Phosphorylation of ATR-Interacting Protein on Ser Mediates an Interaction with Breast-Ovarian Cancer Susceptibility 1 and Checkpoint Function

The signaling of DNA damage and replication stress involves a multitude of proteins, including the kinases ataxia-telangiectasia mutated (ATM) and ATM and Rad3-related (ATR), and proteins with BRCA1 COOH-terminal (BRCT) domains. The BRCT domain–containing proteins facilitate the phosphorylation of ATM/ATR substrates and can be coimmunoprecipitated with ATM or ATR. However, their mode of interaction with the ATM/ATR kinases remains elusive. Here, we show that breastovarian cancer susceptibility 1 (BRCA1) interacts directly with ATR-interacting protein (ATRIP), an obligate partner of ATR. The interaction involves the BRCT domains of BRCA1 and Ser of ATRIP, a residue that is phosphorylated in both irradiated and nonirradiated cells. Consistent with a role of BRCA1 in ATR signaling, substitution of Ser of ATRIP with Ala leads to a G2-M checkpoint defect. We propose that a direct physical interaction between BRCA1 and ATRIP is required for the checkpoint function of ATR. [Cancer Res 2007;67(13):6100–5]