Rinderpest virus C and V proteins interact with the major (L) component of the viral polymerase

Type I and Type II Interferon Antagonism Strategies Used by Paramyxoviridae: Previous and New Discoveries, in Comparison

Paramyxoviridae is a viral family within the order of Mononegavirales; they are negative single-strand RNA viruses that can cause significant diseases in both humans and animals. In order to replicate, paramyxoviruses–as any other viruses–have to bypass an important protective mechanism developed by the host’s cells: the defensive line driven by interferon. Once the viruses are recognized, the cells start the production of type I and type III interferons, which leads to the activation of hundreds of genes, many of which encode proteins with the specific function to reduce viral replication. Type II interferon is produced by active immune cells through a different signaling pathway, and activates a diverse range of genes with the same objective to block viral replication. As a result of this selective pressure, viruses have evolved different strategies to avoid the defensive function of interferons. The strategies employed by the different viral species to fight the interferon system include a number of sophisticated mechanisms. Here we analyzed the current status of the various strategies used by paramyxoviruses to subvert type I, II, and III interferon responses.

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.

Peste des petits ruminants virus non-structural C protein inhibits the induction of interferon-β by potentially interacting with MAVS and RIG-I

Peste des petits ruminants virus (PPRV) causes an acute and highly contagious disease in domestic and wild small ruminants throughout the world, mainly by invoking immunosuppression in its natural hosts. It has been suggested that the non-structural C protein of PPRV helps in evading host responses but the molecular mechanisms by which it antagonizes the host responses have not been fully characterized. Here, we report the antagonistic effect of PPRV C protein on the expression of interferon-β (IFN-β) through both MAVS and RIG-I mediated pathways in vitro. Dual luciferase reporter assay and direct expression of IFN-β mRNA analysis indicated that PPRV C significantly down regulates IFN-β via its potential interaction with MAVS and RIG-I signaling molecules. Results further indicated that PPRV C protein significantly suppresses endogenous and exogenous IFN-β-induced anti-viral effects in PPRV, EMCV and SVS infections in vitro. Moreover, PPRV C protein not only down regulates IFN-β but also the downstream cytokines of interferon stimulated genes 56 (ISG56), ISG15, C-X-C motif chemokine (CXCL10) and RIG-I mediated activation of IFN promoter elements of ISRE and NF-κB. Further, this study deciphers that PPRV C protein could significantly inhibit the phosphorylation of STAT1 and interferes with the signal transmission in JAK-STAT signaling pathway. Collectively, this study indicates that PPRV C protein is important for innate immune evasion and disease progression.

Autophagy enhances the replication of Peste des petits ruminants virus and inhibits caspase-dependent apoptosis in vitro

Peste des petits ruminants (PPR) is an acute and highly contagious disease in small ruminants that causes significant economic losses in developing countries. An increasing number of studies have demonstrated that both autophagy and apoptosis are important cellular mechanisms for maintaining homeostasis, and they participate in the host response to pathogens. However, the crosstalk between apoptosis and autophagy in host cells during PPRV infection has not been clarified. In this study, autophagy was induced upon virus infection in caprine endometrial epithelial cells (EECs), as determined by the appearance of double- and single-membrane autophagy-like vesicles, LC3-I/LC3-II conversion, and p62 degradation. We also found that PPRV infection triggered a complete autophagic response, most likely mediated by the non-structural protein C and nucleoprotein N. Moreover, our results suggest that autophagy not only promotes the replication of PPRV in EECs but also provides a potential mechanism for inhibiting PPRV-induced apoptosis. Inhibiting autophagosome formation by wortmannin and knocking down the essential autophagic proteins Beclin-1 and ATG7 induces caspase-dependent apoptosis in EECs in PPRV infection. However, inhibiting autophagosome and lysosome fusion by NH₄Cl and chloroquine did not increase the number of apoptotic cells. Collectively, these data are the first to indicate that PPRV-induced autophagy inhibits caspase-dependent apoptosis and thus contributes to the enhancement of viral replication and maturity in host cells.

