Membrane-bound antiviral antibodies as receptors for Sendai virions in receptor-depleted erythrocytes

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

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

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

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

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

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

Reconstituted Sendai virus envelopes as biological carriers: dual role of F protein in binding and fusion with liver cells

We have assessed the potential of reconstituted Sendai viral envelopes containing only the fusion protein (F-virosomes) as biological carriers for the delivery of drugs and macromolecules. [125I]lysozyme entrapped in F-virosome is used to study its distribution in various organs of Balb/c mouse in vivo as a function of dose and time. F-virosomes injected intravenously are rapidly cleared from circulation. A major percentage (55-60%) of vesicle contents is delivered to liver at 15 min after injection, showing thereby the liver to be the major site for the accumulation of vesicles. Uptake of virosomes by liver is found to reach a near saturation level at a dose of 0.5 mg F-protein associated with virosomes. In competition studies, the inhibitory effect of asialofetuin on the uptake of F-virosomes suggests the involvement of asialoglycoprotein receptor in its recognition by hepatic parenchymal cells. Incorporation of asialoganglioside-GM1 in the F-virosomes enhanced the uptake by about 1.6-fold. The observed specific interaction of hepatic receptor with F-protein containing a terminal galactose moiety is further supported by degalactosylation of F-virosomes with hard-shelled clam exoglycosidase. The uptake of degalactosylated F-virosomes by liver is found to be significantly reduced. The subcellular radioactivity profile in liver cells exhibits a considerable decrease in cytosolic localisation of the degalactosylated F-virosomal contents with a concomitant increase in their accumulation in lysosomal/mitochondrial fraction as compared to the untreated virosomes. Trypsinized and heat-treated F-virosomes also reflect similar subcellular distribution profile as that of degalactosylated virosomes. Moreover, F-virosomes are able to interact and deliver [125I]lysozyme to the HepG2 cells in culture in the presence of a potent inhibitor of endocytotic process. These results indicate the involvement of specific binding of F-proteins with hepatic receptors followed by their fusion with the membrane of liver cells in the delivery of [125I]lysozyme. The findings reported here open up the possibility of using F-virosomes with defined specificity as fusogenic vehicles for efficient delivery of drugs and biologically active macromolecules both in vivo and in vitro.

Targeted delivery of hygromycin B using reconstituted Sendai viral envelopes lacking hemagglutinin-neuraminidase

Hygromycin B was encapsulated in reconstituted Sendai viral envelopes containing only the fusion (F) protein (F-virosomes). Incubation of loaded F-virosomes with cultured HepG2 cells resulted in fusion mediated delivery of hygromycin B to the cell cytoplasm, as was inferred from inhibition of DNA synthesis. Binding of the F-virosomes to HepG2 cells was mediated by the interaction of terminal beta-galactose residues of fusion protein with asialoglycoprotein receptor on HepG2 cells, subsequently leading to fusion between the two membranes. The cytotoxic effect of hygromycin B enclosed in F-virosomes was comparable with that of F,HN-virosomes containing both hemagglutinin-neuraminidase (HN) and F protein and F,HNred-virosomes containing HN whose disulfide bonds were irreversibly reduced (HNred). Hygromycin B loaded fusogenic liposomes were prepared by coreconstituting the viral envelope containing only fusion protein with exogenous lipids. These fusogenic liposomes were found to be more active than F-virosomes at the same fusion protein concentrations.

Functional interactions between the fusion protein and hemagglutinin-neuraminidase of human parainfluenza viruses

