InterPro domain: IPR013922

Cyclins are eukaryotic proteins that play an active role in controlling nuclear cell division cycles [ 1 ], and regulate cyclin dependent kinases (CDKs). Cyclins, together with the p34 (cdc2) or cdk2 kinases, form the Maturation Promoting Factor (MPF). There are two main groups of cyclins, G1/S cyclins, which are essential for the control of the cell cycle at the G1/S (start) transition, and G2/M cyclins, which are essential for the control of the cell cycle at the G2/M (mitosis) transition. G2/M cyclins accumulate steadily during G2 and are abruptly destroyed as cells exit from mitosis (at the end of the M-phase). In most species, there are multiple forms of G1 and G2 cyclins. For example, in vertebrates, there are two G2 cyclins, A and B, and at least three G1 cyclins, C, D, and E.

Cyclin homologues have been found in various viruses, including Saimiriine herpesvirus 2 (Herpesvirus saimiri) and Human herpesvirus 8 (HHV-8) (Kaposi's sarcoma-associated herpesvirus). These viral homologues differ from their cellular counterparts in that the viral proteins have gained new functions and eliminated others to harness the cell and benefit the virus [ 2 ].

This entry includes cyclin PHO80 and other cyclins that partner with the cyclin-dependent kinase (CDK) PHO85. The PHO80/PHO85 cyclin-cdk complex is used for a regulatory process other than cell-cycle control [ 3 ]. This entry also includes other PHO80-like cyclins that are involved in the cell-cycle control. They belong to the P/U family and interact preferentially with CDKA1 [ 4 ].

1. Cell cycle regulation and neural differentiation. Oncogene 22, 5208-19
2. Cyclin' on the viral path to destruction. Nat. Cell Biol. 2, E203-4
3. Phosphorylation of the transcription factor PHO4 by a cyclin-CDK complex, PHO80-PHO85. Science 263, 1153-6
4. Molecular characterization of Arabidopsis PHO80-like proteins, a novel class of CDKA;1-interacting cyclins. Cell. Mol. Life Sci. 61, 1485-97