Structure and function of a membrane anchor-less form of the hemagglutinin-neuraminidase glycoprotein of Newcastle disease virus

Evaluation of Newcastle disease virus LaSota strain attenuated by codon pair deoptimization of the HN and F genes for in ovo vaccination

In ovo vaccination is an attractive immunization approach for the poultry industry. However, commonly used Newcastle disease virus (NDV) vaccines cannot be administered in ovo because of the reduced hatchability and embryo mortality. The codon pair deoptimization (CPD) approach has been used to efficiently and rapidly attenuate viruses by targeting the virulence genes. In this study, we aimed to attenuate the NDV LaSota (LS) vaccine strain for in ovo vaccination by CPD of the fusion (F) or/and hemagglutinin-neuraminidase (HN) genes with approximately 44 % suboptimal codon substitutions. Three NDV LS recombinants expressing codon deoptimized F (rLS/F-d), HN (rLS/HN-d), or both genes (rLS/F+HN-d) were generated using reverse genetics technology. Biological assays showed that the CPD viruses retained similar hemagglutination activity and growth ability to the parental rLS virus. The CPD of the HN gene slightly attenuated the rLS/HN-d and rLS/F+HN-d viruses, whereas the CPD of the F gene marginally increased the rLS/F-d virus pathogenicity compared to rLS. Nevertheless, all three CPD rLS viruses were still lethal to 10-day-old specific-pathogen-free (SPF) chicken embryos. In ovo inoculation of 18-day-old SPF chicken embryos with the CPD viruses severely reduced chicken's hatch and survival rates. These results suggested that the CPD of the surface glycoprotein genes of the LS strain at the current level of suboptimal codon substitutions could not sufficiently attenuate the virus for use as an in ovo vaccine, and codon deoptimizing a greater proportion of the F and HN genes or additional gene(s) may be required for sufficient attenuation of the LS strain.

Hidden Relationships between N-Glycosylation and Disulfide Bonds in Individual Proteins

N-Glycosylation (NG) and disulfide bonds (DBs) are two prevalent co/post-translational modifications (PTMs) that are often conserved and coexist in membrane and secreted proteins involved in a large number of diseases. Both in the past and in recent times, the enzymes and chaperones regulating these PTMs have been constantly discovered to directly interact with each other or colocalize in the ER. However, beyond a few model proteins, how such cooperation affects N-glycan modification and disulfide bonding at selective sites in individual proteins is largely unknown. Here, we reviewed the literature to discover the current status in understanding the relationships between NG and DBs in individual proteins. Our results showed that more than 2700 human proteins carry both PTMs, and fewer than 2% of them have been investigated in the associations between NG and DBs. We summarized both these proteins with the reported relationships in the two PTMs and the tools used to discover the relationships. We hope that, by exposing this largely understudied field, more investigations can be encouraged to unveil the hidden relationships of NG and DBs in the majority of membranes and secreted proteins for pathophysiological understanding and biotherapeutic development.

The Hemagglutinin-Neuraminidase (HN) Head Domain and the Fusion (F) Protein Stalk Domain of the Parainfluenza Viruses Affect the Specificity of the HN-F Interaction

Membrane fusion by the parainfluenza viruses is induced by virus-specific functional interaction between the attachment protein (HN) and the fusion (F) protein. This interaction is thought to be mediated by transient contacts between particular amino acids in the HN stalk domain and those in the F head domain. However, we recently reported that replacement of specified amino acids at or around the dimer interface of the HN head domain remarkably affected the F protein specificity. We then intended to further investigate this issue in the present study and revealed that the HPIV2 HN protein can be converted to an SV41 HN-like protein by substituting at least nine amino acids in the HPIV2 HN head domain with the SV41 HN counterparts in addition to the replacement of the stalk domain, indicating that specified amino acids in the HN head domain play very important roles in determining the specificity of the HN-F interaction. On the other hand, we previously reported that the PIV5 F protein can be converted to an SV41 F-like protein by replacing 21 amino acids in the head domain of the PIV5 F protein with those of the SV41 F protein. We then intended to further investigate this issue in the present study and found that replacement of 15 amino acids in the stalk domain in addition to the replacement of the 21 amino acids in the head domain of the PIV5 F protein resulted in creation of a more SV41 F-like protein, indicating that specified amino acids in the F stalk domain play important roles in determining the specificity of the HN-F interaction. These results suggest that the conformations of the HN stalk domain and the F head domain are dependent on the structures of the HN head domain and the F stalk domain, respectively. Presumably, the conformations of the former domains, which are considered directly involved in the HN-F interaction, can be modified by subtle changes in the structure of the latter domains, resulting in an altered specificity for the interacting partners.

