Complete nucleotide sequence of the neuraminidase gene of human influenza virus A/WSN/33

Neuraminidase-associated plasminogen recruitment enables systemic spread of natural avian Influenza viruses H3N1

Repeated outbreaks due to H3N1 low pathogenicity avian influenza viruses (LPAIV) in Belgium were associated with unusually high mortality in chicken in 2019. Those events caused considerable economic losses and prompted restriction measures normally implemented for eradicating high pathogenicity avian influenza viruses (HPAIV). Initial pathology investigations and infection studies suggested this virus to be able to replicate systemically, being very atypical for H3 LPAIV. Here, we investigate the pathogenesis of this H3N1 virus and propose a mechanism explaining its unusual systemic replication capability. By intravenous and intracerebral inoculation in chicken, we demonstrate systemic spread of this virus, extending to the central nervous system. Endoproteolytic viral hemagglutinin (HA) protein activation by either tissue-restricted serine peptidases or ubiquitous subtilisin-like proteases is the functional hallmark distinguishing (H5 or H7) LPAIV from HPAIV. However, luciferase reporter assays show that HA cleavage in case of the H3N1 strain in contrast to the HPAIV is not processed by intracellular proteases. Yet the H3N1 virus replicates efficiently in cell culture without trypsin, unlike LPAIVs. Moreover, this trypsin-independent virus replication is inhibited by 6-aminohexanoic acid, a plasmin inhibitor. Correspondingly, in silico analysis indicates that plasminogen is recruitable by the viral neuraminidase for proteolytic activation due to the loss of a strongly conserved N-glycosylation site at position 130. This mutation was shown responsible for plasminogen recruitment and neurovirulence of the mouse brain-passaged laboratory strain A/WSN/33 (H1N1). In conclusion, our findings provide good evidence in natural chicken strains for N1 neuraminidase-operated recruitment of plasminogen, enabling systemic replication leading to an unusual high pathogenicity phenotype. Such a gain of function in naturally occurring AIVs representing an established human influenza HA-subtype raises concerns over potential zoonotic threats.

Organic synthesis and anti-influenza A virus activity of cyclobakuchiols A, B, C, and D

Novel antiviral agents for influenza, which poses a substantial threat to humans, are required. Cyclobakuchiols A and B have been isolated from Psoralea glandulosa, and cyclobakuchiol C has been isolated from P. corylifolia. The structural differences between cyclobakuchiol A and C arise due to the oxidation state of isopropyl group, and these compounds can be derived from (+)-(S)-bakuchiol, a phenolic isoprenoid compound present in P. corylifolia seeds. We previously reported that bakuchiol induces enantiospecific anti-influenza A virus activity involving nuclear factor erythroid 2-related factor 2 (Nrf2) activation. However, it remains unclear whether cyclobakuchiols A–C induce anti-influenza A virus activity. In this study, cyclobakuchiols A, B, and C along with cyclobakuchiol D, a new artificial compound derived from cyclobakuchiol B, were synthesized and examined for their anti-influenza A virus activities using Madin-Darby canine kidney cells. As a result, cyclobakuchiols A–D were found to inhibit influenza A viral infection, growth, and the reduction of expression of viral mRNAs and proteins in influenza A virus-infected cells. Additionally, these compounds markedly reduced the mRNA expression of the host cell influenza A virus-induced immune response genes, interferon-β and myxovirus-resistant protein 1. In addition, cyclobakuchiols A–D upregulated the mRNA levels of NAD(P)H quinone oxidoreductase 1, an Nrf2-induced gene, in influenza A virus-infected cells. Notably, cyclobakuchiols A, B, and C, but not D, induced the Nrf2 activation pathway. These findings demonstrate that cyclobakuchiols have anti-influenza viral activity involving host cell oxidative stress response. In addition, our results suggest that the suitably spatial configuration between oxidized isopropyl group and phenol moiety in the structure of cyclobakuchiols is required for their effect.

Galectin-1 binds to influenza virus and ameliorates influenza virus pathogenesis

Innate immune response is important for viral clearance during influenza virus infection. Galectin-1, which belongs to S-type lectins, contains a conserved carbohydrate recognition domain that recognizes galactose-containing oligosaccharides. Since the envelope proteins of influenza virus are highly glycosylated, we studied the role of galectin-1 in influenza virus infection in vitro and in mice. We found that galectin-1 was upregulated in the lungs of mice during influenza virus infection. There was a positive correlation between galectin-1 levels and viral loads during the acute phase of viral infection. Cells treated with recombinant human galectin-1 generated lower viral yields after influenza virus infection. Galectin-1 could directly bind to the envelope glycoproteins of influenza A/WSN/33 virus and inhibit its hemagglutination activity and infectivity. It also bound to different subtypes of influenza A virus with micromolar dissociation constant (K(d)) values and protected cells against influenza virus-induced cell death. We used nanoparticle, surface plasmon resonance analysis and transmission electron microscopy to further demonstrate the direct binding of galectin-1 to influenza virus. More importantly, we show for the first time that intranasal treatment of galectin-1 could enhance survival of mice against lethal challenge with influenza virus by reducing viral load, inflammation, and apoptosis in the lung. Furthermore, galectin-1 knockout mice were more susceptible to influenza virus infection than wild-type mice. Collectively, our results indicate that galectin-1 has anti-influenza virus activity by binding to viral surface and inhibiting its infectivity. Thus, galectin-1 may be further explored as a novel therapeutic agent for influenza.

