1
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Herwig S, Kleinschmidt JH. The Formation of β-Strand Nine ( β9) in the Folding and Insertion of BamA from an Unfolded Form into Lipid Bilayers. MEMBRANES 2023; 13:247. [PMID: 36837750 PMCID: PMC9964827 DOI: 10.3390/membranes13020247] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 01/02/2023] [Revised: 02/10/2023] [Accepted: 02/14/2023] [Indexed: 06/18/2023]
Abstract
Transmembrane proteins span lipid bilayer membranes and serve essential functions in all living cells. Membrane-inserted domains are of either α-helical or β-barrel structure. Despite their biological importance, the biophysical mechanisms of the folding and insertion of proteins into membranes are not well understood. While the relative composition of the secondary structure has been examined by circular dichroism spectroscopy in folding studies for several outer membrane proteins, it is currently not known how individual β-strands fold. Here, the folding and insertion of the β-barrel assembly machinery protein A (BamA) from the outer membrane of Escherichia coli into lipid bilayers were investigated, and the formation of strand nine (β9) of BamA was examined. Eight single-cysteine mutants of BamA were overexpressed and isolated in unfolded form in 8 M urea. In each of these mutants, one of the residues of strand β9, from R572 to V579, was replaced by a cysteine and labeled with the fluorophore IAEDANS for site-directed fluorescence spectroscopy. Upon urea-dilution, the mutants folded into the native structure and were inserted into lipid bilayers of dilauroylphosphatidylcholine, similar to wild-type BamA. An aqueous and a membrane-adsorbed folding intermediate of BamA could be identified by strong shifts in the intensity maxima of the IAEDANS fluorescence of the labeled mutants of BamA towards shorter wavelengths, even in the absence of lipid bilayers. The shifts were greatest for membrane-adsorbed mutants and smaller for the inserted, folded mutants or the aqueous intermediates. The spectra of the mutants V573C-, L575C-, G577C-, and V579C-BamA, facing the lipid bilayer, displayed stronger shifts than the spectra recorded for the mutants R572C-, N574C-, T576C-, and K578C-BamA, facing the β-barrel lumen, in both the membrane-adsorbed form and the folded, inserted form. This alternating pattern was neither observed for the IAEDANS spectra of the unfolded forms nor for the water-collapsed forms, indicating that strand β9 forms in a membrane-adsorbed folding intermediate of BamA. The combination of cysteine scanning mutagenesis and site-directed fluorescence labeling is shown to be a valuable tool in examining the local secondary structure formation of transmembrane proteins.
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Affiliation(s)
- Sascha Herwig
- Institut für Biologie, FB 10 Mathematik und Naturwissenschaften, Universität Kassel, Heinrich-Plett-Str. 40, D-34132 Kassel, Germany
| | - Jörg H. Kleinschmidt
- Institut für Biologie, FB 10 Mathematik und Naturwissenschaften, Universität Kassel, Heinrich-Plett-Str. 40, D-34132 Kassel, Germany
- Center of Interdisciplinary Nanostructure Science and Technology, Universität Kassel, Heinrich-Plett-Str. 40, D-34132 Kassel, Germany
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2
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Hermansen S, Ryoo D, Orwick-Rydmark M, Saragliadis A, Gumbart JC, Linke D. The Role of Extracellular Loops in the Folding of Outer Membrane Protein X (OmpX) of Escherichia coli. Front Mol Biosci 2022; 9:918480. [PMID: 35911955 PMCID: PMC9329534 DOI: 10.3389/fmolb.2022.918480] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/12/2022] [Accepted: 06/16/2022] [Indexed: 11/25/2022] Open
Abstract
The outer membrane of Gram-negative bacteria acts as an additional diffusion barrier for solutes and nutrients. It is perforated by outer membrane proteins (OMPs) that function most often as diffusion pores, but sometimes also as parts of larger cellular transport complexes, structural components of the cell wall, or even as enzymes. These OMPs often have large loops that protrude into the extracellular environment, which have promise for biotechnological applications and as therapeutic targets. Thus, understanding how modifications to these loops affect OMP stability and folding is critical for their efficient application. In this work, the small outer membrane protein OmpX was used as a model system to quantify the effects of loop insertions on OMP folding and stability. The insertions were varied according to both hydrophobicity and size, and their effects were determined by assaying folding into detergent micelles in vitro by SDS-PAGE and in vivo by isolating the outer membrane of cells expressing the constructs. The different insertions were also examined in molecular dynamics simulations to resolve how they affect OmpX dynamics in its native outer membrane. The results indicate that folding of OMPs is affected by both the insert length and by its hydrophobic character. Small insertions sometimes even improved the folding efficiency of OmpX, while large hydrophilic inserts reduced it. All the constructs that were found to fold in vitro could also do so in their native environment. One construct that could not fold in vitro was transported to the OM in vivo, but remained unfolded. Our results will help to improve the design and efficiency of recombinant OMPs used for surface display.
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Affiliation(s)
- Simen Hermansen
- Section for Genetics and Evolutionary Biology, Department of Biosciences, University of Oslo, Oslo, Norway
| | - David Ryoo
- Interdisciplinary Bioengineering Graduate Program, Georgia Institute of Technology, Atlanta, GA, United States
| | - Marcella Orwick-Rydmark
- Section for Genetics and Evolutionary Biology, Department of Biosciences, University of Oslo, Oslo, Norway
| | - Athanasios Saragliadis
- Section for Genetics and Evolutionary Biology, Department of Biosciences, University of Oslo, Oslo, Norway
| | - James C. Gumbart
- School of Physics, Georgia Institute of Technology, Atlanta, GA, United States
| | - Dirk Linke
- Section for Genetics and Evolutionary Biology, Department of Biosciences, University of Oslo, Oslo, Norway
- *Correspondence: Dirk Linke,
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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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4
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Licari G, Dehghani-Ghahnaviyeh S, Tajkhorshid E. Membrane Mixer: A Toolkit for Efficient Shuffling of Lipids in Heterogeneous Biological Membranes. J Chem Inf Model 2022; 62:986-996. [PMID: 35104125 PMCID: PMC8892574 DOI: 10.1021/acs.jcim.1c01388] [Citation(s) in RCA: 15] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Molecular dynamics (MD) simulations of biological membranes have achieved such levels of sophistication that are commonly used to predict unresolved structures and various properties of lipids and to substantiate experimental data. While achieving sufficient sampling of lipid dynamics remains a major challenge, a commonly used method to improve lipid sampling, e.g., in terms of specific interactions with membrane-associated proteins, is to randomize the initial arrangement of lipid constituents in multiple replicas of simulations, without changing the overall lipid composition of the membrane of interest. Here, we introduce a method that can rapidly generate multiple replicas of lipid bilayers with different spatial and conformational configurations for any given lipid composition. The underlying algorithm, which allows one to shuffle lipids at any desired level, relies on the application of an external potential, here referred to as the "carving potential", that removes clashes/entanglements before lipid positions are exchanged (shuffled), thereby minimizing the energy penalty due to abrupt lipid repositioning. The method is implemented as "Membrane Mixer Plugin (MMP) 1.0" in VMD, with a convenient graphical user interface that guides the user in setting various options and parameters. The plugin is fully automated and generates new membrane replicas more rapidly and conveniently than other analogous tools. The plugin and its capabilities introduced here can be extended to include additional features in future versions.
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Affiliation(s)
- Giuseppe Licari
- Theoretical and Computational Biophysics Group, NIH Center for Macromolecular Modeling and Bioinformatics, Beckman Institute for Advanced Science and Technology, Department of Biochemistry, and Center for Biophysics and Quantitative Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States,Current address: Pharmaceutical Development Biologicals, Boehringer Ingelheim Pharmaceuticals, Inc., Biberach An Der Riß, Germany,Contributed equally to this work
| | - Sepehr Dehghani-Ghahnaviyeh
- Theoretical and Computational Biophysics Group, NIH Center for Macromolecular Modeling and Bioinformatics, Beckman Institute for Advanced Science and Technology, Department of Biochemistry, and Center for Biophysics and Quantitative Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States,Contributed equally to this work
| | - Emad Tajkhorshid
- Theoretical and Computational Biophysics Group, NIH Center for Macromolecular Modeling and Bioinformatics, Beckman Institute for Advanced Science and Technology, Department of Biochemistry, and Center for Biophysics and Quantitative Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
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5
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Tiwari PB, Mahalakshmi R. Interplay of protein primary sequence, lipid membrane, and chaperone in β-barrel assembly. Protein Sci 2021; 30:624-637. [PMID: 33410567 DOI: 10.1002/pro.4022] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/11/2020] [Revised: 12/25/2020] [Accepted: 12/30/2020] [Indexed: 02/06/2023]
Abstract
The outer membrane of a Gram-negative bacterium is a crucial barrier between the external environment and its internal physiology. This barrier is bridged selectively by β-barrel outer membrane proteins (OMPs). The in vivo folding and biogenesis of OMPs necessitates the assistance of the outer membrane chaperone BamA. Nevertheless, OMPs retain the ability of independent self-assembly in vitro. Hence, it is unclear whether substrate-chaperone dynamics is influenced by the intrinsic ability of OMPs to fold, the magnitude of BamA-OMP interdependence, and the contribution of BamA to the kinetics of OMP assembly. We addressed this by monitoring the assembly kinetics of multiple 8-stranded β-barrel OMP substrates with(out) BamA. We also examined whether BamA is species-specific, or nonspecifically accelerates folding kinetics of substrates from independent species. Our findings reveal BamA as a substrate-independent promiscuous molecular chaperone, which assists the unfolded OMP to overcome the kinetic barrier imposed by the bilayer membrane. We additionally show that while BamA kinetically accelerates OMP folding, the OMP primary sequence remains a vital deciding element in its assembly rate. Our study provides unexpected insights on OMP assembly and the functional relevance of BamA in vivo.
