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Updated: Apr 12, 2026
From Constructs to Crystals – Towards Structure Determination of β-barrel Outer Membrane Proteins
Published on: July 4, 2016
Purification, Refolding, and Crystallization of the Outer Membrane Protein OmpG from Escherichia coli
Stefan Köster1, Katharina van Pee1, Özkan Yildiz1
1Department of Structural Biology, Max Planck Institute of Biophysics, Frankfurt am Main, Germany.
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.
Insights
The outer membrane protein G (OmpG) from E. coli functions as a pH-gated channel. Its activity is regulated by loop L6, which blocks the pore at acidic pH, revealing key channel mechanisms.
Area of Science:
- Structural biology
- Microbial physiology
- Biophysics
Background:
- OmpG is a monomeric beta-barrel protein in E. coli outer membranes.
- Unlike trimeric porins, OmpG's structure features 14 antiparallel beta-strands.
- Its channel activity is notably pH-dependent.
Purpose of the Study:
- To elucidate the structural and mechanistic basis of OmpG channel gating.
- To understand how pH influences OmpG's pore activity.
- To reveal the role of extracellular loop L6 in channel regulation.
Main Methods:
- Protein purification via refolding from inclusion bodies.
- Crystallization in two and three dimensions.
- Analysis using electron microscopy and X-ray crystallography.
Main Results:
- OmpG forms a monomeric beta-barrel channel.
- Extracellular loop L6 acts as a pH-sensitive gate.
- Protonation of histidine residues triggers loop L6 rearrangement and pore closure at acidic pH.
- The open channel permits passage of molecules up to 900 Da.
Conclusions:
- OmpG's unique monomeric structure and pH-gated mechanism are revealed.
- Protonation-induced histidine repulsion drives loop L6-mediated channel closure.
- Structural insights provide a fundamental understanding of OmpG channel activity.
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