1
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Mittal A, Chauhan A. Aspects of Biological Replication and Evolution Independent of the Central Dogma: Insights from Protein-Free Vesicular Transformations and Protein-Mediated Membrane Remodeling. J Membr Biol 2022; 255:185-209. [PMID: 35333977 PMCID: PMC8951669 DOI: 10.1007/s00232-022-00230-4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/22/2022] [Accepted: 03/06/2022] [Indexed: 11/21/2022]
Abstract
Biological membrane remodeling is central to living systems. In spite of serving as “containers” of whole-living systems and functioning as dynamic compartments within living systems, biological membranes still find a “blue collar” treatment compared to the “white collar” nucleic acids and proteins in biology. This may be attributable to the fact that scientific literature on biological membrane remodeling is only 50 years old compared to ~ 150 years of literature on proteins and a little less than 100 years on nucleic acids. However, recently, evidence for symbiotic origins of eukaryotic cells from data only on biological membranes was reported. This, coupled with appreciation of reproducible amphiphilic self-assemblies in aqueous environments (mimicking replication), has already initiated discussions on origins of life beyond nucleic acids and proteins. This work presents a comprehensive compilation and meta-analyses of data on self-assembly and vesicular transformations in biological membranes—starting from model membranes to establishment of Influenza Hemagglutinin-mediated membrane fusion as a prototypical remodeling system to a thorough comparison between enveloped mammalian viruses and cellular vesicles. We show that viral membrane fusion proteins, in addition to obeying “stoichiometry-driven protein folding”, have tighter compositional constraints on their amino acid occurrences than general-structured proteins, regardless of type/class. From the perspective of vesicular assemblies and biological membrane remodeling (with and without proteins) we find that cellular vesicles are quite different from viruses. Finally, we propose that in addition to pre-existing thermodynamic frameworks, kinetic considerations in de novo formation of metastable membrane structures with available “third-party” constituents (including proteins) were not only crucial for origins of life but also continue to offer morphological replication and/or functional mechanisms in modern life forms, independent of the central dogma.
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Affiliation(s)
- Aditya Mittal
- Kusuma School of Biological Sciences, Indian Institute of Technology Delhi (IIT Delhi), Hauz Khas, New Delhi, 110016, India. .,Supercomputing Facility for Bioinformatics and Computational Biology (SCFBio), IIT Delhi, Hauz Khas, New Delhi, 110016, India.
| | - Akanksha Chauhan
- Kusuma School of Biological Sciences, Indian Institute of Technology Delhi (IIT Delhi), Hauz Khas, New Delhi, 110016, India
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2
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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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3
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Abstract
Influenza A virus (IAV) is an enveloped virus of the Orthomyxoviridae with a negative-sense single-stranded RNA genome. During virus cell entry, viral and cellular cues are delivered in a stepwise manner within two distinct cellular compartments-the endosomes and the cytosol. Endosome maturation primes the viral core for uncoating by cytosolic host proteins and host-mediated virus disaggregation is essential for genome import and replication in the nucleus. Recent evidence shows that two well-known cellular proteins-histone deacetylase 6 (HDAC6) and karyopherin-β2 (kapβ2)-uncoat influenza virus. HDAC6 is 1 of 11 HDACs and an X-linked, cytosolic lysine deacetylase. Under normal cellular conditions HDAC6 is the tubulin deacetylase. Under proteasomal stress HDAC6 binds unanchored ubiquitin, dynein and myosin II to sequester misfolded protein aggregates for autophagy. Kapβ2 is a member of the importin β family that transports RNA-binding proteins into the nucleus by binding to disordered nuclear localization signals (NLSs) known as PY-NLS. Kapβ2 is emerging as a universal uncoating factor for IAV and human immunodeficiency virus type 1 (HIV-1). Kapβ2 can also reverse liquid-liquid phase separation (LLPS) of RNA-binding proteins by promoting their disaggregation. Thus, it is becoming evident that key players in the management of cellular condensates and membraneless organelles are potent virus uncoating factors. This emerging concept reveals implications in viral pathogenesis, as well as, the promise for cell-targeted therapeutic strategies to block universal virus uncoating pathways hijacked by enveloped RNA viruses.
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Affiliation(s)
- Yohei Yamauchi
- School of Cellular & Molecular Medicine, University of Bristol, Bristol, United Kingdom.
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4
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Bashkirov PV, Chekashkina KV, Shnyrova AV, Frolov VA. Electrophysiological Methods for Detection of Membrane Leakage and Hemifission by Dynamin 1. Methods Mol Biol 2020; 2159:141-162. [PMID: 32529369 DOI: 10.1007/978-1-0716-0676-6_11] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
Membrane fusion and fission are indispensable parts of intracellular membrane recycling and transport. Electrophysiological techniques have been instrumental in discovering and studying fusion and fission pores, the key intermediates shared by both processes. In cells, electrical admittance measurements are used to assess in real time the dynamics of the pore conductance, reflecting the nanoscale transformations of the pore, simultaneously with membrane leakage. Here, we described how this technique is adapted to in vitro mechanistic analyses of membrane fission by dynamin 1 (Dyn1), the protein orchestrating membrane fission in endocytosis. We reconstitute the fission reaction using purified Dyn1 and biomimetic lipid membrane nanotubes of defined geometry. We provide a comprehensive protocol describing simultaneous measurements of the ionic conductance through the nanotube lumen and across the nanotube wall, enabling spatiotemporal correlation between the nanotube constriction by Dyn1, leading to fission and membrane leakage. We present examples of "leaky" and "tight" fission reactions, specify the resolution limits of our method, and discuss how our results support the hemi-fission conjecture.
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Affiliation(s)
- Pavel V Bashkirov
- Federal Research and Clinical Center of Physical-Chemical Medicine, Moscow, Russia
- A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia
| | - Ksenia V Chekashkina
- Federal Research and Clinical Center of Physical-Chemical Medicine, Moscow, Russia
- A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia
| | - Anna V Shnyrova
- Department of Biochemistry and Molecular Biology, Biophysics Institute (CSIC, UPV/EHU), University of the Basque Country, Leioa, Spain
| | - Vadim A Frolov
- Department of Biochemistry and Molecular Biology, Biophysics Institute (CSIC, UPV/EHU), University of the Basque Country, Leioa, Spain.
- IKERBASQUE, Basque Foundation for Science, Bilbao, Spain.
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5
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D'Agostino M, Risselada HJ, Endter LJ, Comte-Miserez V, Mayer A. SNARE-mediated membrane fusion arrests at pore expansion to regulate the volume of an organelle. EMBO J 2018; 37:embj.201899193. [PMID: 30120144 DOI: 10.15252/embj.201899193] [Citation(s) in RCA: 26] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/07/2018] [Revised: 06/28/2018] [Accepted: 07/03/2018] [Indexed: 11/09/2022] Open
Abstract
Constitutive membrane fusion within eukaryotic cells is thought to be controlled at its initial steps, membrane tethering and SNARE complex assembly, and to rapidly proceed from there to full fusion. Although theory predicts that fusion pore expansion faces a major energy barrier and might hence be a rate-limiting and regulated step, corresponding states with non-expanding pores are difficult to assay and have remained elusive. Here, we show that vacuoles in living yeast are connected by a metastable, non-expanding, nanoscopic fusion pore. This is their default state, from which full fusion is regulated. Molecular dynamics simulations suggest that SNAREs and the SM protein-containing HOPS complex stabilize this pore against re-closure. Expansion of the nanoscopic pore to full fusion can thus be triggered by osmotic pressure gradients, providing a simple mechanism to rapidly adapt organelle volume to increases in its content. Metastable, nanoscopic fusion pores are then not only a transient intermediate but can be a long-lived, physiologically relevant and regulated state of SNARE-dependent membrane fusion.