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Affiliation(s)
- Pankaj B Tiwari
- Molecular Biophysics Laboratory, Department of Biological Sciences, Indian Institute of Science Education and Research, Bhopal, India
| | - Radhakrishnan Mahalakshmi
- Molecular Biophysics Laboratory, Department of Biological Sciences, Indian Institute of Science Education and Research, Bhopal, India
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6
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Gerlach L, Gholami O, Schürmann N, Kleinschmidt JH. Folding of β-Barrel Membrane Proteins into Lipid Membranes by Site-Directed Fluorescence Spectroscopy. Methods Mol Biol 2020; 2003:465-492. [PMID: 31218630 DOI: 10.1007/978-1-4939-9512-7_20] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/09/2023]
Abstract
Protein-lipid interactions are important for folding and membrane insertion of integral membrane proteins that are composed either of α-helical or of β-barrel structure in their transmembrane domains. While α-helical transmembrane proteins fold co-translationally while they are synthesized by a ribosome, β-barrel transmembrane proteins (β-TMPs) fold and insert posttranslationally-in bacteria after translocation across the cytoplasmic membrane, in cell organelles of eukaryotes after import across the outer membrane of the organelle. β-TMPs can be unfolded in aqueous solutions of chaotropic denaturants like urea and spontaneously refold upon denaturant dilution in the presence of preformed lipid bilayers. This facilitates studies on lipid interactions during folding into lipid bilayers. For several β-TMPs, the kinetics of folding has been reported as strongly dependent on protein-lipid interactions. The kinetics of adsorption/insertion and folding of β-TMPs can be monitored by fluorescence spectroscopy. These fluorescence methods are even more powerful when combined with site-directed mutagenesis for the preparation of mutants of a β-TMP that are site-specifically labeled with a fluorophore or a fluorophore and fluorescence quencher or fluorescence resonance energy acceptor. Single tryptophan or single cysteine mutants of the β-TMP allow for the investigation of local protein-lipid interactions, at specific regions within the protein. To examine the structure formation of β-TMPs in a lipid environment, fluorescence spectroscopy has been used for double mutants of β-TMPs that contain a fluorescent tryptophan and a spin-label, covalently attached to a cysteine as a fluorescence quencher. The sites of mutation are selected so that the tryptophan is in close proximity to the quencher at the cysteine only when the β-TMP is folded. In a folding experiment, the evolution of fluorescence quenching as a function of time at specific sites within the protein can provide important information on the folding mechanism of the β-TMP. Here, we report protocols to examine membrane protein folding for two β-TMPs in a lipid environment, the outer membrane protein A from Escherichia coli, OmpA, and the voltage-dependent anion-selective channel, human isoform 1, hVDAC1, from mitochondria.
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Affiliation(s)
- Lisa Gerlach
- Department of Biophysics, Institute of Biology, FB 10 and Center for Interdisciplinary Nanostructure Science and Technology (CINSaT), University of Kassel, Kassel, Germany
| | - Omkolsum Gholami
- Department of Biophysics, Institute of Biology, FB 10 and Center for Interdisciplinary Nanostructure Science and Technology (CINSaT), University of Kassel, Kassel, Germany
| | - Nicole Schürmann
- Department of Biophysics, Institute of Biology, FB 10 and Center for Interdisciplinary Nanostructure Science and Technology (CINSaT), University of Kassel, Kassel, Germany
| | - Jörg H Kleinschmidt
- Department of Biophysics, Institute of Biology, FB 10 and Center for Interdisciplinary Nanostructure Science and Technology (CINSaT), University of Kassel, Kassel, Germany.
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7
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Asamoto DK, Kang G, Kim JE. Folding of the β-Barrel Membrane Protein OmpA into Nanodiscs. Biophys J 2019; 118:403-414. [PMID: 31843264 DOI: 10.1016/j.bpj.2019.11.3381] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/04/2019] [Revised: 11/08/2019] [Accepted: 11/20/2019] [Indexed: 01/19/2023] Open
Abstract
Nanodiscs (NDs) are an excellent alternative to small unilamellar vesicles (SUVs) for studies of membrane protein structure, but it has not yet been shown that membrane proteins are able to spontaneously fold and insert into a solution of freely diffusing NDs. In this article, we present SDS-PAGE differential mobility studies combined with fluorescence, circular dichroism, and ultraviolet resonance Raman spectroscopy to confirm the spontaneous folding of outer membrane protein A (OmpA) into preformed NDs. Folded OmpA in NDs was incubated with Arg-C protease, resulting in the digestion of OmpA to membrane-protected fragments with an apparent molecular mass of ∼26 kDa (major component) and ∼24 kDa (minor component). The OmpA folding yields were greater than 88% in both NDs and SUVs. An OmpA adsorbed intermediate on NDs could be isolated at low temperature and induced to fold via an increase in temperature, analogous to the temperature-jump experiments on SUVs. The circular dichroism spectra of OmpA in NDs and SUVs were similar and indicated β-barrel secondary structure. Further evidence of OmpA folding into NDs was provided by ultraviolet resonance Raman spectroscopy, which revealed the intense 785 cm-1 structural marker for folded OmpA in NDs. The primary difference between folding in NDs and SUVs was the kinetics; the rate of folding was two- to threefold slower in NDs compared to in SUVs, and this decreased rate can tentatively be attributed to the properties of NDs. These data indicate that NDs may be an excellent alternative to SUVs for folding experiments and offer benefits of optical clarity, sample homogeneity, control of ND:protein ratios, and greater stability.
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Affiliation(s)
- DeeAnn K Asamoto
- Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California
| | - Guipeun Kang
- Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California
| | - Judy E Kim
- Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California.
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8
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Naberezhnykh GA, Karpenko AA, Khomenko VA, Solov’eva TF, Novikova OD. The Formation of Ordered structures of Bacterial Porins in a Lipid Bilayer and the Analysis of their Morphology by Atomic Force Microscopy. Biophysics (Nagoya-shi) 2019. [DOI: 10.1134/s0006350919060162] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022] Open
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9
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Gupta A, Mahalakshmi R. Reversible folding energetics of Yersinia Ail barrel reveals a hyperfluorescent intermediate. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2019; 1862:183097. [PMID: 31672545 DOI: 10.1016/j.bbamem.2019.183097] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/09/2019] [Revised: 10/01/2019] [Accepted: 10/10/2019] [Indexed: 12/17/2022]
Abstract
Deducing the molecular details of membrane protein folding has lately become an important area of research in biology. Using Ail, an outer membrane protein (OMP) from Yersina pestis as our model, we explore details of β-barrel folding, stability, and unfolding. Ail displays a simple transmembrane β-barrel topology. Here, we find that Ail follows a simple two-state mechanism in its folding and unfolding thermodynamics. Interestingly, Ail displays multi-step folding kinetics. The early kinetic intermediates in the folding pathway populate near the unfolded state (βT ≈ 0.20), and do not display detectable changes in the local environment of the two interface indoles. Interestingly, tryptophans regulate the late events of barrel rearrangement, and Ail thermodynamic stability. We show that W149 → Y/F/A substitution destabilizes Ail by ~0.13-1.7 kcal mol-1, but retains path-independent thermodynamic equilibrium of Ail. In surprising contrast, substituting W42 and retaining W149 shifts the thermodynamic equilibrium to an apparent kinetic retardation of only the unfolding process, which gives rise to an associated increase in scaffold stability by ~0.3-1.1 kcal mol-1. This is accompanied by the formation of an unusual hyperfluorescent state in the unfolding pathway that is more structured, and represents a conformationally dynamic unfolding intermediate with the interface W149 now lipid solvated. The defined role of each tryptophan and poorer folding efficiency of Trp mutants together presents compelling evidence for the importance of interface aromatics in the unique (un)folding pathway of Ail, and offers interesting insight on alternative pathways in generalized OMP assembly and unfolding mechanisms.
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Affiliation(s)
- Ankit Gupta
- Molecular Biophysics Laboratory, Department of Biological Sciences, Indian Institute of Science Education and Research, Bhopal 462066. India
| | - Radhakrishnan Mahalakshmi
- Molecular Biophysics Laboratory, Department of Biological Sciences, Indian Institute of Science Education and Research, Bhopal 462066. India.
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10
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Lavi I, Goudarzi M, Raz E, Gov NS, Voituriez R, Sens P. Cellular Blebs and Membrane Invaginations Are Coupled through Membrane Tension Buffering. Biophys J 2019; 117:1485-1495. [PMID: 31445681 DOI: 10.1016/j.bpj.2019.08.002] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/21/2018] [Revised: 07/26/2019] [Accepted: 08/01/2019] [Indexed: 01/06/2023] Open
Abstract
Bleb-type cellular protrusions play key roles in a range of biological processes. It was recently found that bleb growth is facilitated by a local supply of membrane from tubular invaginations, but the interplay between the expanding bleb and the membrane tubes remains poorly understood. On the one hand, the membrane area stored in tubes may serve as a reservoir for bleb expansion. On the other hand, the sequestering of excess membrane in stabilized invaginations may effectively increase the cell membrane tension, which suppresses spontaneous protrusions. Here, we investigate this duality through physical modeling and in vivo experiments. In agreement with observations, our model describes the transition into a tube-flattening mode of bleb expansion while also predicting that the blebbing rate is impaired by elevating the concentration of the curved membrane proteins that form the tubes. We show both theoretically and experimentally that the stabilizing effect of tubes could be counterbalanced by the cortical myosin contractility. Our results largely suggest that proteins able to induce membrane tubulation, such as those containing N-BAR domains, can buffer the effective membrane tension-a master regulator of all cell deformations.
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Affiliation(s)
- Ido Lavi
- Laboratoire Jean Perrin, UMR 8237 CNRS, Sorbonne University, Paris, France.
| | - Mohammad Goudarzi
- Institute of Cell Biology, Center for Molecular Biology of Inflammation, University of Münster, Münster, Germany
| | - Erez Raz
- Institute of Cell Biology, Center for Molecular Biology of Inflammation, University of Münster, Münster, Germany
| | - Nir S Gov
- Department of Chemical Physics, Weizmann Institute of Science, Rehovot, Israel
| | - Raphael Voituriez
- Laboratoire Jean Perrin, UMR 8237 CNRS, Sorbonne University, Paris, France
| | - Pierre Sens
- Institut Curie, PSL Research University, CNRS, UMR 168, Paris, France
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11
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Pushing beyond the Envelope: the Potential Roles of OprF in Pseudomonas aeruginosa Biofilm Formation and Pathogenicity. J Bacteriol 2019; 201:JB.00050-19. [PMID: 31010902 DOI: 10.1128/jb.00050-19] [Citation(s) in RCA: 16] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
The ability of Pseudomonas aeruginosa to form biofilms, which are communities of cells encased in a self-produced extracellular matrix, protects the cells from antibiotics and the host immune response. While some biofilm matrix components, such as exopolysaccharides and extracellular DNA, are relatively well characterized, the extracellular matrix proteins remain understudied. Multiple proteomic analyses of the P. aeruginosa soluble biofilm matrix and outer membrane vesicles, which are a component of the matrix, have identified OprF as an abundant matrix protein. To date, the few reports on the effects of oprF mutations on biofilm formation are conflicting, and little is known about the potential role of OprF in the biofilm matrix. The majority of OprF studies focus on the protein as a cell-associated porin. As a component of the outer membrane, OprF assumes dual conformations and is involved in solute transport, as well as cell envelope integrity. Here, we review the current literature on OprF in P. aeruginosa, discussing how the structure and function of the cell-associated and matrix-associated protein may affect biofilm formation and pathogenesis in order to inform future research on this understudied matrix protein.
