Mutations in the Ectodomain of Newcastle Disease Virus Fusion Protein Confer a Hemagglutinin-Neuraminidase-Independent Phenotype

The entry of enveloped viruses into host cells is preceded by membrane fusion, which in paramyxoviruses is triggered by the fusion (F) protein. Refolding of the F protein from a metastable conformation to a highly stable postfusion form is critical for the promotion of fusion, although the mechanism is still not well understood. Here we examined the effects of mutations of individual residues of the F protein of Newcastle disease virus, located at critical regions of the protein, such as the C terminus of the N-terminal heptad repeat (HRA) and the N terminus of the C-terminal heptad repeat (HRB). Seven of the mutants were expressed at the cell surface, showing differences in antibody reactivity in comparison with the F wild type. The N211A, L461A, I463A, and I463F mutants showed a hyperfusogenic phenotype both in syncytium and in dye transfer assays. The four mutants promoted fusion more efficiently at lower temperatures than the wild type did, meaning they probably had lower energy requirements for activation. Moreover, the N211A, I463A, and I463F mutants exhibited hemagglutinin-neuraminidase (HN)-independent activity when influenza virus hemagglutinin (HA) was coexpressed as an attachment protein. The data are discussed in terms of alterations of the refolding pathway and/or the stability of the prefusion and fusion conformations.Newcastle disease virus (NDV) is an avian enveloped virus belonging to the family Paramyxoviridae. Two viral membrane-associated proteins are responsible for the entry of the virus into the host cell: they are hemagglutinin-neuraminidase (HN), a receptor-binding protein that interacts with sialoglycoconjugates at the cell surface, and F, a trimeric class I fusion protein that, upon activation, triggers the fusion of the viral and target membranes. F protein is activated after the attachment of its homotypic HN protein to the proper receptor; however, how HN activates F is not well understood. F protein is synthesized as an inactive precursor, F₀, that is activated by proteolytic cleavage to the disulfide-linked F₁-F₂ fusion-competent form (Fig. ​(Fig.1)1) (10). The crystal structures of several paramyxoviral fusion proteins, in both the prefusion and postfusion conformations (3, 26, 27), have revealed that these proteins undergo major conformational changes, from a metastable conformation to a highly stable, postfusion form. Several regions in the ectodomain of class I viral fusion proteins are involved in these conformational conversions, including a hydrophobic fusion peptide at the N terminus of the F1 protein and two hydrophobic heptad repeat motifs, HRA and HRB, located at its N and C termini, respectively (Fig. ​(Fig.1).1). In the prefusion form, HRB shows a triple-stranded coiled-coil conformation forming the stalk of the mushroom-like protein (3, 19, 27). Its globular head contains three domains, DI to DIII (Fig. ​(Fig.1),1), with the base of the head being formed by the DI and DII domains, with residues predominantly located between HRA and HRB. The top of the head is formed by DIII, consisting mainly of HRA and the fusion peptide, located on the side of the head sequestered between adjacent subunits. In this prefusion state, HRA is folded as two antiparallel β-strands and four (h1 to h4) helices (27) (see Fig. ​Fig.6).6). The DIII domain undergoes major structural changes from the prefusion to the final postfusion conformation. HRA refolds as an α-helix, propelling the fusion peptide into the target membrane and generating a prehairpin intermediate (see Fig. ​Fig.6).6). The final, stable conformation consists of a six-helical bundle (6HB), comprising a dimer of trimers in which the trimeric HRA coiled coil forms the core, packed along the outside by three antiparallel HRB α-helices (1, 3, 19, 27).Open in a separate windowFIG. 1.Schematic representation of the structure of the NDV fusion protein. (A) Domain structure of F protein (27). (B) Locations of the fusion peptide, HR regions, and sequences studied. Mutated residues are indicated in bold.Open in a separate windowFIG. 6.Scheme of conformational changes in HRA from prefusion to postfusion state. (A) Ribbon model of PIV5 F protein in its metastable prefusion conformation (PDB accession number 2b9b) (27), showing some residues (named in white) from the A subunit and the corresponding residues in the NDV F protein (named in yellow). Subunits B and C are depicted in gray for clarity. (B) In the metastable, prefusion conformation, HRA is folded as a spring-loaded mixture of α-helices, turns, and β-strands, comprising 11 segments in the DIII head domain of the trimer (27). (C) After fusion, HRA is presented as a single long helix that allows the fusion peptide to be buried in the target membrane. The approximate positions of HRC and the core β-sheet are shown as dashed lines for both conformations.The refolding mechanism that triggers F protein activation is still not well understood. Mutational analysis of the HRA and HRB domains of paramyxovirus F proteins (3, 13, 18, 19, 22, 23), as well as the use of HRA- and HRB-derived peptides (6, 17), has led to the proposal of a series of discrete refolding intermediates of the F protein, from the metastable native conformation, through the prehairpin intermediate, and to the final postfusion hairpin structure (6HB) (17, 19, 27). To gain further insight into the individual residues critical for this mechanism, in this work we mutated several residues of the head and stalk of the NDV F protein (Fig. ​(Fig.1).1). The mutations disrupted F protein antibody reactivity, fusogenicity, and HN dependence in different ways. Interestingly, a mutant of the C-terminal h4 α-helix of HRA (N211A mutant) and two mutants of a residue located at the most N-terminal position of HRB (I463A and I463F mutants) exhibited a hyperfusogenic phenotype and HN-independent activity when influenza virus hemagglutinin (HA) was coexpressed as an attachment protein. The data are discussed in terms of alterations of the refolding pathway and/or the stability of the prefusion and fusion conformations.     

