Kss1

Kss1, also written Kss1p, is a mitogen-activated protein kinase (MAPK) found in the budding yeast Saccharomyces cerevisiae. It participates in several signaling pathways, most notably the pheromone response and the regulation of invasive (filamentous) growth under nutrient limitation. As a MAPK, Kss1 acts within a three-tier signaling cascade that typically begins with the MAP kinase kinase kinase Ste11 and the MAP kinase kinase Ste7, culminating in activation of Kss1. Once active, Kss1 phosphorylates downstream targets, including the transcription factor Ste12, which controls expression of genes involved in mating, adhesion, and filamentous growth. Kss1 has functional overlap with Fus3, the other major MAPK in the pheromone pathway; in certain conditions, Kss1 can contribute to pheromone responses when Fus3 activity is reduced.

Regulation of Kss1 signaling involves phosphatases and cross-talk with other MAPK pathways, helping to constrain inappropriate pathway activation. Structurally, Kss1 is a serine/threonine kinase with the conserved MAPK activation loop containing the TEY motif, and it shares signatures common to eukaryotic MAPKs. In research, Kss1 serves as a model for understanding MAPK signaling dynamics, pathway specificity, and the balance between mating and invasive growth programs. Alterations in KSS1 can affect mating efficiency, colony morphology, and stress responses in laboratory strains. See also Saccharomyces cerevisiae signaling pathways and MAP kinase.