The ectodomain of HA2 of influenza virus promotes rapid pH dependent membrane fusion

M6PR interacts with the HA2 subunit of influenza A virus to facilitate the fusion of viral and endosomal membranes

Influenza A virus (IAV) commandeers numerous host cellular factors for successful replication. However, very few host factors have been revealed to be involved in the fusion of viral envelope and late endosomal membranes. In this study, we identified cation-dependent mannose-6-phosphate receptor (M6PR) as a crucial host factor for the replication of IAV. We found that siRNA knockdown of M6PR expression significantly reduced the growth titers of different subtypes of IAV, and that the inhibitory effect of M6PR siRNA treatment on IAV growth was overcome by the complement of exogenously expressed M6PR. When A549 cells were treated with siRNA targeting M6PR, the nuclear accumulation of viral nucleoprotein (NP) was dramatically inhibited at early timepoints post-infection, indicating that M6PR engages in the early stage of the IAV replication cycle. By investigating the role of M6PR in the individual entry and post-entry steps of IAV replication, we found that the downregulation of M6PR expression had no effect on attachment, internalization, early endosome trafficking, or late endosome acidification. However, we found that M6PR expression was critical for the fusion of viral envelope and late endosomal membranes. Of note, M6PR interacted with the hemagglutinin (HA) protein of IAV, and further studies showed that the lumenal domain of M6PR and the ectodomain of HA2 mediated the interaction and directly promoted the fusion of the viral and late endosomal membranes, thereby facilitating IAV replication. Together, our findings highlight the importance of the M6PR-HA interaction in the fusion of viral and late endosomal membranes during IAV replication.

The scientific adventures of Richard Epand

This essay summarizes the many areas of science that my career has contributed to. It attempts to highlight some of the innovative concepts that developed from this work. The discussion encompasses studies I undertook from graduate school to the present but it will not attempt to be comprehensive. I apologize to individuals whose work I omitted. Because of space I cannot acknowledge all the contributions from other individuals that made these achievements possible.

Nanodisc-Mediated Conversion of Virustatic Antiviral Antibody to Disrupt Virus Envelope in Infected Cells

Many antibody-based antivirals, including broadly neutralizing antibodies (bnAbs) against various influenza virus strains, suffer from limited potency. A booster of the antiviral activity of an antibody is expected to facilitate development of antiviral therapeutics. In this study, a nanodisc (ND), a discoidal lipid bilayer encircled by membrane scaffold proteins, is engineered to provide virucidal properties to antibodies, thereby augmenting their antiviral activity. NDs carrying the Fc-binding peptide sequence form an antibody-ND complex (ANC), which can co-endocytose into cells infected with influenza virus. ANC efficiently inhibits endosome escape of viral RNA by dual complimentary mode of action. While the antibody moiety in an ANC inhibits hemagglutinin-mediated membrane fusion, its ND moiety destroys the viral envelope using free hemagglutinins that are not captured by antibodies. Providing virus-infected host cells with the ability to self-eliminate by the synergistic effect of ANC components dramatically amplifies the antiviral efficacy of a bnAb against influenza virus. When the efficacy of ANC is assessed in mouse models, administration of ANCs dramatically reduces morbidity and mortality compared to bnAb alone. This study is the first to demonstrate the novel nanoparticle ANC and its role in combating viral infections, suggesting that ANC is a versatile platform applicable to various viruses.

Membrane-Bound Configuration and Lipid Perturbing Effects of Hemagglutinin Subunit 2 N-Terminus Investigated by Computer Simulations

Hemagglutinin (HA) mediated fusion of influenza virus envelope with host lipid membrane is a critical step warrantying virus entry to the cell. Despite tremendous advances in structural biology methods, the knowledge concerning the details of HA2 subunit insertion into the target membrane and its subsequent bilayer perturbing effect is still rather limited. Herein, based on a set of molecular dynamics simulations, we investigate the structure and interaction with lipid membrane of the N-terminal HA2 region comprising a trimer of fusion peptides (HAfps) tethered by flexible linkers to a fragment of coiled-coil stem structure. We find that, prior to insertion into the membrane, HAfps within the trimers do not sample space individually but rather associate into a compact hydrophobic aggregate. Once within the membrane, they fold into tight helical hairpins, which remain at the lipid-water interface. However, they can also assume stable, membrane-spanning configurations of significantly increased membrane-perturbing potential. In this latter case, HAfps trimers centre around the well-hydrated transmembrane channel-forming distinct, symmetric assemblies, whose wedge-like shape may play a role in promoting membrane curvature. We also demonstrate that, following HAfps insertion, the coiled-coil stem spontaneously tilts to almost membrane-parallel orientation, reflecting experimentally observed configuration adopted in the course of membrane fusion by complete HA2 units at the rim of membrane contact zones.

Effect of pH on the influenza fusion peptide properties unveiled by constant-pH molecular dynamics simulations combined with experiment

The influenza virus fusion process, whereby the virus fuses its envelope with the host endosome membrane to release the genetic material, takes place in the acidic late endosome environment. Acidification triggers a large conformational change in the fusion protein, hemagglutinin (HA), which enables the insertion of the N-terminal region of the HA2 subunit, known as the fusion peptide, into the membrane of the host endosome. However, the mechanism by which pH modulates the molecular properties of the fusion peptide remains unclear. To answer this question, we performed the first constant-pH molecular dynamics simulations of the influenza fusion peptide in a membrane, extending for 40 µs of aggregated time. The simulations were combined with spectroscopic data, which showed that the peptide is twofold more active in promoting lipid mixing of model membranes at pH 5 than at pH 7.4. The realistic treatment of protonation introduced by the constant-pH molecular dynamics simulations revealed that low pH stabilizes a vertical membrane-spanning conformation and leads to more frequent contacts between the fusion peptide and the lipid headgroups, which may explain the increase in activity. The study also revealed that the N-terminal region is determinant for the peptide’s effect on the membrane.

