Serine/threonine protein kinase involved in a signal transduction pathway that plays a role in yeast cell morphogenesis and cell growth. This pathway seems to start by SMP3; then involves the kinase PKC1 that may act on the BCK1 kinase that then phosphorylates MKK1 and MKK2 which themselves phosphorylate the MPK1 kinase. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Present with 1950 molecules/cell in log phase SD medium. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase subfamily.