Sequencing and mutational analysis of the non-coding regions of influenza A virus

The genome of influenza A virus consists of eight negative-stranded RNA segments which contain one or two coding regions flanked by the 3' and 5' non-coding regions (NCRs). Despite the importance of NCRs in replication and pathogenesis of influenza virus, sequencing of influenza virus genome has mainly been focused on coding regions of the individual genes and very limited NCR sequences are available. In this study, we sequenced the NCRs of seven influenza A virus strains of different host origin and varying pathogenicity using two recently developed methods [de Wit, E., Bestebroer, T.M., Spronken, M.I., Rimmelzwaan, G.F., Osterhaus, A.D., Fouchier, R.A., 2007. Rapid sequencing of the non-coding regions of influenza A virus. J. Virol. Methods 139, 85-89; Szymkowiak, C., Kwan, W.S., Su, Q., Toner, T.J., Shaw, A.R., Youil, R., 2003. Rapid method for the characterization of 3' and 5' UTRs of influenza viruses. J. Virol. Methods 107, 15-20]. In addition to sequence and length variation present in the segment-specific NCRs among different influenza strains, we also observed sequence variations at the fourth nucleotide of 3' NCR of polymerase genes. To evaluate the role of sequence change in the NCRs in reporter gene expression, we introduced mutations at the NCRs of two polymerase gene segments, PB1 and PA, and created the green fluorescent protein (GFP) reporter plasmids. By measuring the GFP expression level, we confirmed that single or two mutations introduced at the 3' and 5' NCRs of PB1 and PA gene could alter the protein expression levels. Our study reaffirms the importance of NCRs in influenza virus replication and further analysis of their roles will lead to better understanding of influenza pathogenesis.

Hydrophobic polycationic coatings inactivate wild-type and zanamivir- and/or oseltamivir-resistant human and avian influenza viruses

Glass slides painted with the hydrophobic long-chained polycation N,N-dodecyl,methyl-polyethylenimine (N,N-dodecyl,methyl-PEI) are highly lethal to waterborne influenza A viruses, including not only wild-type human and avian strains but also their neuraminidase mutants resistant to currently used anti-influenza drugs.

The cytoplasmic tails of the influenza virus spike glycoproteins are required for normal genome packaging

Deletion of the cytoplasmic tails of the influenza A virus spike glycoproteins, hemagglutinin (HA) and neuraminidase (NA), has previously been shown to result in markedly defective virion morphogenesis (Jin et al., 1997, EMBO J. 16, 1236-1247). We have found that influenza A virus preparations lacking the HA and NA cytoplasmic tails (HAt-/NAt-) have a reduced vRNA to protein content, contain an increase in cellular RNA contaminants, and exhibit increased resistance to ultraviolet (UV) inactivation. There is also a direct correlation between abnormal virion morphology and reduced infectivity. The data suggest that the HAt-/NAt- virion population contains a broader range of number of packaged RNA segments than wild-type (wt) virus. Sucrose gradient centrifugation analysis indicated the presence of a subpopulation of virions with pronounced deformation in virion morphology and reduced infectivity. The role of the HA and NA cytoplasmic tails was examined further by using a trans-complementation assay and it was found that expression of wt HA and NA from cDNAs followed by HAt-/NAt- virus infection caused the formation of a pseudotype virus with wt sedimentation properties. Taken together the data indicate that the HA and NA cytoplasmic tails affect not only virion morphology but also proper genome packaging.

Mutation of neuraminidase cysteine residues yields temperature-sensitive influenza viruses

The influenza virus neuraminidase (NA) is a tetrameric, virus surface glycoprotein possessing receptor-destroying activity. This enzyme facilitates viral release and is a target of anti-influenza virus drugs. The NA structure has been extensively studied, and the locations of disulfide bonds within the NA monomers have been identified. Because mutation of cysteine residues in other systems has resulted in temperature-sensitive (ts) proteins, we asked whether mutation of cysteine residues in the influenza virus NA would yield ts mutants. The ability to rationally design tight and stable ts mutations could facilitate the creation of efficient helper viruses for influenza virus reverse genetics experiments. We generated a series of cysteine-to-glycine mutants in the influenza A/WSN/33 virus NA. These were assayed for neuraminidase activity in a transient expression system, and active mutants were rescued into infectious virus by using established reverse genetics techniques. Mutation of two cysteines not involved in intrasubunit disulfide bonds, C49 and C146, had modest effects on enzymatic activity and on viral replication. Mutation of two cysteines, C303 and C320, which participate in a single disulfide bond located in the beta5L0,1 loop, produced ts enzymes. Additionally, the C303G and C320G transfectant viruses were found to be attenuated and ts. Because both the C303G and C320G viruses exhibited stable ts phenotypes, they were tested as helper viruses in reverse genetics experiments. Efficiently rescued were an N1 neuraminidase from an avian H5N1 virus, an N2 neuraminidase from a human H3N2 virus, and an N7 neuraminidase from an H7N7 equine virus. Thus, these cysteine-to-glycine NA mutants allow the rescue of a variety of wild-type and mutant NAs into influenza virus.

Mutations affecting the sensitivity of the influenza virus neuraminidase to 4-guanidino-2,4-dideoxy-2,3-dehydro-N-acetylneuraminic acid

4-Guanidino-2,4-dideoxy-2,3-dehydro-N-acetylneuraminic acid (4-guanidino-Neu5Ac2en) specifically inhibits the influenza virus neuraminidase (NA) through interaction of the guanidino group with conserved Glu 119 and Glu 227 residues in the substrate binding pocket of the enzyme. To understand the mechanism by which influenza viruses become resistant to 4-guanidino-Neu5Ac2en, we investigated mutations at amino acid residues 119 and 227 in the influenza virus NA for their effects on this compound and on NA activity. The NA gene was cloned from the NWS-G70c virus, and mutations were introduced at the codon for amino acid residue 119 or 227. All of the 13 mutants containing a change at residue 119 were transported to the cell surface, although their expression levels ranged from 68.2 to 91.3% of wild type. Mutant NAs that retained at least 20% of the wild-type enzymatic activity were tested for their sensitivity to 4-guanidino-Neu5Ac2en and found to be sevenfold less sensitive to this compound than was the wild-type NA. By contrast, only 6 of 13 mutants defined by modifications at residue 227 were transported to the cell surface, and those NAs lacked substantial enzymatic activity (9% of wild type, at most). These results suggest that only a limited number of resistant viruses arise through mutations at Glu 119 and Glu 227 under selective pressure from 4-guanidino-Neu5Ac2en and that the development of compounds which interact with 227 Glu more strongly than does 4-guanidino-Neu5Ac2en may reduce the likelihood of drug-resistant viruses still further.

