Imported:YPM/Other yeast pathways
Yeast has six MAPK cascades but we restrict attention to three of them: Fus3, Kss1 and Hog1. Kss1 can partially compensate for loss of Fus3 but plays its predominant role in the "filamentous growth" response. Hog1 modulates the cell's behaviour in the presence of high external salt concentration.
Fus3 and Kss1 share the same MAPK cascade: Ste11 -> Ste7 -> Fus3/Kss1. This raises a "specificity" question: how do upstream events get faithfully represented downstream of the shared components? In this case, Fus3 activation depends on membrane recruitment of a scaffolding protein, Ste5, that thereby creates "pools" of Ste11/Ste7/Fus3 in the vicinity of activated Ste20. Kss1 doesn't bind (or with only low affinity?) Ste5 and so remains largely inactivated. In the absence of Ste5 membrane recruitment, no "pools" form (so no kinetic deformation) and the natural preference of Ste7 for Kss1 (over Fus3) as substrate prevails.
Pheromone reception leads to activation of a GPCR that promotes membrane recruitment of Ste5 whereas filamentous growth does not recruit Ste5 to the membrane. Thus, the two distinct upstream processes provoked by pheromones and filamentous growth get faithfully represented downstream by activation of respectively Fus3 and Kss1.
How does this allow a cell to make decisions like "I don't want to mate if there's no food around"? (Regulation of transcription factors?) More generally, how do the three MAPK cascades behave in the presence of various cocktails of upstream activation?