Influenza virus proteins. I. Analysis of polypeptides of the virion and identification of spike glycoproteins

Influenza virus-model membrane interaction. A morphological approach using modern cryotechniques

The membrane fusion activity of influenza virus was characterized morphologically using a model system composed of a highly purified influenza B virus suspension and ganglioside-containing zwitterionic liposomes. Electron microscopical analysis was performed after a combination of fast-freezing with either freeze-fracture or freeze-substitution-thin sectioning, ensuring maximal time resolution and avoiding preparation artifacts. In a parallel fluorescence 'lipid mixing' fusion assay, influenza virus-membrane fusion was characterized biochemically. Biochemical and morphological data are in full agreement, indicating negligible membrane fusion activity at neutral pH and high fusion activity at low pH. The freeze-fracture morphology strongly suggests a local point contact between viral and liposomal membrane at neutral pH, and a local point fusion mechanism for influenza virus-membrane fusion upon lowering of the pH. Fusion is followed by lipid mixing, lateral diffusion of viral spike proteins and exposure of viral contents at the inner liposomal surface.

Antigen-presenting B cells and helper T cells cooperatively mediate intravirionic antigenic competition between influenza A virus surface glycoproteins

Parenteral vaccination of BALB/c mice primed by infection with H3N2 variants of influenza A virus results in a reduced production of N2 antibody in response to homologous (H3N2) vaccine compared with the response to an H7N2 vaccine equal in N2 immunologenicity. We now have studied the interaction in vitro of purified splenic B and T lymphocytes from variably immunized mice to ascertain the cellular basis of the hemagglutinin (HA)-influenced antibody response to neuraminidase (NA). Assay of the proliferative response of T cells in B/T-cell mixtures stimulated by H3N1 (HA-specific) and H6N2 (NA-specific) reassortant (recombinant) viruses in vitro has enabled us to differentiate cellular responses to HA and NA antigens. Using a factorial design in analysis of B/T-cell mixtures, we have shown that: (i) intravirionic HA is dominant over NA in both B- and T-cell priming; (ii) an increase in H3-specific B cells occurs in mice administered boosters of H3N2 vaccine, and an increase in N2-specific B cells occurs in those given a booster of H7N2 vaccine; and (iii) memory B cells function as antigen-presenting cells and interact with memory helper T cells in the mediation of intravirionic HA-NA antigenic competition in favor of HA. The damping of response to the NA antigen in favor of HA with reinfection prohibits balanced immunologic response to the two antigens. The present studies define further the complex immunology of influenza virus infection.

Characterisation of lymphoproliferative disease virus of turkeys. Structural polypeptides of the C-type particles

Lymphoproliferative disease virus of turkeys (LPDV), a C-type retrovirus, was shown to contain 3 major [32 kilodaltons (kd, p 32), 26 kd, 22/21 kd] and 2 minor (41 kd and 12 kd) polypeptides. Preliminary evidence suggests a glycoprotein of 76 kd (GP 76) and a major doublet polypeptide of 13.5/13 kd to be also of viral origin. Of these GP 76 was susceptible to bromelain action implying its surface location in the virion, while p 32, p 26 and p 13.5/13 were the main constituents of viral cores. p 13.5/13 bound an RNA probe, suggesting it to be the main constituent of viral ribonucleoprotein. p 22/21 was not cleaved by bromelain, and was absent in viral cores suggesting its intramembrane location between virion envelope and core. The polypeptide profile of LPDV is distinct from those of avian sarcoma-leukosis viruses and avian reticuloendotheliosis viruses.

Identification of the glycoproteins of lymphocystis disease virus (LDV) of fish

Analysis of highly purified fish Lymphocystis Disease Virus (LDV), strain Leetown NFH, by three different methods, namely periodic Acid Schiff reaction, radiolabelling with tritiated fucose and N-acetyl-D-glucosamine and staining with three lectins, indicated that ten glycoproteins were associated with the virus structure. Six of them were detected by all of the three methods, three by both radiolabelling and lectin staining but only one by the lectin technique. Localization of these glycoproteins at the surface or inside the virion is discussed.

