1
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Drabik D, Hinc P, Stephan M, Cavalcanti RRM, Czogalla A, Dimova R. Effect of leaflet asymmetry on the stretching elasticity of lipid bilayers with phosphatidic acid. Biophys J 2024:S0006-3495(24)00380-1. [PMID: 38822521 DOI: 10.1016/j.bpj.2024.05.031] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/26/2024] [Revised: 05/03/2024] [Accepted: 05/29/2024] [Indexed: 06/03/2024] Open
Abstract
The asymmetry of membranes has a significant impact on their biophysical characteristics and behavior. This study investigates the composition and mechanical properties of symmetric and asymmetric membranes in giant unilamellar vesicles (GUVs) made of palmitoyloleoyl phosphatidylcholine (POPC) and palmitoyloleoyl phosphatidic acid (POPA). A combination of fluorescence quantification, zeta potential measurements, micropipette aspiration, and bilayer molecular dynamics simulations are used to characterize these membranes. The outer leaflet composition in vesicles is found consistent across the two preparation methods we employed, namely electroformation and inverted emulsion transfer. However, characterizing the inner leaflet poses challenges. Micropipette aspiration of GUVs show that oil residues do not substantially alter membrane elasticity, but simulations reveal increased membrane thickness and decreased interleaflet coupling in the presence of oil. Asymmetric membranes with a POPC:POPA mixture in the outer leaflet and POPC in the inner leaflet display similar stretching elasticity values to symmetric POPC:POPA membranes, suggesting potential POPA insertion into the inner leaflet during vesicle formation and suppressed asymmetry. The inverse compositional asymmetry, with POPC in the outer leaflet and POPC:POPA in the inner one yield less stretchable membranes with higher compressibility modulus compared with their symmetric counterparts. Challenges in achieving and predicting compositional correspondence highlight the limitations of phase-transfer-based methods. In addition, caution is advised when using fluorescently labeled lipids (even at low fractions of 0.5 mol %), as unexpected gel-like domains in symmetric POPC:POPA membranes were observed only with a specific type of labeled DOPE (dioleoylphosphatidylethanolamine) and the same fraction of unlabeled DOPE. The latter suggest that such domain formation may result from interactions between lipids and membrane fluorescent probes. Overall, this study underscores the complexity of factors influencing GUV membrane asymmetry, emphasizing the need for further research and improvement of characterization techniques.
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Affiliation(s)
- Dominik Drabik
- Department of Cytobiochemistry, Faculty of Biotechnology, University of Wroclaw, Wroclaw, Poland; Max Planck Institute of Colloids and Interfaces, Potsdam, Germany; Department of Biomedical Engineering, Wroclaw University of Science and Technology, Wroclaw, Poland.
| | - Piotr Hinc
- Department of Cytobiochemistry, Faculty of Biotechnology, University of Wroclaw, Wroclaw, Poland
| | - Mareike Stephan
- Max Planck Institute of Colloids and Interfaces, Potsdam, Germany
| | | | - Aleksander Czogalla
- Department of Cytobiochemistry, Faculty of Biotechnology, University of Wroclaw, Wroclaw, Poland.
| | - Rumiana Dimova
- Max Planck Institute of Colloids and Interfaces, Potsdam, Germany.
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2
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Doktorova M, Levental I, Heberle FA. Seeing the Membrane from Both Sides Now: Lipid Asymmetry and Its Strange Consequences. Cold Spring Harb Perspect Biol 2023; 15:a041393. [PMID: 37604588 PMCID: PMC10691478 DOI: 10.1101/cshperspect.a041393] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 08/23/2023]
Abstract
Almost all biomembranes are constructed as lipid bilayers and, in almost all of these, the two opposing monolayers (leaflets) have distinct lipid compositions. This lipid asymmetry arises through the concerted action of a suite of energy-dependent enzymes that maintain living bilayers in a far-from-equilibrium steady-state. Recent discoveries reveal that lipid compositional asymmetry imparts biophysical asymmetries and that this dualistic organization may have major consequences for cellular physiology. Importantly, while transbilayer asymmetry appears to be an essential, near-ubiquitous characteristic of biological membranes, it has been challenging to reproduce in reconstituted or synthetic systems. Although recent methodological developments have overcome some critical challenges, it remains difficult to extrapolate results from available models to biological systems. Concurrently, there are few experimental approaches for targeted, controlled manipulation of lipid asymmetry in living cells. Thus, the biophysical and functional consequences of membrane asymmetry remain almost wholly unexplored. This perspective summarizes the current state of knowledge and highlights emerging themes that are beginning to make inroads into the fundamental question of why life tends toward asymmetry in its bilayers.
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Affiliation(s)
- Milka Doktorova
- Department of Molecular Physiology and Pharmacology, University of Virginia, Center for Membrane and Cell Physiology, Charlottesville, Virginia 22908, USA
| | - Ilya Levental
- Department of Molecular Physiology and Pharmacology, University of Virginia, Center for Membrane and Cell Physiology, Charlottesville, Virginia 22908, USA
| | - Frederick A Heberle
- Department of Chemistry, University of Tennessee Knoxville, Knoxville, Tennessee 37996, USA
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3
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Konar S, Arif H, Allolio C. Mitochondrial membrane model: Lipids, elastic properties, and the changing curvature of cardiolipin. Biophys J 2023; 122:4274-4287. [PMID: 37798880 PMCID: PMC10645570 DOI: 10.1016/j.bpj.2023.10.002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/07/2023] [Revised: 06/12/2023] [Accepted: 10/02/2023] [Indexed: 10/07/2023] Open
Abstract
Mammalian and Drosophila melanogaster model mitochondrial membrane compositions are constructed from experimental data. Simplified compositions for inner and outer mitochondrial membranes are provided, including an asymmetric inner mitochondrial membrane. We performed atomistic molecular dynamics simulations of these membranes and computed their material properties. When comparing these properties to those obtained by extrapolation from their constituting lipids, we find good overall agreement. Finally, we analyzed the curvature effect of cardiolipin, considering ion concentration effects, oxidation, and pH. We draw the conclusion that cardiolipin-negative curvature is most likely due to counterion effects, such as cation adsorption, in particular of H3O+. This oft-neglected effect might account for the puzzling behavior of this lipid.
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Affiliation(s)
- Sukanya Konar
- Faculty of Mathematics and Physics, Mathematical Institute, Charles University, Prague, Czech Republic
| | - Hina Arif
- Faculty of Mathematics and Physics, Mathematical Institute, Charles University, Prague, Czech Republic
| | - Christoph Allolio
- Faculty of Mathematics and Physics, Mathematical Institute, Charles University, Prague, Czech Republic.
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4
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Biriukov D, Javanainen M. Efficient Simulations of Solvent Asymmetry Across Lipid Membranes Using Flat-Bottom Restraints. J Chem Theory Comput 2023; 19:6332-6341. [PMID: 37651714 PMCID: PMC10537000 DOI: 10.1021/acs.jctc.3c00614] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/08/2023] [Indexed: 09/02/2023]
Abstract
The routinely employed periodic boundary conditions complicate molecular simulations of physiologically relevant asymmetric lipid membranes together with their distinct solvent environments. Therefore, separating the extracellular fluid from its cytosolic counterpart has often been performed using a costly double-bilayer setup. Here, we demonstrate that the lipid membrane and solvent asymmetry can be efficiently modeled with a single lipid bilayer by applying an inverted flat-bottom potential to ions and other solute molecules, thereby restraining them to only interact with the relevant leaflet. We carefully optimized the parameters of the suggested method so that the results obtained using the flat-bottom and double-bilayer approaches become mutually indistinguishable. Then, we apply the flat-bottom approach to lipid bilayers with various compositions and solvent environments, covering ions and cationic peptides to validate the approach in a realistic use case. We also discuss the possible limitations of the method as well as its computational efficiency and provide a step-by-step guide on how to set up such simulations in a straightforward manner.
