Mutation in the matrix protein of Newcastle disease virus can result in decreased fusion glycoprotein incorporation into particles and decreased infectivity

Multifunctionality of matrix protein in the replication and pathogenesis of Newcastle disease virus: A review

As an important structural protein in virion morphogenesis, the matrix (M) protein of Newcastle disease virus (NDV) is demonstrated to be a nuclear-cytoplasmic trafficking protein and plays essential roles in viral assembly and budding. In recent years, increasing lines of evidence have indicated that the M protein has obvious influence on the pathotypes of NDV, and the interaction of M protein with cellular proteins is also closely associated with the replication and pathogenicity of NDV. Although substantial progress has been made in the past 40 years towards understanding the structure and function of NDV M protein, the available information is scattered. Therefore, this review article summarizes and updates the research progress on the structural feature, virulence and pathotype correlation, and nucleocytoplasmic transport mechanism of NDV M protein, as well as the functions of M protein and cellular protein interactions in M's intracellular localization, viral RNA synthesis and transcription, viral protein synthesis, viral immune evasion, and viral budding and release, which will provide an in-depth understanding of the biological functions of M protein in the replication and pathogenesis of NDV, and also contribute to the development of effective antiviral strategies aiming at blocking the early or late steps of NDV lifecycles.

Multiple potential recombination events among Newcastle disease virus genomes in China between 1946 and 2020

Introduction

Newcastle Disease Virus (NDV) is a highly adaptable virus with large genetic diversity that has been widely studied for its oncolytic activities and potential as a vector vaccine. This study investigated the molecular characteristics of 517 complete NDV strains collected from 26 provinces across China between 1946-2020.

Methods

Herein, phylogenetic, phylogeographic network, recombination, and amino acid variability analyses were performed to reveal the evolutionary characteristics of NDV in China.

Results and discussions

Phylogenetic analysis revealed the existence of two major groups: GI, which comprises a single genotype Ib, and GII group encompassing eight genotypes (I, II, III, VI. VII. VIII, IX and XII). The Ib genotype is found to dominate China (34%), particularly South and East China, followed by VII (24%) and VI (22%). NDV strains from the two identified groups exhibited great dissimilarities at the nucleotide level of phosphoprotein (P), matrix protein (M), fusion protein (F), and haemagglutinin-neuraminidase (HN) genes. Consistently, the phylogeographic network analysis revealed two main Network Clusters linked to a possible ancestral node from Hunan (strain MH289846.1). Importantly, we identified 34 potential recombination events that involved mostly strains from VII and Ib genotypes. A recombinant of genotype XII isolated in 2019 seems to emerge newly in Southern China. Further, the vaccine strains are found to be highly involved in potential recombination. Therefore, since the influence of recombination on NDV virulence cannot be predicted, this report's findings need to be considered for the security of NDV oncolytic application and the safety of NDV live attenuated vaccines.

Cleavage of a Neuroinvasive Human Respiratory Virus Spike Glycoprotein by Proprotein Convertases Modulates Neurovirulence and Virus Spread within the Central Nervous System

Human coronaviruses (HCoV) are respiratory pathogens that may be associated with the development of neurological diseases, in view of their neuroinvasive and neurotropic properties. The viral spike (S) glycoprotein is a major virulence factor for several coronavirus species, including the OC43 strain of HCoV (HCoV-OC43). In an attempt to study the role of this protein in virus spread within the central nervous system (CNS) and neurovirulence, as well as to identify amino acid residues important for such functions, we compared the sequence of the S gene found in the laboratory reference strain HCoV-OC43 ATCC VR-759 to S sequences of viruses detected in clinical isolates from the human respiratory tract. We identified one predominant mutation at amino acid 758 (from RRSR↓ G₇₅₈ to RRSR↓R₇₅₈), which introduces a putative furin-like cleavage (↓) site. Using a molecular cDNA infectious clone to generate a corresponding recombinant virus, we show for the first time that such point mutation in the HCoV-OC43 S glycoprotein creates a functional cleavage site between the S1 and S2 portions of the S protein. While the corresponding recombinant virus retained its neuroinvasive properties, this mutation led to decreased neurovirulence while potentially modifying the mode of virus spread, likely leading to a limited dissemination within the CNS. Taken together, these results are consistent with the adaptation of HCoV-OC43 to the CNS environment, resulting from the selection of quasi-species harboring mutations that lead to amino acid changes in viral genes, like the S gene in HCoV-OC43, which may contribute to a more efficient establishment of a less pathogenic but persistent CNS infection. This adaptative mechanism could potentially be associated with human encephalitis or other neurological degenerative pathologies.

