Isolation and structural analysis of influenza C virion glycoproteins

Structural and functional analysis of the roles of influenza C virus membrane proteins in assembly and budding

Assembly and budding of the influenza C virus is mediated by three membrane proteins: the hemagglutinin-esterase-fusion glycoprotein (HEF), the matrix protein (CM1), and the ion channel (CM2). Here we investigated whether the formation of the hexagonal HEF arrangement, a distinctive feature of influenza C virions is important for virus budding. We used super resolution microscopy and found 250-nm sized HEF clusters at the plasma membrane of transfected cells, which were insensitive to cholesterol extraction and cytochalasin treatment. Overexpression of either CM1, CM2, or HEF caused the release of membrane-enveloped particles. Cryo-electron microscopy of the latter revealed spherical vesicles exhibiting the hexagonal HEF clusters. We subsequently used reverse genetics to identify elements in HEF required for this clustering. We found that deletion of the short cytoplasmic tail of HEF reduced virus titer and hexagonal HEF arrays, suggesting that an interaction with CM1 stabilizes the HEF clusters. In addition, we substituted amino acids at the surface of the closed HEF conformation and identified specific mutations that prevented virus rescue, others reduced virus titers and the number of HEF clusters in virions. Finally, mutation of two regions that mediate contacts between trimers in the in-situ structure of HEF was shown to prevent rescue of infectious virus particles. Mutations at residues thought to mediate lateral interactions were revealed to promote intracellular trafficking defects. Taken together, we propose that lateral interactions between the ectodomains of HEF trimers are a driving force for virus budding, although CM2 and CM1 also play important roles in this process.

Temporal and Gene Reassortment Analysis of Influenza C Virus Outbreaks in Hong Kong, SAR, China

From 2014 to week 07/2020 the Centre for Health Protection in Hong Kong conducted screening for influenza C virus (ICV). A retrospective analysis of ICV detections to week 26/2019 revealed persistent low-level circulation with outbreaks occurring biennially in the winters of 2015 to 2016 and 2017 to 2018 (R. S. Daniels et al., J Virol 94:e01051-20, 2020, https://doi.org/10.1128/JVI.01051-20). Here, we report on an outbreak occurring in 2019 to 2020, reinforcing the observation of biennial seasonality in Hong Kong. All three outbreaks occurred in similar time frames, were subsequently dwarfed by seasonal epidemics of influenza types A and B, and were caused by similar proportions of C/Kanagawa/1/76 (K)-lineage and C/São Paulo/378/82 S1- and S2-sublineage viruses. Ongoing genetic drift was observed in all genes, with some evidence of amino acid substitution in the hemagglutinin-esterase-fusion (HEF) glycoprotein possibly associated with antigenic drift. A total of 61 ICV genomes covering the three outbreaks were analyzed for reassortment, and 9 different reassortant constellations were identified, 1 K-lineage, 4 S1-sublineage, and 4 S2-sublineage, with 6 of these being identified first in the 2019-1920 outbreak (2 S2-lineage and 4 S1-lineage). The roles that virus interference/enhancement, ICV persistent infection, genome evolution, and reassortment might play in the observed seasonality of ICV in Hong Kong are discussed. IMPORTANCE Influenza C virus (ICV) infection of humans is common, with the great majority of people being infected during childhood, though reinfection can occur throughout life. While infection normally results in "cold-like" symptoms, severe disease cases have been reported in recent years. However, knowledge of ICV is limited due to poor systematic surveillance and an inability to propagate the virus in large amounts in the laboratory. Following recent systematic surveillance in Hong Kong SAR, China, and direct ICV gene sequencing from clinical specimens, a 2-year cycle of disease outbreaks (epidemics) has been identified, with gene mixing playing a significant role in ICV evolution. Studies like those reported here are key to developing an understanding of the impact of influenza C virus infection in humans, notably where comorbidities exist and severe respiratory disease can develop.

In situ structure and organization of the influenza C virus surface glycoprotein

The lipid-enveloped influenza C virus contains a single surface glycoprotein, the haemagglutinin-esterase-fusion (HEF) protein, that mediates receptor binding, receptor destruction, and membrane fusion at the low pH of the endosome. Here we apply electron cryotomography and subtomogram averaging to describe the structural basis for hexagonal lattice formation by HEF on the viral surface. The conformation of the glycoprotein in situ is distinct from the structure of the isolated trimeric ectodomain, showing that a splaying of the membrane distal domains is required to mediate contacts that form the lattice. The splaying of these domains is also coupled to changes in the structure of the stem region which is involved in membrane fusion, thereby linking HEF's membrane fusion conformation with its assembly on the virus surface. The glycoprotein lattice can form independent of other virion components but we show a major role for the matrix layer in particle formation.

Development and characterization of standard reagents for cell-based prepandemic influenza vaccine products

Outbreaks of infection by novel avian influenza virus strains in humans cause public health issues worldwide, and the development of vaccines against such novel strains is the most effective method for the prevention of these virus outbreaks. All types of vaccines must be tested for potency before use; thus, quantitative potency assays are needed for influenza vaccines. The single radial immunodiffusion (SRID) assay is considered the gold standard for quantification of influenza virus antigens, and the SRID reference reagents are essential for the determination of vaccine potency. However, it remains debatable whether reference reagents derived from egg-based vaccine platforms can be used to precisely quantify non-egg-derived vaccines; thus, influenza vaccine production using cell-based platforms has attracted increasing attention. To evaluate the utility of reference reagents derived from a cell-based influenza vaccine platform, we prepared cell-based reference reagents from MDCK cell-grown viruses and compared them with egg-derived reference reagents. A primary liquid standard (PLS) was purified from cell-derived candidate influenza vaccine viruses, and hemagglutinin (HA) antigen content was determined by a densitometric method. The produced PLS could be stored at 4°C for more than 10 months. We also established a simple HA protein purification method for goat antiserum preparation, and the performance of the resulting antiserum was compared to that of standard reagents obtained using different production platforms. The results of this study indicate that these reference reagents can be used for both cell-based and egg-based production platforms and that the differences between these two types of platforms are negligible.

