DNA double-strand break (DSB) repair by homologous recombination is initiated by DNA end resection, a process involving the controlled degradation of the 5′-terminated strands at DSB sites^1,2. The breast cancer suppressor BRCA1–BARD1 not only promotes resection and homologous recombination, but it also protects DNA upon replication stress^{1,3,4,5,6,7,8,9}. BRCA1–BARD1 counteracts the anti-resection and pro-non-homologous end-joining factor 53BP1, but whether it functions in resection directly has been unclear^{10,11,12,13,14,15,16}. Using purified recombinant proteins, we show here that BRCA1–BARD1 directly promotes long-range DNA end resection pathways catalysed by the EXO1 or DNA2 nucleases. In the DNA2-dependent pathway, BRCA1–BARD1 stimulates DNA unwinding by the Werner or Bloom helicase. Together with MRE11–RAD50–NBS1 and phosphorylated CtIP, BRCA1–BARD1 forms the BRCA1–C complex^17,18, which stimulates resection synergistically to an even greater extent. A mutation in phosphorylated CtIP (S327A), which disrupts its binding to the BRCT repeats of BRCA1 and hence the integrity of the BRCA1–C complex^19,20,21, inhibits resection, showing that BRCA1–C is a functionally integrated ensemble. Whereas BRCA1–BARD1 stimulates resection in DSB repair, it paradoxically also protects replication forks from unscheduled degradation upon stress, which involves a homologous recombination-independent function of the recombinase RAD51 (refs. ^4,5,6,8). We show that in the presence of RAD51, BRCA1–BARD1 instead inhibits DNA degradation. On the basis of our data, the presence and local concentration of RAD51 might determine the balance between the pronuclease and the DNA protection functions of BRCA1–BARD1 in various physiological contexts.

通过同源重组进行的 DNA 双链断裂 （DSB） 修复由 DNA 末端切除启动，该过程涉及 DSB 位点 5′ 末端链的受控降解^1,2。乳腺癌抑制因子 BRCA1-BARD1 不仅促进切除和同源重组，而且还在复制应激时保护 DNA^{1,3,4,5,6,7,8,9}。BRCA1-BARD1 抵消抗切除和促非同源末端连接因子 53BP1，但它是否直接在切除中发挥作用尚不清楚^{10,11,12,13,14,15,16}。使用纯化的重组蛋白，我们在这里表明 BRCA1-BARD1 直接促进由 EXO1 或 DNA2 核酸酶催化的长程 DNA 末端切除途径。在 DNA2 依赖性通路中，BRCA1-BARD1 刺激 DNA 通过 Werner 或 Bloom 解旋酶解旋。BRCA1-BARD1 与 MRE11-RAD50-NBS1 和磷酸化 CtIP 一起形成 BRCA1-C 复合体^17,18，其协同刺激切除程度更高。磷酸化 CtIP （S327A） 突变会破坏其与 BRCA1 的 BRCT 重复序列的结合，从而破坏 BRCA1-C 复合体^19,20,21 的完整性，从而抑制切除，表明 BRCA1-C 是一个功能整合的集合。虽然 BRCA1-BARD1 在 DSB 修复中刺激切除，但自相矛盾的是，它也保护复制叉在应激时免受计划外降解，这涉及重组酶 RAD51 的同源重组非依赖性功能（参考文献。^我们表明，在 RAD51 存在的情况下，BRCA1-BARD1 反而抑制 DNA 降解。 根据我们的数据，RAD51 的存在和局部浓度可能决定了在各种生理环境中 BRCA1-BARD1 的原核酸酶和 DNA 保护功能之间的平衡。