Interactions amongst rabies virus nucleoprotein, phosphoprotein and genomic RNA in virus-infected and transfected cells

Structural Determination of the Australian Bat Lyssavirus Nucleoprotein and Phosphoprotein Complex

Australian bat lyssavirus (ABLV) shows similar clinical symptoms as rabies, but there are currently no protein structures available for ABLV proteins. In lyssaviruses, the interaction between nucleoprotein (N) and phosphoprotein (N) in the absence of RNA generates a complex (N0P) that is crucial for viral assembly, and understanding the interface between these two proteins has the potential to provide insight into a key feature: the viral lifecycle. In this study, we used recombinant chimeric protein expression and X-ray crystallography to determine the structure of ABLV nucleoprotein bound to residues 1-40 of its phosphoprotein chaperone. Comparison of our results with the recently generated structure of RABV CVS-11 N0P demonstrated a highly conserved interface in this complex. Because the N0P interface is conserved in the lyssaviruses of phylogroup I, it is an attractive therapeutic target for multiple rabies-causing viral species.

Innate Immune Signaling and Role of Glial Cells in Herpes Simplex Virus- and Rabies Virus-Induced Encephalitis

The environment of the central nervous system (CNS) represents a double-edged sword in the context of viral infections. On the one hand, the infectious route for viral pathogens is restricted via neuroprotective barriers; on the other hand, viruses benefit from the immunologically quiescent neural environment after CNS entry. Both the herpes simplex virus (HSV) and the rabies virus (RABV) bypass the neuroprotective blood-brain barrier (BBB) and successfully enter the CNS parenchyma via nerve endings. Despite the differences in the molecular nature of both viruses, each virus uses retrograde transport along peripheral nerves to reach the human CNS. Once inside the CNS parenchyma, HSV infection results in severe acute inflammation, necrosis, and hemorrhaging, while RABV preserves the intact neuronal network by inhibiting apoptosis and limiting inflammation. During RABV neuroinvasion, surveilling glial cells fail to generate a sufficient type I interferon (IFN) response, enabling RABV to replicate undetected, ultimately leading to its fatal outcome. To date, we do not fully understand the molecular mechanisms underlying the activation or suppression of the host inflammatory responses of surveilling glial cells, which present important pathways shaping viral pathogenesis and clinical outcome in viral encephalitis. Here, we compare the innate immune responses of glial cells in RABV- and HSV-infected CNS, highlighting different viral strategies of neuroprotection or Neuroinflamm. in the context of viral encephalitis.

Brothers in Arms: Structure, Assembly and Function of Arenaviridae Nucleoprotein

Arenaviridae is a family of viruses harbouring important emerging pathogens belonging to the Bunyavirales order. Like in other segmented negative strand RNA viruses, the nucleoprotein (NP) is a major actor of the viral life cycle being both (i) the necessary co-factor of the polymerase present in the L protein, and (ii) the last line of defence of the viral genome (vRNA) by physically hiding its presence in the cytoplasm. The NP is also one of the major players interfering with the immune system. Several structural studies of NP have shown that it features two domains: a globular RNA binding domain (NP-core) in its N-terminal and an exonuclease domain (ExoN) in its C-terminal. Further studies have observed that significant conformational changes are necessary for RNA encapsidation. In this review we revisited the most recent structural and functional data available on Arenaviridae NP, compared to other Bunyavirales nucleoproteins and explored the structural and functional implications. We review the variety of structural motif extensions involved in NP–NP binding mode. We also evaluate the major functional implications of NP interactome and the role of ExoN, thus making the NP a target of choice for future vaccine and antiviral therapy.

Rabies viruses leader RNA interacts with host Hsc70 and inhibits virus replication

Viruses have been shown to be equipped with regulatory RNAs to evade host defense system. It has long been known that rabies virus (RABV) transcribes a small regulatory RNA, leader RNA (leRNA), which mediates the transition from viral RNA transcription to replication. However, the detailed molecular mechanism remains enigmatic. In the present study, we determined the genetic architecture of RABV leRNA and demonstrated its inhibitory effect on replication of wild-type rabies, DRV-AH08. The RNA immunoprecipitation results suggest that leRNA inhibits RABV replication via interfering the binding of RABV nucleoprotein with genomic RNA. Furthermore, we identified heat shock cognate 70 kDa protein (Hsc70) as a leRNA host cellular interacting protein, of which the expression level was dynamically regulated by RABV infection. Notably, our data suggest that Hsc70 was involved in suppressing RABV replication by leader RNA. Finally, our experiments imply that leRNA might be potentially useful as a novel drug in rabies post-exposure prophylaxis. Together, this study suggested leRNA in concert with its host interacting protein Hsc70, dynamically down-regulate RABV replication.