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Affiliation(s)
- Massimo D'Agostino
- Département de Biochimie, Université de Lausanne, Epalinges, Switzerland
| | - Herre Jelger Risselada
- Department of Theoretical Physics, Georg-August University, Göttingen, Germany.,Leiden Institute of Chemistry, Leiden University, Leiden, The Netherlands
| | - Laura J Endter
- Leiden Institute of Chemistry, Leiden University, Leiden, The Netherlands
| | | | - Andreas Mayer
- Département de Biochimie, Université de Lausanne, Epalinges, Switzerland
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6
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Basu A, Komazin-Meredith G, McCarthy C, Antanasijevic A, Cardinale SC, Mishra RK, Barnard DL, Caffrey M, Rong L, Bowlin TL. Molecular Mechanism Underlying the Action of Influenza A Virus Fusion Inhibitor MBX2546. ACS Infect Dis 2017; 3:330-335. [PMID: 28301927 DOI: 10.1021/acsinfecdis.6b00194] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Abstract
Influenza A virus envelop protein hemagglutinin (HA) plays important roles in viral entry. We previously have reported that MBX2546, a novel influenza A virus inhibitor, binds to HA and inhibits HA-mediated membrane fusion. In this report, we show that (i) both binding and stabilization of HA by MBX2546 are required for the inhibition of viral infection and (ii) the binding of HA by MBX2546 represses the low-pH-induced conformational change in the HA, which is a prerequisite for membrane fusion. Mutations in MBX2546-resistant influenza A/PR/8/34 (H1N1) viruses are mapped in the HA stem region near the amino terminus of HA2. Finally, we have modeled the binding site of MBX2546 using molecular dynamics and find that the resulting structure is in good agreement with our results. Together, these studies underscore the importance of the HA stem loop region as a potential target for therapeutic intervention.
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Affiliation(s)
- Arnab Basu
- Microbiotix Inc., One Innovation Drive, Worcester, Massachusetts 01605, United States
| | | | - Courtney McCarthy
- Microbiotix Inc., One Innovation Drive, Worcester, Massachusetts 01605, United States
| | - Aleksandar Antanasijevic
- Department
of Biochemistry and Molecular Genetics, University of Illinois at Chicago, Chicago, Illinois 60607, United States
| | - Steven C. Cardinale
- Microbiotix Inc., One Innovation Drive, Worcester, Massachusetts 01605, United States
| | - Rama K. Mishra
- Center for Molecular
Innovation and Drug Discovery, Northwestern University, Evanston, Illinois 60208, United States
| | - Dale L. Barnard
- Institute
for
Antiviral Research, Utah State University, Logan, Utah 84322, United States
| | - Michael Caffrey
- Department
of Biochemistry and Molecular Genetics, University of Illinois at Chicago, Chicago, Illinois 60607, United States
| | - Lijun Rong
- Department
of Microbiology and Immunology, College of Medicine, University of Illinois at Chicago, Chicago, Illinois 60612, United States
| | - Terry L. Bowlin
- Microbiotix Inc., One Innovation Drive, Worcester, Massachusetts 01605, United States
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7
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Cholesterol Increases the Openness of SNARE-Mediated Flickering Fusion Pores. Biophys J 2016; 110:1538-1550. [PMID: 27074679 DOI: 10.1016/j.bpj.2016.02.019] [Citation(s) in RCA: 51] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/12/2014] [Revised: 02/03/2016] [Accepted: 02/04/2016] [Indexed: 11/22/2022] Open
Abstract
Flickering of fusion pores during exocytotic release of hormones and neurotransmitters is well documented, but without assays that use biochemically defined components and measure single-pore dynamics, the mechanisms remain poorly understood. We used total internal reflection fluorescence microscopy to quantify fusion-pore dynamics in vitro and to separate the roles of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins and lipid bilayer properties. When small unilamellar vesicles bearing neuronal v-SNAREs fused with planar bilayers reconstituted with cognate t-SNARES, lipid and soluble cargo transfer rates were severely reduced, suggesting that pores flickered. From the lipid release times we computed pore openness, the fraction of time the pore is open, which increased dramatically with cholesterol. For most lipid compositions tested, SNARE-mediated and nonspecifically nucleated pores had similar openness, suggesting that pore flickering was controlled by lipid bilayer properties. However, with physiological cholesterol levels, SNAREs substantially increased the fraction of fully open pores and fusion was so accelerated that there was insufficient time to recruit t-SNAREs to the fusion site, consistent with t-SNAREs being preclustered by cholesterol into functional docking and fusion platforms. Our results suggest that cholesterol opens pores directly by reducing the fusion-pore bending energy, and indirectly by concentrating several SNAREs into individual fusion events.
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8
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Smrt ST, Lorieau JL. Membrane Fusion and Infection of the Influenza Hemagglutinin. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2016; 966:37-54. [PMID: 27966108 DOI: 10.1007/5584_2016_174] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/05/2022]
Abstract
The influenza virus is a major health concern associated with an estimated 5000 to 30,000 deaths every year (Reed et al. 2015) and a significant economic impact with the development of treatments, vaccinations and research (Molinari et al. 2007). The entirety of the influenza genome is comprised of only eleven coding genes. An enormous degree of variation in non-conserved regions leads to significant challenges in the development of inclusive inhibitors for treatment. The fusion peptide domain of the influenza A hemagglutinin (HA) is a promising candidate for treatment since it is one of the most highly conserved sequences in the influenza genome (Heiny et al. 2007), and it is vital to the viral life cycle. Hemagglutinin is a class I viral fusion protein that catalyzes the membrane fusion process during cellular entry and infection. Impediment of the hemagglutinin's function, either through incomplete post-translational processing (Klenk et al. 1975; Lazarowitz and Choppin 1975) or through mutations (Cross et al. 2001), leads to non-infective virus particles. This review will investigate current research on the role of hemagglutinin in the virus life cycle, its structural biology and mechanism as well as the central role of the hemagglutinin fusion peptide (HAfp) to influenza membrane fusion and infection.
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Affiliation(s)
- Sean T Smrt
- Department of Chemistry, University of Illinois at Chicago, Chicago, IL, 60607, USA
| | - Justin L Lorieau
- Department of Chemistry, University of Illinois at Chicago, Chicago, IL, 60607, USA.
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9
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Desai TM, Marin M, Chin CR, Savidis G, Brass AL, Melikyan GB. IFITM3 restricts influenza A virus entry by blocking the formation of fusion pores following virus-endosome hemifusion. PLoS Pathog 2014; 10:e1004048. [PMID: 24699674 PMCID: PMC3974867 DOI: 10.1371/journal.ppat.1004048] [Citation(s) in RCA: 242] [Impact Index Per Article: 24.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/17/2013] [Accepted: 02/18/2014] [Indexed: 02/04/2023] Open
Abstract
Interferon-induced transmembrane proteins (IFITMs) inhibit infection of diverse enveloped viruses, including the influenza A virus (IAV) which is thought to enter from late endosomes. Recent evidence suggests that IFITMs block virus hemifusion (lipid mixing in the absence of viral content release) by altering the properties of cell membranes. Consistent with this mechanism, excess cholesterol in late endosomes of IFITM-expressing cells has been reported to inhibit IAV entry. Here, we examined IAV restriction by IFITM3 protein using direct virus-cell fusion assay and single virus imaging in live cells. IFITM3 over-expression did not inhibit lipid mixing, but abrogated the release of viral content into the cytoplasm. Although late endosomes of IFITM3-expressing cells accumulated cholesterol, other interventions leading to aberrantly high levels of this lipid did not inhibit virus fusion. These results imply that excess cholesterol in late endosomes is not the mechanism by which IFITM3 inhibits the transition from hemifusion to full fusion. The IFITM3's ability to block fusion pore formation at a post-hemifusion stage shows that this protein stabilizes the cytoplasmic leaflet of endosomal membranes without adversely affecting the lumenal leaflet. We propose that IFITM3 interferes with pore formation either directly, through partitioning into the cytoplasmic leaflet of a hemifusion intermediate, or indirectly, by modulating the lipid/protein composition of this leaflet. Alternatively, IFITM3 may redirect IAV fusion to a non-productive pathway, perhaps by promoting fusion with intralumenal vesicles within multivesicular bodies/late endosomes. Interferon-induced transmembrane proteins (IFITMs) block infection of many enveloped viruses, including the influenza A virus (IAV) that enters from late endosomes. IFITMs are thought to prevent virus hemifusion (merger of contacting leaflets without formation of a fusion pore) by altering the properties of cell membranes. Here we performed single IAV imaging and found that IFITM3 did not interfere with hemifusion, but prevented complete fusion. Also, contrary to a current view that excess cholesterol in late endosomes of IFITM3-expressing cells inhibits IAV entry, we show that cholesterol-laden endosomes are permissive for virus fusion. The ability of IFITM3 to block the formation of fusion pores implies that this protein stabilizes the cytoplasmic leaflet of endosomal membranes, either directly or indirectly, through altering its physical properties. IFITM3 may also redirect IAV to a non-productive pathway by promoting fusion with intralumenal vesicles of late endosomes instead of their limiting membrane.
