Isolation of paramyxovirus glycoproteins. Association of both hemagglutinating and neuraminidase activities with the larger SV5 glycoprotein

Inhibition of measles virus-induced cell-cell fusion with a monoclonal antibody directed against the haemagglutinin

A neutralizing monoclonal antibody (C26-15) against the haemagglutinin (H protein) of measles virus was generated which caused cell-cell fusion inhibition in cultures of measles virus-infected cells. It was shown that this phenomenon coincided with a down-regulation of the expression of both the H protein and the fusion (F) protein. We also showed cell-cell fusion inhibition with a polyclonal rabbit serum directed against Tween-ether inactivated measles virus, which did not contain biologically active antibodies against the F protein. Cell-cell fusion inhibition caused by anti-H antibodies is distinct from cell-cell fusion inhibition induced by a direct interaction of anti-F antibody with the F protein in the membrane of infected cells. Since both mechanisms may also be involved in the in vivo situation, the exclusive role for the generation of anti-F antibody to prevent virus spread by cell-cell fusion in vivo is questioned. It is speculated that the observed down-regulation of both glycoproteins may lead to a less efficient killing of infected cells by cytotoxic T-lymphocytes, which may constitute an alternative explanation for the insufficient protection after vaccination with an inactivated measles vaccine.

Determination of the orientation of an integral membrane protein and sites of glycosylation by oligonucleotide-directed mutagenesis: influenza B virus NB glycoprotein lacks a cleavable signal sequence and has an extracellular NH2-terminal region

The membrane orientation of the NB protein of influenza B virus, a small (Mr, approximately 18,000) glycoprotein with a single internal hydrophobic domain, was investigated by biochemical and genetic means. Cell fractionation and protein solubility studies indicate NB is an integral membrane protein, and NB has been shown to be a dimer under nonreducing conditions. Treatment of infected-cell surfaces with proteinase K and endoglycosidase F and immunoprecipitation with a site-specific antibody suggests that the 18-amino-acid NH2-terminal region of NB is exposed at the cell surface. Oligonucleotide-directed mutagenesis to eliminate each of the four potential sites of N-linked glycosylation and expression of the mutant NB proteins in eucaryotic cells suggest that the two sites adjacent to the NH2 terminus are glycosylated. This provides further evidence that NB, which lacks a cleavable NH2-terminal signal sequence, has an exposed NH2 terminus at the cell surface.

Expression of gangliosides as receptors at the cell surface controls infection of NCTC 2071 cells by Sendai virus

The involvement of gangliosides as receptors for Sendai virus was established previously using experimentally produced receptor-deficient cells. In the search for a naturally occurring counterpart, NCTC 2071 cells emerged as a likely candidate. These cells in their native state were not agglutinated nor infected by Sendai virus, but were infected by the virus when the gangliosides GD1a, GT1b, or GQ1b were supplied in the culturing medium. Preliminary analysis indicated that NCTC 2071 cells contained an unusually high ratio of sialoglycoproteins to gangliosides. A brief treatment of the cell surface with the protease trypsin made greater than 99% of the native monolayer susceptible to infection by the wild-type virus which contains the viral attachment protein HN. (Incubation of the trypsin-treated cells with a temperature-sensitive mutant missing HN produced no detectable infection.) The increased binding of cholera toxin, a ganglioside-specific probe, after incubation of the cells with trypsin and sialidase, was consistent with the hypothesis that gangliosides more complex than GM1 are on the surface of NCTC 2071 cells and that trypsin treatment increases their accessibility. The presence of receptor gangliosides in lipid extracts of NCTC 2071 cells was confirmed by thin-layer chromatography of the ganglioside fraction and by the binding of cholera toxin. These results demonstrate that cells containing receptor gangliosides may still be resistant to infection because these are not expressed properly at the cell surface as receptors for interaction with the HN protein of Sendai virus.

Converting Sendai virus into a specific fusogen whose cell target can be selected

Covalent intermolecular hybrids of Fab anti-hemagglutinin-neuraminidase (HN) monoclonal antibody and avidin were prepared and characterized. These conjugates were used to block and redirect the fusion activity of Sendai virus (SV). After incubation of SV with Fab anti-HN: avidin conjugate on ice for 1-2 h, the SV fused only those P815 or BW5147 cells which were labeled with biotin-modified anti-cell surface immunoglobulin. The levels of cell-cell fusion obtained were at least as high as those achieved with unmodified SV and unlabeled P815 or BW5147 cells. These results demonstrate that it is possible to block the normal agglutinating activity of the HN molecules of SV and to introduce a new cell recognition feature without negating the fusogenic potential of the virus. Such an approach may be useful in harnessing the fusion activity of SV to a targeted delivery system for microinjection of macromolecules into selected cell populations.

Identification of the glycoproteins of lymphocystis disease virus (LDV) of fish

Analysis of highly purified fish Lymphocystis Disease Virus (LDV), strain Leetown NFH, by three different methods, namely periodic Acid Schiff reaction, radiolabelling with tritiated fucose and N-acetyl-D-glucosamine and staining with three lectins, indicated that ten glycoproteins were associated with the virus structure. Six of them were detected by all of the three methods, three by both radiolabelling and lectin staining but only one by the lectin technique. Localization of these glycoproteins at the surface or inside the virion is discussed.