Incidence of human rabies and characterization of rabies virus nucleoprotein gene in dogs in Fujian Province, Southeast China, 2002-2012

Background

Rabies is a global fatal infectious viral disease that is characterized by a high mortality after onset of clinical symptoms. Recently, there has been an increase in the incidence of rabies in China. The aim of this study was to investigate the incidence of human rabies and characterize the rabies virus nucleoprotein gene in dogs sampled from Fujian Province, Southeast China from 2002 to 2012.

Methods

Data pertaining to human rabies cases in Fujian Province during the period from 2002 through 2012 were collected, and the epidemiological profiles were described. The saliva and brain specimens were collected from dogs in Quanzhou, Longyan and Sanming cities of the province, and the rabies virus antigen was determined in the canine saliva specimens using an ELISA assay. Rabies virus RNA was extracted from canine brain specimens, and rabies virus nucleoprotein gene was amplified using a nested RT-PCR assay, followed by sequencing and genotyping.

Results

A total of 226 human rabies cases were reported in Fujian Province from 2002 to 2012, in which 197 cases were detected in three cities of Quanzhou, Longyan and Sanming. ELISA assay revealed positive rabies virus antigen in six of eight rabid dogs and 165 of 3492 seemingly healthy dogs. The full-length gene fragment of the rabies virus nucleoprotein gene was amplified from the brain specimens of seven rabid dogs and 12 seemingly healthy dogs. Sequence alignment and phylogenetic analysis revealed that these 19 rabies virus nucleoprotein genes all belonged to genotype I, and were classified into three genetic groups. Sequencing analysis showed a 99.7% to 100% intra-group and an 86.4% to 89.3% inter-group homology.

Conclusions

This study is the first description pertaining to the epidemiological characteristics of human rabies cases and characterization of the rabies virus nucleoprotein gene in dogs in Fujian Province, Southeast China. Our findings may provide valuable knowledge for the development of strategies targeting the prevention and control of rabies.

Phenotypic Consequences In vivo and In vitro of Rearranging the P Gene of RABV HEP-Flury

Phosphoprotein (P) of the Rabies virus (RABV) is critically required for viral replication and pathogenicity. Here we manipulated infectious cDNA clones of the RABV HEP-Flury to translocate the P gene from its wild-type position 2 to 1, 3, or 4 in gene order, using an approach which left the viral nucleotide sequence unaltered. The recovered viruses were evaluated for the levels of gene expression, growth kinetics in cell culture, lethality in suckling mice and protection of mice. The results showed that viral replication was affected by the absolute value of N protein which was regulated by P protein. Viral lethality in suckling mice was consistent with the ratio of P mRNA in one complete transcription. The protection of mice induced by viruses was related to the antibody titer 5 weeks post-infection which might be regulated by G protein. However, the ability to induce cell apoptosis and viral spread were not only related to the viral replication but also to the ratio of related gene which affected by the gene position. These findings might not only improve the understanding of phenotype of RABV and P gene rearrangement, but also help rabies vaccine candidate construction.

An In Vitro RNA Synthesis Assay for Rabies Virus Defines Ribonucleoprotein Interactions Critical for Polymerase Activity

We report an in vitro RNA synthesis assay for the RNA-dependent RNA polymerase (RdRP) of rabies virus (RABV). We expressed RABV large polymerase protein (L) in insect cells from a recombinant baculovirus vector and the phosphoprotein cofactor (P) in Escherichia coli and purified the resulting proteins by affinity and size exclusion chromatography. Using chemically synthesized short RNA corresponding to the first 19 nucleotides (nt) of the rabies virus genome, we demonstrate that L alone initiates synthesis on naked RNA and that P serves to enhance the initiation and processivity of the RdRP. The L-P complex lacks full processivity, which we interpret to reflect the lack of the viral nucleocapsid protein (N) on the template. Using this assay, we define the requirements in P for stimulation of RdRP activity as residues 11 to 50 of P and formally demonstrate that ribavirin triphosphate (RTP) inhibits the RdRP. By comparing the properties of RABV RdRP with those of the related rhabdovirus, vesicular stomatitis virus (VSV), we demonstrate that both polymerases can copy the heterologous promoter sequence. The requirements for engagement of the N-RNA template of VSV by its polymerase are provided by the C-terminal domain (CTD) of P. A chimeric RABV P protein in which the oligomerization domain (OD) and the CTD were replaced by those of VSV P stimulated RABV RdRP activity on naked RNA but was insufficient to permit initiation on the VSV N-RNA template. This result implies that interactions between L and the template N are also required for initiation of RNA synthesis, extending our knowledge of ribonucleoprotein interactions that are critical for gene expression.

