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Cyclins are a family of proteins that control the progression of cells through the cell cycle by activating cyclin-dependent kinase (Cdk) enzymes.[1]


Cyclins were originally discovered by R. Timothy Hunt in 1982 while studying the cell cycle of sea urchins.[2][3]

In an interview for "The Life Scientific" (aired on 13/12/2011) hosted by Jim Al-Khalili, R. Timothy Hunt explained that the name "cyclin" was originally named after his hobby cycling. It was only after the naming did its importance in the cell cycle become apparent. As it was appropriate the name stuck.[4] R. Timothy Hunt: "By the way, the name cyclin, which I coined, was really a joke, it's because I liked cycling so much at the time, but they did come and go in the cell..."[4]


Expression of human cyclins through the cell cycle.

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.[5]) The oscillations of the cyclins, namely fluctuations in cyclin gene expression and destruction by the ubiquitin mediated proteasome pathway, 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.[5]

Cyclins, when bound with the dependent kinases, such as the p34/cdc2/cdk1 protein, 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 cyclins, G1/S cyclins, S cyclins, and M 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, such as 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.

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.

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.[6]

Domain structure[edit]

Cyclins are generally very different from each other in primary structure, or amino acid sequence. However, all members of the cyclin family are similar in 100 amino acids that make up the cyclin box. Cyclins contain two domains of a 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
PDB 1vin EBI.jpg
Structure of bovine cyclin A.[7]
Symbol Cyclin_N
Pfam PF00134
Pfam clan CL0065
InterPro IPR006671
SCOP 1vin
Cyclin, C-terminal domain
PDB 1e9h EBI.jpg
Structure of CDK2-cyclin A/indirubin-5-sulphonate.[8]
Symbol Cyclin_C
Pfam PF02984
Pfam clan CL0065
InterPro IPR004367
SCOP 1vin
K cyclin, C terminal
PDB 1g3n EBI.jpg
structure of a p18(ink4c)-cdk6-k-cyclin ternary complex
Symbol K-cyclin_vir_C
Pfam PF09080
InterPro IPR015164
SCOP 1g3n


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.[9][10] A study in C. elegans revealed the specific roles of mitotic cyclins.[11][12] Notably, recent studies have shown that cyclin A creates a cellular environment that promotes microtubule detachment from kinetochores in prometaphase to ensure efficient error correction and faithful chromosome segregation. Cells must separate their chromosomes precisely, an event that relies on the bi-oriented attachment of chromosomes to spindle microtubules through specialized structures called kinetochores. In the early phases of division, there are numerous errors in how kinetochores bind to spindle microtubules. The unstable attachments promote the correction of errors by causing a constant detachment, realignment and reattachment of microtubules from kinetochores in the cells as they try to find the correct attachment. Protein cyclin A governs this process by keeping the process going until the errors are eliminated. In normal cells, persistent cyclin A expression prevents the stabilization of microtubules bound to kinetochores even in cells with aligned chromosomes. As levels of cyclin A decline, microtubule attachments become stable, allowing the chromosomes to be divided correctly as cell division proceeds. In contrast, in cyclin A-deficient cells, microtubule attachments are prematurely stabilized. Consequently, these cells may fail to correct errors, leading to higher rates of chromosome mis-segregation.[13]

Main groups[edit]

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).
    • Cyclin B / CDK1 – regulates progression from G2 to 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, Cdk6) cyclin E (Cdk2) cyclin A (Cdk2, Cdk1) cyclin B (Cdk1)
family members

Other proteins containing this domain[edit]

In addition, the following human proteins contain a cyclin domain:



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.[14]


  1. ^ Galderisi U, Jori FP, Giordano A (August 2003). "Cell cycle regulation and neural differentiation". Oncogene. 22 (33): 5208–19. doi:10.1038/sj.onc.1206558. PMID 12910258. 
  2. ^ Evans et al., 1983, Cell 33, p389-396
  3. ^
  4. ^ a b "The Life Scientific". BBC Radio 4. BBC. Retrieved 13 December 2011. 
  5. ^ a b Morgan, DO (2007) 'The Cell Cycle: Principles of Control, Oxford University Press
  6. ^ Clute and Pines, (1999) Nature Cell Biology, 1, p82-87
  7. ^ 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. 
  8. ^ 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. 
  9. ^ 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. 
  10. ^ 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. 
  11. ^ van der Voet, Monique; Lorson, Monique; Srinivasan, Dayalan G.; Bennett, Karen L.; van den Heuvel, Sander (2009). "C. elegans mitotic cyclins have distinct as well as overlapping functions in chromosome segregation". Cell Cycle. 8 (24): 4091–4102. doi:10.4161/cc.8.24.10171. ISSN 1538-4101. PMC 3614003Freely accessible. PMID 19829076. 
  12. ^ Rahman, Mohammad M.; Kipreos, Edward (2010). "The specific roles of mitotic cyclins revealed". Cell Cycle. 9 (1): 22–27. doi:10.4161/cc.9.1.10577. ISSN 1538-4101. 
  13. ^ "Cell cycle: Cyclin A corrections". Nature Reviews Molecular Cell Biology. 14: 692. doi:10.1038/nrm3680. 
  14. ^ "The Nobel Prize in Physiology or Medicine 2001". The Nobel Foundation. Retrieved 2009-03-15. 

External links[edit]

Further reading[edit]

  • 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. 

This article incorporates text from the public domain Pfam and InterPro IPR006671