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Hepatitis C Virus NS2-NS3 cleavage is mediated by NS2 autoprotease (NS2 pro ) and this cleavage is important for genome replication and virus assembly. Efficient NS2-NS3 cleavage relies on the stimulation of an intrinsic NS2 pro activity by the NS3 protease domain. NS2 pro activation depends on conserved hydrophobic NS3 surface residues and yet unknown NS2-NS3 surface interactions. Guided by an in silico NS2-NS3 precursor model, we experimentally identified two NS2 surface residues, F103 and L144, that are important for NS2 pro activation by NS3. When analyzed in the absence of NS3, a combination of defined amino acid exchanges, namely F103A and L144I, acts together to increase intrinsic NS2 pro activity. This effect is conserved between different HCV genotypes. For mutation L144I its stimulatory effect on NS2 pro could be also demonstrated for two other mammalian hepaciviruses, highlighting the functional significance of this finding. We hypothesize that the two exchanges stimulating the intrinsic NS2 pro activity mimic structural changes occurring during NS3-mediated NS2 pro activation. Introducing these activating NS2 pro mutations into a NS2-NS5B replicon reduced NS2-NS3 cleavage and RNA replication, indicating their interference with NS2-NS3 surface interactions pivotal for NS2 pro activation by NS3. Data from chimeric hepaciviral NS2-NS3 precursor constructs, suggest that NS2 F103 is involved in the reception or transfer of the NS3 stimulus by NS3 P115. Accordingly, fine-tuned NS2-NS3 surface interactions are a salient feature of HCV NS2-NS3 cleavage. Together, these novel insights provide an exciting basis to dissect molecular mechanisms of NS2 pro activation by NS3.

The hepatitis C virus (HCV) life cycle is highly regulated and characterized by a step-wise succession of interactions between viral and host cell proteins resulting in the assembly of macromolecular complexes, which catalyse genome replication and/or virus production. Non-structural (NS) protein 3, comprising a protease and a helicase domain, is involved in orchestrating these processes by undergoing protein interactions in a temporal fashion. Recently, we identified a multifunctional NS3 protease surface patch promoting pivotal protein-protein interactions required for early steps of the HCV life cycle, including NS3-mediated NS2 protease activation and interactions required for replicase assembly. In this work, we extend this knowledge by identifying further NS3 surface determinants important for NS5A hyperphosphorylation, replicase assembly or virion morphogenesis, which map to protease and helicase domain and form a contiguous NS3 surface area. Functional interrogation led to the identification of phylogenetically conserved amino acid positions exerting a critical function in virion production without affecting RNA replication. These findings illustrate that NS3 uses a multipurpose protein surface to orchestrate the step-wise assembly of functionally distinct multiprotein complexes. Taken together, our data provide a basis to dissect the temporal formation of viral multiprotein complexes required for the individual steps of the HCV life cycle.

The first marine pestivirus, Phocoena pestivirus (PhoPeV), isolated from harbor porpoise, has been recently described. To further characterize this unique pestivirus, its host cell tropism and growth kinetics were determined in different cell lines. In addition, the interaction of PhoPeV with innate immunity in porcine epithelial cells and the role of selected cellular factors involved in the viral entry and RNA replication of PhoPeV were investigated in comparison to closely and distantly related pestiviruses. While Bungowannah pestivirus (BuPV), a unique porcine pestivirus closely related to PhoPeV, exhibits a broad cell tropism, PhoPeV only infects cells from pigs, cattle, sheep, and cats, as has been described for classical swine fever virus (CSFV). Viral titers correlate with the amount of intracellular PhoPeV-specific RNA detected in the tested cell lines. PhoPeV replicates most efficiently in the porcine kidney cell line SK6. Pestiviruses generally counteract the cellular innate immune response by degradation of interferon regulatory factor 3 (IRF3) mediated by the viral N-terminal protease (Npro). No degradation of IRF3 and an increased expression of the type 1 interferon-stimulated antiviral protein Mx1 was observed in porcine cells infected with PhoPeV whose genome lacks the Npro encoding region. Infection of a CD46-deficient porcine cell line suggested that CD46, which is implicated in the viral entry of several pestiviruses, is not a major factor for the viral entry of PhoPeV. Moreover, the results of this study confirmed that the cellular factor DNAJC14 plays a crucial role in viral RNA replication of non-cytopathic pestiviruses, including PhoPeV.

