MAP4K component of the MAPK pathway required for the mating pheromone response, and the regulation of cell polarity and cell cycle. Phosphorylates histone H2B to form H2BS10ph (By similarity). Required for hyphal formation and virulence. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.