Regulation of the Cdc42 GTPase module during cell fusion in S. cerevisiae.
Sophie Barale (1), Derek McCusker (2), Robert A. Arkowitz (1)
(1) Inst. Sig., Dev. Bio, & Cancer, CNRS UMR 6543, Parc Valrose, Nice, 06108, France; (2) Dept of Biology, Sinsheimer Laboratories, University of California at Santa Cruz, Santa Cruz, California 95064, USA
During yeast mating, chemotropic growth and cell fusion are critical for zygote formation. Both the guanine nucleotide exchange factor Cdc24p and the G-protein Cdc42p are involved in the mating process. In particular, Cdc24p is necessary for oriented growth along a pheromone gradient. To further investigate functions of this critical GTPase module during mating we have identified and characterized additional cdc24 and cdc42 mating mutants. Two mating specific cdc24 mutants, ( cdc24-m5 and cdc24-m6 ) were isolated each with a conserved residue in the catalytic domain that was altered. In addition, a mating specific cdc42 mutant was isolated in which a conserved residue in effector loop 1 was mutated. These mutants respond normally to pheromone, yet accumulate prezygotes during mating. At a molecular level, cdc24-m6 mutants are defective in maintaining or restricting specific proteins required for cell fusion to the cell contact region during mating. Genetic analyses suggest that cycling of Cdc42 between GDP and GTP states is critical for efficient cell fusion. We are currently examining the levels of activated Cdc42 during the mating process in wild-type cells and cdc24 and cdc42 mutants.