1
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Golani G, Leikina E, Melikov K, Whitlock JM, Gamage DG, Luoma-Overstreet G, Millay DP, Kozlov MM, Chernomordik LV. Myomerger promotes fusion pore by elastic coupling between proximal membrane leaflets and hemifusion diaphragm. Nat Commun 2021; 12:495. [PMID: 33479215 PMCID: PMC7820291 DOI: 10.1038/s41467-020-20804-x] [Citation(s) in RCA: 30] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2020] [Accepted: 12/08/2020] [Indexed: 01/09/2023] Open
Abstract
Myomerger is a muscle-specific membrane protein involved in formation of multinucleated muscle cells by mediating the transition from the early hemifusion stage to complete fusion. Here, we considered the physical mechanism of the Myomerger action based on the hypothesis that Myomerger shifts the spontaneous curvature of the outer membrane leaflets to more positive values. We predicted, theoretically, that Myomerger generates the outer leaflet elastic stresses, which propagate into the hemifusion diaphragm and accelerate the fusion pore formation. We showed that Myomerger ectodomain indeed generates positive spontaneous curvature of lipid monolayers. We substantiated the mechanism by experiments on myoblast fusion and influenza hemagglutinin-mediated cell fusion. In both processes, the effects of Myomerger ectodomain were strikingly similar to those of lysophosphatidylcholine known to generate a positive spontaneous curvature of lipid monolayers. The control of post-hemifusion stages by shifting the spontaneous curvature of proximal membrane monolayers may be utilized in diverse fusion processes.
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Affiliation(s)
- Gonen Golani
- Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, 69978, Israel
| | - Evgenia Leikina
- Section on Membrane Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, 20892, USA
| | - Kamran Melikov
- Section on Membrane Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, 20892, USA
| | - Jarred M Whitlock
- Section on Membrane Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, 20892, USA
| | - Dilani G Gamage
- Division of Molecular Cardiovascular Biology, Cincinnati Children's Hospital Medical Center, Cincinnati, OH, 45229, USA
| | - Gracia Luoma-Overstreet
- Section on Membrane Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, 20892, USA
| | - Douglas P Millay
- Division of Molecular Cardiovascular Biology, Cincinnati Children's Hospital Medical Center, Cincinnati, OH, 45229, USA
- Department of Pediatrics, University of Cincinnati, Cincinnati, OH, 45229, USA
| | - Michael M Kozlov
- Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, 69978, Israel.
| | - Leonid V Chernomordik
- Section on Membrane Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, 20892, USA.
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2
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Chen B, You W, Wang Y, Shan T. The regulatory role of Myomaker and Myomixer-Myomerger-Minion in muscle development and regeneration. Cell Mol Life Sci 2020; 77:1551-1569. [PMID: 31642939 PMCID: PMC11105057 DOI: 10.1007/s00018-019-03341-9] [Citation(s) in RCA: 33] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2019] [Revised: 10/07/2019] [Accepted: 10/10/2019] [Indexed: 12/12/2022]
Abstract
Skeletal muscle plays essential roles in motor function, energy, and glucose metabolism. Skeletal muscle formation occurs through a process called myogenesis, in which a crucial step is the fusion of mononucleated myoblasts to form multinucleated myofibers. The myoblast/myocyte fusion is triggered and coordinated in a muscle-specific way that is essential for muscle development and post-natal muscle regeneration. Many molecules and proteins have been found and demonstrated to have the capacity to regulate the fusion of myoblast/myocytes. Interestingly, two newly discovered muscle-specific membrane proteins, Myomaker and Myomixer (also called Myomerger and Minion), have been identified as fusogenic regulators in vertebrates. Both Myomaker and Myomixer-Myomerger-Minion have the capacity to directly control the myogenic fusion process. Here, we review and discuss the latest studies related to these two proteins, including the discovery, structure, expression pattern, functions, and regulation of Myomaker and Myomixer-Myomerger-Minion. We also emphasize and discuss the interaction between Myomaker and Myomixer-Myomerger-Minion, as well as their cooperative regulatory roles in cell-cell fusion. Moreover, we highlight the areas for exploration of Myomaker and Myomixer-Myomerger-Minion in future studies and consider their potential application to control cell fusion for cell-therapy purposes.
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Affiliation(s)
- Bide Chen
- College of Animal Sciences, Zhejiang University, 866 Yuhangtang Road, Hangzhou, China
- The Key Laboratory of Molecular Animal Nutrition, Ministry of Education, Hangzhou, China
- Zhejiang Provincial Laboratory of Feed and Animal Nutrition, Hangzhou, China
| | - Wenjing You
- College of Animal Sciences, Zhejiang University, 866 Yuhangtang Road, Hangzhou, China
- The Key Laboratory of Molecular Animal Nutrition, Ministry of Education, Hangzhou, China
- Zhejiang Provincial Laboratory of Feed and Animal Nutrition, Hangzhou, China
| | - Yizhen Wang
- College of Animal Sciences, Zhejiang University, 866 Yuhangtang Road, Hangzhou, China
- The Key Laboratory of Molecular Animal Nutrition, Ministry of Education, Hangzhou, China
- Zhejiang Provincial Laboratory of Feed and Animal Nutrition, Hangzhou, China
| | - Tizhong Shan
- College of Animal Sciences, Zhejiang University, 866 Yuhangtang Road, Hangzhou, China.
- The Key Laboratory of Molecular Animal Nutrition, Ministry of Education, Hangzhou, China.
- Zhejiang Provincial Laboratory of Feed and Animal Nutrition, Hangzhou, China.
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3
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Leikina E, Gamage DG, Prasad V, Goykhberg J, Crowe M, Diao J, Kozlov MM, Chernomordik LV, Millay DP. Myomaker and Myomerger Work Independently to Control Distinct Steps of Membrane Remodeling during Myoblast Fusion. Dev Cell 2018; 46:767-780.e7. [PMID: 30197239 DOI: 10.1016/j.devcel.2018.08.006] [Citation(s) in RCA: 96] [Impact Index Per Article: 16.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/21/2018] [Revised: 06/27/2018] [Accepted: 08/08/2018] [Indexed: 02/03/2023]
Abstract
Classic mechanisms for membrane fusion involve transmembrane proteins that assemble into complexes and dynamically alter their conformation to bend membranes, leading to mixing of membrane lipids (hemifusion) and fusion pore formation. Myomaker and Myomerger govern myoblast fusion and muscle formation but are structurally divergent from traditional fusogenic proteins. Here, we show that Myomaker and Myomerger independently mediate distinct steps in the fusion pathway, where Myomaker is involved in membrane hemifusion and Myomerger is necessary for fusion pore formation. Mechanistically, we demonstrate that Myomerger is required on the cell surface where its ectodomains stress membranes. Moreover, we show that Myomerger drives fusion completion in a heterologous system independent of Myomaker and that a Myomaker-Myomerger physical interaction is not required for function. Collectively, our data identify a stepwise cell fusion mechanism in myoblasts where different proteins are delegated to perform unique membrane functions essential for membrane coalescence.
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Affiliation(s)
- Evgenia Leikina
- Section on Membrane Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA
| | - Dilani G Gamage
- Division of Molecular Cardiovascular Biology, Cincinnati Children's Hospital Medical Center, Cincinnati, OH 45229, USA
| | - Vikram Prasad
- Division of Molecular Cardiovascular Biology, Cincinnati Children's Hospital Medical Center, Cincinnati, OH 45229, USA
| | - Joanna Goykhberg
- Section on Membrane Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA
| | - Michael Crowe
- Department of Cancer Biology, University of Cincinnati, Cincinnati, OH 45229, USA
| | - Jiajie Diao
- Department of Cancer Biology, University of Cincinnati, Cincinnati, OH 45229, USA
| | - Michael M Kozlov
- Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv 69978, Israel
| | - Leonid V Chernomordik
- Section on Membrane Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA.
| | - Douglas P Millay
- Division of Molecular Cardiovascular Biology, Cincinnati Children's Hospital Medical Center, Cincinnati, OH 45229, USA; Department of Pediatrics, University of Cincinnati College of Medicine, Cincinnati, OH 45229, USA.
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4
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Boonstra S, Blijleven JS, Roos WH, Onck PR, van der Giessen E, van Oijen AM. Hemagglutinin-Mediated Membrane Fusion: A Biophysical Perspective. Annu Rev Biophys 2018; 47:153-173. [PMID: 29494252 DOI: 10.1146/annurev-biophys-070317-033018] [Citation(s) in RCA: 42] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Influenza hemagglutinin (HA) is a viral membrane protein responsible for the initial steps of the entry of influenza virus into the host cell. It mediates binding of the virus particle to the host-cell membrane and catalyzes fusion of the viral membrane with that of the host. HA is therefore a major target in the development of antiviral strategies. The fusion of two membranes involves high activation barriers and proceeds through several intermediate states. Here, we provide a biophysical description of the membrane fusion process, relating its kinetic and thermodynamic properties to the large conformational changes taking place in HA and placing these in the context of multiple HA proteins working together to mediate fusion. Furthermore, we highlight the role of novel single-particle experiments and computational approaches in understanding the fusion process and their complementarity with other biophysical approaches.
