Structural components of Sendai virus. Serological and physicochemical characterization of hemagglutinin subunit associated with neuraminidase activity

Solid-Phase Binding Assay for Ganglioside Binding of Human Respiroviruses

Human parainfluenza virus types 1 (hPIV-1) and 3 (hPIV-3) belong to the family Paramyxoviridae, subfamily Paramyxoviridae, and genus Respirovirus. The viruses enter by utilizing glycoproteins or glycosphingolipids (gangliosides) containing sialic acid on the cell membrane. We developed a solid-phase binding assay to evaluate hPIV-1, hPIV-3, and Sendai virus' abilities to bind to different types of gangliosides. hPIV1 and hPIV3 show strong binding to neolacto-series gangliosides containing a non-reducing terminal sialic acid residue and different specificity regarding the sialic acid linkages. This solid-phase binding assay is suitable to evaluate other orthomyxoviruses and paramyxoviruses' binding specificities utilizing sialic acids.

Flexibility of the Head-Stalk Linker Domain of Paramyxovirus HN Glycoprotein Is Essential for Triggering Virus Fusion

The Paramyxoviridae comprise a large family of enveloped, negative-sense, single-stranded RNA viruses with significant economic and public health implications. For nearly all paramyxoviruses, infection is initiated by fusion of the viral and host cell plasma membranes in a pH-independent fashion. Fusion is orchestrated by the receptor binding protein hemagglutinin-neuraminidase (HN; also called H or G depending on the virus type) protein and a fusion (F) protein, the latter undergoing a major refolding process to merge the two membranes. Mechanistic details regarding the coupling of receptor binding to F activation are not fully understood. Here, we have identified the flexible loop region connecting the bulky enzymatically active head and the four-helix bundle stalk to be essential for fusion promotion. Proline substitution in this region of HN of parainfluenza virus 5 (PIV5) and Newcastle disease virus HN abolishes cell-cell fusion, whereas HN retains receptor binding and neuraminidase activity. By using reverse genetics, we engineered recombinant PIV5-EGFP viruses with mutations in the head-stalk linker region of HN. Mutations in this region abolished virus recovery and infectivity. In sum, our data suggest that the loop region acts as a "hinge" around which the bulky head of HN swings to-and-fro to facilitate timely HN-mediate F-triggering, a notion consistent with the stalk-mediated activation model of paramyxovirus fusion.

IMPORTANCE

Paramyxovirus fusion with the host cell plasma membrane is essential for virus infection. Membrane fusion is orchestrated via interaction of the receptor binding protein (HN, H, or G) with the viral fusion glycoprotein (F). Two distinct models have been suggested to describe the mechanism of fusion: these include "the clamp" and the "provocateur" model of activation. By using biochemical and reverse genetics tools, we have obtained strong evidence in favor of the HN stalk-mediated activation of paramyxovirus fusion. Specifically, our data strongly support the notion that the short linker between the head and stalk plays a role in "conformational switching" of the head group to facilitate F-HN interaction and triggering.

Sendai virus-based liposomes enable targeted cytosolic delivery of nanoparticles in brain tumor-derived cells

Background

Nanotechnology-based bioassays that detect the presence and/or absence of a combination of cell markers are increasingly used to identify stem or progenitor cells, assess cell heterogeneity, and evaluate tumor malignancy and/or chemoresistance. Delivery methods that enable nanoparticles to rapidly detect emerging, intracellular markers within cell clusters of biopsies will greatly aid in tumor characterization, analysis of functional state and development of treatment regimens.

Results

Experiments utilized the Sendai virus to achieve in vitro, cytosolic delivery of Quantum dots in cells cultured from Human brain tumors. Using fluorescence microscopy and Transmission Electron Microscopy, in vitro experiments illustrated that these virus-based liposomes decreased the amount of non-specifically endocytosed nanoparticles by 50% in the Human glioblastoma and medulloblastoma samples studied. Significantly, virus-based liposome delivery also facilitated targeted binding of Quantum dots to cytosolic Epidermal Growth Factor Receptor within cultured cells, focal to the early detection and characterization of malignant brain tumors.

Conclusions

These findings are the first to utilize the Sendai virus to achieve cytosolic, targeted intracellular binding of Qdots within Human brain tumor cells. The results are significant to the continued applicability of nanoparticles used for the molecular labeling of cancer cells to determine tumor heterogeneity, grade, and chemotherapeutic resistivity.

HVJ envelope vector, a versatile delivery system: its development, application, and perspectives

An efficient and minimally invasive vector system is the "bottle neck" of both gene transfer and drug delivery. Numerous viral and non-viral (synthetic) delivery systems have been developed and improved. Hemagglutinating virus of Japan (HVJ, Sendai virus) envelope vector is a novel and unique system which combined the advantages of viral and non-viral vectors with the following features and advantages: (1) safe and easy as a "non-viral" transfection reagent; (2) delivery of various molecules including plasmid DNA, siRNA, protein, antisense oligonucleotide; (3) wide usability from in vitro to in vivo. In this review, the development, application, and perspectives of the HVJ envelope vector will be discussed.