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Affiliation(s)
- Tanay M. Desai
- Division of Pediatric Infectious Diseases, Emory University Children's Center, Atlanta, Georgia, United States of America
| | - Mariana Marin
- Division of Pediatric Infectious Diseases, Emory University Children's Center, Atlanta, Georgia, United States of America
| | - Christopher R. Chin
- Microbiology and Physiological Systems (MaPS) Department, University of Massachusetts Medical School, Worcester, Massachusetts, United States of America
| | - George Savidis
- Microbiology and Physiological Systems (MaPS) Department, University of Massachusetts Medical School, Worcester, Massachusetts, United States of America
| | - Abraham L. Brass
- Microbiology and Physiological Systems (MaPS) Department, University of Massachusetts Medical School, Worcester, Massachusetts, United States of America
| | - Gregory B. Melikyan
- Division of Pediatric Infectious Diseases, Emory University Children's Center, Atlanta, Georgia, United States of America
- Children's Healthcare of Atlanta, Atlanta, Georgia, United States of America
- * E-mail:
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10
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Ivanovic T, Choi JL, Whelan SP, van Oijen AM, Harrison SC. Influenza-virus membrane fusion by cooperative fold-back of stochastically induced hemagglutinin intermediates. eLife 2013; 2:e00333. [PMID: 23550179 PMCID: PMC3578201 DOI: 10.7554/elife.00333] [Citation(s) in RCA: 121] [Impact Index Per Article: 11.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/23/2012] [Accepted: 12/25/2012] [Indexed: 12/20/2022] Open
Abstract
Influenza virus penetrates cells by fusion of viral and endosomal membranes catalyzed by the viral hemagglutinin (HA). Structures of the initial and final states of the HA trimer define the fusion endpoints, but do not specify intermediates. We have characterized these transitions by analyzing low-pH-induced fusion kinetics of individual virions and validated the analysis by computer simulation. We detect initial engagement with the target membrane of fusion peptides from independently triggered HAs within the larger virus-target contact patch; fusion then requires engagement of three or four neighboring HA trimers. Effects of mutations in HA indicate that withdrawal of the fusion peptide from a pocket in the pre-fusion trimer is rate-limiting for both events, but the requirement for cooperative action of several HAs to bring the fusing membranes together leads to a long-lived intermediate state for single, extended HA trimers. This intermediate is thus a fundamental aspect of the fusion mechanism. DOI:http://dx.doi.org/10.7554/eLife.00333.001.
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Affiliation(s)
- Tijana Ivanovic
- Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, United States
- Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder, Colorado, United States
| | - Jason L Choi
- Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, United States
| | - Sean P Whelan
- Department of Microbiology and Immunology, Harvard Medical School, Boston, United States
| | - Antoine M van Oijen
- Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, United States
| | - Stephen C Harrison
- Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, United States
- Howard Hughes Medical Institute, Harvard Medical School, Boston, United States
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11
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Abstract
Viruses are intracellular parasites that hijack the cellular machinery for their own replication. Therefore, an obligatory step in the virus life cycle is the delivery of the viral genome inside the cell. Enveloped viruses (i.e., viruses with a lipid envelope) use a two-step procedure to release their genetic material into the cell: (i) they first bind to specific surface receptors of the target cell membrane and then, (ii) they fuse the viral and cell membranes. This last step may occur at the cell surface or after internalization of the virus particle by endocytosis or by some other route (e.g., macropinocytosis). Remarkably, the virus-cell membrane fusion process goes essentially along the same intermediate steps as other membrane fusions that occur for instance in vesicular fusion at the nerve synapsis or cell-cell fusion in yeast mating. Specialized viral proteins, fusogens, promote virus-cell membrane fusion. The viral fusogens experience drastic structural rearrangements during fusion, liberating the energy required to overcome the repulsive forces that prevent spontaneous fusion of the two membranes. This chapter describes the different types of viral fusogens and their mode of action, as are currently known.
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12
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Garg P, Nemec KN, Khaled AR, Tatulian SA. Transmembrane pore formation by the carboxyl terminus of Bax protein. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2013; 1828:732-42. [DOI: 10.1016/j.bbamem.2012.08.006] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/28/2012] [Revised: 07/29/2012] [Accepted: 08/07/2012] [Indexed: 12/29/2022]
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13
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Abstract
HIV entry involves binding of the trimeric viral envelope glycoprotein (Env) gp120/gp41 to cell surface receptors, which triggers conformational changes in Env that drive the membrane fusion reaction. The conformational landscape that the lipids and Env navigate en route to fusion has been examined by biophysical measurements on the microscale, whereas electron tomography, x-rays, and NMR have provided insights into the process on the nanoscale and atomic scale. However, the coupling between the lipid and protein pathways that give rise to fusion has not been resolved. Here, we discuss the known and unknown about the overall HIV Env-mediated fusion process.
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Affiliation(s)
| | - Stewart Durell
- Laboratory of Cell Biology, Center for Cancer Research, NCI, National Institutes of Health, and
| | - Mathias Viard
- From the Nanobiology Program and
- the Basic Science Program, SAIC-Frederick, Inc., Center for Cancer Research Nanobiology Program (CCRNP), Frederick National Lab, Frederick, Maryland 21702
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14
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Hamilton BS, Whittaker GR, Daniel S. Influenza virus-mediated membrane fusion: determinants of hemagglutinin fusogenic activity and experimental approaches for assessing virus fusion. Viruses 2012; 4:1144-68. [PMID: 22852045 PMCID: PMC3407899 DOI: 10.3390/v4071144] [Citation(s) in RCA: 125] [Impact Index Per Article: 10.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2012] [Revised: 07/11/2012] [Accepted: 07/17/2012] [Indexed: 12/15/2022] Open
Abstract
Hemagglutinin (HA) is the viral protein that facilitates the entry of influenza viruses into host cells. This protein controls two critical aspects of entry: virus binding and membrane fusion. In order for HA to carry out these functions, it must first undergo a priming step, proteolytic cleavage, which renders it fusion competent. Membrane fusion commences from inside the endosome after a drop in lumenal pH and an ensuing conformational change in HA that leads to the hemifusion of the outer membrane leaflets of the virus and endosome, the formation of a stalk between them, followed by pore formation. Thus, the fusion machinery is an excellent target for antiviral compounds, especially those that target the conserved stem region of the protein. However, traditional ensemble fusion assays provide a somewhat limited ability to directly quantify fusion partly due to the inherent averaging of individual fusion events resulting from experimental constraints. Inspired by the gains achieved by single molecule experiments and analysis of stochastic events, recently-developed individual virion imaging techniques and analysis of single fusion events has provided critical information about individual virion behavior, discriminated intermediate fusion steps within a single virion, and allowed the study of the overall population dynamics without the loss of discrete, individual information. In this article, we first start by reviewing the determinants of HA fusogenic activity and the viral entry process, highlight some open questions, and then describe the experimental approaches for assaying fusion that will be useful in developing the most effective therapies in the future.
