Pneumotropic revertants derived from a pantropic mutant, F1-R, of Sendai virus

Influenza A Virus M2 Protein Apical Targeting Is Required for Efficient Virus Replication

The influenza A virus (IAV) M2 protein is a multifunctional protein with critical roles in virion entry, assembly, and budding. M2 is targeted to the apical plasma membrane of polarized epithelial cells, and the interaction of the viral proteins M2, M1, HA, and NA near glycolipid rafts in the apical plasma membrane is hypothesized to coordinate the assembly of infectious virus particles. To determine the role of M2 protein apical targeting in IAV replication, a panel of M2 proteins with basolateral plasma membrane (M2-Baso) or endoplasmic reticulum (M2-ER) targeting sequences was generated. MDCK II cells stably expressing M2-Baso, but not M2-ER, complemented the replication of M2-stop viruses. However, in primary human nasal epithelial cell (hNEC) cultures, viruses encoding M2-Baso and M2-ER replicated to negligible titers compared to those of wild-type virus. M2-Baso replication was negatively correlated with cell polarization. These results demonstrate that M2 apical targeting is essential for IAV replication: targeting M2 to the ER results in a strong, cell type-independent inhibition of virus replication, and targeting M2 to the basolateral membrane has greater effects in hNECs than in MDCK cells.IMPORTANCE Influenza A virus assembly and particle release occur at the apical membrane of polarized epithelial cells. The integral membrane proteins encoded by the virus, HA, NA, and M2, are all targeted to the apical membrane and believed to recruit the other structural proteins to sites of virus assembly. By targeting M2 to the basolateral or endoplasmic reticulum membranes, influenza A virus replication was significantly reduced. Basolateral targeting of M2 reduced the infectious virus titers with minimal effects on virus particle release, while targeting to the endoplasmic reticulum resulted in reduced infectious and total virus particle release. Therefore, altering the expression and the intracellular targeting of M2 has major effects on virus replication.

Influenza virus m2 ion channel protein is necessary for filamentous virion formation

Influenza A virus buds from cells as spherical (approximately 100-nm diameter) and filamentous (approximately 100 nm x 2 to 20 microm) virions. Previous work has determined that the matrix protein (M1) confers the ability of the virus to form filaments; however, additional work has suggested that the influenza virus M2 integral membrane protein also plays a role in viral filament formation. In examining the role of the M2 protein in filament formation, we observed that the cytoplasmic tail of M2 contains several sites that are essential for filament formation. Additionally, whereas M2 is a nonraft protein, expression of other viral proteins in the context of influenza virus infection leads to the colocalization of M2 with sites of virus budding and lipid raft domains. We found that an amphipathic helix located within the M2 cytoplasmic tail is able to bind cholesterol, and we speculate that M2 cholesterol binding is essential for both filament formation and the stability of existing viral filaments.

Viral protein determinants of Lassa virus entry and release from polarized epithelial cells

The epithelium plays a key role in the spread of Lassa virus. Transmission from rodents to humans occurs mainly via inhalation or ingestion of droplets, dust, or food contaminated with rodent urine. Here, we investigated Lassa virus infection in cultured epithelial cells and subsequent release of progeny viruses. We show that Lassa virus enters polarized Madin-Darby canine kidney (MDCK) cells mainly via the basolateral route, consistent with the basolateral localization of the cellular Lassa virus receptor alpha-dystroglycan. In contrast, progeny virus was efficiently released from the apical cell surface. Further, we determined the roles of the glycoprotein, matrix protein, and nucleoprotein in directed release of nascent virus. To do this, a virus-like-particle assay was developed in polarized MDCK cells based on the finding that, when expressed individually, both the glycoprotein GP and matrix protein Z form virus-like particles. We show that GP determines the apical release of Lassa virus from epithelial cells, presumably by recruiting the matrix protein Z to the site of virus assembly, which is in turn essential for nucleocapsid incorporation into virions.

