Homologous-pairing activity of the human DNA-repair proteins Xrcc3 center dot Rad51C

The human Xrcc3 protein is involved in the repair of damaged DNA through ho mologous recombination, in which homologous pairing is a key step. The Rad5 1 protein is believed to be the only protein factor that promotes homologou s pairing in recombinational DNA repair in mitotic cells. In the brain, how ever. Rad51 expression is extremely low, whereas XRCC3, a human homologue o f Saccharomyces cerevisiae RAD57 that activates the Rad51-dependent homolog ous pairing with the yeast Rad55 protein, is expressed. In this study, a tw o-hybrid analysis conducted with the use of a human brain cDNA library reve aled that the major Xrcc3-interacting protein is a Rad51 paralog, Rad51C/Ra d51L2. The purified Xrcc3 Rad51C complex, which shows apparent 1:1 stoichio metry, was found to catalyze the homologous pairing. Although the activity is reduced, the Rad51C protein alone also catalyzed homologous pairing, sug gesting that Rad51C is a catalytic subunit for homologous pairing. The DNA- binding activity of Xrcc3Rad51C was drastically decreased in the absence of Xrcc3, indicating that Xrcc3 is important for the DNA binding of Xrcc3 Rad 51C. Electron microscopic observations revealed that Xrcc3Rad51C and Rad51C formed similar filamentous structures with circular single-stranded DNA.