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12
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Puth S, Hong SH, Na HS, Lee HH, Lee YS, Kim SY, Tan W, Hwang HS, Sivasamy S, Jeong K, Kook JK, Ahn SJ, Kang IC, Ryu JH, Koh JT, Rhee JH, Lee SE. A built-in adjuvant-engineered mucosal vaccine against dysbiotic periodontal diseases. Mucosal Immunol 2019; 12:565-579. [PMID: 30487648 DOI: 10.1038/s41385-018-0104-6] [Citation(s) in RCA: 17] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/27/2018] [Revised: 09/27/2018] [Accepted: 10/15/2018] [Indexed: 02/04/2023]
Abstract
Periodontitis is associated with a dysbiotic shift in the oral microbiome. Vaccine approaches to prevent microbial shifts from healthy to diseased state in oral biofilms would provide a fundamental therapeutic strategy against periodontitis. Since dental plaque formation is a polymicrobial and multilayered process, vaccines targeting single bacterial species would have limited efficacy in clinical applications. In this study, we developed a divalent mucosal vaccine consisting of a mixture of FlaB-tFomA and Hgp44-FlaB fusion proteins targeting virulence factors of inflammophilic bacteria Fusobacterium nucleatum and Porphyromonas gingivalis, respectively. Introduction of peptide linkers between FlaB and antigen improved the stability and immunogenicity of engineered vaccine antigens. The intranasal immunization of divalent vaccine induced protective immune responses inhibiting alveolar bone loss elicited by F. nucleatum and P. gingivalis infection. The built-in flagellin adjuvant fused to protective antigens enhanced antigen-specific antibody responses and class switch recombination. The divalent vaccine antisera recognized natural forms of surface antigens and reacted with diverse clinical isolates of Fusobacterium subspecies and P. gingivalis. The antisera inhibited F. nucleatum-mediated biofilm formation, co-aggregation of P. gingivalis and Treponema denticola, and P. gingivalis-host cell interactions. Taken together, the built-in adjuvant-engineered mucosal vaccine provides a technological platform for multivalent periodontitis vaccines targeting dysbiotic microbiome.
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Affiliation(s)
- Sao Puth
- Clinical Vaccine R&D Center, Chonnam National University, Hwasun-gun, Jeonnam, 58128, Republic of Korea.,Department of Microbiology, Chonnam National University Medical School, Hwasun-gun, Jeonnam, 58128, Republic of Korea
| | - Seol Hee Hong
- Clinical Vaccine R&D Center, Chonnam National University, Hwasun-gun, Jeonnam, 58128, Republic of Korea.,Department of Pharmacology and Dental Therapeutics, School of Dentistry, Chonnam National University, Gwangju, 61186, Republic of Korea
| | - Hee Sam Na
- Department of Oral Microbiology, School of Dentistry, Pusan National University, Yangsan, 50612, Republic of Korea
| | - Hye Hwa Lee
- Clinical Vaccine R&D Center, Chonnam National University, Hwasun-gun, Jeonnam, 58128, Republic of Korea.,Department of Microbiology, Chonnam National University Medical School, Hwasun-gun, Jeonnam, 58128, Republic of Korea
| | - Youn Suhk Lee
- Clinical Vaccine R&D Center, Chonnam National University, Hwasun-gun, Jeonnam, 58128, Republic of Korea.,Department of Pharmacology and Dental Therapeutics, School of Dentistry, Chonnam National University, Gwangju, 61186, Republic of Korea
| | - Soo Young Kim
- Clinical Vaccine R&D Center, Chonnam National University, Hwasun-gun, Jeonnam, 58128, Republic of Korea.,Department of Microbiology, Chonnam National University Medical School, Hwasun-gun, Jeonnam, 58128, Republic of Korea
| | - Wenzhi Tan
- Clinical Vaccine R&D Center, Chonnam National University, Hwasun-gun, Jeonnam, 58128, Republic of Korea.,Department of Microbiology, Chonnam National University Medical School, Hwasun-gun, Jeonnam, 58128, Republic of Korea
| | - Hye Suk Hwang
- Clinical Vaccine R&D Center, Chonnam National University, Hwasun-gun, Jeonnam, 58128, Republic of Korea.,Department of Microbiology, Chonnam National University Medical School, Hwasun-gun, Jeonnam, 58128, Republic of Korea
| | - Sethupathy Sivasamy
- Clinical Vaccine R&D Center, Chonnam National University, Hwasun-gun, Jeonnam, 58128, Republic of Korea.,Department of Microbiology, Chonnam National University Medical School, Hwasun-gun, Jeonnam, 58128, Republic of Korea
| | - Kwangjoon Jeong
- Clinical Vaccine R&D Center, Chonnam National University, Hwasun-gun, Jeonnam, 58128, Republic of Korea.,Department of Microbiology, Chonnam National University Medical School, Hwasun-gun, Jeonnam, 58128, Republic of Korea
| | - Joong-Ki Kook
- Korean Collection for Oral Microbiology and Department of Oral Biochemistry, School of Dentistry, Chosun University, Gwangju, 61452, Republic of Korea
| | - Sug-Joon Ahn
- Dental Research Institute and Department of Orthodontics, School of Dentistry, Seoul National University, Seoul, 03080, Republic of Korea
| | - In-Chol Kang
- Department of Oral Microbiology, School of Dentistry, Chonnam National University, Gwangju, 61186, Republic of Korea
| | - Je-Hwang Ryu
- Department of Pharmacology and Dental Therapeutics, School of Dentistry, Chonnam National University, Gwangju, 61186, Republic of Korea
| | - Jeong Tae Koh
- Department of Pharmacology and Dental Therapeutics, School of Dentistry, Chonnam National University, Gwangju, 61186, Republic of Korea
| | - Joon Haeng Rhee
- Clinical Vaccine R&D Center, Chonnam National University, Hwasun-gun, Jeonnam, 58128, Republic of Korea. .,Department of Microbiology, Chonnam National University Medical School, Hwasun-gun, Jeonnam, 58128, Republic of Korea.
| | - Shee Eun Lee
- Clinical Vaccine R&D Center, Chonnam National University, Hwasun-gun, Jeonnam, 58128, Republic of Korea. .,Department of Pharmacology and Dental Therapeutics, School of Dentistry, Chonnam National University, Gwangju, 61186, Republic of Korea.
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13
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Schüßler A, Herwig S, Kleinschmidt JH. Kinetics of Insertion and Folding of Outer Membrane Proteins by Gel Electrophoresis. Methods Mol Biol 2019; 2003:145-162. [PMID: 31218617 DOI: 10.1007/978-1-4939-9512-7_7] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/03/2022]
Abstract
To examine the mechanisms of folding and insertion of TMPs into membranes, kinetic studies are instrumental, for example, for the analysis of folding steps and involved intermediates or for the determination of activation energies. For many β-barrel transmembrane proteins (β-TMPs) it has been shown that the folded, functional form can be separated from the unfolded form by a simple electrophoretic mobility assay. The only requirements for a separation by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) are that the folded form is sufficiently stable and that the samples are not heat-denatured before the electrophoresis is performed. Many folded β-TMPs resist the treatment with SDS at room temperature and are stable against forces during electrophoresis. On the other side, SDS also binds to unfolded forms of β-TMPs and prevents their folding into β-barrel structure. These observations have been used to develop a simple assay to monitor the kinetics of β-barrel tertiary structure formation in a membrane environment by electrophoresis. A folding reaction of a β-TMP is initiated by dilution of the denaturant in the presence of preformed lipid bilayers, proteoliposomes or membrane vesicles. At selected times, samples are taken from the reaction. In these samples, folding is stopped by addition of SDS. At the end of the entire folding reaction, all samples are analyzed by SDS-PAGE and the fractions of folded β-TMP that they contain are determined by densitometry.An advantage of this kinetic assay is that it not only allows a direct determination of fractions of folded and unfolded forms at a selected time during folding of the β-TMP into a membrane, but also facilitates the determination of the impact of folding factors (e.g., molecular chaperones) or folding machinery that most often have a different molecular mass and electrophoretic mobility. The assay has been very useful to examine how folding and insertion is affected by the structure of the phospholipids in the lipid bilayer and how folding machinery compensates for the presence of membrane lipids that retard folding and insertion of β-TMPs.
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Affiliation(s)
- Andre Schüßler
- Department of Biophysics, Institute of Biology, FB10 and Center for Interdisciplinary Nanostructure Science and Technology (CINSaT), University of Kassel, Kassel, Germany
| | - Sascha Herwig
- Department of Biophysics, Institute of Biology, FB10 and Center for Interdisciplinary Nanostructure Science and Technology (CINSaT), University of Kassel, Kassel, Germany
| | - Jörg H Kleinschmidt
- Department of Biophysics, Institute of Biology, FB10 and Center for Interdisciplinary Nanostructure Science and Technology (CINSaT), University of Kassel, Kassel, Germany.
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14
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Iyer BR, Vetal PV, Noordeen H, Zadafiya P, Mahalakshmi R. Salvaging the Thermodynamic Destabilization of Interface Histidine in Transmembrane β-Barrels. Biochemistry 2018; 57:6669-6678. [PMID: 30284812 PMCID: PMC6284319 DOI: 10.1021/acs.biochem.8b00805] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
The ability of histidine to participate in a wide range of stabilizing polar interactions preferentially populates this residue in functionally important sites of proteins. Histidine possesses an amphiphilic and electrostatic nature that is essential for amino acids residing at membrane interfaces. However, the frequency of occurrence of histidine at membrane interfaces, particularly transmembrane β-barrels, is lower than those of other aromatic residues. Here, we carry out comprehensive energetic measurements using equilibrium folding of the outer membrane enzyme PagP to address the contribution of a C-terminal interface histidine to barrel stability. We show that placing histidine at the C-terminus universally destabilizes PagP by 4.0-8.0 kcal mol-1 irrespective of the neighboring residue. Spectroscopic and electrophoretic measurements indicate that the altered stability may arise from a loss of barrel compaction. Isoleucine, methionine, and valine salvage this destabilization marginally (in addition to tyrosine, which shows an exceptionally high folding free energy value), when placed at the penultimate position, at the expense of an altered folding pathway. Double-mutant cycle analysis indicates that the coupling energy between the terminal and penultimate residues in PagP-X160H161 increases when the level of intrinsic destabilization by the terminal H161 is high. Our observations that neighboring residues cannot salvage the energetic destabilization of histidine may explain why histidine is less abundant at membrane interfaces.