Human coronaviruses (HCoV) are respiratory pathogens involved in a sizable proportion of common colds. They have over the years been associated with the development of neurological diseases, given their demonstrated neuroinvasive and neurotropic properties. The viral spike (S) glycoprotein appears to be associated with these neurologic features and is a major factor of virulence for several coronavirus species, including HCoV-OC43. To further characterize the role of this protein in neurovirulence and virus spread within the CNS, we sought to identify amino acid residues that may be important for this function. Our data revealed that one of them, G758R, introduces a functional furin-like cleavage site in the S protein (RRSR↓R₇₅₈). This change in S protein mostly impacts neurovirulence, which seems associated with a modified viral dissemination, without significantly affecting its neuroinvasive capacity. This mutation, found in all characterized contemporary human clinical respiratory isolates, underlines previous findings that naturally existing field isolates of HCoV-OC43 variants still possess the capacity to invade the CNS where they could eventually adapt and establish a persistent human CNS infection, a mechanism potentially associated with human encephalitis or neurodegenerative pathologies of unknown etiologies.

Newcastle disease virus: current status and our understanding

Newcastle disease (ND) is one of the highly pathogenic viral diseases of avian species. ND is economically significant because of the huge mortality and morbidity associated with it. The disease is endemic in many third world countries where agriculture serves as the primary source of national income. Newcastle disease virus (NDV) belongs to the family Paramyxoviridae and is well characterized member among the avian paramyxovirus serotypes. In recent years, NDV has lured the virologists not only because of its pathogenic potential, but also for its oncolytic activity and its use as a vaccine vector for both humans and animals. The NDV based recombinant vaccine offers a pertinent choice for the construction of live attenuated vaccine due to its modular nature of transcription, minimum recombination frequency, and lack of DNA phase during replication. Our current understanding about the NDV biology is expanding rapidly because of the availability of modern molecular biology tools and high-throughput complete genome sequencing.

Heterologous expression, characterization and evaluation of the matrix protein from Newcastle disease virus as a target for antiviral therapies

Newcastle disease virus (NDV) is an infectious agent of a large variety of birds, including chicken, which poses a real threat to the agriculture industry. Matrix (M) proteins of NDV and many other viruses perform critical functions during viral assembly and budding from the host cell. M-proteins are well conserved and therefore are potential targets for antiviral therapies. To validate this, we expressed the NDV M-protein in its native form in Saccharomyces cerevisiae and in inclusion bodies in Escherichia coli. Proper refolding of the recombinant protein produced in E. coli was verified using circular dichroism and infrared spectroscopies and electron microscopy. Immunization of chickens with the NDV M-protein elicited significant serum antibody titers. However, the antibodies conferred little protection against the ND following lethal viral challenges. We conclude that the M-protein is not exposed on the surface of the host cell or the virus at any stage during its life cycle. We discuss how the conserved M-protein can further be exploited as an antiviral drug target.

Influenza virus hemagglutinin and neuraminidase glycoproteins stimulate the membrane association of the matrix protein

We have analyzed the mechanism by which the matrix (M1) protein associates with cellular membranes during influenza A virus assembly. Interaction of the M1 protein with the viral hemagglutinin (HA) or neuraminidase (NA) glycoprotein was extensively analyzed by using wild-type and transfectant influenza viruses as well as recombinant vaccinia viruses expressing the M1 protein, HA, or NA. Membrane binding of the M1 protein was significantly stimulated at the late stage of virus infection. Using recombinant vaccinia viruses, we found that a relatively small fraction (20 to 40%) of the cytoplasmic M1 protein associated with cellular membranes in the absence of other viral proteins, while coexpression of the HA and the NA stimulated membrane binding of the M1 protein. The stimulatory effect of the NA (>90%) was significant and higher than that of the HA (>60%). Introduction of mutations into the cytoplasmic tail of the NA interfered with its stimulatory effect. Meanwhile, the HA may complement the defective NA and facilitate virus assembly in cells infected with the NA/TAIL(-) transfectant. In conclusion, the highly conserved cytoplasmic tails of the HA and NA play an important role in virus assembly.

