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Shendrik P, Golani G, Dharan R, Schwarz US, Sorkin R. Membrane Tension Inhibits Lipid Mixing by Increasing the Hemifusion Stalk Energy. ACS NANO 2023; 17:18942-18951. [PMID: 37669531 PMCID: PMC7615193 DOI: 10.1021/acsnano.3c04293] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/14/2023] [Accepted: 08/23/2023] [Indexed: 09/07/2023]
Abstract
Fusion of biological membranes is fundamental in various physiological events. The fusion process involves several intermediate stages with energy barriers that are tightly dependent on the mechanical and physical properties of the system, one of which is membrane tension. As previously established, the late stages of fusion, including hemifusion diaphragm and pore expansions, are favored by membrane tension. However, a current understanding of how the energy barrier of earlier fusion stages is affected by membrane tension is lacking. Here, we apply a newly developed experimental approach combining micropipette-aspirated giant unilamellar vesicles and optically trapped membrane-coated beads, revealing that membrane tension inhibits lipid mixing. We show that lipid mixing is 6 times slower under a tension of 0.12 mN/m compared with tension-free membranes. Furthermore, using continuum elastic theory, we calculate the dependence of the hemifusion stalk formation energy on membrane tension and intermembrane distance and find the increase in the corresponding energy barrier to be 1.6 kBT in our setting, which can explain the increase in lipid mixing time delay. Finally, we show that tension can be a significant factor in the stalk energy if the pre-fusion intermembrane distance is on the order of several nanometers, while for membranes that are tightly docked, tension has a negligible effect.
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Affiliation(s)
- Petr Shendrik
- School
of Chemistry, Raymond & Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, 6997801, Israel
- Center
of Physics and Chemistry of Living Systems, Tel Aviv University, Tel Aviv, 6997801, Israel
| | - Gonen Golani
- Institute
for Theoretical Physics and BioQuant Center for Quantitative Biology, Heidelberg University, 69120, Heidelberg, Germany
| | - Raviv Dharan
- School
of Chemistry, Raymond & Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, 6997801, Israel
- Center
of Physics and Chemistry of Living Systems, Tel Aviv University, Tel Aviv, 6997801, Israel
| | - Ulrich S. Schwarz
- Institute
for Theoretical Physics and BioQuant Center for Quantitative Biology, Heidelberg University, 69120, Heidelberg, Germany
| | - Raya Sorkin
- School
of Chemistry, Raymond & Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, 6997801, Israel
- Center
of Physics and Chemistry of Living Systems, Tel Aviv University, Tel Aviv, 6997801, Israel
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2
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Golani G, Schwarz US. High curvature promotes fusion of lipid membranes: Predictions from continuum elastic theory. Biophys J 2023; 122:1868-1882. [PMID: 37077047 PMCID: PMC10209146 DOI: 10.1016/j.bpj.2023.04.018] [Citation(s) in RCA: 5] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2022] [Revised: 03/19/2023] [Accepted: 04/14/2023] [Indexed: 04/21/2023] Open
Abstract
The fusion of lipid membranes progresses through a series of hemifusion intermediates with two significant energy barriers related to the formation of stalk and fusion pore, respectively. These energy barriers determine the speed and success rate of many critical biological processes, including the fusion of highly curved membranes, for example synaptic vesicles and enveloped viruses. Here we use continuum elastic theory of lipid monolayers to determine the relationship between membrane shape and energy barriers to fusion. We find that the stalk formation energy decreases with curvature by up to 31 kBT in a 20-nm-radius vesicle compared with planar membranes and by up to 8 kBT in the fusion of highly curved, long, tubular membranes. In contrast, the fusion pore formation energy barrier shows a more complicated behavior. Immediately after stalk expansion to the hemifusion diaphragm, the fusion pore formation energy barrier is low (15-25 kBT) due to lipid stretching in the distal monolayers and increased tension in highly curved vesicles. Therefore, the opening of the fusion pore is faster. However, these stresses relax over time due to lipid flip-flop from the proximal monolayer, resulting in a larger hemifusion diaphragm and a higher fusion pore formation energy barrier, up to 35 kBT. Therefore, if the fusion pore fails to open before significant lipid flip-flop takes place, the reaction proceeds to an extended hemifusion diaphragm state, which is a dead-end configuration in the fusion process and can be used to prevent viral infections. In contrast, in the fusion of long tubular compartments, the surface tension does not accumulate due to the formation of the diaphragm, and the energy barrier for pore expansion increases with curvature by up to 11 kBT. This suggests that inhibition of polymorphic virus infection could particularly target this feature of the second barrier.
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Affiliation(s)
- Gonen Golani
- Institute for Theoretical Physics and BioQuant Center for Quantitative Biology, Heidelberg University, Heidelberg, Germany
| | - Ulrich S Schwarz
- Institute for Theoretical Physics and BioQuant Center for Quantitative Biology, Heidelberg University, Heidelberg, Germany.
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3
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Poojari CS, Scherer KC, Hub JS. Free energies of membrane stalk formation from a lipidomics perspective. Nat Commun 2021; 12:6594. [PMID: 34782611 PMCID: PMC8593120 DOI: 10.1038/s41467-021-26924-2] [Citation(s) in RCA: 27] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/03/2021] [Accepted: 10/18/2021] [Indexed: 11/21/2022] Open
Abstract
Many biological membranes are asymmetric and exhibit complex lipid composition, comprising hundreds of distinct chemical species. Identifying the biological function and advantage of this complexity is a central goal of membrane biology. Here, we study how membrane complexity controls the energetics of the first steps of membrane fusions, that is, the formation of a stalk. We first present a computationally efficient method for simulating thermodynamically reversible pathways of stalk formation at coarse-grained resolution. The method reveals that the inner leaflet of a typical plasma membrane is far more fusogenic than the outer leaflet, which is likely an adaptation to evolutionary pressure. To rationalize these findings by the distinct lipid compositions, we computed ~200 free energies of stalk formation in membranes with different lipid head groups, tail lengths, tail unsaturations, and sterol content. In summary, the simulations reveal a drastic influence of the lipid composition on stalk formation and a comprehensive fusogenicity map of many biologically relevant lipid classes. Fusion of cellular membranes begins with the formation of a stalk. Here, the authors develop a computationally efficient method for coarse-grained simulations of stalk formation and apply this approach to comprehensively analyse how stalk formation is influenced by the membrane lipid composition.
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Affiliation(s)
- Chetan S Poojari
- Saarland University, Theoretical Physics and Center for Biophysics, Saarland University, Saarbrücken, Germany
| | - Katharina C Scherer
- Saarland University, Theoretical Physics and Center for Biophysics, Saarland University, Saarbrücken, Germany
| | - Jochen S Hub
- Saarland University, Theoretical Physics and Center for Biophysics, Saarland University, Saarbrücken, Germany.