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Affiliation(s)
- Bharat Ramasubramanian Iyer
- Molecular Biophysics Laboratory, Department of Biological Sciences , Indian Institute of Science Education and Research , Bhopal 462066 , India
| | - Pallavi Vijay Vetal
- Molecular Biophysics Laboratory, Department of Biological Sciences , Indian Institute of Science Education and Research , Bhopal 462066 , India
| | - Henna Noordeen
- Molecular Biophysics Laboratory, Department of Biological Sciences , Indian Institute of Science Education and Research , Bhopal 462066 , India
| | - Punit Zadafiya
- Molecular Biophysics Laboratory, Department of Biological Sciences , Indian Institute of Science Education and Research , Bhopal 462066 , India
| | - Radhakrishnan Mahalakshmi
- Molecular Biophysics Laboratory, Department of Biological Sciences , Indian Institute of Science Education and Research , Bhopal 462066 , India
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15
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Schiffrin B, Brockwell DJ, Radford SE. Outer membrane protein folding from an energy landscape perspective. BMC Biol 2017; 15:123. [PMID: 29268734 PMCID: PMC5740924 DOI: 10.1186/s12915-017-0464-5] [Citation(s) in RCA: 50] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023] Open
Abstract
The cell envelope is essential for the survival of Gram-negative bacteria. This specialised membrane is densely packed with outer membrane proteins (OMPs), which perform a variety of functions. How OMPs fold into this crowded environment remains an open question. Here, we review current knowledge about OMP folding mechanisms in vitro and discuss how the need to fold to a stable native state has shaped their folding energy landscapes. We also highlight the role of chaperones and the β-barrel assembly machinery (BAM) in assisting OMP folding in vivo and discuss proposed mechanisms by which this fascinating machinery may catalyse OMP folding.
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Affiliation(s)
- Bob Schiffrin
- Astbury Centre for Structural Molecular Biology, School of Molecular and Cellular Biology, Faculty of Biological Sciences, University of Leeds, Leeds, LS2 9JT, UK
| | - David J Brockwell
- Astbury Centre for Structural Molecular Biology, School of Molecular and Cellular Biology, Faculty of Biological Sciences, University of Leeds, Leeds, LS2 9JT, UK.
| | - Sheena E Radford
- Astbury Centre for Structural Molecular Biology, School of Molecular and Cellular Biology, Faculty of Biological Sciences, University of Leeds, Leeds, LS2 9JT, UK.
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16
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Goudarzi M, Tarbashevich K, Mildner K, Begemann I, Garcia J, Paksa A, Reichman-Fried M, Mahabaleshwar H, Blaser H, Hartwig J, Zeuschner D, Galic M, Bagnat M, Betz T, Raz E. Bleb Expansion in Migrating Cells Depends on Supply of Membrane from Cell Surface Invaginations. Dev Cell 2017; 43:577-587.e5. [PMID: 29173819 PMCID: PMC5939956 DOI: 10.1016/j.devcel.2017.10.030] [Citation(s) in RCA: 25] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/30/2017] [Revised: 09/27/2017] [Accepted: 10/26/2017] [Indexed: 01/14/2023]
Abstract
Cell migration is essential for morphogenesis, organ formation, and homeostasis, with relevance for clinical conditions. The migration of primordial germ cells (PGCs) is a useful model for studying this process in the context of the developing embryo. Zebrafish PGC migration depends on the formation of cellular protrusions in form of blebs, a type of protrusion found in various cell types. Here we report on the mechanisms allowing the inflation of the membrane during bleb formation. We show that the rapid expansion of the protrusion depends on membrane invaginations that are localized preferentially at the cell front. The formation of these invaginations requires the function of Cdc42, and their unfolding allows bleb inflation and dynamic cell-shape changes performed by migrating cells. Inhibiting the formation and release of the invaginations strongly interfered with bleb formation, cell motility, and the ability of the cells to reach their target.
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Affiliation(s)
- Mohammad Goudarzi
- Institute for Cell Biology, ZMBE, Von-Esmarch-Strasse 56, 48149 Münster, Germany
| | | | - Karina Mildner
- Electron Microscopy Unit, Max Planck Institute for Molecular Biomedicine, Röntgenstrasse 20, 48149 Münster, Germany
| | - Isabell Begemann
- Workgroup Nanoforces in Cells, Institute of Medical Physics und Biophysics, DFG Cluster of Excellence 'Cells in Motion' (EXC 1003), Robert-Koch-Strasse 31, 48149 Münster, Germany
| | - Jamie Garcia
- Department of Cell Biology, Duke University, 333B Nanaline Duke Building, Box 3709, Durham, NC 27710, USA
| | - Azadeh Paksa
- Institute for Cell Biology, ZMBE, Von-Esmarch-Strasse 56, 48149 Münster, Germany
| | | | - Harsha Mahabaleshwar
- Institute for Cell Biology, ZMBE, Von-Esmarch-Strasse 56, 48149 Münster, Germany
| | - Heiko Blaser
- Germ Cell Development, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37070 Göttingen, Germany
| | - Johannes Hartwig
- Institute for Cell Biology, ZMBE, Von-Esmarch-Strasse 56, 48149 Münster, Germany
| | - Dagmar Zeuschner
- Electron Microscopy Unit, Max Planck Institute for Molecular Biomedicine, Röntgenstrasse 20, 48149 Münster, Germany
| | - Milos Galic
- Workgroup Nanoforces in Cells, Institute of Medical Physics und Biophysics, DFG Cluster of Excellence 'Cells in Motion' (EXC 1003), Robert-Koch-Strasse 31, 48149 Münster, Germany
| | - Michel Bagnat
- Department of Cell Biology, Duke University, 333B Nanaline Duke Building, Box 3709, Durham, NC 27710, USA
| | - Timo Betz
- Institute for Cell Biology, ZMBE, Von-Esmarch-Strasse 56, 48149 Münster, Germany
| | - Erez Raz
- Institute for Cell Biology, ZMBE, Von-Esmarch-Strasse 56, 48149 Münster, Germany.
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17
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Henke K, Welte W, Hauser K. Direct Monitoring of β-Sheet Formation in the Outer Membrane Protein TtoA Assisted by TtOmp85. Biochemistry 2016; 55:4333-43. [PMID: 27400268 DOI: 10.1021/acs.biochem.6b00691] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy was applied to investigate the folding of an outer membrane protein, TtoA, assisted by TtOmp85, both from the thermophilic eubacterium Thermus thermophilus. To directly monitor the formation of β-sheet structure in TtoA and to analyze the function of TtOmp85, we immobilized unfolded TtoA on an ATR crystal. Interaction with TtOmp85 initiated TtoA folding as shown by time-dependent spectra recorded during the folding process. Our ATR-FTIR experiments prove that TtOmp85 possesses specific functionality to assist β-sheet formation of TtoA. We demonstrate the potential of this spectroscopic approach to study the interaction of outer membrane proteins in vitro and in a time-resolved manner.
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Affiliation(s)
- Katharina Henke
- Department of Chemistry, ‡Department of Biology, and §Konstanz Research School Chemical Biology, University of Konstanz , 78457 Konstanz, Germany
| | - Wolfram Welte
- Department of Chemistry, ‡Department of Biology, and §Konstanz Research School Chemical Biology, University of Konstanz , 78457 Konstanz, Germany
| | - Karin Hauser
- Department of Chemistry, ‡Department of Biology, and §Konstanz Research School Chemical Biology, University of Konstanz , 78457 Konstanz, Germany
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18
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Abstract
The assembly of β-barrel proteins into membranes is mediated by an evolutionarily conserved machine. This process is poorly understood because no stable partially folded barrel substrates have been characterized. Here, we slowed the folding of the Escherichia coli β-barrel protein, LptD, with its lipoprotein plug, LptE. We identified a late-stage intermediate in which LptD is folded around LptE, and both components interact with the two essential β-barrel assembly machine (Bam) components, BamA and BamD. We propose a model in which BamA and BamD act in concert to catalyze folding, with the final step in the process involving closure of the ends of the barrel with release from the Bam components. Because BamD and LptE are both soluble proteins, the simplest model consistent with these findings is that barrel folding by the Bam complex begins in the periplasm at the membrane interface.
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19
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Classifying β-Barrel Assembly Substrates by Manipulating Essential Bam Complex Members. J Bacteriol 2016; 198:1984-92. [PMID: 27161117 DOI: 10.1128/jb.00263-16] [Citation(s) in RCA: 35] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/05/2016] [Accepted: 04/29/2016] [Indexed: 11/20/2022] Open
Abstract
UNLABELLED The biogenesis of the outer membrane (OM) of Escherichia coli is a conserved and vital process. The assembly of integral β-barrel outer membrane proteins (OMPs), which represent a major component of the OM, depends on periplasmic chaperones and the heteropentameric β-barrel assembly machine (Bam complex) in the OM. However, not all OMPs are affected by null mutations in the same chaperones or nonessential Bam complex members, suggesting there are categories of substrates with divergent requirements for efficient assembly. We have previously demonstrated two classes of substrates, one comprising large, low-abundance, and difficult-to-assemble substrates that are heavily dependent on SurA and also Skp and FkpA, and the other comprising relatively simple and abundant substrates that are not as dependent on SurA but are strongly dependent on BamB for assembly. Here, we describe novel mutations in bamD that lower levels of BamD 10-fold and >25-fold without altering the sequence of the mature protein. We utilized these mutations, as well as a previously characterized mutation that lowers wild-type BamA levels, to reveal a third class of substrates. These mutations preferentially cause a marked decrease in the levels of multimeric proteins. This susceptibility of multimers to lowered quantities of Bam machines in the cell may indicate that multiple Bam complexes are needed to efficiently assemble multimeric proteins into the OM. IMPORTANCE The outer membrane (OM) of Gram-negative bacteria, such as Escherichia coli, serves as a selective permeability barrier that prevents the uptake of toxic molecules and antibiotics. Integral β-barrel proteins (OMPs) are assembled by the β-barrel assembly machine (Bam), components of which are conserved in mitochondria, chloroplasts, and all Gram-negative bacteria, including many clinically relevant pathogenic species. Bam is essential for OM biogenesis and accommodates a diverse array of client proteins; however, a mechanistic model that accounts for the selectivity and broad substrate range of Bam is lacking. Here, we show that the assembly of multimeric OMPs is more strongly affected than that of monomeric OMPs when essential Bam complex components are limiting, suggesting that multiple Bam complexes are needed to assemble multimeric proteins.
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20
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Assemblies of pore-forming toxins visualized by atomic force microscopy. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2015; 1858:500-11. [PMID: 26577274 DOI: 10.1016/j.bbamem.2015.11.005] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/25/2015] [Revised: 10/23/2015] [Accepted: 11/09/2015] [Indexed: 02/05/2023]
Abstract
A number of pore-forming toxins (PFTs) can assemble on lipid membranes through their specific interactions with lipids. The oligomeric assemblies of some PFTs have been successfully revealed either by electron microscopy (EM) and/or atomic force microscopy (AFM). Unlike EM, AFM imaging can be performed under physiological conditions, enabling the real-time visualization of PFT assembly and the transition from the prepore state, in which the toxin does not span the membrane, to the pore state. In addition to characterizing PFT oligomers, AFM has also been used to examine toxin-induced alterations in membrane organization. In this review, we summarize the contributions of AFM to the understanding of both PFT assembly and PFT-induced membrane reorganization. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.
