Fusion activity of influenza virus PR8/34 correlates with a temperature-induced conformational change within the hemagglutinin ectodomain detected by photochemical labeling

Planar aggregation of the influenza viral fusion peptide alters membrane structure and hydration, promoting poration

To infect, enveloped viruses employ spike protein, spearheaded by its amphipathic fusion peptide (FP), that upon activation extends out from the viral surface to embed into the target cellular membrane. Here we report that synthesized influenza virus FPs are membrane active, generating pores in giant unilamellar vesicles (GUV), and thus potentially explain both influenza virus' hemolytic activity and the liposome poration seen in cryo-electron tomography. Experimentally, FPs are heterogeneously distributed on the GUV at the time of poration. Consistent with this heterogeneous distribution, molecular dynamics (MD) simulations of asymmetric bilayers with different numbers of FPs in one leaflet show FP aggregation. At the center of FP aggregates, a profound change in the membrane structure results in thinning, higher water permeability, and curvature. Ultimately, a hybrid bilayer nanodomain forms with one lipidic leaflet and one peptidic leaflet. Membrane elastic theory predicts a reduced barrier to water pore formation when even a dimer of FPs thins the membrane as above, and the FPs of that dimer tilt, to continue the leaflet bending initiated by the hydrophobic mismatch between the FP dimer and the surrounding lipid.

Photolabeling Strategies to Study Membranotropic Peptides Interacting with Lipids and Proteins in Membranes

Membranotropic peptides is a class of peptides that exert their biological action at the level of cell membranes. Understanding how they interact with their different membrane binding partners (lipids, proteins, and/or glycoconjugates) is important to decipher their mechanism of action. Affinity photolabeling is a powerful method to study noncovalent interactions and provide a submolecular picture of the contacts between two interacting partners. In this review, we give a panorama of photolabeling-based studies of the interactions between membranotropic peptides and membranes using either photoreactive lipids or peptides.

Protein profiling and pseudo-parallel reaction monitoring to monitor a fusion-associated conformational change in hemagglutinin

Influenza infection requires viral escape from early endosomes into the cytosol, which is enabled by an acid-induced irreversible conformational transformation in the viral protein hemagglutinin. Despite the direct relationship between this conformational change and infectivity, label-free methods for characterizing this and other protein conformational changes in biological mixtures are limited. While the chemical reactivity of the protein backbone and side-chain residues is a proxy for protein conformation, coupling this reactivity to quantitative mass spectrometry is a challenge in complex environments. Herein, we evaluate whether electrophilic amidination coupled with pseudo-parallel reaction monitoring is an effective label-free approach to detect the fusion-associated conformational transformation in recombinant hemagglutinin (rHA). We identified rHA peptides that are differentially amidinated between the pre- and post-fusion states, and validated that this difference relies upon the fusion-associated conformational switch. We further demonstrate that we can distinguish the fusion profile in a matrix of digested cellular lysate. This fusion assay can be used to evaluate fusion competence for modified HA. Graphical abstract.

Contributions of Influenza Virus Hemagglutinin and Host Immune Responses Toward the Severity of Influenza Virus: Streptococcus pyogenes Superinfections

Influenza virus infections can be complicated by bacterial superinfections, which are medically relevant because of a complex interaction between the host, the virus, and the bacteria. Studies to date have implicated several influenza virus genes, varied host immune responses, and bacterial virulence factors, however, the host-pathogen interactions that predict survival versus lethal outcomes remain undefined. Previous work by our group showed that certain influenza viruses could yield a survival phenotype (A/swine/Texas/4199-2/98-H3N2, TX98), whereas others were associated with a lethal phenotype (A/Puerto Rico/8/34-H1N1, PR8). Based on this observation, we developed the hypothesis that individual influenza virus genes could contribute to a superinfection, and that the host response after influenza virus infection could influence superinfection severity. The present study analyzes individual influenza virus gene contributions to superinfection severity using reassortant viruses created using TX98 and PR8 viral genes. Host and pathogen interactions, relevant to survival and lethal phenotypes, were studied with a focus on pathogen clearance, host cellular infiltrates, and cytokine levels after infection. Specifically, we found that the hemagglutinin gene expressed by an influenza virus can contribute to the severity of a secondary bacterial infection, likely through modulation of host proinflammatory responses. Altogether, these results advance our understanding of molecular mechanisms underlying influenza virus-bacteria superinfections and identify viral and corresponding host factors that may contribute to morbidity and mortality.