Influenza virus hemagglutinin and neuraminidase glycoproteins stimulate the membrane association of the matrix protein

We have analyzed the mechanism by which the matrix (M1) protein associates with cellular membranes during influenza A virus assembly. Interaction of the M1 protein with the viral hemagglutinin (HA) or neuraminidase (NA) glycoprotein was extensively analyzed by using wild-type and transfectant influenza viruses as well as recombinant vaccinia viruses expressing the M1 protein, HA, or NA. Membrane binding of the M1 protein was significantly stimulated at the late stage of virus infection. Using recombinant vaccinia viruses, we found that a relatively small fraction (20 to 40%) of the cytoplasmic M1 protein associated with cellular membranes in the absence of other viral proteins, while coexpression of the HA and the NA stimulated membrane binding of the M1 protein. The stimulatory effect of the NA (>90%) was significant and higher than that of the HA (>60%). Introduction of mutations into the cytoplasmic tail of the NA interfered with its stimulatory effect. Meanwhile, the HA may complement the defective NA and facilitate virus assembly in cells infected with the NA/TAIL(-) transfectant. In conclusion, the highly conserved cytoplasmic tails of the HA and NA play an important role in virus assembly.

Changes in the neuraminidase of neurovirulent influenza virus strains

The influenza virus A/WS/33 has been adapted to mouse brain to produce two neurovirulent derivatives, A/NWS/33 (NWS) and A/WSN/33 (WSN), with the viral neuraminidase gene shown to be the major determinant of neurovirulence. The complete nucleotide sequence of the NA genes from each strain has been determined, which has allowed the identification of changes that have occurred during adaptation to mouse brain. Five changes are shared by the neurovirulent strains. Comparison to the known neuraminidase structure has identified four of these that may affect the active site of the enzyme. In addition, significant differences in the properties of the neuraminidase from the neurovirulent strains were observed relative to the parent strain. While no correlation was observed between neurovirulence and overall neuraminidase activity or preference for a particular N-substitution, the enzymes from both neurovirulent strains showed an increased preference for small substrates and those with 2-->3 linkages, and their activity was potentiated by Ca2+ ions.

The cytoplasmic tail of the neuraminidase protein of influenza A virus does not play an important role in the packaging of this protein into viral envelopes

We have rescued a transfectant influenza virus, NA/TAIL(-), whose neuraminidase (NA) protein lacks the predicted cytoplasmic tail. The virus was attenuated (one log10 reduction) both in tissue culture and in mouse lungs. Attenuation correlated with a 50% reduction of the level of NA in infected cells and levels of incorporation of the tail-less NA protein into viral particles paralleled that in infected cells. This result indicates that the signal for packaging of the NA protein into the viral envelope is not located in its cytoplasmic domain.

Use of a mammalian internal ribosomal entry site element for expression of a foreign protein by a transfectant influenza virus

The ribonucleoprotein transfection system for influenza virus allowed us to construct two influenza A viruses, GP2/BIP-NA and HGP2/BIP-NA, which contained bicistronic neuraminidase (NA) genes. The mRNAs derived from the bicistronic NA genes have two different open reading frames (ORFs). The first ORF encodes a foreign polypeptide (GP2 or HGP2) containing amino acid sequences derived from the gp41 protein of human immunodeficiency virus type 1. The second ORF encodes the NA protein; its translation is achieved via an internal ribosomal entry site which is derived from the 5' noncoding region of the human immunoglobulin heavy-chain-binding protein mRNA. The GP2 (79 amino acids) and HGP2 (91 amino acids) polypeptides are expressed in cells infected with the corresponding transfectant virus. The HGP2 polypeptide, which contains transmembrane and cytoplasmic domains identical to those of the hemagglutinin (HA) protein of influenza A/WSN/33 virus, is packaged into virus particles. This novel influenza virus system involving an internal ribosomal entry site element may afford a way to express a variety of foreign genes in mammalian cells.

Rapid detection of influenza A neuraminidase subtypes by cDNA amplification coupled to a simple DNA enzyme immunoassay

A newly developed colorimetric method, DNA enzyme immunoassay (DEIA), was applied to the detection of neuraminidase subtypes N1 and N2 of influenza A viruses. Reverse transcription and polymerase chain reaction with universal primers were used for genomic amplification of H1N1, H2N2, and H3N2 strains. Following amplification, an aliquot of the PCR product was hybridized to biotinylated DNA sequences (N1/N2 probes) immobilized on microtiter wells. The hybridization event was revealed by monoclonal antibodies to double stranded DNA in a standard ELISA reaction. The assay described here was able to distinguish accurately between the two neuraminidase subtypes of human influenza A viruses. It is a simple and rapid method facilitating the handling of a large number of samples and therefore seems to be easily applicable to diagnostic laboratories.

Paramyxovirus mediated cell fusion requires co-expression of both the fusion and hemagglutinin-neuraminidase glycoproteins

Syncytia formation in either CV-1 or HeLa T4+ cells required recombinant expression of both fusion (F) and hemagglutinin-neuraminidase (HN) glycoproteins from the human parainfluenza virus type 3 (HPIV3), human parainfluenza virus type 2 (HPIV2), and simian virus 5 (SV5). In this system, recombinant T7 transcription vectors (pT7-5 or pGEM) containing F or HN, were transfected individually or in combination into cells previously infected with a recombinant vaccinia virus expressing T7 RNA polymerase (vTF7-3). While both proteins were processed and expressed at the cell surface, syncytia formation occurred only when both glycoproteins were co-expressed. The function of HN in the fusion process could not be replaced using lectins or by co-expression of heterologous F and HN proteins. Further, cell fusion was not observed when experiments were performed using individually expressed F and HN proteins in adjacent cells. The data presented in this report support the notion that a specific interaction between both paramyxoviral glycoproteins is required for the formation of syncytia in tissue culture monolayers.

Glycosylation of neuraminidase determines the neurovirulence of influenza A/WSN/33 virus

The neuraminidase (NA) gene of influenza A/WSN/33 (WSN) virus has previously been shown to be associated with neurovirulence in mice and growth in Madin-Darby bovine kidney (MDBK) cells. Nucleotide sequence analysis has indicated that the NA of WSN virus lacks a conserved glycosylation site at position 130 (corresponding to position 146 in the N2 subtype). To investigate the role of this carbohydrate in viral pathogenicity, we used reverse genetics methods to generate a Glyc+ mutant virus, in which the glycosylation site Asn-130 was introduced into the WSN virus NA. Unlike the wild-type WSN virus, the Glyc+ mutant virus did not undergo multicycle replication in MDBK cells in the absence of trypsin, presumably because of lack of cleavage activation of infectivity. In contrast, revertant viruses derived from the Glyc+ mutant were able to replicate in MDBK cells without exogenous protease. Nucleotide sequence analysis revealed that the NAs of the revertant viruses had lost the introduced glycosylation site. In contrast to wild-type and revertant viruses, the Glyc+ mutant virus was not able to multiply in mouse brain. These results suggest that the absence of a glycosylation site at position 130 of the NA plays a key role in the neurovirulence of WSN virus in mice.