2H nuclear magnetic resonance spectroscopy supports larger amplitude fast motion and interference with lipid chain ordering for membrane that contains β sheet human immunodeficiency virus gp41 fusion peptide or helical hairpin influenza virus hemagglutinin fusion peptide at fusogenic pH

Enveloped viruses are surrounded by a membrane which is obtained from an infected host cell during budding. Infection of a new cell requires joining (fusion) of the virus and cell membranes. This process is mediated by a monotopic viral fusion protein with a large ectodomain outside the virus. The ectodomains of class I enveloped viruses have a N-terminal "fusion peptide" (fp) domain that is critical for fusion and binds to the cell membrane. In this study, ²H NMR spectra are analyzed for deuterated membrane with fp from either HIV gp41 (GP) or influenza hemagglutinin (HA) fusion proteins. In addition, the HAfp samples are studied at more fusogenic pH 5 and less fusogenic pH 7. GPfp adopts intermolecular antiparallel β sheet structure whereas HAfp is a monomeric helical hairpin. The data are obtained for a set of temperatures between 35 and 0 °C using DMPC-d54 lipid with perdeuterated acyl chains. The DMPC has liquid-crystalline (L_α) phase with disordered chains at higher temperature and rippled gel (P_β') or gel phase (L_β') with ordered chains at lower temperature. At given temperature T, the no peptide and HAfp, pH 7 samples exhibit similar spectral lineshapes. Spectral broadening with reduced temperature correlates with the transition from L_α to P_β' and then L_β' phases. At given T, the lineshapes are narrower for HAfp, pH 5 vs. no peptide and HAfp, pH 7 samples, and even narrower for the GPfp sample. These data support larger-amplitude fast (>10⁵ Hz) lipid acyl chain motion for samples with fusogenic peptides, and peptide interference with chain ordering. The NMR data of the present paper correlate with insertion of these peptides into the hydrocarbon core of the membrane and support a significant fusion contribution from the resultant lipid acyl chain disorder, perhaps because of reduced barriers between the different membrane topologies in the fusion pathway. Membrane insertion and lipid perturbation appear common to both β sheet and helical hairpin peptides.

Virucidal nano-perforator of viral membrane trapping viral RNAs in the endosome

Membrane-disrupting agents that selectively target virus versus host membranes could potentially inhibit a broad-spectrum of enveloped viruses, but currently such antivirals are lacking. Here, we develop a nanodisc incorporated with a decoy virus receptor that inhibits virus infection. Mechanistically, nanodiscs carrying the viral receptor sialic acid bind to influenza virions and are co-endocytosed into host cells. At low pH in the endosome, the nanodiscs rupture the viral envelope, trapping viral RNAs inside the endolysosome for enzymatic decomposition. In contrast, liposomes containing a decoy receptor show weak antiviral activity due to the lack of membrane disruption. The nanodiscs inhibit influenza virus infection and reduce morbidity and mortality in a mouse model. Our results suggest a new class of antivirals applicable to other enveloped viruses that cause irreversible physical damage specifically to virus envelope by viruses' own fusion machine. In conclusion, the lipid nanostructure provides another dimension for antiviral activity of decoy molecules.

The Stabilities of the Soluble Ectodomain and Fusion Peptide Hairpins of the Influenza Virus Hemagglutinin Subunit II Protein Are Positively Correlated with Membrane Fusion

Cellular entry of influenza virus is mediated by the viral protein hemagglutinin (HA), which forms an initial complex of three HA1 and three HA2 subunits. Each HA2 includes a fusion peptide (FP), a soluble ectodomain (SE), and a transmembrane domain. HA1 binds to cellular sialic acids, followed by virus endocytosis, pH reduction, dissociation of HA1, and structural rearrangement of HA2 into a final trimer-of-SE hairpins. A decrease in pH also triggers HA2-mediated virus/endosome membrane fusion. SE hairpins have an interior parallel helical bundle and C-terminal strands in the grooves of the exterior of the bundle. FPs are separate helical hairpins. This study compares wild-type HA2 (WT-HA2) with G1E(FP) and I173E(SE strand) mutants. WT-HA2 induces vesicle fusion at pH 5.0, whereas the extent of fusion is greatly reduced for both mutants. Circular dichroism for HA2 and FHA2≡FP+SE constructs shows dramatic losses of stability for the mutants, including a T_m reduced by 40 °C for I173E-FHA2. This is evidence of destabilization of SE hairpins via dissociation of strands from the helical bundle, which is also supported by larger monomer fractions for mutant versus WT proteins. The G1E mutant may have disrupted FP hairpins, with consequent non-native FP binding to dissociated SE strands. It is commonly proposed that free energy released by the HA2 structural rearrangement catalyzes HA-mediated fusion. This study supports an alternate mechanistic model in which fusion is preceded by FP insertion in the target membrane and formation of the final SE hairpin. Less fusion by the mutants is due to the loss of hairpin stability and consequent reduced level of membrane apposition of the virus and target membranes.

Full-length trimeric influenza virus hemagglutinin II membrane fusion protein and shorter constructs lacking the fusion peptide or transmembrane domain: Hyperthermostability of the full-length protein and the soluble ectodomain and fusion peptide make significant contributions to fusion of membrane vesicles

Influenza virus is a class I enveloped virus which is initially endocytosed into a host respiratory epithelial cell. Subsequent reduction of the pH to the 5-6 range triggers a structural change of the viral hemagglutinin II (HA2) protein, fusion of the viral and endosomal membranes, and release of the viral nucleocapsid into the cytoplasm. HA2 contains fusion peptide (FP), soluble ectodomain (SE), transmembrane (TM), and intraviral domains with respective lengths of ∼ 25, ∼ 160, ∼ 25, and ∼ 10 residues. The present work provides a straightforward protocol for producing and purifying mg quantities of full-length HA2 from expression in bacteria. Biophysical and structural comparisons are made between full-length HA2 and shorter constructs including SHA2 ≡ SE, FHA2 ≡ FP+SE, and SHA2-TM ≡ SE+TM constructs. The constructs are helical in detergent at pH 7.4 and the dominant trimer species. The proteins are highly thermostable in decylmaltoside detergent with Tm>90 °C for HA2 with stabilization provided by the SE, FP, and TM domains. The proteins are likely in a trimer-of-hairpins structure, the final protein state during fusion. All constructs induce fusion of negatively-charged vesicles at pH 5.0 with much less fusion at pH 7.4. Attractive protein/vesicle electrostatics play a role in fusion, as the proteins are positively-charged at pH 5.0 and negatively-charged at pH 7.4 and the pH-dependence of fusion is reversed for positively-charged vesicles. Comparison of fusion between constructs supports significant contributions to fusion from the SE and the FP with little effect from the TM.

