Membrane fusion proteins of enveloped animal viruses

Assembly of influenza hemagglutinin trimers and its role in intracellular transport

The hemagglutinin (HA) of influenza virus is a homotrimeric integral membrane glycoprotein. It is cotranslationally inserted into the endoplasmic reticulum as a precursor called HA0 and transported to the cell surface via the Golgi complex. We have, in this study, investigated the kinetics and cellular location of the assembly reaction that results in HA0 trimerization. Three independent criteria were used for determining the formation of quaternary structure: the appearance of an epitope recognized by trimer-specific monoclonal antibodies; the acquisition of trypsin resistance, a characteristic of trimers; and the formation of stable complexes which cosedimented with the mature HA0 trimer (9S20,w) in sucrose gradients containing Triton X-100. The results showed that oligomer formation is a posttranslational event, occurring with a half time of approximately 7.5 min after completion of synthesis. Assembly occurs in the endoplasmic reticulum, followed almost immediately by transport to the Golgi complex. A stabilization event in trimer structure occurs when HA0 leaves the Golgi complex or reaches the plasma membrane. Approximately 10% of the newly synthesized HA0 formed aberrant trimers which were not transported from the endoplasmic reticulum to the Golgi complex or the plasma membrane. Taken together the results suggested that formation of correctly folded quaternary structure constitutes a key event regulating the transport of the protein out of the endoplasmic reticulum. Further changes in subunit interactions occur as the trimers move along the secretory pathway.

Membrane vesicles containing the Sendai virus binding glycoprotein, but not the viral fusion protein, fuse with phosphatidylserine liposomes at low pH

Membrane vesicles containing the Sendai virus hemagglutinin/neuraminidase (HN) glycoprotein were able to induce carboxyfluorescein (CF) release from loaded phosphatidylserine (PS) but not loaded phosphatidylcholine (PC) liposomes. Similarly, fluorescence dequenching was observed only when HN vesicles, bearing self-quenched N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine (N-NBD-PE), were incubated with PS but not PC liposomes. Thus, fusion between Sendai virus HN glycoprotein vesicles and the negatively charged PS liposomes is suggested. Induction of CF release and fluorescence dequenching were not observed when Pronase-treated HN vesicles were incubated with the PS liposomes. On the other hand, the fusogenic activity of the HN vesicles was not inhibited by treatment with dithiothreitol (DTT) or phenylmethanesulfonyl fluoride (PMSF), both of which are known to inhibit the Sendai virus fusogenic activity. Fusion was highly dependent on the pH of the medium, being maximal after an incubation of 60-90 s at pH 4.0. Electron microscopy studies showed that incubation at pH 4.0 of the HN vesicles with PS liposomes, both of which are of an average diameter of 150 nm, resulted in the formation of large unilamellar vesicles, the average diameter of which reached 450 nm. The relevance of these observations to the mechanism of liposome-membrane and virus-membrane fusion is discussed.

Fusion of Sendai virus with negatively charged liposomes as studied by pyrene-labelled phospholipid liposomes

Sendai virus particles fuse with negatively charged liposomes but not with vesicles made of zwitterionic phospholipids. The liposome-virus fusion process was studied by dilution of the concentration-dependent excimer-forming fluorophore 2-pyrenyldodecanoylphosphatidylcholine contained in the liposomes by the viral lipids. The data were analyzed in the framework of a mass action kinetic model. This provided analytical solutions for the final levels of probe dilution and numerical solutions for the kinetics of the overall fusion process, in terms of rate constants for the liposome-virus adhesion, deadhesion and fusion. This analysis led to the following conclusions: At neutral pH and 37 degrees C, only 15% of the virus particles can fuse with the phospholipid vesicles, although all the virions may aggregate with the liposomes. The rate constants for aggregation, fusion and deadhesion are of the orders of magnitude of 10(7) M-1 X s-1, 10(-3) s-1 and 10(-2), s-1, respectively. The fraction of active virus increases with temperature. At acidic pH, both the fraction of 'fusable' virus and the rate of fusion increase markedly. The optimal pH for fusion is 3-4, where most of the virus particles are active. At higher pH values, an increasing fraction of the virus particles become inactive, probably due to ionization of viral glycoproteins, whereas at pH values below 3.0 the fusion is markedly reduced, most likely due to protonation of the negatively charged vesicles. While only 15% of the virions fuse with the liposomes at pH 7.4 and 37 degrees C, all the liposomes lose their content (Amselem, S., Loyter, A. Lichtenberg, D. and Barenholz, Y. (1985) Biochim. Biophys. Acta 820, 1-10). We therefore propose that release of entrapped solutes is due to liposome-virus aggregation, and not to fusion. Both trypsinization and heat inactivation of the virus particles inhibit not only the fusion process but also the release of carboxyfluorescein. This demonstrates the obligatory role of viral membrane proteins in liposome-virus aggregation. Reconstituted vesicles made of the viral lipid and the hemagglutinin/neuraminidase (HN) glycoprotein fuse with negatively charged liposomes similar to the intact virions. This suggests that the fusion of virions with negatively charged vesicles, unlike the fusion of the virus with biological membranes, requires only the HN and not the fusion glycoprotein.

Fluorescence photobleaching recovery as a method to quantitate viral envelope-cell fusion: application to study fusion of Sendai virus envelopes with cells

A method to quantitate viral envelope-cell fusion at the single-cell level is presented. The method is based on the incorporation of nonquenching concentrations of a fluorescent lipid probe into the viral envelope; fluorescence photobleaching recovery (FPR) is then applied to measure the lateral mobilization of the probe in the cell membrane following fusion. In adsorbed (unfused) viral envelopes, the probe is constricted to the envelope and is laterally immobile on the micrometer scale of FPR. After fusion, the envelope lipids intermix with the plasma membrane, the probe becomes laterally mobile, and the fraction of fused viral envelopes can be extracted from the fraction of mobile probe molecules. The method has several advantages: (i) It clearly distinguishes fused from internalized envelopes, as probes in the latter are immobile in FPR studies; (ii) focusing the laser beam on specific regions of the cell enables region-specific measurements of the fusion level; (iii) one cell is measured at a time, enabling studies on the distribution of the fusion level within the cell population. The new method was employed to study fusion of reconstituted Sendai virus envelopes (RSVE) containing N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine with several cell types. Experiments with human erythrocytes demonstrated that the lateral mobilization measured is due to fusion and not the result of exchange processes. The extent of RSVE-erythrocyte fusion determined by FPR was similar to that measured by two other independent methods (fluorescence dequenching and removal of adsorbed RSVE by dithiothreitol).(ABSTRACT TRUNCATED AT 250 WORDS)

