FIG. 7.
Model for specific signaling by the selective activation of the Ste5 scaffold protein combined with MAPKs with different scaffold requirements. (A) Fus3MAPK requires active Ste5 in order to effectively receive any signals. Hence, in nonmating cells, Fus3 mostly ignores signals from the invasive growth and osmostress pathways because Ste5 is inactive. Kss1MAPK uses an unknown scaffold or is scaffold independent. Hog1MAPK may use Pbs2MEK as a scaffold (37, 50). When inactive, Ste5 is presumably unable to organize Ste11MEKK, Ste7MEK, and Fus3 into a signaling-competent configuration. Here we have pictured this as a lack of Ste5-kinase association (48). (B and C) In mating cells, activated Ste5 relays the pheromone signal via activated Ste11 to Fus3. However, activated Ste11 can apparently also leak out and transmit signals in a scaffold-independent fashion (B). Furthermore, active Ste5 can also accept Ste11 activated by the other pathways and channel those signals to Fus3, promoting leaking (C).