101
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Xu J, Camacho M, Xu Y, Esser V, Liu X, Trimbuch T, Pan YZ, Ma C, Tomchick DR, Rosenmund C, Rizo J. Mechanistic insights into neurotransmitter release and presynaptic plasticity from the crystal structure of Munc13-1 C 1C 2BMUN. eLife 2017; 6. [PMID: 28177287 PMCID: PMC5344669 DOI: 10.7554/elife.22567] [Citation(s) in RCA: 89] [Impact Index Per Article: 12.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/21/2016] [Accepted: 02/07/2017] [Indexed: 12/25/2022] Open
Abstract
Munc13–1 acts as a master regulator of neurotransmitter release, mediating docking-priming of synaptic vesicles and diverse presynaptic plasticity processes. It is unclear how the functions of the multiple domains of Munc13–1 are coordinated. The crystal structure of a Munc13–1 fragment including its C1, C2B and MUN domains (C1C2BMUN) reveals a 19.5 nm-long multi-helical structure with the C1 and C2B domains packed at one end. The similar orientations of the respective diacyglycerol- and Ca2+-binding sites of the C1 and C2B domains suggest that the two domains cooperate in plasma-membrane binding and that activation of Munc13–1 by Ca2+ and diacylglycerol during short-term presynaptic plasticity are closely interrelated. Electrophysiological experiments in mouse neurons support the functional importance of the domain interfaces observed in C1C2BMUN. The structure imposes key constraints for models of neurotransmitter release and suggests that Munc13–1 bridges the vesicle and plasma membranes from the periphery of the membrane-membrane interface. DOI:http://dx.doi.org/10.7554/eLife.22567.001 The human brain contains billions of cells called neurons that communicate with each other using molecules called neurotransmitters. An electrical signal in one neuron triggers the release of neurotransmitters from the cell, which then activate or inhibit electrical signals in neighboring neurons. Inside the cell, neurotransmitters are stored in small bubble-like structures called synaptic vesicles. The vesicles fuse with the membrane that surrounds the cell to release the neurotransmitters. This process must be tightly controlled to ensure that neurotransmitters are released rapidly and at the right time. A protein called Munc13 is a key component of the machinery that regulates the fusion of synaptic vesicles. It helps the synaptic vesicle to dock onto the cell membrane and get ready for fusion. Munc13 is a large protein and contains several different regions, including three domains called C1, C2B and MUN. These three domains control the release of neurotransmitters, but how they do so is poorly understood. Xu, Camacho et al. used a technique called X-ray crystallography to analyse the three-dimensional shape of the part of Munc13 that contains the three domains. The experiments reveal that the MUN domain forms a long rod-like shape with the C1 and C2B domains packed at one end. Several mutations that reduce the ability of the domains to interact with each other altered the release of neurotransmitters from mouse neurons to different extents. These findings suggest that the overall architecture of the region containing the C1, C2B and MUN domains is important for the normal activity of Munc13. The structure revealed by Xu, Camacho et al. sets a framework for understanding how Munc13 controls neurotransmitter release, and thus mediates diverse forms of information processing in the brain. DOI:http://dx.doi.org/10.7554/eLife.22567.002
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Affiliation(s)
- Junjie Xu
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, United States
| | - Marcial Camacho
- Department of Neurophysiology, NeuroCure Cluster of Excellence, Charité-Universitätsmedizin Berlin, Berlin, Germany
| | - Yibin Xu
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, United States
| | - Victoria Esser
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, United States
| | - Xiaoxia Liu
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, United States
| | - Thorsten Trimbuch
- Department of Neurophysiology, NeuroCure Cluster of Excellence, Charité-Universitätsmedizin Berlin, Berlin, Germany
| | - Yun-Zu Pan
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, United States
| | - Cong Ma
- Key Laboratory of Molecular Biophysics of the Ministry of Education, Huazhong University of Science and Technology, Wuhan, China.,College of Life Science and Technology, Huazhong University of Science and Technology, Wuhan, China
| | - Diana R Tomchick
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, United States
| | - Christian Rosenmund
- Department of Neurophysiology, NeuroCure Cluster of Excellence, Charité-Universitätsmedizin Berlin, Berlin, Germany
| | - Josep Rizo
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, United States
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102
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Guo T, Duan Z, Chen J, Xie C, Wang Y, Chen P, Wang X. Pull-down combined with proteomic strategy reveals functional diversity of synaptotagmin I. PeerJ 2017; 5:e2973. [PMID: 28194317 PMCID: PMC5301975 DOI: 10.7717/peerj.2973] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/2016] [Accepted: 01/10/2017] [Indexed: 12/05/2022] Open
Abstract
Synaptotagmin I (Syt I) is most abundant in the brain and is involved in multiple cellular processes. Its two C2 domains, C2A and C2B, are the main functional regions. Our present study employed a pull-down combined with proteomic strategy to identify the C2 domain-interacting proteins to comprehensively understand the biological roles of the C2 domains and thus the functional diversity of Syt I. A total of 135 non-redundant proteins interacting with the C2 domains of Syt I were identified. Out of them, 32 and 64 proteins only bound to C2A or C2B domains, respectively, and 39 proteins bound to both of them. Compared with C2A, C2B could bind to many more proteins particularly those involved in synaptic transmission and metabolic regulation. Functional analysis indicated that Syt I may exert impacts by interacting with other proteins on multiple cellular processes, including vesicular membrane trafficking, synaptic transmission, metabolic regulation, catalysis, transmembrane transport and structure formation, etc. These results demonstrate that the functional diversity of Syt I is higher than previously expected, that its two domains may mediate the same and different cellular processes cooperatively or independently, and that C2B domain may play even more important roles than C2A in the functioning of Syt I. This work not only further deepened our understanding of the functional diversity of Syt I and the functional differences between its two C2 domains, but also provided important clues for the further related researches.
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Affiliation(s)
- Tianyao Guo
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
| | - Zhigui Duan
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
| | - Jia Chen
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
| | - Chunliang Xie
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
| | - Ying Wang
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
| | - Ping Chen
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
| | - Xianchun Wang
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
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103
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Abstract
Membrane fusion is the cell's delivery process, enabling its many compartments to receive cargo and machinery for cell growth and intercellular communication. The overall activation energy of the process must be large enough to prevent frequent and nonspecific spontaneous fusion events, yet must be low enough to allow it to be overcome upon demand by specific fusion proteins [such as soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs)]. Remarkably, to the best of our knowledge, the activation energy for spontaneous bilayer fusion has never been measured. Multiple models have been developed and refined to estimate the overall activation energy and its component parts, and they span a very broad range from 20 kBT to 150 kBT, depending on the assumptions. In this study, using a bulk lipid-mixing assay at various temperatures, we report that the activation energy of complete membrane fusion is at the lowest range of these theoretical values. Typical lipid vesicles were found to slowly and spontaneously fully fuse with activation energies of ∼30 kBT Our data demonstrate that the merging of membranes is not nearly as energy consuming as anticipated by many models and is ideally positioned to minimize spontaneous fusion while enabling rapid, SNARE-dependent fusion upon demand.
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104
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Han J, Pluhackova K, Böckmann RA. The Multifaceted Role of SNARE Proteins in Membrane Fusion. Front Physiol 2017; 8:5. [PMID: 28163686 PMCID: PMC5247469 DOI: 10.3389/fphys.2017.00005] [Citation(s) in RCA: 178] [Impact Index Per Article: 25.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/25/2016] [Accepted: 01/04/2017] [Indexed: 12/28/2022] Open
Abstract
Membrane fusion is a key process in all living organisms that contributes to a variety of biological processes including viral infection, cell fertilization, as well as intracellular transport, and neurotransmitter release. In particular, the various membrane-enclosed compartments in eukaryotic cells need to exchange their contents and communicate across membranes. Efficient and controllable fusion of biological membranes is known to be driven by cooperative action of SNARE proteins, which constitute the central components of the eukaryotic fusion machinery responsible for fusion of synaptic vesicles with the plasma membrane. During exocytosis, vesicle-associated v-SNARE (synaptobrevin) and target cell-associated t-SNAREs (syntaxin and SNAP-25) assemble into a core trans-SNARE complex. This complex plays a versatile role at various stages of exocytosis ranging from the priming to fusion pore formation and expansion, finally resulting in the release or exchange of the vesicle content. This review summarizes current knowledge on the intricate molecular mechanisms underlying exocytosis triggered and catalyzed by SNARE proteins. Particular attention is given to the function of the peptidic SNARE membrane anchors and the role of SNARE-lipid interactions in fusion. Moreover, the regulatory mechanisms by synaptic auxiliary proteins in SNARE-driven membrane fusion are briefly outlined.
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Affiliation(s)
- Jing Han
- Department of Biochemistry and Molecular Biology, School of Basic Medical Sciences, Xi'an Jiaotong University Health Science CenterXi'an, China; Computational Biology, Department of Biology, Friedrich-Alexander Universität Erlangen-NürnbergErlangen, Germany
| | - Kristyna Pluhackova
- Computational Biology, Department of Biology, Friedrich-Alexander Universität Erlangen-Nürnberg Erlangen, Germany
| | - Rainer A Böckmann
- Computational Biology, Department of Biology, Friedrich-Alexander Universität Erlangen-Nürnberg Erlangen, Germany
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105
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Mattie S, McNally EK, Karim MA, Vali H, Brett CL. How and why intralumenal membrane fragments form during vacuolar lysosome fusion. Mol Biol Cell 2017; 28:309-321. [PMID: 27881666 PMCID: PMC5231899 DOI: 10.1091/mbc.e15-11-0759] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/12/2015] [Revised: 11/04/2016] [Accepted: 11/18/2016] [Indexed: 11/11/2022] Open
Abstract
Lysosomal membrane fusion mediates the last step of the autophagy and endocytosis pathways and supports organelle remodeling and biogenesis. Because fusogenic proteins and lipids concentrate in a ring at the vertex between apposing organelle membranes, the encircled area of membrane can be severed and internalized within the lumen as a fragment upon lipid bilayer fusion. How or why this intralumenal fragment forms during fusion, however, is not entirely clear. To better understand this process, we studied fragment formation during homotypic vacuolar lysosome membrane fusion in Saccharomyces cerevisiae Using cell-free fusion assays and light microscopy, we find that GTPase activation and trans-SNARE complex zippering have opposing effects on fragment formation and verify that this affects the morphology of the fusion product and regulates transporter protein degradation. We show that fragment formwation is limited by stalk expansion, a key intermediate of the lipid bilayer fusion reaction. Using electron microscopy, we present images of hemifusion diaphragms that form as stalks expand and propose a model describing how the fusion machinery regulates fragment formation during lysosome fusion to control morphology and protein lifetimes.