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21
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Abstract
The major class of integral proteins found in the outer membrane (OM) of E. coli and Salmonella adopt a β-barrel conformation (OMPs). OMPs are synthesized in the cytoplasm with a typical signal sequence at the amino terminus, which directs them to the secretion machinery (SecYEG) located in the inner membrane for translocation to the periplasm. Chaperones such as SurA, or DegP and Skp, escort these proteins across the aqueous periplasm protecting them from aggregation. The chaperones then deliver OMPs to a highly conserved outer membrane assembly site termed the Bam complex. In E. coli, the Bam complex is composed of an essential OMP, BamA, and four associated OM lipoproteins, BamBCDE, one of which, BamD, is also essential. Here we provide an overview of what we know about the process of OMP assembly and outline the various hypotheses that have been proposed to explain how proteins might be integrated into the asymmetric OM lipid bilayer in an environment that lacks obvious energy sources. In addition, we describe the envelope stress responses that ensure the fidelity of OM biogenesis and how factors, such as phage and certain toxins, have coopted this essential machine to gain entry into the cell.
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22
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Krishnan S, Chang AC, Hodges J, Couraud PO, Romero IA, Weksler B, Nicholson BA, Nolan LK, Prasadarao NV. Serotype O18 avian pathogenic and neonatal meningitis Escherichia coli strains employ similar pathogenic strategies for the onset of meningitis. Virulence 2015; 6:777-86. [PMID: 26407066 DOI: 10.1080/21505594.2015.1091914] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022] Open
Abstract
Neonatal meningitis Escherichia coli K1 (NMEC) are thought to be transmitted from mothers to newborns during delivery or by nosocomial infections. However, the source of E. coli K1 causing these infections is not clear. Avian pathogenic E. coli (APEC) have the potential to cause infection in humans while human E. coli have potential to cause colibacillosis in poultry, suggesting that these strains may lack host specificity. APEC strains are capable of causing meningitis in newborn rats; however, it is unclear whether these bacteria use similar mechanisms to that of NMEC to establish disease. Using four representative APEC and NMEC strains that belong to serotype O18, we demonstrate that these strains survive in human serum similar to that of the prototypic NMEC strain E44, a derivative of RS218. These bacteria also bind and enter both macrophages and human cerebral microvascular endothelial cells (HCMEC/D3) with similar frequency as that of E44. The amino acid sequences of the outer membrane protein A (OmpA), an important virulence factor in the pathogenesis of meningitis, are identical within these representative APEC and NMEC strains. Further, these strains also require FcγRI-α chain (CD64) and Ecgp96 as receptors for OmpA in macrophages and HCMEC/D3, respectively, to bind and enter these cells. APEC and NMEC strains induce meningitis in newborn mice with varying degree of pathology in the brains as assessed by neutrophil recruitment and neuronal apoptosis. Together, these results suggest that serotype O18 APEC strains utilize similar pathogenic mechanisms as those of NMEC strains in causing meningitis.
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Affiliation(s)
- Subramanian Krishnan
- a Division of Infectious Diseases and Department of Pediatrics; Children's Hospital Los Angeles , CA USA
| | - Alexander C Chang
- a Division of Infectious Diseases and Department of Pediatrics; Children's Hospital Los Angeles , CA USA
| | - Jacqueline Hodges
- a Division of Infectious Diseases and Department of Pediatrics; Children's Hospital Los Angeles , CA USA
| | - Pierre-Olivier Couraud
- b Inserm; Institut Cochin, Paris, France; Université Paris Descartes; Sorbonne Paris Cité , Paris , France
| | - Ignacio A Romero
- c Department of Life ; Health and Chemical Sciences; Open University ; Milton Keynes , UK
| | - Babette Weksler
- d Division of Hematology and Medical Oncology; Weill Cornell Medical College ; New York , NY USA
| | - Bryon A Nicholson
- e Department of Veterinary Microbiology and Preventive Medicine ; College of Veterinary Medicine; Iowa State University ; Ames , IA USA
| | - Lisa K Nolan
- e Department of Veterinary Microbiology and Preventive Medicine ; College of Veterinary Medicine; Iowa State University ; Ames , IA USA
| | - Nemani V Prasadarao
- a Division of Infectious Diseases and Department of Pediatrics; Children's Hospital Los Angeles , CA USA.,f Department of Surgery ; Children's Hospital Los Angeles; University of Southern California ; Los Angeles , CA USA
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23
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Kleinschmidt JH. Folding of β-barrel membrane proteins in lipid bilayers - Unassisted and assisted folding and insertion. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2015; 1848:1927-43. [PMID: 25983306 DOI: 10.1016/j.bbamem.2015.05.004] [Citation(s) in RCA: 65] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/22/2015] [Revised: 05/06/2015] [Accepted: 05/07/2015] [Indexed: 01/08/2023]
Abstract
In cells, β-barrel membrane proteins are transported in unfolded form to an outer membrane into which they fold and insert. Model systems have been established to investigate the mechanisms of insertion and folding of these versatile proteins into detergent micelles, lipid bilayers and even synthetic amphipathic polymers. In these experiments, insertion into lipid membranes is initiated from unfolded forms that do not display residual β-sheet secondary structure. These studies therefore have allowed the investigation of membrane protein folding and insertion in great detail. Folding of β-barrel membrane proteins into lipid bilayers has been monitored from unfolded forms by dilution of chaotropic denaturants that keep the protein unfolded as well as from unfolded forms present in complexes with molecular chaperones from cells. This review is aimed to provide an overview of the principles and mechanisms observed for the folding of β-barrel transmembrane proteins into lipid bilayers, the importance of lipid-protein interactions and the function of molecular chaperones and folding assistants. This article is part of a Special Issue entitled: Lipid-protein interactions.
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Affiliation(s)
- Jörg H Kleinschmidt
- Abteilung Biophysik, Institut für Biologie, FB 10, Universität Kassel and Center for Interdisciplinary Nanostructure Science and Technology (CINSaT), Heinrich-Plett-Str. 40, D-34132 Kassel, Germany.
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24
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Chaturvedi D, Mahalakshmi R. Juxtamembrane tryptophans have distinct roles in defining the OmpX barrel-micelle boundary and facilitating protein-micelle association. FEBS Lett 2015; 588:4464-71. [PMID: 25448987 DOI: 10.1016/j.febslet.2014.10.017] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/09/2014] [Revised: 10/15/2014] [Accepted: 10/16/2014] [Indexed: 10/24/2022]
Abstract
Defining the span of the transmembrane region, a key requirement to ensure correct folding, stability and function of bacterial outer membrane β-barrels, is assisted by the amphipathic property of tryptophan. We demonstrate the unique and distinctive properties of the interface Trp76 and Trp140 of outer membrane protein X, and map their positional relevance to the refolding process, barrel formation and the resulting stability in dodecylphosphocholine micelles. The solvent-exposed Trp76 displays a rigid interfacial localization, whereas Trp140 is relatively micelle-solvated and contributes to barrel folding and global OmpX stability. Kinetic contribution to OmpX stability is influenced by the two tryptophans. Differential associations of the indoles with the detergent milieu therefore contribute to micelle-assisted β-barrel folding and concomitant OmpX stability.
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Affiliation(s)
- Deepti Chaturvedi
- Molecular Biophysics Laboratory, Department of Biological Sciences, Indian Institute of Science Education and Research, Bhopal 462023, India
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25
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McMorran LM, Brockwell DJ, Radford SE. Mechanistic studies of the biogenesis and folding of outer membrane proteins in vitro and in vivo: what have we learned to date? Arch Biochem Biophys 2014; 564:265-80. [PMID: 24613287 PMCID: PMC4262575 DOI: 10.1016/j.abb.2014.02.011] [Citation(s) in RCA: 43] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/10/2014] [Revised: 02/16/2014] [Accepted: 02/20/2014] [Indexed: 11/17/2022]
Abstract
Research into the mechanisms by which proteins fold into their native structures has been on-going since the work of Anfinsen in the 1960s. Since that time, the folding mechanisms of small, water-soluble proteins have been well characterised. By contrast, progress in understanding the biogenesis and folding mechanisms of integral membrane proteins has lagged significantly because of the need to create a membrane mimetic environment for folding studies in vitro and the difficulties in finding suitable conditions in which reversible folding can be achieved. Improved knowledge of the factors that promote membrane protein folding and disfavour aggregation now allows studies of folding into lipid bilayers in vitro to be performed. Consequently, mechanistic details and structural information about membrane protein folding are now emerging at an ever increasing pace. Using the panoply of methods developed for studies of the folding of water-soluble proteins. This review summarises current knowledge of the mechanisms of outer membrane protein biogenesis and folding into lipid bilayers in vivo and in vitro and discusses the experimental techniques utilised to gain this information. The emerging knowledge is beginning to allow comparisons to be made between the folding of membrane proteins with current understanding of the mechanisms of folding of water-soluble proteins.
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Affiliation(s)
- Lindsay M McMorran
- Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds LS2 9JT, UK; School 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; School of Molecular and Cellular Biology, University of Leeds, Leeds LS2 9JT, UK
| | - Sheena E Radford
- Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds LS2 9JT, UK; School of Molecular and Cellular Biology, University of Leeds, Leeds LS2 9JT, UK.
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26
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Folding and stability of integral membrane proteins in amphipols. Arch Biochem Biophys 2014; 564:327-43. [PMID: 25449655 DOI: 10.1016/j.abb.2014.10.013] [Citation(s) in RCA: 44] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/30/2014] [Revised: 10/11/2014] [Accepted: 10/22/2014] [Indexed: 11/23/2022]
Abstract
Amphipols (APols) are a family of amphipathic polymers designed to keep transmembrane proteins (TMPs) soluble in aqueous solutions in the absence of detergent. APols have proven remarkably efficient at (i) stabilizing TMPs, as compared to detergent solutions, and (ii) folding them from a denatured state to a native, functional one. The underlying physical-chemical mechanisms are discussed.
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27
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Höhr AIC, Straub SP, Warscheid B, Becker T, Wiedemann N. Assembly of β-barrel proteins in the mitochondrial outer membrane. BIOCHIMICA ET BIOPHYSICA ACTA-MOLECULAR CELL RESEARCH 2014; 1853:74-88. [PMID: 25305573 DOI: 10.1016/j.bbamcr.2014.10.006] [Citation(s) in RCA: 54] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/21/2014] [Revised: 09/25/2014] [Accepted: 10/01/2014] [Indexed: 12/15/2022]
Abstract
Mitochondria evolved through endosymbiosis of a Gram-negative progenitor with a host cell to generate eukaryotes. Therefore, the outer membrane of mitochondria and Gram-negative bacteria contain pore proteins with β-barrel topology. After synthesis in the cytosol, β-barrel precursor proteins are first transported into the mitochondrial intermembrane space. Folding and membrane integration of β-barrel proteins depend on the mitochondrial sorting and assembly machinery (SAM) located in the outer membrane, which is related to the β-barrel assembly machinery (BAM) in bacteria. The SAM complex recognizes β-barrel proteins by a β-signal in the C-terminal β-strand that is required to initiate β-barrel protein insertion into the outer membrane. In addition, the SAM complex is crucial to form membrane contacts with the inner mitochondrial membrane by interacting with the mitochondrial contact site and cristae organizing system (MICOS) and shares a subunit with the endoplasmic reticulum-mitochondria encounter structure (ERMES) that links the outer mitochondrial membrane to the endoplasmic reticulum (ER).
