Department of Microbiology, University of California, Davis, Davis, California 95616-8665, USA.
Nature. 2011 Oct 23;479(7372):245-8. doi: 10.1038/nature10522.
Homologous recombination is a high-fidelity DNA repair pathway. Besides a critical role in accurate chromosome segregation during meiosis, recombination functions in DNA repair and in the recovery of stalled or broken replication forks to ensure genomic stability. In contrast, inappropriate recombination contributes to genomic instability, leading to loss of heterozygosity, chromosome rearrangements and cell death. The RecA/UvsX/RadA/Rad51 family of proteins catalyses the signature reactions of recombination, homology search and DNA strand invasion. Eukaryotes also possess Rad51 paralogues, whose exact role in recombination remains to be defined. Here we show that the Saccharomyces cerevisiae Rad51 paralogues, the Rad55-Rad57 heterodimer, counteract the antirecombination activity of the Srs2 helicase. The Rad55-Rad57 heterodimer associates with the Rad51-single-stranded DNA filament, rendering it more stable than a nucleoprotein filament containing Rad51 alone. The Rad51-Rad55-Rad57 co-filament resists disruption by the Srs2 antirecombinase by blocking Srs2 translocation, involving a direct protein interaction between Rad55-Rad57 and Srs2. Our results demonstrate an unexpected role of the Rad51 paralogues in stabilizing the Rad51 filament against a biologically important antagonist, the Srs2 antirecombination helicase. The biological significance of this mechanism is indicated by a complete suppression of the ionizing radiation sensitivity of rad55 or rad57 mutants by concomitant deletion of SRS2, as expected for biological antagonists. We propose that the Rad51 presynaptic filament is a meta-stable reversible intermediate, whose assembly and disassembly is governed by the balance between Rad55-Rad57 and Srs2, providing a key regulatory mechanism controlling the initiation of homologous recombination. These data provide a paradigm for the potential function of the human RAD51 paralogues, which are known to be involved in cancer predisposition and human disease.
同源重组是一种高保真的 DNA 修复途径。除了在减数分裂过程中对准确的染色体分离具有关键作用外，重组还在 DNA 修复和停滞或断裂复制叉的恢复中发挥作用，以确保基因组稳定性。相比之下，不适当的重组会导致基因组不稳定，导致杂合性丢失、染色体重排和细胞死亡。RecA/UvsX/RadA/Rad51 蛋白家族催化重组的特征反应，包括同源性搜索和 DNA 链入侵。真核生物还具有 Rad51 同源物，其在重组中的确切作用仍有待确定。在这里，我们表明酿酒酵母的 Rad51 同源物，即 Rad55-Rad57 异二聚体，可拮抗 Srs2 解旋酶的抗重组活性。Rad55-Rad57 异二聚体与 Rad51 单链 DNA 丝结合，使其比仅包含 Rad51 的核蛋白丝更稳定。Rad51-Rad55-Rad57 共丝通过阻止 Srs2 易位来抵抗 Srs2 抗重组酶的破坏，涉及 Rad55-Rad57 和 Srs2 之间的直接蛋白质相互作用。我们的结果表明，Rad51 同源物在稳定 Rad51 丝对抗生物学上重要的拮抗剂，即 Srs2 抗重组解旋酶方面发挥了意想不到的作用。这种机制的生物学意义通过同时缺失 SRS2 来完全抑制 rad55 或 rad57 突变体对电离辐射的敏感性来表明，这与生物学拮抗剂一致。我们提出 Rad51 前突触丝是一种亚稳态可逆中间物，其组装和解组装受 Rad55-Rad57 和 Srs2 之间的平衡控制，为控制同源重组起始的关键调节机制提供了一个范例。这些数据为人类 RAD51 同源物的潜在功能提供了一个范例，已知它们参与癌症易感性和人类疾病。