Functions of surface glycoproteins of myxoviruses and paramyxoviruses and their inhibition

Two glycoproteins, HN and F, are present on the surface of paramyxoviruses. HN has receptor-binding amd neuraminidase activities. F is involved in viral penetration, cell fusion and haemolysis and is activated by proteolytic cleavage by a host enzyme into two disulphide-bonded subunits (f1 and F2). The ability of the virus to initiate infection and undergo multiple cycle replication depends on the presence of an activating protease in the host; thus cleavage of F is a major determinant of pathogenesis. The new N-terminus generated on F1 by cleavage is involved in biological activity, and the amino acid sequence of this region of F1 by cleavage is involved in biological activity, and the amino acid sequence of this region of F1 is hydrophobic and highly conserved among para-myxoviruses. In an attempt to design specific inhibitors, oligopeptides and analogous to this region were synthesized and found to be highly active, specific inhibitors of viral penetration, cell fusion and haemolysis. Inhibition is amino-acid-sequence-specific and affected by peptide length, steric configuration and addition of groups to the n-terminal and C-terminal amino acids. Replication of influenza virus was also specifically inhibited by oligopeptides resembling the N-terminus of the HA2 polypeptide. Like that of F1 protein the N-terminus of HA2 is generated by a proteolytic cleavage that activates infectivity. These results have provided information on the action of proteins in viral penetration and membrane fusion and they suggest a possible new approach to chemical inhibition of viral replication. Studies with specific antibodies to each of the paramyxovirus glycoproteins have shown that antibodies to the F protein are essential for effective prevention of the spread of infection. Antibodies to the HN protein, although capable of neutralizing released virus, do not prevent spread to adjacent cells through membrane fusion mediated by the F protein. These findings have implications for the design of effective vaccines against paramyxoviruses and also provided additional insight into the mechanisms involved in the atypical and severe infections observed in individuals who received inactivated paramyxovirus vaccines and were later infected.

Receptor specificities of human respiroviruses

Through their hemagglutinin-neuraminidase glycoprotein, parainfluenza viruses bind to sialic acid-containing glycoconjugates to initiate infection. Although the virus-receptor interaction is a key factor of infection, the exact nature of the receptors that human parainfluenza viruses recognize has not been determined. We evaluated the abilities of human parainfluenza virus types 1 (hPIV-1) and 3 (hPIV-3) to bind to different types of gangliosides. Both hPIV-1 and hPIV-3 preferentially bound to neolacto-series gangliosides containing a terminal N-acetylneuraminic acid (NeuAc) linked to N-acetyllactosamine (Galbeta1-4GlcNAc) by the alpha2-3 linkage (NeuAcalpha2-3Galbeta1-4GlcNAc). Unlike hPIV-1, hPIV-3 bound to gangliosides with a terminal NeuAc linked to Galbeta1-4GlcNAc through an alpha2-6 linkage (NeuAcalpha2-6Galbeta1-4GlcNAc) or to gangliosides with a different sialic acid, N-glycolylneuraminic acid (NeuGc), linked to Galbeta1-4GlcNAc (NeuGcalpha2-3Galbeta1-4GlcNAc). These results indicate that the molecular species of glycoconjugate that hPIV-1 recognizes are more limited than those recognized by hPIV-3. Further analysis using purified gangliosides revealed that the oligosaccharide core structure is also an important element for binding. Gangliosides that contain branched N-acetyllactosaminoglycans in their core structure showed higher avidity than those without them. Agglutination of human, cow, and guinea pig erythrocytes but not equine erythrocytes by hPIV-1 and hPIV-3 correlated well with the presence or the absence of sialic acid-linked branched N-acetyllactosaminoglycans on the cell surface. Finally, NeuAcalpha2-3I, which bound to both viruses, inhibited virus infection of Lewis lung carcinoma-monkey kidney cells in a dose-dependent manner. We conclude that hPIV-1 and hPIV-3 preferentially recognize oligosaccharides containing branched N-acetyllactosaminoglycans with terminal NeuAcalpha2-3Gal as receptors and that hPIV-3 also recognizes NeuAcalpha2-6Gal- or NeuGcalpha2-3Gal-containing receptors. These findings provide important information that can be used to develop inhibitors that prevent human parainfluenza virus infection.

