Trypsin action on the growth of Sendai virus in tissue culture cells. 3. Structural difference of Sendai viruses grown in eggs and tissue culture cells

The E3 protein of bovine coronavirus is a receptor-destroying enzyme with acetylesterase activity

In addition to members of the Orthomyxoviridae and Paramyxoviridae, several coronaviruses have been shown to possess receptor-destroying activities. Purified bovine coronavirus (BCV) preparations have an esterase activity which inactivates O-acetylsialic acid-containing receptors on erythrocytes. Diisopropyl fluorophosphate (DFP) completely inhibits this receptor-destroying activity of BCV, suggesting that the viral enzyme is a serine esterase. Treatment of purified BCV with [3H]DFP and subsequent sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the proteins revealed that the E3 protein was specifically phosphorylated. This finding suggests that the esterase/receptor-destroying activity of BCV is associated with the E3 protein. Furthermore, treatment of BCV with DFP dramatically reduced its infectivity in a plaque assay. It is assumed that the esterase activity of BCV is required in an early step of virus replication, possibly during virus entry or uncoating.

Identification of amino acids relevant to three antigenic determinants on the fusion protein of Newcastle disease virus that are involved in fusion inhibition and neutralization

Nucleotide sequence analysis of F protein antigenic variants of Newcastle disease virus mapped three distinct antigenic determinants to positions 343, 72, and 161 on the protein. The high fusion-inhibiting and neutralizing capacities of all of the monoclonal antibodies used for selection suggested close functional and structural relationships of the three positions with the fusion-inducing N-terminal region of the F1 subunit. The former two positions were located at the cysteine cluster domain near the C terminus of the F1 subunit and at the major hydrophilic domain in the F2 subunit, respectively, and both domains appeared to represent the major antigenic determinants of paramyxovirus F protein.

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.

Introduction of macromolecules into mammalian cells by cell fusion

Proteins with molecular weights of up to 500K can be enclosed in erythrocyte ghosts by exposing the ghosts to hypotonic solution containing these proteins. The proteins can then be introduced into recipient cells by fusing the ghosts with the cells using HVJ, PEG, or influenza virus. Some applications of this method are described. By an improved method, 15 kbp DNA and IgM (900 kDa) can be entrapped in erythrocyte membranes and these are then treated with liposomes containing gangliosides and HVJ. These treated membranes containing large macromolecules fuse with almost 100% of the recipient cells used. Naked liposomes infrequently fuse with cultured cells, so introduction of their contents into cells is very inefficient. However, liposomes constituted from lipid and glycoproteins (HN and F) of HVJ (Sendai virus), by removing a nonionic detergent, fuse with cells about 200 times more efficiently than naked liposomes. Naked liposomes can fuse with specific cells, such as cells infected with subacute sclerosing panencephalitis virus or with human immunodeficiency virus. Plasmid DNA and mRNA of up to about 40 kbp can be entrapped efficiently in liposomes associated with gangliosides formed by reverse-phase evaporation, and then reacted with HVJ. The contents of the resulting liposomes with HVJ can be introduced efficiently into cultured cells in a suspended or plated state, and nearly all the cells then express the gene transiently. This procedure is also effective for obtaining stable transformants of many kinds of cultured cells.

Pneumopathogenicity in mice of a Sendai virus mutant, TSrev-58, is accompanied by in vitro activation with trypsin

The pneumopathogenicity of a trypsin-sensitive revertant of Sendai virus, TSrev-58, which was derived from a trypsin-resistant mutant, TR-5, was examined in mice. In comparison with TR-5, the revertant had a single amino acid substitution at residue 116 (Ile----Arg) on F protein, which was the cleavage site, and had the same trypsin sensitivity as the wild-type virus. However, TSrev-58 still had a single amino acid difference from the wild-type virus at residue 109 (Asn----Asp) (M. Itoh, H. Shibuta, and M. Homma, J. Gen. Virol. 68:2939-2943, 1987). Nevertheless, the present study revealed that TSrev-58 had the same pneumopathogenicity in mice as the wild-type virus. This result indicates that the activating protease of Sendai virus present in the lungs of mice is quite similar to trypsin and also that the in vitro trypsin sensitivity of Sendai virus can be a good marker of pneumopathogenicity in mice.

