Identification of the Subunit–Subunit Interface of Xenopus Rad51.1 Protein: Similarity to RecA

Rad51, like its prokaryotic homolog RecA, forms a helical filament for homologous DNA recombination and recombinational DNA repair. Comparison of the three-dimensional structures of human Rad51 and Escherichia coli RecA indicated that the tyrosine residue at position 191 in human Rad51 lies at the centre of a putative subunit–subunit contact interface. We inserted a tryptophan residue as a fluorescent probe at the corresponding position in Xenopus Rad51.1 and found that its fluorescence depended upon the protein concentration, indicating that the residue is truly in the subunit–subunit interface. We also found that 3 M urea, which promoted the dissociation of Rad51 filament without complete unfolding of the protein, exposed the tryptophan residue to solvent. The fluorescence was not modified by binding to DNA and only slightly modified by ATP, indicating that the same site is used for formation of the active ATP-Rad51-DNA filament. The slight changes in fluorescence caused by ATP and ADP suggest that the subunit–subunit contact is altered, leading to the elongation of the filament by these nucleotides, as with the RecA filament. Thus, Rad51 forms filaments by subunit–subunit contact much like RecA does.