Localization of the influenza virus nucleoprotein: cell-associated and extracellular non-virion forms

Both the supernatant of influenza virus-infected chick embryo cells and allantoic fluid containing influenza virus were shown to contain non-virion nucleoprotein (NP), which reacted readily with anti-NP monoclonal antibodies. Adsorption onto erythrocytes and centrifugation at 70,000 g for 2 h resulted in the removal of about 20% of the extracellular NP, whereas centrifugation at 100,000 g for 4 h eliminated about 50%, and practically all [3H]uridine-labelled virions. These results suggest that of the extracellular NP about 30% exists in the form of ribonucleoprotein, about 20% is precipitated with virions and about 50% occurs as free molecules. Comparative analysis of the kinetics of the accumulation of NP in the supernatant of infected cells, on the cell surface and inside the cells in relation to virus production, showed that there is a significant correlation between them.

Influenza virus M2 integral membrane protein is a homotetramer stabilized by formation of disulfide bonds

The oligomeric structure of the influenza A virus M2 integral membrane protein was determined. On SDS-polyacrylamide gels under nonreducing conditions, the influenza A/Udorn/72 virus M2 forms disulfide-linked dimers (30 kDa) and tetramers (60 kDa). Sucrose gradient analysis and chemical cross-linking analysis indicated that the oligomeric form of M2 is a tetramer consisting of either a pair of disulfide-linked dimers or disulfide-linked tetramers. In addition, a small amount of a cross-linked species of 150-180,000 kDa, which the available data suggest contains only M2 polypeptides, was observed. The role of M2 cysteine residues in disulfide bond formation and their role in forming oligomers were examined by converting each of the two extracellular and single cytoplasmic cysteine residues to serine residues and expressing the altered M2 proteins in eukaryotic cells. Removal of either one of the N-terminal cysteines at residues 17 or 19 indicated that tetramers formed that consisted of a pair of noncovalently associated disulfide-linked dimers, suggesting that each of the cysteine residues is equally competent for forming disulfide bonds. When both cysteine residues were removed from the M2 N-terminal domain, no disulfide-linked forms were observed. When solubilized in detergent this double-cysteine mutant lost reactivity with a M2-specific mAb and exhibited an altered sedimentation pattern on sucrose gradients. However, chemical cross-linking of this double-cysteine mutant in membranes indicated that it can form tetramers. Taken together, these data suggest that disulfide bond formation, although not essential for oligomeric assembly, stabilizes the M2 tetramer from disruption by detergent solubilization.

Comparison of complete amino acid sequences and receptor-binding properties among 13 serotypes of hemagglutinins of influenza A viruses

We determined the sequences of 7 serotypes (H4, H6, H8, H9, H11, H12, and H13) of hemagglutinin (HA) genes, which have not been reported so far. The coding regions consisted of 1692 nucleotides in H4, 1698 in H6, 1695 in H8, 1680 in H9, 1695 in H11, 1692 in H12, and 1698 in H13, and specified 564, 566, 565, 560, 565, 564, and 566 amino acids, respectively. By comparison of amino acid sequences, 13 HA serotypes could be divided into two families, i.e., an H1 group (H1, H2, H5, H6, H8, H9, H11, H12, and H13) and an H3 group (H3, H4, H7, and H10). The relationship was essentially similar to that reported by Air from the comparison of 80 amino-terminal amino acid sequence of 12 HA serotypes (G.M. Air, 1981, Proc. Natl. Acad. Sci. USA 78, 7639-7643). Though a considerable amino acid sequence difference exists between certain HA serotypes, several amino acid residues in fusion peptides (HA2(1-11)) and receptor-binding sites (HA1(98), -134, -138, -153, -183, and -195) were shown to be conserved among the 13 HA serotypes. Human H1 and avian H3, H4, H8, and H10 viruses preferentially bound NeuAc alpha 2,3Gal sequences, whereas human H2 and H3 and avian H6 and H9 viruses bound NeuAc alpha 2,6Gal sequences, although the amino acid residues at position 226 of human H2 and avian H6 and H9 serotype HAs are glutamine. These results show that the amino acid residue at position 226 is not necessarily a determinant of receptor specificity for all serotypes.

[The anomalous isoelectric properties of influenza virus matrix protein M1]

The isoelectric point (pI) values of matrix protein M1 of influenza A, B, and C viruses, calculated theoretically on the basis of its primary structure, were found to be about 10.0. However, experimental pI determination by electrofocusing in ampholyte-containing polyacrylamide gel showed it to be 5.0 for M1 protein isolated from A/WSN/33 (H1N1) and A/Aichi/2/68 (H3N2) viruses by mild deproteinization with nonionic detergents. This marked discrepancy between experimental and theoretical pI values indicated that influenza virus matrix protein M1 possessed an unusual tertiary structure and/or intensive posttranslational addition of charged residues.

NS2 protein of influenza virus is found in purified virus and phosphorylated in infected cells

Purified viral preparations of influenza A virus were examined for the presence of NS2 protein hitherto considered as a viral nonstructural protein that is present only in infected cells. Analysis of purified virus by radioimmunoprecipitation with monospecific antisera to NS2 revealed its presence in the virus particle suggesting that it is a viral structural protein. NS2 protein was also shown to be phosphorylated in infected cells in this study. This brings the number of influenza virus phosphoproteins to three which include NP, NS1, and NS2. These observations raise important questions about the role of NS2 in the replication of influenza virus.

