Dual topology of the processed hepatitis C virus protein NS4B is influenced by the NS5A protein

To Explore the Potential Targets and Current Structure-based Design Strategies Utilizing Co-crystallized Ligand to Combat HCV

BACKGROUND

Hepatitis C Virus (HCV) belongs to the Hepacivirus family. HCV has been designated as a very dreadful virus as it can attack the liver, causing inflammation and even may lead to cancer in chronic conditions. It was estimated that 71 million people around the world have chronic HCV infection. World Health Organization (WHO) reported that about 399000 people died because of chronic cirrhosis and liver cancer globally. In spite of the abundance of availability of drugs for the treatment of HCV, however, the issue of drug resistance surpasses all the possibilities of therapeutic management of HCV. Therefore, to address this issue of 'drug-resistance', various HCV targets were explored to quest the evaluation of the mechanism of the disease progression.

METHODS

An attempt has been made in the present study to explore the various targets of HCV involved in the mechanism(s) of the disease initiation and progression and to focus on the mode of binding of ligands, which are co-crystallized at the active cavity of different HCV targets.

CONCLUSION

The present study could predict some crucial features of these ligands, which possibly interacted with various amino acid residues responsible for their biological activity and molecular signaling pathway(s). Such binding mode may be considered as a template for the high throughput screening and designing of active congeneric ligands to combat HCV.

Bioorthogonal dissection of the replicase assembly of hepatitis C virus

Positive-strand RNA viruses such as hepatitis C virus (HCV), flaviviruses, and coronaviruses are medically important. Assembly of replicase on host membranes is a conserved replication strategy and an attractive antiviral target. The mechanisms of replicase assembly are largely unknown, due to the technical difficulties in purifying the replicase and carrying out structural studies. Here, with an HCV replicase assembly surrogate system, we employed a bioorthogonal system to introduce the photolabile unnatural amino into each residue in the cytosolic regions of NS4B and the amphipathic helix (AH) of NS5A. Photocrosslinking enabled visualization of NS4B oligomerization and NS5A dimerization at pinpointed interacting residues and identifying contacting sites among the replicase components. Characterization of the interacting sites revealed hub elements in replicase assembly by docking replicase components to prompt protein-protein interactions. The results provide information about the molecular architecture of the replicase, advancing understanding of the mechanism of replicase assembly.

Hepatitis C Viral Replication Complex

The life cycle of the hepatitis C virus (HCV) can be divided into several stages, including viral entry, protein translation, RNA replication, viral assembly, and release. HCV genomic RNA replication occurs in the replication organelles (RO) and is tightly linked to ER membrane alterations containing replication complexes (proteins NS3 to NS5B). The amplification of HCV genomic RNA could be regulated by the RO biogenesis, the viral RNA structure (i.e., cis-acting replication elements), and both viral and cellular proteins. Studies on HCV replication have led to the development of direct-acting antivirals (DAAs) targeting the replication complex. This review article summarizes the viral and cellular factors involved in regulating HCV genomic RNA replication and the DAAs that inhibit HCV replication.

Study of membrane deformations induced by Hepatitis C protein NS4B and its terminal amphipathic peptides

Many viruses destabilize cellular membranous compartments to form their replication complexes, but the mechanism(s) underlying membrane perturbation remains unknown. Expression in eukaryotic cells of NS4B, a protein of the hepatitis C virus (HCV), alters membranous complexes and induces structures similar to the so-called membranous web that appears crucial to the formation of the HCV replication complex. As over-expression of the protein is lethal to both prokaryotic and eukaryotic cells, NS4B was produced in large quantities in a "cell-free" system in the presence of detergent, after which it was inserted into lipid membranes. X-ray diffraction revealed that NS4B modifies the phase diagram of synthetic lipid aqueous phases considerably, perturbing the transition temperature and cooperativity. Cryo-electron microscopy demonstrated that NS4B introduces significant disorder in the synthetic membrane as well as discontinuities that could be interpreted as due to the formation of pores and membrane merging events. C- and N-terminal fragments of NS4B are both able to destabilize liposomes. While most NS4B amphipathic peptides perforate membranes, one NS4B peptide induces membrane fusion. Cryo-electron microscopy reveals a particular structure that can be interpreted as arising from hemi-fusion-like events. Amphipathic domains are present in many proteins, and if exposed to the aqueous cytoplasmic medium are sufficient to destabilize membranes in order to form viral replication complexes. These domains have important functions in the viral replication cycle, and thus represent potential targets for the development of anti-viral molecules.

