Cyclins

Cyclins by their names are known as proteins whose abundance oscillates in the cell division cycle. There are numerous cyclin proteins each of which is associated with a certain stage of the cell cycle. Cyclin B is one of them and is associated with mitosis (hence named mitotic cyclin). Cyclin B is synthesized in the S/G2 phases and its abundance peaks at G2/M transition. Cyclin B binds to p34cdc2 and induces activation of p34cdc2 by triggering the critical dephosphorylation (removing a phosphate group from some of the phosphorylated sites) of the kinase. The active p34cdc2-cyclin B complex functions as the mitosis promoting factor (MPF). Once mitosis is finished, cyclin B is dessociated from p34cdc2 and is degraded through a ubiquitin-dependent mechanism. Cyclin B is then synthesized again in the next cell cycle.

Cyclin B is conserved across wide range of taxa. A homolog in marine phytoplankton was recently detected and its encoded gene isolated (Lin et al. 1995; Lin and Zhang, in preparation