Complete dissociation of the HIV-1 gp41 ectodomain and membrane proximal regions upon phospholipid binding

The envelope glycoprotein gp41 mediates the process of membrane fusion that enables entry of the HIV-1 virus into the host cell. Strong lipid affinity of the ectodomain suggests that its heptad repeat regions play an active role in destabilizing membranes by directly binding to the lipid bilayers and thereby lowering the free-energy barrier for membrane fusion. In such a model, immediately following the shedding of gp120, the N-heptad and C-heptad helices dissociate and melt into the host cell and viral membranes, respectively, pulling the destabilized membranes into juxtaposition, ready for fusion. Post-fusion, reaching the final 6-helix bundle (6 HB) conformation then involves competition between intermolecular interactions needed for formation of the symmetric 6 HB trimer and the membrane affinity of gp41's ectodomain, including its membrane-proximal regions. Our solution NMR study of the structural and dynamic properties of three constructs containing the ectodomain of gp41 with and without its membrane-proximal regions suggests that these segments do not form inter-helical interactions until the very late steps of the fusion process. Interactions between the polar termini of the heptad regions, which are not associating with the lipid surface, therefore may constitute the main driving force initiating formation of the final post-fusion states. The absence of significant intermolecular ectodomain interactions in the presence of dodecyl phosphocholine highlights the importance of trimerization of gp41's transmembrane helix to prevent complete dissociation of the trimer during the course of fusion.

A nonfusogenic antigen mimic of influenza hemagglutinin glycoproteins constituted with soluble full-length HA1 and truncated HA2 proteins expressed in E. coli

A novel method is proposed to produce a soluble recombinant antigen mimic, constituted with full-length HA1 and truncated HA2 individually expressed in E. coli, instead of a precursor form of hemagglutinin protein, that is similar to the naturally processed and disulfide-linked HA1/HA2 on the envelope of the influenza A virus strain X-31 (H3N2). A truncated ectodomain of HA2 subunit, HA2(23-185)/C137S, lacked two membrane-interacting sequences, i.e., the N-terminal fusion peptide as well as the transmembrane domain and short cytoplasmic segment at the C terminus. A recombinant HA1 (rHA1) subunit protein, HA1(1-328)/C14S/L157S, lacked the signal peptide. Mutations C137S and C14S in the HA2 and HA1 subunits, respectively, were introduced to prevent any possible disulfide linkage between the two subunit proteins. The rHA antigen mimic would be nonfusogenic mainly due to the absence of the N-terminal fusion peptide as well as the C-terminal transmembrane domain in the truncated HA2, and eventually less cytotoxic as well. Antibody responses induced by two soluble rHA antigens were evaluated by ELISA assays to detect rHA antigens injected and to validate both anti-HA1 and anti-HA2 antibodies produced in the mice sera. Antigenic rHA proteins also elicited neutralizing antibodies against homologous H3N2 influenza virus in the immunized mice, without severe body weight loss or any other adverse symptoms.

The three lives of viral fusion peptides

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The presence of a fusion peptide (FP) is a hallmark of viral fusion glycoproteins.

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Structure–function relationships underlying FP conservation remain greatly unknown.

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FPs establish interactions satisfying their folding within pre-fusion glycoproteins.

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Upon fusion activation FPs insert into and restructure target membranes.

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FPs can finally combine with transmembrane domains to form integral membrane bundles.

Fusion peptides comprise conserved hydrophobic domains absolutely required for the fusogenic activity of glycoproteins from divergent virus families. After 30 years of intensive research efforts, the structures and functions underlying their high degree of sequence conservation are not fully elucidated. The long-hydrophobic viral fusion peptide (VFP) sequences are structurally constrained to access three successive states after biogenesis. Firstly, the VFP sequence must fulfill the set of native interactions required for (meta) stable folding within the globular ectodomains of glycoprotein complexes. Secondly, at the onset of the fusion process, they get transferred into the target cell membrane and adopt specific conformations therein. According to commonly accepted mechanistic models, membrane-bound states of the VFP might promote the lipid bilayer remodeling required for virus-cell membrane merger. Finally, at least in some instances, several VFPs co-assemble with transmembrane anchors into membrane integral helical bundles, following a locking movement hypothetically coupled to fusion-pore expansion. Here we review different aspects of the three major states of the VFPs, including the functional assistance by other membrane-transferring glycoprotein regions, and discuss briefly their potential as targets for clinical intervention.

Two conserved residues are important for inducing highly ordered membrane domains by the transmembrane domain of influenza hemagglutinin

The interaction with lipids of a synthetic peptide corresponding to the transmembrane domain of influenza hemagglutinin was investigated by means of electron spin resonance. A detailed analysis of the electron spin resonance spectra from spin-labeled phospholipids revealed that the major effect of the peptide on the dynamic membrane structure is to induce highly ordered membrane domains that are associated with electrostatic interactions between the peptide and negatively charged lipids. Two highly conserved residues in the peptide were identified as being important for the membrane ordering effect. Aggregation of large unilamellar vesicles induced by the peptide was also found to be correlated with the membrane ordering effect of the peptide, indicating that an increase in membrane ordering, i.e., membrane dehydration, is important for vesicle aggregation. The possibility that hydrophobic interaction between the highly ordered membrane domains plays a role in vesicle aggregation and viral fusion is discussed.

The final conformation of the complete ectodomain of the HA2 subunit of influenza hemagglutinin can by itself drive low pH-dependent fusion

One of the best characterized fusion proteins, the influenza virus hemagglutinin (HA), mediates fusion between the viral envelope and the endosomal membrane during viral entry into the cell. In the initial conformation of HA, its fusogenic subunit, the transmembrane protein HA2, is locked in a metastable conformation by the receptor-binding HA1 subunit of HA. Acidification in the endosome triggers HA2 refolding toward the final lowest energy conformation. Is the fusion process driven by this final conformation or, as often suggested, by the energy released by protein restructuring? Here we explored structural properties as well as the fusogenic activity of the full sized trimeric HA2(1-185) (here called HA2*) that presents the final conformation of the HA2 ectodomain. We found HA2* to mediate fusion between lipid bilayers and between biological membranes in a low pH-dependent manner. Two mutations known to inhibit HA-mediated fusion strongly inhibited the fusogenic activity of HA2*. At surface densities similar to those of HA in the influenza virus particle, HA2* formed small fusion pores but did not expand them. Our results confirm that the HA1 subunit responsible for receptor binding as well as the transmembrane and cytosolic domains of HA2 is not required for fusion pore opening and substantiate the hypothesis that the final form of HA2 is more important for fusion than the conformational change that generates this form.