The viruses of the family Flaviviridae possess a positive-strand RNA genome and express a single polyprotein which is processed into functional proteins. Initially, the nonstructural (NS) proteins, which are not part of the virions, form complexes capable of genome replication. Later on, the NS proteins also play a critical role in virion formation. The molecular basis to understand how the same proteins form different complexes required in both processes is so far unknown. For pestiviruses, uncleaved NS2-3 is essential for virion morphogenesis while NS3 is required for RNA replication but is not functional in viral assembly. Recently, we identified two gain of function mutations, located in the C-terminal region of NS2 and in the serine protease domain of NS3 (NS3 residue 132), which allow NS2 and NS3 to substitute for uncleaved NS2-3 in particle assembly. We report here the crystal structure of pestivirus NS3-4A showing that the NS3 residue 132 maps to a surface patch interacting with the C-terminal region of NS4A (NS4A-kink region) suggesting a critical role of this contact in virion morphogenesis. We show that destabilization of this interaction, either by alanine exchanges at this NS3/4A-kink interface, led to a gain of function of the NS3/4A complex in particle formation. In contrast, RNA replication and thus replicase assembly requires a stable association between NS3 and the NS4A-kink region. Thus, we propose that two variants of NS3/4A complexes exist in pestivirus infected cells each representing a basic building block required for either RNA replication or virion morphogenesis. This could be further corroborated by trans-complementation studies with a replication-defective NS3/4A double mutant that was still functional in viral assembly. Our observations illustrate the presence of alternative overlapping surfaces providing different contacts between the same proteins, allowing the switch from RNA replication to virion formation.

A serum neutralization test (SNT) is an essential method for the serological diagnosis of pestivirus infections, including classical swine fever, because of the cross reactivity of antibodies against pestiviruses and the non-quantitative properties of antibodies in an enzyme-linked immunosorbent assay. In conventional SNTs, an immunoperoxidase assay or observation of cytopathic effect after incubation for 3 to 7 days is needed to determine the SNT titer, which requires labor-intensive or time-consuming procedures. Therefore, a new SNT, based on the luciferase system and using classical swine fever virus, bovine viral diarrhea virus, and border disease virus possessing the 11-amino-acid subunit derived from NanoLuc luciferase was developed and evaluated; this approach enabled the rapid and easy determination of the SNT titer using a luminometer. In the new method, SNT titers can be determined tentatively at 2 days post-infection (dpi) and are comparable to those obtained by conventional SNTs at 3 or 4 dpi. In conclusion, the luciferase-based SNT can replace conventional SNTs as a high-throughput antibody test for pestivirus infections.

Hepatitis C virus (HCV) infection is a leading cause of liver disease worldwide. The HCV RNA genome is translated into a single polyprotein. Most of the cleavage sites in the non-structural (NS) polyprotein region are processed by the NS3/NS4A serine protease. The vital NS2-NS3 cleavage is catalyzed by the NS2 autoprotease. For efficient processing at the NS2/NS3 site, the NS2 cysteine protease depends on the NS3 serine protease domain. Despite its importance for the viral life cycle, the molecular details of the NS2 autoprotease activation by NS3 are poorly understood. Here, we report the identification of a conserved hydrophobic NS3 surface patch that is essential for NS2 protease activation. One residue within this surface region is also critical for RNA replication and NS5A hyperphosphorylation, two processes known to depend on functional replicase assembly. This dual function of the NS3 surface patch prompted us to reinvestigate the impact of the NS2-NS3 cleavage on NS5A hyperphosphorylation. Interestingly, NS2-NS3 cleavage turned out to be a prerequisite for NS5A hyperphosphorylation, indicating that this cleavage has to occur prior to replicase assembly. Based on our data, we propose a sequential cascade of molecular events: in uncleaved NS2-NS3, the hydrophobic NS3 surface patch promotes NS2 protease stimulation; upon NS2-NS3 cleavage, this surface region becomes available for functional replicase assembly. This model explains why efficient NS2-3 cleavage is pivotal for HCV RNA replication. According to our model, the hydrophobic surface patch on NS3 represents a module critically involved in the temporal coordination of HCV replicase assembly.

[This corrects the article DOI: 10.1371/journal.ppat.1004736.].

