Interaction of fluorescently labeled analogues of the amino-terminal fusion peptide of Sendai virus with phospholipid membranes

Identification and Characteristics of Fusion Peptides Derived From Enveloped Viruses

Membrane fusion events allow enveloped viruses to enter and infect cells. The study of these processes has led to the identification of a number of proteins that mediate this process. These proteins are classified according to their structure, which vary according to the viral genealogy. To date, three classes of fusion proteins have been defined, but current evidence points to the existence of additional classes. Despite their structural differences, viral fusion processes follow a common mechanism through which they exert their actions. Additional studies of the viral fusion proteins have demonstrated the key role of specific proteinogenic subsequences within these proteins, termed fusion peptides. Such peptides are able to interact and insert into membranes for which they hold interest from a pharmacological or therapeutic viewpoint. Here, the different characteristics of fusion peptides derived from viral fusion proteins are described. These criteria are useful to identify new fusion peptides. Moreover, this review describes the requirements of synthetic fusion peptides derived from fusion proteins to induce fusion by themselves. Several sequences of the viral glycoproteins E1 and E2 of HCV were, for example, identified to be able to induce fusion, which are reviewed here.

Structure and interaction with lipid membrane models of Semliki Forest virus fusion peptide

Semliki Forest virus (SFV) is a well-characterized alphavirus that infects cells via endocytosis and an acid-triggered fusion step using class II fusion proteins. Membrane fusion is mediated by the viral spike protein, a heterotrimer of two transmembrane subunits, E1 and E2, and a peripheral protein, E3. Sequence analysis of the E1 ectodomain of a number of alphaviruses demonstrated the presence of a highly conserved hydrophobic domain on the E1 ectodomain. This sequence was proposed to be the fusion peptide of SFV and is believed to be the domain of E1 that interacts with the target membrane and triggers fusion. Here, we investigate the structure and the interaction with lipid membrane models of ₇₆YQCKVYTGVYPFMWGGAYCFC₉₆ sequence from SFV, named SFV21, using optical method (ellipsometry) and vibrational spectroscopiy approaches (Polarization Modulation infra-Red Reflection Absorption Spectroscopy, PMIRRAS, and polarized ATR-FTIR). We demonstrate a structural flexibility of SFV21 sequence whether the lateral pressure and the lipid environment. In a lipid environment that mimics eukaryotic cell membranes, a conformational transition from an α-helix to a β-sheet is induced in the presence of lipid by increasing the peptide to lipid ratio, which leads to important perturbations in the membrane organisation.

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SFV21 fusion peptide displays structural flexibility between α-helix and β-sheets.

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A conformational transition from an α-helix to a β-sheet is induced by the increase of the peptide to lipid ratio.

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SFV21 fusion peptide leads to important perturbations in the membrane organisation.

Study of the putative fusion regions of the preS domain of hepatitis B virus

In a previous study, it was shown that purified preS domains of hepatitis B virus (HBV) could interact with acidic phospholipid vesicles and induce aggregation, lipid mixing and leakage of internal contents which could be indicative of their involvement in the fusion of the viral and cellular membranes (Núñez, E. et al. 2009. Interaction of preS domains of hepatitis B virus with phospholipid vesicles. Biochim. Biophys. Acta 17884:417-424). In order to locate the region responsible for the fusogenic properties of preS, five mutant proteins have been obtained from the preS1 domain of HBV, in which 40 amino acids have been deleted from the sequence, with the starting point of each deletion moving 20 residues along the sequence. These proteins have been characterized by fluorescence and circular dichroism spectroscopy, establishing that, in all cases, they retain their mostly non-ordered conformation with a high percentage of β structure typical of the full-length protein. All the mutants can insert into the lipid matrix of dimyristoylphosphatidylglycerol vesicles. Moreover, we have studied the interaction of the proteins with acidic phospholipid vesicles and each one produces, to a greater or lesser extent, the effects of destabilizing vesicles observed with the full-length preS domain. The ability of all mutants, which cover the complete sequence of preS1, to destabilize the phospholipid bilayers points to a three-dimensional structure and/or distribution of amino acids rather than to a particular amino acid sequence as being responsible for the membrane fusion process.

Peste des petits ruminants virus infection of small ruminants: a comprehensive review

Peste des petits ruminants (PPR) is caused by a Morbillivirus that belongs to the family Paramyxoviridae. PPR is an acute, highly contagious and fatal disease primarily affecting goats and sheep, whereas cattle undergo sub-clinical infection. With morbidity and mortality rates that can be as high as 90%, PPR is classified as an OIE (Office International des Epizooties)-listed disease. Considering the importance of sheep and goats in the livelihood of the poor and marginal farmers in Africa and South Asia, PPR is an important concern for food security and poverty alleviation. PPR virus (PPRV) and rinderpest virus (RPV) are closely related Morbilliviruses. Rinderpest has been globally eradicated by mass vaccination. Though a live attenuated vaccine is available against PPR for immunoprophylaxis, due to its instability in subtropical climate (thermo-sensitivity), unavailability of required doses and insufficient coverage (herd immunity), the disease control program has not been a great success. Further, emerging evidence of poor cross neutralization between vaccine strain and PPRV strains currently circulating in the field has raised concerns about the protective efficacy of the existing PPR vaccines. This review summarizes the recent advancement in PPRV replication, its pathogenesis, immune response to vaccine and disease control. Attempts have also been made to highlight the current trends in understanding the host susceptibility and resistance to PPR.

Role of sequence and structure of the Hendra fusion protein fusion peptide in membrane fusion

Viral fusion proteins are intriguing molecular machines that undergo drastic conformational changes to facilitate virus-cell membrane fusion. During fusion a hydrophobic region of the protein, termed the fusion peptide (FP), is inserted into the target host cell membrane, with subsequent conformational changes culminating in membrane merger. Class I fusion proteins contain FPs between 20 and 30 amino acids in length that are highly conserved within viral families but not between. To examine the sequence dependence of the Hendra virus (HeV) fusion (F) protein FP, the first eight amino acids were mutated first as double, then single, alanine mutants. Mutation of highly conserved glycine residues resulted in inefficient F protein expression and processing, whereas substitution of valine residues resulted in hypofusogenic F proteins despite wild-type surface expression levels. Synthetic peptides corresponding to a portion of the HeV F FP were shown to adopt an α-helical secondary structure in dodecylphosphocholine micelles and small unilamellar vesicles using circular dichroism spectroscopy. Interestingly, peptides containing point mutations that promote lower levels of cell-cell fusion within the context of the whole F protein were less α-helical and induced less membrane disorder in model membranes. These data represent the first extensive structure-function relationship of any paramyxovirus FP and demonstrate that the HeV F FP and potentially other paramyxovirus FPs likely require an α-helical structure for efficient membrane disordering and fusion.

Studies on viral fusion peptides: the distribution of lipophilic and electrostatic potential over the peptide determines the angle of insertion into a membrane

The oblique insertion of type 1 viral fusion peptides into the cell membrane of the host cell has been shown previously to be an essential element of viral fusion. The actual physical explanation of the cause of the oblique insertion has been the subject of speculation. In this study the physical properties of the fusion peptide surface have been determined computationally and compared to the tilt angles determined both experimentally and by the use of molecular dynamics. It has been shown that the relationship between the distribution of lipophilic potential over the peptide surface and the peptide geometry control the tilt angle of the peptide in a biomimetic DMPC bilayer whereas the depth of penetration into the bilayer appears to be determined by the electrostatic potential and hydrogen bonding at the C-terminus.

