Pheromone signalling in Saccharomyces cerevisiae is mediated by the STE4-STE18 G-protein ␥ subunits. A possible target for the subunits is Ste20p, whose structural homolog, the serine/threonine kinase PAK, is activated by GTP-binding p21s Cdc42 and Rac1. The putative Cdc42p-binding domain of Ste20p, expressed as a fusion protein, binds human and yeast GTP-binding Cdc42p. Cdc42p is required for ␣-factor-induced activation of FUS1. cdc24 ts strains defective for Cdc42p GDP/GTP exchange show no pheromone induction at restrictive temperatures but are partially rescued by overexpression of Cdc42p, which is potentiated by Cdc42p 12V mutants. Epistatic analysis indicates that CDC24 and CDC42 lie between STE4 and STE20 in the pathway. The two-hybrid system revealed that Ste4p interacts with Cdc24p. We propose that Cdc42p plays a pivotal role both in polarization of the cytoskeleton and in pheromone signalling.In eukaryotic cells, many growth and differentiation signals are channelled through protein phosphorylation cascades that involve the mitogen-activated protein (MAP) kinase family. The similarity of MAP kinase cascades in multicellular organisms to those in the yeast Saccharomyces cerevisiae is a subject of great interest (7,17,42,50). One of the best studied of these cascades at the genetic level is the pheromone-dependent pathway in S. cerevisiae, which links occupation of pheromone receptors by a-or ␣-factor to a variety of events that lead to mating (for reviews, see references 3, 27, and 41). The pheromone-induced transcriptional activation of mating-specific genes requires a kinase cascade that includes the protein kinases encoded by STE20, STE11, STE7, KSS1, and FUS3. Activation of the pathway is initiated by the heterotrimeric G-protein ␥ subunits encoded by STE4 and STE18 (71, 72); the STE20 gene product is a kinase which may lie at the top of the kinase cascade and has been implicated as a target of the STE4-encoded  subunit (32). In mammalian cells, the activation of MAP kinase by various proto-oncogenic tyrosine kinases requires small GTP-binding proteins (p21s) of the Ras family; more recently, heterotrimeric G-protein signalling to MAP kinase has been shown to be mediated by ␥ subunits via Ras (13). While the mating pathway in Schizosaccharomyces pombe parallels this pathway in requiring ras1 (73), there has been no evidence for the involvement of a p21 in the S. cerevisiae pheromone-induced kinase cascade.We have described a number of proteins that appear to be the target of the Rho subfamily of p21s (39,40). One of these proteins was purified and shown to be a serine/threonine kinase that is directly activated by GTP-binding Cdc42 (GTPCdc42p) and GTP-Rac1. This brain-enriched kinase, designated PAK (40), is related to Ste20p both in the kinase domain and in a separate region identified as the p21-binding domain. These findings suggested that Cdc42p, a p21 first identified as a gene product required for polarized budding (29), participates in the mating response (Rac has not been found in S. cere...