Evidence of independent evolution of genotype XIII Newcastle disease viruses in India

Despite the prevalence of Newcastle disease virus (NDV) outbreaks in India through the decades, there has been little genetic characterisation of the virulent strains circulating in Northeast India. In 2014, a poultry farm in the Kamrup district of Assam reported an ND outbreak. In this study, genetic analysis and clinicopathological tests showed the virulent nature of the isolate Kamrup. Based on prudent classification criteria, the virulent strain Kamrup was found to be most closely related to members of genotype XIII of class II NDV. A phylogenetic analysis of NDV strains suggested three sub-genotypes: XIIIa, XIIIb and XIIIc. NDV strain Kamrup belonged to sub-genotype XIIIc. Sub-genotype XIIIc isolates were similar to the 1982 isolate from cockatoo and appeared to have evolved parallel to the preceding genotype XIII viruses circulating in India. The high genetic diversity and frequency of mutations observed in the envelope glycoproteins of strain Kamrup demonstrate the evolution of the pandemic genotype XIII NDV in India, which further undermines and complicates of NDV management in India.

Contribution of HN protein length diversity to Newcastle disease virus virulence, replication and biological activities

To evaluate the contribution of length diversity in the hemagglutinin-neuraminidase (HN) protein to the pathogenicity, replication and biological characteristics of Newcastle disease virus (NDV), we used reverse genetics to generate a series of recombinant NDVs containing truncated or extended HN proteins based on an infectious clone of genotype VII NDV (SG10 strain). The mean death times and intracerebral pathogenicity indices of these viruses showed that the different length mutations in the HN protein did not alter the virulence of NDV. In vitro studies of recombinant NDVs containing truncated or extended HN proteins revealed that the extension of HN protein increased its hemagglutination titer, receptor-binding ability and impaired its neuraminidase activity, fusogenic activity and replication ability. Furthermore, the hemadsorption, neuraminidase and fusogenic promotion activities at the protein level were consistent with those of viral level. Taken together, our results demonstrate that the HN biological activities affected by the C-terminal extension are associated with NDV replication but not the virulence.

The Fusion Protein Specificity of the Parainfluenza Virus Hemagglutinin-Neuraminidase Protein Is Not Solely Defined by the Primary Structure of Its Stalk Domain

Virus-specific interaction between the attachment protein (HN) and the fusion protein (F) is prerequisite for the induction of membrane fusion by parainfluenza viruses. This HN-F interaction presumably is mediated by particular amino acids in the HN stalk domain and those in the F head domain. We found in the present study, however, that a simian virus 41 (SV41) F-specific chimeric HPIV2 HN protein, SCA, whose cytoplasmic, transmembrane, and stalk domains were derived from the SV41 HN protein, could not induce cell-cell fusion of BHK-21 cells when coexpressed with an SV41 HN-specific chimeric PIV5 F protein, no. 36. Similarly, a headless form of the SV41 HN protein failed to induce fusion with chimera no. 36, whereas it was able to induce fusion with the SV41 F protein. Interestingly, replacement of 13 amino acids of the SCA head domain, which are located at or around the dimer interface of the head domain, with SV41 HN counterparts resulted in a chimeric HN protein, SCA-RII, which induced fusion with chimera no. 36 but not with the SV41 F protein. More interestingly, retroreplacement of 11 out of the 13 amino acids of SCA-RII with the SCA counterparts resulted in another chimeric HN protein, IM18, which induced fusion either with chimera no. 36 or with the SV41 F protein, similar to the SV41 HN protein. Thus, we conclude that the F protein specificity of the HN protein that is observed in the fusion event is not solely defined by the primary structure of the HN stalk domain.

IMPORTANCE

It is appreciated that the HN head domain initially conceals the HN stalk domain but exposes it after the head domain has bound to the receptors, which allows particular amino acids in the stalk domain to interact with the F protein and trigger it to induce fusion. However, other regulatory roles of the HN head domain in the fusion event have been ill defined. We have shown in the current study that removal of the head domain or amino acid substitutions in a particular region of the head domain drastically change the F protein specificity of the HN protein, suggesting that the ability of a given HN protein to interact with an F protein is defined not only by the primary structure of the HN stalk domain but also by its conformation. This notion seems to account for the unidirectional substitutability among rubulavirus HN proteins in triggering noncognate F proteins.