The actin cytoskeleton inhibits pore expansion during PIV5 fusion protein-promoted cell-cell fusion

Paramyxovirus fusion (F) proteins promote both virus-cell fusion, required for viral entry, and cell-cell fusion, resulting in syncytia formation. We used the F-actin stabilizing drug, jasplakinolide, and the G-actin sequestrant, latrunculin A, to examine the role of actin dynamics in cell-cell fusion mediated by the parainfluenza virus 5 (PIV5) F protein. Jasplakinolide treatment caused a dose-dependent increase in cell-cell fusion as measured by both syncytia and reporter gene assays, and latrunculin A treatment also resulted in fusion stimulation. Treatment with jasplakinolide or latrunculin A partially rescued a fusion pore opening defect caused by deletion of the PIV5 F protein cytoplasmic tail, but these drugs had no effect on fusion inhibited at earlier stages by either temperature arrest or by a PIV5 heptad repeat peptide. These data suggest that the cortical actin cytoskeleton is an important regulator of fusion pore enlargement, an energetically costly stage of viral fusion protein-mediated membrane merger.

Lipid-destabilising properties of a peptide with structural plasticity

The Chameleon peptide (Cham) is a peptide designed from two regions of the GB1 protein, one folded as an alpha-helix and the other as a beta structure. Depending on the environment, the Cham peptide adopts an alpha or a beta conformation when inserted in different locations of GB1. This environment dependence is also observed for tilted peptides. These short protein fragments, able to destabilise organised system, are mainly folded in beta structure in water and in alpha helix in a hydrophobic environment, like the lipid bilayer. In this paper, we tested whether the Cham peptide can be qualified as a tilted peptide. For this, we have compared the properties of Cham peptide (hydrophobicity, destabilising properties, conformation) to those of tilted peptides. The results suggest that Cham is a tilted peptide. Our study, together the presence of tilted fragments in transconformational proteins, suggests a relationship between tilted peptides and structural lability.

Characterization of a putative fusogenic sequence in the E2 hepatitis G virus protein

With the aim of better understanding the fusion process mediated by the envelope proteins of the hepatitis G virus (HGV/GBV-C), we have investigated the interaction with model membranes of two overlapping peptides [(267-284) and (279-298)] belonging to the E2 structural protein. The peptides were compared for their ability to perturb lipid bilayers by means of different techniques such as differential scanning calorimetry and fluorescence spectroscopy. Furthermore, the conformational behaviour of the peptides in different membrane environments was studied by Fourier-transform infrared spectroscopy and circular dichroism. The results showed that only the E2(279-298) peptide sequence was able to bind with high affinity to negatively charged membranes, to permeabilize efficiently negative lipid bilayers, to induce haemolysis, and to promote inter-vesicle fusion. This fusogenic activity could be related to the induced peptide conformation upon interaction with the target membrane.

Structure and orientation study of fusion peptide FP23 of gp41 from HIV-1 alone or inserted into various lipid membrane models (mono-, bi- and multibi-layers) by FT-IR spectroscopies and Brewster angle microscopy

In the present work, we study the structure and the orientation of the 23 N-terminal peptide of the HIV-1 gp 41 protein (AVGIGALFLGFLGAAGSTMGARS) called FP23. The behaviour of FP23 was investigated alone at the air/water interface and inserted into various lipid model systems: in monolayer or multibilayers of a DOPC/cholesterol/DOPE/DOPG (6/5/3/2) and in a DMPC bilayer. PMIRRAS and polarized ATR spectroscopy coupled with Brewster angle microscopy and spectral simulations were used to precisely determine the structure and the orientation of the peptide in its environment as well as the lipid perturbations induced by the FP23 insertion. The infra-red results show the structural polymorphism of the FP23 and its ability to transit quasi irreversibly from an alpha-helix to antiparallel beta-sheets. At the air/water interface, the transition is induced by compression of the peptide alone and is modulated by compression and lipid to peptide ratio (Ri) when FP23 is inserted into a lipid monolayer. In multibilayers and in a single bilayer, there is coexistence in quasi equal proportions of alpha-helix and antiparallel beta-sheets of FP23 at low peptide content (Ri=100, 200) while antiparallel beta-sheets are predominant at high FP23 concentration (Ri=50). In (multi)bilayer systems, evaluation of dichroic ratios and sprectral simulations show that both the alpha-helix and the antiparallel beta-sheets are tilted at diluted FP23 concentrations (tilt angle of alpha-helix with respect to the normal of the interface=36.5+/-3.0 degrees for FP23 in multibilayers of DOPC/Chol/DOPE/DOPG at Ri=200 and 39.0+/-5.0 degrees in a single bilayer of DMPC at Ri=100 and tilt angle of the beta-sheets=36.0+/-2.0 degrees for the beta-sheets in multibilayers and 30.0+/-2.0 degrees in the lipid bilayer). In parallel, the FP23 induces an increase of the lipid chain disorder which shows both by an increase of the methylene stretching frequencies and an increase of the average C-C-C angle of the acyl chains. At high FP23 content (Ri=50), the antiparallel beta-sheets induce a complete disorganization of the lipid chains in (multi)bilayers.

The energetics of membrane fusion from binding, through hemifusion, pore formation, and pore enlargement

The main steps of viral membrane fusion are local membrane approach, hemifusion, pore formation, and pore enlargement. Experiments and theoretical analyses have helped determine the relative energies required for each step. Key protein structures and conformational changes of the fusion process have been identified. The physical deformations of monolayer bending and lipid tilt have been applied to the steps of membrane fusion. Experiment and theory converge to strongly indicate that, contrary to former conceptions, the fusion process is progressively more energetically difficult: hemifusion has a relatively low energy barrier, pore formation is more energy-consuming, and pore enlargement is the most difficult to achieve.

Effect of nanomolar concentrations of sodium dodecyl sulfate, a catalytic inductor of alpha-helices, on human calcitonin incorporation and channel formation in planar lipid membranes

Human Calcitonin (hCt) is a peptide hormone which has a regulatory action in calcium-phosphorus metabolism. It is currently used as a therapeutic tool in bone pathologies such as osteoporosis and Paget's disease. However, due to its amphiphilic property tends to form a gelatinous solution in water which consists of fibrils that limits its therapeutic use. Here we show that sodium dodecyl sulfate (SDS), an anionic detergent able to induce and stabilize alpha-helices in polypeptides, at a monomeric concentration ranging between 0.26 mM-5 pM (all concentrations are below the CMC), increases the rate and number of hCt channel formation in planar lipid membranes, at both high and low hCt concentrations, with a maximum increase at a molecular hCt/SDS ratio of 1000:1. This effect could be interpreted as a counteraction to the fibrillation process of hCt molecules by removing molecules available for aggregation from the fluid; furthermore, this action, independently of channel formation in the cell membrane, could improve the peptide-receptor interaction. The action of SDS could be attributable to the strength of the sulfate negative charge and the hydrophobic chain; in fact, a similar effect was obtained with lauryl sarcosine and not with a neutral detergent such as n-dodecyl-beta-D-maltoside. The very low molecular ratio between SDS and peptide is suggestive of a possible catalytic action of SDS that could induce alpha-helices, the appropriate structures for interacting with the membrane. Moreover, in the experimental conditions investigated, the addition of SDS does not modify the membrane's electrical properties and most of the channel properties. This finding may contribute to the knowledge of environment-folding diseases due to protein and peptides.

