Characterization of human parainfluenza viruses. I. The structural proteins of parainfluenza virus 2 and their synthesis in infected cells

Synthesis of human parainfluenza virus 2 nucleocapsid protein in yeast as nucleocapsid-like particles and investigation of its antigenic structure

The aim of this study was to investigate the suitability of yeast Saccharomyces cerevisiae expression system for the production of human parainfluenza virus type 2 (HPIV2) nucleocapsid (N) protein in the form of nucleocapsid-like particles (NLPs) and to characterize its antigenic structure. The gene encoding HPIV2 N amino acid (aa) sequence RefSeq NP_598401.1 was cloned into the galactose-inducible S. cerevisiae expression vector and its high-level expression was achieved. However, this recombinant HPIV2 N protein did not form NLPs. The PCR mutagenesis was carried out to change the encoded aa residues to the ones conserved across HPIV2 isolates. Synthesis of the modified proteins in yeast demonstrated that the single aa substitution NP_598401.1:p.D331V was sufficient for the self-assembly of NLPs. The significance of certain aa residues in this position was confirmed by analysing HPIV2 N protein structure models. To characterize the antigenic structure of NLP-forming HPIV2 N protein, a panel of monoclonal antibodies (MAbs) was generated. The majority of the MAbs raised against the recombinant NLPs recognized HPIV2-infected cells suggesting the antigenic similarity between the recombinant and virus-derived HPIV2 N protein. Fine epitope mapping revealed the C-terminal part (aa 386-504) as the main antigenic region of the HPIV2 N protein. In conclusion, the current study provides new data on the impact of HPIV2 N protein sequence variants on the NLP self-assembly and demonstrates an efficient production of recombinant HPIV2 N protein in the form of NLPs.

Budding of Marburgvirus is associated with filopodia

Viruses exploit the cytoskeleton of host cells to transport their components and spread to neighbouring cells. Here we show that the actin cytoskeleton is involved in the release of Marburgvirus (MARV) particles. We found that peripherally located nucleocapsids and envelope precursors of MARV are located either at the tip or at the side of filopodial actin bundles. Importantly, viral budding was almost exclusively detected at filopodia. Inhibiting actin polymerization in MARV-infected cells significantly diminished the amount of viral particles released into the medium. This suggested that dynamic polymerization of actin in filopodia is essential for efficient release of MARV. The viral matrix protein VP40 plays a key role in the release of MARV particles and we found that the intracellular localization of recombinant VP40 and its release in form of virus-like particles were strongly influenced by overexpression or inhibition of myosin 10 and Cdc42, proteins important in filopodia formation and function. We suggest that VP40, which is capable of interacting with viral nucleocapsids, provides an interface of MARV subviral particles and filopodia. As filopodia are in close contact with neighbouring cells, usurpation of these structures may facilitate spread of MARV to adjacent cells.

Parainfluenza viruses

Human parainfluenza viruses (HPIV) were first discovered in the late 1950s. Over the last decade, considerable knowledge about their molecular structure and function has been accumulated. This has led to significant changes in both the nomenclature and taxonomic relationships of these viruses. HPIV is genetically and antigenically divided into types 1 to 4. Further major subtypes of HPIV-4 (A and B) and subgroups/genotypes of HPIV-1 and HPIV-3 have been described. HPIV-1 to HPIV-3 are major causes of lower respiratory infections in infants, young children, the immunocompromised, the chronically ill, and the elderly. Each subtype can cause somewhat unique clinical diseases in different hosts. HPIV are enveloped and of medium size (150 to 250 nm), and their RNA genome is in the negative sense. These viruses belong to the Paramyxoviridae family, one of the largest and most rapidly growing groups of viruses causing significant human and veterinary disease. HPIV are closely related to recently discovered megamyxoviruses (Hendra and Nipah viruses) and metapneumovirus.