Structural proteins of human respiratory coronavirus OC43

The human respiratory coronavirus OC43 was grown on a human rectal tumor cell line and was isotopically labeled with amino acids, glucosamine, and orthophosphate to analyze virion structural proteins. Four major protein species were resolved by electrophoresis and many of their properties were deduced from digestion studies using proteolytic enzymes. The four proteins are: A 190 kDa protein, the presumed peplomeric protein, that was glycosylated and proteolytically cleavable by trypsin into subunits of 110 and 90 kDa. The subunits each represent a different amino acid sequence on the basis of peptide mapping; a 130 kDa protein that was glycosylated and behaved as a disulfide-linked dimer of 65 kDa molecules. It is the apparent virion hemagglutinin on the basis of digestion studies with trypsin, bromelain and pronase; a 55 kDa nucleocapsid protein that was phosphorylated; a 26 kDa matrix protein that was glycosylated. The 190, 130, 55 and 26 kDa species can therefore be designated P, H, N and M, respectively. They exist in molar ratios of 4:1:33:33, and are calculated to be present at the rate of 88, 22, 726, and 726 molecules per virion, respectively.

Host cell-mediated selection of a mutant influenza A virus that has lost a complex oligosaccharide from the tip of the hemagglutinin

During serial passage in Madin-Darby bovine kidney (MDBK) cells, a substrain of influenza virus A/WSN is lost from the population and is replaced by a mutant virus with altered host cell binding properties. This selection does not occur during growth in chicken embryo fibroblasts (CEF). It occurs during growth in MDBK cells because the parental virus produced by these cells has a dramatically reduced affinity for cellular receptors [Crecelius, D.M., Deom, C. M. & Schulze, I. T. (1984) Virology 139, 164-177]. We have now compared the hemagglutinin (HA) subunits, HA1 and HA2, of the parent and mutant viruses by NaDodSO4/PAGE and have found that when the viruses are grown in either host cell the HA1 subunit of the mutant is smaller than that of the parent virus. The nonglycosylated HAs, made in the presence of tunicamycin, have the same apparent molecular weight, indicating that the HA1 subunit of the mutant virus contains less carbohydrate than that of the parent. This reduction in carbohydrate content was observed with 11 independently derived mutants that had been selected by growth in MDBK cells. The nucleotide sequence of the HA gene of the parent and mutant viruses indicates that there are five potential glycosylation sites on the parent HA1 subunit and four on the mutant and that the mutation responsible for this difference is a single base change that eliminates the glycosylation site at amino acid 125 of the parent HA1 subunit. Treatment of the parent and mutant HAs from both cell sources with endo-beta-N-acetylglucosaminidases F and H showed that the HA1 of the parent virus has four complex and one high-mannose oligosaccharides, whereas that of the mutant virus has three complex and one high-mannose oligosaccharides. Thus, all of the potential sites on both HA1 subunits are glycosylated. We conclude that the oligosaccharide attached to amino acid 125 of the parent HA by MDBK cells can reduce the affinity of the virus for cellular receptors and that the mutant virus has a higher affinity than the parent because the mutant HA is not glycosylated at that site. Since amino acid 125 of the parent HA is glycosylated by both CEF and MDBK cells, we further conclude that the host-determined structure of the oligosaccharide at that site affects the affinity of the parent virus for cellular receptors and, thereby, determines whether the mutant virus will have a growth advantage.

Mutants of the membrane-binding region of Semliki Forest virus E2 protein. I. Cell surface transport and fusogenic activity

Three mutations of the membrane-binding region of the Semliki Forest virus (SFV) p62 polypeptide (the precursor for virion E3 and E2) have been made by oligonucleotide-directed mutagenesis of a cDNA clone encoding the SFV structural proteins. One of the mutations (A2) substitutes a Glu for an Ala in the middle of the hydrophobic stretch which spans the bilayer. A1 and A3 alter the two basic charged amino acids in the cytoplasmic domain next to the hydrophobic region. The wild-type charge cluster of Arg-Ser-Lys (+2) has been changed to Gly-Ser-Met (0;A3) or to Gly-Ser-Glu (-1;A1). The mutant p62 proteins have been analyzed both in the presence and the absence of E1, the other half of the heterodimer spike complex of SFV. The mutant proteins expressed in COS-7 cells are glycosylated and are of the expected sizes. When co-expressed with E1, all three mutants are cleaved to yield the E2 protein and transported to the surface of COS-7 cells. When expressed in the absence of E1, the mutant p62 proteins remain uncleaved but still reach the cell surface. Once at the cell surface, all three mutants, when co-expressed with E1, can promote low pH-triggered cell-cell fusion. These results show that the three mutant p62/E2 proteins are still membrane associated in a functionally unaltered way.