Protection of mice from respiratory Sendai virus infections by recombinant vaccinia viruses

Mechanisms of protection of mice from Sendai virus, which is exclusively pneumotropic and causes a typical respiratory disease, by immunization with recombinant vaccinia viruses (RVVs) were investigated. Although the RVV carrying a hemagglutinin-neuraminidase gene of Sendai virus (Vac-HN) propagated in the noses and lungs of mice by either intranasal (i.n.) or intraperitoneal (i.p.) inoculation, no vaccinia virus antigens were detected in the mucosal layer of upper and lower airways of the i.p.-inoculated mice. The mice immunized i.n. with Vac-HN or Vac-F (the RVV carrying a fusion protein gene of Sendai virus) demonstrated the strong resistance to Sendai virus challenge both in the lung and in the nose, whereas the i.p.-immunized mice showed almost no resistance in the nose but showed a partial resistance in the lung. Titration of Sendai virus-specific antibodies in the nasal wash (NW), bronchoalveolar lavage (BAL), and serum collected from the Vac-F-immunized mice showed that the NW from the i.n.-immunized mice contained immunoglobulin A (IgA) antibodies but no IgG and the BAL from the mice contained both IgA and IgG antibodies. On the other hand, neither IgA nor IgG antibodies were detected in the NW from the i.p.-immunized mice and only IgG antibodies were detected in the BAL, although both i.n.- and i.p.-immunized mice exhibited similar levels of serum IgG, IgA, and neutralizing antibodies. The resistance to Sendai virus in the noses of i.n.-immunized mice could be abrogated by the intranasal instillation of anti-mouse IgA but not of anti-IgG antiserum, while the resistance in the lung was not significantly abrogated by such treatments. These results demonstrate that IgA is a major mediator for the immunity against Sendai virus induced by the RVVs and IgG is a supplementary one, especially in the lung, and that the RVV should be intranasally inoculated to induce an efficient mucosal immunity even if it has a pantropic nature.

F0-containing noninfectious Sendai virus can initiate replication in mouse lungs but requires a relatively long incubation period

The replication of LLC-MK2-grown noninfectious Sendai virus, containing exclusively fusion (F) glycoprotein precursors, was examined in the mouse lung to study the accessibility of virus inoculated intranasally to the virus activator present in the lung. When mice were intranasally inoculated with various doses of the virus after in vitro activation with trypsin, the 50% mouse infectious dose (MID50) was determined to be 0.7 cell-infectious units (CIU) per mouse, indicating that one infectious unit of Sendai virus is enough to initiate replication in the mouse lung and that the present experimental system is highly sensitive. On the other hand, in mice inoculated with virus not treated with trypsin, virus replication in the lung was recognized even in mice inoculated with samples containing no infectious virus, and the MID50 was determined to be 67.5 CIU per mouse (here, CIU were assayed after in vitro trypsin treatment). When mice were infected with 20 MID50 of trypsin-treated infectious and untreated noninfectious viruses (an approximately 100-fold greater amount of noninfectious virus than of infectious virus was used), the noninfectious virus was found to require 2 more days of incubation than the infectious virus, and many of the F proteins synthesized in the lungs of mice infected with the F0-containing virus were present in the cleaved form. In addition, the infection of mice with noninfectious virus was strongly suppressed by aprotinin, a serine protease inhibitor. These results indicate that Sendai virus can initiate replication in the mouse lung even with the F0-containing noninfectious virus and strongly suggest that this infection process is mediated by cleavage activation of the F0 proteins of inoculated viruses by a serine protease(s) present in the lumen of the mouse respiratory tract but that activation of the noninfectious virus is an inefficient process.

Crystallization of biologically active hemagglutinin-neuraminidase glycoprotein dimers proteolytically cleaved from human parainfluenza virus type 1

We isolated, purified, and characterized the hemagglutinin-neuraminidase (HN) of human parainfluenza virus type 1, with the ultimate goal of producing crystals suitable for three-dimensional X-ray structure analysis. Pronase was used to cleave the globular head of the HN molecule directly from virus particles, forming HN monomers and dimers. The purified dimers retained neuraminidase and hemadsorption activity and were recognized by 14 anti-HN monoclonal antibodies, demonstrating intact HN antigenic structure and function. N-terminal sequence analysis of the dimers showed that cleavage had occurred at amino acid 136 or 137, freeing the C-terminal 438 or 439 amino acids. On electron micrography, the dimer appeared as two box-shaped structures, each approximately 5 by 5 nm. When the purified HN dimers were crystallized in hanging drops by vapor diffusion against 20% polyethylene glycol 3350, they formed both rectangular plates and needlelike crystals. The rectangular crystals diffracted X-rays, indicating an ordered atomic structure. However, the resolution was approximately 10 A (1 nm), insufficient for three-dimensional structural analysis. Experiments to improve the resolution by increasing the size and quality of the crystals are in progress.