Different Regions of the Newcastle Disease Virus Fusion Protein Modulate Pathogenicity

Newcastle disease virus (NDV), also designated as Avian paramyxovirus type 1 (APMV-1), is the causative agent of a notifiable disease of poultry but it exhibits different pathogenicity dependent on the virus strain. The molecular basis for this variability is not fully understood. The efficiency of activation of the fusion protein (F) is determined by presence or absence of a polybasic amino acid sequence at an internal proteolytic cleavage site which is a major determinant of NDV virulence. However, other determinants of pathogenicity must exist since APMV-1 of high (velogenic), intermediate (mesogenic) and low (lentogenic) virulence specify a polybasic F cleavage site. We aimed at elucidation of additional virulence determinants by constructing a recombinant virus that consists of a lentogenic NDV Clone 30 backbone and the F protein gene from a mesogenic pigeon paramyxovirus-1 (PPMV-1) isolate with an intracerebral pathogenicity index (ICPI) of 1.1 specifying the polybasic sequence R-R-K-K-R*F motif at the cleavage site. The resulting virus was characterized by an ICPI of 0.6, indicating a lentogenic pathotype. In contrast, alteration of the cleavage site G-R-Q-G-R*L of the lentogenic Clone 30 to R-R-K-K-R*F resulted in a recombinant virus with an ICPI of 1.36 which was higher than that of parental PPMV-1. Substitution of different regions of the F protein of Clone 30 by those of PPMV-1, while maintaining the polybasic amino acid sequence at the F cleavage site, resulted in recombinant viruses with ICPIs ranging from 0.59 to 1.36 suggesting that virulence is modulated by regions of the F protein other than the polybasic cleavage site.     

Role of the Cytoplasmic Tail Amino Acid Sequences of Newcastle Disease Virus Hemagglutinin-Neuraminidase Protein in Virion Incorporation,Cell Fusion,and Pathogenicity