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4
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Akimov SA, Molotkovsky RJ, Kuzmin PI, Galimzyanov TR, Batishchev OV. Continuum Models of Membrane Fusion: Evolution of the Theory. Int J Mol Sci 2020; 21:E3875. [PMID: 32485905 PMCID: PMC7312925 DOI: 10.3390/ijms21113875] [Citation(s) in RCA: 21] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2020] [Revised: 05/24/2020] [Accepted: 05/27/2020] [Indexed: 12/18/2022] Open
Abstract
Starting from fertilization, through tissue growth, hormone secretion, synaptic transmission, and sometimes morbid events of carcinogenesis and viral infections, membrane fusion regulates the whole life of high organisms. Despite that, a lot of fusion processes still lack well-established models and even a list of main actors. A merger of membranes requires their topological rearrangements controlled by elastic properties of a lipid bilayer. That is why continuum models based on theories of membrane elasticity are actively applied for the construction of physical models of membrane fusion. Started from the view on the membrane as a structureless film with postulated geometry of fusion intermediates, they developed along with experimental and computational techniques to a powerful tool for prediction of the whole process with molecular accuracy. In the present review, focusing on fusion processes occurring in eukaryotic cells, we scrutinize the history of these models, their evolution and complication, as well as open questions and remaining theoretical problems. We show that modern approaches in this field allow continuum models of membrane fusion to stand shoulder to shoulder with molecular dynamics simulations, and provide the deepest understanding of this process in multiple biological systems.
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Affiliation(s)
- Sergey A. Akimov
- Laboratory of Bioelectrochemistry, A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, 31/4 Leninskiy Prospekt, 119071 Moscow, Russia; (R.J.M.); (P.I.K.); (T.R.G.); (O.V.B.)
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5
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Functional role of PGAM5 multimeric assemblies and their polymerization into filaments. Nat Commun 2019; 10:531. [PMID: 30705304 PMCID: PMC6355839 DOI: 10.1038/s41467-019-08393-w] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/02/2018] [Accepted: 01/08/2019] [Indexed: 01/01/2023] Open
Abstract
PGAM5 is a mitochondrial protein phosphatase whose genetic ablation in mice results in mitochondria-related disorders, including neurodegeneration. Functions of PGAM5 include regulation of mitophagy, cell death, metabolism and aging. However, mechanisms regulating PGAM5 activation and signaling are poorly understood. Using electron cryo-microscopy, we show that PGAM5 forms dodecamers in solution. We also present a crystal structure of PGAM5 that reveals the determinants of dodecamer formation. Furthermore, we observe PGAM5 dodecamer assembly into filaments both in vitro and in cells. We find that PGAM5 oligomerization into a dodecamer is not only essential for catalytic activation, but this form also plays a structural role on mitochondrial membranes, which is independent of phosphatase activity. Together, these findings suggest that modulation of the oligomerization of PGAM5 may be a regulatory switch of potential therapeutic interest. PGAM5 is a mitochondrial protein phosphatase whose functions include regulation of mitophagy and cell death. Here, the authors use x-ray crystallography and EM to show that PGAM5 forms dodecameric rings and filaments in solution, and find that PGAM5 rings are essential for catalysis and for a structural effect PGAM5 has on mitochondrial membranes, independently of catalytic activity.
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6
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Thermodynamically reversible paths of the first fusion intermediate reveal an important role for membrane anchors of fusion proteins. Proc Natl Acad Sci U S A 2019; 116:2571-2576. [PMID: 30700547 DOI: 10.1073/pnas.1818200116] [Citation(s) in RCA: 46] [Impact Index Per Article: 9.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
Biological membrane fusion proceeds via an essential topological transition of the two membranes involved. Known players such as certain lipid species and fusion proteins are generally believed to alter the free energy and thus the rate of the fusion reaction. Quantifying these effects by theory poses a major challenge since the essential reaction intermediates are collective, diffusive and of a molecular length scale. We conducted molecular dynamics simulations in conjunction with a state-of-the-art string method to resolve the minimum free-energy path of the first fusion intermediate state, the so-called stalk. We demonstrate that the isolated transmembrane domains (TMDs) of fusion proteins such as SNARE molecules drastically lower the free energy of both the stalk barrier and metastable stalk, which is not trivially explained by molecular shape arguments. We relate this effect to the local thinning of the membrane (negative hydrophobic mismatch) imposed by the TMDs which favors the nearby presence of the highly bent stalk structure or prestalk dimple. The distance between the membranes is the most crucial determinant of the free energy of the stalk, whereas the free-energy barrier changes only slightly. Surprisingly, fusion enhancing lipids, i.e., lipids with a negative spontaneous curvature, such as PE lipids have little effect on the free energy of the stalk barrier, likely because of its single molecular nature. In contrast, the lipid shape plays a crucial role in overcoming the hydration repulsion between two membranes and thus rather lowers the total work required to form a stalk.
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7
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Pannuzzo M, McDargh ZA, Deserno M. The role of scaffold reshaping and disassembly in dynamin driven membrane fission. eLife 2018; 7:39441. [PMID: 30561335 PMCID: PMC6355196 DOI: 10.7554/elife.39441] [Citation(s) in RCA: 30] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/21/2018] [Accepted: 12/13/2018] [Indexed: 12/11/2022] Open
Abstract
The large GTPase dynamin catalyzes membrane fission in eukaryotic cells, but despite three decades of experimental work, competing and partially conflicting models persist regarding some of its most basic actions. Here we investigate the mechanical and functional consequences of dynamin scaffold shape changes and disassembly with the help of a geometrically and elastically realistic simulation model of helical dynamin-membrane complexes. Beyond changes of radius and pitch, we emphasize the crucial role of a third functional motion: an effective rotation of the filament around its longitudinal axis, which reflects alternate tilting of dynamin’s PH binding domains and creates a membrane torque. We also show that helix elongation impedes fission, hemifission is reached via a small transient pore, and coat disassembly assists fission. Our results have several testable structural consequences and help to reconcile mutual conflicting aspects between the two main present models of dynamin fission—the two-stage and the constrictase model. When cells take up material from their surroundings, they must first transport this cargo across their outer membrane, a flexible sheet of tightly organized fat molecules that act as a barrier to the environment. Cells can achieve this by letting their membrane surround the object, pulling it inwards until it is contained in a pouch that bulges into the cell. This bag is then corded up so it splits off from the outer membrane. The ‘cord’ is a protein called dynamin, which is thought to form a tight spiral around the bag’s neck, closing it over and pinching it away. The structure of dynamin is fairly well known, and yet several theories compete to explain how it may snap the bag off the outer membrane. Here, Pannuzzo et al. have created a computer simulation that faithfully replicates the geometry and the elasticity of the membrane and of dynamin, and used it to test different ways the protein could work. The first test featured simple constriction, where the dynamin spiral contracts around the membrane to pinch it; this only separated the bag from the membrane after implausibly tight constriction. The second test added elongation, with the spiral lengthening as well as reducing its diameter, but this further reduced the ability for the protein to snap off the membrane. The final test combined constriction and rotation, whereby dynamin ‘twirls’ as it presses on the neck of the bag: this succeeded in efficiently severing the membrane once the dynamin spiral disassembled. Indeed, the simulations suggested that dynamin might start to dismantle while it constricts, without compromising its role. In fact, getting rid of excess length as the protein contracts helps to dissolve any remnants of a membrane connection. Defects in dynamin are associated with conditions such as centronuclear myopathy and Charcot‐Marie‐Tooth peripheral neuropathy. Recent research also indicates that the protein is involved in a much wider range of neurological disorders that include Alzheimer's, Parkinson's, Huntington's, and amyotrophic lateral sclerosis. The models created by Pannuzzo et al. are useful tools to understand how dynamin and similar proteins work and sometimes fail.
