Although MAP kinase is often activated in multicellular animals by RTKs or cytokine receptors, signaling from other receptors can activate MAP kinase in other eukaryotic cells (see Figure 15-32). To illustrate, we consider the mating pathway in the yeast S. cerevisiae, a well-studied example of a MAP kinase cascade linked to G protein–coupled receptors (GPCRs), in this case for two secreted peptide pheromones, the a and α factors.

Haploid yeast cells are either of the a or α mating type. They secrete protein signals known as pheromones, which induce mating between haploid yeast cells of the opposite mating types. An a haploid cell secretes the a mating factor and has GPCRs for the α factor on its cell surface; an α cell secretes the α factor and has GPCRs for the a factor (see Figure 1-23). Thus each type of cell recognizes the mating factor produced by the opposite type. Activation of the MAP kinase pathway by either the a or the α GPCRs induces transcription of genes that inhibit progression of the cell cycle and others that enable cells of opposite mating type to fuse together and ultimately form a diploid cell.

Ligand binding to either of the two yeast pheromone GPCRs triggers the exchange of GTP for GDP on the G_α subunit and dissociation of G_α·GTP from the G_βγ complex. This activation process is identical to that for the GPCRs discussed in the previous chapter (see Figure 15-14). In many mammalian GPCR-initiated pathways, the active G_α transduces the signal. In contrast, the dissociated G_βγ complex mediates all the physiological responses induced by activation of the yeast pheromone GPCRs (Figure 16-27a). As evidence, in yeast cells that lack G_α, the G_βγ subunit is always free. Such cells can mate in the absence of mating factors; that is, the mating response is constitutively on. However, in cells defective for the G_β or G_γ subunit, the mating pathway cannot be induced at all. If dissociated G_α were the transducer, the pathway would be expected to be constitutively active in these mutant cells.

In yeast mating pathways, G_βγ functions by triggering a kinase cascade that is analogous to the one downstream from Ras; each protein in the cascade has a yeast-specific name, but shares sequences with, and is analogous in structure and function to, the corresponding mammalian protein shown in Figure 16-24. The components of this cascade were uncovered mainly through analyses of mutants that possess functional a and α receptors and G proteins, but are sterile (Ste) or defective in mating responses. The physical interactions between the components of the cascade were assessed through immunoprecipitation experiments with extracts of yeast cells and other types of studies. Based on these studies, scientists have proposed the kinase cascade shown in Figure 16-27a. Free G_βγ, which is tethered to the membrane via the lipid bound to the γ subunit, binds the Ste5 protein, thus recruiting it and its bound kinases to the plasma membrane. Ste5, which has no obvious catalytic function, acts as a scaffold for assembling other components in the cascade (Ste11, Ste7, and Fus3). G_βγ also activates cdc24, a GEF for the Ras-like protein Cdc42; GTP·Cdc42 in turn activates the Ste20 protein kinase. Ste20 phosphorylates and activates Ste11, a serine/threonine kinase analogous to Raf and other mammalian MEK kinase (MEKK) proteins. Activated Ste11 then phosphorylates Ste7, a dual-specificity MEK, which then phosphorylates and activates Fus3, a serine/threonine kinase equivalent to MAP kinase. After translocation to the nucleus, Fus3 phosphorylates two proteins, Dig1 and Dig2, relieving their inhibition of the Ste12 transcription factor. Activated Ste12, in turn, induces expression of proteins involved in mating-specific cellular responses. Fus3 also affects gene expression by phosphorylating other proteins.