To determine the role of amino acid sequences of the hemagglutinin-neuraminidase (HN) cytoplasmic tail in Newcastle disease virus (NDV) replication and pathogenicity, we generated recombinant NDVs with a deletion or point mutation in the N-terminal cytoplasmic tail. The first 2-amino-acid deletion in the cytoplasmic tail did not affect the biological characteristics of NDV. However, a 4-amino-acid deletion and the substitution of alanine for serine at position 6 affected cell fusion, pathogenicity, and colocalization of the HN and M proteins of NDV, indicating that these residues of the HN cytoplasmic tail are critical for its specific incorporation into virions.Newcastle disease virus (NDV) causes a highly contagious respiratory and neurologic disease in chickens, leading to severe economic losses in the poultry industry worldwide (1). NDV is a member of the family Paramyxoviridae and has a nonsegmented, negative-sense RNA genome consisting of six genes (3′-NP-P-M-F-HN-L-5′) (7). Infection of host cells by NDV is accomplished through the interaction of two surface glycoproteins, the fusion (F) and hemagglutinin-neuraminidase (HN) proteins. The F protein directs the membrane fusion between the viral and cellular membranes, while the HN protein mediates attachment to sialic acid, has neuraminidase activity, and plays a role in fusion promotion (4).The HN protein of NDV is a type II transmembrane glycoprotein and possesses three spatially distinct domains: the ectodomain, transmembrane domain, and cytoplasmic tail. The globular ectodomain contains the sites for receptor binding and neuraminidase activity, and the transmembrane domain anchors to viral envelopes (8). The cytoplasmic tail domain contains 26 highly conserved amino acids whose functions are not well-known. In a plasmid-based expression system, truncation (23 amino acids) of the cytoplasmic tail caused improper orientation of the HN protein in the membrane insertion (13). In other paramyxoviruses, cytoplasmic tails of the HN proteins are known to play crucial roles in virus budding and assembly (10, 12). Our unsuccessful attempt to recover a recombinant NDV (rNDV) with complete deletion of the HN cytoplasmic tail also suggested that the cytoplasmic tail is required for assembly and budding of NDV. Therefore, in this study, we determined the role of amino acid sequences of the cytoplasmic tail in the NDV replication cycle. Since essential regions of the HN cytoplasmic tail for virus replication are unknown, we consecutively deleted the first 6 nucleotides (nt), 12 nt, or 18 nt of the HN cytoplasmic tail in a full-length antigenomic cDNA of NDV intermediate virulent (mesogenic) strain Beaudette C (BC) (6), thus maintaining the “rule of six” for the NDV genome (Fig. ​(Fig.1A).1A). rNDVs were recovered using our standard protocol (6). We recovered rNDVs containing 2-amino-acid deletion and 4-amino-acid deletion of the HN cytoplasmic tail (rBC/HNΔ2 and rBC/HNΔ4, respectively), indicating that only these 4 amino acids are dispensable in generating infectious virions. Since rNDV containing 6-amino-acid deletion of the HN cytoplasmic tail could not be recovered, we wanted to know the role of amino acids at positions 5 and 6 in NDV replications. The serine residue at position 6 is a potential phosphorylation site. Therefore, to determine whether phosphorylation at this site is crucial for recovery of NDV, we additionally generated rNDVs with substitution of alanine and glutamic acid for serine (rBC/HNS6A and rBC/HNS6E, respectively) to confirm its crucial role in the recovery of rNDV.Open in a separate windowFIG. 1.Constructs of recombinant NDVs containing a deletion or point mutation in the N-terminal cytoplasmic tail of the HN protein and replication and fusion index of recovered viruses in infected cells. (A) Consecutively, 6 nt, 12 nt, or 18 nt of mRNA of the HN cytoplasmic tail in a full-length antigenomic cDNA of NDV was deleted. Deletions in the HN cytoplasmic tails are indicated by the large boldface dashes. In addition, serine at position 6 was substituted with alanine and glutamic acid was substituted by changing guanine to cytidine and adenosine, respectively. (B) In vitro replication of the mutant viruses was determined in virus-infected DF-1 cells at an MOI of 0.01. The viral titers were determined by plaque assay. (C) The fusion index was determined in virus-infected Vero cells at an MOI of 0.1. Cells were stained with hematoxylin-eosin, and the fusion index was calculated as a mean number of nuclei per cell. The assay was performed three times.In vitro replication of recovered viruses was determined by plaque assay in virus-infected DF-1 cells at a multiplicity of infection (MOI) of 0.01 (5). All mutant viruses and the parental virus, rBC, grew to similar titers, indicating that alteration of the HN cytoplasmic tails did not affect their in vitro replication (Fig. ​(Fig.1B).1B). Although the rBC/HNΔ4 mutant had grown well up to 24 h postinfection, a reduction of the viral titer was detected thereafter with rapid and extensive induction of syncytia. Therefore, we determined fusion promotion activity of the mutant viruses by quantitating syncytia in virus-infected Vero cells at an MOI of 0.1 at 30 h postinfection (8) and confirmed increased fusion promotion