Centrosomal and cytoplasmic Cdc2/cyclin B1 activation precedes nuclear mitotic events.
The activation of cdc2/cyclin B is the trigger for entry into mitosis. The mechanism of cdc2/cyclin B activation is complex, but the final step is the dephosphorylation of the Thr14 and Tyr15 residues on the cdc2 subunit, catalyzed by a member of the Cdc25 family of phosphatases. Cdc2/cyclin B1 accumulates at the centrosome in late G2 phase and has been implicated in the conversion of the centrosome from an interphase to a mitotic microtubule organizing center. Here we demonstrate biochemically that cdc2/cyclin B1 accumulates at the centrosome in late G2 as the inactive, phosphotyrosine 15 form and that the centrosomal cdc2/cyclin B1 can be activated in vitro by recombinant cdc25B. We provide evidence that a portion of the cdc2/cyclin B1 translocated into the nucleus in prophase is the inactive tyrosine-15-phosphorylated form. At this time the centrosomal and cytoplasmic cdc2/cyclin B1 is already active. This provides evidence that the activation of cdc2/cyclin B1 is initiated in the cytoplasm and that full activation of the translocated pool occurs in the nucleus.
De Souza
C P
CP
Queensland Cancer Fund Research Laboratories, Queensland Institute of Medical Research, Brisbane, Australia.
Ellem
K A
KA
Gabrielli
B G
BG
D016203:CDC2 Protein Kinase / Q000378:metabolism*; D002467:Cell Nucleus / Q000378:metabolism / Q000648:ultrastructure; D018385:Centrosome / Q000378:metabolism* / Q000648:ultrastructure; D019926:Cyclin B / Q000378:metabolism*; D056744:Cyclin B1; D004789:Enzyme Activation; D006801:Humans; D008938:Mitosis; D015398:Signal Transduction; D013601:T-Lymphocytes / Q000166:cytology* / Q000378:metabolism*; D014407:Tumor Cells, Cultured
C534699:CCNB1 protein, human; D019926:Cyclin B; D056744:Cyclin B1; D016203:CDC2 Protein Kinase