Sorting of the respiratory syncytial virus matrix protein into detergent-resistant structures is dependent on cell-surface expression of the glycoproteins

The interaction of the respiratory syncytial virus (RSV) Matrix (M) protein with the plasma membrane was investigated using polyclonal and monoclonal antisera raised against recombinant M expressed in bacteria. M bound mainly to the plasma membrane, although a significant proportion bound to internal membranes. However, no localisation of M with the Golgi was observed, suggesting that transport of M to the plasma membrane was independent of the transport mechanism for the viral glycoproteins. Expression from a recombinant baculovirus demonstrated the ability of M to bind membranes in the absence of viral glycoprotein expression. When cell-surface expression of the viral glycoproteins was prevented using Brefeldin A, M was still found in association with the plasma membrane, but the characteristics of M's membrane-binding ability were different to that found in untreated infected cells. In the presence of normal glycoprotein expression, M was sorted into lipid rafts and, in addition, formed structures that could only be disrupted by treatment with high salt buffers, a feature suggesting an interaction with the cytoskeleton or the formation of strong intramolecular associations. Brefeldin A prevented M from being sorted into lipid rafts or from forming strong intramolecular associations. Brefeldin A also affected the stability of M bound to the plasma membrane, as M was more readily dissociated in the presence of the inhibitor. Coexpression of M and F resulted in the incorporation of M into lipid rafts but did not cause the formation of the strong intramolecular bonds, suggesting that additional factors are required for this phenomena.

Characterization of human parainfluenza virus type 2 RNAs in infected cells and by in vitro synthesis

The RNA species synthesized in HPIV-2 infected CV-1 cells were identified and characterized. The largest RNA of approximately 5.5 x 10(6) in molecular weight (MW) based on electrophoretic mobility, was identified as the genomic RNA. The other small RNA species of MWs 2.4 x 10(6), 1.1 x 10(6), 0.77 x 10(6), 0.68 x 10(6) and 0.5 x 10(6) were identified as mRNAs. The five smallest RNAs were also synthesized in vitro and comigrated with RNAs synthesized in virus-infected cells. mRNAs synthesized both in vitro and in virus-infected cells were translated in vitro. NP, P, M and V proteins synthesized in vitro comigrated, when analyzed by SDS-PAGE, with the authentic proteins synthesized in virus-infected cells. Additionally, peptide mapping showed that the NP, P and M proteins synthesized in vitro were indistinguishable from their counterparts synthesized in infected cells. Analysis of the proteins from virions identified L, HN, NP, F (F1, F2), P, M and V proteins as virion structural proteins. Electrophoretic mobility of reduced and nonreduced F proteins was found to be different due to the conformational changes conferred by disulfide bonds.

Distinct hemagglutinin and neuraminidase epitopes involved in antigenic variation of recent human parainfluenza virus type 2 isolates

A panel of fourteen neutralizing anti-HN monoclonal antibodies (mAbs) to the prototype Greer strain of human parainfluenza virus type 2 (PI2) was used to determine the extent of antigenic variation in recent virus isolates. Competitive binding analysis with the mAbs indicated the presence of at least five distinct antigenic sites (I to V) on the HN glycoprotein molecule. MAbs recognizing different antigenic sites were found to be associated with the hemagglutinin (sites I, IV and V), hemagglutinin and neuraminidase (site II), or neuraminidase (site III) activities. The location of two distinct epitopes identifying the neuraminidase sites (II and III) was further verified from the generation of escape mutants. Antibodies directed to sites I and III failed to show any detectable binding or neutralizing activity against a number of natural PI2 virus isolates collected in Texas between 1986 and 1987. Interestingly, these natural variants, unlike the prototype virus, did not show any detectable neuraminidase activity with fetuin as a substrate and the enzyme activity was only detected with N-acetylneuramin-lactose as an alternative substrate. Despite the observed variation in the antigenic sites, primary infection with the prototype virus or the natural variants generated a protective immune response against challenge infection with the other virus strains.

Sequence determination of the hemagglutinin-neuraminidase (HN) gene of human parainfluenza type 2 virus and the construction of a phylogenetic tree for HN proteins of all the paramyxoviruses that are infectious to humans

The nucleotide sequence of the hemagglutinin-neuraminidase (HN) gene of human parainfluenza type 2 virus (PIV-2) was determined. The PIV-2 HN gene was 2112 nucleotides excluding poly(A) tail. There was a single large open reading frame in the mRNA which encoded a protein of 571 amino acids with a calculated molecular weight of 63,262. Analysis of the deduced amino acid sequence revealed that there were fourteen potential glycosylation sites and a major hydrophobic region near the N-terminus, which would anchor the protein in the viral membrane. Comparisons of the HN protein sequences of PIV-2 with those of Simian virus 5 (SV5), Sendai virus (SV, parainfluenza virus type 1), human parainfluenza virus type 3 (PIV-3), type 4 (PIV-4), bovine parainfluenza virus type 3 (BPIV-3), mumps virus (MuV), and Newcastle disease virus (NDV) showed definite amino acid sequence relatedness, indicating a common ancestor for these viruses. Furthermore, statistical analysis of the protein sequences suggested a possible evolutionary relatedness among the paramyxoviruses. This is the first time that a phylogenetic tree has been constructed for all the parainfluenza viruses and mumps virus which are infectious to humans. In addition, amino acid sequences involved in hemagglutinating and neuraminidase activities of paramyxovirus were discussed.