Conserved glycine residues in the fusion peptide of the paramyxovirus fusion protein regulate activation of the native state

Hydrophobic fusion peptides (FPs) are the most highly conserved regions of class I viral fusion-mediating glycoproteins (vFGPs). FPs often contain conserved glycine residues thought to be critical for forming structures that destabilize target membranes. Unexpectedly, a mutation of glycine residues in the FP of the fusion (F) protein from the paramyxovirus simian parainfluenza virus 5 (SV5) resulted in mutant F proteins with hyperactive fusion phenotypes (C. M. Horvath and R. A. Lamb, J. Virol. 66:2443-2455, 1992). Here, we constructed G3A and G7A mutations into the F proteins of SV5 (W3A and WR isolates), Newcastle disease virus (NDV), and human parainfluenza virus type 3 (HPIV3). All of the mutant F proteins, except NDV G7A, caused increased cell-cell fusion despite having slight to moderate reductions in cell surface expression compared to those of wild-type F proteins. The G3A and G7A mutations cause SV5 WR F, but not NDV F or HPIV3 F, to be triggered to cause fusion in the absence of coexpression of its homotypic receptor-binding protein hemagglutinin-neuraminidase (HN), suggesting that NDV and HPIV3 F have stricter requirements for homotypic HN for fusion activation. Dye transfer assays show that the G3A and G7A mutations decrease the energy required to activate F at a step in the fusion cascade preceding prehairpin intermediate formation and hemifusion. Conserved glycine residues in the FP of paramyxovirus F appear to have a primary role in regulating the activation of the metastable native form of F. Glycine residues in the FPs of other class I vFGPs may also regulate fusion activation.

Inhibition of HIV-1 envelope glycoprotein-mediated cell fusion by a DL-amino acid-containing fusion peptide: possible recognition of the fusion complex

The N-terminal fusion peptide (FP) of human immunodeficiency virus-1 (HIV-1) is a potent inhibitor of cell-cell fusion, possibly because of its ability to recognize the corresponding segments inside the fusion complex within the membrane. Here we show that a fusion peptide in which the highly conserved Ile(4), Phe(8), Phe(11), and Ala(14) were replaced by their d-enantiomers (IFFA) is a potent inhibitor of cell-cell fusion. Fourier transform infrared spectroscopy confirmed that despite these drastic modifications, the peptide preserved most of its structure within the membrane. Fluorescence energy transfer studies demonstrated that the diastereomeric peptide interacted with the wild type FP, suggesting this segment as the target site for inhibition of membrane fusion. This is further supported by the similar localization of the wild type and IFFA FPs to microdomains in T cells and the preferred partitioning into ordered regions within sphingomyelin/phosphatidyl-choline/cholesterol giant vesicles. These studies provide insight into the mechanism of molecular recognition within the membrane milieu and may serve in designing novel HIV entry inhibitors.

Oligomerization of fusogenic peptides promotes membrane fusion by enhancing membrane destabilization

A key element of membrane fusion reactions in biology is the involvement of specific fusion proteins. In many viruses, the proteins that mediate membrane fusion usually exist as homotrimers. Furthermore, they contain extended triple-helical coiled-coil domains and fusogenic peptides. It has been suggested that the coiled-coil domains present the fusogenic peptide in a conformation or geometry favorable for membrane fusion. To test the hypothesis that trimerization of fusogenic peptide is related to optimal fusion, we have designed and synthesized a triple-stranded coiled-coil X31 peptide, also known as the ccX31, which mimics the influenza virus hemagglutinin fusion peptide in the fusion-active state. We compared the membrane interactive properties of ccX31 versus the monomeric X31 fusogenic peptide. Our data show that trimerization enhances peptide-induced leakage of liposomal contents and lipid mixing. Furthermore, studies using micropipette aspiration of single vesicles reveal that ccX31 decreases lysis tension, tau(lysis), but not area expansion modulus, Ka, of phospholipid bilayers, whereas monomeric X31 peptide lowers both tau(lysis) and Ka. Our results are consistent with the hypothesis that oligomerization of fusogenic peptide promotes membrane fusion, possibly by enhancing localized destabilization of lipid bilayers.

Structural and functional properties of an unusual internal fusion peptide in a nonenveloped virus membrane fusion protein

The avian and Nelson Bay reoviruses are two of only a limited number of nonenveloped viruses capable of inducing cell-cell membrane fusion. These viruses encode the smallest known membrane fusion proteins (p10). We now show that a region of moderate hydrophobicity we call the hydrophobic patch (HP), present in the small N-terminal ectodomain of p10, shares the following characteristics with the fusion peptides of enveloped virus fusion proteins: (i) an abundance of glycine and alanine residues, (ii) a potential amphipathic secondary structure, (iii) membrane-seeking characteristics that correspond to the degree of hydrophobicity, and (iv) the ability to induce lipid mixing in a liposome fusion assay. The p10 HP is therefore predicted to provide a function in the mechanism of membrane fusion similar to those of the fusion peptides of enveloped virus fusion peptides, namely, association with and destabilization of opposing lipid bilayers. Mutational and biophysical analysis suggested that the internal fusion peptide of p10 lacks alpha-helical content and exists as a disulfide-stabilized loop structure. Similar kinked structures have been reported in the fusion peptides of several enveloped virus fusion proteins. The preservation of a predicted loop structure in the fusion peptide of this unusual nonenveloped virus membrane fusion protein supports an imperative role for a kinked fusion peptide motif in biological membrane fusion.

Structural and dynamical changes of the bindin B18 peptide upon binding to lipid membranes. A solid-state NMR study

Structural and dynamical features of the B18 peptide from the sea urchin sperm binding protein were determined in the crystalline state and in zwitterionic lipid bilayers at a peptide:lipid molar ratio of 1:12 using solid-state NMR spectroscopy. The study was focused on three (13)C and (15)N uniformly labeled leucine residues, which were introduced into three different B18 peptides at positions evenly distributed along the B18 primary structure. Isotropic (13)C and (15)N chemical shift measurements showed that while B18 possesses a nonhelical and non-sheet-like structure in the crystalline state, the peptide adopts an oligomeric beta-sheet structure in the membrane in the presence of Zn(2+) ions at high peptide:lipid ratio. Torsion angle measurements for the three leucine sites supported these results, with phi torsion angles between -80 degrees and -90 degrees in the crystalline state and between -110 degrees and -120 degrees in the membrane-bound form. These phi torsion angles determined for membrane-bound B18 are consistent with a parallel beta-sheet secondary structure. Analysis of motionally averaged dipolar coupling measurements established an increase of the mobility in the leucine side chains upon binding to the membrane, whereas the backbone mobility remained essentially unchanged, except in the binding site of Zn(2+) ions. This difference in mobility was related to the H-bond network in the parallel beta-sheet structure, which involves the backbone and excludes the side chains of leucine residues. The parallel beta-sheet structure of B18 in the membrane in the presence of Zn(2+) appears to be an active state for the fusion of zwitterionic membranes in the presence of Zn(2+). A fluorescence fusion assay indicated that high B18 concentrations are required to induce fusion in these systems. Therefore, it was hypothesized that the oligomeric beta-sheet secondary structure revealed in the study represents an active state of the peptide in a membrane environment during fusion.

Attenuated total reflection IR spectroscopy as a tool to investigate the orientation and tertiary structure changes in fusion proteins

Membrane fusion proceeds via a merging of two lipid bilayers and a redistribution of aqueous contents and bilayer components. It involves transition states in which the phospholipids are not arranged in bilayers and in which the monolayers are highly curved. Such transition states are energetically unfavourable since biological membranes are submitted to strong repulsive hydration electrostatic and steric barriers. Viral membrane proteins can help to overcome these barriers. Viral proteins involved in membrane fusion are membrane associated and the presence of lipids restricts drastically the potential of methods (RMN, X-ray crystallography) that have been used successfully to determine the tertiary structure of soluble proteins. We describe here how IR spectroscopy allows to solve some of the problems related to the lipid environment. The principles of the method, the experimental setup and the preparation of the samples are briefly described. A few examples illustrate how attenuated total reflection Fourier-transform IR (ATR-FTIR) spectroscopy can be used to gain information on the orientation and the accessibility to the water phase of the fusogenic domain of viral proteins. Recent developments suggest that the method could also be used to detect changes located in the membrane domains and to identify intermediate structural states involved in the fusion process.

Are fusion peptides a good model to study viral cell fusion?