Epidemiology and Clinical Characteristics of Influenza C Virus

Influenza C virus (ICV) is a common yet under-recognized cause of acute respiratory illness. ICV seropositivity has been found to be as high as 90% by 7-10 years of age, suggesting that most people are exposed to ICV at least once during childhood. Due to difficulty detecting ICV by cell culture, epidemiologic studies of ICV likely have underestimated the burden of ICV infection and disease. Recent development of highly sensitive RT-PCR has facilitated epidemiologic studies that provide further insights into the prevalence, seasonality, and course of ICV infection. In this review, we summarize the epidemiology and clinical characteristics of ICV.

Influenza C and D Viruses Package Eight Organized Ribonucleoprotein Complexes

Influenza A and B viruses have eight-segmented, single-stranded, negative-sense RNA genomes, whereas influenza C and D viruses have seven-segmented genomes. Each genomic RNA segment exists in the form of a ribonucleoprotein complex (RNP) in association with nucleoproteins and an RNA-dependent RNA polymerase in virions. Influenza D virus was recently isolated from swine and cattle, but its morphology is not fully studied. Here, we examined the morphological characteristics of D/bovine/Yamagata/10710/2016 (D/Yamagata) and C/Ann Arbor/50 (C/AA), focusing on RNPs packaged within the virions. By scanning transmission electron microscopic tomography, we found that more than 70% of D/Yamagata and C/AA virions packaged eight RNPs arranged in the "1+7" pattern as observed in influenza A and B viruses, even though type C and D virus genomes are segmented into only seven segments. These results imply that influenza viruses generally package eight RNPs arranged in the "1+7" pattern regardless of the number of RNA segments in their genome.IMPORTANCE The genomes of influenza A and B viruses are segmented into eight segments of negative-sense RNA, and those of influenza C and D viruses are segmented into seven segments. For progeny virions to be infectious, each virion needs to package all of their genomic segments. Several studies support the conclusion that influenza A and B viruses selectively package eight distinct genomic RNA segments; however, the packaging of influenza C and D viruses, which possess seven segmented genomes, is less understood. By using electron microscopy, we showed that influenza C and D viruses package eight RNA segments just as influenza A and B viruses do. These results suggest that influenza viruses prefer to package eight RNA segments within virions independent of the number of genome segments.

Novel Influenza D virus: Epidemiology, pathology, evolution and biological characteristics

In 2011, a new virus was isolated from pigs with influenza-like symptoms and subsequently also from cattle, which are the main reservoir of the virus. It is similar to Influenza C virus (ICV), a (predominantly) human pathogen, causing respiratory disease in children. Since the virus is unable to reassort with ICV (and based on several other criteria as discussed in the text) it is now officially named as Influenzavirus D (IDV), a new genus of the Orthomyxoviridae. We summarize the epidemiology, pathology and evolution of IDV and its biological characteristics with emphasis on the only glycoprotein HEF. Based on the limited data available we finally consider whether IDV represent a public health threat.

The role of stearate attachment to the hemagglutinin-esterase-fusion glycoprotein HEF of influenza C virus

The only spike of influenza C virus, the hemagglutinin-esterase-fusion glycoprotein (HEF) combines receptor binding, receptor hydrolysis and membrane fusion activities. Like other hemagglutinating glycoproteins of influenza viruses HEF is S-acylated, but only with stearic acid at a single cysteine located at the cytosol-facing end of the transmembrane region. Previous studies established the essential role of S-acylation of hemagglutinin for replication of influenza A and B virus by affecting budding and/or membrane fusion, but the function of acylation of HEF was hitherto not investigated. Using reverse genetics we rescued a virus containing non-stearoylated HEF, which was stable during serial passage and showed no competitive fitness defect, but the growth rate of the mutant virus was reduced by one log. Deacylation of HEF does neither affect the kinetics of its plasma membrane transport nor the protein composition of virus particles. Cryo-electron microscopy showed that the shape of viral particles and the hexagonal array of spikes typical for influenza C virus were not influenced by this mutation indicating that virus budding was not disturbed. However, the extent and kinetics of haemolysis were reduced in mutant virus at 37°C, but not at 33°C, the optimal temperature for virus growth, suggesting that non-acylated HEF has a defect in membrane fusion under suboptimal conditions.

Hemagglutinin-esterase-fusion (HEF) protein of influenza C virus

Influenza C virus, a member of the Orthomyxoviridae family, causes flu-like disease but typically only with mild symptoms. Humans are the main reservoir of the virus, but it also infects pigs and dogs. Very recently, influenza C-like viruses were isolated from pigs and cattle that differ from classical influenza C virus and might constitute a new influenza virus genus. Influenza C virus is unique since it contains only one spike protein, the hemagglutinin-esterase-fusion glycoprotein HEF that possesses receptor binding, receptor destroying and membrane fusion activities, thus combining the functions of Hemagglutinin (HA) and Neuraminidase (NA) of influenza A and B viruses. Here we briefly review the epidemiology and pathology of the virus and the morphology of virus particles and their genome. The main focus is on the structure of the HEF protein as well as on its co- and post-translational modification, such as N-glycosylation, disulfide bond formation, S-acylation and proteolytic cleavage into HEF1 and HEF2 subunits. Finally, we describe the functions of HEF: receptor binding, esterase activity and membrane fusion.

Role of the CM2 protein in the influenza C virus replication cycle

CM2 is the second membrane protein of influenza C virus. Although its biochemical characteristics, coding strategy, and properties as an ion channel have been extensively studied, the role(s) of CM2 in the virus replication cycle remains to be clarified. In order to elucidate this role, in the present study we generated CM2-deficient influenza C virus-like particles (VLPs) and examined the VLP-producing 293T cells, VLPs, and VLP-infected HMV-II cells. Quantification of viral RNA (vRNA) in the VLPs by real-time PCR revealed that the CM2-deficient VLPs contain approximately one-third of the vRNA found in wild-type VLPs although no significant differences were detected in the expression levels of viral components in VLP-producing cells or in the number and morphology of the generated VLPs. This finding suggests that CM2 is involved in the genome packaging process into VLPs. Furthermore, HMV-II cells infected with CM2-deficient VLPs exhibited significantly reduced reporter gene expression. Although CM2-deficient VLPs could be internalized into HMV-II cells as efficiently as wild-type VLPs, a smaller amount of vRNA was detected in the nuclear fraction of CM2-deficient VLP-infected cells than in that of wild-type VLP-infected cells, suggesting that the uncoating process of the CM2-deficient VLPs in the infected cells did not proceed in an appropriate manner. Taken together, the data obtained in the present study indicate that CM2 has a potential role in the genome packaging and uncoating processes of the virus replication cycle.

