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The Thermodynamic Stability of Membrane Proteins in Micelles and Lipid Bilayers Investigated with the Ferrichrom Receptor FhuA. J Membr Biol 2022; 255:485-502. [PMID: 35552784 PMCID: PMC9581862 DOI: 10.1007/s00232-022-00238-w] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/19/2022] [Accepted: 04/05/2022] [Indexed: 12/03/2022]
Abstract
Extraction of integral membrane proteins into detergents for structural and functional studies often leads to a strong loss in protein stability. The impact of the lipid bilayer on the thermodynamic stability of an integral membrane protein in comparison to its solubilized form in detergent was examined and compared for FhuA from Escherichia coli and for a mutant, FhuAΔ5-160, lacking the N-terminal cork domain. Urea-induced unfolding was monitored by fluorescence spectroscopy to determine the effective free energies \documentclass[12pt]{minimal}
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\begin{document}$$ \Delta G{^\text{o}_{\rm u}} $$\end{document}ΔGuo of unfolding. To obtain enthalpic and entropic contributions of unfolding of FhuA, \documentclass[12pt]{minimal}
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\begin{document}$$ \Delta G{^\text{o}_{\rm u}} $$\end{document}ΔGuo were determined at various temperatures. When solubilized in LDAO detergent, wt-FhuA and FhuAΔ5-160 unfolded in a single step. The 155-residue cork domain stabilized wt-FhuA by \documentclass[12pt]{minimal}
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\begin{document}$$ \Delta\Delta G{^\text{o}_{\rm u}} $$\end{document}ΔΔGuo~ 40 kJ/mol. Reconstituted into lipid bilayers, wt-FhuA unfolded in two steps, while FhuAΔ5-160 unfolded in a single step, indicating an uncoupled unfolding of the cork domain. For FhuAΔ5-160 at 35 °C, \documentclass[12pt]{minimal}
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\begin{document}$$ \Delta G{^\text{o}_{\rm u}} $$\end{document}ΔGuo increased from ~ 5 kJ/mol in LDAO micelles to about ~ 20 kJ/mol in lipid bilayers, while the temperature of unfolding increased from TM ~ 49 °C in LDAO micelles to TM ~ 75 °C in lipid bilayers. Enthalpies \documentclass[12pt]{minimal}
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\begin{document}$$\Delta H{_{\rm M}^\text{o}}$$\end{document}ΔHMowere much larger than free energies \documentclass[12pt]{minimal}
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\begin{document}$$ \Delta G{^\text{o}_{\rm u}} $$\end{document}ΔGuo, for FhuAΔ5-160 and for wt-FhuA, and compensated by a large gain of entropy upon unfolding. The gain in conformational entropy is expected to be similar for unfolding of FhuA from micelles or bilayers. The strongly increased TM and \documentclass[12pt]{minimal}
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\begin{document}$$\Delta H{_{\rm M}^\text{o}}$$\end{document}ΔHMo observed for the lipid bilayer-reconstituted FhuA in comparison to the LDAO-solubilized forms, therefore, very likely arise from a much-increased solvation entropy of FhuA in bilayers.
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2
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Barberi L, Livolant F, Leforestier A, Lenz M. Local structure of DNA toroids reveals curvature-dependent intermolecular forces. Nucleic Acids Res 2021; 49:3709-3718. [PMID: 33784405 PMCID: PMC8053110 DOI: 10.1093/nar/gkab197] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/11/2020] [Revised: 02/17/2021] [Accepted: 03/12/2021] [Indexed: 01/15/2023] Open
Abstract
In viruses and cells, DNA is closely packed and tightly curved thanks to polyvalent cations inducing an effective attraction between its negatively charged filaments. Our understanding of this effective attraction remains very incomplete, partly because experimental data is limited to bulk measurements on large samples of mostly uncurved DNA helices. Here we use cryo electron microscopy to shed light on the interaction between highly curved helices. We find that the spacing between DNA helices in spermine-induced DNA toroidal condensates depends on their location within the torus, consistent with a mathematical model based on the competition between electrostatic interactions and the bending rigidity of DNA. We use our model to infer the characteristics of the interaction potential, and find that its equilibrium spacing strongly depends on the curvature of the filaments. In addition, the interaction is much softer than previously reported in bulk samples using different salt conditions. Beyond viruses and cells, our characterization of the interactions governing DNA-based dense structures could help develop robust designs in DNA nanotechnologies.
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Affiliation(s)
- Luca Barberi
- Université Paris-Saclay, CNRS, LPTMS, 91405, Orsay, France
| | | | | | - Martin Lenz
- Université Paris-Saclay, CNRS, LPTMS, 91405, Orsay, France.,PMMH, CNRS, ESPCI Paris, PSL University, Sorbonne Université, Université de Paris, F-75005 Paris, France
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3
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Breyton C, Javed W, Vermot A, Arnaud CA, Hajjar C, Dupuy J, Petit-Hartlein I, Le Roy A, Martel A, Thépaut M, Orelle C, Jault JM, Fieschi F, Porcar L, Ebel C. Assemblies of lauryl maltose neopentyl glycol (LMNG) and LMNG-solubilized membrane proteins. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2019; 1861:939-957. [PMID: 30776334 DOI: 10.1016/j.bbamem.2019.02.003] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/01/2018] [Revised: 02/08/2019] [Accepted: 02/08/2019] [Indexed: 12/11/2022]
Abstract
Laurylmaltose neopentylglycol (LMNG) bears two linked hydrophobic chains of equal length and two hydrophilic maltoside groups. It arouses a strong interest in the field of membrane protein biochemistry, since it was shown to efficiently solubilize and stabilize membrane proteins often better than the commonly used dodecylmaltopyranoside (DDM), and to allow structure determination of some challenging membrane proteins. However, LMNG was described to form large micelles, which could be unfavorable for structural purposes. We thus investigated its auto-assemblies and the association state of different membrane proteins solubilized in LMNG by analytical ultracentrifugation, size exclusion chromatography coupled to light scattering, centrifugation on sucrose gradient and/or small angle scattering. At high concentrations (in the mM range), LMNG forms long rods, and it stabilized the membrane proteins investigated herein, i.e. a bacterial multidrug transporter, BmrA; a prokaryotic analogous of the eukaryotic NADPH oxidases, SpNOX; an E. coli outer membrane transporter, FhuA; and the halobacterial bacteriorhodopsin, bR. BmrA, in the Apo and the vanadate-inhibited forms showed reduced kinetics of limited proteolysis in LMNG compared to DDM. Both SpNOX and BmrA display an increased specific activity in LMNG compared to DDM. The four proteins form LMNG complexes with their usual quaternary structure and with usual amount of bound detergent. No heterogeneous complexes related to the large micelle size of LMNG alone were observed. In conditions where LMNG forms assemblies of large size, FhuA crystals diffracting to 4.0 Å were obtained by vapor diffusion. LMNG large micelle size thus does not preclude membrane protein homogeneity and crystallization.
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Affiliation(s)
- Cécile Breyton
- Univ. Grenoble Alpes, CNRS, CEA, Institute for Structural Biology (IBS), 38000 Grenoble, France
| | - Waqas Javed
- Univ. Grenoble Alpes, CNRS, CEA, Institute for Structural Biology (IBS), 38000 Grenoble, France; University of Lyon, CNRS, UMR5086, Molecular Microbiology and Structural Biochemistry, IBCP, Lyon 69367, France
| | - Annelise Vermot
- Univ. Grenoble Alpes, CNRS, CEA, Institute for Structural Biology (IBS), 38000 Grenoble, France
| | - Charles-Adrien Arnaud
- Univ. Grenoble Alpes, CNRS, CEA, Institute for Structural Biology (IBS), 38000 Grenoble, France
| | - Christine Hajjar
- Univ. Grenoble Alpes, CNRS, CEA, Institute for Structural Biology (IBS), 38000 Grenoble, France
| | - Jérôme Dupuy
- Univ. Grenoble Alpes, CNRS, CEA, Institute for Structural Biology (IBS), 38000 Grenoble, France
| | - Isabelle Petit-Hartlein
- Univ. Grenoble Alpes, CNRS, CEA, Institute for Structural Biology (IBS), 38000 Grenoble, France
| | - Aline Le Roy
- Univ. Grenoble Alpes, CNRS, CEA, Institute for Structural Biology (IBS), 38000 Grenoble, France
| | - Anne Martel
- Institut Max Von Laue Paul Langevin, 38042 Grenoble, France
| | - Michel Thépaut
- Univ. Grenoble Alpes, CNRS, CEA, Institute for Structural Biology (IBS), 38000 Grenoble, France
| | - Cédric Orelle
- University of Lyon, CNRS, UMR5086, Molecular Microbiology and Structural Biochemistry, IBCP, Lyon 69367, France
| | - Jean-Michel Jault
- University of Lyon, CNRS, UMR5086, Molecular Microbiology and Structural Biochemistry, IBCP, Lyon 69367, France
| | - Franck Fieschi
- Univ. Grenoble Alpes, CNRS, CEA, Institute for Structural Biology (IBS), 38000 Grenoble, France
| | - Lionel Porcar
- Institut Max Von Laue Paul Langevin, 38042 Grenoble, France
| | - Christine Ebel
- Univ. Grenoble Alpes, CNRS, CEA, Institute for Structural Biology (IBS), 38000 Grenoble, France.
