The human Rad52 protein forms a heptameric ring that catalyzes homologous pairing. The N-terminal half of Rad52 is the catalytic domain for homologous pairing, and the ring formed by the domain fragment was reported to be approximately decameric. Splicing variants of Rad52 and a yeast homolog (Rad59) are composed mostly of this domain. In this study, we determined the crystal structure of the homologous-pairing domain of human Rad52 and revealed that the domain forms an undecameric ring. Each monomer has a β-β-β-α fold, which consists of highly conserved amino acid residues among Rad52 homologs. A mutational analysis revealed that the amino acid residues located between the β-β-β-α fold and the characteristic hairpin loop are essential for ssDNA and dsDNA binding.
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