Chlamydia trachomatis and parainfluenza 2 virus: a shared antigenic determinant?

A tracheal aspirate from which parainfluenza 2 (PI-2) virus but not chlamydiae was isolated demonstrated positive immunofluorescence of elementary body- and reticulate body-like particles on direct examination with anti-chlamydia monoclonal antibodies (Syva Co.). In subsequent studies, we found this reagent to show specific staining of this strain of PI-2 as well as of 3 of 18 additional PI-2 strains that were evaluated.

Comparison of monoclonal antibody time-resolved fluoroimmunoassay with monoclonal antibody capture-biotinylated detector enzyme immunoassay for respiratory syncytial virus and parainfluenza virus antigen detection

An all-monoclonal antibody, time-resolved fluoroimmunoassay was compared with several enzyme immunoassays for the detection of respiratory syncytial virus and parainfluenza virus type 1, 2, and 3 antigens in clinical specimens. The most sensitive enzyme immunoassay for parainfluenza virus type 1 was an all-monoclonal antibody assay with biotin-labeled detector antibody and streptavidin-peroxidase conjugate, but for respiratory syncytial virus and parainfluenza virus types 2 and 3 the most sensitive assay was a polyclonal antibody assay with horse capture antibodies and bovine or rabbit detector antibodies with anti-species peroxidase. All tests were evaluated with nasopharyngeal aspirate specimens from respiratory illnesses and with cell culture harvests of multiple strains of each virus isolated over many years. The time-resolved fluoroimmunoassay detected respiratory syncytial virus antigen in 92% of the specimens positive by culture, which was a decidedly higher sensitivity than either the monoclonal or polyclonal antibody enzyme immunoassay format (62 and 76%, respectively). For the parainfluenza viruses the time-resolved fluoroimmunoassay detected type-specific antigen in 94 to 100% of culture-positive specimens and again was more sensitive than the all-monoclonal antibody enzyme immunoassays (75 to 89%) or all-polyclonal antibody enzyme immunoassays (66 to 95%). Combined with results from a previously reported adenovirus time-resolved fluoroimmunoassay, these tests identified respiratory antigens in large numbers of clinical specimens.

Cross-neutralization of genetic reassortants of bluetongue virus serotypes 20 and 21

Genetic reassortment studies of bluetongue virus (BTV) Types 20 and 21 have revealed a reassortant genotype that was not neutralized serotype-specifically. In reciprocal neutralization tests, BTV 20 and 21 were neutralized specifically by homologous antiserum. Similarly, reassortants that possessed both outer capsid proteins (i.e., VP2 and VP5) from the same parent virus reacted with that antiserum specifically. However, two reassortants, 16(9) and 19(1), with VP2 of BTV 20 and VP5 of BTV 21 had intermediate neutralization characteristics. These reassortants were neutralized to high titres by antiserum to BTV 20 and to lower, but significant titres by antiserum to BTV 21. In addition, antiserum to BTV 20 induced 10-16-fold higher titres in plaque reduction neutralization (PRN) tests with these two reassortants compared with BTV 20 itself. Evidence of the serological cross-reactivity of Reassortants 16(9) and 19(1) was also found with respect to reductions in plaque sizes observed in the PRN tests. The average plaque sizes of these reassortants were reduced to differing extents by antiserum to BTV 20 and 21, while those formed by the parent viruses were reduced in size by homologous antiserum only. Immunoblotting analysis of the structural proteins of BTV 20 and 21 demonstrated that VP2 alone was antigenically distinct, therefore confirming its role in determining serotype specificity in virus-neutralization tests. Electrophoretic analysis revealed considerable migrational differences between VP2 and VP5 of the parent viruses, suggesting that there was some divergence in their molecular weights, intrinsic charges or structural compositions. Taken together, the data suggest that the intermediate neutralization characteristics of the reassortants that contain VP2 and VP5 from different parent viruses are due to conformational alterations in their outer capsid structure which allow antibody recognition of common neutralizing epitopes that are not exposed on BTV 20 or BTV 21.

