FIG. 4.

Signaling specificity during mating and pseudohyphal growth in S. cerevisiae. Two related MAP kinase signaling cascades allow haploid and diploid yeast cells to respond to two different environmental signs, pheromone and nitrogen limitation, and give rise to two completely different developmental fates, mating in haploid cells and filamentous growth in diploid cells. Signaling specificity is achieved by at least four mechanisms. First, pheromone activates a G protein whose βγ subunits activate the MAP kinase cascade, and these components are not expressed in diploid yeast cells and do not regulate filamentous growth. Second, the Ste5 scaffold tethers the components during mating but does not play a role in diploid filamentous growth. Third, the MAP kinase has diverged and specialized: Fus3 to regulate mating and Kss1 to regulate filamentous growth. Finally, Ste12 homodimers or heterodimers with Mcm1 activate pheromone response element-regulated genes in mating or with Tec1 to regulate filamentation response element-driven genes during diploid filamentous growth.