Completion of the amino acid sequence of a Hong Kong influenza hemagglutinin heavy chain: sequence of cyanogen bromide fragment CN1

Structural studies on membrane-embedded influenza hemagglutinin and its fragments

The mechanism of influenza virus hemagglutinin (HA)-mediated membrane fusion has been inferred in part from studies examining pH-induced structural changes in soluble HA derivatives lacking the viral membrane anchor and, sometimes, the fusion peptide (the C- and N-terminal residues of the HA2 chain, respectively). To reconcile structure-based mechanisms of HA-mediated membrane fusion with structural implications of functional studies performed on membrane-embedded HA, we have undertaken attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopic analyses of membrane-embedded HA (strain X:31) and its fragments reconstituted into supported lipid bilayers. The fragments correspond to proteolytic products with the majority of the HA1 chain and, in some cases, the fusion peptide removed (THA2 and THA2F-, respectively). In combination with R18 fluorescence dequenching to monitor the functional implications of HA1 subunit removal, we have assessed the influence of pH and target membrane presentation on the secondary structures, orientations relative to the membrane, and dynamics of these molecules. We find that X:31 HA is more tilted towards the plane of the membrane under fusion than under resting conditions, that the fitting of HA depends on the presence of the HA1 chain, that the residues connecting the membrane-inserted fusion peptide with the crystallographically determined coiled coil probably adopt an alpha-helical conformation, and that several changes in the secondary structure and the amide H/D exchange kinetics occur as a result of acidification and target membrane presentation, which can be interpreted as small changes and a release of strain in the static and dynamic structure of membrane-bound HA. THA2 mediatcs fusion, but less efficiently and with less pH-selectivity than HA.

Evolution of H3N8 equine influenza virus from 1963 to 1991

The antigenic properties of H3N8 influenza viruses isolated from outbreaks of equine influenza in Sweden between 1979 and 1991 have been studied in hemagglutination inhibition tests with polyclonal and monoclonal antisera, and antigenic drift of the virus has been demonstrated. To clarify the basis of the antigenic drift, amino acid sequences of the globular head regions (HA1) of the hemagglutinin membrane glycoproteins of virus strains from 1979, 1984, 1988 and 1990 have been deduced from the nucleotide sequences of the hemagglutinin genes, and the sequence information has been used to construct a phylogenetic tree of H3N8 equine influenza strains. Several strains from previous studies have been included to give a clearer picture of viral evolution in an international context.

Equine influenza virus from the 1991 Swedish epizootic shows major genetic and antigenic divergence from the prototype virus

The antigenic properties of H3N8 equine influenza virus from the Swedish epizootic of 1991 differ from those of A/eq 2/Fontainebleau/79 (representative of the Swedish vaccine strain) in hemagglutination inhibition tests. The amino acid sequence of the hemagglutinin (HA) of an isolate from the 1991 outbreak was deduced from the nucleotide sequence and comparison was made to the A/eq 2/Fontainebleau/79 strain. Twenty-three amino acid substitutions were found, 10 mapping onto areas of the HA known to bind antibodies in human H3 influenza viruses. The amino acid changes together with the serological data suggest that a major antigenic drift has taken place in equine H3N8 viruses in Sweden and we conclude that recent strains of the virus must be incorporated into vaccines on a regular basis if epizootics of equine influenza are to be controlled in the future.

The signal peptide of the rotavirus glycoprotein VP7 is essential for its retention in the ER as an integral membrane protein

The rotavirus glycoprotein VP7 has a cleavable signal peptide and is normally resident as an integral membrane protein in the ER of infected cells. A gene was constructed in which the VP7 H2 signal peptide was replaced by one from influenza hemagglutinin. COS cells transfected with this gene produced VP7 with the correct amino terminus, but the protein was rapidly secreted. Uncleaved VP7 from either precursor was not detected in cells after brief pulse-labeling, suggesting that the signal peptide was not acting as a temporary anchor; rather, it exerted its effect despite rapid cleavage. By splicing the H2 signal peptide onto another reporter protein, the malaria S-antigen, we demonstrated that H2 was necessary, but not itself sufficient, for targeting and retention. We propose that an interaction between the cleaved signal peptide and other downstream sequences in VP7 is required for retention of this protein in the ER as an integral membrane polypeptide.

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.

Membrane fusion proteins of enveloped animal viruses

In a living cell membrane-bound compartments are continuously either separated or united through fusion reactions, and literally thousands of such reactions take place every minute. The formation of membrane vesicles from pre-existing membranes, and their fusion with specific acceptor membranes, constitute a prerequisite for the transport of most impermeant molecules and macromolecules into the cell by endocytosis, out of the cell by exocytosis, and between the cellular organelles (Palade, 1975; Silverstein, 1978; Evered & Collins, 1982). Less frequent, but equally crucial, are fusion events in fertilization, cell division, polykaryon formation, enucleation, etc. (for reviews see Poste & Nicholson, 1978). Although a great deal is known about the properties and consequences of individual forms of membrane fusion in cellular systems, and about fusion in artificial lipid membranes, the molecular basis for the reactions remain largely unclear.

