Identification of amino acids involved in the sialidase activity of the mumps virus hemagglutinin-neuraminadase protein

Effects of hemagglutinin-neuraminidase protein mutations on cell-cell fusion mediated by human parainfluenza type 2 virus

The monoclonal antibody M1-1A, specific for the hemagglutinin-neuraminidase (HN) protein of human parainfluenza type 2 virus (HPIV2), blocks virus-induced cell-cell fusion without affecting the hemagglutinating and neuraminidase activities. F13 is a neutralization escape variant selected with M1-1A and contains amino acid mutations N83Y and M186I in the HN protein, with no mutation in the fusion protein. Intriguingly, F13 exhibits reduced ability to induce cell-cell fusion despite its multistep replication. To investigate the potential role of HPIV2 HN protein in the regulation of cell-cell fusion, we introduced these mutations individually or in combination to the HN protein in the context of recombinant HPIV2. Following infection at a low multiplicity, Vero cells infected with the mutant virus H-83/186, which carried both the N83Y and M186I mutations, remained as nonfused single cells at least for 24 h, whereas most of the cells infected with wild-type virus mediated prominent cell-cell fusion within 24 h. On the other hand, the cells infected with the mutant virus, carrying either the H-83 or H-186 mutation, mediated cell-cell fusion but less efficiently than those infected with wild-type virus. Irrespective of the ability to cause cell-cell fusion, however, every virus could infect all the cells in the culture within 48 h after the initial infection. These results indicated that both the N83Y and M186I mutations in the HN protein are involved in the regulation of cell-cell fusion. Notably, the limited cell-cell fusion by H-83/186 virus was greatly promoted by lysophosphatidic acid, a stimulator of the Ras and Rho family GTPases.

The F gene of rodent brain-adapted mumps virus is a major determinant of neurovirulence

Prior to the introduction of live-attenuated vaccines, mumps virus (MuV) was the leading cause of virus-induced meningitis. Although vaccination has been effective at controlling the disease, the use of insufficiently attenuated strains has been associated with high rates of aseptic meningitis in vaccinees. The molecular basis of MuV attenuation is poorly understood, and no reliable molecular markers of virulence have been identified. In this study, reverse genetics has been used to identify molecular determinants of MuV neuropathogenesis. Recombinant viruses, containing the envelope-associated genes from the Kilham (MuV(KH)) rodent brain-adapted strain of MuV, were generated in the Jeryl Lynn 5 (MuV(JL5)) vaccine strain background. The syncytium phenotypes of the recombinant viruses on Vero cells differed depending on the source of the fusion (F) and hemagglutinin-neuraminidase (HN) glycoproteins, with heterologous combinations showing either an increase or a decrease in the level of cell fusion compared to that of the homologous parental combinations. This was confirmed by transiently cotransfecting eukaryotic F and HN glycoprotein expression constructs. A Lewis rat model that discriminates between neurovirulent and nonneurovirulent MuV strains based on the extent of hydrocephalus induced in the rat brain after intracerebral inoculation was used to assess the phenotype of the recombinant viruses. Expression of the matrix (M), small hydrophobic (SH), or HN gene in isolation did not confer a neurovirulent phenotype. Expression of the F gene of the neurovirulent strain alone was sufficient to induce significant levels of hydrocephalus. Coexpression of the homologous HN gene led to a marginal increase in the level of hydrocephalus.

Mutation at residue 523 creates a second receptor binding site on human parainfluenza virus type 1 hemagglutinin-neuraminidase protein

