Protein ubiquitination at sites of DNA double-strand breaks (DSBs) by RNF168 recruits BRCA1 and 53BP1^1,2, which are mediators of the homologous recombination and non-homologous end joining DSB repair pathways, respectively³. Non-homologous end joining relies on 53BP1 binding directly to ubiquitinated lysine 15 on H2A-type histones (H2AK15ub)^4,5 (which is an RNF168-dependent modification⁶), but how RNF168 promotes BRCA1 recruitment and function remains unclear. Here we identify a tandem BRCT-domain-associated ubiquitin-dependent recruitment motif (BUDR) in BRCA1-associated RING domain protein 1 (BARD1) (the obligate partner protein of BRCA1) that, by engaging H2AK15ub, recruits BRCA1 to DSBs. Disruption of the BUDR of BARD1 compromises homologous recombination and renders cells hypersensitive to PARP inhibition and cisplatin. We further show that BARD1 binds nucleosomes through multivalent interactions: coordinated binding of H2AK15ub and unmethylated H4 lysine 20 by its adjacent BUDR and ankyrin repeat domains, respectively, provides high-affinity recognition of DNA lesions in replicated chromatin and promotes the homologous recombination activities of the BRCA1–BARD1 complex. Finally, our genetic epistasis experiments confirm that the need for BARD1 chromatin-binding activities can be entirely relieved upon deletion of RNF168 or 53BP1. Thus, our results demonstrate that by sensing DNA-damage-dependent and post-replication histone post-translation modification states, BRCA1–BARD1 complexes coordinate the antagonization of the 53BP1 pathway with promotion of homologous recombination, establishing a simple paradigm for the governance of the choice of DSB repair pathway.

RNF168 在 DNA 双链断裂 (DSB) 位点上的蛋白质泛素化会募集 BRCA1 和 53BP1 ^1,2 ，它们分别是同源重组和非同源末端连接 DSB 修复途径的介体³ 。非同源末端连接依赖于 53BP1 直接与 H2A 型组蛋白 (H2AK15ub) ^4,5上的泛素化赖氨酸 15 结合（这是一种依赖于 RNF168 的修饰⁶ ），但 RNF168 如何促进 BRCA1 募集和功能仍不清楚。在这里，我们在 BRCA1 相关 RING 结构域蛋白 1 (BARD1)（BRCA1 的专性伴侣蛋白）中鉴定出串联 BRCT 结构域相关的泛素依赖性招募基序 (BUDR)，通过接合 H2AK15ub，将 BRCA1 招募到 DSB。 BARD1 BUDR 的破坏会损害同源重组，并使细胞对 PARP 抑制和顺铂过敏。我们进一步表明，BARD1 通过多价相互作用结合核小体：H2AK15ub 和未甲基化的 H4 赖氨酸 20 分别通过其相邻的 BUDR 和锚蛋白重复结构域协调结合，提供对复制染色质中 DNA 损伤的高亲和力识别，并促进BRCA1-BARD1 复合体。最后，我们的遗传上位实验证实，删除 RNF168 或 53BP1 后，可以完全缓解对 BARD1 染色质结合活性的需求。因此，我们的结果表明，通过感测 DNA 损伤依赖性和复制后组蛋白翻译后修饰状态，BRCA1-BARD1 复合物协调 53BP1 通路的拮抗作用并促进同源重组，为管理DSB修复途径的选择