Nuclear matrix binding site in the Rad51 recombinase.

It has been shown that the key homologous recombination protein Rad51 accumulates in DNA damage-induced nuclear foci that are attached to the nuclear matrix. In the present communication we attempted to find whether Rad51 contains a functional domain responsible for nuclear matrix binding. By alignments of the sequences encoding nuclear matrix targeting signals of human nuclear matrix binding proteins with the whole length human Rad51sequence a putative nuclear matrix targeting signal was identified. To prove that it is responsible for the nuclear matrix association of Rad51 18 base pairs encoding a cluster of hydrophobic amino acids in the human Rad51 Flag-tagged gene were deleted. The formation of damage-induced Rad51 foci and their association with the nuclear matrix were monitored in HeLa cells transfected with the wild-type and the mutated Rad51gene after treatment with mitomycin C. The results showed that while the wild-type protein formed Rad51 foci attached to the nuclear matrix, the mutated Rad51 failed to form DNA damage-induced nuclear foci. The loss of foci formation activity of the mutated protein was not due to impaired ability to bind double-stranded DNA in an ATP-dependent way in vitro and to bind chromatin in vivo. These data suggest that the assembly of Rad51 into nuclear foci is assisted by association with the nuclear matrix, which may support the spatial organization of the process of repair by homologous recombination.