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.

Progression through the cell cycle is monitored at two major points: during the G1/S and the G2/M transitions. In most cells, the G2/M transition is regulated by the timing of p34cdc2 dephosphorylation which results in the activation of the kinase activity of the cdc2‐cyclin B complex. The timing of p34cdc2 dephosphorylation is determined by the balance between the activity of the kinase that phosphorylates p34cdc2 (wee1 in human cells) and the opposing phosphatase (cdc25C). Both enzymes are regulated and it has been shown that cdc25C is phosphorylated and activated by the cdc2‐cyclin B complex. This creates a positive feed‐back loop providing a switch used to control the onset of mitosis. Here, we show that another member of the human cdc25 family, cdc25A, undergoes phosphorylation during S phase, resulting in an increase of its phosphatase activity. The phosphorylation of cdc25A is dependent on the activity of the cdc2‐cyclin E kinase. Microinjection of anti‐cdc25A antibodies into G1 cells blocks entry into S phase. These results indicate that the cdc25A phosphatase is required to enter S phase in human cells and suggest that this enzyme is part of an auto‐amplification loop analogous to that described at the G2/M transition. We discuss the nature of the in vivo substrate of the cdc25A phosphatase in S phase and the possible implications for the regulation of S phase entry.

Cep152 interacts with the cryptic Polo-box of Plk4 and is required for Plk4-induced centriole overduplication.

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