How order and disorder within paramyxoviral nucleoproteins and phosphoproteins orchestrate the molecular interplay of transcription and replication

In this review, we summarize computational and experimental data gathered so far showing that structural disorder is abundant within paramyxoviral nucleoproteins (N) and phosphoproteins (P). In particular, we focus on measles, Nipah, and Hendra viruses and highlight both commonalities and differences with respect to the closely related Sendai virus. The molecular mechanisms that control the disorder-to-order transition undergone by the intrinsically disordered C-terminal domain (NTAIL) of their N proteins upon binding to the C-terminal X domain (XD) of the homologous P proteins are described in detail. By having a significant residual disorder, NTAIL-XD complexes are illustrative examples of "fuzziness", whose possible functional significance is discussed. Finally, the relevance of N-P interactions as promising targets for innovative antiviral approaches is underscored, and the functional advantages of structural disorder for paramyxoviruses are pinpointed.

Control of the induction of type I interferon by Peste des petits ruminants virus

Peste des petits ruminants virus (PPRV) is a morbillivirus that produces clinical disease in goats and sheep. We have studied the induction of interferon-β (IFN-β) following infection of cultured cells with wild-type and vaccine strains of PPRV, and the effects of such infection with PPRV on the induction of IFN-β through both MDA-5 and RIG-I mediated pathways. Using both reporter assays and direct measurement of IFN-β mRNA, we have found that PPRV infection induces IFN-β only weakly and transiently, and the virus can actively block the induction of IFN-β. We have also generated mutant PPRV that lack expression of either of the viral accessory proteins (V&C) to characterize the role of these proteins in IFN-β induction during virus infection. Both PPRV_ΔV and PPRV_ΔC were defective in growth in cell culture, although in different ways. While the PPRV V protein bound to MDA-5 and, to a lesser extent, RIG-I, and over-expression of the V protein inhibited both IFN-β induction pathways, PPRV lacking V protein expression can still block IFN-β induction. In contrast, PPRV C bound to neither MDA-5 nor RIG-I, but PPRV lacking C protein expression lost the ability to block both MDA-5 and RIG-I mediated activation of IFN-β. These results shed new light on the inhibition of the induction of IFN-β by PPRV.

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.

Structural Disorder within Paramyxoviral Nucleoproteins and Phosphoproteins in Their Free and Bound Forms: From Predictions to Experimental Assessment

We herein review available computational and experimental data pointing to the abundance of structural disorder within the nucleoprotein (N) and phosphoprotein (P) from three paramyxoviruses, namely the measles (MeV), Nipah (NiV) and Hendra (HeV) viruses. We provide a detailed molecular description of the mechanisms governing the disorder-to-order transition that the intrinsically disordered C-terminal domain (NTAIL) of their N proteins undergoes upon binding to the C-terminal X domain (PXD) of the homologous P proteins. We also show that NTAIL-PXD complexes are "fuzzy", i.e., they possess a significant residual disorder, and discuss the possible functional significance of this fuzziness. Finally, we emphasize the relevance of N-P interactions involving intrinsically disordered proteins as promising targets for new antiviral approaches, and end up summarizing the general functional advantages of disorder for viruses.

Antibodies to the core proteins of Nairobi sheep disease virus/Ganjam virus reveal details of the distribution of the proteins in infected cells and tissues

Nairobi sheep disease virus (NSDV; also called Ganjam virus in India) is a bunyavirus of the genus Nairovirus. It causes a haemorrhagic gastroenteritis in sheep and goats with mortality up to 90%. The virus is closely related to the human pathogen Crimean-Congo haemorrhagic fever virus (CCHFV). Little is currently known about the biology of NSDV. We have generated specific antibodies against the virus nucleocapsid protein (N) and polymerase (L) and used these to characterise NSDV in infected cells and to study its distribution during infection in a natural host. Due to its large size and the presence of a papain-like protease (the OTU-like domain) it has been suggested that the L protein of nairoviruses undergoes an autoproteolytic cleavage into polymerase and one or more accessory proteins. Specific antibodies which recognise either the N-terminus or the C-terminus of the NSDV L protein showed no evidence of L protein cleavage in NSDV-infected cells. Using the specific anti-N and anti-L antibodies, it was found that these viral proteins do not fully colocalise in infected cells; the N protein accumulated near the Golgi at early stages of infection while the L protein was distributed throughout the cytoplasm, further supporting the multifunctional nature of the L protein. These antibodies also allowed us to gain information about the organs and cell types targeted by the virus in vivo. We could detect NSDV in cryosections prepared from various tissues collected post-mortem from experimentally inoculated animals; the virus was found in the mucosal lining of the small and large intestine, in the lungs, and in mesenteric lymph nodes (MLN), where NSDV appeared to target monocytes and/or macrophages.

