Abstract: Human Rad51 (HsRad51) catalyzes the strand exchange reaction, a crucial step in homologous recombination, by forming a filamentous complex with DNA. The structure of this filament is modified by ATP, which is required and hydrolyzed for the reaction. We analyzed the structure and the ATP-promoted conformational change of this filament. We systematically replaced aromatic residues in the protein, one at a time, with tryptophan, a fluorescent probe, and examined its effect on the activities (DNA binding, ATPase, ATP-promoted conformational change, and strand exchange reaction) and the fluorescence changes upon binding of ATP and DNA. Some residues were also replaced with alanine. We thus obtained structural information about various positions of the protein in solution. All the proteins conserved, at least partially, their activities. However, the replacement of histidine at position 294 (H294) and phenylalanine at 129 (F129) affected the ATP-induced conformational change of the DNA-HsRad51 filament, although it did not prevent DNA binding. F129 is considered to be close to the ATP-binding site and to H294 of a neighboring subunit. ATP probably modifies the structure around F129 and affects the subunit/subunit contact around H294 and the structure of the DNA-binding site. The replacement also reduced the DNA-dependent ATPase activity, suggesting that these residues are also involved in the transmission of the allosteric effect of DNA to the ATP-binding site, which is required for the stimulation of ATPase activity by DNA. The fluorescence analyses supported the structural change of the DNA-binding site by ATP and that of the ATP-binding site by DNA. This information will be useful to build a molecular model of the Rad51-DNA complex and to understand the mechanism of activation of Rad51 by ATP and that of the Rad51-promoted strand exchange reaction.
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