Catalytic domain of the Serine/Threonine Kinases, Mitogen-Activated Protein Kinases 4 (also called ERK4) and 6 (also called ERK3)
STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. MAPK4 (also called ERK4 or p63MAPK) and MAPK6 (also called ERK3 or p97MAPK) are atypical MAPKs that are not regulated by MAPK kinases. MAPK6 is expressed ubiquitously with highest amounts in brain and skeletal muscle. It may be involved in the control of cell differentiation by negatively regulating cell cycle progression in certain conditions. It may also play a role in glucose-induced insulin secretion. MAPK6 and MAPK4 cooperate to regulate the activity of MAPK-activated protein kinase 5 (MK5), leading to its relocation to the cytoplasm and exclusion from the nucleus. The MAPK6/MK5 and MAPK4/MK5 pathways may play critical roles in embryonic and post-natal development. MAPKs are important mediators of cellular responses to extracellular signals. The MAPK4/6 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.