Initiation and regulation of paramyxovirus transcription and replication

The paramyxovirus family has a genome consisting of a single strand of negative sense RNA. This genome acts as a template for two distinct processes: transcription to generate subgenomic, capped and polyadenylated mRNAs, and genome replication. These viruses only encode one polymerase. Thus, an intriguing question is, how does the viral polymerase initiate and become committed to either transcription or replication? By answering this we can begin to understand how these two processes are regulated. In this review article, we present recent findings from studies on the paramyxovirus, respiratory syncytial virus, which show how its polymerase is able to initiate transcription and replication from a single promoter. We discuss how these findings apply to other paramyxoviruses. Then, we examine how trans-acting proteins and promoter secondary structure might serve to regulate transcription and replication during different phases of the paramyxovirus replication cycle.

Order and Disorder in the Replicative Complex of Paramyxoviruses

In this review we summarize available data showing the abundance of structural disorder within the nucleoprotein (N) and phosphoprotein (P) from three paramyxoviruses, namely the measles (MeV), Nipah (NiV) and Hendra (HeV) viruses. We provide a detailed description of the molecular mechanisms that govern the disorder-to-order transition that the intrinsically disordered C-terminal domain (NTAIL) of their N proteins undergoes upon binding to the C-terminal X domain (XD) of the homologous P proteins. We also show that a significant flexibility persists within NTAIL-XD complexes, which therefore provide illustrative examples of "fuzziness". The functional implications of structural disorder for viral transcription and replication are discussed in light of the ability of disordered regions to establish a complex molecular partnership and to confer a considerable reach to the elements of the replicative machinery.

Peste des petits ruminants virus infection of small ruminants: a comprehensive review

Peste des petits ruminants (PPR) is caused by a Morbillivirus that belongs to the family Paramyxoviridae. PPR is an acute, highly contagious and fatal disease primarily affecting goats and sheep, whereas cattle undergo sub-clinical infection. With morbidity and mortality rates that can be as high as 90%, PPR is classified as an OIE (Office International des Epizooties)-listed disease. Considering the importance of sheep and goats in the livelihood of the poor and marginal farmers in Africa and South Asia, PPR is an important concern for food security and poverty alleviation. PPR virus (PPRV) and rinderpest virus (RPV) are closely related Morbilliviruses. Rinderpest has been globally eradicated by mass vaccination. Though a live attenuated vaccine is available against PPR for immunoprophylaxis, due to its instability in subtropical climate (thermo-sensitivity), unavailability of required doses and insufficient coverage (herd immunity), the disease control program has not been a great success. Further, emerging evidence of poor cross neutralization between vaccine strain and PPRV strains currently circulating in the field has raised concerns about the protective efficacy of the existing PPR vaccines. This review summarizes the recent advancement in PPRV replication, its pathogenesis, immune response to vaccine and disease control. Attempts have also been made to highlight the current trends in understanding the host susceptibility and resistance to PPR.

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.