The paramyxovirus hemagglutinin-neuraminidase (HN) is a multifunctional protein mediating hemagglutination (HA), neuraminidase (NA), and fusion promotion activities. It has been a matter of debate whether HN contains combined or separate sites for HA and NA activities. To clear the issue, we determined the presence of the second binding site on human parainfluenza virus (hPIV) type 1, 2, and 3 and Sendai virus (SeV) HN proteins. Results of virus elution from erythrocytes at an elevated temperature and HA inhibition by NA inhibitor BCX-2798 suggest that all hPIVs bind to the receptor only through the NA catalytic site, while SeV HN has an additional receptor binding site. Comparison of SeV and hPIV1 HN sequences revealed two amino acid differences at residues 521 and 523 in the region close to the second binding site identified in Newcastle disease virus HN. We mutated hPIV1 HN at position 523 from Asn to the residue of SeV HN, Asp, and rescued a recombinant SeV that carries the mutated hPIV1 HN by a reverse genetics system. The hPIV1 HN with Asp at position 523 hemagglutinated in the presence of BCX-2798, suggesting that the amino acid difference at position 523 is critical for the formation of a second binding site. Creation of the second binding site on hPIV1 HN, however, did not significantly affect the growth or fusion activity of the recombinant virus. Our study indicates that the presence and requirement of a second binding site vary among paramyxoviruses.

Crystallization and preliminary X-ray diffraction analysis of central structure domains from mumps virus F protein

Fusion of members of the Paramyxoviridae family involves two glycoproteins: the attachment protein and the fusion protein. Changes in the fusion-protein conformation were caused by binding of the attachment protein to the cellular receptor. In the membrane-fusion process, two highly conserved heptad-repeat (HR) regions, HR1 and HR2, are believed to form a stable six-helix coiled-coil bundle. However, no crystal structure has yet been determined for this state in the mumps virus (MuV, a member of the Paramyxoviridae family). In this study, a single-chain protein consisting of two HR regions connected by a flexible amino-acid linker (named 2-Helix) was expressed, purified and crystallized by the hanging-drop vapour-diffusion method. A complete X-ray data set was obtained in-house to 2.2 A resolution from a single crystal. The crystal belongs to space group C2, with unit-cell parameters a = 161.2, b = 60.8, c = 40.1 A, beta = 98.4 degrees. The crystal structure will help in understanding the molecular mechanism of Paramyxoviridae family membrane fusion.

Influence of the human parainfluenza virus 3 attachment protein's neuraminidase activity on its capacity to activate the fusion protein

In order to examine functions of the hemagglutinin-neuraminidase (HN) protein that quantitatively influence fusion promotion, human parainfluenza virus 3 (HPIV3) variants with alterations in HN were studied. The variant HNs have mutations that affect either receptor binding avidity, neuraminidase activity, or fusion protein (F) activation. Neuraminidase activity was regulated by manipulation of temperature and pH. F activation was assessed by quantitating the irreversible binding of target erythrocytes (RBC) to HN/F-coexpressing cells in the presence of 4-GU-DANA (zanamivir) to release target cells bound only by HN-receptor interactions; the remaining, irreversibly bound target cells are retained via the fusion protein. In cells coexpressing wild-type (wt) or variant HNs with wt F, the fusion promotion capacity of HN was distinguished from target cell binding by measuring changes with time in the amounts of target RBC that were (i) reversibly bound by HN-receptor interaction (released only upon the addition of 4-GU-DANA), (ii) released by HN's neuraminidase, and (iii) irreversibly bound by F-insertion or fusion (F triggered). For wt HN, lowering the pH (to approach the optimum for HPIV3 neuraminidase) decreased F triggering via release of HN from its receptor. An HN variant with increased receptor binding avidity had F-triggering efficiency like that of wt HN at pH 8.0, but this efficiency was not decreased by lowering the pH to 5.7, which suggested that the variant HN's higher receptor binding activity counterbalanced the receptor dissociation promoted by increased neuraminidase activity. To dissect the specific contribution of neuraminidase to triggering, two variant HNs that are triggering-defective due to a mutation in the HN stalk were evaluated. One of these variants has, in addition, a mutation in the globular head that renders it neuraminidase dead, while the HN with the stalk mutation alone has 30% of wt neuraminidase. While the variant without neuraminidase activity triggered F effectively at 37 degrees C irrespective of pH, the variant possessing effective neuraminidase activity completely failed to activate F at pH 5.7 and was capable of only minimal triggering activity even at pH 8.0. These results demonstrate that neuraminidase activity impacts the extent of HPIV3-mediated fusion by releasing HN from contact with receptor. Any particular HN's competence to promote F-mediated fusion depends on the balance between its inherent F-triggering efficacy and its receptor-attachment regulatory functions (binding and receptor cleavage).

