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.

>P14681|KSS1|Saccharomyces cerevisiae (strain ATCC 204508 / S288c)(Baker's yeast)
MARTITFDIPSQYKLVDLIGEGAYGTVCSAIHKPSGIKVAIKKIQPFSKKLFVTRTIREIKLLRYFHEHENIISILDKVRPVSIDKLNAVYLVEELMETDLQKVINNQNSGFSTLSDDHVQYFTYQILRALKSIHSAQVIHRDIKPSNLLLNSNCDLKVCDFGLARCLASSSDSRETLVGFMTEYVATRWYRAPEIMLTFQEYTTAMDIWSCGCILAEMVSGKPLFPGRDYHHQLWLILEVLGTPSFEDFNQIKSKRAKEYIANLPMRPPLPWETVWSKTDLNPDMIDLLDKMLQFNPDKRISAAEALRHPYLAMYHDPSDEPEYPPLNLDDEFWKLDNKIMRPEEEEEVPIEMLKDMLYDELMKTME