Characterization of a pantropic variant of Sendai virus derived from a host range mutant

A variant (F1-R) was isolated from a temperature-sensitive host range mutant (ts-f1) of Sendai virus. F1-R was no longer temperature-sensitive but it retained the host range phenotype. Unlike wild-type virus, F1-R and ts-f1 undergo multiple cycles of replication in several cell lines in the absence of trypsin. This was attributed to proteolytic activation of the fusion (F) glycoprotein of the host range mutants, in cell nonpermissive to wild-type virus. In mice infected intranasally the variant F1-R caused a generalized infection. This was shown by immunohistology and with infectious virus being recovered from several organs whereas infection with wild-type virus was restricted to the lung. These observations indicate that the pantropic property of F1-R is the result of proteolytic activation of the virus by ubiquitous proteases. Nucleotide sequence analyses revealed that ts-f1 and F1-R differed from the wild-type virus by mutations at the region of the cleavage site of F and at the glycosylation site of the F2 subunit. The findings indicated that these mutations are responsible for the increased cleavability of the F protein of ts-f1 and F1-R and therefore are important determinants for the pantropism of F1-R.

Cell-mediated immunity induced in mice after vaccination with a protease activation mutant, TR-2, of Sendai virus

Our previous study has shown that, although a trypsin-resistant mutant of Sendai virus, TR-2, replicates only in a single cycle in mouse lung with a negligible lesion, the animal acquires a strong immunity against lethal infection with wild-type Sendai virus, suggesting that TR-2 could be used as a new type of live vaccine (M. Tashiro and M. Homma, J. Virol. 53:228-234, 1985). In the present study, we investigated the immunological response elicited in TR-2-infected mice, particularly with respect to cell-mediated immunity. Analyses of cytotoxic activities of spleen cells with 51Cr release assays revealed that Sendai virus-specific T lymphocytes (CTL), in addition to natural killer activity and antiviral antibodies, were induced in DBA/2 and C3H/He mice infected intranasally with TR-2. Proteolytic activation of the fusion glycoprotein F was required for the primary induction of CTL, though not necessarily for stimulation of natural killer and antibody responses. Memory of the CTL induced by TR-2 was long-lasting and was recalled in vivo immediately after challenge with wild-type Sendai virus. In contrast to TR-2, immunization with inactive split vaccine failed to induce the CTL response, but it elicited a high titer of serum antibody and a low level of natural killer activity.

Fusion (F) protein gene of Newcastle disease virus: sequence and hydrophobicity comparative analysis between virulent and avirulent strains

The nucleotide and predicted amino acid sequences have been obtained for the fusion (F) protein gene of the avirulent strain La Sota of Newcastle disease virus (NDV). The F1 N-terminus begins with the tripeptide Leu-Ileu-Gly instead of Phe-X-Gly as usually observed in fusion peptide. It was found that the cleavage-activation domain of the avirulent La Sota strain contained single (but no pairs of) basic residues in the sequence Gly-Arg-Gln-Gly-Arg. Hydrophobicity analysis suggested that the cleavage-activation domain became more hydrophobic and could be less accessible for host-specific protease(s); dibasic residues next to the F1 N-terminus were shown to be important for keeping the cleavage-activation site in exposed positioning, suitable for F protein activation. Comparative sequence analysis of the NDV F proteins revealed a striking homology between lentogenic La Sota and mesogenic Beaudette C strains. Furthermore, 58 variable positions were recorded in the NDV F protein, excluding signal sequence; some of these mutations, in the cysteine-clustered region, were surmised to alter virulence.

