Post-translational folding of the influenza C virus glycoprotein HEF: defective processing in cells expressing the cloned gene

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

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

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

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

Genomic and evolutionary characterization of a novel influenza-C-like virus from swine

We recently described the isolation of a novel influenza virus from swine exhibiting respiratory disease in the United States that is distantly related to human influenza C virus. Based on genetic, biochemical and morphological analysis, the virus was provisionally classified as C/swine/Oklahoma/1334/2011 (C/OK). To further understand the genetics and evolution of this novel pathogen, we performed a comprehensive analysis of its sequence and phylogeny. The results demonstrated that C/OK and human influenza C viruses share a conserved array of predicted functional domains in the viral RNA genome replication and viral entry machinery but vary at key functional sites. Furthermore, our evolutionary analysis showed that homologous genes of C/OK and human influenza C viruses diverged from each other an estimated several hundred to several thousand years ago. Taken together, the findings described in this study support and extend our previous observations that C/OK is a genetically and evolutionarily distinct influenza virus in the family Orthomyxoviridae.

Structure and functions of the human amyloid precursor protein: the whole is more than the sum of its parts

The amyloid precursor protein (APP) is a transmembrane protein that plays major roles in the regulation of several important cellular functions, especially in the nervous system, where it is involved in synaptogenesis and synaptic plasticity. The secreted extracellular domain of APP, sAPPalpha, acts as a growth factor for many types of cells and promotes neuritogenesis in post-mitotic neurons. Alternative proteolytic processing of APP releases potentially neurotoxic species, including the amyloid-beta (Abeta) peptide that is centrally implicated in the pathogenesis of Alzheimer's disease (AD). Reinforcing this biochemical link to neuronal dysfunction and neurodegeneration, APP is also genetically linked to AD. In this review, we discuss the biological functions of APP in the context of tissue morphogenesis and restructuring, where APP appears to play significant roles both as a contact receptor and as a diffusible factor. Structural investigation of APP, which is necessary for a deeper understanding of its roles at a molecular level, has also been advancing rapidly. We summarize recent progress in the determination of the structure of isolated APP fragments and of the conformations of full-length sAPPalpha, in both monomeric and dimeric states. The potential role of APP dimerization for the regulation of its biological functions is also discussed.

Use of influenza C virus glycoprotein HEF for generation of vesicular stomatitis virus pseudotypes

Influenza C virus contains two envelope glycoproteins: CM2, a putative ion channel protein; and HEF, a unique multifunctional protein that performs receptor-binding, receptor-destroying and fusion activities. Here, it is demonstrated that expression of HEF is sufficient to pseudotype replication-incompetent vesicular stomatitis virus (VSV) that lacks the VSV glycoprotein (G) gene. The pseudotyped virus showed characteristic features of influenza C virus with respect to proteolytic activation, receptor usage and cell tropism. Chimeric glycoproteins composed of HEF ectodomain and VSV-G C-terminal domains were efficiently incorporated into VSV particles and showed receptor-binding and receptor-destroying activities but, unlike authentic HEF, did not mediate efficient infection, probably because of impaired fusion activity. HEF-pseudotyped VSV efficiently infected polarized Madin-Darby canine kidney cells via the apical plasma membrane, whereas entry of VSV-G-complemented virus was restricted to the basolateral membrane. These findings suggest that pseudotyping of viral vectors with HEF might be useful for efficient apical gene transfer into polarized epithelial cells and for targeting cells that express 9-O-acetylated sialic acids.

Expression of unmodified hepatitis C virus envelope glycoprotein-coding sequences leads to cryptic intron excision and cell surface expression of E1/E2 heterodimers comprising full-length and partially deleted E1

Hepatitis C virus (HCV) is a positive-strand RNA virus that replicates exclusively in the cytoplasm of infected cells. The viral envelope glycoproteins, E1 and E2, appear to be retained in the endoplasmic reticulum, where viral budding is thought to occur. Surprisingly, we found that the expression system used to generate HCV envelope glycoproteins influences their subcellular localization and processing. These findings have important implications for optimizing novel HCV fusion and entry assays as well as for budding and virus particle formation.

