1. Centre for Biomolecular Sciences, The University of St Andrews, Fife KY16 9RH, UK
2. Department of Molecular and Cellular Biology, The University of Guelph, Guelph, Ontario N1G 2W1, Canada
3. These authors contributed equally to this work.
4. Present addresses: Department of Biochemistry, Imperial College of Science, Faculty of Life Sciences, Division of Molecular Biosciences, Wolfson Building, London SW7 2AZ, UK (K.B.); Department of Biology, University of Konstanz, 78457 Konstanz, Germany (J.N.).

Correspondence to: James H. Naismith¹ Correspondence and requests for materials should be addressed to J.H.N. (Email: naismith@st-and.ac.uk).

Coordinates and data are available from the Worldwide Protein Data Bank, accession code 2j58.

Abstract

Many types of bacteria produce extracellular polysaccharides (EPSs). Some are secreted polymers and show only limited association with the cell surface, whereas others are firmly attached to the cell surface and form a discrete structural layer, the capsule, which envelopes the cell and allows the bacteria to evade or counteract the host immune system¹. EPSs have critical roles in bacterial colonization of surfaces², such as epithelia and medical implants; in addition some EPSs have important industrial and biomedical applications in their own right³. Here we describe the 2.26 Å resolution structure of the 340 kDa octamer of Wza, an integral outer membrane lipoprotein, which is essential for group 1 capsule export in Escherichia coli. The transmembrane region is a novel -helical barrel. The bulk of the Wza structure is located in the periplasm and comprises three novel domains forming a large central cavity. Wza is open to the extracellular environment but closed to the periplasm. We propose a route and mechanism for translocation of the capsular polysaccharide. This work may provide insight into the export of other large polar molecules such as DNA and proteins.

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