HIV gp41 six-helix bundle constructs induce rapid vesicle fusion at pH 3.5 and little fusion at pH 7.0: understanding pH dependence of protein aggregation, membrane binding, and electrostatics, and implications for HIV-host cell fusion

The HIV gp41 protein catalyzes fusion between HIV and target cell membranes. The fusion states of the gp41 ectodomain include early coiled-coil (CC) structure and final six-helix bundle (SHB) structure. The ectodomain has an additional N-terminal apolar fusion peptide (FP) sequence which binds to target cell membranes and plays a critical role in fusion. One approach to understanding gp41 function is study of vesicle fusion induced by constructs that encompass various regions of gp41. There are apparent conflicting literature reports of either rapid or no fusion of negatively charged vesicles by SHB constructs. These reports motivated the present study, which particularly focused on effects of pH because the earlier high and no fusion results were at pH 3.0 and 7.2, respectively. Constructs include "Hairpin," which has SHB structure but lacks the FP, "FP-Hairpin" with FP + SHB, and "N70," which contains the FP and part of the CC but does not have SHB structure. Aqueous solubility, membrane binding, and vesicle fusion function were measured at a series of pHs and much of the pH dependences of these properties were explained by protein charge. At pH 3.5, all constructs were positively charged, bound negatively charged vesicles, and induced rapid fusion. At pH 7.0, N70 remained positively charged and induced rapid fusion, whereas Hairpin and FP-Hairpin were negatively charged and induced no fusion. Because viral entry occurs near pH 7 rather than pH 3, our results are consistent with fusogenic function of early CC gp41 and with fusion arrest by final SHB gp41.

Hairpin folding of HIV gp41 abrogates lipid mixing function at physiologic pH and inhibits lipid mixing by exposed gp41 constructs

Conformational changes in the HIV gp41 protein are directly correlated with fusion between the HIV and target cell plasma membranes, which is the initial step of infection. Key gp41 fusion conformations include an early extended conformation termed prehairpin which contains exposed regions and a final low-energy conformation termed hairpin which has a compact six-helix bundle structure. Current fusion models debate the roles of hairpin and prehairpin conformations in the process of membrane merger. In the present work, gp41 constructs have been engineered which correspond to fusion relevant parts of both prehairpin and hairpin conformations and have been analyzed for their ability to induce lipid mixing between membrane vesicles. The data correlate membrane fusion function with the prehairpin conformation and suggest that one of the roles of the final hairpin conformation is sequestration of membrane-perturbing gp41 regions with consequent loss of the membrane disruption induced earlier by the prehairpin structure. To our knowledge, this is the first biophysical study to delineate the membrane fusion potential of gp41 constructs modeling key fusion conformations.

Membrane insertion of the three main membranotropic sequences from SARS-CoV S2 glycoprotein

In order to complete the fusion process of SARS-CoV virus, several regions of the S2 virus envelope glycoprotein are necessary. Recent studies have identified three membrane-active regions in the S2 domain of SARS-CoV glycoprotein, one situated downstream of the minimum furin cleavage, which is considered the fusion peptide (SARS_FP), an internal fusion peptide located immediately upstream of the HR1 region (SARS_IFP) and the pre-transmembrane domain (SARS_PTM). We have explored the capacity of these selected membrane-interacting regions of the S2 SARS-CoV fusion protein, alone or in equimolar mixtures, to insert into the membrane as well as to perturb the dipole potential of the bilayer. We show that the three peptides interact with lipid membranes depending on lipid composition and experiments using equimolar mixtures of these peptides show that different segments of the protein may act in a synergistic way suggesting that several membrane-active regions could participate in the fusion process of the SARS-CoV.

Experimental evidence to support a theory of lipid membrane fusion

Membrane fusion between two lipid membranes with different curvatures was measured by using a fluorescence fusion assay for lipid vesicle systems and was also obtained by measuring lipid monolayer surface tension upon the fusion of vesicles to monolayer membranes. For such membrane systems, it was found that when lysolipid was incorporated only in the membrane with a greater curvature, membrane fusion was more suppressed than those for the case where the same amount (molar ratio of lysolipid to non-lysolipids) of lysolipid was incorporated only in the membrane with a lower curvature. When lysolipid was incorporated only in a flat membrane (e.g., monolayer) and the fusion of small vesicles (SUV) to the monolayer was measured, suppression of membrane fusion by lysolipid was minimal. It is known that lysolipid lowers the surface energy of curved membranes, which stabilizes energetically such membrane surfaces, and thus suppresses membrane fusion. Our results support our theory of lipid membrane fusion where the membrane fusion occurs through the most curved membrane region at the contact area of two interacting membranes.

Fatty acids can substitute the HIV fusion peptide in lipid merging and fusion: an analogy between viral and palmitoylated eukaryotic fusion proteins

Various fusion proteins from eukaryotes and viruses share structural similarities such as a coiled coil motif. However, compared with eukaryotic proteins, a viral fusion protein contains a fusion peptide (FP), which is an N-terminal hydrophobic fragment that is primarily involved in directing fusion via anchoring the protein to the target cell membrane. In various eukaryotic fusion proteins the membrane targeting domain is cysteine-rich and must undergo palmitoylation prior to the fusion process. Here we examined whether fatty acids can replace the FP of human immunodeficiency virus type 1 (HIV-1), thereby discerning between the contributions of the sequence versus hydrophobicity of the FP in the lipid-merging process. For that purpose, we structurally and functionally characterized peptides derived from the N terminus of HIV fusion protein - gp41 in which the FP is lacking or replaced by fatty acids. We found that fatty acid conjugation dramatically enhanced the capability of the peptides to induce lipid mixing and aggregation of zwitterionic phospholipids composing the outer leaflet of eukaryotic cell membranes. The enhanced effect of the acylated peptides on membranes was further supported by real-time atomic force microscopy (AFM) showing nanoscale holes in zwitterionic membranes. Membrane-binding experiments revealed that fatty acid conjugation did not increase the affinity of the peptides to the membrane significantly. Furthermore, all free and acylated peptides exhibited similar alpha-helical structures in solution and in zwitterionic membranes. Interestingly, the fusogenic active conformation of N36 in negatively charged membranes composing the inner leaflet of eukaryotic cells is beta-sheet. Apparently, N-terminal heptad repeat (NHR) can change its conformation as a response to a change in the charge of the membrane head group. Overall, the data suggest an analogy between the eukaryotic cysteine-rich domains and the viral fusion peptide, and mark the hydrophobic nature of FP as an important characteristic for its role in lipid merging.