Role of individual oligosaccharide chains in antigenic properties, intracellular transport, and biological activities of influenza C virus hemagglutinin-esterase protein

The hemagglutinin-esterase (HE) glycoprotein of influenza C virus is composed of three domains: a stem domain active in membrane fusion (F), an acetylesterase domain (E), and a receptor-binding domain (R). The protein contains eight N-linked glycosylation sites, four (positions 26, 395, 552, and 603) in the F domain, three (positions 61, 131, and 144) in the E domain, and one (position 189) in the R domain. Here, we investigated the role of the individual oligosaccharide chains in antigenic properties, intracellular transport, and biological activities of the HE protein by eliminating each of the glycosylation sites by site-specific mutagenesis. Comparison of electrophoretic mobility between the wild-type and the mutant proteins showed that while seven of the glycosylation sites are used, one (position 131) is not. Analysis of reactivity of the mutants with anti-HE monoclonal antibodies demonstrated that glycosylation at position 144 is essential for the formation of conformation-dependent epitopes. It was also evident that glycosylation at the two sites in the F domain (positions 26 and 603), in addition to that in the E domain (position 144), is required for the HE molecule to be transported from the endoplasmic reticulum and that mutant HEs lacking one of these three sites failed to undergo the trimer assembly. Removal of an oligosaccharide chain at position 144 or 189 resulted in a decrease in the esterase activity. By contrast, two mutants lacking an oligosaccharide chain at position 26 or 603, which were defective not only in cell surface expression but in trimerization, possessed full-enzyme activity, suggesting that the HE monomers present within the cell have acetylesterase activity. Fusion activity of cells expressing each of mutant HEs was found to be comparable with the ability of the protein to be transported to the cell surface, suggesting that there is no specific oligosaccharide chain that plays a critical role in promoting membrane fusion.

Cell surface expression of biologically active influenza C virus HEF glycoprotein expressed from cDNA

The hemagglutinin, esterase, and fusion (HEF) glycoprotein of influenza C virus possesses receptor binding, receptor destroying, and membrane fusion activities. The HEF cDNAs from influenza C/Ann Arbor/1/50 (HEF-AA) and influenza C/Taylor/1223/47 (HEF-Tay) viruses were cloned and expressed, and transport of HEF to the cell surface was monitored by susceptibility to cleavage by exogenous trypsin, indirect immunofluorescence microscopy, and flow cytometry. Previously it has been found in studies with the C/Johannesburg/1/66 strain of influenza C virus (HEF-JHB) that transport of HEF to the cell surface is severely inhibited, and it is thought that the short cytoplasmic tail, Arg-Thr-Lys, is involved in blocking HEF cell surface expression (F. Oeffner, H.-D. Klenk, and G. Herrler, J. Gen. Virol. 80:363-369, 1999). As the cytoplasmic tail amino acid sequences of HEF-AA and HEF-Tay are identical to that of HEF-JHB, the data indicate that cell surface expression of HEF-AA and HEF-Tay is not inhibited by this amino acid sequence. Furthermore, the abundant cell surface transport of HEF-AA and HEF-Tay indicates that their cell surface expression does not require coexpression of another viral protein. The HEF-AA and HEF-Tay HEF glycoproteins bound human erythrocytes, promoted membrane fusion in a low-pH and trypsin-dependent manner, and displayed esterase activity, indicating that the HEF glycoprotein alone mediates all three known functions at the cell surface.

Location of a linear epitope recognized by monoclonal antibody S16 on the hemagglutinin-esterase glycoprotein of influenza C virus

We reported previously that monoclonal antibody S16, which had been raised against the hemagglutinin-esterase (HE) glycoprotein of influenza C/Ann Arbor/1/50 (AA/50) virus, recognizes a linear epitope present on the HE molecules of all influenza C viruses examined except for viruses belonging to a lineage represented by Aichi/1/81 (AI/81). Comparison of the deduced amino acid sequence of HE between viruses on the AI/81-related lineage and those on the others suggests that the epitope recognized by S16 is located in a region containing amino acid residue 403 and that a change from Glu to Lys at this position causes the loss of reactivity with the antibody. To prove it, the wild type (WT) HEs of AA/50 and AI/81 as well as their mutants with an amino acid substitution at residue 403 were expressed in CV-1 cells from the recombinant simian virus 40 (SV40) and tested for reactivity with S16 by immunoprecipitation. The results showed that the AA/50 virus WT and AI/81 virus mutant HEs (both having Glu at residue 403) were reactive with S16 whereas the AI/81 virus WT and AA/50 virus mutant HEs (both having Lys at residue 403) were not. Furthermore, we examined the reactivity of S16 with two synthetic peptides (corresponding to residues 397-409) that possess Glu and Lys at position 403, respectively, by enzyme-linked immunosorbent assays. The results demonstrated that the former peptide but not the latter was reactive with S16. These observations support strongly the notion described above. During this study it was also found that S16 cross-reacts with large T antigen of SV40.

