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Proc Natl Acad Sci U S A. 2002 Nov 12;99(23):14795-800. Epub 2002 Oct 24.

Disruption of the checkpoint kinase 1/cell division cycle 25A pathway abrogates ionizing radiation-induced S and G2 checkpoints.

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  • 1Department of Cell Biology and Physiology, Howard Hughes Medical Institute, Washington University School of Medicine, Box 8228, 660 South Euclid Avenue, St. Louis, MO 63110-1093, USA.


Checkpoint kinase (Chk)1 is an evolutionarily conserved protein kinase that was first identified in fission yeast as an essential component of the DNA damage checkpoint. In mice, Chk1 provides an essential function in the absence of environmentally imposed genotoxic stress. Here we show that human cells lacking Chk1 exhibit defects in both the ionizing radiation (IR)-induced S and G(2) checkpoints. In addition, loss of Chk1 resulted in the accumulation of a hypophosphorylated form of the Cdc25A protein phosphatase, and Chk1-deficient cells failed to degrade Cdc25A after IR. The IR-induced S and G(2) checkpoints were partially restored in Chk1-deficient cells when Cdc25A accumulation was interfered with. Finally, Cdc25A was phosphorylated by Chk1 in vitro on similar sites phosphorylated in vivo, including serine-123. These findings indicate that Chk1 directly phosphorylates Cdc25A during an unperturbed cell cycle, and that phosphorylation of Cdc25A by Chk1 is required for cells to delay cell cycle progression in response to double-strand DNA breaks.

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