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Affiliation(s)
- Denys Biriukov
- Institute
of Organic Chemistry and Biochemistry, Academy of Sciences of the
Czech Republic, Flemingovo
nam. 2, Prague 6 CZ-16610, Czech Republic
- Central
European Institute of Technology, Masaryk
University, Kamenice
5, Brno CZ-62500, Czech Republic
| | - Matti Javanainen
- Institute
of Organic Chemistry and Biochemistry, Academy of Sciences of the
Czech Republic, Flemingovo
nam. 2, Prague 6 CZ-16610, Czech Republic
- Institute
of Biotechnology, University of Helsinki, Helsinki FI-00790, Finland
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5
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Chaisson EH, Heberle FA, Doktorova M. Building Asymmetric Lipid Bilayers for Molecular Dynamics Simulations: What Methods Exist and How to Choose One? MEMBRANES 2023; 13:629. [PMID: 37504995 PMCID: PMC10384462 DOI: 10.3390/membranes13070629] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/07/2023] [Revised: 06/20/2023] [Accepted: 06/25/2023] [Indexed: 07/29/2023]
Abstract
The compositional asymmetry of biological membranes has attracted significant attention over the last decade. Harboring more differences from symmetric membranes than previously appreciated, asymmetric bilayers have proven quite challenging to study with familiar concepts and techniques, leaving many unanswered questions about the reach of the asymmetry effects. One particular area of active research is the computational investigation of composition- and number-asymmetric lipid bilayers with molecular dynamics (MD) simulations. Offering a high level of detail into the organization and properties of the simulated systems, MD has emerged as an indispensable tool in the study of membrane asymmetry. However, the realization that results depend heavily on the protocol used for constructing the asymmetric bilayer models has sparked an ongoing debate about how to choose the most appropriate approach. Here we discuss the underlying source of the discrepant results and review the existing methods for creating asymmetric bilayers for MD simulations. Considering the available data, we argue that each method is well suited for specific applications and hence there is no single best approach. Instead, the choice of a construction protocol-and consequently, its perceived accuracy-must be based primarily on the scientific question that the simulations are designed to address.
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Affiliation(s)
- Emily H. Chaisson
- Department of Chemistry, University of Tennessee Knoxville, Knoxville, TN 37916, USA
| | - Frederick A. Heberle
- Department of Chemistry, University of Tennessee Knoxville, Knoxville, TN 37916, USA
| | - Milka Doktorova
- Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, VA 22903, USA
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6
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Pirhadi E, Vanegas JM, Farin M, Schertzer JW, Yong X. Effect of Local Stress on Accurate Modeling of Bacterial Outer Membranes Using All-Atom Molecular Dynamics. J Chem Theory Comput 2023; 19:363-372. [PMID: 36579901 DOI: 10.1021/acs.jctc.2c01026] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
Biological membranes are fundamental components of living organisms that play an undeniable role in their survival. Molecular dynamics (MD) serves as an essential computational tool for studying biomembranes on molecular and atomistic scales. The status quo of MD simulations of biomembranes studies a nanometer-sized membrane patch periodically extended under periodic boundary conditions (PBCs). In nature, membranes are usually composed of different lipids in their two layers (referred to as leaflets). This compositional asymmetry imposes a fixed ratio of lipid numbers between the two leaflets in a periodically constrained membrane, which needs to be set appropriately. The widely adopted methods of defining a leaflet lipid ratio suffer from the lack of control over the mechanical tension of each leaflet, which could significantly influence research findings. In this study, we investigate the role of membrane-building protocol and the resulting initial stress state on the interaction between small molecules and asymmetric membranes. We model the outer membrane of Pseudomonas aeruginosa bacteria using two different building protocols and probe their interactions with the Pseudomonas quinolone signal (PQS). Our results show that differential stress could shift the position of free energy minimum for the PQS molecule between the two leaflets of the asymmetric membrane. This work provides critical insights into the relationship between the initial per-leaflet tension and the spontaneous intercalation of PQS.
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Affiliation(s)
- Emad Pirhadi
- Department of Mechanical Engineering, Binghamton University, Binghamton, New York 13902-6000, United States
| | - Juan M Vanegas
- Department of Biochemistry and Biophysics, Oregon State University, Corvallis, Oregon 97331-4003, United States
| | - Mithila Farin
- Department of Mechanical Engineering, Binghamton University, Binghamton, New York 13902-6000, United States
| | - Jeffrey W Schertzer
- Department of Biological Sciences, Binghamton University, Binghamton, New York 13902-6000, United States
| | - Xin Yong
- Department of Mechanical Engineering, Binghamton University, Binghamton, New York 13902-6000, United States
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7
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da Hora GCA, Nguyen JDM, Swanson JMJ. Can membrane composition traffic toxins? Mycolactone and preferential membrane interactions. Biophys J 2022; 121:4260-4270. [PMID: 36258678 PMCID: PMC9703097 DOI: 10.1016/j.bpj.2022.10.019] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/26/2022] [Revised: 08/31/2022] [Accepted: 10/13/2022] [Indexed: 12/14/2022] Open
Abstract
Mycolactone is a cytotoxic and immunosuppressive macrolide produced by Mycobacterium ulcerans and the sole causative agent of the neglected tropical skin disease Buruli ulcer. The toxin acts by invading host cells and interacting with intracellular targets to disrupt multiple fundamental cellular processes. Mycolactone's amphiphilic nature enables strong interactions with lipophilic environments, including cellular membranes; however, the specificity of these interactions and the role of membranes in the toxin's pathogenicity remain unknown. It is likely that preferential interactions with lipophilic carriers play a key role in the toxin's distribution in the host, which, if understood, could provide insights to aid in the development of needed diagnostics for Buruli ulcer disease. In this work, molecular dynamics simulations were combined with enhanced free-energy sampling to characterize mycolactone's association with and permeation through models of the mammalian endoplasmic reticulum (ER) and plasma membranes (PMs). We find that increased order in the PMs not only leads to a different permeation mechanism compared with that in the ER membrane but also an energetic driving force for ER localization. Increased hydration, membrane deformation, and preferential interactions with unsaturated lipid tails stabilize the toxin in the ER membrane, while disruption of lipid packing is a destabilizing force in the PMs.
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Affiliation(s)
| | - John D M Nguyen
- Department of Chemistry, University of Utah, Salt Lake City, Utah
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8
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Varma M, Deserno M. Distribution of cholesterol in asymmetric membranes driven by composition and differential stress. Biophys J 2022; 121:4001-4018. [PMID: 35927954 PMCID: PMC9674969 DOI: 10.1016/j.bpj.2022.07.032] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2022] [Revised: 07/11/2022] [Accepted: 07/27/2022] [Indexed: 11/25/2022] Open
Abstract
Many lipid membranes of eukaryotic cells are asymmetric, which means the two leaflets differ in at least one physical property, such as lipid composition or lateral stress. Maintaining this asymmetry is helped by the fact that ordinary phospholipids rarely transition between leaflets, but cholesterol is an exception: its flip-flop times are in the microsecond range, so that its distribution between leaflets is determined by a chemical equilibrium. In particular, preferential partitioning can draw cholesterol into a more saturated leaflet, and phospholipid number asymmetry can force it out of a compressed leaflet. Combining highly coarse-grained membrane simulations with theoretical modeling, we investigate how these two driving forces play against each other until cholesterol's chemical potential is equilibrated. The theory includes two coupled elastic sheets and a Flory-Huggins mixing free energy with a χ parameter. We obtain a relationship between χ and the interaction strength between cholesterol and lipids in either of the two leaflets, and we find that it depends, albeit weakly, on lipid number asymmetry. The differential stress measurements under various asymmetry conditions agree with our theoretical predictions. Using the two kinds of asymmetries in combination, we find that it is possible to counteract the phospholipid number bias, and the resultant stress in the membrane, via the control of cholesterol mixing in the leaflets.
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Affiliation(s)
- Malavika Varma
- Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania
| | - Markus Deserno
- Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania.