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Affiliation(s)
- Sander Boonstra
- Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands; , , , ,
| | - Jelle S Blijleven
- Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands; , , , ,
| | - Wouter H Roos
- Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands; , , , ,
| | - Patrick R Onck
- Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands; , , , ,
| | - Erik van der Giessen
- Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands; , , , ,
| | - Antoine M van Oijen
- School of Chemistry; Faculty of Science, Medicine and Health; University of Wollongong, Wollongong, New South Wales 2522, Australia;
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5
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Abstract
Cell-cell fusion is essential for fertilization and organ development. Dedicated proteins known as fusogens are responsible for mediating membrane fusion. However, until recently, these proteins either remained unidentified or were poorly understood at the mechanistic level. Here, we review how fusogens surmount multiple energy barriers to mediate cell-cell fusion. We describe how early preparatory steps bring membranes to a distance of ∼10 nm, while fusogens act in the final approach between membranes. The mechanical force exerted by cell fusogens and the accompanying lipidic rearrangements constitute the hallmarks of cell-cell fusion. Finally, we discuss the relationship between viral and eukaryotic fusogens, highlight a classification scheme regrouping a superfamily of fusogens called Fusexins, and propose new questions and avenues of enquiry.
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Affiliation(s)
- Javier M Hernández
- Department of Structural Biochemistry, Max Planck Institute of Molecular Physiology, D-44227 Dortmund, Germany
| | - Benjamin Podbilewicz
- Department of Biology, Technion - Israel Institute of Technology, Haifa 32000, Israel
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6
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Blijleven JS, Boonstra S, Onck PR, van der Giessen E, van Oijen AM. Mechanisms of influenza viral membrane fusion. Semin Cell Dev Biol 2016; 60:78-88. [PMID: 27401120 DOI: 10.1016/j.semcdb.2016.07.007] [Citation(s) in RCA: 34] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/20/2016] [Revised: 06/28/2016] [Accepted: 07/07/2016] [Indexed: 11/18/2022]
Abstract
Influenza viral particles are enveloped by a lipid bilayer. A major step in infection is fusion of the viral and host cellular membranes, a process with large kinetic barriers. Influenza membrane fusion is catalyzed by hemagglutinin (HA), a class I viral fusion protein activated by low pH. The exact nature of the HA conformational changes that deliver the energy required for fusion remains poorly understood. This review summarizes our current knowledge of HA structure and dynamics, describes recent single-particle experiments and modeling studies, and discusses their role in understanding how multiple HAs mediate fusion. These approaches provide a mechanistic picture in which HAs independently and stochastically insert into the target membrane, forming a cluster of HAs that is collectively able to overcome the barrier to membrane fusion. The new experimental and modeling approaches described in this review hold promise for a more complete understanding of other viral fusion systems and the protein systems responsible for cellular fusion.
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Affiliation(s)
- Jelle S Blijleven
- Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands
| | - Sander Boonstra
- Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands
| | - Patrick R Onck
- Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands
| | - Erik van der Giessen
- Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands
| | - Antoine M van Oijen
- School of Chemistry, Faculty of Science, Medicine and Health, University of Wollongong, NSW 2522, Australia.
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7
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Ryham RJ, Klotz TS, Yao L, Cohen FS. Calculating Transition Energy Barriers and Characterizing Activation States for Steps of Fusion. Biophys J 2016; 110:1110-24. [PMID: 26958888 PMCID: PMC4788739 DOI: 10.1016/j.bpj.2016.01.013] [Citation(s) in RCA: 50] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2015] [Revised: 01/05/2016] [Accepted: 01/13/2016] [Indexed: 12/29/2022] Open
Abstract
We use continuum mechanics to calculate an entire least energy pathway of membrane fusion, from stalk formation, to pore creation, and through fusion pore enlargement. The model assumes that each structure in the pathway is axially symmetric. The static continuum stalk structure agrees quantitatively with experimental stalk architecture. Calculations show that in a stalk, the distal monolayer is stretched and the stored stretching energy is significantly less than the tilt energy of an unstretched distal monolayer. The string method is used to determine the energy of the transition barriers that separate intermediate states and the dynamics of two bilayers as they pass through them. Hemifusion requires a small amount of energy independently of lipid composition, while direct transition from a stalk to a fusion pore without a hemifusion intermediate is highly improbable. Hemifusion diaphragm expansion is spontaneous for distal monolayers containing at least two lipid components, given sufficiently negative diaphragm spontaneous curvature. Conversely, diaphragms formed from single-component distal monolayers do not expand without the continual injection of energy. We identify a diaphragm radius, below which central pore expansion is spontaneous. For larger diaphragms, prior studies have shown that pore expansion is not axisymmetric, and here our calculations supply an upper bound for the energy of the barrier against pore formation. The major energy-requiring deformations in the steps of fusion are: widening of a hydrophobic fissure in bilayers for stalk formation, splay within the expanding hemifusion diaphragm, and fissure widening initiating pore formation in a hemifusion diaphragm.
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Affiliation(s)
- Rolf J Ryham
- Department of Mathematics, Fordham University, Bronx, New York.
| | - Thomas S Klotz
- Department of Computational and Applied Mathematics, Rice University, Houston, Texas
| | - Lihan Yao
- Department of Mathematics, Fordham University, Bronx, New York
| | - Fredric S Cohen
- Department of Molecular Biophysics and Physiology, Rush University, Chicago, Illinois
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8
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Read J, Clancy EK, Sarker M, de Antueno R, Langelaan DN, Parmar HB, Shin K, Rainey JK, Duncan R. Reovirus FAST Proteins Drive Pore Formation and Syncytiogenesis Using a Novel Helix-Loop-Helix Fusion-Inducing Lipid Packing Sensor. PLoS Pathog 2015; 11:e1004962. [PMID: 26061049 PMCID: PMC4464655 DOI: 10.1371/journal.ppat.1004962] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/29/2015] [Accepted: 05/18/2015] [Indexed: 02/07/2023] Open
Abstract
Pore formation is the most energy-demanding step during virus-induced membrane fusion, where high curvature of the fusion pore rim increases the spacing between lipid headgroups, exposing the hydrophobic interior of the membrane to water. How protein fusogens breach this thermodynamic barrier to pore formation is unclear. We identified a novel fusion-inducing lipid packing sensor (FLiPS) in the cytosolic endodomain of the baboon reovirus p15 fusion-associated small transmembrane (FAST) protein that is essential for pore formation during cell-cell fusion and syncytiogenesis. NMR spectroscopy and mutational studies indicate the dependence of this FLiPS on a hydrophobic helix-loop-helix structure. Biochemical and biophysical assays reveal the p15 FLiPS preferentially partitions into membranes with high positive curvature, and this partitioning is impeded by bis-ANS, a small molecule that inserts into hydrophobic defects in membranes. Most notably, the p15 FLiPS can be functionally replaced by heterologous amphipathic lipid packing sensors (ALPS) but not by other membrane-interactive amphipathic helices. Furthermore, a previously unrecognized amphipathic helix in the cytosolic domain of the reptilian reovirus p14 FAST protein can functionally replace the p15 FLiPS, and is itself replaceable by a heterologous ALPS motif. Anchored near the cytoplasmic leaflet by the FAST protein transmembrane domain, the FLiPS is perfectly positioned to insert into hydrophobic defects that begin to appear in the highly curved rim of nascent fusion pores, thereby lowering the energy barrier to stable pore formation. The fusogenic ortho- and aquareoviruses are the only known nonenveloped viruses that induce syncytium formation. Cell-cell fusion is a virulence determinant of fusogenic reoviruses, and is mediated by a singular family of fusion-associated small transmembrane (FAST) proteins, the smallest known viral fusogens. Unlike their enveloped virus counterparts, reovirus FAST proteins have exceptionally small ectodomains and considerable larger cytoplasmic endodomains, suggesting FAST protein interactions with the cytoplasmic leaflet of the plasma membrane likely play a prominent role in the fusion process. We determined that the baboon reovirus p15 FAST protein endodomain contains a novel type of helix-loop-helix lipid packing sensor that partitions into hydrophobic defects present in highly curved membranes. This fusion-inducing lipid packing sensor (FLiPS) is required for pore formation, and can be functionally replaced by heterologous lipid packing sensors. By masking hydrophobic defects appearing in the highly curved rim of nascent fusion pores, the FliPS would make the forward reaction to pore formation a more energetically favored means of resolving an unstable hemifusion intermediate. These results define a new role for curvature sensing motifs, and reveal how viral fusion proteins can drive pore formation without having to rely on membrane stresses induced by complex refolding of large ectodomains.
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Affiliation(s)
- Jolene Read
- Department of Microbiology and Immunology, Dalhousie University, Halifax, Nova Scotia, Canada
| | - Eileen K. Clancy
- Department of Microbiology and Immunology, Dalhousie University, Halifax, Nova Scotia, Canada
| | - Muzaddid Sarker
- Department of Microbiology and Immunology, Dalhousie University, Halifax, Nova Scotia, Canada
- Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia, Canada
| | - Roberto de Antueno
- Department of Microbiology and Immunology, Dalhousie University, Halifax, Nova Scotia, Canada
| | - David N. Langelaan
- Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia, Canada
| | - Hiren B. Parmar
- Department of Microbiology and Immunology, Dalhousie University, Halifax, Nova Scotia, Canada
| | - Kyungsoo Shin
- Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia, Canada
| | - Jan K. Rainey
- Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia, Canada
- Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada
| | - Roy Duncan
- Department of Microbiology and Immunology, Dalhousie University, Halifax, Nova Scotia, Canada
- Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia, Canada
- Department of Pediatrics, Dalhousie University, Halifax, Nova Scotia, Canada
- * E-mail:
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9
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Affiliation(s)
- Benjamin Podbilewicz
- Department of Biology, Technion–Israel Institute of Technology, Haifa 32000, Israel;
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10
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Lysophosphatidylcholine reversibly arrests pore expansion during syncytium formation mediated by diverse viral fusogens. J Virol 2014; 88:6528-31. [PMID: 24672027 DOI: 10.1128/jvi.00314-14] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023] Open
Abstract
Using lysophosphatidylcholine, a curvature-inducing lysolipid, we have isolated a reversible, "stalled pore" phenotype during syncytium formation induced by the p14 fusion-associated small transmembrane (FAST) protein and influenza virus hemagglutinin (HA) fusogens. This is the first evidence that lateral propagation of stable fusion pores leading to syncytiogenesis mediated by diverse viral fusogens is inhibited by promotion of positive membrane curvature in the outer leaflets of the lipid bilayer surrounding intercellular fusion pores.
