The C protein of wild-type measles virus has the ability to shuttle between the nucleus and the cytoplasm

Discovery of plant-based phytochemical as effective antivirals that target the non-structural protein C of the Nipah virus through computational methods

Nipah Virus (NiV) belongs to the Paramyxoviridae family and was first identified during an outbreak in Malaysia. Some initial symptoms include mild fever, headache and sore throat, which could escalate to respiratory illness and brain inflammation. The mortality rate of NiV infection can range from 40% to 75%, which is quite high. This is mainly due to the lack of efficient drugs and vaccines. In most instances, NiV is transmitted from animals to humans. Non-Structural Proteins (C, V and W) of the Nipah virus impede the host immune response by obstructive the JAK/STAT pathway. However, Non-Structural Proteins - C (NSP-C) plays a vital role in NiV pathogenesis, which includes IFN antagonist activity and viral RNA production. In the present study, the full-length structure of NiV-NSP-C was predicted using computational modelling, and the stability of the structure was analysed using 200 ns molecular dynamic (MD) simulation. Further, the structure-based virtual screening identified five potent phytochemicals (PubChem CID: 9896047, 5885, 117678, 14887603 and 5461026) with better binding affinity against NiV-NSP-C. DFT studies clearly showed that the phytochemicals had higher chemical reactivity, and the complex MD simulation depicted that the identified inhibitors exhibited stable binding with NiV-NSP-C. Furthermore, experimental validation of these identified phytochemicals would likely control the infection of NiV.Communicated by Ramaswamy H. Sarma.

C Proteins: Controllers of Orderly Paramyxovirus Replication and of the Innate Immune Response

Particles of many paramyxoviruses include small amounts of proteins with a molecular weight of about 20 kDa. These proteins, termed “C”, are basic, have low amino acid homology and some secondary structure conservation. C proteins are encoded in alternative reading frames of the phosphoprotein gene. Some viruses express nested sets of C proteins that exert their functions in different locations: In the nucleus, they interfere with cellular transcription factors that elicit innate immune responses; in the cytoplasm, they associate with viral ribonucleocapsids and control polymerase processivity and orderly replication, thereby minimizing the activation of innate immunity. In addition, certain C proteins can directly bind to, and interfere with the function of, several cytoplasmic proteins required for interferon induction, interferon signaling and inflammation. Some C proteins are also required for efficient virus particle assembly and budding. C-deficient viruses can be grown in certain transformed cell lines but are not pathogenic in natural hosts. C proteins affect the same host functions as other phosphoprotein gene-encoded proteins named V but use different strategies for this purpose. Multiple independent systems to counteract host defenses may ensure efficient immune evasion and facilitate virus adaptation to new hosts and tissue environments.

Proteomic Analysis Uncovers Measles Virus Protein C Interaction With p65-iASPP Protein Complex

Viruses manipulate the central machineries of host cells to their advantage. They prevent host cell antiviral responses to create a favorable environment for their survival and propagation. Measles virus (MV) encodes two nonstructural proteins MV-V and MV-C known to counteract the host interferon response and to regulate cell death pathways. Several molecular mechanisms underlining MV-V regulation of innate immunity and cell death pathways have been proposed, whereas MV-C host-interacting proteins are less studied. We suggest that some cellular factors that are controlled by MV-C protein during viral replication could be components of innate immunity and the cell death pathways. To determine which host factors are targeted by MV-C, we captured both direct and indirect host-interacting proteins of MV-C protein. For this, we used a strategy based on recombinant viruses expressing tagged viral proteins followed by affinity purification and a bottom-up mass spectrometry analysis. From the list of host proteins specifically interacting with MV-C protein in different cell lines, we selected the host targets that belong to immunity and cell death pathways for further validation. Direct protein interaction partners of MV-C were determined by applying protein complementation assay and the bioluminescence resonance energy transfer approach. As a result, we found that MV-C protein specifically interacts with p65–iASPP protein complex that controls both cell death and innate immunity pathways and evaluated the significance of these host factors on virus replication.

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Measles virus controls immune response and cell death pathways to achieve replication.

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Host proteins interaction network with measles virulence factor C protein.

