Initial activation of cyclin‐B1–cdc2 kinase requires phosphorylation of cyclin B1

At the G 2 /M transition of the cell cycle, the cdc25c phosphatase dephosphorylates inhibitory residues of cdc2, and cyclin‐B–cdc2 kinase (MPF) is activated. Phosphorylation of cyclin B1 induces its nuclear accumulation, and, since cdc25c is also believed to accumulate and activate shortly before G 2 /M in the nucleus, it has been proposed that this induces cyclin‐B1–cdc2 kinase activation. We demonstrate that cyclin B1 phosphorylation has another essential function in vivo : it is required for cdc25c and MPF activation, which does not require nuclear accumulation of cyclin B1, and occurs in the cytoplasm.