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11
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Markosyan RM, Cohen FS. The transmembrane domain and acidic lipid flip-flop regulates voltage-dependent fusion mediated by class II and III viral proteins. PLoS One 2013; 8:e76174. [PMID: 24124539 PMCID: PMC3790697 DOI: 10.1371/journal.pone.0076174] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/01/2013] [Accepted: 08/12/2013] [Indexed: 11/18/2022] Open
Abstract
Voltage dependence of fusion induced by class II and class III viral fusion proteins was investigated. Class II proteins from Ross River and Sindbus virus and a mutant class III protein from Epstein Barr virus were found to induce cell-cell fusion that is voltage dependent. Combined with previous studies, in all, four class II and two class III protein have now been shown to exhibit voltage-dependent fusion, demonstrating that this is probably a general phenomenon for these two classes of viral fusion proteins. In the present study, monitoring fusion of pseudovirus expressing Vesicular Stomatitis virus (VSV) G within endosomes shows that here, too, fusion is voltage dependent. This supports the claim that voltage dependence of fusion is biologically relevant and that cell-cell fusion reliably models the voltage dependence. Fusion induced by class I viral proteins is independent of voltage; chimeras expressing the ectodomain of a class I fusion protein and the transmembrane domain of VSV G could therefore be used to explore the location within the protein responsible for voltage dependence. Results showed that the transmembrane domain is the region associated with voltage dependence. Experiments in which cells were enriched with acidic lipids led to the conclusion that it is the flip-flop of acidic lipids that carries the charge responsible for the observed voltage dependence of fusion. This flip-flop occurred downstream of hemifusion, in accord with previous findings that the voltage dependent steps of fusion occur at a stage subsequent to hemifusion.
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Affiliation(s)
- Ruben M. Markosyan
- Department of Molecular Biophysics and Physiology, Rush University Medical Center, Chicago, Illinois, United States of America
| | - Fredric S. Cohen
- Department of Molecular Biophysics and Physiology, Rush University Medical Center, Chicago, Illinois, United States of America
- * E-mail:
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12
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Li K, Markosyan RM, Zheng YM, Golfetto O, Bungart B, Li M, Ding S, He Y, Liang C, Lee JC, Gratton E, Cohen FS, Liu SL. IFITM proteins restrict viral membrane hemifusion. PLoS Pathog 2013; 9:e1003124. [PMID: 23358889 PMCID: PMC3554583 DOI: 10.1371/journal.ppat.1003124] [Citation(s) in RCA: 275] [Impact Index Per Article: 25.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/15/2012] [Accepted: 11/21/2012] [Indexed: 12/20/2022] Open
Abstract
The interferon-inducible transmembrane (IFITM) protein family represents a new class of cellular restriction factors that block early stages of viral replication; the underlying mechanism is currently not known. Here we provide evidence that IFITM proteins restrict membrane fusion induced by representatives of all three classes of viral membrane fusion proteins. IFITM1 profoundly suppressed syncytia formation and cell-cell fusion induced by almost all viral fusion proteins examined; IFITM2 and IFITM3 also strongly inhibited their fusion, with efficiency somewhat dependent on cell types. Furthermore, treatment of cells with IFN also markedly inhibited viral membrane fusion and entry. By using the Jaagsiekte sheep retrovirus envelope and influenza A virus hemagglutinin as models for study, we showed that IFITM-mediated restriction on membrane fusion is not at the steps of receptor- and/or low pH-mediated triggering; instead, the creation of hemifusion was essentially blocked by IFITMs. Chlorpromazine (CPZ), a chemical known to promote the transition from hemifusion to full fusion, was unable to rescue the IFITM-mediated restriction on fusion. In contrast, oleic acid (OA), a lipid analog that generates negative spontaneous curvature and thereby promotes hemifusion, virtually overcame the restriction. To explore the possible effect of IFITM proteins on membrane molecular order and fluidity, we performed fluorescence labeling with Laurdan, in conjunction with two-photon laser scanning and fluorescence-lifetime imaging microscopy (FLIM). We observed that the generalized polarizations (GPs) and fluorescence lifetimes of cell membranes expressing IFITM proteins were greatly enhanced, indicating higher molecularly ordered and less fluidized membranes. Collectively, our data demonstrated that IFITM proteins suppress viral membrane fusion before the creation of hemifusion, and suggested that they may do so by reducing membrane fluidity and conferring a positive spontaneous curvature in the outer leaflets of cell membranes. Our study provides novel insight into the understanding of how IFITM protein family restricts viral membrane fusion and infection. Many pathogenic viruses contain an envelope that must fuse with the cell membrane in order to gain entry and initiate infection. This process is mediated by one or more glycoproteins present on the surface of the virions, known as viral fusion proteins. Recently, a family of interferon-inducible transmembrane (IFITM) protein has been shown to block viral infection, including those of highly pathogenic viruses. Here we provide evidence that these IFITM proteins potently suppress membrane fusion induced by representatives of all three classes of viral fusion proteins. Interestingly, we found that the block is not at the steps of receptor binding or low pH that triggers conformational changes of viral fusion proteins required for membrane fusion. Rather, we discovered that the creation of hemifusion, an intermediate in which the outer membranes of the two lipid bilayers have merged but the inner membranes still remain intact is blocked by IFITM proteins. We further demonstrated that overexpression of IFITM proteins rigidify the cell membrane, thereby reducing membrane fluidity and fusion potential. Our study provides novel insight into the understanding of how IFITM proteins restrict viral entry and infection.
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Affiliation(s)
- Kun Li
- Department of Molecular Microbiology and Immunology, Bond Life Sciences Center, University of Missouri, Columbia, Missouri, United States of America
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13
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Kasai H, Takahashi N, Tokumaru H. Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis. Physiol Rev 2012; 92:1915-64. [DOI: 10.1152/physrev.00007.2012] [Citation(s) in RCA: 120] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.
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Affiliation(s)
- Haruo Kasai
- Laboratory of Structural Physiology, Center for Disease Biology and Integrative Medicine, Graduate School of Medicine, The University of Tokyo, Tokyo, Japan; and Faculty of Pharmaceutical Sciences at Kagawa, Tokushima Bunri University, Kagawa, Japan
| | - Noriko Takahashi
- Laboratory of Structural Physiology, Center for Disease Biology and Integrative Medicine, Graduate School of Medicine, The University of Tokyo, Tokyo, Japan; and Faculty of Pharmaceutical Sciences at Kagawa, Tokushima Bunri University, Kagawa, Japan
| | - Hiroshi Tokumaru
- Laboratory of Structural Physiology, Center for Disease Biology and Integrative Medicine, Graduate School of Medicine, The University of Tokyo, Tokyo, Japan; and Faculty of Pharmaceutical Sciences at Kagawa, Tokushima Bunri University, Kagawa, Japan
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14
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Nikolaus J, Warner JM, O'Shaughnessy B, Herrmann A. The pathway to membrane fusion through hemifusion. CURRENT TOPICS IN MEMBRANES 2011; 68:1-32. [PMID: 21771493 DOI: 10.1016/b978-0-12-385891-7.00001-5] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Affiliation(s)
- Jörg Nikolaus
- Department of Biology, Faculty of Mathematics and Natural Sciences I, Humboldt-University Berlin, Berlin, Germany
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15
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Markosyan RM, Cohen FS. Negative potentials across biological membranes promote fusion by class II and class III viral proteins. Mol Biol Cell 2010; 21:2001-12. [PMID: 20427575 PMCID: PMC2883944 DOI: 10.1091/mbc.e09-10-0904] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022] Open
Abstract
Fusion of virions pseudotyped with a class II, SFV E1 or VEEV E, or a class III protein, VSV G is promoted by negative potentials and hindered by positive potentials across the target cell. Hemifusion is independent of polarity. Reversion of hemifused membranes into two distinct ones is responsible for voltage-dependence and inhibition of fusion. Voltage was investigated as a factor in the fusion of virions. Virions, pseudotyped with a class II, SFV E1 or VEEV E, or a class III protein, VSV G, were prepared with GFP within the core and a fluorescent lipid. This allowed both hemifusion and fusion to be monitored. Voltage clamping the target cell showed that fusion is promoted by a negative potential and hindered by a positive potential. Hemifusion occurred independent of polarity. Lipid dye movement, in the absence of content mixing, ceased before complete transfer for positive potentials, indicating that reversion of hemifused membranes into two distinct membranes is responsible for voltage dependence and inhibition of fusion. Content mixing quickly followed lipid dye transfer for a negative potential, providing a direct demonstration that hemifusion induced by class II and class III viral proteins is a functional intermediate of fusion. In the hemifused state, virions that fused exhibited slower lipid transfer than did nonfusing virions. All viruses with class II or III fusion proteins may utilize voltage to achieve infection.
