Human influenza virus hemagglutinin with high sensitivity to proteolytic activation

Hemagglutinin Subtype Specificity and Mechanisms of Highly Pathogenic Avian Influenza Virus Genesis

Highly Pathogenic Avian Influenza Viruses (HPAIVs) arise from low pathogenic precursors following spillover from wild waterfowl into poultry populations. The main virulence determinant of HPAIVs is the presence of a multi-basic cleavage site (MBCS) in the hemagglutinin (HA) glycoprotein. The MBCS allows for HA cleavage and, consequently, activation by ubiquitous proteases, which results in systemic dissemination in terrestrial poultry. Since 1959, 51 independent MBCS acquisition events have been documented, virtually all in HA from the H5 and H7 subtypes. In the present article, data from natural LPAIV to HPAIV conversions and experimental in vitro and in vivo studies were reviewed in order to compile recent advances in understanding HA cleavage efficiency, protease usage, and MBCS acquisition mechanisms. Finally, recent hypotheses that might explain the unique predisposition of the H5 and H7 HA sequences to obtain an MBCS in nature are discussed.

The Emergence of H7N7 Highly Pathogenic Avian Influenza Virus from Low Pathogenicity Avian Influenza Virus Using an in ovo Embryo Culture Model

Outbreaks of highly pathogenic avian influenza virus (HPAIV) often result in the infection of millions of poultry, causing up to 100% mortality. HPAIV has been shown to emerge from low pathogenicity avian influenza virus (LPAIV) in field outbreaks. Direct evidence for the emergence of H7N7 HPAIV from a LPAIV precursor with a rare di-basic cleavage site (DBCS) was identified in the UK in 2008. The DBCS contained an additional basic amino acid compared to commonly circulating LPAIVs that harbor a single-basic amino acid at the cleavage site (SBCS). Using reverse genetics, outbreak HPAIVs were rescued with a DBCS (H7N7_DB), as seen in the LPAIV precursor or an SBCS representative of common H7 LPAIVs (H7N7_SB). Passage of H7N7_DB in chicken embryo tissues showed spontaneous evolution to a HPAIV. In contrast, deep sequencing of extracts from embryo tissues in which H7N7_SB was serially passaged showed retention of the LPAIV genotype. Thus, in chicken embryos, an H7N7 virus containing a DBCS appears naturally unstable, enabling rapid evolution to HPAIV. Evaluation in embryo tissue presents a useful approach to study AIV evolution and allows a laboratory-based dissection of molecular mechanisms behind the emergence of HPAIV.

Role of host trypsin-type serine proteases and influenza virus-cytokine-trypsin cycle in influenza viral pathogenesis. Pathogenesis-based therapeutic options

Influenza A virus (IAV) is one of the most common infectious pathogen and associated with significant morbidity and mortality. Although processing the IAV hemagglutinin (HA) envelope glycoprotein precursor is a pre-requisite for viral membrane fusion activity, viral entry and transmission, HA-processing protease is not encoded in the IAV genome and thus the cellular trypsin-type serine HA-processing proteases determine viral infectious tropism and viral pathogenicity. The initial process of IAV infection of the airway is followed by marked upregulation of ectopic trypsin in various organs and endothelial cells through the induction of various proinflammatory cytokines, and this process has been termed the "influenza virus-cytokine-trypsin" cycle. In the advanced stage of IAV infection, the cytokine storm induces disorders of glucose and lipid metabolism and the "metabolic disorders-cytokine" cycle is then linked with the "influenza virus-cytokine-trypsin" cycle, to advance the pathogenic process into energy crisis and multiple organ failure. Application of protease inhibitors and treatment of metabolic disorders that break these cycles and their interconnection is therefore a promising therapeutic approach against influenza. This review discusses IAV pathogenicity on trypsin type serine HA-processing proteases, cytokines, metabolites and therapeutic options.

A mutant H3N2 influenza virus uses an alternative activation mechanism in TMPRSS2 knockout mice by loss of an oligosaccharide in the hemagglutinin stalk region

The host protease TMPRSS2 plays an essential role in proteolytic activation of the influenza A virus (IAV) hemagglutinin (HA) protein possessing a monobasic cleavage site. However, after passages in TMPRSS2 knockout mice, an H3N2 subtype IAV began to undergo cleavage activation of HA, showing high virulence in the mice due to the loss of an oligosaccharide at position 8 in the HA stalk region. Thus, the H3N2 IAV acquired cleavability by an alternative HA activation mechanism/protease(s).

The hemagglutinin: a determinant of pathogenicity

The hemagglutinin (HA) is a prime determinant of the pathogenicity of influenza A viruses. It initiates infection by binding to cell surface receptors and by inducing membrane fusion. The fusion capacity of HA depends on cleavage activation by host proteases, and it has long been known that highly pathogenic avian influenza viruses displaying a multibasic cleavage site differ in protease sensitivity from low pathogenic avian and mammalian influenza viruses with a monobasic cleavage site. Evidence is increasing that there are also variations in proteolytic activation among the viruses with a monobasic cleavage site, and several proteases have been identified recently that activate these viruses in a natural setting. Differences in protease sensitivity of HA and in tissue specificity of the enzymes are important determinants for virus tropism in the respiratory tract and for systemic spread of infection. Protease inhibitors that interfere with cleavage activation have the potential to be used for antiviral therapy and attenuated viruses have been generated by mutation of the cleavage site that can be used for the development of inactivated and live vaccines. It has long been known that human and avian influenza viruses differ in their specificity for sialic acid-containing cell receptors, and it is now clear that human tissues contain also receptors for avian viruses. Differences in receptor-binding specificity of seasonal and zoonotic viruses and differential expression of receptors for these viruses in the human respiratory tract account, at least partially, for the severity of disease. Receptor binding and fusion activation are modulated by HA glycosylation, and interaction of the glycans of HA with cellular lectins also affects virus infectivity. Interestingly, some of the mechanisms underlying pathogenicity are determinants of host range and transmissibility, as well.