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4
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Mertens MAS, Sauer DF, Markel U, Schiffels J, Okuda J, Schwaneberg U. Chemoenzymatic cascade for stilbene production from cinnamic acid catalyzed by ferulic acid decarboxylase and an artificial metathease. Catal Sci Technol 2019. [DOI: 10.1039/c9cy01412h] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
We report a chemoenzymatic cascade reaction for stilbene production combining decarboxylation and olefin metathesis with efficient removal of metal contamination.
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Affiliation(s)
| | - Daniel F. Sauer
- Institute of Biotechnology
- RWTH Aachen University
- 52074 Aachen
- Germany
| | - Ulrich Markel
- Institute of Biotechnology
- RWTH Aachen University
- 52074 Aachen
- Germany
| | | | - Jun Okuda
- Institute of Inorganic Chemistry
- RWTH Aachen University
- 52074 Aachen
- Germany
| | - Ulrich Schwaneberg
- Institute of Biotechnology
- RWTH Aachen University
- 52074 Aachen
- Germany
- DWI-Leibniz Institute for Interactive Materials
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5
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Kinzel J, Sauer DF, Bocola M, Arlt M, Mirzaei Garakani T, Thiel A, Beckerle K, Polen T, Okuda J, Schwaneberg U. 2-Methyl-2,4-pentanediol (MPD) boosts as detergent-substitute the performance of ß-barrel hybrid catalyst for phenylacetylene polymerization. Beilstein J Org Chem 2017; 13:1498-1506. [PMID: 28845193 PMCID: PMC5550818 DOI: 10.3762/bjoc.13.148] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2017] [Accepted: 07/13/2017] [Indexed: 02/02/2023] Open
Abstract
Covering hydrophobic regions with stabilization agents to solubilize purified transmembrane proteins is crucial for their application in aqueous media. The small molecule 2-methyl-2,4-pentanediol (MPD) was used to stabilize the transmembrane protein Ferric hydroxamate uptake protein component A (FhuA) utilized as host for the construction of a rhodium-based biohybrid catalyst. Unlike commonly used detergents such as sodium dodecyl sulfate or polyethylene polyethyleneglycol, MPD does not form micelles in solution. Molecular dynamics simulations revealed the effect and position of stabilizing MPD molecules. The advantage of the amphiphilic MPD over micelle-forming detergents is demonstrated in the polymerization of phenylacetylene, showing a ten-fold increase in yield and increased molecular weights.
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Affiliation(s)
- Julia Kinzel
- Institute of Biotechnology, RWTH Aachen University, Worringer Weg 3, 52074 Aachen, Germany
| | - Daniel F Sauer
- Institut für Anorganische Chemie, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
| | - Marco Bocola
- Institute of Biotechnology, RWTH Aachen University, Worringer Weg 3, 52074 Aachen, Germany
| | - Marcus Arlt
- Institute of Biotechnology, RWTH Aachen University, Worringer Weg 3, 52074 Aachen, Germany
| | - Tayebeh Mirzaei Garakani
- Institute of Biotechnology, RWTH Aachen University, Worringer Weg 3, 52074 Aachen, Germany.,DWI - Leibniz Institute for Interactive Materials e.V., Forckenbeckstr. 50, 52056, Aachen, Germany
| | - Andreas Thiel
- Institut für Anorganische Chemie, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
| | - Klaus Beckerle
- Institut für Anorganische Chemie, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
| | - Tino Polen
- Institute of Bio- und Geosciences, IBG-1: Biotechnology, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
| | - Jun Okuda
- Institut für Anorganische Chemie, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
| | - Ulrich Schwaneberg
- Institute of Biotechnology, RWTH Aachen University, Worringer Weg 3, 52074 Aachen, Germany.,DWI - Leibniz Institute for Interactive Materials e.V., Forckenbeckstr. 50, 52056, Aachen, Germany
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Thoma J, Ritzmann N, Wolf D, Mulvihill E, Hiller S, Müller DJ. Maltoporin LamB Unfolds β Hairpins along Mechanical Stress-Dependent Unfolding Pathways. Structure 2017. [DOI: 10.1016/j.str.2017.05.010] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
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7
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Charan H, Glebe U, Anand D, Kinzel J, Zhu L, Bocola M, Garakani TM, Schwaneberg U, Böker A. Nano-thin walled micro-compartments from transmembrane protein-polymer conjugates. SOFT MATTER 2017; 13:2866-2875. [PMID: 28352880 DOI: 10.1039/c6sm02520j] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
The high interfacial activity of protein-polymer conjugates has inspired their use as stabilizers for Pickering emulsions, resulting in many interesting applications such as synthesis of templated micro-compartments and protocells or vehicles for drug and gene delivery. In this study we report, for the first time, the stabilization of Pickering emulsions with conjugates of a genetically modified transmembrane protein, ferric hydroxamate uptake protein component A (FhuA). The lysine residues of FhuA with open pore (FhuA ΔCVFtev) were modified to attach an initiator and consequently controlled radical polymerization (CRP) carried out via the grafting-from technique. The resulting conjugates of FhuA ΔCVFtev with poly(N-isopropylacrylamide) (PNIPAAm) and poly((2-dimethylamino)ethyl methacrylate) (PDMAEMA), the so-called building blocks based on transmembrane proteins (BBTP), have been shown to engender larger structures. The properties such as pH-responsivity, temperature-responsivity and interfacial activity of the BBTP were analyzed using UV-Vis spectrophotometry and pendant drop tensiometry. The BBTP were then utilized for the synthesis of highly stable Pickering emulsions, which could remain non-coalesced for well over a month. A new UV-crosslinkable monomer was synthesized and copolymerized with NIPAAm from the protein. The emulsion droplets, upon crosslinking of polymer chains, yielded micro-compartments. Fluorescence microscopy proved that these compartments are of micrometer scale, while cryo-scanning electron microscopy and scanning force microscopy analysis yielded a thickness in the range of 11.1 ± 0.6 to 38.0 ± 18.2 nm for the stabilizing layer of the conjugates. Such micro-compartments would prove to be beneficial in drug delivery applications, owing to the possibility of using the channel of the transmembrane protein as a gate and the smart polymer chains as trigger switches to tune the behavior of the capsules.
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Affiliation(s)
- Himanshu Charan
- Fraunhofer Institute for Applied Polymer Research IAP, Geiselbergstr. 69, 14476 Potsdam-Golm, Germany.