Structure, function and evolution of the hemagglutinin-esterase proteins of corona- and toroviruses

Virus attachment to host cells is mediated by dedicated virion proteins, which specifically recognize one or, at most, a limited number of cell surface molecules. Receptor binding often involves protein-protein interactions, but carbohydrates may serve as receptor determinants as well. In fact, many different viruses use members of the sialic acid family either as their main receptor or as an initial attachment factor. Sialic acids (Sias) are 9-carbon negatively-charged monosaccharides commonly occurring as terminal residues of glycoconjugates. They come in a large variety and are differentially expressed in cells and tissues. By targeting specific Sia subtypes, viruses achieve host cell selectivity, but only to a certain extent. The Sia of choice might still be abundantly present on non-cell associated molecules, on non-target cells (including cells already infected) and even on virus particles themselves. This poses a hazard, as high-affinity virion binding to any of such "false'' receptors would result in loss of infectivity. Some enveloped RNA viruses deal with this problem by encoding virion-associated receptor-destroying enzymes (RDEs). These enzymes make the attachment to Sia reversible, thus providing the virus with an escape ticket. RDEs occur in two types: neuraminidases and sialate-O-acetylesterases. The latter, originally discovered in influenza C virus, are also found in certain nidoviruses, namely in group 2 coronaviruses and in toroviruses, as well as in infectious salmon anemia virus, an orthomyxovirus of teleosts. Here, the structure, function and evolution of viral sialate-O-acetylesterases is reviewed with main focus on the hemagglutinin-esterases of nidoviruses.

Identification of an amino acid residue on influenza C virus M1 protein responsible for formation of the cord-like structures of the virus

Influenza C virus-like particles (VLPs) have been generated from cloned cDNAs. A cDNA of the green fluorescent protein (GFP) gene in antisense orientation was flanked by the 5' and 3' non-coding regions of RNA segment 5 of the influenza C virus. The cDNA cassette was inserted between an RNA polymerase I promoter and terminator of the Pol I vector. This plasmid DNA was transfected into 293T cells together with plasmids encoding virus proteins of C/Ann Arbor/1/50 or C/Yamagata/1/88. Transfer of the supernatants of the transfected 293T cells to HMV-II cells resulted in GFP expression in the HMV-II cells. The quantification of the GFP-positive HMV-II cells indicated the presence of approximately 10(6) VLPs (ml supernatant)(-1). Cords 50-300 microm in length were observed on transfected 293T cells, although the cords were not observed when the plasmid for M1 protein of C/Ann Arbor/1/50 was replaced with that of C/Taylor/1233/47. A series of transfection experiments with plasmids encoding M1 mutants of C/Ann Arbor/1/50 or C/Taylor/1233/47 showed that an amino acid at residue 24 of the M1 protein is responsible for cord formation. This finding provides direct evidence for a previous hypothesis that M1 protein is involved in the formation of cord-like structures protruding from the C/Yamagata/1/88-infected cells. Evidence was obtained by electron microscopy that transfected cells bearing cords produced filamentous VLPs, suggesting the potential role of the M1 protein in determining the filamentous/spherical morphology of influenza C virus.

Hemagglutinin-esterase, a novel structural protein of torovirus

We have characterized the 3'-most 3 kb of the genome of bovine torovirus (BoTV) strain Breda. A novel 1.2-kb gene, located between the genes for the membrane and nucleocapsid proteins, was identified. This gene, the 3'-most 0.5 kb of which is also present in the genome of the equine torovirus isolate Berne virus (BEV), codes for a class I membrane protein displaying 30% sequence identity with the hemagglutinin-esterases (HEs) of coronaviruses and influenza C viruses. Heterologous expression of the BoTV HE gene yielded a 65,000-molecular weight N-glycosylated protein displaying acetylesterase activity. Serologic evidence indicates that the HE homolog is expressed during the natural infection and represents a prominent antigen. By using an antiserum raised against residues 13 to 130 of HE, the HE protein was detected in radioiodinated, sucrose gradient-purified BoTV preparations. Formal evidence that HE is a structural protein was provided by immunoelectron microscopy. In addition to the large, 17- to 20-nm spikes, BoTV virions possess shorter surface projections (6 nm on average). We postulate that these surface projections, which are absent from the BEV virion, are composed of the BoTV HE homolog. The HE gene, which has now been demonstrated in three different virus genera, is a showpiece example of modular evolution.

Identification of a 40-kDa cell surface sialoglycoprotein with the characteristics of a major influenza C virus receptor in a Madin-Darby canine kidney cell line

Infection of cells by influenza C virus is known to be initiated by virus attachment to cell surface glycoconjugates containing N-acetyl-9-O-acetylneuraminic acid. Using an in vitro virus binding assay, we have detected this carbohydrate on several glycoproteins of Madin-Darby canine kidney cells (type I), a polarized epithelial cell line permissive for infection with influenza C virus. Among these proteins, only one was found to be present to a significant extent on the cell surface. This protein, gp40, was characterized as an O-glycosylated (mucin-type) integral membrane protein of 40 kDa, which was predominantly localized on the apical plasma membrane of filter-grown cells. It is a major cell surface sialoglycoprotein in this cell line and was shown to be subject to constitutive and rapid endocytosis. Thus, this glycoprotein can mediate not only the binding of influenza C virus to the cell surface, but also its delivery to endosomes, where penetration occurs by membrane fusion. Other highly sialylated cell surface glycoproteins were also detected but did not mediate influenza C virus binding to a significant extent, indicating that only gp40 contains 9-O-acetylated sialic acids.