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9
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Doktorova M, Levental I. Cholesterol's balancing act: Defying the status quo. Biophys J 2022; 121:3771-3773. [PMID: 36084632 PMCID: PMC9674974 DOI: 10.1016/j.bpj.2022.08.036] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/17/2022] [Revised: 08/23/2022] [Accepted: 08/24/2022] [Indexed: 11/18/2022] Open
Affiliation(s)
- Milka Doktorova
- Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, Virginia.
| | - Ilya Levental
- Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, Virginia
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10
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Park S, Im W, Pastor RW. Developing initial conditions for simulations of asymmetric membranes: a practical recommendation. Biophys J 2021; 120:5041-5059. [PMID: 34653389 DOI: 10.1016/j.bpj.2021.10.009] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/13/2021] [Revised: 05/09/2021] [Accepted: 10/08/2021] [Indexed: 01/03/2023] Open
Abstract
It has been proposed that the surface tension difference between leaflets (or differential stress) in asymmetric bilayers is generally nonvanishing. This implies that there is no unique approach to generate initial conditions for simulations of asymmetric bilayers in the absence of experimentally derived constraints. Current generation methods include individual area per lipid (APL) based, leaflet surface area (SA) matching, and zero leaflet tension based (0-DS). This work adds a bilayer-based approach that aims for achieving partial chemical equilibrium by interleaflet switching of selected lipids via P21 periodic boundary conditions. Based on a recently proposed theoretical framework, we obtained expressions for tensions in asymmetric bilayers from both the bending and area strains. We also developed a quantitative measure for the energetic penalty from the differential stress. The impacts of APL-, SA-, and 0-DS-based approaches on mechanical properties are assessed for two different asymmetric bilayers. The lateral pressure profile and its moments differ significantly for each method, whereas the area compressibility modulus is relatively insensitive. Application of P21 periodic boundary conditions (APL/P21, SA/P21, and 0-DS/P21) results in better agreement in mechanical properties between asymmetric bilayers generated by APL-, SA-, and 0-DS-based approaches, in which changes are the smallest for bilayers from the SA-based method. The estimated differential stress from the theory shows good agreement with that from the simulations. These simulation results and the good agreement between the predicted and observed differential stress further support the theoretical framework in which bilayer mechanical properties are outcomes of the interplay between intrinsic bending and asymmetric lipid packing. Based on the simulation results and theoretical predictions, the SA/P21-based, or at least the SA-based (when the differential stress is small), approach is recommended as a practical method for developing initial conditions for asymmetric bilayer simulations.
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Affiliation(s)
- Sooyhung Park
- Department of Biological Sciences, Bethlehem, Pennsylvania; Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania.
| | - Wonpil Im
- Department of Biological Sciences, Bethlehem, Pennsylvania; Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania
| | - Richard W Pastor
- Laboratory of Computational Biology, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland
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11
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Jiang W, Lin YC, Luo YL. Mechanical properties of anionic asymmetric bilayers from atomistic simulations. J Chem Phys 2021; 154:224701. [PMID: 34241213 DOI: 10.1063/5.0048232] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Mechanotransduction, the biological response to mechanical stress, is often initiated by activation of mechanosensitive (MS) proteins upon mechanically induced deformations of the cell membrane. A current challenge in fully understanding this process is in predicting how lipid bilayers deform upon the application of mechanical stress. In this context, it is now well established that anionic lipids influence the function of many proteins. Here, we test the hypothesis that anionic lipids could indirectly modulate MS proteins by alteration of the lipid bilayer mechanical properties. Using all-atom molecular dynamics simulations, we computed the bilayer bending rigidity (KC), the area compressibility (KA), and the surface shear viscosity (ηm) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (PC) lipid bilayers with and without phosphatidylserine (PS) or phosphatidylinositol bisphosphate (PIP2) at physiological concentrations in the lower leaflet. Tensionless leaflets were first checked for each asymmetric bilayer model, and a formula for embedding an asymmetric channel in an asymmetric bilayer is proposed. Results from two different sized bilayers show consistently that the addition of 20% surface charge in the lower leaflet of the PC bilayer with PIP2 has minimal impact on its mechanical properties, while PS reduced the bilayer bending rigidity by 22%. As a comparison, supplementing the PIP2-enriched PC membrane with 30% cholesterol, a known rigidifying steroid lipid, produces a significant increase in all three mechanical constants. Analysis of pairwise splay moduli suggests that the effect of anionic lipids on bilayer bending rigidity largely depends on the number of anionic lipid pairs formed during simulations. The potential implication of bilayer bending rigidity is discussed in the framework of MS piezo channels.
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Affiliation(s)
- Wenjuan Jiang
- Department of Pharmaceutical Sciences, College of Pharmacy, Western University of Health Sciences, Pomona, California 91766, USA
| | - Yi-Chun Lin
- Department of Pharmaceutical Sciences, College of Pharmacy, Western University of Health Sciences, Pomona, California 91766, USA
| | - Yun Lyna Luo
- Department of Pharmaceutical Sciences, College of Pharmacy, Western University of Health Sciences, Pomona, California 91766, USA
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12
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Weigle AT, Carr M, Shukla D. Impact of Increased Membrane Realism on Conformational Sampling of Proteins. J Chem Theory Comput 2021; 17:5342-5357. [PMID: 34339605 DOI: 10.1021/acs.jctc.1c00276] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The realism and accuracy of lipid bilayer simulations through molecular dynamics (MD) are heavily dependent on the lipid composition. While the field is pushing toward implementing more heterogeneous and realistic membrane compositions, a lack of high-resolution lipidomic data prevents some membrane protein systems from being modeled with the highest level of realism. Given the additional diversity of real-world cellular membranes and protein-lipid interactions, it is still not fully understood how altering membrane complexity affects modeled membrane protein functions or if it matters over long-timescale simulations. This is especially true for organisms whose membrane environments have little to no computational study, such as the plant plasma membrane. Tackling these issues in tandem, a generalized, realistic, and asymmetric plant plasma membrane with more than 10 different lipid species is constructed herein. Classical MD simulations of pure membrane constructs were performed to evaluate how altering the compositional complexity of the membrane impacted the plant membrane properties. The apo form of a plant sugar transporter, OsSWEET2b, was inserted into membrane models where lipid diversity was calculated in either a size-dependent or size-independent manner. An adaptive sampling simulation regime validated by Markov-state models was performed to capture the gating dynamics of OsSWEET2b in each of these membrane constructs. In comparison to previous OsSWEET2b simulations performed in a pure POPC bilayer, we confirm that simulations performed within a native-like membrane composition alter the stabilization of apo OsSWEET2b conformational states by ∼1 kcal/mol. The free-energy barriers of intermediate conformational states decrease when realistic membrane complexity is simplified, albeit roughly within sampling error, suggesting that protein-specific responses to membranes differ due to altered packing caused by compositional fluctuations. This work serves as a case study where a more realistic bilayer composition makes unbiased conformational sampling easier to achieve than with simplified bilayers.
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Affiliation(s)
- Austin T Weigle
- Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States
| | - Matthew Carr
- Independent Software Development Provider310 East Marlette Avenue, Phoenix, Arizona 85012, United States
| | - Diwakar Shukla
- Department of Chemical & Biomolecular Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.,Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.,Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.,Cancer Center at Illinois, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.,Center for Digital Agriculture, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.,Department of Plant Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.,National Center for Supercomputing Applications, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.,NIH Center for Macromolecular Modeling and Bioinformatics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States
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13
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Scott HL, Kennison KB, Enoki TA, Doktorova M, Kinnun JJ, Heberle FA, Katsaras J. Model Membrane Systems Used to Study Plasma Membrane Lipid Asymmetry. Symmetry (Basel) 2021; 13. [PMID: 35498375 PMCID: PMC9053528 DOI: 10.3390/sym13081356] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022] Open
Abstract
It is well known that the lipid distribution in the bilayer leaflets of mammalian plasma membranes (PMs) is not symmetric. Despite this, model membrane studies have largely relied on chemically symmetric model membranes for the study of lipid–lipid and lipid–protein interactions. This is primarily due to the difficulty in preparing stable, asymmetric model membranes that are amenable to biophysical studies. However, in the last 20 years, efforts have been made in producing more biologically faithful model membranes. Here, we review several recently developed experimental and computational techniques for the robust generation of asymmetric model membranes and highlight a new and particularly promising technique to study membrane asymmetry.