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Affiliation(s)
- Martina Pannuzzo
- Department of Physics, Carnegie Mellon University, Pittsburgh, United States
| | - Zachary A McDargh
- Department of Physics, Carnegie Mellon University, Pittsburgh, United States
| | - Markus Deserno
- Department of Physics, Carnegie Mellon University, Pittsburgh, United States
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8
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Bu B, Crowe M, Diao J, Ji B, Li D. Cholesterol suppresses membrane leakage by decreasing water penetrability. SOFT MATTER 2018; 14:5277-5282. [PMID: 29896597 DOI: 10.1039/c8sm00644j] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Membrane fusion is a fundamental biological process that lies at the heart of enveloped virus infection, synaptic signaling, intracellular vesicle trafficking, gamete fertilization, and cell-cell fusion. Membrane fusion is initiated as two apposed membranes merge to a single bilayer called a hemifusion diaphragm. It is believed that the contents of the two fusing membranes are released through a fusion pore formed at the hemifusion diaphragm, and yet another possible pathway has been proposed in which an undefined pore may form outside the hemifusion diaphragm at the apposed membranes, leading to the so-called leaky fusion. Here, we performed all-atom molecular dynamics simulations to study the evolution of the hemifusion diaphragm structure with various lipid compositions. We found that the lipid cholesterol decreased water penetrability to inhibit leakage pore formation. Biochemical leakage experiments support these simulation results. This study may shed light on the underlying mechanism of the evolution pathways of the hemifusion structure, especially the understanding of content leakage during membrane fusion.
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Affiliation(s)
- Bing Bu
- Biomechanics and Biomaterials Laboratory, Department of Applied Mechanics, Beijing Institute of Technology, Beijing 100081, China.
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9
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Liu X, Tian F, Yue T, Zhang X, Zhong C. Pulling force and surface tension drive membrane fusion. J Chem Phys 2017; 147:194703. [PMID: 29166098 DOI: 10.1063/1.4997393] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Despite catalyzed by fusion proteins of quite different molecular architectures, intracellular, viral, and cell-to-cell fusions are found to have the essential common features and the nearly same nature of transition states. The similarity inspires us to find a more general catalysis mechanism for membrane fusion that minimally depends on the specific structures of fusion proteins. In this work, we built a minimal model for membrane fusion, and by using dissipative particle dynamics simulations, we propose a mechanism that the pulling force generated by fusion proteins initiates the fusion process and the membrane tension regulates the subsequent fusion stages. The model shows different features compared to previous computer simulation studies: the pulling force catalyzes membrane fusion through lipid head overcrowding in the contacting region, leading to an increase in the head-head repulsion and/or the unfavorable head-tail contacts from opposing membranes, both of which destabilize the contacting leaflets and thus promote membrane fusion or vesicle rupture. Our simulations produce a variety of shapes and intermediates, closely resembling cases seen experimentally. Our work strongly supports the view that the tight pulling mechanism is a conserved feature of fusion protein-mediated fusion and that the membrane tension plays an essential role in fusion.
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Affiliation(s)
- Xuejuan Liu
- State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People's Republic of China
| | - Falin Tian
- State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People's Republic of China
| | - Tongtao Yue
- State Key Laboratory of Heavy Oil Processing, Center for Bioengineering and Biotechnology, China University of Petroleum (East China), Qingdao 266580, People's Republic of China
| | - Xianren Zhang
- State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People's Republic of China
| | - Chongli Zhong
- State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People's Republic of China
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10
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Smirnova YG, Müller M. Calculation of membrane bending rigidity using field-theoretic umbrella sampling. J Chem Phys 2016; 143:243155. [PMID: 26723640 DOI: 10.1063/1.4938383] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
The free-energy change of membrane shape transformations can be small, e.g., as in the case of membrane bending. Therefore, the calculation of the free-energy difference between different membrane morphologies is a challenge. Here, we discuss a computational method - field-theoretic umbrella sampling - to compute the local chemical potential of a non-equilibrium configuration and illustrate how one can apply it to study free-energy changes of membrane transformations using simulations. Specifically, the chemical potential profile of the bent membrane and the bending rigidity of membrane are calculated for a soft, coarse-grained amphiphile model and the MARTINI model of a dioleoylphosphatidylcholine (DOPC) membrane.
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Affiliation(s)
- Y G Smirnova
- Institute for Theoretical Physics, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
| | - M Müller
- Institute for Theoretical Physics, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
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11
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Truszkowski A, van den Broek K, Kuhn H, Zielesny A, Epple M. Mesoscopic Simulation of Phospholipid Membranes, Peptides, and Proteins with Molecular Fragment Dynamics. J Chem Inf Model 2015; 55:983-97. [DOI: 10.1021/ci5006096] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Affiliation(s)
- Andreas Truszkowski
- Inorganic
Chemistry and Center for Nanointegration Duisburg−Essen (CENIDE), University of Duisburg−Essen, 45141 Essen, Germany
- Institute
for Bioinformatics and Cheminformatics, Westphalian University of Applied Sciences, 45665 Recklinghausen, Germany
| | - Karina van den Broek
- Department
of Pharmacy−Center for Drug Research, Ludwig-Maximilians University Munich, 80539 Munich, Germany
| | - Hubert Kuhn
- Inorganic
Chemistry and Center for Nanointegration Duisburg−Essen (CENIDE), University of Duisburg−Essen, 45141 Essen, Germany
- CAM-D Technologies, 45127 Essen, Germany
| | - Achim Zielesny
- Institute
for Bioinformatics and Cheminformatics, Westphalian University of Applied Sciences, 45665 Recklinghausen, Germany
| | - Matthias Epple
- Inorganic
Chemistry and Center for Nanointegration Duisburg−Essen (CENIDE), University of Duisburg−Essen, 45141 Essen, Germany
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12
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Dehghan A, Pastor KA, Shi AC. Line tension of multicomponent bilayer membranes. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2015; 91:022713. [PMID: 25768537 DOI: 10.1103/physreve.91.022713] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/26/2014] [Indexed: 06/04/2023]
Abstract
The line tension or edge energy of bilayer membranes self-assembled from binary amphiphilic molecules is studied using self-consistent-field theory (SCFT). Specifically, solutions of the SCFT equations corresponding to an infinite membrane with a circular pore, or an open membrane, are obtained for a coarse-grained model in which the amphiphilic species and hydrophilic solvents are represented by ABandED diblock copolymers and C homopolymers, respectively. The edge energy of the membrane is extracted from the free energy of the open membranes. Results for membranes composed of mixtures of symmetric and cone- or inverse cone-shaped amphiphilic molecules with neutral and/or repulsive interactions are obtained and analyzed. It is observed that an increase in the concentration of the cone-shaped species leads to a decrease of the line tension. In contrast, adding inverse cone-shaped copolymers results in an increase of the line tension. Furthermore, the density profile of the copolymers reveals that the line tension is regulated by the distribution of the amphiphiles at the bilayer edge.