Nipah virus fusion protein: influence of cleavage site mutations on the cleavability by cathepsin L, trypsin and furin

Nipah virus (NiV), a highly pathogenic member of the Paramyxoviridae which originated from bats, encodes for a fusion (F) protein which is proteolytically processed within endosomes by cathepsin L. We show here that sequence requirements for NiV F activation differ markedly from other para- or orthomyxoviral fusion proteins. In contrast to other viral fusion proteins with monobasic cleavage sites, processing of NiV F proteins with one single basic amino acid in the cleavage peptide by exogenous trypsin is very inefficient, and introduction of a consensus sequence for furin does not result in cleavage by this ubiquitous protease. In contrast, a multibasic cleavage peptide in the NiV F protein completely impairs proteolytic processing and the generation of biological activity.

Molecular characterization and complete genome sequence of avian paramyxovirus type 4 prototype strain duck/Hong Kong/D3/75

BACKGROUND

Avian paramyxoviruses (APMVs) are frequently isolated from domestic and wild birds throughout the world. All APMVs, except avian metapneumovirus, are classified in the genus Avulavirus of the family Paramyxoviridae. At present, the APMVs of genus Avulavirus are divided into nine serological types (APMV 1-9). Newcastle disease virus represents APMV-1 and is the most characterized among all APMV types. Very little is known about the molecular characteristics and pathogenicity of APMV 2-9.

RESULTS

As a first step towards understanding the molecular genetics and pathogenicity of APMV-4, we have sequenced the complete genome of APMV-4 strain duck/Hong Kong/D3/75 and determined its pathogenicity in embryonated chicken eggs. The genome of APMV-4 is 15,054 nucleotides (nt) in length, which is consistent with the "rule of six". The genome contains six non-overlapping genes in the order 3'-N-P/V-M-F-HN-L-5'. The genes are flanked on either side by highly conserved transcription start and stop signals and have intergenic sequences varying in length from 9 to 42 nt. The genome contains a 55 nt leader region at 3' end. The 5' trailer region is 17 nt, which is the shortest in the family Paramyxoviridae. Analysis of mRNAs transcribed from the P gene showed that 35% of the transcripts were edited by insertion of one non-templated G residue at an editing site leading to production of V mRNAs. No message was detected that contained insertion of two non-templated G residues, indicating that the W mRNAs are inefficiently produced in APMV-4 infected cells. The cleavage site of the F protein (DIPQR downward arrowF) does not conform to the preferred cleavage site of the ubiquitous intracellular protease furin. However, exogenous proteases were not required for the growth of APMV-4 in cell culture, indicating that the cleavage does not depend on a furin site.

CONCLUSION

Phylogenic analysis of the nucleotide sequences of viruses of all five genera of the family Paramyxoviridae showed that APMV-4 is more closely related to the APMVs than to other paramyxoviruses, reinforcing the classification of all APMVs in the genus Avulavirus of the family Paramyxoviridae.

Molecular mechanism of paramyxovirus budding

Components of paramyxoviruses are assembled at the plasma membrane of infected cells, and progeny viruses are formed by the budding process. Although the molecular mechanisms that drive budding (membrane curving and "pinching-off" reaction) are not well understood, the viral matrix (M) protein is thought to play a major role in the process. The M protein forms a dense layer tightly associated with the inner leaflet of the plasma membrane of infected cells. Expression of the M protein of some paramyxoviruses results in the formation and release of virus-like particles that contain the M protein; thus, in these viruses, the M protein alone can apparently trigger all steps required for the formation and release of virus-like particles. M also interacts specifically with viral envelope glycoproteins and nucleocapsids and is involved in directed transport of viral components to the budding site at the apical surface of polarized cells. In addition, protein-protein interactions between M and the cytoplasmic tail of viral glycoproteins and between M and the nucleocapsid affect the efficiency of virus production. The structural organization of the virion and the functions of the M protein clearly indicate that this protein orchestrates the budding of paramyxovirus.