Virus replication inhibitory peptide inhibits the conversion of phospholipid bilayers to the hexagonal phase

Virus replication inhibitory peptide (carbobenzoxy-D-Phe-L-PheGly) was shown to be a potent specific inhibitor of the replication of paramyxovirus and myxovirus (Richardson, Scheid and Choppin (1980), Virology 105, 205-222). This peptide inhibits the membrane fusing activity of a viral glycoprotein. Many agents which promote the formation of the hexagonal phase in membranes also accelerate membrane fusion. At a mole fraction of 0.1, viral replication inhibitory peptide can raise the bilayer to hexagonal phase transition temperature of dielaidoylphosphatidylethanolamine by almost 10 degrees. Two related peptides, carbobenzoxy-L-PheGly and carbobenzoxy-L-GlyPhe, are less potent in raising the bilayer to hexagonal phase transition temperature, with the latter peptide being the least effective of the three. This order of potency is the same as the order of potency in inhibiting viral replication. Substances which inhibit hexagonal phase formation of pure lipids may also inhibit membrane fusion.

Membrane fusion: lipid vesicles as a model system

In many cellular functions the process of membrane fusion is of vital importance. It occurs in a highly specific and strictly controlled fashion. Proteins are likely to play a key role in the induction and modulation of membrane fusion reactions. Aimed at providing insight into the molecular mechanisms of membrane fusion, numerous studies have been carried out on model membrane systems. For example, the divalent-cation induced aggregation and fusion of vesicles consisting of negatively charged phospholipids, such as phosphatidylserine (PS) or cardiolipin (CL), have been characterized in detail. It is important to note that these systems largely lack specificity and control. Therefore conclusions derived from their investigation can not be extrapolated directly to a seemingly comparable counterpart in biology. Yet, the study of model membrane systems does reveal the general requirements of lipid bilayer fusion. The most prominent barrier to molecular contact between two apposing bilayers appears to be due to the hydration of the polar groups of the lipid molecules. Thus, dehydration of the bilayer surface and fluctuations in lipid packing, allowing direct hydrophobic interactions, are critical to the induction of membrane fusion. These membrane alterations are likely to occur only locally, at the site of intermembrane contact. Current views on the way membrane proteins may induce fusion under physiological conditions also emphasize the notion of local surface dehydration and perturbation of lipid packing, possibly through penetration of apolar amino acid segments into the hydrophobic membrane interior.

A new mechanism for the neutralization of enveloped viruses by antiviral antibody

Despite the considerable research that has been carried out into viral neutralization by antiviral antibody, its mechanisms remain poorly understood. Cases have been reported in which antiviral antibody can inhibit viral replication without inhibiting the binding and uptake of virus by susceptible cells. It has been shown that many enveloped viruses enter their target cells by endocytosis and are subsequently located in cellular compartments of increasing acidity. With several enveloped viruses this acidic pH can catalyse a fusion reaction between the membrane of the virus particle and that of a prelysosomal endosome, thus enabling the viral core to enter the cytosol and replication to commence. We have recently demonstrated that such an endosomal fusion event at mild acidic pH is involved in the entry pathway of the enveloped flavivirus, West Nile virus (WNV), into macrophages. We now show that antiviral antibody can neutralize WNV by inhibiting this intraendosomal acid-catalysed fusion step and we speculate on possible implications for the future design of antiviral vaccines.

A mechanism for the insertion of complement component C9 into target membranes

Complement component C9 is a globular serum protein which can insert and polymerise in a target membrane to form a large membrane channel. The ability to insert in the membrane is conferred by amphipathetic elements of secondary structure in the central part of the molecule. Towards each end high cysteine domains are found, one of which is homologous to the apoprotein binding domains of the LDL receptor. From the sequence and topological data for C9 we present a model for its structure and insertion into the membrane.

Trifluoperazine inhibits Sendai virus-induced hemolysis

Sendai virus-induced hemolysis, a manifestation of virus-red cell fusion, is inhibited by exposure of the virus to 50 microM and higher concentrations of trifluoperazine. Trifluoperazine does not disrupt the virus, since trifluoperazine-treated virus with no hemolytic activity sediments slightly faster than untreated virus on sucrose density gradients and contains viral proteins in proportions characteristic of untreated virus. Trifluoperazine affects the fusion protein to a greater extent than the hemagglutinin, since trifluoperazine-treated virus with no hemolytic activity is as active or nearly as active in agglutinating red cells. The partition coefficient of trifluoperazine between the virus membrane and buffer is lower at 4 degrees C than, but the same at 37 degrees C, as that between the red cell membrane and buffer. Nevertheless, virus-independent red cell lysis and inactivation of virus-mediated hemolysis occur when the red cell and viral membranes, respectively, contain similar concentrations of trifluoperazine. Furthermore, 13-28% more trifluoperazine is necessary to achieve either effect at 4 degrees C or at 25 degrees C than at 37 degrees C. Changes in the surface activity of trifluoperazine do not explain these results, insofar as the critical micellar concentration of (0.75 mM) and maximal reduction in surface tension by (40 dyn/cm) trifluoperazine are the same at 25 degrees C and 37 degrees C. The fluorescence of viral tryptophan decreases by approx. 25% when viral hemolysis is inactivated by trifluoperazine, by trypsin treatment or by heating at 100 degrees C for 5 min.

Effect of cholesterol on Ca2+-induced aggregation of liposomes and calcium diphosphatidate membrane traversal

Sonicated cholesterol-phosphatidylcholine (PC) liposomes containing 4 mol % phosphatidic acid (PA) aggregate in 10 mM Ca2+, slowly at low molar fractions of cholesterol (up to 30%) and 15 times faster at higher concentrations; the inflection point is at ca. 35 mol % bilayer cholesterol. O-[[(Methoxyethoxy)ethoxy]ethyl]cholesterol (OH-blocked cholesterol) does not give this rate enhancement. If PC is replaced by diether PC (CO groups abolished), cholesterol does not accelerate aggregation at concentrations in the bilayer below 50 mol %. No change in Ca2+-induced aggregation rates was observed if the ester CO groups of the bridge-forming PA only were replaced by CH2 (diether PA) in liposomes containing PC and cholesterol. PA-mediated Ca2+ membrane traversal seems to be accelerated by the addition of cholesterol to the PC-PA membrane, but analysis shows that the effect is due to the bilayer condensation effect of cholesterol resulting in an increase in the surface concentration of PA and that membrane cholesterol in fact slightly reduces the rate of Ca(PA)2 traversal; OH-blocked cholesterol, however, increases this rate 3-fold. It appears that lipid OH and CO groups interact, directly or with the mediation of water, in establishing the structure of the membrane "hydrogen belts", i.e., the strata containing those hydrogen-bond donors and acceptors. Cholesterol hydroxyl above 33 mol % (saturation of a 2:1 PC/cholesterol complex?) causes a restructuring of the hydrogen belts that facilitates membrane-water-membrane dehydration, the prerequisite for liposome aggregation by trans-Ca(PA)2 formation. On the other hand, the formation of the dehydrated cis-Ca(PA)2 complex that precedes Ca2+ membrane traversal is not accelerated by presence of the cholesterol hydroxyl group.