Chemical and antigenic characterization of the carbohydrate side chains of an Asian (N2) influenza virus neuraminidase

The Pronase-released neuraminidase heads from the Asian influenza virus A/Tokyo/3/67 contain four oligosaccharide units attached at asparagine residues 86, 146, 200, and 234. Chemical analysis of the isolated tryptic, chymotryptic, or thermolytic glycopeptides shows that the oligosaccharide side chains attached at residues 86 and 200 are essentially of the oligomannoside (simple or Type II) variety containing two residues of N-acetylglucosamine, five residues of mannose, and less than molar ratios of galactose and fucose. The carbohydrate side chains attached at residues 146 and 234 are of the N-acetyllactosamine (complex or Type I) type and contain N-acetylglucosamine, mannose, galactose, and fucose. The complex oligosaccharide unit at residue 146 is unusual in that it also contains N-acetylgalactosamine, a sugar residue rarely found in N-glycosidically linked carbohydrates. Antigenic analysis of these four isolated glycopeptides showed that only the N-acetyllactosamine oligosaccharide unit at asparagine residue 146 was capable of binding to antibodies raised against uninfected chick chorioallantoic membranes and is hence antigenically related to chick embryo host antigen.

A computer program for predicting protein antigenic determinants

A computerized method for predicting the locations of protein antigenic determinants is presented, which requires only the amino acid sequence of a protein, and no other information. This procedure has been used to predict the major antigenic determinant of the hepatitis B surface antigen, as well as antigenic sites on a series of test proteins of known antigenic structure [Hopp & Woods (1981) Proc. natn. Acad. Sci. U.S.A. 78, 3824-3828.] The method is suitable for use in smaller personal computers, and is written in the BASIC language, in order to make it available to investigators with limited computer experience and/or resources. A means of locating multiple antigenic sites on a homologous series of proteins is demonstrated using the influenza hemagglutinin as an example.

Disulfide bonding in influenza virus proteins as revealed by polyacrylamide gel electrophoresis

Disulfide bonding in the major proteins of influenza virus A, WSN strain, was studied by electrophoresis in sodium dodecyl sulfate-polyacrylamide gels under reducing and nonreducing conditions. The electrophoretic behavior of the proteins correlated with their localization in the virions and their chemical composition. The internal proteins of the viral particles, i.e. matrix and nucleoproteins, were shown to contain a relatively small number of cysteine residues. Electrophoresis under nonreducing conditions yielded multiple forms of the proteins which could be discriminated by small but readily observable, reproducible differences in their migration rates in the gel. the multiplicity of the protein forms was caused by the formation of intramolecular disulfide bonds in matrix and nucleoproteins that arose during or after solubilization in sodium dodecyl sulfate. On the other hand, we failed to detect native inter- and intramolecular linkages in matrix and nucleoproteins. External glycoproteins of the virions (HA and NA) had, in contrast to the internal ones, a higher number of cysteine residues and native disulfide bonds. At least three disulfide linkages were revealed in HA and NA in our experiments. In uncleaved HA all of the linkages were intramolecular. In NA at least one disulfide bond linked two identical polypeptides into a dimer. It was established that the reduction of the different disulfide linkages in HA and NA required different concentrations of the reducing agent.

Changes in the conformation of influenza virus hemagglutinin at the pH optimum of virus-mediated membrane fusion

A conformational change in the hemagglutinin glycoprotein of influenza virus has been observed to occur to pH values corresponding to those optimal for the membrane fusion activity of the virus. CD, electron microscopic, and sedimentation analyses show that, in the pH range 5.2-4.9, bromelain-solubilized hemagglutinin (BHA) aggregates as protein-protein rosettes and acquires the ability to bind both lipid vesicles and nonionic detergent. Trypsin treatment of BHA in the pH 5.0-induced conformation indicates that aggregation is a property of the BHA2 component and that the conformation change also involves BHA1. The implications of these observations for the role of the glycoprotein in membrane fusion are discussed.

Cell-surface expression of influenza haemagglutinin from a cloned DNA copy of the RNA gene

By replacing either the eight early or the late genes of SV40 with a cloned copy of the influenza virus haemagglutinin gene we have constructed recombinant viruses which, in infected cells, express large quantities of haemagglutinin. This glycoprotein, over 10(8) molecules of which are produced per cell, is identical in molecular weight to authentic influenza virus haemagglutinin, accumulates at the cell surface and displays haemabsorbing activity.