Fusion of Newcastle disease virus with liposomes: role of the lipid composition of liposomes

We demonstrate here that fusion occurs between the membrane of the Newcastle disease virus (NDV) and liposomes. Fluorescence dequenching studies (using Rhodamine-bearing viral envelopes) revealed the mixing of the lipids constituting the viral and liposomal membrane. The digestion of internal viral proteins by trypsin-containing liposomes indicated the mixing of the internal aqueous compartments. This last assay is independent of exchange of lipids between liposomal and viral membrane in the absence of fusion. Investigation of the effects of liposomal composition indicated that the presence of phosphatidylethanolamine and gangliosides are essential to optimize fusion. The fact that the Newcastle disease virus membrane can fuse with liposome also confirms that fusion must be determined by the viral proteins and could be mostly independent of the nature or presence of the host proteins.

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.

Antigenic relationships between avian paramyxoviruses. I. Quantitative characteristics based on hemagglutination and neuraminidase inhibition tests

Comprehensive hemagglutination inhibition (HI) and neuraminidase inhibition (NI) cross reaction tests were performed using 8 of 9 serotypes of avian paramyxoviruses (PMV). The studies were designed as full scale repeating experiments which permitted an adequate statistical treatment and elaboration of quantitative criteria of antigenic kinship. The results have shown diverse antigenic relationships between different avian paramyxovirus (PMV) serotypes which were asymmetric in some cases. The antigenic relationships found by HI test did not always parallel those found by NI tests. The antigenic inter-relationships have been displayed quantitatively in a diagram. This has given a basis for some suggestions concerning: the independent antigenic drift of the HA and Nase antigenic sites of hemagglutinin-neuraminidase (HN) glycoprotein of avian PMVs; a tentative subdivision of the whole group of avian PMVs into two subgroups: the first including PMV-2 and PMV-6 serotypes and the second including PMV-1, PMV-3, PMV-4, PMV-7, PMV-8 and PMV-9 serotypes; the conception that genomic material coding for the HN glycoprotein consists of a "common-to-all-the-PMVs" portion and a "serotype-specific" portion, on one hand, and of a "conserved" portion and a "variable" portion, on the other; the ratios between the portions have been shown to be different for, at least, certain PMV serotypes; the evolutionary pathways of the avain PMV HN antigenic drift.

Neutralizing activity of the antibodies against two kinds of envelope glycoproteins of Sendai virus

Murine monoclonal antibodies against the fusion (F) and hemagglutinin-neuraminidase (HN) proteins of Sendai virus (SV) were prepared and studied on their antiviral activities, particularly on the neutralization of infectivity. On the analysis with solid phase competitive ELISA, 26 anti-HN antibodies were divided into at least four groups (HN-I, -II, -III and -IV). Antigenic sites recognized by the HN-I, -II, and -III group antibodies topographically separate from each other. Sites recognized by the HN-IV group antibodies overlaps partially with ones recognized by the HN-I, HN-II and -III group antibodies. The antibodies belonging to the HN-III group highly neutralize the infectivity of SV and weakly or not at all inhibit the hemagglutination (HA). In contrast, the HN-IV group antibodies strongly inhibit HA, but weakly neutralize the infectivity. Adsorption of SV to chicken red blood cells or L cells is inhibited by the HN-IV antibodies, but scarcely by the HN-III antibodies. On the other hand, incubation with HN-III antibodies of HeLa cells that have been preadsorbed with SV at 4 degrees C, followed by culture at 37 degrees C, causes inhibition of infection, but the HN-IV antibodies do not effectively interfere with such infection. The competitive ELISA showed that 17 anti-F antibodies were divided into two groups (F-I and -II). Two antigenic sites recognized by the antibodies, however, seem to be near to each other because a certain competition is observed between the antibodies of both groups. Two of the seven antibodies belonging to the F-II group inhibit the hemolysis activity and also neutralize the infectivity of SV, but the other five F-II antibodies do not. One of the anti-F antibodies has a low HI activity, and, in competition tests, competes with one of the anti-HN antibodies (HN-IV).

Human parainfluenza type 3 virus hemagglutinin-neuraminidase glycoprotein: nucleotide sequence of mRNA and limited amino acid sequence of the purified protein

The nucleotide sequence of mRNA for the hemagglutinin-neuraminidase (HN) protein of human parainfluenza type 3 virus obtained from the corresponding cDNA clone had a single long open reading frame encoding a putative protein of 64,254 daltons consisting of 572 amino acids. The deduced protein sequence was confirmed by limited N-terminal amino acid microsequencing of CNBr cleavage fragments of native HN that was purified by immunoprecipitation. The HN protein is moderately hydrophobic and has four potential sites (Asn-X-Ser/Thr) of N-glycosylation in the C-terminal half of the molecule. It is devoid of both the N-terminal signal sequence and the C-terminal membrane anchorage domain characteristic of the hemagglutinin of influenza virus and the fusion (F0) protein of the paramyxoviruses. Instead, it has a single prominent hydrophobic region capable of membrane insertion beginning at 32 residues from the N terminus. This N-terminal membrane insertion is similar to that of influenza virus neuraminidase and the recently reported structures of HN proteins of Sendai virus and simian virus 5.

