Architecture of a channel-forming O-antigen polysaccharide ABC transporter

• Published in: Nature (London) Vol. 553; no. 7688; pp. 361 - 365
• Main Authors: Bi, Yunchen, Mann, Evan, Whitfield, Chris, Zimmer, Jochen
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 18.01.2018; Nature Publishing Group
• Subjects: 631/326/421; 631/535/1266; 631/57/2270; ABC transporter; ABC transporters; Adenosine triphosphate; Antigens; Binding; Biosynthesis; Cell surface; Cellulose; Chemical properties; Chemical synthesis; Conformation; Crystal structure; E coli; Health aspects; Helices; Homology; Humanities and Social Sciences; Immune response; Innate immunity; Interfaces; Klebsiella pneumoniae; letter; Lipid A; Lipids; Lipopolysaccharides; multidisciplinary; Periplasm; Physiological aspects; Polymers; Polysaccharides; Proteins; Saccharides; Science; Site-directed mutagenesis; Structure-function relationships; Substrates; Translocation; Transport; Transport proteins; United States
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature25190

The crystal structure of a channel-forming O-antigen polysaccharide ABC transporter suggests a novel biopolymer translocation mechanism. Structure of an O-antigen polysaccharide transporter Bacterial cells are decorated with polysaccharides such as O-antigens, which help them to evade the innate immune responses of the host. In Gram-negative bacteria, these polysaccharides are transported from the cytoplasm to the periplasm before being incorporated into the outer membrane. In this paper, Jochen Zimmer and colleagues report the crystal structure of a bacterial O-antigen polysaccharide transporter. This represents a key structure in bacterial cell envelope biogenesis. Unusually for ATP-binding cassette (ABC) transporters, which usually operate by an alternating access model, the O-antigen transporter in its open state forms a continuous channel which spans the entire membrane. As a result, the authors suggest that the polysaccharides are transported via a processive mechanism whereby they thread through the open channel. O-antigens are cell surface polysaccharides of many Gram-negative pathogens that aid in escaping innate immune responses 1 . A widespread O-antigen biosynthesis mechanism involves the synthesis of the lipid-anchored polymer on the cytosolic face of the inner membrane, followed by transport to the periplasmic side where it is ligated to the lipid A core to complete a lipopolysaccharide molecule 2 . In this pathway, transport to the periplasm is mediated by an ATP-binding cassette (ABC) transporter, called Wzm–Wzt. Here we present the crystal structure of the Wzm–Wzt homologue from Aquifex aeolicus in an open conformation. The transporter forms a transmembrane channel that is sufficiently wide to accommodate a linear polysaccharide. Its nucleotide-binding domain and a periplasmic extension form ‘gate helices’ at the cytosolic and periplasmic membrane interfaces that probably serve as substrate entry and exit points. Site-directed mutagenesis of the gates impairs in vivo O-antigen secretion in the Escherichia coli prototype. Combined with a closed structure of the isolated nucleotide-binding domains, our structural and functional analyses suggest a processive O-antigen translocation mechanism, which stands in contrast to the classical alternating access mechanism of ABC transporters.