Similarity of the conformation of diphtheria toxin at high temperature to that in the membrane-penetrating low-pH state

At high temperature, nicked free monomers of diphtheria toxin undergo a transition to a thermally denatured state, with a midpoint of 45-50 degrees C. In this report, the high-temperature (60 degrees C) conformation has been compared to the native (neutral pH) and low-pH (pH less than 5) conformations. The low-pH and high-temperature conformations are similar although not identical. As in the conformation at low pH, and unlike the toxin in its native conformation, the protein in its conformation at high temperature is hydrophobic, has low fluorescence intensity, and has increased exposure of tryptophan to aqueous solution. As at low pH, at high temperature the circular dichroism spectrum shows at most only partial unfolding of secondary structure. In contrast, the conformation of the toxin in guanidinium chloride is much closer to a random coil. The effects of high temperature and low pH interact in the sense that sensitivity of the native conformation to one is increased by the other. That is, the transition temperature between native and thermally denatured states is decreased as pH is decreased, and the transition pH between neutral-pH and low-pH states is increased as temperature is increased. This implies that there is some region on the protein where high temperature and low pH can disrupt conformation in a similar manner. Taken together, these results indicate that the low-pH and high-temperature conformations can both be defined as partially denatured states, even though unfolding may not be extensive at low pH. Similar behavior may occur in other proteins that undergo functionally important conformational disruption at low pH.

Sendai virus-erythrocyte membrane interaction: quantitative and kinetic analysis of viral binding, dissociation, and fusion

A kinetic and quantitative analysis of the binding and fusion of Sendai virus with erythrocyte membranes was performed by using a membrane fusion assay based on the relief of fluorescence self-quenching. At 37 degrees C, the process of virus association displayed a half time of 2.5 min; at 4 degrees C, the half time was 3.0 min. The fraction of the viral dose which became cell associated was independent of the incubation temperature and increased with increasing target membrane concentration. On the average, one erythrocyte ghost can accommodate ca. 1,200 Sendai virus particles. The stability of viral attachment was sensitive to a shift in temperature: a fraction of the virions (ca. 30%), attached at 4 degrees C, rapidly (half time, ca. 2.5 min) eluted from the cell surface at 37 degrees C, irrespective of the presence of free virus in the medium. The elution can be attributed to a spontaneous, temperature-induced release, rather than to viral neuraminidase activity. Competition experiments with nonlabeled virus revealed that viruses destined to fuse do not exchange with free particles in the medium but rather bind in a rapid and irreversible manner. The fusion rate of Sendai virus was affected by the density of the virus particles on the cell surface and became restrained when more than 170 virus particles were attached per ghost. In principle, all virus particles added displayed fusion activity. However, at high virus-to-ghost ratios, only a fraction actually fused, indicating that a limited number of fusion sites exist on the erythrocyte membrane. We estimate that ca. 180 virus particles maximally can fuse with one erythrocyte ghost.

A fluorescence-based detergent binding assay for protein hydrophobicity

Protein hydrophobicity is often detected by binding of protein to micelles of a mild detergent. A new fluorescence method for detection of this binding is presented. The method is based on a long-range quenching of tryptophan fluorescence by energy transfer to a pyrene-labeled phospholipid probe incorporated into micelles of Brij 96. The method is rapid, simple, and requires only a few micrograms of protein. Strongest quenching is obtained when both pyrene probe and brominated Brij 96, a short-range quencher, are combined. To define the best assay conditions the physical properties and quenching behavior of micelles with or without these probes have been compared. It is shown that both quenchers accurately measure binding of model compounds and protein toxins to micelles. Comparison of quenching by the different probes can be used to derive information on tryptophan location relative to the micelle core.

Ricinus communis agglutinin-mediated agglutination and fusion of glycolipid-containing phospholipid vesicles: effect of carbohydrate head group size, calcium ions, and spermine

The glycolipids galactosylcerebroside (GalCer), lactosylceramide (LacCer), and trihexosylceramide (Gb3) were inserted into phospholipid vesicles, consisting of phosphatidylethanolamine and phosphatidic acid. The extent to which their carbohydrate head groups protruded beyond the vesicle surface and their interference with membrane approach were examined by determining vesicle susceptibility toward type I Ricinus communis agglutinin (RCA1) induced agglutination and Ca2+- and spermine-induced aggregation and fusion either in the presence or in the absence of the lectin. The initial agglutination rates increased in the order GalCer much less than LacCer less than Gb3, while a reversed order was obtained for Ca2+- and spermine-induced aggregation and fusion, indicating an enhanced steric interference on close approach of bilayers with increasing head group size. The lectin-mediated agglutination rates for LacCer- and Gb3-containing vesicles increased by an order of magnitude when Ca2+ was also included in the medium, at a concentration that did not induce aggregation per se. Charge neutralization could not account for this observation as the polyvalent cation spermine did not display this synergistic effect with RCA1. Addition of Ca2+ to preagglutinated vesicles substantially reduced the threshold cation concentration for fusion (micromolar vs. millimolar). Quantitatively, this concentration decreased with decreasing carbohydrate head group size, indicating that the head group protrusion determined the interbilayer distance within the vesicle aggregate. The distinct behavior of Ca2+ vs. spermine on RCA1-induced agglutination on the one hand and fusion on the other indicated that Ca2+ regulates the steric orientation of the carbohydrate head group, which appears to be related to its ability to dehydrate the bilayer. As a result, lectin agglutinability becomes enhanced while fusion will be interrupted as the interbilayer distance increases, the threshold head group size being three carbohydrate residues (Gb3). Finally, GalCer-containing vesicles were not agglutinated by RCA1 at ambient temperature, irrespective of the presence of Ca2+. Above 25 degrees C, RCA1 facilitated Ca2+-induced fusion of the vesicles, which was abolished by the haptenic sugar lactose. Since Gb3- and LacCer-containing vesicles displayed a similar behavior, a temperature-induced alteration in the supporting lipid matrix is suggested, which apparently affects lectin/glycolipid interaction.