The interaction of Sendai virus glycoprotein-bearing recombinant vesicles with cell surfaces

Sendai virus glycoproteins HN and F were purified by immunoaffinity chromatography from virions disrupted by beta-D-octylglucoside. The purified glycoproteins were reconstituted in recombinant vesicles with phosphatidylcholine or phosphatidylethanolamine and phosphatidylserine. P815 or EL-4 cells treated with glycoprotein HN/F-phosphatidylcholine recombinant vesicles acquired the glycoproteins and retained them in the plasma membrane for 4 h as demonstrated by surface immunofluorescence specific for each protein. Cells treated with glycoprotein HN-phosphatidylcholine recombinant vesicles initially bore glycoprotein HN on the surface but the protein eluted within 2 h. Surfaces of cells treated with glycoprotein F-phosphatidylcholine recombinant vesicles did not acquire the glycoprotein. Cells treated with glycoprotein HN-phosphatidylethanolamine: phosphatidylserine recombinant vesicles or glycoprotein F-phosphatidylethanolamine: phosphatidylserine recombinant vesicles in the presence of 5 mM Ca2+ acquired each protein for at least 2 h. Experiments showed that the acquired glycoproteins capped with antibody and that when glycoproteins HN and F were together on the surface they co-capped. Acquired viral glycoproteins did not co-cap with intrinsic H-2 glycoproteins.

Respiratory syncytial virus envelope glycoprotein (G) has a novel structure

Amino acid sequence of human respiratory syncytial virus envelope glycoprotein (G) was deduced from the DNA sequence of a recombinant plasmid and confirmed by limited amino acid microsequencing of purified 90K G protein. The calculated molecular mass of the protein encoded by the only long open reading frame of 298 amino acids was 32,588 daltons and was somewhat smaller than the 36K polypeptide translated in vitro from mRNA selected by this plasmid. Inspection of the sequence revealed a single hydrophobic domain of 23 amino acids capable of membrane insertion at 41 residues from the N-terminus. There was no N-terminal signal sequence and the hydrophilic N-terminal 20 residues probably represent the cytoplasmic tail of the protein. The N-terminally oriented membrane insertion was somewhat analogous to paramyxovirus hemagglutinin-neuraminidase (HN) and influenza neuraminidase (NA). The protein was moderately hydrophilic and rich in hydroxy-amino acids. It was both N- and O-glycosylated with the latter contributing significantly to the net molecular mass 90K.

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.

Isolation and characterization of the measles virus F1 polypeptide: comparison with other paramyxovirus fusion proteins

Measles virus fusion (F) protein has been isolated by immunoadsorption to a complex of monoclonal antibodies bound to protein A-Sepharose. The 41-kDa F1 component of the fusion protein was obtained pure in high yield by preparative SDS-polyacrylamide gel electrophoresis. The amino acid composition of the F1 chain was determined and the N-terminal sequence was analyzed for 40 residues. The structure determined is largely hydrophobic, with 24 residues of Val, Ile, Leu, Met, Phe, or Ala. Comparison with previously published data on the F1 polypeptide of Sendai virus showed considerable similarity in amino acid composition. Extensive N-terminal sequence homologies with F1 polypeptides of different paramyxoviruses are also noticed, including a nine-residue segment strictly conserved among four F1 polypeptides studied, as well as a weaker but distinct and Gly-rich sequence homology with the influenza A and B virus HA2 polypeptides. The evolutionary conservation of the N-terminal region at the site of cleavage of surface glycoproteins of the two families of myxoviruses highlights its specialized function in membrane fusion.

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.

Proteins associated with human parainfluenza virus type 3

The polypeptides associated with human parainfluenza virus type 3 were identified. Five proteins were present in detergent- and salt-resistant viral cores. Of these, three proteins designated NP0, NP1, and NP2 of 68,000, 58,000, and 52,000 daltons, respectively, were stably associated with 50S RNA in CsCl gradient-purified nucleocapsids. The amounts of NP1 and NP2 were variable, and these proteins were shown to be structurally related to the major nucleocapsid protein (NP0) by partial Staphylococcus aureus V8 protease mapping. The other core proteins included a 240K protein designated L (candidate for the viral polymerase) and an 84K protein designated as the phosphoprotein (P) on the basis of a predominant incorporation of Pi. The viral envelope had four prominent proteins (72, 53, 40, and 12K) under reducing conditions of electrophoresis. The 72 and 53K proteins were specifically labeled with [3H]glucosamine and [3H]mannose. When sulfhydryl reagents were removed, a new 62K protein was visualized in place of the 72, 53, and 12K proteins. The 53 and 12K proteins were interpreted to be the two subunits (F1 and F2) of the fusion protein, and the 72K protein was designated as the HN (hemagglutinin-neuraminidase) glycoprotein. The unglycosylated 40K protein represented the viral matrix protein (M). Immunoprecipitation of infected cell lysates with rabbit hyperimmune antiserum against purified virus confirmed the viral origin of these polypeptides.

Identification and predicted sequence of a previously unrecognized small hydrophobic protein, SH, of the paramyxovirus simian virus 5

A previously unrecognized gene (SH) has been identified on the virion RNA of the paramyxovirus simian virus 5 between the genes for the fusion protein and the hemagglutinin-neuraminidase. An SH mRNA of 292 nucleotides (plus polyadenylate residues), transcribed from the SH gene, has been identified. The SH mRNA contains a single open reading frame which encodes a polypeptide of 44 amino acids with a molecular weight of 5,012. The SH polypeptide is predicted to contain an extensive hydrophobic region. This protein has been identified in simian virus 5-infected cells, and it has been shown to be encoded by the SH mRNA by in vitro translation of size-fractionated mRNAs, hybrid-arrest translation, and hybrid-selection translation.