Influence of calcium on lipid mixing mediated by influenza hemagglutinin

We studied the influence of calcium on lipid mixing mediated by influenza hemagglutinin (HA). Lipid mixing between HA-expressing cells and liposomes containing disialoganglioside, influenza virus receptor, was studied at 37 degrees C and neutral pH after a low-pH pulse at 4 degrees C. With DSPC/cholesterol liposomes, calcium present after raising the temperature significantly promoted lipid mixing only when it was triggered by a short low-pH application. In case of DOPC/cholesterol liposomes, calcium promotion was observed regardless of the duration of the low-pH pulse. Calcium present during a short, but not long, low-pH application to HA-expressing cells with bound DSPC/cholesterol liposomes at 4 degrees C inhibited subsequent lipid mixing. We hypothesize that calcium influences lipid mixing because it binds to a vestigial esterase domain of hemagglutinin or causes expulsion of the fusion peptide from an electronegative cavity. We suggest that calcium promotes the transition from early and reversible conformation(s) of low pH-activated HA towards an irreversible conformation that underlies both HA-mediated lipid mixing and HA inactivation.

Interaction of influenza virus fusion peptide with lipid membranes: effect of lysolipid

The effect of lysophosphatidylcholine (LPC) on lipid vesicle fusion and leakage induced by influenza virus fusion peptides and the peptide interaction with lipid membranes were studied by using fluorescence spectroscopy and monolayer surface tension measurements. It was confirmed that the wild-type fusion peptide-induced vesicle fusion rate increased several-fold between pH 7 and 5, unlike a mutated peptide, in which valine residues were substituted for glutamic acid residues at positions 11 and 15. This mutated peptide exhibited a much greater ability to induce lipid vesicle fusion and leakage but in a less pH-dependent manner compared to the wild-type fusion peptide. The peptide-induced vesicle fusion and leakage were well correlated with the degree of interaction of these peptides with lipid membranes, as deduced from the rotational correlation time obtained for the peptide tryptophan fluorescence. Both vesicle fusion and leakage induced by the peptides were suppressed by LPC incorporated into lipid vesicle membranes in a concentration-dependent manner. The rotational correlation time associated with the peptide's tryptophan residue, which interacts with lipid membranes containing up to 25 mole % LPC, was virtually the same compared to lipid membranes without LPC, indicating that LPC-incorporated membrane did not affect the peptide interaction with the membrane. The adsorption of peptide onto a lipid monolayer also showed that the presence of LPC did not affect peptide adsorption.

Conformational flexibility and strand arrangements of the membrane-associated HIV fusion peptide trimer probed by solid-state NMR spectroscopy

The human immunodeficiency virus (HIV) fusion peptide (HFP) is the N-terminal apolar region of the HIV gp41 fusion protein and interacts with target cell membranes and promotes membrane fusion. The free peptide catalyzes vesicle fusion at least to the lipid mixing stage and serves as a useful model fusion system. For gp41 constructs which lack the HFP, high-resolution structures show trimeric protein and suggest that at least three HFPs interact with the membrane with their C-termini in close proximity. In addition, previous studies have demonstrated that HFPs which are cross-linked at their C-termini to form trimers (HFPtr) catalyze fusion at a rate which is 15-40 times greater than that of non-cross-linked HFP. In the present study, the structure of membrane-associated HFPtr was probed with solid-state nuclear magnetic resonance (NMR) methods. Chemical shift and intramolecular (13)CO-(15)N distance measurements show that the conformation of the Leu-7 to Phe-11 region of HFPtr has predominant helical conformation in membranes without cholesterol and beta strand conformation in membranes containing approximately 30 mol % cholesterol. Interstrand (13)CO-(13)CO and (13)CO-(15)N distance measurements were not consistent with an in-register parallel strand arrangement but were consistent with either (1) parallel arrangement with adjacent strands two residues out-of-register or (2) antiparallel arrangement with adjacent strand crossing between Phe-8 and Leu-9. Arrangement 1 could support the rapid fusion rate of HFPtr because of placement of the apolar N-terminal regions of all strands on the same side of the oligomer while arrangement 2 could support the assembly of multiple fusion protein trimers.

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.

Class II fusion protein of alphaviruses drives membrane fusion through the same pathway as class I proteins

Viral fusion proteins of classes I and II differ radically in their initial structures but refold toward similar conformations upon activation. Do fusion pathways mediated by alphavirus E1 and influenza virus hemagglutinin (HA) that exemplify classes II and I differ to reflect the difference in their initial conformations, or concur to reflect the similarity in the final conformations? Here, we dissected the pathway of low pH–triggered E1-mediated cell–cell fusion by reducing the numbers of activated E1 proteins and by blocking different fusion stages with specific inhibitors. The discovered progression from transient hemifusion to small, and then expanding, fusion pores upon an increase in the number of activated fusion proteins parallels that established for HA-mediated fusion. We conclude that proteins as different as E1 and HA drive fusion through strikingly similar membrane intermediates, with the most energy-intensive stages following rather than preceding hemifusion. We propose that fusion reactions catalyzed by all proteins of both classes follow a similar pathway.

A Trimeric HIV-1 Fusion Peptide Construct Which Does Not Self-Associate in Aqueous Solution and Which Has 15-Fold Higher Membrane Fusion Rate

A peptide construct (FPtr) was synthesized which mimics the biologically relevant topology of fusion peptide (FP) domains of the trimeric HIV-1 gp41 envelope protein. The FP domains play a critical role in gp41-catalyzed fusion of viral and host cell membranes which is a key step in viral infection. The FPtr construct contains three FP strands chemically bonded at their C-termini through lysine side chains. Analytical ultracentrifugation demonstrated that FPtr does not self-associate in aqueous solution and therefore models the expected FP topology of gp41. Comparative functional fusion assays were carried out using FPtr, FPdm (a cross-linked FP dimer construct), and FPmn (FP monomer). The derived fusion rate constants order ktr > kdm > kmn, and the ratio ktr/kmn has values in the range of 15-40. These results suggest that there is strong correlation of the fusion rate with the biologically relevant trimeric FP topology.

