Mitogen-activated protein kinases function in at least five, physiologically distinct, signaling pathways in yeast. These include pathways that mediate response to mating pheromone, pseudohyphal development and invasive growth, cell integrity, sporulation, and response to high extracellular osmolarity. These kinases and their upstream activating kinases comprise signaling modules that, in at least some cases, exist as multiprotein complexes. Studies during the past year have revealed that the Ste5 protein of the mating pheromone response pathway serves as a scaffold to promote interactions among the protein kinases in this pathway, and to prevent their interaction with kinases of other modules.