Isolation and studies of myxovirus glycoproteins

The isolation of ortho- and paramyxovirus glycoproteins using a new nonionic detergent (MESK) is reported. MESK was shown to solubilize most of the viral envelope glycoproteins without decreasing their biologic activity. Solubilized glycoproteins are not contaminated by any internal viral proteins or by appreciable quantities of viral envelope lipids. The removal of MESK by dialysis resulted in the formation of glycoprotein micelles. The immunogenic activity of isolated glycoproteins was compared to that of virus particles. Immunization with isolated glycoproteins was shown to protect mice against a lethal influenza infection. Virions were treated with MESK in the presence of exogenous egg phosphatidylcholine, detergent was removed by dialysis and the glycoprotein was reconstituted in the vesicles. This reconstitution was accompanied by restoration of the haemolytic activity of Sendai virus proteins up to that of native virus particles. The level of activity, also the morphology and buoyant density of the vesicle were dependent on the protein/lipid ratio. MESK proved to be of value for the selective solubilization of the surface glycoproteins of animal enveloped viruses and their reconstitution in liposomes.

Reconstitution and fusogenic properties of Sendai virus envelopes

Sendai virus membranes were reconstituted by detergent dialysis, using the non-ionic detergents Triton X-100 and octyl glucoside. Membrane reassembly was determined by measuring the surface-density-dependent efficiency of resonance energy transfer between two fluorescent phospholipid analogues, which were co-reconstituted with the viral envelopes. The functional incorporation of the viral proteins was established by monitoring the ability of the reconstitution products to fuse with erythrocyte membranes, utilizing assays based on either resonance energy transfer or on relief of fluorescence selfquenching. The persistent adherence of residual Triton X-100 with the reconstituted membrane was revealed by an artificial detergent-effect on the resonance energy transfer efficiency and the occurrence of hemolysis of human erythrocytes under conditions where fusion does not occur. Properly reconstituted Sendai virus envelopes were obtained with octyl glucoside. The fusion activity of the viral envelopes was dependent on the initial concentration of octyl glucoside used to disrupt the virus and the rate of detergent removal. Rapid removal of detergent by dialysis against large volumes of dialysis buffer (ratio 1:850) or by gel filtration produced reconstituted membranes capable of inducing hemagglutination but significant fusion activity was not detected. By decreasing the volume ratio of dialysate versus dialysis buffer to 1:250 or 1:25, fusogenic viral envelopes were obtained. The initial fusion kinetics of the reconstituted viral membrane and the parent virus were different in that both the onset and the initial rate of fusion of the reconstituted membranes were faster, whereas the extents to which both particles eventually fused with the target membrane were similar. The differences in the initial fusion kinetics lead us to suggest that the details of the fusion mechanism between Sendai virus and the target membrane involve factors other than the mere presence of glycoproteins F and HN in the viral bilayer. Finally, the results also indicate that determination of the viral fusion activity in a direct manner, rather than by an indirect assay, such as hemolysis, is imperative for a proper evaluation of the functional properties retained upon viral reconstitution.

Subunit vaccines against enveloped viruses: virosomes, micelles and other protein complexes

The envelope proteins (the peplomers) of enveloped viruses are the components that are important for induction of protective immunity. This article reviews methods and problems of making subunit vaccines of peplomers. In the first section, the solubilization of enveloped viruses with detergent is discussed. The preparation of envelope proteins into defined different physical forms is described, i.e. monomeric and micelle forms and the reconstitution of the protein into lipid vesicles (virosomes). Finally, the preparation of a new type of complex is described (named iscom), which is highly immunogenic. In the following sections the efficacy of the different physical forms are reviewed and it is concluded that monomeric forms must be avoided since they are poorly immunogenic and they may even have a suppressive effect on the immune response. The multimeric micelles, virosomes and iscoms are all immunogenic. The iscom is an interesting new concept that can be used to produce efficient subunit vaccines.

Hemagglutinin-neuraminidase protein of the paramyxovirus simian virus 5: nucleotide sequence of the mRNA predicts an N-terminal membrane anchor

The nucleotide sequence of a cloned cDNA copy of the mRNA coding for the hemagglutinin-neuraminidase of the paramyxovirus SV5 was determined. There was a single large open reading frame on the mRNA which encoded a protein of 565 amino acids with a molecular weight of 62,134. The deduced amino acid sequence indicated that the only major hydrophobic region in the protein sufficiently long to anchor the protein in the membrane is located near the N terminus (amino acids 18 to 36). It is suggested that, like the influenza virus neuraminidase, hemagglutinin-neuraminidase of paramyxoviruses is oriented with its N terminus inserted into the membrane.

