OVEREXPRESSION OF A KINASE-INACTIVE ATR PROTEIN CAUSES SENSITIVITY TODNA-DAMAGING AGENTS AND DEFECTS IN CELL-CYCLE CHECKPOINTS

ATR, a phosphatidylinositol kinase-related protein homologous to ataxi a telangiectasia mutated (ATM), is important for the survival of human cells following many forms of DNA damage. Expression of a kinase-inac tive allele of ATR (ATRkd) in human fibroblasts causes increased sensi tivity to ionizing radiation (IR), cis-platinum and methyl methanesulf onate, but only slight UV radiation sensitivity. ATRkd overexpression abrogates the G(2)/M arrest after exposure to IR, and overexpression o f wild-type ATR complements the radioresistant DNA synthesis phenotype of cells lacking ATM, suggesting a potential functional overlap betwe en these proteins, ATRkd overexpression also causes increased sensitiv ity to hydroxyurea that is associated with microtubule-mediated nuclea r abnormalities. These observations are consistent with uncoupling of certain mitotic events from the completion of S-phase. Thus, ATR is an important component of multiple DNA damage response pathways and may be involved in the DNA replication (S/M) checkpoint.