Heterogeneous forms of polymerase proteins exist in influenza A virus-infected cells

In influenza virus-infected cells a virus coded polymerase that consists of three polypeptide subunits, namely PB1, PB2 and PA, mediates both transcription and replication. Radioimmunoprecipitation with monospecific antisera to each of the polymerase proteins revealed additional forms of PB1 and PA proteins in infected cells. PA antiserum detected two additional proteins of 62k and 60k and PB1 antiserum recognized two additional proteins of 85k and 70k. Further investigation was carried out on the 62k PA and 85k PB1 related proteins. Limited proteolysis peptide mapping showed that these proteins are subsets of their normal counter-parts. These new forms of polymerase proteins are designated as "b" forms (PAb and PB1b) to distinguish them from the previously recognized forms designated as "a" forms (PAa and PB1a). Both PAb and PB1b proteins were found in cells infected with all the influenza type A viruses tested indicating that they are evolutionarily conserved. Pulse chase experiments showed that the "b" forms are not derived from "a" forms. This suggested that "b" forms are translated independently. The "b" forms were not detected in purified virus but were found to be associated with intracellular RNP templates, suggesting a role for these proteins in intracellular virus replication events.

The phosphorylation of the integral membrane (M1) protein of influenza virus

The phosphorylation of the internal and integral membrane (M1) protein of influenza virus was studied. Four points can be made based on the data: (1) The M1 contains at least two moles of phosphate per mole of M1. (2) Phosphorylation of M1 is conserved between influenza A, B and C viruses. Other characteristics of the M1 are also conserved, such as solubility in organic solvent, heterogeneity and ability to partition into lipid vesicles. (3) M1 is phosphorylated in cells infected with a vaccinia recombinant (vP273) containing only the gene of M1, either as a result of a vaccinia virus associated kinase or a cellular one. (4) The phosphate is located within or in close proximity to the major stretch of neutral and hydrophobic amino acids found in M1, as determined by analyzing cyanogen bromide fragments.

[Standardization of conditions for detecting the internal proteins of the influenza virus in studying their antigenic properties in solid-phase immunoenzyme analysis]

The influence of the conditions of adsorption and virion destruction by freezing-thawing and detergents on the detection of M1 and NP proteins of different influenza virus strains by solid-phase enzyme immunoassay with direct virion adsorption on polystyrene was studied. It was found that for the detection of M1 protein the optimal conditions included virion disruption with detergent and adsorption to polystyrene at 4 degrees C, and for NP protein disruption by freezing-thawing at adsorption to polystyrene at 37 degrees C. In the study of the antigenic properties of protein M1 of different influenza virus strains using monoclonal antibodies it was shown to be necessary, first, to achieve maximum detection of proteins and, second, to standardize the amount of the adsorbed antigen with polyclonal antibodies.

[The structure of oligosaccharide fragments of glycoproteins from influenza virus A/Krasnodar/101/59/(H2N2)--heavy and light chains of hemagglutinin and neuraminidase]

The structure and heterogeneity of carbohydrate chains of hemagglutinin (HA) and neuraminidase (NA), the surface glycoproteins of influenza virus A/Krasnodar/101/59 (H2N2), were investigated. Hemagglutinin was reduced with beta-mercaptoethanol and its heavy (HA1) and light (HA2) chains were separated by gel chromatography. Amino acid and sugar composition of HA1, HA2 and NA was elucidated. The carbohydrate chains of the glycoproteins were cleaved off by the alkaline LiBH4 treatment and oligosaccharides were reduced with NaB[3H]4. They were fractionated by subsequent two-step HPLC on Ultrasphere-C8 and Zorbax-NH2 columns with simultaneous identification using nonlabelled oligosaccharides of known structures. Some of the major oligosaccharides isolated from HA1, HA2 and NA were thus identified as high mannose chains, containing 5-9 mannose residues, and complex chains, first of all biantennary chains having or not having bisecting N-acetylglucosamine and/or fucose residues. The approach which has been developed enables one to study the structure and heterogeneity of carbohydrate chains starting from one nmole of a desialylated N-glycoprotein.

Differential phosphorylation of the nucleoprotein of influenza A viruses

An analysis of the nucleoprotein (NP) of 29 different influenza A viruses by phosphopeptide fingerprinting revealed three prototype patterns. The first, which was a complex pattern consisting of six to seven phosphopeptides, another which was relatively simple consisted of two or three phosphopeptides, and a third one which was complex but was missing the main phosphopeptide shared by the two other patterns. Phosphoserine was the only labelled phosphamino acid detected. A tentative deduction of two of the phosphate attachment sites (serine residues at positions 3 and 473) could be made by comparison of the known amino acid sequences of the NPs of 25 strains. No correlation was found between species specificity or subtype or year of isolation of the strains. During the infectious cycle the fingerprint underwent significant changes, indicating subtle phosphorylation and dephosphorylation of the NP at various stages during viral multiplication. Most of the phosphopeptides were metabolically stable; however one major phosphopeptide, which was not found in the NP of mature virions, exhibited a high turnover (presumably serine at position 3). The phosphopeptide fingerprint could be significantly influenced in vivo by the specific stimulation of cellular protein kinase C by the phorbol ester 12-O-tetradecanoylphorbol 13-acetate or by its inhibition with the isoquinoline sulphonamide H7.H7 specifically inhibited the replication of influenza A viruses by deregulation of viral protein synthesis without interfering with the multiplication of a parainfluenza virus (Newcastle disease virus), an alphavirus (Semliki Forest virus) or a flavivirus (West Nile). Therefore the correct phosphorylation of the NP of influenza viruses appears to be essential for influenza virus replication.