activity of rBC/HNΔ4 followed by rBC/HNS6A compared to that of rBC (Fig. ​(Fig.1C).1C). Similarly, enhanced fusion activity was observed in other cytoplasmic tail-truncated paramyxoviruses, such as simian virus 5 and measles virus (2, 9). It has been postulated that interaction of matrix (M) protein with the cytoplasmic tails of the glycoproteins involves in a fusion-refractory conformation at the early stage of viral maturation (2). Therefore, these altered HN cytoplasmic tails could assist NDV in gaining its cell fusion competence by modulating this fusion-refractory conformation.In general, the levels of the HN protein contents on the surfaces of virus-infected cells and in the virus particles were more affected by point mutation of serine than by truncation of the cytoplasmic tail. We analyzed surface expression of the HN protein on virus-infected DF-1 cells at an MOI of 0.1. At 24 h postinfection, the cells were labeled with a monoclonal antibody against the NDV HN protein followed by anti-Alexa Fluor 488 conjugate, fixed with 4% paraformaldehyde, and analyzed by a fluorescence-activated cell sorter (AriaII; BD Bioscience) with Flowjo program (Tree Star, Inc.) (Fig. ​(Fig.2A).2A). The percentages of cells expressing the HN proteins were 89 (rBC), 78 (rBC/HNΔ2), 71 (rBC/HNΔ4), 64 (rBC/HNS6A), and 53 (rBC/HNS6E). To analyze incorporation of the HN proteins into the viral particles, the parental and mutant viruses harvested from allantoic fluid samples were purified through a 30% sucrose cushion. The viral proteins were separated on an 8% sodium dodecyl sulfate-polyacrylamide gel (Fig. ​(Fig.2B).2B). We first examined whether the mutant viruses incorporated the same levels of other viral proteins. This assay was performed by determining the ratios of the P protein to M protein. We found that similar levels of the P and M proteins were present among the different mutant viruses (Fig. ​(Fig.2B).2B). We then measured the levels of the HN proteins incorporated into the virus particles by determining the ratios of the HN protein to M protein (Fig. ​(Fig.2C).2C). The pattern of incorporation of the HN proteins into the virus particles was similar with their cell surface expression. The HN protein contents of rBC/HNΔ2 and rBC/HNΔ4 were not significantly different from that of the parental virus (P > 0.05), indicating that truncation of the cytoplasmic tail did not impair its incorporation into the viral particles. In contrast, substitution of glutamic acid for serine decreased incorporation of the HN protein into the viral particles, indicating that serine plays an important role in both cell surface expression of the HN protein and its incorporation into the viral particles.Open in a separate windowFIG. 2.Effect of alteration of the HN cytoplasmic tail on incorporation of the HN proteins into viral particles and their surface expression in DF-1 cells. (A) Surface expression of the NDV HN protein in DF-1 cells was analyzed by a fluorescence-activated cell sorter. At 24 h postinfection, DF-1 cells infected with each virus were stained with monoclonal antibody against the HN protein followed by anti-Alexa Fluor 488 conjugate. (B) Ultracentrifuge-purified viruses from infected allantoic fluid samples were separated by electrophoresis, and the gel was then stained with Coomassie brilliant blue. (C) Ratios of HN protein to M-protein levels from the parental virus and the HN cytoplasmic tail mutant viruses were quantified.We further determined the effect of cytoplasmic tail alteration on the pathogenicity of NDV in embryonated eggs and chicks (Table ​(Table1).1). The mean death time (MDT) was determined as the mean time (h) for the minimum lethal dose of virus to kill all the embryos after inoculation of 9-day-old specific-pathogen-free (SPF) embryonated chicken eggs with virus (1). The criteria for classifying the virulence of NDV strains are as follows: virulent strains take <60 h to kill embryos, intermediate virulent strains take 60 to 90 h to kill embryos, and avirulent strains take >90 h to kill embryos. Two mutant viruses (rBC/HNΔ2 and rBC/HNS6E) showed similar values of MDT compared to rBC (59 h). In contrast, the MDTs of rBC/HNΔ4 and rBC/HNS6A were 50 h and 51 h, respectively. Increased pathogenicity of these two mutants was also confirmed by an intracerebral pathogenicity index (ICPI) test in 1-day-old SPF chicks (1). The scale of the ICPI value in evaluating the virulence of NDV strains is from 0.00 (avirulent strains) to 2.00 (highly virulent NDV strains). The rBC/HNΔ4 virus had the highest ICPI value (1.61), followed by rBC/HNS6A (ICPI value of 1.58), among the parental and mutant viruses, probably due to their enhanced fusion promotion activity. In contrast, rBC/HNS6E had the lowest ICPI value (1.41), which would be associated with decreased HN protein contents detected in the viral particles and virus-infected cells. In our previous study, decreased HN protein contents in virus particles due to complete deletion of 5′ untranslated regions of the HN gene also resulted in attenuation of the virus in chickens (14). Consistently, rBC/HNΔ2 showed biological characteristics and pathogenicity similar to those of the parental virus, suggesting that aspartic acid and arginine are indispensable for the HN cytoplasmic tail of NDV.