IMPORTANCE

The current understanding of the structural and functional significance of the components of the rabies virus replication machinery is incomplete. Although structures are available for the nucleocapsid protein in complex with RNA, and also for portions of P, information on both the structure and function of the L protein is lacking. This study reports the expression and purification of the full-length L protein of RABV and the characterization of its RdRP activity in vitro The study provides a new assay that has utility for screening inhibitors and understanding their mechanisms of action, as well as defining new interactions that are required for RdRP activity.

Everything You Always Wanted to Know About Rabies Virus (But Were Afraid to Ask)

The cultural impact of rabies, the fatal neurological disease caused by infection with rabies virus, registers throughout recorded history. Although rabies has been the subject of large-scale public health interventions, chiefly through vaccination efforts, the disease continues to take the lives of about 40,000-70,000 people per year, roughly 40% of whom are children. Most of these deaths occur in resource-poor countries, where lack of infrastructure prevents timely reporting and postexposure prophylaxis and the ubiquity of domestic and wild animal hosts makes eradication unlikely. Moreover, although the disease is rarer than other human infections such as influenza, the prognosis following a bite from a rabid animal is poor: There is currently no effective treatment that will save the life of a symptomatic rabies patient. This review focuses on the major unanswered research questions related to rabies virus pathogenesis, especially those connecting the disease progression of rabies with the complex dysfunction caused by the virus in infected cells. The recent applications of cutting-edge research strategies to this question are described in detail.

Analysis of the complete genome of the first Irkut virus isolate from China: comparison across the Lyssavirus genus

The genome of Irkut virus, isolate IRKV-THChina12, the first non-rabies lyssavirus from China (of bat origin), has been completely sequenced. In general, coding and non-coding regions of this viral genome are similar to those of other lyssaviruses. However, alignment of the deduced amino acid sequences of the structural proteins of IRKV-THChina12 with those of other lyssavirus representatives revealed significant variability between viral species. The nucleoprotein and matrix protein were found to be the most conserved, followed by the large protein, glycoprotein and phosphoprotein. Differences in the antigenic sites in glycoprotein may result in only partial protection of the available rabies biologics against Irkut virus, which is of particular concern for pre- and post-exposure rabies prophylaxis.

Host-rabies virus protein-protein interactions as druggable antiviral targets

We present an unconventional approach to antiviral drug discovery, which is used to identify potent small molecules against rabies virus. First, we conceptualized viral capsid assembly as occurring via a host-catalyzed biochemical pathway, in contrast to the classical view of capsid formation by self-assembly. This suggested opportunities for antiviral intervention by targeting previously unappreciated catalytic host proteins, which were pursued. Second, we hypothesized these host proteins to be components of heterogeneous, labile, and dynamic multi-subunit assembly machines, not easily isolated by specific target protein-focused methods. This suggested the need to identify active compounds before knowing the precise protein target. A cell-free translation-based small molecule screen was established to recreate the hypothesized interactions involving newly synthesized capsid proteins as host assembly machine substrates. Hits from the screen were validated by efficacy against infectious rabies virus in mammalian cell culture. Used as affinity ligands, advanced analogs were shown to bind a set of proteins that effectively reconstituted drug sensitivity in the cell-free screen and included a small but discrete subfraction of cellular ATP-binding cassette family E1 (ABCE1), a host protein previously found essential for HIV capsid formation. Taken together, these studies advance an alternate view of capsid formation (as a host-catalyzed biochemical pathway), a different paradigm for drug discovery (whole pathway screening without knowledge of the target), and suggest the existence of labile assembly machines that can be rendered accessible as next-generation drug targets by the means described.