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Affiliation(s)
- Haden L. Scott
- Large Scale Structures Group, Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
- Shull-Wollan Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
- Correspondence: (H.L.S.); (K.B.K.); (T.A.E.); (M.D.); (J.J.K.); (F.A.H.); (J.K.)
| | - Kristen B. Kennison
- Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA
- Correspondence: (H.L.S.); (K.B.K.); (T.A.E.); (M.D.); (J.J.K.); (F.A.H.); (J.K.)
| | - Thais A. Enoki
- Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA
- Department of Molecular Biology and Genetics, Cornell University, Ithaca, NY 14850, USA
- Correspondence: (H.L.S.); (K.B.K.); (T.A.E.); (M.D.); (J.J.K.); (F.A.H.); (J.K.)
| | - Milka Doktorova
- Department of Molecular Physiology and Biological Physics, University of Virginia School of Medicine, Charlottesville, VA 22903, USA
- Correspondence: (H.L.S.); (K.B.K.); (T.A.E.); (M.D.); (J.J.K.); (F.A.H.); (J.K.)
| | - Jacob J. Kinnun
- Large Scale Structures Group, Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
- Shull-Wollan Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
- Correspondence: (H.L.S.); (K.B.K.); (T.A.E.); (M.D.); (J.J.K.); (F.A.H.); (J.K.)
| | - Frederick A. Heberle
- Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA
- Correspondence: (H.L.S.); (K.B.K.); (T.A.E.); (M.D.); (J.J.K.); (F.A.H.); (J.K.)
| | - John Katsaras
- Shull-Wollan Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
- Sample Environment Group, Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
- Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996, USA
- Correspondence: (H.L.S.); (K.B.K.); (T.A.E.); (M.D.); (J.J.K.); (F.A.H.); (J.K.)
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14
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Pezeshkian W, Marrink SJ. Simulating realistic membrane shapes. Curr Opin Cell Biol 2021; 71:103-111. [PMID: 33721706 DOI: 10.1016/j.ceb.2021.02.009] [Citation(s) in RCA: 28] [Impact Index Per Article: 9.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2021] [Revised: 02/08/2021] [Accepted: 02/11/2021] [Indexed: 12/12/2022]
Abstract
Biological membranes exhibit diversity in their shapes and complexity in chemical compositions that are linked to many cellular functions. These two central features of biomembranes have been the subject of numerous simulation studies, using a diverse range of computational techniques. Currently, the field is able to capture this complexity at increasing levels of realism and connect the microscopic view on protein-lipid interactions to cellular morphologies at the level of entire organelles. Here we highlight recent advances in this topic, identify current bottlenecks, and sketch possible ways ahead.
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Affiliation(s)
- Weria Pezeshkian
- Groningen Biomolecular Sciences and Biotechnology Institute and Zernike Institute for Advanced Materials, University of Groningen, Groningen, the Netherlands
| | - Siewert J Marrink
- Groningen Biomolecular Sciences and Biotechnology Institute and Zernike Institute for Advanced Materials, University of Groningen, Groningen, the Netherlands.
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15
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Khelashvili G, Plante A, Doktorova M, Weinstein H. Ca 2+-dependent mechanism of membrane insertion and destabilization by the SARS-CoV-2 fusion peptide. Biophys J 2021; 120:1105-1119. [PMID: 33631204 PMCID: PMC7899928 DOI: 10.1016/j.bpj.2021.02.023] [Citation(s) in RCA: 40] [Impact Index Per Article: 13.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/03/2020] [Revised: 01/10/2021] [Accepted: 02/02/2021] [Indexed: 01/05/2023] Open
Abstract
Cell penetration after recognition of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus by the ACE2 receptor and the fusion of its viral envelope membrane with cellular membranes are the early steps of infectivity. A region of the Spike protein of the virus, identified as the "fusion peptide" (FP), is liberated at its N-terminal site by a specific cleavage occurring in concert with the interaction of the receptor-binding domain of the Spike. Studies have shown that penetration is enhanced by the required binding of Ca2+ ions to the FPs of coronaviruses, but the mechanisms of membrane insertion and destabilization remain unclear. We have predicted the preferred positions of Ca2+ binding to the SARS-CoV-2-FP, the role of Ca2+ ions in mediating peptide-membrane interactions, the preferred mode of insertion of the Ca2+-bound SARS-CoV-2-FP, and consequent effects on the lipid bilayer from extensive atomistic molecular dynamics simulations and trajectory analyses. In a systematic sampling of the interactions of the Ca2+-bound peptide models with lipid membranes, SARS-CoV-2-FP penetrated the bilayer and disrupted its organization only in two modes involving different structural domains. In one, the hydrophobic residues F833/I834 from the middle region of the peptide are inserted. In the other, more prevalent mode, the penetration involves residues L822/F823 from the LLF motif, which is conserved in CoV-2-like viruses, and is achieved by the binding of Ca2+ ions to the D830/D839 and E819/D820 residue pairs. FP penetration is shown to modify the molecular organization in specific areas of the bilayer, and the extent of membrane binding of the SARS-CoV-2 FP is significantly reduced in the absence of Ca2+ ions. These findings provide novel mechanistic insights regarding the role of Ca2+ in mediating SARS-CoV-2 fusion and provide a detailed structural platform to aid the ongoing efforts in rational design of compounds to inhibit SARS-CoV-2 cell entry.
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Affiliation(s)
- George Khelashvili
- Department of Physiology and Biophysics, Weill Cornell Medical College of Cornell University, New York, New York; Institute for Computational Biomedicine, Weill Cornell Medical College of Cornell University, New York, New York.
| | - Ambrose Plante
- Department of Physiology and Biophysics, Weill Cornell Medical College of Cornell University, New York, New York
| | - Milka Doktorova
- Department of Molecular Physiology and Biological Physics, University of Virginia School of Medicine, Charlottesville, Virginia
| | - Harel Weinstein
- Department of Physiology and Biophysics, Weill Cornell Medical College of Cornell University, New York, New York; Institute for Computational Biomedicine, Weill Cornell Medical College of Cornell University, New York, New York.
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16
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Khelashvili G, Plante A, Doktorova M, Weinstein H. Ca 2+ -dependent mechanism of membrane insertion and destabilization by the SARS-CoV-2 fusion peptide. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2021. [PMID: 33299996 DOI: 10.1101/2020.12.03.410472] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
Cell penetration after recognition of the SARS-CoV-2 virus by the ACE2 receptor, and the fusion of its viral envelope membrane with cellular membranes, are the early steps of infectivity. A region of the Spike protein (S) of the virus, identified as the "fusion peptide" (FP), is liberated at its N-terminal site by a specific cleavage occurring in concert with the interaction of the receptor binding domain of the Spike. Studies have shown that penetration is enhanced by the required binding of Ca 2+ ions to the FPs of corona viruses, but the mechanisms of membrane insertion and destabilization remain unclear. We have predicted the preferred positions of Ca 2+ binding to the SARS-CoV-2-FP, the role of Ca 2+ ions in mediating peptide-membrane interactions, the preferred mode of insertion of the Ca 2+ -bound SARS-CoV-2-FP and consequent effects on the lipid bilayer from extensive atomistic molecular dynamics (MD) simulations and trajectory analyses. In a systematic sampling of the interactions of the Ca 2+ -bound peptide models with lipid membranes SARS-CoV-2-FP penetrated the bilayer and disrupted its organization only in two modes involving different structural domains. In one, the hydrophobic residues F833/I834 from the middle region of the peptide are inserted. In the other, more prevalent mode, the penetration involves residues L822/F823 from the LLF motif which is conserved in CoV-2-like viruses, and is achieved by the binding of Ca 2+ ions to the D830/D839 and E819/D820 residue pairs. FP penetration is shown to modify the molecular organization in specific areas of the bilayer, and the extent of membrane binding of the SARS-CoV-2 FP is significantly reduced in the absence of Ca 2+ ions. These findings provide novel mechanistic insights regarding the role of Ca 2+ in mediating SARS-CoV-2 fusion and provide a detailed structural platform to aid the ongoing efforts in rational design of compounds to inhibit SARS-CoV-2 cell entry. STATEMENT OF SIGNIFICANCE SARS-CoV-2, the cause of the COVID-19 pandemic, penetrates host cell membranes and uses viral-to-cellular membrane fusion to release its genetic material for replication. Experiments had identified a region termed "fusion peptide" (FP) in the Spike proteins of coronaviruses, as the spearhead in these initial processes, and suggested that Ca 2+ is needed to support both functions. Absent structure and dynamics-based mechanistic information these FP functions could not be targeted for therapeutic interventions. We describe the development and determination of the missing information from analysis of extensive MD simulation trajectories, and propose specific Ca 2+ -dependent mechanisms of SARS-CoV-2-FP membrane insertion and destabilization. These results offer a structure-specific platform to aid the ongoing efforts to use this target for the discovery and/or of inhibitors.
