May regulate cell morphology, cell wall integrity, salt resistance, cell cycle reentry from stationary-phase arrest, and filamentous growth in response to stress. Activates the MAP kinase kinase skh1/pek1 by phosphorylation. 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. MAP kinase kinase kinase subfamily.