Bugreev Dmitry V, Mazina Olga M, Mazin Alexander V
Department of Biochemistry and Molecular Biology, Drexel University College of Medicine, Philadelphia, Pennsylvania 19102-1192, USA.
Nature. 2006 Aug 3;442(7102):590-3. doi: 10.1038/nature04889. Epub 2006 Jul 2.
Homologous recombination has a crucial function in the repair of DNA double-strand breaks and in faithful chromosome segregation. The mechanism of homologous recombination involves the search for homology and invasion of the ends of a broken DNA molecule into homologous duplex DNA to form a cross-stranded structure, a Holliday junction (HJ). A HJ is able to undergo branch migration along DNA, generating increasing or decreasing lengths of heteroduplex. In both prokaryotes and eukaryotes, the physical evidence for HJs, the key intermediate in homologous recombination, was provided by electron microscopy. In bacteria there are specialized enzymes that promote branch migration of HJs. However, in eukaryotes the identity of homologous recombination branch-migration protein(s) has remained elusive. Here we show that Rad54, a Swi2/Snf2 protein, binds HJ-like structures with high specificity and promotes their bidirectional branch migration in an ATPase-dependent manner. The activity seemed to be conserved in human and yeast Rad54 orthologues. In vitro, Rad54 has been shown to stimulate DNA pairing of Rad51, a key homologous recombination protein. However, genetic data indicate that Rad54 protein might also act at later stages of homologous recombination, after Rad51 (ref. 13). Novel DNA branch-migration activity is fully consistent with this late homologous recombination function of Rad54 protein.
同源重组在DNA双链断裂修复和准确的染色体分离过程中发挥着关键作用。同源重组机制包括寻找同源序列以及断裂DNA分子末端侵入同源双链DNA以形成交联结构，即霍利迪连接体（HJ）。HJ能够沿着DNA进行分支迁移，产生长度不断增加或减少的异源双链。在原核生物和真核生物中，电子显微镜为同源重组的关键中间体HJ提供了物理证据。在细菌中，存在促进HJ分支迁移的特殊酶。然而，在真核生物中，同源重组分支迁移蛋白的身份一直难以确定。在此，我们表明，Swi2/Snf2蛋白Rad54能以高特异性结合类HJ结构，并以ATP酶依赖的方式促进其双向分支迁移。该活性在人类和酵母的Rad54直系同源物中似乎是保守的。在体外，Rad54已被证明能刺激关键同源重组蛋白Rad51的DNA配对。然而，遗传数据表明，Rad54蛋白可能也在同源重组的后期发挥作用，即在Rad51之后（参考文献13）。新发现的DNA分支迁移活性与Rad54蛋白的这种后期同源重组功能完全一致。
