Rad51 is a key element of recombinational DNA repair and its activity is regulated by phosphorylation of the tyrosine residue at position 315 by cAbl kinase. This phosphorylation could be involved in the resistance of cancer cells to chemotherapy. We have investigated the role of this residue by comparing the three-dimensional structures of human Rad51 and its prokaryotic homologue, Escherichia coli RecA. The residue appeared to be on the edge of the subunit-subunit interacting site. The fluorescence intensity of the tryptophan residue inserted at position 315 of human Rad51 in the place of tyrosine was decreased by adding 3 M urea, although the protein was not unfolded as there was no large change in the fluorescence peak position or circular dichroism signal. This change in fluorescence occurred at a lower urea concentration when the protein was diluted, which favours dissociation. These results indicate that the change is related to the dissociation of Rad51 polymer and that residue 315 is close to the subunit-subunit interacting site. ATP and ADP, which affect the filament structure, caused a blue shift in the fluorescence peak. These nucleotides probably altered the subunit-subunit contacts and may thus affect the filament structure. Phosphorylation of this residue could therefore affect the formation and structure of the Rad51 filament. Correct prediction of subunit-subunit interface of Rad51 by simple comparison of structures of Rad51 and RecA supports the idea that Rad51 forms the filament in a similar way as does RecA.
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