Ultrastructural evidence of an interaction between Env and Gag proteins during assembly of HIV type 1

Assembly and budding of retroviruses is believed to involve a complex interaction of envelope and capsid proteins at the host cell membrane. The nature of these interactions is, however, incompletely understood. Studies of the topography of the surface of HIV-1 have shown that the envelope glycoprotein projections (knobs) are arranged in a T = 7 levo rotational symmetry. Similarly, an icosahedral structure has been suggested for the p17 matrix of HIV-1. In an effort to investigate whether there is a structural interaction between these molecules, virions whose maturation was blocked by an inhibitor of HIV protease were studied using cytochemistry, morphometry, and 2D fast Fourier transform image enhancement. Analysis of the relationship between core morphology and the topographic distribution of envelope glycoprotein projections on HIV-1 provided structural evidence of an interaction between Env and Gag proteins. Furthermore, image enhancement revealed a periodic substructure in the Pr55gag plaque. Taken together, the data suggest an interaction between Pr55gag and the gp120-gp41 complex during assembly and budding of HIV-1. This interaction may, in part, contribute to determining the amount of Env glycoprotein that will be incorporated into a virion, and therefore play a role in the biology of HIV-1.

Increased influenza A virus sialidase activity with N-acetyl-9-O-acetylneuraminic acid-containing substrates resulting from influenza C virus O-acetylesterase action

Influenza virus type C (Johannesburg/1/66) was used as a source for the enzyme O-acetylesterase (EC 3.1.1.53) with several natural sialoglycoconjugates as substrates. The resulting products were immediately employed as substrates using influenza virus type A [(Singapore/6/86) (H1N1) or Shanghai/11/87 (H3N2)] as a source for sialidase (neuraminidase, EC 3.2.1.18). A significant increase in the percentage of sialic acid released was found when the O-acetyl group was cleaved by O-acetylesterase activity from certain substrates (bovine submandibular gland mucin, rat serum glycoproteins, human saliva glycoproteins, mouse erythrocyte stroma, chick embryonic brain gangliosides and bovine brain gangliosides). A common feature of all these substrates is that they contain N-acetyl-9-O-acetylneuraminic acid residues. By contrast, no significant increase in the release of sialic acid was detected when certain other substrates could not be de-O-acetylated by the action of influenza C esterase, either because they lacked O-acetylsialic acid (human glycophorin A, alpha 1-acid glycoprotein from human serum, fetuin and porcine submandibular gland mucin) or because the 4-O-acetyl group was scarcely cleaved by the viral O-acetylesterase (equine submandibular gland mucin). The biological significance of these facts is discussed, relative to the infective capacity of influenza C virus.

Location of neutralizing epitopes on the hemagglutinin-esterase protein of influenza C virus

Neutralization-resistant variants of influenza C/Ann Arbor/1/50 virus were selected with monoclonal antibodies against four different antigenic sites on the hemagglutinin-esterase (HE) glycoprotein, and their HE genes were sequenced to identify amino acid residues important for the integrity of each site. Twelve different amino acid substitutions in a total of 18 antigenic variants were all located on the HE1 subunit. Although variants for antigenic site A-2 had a change at position 367, all substitutions in the variants for sites A-1, A-3, and A-4 occurred in the central region of the HE1 spanning amino acid positions 178 to 283. Furthermore, it was found that many of the substitutions in the variants selected with antibodies to sites A-1 and A-3 were clustered within or near one of the three variable regions revealed previously by comparing amino acid sequences of the HEs among various influenza C isolates (Buonagurio, D. A., Nakada, S., Fitch, W. M., and Palese, P., Virology 146, 221-232, 1985). The antigenic variants were also examined for their ability to agglutinate chicken and human erythrocytes in order to obtain information concerning the receptor-binding site on the HE molecule. The results suggested that the amino acid changes at residues 178, 186, 187, 190, 206, 212, and 226 decreased the hemagglutinating activity whereas those at residues 245, 266, and 283 produced an opposite effect.

A single point mutation of the influenza C virus glycoprotein (HEF) changes the viral receptor-binding activity

From strain JHB/1/66 of influenza C virus a mutant was derived with a change in the cell tropism. The mutant was able to grow in a subline of Madin-Darby canine kidney cells (MDCK II) which is resistant to infection by the parent virus due to a lack of receptors. Inactivation of cellular receptors by either neuraminidase or acetylesterase and generation of receptors by resialylation of cells with N-acetyl-9-O-acetylneuraminic acid (Neu5,9Ac2) indicated that 9-O-acetylated sialic acid is a receptor determinant for both parent and mutant virus. However, the mutant required less Neu5,9Ac2 on the cell surface for virus attachment than the parent virus. The increased binding efficiency enabled the mutant to infect cells with a low content of 9-O-acetylated sialic acid which were resistant to the parent virus. By comparing the nucleotide sequences of the glycoprotein (HEF) genes of the parent and the mutant virus only a single point mutation could be identified on the mutant gene. This mutation at nucleotide position 872 causes an amino acid exchange from threonine to isoleucine at position 284 on the amino acid sequence. Sequence similarity with a stretch of amino acids involved in the receptor-binding pocket of the influenza A hemagglutinin suggests that the mutation site on the influenza C glycoprotein (HEF) is part of the receptor-binding site.