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8
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Engineering a Novel Porin OmpGF Via Strand Replacement from Computational Analysis of Sequence Motif. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2017; 1859:1180-1189. [PMID: 28341438 DOI: 10.1016/j.bbamem.2017.03.012] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/30/2016] [Revised: 02/20/2017] [Accepted: 03/18/2017] [Indexed: 12/12/2022]
Abstract
β-Barrelmembrane proteins (βMPs) form barrel-shaped pores in the outer membrane of Gram-negative bacteria, mitochondria, and chloroplasts. Because of the robustness of their barrel structures, βMPs have great potential as nanosensors for single-molecule detection. However, natural βMPs currently employed have inflexible biophysical properties and are limited in their pore geometry, hindering their applications in sensing molecules of different sizes and properties. Computational engineering has the promise to generate βMPs with desired properties. Here we report a method for engineering novel βMPs based on the discovery of sequence motifs that predominantly interact with the cell membrane and appear in more than 75% of transmembrane strands. By replacing β1-β6 strands of the protein OmpF that lack these motifs with β1-β6 strands of OmpG enriched with these motifs and computational verification of increased stability of its transmembrane section, we engineered a novel βMP called OmpGF. OmpGF is predicted to form a monomer with a stable transmembrane region. Experimental validations showed that OmpGF could refold in vitro with a predominant β-sheet structure, as confirmed by circular dichroism. Evidence of OmpGF membrane insertion was provided by intrinsic tryptophan fluorescence spectroscopy, and its pore-forming property was determined by a dye-leakage assay. Furthermore, single-channel conductance measurements confirmed that OmpGF function as a monomer and exhibits increased conductance than OmpG and OmpF. These results demonstrated that a novel and functional βMP can be successfully engineered through strand replacement based on sequence motif analysis and stability calculation.
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9
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Lantez V, Nikolaidis I, Rechenmann M, Vernet T, Noirclerc-Savoye M. Rapid automated detergent screening for the solubilization and purification of membrane proteins and complexes. Eng Life Sci 2015. [DOI: 10.1002/elsc.201400187] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Affiliation(s)
- Violaine Lantez
- Université Grenoble Alpes, IBS; Grenoble France
- CEA, IBS; Grenoble France
- CNRS, IBS; Grenoble France
| | - Ioulia Nikolaidis
- Université Grenoble Alpes, IBS; Grenoble France
- CEA, IBS; Grenoble France
- CNRS, IBS; Grenoble France
- Department of Biochemistry of Membranes; Bijvoet Center for Biomolecular Research, Utrecht University; The Netherlands
| | - Mathias Rechenmann
- Université Grenoble Alpes, IBS; Grenoble France
- CEA, IBS; Grenoble France
- CNRS, IBS; Grenoble France
| | - Thierry Vernet
- Université Grenoble Alpes, IBS; Grenoble France
- CEA, IBS; Grenoble France
- CNRS, IBS; Grenoble France
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10
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11
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Mechanistic Explanation of Different Unfolding Behaviors Observed for Transmembrane and Soluble β-Barrel Proteins. Structure 2013; 21:1317-24. [DOI: 10.1016/j.str.2013.06.001] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2013] [Revised: 06/01/2013] [Accepted: 06/05/2013] [Indexed: 01/31/2023]
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12
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Tomita N, Mohammad MM, Niedzwiecki DJ, Ohta M, Movileanu L. Does the lipid environment impact the open-state conductance of an engineered β-barrel protein nanopore? BIOCHIMICA ET BIOPHYSICA ACTA 2013; 1828:1057-65. [PMID: 23246446 PMCID: PMC3560310 DOI: 10.1016/j.bbamem.2012.12.003] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/06/2012] [Revised: 11/16/2012] [Accepted: 12/04/2012] [Indexed: 12/11/2022]
Abstract
Using rational membrane protein design, we were recently able to obtain a β-barrel protein nanopore that was robust under an unusually broad range of experimental circumstances. This protein nanopore was based upon the native scaffold of the bacterial ferric hydroxamate uptake component A (FhuA) of Escherichia coli. In this work, we expanded the examinations of the open-state current of this engineered protein nanopore, also called FhuA ΔC/Δ4L, employing an array of lipid bilayer systems that contained charged and uncharged as well as conical and cylindrical lipids. Remarkably, systematical single-channel analysis of FhuA ΔC/Δ4L indicated that most of its biophysical features, such as the unitary conductance and the stability of the open-state current, were not altered under the conditions tested in this work. However, electrical recordings at high transmembrane potentials revealed that the presence of conical phospholipids within the bilayer catalyzes the first, stepwise current transition of the FhuA ΔC/Δ4L protein nanopore to a lower-conductance open state. This study reinforces the stability of the open-state current of the engineered FhuA ΔC/Δ4L protein nanopore under various experimental conditions, paving the way for further critical developments in biosensing and molecular biomedical diagnosis.
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Affiliation(s)
- Noriko Tomita
- Department of Physics, Syracuse University, Syracuse, New York 13244-1130, USA
- Institute of Fluid Science, Tohoku University, Sendai, Miyagi 980-8577, Japan
| | | | | | - Makoto Ohta
- Institute of Fluid Science, Tohoku University, Sendai, Miyagi 980-8577, Japan
| | - Liviu Movileanu
- Department of Physics, Syracuse University, Syracuse, New York 13244-1130, USA
- Structural Biology, Biochemistry, and Biophysics Program, Syracuse University, Syracuse, New York 13244-4100, USA
- Syracuse Biomaterials Institute, Syracuse University, Syracuse, New York 13244, USA
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13
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Flayhan A, Wien F, Paternostre M, Boulanger P, Breyton C. New insights into pb5, the receptor binding protein of bacteriophage T5, and its interaction with its Escherichia coli receptor FhuA. Biochimie 2012; 94:1982-9. [PMID: 22659573 DOI: 10.1016/j.biochi.2012.05.021] [Citation(s) in RCA: 49] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/11/2012] [Accepted: 05/22/2012] [Indexed: 01/13/2023]
Abstract
The majority of bacterial viruses are bacteriophages bearing a tail that serves to recognise the bacterial surface and deliver the genome into the host cell. Infection is initiated by the irreversible interaction between the viral receptor binding protein (RBP) and a receptor at the surface of the bacterium. This interaction results ultimately in the phage DNA release in the host cytoplasm. Phage T5 infects Escherichia coli after binding of its RBP pb5 to the outer membrane ferrichrome transporter FhuA. Here, we have studied the complex formed by pb5 and FhuA by a variety of biophysical and biochemical techniques. We show that unlike RBPs of known structures, pb5 probably folds as a unique domain fulfilling both functions of binding to the host receptor and interaction with the rest of the phage. Pb5 likely binds to the domain occluding the β-barrel of FhuA as well as to external loops of the barrel. Furthermore, upon binding to FhuA, pb5 undergoes conformational changes, at the secondary and tertiary structure level that would be the key to the transmission of the signal through the tail to the capsid, triggering DNA release. This is the first structural information regarding the binding of a RBP to a proteic receptor.
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Affiliation(s)
- Ali Flayhan
- CEA, Institut de Biologie Structurale Jean-Pierre Ebel, Grenoble, France
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14
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Flores Jiménez RH, Cafiso DS. The N-terminal domain of a TonB-dependent transporter undergoes a reversible stepwise denaturation. Biochemistry 2012; 51:3642-50. [PMID: 22497281 DOI: 10.1021/bi300118a] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Gram-negative bacteria contain a family of outer membrane transport proteins that function in the uptake of rare nutrients, such as iron and vitamin B(12). These proteins are termed TonB-dependent because transport requires an interaction with the inner-membrane protein TonB. Using a combination of site-directed spin labeling and chemical denaturation, we examined the site-specific unfolding of regions of the Escherichia coli vitamin B(12) transporter, BtuB. The data indicate that a portion of the N-terminal region of the protein, which occupies the lumen of the BtuB barrel, denatures prior to the unfolding of the barrel and that the free energy of folding for the N-terminus is smaller than that typically seen for globular proteins. Moreover, the data indicate that the N-terminal domain does not unfold in a single event but unfolds in a series of independent steps. The unfolding of the N-terminus is reversible, and removal of denaturant restores the native fold of the protein. These data are consistent with proposed transport mechanisms that involve a transient rearrangement or unfolding of the N-terminus of the protein, and they provide evidence of a specific protein conformation that might be an intermediate accessed during transport.