Receptor-mediated endocytosis

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Mutants of the membrane-binding region of Semliki Forest virus E2 protein. I. Cell surface transport and fusogenic activity

Three mutations of the membrane-binding region of the Semliki Forest virus (SFV) p62 polypeptide (the precursor for virion E3 and E2) have been made by oligonucleotide-directed mutagenesis of a cDNA clone encoding the SFV structural proteins. One of the mutations (A2) substitutes a Glu for an Ala in the middle of the hydrophobic stretch which spans the bilayer. A1 and A3 alter the two basic charged amino acids in the cytoplasmic domain next to the hydrophobic region. The wild-type charge cluster of Arg-Ser-Lys (+2) has been changed to Gly-Ser-Met (0;A3) or to Gly-Ser-Glu (-1;A1). The mutant p62 proteins have been analyzed both in the presence and the absence of E1, the other half of the heterodimer spike complex of SFV. The mutant proteins expressed in COS-7 cells are glycosylated and are of the expected sizes. When co-expressed with E1, all three mutants are cleaved to yield the E2 protein and transported to the surface of COS-7 cells. When expressed in the absence of E1, the mutant p62 proteins remain uncleaved but still reach the cell surface. Once at the cell surface, all three mutants, when co-expressed with E1, can promote low pH-triggered cell-cell fusion. These results show that the three mutant p62/E2 proteins are still membrane associated in a functionally unaltered way.

Coronavirus IBV: partial amino terminal sequencing of spike polypeptide S2 identifies the sequence Arg-Arg-Phe-Arg-Arg at the cleavage site of the spike precursor propolypeptide of IBV strains Beaudette and M41

The spike protein of avian infectious bronchitis coronavirus comprises two glycopolypeptides S1 and S2 derived by cleavage of a proglycopolypeptide So, the nucleotide sequence of which has recently been determined for the Beaudette strain (Binns, M.M. et al., 1985, J. Gen. Virol. 66, 719-726). The order of the two glycopolypeptides within So is aminoterminus(N)-S1-S2-carboxyterminus(C). To locate the N-terminus of S2 we have performed partial amino acid sequencing on S2 from IBV-Beaudette labelled with [3H]serine and from the related strain labelled with [3H]valine, leucine and isoleucine. The residues identified and their positions relative to the N-terminus of S2 were: serine, 13; valine, 6, 12; leucine, none in the first 20 residues; isoleucine, 2, 19. These results identified the N-terminus of S2 of IBV-Beaudette as serine, 520 residues from the N-terminus of S1, excluding the signal sequence. Immediately to the N-terminal side of residue 520 So has the sequence Arg-Arg-Phe-Arg-Arg; similar basic connecting peptides are a feature of several other virus spike glycoproteins. It was deduced that for IBV-Beaudette S1 comprises 519 residues (Mr 57.0K) or 514 residues (56.2K) if the connecting peptide was to be removed by carboxypeptidase-like activity in vivo while S2 has 625 residues (69.2K). Nucleotide sequencing of the cleavage region of the So gene of IBV-M41 revealed the same connecting peptide as IBV-Beaudette and that the first 20 N-terminal residues of S2 of IBV-M41 were identical to those of the Beaudette strain. IBV-Beaudette grown in Vero cells had some uncleaved So; this was cleavable by 10 micrograms/ml of trypsin and of chymotrypsin. Partial N-terminal analysis of S1 from IBV-M41 identified leucine and valine residues at positions 2 and 9 respectively from the N-terminus. This confirms the identification, made by Binns et al. (1985), of the N-terminus of S1 and the end of the signal sequence of the IBV-Beaudette spike propolypeptide. N-terminal sequencing of [3H]leucine-labelled IBV-Beaudette membrane (M) polypeptide showed leucine residues at positions 8, 16 and 22 from the N-terminus; these results confirm the open reading frame identified by M.E.G. Boursnell et al. (1984, Virus Res. 1, 303-313) in the nucleotide sequence of M. The N-terminus of the nucleocapsid (N) polypeptide appeared to be blocked.

Studies of influenza haemagglutinin-mediated membrane fusion

A resonance energy transfer assay of membrane fusion developed by P. S. Uster and D. W. Deamer (Biochemistry 24, 1-8 (1985)) was used in a study of influenza haemagglutinin-mediated fusion. The characteristics of fusion and haemolysis by X-31 (H3N2) virus, a number of mutants of X-31 which fuse membranes at higher pH, and purified haemagglutinins released from virus particles either by detergent dissociation or by bromelain digestion were compared with particular regard to pH and temperature dependence. The finding that membrane fusion activity, haemolysis, and changes in haemagglutinin conformation covary with pH and temperature provide support for the role of haemagglutinin in fusion and are discussed in relation to the stability of its structure.

Variant influenza virus hemagglutinin that induces fusion at elevated pH

The hemagglutinin (HA) glycoprotein of influenza virus performs two critical roles during infection: it binds virus to cell surface sialic acids, and under mildly acidic conditions it induces fusion of the virion with intracellular membranes, liberating the genome into the cytoplasm. The pH dependence of fusion varies for different influenza virus strains. Here we report the isolation and characterization of a naturally occurring variant of the X31 strain that fuses at a pH 0.2 units higher than the parent strain does and that is less sensitive to the effects of ammonium chloride, a compound known to elevate endosomal pH. The bromelain-solubilized ectodomain of the variant HA displayed a corresponding shift in the pH at which it changed conformation and bound to liposomes. Cloning and sequencing of the variant HA gene revealed amino acid substitutions at three positions in the polypeptide. Two substitutions were in antigenic determinants in the globular region of HA1, and the third occurred in HA2 near the base of the molecule. By using chimeric HA molecules expressed in CV-1 cells from simian virus 40-based vectors, we demonstrated that the change in HA2 was solely responsible for the altered fusion phenotype. This substitution, asparagine for aspartic acid at position 132, disrupted a highly conserved interchain salt bridge between adjacent HA2 subunits. The apparent role of this residue in stabilizing the HA trimer is consistent with the idea that the trimer dissociates at low pH. Furthermore, the results demonstrate that influenza virus populations contain fusion variants, raising the possibility that such variants may play a role in the evolution of the virus.