TABLE 1.

Pathogenicity of the HN cytoplasmic tail mutant viruses in embryonated eggs and chicks

A Y526Q Mutation in the Newcastle Disease Virus HN Protein Reduces Its Functional Activities and Attenuates Virus Replication and Pathogenicity

The hemagglutinin-neuraminidase (HN) protein of Newcastle disease virus (NDV) is a multifunctional protein that plays a crucial role in virus infectivity. In this study, using the mesogenic strain Beaudette C (BC), we mutated three conserved amino acids thought to be part of the binding/catalytic active site in the HN protein. We also mutated five additional residues near the proposed active site that are nonconserved between BC and the avirulent strain LaSota. The eight recovered NDV HN mutants were assessed for effects on biological activities. While most of the mutations had surprisingly little effect, mutation at conserved residue Y526 reduced the neuraminidase, receptor binding, and fusion activities and attenuated viral virulence in eggs and young birds.Newcastle disease virus (NDV) is an avian pathogen of the genus Avulavirus in the family Paramyxoviridae (10). The envelope of NDV contains two surface glycoproteins, the fusion (F) protein and the HN (hemagglutinin-neuraminidase [NA]) protein. The F protein mediates viral penetration and requires cleavage-activation by host protease. Cleavability of the F protein is a major determinant of virulence. However, other viral proteins, including HN, also contribute to virulence (5). HN is a multifunctional glycoprotein. It recognizes sialic acid-containing receptors on cell surfaces; promotes the fusion activity of F protein, thereby allowing the virus to penetrate the cell surface; and acts as an NA that removes sialic acid from progeny virus particles to prevent viral self-aggregation (9).HN is a type II homotetrameric glycoprotein with a monomer length of 577 amino acids for most NDV strains (14). The ectodomain of the HN protein consists of a 95-amino-acid stalk region supporting a 428-amino-acid terminal globular head. Although mutations in the transmembrane and stalk regions of the HN protein can affect the structure and activities of the protein (11, 15), the antigenic, receptor recognition, and NA active sites are all localized in the globular head (12, 16). The X-ray crystal structure of the globular head of the NDV HN protein has identified residues that appear to contribute to receptor recognition, NA, and fusion activities (4). Previous studies have proposed that conserved residues R174, I175, D198, K236, R416, R498, Y526, and E547 are important in receptor recognition and NA activities and that residues R174 and E547 influence the fusion promotion activity of the HN protein (3, 4, 6). Although transfection studies using plasmids expressing HN mutants of NDV have highlighted the importance of these residues in different biological functions of the HN protein, their contribution to NDV biology and pathogenesis in the context of the complete virus was not known.In this study, we examined the roles of three of the above-named conserved residues, R416, R498, and Y526 (all located near the sialic acid binding site), in the biological activities and pathogenesis of the HN protein of NDV in the context of infectious virus. In addition, comparison of the HN protein sequence between the avirulent strain LaSota and the moderately virulent strain Beaudette C (BC) identified 12 amino acid differences in the globular head region of the HN protein (H203, T214, I219, S228, L269, A271, E293, G310, S494, E495, T502, and N568, named according to the BC amino acid assignment). We also examined five of these nonconserved residues, T214, I219, S494, E495, and N568, located in close proximity to residues identified earlier by crystal structure studies, to determine whether these might affect HN function and contribute to the difference in pathogenicity between the LaSota and BC strains (Fig. ​(Fig.11).Open in a separate windowFIG. 1.Three-dimensional structure of the NDV HN protein showing the positions of amino acid residues that were substituted in the present study. The residues are shown in space-filling mode and represented in different colors. The MacPymol (DeLano Scientific) software was used to generate the model of the globular domain of the NDV HN monomer. The structure was derived from the crystal structure of the NDV HN protein reported by Crennell et al. (4).We used site-directed mutagenesis (2) to introduce individual amino acid substitutions into a cDNA of the HN gene of strain BC. For the conserved residues, we changed arginine at positions 416 and 498 and tyrosine at position 526 to polar glutamine. For the nonconserved residues, the assignments T214, I219, S494, E495, and N568 of strain BC were altered to the corresponding assignments of strain LaSota: S214, V219, G494, V495, and D568, respectively. Each mutagenized HN gene was then inserted into a full-length cDNA clone of the BC antigenome. These clones were transfected into HEp2 cells, and mutant viruses were recovered as previously described (8). These viruses were designated according to the substitutions introduced: T214S, I219V, R416Q, S494G, E495V, R498Q, Y526Q, and N568D. The HN genes from recovered viruses were sequenced. This confirmed the presence of each introduced mutation and the lack of adventitious mutations in the HN gene. To determine the stability of each HN mutation, the recovered viruses were passaged five times in 9-day-old embryonated chicken eggs and five times in chicken embryo fibroblast DF-1 cells. Sequence analysis of the HN gene of the mutant viruses at each passage showed that the introduced mutations were unaltered (data not shown). To rule out the possibility that change in the HN protein sequence could be compensated for by a mutation in the F protein, the F gene from each recovered virus was sequenced. No compensatory mutations in the F gene were observed (data not shown). The HN protein content of each mutant virus, determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Coomassie staining, was very similar to that of the parental BC virus (pBC) (Table ​(Table1).1). The multicycle growth kinetics of the recombinant HN mutant viruses in DF-1 cells (Fig. ​(Fig.2)2) showed that the replication kinetics of all of the HN mutant viruses were similar to those of pBC, with the exception of the Y526Q mutant, which showed delayed growth and had a lower virus yield (1.5 to 2.0 log₁₀ PFU/ml) than the parental and other mutant viruses. In addition, the Y526Q mutant produced syncytia at 72 h, whereas the parental and other mutant viruses initiated syncytia at 24 h postinfection. These studies showed the importance of amino acid residue Y526 at the active site of the HN protein of NDV.Open in a separate windowFIG. 2.Multicycle growth kinetics of HN mutants of NDV in chicken embryo fibroblast (DF-1) cells. Cells were infected with the indicated parental or mutant virus at an multiplicity of infection of 0.01. Supernatant samples were collected at 8-h intervals until 64 h postinfection, and virus titers were determined at different time points by plaque assay. Values are averages from three independent experiments.

TABLE 1.

Biological activities of HN mutants of NDV

新城疫病毒F蛋白细胞融合活性位点中保守氨基酸基因突变分析

为了确定新城疫病毒融合蛋白(F)分子上活性位点中保守氨基酸在F蛋白的细胞融合作用,弄清F细胞融合的分子机理,采用基因定点突变法,创造一个酶切位点,用酶切反应初步筛选突变株,然后用DNA序列分析进一步确定,并于真核细胞内进行表达,Giemsa染色定性和指示基因法定量检测细胞融合功能,荧光强度分析(FACS)检测表达效率情况。结果表明,NDV F第117位苯丙氨酸(F)突变成亮氨酸(L)时对细胞融合作用没有显著影响。R112和K115同为保守序列,分别突变为G时,细胞融合活性只有原来的44%,下降了56%。细胞表面表达效率没有明显的改变。N147突变为K时,细胞融合活性明显下降,只有原来的15%,而细胞表面表达效率没有明显的改变。L154为保守序列,突变为K时,细胞融合活性消失,说明L154是一个非常关键的氨基酸,对维持F蛋白的细胞融合活性非常重要。细胞表面表达效率也有所下降(为原来的94%)。D462属于高度保守氨基酸,当突变为N时,细胞融合活性消失,但经细胞表面表达效率分析证明,此突变蛋白未表达于细胞表面,证明在细胞浆转运至细胞表面的过程中发生了问题。当突变为R和E时,细胞融合活性未发生改变,但细胞表面表达效率有所下降,分别为野毒株的63%和44%。说明NDV F分子上与HN相互作用的特异性区域中的某些保守氨基酸在细胞融合中发挥着重要作用,对F蛋白的折叠、加工、转运等,发挥着不同作用,从而影响F蛋白的细胞融合作用和/或在细胞表面的表达量。     