Six-helix bundle assembly and analysis of the central core of mumps virus fusion protein

The fusion protein of enveloped viruses mediates the fusion between the viral and cellular membranes, allowing the penetration of the viral genomes into the host cell. Many of these proteins share a common fold comprising a central core trimer of anti-parallel coiled-coil heterodimers, which are formed by two discontinuous heptad repeat (HR) motifs located at the ectodomain of the fusion proteins. In this study, we constructed and purified the corresponding regions (HR1 and HR2) of mumps virus fusion protein that are predicted to form coiled coil. The HR1 and HR2 were expressed and purified separately or as a single chain connected by an amino acid linker (HR1-linker-HR2, named 2-Helix). Series of biochemical and biophysical analyses of the expressed proteins have shown that HR1 and HR2 of mumps virus fusion protein share the common features of other enveloped virus fusion proteins. CD spectral results show that HR1 forms an alpha-helical coil structure while HR2 exists as an unstructured monomer in PBS in nature. Mixtures of HR1 and HR2 could form a stable six-helix bundle, indicating the interaction of HR1 and HR2. The 2-Helix protein also shows characteristic properties of the 6-helix bundle. Therefore, mumps virus fusion protein has a common core architecture and its HR regions could be used as a drug target for virus fusion inhibitors.

Human parainfluenza virus type 3 HN-receptor interaction: effect of 4-guanidino-Neu5Ac2en on a neuraminidase-deficient variant

The envelope of human parainfluenza virus type 3 (HPF3) contains two viral glycoproteins, the hemagglutinin-neuraminidase (HN) and the fusion protein (F). HN, which is responsible for receptor attachment and for promoting F-mediated fusion, also possesses neuraminidase (receptor-destroying) activity. We reported previously that 4-guanidino-neu5Ac2en (4-GU-DANA) and related sialic acid-based inhibitors of HPF3 neuraminidase activity also inhibit HN-mediated receptor binding and fusion processes not involving neuraminidase activity. We have now examined this mechanism, as well as neuraminidase's role in the viral life cycle, using a neuraminidase-deficient HPF3 variant (C28a) and stable cell lines expressing C28a or wild-type (wt) HN. C28a, which has a wt F sequence and two point mutations in the HN gene corresponding to two amino acid changes in the HN protein, is the first HPF3 variant with insignificant neuraminidase activity. Cells expressing C28a HN did not bind erythrocytes at 4 degrees C unless pretreated with neuraminidase, but no such pretreatment was required for hemadsorption activity (HAD) at 22 or 37 degrees C. HAD was blocked by 4-GU-DANA, attesting to the ability of this compound to inhibit HN's receptor-binding activity. C28a or wt plaque enlargement, a process that involves cell-cell fusion and does not depend on virion release, is diminished by the presence of 4-GU-DANA, confirming the inhibitory effect of 4-GU-DANA on the fusogenic function of C28a HN. In C28a-infected cell monolayers, virion release and thus multicycle replication are severely restricted. This defect was corrected by supplementation of exogenous neuraminidase and also by the addition of 4-GU-DANA; neuraminidase destroys the receptors whereby newly formed C28a virions would remain attached to the cell surface, whereas 4-GU-DANA prevents the attachment itself, obviating the need for receptor cleavage. In accord with the ability of 4-GU-DANA to prevent attachment, the neuraminidase inhibitory effect of 4-GU-DANA on wt HPF3 did not diminish virion release into the medium. Thus, it is by inhibition of viral entry and syncytium formation that sialic acid analogs like 4-GU-DANA may counteract wt HPF3 infection.