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Affiliation(s)
- Ricardo H Flores Jiménez
- Department of Chemistry, Biophysics Program, and Center for Membrane Biology, University of Virginia, Charlottesville, Virginia 22904-4319, United States
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15
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Tenne SJ, Schwaneberg U. First insights on organic cosolvent effects on FhuA wildtype and FhuA Δ1-159. Int J Mol Sci 2012; 13:2459-2471. [PMID: 22408464 PMCID: PMC3292033 DOI: 10.3390/ijms13022459] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/06/2012] [Revised: 02/09/2012] [Accepted: 02/13/2012] [Indexed: 11/24/2022] Open
Abstract
Circular dichroism (CD) and deconvolution were used to study the structural integrity of a "plugged" and an "open" FhuA transmembrane channel protein in the presence of varied concentrations of tetrahydrofuran (THF), ethanol (EtOH) and chloroform/methanol (C/M). FhuA is an Escherichia coli outer membrane protein (78.9 kDa) consisting of 22 β-sheets and an internal globular cork domain which acts as an iron transporter. FhuA and the deletion variant FhuA Δ1-159 showed comparable and remarkable resistance in the presence of THF (≤40 vol%) and EtOH (≤10 vol%). In C/M, significant differences in structural resistance were observed (FhuA stable ≤10 vol%; FhuA Δ1-159 ≤1 vol%). Deconvolution of CD-spectra for FhuA and FhuA Δ1-159 yielded β-sheet contents of 61 % (FhuA) and 58% (FhuA Δ1-159). Interestingly, FhuA and FhuA Δ1-159 had comparable β-sheet contents in the presence and absence of all three organic cosolvents. Additionally, precipitated FhuA and FhuA Δ1-159 (in 40 vol% C/M or 65 vol% THF) redissolved by supplementing the detergent n-octyl-oligo-oxyethylene (oPOE).
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Affiliation(s)
- Stefanie-Joana Tenne
- Lehrstuhl für Biotechnologie, RWTH Aachen University, Worringer Weg 1, Aachen, D-52074, Germany; E-Mail:
| | - Ulrich Schwaneberg
- Lehrstuhl für Biotechnologie, RWTH Aachen University, Worringer Weg 1, Aachen, D-52074, Germany; E-Mail:
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Naveed H, Xu Y, Jackups R, Liang J. Predicting three-dimensional structures of transmembrane domains of β-barrel membrane proteins. J Am Chem Soc 2012; 134:1775-81. [PMID: 22148174 DOI: 10.1021/ja209895m] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
β-Barrel membrane proteins are found in the outer membrane of gram-negative bacteria, mitochondria, and chloroplasts. They are important for pore formation, membrane anchoring, and enzyme activity. These proteins are also often responsible for bacterial virulence. Due to difficulties in experimental structure determination, they are sparsely represented in the protein structure databank. We have developed a computational method for predicting structures of the transmembrane (TM) domains of β-barrel membrane proteins. Based on physical principles, our method can predict structures of the TM domain of β-barrel membrane proteins of novel topology, including those from eukaryotic mitochondria. Our method is based on a model of physical interactions, a discrete conformational state space, an empirical potential function, as well as a model to account for interstrand loop entropy. We are able to construct three-dimensional atomic structure of the TM domains from sequences for a set of 23 nonhomologous proteins (resolution 1.8-3.0 Å). The median rmsd of TM domains containing 75-222 residues between predicted and measured structures is 3.9 Å for main chain atoms. In addition, stability determinants and protein-protein interaction sites can be predicted. Such predictions on eukaryotic mitochondria outer membrane protein Tom40 and VDAC are confirmed by independent mutagenesis and chemical cross-linking studies. These results suggest that our model captures key components of the organization principles of β-barrel membrane protein assembly.
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Affiliation(s)
- Hammad Naveed
- Department of Bioengineering, University of Illinois at Chicago, 835 South Wolcott Avenue, Chicago, Illinois 60607, USA
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Odahara T, Ishii N, Ooishi A, Honda S, Uedaira H, Hara M, Miyake J. Thermostability of Rhodopseudomonas viridis and Rhodospirillum rubrum chromatophores reflecting physiological conditions. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2011; 1808:1645-53. [DOI: 10.1016/j.bbamem.2011.02.013] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/29/2010] [Revised: 01/24/2011] [Accepted: 02/15/2011] [Indexed: 11/29/2022]
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18
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Mohammad MM, Howard KR, Movileanu L. Redesign of a plugged beta-barrel membrane protein. J Biol Chem 2011; 286:8000-8013. [PMID: 21189254 PMCID: PMC3048687 DOI: 10.1074/jbc.m110.197723] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2010] [Revised: 12/16/2010] [Indexed: 11/06/2022] Open
Abstract
The redesign of biological nanopores is focused on bacterial outer membrane proteins and pore-forming toxins, because their robust β-barrel structure makes them the best choice for developing stochastic biosensing elements. Using membrane protein engineering and single-channel electrical recordings, we explored the ferric hydroxamate uptake component A (FhuA), a monomeric 22-stranded β-barrel protein from the outer membrane of Escherichia coli. FhuA has a luminal cross-section of 3.1 × 4.4 nm and is filled by a globular N-terminal cork domain. Various redesigned FhuA proteins were investigated, including single, double, and multiple deletions of the large extracellular loops and the cork domain. We identified four large extracellular loops that partially occlude the lumen when the cork domain is removed. The newly engineered protein, FhuAΔC/Δ4L, was the result of a removal of almost one-third of the total number of amino acids of the wild-type FhuA (WT-FhuA) protein. This extensive protein engineering encompassed the entire cork domain and four extracellular loops. Remarkably, FhuAΔC/Δ4L forms a functional open pore in planar lipid bilayers, with a measured unitary conductance of ∼4.8 nanosiemens, which is much greater than the values recorded previously with other engineered FhuA protein channels. There are numerous advantages and prospects of using such an engineered outer membrane protein not only in fundamental studies of membrane protein folding and design, and the mechanisms of ion conductance and gating, but also in more applicative areas of stochastic single-molecule sensing of proteins and nucleic acids.
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Affiliation(s)
- Mohammad M Mohammad
- From the Department of Physics, Syracuse University, Syracuse, New York 13244-1130
| | - Khalil R Howard
- the Structural Biology, Biochemistry, and Biophysics Program, Syracuse University, Syracuse, New York 13244-4100, and
| | - Liviu Movileanu
- From the Department of Physics, Syracuse University, Syracuse, New York 13244-1130,; the Structural Biology, Biochemistry, and Biophysics Program, Syracuse University, Syracuse, New York 13244-4100, and; the Syracuse Biomaterials Institute, Syracuse University, Syracuse, New York 13244.
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19
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Reconstitution of bacterial outer membrane TonB-dependent transporters in planar lipid bilayer membranes. Proc Natl Acad Sci U S A 2009; 106:21990-5. [PMID: 19959664 DOI: 10.1073/pnas.0910023106] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Micronutrients such as siderophore-bound iron and vitamin B(12) cross the outer membrane of gram-negative bacteria through a group of 22-stranded beta-barrel proteins. They share the unusual feature that their N-terminal end inserts from the periplasmic side into the beta-barrel and plugs the lumen. Transport results from energy-driven movement of TonB protein, which either pulls the plug out of the barrel or causes it to rearrange within the barrel. Attempts to reconstitute native plugged channels in an ion-conducting state in lipid bilayer membranes have so far been unsuccessful. We, however, have discovered that if the cis solution contained 4 M urea, then, with the periplasmic side of the channel facing that solution, macroscopic conductances and single channel events could be observed. These results were obtained with FhuA, Cir, and BtuB; for the former two, the channels were closed by removing the 4 M urea. Channels generated by 4 M urea exposure were not a consequence of general protein denaturation, as their ligand-binding properties were preserved. Thus, with FhuA, addition of ferrichrome (its siderophore) to the trans, extracellular-facing side reversibly inhibited 4 M urea-induced channel opening and blocked the channels. With Cir, addition of colicin Ia (the microbial toxin that targets Cir) to the trans, extracellular-facing side prevented 4 M urea-induced channel opening. We hypothesize that 4 M urea reversibly unfolds the FhuA and Cir plugs, thereby opening an ion-conducting pathway through these channels, and that this mimics to some extent the in vivo action of TonB on these plugs.