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Cellular p65–iASPP complex is targeted by measles virus C protein.

Genomic characterization, phylogenetic position and in situ localization of a novel putative mononegavirus in Lepeophtheirus salmonis

The complete genome sequence of a novel mononegavirus, Lepeophtheirus salmonis negative-stranded RNA virus 1 (LsNSRV-1), obtained from a salmonid ectoparasite, Lepeophtheirus salmonis was determined. The viral genome contains five open reading frames encoding three unknown proteins (ORF I, II and III), a putative glycoprotein (G), and a large (L) protein. Phylogenetic analysis placed LsNSRV-1 in the recently established mononegaviral family Artoviridae. LsNSRV-1 showed a prevalence of around 97% and was detected in all L. salmonis developmental stages. Viral genomic and antigenomic RNA was localized to nerve tissue, connective tissue, epithelial cells of the gut, subepidermal tissue, exocrine and cement glands, as well as the testis, vas deferens and spermatophore sac of male L. salmonis and the ovaries and oocytes of females. Viral RNA was detected in both the cytoplasm and the nucleoli of infected cells, and putative nuclear export and localization signals were found within the ORF I, III and L proteins, suggesting nuclear replication of LsNSRV-1. RNA interference (RNAi) was induced twice during development by the introduction of a double-stranded RNA fragment of ORF I, resulting in a transient knockdown of viral RNA. A large variation in the knockdown level was seen in adult males and off springs of knockdown animals, whereas the RNA level was more stable in adult females. Together with the localization of viral RNA within the male spermatophore and female oocytes and the amplification of viral RNA in developing embryos, this suggests that LsNSRV-1 is transmitted both maternally and paternally. Small amounts of viral RNA were detected at the site where chalimi were attached to the skin of Atlantic salmon (Salmo salar). However, as the RNAi-mediated treatment did not result in LsNSRV-1-negative offspring and the virus failed to replicate in the tested fish cell cultures, it is difficult to investigate the influence of secreted LsNSRV-1 on the salmon immune response.

Roles of nuclear trafficking in infection by cytoplasmic negative-strand RNA viruses: paramyxoviruses and beyond

Genome replication and virion production by most negative-sense RNA viruses (NSVs) occurs exclusively in the cytoplasm, but many NSV-expressed proteins undergo active nucleocytoplasmic trafficking via signals that exploit cellular nuclear transport pathways. Nuclear trafficking has been reported both for NSV accessory proteins (including isoforms of the rabies virus phosphoprotein, and V, W and C proteins of paramyxoviruses) and for structural proteins. Trafficking of the former is thought to enable accessory functions in viral modulation of antiviral responses including the type I IFN system, but the intranuclear roles of structural proteins such as nucleocapsid and matrix proteins, which have critical roles in extranuclear replication and viral assembly, are less clear. Nevertheless, nuclear trafficking of matrix protein has been reported to be critical for efficient production of Nipah virus and Respiratory syncytial virus, and nuclear localization of nucleocapsid protein of several morbilliviruses has been linked to mechanisms of immune evasion. Together, these data point to the nucleus as a significant host interface for viral proteins during infection by NSVs with otherwise cytoplasmic life cycles. Importantly, several lines of evidence now suggest that nuclear trafficking of these proteins may be critical to pathogenesis and thus could provide new targets for vaccine development and antiviral therapies.

Region of Nipah virus C protein responsible for shuttling between the cytoplasm and nucleus

Nipah virus (NiV) causes severe encephalitis in humans, with high mortality. NiV nonstructural C protein (NiV-C) is essential for its pathogenicity, but its functions are unclear. In this study, we focused on NiV-C trafficking in cells and found that it localizes predominantly in the cytoplasm but partly in the nucleus. An analysis of NiV-C mutants showed that amino acids 2, 21-24 and 110-139 of NiV-C are important for its localization in the cytoplasm. Inhibitor treatment indicates that the nuclear export determinant is not a classical CRM1-dependent nuclear export signal. We also determined that amino acids 60-75 and 72-75 were important for nuclear localization of NiV-C. Furthermore, NiV-C mutants that had lost their capacity for nuclear localization inhibited the interferon (IFN) response more strongly than complete NiV-C. These results indicate that the IFN-antagonist activity of NiV-C occurs in the cytoplasm.

