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Updated: Sep 28, 2026
From Constructs to Crystals – Towards Structure Determination of β-barrel Outer Membrane Proteins
Published on: July 4, 2016
The core of the tetrameric mycobacterial porin MspA is an extremely stable beta-sheet domain
Christian Heinz1, Harald Engelhardt, Michael Niederweis
1Lehrstuhl für Mikrobiologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Staudtstr. 5, Germany.
Abstract:
MspA is the major porin of Mycobacterium smegmatis mediating the exchange of hydrophilic solutes across the cell wall and is the prototype of a new family of tetrameric porins with a single central pore of 10 nm in length. Infrared and circular dichroism spectroscopy revealed that MspA consists mainly of antiparallel beta-strands organized in a coherent domain. Heating to 92 and 112 degrees C was required to dissociate the MspA tetramer and to unfold the beta-sheet domain in the monomer, respectively. The stability of the MspA tetramer exceeded the remarkable stability of the porins of Gram-negative bacteria for every condition tested and was not reduced in the presence of 2% SDS and at any pH from 2 to 14. These results indicated that the interactions between the MspA subunits are different from those in the porins of Gram-negative bacteria and are discussed in the light of a channel-forming beta-barrel as a core structure of MspA. Surprisingly, the channel activity of MspA in 2% SDS and 7.6 m urea at 50 degrees C was reduced 13- and 30-fold, respectively, although the MspA tetramer and the beta-sheet domain were stable under those conditions. Channel closure by conformational changes of extracellular loops under those conditions is discussed to explain these observations. This study presents the first experimental evidence that outer membrane proteins not only from Gram-negative bacteria but also from mycobacteria are beta-sheet proteins and demonstrates that MspA constitutes the most stable transmembrane channel protein known so far. Thus, MspA may be of special interest for biotechnological applications.
Insights
Mycobacterium smegmatis porin A (MspA) is a highly stable transmembrane channel protein. This beta-barrel protein exhibits remarkable stability, suggesting potential biotechnological applications.
Area of Science:
- Structural biology
- Membrane protein biophysics
- Mycobacterial outer membrane proteins
Background:
- MspA is the primary porin in Mycobacterium smegmatis, facilitating hydrophilic solute transport across the cell wall.
- It represents a novel family of tetrameric porins with a unique central pore.
- Understanding MspA's structure and stability is crucial for comprehending mycobacterial cell envelope function.
Purpose of the Study:
- To investigate the structural characteristics and stability of MspA.
- To compare MspA's stability with porins from Gram-negative bacteria.
- To explore the implications of MspA's stability for potential biotechnological uses.
Main Methods:
- Infrared spectroscopy and circular dichroism spectroscopy were employed to analyze MspA's secondary structure.
- Thermal denaturation studies were conducted to determine the dissociation and unfolding temperatures.
- MspA stability was assessed under various conditions, including SDS, pH, urea, and temperature.
Main Results:
- MspA is predominantly composed of antiparallel beta-strands, forming a stable beta-sheet domain.
- The MspA tetramer dissociates at 92°C, and the monomer unfolds at 112°C.
- MspA exhibits exceptional stability, surpassing Gram-negative porins across tested conditions (pH 2-14, 2% SDS).
- Despite tetramer and beta-sheet stability, channel activity significantly decreased in 2% SDS and 7.6 M urea at 50°C, attributed to conformational changes in extracellular loops.
- This study provides the first experimental evidence of beta-sheet outer membrane proteins in mycobacteria.
Conclusions:
- MspA is the most stable transmembrane channel protein identified to date.
- Its unique stability and beta-barrel structure suggest distinct subunit interactions compared to Gram-negative porins.
- MspA's remarkable stability makes it a promising candidate for diverse biotechnological applications.
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