Mammalian Chk2 is a downstream effector of the ATM-dependent DNA damage checkpoint pathway

In response to DNA damage and replication blocks, cells activate pathways t hat arrest the cell cycle and induce the transcription of genes that facili tate repair. In mammals, ATM (ataxia telangiectasia mutated) kinase togethe r with other checkpoint kinases are important components in this response. We have cloned the rat and human homologs of Saccharomyces cerevisiae Rad 5 3 and Schizosaccharomyces pombe Cds1, called checkpoint kinase 2 (chk2). Co mplementation studies suggest that Chk2 can partially replace the function of the defective checkpoint kinase in the Cds1 deficient yeast strain. Chk2 was phosphorylated and activated in response to DNA damage in an ATM depen dent manner. Its activation in response to replication blocks by hydroxyure a (HU) treatment, however, was independent of ATM, Using mass spectrometry, me found that, similar to Chk1, Chk2 can phosphorylate serine 216 in Cdc25 C, a site known to be involved in negative regulation of Cdc25C, These resu lts suggest that Chk2 is a downstream effector of the ATM-dependent DNA dam age checkpoint pathway. Activation of Chk2 might not only delay mitotic ent ry, but also increase the capacity of cultured cells to survive after treat ment with gamma-radiation or with the topoisomerase-I inhibitor topotecan.