The fusion glycoprotein (F) and hemagglutinin-neuraminidase (HN) genes of human parainfluenza virus type 2 (PI2) were molecularly cloned and expressed in HeLa-T4 cells by using the vaccinia virus-T7 transient expression system. Expression of the F and HN proteins was detected by using immunoprecipitation and surface immunofluorescence staining. Although the F protein was found to be cleaved into F1 and F2 and expressed on cell surfaces, no cell fusion was observed. However, cotransfection of the F-protein gene together with the P12 HN gene resulted in significant levels of cell fusion. Cell fusion was also observed when separate cell cultures were transfected with the HN and F genes and the F-expressing cells were mixed with the HN-expressing cells. Surprisingly, when the PI2 F protein was expressed together with the parainfluenza virus type 3 (PI3) HN protein, no fusion was detectable in the transfected cells. Similarly, no fusion was found upon coexpression of the PI2 HN and PI3 F proteins. However, coexpression of the PI3 F and HN proteins resulted in extensive cell fusion, which resembled the PI2 coexpression result. These results indicate that under the conditions used, the F protein is unable to cause fusion by itself and the HN protein provides a specific function in cell fusion which cannot be provided by another paramyxovirus attachment protein. Further, the results suggest that a type-specific functional interaction between the F and HN proteins is involved in mediating cell fusion.

Complementation between avirulent Newcastle disease virus and a fusion protein gene expressed from a retrovirus vector: requirements for membrane fusion

The cDNA derived from the fusion gene of the virulent AV strain of Newcastle disease virus (NDV) was expressed in chicken embryo cells by using a retrovirus vector. The fusion protein expressed in this system was transported to the cell surface and was efficiently cleaved into the disulfide-linked F1-F2 form found in infectious virions. The cells expressing the fusion gene grew normally and could be passaged many times. Monolayers of these cells would plaque, in the absence of trypsin, avirulent NDV strains (strains which encode a fusion protein which is not cleaved in tissue culture). Fusion protein-expressing cells would not fuse if mixed with uninfected cells or uninfected cells expressing the hemagglutinin-neuraminidase (HN) protein. However, the fusion protein-expressing cells, if infected with avirulent strains of NDV, would fuse with uninfected cells, suggesting that fusion requires both the fusion protein and another viral protein expressed in the same cell. Fusion was also seen after transfection of the HN protein gene into fusion protein-expressing cells. Thus, the expressed fusion protein gene is capable of complementing the virus infection, providing an active cleaved fusion protein required for the spread of infection. However, the fusion protein does not mediate cell fusion unless the cell also expresses the HN protein. Fusion protein-expressing cells would not plaque influenza virus in the absence of trypsin, nor would influenza virus-infected fusion protein-expressing cells fuse with uninfected cells. Thus, the influenza virus HA protein will not substitute for the NDV HN protein in cell-to-cell fusion.

Binding and entry of animal viruses

Viruses are infectious agents capable of packaging and delivering nucleic acids and proteins to specific populations of cells. To initiate infection, viruses bind to sites, or receptors, on the cell surface and transfer their genome across the limiting membrane of the cell. The mechanisms underlying these events, and viral tropism for particular host cells, are becoming increasingly well understood. Several cell surface proteins have now been identified as viral receptors, and analyses of intact virus particles and sub-viral components are revealing the structures of the binding determinants on the viruses themselves. For many viruses, the events leading to penetration and delivery involve constitutive endocytic properties of the host cell, and the low pH environment in endocytic compartments is a crucial trigger in the penetration process. The knowledge of viral tropism, binding and entry suggests strategies which may be applied to the design of targeted therapeutic agents with appropriate specificities and effective delivery mechanisms.

Entry mechanisms of enveloped viruses. Implications for fusion of intracellular membranes

Enveloped viruses infect cells by a mechanism involving membrane fusion. This process is mediated and triggered by specific viral membrane glycoproteins. Evidence is accumulating that fusion of intracellular membranes, as occurs during endocytosis and transport between intracellular organelles, also requires the presence of specific proteins. The relevance of elucidating the mechanisms of virus fusion for a better understanding of fusion of intracellular membranes is discussed.

Selective extraction of haemagglutinin and matrix protein from Sendai virions by employing trifluoperazine as a detergent

Incubation of trifluoperazine, a local anaesthetic, at concentrations higher than the cmc with Sendai virus particles produces the selective solubilization of the haemagglutinin neuraminidase (HN) and matrix (M) proteins. This phenomenon involves aggregation of the Sendai virions and therefore the separation of HN and M from the rest of the particle can be performed by bench centrifugation. The supernatant contains the HN and M proteins and HN, once inserted into liposomes, elicits its own biological activities. Therefore, the method seems suitable for purifying large amounts of HN.