Two single amino acid substitutions in the intervening region of Newcastle disease virus HN protein attenuate viral replication and pathogenicity

Among the proteins encoded by Newcastle disease virus (NDV), the attachment protein (HN) is an important determinant of virulence and pathogenicity. HN has been molecularly characterized at the protein level; however, the relationship between the molecular character of HN and the animal pathotype it causes has not been well explored. Here, we revisited the intervening region (IR) of the HN stalk and extended the known biological functions of HN. Three distinct substitutions (A89Q, P93A, and L94A) in the IR of genotype VII NDV (G7 strain) HN protein were analyzed. The A89Q and L94A mutations weakened the fusion promotion activity of HN to 44% and 41% of that of wild type, respectively, whereas P93A decreased the neuraminidase activity to 21% of the parental level. At the virus level, P93A and L94A-bearing viruses displayed impaired receptor recognition ability, neuraminidase activity, and fusion-promoting activity, all of which led to virus attenuation. In addition, the L94A-mutated virus showed a dramatic decline in replication and was attenuated in cells and in chickens. Our data demonstrate that the HN biological activities and functions modulated by these specific amino acids in the IR are associated with NDV replication and pathogenicity.

Assembly and immunological properties of a bivalent virus-like particle (VLP) for avian influenza and Newcastle disease

Avian influenza virus (AIV) and Newcastle disease virus (NDV) are both important pathogens in poultry worldwide. The protection of poultry from avian influenza and Newcastle disease can be achieved through vaccination. We embarked on the development of a bivalent vaccine that would allow for a single immunization against both avian influenza and Newcastle disease. We constructed a chimeric virus-like particle (VLP) that is composed of the M1 protein and HA protein of avian influenza virus and a chimeric protein containing the cytoplasmic and transmembrane domains of AIV neuraminidase protein (NA) and the ectodomain of the NDV hemagglutinin-neuraminidase (HN) protein (NA/HN). The single immunization of chickens with the chimeric VLP vaccine induced both AIV H5- and NDV-specific antibodies. The HI titers and specific antibodies elicited by the chimeric VLPs were statistically similar to those elicited in animals vaccinated with the corresponding commercial monovalent vaccines. Chickens vaccinated with chimeric VLP vaccine and then challenged with the Newcastle disease F48E9 virus displayed complete protection. Overall, the chimeric VLP vaccine elicits strong immunity and can protect against Newcastle disease virus challenge.

A mutation in the stalk of the newcastle disease virus hemagglutinin-neuraminidase (HN) protein prevents triggering of the F protein despite allowing efficient HN-F complex formation

Newcastle disease virus (NDV)-induced membrane fusion requires formation of a complex between the hemagglutinin-neuraminidase (HN) and fusion (F) proteins. Substitutions for NDV HN stalk residues A89, L90, and L94 block fusion by modulating formation of the HN-F complex. Here, we demonstrate that a nearby L97A substitution, though previously shown to block fusion, allows efficient HN-F complex formation and likely acts by preventing changes in the HN stalk required for triggering of the bound F protein.

Full conversion of the hemagglutinin-neuraminidase specificity of the parainfluenza virus 5 fusion protein by replacement of 21 amino acids in its head region with those of the simian virus 41 fusion protein

For most parainfluenza viruses, a virus type-specific interaction between the hemagglutinin-neuraminidase (HN) and fusion (F) proteins is a prerequisite for mediating virus-cell fusion and cell-cell fusion. The molecular basis of this functional interaction is still obscure partly because it is unknown which region of the F protein is responsible for the physical interaction with the HN protein. Our previous cell-cell fusion assay using the chimeric F proteins of parainfluenza virus 5 (PIV5) and simian virus 41 (SV41) indicated that replacement of two domains in the head region of the PIV5 F protein with the SV41 F counterparts bestowed on the PIV5 F protein the ability to induce cell-cell fusion on coexpression with the SV41 HN protein while retaining its ability to induce fusion with the PIV5 HN protein. In the study presented here, we furthered the chimeric analysis of the F proteins of PIV5 and SV41, finding that the PIV5 F protein could be converted to an SV41 HN-specific chimeric F protein by replacing five domains in the head region with the SV41 F counterparts. The five SV41 F-protein-derived domains of this chimera were then divided into 16 segments; 9 out of 16 proved to be not involved in determining its specificity for the SV41 HN protein. Finally, mutational analyses of a chimeric F protein, which harbored seven SV41 F-protein-derived segments, revealed that replacement of at most 21 amino acids of the PIV5 F protein with the SV41 F-protein counterparts was enough to convert its HN protein specificity.