Fusion peptides are hydrophobic and conserved sequences located within glycoprotein ectodomains that protrude from the virion surface. Direct participation of fusion peptides in the viral membrane fusion phenomenon has been inferred from genetic analyses showing that even a single residue substitution or a deletion within these sequences may completely block the process. However, the specific fusion peptide activities associated to the multi-step fusion mechanism are not well defined. Based on the assumption that fusion peptides are transferred into target membranes, biophysical methodologies have been applied to study integration into model membranes of synthetic fragments representing functional and non-functional sequences. From these studies, it is inferred that, following insertion, functional sequences generate target membrane perturbations and adopt specific structural arrangements within. Further characterization of these artificial systems may help in understanding the molecular processes that bring initial bilayer destabilizations to the eventual opening of a fusion pore.

The fusion core complex of the peste des petits ruminants virus is a six-helix bundle assembly

We describe the properties of the two heptad repeats (HR1 and HR2) of the Peste des petits ruminants virus (PPRV) fusion protein (F) to obtain insights into the mechanism by which these repeats influence PPRV-mediated cell fusion. Both HR1 and HR2 inhibit PPRV-mediated syncytia formation in Vero cells in vitro. Of these, HR2 was found to be more effective than HR1. We studied the mechanism of fusion inhibition by these two repeats by using various biophysical and biochemical methods either separately or together. CD spectral analysis of these repeats revealed that the alpha-helical content of HR1 and HR2 when used together is higher than that of their simulated spectrum in the mixture, suggesting the formation of a highly structured complex by these repeats. Protease protection assays confirmed that such a complex is highly stable. Electrospray mass spectrometry of protease-digested products of the HR1-HR2 complex showed protection of fragments corresponding to both HR1 and HR2 sequences involved in complex formation. By employing size-exclusion chromatography and chemical cross-linking experiments, we show that three units each of HR1 and HR2 form a complex in which HR1 is a trimer and HR2 is a monomer. Homology-based three-dimensional modeling of this complex showed that HR1 and HR2 together form a six-helix and trimeric coiled-coil bundle. In this model, the HR1 trimer forms the core whereas HR2, while interacting with HR1 in an antiparallel orientation, forms a two-stranded coiled-coil structure and lies at the periphery of the structure. These results are discussed in the context of a common fusion mechanism among paramyxoviruses.

Sendai virus N-terminal fusion peptide consists of two similar repeats, both of which contribute to membrane fusion

The N-terminal fusion peptide of Sendai virus F1 envelope glycoprotein is a stretch of 14 amino acids, most of which are hydrophobic. Following this region, we detected a segment of 11 residues that are strikingly similar to the N-terminal fusion peptide. We found that, when anchored to the membrane by palmitoylation of its N-terminus, this segment (WT-palm-19-33) induces membrane fusion of large unilamellar liposomes to almost the same extent as a segment that includes the N-terminal fusion peptide. The activity of WT-palm-19-33 was dependent on its specific sequence, as a palmitoylated peptide with the same amino-acid composition but a scrambled sequence was inactive. Interestingly, two mutations (G7A and G12A) known to increase F1- induced cell-cell fusion, also increased the homology between the N-terminal fusion peptide and WT-palm-19-33. The role of the amino-acid sequence on the fusogenicity, secondary structure, and mechanism of membrane fusion was analyzed by comparing a peptide comprising both homologous segments (WT 1-33), a G12A mutant (G12A 1-33), a G7A-G12A double mutant (G7A-G12A 1-33), and a peptide with a scrambled sequence (SC 1-33). Based on these experiments, we postulate that replacement of Gly 7 and Gly12 by Ala increases the alpha helical content of the N-terminal region, with a concomitant increase in its fusogenic activity. Furthermore, the dissimilar abilities of the different peptides to induce membrane negative curvature as well as to promote isotropic 31P NMR signals, suggest that these mutations might also alter the extent of membrane penetration of the 33-residue peptide. Interestingly, our results serve to explain the effect of the G7A and G12A mutations on the fusogenic activity of the parent F1 protein in vivo.

The HIV-1 gp41 N-terminal heptad repeat plays an essential role in membrane fusion

For many different enveloped viruses the crystal structure of the fusion protein core has been established. A striking conservation in the tertiary and quaternary arrangement of these core structures is repeatedly revealed among members of diverse families. It has been proposed that the primary role of the core involves structural rearrangements which facilitate apposition between viral and target cell membranes. Forming the internal trimeric coiled coil of the core, the N-terminal heptad repeat (NHR) of HIV-1 gp41 was suggested to have additional roles, due to its ability to bind biological membranes. The NHR is adjacent to the N-terminal hydrophobic fusion peptide (FP), which alone can fuse biological membranes. To investigate the role of the NHR in membrane fusion, we synthesized and functionally characterized HIV-1 gp41 peptides corresponding to the FP and NHR alone, as well as continuous peptides made of both FP and NHR (wild type and mutant). We show here that a consecutive, 70-residue peptide consisting of both the FP and NHR (gp41/1-70) has dramatic fusogenic properties. The effect of including the complete NHR, as compared to shorter 23-, 33-, or 52-residue N-terminal peptides, is illustrated by a leap in lipid mixing of phosphatidylcholine (PC) large unilamellar vesicles (LUV) and clearly delineates the synergistic role of the NHR in the fusion event. Furthermore, a mutation in the NHR that renders the virus noninfectious is reflected by a significant reduction in in vitro lipid mixing induced by the mutant, gp41/1-70 (I62D). Additional spectroscopic studies, characterizing membrane binding and apposition induced by the peptides, help to clarify the role of the NHR in membrane fusion.

Cellular membrane-binding ability of the C-terminal cytoplasmic domain of human immunodeficiency virus type 1 envelope transmembrane protein gp41

The amphipathic alpha-helices located in the cytoplasmic tail of the envelope (Env) transmembrane glycoprotein gp41 of human immunodeficiency virus type 1 have been implicated in membrane association and cytopathicity. Deletion of the last 12 amino acids in the C terminus of this domain severely impairs infectivity. However, the nature of the involvement of the cytoplasmic tail in Env-membrane interactions in cells and the molecular basis for the defect in infectivity of this mutant virus are still poorly understood. In this study we examined the interaction of the cytoplasmic tail with membranes in living mammalian cells by expressing a recombinant cytoplasmic tail fragment and an Escherichia coli beta-galactosidase/cytoplasmic tail fusion protein, both of them lacking gp120, the gp41 ectodomain, and the transmembrane region. We found through cell fractionation, in vivo membrane flotation, and confocal immunofluorescence studies that the cytoplasmic tail contained determinants to be routed to a perinuclear membrane region in cells. Further mapping showed that each of the three lentivirus lytic peptide (LLP-1, LLP-2, and LLP-3) sequences conferred this cellular membrane-targeting ability. Deletion of the last 12 amino acids from the C terminus abolished the ability of the LLP-1 motif to bind to membranes. High salt extraction, in vitro transcription and translation, and posttranslational membrane binding analyses indicated that the beta-galactosidase/LLP fusion proteins were inserted into membranes via the LLP sequences. Subcellular fractionation and confocal microscopy studies revealed that each of the LLP motifs, acting in a position-independent manner, targeted non-endoplasmic reticulum (ER)-associated beta-galactosidase and enhanced green fluorescence protein to the ER. Our study provides a basis for the involvement of the gp41 cytoplasmic tail during Env maturation and also supports the notion that the membrane apposition of the C-terminal cytoplasmic tail plays a crucial role in virus-host interaction.

