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Zeinert R, Zhou F, Franco P, Zöller J, Lessen HJ, Aravind L, Langer JD, Sodt AJ, Storz G, Matthies D. Magnesium Transporter MgtA revealed as a Dimeric P-type ATPase. bioRxiv 2024:2024.02.28.582502. [PMID: 38464158 PMCID: PMC10925321 DOI: 10.1101/2024.02.28.582502] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 03/12/2024]
Abstract
Magnesium (Mg2+) uptake systems are present in all domains of life given the vital role of this ion. Bacteria acquire Mg2+ via conserved Mg2+ channels and transporters. The transporters are required for growth when Mg2+ is limiting or during bacterial pathogenesis, but, despite their significance, there are no known structures for these transporters. Here we report the first structure of the Mg2+ transporter MgtA solved by single particle cryo-electron microscopy (cryo-EM). Using mild membrane extraction, we obtained high resolution structures of both a homodimeric form (2.9 Å), the first for a P-type ATPase, and a monomeric form (3.6 Å). Each monomer unit of MgtA displays a structural architecture that is similar to other P-type ATPases with a transmembrane domain and two soluble domains. The dimer interface consists of contacts between residues in adjacent soluble nucleotide binding and phosphotransfer regions of the haloacid dehalogenase (HAD) domain. We suggest oligomerization is a conserved structural feature of the diverse family of P-type ATPase transporters. The ATP binding site and conformational dynamics upon nucleotide binding to MgtA were characterized using a combination of cryo-EM, molecular dynamics simulations, hydrogen-deuterium exchange mass spectrometry, and mutagenesis. Our structure also revealed a Mg2+ ion in the transmembrane segments, which, when combined with sequence conservation and mutagenesis studies, allowed us to propose a model for Mg2+ transport across the lipid bilayer. Finally, our work revealed the N-terminal domain structure and cytoplasmic Mg2+ binding sites, which have implications for related P-type ATPases defective in human disease.
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Affiliation(s)
- Rilee Zeinert
- Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda MD 20892, USA
| | - Fei Zhou
- Unit on Structural Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda MD 20892, USA
| | - Pedro Franco
- Max Planck Institute of Biophysics, 60438 Frankfurt am Main, Germany
| | - Jonathan Zöller
- Max Planck Institute of Biophysics, 60438 Frankfurt am Main, Germany
| | - Henry J. Lessen
- Unit on Membrane Chemical Physics, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda MD 20892, USA
| | - L. Aravind
- National Center for Biotechnology Information, National Institutes of Health, Bethesda MD 20892, USA
| | - Julian D. Langer
- Max Planck Institute of Biophysics, 60438 Frankfurt am Main, Germany
| | - Alexander J. Sodt
- Unit on Membrane Chemical Physics, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda MD 20892, USA
| | - Gisela Storz
- Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda MD 20892, USA
| | - Doreen Matthies
- Unit on Structural Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda MD 20892, USA
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Gupta S, Lessen HJ, Heberle FA, Sodt AJ, Ashkar R. Domain-induced dynamics in phase-separating lipid membranes. Biophys J 2023; 122:364a. [PMID: 36783851 DOI: 10.1016/j.bpj.2022.11.2011] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/12/2023] Open
Affiliation(s)
- Sudipta Gupta
- Department of Physics, Virginia Tech, Blacksburg, VA, USA; Center for Soft Matter and Biological Physics, Virginia Tech, Blacksburg, VA, USA
| | - Henry J Lessen
- Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, USA
| | | | - Alexander J Sodt
- Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, USA
| | - Rana Ashkar
- Department of Physics, Virginia Tech, Blacksburg, VA, USA; Center for Soft Matter and Biological Physics, Virginia Tech, Blacksburg, VA, USA
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Rosencrans WM, Queralt-Martin M, Lessen HJ, Larimi MG, Rajendran M, Chou TF, Mahalakshmi R, Sodt AJ, Yu TY, Bezrukov SM, Rostovtseva TK. Defining the roles and regulation of the mitochondrial VDAC isoforms one molecule at a time. Biophys J 2023; 122:93a. [PMID: 37220506 PMCID: PMC7614561 DOI: 10.1016/j.bpj.2022.11.700] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/12/2023] Open
