Mechanics of membrane fusion

Structure and working of viral fusion machinery

This chapter discusses the structure and working of viral fusion machinery. The entry of enveloped viruses into cells requires the fusion of viral and cellular membranes, driven by conformational changes in viral glycoproteins. Structural studies have defined three classes of viral membrane fusion proteins. Despite their different structural organizations, all seem to have a common mechanism of action that generates the same lipid organizations during the fusion pathway. The entry of enveloped viruses into host cells requires binding of the virus to one or more receptors present at the cell surface, followed by fusion of the viral envelope with a cellular membrane. These steps are mediated by virally encoded glycoproteins that promote both receptor recognition and membrane fusion. The first crystal structure of a viral fusion protein ectodomain that has been determined is that of influenza virus hemagglutinin (HA) in its prefusion conformation. The structures of viral fusion glycoproteins, of which the conformational change is triggered at low pH, has allowed the identification of amino acid residues that play the role of pH-sensitive molecular switches.

Cell entry of enveloped viruses

Enveloped viruses penetrate their cell targets following the merging of their membrane with that of the cell. This fusion process is catalyzed by one or several viral glycoproteins incorporated on the membrane of the virus. These envelope glycoproteins (EnvGP) evolved in order to combine two features. First, they acquired a domain to bind to a specific cellular protein, named "receptor." Second, they developed, with the help of cellular proteins, a function of finely controlled fusion to optimize the replication and preserve the integrity of the cell, specific to the genus of the virus. Following the activation of the EnvGP either by binding to their receptors and/or sometimes the acid pH of the endosomes, many changes of conformation permit ultimately the action of a specific hydrophobic domain, the fusion peptide, which destabilizes the cell membrane and leads to the opening of the lipidic membrane. The comprehension of these mechanisms is essential to develop medicines of the therapeutic class of entry inhibitor like enfuvirtide (Fuzeon) against human immunodeficiency virus (HIV). In this chapter, we will summarize the different envelope glycoprotein structures that viruses develop to achieve membrane fusion and the entry of the virus. We will describe the different entry pathways and cellular proteins that viruses have subverted to allow infection of the cell and the receptors that are used. Finally, we will illustrate more precisely the recent discoveries that have been made within the field of the entry process, with a focus on the use of pseudoparticles. These pseudoparticles are suitable for high-throughput screenings that help in the development of natural or artificial inhibitors as new therapeutics of the class of entry inhibitors.

Single SNARE-mediated vesicle fusion observed in vitro by polarized TIRFM

Single-vesicle fusion assays in vitro are useful tools for examining mechanisms of membrane fusion at the molecular level mediated by soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). This approach allows the experimentalist to define the lipid and protein composition of the two fusing membranes and perform experiments under highly controlled conditions. In previous experiments, in which we reconstituted a SNARE acceptor complex into supported membranes and observed the docking and fusion of fluorescently labeled synaptobrevin proteoliposomes by total internal reflection fluorescence microscopy with millisecond time resolution, we were able to determine the optimal number of SNARE complexes needed for fast fusion. Here, we utilize this assay in combination with polarized total internal reflection fluorescence microscopy to investigate topology changes that vesicles undergo after the onset of fusion. The theory that describes the fluorescence intensity during the transformation of a single vesicle from a spherical particle to a flat membrane patch is developed and confirmed by experiments with three different fluorescent probes. Our results show that on average, the fusing vesicles flatten and merge into the planar membrane within 8 ms after fusion starts.

All-or-none versus graded: single-vesicle analysis reveals lipid composition effects on membrane permeabilization

We report a single-vesicle approach to compare the all-or-none and graded mechanisms of lipid bilayer permeabilization by CpreTM and NpreTM, two peptides derived from the membrane-proximal external region of the HIV fusion glycoprotein gp41 subunit. According to bulk requenching assays, these peptides permeabilize large unilamellar vesicles via all-or-none and graded mechanisms, respectively. Visualization of the process using giant unilamellar vesicles shows that the permeabilization of individual liposomes by these two peptides differs in kinetics, degree of dye filling, and stability of the permeabilized state. All-or-none permeabilization by CpreTM is characterized by fast and total filling of the individual vesicles. This process is usually accompanied by the formation of stably open pores, as judged from the capacity of the vesicles to incorporate a second dye added after several hours. In contrast, graded permeabilization by NpreTM is transient and exhibits slower kinetics, which leads to partial filling of the individual liposomes. Of importance, quantitative analysis of vesicle population distribution allowed the identification of mixed mechanisms of membrane permeabilization and the assessment of cholesterol effects. Specifically, the presence of this viral envelope lipid increased the stability of the permeating structures, which may have implications for the fusogenic activity of gp41.

