Comparison of RNA synthesis initiation properties of non-segmented negative strand RNA virus polymerases.

It is generally thought that the promoters of non-segmented, negative strand RNA viruses (nsNSVs) direct the polymerase to initiate RNA synthesis exclusively opposite the 3´ terminal nucleotide of the genome RNA by a de novo (primer independent) initiation mechanism. However, recent studies have revealed that there is diversity between different nsNSVs with pneumovirus promoters directing the polymerase to initiate at positions 1 and 3 of the genome, and ebolavirus polymerases being able to initiate at position 2 on the template. Studies with other RNA viruses have shown that polymerases that engage in de novo initiation opposite position 1 typically have structural features to stabilize the initiation complex and ensure efficient and accurate initiation. This raised the question of whether different nsNSV polymerases have evolved fundamentally different structural properties to facilitate initiation at different sites on their promoters. Here we examined the functional properties of polymerases of respiratory syncytial virus (RSV), a pneumovirus, human parainfluenza virus type 3 (PIV-3), a paramyxovirus, and Marburg virus (MARV), a filovirus, both on their cognate promoters and on promoters of other viruses. We found that in contrast to the RSV polymerase, which initiated at positions 1 and 3 of its promoter, the PIV-3 and MARV polymerases initiated exclusively at position 1 on their cognate promoters. However, all three polymerases could recognize and initiate from heterologous promoters, with the promoter sequence playing a key role in determining initiation site selection. In addition to examining de novo initiation, we also compared the ability of the RSV and PIV-3 polymerases to engage in back-priming, an activity in which the promoter template is folded into a secondary structure and nucleotides are added to the template 3´ end. This analysis showed that whereas the RSV polymerase was promiscuous in back-priming activity, the PIV-3 polymerase generated barely detectable levels of back-primed product, irrespective of promoter template sequence. Overall, this study shows that the polymerases from these three nsNSV families are fundamentally similar in their initiation properties, but have differences in their abilities to engage in back-priming.

The catalytic mechanism of human parainfluenza virus type†3 haemagglutinin-neuraminidase revealed.

Human parainfluenza virus type 3 (hPIV-3) is one of the leading causes for lower respiratory tract disease in children, with neither an approved antiviral drug nor vaccine available to date. Understanding the catalytic mechanism of human parainfluenza virus haemagglutinin-neuraminidase (HN) protein is key to the design of specific inhibitors against this virus. Herein, we used (1) H NMR spectroscopy, X-ray crystallography, and virological assays to study the catalytic mechanism of the HN enzyme activity and have identified the conserved Tyr530 as a key amino acid involved in catalysis. A novel 2,3-difluorosialic acid derivative showed prolonged enzyme inhibition and was found to react and form a covalent bond with Tyr530. Furthermore, the novel derivative exhibited enhanced potency in virus blockade assays relative to its Neu2en analogue. These outcomes open the door for a new generation of potent inhibitors against hPIV-3 HN.

Exposing the flexibility of human parainfluenza virus hemagglutinin-neuraminidase.

Human parainfluenza virus type 3 (hPIV-3) is a clinically significant pathogen and is the causative agent of pneumonia and bronchiolitis in children. In this study the solution dynamics of human parainfluenza type 3 hemagglutinin-neuraminidase (HN) have been investigated. A flexible loop around Asp216 that adopts an open conformation in direct vicinity of the active site of the apo-form of the protein and closes upon inhibitor binding has been identified. To date, no available X-ray crystal structure has shown the molecular dynamics simulation-derived predominant loop-conformation states found in the present study. The outcomes of this study provide additional insight into the dynamical properties of hPIV-3 HN and may have important implications in defining HN glycan recognition events, receptor specificity, and antiparainfluenza virus drug discovery.

Structural Insights into Human Parainfluenza Virus 3 Hemagglutinin-Neuraminidase Using Unsaturated 3- N-Substituted Sialic Acids as Probes.