Temperature and pH dependence of the haemolytic activity of influenza virus and of the rotational mobility of the spike glycoproteins

Influenza virus (strain X-47) was labeled with the triplet probe, eosin 5-isothiocyanate. Most of the label was found to be associated with haemagglutinin, the major glycoprotein of the viral envelope. Rotational diffusion of the glycoprotein was investigated by measuring flash-induced transient dichroism of the eosin probe. The anisotropy decay curves showed that mobility of haemagglutinin measured at pH 7.3 increased considerably with temperature with the greatest change occurring over the range 20-30 degrees C. However, at pH 5.2 no mobility was detectable over the time range of the experiment. The activity of the virus was determined by assaying haemolysis of human erythrocytes. The haemolytic activity showed an optimum at pH 5.2 and increased markedly with temperature, being negligible below 20 degrees C. In addition, inactivation of the virus by incubation at pH 5.2 was also strongly temperature dependent. A 15 min incubation at pH 5.2 inactivated the virus above 30 degrees C but had no effect below 20 degrees C. On the basis of these results, it is proposed that mobility of haemagglutinin is significant for its functional properties. When the pH is reduced from 7.3 to 5.2, the mobility observed at higher temperatures is required for the molecular rearrangements which accompany the fusion event. In the absence of an apposing membrane, these rearrangements result in irreversible aggregation of haemagglutinin in the viral membrane, and hence loss of mobility and activity.

Fusion of influenza virus with cardiolipin liposomes at low pH: mass action analysis of kinetics and extent

The kinetics and extent of low pH induced fusion between influenza virus and large unilamellar cardiolipin liposomes were investigated with an assay for lipid mixing based on fluorescence resonance energy transfer. The results were analyzed in terms of a mass action kinetic model, which views the overall fusion reaction as a sequence of a second-order process of virus-liposome adhesion or aggregation followed by the first-order fusion reaction itself. The fluorescence development during the course of the fusion process was calculated by numerical integration, employing separate rate constants for the initial aggregation step and for the subsequent fusion reaction. Analytical solutions were found for several limiting cases. Deaggregation of virus--liposome aggregates was explicitly taken into account but was found to be a minor effect under the conditions studied. The calculations gave good simulations and predictions for the kinetics and extent of fusion at different virus/liposome concentrations and ratios. At pH 5.0 and 37 degrees C, very high rate constants for aggregation and fusion were obtained, and essentially all of the virus particles were involved in the fusion process. Experiments at different virus/liposome ratios showed that fusion products may consist of a single virus particle and several liposomes but not of a single liposome and several virus particles. At pH 6.0, the rate constant for aggregation was the same as at pH 5.0, but the rate constant of fusion was about 5-fold lower, and only 25-40% of the virus particles were capable of fusing with the liposomes. The analytical procedure presented enables elucidation of the crucial role of the composition of target membrane vesicles in the initial adhesion and subsequent fusion of the virus at various pH values.

Electron microscopy of the low pH structure of influenza virus haemagglutinin

Influenza virus haemagglutinin mediates infection of cells by fusion of viral and endosomal membranes, triggered by low pH which induces a conformational change in the protein. We report studies of this change by electron microscopy, neutron scattering, sedimentation and photon correlation on X-31 (H3N2) haemagglutinin, both intact and bromelain cleaved, in various assemblies. HAs in all preparations showed a thinning at low pH, and a marked elongation which was removed on tryptic digestion, revealing altered features in the remaining stem portion of the molecule. A tentative model of the change is proposed, with reference to the known X-ray structure at neutral pH, in which major changes occur in the stem tertiary structure, while the top portion is only affected in its quaternary structure.

The lateral mobility of cell membrane components is not altered following cell fusion induced by Sendai virus

The interaction of Sendai virus glycoproteins with cell membranes was proposed to increase the lateral mobility of membrane proteins, enabling membrane fusion and the aggregation of intramembrane particles by thermotropic separation (Volsky, DJ & Loyter, A, Biochim biophys acta 514 (1978) 213 [13]; Maeda, T et al. Exp cell res 123 (1979) 333 [15]; and Kim, J & Okada, Y, Exp cell res 132 (1981) 125 [44]). In order to test this hypothesis, we employed fluorescence photobleaching recovery to investigate the effects of Sendai virus-induced fusion on the lateral mobility of membrane proteins and lipids in a variety of cell types (human erythrocytes, BHK21, HeLa, 3T3 NIH, and mouse spleen lymphocytes). The results of the lateral diffusion measurements demonstrate that no significant alterations occur in the lateral motion of membrane proteins or a fluorescent phospholipid on all the cell types examined, including cells which revealed high susceptibility to the virally mediated fusion (human erythrocytes and BHK21 cells). These findings suggest that a permanent increase in the lateral mobility of cell surface components does not generally occur during Sendai virus-induced cell fusion, and thus cannot play a role in the fusion mechanism. The possible involvement of transient alterations in the lateral mobility of membrane components in the fusion mechanism is discussed.

Structural alterations of the human erythrocyte membrane upon influenza virus attachment

Molecular events on the human erythrocyte membrane subsequent to influence virus binding were investigated by electron spin resonance (ESR) measurements after spin labeling of the cell membrane at different positions. Virus binding affected the glycocalyx structure as well as the physical state of the cytoskeleton at the inner leaflet, but not the lipid phase. A lateral reorganization of spin-labeled glycophorin was not indicated after virus attachment.

Studies on the mechanism of membrane fusion: site-specific mutagenesis of the hemagglutinin of influenza virus

Oligonucleotide-directed mutagenesis of a cDNA encoding the hemagglutinin of influenza virus has been used to introduce single base changes into the sequence that codes for the conserved apolar "fusion peptide" at the amino-terminus of the HA2 subunit. The mutant sequences replaced the wild-type gene in SV40-HA recombinant virus vectors, and the altered HA proteins were expressed in simian cells. Three mutants have been constructed that introduce single, nonconservative amino acid changes in the fusion peptide, and three fusion phenotypes were observed: substitution of glutamic acid for the glycine residue at the amino-terminus of HA2 abolished all fusion activity; substitution of glutamic acid for the glycine residue at position 4 in HA2 raised the threshold pH and decreased the efficiency of fusion; and, finally, extension of the hydrophobic stretch by replacement of the glutamic acid at position 11 with glycine yielded a mutant protein that induced fusion of erythrocytes with cells with the same efficiency and pH profile as the wild-type protein. However, the ability of this mutant to induce polykaryon formation was greatly impaired. Nevertheless, all the mutant proteins underwent a pH-dependent conformational change and bound to liposomes. These results are discussed in terms of the mechanism of HA-induced membrane fusion.