Capture and imaging of a prehairpin fusion intermediate of the paramyxovirus PIV5

During cell entry, enveloped viruses fuse their viral membrane with a cellular membrane in a process driven by energetically favorable, large-scale conformational rearrangements of their fusion proteins. Structures of the pre- and postfusion states of the fusion proteins including paramyxovirus PIV5 F and influenza virus hemagglutinin suggest that this occurs via two intermediates. Following formation of an initial complex, the proteins structurally elongate, driving a hydrophobic N-terminal "fusion peptide" away from the protein surface into the target membrane. Paradoxically, this first conformation change moves the viral and cellular bilayers further apart. Next, the fusion proteins form a hairpin that drives the two membranes into close opposition. While the pre- and postfusion hairpin forms have been characterized crystallographically, the transiently extended prehairpin intermediate has not been visualized. To provide evidence for this extended intermediate we measured the interbilayer spacing of a paramyxovirus trapped in the process of fusing with solid-supported bilayers. A gold-labeled peptide that binds the prehairpin intermediate was used to stabilize and specifically image F-proteins in the prehairpin intermediate. The interbilayer spacing is precisely that predicted from a computational model of the prehairpin, providing strong evidence for its structure and functional role. Moreover, the F-proteins in the prehairpin conformation preferentially localize to a patch between the target and viral membranes, consistent with the fact that the formation of the prehairpin is triggered by local contacts between F- and neighboring viral receptor-binding proteins (HN) only when HN binds lipids in its target membrane.

Impact of pollution, climate, and sociodemographic factors on spatiotemporal dynamics of seasonal respiratory viruses

Seasonal viruses present a major cause of morbidity and mortality in temperate climates. Through major pandemics and smaller annual epidemics, viruses such as influenza, respiratory syncytial virus (RSV) and human rhinovirus (HRV) result in lost school and work days for most that are infected and more serious complications for the immunocompromised. The reasons for these viruses showing strict seasonality include but are not limited to the influence of cold weather and humidity on virus particles, human physiology, and human behavior. The relative importance of each is dependent on what geographic scale is being explored as well as the individual region and time period. Theoretical mathematics has also revealed that climatic changes are likely not the only reasons for strong seasonal cycles, but these are also based in periodic resonance with the natural cycles of immunity and antigenic variance, as well as nationwide synchrony through transportation networks. Investigations of seasonality will aid in understanding disease transmission, and thereby effective prevention strategies. The authors present a review of the literature on seasonal viruses, their annual diffusion through populations, and factors that reduce or enhance their seasonal spread. They also offer suggestions for targeted interventions to reduce the disease burden from seasonal viruses.

Development and application of diazirines in biological and synthetic macromolecular systems

Many different reagents and methodologies have been utilised for the modification of synthetic and biological macromolecular systems. In addition, an area of intense research at present is the construction of hybrid biosynthetic polymers, comprised of biologically active species immobilised or complexed with synthetic polymers. One of the most useful and widely applicable techniques available for functionalisation of macromolecular systems involves indiscriminate carbene insertion processes. The highly reactive and non-specific nature of carbenes has enabled a multitude of macromolecular structures to be functionalised without the need for specialised reagents or additives. The use of diazirines as stable carbene precursors has increased dramatically over the past twenty years and these reagents are fast becoming the most popular photophors for photoaffinity labelling and biological applications in which covalent modification of macromolecular structures is the basis to understanding structure-activity relationships. This review reports the synthesis and application of a diverse range of diazirines in macromolecular systems.

The HIV Env-mediated fusion reaction

The current general model of HIV viral entry involves the binding of the trimeric viral envelope glycoprotein gp120/gp41 to cell surface receptor CD4 and chemokine co-receptor CXCR4 or CCR5, which triggers conformational changes in the envelope proteins. Gp120 then dissociates from gp41, allowing for the fusion peptide to be inserted into the target membrane and the pre-hairpin configuration of the ectodomain to form. The C-terminal heptad repeat region and the leucine/isoleucine zipper region then form the thermostable six-helix coiled-coil, which drives the membrane merger and eventual fusion. This model needs updating, as there has been a wealth of data produced in the last few years concerning HIV entry, including target cell dependencies, fusion kinetic data, and conformational intermediates. A more complete model must include the involvement of membrane microdomains, actin polymerization, glycosphingolipids, and possibly CD4 and chemokine signaling in entry. In addition, kinetic experiments involving the addition of fusion inhibitors have revealed some of the rate-limiting steps in this process, adding a temporal component to the model. A review of these data that may require an updated version of the original model is presented here.

