An electron microscopic study of the presence or absence of neuraminic acid in enveloped viruses

Nanomaterials-based immunosensors for avian influenza virus detection

Avian influenza viruses (AIV) are capable of infecting a considerable proportion of the world's population each year, leading to severe epidemics with high rates of morbidity and mortality. The methods now used to diagnose influenza virus A include the Western blot test (WB), hemagglutination inhibition (HI), and enzyme-linked immunosorbent assays (ELISAs). But because of their labor-intensiveness, lengthy procedures, need for costly equipment, and inexperienced staff, these approaches are considered inappropriate. The present review elucidates the recent advancements in the field of avian influenza detection through the utilization of nanomaterials-based immunosensors between 2014 and 2024. The classification of detection techniques has been taken into account to provide a comprehensive overview of the literature. The review encompasses a detailed illustration of the commonly employed detection mechanisms in immunosensors, namely, colorimetry, fluorescence assay, surface plasmon resonance (SPR), surface-enhanced Raman spectroscopy (SERS), electrochemical detection, quartz crystal microbalance (QCM) piezoelectric, and field-effect transistor (FET). Furthermore, the challenges and future prospects for the immunosensors have been deliberated upon. The present review aims to enhance the understanding of immunosensors-based sensing platforms for virus detection and to stimulate the development of novel immunosensors by providing novel ideas and inspirations. Therefore, the aim of this paper is to provide an updated information about biosensors, as a recent detection technique of influenza with its details regarding the various types of biosensors, which can be used for this review.

Influenza virus budding from the tips of cellular microvilli in differentiated human airway epithelial cells

The epithelium of conducting airways represents the main target for influenza virus in mammals. However, the peculiarities of virus interactions with differentiated airway epithelial cells remain largely unknown. Here, influenza virus budding was studied in differentiated cultures of human tracheobronchial epithelial cells using transmission electron microscopy. Budding of spherical and filamentous virions was observed on the apical surfaces of cells with no association with cilia and secretory granules. Quantitative analysis of the distribution of viral buds on the cell surface indicated that the tips of the microvilli represented a prominent site of influenza virus budding in the human airway epithelium. As the microvilli of differentiated cells are involved in many fundamental cell functions, these data will prompt further studies on the biological significance of microvilli-associated budding for virus replication, transmission and pathogenicity.

Postreassortment amino acid substitutions in influenza A viruses

The genome of the influenza A virus consists of eight single-stranded negative sense RNA segments. Segmentation allows reassortment of genes between influenza A virus strains when two strains infect one host cell. Reassortment may lead to the emergence of pandemic influenza viruses. The process of reassortment is limited by the necessity of a functional balance among viral genes. The nature of the functional constraint on reassortment is currenty not well understood. An insight into the basis of functional matching of virus genes, its restrictions and its restoration after reassortment may be provided by the analysis of postreassortment mutations in model systems. This article summarizes the data on postreassortment amino acid changes in virus glycoproteins and polymerase proteins and their effect on the intergenic functional match.

Membrane Glycoproteins of Enveloped Viruses

This chapter focuses on the recent information of the glycoprotein components of enveloped viruses and points out specific findings on viral envelopes. Although enveloped viruses of different major groups vary in size and shape, as well as in the molecular weight of their structural polypeptides, there are general similarities in the types of polypeptide components present in virions. The types of structural components found in viral membranes are summarized briefly in the chapter. All the enveloped viruses studied to date possess one or more glycoprotein species and lipid as a major structural component. The presence of carbohydrate covalently linked to proteins is demonstrated by the incorporation of a radioactive precursor, such as glucosamine or fucose, into viral polypeptides, which is resolved by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis. Enveloped viruses share many common features in the organization of their structural components, as indicated by several approaches, including electron microscopy, surface-labeling, and proteolytic digestion experiments, and the isolation of subviral components. The chapter summarizes the detailed structure of the glycoproteins of four virus groups: (1) influenza virus glycoproteins, (2) rhabdovirus G protein, (3) togavirus glycoprotein, and (4) paramyxovirus glycoproteins The information obtained includes the size and shape of viral glycoproteins, the number of polypeptide chains in the complete glycoprotein structure, and compositional data on the polypeptide and oligosaccharide portions of the molecules.