A novel approach to human parainfluenza virus 3 (hPIV-3) inhibitor design has been evaluated by targeting an unexplored pocket within the active site region of the hemagglutinin-neuraminidase (HN) of the virus that is normally occluded upon ligand engagement. To explore this opportunity, we developed a highly efficient route to introduce nitrogen-based functionalities at the naturally unsubstituted C-3 position on the neuraminidase inhibitor template N-acyl-2,3-dehydro-2-deoxy-neuraminic acid ( N-acyl-Neu2en), via a regioselective 2,3-bromoazidation. Introduction of triazole substituents at C-3 on this template provided compounds with low micromolar inhibition of hPIV-3 HN neuraminidase activity, with the most potent having 48-fold improved potency over the corresponding C-3 unsubstituted analogue. However, the C-3-triazole N-acyl-Neu2en derivatives were significantly less active against the hemagglutinin function of the virus, with high micromolar IC50 values determined, and showed insignificant in vitro antiviral activity. Given the different pH optima of the HN protein's neuraminidase (acidic pH) and hemagglutinin (neutral pH) functions, the influence of pH on inhibitor binding was examined using X-ray crystallography and STD NMR spectroscopy, providing novel insights into the multifunctionality of hPIV-3 HN. While the 3-phenyltriazole- N-isobutyryl-Neu2en derivative could bind HN at pH 4.6, suitable for neuraminidase inhibition, at neutral pH binding of the inhibitor was substantially reduced. Importantly, this study clearly demonstrates for the first time that potent inhibition of HN neuraminidase activity is not necessarily directly correlated with a strong antiviral activity, and suggests that strong inhibition of the hemagglutinin function of hPIV HN is crucial for potent antiviral activity. This highlights the importance of designing hPIV inhibitors that primarily target the receptor-binding function of hPIV HN.

Antiviral effects of glycosylation and glucose trimming inhibitors on human parainfluenza virus type 3.

Endoplasmic reticulum (ER) alpha-glucosidase inhibitors block the trimming of N-linked glycosylation and thus prevent the production of several viruses. The present study investigates the antiviral effects of the alpha-glucosidase and alpha-mannosidase inhibitors (castanospermine, 1-deoxynojirimycin, bromoconduritol, deoxymannojirimycin and swainsonine) on human parainfluenza virus type 3 (HPIV3). The alpha-glucosidase inhibitors (castanospermine, 1-deoxynojirimycin) in recombinant expression systems reduced the surface and intracellular expression of both HPIV3 F and HN proteins. On the other hand, alpha-mannosidase inhibitors prevented processing of the oligosaccharides on HPIV3 glycoproteins into the complex form. Consequently, alpha-glycosidase inhibitors (castanospermine and 1-deoxynojirimycin) significantly inhibited viral fusion activity. We demonstrated that the alpha-glucosidase inhibitors (castanospermine and 1-deoxynojirimycin) reduced the infectivity of newly released viral particles. We postulate that alpha-glucosidase inhibitors can prevent the first steps of HPIV3 envelope glycoprotein processing and that the inhibition of glucose trimming has antiviral effects.

Human parainfluenza virus 3 neuraminidase activity contributes to dendritic cell maturation.

Mechanisms of dendritic cells (DCs) immunomodulation by parainfluenza viruses have not been characterized. We analyzed whether the human parainfluenza 3 (HPF3) virus hemagglutinin-neuraminidase glycoprotein (HN) might influence DC maturation. HN possesses a receptor binding function and a neuraminidase or desialidating activity. To assess whether the neuraminidase activity of HN affects DC maturation, human myeloid DCs were exposed to either live or UV-inactivated HPF3 viruses containing wild type or a mutated form of HN with decreased neuraminidase activity. Exposure of human DCs to either UV-inactivated or live virus induced up-regulation of CD83 and CD86 surface markers, morphological changes, and a cytokine expression pattern consistent with maturation. However, the level of maturation was found to be lower in DCs infected with the neuraminidase deficient variant as compared to the wild type. These results suggest that during the course of viral infection, HN's neuraminidase activity may play an important role contributing to maturation and activation of DCs.

Catalytic mechanism and novel receptor binding sites of human parainfluenza virus type 3 hemagglutinin-neuraminidase (hPIV3 HN).