Influenza HA subtypes demonstrate divergent phenotypes for cleavage activation and pH of fusion: implications for host range and adaptation

The influenza A virus (IAV) HA protein must be activated by host cells proteases in order to prime the molecule for fusion. Consequently, the availability of activating proteases and the susceptibility of HA to protease activity represents key factors in facilitating virus infection. As such, understanding the intricacies of HA cleavage by various proteases is necessary to derive insights into the emergence of pandemic viruses. To examine these properties, we generated a panel of HAs that are representative of the 16 HA subtypes that circulate in aquatic birds, as well as HAs representative of the subtypes that have infected the human population over the last century. We examined the susceptibility of the panel of HA proteins to trypsin, as well as human airway trypsin-like protease (HAT) and transmembrane protease, serine 2 (TMPRSS2). Additionally, we examined the pH at which these HAs mediated membrane fusion, as this property is related to the stability of the HA molecule and influences the capacity of influenza viruses to remain infectious in natural environments. Our results show that cleavage efficiency can vary significantly for individual HAs, depending on the protease, and that some HA subtypes display stringent selectivity for specific proteases as activators of fusion function. Additionally, we found that the pH of fusion varies by 0.7 pH units among the subtypes, and notably, we observed that the pH of fusion for most HAs from human isolates was lower than that observed from avian isolates of the same subtype. Overall, these data provide the first broad-spectrum analysis of cleavage-activation and membrane fusion characteristics for all of the IAV HA subtypes, and also show that there are substantial differences between the subtypes that may influence transmission among hosts and establishment in new species.

IAV is associated with significant morbidity and mortality, and represents a challenging public health threat that affects social and economic welfare each year, particularly during IAV pandemics. Although we know that all human strains derive, either directly or via intermediate hosts, from avian viral sources, we know very little about the phenotypic characteristics of the 16 HA subtypes that circulate in aquatic birds and have potential to infect mammals. HA membrane fusion properties, in conjunction with the characteristics for protease activation of HA, a requirement for fusion, are critical factors involved in the ecology and transmission of IAVs, and need to be understood if we are to derive explanations for how pandemic viruses emerge in humans. We examined the cleavage-activation and membrane fusion characteristics for the 16 HA subtypes by transiently expressing HA proteins in cells. Our findings show that the cleavability of the HAs vary considerably between subtypes and depending on the protease. Additionally, analysis of the pH of fusion for each subtype showed that HA stability varied significantly among the subtypes, as well as within subtypes from viruses isolated from different species. Overall, these data have implications for host range, potential for adaptation, and persistence in natural environments.

H9 avian influenza reassortant with engineered polybasic cleavage site displays a highly pathogenic phenotype in chicken

In the field, highly pathogenic avian influenza viruses (HPAIV) originate from low-pathogenic strains of the haemagglutinin (HA) serotypes H5 and H7 that have acquired a polybasic HA cleavage site. This observation suggests the presence of a cryptic virulence potential of H5 and H7 low-pathogenic avian influenza viruses (LPAIV). Among all other LPAIV, the H9N2 strains are of particular relevance as they have become widespread across many countries in several avian species and have been transmitted to humans. To assess the potential of these strains to transform into an HPAIV, we introduced a polybasic cleavage site into the HA of a contemporary H9N2 isolate. Whereas the engineered polybasic HA cleavage site mutant remained a low-pathogenic strain like its parent virus, a reassortant expressing the modified H9 HA with engineered polybasic cleavage site and all the other genes from an H5N1 HPAIV became highly pathogenic in chicken with an intravenous pathogenicity index of 1.23. These results suggest that an HPAIV with a subtype other than H5 or H7 would only emerge under conditions where the HA gene could acquire a polybasic cleavage site and the other viral genes carry additional virulence determinants.

Aprotinin and similar protease inhibitors as drugs against influenza

Efforts to develop new antiviral chemotherapeutic approaches are focusing on compounds that target either influenza virus replication itself or host factor(s) that are critical to influenza replication. Host protease mediated influenza hemagglutinin (HA) cleavage is critical for activation of virus infectivity and as such is a chemotherapeutic target. Influenza pathogenesis involves a "vicious cycle" in which host proteases activate progeny virus which in turn amplifies replication and stimulates further protease activities which may be detrimental to the infected host. Aprotinin, a 58 amino acid polypeptide purified from bovine lung that is one of a family of host-targeted antivirals that inhibit serine proteases responsible for influenza virus activation. This drug and similar agents, such as leupeptin and camostat, suppress virus HA cleavage and limit reproduction of human and avian influenza viruses with a single arginine in the HA cleavage site. Site-directed structural modifications of aprotinin are possible to increase its intracellular targeting of cleavage of highly virulent H5 and H7 hemagglutinins possessing multi-arginine/lysine cleavage site. An additional mechanism of action for serine protease inhibitors is to target a number of host mediators of inflammation and down regulate their levels in virus-infected hosts. Aprotinin is a generic drug approved for intravenous use in humans to treat pancreatitis and limit post-operative bleeding. As an antiinfluenzal compound, aprotinin might be delivered by two routes: (i) a small-particle aerosol has been approved in Russia for local respiratory application in mild-to-moderate influenza and (ii) a proposed intravenous administration for severe influenza to provide both an antiviral effect and a decrease in systemic pathology and inflammation.

Insertion of a multibasic cleavage site in the haemagglutinin of human influenza H3N2 virus does not increase pathogenicity in ferrets

A multibasic cleavage site (MBCS) in the haemagglutinin (HA) protein of influenza A virus is a key determinant of pathogenicity in chickens, and distinguishes highly pathogenic avian influenza (HPAI) viruses from low pathogenic avian influenza viruses (LPAI). An MBCS has only been detected in viruses of the H5 and H7 subtypes. Here we investigated the phenotype of a human H3N2 virus with an MBCS in HA. Insertion of an MBCS in the H3N2 virus resulted in cleavage of HA and efficient replication in Madin-Darby canine kidney cells in the absence of exogenous trypsin in vitro, similar to HPAI H5N1 virus. However, studies in ferrets demonstrated that insertion of the MBCS into HA did not result in increased virus shedding, cellular host range, systemic replication or pathogenicity, as compared with wild-type virus. This study indicates that acquisition of an MBCS alone is insufficient to increase pathogenicity of a prototypical seasonal human H3N2 virus.

Novel insights into proteolytic cleavage of influenza virus hemagglutinin

The influenza virus hemagglutinin (HA) mediates the first essential step in the viral life cycle, virus entry into target cells. Influenza virus HA is synthesised as a precursor protein in infected cells and requires cleavage by host cell proteases to transit into an active form. Cleavage is essential for influenza virus infectivity and the HA‐processing proteases are attractive targets for therapeutic intervention. It is well established that cleavage by ubiquitously expressed subtilisin‐like proteases is a hallmark of highly pathogenic avian influenza viruses (HPAIV). In contrast, the nature of the proteases responsible for cleavage of HA of human influenza viruses and low pathogenic avian influenza viruses (LPAIV) is not well understood. Recent studies suggest that cleavage of HA of human influenza viruses might be a cell‐associated event and might be facilitated by the type II transmembrane serine proteases (TTSPs) TMPRSS2, TMPRSS4 and human airway trypsin‐like protease (HAT). Here, we will introduce the different concepts established for proteolytic activation of influenza virus HA, with a particular focus on the role of TTSPs, and we will discuss their implications for viral tropism, pathogenicity and antiviral intervention. Copyright © 2010 John Wiley & Sons, Ltd.