Mutation of the HANA protein of Sendai virus by passage in eggs

A study was made to elucidate the effect of host cells on the HANA protein of Sendai virus. Two strains of Sendai virus were isolated from an epidemic in an animal laboratory by inoculating the lung homogenate of a moribund mouse either into LLC-MK2 cells (Oh-L) or into the allantoic cavity of embryonated eggs (Oh-E). Oh-E agglutinated chicken red blood cells at 37 degrees (HA37+), while Oh-L did not (HA37-). When Oh-L was passaged in eggs, conversion of the HA37- virus to the HA37+ virus readily occurred. A single point mutation was recognized on the HANA protein of the HA37+ virus either at position 525 (Gln----Arg) or at position 198 (Leu----Phe). Hl test with monoclonal antibody revealed conformational changes around the receptor binding site. Neuraminidase activity was also affected by these mutations. The changes in these biological activities of the HANA protein seemed to allow the HA37+ virus to replicate in eggs. On the contrary, the HA37+ virus replicates as efficiently as the HA37- virus in LLC-MK2 cells and no reversion to the HA37- virus was observed. The overall results indicate that the passage of Sendai virus in eggs resulted in selection of viruses possessing a specific mutation on the HANA protein. The pneumopathogenicity in mice was not significantly different between the HA37- virus and the HA37+ virus, suggesting the existence of genes other than the HANA gene that determine mouse pathogenicity.

Evaluation of a protease activation mutant of Sendai virus as a potent live vaccine

A protease activation mutant of Sendai virus, TR-5, was investigated as a candidate for a live vaccine. Vaccination with TR-5 which had been activated by chymotrypsin beforehand (active TR-5) elicited protective immunity against otherwise lethal challenge infection with wild-type Sendai virus in DBA/2, C3H and ICR strains of mice. Less of the active TR-5 was required to confer protection on mice compared with an ordinary ether-inactivated Sendai virus vaccine (split vaccine). The protective immunity elicited by TR-5 lasted longer and the booster effect was more prominent compared to the split vaccine. No seroconversion was observed with contact mice when housed in a cage with mice vaccinated with the active TR-5. The overall results show that the active TR-5 is an effective and safe live vaccine of Sendai virus in mice.

Protection of mice against Sendai virus pneumonia by non-neutralizing anti-F monoclonal antibodies

Nine monoclonal antibodies (MAbs) directed to F protein of Sendai virus were obtained and characterized for their protective ability against Sendai virus infection in mice. None of the MAbs showed hemagglutination-inhibition (HI), hemolysis-inhibition (HLI), or neutralization (NT) activities in vitro when assayed by standard methods. Some of the MAbs, however, showed complement-requiring NT (C-NT) and complement-requiring hemolysis (C-HL) activities when assayed in the presence of complement. Passive immunization experiments revealed that the MAbs with higher C-NT and C-HL activities showed protective activity against Sendai virus pneumonia in mice, and that some MAbs with IgG1 isotype having neither C-NT nor C-HL activity also showed the protective activity. Digestion of the MAbs with pepsin which split immunoglobulin molecules into F(ab')2 and Fc fragments greatly suppressed the protective activity. These results suggest that not only complement-mediated immunological responses such as immune virolysis but also antibody-dependent cellular cytotoxicity (ADCC) and/or immune phagocytosis, in which complement system is not necessarily involved, play an important role in the protection of mice from Sendai virus infection.

Virus-specific polypeptides of human parainfluenza virus type 4 and their synthesis in infected cells

We have studied the structural components of human parainfluenza virus type 4A (PIV-4A) and identified some virus-specific polypeptides by immunoprecipitation with polyclonal and monoclonal antibodies followed by one- or two-dimensional SDS-PAGE. HN polypeptides existed as monomer, disulfide-linked dimer, and disulfide-linked larger oligomer in cells infected with PIV-4A. Interestingly, the nonreduced NP, the nonreduced fusion, and the reduced F1 proteins migrated as doublets. Two F1 polypeptides were derived from different F1 + 2 proteins which migrated separately under nonreducing condition. In Vero cells infected with two strains of PIV-4A, two lower-molecular-weight proteins related to NP were detected. Oligopeptide patterns of the lower-molecular-weight protein were similar to those of NP protein synthesized in primary monkey kidney cells. The NP-related low-molecular-weight protein(s) was immunoprecipitated by 1 of 11 monoclonal antibodies against mumps virus NP protein. The MAb also reacted with NP proteins of PIV-2 and SV5. Thus, the epitope recognized by the MAb was common among PIV-2, PIV-4, mumps virus, and SV5, suggesting that the epitope might have an important biological function. However, the MAb did not react with the intact NP protein from cells infected with PIV-4, indicating that the epitope of PIV-4A was presented only when NP was cleaved. Phosphorylation was demonstrated for NP and P proteins.