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Affiliation(s)
- Ashkan Dehghan
- Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, Canada L8S 4M1
| | - Kyle A Pastor
- Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, Canada L8S 4M1
| | - An-Chang Shi
- Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, Canada L8S 4M1
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13
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Fuhrmans M, Marelli G, Smirnova YG, Müller M. Mechanics of membrane fusion/pore formation. Chem Phys Lipids 2015; 185:109-28. [DOI: 10.1016/j.chemphyslip.2014.07.010] [Citation(s) in RCA: 39] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/27/2014] [Revised: 07/07/2014] [Accepted: 07/24/2014] [Indexed: 11/27/2022]
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14
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Akimov SA, Molotkovsky RJ, Galimzyanov TR, Radaev AV, Shilova LA, Kuzmin PI, Batishchev OV, Voronina GF, Chizmadzhev YA. Model of membrane fusion: Continuous transition to fusion pore with regard of hydrophobic and hydration interactions. BIOCHEMISTRY (MOSCOW) SUPPLEMENT SERIES A: MEMBRANE AND CELL BIOLOGY 2014. [DOI: 10.1134/s1990747814010024] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
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15
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Lin YL, Chang HY, Sheng YJ, Tsao HK. The fusion mechanism of small polymersomes formed by rod-coil diblock copolymers. SOFT MATTER 2014; 10:1500-1511. [PMID: 24652278 DOI: 10.1039/c3sm52387j] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
The fusion mechanism of polymersomes self-assembled by rod-coil copolymers is intrinsically different from that of liposomes due to the effect of chain topology on conformational entropy and molecular packing. The influences of membrane tension, coil-block length, rod-block length, mutual compatibility between the solvent and the rod-coil block, and π-π interaction strength on the fusion pathway are explored by dissipative particle dynamics. The fusion process of spontaneously formed polymersomes generally consists of four stages. In the kissing stage, hopping of rod-blocks forms a connection between two vesicles of a one-legged rod-coil copolymer. In the adhesion stage, a stalk is developed by a few link-up rods and then a stretched diaphragm with rods lying parallel to the stretching direction is formed in the hemi-fusion stage. Eventually, a pore is developed and expanded in the fusion stage. If the membrane tension (τ) is adjusted by deflation/inflation of the polymersomes, the hemi-fusion diaphragm disappears. As τ is reduced, multiple stalks take shape and lead to the formation of inverted micelles, which is the rate-determining step and raises the fusion time substantially. As τ is elevated, a neck is developed after the stalk formation. The fusion process is significantly accelerated. τ of spontaneously formed vesicles varies with the coil-block length, rod-block length, solvent quality, and π-π interaction strength. There exists a critical value of τ below which the fusion process cannot be completed and a hemi-fused polymersome is formed. In addition to τ, the anisotropic steric interactions within the rod layers also resist hopping of longer rod-blocks. The coil layers develop a barrier impeding fusion between vesicles with longer coil-blocks. Consequently, lowering the solvent quality for the coil-block or rod-block facilities the fusion process due to the formation of a thinner coil layer.
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Affiliation(s)
- Yung-Lung Lin
- Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106, Republic of China.
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16
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Chang HY, Lin YL, Sheng YJ, Tsao HK. Structural Characteristics and Fusion Pathways of Onion-Like Multilayered Polymersome Formed by Amphiphilic Comb-Like Graft Copolymers. Macromolecules 2013. [DOI: 10.1021/ma400667n] [Citation(s) in RCA: 51] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023]
Affiliation(s)
- Hung-Yu Chang
- Department
of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106, R.O.C
| | - Yung-Lung Lin
- Department
of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106, R.O.C
| | - Yu-Jane Sheng
- Department
of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106, R.O.C
| | - Heng-Kwong Tsao
- Department of Chemical and Materials Engineering, Department
of Physics, National Central University, Jhongli, Taiwan 320, R.O.C
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17
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Study of the micelle-to-vesicle transition and smallest possible vesicle size by temperature-jumps. J Colloid Interface Sci 2013; 396:173-7. [DOI: 10.1016/j.jcis.2012.12.070] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/08/2012] [Revised: 12/26/2012] [Accepted: 12/27/2012] [Indexed: 11/23/2022]
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18
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Papanicolaou KN, Phillippo MM, Walsh K. Mitofusins and the mitochondrial permeability transition: the potential downside of mitochondrial fusion. Am J Physiol Heart Circ Physiol 2012; 303:H243-55. [PMID: 22636681 DOI: 10.1152/ajpheart.00185.2012] [Citation(s) in RCA: 43] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
Mitofusins (Mfn-1 and Mfn-2) are transmembrane proteins that bind and hydrolyze guanosine 5'-triphosphate to bring about the merging of adjacent mitochondrial membranes. This event is necessary for mitochondrial fusion, a biological process that is critical for organelle function. The broad effects of mitochondrial fusion on cell bioenergetics have been extensively studied, whereas the local effects of mitofusin activity on the structure and integrity of the fusing mitochondrial membranes have received relatively little attention. From the study of fusogenic proteins, theoretical models, and simulations, it has been noted that the fusion of biological membranes is associated with local perturbations on the integrity of the membrane that present in the form of lipidic holes which open on the opposing bilayers. These lipidic holes represent obligate intermediates that make the fusion process thermodynamically more favorable and at the same time induce leakage to the fusing membranes. In this perspectives article we present the relevant evidence selected from a spectrum of membrane fusion/leakage models and attempt to couple this information with observations conducted with cardiac myocytes or mitochondria deficient in Mfn-1 and Mfn-2. More specifically, we argue in favor of a situation whereby mitochondrial fusion in cardiac myocytes is coupled with outer mitochondrial membrane destabilization that is opportunistically employed during the process of mitochondrial permeability transition. We hope that these insights will initiate research on this new hypothesis of mitochondrial permeability transition regulation, a poorly understood mitochondrial function with significant consequences on myocyte survival.
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Affiliation(s)
- Kyriakos N Papanicolaou
- Whitaker Cardiovascular Institute, Boston University School of Medicine, Massachusetts, 02118, USA
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Fuhrmans M, Marrink SJ. Molecular View of the Role of Fusion Peptides in Promoting Positive Membrane Curvature. J Am Chem Soc 2012; 134:1543-52. [DOI: 10.1021/ja207290b] [Citation(s) in RCA: 65] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Marc Fuhrmans
- Groningen Biomolecular Sciences and Biotechnology Institute & Zernike Institute for Advanced Materials, University of Groningen, Groningen, The Netherlands
- Universität Göttingen, Göttingen, Germany
| | - Siewert J. Marrink
- Groningen Biomolecular Sciences and Biotechnology Institute & Zernike Institute for Advanced Materials, University of Groningen, Groningen, The Netherlands
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20
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Noguchi H. Solvent-free coarse-grained lipid model for large-scale simulations. J Chem Phys 2011; 134:055101. [DOI: 10.1063/1.3541246] [Citation(s) in RCA: 40] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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21
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Almquist BD, Verma P, Cai W, Melosh NA. Nanoscale patterning controls inorganic-membrane interface structure. NANOSCALE 2011; 3:391-400. [PMID: 20931126 DOI: 10.1039/c0nr00486c] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
The ability to non-destructively integrate inorganic structures into or through biological membranes is essential to realizing full bio-inorganic integration, including arrayed on-chip patch-clamps, drug delivery, and biosensors. Here we explore the role of nanoscale patterning on the strength of biomembrane-inorganic interfaces. AFM measurements show that inorganic probes functionalized with hydrophobic bands with thicknesses complimentary to the hydrophobic lipid bilayer core exhibit strong attachment in the bilayer. As hydrophobic band thickness increases to 2-3 times the bilayer core the interfacial strength decreases, comparable to homogeneously hydrophobic probes. Analytical calculations and molecular dynamics simulations predict a transition between a 'fused' interface and a 'T-junction' that matches the experimental results, showing lipid disorder and defect formation for thicker bands. These results show that matching biological length scales leads to more intimate bio-inorganic junctions, enabling rational design of non-destructive membrane interfaces.
