Structural characterization of viral fusion proteins

An epitope of the Semliki Forest virus fusion protein exposed during virus-membrane fusion

Semliki Forest virus (SFV) is an enveloped alphavirus that infects cells via a membrane fusion reaction triggered by acidic pH in the endocytic pathway. Fusion is mediated by the spike protein E1 subunit, an integral membrane protein that contains the viral fusion peptide and forms a stable homotrimer during fusion. We have characterized four monoclonal antibodies (MAbs) specific for the acid conformation of E1. These MAbs did not inhibit fusion, suggesting that they bind to an E1 region different from the fusion peptide. Competition analyses demonstrated that all four MAbs bound to spatially related sites on acid-treated virions or isolated spike proteins. To map the binding site, we selected for virus mutants resistant to one of the MAbs, E1a-1. One virus isolate, SFV 4-2, showed reduced binding of three acid-specific MAbs including E1a-1, while its binding of one acid-specific MAb as well as non-acid-specific MAbs to E1 and E2 was unchanged. The SFV 4-2 mutant was fully infectious, formed the E1 homotrimer, and had the wild-type pH dependence of infection. Sequence analysis demonstrated that the relevant mutation in SFV 4-2 was a change of E1 glycine 157 to arginine (G157R). Decreased binding of MAb E1a-1 was observed under a wide range of assay conditions, strongly suggesting that the E1 G157R mutation directly affects the MAb binding site. These data thus localize an E1 region that is normally hidden in the neutral pH structure and becomes exposed as part of the reorganization of the spike protein to its fusion-active conformation.

X-ray diffraction study of feline leukemia virus fusion peptide and lipid polymorphism

The structural effects of the fusion peptide of feline leukemia virus (FeLV) on the lipid polymorphism of N-methylated dioleoylphosphatidylethanolamine were studied using a temperature ramp with sequential X-ray diffraction. This peptide, the hydrophobic amino-terminus of p15E, has been proven to be fusogenic and to promote the formation of highly curved, intermediate structures on the lamellar liquid-crystal to inverse hexagonal phase transition pathway. The FeLV peptide produces marked effects on the thermotropic mesomorphic behaviour of MeDOPE, a phospholipid with an intermediate spontaneous radius of curvature. The peptide is shown to reduce the lamellar repeat distance of the membrane prior to the onset of an inverted cubic phase. This suggests that membrane thinning may play a role in peptide-induced membrane fusion and strengthens the link between the fusion pathway and inverted cubic phase formation. The results of this study are interpreted in relation to models of the membrane fusion mechanism.

Biological barriers to cellular delivery of lipid-based DNA carriers

Although lipid-based DNA delivery systems are being assessed in gene therapy clinical trials, many investigators in this field are concerned about the inefficiency of lipid-based gene transfer technology, a criticism directed at all formulations used to enhance transfer of plasmid expression vectors. It is important to recognize that many approaches have been taken to improve transfection efficiency, however because of the complex nature of the formulation technology being developed, it has been extremely difficult to define specific carrier attributes that enhance transfection. We believe that these optimization processes are flawed for two reasons. First, a very defined change in formulation components affects the physical and chemical characteristics of the carrier in many ways. As a consequence, it has not been possible to define structure/activity relationships. Second, the primary endpoint used to assess plasmid delivery has been transgene expression, an activity that is under the control of cellular processes that have nothing to do with delivery. Gene expression following administration of a plasmid expression vector involves a number of critical steps: (i) DNA protection, (ii) binding to a specific cell population, (iii) DNA transfer across the cell membrane, (iv) release of DNA into the cytoplasm, (v) transport through the cell and across the nuclear membrane as well as (vi) transcription and translation of the gene. The objective of this review is to describe lipid-based DNA carrier systems and the attributes believed to be important in regulating the transfection activity of these formulations. Although membrane destabilization activity of the lipid-based carriers plays an important role, we suggest here that a critical element required for efficient transfection is dissociation of lipids bound to the plasmid expression vector following internalization.

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.

