Fig. 3: Structural basis for RAD51–X3C octamer assembly.

a, The four RAD51 molecules of the RAD51–X3C octamer that pack into a loosely associated ring-like arrangement are shown as surface representations. The X3C modules that emanate from the RAD51 base are shown as cartoon representations. Insets: magnified views of the interaction between the XRCC3 loop insertion with RAD51 at both ends of the structure. b, Top left: cartoon representation of XRCC3 with L1, L2 and the loop insertion labeled. Top right: cartoon representations of RAD51 and RAD51 paralogs superimposed at their ATPase domains. Bottom: sequence alignment of the region corresponding to the XRCC3 loop insertion. c, XRCC3.B, RAD51.A1 and RAD51.A2 from the RAD51–X3C structure are shown as cartoons, colored as in a. The unique XRCC3 loop insertion is colored red. RAD51.A1 residues that interact with DNA in previously determined RAD51–DNA structures are shown as sticks and labeled. d, The ATPase domains of RAD51.A1 and RAD51.A2 from the RAD51–X3C structure and RAD51 from the presynaptic filament structure were superimposed. ATP and nonhydrolyzable ATP analogs are shown as spheres. e, The RAD51 filament was superimposed onto RAD51.A1 of the RAD51–X3C complex. The RAD51 filament is shown in surface representation and DNA is shown as sticks. XRCC3.B from the RAD51–X3C complex is shown as a cartoon. f, DNA from the alignment described in e is shown as sticks in the context of the RAD51–X3C complex. g, A RAD51 molecule from RAD51 filament (red) superimposed onto RAD51.A1 (left) and RAD51.A2 (right) from the RAD51–X3C structure.