Dynamin self-assembly and the vesicle scission mechanism: how dynamin oligomers cleave the membrane neck of clathrin-coated pits during endocytosis

Recently, Gao et al. and Chappie et al. elucidated the crystal structures of the polytetrameric stalk domain of the dynamin-like virus resistance protein, MxA, and of the G-domain dimer of the large, membrane-deforming GTPase, dynamin, respectively. Combined, they provide a hypothetical oligomeric structure for the complete dynamin protein. Here, it is discussed how the oligomers are expected to form and how they participate in dynamin mediated vesicle fission during the process of endocytosis. The proposed oligomeric structure is compared with the novel mechanochemical model of dynamin function recently proposed by Bashkirov et al. and Pucadyil and Schmid. In conclusion, the new model of the dynamin oligomer has the potential to explain how short self-limiting fissogenic dynamin assemblies are formed and how concerted GTP hydrolysis is achieved. The oligomerisation of two other dynamin superfamily proteins, the guanylate binding proteins (GBPs) and the immunity-related GTPases (IRGs), is addressed briefly.

Membrane remodeling induced by the dynamin-related protein Drp1 stimulates Bax oligomerization

In response to many apoptotic stimuli, oligomerization of Bax is essential for mitochondrial outer membrane permeabilization and the ensuing release of cytochrome c. These events are accompanied by mitochondrial fission that appears to require Drp1, a large GTPase of the dynamin superfamily. Loss of Drp1 leads to decreased cytochrome c release by a mechanism that is poorly understood. Here we show that Drp1 stimulates tBid-induced Bax oligomerization and cytochrome c release by promoting tethering and hemifusion of membranes in vitro. This function of Drp1 is independent of its GTPase activity and relies on arginine 247 and the presence of cardiolipin in membranes. In cells, overexpression of Drp1 R247A/E delays Bax oligomerization and cell death. Our findings uncover a function of Drp1 and provide insight into the mechanism of Bax oligomerization.

Direct simulation of protein-mediated vesicle fusion: lung surfactant protein B

We simulated spontaneous fusion of small unilamellar vesicles mediated by lung surfactant protein B (SP-B) using the MARTINI force field. An SP-B monomer triggers fusion events by anchoring two vesicles and facilitating the formation of a lipid bridge between the proximal leaflets. Once a lipid bridge is formed, fusion proceeds via a previously described stalk - hemifusion diaphragm - pore-opening pathway. In the absence of protein, fusion of vesicles was not observed in either unbiased simulations or upon application of a restraining potential to maintain the vesicles in close proximity. The shape of SP-B appears to enable it to bind to two vesicles at once, forcing their proximity, and to facilitate the initial transfer of lipids to form a high-energy hemifusion intermediate. Our results may provide insight into more general mechanisms of protein-mediated membrane fusion, and a possible role of SP-B in the secretory pathway and transfer of lung surfactant to the gas exchange interface.

Arg206 of SNAP-25 is essential for neuroexocytosis at the Drosophila melanogaster neuromuscular junction

An analysis of SNAP-25 isoform sequences indicates that there is a highly conserved arginine residue (198 in vertebrates, 206 in the genus Drosophila) within the C-terminal region, which is cleaved by botulinum neurotoxin A, with consequent blockade of neuroexocytosis. The possibility that it may play an important role in the function of the neuroexocytosis machinery was tested at neuromuscular junctions of Drosophila melanogaster larvae expressing SNAP-25 in which Arg206 had been replaced by alanine. Electrophysiological recordings of spontaneous and evoked neurotransmitter release under different conditions as well as testing for the assembly of the SNARE complex indicate that this residue, which is at the P1′ position of the botulinum neurotoxin A cleavage site, plays an essential role in neuroexocytosis. Computer graphic modelling suggests that this arginine residue mediates protein–protein contacts within a rosette of SNARE complexes that assembles to mediate the fusion of synaptic vesicles with the presynaptic plasma membrane.

Membrane bending energy and fusion pore kinetics in Ca(2+)-triggered exocytosis

A fusion pore composed of lipid is an obligatory kinetic intermediate of membrane fusion, and its formation requires energy to bend membranes into highly curved shapes. The energetics of such deformations in viral fusion is well established, but the role of membrane bending in Ca(2+)-triggered exocytosis remains largely untested. Amperometry recording showed that during exocytosis in chromaffin and PC12 cells, fusion pores formed by smaller vesicles dilated more rapidly than fusion pores formed by larger vesicles. The logarithm of 1/(fusion pore lifetime) varied linearly with vesicle curvature. The vesicle size dependence of fusion pore lifetime quantitatively accounted for the nonexponential fusion pore lifetime distribution. Experimentally manipulating vesicle size failed to alter the size dependence of fusion pore lifetime. Manipulations of membrane spontaneous curvature altered this dependence, and applying the curvature perturbants to the opposite side of the membrane reversed their effects. These effects of curvature perturbants were opposite to those seen in viral fusion. These results indicate that during Ca(2+)-triggered exocytosis membrane bending opposes fusion pore dilation rather than fusion pore formation. Ca(2+)-triggered exocytosis begins with a proteinaceous fusion pore with less stressed membrane, and becomes lipidic as it dilates, bending membrane into a highly curved shape.