Role of hemagglutinin cleavage for the pathogenicity of influenza virus

Although human epidemics of influenza occur on nearly an annual basis and result in a significant number of "excess deaths," the viruses responsible are not generally considered highly pathogenic. On occasion, however, an outbreak occurs that demonstrates the potential lethality of influenza viruses. The human pandemic of 1918 spread worldwide and killed millions, and the limited human outbreak of highly pathogenic avian viruses in Hong Kong at the end of 1997 is a warning that this could happen again. In avian species such as chickens and turkeys, several outbreaks of highly pathogenic influenza viruses have been documented. Although the reason for the lethality of the human 1918 viruses remains unclear, the pathogenicity of the avian viruses, including those that caused the human 1997 outbreak, relates primarily to properties of the hemagglutinin glycoprotein (HA). Cleavage of the HA precursor molecule HA0 is required to activate virus infectivity, and the distribution of activating proteases in the host is one of the determinants of tropism and, as such, pathogenicity. The HAs of mammalian and nonpathogenic avian viruses are cleaved extracellularly, which limits their spread in hosts to tissues where the appropriate proteases are encountered. On the other hand, the HAs of pathogenic viruses are cleaved intracellularly by ubiquitously occurring proteases and therefore have the capacity to infect various cell types and cause systemic infections. The x-ray crystal structure of HA0 has been solved recently and shows that the cleavage site forms a loop that extends from the surface of the molecule, and it is the composition and structure of the cleavage loop region that dictate the range of proteases that can potentially activate infectivity. Here influenza virus pathogenicity is discussed, with an emphasis on the role of HA0 cleavage as a determining factor.

Essential genes for myoblast fusion in Drosophila embryogenesis

In Drosophila, as in vertebrates, each muscle is a syncytium and arises from mesodermal cells by successive fusion. This requires cell-cell recognition, alignment, formation of prefusion complexes, followed by electron-dense plaques and membrane breakdown. Because muscle development in Drosophila is rapid and well-documented, it has been possible to identify several genes essential for fusion. Molecular analysis of two of these genes revealed the importance of cytoplasmic components. One of these, Myoblast city, is expressed in several tissues and is homologous to the mammalian protein DOCK180. Myoblast city is presumably involved in cell recognition and cell adhesion. Blown fuse, the second cytoplasmic component, is selectively expressed in the mesoderm and essential in order to proceed from the prefusion complex to electron-dense plaques at opposed membranes between adjacent myoblasts. The rolling stone gene is transiently expressed during myoblast fusion. The Rost protein is located in the membrane and thus might be a key component for cell recognition. The molecular characterization of further genes relevant for fusion such as singles bar and sticks and stones will help to elucidate the mechanism of myoblast fusion in Drosophila.

The cholesterol requirement for sindbis virus entry and exit and characterization of a spike protein region involved in cholesterol dependence

Semliki Forest virus (SFV) and Sindbis virus (SIN) are enveloped alphaviruses that enter cells via low-pH-triggered fusion in the endocytic pathway and exit by budding from the plasma membrane. Previous studies with cholesterol-depleted insect cells have shown that SFV requires cholesterol in the cell membrane for both virus fusion and efficient exit of progeny virus. An SFV mutant, srf-3, shows efficient fusion and exit in the absence of cholesterol due to a single point mutation in the E1 spike subunit, proline 226 to serine. We have here characterized the role of cholesterol in the entry and exit of SIN, an alphavirus quite distantly related to SFV. Growth, primary infection, fusion, and exit of SIN were all dramatically inhibited in cholesterol-depleted cells compared to control cells. Based on sequence differences within the E1 226 region between SFV, srf-3, and SIN, we constructed six SIN mutants with alterations within this region and characterized their cholesterol dependence. A SIN mutant, SGM, that had the srf-3 amino acid sequence from E1 position 224 to 235 showed increases of approximately 100-fold in infection and approximately 250-fold in fusion with cholesterol-depleted cells compared with infection and fusion of wild-type SIN. Pulse-chase analysis demonstrated that SGM exit from cholesterol-depleted cells was markedly more efficient than that of wild-type SIN. Thus, similar to SFV, SIN was cholesterol dependent for both virus entry and exit, and the cholesterol dependence of both steps could be modulated by sequences within the E1 226 region.

Alpha-granule secretion from alpha-toxin permeabilized, MgATP-exposed platelets is induced independently by H+ and Ca2+

In order to better understand granule release from platelets, we developed an alpha-toxin permeabilized platelet model to study alpha-granule secretion. Secretion of alpha-granules was analyzed by flow cytometry using P-selectin as a marker for alpha-granule release. P-selectin surface expression occurred when platelets were permeabilized in the presence of Ca2+. Responsiveness to Ca2+ was lost 30 min after permeabilization but could be reconstituted with MgATP. Alpha-toxin-permeabilized, MgATP-exposed platelets also degranulated within a pH range of 5.4-5.9 without exposure to and independent of Ca2+. ATP, GTP, CTP, UTP, and ITP supported Ca2+-induced alpha-granule secretion, while H+-induced alpha-granule secretion occurred only with ATP and GTP. Both Ca2+- and H+-induced alpha-granule secretion required ATP hydrolysis. Kinase inhibitors blocked both Ca2+- and H+-induced secretion. These data suggest that alpha-granule secretion in this permeabilized platelet system shares many characteristics with granule secretion studied in other permeabilized cell models. Furthermore, these results show that H+ can trigger alpha-granule release independent of Ca2+.

