Cdc24 Background Information One of the best characterized MAP kinase pathways is the yeast pheromone response pathway (1). Extracellular pheromones bind to cell surface receptors and lead to the activation of the MAPKKK Ste11 and the subsequent members of this cascade, Ste7, FusS3 (also designated Dac2) and Kss1 (2,3). These MAP kinases activate Ste12 and Far1, which effect transcriptional and morphological changes necessary for mating (4,5). Cdc42, a small GTP-binding protein, is thought to play a critical role in this pathway (6). Activity of Cdc42 is regulated in part by the guanylyl-nucleotide-exchange factor Cdc24 (also designated Cls4) (7). Cells lacking Cdc24 are unable to respond to pheromone and also show defects in bud-site selection and bud emergence (8).
