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Title: A conformational landscape for alginate secretion across the outer membrane of Pseudomonas aeruginosa

Crystal structures of the β-barrel porin AlgE reveal a mechanism whereby alginate is exported from P. aeruginosa for biofilm formation. The exopolysaccharide alginate is an important component of biofilms produced by Pseudomonas aeruginosa, a major pathogen that contributes to the demise of cystic fibrosis patients. Alginate exits the cell via the outer membrane porin AlgE. X-ray structures of several AlgE crystal forms are reported here. Whilst all share a common β-barrel constitution, they differ in the degree to which loops L2 and T8 are ordered. L2 and T8 have been identified as an extracellular gate (E-gate) and a periplasmic gate (P-gate), respectively, that reside on either side of an alginate-selectivity pore located midway through AlgE. Passage of alginate across the membrane is proposed to be regulated by the sequential opening and closing of the two gates. In one crystal form, the selectivity pore contains a bound citrate. Because citrate mimics the uronate monomers of alginate, its location is taken to highlight a route through AlgE taken by alginate as it crosses the pore. Docking and molecular-dynamics simulations support and extend the proposed transport mechanism. Specifically, the P-gate and E-gate are flexible and move between open and closed states. Citrate canmore » leave the selectivity pore bidirectionally. Alginate docks stably in a linear conformation through the open pore. To translate across the pore, a force is required that presumably is provided by the alginate-synthesis machinery. Accessing the open pore is facilitated by complex formation between AlgE and the periplasmic protein AlgK. Alginate can thread through a continuous pore in the complex, suggesting that AlgK pre-orients newly synthesized exopolysaccharide for delivery to AlgE.« less
Authors:
 [1] ;  [2] ; ; ; ; ;  [1] ; ;  [3] ;  [4] ;  [2] ;  [1]
  1. Trinity College, Dublin (Ireland)
  2. University of Oxford, South Parks Road, Oxford (United Kingdom)
  3. The Hospital for Sick Children, Toronto, Ontario (Canada)
  4. (Canada)
Publication Date:
OSTI Identifier:
22347775
Resource Type:
Journal Article
Resource Relation:
Journal Name: Acta Crystallographica. Section D: Biological Crystallography; Journal Volume: 70; Journal Issue: Pt 8; Other Information: PMCID: PMC4118822; PMID: 25084326; PUBLISHER-ID: be5250; OAI: oai:pubmedcentral.nih.gov:4118822; Copyright (c) Tan et al. 2014; This is an open-access article distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
Denmark
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; CITRATES; CRYSTAL STRUCTURE; CRYSTALS; DELIVERY; FILMS; MEMBRANE PROTEINS; MEMBRANES; MONOMERS; SIMULATION; SYNTHESIS