Fus3

Fus3 is a MAPK protein involved in the mating decision of yeast. The dissociation of Fus3 from scaffold protein Ste5 results in the switch-like mating decision observed in yeast. During this process, Fus3 competes with a phosphatase Pte1, attempting to phosphorylate 4 key phosphorylation sites on Ste5. When all 4 sites on Ste5 have been dephosphorylated by Pte1, Fus3 dissociates from Ste5 and trans locates to the nucleus.

One regulator of Fus3 is Ste5. Ste5 causes autophosphorylation of one of two locations modulated by the MAPK kinase Ste7 (the main activator of Fus3). This single phosphorylation causes Fus3 to phosphorylate Ste5 leading to a decrease in signal. However, Ste5 also selectively catalytically unlocks Fus3 for phosphorylation by Ste7. Both the catalytic domain on Ste5 as well as Ste7 must be present in order to activate Fus3, which helps to explain why Fus3 is only activated during the mating pathway, and remains inactive in other situations which use Ste7. When binding to Ste5 is disrupted, Fus3 behaves like its homologue Kss1, and the cells no longer respond to a gradient or mate efficiently with distant partners.