Figure 6.

Chk2/hCds1 functions as a G₁ checkpoint in response to DNA damage. (A) Diagram of Chk2/hCds1 protein and deletion mutants. (SQ) Region that contains the SQ/TQ motifs that are putatively phosphorylated by ATM; (FHA) FHA domain; (KD) kinase domain. (B) Expression of Flag-tagged Chk2/hCds1 proteins in transiently transfected U2–OS cells exposed to IR. Extracts from transfected cells were resolved by denaturing gel electrophoresis and immunoblotted with an antibody that recognizes the Flag tag (p53W248, which does not have a Flag tag, is not recognized by this antibody). (C) Chk2/hCds1 induces cell cycle arrest in G₁. U2–OS cells were either nontransfected (−) or transiently transfected with plasmids expressing the indicated Chk2/hCds1 and p53 checkpoint proteins and a plasmid expressing GFP (as a marker). Cell cycle profile was determined 12 hr after exposure of the cells to IR. GFP− cells do not express proteins from the transfected plasmids and serve as negative controls. 2N and 4N DNA contents correspond to cells in G₁ and G₂/M, respectively.