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17
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Rosário-Ferreira N, Marques-Pereira C, Gouveia RP, Mourão J, Moreira IS. Guardians of the Cell: State-of-the-Art of Membrane Proteins from a Computational Point-of-View. Methods Mol Biol 2021; 2315:3-28. [PMID: 34302667 DOI: 10.1007/978-1-0716-1468-6_1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
Membrane proteins (MPs) encompass a large family of proteins with distinct cellular functions, and although representing over 50% of existing pharmaceutical drug targets, their structural and functional information is still very scarce. Over the last years, in silico analysis and algorithm development were essential to characterize MPs and overcome some limitations of experimental approaches. The optimization and improvement of these methods remain an ongoing process, with key advances in MPs' structure, folding, and interface prediction being continuously tackled. Herein, we discuss the latest trends in computational methods toward a deeper understanding of the atomistic and mechanistic details of MPs.
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Affiliation(s)
- Nícia Rosário-Ferreira
- Coimbra Chemistry Center, Department of Chemistry, University of Coimbra, Coimbra, Portugal.,Center for Neuroscience and Cell Biology, Center for Innovative Biomedicine and Biotechnology, University of Coimbra, Coimbra, Portugal
| | - Catarina Marques-Pereira
- Center for Neuroscience and Cell Biology, Center for Innovative Biomedicine and Biotechnology, University of Coimbra, Coimbra, Portugal.,PhD Programme in Experimental Biology and Biomedicine, Institute for Interdisciplinary Research (IIIUC), University of Coimbra, Coimbra, Portugal
| | - Raquel P Gouveia
- Center for Neuroscience and Cell Biology, Center for Innovative Biomedicine and Biotechnology, University of Coimbra, Coimbra, Portugal
| | - Joana Mourão
- Center for Neuroscience and Cell Biology, University of Coimbra, Coimbra, Portugal
| | - Irina S Moreira
- Department of Life Sciences, University of Coimbra, Coimbra, Portugal.
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18
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Karlsen ML, Bruhn DS, Pezeshkian W, Khandelia H. Long chain sphingomyelin depletes cholesterol from the cytoplasmic leaflet in asymmetric lipid membranes. RSC Adv 2021; 11:22677-22682. [PMID: 35480443 PMCID: PMC9034350 DOI: 10.1039/d1ra01464a] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/23/2021] [Accepted: 06/16/2021] [Indexed: 12/28/2022] Open
Abstract
The transbilayer distribution of cholesterol (CHL) in complex asymmetric lipid membranes remains controversial, with contrasting investigations suggesting that there is more CHL either in the exoplasmic, outer leaflet (OL) or the cytoplasmic, inner leaflet (IL) depending on cell type or model, membrane composition, and method of investigation. Here, we launch systematic coarse-grained molecular dynamics simulations to investigate the impact of the sphingomyelin (SM) acyl chain length upon CHL distribution in asymmetric lipid membrane mixtures which account for the variation of the most abundant headgroups and acyl chain unsaturation in the two membrane leaflets. We find that there is always more CHL in the OL, but longer chain SM depletes more CHL from the IL than short chain SM in simple membrane mixtures containing SM and 16 : 0, 18 : 1 phospholipids. The difference between longer and shorter chain SM is neutralised in a more complex asymmetric membrane, where there are more saturated tails in the outer leaflet. We propose that interdigitation of long-chain SM into the opposing IL pushes cytoplasmic CHL towards the OL, but higher chain saturation of the outer leaflet compensates for the effect of SM chain length. Long acyl chain sphingomyelin and saturated phospholipid tails in the outer membrane leaflet deplete cholesterol from the inner leaflet in mammalian membranes.![]()
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Affiliation(s)
- Maria Lyngby Karlsen
- PHYLIFE: Physical Life Science
- Department of Physics, Chemistry and Pharmacy
- University of Southern Denmark
- Odense 5230 M
- Denmark
| | - Dennis S. Bruhn
- PHYLIFE: Physical Life Science
- Department of Physics, Chemistry and Pharmacy
- University of Southern Denmark
- Odense 5230 M
- Denmark
| | - Weria Pezeshkian
- PHYLIFE: Physical Life Science
- Department of Physics, Chemistry and Pharmacy
- University of Southern Denmark
- Odense 5230 M
- Denmark
| | - Himanshu Khandelia
- PHYLIFE: Physical Life Science
- Department of Physics, Chemistry and Pharmacy
- University of Southern Denmark
- Odense 5230 M
- Denmark
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19
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Jiang X, Yang K, Han ML, Yuan B, Li J, Gong B, Velkov T, Schreiber F, Wang L, Li J. Outer Membranes of Polymyxin-Resistant Acinetobacter baumannii with Phosphoethanolamine-Modified Lipid A and Lipopolysaccharide Loss Display Different Atomic-Scale Interactions with Polymyxins. ACS Infect Dis 2020; 6:2698-2708. [PMID: 32871077 DOI: 10.1021/acsinfecdis.0c00330] [Citation(s) in RCA: 19] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
Resistance to the last-line polymyxins is increasingly reported in multidrug-resistant Gram-negative pathogens, including Acinetobacter baumannii, which develops resistance via either lipid A modification (e.g., with phosphoethanolamine [pEtN]) or even lipopolysaccharide (LPS) loss in the outer membrane (OM). Considering these two different mechanisms, quantitative membrane lipidomics data were utilized to develop three OM models representing polymyxin-susceptible and -resistant A. baumannii strains. Through all-atom molecular simulations with enhanced sampling techniques, the effect of lipid A-pEtN modification and LPS loss on the action of colistin (i.e., polymyxin E) was examined for the first time, with a focus on the dynamics and energetics of colistin penetration into these OMs. Lipid A-pEtN modification improved the OM stability, impeding the penetration of colistin into the OM; this differed from the current literature that lipid A-pEtN modification confers resistance by diminishing the initial interaction with polymyxins. In contrast, the LPS deficiency significantly reduced the negative charges on the OM surface, diminishing the binding of colistin. Moreover, both lipid A-pEtN modification and LPS loss also constituted colistin resistance through disturbing the conformational transitions of the colistin molecule. Collectively, atomic-scale interactions between polymyxins and different bacterial OMs are very different and the findings may facilitate the discovery of new-generation polymyxins against Gram-negative 'superbugs'.
