Cyclin

Cyclins are a family of proteins that control the progression of cells through the cell cycle by activating cyclin-dependent kinase (Cdk) enzymes.

Function

Cyclins were originally named because their concentration varies in a cyclical fashion during the cell cycle. (Note that the cyclins are now classified according to their conserved cyclin box structure, and not all these cyclins alter in level through the cell cycle.) The oscillations of the cyclins, namely fluctuations in cyclin gene expression and destruction by the ubiquitin mediated proteasome pathway, induce oscillations in Cdk activity to drive the cell cycle. A cyclin forms a complex with Cdk, which begins to activate the Cdk, but the complete activation requires phosphorylation, as well. Complex formation results in activation of the Cdk active site. Cyclins themselves have no enzymatic activity but have binding sites for some substrates and target the Cdks to specific subcellular locations.

They were discovered by R. Timothy Hunt in 1982 while studying the cell cycle of sea urchins.