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Hong H, Joh NH, Bowie JU, Tamm LK. Chapter 8 Methods for Measuring the Thermodynamic Stability of Membrane Proteins. Methods Enzymol 2009; 455:213-36. [DOI: 10.1016/s0076-6879(08)04208-0] [Citation(s) in RCA: 63] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
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21
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Hullmann J, Patzer SI, Römer C, Hantke K, Braun V. Periplasmic chaperone FkpA is essential for imported colicin M toxicity. Mol Microbiol 2008; 69:926-37. [PMID: 18554332 PMCID: PMC2615193 DOI: 10.1111/j.1365-2958.2008.06327.x] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Chaperones facilitate correct folding of newly synthesized proteins. We show here that the periplasmic FkpA chaperone is required for killing Escherichia coli by colicin M entering cells from the outside. Highly active colicin M preparations were inactive against fkpA mutant cells; 104-fold dilutions killed fkpA+ cells. Three previously isolated spontaneous mutants tolerant to colicin M carried a stop codon or an IS1 insertion in the peptidyl-prolyl-cis-trans-isomerase (PPIase) domain (C-domain) of FkpA, which resulted in deletion of the domain. A randomly generated mutant carried a G148D mutation in the C-domain. A temperature-sensitive mutant tolerant to colicin M carried a Y25N mutation in the FkpA N-domain. Mutants transformed with wild-type fkpA were colicin M-sensitive. Isolated FkpA-His reduced colicin M-His cleavage by proteinase K and renatured denatured colicin M-His in vitro; renaturation was prevented by the PPIase inhibitor FK506. In both assays, periplasmic SurA-His had no effect. No other tested periplasmic chaperone could activate colicin M. Among the tested colicins, only colicin M required FkpA for activity. Colicin M bound to cells via FhuA was inactivated by trypsin; unbound colicin M retained activity. We propose that colicin M unfolds during import across the outer membrane, FkpA specifically assists in folding colicin M into an active toxin in the periplasm and PPIase is essential for colicin M activity. Colicin M is a suitable tool for the isolation of FkpA mutants used to elucidate the functions of the FkpA N- and C-domains.
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Affiliation(s)
- Julia Hullmann
- Microbiology/Membrane Physiology, University of Tübingen, Auf der Morgenstelle 28, D-72076 Tübingen, Germany
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22
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Serwer P, Wright ET, Hakala KW, Weintraub ST. Evidence for bacteriophage T7 tail extension during DNA injection. BMC Res Notes 2008; 1:36. [PMID: 18710489 PMCID: PMC2525648 DOI: 10.1186/1756-0500-1-36] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/21/2008] [Accepted: 06/26/2008] [Indexed: 11/10/2022] Open
Abstract
BACKGROUND Electron micrographs of bacteriophage T7 reveal a tail shorter than needed to reach host cytoplasm during infection-initiating injection of a T7 DNA molecule through the tail and cell envelope. However, recent data indicate that internal T7 proteins are injected before the DNA molecule is injected. Thus, bacteriophage/host adsorption potentially causes internal proteins to become external and lengthen the tail for DNA injection. But, the proposed adsorption-induced tail lengthening has never been visualized. FINDINGS In the present study, electron microscopy of particles in T7 lysates reveals a needle-like capsid extension that attaches partially emptied bacteriophage T7 capsids to non-capsid vesicles and sometimes enters an attached vesicle. This extension is 40-55 nm long, 1.7-2.4x longer than the T7 tail and likely to be the proposed lengthened tail. The extension is 8-11 nm in diameter, thinner than most of the tail, with an axial hole 3-4 nm in diameter. Though the bound vesicles are not identified by microscopy, these vesicles resemble the major vesicles in T7 lysates, found to be E. coli outer membrane vesicles by non-denaturing agarose gel electrophoresis, followed by mass spectrometry. CONCLUSION The observed lengthened tail is long enough to reach host cytoplasm during DNA injection. Its channel is wide enough to be a conduit for DNA injection and narrow enough to clamp DNA during a previously observed stalling/re-starting of injection. However, its outer diameter is too large to explain formation by passing of an intact assembly through any known capsid hole unless the hole is widened.
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Affiliation(s)
- Philip Serwer
- Department of Biochemistry, The University of Texas Health Science Center, San Antonio, Texas 78229-3900, USA.
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23
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Expression, purification, and structural characterization of CfrA, a putative iron transporter from Campylobacter jejuni. J Bacteriol 2008; 190:5650-62. [PMID: 18556796 DOI: 10.1128/jb.00298-08] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The gene for the Campylobacter ferric receptor (CfrA), a putative iron-siderophore transporter in the enteric food-borne pathogen Campylobacter jejuni, was cloned, and the membrane protein was expressed in Escherichia coli, affinity purified, and then reconstituted into model lipid membranes. Fourier transform infrared spectra recorded from the membrane-reconstituted CfrA are similar to spectra that have been recorded from other iron-siderophore transporters and are highly characteristic of a beta-sheet protein (approximately 44% beta-sheet and approximately 10% alpha-helix). CfrA undergoes relatively extensive peptide hydrogen-deuterium exchange upon exposure to (2)H(2)O and yet is resistant to thermal denaturation at temperatures up to 95 degrees C. The secondary structure, relatively high aqueous solvent exposure, and high thermal stability are all consistent with a transmembrane beta-barrel structure containing a plug domain. Sequence alignments indicate that CfrA contains many of the structural motifs conserved in other iron-siderophore transporters, including the Ton box, PGV, IRG, RP, and LIDG motifs of the plug domain. Surprisingly, a homology model reveals that regions of CfrA that are expected to play a role in enterobactin binding exhibit sequences that differ substantially from the sequences of the corresponding regions that play an essential role in binding/transport by the E. coli enterobactin transporter, FepA. The sequence variations suggest that there are differences in the mechanisms used by CfrA and FepA to interact with bacterial siderophores. It may be possible to exploit these structural differences to develop CfrA-specific therapeutics.
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24
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Huysmans GH, Radford SE, Brockwell DJ, Baldwin SA. The N-terminal helix is a post-assembly clamp in the bacterial outer membrane protein PagP. J Mol Biol 2007; 373:529-40. [PMID: 17868697 PMCID: PMC2887491 DOI: 10.1016/j.jmb.2007.07.072] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/02/2007] [Revised: 07/23/2007] [Accepted: 07/29/2007] [Indexed: 12/04/2022]
Abstract
The Escherichia coli outer membrane beta-barrel enzyme PagP and its homologues are unique in that the eight-stranded barrel is tilted by about 25 degrees with respect to the membrane normal and is preceded by a 19-residue amphipathic alpha-helix. To investigate the role of this helix in the folding and stability of PagP, mutants were generated in which the helix was deleted (Delta(1-19)), or in which residues predicted to be involved in helix-barrel interactions were altered (W17A or R59L). The ability of the variants to insert into detergent micelles or liposomes was studied in vitro using circular dichroism, fluorescence, Fourier transform infrared spectroscopy, electrophoretic mobility and gain of enzyme activity. The data show that PagP, initially unfolded in 5% (w/v) perfluoro-octanoic acid or 6 M guanidinium chloride, inserts spontaneously and folds quantitatively to an active conformation into detergent micelles of cyclofos-7 or into large vesicles of diC(12:0)-phosphatidylcholine (diC(12:0)PC), respectively, the latter in the presence of 7 M urea. Successful refolding of all variants into both micelles and liposomes ruled out an essential role for the helix or helix-barrel interactions in folding and membrane insertion. Measurements of thermal stability indicated that the variants R59L, W17A/R59L and Delta(1-19) were destabilised substantially compared with wild-type PagP. However, in contrast to the other variants, destabilisation of the W17A variant relative to wild-type PagP was much greater in liposomes than in micelles. Analysis of the kinetics of folding and unfolding of all variants in diC(12:0)PC liposomes suggested that this destabilisation arises predominantly from an increased dissociation of the refolded variant proteins from the lipid-inserted state. The data support the view that the helix of PagP is not required for folding and assembly, but instead acts as a clamp, stabilising membrane-inserted PagP after folding and docking with the membrane are complete.