Respiratory syncytial virus fusion glycoprotein: nucleotide sequence of mRNA, identification of cleavage activation site and amino acid sequence of N-terminus of F1 subunit

The amino acid sequence of respiratory syncytial virus fusion protein (Fo) was deduced from the sequence of a partial cDNA clone of mRNA and from the 5' mRNA sequence obtained by primer extension and dideoxysequencing. The encoded protein of 574 amino acids is extremely hydrophobic and has a molecular weight of 63371 daltons. The site of proteolytic cleavage within this protein was accurately mapped by determining a partial amino acid sequence of the N-terminus of the larger subunit (F1) purified by radioimmunoprecipitation using monoclonal antibodies. Alignment of the N-terminus of the F1 subunit within the deduced amino acid sequence of Fo permitted us to identify a sequence of lys-lys-arg-lys-arg-arg at the C-terminus of the smaller N-terminal F2 subunit that appears to represent the cleavage/activation domain. Five potential sites of glycosylation, four within the F2 subunit, were also identified. Three extremely hydrophobic domains are present in the protein; a) the N-terminal signal sequence, b) the N-terminus of the F1 subunit that is analogous to the N-terminus of the paramyxovirus F1 subunit and the HA2 subunit of influenza virus hemagglutinin, and c) the putative membrane anchorage domain near the C-terminus of F1.

An alternative route of infection for viruses: entry by means of the asialoglycoprotein receptor of a Sendai virus mutant lacking its attachment protein

During the first stage of infection, the paramyxovirus Sendai virus attaches to host cells by recognizing specific receptors on the cell surface. Productive virus-cell interactions result in membrane fusion between the viral envelope and the cell surface membrane. It has recently been shown that the ganglioside GD1a and its more complex homologs GT1b and GQ1b are cell surface receptors for Sendai virus. We report in this paper that the temperature-sensitive mutant ts271 of the Enders strain of Sendai virus lacks the viral attachment protein HN and the biological activities of hemagglutination and sialidase activity associated with it when the virus is grown at 38 degrees C. This HN- virus was unable to infect or agglutinate conventional host cells that contained receptor gangliosides and were readily infected by the parental wild-type virus. The HN- virus did, however, attach to and infect Hep G2 cells, a line of hepatoma cells that retains the asialoglycoprotein receptor (ASGP-R) upon continuous culture. This receptor is a mammalian lectin that recognizes galactose- or N-acetylgalactosamine-terminated proteins. In accordance with the known properties of this receptor, infection by the HN- virus was abolished by treatment of Hep G2 cells with sialidase, by the presence of Ca2+ chelators, and by competition with N-acetylgalactosamine, asialoorosomucoid, and antibody to the receptor. F, the only glycoprotein on the HN- virus, was shown to compete with the galactose-terminated protein asialoorosomucoid for the ASGP-R. The ability of the HN- virus to cause cell-cell fusion of Hep G2 cells indicated that attachment of this virus to the ASGP-R still permitted viral entry by its usual mode--i.e., membrane fusion at the cell surface. These results open up the possibility that enveloped viruses, which contain glycosylated proteins or lipids, may make use of naturally occurring lectins in addition to their normal receptors as a means of attachment to host cells.

Analysis and gene assignment of mRNAs of a paramyxovirus, simian virus 5

Polypeptides synthesized by the paramyxovirus SV5 in infected CV-1 cells were readily identified when the host cell was treated with actinomycin D. The unglycosylated forms of HN and Fo synthesized in infected cells in the presence of tunicamycin and HN and Fo synthesized in vitro were identified by immunoprecipitation with specific antibodies. Separation of SV5-specific poly(A)-containing RNAs on methyl-mercury agarose gels and in vitro translation of fractions, indicated that the viral polypeptides were translated from individual mRNAs except P (Mr approximately 44K) and the nonstructural polypeptide V (Mr approximately 24K) for which the mRNAs could not be separated. cDNA copies of SV5-specific mRNAs were synthesized and cloned in plasmid pBR322. Clones to NP, P + V, M, F, and HN were identified by hybrid-arrest and hybrid-selection translation of SV5 mRNAs. Tryptic peptide mapping of polypeptides P and V indicated that the peptides of V were a subset of those of P. Hybridization of cDNA probes to infected cell mRNAs separated on agarose gels permitted identification of the NP, P + V, M, F, and HN mRNAs and presumptive polycistronic mRNAs. The sizes and sequence homologies of these polycistronic mRNAs were used to derive a likely gene order on the SV5 50 S genome RNA.

Structural characterization of virion proteins and genomic RNA of human parainfluenza virus 3

The virion proteins and genomic RNA of human parainfluenza virus 3 have been characterized. The virion contains seven major and two minor proteins. Three proteins of 195 X 10(3) molecular weight (195K), 87K, and 67K are associated with the nucleocapsid of the virion and have been designated L, P, and NP, respectively. Three proteins can be labeled with [14C]glucosamine and have molecular weights of 69K, 60K, and 46K. We have designated these proteins as HN, F0, and F1, respectively. HN protein has interchain disulfide bonds, but does not participate in disulfide bonding to form homomultimeric forms. F1 appears to be derived from a complex, F1,2, that has an electrophoretic mobility similar to that of F0 under nonreducing conditions. A protein of 35K is associated with the envelope components of the virion and aggregates under low-salt conditions; this protein has been designated M. The genome of human parainfluenza virus 3 is a linear RNA molecule with a molecular weight of approximately 4.6 X 10(6).