Cell surface influenza haemagglutinin can mediate infection by other animal viruses

We have used filter-grown Madin-Darby canine kidney (MDCK) cells to explore the mechanism by which influenza virus facilitates secondary virus infection. Vesicular stomatitis virus (VSV) and Semliki Forest virus (SFV) infect only through the basolateral surface of these polarized epithelial cells and not through the apical surface. Prior infection with influenza virus rendered the cell susceptible to infection by VSV or SFV through either surface. The presence of both a permissive and a restrictive surface for virus entry in the same cell allowed us to determine how the influenza infection enhanced the subsequent infection of a second virus. Biochemical and morphological evidence showed that influenza haemagglutinin on the apical surface serves as a receptor for the superinfecting virus by binding to its sialic acid-bearing envelope proteins. Influenza virus also facilitates secondary virus infection in non-epithelial cells; baby hamster kidney cells (BHK-21), which are normally resistant to infection by the coronavirus (mouse hepatitis virus MHV-A59), could be infected via the haemagglutinin-sialic acid interaction. Facilitation of secondary virus infection requires only the sialic acid-binding properties of the haemagglutinin since the uncleaved haemagglutinin could also mediate virus entry.

Cholesterol and the cell membrane

Recent studies concerning cholesterol, its behavior and its roles in cell growth provide important new clues to the role of this fascinating molecule in normal and pathological states.

Structure and composition of influenza virus. A small-angle neutron scattering study

A detailed analysis is presented of the small-angle neutron scattering curves of homogeneous solutions of influenza B virus, both intact and after treatment with bromelain, which removes the external glycoprotein spikes. The two sets of data are consistent with the following low-resolution structure: the virus particles are spherical, about 1200 A in diameter and of Mr about 180 X 10(6). The lipid bilayer is centred at a radius of 425 A, is 40 A to 50 A thick and constitutes 25% to 28% of the virus mass. The surface glycoproteins, predominantly haemagglutinin, contribute 40% to 46% of the total mass. Surprisingly little protein is found in the interior of the virus. It is suggested that the reason for this is that many particles do not contain the full complement of ribonucleoprotein complexes. These results are in good agreement with recent scanning transmission electron microscopic measurements of molecular mass and cryo-electron microscopic observations of the same preparations. Appendix 1 describes a new method of deriving spherical shell models from contrast variation neutron scattering data on viruses, in which scattering curves from all measured contrasts are used simultaneously. There is also a discussion of the assumptions and limitations implicit in the structural interpretation of such models, with emphasis on viruses containing lipid bilayers. Appendix 2 examines the effect on the scattering curves of various arrangements of the surface glycoproteins.

Fusion of Sendai virions with phosphatidylcholine-cholesterol liposomes reflects the viral activity required for fusion with biological membranes

Sendai virus envelopes were reconstituted after solubilization of intact virions with either Triton X-100 or octylglucoside. Envelopes obtained from Triton X-100, but not from octylglucoside solubilized virions, were hemolytic and promoted cell-cell fusion. Fluorescence dequenching studies [using N-4-nitrobenzo-2-oxa-1,3-diazole phosphatidylethanolamine-bearing viral envelopes] revealed that both preparations fused with negatively charged phospholipids. Fusion with phosphatidylcholine (PC)/cholesterol (chol) liposomes was promoted only by the hemolytic viral envelopes. Fluorescence dequenching studies, using intact virions bearing octadecylrhodamine B chloride, revealed that intact virions fused with PC/chol as well as with negatively charged phospholipids. Only fusion with PC/chol liposomes was inhibited by phenylmethylsulfonyl fluoride and dithiothreitol, reagents which are known to block the viral ability to fuse with biological membranes.

pH-induced alterations in the fusogenic spike protein of Semliki Forest virus

The spike glycoproteins of Semliki Forest virus mediate membrane fusion between the viral envelope and cholesterol-containing target membranes under conditions of mildly acidic pH (pH less than 6.2). The fusion reaction is critical for the infectious cycle, catalyzing virus penetration from the acidic endosome compartment. To define the role of the viral spike glycoproteins in the fusion reaction, conformational changes in the spikes at acid pH were studied using protease digestion and binding assays to liposomes and nonionic detergent. A method was also developed to prepare fragments of both transmembrane subunit glycopolypeptides of the spike, E1 and E2, which lacked the hydrophobic anchor peptides. Unlike the intact spikes the fragments were monomeric and therefore useful for obtaining information on conformational changes in individual subunits. The results showed that both E1 and E2 undergo irreversible conformational changes at the pH of fusion, that the conformational change of E1 depends, in addition to acidic pH, on the presence of cholesterol, and that no major changes in the solubility properties of the spikes takes place. On the basis of these findings it was concluded that fusion involves both subunits of the spike and that E1 confers the stereo-specific sterol requirement. The results indicated, moreover, that acid-induced fusion of Semliki Forest virus differs in important respects from that of influenza virus, another well-defined model system for protein-mediated membrane fusion.

Normal expression of polymorphic endogenous retroviral RNA containing segments identical to mink cell focus-forming virus

In the absence of infectious virus, strains of mice express polyadenylated RNA transcripts homologous to the genome of murine leukemia virus. In addition to transcripts consistent with full-length and spliced env retroviral RNAs, several unique RNA species which lack the env sequence accumulate in a tissue-specific manner. These RNA species are presumed to be transcribed from endogenous retroviral sequences that constitute the bulk of the murine leukemia virus-related sequences in the murine genome. To determine the relationship of these RNA transcripts to infectious murine leukemia virus and the precise structural basis of the heterogeneity observed for the env-lacking transcripts, we isolated and sequenced cDNA recombinants representing the RNAs expressed in strain 129 GIX+ mice. Comparisons of the nucleotide sequences demonstrated that the endogenous retroviral transcripts differed in pol, p15E, and R-peptide regions by single nucleotide changes. In contrast, the gp70-coding regions of two cDNA clones derived from epididymis and liver were completely homologous over a 599-nucleotide overlapping sequence. The structures of env-lacking transcripts were examined in two independent cDNA clones, and each was found to contain a different deletion that was potentially mediated by seven-base pair direct repeats in the intact sequence. The extensive sequence homology between cDNAs allowed construction of a cumulative sequence map of the 3' end of an intact endogenous retroviral transcript. A comparison of this sequence with infectious ecotropic and mink cell focus-forming viruses revealed that the endogenous transcripts are highly homologous with the substituted portions of leukemogenic mink cell focus-forming viruses and therefore further define the boundaries of recombination required to generate these viruses.