Distinct and overlapping roles of Nipah virus P gene products in modulating the human endothelial cell antiviral response

Nipah virus (NiV) is a highly pathogenic zoonotic paramyxovirus that causes fatal encephalitis in up to 75% of infected humans. Like other paramyxoviruses, NiV employs co-transcriptional mRNA editing during transcription of the phosphoprotein (P) gene to generate additional mRNAs encoding the V and W proteins. The C protein is translated from the P mRNA, but in an alternative reading frame. There is evidence from both in vitro and in vivo studies to show that the P gene products play a role in NiV pathogenesis. We have developed a reverse genetic system to dissect the individual roles of the NiV P gene products in limiting the antiviral response in primary human microvascular lung endothelial cells, which represent important targets in human NiV infection. By characterizing growth curves and early antiviral responses against a number of recombinant NiVs with genetic modifications altering expression of the proteins encoded by the P gene, we observed that multiple elements encoded by the P gene have both distinct and overlapping roles in modulating virus replication as well as in limiting expression of antiviral mediators such as IFN-β, CXCL10, and CCL5. Our findings corroborate observations from in vivo hamster infection studies, and provide molecular insights into the attenuation and the histopathology observed in hamsters infected with C, V, and W-deficient NiVs. The results of this study also provide an opportunity to verify the results of earlier artificial plasmid expression studies in the context of authentic viral infection.

Mutations within the human parainfluenza virus type 3 (HPIV 3) C protein affect viral replication and host interferon induction

Human parainfluenza virus type 3 (HPIV 3) encodes a multifunctional C protein that is capable of inhibiting viral replication and counteracting the host interferon (IFN) signaling pathway. We recently demonstrated that the C protein is phosphorylated both in vitro and in vivo and mutations within the phosphorylation sites exhibit differential inhibitory activities in vitro. In this study, we report for the first time the successful recovery of mutant HPIV 3 viruses containing mutations within the C protein. Three mutant viruses, Cm-1, Cm-3 and Cm-4, harboring individual mutations of S7, S47T48 and S81 residues, respectively, were examined for their replication profiles and their ability to abrogate host IFN induction. Viral transcription was similar for all viruses; however Cm-3 displayed a relatively higher replication. Infection of cells with Cm-1 and Cm-3 led to the activation of IFN regulatory transcription factor 3 (IRF-3) and subsequent increase in IFN-β mRNA levels as determined by immunofluorescence assay and RT-PCR analyses, respectively. Moreover, Cm-3 was able to partially resist the interferon induced antiviral state in Vero cells. Taken together, these results suggest that mutations within the C protein differentially affect viral replication and host interferon induction.

The nucleocapsid protein of measles virus blocks host interferon response

Measles virus (MV) belongs to the genus Morbillivirus of the family Paramyxoviridae. A number of paramyxoviruses inhibit host interferon (IFN) signaling pathways in host immune systems by various mechanisms. Inhibition mechanisms have been described for many paramyxoviruses. Although there are inconsistencies among previous reports concerning MV, it appears that P/V/C proteins interfere with the pathways. In this study, we confirmed the effects of MV P gene products of a wild MV strain on IFN pathways and examined that of other viral proteins on it. Interestingly, we found that N protein acts as an IFN-α/β and γ-antagonist as strong as P gene products. We further investigated the mechanisms of MV-N inhibition, and revealed that MV-N blocks the nuclear import of activated STAT without preventing STAT and Jak activation or STAT degradation, and that the nuclear translocation of MV-N is important for the inhibition. The inhibitory effect of the N protein was observed as a common feature of other morbilliviruses. The results presented in this report suggest that N protein of MV as well as P/V/C proteins is involved in the inhibition of host IFN signaling pathways.

Evidence for phosphorylation of human parainfluenza virus type 3 C protein: mutant C proteins exhibit variable inhibitory activities in vitro

The P mRNA of human parainfluenza virus type 3, like other members of the subfamily Paramyxovirinae, gives rise to several polypeptides, one amongst them, the C protein, which is involved in inhibition of viral RNA synthesis as well as counteracting the host interferon signaling pathway. As a further step towards characterizing the function of C protein we present evidence to demonstrate the phosphorylation of C protein. Evidence for this observation emerged from deletion mapping studies coupled with mass spectroscopy analysis confirming residues S7, S22, S47T48 and S81 residues as the phosphorylation sites within the NH(2)-terminus of C protein. Here, we utilized a HPIV 3 minigenome replication assay and real time RT-PCR analysis to measure the relative RNA levels synthesized in the presence of mutant C proteins. Mutants S7A and S81A displayed low levels of RNA while mutant 5A that was devoid of all these phosphorylation sites exhibited high RNA level in comparison to wild type C during transcription. Interestingly, high levels of RNA were observed in the presence of S81A and mutant 5A during replication. Taken together, our results indicate that phosphorylation may differentially affect the inhibitory activity of C protein thereby regulating viral RNA synthesis.

