Response of Xenopus Cds1 in Cell-free Extracts to DNA Templates with Double-stranded Ends

Although homologues of the yeast checkpoint kinases Cds1 and Chk1 have been identified in various systems, the respective roles of these kinases in the responses to damaged and/or unreplicated DNA in vertebrates have not been delineated precisely. Likewise, it is largely unknown how damaged DNA and unreplicated DNA trigger the pathways that contain these effector kinases. We report that Xenopus Cds1 (Xcds1) is phosphorylated and activated by the presence of some simple DNA molecules with double-stranded ends in cell-free Xenopus egg extracts. Xcds1 is not affected by aphidicolin, an agent that induces DNA replication blocks. In contrast, Xenopus Chk1 (Xchk1) responds to DNA replication blocks but not to the presence of double-stranded DNA ends. Immunodepletion of Xcds1 (and/or Xchk1) from egg extracts did not attenuate the cell cycle delay induced by double-stranded DNA ends. These results imply that the cell cycle delay triggered by double-stranded DNA ends either does not involve Xcds1 or uses a factor(s) that can act redundantly with Xcds1.