Why fibrous proteins are romantic

Here I give a personal account of the great history of fibrous protein structure. I describe how Astbury first recognized the essential simplicity of fibrous proteins and their paradigmatic role in protein structure. The poor diffraction patterns yielded by these proteins were then deciphered by Pauling, Crick, Ramachandran and others (in part by model building) to reveal alpha-helical coiled coils, beta-sheets, and the collagen triple helical coiled coil-all characterized by different local sequence periodicities. Longer-range sequence periodicities (or "magic numbers") present in diverse fibrous proteins, such as collagen, tropomyosin, paramyosin, myosin, and were then shown to account for the characteristic axial repeats observed in filaments of these proteins. More recently, analysis of fibrous protein structure has been extended in many cases to atomic resolution, and some systems, such as "leucine zippers," are providing a deeper understanding of protein design than similar studies of globular proteins. In the last sections, I provide some dramatic examples of fibrous protein dynamics. One example is the so-called "spring-loaded" mechanism for viral fusion by the hemagglutinin protein of influenza. Another is the possible conformational changes in prion proteins, implicated in "mad cow disease," which may be related to similar transitions in a variety of globular and fibrous proteins.

A mechanism of protein-mediated fusion: coupling between refolding of the influenza hemagglutinin and lipid rearrangements

Although membrane fusion mediated by influenza virus hemagglutinin (HA) is the best characterized example of ubiquitous protein-mediated fusion, it is still not known how the low-pH-induced refolding of HA trimers causes fusion. This refolding involves 1) repositioning of the hydrophobic N-terminal sequence of the HA2 subunit of HA ("fusion peptide"), and 2) the recruitment of additional residues to the alpha-helical coiled coil of a rigid central rod of the trimer. We propose here a mechanism by which these conformational changes can cause local bending of the viral membrane, priming it for fusion. In this model fusion is triggered by incorporation of fusion peptides into viral membrane. Refolding of a central rod exerts forces that pull the fusion peptides, tending to bend the membrane around HA trimer into a saddle-like shape. Elastic energy drives self-assembly of these HA-containing membrane elements in the plane of the membrane into a ring-like cluster. Bulging of the viral membrane within such cluster yields a dimple growing toward the bound target membrane. Bending stresses in the lipidic top of the dimple facilitate membrane fusion. We analyze the energetics of this proposed sequence of membrane rearrangements, and demonstrate that this simple mechanism may explain some of the known phenomenological features of fusion.

The role of low pH and disulfide shuffling in the entry and fusion of Semliki Forest virus and Sindbis virus

Semliki Forest virus (SFV), an enveloped alphavirus, infects cells via a membrane fusion reaction that is induced by the low pH in endocytic vesicles. The role of low pH in the entry of the alphavirus Sindbis virus (SIN) is unclear, and an alternative fusion mechanism involving receptor-induced disulfide bond rearrangements at neutral pH has been proposed. The entry properties of SFV and SIN were here compared in parallel using treatment with the weak base NH4Cl or the vacuolar ATPase inhibitors bafilomycin A-1 or concanamycin to neutralize endosome pH. Three membrane impermeant thiol modifying reagents, 5,5'-dithio-bis(2-nitrobenzoic acid) (DTNB), p-chloromercuriphenylsulfonic acid (pCMBS), and monobromotrimethylammoniobimane (Thiolyte MQ), were used to inhibit thiol-disulfide exchange reactions. Primary infection by both SFV and SIN was inhibited by neutralization of endosome pH using NH4Cl, bafilomycin, or concanamycin. The concentration of NH4Cl or bafilomycin required for inhibition correlated with the pH dependence of membrane fusion for SFV, SIN, and a pH-shift mutant of SFV. SFV and SIN infection were partially inhibited by the thiol blocker DTNB, but not by pCMBS or Thiolyte MQ. Our data suggest that acidic endosomal pH induces the fusion activity of both SFV and SIN during virus infection.

