GENETIC-EVIDENCE FOR A FUNCTIONAL INTERACTION BETWEEN SACCHAROMYCES-CEREVISIAE CDC24 AND CDC42

Cdc24p and Cdc42p are involved in the control of cell polarity during the Saccharomyces cerevisiae cell cycle. Cdc42p is a member of the Ras superfamily of GTPases and Cdc24p displays limited amino-acid sequenc e similarity with the Dbl proto-oncoprotein, which acts to stimulate g uanine-nucleotide exchange on human Cdc42p. We have performed several genetic experiments to test whether Cdc24p and Cdc42p interact within the cell. First, overexpression of Cdc24p suppressed the dominant-nega tive cdc42(D118A) allele. Second, overexpression of wild-type CDC24 an d CDC42 genes together was a lethal event resulting in a morphological phenotype of large, round, unbudded cells, indicating a loss of cell polarity. Third, a cdc24(ts) cdc42(ts) double mutant exhibited a synth etic-lethal phenotype at the semi-permissive temperature of 30 degrees C. These data suggest that Cdc24p and Cdc42p interact within the cell and that Cdc24p may be involved in the regulation of Cdc42p activity.