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Affiliation(s)
- Alexandra I C Höhr
- Institut für Biochemie und Molekularbiologie, ZBMZ, Universität Freiburg, 79104 Freiburg, Germany; Fakultät für Biologie, Universität Freiburg, 79104 Freiburg, Germany
| | - Sebastian P Straub
- Institut für Biochemie und Molekularbiologie, ZBMZ, Universität Freiburg, 79104 Freiburg, Germany; Fakultät für Biologie, Universität Freiburg, 79104 Freiburg, Germany
| | - Bettina Warscheid
- BIOSS Centre for Biological Signalling Studies, Universität Freiburg, 79104 Freiburg, Germany; Abteilung Biochemie und Funktionelle Proteomik, Institut für Biologie II, Fakultät für Biologie, Universität Freiburg, 79104 Freiburg, Germany
| | - Thomas Becker
- Institut für Biochemie und Molekularbiologie, ZBMZ, Universität Freiburg, 79104 Freiburg, Germany; BIOSS Centre for Biological Signalling Studies, Universität Freiburg, 79104 Freiburg, Germany
| | - Nils Wiedemann
- Institut für Biochemie und Molekularbiologie, ZBMZ, Universität Freiburg, 79104 Freiburg, Germany; BIOSS Centre for Biological Signalling Studies, Universität Freiburg, 79104 Freiburg, Germany.
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28
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Correia S, Nunes-Miranda JD, Pinto L, Santos HM, de Toro M, Sáenz Y, Torres C, Capelo JL, Poeta P, Igrejas G. Complete proteome of a quinolone-resistant Salmonella Typhimurium phage type DT104B clinical strain. Int J Mol Sci 2014; 15:14191-219. [PMID: 25196519 PMCID: PMC4159846 DOI: 10.3390/ijms150814191] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/08/2014] [Revised: 06/27/2014] [Accepted: 07/25/2014] [Indexed: 11/26/2022] Open
Abstract
Salmonellosis is one of the most common and widely distributed foodborne diseases. The emergence of Salmonella strains that are resistant to a variety of antimicrobials is a serious global public health concern. Salmonella enterica serovar Typhimurium definitive phage type 104 (DT104) is one of these emerging epidemic multidrug resistant strains. Here we collate information from the diverse and comprehensive range of experiments on Salmonella proteomes that have been published. We then present a new study of the proteome of the quinolone-resistant Se20 strain (phage type DT104B), recovered after ciprofloxacin treatment and compared it to the proteome of reference strain SL1344. A total of 186 and 219 protein spots were recovered from Se20 and SL1344 protein extracts, respectively, after two-dimensional gel electrophoresis. The signatures of 94% of the protein spots were successfully identified through matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF MS). Three antimicrobial resistance related proteins, whose genes were previously detected by polymerase chain reaction (PCR), were identified in the clinical strain. The presence of these proteins, dihydropteroate synthase type-2 (sul2 gene), aminoglycoside resistance protein A (strA gene) and aminoglycoside 6'-N-acetyltransferase type Ib-cr4 (aac(6')-Ib-cr4 gene), was confirmed in the DT104B clinical strain. The aac(6')-Ib-cr4 gene is responsible for plasmid-mediated aminoglycoside and quinolone resistance. This is a preliminary analysis of the proteome of these two S. Typhimurium strains and further work is being developed to better understand how antimicrobial resistance is developing in this pathogen.
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Affiliation(s)
- Susana Correia
- Institute for Biotechnology and Bioengineering, Centre of Genomics and Biotechnology, University of Trás-os-Montes and Alto Douro, 5001-801 Vila Real, Portugal.
| | - Júlio D Nunes-Miranda
- Institute for Biotechnology and Bioengineering, Centre of Genomics and Biotechnology, University of Trás-os-Montes and Alto Douro, 5001-801 Vila Real, Portugal.
| | - Luís Pinto
- Institute for Biotechnology and Bioengineering, Centre of Genomics and Biotechnology, University of Trás-os-Montes and Alto Douro, 5001-801 Vila Real, Portugal.
| | - Hugo M Santos
- BIOSCOPE group, REQUIMTE-CQFB, Chemistry Department, Faculty of Science and Technology, University NOVA of Lisbon, 2829-516 Monte de Caparica, Portugal.
| | - María de Toro
- Departamento de Biología Molecular (Universidad de Cantabria) and Instituto de Biomedicina y Biotecnología de Cantabria IBBTEC (UC-SODERCAN-CSIC), Santander 39011, Spain.
| | - Yolanda Sáenz
- Microbiología Molecular, Centro de Investigación Biomédica de La Rioja, C/Piqueras 98, 26006 Logroño, La Rioja, Spain.
| | - Carmen Torres
- Microbiología Molecular, Centro de Investigación Biomédica de La Rioja, C/Piqueras 98, 26006 Logroño, La Rioja, Spain.
| | - José Luis Capelo
- BIOSCOPE group, REQUIMTE-CQFB, Chemistry Department, Faculty of Science and Technology, University NOVA of Lisbon, 2829-516 Monte de Caparica, Portugal.
| | - Patrícia Poeta
- Centre of Studies of Animal and Veterinary Sciences, University of Trás-os-Montes and Alto Douro, 5001-801 Vila Real, Portugal.
| | - Gilberto Igrejas
- Institute for Biotechnology and Bioengineering, Centre of Genomics and Biotechnology, University of Trás-os-Montes and Alto Douro, 5001-801 Vila Real, Portugal.
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29
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Selkrig J, Leyton DL, Webb CT, Lithgow T. Assembly of β-barrel proteins into bacterial outer membranes. BIOCHIMICA ET BIOPHYSICA ACTA-MOLECULAR CELL RESEARCH 2014; 1843:1542-50. [DOI: 10.1016/j.bbamcr.2013.10.009] [Citation(s) in RCA: 62] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/26/2013] [Revised: 10/05/2013] [Accepted: 10/08/2013] [Indexed: 12/30/2022]
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30
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Renaturing membrane proteins in the lipid cubic phase, a nanoporous membrane mimetic. Sci Rep 2014; 4:5806. [PMID: 25055873 PMCID: PMC4108929 DOI: 10.1038/srep05806] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/21/2014] [Accepted: 06/23/2014] [Indexed: 12/31/2022] Open
Abstract
Membrane proteins play vital roles in the life of the cell and are important therapeutic targets. Producing them in large quantities, pure and fully functional is a major challenge. Many promising projects end when intractable aggregates or precipitates form. Here we show how such unfolded aggregates can be solubilized and the solution mixed with lipid to spontaneously self-assemble a bicontinuous cubic mesophase into the bilayer of which the protein, in a confined, chaperonin-like environment, reconstitutes with 100% efficiency. The test protein, diacylglycerol kinase, reconstituted in the bilayer of the mesophase, was then crystallized in situ by the in meso or lipid cubic phase method providing an X-ray structure to a resolution of 2.55 Å. This highly efficient, inexpensive, simple and rapid approach should find application wherever properly folded, membrane reconstituted and functional proteins are required where the starting material is a denatured aggregate.
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31
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Diéguez-Casal E, Freixeiro P, Costoya L, Criado MT, Ferreirós C, Sánchez S. High resolution clear native electrophoresis is a good alternative to blue native electrophoresis for the characterization of the Escherichia coli membrane complexes. J Microbiol Methods 2014; 102:45-54. [PMID: 24845470 DOI: 10.1016/j.mimet.2014.05.003] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/21/2014] [Revised: 05/09/2014] [Accepted: 05/09/2014] [Indexed: 10/25/2022]
Abstract
Blue native electrophoresis (BNE) has become the most popular method for the global analysis of membrane protein complexes. Although it has been shown to be very useful for that purpose, it can produce the dissociation of complexes with weak interactions and, due to the use of Coomassie Brilliant Blue, does not allow the subsequent application of fluorimetric and/or enzymatic techniques. Recently, we have successfully used the high resolution clear native electrophoresis (hrCNE) for the analysis of Neisseria meningitidis outer membrane porin complexes. The aim of this study was to determine the composition of the complexome of the Escherichia coli envelope by using hrCNE and to compare our results with those previously obtained using BNE. The bidimensional electrophoresis approaches used, hrCN/hrCNE and hrCN/SDS-PAGE, coupled to mass spectrometry allowed a detailed analysis of the complexome of E. coli membranes. For the first time, the three subunits of the formate dehydrogenase FDH-O were identified forming a single complex and hrCNE also allowed the identification of both the HflK and HflC proteins as components of the HflA complex. This technique also allowed us to suggest a relationship between OmpF and DLDH and, although OmpA is considered to be monomeric in vivo, we found this protein structured as homodimers. Thus hrCNE provides a good tool for future analyses of bacterial membrane proteins and complexes and is an important alternative to the commonly used BNE.
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Affiliation(s)
- Ernesto Diéguez-Casal
- Departamento de Microbiología y Parasitología, Facultad de Farmacia, Campus Vida, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain
| | - Paula Freixeiro
- Departamento de Microbiología y Parasitología, Facultad de Farmacia, Campus Vida, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain
| | - Liliana Costoya
- Departamento de Microbiología y Parasitología, Facultad de Farmacia, Campus Vida, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain
| | - M Teresa Criado
- Departamento de Microbiología y Parasitología, Facultad de Farmacia, Campus Vida, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain
| | - Carlos Ferreirós
- Departamento de Microbiología y Parasitología, Facultad de Farmacia, Campus Vida, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain
| | - Sandra Sánchez
- Departamento de Microbiología y Parasitología, Facultad de Farmacia, Campus Vida, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain.
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32
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Abstract
The Bam machine assembles β-barrel membrane proteins into the outer membranes of Gram-negative bacteria. The central component of the Bam complex, BamA, is a β-barrel that is conserved in prokaryotes and eukaryotes. We have previously reported an in vitro assay for studying the assembly of β-barrel proteins by the Bam complex and now apply this assay to identify the specific components that are required for BamA assembly. We establish that BamB and BamD, two lipoprotein components of the complex, bind to the unfolded BamA substrate and are sufficient to accelerate its assembly into the membrane.