Sendai virus M protein binds independently to either the F or the HN glycoprotein in vivo

We have analyzed the mechanism by which M protein interacts with components of the viral envelope during Sendai virus assembly. Using recombinant vaccinia viruses to selectively express combinations of Sendai virus F, HN, and M proteins, we have successfully reconstituted M protein-glycoprotein interaction in vivo and determined the molecular interactions which are necessary and sufficient to promote M protein-membrane binding. Our results showed that M protein accumulates on cellular membranes via a direct interaction with both F and HN proteins. Specifically, our data demonstrated that a small fraction (8 to 16%) of M protein becomes membrane associated in the absence of Sendai virus glycoproteins, while > 75% becomes membrane bound in the presence of both F and HN proteins. Selective expression of M protein together with either F or HN protein showed that each viral glycoprotein is individually sufficient to promote efficient (56 to 73%) M protein-membrane binding. Finally, we observed that M protein associates with cellular membranes in a time-dependent manner, implying a need for either maturation or transport before binding to glycoproteins.

The matrix proteins of neurovirulent subacute sclerosing panencephalitis virus and its acute measles virus progenitor are functionally different

Persistence of measles virus in the brains of patients with subacute sclerosing panencephalitis (SSPE) is accompanied by changes in the viral matrix (M) protein. To understand the significance of these changes, cell culture and cell-free assays were developed to compare the functions of the M proteins of an SSPE virus Biken strain and its acute measles virus progenitor Nagahata strain. The Nagahata viral M protein is associated with the intracellular viral nucleocapsids and the plasma membrane, whereas the Biken viral M protein is localized mainly in the cytosol. The lack of M protein in the Biken viral nucleocapsids is due to a failure of the Biken M protein to bind to the viral nucleocapsids. The Biken M protein also fails to bind to the Nagahata viral nucleocapsids. Conversely, the Nagahata M protein can bind to the Biken viral nucleocapsids, although this association is not as stable at physiological salt concentration. These results offer concrete evidence that the M protein of an SSPE virus is functionally different from that of its progenitor acute measles virus.

Intracellular processing of the paramyxovirus F protein: critical role of the predicted amphipathic alpha helix adjacent to the fusion domain

At a nonpermissive temperature, the group D temperature-sensitive mutants of Newcastle disease virus strain Australia-Victoria (AV) are defective in plaque formation, in inducing infected cells to fuse, and in incorporating the cleaved fusion glycoprotein, F1 + F2, into virus particles. In this study, the F protein of AV, expressed in chicken embryo cells, was able to complement these mutants in a plaque assay, identifying the F gene as the gene containing the group D temperature-sensitive lesions. The F genes of mutants D1, D2, and D3 were found to contain single mutations relative to the AV sequence, clustered within a predicted amphipathic alpha helix (AAH) adjacent to the hydrophobic amino terminus of F1. These mutant F proteins were inefficiently processed at the permissive temperature, a problem that was exacerbated at the nonpermissive temperature. Surprisingly, the AV F protein was also found to be partially temperature sensitive in processing. Its AAH is predicted to contain a break in the helix close to the D mutation sites, which are themselves predicted to further weaken the helix at this point. Interestingly, six revertants of the group D mutants were found to have an additional lesion in the AAH, repairing both the AV and mutant helices, resulting in a predicted perfect helix. The F protein of these revertants had overcome both the processing defects of the mutants and the temperature sensitivity of AV, indicating that the AAH region is critical for F protein processing. The lesions of a second group of revertants were localized within F2, suggesting an interaction with the F1 AAH region containing the original lesion.