Triggering of the newcastle disease virus fusion protein by a chimeric attachment protein that binds to Nipah virus receptors

The fusion (F) proteins of Newcastle disease virus (NDV) and Nipah virus (NiV) are both triggered by binding to receptors, mediated in both viruses by a second protein, the attachment protein. However, the hemagglutinin-neuraminidase (HN) attachment protein of NDV recognizes sialic acid receptors, whereas the NiV G attachment protein recognizes ephrinB2/B3 as receptors. Chimeric proteins composed of domains from the two attachment proteins have been evaluated for fusion-promoting activity with each F protein. Chimeras having NiV G-derived globular domains and NDV HN-derived stalks, transmembranes, and cytoplasmic tails are efficiently expressed, bind ephrinB2, and trigger NDV F to promote fusion in Vero cells. Thus, the NDV F protein can be triggered by binding to the NiV receptor, indicating that an aspect of the triggering cascade induced by the binding of HN to sialic acid is conserved in the binding of NiV G to ephrinB2. However, the fusion cascade for triggering NiV F by the G protein and that of triggering NDV F by the chimeras can be distinguished by differential exposure of a receptor-induced conformational epitope. The enhanced exposure of this epitope marks the triggering of NiV F by NiV G but not the triggering of NDV F by the chimeras. Thus, the triggering cascade for NiV G-F fusion may be more complex than that of NDV HN and F. This is consistent with the finding that reciprocal chimeras having NDV HN-derived heads and NiV G-derived stalks, transmembranes, and tails do not trigger either F protein for fusion, despite efficient cell surface expression and receptor binding.

Identification of domains on the fusion (F) protein trimer that influence the hemagglutinin-neuraminidase specificity of the f protein in mediating cell-cell fusion

For most paramyxoviruses, virus type-specific interaction between fusion (F) protein and attachment protein (hemagglutinin-neuraminidase [HN], hemagglutinin [H], or glycoprotein [G]) is a prerequisite for mediating virus-cell fusion and cell-cell fusion. Our previous cell-cell fusion assay using the chimeric F proteins of human parainfluenza virus 2 (HPIV2) and simian virus 41 (SV41) suggested that the middle region of the HPIV2 F protein contains the site(s) that determines its specificity for the HPIV2 HN protein. In the present study, we further investigated the sites of the F protein that could be critical for determining the HN protein specificity. By analyzing the reported structure of the F protein of parainfluenza virus 5 (PIV5), we found that four major domains (M1, M2, M3, and M4) and five minor domains (A to E) in the middle region of the PIV5 F protein were exposed on the trimer surface. We then replaced these domains with the SV41 F counterparts individually or in combination and examined whether the resulting chimeras could mediate cell-cell fusion when coexpressed with the SV41 HN protein. The results showed that a chimera designated M(1+2), which harbored SV41 F-derived domains M1 and M2, mediated cell-cell fusion with the coexpressed SV41 HN protein, suggesting that these domains are involved in determining the HN protein specificity. Intriguingly, another chimera which harbored the SV41 F-derived domain B in addition to domains M1 and M2 showed increased specificity for the SV41 HN protein compared to that of M(1+2), although it was capable of mediating cell-cell fusion by itself.

Analysis of sequence and haemagglutinin activity of the HN glycoprotein of Newcastle disease virus

Reverse transcription-polymerase chain reaction (RT-PCR) was used to generate sequence data for recent Taiwanese strains of Newcastle disease virus (NDV) isolated from 1999 to 2003, covering the full length of the haemagglutinin-neuraminidase (HN) gene and protein. Nucleotide sequence analysis of the HN gene of these recent isolates revealed that the whole HN gene carries an open reading frame encoding 571 amino acids and possesses a shorter C-terminal extension. Six amino acid substitutions in epitopes on the HN glycoprotein of the recent Taiwanese NDV isolates were also found. All the recent Taiwanese NDV isolates have the amino acid sequence (112)RRQKRF(117) for the F protein. A phylogenetic tree analysis based on the nucleotide sequences of the F gene revealed that all recent Taiwanese isolates were related to genotype VII viruses. Since the recent Taiwanese NDV isolates exhibited a low level of haemagglutination (HA) activity, we generated two sets of mutants to elucidate whether mutations in the heptad repeat region of the HN protein could affect the HA activity. To demonstrate the presence of the viruses used in the HA test, a real-time RT-PCR was established to determine the copy number of NDV isolates. From sequence analysis, site-directed mutagenesis, and haemadsorption assays, it was found that the HN glycoprotein of recent Taiwanese NDV isolates carrying a substitution at the amino acid residue 81 (I to M) in the heptad repeat region in the stalk domain showed a dramatic decrease in the activity of HA. We infer from these results that a specific amino acid sequence within the heptad repeat region of the stalk is important for the HA activity of the HN glycoprotein.