Fluorescence anisotropy of UV-irradiated viruses

The steady-state fluorescence anisotropy measurements on influenza and parainfluenza viruses, showed no changes in the microviscosity of the viral membranes after exposure to UV-irradiation, when a fluorescent probe was used, but the intrinsic fluorescence of viral proteins presented, under the same experimental conditions, a significant difference of anisotropy behaviour in the two viruses used.

[The spatial interrelationships between influenza virus hemagglutinin and the lipid phase in the liposomal membrane]

A scheme has been proposed demonstrating the location of tryptophan residues of hemagglutinin molecule in relation to the middle of the lipid layer 1.2 nm thick with a fluorescent probe pyrene. In the immediate proximity to it, one tryptophanyl of protein molecule is located in a hydrophobic "pocket". At a distance of 2.85 nm from the middle of the lipid zone 3 tryptophanyls are located and the remaining five at a distance over 3.6 nm. After treatment with proteolytic enzyme bromelin of the liposomes with hemagglutinin incorporated into their bilayer, the hydrophobic "anchor" of protein molecule contains one tryptophanyl which is raised by 0.3 nm and its hydrophobic environment is changed.

[The structure of complex carbohydrate chains of hemagglutinin from influenza viruses A/Kiev/59/79 (H1N1), A/Chile/1/83/25(H1N1) and X/79(H3N2)]

An earlier developed method of identification of oligosaccharides by HPLC was used for studying the carbohydrate chains of three hemagglutinins from various influenza virus strains. The structures of main oligosaccharides of the complex type were elucidated on the basis of their chromatographic characteristics and monosaccharide composition. Oligosaccharide patterns varied in the above hemagglutinin samples but in all cases the major complex chains were fucosylated and nonfucosylated biatennary chains; bisected and triantennary chains were also found.

Separation of hemagglutinin and neuraminidase from influenza virus membrane by column displacement electrophoresis (isotachophoresis) with preservation of their activities

Triton X-100-solubilized membrane glycoproteins (neuraminidase and hemagglutinin) from purified equine influenza virus particles were separated by column displacement electrophoresis (isotachophoresis) in the presence of Pharmalyte spacers. Electrophoresis was performed in a 1.80 cm glass electrophoresis column with Sephadex G-25 Fine serving as supporting medium. Triton X-100 was present in the system to suppress protein aggregation. Neuraminidase and hemagglutinin activities were preserved and appeared in the electropherogram as separate peaks with some overlapping.

Mixed anionic detergent/aliphatic alcohol-polyacrylamide gel electrophoresis alters the separation of proteins relative to conventional sodium dodecyl sulfate-polyacrylamide gel electrophoresis

The order and relative mobility of proteins on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) is affected by unknown components that are differentially present in SDS preparations obtained from different sources [J.B. Swaney, G.F. Vande Woude, and H.L. Bachrach (1974) Anal. Biochem. 58, 337-346]. The modified separation capabilities of such SDS preparations are useful but the use of this phenomenon in a controlled manner requires that the components responsible for the altered separation be identified. Accordingly, this paper describes a polyacrylamide gel electrophoresis system [mixed alcohol/detergent-polyacrylamide gel electrophoresis (MAD-PAGE)] that employs a mixture of alcohol and detergent instead of SDS alone to modify and enhance protein separation relative to conventional SDS-PAGE. A defined mixture consisting of four sulfated alkyl detergents (dodecyl sulfate, tetradecyl sulfate, hexadecyl sulfate, octadecyl sulfate) as well as the four alcohols of corresponding aliphatic chain length was found to be effective at duplicating the electrophoretic effect of USP-grade SDS and thus changed the relative order and position of polypeptides on electrophoresis relative to conventional SDS-PAGE. This method serves as an adjunct to conventional SDS-PAGE by providing another means of resolving proteins that are not normally resolved by SDS-PAGE. Further, it was found that MAD-PAGE is capable of resolving the NS1 protein of influenza virus into three fractions, whereas conventional SDS-PAGE yields one electrophoretic species. Reelectrophoresis of these novel NS1 bands by conventional SDS-PAGE indicated that they were not modified during MAD-PAGE and probably represented distinct molecular forms present in infected cells.(ABSTRACT TRUNCATED AT 250 WORDS)

The intracellular distribution of influenza virus matrix protein and nucleoprotein in infected cells and their relationship to haemagglutinin in the plasma membrane