Ubiquitous activation of the Nipah virus fusion protein does not require a basic amino acid at the cleavage site

Nipah virus (NiV), a highly pathogenic paramyxovirus, causes a systemic infection in vivo and is able to replicate in cultured cells of many species and organs. Such pantropic paramyxoviruses generally encode fusion (F) proteins with multibasic cleavage sites activated by furin or other ubiquitous intracellular host cell proteases. In contrast, NiV has an F protein with a single arginine (R109) at the cleavage site, as is the case with paramyxoviruses that are activated by trypsin-like proteases only present in specific cells or tissues and therefore only cause localized infections. Unlike these viruses, cleavage of the NiV F protein is ubiquitous and does not require the addition of exogenous proteases in cell culture. To determine the importance of the amino acid sequence at the NiV F protein cleavage site for ubiquitous activation, we generated NiV F proteins with mutations around R109. Surprisingly, neither the exchange of amino acids upstream of R109 nor replacement of the basic residue itself interfered with F cleavage. Thus, R109 is not essential for F cleavage and activation. Our data demonstrate that NiV F-protein activation depends on a novel type of proteolytic cleavage that has not yet been described for any other paramyxovirus F protein. NiV F activation is mediated by a ubiquitous protease that requires neither a monobasic nor a multibasic cleavage site and therefore differs from the furin- or trypsin-like proteases known to activate other ortho- and paramyxovirus fusion proteins.

Mutations in Sendai virus variant F1-R that correlate with plaque formation in the absence of trypsin

With the emergence of new viruses, such as the SARS virus and the avian influenza virus, the importance of investigations on the genetic basis of viral infections becomes clear. Sendai virus causes a localized respiratory tract infection in rodents, while a mutant, F1-R, causes a systemic infection. It has been suggested that two determinants are responsible for the systemic infection caused by F1-R [Okada et al (1998) Arch Virol 143:2343–2352]. The primary determinant of the pantropism is the enhanced proteolytic cleavability of the fusion (F) protein of F1-R, which allows the virus to undergo multiple rounds of replication in many different organs, whereas wild-type virus can only undergo multiple rounds of replication in the lungs. The enhanced cleavability of F1-R F was previously attributed to an amino acid change at F115 that is adjacent to the cleavage site at amino acid 116. Secondly, wild-type virus buds only from the apical domain of bronchial epithelium, releasing virus into the lumen of the respiratory tract, whereas F1-R buds from both apical and basolateral domains. Thus, virus is released into the basement membrane where it can easily gain access to the bloodstream for dissemination. The microtubule disruption is attributed to two amino acid differences in M protein. To confirm that the F and M gene mutations described above are solely responsible for the phenotypic differences seen in wld-type versus F1-R infections, reverse genetics was used to construct recombinant Sendai viruses with various combinations of the mutations found in the M and F genes of F1-R. Plaque assays were performed with or without trypsin addition. A recombinant virus containing all F1-R M and F mutations formed plaques in LLC-MK2 cells and underwent multiple cycles of replication without trypsin addition. To clarify which mutation(s) are necessary for plaque formation, plaque assays were done using other recombinant viruses. A virus with only the F115 change, which was previously thought to be the only change important for plaque formation of F 1-R F, did not confer upon the virus the ability to form plaques without the addition of trypsin. Another virus with the F115 and both M changes gave the same result. Therefore, more than one mutation in the F gene contributes to the ability of F1-R to form plaques without trypsin addition.

Polarized glycoprotein targeting affects the spread of measles virus in vitro and in vivo

We have shown previously that basolateral targeting of plasmid-encoded measles virus (MV) F and H protein is dependent on single tyrosine residues in the cytoplasmic tails of the glycoproteins and is essential for fusion activity in polarized epithelial cells. Here, we present data on the functional importance of polarized glycoprotein expression for the cytopathic properties of infectious MV in culture and for pathogenesis in vivo. By the introduction of single point mutations, we generated recombinant viruses in which the basolateral targeting signal of either one or both glycoproteins was destroyed (tyrosine mutants). As a consequence, the mutated glycoproteins were predominantly expressed on the apical membrane of polarized Madin-Darby canine kidney cells. In contrast to parental MV, none of these virus mutants was able to spread by syncytia formation in polarized cells showing that the presence of both MV glycoproteins at the basolateral cell surface is required for cell-to-cell fusion in vitro. Using cotton rats as an animal model that allows MV replication in the respiratory tract, we showed that basolateral glycoprotein targeting is also of importance for the spread of infection in vivo. Whereas parental MV was able to spread laterally within the respiratory epithelium and from there to cells in the underlying tissue, tyrosine mutants infected only single epithelial and very few subepithelial cells. These data strongly suggest that basolateral targeting of MV glycoproteins helps to overcome the epithelial barrier and thereby facilitates the systemic spread of MV infection in vivo.