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Affiliation(s)
- Brian S. Hamilton
- Department of Microbiology and Immunology, Cornell University, Ithaca, NY 14853, USA;
| | - Gary R. Whittaker
- Department of Microbiology and Immunology, Cornell University, Ithaca, NY 14853, USA;
| | - Susan Daniel
- School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY 14853, USA;
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15
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Essid M, Gopaldass N, Yoshida K, Merrifield C, Soldati T. Rab8a regulates the exocyst-mediated kiss-and-run discharge of the Dictyostelium contractile vacuole. Mol Biol Cell 2012; 23:1267-82. [PMID: 22323285 PMCID: PMC3315810 DOI: 10.1091/mbc.e11-06-0576] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022] Open
Abstract
A molecular dissection of contractile vacuole (CV) discharge shows that Rab8a is recruited to the CV a few seconds before the exocyst. Together they tether it to the plasma membrane and commit it to fusion. GTP hydrolysis is necessary for vacuole detethering, a process in which LvsA, a protein of the Chédiak–Higashi family, plays a crucial role. Water expulsion by the contractile vacuole (CV) in Dictyostelium is carried out by a giant kiss-and-run focal exocytic event during which the two membranes are only transiently connected but do not completely merge. We present a molecular dissection of the GTPase Rab8a and the exocyst complex in tethering of the contractile vacuole to the plasma membrane, fusion, and final detachment. Right before discharge, the contractile vacuole bladder sequentially recruits Drainin, a Rab11a effector, Rab8a, the exocyst complex, and LvsA, a protein of the Chédiak–Higashi family. Rab8a recruitment precedes the nucleotide-dependent arrival of the exocyst to the bladder by a few seconds. A dominant-negative mutant of Rab8a strongly binds to the exocyst and prevents recruitment to the bladder, suggesting that a Rab8a guanine nucleotide exchange factor activity is associated with the complex. Absence of Drainin leads to overtethering and blocks fusion, whereas expression of constitutively active Rab8a allows fusion but blocks vacuole detachment from the plasma membrane, inducing complete fragmentation of tethered vacuoles. An indistinguishable phenotype is generated in cells lacking LvsA, implicating this protein in postfusion detethering. Of interest, overexpression of a constitutively active Rab8a mutant reverses the lvsA-null CV phenotype.
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Affiliation(s)
- Miriam Essid
- Départment de Biochimie, Faculté des Sciences, Université de Genève, Geneva, Switzerland
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16
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Abstract
Cell-to-cell fusion plays an important role in normal physiology and in different pathological conditions. Early fusion stages mediated by specialized proteins and yielding fusion pores are followed by a pore expansion stage that is dependent on cell metabolism and yet unidentified machinery. Because of a similarity of membrane bending in the fusion pore rim and in highly curved intracellular membrane compartments, in the present study we explored whether changes in the activity of the proteins that generate these compartments affect cell fusion initiated by protein fusogens of influenza virus and baculovirus. We raised the intracellular concentration of curvature-generating proteins in cells by either expressing or microinjecting the ENTH (epsin N-terminal homology) domain of epsin or by expressing the GRAF1 (GTPase regulator associated with focal adhesion kinase 1) BAR (Bin/amphiphysin/Rvs) domain or the FCHo2 (FCH domain-only protein 2) F-BAR domain. Each of these treatments promoted syncytium formation. Cell fusion extents were also influenced by treatments targeting the function of another curvature-generating protein, dynamin. Cell-membrane-permeant inhibitors of dynamin GTPase blocked expansion of fusion pores and dominant-negative mutants of dynamin influenced the syncytium formation extents. We also report that syncytium formation is inhibited by reagents lowering the content and accessibility of PtdIns(4,5)P2, an important regulator of intracellular membrane remodelling. Our findings indicate that fusion pore expansion at late stages of cell-to-cell fusion is mediated, directly or indirectly, by intracellular membrane-shaping proteins.
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17
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18
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Joo KI, Tai A, Lee CL, Wong C, Wang P. Imaging multiple intermediates of single-virus membrane fusion mediated by distinct fusion proteins. Microsc Res Tech 2010; 73:886-900. [PMID: 20232362 DOI: 10.1002/jemt.20841] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Abstract
Membrane fusion plays an essential role in the entry of enveloped viruses into target cells. The merging of viral and target cell membranes is catalyzed by viral fusion proteins, which involves multiple sequential steps in the fusion process. However, the fusion mechanisms mediated by different fusion proteins involve multiple transient intermediates that have not been well characterized. Here, we report a synthetic virus platform that allows us to better understand the different fusion mechanisms driven by the diverse types fusion proteins. The platform consists of lentiviral particles coenveloped with a surface antibody, which serves as the binding protein, along with a fusion protein derived from either influenza virus (HAmu) or Sindbis virus (SINmu). By using a single virus tracking technique, we demonstrated that both HAmu- and SINmu-bearing viruses enter cells through clathrin-dependent endocytosis, but they required different endosomal trafficking routes to initiate viral fusion. Direct observation of single viral fusion events clearly showed that hemifusion mediated by SINmu upon exposure to low pH occurs faster than that mediated by HAmu. Monitoring sequential fusion processes by dual labeling the outer and inner leaflets of viral membranes also revealed that the SINmu-mediated hemifusion intermediate is relatively long-lived as compared with that mediated by HAmu. Taken together, we have demonstrated that the combination of this versatile viral platform with the techniques of single virus tracking can be a powerful tool for revealing molecular details of fusion mediated by various fusion proteins.
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Affiliation(s)
- Kye-Il Joo
- Department of Chemical Engineering and Materials Science, Viterbi School of Engineering, University of Southern California, Los Angeles, California 90089, USA
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19
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Dominak LM, Omiatek DM, Gundermann EL, Heien ML, Keating CD. Polymeric crowding agents improve passive biomacromolecule encapsulation in lipid vesicles. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2010; 26:13195-200. [PMID: 20695558 PMCID: PMC2919175 DOI: 10.1021/la101903r] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/12/2010] [Revised: 06/24/2010] [Indexed: 05/19/2023]
Abstract
Large solutes such as high molecular weight proteins can be difficult to encapsulate in lipid vesicles. Passive trapping of these macromolecular solutes during vesicle formation typically results in concentrations inside the vesicles that are much lower than in the external solution. Here, we investigated the effect of macromolecular crowding on passive encapsulation of biological macromolecules with molecular weights ranging from 52 kDa to 660 kDa within both individual giant lipid vesicles (GVs, > 3 microm diameter) and populations of 200 nm diameter large unilamellar vesicles (LUVs). Fluorescently labeled biomacromolecules were encapsulated during vesicle formation in the presence or absence of three weight percent poly(ethylene glycol) (PEG; 8 kDa) or dextran 500 kDa, which served as crowding agents. Encapsulation efficiency of the labeled biomolecules was higher for the lower molecular weight solutes, with internal concentrations essentially equal to external concentrations for labeled biomacromolecules with hydrodynamic radii (r(h)) less than 10 nm. In contrast, internal concentrations were reduced markedly for larger solutes with r(h) > or = 10 nm. Addition of PEG or dextran during vesicle formation improved encapsulation of these larger proteins up to the same levels as observed for the smaller proteins, such that internal and external concentrations were equal. This observation is consistent with PEG and dextran acting as volume excluders, reducing the hydrodynamic radius of the biomacromolecules and increasing their encapsulation. This work demonstrates a simple and general route to improved encapsulation of otherwise poorly encapsulated macromolecular solutes in both GV and LUVs up to their concentration in the solution present during vesicle formation.