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Affiliation(s)
- Xukai Jiang
- Biomedicine Discovery Institute, Infection & Immunity Program, Department of Microbiology, Monash University, Melbourne, VIC 3800, Australia
| | - Kai Yang
- Center for Soft Condensed Matter Physics and Interdisciplinary Research, School of Physical Science and Technology, Soochow University, Suzhou 215006, China
| | - Mei-Ling Han
- Biomedicine Discovery Institute, Infection & Immunity Program, Department of Microbiology, Monash University, Melbourne, VIC 3800, Australia
| | - Bing Yuan
- Center for Soft Condensed Matter Physics and Interdisciplinary Research, School of Physical Science and Technology, Soochow University, Suzhou 215006, China
| | - Jingliang Li
- Institute for Frontier Materials, Deakin University, Geelong, VIC 3217, Australia
| | - Bin Gong
- School of Software, Shandong University, Jinan 250101, China
| | - Tony Velkov
- Department of Pharmacology & Therapeutics, University of Melbourne, Melbourne, VIC 3010, Australia
| | - Falk Schreiber
- Department of Computer and Information Science, University of Konstanz, Konstanz 78467, Germany
| | - Lushan Wang
- State Key Laboratory of Microbial Technology, Shandong University, Qingdao 266237, China
| | - Jian Li
- Biomedicine Discovery Institute, Infection & Immunity Program, Department of Microbiology, Monash University, Melbourne, VIC 3800, Australia
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20
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Mohideen N, Weiner MD, Feigenson GW. Bilayer compositional asymmetry influences the nanoscopic to macroscopic phase domain size transition. Chem Phys Lipids 2020; 232:104972. [PMID: 32941827 DOI: 10.1016/j.chemphyslip.2020.104972] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/28/2020] [Revised: 08/18/2020] [Accepted: 09/04/2020] [Indexed: 10/23/2022]
Abstract
The eukaryotic plasma membrane (PM) exhibits lipid mixing heterogeneities known as lipid rafts. These lipid rafts, the result of liquid-liquid phase separation, can be modeled by coexisting liquid ordered (Lo) and liquid disordered (Ld) domains. Four-lipid component systems with a high-melting lipid, a nanodomain-inducing low-melting lipid, a macrodomain-inducing low-melting lipid, and cholesterol (chol) can give rise to domains of different sizes. These four-component systems have been characterized in experiments, yet there are few studies that model the asymmetric distribution of lipids actually found in the PM. We used molecular dynamics (MD) simulations to analyze the transition from nanoscopic to macroscopic domains in symmetric and in asymmetric model membranes. Using coarse-grained MD simulations, we found that asymmetry promotes macroscopic domain growth in a case where symmetric systems exhibit nanoscopic domains. Also, macroscopic domain formation in symmetric systems is highly dependent on registration of like phases in the cytoplasmic and exoplasmic leaflets. Using united-atom MD simulations, we found that symmetric Lo domains are only slightly more ordered than asymmetric Lo domains. We also found that large Lo domains in our asymmetric systems induce a slight chain ordering in the apposed cytoplasmic regions. The chol fractions of phase-separated Lo and Ld domains of the exoplasmic leaflet were unchanged whether the system was symmetric or asymmetric.
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Affiliation(s)
- Naveen Mohideen
- Cornell University Department of Physics, 117 Clark Hall, Ithaca, New York, 14853, United States; Johns Hopkins University Department of Molecular Biophysics, 101 Jenkins Hall, 3400 N. Charles Street, Baltimore, Maryland, 21218, United States.
| | - Michael D Weiner
- Cornell University Department of Physics, 117 Clark Hall, Ithaca, New York, 14853, United States; Georgia Institute of Technology Partnership for an Advanced Computing Environment, 756 W. Peachtree St. NW, Atlanta, Georgia, 30332, United States.
| | - Gerald W Feigenson
- Cornell University Department of Molecular Biology and Genetics, Room 201 215 Tower Rd. Ithaca, New York, 14853, United States.
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21
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Horne JE, Brockwell DJ, Radford SE. Role of the lipid bilayer in outer membrane protein folding in Gram-negative bacteria. J Biol Chem 2020; 295:10340-10367. [PMID: 32499369 PMCID: PMC7383365 DOI: 10.1074/jbc.rev120.011473] [Citation(s) in RCA: 75] [Impact Index Per Article: 18.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/04/2020] [Revised: 06/03/2020] [Indexed: 01/09/2023] Open
Abstract
β-Barrel outer membrane proteins (OMPs) represent the major proteinaceous component of the outer membrane (OM) of Gram-negative bacteria. These proteins perform key roles in cell structure and morphology, nutrient acquisition, colonization and invasion, and protection against external toxic threats such as antibiotics. To become functional, OMPs must fold and insert into a crowded and asymmetric OM that lacks much freely accessible lipid. This feat is accomplished in the absence of an external energy source and is thought to be driven by the high thermodynamic stability of folded OMPs in the OM. With such a stable fold, the challenge that bacteria face in assembling OMPs into the OM is how to overcome the initial energy barrier of membrane insertion. In this review, we highlight the roles of the lipid environment and the OM in modulating the OMP-folding landscape and discuss the factors that guide folding in vitro and in vivo We particularly focus on the composition, architecture, and physical properties of the OM and how an understanding of the folding properties of OMPs in vitro can help explain the challenges they encounter during folding in vivo Current models of OMP biogenesis in the cellular environment are still in flux, but the stakes for improving the accuracy of these models are high. OMP folding is an essential process in all Gram-negative bacteria, and considering the looming crisis of widespread microbial drug resistance it is an attractive target. To bring down this vital OMP-supported barrier to antibiotics, we must first understand how bacterial cells build it.
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Affiliation(s)
- Jim E Horne
- Astbury Centre for Structural Molecular Biology, School of Molecular and Cellular Biology, Faculty of Biological Sciences, University of Leeds, Leeds, United Kingdom
- Department of Biochemistry, University of Oxford, Oxford, United Kingdom
| | - David J Brockwell
- Astbury Centre for Structural Molecular Biology, School of Molecular and Cellular Biology, Faculty of Biological Sciences, University of Leeds, Leeds, United Kingdom
| | - Sheena E Radford
- Astbury Centre for Structural Molecular Biology, School of Molecular and Cellular Biology, Faculty of Biological Sciences, University of Leeds, Leeds, United Kingdom
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22
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Blumer M, Harris S, Li M, Martinez L, Untereiner M, Saeta PN, Carpenter TS, Ingólfsson HI, Bennett WFD. Simulations of Asymmetric Membranes Illustrate Cooperative Leaflet Coupling and Lipid Adaptability. Front Cell Dev Biol 2020; 8:575. [PMID: 32850783 PMCID: PMC7396604 DOI: 10.3389/fcell.2020.00575] [Citation(s) in RCA: 16] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2020] [Accepted: 06/15/2020] [Indexed: 01/31/2023] Open
Abstract
Biological membranes are composed of lipid bilayers that are often asymmetric with regards to the lipid composition and/or aqueous solvent they separate. Studying lipid asymmetry both experimentally and computationally is challenging. Molecular dynamics simulations of lipid bilayers with asymmetry are difficult due to finite system sizes and time scales accessible to simulations. Due to the very slow flip-flop rate for phospholipids, one must first choose how many lipids are on each side of the bilayer, but the resulting bilayer may be unstable (or metastable) due to differing tensile and compressive forces between leaflets. Here we use molecular dynamics simulations to investigate a number of different asymmetric membrane systems, both with atomistic and coarse-grained models. Asymmetries studied include differences in number of lipids, lipid composition (unsaturated and saturated tails and different headgroups), and chemical gradients between the aqueous phases. Extensive analysis of the bilayers' properties such as area per lipid, density, and lateral pressure profiles are used to characterize bilayer asymmetry. We also address how cholesterol (which flip-flops relatively quickly) influences membrane asymmetries. Our results show how each leaflet is influenced by the other and can mitigate the structural changes to the bilayer overall structure. Cholesterol can respond to changes in bilayer asymmetry to alleviate some of the effect on the bilayer structure, but that will alter its leaflet distribution, which in turn affects its chemical potential. Ionic imbalances are shown to have a modest change in bilayer structure, despite large changes in the electrostatic potential. Bilayer asymmetry can also induce a modest electrostatic potential across the membrane. Our results highlight the importance of membrane asymmetry on bilayer properties, the influence of lipid headgroups, tails and cholesterol on asymmetry, and the ability of lipids to adapt to different environments.