Viral ion channels: structure and function

Viral ion channels are short auxiliary membrane proteins with a length of ca. 100 amino acids. They are found in enveloped viruses from influenza A, influenza B and influenza C (Orthomyxoviridae), and the human immunodeficiency virus type 1 (HIV-1, Retroviridae). The channels are called M2 (influenza A), NB (influenza B), CM2 (influenza C) and Vpu (HIV-1). Recently, in Paramecium bursaria chlorella virus (PBCV-1, Phycodnaviridae), a K+ selective ion channel has been discovered. The viral channels form homo oligomers to allow an ion flux and represent miniaturised systems. Proton conductivity of M2 is established; NB, Vpu and the potassium channel from PBC-1 conduct ions; for CM2 ion conductivity is still under proof. This review summarises the current knowledge of these short viral membrane proteins. Their discovery is outlined and experimental evidence for their structure and function is discussed. Studies using computational methods are presented as well as investigations of drug-protein interactions.

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.

Genetic characterization of parainfluenza virus 3 derived from guinea pigs

To understand the relationship between novel parainfluenza virus 3 (PIV-3), which has recently been isolated from the lungs of guinea pigs, and other PIV-3 strains, we determined the complete nucleotide sequence of the novel PIV-3 (GPv) genome. A comparison of the nucleotide sequence among PIV-3s, including bovine PIV-3, revealed that GPv is closely related to human PIV-3. The results of the phylogenetic analysis clearly showed that GPv is a lineage of human PIV-3, suggesting that GPv has probably been introduced into guinea pig colonies via infected humans.

Involvement of the mutated M protein in altered budding polarity of a pantropic mutant, F1-R, of Sendai virus

Wild-type Sendai virus buds at the apical plasma membrane domain of polarized epithelial MDCK cells, whereas a pantropic mutant, F1-R, buds at both the apical and basolateral domains. In F1-R-infected cells, polarized protein transport and the microtubule network are impaired. It has been suggested that the mutated F and/or M proteins in F1-R are responsible for these changes (M. Tashiro, J. T. Seto, H.-D. Klenk, and R. Rott, J. Virol. 67:5902-5910, 1993). To clarify which gene or mutation(s) was responsible for the microtubule disruption which leads to altered budding of F1-R, MDCK cell lines containing the M gene of either the wild type or F1-R were established. When wild-type M protein was expressed at a level corresponding to that synthesized in virus-infected cells, cellular polarity and the integrity of the microtubules were affected to some extent. On the other hand, expression of the mutated F1-R M protein resulted in the formation of giant cells about 40 times larger than normal MDCK cells. Under these conditions, the effects on the microtubule network were enhanced. The microtubules were disrupted and polarized protein transport was impaired as indicated by the nonpolarized secretion of gp80, a host cell glycoprotein normally secreted from the apical domain, and bipolar budding of wild-type and F1-R Sendai viruses. The mutated F glycoprotein of F1-R was transported bipolarly in cells expressing the F1-R M protein, whereas it was transported predominantly to the apical domain when expressed alone or in cells coexpressing the wild-type M protein. These findings indicate that the M protein of F1-R is involved in the disruption of the microtubular network, leading to impairment of cellular polarity, bipolar transport of the F glycoprotein, and bipolar budding of the virus.