Cloning and sequencing of the matrix protein (M) gene of turkey rhinotracheitis virus reveal a gene order different from that of respiratory syncytial virus

Several biochemical properties and the sequence of the fusion glycoprotein (F) have indicated that turkey rhinotracheitis virus (TRTV) is a pneumovirus, subfamily Pneumovirinae of the Paramyxoviridae family. As TRTV was known to generate polycistronic mRNAs, cDNA was generated from TRTV strain UK/3BV/85-infected Vero cell mRNAs using an oligonucleotide primer corresponding to a region of the F gene. Sequencing of four cDNAs revealed that the gene adjacent to the beginning (3' end) of the F gene was that for the matrix (M) protein, i.e., that TRTV had the partial gene order 3'-M-F-5'. This was unexpected as human respiratory syncytial (RS) virus, the type species of the genus Pneumovirus, has the partial gene order 3'-M-SH-G-F-5', where SH and G are the small hydrophobic protein and attachment glycoprotein, respectively. Instead TRTV resembled the Morbillivirus and Paramyxovirus genera of the Paramyxoviridae (subfamily Paramyxovirinae) which have the partial gene order 3'-M-F-5'. Two further oligonucleotides, one corresponding to a sequence near the end of the M gene and the other (oligo B) to a sequence near the beginning of the F gene, with their 5' ends spaced 300 nucleotides apart on the basis of the cDNA sequence, were used in a polymerase chain reaction (PCR) using genomic RNA as template. Only a PCR product of 0.3 kb was obtained. The same sized product was also obtained using these oligonucleotides and genomic RNA from three other TRTV strains (SA/91/78, UK/8544/85, and SA/2381/88) which had been grown in chicken tracheal organ cultures. In addition PCR was performed using genomic RNA from TRTV-3BV and SA/2381/88 with oligo B and another oligonucleotide near the 5' end of the gene upstream from M, spaced 1141 nucleotides apart on the basis of the sequence data. Only a 1.14-kb PCR product was obtained. Larger products would have been expected if another gene had been situated between M and F. The absence of such larger products, plus the demonstration that infected cells contained M-F dicistronic mRNAs, supported the conclusion that in the TRTV genome the M gene is adjacent to the F gene in the order 3'-M-F-5'. The 5' termini of the M and F mRNAs were confirmed by mRNA mapping. The TRTV M gene encoded a protein of 254 amino acids, very similar to that of RS virus (256 residues; 37% amino acid identity) but very different from that of the morbilliviruses and paramyxoviruses (approximately 350 residues).(ABSTRACT TRUNCATED AT 400 WORDS)

Complete nucleotide sequence of the matrix gene of human parainfluenza type 2 virus and expression of the M protein in bacteria

The sequence of the M gene of human parainfluenza virus type 2 (PIV-2) has been determined. The sequence contained a large open reading frame with 1131 nucleotides encoding a protein with a calculated molecular weight of 42,312. Comparison of M protein sequence indicated that PIV-2 was more closely related to mumps virus and Newcastle disease virus than to other parainfluenza viruses, Sendai virus (SV), and parainfluenza virus type 3 (PIV-3), indicating a possible subdividing of the Paramyxovirus into two groups. This grouping is consistent with that obtained from analysis of the HN gene. Measles virus and canine distemper virus definitely belong to the subgroup composed of SV and PIV-3. No homology region was found in all the paramyxoviruses compared. However, a tertiary structure may be conserved in each subgroup of paramyxovirus. The M protein of PIV-2 was expressed in bacteria, and the product was recognized by a monoclonal antibody specific for the PIV-2 M protein. The bacterial-expressed protein, however, was heterogeneous and smaller in size.

Sequence characterization of the membrane protein gene of paramyxovirus simian virus 5

The complete nucleotide sequence of the membrane (M) protein gene of the paramyxovirus simian virus 5 (SV5) was determined from cDNA clones of viral mRNAs. The M gene boundaries were determined by (i) primer extension sequencing on M mRNA; (ii) nuclease S1 analysis; and (iii) primer extension sequencing on viral genomic RNA. The M gene mRNA consisted of 1371 templated nucleotides. It contains a single large open reading frame that can encode a protein of 377 amino acids with a predicted Mr = 42,253. The authenticity of the predicted M protein coding sequence was confirmed by synthesis of the M protein from mRNA synthesized from cDNA. The predicted M amino acid sequence indicated it is an overall hydrophobic protein carrying a net positive charge. Alignment of the SV5 protein amino acid sequence with the M protein sequences of other paramyxoviruses indicated that these viruses fall into the following two groups: (1) SV5, mumps virus, and Newcastle disease virus; or (2) Sendai, parainfluenza virus type 3, measles virus, and canine distemper virus, with mumps virus M sequence being the most closely related to SV5.