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Affiliation(s)
- Gerard H.M. Huysmans
- Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds, LS2 9JT, UK
- Institute of Membrane and Systems Biology, University of Leeds, Leeds, LS2 9JT, UK
| | - Sheena E. Radford
- Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds, LS2 9JT, UK
- Institute of Molecular and Cellular Biology, University of Leeds, Leeds, LS2 9JT, UK
| | - David J. Brockwell
- Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds, LS2 9JT, UK
- Institute of Molecular and Cellular Biology, University of Leeds, Leeds, LS2 9JT, UK
| | - Stephen A. Baldwin
- Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds, LS2 9JT, UK
- Institute of Membrane and Systems Biology, University of Leeds, Leeds, LS2 9JT, UK
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25
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Peacock RS, Andrushchenko VV, Demcoe AR, Gehmlich M, Lu LS, Herrero AG, Vogel HJ. Characterization of TonB Interactions with the FepA Cork Domain and FecA N-terminal Signaling Domain. Biometals 2006; 19:127-42. [PMID: 16718599 DOI: 10.1007/s10534-005-5420-0] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2005] [Accepted: 11/23/2005] [Indexed: 10/24/2022]
Abstract
The mechanism of TonB dependent siderophore uptake through outer membrane transporters in Gram-negative bacteria is poorly understood. In an effort to expand our knowledge of the interaction between TonB and the outer membrane transporters, we have cloned and expressed the FepA cork domain (11-154) from Salmonella typhimurium and characterized its interaction with the periplasmic C-terminal domain of TonB (103-239) by isotope assisted FTIR and NMR spectroscopy. For comparison we also performed similar experiments using the FecA N-terminal domain (1-96) from Escherichia coli which includes the conserved TonB box. The FepA cork domain was completely unfolded in solution, as observed for the E. coli cork domain previously [Usher et al. (2001) Proc Natl Acad Sci USA 98, 10676-10681]. The FepA cork domain was found to bind to TonB, eliciting essentially the same chemical shift changes in TonB C-terminal domain as was observed in the presence of TonB box peptides. The FecA construct did not cause this same structural change in TonB. The binding of the FepA cork domain to TonB-CTD was found to decrease the amount of ordered secondary structure in TonB-CTD. It is likely that the FecA N-terminal domain interferes with TonB-CTD binding to the TonB box. Binding of the FepA cork domain induces a loss of secondary structure in TonB, possibly exposing TonB surface area for additional intermolecular interactions such as potential homodimerization or additional interactions with the barrel of the outer membrane transporter.
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Affiliation(s)
- R Sean Peacock
- Structural Biology Research Group, Department of Biological Sciences, University of Calgary, Calgary, Alberta, T2N 1N4, Canada
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26
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Mdzinarashvili T, Khvedelidze M, Ivanova A, Mrevlishvili G, Kutateladze M, Balarjishvili N, Celia H, Pattus F. Biophysics of T5, IRA phages, Escherichia coli outer membrane protein FhuA and T5-FhuA interaction. EUROPEAN BIOPHYSICS JOURNAL: EBJ 2005; 35:231-8. [PMID: 16341727 DOI: 10.1007/s00249-005-0029-3] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/20/2005] [Revised: 06/29/2005] [Accepted: 07/18/2005] [Indexed: 10/25/2022]
Abstract
In spite of the similarities in a structural organization of T5 and IRA phages their thermal and hydrodynamical peculiarities are completely different. One of the significant differences is observed in temperature value at which thermally induced DNA ejection starts. If in the case of physiological conditions this difference equals to 30 degrees capital ES, Cyrillic, then it decreases as ionic strength of the solvent decreases. Also, from our experimental results follows that in the opening of phage tail channel for T5 phage (at pH7) significant role-play electrostatic forces. In spite of that both of these phages grow on the same Escherichia coli strain, we have shown that these phages need different receptors to penetrate into the bacterial cell precisely FhuA serves as receptor only for T5 phage. The higher FhuA concentration in T5 phage suspension is, the more intensive DNA ejection in environment is. The minimal FhuA/T5 ratio, which is 300/1, correspondingly, necessary for effective DNA ejection from the phage head was experimentally determined. For the first time the ejection of T5 phage DNA induced by FhuA was observed in an incessant regime. The deconvolution of calorimetric curve of FhuA's denaturation has been shown that in a chosen condition there are four thermodynamically independent domains in the structure of FhuA.
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Affiliation(s)
- T Mdzinarashvili
- Institute of Physics, Department of Exact and Natural Sciences, Tbilisi State University, 3 Chavchavadze ave., Tbilisi, Republic of Georgia.
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27
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Destoumieux-Garzón D, Duquesne S, Peduzzi J, Goulard C, Desmadril M, Letellier L, Rebuffat S, Boulanger P. The iron-siderophore transporter FhuA is the receptor for the antimicrobial peptide microcin J25: role of the microcin Val11-Pro16 beta-hairpin region in the recognition mechanism. Biochem J 2005; 389:869-76. [PMID: 15862112 PMCID: PMC1180738 DOI: 10.1042/bj20042107] [Citation(s) in RCA: 91] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
The role of the outer-membrane iron transporter FhuA as a potential receptor for the antimicrobial peptide MccJ25 (microcin J25) was studied through a series of in vivo and in vitro experiments. The requirement for both FhuA and the inner-membrane TonB-ExbB-ExbD complex was demonstrated by antibacterial assays using complementation of an fhuA(-) strain and by using isogenic strains mutated in genes encoding the protein complex respectively. In addition, MccJ25 was shown to block phage T5 infection of Escherichia coli, in vivo, by inhibiting phage adhesion, which suggested that MccJ25 prevents the interaction between the phage and its receptor FhuA. This in vivo activity was confirmed in vitro, as MccJ25 inhibited phage T5 DNA ejection triggered by purified FhuA. Direct interaction of MccJ25 with FhuA was demonstrated for the first time by size-exclusion chromatography and isothermal titration calorimetry. MccJ25 bound to FhuA with a 2:1 stoichiometry and a K(d) of 1.2 microM. Taken together, our results demonstrate that FhuA is the receptor for MccJ25 and that the ligand-receptor interaction may occur in the absence of other components of the bacterial membrane. Finally, both differential scanning calorimetry and antimicrobial assays showed that MccJ25 binding involves external loops of FhuA. Unlike native MccJ25, a thermolysin-cleaved MccJ25 variant was unable to bind to FhuA and failed to prevent phage T5 infection of E. coli. Therefore the Val11-Pro16 beta-hairpin region of MccJ25, which is disrupted upon cleavage by thermolysin, is required for microcin recognition.
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Affiliation(s)
- Delphine Destoumieux-Garzón
- Chimie et Biochimie des Substances Naturelles, CNRS UMR 5154, Muséum National d'Histoire Naturelle USM 502, Département Régulations Développement et Diversité Moléculaire, 63 rue Buffon, 75005 Paris, France.
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28
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Eisenhauer HA, Shames S, Pawelek PD, Coulton JW. Siderophore Transport through Escherichia coli Outer Membrane Receptor FhuA with Disulfide-tethered Cork and Barrel Domains. J Biol Chem 2005; 280:30574-80. [PMID: 15994322 DOI: 10.1074/jbc.m506708200] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The hydroxamate siderophore receptor FhuA is a TonB-dependent outer membrane protein of Escherichia coli composed of a C-terminal 22-stranded beta-barrel occluded by an N-terminal globular cork domain. During siderophore transport into the periplasm, the FhuA cork domain has been proposed to undergo conformational changes that allow transport through the barrel lumen; alternatively, the cork may be completely displaced from the barrel. To probe such changes, site-directed cysteine mutants in the cork domain (L109C and Q112C) and in the barrel domain (S356C and M383C) were created within the putative siderophore transport pathway. Molecular modeling predicted that the double cysteine mutants L109C/S356C and Q112C/M383C would form disulfide bonds, thereby tethering the cork and barrel domains. The double cysteine FhuA mutants were denatured under nonreducing conditions and fluorescently labeled with thiol-specific Oregon Green maleimide. Subsequent SDS-PAGE analysis revealed two distinct species: FhuA containing a disulfide bond and FhuA with free sulfhydryl groups. To address the role of the putative siderophore transport pathway and to evaluate possible rearrangements of the cork domain during ferricrocin transport, disulfide bond formation was enhanced by an oxidative catalyst. Cells containing double cysteine FhuA mutants that were subjected to oxidation during ferricrocin transport exhibited disulfide bond formation to near completion. After disulfide tethering of the cork to the barrel, ferricrocin transport was equivalent to transport by untreated cells. These results demonstrate that blocking the putative siderophore transport pathway does not abrogate ferricrocin uptake. We propose that, during siderophore transport through FhuA, the cork domain remains within the barrel rather than being displaced.
