Expression of cyclins through the cell cycle.

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.^[1][2]

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.^[3][4]

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.

References

• Lodish,Berk, Matsudaira, Kaiser, Kreiger, Scott, Zipursky & Darnell,Molecular Cell Biology, 5th edition

Cyclin/CDK complexes controlling cell cycle Cyclin B/CDK1-regulates transmission from S to G2 phase. Cyclin D/CDK4;Cyclin d/CDK 6;Cyclin E/CDK2-regulates transition from G1 to S phase. Cyclin A/CDK2 and Cyclin B/CDK1-active in G1 phase.

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