Molecular cloning and complete nucleotide sequence of the attenuated rabies virus SAD B19

Complementary DNA spanning the entire genome of the attenuated rabies virus strain SAD B19 which is used for oral immunization of foxes in Europe was cloned and sequenced. The viral genome comprises 11,928 nucleotides and encodes the five viral proteins N, NS, M, G, and L. Deduced protein sequences are highly similar to those of the pathogenic PV strain, homologies ranging from 90.6% for the M to 98.6% for the L protein. The five cistrons are separated by intergenic regions of 2, 5, 5, and 24 nucleotides, respectively. The G transcription stop/polyadenylation consensus signal in SAD B19 is destroyed by a deletion of three A residues. The strong conservation of both noncoding and coding nucleotide sequences indicates a high selective pressure on the primary structure of rabies virus genomic RNA.

Molecular cloning of the rinderpest virus matrix gene: comparative sequence analysis with other paramyxoviruses

The nucleotide sequence of the gene encoding the matrix or membrane (M) protein of the virulent (Kabete-O) strain of rinderpest virus (RPV) has been determined. The M gene is 1457 nucleotides long with a single, large open reading frame. The derived polypeptide has 335 amino acids, corresponding to a calculated molecular weight of 38,289 and contains both small hydrophobic regions and many basic residues. The predicted amino acid sequence was compared to the M proteins of paramyxoviruses. Sequence comparison and hydropathy profiles among the morbilliviruses revealed that the M protein of RPV exhibits features similar to those of the M protein of MV and CDV. There is 78.2% homology at the amino acid level between the M protein of RPV and MV, and 77.6% between RPV and CDV. This indicates that a high degree of homology exists among the members of the genus Morbillivirus. In contrast, there is only 37.3 and 18% homology between RPV and bovine parainfluenza type 3 (BPV3), and RPV and Newcastle disease virus (NDV) M proteins, respectively. Thus the M proteins of the morbilliviruses are highly conserved whereas the M proteins of the genus Paramyxovirus show more divergence.

Proteins of Newcastle disease virus envelope: interaction between the outer hemagglutinin-neuraminidase glycoprotein and the inner non-glycosylated matrix protein

Using linear sucrose-density ultracentrifugation analysis of Triton-solubilized Newcastle Disease Virus envelopes, we have evidenced, for the first time, the existence of interactions between the outer hemagglutinin-neuraminidase transmembrane glycoprotein and the inner non-glycosylated peripheral matrix protein. Such interactions seem to be electrostatic. These conclusions are based on the behavior of both proteins at different ionic strengths. When in low ionic strength buffer, hemagglutinin-neuraminidase and matrix proteins band together in the sucrose gradient, whereas at high ionic strength both proteins band at different rates in the gradient. The behavior of the inner matrix protein in our conditions was the expected one for a peripheral protein. The results of these 'in vitro' studies are also discussed in terms of the possible 'in vivo' role of such interactions.

Complete nucleotide sequence of the matrix protein mRNA of mumps virus

The complete nucleotide sequence of the mumps virus membrane protein or matrix protein (M) has been determined by sequencing cDNA clones and confirmed by partially sequencing the M mRNA and the genome. The mRNA is 1248 nucleotides long excluding the poly(A) and encodes a protein of 375 amino acids. The molecular weight (38,670), deduced from the amino acid sequence, is in agreement with the molecular weight of the viral M protein estimated by polyacrylamide gel electrophoresis (39-40 kDa). The mumps virus M protein shows 23-27% homology with M proteins of Newcastle disease virus (NDV), measles virus, canine distemper virus (CDV), parainfluenza virus type 3, and Sendai virus, respectively. A comparison of the M protein sequences of the above six paramyxoviruses did not reveal any conserved area of homology common among all paramyxovirus M proteins.

Association of soluble matrix protein of Newcastle disease virus with liposomes is independent of ionic conditions

An immunoaffinity method was designed for purification of a soluble form of the matrix (M) protein of Newcastle disease virus. The resulting M protein sedimented in a sucrose gradient as a small complex. This purified M protein associated with liposomes containing a net neutral, negative, or positive charge. The liposomes were composed of phosphatidylcholine, cholesterol, and a third lipid which provided the charge. The M protein-liposome associations were not prevented by high salt conditions. These observations are consistent with a nonelectrostatic association between the M protein and liposomes. Monoclonal antibodies to three separate epitopes of the M protein were all able to bind M protein complexed with liposomes, suggesting that the three M protein epitopes are not directly involved in the interaction between the M protein and liposomes. The M protein was also able to associate with liposomes lacking cholesterol implying that cholesterol does not play a substantial role in the M protein-liposome interaction.

