The Xrcc2 and Rad51D/Rad51L3 proteins, which belong to the Rad51 paralogs, are required for homologous recombinational repair (HRR) in vertebrates. The Xrcc2 and Rad51D/Rad51L3 genes, whose products interact with each other, have essential roles in ensuring normal embryonic development. In the present study, we coexpressed the human Xrcc2 and Rad51D/Rad51L3 proteins (Xrcc2 and Rad51D, respectively) in Escherichia coli, and purified the Xrcc2·Rad51D complex to homogeneity. The Xrcc2·Rad51D complex catalyzed homologous pairing between single-stranded and double-stranded DNA, similar to the function of the Xrcc3· Rad51C complex, which is another complex of the Rad51 paralogs. An electron microscopic analysis showed that Xrcc2·Rad51D formed a multimeric ring structure in the absence of DNA. In the presence of ssDNA, Xrcc2·Rad-51D formed a filamentous structure, which is commonly observed among the human homologous pairing proteins, Rad51, Rad52, and Xrcc3·Rad51C.
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