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Affiliation(s)
- Benjamin D Almquist
- Department of Materials Science and Engineering, Stanford University, 476 Lomita Mall, Stanford, California 94305, USA
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22
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Müller M, Schick M. An Alternate Path for Fusion and its Exploration by Field-Theoretic Means. CURRENT TOPICS IN MEMBRANES 2011; 68:295-323. [DOI: 10.1016/b978-0-12-385891-7.00012-x] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/12/2023]
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23
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Markvoort AJ, Marrink SJ. Lipid acrobatics in the membrane fusion arena. CURRENT TOPICS IN MEMBRANES 2011; 68:259-94. [PMID: 21771503 DOI: 10.1016/b978-0-12-385891-7.00011-8] [Citation(s) in RCA: 49] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Affiliation(s)
- Albert J Markvoort
- Institute for Complex Molecular Systems & Biomodeling and Bioinformatics Group, Eindhoven University of Technology, Eindhoven, The Netherlands
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24
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Wu S, Lu T, Guo H. Dissipative particle dynamic simulation study of lipid membrane. ACTA ACUST UNITED AC 2010. [DOI: 10.1007/s11458-009-0210-9] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/19/2022]
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25
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Abstract
Lipid bilayer fusion is a complex process requiring several intermediate steps. Initially, the two bilayers are brought into close contact following removal of intervening water layers and overcoming electrostatic repulsions between opposing bilayer head groups. In this study we monitor by light scattering the reversible aggregation of phosphatidylcholine single shell vesicles during which adhesion occurs but stops prior to a fusion process. Light scattering measurements of dimyristoyl-sn-glycero-3-phosphocholine (DMPC), dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) in water show that lowering the temperature of about 0.14 micron single shell vesicles of DPPC (from 20 °C to 5 °C) and about 2 micron vesicles of DSPC (from 20 °C to 15 °C), but not of 1 micron vesicles of DMPC, results in extensive aggregation within 24 hours that is reversible by an increase in temperature. Aggregation of DSPC vesicles was confirmed by direct visual observation. Orientation of lipid head groups parallel to the plane of the bilayer and consequent reduction of the negative surface charge can account for the ability of DPPC and DSPC vesicles to aggregate. Retention of negatively charged phosphates on the surface and the burial of positively charged cholines within the bilayer offer an explanation for the failure of DMPC vesicles to aggregate. Lowering the temperature of 1,2-dipalmitoyl-sn-glycero-3-phosphoserine (DPPS) vesicles from 20 °C to 5 °C failed to increase aggregation within 24 hours at Mg++/DPPS ratios that begin to initiate aggregation and fusion.
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26
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Raudino A, Pannuzzo M. Nucleation theory with delayed interactions: An application to the early stages of the receptor-mediated adhesion/fusion kinetics of lipid vesicles. J Chem Phys 2010; 132:045103. [DOI: 10.1063/1.3290823] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
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27
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Norizoe Y, Daoulas KC, Müller M. Measuring excess free energies of self-assembled membrane structures. Faraday Discuss 2010; 144:369-91; discussion 445-81. [DOI: 10.1039/b901657k] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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28
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Grafmüller A, Shillcock J, Lipowsky R. Dissipative particle dynamics of tension-induced membrane fusion. MOLECULAR SIMULATION 2009. [DOI: 10.1080/08927020802610296] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/21/2022]
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29
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Grafmüller A, Shillcock J, Lipowsky R. The fusion of membranes and vesicles: pathway and energy barriers from dissipative particle dynamics. Biophys J 2009; 96:2658-75. [PMID: 19348749 PMCID: PMC2711276 DOI: 10.1016/j.bpj.2008.11.073] [Citation(s) in RCA: 116] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/11/2008] [Revised: 11/14/2008] [Accepted: 11/19/2008] [Indexed: 10/20/2022] Open
Abstract
The fusion of lipid bilayers is studied with dissipative particle dynamics simulations. First, to achieve control over membrane properties, the effects of individual simulation parameters are studied and optimized. Then, a large number of fusion events for a vesicle and a planar bilayer are simulated using the optimized parameter set. In the observed fusion pathway, configurations of individual lipids play an important role. Fusion starts with individual lipids assuming a splayed tail configuration with one tail inserted in each membrane. To determine the corresponding energy barrier, we measure the average work for interbilayer flips of a lipid tail, i.e., the average work to displace one lipid tail from one bilayer to the other. This energy barrier is found to depend strongly on a certain dissipative particle dynamics parameter, and, thus, can be adjusted in the simulations. Overall, three subprocesses have been identified in the fusion pathway. Their energy barriers are estimated to lie in the range 8-15 k(B)T. The fusion probability is found to possess a maximum at intermediate tension values. As one decreases the tension, the fusion probability seems to vanish before the tensionless membrane state is attained. This would imply that the tension has to exceed a certain threshold value to induce fusion.
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Affiliation(s)
- Andrea Grafmüller
- Theory and Bio-Systems, Max Planck Institute for Colloids and Interfaces, Potsdam, Germany.
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30
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Daoulas KC, Müller M. Comparison of Simulations of Lipid Membranes with Membranes of Block Copolymers. ADVANCES IN POLYMER SCIENCE 2009. [DOI: 10.1007/978-3-642-10479-4_7] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
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31
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Lee J, Schick M. Calculation of free energy barriers to the fusion of small vesicles. Biophys J 2008; 94:1699-706. [PMID: 18024495 PMCID: PMC2242767 DOI: 10.1529/biophysj.107.119511] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/13/2007] [Accepted: 11/01/2007] [Indexed: 11/18/2022] Open
Abstract
The fusion of small vesicles, either with a planar bilayer or with one another, is studied using a microscopic model in which the bilayers are composed of hexagonal- and lamellar-forming amphiphiles. The free energy of the system is obtained within the self-consistent field approximation. We find that the free energy barrier to form the initial stalk is hardly affected by the radius of the vesicle, but that the barrier to expand the hemifusion diaphragm and form a fusion pore decreases rapidly as the radius decreases. As a consequence, once the initial barrier to stalk formation is overcome, one which we estimate at 13 k(B)T for biological membranes, fusion involving small vesicles should proceed with little or no further input of energy.
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Affiliation(s)
| | - M. Schick
- Department of Physics, University of Washington, Seattle, Washington
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32
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Lee JY, Schick M. Dependence of the energies of fusion on the intermembrane separation: optimal and constrained. J Chem Phys 2007; 127:075102. [PMID: 17718633 DOI: 10.1063/1.2766945] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We calculate the characteristic energies of fusion between planar bilayers as a function of the distance between them, measured from the hydrophobic/hydrophilic interface of one of the two nearest, cis, leaves to the other. The two leaves of each bilayer are of equal composition: 0.6 volume fraction of a lamellar-forming amphiphile, such as dioleoylphosphatidylcholine, and 0.4 volume fraction of a hexagonal-forming amphiphile, such as dioleoylphosphatidylethanolamine. Self-consistent field theory is employed to solve the model. We find that the largest barrier to fusion is that to create the metastable stalk. This barrier is the smallest, about 14.6k(B)T, when the bilayers are at a distance about 20% greater than the thickness of a single leaf, a distance which would correspond to between 2 and 3 nm for typical bilayers. The very size of the protein machinery which brings the membranes together can prevent them from reaching this optimum separation. For even modestly larger separations, we find a linear rate of increase of the free energy with distance between bilayers for the metastable stalk itself and for the barrier to the creation of this stalk. We estimate these rates for biological membranes to be about 7.1k(B)Tnm and 16.7 k(B)Tnm, respectively. The major contribution to this rate comes from the increased packing energy associated with the hydrophobic tails. From this we estimate, for the case of hemagglutinin, a free energy of 38k(B)T for the metastable stalk itself and a barrier to create it of 73 k(B)T. Such a large barrier would require that more than a single hemagglutinin molecule be involved in the fusion process, as is observed.
