NUCLEOTIDE COFACTOR-DEPENDENT STRUCTURAL-CHANGE OF XENOPUS-LAEVIS RAD51 PROTEIN FILAMENT DETECTED BY SMALL-ANGLE NEUTRON-SCATTERING MEASUREMENTS IN SOLUTION

Rad51 protein, a eukaryotic homologue of RecA protein, forms a filamen tous complex with DNA and catalyzes homologous recombination. We have analyzed the structure of Xenopus Rad51 protein (XRad51.1) in solution by small-angle neutron scattering (SANS). The measurements showed tha t XRad51.1 forms a helical filament independently of DNA. The sizes of the cross-sectional and helical pitch of the filament could be determ ined, respectively, from a Guinier plot and the position of the subsid iary maximum of SANS data. We observed that the helical structure is m odified by nucleotide binding as in the case of RecA. Upon ATP binding under high-salt conditions (600 mM NaCl), the helical pitch of XRad51 .1 filament was increased from 8 to 10 nm and the cross-sectional diam eter decreased from 7 to 6 nm. The pitch sizes of XRad51.1 are similar to, though slightly larger than, those of RecA filament under corresp onding conditions. A similar helical pitch size was observed by electr on microscopy for budding yeast Rad51 [Ogawa, T., et al. (1993) Scienc e 259, 1896-1899]. In contrast to the RecA filament, the structure of XRad51.1 filament with ADP is not significantly different from that wi th ATP. Thus, the hydrolysis of ATP to ADP does not modify the helical filament of XRad51.1. Together with our recent observation that ADP d oes not weaken the XRad51.1/DNA interaction, the effect of ATP hydroly sis on XRad51.1 nucleofilament should be very different from that on R ecA.