Preparation of recombinant viral glycoproteins for novel and therapeutic antibody discovery

Neutralizing antibodies are a critical component in the protection or recovery from viral infections. In the absence of available vaccines or antiviral drugs for many important human viral pathogens, the identification and characterization of new human monoclonal antibodies (hmAbs) that are able to neutralize viruses offers the possibility for effective pre- and/or post-exposure therapeutic modalities. Such hmAbs may also help in our understanding of the virus entry process, the mechanisms of virus neutralization, and in the eventual development of specific entry inhibitors, vaccines, and research tools. The majority of the more recently developed antiviral hmAbs have come from the use of antibody phage-display technologies using both naïve and immune libraries. Many of these agents are also enveloped viruses possessing important neutralizing determinants within their membrane-anchored envelope glycoproteins, and the use of recombinant, soluble versions of these viral glycoproteins is often critical in the isolation and development of antiviral hmAbs. This chapter will detail several methods that have been successfully employed to produce, purify, and characterize soluble and secreted versions of several viral envelope glycoproteins which have been successfully used as antigens to capture and isolate human phage-displayed monoclonal antibodies.

Engineered intermonomeric disulfide bonds in the globular domain of Newcastle disease virus hemagglutinin-neuraminidase protein: implications for the mechanism of fusion promotion

The promotion of membrane fusion by Newcastle disease virus (NDV) requires an interaction between the viral hemagglutinin-neuraminidase (HN) and fusion (F) proteins, although the mechanism by which this interaction regulates fusion is not clear. The NDV HN protein exists as a tetramer composed of a pair of dimers. Based on X-ray crystallographic studies of the NDV HN globular domain (S. Crennell et al., Nat. Struct. Biol. 7:1068-1074, 2000), it was proposed that the protein undergoes a significant conformational change from an initial structure having minimal intermonomeric contacts to a structure with a much more extensive dimer interface. This conformational change was predicted to be integral to fusion promotion with the minimal interface form required to maintain F in its prefusion state until HN binds receptors. However, no evidence for such a conformational change exists for any other paramyxovirus attachment protein. To test the NDV model, we have engineered a pair of intermonomeric disulfide bonds across the dimer interface in the globular domain of an otherwise non-disulfide-linked NDV HN protein by the introduction of cysteine substitutions for residues T216 and D230. The disulfide-linked dimer is formed both intracellularly and in the absence of receptor binding and is efficiently expressed at the cell surface. The disulfide bonds preclude formation of the minimal interface form of the protein and yet enhance both receptor-binding activity at 37 degrees C and fusion promotion. These results confirm that neither the minimal interface form of HN nor the proposed drastic conformational change in the protein is required for fusion.

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.

Enhancement of pathogenicity of Newcastle disease virus by alteration of specific amino acid residues in the surface glycoproteins F and HN

Recombinant viruses were rescued after site-specific mutagenesis of a full-length clone of the lentogenic Newcastle disease virus (NDV) strain Clone 30. To assess the contribution of different amino acids to virulence, specific alterations were introduced into the fusion (F) protein and in the hemagglutinin-neuraminidase (HN) protein based on sequence comparison between NDV strains of different virulence. Modification of the proteolytic cleavage site in the F protein to a polybasic motif increased the intracerebral pathogenicity index (ICPI) from 0.0 to 1.28. Moreover, the additional exchange of amino acid 123 of the HN protein from tryptophan to cysteine in combination with alteration of amino acid 27 of the F protein from cysteine to arginine increased the ICPI to 1.5. The HN mutation visibly altered conformation of the protein, resulting in the formation of disulfide-linked HN dimers that may indicate that this HN conformation is beneficial for the virulent phenotype.

Addition of N-glycans in the stalk of the Newcastle disease virus HN protein blocks its interaction with the F protein and prevents fusion

Most paramyxovirus fusion (F) proteins require the coexpression of the homologous attachment (HN) protein to promote membrane fusion, consistent with the existence of a virus-specific interaction between the two proteins. Analysis of the fusion activities of chimeric HN proteins indicates that the stalk region of the HN spike determines its F protein specificity, and analysis of a panel of site-directed mutants indicates that the F-interactive site resides in this region. Here, we use the addition of oligosaccharides to further explore the role of the HN stalk in the interaction with F. N-glycans were individually added at several positions in the stalk to determine their effects on the activities of HN, as well as its structure. N-glycan addition at positions 69 and 77 in the stalk specifically blocks fusion and the HN-F interaction without affecting either HN structure or its other activities. N-glycans added at other positions in the stalk modulate activities that reside in the globular head of HN. This correlates with an alteration of the tetrameric structure of the protein, as indicated by sucrose gradient sedimentation analyses. Finally, N-glycan addition in another region of HN (residues 124 to 152), predicted by a peptide-based analysis to mediate the interaction with F, does not significantly reduce the level of fusion, arguing strongly against this site being part of the F-interactive domain in HN. Our data support the idea that the F-interactive site on HN is defined by the stalk region of the protein.