Molecular characterization of KGH, the first human isolate of rabies virus in Korea

The complete genome sequence of the KGH strain of the first human rabies virus, which was isolated from a skin biopsy of a patient with rabies, whose symptoms developed due to bites from a raccoon dog in 2001. The size of the KGH strain genome was determined to be 11,928 nucleotides (nt) with a leader sequence of 58 nt, nucleoprotein gene of 1,353 nt, phosphoprotein gene of 894 nt, matrix protein gene of 609 nt, glycoprotein gene of 1,575 nt, RNA-dependent RNA polymerase gene of 6,384 nt, and trailer region of 69 nt. Sequence similarity was compared with 39 fully sequenced rabies virus genomes currently available, and the result showed 70.6-91.6 % at the nucleotide level, and 82.8-97.9 % at the amino acid level. The deduced amino acids in the viral protein were compared with those of other rabies viruses, and various functional regions were investigated. As a result, we found that the KGH strain only had a unique amino acid substitution that was identified to be associated either with host immune response and pathogenicity in the N protein, or with a related region regulating STAT1 in the P protein, and related to pathogenicity in G protein. Based on phylogenetic analyses using the complete genome of 39 rabies viruses, the KGH strain was determined to be closely related with the NNV-RAB-H strain and transplant rabies virus serotype 1, which are Indian isolates, and was confirmed to belong to the Arctic-like 2 clade. The KGH strain was most closely related to the SKRRD0204HC and SKRRD0205HC strain when compared with Korean animal isolates, which was separated around the same time and place, and belonged to the Gangwon III subgroup.

RNA-binding domain in the nucleocapsid protein of gill-associated nidovirus of penaeid shrimp

Gill-associated virus (GAV) infects Penaeus monodon shrimp and is the type species okavirus in the Roniviridae, the only invertebrate nidoviruses known currently. Electrophoretic mobility shift assays (EMSAs) using His₆-tagged full-length and truncated proteins were employed to examine the nucleic acid binding properties of the GAV nucleocapsid (N) protein in vitro. The EMSAs showed full-length N protein to bind to all synthetic single-stranded (ss)RNAs tested independent of their sequence. The ssRNAs included (+) and (−) sense regions of the GAV genome as well as a (+) sense region of the M RNA segment of Mourilyan virus, a crustacean bunya-like virus. GAV N protein also bound to double-stranded (ds)RNAs prepared to GAV ORF1b gene regions and to bacteriophage M13 genomic ssDNA. EMSAs using the five N protein constructs with variable-length N-terminal and/or C-terminal truncations localized the RNA binding domain to a 50 amino acid (aa) N-terminal sequence spanning Met¹¹ to Arg⁶⁰. Similarly to other RNA binding proteins, the first 16 aa portion of this sequence was proline/arginine rich. To examine this domain in more detail, the 18 aa peptide (M¹¹PVRRPLPPQPPRNARLI²⁹) encompassing this sequence was synthesized and found to bind nucleic acids similarly to the full-length N protein in EMSAs. The data indicate a fundamental role for the GAV N protein proline/arginine-rich domain in nucleating genomic ssRNA to form nucleocapsids. Moreover, as the synthetic peptide formed higher-order complexes in the presence of RNA, the domain might also play some role in protein/protein interactions stabilizing the helical structure of GAV nucleocapsids.

The cell biology of rabies virus: using stealth to reach the brain

Rabies virus, the prototypical neurotropic virus, causes one of the most lethal zoonotic diseases. According to official estimates, over 55,000 people die of the disease annually, but this is probably a severe underestimation. A combination of virulence factors enables the virus to enter neurons at peripheral sites and travel through the spinal cord to the brain of the infected host, where it often induces aggression that facilitates the transfer of the virus to a new host. This Review summarizes the current knowledge of the replication cycle of rabies virus and virus- host cell interactions, both of which are fundamental elements in our quest to understand the life cycle of rabies virus and the pathogenesis of rabies.

