RAD52 is important for the repair of DNA double-stranded breaks^1,2, mitotic DNA synthesis^3,4,5 and alternative telomere length maintenance^6,7. Central to these functions, RAD52 promotes the annealing of complementary single-stranded DNA (ssDNA)^8,9 and provides an alternative to BRCA2/RAD51-dependent homologous recombination repair¹⁰. Inactivation of RAD52 in homologous-recombination-deficient BRCA1- or BRCA2-defective cells is synthetically lethal^11,12, and aberrant expression of RAD52 is associated with poor cancer prognosis^13,14. As a consequence, RAD52 is an attractive therapeutic target against homologous-recombination-deficient breast, ovarian and prostate cancers^15,16,17. Here we describe the structure of RAD52 and define the mechanism of annealing. As reported previously^18,19,20, RAD52 forms undecameric (11-subunit) ring structures, but these rings do not represent the active form of the enzyme. Instead, cryo-electron microscopy and biochemical analyses revealed that ssDNA annealing is driven by RAD52 open rings in association with replication protein-A (RPA). Atomic models of the RAD52–ssDNA complex show that ssDNA sits in a positively charged channel around the ring. Annealing is driven by the RAD52 N-terminal domains, whereas the C-terminal regions modulate the open-ring conformation and RPA interaction. RPA associates with RAD52 at the site of ring opening with critical interactions occurring between the RPA-interacting domain of RAD52 and the winged helix domain of RPA2. Our studies provide structural snapshots throughout the annealing process and define the molecular mechanism of ssDNA annealing by the RAD52–RPA complex.

RAD52 对于 DNA 双链断裂的修复^1,2、有丝分裂 DNA 合成^3,4,5以及替代端粒长度维持^6,7非常重要。作为这些功能的核心，RAD52 促进互补单链 DNA (ssDNA) 的退火^8,9并为 BRCA2/RAD51 依赖性同源重组修复提供替代方案¹⁰。在同源重组缺陷型BRCA1或BRCA2缺陷细胞中，RAD52 失活具有综合致死性^11,12，RAD52 的异常表达与不良癌症预后相关^13,14。因此，RAD52 是针对同源重组缺陷型乳腺癌、卵巢癌和前列腺癌的有吸引力的治疗靶点^15,16,17。在这里，我们描述了 RAD52 的结构并定义了退火机制。如之前报道的^18,19,20，RAD52形成十一聚体（11个亚基）环结构，但这些环并不代表酶的活性形式。相反，冷冻电子显微镜和生化分析表明，ssDNA 退火是由与复制蛋白 A (RPA) 相关的 RAD52 开环驱动的。 RAD52-ssDNA 复合物的原子模型表明，ssDNA 位于环周围带正电荷的通道中。退火由 RAD52 N 端结构域驱动，而 C 端区域调节开环构象和 RPA 相互作用。 RPA 在开环位点与 RAD52 结合，RAD52 的 RPA 相互作用结构域和 RPA2 的翼状螺旋结构域之间发生关键的相互作用。我们的研究提供了整个退火过程的结构快照，并定义了 RAD52-RPA 复合物进行 ssDNA 退火的分子机制。