Amino acid substitutions in the F-specific domain in the stalk of the newcastle disease virus HN protein modulate fusion and interfere with its interaction with the F protein

The hemagglutinin-neuraminidase (HN) protein of Newcastle disease virus mediates attachment to sialic acid receptors, as well as cleavage of the same moiety. HN also interacts with the other viral glycoprotein, the fusion (F) protein, to promote membrane fusion. The ectodomain of the HN spike consists of a stalk and a terminal globular head. The most conserved part of the stalk consists of two heptad repeats separated by a nonhelical intervening region (residues 89 to 95). Several amino acid substitutions for a completely conserved proline residue in this region not only impair fusion and the HN-F interaction but also decrease neuraminidase activity in the globular domain, suggesting that the substitutions may alter HN structure. Substitutions for L94 also interfere with fusion and the HN-F interaction but have no significant effect on any other HN function. Amino acid substitutions at other positions in the intervening region also modulate only fusion. In all cases, diminished fusion correlates with a decreased ability of the mutated HN protein to interact with F at the cell surface. These findings indicate that the intervening region is critical to the role of HN in the promotion of fusion and may be directly involved in its interaction with the homologous F protein.

Loss of N-linked glycosylation from the hemagglutinin-neuraminidase protein alters virulence of Newcastle disease virus

The hemagglutinin-neuraminidase (HN) protein of Newcastle disease virus (NDV) is an important determinant of its virulence. We investigated the role of each of the four functional N-linked glycosylation sites (G1 to G4) of the HN glycoprotein of NDV on its pathogenicity. The N-linked glycosylation sites G1 to G4 at residues 119, 341, 433, and 481, respectively, of a moderately pathogenic NDV strain Beaudette C (BC) were eliminated individually by site-directed mutagenesis on a full-length cDNA clone of BC. A double mutant (G12) was also created by eliminating the first and second glycosylation sites at residues 119 and 341, respectively. Infectious virus was recovered from each of the cDNA clones of the HN glycoprotein mutants, employing a reverse genetics technique. There was a greater delay in the replication of G4 and G12 mutant viruses than in the parental virus. Loss of glycosylation does not affect the receptor recognition by HN glycoprotein of NDV. The neuraminidase activity of G4 and G12 mutant viruses and the fusogenicity of the G4 mutant virus were significantly lower than those of the parental virus. The fusogenicity of the double mutant virus (G12) was significantly higher than that of the parental virus. Cell surface expression of the G4 virus HN was significantly lower than that of the parental virus. The antigenic reactivities of the mutants to a panel of monoclonal antibodies against the HN protein indicated that removal of glycosylation from the HN protein increased (G1, G3, and G12) or decreased (G2 and G4) the formation of antigenic sites, depending on their location. In standard tests to assess virulence in chickens, all of the glycosylation mutants were less virulent than the parental BC virus, but the G4 and G12 mutants were the least virulent.

Mutated form of the Newcastle disease virus hemagglutinin-neuraminidase interacts with the homologous fusion protein despite deficiencies in both receptor recognition and fusion promotion

The Newcastle disease virus (NDV) hemagglutinin-neuraminidase (HN) protein mediates attachment to cellular receptors. The fusion (F) protein promotes viral entry and spread. However, fusion is dependent on a virus-specific interaction between the two proteins that can be detected at the cell surface by a coimmunoprecipitation assay. A point mutation of I175E in the neuraminidase (NA) active site converts the HN of the Australia-Victoria isolate of the virus to a form that can interact with the F protein despite negligible receptor recognition and fusion-promoting activities. Thus, I175E-HN could represent a fusion intermediate in which HN and F are associated and primed for the promotion of fusion. Both the attachment and fusion-promoting activities of this mutant HN protein can be rescued either by NA activity contributed by another HN protein or by a set of four substitutions at the dimer interface. These substitutions were identified by the evaluation of chimeras composed of segments from HN proteins derived from two different NDV strains. These findings suggest that the I175E substitution converts HN to an F-interactive form, but it is one for which receptor binding is still required for fusion promotion. The data also indicate that the integrity of the HN dimer interface is critical to its receptor recognition activity.

