Cyclin

Cyclins are a family of proteins involved in the progression of cells through the cell cycle. They are the "regulatory subunits of the heterodimeric protein kinases that control cell cycle events."

Function
A cyclin forms a complex with its partner cyclin-dependent kinase (Cdk), which activates the latter's protein kinase function.

Cyclins are so named because their concentration varies in a cyclical fashion during the cell cycle; they are produced or degraded as needed in order to drive the cell through the different stages of the cell cycle.

When its concentrations in the cell are low, the cyclin detaches from the Cdk, inhibiting the enzyme's activity, probably by causing a protein chain to block the enzymatic site.

Types
There are several different cyclins which are active in different parts of the cell cycle and which cause the Cdk to phosphorylate different substrates. However, there are several "orphan" cyclins for which no Cdk partner has been identified. For example, cyclin F is an orphan cyclin that is essential for G2/M transition.

Other specific types include:
 * Cyclin D
 * Cyclin E
 * Cyclin A
 * Cyclin B

Domain structure
Cyclins contain two domains of similar all-alpha fold, N- and C-terminal.

Human proteins with cyclin domains
CABLES2;  CCNA1;     CCNA2;     CCNB1;     CCNB2;     CCNB3;     CCNC;      CCND1; CCND2;    CCND3;     CCNE1;     CCNE2;     CCNF;      CCNG1;     CCNG2;     CCNH; CCNI;     CCNJ;      CCNJL;     CCNK;      CCNL1;     CCNT1;     CCNT2;     CCNY; CCNYL1;   CNTD2;     UDG2;

History
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.