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Affiliation(s)
- Ruben M Markosyan
- Department of Molecular Biophysics and Physiology, Rush University Medical Center, Chicago, IL 60612, USA
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16
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Is HAP2-GCS1 an ancestral gamete fusogen? Trends Cell Biol 2010; 20:134-41. [DOI: 10.1016/j.tcb.2009.12.007] [Citation(s) in RCA: 81] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2009] [Revised: 12/17/2009] [Accepted: 12/17/2009] [Indexed: 12/31/2022]
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17
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Clancy EK, Barry C, Ciechonska M, Duncan R. Different activities of the reovirus FAST proteins and influenza hemagglutinin in cell–cell fusion assays and in response to membrane curvature agents. Virology 2010; 397:119-29. [DOI: 10.1016/j.virol.2009.10.039] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2009] [Revised: 09/30/2009] [Accepted: 10/22/2009] [Indexed: 12/12/2022]
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18
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Abstract
Prm1 is a pheromone-induced membrane glycoprotein that promotes plasma membrane fusion in yeast mating pairs. HA-Prm1 migrates at twice its expected molecular weight on non-reducing SDS-PAGE gels and coprecipitates with Prm1-TAP, indicating that Prm1 is a disulfide-linked homodimer. The N terminus of a plasma membrane-localized GFP-Prm1 endocytic mutant projects into the cytoplasm, where it is protected from low pH quenching in live cells and from external protease in spheroplasts. In a revised topological map, Prm1 has four transmembrane domains and two large extracellular loops. Mutation of all four cysteines in the extracellular loops blocked disulfide bond formation and destabilized the Prm1 homodimer without preventing Prm1 transport to contact sites in mating pairs. Cys(120) in loop 1 and Cys(545) in loop 2 form disulfide cross-links in the Prm1 homodimer and are required for fusion activity. Cys(120) lies between a hydrophobic segment formerly thought to be a transmembrane domain and an amphipathic helix. An interaction between either of these regions and the opposing membrane could promote fusion.
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Affiliation(s)
- Valerie N Olmo
- Department of Biochemistry and Molecular Biology, The Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland 21205, USA
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19
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Melikyan GB. Common principles and intermediates of viral protein-mediated fusion: the HIV-1 paradigm. Retrovirology 2008; 5:111. [PMID: 19077194 PMCID: PMC2633019 DOI: 10.1186/1742-4690-5-111] [Citation(s) in RCA: 135] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/11/2008] [Accepted: 12/10/2008] [Indexed: 12/20/2022] Open
Abstract
Enveloped viruses encode specialized fusion proteins which promote the merger of viral and cell membranes, permitting the cytosolic release of the viral cores. Understanding the molecular details of this process is essential for antiviral strategies. Recent structural studies revealed a stunning diversity of viral fusion proteins in their native state. In spite of this diversity, the post-fusion structures of these proteins share a common trimeric hairpin motif in which the amino- and carboxy-terminal hydrophobic domains are positioned at the same end of a rod-shaped molecule. The converging hairpin motif, along with biochemical and functional data, implies that disparate viral proteins promote membrane merger via a universal "cast-and-fold" mechanism. According to this model, fusion proteins first anchor themselves to the target membrane through their hydrophobic segments and then fold back, bringing the viral and cellular membranes together and forcing their merger. However, the pathways of protein refolding and the mechanism by which this refolding is coupled to membrane rearrangements are still not understood. The availability of specific inhibitors targeting distinct steps of HIV-1 entry permitted the identification of key conformational states of its envelope glycoprotein en route to fusion. These studies provided functional evidence for the direct engagement of the target membrane by HIV-1 envelope glycoprotein prior to fusion and revealed the role of partially folded pre-hairpin conformations in promoting the pore formation.
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Affiliation(s)
- Gregory B Melikyan
- Institute of Human Virology, Department of Microbiology and Immunology, University of Maryland School of Medicine, 725 W, Lombard St, Baltimore, MD 21201, USA.
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20
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Abstract
Diverse membrane fusion reactions in biology involve close contact between two lipid bilayers, followed by the local distortion of the individual bilayers and reformation into a single, merged membrane. We consider the structures and energies of the fusion intermediates identified in experimental and theoretical work on protein-free lipid bilayers. On the basis of this analysis, we then discuss the conserved fusion-through-hemifusion pathway of merger between biological membranes and propose that the entire progression, from the close juxtaposition of membrane bilayers to the expansion of a fusion pore, is controlled by protein-generated membrane stresses.
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Affiliation(s)
- Leonid V Chernomordik
- Section on Membrane Biology, Laboratory of Cellular and Molecular Biophysics, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892-1855, USA.
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21
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Soman NR, Lanza GM, Heuser JM, Schlesinger PH, Wickline SA. Synthesis and characterization of stable fluorocarbon nanostructures as drug delivery vehicles for cytolytic peptides. NANO LETTERS 2008; 8:1131-6. [PMID: 18302330 PMCID: PMC2710241 DOI: 10.1021/nl073290r] [Citation(s) in RCA: 104] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
Abstract
The therapeutic potential of cytolytic peptides is plagued by nonspecificity and enzymatic degradation. We report the first stable incorporation of melittin (a 26 amino acid amphipathic peptide) into an outer lipid monolayer of perfluorocarbon nanoparticles. Melittin binds avidly to the nanoparticles (dissociation constant approximately 3.27 nM) and retains its pore-forming activity after contact-mediated delivery to model bilayer membrane (liposomes) thereby demonstrating the effectiveness of perfluorocarbon nanoparticles as unique nanocarriers for cytolytic peptides.
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Affiliation(s)
| | | | | | - Paul H. Schlesinger
- Corresponding authors: Paul H. Schlesinger, Associate Professor of Cell Biology and Physiology, Washington University School of Medicine, Campus Box 8228, 660 South Euclid Ave., St. Louis, MO 63110. Telephone: 1-314-362-2223. E-mail: . Samuel Wickline, Professor of Medicine, Biomedical Engineering, Physics CTRAIN Group, Washington University School of Medicine, Campus Box 8215, 660 South Euclid Ave., St. Louis, MO 63110. E-mail:
| | - Samuel A. Wickline
- Corresponding authors: Paul H. Schlesinger, Associate Professor of Cell Biology and Physiology, Washington University School of Medicine, Campus Box 8228, 660 South Euclid Ave., St. Louis, MO 63110. Telephone: 1-314-362-2223. E-mail: . Samuel Wickline, Professor of Medicine, Biomedical Engineering, Physics CTRAIN Group, Washington University School of Medicine, Campus Box 8215, 660 South Euclid Ave., St. Louis, MO 63110. E-mail:
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22
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Markosyan RM, Kielian M, Cohen FS. Fusion induced by a class II viral fusion protein, semliki forest virus E1, is dependent on the voltage of the target cell. J Virol 2007; 81:11218-25. [PMID: 17686870 PMCID: PMC2045574 DOI: 10.1128/jvi.01256-07] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Cells expressing the low pH-triggered class II viral fusion protein E1 of Semliki Forest virus (SFV) were fused to target cells. Fusion was monitored by electrical capacitance and aqueous dye measurements. Electrical voltage-clamp measurements showed that SFV E1-induced cell-cell fusion occurred quickly after acidification for a trans-negative potential across the target membrane (i.e., negative potential inside the target cell) but that a trans-positive potential eliminated all fusion. Use of an ionophore to control potentials for a large population of cells confirmed the dependence of fusion on voltage polarity. In contrast, fusion induced by the class I fusion proteins of human immunodeficiency virus, avian sarcoma leukosis virus, and influenza virus was independent of the voltage polarity across the target cell. Initial pore size and pore growth were also independent of voltage polarity for the class I proteins. An intermediate of SFV E1-induced fusion was created by transient acidification at low temperature. Membranes were hemifused at this intermediate state, and raising the temperature at neutral pH allowed full fusion to occur. Capacitance measurements showed that maintaining a trans-positive potential definitely blocked fusion at steps following the creation of the hemifusion intermediate and may have inhibited fusion at prior steps. It is proposed that the trans-negative voltage across the endosomal membrane facilitates fusion after low-pH-induced conformational changes of SFV E1 have occurred.
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Affiliation(s)
- Ruben M Markosyan
- Department of Molecular Biophysics and Physiology, Rush University Medical Center, 1653 W. Congress Pkwy., Chicago, IL 60612, USA
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23
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Wong JL, Koppel DE, Cowan AE, Wessel GM. Membrane hemifusion is a stable intermediate of exocytosis. Dev Cell 2007; 12:653-9. [PMID: 17420001 PMCID: PMC1989768 DOI: 10.1016/j.devcel.2007.02.007] [Citation(s) in RCA: 56] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/18/2006] [Revised: 01/17/2007] [Accepted: 02/09/2007] [Indexed: 11/24/2022]
Abstract
Membrane fusion during exocytosis requires that two initially distinct bilayers pass through a hemifused intermediate in which the proximal monolayers are shared. Passage through this intermediate is an essential step in the process of secretion, but is difficult to observe directly in vivo. Here we study membrane fusion in the sea urchin egg, in which thousands of homogeneous cortical granules are associated with the plasma membrane prior to fertilization. Using fluorescence redistribution after photobleaching, we find that these granules are stably hemifused to the plasma membrane, sharing a cytoplasmic-facing monolayer. Furthermore, we find that the proteins implicated in the fusion process-the vesicle-associated proteins VAMP/synaptobrevin, synaptotagmin, and Rab3-are each immobile within the granule membrane. Thus, these secretory granules are tethered to their target plasma membrane by a static, catalytic fusion complex that maintains a hemifused membrane intermediate.