Comparison of the deduced matrix and fusion protein sequences of equine morbillivirus with cognate genes of the Paramyxoviridae

The nucleotide sequence of the matrix protein of equine morbillivirus (EMV) was determined to be 1062 nucleotides and coded for a deduced protein of M(r) 40148 having a net charge of + 19 at neutral pH. The matrix protein gene was separated from the P and F genes by intercistronic regions of 546 and 469 nucleotides, respectively. The nucleotide sequence which coded for the F protein was 1641 nucleotides and coded for a deduced protein of 546 amino acids having an M(r) of 60,447 and a charge + 4 at neutral pH. Partial sequence information was also determined for the P/V proteins. M, P and F protein sequence comparisons revealed that a greater homology existed between EMV and known members of the morbillivirus genus than with other members of the Paramyxoviridae and that this homology resided within the central half of the protein for the fusion protein, the C-terminal half of the matrix protein and at certain sites with the P protein. Far greater homology was seen between the morbilliviruses and EMV than for the other paramyxoviridae. It was inferred from phylogenetic analyses that EMV was a distantly related member of the morbillivirus genus. A conserved sequence of 18 nucleotides (assumed to be the transcriptional editing site) was present in the P gene of EMV. Insertion of a single nucleotide residue within this site generated the C-terminus of a V-like, cysteine rich protein. Likewise, a conserved 'CTT' intergenic region presumed to be the transcription termination and polyadenylation signal was present in EMV between the P-M-F genes. The close sequence homology of these sites with that of morbilliviruses also inferred that EMV was a member of the morbillivirus genus.

Activation of the M2 ion channel of influenza virus: a role for the transmembrane domain histidine residue

To test the hypothesis that transmembrane domain histidine residue 37 of the M2 ion channel of influenza A virus mediates the low pH-induced activation of the channel, the residue was changed to glycine, glutamate, arginine, or lysine. The wild-type and altered M2 proteins were expressed in oocytes of Xenopus laevis and membrane currents were recorded. The mass of protein expressed in individual oocytes was measured using quantitative immunoblotting and correlated with membrane currents. Oocytes expressing the M2-H37G protein had a voltage-independent conductance with current-voltage relationship similar to that of the wild-type M2 channel. The conductance of the M2-H37G protein was reversibly inhibited by the M2 ion channel blocker amantadine and was only very slightly modulated by changes in pHout over the range pH 5.4 to pH 8.2. Oocytes expressing the M2-H37E protein also had a voltage-independent conductance with a current-voltage relationship similar to that of the wild-type M2 channel. The conductance of the M2-H37E protein was reversibly inhibited by amantadine and was also only very slightly modulated by changes in pHout over the range pH 5.4 to pH 8.2. These slight alterations in conductance of the mutant ion channels on changes in pHout are in striking contrast to the 50-fold change in conductance seen for the wild-type M2 channel over the range pH 4.5 to pH 8.2. The specific activity of the M2-H37G protein was 1.36 +/- 0.37 microA/ng and the specific activity of the M2-H37E protein was 30 +/- 3 microA/ng at pH 6.2. These values of specific activity greatly exceed that of the wild-type protein at the same pH (0.16 + 0.01 micro A/ng). Oocytes expressing the M2-H37K and M2-H37R mutant proteins could not be studied because the oocytes did not survive more than a few hours in culture. Oocytes expressing the M2-H37E mutant protein also had a voltage-activated Cl- conductance that was observed only for oocytes that expressed a mass of protein exceeding a large threshold value. These results are consistent with protonation of histidine residue 37 as an essential step in the activation of the wild-type M2 ion channel.

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.

Analysis of the posttranslational modifications of the influenza virus M2 protein

The sites of posttranslational modifications of the influenza A virus M2 protein were examined, and the effect of these modifications on the M2 protein ion channel activity was analyzed. Cysteine residues 17 and 19 in the M2 protein ectodomain form disulfide bonds. The cytoplasmic tail is posttranslationally modified by palmitoylation, and mutagenic studies support the view that cysteine residue 50 is the site for fatty acylation. In addition, the cytoplasmic tail of the M2 protein was found to be posttranslationally modified by the addition of phosphate to specific serine residues. Site-directed mutagenesis of serine residues in the M2 protein cytoplasmic tail, combined with phosphoamino acid analysis, indicated that serine residue 64 is the predominant site for phosphorylation but that serine residues 82, 89, and 93 were also phosphorylated but to much lesser extents. Disulfide-bond formation, palmitoylation, and phosphorylation occurred on M2 protein expressed in mammalian cells infected with influenza virus, in mammalian cells in which the M2 protein was expressed from DNA expression vectors, and when the M2 protein was expressed in oocytes of Xenopus laevis. The membrane currents of oocytes of Xenopus laevis expressing wild-type and site-specifically altered forms of the M2 protein, to ablate posttranslational modifications, indicated that none of the posttranslational modifications significantly affected the ion channel activity of the M2 protein in oocytes. Therefore, these data do not indicate a functional role for posttranslational modifications of the M2 protein in its ion channel activity.