Alterations of the stalk of the influenza virus neuraminidase: deletions and insertions

The neuraminidase (NA) of influenza viruses cleaves sialic acids from receptors, prevents self-aggregation and facilitates release of virus during budding from host cells. Although the structure and function of the globular head of the influenza virus NA has been well studied, much less is known about the stalk of the NA, the region between the viral membrane and the globular head. Applying a reverse genetics system, we altered the stalk of the influenza A/WSN/33 virus NA by making deletions, insertions and mutations in this region of the gene. Our data show that the length of the NA stalk can be variable. Deletions of up to 28 amino acids and insertions of up to 41 amino acids in the stalk region did not abolish formation of infectious progeny virus. The data also indicate that the cysteine at position 76 is essential for formation of infectious virus, and that deletions beyond the cysteine did not result in infectious virus. Interestingly, shortening of the length of the stalk region by 28 amino acids resulted in a virus with a markedly reduced growth rate in MDCK cells as compared to that in MDBK cells. An insertion of 41 extra amino acids into the stalk did not significantly interfere with viral growth in MDCK or MDBK cells, which suggests that the stalk region would tolerate the introduction of long foreign sequences.

Determinants of oligomeric structure in the chicken liver glycoprotein receptor

The oligomeric state of the chicken liver receptor (chicken hepatic lectin), which mediates endocytosis of glycoproteins terminating with N-acetylglucosamine, has been investigated using physical methods as well as chemical cross-linking. Receptor isolated from liver and from transfected rat fibroblasts expressing the full-length polypeptide is a homotrimer immediately following solubilization in non-ionic detergent, but forms the previously observed hexamer during purification. These results are most consistent with the presence of a trimer of receptor polypeptides in liver membranes and in transfected cells. Analysis of truncated receptors reveals that the C-terminal extracellular portion of this type-II transmembrane protein does not form stable oligomers when isolated from the membrane anchor and cytoplasmic tail. The behaviour of chimeric receptors, in which the cytoplasmic tail of the glycoprotein receptor is replaced with the corresponding segments of rat liver asialoglycoprotein receptor or the beta-subunit of Na+,K(+)-ATPase, or with unrelated sequences from globin, indicates that the cytoplasmic tail influences oligomer stability. Replacement of N-terminal portions of the receptor with corresponding segments of influenza virus neuraminidase results in formation of tetramers, suggesting that the membrane anchor and flanking sequences are important determinants of oligomer formation.

Persistence of viral genes in a variant of MDBK cell after productive replication of a mutant of influenza virus A/WSN

The MDBK-R cell line is a variant of the MDBK cell line, which was derived by three consecutive high multiplicity superinfections of MDBK cells with AWBY-140 virus, a mutant of influenza virus A/WSN (H 1N 1). MDBK-R cells are permissive for productive replication of AWBY-140, but resist lysis by the virus and grew normally without producing infectious virus after replication of the mutant occurred there. By polymerase chain reaction (PCR), we demonstrated nucleotide sequences specific to all the 8 genes of AWBY-140 in MDBK-R cells which had been infected with the mutant at a high multiplicity and subsequently received 25 passages. This suggests that the genes of influenza virus mutant persisted in the dividing host cells for a long time after productive infection, when none of the cells was producing virus. We were also able to amplify the M gene related sequence of the mutant from both poly(A)+ and poly(A)- fractions of the RNA extracted from the cells at 27th passage level by PCR, which suggests that the persisting genes were replicated and transcribed, but we failed to demonstrate any viral protein in the cells by Western blotting.

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.

Synthesis and processing of the influenza virus neuraminidase, a type II transmembrane glycoprotein

The influenza virus neuraminidase (NA), a type II transmembrane glycoprotein, is expressed at the surface of infected cells and is a major structural component of the virion. The kinetics of biosynthesis of NA, including modification of N-linked sugar chains, association with GRP78-BiP, oligomerization, and transport to the cell surface, were examined in A/WSN/33 influenza-infected BHK cells. Prior to gaining endoglycosidase H (endo H) resistance, NA was found to transiently associate with GRP78-BiP (t1/2 approximately 5 min). The protein was synthesized as a monomer and within 10 min a significant fraction of it was chased into dimers and tetramers with a t1/2 approximately 15 to 20 min before endo H resistance was acquired suggesting that oligomerization took place in the endoplasmic reticulum. WSN NA remained completely endo H sensitive up to 15 min after synthesis, acquired partial resistance to endo H between 15 and 30 min (t1/2 approximately 25 min) after synthesis and exhibited heterogeneity in endo H-resistant forms. NA was first detected at the cell surface 30 min after synthesis, increased to a maximum at 1 hr, after which it decreased, presumably due to incorporation into virions. The results on the biosynthesis of NA, a type II protein for which the three-dimensional structure is known, will be useful in structure/function and virion assembly studies.

Three-dimensional structure of the neuraminidase of influenza virus A/Tokyo/3/67 at 2.2 A resolution

An atomic model of the tetrameric surface glycoprotein neuraminidase of influenza virus A/Tokyo/3/67 has been built and refined based on X-ray diffraction data at 2.2 A resolution. The crystallographic residual is 0.21 for data between 6 and 2.2 A resolution and the r.m.s. deviations from ideal geometry are 0.02 A for bond lengths and 3.9 degrees for bond angles. The model includes amino acid residues 83 to 469, four oligosaccharide structures N-linked at asparagine residues 86, 146, 200 and 234, a single putative Ca2+ ion site, and 85 water molecules. One of the oligosaccharides participates in a novel crystal contact. The folding pattern is a beta-sheet propeller as described earlier and details of the intramolecular interactions between the six beta-sheets are presented. Strain-invariant residues are clustered around the propeller axis on the upper surface of the molecule where they line the wall of a cavity into which sialic has been observed to bind. Strain-variable residues implicated in binding to antibodies surround this site.