Fusion (F) protein gene of Newcastle disease virus: sequence and hydrophobicity comparative analysis between virulent and avirulent strains

The nucleotide and predicted amino acid sequences have been obtained for the fusion (F) protein gene of the avirulent strain La Sota of Newcastle disease virus (NDV). The F1 N-terminus begins with the tripeptide Leu-Ileu-Gly instead of Phe-X-Gly as usually observed in fusion peptide. It was found that the cleavage-activation domain of the avirulent La Sota strain contained single (but no pairs of) basic residues in the sequence Gly-Arg-Gln-Gly-Arg. Hydrophobicity analysis suggested that the cleavage-activation domain became more hydrophobic and could be less accessible for host-specific protease(s); dibasic residues next to the F1 N-terminus were shown to be important for keeping the cleavage-activation site in exposed positioning, suitable for F protein activation. Comparative sequence analysis of the NDV F proteins revealed a striking homology between lentogenic La Sota and mesogenic Beaudette C strains. Furthermore, 58 variable positions were recorded in the NDV F protein, excluding signal sequence; some of these mutations, in the cysteine-clustered region, were surmised to alter virulence.

Structure, function, and intracellular processing of paramyxovirus membrane proteins

Paramyxoviruses are a fascinating group of viruses with diverse hosts and disease manifestations. They are valuable systems for studying viral pathogenesis, molecular mechanisms of negative strand viral replication, and glycoprotein structure and function. In the past few years this group of viruses has received increased attention and as a result there is a wealth of new information. For example, most of the genes of many paramyxoviruses have been cloned and sequenced. The recent availability of sequence information from a number of paramyxoviruses now allows the direct comparison of the amino acid sequence and determinants of secondary structure of analogous genes across the family of viruses. Such comparisons are revealing for two reasons. First, results provide clues to the evolution of these viruses. Second, and more importantly, comparisons of analogous genes may point to sequences and structural determinants that are central to the function of the individual proteins. Below is a comparison of five of the paramyxovirus genes with a discussion of the implications of common structural determinants for function, intracellular processing, and evolutionary origin. The focus is on the paramyxovirus membrane proteins, although other proteins are discussed briefly.

Temperature-sensitive mutants of Newcastle disease virus altered in HN glycoprotein size, stability, or antigenic maturity

It has been suggested that the 11 group B, C, and BC temperature-sensitive (ts) mutants of Newcastle disease virus (NDV), strain Australia-Victoria (AV-WT), have lesions in the gene for the hemagglutinin/neuraminidase glycoprotein (HN), and that complementation between groups B and C is intracistronic. Virions produced by these mutants even at permissive temperature contain greatly reduced amounts of HN, and the accompanying hemagglutinating and neuraminidase functions. To explore the basis for decreased HN incorporation into virions and the temperature sensitivity of these mutants, infected chick embryo cells were examined for changes in HN characteristics. The HN of two of the mutants was clearly altered in electrophoretic migration rates in both virions and infected cells. The migrational differences were not due to differences in glycosylation because altered migration rates were also observed in the presence of tunicamycin. In all cases, cells infected by these mutants produced as much HN as did AV-WT-infected cells, but the HN of six of these mutants was metabolically unstable. All of the mutants, including those with metabolically stable HN, exhibited greatly restricted ability to convert HN to an antigenically reactive form, indicating an early block in processing. For most of these mutants, the neuraminidase activities of infected cells were somewhat temperature sensitive, but the production of hemadsorbing activities on cell surfaces was not temperature sensitive. In contrast, the hemadsorbing and neuraminidase activities of cells infected by one mutant, BC2, were temperature sensitive, probably a reflection of the previously described extreme thermolability of the HN of this mutant. The relationship between these mutant characteristics, their temperature sensitivity and the virion phenotypes, is discussed. The data presented here confirm the assignment of these 11 group B, C, and BC mutants to defects in HN and begin to separate them into groups with different characteristics.

