Mouse Rad54 affects DNA conformation and DNA-damage-induced Rad51 foci formation

Error-free repair by homologous recombination of DNA double strand breaks i nduced by ionizing radiation (IR) requires the Rad52 group proteins, includ ing Rad51 and Rad54, in the yeast Saccharomyces cerevisiae [1]. The formati on of a 'joint' molecule between the damaged DNA and the homologous repair template is a key step in recombination mediated by Rad51 and stimulated by Rad54 [2-5]. Mammalian homologs of Rad51 and Rad54 have been identified [2 ,3,6]. Here, we demonstrate that mouse Rad54 (mRad54) formed IR-induced nuc lear foci that colocalized with mRad51. Interaction between mRad51 and mRad 54 was induced by genotoxic stress, but only when lesions that required mRa d54 for their repair were formed. Interestingly, mRad54 was essential for t he formation of in-induced mRad51 foci. Rad54 belongs to the SW12/SNF2 prot ein family, members of which modulate protein-DNA interactions in an ATP-dr iven manner [7]. Results of a topological assay suggested that purified hum an Rad54 (hRad54) protein can unwind double-stranded (ds) DNA at the expens e of ATP hydrolysis. Unwinding of the homologous repair template could prom ote the formation or stabilization of hRad51-mediated joint molecules. Rad5 4 appears to be required downstream of other Rad52 group proteins, such as Rad52 and the Rad55-Rad57 heterodimer, that assist Rad51 in interacting wit h the broken DNA [2-4].