An influenza A virus containing influenza B virus 5' and 3' noncoding regions on the neuraminidase gene is attenuated in mice

Influenza A and B viruses have not been shown to form reassortants. It had been assumed that the lack of genotypic mixing between influenza virus types reflected differences in polymerase and packaging specificity. In this study, we show that an influenza A virus polymerase transcribes and replicates a chloramphenicol acetyltransferase (CAT) gene flanked by the nontranslated sequences of an influenza B virus gene. Although the transcription level of this CAT gene was several times lower than that of a CAT gene flanked by the homologous nontranslated sequences of an influenza A virus, we proceeded to construct a chimeric type A/B influenza virus. Using recombinant DNA techniques, a chimeric neuraminidase gene was introduced into the genome of influenza A/WSN/33 virus. The hybrid influenza A/B virus gene contained the coding region of the A/WSN neuraminidase and the 3' and 5' nontranslated sequences of the nonstructural gene of influenza B/Lee virus. The resulting chimeric virus formed plaques in Madin-Darby bovine kidney cells but replicated more slowly and achieved lower titers than wild-type influenza A/WSN/33 virus. The chimeric virus was attenuated for mice as indicated by a 400-fold increase in its LD50. Interestingly, the virus was greatly restricted in replication in the upper respiratory tract and partially restricted in the lungs. Animals infected with the transfectant virus were highly resistant to influenza virus challenge. It appears that this chimeric virus has many of the properties desirable for a live attenuated virus vaccine.

Detection and identification of human influenza viruses by the polymerase chain reaction

A series of oligonucleotide primers are described which hybridize to conserved regions of influenza virus cDNA and prime DNA synthesis in Taq polymerase catalyzed amplification reactions (PCR). Primers were designed to hybridize as nested pairs and, following a two-step amplification, produce uniquely sized DNA fragments diagnostic for viral type and subtype. Influenza A and B matrix-protein genes and the influenza C haemagglutinin gene were targets for the type-specific primers. Subtype-specific primers targeted conserved sequences within the three haemagglutinin or two neuraminidase subtypes of different human influenza isolates. The utility of this method was demonstrated using computer search methods and by accurately amplifying DNA from a variety of influenza A, B, and C strains. Type-specific primer sets showed a broad type specificity and amplified DNA from viral strains of unknown sequence. Restriction mapping and DNA sequencing showed that fragments amplified in this manner derived from the input template, confirming the accuracy of the method and demonstrating how PCR can be used to quickly derive sufficient sequence information for analysis of viral relatedness. Subtyping primers were able to distinguish accurately between the three haemagglutinin (H1, H2, H3) and two neuraminidase (N1, N2) alleles of human influenza A isolates. Again DNA was amplified from viruses of unknown sequence confirming that most of these primer sets may prove useful as broad range subtyping reagents. In order to simplify the work associated with analysis of many samples, we have also devised a rapid method for the isolation of viral RNA and synthesis of cDNA. Using this 'mini-prep' technique, it is possible to detect, amplify, and identify picogram quantities of influenza virus in a single day, confirming that PCR provides a useful alternative to existing methods of influenza detection.

Cell surface transport, oligomerization, and endocytosis of chimeric type II glycoproteins: role of cytoplasmic and anchor domains

We investigated the role of cytoplasmic and anchor domains of type II glycoproteins in intracellular transport, oligomerization, and endocytosis by expressing the wild-type and chimeric genes in mammalian cells. Chimeric genes were constructed by exchanging the DNA segments that encode the cytoplasmic and anchor domains between the human influenza virus (A/WSN/33) neuraminidase (NA) and transferrin receptor (TR). The chimeric proteins in which domains were exchanged precisely were productively targeted to the cell surface. However, the proteins appeared to assemble differently in the intracellular compartment. For example, while TR existed predominantly as a dimer, NATR delta 90, containing the cytoplasmic and signal-anchor domains of NA and the ectodomain of TR, was present as a tetramer, a dimer, and a monomer. Similarly, the influenza virus NA existed predominantly as a tetramer but TRNA delta 35, in which the cytoplasmic and signal-anchor domains of TR were joined to the ectodomain of NA, existed predominantly as a dimer, suggesting that the cytoplasmic and anchor domains of type II glycoproteins affect the subunit assembly of heterologous ectodomains. In addition, we analyzed the role of the cytoplasmic domain in endocytosis. NA and NATR delta 90 did not undergo endocytosis, whereas both TR and TRNA delta 35 were internalized efficiently, demonstrating that the NH2 cytoplasmic domain of TR was capable of internalizing a heterologous ectodomain (NA) from the cell surface.

The major 85-kDa surface antigen of the mammalian-stage forms of Trypanosoma cruzi is a family of sialidases

Trypanosoma cruzi, an intracellular protozoan parasite infecting a wide variety of vertebrates, is the agent responsible for Chagas disease in humans. An estimated 15-20 million people in South and Central America are infected with the parasite. Chagas disease often results in severe autoimmune and inflammatory pathology and is the major cause of heart failure in endemic areas. Nevertheless, little is known about the host-parasite interactions that lead to this pathology. We have previously cloned several members of a large gene family (SA85-1) and shown that these genes encode 85-kDa T. cruzi, mammalian-stage-specific, surface antigens. Here we report that members of the SA85-1 family possess sialidase activity and are shed by the parasite. We suggest that the sialidases may contribute to the pathology during T. cruzi infection by cleaving sialic acid from cells of the immune system.

Cell surface transport, oligomerization, and endocytosis of chimeric type II glycoproteins: role of cytoplasmic and anchor domains

We investigated the role of cytoplasmic and anchor domains of type II glycoproteins in intracellular transport, oligomerization, and endocytosis by expressing the wild-type and chimeric genes in mammalian cells. Chimeric genes were constructed by exchanging the DNA segments that encode the cytoplasmic and anchor domains between the human influenza virus (A/WSN/33) neuraminidase (NA) and transferrin receptor (TR). The chimeric proteins in which domains were exchanged precisely were productively targeted to the cell surface. However, the proteins appeared to assemble differently in the intracellular compartment. For example, while TR existed predominantly as a dimer, NATR delta 90, containing the cytoplasmic and signal-anchor domains of NA and the ectodomain of TR, was present as a tetramer, a dimer, and a monomer. Similarly, the influenza virus NA existed predominantly as a tetramer but TRNA delta 35, in which the cytoplasmic and signal-anchor domains of TR were joined to the ectodomain of NA, existed predominantly as a dimer, suggesting that the cytoplasmic and anchor domains of type II glycoproteins affect the subunit assembly of heterologous ectodomains. In addition, we analyzed the role of the cytoplasmic domain in endocytosis. NA and NATR delta 90 did not undergo endocytosis, whereas both TR and TRNA delta 35 were internalized efficiently, demonstrating that the NH2 cytoplasmic domain of TR was capable of internalizing a heterologous ectodomain (NA) from the cell surface.