Fusion of a Sendai mutant deficient in HN protein (ts271) with cardiolipin liposomes

Sendai mutant ts271 contains less than 5% of the amount of HN glycoprotein found in wild-type Sendai. Fusion of this mutant with cardiolipin liposomes revealed no differences from the wild-type virus with regard to specific activity, pH dependence, or radiation inactivation. Target sizes of both mutant and wild-type viral proteins were determined by the radiation-induced disappearance of each band from an SDS-polyacrylamide gel and no differences were found. Of the viral proteins, only F had a target size corresponding to the monomer molecular weight, ca. 60 kDa, identical to the minimum unit previously determined by functional assay for Sendai virus-erythrocyte membrane fusion (K. Bundo-Morita, S. Gibson, and J. Lenard, Biochemistry 26, 6223-6227 (1987)). This provides additional evidence that F alone is the active protein mediating Sendai-erythrocyte fusion. It is concluded that the HN protein is unlikely to mediate any fusion reactions of the intact virions, either with biological membranes or with cardiolipin liposomes.

Reconstitution of functional influenza virus envelopes and fusion with membranes and liposomes lacking virus receptors

Reconstituted influenza virus envelopes were obtained following solubilization of intact virions with Triton X-100. Quantitative determination revealed that the hemolytic and fusogenic activities of the envelopes prepared by the present method were close or identical to those expressed by intact virions. Hemolysis as well as virus-membrane fusion occurred only at low pH values, while both activities were negligible at neutral pH values. Fusion of intact virions as well as reconstituted envelopes with erythrocyte membranes--and also with liposomes--was determined by the use of fluorescently labeled viral envelopes and fluorescence dequenching measurements. Fusion with liposomes did not require the presence of specific virus receptors, namely sialoglycolipids. Under hypotonic conditions, influenza virions or their reconstituted envelopes were able to fuse with erythrocyte membranes from which virus receptors had been removed by treatment with neuraminidase and pronase. Inactivated intact virions or reconstituted envelopes, namely, envelopes treated with hydroxylamine or glutaraldehyde or incubated at low pH or 85 degrees C, neither caused hemolysis nor possessed fusogenic activity. Fluorescence dequenching measurements showed that only fusion with liposomes composed of neutral phospholipids and containing cholesterol reflected the viral fusogenic activity needed for infection.

Active function of membrane receptors for enveloped viruses. I. Specific requirement for liposome-associated sialoglycolipids, but not sialoglycoproteins, to allow lysis of phospholipid vesicles by reconstituted Sendai viral envelopes

Phospholipid liposomes composed of phosphatidylcholine (PC) and cholesterol (chol), bearing the sialoglycoprotein glycophorin (GP), are able to effectively bind Sendai virus particles, but not to be lysed by them. Incorporation of gangliosides (gangl) into the above phospholipid vesicles (yielding liposomes composed of PC/chol/gangl/GP), although not increasing their ability to interact with Sendai virions, rendered them susceptible to the viral lytic activity. This was inferred from the ability of the virus to induce release of carboxyfluorescein (CF) upon interaction at 37 degrees C with liposomes composed of PC/chol/gangl/GP. Lysis of liposomes required the presence of the two viral envelope glycoproteins, namely the hemagglutinin/neuraminidase (HN) and the fusion (F) polypeptides, and was inhibited by phenylmethyl sulfonylfluoride (PMSF), dithiothreitol (DTT) and trypsin, showing that virus-induced lysis of PC/chol/gangl/GP liposomes reflects the fusogenic activity of the virus. Incubation of Sendai virus particles with liposomes containing the acidic phospholipid dicetylphosphate (DCP) but lacking sialic acid containing receptors, also resulted in release of the liposome content. Lysis of these liposomes was due to the activity of the viral HN glycoprotein, therefore not reflecting the natural viral fusogenic activity. Fluorescence dequenching studies, using fluorescently labeled reconstituted Sendai virus envelopes (RSVE), have shown that the viral envelopes are able to fuse with neutral, almost to the same extent, as with negatively charged liposomes. However, fusion with negatively charged liposomes, as opposed to fusion with neutral liposomes, was mediated by the viral HN glycoprotein and not by the viral fusion polypeptide.

