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.