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Affiliation(s)
- Julian L. Wong
- Department of Molecular Biology, Cellular Biology, and Biochemistry Box G • Brown University • Providence, RI 02912
| | - Dennis E. Koppel
- Department of Molecular, Microbial and Structural Biology and Richard D. Berlin Center for Cell Analysis and Modeling University of Connecticut Health Center • Farmington, CT 06032
| | - Ann E. Cowan
- Department of Molecular, Microbial and Structural Biology and Richard D. Berlin Center for Cell Analysis and Modeling University of Connecticut Health Center • Farmington, CT 06032
| | - Gary M. Wessel
- Department of Molecular Biology, Cellular Biology, and Biochemistry Box G • Brown University • Providence, RI 02912
- Corresponding author phone: (401) 863-1051, fax: (401) 863-1182 e-mail:
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24
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Chernomordik LV, Zimmerberg J, Kozlov MM. Membranes of the world unite! J Cell Biol 2006; 175:201-7. [PMID: 17043140 PMCID: PMC2064561 DOI: 10.1083/jcb.200607083] [Citation(s) in RCA: 173] [Impact Index Per Article: 9.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/17/2006] [Accepted: 09/08/2006] [Indexed: 11/22/2022] Open
Abstract
Despite diverse origins, cellular fusion mechanisms converge at a pathway of phospholipid bilayer fusion. In this mini-review, we discuss how proteins can mediate each of the three major stages in the fusion pathway: contact, hemifusion, and the opening of an expanding fusion pore.
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Affiliation(s)
- Leonid V Chernomordik
- Laboratory of Cellular and Molecular Biophysics, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA.
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25
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Zhukovsky MA, Leikina E, Markovic I, Bailey AL, Chernomordik LV. Heterogeneity of early intermediates in cell-liposome fusion mediated by influenza hemagglutinin. Biophys J 2006; 91:3349-58. [PMID: 16905609 PMCID: PMC1614502 DOI: 10.1529/biophysj.106.088005] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
To explore early intermediates in membrane fusion mediated by influenza virus hemagglutinin (HA) and their dependence on the composition of the target membrane, we studied lipid mixing between HA-expressing cells and liposomes containing phosphatidylcholine (PC) with different hydrocarbon chains. For all tested compositions, our results indicate the existence of at least two types of intermediates, which differ in their lifetimes. The composition of the target membrane affects the stability of fusion intermediates at a stage before lipid mixing. For less fusogenic distearoyl PC-containing liposomes at 4 degrees C, some of the intermediates inactivate, and no intermediates advance to lipid mixing. Fusion intermediates that formed for the more fusogenic dioleoyl PC-containing liposomes did not inactivate and even yielded partial lipid mixing at 4 degrees C. Thus, a more fusogenic target membrane effectively blocks nonproductive release of the conformational energy of HA. Even for the same liposome composition, HA forms two types of fusion intermediates, dissimilar in their stability and propensity to fuse. This diversity of fusion intermediates emphasizes the importance of local membrane composition and local protein concentration in fusion of heterogeneous biological membranes.
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Affiliation(s)
- Mikhail A Zhukovsky
- Laboratory of Cellular and Molecular Biophysics, Section on Membrane Biology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892, USA.
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26
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Markosyan RM, Cohen FS, Melikyan GB. Time-resolved imaging of HIV-1 Env-mediated lipid and content mixing between a single virion and cell membrane. Mol Biol Cell 2005; 16:5502-13. [PMID: 16195349 PMCID: PMC1289397 DOI: 10.1091/mbc.e05-06-0496] [Citation(s) in RCA: 61] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
A method has been developed to follow fusion of individual pseudotyped virus expressing HIV-1 Env to cells by time-resolved fluorescence microscopy. Viral envelopes were labeled with a fluorescent lipid dye (DiD) and virus content was rendered visible by incorporating a Gag-GFP chimera. The Gag-GFP is naturally cleaved to the much smaller NC-GFP fragment in the mature virions. NC-GFP was readily released upon permeabilization of the viral envelope, whereas the capsid was retained. The NC-GFP thus provides a relatively small and mobile aqueous marker to follow viral content transfer. In fusion experiments, virions were bound to cells at low temperature, and fusion was synchronously triggered by a temperature jump. DiD transferred from virions to cells without a significant lag after the temperature jump. Some virions released DiD but retained NC-GFP. Surprisingly, the fraction of lipid mixing events yielding NC-GFP transfer was dependent on the type of target cell: of three infectable cell lines, only one permitted NC-GFP transfer within minutes of raising temperature. NC-GFP release did not correlate with the level of CD4 or coreceptor expression in the target cells. The data indicate that fusion pores formed by HIV-1 Env can remain small for a relatively long time before they enlarge.
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Affiliation(s)
- Ruben M Markosyan
- Department of Molecular Biophysics and Physiology, Rush University Medical Center, Chicago, IL 60612
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27
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Leikina E, Delanoe-Ayari H, Melikov K, Cho MS, Chen A, Waring AJ, Wang W, Xie Y, Loo JA, Lehrer RI, Chernomordik LV. Carbohydrate-binding molecules inhibit viral fusion and entry by crosslinking membrane glycoproteins. Nat Immunol 2005; 6:995-1001. [PMID: 16155572 DOI: 10.1038/ni1248] [Citation(s) in RCA: 189] [Impact Index Per Article: 9.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/13/2005] [Accepted: 08/08/2005] [Indexed: 12/16/2022]
Abstract
Defensins are peptides that protect the host against microorganisms. Here we show that the theta-defensin retrocyclin 2 (RC2) inhibited influenza virus infection by blocking membrane fusion mediated by the viral hemagglutinin. RC2 was effective even after hemagglutinin attained a fusogenic conformation or had induced membrane hemifusion. RC2, a multivalent lectin, prevented hemagglutinin-mediated fusion by erecting a network of crosslinked and immobilized surface glycoproteins. RC2 also inhibited fusion mediated by Sindbis virus and baculovirus. Human beta-defensin 3 and mannan-binding lectin also blocked viral fusion by creating a protective barricade of immobilized surface proteins. This general mechanism might explain the broad-spectrum antiviral activity of many multivalent lectins of the innate immune system.
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Affiliation(s)
- Eugenia Leikina
- Section on Membrane Biology, Laboratory of Cellular and Molecular Biophysics, National Institute of Child Health and Human Development, Bethesda, Maryland 20892-1855, USA
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28
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Abstract
Until recently, cells were thought to be integral and discrete components of tissues, and their state was determined by cell differentiation. However, under some conditions, stem cells or their progeny can fuse with cells of other types, mixing cytoplasmic and even genetic material of different (heterotypic) origins. The fusion of heterotypic cells could be of central importance for development, repair of tissues and the pathogenesis of disease.
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Affiliation(s)
- Brenda M Ogle
- Transplantation Biology and the Department of Physiology, Mayo Clinic, Rochester, Minnesota 55905, USA
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29
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Bailey A, Zhukovsky M, Gliozzi A, Chernomordik LV. Liposome composition effects on lipid mixing between cells expressing influenza virus hemagglutinin and bound liposomes. Arch Biochem Biophys 2005; 439:211-21. [PMID: 15963452 DOI: 10.1016/j.abb.2005.05.010] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/14/2005] [Revised: 05/03/2005] [Accepted: 05/05/2005] [Indexed: 11/23/2022]
Abstract
The involvement of contacting and distal lipid monolayers in different stages of protein-mediated fusion was studied for fusion mediated by influenza virus hemagglutinin. Inclusion of non-bilayer lipids in the composition of the liposomes bound to hemagglutinin-expressing cells affects fusion triggered by low pH. Lysophosphatidylcholine added to the outer membrane monolayers inhibits fusion. The same lipid added to the inner monolayer of the liposomes promotes both lipid and content mixing. In contrast to the inverted cone-shaped lysophosphatidylcholine, lipids of the opposite effective shape, oleic acid or cardiolipin with calcium, present in the inner monolayers inhibit fusion. These results along with fusion inhibition by a bipolar lipid that does not support peeling of one monolayer of the liposomal membrane from the other substantiate the hypothesis that fusion proceeds through a local hemifusion intermediate. The transition from hemifusion to the opening of an expanding fusion pore allows content mixing and greatly facilitates lipid mixing between liposomes and cells.
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Affiliation(s)
- Austin Bailey
- Section on Membrane Biology, Laboratory of Cellular and Molecular Biophysics, NICHD, NIH, Bethesda, MD, USA
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30
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Melikyan GB, Barnard RJO, Abrahamyan LG, Mothes W, Young JAT. Imaging individual retroviral fusion events: from hemifusion to pore formation and growth. Proc Natl Acad Sci U S A 2005; 102:8728-33. [PMID: 15937118 PMCID: PMC1150829 DOI: 10.1073/pnas.0501864102] [Citation(s) in RCA: 77] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Viral fusion proteins catalyze merger of viral and cell membranes through a series of steps that have not yet been well defined. To elucidate the mechanism of virus entry, we have imaged fusion between single virions bearing avian sarcoma and leukosis virus (ASLV) envelope glycoprotein (Env) and the cell membrane. Viral particles were labeled with a lipophilic dye and with palmitylated enhanced YFP that was incorporated into the inner leaflet of the viral membrane. When individual virions were bound to target cells expressing cognate receptors, they transferred their lipids and contents only when exposed to low, but not neutral, pH. These data are consistent with the proposed two-step mechanism of ASLV entry that involves receptor-priming followed by low pH activation. Most importantly, lipid mixing commonly occurred before formation of a small fusion pore that was quickly and sensitively detected by pH-dependent changes in palmitylated enhanced YFP fluorescence. Nascent fusion pores were metastable and irreversibly closed, remained small, or fully enlarged, permitting nucleocapsid delivery into the cytosol. These findings strongly imply that hemifusion and a small pore are the key intermediates of ASLV fusion. When added before low pH treatment, a peptide designed to prevent Env from folding into a final helical-bundle conformation abolished virus-cell fusion and infection. Therefore, we conclude that, after receptor-activation, Env undergoes low pH-dependent refolding into a six-helix bundle and, in doing so, sequentially catalyzes hemifusion, fusion pore opening, and enlargement.
