A human homologue of the checkpoint kinase Cds1 directly inhibits Cdc25 phosphatase

Background: In human-cells, the mitosis-inducing kinase Cdc2 is inhibited b y phosphorylation on Thr14 and Tyr15, Disruption of these phosphorylation s ites abrogates checkpoint-mediated regulation of Cdc2 and renders cells hig hly sensitive to agents that damage DNA, Phosphorylation of these sites is controlled by the opposing activities of the Wee1/Myt1 kinases and the Cdc2 5 phosphatase, The regulation of these enzymes is therefore likely to be cr ucial for the operation of the G2-M DNA-damage checkpoint. Results: Here, we show that the activity of Cdc25 decreased following expos ure to ionizing radiation. The irradiation-induced decrease in Cdc25 activi ty was suppressed by wortmannin, an inhibitor of phosphatidylinositol (PI) 3-kinases, and was dependent on the function of the gene that is mutated in ataxia telangiectasia. We also identified two human kinases that phosphory late and inactivate Cdc25 in vitro, One is the previously characterized Chk 1 kinase, The second is novel and is homologous to the Cds1/Rad53 family of checkpoint kinases in yeast, Human Cds1 was found to be activated in respo nse to DNA damage. Conclusions: These results suggest that, in human cells, the DNA-damage che ckpoint involves direct inactivation of Cdc25 catalyzed by Cds1 and/or Chk1 .