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Affiliation(s)
- Christine L Hagan
- Department of Chemistry and Chemical Biology, Harvard University , Cambridge, Massachusetts 02138, United States
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33
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Patel GJ, Kleinschmidt JH. The lipid bilayer-inserted membrane protein BamA of Escherichia coli facilitates insertion and folding of outer membrane protein A from its complex with Skp. Biochemistry 2013; 52:3974-86. [PMID: 23641708 DOI: 10.1021/bi400103t] [Citation(s) in RCA: 49] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
Abstract
Folding of β-barrel membrane proteins, either from a urea-unfolded form or from chaperone-bound aqueous forms, has been characterized for pure lipid bilayers. The impact of preinserted integral proteins from biomembranes has not been examined in biophysical comparisons, but this knowledge is important for the characterization of protein assembly machinery in membranes to distinguish specific effects from unspecific effects. Here, folding was studied for a β-barrel membrane protein, outer membrane protein A (OmpA) from Escherichia coli, in the absence and presence of two other preinserted integral proteins, BamA of the β-barrel assembly machinery complex (BAM) from E. coli and FomA from Fusobacterium nucleatum. Three different preformed lipid membranes of phosphatidylcholine were prepared to compare the folding kinetics of OmpA, namely, proteoliposomes containing either BamA or FomA and pure liposomes. Urea-unfolded OmpA folded faster into phosphatidylcholine bilayers containing FomA than into pure lipid bilayers, but the kinetics of OmpA folding and insertion were fastest for bilayers containing BamA. Incorporation of BamA into lipid bilayers composed of phosphatidylcholine and phosphatidylethanolamine greatly weakened the inhibiting effect of phosphatidylethanolamine on the folding of OmpA. Folding of OmpA from its complex with the periplasmic chaperone Skp into bilayers composed of phosphatidylethanolamine and phosphatidylcholine was inhibited in the absence of BamA but facilitated when BamA was present, indicating an interaction of Skp-OmpA complexes with BamA.
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Affiliation(s)
- Geetika J Patel
- Fachbereich Biologie, Universität Konstanz, D-78457 Konstanz, Germany
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34
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Folding of outer membrane proteins. Arch Biochem Biophys 2013; 531:34-43. [DOI: 10.1016/j.abb.2012.10.008] [Citation(s) in RCA: 57] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/27/2012] [Revised: 10/11/2012] [Accepted: 10/19/2012] [Indexed: 11/18/2022]
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35
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Pocanschi CL, Popot JL, Kleinschmidt JH. Folding and stability of outer membrane protein A (OmpA) from Escherichia coli in an amphipathic polymer, amphipol A8-35. EUROPEAN BIOPHYSICS JOURNAL: EBJ 2013; 42:103-18. [DOI: 10.1007/s00249-013-0887-z] [Citation(s) in RCA: 60] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/21/2012] [Accepted: 01/02/2013] [Indexed: 11/29/2022]
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36
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Ye C, Chai Q, Zhong M, Wei Y. Effect of crowding by Ficolls on OmpA and OmpT refolding and membrane insertion. Protein Sci 2012; 22:239-45. [PMID: 23225740 DOI: 10.1002/pro.2205] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2012] [Revised: 11/26/2012] [Accepted: 11/29/2012] [Indexed: 11/07/2022]
Abstract
Folding of outer membrane proteins (OMPs) has been studied extensively in vitro. However, most of these studies have been conducted in dilute buffer solution, which is different from the crowded environment in the cell periplasm, where the folding and membrane insertion of OMPs actually occur. Using OmpA and OmpT as model proteins and Ficoll 70 as the crowding agent, here we investigated the effect of the macromolecular crowding condition on OMP membrane insertion. We found that the presence of Ficoll 70 significantly slowed down the rate of membrane insertion of OmpA while had little effect on those of OmpT. To investigate if the soluble domain of OmpA slowed down membrane insertion in the presence of the crowding agent, we created a truncated OmpA construct that contains only the transmembrane domain (OmpA171). In the absence of crowding agent, OmpA171 refolded at a similar rate as OmpA, although with decreased efficiency. However, under the crowding condition, OmpA171 refolded significantly faster than OmpA. Our results suggest that the periplasmic domain slows down the rate, while improves the efficiency, of OmpA folding and membrane insertion under the crowding condition. Such an effect was not obvious when refolding was studied in buffer solution in the absence of crowding.
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Affiliation(s)
- Cui Ye
- Department of Chemistry, University of Kentucky, Lexington, KY 40506, USA
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37
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Webb CT, Heinz E, Lithgow T. Evolution of the β-barrel assembly machinery. Trends Microbiol 2012; 20:612-20. [DOI: 10.1016/j.tim.2012.08.006] [Citation(s) in RCA: 112] [Impact Index Per Article: 9.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/18/2012] [Revised: 08/10/2012] [Accepted: 08/14/2012] [Indexed: 11/29/2022]
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38
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Thoma J, Bosshart P, Pfreundschuh M, Müller D. Out but Not In: The Large Transmembrane β-Barrel Protein FhuA Unfolds but Cannot Refold via β-Hairpins. Structure 2012; 20:2185-90. [DOI: 10.1016/j.str.2012.10.006] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/28/2012] [Revised: 10/15/2012] [Accepted: 10/19/2012] [Indexed: 12/19/2022]
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39
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Tatulian SA, Garg P, Nemec KN, Chen B, Khaled AR. Molecular basis for membrane pore formation by Bax protein carboxyl terminus. Biochemistry 2012; 51:9406-19. [PMID: 23110300 DOI: 10.1021/bi301195f] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
Bax protein plays a key role in mitochondrial membrane permeabilization and cytochrome c release upon apoptosis. Our recent data have indicated that the 20-residue C-terminal peptide of Bax (BaxC-KK; VTIFVAGVLTASLTIWKKMG), when expressed intracellularly, translocates to the mitochondria and exerts lethal effect on cancer cells. Moreover, the BaxC-KK peptide, as well as two mutants where the two lysines are replaced with glutamate (BaxC-EE) or leucine (BaxC-LL), have been shown to form relatively large pores in lipid membranes, composed of up to eight peptide molecules per pore. Here the pore structure is analyzed by polarized Fourier transform infrared, circular dichroism, and fluorescence experiments on the peptides reconstituted in phospholipid membranes. The peptides assume an α/β-type secondary structure within membranes. Both β-strands and α-helices are significantly (by 30-60 deg) tilted relative to the membrane normal. The tryptophan residue embeds into zwitterionic membranes at 8-9 Å from the membrane center. The membrane anionic charge causes a deeper insertion of tryptophan for BaxC-KK and BaxC-LL but not for BaxC-EE. Combined with the pore stoichiometry determined earlier, these structural constraints allow construction of a model of the pore where eight peptide molecules form an "α/β-ring" structure within the membrane. These results identify a strong membranotropic activity of Bax C-terminus and propose a new mechanism by which peptides can efficiently perforate cell membranes. Knowledge on the pore forming mechanism of the peptide may facilitate development of peptide-based therapies to kill cancer or other detrimental cells such as bacteria or fungi.
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Affiliation(s)
- Suren A Tatulian
- Department of Physics, University of Central Florida, Orlando, Florida, United States.
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40
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Andersen KK, Wang H, Otzen DE. A Kinetic Analysis of the Folding and Unfolding of OmpA in Urea and Guanidinium Chloride: Single and Parallel Pathways. Biochemistry 2012; 51:8371-83. [DOI: 10.1021/bi300974y] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Kell K. Andersen
- Interdisciplinary Nanoscience Centre (iNANO), Centre
for Insoluble Protein Structures (inSPIN), Department of Molecular
Biology and Genetics, University of Aarhus, Gustav Wieds Vej 14, DK-8000 Aarhus C, Denmark
| | - Huabing Wang
- Interdisciplinary Nanoscience Centre (iNANO), Centre
for Insoluble Protein Structures (inSPIN), Department of Molecular
Biology and Genetics, University of Aarhus, Gustav Wieds Vej 14, DK-8000 Aarhus C, Denmark
| | - Daniel E. Otzen
- Interdisciplinary Nanoscience Centre (iNANO), Centre
for Insoluble Protein Structures (inSPIN), Department of Molecular
Biology and Genetics, University of Aarhus, Gustav Wieds Vej 14, DK-8000 Aarhus C, Denmark
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41
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Alessandrini A, Viero G, Dalla Serra M, Prévost G, Facci P. γ-Hemolysin oligomeric structure and effect of its formation on supported lipid bilayers: an AFM investigation. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2012; 1828:405-11. [PMID: 23036932 DOI: 10.1016/j.bbamem.2012.09.027] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/26/2012] [Revised: 09/05/2012] [Accepted: 09/25/2012] [Indexed: 11/28/2022]
Abstract
γ-Hemolysins are bicomponent β-barrel pore forming toxins produced by Staphylococcus aureus as water-soluble monomers, which assemble into oligomeric pores on the surface of lipid bilayers. Here, after investigating the oligomeric structure of γ-hemolysins on supported lipid bilayers (SLBs) by atomic force microscopy (AFM), we studied the effect produced by this toxin on the structure of SLBs. We found that oligomeric structures with different number of monomers can assemble on the lipid bilayer being the octameric form the stablest one. Moreover, in this membrane model we found that γ-hemolysins can form clusters of oligomers inducing a curvature in the lipid bilayer, which could probably enhance the aggressiveness of these toxins at high concentrations.
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Affiliation(s)
- Andrea Alessandrini
- Centro S3, CNR-Istituto di Nanoscienze, Via Campi 213/A, 41125 Modena, Italy.
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42
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Leo JC, Grin I, Linke D. Type V secretion: mechanism(s) of autotransport through the bacterial outer membrane. Philos Trans R Soc Lond B Biol Sci 2012; 367:1088-101. [PMID: 22411980 PMCID: PMC3297439 DOI: 10.1098/rstb.2011.0208] [Citation(s) in RCA: 172] [Impact Index Per Article: 14.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Autotransport in Gram-negative bacteria denotes the ability of surface-localized proteins to cross the outer membrane (OM) autonomously. Autotransporters perform this task with the help of a β-barrel transmembrane domain localized in the OM. Different classes of autotransporters have been investigated in detail in recent years; classical monomeric but also trimeric autotransporters comprise many important bacterial virulence factors. So do the two-partner secretion systems, which are a special case as the transported protein resides on a different polypeptide chain than the transporter. Despite the great interest in these proteins, the exact mechanism of the transport process remains elusive. Moreover, different periplasmic and OM factors have been identified that play a role in the translocation, making the term ‘autotransport’ debatable. In this review, we compile the wealth of details known on the mechanism of single autotransporters from different classes and organisms, and put them into a bigger perspective. We also discuss recently discovered or rediscovered classes of autotransporters.