Insertion of a multibasic cleavage motif into the hemagglutinin of a low-pathogenic avian influenza H6N1 virus induces a highly pathogenic phenotype

The highly pathogenic avian influenza (HPAI) virus phenotype is restricted to influenza A viruses of the H5 and H7 hemagglutinin (HA) subtypes. To obtain more information on the apparent subtype-specific nature of the HPAI virus phenotype, a low-pathogenic avian influenza (LPAI) H6N1 virus was generated, containing an HPAI H5 RRRKKR [downward arrow] G multibasic cleavage site (MBCS) motif in HA (the downward arrow indicates the site of cleavage). This insertion converted the LPAI virus phenotype into an HPAI virus phenotype in vitro and in vivo. The H6N1 virus with an MBCS displayed in vitro characteristics similar to those of HPAI H5 viruses, such as cleavage of HA(0) (the HA protein of influenza A virus initially synthesized as a single polypeptide precursor) and virus replication in the absence of exogenous trypsin. Studies of chickens confirmed the HPAI phenotype of the H6N1 virus with an MBCS, with an intravenous pathogenicity index of 1.4 and systemic virus replication upon intranasal inoculation, the hallmarks of HPAI viruses. This study provides evidence that the subtype-specific nature of the emergence of HPAI viruses is not at the molecular, structural, or functional level, since the introduction of an MBCS resulted in a fully functional virus with an HPAI virus genotype and phenotype.

Novel type II transmembrane serine proteases, MSPL and TMPRSS13, Proteolytically activate membrane fusion activity of the hemagglutinin of highly pathogenic avian influenza viruses and induce their multicycle replication

Host cellular proteases induce influenza virus entry into cells by cleaving the viral surface envelope glycoprotein hemagglutinin (HA). However, details on the cellular proteases involved in this event are not fully available. We report here that ubiquitous type II transmembrane serine proteases, MSPL and its splice variant TMPRSS13, are novel candidates for proteases processing HA proteins of highly pathogenic avian influenza (HPAI) viruses, apart from the previously identified furin and proprotein convertases 5 and 6. HAs from all HPAI virus H5 and H7 strains have one of two cleavage site motifs, the R-X-K/R-R motif with R at position P4 and the K-K/R-K/T-R motif with K at position P4. In studies of synthetic 14-residue HPAI virus HA peptides with these cleavage site motifs, furin preferentially cleaved only HA peptides with the R-K-K-R motif in the presence of calcium and not peptides with the other motif, whereas MSPL and TMPRSS13 cleaved both types of HA peptides (those with the R/K-K-K-R motif) efficiently in the absence of calcium. Full-length recombinant HPAI virus HA with the K-K-K-R cleavage motif exhibited poor susceptibility to cleavage in the absence of MSPL or TMPRSS13 and the presence of furin in infected cells, but it was converted to mature HA subunits in transfected cells expressing MSPL or TMPRSS13, with membrane-fused giant-cell formation. This conversion and membrane fusion were suppressed by inhibitors of MSPL and TMPRSS13. Furthermore, infection with and multiplication of genetically modified live HPAI virus A/Crow/Kyoto/53/2004 (H5N1) with the K-K-K-R cleavage site motif were detected only in MSPL- and TMPRSS13-expressing cells.

Acquisition of a polybasic hemagglutinin cleavage site by a low-pathogenic avian influenza virus is not sufficient for immediate transformation into a highly pathogenic strain

Highly pathogenic avian influenza viruses (HPAIV) differ from all other strains by a polybasic cleavage site in their hemagglutinin. All these HPAIV share the H5 or H7 subtype. In order to investigate whether the acquisition of a polybasic cleavage site by an avirulent avian influenza virus strain with a hemagglutinin other than H5 or H7 is sufficient for immediate transformation into an HPAIV, we adapted the hemagglutinin cleavage site of A/Duck/Ukraine/1/1963 (H3N8) to that of the HPAIV A/Chicken/Italy/8/98 (H5N2), A/Chicken/HongKong/220/97 (H5N1), or A/Chicken/Germany/R28/03 (H7N7) and generated the recombinant wild-type and cleavage site mutants. In contrast to the wild type, multicycle replication of these mutants in tissue culture was demonstrated by positive plaque assays and viral multiplication in the absence of exogenous trypsin. Therefore, in vitro all cleavage site mutants resemble an HPAIV. However, in chicken they did not exhibit high pathogenicity, although they could be reisolated from cloacal swabs to some extent, indicating enhanced replication in vivo. These results demonstrate that beyond the polybasic hemagglutinin cleavage site, the virulence of HPAIV in chicken is based on additional pathogenicity determinants within the hemagglutinin itself or in the other viral proteins. Taken together, these observations support the notion that acquisition of a polybasic hemagglutinin cleavage site by an avirulent strain with a non-H5/H7 subtype is only one among several alterations necessary for evolution into an HPAIV.

New avian influenza A virus subtype combination H5N7 identified in Danish mallard ducks

During the past years increasing incidences of influenza A zoonosis have made it of uppermost importance to possess methods for rapid and precise identification and characterisation of influenza A viruses. We present here a convenient one-step RT-PCR method that will amplify full-length haemagglutinin (HA) and neuraminidase (NA) directly from clinical samples and from all known subtypes of influenza A. We applied the method on samples collected in September 2003 from a Danish flock of mallards with general health problems and by this a previously undescribed influenza A subtype combination, H5N7, was identified. The HA gene showed great sequence similarity to the highly pathogenic avian influenza A virus (HPAIV) A/Chicken/Italy/312/97 (H5N2); however, the cleavage site sequence between HA1 and HA2 had a motif typical for low pathogenic avian influenza viruses (LPAIV). The full-length NA sequence was most closely related to the HPAIV A/Chicken/Netherlands/01/03 (H7N7) that infected chickens and humans in the Netherlands in 2003. Ten persons with direct or indirect contact with the Danish mallard ducks showed signs of influenza-like illness 2-3 days following the killing of the ducks, but no evidence of influence infections was detected. To our knowledge this is the first report of an H5N7 influenza A virus.