Analysis of Sendai virus mRNAs with cDNA clones of viral genes and sequences of biologically important regions of the fusion protein

cDNA clones representing five of the genes of Sendai virus (P, HN, NP, F, and M) were isolated and used to identify the viral mRNAs by hybridization. Five mRNAs that were monocistronic transcripts of these genes were identified. A sixth transcript, which was identified on the basis of size and of hybridization to viral RNA but not to the cDNA of the other five genes, is thought to represent the message for the L protein. In addition, polycistronic transcripts of the NP and P genes and of the M and F genes were also found. The latter establishes the position of the F gene adjacent to the M gene; these results confirm and extend the previously reported partial gene order of the virus. Nucleotide sequences and derived amino acid sequences of two biologically important regions of the F protein--approximately 25% of F proximal to its COOH terminus and the region spanning the site of the proteolytic cleavage that activates the fusion activity of the protein--are presented. The F protein has an unusually large "cytoplasmic domain" of 42 amino acids beyond the hydrophobic region by which it is anchored in the viral membrane. A single possible trypsin cleavage site was found at the junction of the F1 and F2 polypeptides, and 26 hydrophobic amino acids extend from this cleavage site at the NH2 terminus of the F1 polypeptide.

The role of viral glycoproteins in mumps virus-mediated antibody-dependent cellular cytotoxity in vitro

Treatment of human peripheral blood lymphocytes with live or UV-inactivated mumps virions enhances antibody-mediated cellular cytotoxicity (ADCC), reflected by increased target cell lysis in a 51Cr-release assay or an increased number of plaque-forming cells on monolayers of bovine erythrocytes (Eb) in the presence of anti-Eb antibodies. Virus treatment of the Eb targets causes a similar enhancement. The role of viral glycoproteins in ADCC enhancement was investigated by using a panel of monoclonal antibodies raised in mice against mumps virions. Most of the lymphocytes bound mumps virions, as ascertained by indirect immunofluorescence. A high proportion of virus-treated lymphocytes also formed rosettes with Eb. Anti-HN antibodies inhibited rosetting to various degrees. Although antibodies with high haemagglutination inhibition titres were most efficient inhibitors, antibodies without this serological activity were also inhibitory. Anti-F antibodies were only weakly inhibitory, and anti-NP antibodies had no effect. Anti-HN antibodies also abrogated target cell lysis in the 51Cr-release assay and effector cell recruitment in the ADCC plaque assay by inhibiting virus-mediated Eb-lymphocyte interactions both at the target cell and at the effector cell level. Anti-F or anti-NP antibodies were only weakly or not at all inhibitory. The results suggest that virus-mediated enhancement of ADCC is caused by the HN glycoprotein, primarily (although perhaps not exclusively) by its improvement of the effector cell-target cell contacts necessary for the efficient execution of target cell lysis.

Fusion protein of the paramyxovirus simian virus 5: nucleotide sequence of mRNA predicts a highly hydrophobic glycoprotein

The nucleotide sequence of the mRNA coding for the fusion glycoprotein (F) of the paramyxovirus, simian virus 5, has been obtained. There is a single large open reading frame on the mRNA that encodes a protein of 529 amino acids with a molecular weight of 56,531. The proteolytic cleavage/activation site of F, to yield F2 and F1, contains five arginine residues. Six potential glycosylation sites were identified in the protein, two on F2 and four on F1. The deduced amino acid sequence indicates that F is extensively hydrophobic over the length of the polypeptide chain. Three regions are very hydrophobic and could interact directly with membranes: these are the NH2-terminal putative signal peptide, the COOH-terminal putative membrane anchorage domain, and the NH2-terminal region of F1.

Influenza virus neuraminidase with hemagglutinin activity

Isolated intact influenza virus neuraminidase (NA) molecules of the N9 subtype have been found to possess hemagglutinin (HA) activity which, at equivalent protein concentration, was fourfold higher than that of isolated hemagglutinin molecules of the H3 subtype. The amino-terminal sequence of the N9 NA is the same as in neuraminidases of the eight other influenza A virus NA subtypes previously reported. Viruses possessing N9 NA therefore have two different HA activities and antibody to either HA or NA alone was incapable of inhibiting hemagglutination by the virus. However, antibody to the HA of an H1N9 virus neutralized its infectivity as effectively as it neutralized H1N1 or H1N2 viruses whose neuraminidases have no HA activity. (Antibodies to N9 NA did not neutralize the infectivity of viruses with N9 neuraminidase). 2-deoxy-2,3-dehydro-N-acetyl-neuraminic acid inhibited N9 NA activity but had no effect on the HA activity of the isolated N9 NA. One interpretation of this result would be that the HA and NA activities are located in separate sites. Pronase-released N9 NA heads form crystals suitable for X-ray diffraction studies and preliminary data to 2.9 A establish the space group as cubic, I432 with cell dimension a = 184 A. Data extend to beyond 1.9 A resolution, and these will be collected in the future.

Resolution of two surface glycoproteins from human parainfluenza-3 virus by crossed immunoelectrophoresis

The technique of two-dimensional crossed immunoelectrophoresis (CIE) was used to resolve two glycoproteins from purified human parainfluenza type 3 virus. Virus preparations were extracted with Triton X-100 and fractionated by centrifugation in a Beckman airfuge. Two immunoprecipitates were detected by CIE in the supernatant fractions, but were not found in the pellets from extracted virus. Viral glycoproteins labeled with [35S]methionine were isolated by affinity chromatography on concanavalin A (Con A) agarose columns, resolved by CIE and detected by autoradiography. Resolution of two glycoprotein peaks from as little as 4.5 micrograms of protein from extracted virus is consistent with results from polyacrylamide gel patterns showing two unique glycoproteins with molecular weights of 48 kd and 65 kd.