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Affiliation(s)
- Lisa M. Dominak
- Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802
| | - Donna M. Omiatek
- Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802
| | - Erica L. Gundermann
- Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802
| | - Michael L. Heien
- Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802
| | - Christine D. Keating
- Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802
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Abstract
Enveloped viruses use specialized protein machinery to fuse the viral membrane with that of the host cell during cell invasion. In influenza virus, hundreds of copies of the haemagglutinin (HA) fusion glycoprotein project from the virus surface. Despite intensive study of HA and its fusion activity, the protein's modus operandi in manipulating viral and target membranes to catalyse their fusion is poorly understood. Here, the three-dimensional architecture of influenza virus-liposome complexes at pH 5.5 was investigated by electron cryo-tomography. Tomographic reconstructions show that early stages of membrane remodeling take place in a target membrane-centric manner, progressing from punctate dimples, to the formation of a pinched liposomal funnel that may impinge on the apparently unperturbed viral envelope. The results suggest that the M1 matrix layer serves as an endoskeleton for the virus and a foundation for HA during membrane fusion. Fluorescence spectroscopy monitoring fusion between liposomes and virions shows that leakage of liposome contents takes place more rapidly than lipid mixing at pH 5.5. The relation of 'leaky' fusion to the observed prefusion structures is discussed.
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Affiliation(s)
- Kelly K Lee
- Department of Medicinal Chemistry, University of Washington, Seattle, WA 98195-7610, USA.
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21
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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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Affiliation(s)
- Jakob B. Sørensen
- Department of Neuroscience and Pharmacology, Faculty of Health Sciences, University of Copenhagen, 2200 Copenhagen N, Denmark;
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23
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Koch P, Lampe M, Godinez WJ, Müller B, Rohr K, Kräusslich HG, Lehmann MJ. Visualizing fusion of pseudotyped HIV-1 particles in real time by live cell microscopy. Retrovirology 2009; 6:84. [PMID: 19765276 PMCID: PMC2762461 DOI: 10.1186/1742-4690-6-84] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/22/2009] [Accepted: 09/18/2009] [Indexed: 11/10/2022] Open
Abstract
Background Most retroviruses enter their host cells by fusing the viral envelope with the plasma membrane. Although the protein machinery promoting fusion has been characterized extensively, the dynamics of the process are largely unknown. Results We generated human immunodeficiency virus-1 (HIV-1) particles pseudotyped with the envelope (Env) protein of ecotropic murine leukemia virus eMLV to study retrovirus entry at the plasma membrane using live-cell microscopy. This Env protein mediates highly efficient pH independent fusion at the cell surface and can be functionally tagged with a fluorescent protein. To detect fusion events, double labeled particles carrying one fluorophor in Env and the other in the matrix (MA) domain of Gag were generated and characterized. Fusion events were defined as loss of Env signal after virus-cell contact. Single particle tracking of >20,000 individual traces in two color channels recorded 28 events of color separation, where particles lost the Env protein, with the MA layer remaining stable at least for a short period. Fourty-five events were detected where both colors were lost simultaneously. Importantly, the first type of event was never observed when particles were pseudotyped with a non-fusogenic Env. Conclusion These results reveal rapid retroviral fusion at the plasma membrane and permit studies of the immediate post-fusion events.
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Affiliation(s)
- Peter Koch
- Department of Virology, Universitätsklinikum Heidelberg, Heidelberg, Germany.
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24
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Richard JP, Leikina E, Chernomordik LV. Cytoskeleton reorganization in influenza hemagglutinin-initiated syncytium formation. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2008; 1788:450-7. [PMID: 18976631 DOI: 10.1016/j.bbamem.2008.09.014] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/03/2008] [Revised: 09/23/2008] [Accepted: 09/24/2008] [Indexed: 11/28/2022]
Abstract
Little is known about the mechanisms of cell-cell fusion in development and diseases and, especially, about fusion stages downstream of an opening of nascent fusion pore(s). Earlier works on different cell-cell fusion reactions have indicated that cytoskeleton plays important role in syncytium formation. However, due to complexity of these reactions and multifaceted contributions of cytoskeleton in cell physiology, it has remained unclear whether cytoskeleton directly drives fusion pore expansion or affects preceding fusion stages. Here we explore cellular reorganization associated with fusion pore expansion in syncytium formation using relatively simple experimental system. Fusion between murine embryonic fibroblasts NIH3T3-based cells is initiated on demand by well-characterized fusogen influenza virus hemagglutinin. We uncouple early fusion stages dependent on protein fusogens from subsequent fusion pore expansion stage and establish that the transition from local fusion to syncytium requires metabolic activity of living cells. Effective syncytium formation for cells with disorganized actin and microtubule cytoskeleton argues against hypothesis that cytoskeleton drives fusion expansion.
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Affiliation(s)
- Jean-Philippe Richard
- 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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25
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Abstract
Membrane fusion is an essential step during entry of enveloped viruses into cells. Conventional fusion assays are generally limited to observation of ensembles of multiple fusion events, confounding more detailed analysis of the sequence of the molecular steps involved. We have developed an in vitro, two-color fluorescence assay to monitor kinetics of single virus particles fusing with a target bilayer on an essentially fluid support. Analysis of lipid- and content-mixing trajectories on a particle-by-particle basis provides evidence for multiple, long-lived kinetic intermediates leading to hemifusion, followed by a single, rate-limiting step to pore formation. We interpret the series of intermediates preceding hemifusion as a result of the requirement that multiple copies of the trimeric hemagglutinin fusion protein be activated to initiate the fusion process.
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26
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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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27
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Abstract
Infection by viruses having lipid-bilayer envelopes proceeds through fusion of the viral membrane with a membrane of the target cell. Viral 'fusion proteins' facilitate this process. They vary greatly in structure, but all seem to have a common mechanism of action, in which a ligand-triggered, large-scale conformational change in the fusion protein is coupled to apposition and merger of the two bilayers. We describe three examples—the influenza virus hemagglutinin, the flavivirus E protein and the vesicular stomatitis virus G protein—in some detail, to illustrate the ways in which different structures have evolved to implement this common mechanism. Fusion inhibitors can be effective antiviral agents.
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Affiliation(s)
- Stephen C Harrison
- Jack and Eileen Connors Structural Biology Laboratory, Harvard Medical School, Laboratory of Molecular Medicine, Children's Hospital Boston, Howard Hughes Medical Institute, 250 Longwood Avenue, Boston, MA 02115, USA.
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28
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Abstract
The aqueous compartment inside a vesicle makes its first connection with the extracellular fluid through an intermediate structure termed the exocytotic fusion pore. Progress in exocytosis can be measured in terms of the formation and growth of the fusion pore. The fusion pore has become a major focus of research in exocytosis; sensitive biophysical measurements have provided various glimpses of what it looks like and how it behaves. Some of the principal questions about the molecular mechanism of exocytosis can be cast explicitly in terms of properties and transitions of fusion pores. This Review will present current knowledge about fusion pores in Ca(2+)-triggered exocytosis, highlight recent advances and relate questions about fusion pores to broader issues concerning how cells regulate exocytosis and how nerve terminals release neurotransmitter.
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Affiliation(s)
- Meyer B Jackson
- Department of Physiology, University of Wisconsin School of Medicine and Public Health, 1300 University Avenue, Madison,WI 53706, USA.
