We have investigated the mechanisms responsible for the sudden activation of the cdc2‐cyclin B protein kinase before mitosis. It has been found previously that cdc25 is the tyrosine phosphatase responsible for dephosphorylating and activating cdc2‐cyclin B. In Xenopus eggs and early embryos a cdc25 homologue undergoes periodic phosphorylation and activation. Here we show that the catalytic activity of human cdc25‐C phosphatase is also activated directly by phosphorylation in mitotic cells. Phosphorylation of cdc25‐C in mitotic HeLa extracts or by cdc2‐cyclin B increases its catalytic activity. cdc25‐C is not a substrate of the cyclin A‐associated kinases. cdc25‐C is able to activate cdc2‐cyclin B1 in Xenopus egg extracts and to induce Xenopus oocyte maturation, but only after stable thiophosphorylation. This demonstrates that phosphorylation of cdc25‐C is required for the activation of cdc2‐cyclin B and entry into M‐phase. Together, these studies offer a plausible explanation for the rapid activation of cdc2‐cyclin B at the onset of mitosis and the self‐amplification of MPF observed in vivo.

The small nuclear GTPase Ran controls the directionality of macromolecular transport between the nucleus and the cytoplasm. Ran also has important roles during mitosis, when the nucleus is dramatically reorganized to allow chromosome segregation. Ran directs the assembly of the mitotic spindle, nuclear-envelope dynamics and the timing of cell-cycle transitions. The mechanisms that underlie these functions provide insights into the spatial and temporal coordination of the changes that occur in intracellular organization during the cell-division cycle.

The balance between cell cycle progression and apoptosis is important for both surveillance against genomic defects and responses to drugs that arrest the cell cycle. In this report, we show that the level of the human anti‐apoptotic protein Mcl‐1 is regulated during the cell cycle and peaks at mitosis. Mcl‐1 is phosphorylated at two sites in mitosis, Ser64 and Thr92. Phosphorylation of Thr92 by cyclin‐dependent kinase 1 (CDK1)–cyclin B1 initiates degradation of Mcl‐1 in cells arrested in mitosis by microtubule poisons. Mcl‐1 destruction during mitotic arrest requires proteasome activity and is dependent on Cdc20/Fizzy, which mediates recognition of mitotic substrates by the anaphase‐promoting complex/cyclosome (APC/C) E3 ubiquitin ligase. Stabilisation of Mcl‐1 during mitotic arrest by mutation of either Thr92 or a D‐box destruction motif inhibits the induction of apoptosis by microtubule poisons. Thus, phosphorylation of Mcl‐1 by CDK1–cyclin B1 and its APC/CCdc20‐mediated destruction initiates apoptosis if a cell fails to resolve mitosis. Regulation of apoptosis, therefore, is linked intrinsically to progression through mitosis and is governed by a temporal mechanism that distinguishes between normal mitosis and prolonged mitotic arrest.