Activity of influenza C virus O-acetylesterase with O-acetyl-containing compounds

Influenza C virus (strain C/Johannesburg/1/66) was grown, harvested, purified and used as source for the enzyme O-acetylesterase (N-acyl-O-acetylneuraminate O-acetylhydrolase; EC 3.1.1.53). This activity was studied and characterized with regard to some new substrates. The pH optimum of the enzyme is around 7.6, its stability at different pH values shows a result similar to that of the pH optimum, and its activity is well maintained in the pH range from 7.0 to 8.5 (all these tests were performed with 4-nitrophenyl acetate as substrate). Remarkable differences were found in the values of both Km and Vmax, with the synthetic substrates 4-nitrophenyl acetate, 2-nitrophenyl acetate, 4-methylumbelliferyl acetate, 1-naphthyl acetate and fluorescein diacetate. The use of 4-nitrophenyl acetate, 4-methylumbelliferyl acetate or 1-naphthyl acetate as substrate seems to be convenient for routine work, but it is better to carry out the measurements in parallel with those on bovine submandibular gland mucin (the latter is a natural and commercially available substrate). It was found that 4-acetoxybenzoic acid, as well as the methyl ester of 2-acetoxybenzoic acid, but not 2-acetoxybenzoic acid itself, are cleaved by this enzyme. Triacetin, di-O-acetyladenosine, tri-O-acetyladenosine, and di-O-acetyl-N-acetyladenosine phosphate, hitherto unreported as substrates for this viral esterase, are hydrolysed at different rates by this enzyme. We conclude that the O-acetylesterase from influenza C virus has a broad specificity towards both synthetic and natural non-sialic acid-containing substrates. Zn2+, Mn2+ and Pb2+ (as their chloride salts), N-acetylneuraminic acid, 4-methyl-umbelliferone and 2-acetoxybenzoic acid (acetylsalicylic acid) did not act as inhibitors.

Structure and function of the HEF glycoprotein of influenza C virus

Soon after the first isolation of an influenza C virus from a patient, it became obvious that this virus differs from other myxoviruses in several aspects. Pronounced differences have been observed in the interactions between the virus and cell surfaces, suggesting that influenza C virus attaches to the receptors different from those recognized by other myxoviruses. While influenza A and B viruses agglutinate erythrocytes from many species, including humans, the spectrum of erythrocytes agglutinated by influenza C virus is much more restricted. Erythrocytes from rats, mice, and adult chickens are suitable for hemagglutination and hemadsorption tests; cells from other species, however, react not at all or only poorly with influenza C virus. Differences are also observed so far as hemagglutination inhibitors are concerned. A variety of glycoproteins have been shown to prevent influenza A and B viruses from agglutinating erythrocytes. In the case of influenza C virus, rat serum was for a long time the only known hemagglutination inhibitor. A difference in the receptors for influenza C virus and other myxo-viruses was also suggested by studies on the receptor-destroying enzyme. The ability of influenza C virus to inactivate its own receptors was reported soon after the first isolation of this virus from a patient. However, the influenza C enzyme did not affect the receptors of other myxoviruses and, conversely, the receptor-destroying enzyme of either of the latter viruses was unable to inactivate the receptors for influenza C virus on erythrocytes. While the enzyme of influenza A and B virus was characterized as a neuraminidase in the 1950s, even with refined methodology no such activity was detectable with influenza C virus. It is now known that both the receptor-binding and receptor-destroying activities, as well as the fusion activity of influenza C virus are mediated by the only glycoprotein present on the surface of the virus particle. The structure and functions of this protein, which is designated as HEF, are reviewed in this chapter.

Characterization of the cord-like structures emerging from the surface of influenza C virus-infected cells

When HMV-II cells (a human malignant melanoma cell line) infected with a newly isolated influenza C strain (Yamagata/1/88) were examined by simple light microscopy, it was found that a large number of cord-like structures which had lengths up to about 500 microns or greater were emerging from the cell surface. The existence of viral glycoproteins (hemagglutinin-esterase, HE) on the surface of these huge structures was confirmed by hemadsorption experiments with erythrocytes from a variety of species as well as by immunofluorescent staining with anti-HE monoclonal antibody. Furthermore, electron microscopy revealed that numerous filamentous particles in the process of budding, each covered with a layer of surface projections approximately 13 nm in length, aggregated with their long axes parallel to form a cord-like structure visible under a light microscope. An electron-dense layer, which presumably consists of membrane protein (M), was seen in cross-sections of all filamentous virions whereas internal nucleocapsids were rarely seen. SDS-polyacrylamide gel electrophoresis of the purified cords also showed that they contained HE and M polypeptides but not nucleoprotein, confirming that long filamentous particles are mostly devoid of nucleocapsids. The emergence of cords on the cell surface was observed in various cell cultures infected with C/Yamagata/1/88 though their number and length varied markedly depending on cell type. The production of cord-like structures was also evident in HMV-II cells infected with any of several different influenza C strains, which suggests that the cord formation is a common feature of influenza C virus group.

The hemagglutinating glycoproteins of influenza B and C viruses are acylated with different fatty acids

We present evidence that the hemagglutinin (HA) of influenza B virus and the glycoprotein of influenza C virus (HEF) are acylated. The fatty acid linkage is sensitive to treatment with hydroxylamine and mercaptoethanol, which points to a labile thioester-type linkage. The HA of influenza B virus contains mainly palmitic acid, whereas the HEF glycoprotein of influenza C virus is acylated with stearic acid which has not been observed before as the prevailing fatty acid in viral or cellular acyl proteins.

Structure and orientation of expressed bovine coronavirus hemagglutinin-esterase protein

The sequence of the hemagglutinin-esterase (HE) gene for the Mebus strain of bovine coronavirus was obtained from cDNA clones, and its deduced product is a 47,700-kilodalton apoprotein of 424 amino acids. Expression of the HE protein in vitro in the presence of microsomes revealed N-terminal signal peptide cleavage and C-terminal anchorage but not disulfide-linked dimerization. Dimerization was observed only after expression in vivo, during which HE was also transported to the cell surface.

Expression of the spleen focus-forming virus envelope gene in a polarized epithelial cell line

Friend spleen focus-forming virus (F-SFFV) encodes a glycoprotein designated gp52, which is defective in its intracellular transport and accumulates in the rough endoplasmic reticulum. Only 3-5% of the mature form of gp52 eventually reaches the cell surface. Compared to transport-competent murine leukemia virus (MuLV) glycoproteins, the gp52 molecule exhibits several structural differences which may have resulted in the possible loss of signals required for transport to the cell surface. To determine the effect of these alterations on the specific sites of surface expression of the molecule, the SFFV env gene was expressed from a vaccinia virus recombinant in a polarized epithelial cell line in which retrovirus glycoproteins are expressed exclusively on basolateral surfaces. We also determined the site of expression of a chimeric env protein which contains the external domain of SFFV gp52 the transmembrane, and the cytoplasmic tail residues of Friend MuLV. The wild-type and chimeric env gene products were defective in transport, and remained primarily in an unprocessed form in MDCK cells or CV-1 cells. However, both glycoproteins were detected at low levels on the basolateral surfaces of MDCK cells, a line of polarized epithelial cells. These results indicate that the presence or absence of a cytoplasmic tail as well as a 585-base deletion in the external domain has no affect on the site of polarized expression of a murine retrovirus glycoprotein.