Inhibition of sendai virus-induced hemolysis by long chain fatty acids

A number of fatty acids were found to inhibit Sendai virus-induced hemolysis. cis-Unsaturated fatty acids such as oleate, as well as the methyl-branched isostearate, completely inhibited viral hemolysis at concentrations as low as 5-10 micrograms/ml, whereas the saturated, normal acids such as palmitate and stearate were comparably inhibitory only at 2-5 times those concentrations. trans-Unsaturated acids, as well as several other amphiphilic compounds, were either not or only weakly inhibitory. In contrast to their disparate effects on viral hemolysis, cis- and trans-unsaturated acids lysed erythrocytes in the same concentration range, which is several times higher than that at which the former compounds inhibited viral hemolysis. The mechanism of inhibition of viral hemolysis by isostearate involves the inactivation of viral hemolytic activity per se, since isostearate neither inhibited viral hemagglutination nor rendered erythrocytes significantly less susceptible to hemolysis. Furthermore, the concentration dependence of hemolysis inhibition by isostearate was biphasic, increasing sharply at the critical micelle concentration from a linear relationship below that concentration. Finally, an inhibitory concentration of isostearate was well below that at which amphiphiles dissolved membranes and did not dissolve Sendai virus, as shown by sucrose gradient centrifugation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It was concluded that low concentrations of fatty acids--particularly cis-unsaturated or fluid-phase types--could block the fusion, as opposed to agglutination, step of viral hemolysis by perturbing hydrophobic regions of the Sendai virus membrane.

A simple procedure for the analysis of the structural proteins of influenza and parainfluenza viruses involving adsorption to erythrocytes

A simple procedure for the analysis of the structural proteins of influenza and parainfluenza viruses utilizing adsorption to erythrocytes is described. The method involves virus growth in the presence of [35S]methionine, adsorption of clarified culture medium with a 0.5% suspension of either guinea-pig or chicken erythrocytes and analysis of the virus-erythrocyte aggregates by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). All of the structural proteins can be detected using this procedure, and the protein profiles of virus-adsorbed erythrocyte complexes compare extremely well with those of sucrose density gradient purified virus preparations.

Monoclonal antibodies to respiratory syncytial virus proteins: identification of the fusion protein

Six monoclonal antibodies directed against respiratory syncytial virus proteins were produced. Each was characterized by immunoprecipitation and indirect immunofluorescence. One was directed against the nucleocapsid protein. NP 44, two were directed against a 37,000-dalton protein, two were directed against the major envelope glycoprotein, GP 90, and one was directed against the 70,000-dalton envelope protein, VP 70. Indirect immunofluorescence stain patterns of infected HEp-2 cells defined GP 90 and VP 70 as viral proteins expressed on the cell surface, whereas NP 44 and the 37,000-dalton protein were detected as intracytoplasmic inclusions. One of the anti-GP 90 antibodies neutralized virus only in the presence of complement but did not inhibit cell-cell fusion. The anti-VP 70 antibody neutralized virus without complement and inhibited cell-cell fusion of previously infected HEp-2 cells, thus identifying VP 70 as the fusion protein.

Monoclonal antibodies against a paramyxovirus isolated from Japanese Sparrow-Hawks

Five monoclonal antibodies against the haemagglutinin-neuraminidase (HN) molecule of Taka virus, a variant of Newcastle disease virus (NDV), were established to compare the antigenicities of several avian paramyxoviruses including NDV. From the results of the cross haemagglutination-inhibition (HI) test with the monoclonal antibodies, the HN molecule of Taka virus seemed to have at least three different antigenic determinants; one was specific for all NDV strains tested, the second was for Taka virus and Komarov strain of NDV and the third was for Taka virus, Komarov strain, Bangor and Yucaipa. Furthermore, the differences in the ratio of HI to neuraminidase-inhibition titers suggested that the separate active sites involved in haemagglutinin and neuraminidase activities might exist at least in close proximity.

Membrane fusion proteins of enveloped animal viruses

In a living cell membrane-bound compartments are continuously either separated or united through fusion reactions, and literally thousands of such reactions take place every minute. The formation of membrane vesicles from pre-existing membranes, and their fusion with specific acceptor membranes, constitute a prerequisite for the transport of most impermeant molecules and macromolecules into the cell by endocytosis, out of the cell by exocytosis, and between the cellular organelles (Palade, 1975; Silverstein, 1978; Evered & Collins, 1982). Less frequent, but equally crucial, are fusion events in fertilization, cell division, polykaryon formation, enucleation, etc. (for reviews see Poste & Nicholson, 1978). Although a great deal is known about the properties and consequences of individual forms of membrane fusion in cellular systems, and about fusion in artificial lipid membranes, the molecular basis for the reactions remain largely unclear.