Nucleocapsid structure and function

Measles virus belongs to the Paramyxoviridae family within the Mononegavirales order. Its nonsegmented, single-stranded, negative-sense RNA genome is encapsidated by the nucleoprotein (N) to form a helical nucleocapsid. This ribonucleoproteic complex is the substrate for both transcription and replication. The RNA-dependent RNA polymerase binds to the nucleocapsid template via its co-factor, the phosphoprotein (P). This chapter describes the main structural information available on the nucleoprotein, showing that it consists of a structured core (N(CORE)) and an intrinsically disordered C-terminal domain (N(TAIL)). We propose a model where the dynamic breaking and reforming of the interaction between N(TAIL) and P would allow the polymerase complex (L-P) to cartwheel on the nucleocapsid template. We also propose a model where the flexibility of the disordered N and P domains allows the formation of a tripartite complex (No-P-L) during replication, followed by the delivery of N monomers to the newly synthesized genomic RNA chain. Finally, the functional implications of structural disorder are also discussed in light of the ability of disordered regions to establish interactions with multiple partners, thus leading to multiple biological effects.

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.

The C, V and W proteins of Nipah virus inhibit minigenome replication

Nipah virus (NiV) is a recently emergent, highly pathogenic, zoonotic paramyxovirus of the genus Henipavirus. Like the phosphoprotein (P) gene of other paramyxoviruses, the P gene of NiV is predicted to encode three additional proteins, C, V and W. When the C, V and W proteins of NiV were tested for their ability to inhibit expression of the chloramphenicol acetyltransferase (CAT) reporter gene in plasmid-based, minigenome replication assays, each protein inhibited CAT expression in a dose-dependent manner. The C, V and W proteins of NiV also inhibited expression of CAT from a measles virus (MV) minigenome, but not from a human parainfluenzavirus 3 (hPIV3) minigenome. Interestingly, the C and V proteins of MV, which have previously been shown to inhibit MV minigenome replication, also inhibited NiV minigenome replication; however, they were not able to inhibit hPIV3 minigenome replication. In contrast, the C protein of hPIV3 inhibited minigenome replication of hPIV3, NiV and MV. Although there is very limited amino acid sequence similarity between the C, V and W proteins within the paramyxoviruses, the heterotypic inhibition of replication suggests that these proteins may share functional properties.

Human parainfluenza virus type 2 V protein inhibits genome replication by binding to the L protein: possible role in promoting viral fitness

The human parainfluenza virus type 2 (hPIV2) V protein plays important roles in inhibiting the host interferon response and promoting virus growth, but its role in hPIV2 replication and transcription is not clear. A green fluorescent protein (GFP)-expressing a negative-sense minigenomic construct of hPIV2 has been established by standard technology, with helper plasmids expressing the nucleocapsid protein (NP), phosphoprotein (P), and large RNA polymerase (L) protein, to examine the role of V protein. We found that the simultaneous expression of wild-type V protein in the minigenome system inhibited GFP expression, at least in part, by inhibiting minigenome replication. In contrast, expression of C terminally truncated or mutant hPIV2 V proteins had no effect. Moreover, the V protein of simian virus 41, the rubulavirus most closely related virus to hPIV2, also inhibited GFP expression, whereas that of PIV5, a more distantly related rubulavirus, did not. Using these other rubulavirus V proteins, as well as various mutant hPIV2 V proteins, we found that the ability of V protein to inhibit GFP expression correlated with its ability to bind to L protein via its C-terminal V protein-specific region, but there was no correlation with NP binding. A possible role for this inhibition of genome replication in promoting viral fitness is discussed.