Affiliation(s)
- William M Rosencrans
- Department of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA
- National Institutes of Health, Bethesda, MD, USA
| | | | - Henry J Lessen
- Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, USA
| | | | - Megha Rajendran
- Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, USA
| | - Tsui-Fen Chou
- Department of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA
| | - Radhakrishnan Mahalakshmi
- Department of Biological Sciences, Indian Institute of Science Education and Research Bhopal, Bhopal, India
| | - Alexander J Sodt
- Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, USA
| | - Tsyr-Yan Yu
- Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
| | - Sergey M Bezrukov
- Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, USA
| | - Tatiana K Rostovtseva
- Section on Molecular Transport, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, USA
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Lessen HJ, Sapp KC, Beaven AH, Ashkar R, Sodt AJ. Molecular mechanisms of spontaneous curvature and softening in complex lipid bilayer mixtures. Biophys J 2022; 121:3188-3199. [PMID: 35927953 PMCID: PMC9463698 DOI: 10.1016/j.bpj.2022.07.036] [Citation(s) in RCA: 8] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/25/2022] [Revised: 06/01/2022] [Accepted: 07/28/2022] [Indexed: 11/21/2022] Open
Abstract
Membrane reshaping is an essential biological process. The chemical composition of lipid membranes determines their mechanical properties and thus the energetics of their shape. Hundreds of distinct lipid species make up native bilayers, and this diversity complicates efforts to uncover what compositional factors drive membrane stability in cells. Simplifying assumptions, therefore, are used to generate quantitative predictions of bilayer dynamics based on lipid composition. One assumption commonly used is that "per lipid" mechanical properties are both additive and constant-that they are an intrinsic property of lipids independent of the surrounding composition. Related to this is the assumption that lipid bulkiness, or "shape," determines its curvature preference, independently of context. In this study, all-atom molecular dynamics simulations on three separate multilipid systems were used to explicitly test these assumptions, applying methodology recently developed to isolate properties of single lipids or nanometer-scale patches of lipids. The curvature preference experienced by populations of lipid conformations were inferred from their redistribution on a dynamically fluctuating bilayer. Representative populations were extracted by both structural similarity and semi-automated hidden Markov model analysis. The curvature preferences of lipid dimers were then determined and compared with an additive model that combines the monomer curvature preference of both the individual lipids. In all three systems, we identified conformational subpopulations of lipid dimers that showed non-additive curvature preference, in each case mediated by a special chemical interaction (e.g., hydrogen bonding). Our study highlights the importance of specific chemical interactions between lipids in multicomponent bilayers and the impact of interactions on bilayer stiffness. We identify two mechanisms of bilayer softening: diffusional softening, driven by the dynamic coupling between lipid distributions and membrane undulations, and conformational softening, driven by the inter-conversion between distinct dimeric conformations.
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Affiliation(s)
- Henry J Lessen
- Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland
| | - Kayla C Sapp
- Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland
| | - Andrew H Beaven
- Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland; Postdoctoral Research Associate Program, National Institute of General Medical Sciences, National Institutes of Health, Bethesda, Maryland
| | - Rana Ashkar
- Department of Physics, Virginia Tech, Blacksburg, Virginia; Center for Soft Matter and Biological Physics, Virginia Tech, Blacksburg, Virginia
| | - Alexander J Sodt
- Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland.