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Affiliation(s)
- J Y Lee
- Department of Physics, University of Washington, P.O. Box 351560, Seattle, Washington 98195-1560, USA
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33
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Vijayan K, Discher DE, Lal J, Janmey P, Goulian M. Interactions of membrane-active peptides with thick, neutral, nonzwitterionic bilayers. J Phys Chem B 2007; 109:14356-64. [PMID: 16852806 PMCID: PMC2532852 DOI: 10.1021/jp050060x] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Alamethicin is a well-studied channel-forming peptide that has a prototypical amphipathic helix structure. It permeabilizes both microbial and mammalian cell membranes, causing loss of membrane polarization and leakage of endogenous contents. Antimicrobial peptide-lipid systems have been studied quite extensively and have led to significant advancements in membrane biophysics. These studies have been performed on lipid bilayers that are generally charged or zwitterionic and restricted to a thickness range of 3-5 nm. Bilayers of amphiphilic diblock copolymers are a relatively new class of membranes that can have significantly different physicochemical properties compared with those of lipid membranes. In particular, they can be made uncharged, nonzwitterionic, and much thicker than their lipid counterparts. In an effort to extend studies of membrane-protein interactions to these synthetic membranes, we have characterized the interactions of alamethicin and several other membrane-active peptides with diblock copolymer bilayers. We find that although alamethicin is too small to span the bilayer, the peptide interacts with, and ruptures, thick polymer membranes.
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Affiliation(s)
- Kandaswamy Vijayan
- Departments of Physics, of Chemical and Biomolecular Engineering, and of Physiology and Institute for Medicine and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
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34
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Lee JY, Schick M. Field theoretic study of bilayer membrane fusion III: membranes with leaves of different composition. Biophys J 2007; 92:3938-48. [PMID: 17351002 PMCID: PMC1868991 DOI: 10.1529/biophysj.106.097063] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2006] [Accepted: 12/04/2006] [Indexed: 11/18/2022] Open
Abstract
We extend previous work on homogeneous bilayers to calculate the barriers to fusion of planar bilayers that contain two different amphiphiles, a lamellar former and a hexagonal former, with different compositions of the two in each leaf. Self-consistent field theory is employed, and both standard and alternative pathways are explored. We first calculate these barriers as the amount of hexagonal former is increased equally in both leaves to levels appropriate to the plasma membrane of human red blood cells. We follow these barriers as the composition of hexagonal formers is then increased in the cis layer and decreased in the trans layer, again to an extent comparable to the biological system. We find that, while the fusion pathway exhibits two barriers in both the standard and alternative pathways, in both cases the magnitudes of these barriers are comparable to one another, and small, on the order of 13 k(B)T. As a consequence, one expects that once the bilayers are brought sufficiently close to one another to initiate the process, fusion should occur rapidly.
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Affiliation(s)
- J Y Lee
- Department of Physics, University of Washington, Seattle, Washington, USA
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35
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Grafmüller A, Shillcock J, Lipowsky R. Pathway of membrane fusion with two tension-dependent energy barriers. PHYSICAL REVIEW LETTERS 2007; 98:218101. [PMID: 17677811 DOI: 10.1103/physrevlett.98.218101] [Citation(s) in RCA: 137] [Impact Index Per Article: 8.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/02/2007] [Indexed: 05/16/2023]
Abstract
Fusion of bilayer membranes is studied via dissipative particle dynamics (DPD) simulations. A new set of DPD parameters is introduced which leads to an energy barrier for flips of lipid molecules between adhering membranes. A large number of fusion events is monitored for a vesicle in contact with a planar membrane. Several time scales of the fusion process are found to depend exponentially on the membrane tension. This implies an energy barrier of about 10k(B)T for intermembrane flips and a second size-dependent barrier for the nucleation of a small hemifused membrane patch.
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Affiliation(s)
- Andrea Grafmüller
- Max Planck Institute of Colloids and Interfaces, Science Park Golm, 14424 Potsdam, Germany
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36
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Sevink GJA, Zvelindovsky A. Mesoscopic dynamics of complex vesicle formation: kinetic versus thermodynamic factors. MOLECULAR SIMULATION 2007. [DOI: 10.1080/08927020601133391] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
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37
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Li DW, Liu XY. Examination of membrane fusion by dissipative particle dynamics simulation and comparison with continuum elastic models. J Chem Phys 2007; 122:174909. [PMID: 15910071 DOI: 10.1063/1.1889433] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
A dissipative particle dynamics model is applied to probe the lipidic membrane fusion. This model is verified by reproducing the lipid phase behavior. The classical stalk model has been visited and modified. The tilt deformation of the lipids and the noncircular shape of the stalk are supported. The stalk is shown to undergo asymmetric expansion to form the trans-monolayers contact (TMC). Unlike previous models, an energy barrier between the stalk and the TMC has been identified, implying that the TMC should be a metastable formation. This shows good agreement with the fusion experiments. Two typical elastic continuum models are compared with our result and possible modifications to the two elastic models are suggested. The effect of spontaneous curvature of lipid on selection of fusion pathway is also examined. It is observed that a bent stalk with pore or an inverted micellar intermediate will have more chance to occur than traditional stalk when the spontaneous curvature of the lipid becomes more negative.
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Affiliation(s)
- Da-Wei Li
- Department of Physics, Faculty of Science, National University of Singapore
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38
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Loison C, Mareschal M, Schmid F. Pores in bilayer membranes of amphiphilic molecules: coarse-grained molecular dynamics simulations compared with simple mesoscopic models. J Chem Phys 2006; 121:1890-900. [PMID: 15260741 DOI: 10.1063/1.1752884] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Abstract
We investigate pores in fluid membranes by molecular dynamics simulations of an amphiphile-solvent mixture, using a molecular coarse-grained model. The amphiphilic membranes self-assemble into a lamellar stack of amphiphilic bilayers separated by solvent layers. We focus on the particular case of tensionless membranes, in which pores spontaneously appear because of thermal fluctuations. Their spatial distribution is similar to that of a random set of repulsive hard disks. The size and shape distribution of individual pores can be described satisfactorily by a simple mesoscopic model, which accounts only for a pore independent core energy and a line tension penalty at the pore edges. In particular, the pores are not circular: their shapes are fractal and have the same characteristics as those of two-dimensional ring polymers. Finally, we study the size-fluctuation dynamics of the pores, and compare the time evolution of their contour length to a random walk in a linear potential.
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Affiliation(s)
- C Loison
- Max Planck Institut für Chemische Physik fester Stoffe, Nöthnitzer str. 40, D-01187 Dresden, Germany.
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39
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Tolpekina TV, den Otter WK, Briels WJ. Simulations of stable pores in membranes: system size dependence and line tension. J Chem Phys 2006; 121:8014-20. [PMID: 15485265 DOI: 10.1063/1.1796254] [Citation(s) in RCA: 85] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Amphiphilic bilayers with a pore were simulated using a coarse grained model. By stretching the bilayer to 70% beyond its equilibrium surface area, we established the phase diagram of pores, identifying regions where pores are stable, metastable, or unstable. A simple theoretical model is proposed to explain the phase diagram, and to calculate the critical and equilibrium relative stretches. Interestingly, these are found to scale with the inverse cubic root of the number of amphiphiles in the bilayer, thus explaining the order of magnitude difference between the simulated and the measured values. Three different methods are used to calculate a line tension coefficient of (3.5-4.0) x 10(-11) J/m, in good agreement with experimental data.