Fusion deficiency induced by mutations at the dimer interface in the Newcastle disease virus hemagglutinin-neuraminidase is due to a temperature-dependent defect in receptor binding

The tetrameric paramyxovirus hemagglutinin-neuraminidase (HN) protein mediates attachment to sialic acid-containing receptors as well as cleavage of the same moiety via its neuraminidase (NA) activity. The X-ray crystallographic structure of an HN dimer from Newcastle disease virus (NDV) suggests that a single site in two different conformations mediates both of these activities. This conformational change is predicted to involve an alteration in the association between monomers in each HN dimer and to be part of a series of changes in the structure of HN that link its recognition of receptors to the activation of the other viral surface glycoprotein, the fusion protein. To explore the importance of the dimer interface to HN function, we performed a site-directed mutational analysis of residues in a domain defined by residues 218 to 226 at the most membrane-proximal part of the dimer interface in the globular head. Proteins carrying substitutions for residues F220, S222, and L224 in this domain were fusion deficient. However, this fusion deficiency was not due to a direct effect of the mutations on fusion. Rather, the fusion defect was due to a severely impaired ability to mediate receptor recognition at 37 degrees C, a phenotype that is not attributable to a change in NA activity. Since each of these mutated proteins efficiently mediated attachment in the cold, it was also not due to an inherent inability of the mutated proteins to recognize receptors. Instead, the interface mutations acted by weakening the interaction between HN and its receptor(s). The phenotype of these mutants correlates with the disruption of intermonomer subunit interactions.

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.

Determination of the disulfide bond arrangement of Newcastle disease virus hemagglutinin neuraminidase. Correlation with a beta-sheet propeller structural fold predicted for paramyxoviridae attachment proteins

Disulfide bonds stabilize the structure and functions of the hemagglutinin neuraminidase attachment glycoprotein (HN) of Newcastle disease virus. Until this study, the disulfide linkages of this HN and structurally similar attachment proteins of other members of the paramyxoviridae family were undefined. To define these linkages, disulfide-linked peptides were produced by peptic digestion of purified HN ectodomains of the Queensland strain of Newcastle disease virus, isolated by reverse phase high performance liquid chromatography, and analyzed by mass spectrometry. Analysis of peptides containing a single disulfide bond revealed Cys(531)-Cys(542) and Cys(172)-Cys(196) linkages and that HN ectodomains dimerize via Cys(123). Another peptide, with a chain containing Cys(186) linked to a chain containing Cys(238), Cys(247), and Cys(251), was cleaved at Met(249) with cyanogen bromide. Subsequent tandem mass spectrometry established Cys(186)-Cys(247) and Cys(238)-Cys(251) linkages. A glycopeptide with a chain containing Cys(344) linked to a chain containing Cys(455), Cys(461), and Cys(465) was treated sequentially with peptide-N-glycosidase F and trypsin. Further treatment of this peptide by one round of manual Edman degradation or tandem mass spectrometry established Cys(344)-Cys(461) and Cys(455)-Cys(465) linkages. These data, establishing the disulfide linkages of all thirteen cysteines of this protein, are consistent with published predictions that the paramyxoviridae HN forms a beta-propeller structural fold.

Functional chimeric HN glycoproteins derived from Newcastle disease virus and human parainfluenza virus-3

Newcastle disease virus (NDV) is primarily a respiratory tract pathogen of birds, particularly chickens, but it occasionally produces infection in man. Human parainfluenza virus type 3 (hPIV3) is a common respiratory pathogen, particularly in young children. These two viruses gain entry to host cells via direct fusion between the viral envelope and the cell membrane, mediated by the two surface glycoproteins: the hemagglutinin-neuraminidase (HN) and fusion (F) proteins. Promotion of fusion by HN and F requires that they are derived from homologous viruses. We have constructed chimeric proteins composed of domains from heterologous HN proteins. Their ability to bind cellular receptors and to complement the F protein of each virus in the promotion of fusion were evaluated in a transient expression system. The fusion specificity was found to segregate with a segment extending from the middle of the transmembrane anchor to the top of the putative stalk region of the ectodomain. All of the chimeras, in which the globular domain is derived from the NDV HN and various lengths of the stalk region are derived from the hPIV3 HN maintain receptor binding activity, but some have markedly reduced neuraminidase (NA) activity. Decrease in the NA activity of the chimeras correlates with alteration in the antigenic structure of the globular domain. This suggests that the stalk region of the HN spike is important for maintenance of the structure and function of the globular domain of the HN protein spike.