Role of sialic acid-containing molecules in paramyxovirus entry into the host cell: A minireview

Sialic acid-containing compounds play a key role in the initial steps of the paramyxovirus life cycle. As enveloped viruses, their entry into the host cell consists of two main events: binding to the host cell and membrane fusion. Virus adsorption occurs at the surface of the host cell with the recognition of specific receptor molecules located at the cell membrane by specific viral attachment proteins. The viral attachment protein present in some paramyxoviruses (Respirovirus, Rubulavirus and Avulavirus) is the HN glycoprotein, which binds to cellular sialic acid-containing molecules and exhibits sialidase and fusion promotion activities. Gangliosides of the gangliotetraose series bearing the sialic acid N-acetylneuraminic (Neu5Ac) on the terminal galactose attached in alpha2-3 linkage, such as GD1a, GT1b, and GQ1b, and neolacto-series gangliosides are the major receptors for Sendai virus. Much less is known about the receptors for other paramyxoviruses than for Sendai virus. Human parainfluenza viruses 1 and 3 preferentially recognize oligosaccharides containing N-acetyllactosaminoglycan branches with terminal Neu5Acalpha2-3Gal. In the case of Newcastle disease virus, has been reported the absence of a specific pattern of the gangliosides that interact with the virus. Additionally, several works have described the use of sialylated glycoproteins as paramyxovirus receptors. Accordingly, the design of specific sialic acid analogs to inhibit the sialidase and/or receptor binding activity of viral attachment proteins is an important antiviral strategy. In spite of all these data, the exact nature of paramyxovirus receptors, apart from their sialylated nature, and the mechanism(s) of viral attachment to the cell surface are poorly understood.

Postreassortment changes in influenza A virus hemagglutinin restoring HA-NA functional match

An important function of influenza virus neuraminidase (NA) is the removal of sialic acid residues from virion components in order to prevent the aggregation of virus particles. In previous communications we have reported that reassortant viruses containing the NA gene of A/USSR/90/77 (H1N1) virus and HA genes of H3, H4, H10, or H13 subtypes had a tendency to virion aggregation at 4 degrees C and that the virion clusters irreversibly dissociated after the treatment with bacterial neuraminidase. It was concluded that in such reassortants the removal of sialic acid residues is inefficient. Nonaggregating variants of the reassortants were selected in the course of serial passages in embryonated chicken eggs. In the present paper a reassortant virus, R2, having the HA gene of A/Duck/Ukraine/1/63 (H3N8) virus and the other genes of A/USSR/90/77 (H1N1) virus, as well as its non-aggregating passage variants and both parent viruses, have been studied in order to reveal the presence of unremoved sialic acid residues in the virions. An assay of sialic acid content by high-performance liquid chromatography with fluorescent detection has revealed the presence of sialic acid in the purified virus preparations of A/USSR/90/77 (H1N1) virus and the R2 reassortant and its nonaggregating variants, whereas only trace amounts of sialic acid have been detected in the A/Duck/Ukraine/1/63 (H3N8) parent virus. The data obtained with the use of the labeled "indicator" virus suggest that the unremoved sialic acid residues are present at the virion surface. The nonaggregating variants have been shown to possess a lower affinity toward high-molecular-weight sialic acid-containing substrates compared to the initial reassortant R2. Sequencing of HA genes has revealed amino acid changes in the nonaggregating variants compared to the initial reassortant. One substitution, N248D in HA1, is the same in two independently selected nonaggregating variants. The presented data suggest that the complete removal of sialic acid residues by viral NA from the virion components is not obligatory for the absence of virus particle aggregation: the latter may be achieved (in the reassortants and, presumably, in the wild-type virus) through a balance between the degree of HA affinity toward the sialic acid-containing receptors and the extent of the removal of sialic acid residues by NA.

