Abstract
Gram-negative bacteria are surrounded by an outer membrane composed of phospholipids and lipopolysaccharide, which acts as a barrier and contributes to antibiotic resistance. The transport mechanisms of phospholipid translocation across the periplasm, such as the MCE (Mammalian Cell Entry) systems, have not been well characterized. Our ~3.5A cryo-EM structure of the E. coli MCE protein LetB reveals a ~0.6 megadalton complex in the periplasm that consists of seven stacked rings, with a central hydrophobic tunnel sufficiently long to span the periplasm. Lipids bind inside the tunnel, suggesting that it functions as a pathway for lipid transport. Cryo-EM structures in the open and closed states reveal a dynamic tunnel lining, with implications for gating or substrate translocation. Our results support a model in which LetB establishes a physical link between the two membranes, and creates a hydrophobic pathway for the translocation of lipids across the periplasm.
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