EXD2 is required for repair of damage to DNA
In an effort to identify factors required to promote HR, we carried out an unbiased proteomic approach to define the CtIP interactome. Here, we have identified EXD2, a largely uncharacterized protein with a putative exonuclease domain, as a candidate CtIP binding partner (Fig. 1a). We validated this interaction by co-immunoprecipitations from human cell extracts and found that we could readily detect endogenous EXD2 by western blotting of GFP-CtIP immunoprecipitates (Fig. 1b). Endogenous CtIP, as well as its known interactors MRE11 and BRCA1 were detected in a reciprocal FLAG-EXD2 immunoprecipitates (Fig. 1c; lysates were treated with benzonase to prevent DNA bridging). Therefore, we conclude that the two proteins likely exist in the same complex in cells.
EXD2 is highly conserved across vertebrates (Supplementary Fig. 1) and was recently identified in the screen for suppression of sensitivity to mitomycin C 24. However, the biological and biochemical features of this protein are unknown. Since we identified EXD2 as an interactor of DBS-repair factors we tested its requirement in response to a range of DSBs-inducing agents namely, ionizing radiation (IR), campthotecin (CPT) and phleomycin. We found that depletion of EXD2 by two different siRNAs sensitized U2OS cells to these agents (Fig. 1d, e, f and Supplementary Fig. 2a and b). Taken together these results suggest a putative role for this protein in the repair of damaged DNA.