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Affiliation(s)
- Madison Blumer
- Harvey Mudd College, Claremont, CA, United States.,Scripps College, Claremont, CA, United States
| | | | - Mengzhe Li
- Harvey Mudd College, Claremont, CA, United States.,Claremont McKenna College, Claremont, CA, United States
| | | | - Michael Untereiner
- Harvey Mudd College, Claremont, CA, United States.,Pomona College, Claremont, CA, United States
| | | | - Timothy S Carpenter
- Biochemical and Biophysical Systems Group, Biosciences and Biotechnology Division, Lawrence Livermore National Laboratory, Livermore, CA, United States
| | - Helgi I Ingólfsson
- Biochemical and Biophysical Systems Group, Biosciences and Biotechnology Division, Lawrence Livermore National Laboratory, Livermore, CA, United States
| | - W F Drew Bennett
- Biochemical and Biophysical Systems Group, Biosciences and Biotechnology Division, Lawrence Livermore National Laboratory, Livermore, CA, United States
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23
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Pezeshkian W, König M, Wassenaar TA, Marrink SJ. Backmapping triangulated surfaces to coarse-grained membrane models. Nat Commun 2020; 11:2296. [PMID: 32385270 PMCID: PMC7210967 DOI: 10.1038/s41467-020-16094-y] [Citation(s) in RCA: 68] [Impact Index Per Article: 17.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/25/2020] [Accepted: 04/07/2020] [Indexed: 12/14/2022] Open
Abstract
Many biological processes involve large-scale changes in membrane shape. Computer simulations of these processes are challenging since they occur across a wide range of spatiotemporal scales that cannot be investigated in full by any single current simulation technique. A potential solution is to combine different levels of resolution through a multiscale scheme. Here, we present a multiscale algorithm that backmaps a continuum membrane model represented as a dynamically triangulated surface (DTS) to its corresponding molecular model based on the coarse-grained (CG) Martini force field. Thus, we can use DTS simulations to equilibrate slow large-scale membrane conformational changes and then explore the local properties at CG resolution. We demonstrate the power of our method by backmapping a vesicular bud induced by binding of Shiga toxin and by transforming the membranes of an entire mitochondrion to near-atomic resolution. Our approach opens the way to whole cell simulations at molecular detail. Computer simulations of large-scale changes in membrane shape are challenging since they occur across a wide range of spatiotemporal scales. Here, authors present a multiscale algorithm that backmaps a continuum membrane model represented as a dynamically triangulated surface to its corresponding molecular model based on the coarse-grained Martini force field.
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Affiliation(s)
- Weria Pezeshkian
- Groningen Biomolecular Sciences and Biotechnology Institute and Zernike Institute for Advanced Materials, University of Groningen, Groningen, Netherlands.
| | - Melanie König
- Groningen Biomolecular Sciences and Biotechnology Institute and Zernike Institute for Advanced Materials, University of Groningen, Groningen, Netherlands
| | - Tsjerk A Wassenaar
- Groningen Biomolecular Sciences and Biotechnology Institute and Zernike Institute for Advanced Materials, University of Groningen, Groningen, Netherlands
| | - Siewert J Marrink
- Groningen Biomolecular Sciences and Biotechnology Institute and Zernike Institute for Advanced Materials, University of Groningen, Groningen, Netherlands.
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24
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Pinigin KV, Kondrashov OV, Jiménez-Munguía I, Alexandrova VV, Batishchev OV, Galimzyanov TR, Akimov SA. Elastic deformations mediate interaction of the raft boundary with membrane inclusions leading to their effective lateral sorting. Sci Rep 2020; 10:4087. [PMID: 32139760 PMCID: PMC7058020 DOI: 10.1038/s41598-020-61110-2] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2019] [Accepted: 02/18/2020] [Indexed: 02/06/2023] Open
Abstract
Liquid-ordered lipid domains represent a lateral inhomogeneity in cellular membranes. These domains have elastic and physicochemical properties different from those of the surrounding membrane. In particular, their thickness exceeds that of the disordered membrane. Thus, elastic deformations arise at the domain boundary in order to compensate for the thickness mismatch. In equilibrium, the deformations lead to an incomplete register of monolayer ordered domains: the elastic energy is minimal if domains in opposing monolayers lie on the top of each other, and their boundaries are laterally shifted by about 3 nm. This configuration introduces a region, composed of one ordered and one disordered monolayers, with an intermediate bilayer thickness. Besides, a jump in a local monolayer curvature takes place in this intermediate region, concentrating here most of the elastic stress. This region can participate in a lateral sorting of membrane inclusions by offering them an optimal bilayer thickness and local curvature conditions. In the present study, we consider the sorting of deformable lipid inclusions, undeformable peripheral and deeply incorporated peptide inclusions, and undeformable transmembrane inclusions of different molecular geometry. With rare exceptions, all types of inclusions have an affinity to the ordered domain boundary as compared to the bulk phases. The optimal lateral distribution of inclusions allows relaxing the elastic stress at the boundary of domains.
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Affiliation(s)
- Konstantin V Pinigin
- A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, 31/4 Leninskiy prospekt, Moscow, 119071, Russia
| | - Oleg V Kondrashov
- A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, 31/4 Leninskiy prospekt, Moscow, 119071, Russia
| | - Irene Jiménez-Munguía
- National University of Science and Technology "MISiS", 4 Leninskiy prospect, Moscow, 119049, Russia
| | | | - Oleg V Batishchev
- A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, 31/4 Leninskiy prospekt, Moscow, 119071, Russia
| | - Timur R Galimzyanov
- A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, 31/4 Leninskiy prospekt, Moscow, 119071, Russia
| | - Sergey A Akimov
- A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, 31/4 Leninskiy prospekt, Moscow, 119071, Russia.
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25
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Hossein A, Deserno M. Spontaneous Curvature, Differential Stress, and Bending Modulus of Asymmetric Lipid Membranes. Biophys J 2019; 118:624-642. [PMID: 31954503 DOI: 10.1016/j.bpj.2019.11.3398] [Citation(s) in RCA: 89] [Impact Index Per Article: 17.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/27/2019] [Revised: 11/20/2019] [Accepted: 11/25/2019] [Indexed: 12/24/2022] Open
Abstract
Lipid bilayers can exhibit asymmetric states, in which the physical characteristics of one leaflet differ from those of the other. This most visibly manifests in a different lipid composition, but it can also involve opposing lateral stresses in each leaflet that combine to an overall vanishing membrane tension. Here, we use theoretical modeling and coarse-grained simulation to explore the interplay between a compositional asymmetry and a nonvanishing differential stress. Minimizing the total elastic energy leads to a preferred spontaneous curvature that balances torques due to both bending moments and differential stress, with sometimes unexpected consequences. For instance, asymmetric flat bilayers, whose specific areas in each leaflet are matched to those of corresponding tensionless symmetric flat membranes, still exhibit a residual differential stress because the conditions of vanishing area strain and vanishing bending moment differ. We also measure the curvature rigidity of asymmetric bilayers and find that a sufficiently strong differential stress, but not compositional asymmetry alone, can increase the bending modulus. The likely cause is a stiffening of the compressed leaflet, which appears to be related to its gel transition but not identical with it. We finally show that the impact of cholesterol on differential stress depends on the relative strength of elastic and thermodynamic driving forces: if cholesterol solvates equally well in both leaflets, it will redistribute to cancel both leaflet tensions almost completely, but if its partitioning free energy prefers one leaflet over the other, the resulting distribution bias may even create differential stress. Because cells keep most of their lipid bilayers in an asymmetric nonequilibrium steady state, our findings suggest that biomembranes are elastically more complex than previously thought: besides a spontaneous curvature, they might also exhibit significant differential stress, which could strongly affect their curvature energetics.
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Affiliation(s)
- Amirali Hossein
- Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania
| | - Markus Deserno
- Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania.