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Affiliation(s)
- T V Tolpekina
- Computational Dispersion Rheology, Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
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40
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Katsov K, Müller M, Schick M. Field theoretic study of bilayer membrane fusion: II. Mechanism of a stalk-hole complex. Biophys J 2006; 90:915-26. [PMID: 16272437 PMCID: PMC1367116 DOI: 10.1529/biophysj.105.071092] [Citation(s) in RCA: 75] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/20/2005] [Accepted: 10/12/2005] [Indexed: 11/18/2022] Open
Abstract
We use self-consistent field theory to determine structural and energetic properties of intermediates and transition states involved in bilayer membrane fusion. In particular, we extend our original calculations from those of the standard hemifusion mechanism, which was studied in detail in the first article of this series, to consider a possible alternative to it. This mechanism involves non-axial stalk expansion, in contrast to the axially symmetric evolution postulated in the classical mechanism. Elongation of the initial stalk facilitates the nucleation of holes and leads to destabilization of the fusing membranes via the formation of a stalk-hole complex. We study properties of this complex in detail, and show how transient leakage during fusion, previously predicted and recently observed in experiment, should vary with lipid architecture and tension. We also show that the barrier to fusion in the alternative mechanism is lower than that of the standard mechanism by a few k(B)T over most of the relevant region of system parameters, so that this alternative mechanism is a viable alternative to the standard pathway. We emphasize that any mechanism, such as this alternative one, which affects, even modestly, the line tension of a hole in a membrane, affects greatly the ability of that membrane to undergo fusion.
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Affiliation(s)
- K Katsov
- Materials Research Laboratory, University of California, Santa Barbara, California, USA
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41
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Schick * M, Katsov K, Müller M. The central role of line tension in the fusion of biological membranes. Mol Phys 2005. [DOI: 10.1080/00268970500151486] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
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42
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Gilbert RJC. Inactivation and Activity of Cholesterol-Dependent Cytolysins: What Structural Studies Tell Us. Structure 2005; 13:1097-106. [PMID: 16084382 DOI: 10.1016/j.str.2005.04.019] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/08/2005] [Revised: 04/21/2005] [Accepted: 04/26/2005] [Indexed: 10/25/2022]
Abstract
The homologous bacterially expressed cholesterol-dependent cytolysins (CDCs) form pores via oligomerization; this must occur preferentially once the target membrane has been engaged. Conformational changes in CDCs then drive partition from an aqueous environment to a lipidic one. This review addresses how premature oligomerization is prevented, how conformational changes are triggered, and how cooperativity between subunits brings about new functionality absent from isolated protomers. Variations are found in the answers provided by the CDCs to these issues. Some toxins use pH as a trigger of activity, but recent results have shown that dimerization in solution is an alternative way of preventing premature oligomerization, in particular for the CDC from Clostridium perfringens, perfringolysin. More controversially, there is still no resolution to the debate as to whether incomplete (arciform) oligomers form pores: recent results again suggest that they do.
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Affiliation(s)
- Robert J C Gilbert
- Division of Structural Biology, Henry Wellcome Building for Genomic Medicine, University of Oxford, Roosevelt Drive, Oxford, OX3 7BN, United Kingdom.
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43
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Knoll A, Lyakhova KS, Horvat A, Krausch G, Sevink GJA, Zvelindovsky AV, Magerle R. Direct imaging and mesoscale modelling of phase transitions in a nanostructured fluid. NATURE MATERIALS 2004; 3:886-891. [PMID: 15568030 DOI: 10.1038/nmat1258] [Citation(s) in RCA: 60] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/02/2004] [Accepted: 09/15/2004] [Indexed: 05/24/2023]
Abstract
The kinetics of phase transitions is essential for understanding pattern formation in structured fluids. These fluids play a key role in the morphogenesis of biological cells, and they are very common in pharmaceutical products and plastic materials. Until now, it has not been possible to follow phase transitions in structured fluids experimentally in real time and with high spatial resolution. Previous work has relied on static images and indirect experimental evidence from spatially averaging scattering experiments. Simulating the processes with computer models is a further challenge because of the multiple time and length scales involved. Our movies based on in situ scanning force microscopy show the time sequence of the elementary steps of a phase transition in a fluid film of block copolymer from the cylinder to the perforated lamella phase. The movies validate a versatile simulation model that gives physical insight into the nature of the process. Our approach provides a means of improving the study and understanding of pattern formation processes in nanostructured fluids. We expect a significant impact on nanotechnology where block copolymers serve as self-organized templates for the synthesis of inorganic nanostructured materials.
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Affiliation(s)
- A Knoll
- Physikalische Chemie II, Universität Bayreuth, D-95440 Bayreuth, Germany
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44
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Malinin VS, Lentz BR. Energetics of vesicle fusion intermediates: comparison of calculations with observed effects of osmotic and curvature stresses. Biophys J 2004; 86:2951-64. [PMID: 15111411 PMCID: PMC1304163 DOI: 10.1016/s0006-3495(04)74346-5] [Citation(s) in RCA: 72] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
We reported previously the effects of both osmotic and curvature stress on fusion between poly(ethylene glycol)-aggregated vesicles. In this article, we analyze the energetics of fusion of vesicles of different curvature, paying particular attention to the effects of osmotic stress on small, highly curved vesicles of 26 nm diameter, composed of lipids with negative intrinsic curvature. Our calculations show that high positive curvature of the outer monolayer "charges" these vesicles with excess bending energy, which then releases during stalk expansion (increase of the stalk radius, r(s)) and thus "drives" fusion. Calculations based on the known mechanical properties of lipid assemblies suggest that the free energy of "void" formation as well as membrane-bending free energy dominate the evolution of a stalk to an extended transmembrane contact. The free-energy profile of stalk expansion (free energy versus r(s)) clearly shows the presence of two metastable intermediates (intermediate 1 at r(s) approximately 0 - 1.0 nm and intermediate 2 at r(s) approximately 2.5 - 3.0 nm). Applying osmotic gradients of +/-5 atm, when assuming a fixed trans-bilayer lipid mass distribution, did not significantly change the free-energy profile. However, inclusion in the model of an additional degree of freedom, the ability of lipids to move into and out of the "void", made the free-energy profile strongly dependent on the osmotic gradient. Vesicle expansion increased the energy barrier between intermediates by approximately 4 kT and the absolute value of the barrier by approximately 7 kT, whereas compression decreased it by nearly the same extent. Since these calculations, which are based on the stalk hypothesis, correctly predict the effects of both membrane curvature and osmotic stress, they support the stalk hypothesis for the mechanism of membrane fusion and suggest that both forms of stress alter the final stages, rather than the initial step, of the fusion process, as previously suggested.