The polypeptides of human parainfluenza type 2 virus and their synthesis in infected cells

We have studied the structural components of three strains of human parainfluenza virus type 2 (HPI-2) and identified the virus-specific polypeptides. Molecular weight of P and F1 polypeptides determined by us, when compared with that reported previously, was in reversed order. HN polypeptide existed chiefly as disulfide-linked dimers in cells infected with Toshiba strain, while as monomers and dimers in nearly equal proportion in cells infected with two other strains. Similar disulfide-linked NP oligomers were found in cells infected with all three strains. F1 and F2, cleaved forms of F protein, could be detected in cells infected with all three strains, but the ratio of cleaved (F1 and F2) to uncleaved (F0) forms was markedly lower in 62-M 786- and 63-M 1-infected than in Toshiba strain-infected cells. However, there was no difference of oligopeptide mapping patterns and isoelectric point of F polypeptide between Toshiba and 62-M 786 strains. By contrast, oligopeptide mapping patterns of HN protein of Toshiba strain differed from those of the two other strains. Furthermore, the HN polypeptide of Toshiba strain was phosphorylated in the infected Vero cells, but that of the other two strains was not.

Genetic reassortants for identification of the genome segment coding for the bluetongue virus hemagglutinin

Two bluetongue virus (BTV) serotypes isolated in Australia and two selected reassortants derived from cells coinfected with these viruses have been used to identify the gene coding for the virus hemagglutinin. The parent viruses had characteristic hemagglutination patterns: BTV type 20 agglutinated sheep erythrocytes only; and BTV type 21 agglutinated sheep, bovine, human, and goose erythrocytes. Analysis of the two virus clones that had reassorted in genes coding for the outer capsid polypeptides demonstrated that hemagglutination and hemagglutination inhibition are functions associated with the outer capsid protein (VP2), which is encoded by genome segment 2.

Studies on human parainfluenza virus 3: characterization of the structural proteins and in vitro synthesized proteins coded by mRNAs isolated from infected cells

The structural proteins of human parainfluenza virus 3, a member of the paramyxovirus family, were characterized by SDS-polyacrylamide gel electrophoresis of radiolabeled virus. The purified virion contains at least eight structural proteins, with estimated molecular weights of 251K, 90K, 71K, 68K, 65K, 51K, 35K, and 21K, respectively. Three of the polypeptides (71K, 65K, and 51K) were identified as glycoproteins based on their incorporation of [3H]glucosamine. Disruption of the virus by Triton X-100 in the presence of increasing salt concentrations indicated that the polypeptides of molecular weights 251K, 90K, 68K, and 21K were components of the nucleocapsid. In parainfluenza virus 3 infected BS-C-1 cells, seven virus structural polypeptides were identified. Six structural proteins (90K, 71K, 68K, 51K, 35K, and 21K) were detected in the cell lysate at 7 hr after infection, while at 10 hr an additional polypeptide (251K) was also observed. At least two nonstructural polypeptides of molecular weights 30K and 25K were also detected in infected cells. mRNAs isolated from virus-infected cells were translated in a cell-free protein-synthesizing system. The in vitro translation products were identical to the authentic virion polypeptides as determined by partial digestion with staphylococcal V8 protease.

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

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

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

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

Serological diagnosis of parainfluenza virus infections by enzyme immunoassay with special emphasis on purity of viral antigens

Enzyme immunoassay (EIA) for parainfluenza virus type 1, 2, and 3 antibodies was compared with the complement fixation test (CF) as a diagnostic method in 180 patients with respiratory symptoms. The CF test detected rises in parainfluenza virus antibodies in 30 cases, whereas EIA detected 47 rises. Patients with antibody rises in parainfluenza or mumps virus antibodies were studied for cross-reactions by CF, hemagglutination inhibition (HAI), and EIA using purified viral envelope glycoprotein and nucleocapsid preparations. All methods showed marked cross-reactivity between parainfluenza virus type 1 and 3 antibodies. Mumps virus infection often raised heterologous antibodies even against purified viral antigens. Rabbit antisera produced against viral envelope glycoproteins showed heterologous antibody responses between parainfluenza 1 and 3 antibodies and between parainfluenza 1 and mumps antibodies by HAI, EIA, and immunoprecipitation. The cross-reactive antibodies were usually directed against both of the envelope glycoproteins, HN and F proteins, of the viruses.