Novel assay for the influenza virus M2 channel activity

The influenza A M2 ion channel was expressed and activity characterized in Xenopus oocytes. Based on the activation properties of the channels, a high throughput, non-electrophysiological screening assay was developed in order to identify novel inhibitors of the channel. This will facilitate discovery of novel agents to treat influenza viral infections.

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.

Possible involvement of microtubule disruption in bipolar budding of a Sendai virus mutant, F1-R, in epithelial MDCK cells

Envelope glycoproteins F and HN of wild-type Sendai virus are transported to the apical plasma membrane domain of polarized epithelial MDCK cells, where budding of progeny virus occurs. On the other hand, a pantropic mutant, F1-R, buds bipolarly at both the apical and basolateral domains, and the viral glycoproteins have also been shown to be transported to both of these domains (M. Tashiro, M. Yamakawa, K. Tobita, H.-D. Klenk, R. Rott, and J.T. Seto, J. Virol. 64:4672-4677, 1990). MDCK cells were infected with wild-type virus and treated with the microtubule-depolymerizing drugs colchicine and nocodazole. Budding of the virus and surface expression of the glycoproteins were found to occur in a nonpolarized fashion similar to that found in cells infected with F1-R. In uninfected cells, the drugs were shown to interfere with apical transport of a secretory cellular glycoprotein, gp80, and basolateral uptake of [35S]methionine as well as to disrupt microtubule structure, indicating that cellular polarity of MDCK cells depends on the presence of intact microtubules. Infection by the F1-R mutant partially affected the transport of gp80, uptake of [35S]methionine, and the microtubule network, whereas wild-type virus had a marginal effect. These results suggest that apical transport of the glycoproteins of wild-type Sendai virus in MDCK cells depends on intact microtubules and that bipolar budding by F1-R is possibly due, at least in part, to the disruption of microtubules. Nucleotide sequence analyses of the viral genes suggest that the mutated M protein of F1-R might be involved in the alteration of microtubules.

Sendai virus assembly: M protein binds to viral glycoproteins in transit through the secretory pathway

We have examined the relative ability of Sendai virus M (matrix) protein to associate with membranes containing viral glycoproteins at three distinct stages of the exocytic pathway prior to cell surface appearance. By the use of selective low-temperature incubations or the ionophore monensin, the transport of newly synthesized viral glycoproteins was restricted to either the pre-Golgi intermediate compartment (by incubation at 15 degrees C), the medial Golgi (in the presence of monensin), or the trans-Golgi network (by incubation at 20 degrees C). All three of these treatments resulted in a marked accumulation of the M protein on perinuclear Golgi-like membranes which in each case directly reflected the distribution of the viral F protein. Subsequent redistribution of the F protein to the plasma membrane by removal of the low-temperature (20 degrees C) block resulted in a concomitant redistribution of the M protein, thus implying association of the two components during intracellular transit. The extent of M protein-glycoprotein association was further examined by cell fractionation studies performed under each of the three restrictive conditions. Following equilibrium sedimentation of membranes derived from monensin-treated cells, approximately 40% of the recovered M protein was found to cofractionate with membranes containing the viral glycoproteins. Also, by flotation analyses, a comparable subpopulation of M protein was found to be membrane associated whether viral glycoproteins were restricted to the trans-Golgi network, the medial Golgi, or the pre-Golgi intermediate compartment. Additionally, transient expression of M protein alone from cloned cDNA showed that neither membrane association nor Golgi localization occurs in the absence of Sendai virus glycoproteins.