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29
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Plonsky I, Kingsley DH, Rashtian A, Blank PS, Zimmerberg J. Initial size and dynamics of viral fusion pores are a function of the fusion protein mediating membrane fusion. Biol Cell 2008; 100:377-86. [PMID: 18208404 PMCID: PMC3650648 DOI: 10.1042/bc20070040] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
BACKGROUND INFORMATION Protein-mediated merger of biological membranes, membrane fusion, is an important process. To investigate the role of fusogenic proteins in the initial size and dynamics of the fusion pore (a narrow aqueous pathway, which widens to finalize membrane fusion), two different fusion proteins expressed in the same cell line were investigated: the major glycoprotein of baculovirus Autographa californica (GP64) and the HA (haemagglutinin) of influenza X31. RESULTS The host Sf9 cells expressing these viral proteins, irrespective of protein species, fused to human RBCs (red blood cells) upon acidification of the medium. A high-time-resolution electrophysiological study of fusion pore conductance revealed fundamental differences in (i) the initial pore conductance; pores created by HA were smaller than those created by GP64; (ii) the ability of pores to flicker; only HA-mediated pores flickered; and (iii) the time required for pore formation; HA-mediated pores took much longer to form after acidification. CONCLUSION HA and GP64 have divergent electrophysiological phenotypes even when they fuse identical membranes, and fusion proteins play a crucial role in determining initial fusion pore characteristics. The structure of the initial fusion pore detected by electrical conductance measurements is sensitive to the nature of the fusion protein.
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Affiliation(s)
- Ilya Plonsky
- 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, U.S.A
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30
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Biswas S, Yin SR, Blank PS, Zimmerberg J. Cholesterol promotes hemifusion and pore widening in membrane fusion induced by influenza hemagglutinin. J Gen Physiol 2008; 131:503-13. [PMID: 18443361 PMCID: PMC2346574 DOI: 10.1085/jgp.200709932] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2007] [Accepted: 04/02/2008] [Indexed: 01/01/2023] Open
Abstract
Cholesterol-specific interactions that affect membrane fusion were tested for using insect cells; cells that have naturally low cholesterol levels (< 4 mol %). Sf9 cells were engineered (HAS cells) to express the hemagglutinin (HA) of the influenza virus X-31 strain. Enrichment of HAS cells with cholesterol reduced the delay between triggering and lipid dye transfer between HAS cells and human red blood cells (RBC), indicating that cholesterol facilitates membrane lipid mixing prior to fusion pore opening. Increased cholesterol also increased aqueous content transfer between HAS cells and RBC over a broad range of HA expression levels, suggesting that cholesterol also favors fusion pore expansion. This interpretation was tested using both trans-cell dye diffusion and fusion pore conductivity measurements in cholesterol-enriched cells. The results of this study support the hypothesis that host cell cholesterol acts at two stages in membrane fusion: (1) early, prior to fusion pore opening, and (2) late, during fusion pore expansion.
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Affiliation(s)
- Subrata Biswas
- Laboratory of Cellular and Molecular Biophysics, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA
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31
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Wessels L, Elting MW, Scimeca D, Weninger K. Rapid membrane fusion of individual virus particles with supported lipid bilayers. Biophys J 2007; 93:526-38. [PMID: 17449662 PMCID: PMC1896232 DOI: 10.1529/biophysj.106.097485] [Citation(s) in RCA: 60] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Many enveloped viruses employ low-pH-triggered membrane fusion during cell penetration. Solution-based in vitro assays in which viruses fuse with liposomes have provided much of our current biochemical understanding of low-pH-triggered viral membrane fusion. Here, we extend this in vitro approach by introducing a fluorescence assay using single particle tracking to observe lipid mixing between individual virus particles (influenza or Sindbis) and supported lipid bilayers. Our single-particle experiments reproduce many of the observations of the solution assays. The single-particle approach naturally separates the processes of membrane binding and membrane fusion and therefore allows measurement of details that are not available in the bulk assays. We find that the dynamics of lipid mixing during individual Sindbis fusion events is faster than 30 ms. Although neither virus binds membranes at neutral pH, under acidic conditions, the delay between membrane binding and lipid mixing is less than half a second for nearly all virus-membrane combinations. The delay between binding and lipid mixing lengthened only for Sindbis virus at the lowest pH in a cholesterol-dependent manner, highlighting the complex interaction between lipids, virus proteins, and buffer conditions in membrane fusion.
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Affiliation(s)
- Laura Wessels
- Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, USA
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32
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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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33
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Lentz BR. PEG as a tool to gain insight into membrane fusion. EUROPEAN BIOPHYSICS JOURNAL: EBJ 2006; 36:315-26. [PMID: 17039359 DOI: 10.1007/s00249-006-0097-z] [Citation(s) in RCA: 96] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/25/2006] [Revised: 07/30/2006] [Accepted: 08/30/2006] [Indexed: 10/24/2022]
Abstract
Thirty years ago, Klaus Arnold and others showed that the action of PEG in promoting cell-cell fusion was not due to such effects as surface absorption, cross-linking, solubilization, etc. Instead PEG acted simply by volume exclusion, resulting in an osmotic force driving membranes into close contact in a dehydrated region. This simple observation, based on a number of physical measurements and the use of PEG-based detergents that insert into membranes, spawned several important areas of research. One such area is the use of PEG to bring membranes into contact so that the role of different lipids and fusion proteins in membrane fusion can be examined in detail. We have summarized here insights into the fusion mechanism that have been obtained by this approach. This evidence indicates that fusion of model membranes (and probably cell membranes) occurs via severely bent lipidic structures formed at the point of sufficiently close contact between membranes of appropriate lipid composition. This line of research has also suggested that fusion proteins seem to catalyze fusion in part by reducing the free energy of hydrophobic interstices inherent to the lipidic fusion intermediate structures.
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Affiliation(s)
- Barry R Lentz
- Department of Biochemistry and Biophysics, Program in Molecular and Cellular Biophysics, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7260, USA.
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34
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Kasai H, Kishimoto T, Nemoto T, Hatakeyama H, Liu TT, Takahashi N. Two-photon excitation imaging of exocytosis and endocytosis and determination of their spatial organization. Adv Drug Deliv Rev 2006; 58:850-77. [PMID: 16996640 DOI: 10.1016/j.addr.2006.07.008] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/07/2006] [Accepted: 07/13/2006] [Indexed: 12/17/2022]
Abstract
Two-photon excitation imaging is the least invasive optical approach to study living tissues. We have established two-photon extracellular polar-tracer (TEP) imaging with which it is possible to visualize and quantify all exocytic events in the plane of focus within secretory tissues. This technology also enables estimate of the precise diameters of vesicles independently of the spatial resolution of the optical microscope, and determination of the fusion pore dynamics at nanometer resolution using TEP-imaging based quantification (TEPIQ). TEP imaging has been applied to representative secretory glands, e.g., exocrine pancreas, endocrine pancreas, adrenal medulla and a pheochromocytoma cell line (PC12), and has revealed unexpected diversity in the spatial organization of exocytosis and endocytosis crucial for the physiology and pathology of secretory tissues and neurons. TEP imaging and TEPIQ analysis are powerful tools for elucidating the molecular and cellular mechanisms of exocytosis and certain related diseases, such as diabetes mellitus, and the development of new therapeutic agents and diagnostic tools.
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Affiliation(s)
- Haruo Kasai
- Division of Biophysics, Center for Disease Biology and Integrative Medicine, Faculty of Medicine, The University of Tokyo, Tokyo 113-0033, Japan.
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35
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Harata NC, Aravanis AM, Tsien RW. Kiss-and-run and full-collapse fusion as modes of exo-endocytosis in neurosecretion. J Neurochem 2006; 97:1546-70. [PMID: 16805768 DOI: 10.1111/j.1471-4159.2006.03987.x] [Citation(s) in RCA: 138] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Neurotransmitters and hormones are released from neurosecretory cells by exocytosis (fusion) of synaptic vesicles, large dense-core vesicles and other types of vesicles or granules. The exocytosis is terminated and followed by endocytosis (retrieval). More than fifty years of research have established full-collapse fusion and clathrin-mediated endocytosis as essential modes of exo-endocytosis. Kiss-and-run and vesicle reuse represent alternative modes, but their prevalence and importance have yet to be elucidated, especially in neurons of the mammalian CNS. Here we examine various modes of exo-endocytosis across a wide range of neurosecretory systems. Full-collapse fusion and kiss-and-run coexist in many systems and play active roles in exocytotic events. In small nerve terminals of CNS, kiss-and-run has an additional role of enabling nerve terminals to conserve scarce vesicular resources and respond to high-frequency inputs. Full-collapse fusion and kiss-and-run will each contribute to maintaining cellular communication over a wide range of frequencies.