The CM2 protein of influenza C virus is an oligomeric integral membrane glycoprotein structurally analogous to influenza A virus M2 and influenza B virus NB proteins

We have undertaken a characterization of the CM2 protein of influenza C virus. The CM2 coding region of RNA segment 6 (nucleotides 731-1147) was cloned from two strains of influenza C virus and expressed using the vaccinia virus-bacteriophage T7 RNA polymerase (vac-T7) system. An antiserum raised to a C-terminal peptide in the CM2 open reading frame recognized the CM2 protein in influenza C virus-infected cells and after vac-T7 expression of the CM2 open reading frame. CM2 is posttranslationally modified by addition of high-mannose carbohydrate chains (Mr approximately 18 kDa) and by further addition of polylactosaminoglycans (Mr approximately 21-35 kDa). The available data indicate that CM2 has a cleavable signal peptide at the N-terminus of the protein. Site-directed mutagenesis eliminated the single potential N-linked carbohydrate attachment site on CM2 and indicated that the protein has an NoutCin orientation in membranes. Nonreducing SDS-PAGE indicated that the protein was expressed as disulfide-linked dimers and tetramers. Cell surface biotinylation and indirect immunofluorescence showed the protein to be expressed at the cell surface. Elimination of the N-linked carbohydrate attachment site and addition of a C-terminal HA epitope tag did not adversely affect surface expression of CM2. The NoutCin membrane orientation of CM2, the size of the ectodomain and cytoplasmic tail of CM2, and its ability to form disulfide-linked oligomers are reminiscent of the structural properties of influenza A virus M2 and influenza B virus NB proteins.

Differential fatty acid selection during biosynthetic S-acylation of a transmembrane protein (HEF) and other proteins in insect cells (Sf9) and in mammalian cells (CV1)

The transmembrane glycoprotein HEF and its acylation deficient mutant M1 were expressed in Sf9 insect cells infected with recombinant baculovirus and in CV1 mammalian cells using the vaccinia T7 system. In insect cells (Sf9), both wild type HEF and HEF(M1) are synthesized in their precursor form HEF0, which appears as a double band in SDS gels. Digestion with glycopeptidase F and endoglycosidase H reveals that the larger 84-kDa form is modified by the attachment of unprocessed carbohydrates of the high mannose type whereas the smaller 76-kDa form is non-glycosylated. As revealed by in vitro labeling experiments with palmitic acid another modification of HEF is the attachment of a long chain fatty acid to cysteine residue Cys-652 which is located at the internal border of the cytoplasmic membrane. After labeling with [3H]palmitic acid in both systems only HEF(WT) is acylated, whereas HEF(M1) is not. High performance liquid chromatography analysis of the fatty acids bound to HEF(WT) expressed in Sf9 insect cells reveals nearly 80% of palmitic acid. In contrast to this finding, the acylation pattern of HEF expressed in CV1 cells shows nearly the same amounts of stearic and palmitic acid (40%). Since the interconversion of the input [3H]palmitic acid to stearic acid is even lower in CV1 cells than in insect cells, it follows that only HEF expressed in mammalian, but not in insect cells selects for stearic acid during its biosynthetic acylation. We extended our study to acylation of endogenous proteins in Sf9 cells. In finding only palmitate linked to protein we present evidence that, in contrast to mammalian cells, insect cells (Sf9) cannot transfer stearic acid to polypeptide. This finding favors the hypothesis of enzymatic acylation over non-enzymatic mechanisms of acyl transfer to protein.

Cytoplasmic tail length influences fatty acid selection for acylation of viral glycoproteins

We report remarkable differences in the fatty acid content of thioester-type acylated glycoproteins of enveloped viruses from mammalian cells. The E2 glycoprotein of Semliki Forest virus contains mainly palmitic acid like most other palmitoylated proteins analysed so far. However, the other glycoprotein (E1) of the same virus, as well as the HEF (haemagglutinin esterase fusion) glycoprotein of influenza C virus, are unique in this respect because they are acylated primarily with stearic acid. Comparative radiolabelling of uninfected cells with different fatty acids suggests that stearate may also be the prevailing fatty acid in some cellular acylproteins. To look for further differences between palmitoylated and stearoylated glycoproteins we characterized stearoylation in more detail. We identified the acylation site of HEF as a cysteine residue located at the boundary between the transmembrane region and the cytoplasmic tail. The attachment of stearate to HEF and E1 occurs post-translationally in a pre-Golgi compartment. Thus, stearoylated and palmitoylated proteins cannot be discriminated on the basis of the fatty acid linkage site or the intracellular compartment, where acylation occurs. However, stearoylated acylproteins contain a very short, positively charged cytoplasmic tail, whereas in palmitoylated proteins this molecular region is longer. Replacing the short cytoplasmic tail of stearoylated HEF with the long influenza A virus haemagglutinin (HA) tail in an HEF-HA chimera, and subsequent vaccinia T7 expression in CV-1 cells, yielded proteins with largely palmitic acid bound. The reverse chimera, HA-HEF with a short cytoplasmic tail was not fatty acylated at all during expression, indicating that conformational or topological constraints control fatty acid transfer.