A single amino acid alteration in the human parainfluenza virus type 3 hemagglutinin-neuraminidase glycoprotein confers resistance to the inhibitory effects of zanamivir on receptor binding and neuraminidase activity

Entry and fusion of human parainfluenza virus type 3 (HPF3) requires interaction of the viral hemagglutinin-neuraminidase (HN) glycoprotein with its sialic acid receptor. 4-Guanidino-2,4-dideoxy-2,3-dehydro-N-acetylneuraminic acid (4-GU-DANA; zanamivir), a sialic acid transition-state analog designed to fit the influenza virus neuraminidase catalytic site, possesses antiviral activity at nanomolar concentrations in vitro. We have shown previously that 4-GU-DANA also inhibits both HN-mediated binding of HPF3 to host cell receptors and HN's neuraminidase activity. In the present study, a 4-GU-DANA-resistant HPF3 virus variant (ZM1) was generated by serial passage in the presence of 4-GU-DANA. ZM1 exhibited a markedly fusogenic plaque morphology and harbored two HN gene mutations resulting in two amino acid alterations, T193I and I567V. Another HPF3 variant studied in parallel, C-0, shared an alteration at T193 and exhibited similar plaque morphology but was not resistant to 4-GU-DANA. Neuraminidase assays revealed a 15-fold reduction in 4-GU-DANA sensitivity for ZM1 relative to the wild type (WT) and C-0. The ability of ZM1 to bind sialic acid receptors was inhibited 10-fold less than for both WT and C-0 in the presence of 1 mM 4-GU-DANA. ZM1 also retained infectivity at 15-fold-higher concentrations of 4-GU-DANA than WT and C-0. A single amino acid alteration at HN residue 567 confers these 4-GU-DANA-resistant properties. An understanding of ZM1 and other escape variants provides insight into the effects of this small molecule on HN function as well as the role of the HN glycoprotein in HPF3 pathogenesis.

Structural and functional relationship between the receptor recognition and neuraminidase activities of the Newcastle disease virus hemagglutinin-neuraminidase protein: receptor recognition is dependent on neuraminidase activity

The terminal globular domain of the paramyxovirus hemagglutinin-neuraminidase (HN) glycoprotein spike has a number of conserved residues that are predicted to form its neuraminidase (NA) active site, by analogy to the influenza virus neuraminidase protein. We have performed a site-directed mutational analysis of the role of these residues in the functional activity of the Newcastle disease virus (NDV) HN protein. Substitutions for several of these residues result in a protein lacking both detectable NA and receptor recognition activity. Contribution of NA activity, either exogenously or by coexpression with another HN protein, partially rescues the receptor recognition activity of these proteins, indicating that the receptor recognition deficiencies of the mutated HN proteins result from their lack of detectable NA activity. In addition to providing support for the homology-based predictions for the structure of HN, these findings argue that (i) the HN residues that mediate its NA activity are not critical to its attachment function and (ii) NA activity is required for the protein to mediate binding to receptors.

Biological characteristics of genetic variants of Urabe AM9 mumps vaccine virus

The Urabe AM9 mumps vaccine is composed of a mixture of variants distinguishable by a difference at nucleotide (nt) 1081 of the hemagglutinin-neuraminidase (HN) gene (Brown, E.G., Dimock, K., Wright, K.E., 1996. The Urabe AM9 mumps vaccine is a mixture of viruses differing at amino acid (aa) 335 of the hemagglutinin-neuraminidase gene with one form associated with disease. J. Infect. Dis. 174, 619-622.). Further genetic and biological variation was detected in plaque purified viruses from the Urabe AM9 vaccine by examining the HN gene sequence, plaque morphology, cytopathic effects and growth in Vero cells, and temperature sensitivity (ts). Infection of Vero cells with plaque purified viruses with a G at nt 1081 of the HN gene produced large, clear plaques, caused significant CPE early after infection but yielded lower titres of virus than other purified viruses. None of these viruses were ts. In contrast, half of the plaque purified viruses with an A at nt 1081 were sensitive to a temperature of 39.5 degrees C. These viruses produced small plaques, caused significant CPE and grew to low titres. Two ts viruses possessed a unique aa substitution at aa 468 of HN. The remaining A(1081) viruses were not ts, produced large plaques but little CPE, and grew to titres 10-fold higher than the G(1081) viruses. Isolates of Urabe AM9 associated with post-vaccination illness were similar to these non-ts A(1081) viruses, but could be further sub-divided into two groups on the basis of a difference at aa 464 of HN. The post-vaccination isolates may represent insufficiently attenuated components of the vaccine, while the G(1081) and ts subset of A(1081) viruses may be more fully attenuated.