Tryptase Clara, an activating protease for Sendai virus in rat lungs, is involved in pneumopathogenicity

Tryptase Clara is an arginine-specific serine protease localized exclusively in and secreted from Clara cells of the bronchial epithelium of rats (H. Kido, Y. Yokogoshi, K. Sakai, M. Tashiro, Y. Kishino, A. Fukutomi, and N. Katunuma, J. Biol. Chem. 267:13573-13579, 1992). The purified protease was shown in vitro to behave similarly to trypsin, cleaving the precursor glycoprotein F of Sendai virus at residue Arg-116 and activating viral infectivity in a dose-dependent manner. Anti-tryptase Clara antibody inhibited viral activation by the protease in vitro in lung block cultures and in vivo in infected rats. When the enzyme-specific antibody was administered intranasally to rats that were also infected intranasally with Sendai virus, activation of progeny virus in the lungs was significantly inhibited. Thus, multiple cycles of viral replication were suppressed, resulting in a reduction in lung lesions and in the mortality rate. These findings indicate that tryptase Clara is an activating protease for Sendai virus in rat lungs and is therefore involved in pulmonary pathogenicity of the virus in rats.

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.

Budding site of Sendai virus in polarized epithelial cells is one of the determinants for tropism and pathogenicity in mice

Wild-type Sendai virus fusion (F) glycoprotein requires trypsin or a trypsin-like protease for cleavage-activation in vitro and in vivo, respectively. The virus is pneumotropic in mice and buds at the apical domain of bronchial epithelial cells. On the other hand, the F protein of the protease-activation host range mutant, F1-R, is cleaved by ubiquitous proteases present in different cell lines and in various organs of mice. F1-R causes a systemic infection in mice and the mutant buds bipolarly at the apical and basolateral domains of infected epithelial cells. The enhanced cleavability of the F protein of F1-R has been shown to be a primary determinant for pantropism. Additionally, it has been postulated that bipolar budding of F1-R is required for the systemic spread of the virus and it has been attributed to mutations in the matrix (M) protein of F1-R (Tashiro et al., Virology 184, 227-234, 1991). In this study protease-activation mutants (KD series) were isolated from wild-type virus. They were revealed to bud at the apical domain, and the F protein was cleaved by ubiquitous proteases in mouse organs. The KD mutants were exclusively pneumotropic in mice following intranasal infection, whereas they caused a generalized infection when inoculated directly into the circulatory system. Comparative nucleotide sequence analysis of the F gene of the KD mutants revealed that the deduced amino acid substitutions responsible for enhanced cleavability of the F protein occurred removed from the cleavage site. Mutations were not at all found in the M gene of the KD mutants analyzed, in support of the role of the M protein of F1-R and of a revertant T-9 derived from the latter in bipolar budding. These results suggest that bipolar budding is necessary for the systemic spread of F1-R from the lungs and that apical budding by wild-type virus and the KD mutants leads to respiratory infections. Differential budding at the primary target of infection, in addition to the cleavage-activation of the F protein in mouse organs, is therefore also a determinant for tropism and pathogenicity of Sendai virus in mice.

Significance of basolateral domain of polarized MDCK cells for Sendai virus-induced cell fusion

Fusion (fusion from within) of polarized MDCK monolayer cells grown on porous membranes was examined after infection with Sendai viruses. Wild-type virus, that buds at the apical membrane domain, did not induce cell fusion even when the F glycoprotein expressed at the apical domain was activated with trypsin. On the other hand, a protease activation mutant, F1-R, with F protein in the activated form and that buds bipolarly at the apical and basolateral domains, caused syncytia formation in the absence of exogenous protease. Anti-Sendai virus antibodies added to the basolateral side, but not at the apical side, inhibited cell fusion induced by F1-R. In addition, T-9, a mutant with bipolar budding phenotype of F1-R but with an uncleavable F protein phenotype like wild-type virus, induced cell fusion exclusively when trypsin was added to the basolateral medium. By electron microscopy, cell-to-cell fusion was shown to occur at the lateral domain of the plasma membrane. These results indicate that in addition to proteolytic activation of the F protein, basolateral expression of Sendai virus envelope glycoproteins is required to induce cell fusion.