Pre- and post-embedding immune electron microscopy techniques employing ferritin and large and small gold markers to detect cell surface and intracellular antigens respectively, have been combined in a study of influenza virus-infected cells. This has permitted, for the first time, the simultaneous detection of intracellular virus matrix protein (M), nucleoprotein (NP) and membrane haemagglutinin (HA). The technique facilitated an investigation of the possible physical interrelationship between these three proteins both in the infected cell, and on the infected cell membrane. Electron-dense bodies uniformly labelled by antibody to M protein were observed in the nucleus and cytoplasm. Similarly, NP was detected in both the nucleus and cytoplasm. Approximately 50% of the nuclear NP was located in close proximity to the M protein-containing dense bodies but mainly on the perimeter of the structures. A similar relationship of NP to the M-containing dense bodies was observed in the cytoplasm. M protein and NP were readily detected in sections of budding virions. Labelling of these proteins was also observed on the cytoplasmic face of the plasma membrane but the density of labelling only occasionally approached that of newly formed virions. These findings suggest that budding occurs very quickly after the internal proteins arrive at the plasma membrane. Double labelling experiments on the cell surface indicate that NP and HA behave as independent molecules and do not form tight complexes with each other.

Functional and structural analysis of the ribonucleoprotein complexes of different human influenza virus strains

Ribonucleoprotein (RNP) cores were prepared from various strains of human influenza virus by treating the purified or spikeless virus particles with non-ionic detergents such as Nonidet P-40 and centrifugation in continuous linear glycerol gradients. In addition to RNA, the purified complexes contained viral nucleoprotein (NP) and the three P proteins (PA, PB1, PB2) as determined by polyacrylamide gel electrophoresis (PAGE) under denaturing conditions. Contaminations with other viral polypeptides, especially HA1, HA2 and M were below 1%. All RNP complexes were transcriptionally active in vitro. Comparison of the polymerase activity of purified complexes revealed considerable differences depending not only on the content of polymerase proteins. The activity of RNP complexes was enhanced for all strains tested by adding of ApG.

[Identification of the antigenic components of the host cell in isolated glycoproteins of the influenza virus]

Immunological analysis performed by solid-phase enzyme immunoassay and complement fixation test revealed the presence of host cell antigenic components in purified intact influenza virions and isolated hemagglutinins and glycoproteins. Three antigens inherent to chick embryo cells were identified in virus preparations: the species-specific antigen, heterogenetic Forssman antigen, and one similar to human group A antigen. Antisera to purified whole-virion influenza vaccines, to glycoproteins and to hemagglutinin isolated from them were found to contain antibody populations which were conducive to broad cross reactions between them and heterotypic virus antigens. To avoid erroneous results, these antibody populations must be removed from the sera by pre-adsorption with host cell antigens.

Nucleotide sequence of RNA segment 7 and the predicted amino sequence of M1 and M2 proteins of FPV/Weybridge (H7N7) and WSN (H1N1) influenza viruses

Since the gene products (M1 and M2) of influenza virus RNA segment 7 have been implicated in host range restriction, sensitivity to the drug amantadine, virus yield in chicken embryos as well as in virus assembly and morphology, we have determined the nucleotide sequence of this RNA segment for an avian [A/FPV/Weybridge (H7N7)] and a human [A/WSN/33 (H1N1)] virus and compared it to that of the other influenza A virus strains. The results show that all ten strains of influenza A virus contain an identical number of nucleotides (1027 bases) in RNA segment 7 and an identical number of amino acids in M1 (252 aa) and M2 (97 aa) proteins. The observed amino acid changes are conservative in nature suggesting the requirement of a critical structure of both proteins in virus assembly. Furthermore, the presence of some consistent amino acid substitutions among different human and avian strains also supports the possible existence of host range and drug resistance determinants in M1 and M2 proteins.

Influenza A viral nucleoprotein detection in isolates from human and various animal species

A double antibody sandwich, enzyme-linked immunosorbent assay (DAS-ELISA) was developed to detect influenza A viral antigen, employing a monoclonal antibody directed against type-specific influenza A nucleoprotein (McAb anti-NP). McAb anti-NP was used to coat ELISA plates as well as to prepare the peroxidase conjugate. Influenza A viruses of avian, equine, swine, and human origin were detected in allantoic fluids of inoculated eggs with higher sensitivity by the DAS-ELISA than by hemagglutination (HA) assays. Minimal concentrations of 8 ng/ml influenza virus protein were detected in Nonidet P40-treated virus preparations. Viral antigen detection in tissues of experimentally infected chickens and pigs was successful, but in pigs yielded a lower positive score than the conventional method of virus isolation in eggs. The test is sensitive, rapid, and easy to perform, but does not permit influenza A subtyping. In avian species, the McAb anti-NP DAS-ELISA differentiates between influenza and Newcastle disease viruses. In pigs, the test distinguishes between influenza and Aujeszky's disease.

