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Catalytic domain of the Serine/Threonine Kinase, Apoptosis signal-regulating kinase STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. Members of this subfamily are mitogen-activated protein kinase (MAPK) kinase kinases (MAPKKKs or MKKKs) and include ASK1, ASK2, and MAPKKK15. ASK1 (also called MAPKKK5) functions in the c-Jun N-terminal kinase (JNK) and p38 MAPK signaling pathways by directly activating their respective MAPKKs, MKK4/MKK7 and MKK3/MKK6. It plays important roles in cytokine and stress responses, as well as in reactive oxygen species-mediated cellular responses. ASK1 is implicated in various diseases mediated by oxidative stress including inschemic heart disease, hypertension, vessel injury, brain ischemia, Fanconi anemia, asthma, and pulmonary edema, among others. ASK2 (also called MAPKKK6) functions only in a heteromeric complex with ASK1, and can activate ASK1 by direct phosphorylation. The function of MAPKKK15 is still unknown. The ASK 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.
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