CyclinCDK
Cyclin-dependent kinases (CDKs) are a family of serine/threonine kinases that, when bound to a regulatory cyclin subunit, drive progression through the eukaryotic cell cycle and coordinate other cellular processes. The complex formed by a CDK and its cyclin—often called a cyclin-CDK complex—exhibits kinase activity only when the cyclin partner is present, which provides substrate specificity and timing. Activation typically requires phosphorylation of the CDK by a CDK-activating kinase (CAK) and relief of autoinhibition, as well as removal of inhibitory phosphates by phosphatases such as Cdc25. Cyclin levels rise and fall during the cell cycle, ensuring the orderly sequence of events.
Regulation of cyclin-CDK activity involves multiple layers. Cyclin synthesis and targeted destruction by the ubiquitin–proteasome system
In humans, distinct cyclin-CDK pairs govern major cell-cycle transitions: Cyclin D–CDK4/6 promotes G1 progression and Rb