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Affiliation(s)
- H Anne Eisenhauer
- Department of Microbiology and Immunology, McGill University, Montreal, Quebec H3A 2B4, Canada
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29
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Hoegy F, Celia H, Mislin GL, Vincent M, Gallay J, Schalk IJ. Binding of iron-free siderophore, a common feature of siderophore outer membrane transporters of Escherichia coli and Pseudomonas aeruginosa. J Biol Chem 2005; 280:20222-30. [PMID: 15784620 DOI: 10.1074/jbc.m500776200] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
TonB-dependent iron transporters present in the outer membranes of Gram-negative bacteria transport ferric-siderophore complexes into the periplasm. This requires proton motive force and an integral inner membrane complex, TonB-ExbB-ExbD. Recognition of iron-free siderophores by TonB-dependent outer membrane transporters (OMT) has only been described for a subfamily called OMT(N). These OMT(N)s have an additional domain at the N terminus, which interacts with an inner membrane regulatory protein to activate a cytoplasmic sigma factor. This induces transcription of iron transport genes. Here we showed that the ability to bind aposiderophores is not specific to the OMT(N) subfamily but may be a more general feature of OMTs. FhuA, the ferrichrome OMT in Escherichia coli, and FptA, the pyochelin (Pch) OMT in Pseudomonas aeruginosa, were both able to bind in vitro and in vivo the apo-forms and the ferric forms of their corresponding siderophore at a common binding site. FptA produced in P. aeruginosa cells grown in an iron-deficient medium copurifies with a ligand that, as characterized by fluorescence, is iron-free Pch. As described previously for the FpvA transporter (pyoverdine OMT in P. aeruginosa), it appears that in conditions of iron limitation all the FptA receptors at the cell surface are loaded with apoPch. This FptA-Pch complex is less stable in vitro than the previously described copurified FpvA-Pvd complex and can be loaded with iron in vitro in the presence of Pch-Fe, citrate-Fe, or ferrichrome-Fe. These findings improved our understanding of the iron uptake mechanism via siderophores in Gram-negative bacteria.
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Affiliation(s)
- Françoise Hoegy
- Département des Récepteurs et Protéines Membranaires, UPR 9050 CNRS, ESBS, Bld. Sébastien Brant, F-67413 Illkirch, Strasbourg, France
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Tamm LK, Hong H, Liang B. Folding and assembly of beta-barrel membrane proteins. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2005; 1666:250-63. [PMID: 15519319 DOI: 10.1016/j.bbamem.2004.06.011] [Citation(s) in RCA: 236] [Impact Index Per Article: 12.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/20/2004] [Accepted: 06/24/2004] [Indexed: 11/20/2022]
Abstract
Beta-barrel membrane proteins occur in the outer membranes of Gram-negative bacteria, mitochondria and chloroplasts. The membrane-spanning sequences of beta-barrel membrane proteins are less hydrophobic than those of alpha-helical membrane proteins, which is probably the main reason why completely different folding and membrane assembly pathways have evolved for these two classes of membrane proteins. Some beta-barrel membrane proteins can be spontaneously refolded into lipid bilayer model membranes in vitro. They may also have this ability in vivo although lipid and protein chaperones likely assist with their assembly in appropriate target membranes. This review summarizes recent work on the thermodynamic stability and the mechanism of membrane insertion of beta-barrel membrane proteins in lipid model and biological membranes. How lipid compositions affect folding and assembly of beta-barrel membrane proteins is also reviewed. The stability of these proteins in membranes is not as large as previously thought (<10 kcal/mol) and is modulated by elastic forces of the lipid bilayer. Detailed kinetic studies indicate that beta-barrel membrane proteins fold in distinct steps with several intermediates that can be characterized in vitro. Formation of the barrel is synchronized with membrane insertion and all beta-hairpins insert simultaneously in a concerted pathway.
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Affiliation(s)
- Lukas K Tamm
- Department of Molecular Physiology and Biological Physics, University of Virginia Health Science Center, P.O. Box 800736, 1300 Jefferson Park Avenue, Charlottesville, VA 22908-0736, USA.
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Abstract
Iron is an essential element for most organisms, including bacteria. The oxidized form is insoluble, and the reduced form is highly toxic for most macromolecules and, in biological systems, is generally sequestrated by iron- and heme-carrier proteins. Thus, despite its abundance on earth, there is practically no free iron available for bacteria whatever biotope they colonize. To fulfill their iron needs, bacteria have multiple iron acquisition systems, reflecting the diversity of their potential biotopes. The iron/heme acquisition systems in bacteria have one of two general mechanisms. The first involves direct contact between the bacterium and the exogenous iron/heme sources. The second mechanism relies on molecules (siderophores and hemophores) synthesized and released by bacteria into the extracellular medium; these molecules scavenge iron or heme from various sources. Recent genetic, biochemical, and crystallographic studies have allowed substantial progress in describing molecular mechanisms of siderophore and hemophore interactions with the outer membrane receptors, transport through the inner membrane, iron storage, and regulation of genes encoding biosynthesis and uptake proteins.
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Affiliation(s)
- Cécile Wandersman
- Unité des Membranes Bactériennes, Département de Microbiologie Fondamentale et Médicale, Institut Pasteur, 75724 Paris Cedex 15, France.
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Oke M, Sarra R, Ghirlando R, Farnaud S, Gorringe AR, Evans RW, Buchanan SK. The plug domain of a neisserial TonB-dependent transporter retains structural integrity in the absence of its transmembrane beta-barrel. FEBS Lett 2004; 564:294-300. [PMID: 15111112 DOI: 10.1016/s0014-5793(04)00196-6] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/15/2003] [Accepted: 02/03/2004] [Indexed: 10/26/2022]
Abstract
Transferrin binding protein A (TbpA) is a TonB-dependent outer membrane protein expressed by pathogenic bacteria for iron acquisition from human transferrin. The N-terminal 160 residues (plug domain) of TbpA were overexpressed in both the periplasm and cytoplasm of Escherichia coli. We found this domain to be soluble and monodisperse in solution, exhibiting secondary structure elements found in plug domains of structurally characterized TonB-dependent transporters. Although the TbpA plug domain is apparently correctly folded, we were not able to observe an interaction with human transferrin by isothermal titration calorimetry or nitrocellulose binding assays. These experiments suggest that the plug domain may fold independently of the beta-barrel, but extracellular loops of the beta-barrel are required for ligand binding.
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Affiliation(s)
- M Oke
- National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Department of Health and Human Services, Bethesda, MD 20892-8030, USA
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Koedding J, Howard P, Kaufmann L, Polzer P, Lustig A, Welte W. Dimerization of TonB Is Not Essential for Its Binding to the Outer Membrane Siderophore Receptor FhuA of Escherichia coli. J Biol Chem 2004; 279:9978-86. [PMID: 14665631 DOI: 10.1074/jbc.m311720200] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
FhuA belongs to a family of specific siderophore transport systems located in the outer membrane of Escherichia coli. The energy required for the transport process is provided by the proton motive force of the cytoplasmic membrane and is transmitted to FhuA by the protein TonB. Although the structure of full-length TonB is not known, the structure of the last 77 residues of a fragment composed of the 86 C-terminal amino acids was recently solved and shows an intertwined dimer (Chang, C., Mooser, A., Pluckthun, A., and Wlodawer, A. (2001) J. Biol. Chem. 276, 27535-27540). We analyzed the ability of truncated C-terminal TonB fragments of different lengths (77, 86, 96, 106, 116, and 126 amino acid residues, respectively) to bind to the receptor FhuA. Only the shortest TonB fragment, TonB-77, could not effectively interact with FhuA. We have also observed that the fragments TonB-77 and TonB-86 form homodimers in solution, whereas the longer fragments remain monomeric. TonB fragments that bind to FhuA in vitro also inhibit ferrichrome uptake via FhuA in vivo and protect cells against attack by bacteriophage Phi80.