Mutations in the Fusion Protein Cleavage Site of Avian Paramyxovirus Serotype 4 Confer Increased Replication and Syncytium Formation In Vitro but Not Increased Replication and Pathogenicity in Chickens and Ducks

To evaluate the role of the F protein cleavage site in the replication and pathogenicity of avian paramyxoviruses (APMVs), we constructed a reverse genetics system for recovery of infectious recombinant APMV-4 from cloned cDNA. The recovered recombinant APMV-4 resembled the biological virus in growth characteristics in vitro and in pathogenicity in vivo. The F cleavage site sequence of APMV-4 (DIQPR↓F) contains a single basic amino acid, at the -1 position. Six mutant APMV-4 viruses were recovered in which the F protein cleavage site was mutated to contain increased numbers of basic amino acids or to mimic the naturally occurring cleavage sites of several paramyxoviruses, including neurovirulent and avirulent strains of NDV. The presence of a glutamine residue at the -3 position was found to be important for mutant virus recovery. In addition, cleavage sites containing the furin protease motif conferred increased replication and syncytium formation in vitro. However, analysis of viral pathogenicity in 9-day-old embryonated chicken eggs, 1-day-old and 2-week-old chickens, and 3-week-old ducks showed that none the F protein cleavage site mutations altered the replication, tropism, and pathogenicity of APMV-4, and no significant differences were observed among the parental and mutant APMV-4 viruses in vivo. Although parental and mutant viruses replicated somewhat better in ducks than in chickens, they all were highly restricted and avirulent in both species. These results suggested that the cleavage site sequence of the F protein is not a limiting determinant of APMV-4 pathogenicity in chickens and ducks.     

Foot-and-Mouth Disease Virus Nonstructural Protein 2C Interacts with Beclin1, Modulating Virus Replication

Foot-and-mouth disease virus (FMDV), the causative agent of foot-and-mouth disease, is an Apthovirus within the Picornaviridae family. Replication of the virus occurs in association with replication complexes that are formed by host cell membrane rearrangements. The largest viral protein in the replication complex, 2C, is thought to have multiple roles during virus replication. However, studies examining the function of FMDV 2C have been rather limited. To better understand the role of 2C in the process of virus replication, we used a yeast two-hybrid approach to identify host proteins that interact with 2C. We report here that cellular Beclin1 is a specific host binding partner for 2C. Beclin1 is a regulator of the autophagy pathway, a metabolic pathway required for efficient FMDV replication. The 2C-Beclin1 interaction was further confirmed by coimmunoprecipitation and confocal microscopy to actually occur in FMDV-infected cells. Overexpression of either Beclin1 or Bcl-2, another important autophagy factor, strongly affects virus yield in cell culture. The fusion of lysosomes to autophagosomes containing viral proteins is not seen during FMDV infection, a process that is stimulated by Beclin1; however, in FMDV-infected cells overexpressing Beclin1 this fusion occurs, suggesting that 2C would bind to Beclin1 to prevent the fusion of lysosomes to autophagosomes, allowing for virus survival. Using reverse genetics, we demonstrate here that modifications to the amino acids in 2C that are critical for interaction with Beclin1 are also critical for virus growth. These results suggest that interaction between FMDV 2C and host protein Beclin1 could be essential for virus replication.     

新城疫病毒F蛋白中两段七肽重复序列的克隆和表达

从新城疫病毒 (NDV)中国强毒株F4 8E9和弱毒株长春株F蛋白的cDNA中亚克隆出两段七肽重复序列(HeptadRepeatRegion ,HR1,HR2 ) ,将HR1和HR2分别插入表达载体pGEX 6p 1,在大肠杆菌BL2 1(DE3 )中表达 ,将与载体中的GST(GlutathioneS Trasferase)融合表达的可溶性融合蛋白用GST亲和层析柱纯化。纯化的融合蛋白用蛋白酶酶切后 ,先用GST亲和层析柱除去GST ,再加热进一步纯化。纯化的HR1和HR2质谱分析其分子量 ,结果表明 ,强株的HR1和HR2的分子量分别为 7 10 3kD和 6 3 0 1kD ,弱株的HR1和HR2的分子量分别为 7 10 7kD和6 3 0 9kD ,强弱株HR1和HR2的分子量都基本一致。本工作为研究HR1、HR2的结构以及它们在NDV与宿主细胞融合中的作用奠定了基础。     

Species Based Synonymous Codon Usage in Fusion Protein Gene of Newcastle Disease Virus

Newcastle disease is highly pathogenic to poultry and many other avian species. However, the Newcastle disease virus (NDV) has also been reported from many non-avian species. The NDV fusion protein (F) is a major determinant of its pathogenicity and virulence. The functionalities of F gene have been explored for the development of vaccine and diagnostics against NDV. Although the F protein is well studied but the codon usage and its nucleotide composition from NDV isolated from different species have not yet been explored. In present study, we have analyzed the factors responsible for the determination of codon usage in NDV isolated from four major avian host species. The F gene of NDV is analyzed for its base composition and its correlation with the bias in codon usage. Our result showed that random mutational pressure is responsible for codon usage bias in F protein of NDV isolates. Aromaticity, GC3s, and aliphatic index were not found responsible for species based synonymous codon usage bias in F gene of NDV. Moreover, the low amount of codon usage bias and expression level was further confirmed by a low CAI value. The phylogenetic analysis of isolates was found in corroboration with the relatedness of species based on codon usage bias. The relationship between the host species and the NDV isolates from the host does not represent a significant correlation in our study. The present study provides a basic understanding of the mechanism involved in codon usage among species.     