The role of M cells of human nasopharyngeal lymphoid tissue in influenza virus sampling

Little is known about the role of the M cells of human nasopharyngeal lymphoid tissue in the sampling of viruses that cause respiratory infections. To clarify whether M cells could function as a gateway for influenza virus into human nasopharyngeal lymphoid tissue, excised adenoid tissue was incubated in media containing influenza A virus for 30, 60, and 90 min, respectively. Transmission electron microscopic observation revealed that many influenza viruses adhered to M cell surfaces and were taken up into the cytoplasmic vesicles of M cells after 30 min incubation; the viruses had been transported into enfolded lymphoid cells after 60 min incubation. By staining M cells with Sambucus nigra lectin, which specifically recognizes the NeuAcalpha2,6 Gal linkage of sialoprotein, it was also found that abundant receptors for the human influenza virus are present on the M cell surface. Our findings indicated that M cells of human nasopharyngeal tonsils function as a major port for influenza A virus entry and that the virus could be efficiently transferred to enfolded macrophages and lymphoid cells by M cells. The transport of influenza viruses to lymphoid cells by M cells may promote antigen delivery to the immune system, and these findings may be important for systemic delivery of those influenza viruses that have the capacity to productively infect cells outside of the respiratory tract.

Human influenza A viruses are proteolytically activated and do not induce apoptosis in CACO-2 cells

Replication of human influenza A/H3N2 and A/H1N1 viruses was studied in human CACO-2 cells, a continuous line of intestinal epithelial differentiated cells. Hemagglutinin (HA) was cleaved in these cells by an endogenous protease. Thus, infectious virus was produced that underwent multiple cycle replication and plaque formation in the absence of trypsin added to the media. Cleavage of de novo-synthesized HA occurred at a late stage of the exocytic pathway as indicated by pulse-chase labeling and by experiments employing endoglycosidase H and brefeldin A treatment. However, surface-labeling experiments employing biotinylation suggested that there is no cleavage at the plasma membrane. Unlike HA of serotypes H5 and H7 cleaved at multibasic cleavage sites by furin, the HAs with monobasic cleavage sites analyzed here were not cleaved in CACO-2 cells in the presence of aprotinin, a natural inhibitor of trypsinlike proteases. Growing CACO-2 cells were able to cleave HA of incoming virus, although influenza virus activating protease was not detected in culture medium. These observations indicate that the activating enzyme of CACO-2 cells is a trypsinlike protease functioning in the trans-Golgi network and presumably endosomes. In support of this concept immune staining with antibodies specific to human and bovine trypsin revealed the presence of a trypsinlike protease in CACO-2 cells. Unlike MDCK and CV-1 cells undergoing rapid apoptosis after influenza virus infection, CACO-2 cells showed no apoptosis but displayed cytopathic effects with necrotic signs significantly later after infection. It follows from these data that, depending on the cell type, influenza virus may kill cells either by apoptosis or by necrosis.

Cleavage of influenza a virus hemagglutinin in human respiratory epithelium is cell associated and sensitive to exogenous antiproteases

Proteolytic cleavage of the hemagglutinin (HA) of human influenza viruses A/Aichi/2/68 (H3N2) and A/WSN/34 (H1N1) from HA0 to HA1/HA2 was studied in primary human adenoid epithelial cells (HAEC). HAEC contain a mixture of ciliated and nonciliated secretory cells and mimic the epithelium membrane of the human respiratory tract. Pulse-chase labeling with [(35)S]methionine and Western blot analysis with anti-HA antibodies of cellular and virion polypeptides showed that HAEC cleaved newly synthesized HA0 to HA1/HA2 ("cleavage from within") and significant amounts of cleaved HA accumulated within cells. It was also shown that HAEC was able to cleave HA0 of incoming virions ("cleavage from without"), whereas the HA0 of nonabsorbed virions free in extracellular fluid were not cleaved, supporting the conclusion that HA0 cleavage in HAEC is cell associated. Low-molecular-weight inhibitors of serine proteases, aprotinin and leupeptin, when added to influenza virus-infected HAEC suppressed HA0 cleavage and reduced the amount of cleaved HA1/HA2 both in cells and in progeny virions and thus diminished the infectivity of the virus. In contrast, the addition of fetal bovine serum, containing a number of high-molecular-weight antiproteases that compete for proteases in the extracellular environment, did not inhibit influenza virus growth in HAEC. These data suggest that in human respiratory epithelium the cleavage of influenza virus HA containing a single arginine in the proteolytic site (i) is a cell-associated process accomplished by serine-type protease(s) and (ii) is sensitive to low-molecular-weight exogenous inhibitors of serine proteases.

[Factors affecting the virulence and pathogenicity of avian and human viral strains (influenza virus type A)]

Most studies performed in avian viral strains seem to indicate that virulence is a polygenic phenomenon. However, hemagglutinin and neuraminidase and the genes codifying these substances (genes 4 and 6) play an essential role in viral pathogenesis. Avian strains can be classified as avirulent or virulent according to the ability of hemagglutinin to be activated by endoproteases of the respiratory tract only or by proteases from other tissues. This ability is based on the progressive development of mutations that lead to the substitution of the normal amino acids at the point of hemagglutinin hydrolysis by the other basic amino acids that determine the amplification of the spectrum of hydrolysis and activation. Neuraminidase participates in the acquisition of virulence through its capacity to bind to plasminogen and by increasing the concentration of activating proteases. Adaptation to the host, through recognition of the cell receptor, is another factor determining the virulence and interspecies transmission of avian strains. From an epidemiological point of view, viral strains should be subtyped and the activating capacity of hemagglutinin should be determined to identify their degree of virulence.

Inhibition of release of lentivirus particles with incorporated human influenza virus haemagglutinin by binding to sialic acid-containing cellular receptors

Mutants of the haemagglutinin (HA) gene of human influenza virus A/Aichi/2/68 (H3N2) encoding HA proteins that are proteolytically cleaved intracellularly, defective in binding to cellular receptors or defective for acylation within the cytoplasmic C terminus have been generated. Here, the properties of these mutated HA molecules are described and their incorporation into the lipid membrane of released human immunodeficiency virus (HIV)-like particles is analysed. It is demonstrated that, when produced from cells coexpressing any of the binding-competent Aichi-HA molecules, release of HIV-like particles into the extracellular medium is reduced and the particles that are released fail to incorporate Aichi-HA. These blocks in release and incorporation, respectively, can both be overcome. The release of normal amounts of particles with incorporated HA can be achieved either by mutation of the receptor-binding site on the Aichi-HA molecule or by removal of sialic acid from surface proteins with neuraminidase. In contrast, as a result of blockage of the sialic acid-binding site by sialidated oligosaccharides on the HA itself, the HA of influenza virus A/FPV/Rostock/34 (H7N1) is efficiently incorporated into HIV-like particles. These results, namely that particle release can be inhibited by interactions between the incorporated glycoprotein and the cell surface and/or that interactions with other cellular components can be inhibitory to incorporation into retrovirus envelopes, probably reflect general principles that may hold for many viral and cellular glycoproteins.