Fusion glycoprotein (F) of rinderpest virus: entire nucleotide sequence of the F mRNA, and several features of the F protein

The full-length cDNA corresponding to the mRNA for the fusion protein of rinderpest virus (RV) was cloned and its complete nucleotide sequence was determined. The mRNA for the F protein was composed of 2359 nucleotides and contained a single large open reading frame which was capable of encoding 566 amino acids with a molecular weight (MW) of 58,929. The RV-F mRNA had a long noncoding region at the 5' end (586 bases) which was C-rich like the measles virus (MV)-F mRNA but they did not appear to be homologous with each other. Their secondary structure with long G-C stems suggested that they are easily folded. The coding region of RV-F mRNA was significantly homologous with that of MV-F; 74% of the nucleotides and 79.0% [corrected] of the amino acids were identical. The predicted RV-F protein had a basic amino acid region (104-108) which may be cleaved by protease to yield an activated form of F1,2. Three regions (1-19, 109-133, 418-513) were highly hydrophobic, and the N-terminal hydrophobic region of F1 or the positions of cysteines were significantly conserved compared with those of the other paramyxovirus F proteins. Three potential sites for glycosylation existed only in the F2 protein. Several features of the predicted RV-F protein were confirmed in polyacrylamide gel electrophoresis.

Structure, function, and intracellular processing of paramyxovirus membrane proteins

Paramyxoviruses are a fascinating group of viruses with diverse hosts and disease manifestations. They are valuable systems for studying viral pathogenesis, molecular mechanisms of negative strand viral replication, and glycoprotein structure and function. In the past few years this group of viruses has received increased attention and as a result there is a wealth of new information. For example, most of the genes of many paramyxoviruses have been cloned and sequenced. The recent availability of sequence information from a number of paramyxoviruses now allows the direct comparison of the amino acid sequence and determinants of secondary structure of analogous genes across the family of viruses. Such comparisons are revealing for two reasons. First, results provide clues to the evolution of these viruses. Second, and more importantly, comparisons of analogous genes may point to sequences and structural determinants that are central to the function of the individual proteins. Below is a comparison of five of the paramyxovirus genes with a discussion of the implications of common structural determinants for function, intracellular processing, and evolutionary origin. The focus is on the paramyxovirus membrane proteins, although other proteins are discussed briefly.

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.

Endoproteolytic cleavage of gp160 is required for the activation of human immunodeficiency virus

The envelope protein of human immunodeficiency virus (HIV) is synthesized as a polyprotein (gp160) and cleaved intracellularly to a gp120-gp41 heterodimer. In this study, the tryptic-like endoproteolytic cleavage site was removed by site-directed mutagenesis and replaced with a chymotryptic-like site. The resultant mutant, RIP7/mut10, was found to be indistinguishable from wild-type HIV when analyzed at the level of proviral replication, RNA processing, protein expression, and viral assembly. However, the gp160 polyprotein was not cleaved and the mutated virions were biologically inactive, until and unless they were exposed to limiting concentrations of chymotrypsin. As is the case for other enveloped mammalian viruses, endoproteolytic cleavage of the HIV envelope protein and release of a unique hydrophobic domain appear to be necessary for the full expression of viral infectivity.

Role of individual glycoproteins of human parainfluenza virus type 3 in the induction of a protective immune response

Affinity-purified hemagglutinin-neuraminidase (HN) and fusion (F) glycoproteins of human parainfluenza virus type 3 (P13 virus) were used to investigate their role in the induction of a protective immune response following immunization of hamsters. The efficacy of immunization with the glycoprotein antigens was tested by challenge infection. Results of virus recovery from lungs and trachea demonstrated that although immunization with HN or F alone induced an antibody response to the respective glycoproteins, it did not provide a significant level of protection. However, immunization with a mixture of both purified glycoproteins induced higher virus-neutralizing activity in bronchial lavages and afforded complete protection from challenge infection. Similarly, incomplete protection was observed after passive transfer of monospecific rabbit antibody to the purified HN or F in baby hamsters. On the other hand, passive transfer of a mixture of antibodies to HN and F conferred a higher level of protection. Thus, the presence of antibody to both glycoproteins of P13 virus may be essential for protective immunity.

