Cyclin
Cyclin
Bruce Perens' Open Source Series
Category="Cell cycle"Category="Proteins"Cyclin is a Protein involved in the progression of cells through the Cell cycle. It forms a complex with the 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, cyclin detaches from Cdk, inhibiting the enzyme's activity, probably by causing a protein chain to block the enzymatic site (1,2).There are several different cyclins which are active in different parts of the cell cycle and which cause the Cdk to phosphorylate different substrates.
One major cyclin is cyclin B, a mitotic cyclin. The amount of cyclin B, and the activity of the cyclin B-Cdk complex, rises through the cell cycle until mitosis, when it falls abruptly due to degradation. The complex of Cdk and cyclin B is called Maturation promoting factor (MPF). Other cyclins include cyclin E (binds to G1 phase Cdk), which is required for the transition from G1 to S phase and cyclin A (binds to S phase Cdk) and is required for the cell to progress through the S phase.
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
1. Fung, T. K. and Poon R.Y.C. (2005). A roller coaster ride with the mitotic cyclins. Seminars in Cell & Developmental Biology. 16 (3):335-342. Available via the World Wide Web: <;/science?_ob=ArticleURL&_aset=V-WA-A-W-AZA-MsSAYZW-UUW-U-AAAYCAUBUC-AAAZUEAAUC-AUYEVWCZA-AZA-U&_rdoc=1&_fmt=full&_udi=B6WX0-4FMK8MY-1&_coverDate=06/30/2005&_cdi=7144&_orig=search&_st=13&_sort=d&view=c&_acct=C000054278&_version=1&_urlVersion=0&_userid=2139826&md5=9c1383f7b196b132e10bd4e8a8e19448>2. Karp G. (2005). Cell and Molecular Biology: Concepts and Experiments, Fourth ed, pp. 148, 165-170, and 624-664. John Wiley and Sons, Hoboken, NJ.