The human parainfluenza virus type 3 (hPIV3) hemagglutinin-neuraminidase (HN) has opposing functions of binding sialic acid receptors and cleaving them, facilitating virus release. The crystal structure of hPIV3 HN complexed with the substrate analogue difluorosialic acid (DFSA) revealed that catalysis by HN involves the formation of a covalently linked sialosyl-enzyme intermediate which was trapped along with a transition-state analogue resembling an oxocarbenium ion. This mechanism of enzyme catalysis was also confirmed in the crystal structure of the influenza N9 neuraminidase complexed with DFSA. Additionally, novel secondary receptor binding sites were identified in the hPIV3 HN-DFSA complex including one near the catalytic cavity which upon binding DFSA imposes subtle changes and may help the HN balance the opposing functions. Multiple receptor binding sites may increase avidity to facilitate cell binding and fusion promotion. The secondary receptor binding sites in the paramyxoviruses are so far unique to each virus type.

Amino acid substitutions contributing to α2,6-sialic acid linkage binding specificity of human parainfluenza virus type 3 hemagglutinin-neuraminidase.

Human parainfluenza virus type 3 (hPIV3) recognizes both α2,3- and α2,6-linked sialic acids, whereas human parainfluenza virus type 1 (hPIV1) recognizes only α2,3-linked sialic acids. To identify amino acid residues that confer α2,6-linked sialic acid recognition of hPIV3, amino acid residues in or neighboring the sialic acid binding pocket of the hPIV3 hemagglutinin-neuraminidase (HN) glycoprotein were substituted for the corresponding residues of hPIV1 HN. Hemadsorption assay with sialyl linkage-modified red blood cells indicated that amino acid residues at positions 275, 277, 372, and 426 contribute to α2,6-linked sialic acid recognition of the HN3 glycoprotein.

Human parainfluenza virus type 3: analysis of the cytoplasmic tail and transmembrane anchor of the hemagglutinin-neuraminidase protein in promoting cell fusion.

The role of the cytoplasmic tail and transmembrane anchor of the human parainfluenza virus type 3 (HPIV3) hemagglutinin-neuraminidase (HN) glycoprotein in promoting cell fusion was investigated. A series of amino terminal deletion mutants (d10, d20, d27, d31, d40, d44, and d73) were compared for processing, cell surface expression, and maintenance of their biological attributes by recombinant expression of mutant genes using a plasmid vector (pcDL-SR alpha-296) in CV-1 and HeLa cells. To determine the fusion promoting activity (FPA) of the various mutant proteins, a simple assay was developed which quantified the fusion of two different HeLa cell types. One of the cell types, HeLa-tat, constitutively expressed the human immunodeficiency virus type I (HIV-1) tat protein from a Moloney murine leukemia virus long terminal repeat (LTR), while the second cell type, HeLa beta-gal, contained a reporter gene, beta-galactosidase, under the control of an HIV1-LTR. Fusion of mixed HeLa cell monolayers (50:50, HeLa-tat: HeLa beta-gal), following transfection with appropriate plasmids, resulted in transactivation of the reporter gene which was then measured by direct staining of cells or using cell lysates with appropriate substrates. Cell fusion was observed only when both the HPIV3 F and functional HN proteins were both co-transfected into cells. Of the seven deletion mutants examined, only d10, d20, d27 and d31 were expressed to significant levels on the cell surface and only these four mutant proteins maintained FPA. Compared with the wt HN at 48 h post transfection, d10 and d20 had enhanced FPA (119% and 158%, respectively), while d27 and d31 were diminished (74% and > 4%, respectively). Analysis of protein expression suggested that the reason for the increase in FPA of the mutant proteins was that the levels of protein expressed at the cell surface was twofold or threefold higher for d10 and d20, respectively, compared to the wt HN.

The importance of IMP dehydrogenase inhibition in the broad spectrum antiviral activity of ribavirin and selenazofurin.

