Rad54 protein promotes branch migration of Holliday junctions

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Abstract
Rad54, a translocase that uses the energy of ATP hydrolysis to move along a duplex, promotes Holliday junction branch migration. Genetic studies in yeast have implicated Rad54 as acting late in recombination, when Holliday junctions are present. Homologous recombination has a crucial function in the repair of DNA double-strand breaks and in faithful chromosome segregation1,2,3. 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)4,5,6,7. 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 microscopy8. In bacteria there are specialized enzymes that promote branch migration of HJs7. However, in eukaryotes the identity of homologous recombination branch-migration protein(s) has remained elusive. Here we show that Rad54, a Swi2/Snf2 protein9, 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 protein10,11,12. 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.