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Affiliation(s)
- Sevan Mattie
- Department of Biology, Concordia University, Montréal, QC H4B 1R6, Canada
| | - Erin K McNally
- Department of Biology, Concordia University, Montréal, QC H4B 1R6, Canada
| | - Mahmoud A Karim
- Department of Biology, Concordia University, Montréal, QC H4B 1R6, Canada
| | - Hojatollah Vali
- Department of Anatomy and Cell Biology, McGill University, Montréal, QC H3A 0C7, Canada
| | - Christopher L Brett
- Department of Biology, Concordia University, Montréal, QC H4B 1R6, Canada
- Department of Anatomy and Cell Biology, McGill University, Montréal, QC H3A 0C7, Canada
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106
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Hernandez JM. Reconstitution of Synaptic SNAREs into Large Liposomes with Reduced Curvature Stress. Methods Mol Biol 2017; 1538:93-105. [PMID: 27943186 DOI: 10.1007/978-1-4939-6688-2_8] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
Liposomes constitute a convenient biochemical model system to investigate mechanistic aspects of the membrane fusion of synaptic vesicles. The proteins responsible for mediating fusion are the SNAREs that belong to a highly conserved family of transmembrane proteins. Reconstituting SNAREs into liposomes using detergents has become a common approach not only to understand how SNAREs work, but also how fusion is regulated by the vast array of accessory proteins present at the presynapse. However, a concern has been that the high curvature stress of the small liposomes (diameters of ~40 nm) frequently used in many studies renders them prone to spontaneous fusion, bringing into question whether the measurements obtained faithfully represent SNARE-mediated fusion. By systematically varying the detergent concentration and characterizing the SNARE-liposome size distributions by light scattering, we describe a detailed procedure to reconstitute SNAREs into large liposomes with considerably reduced curvature stress.
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Affiliation(s)
- Javier M Hernandez
- Department of Structural Biochemistry, Max Planck Institute for Molecular Physiology, Otto-Hahn-Str. 11, 44202, Dortmund, Germany.
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107
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Ries O, Löffler PMG, Rabe A, Malavan JJ, Vogel S. Efficient liposome fusion mediated by lipid–nucleic acid conjugates. Org Biomol Chem 2017; 15:8936-8945. [DOI: 10.1039/c7ob01939d] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
Highly efficient fusion and content mixing of liposomes encoded by lipidated oligonucleotides (LiNAs). “Hot fusion of biomembranes” – a low leakage process at elevated temperature.
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Affiliation(s)
- O. Ries
- Biomolecular Nanoscale Engineering Center
- Department of Physics
- Chemistry and Pharmacy
- University of Southern Denmark
- 5230 Odense
| | - P. M. G. Löffler
- Biomolecular Nanoscale Engineering Center
- Department of Physics
- Chemistry and Pharmacy
- University of Southern Denmark
- 5230 Odense
| | - A. Rabe
- Biomolecular Nanoscale Engineering Center
- Department of Physics
- Chemistry and Pharmacy
- University of Southern Denmark
- 5230 Odense
| | - J. J. Malavan
- Biomolecular Nanoscale Engineering Center
- Department of Physics
- Chemistry and Pharmacy
- University of Southern Denmark
- 5230 Odense
| | - Stefan Vogel
- Biomolecular Nanoscale Engineering Center
- Department of Physics
- Chemistry and Pharmacy
- University of Southern Denmark
- 5230 Odense
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108
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Stability, folding dynamics, and long-range conformational transition of the synaptic t-SNARE complex. Proc Natl Acad Sci U S A 2016; 113:E8031-E8040. [PMID: 27911771 DOI: 10.1073/pnas.1605748113] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022] Open
Abstract
Synaptic soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) couple their stepwise folding to fusion of synaptic vesicles with plasma membranes. In this process, three SNAREs assemble into a stable four-helix bundle. Arguably, the first and rate-limiting step of SNARE assembly is the formation of an activated binary target (t)-SNARE complex on the target plasma membrane, which then zippers with the vesicle (v)-SNARE on the vesicle to drive membrane fusion. However, the t-SNARE complex readily misfolds, and its structure, stability, and dynamics are elusive. Using single-molecule force spectroscopy, we modeled the synaptic t-SNARE complex as a parallel three-helix bundle with a small frayed C terminus. The helical bundle sequentially folded in an N-terminal domain (NTD) and a C-terminal domain (CTD) separated by a central ionic layer, with total unfolding energy of ∼17 kBT, where kB is the Boltzmann constant and T is 300 K. Peptide binding to the CTD activated the t-SNARE complex to initiate NTD zippering with the v-SNARE, a mechanism likely shared by the mammalian uncoordinated-18-1 protein (Munc18-1). The NTD zippering then dramatically stabilized the CTD, facilitating further SNARE zippering. The subtle bidirectional t-SNARE conformational switch was mediated by the ionic layer. Thus, the t-SNARE complex acted as a switch to enable fast and controlled SNARE zippering required for synaptic vesicle fusion and neurotransmission.
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109
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SNARE-mediated membrane fusion trajectories derived from force-clamp experiments. Proc Natl Acad Sci U S A 2016; 113:13051-13056. [PMID: 27807132 DOI: 10.1073/pnas.1615885113] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
Fusion of lipid bilayers is usually prevented by large energy barriers arising from removal of the hydration shell, formation of highly curved structures, and, eventually, fusion pore widening. Here, we measured the force-dependent lifetime of fusion intermediates using membrane-coated silica spheres attached to cantilevers of an atomic-force microscope. Analysis of time traces obtained from force-clamp experiments allowed us to unequivocally assign steps in deflection of the cantilever to membrane states during the SNARE-mediated fusion with solid-supported lipid bilayers. Force-dependent lifetime distributions of the various intermediate fusion states allowed us to propose the likelihood of different fusion pathways and to assess the main free energy barrier, which was found to be related to passing of the hydration barrier and splaying of lipids to eventually enter either the fully fused state or a long-lived hemifusion intermediate. The results were compared with SNARE mutants that arrest adjacent bilayers in the docked state and membranes in the absence of SNAREs but presence of PEG or calcium. Only with the WT SNARE construct was appreciable merging of both bilayers observed.
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110
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Lin CC, Hsu HF, Walla PJ. A One Donor-Two Acceptor Lipid Bilayer FRET Assay Based on Asymmetrically Labeled Liposomes. J Phys Chem B 2016; 120:11085-11092. [PMID: 27762543 DOI: 10.1021/acs.jpcb.6b05654] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Abstract
The fusion of two opposing membranes is essential in biological functions such as fertilization, viral entry, membrane trafficking and synaptic transmission. Before the membrane bilayers are fully connected, at some stage a hemifusion intermediate-when the outer leaflets are merged but not the inner leaflets-is formed. However, the position of hemifusion in the energy landscape and the duration of it vary and have not been fully mapped out. To date, there has not been a way to differentiate lipid mixing of the two leaflets directly in a single experiment. Herein we demonstrate labeling of the outer and inner leaflets with different fluorophores, which can be distinguished by their fluorescence lifetimes. As a proof of concept, the asymmetrically labeled liposomes were used as acceptor liposomes in a novel one donor-two acceptor Förster resonance energy transfer (FRET) assay to monitor membrane fusion reactions mediated by the synaptic proteins soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) in microfluidic devices. Initial hemifusion was clearly indicated by the acceptor fluorescence lifetime originating solely from FRET acceptors on the outer leaflet (Oregon Green 488; τFl ∼ 4.8 ns). Progression to full fusion was then indicated by the significantly increasing lifetime contribution from acceptors on the inner leaflet (nitrobenzoxadiazole; τFl ∼ 6.7 ns). The new labeling strategy creates many possibilities in the design of bulk and single-molecule experiments.
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Affiliation(s)
- Chao-Chen Lin
- Research Group Biomolecular Spectroscopy and Single-Molecule Detection, Max Planck Institute for Biophysical Chemistry , Am Faßberg 11, 37077 Göttingen, Germany
| | - Hsin-Fang Hsu
- Laboratory for Fluid Dynamics, Pattern Formation and Biocomplexity, Max Planck Institute for Dynamics and Self-Organization , Am Faßberg 17, 37077 Göttingen, Germany
| | - Peter Jomo Walla
- Research Group Biomolecular Spectroscopy and Single-Molecule Detection, Max Planck Institute for Biophysical Chemistry , Am Faßberg 11, 37077 Göttingen, Germany.,Department of Biophysical Chemistry, Institute for Physical and Theoretical Chemistry, Technical University of Braunschweig , Hans-Sommer-Straße 10, 38106 Braunschweig, Germany
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111
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Bunker A, Magarkar A, Viitala T. Rational design of liposomal drug delivery systems, a review: Combined experimental and computational studies of lipid membranes, liposomes and their PEGylation. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2016; 1858:2334-2352. [DOI: 10.1016/j.bbamem.2016.02.025] [Citation(s) in RCA: 103] [Impact Index Per Article: 12.9] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/21/2015] [Revised: 02/09/2016] [Accepted: 02/10/2016] [Indexed: 01/22/2023]
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112
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D'Agostino M, Risselada HJ, Mayer A. Steric hindrance of SNARE transmembrane domain organization impairs the hemifusion-to-fusion transition. EMBO Rep 2016; 17:1590-1608. [PMID: 27644261 DOI: 10.15252/embr.201642209] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/15/2016] [Accepted: 08/12/2016] [Indexed: 11/09/2022] Open
Abstract
SNAREs fuse membranes in several steps. Trans-SNARE complexes juxtapose membranes, induce hemifused stalk structures, and open the fusion pore. A recent penetration model of fusion proposed that SNAREs force the hydrophilic C-termini of their transmembrane domains through the hydrophobic core of the membrane(s). In contrast, the indentation model suggests that the C-termini open the pore by locally compressing and deforming the stalk. Here we test these models in the context of yeast vacuole fusion. Addition of small hydrophilic tags renders bilayer penetration by the C-termini energetically unlikely. It preserves fusion activity, however, arguing against the penetration model. Addition of large protein tags to the C-termini permits SNARE activation, trans-SNARE pairing, and hemifusion but abolishes pore opening. Fusion proceeds if the tags are detached from the membrane by a hydrophilic spacer or if only one side of the trans-SNARE complex carries a protein tag. Thus, both sides of a trans-SNARE complex can drive pore opening. Our results are consistent with an indentation model in which multiple SNARE C-termini cooperate in opening the fusion pore by locally deforming the inner leaflets.
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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, Goettingen, Germany.,Leibniz-Institut für Oberflächenmodifizierung, Leipzig, Germany
| | - Andreas Mayer
- Département de Biochimie, Université de Lausanne, Epalinges, Switzerland
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113
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Nikolaus J, Karatekin E. SNARE-mediated Fusion of Single Proteoliposomes with Tethered Supported Bilayers in a Microfluidic Flow Cell Monitored by Polarized TIRF Microscopy. J Vis Exp 2016. [PMID: 27585113 DOI: 10.3791/54349] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
Abstract
In the ubiquitous process of membrane fusion the opening of a fusion pore establishes the first connection between two formerly separate compartments. During neurotransmitter or hormone release via exocytosis, the fusion pore can transiently open and close repeatedly, regulating cargo release kinetics. Pore dynamics also determine the mode of vesicle recycling; irreversible resealing results in transient, "kiss-and-run" fusion, whereas dilation leads to full fusion. To better understand what factors govern pore dynamics, we developed an assay to monitor membrane fusion using polarized total internal reflection fluorescence (TIRF) microscopy with single molecule sensitivity and ~15 msec time resolution in a biochemically well-defined in vitro system. Fusion of fluorescently labeled small unilamellar vesicles containing v-SNARE proteins (v-SUVs) with a planar bilayer bearing t-SNAREs, supported on a soft polymer cushion (t-SBL, t-supported bilayer), is monitored. The assay uses microfluidic flow channels that ensure minimal sample consumption while supplying a constant density of SUVs. Exploiting the rapid signal enhancement upon transfer of lipid labels from the SUV to the SBL during fusion, kinetics of lipid dye transfer is monitored. The sensitivity of TIRF microscopy allows tracking single fluorescent lipid labels, from which lipid diffusivity and SUV size can be deduced for every fusion event. Lipid dye release times can be much longer than expected for unimpeded passage through permanently open pores. Using a model that assumes retardation of lipid release is due to pore flickering, a pore "openness", the fraction of time the pore remains open during fusion, can be estimated. A soluble marker can be encapsulated in the SUVs for simultaneous monitoring of lipid and soluble cargo release. Such measurements indicate some pores may reseal after losing a fraction of the soluble cargo.