Viruses as model systems in cell biology

This chapter focuses on the contributions that studies with viruses have made to current concepts in cell biology. Among the important advantages that viruses provide in such studies is their structural and genetic simplicity. The chapter describes the methods for growth, assay, and purification of viruses and infection of cells by several viruses that have been widely utilized for studies of cellular processes. Most investigations of virus replication at the cellular level are carried out using animal cells in culture. For the events in individual cells to occur with a high level of synchrony, single cycle growth conditions are used. Cells are infected using a high multiplicity of infectious virus particles in a low volume of medium to enhance the efficiency of virus adsorption to cell surfaces. After the adsorption period, the residual inoculum is removed and replaced with an appropriate culture medium. During further incubation, each individual cell in the culture is at a similar temporal stage in the viral replication process. Therefore, experimental procedures carried out on the entire culture reflect the replicative events occurring within an individual cell. The length of a single cycle of virus growth can range from a few hours to several days, depending on the virus type.

Inhibition of sialidases from viral, bacterial and mammalian sources by analogues of 2-deoxy-2,3-didehydro-N-acetylneuraminic acid modified at the C-4 position

The inhibition of sialidase activity from influenza viruses A and B, parainfluenza 2 virus, Vibrio cholerae, Arthrobacter ureafaciens, Clostridium perfringens, and sheep liver by a range of 2-deoxy-2,3-didehydro-N-acetylneuraminic acid analogues modified at the C-4 position has been studied. All substitutions tested resulted in a decrease in the degree of inhibition of the bacterial and mammalian sialidases. For sialidases from influenza viruses A and B, on the other hand, most of the substitutions tested either had no significant effect on binding or, in the case of the basic amino and guanidino substituents, resulted in significantly stronger inhibition. The results for parainfluenza 2 virus sialidase were mostly intermediate, in that inhibition was neither significantly increased nor decreased by most of the modifications. We conclude that only the influenza A and B sialidase active sites possess acid groups correctly positioned to participate in charge-charge interactions in the region of C-4 of bound substrate, and that the C-4 binding pockets of the bacterial and mammalian sialidases examined are considerably smaller than is observed for either the influenza virus or parainfluenza virus sialidases.

Nuclear retention of M1 protein in a temperature-sensitive mutant of influenza (A/WSN/33) virus does not affect nuclear export of viral ribonucleoproteins

We investigated the properties of ts51, an influenza virus (A/WSN/33) temperature-sensitive RNA segment 7 mutant. Nucleotide sequence analysis revealed that ts51 possesses a single nucleotide mutation, T-261----C, in RNA segment 7, resulting in a single amino acid change. Phenylalanine (position 79) in the wild-type M1 protein was substituted by serine in ts51. This mutation was phenotypically characterized by dramatic nuclear accumulation of the M1 protein and interfered with some steps at the late stage of virus replication, possibly affecting the assembly and/or budding of viral particles. However, although M1 protein was retained within the nucleus, export of the newly synthesized viral ribonucleoprotein containing the minus-strand RNA into the cytoplasm was essentially the same at both permissive and nonpermissive temperatures. The roles of M1 in the export of viral ribonucleoproteins from the nucleus into the cytoplasm and in the virus particle assembly process are discussed.

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.