Isolation of the influenza C virus glycoprotein in a soluble form by bromelain digestion

The spike glycoprotein of influenza C/Johannesburg/1/66 was isolated in a soluble form by digestion of MDCK cell-grown virions with bromelain. The whole ectodomain of the glycoprotein could be recovered with an apparent molecular weight of 75,000 daltons determined in SDS-PAGE. Comparison to Triton X-100-isolated glycoprotein revealed that a C-terminal peptide of 3000-4500 daltons must have remained in the viral membrane. When purified by sucrose density gradient centrifugation the glycoprotein sedimented with a sedimentation coefficient of 10 S, indicating a molecular weight of 206,000 daltons, which is consistent with a trimeric structure of the spike molecule. The trimeric form was stabilized in sucrose gradients by Ca2+ ions. Bromelain digestion of virions with uncleaved glycoprotein, grown in MDCK cells without trypsin, produced two disulphide-linked subunits with similar electrophoretic mobilities in SDS-PAGE to the biologically active glycoprotein. The smaller subunit differed from the product cleaved in vivo (gp 30) by the presence of an additional arginine residue at the N-terminus. The soluble glycoprotein appears to possess both receptor-binding and receptor-destroying enzyme activities, as isolated glycoprotein inhibited hemagglutination of intact influenza C virions and showed RDE activity in an in vitro test. Glycoprotein exposed to low pH, which was sensitive to trypsin digestion, also demonstrated both these biological activities. Glycoprotein-mediated hemolysis could not be observed.

Serine 71 of the glycoprotein HEF is located at the active site of the acetylesterase of influenza C virus

The acetylesterase of influenza C virus has been reported recently to be inhibited by diisopropylfluorophosphate (DFP) [Muchmore EA, Varki A (1987) Science 236: 1293-1295]. As this inhibitor is known to bind covalently to the serine in the active site of serine esterases, we attempted to determine the serine in the active site of the influenza C acetylesterase. Incubation of purified influenza C virus with 3H-DFP resulted in the selective labelling of the influenza C glycoprotein HEF. The labelled glycoprotein was isolated from a SDS-polyacrylamide gel. Following reduction and carboxymethylation, tryptic peptides of HEF were prepared and analyzed by reversed phase HPLC. The peptide containing the 3H-DFP was subjected to sequence analysis. The amino acids determined from the NH2-terminus were used to locate the peptide on the HEF polypeptide. Radiosequencing revealed that 3H-DFP is attached to amino acid 17 of the tryptic peptide. These results indicate that serine 71 is the active-site serine of the acetylesterase of influenza C virus.

The molecular biology of influenza virus pathogenicity

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

The influenza C virus glycoprotein (HE) exhibits receptor-binding (hemagglutinin) and receptor-destroying (esterase) activities

A cDNA copy of RNA segment 4 of influenza C/Cal/78 virus was cloned into an SV40 vector and expressed in CV-1 cells. The gene product expressed from the SV40 recombinant virus was immunoprecipitated by monoclonal antibodies directed against the influenza C virus glycoprotein. Cells infected with the recombinant virus also exhibited C virus-specific hemagglutinin and O-acetylesterase activity. This suggests that the same C virus protein is associated with receptor-binding as well as receptor-destroying activity. The latter viral activity was measured using as substrates bovine submaxillary mucin or a low molecular weight compound p-nitrophenylacetate. In analogy to the parainfluenza virus HN protein, the influenza C virus glycoprotein was termed HE, because it possesses hemagglutinin and esterase (receptor-destroying) activity.

The large external domain is sufficient for the correct sorting of secreted or chimeric influenza virus hemagglutinins in polarized monkey kidney cells

MA104.11 rhesus kidney cells express several characteristics of polarized epithelial cells, including the formation of "domes" on impermeable substrates, the establishment of a transmonolayer electrical resistance when grown on collagen gels, the polarized maturation of influenza and vesicular stomatitis viruses, and the expression of the glycoproteins of those viruses at a single surface domain. The polarized expression of the influenza virus hemagglutinin (HA) is maintained in MA104.11 cells infected with SV40-derived vectors carrying a cDNA gene for either the wild-type influenza virus HA, a truncated HA gene encoding a secreted form of HA (HAsec), or a chimeric gene encoding a hybrid protein with the external domain of the HA and the transmembrane and cytoplasmic domains of the vesicular stomatitis virus G protein (HAG). Thus, the recognition event separating glycoproteins, such as HA, destined for the apical surface from proteins, such as G, destined for the basolateral membranes involves features of the external domains of the proteins. The transmembrane and cytoplasmic domains of HA have no role in this process.

Nonpolarized expression of a secreted murine leukemia virus glycoprotein in polarized epithelial cells

Vaccinia virus recombinants were generated which express the intact gp70/p15E of Friend mink cell focus inducing virus (F-MCFV) or truncated forms of the glycoprotein that lack the transmembrane and cytoplasmic domains. The transport of the intact and truncated envelope glycoproteins to apical or basolateral surfaces was studied in the polarized epithelial MDCK cell line. Infection of MDCK cells with the recombinant expressing the intact F-MCFV envelope glycoprotein resulted in transport exclusively to the basolateral surfaces, whereas the recombinant expressing the truncated glycoprotein was found to be secreted from both the apical and basolateral surfaces. Thus removal of the transmembrane and cytoplasmic domains of the p15E protein results in a loss of directional transport to the basolateral membrane of polarized epithelial cells.

