Catalytic domain of the Serine/Threonine protein kinase, Casein Kinase 1 gamma
STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. CK1 phosphorylates a variety of substrates including enzymes, transcription and splice factors, cytoskeletal proteins, viral oncogenes, receptors, and membrane-associated proteins. There are mutliple isoforms of CK1 and in mammals, seven isoforms (alpha, beta, gamma1-3, delta, and epsilon) have been characterized. These isoforms differ mainly in the length and structure of their C-terminal non-catalytic region. CK1gamma proteins are unique within the CK1 subfamily in that they are palmitoylated at the C-termini and are anchored to the plasma membrane. CK1gamma is involved in transducing the signaling of LDL-receptor-related protein 6 (LRP6) through direct phosphorylation following Wnt stimulation, resulting in the recruitment of the scaffold protein Axin. In Xenopus embryos, CK1gamma is required during anterio-posterior patterning. In higher vertebrates, three CK1gamma (gamma1-3) isoforms exist. In mammalian cells, CK1gamma2 has been implicated in regulating the synthesis of sphingomyelin, a phospholipid that is found in the outer leaflet of the plasma membrane, by hyperphosphorylating and inactivating the ceramide transfer protein CERT. CK1gamma2 also phosphorylates the transcription factor Smad-3 resulting in its ubiquitination and degradation. It inhibits Smad-3 mediated responses of Transforming Growth Factor-beta (TGF-beta) including cell growth arrest. The CK1 gamma subfamily is part of a larger superfamily that includes the catalytic domains of other protein STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.
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