Specific single or double proline substitutions in the "spring-loaded" coiled-coil region of the influenza hemagglutinin impair or abolish membrane fusion activity

We tested the role of the "spring-loaded" conformational change in the fusion mechanism of the influenza hemagglutinin (HA) by assessing the effects of 10 point mutants in the region of high coiled-coil propensity, HA2 54-81. The mutants included proline substitutions at HA2 55, 71, and 80, as well as a double proline substitution at residues 55 and 71. Mutants were expressed in COS or 293T cells and assayed for cell surface expression and structural features as well as for their ability to change conformation and induce fusion at low pH. We found the following: Specific mutations affected the precise carbohydrate structure and folding of the HA trimer. All of the mutants, however, formed trimers that could be expressed at the cell surface in a form that could be proteolytically cleaved from the precursor, HA0, to the fusion-permissive form, HA1-S-S-HA2. All mutants reacted with an antibody against the major antigenic site and bound red blood cells. Seven out of ten mutants displayed a wild-type (wt) or moderately elevated pH dependence for the conformational change. V55P displayed a substantial reduction (approximately 60- 80%) in the initial rate of lipid mixing. The other single mutants displayed efficient fusion with the same pH dependence as wt-HA. The double proline mutant V55P/ S71P displayed no fusion activity despite being well expressed at the cell surface as a proteolytically cleaved trimer that could bind red blood cells and change conformation at low pH. The impairment in fusion for both V55P and V55P/S71P was at the level of outer leaflet lipid mixing. We interpret our results in support of the hypothesis that the spring-loaded conformational change is required for fusion. An alternate model is discussed.

SIRE-1, a copia/Ty1-like retroelement from soybean, encodes a retroviral envelope-like protein

The soybean genome hosts a family of several hundred, relatively homogeneous copies of a large, copia/Ty1-like retroelement designated SIRE-1. A copy of this element has been recovered from a Glycine max genomic library. DNA sequence analysis of two SIRE-1 subclones revealed that SIRE-1 contains a long, uninterrupted, ORF between the 3' end of the pol ORF and the 3' long terminal repeat (LTR), a region that harbors the env gene in retroviral genomes. Conceptual translation of this second ORF produces a 70-kDa protein. Computer analyses of the amino acid sequence predicted patterns of transmembrane domains, alpha-helices, and coiled coils strikingly similar to those found in mammalian retroviral envelope proteins. In addition, a 65-residue, proline-rich domain is characterized by a strong amino acid compositional bias virtually identical to that of the 60-amino acid, proline-rich neutralization domain of the feline leukemia virus surface protein. The assignment of SIRE-1 to the copia/Ty1 family was confirmed by comparison of the conceptual translation of its reverse transcriptase-like domain with those of other retroelements. This finding suggests the presence of a proretrovirus in a plant genome and is the strongest evidence to date for the existence of a retrovirus-like genome closely related to copia/Ty1 retrotransposons.

fus-1, a pH shift mutant of Semliki Forest virus, acts by altering spike subunit interactions via a mutation in the E2 subunit

Semliki Forest virus (SFV), an enveloped alphavirus, is a well-characterized paradigm for viruses that infect cells via endocytic uptake and low-pH-triggered fusion. The SFV spike protein is composed of a dimer of E1 and E2 transmembrane subunits, which dissociate upon exposure to low pH, liberating E2 and the fusogenic E1 subunit to undergo independent conformational changes. SFV fusion and infection are blocked by agents such as ammonium chloride, which act by raising the pH in the endosome and inhibiting the low-pH-induced conformational changes in the SFV spike protein. We have previously isolated an SFV mutant, fus-1, that requires more acidic pH to trigger its fusion activity and is therefore more sensitive to inhibition by ammonium chloride. The acid shift in the fusion activity of fus-1 was here shown to be due to a more acidic pH threshold for the initial dissociation of the fus-1 spike dimer, thereby resulting in a more acidic pH requirement for the subsequent conformational changes in both fus-1 E1 and fus-1 E2. Sequence analysis demonstrated that the fus-1 phenotype was due to a mutation in the E2 spike subunit, threonine 12 to isoleucine. fus-1 revertants that have regained the parental fusion phenotype and ammonium chloride sensitivity were shown to have also regained E2 threonine 12. Our results identify a region of the SFV E2 spike protein subunit that regulates the pH dependence of E1-catalyzed fusion by controlling the dissociation of the E1/E2 dimer.