Rinderpest virus blocks type I and type II interferon action: role of structural and nonstructural proteins

Rinderpest virus (RPV) is a paramyxovirus closely related to the human pathogen Measles virus. It causes severe disease in cattle, buffalo, and some wild animals; although it can infect humans, it does not cause disease. Here, we demonstrate that RPV blocks the action of both type I (alpha) and type II (gamma) interferons (IFNs) by blocking the phosphorylation and nuclear translocation of STAT1 and STAT2 and that this block is not related to species specificity. In addition, both wild-type virulent and vaccine strains of the virus blocked IFN action. Unlike the case with some other paramyxoviruses, neither STAT1 nor STAT2 is degraded upon virus infection. STAT1 is bound by both the viral structural protein P, and thereby recruited to concentrations of viral protein in the cell, and the nonstructural protein V. Although both P and V proteins bind to STAT1 and can block IFN action when expressed in transfected cells, the IFN antagonist activity of the P protein is weaker than that of the V protein. The viral C protein also seems to weakly block IFN-induced activation of STAT1 in transfection experiments. However, studies with knockout viruses showed that the viral V protein appears to be the dominant inhibitor of IFN signaling in the context of virus infection, since prevention of viral V expression restored the IFN sensitivity of infected cells. Although a change in the distribution pattern of STAT2 was observed in virus-infected cells, STAT2 was not bound by any viral protein.

Inhibition of measles virus minireplicon-encoded reporter gene expression by V protein

Measles virus V protein is a Cys-rich polypeptide that is dispensable for virus propagation in continuous cell lines, but necessary for efficient viral replication in animals. Those functions modulating virus propagation in vivo are not understood completely, although V protein is known to interfere with the host interferon response and control of viral gene expression. The ability to modulate gene expression was investigated further with a minireplicon transient expression system in which V protein was found to repress reporter activity. Two regions of the polypeptide contributed to this repressive effect including the carboxy-terminus and a region conserved in morbillivirus V proteins located between amino acids 110-131, whereas domains known to mediate the interaction between V and the nucleocapsid (N) protein were not essential. Accumulation of encapsidated minigenome in transfected cells was inhibited by V protein suggesting that it acted as a repressor of genome replication thereby limiting availability of template for reporter gene mRNA transcription.

Structural disorder within the replicative complex of measles virus: functional implications

Measles virus belongs to the Paramyxoviridae family within the Mononegavirales order. Its non-segmented, single stranded, negative sense RNA genome is encapsidated by the nucleoprotein (N) to form a helical nucleocapsid. This ribonucleoproteic complex is the substrate for both transcription and replication. The RNA-dependent RNA polymerase binds to the nucleocapsid template via its co-factor, the phosphoprotein (P). In this review, we summarize the main experimental data pointing out the abundance of structural disorder within measles virus N and P. We also describe studies indicating that structural disorder is a widespread property in the replicative complex of Paramyxoviridae and, more generally, of Mononegavirales. The functional implications of structural disorder are also discussed. Finally, we propose a model where the flexibility of the disordered N and P domains allows the formation of a tripartite complex (N degrees-P-L) during replication, followed by the delivery of N monomers to the newly synthesized genomic RNA chain.

Identification of naturally occurring amino acid variations that affect the ability of the measles virus C protein to regulate genome replication and transcription

The C protein of measles virus (MV C) is a basic protein of 186 amino acids (aa) that plays at least two roles in infected cells, interference with the innate immune response and modulation of viral polymerase activity. In this study, Northern blots were used to demonstrate that C proteins from three vaccine strains and three wild-type isolates of MV downregulated both mRNA transcription and genome replication in a plasmid-based mini-genome assay. The effect on transcription always paralleled the effect on replication; however, the six MV C proteins varied considerably in their ability to inhibit polymerase activity. Though the amino-terminal 45 aa of the C protein are more variable among different MV strains than the remaining 75% of the protein, the ability of the MV C proteins to inhibit polymerase activity was not regulated by substitutions in the amino terminus, but rather by the more conserved region containing aa 46-167. Naturally occurring substitutions at positions 147 and 166, but not 88 and 186, were found to regulate MV C protein activity. Deletion of the carboxyl-terminal 19 aa did not affect the polymerase-modulating activity. Though we did not find a link between the aa changes in MV C and attenuation, these data provide new information regarding the functions of this non-structural protein.