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Affiliation(s)
- Jiri Koedding
- Fakultaet fuer Biologie, Universitaet Konstanz, Germany
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Abstract
Transport systems of Gram-negative bacteria coordinate the passage of metabolites through the outer membrane, periplasm, and the cytoplasmic membrane without compromising the protective properties of the cell envelope. Active transporters orchestrate the import of metals against concentration gradients. These thermodynamically unfavorable processes are coupled to both an electrochemical proton gradient and the hydrolysis of ATP. Crystallographic structures of transport proteins now define in molecular detail most components of an active metal import pathway from Escherichia coli.
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Affiliation(s)
- Andrew D Ferguson
- Department of Biochemistry, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390, USA
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Fyfe PK, Isaacs NW, Cogdell RJ, Jones MR. Disruption of a specific molecular interaction with a bound lipid affects the thermal stability of the purple bacterial reaction centre. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2004; 1608:11-22. [PMID: 14741581 DOI: 10.1016/j.bbabio.2003.09.014] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
Relatively little is known about the functions of specific molecular interactions between membrane proteins and membrane lipids. The structural and functional consequences of disrupting a previously identified interaction between a molecule of the diacidic lipid cardiolipin and the purple bacterial reaction centre were examined. Mutagenesis of a highly conserved arginine (M267) that is responsible for binding the head-group of the cardiolipin (to leucine) did not affect the rate of photosynthetic growth, the functional properties of the reaction centre, or the X-ray crystal structure of the complex (determined to a resolution of 2.8 A). However, the thermal stability of the protein was compromised by this mutation, part of the reaction centre population showing an approximately 5 degrees C decrease in melting temperature in response to the arginine to leucine mutation. The crystallised mutant reaction centre also no longer bound detectable amounts of cardiolipin at this site. Taken together, these observations suggest that this particular protein-lipid interaction contributes to the thermal stability of the complex, at least when in detergent micelles. These findings are discussed in the light of proposals concerning the unfolding processes that occur when membrane proteins are heated, and we propose that one function of the cardiolipin is to stabilise the interaction between adjacent membrane-spanning alpha-helices in a region where there are no direct protein-protein interactions.
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Affiliation(s)
- Paul K Fyfe
- Department of Biochemistry, School of Medical Sciences, University of Bristol, University Walk, BS8 1TD Bristol, UK
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Heinz C, Engelhardt H, Niederweis M. The core of the tetrameric mycobacterial porin MspA is an extremely stable beta-sheet domain. J Biol Chem 2003; 278:8678-85. [PMID: 12501242 DOI: 10.1074/jbc.m212280200] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
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.
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Affiliation(s)
- Christian Heinz
- Lehrstuhl für Mikrobiologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Staudtstr. 5, Germany
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Faraldo-Gómez JD, Sansom MSP. Acquisition of siderophores in gram-negative bacteria. Nat Rev Mol Cell Biol 2003; 4:105-16. [PMID: 12563288 DOI: 10.1038/nrm1015] [Citation(s) in RCA: 251] [Impact Index Per Article: 12.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
The outer membrane of Gram-negative bacteria constitutes a permeability barrier that protects the cell from exterior hazards, but also complicates the uptake of nutrients. In the case of iron, the challenge is even greater, because of the scarcity of this indispensable element in the cell's surroundings. To solve this dilemma, bacteria have evolved sophisticated mechanisms whereby the concerted actions of receptor, transporter and energy-transducing proteins ensure that there is a sufficient supply of iron-containing compounds, such as siderophores.
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Affiliation(s)
- José D Faraldo-Gómez
- Department of Biochemistry, Weill Medical College of Cornell University, 1300 York Avenue, New York, NY 10021, USA.
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38
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Vakharia HL, Postle K. FepA with globular domain deletions lacks activity. J Bacteriol 2002; 184:5508-12. [PMID: 12218040 PMCID: PMC135345 DOI: 10.1128/jb.184.19.5508-5512.2002] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2002] [Accepted: 06/30/2002] [Indexed: 11/20/2022] Open
Abstract
TonB-gated transporters have beta-barrels containing an amino-terminal globular domain that occludes the interior of the barrel. Mutations in the globular domain prevent transport of ligands across the outer membrane. Surprisingly, FepA with deletions of the globular domain (amino acids 3 to 150 and 17 to 150) was previously reported to retain significant sensitivity to colicins B and D and to use ferric enterochelin, all in a TonB-dependent fashion. To further understand TonB interaction with the beta-barrel, in the present study, proteins with deletions of amino acids 1 to 152, 7 to 152, 20 to 152, and 17 to 150 in fepA were constructed and expressed in a deltafepA strain. In contrast to previous studies of fepA globular domain deletions, constructs in this study did not retain sensitivity to colicin B and conferred only marginal sensitivity to colicin D. Consistent with these observations, they failed to bind colicin B and detectably cross-link to TonB in vivo. To address this discrepancy, constructs were tested in other strains, one of which (RWB18-60) did support activity of the FepA globular domain deletion proteins constructed in this study. The characteristics of that strain, as well as the strain in which the deltaFhuA globular domain mutants were seen to be active, suggests the hypothesis that interprotein complementation by two individually nonfunctional proteins restores TonB-dependent activity.
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Affiliation(s)
- Hema L Vakharia
- School of Molecular Biosciences, Washington State University, Pullman, Washington 99164-4234, USA
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Plançon L, Janmot C, le Maire M, Desmadril M, Bonhivers M, Letellier L, Boulanger P. Characterization of a high-affinity complex between the bacterial outer membrane protein FhuA and the phage T5 protein pb5. J Mol Biol 2002; 318:557-69. [PMID: 12051859 DOI: 10.1016/s0022-2836(02)00089-x] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
Binding of bacteriophage T5 to Escherichia coli cells is mediated by specific interactions between the receptor-binding protein pb5 (67.8 kDa) and the outer membrane iron-transporter FhuA. A histidine-tagged form of pb5 was overproduced and purified. Isolated pb5 is monomeric and organized mostly as beta-sheets (51%). pb5 functionality was attested in vivo by its ability to impair infection of E. coli cells by phage T5 and Phi80, and to prevent growth of bacteria on iron-ferrichrome as unique iron source. pb5 was functional in vitro, since addition of an equimolar concentration of pb5 to purified FhuA prevented DNA release from phage T5. However, pb5 alone was not sufficient for the conversion of FhuA into an open channel. Direct interaction of pb5 with FhuA was demonstrated by isolating a pb5/FhuA complex using size-exclusion chromatography. The stoichiometry, 1 mol of pb5/1 mol of FhuA, was deduced from its molecular mass, established by analytical ultracentrifugation after determination of the amount of bound detergent. SDS-PAGE and differential scanning calorimetry experiments highlighted the great stability of the complex: (i) it was not dissociated by 2% SDS even when the temperature was raised to 70 degrees C; (ii) thermal denaturation of the complex occurred at 85 degrees C, while pb5 and FhuA were denatured at 45 degrees C and 74 degrees C, respectively. The stability of the complex renders it suitable for high-resolution structural studies, allowing future analysis of conformational changes into both FhuA and pb5 upon adsorption of the virus to its host.
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Affiliation(s)
- L Plançon
- Institut de Biochimie et Biophysique Moléculaire et Cellulaire, UMR CNRS 8619, Université Paris Sud, Bât 430, 91 405 Orsay Cedex, France
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40
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Rosenbusch JP. Stability of membrane proteins: relevance for the selection of appropriate methods for high-resolution structure determinations. J Struct Biol 2001; 136:144-57. [PMID: 11886216 DOI: 10.1006/jsbi.2001.4431] [Citation(s) in RCA: 68] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
High stability is a prominent characteristic of integral membrane proteins of known atomic structure. But rather than being an intrinsic property, it may be due to a selection exerted by biochemical procedures prior to structure determination, since solubilization results in the transient exposure of membrane proteins to solution conditions. This may cause structural perturbations that interfere with 3D crystallization and hence with X-ray analysis. This problem also affects the preparation of samples for electron crystallography and NMR studies and may account for the fact that high-resolution structures of representatives of whole groups, such as transport proteins and signal transducers, have not been elucidated so far by any method. A knowledge of the proportion of labile proteins among membrane proteins, and of the kinetics of their denaturation, is therefore necessary. Establishing stability profiles, developing methods to maintain lateral pressure, or preventing contact with water (or both) should prove significant in establishing the structures of conformationally flexible proteins.
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