Proteins of Newcastle Disease Virus and of the Viral Nucleocapsid

Newcastle disease virus was found to contain three major proteins. The structure unit of the viral nucleocapsid appears to be monomeric and to consist of a single large protein of an approximate molecular weight of 62,000.     

转新城疫病毒融合蛋白基因水稻植株的获得

以编码新城疫病毒融合蛋白(NDV—F)基因为外源基因,与玉米泛素蛋白(Ubi)启动子和农杆菌胭脂碱合成酶基因(NOS)终止子构建成嵌合基因,构建了适宜于农杆菌介导转化水稻的表达质粒pUNDV;并以潮霉素磷酸转移酶(HPT)基因作选择标记基因、β-半乳糖苷酸酶(GUS)基因作报告基因,借助于农杆菌介导转化水稻,获得了多株转基因植株。PCR分析和GUS活性检测结果证实含有NDV—F基冈的T—DNA已整合到水稻基因组中,为研制廉价的转基因水稻新城疫基因工程疫苗奠定了基础。     

Cytoplasmic Translocation of Polypyrimidine Tract-Binding Protein and Its Binding to Viral RNA during Japanese Encephalitis Virus Infection Inhibits Virus Replication

Japanese encephalitis virus (JEV) has a single-stranded, positive-sense RNA genome containing a single open reading frame flanked by the 5′- and 3′-non-coding regions (NCRs). The virus genome replicates via a negative-sense RNA intermediate. The NCRs and their complementary sequences in the negative-sense RNA are the sites for assembly of the RNA replicase complex thereby regulating the RNA synthesis and virus replication. In this study, we show that the 55-kDa polypyrimidine tract-binding protein (PTB) interacts in vitro with both the 5′-NCR of the positive-sense genomic RNA - 5NCR(+), and its complementary sequence in the negative-sense replication intermediate RNA - 3NCR(-). The interaction of viral RNA with PTB was validated in infected cells by JEV RNA co-immunoprecipitation and JEV RNA-PTB colocalization experiments. Interestingly, we observed phosphorylation-coupled translocation of nuclear PTB to cytoplasmic foci that co-localized with JEV RNA early during JEV infection. Our studies employing the PTB silencing and over-expression in cultured cells established an inhibitory role of PTB in JEV replication. Using RNA-protein binding assay we show that PTB competitively inhibits association of JEV 3NCR(-) RNA with viral RNA-dependent RNA polymerase (NS5 protein), an event required for the synthesis of the plus-sense genomic RNA. cAMP is known to promote the Protein kinase A (PKA)-mediated PTB phosphorylation. We show that cells treated with a cAMP analogue had an enhanced level of phosphorylated PTB in the cytoplasm and a significantly suppressed JEV replication. Data presented here show a novel, cAMP-induced, PTB-mediated, innate host response that could effectively suppress JEV replication in mammalian cells.     

Newcastle Disease Virus Fusion Protein Is the Major Contributor to Protective Immunity of Genotype-Matched Vaccine

Virulent strains of Newcastle disease virus (NDV) can cause devastating disease in chickens worldwide. Although the current vaccines are substantially effective, they do not completely prevent infection, virus shedding and disease. To produce genotype-matched vaccines, a full-genome reverse genetics system has been used to generate a recombinant virus in which the F protein cleavage site has been changed to that of avirulent vaccine virus. In the other strategy, the vaccines have been generated by replacing the F and HN genes of a commercial vaccine strain with those from a genotype-matched virus. However, the protective efficacy of a chimeric virus vaccine has not been directly compared with that of a full-genome virus vaccine developed by reverse genetics. Therefore, in this study, we evaluated the protective efficacy of genotype VII matched chimeric vaccines by generating three recombinant viruses based on avirulent LaSota (genotype II) strain in which the open reading frames (ORFs) encoding the F and HN proteins were replaced, individually or together, with those of the circulating and highly virulent Indonesian NDV strain Ban/010. The cleavage site of the Ban/010 F protein was mutated to the avirulent motif found in strain LaSota. In vitro growth characteristics and a pathogenicity test indicated that all three chimeric viruses retained the highly attenuated phenotype of the parental viruses. Immunization of chickens with chimeric and full-length genome VII vaccines followed by challenge with virulent Ban/010 or Texas GB (genotype II) virus demonstrated protection against clinical disease and death. However, only those chickens immunized with chimeric rLaSota expressing the F or F plus HN proteins of the Indonesian strain were efficiently protected against shedding of Ban/010 virus. Our findings showed that genotype-matched vaccines can provide protection to chickens by efficiently preventing spread of virus, primarily due to the F protein.     

