title: Purification, Refolding, and Crystallization of the Outer Membrane Protein OmpG from Escherichia coli.
abstract: OmpG is a pore-forming protein from E. coli outer membranes. Unlike the classical outer membrane porins, which are trimers, the OmpG channel is a monomeric β-barrel made of 14 antiparallel β-strands with short periplasmic turns and longer extracellular loops. The channel activity of OmpG is pH dependent and the channel is gated by the extracellular loop L6. At neutral/high pH, the channel is open and permeable for substrate molecules with a size up to 900 Da. At acidic pH, loop L6 folds across the channel and blocks the pore. The channel blockage at acidic pH appears to be triggered by the protonation of a histidine pair on neighboring β-strands, which repel one another, resulting in the rearrangement of loop L6 and channel closure. OmpG was purified by refolding from inclusion bodies and crystallized in two and three dimensions. Crystallization and analysis by electron microscopy and X-ray crystallography revealed the fundamental mechanisms essential for the channel activity.
authors.affiliation: Department of Structural Biology, Max Planck Institute of Biophysics, Frankfurt am Main, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Köster
authors.affiliation: Department of Structural Biology, Max Planck Institute of Biophysics, Frankfurt am Main, Germany.
authors.forename: Katharina
authors.initials: K
authors.lastname: van Pee
authors.affiliation: Department of Structural Biology, Max Planck Institute of Biophysics, Frankfurt am Main, Germany. Electronic address: oezkan.yildiz@biophys.mpg.de.
authors.forename: Özkan
authors.initials: Ö
authors.lastname: Yildiz
mesh_terms: D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism / Q000648:ultrastructure; D002850:Chromatography, Gel; D002852:Chromatography, Ion Exchange; D002942:Circular Dichroism; D020285:Cryoelectron Microscopy / Q000379:methods*; D003460:Crystallization / Q000379:methods; D018360:Crystallography, X-Ray / Q000379:methods*; D004586:Electrophoresis; D004926:Escherichia coli / Q000737:chemistry* / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism / Q000648:ultrastructure; D008958:Models, Molecular; D018272:Porins / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism / Q000648:ultrastructure; D058849:Protein Refolding; D017433:Protein Structure, Secondary
chemical_list: D001425:Bacterial Outer Membrane Proteins; D029968:Escherichia coli Proteins; C114347:OmpG protein, E coli; D018272:Porins
keywords: 2D crystallization; 3D crystallization; Membrane protein; Porin; Protein purification; Protein refolding; β-Barrel