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Description

G2/mitotic-specific cyclin-B1 (UniProt: P14635, also known as Cyclin B1) is encoded by the CCNB1 (also known as CCNB) gene (Gene ID: 891) in human. Cyclins are the regulatory subunits of the cell cycle-dependent kinases (CDKs) that are responsible for the phosphorylation of several cellular targets. Cyclins contain the nuclear localization sequence (NLS) that help move CDKs into the nucleus. They also contain PEST (Pro, Glu, Ser, and Thr) sequences that target them for degradation by the ubiquitin-proteasomal pathway. Once the CDKs have completed their role, they undergo a rapid programmed proteolysis via ubiquitin-mediated delivery to the proteasome complex. Cyclin B1, a regulatory protein involved in mitosis, complexes with CDK1 to form the maturation-promoting factor (MPF). It is shown to be essential for the control of the cell cycle at the G2/M (mitosis) transition. It accumulates steadily during G2 phase and is abruptly destroyed at mitosis. Hence, the cyclin B1-CDK1 complex is considered to be a key regulator for mitotic entry. This complex phosphorylates a number of proteins prior to mitotic entry. Although five serine phosphorylation sites are described for cyclin B1, (Ser 116, 126, 128, 133, and 147), serine 133 phosphorylation by PLK1 regulates the entry of Cyclin B1-CDK1 complex into the nucleus during prophase. At the end of mitosis, cyclin B1 is rapidly removed by a ubiquitin ligase (anaphase-promoting complex/cyclosome) loaded with the targeting subunit CDC20. Activated cyclin B1-CDK1 complex is reported to catalyze its own destruction by stimulating the activity of APC. (Ref.: Van Zon, W., et al. (2010). J. Cell. Biol. 190(4), 587-602, Yuan, J., et al. (2004). Oncogene 23(34), 5843-5852).