Protease cleavage of reovirus capsid protein mu1/mu1C is blocked by alkyl sulfate detergents, yielding a new type of infectious subvirion particle

Mammalian reovirus virions undergo partial disassembly of the outer capsid upon exposure to proteases in vitro, producing infectious subvirion particles (ISVPs) that lack protein sigma3 and contain protein mu1/mu1C as endoprotease-generated fragments mu1delta/delta and phi. ISVPs are thought to be required for two early steps in reovirus infection: membrane penetration and activation of the particle-bound viral transcriptase complexes. Genetic and biochemical evidence implicates outer-capsid protein mu1 in both these steps. To determine whether the cleavage of mu1/mu1C is relevant to the unique properties of ISVPs, we analyzed the properties of novel subvirion particles that lacked sigma3 yet retained mu1/mu1C in an uncleaved but cleavable form. These detergent-plus-protease subvirion particles (dpSVPs) were produced by treating virions with chymotrypsin in the presence of micelle-forming concentrations of alkyl sulfate detergents. Infections with dpSVPs in murine L or canine MDCK cells provided evidence that the cleavage of mu1/mu1C during viral entry into these cells is dispensable for reovirus infection. Additionally, dpSVPs behaved like ISVPs in their capacity to permeabilize lipid bilayers and to undergo transcriptase activation in vitro, supporting the conclusion that cleavage of mu1/mu1C to mu1delta/delta and phi during viral entry is not required for either membrane penetration or transcriptase activation in cells. The capacity of alkyl sulfate detergents to inhibit the cleavage of mu1/mu1C in a reversible fashion suggests a specific association between virus particle and detergent micelles that may mimic virus particle-phospholipid membrane interactions during reovirus entry into cells.

Pores formed by influenza hemagglutinin

Low pH-induced fusion mediated by the hemagglutinin (HA) of influenza virus involves a conformational change in the protein that leads to the insertion of a "fusion peptide" of the protein into the target membrane. It has been suggested that this insertion, aided by the formation of a complex of multiple HA trimers, would lead to perturbation of the bilayer structure of the membrane, initiating fusion. Here we present data showing that the interaction of the bromelain released ectodomain of the protein (BHA) with liposomal membranes at low pH leads to pore formation, at least at low temperatures. Strongly temperature-dependent low pH-induced inactivation of BHA resulted in a complete lack of activity of BHA above 10 degrees C. Even at 0 degrees C, only about 5% of the BHA participated in pore formation. Viral HA was less rapidly inactivated and still induced pores at 37 degrees C. BHA-induced pore formation showed a sigmoidal time course. Once BHA had formed a pore in one liposome, it did not form a pore in a further liposome. Quantitative analysis of pore formation indicated that one single BHA trimer sufficed to produce a pore. These data indicate that fusion peptide insertion perturbs the membrane and that the formation of a complex of trimers is not a prerequisite for the perturbation.

How lysophosphatidylcholine inhibits cell-cell fusion mediated by the envelope glycoprotein of human immunodeficiency virus

We have investigated the effect of lysophosphatidylcholine (LPC) on fusion of cells expressing the envelope glycoprotein of HIV-1, gp120/gp41, with cells expressing the receptor for this virus, CD4. Fusion was inhibited by micromolar concentrations of LPC added from an aqueous stock solution, but not by membrane-associated LPC or LPC produced in the membrane by the action of phospholipase A2. No inhibition was found for LPCs with acyl chain lengths shorter than 12 carbon atoms, and the inhibitory effect of longer molecules increased with their length. gp120-CD4 specific cell-cell binding was inhibited with a chain length dependence corresponding to that of fusion, but with palmitoyl-LPC, fusion was more strongly affected than binding. These data indicate that gp120/gp41-induced fusion is inhibited by LPC because LPC affects viral protein-host cell binding and not because LPC prevented the formation of lipid intermediates required for fusion.

Structure-activity relationships in the fusion of small unilamellar phosphatidylcholine vesicles induced by a model peptide

Limited proteolysis of fatty acid-free bovine serum albumin by pepsin yields several well characterized peptides, one of which (P9, M(r) 9,000), induces fusion of small unilamellar vesicles (SUV) of phosphatidylcholine at pH 3.6. Circular dichroism (CD) of P9 solutions confirmed that the peptide undergoes a reversible transition between pH 7 and pH 3.6. The spectral changes observed with CD suggest that in the low pH conformation there is a decrease in the alpha-helical contents and an exposure of hydrophobic residues. CD and differential ultraviolet spectroscopy demonstrated that P9 binds to micelles of hexadecylphosphorylcholine and the binding produces changes in the tertiary structure of the peptide. Reduction and carboxymethylation of the two disulfide bridges of P9 produced loss of the ability to induce fusion of SUV, although the reduced peptide binds to vesicles, induces loss of entrapped marker and produces vesicle disruption. In the active form P9 exposes hydrophobic groups, one amphiphilic alpha-helix and requires the integrity of the disulfide bridge-stabilized tertiary structure.