Interaction of Newcastle disease virus strains differing in virulence with chicken red blood cell receptors

Nine NDV strains belonging to lentogenic, mesogenic and velogenic groups were studied. Virus adsorption to chicken red blood cell (RBC) surface was performed at 4 degrees C, and after a temperature shift from 4 degrees to 37 degrees C elution of pre-adsorbed virus and accumulation of free N-acetyl-neuraminic acid (NANA) split from RBC receptors as a result of neuraminidase (Nase) activity was detected. In the case of high multiplicity of adsorption the elution was very fast (complete elution within 5 minutes) for all the strains irrespective of their virulence. Although physical saturation of RBC surface with the adsorbed virus was not achieved, a certain minimal (strain-specific) amount of the pre-adsorbed virus which splits a maximally possible (for a given strain) quantity of the NANA was found (a state of "enzymatic saturation"). Below a certain low multiplicity of adsorption elution was delayed for about 20-30 minutes while the accumulation of the split NANA began immediately after the temperature shift. This phenomenon was interpreted as a result of "crawling" of the adsorbed virions upon the RBC surface followed by "browsing" of RBC receptors and liberation of NANA. Thus, the Nase activity of the attached virus ("in situ Nase activity") is a factor providing both elution and "crawling" of the virus (depending on the multiplicity of adsorption). The in situ Nase activity of all the strains used was determined quantitatively by (1) parameters of enzymatic kinetics (Vmax, Km and Km/Vmax) and (2) parameters of enzymatic efficiency related to a certain quantity of the adsorbed virus, namely, per amount of: a) "crawling" virus, b) that providing "enzymatic saturation", and c) that equal to Km. Computation of these parameters revealed inverse correlation between the in situ Nase activity and the strain virulence. Thus, these indications can be in vitro markers of the in vivo virulence.

The major oligosaccharides in the large subunit of the hemagglutinin from fowl plague virus, strain Dutch. Structure elucidation by one-dimensional and two-dimensional 1H nuclear magnetic resonance and by methylation analysis

The N-glycosidically linked glycans in the large subunit (HA1) of the hemagglutinin from fowl plague virus, strain Dutch (containing about 15%, w/w, of carbohydrates), were liberated by alkaline hydrolysis, and were filtrated through Bio-Gel as the re-N-acetylated oligosaccharide alditols. One major fraction (90%, mol/mol) was obtained. It was subfractionated by concanavalin A affinity chromatography and was analyzed by methylation/capillary gas chromatography/mass fragmentography and especially by one-dimensional and two-dimensional 1H nuclear magnetic resonance. The major HA1 glycans, which are not sialylated, were thus found to comprise about 40%, 30% and 20% (mol/mol), respectively, of biantennary intersected, biantennary, and triantennary N-acetyllactosaminic ('complex') oligosaccharides. About two thirds of the internal GlcNAc residues in these glycans are substituted by Fuc(alpha 1----6), all the triantennary species carry the third Gal(beta 1----4)GlcNAc(beta 1----unit at the Man(alpha 1----6)-branch, and roughly one fourth of the N-acetyllactosamine units in the non-intersected biantennary oligosaccharides are incomplete.

Sialic acid is incorporated into influenza hemagglutinin glycoproteins in the absence of viral neuraminidase

We have analyzed the pronase-derived glycopeptides of the hemagglutinin glycoproteins expressed from SV40 vectors carrying cloned cDNA copies of the HA gene and of HA isolated from influenza virions (A/Jap/305/57). The glycopeptides derived from he HA glycoprotein obtained from cloned genes were heterogeneous, ranging in size from 3800 to 2800 daltons. Upon treatment with neuraminidase, sialic acid was released from the glycopeptides and their size was reduced to 2900-2400 daltons. However, under the same conditions, no sialic acid was detected in the virion HA. The presence of sialic acid was confirmed by monosaccharide analysis of the HA glycoprotein derived from products of cloned genes. These results support the idea that during replication of influenza virus, the viral neuraminidase cleaves sialic acid from the HA glycoprotein in infected cells.