Oligomeric beta-structure of the membrane-bound HIV-1 fusion peptide formed from soluble monomers

The human immunodeficiency virus type 1 (HIV-1) fusion peptide serves as a useful model system for understanding viral/target cell fusion, at least to the lipid mixing stage. Previous solid-state NMR studies have shown that the peptide adopts an oligomeric beta-strand structure when associated with a lipid and cholesterol mixture close to that of membranes of host cells of the virus. In this study, this structure was further investigated using four different peptide constructs. In aqueous buffer solution, two of the constructs were primarily monomeric whereas the other two constructs had significant populations of oligomers/aggregates. NMR measurements for all membrane-associated peptide constructs were consistent with oligomeric beta-strand structure. Thus, constructs that are monomeric in solution can be converted to oligomers as a result of membrane association. In addition, samples prepared by very different methods had very similar NMR spectra, which indicates that the beta-strand structure is an equilibrium rather than a kinetically trapped structure. Lipid mixing assays were performed to assess the fusogenicities of the different constructs, and there was not a linear correlation between the solution oligomeric state and fusogenicity. However, the functional assays do suggest that small oligomers may be more fusogenic than either monomers or large aggregates.

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.

Solid-State Nuclear Magnetic Resonance Evidence for Parallel and Antiparallel Strand Arrangements in the Membrane-Associated HIV-1 Fusion Peptide

The HIV-1 fusion peptide serves as a useful model system for understanding viral/target cell fusion, at least to the lipid-mixing stage. Previous solid-state NMR studies have shown that the membrane-bound HIV-1 fusion peptide adopts an extended conformation in a lipid mixture close to that of host cells of the virus. In the present study, solid-state NMR REDOR methods were applied for detection of oligomeric beta strand structure. The samples were prepared under fusogenic conditions and contained equimolar amounts of two peptides, one with selective [(13)C]carbonyl labeling and the other with selective [(15)N]amide labeling. In the REDOR measurements, observation of reduced (13)C intensity due to hydrogen-bonded amide (15)N provides strong experimental evidence of oligomer formation by the membrane-associated peptide. Comparison of REDOR spectra on samples that were labeled at different residue positions suggests that there are both parallel and antiparallel arrangements of peptide strands. In the parallel arrangement, interpeptide hydrogen bonding decreases toward the C-terminus, while in the antiparallel arrangement, hydrogen bonds are observed along the entire length of residues which was probed (Gly-5 to Gly-16). For the parallel arrangement, these observations are consistent with the model in which the apolar N-terminal and central regions of the peptides penetrate into the membrane and hydrogen bond with one another while the polar C-terminus of the peptide is outside the membrane and hydrogen bonds with water. These measurements show that, at least at the end state of fusion, the peptide can adopt an oligomeric beta strand structure.

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.

Viral fusion proteins: multiple regions contribute to membrane fusion

In recent years, the simple picture of a viral fusion protein interacting with the cell and/or viral membranes by means of only two localized segments (i.e. the fusion peptide and the transmembrane domain) has given way to a more complex picture in which multiple regions from the viral proteins interact with membranes. Indeed, possible roles in membrane binding and/or destabilization have been postulated for the N-terminal heptad repeats, pre-transmembrane segments, and other internal regions of fusion proteins from distant viruses (such as orthomyxo-, retro-, paramyxo-, or flaviviruses). This review focuses on the experimental evidence and functional models postulated so far about the role of these regions in the process of virus-induced membrane fusion.

Early steps of the conformational change of influenza virus hemagglutinin to a fusion active state: stability and energetics of the hemagglutinin

A conformational change of the homotrimeric glycoprotein hemagglutinin (HA) of influenza virus mediates fusion between the viral envelope and the endosome membrane. The conformational change of the HA ectodomain is triggered by the acidic pH of the endosome lumen. An essential step of the conformational change is the formation of an extended coiled-coil motif exposing the hydrophobic fusion peptide toward the target membrane. The structures of the neutral-pH, non-fusion active conformation of the HA ectodomain and of a fragment of the ectodomain containing the coiled-coil motif are known. However, it is not known by which mechanism protonation triggers the conformational change of the stable neutral-pH conformation of the ectodomain. Here, recent studies on the stability of the HA ectodomain at neutral pH, the energetics of the conformational change toward the fusion-active state and of the unfolding of the HA ectodomain are summarised. A model for the early steps of the conformational change of the HA ectodomain is presented. The model implicates that protonation leads to a partial dissociation of the distal domains of the HA monomers that is driven by electrostatic repulsion. The opening of the ectodomain enables water to enter the ectodomain. The interaction of water with respective sequences originally shielded from contact with water drives the formation of the coiled-coil structure.

Architecture of the influenza hemagglutinin membrane fusion site

The mechanism of influenza hemagglutinin (HA) mediated membrane fusion has been intensively studied for over 20 years after the bromelain-released ectodomain of HA at neutral pH was first crystallized. Nearly 10 years ago, the low-pH-induced "spring coiled" conformational change of HA was predicted from peptide chemistry and confirmed by crystallography. Other work has yielded a wealth of knowledge on the observed changes in HA fusion/hemifusion phenotypes as a function of site-specific mutations of HA, or added amphipathic molecules or particular IgGs. It is becoming clear that the conformational changes predicted by the crystallography are necessary to cause fusion and that interfering with these changes can block fusion or reduce it to hemifusion. What is not known is how the conformational changes cause fusion. In particular, while it is generally agreed that fusion requires an aggregate of HAs, how the aggregate may act to transduce the energy of the HA conformational changes to creating the initial fusion defect is not known. We have used a comprehensive mass action kinetic model of HA-mediated fusion to carry out a "meta-analysis" of several key data sets, using HA-expressing cells and using virions. The consensus result of these detailed kinetic studies was that the fusion site of influenza hemagglutinin (HA) is an aggregate with at least eight HAs. The high-energy conformational change of only two of these HAs within the aggregate permits the formation of the first fusion pore. This "8 and 2" result was required to best fit all the data. We review these studies and how this kinetic result can guide and constrain HA fusion models. The kinetic analysis suggests that the sequence of fusion intermediates starts with protein control and ends with lipid control, which makes sense. While curvature intermediates, e.g. the lipid stalk, are almost certainly within the fusion sequence, the "8 and 2" result does not suggest that they are the first step after HA aggregation. The stabilized hydrophobic defect model we have proposed as a precursor to the lipid stalk can form and is consistent with the "8 and 2" result.

