Résumé
The serine-threonine protein kinase p34cdc2 is a crucial component of the regulatory mechanisms that bring about coordinate division in eukaryotic cells (1). Whilst p34cdc2 protein level is constant throughout the cell division cycle, its kinase activity reaches a maximum at the G2/M transition, coincident with mitotic entry. The complete activation of p34cdc2 kinase at mitosis involves two distinct mechanims. First, p34cdc2 binds a regulatory subunit, named cyclin B, whose expression oscillates during the cell cycle. A number of different cyclins have been cloned, but their precise function, in particular the nature of the cdk protein they bind or the time of the cell cycle when these cyclin-dependent kinases are activated, is still not fully elucidated (2). Secondly, p34cdc2 kinase activation involves changes in the phosphorylation state of p34, in particular the dephosphorylation of the residues thr14 and tyr15, located in the amino terminus of the molecule and within the ATP-binding site of p34cdc2 (3).These specific dephosphorylations are under the control of the cdc25 protein phosphatase, which is the first member of a new phosphatase class showing a dual specificity for both tyrosine and threonine residues (4,5). Homologs of the fission yeast cdc25 gene have now been identified in a variety of organisms, including human (5). Interestingly, three cdc25 genes named A, B and C have been cloned in human cells (6,7), but their substrate specificity towards the different cylin-dependent kinases and their function during the cell cycle is still unknown.