The nucleosides ribavirin, selenazofurin, tiazofurin and bredinin all exhibit the lowering of guanylate pools in vitro and in vivo by the inhibition of IMP dehydrogenase. However, each of these nucleosides has a separate profile of antiviral and antitumor activity. The IMP dehydrogenase inhibition in the case of ribavirin and bredinin appears to be due to the nucleoside 5'-monophosphate and in the case of selenazofurin and tiazofurin to the NAD analogs formed intracellularly. With regard to the antiviral activity of these nucleosides, although selenazofurin was the most potent antiviral agent in vitro, its antiviral activity was also most readily reversed by exogenous guanosine. The antiviral effects of ribavirin were only partially reversed under the conditions studied. These and related studies show that each of these nucleosides form nucleotide metabolites which act as enzyme inhibitors at additional sites other than IMP dehydrogenase. As in the case of ribavirin such inhibition of IMP dehydrogenase may result in an increased "self potentiation" by the lowering of guanylate pools in those instances where guanylate analogs are involved as inhibitors of viral specific or viral induced enzymes. Further studies should more clearly elucidate the importance of the simultaneous inhibition of various enzyme sites by different metabolic nucleotide forms of the same nucleoside analog.

Analysis of the mutations in the active site of the RNA-dependent RNA polymerase of human parainfluenza virus type 3 (HPIV3).

The large protein (L) of the human parainfluenza virus type 3 (HPIV3) is the functional RNA-dependent RNA polymerase, which possesses highly conserved residues QGDNQ located within motif C of domain III comprising the putative polymerase active site. We have characterized the role of the QGDNQ residues as well as the residues flanking this region in the polymerase activity of the L protein by site-directed mutagenesis and examining the polymerase activity of the wild-type and mutant L proteins by an in vivo minigenome replication assay and an in vitro mRNA transcription assay. All mutations in the QGDNQ residues abolished transcription while mutations in the flanking residues gave rise to variable polymerase activities. These observations support the contention that the QGDNQ sequence is absolutely required for the polymerase activity of the HPIV3 RNA-dependent RNA polymerase.

Parainfluenza-3 virus: difference in capacity of neuraminidase-weak and strong strains to infect young calves and to elicit cellular immune response.

Calves less than four weeks old could not be infected with a neuraminidase-weak strain of parainfluenza-3 virus (Pi3) but were successfully infected with either of two neuraminidase-strong strains. The criteria for infection were virus excretion, cell-mediated and antibody-mediated immune responses. In the lymphocyte stimulation test, calves infected with the neuraminidase-strong Pi3 strain Tüb-E6 responded more strongly to antigen prepared from this strain than to antigen from the heterologous Pi3 strain Um-23. The non-immunoglobulin haemagglutination inhibition activity of the liquid phase of nasal secretions of newborn calves decreased after treatment with Vibrio cholerae neuraminidase. For virus-bound neuraminidase the liquid phase from newborn calves was a richer substrate than the liquid phase from older animals.

Adenosine triphosphate phosphoydrolase activity associated with purified parainfluenza type 3 virions.

An adenosine triphosphate phosphohydrolase associated with purified parainfluenza type 3 virions has been characterized. It hydrolyzed ATP to ADP and AMP when activated with Mg-2+ ions. Using Ca-2+ the production of ADP was inhibited but not that of AMP. Neither K+ NOR Na+ ions were required for the expression of maximal activity. Ouabain had no inhibitory effect on enzyme activity even at 10-3M. After exposure of virus preparations to Tween 20, enzyme activity was not affected. A linear relationship between enzyme activity and concentration of virus was observed.

[Study on functions of N-carbohydrate chains in human parainfluenza virus type 3 hemagglutinin-neuraminidase protein].

