Structure of the monomeric outer-membrane porin OmpG in the open and closed conformation

Authors

  • Özkan Yildiz,

  • Kutti R Vinothkumar,

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    • Present address: Structural Studies Division, Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK
  • Panchali Goswami,

  • Werner Kühlbrandt

    Corresponding author
    1. Department of Structural Biology, Max Planck Institute of Biophysics, Frankfurt am Main, Germany
    • Corresponding author. Department of Structural Biology, Max Planck Institute of Biophysics, Max-von-Laue-Str. 3, Frankfurt am Main 60438, Germany. Tel.: +49 69 6303 3000; Fax: +49 69 6303 3002; E-mail: werner.kuehlbrandt@mpibp-frankfurt.mpg.de

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Abstract

OmpG, a monomeric pore-forming protein from Escherichia coli outer membranes, was refolded from inclusion bodies and crystallized in two different conformations. The OmpG channel is a 14-stranded β-barrel, with short periplasmic turns and seven extracellular loops. Crystals grown at neutral pH show the channel in the open state at 2.3 Å resolution. In the 2.7 Å structure of crystals grown at pH 5.6, the pore is blocked by loop 6, which folds across the channel. The rearrangement of loop 6 appears to be triggered by a pair of histidine residues, which repel one another at acidic pH, resulting in the breakage of neighbouring H-bonds and a lengthening of loop 6 from 10 to 17 residues. A total of 151 ordered LDAO detergent molecules were found in the 2.3 Å structure, mostly on the hydrophobic outer surface of OmpG, mimicking the outer membrane lipid bilayer, with three LDAO molecules in the open pore. In the 2.7 Å structure, OmpG binds one OG and one glucose molecule as sugar substrates in the closed pore.

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