Replication of H1N1 influenza viruses in cultured mouse embryo brain cells: virus strain and cell differentiation affect synthesis of proteins encoded in RNA segments 7 and 8 and efficiency of mRNA splicing

The aims of these studies are (1) to determine whether, and by what mechanism(s), underexpression of M1 and/or NS1 protein restricts replication and cytopathogenicity in mouse brain cells of human influenza viruses which are closely related to the neurovirulent WSN variant but not selected for the neurovirulent phenotype; (2) to learn, ultimately, whether similarly restricted replication in natural infections might be enough to cause direct or indirect, immunologically mediated, neuropathology. On the basis of immunostaining, we have suggested that, in "aged" mouse embryo brain (MEB) cell cultures infected with A/PR/8/34 (PR8) or A/WS/33 (WS), M1 protein expression is restricted mainly in mature astrocytes (the dominant cell type in such cultures), but not in mature oligodendrocytes or neurons. Here we show that amounts of radiolabeled M1 protein in lysates of MEB cultures infected with PR8, WS, or WSN differ in proportion to previously reported single-cycle yields of trypsin-activated infectious virions. Low or undetectable cell-associated M1 does not reflect accelerated degradation, but tends to be accompanied by increased M2 protein (a product of spliced mRNA7). Radiolabeled NS1 is reduced, NS2 relatively increased, in "aged" MEB cultures infected at low m.o.i. with PR8, at high m.o.i. with WS as well, but not with WSN. In contrast, actively dividing and differentiating astrocyte-enriched or "young" MEB cultures tend to produce greatly increased amounts of NS2 even though NS1 may be at "normal" levels, both relative to those in similarly infected CEF cultures. We show, in extension of comparative studies by others on permissive and abortive FPV-infected cell systems, that virus strain-, cell type-, and perhaps differentiation-dependent variations in efficiency of mRNA 7 and 8 transcription and/or splicing are primary factors controlling variable expression of M and NS proteins in mouse brain cell cultures.

Introduction of site-specific mutations into the genome of influenza virus

We succeeded in rescuing infectious influenza virus by transfecting cells with RNAs derived from specific recombinant DNAs. RNA corresponding to the neuraminidase (NA) gene of influenza A/WSN/33 (WSN) virus was transcribed in vitro from plasmid DNA and, following the addition of purified influenza virus RNA polymerase complex, was transfected into MDBK cells. Superinfection with helper virus lacking the WSN NA gene resulted in the release of virus containing the WSN NA gene. We then introduced five point mutations into the WSN NA gene by cassette mutagenesis of the plasmid DNA. Sequence analysis of the rescued virus revealed that the genome contained all five mutations present in the mutated plasmid. The ability to create viruses with site-specific mutations will allow the engineering of influenza viruses with defined biological properties.

Characterization of a temperature-sensitive mutant in the RNA polymerase PB2 subunit gene of influenza A/WSN/33 virus

The temperature-sensitive mutant ts-1 of influenza virus A/WSN/33 carries mutations in the gene encoding RNA polymerase PB2 subunit. Effect of temperature on various steps of viral RNA synthesis was examined using disrupted virions of ts-1 mutant. The initiation of RNA synthesis with dinucleotide ApG primer was not affected by elevated temperature, whereas that with primer RNA containing 5'-terminal cap-1 structure was temperature-sensitive. The result supports the previous notion deduced from the UV-crosslinking experiments, that PB2 is involved in the cap-1 dependent initiation of RNA synthesis. In addition, the ts-1 mutant showed a defect in RNA chain elongation. Nucleotide sequence analysis of RNA segment 1 of ts-1 mutant revealed that the amino acid number 417 is essential for the recognition of cap-1 structures and/or the interaction with catalytic unit of the RNA polymerase.

Role of hemagglutinin cleavage and expression of M1 protein in replication of A/WS/33, A/PR/8/34, and WSN influenza viruses in mouse brain

The combined presence of WSN gene segments 6 (neuraminidase), 7 (M1 and M2), and 8 (NS1 and NS2) in reassortants of WSN with A/Aichi/2/68 (H3N2) has been found by others to be necessary for full expression of neurovirulence in mice. We are examining the expression of the analogous three gene segments in brains of mice after intracerebral infection with non-neuroadapted strains A/WS/33 (WS) (from which WSN was derived) and A/PR/8/34 (PR8). Our aim is to determine possible mechanisms by which one or more of the five gene products may restrict replication of these strains in mouse brain cells to a single cycle, yielding noninfectious hemagglutinating particles (incomplete growth cycle). We found that minority subsets of such particles did produce plaques, provided they were activated by trypsin (analogous to other abortive systems producing virions with uncleaved HA), a step obviated for some WSN virions by indirect promotion of hemagglutinin cleavage by the neuraminidase of that strain. The percentage of such potentially infectious virions, relative to total hemagglutinating particles, was significantly lower in WS- or PR8-infected than in WSN-infected brains, suggesting possible defects in synthesis or function of M1 protein in the former. Cells in immunostained sections and appropriate bands in Western blots (immunoblots) of viral proteins electrophoretically separated from lysates of PR8-infected brains reacted with antibody to nucleoprotein but not to M1 protein. Either method revealed the presence of both proteins in WSN-infected brains. In contrast, Western blot analyses of particles concentrated from PR8-, WS-, or WSN-infected brains by hemadsorption, elution, and pelleting did reveal NP and M1 bands with comparable relative peroxidase-antiperoxidase staining intensities. The findings suggest that availability of M1 protein is a factor influencing the extent or rate of assembly of potentially infectious (i.e., trypsin-activated) progeny virions in mouse brains and that in this respect the two non-neurovirulent strains differ from WSN quantitatively rather than qualitatively.