To determine the functions of N-carbohydrate chains in human parainfluenza virus type 3 hemagglutinin-neuraminidase(HN) protein, a PCR-based site-directed mutagenesis method was used to obtain N-glycan mutants. Protein electrophoresis rate, cell surface expression,receptor binding activity, neuraminidase activity and cell fusion promotion activity were determined. The HN proteins of single mutants (G1, G2, and G4) and multiple mutants (G12, G14, G24 and G124) migrated faster than the wild-type (wt) HN protein on polyacrylamide gels, while G3-mutated protein and wt HN protein migrated at the same position. There was no statistic difference in cell surface expression and neuraminidase activity between wt and each mutant HN protein (P>0.05), but receptor binding activity and cell fusion promotion activity of each mutant protein was reduced to significant extent (P<0.05). G1, G2 and G4 mutants exhibited re duced receptor binding activity, which was 83.94%, 76.45% and 55.32% of the wt level, respectively. G1, G2 and G4-mutated proteins also showed reductions in fusion promotion activity, which was 80.84%, 77.83% and 64.16%, respectively. Multiple mutants with G12-, G14-, G24- and G124- substitutions could further reduce receptor binding activities, 33.07%, 20.67%, 19.96% and 15.11% of the wt HN level, respectively. G12, G14, G24 and G124 mutants exhibited levels of fusion promotion activity that were only 46.360, 12.04%, 13.43% and 4.05% of the wt amount, respectively. As N-glycans of hPIV3 HN protein play an important role in receptor binding activity and cell fusion promotion activity of HN protein. We propose that the loss of N-glycans change the conformation or orientation of globular domain that is responsible for receptor binding and lower receptor binding activity and cell fusion promotion activi ty.

Interaction between the hemagglutinin-neuraminidase and fusion glycoproteins of human parainfluenza virus type III regulates viral growth in vivo.

UNLABELLED: Paramyxoviruses, enveloped RNA viruses that include human parainfluenza virus type 3 (HPIV3), cause the majority of childhood viral pneumonia. HPIV3 infection starts when the viral receptor-binding protein engages sialic acid receptors in the lung and the viral envelope fuses with the target cell membrane. Fusion/entry requires interaction between two viral surface glycoproteins: tetrameric hemagglutinin-neuraminidase (HN) and fusion protein (F). In this report, we define structural correlates of the HN features that permit infection in vivo. We have shown that viruses with an HN-F that promotes growth in cultured immortalized cells are impaired in differentiated human airway epithelial cell cultures (HAE) and in vivo and evolve in HAE into viable viruses with less fusogenic HN-F. In this report, we identify specific structural features of the HN dimer interface that modulate HN-F interaction and fusion triggering and directly impact infection. Crystal structures of HN, which promotes viral growth in vivo, show a diminished interface in the HN dimer compared to the reference strain's HN, consistent with biochemical and biological data indicating decreased dimerization and decreased interaction with F protein. The crystallographic data suggest a structural explanation for the HN's altered ability to activate F and reveal properties that are critical for infection in vivo. IMPORTANCE: Human parainfluenza viruses cause the majority of childhood cases of croup, bronchiolitis, and pneumonia worldwide. Enveloped viruses must fuse their membranes with the target cell membranes in order to initiate infection. Parainfluenza fusion proceeds via a multistep reaction orchestrated by the two glycoproteins that make up its fusion machine. In vivo, viruses adapt for survival by evolving to acquire a set of fusion machinery features that provide key clues about requirements for infection in human beings. Infection of the lung by parainfluenzavirus is determined by specific interactions between the receptor binding molecule (hemagglutinin-neuraminidase [HN]) and the fusion protein (F). Here we identify specific structural features of the HN dimer interface that modulate HN-F interaction and fusion and directly impact infection. The crystallographic and biochemical data point to a structural explanation for the HN's altered ability to activate F for fusion and reveal properties that are critical for infection by this important lung virus in vivo.

Synthesis and evaluation of 4-O-alkylated 2-deoxy-2,3-didehydro-N-acetylneuraminic acid derivatives as inhibitors of human parainfluenza virus type-3 sialidase activity.

The X-ray crystal structure of the paramyxoviral surface glycoprotein haemagglutinin-neuraminidase (HN) from Newcastle Disease virus was used as a template to design inhibitors of the HN from human parainfluenza virus type-3 (hPIV-3). 4-O-Alkylated derivatives of 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (Neu5Ac2en), accessed from 8,9-O-isopropylidenated-Neu5Ac2en1Me, were found to inhibit the sialidase (neuraminidase) activity of hPIV-3 (strain C243) in the range of 3-30muM. This is comparable or improved activity compared to the parent 4-hydroxy compound.