Pestiviruses incur significant economic and welfare burdens on global livestock production. We used gene editing to produce pigs that were fully resistant to the pestivirus classical swine fever virus.DNAJC14 is essential for replication of classical pestiviruses in cell lines in vitro, but its importance for these viruses in the context of an animal was unknown.We used CRISPR/Cas to edit pig DNAJC14 in zygotes, which developed to produce healthy animals.Primary cells isolated from DNAJC14-edited pigs were resistant to infection with two different pestiviruses: classical swine fever virus and bovine viral diarrhoea virus.No signs of infection were detected when young adult pigs with edited DNAJC14 were inoculated with classical swine fever virus, demonstrating gene editing as a viable option for control of these devastating pathogens. Infectious diseases remain a major impediment to livestock production, negatively impacting both productivity and welfare. Where key interactions between viruses and host proteins have been identified, it is possible to rationally devise intervention strategies. In vitro studies have identified the host protein DNAJC14 as a core component of the replicative cycle of classical pestiviruses. Outbreaks caused by this group of viruses cause enormous losses in stock farming due to culling and export restrictions. Using CRISPR/Cas9 gene editing, we produced a cohort of pigs with altered DNAJC14. Primary cells from these animals did not support replication of either classical swine fever virus (CSFV) or bovine viral diarrhoea virus (BVDV) in vitro. In vivo challenge with CSFV revealed that the edited pigs displayed complete resistance to infection. This establishes gene editing as an additional strategy that can contribute to the control of classical pestiviruses. [Display omitted] We demonstrate that pigs with altered DNAJC14 are resistant to infection with classical swine fever virus under laboratory conditions, placing this discovery at a Technology Readiness Level of 4. While no obvious phenotypic deficits were observed, further work is required to confirm that important welfare and production parameters have not been altered, before commercialization. The closest related prior example is editing of CD163 in pigs to produce animals that are resistant to porcine reproductive and respiratory syndrome virus (PRRSV), another major pathogen. Thus far, gene-edited pigs with PRRSV resistance have been approved in Colombia, Brazil, Dominican Republic, and, most recently, the USA. This reflects maturing regulatory pathways for such products in many jurisdictions. The pigs presented in the current work will provide a valuable additional example to aid regulatory discourse. CRISPR/Cas9 editing of pig DNAJC14 resulted in animals that were completely resistant to in vivo challenge with classical swine fever virus, highlighting the pivotal role of this host protein in classical pestivirus infections and providing a novel strategy to improve livestock health, enhance animal welfare, and reduce economic losses.

Hepatitis C virus (HCV) infection is a leading cause of liver disease worldwide. The HCV RNA genome is translated into a single polyprotein. Most of the cleavage sites in the non-structural (NS) polyprotein region are processed by the NS3/NS4A serine protease. The vital NS2-NS3 cleavage is catalyzed by the NS2 autoprotease. For efficient processing at the NS2/NS3 site, the NS2 cysteine protease depends on the NS3 serine protease domain. Despite its importance for the viral life cycle, the molecular details of the NS2 autoprotease activation by NS3 are poorly understood. Here, we report the identification of a conserved hydrophobic NS3 surface patch that is essential for NS2 protease activation. One residue within this surface region is also critical for RNA replication and NS5A hyperphosphorylation, two processes known to depend on functional replicase assembly. This dual function of the NS3 surface patch prompted us to reinvestigate the impact of the NS2-NS3 cleavage on NS5A hyperphosphorylation. Interestingly, NS2-NS3 cleavage turned out to be a prerequisite for NS5A hyperphosphorylation, indicating that this cleavage has to occur prior to replicase assembly. Based on our data, we propose a sequential cascade of molecular events: in uncleaved NS2-NS3, the hydrophobic NS3 surface patch promotes NS2 protease stimulation; upon NS2-NS3 cleavage, this surface region becomes available for functional replicase assembly. This model explains why efficient NS2-3 cleavage is pivotal for HCV RNA replication. According to our model, the hydrophobic surface patch on NS3 represents a module critically involved in the temporal coordination of HCV replicase assembly.

The members of the genus Pestivirus in the family Flaviviridae comprise economically important pathogens of life stock like classical swine fever (CSFV) and bovine viral diarrhea virus (BVDV). Intense research over the last years revealed that at least 11 recognized and eight proposed pestivirus species exist. The single-stranded, positive-sense RNA genome encodes for one large polyprotein which is processed by viral and cell-derived proteases into 12 mature proteins. Besides its protein-coding function, the RNA genome also contains RNA secondary structures with critical importance for various stages of the viral life cycle. Some of those RNA secondary structures, like the internal ribosome entry site (IRES) and a 3’ stem-loop essential for genome replication, had already been studied for a few individual pestiviruses. In this study, we provide the first genome-wide multiple sequence alignment (MSA) including all known pestivirus species (accepted and tentative). Moreover, we performed a comprehensive analysis of RNA secondary structures phylogenetically conserved across the complete genus. While showing well-described structures, like a 5’ stem-loop structure, the IRES element, and the 3’ stem loop SL I to be conserved between all pestiviruses, other RNA secondary structures in the 3’ untranslated region (UTR) were only conserved in subsets of the species. We identified 29 novel phylogenetically conserved RNA secondary structures in the protein-coding region, with so far unresolved functional importance. The microRNA binding site for miR-17 was previously known in species A, B, and C; in this study, we identified it in ten additional species, but not in species K, S, Q, and R. Another interesting finding is the identification of a putative long-distance RNA interaction between the IRES and the 3’ end of the genome. These results together with the now available comprehensive multiple sequence alignment including all 19 pestivirus species, represent a valuable resource for future research and diagnostic purposes.