Redundancy of signal and anchor functions in the NH2-terminal uncharged region of influenza virus neuraminidase, a class II membrane glycoprotein

Class II membrane glycoproteins share a common topology of the NH2 terminus inside and the COOH terminus outside the cell. Their transport to the cell surface is initiated by the function of a single hydrophobic domain near the NH2 terminus. This functional domain serves both as an uncleaved signal sequence and as a transmembrane anchor. We examined the signal and anchor functions of influenza virus neuraminidase, a prototype class II membrane glycoprotein, by deletion analysis of its long, uncharged amino-terminal region. The results presented here show that the entire stretch of 29 uncharged amino acids (7 to 35) is not required for either a signal sequence or an anchor sequence function. On the basis of translocation and membrane stability data for different mutants, we suggest that the first 20 amino acid residues (7 to 27) are likely to provide the hydrophobic core for these functions and that within this putative subdomain some sequences are more efficient than the other sequences in providing a translocation function. Finally, it appears that neuraminidase and its mutant proteins are translocated with the proper orientation, regardless of the characteristics of the flanking sequences.

Complete nucleotide sequence of the neuraminidase gene of the human influenza virus A/Chile/1/83 (H1N1). Brief report

The complete nucleotide sequence of the neuraminidase (NA) gene of influenza virus A/Chile/1/83 (H1N1) has been determined after reverse transcription and cloning into the plasmid pAT 153/PvuII/8. The gene is 1461 nucleotides long and codes for a protein of 470 amino acids. The overall nucleotide and predicted amino acid sequence of the A/Chile/1/83 NA exhibits a high homology with other N1 neuraminidases. Hyper-variable regions concerning A to G exchanges 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.

Mutational analysis of the signal-anchor domain of influenza virus neuraminidase

Influenza virus neuraminidase (NA; EC 3.2.1.18) possesses a signal-anchor hydrophobic domain at the amino terminus. To characterize the nature of this signal-anchor domain we have introduced single amino acid changes in this domain by oligonucleotide-directed mutagenesis. Three mutant NA proteins that were synthesized contained a single charged amino acid residue in place of a hydrophobic amino acid residue at position 11, 17, or 26 of the signal-anchor domain. When the altered NA proteins were expressed in CV-1 cells, two phenotypes were observed: substitution of arginine in place of glycine at position 11 and substitution of aspartic acid for valine at position 17 did not abolish the signal, the anchor, or the transport functions. On the other hand, substitution of arginine for isoleucine at position 26 blocked the migration of the NA protein from the Golgi complex to the cell surface. Double mutants were constructed from these single point mutations and they exhibited two phenotypes: one double mutant (aspartic acid at position 17 and arginine at position 26) was present mostly in the cytoplasm and the other (arginine at positions 11 and 26) was present mostly in the rough endoplasmic reticulum. These results indicate that the hydrophobic amino acids at positions 11, 17, and 26 are required for intracellular transport. Furthermore, the accumulation of the mutant proteins in the rough endoplasmic reticulum or the Golgi apparatus suggests the existence of putative intracellular transport (or traffic) signals in the signal-anchor domain of NA.

Identification of defects in the neuraminidase gene of four temperature-sensitive mutants of A/WSN/33 influenza virus

Four influenza (A/WSN/33) mutants, temperature sensitive (ts) for neuraminidase (NA) (Sugiura et al., 1972, 1975) were analyzed. All four ts mutants were found to be defective at the nonpermissive temperature (39.5 degrees) both in enzymatic activity and in transport to the cell surface. Upon shift down to the permissive temperature (33 degrees), enzymatic activity and transport to the cell surface were both restored suggesting that the mutational defect is reversible. Comparative sequence analysis of the NA gene from ts mutants, their revertants and wild type WSN viruses revealed that in each case single point mutations causing amino acid substitutions were associated with the ts defect. The positions of each point mutation when mapped in the three-dimensional structure of NA varied. However, all four amino acid substitutions were located in beta-sheet strands of the head region. Several other amino acid changes not essential for the ts phenotype were found in each mutant NA. The nonessential changes were localized either in the stalk region or in the loop structures of the head, but none in the beta-sheet strands. Because both enzymatic activity and transport of NA were affected in all four mutants, we propose that the mutational phenotype is caused by a change in overall conformation rather than a localized change in the sialic acid binding site.

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.

Biological and immunological properties of haemagglutinin and neuraminidase expressed from cloned cDNAs in prokaryotic and eukaryotic cells

To study the biological and immunological properties of influenza virus surface glycoproteins, cDNA copies of the haemagglutinin (HA) and the neuraminidase (NA) genes of A/WSN/33 influenza virus were cloned and expressed in prokaryotic and eukaryotic cells. In Escherichia coli, maximum expression of HA is obtained only as a fusion protein in which the NH2-terminal portion is provided by a bacterial protein (i.e. beta gal or trpLE'). The HA expressed in bacteria (bacterial HA) is recognized by polyclonal anti-WSN antibodies but not by neutralizing monoclonal antibodies. The antibodies made against the bacterial HA bind to the detergent-treated viral HA, intact virus and live influenza infected cells, but fail to show either haemagglutination inhibition (HI) or virus neutralization. These results suggest that the three-dimensional structure as well as the antigenic epitopes of the bacterial HA are different from that of native viral HA. HA, expressed from cDNA in cultured animal cells, is shown to possess the structural features of the native viral HA. It is glycosylated, transported to the apical domain of the plasma membrane of polarized cells, causes haemadsorption and can induce cell to cell fusion at low pH after proteolytic cleavage. An attempt was made to define the structural features of HA required for sorting and directional transport by making chimeras with vesicular stomatitis virus G (VSV G) proteins either by switching the amino terminus or the carboxy terminus of HA with that of VSV G. These chimeric proteins were translocated across the rough endoplasmic reticulum (RER) but were blocked in transport between the RER and cell membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Sequence of the neuraminidase gene of an avian influenza A virus (A/parrot/ulster/73, H7N1)

The complete sequence of the neuraminidase (NA) gene of the influenza A strain A/parrot/ Ulster /73 ( H7N1 ) has been determined after reverse transcribing and cloning it into the pBR322 plasmid, followed by subcloning into M13 vectors and sequencing with dideoxynucleotide chain terminators. The gene consists of 1458 nucleotides and codes for a protein of 469 amino acids. The neuraminidase has seven potential glycosylation sites. According to the molecular weight as determined by electrophoretic migration in polyacrylamide gel all of these sites might carry a carbohydrate side chain. When the parrot Ulster NA was compared with two other N1 neuraminidases, those of the human PR8 and WSN strains, deletions in the stalk region of 15 amino acids for PR8 NA and of 16 amino acids for WSN NA were apparent. No further rearrangements were found within N1 neuraminidases. Although the parrot Ulster strain was isolated 40 years after the two human strains, the base sequence homology of their NA genes is still 83 or 82%, respectively.