Fusion peptides and the mechanism of viral fusion

Segments of viral fusion proteins play an important role in viral fusion. They are defined by a number of criteria, including the sensitivity of this region of the viral fusion protein to loss of function as a consequence of mutation. In addition, small model peptides designed to mimic this segment of viral fusion proteins often have some membrane perturbing activity. The properties of viral fusion peptides are quite varied. Many are found at the amino terminus of viral fusion proteins. As isolated peptides, they have been found to form both alpha-helical as well as beta-structure. In addition, some viruses have internal fusion peptides. Just as there are several structural motifs for viral fusion peptides, there are also several mechanisms by which they accelerate the process of membrane fusion. These include the promotion of negative curvature, lowering the rupture tension of the lipid monolayer, acting as an anchor to join the fusion membranes, transmitting a force to the membrane or imparting energy to the system by other means. It is not likely that the fusion peptide can fulfill all of these diverse roles and future studies will elucidate which of these mechanisms is most important for the action of individual viral fusion peptides.

Synthesis, enhanced fusogenicity, and solid state NMR measurements of cross-linked HIV-1 fusion peptides

In the HIV-1 gp41 and other viral fusion proteins, the minimal oligomerization state is believed to be trimeric with three N-terminal fusion peptides inserting into the membrane in close proximity. Previous studies have demonstrated that the fusion peptide by itself serves as a useful model fusion system, at least to the hemifusion stage in which the viral and target cell lipids are mixed. In the present study, HIV-1 fusion peptides were chemically synthesized and cross-linked at their C-termini to form dimers or trimers. C-terminal trimerization is their likely topology in the fusogenic form of the intact gp41 protein. The fusogenicity of the peptides was then measured in an intervesicle lipid mixing assay, and the assay results were compared to those of the monomer. For monomer, dimer, and trimer at peptide strand/lipid mol ratios between 0.0050 and 0.010, the final extent of lipid mixing for the dimer and trimer was 2-3 times greater than for the monomer. These data suggest that the higher local concentration of peptide strands in the cross-linked peptides enhances fusogenicity and that oligomerization of the fusion peptide in gp41 may enhance the rate of viral/target cell membrane fusion. For gp41, this effect is in addition to the role of the trimeric coiled-coil structure in bringing about apposition of viral and target cell membranes. NMR measurements on the membrane-associated dimeric fusion peptide were consistent with an extended structure at Phe-8, which is the same as has been observed for the membrane-bound monomer in the same lipid composition.

On the interaction between gp41 and membranes: the immunodominant loop stabilizes gp41 helical hairpin conformation

gp41 is the protein responsible for the process of membrane fusion that allows primate lentiviruses (HIV and SIV) to enter into their host cells. gp41 ectodomain contains an N-terminal and a C-terminal heptad repeat region (NHR and CHR) connected by an immunodominant loop. In the absence of membranes, the NHR and CHR segments fold into a protease-resistant core with a trimeric helical hairpin structure. However, when the immunodominant loop is not present (either in a complex formed by HIV-1 gp41-derived NHR and CHR peptides or by mild treatment with protease of recombinant constructs of HIV-1 gp41 ectodomain, which also lack the N-terminal fusion peptide and the C-terminal Trp-rich region) membrane binding induces a conformational change in the gp41 core structure. Here, we further investigated whether covalently linking the NHR and CHR segments by the immunodominant loop affects this conformational change. Specifically, we analyzed a construct corresponding to a fragment of SIVmac239 gp41ectodomain (residues 27-149, named e-gp41) by means of surface plasmon resonance, Trp and rhodamine fluorescence, ATR-FTIR spectroscopy, and differential scanning calorimetry. Our results suggest that the presence of the loop stabilizes the trimeric helical hairpin both when e-gp41 is in aqueous solution and when it is bound to the membrane surface. Bearing in mind possible differences between HIV-1 and SIV gp41, and considering that the gp41 ectodomain constructs analyzed to date lack the N-terminal fusion peptide and the C-terminal Trp-rich region, we discuss our observations in relation to the mechanism of virus-induced membrane fusion.

Tight binding of influenza virus hemagglutinin to its receptor interferes with fusion pore dilation

Deletion of oligosaccharide side chains near the receptor binding site of influenza virus A/USSR/90/77 (H1N1) hemagglutinin (HA) enhanced the binding of HA to erythrocyte receptors, as was also observed with A/FPV/Rostock/34 (H7N1). Correlated with the enhancement of binding activity, the cell fusion activity of HA was reduced. A mutant HA in which three oligosaccharide side chains were deleted showed the highest level of binding and the lowest level of fusion among the HAs tested. The cell fusion activity of the oligosaccharide deletion mutant of HA, however, was drastically elevated when the binding activity was reduced by deletion of four amino acids adjacent to the receptor binding site. Thus, a reciprocal relationship was observed between the receptor binding and the cell fusion activities of H1/USSR HA. No difference was observed, however, in lipid mixing activity, so-called hemifusion, between wild-type (WT) and oligosaccharide deletion mutant HAs. Soluble dye transfer testing showed that even the HA with the lowest cell fusion activity was able to form fusion pores through which a small molecule such as calcein could pass. However, electron microscopic studies revealed that a large molecule such as hemoglobin hardly passed through the fusion pores formed by the mutant HA, whereas hemoglobin did efficiently pass through those formed by the WT HA. These results suggested that interference in the process of dilation of fusion pores occurs when the binding of HA to the receptor is too tight. Since the viral nucleocapsid is far larger than hemoglobin, appropriate receptor binding affinity is important for virus entry.

Reversible stages of the low-pH-triggered conformational change in influenza virus hemagglutinin

The refolding of the prototypic fusogenic protein hemagglutinin (HA) at the pH of fusion is considered to be a concerted and irreversible discharge of a loaded spring, with no distinct intermediates between the initial and final conformations. Here, we show that HA refolding involves reversible conformations with a lifetime of minutes. After reneutralization, low pH-activated HA returns from the conformations wherein both the fusion peptide and the kinked loop of the HA2 subunit are exposed, but the HA1 subunits have not yet dissociated, to a structure indistinguishable from the initial one in functional, biochemical and immunological characteristics. The rate of the transition from reversible conformations to irreversible refolding depends on the pH and on the presence of target membrane. Importantly, recovery of the initial conformation is blocked by the interactions between adjacent HA trimers. The existence of the identified reversible stage of refolding can be crucial for allowing multiple copies of HA to synchronize their release of conformational energy, as required for fusion.