Enveloped viruses: a common mode of membrane fusion?

Viruses use elaborate stratagems to enter cells. The HIV-1 envelope glycoprotein, which mediates both attachment and membrane fusion, has grudgingly begun to yield high-resolution structural information that suggests mechanistic similarities with the hemagglutinin protein of influenza virus.

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.

Structural features of membrane fusion between influenza virus and liposome as revealed by quick-freezing electron microscopy

The structure of membrane fusion intermediates between the A/PR/8(H1N1) strain of influenza virus and a liposome composed of egg phosphatidylcholine, cholesterol, and glycophorin was studied using quick-freezing electron microscopy. Fusion by viral hemagglutinin protein was induced at pH 5.0 and 23 degrees C. After a 19-s incubation under these conditions, small protrusions with a diameter of 10-20 nm were found on the fractured convex faces of the liposomal membranes, and small pits complementary to the protrusions were found on the concave faces. The protrusions and pits corresponded to fractured parts of outward bendings of the lipid bilayer or "microprotrusions of the lipid bilayer." At the loci of the protrusions and pits, liposomal membranes had local contacts with viral membranes. In many cases both the protrusions and the pits were aligned in regular polygonal arrangements, which were thought to reflect the array of hemagglutinin spikes on the viral surface. These structures were induced only when the medium was acidic with the virus present. Based on these observations, it was concluded that the microprotrusions of the lipid bilayer are induced by hemagglutinin protein. Furthermore, morphological evidence for the formation of the "initial fusion pore" at the microprotrusion was obtained. The protrusion on the convex face sometimes had a tiny hole with a diameter of <4 nm in the center. The pits transformed into narrow membrane connections <10 nm in width, bridging viruses and liposomes. The structures of the fusion pore and fusion neck with larger sizes were also observed, indicating growth of the protrusions and pits to distinct fusion sites. We propose that the microprotrusion of the lipid bilayer is a fusion intermediate induced by hemagglutinin protein, and suggest that the extraordinarily high curvature of this membrane structure is a clue to the onset of fusion. The possible architecture of the fusion intermediate is discussed with regard to the localization of intramembrane particles at the microprotrusion.

Fusion peptides derived from the HIV type 1 glycoprotein 41 associate within phospholipid membranes and inhibit cell-cell Fusion. Structure-function study

The fusion domain of human immunodeficiency virus (HIV-1) envelope glycoprotein (gp120-gp41) is a conserved hydrophobic region located at the N terminus of the transmembrane glycoprotein (gp41). A V2E mutant has been shown to dominantly interfere with wild-type envelope-mediated syncytium formation and virus infectivity. To understand this phenomenon, a 33-residue peptide (wild type, WT) identical to the N-terminal segment of gp41 and its V2E mutant were synthesized, fluorescently labeled, and characterized. Both peptides inhibited HIV-1 envelope-mediated cell-cell fusion and had similar alpha-helical content in membrane mimetic environments. Studies with fluorescently labeled peptide analogues revealed that both peptides have high affinity for phospholipid membranes, are susceptible to digestion by proteinase-K in their membrane-bound state, and tend to self- and coassemble in the membranes. In SDS-polyacrylamide gel electrophoresis the WT peptide formed dimers as well as higher order oligomers, whereas the V2E mutant only formed dimers. The WT, but not the V2E mutant, induced liposome aggregation, destabilization, and fusion. Moreover, the V2E mutant inhibited vesicle fusion induced by the WT peptide, probably by forming inactive heteroaggregates. These data form the basis for an explanation of the mechanism by which the gp41 V2E mutant inhibits HIV-1 infectivity in cells when co-expressed with WT gp41.

