Structural basis of unidirectional export of lipopolysaccharide to the cell surface

• Published in: Nature (London) 2019-03, Vol.567 (7749), p.550-553
• Main Authors: Owens, Tristan W., Taylor, Rebecca J., Pahil, Karanbir S., Bertani, Blake R., Ruiz, Natividad, Kruse, Andrew C., Kahne, Daniel
• Format: Article
• Language: English
• Subjects: 38/70; 631/326/421; 631/45/287/1190; 631/45/612/1237; 631/535/1266; 82/1; 82/80; 82/83; ABC transporters; Adenosine Triphosphate - metabolism; Antibiotics; Asymmetry; ATP; ATP-Binding Cassette Transporters - chemistry; ATP-Binding Cassette Transporters - metabolism; Bacteria; Bacterial Proteins - chemistry; Bacterial Proteins - metabolism; BASIC BIOLOGICAL SCIENCES; Biological Transport; Bridges; Cell Membrane - metabolism; Cell receptors; Cell surface; Concentration gradient; Crystal structure; Crystallography, X-Ray; Cytoplasm; E coli; Escherichia coli; Escherichia coli Proteins - chemistry; Gram-negative bacteria; Humanities and Social Sciences; Klebsiella pneumoniae; Letter; Lipopolysaccharides; Lipopolysaccharides - chemistry; Lipopolysaccharides - metabolism; Membrane proteins; Membrane Proteins - chemistry; Membranes; Models, Molecular; multidisciplinary; Multiprotein Complexes - chemistry; Multiprotein Complexes - metabolism; Observations; Pathogens; Protein Subunits - chemistry; Protein Subunits - metabolism; Proteins; Pseudomonas aeruginosa; Science; Science (multidisciplinary); Structure; Substrates; Sugar; Transport; Vibrio cholerae - chemistry; Vibrio cholerae - cytology; Vibrio cholerae - metabolism; X-ray crystallography
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-019-1039-0

Gram-negative bacteria are surrounded by an inner cytoplasmic membrane and by an outer membrane, which serves as a protective barrier to limit entry of many antibiotics. The distinctive properties of the outer membrane are due to the presence of lipopolysaccharide 1 . This large glycolipid, which contains numerous sugars, is made in the cytoplasm; a complex of proteins forms a membrane-to-membrane bridge that mediates transport of lipopolysaccharide from the inner membrane to the cell surface 1 . The inner-membrane components of the protein bridge comprise an ATP-binding cassette transporter that powers transport, but how this transporter ensures unidirectional lipopolysaccharide movement across the bridge to the outer membrane is unknown 2 . Here we describe two crystal structures of a five-component inner-membrane complex that contains all the proteins required to extract lipopolysaccharide from the membrane and pass it to the protein bridge. Analysis of these structures, combined with biochemical and genetic experiments, identifies the path of lipopolysaccharide entry into the cavity of the transporter and up to the bridge. We also identify a protein gate that must open to allow movement of substrate from the cavity onto the bridge. Lipopolysaccharide entry into the cavity is ATP-independent, but ATP is required for lipopolysaccharide movement past the gate and onto the bridge. Our findings explain how the inner-membrane transport complex controls efficient unidirectional transport of lipopolysaccharide against its concentration gradient. Crystal structures of a five-protein complex comprising the inner-membrane components of the bacterial lipopolysaccharide transporter provide insight into the mechanism of extraction of lipopolysaccharide from the inner membrane and its transport to the outer membrane.