Relevance of the Measles Virus Expression in Cancer - an Update

Evidence of an association between classical Hodgkin lymphoma and the measles virus has previously been presented by our group. Arguments held against our thesis were reevaluated. Substantiation of a relationship between the measles virus and additional solid tumors was submitted. Moreover, a pathogenic pathway was suggested to support a possible contribution of the measles virus to the development of classical Hodgkin lymphoma. We have chosen to exclude a discussion of measles virotherapy, since this carries distinct implications. We now add new evidence regarding the expression of the measles virus phosphoprotein in a few cancers. We also suggest a role in this context for atypical measles syndrome in malignant tumors. Last, we propose a collaboration which may make the best, on the one hand of our cohort of classical Hodgkin lymphoma, half of which carry the measles virus expression in their tumor cells. The planned study will also look into the patients vaccination records and into a previous history of the measles disease. On the other hand, cohorts of patients diagnosed with late onset measles will be assessed for the eventual diagnosis of atypical measles syndrome and will be followed up for the subsequent development of a malignant tumor.

Measles virus C protein facilitates transcription by the control of N protein-viral genomic RNA interaction in early phases of infection

Measles virus (MV) C protein has been known a multifunctional protein involved in anti-IFN response, viral RNA synthesis, and so on. Recent studies have clarified that double-stranded RNA (dsRNA) is derived from viral genomic RNA (vRNA), and the amount of dsRNA was increased in an MV lacking C protein (MV(C-))-infected cells, suggesting that C protein blocks viral RNA synthesis. However, detailed roles of C protein in viral RNA synthesis remain unknown. Here, we have confirmed through time course experiments using Vero/hSLAM cells that as reported previously, the amount of mRNA is increased in MV(C-)-infected cells at 36 h post infection (hpi). In contrast, we found that the transcription level is lower in MV(C-)-infected cells than wild-type MV-infected cells in early phases of infection. Immunoprecipitation assays were performed to find an interactor(s) of C protein, revealed that C protein interacts with N protein in the absence of vRNA and P protein. RNA immunoprecipitation (RIP) assays showed that the interaction between N protein and vRNA was increased in MV(C-)-infected cells. These results suggest that in early phases of infection C protein facilitates viral transcription to control the formation of nascent nucleocapsid composed of N protein and vRNA.

Evolution and structural organization of the C proteins of paramyxovirinae

The phosphoprotein (P) gene of most Paramyxovirinae encodes several proteins in overlapping frames: P and V, which share a common N-terminus (PNT), and C, which overlaps PNT. Overlapping genes are of particular interest because they encode proteins originated de novo, some of which have unknown structural folds, challenging the notion that nature utilizes only a limited, well-mapped area of fold space. The C proteins cluster in three groups, comprising measles, Nipah, and Sendai virus. We predicted that all C proteins have a similar organization: a variable, disordered N-terminus and a conserved, α-helical C-terminus. We confirmed this predicted organization by biophysically characterizing recombinant C proteins from Tupaia paramyxovirus (measles group) and human parainfluenza virus 1 (Sendai group). We also found that the C of the measles and Nipah groups have statistically significant sequence similarity, indicating a common origin. Although the C of the Sendai group lack sequence similarity with them, we speculate that they also have a common origin, given their similar genomic location and structural organization. Since C is dispensable for viral replication, unlike PNT, we hypothesize that C may have originated de novo by overprinting PNT in the ancestor of Paramyxovirinae. Intriguingly, in measles virus and Nipah virus, PNT encodes STAT1-binding sites that overlap different regions of the C-terminus of C, indicating they have probably originated independently. This arrangement, in which the same genetic region encodes simultaneously a crucial functional motif (a STAT1-binding site) and a highly constrained region (the C-terminus of C), seems paradoxical, since it should severely reduce the ability of the virus to adapt. The fact that it originated twice suggests that it must be balanced by an evolutionary advantage, perhaps from reducing the size of the genetic region vulnerable to mutations.