Intranasal immunization against influenza

Nasalflu is a novel influenza subunit vaccine, which is administered by the intranasal route using a spray device. Nasalflu is based on the virosomal concept which is registered in the EU as Epaxal Berna, a vaccine against Hepatitis A, and Inflexal Berna V, a subunit influenza vaccine. The virosome is a carrier system which delivers antigens to cells and is able to induce both B- and T-cell immunity. When virosomal vaccines are given parenterally, an immune response is elicited fast and sufficiently.

Self-assembly of influenza hemagglutinin: studies of ectodomain aggregation by in situ atomic force microscopy

We have used in situ tapping mode atomic force microscopy (AFM) to study the structural morphology of two fragments of the influenza hemagglutinin protein bound to supported bilayers. The two proteins that we studied are the bromelain-cleaved hemagglutinin (BHA), corresponding to the full ectodomain of the hemagglutinin protein, and FHA2, the 127 amino acid N-terminal fragment of the HA2 subunit of the hemagglutinin protein. While BHA is water soluble at neutral pH and is known to bind to membranes via specific interactions with a viral receptor, FHA2 can only be solubilized in water with an appropriate detergent. Furthermore, FHA2 is known to readily bind to membranes at neutral pH in the absence of a receptor. Our in situ AFM studies demonstrated that, when bound to supported bilayers at neutral pH, both these proteins are self-assembled as single trimeric molecules. In situ acidification resulted in further lateral association of the FHA2 without a large perturbation of the bilayer. In contrast, BHA remained largely unaffected by acidification, except in areas of exposed mica where it is aggregated. Remarkably, these results are consistent with previous observations that FHA2 promotes membrane fusion while BHA only induces liposome leakage at low pH. The results presented here are the first example of in situ imaging of the ectodomain of a viral envelope protein allowing characterization of the real-time self-assembly of a membrane fusion protein.

Modulation of membrane curvature by peptides

The fusion of two stable bilayers likely proceeds through intermediates in which the membrane acquires curvature. The insertion of peptides into the membrane will affect its curvature tendency. Studies with a number of small viral fusion peptides indicate that these peptides promote negative curvature at low concentration. This is in accord with the curvature requirements to initiate membrane fusion according to the stalk-pore model. Although a characteristic of fusion peptides, the promotion of negative curvature is only one of several mechanisms by which fusion proteins accelerate the rate of fusion. In addition, the fusion peptide itself, as well as other regions in the viral fusion protein, facilitates membrane fusion by mechanisms that are largely independent of curvature. Leakage of the internal aqueous contents of liposomes is another manifestation of the alteration of membrane properties. Peptides exhibit quite different relative potencies between fusion and leakage that is determined by the structure and mode of insertion of the peptide into the membrane.

Common properties of fusion peptides from diverse systems

Although membrane fusion occurs ubiquitously and continuously in all eukaroytic cells, little is known about the mechanism that governs lipid bilayer fusion associated with any intracellular fusion reactions. Recent studies of the fusion of enveloped viruses with host cell membranes have helped to define the fusion process. The identification and characterization of key proteins involved in fusion reactions have mainly driven recent advances in our understanding of membrane fusion. The most important denominator among the fusion proteins is the fusion peptide. In this review, work done in the last few years on the molecular mechanism of viral membrane fusion will be highlighted, focusing in particular on the role of the fusion peptide and the modification of the lipid bilayer structure. Much of what is known regarding the molecular mechanism of viral membrane fusion has been gained using liposomes as model systems in which the molecular components of the membrane and the environment are strictly controlled. Many amphilphilic peptides have a high affinity for lipid bilayers, but only a few sequences are able to induce membrane fusion. The presence of alpha-helical structure in at least part of the fusion peptide is strongly correlated with activity whereas, beta-structure tends to be less prevalent, associated with non-native experimental conditions, and more related to vesicle aggregation than fusion. The specific angle of insertion of the peptides into the membrane plane is also found to be an important characteristic for the fusion process. A shallow penetration, extending only to the central aliphatic core region, is likely responsible for the destabilization of the lipids required for coalescence of the apposing membranes and fusion.

