The exchange factor Cdc24 is required for cell fusion during yeast mating.

During Saccharomyces cerevisiae mating, chemotropic growth and cell fusion are critical for zygote formation. Cdc24p, the guanine nucleotide exchange factor for the Cdc42 G protein, is necessary for oriented growth along a pheromone gradient during mating. To understand the functions of this critical Cdc42p activator, we identified additional cdc24 mating mutants. Two mating-specific mutants, the cdc24-m5 and cdc24-m6 mutants, each were isolated with a mutated residue in the conserved catalytic domain. The cdc24-m6 mutant responds normally to pheromone and orients its growth towards a mating partner yet accumulates prezygotes during mating. cdc24-m6 prezygotes have two apposed intact cell walls and do not correctly localize proteins required for cell fusion, despite normal exocytosis. Our results indicate that the exchange factor Cdc24p is necessary for maintaining or restricting specific proteins required for cell fusion to the cell contact region during mating.