VirB11 ATPases are hexameric assemblies that power type IV secretion systems in bacteria. The hexamer of Brucella suis VirB11 (BsB11), like that of the Helicobacter pylori VirB11 (Hp0525), consists of a double ring structure formed by the N-terminal and C-terminal domains of each monomer. However, the monomer differs dramatically from that of Hp0525 by a large domain swap that leaves the hexameric assembly intact but profoundly alters the nucleotide-binding site and the interface between subunits