The bZIP Proteins of Oncogenic Viruses

Basic leucine zipper (bZIP) transcription factors (TFs) govern diverse cellular processes and cell fate decisions. The hallmark of the leucine zipper domain is the heptad repeat, with leucine residues at every seventh position in the domain. These leucine residues enable homo- and heterodimerization between ZIP domain α-helices, generating coiled-coil structures that stabilize interactions between adjacent DNA-binding domains and target DNA substrates. Several cancer-causing viruses encode viral bZIP TFs, including human T-cell leukemia virus (HTLV), hepatitis C virus (HCV) and the herpesviruses Marek’s disease virus (MDV), Epstein–Barr virus (EBV) and Kaposi’s sarcoma-associated herpesvirus (KSHV). Here, we provide a comprehensive review of these viral bZIP TFs and their impact on viral replication, host cell responses and cell fate.

Hepatitis C Virus Replication

Replication and amplification of the viral genome is a key process for all viruses. For hepatitis C virus (HCV), a positive-strand RNA virus, amplification of the viral genome requires the synthesis of a negative-sense RNA template, which is in turn used for the production of new genomic RNA. This process is governed by numerous proteins, both host and viral, as well as distinct lipids and specific RNA elements within the positive- and negative-strand RNAs. Moreover, this process requires specific changes to host cell ultrastructure to create microenvironments conducive to viral replication. This review will focus on describing the processes and factors involved in facilitating or regulating HCV genome replication.

Hepatitis C virus NS5A inhibitor daclatasvir allosterically impairs NS4B-involved protein-protein interactions within the viral replicase and disrupts the replicase quaternary structure in a replicase assembly surrogate system

Daclatasvir (DCV) is a highly potent direct-acting antiviral that targets the non-structural protein 5A (NS5A) of hepatitis C virus (HCV) and has been used with great clinical success. Previous studies have demonstrated its impact on viral replication complex assembly. However, the precise mechanisms by which DCV impairs the replication complex assembly remains elusive. In this study, by using HCV subgenomic replicons and a viral replicase assembly surrogate system in which the HCV NS3-5B polyprotein is expressed to mimic the viral replicase assembly, we assessed the impact of DCV on the aggregation and tertiary structure of NS5A, the protein-protein interactions within the viral replicase and the quaternary structure of the viral replicase. We found that DCV did not affect aggregation and tertiary structure of NS5A. DCV induced a quaternary structural change of the viral replicase, as evidenced by selective increase of NS4B's sensitivity to proteinase K digestion. Mechanically, DCV impaired the NS4B-involved protein-protein interactions within the viral replicase. These phenotypes were consistent with the phenotypes of several reported NS4B mutants that abolish the viral replicase assembly. The DCV-resistant mutant Y93H was refractory to the DCV-induced reduction of the NS4B-involved protein interactions and the quaternary structural change of the viral replicase. In addition, Y93H reduced NS4B-involved protein-protein interactions within the viral replicase and attenuated viral replication. We propose that DCV may induce a positional change of NS5A, which allosterically affects protein interactions within the replicase components and disrupts replicase assembly.

Modulation of Lipid Droplet Metabolism-A Potential Target for Therapeutic Intervention in Flaviviridae Infections

Lipid droplets (LDs) are endoplasmic reticulum (ER)-related dynamic organelles that store and regulate fatty acids and neutral lipids. They play a central role in cellular energy storage, lipid metabolism and cellular homeostasis. It has become evident that viruses have co-evolved in order to exploit host lipid metabolic pathways. This is especially characteristic of the Flaviviridae family, including hepatitis C virus (HCV) and several flaviviruses. Devoid of an appropriate lipid biosynthetic machinery of their own, these single-strand positive-sense RNA viruses can induce dramatic changes in host metabolic pathways to establish a favorable environment for viral multiplication and acquire essential components to facilitate their assembly and traffic. Here we have reviewed the current knowledge on the intracellular life cycle of those from the Flaviviridae family, with particular emphasis on HCV and dengue virus (DENV), and their association with the biosynthesis and metabolism of LDs, with the aim to identify potential antiviral targets for development of novel therapeutic interventions.

PTC725, an NS4B-Targeting Compound, Inhibits a Hepatitis C Virus Genotype 3 Replicon, as Predicted by Genome Sequence Analysis and Determined Experimentally

PTC725 is a small molecule NS4B-targeting inhibitor of hepatitis C virus (HCV) genotype (gt) 1 RNA replication that lacks activity against HCV gt2. We analyzed the Los Alamos HCV sequence database to predict susceptible/resistant HCV gt's according to the prevalence of known resistance-conferring amino acids in the NS4B protein. Our analysis predicted that HCV gt3 would be highly susceptible to the activity of PTC725. Indeed, PTC725 was shown to be active against a gt3 subgenomic replicon with a 50% effective concentration of ∼5 nM. De novo resistance selection identified mutations encoding amino acid substitutions mapping to the first predicted transmembrane region of NS4B, a finding consistent with results for PTC725 and other NS4B-targeting compounds against HCV gt1. This is the first report of the activity of an NS4B targeting compound against HCV gt3. In addition, we have identified previously unreported amino acid substitutions selected by PTC725 treatment which further demonstrate that these compounds target the NS4B first transmembrane region.