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Affiliation(s)
- Joerg Nikolaus
- Department of Cellular and Molecular Physiology, Yale University School of Medicine; Nanobiology Institute, Yale University
| | - Erdem Karatekin
- Department of Cellular and Molecular Physiology, Yale University School of Medicine; Nanobiology Institute, Yale University; Department of Molecular Biophysics and Biochemistry, Yale University; Laboratoire de Neurophotonique, Université Paris Descartes, Faculté des Sciences Fondamentales et Biomédicales, Centre National de la Recherche Scientifique (CNRS);
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114
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Calder LJ, Rosenthal PB. Cryomicroscopy provides structural snapshots of influenza virus membrane fusion. Nat Struct Mol Biol 2016; 23:853-8. [PMID: 27501535 PMCID: PMC6485592 DOI: 10.1038/nsmb.3271] [Citation(s) in RCA: 60] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/23/2016] [Accepted: 07/07/2016] [Indexed: 01/01/2023]
Abstract
The lipid-enveloped influenza virus enters host cells during infection by binding cell surface receptors and, following receptor-mediated endocytosis, fusing with the membrane of the endosome, delivering the viral genome and transcription machinery into the host cell. These events are mediated by the haemagglutinin (HA) surface glycoprotein. At the low pH of the endosome, an irreversible conformational change in the HA, including the exposure of the hydrophobic fusion peptide, activates membrane fusion. Here we use electron cryomicroscopy and cryotomography to image influenza virus fusion with target membranes at low pH. We visualize structural intermediates of HA and their interactions with membranes during the course of membrane fusion as well as ultra-structural changes in the virus that accompany membrane fusion. Our observations are relevant to a wide range of protein-mediated membrane fusion processes and demonstrate how dynamic membrane events may be studied by cryomicroscopy.
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115
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Direct targeting of membrane fusion by SNARE mimicry: Convergent evolution of Legionella effectors. Proc Natl Acad Sci U S A 2016; 113:8807-12. [PMID: 27436892 DOI: 10.1073/pnas.1608755113] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Legionella pneumophila, the Gram-negative pathogen causing Legionnaires' disease, infects host cells by hijacking endocytic pathways and forming a Legionella-containing vacuole (LCV) in which the bacteria replicate. To promote LCV expansion and prevent lysosomal targeting, effector proteins are translocated into the host cell where they alter membrane traffic. Here we show that three of these effectors [LegC2 (Legionella eukaryotic-like gene C2)/YlfB (yeast lethal factor B), LegC3, and LegC7/YlfA] functionally mimic glutamine (Q)-SNARE proteins. In infected cells, the three proteins selectively form complexes with the endosomal arginine (R)-SNARE vesicle-associated membrane protein 4 (VAMP4). When reconstituted in proteoliposomes, these proteins avidly fuse with liposomes containing VAMP4, resulting in a stable complex with properties resembling canonical SNARE complexes. Intriguingly, however, the LegC/SNARE hybrid complex cannot be disassembled by N-ethylmaleimide-sensitive factor. We conclude that LegCs use SNARE mimicry to divert VAMP4-containing vesicles for fusion with the LCV, thus promoting its expansion. In addition, the LegC/VAMP4 complex avoids the host's disassembly machinery, thus effectively trapping VAMP4 in an inactive state.
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Malmersjö S, Di Palma S, Diao J, Lai Y, Pfuetzner RA, Wang AL, McMahon MA, Hayer A, Porteus M, Bodenmiller B, Brunger AT, Meyer T. Phosphorylation of residues inside the SNARE complex suppresses secretory vesicle fusion. EMBO J 2016; 35:1810-21. [PMID: 27402227 PMCID: PMC5010044 DOI: 10.15252/embj.201694071] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/08/2016] [Accepted: 06/09/2016] [Indexed: 12/22/2022] Open
Abstract
Membrane fusion is essential for eukaryotic life, requiring SNARE proteins to zipper up in an α‐helical bundle to pull two membranes together. Here, we show that vesicle fusion can be suppressed by phosphorylation of core conserved residues inside the SNARE domain. We took a proteomics approach using a PKCB knockout mast cell model and found that the key mast cell secretory protein VAMP8 becomes phosphorylated by PKC at multiple residues in the SNARE domain. Our data suggest that VAMP8 phosphorylation reduces vesicle fusion in vitro and suppresses secretion in living cells, allowing vesicles to dock but preventing fusion with the plasma membrane. Markedly, we show that the phosphorylation motif is absent in all eukaryotic neuronal VAMPs, but present in all other VAMPs. Thus, phosphorylation of SNARE domains is a general mechanism to restrict how much cells secrete, opening the door for new therapeutic strategies for suppression of secretion.
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Affiliation(s)
- Seth Malmersjö
- Department of Chemical and Systems Biology, Stanford University, Stanford, CA, USA
| | - Serena Di Palma
- Institute of Molecular Life Sciences, University of Zurich, Zurich, Switzerland Functional Genomics Center Zurich, ETH Zurich/University of Zurich, Zurich, Switzerland
| | - Jiajie Diao
- Departments of Molecular and Cellular Physiology, Neurology and Neurological Sciences, Photon Science, and Structural Biology, Stanford University, Stanford, CA, USA Howard Hughes Medical Institute, Stanford, CA, USA
| | - Ying Lai
- Departments of Molecular and Cellular Physiology, Neurology and Neurological Sciences, Photon Science, and Structural Biology, Stanford University, Stanford, CA, USA Howard Hughes Medical Institute, Stanford, CA, USA
| | - Richard A Pfuetzner
- Departments of Molecular and Cellular Physiology, Neurology and Neurological Sciences, Photon Science, and Structural Biology, Stanford University, Stanford, CA, USA Howard Hughes Medical Institute, Stanford, CA, USA
| | - Austin L Wang
- Departments of Molecular and Cellular Physiology, Neurology and Neurological Sciences, Photon Science, and Structural Biology, Stanford University, Stanford, CA, USA Howard Hughes Medical Institute, Stanford, CA, USA
| | - Moira A McMahon
- Department of Pediatrics, Stanford University, Stanford, CA, USA
| | - Arnold Hayer
- Department of Chemical and Systems Biology, Stanford University, Stanford, CA, USA
| | - Matthew Porteus
- Department of Pediatrics, Stanford University, Stanford, CA, USA
| | - Bernd Bodenmiller
- Institute of Molecular Life Sciences, University of Zurich, Zurich, Switzerland
| | - Axel T Brunger
- Departments of Molecular and Cellular Physiology, Neurology and Neurological Sciences, Photon Science, and Structural Biology, Stanford University, Stanford, CA, USA Howard Hughes Medical Institute, Stanford, CA, USA
| | - Tobias Meyer
- Department of Chemical and Systems Biology, Stanford University, Stanford, CA, USA
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117
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Visualization and Sequencing of Membrane Remodeling Leading to Influenza Virus Fusion. J Virol 2016; 90:6948-6962. [PMID: 27226364 DOI: 10.1128/jvi.00240-16] [Citation(s) in RCA: 50] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2016] [Accepted: 05/14/2016] [Indexed: 12/31/2022] Open
Abstract
UNLABELLED Protein-mediated membrane fusion is an essential step in many fundamental biological events, including enveloped virus infection. The nature of protein and membrane intermediates and the sequence of membrane remodeling during these essential processes remain poorly understood. Here we used cryo-electron tomography (cryo-ET) to image the interplay between influenza virus and vesicles with a range of lipid compositions. By following the population kinetics of membrane fusion intermediates imaged by cryo-ET, we found that membrane remodeling commenced with the hemagglutinin fusion protein spikes grappling onto the target membrane, followed by localized target membrane dimpling as local clusters of hemagglutinin started to undergo conformational refolding. The local dimples then transitioned to extended, tightly apposed contact zones where the two proximal membrane leaflets were in most cases indistinguishable from each other, suggesting significant dehydration and possible intermingling of the lipid head groups. Increasing the content of fusion-enhancing cholesterol or bis-monoacylglycerophosphate in the target membrane led to an increase in extended contact zone formation. Interestingly, hemifused intermediates were found to be extremely rare in the influenza virus fusion system studied here, most likely reflecting the instability of this state and its rapid conversion to postfusion complexes, which increased in population over time. By tracking the populations of fusion complexes over time, the architecture and sequence of membrane reorganization leading to efficient enveloped virus fusion were thus resolved. IMPORTANCE Enveloped viruses employ specialized surface proteins to mediate fusion of cellular and viral membranes that results in the formation of pores through which the viral genetic material is delivered to the cell. For influenza virus, the trimeric hemagglutinin (HA) glycoprotein spike mediates host cell attachment and membrane fusion. While structures of a subset of conformations and parts of the fusion machinery have been characterized, the nature and sequence of membrane deformations during fusion have largely eluded characterization. Building upon studies that focused on early stages of HA-mediated membrane remodeling, here cryo-electron tomography (cryo-ET) was used to image the three-dimensional organization of intact influenza virions at different stages of fusion with liposomes, leading all the way to completion of the fusion reaction. By monitoring the evolution of fusion intermediate populations over the course of acid-induced fusion, we identified the progression of membrane reorganization that leads to efficient fusion by an enveloped virus.
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118
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Affiliation(s)
- Tolga Soykan
- Department of Molecular Pharmacology and Cell Biology, Leibniz Institut für Molekulare Pharmakologie, 13125 Berlin, Germany
| | - Volker Haucke
- Department of Molecular Pharmacology and Cell Biology, Leibniz Institut für Molekulare Pharmakologie, 13125 Berlin, Germany
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119
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Zhao WD, Hamid E, Shin W, Wen PJ, Krystofiak ES, Villarreal SA, Chiang HC, Kachar B, Wu LG. Hemi-fused structure mediates and controls fusion and fission in live cells. Nature 2016; 534:548-52. [PMID: 27309816 PMCID: PMC4930626 DOI: 10.1038/nature18598] [Citation(s) in RCA: 99] [Impact Index Per Article: 12.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/13/2015] [Accepted: 05/24/2016] [Indexed: 11/09/2022]
Abstract
Membrane fusion and fission are vital for eukaryotic life. For three decades, it has been proposed that fusion is mediated by fusion between the proximal leaflets of two bilayers (hemi-fusion) to produce a hemi-fused structure, followed by fusion between the distal leaflets, whereas fission is via hemi-fission, which also produces a hemi-fused structure, followed by full fission. This hypothesis remained unsupported owing to the lack of observation of hemi-fusion or hemi-fission in live cells. A competing fusion hypothesis involving protein-lined pore formation has also been proposed. Here we report the observation of a hemi-fused Ω-shaped structure in live neuroendocrine chromaffin cells and pancreatic β-cells, visualized using confocal and super-resolution stimulated emission depletion microscopy. This structure is generated from fusion pore opening or closure (fission) at the plasma membrane. Unexpectedly, the transition to full fusion or fission is determined by competition between fusion and calcium/dynamin-dependent fission mechanisms, and is notably slow (seconds to tens of seconds) in a substantial fraction of the events. These results provide key missing evidence in support of the hemi-fusion and hemi-fission hypothesis in live cells, and reveal the hemi-fused intermediate as a key structure controlling fusion and fission, as fusion and fission mechanisms compete to determine the transition to fusion or fission.