Measles virus C protein interferes with Beta interferon transcription in the nucleus

Transcriptional induction of beta interferon (IFN-β) through pattern recognition receptors is a key event in the host defense against invading viruses. Infection of cells by paramyxoviruses, like measles virus (MV) (genus Morbillivirus), is sensed predominantly by the ubiquitous cytoplasmic helicase RIG-I, recognizing viral 5'-triphosphate RNAs, and to some degree by MDA5. While MDA5 activation is effectively prevented by the MV V protein, the viral mechanisms for inhibition of MDA5-independent induction of IFN-β remained obscure. Here, we identify the 186-amino-acid MV C protein, which shuttles between the nucleus and the cytoplasm, as a major viral inhibitor of IFN-β transcription in human cells. Activation of the transcription factor IRF3 by upstream kinases and nuclear import of activated IRF3 were not affected in the presence of C protein, suggesting a nuclear target. Notably, C proteins of wild-type MV isolates, which are poor IFN-β inducers, were found to comprise a canonical nuclear localization signal (NLS), whereas the NLSs of all vaccine strains, irrespective of their origins, were mutated. Site-directed mutagenesis of the C proteins from an MV wild-type isolate and from the vaccine virus strain Schwarz confirmed a correlation of nuclear localization and inhibition of IFN-β transcription. A functional NLS and efficient nuclear accumulation are therefore critical for MV C to retain its potential to downregulate IFN-β induction. We suggest that a defect in efficient nuclear import of C protein contributes to attenuation of MV vaccine strains.

Interplay between innate immunity and negative-strand RNA viruses: towards a rational model

The discovery of a new class of cytosolic receptors recognizing viral RNA, called the RIG-like receptors (RLRs), has revolutionized our understanding of the interplay between viruses and host cells. A tremendous amount of work has been accumulating to decipher the RNA moieties required for an RLR agonist, the signal transduction pathway leading to activation of the innate immunity orchestrated by type I interferon (IFN), the cellular and viral regulators of this pathway, and the viral inhibitors of the innate immune response. Previous reviews have focused on the RLR signaling pathway and on the negative regulation of the interferon response by viral proteins. The focus of this review is to put this knowledge in the context of the virus replication cycle within a cell. Likewise, there has been an expansion of knowledge about the role of innate immunity in the pathophysiology of viral infection. As a consequence, some discrepancies have arisen between the current models of cell-intrinsic innate immunity and current knowledge of virus biology. This holds particularly true for the nonsegmented negative-strand viruses (Mononegavirales), which paradoxically have been largely used to build presently available models. The aim of this review is to bridge the gap between the virology and innate immunity to favor the rational building of a relevant model(s) describing the interplay between Mononegavirales and the innate immune system.

RNA splicing in a new rhabdovirus from Culex mosquitoes

Among members of the order Mononegavirales, RNA splicing events have been found only in the family Bornaviridae. Here, we report that a new rhabdovirus isolated from the mosquito Culex tritaeniorhynchus replicates in the nuclei of infected cells and requires RNA splicing for viral mRNA maturation. The virus, designated Culex tritaeniorhynchus rhabdovirus (CTRV), shares a similar genome organization with other rhabdoviruses, except for the presence of a putative intron in the coding region for the L protein. Molecular phylogenetic studies indicated that CTRV belongs to the family Rhabdoviridae, but it is yet to be assigned a genus. Electron microscopic analysis revealed that the CTRV virion is extremely elongated, unlike virions of rhabdoviruses, which are generally bullet shaped. Northern hybridization confirmed that a large transcript (approximately 6,500 nucleotides [nt]) from the CTRV L gene was present in the infected cells. Strand-specific reverse transcription-PCR (RT-PCR) analyses identified the intron-exon boundaries and the 76-nt intron sequence, which contains the typical motif for eukaryotic spliceosomal intron-splice donor/acceptor sites (GU-AG), a predicted branch point, and a polypyrimidine tract. In situ hybridization exhibited that viral RNAs are primarily localized in the nucleus of infected cells, indicating that CTRV replicates in the nucleus and is allowed to utilize the host's nuclear splicing machinery. This is the first report of RNA splicing among the members of the family Rhabdoviridae.