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Katsov K, Müller M, Schick M. Field theoretic study of bilayer membrane fusion. I. Hemifusion mechanism. Biophys J 2004; 87:3277-90. [PMID: 15326031 PMCID: PMC1304796 DOI: 10.1529/biophysj.103.038943] [Citation(s) in RCA: 110] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/18/2003] [Accepted: 08/13/2004] [Indexed: 11/18/2022] Open
Abstract
Self-consistent field theory is used to determine structural and energetic properties of metastable intermediates and unstable transition states involved in the standard stalk mechanism of bilayer membrane fusion. A microscopic model of flexible amphiphilic chains dissolved in hydrophilic solvent is employed to describe these self-assembled structures. We find that the barrier to formation of the initial stalk is much smaller than previously estimated by phenomenological theories. Therefore its creation it is not the rate-limiting process. The relevant barrier is associated with the rather limited radial expansion of the stalk into a hemifusion diaphragm. It is strongly affected by the architecture of the amphiphile, decreasing as the effective spontaneous curvature of the amphiphile is made more negative. It is also reduced when the tension is increased. At high tension the fusion pore, created when a hole forms in the hemifusion diaphragm, expands without bound. At very low membrane tension, small fusion pores can be trapped in a flickering metastable state. Successful fusion is severely limited by the architecture of the lipids. If the effective spontaneous curvature is not sufficiently negative, fusion does not occur because metastable stalks, whose existence is a seemingly necessary prerequisite, do not form at all. However if the spontaneous curvature is too negative, stalks are so stable that fusion does not occur because the system is unstable either to a phase of stable radial stalks, or to an inverted-hexagonal phase induced by stable linear stalks. Our results on the architecture and tension needed for successful fusion are summarized in a phase diagram.
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Affiliation(s)
- K Katsov
- Department of Physics, University of Washington, Seattle, Washington 98195-1560, USA
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Kozlovsky Y, Efrat A, Siegel DP, Siegel DA, Kozlov MM. Stalk phase formation: effects of dehydration and saddle splay modulus. Biophys J 2004; 87:2508-21. [PMID: 15454446 PMCID: PMC1304670 DOI: 10.1529/biophysj.103.038075] [Citation(s) in RCA: 77] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2003] [Accepted: 07/23/2004] [Indexed: 11/18/2022] Open
Abstract
One of the earliest lipid intermediates forming in the course of membrane fusion is the lipid stalk. Although many aspects of the stalk hypothesis were elaborated theoretically and confirmed by experiments it remained unresolved whether stalk formation is always an energy consuming process or if there are conditions where the stalks are energetically favorable and form spontaneously resulting in an equilibrium stalk phase. Motivated by a recent breakthrough experiments we analyze the physical factors determining the spontaneous stalk formation. We show that this process can be driven by interplay between two factors: the elastic energy of lipid monolayers including a contribution of the saddle splay deformation and the energy of hydration repulsion acting between apposing membranes. We analyze the dependence of stalk formation on the saddle splay (Gaussian) modulus of the lipid monolayers and estimate the values of this modulus based on the experimentally established phase boundary between the lamellar and the stalk phases. We suggest that fusion proteins can induce stalk formation just by bringing the membranes into close contact, and accumulating, at least locally, a sufficiently large energy of the hydration repulsion.
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Affiliation(s)
- Yonathan Kozlovsky
- Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel
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Jiang FY, Bouret Y, Kindt JT. Molecular dynamics simulations of the lipid bilayer edge. Biophys J 2004; 87:182-92. [PMID: 15240456 PMCID: PMC1304341 DOI: 10.1529/biophysj.103.031054] [Citation(s) in RCA: 79] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/23/2003] [Accepted: 03/29/2004] [Indexed: 11/18/2022] Open
Abstract
Phospholipid bilayers have been intensively studied by molecular dynamics (MD) simulation in recent years. The properties of bilayer edges are important in determining the structure and stability of pores formed in vesicles and biomembranes. In this work, we use molecular dynamics simulation to investigate the structure, dynamics, and line tension of the edges of bilayer ribbons composed of pure dimyristoylphosphatidylcholine (DMPC) or palmitoyl-oleoylphosphatidylethanolamine (POPE). As expected, we observe a significant reorganization of lipids at and near the edges. The treatment of electrostatic effects is shown to have a qualitative impact on the structure and stability of the edge, and significant differences are observed in the dynamics and structure of edges formed by DMPC and palmitoyl-oleoylphosphatidylethanolamine. From the pressure anisotropy in the simulation box, we calculate a line tension of approximately 10-30 pN for the DMPC edge, in qualitative agreement with experimental estimates for similar lipids.
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Affiliation(s)
- Frank Y Jiang
- Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA
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Loison C, Mareschal M, Kremer K, Schmid F. Thermal fluctuations in a lamellar phase of a binary amphiphile–solvent mixture: A molecular-dynamics study. J Chem Phys 2003. [DOI: 10.1063/1.1626634] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Frolov VA, Dunina-Barkovskaya AY, Samsonov AV, Zimmerberg J. Membrane permeability changes at early stages of influenza hemagglutinin-mediated fusion. Biophys J 2003; 85:1725-33. [PMID: 12944287 PMCID: PMC1303346 DOI: 10.1016/s0006-3495(03)74602-5] [Citation(s) in RCA: 69] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022] Open
Abstract
While biological membrane fusion is classically defined as the leak-free merger of membranes and contents, leakage is a finding in both experimental and theoretical studies. The fusion stages, if any, that allow membrane permeation are uncharted. In this study we monitored membrane ionic permeability at early stages of fusion mediated by the fusogenic protein influenza hemagglutinin (HA). HAb2 cells, expressing HA on their plasma membrane, fused with human red blood cells, cultured liver cells PLC/PRF/5, or planar phospholipid bilayer membranes. With a probability that depended upon the target membrane, an increase of the electrical conductance of the fusing membranes (leakage) by up to several nS was generally detected. This leakage was recorded at the initial stages of fusion, when fusion pores formed. This leakage usually accompanied the "flickering" stage of the early fusion pore development. As the pore widened, the leakage reduced; concomitantly, the lipid exchange between the fusing membranes accelerated. We conclude that during fusion pore formation, HA locally and temporarily increases the permeability of fusing membranes. Subsequent rearrangement in the fusion complex leads to the resealing of the leaky membranes and enlargement of the pore.
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Affiliation(s)
- V A Frolov
- A. N. Frumkin Institute of Electrochemistry, Russian Academy of Sciences, Moscow, Russia
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Müller M, Katsov K, Schick M. A new mechanism of model membrane fusion determined from Monte Carlo simulation. Biophys J 2003; 85:1611-23. [PMID: 12944277 PMCID: PMC1303336 DOI: 10.1016/s0006-3495(03)74592-5] [Citation(s) in RCA: 127] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2003] [Accepted: 04/10/2003] [Indexed: 11/28/2022] Open
Abstract
We have carried out extensive Monte Carlo simulations of the fusion of tense apposed bilayers formed by amphiphilic molecules within the framework of a coarse-grained lattice model. The fusion pathway differs from the usual stalk mechanism. Stalks do form between the apposed bilayers, but rather than expand radially to form an axial-symmetric hemifusion diaphragm of the trans leaves of both bilayers, they promote in their vicinity the nucleation of small holes in the bilayers. Two subsequent paths are observed. 1) The stalk encircles a hole in one bilayer creating a diaphragm comprised of both leaves of the other intact bilayer, which ruptures to complete the fusion pore. 2) Before the stalk can encircle a hole in one bilayer, a second hole forms in the other bilayer, and the stalk aligns and encircles them both to complete the fusion pore. Both pathways give rise to mixing between the cis and trans leaves of the bilayer and allow for transient leakage.
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Affiliation(s)
- M Müller
- Institute for Physics, Johannes Gutenberg University, Mainz, Germany
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