Genetic variants of influenza A/Taiwan/1/86 cocirculating in Canada during the winter of 1986 to 1987

The first isolate of influenza virus in Canada during the winter of 1986 to 1987 was a genetic variant of A/Taiwan/1/86. This genetic variant type was the predominant strain obtained from several of the western provinces. The variant strains were antigenically indistinguishable from A/Taiwan/1/86 but were remarkably distinct by T1 oligonucleotide mapping. T1 mapping of individual genome segments indicated that the variants evolved from an A/Taiwan/1/86-like virus through the accumulation of point mutation or deletion or insertion events and probably do not contain foreign genes. The relative distribution of genetic variation was approximately equal among the individual genes, with the possible exception of segments 1 or 2 that were analyzed in combination and thus could not be individually associated with the observed variation.

[Comparative study of carbohydrate chains of H1 hemagglutinins of influenza viruses A/Kiev/59/79 and X/Leningrad/54/1 using oligosaccharide maps]

For comparative study of carbohydrate chains of N-glycoproteins, method of "oligosaccharide maps" has been developed. It consists in fractionation of reduced oligosaccharide fragments by gel-chromatography and HPLC on reverse phase and amino columns. Using two HPLC retention time values for each oligosaccharide, two-dimensional maps for both variants of H1 hemagglutinin were constructed. The monosaccharide composition of the majority of oligosaccharides isolated was also elucidated. The carbohydrate chain's patterns for the H1 hemagglutinin variants were found to be similar but to differ considerably from those for H3 hemagglutinin. The data obtained show that the glycosylation pattern depends on virus strain, i.e. on the structure of the polypeptide chain of hemagglutinin.

Monoclonal antibodies detect M-protein epitopes on the surface of influenza virions

Various data obtained with activable hydrophobic probes, proteolytic treatments and anti M-protein polyclonal antibodies strongly suggest that M-protein of influenza A is an integral part of the lipid bilayer of native virions and somehow spans at the surface of the virions. Therefore we have looked for the presence of M-protein epitopes on the surface of influenza A virion by using four type A M-protein monoclonal antibodies. We developed a specific and sensitive competition ELISA where intact virions, dodecyl-sulfate disrupted virions and spikeless particles obtained after proteolytic treatment with caseinase C were used to test their ability to inhibit the reaction between these monoclonal antibodies and pure M-protein. Intact virions or SDS disrupted virions prevented three monoclonal antibodies from reacting with the M-protein. Spikeless particles also inhibited the specific binding of two of these antibodies, whereas the other fourth antibody was inhibited by contact with SDS disrupted particles only. Data presented show that at least three distinct M-protein epitopes were detected, of which at least two are exposed on the surface of intact virions. Of these two epitopes, one is inactivated by the proteolytic treatment. The third epitope could only react with its monoclonal antibody when the virus particles were solubilized with SDS. This work provides a clear demonstration that a substantial part of the M-protein spans the lipid bilayer and that the rest, protected by lipids, resists proteolytic enzymes and is prevented from binding with anti M-protein monoclonal antibodies.

[Antigenic and biological characteristics of influenza virus A strains isolated in 1985]

The antigenic structure of hemagglutinin of influenza A virus (H3N2) strains isolated in 1985 was studied using a series of monoclonal antibody to A/Dunedin/4/73/A (H3N2) and A/Bangkok/1/79/A (H3N2), and biological and physico-chemical properties of these strains were compared with those of influenza A (H3N2) virus of 1983 and reference A (H3N2) of 1979-1984 (the rate of adsorption on chick erythrocytes and eluting activity, thermostability of hemagglutinin and neuraminidase, sensitivity to nonionic detergents, sensitivity to remantadine, analysis of virion polypeptide composition). A high degree of heterogeneity of the 1985 strain population of influenza A (H3N2) virus was revealed both in the antigenic structure of hemagglutinin and in all the biological and physico-chemical parameters tested. It was suggested that the A/Caen/1/84 strain originated not from A/Philippines/2/82 but directly from A/Texas/1/77.

Sequence analysis of the haemagglutinin of A/Taiwan/1/86, a new variant of human influenza A(H1N1) virus

A/Taiwan/1/86 is representative of newly emerged antigenic variants of influenza A(H1N1) viruses which are readily distinguishable from all previous A(H1N1) isolates. Nucleotide sequence analysis of the haemagglutinin HA1 coding region of A/Taiwan/1/86 suggests that this virus has evolved from viruses circulating in the Hong Kong region in 1982 to 1983. The considerable alteration in antigenicity of this new isolate is likely to have arisen from five amino acid substitutions in a nine amino acid stretch, situated on the outer surface of a short alpha-helix on the tip of the HA molecule in antigenic site Sb.

[Cell-dependent and strain variations in the nucleocapsid protein P2(PA) of the influenza virus]

The electrophoretic pattern of influenza virus nucleocapsid proteins, P1 (PB1), P2 (PA), and P3 (PB2), was examined by high resolution electrophoresis in gradient polyacrylamide gel. The electrophoretic mobility of P polypeptides varied from strain to strain, and more marked variability was revealed in protein P2 (PA). In addition, P2 (PA) polypeptides exhibited diffuse distribution in gel, in contrast to those of P1 (PB1) and P3 (PB2) which had compact electrophoretic bands. This P2 electrophoretic heterogeneity was found to be host cell-dependent. From this observation, host-dependent modification of influenza virus nucleocapsid protein P2 (PA) was suggested.