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.

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.

Use of reconstituted Sendai virus envelopes for fusion-mediated microinjection of double-stranded RNA: inhibition of protein synthesis in interferon-treated cells

Poly(I).poly(C) molecules were trapped with reconstituted Sendai virus envelopes when added to the reconstitution system. A quantitative estimation indicated that about 10% of the added poly(I).poly(C) remained associated with the fusogenic viral envelopes. About 50% of the associated poly(I).poly(C) were found to be RNAase A resistant, enclosed within the viral envelopes. Incubation of loaded viral envelopes with HeLa or L-cells resulted in strong inhibition of protein synthesis, indicating fusion-mediated microinjection of the enclosed poly(I).poly(C). Introduction of poly(I).poly(C) into cultured cells by the use of reconstituted Sendai virus envelopes was as efficient as the introduction of these polynucleotides using the calcium phosphate coprecipitation technique. The inhibition of protein synthesis in L-cells but not in HeLa cells was dependent upon pretreatment with interferon. Incubation of poly(I).poly(C)-loaded viral envelopes with interferon-treated variant cells of the NIH 3T3 line, which possess a very low amount of RNAase L, resulted in only 25% inhibition of protein synthesis, compared to 85% inhibition observed in L-cells.

Protein blot analysis of virus receptors: identification and characterization of the Sendai virus receptor

Receptors for Sendai virions in human erythrocyte ghost membranes were identified by virus overlay of protein blots. Among the various erythrocyte polypeptides, only glycophorin was able to bind Sendai virions effectively. The detection of Sendai virions bound to glycophorin was accomplished either by employing anti-Sendai virus antibodies or by autoradiography, when 125I-labeled Sendai virions were used. The binding activity was associated with the viral hemagglutinin/neuraminidase (HN) glycoprotein, as inferred from the observation that the binding pattern of purified HN glycoprotein to human erythrocyte membranes was identical to that of intact Sendai virions. No binding was observed when blots, containing either human erythrocyte membranes or purified glycophorin, were probed with the viral fusion factor (F glycoprotein). Active virions competed effectively with the binding of 125I-labeled Sendai virions (or purified HN glycoprotein), whereas no competition was observed with inactivated Sendai virus. The results of the present work clearly show that protein blotting can be used to identify virus receptors in cell membrane preparations.

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.

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.

Use of virus-attached antibodies or insulin molecules to mediate fusion between Sendai virus envelopes and neuraminidase-treated cells

Anti-human erythrocyte antibodies or insulin molecules were covalently coupled to the glycoproteins (the hemagglutinin/neuraminidase and the fusion polypeptides) of Sendai virus envelopes with N-succinimidyl 3-(2-pyridyldithio)propionate and succinimidyl 4-(p-maleimidophenyl)butyrate as cross-linking reagents. Reconstituted Sendai virus envelopes, bearing covalently attached anti-human erythrocyte antibodies or insulin molecules, were able to bind to but not fuse with virus receptor depleted human erythrocytes (neuraminidase-treated human erythrocytes). Only coreconstitution of Sendai virus glycoproteins, bearing attached anti-human erythrocyte antibodies or insulin molecules with intact, untreated viral glycoproteins, led to the formation of fusogenic, targeted reconstituted Sendai virus envelopes. Binding and fusion of reconstituted Sendai virus envelopes, bearing anti-human erythrocyte antibodies or insulin molecules, with neuraminidase-treated human erythrocytes were blocked by the monovalent fraction, obtained after papain digestion of immunoglobulins, made of anti-human erythrocyte antibodies or free insulin molecules, respectively. The results of this work demonstrate an active role of the viral binding protein (hemagglutinin/neuraminidase polypeptide) in the virus membrane fusion process and show a novel and efficient method for the construction of targeted, fusogenic Sendai virus envelopes.