Role of the N-terminal peptides of viral envelope proteins in membrane fusion

Membrane fusion is an important biological process that is observed in a wide variety of intra and intercellular events. In this review, work done in the last few years on the molecular mechanism of viral membrane fusion is highlighted, focusing in particular on the role of the fusion peptide and the modification of the lipid bilayer structure. While the Influenza hemagglutinin is currently the best understand fusion protein, there is still much to be learned about the key events in enveloped virus fusion reactions. This review compares our current understanding of the membrane fusion activity of Influenza and retrovirus viruses. We shall be concerned especially with the studies that lead to interpretations at the molecular level, so we shall concentrate on model membrane systems where the molecular components of the membrane and the environment are strictly controlled.

pH-dependent changes in photoaffinity labeling patterns of the H1 influenza virus hemagglutinin by using an inhibitor of viral fusion

The hemagglutinin (HA) protein undergoes a low-pH-induced conformational change in the acidic milieu of the endosome, resulting in fusion of viral and cellular membranes. A class of compounds that specifically interact with the HA protein of H1 and H2 subtype viruses and inhibit this conformational change was recently described (G. X. Luo et al., Virology 226:66-76, 1996, and J. Virol. 71:4062-4070, 1997). In this study, purified HA trimers (bromelain-cleaved HA [BHA]) are used to examine the properties and binding characteristics of these inhibitors. Compounds were able to inhibit the low-pH-induced change of isolated trimers, as detected by resistance to digestion with trypsin. Protection from digestion was extremely stable, as BHA-inhibitor complexes could be incubated for 24 h in low pH with almost no change in BHA structure. One inhibitor was prepared as a radiolabeled photoaffinity analog and used to probe for specific drug interactions with the HA protein. Analysis of BHA after photoaffinity analog binding and UV cross-linking revealed that the HA2 subunit of the HA was specifically radiolabeled. Cross-linking of the photoaffinity analog to BHA under neutral (native) pH conditions identified a stretch of amino acids within the alpha-helix of HA2 that interact with the inhibitor. Interestingly, cross-linking of the analog under acidic conditions identified a different region within the HA2 N terminus which interacts with the photoaffinity compound. These attachment sites help to delineate a potential binding pocket and suggest a model whereby the BHA is able to undergo a partial, reversible structural change in the presence of inhibitor compound.

Conformational changes and fusion activity of vesicular stomatitis virus glycoprotein: [125I]iodonaphthyl azide photolabeling studies in biological membranes

The interaction of VSV glycoprotein (VSV G) with biological membranes was studied by photosensitized labeling. The method is based on photosensitized activation by the fluorescent lipid analog 3,3'-dioctadecyloxacarbocyanine (DiO) of a hydrophobic probe, [125I]iodonaphthyl azide (125INA), that rapidly partitions into the membrane bilayer of virus and cells. 125INA labeling of proteins and lipids can be confined to the site of chromophore localization by photosensitized labeling. Photoactivation using visible light of target membrane labeled with DiO and 125INA, to which unlabeled virions are bound, results in exclusive labeling of envelope glycoproteins inserted into the target membrane [Pak et al. (1994) J. Biol. Chem. 269, 14614]. In this study, we labeled lipid symmetric erythrocyte ghosts with 125INA and DiO. Photosensitized activation of VSV prebound to labeled ghosts with visible light resulted in VSV G labeling under fusogenic conditions. Photoactivation of 125INA by UV light, which is nonspecific, produced labeled VSV G at both acidic and neutral pH. Photosensitized labeling of VSV G by DiO-125INA-ghosts was also observed at pH 5.5, 4 degrees C, in the absence of mixing between viral and cellular lipids, suggesting insertion of the ectodomain of VSV G. Soluble VSV G lacking the transmembrane domain inserted into DiO-125INA-ghosts under the same conditions as intact VSV G. DiO inserted into intact VSV appeared to be a suitable fluorophore for continuous kinetic measurements of membrane fusion by fluorescence dequenching. Our photosensitized labeling results establish biochemical correlates for the three states of VSV G, which we had proposed based on kinetic data [Clague et al., Biochemistry 29, 1303]. In addition, we found that VSV G insertion into the target membrane is reversible, suggesting a "velcro"-like attachment of the fusogenic domain with the target membrane.