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Lessen HJ, Beaven AH, Sapp K, Sodt AJ. Analysis of distinct multi-lipid configurations reveals a mechanism for bilayer softening. Biophys J 2022. [DOI: 10.1016/j.bpj.2021.11.2539] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022] Open
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Lessen HJ, Sodt AJ. A Reductionist Model of Mixed-Bilayer Mechanics Built using Molecular Simulation. Biophys J 2021. [DOI: 10.1016/j.bpj.2020.11.1537] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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Marx DC, Plummer AM, Faustino AM, Devlin T, Roskopf MA, Leblanc MJ, Lessen HJ, Amann BT, Fleming PJ, Krueger S, Fried SD, Fleming KG. SurA is a cryptically grooved chaperone that expands unfolded outer membrane proteins. Proc Natl Acad Sci U S A 2020; 117:28026-28035. [PMID: 33093201 PMCID: PMC7668074 DOI: 10.1073/pnas.2008175117] [Citation(s) in RCA: 14] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2020] [Accepted: 08/26/2020] [Indexed: 11/18/2022] Open
Abstract
The periplasmic chaperone network ensures the biogenesis of bacterial outer membrane proteins (OMPs) and has recently been identified as a promising target for antibiotics. SurA is the most important member of this network, both due to its genetic interaction with the β-barrel assembly machinery complex as well as its ability to prevent unfolded OMP (uOMP) aggregation. Using only binding energy, the mechanism by which SurA carries out these two functions is not well-understood. Here, we use a combination of photo-crosslinking, mass spectrometry, solution scattering, and molecular modeling techniques to elucidate the key structural features that define how SurA solubilizes uOMPs. Our experimental data support a model in which SurA binds uOMPs in a groove formed between the core and P1 domains. This binding event results in a drastic expansion of the rest of the uOMP, which has many biological implications. Using these experimental data as restraints, we adopted an integrative modeling approach to create a sparse ensemble of models of a SurA•uOMP complex. We validated key structural features of the SurA•uOMP ensemble using independent scattering and chemical crosslinking data. Our data suggest that SurA utilizes three distinct binding modes to interact with uOMPs and that more than one SurA can bind a uOMP at a time. This work demonstrates that SurA operates in a distinct fashion compared to other chaperones in the OMP biogenesis network.
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Affiliation(s)
- Dagan C Marx
- Thomas C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, MD 21218
| | - Ashlee M Plummer
- Thomas C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, MD 21218
| | | | - Taylor Devlin
- Thomas C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, MD 21218
| | - Michaela A Roskopf
- Thomas C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, MD 21218
| | - Mathis J Leblanc
- Thomas C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, MD 21218
| | - Henry J Lessen
- Thomas C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, MD 21218
| | - Barbara T Amann
- Thomas C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, MD 21218
| | - Patrick J Fleming
- Thomas C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, MD 21218
| | - Susan Krueger
- National Institute of Standards and Technology, Gaithersburg, MD 20899
| | - Stephen D Fried
- Department of Chemistry, Johns Hopkins University, Baltimore, MD 21218
| | - Karen G Fleming
- Thomas C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, MD 21218;
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O'Brien ES, Fuglestad B, Lessen HJ, Stetz MA, Lin DW, Marques BS, Gupta K, Fleming KG, Wand AJ. Membrane Proteins Have Distinct Fast Internal Motion and Residual Conformational Entropy. Angew Chem Int Ed Engl 2020; 59:11108-11114. [PMID: 32277554 PMCID: PMC7318686 DOI: 10.1002/anie.202003527] [Citation(s) in RCA: 14] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2020] [Indexed: 12/31/2022]
Abstract
The internal motions of integral membrane proteins have largely eluded comprehensive experimental characterization. Here the fast side-chain dynamics of the α-helical sensory rhodopsin II and the β-barrel outer membrane protein W have been investigated in lipid bilayers and detergent micelles by solution NMR relaxation techniques. Despite their differing topologies, both proteins have a similar distribution of methyl-bearing side-chain motion that is largely independent of membrane mimetic. The methyl-bearing side chains of both proteins are, on average, more dynamic in the ps-ns timescale than any soluble protein characterized to date. Accordingly, both proteins retain an extraordinary residual conformational entropy in the folded state, which provides a counterbalance to the absence of the hydrophobic effect. Furthermore, the high conformational entropy could greatly influence the thermodynamics underlying membrane-protein functions, including ligand binding, allostery, and signaling.