A recent field isolate of Sendai virus has a temperature-sensitive HN glycoprotein

A recent field isolate of Sendai virus was found to have a temperature-sensitive (ts) hemagglutinin-neuraminidase (HN) glycoprotein. The ts phenotype was manifested as a loss of cell binding, reduced replication, and virions that were lacking surface HN after growth at the nonpermissive temperature (38 degrees). Low neuraminidase activity and failure of the field isolate to remove sialic acid from receptors on the surface of erythrocytes indicated that rapid elution of the field isolate virions from erythrocytes at the nonpermissive temperature was not due to neuraminidase activity but to a proposed conformational change in the HN molecule. The specific amino acids responsible for the ts phenotype could not be determined due to the number of amino acid differences between the field isolate and Enders strain. Heat inactivation and monoclonal antibody inhibition of HN functions indicated that the HN protein of this isolate was, in addition to ts, an unstable molecule.

Adsorption of LLCMK2 cell-grown Sendai virus onto human red blood cells and its release from the virus adsorbed cells

An early stage of virus adsorption was studied in a system of Sendai virus metabolically labeled with [3H]leucine in LLCMK2 cells and of human red blood cells (RBCs). The efficiency of viral release from the virus-bound RBCs by incubation at 37 C depended on the number of virus particles which had been used for adsorption onto the RBC at 4 C. When 7.8 x 10(2) virus particles were previously adsorbed onto the RBC at 4 C, most of the viruses were dissociated from the RBC at 37 C. In the case of adsorption of 3 to 12 virus particles per RBC, however, most of the viruses were not dissociated from the RBC by incubation at 37 C. Such RBC-bound viruses were released by incubation with various bacterial neuraminidases (Clostridium perfringens, etc.) or with a large number of LLCMK2 cell-grown Sendai virus (LLCMK2-Sendai) particles, but not released by treatment with hemagglutinin-neuraminidase protein (Sendai-gp) isolated from egg-grown Sendai virus.

Isolation of a biologically active soluble form of the hemagglutinin-neuraminidase protein of Sendai virus

As a first step in establishing the three-dimensional structure of the Sendai virus hemagglutinin-neuraminidase (HN), we have isolated and characterized a potentially crystallizable form of the molecule. The sequence of HN, a surface glycoprotein, predicts a protein with an uncharged hydrophobic region near the amino terminus which is responsible for anchorage in the viral envelope. To avoid rosette formation (aggregation), which would preclude crystallization, this hydrophobic tail was removed from a membrane-free form of HN by proteolytic digestion. This digestion resulted in a single product with a molecular weight of about 10,000 less than native HN. N-terminal amino acid sequence analysis of cleaved HN (C-HN) indicated a single cleavage site at amino acid residue 131, resulting in a product consisting of the carboxyl-terminal 444 amino acids of HN. Functional analyses revealed that C-HN retained full neuraminidase activity and was able to bind erythrocytes, indicating that the N-terminal 131 residues were not necessary for these biological activities. Furthermore, this cleavage product retained the antigenic structure of intact HN, since monoclonal antibodies still bound to C-HN in enzyme-linked immunosorbent assay and Western (immuno-) blot analysis. Viewed by electron microscopy, the dimeric and tetrameric forms of intact HN form rosettes while C-HN maintains the oligomeric structure but no longer aggregates. Furthermore, the electron micrographs revealed a C-HN tetramer strikingly similar to the influenza virus neuraminidase in both size and gross structural features.

The role of proteases in 4-ipomeanol-induced enhancement of Sendai viral pneumonia in mice

The ability of reconstituted, concentrated lyophilized lavage fluid to activate noninfectious Sendai virus (NISV) in vitro was examined. Lavage fluid was obtained from 4-ipomeanol (4-IP)-injured lungs at 1, 3, 5, 7, 9, and 13 days after treatment, and 1, 7, and 13 days after sham treatment (controls). Significantly higher viral titers were obtained using lavage fluid collected 1 day after 4-IP treatment. Higher protein concentrations were present in lavage fluid obtained at day 1 and 3 after 4-IP treatment. It is concluded that local viral-activating protease concentrations resulting from 4-IP-induced pulmonary injury is a likely microenvironmental modulator of paramyxoviral replication and can play an important role in paramyxoviral-induced lung injury.

Antigenic relationships between avian paramyxoviruses. III. A mathematical model of antigenic drift and a computer-assisted approach for construction of a phylogenetic tree

The suggested model of antigenic kinship between related paramyxoviruses is based on another concept of antigenic determinant, as compared to the previously suggested combinatorial mathematical model by the authors. According to it, antigenic changes of any determinant do not proceed by "leaps" but can be changed gradually. Such changed determinant can induce a correspondingly changed type of antibodies which still preserve a certain kinship to the original type of the determinant (before its changing) revealed by cross reaction serological tests. Accordingly, there can be "families" of the determinants differing by degree of relatedness to (or, reversely, by antigenic distance from) the "original" ("ancestor") determinant. In addition to another interpretation of the antigenic kinship, the new mathematical model was used as an approach for revealing phylogenetic relationships between antigenically related viruses.