Molecular mechanism underlying the action of a novel fusion inhibitor of influenza A virus

In the initial stages of influenza virus infection, the hemagglutinin (HA) protein of influenza virus mediates both adsorption and penetration of the virus into the host cell. Recently, we identified and characterized BMY-27709 as an inhibitor of the H1 and H2 subtypes of influenza A virus that specifically inhibits the HA function necessary for virus-cell membrane fusion (G.-X. Luo, R. Colonno, and M. Krystal, Virology 226:66-76, 1996). Studies presented herein show that the inhibition is mediated through specific interaction with the HA protein. This binding represses the low-pH-induced conformational change of the HA protein which is a prerequisite for membrane fusion. In an attempt to define the binding pocket within the HA molecule, a number of drug-resistant viruses have been isolated and characterized. Sequence analyses of the HA gene of these drug-resistant viruses mapped amino acid changes responsible for drug resistance to a region located near the amino terminus of HA2. In addition, we have identified inactive analogs of BMY-27709 which are able to compete out the inhibitory activity of BMY-27709. This finding suggests that inhibition of the HA-mediated membrane fusion by this class of compounds is not solely the result of binding within the HA molecule but requires specific interactions.

Transient changes of the conformation of hemagglutinin of influenza virus at low pH detected by time-resolved circular dichroism spectroscopy

Membrane fusion of influenza virus is mediated by a conformational change of the viral membrane protein hemagglutinin (HA) triggered by low pH. By near UV CD spectroscopy, which is sensitive to the arrangement and mobility of aromatic amino acids in proteins, we have monitored continuously with a time resolution of 5 s the kinetics of structural alterations of the ectodomain of HA isolated from different influenza virus strains (H1 (A/PR 8/34), H2 (A/Japan), and H3 (X31)). To establish a functional correlation to structural alterations of the HA ectodomain reflected by the CD, we have measured the kinetics of the virus-erythrocyte fusion and of the inactivation of fusion by low pH preincubation of viruses. At acidic pH we found a multiphasic behavior of the CD signal recorded at 283 nm. Upon lowering the pH we detected first an increase of the CD amplitude, which is associated with the formation of a fusion-competent state of HA. The initial increase was followed by a continuous decline of CD amplitude, which can be ascribed to a transformation into a fusion-inactivated conformation that is in its early phase reversible as found for A/Japan. The half-time of the different phases of the CD signal depended on the virus strain, the temperature, and the acidic pH. The results support recent hypotheses that the fusion-competent conformation is an intermediate of the fusion-inactivated structure of HA.

Inhibition of influenza virus fusion by polyanionic proteins

Anionic charge-modified human serum albumin (HSA) has previously been shown to exert potent in vitro activity against human immunodeficiency virus type 1 (HIV-1). In these studies, introduction of the additional negative charges was performed by derivatizing the epsilon-amino groups of lysine residues with succinic (Suc-HSA) or cis-aconitic anhydride (Aco-HSA), by which primary amino groups are replaced with carboxylic acids. The anti-HIV-1 activity was related to inhibition of gp41-mediated membrane fusion. Here, we investigated the activity of aconitylated and succinylated proteins on influenza virus membrane fusion, which is mediated by the viral membrane glycoprotein hemagglutinin (HA). Aco-HSA and Suc-HSA markedly inhibited the rates and extents of fusion of fluorescently labeled virosomes bearing influenza HA, with target membranes derived from erythrocytes. The inhibitory activity was dependent on the overall negative-charge density; HSA modified with 36 or less extra negative charges failed to inhibit fusion. The inhibition of fusion showed a certain degree of specificity for the protein carrying the negative charges: polyanionic HSA and beta-lactoglobulin A derivatives had fusion-inhibitory activity, whereas succinylated BSA, lactalbumin, lactoferrin, lysozyme, and transferrin were inactive. Aco60-HSA and Aco-beta-lactoglobulin A inhibited influenza virus membrane fusion in a concentration-dependent manner, IC50 values being about 4 and 10 microg/mL, respectively. HA-mediated membrane fusion is pH dependent. Aco60-HSA did not induce a shift in the pH threshold or in the pH optimum. Fusion with liposomes of another low pH-dependent virus, Semliki Forest virus, was not specifically affected by any of the compounds reported here. In view of some structural and functional similarities between influenza HA and the HIV-1 gp120/gp41 complex, it is tempting to postulate that the current results might have some implications for the anti-HIV-1 mechanism of polyanionic proteins.

Virus-cell and cell-cell fusion

Significant progress has been made in elucidating the mechanisms of viral membrane fusion proteins; both those that function at low, as well as those that function at neutral, pH. For many viral fusion proteins evidence now suggests that a triggered conformational change that exposes a previously cryptic fusion peptide, along with a rearrangement of the fusion protein oligomer, allows the fusion peptide to gain access to the target bilayer and thus initiate the fusion reaction. Although the topologically equivalent process of cell-cell fusion is less well understood, several cell surface proteins, including members of the newly described ADAM gene family, have emerged as candidate adhesion/fusion proteins.