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).

The predominant mechanism by which ribavirin exerts its antiviral activity in vitro against flaviviruses and paramyxoviruses is mediated by inhibition of IMP dehydrogenase.

It is not yet clear to what extent depletion of intracellular GTP pools contributes to the antiviral activity of ribavirin. Therefore, the antiviral activities of (i) ribavirin, (ii) its 5-ethynyl analogue, 5-ethynyl-1-beta-D-ribofuranosylimidazole-4-carboxamide (EICAR), and (iii) mycophenolic acid (MPA) (a compound that inhibits only cellular IMP dehydrogenase activity) were studied on the replication of flaviviruses and paramyxoviruses. In addition, the effects of these three compounds on intracellular GTP pools were assessed. A linear correlation was observed over a broad concentration range between the antiviral activities of ribavirin, EICAR, and MPA and the effects of these compounds on GTP pool depletion. When the 50% effective concentrations (EC50s) for the antiviral activities of ribavirin, EICAR, and MPA were plotted against the respective EC50 values for GTP pool depletion, a linear correlation was calculated. These data provide compelling evidence that the predominant mechanism of action of ribavirin in vitro against flavi- and paramyxoviruses is based on inhibition of cellular IMP dehydrogenase activity.

Comparison of identical temperature-sensitive mutations in the L polymerase proteins of sendai and parainfluenza3 viruses.

The L subunit of the RNA-dependent RNA polymerase of negative strand RNA viruses is believed to possess all the enzymatic activities necessary for viral transcription and replication. Mutations in the L proteins of human parainfluenza virus type 3 (PIV3) and vesicular stomatitis virus (VSV) have been shown to confer temperature sensitivity to the viruses; however, their specific defects have not been determined. Mutant PIV3 L proteins expressed from plasmids were tested for temperature sensitivity in transcription and replication in a minigenome reporter system in cells and for in vitro transcription from purified PIV3 template. The single L mutants, Y942H and L992F, were temperature sensitive (ts) in both assays, although viral RNA synthesis was not completely abolished at the nonpermissive temperature. Surprisingly, the T1558I L mutant was not ts, although its cognate virus was ts. Thus the ts defect in this virus may be due to the abrogation of an essential interaction of the mutant polymerase with a host cell component, which is not measured by the RNA synthesis assays. Most of the combinations of the PIV3 L mutations were not additive and did not show temperature sensitivity in in vitro transcription. Since they were ts in the minigenome assay in vivo, replication appears to be specifically defective. The ts mutations in PIV3 and VSV L proteins were also substituted into the Sendai L protein to compare the defects in related systems. Only Sendai Y942H L was ts in both transcription and replication. One Sendai L mutant, L992F, gave much better replication than transcription. Several other mutants could transcribe but not replicate in vitro, while replication in vivo was normal.

Hemagglutinin-neuraminidase of human parainfluenza 3: role of the neuraminidase in the viral life cycle.

The function of neuraminidase in the life cycle and pathogenesis of human parainfluenza virus type 3 (HPF3) was studied by analyzing a variant of HPF3 that has decreased neuraminidase enzymatic activity. The variant virus is more fusogenic than the wild-type virus during an acute infection. Cloning and sequencing of the fusion (F) and hemagglutinin-neuraminidase (HN) genes from this variant revealed a single amino acid change in the HN protein and no alterations in the F protein sequence. Analysis of the growth properties of this variant revealed a delay in release of virus particles into the supernatant. Addition of exogenous neuraminidase to the culture resulted in increased release of infections viral particles, suggesting that the viral neuraminidase is important for release of HPF3 from the infected cell surface. In addition, the behavior of the variant virus during high-multiplicity infection and in the presence of exogenous neuraminidase provided evidence that the neuraminidase of HPF3 determines the outcome of viral infection (cytopathic versus persistent) in cell culture.