[Study of the host glycosyl component of the influenza virus using monoclonal antibodies]

The crossreacting glycosylated host component (COHC) of influenza virus propagated in chick embryo has been studied using the monoclonal antibodies (MA). The specificity of MA to COHC has been demonstrated by different methods. Both MA of clones A9 and B7 react with the spatially overlapping antigenic sites of oligosaccharide chain, but only MA of B7 are active in hemagglutination inhibition test. The COHC is shown to be associated with both surface glycoprotein adsorbing on the surface of the virion. The study of the components from the allantonic fluid has shown the COHC to be associated with the sole embryo glycoprotein with the mol. mass around 150 kD and p1 9.0-9.5. Thus, the glycoprotein is possibly identical with the glycoprotein adsorbing on the virion surface. The content of COHC in various preparations of influenza virions has been also studied. The relative content of COHC is shown to increase during the purification of viral preparation. The COHC effect on the characteristics of vaccine preparations is discussed.

[Intracellular proteolytic cleavage of the influenza virus protein NP as a sign of the epidemicity of virus strains?]

The main nucleocapsid protein (NP) of human epidemic viruses was found to be cleaved via NP56----HP53 mol. wt. reduction in infected cells, while the NP of animal influenza viruses was refractory to analogous intracellular modification. Like animal influenza viruses, the strain A/Baku/799/82(H1N3) isolated from a sick child has been observed to exhibit the intracellular resistance of NP to intracellular proteolysis. The similar NP resistance has been revealed for A/New Jersey/8/76(H1N1) and A/seal/Massachusetts/81 (H7N7) viruses, which are able to induce only a sporadic human influenza viral infection. Thus, the results reveal a correlation between the viral strains epidemicity and intracellular cleavability of their NPs. The influenza viral strains epidemic for humans are characterized by cleavable NP, whereas the strains, which are known to induce the sporadic influenza human infection are found to exhibit the resistance of NP to intracellular proteolysis. It is reasonable to consider the phenomenon of NP56----NP53 proteolytic modification as a sign of viral strain epidemicity for humans.

Functional and antigenic domains of the matrix (M1) protein of influenza A virus

The membrane- and ribonucleocapsid (RNP)-binding domains of the matrix (M1) protein of influenza A virus (WSN strain) were partially mapped and characterized by reactivity with monoclonal antibodies (MAb) as well as by proteolytic cleavages and amino acid sequencing of the resulting peptides. Of two peptides formed by formic acid hydrolysis, a 9-kilodalton fragment at the amino-terminal third of the M1 protein was recognized by MAb M2-1C6 (to epitope 1), and a 15-kilodalton fragment at the carboxy-terminal two-thirds was recognized by MAb 289/4 (to epitope 2). Partial cleavage by staphylococcal V8 protease gave rise to a 16-kilodalton peptide, mapping to amino acid 8, which was recognized by MAbs to all three epitopes but rather weakly by MAb 904/6 to epitope 3. These studies suggest that epitope 1 of the M1 protein resides between amino acids 8 and 89, whereas epitopes 2 and possibly 3 are located between amino acids 89 and 141 or somewhat more carboxy distal. The intact M1 protein and its N-terminal 9- and 10-kilodalton peptides generated by formic acid or V8 protease cleavage, respectively, reconstituted with dipalmitoylphosphatidylcholine vesicles, but these N-terminal peptides had little effect on in vitro transcription of the RNP core. In sharp contrast, both intact M1 protein and the C-terminal 15-kilodalton formic acid fragment were able to inhibit viral transcription markedly. Moreover, MAb 289/4 (to epitope 2) reversed this inhibited transcription significantly. These studies suggest that the lipid-binding domain of the M1 protein is located within the amino-terminal third, whereas the site involved in the interaction of the M1 protein with RNP cores is located within the carboxy-terminal two-thirds.

Purified influenza virus nucleoprotein protects mice from lethal infection

Local administration of nucleoprotein purified from X31 (H3N2) influenza A virus primed for A virus cross-reactive cytotoxic T cells and resulted in substantial protection (75%) of mice from a lethal challenge with the heterologous mouse-adapted A/PR/8/34 (H1N1) virus. By following the course of a lethal virus challenge we found that nucleoprotein priming did not prevent virus infection but rather aided recovery. Nucleoprotein-primed mice suffered initial symptoms of infection, i.e. weight loss and surface temperature changes, but started to recover after approximately 7 days. We suggest that such heterotypic protection can be attributed to priming of A virus cross-reactive cytotoxic T cells.

Antigenic and amino acid sequence analysis of the variants of H1N1 influenza virus in 1986

Since their reintroduction to human populations in 1977, influenza A viruses of the H1N1 subtype have undergone antigenic drift. Recently a distinct antigenic variant, A/Singapore/6/86, has been almost exclusively isolated internationally, and the antigenic properties and amino acid sequence of its haemagglutinin have been determined and compared with those of the haemagglutinins of other H1N1 viruses, in particular A/Chile/1/83. Fourteen amino acid sequence differences are detected between the HA1 components of these two viruses, ten of which are different from equivalent residues in the haemagglutinins of all H1N1 viruses isolated between 1982 and 1983, and seven of which are novel in the haemagglutinins of all H1N1 viruses sequenced to date. The results are discussed in relation to the three-dimensional structure of the haemagglutinin and the location of the previously defined antigenically important regions.