Separation and sugar component analysis of the oligosaccharides in the surface glycoproteins of Newcastle disease virus

The precursor glycoproteins HN0 and F0 in the surface spikes of Newcastle Disease Virus strain Ulster as produced by MDBK cells, were found to contain 10.4 and 11.9 weight per cent, respectively, of the sugars typical for N-glycosidically linked glycoprotein glycans. A molar ratio of D-mannose:D-galactose: L-fucose:N-acetyl-D-glucosamine approaching 1.0:1.1:0.5:1.0 was found for HN0, and of 1.0:0.7:0.3:0.6 for F0. By a sequence of degradation (with pronase, with endo-beta-N-acetylglucosaminidase H [endo H], and by hydrazinolysis) and separation procedures (Concanavalin A-affinity and Biogel P-4 chromatography), the radiolabelled carbohydrate moieties of NDV HN0 and F0 (as oligosaccharitols) were separated into (at least) ten and eight fractions, respectively. Separate in vivo labelling with tritiated derivatives of the four sugars showed that both glycoproteins contain oligosaccharides of the oligomannosidic ("high mannose"), of the N-acetyllactosaminic ("complex"), as well as of the "mixed" type. The majority of the oligosaccharides in F0, but not of those in HN0, was found to be endo H-sensitive.

Biological consequences of neuraminidase deficiency in Newcastle disease virus

A second-step revertant (L1) of a temperature-sensitive mutant (C1) of Newcastle disease virus agglutinated erythrocytes normally but had less than 3% of the wild-type (strain AV) levels of neuraminidase activity. Revertant L1 had seven times more virion-associated N-acetylneuraminic acid (NANA) than strain AV. NANA residues on purified virions were specifically labeled with periodate and tritiated borohydride. Analyses of radiolabeled L1 virions on sodium dodecyl sulfate-polyacrylamide gels showed that most of the virion-associated NANA was in a high-molecular-weight component with an electrophoretic mobility different from that of any known viral protein. NANA was also detected in molecules with the electrophoretic mobility of the viral glycoproteins HN and F1. Revertant L1 had a twofold lower rate constant of attachment to HeLa cells than that of the wild-type. Treatment of L1 virions with Vibrio cholerae neuraminidase removed the excess NANA and returned L1 attachment kinetics to normal. Revertant N1, which has 10-fold more neuraminidase activity than L1, penetrated host cells at the same rate as L1. L1 was impaired in elution from erythrocytes. Removal of virion-associated NANA exacerbated this defect. Despite a small disadvantage in attachment and a major defect in elution relative to strain AV, revertant L1 enjoyed a slight advantage over the wild-type during a single reproductive cycle in cultured chicken embryo cells.

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.

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.

Glycosylation does not determine segregation of viral envelope proteins in the plasma membrane of epithelial cells

Enveloped viruses are excellent tools for the study of the biogenesis of epithelial polarity, because they bud asymmetrically from confluent monolayers of epithelial cells and because polarized budding is preceded by the accumulation of envelope proteins exclusively in the plasma membrane regions from which the viruses bud. In this work, three different experimental approaches showed that the carbohydrate moieties do not determine the final surface localization of either influenza (WSN strain) or vesicular stomatitis virus (VSV) envelope proteins in infected Madin-Darby Canine Kidney (MDCK) cells, as determined by immunofluorescence and immunoelectron microscopy, using ferritin as a marker. Infected concanavalin A- and ricin 1-resistant mutants of MDCK cells, with alterations in glycosylation, exhibited surface distributions of viral glycoproteins identical to those of the parental cell line, i.e., influenza envelope proteins were exclusively found in the apical surface, whereas VSV G protein was localized only in the basolateral region. MDCK cells treated with tunicamycin, which abolishes the glycosylation of viral glycoproteins, exhibited the same distribution of envelope proteins as control cells, after infection with VSF or influenza. A temperature-sensitive mutant of influenza WSN, ts3, which, when grown at the nonpermissive temperature of 39.5 degrees C, retains the sialic acid residues in the envelope glycoproteins, showed, at both 32 degrees C (permissive temperature) and 39.5 degrees C, budding polarity and viral glycoprotein distribution identical to those of the parental WSN strain, when grown in MDCK cells. These results demonstrate that carbohydrate moieties are not components of the addressing signals that determine the polarized distribution of viral envelope proteins, and possibly of the intrinsic cellular plasma membrane proteins, in the surface of epithelial cells.