The molecular biology of influenza virus pathogenicity

It is an accepted concept that the pathogenicity of a virus is of polygenic nature. Because of their segmented genome, influenza viruses provide a suitable system to prove this concept. The studies employing virus mutants and reassortants have indicated that the pathogenicity depends on the functional integrity of each gene and on a gene constellation optimal for the infection of a given host. As a consequence, virtually every gene product of influenza virus has been reported to contribute to pathogenicity, but evidence is steadily growing that a key role has to be assigned to hemagglutinin. As the initiator of infection, hemagglutinin has a double function: (1) promotion of adsorption of the virus to the cell surface, and (2) penetration of the viral genome through a fusion process among viral and cellular membranes. Adsorption is based on the binding to neuraminic acid-containing receptors, and different virus strains display a distinct preference for specific oligosaccharides. Fusion capacity depends on proteolytic cleavage by host proteases, and variations in amino acid sequence at the cleavage site determine whether hemagglutinin is activated in a given cell. Differences in cleavability and presumably also in receptor specificity are important determinants for host tropism, spread of infection, and pathogenicity. The concept that proteolytic activation is a determinant for pathogenicity was originally derived from studies on avian influenza viruses, but there is now evidence that it may also be relevant for the disease in humans because bacterial proteases have been found to promote the development of influenza pneumonia in mammals.

Dynamics of functional maturation and inactivation of HN glycoprotein in NDV-infected chick embryo fibroblasts

In avirulent NDV strain-infected chick embryo cells treated with cycloheximide at different intervals post infection a decrease of the level of hemagglutinating (HA) and neuraminidase (Nase) activities was observed. Studies on this system led to conclusion that the HA-Nase (HN) glycoprotein molecules are unstable and the actual amount of the functionally active (mature) HN entities is determined by a dynamic equilibrium between the antidromic processes of the HN functional maturation and inactivation. Kinetic studies on the actual intracellular levels of the HA and Nase activities using 5 min intervals of their detection after the cycloheximide treatment permitted to uncouple the processes of the HN maturation and inactivation. Analytical part of the studies made it possible to compute quantitative parameters of the involved processes: (a) pool size of the functionally nonactive HN precursors, (b) time needed for their functional maturation, and (c) rate of their inactivation.

In vitro mutagenesis identifies a region within the envelope gene of the human immunodeficiency virus that is critical for infectivity

Site-specific mutagenesis was used to introduce amino acid substitutions at the asparagine codons of four conserved potential N-linked glycosylation sites within the gp120 envelope protein of human immunodeficiency virus (HIV). One of these alterations resulted in the production of noninfectious virus particles. The amino acid substitution did not interfere with the synthesis, processing, and stability of the env gene polypeptides gp120 and gp41 or the binding of gp120 to its cellular receptor, the CD4 (T4) molecule. Vaccinia virus recombinants containing wild-type or mutant HIV env genes readily induced syncytia in CD4+ HeLa cells. These results suggest that alterations involving the second conserved domain of the HIV gp120 may interfere with an essential early step in the virus replication cycle other than binding to the CD4 receptor. In long-term cocultures of a T4+ lymphocyte cell line and colon carcinoma cells producing the mutant virus, revertant infectious virions were detected. Molecular characterization of two revertant proviral clones revealed the presence of the original mutation as well as a compensatory amino acid change in another region of HIV gp120.