What studies of fusion peptides tell us about viral envelope glycoprotein-mediated membrane fusion (review)

This review describes the numerous and innovative methods used to study the structure and function of viral fusion peptides. The systems studied include both intact fusion proteins and synthetic peptides interacting with model membranes. The strategies and methods include dissecting the fusion process into intermediate stages, comparing the effects of sequence mutations, electrophysiological patch clamp methods, hydrophobic photolabelling, video microscopy of the redistribution of both aqueous and lipophilic fluorescent probes between cells, standard optical spectroscopy of peptides in solution (circular dichroism and fluorescence) and attenuated total reflection-Fourier transform infrared spectroscopy of peptides bound to planar bilayers. Although the goal of a detailed picture of the fusion pore has not been achieved for any of the intermediate stages, important properties useful for constraining the development of models are emerging. For example, the presence of alpha-helical structure in at least part of the fusion peptide is strongly correlated with activity; whereas, beta-structure tends to be less prevalent, associated with non-native experimental conditions, and more related to vesicle aggregation than fusion. The specific angle of insertion of the peptides into the membrane plane is also found to be an important characteristic for the fusion process. A shallow penetration, extending only to the central aliphatic core region, is likely responsible for the destabilization of the lipids required for coalescence of the apposing membranes and fusion. The functional role of the fusion peptides (which tend to be either nonpolar or aliphatic) is then to bind to and dehydrate the outer bilayers at a localized site; and thus reduce the energy barrier for the formation of highly curved, lipidic 'stalk' intermediates. In addition, the importance of the formation of specific, 'higher-order' fusion peptide complexes has also been shown. Recent crystallographic structures of core domains of two more fusion proteins (in addition to influenza haemagglutinin) has greatly facilitated the development of prototypic models of the fusion site. This latter effort will undoubtedly benefit from the insights and constraints gained from the studies of fusion peptides.

H+-induced membrane insertion of influenza virus hemagglutinin involves the HA2 amino-terminal fusion peptide but not the coiled coil region

Fusion of influenza virus with target membranes is induced by acid and involves complex changes in the viral envelope protein hemagglutinin (HA). In a first, kinetically distinct step, the HA polypeptide chain 2 (HA2) is inserted into the target membrane bilayer. Using hydrophobic photolabeling with the phospholipid analogue 1-O-hexadecanoyl-2-O-[9-[[[2-[125I]iodo-4(trifluoromethyl-3H-diazirin -3-yl)benzyl]oxy]carbonyl]nonanoyl]-sn-glycero-3-phosphocholine, we identified the segment within HA2 that interacts with the membrane. The sole part of the HA2 ectodomain that was labeled with the membrane-restricted reagent is the NH2-terminal fusion peptide (residues 1-22). No labeling occurred within the long coiled coil region generated during the acid-induced conformational transition (Bullough, P. A., Hughson, F. M., Skehel, J. J., and Wiley, D. C. (1994) Nature 371, 37-43). These data strongly suggest that the coiled coil region of HA2 does not insert into the lipid bilayer. This conclusion is at variance with the recent suggestion (Yu, Y. G., King, D. S., and Shin, Y.-K.(1994) Science 266, 274-276) that the coiled coil of HA may splay apart and insert into the target membrane, providing a mechanism by which the viral and the target membrane may come in close apposition.

Membrane fusion mediated by the influenza virus hemagglutinin requires the concerted action of at least three hemagglutinin trimers