Secondary structure of the pore-forming colicin A and its C-terminal fragment. Experimental fact and structure prediction

Conformational investigations, using circular dichroism, on the pore-forming protein, colicin A (Mr 60 000), and a C-terminal bromelain fragment (Mr 20 000) were undertaken to estimate their secondary structure and to search for pH-dependent conformational changes. Colicin A and the bromelain peptide are mainly alpha-helical with an enrichment of the alpha-helical content in the C-terminal domain carrying the ionophoric activity. The non-negligible beta-sheet structure in the C-terminal domain is unstable and is easily transformed into alpha-helix upon decreasing the polarity of the solvent. No evidence of pH-dependent conformational modification, correlated with modification of colicin A activity, could be obtained. The secondary structure estimated on the basis of experimental data favoured a model in which the pore is built of a minimal number of six transmembrane alpha-helical segments. Search for such segments in the amino acid sequence of the C-terminal domain of colicin A was carried out by combining secondary structure prediction methods with hydrophobicity and hydrophobic movement calculations. Similar calculations on the C-terminal domains of colicin E1 and IB indicate a common structure of the pores formed by colicin A, E1 and IB. Only two or three putative transmembrane segments could be selected in the sequences of colicin A, IB or E1. As a result, it is concluded that the channel is probably not built by a single colicin molecule but more likely by an oligomer.

Expression at the cell surface of biologically active fusion and hemagglutinin/neuraminidase proteins of the paramyxovirus simian virus 5 from cloned cDNA

cDNAs encoding the mRNAs for the fusion protein (F) and the hemagglutinin/neuraminidase protein (HN) of the paramyxovirus simian virus 5 have been inserted into a eukaryotic expression vector under the control of the simian virus 40 late promoter. The F and HN proteins synthesized in recombinant infected cells are indistinguishable in terms of electrophoretic mobility and glycosylation from the proteins synthesized in simian virus 5-infected cells. In addition, the expressed F and HN proteins have been shown to be anchored in the plasma membrane in a biologically active form by indirect live cell immunofluorescence, the F-mediated formation of syncytia, and the ability of HN to cause the hemadsorption of erythrocytes to the infected cell surface.

Revelation through exploitation: the viral model for intracellular traffic

Enveloped viruses exploit the existing routes of membrane traffic to enter and leave the host cell. The similarity between viral envelopes and cellular membranes has allowed the use of animal viruses as probes to examine aspects of intracellular traffic.

The interaction of Sendai virus with negatively charged liposomes: virus-induced lysis of carboxyfluorescein-loaded small unilamellar vesicles

The interaction of Sendai virus with small, unilamellar vesicles, lacking virus receptors and loaded with self-quenched 6-carboxyfluorescein, was studied. Sendai virions induced release of carboxyfluorescein from vesicles composed of negative charged phospholipids, despite the fact that they did not contain virus receptors. Preliminary experiments indicate that the carboxyfluorescein release is accompanied by mixing of the virus and liposome lipids and their entrapped contents, suggesting liposome-virus fusion. No release of carboxyfluorescein was observed with vesicles containing only phosphatidylcholine. The rate of virus-induced carboxyfluorescein release was temperature dependent; the lytic activity of the virus was greatly enhanced above 25 degrees C. This effect was not due to a thermal phase transition of the lipids in either the lipid vesicles or the virions. Virus-induced carboxyfluorescein release was inhibited by the presence of calcium ions in the medium and of cholesterol in the lipid vesicles. It increased with increasing concentrations of either the lipid vesicles or the virions. pretreatment of virions with increasing concentrations of three different proteolytic enzymes (trypsin, chymotrypsin and proteinase) inhibited the virus' ability to cause release of carboxyfluorescein from negatively charged liposomes. Inhibition of the viral lytic activity was also observed after virions were incubated above 56 degrees C.

pH-dependence of the phospholipid interaction of diphtheria-toxin fragments

Photoreactive phospholipids have been used to probe the lipid interaction of diphtheria toxin. Low pH values induce the membrane insertion of both the binding and enzymic fragments of the toxin. The efficiency of this process is much higher with asolectin than with egg lecithin (phosphatidylcholine)/cholesterol liposomes. The low-pH-induced interaction of the toxin fragments with the membrane hydrocarbon phase is more evident for the enzymic A-chain than for the binding B-chain, and it is fully reversed by returning the pH to neutrality.

Sendai virus envelope glycoproteins become laterally mobile on the surface of human erythrocytes following fusion

Fluorescence photobleaching recovery has been employed to study the lateral mobility of the Sendai virus envelope glycoproteins (HN, neuraminidase/hemagglutinin protein (HN) fusion protein (F) on the surface of human erythrocytes. Our results indicate that the two viral glycoproteins are laterally immobile on the cell surface prior to fusion, and become mobile during the fusion process. The two fused glycoproteins are mobilized to the same extent (diffusion coefficients of 3.1-3.3 X 10(-10) cm2/sec with mobile fractions of 0.53-0.57 for both HN and F). Their mobilization is blocked under conditions that allow virus adsorption and hemagglutination, but not virus-cell or cell-cell fusion. These findings suggest a possible role for the lateral diffusion of the viral glycoproteins in the mechanism of cell-cell fusion, enabling them to perturb the membranes of adjacent cells and lead to cell-cell fusion.

Processes controlling sperm-egg fusion

Sperm interaction with the egg envelopes triggers the acrosome reaction. Indeed, sperm-egg fusion is accomplished by the fusion of the acrosomal process (or of the exposed inner acrosomal membrane in mammals) with the egg plasma membrane. Fusion must be preceded by the establishment of molecular contact between the two membranes. It is suggested that, as in the case of artificial phospholipid membranes, the two major obstacles to the establishment of molecular contact are electrostatic repulsion and the hydration barrier. It is argued that morphology of the acrosome is such as to favour the overcoming of such barriers. By analogy with the conditions governing fusion of artificial phospholipid membranes and cell fusion, it is proposed that the following processes play a role in sperm-egg fusion. The large calcium uptake accompanying the acrosome reaction may help fusion either through the known effect of calcium on fusion of phospholipid membranes or by shielding the surface charges of the acrosomal process. Fusogenic proteins at the surface of the acrosomal process are likely to play a role in the fusion of the acrosomal process with the egg plasma membrane. The activation of phospholipases in conjunction with the acrosome reaction may also be instrumental in sperm-egg fusion through the transient production of lysophosphatides. Clearance or translocation of intramembraneous proteins in the egg plasma membrane at the site of contact with the acrosomal process may also be required for fusion. Lastly it is suggested that a translocation or a conformational change of some proteins of the egg plasma membrane, which is required for fusion, may be induced by the depolarization of the egg plasma membrane that follows molecular contact with the acrosomal process.