Flaviviridae Replication Organelles: Oh, What a Tangled Web We Weave

Replication of positive-strand RNA viruses occurs in tight association with reorganized host cell membranes. In a concerted fashion, viral and cellular factors generate distinct organelle-like structures, designated viral replication factories. These virus-induced compartments promote highly efficient genome replication, allow spatiotemporal coordination of the different steps of the viral replication cycle, and protect viral RNA from the hostile cytoplasmic environment. The combined use of ultrastructural and functional studies has greatly increased our understanding of the architecture and biogenesis of viral replication factories. Here, we review common concepts and distinct differences in replication organelle morphology and biogenesis within the Flaviviridae family, exemplified by dengue virus and hepatitis C virus. We discuss recent progress made in our understanding of the complex interplay between viral determinants and subverted cellular membrane homeostasis in biogenesis and maintenance of replication factories of this virus family.

Interaction between Nonstructural Proteins NS4B and NS5A Is Essential for Proper NS5A Localization and Hepatitis C Virus RNA Replication

The hepatitis C virus NS5A protein is tethered to cellular membranes via an amphipathic amino-terminal helix that is inserted in-plane into the outer endoplasmic reticulum (ER)-derived membrane leaflet. The charged face of the helix faces the cytoplasm and may contribute to interactions involved in replicase assembly and function. Using an aggressive charge flip mutagenesis strategy, we identified a number of essential residues for replication on the charged face of the NS5A anchor and identified a double charge face mutant that is lethal for RNA replication but generates suppressor mutations in the carboxy-terminal helix of the NS4B protein. This suppressor restores RNA replication of the NS5A helix double flip mutant (D1979K/D1982K) and, interestingly, seems to function by restoring the proper localization of NS5A to the viral replicase. These data add to our understanding of the complex organization and assembly of the viral replicase via NS4B-NS5A interactions.

IMPORTANCE

Information about the functional role of the cytosolic face of the NS5A anchoring helix remains obscure. In this study, we show that while the hydrophobic face of the NS5A anchor helix mediates membrane association, the polar cytosolic face of the helix plays a key role during hepatitis C virus (HCV) replication by mediating the interaction of NS5A with other HCV nonstructural proteins via NS4B. Such an interaction determines the subcellular localization of NS5A by engaging NS5A in the HCV replication process during the formation of a functional HCV replication complex. Thus, collectively, it can be stated that the findings in the present study provide further information about the interactions between the HCV nonstructural proteins during HCV RNA replication and provide a platform to gain more insights about the molecular architecture of HCV replication complexes.

Cell-free expression, purification, and membrane reconstitution for NMR studies of the nonstructural protein 4B from hepatitis C virus

We describe the expression of the hepatitis C virus nonstructural protein 4B (NS4B), which is an integral membrane protein, in a wheat germ cell-free system, the subsequent purification and characterization of NS4B and its insertion into proteoliposomes in amounts sufficient for multidimensional solid-state NMR spectroscopy. First spectra of the isotopically [(2)H,(13)C,(15)N]-labeled protein are shown to yield narrow (13)C resonance lines and a proper, predominantly α-helical fold. Clean residue-selective leucine, isoleucine and threonine-labeling is demonstrated. These results evidence the suitability of the wheat germ-produced integral membrane protein NS4B for solid-state NMR. Still, the proton linewidth under fast magic angle spinning is broader than expected for a perfect sample and possible causes are discussed.

A Journey around the Medicinal Chemistry of Hepatitis C Virus Inhibitors Targeting NS4B: From Target to Preclinical Drug Candidates

Hepatitis C virus (HCV) infection is a global health burden with an estimated 130-170 million chronically infected individuals and is the cause of serious liver diseases such as cirrhosis and hepatocellular carcinoma. HCV NS4B protein represents a validated target for the identification of new drugs to be added to the combination regimen recently approved. During the last years, NS4B has thus been the object of impressive medicinal chemistry efforts, which led to the identification of promising preclinical candidates. In this context, the present review aims to discuss research published on NS4B functional inhibitors focusing the attention on hit identification, hit-to-lead optimization, ADME profile evaluation, and the structure-activity relationship data raised for each compound family taken into account. The information delivered in this review will be a useful and valuable tool for those medicinal chemists dealing with research programs focused on NS4B and aimed at the identification of innovative anti-HCV compounds.