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Affiliation(s)
- Wei-Dong Zhao
- National Institute of Neurological Disorders and Stroke, 35 Convent Drive, Room 2B-1012, Bethesda, Maryland 20892, USA
| | - Edaeni Hamid
- National Institute of Neurological Disorders and Stroke, 35 Convent Drive, Room 2B-1012, Bethesda, Maryland 20892, USA
| | - Wonchul Shin
- National Institute of Neurological Disorders and Stroke, 35 Convent Drive, Room 2B-1012, Bethesda, Maryland 20892, USA
| | - Peter J Wen
- National Institute of Neurological Disorders and Stroke, 35 Convent Drive, Room 2B-1012, Bethesda, Maryland 20892, USA
| | - Evan S Krystofiak
- National Institute on Deafness and other Communication Disorders, 35A Convent Drive, Room 3D-824, Bethesda, Maryland 20892, USA
| | - Seth A Villarreal
- National Institute of Neurological Disorders and Stroke, 35 Convent Drive, Room 2B-1012, Bethesda, Maryland 20892, USA
| | - Hsueh-Cheng Chiang
- National Institute of Neurological Disorders and Stroke, 35 Convent Drive, Room 2B-1012, Bethesda, Maryland 20892, USA
| | - Bechara Kachar
- National Institute on Deafness and other Communication Disorders, 35A Convent Drive, Room 3D-824, Bethesda, Maryland 20892, USA
| | - Ling-Gang Wu
- National Institute of Neurological Disorders and Stroke, 35 Convent Drive, Room 2B-1012, Bethesda, Maryland 20892, USA
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120
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Choi UB, Zhao M, Zhang Y, Lai Y, Brunger AT. Complexin induces a conformational change at the membrane-proximal C-terminal end of the SNARE complex. eLife 2016; 5. [PMID: 27253060 PMCID: PMC4927292 DOI: 10.7554/elife.16886] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/13/2016] [Accepted: 06/01/2016] [Indexed: 01/14/2023] Open
Abstract
Complexin regulates spontaneous and activates Ca2+-triggered neurotransmitter release, yet the molecular mechanisms are still unclear. Here we performed single molecule fluorescence resonance energy transfer experiments and uncovered two conformations of complexin-1 bound to the ternary SNARE complex. In the cis conformation, complexin-1 induces a conformational change at the membrane-proximal C-terminal end of the ternary SNARE complex that specifically depends on the N-terminal, accessory, and central domains of complexin-1. The complexin-1 induced conformation of the ternary SNARE complex may be related to a conformation that is juxtaposing the synaptic vesicle and plasma membranes. In the trans conformation, complexin-1 can simultaneously interact with a ternary SNARE complex via the central domain and a binary SNARE complex consisting of syntaxin-1A and SNAP-25A via the accessory domain. The cis conformation may be involved in activation of synchronous neurotransmitter release, whereas both conformations may be involved in regulating spontaneous release. DOI:http://dx.doi.org/10.7554/eLife.16886.001 Nerve cells communicate via electrical signals that travel at high speeds. However, these signals cannot pass across the gaps – called synapses – that separate one nerve cell from the next. Instead, signals pass between nerve cells via molecules called neurotransmitters that are released from the membrane of the first cell and recognized by receptors in the membrane of the next. Prior to being released, neurotransmitters are packaged inside bubble-like structures called vesicles. The synaptic vesicles must fuse with the cell membrane in order to release their contents into the synaptic cleft. Proteins called SNAREs work together with other proteins to allow this membrane fusion to occur rapidly after the electrical signal arrives. Complexin is a synaptic protein that binds tightly to a complex of SNARE proteins to regulate membrane fusion. This protein activates the quick release of neurotransmitters, which is triggered by an increase in calcium ions as the electrical signal reachess the synapse. Complexin also regulates a different type of neurotransmitter release, which is known as “spontaneous release”. The complexin protein is made up of different regions, each of which is required for one or more of the protein’s activities. However, it is not clear how these regions, or domains, interact with SNAREs and other proteins to enable complexin to perform these roles. Choi et al. have now investigated whether the different activities of mammalian complexin are related to the structure that it adopts when it interacts with the SNARE complex. Complexes of SNARE proteins were assembled with one of the SNARE proteins tethered to a surface for imaging. Next, a light-based imaging technique called single molecule Förster resonance energy transfer (or FRET) was used to monitor how complexin interacts with the SNARE complex. This technique allows individual proteins that have been labeled with fluorescent markers to be followed under a microscope and can show how they interact in real-time. Using this approach, Choi et al. showed that complexin could adopt two different shapes or conformations when it binds to the SNARE complex. In one, complexin interacted closely with the SNARE complex so that it made part of the complex change shape. In the other, complexin was able to bridge two SNARE complexes. Complexin can therefore interact with SNARE complexes in different ways by using different regions of the protein. These findings provide insight into how complexin may regulate membrane fusion via the SNARE complex. In the future, single molecule FRET could be used to study other proteins found at synapses and understand the other steps that regulate the release of neurotransmitters. DOI:http://dx.doi.org/10.7554/eLife.16886.002
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Affiliation(s)
- Ucheor B Choi
- Department of Molecular and Cellular Physiology, Howard Hughes Medical Institute, Stanford University, Stanford, United States.,Department of Neurology and Neurological Sciences, Howard Hughes Medical Institute, Stanford University, Stanford, United States.,Department of Photon Science, Howard Hughes Medical Institute, Stanford University, Stanford, United States.,Department of Structural Biology, Howard Hughes Medical Institute, Stanford University, Stanford, United States
| | - Minglei Zhao
- Department of Molecular and Cellular Physiology, Howard Hughes Medical Institute, Stanford University, Stanford, United States.,Department of Neurology and Neurological Sciences, Howard Hughes Medical Institute, Stanford University, Stanford, United States.,Department of Photon Science, Howard Hughes Medical Institute, Stanford University, Stanford, United States.,Department of Structural Biology, Howard Hughes Medical Institute, Stanford University, Stanford, United States
| | - Yunxiang Zhang
- Department of Molecular and Cellular Physiology, Howard Hughes Medical Institute, Stanford University, Stanford, United States.,Department of Neurology and Neurological Sciences, Howard Hughes Medical Institute, Stanford University, Stanford, United States.,Department of Photon Science, Howard Hughes Medical Institute, Stanford University, Stanford, United States.,Department of Structural Biology, Howard Hughes Medical Institute, Stanford University, Stanford, United States
| | - Ying Lai
- Department of Molecular and Cellular Physiology, Howard Hughes Medical Institute, Stanford University, Stanford, United States.,Department of Neurology and Neurological Sciences, Howard Hughes Medical Institute, Stanford University, Stanford, United States.,Department of Photon Science, Howard Hughes Medical Institute, Stanford University, Stanford, United States.,Department of Structural Biology, Howard Hughes Medical Institute, Stanford University, Stanford, United States
| | - Axel T Brunger
- Department of Molecular and Cellular Physiology, Howard Hughes Medical Institute, Stanford University, Stanford, United States.,Department of Neurology and Neurological Sciences, Howard Hughes Medical Institute, Stanford University, Stanford, United States.,Department of Photon Science, Howard Hughes Medical Institute, Stanford University, Stanford, United States.,Department of Structural Biology, Howard Hughes Medical Institute, Stanford University, Stanford, United States
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Patience GS, Srinivisan B, Perrier M, Boffito DC, Patience PA. How do you write and present research well? 9-show and state what error bars represent. CAN J CHEM ENG 2016. [DOI: 10.1002/cjce.22509] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Affiliation(s)
- Gregory S. Patience
- Department of Chemical Engineering; Polytechnique Montréal; C.P. 6079, Succ. CV Montréal, QC, H3C 3A7 Canada
| | - Bala Srinivisan
- Department of Chemical Engineering; Polytechnique Montréal; C.P. 6079, Succ. CV Montréal, QC, H3C 3A7 Canada
| | - Michel Perrier
- Department of Chemical Engineering; Polytechnique Montréal; C.P. 6079, Succ. CV Montréal, QC, H3C 3A7 Canada
| | - Daria C. Boffito
- Department of Chemical Engineering; Polytechnique Montréal; C.P. 6079, Succ. CV Montréal, QC, H3C 3A7 Canada
| | - Paul A. Patience
- Department of Electrical Engineering; Polytechnique Montréal; C.P. 6079, Succ. CV Montréal, QC, H3C 3A7 Canada
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122
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Han J, Pluhackova K, Böckmann RA. Exploring the Formation and the Structure of Synaptobrevin Oligomers in a Model Membrane. Biophys J 2016; 110:2004-15. [PMID: 27166808 PMCID: PMC4939486 DOI: 10.1016/j.bpj.2016.04.006] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/30/2015] [Revised: 03/03/2016] [Accepted: 04/06/2016] [Indexed: 11/28/2022] Open
Abstract
SNARE complexes have been shown to act cooperatively to enable the synaptic vesicle fusion in neuronal transmission at millisecond timescale. It has previously been suggested that the oligomerization of SNARE complexes required for cooperative action in fusion is mediated by interactions between transmembrane domains (TMDs). We study the oligomerization of synaptobrevin TMD using ensembles of molecular dynamics (MD) simulations at coarse-grained resolution for both the wild-type (WT) and selected mutants. Trimerization and tetramerization of the sybII WT and mutants displayed distinct kinetics depending both on the rate of dimerization and the availability of alternative binding interfaces. Interestingly, the tetramerization kinetics and propensity for the sybII W89A-W90A mutant was significantly increased as compared with the WT; the tryptophans in WT sybII impose sterical restraints on oligomer packing, thereby maintaining an appropriate plasticity and accessibility of sybII to the binding of its cognate SNARE partners during membrane fusion. Higher-order oligomeric models (ranging from pentamer to octamer), built by incremental addition of peptides to smaller oligomers, revealed substantial stability and high compactness. These larger sybII oligomers may induce membrane deformation, thereby possibly facilitating fast fusion exocytosis.
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Affiliation(s)
- Jing Han
- Computational Biology, Department of Biology, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany
| | - Kristyna Pluhackova
- Computational Biology, Department of Biology, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany
| | - Rainer A Böckmann
- Computational Biology, Department of Biology, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany.
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123
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Abstract
SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins are a highly conserved set of membrane-associated proteins that mediate intracellular membrane fusion. Cognate SNAREs from two separate membranes zipper to facilitate membrane apposition and fusion. Though the stable post-fusion conformation of SNARE complex has been extensively studied with biochemical and biophysical means, the pathway of SNARE zippering has been elusive. In this review, we describe some recent progress in understanding the pathway of SNARE zippering. We particularly focus on the half-zippered intermediate, which is most likely to serve as the main point of regulation by the auxiliary factors.