The functions of oligosaccharide chains associated with influenza C viral glycoproteins. I. The formation of influenza C virus particles in the absence of glycosylation

The effect of a glycosylation inhibitor, tunicamycin (TM) on the replication of influenza C virus was investigated. Incorporation of [3H]-glucosamine into the gp88 glycoproteins of this virus was completely inhibited by TM at the concentrations higher than 0.25 microgram/ml. Under these conditions, the synthesis of internal proteins NP and M was shown in TM-treated cells but the synthesis of gp88 was not. The disappearance of gp88 was however accompanied with the appearance of two new polypeptides with molecular weights of 80,000 (T80) and 76,000 (T76). While T80 was identified by peptide mapping as a host cell protein whose synthesis was enhanced by TM, T76 was shown to correspond to a nonglycosylated form of gp88. Pulse-chase experiments revealed that there was no significant difference in the intracellular stability of T76 and gp88. Although TM depressed the production of infectious progeny virus greater than 100-fold, only a five-fold decrease was observed in the release of noninfectious physical particles, suggesting that glycosylation is not essential for the formation of influenza C virus particles. However, the virions from TM-treated cells had a lower buoyant density in isopycnic sucrose gradients and lacked surface proteins in either glycosylated or nonglycosylated form.

The functions of oligosaccharide chains associated with influenza C viral glycoproteins. II. The role of carbohydrates in the antigenic properties of influenza C viral glycoproteins

The antigenic properties of influenza C viral glycoprotein gp88 were compared with those of its nonglycosylated counterpart T76 synthesized in infected cells treated with tunicamycin. Radioimmunoprecipitation experiments with three different monoclonal antibodies against gp88 revealed that an antibody designated Q-5 precipitated gp88 but not T76, indicating the requirement for glycosylation for the binding of this antibody to gp88. It is unlikely, however, that the antigenic determinant recognized by Q-5 is carbohydrate moiety since the ability of the antibody to bind to gp88 varied depending on the virus strain, and trypsin-treatment of gp88 eliminated its reactivity with Q-5. Gel electrophoretic analysis under nonreducing conditions showed that T76 underwent the formation of disulfide-linked multimers in the absence of reducing agent while gp88 behaved as monomers, suggesting that glycosylation is required for gp88 molecules to attain an appropriate conformation. These observations, altogether, suggests that glycosylation is important in determining the immunological specificity of gp88 presumably by influencing the folding of this glycoprotein.

Structure and composition of influenza virus. A small-angle neutron scattering study

A detailed analysis is presented of the small-angle neutron scattering curves of homogeneous solutions of influenza B virus, both intact and after treatment with bromelain, which removes the external glycoprotein spikes. The two sets of data are consistent with the following low-resolution structure: the virus particles are spherical, about 1200 A in diameter and of Mr about 180 X 10(6). The lipid bilayer is centred at a radius of 425 A, is 40 A to 50 A thick and constitutes 25% to 28% of the virus mass. The surface glycoproteins, predominantly haemagglutinin, contribute 40% to 46% of the total mass. Surprisingly little protein is found in the interior of the virus. It is suggested that the reason for this is that many particles do not contain the full complement of ribonucleoprotein complexes. These results are in good agreement with recent scanning transmission electron microscopic measurements of molecular mass and cryo-electron microscopic observations of the same preparations. Appendix 1 describes a new method of deriving spherical shell models from contrast variation neutron scattering data on viruses, in which scattering curves from all measured contrasts are used simultaneously. There is also a discussion of the assumptions and limitations implicit in the structural interpretation of such models, with emphasis on viruses containing lipid bilayers. Appendix 2 examines the effect on the scattering curves of various arrangements of the surface glycoproteins.

Natural heterogeneity of shape, infectivity and protein composition in an influenza A (H3N2) virus preparation

Influenza A (X31) virus was purified over a zonal sucrose gradient. The resulting gradient fractions were examined by electron microscopy, HA and infectivity titrations and gel electrophoresis. The fractions containing a homogeneous suspension of spherical particles had the highest infectivity per amount of viral protein and a much higher HA:M ratio than the unfractionated preparation. These results explain differences in the proportions of HA and M protein we have reported elsewhere on monodisperse virus suspensions compared with earlier measurements on crude suspensions.

Inhibition of proteolytic cleavage of the hemagglutinin of influenza virus by the calcium-specific ionophore A23187

At calcium-specific ionophore A23187 concentrations of approximately 0.25 microM [which still allow assembly and release of fowl plague virus (FPV) particles] post-translational proteolytic cleavage of the viral hemagglutinin precursor HA into the fragments HA1 and HA2 is inhibited. The resulting virus particles with uncleaved hemagglutinin, that cannot be obtained under normal conditions, provide a suitable substrate for in vitro assays of the protease sensitivity of the FPV hemagglutinin. Proteolytic activation is accomplished with trypsin. Treatment with cathepsin B at low pH yields aberrant cleavage products suggesting that the cellular cleavage enzyme is not of lysosomal origin. A protease that cleaves the FPV hemagglutinin in the correct place can be detected in lysates of MDBK cells. This enzyme is calcium dependent and has a neutral pH optimum.