Relative affinity of the human parainfluenza virus type 3 hemagglutinin-neuraminidase for sialic acid correlates with virus-induced fusion activity

The ability of enveloped viruses to cause disease depends on their ability to enter the host cell via membrane fusion events. An understanding of these early events in infection, crucial for the design of methods of blocking infection, is needed for viruses that mediate membrane fusion at neutral pH, such as paramyxoviruses and human immunodeficiency virus. Sialic acid is the receptor for the human parainfluenza virus type 3 (HPF3) hemagglutinin-neuraminidase (HN) glycoprotein, the molecule responsible for binding of the virus to cell surfaces. In order for the fusion protein (F) of HPF3 to promote membrane fusion, the HN must interact with its receptor. In the present report, two variants of HPF3 with increased fusion-promoting phenotypes were selected and used to study the function of the HN glycoprotein in membrane fusion. Increased fusogenicity correlated with single amino acid changes in the HN protein that resulted in increased binding of the variant viruses to the sialic acid receptor. These results suggest that the avidity of binding of the HN protein to its receptor regulates the level of F protein-mediated fusion and begin to define one role of the receptor-binding protein of a paramyxovirus in the membrane fusion process.

Sequence and structure alignment of paramyxovirus hemagglutinin-neuraminidase with influenza virus neuraminidase

A model is proposed for the three-dimensional structure of the paramyxovirus hemagglutinin-neuraminidase (HN) protein. The model is broadly similar to the structure of the influenza virus neuraminidase and is based on the identification of invariant amino acids among HN sequences which have counterparts in the enzyme-active center of influenza virus neuraminidase. The influenza virus enzyme-active site is constructed from strain-invariant functional and framework residues, but in this model of HN, it is primarily the functional residues, i.e., those that make direct contact with the substrate sialic acid, which have identical counterparts in neuraminidase. The framework residues of the active site are different in HN and in neuraminidase and appear to be less strictly conserved within HN sequences than within neuraminidase sequences.

Identification of an amino acid that defines the fusogenicity of mumps virus

Recombinant cDNA clones representing the fusion (F) and hemagglutinin-neuraminidase (HN) proteins of two mumps virus strains different in fusogenicity were constructed. Upon transfection of COS7 cells, extensive cell fusion was observed only when cells expressed the F protein of the fusing strain together with the HN protein derived from either strain. Mutational analyses further showed that the amino acid at position 195 of the F protein plays a critical role in determining the extent of cell fusion induced by mumps virus, since replacement of Ser-195 by Tyr significantly reduced the fusion inducibility of otherwise fusion-competent F protein.

Fusion mutants of Newcastle disease virus selected with monoclonal antibodies to the hemagglutinin-neuraminidase

The Australia-Victoria (AV) isolate of Newcastle disease virus (NDV) induces fusion from within but not fusion from without. L1, a neuraminidase (NA)-deficient virus derived from AV, has the opposite fusion phenotype from the wild-type virus. It fails to induce the former mode of fusion, but has gained a limited ability to promote the latter. Monoclonal antibodies to antigenic site 23 on the hemagglutinin-neuraminidase (HN) glycoprotein have previously been shown to select variants of the AV isolate that have altered NA activity or receptor-binding affinity. By using an antibody to this site, variants of L1 have been selected. Three of the variants have gained an increased affinity for sialic acid-containing receptors, as evidenced by the resistance of their hemagglutinating activity to the presence of reduced amounts of sialic acid on the surface of chicken erythrocytes. All four variants still have very low levels of NA activity, comparable to that of the parent virus, L1. The alteration in receptor-binding affinity results in a decreased potential for elution from cellular receptors and correlates with an increased ability to promote both modes of fusion. A single amino acid substitution in the HN protein of each variant, responsible for its escape from neutralization, has been identified. These studies identify two HN residues, 193 and 203, at which monoclonal antibody-selected substitution influences the receptor recognition properties of NDV and may influence its ability to promote syncytium formation.