In this study we tested the hypothesis that fusion mediated by the influenza virus hemagglutinin (HA) is a cooperative event. To so this we characterized 3T3 cell lines that express HA at nine different defined surface densities. HA densities ranged from 1.0 to 12.6 x 10(3) HA trimers/microns2 as determined by quantitative fluorescent antibody binding. The lateral mobility and percent mobile fraction of HA did not vary significantly among these cells, nor did the contact area between HA-expressing cells and target RBCs. The fusion reaction of each HA-expressing cell line was analyzed using a fluorescence dequenching assay that uses octadecylrhodamine (R18)-labeled RBCs. For each cell line we measured the lag time preceding the onset of fusion, the initial rate of fusion, and final extent of fusion. The final extent of fusion was similar for all cell lines, and the initial rate of fusion as a function of HA surface density displayed a Michaelis-Menten-type dependence. However, the dependence of the lag time preceding the onset of fusion on HA surface density was clearly sigmoidal. Kinetic analysis of the data for the reciprocal lag time vs HA surface density, by both a log/log plot and a Hill plot, suggested that the observed sigmoidicity does not reflect cooperativity at the level of formation of HA aggregates as a prerequisite to fusion. Rather, the cooperativity of the process(es) that occur(s) during the lag time arises at a later step and involves a minimum of three, and most likely four, HA trimers. A model is proposed to explain HA cooperativity during fusion.

Effect of X31 influenza virus fusion on phosphatidylserine asymmetry in erythrocytes

Influenza virus fusion is mediated by its fusion protein, hemagglutinin (HA). HA undergoes a low pH dependent conformational change that results in insertion into the cell membrane bilayer, formation of a fusion pore, and merging of membrane lipids and establishment of cytoplasmic continuity. Erythrocytes, which can serve as targets of influenza virus fusion, display an asymmetric transbilayer arrangement of their phospholipids. The effect of influenza virus fusion on erythrocyte phosphatidylserine asymmetry was determined. Influenza virus were bound to erythrocytes containing the fluorescent membrane probe NBD-PS in the inner leaflet. Induction of fusion by exposure to a low pH environment resulted in movement of PS to the outer leaflet of the cell as well as hemolysis. Insertion of the fusion protein into erythrocytes and subsequent fusion can be distinguished from hemolysis by examining the interaction of a soluble form of HA (BHA) with cells and by monitoring viral fusion at low temperatures. No hemolysis was observed under either condition. BHA binding and insertion into cells did not affect the asymmetry of PS. Incubation of influenza virus fusion at pH 5, 0 degrees C resulted in complete fusion but no outward movement of PS was observed. These findings suggest the viral fusion pore does not involve a rearrangement of the transbilayer phospholipid organization of the target membrane.

Lysophosphatidylcholine inhibits vesicles fusion induced by the NH2-terminal extremity of SIV/HIV fusogenic proteins

Intermediate lipid structures such as inverted micelles and interlamellar attachments are thought to play a crucial role in different biological processes like exocytosis, intracellular trafficking and viral infection. In the present study, we provide evidence that lipid mixing of large unilamellar lipid vesicles (LUV) mediated by the NH2-terminal sequence of the SIV gp32 and of HIV gp41 is inhibited by external addition of lysophosphatidylcholine (lysoPC) to LUV containing phosphatidylethanolamine in their lipid bilayer. Leakage experiments confirm that lysoPC enhances the stability of the lipids organization. The temperature dependence of the two processes as well as the complementary shape of PE and lysoPC suggest that the PE-lysoPC interaction is involved in the fusion inhibition and stabilization of the bilayer.

pH-dependent binding of the fluorophore bis-ANS to influenza virus reflects the conformational change of hemagglutinin

Binding of the fluorophore 1,1'-bis(4-anilino)naphthalene-5,5'-disulfonic acid (bis-ANS) to influenza virus A/PR 8/34 is strongly enhanced at low pH. Binding is accompanied by a significant increase in fluorescence intensity. The binding and the fluorescence increase are associated with the low-pH induced conformational change of the viral spike protein, hemagglutinin, exposing hydrophobic binding sites. The data indicate that in addition to the hydrophobic N-terminus of HA2 other hydrophobic sequences of the HA ectodomain become accessible to bis-ANS at low pH. It is shown that the time course of the fluorescence increase of bis-ANS at low pH is determined by the conformational change of HA. The application of this assay for continuously monitoring the kinetics of the structural alteration in HA is discussed and its relevance for elucidating the temporal relationship between the conformational change of HA and virus-membrane fusion is outlined.