The Plowright vaccine strain of Rinderpest virus has attenuating mutations in most genes

The currently used vaccine strain of Rinderpest virus was derived by serial passage of the highly virulent Kabete ‘O’ strain (KO). A full-length cDNA copy of the KO strain was made from which a virus identical in pathogenicity to the wild-type virus was rescued. A series of chimeric viruses was prepared in which the coding sequences for the N, P, F, H or L proteins were replaced with the corresponding sequences from the vaccine strain. The KO-based virus with the vaccine strain H gene and that with the carboxy-terminal half of the L gene replaced with the corresponding sequence from the vaccine strain retained all or almost all of the virulence of the original KO virus. Animals infected with the KO-based virus containing the vaccine strain N, P or F gene, or the amino-terminal half of the L gene, developed high and prolonged pyrexia and leukopenia, but with reduced or absent lesions and other clinical signs; although partially attenuated, none was nearly as attenuated as the vaccine strain itself. These data indicate that the high attenuation and stability of the current vaccine are due to the accumulation of a number of separate mutations, none of which is itself so sufficiently debilitating that there is strong selective pressure in favour of the revertant.

The polymerase (L) protein of rinderpest virus interacts with the host cell protein striatin

Rinderpest virus (RPV) is a morbillivirus that causes a highly contagious disease affecting members of the order Artiodactyla. The viral L protein is the catalytic subunit of the RNA-dependent RNA polymerase. To search for host cell proteins with which L interacts, a library screen was performed using the yeast two-hybrid system. Several host cell proteins were recovered from the library screen as putative L-interactors; one of these was identified as striatin. A direct interaction between RPV L and striatin was confirmed using both co-immunoprecipitation assays and co-localisation studies using confocal microscopy. Striatin was also shown to co-localise with the RPV L protein in infected cells. The L proteins of morbilliviruses consist of three long highly conserved domains separated by short unconserved stretches of amino acids. The L domain with which striatin interacts was investigated by co-immunoprecipitation and striatin was shown to interact primarily with the central conserved domain.

The L polymerase protein of parainfluenza virus 3 forms an oligomer and can interact with the heterologous Sendai virus L, P and C proteins

We recently showed that the L protein of Sendai virus is present as an oligomer in the active P-L polymerase complex [Smallwood et al., Virology 304 (2002) 235]. We now demonstrate using two different epitope tags that the L protein of a second respirovirus, human parainfluenza type 3 virus (PIV3), also forms an L-L complex. L oligomerization requires the coexpression of the differentially epitope tagged L proteins. By exploiting a series of C-terminal truncations the L-L binding site maps to the N-terminal half of L. There is some complex formation between the heterologous PIV3 and Sendai L and P proteins; however, the heterologous L protein does not function in transcription of either the PIV3 or Sendai template. The PIV3 C protein binds PIV3 L and inhibits RNA synthesis in vitro and in vivo. Significant homology exists between the C proteins of PIV3 and Sendai and complex formation occurs between the PIV3 and Sendai heterologous C and L proteins. In addition, the heterologous C proteins can inhibit transcription at approximately 50% of the level of the homologous protein. These data suggest that while the C proteins may be functionally somewhat interchangeable, the L and P proteins are specific for each virus.

The N-terminal domain of the phosphoprotein of Morbilliviruses belongs to the natively unfolded class of proteins

We report the bacterial expression, purification, and characterization of the N-terminal domain (PNT) of the measles virus phosphoprotein. Using nuclear magnetic resonance, circular dichroism, gel filtration, and light scattering, we show that PNT is not structured in solution. We show by two complementary computational approaches that PNT belongs to the recently described class of natively unfolded proteins, further confirming its reported similarity with acidic activation domains of cellular transcription factors. We extend these results to the N-terminal domains of other Morbillivirus phosphoproteins and to the corresponding protein W of Sendai virus, a Paramyxovirus. Unstructured proteins may undergo some degree of folding upon binding to their partners, a process termed "induced folding." Using limited proteolysis in the presence of trifluoroethanol, we identified residues 27 to 38 as a putative secondary structure element of PNT arising upon induced folding.