Protection against lethal measles virus infection in mice by immune-stimulating complexes containing the hemagglutinin or fusion protein

The importance of each of the two surface glycoproteins of measles virus in active and passive immunization was examined in mice. Infected-cell lysates were depleted of either the hemagglutinin (H) or fusion (F) glycoprotein by using multiple cycles of immunoaffinity chromatography. The products were used to prepare immune-stimulating complexes (iscoms) containing either F or H glycoprotein. Such complexes are highly immunogenic, possibly as a result of effective presentation of viral proteins to the immune system [B. Morein, B. Sundquist, S. Höglund, K. Dalsgaard, and A. Osterhaus, Nature (London) 308:457-460, 1984]. Groups of 3-week-old BALB/c mice were inoculated with the iscom preparations. All animals developed hemolysis-inhibiting antibodies, whereas only sera of animals immunized with the iscoms containing the H glycoprotein had hemagglutination-inhibiting antibodies. Sera from animals immunized with the H or F preparation only precipitated the homologous glycoprotein in radioimmune precipitation assays. The immunized animals were challenged with a lethal dose of the hamster neurotropic variant of measles virus. Of the 7-week-old animals in the nonimmunized control group, 50% died within 10 days after challenge. No animals in the immunized groups showed symptoms of disease throughout the observation period of 3 months. Passive administration of anti-H monoclonal antibodies gave full protection against the 100% lethal acute infection with the hamster neurotropic variant of measles virus in newborn mice, whereas anti-F monoclonal antibodies failed to protect the animals. This study emphasizes that both H and F glycoproteins need to be considered in the development of measles virus subunit vaccines.

Expression of the F and HN glycoproteins of human parainfluenza virus type 3 by recombinant vaccinia viruses: contributions of the individual proteins to host immunity

cDNA clones containing the complete coding sequences for the human parainfluenza virus type 3 (PIV3) fusion (F) and hemagglutinin-neuraminidase (HN) glycoprotein genes were inserted into the thymidine kinase gene of vaccinia virus (WR strain) under the control of the P7.5 early-late vaccinia virus promotor. The recombinant vaccinia viruses, designated vaccinia-F and vaccinia-HN, expressed glycoproteins in cell culture that appeared to be authentic with respect to glycosylation, disulfide linkage, electrophoretic mobility, cell surface expression, and, in the case of the HN protein, biological activity. Cotton rats inoculated intradermally with vaccinia-HN developed serum neutralizing antibody titers equal to that induced by respiratory tract infection with PIV3, whereas animals receiving vaccinia-F had threefold lower neutralizing antibody titers. A single immunization with either recombinant vaccinia virus induced nearly complete resistance in the lower respiratory tract of these animals. With regard to protection in the upper respiratory tract, animals immunized with vaccinia-HN or vaccinia-F exhibited reductions in PIV3 replication of greater than 3,000-fold and 6-fold, respectively. This large difference (greater than 500-fold) in reduction of PIV3 replication in the upper respiratory tract was in contrast to the relatively modest difference (3-fold) in serum neutralizing antibody titers induced by vaccinia-HN versus vaccinia-F. This dissociation between the level of neutralizing antibodies and protection suggested that immunity to PIV3 is complex, and that immune mechanisms other than serum neutralizing antibodies make important contributions to resistance to infection. Overall, under these experimental conditions, vaccinia-HN induced a substantially more protective immune response than did vaccinia-F.

Difference in capacities for virion-to-virion fusion of young and aged HVJ (Sendai virus): a model of membrane fusion

Young and aged HVJ virions differ structurally and morphologically due to changes that occur during aging in vitro or in ovo. Young virions soon after their budding off are rod-shaped, rigid and relatively uniform in size, whereas virions that have aged in vitro after their formation are round, nonrigid and variable in size. These changes during aging seem to be due to the variation of M protein, a "skeletal" protein that is associated with both the envelope membrane proteins and nucleocapsid strands in the virions. The capacities for virion-to-virion fusion of young and aged virions were compared to clarify the relation between the membrane fusion and membrane-associating skeletal proteins. On treatment with polyethylene glycol (PEG), aged virions readily fused, forming large virion vesicles, but young virions were resistant to fusion. Further, aged virions fused even on incubation at 37 degrees C without the fusogen. Thus the capacity for virion-to-virion fusion evidently increases during aging of virions. This result suggests that skeletal proteins associating with the biological membrane are important for preventing membrane fusion, and that virion-to-virion fusion is a good model system for use in studies on the mechanism of membrane fusion.

