title: Phosphorylation and activation of the Xenopus Cdc25 phosphatase in the absence of Cdc2 and Cdk2 kinase activity.
abstract: The M-phase inducer, Cdc25C, is a dual-specificity phosphatase that directly phosphorylates and activates the cyclin B/Cdc2 kinase complex, leading to initiation of mitosis. Cdc25 itself is activated at the G2/M transition by phosphorylation on serine and threonine residues. Previously, it was demonstrated that Cdc2 kinase is capable of phosphorylating and activating Cdc25, suggesting the existence of a positive feedback loop. In the present study, kinases other than Cdc2 that can phosphorylate and activate Cdc25 were investigated. Cdc25 was found to be phosphorylated and activated by cyclin A/Cdk2 and cyclin E/Cdk2 in vitro. However, in interphase Xenopus egg extracts with no detectable Cdc2 and Cdk2, treatment with the phosphatase inhibitor microcystin activated a distinct kinase that could phosphorylate and activate Cdc25. Microcystin also induced other mitotic phenomena such as chromosome condensation and nuclear envelope breakdown in extracts containing less than 5% of the mitotic level of Cdc2 kinase activity. These findings implicate a kinase other than Cdc2 and Cdk2 that may initially activate Cdc25 in vivo and suggest that this kinase may also phosphorylate M-phase substrates even in the absence of Cdc2 kinase.
authors.affiliation: Howard Hughes Medical Institute, University of Colorado School of Medicine, Denver 80262, USA.
authors.forename: T
authors.initials: T
authors.lastname: Izumi
authors.forename: J L
authors.initials: JL
authors.lastname: Maller
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D016203:CDC2 Protein Kinase / Q000378:metabolism*; D042846:CDC2-CDC28 Kinases; D002453:Cell Cycle / Q000502:physiology*; D018797:Cell Cycle Proteins / Q000378:metabolism*; D002467:Cell Nucleus / Q000502:physiology / Q000648:ultrastructure; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D004789:Enzyme Activation; D005260:Female; D007700:Kinetics; D008297:Male; D008938:Mitosis; D008954:Models, Biological; D008969:Molecular Sequence Data; D009865:Oocytes / Q000166:cytology* / Q000502:physiology*; D010749:Phosphoprotein Phosphatases / Q000378:metabolism*; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D013084:Sperm-Ovum Interactions; D014981:Xenopus; D029867:Xenopus Proteins; D020687:cdc25 Phosphatases
chemical_list: D018797:Cell Cycle Proteins; D029867:Xenopus Proteins; D017346:Protein Serine-Threonine Kinases; D016203:CDC2 Protein Kinase; D042846:CDC2-CDC28 Kinases; C495891:Cdk2 protein, Xenopus; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases; D010749:Phosphoprotein Phosphatases; D020687:cdc25 Phosphatases