IDENTIFICATION OF A NOVEL VERTEBRATE CYCLIN - CYCLIN-B3 SHARES PROPERTIES WITH BOTH A-TYPE AND B-TYPE CYCLINS

Cyclins play a key role in controlling progression through the cell cy cle. They act as regulatory subunits of p34cdc2/CDC28 and related cycl in-dependent protein kinases (cdks). In vertebrates, cyclins B1 and B2 function during M phase, whereas cyclin A is required for S phase as well as the G2 to M phase transition. Here, we describe the identifica tion and characterization of a novel vertebrate cyclin, termed cyclin B3. The assignment of this cyclin to the B-type subfamily is based on its cDNA-derived sequence and its pattern of expression in synchronize d cells, both suggesting a distant relationship to other B-type cyclin s. Interestingly, however, cyclin B3 also displays properties that res emble those of A- rather than B-type cyclins. Specifically, cyclin B3 localizes to the cell nucleus throughout the cell cycle, and is able t o associate in vivo with at least two kinase subunits, p34cdc2 and p33 cdk2. Furthermore, deletion of 26 amino acids from the C-terminus of c yclin B3 impairs both its interaction with kinase catalytic subunits a nd its nuclear localization, reminiscent of recent results obtained wi th cyclin A. Based on,these observations, we conclude that cyclin B3 m ay share functional properties with both A- and B-type cyclins.