Singularity in budding: A role for the evolutionarily conserved small GTPase Cdc42p

The budding yeast Saccharomyces cerevisiae initiates polarized growth or budding once per cell cycle at a specific time of the cell cycle and at a specific location on the cell surface. Little is known about the molecular nature of the temporal and spatial regulatory mechanisms. It is also unclear what factors, if any, among the numerous proteins required to make a bud are involved in the determination of budding frequency. Here we describe a class of cdc42 mutants that produce multiple buds at random locations on the cell surface within one nuclear cycle. The critical mutation responsible for this phenotype affects amino acid residue 60, which is located in a domain required for GTIP binding and hydrolysis. This mutation bypasses the requirement for the essential guanine-nucleotide-exchange factor Cdc24p, suggesting that the alteration at residue 60 makes Cdc42p hyperactive, which was confirmed biochemically. This result also suggests that the only essential function of Cdc24p is to activate Cdc42p. Together, these data suggest that the temporal and spatial regulation of polarized growth converges at the level of Cdc42p and that the activity of Cdc42p determines the budding frequency.