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124
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High cholesterol obviates a prolonged hemifusion intermediate in fast SNARE-mediated membrane fusion. Biophys J 2016. [PMID: 26200867 DOI: 10.1016/j.bpj.2015.06.022] [Citation(s) in RCA: 40] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022] Open
Abstract
Cholesterol is essential for exocytosis in secretory cells, but the exact molecular mechanism by which it facilitates exocytosis is largely unknown. Distinguishing contributions from the lateral organization and dynamics of membrane proteins to vesicle docking and fusion and the promotion of fusion pores by negative intrinsic spontaneous curvature and other mechanical effects of cholesterol have been elusive. To shed more light on this process, we examined the effect of cholesterol on SNARE-mediated membrane fusion in a single-vesicle assay that is capable of resolving docking and elementary steps of fusion with millisecond time resolution. The effect of cholesterol on fusion pore formation between synaptobrevin-2 (VAMP-2)-containing proteoliposomes and acceptor t-SNARE complex-containing planar supported bilayers was examined using both membrane and content fluorescent markers. This approach revealed that increasing cholesterol in either the t-SNARE or the v-SNARE membrane favors a mechanism of direct fusion pore opening, whereas low cholesterol favors a mechanism leading to a long-lived (>5 s) hemifusion state. The amount of cholesterol in the target membrane had no significant effect on docking of synaptobrevin vesicles. Comparative studies with α-tocopherol (vitamin E) show that the negative intrinsic spontaneous curvature of cholesterol and its presumed promotion of a very short-lived (<50 ms) lipid stalk intermediate is the main factor that favors rapid fusion pore opening at high cholesterol. This study also shows that this single-vesicle fusion assay can distinguish between hemifusion and full fusion with only a single lipid dye, thereby freeing up a fluorescence channel for the simultaneous measurement of another parameter in fast time-resolved fusion assays.
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125
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Heo P, Yang Y, Han KY, Kong B, Shin JH, Jung Y, Jeong C, Shin J, Shin YK, Ha T, Kweon DH. A Chemical Controller of SNARE-Driven Membrane Fusion That Primes Vesicles for Ca(2+)-Triggered Millisecond Exocytosis. J Am Chem Soc 2016; 138:4512-21. [PMID: 26987363 PMCID: PMC4852477 DOI: 10.1021/jacs.5b13449] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Membrane fusion is mediated by the SNARE complex which is formed through a zippering process. Here, we developed a chemical controller for the progress of membrane fusion. A hemifusion state was arrested by a polyphenol myricetin which binds to the SNARE complex. The arrest of membrane fusion was rescued by an enzyme laccase that removes myricetin from the SNARE complex. The rescued hemifusion state was metastable and long-lived with a decay constant of 39 min. This membrane fusion controller was applied to delineate how Ca(2+) stimulates fusion-pore formation in a millisecond time scale. We found, using a single-vesicle fusion assay, that such myricetin-primed vesicles with synaptotagmin 1 respond synchronously to physiological concentrations of Ca(2+). When 10 μM Ca(2+) was added to the hemifused vesicles, the majority of vesicles rapidly advanced to fusion pores with a time constant of 16.2 ms. Thus, the results demonstrate that a minimal exocytotic membrane fusion machinery composed of SNAREs and synaptotagmin 1 is capable of driving membrane fusion in a millisecond time scale when a proper vesicle priming is established. The chemical controller of SNARE-driven membrane fusion should serve as a versatile tool for investigating the differential roles of various synaptic proteins in discrete fusion steps.
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Affiliation(s)
- Paul Heo
- Department of Genetic Engineering, College of Biotechnology and Bioengineering, and Center for Human Interface Nano Technology, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, South Korea
| | - Yoosoo Yang
- Department of Genetic Engineering, College of Biotechnology and Bioengineering, and Center for Human Interface Nano Technology, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, South Korea
- Biomedical Research Institute, Korea Institute of Science and Technology, Seoul 136-791, South Korea
| | - Kyu-Young Han
- Howard Hughes Medical Institute, Baltimore, MD 21205, United States
- CREOL, The College of Optics & Photonics, University of Central Florida, Orlando, Florida 32816, United States
| | - Byoungjae Kong
- Department of Genetic Engineering, College of Biotechnology and Bioengineering, and Center for Human Interface Nano Technology, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, South Korea
| | - Jong-Hyeok Shin
- Department of Genetic Engineering, College of Biotechnology and Bioengineering, and Center for Human Interface Nano Technology, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, South Korea
| | - Younghoon Jung
- Department of Genetic Engineering, College of Biotechnology and Bioengineering, and Center for Human Interface Nano Technology, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, South Korea
| | - Cherlhyun Jeong
- Biomedical Research Institute, Korea Institute of Science and Technology, Seoul 136-791, South Korea
| | - Jaeil Shin
- Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University, Ames, IA 50011, United States
| | - Yeon-Kyun Shin
- Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University, Ames, IA 50011, United States
| | - Taekjip Ha
- Howard Hughes Medical Institute, Baltimore, MD 21205, United States
- Department of Biophysics and Biophysical Chemistry Johns Hopkins University School of Medicine, Baltimore, MD 21205 United States
- Department of Biophysics, Johns Hopkins University, Baltimore, MD 21218 United States
- Department of Biomedical Engineering, Johns Hopkins University, Baltimore, MD 21205, United States
| | - Dae-Hyuk Kweon
- Department of Genetic Engineering, College of Biotechnology and Bioengineering, and Center for Human Interface Nano Technology, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, South Korea
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Xu W, Nathwani B, Lin C, Wang J, Karatekin E, Pincet F, Shih W, Rothman JE. A Programmable DNA Origami Platform to Organize SNAREs for Membrane Fusion. J Am Chem Soc 2016; 138:4439-47. [PMID: 26938705 DOI: 10.1021/jacs.5b13107] [Citation(s) in RCA: 67] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023]
Abstract
Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes are the core molecular machinery of membrane fusion, a fundamental process that drives inter- and intracellular communication and trafficking. One of the questions that remains controversial has been whether and how SNAREs cooperate. Here we show the use of self-assembled DNA-nanostructure rings to template uniform-sized small unilamellar vesicles containing predetermined maximal number of externally facing SNAREs to study the membrane-fusion process. We also incorporated lipid-conjugated complementary ssDNA as tethers into vesicle and target membranes, which enabled bypass of the rate-limiting docking step of fusion reactions and allowed direct observation of individual membrane-fusion events at SNARE densities as low as one pair per vesicle. With this platform, we confirmed at the single event level that, after docking of the templated-SUVs to supported lipid bilayers (SBL), one to two pairs of SNAREs are sufficient to drive fast lipid mixing. Modularity and programmability of this platform makes it readily amenable to studying more complicated systems where auxiliary proteins are involved.
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Affiliation(s)
| | - Bhavik Nathwani
- Wyss Institute for Biologically Inspired Engineering and Biological Chemistry and Molecular Pharmacology, Harvard Medical School, and Department of Cancer Biology, Dana Farber Cancer Institute , Boston, Massachusetts 02115, United States
| | - Chenxiang Lin
- Wyss Institute for Biologically Inspired Engineering and Biological Chemistry and Molecular Pharmacology, Harvard Medical School, and Department of Cancer Biology, Dana Farber Cancer Institute , Boston, Massachusetts 02115, United States
| | | | - Erdem Karatekin
- Laboratoire de Neurophotonique, Université Paris Descartes, Faculté des Sciences Fondamentales et Biomédicales, Centre National de la Recherche Scientifique (CNRS) UMR8250, 45, rue des Saints Pères, 75270 Cedex 06 Paris, France
| | - Frederic Pincet
- Laboratoire de Physique Statistique, Ecole Normale Supérieure de Paris, Université Pierre et Marie Curie, Université Paris Diderot, Centre National de la Recherche Scientifique, UMR 8550, 24 rue Lhomond, 75005 Paris, France
| | - William Shih
- Wyss Institute for Biologically Inspired Engineering and Biological Chemistry and Molecular Pharmacology, Harvard Medical School, and Department of Cancer Biology, Dana Farber Cancer Institute , Boston, Massachusetts 02115, United States
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Bleicken S, Hofhaus G, Ugarte-Uribe B, Schröder R, García-Sáez AJ. cBid, Bax and Bcl-xL exhibit opposite membrane remodeling activities. Cell Death Dis 2016; 7:e2121. [PMID: 26913610 PMCID: PMC4849160 DOI: 10.1038/cddis.2016.34] [Citation(s) in RCA: 26] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/18/2015] [Revised: 01/20/2016] [Accepted: 01/25/2016] [Indexed: 12/19/2022]
Abstract
The proteins of the Bcl-2 family have a crucial role in mitochondrial outer membrane permeabilization during apoptosis and in the regulation of mitochondrial dynamics. Current models consider that Bax forms toroidal pores at mitochondria that are responsible for the release of cytochrome c, whereas Bcl-xL inhibits pore formation. However, how Bcl-2 proteins regulate mitochondrial fission and fusion remains poorly understood. By using a systematic analysis at the single vesicle level, we found that cBid, Bax and Bcl-xL are able to remodel membranes in different ways. cBid and Bax induced a reduction in vesicle size likely related to membrane tethering, budding and fission, besides membrane permeabilization. Moreover, they are preferentially located at highly curved membranes. In contrast, Bcl-xL not only counterbalanced pore formation but also membrane budding and fission. Our findings support a mechanism of action by which cBid and Bax induce or stabilize highly curved membranes including non-lamellar structures. This molecular activity reduces the energy for membrane remodeling, which is a necessary step in toroidal pore formation, as well as membrane fission and fusion, and provides a common mechanism that links the two main functions of Bcl-2 proteins.
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Affiliation(s)
- S Bleicken
- Membrane Biophysics, Max Planck Institute for Intelligent Systems, Heisenbergstrasse 3, Stuttgart 70569, Germany.,German Cancer Research Center, Im Neuenheimer Feld 267, Heidelberg 69120, Germany.,Membrane Biophysics, Interfaculty Institute of Biochemistry, Eberhard Karls University Tübingen, Hoppe-Seyler-Strasse 4, Tübingen 72076, Germany
| | - G Hofhaus
- CellNetworks, Bioquant, Heidelberg University, Im Neuenheimer Feld 267, Heidelberg, 69120, Germany
| | - B Ugarte-Uribe
- Membrane Biophysics, Max Planck Institute for Intelligent Systems, Heisenbergstrasse 3, Stuttgart 70569, Germany.,German Cancer Research Center, Im Neuenheimer Feld 267, Heidelberg 69120, Germany.,Membrane Biophysics, Interfaculty Institute of Biochemistry, Eberhard Karls University Tübingen, Hoppe-Seyler-Strasse 4, Tübingen 72076, Germany
| | - R Schröder
- CellNetworks, Bioquant, Heidelberg University, Im Neuenheimer Feld 267, Heidelberg, 69120, Germany
| | - A J García-Sáez
- Membrane Biophysics, Max Planck Institute for Intelligent Systems, Heisenbergstrasse 3, Stuttgart 70569, Germany.,German Cancer Research Center, Im Neuenheimer Feld 267, Heidelberg 69120, Germany.,Membrane Biophysics, Interfaculty Institute of Biochemistry, Eberhard Karls University Tübingen, Hoppe-Seyler-Strasse 4, Tübingen 72076, Germany
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128
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Fortoul N, Singh P, Hui CY, Bykhovskaia M, Jagota A. Coarse-Grained Model of SNARE-Mediated Docking. Biophys J 2016; 108:2258-69. [PMID: 25954883 DOI: 10.1016/j.bpj.2015.03.053] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/15/2014] [Revised: 03/24/2015] [Accepted: 03/24/2015] [Indexed: 12/11/2022] Open
Abstract
Synaptic transmission requires that vesicles filled with neurotransmitter molecules be docked to the plasma membrane by the SNARE protein complex. The SNARE complex applies attractive forces to overcome the long-range repulsion between the vesicle and membrane. To understand how the balance between the attractive and repulsive forces defines the equilibrium docked state we have developed a model that combines the mechanics of vesicle/membrane deformation with an apparently new coarse-grained model of the SNARE complex. The coarse-grained model of the SNARE complex is calibrated by comparison with all-atom molecular dynamics simulations as well as by force measurements in laser tweezer experiments. The model for vesicle/membrane interactions includes the forces produced by membrane deformation and hydration or electrostatic repulsion. Combining these two parts, the coarse-grained model of the SNARE complex with membrane mechanics, we study how the equilibrium docked state varies with the number of SNARE complexes. We find that a single SNARE complex is able to bring a typical synaptic vesicle to within a distance of ∼ 3 nm from the membrane. Further addition of SNARE complexes shortens this distance, but an overdocked state of >4-6 SNAREs actually increases the equilibrium distance.