Surface expression of influenza virus neuraminidase, an amino-terminally anchored viral membrane glycoprotein, in polarized epithelial cells

We have investigated the site of surface expression of the neuraminidase (NA) glycoprotein of influenza A virus, which, in contrast to the hemagglutinin, is bound to membranes by hydrophobic residues near the NH2-terminus. Madin-Darby canine kidney or primary African green monkey kidney cells infected with influenza A/WSN/33 virus and subsequently labeled with monoclonal antibody to the NA and then with a colloidal gold- or ferritin-conjugated second antibody exhibited specific labeling of apical surfaces. Using simian virus 40 late expression vectors, we also studied the surface expression of the complete NA gene (SNC) and a truncated NA gene (SN10) in either primary or a polarized continuous line (MA104) of African green monkey kidney cells. The polypeptides encoded by the cloned NA cDNAs were expressed on the surface of both cell types. Analysis of [3H]mannose-labeled polypeptides from recombinant virus-infected MA104 cells showed that the products of cloned NA cDNA comigrated with glycosylated NA from influenza virus-infected cells. Both the complete and the truncated glycoproteins were found to be preferentially expressed on apical plasma membranes, as detected by immunogold labeling. These results indicate that the NA polypeptide contains structural features capable of directing the transport of the protein to apical cell surfaces and the first 10 amino-terminal residues of the NA polypeptide are not involved in this process.

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.

Structure and variation of the influenza C glycoprotein

The amino acid sequence of the influenza C/JHB/1/66 glycoprotein was predicted from sequence analysis of cloned DNA. The glycoprotein exhibited several similarities to the haemagglutinin (HA) glycoproteins of influenza A and B viruses, although its overall homology to these glycoproteins was low. A comparison between the sequences of C/JHB/1/66 and C/Cal/78 strains revealed 96% homology. The nucleotide and amino acid changes appeared to be non-random, with a high proportion occurring between amino acid positions 182-212. A comparison between the JHB/1/66 glycoprotein and the influenza A/X31 HA sequence suggested that this region may be structurally analogous to the 'A' antigenic site of the HA glycoprotein.

Protein and nucleic acid analyses of influenza C viruses isolated from pigs and man

The virus-coded proteins and the genomes of influenza C virus isolates obtained from Chinese pigs in 1981-1982 and of human influenza C virus strains isolated between 1947 and 1981 were compared. Using SDS polyacrylamide gel electrophoresis and one-dimensional peptide mapping we found the virus-coded proteins of the pig influenza C viruses to be similar to those of human influenza C virus strains. The sizes of the genomes of human and pig influenza C viruses were indistinguishable. Genome analysis by oligonucleotide (ON) mapping revealed that the genomes of the pig influenza C viruses were very similar to but not identical with those of human influenza C virus strains. ON changes were found scattered over the whole genome. ON mapping of isolated segments of several influenza C virus strains suggested that two pig strains (C/P/B/10/81 and C/P/B/32/81) are related by a reassortment event which is likely to have occurred in nature. The rate of genome variation in influenza C viruses seemed to be similar to that seen in influenza B, and slower than that recorded for influenza A viruses.

Surface expression of influenza virus neuraminidase, an amino-terminally anchored viral membrane glycoprotein, in polarized epithelial cells

We have investigated the site of surface expression of the neuraminidase (NA) glycoprotein of influenza A virus, which, in contrast to the hemagglutinin, is bound to membranes by hydrophobic residues near the NH2-terminus. Madin-Darby canine kidney or primary African green monkey kidney cells infected with influenza A/WSN/33 virus and subsequently labeled with monoclonal antibody to the NA and then with a colloidal gold- or ferritin-conjugated second antibody exhibited specific labeling of apical surfaces. Using simian virus 40 late expression vectors, we also studied the surface expression of the complete NA gene (SNC) and a truncated NA gene (SN10) in either primary or a polarized continuous line (MA104) of African green monkey kidney cells. The polypeptides encoded by the cloned NA cDNAs were expressed on the surface of both cell types. Analysis of [3H]mannose-labeled polypeptides from recombinant virus-infected MA104 cells showed that the products of cloned NA cDNA comigrated with glycosylated NA from influenza virus-infected cells. Both the complete and the truncated glycoproteins were found to be preferentially expressed on apical plasma membranes, as detected by immunogold labeling. These results indicate that the NA polypeptide contains structural features capable of directing the transport of the protein to apical cell surfaces and the first 10 amino-terminal residues of the NA polypeptide are not involved in this process.

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.

Immunochemistry of variants of influenza virus hemagglutinin and neuraminidase

Influenza is impossible to control by vaccination because of variation in the two surface antigens of the virus, the hemagglutinin (HA) and neuraminidase (NA). This variation is caused by sequence changes in the genes coding these antigens and this article summarizes recent information on the structure of the HA and NA, the way in which these glycoproteins vary and the effects of the changes on the antigenic properties of the virus.

Low resolution structure of the influenza C glycoprotein determined by electron microscopy

The influenza C glycoprotein is clearly seen to be a trimer in specimens prepared with uranyl stains. Three-dimensional reconstructions from naturally occurring hexagonal arrays show that at low resolution (approximately 30 A) the influenza C glycoprotein exhibits similar features to the haemagglutinin glycoprotein of influenza A. Both have a triangular stalk near the membrane. Further from the membrane, the stalk becomes broader and the monomers more separated, leaving an open centre. The molecule narrows at the top. The regions of greatest contact between adjacent trimers in the arrays are situated nearer the distal end of the molecule. These contact zones can be related to equivalent zones on the influenza A haemagglutinin. Differences between the structure of the influenza A haemagglutinin glycoprotein determined by X-ray analysis and reconstructions of the influenza C glycoprotein are greatest at either end of the molecule, where the reconstructions are least reliable. Ordered glycoprotein arrays have not been observed on influenza C virions incubated at low pH. The staining patterns of glycoproteins on intact virions are essentially determined by the pH at which the virus is incubated, and the stain type, but not the pH of the stain.