Hybridoma antibodies produced against bromelain derived cores of influenza virus

Splenic lymphocytes from BALB/c mice immunized with "cores" of influenza virus, obtained after bromelain cleavage of the surface glycoprotein, were fused with the P3-NS1/1-Ag-1 mouse cell line to yield hybridoma cultures. Among 20 stable cloned hybrid cells secreting monoclonal antibodies, one was specific for the nucleoprotein (NP), 11 were specific for the membrane (M) protein and eight were specific for the hemagglutinin (HA). These "cores" used as immunogen contained only the internal proteins of the influenza virus, namely the three polymerases, the NP and the M protein and no HA when examined by standard procedures of SDS-PAGE, electron microscopy and hemagglutination activity. It thus appeared that a small amount of contaminating antigens can sensitize a sufficient number of mouse B cells to be selected as hybrid partners. These antibodies were provisionally assigned as anti-carbohydrate attached to the HA.

Bovine coronavirus hemagglutinin protein

Treatment of purified bovine coronavirus (Mebus strain) with pronase destroyed the integrity of virion surface glycoproteins gp140, gp120, gp100, reduced the amount of gp26 and destroyed the hemagglutinating activity of the virus. Bromelain, on the other hand, destroyed the integrity of gp120, gp100 and gp26 but failed to remove gp140 and failed to destroy viral hemagglutinating activity. These experiments suggest that gp140 is the virion hemagglutinin. Immunoblotting studies using monospecific antiserum demonstrate that gp140 is a disulfide-linked dimeric structure reducible to monomers of 65 kDa.

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

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

Antigenic characterization of human coronaviruses 229E and OC43 by enzyme-linked immunosorbent assay

Human coronaviruses 229E and OC43 possess three distinct antigens each which are located in peplomer, membrane, and nucleoprotein virion subcomponents. Although specific antigens are associated with similar polypeptides in both viruses, neither shared antigens nor serological cross-reactions have been observed. These findings were confirmed by enzyme-linked immunosorbent assay; rabbit whole-virus-specific antisera reacted with dissociated homologous virus and each of its subcomponents, whereas antisera monospecific to separate subcomponents (peplomers, membrane, or core) recognized only their respective components. Since neither shared antigens nor serological cross-reactions were seen between the two viruses, the specificity of the assay was similar to that of crossed-immunoelectrophoresis, virus neutralization, and complement fixation assays. However, sensitivity was increased at least 1,000-fold; complement fixation antibody titers of 2,000 corresponded to enzyme-linked immunosorbent assay titers of 3,200,000. Similar results were obtained with human convalescent-phase sera. In addition, the most prevalent human antibodies were found to be directed against virion peplomers. However, specific antibodies to core antigens and lesser amounts to membrane antigens were found in the sera of patients, which showed significant antibody rises when purified virion subcomponents were used as antigens. Importantly, rises and declines in titers of antibody to one virus and its specific antigens were independent from levels of titers of antibody to the other virus.

Inhibition of T cell proliferation by antibodies to synthetic peptides

While T cell proliferation to antigen in the presence of antigen-presenting cells is well known to be readily inhibited by antibodies directed against Class II major histocompatibility complex (MHC) (Ia/HLA-DR) products, it has not been possible to inhibit proliferation by antibodies directed against the antigen. Because of the implications of these observations for targets of T cell recognition, this phenomenon was reinvestigated using human T cell clones, recognizing a small (24 amino acid) synthetic peptide (termed p20) derived from the influenza hemagglutinin-1 molecule. It was found that proliferation of clones to p20 was inhibited efficiently (less than 90%), using p20 as antigen, and rabbit anti-p20. Inhibition was possible either by coculturing p20 antigen and antibody to p20 with cloned T cells and antigen-presenting (E-) cells, or by pulsing antigen-presenting cells with antigen prior to a brief incubation with antibody before washing the E- cells and using them to stimulate cloned T cells. These results do not indicate why previous attempts had failed, but in view of the different techniques available now (cloned T cells, small synthetic polypeptides, and antibody raised against polypeptide) we investigated the influence of these parameters. It was found that, using cloned T cells, the form of the antigen was of importance, as antibody inhibition of the response to hemagglutinin or whole influenza A was much less apparent. These differences were interpreted as being due to greater access of anti-p20 to p20 than to hemagglutinin or influenza. If uncloned T cell lines were used, inhibition was also much harder to detect. This was interpreted as masking of inhibition of the response of some clones in the line by interleukin 2-induced recruitment.

