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 proteolysis, 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. 
Cyclins, when bound with the dependent kinases, such as the p34 (cdc2) or cdk1 proteins, form the maturation-promoting factor. MPFs activate other proteins through phosphorylation. These phosphorylated proteins, in turn, are responsible for specific events during cycle division such as microtubule formation and chromatin remodeling. Cyclins can be divided into four classes based on their behavior in the cell cycle of vertebrate somatic cells and yeast cells: G1/S cyclins, S cyclins, M cyclins, G1 cyclins. This division is useful when talking about most cell cycles, but it is not universal as some cyclins have different functions or timing in different cell types.
G1/S Cyclins rise in late G1 and fall in early S phase. The Cdk- G1/S cyclin complex begins to induce the initial processes of DNA replication, primarily by arresting systems that prevent S phase Cdk activity in G1. The cyclins also promote other activities to progress the cell cycle, like centrosome duplication in vertebrates or spindle pole body in yeast. The rise in presence of G1/S cyclins is paralleled by a rise in S cyclins.
S cyclins bind to Cdk and the complex directly induces DNA replication. The levels of S cyclins remain high, not only throughout S phase, but through G2 and early mitosis as well to promote early events in mitosis.
M cyclin concentrations rise as the cell begins to enter mitosis and the concentrations peak at metaphase. Cell changes in the cell cycle like the assembly of mitotic spindles and alignment of sister-chromatids along the spindles are induced by M cyclin- Cdk complexes. The destruction of M cyclins during metaphase and anaphase, after the Spindle Assembly Checkpoint is satisfied, causes the exit of mitosis and cytokinesis .
G1 cyclins do not behave like the other cyclins, in that the concentrations increase gradually (with no oscillation), throughout the cell cycle based on cell growth and the external growth-regulatory signals. The presence of G cyclins coordinate cell growth with the entry to a new cell cycle.
Cyclins are generally very different from each other in primary structure, or amino acid sequence. The similarity between members of the cyclin family are similar in the 100 amino acids that make up the cyclin box. Cyclins contain two domains of similar all-α fold, the first located at the N-terminus and the second at the C-terminus. All cyclins are believed to contain a similar tertiary structure of two compact domains of 5 α helices. The first of which is the conserved cyclin box, outside of which cyclins are divergent. For example, the amino-terminal regions of S and M cyclins contain short destruction-box motifs that target these proteins for proteolysis in mitosis.
Cyclin, N-terminal domain Structure of bovine cyclin A. Identifiers Symbol Cyclin_N Pfam PF00134 Pfam clan CL0065 InterPro IPR006671 PROSITE PDOC00264 SCOP 1vin Available protein structures: Pfam structures PDB RCSB PDB; PDBe PDBsum structure summary Cyclin, C-terminal domain Structure of CDK2-cyclin A/indirubin-5-sulphonate. Identifiers Symbol Cyclin_C Pfam PF02984 Pfam clan CL0065 InterPro IPR004367 PROSITE PDOC00264 SCOP 1vin Available protein structures: Pfam structures PDB RCSB PDB; PDBe PDBsum structure summary
There are several different cyclins that are active in different parts of the cell cycle and that 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.
There are two main groups of cyclins:
- G1/S cyclins – essential for the control of the cell cycle at the G1/S transition,
- G2/M cyclins – essential for the control of the cell cycle at the G2/M transition (mitosis). G2/M cyclins accumulate steadily during G2 and are abruptly destroyed as cells exit from mitosis (at the end of the M-phase).
Specific cyclin subtypes include:
Species G1 G1/S S M S. cerevisiae Cln3 (Cdk1) Cln 1,2 (Cdk1) Clb 5,6 (Cdk1) Clb 1,2,3,4 (Cdk 1) S. pombe Puc1? (Cdk1) Puc1, Cig1? (Cdk1) Cig2, Cig1? (Cdk1) Cdc13 (Cdk1) D. melanogaster cyclin D (Cdk4) cyclin E (Cdk2) cyclin E, A (Cdk2,1) cyclin A, B, B3 (Cdk1) X. laevis either not known or not present cyclin E (Cdk2) cyclin E, A (Cdk2,1) cyclin A, B, B3 (Cdk1) H. sapiens cyclin D 1,2,3 (Cdk4,6) cyclin E (Cdk2) cyclin A (Cdk2,1) cyclin B (Cdk1) 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
Other proteins containing this domain
In addition, the following human proteins contain a cyclin domain:
CABLES2, CNTD1, CNTD2
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- ^ Evans et al., 1983, Cell 33, p389-396
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- ^ Clute and Pines, (1999) Nature Cell Biology, 1, p82-87
- ^ Brown NR, Noble ME, Endicott JA, et al. (November 1995). "The crystal structure of cyclin A". Structure 3 (11): 1235–47. doi:10.1016/S0969-2126(01)00259-3. PMID 8591034.
- ^ Davies TG, Tunnah P, Meijer L, et al. (May 2001). "Inhibitor binding to active and inactive CDK2: the crystal structure of CDK2-cyclin A/indirubin-5-sulphonate". Structure 9 (5): 389–97. doi:10.1016/S0969-2126(01)00598-6. PMID 11377199.
- ^ Fung TK, Poon RY (2005). "A roller coaster ride with the mitotic cyclins". Semin. Cell Dev. Biol. 16 (3): 335–42. doi:10.1016/j.semcdb.2005.02.014. PMID 15840442.
- ^ Gerald Karp, (2007). Cell and Molecular Biology: Concepts and Experiments. New York: Wiley. pp. 148, 165–170, and 624–664. ISBN 0-470-04217-6.
- ^ "The Nobel Prize in Physiology or Medicine 2001". The Nobel Foundation. http://nobelprize.org/nobel_prizes/medicine/laureates/2001/index.html. Retrieved 2009-03-15.
- Eukaryotic Linear Motif resource motif class LIG_CYCLIN_1
- Monty Krieger; Matthew P Scott; Matsudaira, Paul T.; Lodish, Harvey F.; Darnell, James E.; Lawrence Zipursky; Kaiser, Chris; Arnold Berk (2004). Molecular cell biology (Fifth ed.). New York: W.H. Freeman and CO. ISBN 0-7167-4366-3.
Cell cycle proteins Cyclin CDK CDK inhibitor P53 p63 p73 family Phases and
checkpointsOther cellular phases
B bsyn: dna (repl, cycl, reco, repr) · tscr (fact, tcrg, nucl, rnat, rept, ptts) · tltn (risu, pttl, nexn) · dnab, rnab/runp · stru (domn, 1°, 2°, 3°, 4°)
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