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Affiliation(s)
- Nicole Fortoul
- Department of Chemical and Biomolecular Engineering, Lehigh University, Bethlehem, Pennsylvania
| | - Pankaj Singh
- Department of Mechanical & Aerospace Engineering, Cornell University, Ithaca, New York
| | - Chung-Yuen Hui
- Department of Mechanical & Aerospace Engineering, Cornell University, Ithaca, New York
| | - Maria Bykhovskaia
- Neuroscience Department, Universidad Central del Caribe, Bayamon, Puerto Rico
| | - Anand Jagota
- Department of Chemical and Biomolecular Engineering, Lehigh University, Bethlehem, Pennsylvania; Bioengineering Program, Lehigh University, Bethlehem, Pennsylvania.
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129
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Abstract
Extensive research has yielded crucial insights into the mechanism of neurotransmitter release, and working models for the functions of key proteins involved in release. The SNAREs Syntaxin-1, Synaptobrevin, and SNAP-25 play a central role in membrane fusion, forming SNARE complexes that bridge the vesicle and plasma membranes and that are disassembled by NSF-SNAPs. Exocytosis likely starts with Syntaxin-1 folded into a self-inhibited closed conformation that binds to Munc18-1. Munc13s open Syntaxin-1, orchestrating SNARE complex assembly in an NSF-SNAP-resistant manner together with Munc18-1. In the resulting primed state, with partially assembled SNARE complexes, fusion is inhibited by Synaptotagmin-1 and Complexins, which also perform active functions in release. Upon influx of Ca(2+), Synaptotagmin-1 activates fast release, likely by relieving the inhibition caused by Complexins and cooperating with the SNAREs in bringing the membranes together. Although alternative models exist and fundamental questions remain unanswered, a definitive description of the basic release mechanism may be available soon.
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Affiliation(s)
- Josep Rizo
- Departments of Biophysics, Biochemistry, and Pharmacology, University of Texas Southwestern Medical Center, Dallas, Texas 75390;
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130
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Membrane Trafficking in Neuronal Development: Ins and Outs of Neural Connectivity. INTERNATIONAL REVIEW OF CELL AND MOLECULAR BIOLOGY 2016; 322:247-80. [PMID: 26940520 DOI: 10.1016/bs.ircmb.2015.10.003] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
During development, neurons progress through rapid yet stereotypical shape changes to achieve proper neuronal connectivity. This morphological progression requires carefully orchestrated plasma membrane expansion, insertion of membrane components including receptors for extracellular cues into the plasma membrane and removal and trafficking of membrane materials and proteins to specific locations. This review outlines the cellular machinery of membrane trafficking that play an integral role in neuronal cell shape change and function from initial neurite formation to pathway navigation and synaptogenesis.
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131
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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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132
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Yan L, Sun S, Wang W, Shi J, Hu X, Wang S, Su D, Rao Z, Hu J, Lou Z. Structures of the yeast dynamin-like GTPase Sey1p provide insight into homotypic ER fusion. J Cell Biol 2015; 210:961-72. [PMID: 26370501 PMCID: PMC4576867 DOI: 10.1083/jcb.201502078] [Citation(s) in RCA: 36] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
The crystal structures of the N-terminal cytosolic domain of Sey1p shed light on the mechanism of Sey1p-mediated ER membrane fusion. Homotypic membrane fusion of the endoplasmic reticulum is mediated by dynamin-like guanosine triphosphatases (GTPases), which include atlastin (ATL) in metazoans and Sey1p in yeast. In this paper, we determined the crystal structures of the cytosolic domain of Sey1p derived from Candida albicans. The structures reveal a stalk-like, helical bundle domain following the GTPase, which represents a previously unidentified configuration of the dynamin superfamily. This domain is significantly longer than that of ATL and critical for fusion. Sey1p forms a side-by-side dimer in complex with GMP-PNP or GDP/AlF4− but is monomeric with GDP. Surprisingly, Sey1p could mediate fusion without GTP hydrolysis, even though fusion was much more efficient with GTP. Sey1p was able to replace ATL in mammalian cells, and the punctate localization of Sey1p was dependent on its GTPase activity. Despite the common function of fusogenic GTPases, our results reveal unique features of Sey1p.
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Affiliation(s)
- Liming Yan
- School of Medicine, Tsinghua University, Beijing, China 100084
| | - Sha Sun
- Department of Genetics and Cell Biology, College of Life Sciences, Nankai University and Tianjin Key Laboratory of Protein Sciences, Tianjin, China 300071
| | - Wei Wang
- School of Medicine, Tsinghua University, Beijing, China 100084 State Key Laboratory of Cotton Biology, Key Laboratory of Plant Stress Biology, Henan University, Kaifeng, China 475004
| | - Juanming Shi
- Department of Genetics and Cell Biology, College of Life Sciences, Nankai University and Tianjin Key Laboratory of Protein Sciences, Tianjin, China 300071
| | - Xiaoyu Hu
- Department of Genetics and Cell Biology, College of Life Sciences, Nankai University and Tianjin Key Laboratory of Protein Sciences, Tianjin, China 300071
| | - Shiyan Wang
- State Key Laboratory of Cotton Biology, Key Laboratory of Plant Stress Biology, Henan University, Kaifeng, China 475004
| | - Dan Su
- Collaborative Innovation Center for Biotherapy, State Key Laboratory of Biotherapy and Cancer Center, West China Hospital, West China Medical School, Sichuan University, Chengdu, China 610041
| | - Zihe Rao
- School of Medicine, Tsinghua University, Beijing, China 100084 National Laboratory of Macromolecules, Institute of Biophysics, Chinese Academy of Science, Beijing, China 100101 Collaborative Innovation Center for Biotherapy, West China Hospital, West China Medical School, Sichuan University, Chengdu, China 610041
| | - Junjie Hu
- Department of Genetics and Cell Biology, College of Life Sciences, Nankai University and Tianjin Key Laboratory of Protein Sciences, Tianjin, China 300071 National Laboratory of Macromolecules, Institute of Biophysics, Chinese Academy of Science, Beijing, China 100101
| | - Zhiyong Lou
- School of Medicine, Tsinghua University, Beijing, China 100084 Collaborative Innovation Center for Biotherapy, West China Hospital, West China Medical School, Sichuan University, Chengdu, China 610041
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133
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Ma L, Rebane AA, Yang G, Xi Z, Kang Y, Gao Y, Zhang Y. Munc18-1-regulated stage-wise SNARE assembly underlying synaptic exocytosis. eLife 2015; 4. [PMID: 26701912 PMCID: PMC4744192 DOI: 10.7554/elife.09580] [Citation(s) in RCA: 63] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2015] [Accepted: 12/22/2015] [Indexed: 12/20/2022] Open
Abstract
Synaptic-soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) proteins couple their stage-wise folding/assembly to rapid exocytosis of neurotransmitters in a Munc18-1-dependent manner. The functions of the different assembly stages in exocytosis and the role of Munc18-1 in SNARE assembly are not well understood. Using optical tweezers, we observed four distinct stages of assembly in SNARE N-terminal, middle, C-terminal, and linker domains (or NTD, MD, CTD, and LD, respectively). We found that SNARE layer mutations differentially affect SNARE assembly. Comparison of their effects on SNARE assembly and on exocytosis reveals that NTD and CTD are responsible for vesicle docking and fusion, respectively, whereas MD regulates SNARE assembly and fusion. Munc18-1 initiates SNARE assembly and structures t-SNARE C-terminus independent of syntaxin N-terminal regulatory domain (NRD) and stabilizes the half-zippered SNARE complex dependent upon the NRD. Our observations demonstrate distinct functions of SNARE domains whose assembly is intimately chaperoned by Munc18-1. DOI:http://dx.doi.org/10.7554/eLife.09580.001 Plants, animals and other eukaryotes transport many large molecules within their cells inside membrane-bound packages called vesicles. These vesicles can fuse with the membrane of a target compartment in the cell to deliver their contents inside, or fuse with the cell’s membrane to release the contents outside of the cell. Membrane fusion is carried out by a group of proteins called SNAREs. These proteins are embedded on the membranes of both the vesicle and its target, and they bind to each other to form a tight complex. This complex docks the vesicle to the target and then acts like a “zipper” to pull the two membranes close enough to fuse. The best-studied SNARE proteins act in nerve cells and fuse vesicles to the cell’s membrane in order to release molecules called neurotransmitters. This process is essential for communication between nerve cells, and relies on a protein called Munc18-1. However, it is not well understood how SNARE proteins assemble into the complex and how Munc18-1 regulates this process. Ma et al. have now used a tool called “optical tweezers” to pull an assembled SNARE complex apart in the laboratory and then observe how it folds and assembles in a step-by-step process. These experiments showed that the complex assembled in four stages and not three as has been reported in previous work. SNARE proteins are made up of four parts called domains, and Ma et al. observed that the N-terminal domains were the first to bind to each other. Next, the binding progressed to the middle domain, then to the C-terminal domain and finally to the linker domain. An intermediate, half-zippered form was also observed. Ma et al. next analysed each domain in more detail and found that the N-terminal and C-terminal domains drive the docking of vesicles to the target membrane, the middle domain is crucial for assembling the SNARE complex correctly, and all three domains regulate the fusing of the membranes. Further experiments showed that Munc18-1 promoted the assembly of new SNARE complexes and stabilized the half-zippered form, rather than stabilizing the complex after it had fully assembled. This study will provide a new tool to examine many other proteins that regulate SNARE assembly, and a basis to understand the role of SNARE proteins in brain activity. DOI:http://dx.doi.org/10.7554/eLife.09580.002
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Affiliation(s)
- Lu Ma
- Department of Cell Biology, Yale School of Medicine, New Haven, United States
| | - Aleksander A Rebane
- Department of Cell Biology, Yale School of Medicine, New Haven, United States.,Integrated Graduate Program in Physical and Engineering Biology, Yale University, New Haven, United States.,Department of Physics, Yale University, New Haven, United States
| | - Guangcan Yang
- Department of Cell Biology, Yale School of Medicine, New Haven, United States.,Department of Physics, Wenzhou University, Wenzhou, China
| | - Zhiqun Xi
- Department of Cell Biology, Yale School of Medicine, New Haven, United States
| | - Yuhao Kang
- Department of Cell Biology, Yale School of Medicine, New Haven, United States
| | - Ying Gao
- Department of Cell Biology, Yale School of Medicine, New Haven, United States
| | - Yongli Zhang
- Department of Cell Biology, Yale School of Medicine, New Haven, United States
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134
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Affiliation(s)
- Carlo Breda
- Department of Genetics, University of Leicester, University Road, Leicester LE1 7RH, UK
| | - Flaviano Giorgini
- Department of Genetics, University of Leicester, University Road, Leicester LE1 7RH, UK
| | - Joern R. Steinert
- MRC Toxicology Unit, University of Leicester, Hodgkin Building, Lancaster Road, Leicester LE1 9HN, UK
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135
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Schwenen LLG, Hubrich R, Milovanovic D, Geil B, Yang J, Kros A, Jahn R, Steinem C. Resolving single membrane fusion events on planar pore-spanning membranes. Sci Rep 2015; 5:12006. [PMID: 26165860 PMCID: PMC4499801 DOI: 10.1038/srep12006] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/02/2015] [Accepted: 06/15/2015] [Indexed: 12/15/2022] Open
Abstract
Even though a number of different in vitro fusion assays have been developed to analyze protein mediated fusion, they still only partially capture the essential features of the in vivo situation. Here we established an in vitro fusion assay that mimics the fluidity and planar geometry of the cellular plasma membrane to be able to monitor fusion of single protein-containing vesicles. As a proof of concept, planar pore-spanning membranes harboring SNARE-proteins were generated on highly ordered functionalized 1.2 μm-sized pore arrays in Si3N4. Full mobility of the membrane components was demonstrated by fluorescence correlation spectroscopy. Fusion was analyzed by two color confocal laser scanning fluorescence microscopy in a time resolved manner allowing to readily distinguish between vesicle docking, intermediate states such as hemifusion and full fusion. The importance of the membrane geometry on the fusion process was highlighted by comparing SNARE-mediated fusion with that of a minimal SNARE fusion mimetic.