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Affiliation(s)
- Gregory B Melikyan
- Department of Molecular Biophysics and Physiology, Rush University Medical Center, Chicago, IL 60612, USA.
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31
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Zaitseva E, Mittal A, Griffin DE, Chernomordik LV. Class II fusion protein of alphaviruses drives membrane fusion through the same pathway as class I proteins. ACTA ACUST UNITED AC 2005; 169:167-77. [PMID: 15809312 PMCID: PMC2171914 DOI: 10.1083/jcb.200412059] [Citation(s) in RCA: 57] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/04/2022]
Abstract
Viral fusion proteins of classes I and II differ radically in their initial structures but refold toward similar conformations upon activation. Do fusion pathways mediated by alphavirus E1 and influenza virus hemagglutinin (HA) that exemplify classes II and I differ to reflect the difference in their initial conformations, or concur to reflect the similarity in the final conformations? Here, we dissected the pathway of low pH–triggered E1-mediated cell–cell fusion by reducing the numbers of activated E1 proteins and by blocking different fusion stages with specific inhibitors. The discovered progression from transient hemifusion to small, and then expanding, fusion pores upon an increase in the number of activated fusion proteins parallels that established for HA-mediated fusion. We conclude that proteins as different as E1 and HA drive fusion through strikingly similar membrane intermediates, with the most energy-intensive stages following rather than preceding hemifusion. We propose that fusion reactions catalyzed by all proteins of both classes follow a similar pathway.
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Affiliation(s)
- Elena Zaitseva
- Section on Membrane Biology, Laboratory of Cellular and Molecular Biophysics, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA
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32
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Starai VJ, Thorngren N, Fratti RA, Wickner W. Ion regulation of homotypic vacuole fusion in Saccharomyces cerevisiae. J Biol Chem 2005; 280:16754-62. [PMID: 15737991 DOI: 10.1074/jbc.m500421200] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Biological membrane fusion employs divalent cations as protein cofactors or as signaling ligands. For example, Mg2+ is a cofactor for the N-ethylmaleimide-sensitive factor (NSF) ATPase, and the Ca2+ signal from neuronal membrane depolarization is required for synaptotagmin activation. Divalent cations also regulate liposome fusion, but the role of such ion interactions with lipid bilayers in Rab- and soluble NSF attachment protein receptor (SNARE)-dependent biological membrane fusion is less clear. Yeast vacuole fusion requires Mg2+ for Sec18p ATPase activity, and vacuole docking triggers an efflux of luminal Ca2+. We now report distinct reaction conditions where divalent or monovalent ions interchangeably regulate Rab- and SNARE-dependent vacuole fusion. In reactions with 5 mm Mg2+, other free divalent ions are not needed. Reactions containing low Mg2+ concentrations are strongly inhibited by the rapid Ca2+ chelator BAPTA. However, addition of the soluble SNARE Vam7p relieves BAPTA inhibition as effectively as Ca2+ or Mg2+, suggesting that Ca2+ does not perform a unique signaling function. When the need for Mg2+, ATP, and Sec18p for fusion is bypassed through the addition of Vam7p, vacuole fusion does not require any appreciable free divalent cations and can even be stimulated by their chelators. The similarity of these findings to those with liposomes, and the higher ion specificity of the regulation of proteins, suggests a working model in which ion interactions with bilayer lipids permit Rab- and SNARE-dependent membrane fusion.
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Affiliation(s)
- Vincent J Starai
- Department of Biochemistry, Dartmouth Medical School, Hanover, New Hampshire 03755-3844, USA
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33
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Roche S, Gaudin Y. Evidence that rabies virus forms different kinds of fusion machines with different pH thresholds for fusion. J Virol 2004; 78:8746-52. [PMID: 15280482 PMCID: PMC479077 DOI: 10.1128/jvi.78.16.8746-8752.2004] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Fusion of rabies virus with membranes is triggered at a low pH and is mediated by a viral glycoprotein (G). Fusion of rabies virus with liposomes was monitored by using a lipid mixing assay based on fluorescence resonance energy transfer. Fusion was detected below pH 6.4, and its extent increased with H(+) concentrations to be maximal around pH 6.15. The origin of the partial fusion activity of rabies virus under suboptimal pH conditions (i.e., between pH 6.15 and 6.4) was investigated. We demonstrate unambiguously that fusion at a suboptimal pH is distinct from the phenomenon of low-pH-induced inactivation and that it is not due to heterogeneity of the virus population. We also show that viruses that do not fuse under suboptimal pH conditions are indeed bound to the target liposomes and that the fusion complexes they have formed are blocked at an early stage of the fusion pathway. Our conclusion is that along the fusion reaction, different kinds of fusion machines with different pH thresholds for fusion can be formed. Possible explanations of this difference of pH sensitivity are discussed.
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Affiliation(s)
- Stéphane Roche
- Laboratoire de Virologie Moléculaire et Structurale, CNRS, 91198 Gif sur Yvette Cedex, France
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Matsuyama S, Delos SE, White JM. Sequential roles of receptor binding and low pH in forming prehairpin and hairpin conformations of a retroviral envelope glycoprotein. J Virol 2004; 78:8201-9. [PMID: 15254191 PMCID: PMC446138 DOI: 10.1128/jvi.78.15.8201-8209.2004] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/18/2003] [Accepted: 03/23/2004] [Indexed: 11/20/2022] Open
Abstract
A general model has been proposed for the fusion mechanisms of class I viral fusion proteins. According to this model a metastable trimer, anchored in the viral membrane through its transmembrane domain, transits to a trimeric prehairpin intermediate, anchored at its opposite end in the target membrane through its fusion peptide. A subsequent refolding event creates a trimer of hairpins (often termed a six-helix bundle) in which the previously well-separated transmembrane domain and fusion peptide (and their attached membranes) are brought together, thereby driving membrane fusion. While there is ample biochemical and structural information on the trimer-of-hairpins conformation of class I viral fusion proteins, less is known about intermediate states between native metastable trimers and the final trimer of hairpins. In this study we analyzed conformational states of the transmembrane subunit (TM), the fusion subunit, of the Env glycoprotein of the subtype A avian sarcoma and leukosis virus (ASLV-A). By analyzing forms of EnvA TM on mildly denaturing sodium dodecyl sulfate gels we identified five conformational states of EnvA TM. Following interaction of virions with a soluble form of the ASLV-A receptor at 37 degrees C, the metastable form of EnvA TM (which migrates at 37 kDa) transits to a 70-kDa and then to a 150-kDa species. Following subsequent exposure to a low pH (or an elevated temperature or the fusion promoting agent chlorpromazine), an additional set of bands at >150 kDa, and then a final band at 100 kDa, forms. Both an EnvA C-helix peptide (which inhibits virus fusion and infectivity) and the fusion-inhibitory agent lysophosphatidylcholine inhibit the formation of the >150- and 100-kDa bands. Our data are consistent with the 70- and 150-kDa bands representing precursor and fully formed prehairpin conformations of EnvA TM. Our data are also consistent with the >150-kDa bands representing higher-order oligomers of EnvA TM and with the 100-kDa band representing the fully formed six-helix bundle. In addition to resolving fusion-relevant conformational intermediates of EnvA TM, our data are compatible with a model in which the EnvA protein is activated by its receptor (at neutral pH and a temperature greater than or equal to room temperature) to form prehairpin conformations of EnvA TM, and in which subsequent exposure to a low pH is required to stabilize the final six-helix bundle, which drives a later stage of fusion.
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Affiliation(s)
- Shutoku Matsuyama
- Department of Cell Biology, University of Virginia, Charlottesville, VA 22908-0732, USA
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35
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Blanco J, Barretina J, Clotet B, Esté JA. R5 HIV gp120-mediated cellular contacts induce the death of single CCR5-expressing CD4 T cells by a gp41-dependent mechanism. J Leukoc Biol 2004; 76:804-11. [PMID: 15258189 DOI: 10.1189/jlb.0204100] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
Abstract
The use of CXC chemokine receptor 4 (CXCR4) and CC chemokine receptor 5 (CCR5) by X4 and R5 human immunodeficiency virus (HIV) envelopes (Env) influences HIV cytopathicity. Here, we have evaluated the role of CCR5 and gp41 in Env-induced cell death occurring during the contacts of uninfected, primary cells with MOLT cells infected with different R5 and X4 HIV isolates. As reported for X4-Env, R5 HIV-infected cells destroyed CD4 T cells expressing the appropriate coreceptor by inducing the formation of syncytia and the death of single target cells. Therefore, only the small (<10%) CCR5+ subset of primary CD4 T cells was sensitive to cellular presentation of R5-Env, and CCR5-CD4 T cells showed complete resistance to R5-Env-mediated cell death. X4- and R5-infected cells killed single primary cells by a common mechanism that was dependent on gp41 function and induced a rapid loss of mitochondrial membrane potential and plasma membrane integrity in target cells. Single-cell death was not affected by the blockade of HIV replication in target cells or G-protein signaling through CXCR4/CCR5. In contrast, caspase inhibition (Z-Val-Ala-Asp-fluoromethylketone) profoundly changed the outcome of cell-to-cell contacts by reducing the number of single dead CD4 T cells and increasing the rate of syncytium formation. In conclusion, X4 and R5 HIV Env share a common gp41-dependent mechanism to kill CD4 T cells during cellular contacts. Env tropism and coreceptor expression but not differential killing mechanisms seem to govern the extent of cytopathic effects induced by HIV infection.
