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Catalytic domain of the Serine/Threonine Kinases, Salt-Inducible kinases STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. SIKs are part of a complex network that regulates Na,K-ATPase to maintain sodium homeostasis and blood pressure. Vertebrates contain three forms of SIKs (SIK1-3) from three distinct genes, which display tissue-specific effects. SIK1, also called SNF1LK, controls steroidogenic enzyme production in adrenocortical cells. In the brain, both SIK1 and SIK2 regulate energy metabolism. SIK2, also called QIK or SNF1LK2, is involved in the regulation of gluconeogenesis in the liver and lipogenesis in adipose tissues, where it phosphorylates the insulin receptor substrate-1. In the liver, SIK3 (also called QSK) regulates cholesterol and bile acid metabolism. In addition, SIK2 plays an important role in the initiation of mitosis and regulates the localization of C-Nap1, a centrosome linker protein. The SIK 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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