Structure of the nucleoprotein binding domain of Mokola virus phosphoprotein

Mokola virus (MOKV) is a nonsegmented, negative-sense RNA virus that belongs to the Lyssavirus genus and Rhabdoviridae family. MOKV phosphoprotein P is an essential component of the replication and transcription complex and acts as a cofactor for the viral RNA-dependent RNA polymerase. P recruits the viral polymerase to the nucleoprotein-bound viral RNA (N-RNA) via an interaction between its C-terminal domain and the N-RNA complex. Here we present a structure for this domain of MOKV P, obtained by expression of full-length P in Escherichia coli, which was subsequently truncated during crystallization. The structure has a high degree of homology with P of rabies virus, another member of Lyssavirus genus, and to a lesser degree with P of vesicular stomatitis virus (VSV), a member of the related Vesiculovirus genus. In addition, analysis of the crystal packing of this domain reveals a potential binding site for the nucleoprotein N. Using both site-directed mutagenesis and yeast two-hybrid experiments to measure P-N interaction, we have determined the relative roles of key amino acids involved in this interaction to map the region of P that binds N. This analysis also reveals a structural relationship between the N-RNA binding domain of the P proteins of the Rhabdoviridae and the Paramyxoviridae.

Peptides that mimic the amino-terminal end of the rabies virus phosphoprotein have antiviral activity

We wanted to develop a therapeutic approach against rabies disease by targeting the lyssavirus transcription/replication complex. Because this complex (nucleoprotein N-RNA template processed by the L polymerase and its cofactor, the phosphoprotein P) is similar to that of other negative-strand RNA viruses, we aimed to design broad-spectrum antiviral drugs that could be used as a complement to postexposure vaccination and immunotherapy. Recent progress in understanding the structure/function of the rabies virus P, N, and L proteins predicts that the amino-terminal end of P is an excellent target for destabilizing the replication complex because it interacts with both L (for positioning onto the N-RNA template) and N (for keeping N soluble, as needed for viral RNA encapsidation). Thus, peptides mimicking various lengths of the amino-terminal end of P have been evaluated, as follows: (i) for binding properties to the N-P-L partners by the two-hybrid method; (ii) for their capacity to inhibit the transcription/replication of a rabies virus minigenome encoding luciferase in BHK-21-T7 cells; and (iii) for their capacity to inhibit rabies virus infection of BHK-21-T7 cells and of two derivatives of the neuronal SK-N-SH cell line. Peptides P60 and P57 (the first 60 and first 57 NH2 residues of P, respectively) exhibited a rapid, strong, and long-lasting inhibitory potential on luciferase expression (>95% from 24 h to 55 h). P42 was less efficient in its inhibition level (75% for 18 to 30 h) and duration (40% after 48 h). The most promising peptides were synthesized in tandem with the Tat sequence, allowing cell penetration. Their inhibitory effects were observed on BHK-21-T7 cells infected with rabies virus and Lagos bat virus but not with vesicular stomatitis virus. In neuronal cells, a significant inhibition of both nucleocapsid inclusions and rabies virus release was observed.

Molecular characterization of the complete genome of a street rabies virus isolated in China

In this study, the complete genomic sequence of a rabies virus isolate HN10, recovered from brain tissue of a rabid patient in China, was determined. This is the first Chinese street isolate that has been fully characterized. The overall organization of this virus is typical of that observed for all other rabies viruses. Alignments of amino acid sequences of the phosphoprotein, glycoprotein and large protein of HN10 with those of other rabies viruses were used to examine the extent of conservation of known functional regions. Phylogenetic analysis using either the complete or partial genomic sequence of HN10 determined that this isolate is most closely associated with viruses previously shown to circulate in Guangxi and Hunan provinces. In addition, of all vaccine strains used for comparison, the attenuated Chinese vaccine strain CTN181 is most closely related to HN10.

Severe acute respiratory syndrome coronavirus nsp9 dimerization is essential for efficient viral growth

The severe acute respiratory syndrome coronavirus (SARS-CoV) devotes a significant portion of its genome to producing nonstructural proteins required for viral replication. SARS-CoV nonstructural protein 9 (nsp9) was identified as an essential protein with RNA/DNA-binding activity, and yet its biological function within the replication complex remains unknown. Nsp9 forms a dimer through the interaction of parallel alpha-helices containing the protein-protein interaction motif GXXXG. In order to study the role of the nsp9 dimer in viral reproduction, residues G100 and G104 at the helix interface were targeted for mutation. Multi-angle light scattering measurements indicated that G100E, G104E, and G104V mutants are monomeric in solution, thereby disrupting the dimer. However, electrophoretic mobility assays revealed that the mutants bound RNA with similar affinity. Further experiments using fluorescence anisotropy showed a 10-fold reduction in RNA binding in the G100E and G104E mutants, whereas the G104V mutant had only a 4-fold reduction. The structure of G104E nsp9 was determined to 2.6-A resolution, revealing significant changes at the dimer interface. The nsp9 mutations were introduced into SARS-CoV using a reverse genetics approach, and the G100E and G104E mutations were found to be lethal to the virus. The G104V mutant produced highly debilitated virus and eventually reverted back to the wild-type protein sequence through a codon transversion. Together, these data indicate that dimerization of SARS-CoV nsp9 at the GXXXG motif is not critical for RNA binding but is necessary for viral replication.

