Catalytic domain of the Serine/Threonine Kinase, Fungal Mitogen-Activated Protein Kinase MPK1STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. This subfamily is composed of the MAPKs MPK1 from Saccharomyces cerevisiae, Pmk1 from Schizosaccharomyces pombe, and similar proteins. MPK1 (also called Slt2) and Pmk1 (also called Spm1) are stress-activated MAPKs that regulate the cell wall integrity pathway, and are therefore important in the maintainance of cell shape, cell wall construction, morphogenesis, and ion homeostasis. MPK1 is activated in response to cell wall stress including heat stimulation, osmotic shock, UV irradiation, and any agents that interfere with cell wall biogenesis such as chitin antagonists, caffeine, or zymolase. MPK1 is regulated by the MAP2Ks Mkk1/2, which are regulated by the MAP3K Bck1. Pmk1 is also activated by multiple stresses including elevated temperatures, hyper- or hypotonic stress, glucose deprivation, exposure to cell-wall damaging compounds, and oxidative stress. It is regulated by the MAP2K Pek1, which is regulated by the MAP3K Mkh1. MAPKs are important mediators of cellular responses to extracellular signals. The MPK1 subfamily is part of a larger superfamily that includes the catalytic domains of other STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.