DESCRIPTION (Verbatim from the applicant's abstract): S. cerevisiae is an excellent organism for studies of cell polarity development due to its powerful genetics, its simplicity, the ability to observe polarity development synchronously in a cell population, and existence of numerous mutations and molecular markers for cell polarity. Studies will determine how Cdc42 GTPase controls actin organization, and how actin mediates polarized endocytosis. Thirty-seven site-directed cdc42 mutants will be characterized biochemically in permeabilized yeast and in actin-assembly competent yeast extracts to relate biochemical activities to in vivo functions, and used in genetic screens to identify components of downstream pathways. Roles for Cdc42 effector Pak kinases in morphogenesis will be elucidated by genetic and chemical-genetic strategies. Special cdc42 alleles defective in pseudohyphal growth will be studied to provide a deeper understanding of the molecular basis for fungal dimorphism, a characteristic of many fungal pathogens. Finally, how a complex set of interacting proteins links endocytic membranes to the Arp2/3 complex, and how these proteins are regulated by novel Ark kinases discovered in this laboratory, will be investigated.