Evolution of the Hong Kong influenza A sub-type. Structural relationships between the haemagglutinin from A/duck/Ukraine/1/63 (Hav 7) and the Hong Kong (H3) haemagglutinins

The relationship between the haemagglutinin from the influenza virus A/duck/Ukraine/1/63 (Hav 7) and the human Hong Kong variants (H3) has been investigated. Amino-acid-sequence analysis shows that the Hav 7 haemagglutinin closely resembles the 1968 human H3 haemagglutinin in structure. However, the number of amino-acid-sequence differences (23) suggest that the Hong Kong haemagglutinin gene did not come directly from A/duck/Ukraine/1/63 but from a virus derived from it by antigenic drift during the period 1963-1968.

Antigenic drift in the hemagglutinin of the Hong Kong influenza subtype: correlation of amino acid changes with alterations in viral antigenicity

The nucleotide sequence of the gene coding for the large subunit of influenza virus hemagglutinin (HA1) was determined for strains A/NT/60/68, A/Eng/878/69, and A/Qu/7/70, three early isolates of the Hong Kong subtype. Sequences were obtained by the dideoxy chain termination method, using reverse transcriptase to synthesize partial DNA copies of the RNA gene. HA1 amino acid sequences predicted from the gene sequences were compared with published data for strains A/Aichi/2/68 and A/Vic/3/75. Compared with earlier strains, the HA1s of A/eng/878/69 and A/Qu/7/70 each contained three amino acid changes. Some of these were also found in A/Vic/3/75, but some were unique to the particular strain. When all of the strains were titrated with a panel of monoclonal antibodies directed against A/NT/60/68, alterations in viral antigenicity could be correlated with particular amino acid changes. The existence of multiple pathways for viral evolution during antigenic drift is discussed.

Amino acid sequence and oligosaccharide distribution of the haemagglutinin from an early Hong Kong influenza virus variant A/Aichi/2/68 (X-31)

The amino acid sequence and oligosaccharide distribution for the haemagglutinin from the early Hong Kong influenza virus A/Aichi/2/68 (X-31) was investigated. The two polypeptide chains, HA1 and HA2, were fragmented by CNBr and enzymic digestion, and the amino acid sequence of each small peptide was deduced by comparing its chromatographic behaviour, electrophoretic mobility, amino acid composition and N-terminus with that of the corresponding peptide of the haemagglutinin of known structure from the influenza-virus variant A/Memphis/102/72. Those peptides in which changes were detected were sequenced fully. The complete amino acid sequence of the haemagglutinin HA1 chain (328 residues) and 188 of the 221 residues of the HA2 chain were established by this approach, and revealed only twelve differences between the amino acid sequences of variant-A/Aichi/68 and -A/Memphis/72 haemagglutinins. These occurred at positions 2, 3, 122, 144, 155, 158, 188, 207, 242 and 275 in the HA1 chain and 150 and 216 in the HA2 chain. The highly aggregated hydrophobic region (residues 180-121) near the C-terminal end of the HA2 chain was not resolved by peptide sequencing. The oligosaccharide distribution in variant-A/Aichi/68 haemagglutinin was identical with that found in that of A/Memphis/72, with sugar units attached at asparagine residues 8, 22 38, 81, 165 and 285 in the HA1 chain and 154 on the HA2 chain. The monosaccharide compositions of the individual carbohydrate units on variant-A/Aichi/68 haemagglutinin differed from those of the corresponding units in variant-A/Memphis/72 haemagglutinin, and evidence was found for heterogeneity in the oligosaccharide units attached at single glycosylation sites.

Cloning and DNA sequence of double-stranded copies of haemagglutinin genes from H2 and H3 strains elucidates antigenic shift and drift in human influenza virus

Double-stranded DNA copies of the RNA gene coding for the haemagglutinin glycoproteins from human H2 and H3 pandemic strains of influenza virus have been cloned. DNA sequence analysis provides the first reported complete nucleotide sequence of an H2 haemagglutinin gene and a partial sequence (45%) of the H3 gene. The H2 haemagglutinin gene consists of 1,773 nucleotides containing an uninterrupted coding sequence of 1,686 nucleotides specifying a protein of 562 amino acids. Comparison of the amino acid sequences of these haemagglutinins with those of other H3 and avian strains reveals the extent of sequence changes in antigenic shifts and drifts.

Antigenic drift between the haemagglutinin of the Hong Kong influenza strains A/Aichi/2/68 and A/Victoria/3/75

A DNA copy of the gene coding for the influenza A/Aichi/2/68 haemagglutinin protein was cloned in the plasmid pBR322 and the complete nucleotide sequence determined. Comparison of this primary structure and the deduced amino acid sequence with the haemagglutinin gene and protein of strains belonging to the same (H3) subtype and to different subtypes, of both human (H2) and avian (Hav1) origin, documents further at the molecular level the two independent modes of antigenic variation of the virus--drift and shift.