Structure and interaction with membrane model systems of a peptide derived from the major epitope region of HIV protein gp41: implications on viral fusion mechanism

The HIV-1 gp41 envelope protein mediates entry of the virus into the target cell by promoting membrane fusion. With a view toward possible new insights into viral fusion mechanisms, we have investigated by infrared, fluorescence, and nuclear magnetic resonance spectroscopies and calorimetry a fragment of 19 amino acids corresponding to the immunodominant region of the gp41 ectodomain, a highly conserved sequence and major epitope. Information on the structure of the peptide both in solution and in the presence of model membranes, its incorporation and location in the phospholipid bilayer, and the modulation of the phase behavior of the membrane has been gathered. Here we demonstrate that the peptide binds and interacts with negatively charged phospholipids, changes its conformation in the presence of a membraneous medium, and induces leakage of vesicle contents as well as a new phospholipid phase. These characteristics might be important for the formation of the fusion-active gp41 core structure, promoting the close apposition of the two viral and target-cell membranes and therefore provoking fusion.

Participation of two fusion peptides in measles virus-induced membrane fusion: emerging similarity with other paramyxoviruses

Paramyxoviruses penetrate into their host cells by fusing their membranes with the plasma membrane. The hydrophobic N terminus of their F1 protein, termed the 'fusion peptide', is thought to be responsible for this process. Recently, an additional internal fusion peptide, homologous in sequence to the N-terminal fusion peptide of HIV-1, was identified in the Sendai virus F1 protein. Here, we investigated whether the presence of an additional internal fusion peptide is a general feature of paramyxoviridae. To this end, we synthesized and structurally and functionally characterized three peptides: (i) MV-197, which corresponds to an internal segment of the F1 protein of the measles virus (amino acids 197-225), homologous in location but not in sequence to the internal fusion peptide of the Sendai virus, (ii) Mu-MV-197, a randomized version of MV-197, and (iii) the 33 amino acid N-terminal fusion peptide of the measles virus. Remarkably, only MV-197 was highly fusogenic toward large unilamellar vesicles composed of either zwitterionic (phosphatidylcholine or phosphatidylcholine/sphingomyelin/cholesterol, a composition similar to that of human cell membranes) or negatively charged phospholipids. Binding experiments, circular dichroism spectroscopy in phospholipid membranes, and homo energy-transfer studies with fluorescently labeled peptides revealed that MV-197 adopts a predominant alpha-helical structure and shares properties similar to those reported for known fusion peptides. These results suggest that the presence of two fusion peptides in the F1 protein is a general feature of paramyxoviruses.

Sendai virus internal fusion peptide: structural and functional characterization and a plausible mode of viral entry inhibition

Viral glycoproteins catalyze the fusion between viral and cellular membranes. The fusion protein (F(1)) of Sendai virus has two fusion peptides. One is located at its N-terminus and the other, highly homologous to the HIV-1 and RSV fusion peptides, in the interior of the F(1) protein. A synthetic peptide corresponding to the internal fusogenic domain, namely, SV-201, was found to inhibit virus-cell fusion without interfering with the binding of the virus to the target cells, thus highlighting the importance of this region in Sendai virus-induced membrane fusion. However, its detailed mechanism of inhibition remains unknown. Here, we synthesized a shorter version of SV-201, namely, SV-208, an elongated one, SV-197, and two mutants of SV-201, and compared them functionally and structurally with SV-201. In contrast to SV-201, SV-208 and the two mutants do not inhibit virus-cell fusion. The differences in the oligomerization state of these peptides in aqueous solution and within the membrane, and in their ability to bind to Sendai virions, enabled us to postulate a possible mechanism of viral entry inhibition: SV-201 binds to its target in Sendai virions before the F(1) internal fusion peptide binds to the membrane, therefore blocking the F(1) conformational change required for fusion. In addition, we further characterized the fusogenic activity of the internal fusion peptide, compared to the N-terminal one, and determined its structure in the membrane-bound state by means of fluorescence, CD, and ATR-FTIR spectroscopy. Remarkably, we found that SV-201 and its elongated form, SV-197, are highly potent in inducing fusion of the highly stable large unilamellar vesicles composed of egg phosphatidylcholine, a property found only in an extended version of the HIV-1 fusion peptide. The inhibitory activity of SV-201 and its fusogenic ability are discussed in terms of the "umbrella" model of Sendai virus-induced membrane fusion.

Binding of a fluorescent dansylcadaverine-substance P analogue to negatively charged phospholipid membranes

We have investigated the binding of a new dansylcadaverine derivative of substance P (DNC-SP) with negatively charged small unilamellar vesicles composed of a mixture of phosphatidylcholine (PC) and either phosphatidylglycerol (PG) or phosphatidylserine (PS) using fluorescence spectroscopic techniques. The changes in fluorescence properties were used to obtain association isotherms at variable membrane negative charges and at different ionic strengths. The experimental association isotherms were analyzed using two binding approaches: (i) the Langmuir adsorption isotherm and the partition equilibrium model, that neglect the activity coefficients; and (ii) the partition equilibrium model combined with the Gouy-Chapman formalism that considers electrostatic effects. A consistent quantitative analysis of each DNC-SP binding curve at different lipid composition was achieved by means of the Gouy-Chapman approach using a peptide effective interfacial charge (v) value of (0.95 +/- 0.02), which is lower than the physical charge of the peptide. For PC/PG membranes, the partition equilibrium constant were 7.8 x 10(3) M(-1) (9/1, mol/mol) and 6.9 x 10(3) M(-1) (7/3, mol/mol), whereas for PC/PS membranes an average value of 6.8 x 10(3) M(-1) was estimated. These partition equilibrium constants were similar to those obtained for the interaction of DNC-SP with neutral PC membranes (4.9 x 10(3) M(-1)), as theoretically expected. We demonstrate that the v parameter is a determinant factor to obtain a unique value of the binding constant independently of the surface charge density of the vesicles. Also, the potential of fluorescent dansylated SP analogue in studies involving interactions with cell membranes is discussed.

Mutational analysis of the subgroup A avian sarcoma and leukosis virus putative fusion peptide domain

Short hydrophobic regions referred to as fusion peptide domains (FPDs) at or near the amino terminus of the membrane-anchoring subunit of viral glycoproteins are believed to insert into the host membrane during the initial stage of enveloped viral entry. Avian sarcoma and leukosis viruses (ASLV) are unusual among retroviruses in that the region in the envelope glycoprotein (EnvA) proposed to be the FPD is internal and contains a centrally located proline residue. To begin analyzing the function of this region of EnvA, 20 substitution mutations were introduced into the putative FPD. The mutant envelope glycoproteins were evaluated for effects on virion incorporation, receptor binding, and infection. Interestingly, most of the single-substitution mutations had little effect on any of these processes. In contrast, a bulky hydrophobic substitution for the central proline reduced viral titers 15-fold without affecting virion incorporation or receptor binding, whereas substitution of glycine for the proline had only a nominal effect on EnvA function. Similar to other viral FPDs, the putative ASLV FPD has been modeled as an amphipathic helix where most of the bulky hydrophobic residues form a patch on one face of the helix. A series of alanine insertion mutations designed to interrupt the hydrophobic patch on the helix had differential effects on infectivity, and the results of that analysis together with the results observed with the substitution mutations suggest no correlation between maintenance of the hydrophobic patch and glycoprotein function.

Paramyxovirus F1 protein has two fusion peptides: implications for the mechanism of membrane fusion

Viral fusion proteins contain a highly hydrophobic segment, named the fusion peptide, which is thought to be responsible for the merging of the cellular and viral membranes. Paramyxoviruses are believed to contain a single fusion peptide at the N terminus of the F1 protein. However, here we identified an additional internal segment in the Sendai virus F1 protein (amino acids 214-226) highly homologous to the fusion peptides of HIV-1 and RSV. A synthetic peptide, which includes this region, was found to induce membrane fusion of large unilamellar vesicles, at concentrations where the known N-terminal fusion peptide is not effective. A scrambled peptide as well as several peptides from other regions of the F1 protein, which strongly bind to membranes, are not fusogenic. The functional and structural characterization of this active segment suggest that the F1 protein has an additional internal fusion peptide that could participate in the actual fusion event. The presence of homologous regions in other members of the same family suggests that the concerted action of two fusion peptides, one N-terminal and the other internal, is a general feature of paramyxoviruses.