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Affiliation(s)
- Julià Blanco
- Retrovirology Laboratory, Fundació irsiCaixa, Hospital Universitari Germans Trias i Pujol, Universitat Autònoma de Barcelona, Catalonia, Spain.
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Leikina E, Mittal A, Cho MS, Melikov K, Kozlov MM, Chernomordik LV. Influenza hemagglutinins outside of the contact zone are necessary for fusion pore expansion. J Biol Chem 2004; 279:26526-32. [PMID: 15078874 DOI: 10.1074/jbc.m401883200] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Current models for membrane fusion in diverse biological processes are focused on the local action of fusion proteins present in the contact zone where the proteins anchored in one membrane might interact directly with the other membrane. Are the fusion proteins outside of the contact zone just bystanders? Here we assess the role of these "outsider" proteins in influenza virus hemagglutinin-mediated fusion between red blood cells and either hemagglutinin-expressing cells or viral particles. To selectively inhibit or enhance the actions of hemagglutinin outsiders, the antibodies that bind to hemagglutinin and proteases that cleave it were conjugated to polystyrene microspheres too large to enter the contact zone. We also involved hemagglutinin outsiders into interactions with additional red blood cells. We find the hemagglutinin outsiders to be necessary and sufficient for fusion. Interfering with the activity of the hemagglutinin outsiders inhibited fusion. Selective conversion of hemagglutinin outsiders alone into fusion-competent conformation was sufficient to achieve fusion. The discovered functional role of fusion proteins located outside of the contact zone suggests a tempting analogy to mechanisms by which proteins mediate membrane fission from outside of the fission site.
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Affiliation(s)
- Eugenia Leikina
- Section on Membrane Biology, Laboratory of Cellular and Molecular Biophysics, NICHD, National Institutes of Health, Bethesda, Maryland 20892-1855, USA
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37
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Affiliation(s)
- Stéphane Roche
- Laboratoire de Genetiquie des Virus du CNRS, Gif sur Yvette, France
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38
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Mittal A, Leikina E, Chernomordik LV, Bentz J. Kinetically differentiating influenza hemagglutinin fusion and hemifusion machines. Biophys J 2003; 85:1713-24. [PMID: 12944286 PMCID: PMC1303345 DOI: 10.1016/s0006-3495(03)74601-3] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022] Open
Abstract
Membrane fusion mediated by influenza virus hemagglutinin (HA) yields different phenotypes depending on the surface density of activated HAs. A key question is whether different phenotypes arise from different fusion machines or whether different numbers of identical fusion machines yield different probabilistic outcomes. If fusion were simply a less probable event than hemifusion, requiring a larger number of identical fusion machines to occur first, then two predictions can be made. First, fusion should have a shorter average delay time than hemifusion, since there are more machines. Second, fusion should have a longer execution time of lipid mixing after it begins than hemifusion, since the full event cannot be faster than the partial event. Using a new automated video microscopy technique, we simultaneously monitored many HA-expressing cells fusing with erythrocytes and identified individual cell pairs with either full or only partial redistribution of fluorescent lipids. The full lipid mixing phenotype also showed contents mixing, i.e., fusion. Kinetic screening of the digitized fluorescence data showed that the execution of lipid mixing after the onset is faster for fusion than hemifusion. We found no correlation between the delay times before the onset of lipid mixing and the final fusion phenotype. We also found that the execution time for fusion was faster than that for hemifusion. Thus, we provide the first experimental evidence for fusion and hemifusion arising from different machines.
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Affiliation(s)
- Aditya Mittal
- Department of Bioscience and Biotechnology, Drexel University, Philadelphia, Pennsylvania 19104, USA
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Tamm LK, Crane J, Kiessling V. Membrane fusion: a structural perspective on the interplay of lipids and proteins. Curr Opin Struct Biol 2003; 13:453-66. [PMID: 12948775 DOI: 10.1016/s0959-440x(03)00107-6] [Citation(s) in RCA: 157] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
The fusion of biological membranes is governed by the carefully orchestrated interplay of membrane proteins and lipids. Recently determined structures of fusion proteins, individual domains of fusion proteins and their complexes with regulatory proteins and membrane lipids have yielded much suggestive insight into how viral and intracellular membrane fusion might proceed. These structures may be combined with new knowledge on the fusion of pure lipid bilayer membranes in an attempt to begin to piece together the complex puzzle of how biological membrane fusion machines operate on membranes.
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Affiliation(s)
- Lukas K Tamm
- Department of Molecular Physiology and Biological Physics, University of Virginia, PO Box 800736, Charlottesville, VA 22908-0736, USA.
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40
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Bentz J, Mittal A. Architecture of the influenza hemagglutinin membrane fusion site. BIOCHIMICA ET BIOPHYSICA ACTA 2003; 1614:24-35. [PMID: 12873763 DOI: 10.1016/s0005-2736(03)00160-3] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
The mechanism of influenza hemagglutinin (HA) mediated membrane fusion has been intensively studied for over 20 years after the bromelain-released ectodomain of HA at neutral pH was first crystallized. Nearly 10 years ago, the low-pH-induced "spring coiled" conformational change of HA was predicted from peptide chemistry and confirmed by crystallography. Other work has yielded a wealth of knowledge on the observed changes in HA fusion/hemifusion phenotypes as a function of site-specific mutations of HA, or added amphipathic molecules or particular IgGs. It is becoming clear that the conformational changes predicted by the crystallography are necessary to cause fusion and that interfering with these changes can block fusion or reduce it to hemifusion. What is not known is how the conformational changes cause fusion. In particular, while it is generally agreed that fusion requires an aggregate of HAs, how the aggregate may act to transduce the energy of the HA conformational changes to creating the initial fusion defect is not known. We have used a comprehensive mass action kinetic model of HA-mediated fusion to carry out a "meta-analysis" of several key data sets, using HA-expressing cells and using virions. The consensus result of these detailed kinetic studies was that the fusion site of influenza hemagglutinin (HA) is an aggregate with at least eight HAs. The high-energy conformational change of only two of these HAs within the aggregate permits the formation of the first fusion pore. This "8 and 2" result was required to best fit all the data. We review these studies and how this kinetic result can guide and constrain HA fusion models. The kinetic analysis suggests that the sequence of fusion intermediates starts with protein control and ends with lipid control, which makes sense. While curvature intermediates, e.g. the lipid stalk, are almost certainly within the fusion sequence, the "8 and 2" result does not suggest that they are the first step after HA aggregation. The stabilized hydrophobic defect model we have proposed as a precursor to the lipid stalk can form and is consistent with the "8 and 2" result.
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Affiliation(s)
- Joe Bentz
- Department of Bioscience and Biotechnology, Drexel University, 32nd and Chestnut Streets, Philadelphia, PA 19104, USA.
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41
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Abstract
The ability of two or more cells to unite to form a new syncytial cell has been utilized in metazoans throughout evolution to form many complex organs, such as muscles, bones and placentae. This requires migration, recognition and adhesion between cells together with fusion of their plasma membranes and rearrangement of their cytoplasmic contents. Until recently, understanding of the mechanisms of cell fusion was restricted to fusion between enveloped viruses and their target cells. The identification of new factors that take part in developmental cell fusion in C. elegans opens the way to understanding how cells fuse and what the functions of this process are. In this review, we describe current knowledge on the mechanisms and putative roles of developmental cell fusion in C. elegans and how cell fusion is regulated, together with other intercellular processes to promote organogenesis.
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Affiliation(s)
- Gidi Shemer
- Department of Biology, Technion-Israel Institute of Technology, Haifa, Israel.
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42
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Abstract
One approach to the understanding of fusion in cells and model membranes involves stalk formation and expansion of the hemifusion diaphragm. We predict theoretically the initiation of hemifusion by stalk expansion and the dynamics of mesoscopic hemifusion diaphragm expansion in the light of recent experiments and theory that suggested that hemifusion is driven by intramembrane tension far from the fusion zone. Our predictions include a square-root scaling of the hemifusion zone size on time as well as an estimate of the minimal tension for initiation of hemifusion. Whereas a minimal amount of pressure is evidently needed for stalk formation, it is not necessarily required for stalk expansion. The energy required for tension-induced fusion is much smaller than that required for pressure-driven fusion.
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Affiliation(s)
- Guy Hed
- Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel.
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43
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Abstract
Disparate biological processes involve fusion of two membranes into one and fission of one membrane into two. To formulate the possible job description for the proteins that mediate remodeling of biological membranes, we analyze the energy price of disruption and bending of membrane lipid bilayers at the different stages of bilayer fusion. The phenomenology and the pathways of the well-characterized reactions of biological remodeling, such as fusion mediated by influenza hemagglutinin, are compared with those studied for protein-free bilayers. We briefly consider some proteins involved in fusion and fission, and the dependence of remodeling on the lipid composition of the membranes. The specific hypothetical mechanisms by which the proteins can lower the energy price of the bilayer rearrangement are discussed in light of the experimental data and the requirements imposed by the elastic properties of the bilayer.