The membrane (M1) protein of influenza virus occurs in two forms and is a phosphoprotein

The membrane (M1) protein of influenza virus was found to be heterogenous and to occur in two forms in the virus particle. The two forms of M1 were found in virus which was produced both early and late after infection and in infected cells. The two forms could be separated on polyacrylamide gels under specific conditions. The two components of M1 contained similar tryptic peptides. However, a small proteolytic difference between the two proteins could not be ruled out. Both M1 proteins were present in phosphorylated form in the virus particle. The phosphorylated M1 components were not readily distinguished from phosphorylated nonstructural protein (NS1) when cytoplasm of infected cells was analyzed on polyacrylamide gels. The two phosphorylated M1 components could, however, be detected in infected cells by immunoprecipitation. One M1 component contained only phosphoserine whereas the second contained phosphoserine and a small amount of phosphothreonine as well. In addition to the phosphorylated nucleoprotein and M1, a third phosphorylated protein was routinely detected in virus particles. It was a surface component of the virus, since it could be removed from whole virus with chymotrypsin and contained phosphate at serine residues. The identity of this component was not known.

The synthesis of polypeptides in influenza C virus-infected cells

The synthesis of virus-specific polypeptides was analyzed in MDCK cells infected with the JJ/50 strain of influenza C virus. In addition to three major structural proteins gp88, NP, and M, the synthesis of five polypeptides with molecular weights of 29,500 (C1), 27,500 (C2), 24,000 (C3), 19,000 (C4), and 14,000 (C5) was found in infected cells. None of these polypeptides were detected either in virions or in immunoprecipitates obtained after treatment of infected cell lysates with antiviral serum, suggesting that they are not viral structural proteins. Polypeptides C1-C5 were found to be synthesized in MDCK cells infected with different influenza C virus strains as well as in different host cell types infected with C/JJ/50. Further, it was observed that cellular protein synthesis was greatly reduced under hypertonic conditions, whereas the synthesis of C1-C5 was relatively unaffected. These results suggest that polypeptides C1-C5 are virus coded rather than host cell coded. Peptide mapping studies showed that each of polypeptides C3, C4, and C5 had a peptide composition similar to the M protein. The amount of C2 synthesized in infected cells was insufficient for mapping. This polypeptide was, however, found to rapidly disappear in pulse-chase experiments, suggesting that C2 is probably not unique but biosynthetically related to one of the other proteins. In contrast to these polypeptides, polypeptide C1 showed a map which is largely different from any major structural polypeptide. It therefore appears likely that C1 is a nonstructural protein of influenza C virus similar to the NS1 protein of influenza A and B viruses.

Anomalous electrophoretic behavior of the membrane (M) protein of influenza virus in polyacrylamide gels

The relative position of M and NS 1 on polyacrylamide gels depends on the concentration of cross-linker in the gel. Inversion in position of M and NS 1 occurs at a cross-linker concentration of 1.2 percent.

Studies on the role of glycosylation in the functions and antigenic properties of influenza virus glycoproteins

The biological and antigenic roles of glycosylation were investigated in the influenza hemagglutinin (HA) glycoprotein using the glycosylation inhibitor tunicamycin (TM). Under conditions where only the nonglycosylated form of HA was detected by immunoprecipitation and gel electrophoresis, the migration of glycoproteins to the cell surface was observed by immunofluorescence using either monospecific or monoclonal antibody to the HA polypeptide. Analysis of the surface fluorescence in TM-treated infected cells by a fluorescence-activated cell sorter (FACS) showed that all cells exhibited fluorescence in the complete absence of glycosylation. The relative amount of HA antigen on cell surfaces was found to be reduced by only 30-40% in TM-treated cells, and this reflected a similar reduction in intracellular synthesis. Electron microscopic studies using ferritin labeling also demonstrated that the nonglycosylated HA glycoprotein was present in significant amounts on surfaces of infected cells. Virions with nonglycosylated glycoproteins were purified, and were found to have an approximate 30-fold decrease in both hemagglutinin and neuraminidase specific activities. The possible role of oligosaccharides in antigenic variation among various H1N1 strains was investigated. Immunoprecipitation reactions involving five different monoclonal antibodies and five antigenic variants of A/USSR/90/77 revealed no major antigenic differences between the glycosylated and nonglycosylated forms of HA.