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Affiliation(s)
- Lando L G Schwenen
- Institute for Organic and Biomolecular Chemistry, University of Göttingen, Tammannstr. 2, 37077 Göttingen, Germany
| | - Raphael Hubrich
- Institute for Organic and Biomolecular Chemistry, University of Göttingen, Tammannstr. 2, 37077 Göttingen, Germany
| | - Dragomir Milovanovic
- Max-Planck-Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, Germany
| | - Burkhard Geil
- Institute for Physical Chemistry, University of Göttingen, Tammannstr. 6, 37077 Göttingen, Germany
| | - Jian Yang
- Leiden Institute of Chemistry - Supramolecular and Biomaterials Chemistry, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands
| | - Alexander Kros
- Leiden Institute of Chemistry - Supramolecular and Biomaterials Chemistry, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands
| | - Reinhard Jahn
- Max-Planck-Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, Germany
| | - Claudia Steinem
- Institute for Organic and Biomolecular Chemistry, University of Göttingen, Tammannstr. 2, 37077 Göttingen, Germany
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136
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Two-Photon Correlation Spectroscopy in Single Dendritic Spines Reveals Fast Actin Filament Reorganization during Activity-Dependent Growth. PLoS One 2015; 10:e0128241. [PMID: 26020927 PMCID: PMC4447372 DOI: 10.1371/journal.pone.0128241] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/30/2014] [Accepted: 04/24/2015] [Indexed: 11/19/2022] Open
Abstract
Two-photon fluorescence correlation spectroscopy (2P-FCS) within single dendritic spines of living hippocampal pyramidal neurons was used to resolve various subpopulations of mobile F-actin during activity-dependent structural changes such as potentiation induced spine head growth. Two major classes of mobile F-actin were discovered: very dynamic and about a hundred times less dynamic F-actin. Spine head enlargement upon application of Tetraethylammonium (TEA), a protocol previously used for the chemical induction of long-term potentiation (cLTP) strictly correlated to changes in the dynamics and filament numbers in the different actin filament fractions. Our observations suggest that spine enlargement is governed by a mechanism in which longer filaments are first cut into smaller filaments that cooperate with the second, increasingly dynamic shorter actin filament population to quickly reorganize and expand the actin cytoskeleton within the spine head. This process would allow a fast and efficient spine head enlargement using a major fraction of the actin filament population that was already present before spine head growth.
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137
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The synaptotagmin juxtamembrane domain is involved in neuroexocytosis. FEBS Open Bio 2015; 5:388-96. [PMID: 25973365 PMCID: PMC4427626 DOI: 10.1016/j.fob.2015.04.013] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2015] [Revised: 04/21/2015] [Accepted: 04/23/2015] [Indexed: 12/21/2022] Open
Abstract
The highly cationic juxtamembrane segment of synaptotagmin juxtamembrane domain was synthesized. This peptide inhibits neurotransmitter release at the neuromuscular junction of mice and Drosophila. This peptide localizes mainly on the presynaptic membrane. The synaptotagmin juxtamembrane peptide binds monophosphoinositides in a Ca2+-independent manner. The juxtamembrane segment of synaptotagmin may contribute to the formation of the hemifusion intermediate.
Synaptotagmin is a synaptic vesicle membrane protein which changes conformation upon Ca2+ binding and triggers the fast neuroexocytosis that takes place at synapses. We have synthesized a series of peptides corresponding to the sequence of the cytosolic juxtamembrane domain of synaptotagmin, which is highly conserved among different isoforms and animal species, with or without either a hexyl hydrophobic chain or the hexyl group plus a fluorescein moiety. We show that these peptides inhibit neurotransmitter release, that they localize on the presynaptic membrane of the motor axon terminal at the neuromuscular junction and that they bind monophosphoinositides in a Ca2+-independent manner. Based on these findings, we propose that the juxtamembrane cytosolic domain of synaptotagmin binds the cytosolic layer of the presynaptic membrane at rest. This binding brings synaptic vesicles and plasma membrane in a very close apposition, favouring the formation of hemifusion intermediates that enable rapid vesicle fusion.
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Key Words
- Anionic phospholipids
- JMS, juxtamembrane segment
- Juxtamembrane domain
- NMJ, neuromuscular junction
- Neuroexocytosis
- Neuromuscular junction
- PM, presynaptic membrane
- SV, synaptic vesicles
- Synaptotagmin
- Syt, synaptotagmin
- TM, transmembrane
- h-FJMS, hexyl fluorescent juxtamembrane segment
- h-JMS, hexyl juxtamembrane segment
- h-sJMS, hexyl scrambled juxtamembrane segment
- α-BTX, alpha-bungarotoxin
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138
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Ryu JK, Min D, Rah SH, Kim SJ, Park Y, Kim H, Hyeon C, Kim HM, Jahn R, Yoon TY. Spring-loaded unraveling of a single SNARE complex by NSF in one round of ATP turnover. Science 2015; 347:1485-9. [PMID: 25814585 DOI: 10.1126/science.aaa5267] [Citation(s) in RCA: 68] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
During intracellular membrane trafficking, N-ethylmaleimide-sensitive factor (NSF) and alpha-soluble NSF attachment protein (α-SNAP) disassemble the soluble NSF attachment protein receptor (SNARE) complex for recycling of the SNARE proteins. The molecular mechanism by which NSF disassembles the SNARE complex is largely unknown. Using single-molecule fluorescence spectroscopy and magnetic tweezers, we found that NSF disassembled a single SNARE complex in only one round of adenosine triphosphate (ATP) turnover. Upon ATP cleavage, the NSF hexamer developed internal tension with dissociation of phosphate ions. After latent time measuring tens of seconds, NSF released the built-up tension in a burst within 20 milliseconds, resulting in disassembly followed by immediate release of the SNARE proteins. Thus, NSF appears to use a "spring-loaded" mechanism to couple ATP hydrolysis and unfolding of substrate proteins.
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Affiliation(s)
- Je-Kyung Ryu
- National Creative Research Initiative Center for Single-Molecule Systems Biology, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, South Korea. Department of Physics, KAIST, Daejeon 305-701, South Korea
| | - Duyoung Min
- National Creative Research Initiative Center for Single-Molecule Systems Biology, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, South Korea. Department of Physics, KAIST, Daejeon 305-701, South Korea
| | - Sang-Hyun Rah
- National Creative Research Initiative Center for Single-Molecule Systems Biology, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, South Korea. Department of Physics, KAIST, Daejeon 305-701, South Korea
| | - Soo Jin Kim
- Graduate School of Medical Science and Engineering, KAIST, Daejeon 305-701, South Korea
| | - Yongsoo Park
- Department of Neurobiology, Max-Planck-Institute for Biophysical Chemistry, 37077 Göttingen, Germany
| | - Haesoo Kim
- Graduate School of Medical Science and Engineering, KAIST, Daejeon 305-701, South Korea
| | - Changbong Hyeon
- Korea Institute for Advanced Study, Seoul 130-722, South Korea
| | - Ho Min Kim
- Graduate School of Medical Science and Engineering, KAIST, Daejeon 305-701, South Korea
| | - Reinhard Jahn
- Department of Neurobiology, Max-Planck-Institute for Biophysical Chemistry, 37077 Göttingen, Germany.
| | - Tae-Young Yoon
- National Creative Research Initiative Center for Single-Molecule Systems Biology, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, South Korea. Department of Physics, KAIST, Daejeon 305-701, South Korea.
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139
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Molecular origins of synaptotagmin 1 activities on vesicle docking and fusion pore opening. Sci Rep 2015; 5:9267. [PMID: 25791821 PMCID: PMC4366854 DOI: 10.1038/srep09267] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/24/2014] [Accepted: 02/26/2015] [Indexed: 11/25/2022] Open
Abstract
Synaptotagmin 1 (Syt1), a major Ca2+ sensor in neuroexocytosis, utilizes SNARE- and membrane-binding to regulate vesicle fusion, a required process for neurotransmitter release at the synapse. However, the mechanism by which Syt1 orchestrates SNARE- and membrane- binding to control individual vesicle fusion steps is still unclear. In this study, we used a number of single vesicle assays that can differentiate intermediates of neuroexocytosis, to focus on Syt1 mutants that might impair Syt1-SNARE/PIP2 interaction, Ca2+-binding, or membrane penetration. Our results show that, although putative Syt1-SNARE/PIP2 coupling through the polybasic region of the C2B domain is critical for vesicle docking, its disruption does not affect content release. In contrast, Ca2+-binding and membrane-penetration mutants significantly reduce content release. Our results thus delineate multiple functions of Syt1 along the pathway of Ca2+-triggered exocytosis in unprecedented detail.
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140
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Brunger AT, Cipriano DJ, Diao J. Towards reconstitution of membrane fusion mediated by SNAREs and other synaptic proteins. Crit Rev Biochem Mol Biol 2015; 50:231-41. [PMID: 25788028 PMCID: PMC4673598 DOI: 10.3109/10409238.2015.1023252] [Citation(s) in RCA: 40] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/14/2023]
Abstract
Proteoliposomes have been widely used for in vitro studies of membrane fusion mediated by synaptic proteins. Initially, such studies were made with large unsynchronized ensembles of vesicles. Such ensemble assays limited the insights into the SNARE-mediated fusion mechanism that could be obtained from them. Single particle microscopy experiments can alleviate many of these limitations but they pose significant technical challenges. Here we summarize various approaches that have enabled studies of fusion mediated by SNAREs and other synaptic proteins at a single-particle level. Currently available methods are described and their advantages and limitations are discussed.