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Affiliation(s)
- Nobutoshi C Harata
- Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, California, USA
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36
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Reese C, Mayer A. Transition from hemifusion to pore opening is rate limiting for vacuole membrane fusion. ACTA ACUST UNITED AC 2006; 171:981-90. [PMID: 16365164 PMCID: PMC2171322 DOI: 10.1083/jcb.200510018] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
Fusion pore opening and expansion are considered the most energy-demanding steps in viral fusion. Whether this also applies to soluble N-ethyl-maleimide sensitive fusion protein attachment protein receptor (SNARE)- and Rab-dependent fusion events has been unknown. We have addressed the problem by characterizing the effects of lysophosphatidylcholine (LPC) and other late-stage inhibitors on lipid mixing and pore opening during vacuole fusion. LPC inhibits fusion by inducing positive curvature in the bilayer and changing its biophysical properties. The LPC block reversibly prevented formation of the hemifusion intermediate that allows lipid, but not content, mixing. Transition from hemifusion to pore opening was sensitive to guanosine-5'-(gamma-thio)triphosphate. It required the vacuolar adenosine triphosphatase V0 sector and coincided with its transformation. Pore opening was rate limiting for the reaction. As with viral fusion, opening the fusion pore may be the most energy-demanding step for intracellular, SNARE-dependent fusion reactions, suggesting that fundamental aspects of lipid mixing and pore opening are related for both systems.
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Affiliation(s)
- Christoph Reese
- Département de Biochimie, Université de Lausanne, 1066 Epalinges, Switzerland
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37
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Zimmerberg J, Chernomordik LV. Neuroscience. Synaptic membranes bend to the will of a neurotoxin. Science 2005; 310:1626-7. [PMID: 16339436 DOI: 10.1126/science.1122439] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/03/2022]
Affiliation(s)
- Joshua Zimmerberg
- Laboratory of Cellular and Molecular Biophysics, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892-1855, USA.
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Giraudo CG, Hu C, You D, Slovic AM, Mosharov EV, Sulzer D, Melia TJ, Rothman JE. SNAREs can promote complete fusion and hemifusion as alternative outcomes. ACTA ACUST UNITED AC 2005; 170:249-60. [PMID: 16027221 PMCID: PMC2171417 DOI: 10.1083/jcb.200501093] [Citation(s) in RCA: 110] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
Using a cell fusion assay, we show here that in addition to complete fusion SNAREs also promote hemifusion as an alternative outcome. Approximately 65% of events resulted in full fusion, and the remaining 35% in hemifusion; of those, approximately two thirds were permanent and approximately one third were reversible. We predict that this relatively close balance among outcomes could be tipped by binding of regulatory proteins to the SNAREs, allowing for dynamic physiological regulation between full fusion and reversible kiss-and-run–like events.
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Affiliation(s)
- Claudio G Giraudo
- Department of Physiology and Cellular Biophysics, Columbia University College of Physicians and Surgeons, New York, NY 10032, USA
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Li Y, Han X, Lai AL, Bushweller JH, Cafiso DS, Tamm LK. Membrane structures of the hemifusion-inducing fusion peptide mutant G1S and the fusion-blocking mutant G1V of influenza virus hemagglutinin suggest a mechanism for pore opening in membrane fusion. J Virol 2005; 79:12065-76. [PMID: 16140782 PMCID: PMC1212637 DOI: 10.1128/jvi.79.18.12065-12076.2005] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Influenza virus hemagglutinin (HA)-mediated membrane fusion is initiated by a conformational change that releases a V-shaped hydrophobic fusion domain, the fusion peptide, into the lipid bilayer of the target membrane. The most N-terminal residue of this domain, a glycine, is highly conserved and is particularly critical for HA function; G1S and G1V mutant HAs cause hemifusion and abolish fusion, respectively. We have determined the atomic resolution structures of the G1S and G1V mutant fusion domains in membrane environments. G1S forms a V with a disrupted "glycine edge" on its N-terminal arm and G1V adopts a slightly tilted linear helical structure in membranes. Abolishment of the kink in G1V results in reduced hydrophobic penetration of the lipid bilayer and an increased propensity to form beta-structures at the membrane surface. These results underline the functional importance of the kink in the fusion peptide and suggest a structural role for the N-terminal glycine ridge in viral membrane fusion.
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Affiliation(s)
- Yinling Li
- Department of Molecular Physiology and Biological Physics, University of Virginia, P.O. Box 800736, Charlottesville, VA 22908-0736, USA
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40
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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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41
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Lizunov VA, Matsumoto H, Zimmerberg J, Cushman SW, Frolov VA. Insulin stimulates the halting, tethering, and fusion of mobile GLUT4 vesicles in rat adipose cells. ACTA ACUST UNITED AC 2005; 169:481-9. [PMID: 15866888 PMCID: PMC2171949 DOI: 10.1083/jcb.200412069] [Citation(s) in RCA: 145] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
Glucose transport in adipose cells is regulated by changing the distribution of glucose transporter 4 (GLUT4) between the cell interior and the plasma membrane (PM). Insulin shifts this distribution by augmenting the rate of exocytosis of specialized GLUT4 vesicles. We applied time-lapse total internal reflection fluorescence microscopy to dissect intermediates of this GLUT4 translocation in rat adipose cells in primary culture. Without insulin, GLUT4 vesicles rapidly moved along a microtubule network covering the entire PM, periodically stopping, most often just briefly, by loosely tethering to the PM. Insulin halted this traffic by tightly tethering vesicles to the PM where they formed clusters and slowly fused to the PM. This slow release of GLUT4 determined the overall increase of the PM GLUT4. Thus, insulin initially recruits GLUT4 sequestered in mobile vesicles near the PM. It is likely that the primary mechanism of insulin action in GLUT4 translocation is to stimulate tethering and fusion of trafficking vesicles to specific fusion sites in the PM.
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Affiliation(s)
- Vladimir A Lizunov
- 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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42
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Cohen FS, Melikyan GB. The energetics of membrane fusion from binding, through hemifusion, pore formation, and pore enlargement. J Membr Biol 2005; 199:1-14. [PMID: 15366419 DOI: 10.1007/s00232-004-0669-8] [Citation(s) in RCA: 236] [Impact Index Per Article: 12.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The main steps of viral membrane fusion are local membrane approach, hemifusion, pore formation, and pore enlargement. Experiments and theoretical analyses have helped determine the relative energies required for each step. Key protein structures and conformational changes of the fusion process have been identified. The physical deformations of monolayer bending and lipid tilt have been applied to the steps of membrane fusion. Experiment and theory converge to strongly indicate that, contrary to former conceptions, the fusion process is progressively more energetically difficult: hemifusion has a relatively low energy barrier, pore formation is more energy-consuming, and pore enlargement is the most difficult to achieve.
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Affiliation(s)
- F S Cohen
- Rush University Medical Center, Department of Molecular Biophysics and Physiology, 1653 W Congress Parkway, Chicago, IL 60612, USA.