Evolution of programmed cell fusion: common mechanisms and distinct functions

Eukaryotic cells have evolved diverged mechanisms to merge cells. Here, we discuss three types of cell fusion: (1) Non-self-fusion, cells with different genetic contents fuse to start a new organism and fusion between enveloped viruses and host cells; (2) Self-fusion, genetically identical cells fuse to form a multinucleated cell; and (3) Auto-fusion, a single cell fuses with itself by bringing specialized cell membrane domains into contact and transforming itself into a ring-shaped cell. This is a new type of selfish fusion discovered in C. elegans. We divide cell fusion into three stages: (1) Specification of the cell-fusion fate; (2) Cell attraction, attachment, and recognition; (3) Execution of plasma membrane fusion, cytoplasmic mixing and cytoskeletal rearrangements. We analyze cell fusion in diverse biological systems in development and disease emphasizing the mechanistic contributions of C. elegans to the understanding of programmed cell fusion, a genetically encoded pathway to merge specific cells.

Three-dimensional, flexible nanoscale field-effect transistors as localized bioprobes

Nanoelectronic devices offer substantial potential for interrogating biological systems, although nearly all work has focused on planar device designs. We have overcome this limitation through synthetic integration of a nanoscale field-effect transistor (nanoFET) device at the tip of an acute-angle kinked silicon nanowire, where nanoscale connections are made by the arms of the kinked nanostructure, and remote multilayer interconnects allow three-dimensional (3D) probe presentation. The acute-angle probe geometry was designed and synthesized by controlling cis versus trans crystal conformations between adjacent kinks, and the nanoFET was localized through modulation doping. 3D nanoFET probes exhibited conductance and sensitivity in aqueous solution, independent of large mechanical deflections, and demonstrated high pH sensitivity. Additionally, 3D nanoprobes modified with phospholipid bilayers can enter single cells to allow robust recording of intracellular potentials.

Spatial regulation of membrane fusion controlled by modification of phosphoinositides

Membrane fusion plays a central role in many cell processes from vesicular transport to nuclear envelope reconstitution at mitosis but the mechanisms that underlie fusion of natural membranes are not well understood. Studies with synthetic membranes and theoretical considerations indicate that accumulation of lipids characterised by negative curvature such as diacylglycerol (DAG) facilitate fusion. However, the specific role of lipids in membrane fusion of natural membranes is not well established. Nuclear envelope (NE) assembly was used as a model for membrane fusion. A natural membrane population highly enriched in the enzyme and substrate needed to produce DAG has been isolated and is required for fusions leading to nuclear envelope formation, although it contributes only a small amount of the membrane eventually incorporated into the NE. It was postulated to initiate and regulate membrane fusion. Here we use a multidisciplinary approach including subcellular membrane purification, fluorescence spectroscopy and Förster resonance energy transfer (FRET)/two-photon fluorescence lifetime imaging microscopy (FLIM) to demonstrate that initiation of vesicle fusion arises from two unique sites where these vesicles bind to chromatin. Fusion is subsequently propagated to the endoplasmic reticulum-derived membranes that make up the bulk of the NE to ultimately enclose the chromatin. We show how initiation of multiple vesicle fusions can be controlled by localised production of DAG and propagated bidirectionally. Phospholipase C (PLCγ), GTP hydrolysis and (phosphatidylinsositol-(4,5)-bisphosphate (PtdIns(4,5)P₂) are required for the latter process. We discuss the general implications of membrane fusion regulation and spatial control utilising such a mechanism.

