Structural studies of a novel mediator regulating homologous recombination

Rad 51在重组、组装到单链DNA上作为核蛋白丝并催化同源DNA序列的侵入和交换中具有核心作用。通常，Rad 51需要介体的帮助来克服RPA的抑制。Swi 5-Sfr 1复合物被认为是一种介体，最近的研究表明它能直接激活重组酶，因此该复合物是第一个重组酶激活剂分子。粟酒Swi 5和Sfr 1形成平行卷曲螺旋异二聚体，通过形成两个新鉴定的亮氨酸拉链基序和一个束而牢固地连接在一起。值得注意的是，螺旋线圈是急剧扭结，产生一个细长和弯曲的形状。这种分子形状适合于结合在Rad 51细丝的重构螺旋槽内，通过使Rad 51细丝稳定为具有较长螺距的活性形式而引起Rad 51细丝的活化。生化分析表明，与Swi 5复合的Sfr 1的C-末端区域具有Swi 5-Sfr 1作为重组激活剂的基本功能，而Sfr 1的N-末端区域在Swi 5-Sfr 1复合物的有效募集中起作用。Swi 5-Sfr 1从酵母到人类都是保守的，因此这里呈现的现象在所有真核生物中都是普遍的。

Rad51 has a central role in recombination, assembling onto single-stranded DNA as a nucleoprotein filament and catalyzing the invasion and exchange of homologous DNA sequences. Generally, Rad51 requires the assistance of mediators to overcome the inhibition by RPA. The Swi5-Sfr1 complex has been identified as a mediator, and recent studies have shown that it can activate recombinases directly, and thus this complex is the first molecule for the recombinase activator.We describe the crystal structure of the C-terminal half of Sfr1 in complex with Swi5 from S. pombe. Swi5 and Sfr1 form a parallel coiled-coil heterodimer, joined firmly together by formation of two newly identified leucine-zipper motifs and a bundle. Remarkably, the coiled-coil is sharply kinked, generating an elongated and curved shape. This molecular shape is suitable for binding within the reconstituted helical groove of the Rad51 filament, causing the activation of Rad51 filament by stabilizing it as an active form with a longer helical pitch. Biochemical analysis shows that the C-terminal region of Sfr1 complexed with Swi5 possesses the essential function of Swi5-Sfr1 for a recombination activator, while the N-terminal region of Sfr1 plays a role for the efficient recruitment of the Swi5-Sfr1 complex. Swi5-Sfr1 is conserved from yeast to humans, and therefore the phenomena presented here would be prevalent among all eukaryotic organisms.