Influence of Viral Envelope on Newcastle Disease Virus Infection

The adsorption characteristics of Newcastle disease virus (NDV) propagated in chicken cells (NDV-C) and in human cells (NDV-H) were examined. Adsorption experiments performed at different temperatures indicated that virus propagated in a particular cell infected that cell type more readily than did virus propagated in a different host. For example, NDV-C was more efficient in initiating infection of chicken cells at 22 C than was NDV-H; the reverse was true when human cells were employed. The results indicate that infection of susceptible cells by NDV is influenced by the host cell in which the virus was propagated. The data also suggest that NDV may be useful in studies on homologous and heterologous membrane-membrane interactions.     

Interaction of Peptides with Sequences from the Newcastle Disease Virus Fusion Protein Heptad Repeat Regions

Typical of many viral fusion proteins, the sequence of the Newcastle disease virus (NDV) fusion protein has several heptad repeat regions. One, HR1, is located just carboxyl terminal to the fusion peptide, while the other, HR2, is located adjacent to the transmembrane domain. The structure and function of a synthetic peptide with a sequence from the region of the NDV HR1 region (amino acids 150 to 173) were characterized. The peptide inhibited fusion with a half-maximal concentration of approximately 2 microM; however, inhibition was observed only if the peptide was added prior to protease activation of the fusion protein. This inhibition was virus specific since the peptide had minimal effect on fusion directed by the Sendai virus glycoproteins. To explore the mechanism of action, the potential HR1 peptide interaction with a previously characterized fusion inhibitory peptide with a sequence from the HR2 domain (J. K. Young, R. P. Hicks, G. E. Wright, and T. G. Morrison, Virology 238:291-304, 1997) was characterized. The results demonstrated an interaction between the two peptides both functionally and directly. First, while the individual peptides each inhibit fusion, equimolar mixtures of the two peptides had minimal effect on fusion, suggesting that the two peptides form a complex preventing their interaction with a target protein. Second, an HR2 peptide covalently linked with biotin was found to bind specifically to HR1 peptide in a Western blot. The structure of the HR1 peptide was analyzed by nuclear magnetic resonance spectroscopy and found to be an alpha helix.     

Role of the Varicella-Zoster Virus gB Cytoplasmic Domain in gB Transport and Viral Egress

To study the function of the varicella-zoster virus (VZV) gB cytoplasmic domain during viral infection, we produced a VZV recombinant virus that expresses a truncated form of gB lacking the C-terminal 36 amino acids of its cytoplasmic domain (VZV gB-36). VZV gB-36 replicates in noncomplementing cells and grows at a rate similar to that of native VZV. However, cells infected with VZVgB-36 form extensive syncytia compared to the relatively small syncytia formed during native VZV infection. In addition, electron microscopy shows that very little virus is present on the surfaces of cells infected with VZV gB-36, while cells infected with native VZV exhibit abundant virions on the cell surface. The C-terminal 36 amino acids of the gB cytoplasmic domain have been shown in transfection-based experiments to contain both an endoplasmic reticulum-to-Golgi transport signal (the C-terminal 17 amino acids) and a consensus YXXphi (where Y is tyrosine, X is any amino acid, and phi is any bulky hydrophobic amino acid) signal sequence (YSRV) that mediates the internalization of gB from the plasma membrane. As predicted based on these data, gB-36 expressed during the infection of cultured cells is transported inefficiently to the Golgi. Despite lacking the YSRV signal sequence, gB-36 is internalized from the plasma membrane; however, in contrast to native gB, it fails to localize to the Golgi. Therefore, the C-terminal 36 amino acids of the VZV gB cytoplasmic domain are required for normal viral egress and for both the pre- and post-Golgi transport of gB.     

Autophagy Benefits the Replication of Newcastle Disease Virus in Chicken Cells and Tissues

Newcastle disease virus (NDV) is an important avian pathogen. We previously reported that NDV triggers autophagy in U251 glioma cells, resulting in enhanced virus replication. In this study, we investigated whether NDV triggers autophagy in chicken cells and tissues to enhance virus replication. We demonstrated that NDV infection induced steady-state autophagy in chicken-derived DF-1 cells and in primary chicken embryo fibroblast (CEF) cells, evident through increased double- or single-membrane vesicles, the accumulation of green fluorescent protein (GFP)-LC3 dots, and the conversion of LC3-I to LC3-II. In addition, we measured autophagic flux by monitoring p62/SQSTM1 degradation, LC3-II turnover, and GFP-LC3 lysosomal delivery and proteolysis, to confirm that NDV infection induced the complete autophagic process. Inhibition of autophagy by pharmacological inhibitors and RNA interference reduced virus replication, indicating an important role for autophagy in NDV infection. Furthermore, we conducted in vivo experiments and observed the conversion of LC3-I to LC3-II in heart, liver, spleen, lung, and kidney of NDV-infected chickens. Regulation of the induction of autophagy with wortmannin, chloroquine, or starvation treatment affects NDV production and pathogenesis in tissues of both lung and intestine; however, treatment with rapamycin, an autophagy inducer of mammalian cells, showed no detectable changes in chicken cells and tissues. Moreover, administration of the autophagy inhibitor wortmannin increased the survival rate of NDV-infected chickens. Our studies provide strong evidence that NDV infection induces autophagy which benefits NDV replication in chicken cells and tissues.