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141
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Pronk S, Lindahl E, Kasson PM. Coupled diffusion in lipid bilayers upon close approach. J Am Chem Soc 2015; 137:708-14. [PMID: 25535654 PMCID: PMC4308745 DOI: 10.1021/ja508803d] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/26/2014] [Indexed: 11/28/2022]
Abstract
Biomembrane interfaces create regions of slowed water dynamics in their vicinity. When two lipid bilayers come together, this effect is further accentuated, and the associated slowdown can affect the dynamics of larger-scale processes such as membrane fusion. We have used molecular dynamics simulations to examine how lipid and water dynamics are affected as two lipid bilayers approach each other. These two interacting fluid systems, lipid and water, both slow and become coupled when the lipid membranes are separated by a thin water layer. We show in particular that the water dynamics become glassy, and diffusion of lipids in the apposed leaflets becomes coupled across the water layer, while the "outer" leaflets remain unaffected. This dynamic coupling between bilayers appears mediated by lipid-water-lipid hydrogen bonding, as it occurs at bilayer separations where water-lipid hydrogen bonds become more common than water-water hydrogen bonds. We further show that such coupling occurs in simulations of vesicle-vesicle fusion prior to the fusion event itself. Such altered dynamics at membrane-membrane interfaces may both stabilize the interfacial contact and slow fusion stalk formation within the interface region.
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Affiliation(s)
- Sander Pronk
- 'Department
of Theoretical Physics, KTH Royal Institute
of Technology, AlbaNova, 106 91 Stockholm, Sweden
| | - Erik Lindahl
- 'Department
of Theoretical Physics, KTH Royal Institute
of Technology, AlbaNova, 106 91 Stockholm, Sweden
- Science
for Life Laboratory, 171
21 Stockholm, Sweden
| | - Peter M. Kasson
- Department
of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, Virginia 22908, United States
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142
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Preobraschenski J, Zander JF, Suzuki T, Ahnert-Hilger G, Jahn R. Vesicular Glutamate Transporters Use Flexible Anion and Cation Binding Sites for Efficient Accumulation of Neurotransmitter. Neuron 2014; 84:1287-301. [DOI: 10.1016/j.neuron.2014.11.008] [Citation(s) in RCA: 69] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 11/04/2014] [Indexed: 11/16/2022]
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143
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Ramalingam L, Yoder SM, Oh E, Thurmond DC. Munc18c: a controversial regulator of peripheral insulin action. Trends Endocrinol Metab 2014; 25:601-8. [PMID: 25028245 PMCID: PMC4253632 DOI: 10.1016/j.tem.2014.06.010] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/17/2014] [Revised: 06/12/2014] [Accepted: 06/20/2014] [Indexed: 12/19/2022]
Abstract
Insulin resistance, a hallmark of impaired glucose tolerance and type 2 diabetes (T2D), arises from dysfunction of insulin action and subsequent glucose uptake by peripheral tissues, predominantly skeletal muscle and fat. Exocytosis of glucose transporter (GLUT4)-containing vesicles facilitated by soluble NSF (N-ethylmaleimide-sensitive factor) attachment receptor (SNARE) protein isoforms, and Munc18c (mammalian homolog of Unc-18c) mediates this glucose uptake. Emerging evidences, including recent human clinical studies, point to pivotal roles for Munc18c in peripheral insulin action in adipose and skeletal muscle. Intriguing new advances are also initiating debates regarding the molecular mechanism(s) controlling Munc18c action. The objective of this review is therefore to present a balanced perspective of new continuities and controversies surrounding the regulation and requirement for Munc18c in the regulation of peripheral insulin action.
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Affiliation(s)
- Latha Ramalingam
- Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis, IN 46202, USA
| | - Stephanie M Yoder
- Department of Pediatrics, Indiana University School of Medicine, Indianapolis, IN 46202, USA
| | - Eunjin Oh
- Department of Pediatrics, Indiana University School of Medicine, Indianapolis, IN 46202, USA
| | - Debbie C Thurmond
- Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis, IN 46202, USA; Department of Pediatrics, Indiana University School of Medicine, Indianapolis, IN 46202, USA; Department of Cellular and Integrative Physiology, Indiana University School of Medicine, Indianapolis, IN 46202, USA.
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144
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Murray RZ, Stow JL. Cytokine Secretion in Macrophages: SNAREs, Rabs, and Membrane Trafficking. Front Immunol 2014; 5:538. [PMID: 25386181 PMCID: PMC4209870 DOI: 10.3389/fimmu.2014.00538] [Citation(s) in RCA: 122] [Impact Index Per Article: 12.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/08/2014] [Accepted: 10/10/2014] [Indexed: 12/21/2022] Open
Abstract
Macrophages have the capacity to rapidly secrete a wide range of inflammatory mediators that influence the development and extent of an inflammatory response. Newly synthesized and/or preformed stored cytokines and other inflammatory mediators are released upon stimulation, the timing, and volume of which is highly regulated. To finely tune this process, secretion is regulated at many levels; at the level of transcription and translation and post-translationally at the endoplasmic reticulum (ER), Golgi, and at or near the cell surface. Here, we discuss recent advances in deciphering these cytokine pathways in macrophages, focusing on recent discoveries regarding the cellular machinery and mechanisms implicated in the synthesis, trafficking, and secretion of cytokines. The specific roles of trafficking machinery including chaperones, GTPases, cytoskeletal proteins, and SNARE membrane fusion proteins will be discussed.
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Affiliation(s)
- Rachael Zoe Murray
- Institute of Health and Biomedical Innovation, Queensland University of Technology , Brisbane, QLD , Australia
| | - Jennifer Lea Stow
- Institute for Molecular Bioscience, The University of Queensland , Brisbane, QLD , Australia
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145
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Affiliation(s)
- Benjamin Podbilewicz
- Department of Biology, Technion–Israel Institute of Technology, Haifa 32000, Israel;
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146
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Ultrathin core–sheath fibers for liposome stabilization. Colloids Surf B Biointerfaces 2014; 122:630-637. [DOI: 10.1016/j.colsurfb.2014.07.042] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/15/2014] [Revised: 07/21/2014] [Accepted: 07/28/2014] [Indexed: 11/20/2022]
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147
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Abstract
The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex drives the majority of intracellular and exocytic membrane fusion events. Whether and how SNAREs cooperate to mediate fusion has been a subject of intense study, with estimates ranging from a single SNARE complex to 15. Here we show that there is no universally conserved number of SNARE complexes involved as revealed by our observation that this varies greatly depending on membrane curvature. When docking rates of small (∼40 nm) and large (∼100 nm) liposomes reconstituted with different synaptobrevin (the SNARE present in synaptic vesicles) densities are taken into account, the lipid mixing efficiency was maximal with small liposomes with only one synaptobrevin, whereas 23-30 synaptobrevins were necessary for efficient lipid mixing in large liposomes. Our results can be rationalized in terms of strong and weak cooperative coupling of SNARE complex assembly where each mode implicates different intermediate states of fusion that have been recently identified by electron microscopy. We predict that even higher variability in cooperativity is present in different physiological scenarios of fusion, and we further hypothesize that plasticity of SNAREs to engage in different coupling modes is an important feature of the biologically ubiquitous SNARE-mediated fusion reactions.
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148
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Mehrshahi P, Johnny C, DellaPenna D. Redefining the metabolic continuity of chloroplasts and ER. TRENDS IN PLANT SCIENCE 2014; 19:501-7. [PMID: 24679997 DOI: 10.1016/j.tplants.2014.02.013] [Citation(s) in RCA: 52] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/20/2013] [Revised: 02/20/2014] [Accepted: 02/21/2014] [Indexed: 05/06/2023]
Abstract
As a hub for plant metabolism, plastids extensively exchange metabolites with the extraplastid environment. For polar metabolites, membrane transporters mediate this exchange, but for many plastid-synthesized nonpolar compounds, such transporters remain elusive. Here, we discuss recent data from transorganellar complementation studies that demonstrate that enzymes in one organelle can directly access nonpolar metabolites from a companion organelle. We propose that a mechanism, based on hemifused-membranes at plastid-endoplasmic reticulum (ER) contact sites, facilitates interorganellar interactions and allows enzymes direct, transporter-independent access to a range of nonpolar compounds in both organelle membranes. In a wider context, interorganellar metabolism at hemifusion interfaces would allow evolution of membrane-spanning pathways for the many thousands of nonpolar metabolites in the plant kingdom to be uncoupled from coevolution with nonpolar metabolite transporters.
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Affiliation(s)
- Payam Mehrshahi
- Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824, USA
| | - Cassandra Johnny
- Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824, USA
| | - Dean DellaPenna
- Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824, USA.
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149
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Expansion of the fusion stalk and its implication for biological membrane fusion. Proc Natl Acad Sci U S A 2014; 111:11043-8. [PMID: 25024174 DOI: 10.1073/pnas.1323221111] [Citation(s) in RCA: 77] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Over the past 20 years, it has been widely accepted that membrane fusion proceeds via a hemifusion step before opening of the productive fusion pore. An initial hourglass-shaped lipid structure, the fusion stalk, is formed between the adjacent membrane leaflets (cis leaflets). It remains controversial if and how fusion proteins drive the subsequent transition (expansion) of the stalk into a fusion pore. Here, we propose a comprehensive and consistent thermodynamic understanding in terms of the underlying free-energy landscape of stalk expansion. We illustrate how the underlying free energy landscape of stalk expansion and the concomitant pathway is altered by subtle differences in membrane environment, such as leaflet composition, asymmetry, and flexibility. Nonleaky stalk expansion (stalk widening) requires the formation of a critical trans-leaflet contact. The fusion machinery can mechanically enforce trans-leaflet contact formation either by directly enforcing the trans-leaflets in close proximity, or by (electrostatically) condensing the area of the cis leaflets. The rate of these fast fusion reactions may not be primarily limited by the energetics but by the forces that the fusion proteins are able to exert.
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150
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Hurlock AK, Roston RL, Wang K, Benning C. Lipid trafficking in plant cells. Traffic 2014; 15:915-32. [PMID: 24931800 DOI: 10.1111/tra.12187] [Citation(s) in RCA: 76] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2014] [Revised: 06/12/2014] [Accepted: 06/12/2014] [Indexed: 12/29/2022]
Abstract
Plant cells contain unique organelles such as chloroplasts with an extensive photosynthetic membrane. In addition, specialized epidermal cells produce an extracellular cuticle composed primarily of lipids, and storage cells accumulate large amounts of storage lipids. As lipid assembly is associated only with discrete membranes or organelles, there is a need for extensive lipid trafficking within plant cells, more so in specialized cells and sometimes also in response to changing environmental conditions such as phosphate deprivation. Because of the complexity of plant lipid metabolism and the inherent recalcitrance of membrane lipid transporters, the mechanisms of lipid transport within plant cells are not yet fully understood. Recently, several new proteins have been implicated in different aspects of plant lipid trafficking. While these proteins provide only first insights into limited aspects of lipid transport phenomena in plant cells, they represent exciting opportunities for further studies.
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Affiliation(s)
- Anna K Hurlock
- Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI, 48824, USA; Department of Energy-Plant Research Laboratory, Michigan State University, East Lansing, MI, 48824, USA
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