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Affiliation(s)
- Leonid V Chernomordik
- Section on Membrane Biology, Laboratory of Cellular and Molecular Biophysics, NICHD, National Institutes of Health, 10 Center Drive, Bethesda, Maryland 20892-1855, USA.
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44
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Blanco J, Barretina J, Ferri KF, Jacotot E, Gutiérrez A, Armand-Ugón M, Cabrera C, Kroemer G, Clotet B, Esté JA. Cell-surface-expressed HIV-1 envelope induces the death of CD4 T cells during GP41-mediated hemifusion-like events. Virology 2003; 305:318-29. [PMID: 12573577 DOI: 10.1006/viro.2002.1764] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Cells expressing the HIV-1 envelope glycoprotein complex (gp120/gp41, Env) induce the death of target cells either after cell-to-cell fusion or after cell-to-cell contact in a fusion-independent fashion. Here, we demonstrate that Env-induced death of single cells (including primary CD4 T cells) required gp120 and gp41 function. The gp41 peptide C34, which blocked syncytium formation, completely inhibited the death of single target cells by specifically acting on gp41 function. Moreover, Env-induced single cell death was exclusively observed in CD4 cells and was associated with specific gp41-mediated transfer of lipids from the membrane of Env-expressing cells to the target cell but not with detectable cytoplasm mixing (complete fusion). We conclude that after gp120 function, gp41 mediates close cell-to-cell contacts, thereby triggering cell death in single uninfected cells in the absence of detectable cell-to-cell fusion.
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Affiliation(s)
- Julià Blanco
- Laboratori de Retrovirologia, Fundació irsiCaixa, Hospital Universitari Germans Trias i Pujol, Universitat Autònoma de Barcelona, Catalonia, Spain.
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45
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Affiliation(s)
- Robert Blumenthal
- Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda-Frederick, Maryland, USA.
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46
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Leikina E, Ramos C, Markovic I, Zimmerberg J, Chernomordik LV. Reversible stages of the low-pH-triggered conformational change in influenza virus hemagglutinin. EMBO J 2002; 21:5701-10. [PMID: 12411488 PMCID: PMC131056 DOI: 10.1093/emboj/cdf559] [Citation(s) in RCA: 56] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
The refolding of the prototypic fusogenic protein hemagglutinin (HA) at the pH of fusion is considered to be a concerted and irreversible discharge of a loaded spring, with no distinct intermediates between the initial and final conformations. Here, we show that HA refolding involves reversible conformations with a lifetime of minutes. After reneutralization, low pH-activated HA returns from the conformations wherein both the fusion peptide and the kinked loop of the HA2 subunit are exposed, but the HA1 subunits have not yet dissociated, to a structure indistinguishable from the initial one in functional, biochemical and immunological characteristics. The rate of the transition from reversible conformations to irreversible refolding depends on the pH and on the presence of target membrane. Importantly, recovery of the initial conformation is blocked by the interactions between adjacent HA trimers. The existence of the identified reversible stage of refolding can be crucial for allowing multiple copies of HA to synchronize their release of conformational energy, as required for fusion.
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Affiliation(s)
| | | | - Ingrid Markovic
- Laboratory of Cellular and Molecular Biophysics, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892-1855, USA
Present address: Division of Monoclonal Antibodies, Office of Therapeutics Research and Review, Center for Biologics Evaluation and Research, Food and Drug Administration, Bethesda, MD 20892, USA Corresponding author e-mail:
| | | | - Leonid V. Chernomordik
- Laboratory of Cellular and Molecular Biophysics, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892-1855, USA
Present address: Division of Monoclonal Antibodies, Office of Therapeutics Research and Review, Center for Biologics Evaluation and Research, Food and Drug Administration, Bethesda, MD 20892, USA Corresponding author e-mail:
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47
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Roche S, Gaudin Y. Characterization of the equilibrium between the native and fusion-inactive conformation of rabies virus glycoprotein indicates that the fusion complex is made of several trimers. Virology 2002; 297:128-35. [PMID: 12083843 DOI: 10.1006/viro.2002.1429] [Citation(s) in RCA: 88] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Rabies virus-induced membrane fusion is triggered at low pH and is mediated by the trimeric viral glycoprotein (G). G assumes three conformations: the native state (N) detected above pH 7; the activated state (A), which initiates the fusion process; and the fusion-inactive conformation (I) observed after prolonged incubation at low pH. Differently from other viral fusogenic glycoproteins, G in the I state recovers its native conformation when reincubated above pH 7. Here, we demonstrate that there is a thermodynamic equilibrium between the different states of G between pH 6 and pH 7.5. The study of this equilibrium at various pH values indicated that the conformational change toward I is induced by the protonation of at least three residues per trimer. Finally, studies on the mechanism leading to low pH induced fusion inactivation indicated that a large number of G molecules is required for stable hydrophobic interaction of the virus with the target membrane.
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Affiliation(s)
- Stéphane Roche
- Laboratoire de Génétique des Virus du CNRS, 91198 Gif sur Yvette Cedex, France
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48
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Nishimura T, Nakano T. Vesicles in the subacrosomal space and partial diaphragms in the subacrosomal nuclear envelope of round spermatids of a rat injected intravenously with gold labeled-testosterone-bovine serum albumin conjugate: vesicular trafficking from acrosome to nucleus. Okajimas Folia Anat Jpn 2002; 79:15-23. [PMID: 12199534 DOI: 10.2535/ofaj.79.15] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Colloidal gold labeled-testosterone-bovine serum albumin conjugate (testosterone-BSA-gold) injected into the vascular system of rats is taken up by endocytosis into round spermatids. Based on observation of silver deposits indicating testosterone-BSA-gold with silver enhancement, we have suggested that testosterone-BSA-gold enters the nuclei through not only the postacrosomal nuclear envelope but also the subacrosomal nuclear envelope (SNE) via the acrosome (Nishimura and Nakano, 1997). However, it was unclear how testosterone-BSA-gold in the acrosome entered the nucleoplasm. Spermatids showing silver deposits on the subacrosomal space were observed under electron microscope without silver enhancement, to clarify the courses of translocation. In the spermatids, vesicles with the gold particles were seen in the subacrosomal space. Some of the vesicles were in contact with the SNE. A part of the outer nuclear membrane projected into the space. Furthermore, local single-bilayer nuclear membranes, which seemed to partially lack nuclear lamina, were present in the SNE. These results indicate the possibility that the vesicles mediate the transport of testosterone-BSA-gold from acrosome to nucleus, and that the vesicle membrane fuses with not only the outer nuclear membrane but also a shared bilayer in the SNE.
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Affiliation(s)
- Toshikazu Nishimura
- Department of Anatomy, Aichi Medical University School of Medicine, Yazako, Japan.
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49
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Mittal A, Leikina E, Bentz J, Chernomordik LV. Kinetics of influenza hemagglutinin-mediated membrane fusion as a function of technique. Anal Biochem 2002; 303:145-52. [PMID: 11950214 DOI: 10.1006/abio.2002.5590] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Reliable techniques are required to evaluate the plausibility of proposed membrane fusion mechanisms. Here we have studied the kinetics of establishing the lipidic connection between hemagglutinin-expressing cells (HA-cells) and red blood cells (RBC) labeled with octadecylrhodamine, R18, using three different experimental approaches: (1) the most common approach of monitoring the rate of the R18 dequenching in a cuvette with a suspension of RBC/HA-cell complexes; (2) video fluorescence microscopy (VFM) to detect the waiting times before the onset of R18 redistribution, not dequenching, for each RBC attached to an adherent HA-cell; and (3) a new approach based on blockage of RBC fusion to an adherent HA-cell at different time points by lysophosphatidylcholine (LPC), so that only the cell pairs which, at the time of LPC application, had fused or were irreversibly committed to fusion contributed to the final extent of lipid mixing. The LPC blockage and VFM gave very similar estimates for the fusion kinetics, with LPC monitoring also those sites committed to the lipid mixing process. In contrast, R18 dequenching in the cuvette was much slower, i.e., it monitors a much later stage of dye redistribution.
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Affiliation(s)
- Aditya Mittal
- Department of Bioscience & Biotechnology, Drexel University, Philadelphia, Pennsylvania, 19104, USA
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50
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Abstract
Multiple cell biological processes involve two opposite rearrangements of membrane configuration, referred to as fusion and fission. While membrane intermediates in protein-mediated fusion have been studied in some detail, the global force which drives sequential stages of the fusion reaction from early local intermediates to an expanding fusion pore remains unknown. Fusion proceeds via stages, which are analogous but in the opposite direction to that of membrane budding-off and fission driven by protein coats. On the basis of this analogy, we propose that an interconnected coat formed by membrane-bound activated fusion proteins surrounding the membrane contact zone generates the driving force for fusion. This fusion protein coat has a strongly curved intrinsic shape opposite to that of the protein coat driving fission. To relieve internal stresses, the fusion protein coat spontaneously bends out of the initial shape of the membrane surface. This bending produces elastic stresses in the underlying lipid bilayer and drives its fusion with the apposing membrane. The hypothesis that 'bystander' proteins (i.e. fusion proteins outside the contact zone) generate the driving force for fusion offers a new interpretation for a number of known features of the fusion reaction mediated by the prototype fusion protein, influenza hemagglutinin, and might bring new insights into mechanisms of other fusion reactions.
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Affiliation(s)
- Michael M Kozlov
- Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel Aviv University, 69978, Israel.
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