Nucleotide sequence of the hemagglutinin-neuraminidase gene of Newcastle disease virus avirulent strain D26: evidence for a longer coding region with a carboxyl terminal extension as compared to virulent strains

The nucleotide sequence of DNA clones complementary to the genomic RNA of an extremely avirulent strain D26 of Newcastle disease virus was analyzed, and the sequence of 2102 nucleotides directly following F gene reported previously (Sato et al., 1987, Virus Res. 7, 241-255), and corresponding to HN0 gene was determined. A long open reading frame coding for the HN0 peptide of 616 amino acid residues was found in this sequence. It was flanked by the consensus sequences N1 and N2 (Ishida et al., 1986, Nucleic Acids Res. 14, 6551-6564), and the former was shown by the primer extension method to serve as the transcriptional initiation site. The deduced amino acid sequence of the HN0 peptide was highly homologous to that of the HN peptides of strains Beaudette C and B1, but had a carboxyl terminal extension of 39 amino acid residues with a potential glycosylation site in it. The terminal extension is likely to be excised during the processing, and this is consistent with the observation that unglycosylated HN0 is larger in size than unglycosylated HN. A microheterogeneity among the cDNA clones in the nucleotide sequence was also noted which may be relevant to the synthesis of a small amount of an HN-sized peptide in strain D26-infected cells.

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.

The role of envelope glycoprotein processing in murine leukemia virus infection

The murine leukemia virus envelope protein is synthesized as a precursor molecule, Pr85env, which is proteolytically cleaved at an arginine residue to produce two mature envelope proteins, gp70 and p15(E). The results presented here indicate that mutation to lysine of the arginine found at the envelope precursor cleavage site results in a precursor which is cleaved with an efficiency at least 10-fold lower than the efficiency with which the wild-type protein is cleaved. This mutation has been used to investigate the requirement for envelope protein processing in various aspects of retroviral infection. Viruses produced by cells transfected with mutant proviral clones are approximately 10-fold less infectious than wild-type viruses. Mutant viruses are incapable of inducing XC cell syncytium formation and are 100-fold less efficient than wild-type viruses at rendering cells resistant to superinfection. Envelope glycoproteins bearing the lysine mutation are found in reduced amounts on the surface of infected cells, and as a result mutant virions contain significantly less envelope protein than do wild-type virions. The phenotypic effects of the processing mutation described here are most likely the result of this paucity of envelope glycoproteins in virions carrying the mutation.

Significance of viral glycoproteins for infectivity and pathogenicity

Disease resulting from virus infection is a complex event depending on the close interaction of viral and cellular factors. Through the application of biochemical and genetic methods, it is now possible to gain an insight into the molecular basis of these interactions. Thus, it has been shown that the glycoproteins of enveloped viruses play a central role in the initiation of infection. They are responsible not only for the adsorption of virions to cellular receptors, but are also for the entry of the genome into the cell by the fusion of viral envelopes with cellular membranes. Evidence is growing that the fusogenic glycoproteins are frequently activated by cellular proteases. The structure of the proteins at the cleavage site and the availability of a suitable protease are critical for tissue tropism, spread of the virus in the infected organism and, thus, for pathogenicity. This will be demonstrated here by the example of the haemagglutinin of influenza viruses.

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.

Intracellular digestion of reovirus particles requires a low pH and is an essential step in the viral infectious cycle

Lysosomotropic drugs such as NH4Cl have been useful for studying the role of low pH in early events in virus infection. NH4Cl blocks the production of infectious progeny virus in mammalian reovirus-infected cells. The inhibitory effect of NH4Cl is mediated by an inhibition of intracellular digestion of reovirus outer capsid proteins. In vitro digestion of viral outer capsid proteins produces infectious partially uncoated particles, called intermediate subviral particles, which are no longer inhibited by the presence of NH4Cl. These results indicate that proteolytic processing of reovirus outer capsid proteins takes place in a low pH compartment of the cell and is an essential step in the viral infectious cycle.