Disulfide bond formation is a determinant of glycosylation site usage in the hemagglutinin-neuraminidase glycoprotein of Newcastle disease virus

Determinants of glycosylation site usage were explored by using the hemagglutinin-neuraminidase (HN) glycoprotein of the paramyxovirus Newcastle disease virus. The amino acid sequence of the HN protein, a type II glycoprotein, has six N-linked glycosylation addition sites, G1 to G6, two of which, G5 and G6, are not used for the addition of carbohydrate (L. McGinnes and T. Morrison, Virology 212:398-410, 1995). The sequence of this protein also has 13 cysteine residues in the ectodomain (C2 to C14). Mutation of either cysteine 13 or cysteine 14 resulted in the addition of another oligosaccharide chain to the protein. These cysteine residues flank the normally unused G6 glycosylation addition site, and mutation of the G6 site eliminated the extra glycosylation found in the cysteine mutants. These results suggested that failure to form an intramolecular disulfide bond resulted in the usage of a normally unused glycosylation site. This conclusion was confirmed by preventing cotranslational disulfide bond formation in cells by using dithiothreitol. Under these conditions, the wild-type protein acquired extra glycosylation, which was eliminated by mutation of the G6 site. These results suggest that localized folding events on the nascent chain, such as disulfide bond formation, which block access to the oligosaccharyl transferase are a determinant of glycosylation site usage.

Isolation of the measles virus hemagglutinin protein in a soluble form by protease digestion

The hemagglutinin (H) glycoprotein was isolated in a soluble form by digesting measles virus particles with an endoproteinase, Asp-N (from a Pseudomonas fragi mutant). Digestion of H with Asp-N brought about glycopeptides in three different forms, depending on the cleaving site: AHD, which has an M(r) of 66,000 as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and which formed a disulfide-linked homodimer with an M(r) of 132,000, and two monomeric digestion products, AHM-1 (with an M(r) of 64,000) and AHM-2 (with an M(r) of 58,000). The susceptibility of the H glycoprotein to the protease depended on the enzyme concentration. AHD was readily formed at a low concentration of Asp-N, while AHM-1 and AHM-2 required higher and even higher protease concentrations, respectively. All of the cleavage products reacted with monoclonal antibodies to various epitopes of the H protein; however, only AHD showed a significant hemagglutinin activity on African green monkey erythrocytes. The hemagglutinin activities of AHM-1 and AHM-2 were restored after a monoclonal antibody lacking the hemagglutination-inhibiting activity was added to the reaction mixture. AHDs purified by size-exclusion high-pressure liquid chromatography had two associating forms; one had an M(r) higher than and the other an M(r) as high as that of a tetramer. The former was associated noncovalently in addition to having two intermolecular disulfide bonds, and the latter was associated covalently with a single intermolecular disulfide bond and was also duplicated through a noncovalent association. In addition, both AHM-1 and AHM-2, having no intermolecular disulfide bond, were in a dimer form. These results suggest that AHM-1 and AHM-2 are monovalent in the hemagglutinin activity, while AHDs are divalent. Comparative analyses of the N termini of these soluble glycopeptides with the sequence of H suggested that the cysteine residue at position 139 was responsible for the intermolecular disulfide bonding between the monomeric H glycoproteins. The cysteine at position 154 was also suggested to participate in the forming of the intermolecular disulfide bond.

Modulation of the activities of HN protein of Newcastle disease virus by nonconserved cysteine residues

Comparisons of the sequences of the hemagglutinin-neuraminidase (HN) protein from thirteen different strains of Newcastle disease virus (NDV) show that while 12 cysteine residues are conserved in all strains, two cysteine residues are variably present (Sakaguchi et al. (1989) Virology 169, 260-272). One of these residues, at amino acid 6, is in the cytoplasmic domain. The other cysteine is at amino acid 123 in the ectodomain and is responsible for disulfide-linked HN dimers detected in some NDV strains (McGinnes and Morrison (1994) Virology 200, 470-483). To explore the role of these nonconserved residues in the structure and function of the protein, cysteine residues at amino acid 6 and 123 in the HN protein of the AV strain of NDV were mutated individually and in combination by site specific mutagenesis to serine and tryptophan, respectively. Proteins with mutations in either residue (C6S or C123W) or in both residues (C6S,123W) were transported to the cell surface. However, all three mutants had reduced attachment, neuraminidase, and fusion promotion activities. All three mutant proteins also showed an alteration in an antigenic site specific for oligomers of HN protein while all other antigenic sites were present at wild type levels. These results suggest that the nonconserved cysteine residues in the HN sequence may modulate the biological activities of the protein by affecting the oligomeric structure of the protein.