Fusion properties of cells infected with human parainfluenza virus type 3: receptor requirements for viral spread and virus-mediated membrane fusion

Cells can be persistently infected with human parainfluenza virus type 3 (HPF3) by using a high multiplicity of infection (MOI) (> or = 5 PFU per cell). The persistently infected cells exhibit no cytopathic effects and do not fuse with each other, yet they readily fuse with uninfected cells. We have previously shown that the failure of the persistently infected cells to fuse with each other is due to the lack of a receptor on these cells for the viral hemagglutinin-neuraminidase glycoprotein, and we have established that both fusion and hemagglutinin-neuraminidase proteins are needed for cell fusion mediated by HPF3. We then postulated that the generation of persistent infection and the failure of cells infected with HPF3 at high MOI to form syncytia are both due to the action of viral neuraminidase in the high-MOI inoculum. In this report, we describe experiments to test this hypothesis and further investigate the receptor requirements for HPF3 infection and cell fusion. A normally cytopathic low-MOI HPF3 infection can be converted into a noncytopathic infection by the addition of exogenous neuraminidase, either in the form of a purified enzyme or as UV-inactivated HPF3 virions. Evidence is presented that the receptor requirements for an HPF3 virus particle to infect a cell are different from those for fusion between cells. By treating infected cells in culture with various doses of neuraminidase, we demonstrate that virus spreads from cell to cell in the complete absence of cell-cell fusion. We compare the outcome of HPF3 infection in the presence of excess neuraminidase with that of another paramyxovirus (simian virus 5) and provide evidence that these two viruses differ in their receptor requirements for mediating fusion.

Expression of mumps virus glycoproteins in mammalian cells from cloned cDNAs: both F and HN proteins are required for cell fusion

Two recombinant plasmids were constructed by inserting the cDNAs of either the fusion (F) or the hemagglutinin-neuraminidase (HN) protein genes of mumps virus into the pcDL-SR alpha expression vector. Both the F and the HN proteins expressed in COS7 cells transfected with their respective recombinant plasmids were indistinguishable in terms of electrophoretic mobility from their counterparts synthesized in mumps virus-infected cells. The F protein was cleaved and expressed on the cell surface, but uncleaved forms were also detected. The expressed HN protein was transported to the cell surface and adsorbed guinea pig erythrocytes. Syncytium formation was induced when COS7 cells were transfected with both recombinant plasmid DNAs together, but not with the recombinant plasmid only carrying the F gene. This observation indicates that cell fusion mediated by mumps virus requires both the F and the HN glycoproteins.

Fusion properties of cells persistently infected with human parainfluenza virus type 3: participation of hemagglutinin-neuraminidase in membrane fusion

Cells persistently infected with human parainfluenza virus type 3 (HPF3) exhibit a novel phenotype. They are completely resistant to fusion with each other but readily fuse with uninfected cells. We demonstrate that the inability of these cells to fuse with each other is due to a lack of cell surface neuraminic acid. Neuraminic acid is the receptor for the HPF3 hemagglutinin-neuraminidase (HN) glycoprotein, the molecule responsible for binding of the virus to cell surfaces. Uninfected CV-1 cells were treated with neuraminidase and then tested for their ability to fuse with the persistently infected (pi) cells. Neuraminidase treatment totally abolished cell fusion. To extend this result, we used a cell line deficient in sialic acid and demonstrated that these cells, like the neuraminidase-treated CV-1 cells, were unable to fuse with pi cells. We then tested whether mimicking the agglutinating function of the HN molecule with lectins would result in cell fusion. We added a panel of five lectins to the neuraminic acid-deficient cells and showed that binding of these cells to the pi cells did not result in fusion; the lectins could not substitute for interaction of neuraminic acid with the HN molecule in promoting membrane fusion. These results provide compelling evidence that the HN molecule of HPF3 and its interaction with neuraminic acid participate in membrane fusion and that cell fusion is mediated by an interaction more complex than mere juxtaposition of the cell membranes.