Effects of hexose starvation and the role of sialic acid in influenza virus release

We previously reported that growth of influenza virus in the presence of cytochalasin B (CB), a drug that disrupts microfilaments and blocks hexose transport, yields particles with glycoproteins that are heterogeneous and unlabeled by [3H]glucosamine. When the virus was grown in glucose-free medium, we observed reduced virus titers similar to those produced by CB. In contrast, treatment of cells with cytochalasin D (CD) and dihydrocytochalasin B (H2CB), drugs which are known to inhibit microfilament function without affecting hexose transport, did not cause a reduction in virus titers or a change in the electrophoretic mobility of viral glycoproteins. Partial inhibition of glycosylation of viral glycoproteins resulting from either CB-induced inhibition of hexose transport or from glucose starvation resulted in the formation of aggregates of virions on cell surfaces. These aggregates can be dissociated by exogenous neuraminidase. Under these conditions the virions contained a functional hemagglutinin glycoprotein (HA) but an inactive neuraminidase glycoprotein (NA) which was not able to cleave sialic acid, the HA receptor, from viral glycoproteins, or from cellular glycoproteins and glycolipids. Neuraminidase treatment of membrane fractions of CB-treated cells did not cause a shift in the electrophoretic mobility of HA or in the gel elution profile of HA glycopeptides obtained after extensive pronase digestion from HA synthesized in glucose-free medium. These findings suggest that sialic acid is not present on labeled glycoproteins in either of these preparations. We obtained evidence that the sialic acid to which HA binds when NA is inactive is on glycoproteins and glycolipids of cellular origin. Our results support the idea that even when NA is functional, sialylated cellular components impede influenza virus release.

[Influenza]

Influenza is the last great uncontrolled plague of mankind. Pandemics and epidemics occur at regular time intervals. The influenza viruses are divided into the types A, B and C and show unique variability of their surface antigens (hemagglutinin and neuraminidase). Influenza viruses of type A show the largest degree of antigenic variation which, in turn, resulted in the definition of a number of subtypes, each comprising many strains. By comparison, influenza viruses of types B and C exhibit much less variation of their surface antigens. As a consequence, no subtypes but many different strains have been recognized. The degree of antigenic variation correlates with the epidemiologic significance of the virus types, type A being the most and type C the least important. Two different kinds of antigenic variation have been recognized: In the case of minor variation of one or both surface antigens, the term "antigenic drift" is employed. Antigenic drift occurs with all three types of virus, it is caused by point mutations which increase the chance of survival of mutants in the diseased host. In addition, influenza A viruses show sudden and complete changes of their surface antigens in regular time intervals, resulting in the appearance of new subtypes. This event is called "antigenic shift". The mechanisms responsible for antigenic shift are poorly understood, only. In addition to the recycling of preceding subtypes, reassortment resulting from double infection of cells with strains of human and animal origin are considered possible explanations. By use of modern DNA recombinant technology, the base sequences of a series of virus genes and, as a consequence, the amino acid sequence of the corresponding antigens have been determined. By means of monoclonal antibodies, the antigenic structure of many influenza antigens has been further elucidated. It can be expected that further research on the molecular basis of antigenic variation could finally result in an understanding of the causal mechanisms. It is an outstanding feature of the epidemiology of influenza A viruses that a family of related strains prevails for a certain period of time and disappears abruptly as a new subtype emerges.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Polypeptides and functions of antigens from human coronaviruses 229E and OC43

Coronaviruses possess three major size classes of polypeptides as judged by molecular weight: approximately 180,000, approximately 50,000, and approximately 23,000. Human coronaviruses 229E and OC43 possess not only three similar size classes of polypeptides but also three distinct antigens, none of which cross-react with the heterologous strain. Polypeptides separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were reacted in rocket immunoelectrophoresis with antiserum monospecific to each of the three strain-specific antigens (excised precipitin lines from crossed immunoelectrophoresis profiles were used for immunogens). Monospecific antiserum with neutralizing ability reacted with a polypeptide of 186,000 daltons for 229E and a polypeptide of 190,000 daltons for OC43. The antigen which elicited neutralizing antibody response was located at the surface, associated with the corona of the virion, glycosylated, and bound by concanavalin A. Another less prominent surface antigen was represented by size classes of 23,000 daltons for 229E and 24,000 for OC43. The core antigens of the viruses had molecular weights of 49,000 and 229E and 52,000 and OC43 virus. Thus, the molecular weights and functions of the antigens of human coronaviruses are similar to those of animal coronaviruses. The polypeptides of coronaviruses 229E and OC43 are nearly identical as judged by molecular weight, but the similar polypeptides of the two viruses represent different immunological specificities.

