REV7 associates with ATRIP and inhibits ATR kinase activity.

Ataxia-telangiectasia mutated and RAD3-related (ATR) and its partner ATR-interacting protein (ATRIP) function as a critical proximal sensor and transducer of the DNA damage response (DDR). Several ATR substrates, including p53 and CHK1, are crucial for the coordination of cell cycle phase transitions, transcription, and DNA repair when cells sustain DNA damage. While much is known about ATR activation mechanisms, it is less clear how ATR signaling is negatively regulated in cells. Here, we identify the DNA repair protein REV7 as a novel direct binding partner of ATRIP. We define a REV7-interaction motif in ATRIP, which, when mutated, abrogates the REV7-ATRIP interaction in vitro and in intact cells. Using in vitro kinase assays, we show that REV7 inhibits ATR-mediated phosphorylation of its substrates, including p53. Disruption of the REV7-ATRIP interaction also enhances phosphorylation of CHK1 at Ser317 in intact cells. Taken together, our results establish REV7 as a critical negative regulator of ATR signaling. REV7 has pleiotropic roles in multiple DDR pathways, including Translesion Synthesis, DNA double-strand break resection, and p53 stability and may play a central role in the integration of multiple genome maintenance pathways.