Catalytic domain of the Serine/Threonine protein kinases, Casein Kinase 1 delta and epsilon
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. The delta and epsilon isoforms of CK1 play important roles in circadian rhythm and cell growth. They phosphorylate PERIOD proteins (PER1-3), which are circadian clock proteins that fulfill negative regulatory functions. PER phosphorylation leads to its degradation. However, CRY proteins form a complex with PER and CK1delta/epsilon that protects PER from degradation and leads to nuclear accummulation of the complex, which inhibits BMAL1-CLOCK dependent transcription activation. CK1delta/epsilon also phosphorylate the tumor suppressor p53 and the cellular oncogene Mdm2, which are key regulators of cell growth, genome integrity, and the development of cancer. This subfamily also includes the CK1 fungal proteins Saccharomyces cerevisiae HRR25 and Schizosaccharomyces pombe HHP1. These fungal proteins are involved in DNA repair. The CK1 delta/epsilon 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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