Effect of pH on the conformation of diphtheria toxin and its implications for membrane penetration

The pH-triggered change in diphtheria toxin conformation and the physical properties of the toxin above and below the transition pH have been examined. Exposure to low pH (less than or equal to 5 at 23 degrees C, less than or equal to 5.3 at 37 degrees C) triggers a rapid (t1/2 less than 30 s) change in toxin conformation; the transition occurs over a narrow pH range (0.2 unit). Below the transition pH, buried tryptophans become exposed, and the toxin becomes hydrophobic, binding very tightly to detergent. Aggregation is observed at low pH, probably due to this extreme hydrophobicity. Circular dichroism and fluorescence properties show that the low-pH conformation is not extensively unfolded. Therefore, the toxin "opens" at low pH without becoming a random coil. The conformation change is partly irreversible, and the degree of irreversibility parallels the degree of aggregation. Reduction of the disulfide bonds does not increase hydrophobicity at neutral pH. Furthermore, none of the structural variants of toxin (monomer or dimer, bound to ApUp or free, and nicked between subunits or intact) are hydrophobic at neutral pH or differ in transition pH markedly. Therefore, these factors do not mimic the effect of low pH. These observations are consistent with a functional role for the pH-triggered changes during penetration of the membranes of acidic organelles. The toxin may have adapted a conformational change similar to partial denaturation for a critical role in function. The possible nature of the pH-sensitive interactions and the effects of aggregation are discussed briefly.

Infectious entry of murine retroviruses into mouse cells: evidence of a postadsorption step inhibited by acidic pH

The entry into cells by many enveloped RNA viruses is accomplished by endocytosis and subsequent penetration of the endosomal membrane by an acidic pH-dependent fusion event. In the current study, we examined early events in the infectious entry of mouse retroviruses, using as a framework the observation that infection of a mouse tail skin cell line by the ecotropic virus Friend murine leukemia virus was inhibited at mildly acidic pH (pH 6). This inhibition operated on a postadsorption step, since binding of virus was unaffected at this pH. The rate of penetration of preadsorbed virus, which displayed first-order kinetics, was markedly affected by changes in the pH of the medium. The half-time for disappearance of infectious cell surface virus at 37 degrees C was approximately 10 min at pH 7.6. At pH 6.0, however, greater than 98% of the adsorbed infectivity remained at the cell surface after 45 min. This cell surface virus, though not infecting the cell at pH 6.0, retained its capacity to enter and infect the cell when the pH of the medium was raised. Acidic pH had little effect on the rate of fluid uptake by the cells, as measured by internalization of [3H]sucrose, indicating that global inhibition of endocytosis had not occurred. In contrast, cell fusion induced by Friend murine leukemia virus was optimal at pH 7.6 but markedly inhibited at a pH of less than 6.4. This inhibitory effect of acidic pH on membrane fusion is unique among the enveloped viruses which have been studied and would preclude entry of Friend murine leukemia virus from within acidified endocytic vesicles. Entry of other members of the ecotropic, mink cell focus-forming, and xenotropic host range groups displayed similar pH sensitivity. However, one xenotropic virus was relatively resistant to the effect of acidic pH, suggesting that differences might exist in the requirements for entry of different retroviruses.

Characterization of the fusogenic properties of Sendai virus: kinetics of fusion with erythrocyte membranes

A novel fluorescence assay [Hoekstra, D., De Boer, T., Klappe, K., & Wilschut, J. (1984) Biochemistry 23, 5675-5681] has been used to characterize the fusogenic properties of Sendai virus, using erythrocyte ghosts and liposomes as target membranes. This assay involves the incorporation of the "fusion-reporting" probe in the viral membrane, allowing continuous monitoring of the fusion process in a very sensitive manner. Fusion was inhibited upon pretreatment of Sendai virus with trypsin. Low concentrations of the reducing agent dithiothreitol (1 mM) almost completely abolished viral fusion activity, whereas virus binding was reduced by ca. 50%, indicating that the fusogenic properties of Sendai virus are strongly dependent on the integrity of intramolecular disulfide bonds in the fusion (F) protein. Pretreatment of erythrocyte ghosts with nonlabeled Sendai virus inhibited subsequent fusion of fluorophore-labeled virus irrespective of the removal of nonbound virus, thus suggesting that the initial binding of the virus to the target membrane is largely irreversible. As a function of pH, Sendai virus displayed optimal fusion activity around pH 7.5-8.0. Preincubation of the virus at suboptimal pH values resulted in an irreversible diminishment of its fusion capacity. Since virus binding was not affected by the pH, the results are consistent with a pH-induced irreversible conformational change in the molecular structure of the F protein, occurring under mild acidic and alkaline conditions. In contrast to virus binding, fusion appeared to be strongly dependent on temperature, increasing ca. 25-fold when the temperature was raised from 23 to 37 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Monoclonal antibodies detect different forms of influenza virus hemagglutinin during viral penetration and biosynthesis

Monoclonal antibodies specific for the influenza virus A/PR/8/34 hemagglutinin (HA) were used to examine the structure of the HA glycoprotein by immunofluorescence techniques during infection of MDCK cells. One antibody (Y8-10C2), shown previously to detect conformational alterations in the HA coinciding with the acid induction of viral fusion activity, bound to internalized virus but not to virus adsorbed to the cell surface. The binding of Y8-10C2 was completely inhibited by ammonium chloride treatment of the cells. These findings are consistent with the idea that Y8-10C2 detects conformational changes in the HA which accompany the acid-induced fusion of viral and endosomal membranes. The same antibody also bound to newly synthesized HA but not to later forms of cytoplasmic HA or to HA incorporated into the cell membrane during virus maturation. A possible common denominator of the antigenic changes detected by antibody Y8-10C2 during virus entry and replication may be alterations in the structural relationship among the three HA monomers which form the mature HA molecule.