Budding yeast have at least six three-component MAP-kinase modules involved in a variety of biological processes, including the two responses illustrated here—a mating response and the response to high osmolarity. (A) The mating response is triggered when a mating factor secreted by a yeast of opposite mating type binds to a G-protein-linked receptor. This activates a G protein, and the βγ complex of the G protein indirectly activates the MAP-kinase-kinase-kinase (kinase A), which then relays the response onward. Once activated, the MAP-kinase (kinase C) phosphorylates and thereby activates several proteins that mediate the mating response, in which the yeast cell stops dividing and prepares for fusion. The three kinases in this module are bound to scaffold protein 1. (B) In a second response, a yeast cell exposed to a high-osmolarity environment is induced to synthesize glycerol to increase its internal osmolarity. This response is mediated by a transmembrane, osmolarity-sensing, receptor protein and a different MAP-kinase module bound to a second scaffold protein. (Note that the kinase domain of scaffold 2 provides the MAP-kinase-kinase activity of this module.) Although both pathways use the same MAP-kinase-kinase-kinase (kinase A, green), there is no cross talk between them, because the kinases in each module are tightly bound to different scaffold proteins, and the osmosensor is bound to the same scaffold protein as the particular kinase it activates.