Multiple viral mutations rather than host factors cause defective measles virus gene expression in a subacute sclerosing panencephalitis cell line

A measles virus (MV) genome originally derived from brain cells of a subacute sclerosing panencephalitis patient expressed in IP-3-Ca cells an unstable MV matrix protein and was unable to produce virus particles. Transfection of this MV genome into other cell lines did not relieve these defects, showing that they are ultimately encoded by viral mutations. However, these defects were partially relieved in a weakly infectious virus which emerged from IP-3-Ca cells and which produced a matrix protein of intermediate stability. The sequences of several cDNAs related to the unstable and intermediately stable matrix proteins showed many differences in comparison with a stable matrix protein sequence and even appreciable heterogeneity among themselves. Nevertheless, partial restoration of matrix protein stability could be ascribed to a single additional amino acid change. From an examination of additional genes, we estimated that, on average, each MV genome in IP-3-Ca cells differs from the others in 30 to 40 of its 16,000 bases. The role of extreme variability of RNA virus genomes in persistent viral infections is discussed in the context of the pathogenesis of subacute sclerosing panencephalitis and of other human diseases of suspected viral etiology.

Strain variation and nuclear association of Newcastle disease virus matrix protein

Five monoclonal antibodies to the matrix (M) protein of Newcastle disease virus (NDV) Australia-Victoria (AV) strain were generated and characterized. In competitive antibody-binding assays, the antibodies fell into three discrete groups. The antigenic sites described by these antibody groups were designated M1, M2, and M3. Each antibody reacted with a panel of NDV strains in a manner unique to its group, confirming the grouping by competitive antibody binding. Only site M1 was found on all 12 of the strains tested and may be a "pan-NDV" epitope. A large portion of the M protein of strain AV was detected in the nuclei of infected cells by all five monoclonal antibodies. In addition, the antibodies only stained the nuclei of cells infected with NDV strains expressing M protein containing the corresponding antigenic site. These results confirm that the immunoreactivity in the nucleus is actually caused by the M protein and not by a cross-reacting host protein induced by viral infection.

Direct adverse effects of Sendai virus DI particles on virus budding and on M protein fate and stability

Upon infections of BHK cells with a mixture of Sendai standard and defective interfering (DI) viruses (mixed virus infection), viral budding was found to be restricted by factors ranging from 5 to more than 20. The reduced viral budding correlated with a high intracellular M protein turnover. M appeared to be degraded shortly after its synthesis, and seemed not to be able to self-associate in a stable way under the plasma membrane as it did in St virus-infected cells. These data, added to the previous findings that infection with DI particles allowed infected cell survival and favored the cell-surface turnover of the hemagglutinin-neuraminidase protein, led to the hypothesis that DI genomes directly act by preventing the stable formation inside the cells of a viral structure composed of M/HN/nucleocapsids. When involved in this structure M would be protected from degradation and HN would be stably anchored in the plasma membrane. Formation of this structure would be necessary for viral budding and would be damaging for the cells. Comparison with results published by other authors shows that such a model is consistent with other data. It can integrate, as well, data obtained in the analysis of mutant viruses involved in persistence.

Differential detergent treatment allows immunofluorescent localization of the Newcastle disease virus matrix protein within the nucleus of infected cells

Paramyxoviruses are cytoplasmic viruses and presumably do not require any nuclear function for their replication. However, recent studies using monoclonal antibodies directed against the Newcastle disease virus matrix (M) protein have found a large portion of the M protein apparently associated with the nucleus of infected cells. Whether the M protein is associated with the cytoplasmic surface of the nucleus or whether the M protein is actually located within the nucleus has not been clearly determined. To examine this question, conditions for selectively permeabilizing the cytoplasmic membrane were sought. After treating fixed cells with a low concentration (0.02%) of the nonionic detergent Triton X-100, the cytoplasmic antigen vimentin was stained with a monoclonal antibody, but nuclear antigens were not. Apparently, 0.02% Triton permeabilizes the plasma membrane while leaving the nuclear membrane intact. Under these conditions, monoclonal antibodies directed against the NDV phosphoprotein and hemagglutinin/neuraminidase glycoprotein stained infected cells, but a monoclonal antibody to the M protein did not. The inability of the anti-M monoclonal antibody to stain the nucleus, even though the outer nuclear membrane is accessible under these conditions, indicates that the M protein is not associated with the outer membrane of the nucleus. The nuclei of infected cells treated with a higher concentration (0.05%) of Triton X-100 were stained both with antibodies to nuclear antigens and with the anti-M monoclonal antibody.