Influenza C virus hemagglutinin: comparison with influenza A and B virus hemagglutinins

The complete nucleotide sequence of the influenza C/California/78 virus RNA 4 was obtained by using cloned cDNA derived from the RNA segment. This gene is 2,071 nucleotides long and can code for a polypeptide of 654 amino acids. Although there are no convincing sequence homologies between RNA 4 and the hemagglutinin genes of influenza A and B viruses, we suggest, on the basis of structural features, that RNA 4 of the influenza C virus codes for the hemagglutinin. The structural features which are common to the hemagglutinins of influenza A, B, and C viruses include (i) a hydrophobic signal peptide, (ii) an arginine cleavage site between the hemagglutinin 1 and 2 subunits, (iii) hydrophobic regions at the amino and carboxyl termini of the hemagglutinin 2 subunit, and (iv) several conserved cysteine residues. Additional evidence that RNA 4 of influenza C virus codes for the hemagglutinin is that the tripeptide Ile-Phe-Gly, known to be present at the amino terminus of the hemagglutinin 2 subunit of influenza C virus, is encoded by RNA 4 at a point immediately adjacent to the presumptive arginine cleavage site. The lack of primary sequence homology between the influenza C virus hemagglutinin and the influenza A or B virus hemagglutinins, which all have similar functions, might be attributed to convergent rather than divergent evolution. However, the structural similarities among the influenza A, B, and C virus hemagglutinins strongly suggest that the three hemagglutinin genes have diverged from a common precursor.

Structure of the influenza C glycoprotein gene as determined from cloned DNA

The complete nucleotide sequence of RNA segment 4 of influenza C/JHB/1/66 virus has been determined, utilizing cloned cDNA derived from the viral RNA segment. The gene is 2073 nucleotides in length, and can code for a polypeptide of 655 amino acids, which corresponds to the viral glycoprotein. The predicted polypeptide has a molecular weight of 72 063, not including oligosaccharides linked to eight predicted glycosylation sites. The influenza C glycoprotein shares structural features with hemagglutinin (HA) glycoproteins of influenza A and B viruses, including three stretches of hydrophobic amino acids believed to function as a signal sequence, a fusion function, and a membrane anchor. However, a substantial part of the protein lacks direct sequence homology to these HA glycoproteins. The results suggest a more distant evolutionary relationship between influenza C virus and influenza A and B viruses, compared to that between influenza A and B themselves.

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.

Posttranslational modification and intracellular transport of mumps virus glycoproteins

Analysis of the pronase-derived glycopeptides of isolated mumps virus glycoproteins revealed the presence of both complex and high-mannose-type oligosaccharides on the HN and F1 glycoproteins, whereas only high-mannose-type glycopeptides were detected on F2. Endoglycosidase F, a newly described glycosidase that cleaves N-linked high mannose as well as complex oligosaccharides, appeared to completely cleave the oligosaccharides linked to HN and F2, whereas F1 was resistant to the enzyme. Two distinct cleavage products of F2 were observed, suggesting the presence of two oligosaccharide side chains. Tunicamycin was found to reduce the infectious virus yield and inhibit mumps virus particle formation. The two glycoproteins, HN and F, were not found in the presence of the glycosylation inhibitor. However, two new polypeptides were detected, with molecular weights of 63,000 (HNT) and 53,000 (FT), respectively, which may represent nonglycosylated forms of the glycoproteins. Synthesis of the nonglycosylated virus-coded proteins (L, NP, P, M, pI, and pII) was not affected by tunicamycin. The formation of HN oligomers and the proteolytic cleavage of the F protein were found to occur with the same kinetics. Analysis of the time course of appearance of mumps virus glycoproteins on the cell surface suggested that dimerization of HN and cleavage of F occur immediately after their exposure on the plasma membrane.

Influenza virus hemagglutinin expression is polarized in cells infected with recombinant SV40 viruses carrying cloned hemagglutinin DNA

Primary cell cultures of African Green monkey kidney (AGMK) contain polarized epithelial cells in which influenza virus matures predominantly at the apical surfaces above tight junctions. Influenza virus glycoproteins were found to be localized at the same membrane domain from which the virus budded. When polarized primary AGMK cells were infected with recombinant SV40 viruses containing DNA coding for either an influenza virus H1 or H2 subtype hemagglutinin (HA), the HA proteins were preferentially expressed at the apical surface in a manner identical to that observed in influenza virus-infected cells. Thus, cellular mechanisms for sorting membrane glycoproteins recognize some structural feature of the HA glycoprotein itself, and other viral proteins are not necessary for this process.

Evidence of a soluble substrate for the receptor-destroying enzyme of influenza C virus

Influenza C virus contains a hemagglutinin and a receptor-destroying enzyme (RDE) whose specificities remain undetermined. In rat serum, there is a molecule that binds specifically to C virus, inhibiting its hemagglutinin. The complex between C virus and the rat serum inhibitor (RSI) was determined to be stable at 4 degrees C, but was disrupted within 20 to 90 min at 23 or 37 degrees C. Virus emerged from the complex with numerous functions intact, whereas the RSI at this point was inactivated, i.e., incapable of further inhibitory reactions with C virus. RSI could not be inactivated at these temperatures by nonviral components of allantoic fluid of infected chicken embryos; however, RSI inactivation was achieved by preparations of sucrose gradient-purified virus. Neutralization of viral hemagglutination activity with antiviral antibody protected the RSI from inactivation. RSI inactivation occurred at temperatures at which the viral RDE was active, and inhibition of viral RDE by periodate treatment sharply reduced the ability of virus to inactivate the RSI. One interpretation of the data suggests that RSI is a receptor analog reactive with both the hemagglutinin and RDE of C virus and that RSI inactivation is an assay of influenza C viral RDE.

Demonstration of hemolytic and fusion activities of influenza C virus

Influenza C virus showed a marked hemolytic activity when incubated with murine erythrocytes at 37 degrees C in acidic medium. The virus-specific hemolysis was most efficient at pH 5.0. Extensive cell fusion also occurred when the erythrocytes were treated with the virus at acidic pH. When propagated in MDCK cells, the virus had an extremely low infectivity and did not display hemolytic activity in any pH range. When the inactive virus was subjected to mild trypsin treatment, hemolytic activity was drastically manifested, accompanying a drastic increase in infectivity. The glycoprotein in the inactive virus was cleaved into smaller components by trypsin treatments. These results indicated that the envelope of influenza C virus can fuse with the cellular membrane under acidic conditions and that the activation of influenza C virus by cleavage was due to the appearance of this envelope fusion activity.