Generation of a highly pathogenic avian influenza A virus from an avirulent field isolate by passaging in chickens

Highly virulent avian influenza viruses can arise from avirulent strains maintained in poultry, but evidence to support their generation from viruses in wild birds is lacking. The most likely mechanism for the acquisition of virulence by benign avian viruses is the introduction of mutations by error-prone RNA polymerase, followed by the selection of virulent viruses. To investigate whether this mechanism could apply to wild waterfowl, we studied an avirulent wild-swan virus that replicates poorly in chickens. After 24 consecutive passages by air sac inoculation, followed by five passages in chicken brain, the avirulent virus became highly pathogenic in chickens, producing a 100% mortality rate. Sequence analysis at the hemmaglutinin cleavage site of the original isolate revealed a typical avirulence type of sequence, R-E-T-R, which progressed incrementally to a typical virulence type of sequence, R-R-K-K-R, during repeated passages in chickens. These results demonstrate that avirulent viruses maintained in wild waterfowl in nature and bearing the consensus avirulence type sequence R-E-T-R have the potential to become highly pathogenic while circulating in chickens.

Pandemic threat posed by avian influenza A viruses

Influenza pandemics, defined as global outbreaks of the disease due to viruses with new antigenic subtypes, have exacted high death tolls from human populations. The last two pandemics were caused by hybrid viruses, or reassortants, that harbored a combination of avian and human viral genes. Avian influenza viruses are therefore key contributors to the emergence of human influenza pandemics. In 1997, an H5N1 influenza virus was directly transmitted from birds in live poultry markets in Hong Kong to humans. Eighteen people were infected in this outbreak, six of whom died. This avian virus exhibited high virulence in both avian and mammalian species, causing systemic infection in both chickens and mice. Subsequently, another avian virus with the H9N2 subtype was directly transmitted from birds to humans in Hong Kong. Interestingly, the genes encoding the internal proteins of the H9N2 virus are genetically highly related to those of the H5N1 virus, suggesting a unique property of these gene products. The identification of avian viruses in humans underscores the potential of these and similar strains to produce devastating influenza outbreaks in major population centers. Although highly pathogenic avian influenza viruses had been identified before the 1997 outbreak in Hong Kong, their devastating effects had been confined to poultry. With the Hong Kong outbreak, it became clear that the virulence potential of these viruses extended to humans.

Role of hemagglutinin cleavage for the pathogenicity of influenza virus

Although human epidemics of influenza occur on nearly an annual basis and result in a significant number of "excess deaths," the viruses responsible are not generally considered highly pathogenic. On occasion, however, an outbreak occurs that demonstrates the potential lethality of influenza viruses. The human pandemic of 1918 spread worldwide and killed millions, and the limited human outbreak of highly pathogenic avian viruses in Hong Kong at the end of 1997 is a warning that this could happen again. In avian species such as chickens and turkeys, several outbreaks of highly pathogenic influenza viruses have been documented. Although the reason for the lethality of the human 1918 viruses remains unclear, the pathogenicity of the avian viruses, including those that caused the human 1997 outbreak, relates primarily to properties of the hemagglutinin glycoprotein (HA). Cleavage of the HA precursor molecule HA0 is required to activate virus infectivity, and the distribution of activating proteases in the host is one of the determinants of tropism and, as such, pathogenicity. The HAs of mammalian and nonpathogenic avian viruses are cleaved extracellularly, which limits their spread in hosts to tissues where the appropriate proteases are encountered. On the other hand, the HAs of pathogenic viruses are cleaved intracellularly by ubiquitously occurring proteases and therefore have the capacity to infect various cell types and cause systemic infections. The x-ray crystal structure of HA0 has been solved recently and shows that the cleavage site forms a loop that extends from the surface of the molecule, and it is the composition and structure of the cleavage loop region that dictate the range of proteases that can potentially activate infectivity. Here influenza virus pathogenicity is discussed, with an emphasis on the role of HA0 cleavage as a determining factor.

Regulation of receptor binding affinity of influenza virus hemagglutinin by its carbohydrate moiety

The hemagglutinin (HA) of the fowl plague virus (FPV) strain of influenza A virus has two N-linked oligosaccharides attached to Asn123 and Asn149 in the vicinity of the receptor binding site. The effect of these carbohydrate side chains on the binding of HA to neuraminic acid-containing receptors has been analyzed. When the oligosaccharides were deleted by site-specific mutagenesis, HA expressed from a simian virus 40 vector showed enhanced hemadsorbing activity. Binding was so strong under these conditions that erythrocytes were no longer released by viral neuraminidase and that release was significantly reduced when neuraminidase from Vibrio cholerae was used. Similarly, when these oligosaccharides were removed selectively from purified viruses by N-glycosidase F, such virions were unable to elute from receptors, although they retained neuraminidase activity. Thus, release of FPV from cell receptors depends on the presence of the HA glycans at Asn123 and Asn149. On the other hand, receptor binding was abolished when these oligosaccharides were sialylated after expression in the absence of neuraminidase (M. Ohuchi, A. Feldmann, R. Ohuchi, and H.-D. Klenk, Virology 212:77-83, 1995). These observations indicate that the receptor affinity of FPV HA is controlled by oligosaccharides adjacent to the receptor binding site.

Expression of influenza B virus hemagglutinin containing multibasic residue cleavage sites

The hemagglutinin (HA) protein of influenza B virus contains a single arginine residue at its cleavage site and the HA0 precursor is not cleaved to the HA1 and HA2 subunits by tissue culture cell-associated proteases. To investigate if an HA protein could be obtained that could be cleaved by an endogenous cellular protease, the cDNA for HA of influenza B/MD/59 virus was subjected to site-specific mutagenesis. Three HA mutant proteins were constructed, through substitution or insertion of arginine residues, that have 4, 5, or 6 basic residues at their cleavage sites. Chemical cross-linking studies indicated that all three HA cleavage site mutants could oligomerize to a trimeric species, like WT HA. The three HA cleavage site mutant proteins were efficiently transported to the cell surface and bound erythrocytes in hemadsorption assays. The mutants were cleaved at a low level to HA1 and HA2 by an endogenous host cell protease and cleavage could be increased somewhat by addition of exogenous trypsin. The fusogenic activities of the HA cleavage site mutants were assessed in comparison to the WT HA protein by determining their syncytium formation ability and by using an R18 lipid-mixing assay and a NBD-taurine aqueous-content mixing assay. While the fusion activity of the WT HA protein was dependent on exogenous trypsin to activate HA, the three HA cleavage site mutant proteins were able to induce fusion in the absence of trypsin when assayed with the R18 lipid-mixing and NBD-taurine aqueous-content mixing assays, but were unable to induce syncytium formation in either the presence or absence of exogenous trypsin. Our results suggest that while the presence of a subtilisin-like protease cleavage sequence at the influenza B virus HA1/HA2 boundary does enable some HA0 molecules to be cleaved intracellularly, it alone is not sufficient for efficient cleavage.