Virus-cell and cell-cell fusion mediated by the HIV-1 envelope glycoprotein is inhibited by short gp41 N-terminal membrane-anchored peptides lacking the critical pocket domain

The interactions between the N- and C-terminal heptad repeat (NHR and CHR) regions of the human immunodeficiency virus (HIV-1) glycoprotein gp41 create a structure comprising a 6-helix bundle (SHB). A sequence in the SHB named the "pocket" is crucial for the SHB's stability and for the fusion inhibitory activity of 36-residue NHR peptide N36. We report that a short 27-residue peptide, N27, which lacks the pocket sequence, exhibits potent inhibitory activity in both cell-cell and virus-cell fusion assays when fatty acids were conjugated to its N but not C terminus. Furthermore, mutations in the positions that prevent interaction with the CHR but not with the NHR resulted in a dramatic reduction in N27 activity. These data support a mechanism in which N27 mainly targets the CHR rather than the internal NHR coiled-coil, reveal the N-terminal edge of the endogenous core structure in situ and hence complement our recent findings of the C-terminal edge of the core, and provide a new approach for designing short inhibitors from the NHR region of other lentiviruses due to similarities in their envelope proteins.

SNAREs: could they be the answer to an energy landscape riddle in exocytosis?

During exocytosis, chemical transmitters stored in secretory vesicles can be released upon fusion of these intracellular organelles to the plasma membrane. In this process, SNARE proteins that form a ternary core complex play a central role. This complex could provide the means for generation/storage of energy necessary for driving the fusion of vesicular and plasma membranes. Recently, the amount of energy for (dis)assembly of the ternary complex has been measured using various experimental approaches, including atomic force microscopy, the surface force apparatus, and isothermal titration calorimetry. The obtained measurements are in good agreement with the calculated energy required for membrane fusion achieved by theoretical modeling approaches. Whether the energy expenditure to form the ternary SNARE complex can be utilized towards membrane fusion and/or docking/tethering of vesicles to the plasma membrane still remains one of the key contemporary issues in biophysics and neuroscience.

Atomic-resolution simulations predict a transition state for vesicle fusion defined by contact of a few lipid tails

Membrane fusion is essential to both cellular vesicle trafficking and infection by enveloped viruses. While the fusion protein assemblies that catalyze fusion are readily identifiable, the specific activities of the proteins involved and nature of the membrane changes they induce remain unknown. Here, we use many atomic-resolution simulations of vesicle fusion to examine the molecular mechanisms for fusion in detail. We employ committor analysis for these million-atom vesicle fusion simulations to identify a transition state for fusion stalk formation. In our simulations, this transition state occurs when the bulk properties of each lipid bilayer remain in a lamellar state but a few hydrophobic tails bulge into the hydrophilic interface layer and make contact to nucleate a stalk. Additional simulations of influenza fusion peptides in lipid bilayers show that the peptides promote similar local protrusion of lipid tails. Comparing these two sets of simulations, we obtain a common set of structural changes between the transition state for stalk formation and the local environment of peptides known to catalyze fusion. Our results thus suggest that the specific molecular properties of individual lipids are highly important to vesicle fusion and yield an explicit structural model that could help explain the mechanism of catalysis by fusion proteins.

Membrane fusion is a common underlying process critical to neurotransmitter release, cellular trafficking, and infection by many viruses. Proteins have been identified that catalyze fusion, and mutations to these proteins have yielded important information on how fusion occurs. However, the precise mechanism by which membrane fusion begins is the subject of active investigation. We have used atomic-resolution simulations to model the process of vesicle fusion and to identify a transition state for the formation of an initial fusion stalk. Doing so required substantial technical advances in combining high-performance simulation and distributed computing to analyze the transition state of a complex reaction in a large system. The transition state we identify in our simulations involves specific structural changes by a few lipid molecules. We also simulate fusion peptides from influenza hemagglutinin and show that they promote the same structural changes as are required for fusion in our model. We therefore hypothesize that these changes to individual lipid molecules may explain a portion of the catalytic activity of fusion proteins such as influenza hemagglutinin.

Transmembrane-domain determinants for SNARE-mediated membrane fusion

Neurosecretion involves fusion of vesicles with the plasma membrane. Such membrane fusion is mediated by the SNARE complex, which is composed of the vesicle-associated protein synaptobrevin (VAMP2), and the plasma membrane proteins syntaxin-1A and SNAP-25. Although clearly important at the point of membrane fusion, the precise structural and functional requirements for the transmembrane domains (TMDs) of SNAREs in bringing about neurosecretion remain largely unknown. Here, we used a bimolecular fluorescence complementation (BiFC) approach to study SNARE protein interactions involving TMDs in vivo. VAMP2 molecules were found to dimerise through their TMDs in intact cells. Dimerisation was abolished when replacing a glycine residue in the centre of the TMD with residues of increasing molecular volume. However, such mutations still were fully competent in bringing about membrane-fusion events, suggesting that dimerisation of the VAMP2 TMDs does not have an important functional role. By contrast, a series of deletion or insertion mutants in the C-terminal half of the TMD were largely deficient in supporting neurosecretion, whereas mutations in the N-terminal half did not display severe secretory deficits. Thus, structural length requirements, largely confined to the C-terminal half of the VAMP2 TMD, seem to be essential for SNARE-mediated membrane-fusion events in cells.

