Structural basis of lipopolysaccharide extraction by the LptBFGC complex.

In Gram-negative bacteria, lipopolysaccharide is essential for outer membrane formation and antibiotic resistance. The seven lipopolysaccharide transport (Lpt) proteins A-G move lipopolysaccharide from the inner to the outer membrane. The ATP-binding cassette transporter LptBFG, which tightly associates with LptC, extracts lipopolysaccharide out of the inner membrane. The mechanism of the LptBFG-LptC complex (LptBFGC) and the role of LptC in lipopolysaccharide transport are poorly understood. Here we characterize the structures of LptBFG and LptBFGC in nucleotide-free and vanadate-trapped states, using single-particle cryo-electron microscopy. These structures resolve the bound lipopolysaccharide, reveal transporter-lipopolysaccharide interactions with side-chain details and uncover how the capture and extrusion of lipopolysaccharide are coupled to conformational rearrangements of LptBFGC. LptC inserts its transmembrane helix between the two transmembrane domains of LptBFG, which represents a previously unknown regulatory mechanism for ATP-binding cassette transporters. Our results suggest a role for LptC in achieving efficient lipopolysaccharide transport, by coordinating the action of LptBFG in the inner membrane and Lpt protein interactions in the periplasm.