For full activation of a cyclin-CDK complex, an enzyme called CDK-activating kinase must phosphorylate an amino acid near the CDK active site.
CDK-activating kinase (CAK) actuates the cyclin-CDK complex by phosphorylation of an amino acid i.e. threonine residue 160 near the CDK active site.
CAK is a member of the CDK family and serves as a positive regulator of CDK1, CDK2, CDK4, as well as CDK6.
CDK activation needs two steps. Primarily, the cyclin should attach to the CDK and secondly, CAK should phosphorylate the cyclin-CDK complex on the threonine residue 160, that is situated in the CDK activation section.
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