Structural optimization and de novo design of dengue virus entry inhibitory peptides

Viral fusogenic envelope proteins are important targets for the development of inhibitors of viral entry. We report an approach for the computational design of peptide inhibitors of the dengue 2 virus (DENV-2) envelope (E) protein using high-resolution structural data from a pre-entry dimeric form of the protein. By using predictive strategies together with computational optimization of binding "pseudoenergies", we were able to design multiple peptide sequences that showed low micromolar viral entry inhibitory activity. The two most active peptides, DN57opt and 1OAN1, were designed to displace regions in the domain II hinge, and the first domain I/domain II beta sheet connection, respectively, and show fifty percent inhibitory concentrations of 8 and 7 microM respectively in a focus forming unit assay. The antiviral peptides were shown to interfere with virus:cell binding, interact directly with the E proteins and also cause changes to the viral surface using biolayer interferometry and cryo-electron microscopy, respectively. These peptides may be useful for characterization of intermediate states in the membrane fusion process, investigation of DENV receptor molecules, and as lead compounds for drug discovery.

How pig sperm prepares to fertilize: stable acrosome docking to the plasma membrane

Background

Mammalian sperms are activated in the oviduct. This process, which involves extensive sperm surface remodelling, is required for fertilization and can be mimicked under in vitro fertilization conditions (IVF).

Methodology/Principal Findings

Here we demonstrate that such treatments caused stable docking and priming of the acrosome membrane to the apical sperm head surface without the emergence of exocytotic membrane fusion. The interacting membranes could be isolated as bilamellar membrane structures after cell disruption. These membrane structures as well as whole capacitated sperm contained stable ternary trans-SNARE complexes that were composed of VAMP 3 and syntaxin 1B from the plasma membrane and SNAP 23 from the acrosomal membrane. This trans-SNARE complex was not observed in control sperm.

Conclusions/Significance

We propose that this capacitation driven membrane docking and stability thereof is a preparative step prior to the multipoint membrane fusions characteristic for the acrosome reaction induced by sperm-zona binding. Thus, sperm can be considered a valuable model for studying exocytosis.

Fourth-order curvature energy model for the stability of bicontinuous inverted cubic phases in amphiphile-water systems

The bicontinuous inverted cubic (Q(II)) phases of amphiphiles in water have many practical applications. It is necessary to understand the stability of these phases as a function of composition and ambient conditions in order to make the best use of them. Moreover, many biomembrane lipids and some biomembrane lipid extracts form Q(II) phases. The stability of Q(II) phases in a given lipid composition is closely related to the susceptibility of that composition to membrane fusion: changes in composition that stabilize Q(II) phases usually increase the rate of membrane fusion. However, the factors determining Q(II) phase stability are not fully understood. Previously, an expression was derived for the curvature free energy of Q(II) phases with respect to that of the lamellar (L(alpha)) phase using a model for the curvature energy with terms up to fourth order in curvature as formulated by Mitov. Here this model is extended to account for the effects of water content on Q(II) phase stability. It is shown that the observed L(alpha)/Q(II) phase-transition temperature, transition enthalpy, and transition kinetics are all sensitive to water content. The same observables also become sensitive to small noncurvature energy contributions to the total free-energy difference between the Q(II) and L(alpha) phases, especially the unbinding energy in the L(alpha) phase. These predictions rationalize earlier observations of Q(II) phase formation in N-monomethylated dioleoylphosphatidylethanolamine that otherwise appear to be inconsistent. The model also provides a fundamental explanation of the hysteresis typically observed in transitions between the L(alpha) and Q(II) phases. It is an accurate model of Q(II) phase stability when the ratio of the volume fraction of the lipid in the Q(II) phase unit cell is < or = 0.5.

Lysophospholipids modulate voltage-gated calcium channel currents in pituitary cells; effects of lipid stress

Voltage-gated calcium channels (VGCCs) are osmosensitive. The hypothesis that this property of VGCCs stems from their susceptibility to alterations in the mechanical properties of the bilayer was tested on VGCCs in pituitary cells using cone-shaped lysophospholipids (LPLs) to perturb bilayer lipid stress. LPLs of different head group size and charge were used: lysophosphatidylcholine (LPC), lysophosphatidylinositol (LPI), lysophosphatidylserine (LPS) and lysophosphatidylethanolamine (LPE). Phosphatidylcholine (PC) and LPC (C6:0) were used as controls. We show that partition of both LPC and LPI into the membrane of pituitary cells suppressed L-type calcium channel currents (I(L)). This suppression of I(L) was slow in onset, reversible upon washout with BSA and associated with a depolarizing shift in activation ( approximately 8mV). In contrast to these effects of LPC and LPI on I(L), LPS, LPE, PC and LPC (C6:0) exerted minimal or insignificant effects. This difference may be attributed to the prominent conical shape of LPC and LPI compared to the shapes of LPS and LPE (which have smaller headgroups), and to PC (which is cylindrical). The similar effects of LPC and LPI on I(L), despite differences in the structure and charge of their headgroups suggest a common lipid stress dependent mechanism in their action on VGCCs.

