ACTIVATION OF THE PHOSPHATASE-ACTIVITY OF HUMAN CDC25A BY A CDK2 CYCLIN-E DEPENDENT PHOSPHORYLATION AT THE G(1)/S TRANSITION

Progression through the cell cycle is monitored at two major points: during the G(1)/S and the G(2)/M transitions. In most cells, the G(2)/M transition is regulated by the timing of p34(cdc2) dephosphorylation which results in the activation of the kinase activity of the cdc2-cyclin B complex. The timing of p34(cdc2) dephosphorylation is determined by the balance between the activity of the kinase that phosphorylates p34(cdc2) (wee1 in human cells) and the opposing phosphatase (cdc25C). Both enzymes are regulated and it has been shown that cdc25C is phosphorylated and activated ba the cdc2-cyclin B complex. This creates a positive feedback loop providing a snitch 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 G(1) 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 G(2)/M transition. We discuss the nature of the irt vivo substrate of the cdc25A phosphatase in S phase and the possible implications for the regulation of S phase entry.