[Electrophoretic mobility of polypeptides in different strains on human influenza A and B viruses]

The method of electrophoresis in a single polyacrylamide gel plate was used for comparative study on virion polypeptides mobility in human influenza A and B virus strains. Molecular masses of individual polypeptides and their portion in the virion were determined. No variations in the migration speed of nucleoprotein (NP) and membrane protein (MP) were found in strains belonging to the same genus, but there were differences in the migration speed of these proteins in the genus A and genus B. Significant differences in migration and the content of glycoproteins, particularly of the heavy chain of hemagglutinin (HAI) in the genus A strain having different subtypes of hemagglutinin, including strains A/WS/33 (HONI), A/FM/1/47 (HINI), A/Sing/1/57 (H2N2), A/Port Chalmers/73 (H3N2), as well as in the genus B in strains B/Lee/40, B/Yamagata/1/73, and B/Johanesbourg/56. No differences in the mobility of glycoproteins in strains similar in the antigenic specificity of hemagglutinin and neuraminidase were found. On the basis of the comparative analysis of virion polypeptide electrophoregrams, three new strains on influenza A virus isolated in December, 1977, were identified as belonging to the species A (HINI).

[Determination of the accessibility and localization of tryptophan residues in the influenza virus hemagglutinin molecule]

The accessibility and localization of tryptophane residues in the influenza viral hemagglutinin molecule have been determined by measuring specific quenching of tryptophane fluorescence by neutral (acrylamide), anionic (I-) and cationic (Cs+) quenchers. It has been shown that acrylamide quenches 64% of tryptophane fluorescence in H3-hemagglutinin whereas I- and Cs+ quench only 34%. The tryptophanyl residues have been assumed to be located in the hemagglutinin molecule both in the cationic and anionic environments. 64% of tryptophanyls have been shown to be located on the surface of the protein globule.

[Biological consequences of breaks and reunions of disulfide bonds in influenza virus proteins]

Consequences of chemical breakage of native disulphide bonds in influenza virus neuraminidase and hemagglutinin glycoproteins induced by mercaptoethanol treatment were studied. Under conditions of blocked reoxidation of thiol groups, this treatment led to significant inhibition of hemagglutinating activity and infectivity of virus particles, and to a lesser inhibition of neuraminidase activity, as well as to promotion of endogenous proteolytic activity. Analysis of virus particles proteins by polyacrylamide gel electrophoresis indicated association of these biological effects with breakage of disulphide bridges, mainly in hemagglutinin glycoproteins. Under certain conditions, the proteins were capable of reformation of disulphide bridges as manifested in restoration of virion biological activity and electrophoretic characteristics of proteins.

Analysis of progressive deletions of the transmembrane and cytoplasmic domains of influenza hemagglutinin

Site-directed oligonucleotide mutagenesis has been used to introduce chain termination codons into the cloned DNA sequences encoding the carboxy-terminal transmembrane (27 amino acids) and cytoplasmic (10 amino acids) domains of influenza virus hemagglutinin (HA). Four mutant genes were constructed which express truncated forms of HA that lack the cytoplasmic domain and terminate at amino acids 9, 14, 17, or 27 of the wild-type hydrophobic domain. Analysis of the biosynthesis and intracellular transport of these mutants shows that the cytoplasmic tail is not needed for the efficient transport of HA to the cell surface; the stop-transfer sequences are located in the hydrophobic domain; 17 hydrophobic amino acids are sufficient to anchor HA stably in the membrane; and mutant proteins with truncated hydrophobic domains show drastic alterations in transport, membrane association, and stability.

Assembly of influenza hemagglutinin trimers and its role in intracellular transport

The hemagglutinin (HA) of influenza virus is a homotrimeric integral membrane glycoprotein. It is cotranslationally inserted into the endoplasmic reticulum as a precursor called HA0 and transported to the cell surface via the Golgi complex. We have, in this study, investigated the kinetics and cellular location of the assembly reaction that results in HA0 trimerization. Three independent criteria were used for determining the formation of quaternary structure: the appearance of an epitope recognized by trimer-specific monoclonal antibodies; the acquisition of trypsin resistance, a characteristic of trimers; and the formation of stable complexes which cosedimented with the mature HA0 trimer (9S20,w) in sucrose gradients containing Triton X-100. The results showed that oligomer formation is a posttranslational event, occurring with a half time of approximately 7.5 min after completion of synthesis. Assembly occurs in the endoplasmic reticulum, followed almost immediately by transport to the Golgi complex. A stabilization event in trimer structure occurs when HA0 leaves the Golgi complex or reaches the plasma membrane. Approximately 10% of the newly synthesized HA0 formed aberrant trimers which were not transported from the endoplasmic reticulum to the Golgi complex or the plasma membrane. Taken together the results suggested that formation of correctly folded quaternary structure constitutes a key event regulating the transport of the protein out of the endoplasmic reticulum. Further changes in subunit interactions occur as the trimers move along the secretory pathway.

