A class of proteins that fluctuate in concentration at specific points during the cell cycle and that regulate the cycle by binding to a kinase.
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cy·clin (sī'klĭn) ![]() |
A class of proteins that fluctuate in concentration at specific points during the cell cycle and that regulate the cycle by binding to a kinase.
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Cyclins are a family of proteins which control the progression of cells through the cell cycle by regulation of cyclin-dependent kinases (CDKs).[1] Cyclins themselves have no enzymatic activity.
The individual steps of cell division must carefully be controlled to make sure that all the required molecular components are ready before proceeding to the next step. This control ensures an orderly division of the cell with high fidelity DNA replication. This control includes checking for DNA damage which if present would be propagated during cell division with often catastrophic consequences for the cell progeny. If DNA damage is present, the cell cycle must be paused in order to allow time for DNA repair before proceeding with cell division.
Cyclins are a critical component of this cell cycle control mechanism. Cyclins, together with the p34 (cdc2) or cdk2 kinases, form the Maturation Promoting Factor (MPF). MPFs activate other proteins through phosphorylation. These phosphorylated proteins in turn are responsible for specific events during cycle division such as microtubule formation, chromatin remodeling, etc.
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A cyclin forms a complex with its partner cyclin-dependent kinase (Cdk). Complex formation results in activation of the Cdk 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.[2][3]
Cyclins contain two domains of similar all-α fold, the first located at the N-terminus and the second at the C-terminus.
There are several different cyclins which are active in different parts of the cell cycle and which cause the Cdk to phosphorylate different substrates. There are also 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.[4][5]
There are two main groups of cyclins:
Specific cyclin subtypes include:
| family | members |
|---|---|
| A | CCNA1, CCNA2 |
| B | CCNB1, CCNB2, CCNB3 |
| C | CCNC |
| D | CCND1, CCND2, CCND3 |
| E | CCNE1, CCNE2 |
| F | CCNF |
| G | CCNG1, CCNG2 |
| H | CCNH |
| I | CCNI, CCNI2 |
| J | CCNJ, CCNJL |
| K | CCNK |
| L | CCNL1, CCNL2 |
| O | CCNO |
| T | CCNT1, CCNT2 |
| Y | CCNY, CCNYL1, CCNYL2, CCNYL3 |
In addition, the following human proteins contain a cyclin domain:
CABLES2, CNTD1, CNTD2
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.[6]
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This article includes text from the public domain Pfam and InterPro IPR006671
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
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