Biologic effects of introducing additional basic amino acid residues into the hemagglutinin cleavage site of a virulent avian influenza virus

We mutated the virulent avian influenza virus A/turkey/Ontario/7732/66 (H5N9)[Q-R-R-R-K-K-R\G at the hemagglutinin (HA) cleavage site] to create a mutant, R(MO-0), with additional basic residues at this site (Q-R-R-R-R-R-K-K-R\G) by reverse genetics. When tested in chicken embryo fibroblast culture, this mutant showed reduced HA cleavability compared to that of the wild-type virus, but its plaque size was not appreciably altered. Virulence of the R(MO-0) virus in chickens was lower than that of the wild-type virus. These findings indicate that addition of excessive basic residues to an optimal recognition sequence for HA cleavage enzymes at the cleavage site is deleterious for HA cleavability. Previously, we showed that a mutant containing the suboptimal HA cleavage site sequence for cleavage enzyme recognition also had reduced HA cleavability and virulence compared to the wild-type virus. We conclude that the data presented here further substantiate our belief that the level of HA cleavability correlates with the degree of virulence when all other genetic characteristics are considered equal, irrespective of the mechanisms by which HA cleavability is reduced.

Oligosaccharides in the stem region maintain the influenza virus hemagglutinin in the metastable form required for fusion activity

The influenza A virus hemagglutinin (HA) has three conserved oligosaccharides located in the stem region at asparagine residues 12, 28, and 478. The biological role of these oligosaccharides has been investigated by mutational analysis of HA of fowl plague virus that was expressed from a simian virus 40 vector in the presence of ammonium chloride for protection from acid denaturation in the trans-Golgi network. Resistance to endoglycosidase H and cleavage of HA into the subunits HA1 and HA2 have been analyzed as markers for intracellular transport. Cell surface exposure has been determined by hemadsorption following neuraminidase treatment, by immunofluorescence staining, and by fluorescence-activated cell sorter analysis. When all three stem oligosaccharides were removed, transport was almost completely blocked. When two of the three stem oligosaccharides, particularly those at asparagine residues 12 and 28, were missing, HA was transported to the surface but showed extremely low fusion activity. With mutants lacking one stem oligosaccharide, fusion was reduced to a lesser extent. Removal of stem oligosaccharides resulted also in an increase in the pH optimum required for fusion. On the other hand, no reduction in fusion activity was observed when oligosaccharides in the head region of the HA spike were removed. These results indicate that the conserved oligosaccharides in the stem stabilize HA in the form susceptible to the conformational change necessary for fusion.

Influenza viruses, cell enzymes, and pathogenicity

Proteolytic cleavage of the influenza virus hemagglutinin glycoprotein (HA) by cellular proteases is a prerequisite for virus infectivity, spread of the virus in the infected organism, tissue tropism, and viral pathogenicity. Production of infectious virus depends upon the structure at the HA cleavage site as well as the substrate specificity and the distribution of appropriate enzymes. Differences exist in the specificities of the endoproteases that recognize the different sequence motifs at the cleavage site. With avian influenza viruses that cause lethal systemic infections, the cleavage site consists of multibasic amino acids. Furin, which activates this type of HA, is a member of the subtilisin family and represents the prototype of ubiquitously occurring membrane-bound proteases. On the other hand, serine proteases secreted from a restricted number of cell types and some bacterial enzymes recognize a monobasic cleavage signal at HA of the mammalian and the apathogenic avian influenza viruses. The limited occurrence of these proteases results in only localized infection. Implementation of these defined conditions for virus activation may represent a novel type of disease control.

Mutational analysis of the insulin-like growth factor I prohormone processing site

Insulin-like growth factor I (IGF-I) is a mitogenic peptide that is produced in most tissues and cell lines and plays an important role in embryonic development and postnatal growth. IGF-I is initially synthesized as a prohormone precursor that is converted to mature IGF-I by endoproteolytic removal of the carboxyl-terminal E-domain. Regulation of the conversion of proIGF-I to mature IGF-I is a potential mechanism by which the biological activity of this growth factor might be modulated. Endoproteolysis of the IGF-I prohormone occurs at the unique pentabasic motif Lys-X-X-Lys-X-X-Arg71-X-X-Arg-X-X-Arg. Recently, a family of enzymes which cleave prohormone precursors at sites containing multiple basic residues has been discovered. The goals of this study were 1) to determine which basic residues in the pentabasic proIGF-I processing site were necessary for proper cleavage and 2) to examine the role that subtilisin-related proprotein convertase 1 (SPC1/furin) might play in proIGF-I processing. We have shown that an expression vector coding for an epitope-tagged proIGF-I directs synthesis and secretion of mature IGF-I-(1-70), extended IGF-I-(1-76), proIGF-I, and N-glycosylated proIGF-I in human embryonic kidney 293 cells. Extended IGF-I-(1-76) is produced by cleavage at Arg77 and requires both Arg74 (P4) and Arg77 (P1). Cleavage at Arg77 does not occur in the SPC1-deficient cell lines RPE.40 and LoVo, suggesting that processing at this site is mediated by SPC1. Mature IGF-I-(1-70) is produced by cleavage at Arg71 and requires both Lys68 (P4) and Arg71 (P1). Lys65 in the P7 position is important for efficient cleavage. SPC1 is not required for processing at Arg71 since this cleavage occurs in RPE.40 and LoVo cells. These data suggest the existence of a processing enzyme which is specific for the Lys-X-X-Arg motif of proIGF-I.

Role of basic residues in the proteolytic activation of Sendai virus fusion glycoprotein

Cleavage activation of the Sendai virus (Fushimi strain) fusion (F) protein was analyzed by site-directed mutagenesis of the amino acids proximal to the highly conserved fusion peptide. In addition, the functional properties of the wild-type and mutant proteins were examined to determine their ability to elicit the formation of syncytia when co-expressed with the hemagglutinin-neuraminidase (HN) glycoprotein. Viral genes were expressed from recombinant T7 transcription vectors (pT7/T3 plasmids) containing F or HN genes, after transfection into cells previously infected with a recombinant vaccinia virus expressing T7 RNA polymerase (vTF7-3). The wild-type F protein sequence (112VPQSRF) which contains a monobasic cleavage activation site was altered to include a tribasic, 112VPRKRF (mB3), or a pentabasic sequence, 112RRRKRF (mB5) adjacent to the fusion peptide. Although addition of basic residues to the normal protein sequence resulted in enhanced cleavage activation of the mB3 and mB5 proteins, only the mB5 protein was able to induce syncytia formation in CV-1 or HeLa T4 cells. Further analysis by the introduction of acidic residues upstream of the cleavage activation site was performed to determine whether increased hydrophilicity of the surrounding residues might contribute to cleavage activation. The mutants examined, mAcB1 (104NDDEENAGVPQSRF), mAcB3 (104NDDEENAGVPRKRF), and mAcB5 (104NDDEENAGRRRKRF) all contained DEE in replacement for the wild-type TTQ sequence (104NDTTQNAGVPQSRF). Analysis showed that only mAcB3 was efficiently cleaved by the endogenous cellular proteases, while mAcB1 was minimally cleaved, and mAcB5 not at all. Consequently, only the mAcB3 mutant was able to support fusion of CV-1 or HeLa T4 cells when co-expressed with HN.