Reducing agent-sensitive dimerization of the hemagglutinin-neuraminidase glycoprotein of Newcastle disease virus correlates with the presence of cysteine at residue 123

Viruses within the Newcastle disease virus (NDV) serotype induce a wide array of disease manifestations ranging from an almost apathogenic pattern to the high mortality caused by avirulent or virulent isolates, respectively. A disulfide-linked dimer form of the NDV hemagglutinin-neuraminidase (HN) glycoprotein can be demonstrated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions for only some of these isolates. For others, indeed the majority of those we have studied, no such reducing agent-sensitive dimeric form of HN is demonstrable. Apparently, there is no causal relationship between disulfide-linked dimeric HN and virulence. Using the deduced amino acid sequence of the dimeric HN of isolate AV as a basis for selection of oligonucleotide primers, we sequenced three additional reducing agent-sensitive dimeric HN glycoproteins and eight for which a disulfide-linked dimer has not been identified, using primer extension and dideoxy sequencing. The deduced amino acid sequences reveal a strict correlation between the presence of cysteine at residue 123 and reducing agent-sensitive dimerization of HN.

Expression at the cell surface of native fusion protein of the Newcastle disease virus (NDV) strain Italien from cloned cDNA

A cDNA library was constructed with poly(A)+-mRNAs from NDV-Italien infected BHK-21 cells. A clone, that hybridized to the F gene mRNA, was sequenced. A long open reading frame encodes for a protein of 553 amino acids, with a calculated molecular weight of 59,153, consisting of twelve cysteine residues and six potential glycosylation sites. The protein sequence contains a hydrophobic region at the N-terminus of F1 and a presumptive long transmembrane fragment near the C-terminus. Comparison of the F proteins from NDV strains Italien and Australia-Victoria shows that the sequences are very similar, with conservation of most cysteine residues and of the potential glycosylation sites. The F coding sequence was inserted into the genome of vaccinia virus under the control of vaccinia P7.5 transcriptional regulatory sequences. Expression of F protein was demonstrated by indirect immunofluorescence with five anti-F monoclonal antibodies known to react with conformational epitopes.

Matrix genes of measles virus and canine distemper virus: cloning, nucleotide sequences, and deduced amino acid sequences

The nucleotide sequences encoding the matrix (M) proteins of measles virus (MV) and canine distemper virus (CDV) were determined from cDNA clones containing these genes in their entirety. In both cases, single open reading frames specifying basic proteins of 335 amino acid residues were predicted from the nucleotide sequences. Both viral messages were composed of approximately 1,450 nucleotides and contained 400 nucleotides of presumptive noncoding sequences at their respective 3' ends. MV and CDV M-protein-coding regions were 67% homologous at the nucleotide level and 76% homologous at the amino acid level. Only chance homology was observed in the 400-nucleotide trailer sequences. Comparisons of the M protein sequences of MV and CDV with the sequence reported for Sendai virus (B. M. Blumberg, K. Rose, M. G. Simona, L. Roux, C. Giorgi, and D. Kolakofsky, J. Virol. 52:656-663; Y. Hidaka, T. Kanda, K. Iwasaki, A. Nomoto, T. Shioda, and H. Shibuta, Nucleic Acids Res. 12:7965-7973) indicated the greatest homology among these M proteins in the carboxyterminal third of the molecule. Secondary-structure analyses of this shared region indicated a structurally conserved, hydrophobic sequence which possibly interacted with the lipid bilayer.

Round cell variant of measles virus: spontaneous conversion from productive to cell-associated state of infection

A subacute sclerosing panencephalitis strain of measles virus was found to be composed of two viral variants distinguishable by their cytopathic effects in Vero cells. One of the variants was similar to defective cell-associated measles virus strains, whereas the other was highly productive of viral progeny but did not induce cell fusion. Cloning of the variants by an agarose plaque procedure revealed a consistent and rapid interconversion of the variants into one another. While the mechanism of this interconversion remains unknown, analysis of the expression of viral antigens by the variants using indirect immunofluorescence with monoclonal antibodies specific for measles structural antigens suggested that the interconversion involved modulation of the expression of the viral matrix or M antigen.