Identification of amino acid positions associated with neuraminidase activity of the hemagglutinin-neuraminidase glycoprotein of Sendai virus

Identification of amino acid positions associated with neuraminidase activity on the hemagglutinin-neuraminidase (HN) glycoprotein of paramyxoviruses has been difficult because neuraminidase-inhibiting antibodies are not neutralizing and thus, escape mutants have not been isolated. Instead, many investigators have correlated an altered neuraminidase (NA) activity of natural virus variants, such as plaque-size variants, with sequence changes in the HN protein. To identify regions on the HN glycoprotein of Sendai virus (SV) that are associated with NA activity, we investigated NA activity of three plaque-size variants which potentially differed from the standard SV (SV/std). NA activity was measured by the ability of virus to elute from chicken erythrocytes as a result of cleaving sialic acid receptors, and by the ability of virus to cleave sialic acid from the small trisaccharide neuraminlactose and the larger substrate fetuin in an in vitro assay. Virions purified from each of the isolated plaques had a HN content and hemagglutinating activity similar to that of SV/std, yet each variant eluted much more rapidly from chicken erythrocytes than SV/std. In vitro NA activity of the plaque-size variants was 1.6 to 3.8 times greater than that of SV/std, providing supporting evidence for the elution data. Although all plaque-size variants showed elevated NA activity, there was no correlation of activity with plaque size. Sequence analysis showed that one of the variants had an amino acid change from glutamic acid to valine at position 165 and from lysine to glutamic acid at position 461, while a second variant had only the change at position 461. A third variant had a nearby change at position 468, from threonine to lysine. Taken together, these data support the conclusion that the amino acid residues at positions 461-468 and 165 are involved in neuraminidase activity of SV.

Hemagglutinin-neuraminidase (HN) amino acid alterations in neutralization escape mutants of Kilham mumps virus

The hemagglutinin-neuraminidase genes of the Kilham strain of mumps virus and three neutralization escape mutants (M11, M12 and M13) of this strain (Löve et al., 1985a) were sequenced using their genomes as template. The predicted amino acid sequences were compared. While one mutant had only one amino acid substitution the other two mutants had four and five respectively. A putative region for the epitope of the selected neutralizing monoclonal antibody was identified in a hydrophilic region encompassing amino acids 352-360, since the single amino acid substitution of one mutant occurred in this region and the other two mutants showed non-conserved amino acid changes in this part of the protein. The previously sequenced prototype strain RW, which lacks capacity to react with the selected neutralizing monoclonal antibody also has one non-conserved amino acid change in the region of the proposed neutralizing epitope. The three mutants showed different biological characteristics. These particular characteristics were therefore interpreted to be primarily associated with strain-specific amino acid changes outside the region of the presumed neutralizing epitope. The decrease in molecular weight in one mutant (M11) was shown to be due to a substitution in position 329 of an asparagine for an aspartic acid, leading to abolishment of a potential N-linked glycosylation site. In the other mutants, one substitution in position 239 of a lysine for a methionine was correlated with an increased neuraminidase activity of strain M12, while a substitution in position 360 of an arginine for a cysteine appeared to represent the most likely explanation for the reduced neurovirulence of strain M13.