Multiple modes of endophilin-mediated conversion of lipid vesicles into coated tubes: implications for synaptic endocytosis

Endophilin A1 is a BAR (Bin/amphiphysin/Rvs) protein abundant in neural synapses that senses and induces membrane curvature, contributing to neck formation in presynaptic endocytic vesicles. To investigate its role in membrane remodeling, we used cryoelectron microscopy to characterize structural changes induced in lipid vesicles by exposure to endophilin. The vesicles convert rapidly to coated tubules whose morphology reflects the local concentration of endophilin. Their diameters and curvature resemble those of synaptic vesicles in situ. Three-dimensional reconstructions of quasicylindrical tubes revealed arrays of BAR dimers, flanked by densities that we equate with amphipathic helices whose folding and membrane insertion were attested by EPR. We also observed the compression of bulbous coated tubes into 70-A-wide cylindrical micelles, which appear to mimic the penultimate (hemi-fission) stage of endocytosis. Our findings suggest that the adaptability of endophilin-lipid interactions underlies dynamic changes of endocytic membranes.

Secretion is required for late events in the cell-fusion pathway of mating yeast

Secretory vesicles accumulate adjacent to the contact site between the two cells of a yeast mating pair before they fuse, but there is no direct evidence that secretion is required to complete fusion. In this study, temperature-sensitive secretion (sec(ts)) mutants were used to investigate the role of secretion in yeast cell fusion. Cell fusion arrested less than 5 minutes after inhibiting secretion. This rapid fusion arrest was not an indirect consequence of reduced mating pheromone signaling, mating-pair assembly or actin polarity. Furthermore, secretion was required to complete cell fusion when it was transiently inhibited by addition and removal of the lipophilic styryl dye, FM4-64. These results indicate that ongoing secretion is required for late events in the cell-fusion pathway, which include plasma-membrane fusion and the completion of cell-wall remodeling, and they demonstrate a just-in-time delivery mechanism for the cell-fusion machinery.

Crystal structure of HIV-1 gp41 including both fusion peptide and membrane proximal external regions

The HIV-1 envelope glycoprotein (Env) composed of the receptor binding domain gp120 and the fusion protein subunit gp41 catalyzes virus entry and is a major target for therapeutic intervention and for neutralizing antibodies. Env interactions with cellular receptors trigger refolding of gp41, which induces close apposition of viral and cellular membranes leading to membrane fusion. The energy released during refolding is used to overcome the kinetic barrier and drives the fusion reaction. Here, we report the crystal structure at 2 A resolution of the complete extracellular domain of gp41 lacking the fusion peptide and the cystein-linked loop. Both the fusion peptide proximal region (FPPR) and the membrane proximal external region (MPER) form helical extensions from the gp41 six-helical bundle core structure. The lack of regular coiled-coil interactions within FPPR and MPER splay this end of the structure apart while positioning the fusion peptide towards the outside of the six-helical bundle and exposing conserved hydrophobic MPER residues. Unexpectedly, the section of the MPER, which is juxtaposed to the transmembrane region (TMR), bends in a 90 degrees-angle sideward positioning three aromatic side chains per monomer for membrane insertion. We calculate that this structural motif might facilitate the generation of membrane curvature on the viral membrane. The presence of FPPR and MPER increases the melting temperature of gp41 significantly in comparison to the core structure of gp41. Thus, our data indicate that the ordered assembly of FPPR and MPER beyond the core contributes energy to the membrane fusion reaction. Furthermore, we provide the first structural evidence that part of MPER will be membrane inserted within trimeric gp41. We propose that this framework has important implications for membrane bending on the viral membrane, which is required for fusion and could provide a platform for epitope and lipid bilayer recognition for broadly neutralizing gp41 antibodies.

Negative potentials across biological membranes promote fusion by class II and class III viral proteins

Fusion of virions pseudotyped with a class II, SFV E1 or VEEV E, or a class III protein, VSV G is promoted by negative potentials and hindered by positive potentials across the target cell. Hemifusion is independent of polarity. Reversion of hemifused membranes into two distinct ones is responsible for voltage-dependence and inhibition of fusion.

