CYCLIN-B2 UNDERGOES CELL CYCLE-DEPENDENT NUCLEAR TRANSLOCATION AND, WHEN EXPRESSED AS A NON-DESTRUCTIBLE MUTANT, CAUSES MITOTIC ARREST IN HELA-CELLS

Cyclin proteins form complexes with members of the p34cdc2 kinase fami ly and they are essential components of the cell cycle regulatory mach inery. They are thought to determine the timing of activation, the sub cellular distribution, and/or the substrate specificity of cdc2-relate d kinases, but their precise mode of action remains to be elucidated. Here we report the cloning and sequencing of avian cyclin B2. Based on the use of monospecific antibodies raised against bacterially express ed protein, we also describe the subcellular distribution of cyclin B2 in chick embryo fibroblasts and in DU249 hepatoma cells. By indirect immunofluorescence microscopy we show that cyclin B2 is cytoplasmic du ring interphase of the cell cycle, but undergoes an abrupt translocati on to the cell nucleus at the onset of mitotic prophase. Finally, we h ave examined the phenotypic consequences of expressing wild-type and m utated versions of avian cyclin B2 in HeLa cells. We found that expres sion of cyclin B2 carrying a mutation at arginine 32 (to serine) cause d HeLa cells to arrest in a pseudomitotic state. Many of the arrested cells displayed multiple mitotic spindles, suggesting that the centros ome cycle had continued in spite of the cell cycle arrest.