Cyclin - Definition

Cyclin is a protein involved in the regulation of the cell cycle. It forms a complex with the cyclin-dependent kinase (Cdk), which activates the latter's protein kinase function.

There are several different cyclins which are active in different parts of the cell cycle and which cause the Cdk to phosphorylate different substrates.

One major cyclin is cyclin B, the mitotic cyclin. The amount of cyclin B, and the activity of the cyclin B-Cdk complex, rises through the cell cycle until mitosis. It falls abruptly during mitosis due to the degradation of cyclin B. The complex of Cdk and cyclin B is called maturation promoting factor (MPF).

Leland H. Hartwell, R. Timothy Hunt, and Paul M. Nurse won the 2001 Nobel Prize in Physiology or Medicine for their discovery of cyclin and cyclin-dependent kinase, central molecules in the regulation of the cell cycle.