Human RAD51 protein (HsRad51) catalyses the strand exchange reaction, which is a crucial step of homologous recombination, an evolutionary well conserved and central process of
DNA metabolism. HsRad51 is thus vital for cell survival and maintenance of the genomic information by ensuring an error-free recombinational repair of double-strand breaks, the most severe DNA damage. The protein is also involved in the creation of gene diversity, shuffling homologous paternal and maternal DNA strands, as well as in cell proliferation by assisting DNA segregation. Both the up- and down-regulations of HsRad51 seem to relate to cancer formation. Besides its vital biological roles, the strand exchange reaction can be highly exploited in the medicinal field. It could be exploited in correction and repair of defective genes in gene therapy and due to its relationship with cancer cell proliferation and radiotherapy resistance, it is also a potential target for anticancer treatment.
HsRad51, like its well-studied bacterial homologue RecA, catalyses the strand exchange reaction by first cooperatively assembling around single-stranded DNA (ssDNA) in the presence of ATP, forming a nucleoprotein filament in which the DNA is stretched 50% compared with its canonical B form. This HsRad51/ssDNAfilament engages a double-stranded DNA (dsDNA) with homologous sequence and promotes strand exchange between the two DNA molecules. Finally, HsRad51 is released from the newly formed dsDNA hybrid. Despite extensive studies on both HsRad51 and RecA, the molecular mechanisms involved in both the search for homologous DNA as well as the strand exchange reaction itself remain unclear. Although HsRad51 and RecA have functionalities in common and the overall structur of the nucleoprotein filaments they form are highly similar there are some distinct differences between the two proteins.
Using mainly spectroscopic tools we study the structure and reorganization processes withing the fibrous complexes of DNA with human (and also yeast) Rad51 and bacterial RecA protein. Besides the search for a mechanistic understanding, at an atomistic level, of how these enzymes function, particular questions concern the role of ATP/ADP, divalent ions and various auxiliary proteins controlling and mediating the gene exchange process.