A novel packing arrangement of AcrB in the lipid bilayer membrane

The central component AcrB of the Escherichia coli drug efflux complex AcrA–AcrB–TolC has been extensively investigated by X-ray crystallography of detergent–protein 3-D crystals. In these crystals, AcrB packs as trimers – the functional unit. We visualized the AcrB–AcrB interaction in its native environment by examining E. coli lipid reconstituted 2-D crystals, which were overwhelmingly formed by asymmetric trimers stabilized by strongly-interacting monomers from adjacent trimers. Most interestingly, we observed lattices formed by an arrangement of AcrB monomers distinct from that in traditional trimers. This hitherto unobserved packing, might play a role in the biogenesis of trimeric AcrB.