Lysophosphatidylcholine mediates the mode of insertion of the NH2-terminal SIV fusion peptide into the lipid bilayer

We report here on the interaction of a synthetic 12 residue peptide corresponding to the N-terminal sequence of gp32 from SIV with phospholipid bilayers. This peptide has been shown to induce lipid mixing of PC/PE/SM/Chol LUV (large unilamellar vesicles) at pH 7.4 and 37 degrees C [(1992) in: Advances in Membrane Fluidity, vol. 6, pp. 365-376, Wiley-Liss]. In the present study, this fusion process was inhibited by the addition of lysophosphatidylcholine (lysoPC) to the lipid bilayer of PC/PE/SM/Chol LUV. Fourier transform infrared spectroscopy (FTIR) reveals that the orientation of the SIV fusion peptide with respect to the lipid acyl chains depends on the presence of lysoPC in the lipid bilayer but that the peptide secondary structure and the amount of lipid-associated peptides do not depend on the lipid composition. The peptide is obliquely inserted into the lipid bilayer of vesicles without lysoPC, whereas it is oriented parallel to the lipid-water interface in the vesicles containing lysoPC. The data provide evidence that the orientation of the SIV fusion peptide depends on the lipid composition, and that this mediates its fusogenic activity.

Single event recording shows that docking onto receptor alters the kinetics of membrane fusion mediated by influenza hemagglutinin

The initial steps of membrane fusion, receptor binding and membrane destabilization, are mediated by the envelope glycoprotein hemagglutinin of influenza virus. Interaction between these functions was determined from the time course of individual virion fusions to a planar membrane with and without receptor. With receptor, fusion was described by a Poisson process. In the absence of receptor, the time course was more complicated and could not be described with exponential rate constants. The conversion of a non-Markovian process into a simple Markov chain is direct evidence that receptor binding fundamentally alters the route of fusion.

A common mechanism for influenza virus fusion activity and inactivation

The fusion of influenza virus (A/PR/8/34 strain) with PC-12 cells was monitored by a fluorescence assay, and the results were analyzed with a mass-action model which could explain and predict the kinetics of fusion. The model accounted explicitly for the reduction in the fusion rate constant upon exposure of the virus to low pH, either for the virus alone in suspension or for the virus bound to the cells. When the pH was lowered without previous viral attachment to cells, an optimal fusion activity was detected at pH 5.2. When the virus was prebound to the cells, however, reduction of pH below 5.2 resulted in enhanced fusion activity at the initial stages. These results were explained by the fact that the rate constants of both fusion and inactivation increased severalfold at pH 4.5 or 4, compared to those at pH 5.2. At pH 5.2, lowering the temperature from 37 to 20 or 4 degrees C resulted in a decrease in the fusion rate constant by more than 30- or 1000-fold, respectively. Inactivation of the virus when preincubated in the absence of target membranes at pH 5 was found to be rapid and extensive at 37 degrees C, but was also detected at 0 degrees C. Our results indicate a strong correlation between fusion and inactivation rate constants, suggesting that the rate-limiting step in viral hemagglutinin (HA)-mediated fusion, that is, rearrangement of viral glycoproteins at the contact points with the target membrane, is similar to that involved in fusion inactivation.

Orientation and structure of the NH2-terminal HIV-1 gp41 peptide in fused and aggregated liposomes

For several retroviruses, the N-terminal hydrophobic sequence of the viral envelope glycoprotein has been shown to play a crucial role in the interaction between the virus and the host cell membrane. We report here on the interaction of a synthetic 16 residues peptide corresponding to the gp41 NH2-terminal sequence of Human Immunodeficiency Virus with the phospholipid bilayer. Fluorescence energy transfer measurements show that this peptide can induce lipid mixing of large unilamellar vesicles (LUV) of various compositions at pH 7.4 and 37 degrees C. LUV undergo fusion, provided they contained phosphatidylethanolamine (PE) in their lipid composition. To provide insight into the mechanism of the fusion event, the peptide secondary structure and orientation in the lipid bilayer were determined using Fourier Transform Infrared Spectroscopy (FTIR). The peptide adopts mainly a beta-sheet conformation in the absence of lipids. After interaction with LUV the beta-sheet is partly converted into alpha-helix. The orientation of the peptide with respect to the lipid acyl chains depends on the presence of PE in the lipid bilayer. The peptide is inserted into the lipid bilayer with the helix axis oriented parallel to the lipid acyl chains in the fused vesicles, whereas it is adsorbed parallel to the lipid/water interface in the aggregated vesicles. The role of the two kinds of orientation during the fusion event is discussed.