Reverse genetics provides direct evidence for a correlation of hemagglutinin cleavability and virulence of an avian influenza A virus

To obtain direct evidence for a relationship between hemagglutinin (HA) cleavability and the virulence of avian influenza A viruses, we generated a series of HA cleavage mutants from a virulent virus, A/turkey/Ontario/7732/66 (H5N9), by reverse genetics. A transfectant virus containing the wild-type HA with R-R-R-K-K-R at the cleavage site, which was readily cleaved by endogenous proteases in chicken embryo fibroblasts (CEF), was highly virulent in intramuscularly or intranasally/orally inoculated chickens. By contrast, a mutant containing the HA with an avirulent-like sequence (R-E-T-R) at the cleavage site, which was not cleaved by the proteases in CEF, was avirulent in chickens, indicating that a genetic alteration confined to the HA cleavage site can affect cleavability and virulence. Mutant viruses with HA cleavage site sequences of T-R-R-K-K-R or T-T-R-K-K-R were as virulent as viruses with the wild-type HA, whereas a mutant with a two-amino-acid deletion but retention of four consecutive basic residues (R-K-K-R) was as avirulent as a virus with the avirulent-type HA. Interestingly, although a mutant containing an HA with R-R-R-K-T-R, which has reduced cleavability in CEF, was as virulent as viruses with high HA cleavability when given intramuscularly, it was less virulent when given intranasally/orally. We conclude that the degree of HA cleavability in CEF predicts the virulence of avian influenza viruses.

Proteolytic cleavage of wild type and mutants of the F protein of human parainfluenza virus type 3 by two subtilisin-like endoproteases, furin and Kex2

The fusion (F) protein of human parainfluenza virus type 3 contains the tribasic cleavage site R-T-K-R, which was altered by site-directed mutagenesis. Wild-type F protein and various mutants were expressed by recombinant vaccinia viruses. The endogenous endoprotease present in CV-1 cells cleaves F variants containing the furin recognition motif R-X-K/R-R but not variants containing the dibasic site K-R or a single R at the cleavage site. A similar cleavage pattern was obtained when the subtilisin-like endoproteases Kex2 and furin were coexpressed with the wild type and mutants of the F protein. Peptidylchloromethylketone inhibitors mimicking basic cleavage sites prevent cleavage of the precursor Fo by the endogenous protease only when the furin-specific motif is present in the peptidyl portion. The data support the concept that furin is a cellular protease responsible for the activation of the F protein of human parainfluenza virus type 3.

Rescue of vector-expressed fowl plague virus hemagglutinin in biologically active form by acidotropic agents and coexpressed M2 protein

The hemagglutinin of the Rostock strain of fowl plague virus was expressed in CV-1 cells by a simian virus 40 vector, and its stability in the exocytotic transport process was examined by a fusion assay. A 50-fold increase in the fusion activity of the hemagglutinin was observed when expression occurred in the presence of ammonium chloride, Tris-HCl, or high doses of amantadine. When chloroquine, another acidotropic agent, was used, the hemagglutinin exposed at the cell surface had to be activated by trypsin, because intracellular cleavage was inhibited by this compound. Hemagglutinin mutants resistant to intracellular cleavage did not require acidotropic agents for full expression of fusion activity, when treated with trypsin after arrival at the cell surface. These results indicate that fowl plague virus hemagglutinin expressed by a simian virus 40 vector is denatured in the acidic milieu of the exocytotic pathway and that cleavage is a major factor responsible for the pH instability. Coexpression with the M2 protein also markedly enhanced the fusion activity of the hemagglutinin, and this effect was inhibited by low doses of amantadine. These results support the concept that M2, known to have ion channel function, protects the hemagglutinin from denaturation by raising the pH in the exocytotic transport system. The data also stress the importance of acidotropic agents or coexpressed M2 for the structural and functional integrity of vector-expressed hemagglutinin.

Sequence specificity of furin, a proprotein-processing endoprotease, for the hemagglutinin of a virulent avian influenza virus

The virulence of avian influenza viruses correlates with the sensitivity of their hemagglutinin (HA) to cellular proteases. Furin, a proprotein-processing subtilisin-related endoprotease, is a leading candidate for the enzyme that cleaves the HA of virulent avian viruses. We therefore compared the specificity of furin with those of proteases in a variety of cultured cells and in a rat Golgi fraction, using the HA cleavage mutants of a virulent avian influenza virus, A/Turkey/Ireland/1378/85 (H5N8). The results indicated similar sequence specificities among the endoproteases when purified furin was used. In experiments with the vaccinia virus expression system, overexpressed furin cleaved mutant HAs that were not recognized by the endogenous proteases, resulting in an apparent broader specificity of furin. These findings authenticate the proposed role of furin as an HA-activating protease in vivo and caution against the use of expression vectors to study protease sequence specificity.

Host cell proteases controlling virus pathogenicity

The majority of viral glycoproteins that undergo post-translational proteolysis are cleaved by ubiquitous intracellular proteases; however, a minority are cleaved by secreted proteases available only in a few host systems. The interplay of viral glycoproteins and cellular proteases may have a pivotal role in the spread of infection, host range and pathogenicity.

Processing of viral glycoproteins by the subtilisin-like endoprotease furin and its inhibition by specific peptidylchloroalkylketones

The spike glycoproteins of many enveloped viruses are proteolytically cleaved at the carboxytermini of sequences containing the basic motif R-X-K/R-R. Cleavage is often necessary for the fusion capacity of the glycoproteins and, thus, for virus infectivity. Among these viruses are pathogenic avian influenza viruses, human parainfluenza virus, human cytomegalovirus, and human immunodeficiency virus; it has been demonstrated that these viruses can be activated by furin. Indigenous furin has been identified in T-lymphocytes, which are host cells for HIV. Furin has been localized in the TGN and on the surface of cells after vectorial expression. Peptidylchloroalkylketones have been designed that inhibit with high specificity cleavage and fusion activity of viral glycoproteins, as well as virus replication.