A peptide analogue to a fusion domain within photoreceptor peripherin/rds promotes membrane adhesion and depolarization

Photoreceptor peripherin/rds promotes membrane fusion, through a putative fusion domain located within the C-terminus (Boesze-Battaglia et al., Biochemistry 37 (1998) 9477-9487). A peptide analogue to this region, PP-5, competitively inhibits peripherin/rds mediated fusion in a cell free assay system. To characterize how this region is involved in the fusion process we investigated two of the individual steps in membrane fusion, membrane adhesion and membrane destabilization inferred from depolarization studies. Membrane depolarization was measured as the collapse of a valinomycin induced K(+) diffusion potential in model membranes, using a potential sensitive fluorescent probe, diS-C(2)-5. PP-5 induced membrane depolarization in a concentration dependent manner. PP-5 has been shown by Fourier transform infrared spectroscopy to be an amphiphilic alpha-helix. Therefore, the requirement for an amphiphilic alpha-helix to promote depolarization was tested using two mutant peptides designed to disrupt either the amphiphilic nature of PP-5 (PP-5AB) or the alpha-helical structure (PP-5HB). PP-5AB inhibited PP-5 induced depolarization when added in an equimolar ratio to PP-5. Neither mutant peptide alone or in combination with PP-5 had any effect on calcium dependent vesicle aggregation. Using non-denaturing gel electrophoresis and size exclusion chromatography techniques PP-5 was shown to form a tetrameric complex. Equimolar mixtures of PP-5 and PP-5AB formed a heterotetramer which was unable to promote membrane depolarization. The hypothesis that PP-5 tetramers promote membrane depolarization is consistent with the calculated Hill coefficient of 3.725, determined from a Hill analysis of the depolarization data.

Selective destabilization of acidic phospholipid bilayers performed by the hepatitis B virus fusion peptide

A peptide corresponding to the N-terminal region of the S protein of hepatitis B virus (Met-Glu-Asn-Ile-Thr-Ser-Gly-Phe-Leu-Gly-Pro-Leu-Leu-Val-Leu-Gln) has been previously demonstrated to perform aggregation and destabilization of acidic liposome bilayers and to adopt a highly stable beta-sheet conformation in the presence of phospholipids. The changes in the lipid moiety produced by this peptide have been followed by fluorescence depolarization and electron microscopy. The later was employed to determine the size and shape of the peptide-vesicle complexes, showing the presence of highly aggregated and fused structures only when negatively charged liposomes were employed. 1,6-Diphenyl-1,3,5-hexatriene depolarization measurements showed that the interaction of the peptide with both negatively charged and zwitterionic liposomes was accompanied by a substantial reduction of the transition amplitude without affecting the temperature of the gel-to-liquid crystalline phase transition. These data are indicative of the peptide insertion inside the bilayer of both types of liposomes affecting the acyl chain order, though only the interaction with acidic phospholipids leads to aggregation and fusion. This preferential destabilization of the peptide towards negatively charged phospholipids can be ascribed to the electrostatic interactions between the peptide and the polar head groups, as monitored by 1-(4-(trimethylammoniumphenyl)-6-phenyl-1,3, 5-hexatriene fluorescence depolarization analysis.

Characterization of the sequence of interactions of the fusion domain of the simian immunodeficiency virus with membranes. Role of the membrane dipole potential

The simian immunodeficiency virus fusion peptide constitutes a 12-residue N-terminal segment of the gp32 protein that is involved in the fusion between the viral and cellular membranes, facilitating the penetration of the virus in the host cell. Simian immunodeficiency virus fusion peptide is a hydrophobic peptide that in Me(2)SO forms aggregates that contain beta-sheet pleated structures. When added to aqueous media the peptide forms large colloidal aggregates. In the presence of lipidic membranes, however, the peptide interacts with the membranes and causes small changes of the membrane electrostatic potential as shown by fluorescein phosphatidylethanolamine fluorescence. Thioflavin T fluorescence and Fourier transformed infrared spectroscopy measurements reveal that the interaction of the peptide with the membrane bilayer results in complete disassembly of the aggregates originating from an Me(2)SO stock solution. Above a lipid/peptide ratio of about 5, the membrane disaggregation and water precipitation processes become dependent on the absolute peptide concentration rather than on the lipid/peptide ratio. A schematic mechanism is proposed, which sheds light on how peptide-peptide interactions can be favored with respect to peptide-lipid interactions at various lipid/peptide ratios. These studies are augmented by the use of the fluorescent dye 1-(3-sulfonatopropyl)-4-[beta[2-(di-n-octylamino)-6-naphthyl]vinyl ] pyridinium betaine that shows the interaction of the peptide with the membranes has a clear effect on the magnitude of the so-called dipole potential that arises from dipolar groups located on the lipid molecules and oriented water molecules at the membrane-water interface. It is shown that the variation of the membrane dipole potential affects the extent of the membrane fusion caused by the peptide and implicates the dipolar properties of membranes in their fusion.

Direct evidence that the N-terminal heptad repeat of Sendai virus fusion protein participates in membrane fusion

Recent studies have demonstrated the importance of heptad repeat regions within envelope proteins of viruses in mediating conformational changes at various stages of viral infection. However, it is not clear if heptad repeats have a direct role in the actual fusion event. Here we have synthesized, fluorescently labeled and functionally and structurally characterized a wild-type 70 residue peptide (SV-117) composed of both the fusion peptide and the N-terminal heptad repeat of Sendai virus fusion protein, two of its mutants, as well as the fusion peptide and heptad repeat separately. One mutation was introduced in the fusion peptide (G119K) and another in the heptad repeat region (I154K). Similar mutations have been shown to drastically reduce the fusogenic ability of the homologous fusion protein of Newcastle disease virus. We found that only SV-117 was active in inducing lipid mixing of egg phosphatidylcholine/phosphatidyiglycerol (PC/PG) large unilamellar vesicles (LUV), and not the mutants nor the mixture of the fusion peptide and the heptad repeat. Functional characterization revealed that SV-117, and to a lesser extent its two mutants, were potent inhibitors of Sendai virus-mediated hemolysis of red blood cells, while the fusion peptide and SV-150 were negligibly active alone or in a mixture. Hemagglutinin assays revealed that none of the peptides disturb the binding of virions to red blood cells. Further studies revealed that SV-117 and its mutants oligomerize similarly in solution and in membrane, and have similar potency in inducing vesicle aggregation. Circular dichroism and FTIR spectroscopy revealed a higher helical content for SV-117 compared to its mutants in 40 % tifluorethanol and in PC/PG multibilayer membranes, respectively, ATR-FTIR studies indicated that SV-117 lies more parallel with the surface of the membrane than its mutants. These observations suggest a direct role for the N-terminal heptad repeat in assisting the fusion peptide in mediating membrane fusion.