Fusion of Sendai virus with liposomes: dependence on the viral fusion protein (F) and the lipid composition of liposomes

The characteristics of fusion of the membrane of Sendai virus with that of liposomes has been investigated using two different methods to monitor the fusion reaction. The first method, which permits quantitation of lipid fused with virus, depends on separation by centrifugation of unfused liposomes from those fused with virus. The second involves the digestion after fusion of internal viral proteins by trypsin contained in liposomes; this assay is completely independent of exchange of lipid between liposomal and viral membranes in the absence of fusion. A fusion-inactive mutant virus, pa-cl, with an uncleaved F protein served as the appropriate control in these experiments. It was found that fusion of the virus with liposomes that contained no protein required cleavage of the F protein; such cleavage was previously shown to be required for fusion of the virus with cell membranes. This indicates the relevance of this model system for studies of fusion. Kinetic studies indicated that at neutral pH fusion was 88% complete in 10 min at 37 degrees. Investigation of the effects of liposomal lipid composition indicated that the presence of cholesterol in the liposomal membrane was required for fusion; a 0.3-0.4-mole fraction of cholesterol was optimal. The presence of neuraminic acid in the membrane was not essential for fusion. The results obtained are compatible with previous evidence suggesting a hydrophobic interaction between the cleaved F protein and the target membrane during fusion.

Sendai-virus-induced cell-mediated cytotoxicity in vitro. The role of viral glycoproteins in cell-mediated cytotoxicity

Treatment of peripheral blood lymphocytes from normal donors with small amounts of purified Sendai virions results in enhanced cellular cytotoxicity in vitro to uninfected tissue culture target cells (virus-dependent cellular cytotoxicity (VDCC)), without any obvious correlation to the natural cytotoxicity (NK) displayed by the lymphocytes in the absence of virus. Removal from the virions of the two surface components present in the viral envelope, the HN glycoprotein (gp 71), carrying haemagglutinating and neuraminidase activity, and the F glycoprotein (gp 49), carrying fusion activity, by treatment with pronase abrogated their capacity to induce VDCC. Similar results were obtained when virions lacking the HN glycoprotein after treatment with chymotrypsin were added to the lymphocytes. In contrast, treatment of the virus particles with trypsin, which removed the F glycoprotein, did not affect their capacity to induce VDCC. When the solubilized and separated peplomers were used for lymphocyte treatment, either alone or in combination, the purified HN glycoprotein had full capacity to induce VDCC, whereas the F glycoprotein was inactive. These results suggest that the HM peplomer is solely or primarily responsible for the cytolytic activity arising in non-sensitized lymphocytes when confronted with certain viruses.

Glycoproteins of Sendai virus (HVJ) have a critical ratio for fusion between virus envelopes and cell membranes

The biological activity of two glycoproteins, hemagglutinin and neuraminidase (HN) and fusion (F) proteins, of Sendai virus (HVJ) were studied using purified proteins. The proteins were purified by chromatography on DEAE and CM cellulose in the presence of Nonidet P-40 (NP40). The glycoproteins were reconstituted at various ratios of F to HN into lipid vesicles containing fragment A of diphtheria toxin. The association of HN and F proteins with the vesicles was confirmed by electron microscopy and sucrose density gradient centrifugation. The cytotoxic activity of vesicles containing fragment A on fusion with L cells was determined by measuring colony formation of the cells. It was found that for maximum cytotoxic activity of the vesicles, there was an optimal ratio of F to HN of two. This suggests that HN is not merely the initial binding site to the cell surface, and that interactions between HN and F proteins on the virus surface may be important for the biological activities of these proteins on the cells.

Fatty acid modification of Newcastle disease virus glycoproteins

The fatty acid acylation of Newcastle disease virus hemagglutininin-neuraminidase and fusion glycoproteins was assayed. [3H]palmitate label was associated with cytoplasmic fusion proteins (F0 and F1) and virion-associated F1. In contrast, there was no detectable [3H]palmitate label associated with the hemagglutin-neuraminidase protein in Newcastle disease virus-infected Chinese hamster ovary cells or chicken embryo cells or in virions released from these cells. Thus, fatty acid modification may not be important for the maturation of some glycoproteins.

Interaction of viruses with cell surface receptors

This chapter discusses the interaction of viruses with cell surface receptors. The rigorous characterizations of receptor–ligand interactions have been derived from binding studies of radiolabeled ligands in neuropharmacology and endocrinology. The definition of viral recognition sites as receptors involves three major criteria that are derived from models of ligand–receptor interactions: saturability, specificity, and competition. A variety of approaches have been used to study the interaction of viral particles with cell surface receptors or reception sites. A rigorous study of viral–receptor interactions requires the use of more than one technique as different approaches provide complementary information about viral binding. The chapter discusses membrane components that interact with viruses. The identification of the subviral components that are responsible for the binding of viruses to cell surfaces has preceded the structural understanding of the cellular receptors themselves. The chapter summarizes current data concerning the viral attachment protein (VAP) of selected viruses.

The structural polypeptides of avian paramyxoviruses

The structural polypeptides of twenty-three avian paramyxoviruses from five serotypes were examined by polyacrylamide gel electrophoresis in the presence of sodium dodecylsulphate under reducing and non-reducing conditions. All virus polypeptide profiles consisted of 7--10 polypeptides of which two were glycosylated. Some variation was seen in the profiles of viruses from the same serotype, but groups formed on the basis of their serological relationships in haemagglutination inhibition tests were identical to those formed on the basis of similarities in their polypeptide profiles.