Measles virus C protein suppresses gamma-activated factor formation and virus-induced cell growth arrest

Measles virus (MeV) produces two accessory proteins, V and C, from the P gene. These accessory proteins have been reported to contribute to efficient virus proliferation through the modulation of host cell events. Our previous paper described that Vero cell-adapted strains of MeV led host cells to growth arrest through the upregulation of interferon regulatory factor 1 (IRF-1), and wild strains did not. In the present study, we found that C protein expression levels varied among MeV strains in infected SiHa cells. C protein levels were inversely correlated with IRF-1 expression levels and with cell growth arrest. Forced expression of C protein released cells from growth arrest. C-deficient recombinant virus efficiently upregulated IRF-1 and caused growth arrest more efficiently than the wild-type virus. C protein preferentially bound to phosphorylated STAT1 and suppressed STAT1 dimer formation. We conclude that MeV C protein suppresses IFN-γ signaling pathway via inhibition of phosphorylated STAT1 dimerization.

pp32, an INHAT component, is a transcription machinery recruiter for maximal induction of IFN-stimulated genes

Type I interferon (IFN) plays a crucial role in establishing the cellular antiviral state by inducing transcription of IFN-stimulated genes (ISGs). Generally, histone acetyltransferases (HATs) are positive regulators of transcription, but histone deacetylase (HDAC) activity is essential for transcriptional induction of ISGs. pp32 is known to be a key component of the inhibitor of acetyltransferase (INHAT) complex that inhibits HAT-dependent transcriptional activation. Here, we show that pp32 is involved in the positive regulation of ISG transcription. pp32 interacted with signal transducer and activator of transcription 1 (STAT1) and STAT2 in an IFN-dependent manner. pp32 was not required for tyrosine phosphorylation and nuclear translocation of STATs, but was needed for binding of transcriptional complexes with ISG promoters and, thereby, for maximal transcription activation. pp32 was found to be associated with ISG promoters in IFN-untreated cells, and its binding amount fluctuated as a function of time after IFN treatment. short interfering RNA (siRNA)-mediated knockdown of pp32 expression reduced the histone acetylation level on ISG promoters, suggesting that pp32 plays a role in ISG transcription by a function other than that of INHAT. Taking these findings together, we propose that pp32 is involved in the formation of ISG transcription initiation complexes, possibly as their recruiter.

Role of interferon antagonist activity of rabies virus phosphoprotein in viral pathogenicity

The fixed rabies virus (RV) strain Nishigahara kills adult mice after intracerebral inoculation, whereas the chicken embryo fibroblast cell-adapted strain Ni-CE causes nonlethal infection in adult mice. We previously reported that the chimeric CE(NiP) strain, which has the phosphoprotein (P protein) gene from the Nishigahara strain in the genetic background of the Ni-CE strain, causes lethal infection in adult mice, indicating that the P gene is responsible for the different pathogenicities of the Nishigahara and Ni-CE strains. Previous studies demonstrated that RV P protein binds to the interferon (IFN)-activated transcription factor STAT1 and blocks IFN signaling by preventing its translocation to the nucleus. In this study, we examine the molecular mechanism by which RV P protein determines viral pathogenicity by comparing the IFN antagonist activities of the Nishigahara and Ni-CE P proteins. The results, obtained from both RV-infected cells and cells transfected to express P protein only, show that Ni-CE P protein is significantly impaired for its capacity to block IFN-activated STAT1 nuclear translocation and, consequently, inhibits IFN signaling less efficiently than Nishigahara P protein. Further, it was demonstrated that a defect in the nuclear export of Ni-CE P protein correlates with a defect in its ability to cause the mislocalization of STAT1. These data provide the first evidence that the capacity of the RV P protein to inhibit STAT1 nuclear translocation and IFN signaling correlates with the viral pathogenicity.