Protein glycosylation. Structural and functional aspects

During the last decade, there have been enormous advances in our knowledge of glycoproteins and the stage has been set for the biotechnological production of many of them for therapeutic use. These advances are reviewed, with special emphasis on the structure and function of the glycoproteins (excluding the proteoglycans). Current methods for structural analysis of glycoproteins are surveyed, as are novel carbohydrate-peptide linking groups, and mono- and oligo-saccharide constituents found in these macromolecules. The possible roles of the carbohydrate units in modulating the physicochemical and biological properties of the parent proteins are discussed, and evidence is presented on their roles as recognition determinants between molecules and cells, or cell and cells. Finally, examples are given of changes that occur in the carbohydrates of soluble and cell-surface glycoproteins during differentiation, growth and malignancy, which further highlight the important role of these substances in health and disease.

Protease-dependent virus tropism and pathogenicity

Viral tissue tropism in a susceptible host is often determined by virus-receptor interactions. Nevertheless, closely related viruses utilizing the same receptor molecules can display striking differences in tropism, or a virus can cause a localized infection despite the widespread occurrence of the receptor. These events are now explained by another mechanism of tropism, in which host proteases play a major role by activating viral fusion glycoproteins.

Role of conserved glycosylation sites in maturation and transport of influenza A virus hemagglutinin

The role of three N-linked glycans which are conserved among various hemagglutinin (HA) subtypes of influenza A viruses was investigated by eliminating the conserved glycosylation (cg) sites at asparagine residues 12 (cg1), 28 (cg2), and 478 (cg3) by site-directed mutagenesis. An additional mutant was constructed by eliminating the cg3 site and introducing a novel site 4 amino acids away, at position 482. Expression of the altered HA proteins in eukaryotic cells by a panel of recombinant vaccinia viruses revealed that rates and efficiency of intracellular transport of HA are dependent upon both the number of conserved N-linked oligosaccharides and their respective positions on the polypeptide backbone. Glycosylation at two of the three sites was sufficient for maintenance of transport of the HA protein. Conserved glycosylation at either the cg1 or cg2 site alone also promoted efficient transport of HA. However, the rates of transport of these mutants were significantly reduced compared with the wild-type protein or single-site mutants of HA. The transport of HA proteins lacking all three conserved sites or both amino-terminally located sites was temperature sensitive, implying that a polypeptide folding step had been affected. Analysis of trimer assembly by these mutants indicated that the presence of a single oligosaccharide in the stem domain of the HA molecule plays an important role in preventing aggregation of molecules in the endoplasmic reticulum, possibly by maintaining the hydrophilic properties of this domain. The conformational change observed after loss of all three conserved oligosaccharides also resulted in exposure of a normally mannose-rich oligosaccharide at the tip of the large stem helix that allowed its conversion to a complex type of structure. Evidence was also obtained suggesting that carbohydrate-carbohydrate interactions between neighboring oligosaccharides at positions 12 and 28 influence the accessibility of the cg2 oligosaccharide for processing enzymes. We also showed that terminal glycosylation of the cg3 oligosaccharide is site specific, since shifting of this site 4 amino acids away, to position 482, yielded an oligosaccharide that was arrested in the mannose-rich form. In conclusion, carbohydrates at conserved positions not only act synergistically by promoting and stabilizing a conformation compatible with transport, they also enhance trimerization and/or folding rates of the HA protein.

The pathogenic determinant of influenza virus

Influenza viruses, like other viruses, must exhibit a genome constellation, which permits optimal virus reproduction in a given host. Besides this prerequisite the influenza virus haemagglutinin glycoprotein (HA) has been shown to be an essential determinant for pathogenicity. HA, which is synthesized as a precursor molecule, is activated by posttranslational cleavage by host proteases to obtain its full biological properties. Proteolytic activation is therefore indispensable for effective virus spread in the infected host and thus for pathogenicity. HA of the highly pathogenic avian influenza viruses inducing a systemic infection in birds is cleaved in a broad range of different host cells. On the other hand, HA of all mammalian viruses and the nonpathogenic avian strains, which cause local infection, exhibit a restricted cleavability. The prime determinant for these differences has been found to be the structure of the cleavage site. This concept was corroborated on virus mutants adapted in vitro to a new host.

Hemagglutinin activation of pathogenic avian influenza viruses of serotype H7 requires the protease recognition motif R-X-K/R-R

The hemagglutinin of influenza virus A/FPV/Rostock/34 (H7) was altered at its multibasic cleavage site by site-directed mutagenesis and assayed for proteolytic activation after expression in CV-1 cells. The results indicated that the cellular protease responsible for activation recognizes the tetrapeptide motif R-X-K/R-R that must be presented in the correct sequence position. Studies on plaque variants of influenza virus A/fowl/Victoria/75 (H7N7) showed that alteration of the consensus sequence resulted in a loss of pathogenicity for chickens.

Mutations within the proteolytic cleavage site of the Rous sarcoma virus glycoprotein define a requirement for dibasic residues for intracellular cleavage

We investigated the amino acid sequence requirements for intracellular cleavage of the Rous sarcoma virus glycoprotein precursor by introducing mutations into the region encoding the cleavage recognition site (Arg-Arg-Lys-Arg). In addition to mutants G1 (Arg-Arg-Glu-Arg) and Dr1 (deletion of all four codons) that we have reported on previously (L. G. Perez and E. Hunter, J. Virol. 61:1609-1614, 1987), we constructed two additional mutants, AR1 (Arg-Arg-Arg-Arg), in which the highly conserved lysine is replaced by an arginine, and S19 (Ser-Arg-Glu-Arg), in which no dibasic pairs remain. The results of these studies demonstrate that when the cleavage sequence is deleted (Dr1) or modified to contain unpaired basic residues (S19), intracellular cleavage of the glycoprotein precursor is completely blocked. This demonstrates that the cellular endopeptidase responsible for cleavage has a stringent requirement for the presence of a pair of basic residues (Arg-Arg or Lys-Arg). Furthermore, it implies that the cleavage enzyme is not trypsinlike, since it is unable to recognize arginine residues that are sensitive to trypsin action. Substitution of the mutated genes into a replication-competent avian retrovirus genome showed that cleavage of the glycoprotein precursor was not required for incorporation into virions but was necessary for infectivity. Treatment of BH-RCAN-S19-transfected turkey cells with low levels of trypsin resulted in the release of infectious virus, demonstrating that exogenous cleavage could generate a biologically active glycoprotein molecule.