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Affiliation(s)
- Evan S. O'Brien
- Department of Biochemistry & BiophysicsUniversity of PennsylvaniaPerelman School of MedicinePhiladelphiaPA19104USA
| | - Brian Fuglestad
- Department of Biochemistry & BiophysicsUniversity of PennsylvaniaPerelman School of MedicinePhiladelphiaPA19104USA
- Present address: Department of ChemistryVirginia Commonwealth UniversityRichmondVA23284USA
| | - Henry J. Lessen
- Department of BiophysicsJohns Hopkins UniversityBaltimoreMD21218USA
| | - Matthew A. Stetz
- Department of Biochemistry & BiophysicsUniversity of PennsylvaniaPerelman School of MedicinePhiladelphiaPA19104USA
| | - Danny W. Lin
- Department of Biochemistry & BiophysicsUniversity of PennsylvaniaPerelman School of MedicinePhiladelphiaPA19104USA
| | - Bryan S. Marques
- Department of Biochemistry & BiophysicsUniversity of PennsylvaniaPerelman School of MedicinePhiladelphiaPA19104USA
| | - Kushol Gupta
- Department of Biochemistry & BiophysicsUniversity of PennsylvaniaPerelman School of MedicinePhiladelphiaPA19104USA
| | - Karen G. Fleming
- Department of BiophysicsJohns Hopkins UniversityBaltimoreMD21218USA
| | - A. Joshua Wand
- Department of Biochemistry & BiophysicsTexas A&M UniversityCollege StationTX77843USA
- Department of Biochemistry & BiophysicsUniversity of PennsylvaniaPerelman School of MedicinePhiladelphiaPA19104USA
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O'Brien ES, Fuglestad B, Lessen HJ, Stetz MA, Lin DW, Marques BS, Gupta K, Fleming KG, Wand AJ. Membrane Proteins Have Distinct Fast Internal Motion and Residual Conformational Entropy. Angew Chem Int Ed Engl 2020. [DOI: 10.1002/ange.202003527] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Affiliation(s)
- Evan S. O'Brien
- Department of Biochemistry & Biophysics University of Pennsylvania Perelman School of Medicine Philadelphia PA 19104 USA
| | - Brian Fuglestad
- Department of Biochemistry & Biophysics University of Pennsylvania Perelman School of Medicine Philadelphia PA 19104 USA
- Present address: Department of Chemistry Virginia Commonwealth University Richmond VA 23284 USA
| | - Henry J. Lessen
- Department of Biophysics Johns Hopkins University Baltimore MD 21218 USA
| | - Matthew A. Stetz
- Department of Biochemistry & Biophysics University of Pennsylvania Perelman School of Medicine Philadelphia PA 19104 USA
| | - Danny W. Lin
- Department of Biochemistry & Biophysics University of Pennsylvania Perelman School of Medicine Philadelphia PA 19104 USA
| | - Bryan S. Marques
- Department of Biochemistry & Biophysics University of Pennsylvania Perelman School of Medicine Philadelphia PA 19104 USA
| | - Kushol Gupta
- Department of Biochemistry & Biophysics University of Pennsylvania Perelman School of Medicine Philadelphia PA 19104 USA
| | - Karen G. Fleming
- Department of Biophysics Johns Hopkins University Baltimore MD 21218 USA
| | - A. Joshua Wand
- Department of Biochemistry & Biophysics Texas A&M University College Station TX 77843 USA
- Department of Biochemistry & Biophysics University of Pennsylvania Perelman School of Medicine Philadelphia PA 19104 USA
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Lessen HJ, Majumdar A, Fleming KG. Backbone Hydrogen Bond Energies in Membrane Proteins Are Insensitive to Large Changes in Local Water Concentration. J Am Chem Soc 2020; 142:6227-6235. [PMID: 32134659 DOI: 10.1021/jacs.0c00290] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
A hallmark feature of biological lipid bilayer structure is a depth-dependent polarity gradient largely resulting from the change in water concentration over the angstrom length scale. This gradient is particularly steep as it crosses the membrane interfacial regions where the water concentration drops at least a million-fold along the direction of the bilayer normal. Although local water content is often assumed to be a major determinant of membrane protein stability, the effect of the water-induced polarity gradient upon backbone hydrogen bond strength has not been systematically investigated. We addressed this question by measuring the free energy change for a number of backbone hydrogen bonds in the transmembrane protein OmpW. These values were obtained at 