Mechanisms of protein kinase PKR-mediated amplification of beta interferon induction by C protein-deficient measles virus

The measles virus P gene products V and C antagonize the host interferon (IFN) response, blocking both IFN signaling and production. Using Moraten vaccine strain-derived measles virus and isogenic mutants deficient for either V or C protein production (V(ko) and C(ko), respectively), we observed that the C(ko) virus was a potent inducer of IFN-beta, while induction by V(ko) virus was an order of magnitude lower than that by the C(ko) virus. The parental recombinant Moraten virus did not significantly induce IFN-beta. The enhanced IFN-inducing capacity of the C(ko) virus correlated with an enhanced activation of IFN regulatory factor 3 (IRF-3), NF-kappaB, and ATF-2 in C(ko)-infected compared to V(ko) or parental virus-infected cells. Furthermore, protein kinase PKR and mitochondrial adapter IPS-1 were required for maximal C(ko)-mediated IFN-beta induction, which correlated with the PKR-mediated enhancement of mitogen-activated protein kinase and NF-kappaB activation. Our results reveal multiple consequences of C protein expression and document an important function for PKR as an enhancer of IFN-beta induction during measles virus infection.

A highly attenuated measles virus vaccine strain encodes a fully functional C protein

The P, V, and C proteins of measles virus are encoded in overlapping reading frames of the P gene, which makes it difficult to analyze the functions of the individual proteins in the context of virus infection. We established a system to analyze the C protein independently from the P and V proteins by placing its gene in an additional transcription unit between the H and L genes. Analyses with this system indicated that a highly attenuated Edmonston lineage vaccine strain encodes a fully functional C protein, and the P and/or V protein is involved in the attenuated phenotype.

Dual modes of rabies P-protein association with microtubules: a novel strategy to suppress the antiviral response

Conventional nuclear import is independent of the cytoskeleton, but recent data have shown that the import of specific proteins can be either facilitated or inhibited by microtubules (MTs). Nuclear import of the P-protein from rabies virus involves a MT-facilitated mechanism, but here, we show that P-protein is unique in that it also undergoes MT-inhibited import, with the mode of MT-interaction being regulated by the oligomeric state of the P-protein. This is the first demonstration that a protein can utilise both MT-inhibited and MT-facilitated import mechanisms, and can switch between these different modes of MT interaction to regulate its nuclear trafficking. Importantly, we show that the P-protein exploits MT-dependent mechanisms to manipulate host cell processes by switching the import of the interferon-activated transcription factor STAT1 from a conventional to a MT-inhibited mechanism. This prevents STAT1 nuclear import and signalling in response to interferon, which is vital to the host innate antiviral response. This is the first report of MT involvement in the viral subversion of interferon signalling that is central to virus pathogenicity, and identifies novel targets for the development of antiviral drugs or attenuated viruses for vaccine applications.

The rinderpest virus non-structural C protein blocks the induction of type 1 interferon

The innate immune response, in particular the production of type 1 interferons, is an essential part of the mammalian host response to viral infection. We have previously shown that rinderpest virus, a morbillivirus closely related to the human pathogen measles virus, blocks the actions of type 1 and type 2 interferons. We show here that this virus can also block the induction of type 1 interferon. The viral non-structural C protein appears to be the active agent, since expressing this protein in cells makes them resistant to activation of the interferon-beta promoter while recombinant virus that does not express the C protein activates this promoter much more than virus expressing the C protein. In addition, differences in activation of the interferon-beta promoter by different strains of rinderpest virus are reflected in differing abilities of their respective C proteins to block activation of the promoter by dsRNA. The C protein blocks the activation of this promoter induced by either cytoplasmic dsRNA or by Newcastle disease virus (NDV) infection, as well as activation induced by overexpression of several elements of the signalling pathway, including mda-5, RIG-I and IRF-3. The RPV C protein also blocks transcription from promoters responsive individually to the three transcription factors that make up the interferon-beta promoter enhanceosome, although it does not appear to block the activation of IRF-3.