Studies on neuraminidase from influenza virus A(H3N2) obtained by two procedures

1. Neuraminidase was obtained by (A) bromelain solubilization or (B) by treatment with N-lauroylsarcosine. 2. 5-N-acetyl-2-O-(3-methoxyphenyl)-alpha-D-neuraminic acid, employed as substrate, avoids the interference produced by the thiobarbituric acid method, and is not interfered by the ampholytes. 3. Only about 20% of original enzyme activity was lost after electrofocusing. The sample from procedure A showed two peaks, corresponding to pIs 4.4 and 5.6. The sample from procedure B, having a higher activity, showed only one peak at pI 4.4. 4. Samples A and B showed different Km and hydrolysis rate with N-acetylneuraminyl-lactose and glycophorin A. It was not found significantly different with other substrates: alpha 1-acid glycoprotein, brain gangliosides, 5-N-acetyl-2-O-(3-methoxyphenyl)-alpha-D-neuraminic acid and 2'-(4-methyl umbelliferyl)-alpha-D-N-acetylneuraminic acid.

Insertion of influenza M protein into the viral lipid bilayer and localization of site of insertion

Recent studies with isolated M protein from influenza virus have shown that the protein has a high affinity for lipid. The ability of M to partition into lipid vesicles merely by shaking vesicles and M together is suggestive evidence that the protein could be interacting with the lipid in the virus particle. A more direct analysis was carried our here to determine whether M is in contact with the viral lipid in situ, by using the photoactivatable hydrophobic probe, pyrenesulfonyl azide. Covalent linkage of this probe to M indicated that a segment of M residues with in the virus membrane in contact with the lipid bilayer. M inserted into lipid vesicles at two locations on the molecule. A major insertion into lipid occurred in the middle of the molecule where a large cluster of 20 hydrophobic and neutral amino acids occurs. A second insertion occurred approximately one fourth in from the amino terminus, where a smaller segment of 13 uncharged amino acids is found. Confirmation that M inserted into lipid at these locations came also from results with cyanogen bromide fragments of M. Of the 12 to 13 fragments produced, 3 specifically bound to lipid vesicles. These were the first, second, and third contiguous segments beginnings at the amino terminus and containing the two hydrophobic areas noted above.

Mitogenicity of influenza hemagglutinin glycoproteins and influenza viruses bearing H2-hemagglutinin

The hemagglutinin glycoprotein is responsible for the mitogenic effect of influenza A viruses of the H2N2 subtype. This was indicated by the ability of viruses bearing the H2-hemagglutinin glycoprotein, regardless of its associated neuraminidase, to induce lymphocyte proliferation in normal spleen cell suspensions and by the ability of antisera with specificity for the H2-hemagglutinin to block this response. Moreover, purified hemagglutinin from representative viruses from the H0N1, H1N1, H2N2, H3N2, and influenza B subtypes were also shown to be mitogenic.

Comparative morphology of measles virus and paramyxovirus-like tubules in multiple sclerosis using ruthenium red stain

Morphology of intracytoplasmic nucleocapsid measles virus from mouse brain and tissue culture was studied with the use of ruthenium red by tilting and high resolution electron microscopy and compared with the paramyxovirus-like tubules found in multiple sclerosis (MS). Both in vivo and in vitro the measles nucleo-protein profiles were surrounded by 'fuzzy' material which could be resolved in a pentagon shape and this stained specifically with ruthenium red. The tubules found in MS appeared not to have a 'fuzzy' coat and also did not stain with ruthenium red. The main difference observed between infected tissue culture cells and mouse brain was that in the latter no alignment of measles nucleoprotein was observed under the cell membrane and no budding particles were seen. The virus could not be passed to mice or tissue culture from the infected mouse brain.

Interaction of influenza virus hemagglutinin with target membrane lipids is a key step in virus-induced hemolysis and fusion at pH 5.2

The molecular mechanism of hemolysis and fusion by influenza virus in acidic media was studied. First, the effect of trypsin treatment on the activity of fibroblast-grown influenza virus was studied. The results showed that the split form of viral hemagglutinin, HA1 and HA2, but not the precursor, is responsible for the activity. Second, the interaction of egg-grown influenza virus, which contains the split hemagglutinin, with lipid liposomes was studied by spin labeling and electron microscopy. Phospholipid transfer from the viral envelope to the lipid bilayer membrane occurred within 30 s at pH 4.5-5.4. The transfer is largely independent of the lipid composition and the crystalline vs. liquid/crystalline state of the membrane. Virus-induced lysis of liposomes also took place rapidly in the same pH range. Envelope fusion with liposomes occurred at pH 5.2 but not at pH 7.0. These characteristic interactions were similar to those between influenza virus and erythrocytes reported previously. On the other hand, hemagglutinating virus of Japan did not interact with liposomes at neutral pH. These results suggest that protonation of the NH2-terminal segment of the HA2 form causes interaction of the segment with the lipid core of the target cell membrane, leading to hemolysis and fusion.