Voltage was investigated as a factor in the fusion of virions. Virions, pseudotyped with a class II, SFV E1 or VEEV E, or a class III protein, VSV G, were prepared with GFP within the core and a fluorescent lipid. This allowed both hemifusion and fusion to be monitored. Voltage clamping the target cell showed that fusion is promoted by a negative potential and hindered by a positive potential. Hemifusion occurred independent of polarity. Lipid dye movement, in the absence of content mixing, ceased before complete transfer for positive potentials, indicating that reversion of hemifused membranes into two distinct membranes is responsible for voltage dependence and inhibition of fusion. Content mixing quickly followed lipid dye transfer for a negative potential, providing a direct demonstration that hemifusion induced by class II and class III viral proteins is a functional intermediate of fusion. In the hemifused state, virions that fused exhibited slower lipid transfer than did nonfusing virions. All viruses with class II or III fusion proteins may utilize voltage to achieve infection.

Architecture of a nascent viral fusion pore

Enveloped viruses use specialized protein machinery to fuse the viral membrane with that of the host cell during cell invasion. In influenza virus, hundreds of copies of the haemagglutinin (HA) fusion glycoprotein project from the virus surface. Despite intensive study of HA and its fusion activity, the protein's modus operandi in manipulating viral and target membranes to catalyse their fusion is poorly understood. Here, the three-dimensional architecture of influenza virus-liposome complexes at pH 5.5 was investigated by electron cryo-tomography. Tomographic reconstructions show that early stages of membrane remodeling take place in a target membrane-centric manner, progressing from punctate dimples, to the formation of a pinched liposomal funnel that may impinge on the apparently unperturbed viral envelope. The results suggest that the M1 matrix layer serves as an endoskeleton for the virus and a foundation for HA during membrane fusion. Fluorescence spectroscopy monitoring fusion between liposomes and virions shows that leakage of liposome contents takes place more rapidly than lipid mixing at pH 5.5. The relation of 'leaky' fusion to the observed prefusion structures is discussed.

Features of a spatially constrained cystine loop in the p10 FAST protein ectodomain define a new class of viral fusion peptides

The reovirus fusion-associated small transmembrane (FAST) proteins are the smallest known viral membrane fusion proteins. With ectodomains of only approximately 20-40 residues, it is unclear how such diminutive fusion proteins can mediate cell-cell fusion and syncytium formation. Contained within the 40-residue ectodomain of the p10 FAST protein resides an 11-residue sequence of moderately apolar residues, termed the hydrophobic patch (HP). Previous studies indicate the p10 HP shares operational features with the fusion peptide motifs found within the enveloped virus membrane fusion proteins. Using biotinylation assays, we now report that two highly conserved cysteine residues flanking the p10 HP form an essential intramolecular disulfide bond to create a cystine loop. Mutagenic analyses revealed that both formation of the cystine loop and p10 membrane fusion activity are highly sensitive to changes in the size and spatial arrangement of amino acids within the loop. The p10 cystine loop may therefore function as a cystine noose, where fusion peptide activity is dependent on structural constraints within the noose that force solvent exposure of key hydrophobic residues. Moreover, inhibitors of cell surface thioreductase activity indicate that disruption of the disulfide bridge is important for p10-mediated membrane fusion. This is the first example of a viral fusion peptide composed of a small, spatially constrained cystine loop whose function is dependent on altered loop formation, and it suggests the p10 cystine loop represents a new class of viral fusion peptides.

Membrane curvature in synaptic vesicle fusion and beyond

Recent evidence suggests that the Ca(2+)-sensors synaptotagmin-1 and Doc2b deform synaptic membranes during synaptic vesicle exocytosis. We discuss how local curvature generated by these and other proteins may stimulate membrane fusion and discuss the potential implications of these findings for other cellular fusion events.

HAP2(GCS1)-dependent gamete fusion requires a positively charged carboxy-terminal domain

HAP2(GCS1) is a deeply conserved sperm protein that is essential for gamete fusion. Here we use complementation assays to define major functional regions of the Arabidopsis thaliana ortholog using HAP2(GCS1) variants with modifications to regions amino(N) and carboxy(C) to its single transmembrane domain. These quantitative in vivo complementation studies show that the N-terminal region tolerates exchange with a closely related sequence, but not with a more distantly related plant sequence. In contrast, a distantly related C-terminus is functional in Arabidopsis, indicating that the primary sequence of the C-terminus is not critical. However, mutations that neutralized the charge of the C-terminus impair HAP2(GCS1)-dependent gamete fusion. Our results provide data identifying the essential functional features of this highly conserved sperm fusion protein. They suggest that the N-terminus functions by interacting with female gamete-expressed proteins and that the positively charged C-terminus may function through electrostatic interactions with the sperm plasma membrane.