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Affiliation(s)
- Jack C Leo
- Max Planck Institute for Developmental Biology, Spemannstrasse 35, 72076 Tübingen, Germany
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43
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The FomA porin from Fusobacterium nucleatum is a Toll-like receptor 2 agonist with immune adjuvant activity. CLINICAL AND VACCINE IMMUNOLOGY : CVI 2012; 19:1093-101. [PMID: 22623652 DOI: 10.1128/cvi.00236-12] [Citation(s) in RCA: 39] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
Many bacterial components selectively activate immune and nonhematopoietic target cells via Toll-like receptor (TLR) signaling; modulation of such host responses defines the immune adjuvant properties of these bacterial products. For example, the outer membrane protein porins from Neisseria, Salmonella, and Shigella are known TLR2 agonists with established systemic and mucosal immune adjuvanticity. Early work indicated that the FomA porin from Fusobacterium nucleatum has immune adjuvant activity in mice. Using a purified recombinant FomA, we have verified its immune stimulatory properties and have defined a role for TLR2 signaling in its in vitro and in vivo activity. FomA induces interleukin 8 (IL-8) secretion and NF-κB-dependent luciferase activity in HEK cells expressing TLR2, IL-6 secretion, and cell surface upregulation of CD86 and major histocompatibility complex (MHC) II in primary B cells from wild-type mice, but it fails to activate cells from TLR2 knockout mice. Accordingly, the immune adjuvant activity of FomA is also TLR2 dependent. In a mouse model of immunization with ovalbumin (OVA), FomA induces enhanced production of OVA-specific IgM and IgG, including IgG1 and IgG2b antibodies, as well as enhanced secretion of IL-10 and IL-6, consistent with a Th2-type adjuvant effect. We also observe a moderate production of anti-FomA antibodies, suggesting that FomA is also immunogenic, a quality that is also TLR2 dependent. Therefore, modulation of host immune responses by FomA may be effective for targeting general host immunity not only to pathogens (as a novel TLR2 adjuvant) but also to F. nucleatum itself (as an antigen), expanding its use as a self-adjuvanted antigen in an immunization strategy against polymicrobial infections, including those by F. nucleatum.
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44
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Bosshart PD, Iordanov I, Garzon-Coral C, Demange P, Engel A, Milon A, Müller DJ. The transmembrane protein KpOmpA anchoring the outer membrane of Klebsiella pneumoniae unfolds and refolds in response to tensile load. Structure 2012; 20:121-7. [PMID: 22244761 DOI: 10.1016/j.str.2011.11.002] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/28/2011] [Revised: 10/20/2011] [Accepted: 11/04/2011] [Indexed: 10/14/2022]
Abstract
In Klebsiella pneumoniae the transmembrane β-barrel forming outer membrane protein KpOmpA mediates adhesion to a wide range of immune effector cells, thereby promoting respiratory tract and urinary infections. As major transmembrane protein OmpA stabilizes Gram-negative bacteria by anchoring their outer membrane to the peptidoglycan layer. Adhesion, osmotic pressure, hydrodynamic flow, and structural deformation apply mechanical stress to the bacterium. This stress can generate tensile load to the peptidoglycan-binding domain (PGBD) of KpOmpA. To investigate how KpOmpA reacts to mechanical stress, we applied a tensile load to the PGBD and observed a detailed unfolding pathway of the transmembrane β-barrel. Each step of the unfolding pathway extended the polypeptide connecting the bacterial outer membrane to the peptidoglycan layer and absorbed mechanical energy. After relieving the tensile load, KpOmpA reversibly refolded back into the membrane. These results suggest that bacteria may reversibly unfold transmembrane proteins in response to mechanical stress.
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Affiliation(s)
- Patrick D Bosshart
- Department of Biosystems Science and Engineering, ETH Zurich, CH-4058 Basel, Switzerland
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Proteomic analyses of Ehrlichia ruminantium highlight differential expression of MAP1-family proteins. Vet Microbiol 2012; 156:305-14. [DOI: 10.1016/j.vetmic.2011.11.022] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2011] [Revised: 11/22/2011] [Accepted: 11/24/2011] [Indexed: 11/21/2022]
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VirK is a periplasmic protein required for efficient secretion of plasmid-encoded toxin from enteroaggregative Escherichia coli. Infect Immun 2012; 80:2276-85. [PMID: 22547550 DOI: 10.1128/iai.00167-12] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
Despite the autotransporter (AT) moniker, AT secretion appears to involve the function of periplasmic chaperones. We identified four periplasmic proteins that specifically bound to plasmid-encoded toxin (Pet), an AT produced by enteroaggregative Escherichia coli (EAEC). These proteins include the 17-kDa Skp chaperone and the 37-kDa VirK protein. We found that the virK gene is present in different Enterobacteriaceae. VirK bound to misfolded conformations of the Pet passenger domain, but it did not bind to the folded passenger domain or to the β domain of Pet. Assays with an EAECΔvirK mutant and its complemented version showed that, in the absence of VirK, Pet was not secreted but was instead retained in the periplasm as proteolytic fragments. In contrast, Pet was secreted from a Δskp mutant. VirK was not required for the insertion of porin proteins into the outer membrane but assisted with insertion of the Pet β domain into the outer membrane. Loss of VirK function blocked the EAEC-mediated cytotoxic effect against HEp-2 cells. Thus, VirK facilitates the secretion of the AT Pet by maintaining the passenger domain in a conformation that both avoids periplasmic proteolysis and facilitates β-domain insertion into the outer membrane.
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Huysmans GH, Radford SE, Baldwin SA, Brockwell DJ. Malleability of the folding mechanism of the outer membrane protein PagP: parallel pathways and the effect of membrane elasticity. J Mol Biol 2012; 416:453-64. [PMID: 22245579 PMCID: PMC3314998 DOI: 10.1016/j.jmb.2011.12.039] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2011] [Revised: 12/19/2011] [Accepted: 12/19/2011] [Indexed: 10/29/2022]
Abstract
Understanding the interactions between membrane proteins and the lipid bilayer is key to increasing our ability to predict and tailor the folding mechanism, structure and stability of membrane proteins. Here, we have investigated the effects of changing the membrane composition and the relative concentrations of protein and lipid on the folding mechanism of the bacterial outer membrane protein PagP. The folding pathway, monitored by tryptophan fluorescence, was found to be characterized by a burst phase, representing PagP adsorption to the liposome surface, followed by a time course that reflects the folding and insertion of the protein into the membrane. In 1,2-dilauroyl-sn-glycero-3-phosphocholine (diC(12:0)PC) liposomes, the post-adsorption time course fits well to a single exponential at high lipid-to-protein ratios (LPRs), but at low LPRs, a second exponential phase with a slower folding rate constant is observed. Interrupted refolding assays demonstrated that the two exponential phases reflect the presence of parallel folding pathways. Partitioning between these pathways was found to be modulated by the elastic properties of the membrane. Folding into mixed 1,2-dilauroyl-sn-glycero-3-phosphoethanolamine:diC(12:0)PC liposomes resulted in a decrease in PagP adsorption to the liposomes and a switch to the slower folding pathway. By contrast, inclusion of 1,2-dilauroyl-sn-glycero-3-phosphoserine into diC(12:0)PC liposomes resulted in a decrease in the folding rate of the fast pathway. The results highlight the effect of lipid composition in tailoring the folding mechanism of a membrane protein, revealing that membrane proteins have access to multiple, competing folding routes to a unique native structure.
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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
| | - 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
| | - 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
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A novel method of AquaporinZ incorporation via binary-lipid Langmuir monolayers. Colloids Surf B Biointerfaces 2012; 89:283-8. [DOI: 10.1016/j.colsurfb.2011.09.004] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/28/2011] [Revised: 09/03/2011] [Accepted: 09/04/2011] [Indexed: 11/24/2022]
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Tang G, Kawai T, Komatsuzawa H, Mintz KP. Lipopolysaccharides mediate leukotoxin secretion in Aggregatibacter actinomycetemcomitans. Mol Oral Microbiol 2011; 27:70-82. [PMID: 22394466 DOI: 10.1111/j.2041-1014.2011.00632.x] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
Abstract
We previously reported that lipopolysaccharide (LPS) -related sugars are associated with the glycosylation of the collagen adhesin EmaA, a virulence determinant of Aggregatibacter actinomycetemcomitans. In this study, the role of LPS in the secretion of other virulence factors was investigated. The secretion of the epithelial adhesin Aae, the immunoglobulin Fc receptor Omp34 and leukotoxin were examined in a mutant strain with inactivated TDP-4-keto-6-deoxy-d-glucose 3,5-epimerase (rmlC), which resulted in altered O-antigen polysaccharides (O-PS) of LPS. The secretion of Aae and Omp34 was not affected. However, the leukotoxin secretion, which is mediated by the TolC-dependent type I secretion system, was altered in the rmlC mutant. The amount of secreted leukotoxin in the bacterial growth medium was reduced nine-fold, with a concurrent four-fold increase of the membrane-bound toxin in the mutant compared with the wild-type strain. The altered leukotoxin secretion pattern was restored to the wild-type by complementation of the rmlC gene in trans. Examination of the ltxA mRNA levels indicated that the leukotoxin secretion was post-transcriptionally regulated in the modified O-PS containing strain. The mutant strain also showed increased resistance to vancomycin, an antibiotic dependent on TolC for internalization, indicating that TolC was affected. Overexpression of TolC in the rmlC mutant resulted in an increased TolC level in the outer membrane but did not restore the leukotoxin secretion profile to the wild-type phenotype. The data suggest that O-PS mediate leukotoxin secretion in A. actinomycetemcomitans.
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Affiliation(s)
- G Tang
- Department of Microbiology and Molecular Genetics, University of Vermont, Burlington, VT 05405, USA
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Kang G, López-Peña I, Oklejas V, Gary CS, Cao W, Kim JE. Förster resonance energy transfer as a probe of membrane protein folding. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2011; 1818:154-61. [PMID: 21925139 DOI: 10.1016/j.bbamem.2011.08.029] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/26/2011] [Revised: 08/24/2011] [Accepted: 08/25/2011] [Indexed: 11/27/2022]
Abstract
The folding reaction of a β-barrel membrane protein, outer membrane protein A (OmpA), is probed with Förster resonance energy transfer (FRET) experiments. Four mutants of OmpA were generated in which the donor fluorophore, tryptophan, and acceptor molecule, a naphthalene derivative, are placed in various locations on the protein to report the evolution of distances across the bilayer and across the protein pore during a folding event. Analysis of the FRET efficiencies reveals three timescales for tertiary structure changes associated with insertion and folding into a synthetic bilayer. A narrow pore forms during the initial stage of insertion, followed by bilayer traversal. Finally, a long-time component is attributed to equilibration and relaxation, and may involve global changes such as pore expansion and strand extension. These results augment the existing models that describe concerted insertion and folding events, and highlight the ability of FRET to provide insight into the complex mechanisms of membrane protein folding. This article is part of a Special Issue entitled: Membrane protein structure and function.
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Affiliation(s)
- Guipeun Kang
- Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla, CA 92093, USA
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