The MRE11 GAR motif regulates DNA double-strand break processing and ATR activation

The MRE11/RAD50/NBS1 complex is the primary sensor rapidly recruited to DNA double-strand breaks （DSBs）. MREll is known to be arginine methylated by PRMT1 within its glycine-arginine-rich （GAR） motif. In this study, we report a mouse knock-in allele of Mrell that substitutes the arginines with lysines in the GAR motif and gener- ates the MRE11^RK protein devoid of methylated arginines. The Mre11^RK/RK mice were hypersensitive to γ-irradiation （IR） and the cells from these mice displayed cell cycle checkpoint defects and chromosome instability. Moreover, the Mre11^RK/RK MEFs exhibited ATR/CHK1 signaling defects and impairment in the recruitment of RPA and RAD51 to the damaged sites. The M^RKRN complex formed and localized to the sites of DNA damage and normally activated the ATM pathway in response to IR. The M^RKRN complex exhibited exonuclease and DNA-binding defects in vitro responsible for the impaired DNA end resection and ATR activation observed in vivo in response to IR. Our findings provide genetic evidence for the critical role of the MRE11 GAR motif in DSB repair, and demonstrate a mechanis- tic link between post-translational modifications at the MRE11 GAR motif and DSB processing, as well as the ATR/ CHK1 checkpoint signaling.