Catalytic domain of the Serine/Threonine Kinases, Mammalian STe20-like protein kinase 1 and 2
STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. This subfamily is composed of MST1, MST2, and related proteins including Drosophila Hippo and Dictyostelium discoideum Krs1 (kinase responsive to stress 1). MST1/2 and Hippo are involved in a conserved pathway that governs cell contact inhibition, organ size control, and tumor development. MST1 activates the mitogen-activated protein kinases (MAPKs) p38 and c-Jun N-terminal kinase (JNK) through MKK7 and MEKK1 by acting as a MAPK kinase kinase kinase. Activation of JNK by MST1 leads to caspase activation and apoptosis. MST1 has also been implicated in cell proliferation and differentiation. Krs1 may regulate cell growth arrest and apoptosis in response to cellular stress. The MST1/2 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.