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

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

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

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

Retrovirus envelope domain at 1.7 angstrom resolution

We report the crystal structure of an extraviral segment of a retrovirus envelope protein, the Moloney murine leukemia virus (MoMuLV) transmembrane (TM) subunit. This segment, which comprises a region of the MoMuLV TM protein analogous to that contained within the X-ray crystal structure of low-pH converted influenza hemagglutinin, contains a trimeric coiled coil, with a hydrophobic cluster at its base and a strand that packs in an antiparallel orientation against the coiled coil. This structure gives the first high-resolution insight into the retrovirus surface and serves as a model for a wide range of viral fusion proteins; key residues in this structure are conserved among C- and D-type retroviruses and the filovirus ebola.

Membrane asymmetry is maintained during influenza-induced fusion

We have investigated the influence of influenza-induced membrane fusion on the transverse asymmetry of the viral and target membranes. Large unilamellar vesicles containing headgroup-labeled fluorescent phospholipid analogues in both leaflets of the membrane were treated with phospholipase D, converting all outer membrane phospholipids to phosphatidic acid and leading to the release of the fluorescent label from the outside leaflet. After fusion of virus with these liposomes, addition of the enzyme to the fusion product did not release fluorescent label again, indicating that the phospholipid analogues from the inner leaflet of the membranes had not appeared on the outer leaflet. Moreover, the integral membrane protein hemagglutinin, which is present on the outer leaflet of the viral membrane, was quantitatively digested with protease after fusion, indicating that hemagglutinin remained on the outer leaflet of the fusion product. Therefore, there is no merger of the inner with outer leaflets of the viral or the liposomal membrane during fusion and transverse membrane asymmetry is maintained.

A trimeric structural domain of the HIV-1 transmembrane glycoprotein

Infection with HIV-1 is initiated by fusion of cellular and viral membranes. The gp41 subunit of the HIV-1 envelope plays a major role in this process, but the structure of gp41 is unknown. We have identified a stable, proteinase-resistant structure comprising two peptides, N-51 and C-43, derived from a recombinant protein fragment of the gp41 ectodomain. In isolation, N-51 is predominantly aggregated and C-43 is unfolded. When mixed, however, these peptides associate to form a stable, alpha-helical, discrete trimer of heterodimers. Proteolysis experiments indicate that the relative orientation of the N-51 and C-43 helices in the complex is antiparallel. We propose that N-51 forms an interior, parallel, homotrimeric, coiled-coil core, against which three C-43 helices pack in an antiparallel fashion. We suggest that this alpha-helical, trimeric complex is the core of the fusion-competent state of the HIV-1 envelope.

CD46-mediated measles virus entry: a first key to host-range specificity

Humans are the sole natural host of measles virus. The identification of CD46 as a virus receptor and of the involvement of moesin sheds some light on the molecular events occurring during virus entry into the cell. Knowledge of the key role of CD46 paves the way to creating transgenic mice sensitive to measles virus infection.

Structure and function of fusion pores in exocytosis and ectoplasmic membrane fusion

Several proteins involved in exocytosis have been identified recently, but it is still completely unclear which molecules perform the fusion event itself. Although in viral fusion the fusion proteins are known, even there the molecular mechanism remains controversial. Investigation of single fusion events by electrophysiological techniques together with fluorimetric measurements have now provided some insight into the properties of the first aqueous connection, the fusion pore. This pore has an initial size similar to an ion channel and allows movement of lipids only after it has substantially expanded, indicating that it is initially not a purely lipidic structure, but incorporates lipids when it expands. Although neurotransmitter release may occur through narrow transient fusion pores, the fusion pore of synaptic vesicles probably expands vey rapidly, making it unlikely that secretion is performed by rapid exo/endocytosis without full fusion under normal conditions. Recent recordings from small membrane patches have made it possible to resolve fusion events from vesicles as small as synaptic vesicles. Future experiments using excised patches may provide an approach to identify the molecular machinery of exocytotic membrane fusion.

Molecular mechanisms of protein-mediated membrane fusion

Two recent X-ray structures shed new light on the molecular mechanisms by which viral proteins mediate membrane fusion. In both cases it is clear that the fusion proteins are capable of dramatic conformational rearrangements. Studies of the intracellular fusion machinery used in vesicular transport have also advanced rapidly, although high-resolution structural information is not yet available.