Evidence for simultaneous protein interactions between human Rad51 paralogs

In yeast, the Rad51-related proteins include Rad55 and Rad57, which form a heterodimer that interacts with Rad51. Five human Rad51 paralogs have been identified (XRCC2, XRCC3, RadS1B/Rad51L1, Rad51C/ Rad51L2, and Rad51D/Rad51 L3), and each interacts with one or more of the others. Previously we repor ted that HsRad51 interacts with XRCC3, and Rad51C interacts with XRCC3, Rad 51B, and HsRad51, Here we report that in the yeast two-hybrid system, Rad51 D interacts with XRCC2 and Rad51C, No other interactions, including self-in teractions, were found, indicating that the observed interactions are speci fic. The yeast Rad51 interacts with human Rad51 and XRCC3, suggesting Rad51 conservation since the human yeast divergence. Data from yeast three-hybri d experiments indicate that a number of the pairs of interactions between h uman Rad51 parlogs can occur simultaneously. For example, Rad51B expression enhances the binding of Rad51C to XRCC3 and to HsRad51D, and Rad51C expres sion allows the indirect interaction of Rad51B with Rad51D, Experiments usi ng 6xHis-tagged proteins in the baculovirus system confirm several of our y east results, including Rad51B interaction with Rad51D only when Rad51C is simultaneously expressed and Rad51C interaction with XRCC2 only when Rad51D is present. These results suggest that these proteins may participate in o ne complex or multiple smaller ones.