Polarized distribution of viral envelope proteins in the plasma membrane of infected epithelial cells

The surface distribution of the envelope glycoproteins of influenza, Sendai and Vesicular Stomatitis viruses was studied by immunofluorescence and immunoelectromicroscopy in infected epithelial cell monolayers, from which these viruses bud in a polarized fashion. It was found that before the onset of viral budding, the envelope proteins are exclusively localized into the same plasma membrane domains of the epithelial cells from which the virions ultimately bud: the glycoproteins of influenza and Sendai were detected at the apical surface, while the G protein of Vesicular Stomatitis virus was concentrated at the basolateral region. On the other hand, Sendai virus nucleocapsids, which can be easily identified in the cytoplasm before viral assembly, could be observed throughout the cell, not showing any preferential localization near the surface that the virions utilize for budding. These results are consistent with a model in which the asymmetric distribution of viral envelope proteins, rather than a polarized delivery of nucleocapsids, directs the polarity of viral budding. Furthermore, the asymmetric surface localization of viral glycoproteins suggests that these proteins share with intrinsic surface proteins of epithelial cells common biogenetic mechanisms and informational features or "sorting out" signals that determine their compartmentalization in the plasma membrane.

The glycoprotein of measles virus. External radioactive labelling of its carbohydrate and partial characterization of the glycopeptide

Measles virus was propagated in VERO cells and purified from the culture supernatants by two successive tartrate-density-gradient centrifugations. Surface carbohydrates were labelled both in vitro and in vivo with 3H after treatment with galactose oxidase/NaB3H4 or with [3H]glucosamine. The major labelled glycoprotein in measles virions had a mol.wt. of 79 000. After labelling with periodate/NaB3H4, which would result in specific labelling of sialic acid residues, the 79 000-mol.wt. glycoprotein was very weakly labelled. This suggests that there is no or a very low amount of sialic acid in the virions. Further analysis of the glycoprotein showed that galactose is the terminal carbohydrate unit in the oligosaccharide, and the molecular weight of the glycopeptide obtained after Pronase digestion is about 3000. The oligosaccharide is attached to the polypeptide through an alkali-stable bond, indicating a N-glycosidic asparagine linkage.

Biosynthesis of the oligosaccharides of influenza viral glycoproteins

Glycosylation of influenza viral glycoproteins was investigated by pulse-labeling of infected BHK21-F cells with radioactive sugar precursors and by cell fractionation and analysis of Pronase-digested viral glycopeptides by gel filtration. The results with short pulses of [3H]mannose suggested that the initial event in glycosylation is the en bloc transfer of oligomannosyl cores to viral glycoproteins associated with rough membranes. The molecular weight of the glycopeptides which represent the cores was estimated to be approximately 1600-2200. Some mannose residues appear to be subsequently removed from oligosaccharide cores. [3H]mannose-labeled glycopeptides obtained either from cells pulsed for brief periods or from rough membranes, which contain predominantly oligosaccharide cores, were sensitive to digestion by endo-p-N-acetylglucosaminidase H (endo-H). On the other hand, glycopeptides larger than oligosaccharide cores, which appeared during chases or after migration of viral glycoproteins from rough to smooth membranes, were resistant to endo-H treatment. The branched sugars (glucosamine, galactose, and fucose), which are contained only in the complex (type I) oligosaccharide chains of virions, appear to be added in a stepwise manner to the trimmed oligosaccharide cores primarily on smooth membranes. Mannoserich glycopeptides of virions (type II) are similar in size to oligosaccharide cores detected in infected cells and are totally sensitive to endo-H, suggesting that type II glycopeptides may represent oligomannosyl cores which escape trimming as well as addition of branched sugars. Comparison of glycopeptides of infected and uninfected BHK21-F cells suggests that influenza viral glycoproteins contain oligosaccharide chains similar in size to those of host cells except for the absence of sialic acid in viral glycoproteins. Further, we observed that intracytoplasmic membranes from infected cells contain much less sialic acid than those from uninfected cells, indicating that viral neuraminidase present in the interior of infected cells possesses enzymatic activity.

