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). 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. Extended N-terminus.