Mode of action of two inhibitory peptides from heptad repeat domains of the fusion protein of Newcastle disease virus

Peptides derived from heptad repeat (HR) sequences of viral fusion proteins from several enveloped viruses have been shown to inhibit virus-mediated membrane fusion but the mechanism remains unknown. To further investigate this, the inhibition mechanism of two HR-derived peptides from the fusion protein of the paramyxovirus Newcastle disease virus (NDV) was investigated. Peptide N24 (residues 145-168) derived from HR1 was found to be 145-fold more inhibitory in a syncytium assay than peptide C24 (residues 474-496), derived from HR2. Both peptides failed to block lipid-mixing between R18-labeled virus and cells. None of the peptides interfered with the binding of hemagglutinin-neuraminidase (HN) protein to the target cells, as demonstrated by hemagglutining assays. When both peptides were mixed at equimolar concentrations, their inhibitory effect was abolished. In addition, both peptides induced the aggregation of negatively charged and zwitterionic phospholipid membranes. The ability of the peptides to interact with each other in solution suggests that these peptides may bind to the opposite HR region on the protein whereas their ability to interact with membranes as well as their failure to block lipid transfer suggest a second binding site. Taken together these results, suggest a mode of action for C24 and N24 in which both peptides have two different targets on the F protein: the opposite HR sequence and their corresponding domains.

Influence of gp41 fusion peptide on the kinetics of poly(ethylene glycol)-mediated model membrane fusion

The fusion peptide of the HIV fusion protein gp41 is required for viral fusion and entry into a host cell, but it is unclear whether this 23-residue peptide can fuse model membranes. We address this question for model membrane vesicles in the presence and absence of aggregating concentrations of poly(ethylene glycol) (PEG). PEG had no effect on the physical properties of peptide bound to membranes or free in solution. We tested for fusion of both highly curved and uncurved PC/PE/SM/CH (35:30:15:20 mol %) vesicles and highly curved PC/PE/CH (1:1:1) vesicles treated with peptide in the presence and absence of PEG. Fusion was never observed in the absence of PEG, although high peptide concentrations led to aggregation and rupture, especially in unstable PC/PE/CH (1:1:1) vesicles. When 5 wt % PEG was present to aggregate vesicles, peptide enhanced the rate of lipid mixing between curved PC/PE/SM/CH vesicles in proportion to the peptide concentration, with this effect leveling off at peptide/lipid (P/L) ratios approximately 1:200. Peptide produced an even larger effect on the rate of contents mixing but inhibited contents mixing at P/L ratios >1:200. No fusion enhancement was seen with uncurved vesicles. The rate of fusion was also enhanced by the presence of hexadecane, and peptide-induced rate enhancement was not observed in the presence of hexadecane. We conclude that gp41 fusion peptide does not induce vesicle fusion at subrupturing concentrations but can enhance fusion between highly curved vesicles induced to fuse by PEG. The different effects of peptide on the rates of lipid mixing and fusion pore formation suggest that, while gp41 fusion peptide does affect hemifusion, it mainly affects pore formation.

Thermal denaturation of influenza virus and its relationship to membrane fusion

The X-31 strain of influenza virus was studied by differential scanning calorimetry (DSC), CD and SDS/PAGE analysis as a function of both temperature and pH. A bromelain-treated virus was also studied by these methods. The major transition observed in the intact virus was a result of the denaturation of the haemagglutinin (HA) protein. At pH 7.4, this transition was similar in the intact virus and the isolated HA, but was absent in the bromelain-treated virus. However, at pH 5 the denaturation temperature and enthalpy were both higher for HA in the virus than in the isolated protein, indicating that HA interacts with other molecular components in the intact virus. The transition observed by DSC occurs at a higher temperature than does the thermal transition observed by CD. The temperature of the CD transition coincides with the temperature at which the fusogenicity of the virus increases, and probably corresponds to the formation of an extended coiled-coil conformation. Analysis by SDS/PAGE at neutral pH under non-reducing conditions demonstrates a selective loss of the HA protein trimer, resulting in the formation of aggregates in the range of temperatures of 55 to 70 degrees C. In contrast, at acidic pH, the HA protein is largely in the monomeric form at 25 degrees C, and there is little change with temperature. There is thus a weakening of the quaternary structure of HA at acidic pH prior to heating. At the temperature at which the virus exhibits an increased fusogenicity at neutral pH, there is a loss of secondary structure and a beginning of a destabilization of the trimeric form of HA. This temperature is lower than that required for the major endothermic peak observed in DSC experiments. The results demonstrate that there is no kinetically trapped high-energy form of HA at neutral pH.

New insights into the spring-loaded conformational change of influenza virus hemagglutinin

Influenza virus hemagglutinin undergoes a conformational change in which a loop-to-helix "spring-loaded" conformational change forms a coiled coil that positions the fusion peptide for interaction with the target bilayer. Previous work has shown that two proline mutations designed to disrupt this change disrupt fusion but did not determine the basis for the fusion defect. In this work, we made six additional mutants with single proline substitutions in the region that undergoes the spring-loaded conformational change and two additional mutants with double proline substitutions in this region. All double mutants were fusion inactive. We analyzed one double mutant, F63P/F70P, as an example. We observed that F63P/F70P undergoes key low-pH-induced conformational changes and binds tightly to target membranes. However, limited proteolysis and electron microscopy observations showed that the mutant forms a coiled coil that is only approximately 50% the length of the wild type, suggesting that it is splayed in its N-terminal half. This work further supports the hypothesis that the spring-loaded conformational change is necessary for fusion. Our data also indicate that the spring-loaded conformational change has another role beyond presenting the fusion peptide to the target membrane.

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.

New insights into the mechanism of virus-induced membrane fusion

Infection by enveloped viruses requires fusion between the viral and cellular membranes, a process mediated by specific viral envelope glycoproteins. Information from studies with whole viruses, as well as protein dissection, has suggested that the fusion glycoprotein (F) from Paramyxoviridae, a family that includes major human pathogens, has two hydrophobic segments, termed fusion peptides. These peptides are directly responsible for the membrane fusion event. The recently determined three-dimensional structure of the pre-fusion conformation of the F protein supported these predictions and enabled the formulation of: (1) a detailed model for the initial interaction between F and the target membrane, (2) a new model for Paramyxovirus-induced membrane fusion that can be extended to other viral families, and (3) a novel strategy for developing better inhibitors of paramyxovirus infection.