Active cyclin B1-Cdk1 first appears on centrosomes in prophase.
Cyclin B1-Cdk1 is the key initiator of mitosis, but when and where activation occurs has not been precisely determined in mammalian cells. Activation may occur in the nucleus or cytoplasm, as just before nuclear envelope breakdown, Polo-like kinase1 (Plk1) is proposed to phosphorylate cyclin B1 in its nuclear export sequence (NES), to trigger rapid nuclear import. We raised phospho-specific antibodies against cyclin B1 that primarily recognise the active form of the complex. We show that cyclin B1 is initially phosphorylated on centrosomes in prophase and that Plk1 phosphorylates cyclin B1, but not in the NES. Furthermore, phosphorylation by Plk1 does not cause cyclin B1 to move into the nucleus. We conclude that cyclin B1-Cdk1 is first activated in the cytoplasm and that centrosomes may function as sites of integration for the proteins that trigger mitosis.
Jackman
Mark
M
Wellcome Trust/ Cancer Research UK Institute and Department of Zoology, Tennis Court Road, Cambridge, CB2 1QR, UK.
Lindon
Catherine
C
Nigg
Erich A
EA
Pines
Jonathon
J
D000818:Animals; D000906:Antibodies / Q000378:metabolism; D016203:CDC2 Protein Kinase / Q000378:metabolism*; D018797:Cell Cycle Proteins; D002467:Cell Nucleus / Q000378:metabolism; D018385:Centrosome / Q000378:metabolism*; D019926:Cyclin B / Q000378:metabolism*; D056744:Cyclin B1; D003593:Cytoplasm / Q000378:metabolism; D004789:Enzyme Activation; D006367:HeLa Cells; D006801:Humans; D046911:Macromolecular Substances; D008856:Microscopy, Fluorescence; D010766:Phosphorylation; D011418:Prophase / Q000502:physiology*; D011494:Protein Kinases / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D012694:Serine / Q000378:metabolism; D000097869:Polo-Like Kinase 1
D000906:Antibodies; D016203:CDC2 Protein Kinase; D018797:Cell Cycle Proteins; D019926:Cyclin B; D056744:Cyclin B1; D046911:Macromolecular Substances; D011494:Protein Kinases; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D012694:Serine; D000097869:Polo-Like Kinase 1; C534699:CCNB1 protein, human