Expression of biologically active and antigenically authentic parainfluenza type 3 virus hemagglutinin-neuraminidase glycoprotein by a recombinant baculovirus

The hemagglutinin-neuraminidase (HN) gene of human type 3 parainfluenza virus has been inserted into a baculovirus expression vector under the control of the polyhedrin promoter. HN protein produced in insect cells by the recombinant baculovirus appeared to be glycosylated, was transported to the cell surface, and was biologically active. All of the HN epitopes previously mapped functionally to a region(s) involved in neuraminidase and/or hemagglutination activities were conformationally unaltered on the recombinant protein. The HN produced in this system also induced a protective immune response in immunized cotton rats. From these studies we conclude that the HN expressed in insect cells represents a source of authentic HN glycoprotein suitable for structural analysis and immunization.

Localization of functional sites on the hemagglutinin-neuraminidase glycoprotein of Sendai virus by sequence analysis of antigenic and temperature-sensitive mutants

To locate the various functions associated with the hemagglutinin-neuraminidase (HN) glycoprotein of Sendai virus in the primary structure of the protein, a temperature-sensitive (ts) mutant and seven antigenic mutants were sequenced. The ts mutant was defective in its ability to agglutinate erythrocytes and infect host cells, while its neuraminidase activity was normal. Its sequence revealed two closely spaced amino acid substitutions (residues 262 and 264) and one distant substitution (residue 461). Revertants could not be isolated, suggesting that more than one of the substitutions is responsible for the defective hemagglutinating activity. The antigenic mutants were selected with monoclonal antibodies that delineate four nonoverlapping antigenic sites (I-IV) and separately inhibit hemagglutinating, neuraminidase, and hemolysis activities. Mutants selected with antibodies to antigenic sites I-III were used to map these functions on the primary sequence of HN. Each antigenic mutant had a single point mutation in the HN gene that resulted in an amino acid substitution in the protein. A site II mutant selected with an antibody which inhibits hemolysin activity had a substitution at amino acid 420, while a mutant selected with antibody that inhibits only erythrocyte binding (site III) had a substitution at amino acid 541. Two antigenic mutants selected with an antibody that inhibits hemagglutination and neuraminidase activities (site I) had amino acid substitutions in close proximity (residues 277 and 279) to the two closely spaced substitutions of the ts mutant. These findings suggest that the region defined by the ts mutant and these two antigenic mutants is involved in host cell binding. Antigenic mutants selected with another site I antibody had amino acid changes at residue 184, indicating that antigenic site I is discontinuous in the primary sequence. This antibody blocks only hemagglutination, but mutants selected with it had a decreased neuraminidase activity. This finding supports the idea that the neuraminidase site is close to, but distinct from, the hemagglutination site.

Resistance of a measles virus mutant to fusion inhibitory oligopeptides is not associated with mutations in the fusion peptide

The nucleotide sequence and predicted amino acid sequence has been obtained for the fusion (F) protein gene of the R93 strain of measles virus and compared to that of the parental strain, Edmonston B. The R93 strain is a mutant measles virus which is able to grow and induce cell fusion in the presence of the fusion inhibiting oligopeptide, Z-D-Phe-L-Phe-L-(NO2)Arg (SV4814). Primer extension sequencing on isolated R93 mRNA demonstrated the presence of three nucleotide changes leading to three amino acid changes, none of which are in the hydrophobic NH2-terminal region of the F1 polypeptide.

Distinct functions of antigenic sites of the HN glycoprotein of Sendai virus

Monoclonal antibodies specific for the hemagglutinin-neuraminidase (HN) glycoprotein of Sendai virus were used to examine the antigenic structure of HN and its role in the initiation of infection and immunity. Using 10 anti-HN antibodies, four distinct antigenic sites designated I-IV were topographically mapped on the HN molecule by competitive-binding assays. To relate the biological functions of HN to its antigenic structure, anti-HN antibodies were analyzed for their inhibitory activity in neuraminidase, hemagglutination, and hemolysis inhibition tests. Antibodies to antigenic site I inhibited hemagglutination and one of these antibodies also inhibited neuraminidase activity. Antibodies to site II inhibited neither activity. However, hemolysis an F protein activity was inhibited, suggesting that these antibodies which bind to HN interfere with F-mediated fusion. Antigenic sites III and IV had different effects on the hemagglutinating and neuraminidase functions of HN: Site III antibodies inhibited hemagglutination while antibodies to site IV only inhibited neuraminidase activity. Antibodies to each antigenic site inhibited virus production. Since antibodies to sites I and III inhibited hemagglutination, it is likely that they block virus adsorption. Antibodies to HN site II only inhibited hemolysis, and therefore, may prevent virus penetration. Antibodies reacting with site IV inhibited virus production after virus penetration. Since neuraminidase activity was the only function inhibited, the viral enzyme may be involved in virus release. The fact that site IV antibodies inhibited neuraminidase but not hemagglutination suggests that these sites are distinct.