Antigen requirements and specificity of a microplate enzyme-linked immunosorbent assay (ELISA) for detecting infectious bronchitis viral antibodies in chicken serum

The conditions of a rapid, indirect-enzyme linked immunosorbent assay for Infectious Bronchitis Virus (IBV) antibodies have been established. Optimal sensitivity was obtained using 10 micrograms/ml protein concentration of the Mass 41 strain purified from infected allantoic fluid. Specificity was demonstrated with Newcastle Disease Virus (NDV) antigen-antibody system. Negligible crossreactions were observed. After bromelain or lipase treatment IBV had an ELISA reactivity similar to untreated particles suggesting that peripheral constituents of IBV play a minor role when whole virus is absorbed on solid phase. The method offers a simple and specific antibody assay which could be used for the laboratory diagnosis of avian infectious bronchitis.

Membrane asymmetry. A survey and critical appraisal of the methodology. II. Methods for assessing the unequal distribution of lipids

In the companion paper, I have reviewed the techniques employed for assessment of the asymmetric distribution and orientation of membrane proteins. This article deals with methods applicable to the investigation of the unequal distribution of lipids between the two membrane leaflets. Among the techniques I will discuss are the use of immunological techniques and lectins, chemical reagents, enzymatic isotopic labeling and degradation of membrane lipids, exchange proteins and physical techniques. Whenever appropriate, problems of crypticity and non-availability of lipids to interact with the appropriate ligands, reagents, modifying enzymes or exchange proteins have been envisaged. It appears that in many case, highly discordant results, sometimes with the same biological material, have been obtained. Some of the difficulties encountered presumably stem from the reported existence of non-bilayer arrangements and isotropic movement of lipids as evidenced by freeze-fracture and NMR studies. Other problems may be related to the induction of such arrangements, especially the inverted micellar arrangement, by the modifying agents, particularly degradation enzymes or exchange proteins when they cause severe unilateral modification of the lipids of the exposed leaflet. In addition, the situation is complicated by the role of the induced increase in the flip-flop rate under different experimental conditions and by modification of the rearrangement of lipid molecules as a result of the metabolic state of the cell or ghost preparation and of the reactivity of lipids as a consequence of temperature changes. Here, more so than with proteins, one must be cautious in interpreting experimental results. Moreover, it would appear that the use of different techniques in conjunction and the consequent comparison of results should be recommended. It has been emphasized that 'general rules' do not hold and that each new material should be assay again. To give one example, it is not pertinent to state that proteins enhance the flip-flop rate in lipid vesicles (and hence in membranes). This holds true for glycophorin from erythrocyte membrane, but could not be proved when mitochondrial cytochrome oxidase was used. There seems to be no rule for the distribution of lipids between the two leaflets of different membranes. For example, even for different strains of the same bacterial species, highly divergent results have been reported. It is generally (and probably under the influence of different studies with erythrocytes) believed that in mammalian plasma membranes, choline phospholipids are enriched in the outer leaflet and aminophospholipids in the inner leaflet. Though this contention may prove to be correct, different instances of contradictory results have been given in the text. This shows that if rules do exist, they remain to be discovered or established...

Interaction of influenza M protein with viral lipid and phosphatidylcholine vesicles

The M protein of influenza is the predominant structural component of the virus. The interactions of this protein with the viral lipid or with other proteins are not known. The ability of M to interact with viral or other lipids was investigated. Purified M was mixed with viral lipid or egg phosphatidylcholine and was incorporated into vesicles (i) by addition of sodium deoxycholate followed by dialysis or (ii) by sonication. Between 90 and 100% of the M became firmly associated with the lipid by either of these two methods, whereas nucleoprotein failed to associate with the vesicles. From association also occurred if M was mixed with performed vesicles. Most of the M attached to the vesicles could be hydrolyzed with proteolytic enzymes such as trypsin or thermolysin, except for a small fragment of about 5,000 daltons which remained associated with the lipid vesicles. The ability of fragments of M to interact with lipids was also investigated. Of 13 fragments produced by cleavage with cyanogen bromide, 3 specifically associated with lipid vesicles. The data indicate that a specific portion of the M molecule has a high affinity for lipid bilayers of various origins.