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26
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Lyman E, Sodt AJ. Differential or Curvature Stress? Modus Vivendi. Biophys J 2019; 118:535-537. [PMID: 32023440 DOI: 10.1016/j.bpj.2019.11.3399] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/18/2019] [Accepted: 11/25/2019] [Indexed: 11/15/2022] Open
Affiliation(s)
- Edward Lyman
- Departments of Physics and Astronomy and Chemistry and Biochemistry, University of Delaware, Newark, Delaware.
| | - Alexander J Sodt
- Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland
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27
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Doktorova M. On the Long and Winding Road to a Perfect Membrane Model. Biophys J 2019; 118:273-275. [PMID: 31968235 DOI: 10.1016/j.bpj.2019.11.3386] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/30/2019] [Accepted: 11/08/2019] [Indexed: 01/04/2023] Open
Affiliation(s)
- Milka Doktorova
- Department of Integrative Biology and Pharmacology, University of Texas Health Science Center at Houston, Houston, Texas.
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28
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Marrink SJ, Corradi V, Souza PC, Ingólfsson HI, Tieleman DP, Sansom MS. Computational Modeling of Realistic Cell Membranes. Chem Rev 2019; 119:6184-6226. [PMID: 30623647 PMCID: PMC6509646 DOI: 10.1021/acs.chemrev.8b00460] [Citation(s) in RCA: 408] [Impact Index Per Article: 81.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/23/2018] [Indexed: 12/15/2022]
Abstract
Cell membranes contain a large variety of lipid types and are crowded with proteins, endowing them with the plasticity needed to fulfill their key roles in cell functioning. The compositional complexity of cellular membranes gives rise to a heterogeneous lateral organization, which is still poorly understood. Computational models, in particular molecular dynamics simulations and related techniques, have provided important insight into the organizational principles of cell membranes over the past decades. Now, we are witnessing a transition from simulations of simpler membrane models to multicomponent systems, culminating in realistic models of an increasing variety of cell types and organelles. Here, we review the state of the art in the field of realistic membrane simulations and discuss the current limitations and challenges ahead.
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Affiliation(s)
- Siewert J. Marrink
- Groningen
Biomolecular Sciences and Biotechnology Institute & Zernike Institute
for Advanced Materials, University of Groningen, Nijenborgh 7, 9747 AG Groningen, The Netherlands
| | - Valentina Corradi
- Centre
for Molecular Simulation and Department of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada
| | - Paulo C.T. Souza
- Groningen
Biomolecular Sciences and Biotechnology Institute & Zernike Institute
for Advanced Materials, University of Groningen, Nijenborgh 7, 9747 AG Groningen, The Netherlands
| | - Helgi I. Ingólfsson
- Biosciences
and Biotechnology Division, Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, United States
| | - D. Peter Tieleman
- Centre
for Molecular Simulation and Department of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada
| | - Mark S.P. Sansom
- Department
of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K.
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29
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Shahane G, Ding W, Palaiokostas M, Orsi M. Physical properties of model biological lipid bilayers: insights from all-atom molecular dynamics simulations. J Mol Model 2019; 25:76. [PMID: 30806797 DOI: 10.1007/s00894-019-3964-0] [Citation(s) in RCA: 78] [Impact Index Per Article: 15.6] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/19/2018] [Accepted: 02/07/2019] [Indexed: 02/04/2023]
Abstract
The physical properties of lipid bilayers are sensitive to the specific type and composition of the lipids that make up the many different types of cell membranes. Studying model bilayers of representative heterogeneous compositions can provide key insights into membrane functionality. In this work, we use atomistic molecular dynamics simulations to characterize key properties in a number of bilayer membranes of varying composition. We first examine basic properties, such as lipid area, volume, and bilayer thickness, of simple, homogeneous bilayers comprising several lipid types, which are prevalent in biological membranes. Such lipids are then used in simulations of heterogeneous systems representative of bacterial, mammalian, and cancer membranes. Our analysis is especially focused on depth-dependent, transmembrane profiles; in particular, we calculate lateral pressure and dipole potential profiles, two fundamental properties which play key roles in a large number of biological functions.
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Affiliation(s)
- Ganesh Shahane
- Institute of Bioengineering, Queen Mary University of London, Mile End Road, London, E1 4NS, UK
| | - Wei Ding
- School of Engineering & Materials Science, Queen Mary University of London, Mile End Road, London, E1 4NS, UK
| | - Michail Palaiokostas
- School of Engineering & Materials Science, Queen Mary University of London, Mile End Road, London, E1 4NS, UK
| | - Mario Orsi
- Department of Applied Sciences, University of the West of England, Coldharbour Lane, Bristol, BS16 1QY, UK.
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30
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Doktorova M, Heberle FA, Marquardt D, Rusinova R, Sanford RL, Peyear TA, Katsaras J, Feigenson GW, Weinstein H, Andersen OS. Gramicidin Increases Lipid Flip-Flop in Symmetric and Asymmetric Lipid Vesicles. Biophys J 2019; 116:860-873. [PMID: 30755300 PMCID: PMC6400823 DOI: 10.1016/j.bpj.2019.01.016] [Citation(s) in RCA: 25] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/02/2018] [Revised: 01/03/2019] [Accepted: 01/09/2019] [Indexed: 01/06/2023] Open
Abstract
Unlike most transmembrane proteins, phospholipids can migrate from one leaflet of the membrane to the other. Because this spontaneous lipid translocation (flip-flop) tends to be very slow, cells facilitate the process with enzymes that catalyze the transmembrane movement and thereby regulate the transbilayer lipid distribution. Nonenzymatic membrane-spanning proteins with unrelated primary functions have also been found to accelerate lipid flip-flop in a nonspecific manner and by various hypothesized mechanisms. Using deuterated phospholipids, we examined the acceleration of flip-flop by gramicidin channels, which have well-defined structures and known functions, features that make them ideal candidates for probing the protein-membrane interactions underlying lipid flip-flop. To study compositionally and isotopically asymmetric proteoliposomes containing gramicidin, we expanded a recently developed protocol for the preparation and characterization of lipid-only asymmetric vesicles. Channel incorporation, conformation, and function were examined with small angle x-ray scattering, circular dichroism, and a stopped-flow spectrofluorometric assay, respectively. As a measure of lipid scrambling, we used differential scanning calorimetry to monitor the effect of gramicidin on the melting transition temperatures of the two bilayer leaflets. The two calorimetric peaks of the individual leaflets merged into a single peak over time, suggestive of scrambling, and the effect of the channel on the transbilayer lipid distribution in both symmetric 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and asymmetric 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/1,2-dimyristoyl-sn-glycero-3-phosphocholine vesicles was quantified from proton NMR measurements. Our results show that gramicidin increases lipid flip-flop in a complex, concentration-dependent manner. To determine the molecular mechanism of the process, we used molecular dynamics simulations and further computational analysis of the trajectories to estimate the extent of membrane deformation. Together, the experimental and computational approaches were found to constitute an effective means for studying the effects of transmembrane proteins on lipid distribution in both symmetric and asymmetric model membranes.
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Affiliation(s)
- Milka Doktorova
- Tri-Institutional PhD Program in Computational Biology and Medicine, Weill Cornell Medical College, New York, New York.
| | - Frederick A Heberle
- Department of Integrative Biology and Pharmacology, University of Texas Health Science Center, Houston, Texas; The Bredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee, Knoxville, Tennessee
| | | | - Radda Rusinova
- Department of Physiology and Biophysics, Weill Cornell Medical College, New York, New York
| | - R Lea Sanford
- Department of Physiology and Biophysics, Weill Cornell Medical College, New York, New York
| | - Thasin A Peyear
- Department of Physiology and Biophysics, Weill Cornell Medical College, New York, New York
| | - John Katsaras
- Large Scale Structures Group, Oak Ridge National Laboratory, Oak Ridge, Tennessee, Oak Ridge National Laboratory, Oak Ridge, Tennessee; Shull Wollan Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee
| | - Gerald W Feigenson
- Department of Molecular Biology and Genetics, Cornell University, Ithaca, New York
| | - Harel Weinstein
- Department of Physiology and Biophysics, Weill Cornell Medical College, New York, New York; The HRH Prince Alwaleed Bin Talal Bin Abdulaziz Alsaud Institute for Computational Biomedicine, Weill Greenberg Center, New York, New York
| | - Olaf S Andersen
- Department of Physiology and Biophysics, Weill Cornell Medical College, New York, New York
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