The fusion glycoprotein of Sendai virus: sequence analysis of an epitope involved in fusion and virus neutralization

To localize the amino acid residues on the F glycoprotein that are involved in Sendai virus fusion and virus neutralization, an anti-F monoclonal antibody which inhibits these functions was used to select three antigenic variants. Sequence analysis of the entire F gene of the three variants identified a single mutation that was responsible for the loss of antibody binding. The mutation, a proline to glutamine substitution at residue 399, was at a position in the primary sequence far removed from the hydrophobic F1-NH2 terminus thought to be directly involved in fusion. A synthetic peptide, comprising amino acid sequences in the region of the mutation, bound to the antibody used to select the variants, suggesting that the site of mutation is also the site of antibody binding. This information suggests that in the three-dimensional structure of the F molecule the amino acid residues around proline 399 are located close to the F1-NH2 terminus, and that fusion is directly inhibited by antibody binding. Other less likely alternatives are discussed.

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.

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.

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).

Dissociation of newly synthesized Sendai viral proteins from the cytoplasmic surface of isolated plasma membranes of infected cells

The interaction of Sendai viral proteins with the membranes of infected cells during budding of progeny virions was studied. BHK cells infected with Sendai virus were labeled with [35S]methionine, and the plasma membranes were purified on polycationic polyacrylamide beads. The isolated membranes were incubated with various agents which perturb protein structure to dissociate viral proteins from the membranes. Incubation of membranes with thiocyanate and guanidine removed both the M and nucleocapsid proteins. Urea (6 M) removed the nucleocapsid proteins but removed M protein only in the presence of 0.1 or 1.0 M KCl. In contrast, high salt concentrations alone eluted only the M protein, leaving the nucleocapsid proteins completely membrane bound. About 65% of the M protein was eluted in the presence of 4 M KCl. The remaining membrane-associated M protein was resistant to further extraction by 4 M KCl. Thus, M protein forms two types of interaction with the membrane, one of them being a more extensive association with the membrane than the other.

The activity of membranes reconstituted from HVJ envelope proteins and lipids to induce hemolysis and fusion between liposomes and erythrocytes

A simple method for preparation of lipid-free envelope proteins (HN protein and F protein) of HVJ (Sendai virus) was developed. Reconstituted 'envelopes' were then prepared from envelope proteins and various lipids by the detergent dialysis method, and the activity to induce hemolysis and fusion between liposome and erythrocyte was studied. Lipid-free envelope protein aggregates could induce hemolysis and liposome-erythrocyte fusion. The activity was however greatly augmented by incorporation of envelope proteins into membrane of viral total lipids. Hemolytic and fusogenic activity was somewhat augmented by incorporation of envelope proteins into dipalmitoylphosphatidylcholine/cholesterol (1:1, molar ratio) and dimyristoylphosphatidylcholine/cholesterol (1:1), though the augmentation was lower than that observed with viral total lipids. When 'envelopes' were reconstituted with the proteins and viral total lipids supplemented with phosphatidylethanolamine, two kinds of 'envelopes' were prepared; one was permeable to Dextran (Mr 75000) and hemolytic, and the other was impermeable to Dextran and nonhemolytic. The latter acquired hemolytic activity after subjection to freezing and thawing, and its barrier function was lost concomitantly. The study suggests that envelope proteins (HN protein and F protein) could function without lipids but their activity was greatly influenced by not only the composition of additional lipids but also mode of arrangement of components on the reconstituted membranes.

The effects of monoclonal antibodies against the hemagglutinin-neuraminidase and fusion protein on the release of Sendai virus from infected cells

Vero cell cultures in Leighton tubes were infected with egg-grown Sendai virus at high multiplicity of infection. Four hours after infection, the cultures were labelled with 35S-methionine, after which various concentrations of fourteen and five mouse monoclonal antibodies directed against different antigenic determinants of the hemagglutinin-neuraminidase (HN) and fusion (F) protein, respectively, were added to the medium. Fourty-eight hours after infection radiolabelled virions released into the medium were collected and purified by discontinuous sucrose gradient centrifugations. The amount of virus-bound radioactivity obtained in the various extracellular materials allowed an estimation of the capacity of the different monoclonal antibodies to inhibit the release of Sendai virus. In addition, the release of virions from infected cells was studied ultrastructurally. Based on their serological reactivity the fourteen anti-HN monoclonal antibodies could be divided into four groups. The first group of clones could not inhibit any biological activity of the virus. These clones were binding proximally, near the base of the HN glycoprotein and could not inhibit the release of the virus. The second group blocked hemolysis, but did not block hemagglutination (HA) or neuraminidase (NA) activity. The third group of clones blocked all biological activities of the HN glycoprotein. The fourth group could only block NA activity. With the exception of one of five monoclonal antibodies belonging to the second group, antibodies of the second, third and fourth group were found to bind more distally on the HN glycoprotein. Except for two monoclonal antibodies of the second group they could all effectively inhibit release of the virus from infected cells. Ultrastructurally, these antibodies caused aggregation of virions in contact with the plasma membrane. The five monoclonal antibodies directed against the F protein reacted with four different antigenic sites. These antibodies could not prevent the release of Sendai virus.