33 backbone amides from hydrogen/deuterium fractionation factors by nuclear magnetic resonance spectroscopy. We surprisingly found that OmpW backbone hydrogen bond energies do not vary over a wide range of water concentrations that are characteristic of the solvation environment in the bilayer interfacial region. We validated the interpretation of our results by determining the hydrodynamic and solvation properties of our OmpW-micelle complex using analytical ultracentrifugation and molecular dynamics simulations. The magnitudes of the backbone hydrogen bond free energy changes in our study are comparable to those observed in water-soluble proteins, the H-segment of the leader peptidase helix used in the von Heijne and White biological scale experiments, and several interfacial peptides. Our results agree with those reported for the transmembrane α-helical portion of the amyloid precursor protein after the latter values were adjusted for kinetic isotope effects. Overall, our work suggests that backbone hydrogen bonds provide modest thermodynamic stability to membrane protein structures and that many amides are unaffected by dehydration within the bilayer.
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Affiliation(s)
- Henry J Lessen
- T.C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, Maryland 21218, United States
| | - Ananya Majumdar
- The Johns Hopkins University Biomolecular NMR Center, Johns Hopkins University, Baltimore, Maryland 21218, United States
| | - Karen G Fleming
- T.C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, Maryland 21218, United States
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Lessen HJ, Fleming P, Fleming KG, Sodt AJ. Transmembrane Beta-Barrel Proteins Rigidify the Bacterial Outer Membrane. Biophys J 2019. [DOI: 10.1016/j.bpj.2018.11.1774] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022] Open
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12
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Lessen HJ, Fleming PJ, Fleming KG, Sodt AJ. Building Blocks of the Outer Membrane: Calculating a General Elastic Energy Model for β-Barrel Membrane Proteins. J Chem Theory Comput 2018; 14:4487-4497. [PMID: 29979594 PMCID: PMC6191857 DOI: 10.1021/acs.jctc.8b00377] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The outer membranes of Gram negative bacteria are the first points of contact these organisms make with their environment. Understanding how composition determines the mechanical properties of this essential barrier is of paramount importance. Therefore, we developed a new computational method to measure the elasticity of transmembrane proteins found in the outer membrane. Using all-atom molecular dynamics simulations of these proteins, we apply a set of external forces to mechanically stress the transmembrane β-barrels. Our results from four representative β-barrels show that outer membrane proteins display elastic properties that are approximately 70 to 190 times stiffer than neat lipid membranes. These findings suggest that outer membrane β-barrels are a significant source of mechanical stability in bacteria. Our all-atom approach further reveals that resistance to radial stress is encoded by a general mechanism that includes stretching of backbone hydrogen bonds and tilting of β-strands with respect to the bilayer normal. This computational framework facilitates an increased theoretical understanding of how varying lipid and protein amounts affect the mechanical properties of the bacterial outer membrane.
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Affiliation(s)
- Henry J. Lessen
- T. C. Jenkins Department of Biophysics, Johns Hopkins University, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health
| | - Patrick J. Fleming
- T. C. Jenkins Department of Biophysics, Johns Hopkins University, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health
| | - Karen G. Fleming
- T. C. Jenkins Department of Biophysics, Johns Hopkins University, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health
| | - Alexander J. Sodt
- T. C. Jenkins Department of Biophysics, Johns Hopkins University, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health
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