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Kozlovsky Y, Efrat A, Siegel DP, Siegel DA, Kozlov MM. Stalk phase formation: effects of dehydration and saddle splay modulus. Biophys J 2004; 87:2508-21. [PMID: 15454446 PMCID: PMC1304670 DOI: 10.1529/biophysj.103.038075] [Citation(s) in RCA: 77] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2003] [Accepted: 07/23/2004] [Indexed: 11/18/2022] Open
Abstract
One of the earliest lipid intermediates forming in the course of membrane fusion is the lipid stalk. Although many aspects of the stalk hypothesis were elaborated theoretically and confirmed by experiments it remained unresolved whether stalk formation is always an energy consuming process or if there are conditions where the stalks are energetically favorable and form spontaneously resulting in an equilibrium stalk phase. Motivated by a recent breakthrough experiments we analyze the physical factors determining the spontaneous stalk formation. We show that this process can be driven by interplay between two factors: the elastic energy of lipid monolayers including a contribution of the saddle splay deformation and the energy of hydration repulsion acting between apposing membranes. We analyze the dependence of stalk formation on the saddle splay (Gaussian) modulus of the lipid monolayers and estimate the values of this modulus based on the experimentally established phase boundary between the lamellar and the stalk phases. We suggest that fusion proteins can induce stalk formation just by bringing the membranes into close contact, and accumulating, at least locally, a sufficiently large energy of the hydration repulsion.
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Affiliation(s)
- Yonathan Kozlovsky
- Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel
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44
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Stenovec M, Kreft M, Poberaj I, Betz WJ, Zorec R. Slow spontaneous secretion from single large dense‐core vesicles monitored in neuroendocrine cells. FASEB J 2004; 18:1270-2. [PMID: 15180959 DOI: 10.1096/fj.03-1397fje] [Citation(s) in RCA: 59] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Hormones are released from cells by passing through an exocytotic pore that forms after vesicle and plasma membrane fusion. In stimulated exocytosis vesicle content is discharged swiftly. Although rapid vesicle discharge has also been proposed to mediate basal secretion, this has not been studied directly. We investigated basal hormone release by preloading fluorescent peptides into single vesicles. The hormone discharge, monitored with confocal microscopy, was compared with the simultaneous loading of vesicle by FM styryl dye. In stimulated vesicles FM 4-64 (4 microM), loading and hormone discharge occurs within seconds. In contrast, in approximately 50% of spontaneously releasing vesicles, the vesicle content discharge and the FM 4-64 loading were slow (approximately 3 min). These results show that in peptide secreting neuroendocrine cells the elementary vesicle content discharge differs in basal and in stimulated exocytosis. It is proposed that the view dating back for some decades, which is that, at rest, the vesicle discharge of hormones and neurotransmitters is similar to that occurring after stimulation, needs to be extended. In addition to the classical paradigm that secretory capacity of a cell is determined by controlling the probability of occurrence of elementary exocytotic events, one will have to consider activity modulation of elementary exocytotic events as well.
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Abstract
During exocytosis, the lumen of secretory vesicles connects with the extracellular space. In some vesicles, this connection closes again, causing the vesicle to be recaptured mostly intact. The degree to which the bilayers of such vesicles mix with the plasma membrane is unknown. Work supporting the kiss-and-run model of transient exocytosis implies that synaptic vesicles allow neither lipid nor protein to escape into the plasma membrane, suggesting that the two bilayers never merge. Here, we test whether neuroendocrine granules behave similarly. Using two-color evanescent field microscopy, we imaged the lipid probe FM4-64 and fluorescent proteins in single dense core granules. During exocytosis, granules lost FM4-64 into the plasma membrane in small fractions of a second. Although FM4-64 was lost, granules retained the membrane protein, GFP-phogrin. By using GFP-phogrin as a probe for resealing, it was found that even granules that reseal lose FM4-64. We conclude that the lipid bilayers of the granule and the plasma membrane become continuous even when exocytosis is transient.
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Affiliation(s)
- Justin W Taraska
- Vollum Institute, Oregon Health and Sciences University, 3181 Southwest Sam Jackson Park Road, Portland, OR 97239, USA
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Zavorotinskaya T, Qian Z, Franks J, Albritton LM. A point mutation in the binding subunit of a retroviral envelope protein arrests virus entry at hemifusion. J Virol 2004; 78:473-81. [PMID: 14671127 PMCID: PMC303374 DOI: 10.1128/jvi.78.1.473-481.2004] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The transmembrane subunits of viral envelope proteins are thought to perform all of the functions required for membrane fusion during entry of enveloped viruses. However, changes in a conserved SPHQ motif near the N terminus of the receptor binding subunit of a murine leukemia virus (MLV) envelope protein block infection and induction of cell-cell fusion but not receptor binding. Here we report evidence that a histidine-to-arginine change at position 8 (H8R) in the SPHQ motif of Moloney MLV blocks infection by arresting virus-cell fusion at the hemifusion state. In cell-cell fusion assays, H8R envelope protein induced mixing of membrane outer leaflet lipids but did not lead to content mixing, a finding indicative of fusion pore formation. Kinetic studies of virus-cell fusion showed that lipid mixing of H8R virus membranes begins much later than for wild-type virus. The length of the delay in lipid mixing decreased upon addition of two second-site changes that increase H8R virus infection to 100-fold less than the wild-type virus. Finally, chlorpromazine, dibucaine, and trifluoperazine, agents that induce pores in an arrested hemifusion state, rescued infection by H8R virus to within 2.5-fold of the level of wild-type virus infection and cell-cell fusion to half that mediated by wild-type envelope protein. We interpret these results to indicate that fusion progressed to the hemifusion intermediate but fusion pore formation was inhibited. These results establish that membrane fusion of Moloney MLV occurs via a hemifusion intermediate. We also interpret these findings as evidence that histidine 8 is a key switch-point residue between the receptor-induced conformation changes that expose fusion peptide and those that lead to six-helix bundle formation.
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Affiliation(s)
- Tatiana Zavorotinskaya
- Department of Molecular Sciences, University of Tennessee Health Sciences Center, Memphis, Tennessee 38163
| | - Zhaohui Qian
- Department of Molecular Sciences, University of Tennessee Health Sciences Center, Memphis, Tennessee 38163
| | - John Franks
- Department of Molecular Sciences, University of Tennessee Health Sciences Center, Memphis, Tennessee 38163
| | - Lorraine M. Albritton
- Department of Molecular Sciences, University of Tennessee Health Sciences Center, Memphis, Tennessee 38163
- Corresponding author. Mailing address: Department of Molecular Sciences, University of Tennessee Health Sciences Center, 858 Madison Ave., Memphis, TN 38163. Phone: (901) 448-5521. Fax: (901) 448-7360. E-mail:
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Frolov VA, Dunina-Barkovskaya AY, Samsonov AV, Zimmerberg J. Membrane permeability changes at early stages of influenza hemagglutinin-mediated fusion. Biophys J 2003; 85:1725-33. [PMID: 12944287 PMCID: PMC1303346 DOI: 10.1016/s0006-3495(03)74602-5] [Citation(s) in RCA: 69] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022] Open
Abstract
While biological membrane fusion is classically defined as the leak-free merger of membranes and contents, leakage is a finding in both experimental and theoretical studies. The fusion stages, if any, that allow membrane permeation are uncharted. In this study we monitored membrane ionic permeability at early stages of fusion mediated by the fusogenic protein influenza hemagglutinin (HA). HAb2 cells, expressing HA on their plasma membrane, fused with human red blood cells, cultured liver cells PLC/PRF/5, or planar phospholipid bilayer membranes. With a probability that depended upon the target membrane, an increase of the electrical conductance of the fusing membranes (leakage) by up to several nS was generally detected. This leakage was recorded at the initial stages of fusion, when fusion pores formed. This leakage usually accompanied the "flickering" stage of the early fusion pore development. As the pore widened, the leakage reduced; concomitantly, the lipid exchange between the fusing membranes accelerated. We conclude that during fusion pore formation, HA locally and temporarily increases the permeability of fusing membranes. Subsequent rearrangement in the fusion complex leads to the resealing of the leaky membranes and enlargement of the pore.
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Affiliation(s)
- V A Frolov
- A. N. Frumkin Institute of Electrochemistry, Russian Academy of Sciences, Moscow, Russia
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48
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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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49
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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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50
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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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