MAP4K component of the MAPK pathway required for the mating pheromone response, haploid invasive growth and diploid pseudohyphal development. Links the pheromone response G-protein beta gamma subunits to downstream signaling components. Needed for mating in haploid cells, induction of a mating-specific gene FUS1, induction of mating-specific morphologies, and pheromone-induced proliferation arrest. Required for the regulation of the actin polarization and bud emergence during cell cycle in G1. Involved in the high osmolarity glycerol (HOG) response. Phosphorylates 'Thr-307' and 'Ser-302' or 'Ser-306' of STE11 and 'Ser-357' of MYO3. Phosphorylates histone H2B to form H2BS10ph during meiosis and H(2)O(2)-induced apoptosis. Its interaction with CDC42 is required for both invasive growth and the formation of pseudohyphae. Its interaction with STE4 is required for the pheromone signaling. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with BEM1, CDC42, CLN2, STE4 and the 14-3-3 proteins BMH1 and BMH2. The translocation from the cytoplasm to the nucleus is stimulated by H(2)O(2). Localizes at bud emergence during cell cycle and the shmoo top during mating, both localizations requiring an interaction with CDC42. The CRIB domain is required for the association with CDC42. Autophosphorylated and phosphorylated by the CLN2-CDC28 complex in a cell cycle dependent manner. Autophosphorylated on serine residues. Present with 259 molecules/cell in log phase SD medium. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.