Effect of nonpolar substitutions of the conserved Phe11 in the fusion peptide of HIV-1 gp41 on its function, structure, and organization in membranes

The fusion domain of the HIV-1 envelope glycoprotein (gp120-gp41) is a conserved hydrophobic region located at the N-terminus of the transmembrane subunit (gp41). A prominent feature of this domain is a conserved five-residue "FLGFL" sequence at positions 8-12. Mutation of the highly conserved Phe(11) to Val (F11V), presumed not to significantly affect the hydrophobicity and the structure of this region, has been shown to decrease the level of syncytium formation and virus infectivity. Here we show that the substitution of Gly for Phe(11) (F11G) reduces cell-cell fusion activity by 80-90%. To determine the effect of these mutations on the properties of the fusion peptide, a 33-residue peptide (WT) identical to the extended fusion domain and its F11V and F11G mutants were synthesized, fluorescently labeled, and studied with respect to their function, structure, and organization in phospholipid membranes. The WT peptide alone induced fusion of both zwitterionic (PC/Chol) and negatively charged (PS/PC/Chol and POPG) vesicles, in contrast to a 23-mer fusion peptide lacking the C-terminal domain which has been shown to be inactive with PC vesicles but able to induce fusion of POPG vesicles which had been preaggragated with Ca(2+) or Mg(2+). The F11V peptide preserved 50% activity, and the F11G peptide was virtually inactive. ATR-FTIR spectroscopy indicated similar secondary structure of the peptides in multibilayers that was independent of membrane composition. Furthermore, all the peptides increased the extent of lipid disorder to a similar extent, but the kinetics of amide II H to D exchange was in the following order: F11G > F11V > WT. Fluorescence studies in the presence of membranes, as well as SDS-PAGE, revealed that the WT and F11V peptides self-associate to similar levels while F11G exhibited a decreased level of self-association. The data suggest that the FLGFL motif contributes to the functional organization of the HIV-1 fusion peptide and that the C-terminal domain following the fusion peptide contributes to the membrane fusion process.

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.

Ultrastructural characterization of peptide-induced membrane fusion and peptide self-assembly in the lipid bilayer

The peptide sequence B18, derived from the membrane-associated sea urchin sperm protein bindin, triggers fusion between lipid vesicles. It exhibits many similarities to viral fusion peptides and may have a corresponding function in fertilization. The lipid-peptide and peptide-peptide interactions of B18 are investigated here at the ultrastructural level by electron microscopy and x-ray diffraction. The histidine-rich peptide is shown to self-associate into two distinctly different supramolecular structures, depending on the presence of Zn(2+), which controls its fusogenic activity. In aqueous buffer the peptide per se assembles into beta-sheet amyloid fibrils, whereas in the presence of Zn(2+) it forms smooth globular clusters. When B18 per se is added to uncharged large unilamellar vesicles, they become visibly disrupted by the fibrils, but no genuine fusion is observed. Only in the presence of Zn(2+) does the peptide induce extensive fusion of vesicles, which is evident from their dramatic increase in size. Besides these morphological changes, we observed distinct fibrillar and particulate structures in the bilayer, which are attributed to B18 in either of its two self-assembled forms. We conclude that membrane fusion involves an alpha-helical peptide conformation, which can oligomerize further in the membrane. The role of Zn(2+) is to promote this local helical structure in B18 and to prevent its inactivation as beta-sheet fibrils.

Contribution of the hydrophobicity gradient to the secondary structure and activity of fusogenic peptides

Fusogenic peptides belong to a class of helical amphipathic peptides characterized by a hydrophobicity gradient along the long helical axis. According to the prevailing theory regarding the mechanism of action of fusogenic peptides, this hydrophobicity gradient causes the tilted insertion of the peptides in membranes, thus destabilizing the lipid core and, thereby, enhancing membrane fusion. To assess the role of the hydrophobicity gradient upon the fusogenic activity, two of these fusogenic peptides and several variants were synthesized. The LCAT-(57-70) peptide, which is part of the sequence of the lipolytic enzyme lecithin cholesterol acyltransferase, forms stable beta-sheets in lipids, while the apolipoprotein A-II (53-70) peptide remains predominantly helical in membranes. The variant peptides were designed through amino acid permutations, to be either parallel, perpendicular, or to retain an oblique orientation relative to the lipid-water interface. Peptide-induced vesicle fusion was monitored by lipid-mixing experiments, using fluorescent probes, the extent of peptide-lipid association, the conformation of lipid-associated peptides and their orientation in lipids, were studied by Fourier Transformed Infrared Spectroscopy. A comparison of the properties of the wild-type and variant peptides shows that the hydrophobicity gradient, which determines the orientation of helical peptides in lipids and their fusogenic activity, further influences the secondary structure and lipid binding capacity of these peptides.

Membrane-induced step in the activation of Sendai virus fusion protein

Peptides derived from conserved heptad-repeat regions of several viruses have been shown recently to inhibit virus-cell fusion. To find out their possible role in the fusion process, two biologically active heptad-repeat segments of the fusion protein (F) of Sendai virus, SV-150 (residues 150-186), and SV-473 (residues 473-495) were synthesized, fluorescently labeled and spectroscopically characterized for their structure and organization in solution and within the membrane. SV-150 was found to be 50-fold less active than SV-473 in inhibiting Sendai virus-cell fusion. Circular dichroism (CD) spectroscopy revealed that in aqueous solution, the peptides are self-associated and adopt low alpha-helical structure. However, when the two peptides are mixed together, their alpha-helical content significantly increases. Fluorescence studies, CD, and polarized attenuated total reflection infrared (ATR-FTIR) spectroscopy showed that both peptides, alone or as a complex, bind strongly to negatively charged and zwitterionic phospholipid membranes, dissociate therein into alpha-helical monomers, but do not perturb the lipid packing of the membrane. The ability of the peptides to interact with each other in solution may be correlated with antiviral activity, whereas their ability to interact with the membrane, together with their location near the fusion peptide and the transmembrane domain, suggests a revision to the currently accepted model for viral-induced membrane fusion. In the revised model, in the sequence of events associated with viral entry, the two heptad-repeat sequences may assist in bringing the viral and cellular membranes closer, thus facilitating membrane fusion.

Lipid headgroup spacing and peptide penetration, but not peptide oligomerization, modulate peptide-induced fusion

In this study, the mechanism by which an amphipathic negatively charged peptide consisting of 11 amino acids (WAE) induces fusion of liposomal phosphatidylcholine membranes is investigated. WAE-induced fusion, which only occurs when the peptide is covalently attached to the bilayer, shows a highly remarkable dependence on naturally occurring phosphatidylcholine species. The initial rate of fusion increased in the order 1-palmitoyl 2-arachidonoyl PC (PAPC) > 1-palmitoyl 2-oleoyl PC (POPC) > 1-stearoyl 2-oleoyl PC (SOPC) > dioleoyl PC (DOPC) > egg yolk PC. Interestingly, the susceptibility of the various PC species toward WAE-induced fusion matched a similar order of increase in intrinsic lipid headgroup spacing of the target membrane. The degree of spacing, in turn, was found to be related to the extent by which the fluorescence quantum yield of the Trp residue increased, which occurred upon the interaction of WAE with target membranes. Therefore, these results demonstrate an enhanced ability for WAE to engage in hydrophobic interactions when headgroup spacing increases. Thus, this latter parameter most likely regulates the degree of penetration of WAE into the target membrane. Apart from penetrating, WAE oligomerizes at the site of fusion as revealed by monitoring the self-quenching of the fluorescently derivatized lipid anchor to which WAE is attached. Clustering appears specifically related to the process of membrane fusion and not membrane aggregation. This is indicated by the fact that fusion and clustering, but not aggregation, display the same strict temperature dependence. However, evidence is presented indicating that clustering is an accompanying event rather than a prerequisite for fusion. The notion that various biologically relevant fusion phenomena are accompanied by protein clustering and the specific PC-species-dependent regulation of membrane fusion emphasize the biological significance of the peptide in serving as a model for investigating mechanisms of protein-induced fusion.