Hemagglutination-inhibition: rapid assay for neuraminic acid-containing viruses

Influenza virus particles bind rapidly to vesicular stomatitis, Sindbis, or Rauscher murine leukemia virus particles, forming mixed aggregates demonstrable by electron microscopy. The normal hemagglutinating property of influenza virus is inhibited by these viruses, providing a rapid quantitative assay. Prior treatment with neuraminidase blocks the ability of other viruses to inhibit influenza virus hemagglutination.

Primaquine diphosphate: inhibition of Newcastle disease virus replication

The response of Newcastle disease virus replication to primaquine, an antimalarial drug, was examined in chicken embryo cells (CEC). Virus-induced hemadsorption was completely inhibited by 250 mug of primaquine per ml. At lower concentrations, hemadsorption inhibition was dose dependent. Primaquine retarded virus-induced redistribution of receptor sites on the host cell plasma membrane as shown by the failure of infected, drug-treated CEC to be agglutinated with concanavalin A. The production of infectious progeny virus was substantially inhibited by the addition of primaquine at various times postinfection. When the drug was added early in the virus replication cycle, viral ribonucleic acid (RNA) synthesis was inhibited; however, when the drug was added late in the cycle, stimulation of RNA synthesis was observed. Primaquine was also shown to retard the incorporation of [(14)C]amino acids into proteins of virus-infected CEC. We suggest that the major role of primaquine is inhibition of protein synthesis; this results in changes in: hemadsorption, redistribution of lectin receptors, release of progeny, and virus-induced RNA synthesis.

Inhibition of virus release by antibodies to surface antigens of influenza viruses

When influenza virus was mixed with antisera to its surface subunits before inoculation of cell cultures, anti-hemagglutinin antibodies neutralized infectivity but anti-neuraminidase did not. When the antisera were added after infection of cell cultures, anti-hemagglutinin and anti-neuraminidase antibodies were equally effective in reducing virus titers in culture fluids. Decreased virus titers were not due to interference of antibody with assay and were not accompanied by a reduction in the synthesis of hemagglutinin and neuraminidase subunits. Both antisera also effectively prevented in vitro virus spread. Inhibition of virus release by neuraminidase antibody appeared unrelated to its antienzyme property. Hydrolysis of N-acetyl neuraminic acid residues of infected host cells proceeded unimpaired in the presence of subunit antisera. Anti-hemagglutinin and anti-neuraminidase antibodies may act to prevent virus release by binding newly formed virus subunits to each other and to anti-genically altered cell membranes.

Lipids in Viruses

This chapter discusses lipids in viruses. Lipid forms an integral part of many viruses and exists either in the form of a continuous envelope or in lipoprotein complexes that surround a nucleoprotein core or helix. In general, the envelope can be described as a molecular container for the genetic material of the virus. Viruses are obligate intracellular parasites and are not known to carry genetic coding for enzymes involved in lipid synthesis. Hence, they generally contain the same classes of lipid as are found in the host cell or their membrane of assembly. Lipids make up 20–35% by weight of most viruses; however, there are exceptions such as vaccinia virus, which has only 5% lipid despite having a complex multimembrane envelope structure. Naked herpesvirus capsids closely resemble non-lipid-containing viruses such as adenovirus or polyoma virus, which are also assembled in the nucleus but show full infectivity without any envelope. Both naked and enveloped herpesvirus particles are found in infected cells; however, only enveloped particles are found in extracellular fluids.