Differences of nucleoproteins of human and avian influenza A virus strains shown by polyacrylamide gel electrophoresis and by the peptide mapping technique

Electrophoretic mobility differences in polyacrylamide gels were detected between (35S)-methionine-labelled nucleoproteins (NPs) induced in monolayer cells by 15 human and 4 avian reference strains of influenza viruses. The (35S)-methionine-labelled tryptic peptides of nucleoproteins of these strains were also analyzed by peptide mapping technique. Based on several detectable hydrophilic peptides the NPs could be arranged in 7 clearly differentiable groups. After radioiodination of NPs from 4 human and 3 avian reference strains the tryptic peptide patterns showed one clear difference between human and avian strains.

[Possibility of detecting the matrix protein of the influenza virus in intact and disrupted virions by immunoenzyme analysis and immune electron microscopy]

ELISA and immune electron microscopy were used to study possible causes of the detection of antigenic reactivity of influenza virus matrix protein in purified virus suspension directly adsorbed on polystyrene. No interaction of antibody to M protein with the surface of virus and subvirus particles was observed. The process of virus sorption on polystyrene for a long period was shown not to lead to disruption of intact virus particles, and the detection of the internal matrix protein in preparations of purified influenza virus was due only to the presence of partially or completely destroyed virions in the viral suspension.

[Variations in the NP protein of the influenza virus detected by peptide mapping and polyacrylamide gel electrophoresis]

NP proteins of 19 reference and 42 epidemic strains of influenza A virus were analysed for their mobility in polyacrylamide gel electrophoresis and distribution of tryptic peptides. The strains could be divided into 4 groups by differences in their electrophoretic mobility, and into 9 groups according to reproducible differences of several hydrophilic peptides determined by peptide mapping.

[Quantitative characteristics of the process of solubilizing hemagglutinin isolated from influenza viruses H1N1 and H3N2]

Quantitative characteristics of the process of solubilization of influenza H1N1 and H3N2 virus hemagglutinin treated with bromeline enzyme were studied. After a long-time contact of viruses with the enzyme the hemagglutinin was found to be destroyed significantly. Certain conditions should be observed in order to increase the amount of hemagglutinin separated by the enzyme and its final yield. First, the quality of the original virus concentrate should be evaluated by electron microscopy. Second, the process of hemagglutinin solubilization should be monitored by single radial immunodiffusion method and electron microscopy to determine the optimal period of incubation of the virus material with bromeline. Finally, several (usually 2-3) incubations of optimal duration ("fractionated" incubation) should be carried out.

[Structure of major complex carbohydrate chains of influenza virus A/Leningrad/385/80 (H3N2) hemagglutinin]

The structure of four oligosaccharides which are the main carbohydrate chains of hemagglutinin of influenza virus A/Leningrad/385/80 (H3N2) has been elucidated. It was shown by means of enzymatic and mild acid hydrolysis, Smith degradation and acetolysis that the oligosaccharides have very similar structures (noncomplete triantennary) and differ from each other only in the number (0, 1 or 2) and position of fucose residues. The peculiarities of glycosylation of H3 hemagglutinin from different strains of influenza virus were discussed.

Characterization of two influenza A viruses from a pilot whale

Influenza A viruses of the H13N2 and H13N9 subtypes were isolated from the lung and hilar node of a pilot whale. Serological, molecular, and biological analyses indicate that the whale isolates are closely related to the H13 influenza viruses from gulls.

Electroelution for purification of influenza A matrix protein for use in immunoassay

A new preparative method for isolation of matrix protein from type A influenza virus was developed. Commercially available whole virus or split virus vaccines were lysed, and the soluble proteins separated by electrophoresis on polyacrylamide gel. The matrix protein was located on the gel by precipitation with KCl, and recovered by electroelution. The method was technically simple and required little direct supervision during the two-step recovery process. Yields of A matrix were consistently high, averaging 68.1% in five trials with A/Brazil/X-71. The method was also successful with other A viruses, although not with influenza B virus. Isolated A matrix had less than 0.5% contamination by hemagglutinin or nucleoprotein, as determined by immunoblotting and ELISA. Matrix protein was immunoreactive in Western blots and was detectable in concentrations as low as 1 ng/ml with ELISA. The isolated matrix provided a suitable standard for detection of matrix protein in nasal washes from patients with influenza A virus infection, and could also be used to detect anti-matrix antibodies, including monoclonal antibodies in tissue culture supernatants. The advantages of electroelution for separation of matrix protein compared to other methods were its technical simplicity, applicability to formalin-fixed influenza virus in commercially available vaccines, its consistently high yield, and its very high level of purification.

Inhibition of subtilisin by influenza virus infected allantoic fluids

Allantoic fluids harvested from embryonated chicken eggs infected with reference strains of influenza A viruses were analysed for subtilisin inhibitor activity. While all acid heat-treated and nontreated virus-infected fluids could reduce subtilisin activity, fluids of FM and Bangkok strains had the greatest inhibitory ability. The degree of subtilisin inhibition closely paralleled the appearance of infectious Bangkok and FM virus in allantoic fluid. Maximum levels were achieved at 48 hr post-infection (p.i.) Ultracentrifugation analyses indicated that the bulk of thermostable inhibitor(s) of 48 hr Bangkok and FM infectious fluids remained in the supernatant rather than sedimenting with the viral pellet.