Structural comparison of the cleavage-activation site of the fusion glycoprotein between virulent and avirulent strains of Newcastle disease virus

The nucleotide sequence of the mRNA encoding the fusion (F0) protein of a virulent strain of Newcastle disease virus was determined. A single open reading frame in the sequence encodes a protein of 553 amino acids with a calculated molecular weight of 59058. The amino acid sequence predicted several structural features involving the fusion-inducing hydrophobic stretch (residues 117-142) and the cleavage-activation site (residues 112-116) to generate the disulfide-linked F1 and F2 subunits. The cleavage-activation site as well as a part of the fusion-inducing sequence were compared among a series of virulent and avirulent strains by the chain-termination method using a synthetic oligonucleotide primer. It was found that without exception, the cleavage-activation site of virulent strains consisted of two dibasic residues with an intervening glutamine, Arg-Arg-Gln-Arg-Arg, whereas the corresponding region of avirulent strains was made of a sequence with single basic residues scattered among uncharged residues, Gly-LysArg-Gln-GlySer-Arg. On the basis of these observations and the previous results showing a strict correlation between the pathogenicity and the cleavability of the fusion protein of NDV (Y. Nagai, H-D. Klenk, and R. Rott, Virology, 72, 494-508, 1976), we propose the importance of the dibasic residues for efficient proteolytic activation of the fusion protein and for the pantropic property of NDV. Some strains were found to have Leu-Ile-Gly as the N-terminus of F1, whereas others contained Phe-Ile-Gly, indicating that Phe-X-Gly is not always conserved at F1 N-terminus of paramyxovirus.

Molecular cloning and nucleotide sequence of P, M and F genes of Newcastle disease virus avirulent strain D26

Molecular cloning of most if not all of the genome of an avirulent strain D26 of Newcastle disease virus (NDV) was carried out. cDNA clones were aligned by mutual hybridization and restriction map analysis. The nucleotide sequence of 3672 bases which completed the partial sequence of P gene reported in our previous paper (Ishida, N. et al., 1986, Nucleic Acids Res. 14, 6551-6564), and also covered M and F genes, was determined. Each gene contained one long open reading frame which could code for polypeptides of 395, 364, and 553 amino acid residues, respectively. The deduced amino acid sequences of P and M gene products showed little homology to those of other paramyxoviruses. In contrast, comparison of the amino acid sequence of the F gene product revealed highly conserved regions including the amino terminal sequence of the F1 portion following the putative processing site. There was only one basic amino acid residue at the putative processing site, which would explain the low virulence of this strain.

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.

Ability of the hydrophobic fusion-related external domain of a paramyxovirus F protein to act as a membrane anchor

The hydrophobic NH2 terminus of F1 (FRED) of the simian virus 5 fusion (F) protein is implicated in mediating cell fusion, but in the inactive F0 precursor the FRED is translocated across membranes. Hybrid proteins containing the FRED as a potential membrane anchorage domain and a mutant of F0 lacking the preceding five-arginine cleavage/activation site were used to study the effect of position on the FRED. The experiments indicate that the SV5 F protein has evolved an exquisite control system for biological activity: the FRED is close to the threshold of hydrophobicity required to function as a membrane anchor. The FRED is not sufficiently hydrophobic to halt translocation when in an internal position, but on cleavage/activation the threshold of hydrophobicity is effectively lowered, and the FRED, now the NH2 terminus of F1, is able to interact stably with membranes.

4-Ipomeanol-induced effects on Sendai viral pneumonia in mice

The effects of the pulmonary toxicant, 4-ipomeanol (4-IP), on Sendai viral pneumonia was examined in young adult female C57BL/6J mice. The histologic severity of the pneumonia was closely correlated with increasing doses of the compound. The more severe pneumonia observed in the 4-IP-treated animals was associated with higher pulmonary viral titers and a more diffuse distribution of viral antigen in bronchioles and alveolar parenchyma. There was no difference in the systemic humoral immune response against Sendai virus or pulmonary interferon production in animals that had received 4-IP or vehicle pretreatment. Classification of the antigen-positive cell types in the alveolar parenchyma resulted in a marked relative increase in the numbers of antigen-positive macrophages, as compared with other antigen-positive cell types.