Molecular cloning and sequence analysis of human parainfluenza type 4A virus HN gene: its irregularities on structure and activities

We cloned the cDNA of human parainfluenza type 4A virus (PIV-4A) HN gene by reverse-transcription of virus-specific mRNAs and genomic RNA, and determined the complete nucleotide sequence of the HN gene. The predicted HN protein sequence of PIV-4A showed significant relatedness with those of other paramyxoviruses, SV5, NDV, MuV, PIV-3, BPIV-3, indicating a common ancestor. The homologies between the viruses suggested that PIV-4A is more closely related to NDV, SV5, and MuV than to the parainfluenza viruses, PIV-3, bovine parainfluenza type 3 virus (BPIV-3), and Sendai virus (SV). Sixty amino acids were commonly conserved among the viruses, other than PIV-4A. Two of these amino acids were substituted in PIV-4A HN and are predicted to be located near the active site of the neuraminidase. The analysis of neuraminidase of PIV-4 revealed that the activity is hardly detectable, suggesting the significant effect of the substituted amino acid sites on neuraminidase activity.

Identification of amino acid residues important to the neuraminidase activity of the HN glycoprotein of Newcastle disease virus

Monoclonal antibodies (MAbs) to three overlapping antigenic sites (designated 12, 2, and 23) on the hemagglutinin-neuraminidase glycoprotein (HN) of Newcastle disease virus (NDV) were previously shown to inhibit neuraminidase activity (NA) on neuraminlactose (R. M. Iorio and M. A. Bratt, 1984a, J. Immunol. 133, 2215-2219; R. M. Iorio et al., 1989, Virus Res. 13, 245-262). However, a competitive inhibitor of NA blocks the binding of only MAbs to site 23, suggesting that the domain they recognize may be closely related to the NA site. Antigenic variants selected with site 23 MAbs have single amino acid substitutions at HN residues 192, 193, or 200. Virions of variants, which have a substitution at residue 193 or 200, have alterations in NA which are not attributable to a commensurate change in HN content. A revertant of a temperature-sensitive mutant, which has markedly diminished NA relative to the wild type, has an amino acid substitution at residue 175. A second step revertant having partially restored NA has an additional substitution at residue 192 identical to that in one of the site 23 variants, which, in turn, also makes the revertant resistant to neutralization by site 23 MAbs. Thus, an amino acid substitution at residue 175, 193, or 200 of the HN of NDV can have marked effects on the NA of the protein. The amino acids in the region around residue 175 are highly conserved between the HNs of NDV and other paramyxoviruses, suggesting that this domain is important to the integrity of the NA site in this group of viruses.

Syncytium formation by recombinant vaccinia viruses carrying bovine parainfluenza 3 virus envelope protein genes

The highly syncytium-inducing M strain and the weakly syncytium-inducing SC strain of bovine parainfluenza 3 virus differ by a single amino acid substitution in each of the hemagglutinin-neuraminidase (HN) and membrane (M) proteins, while their fusion (F) proteins are identical (T. Shioda, S. Wakao, S. Suzu, and H. Shibuta, Virology 162:388-396, 1988). We constructed recombinant vaccinia viruses which express separately the M virus HN (Vac-MHN), SC virus HN (Vac-SCHN), M virus M (Vac-MM), SC virus M (Vac-SCM), and common F (Vac-F) proteins. CV-1 cells were infected with the recombinants, singly or in combination, and implanted onto indicator MDBK cells for syncytium formation. Combinations of Vac-MHN plus Vac-F and Vac-SCHN plus Vac-F induced extensive and weak syncytium formation, respectively. Vac-F alone did not induce syncytium formation, and both Vac-MM and Vac-SCM had no effect on syncytium formation. These findings indicated that the syncytium formation by bovine parainfluenza 3 virus requires both the F and HN proteins and that the extensive syncytium formation by the M virus is due to the M virus HN protein. MSC, another weakly syncytium-inducing virus variant, newly isolated from the M virus, was identical to the M virus in the primary structure of the HN and M proteins but differed from the M virus by a single amino acid residue in the F protein. The combination of the recombinant vaccinia virus expressing the MSC virus F protein and Vac-MHN resulted in weak syncytium formation.