Catalytic domain of the Serine/Threonine Kinase, Mitogen-Activated Protein Kinase
STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. MAPKs serve as important mediators of cellular responses to extracellular signals. They control critical cellular functions including differentiation, proliferation, migration, and apoptosis. They are also implicated in the pathogenesis of many diseases including multiple types of cancer, stroke, diabetes, and chronic inflammation. Typical MAPK pathways involve a triple kinase core cascade comprising of the MAPK, which is phosphorylated and activated by a MAPK kinase (MAP2K or MKK), which itself is phosphorylated and activated by a MAPK kinase kinase (MAP3K or MKKK). Each cascade is activated either by a small GTP-binding protein or by an adaptor protein, which transmits the signal either directly to a MAP3K to start the triple kinase core cascade or indirectly through a mediator kinase, a MAP4K. There are three typical MAPK subfamilies: Extracellular signal-Regulated Kinase (ERK), c-Jun N-terminal Kinase (JNK), and p38. Some MAPKs are atypical in that they are not regulated by MAP2Ks. These include MAPK4, MAPK6, NLK, and ERK7. The MAPK 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.