42 may be involved in intracellular transport independently of its roles in the actin cytoskeleton. Cdc42 interacts with Sec21, a subunit of the COPI/coatomer complex that is essential for ER to Golgi retrograde transport. An allele of Cdc42 that specifically disrupts this interaction confers viability to cdc42 null mutants, with no visible affects on the actin cytoskeleton. However, α-factor secretion, which is dependent on Sec21 function, is disrupted.