Abstract
The tumor suppressor protein BRCA1 is a constituent of several different protein complexes and is required for homology-directed repair (HDR) of DNA double strand breaks (DSBs). The most recently discovered BRCA1-RAP80 complex is recruited to ubiquitin structures on chromatin surrounding the break. Deficiency of any member of this complex confers hypersensitivity to DNA-damaging agents by undefined mechanisms. In striking contrast to other BRCA1-containing complexes that are known to promote HDR, we demonstrate that the BRCA1-RAP80 complex restricts end resection in S/G(2) phase of the cell cycle, thereby limiting HDR. RAP80 or BRCC36 deficiency resulted in elevated Mre11-CtIP-dependent 5' end resection with a concomitant increase in HDR mechanisms that rely on 3' single-stranded overhangs. We propose a model in which the BRCA1-RAP80 complex limits nuclease accessibility to DSBs, thus preventing excessive end resection and potentially deleterious homology-directed DSB repair mechanisms that can impair genome integrity.