Structural basis for lipopolysaccharide insertion in the bacterial outer membrane

Lipopolysaccharide, an essential component of the Gram-negative bacteria outer membrane, is inserted by LptD–LptE, a protein complex with a unique ‘barrel and plug’ architecture; the structure of the LptD–LptE complex of Shigella flexneri determined here shows LptD forming a 26-stranded β-barrel with LptE located inside the barrel of LptD, the first two β-strands are distorted by two proline residues, creating a potential portal in the barrel wall that might allow lateral diffusion of lipopolysaccharide into the outer membrane. Haohao Dong et al. and Shuai Qiao et al. present X-ray crystal structures of the complex between the lipopolysaccharide transport proteins LptD and LptE from the bacteria Salmonella typhimurium and Shigella flexneri, respectively. The two papers report a unique two-protein plug-and-barrel architecture for the LptD–LptE complex that reveals the mechanism by which the cell-wall lipopolysaccharide is delivered and inserted into the external leaflet of the outer membrane of Gram-negative bacteria. As well as providing new detail on the nature of outer membrane biogenesis, this work will provide data relevant to the design of new antibiotic strategies targeting the bacterial outer membrane, much needed in the fight against multi-drug resistant pathogens. One of the fundamental properties of biological membranes is the asymmetric distribution of membrane lipids. In Gram-negative bacteria, the outer leaflet of the outer membrane is composed predominantly of lipopolysaccharides (LPS)1. The export of LPS requires seven essential lipopolysaccharide transport (Lpt) proteins to move LPS from the inner membrane, through the periplasm to the surface2. Of the seven Lpt proteins, the LptD–LptE complex is responsible for inserting LPS into the external leaflet of the outer membrane3,4. Here we report the crystal structure of the ∼110-kilodalton membrane protein complex LptD–LptE from Shigella flexneri at 2.4 A resolution. The structure reveals an unprecedented two-protein plug-and-barrel architecture with LptE embedded into a 26-stranded β-barrel formed by LptD. Importantly, the secondary structures of the first two β-strands are distorted by two proline residues, weakening their interactions with neighbouring β-strands and creating a potential portal on the barrel wall that could allow lateral diffusion of LPS into the outer membrane. The crystal structure of the LptD–LptE complex opens the door to new antibiotic strategies targeting the bacterial outer membrane.