Measles virus interaction with host cells and impact on innate immunity

Because viruses are obligate parasites, numerous partnerships between measles virus and cellular molecules can be expected. At the entry level, measles virus uses at least two cellular receptors, CD150 and a yet to be identified epithelial receptor to which the virus H protein binds. This dual receptor strategy illuminates the natural infection and inter-human propagation of this lymphotropic virus. The attenuated vaccine strains use CD46 as an additional receptor, which results in a tropism alteration. Surprisingly, the intracellular viral and cellular protein partnership leading to optimal virus life cycle remains mostly a black box, while the interactions between viral proteins that sustain the RNA-dependant RNA polymerase activity (i.e., transcription and replication), the particle assembly and the polarised virus budding are documented. Hsp72 is the only cellular protein that is known to regulate the virus transcription and replication through its interaction with the viral N protein. The viral P protein is phosphorylated by the casein kinase II with undetermined functional consequences. The cellular partnership that controls the intracellular trafficking of viral components, the assembly and/or the budding of measles virus, remains unknown. The virus to cell innate immunity war is better documented. The 5' triphosphate-ended virus leader transcript is recognised by RIG-I, a cellular helicase, and induces the interferon response. Measles virus V protein binds to the MDAS helicase and prevents the MDA5-mediated activation of interferon. By interacting with STAT1 and Jak1, the viral P and V proteins prevent the type I interferon receptor (IFNAR) signalling. The virus N protein interacts with eIF3-p40 to inhibit the translation of cellular mRNA. The H protein binds to TLR2, which then transduces an activation signal and CD150 expression in monocytes. The P protein activates the expression of the ubiquitin modifier A20, thus blocking the TLR4-mediated signalling. Few other partnerships between measles virus components and cellular proteins have been postulated or demonstrated, and they need further investigations to understand their physiopathological outcome.

The measles virus replication cycle

This review describes the two interrelated and interdependent processes of transcription and replication for measles virus. First, we concentrate on the ribonucleoprotein (RNP) complex, which contains the negative sense genomic template and in encapsidated in every virion. Second, we examine the viral proteins involved in these processes, placing particular emphasis on their structure, conserved sequence motifs, their interaction partners and the domains which mediate these associations. Transcription is discussed in terms of sequence motifs in the template, editing, co-transcriptional modifications of the mRNAs and the phase of the gene start sites within the genome. Likewise, replication is considered in terms of promoter strength, copy numbers and the remarkable plasticity of the system. The review emphasises what is not known or known only by analogy rather than by direct experimental evidence in the MV replication cycle and hence where additional research, using reverse genetic systems, is needed to complete our understanding of the processes involved.

Expression of human parainfluenza virus type 3 PD protein and intracellular localization in virus infected cells

The P gene of human parainfluenza virus type 3 (HPIV 3) encodes a multicistronic P mRNA that gives rise to four polypeptides. The P and C proteins are synthesized from two discrete overlapping AUG codons from the unedited P mRNA, while synthesis of two additional proteins, V and PD, presumably occurs via a unique transcriptional editing mechanism. However, the presence of V and PD proteins in HPIV 3 infected cells and their role in viral replication remains uncertain. Here, in vitro expression of full-length PD protein from an altered P mRNA and generation of a polyclonal antibody to the COOH-terminus of PD was achieved. Confocal immunofluorescence analysis following Leptomycin B (LMB) treatment revealed the presence of PD protein in nuclear and cytoplasmic compartments of HPIV 3 infected cells suggesting the involvement of a nuclear localization signal in this process. These initial results provide new impetus for further characterization of the role of PD in HPIV 3 infection.

Measles virus circumvents the host interferon response by different actions of the C and V proteins

Measles is an acute febrile infectious disease with high morbidity and mortality. The genome of measles virus (MV), the causative agent, encodes two accessory products, V and C proteins, that play important roles in MV virulence. The V but not the C protein of the IC-B strain (a well-characterized virulent strain of MV) has been shown to block the Jak/Stat signaling pathway and counteract the cellular interferon (IFN) response. We have recently shown that a recombinant IC-B strain that lacks C protein expression replicates poorly in certain cell lines, and its growth defect is related to translational inhibition and strong IFN induction. Here, we show that the V protein of the MV IC-B strain also blocks the IFN induction pathway mediated by the melanoma differentiation-associated gene 5 product, thus actively interfering with the host IFN response at two different steps. On the other hand, the C protein per se possesses no activity to block the IFN induction pathway. Our data indicate that the C protein acts as a regulator of viral RNA synthesis, thereby acting indirectly to suppress IFN induction. Since recombinant MVs with C protein defective in modulating viral RNA synthesis or lacking C protein expression strongly stimulate IFN production, in spite of V protein production, both the C and V proteins must be required for MV to fully circumvent the host IFN response.