Recent studies suggest that HAP2(GCS1) is a deeply conserved protein required for gamete membrane fusion, a critical yet poorly understood step in sexual reproduction. HAP2(GCS1) is present in many plant, protist, and animal genomes, and has been shown to be essential for fertilization in Arabidopsis, Chlamydomonas, and Plasmodium. The loss-of-function phenotype in Chlamydomonas suggests a direct role in gamete plasma membrane fusion. HAP2(GCS1) has no known functional domains, making it difficult to predict how it contributes to gamete fusion. We set out to map the critical features of this protein by testing a series of deletions, substitutions, and interspecific chimeras for their ability to rescue the hap2-1 fertilization defect in Arabidopsis. We found that the N-terminus does not tolerate sequence divergence, but the histidine-rich C-terminus does. We propose that the N-terminus of HAP2(GCS1) functions in part by interacting with proteins on the surface of female gametes. The key feature of the C-terminus is positive charge, a characteristic that could favor interactions with the plasma membrane that promote membrane fusion. Our studies provide a description of HAP2(GCS1) functional domains and provide an important framework for defining the role of this essential component of a conserved reproductive mechanism.

Induction of non-lamellar lipid phases by antimicrobial peptides: a potential link to mode of action

Antimicrobial peptides are naturally produced by numerous organisms including insects, plants and mammals. Their non-specific mode of action is thought to involve the transient perturbation of bacterial membranes but the molecular mechanism underlying the rearrangement of the lipid molecules to explain the formation of pores and micelles is still poorly understood. Biological membranes mostly adopt planar lipid bilayers; however, antimicrobial peptides have been shown to induce non-lamellar lipid phases which may be intimately linked to their proposed mechanisms of action. This paper reviews antimicrobial peptides that alter lipid phase behavior in three ways: peptides that induce positive membrane curvature, peptides that induce negative membrane curvature and peptides that induce cubic lipid phases. Such structures can coexist with the bilayer structure, thus giving rise to lipid polymorphism induced upon addition of antimicrobial peptides. The discussion addresses the implications of induced lipid phases for the mode of action of various antimicrobial peptides.

Viruses as vesicular carriers of the viral genome: a functional module perspective

Enveloped viruses and cellular transport vesicles share obvious morphological and functional properties. Both are composed of a closed membrane, which is lined with coat proteins and encases cargo. Transmembrane proteins inserted into the membrane define the target membrane area with which the vesicle or virus is destined to fuse. Here we discuss recent insight into the functioning of enveloped viruses in the framework of the “functional module” concept. Vesicular transport is an exemplary case of a functional module, as defined as a part of the proteome that assembles to perform a specific autonomous function in a living cell. Cellular vesicles serve to transport cargo between membranous organelles inside the cell, while enveloped viruses can be seen as carriers of the viral genome delivering their cargo from an infected to an uninfected cell. The turnover of both vesicles and viruses involves an analogous series of submodular events. This comprises assembly of elements, budding from the donor compartment, uncoating and/or maturation, docking to and finally fusion with the target membrane to release the cargo. This modular perception enables us to define submodular building blocks so that mechanisms and elements can be directly compared. It will be analyzed where viruses have developed their own specific strategy, where they share functional schemes with vesicles, and also where they even have “hijacked” complete submodular schemes from the cell. Such a perspective may also include new and more specific approaches to pharmacological interference with virus function, which could avoid some of the most severe side effects.

Chronic palmitate exposure inhibits insulin secretion by dissociation of Ca(2+) channels from secretory granules

Long-term (72 hr) exposure of pancreatic islets to palmitate inhibited glucose-induced insulin secretion by >50% with first- and second-phase secretion being equally suppressed. This inhibition correlated with the selective impairment of exocytosis evoked by brief (action potential-like) depolarizations, whereas that evoked by long ( approximately 250 ms) stimuli was unaffected. Under normal conditions, Ca(2+) influx elicited by brief membrane depolarizations increases [Ca(2+)](i) to high levels within discrete microdomains and triggers the exocytosis of closely associated insulin granules. We found that these domains of localized Ca(2+) entry become dispersed by long-term (72 hr), but not by acute (2 hr), exposure to palmitate. Importantly, the release competence of the granules was not affected by palmitate. Thus, the location rather than the magnitude of the Ca(2+) increase determines its capacity to evoke exocytosis. In both mouse and human islets, the palmitate-induced secretion defect was reversed when the beta cell action potential was pharmacologically prolonged.