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.

An assay for the receptor-destroying activity of influenza C virus

We have developed a convenient method for assaying the receptor-destroying enzyme (RDE) activity of influenza C virus. This method measures the ability of the RDE to destroy the hemagglutination-inhibition activity of a potent inhibitor present in rat serum. Some physico-chemical properties of the RDE of influenza C virus were investigated by using this method. The temperature optimum for maximal activity of this enzyme was found to be 45 C to 53 C. There was little difference in thermostability between the RDE and hemagglutinating activities of influenza C virus. When influenza C virions were treated with various concentrations of trypsin, the RDE activity decreased in parallel with the decrease in the amount of residual gp88 glycoprotein, suggesting association of RDE with this glycoprotein.

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.

Activity of human erythrocyte gangliosides as a receptor to HVJ

Liposomes could bind and fuse efficiently to human erythrocytes in the presence of HVJ when they contained gangliosides isolated from human erythrocytes. Sialosylparagloboside, which has a terminal sequence of NeuAc alpha 2-3Gal beta 1-4GlcNAc, has a much higher receptor activity to the virus than GD1a, GD1b, GT1b, and GT1a, all of which contain the terminal sequence of NeuAc alpha 2-3Gal beta 1-3GalNAc or NeuAc alpha 2-8NeuAc alpha 2-3Gal beta 1-3GalNAc. The activity of sialosylparagloboside is comparable to that of glycophorin, a major sialoglycoprotein of human erythrocytes, when compared on the basis of the required amount (as sialic acid) of compounds. The high affinity of sialosylparagloboside to the viral HANA protein is also suggested by the finding that it showed high inhibitory activity against HVJ-mediated binding of glycophorin liposomes to erythrocytes. Sialosylparagloboside was also highly susceptible to the viral sialidase, the other biological function of HANA 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.

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-mediated lysis of liposomes requires cholesterol

Vesicles were constituted with glycophorin, the Sendai virus receptor of human erythrocytes, and loaded with calcein, a polar derivative of fluorescein, at self-quenching concentrations. On exposure to Sendai virus and mild hypo-osmotic stress, vesicles of the appropriate composition released a significant portion of their internal contents, as indicated by an increase in calcein fluorescence. Susceptible liposomes were not induced to leak by heat-inactivated virus or by trypsin-treated virus. The response of the vesicles to virus attachment is thus analogous to virus-induced hemolysis and presumably involves fusion of the vesicle and virus membranes. In addition to glycophorin and phosphatidylcholine, cholesterol was absolutely required for the lytic response to the virus. The need for cholesterol was not attributable to inactivation of the virus by liposomes without cholesterol. The presence of gangliosides increased the encapsulated volume of the liposomes, but gangliosides did not effectively substitute for glycophorin. Thin-layer chromatography of lipid extracted from incubated virus and liposomes containing a small amount of a fluorescent phosphatidylcholine indicated that phosphatidylcholine in the vesicle is not chemically altered by functional interaction with the virus.

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.

Thermostabilities of virion activities of Newcastle disease virus: evidence that the temperature-sensitive mutants in complementation groups B, BC, and C have altered HN proteins

Four virion activities of Newcastle disease virus (hemagglutinating, neuraminidase, hemolytic, and infectious activities) were examined before and after heat stress in low-salt buffer and physiological salt buffer (phosphate-buffered saline). The hemagglutinating and neuraminidase activities of the Australia-Victoria wild-type (AV-WT) strain were thermostable at both salt concentrations tested, whereas the thermostabilities of the hemolytic and infectious activities were salt dependent (thermostable in phosphate-buffered saline but not in low-salt buffer). Virions of RNA(+) temperature-sensitive (ts) mutants of AV-WT were tested for the stabilities of the four activities. Some mutants in groups B, BC, and C were as stable as AV-WT in all functions, but others were much less stable in all functions. The unstable mutants in groups B, BC, and C affirmed the assignment of the ts lesions of these mutants to the hemagglutinin/neuraminidase (HN) protein gene because HN function(s) are required for all four activities. The instability of these ts mutants was not related to their decreased virion HN protein content and was not due to physical loss of the HN protein from the virions. Three of four ts(+) plaque-forming revertants of the least stable mutant, BC2, coreverted for stability, confirming that the unstable phenotype is indeed the result of the mutation responsible for the ts phenotype. Group D mutants were approximately as stable as AV-WT in hemagglutinating, neuraminidase, and hemolytic activities; this is consistent with this group representing a lesion in a gene other than the HN protein gene. However, the infectivities of two of the three group D mutants were less stable than the infectivity of AV-WT in low-salt buffer.

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.