Are all lyssavirus genes equal for phylogenetic analyses?

Individual lyssavirus genes were evaluated for phylogenetic studies from available full genome sequences. The full genome of the ERA rabies virus was sequenced and its accuracy was confirmed through virus recovery by reverse genetics. The full length of the ERA is 11,931 nucleotides (nt), with a leader sequence of 58 nt, the nucleoprotein (N) gene of 1350 nt, phosphoprotein (P) gene of 891 nt, matrix protein (M) gene of 606 nt, glycoprotein (G) gene of 1572 nt, RNA-dependent RNA polymerase (L) gene of 6384 nt, Psi-region (or G-L intergenic region) of 400 nt, and a trailer region of 70 nt. The five mono-cistrons are separated by intergenic regions of 2, 5, 5 and 24 nt, respectively. One obvious difference between the ERA and SAD-B19 rabies virus strains was the putative stop/polyadenylation signal of the G gene, with a poly(A(8)) tract for ERA, and a poly(A(5)) for SAD-B19. The TGpoly(A(8)) sequence tract was identified to be a leaky termination signal in the ERA strain. Through analyses of nt diversity, protein co-variations, structural and functional constraints, and reconstruction of phylogenetic trees from comprehensive datasets, we propose lyssavirus genes probably are of similar value for phylogenetic analyses.

Ebola virus VP35-VP40 interaction is sufficient for packaging 3E-5E minigenome RNA into virus-like particles

The packaging of viral genomic RNA into nucleocapsids and subsequently into virions is not completely understood. Phosphoprotein (P) and nucleoprotein (NP) interactions link NP-RNA complexes with P-L (polymerase) complexes to form viral nucleocapsids. The nucleocapsid then interacts with the viral matrix protein, leading to specific packaging of the nucleocapsid into the virion. A mammalian two-hybrid assay and confocal microscopy were used to demonstrate that Ebola virus VP35 and VP40 interact and colocalize in transfected cells. VP35 was packaged into budding virus-like particles (VLPs) as observed by protease protection assays. Moreover, VP40 and VP35 were sufficient for packaging an Ebola virus minigenome RNA into VLPs. Results from immunoprecipitation-reverse transcriptase PCR experiments suggest that VP35 confers specificity of the nucleocapsid for viral genomic RNA by direct VP35-RNA interactions.

Structural difference recognized by a monoclonal antibody #404-11 between the rabies virus nucleocapsid (NC) produced in virus infected cells and the NC-like structures produced in the nucleoprotein (N) cDNA-transfected cells

We investigated structural changes in the rabies virus (HEP-Flury strain) nucleocapsid (NC) during the virus replication, for which we used two anti-nucleoprotein (N) monoclonal antibodies (mAbs), #404-11 (specific for a conformation-dependently exposed linear epitope) and #1-7-11 (specific for a conformational epitope which is exposed after the nucleocapsid formation). Both mAbs recognized the N protein of the viral NC, but not of the RNA-free N-P complex. The 1-7-11 and 404-11 epitopes could be mapped to the N-terminal and the C-terminal regions of N protein, respectively. Immunoprecipitation studies demonstrated that treatment of the NC either with the alkaline phosphatase or sodium deoxycholate (DOC) resulted in dissociation of most P proteins from the NC and in the reduced reactivity to mAb #404-11, but not to mAb #1-7-11. NC-like structures produced in the N cDNA-transfected cells displayed strong reactivity to mAb #1-7-11; however, reactivity to mAb #404-11 was very weak. And, coexpression with viral phosphoprotein (P) resulted in little increase in reactivity to mAb #404-11 of the NC-like structures, while the reactivity was significantly increased by cotransfection with P and the viral minigenome whose 3'- and 5'-end structures were derived from the viral genome. From these results, we assume that, although the 404-11 epitope is a linear one, the epitope-containing region is exposed only when N proteins encapsidate properly the viral RNA in collaboration with the P protein. Further, exposure of the 404-11 epitope region might be function-related, and be regulated by association and dissociation of the P protein.