Nucleotide sequence of the influenza virus A/USSR/90/77 neuraminidase gene

The complete nucleotide sequence of the N1 neuraminidase gene of influenza virus A/USSR/90/77 was determined. Comparison of its predicted amino acid sequence with other N1 and N2 neuraminidases indicates that the N1 neuraminidases share most of the antigenic determinants mapped on the N2 neuraminidase but display at least one additional potentially antigenic region probably as a result of intersubtypic differences in glycosylation.

NH2-terminal hydrophobic region of influenza virus neuraminidase provides the signal function in translocation

Influenza virus neuraminidase (NA), unlike the majority of integral membrane proteins, does not contain a cleavable signal sequence. It contains an NH2-terminal hydrophobic domain that functions as an anchor. We have investigated the signal function for translocation of this NH2-terminal hydrophobic domain of NA by constructing chimeric cDNA clones in which the DNA coding for the first 40 NH2-terminal hydrophobic amino acids of NA was joined to the DNA coding for the signal-minus hemagglutinin (HA) of influenza virus. The chimeric HA (N4OH) containing the NH2 terminus of NA was expressed in CV1 cells by using a simian virus 40 late-expression vector. The chimeric HA is synthesized, translocated into the rough endoplasmic reticulum, and glycosylated, whereas HA lacking the signal sequence is present only in small amounts and is unglycosylated. These results clearly show that the NH2 terminus of NA, in addition to its anchor function, also provides the signal function in translocation. However, the acquisition of complex oligosaccharides and the transport of N4OH to the cell surface are greatly retarded. To determine if the presence of two anchor sequences, one provided by NA at the NH2 terminus and the other provided by HA at the COOH terminus of N4OH, was responsible for the slow transport, the NH2 terminus of NA was fused to an "anchorless" HA. The resulting chimeric HA (N4OH482) contains the hydrophobic domain of NA at the NH2 terminus but lacks the HA anchor at the COOH terminus. N4OH482 was synthesized and glycosylated; however, as with N4OH, the acquisition of complex oligosaccharides and the migration to the cell surface are greatly retarded. Immunofluorescence data also support that, compared to the native HA, only a small amount of chimeric HA proteins is transported to the cell surface. Thus, the hydrophobic NH2 terminus of NA, although capable of providing the signal function in translocation across the rough endoplasmic reticulum, interferes with the transport of the chimeric HA to the cell surface.

Glycosylation and surface expression of the influenza virus neuraminidase requires the N-terminal hydrophobic region

A full-length double-stranded DNA copy of an influenza A virus N2 neuraminidase (NA) gene was cloned into the late region of pSV2330, a hybrid expression vector that includes pBR322 plasmid DNA sequences and the simian virus 40 early region and simian virus 40 late region promoters, splice sequences, and transcription termination sites. The protein encoded by the cloned wild-type NA gene was shown to be present in the cytoplasm of fixed cells and at the surface of "live" or unfixed cells by indirect immunofluorescence with N2 monoclonal antibodies. Immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis of [35S]methionine-labeled proteins from wild-type vector-infected cells with heterospecific N2 antibody showed that the product of the cloned NA DNA comigrated with glycosylated NA from influenza virus-infected cells, remained associated with internal membranes of cells fractionated into membrane and cytoplasmic fractions, and could form an immunoprecipitable dimer. NA enzymatic activity was detectable after simian virus 40 lysis of vector-infected cells. These properties of the product of the cloned wild-type gene were compared with those of the polypeptides produced by three deletion mutant NA DNAs that were also cloned into the late region of the pSV2330 vector. These mutants lacked 7 (dlk), 21 (dlI), or all 23 amino acids (dlZ) of the amino (N)-terminal variable hydrophobic region that anchors the mature wild-type NA tetrameric structure in the infected cell or influenza viral membrane. Comparison of the phenotypes of these mutants showed that this region in the NA molecule also includes sequences that control translocation of the nascent polypeptide into membrane organelles for glycosylation.

Glycosylation and surface expression of the influenza virus neuraminidase requires the N-terminal hydrophobic region

A full-length double-stranded DNA copy of an influenza A virus N2 neuraminidase (NA) gene was cloned into the late region of pSV2330, a hybrid expression vector that includes pBR322 plasmid DNA sequences and the simian virus 40 early region and simian virus 40 late region promoters, splice sequences, and transcription termination sites. The protein encoded by the cloned wild-type NA gene was shown to be present in the cytoplasm of fixed cells and at the surface of "live" or unfixed cells by indirect immunofluorescence with N2 monoclonal antibodies. Immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis of [35S]methionine-labeled proteins from wild-type vector-infected cells with heterospecific N2 antibody showed that the product of the cloned NA DNA comigrated with glycosylated NA from influenza virus-infected cells, remained associated with internal membranes of cells fractionated into membrane and cytoplasmic fractions, and could form an immunoprecipitable dimer. NA enzymatic activity was detectable after simian virus 40 lysis of vector-infected cells. These properties of the product of the cloned wild-type gene were compared with those of the polypeptides produced by three deletion mutant NA DNAs that were also cloned into the late region of the pSV2330 vector. These mutants lacked 7 (dlk), 21 (dlI), or all 23 amino acids (dlZ) of the amino (N)-terminal variable hydrophobic region that anchors the mature wild-type NA tetrameric structure in the infected cell or influenza viral membrane. Comparison of the phenotypes of these mutants showed that this region in the NA molecule also includes sequences that control translocation of the nascent polypeptide into membrane organelles for glycosylation.