Together with closely related FUS3, KSS1 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 KSS1 activates both pathways, whereas activated FUS3 activates the mating but suppresses the filamentation pathway. KSS1 activity is down-regulated by FUS3 during pheromone induction to prevent inappropriate activation of the filamentation pathway. During induction of filamentation, KSS1 activates the transcription factor STE12 resulting in its binding to and activation of filamentation specific genes. Non-activated KSS1 has a kinase-independent repressive effect on STE12 transcriptional activity, that is mediated by direct binding to STE12 and depends on the presence of DIG1 and DIG2, and that is required for the suppression of filamentation under normal growth conditions. SSN3/SRB10 contributes further to the suppression of filamentation under these conditions by reducing STE12 stability independent of KSS1. FUS3 can partially compensate for the lack of KSS1 but filamentation becomes constitutively induced at a low level in the absence of any signal. KSS1 phosphorylates STE7, STE5, FAR1, DIG1, DIG2, STE12, and SST2. 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 or carbon/nitrogen limitation. In the nucleus, KSS1 forms a complex with DIG1, DIG2 and STE12; in contrast to FUS3 the interaction of KSS1 with STE12 does not depend on DIG1 and DIG2. Phosphorylated KSS1 shows reduced interaction with STE12. During pheromone activation and phosphorylation, KSS1 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 POF1, STE7 and STE5 proteins. KSS1 shuttles rapidly between the cytoplasm and the nucleus independent of its activation state. The TXY motif contains the threonine and tyrosine residues whose phosphorylation activates the MAP kinases. Dually phosphorylated on Thr-183 and Tyr-185 by STE7 in response to pheromone or carbon/nitrogen limitation, which activates the enzyme. Activated FUS3 down-regulates KSS1 phosphorylation. Present with 5480 molecules/cell in log phase SD medium. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. MAP kinase subfamily. HOG1 sub-subfamily.