Catalytic domain of Cyclin-Dependent protein Kinase 4 and 6-like Serine/Threonine Kinases
STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. CDK4 and CDK6 partner with D-type cyclins to regulate the early G1 phase of the cell cycle. They are the first kinases activated by mitogenic signals to release cells from the G0 arrested state. CDK4 and CDK6 are both expressed ubiquitously, associate with all three D cyclins (D1, D2 and D3), and phosphorylate the retinoblastoma (pRb) protein. They are also regulated by the INK4 family of inhibitors which associate with either the CDK alone or the CDK/cyclin complex. CDK4 and CDK6 show differences in subcellular localization, sensitivity to some inhibitors, timing in activation, tumor selectivity, and possibly substrate profiles. Although CDK4 and CDK6 seem to show some redundancy, they also have discrete, nonoverlapping functions. CDK6 plays an important role in cell differentiation. CDKs belong to a large family of STKs that are regulated by their cognate cyclins. Together, they are involved in the control of cell-cycle progression, transcription, and neuronal function. The CDK4/6-like 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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