Membrane fusion

Common themes are emerging from the study of viral, cell-cell, intracellular, and liposome fusion. Viral and cellular membrane fusion events are mediated by fusion proteins or fusion machines. Viral fusion proteins share important characteristics, notably a fusion peptide within a transmembrane-anchored polypeptide chain. At least one protein involved in a cell-cell fusion reaction resembles viral fusion proteins. Components of intracellular fusion machines are utilized in multiple membrane trafficking events and are conserved through evolution. Fusion pores develop during and intracellular fusion events suggesting similar mechanisms for many, if not all, fusion events.

Intermediates and kinetics of membrane fusion

Recently, it has become clear that the influenza virus fusion protein, hemagglutinin (HA), produces membrane destabilization and fusion by a multistep process, which involves the aggregation of the HAs to form a fusion site. While the details of this process are under debate, it is important to recognize that proposing any sequence of "microscopic" fusion intermediates encumbers general "macroscopic" kinetic consequences, i.e., with respect to membrane mixing rates. Using a kinetic scheme which incorporates the essential elements of several recently proposed models, some of these measurable properties have been elucidated. First, a rigorous mathematical relationship between fusion intermediates and the fusion event itself is defined. Second, it is shown that what is measured as the macroscopic "fusion rate constant" is a simple function of all of the rate constants governing the transitions between intermediates, whether or not one of the microscopic steps is rate limiting. Third, while this kinetic scheme predicts a delay (or lag) time for fusion, as has been observed, it will be very difficult to extract reliable microscopic information from these data. Furthermore, it is predicted that the delay time can depend upon HA surface density even when the HA aggregation step is very rapid compared with fusion, i.e., the delay time need not be due to HA aggregation. Fourth, the inactivation process observed for influenza virions at low pH can be described within this kinetic scheme simply, yet rigorously, via the loss of the fusion intermediates. Fifth, predicted Arrhenius plots of fusion rates can be linear for this multistep scheme, even though there is no single rate-determining step and even when a branched step is introduced, i.e., where one pathway predominates at low temperature and the other pathway predominates at high temperature. Furthermore, the apparent activation energies obtained from these plots bear little or no quantitative resemblance to the microscopic activation energies used to simulate the data. Overall, these results clearly show that the intermediates of protein mediated fusion can be studied only by using assays sensitive to the formation of each proposed intermediate.

Membrane fusion activity of the influenza virus hemagglutinin: interaction of HA2 N-terminal peptides with phospholipid vesicles

We have investigated the interaction of a number of synthetic 20-residue peptides, corresponding to the HA2 N-terminus of the influenza virus hemagglutinin (X31 strain), with phospholipid vesicles and monolayers. Besides the wild-type sequence, two peptides were studied with mutations corresponding to those previously studied in entire HA's expressed in transfected cells [Gething et al., (1986) J. Cell. Biol. 102, 11-23]. These mutations comprised a single Glu replacement for Gly at the N-terminus ("El" mutant) or at position 4 ("E4") of the HA2 subunit and were shown to produce striking alterations in virus-induced hemolysis and syncytia formation, especially for E1. The X31 "wild-type" (wt) peptide and its E4 variant are shown here to have the capacity to insert into phosphatidylcholine (POPC) large unilamellar vesicle (LUV) membranes in a strictly pH-dependent manner, penetration being marginal at pH 7.4 and significant at pH 5.0. Bilayer insertion was evident from a shift in the intrinsic Trp fluorescence of the wt and E4 peptides and from the induction of calcein leakage from POPC LUV and correlated well with the peptides' ability at pH 5.0 to penetrate into POPC monolayers at initial surface pressures higher than 30 mN/m. By contrast, the E1 peptide was found, at pH 5.0, to bind less tightly to vesicles (assessed by a physical separation method) and to cause much less leakage of POPC LUV than the wt, even under conditions where the peptides were bound to approximately the same extent. Consistent with the correlation between leakage and penetration observed for the wt peptide at pH 5 versus 7, the E1 peptide, even at low pH, showed much less lipid-vesicle-induced shift of its Trp fluorescence than wt, caused a much slower rate of leakage of vesicle contents, and did not insert into POPC monolayers at surface pressures beyond 28.5 mN/m. Circular dichroism spectroscopy measurements of peptides in POPC SUV showed that the conformations of all three peptides are sensitive to pH, but only the wt and E4 peptides became predominantly alpha-helical at acid pH.