Structure-function study of a heptad repeat positioned near the transmembrane domain of Sendai virus fusion protein which blocks virus-cell fusion

A synthetic heptad repeat, SV-473, derived from Sendai virus fusion protein is a potent inhibitor of virus-cell fusion. In order to understand the mechanism of the inhibitory effect, we synthesized and fluorescently labeled SV-465, an extended version of SV-473 by one more heptad, its mutant peptide A17,24-SV-465, in which two heptadic leucines were substituted with two alanines, and its enatiomer D-SV-465, composed entirely of Damino acids. Similar mutations in the homologous fusion protein of the Newcastle disease virus drastically reduced its activity. The data revealed that SV-465, but not A17,24-SV-465 or its enantiomer, is highly active in inhibiting Sendai virus-induced hemolysis of red blood cells. None of the peptides interfere with the binding of virions to the target red blood cells as demonstrated by hemagglutinin assay. Fluorescence and circular dichroism (CD) spectroscopy indicated that: (i) only SV-465 could self-assemble in aqueous environment; (ii) only SV-465 could co-assemble with two other biologically active heptad repeats derived from Sendai virus fusion protein; (iii) SV-465 has a higher helical content than A17,24-SV-465 in solution, and (iv) all the peptides bind strongly to zwitterionic and negatively charged phospholipids. Polarized attenuated total reflection infrared spectroscopy revealed that they bound as monomers onto the surface of zwitterionic membranes with predominantly alpha-helical structures. The functional role of the amino acid 465-497 domain in Sendai virus-mediated membrane fusion is discussed in light of these findings.

Enhanced efficiency of a targeted fusogenic peptide

Membrane targeting was investigated as a potential strategy to increase the fusogenic activity of an isolated fusion peptide. This was achieved by coupling the fusogenic carboxy-terminal part of the beta-amyloid peptide (Abeta, amino acids 29-40), involved in Alzheimer's disease, to a positively charged peptide (PIP2-binding peptide, PBP) interacting specifically with a naturally occurring negatively charged phospholipid, phosphatidylinositol 4, 5-bisphosphate (PIP2). Peptide-induced vesicle fusion was spectroscopically evidenced by: (i) mixing of membrane lipids, (ii) mixing of aqueous vesicular contents, and (iii) an irreversible increase in vesicle size, at concentrations five to six times lower than the Abeta(29-40) peptide. In contrast, at these concentrations the PBP-Abeta(29-40) peptide did not display any significant activity on neutral vesicles, indicating that negatively charged phospholipids included as targets in the membranes, are required to compensate for the lower hydrophobicity of this peptide. When the alpha-helical structure of the chimeric peptide was induced by dissolving it in trifluoroethanol, an increase of the fusogenic potential of the peptide was observed, supporting the hypothesis that the alpha-helical conformation of the peptide is crucial to trigger the lipid-peptide interaction. The specificity of the interaction between PIP2 and the PBP moiety, was shown by the less efficient targeting of the chimeric peptide to membranes charged with phosphatidylserine. These data thus demonstrate that the specific properties of both the Abeta(29-40) and the PBP peptide are conserved in the chimeric peptide, and that a synergetic effect is reached through chemical linkage of these two fragments.

A synthetic all D-amino acid peptide corresponding to the N-terminal sequence of HIV-1 gp41 recognizes the wild-type fusion peptide in the membrane and inhibits HIV-1 envelope glycoprotein-mediated cell fusion

Recent studies demonstrated that a synthetic fusion peptide of HIV-1 self-associates in phospholipid membranes and inhibits HIV-1 envelope glycoprotein-mediated cell fusion, presumably by interacting with the N-terminal domain of gp41 and forming inactive heteroaggregates [Kliger, Y., Aharoni, A., Rapaport, D., Jones, P., Blumenthal, R. & Shai, Y. (1997) J. Biol. Chem. 272, 13496-13505]. Here, we show that a synthetic all D-amino acid peptide corresponding to the N-terminal sequence of HIV-1 gp41 (D-WT) of HIV-1 associates with its enantiomeric wild-type fusion (WT) peptide in the membrane and inhibits cell fusion mediated by the HIV-1 envelope glycoprotein. D-WT does not inhibit cell fusion mediated by the HIV-2 envelope glycoprotein. WT and D-WT are equally potent in inducing membrane fusion. D-WT peptide but not WT peptide is resistant to proteolytic digestion. Structural analysis showed that the CD spectra of D-WT in trifluoroethanol/water is a mirror image of that of WT, and attenuated total reflectance-fourier transform infrared spectroscopy revealed similar structures and orientation for the two enantiomers in the membrane. The results reveal that the chirality of the synthetic peptide corresponding to the HIV-1 gp41 N-terminal sequence does not play a role in liposome fusion and that the peptides' chirality is not necessarily required for peptide-peptide interaction within the membrane environment. Furthermore, studies along these lines may provide criteria to design protease-resistant therapeutic agents against HIV and other viruses.

A peptide derived from a conserved domain of Sendai virus fusion protein inhibits virus-cell fusion. A plausible mode of action

SV-201, a peptide derived from a conserved and potentially amphipathic region (amino acids 201-229) in the Sendai virus ectodomain, specifically inhibited virus-mediated hemolysis only when added to virions prior to their attachment to red blood cells. Sendai virus-mediated hemagglutinin assay in the presence of SV-201 demonstrated that the peptide does not disturb the binding of virions to the target red blood cells. A mutated peptide with 2 amino acids substitution, rendering the peptide neutral, was biologically inactive. A second mutant with 7 amino acids randomized at the N terminus keeping the hydrophobicity of the peptide unaltered was only slightly active. A hydrophobic peptide corresponding to the fusion peptide domain was also inactive. SV-201, the two mutants, and the fusion peptide bind similarly with high affinity to both negatively charged phosphatidylserine/phosphatidylcholine and zwitterionic phosphatidylcholine lipid vesicles, suggesting that the inhibitory effect is not due merely to membrane modulation. Fluorescence studies with rhodamine-labeled peptides and SV-201-induced inhibition assays, demonstrated that the SV-201 binding site is most probably located in the region corresponding to amino acids 201-229 of the Sendai virus fusion protein. The data presented here suggest that SV-201 disturbs a functional domain in the Sendai virus fusion protein, which is most probably associated with the assembly of the fusion protein and/or membrane apposition. The existence of homologous SV-201 regions in other viruses suggests that these regions may have a similar role, and their synthetic counterparts may act as inhibitors for the corresponding viruses.

Membrane fusion induced by 11-mer anionic and cationic peptides: a structure-function study

We recently demonstrated that an amphipathic net-negatively charged peptide consisting of 11 amino acids (WAE 11) strongly promotes fusion of large unilamellar liposomes (LUV) when anchored to a liposomal membrane [Pecheur, E. I., Hoekstra, D., Sainte-Marie, J., Maurin, L., Bienvenue, A., and Philippot, J. R. (1997) Biochemistry 36, 3773-3781]. To elucidate a potential relationship between peptide structure and its fusogenic properties and to test the hypothesis that specific structural motifs are a prerequisite for WAE-induced fusion, three 11-mer WAE-peptide analogues (WAK, WAEPro, and WAS) were synthesized and investigated for their structure and fusion activity. Structural analysis of the synthetic peptides by infrared attenuated total reflection spectroscopy reveals a distinct propensity of each peptide toward a helical structure after their anchorage to a liposomal surface, emphasizing the importance of anchorage on conveying a secondary structure, thereby conferring fusogenicity to these peptides. However, whereas WAE and WAK peptides displayed an essentially nonleaky fusion process, WAS- and WAEPro-induced fusion was accompanied by substantial leakage. It appears that peptide helicity as such is not a sufficient condition to convey optimal fusion properties to these 11-mer peptides. Studies of changes in the intrinsic Trp fluorescence and iodide quenching experiments were carried out and revealed the absence of migration of the Trp residue of WAS and WAEPro to a hydrophobic environment, upon their interaction with the target membranes. These results do not support the penetration of both peptides as their mode of membrane interaction and destabilization but rather suggest their folding along the vesicle surface, posing them as surface-seeking helixes. This is in striking contrast to the behavior observed for WAE and WAK, for which at least partial penetration of the Trp residue was demonstrated. These results indicate that subtle differences in the primary sequence of a fusogenic peptide could induce dramatic changes in the way the peptide interacts with a bilayer, culminating in equally drastic changes in their functional properties. The data also reveal a certain degree of sequence specificity in WAE-induced fusion.

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.