Together with closely related KSS1, FUS3 is the final kinase in the signal transduction cascade regulating activation/repression of the mating and filamentation pathways, induced by pheromone and nitrogen/carbon limitation, respectively. Phosphorylated FUS3 activates the mating but suppresses the filamentation pathway, whereas activated KSS1 activates both pathways. Pheromone-activated FUS3 functions by inhibiting the binding of the transcriptional activator STE12 to filamentation specific genes while inducing its binding to and activity at mating specific genes. Non-activated FUS3 has a repressive effect on STE12 transcriptional activity. KSS1 can partially compensate for the lack of FUS3 but mating efficiency is reduced and the filamentation program is partially activated upon pheromone signaling. FUS3 phosphorylates STE7, STE5, FAR1, DIG1, DIG2 and STE12. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated by tyrosine and threonine phosphorylation after pheromone treatment. In the nucleus, FUS3 forms a complex with DIG1, DIG2 and STE12. The interaction of FUS3 with STE12 depends on the presence of both DIG1 and DIG2. STE12 is lost from FUS3/DIG1/DIG2 complex after pheromone treatment. During its activation and phosphorylation, FUS3 forms a membrane-associated complex with the scaffold protein STE5, the MAPKK STE7, the MAPKKK STE11, and the G-protein beta subunit GBB/STE4; interacting directly with STE7 and STE5. FUS3 shuttles rapidly between the cytoplasm and the nucleus independent of pheromone treatment or FUS3 phosphorylation level. Activated FUS3 translocates rapidly to the nucleus. The TXY motif contains the threonine and tyrosine residues whose phosphorylation activates the MAP kinases. Dually phosphorylated on Thr-180 and Tyr-182 by STE7 in response to pheromone induction, which activates the enzyme. Activated FUS3 initiates a feedback signal, down-regulating phosphorylation of both, FUS3 and KSS1. Present with 8480 molecules/cell in log phase SD medium. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. MAP kinase subfamily.