Protease activation mutants of Sendai virus: sequence analysis of the mRNA of the fusion protein (F) gene and direct identification of the cleavage-activation site

Trypsin cleaves the fusion protein (F) of wild-type Sendai virus into two disulfide-linked polypeptides, F1 and F2, and thereby activates the membrane fusion activity of the virus. A. Scheid and P.W. Choppin [1976). Virology, 265-277) selected mutant viruses of which the F protein could be activated by different proteases, either elastase, chymotrypsin, or plasmin. Herein, we have further characterized five of these mutants. Sequencing of each mutant mRNA encoding the 60-70 amino acids surrounding the cleavage site revealed one or two amino acid changes near or at the cleavage sites. Virions cleaved in vitro by the appropriate proteases were assayed of their fusion activity by hemolysis, and the cleavage sites were determined by amino acid sequencing. In three cases, the change of protease specificity can be accounted for by changed amino acids right at the cleavage site, whereas several other mutations that potentiate cleavage at new sites by new proteases are somewhat removed from the actual cleavage site. We surmise that such mutations might alter local polypeptide conformation, thereby allowing the proteases access to existing sites. Cleavage at new sites produced fusion proteins with novel F1 NH-termini. We found that a mutant with a charged residue at the third position of this normally hydrophobic NH-terminal sequence retains activity in the hemolysis assay, whereas a mutant with a charged residue at the first position does not.

[Sendai virus replication in primary epithelial cells of mice]

Células epiteliais primárias obtidas do trato respiratório de camundongos jovens foram infectadas com o Vírus Hemaglutinante do Japão (HVJ, Sendai Virus) e, a progénie viral, tratada ou não com tripsina foi titulada através do método de Imunofluorescência Indireta. A progénie de Sendai Virus obtida de células epiteliais primárias de camundongo apresentou um título considerável, demonstrando-se que há ativação das partículas virais, capazes de infectar células LLC-MK 2, nas quais, a progénie viral foi titulada.

Direct role of viral hemagglutinin in B-cell mitogenesis by influenza viruses

The mitogenic activity of influenza virus is a function of the hemagglutinin (HA) molecule. Purified HA is mitogenic for murine B lymphocytes but not T lymphocytes. Furthermore, like the intact virus, HA of the H2 (but not H3) subtype is mitogenic only for B cells expressing the class II major histocompatibility complex glycoprotein I-E. Since virus bearing uncleaved HA is as mitogenic as virus bearing cleaved HA, the membrane fusion activity of the HA molecule is not involved.

Differences in bovine parainfluenza 3 virus variants studied by monoclonal antibodies against viral glycoproteins

We previously showed that of three bovine parainfluenza 3 virus strains the M strain, which is neurovirulent for young mice, has an extensive syncytium-inducing activity, whereas avirulent SC and 910N strains are weak in this activity. It was also demonstrated that both M and SC strains have very low hemagglutination and neuraminidase activities, while the 910N strain shows these activities to high levels. In the present study, monoclonal antibodies (Mabs) were raised against the glycoproteins of the 910N strain, and utilized to further characterize these three viral strains. Five Mabs against the hemagglutinin-neuraminidase protein, which were classified into four different epitope-recognizing groups, neutralized the M strain much more effectively than the 910N and SC strains, while the Mabs showed lower hemagglutination inhibition (HI) titers against the M and SC strains than the 910N strain. Three Mabs against the fusion protein neutralized the M strain but not the 910N and SC strains, while they showed no HI activity against any of these strains. These findings suggested that the M strain is considerably different from other strains in the structure of the viral envelope proteins.