Preclinical Characterization and In Vivo Efficacy of GSK8853, a Small-Molecule Inhibitor of the Hepatitis C Virus NS4B Protein

The hepatitis C virus (HCV) NS4B protein is an antiviral therapeutic target for which small-molecule inhibitors have not been shown to exhibit in vivo efficacy. We describe here the in vitro and in vivo antiviral activity of GSK8853, an imidazo[1,2-a]pyrimidine inhibitor that binds NS4B protein. GSK8853 was active against multiple HCV genotypes and developed in vitro resistance mutations in both genotype 1a and genotype 1b replicons localized to the region of NS4B encoding amino acids 94 to 105. A 20-day in vitro treatment of replicons with GSK8853 resulted in a 2-log drop in replicon RNA levels, with no resistance mutation breakthrough. Chimeric replicons containing NS4B sequences matching known virus isolates showed similar responses to a compound with genotype 1a sequences but altered efficacy with genotype 1b sequences, likely corresponding to the presence of known resistance polymorphs in those isolates. In vivo efficacy was tested in a humanized-mouse model of HCV infection, and the results showed a 3-log drop in viral RNA loads over a 7-day period. Analysis of the virus remaining at the end of in vivo treatment revealed resistance mutations encoding amino acid changes that had not been identified by in vitro studies, including NS4B N56I and N99H. Our findings provide an in vivo proof of concept for HCV inhibitors targeting NS4B and demonstrate both the promise and potential pitfalls of developing NS4B inhibitors.

The interaction between the hepatitis C proteins NS4B and NS5A is involved in viral replication

Hepatitis C virus (HCV) replicates in membrane associated, highly ordered replication complexes (RCs). These complexes include viral and host proteins necessary for viral RNA genome replication. The interaction network among viral and host proteins underlying the formation of these RCs is yet to be thoroughly characterized. Here, we investigated the association between NS4B and NS5A, two critical RC components. We characterized the interaction between these proteins using fluorescence resonance energy transfer and a mammalian two-hybrid system. Specific tryptophan residues within the C-terminal domain (CTD) of NS4B were shown to mediate this interaction. Domain I of NS5A, was sufficient to mediate its interaction with NS4B. Mutations in the NS4B CTD tryptophan residues abolished viral replication. Moreover, one of these mutations also affected NS5A hyperphosphorylation. These findings provide new insights into the importance of the NS4B-NS5A interaction and serve as a starting point for studying the complex interactions between the replicase subunits.

Aminoterminal amphipathic α-helix AH1 of hepatitis C virus nonstructural protein 4B possesses a dual role in RNA replication and virus production

Nonstructural protein 4B (NS4B) is a key organizer of hepatitis C virus (HCV) replication complex formation. In concert with other nonstructural proteins, it induces a specific membrane rearrangement, designated as membranous web, which serves as a scaffold for the HCV replicase. The N-terminal part of NS4B comprises a predicted and a structurally resolved amphipathic α-helix, designated as AH1 and AH2, respectively. Here, we report a detailed structure-function analysis of NS4B AH1. Circular dichroism and nuclear magnetic resonance structural analyses revealed that AH1 folds into an amphipathic α-helix extending from NS4B amino acid 4 to 32, with positively charged residues flanking the helix. These residues are conserved among hepaciviruses. Mutagenesis and selection of pseudorevertants revealed an important role of these residues in RNA replication by affecting the biogenesis of double-membrane vesicles making up the membranous web. Moreover, alanine substitution of conserved acidic residues on the hydrophilic side of the helix reduced infectivity without significantly affecting RNA replication, indicating that AH1 is also involved in virus production. Selective membrane permeabilization and immunofluorescence microscopy analyses of a functional replicon harboring an epitope tag between NS4B AH1 and AH2 revealed a dual membrane topology of the N-terminal part of NS4B during HCV RNA replication. Luminal translocation was unaffected by the mutations introduced into AH1, but was abrogated by mutations introduced into AH2. In conclusion, our study reports the three-dimensional structure of AH1 from HCV NS4B, and highlights the importance of positively charged amino acid residues flanking this amphipathic α-helix in membranous web formation and RNA replication. In addition, we demonstrate that AH1 possesses a dual role in RNA replication and virus production, potentially governed by different topologies of the N-terminal part of NS4B.

With an estimated 180 million chronically infected individuals, hepatitis C virus (HCV) is a leading cause of chronic hepatitis, liver cirrhosis and hepatocellular carcinoma worldwide. HCV is a positive-strand RNA virus that builds its replication complex on rearranged intracellular membranes, designated as membranous web. HCV nonstructural protein 4B (NS4B) is a key organizer of HCV membranous web and replication complex formation. Here, we provide a detailed structure-function analysis of an N-terminal amphipathic α-helix of NS4B, named AH1, and demonstrate that it plays key roles in shaping the membranous web as well as in virus production. We also show that the N-terminal part of NS4B adopts a dual membrane topology in a replicative context, possibly reflecting the different roles of this protein in the viral life cycle.

Hepatitis C virus RNA replication and assembly: living on the fat of the land

Hepatitis C virus (HCV) is a major global health burden accounting for around 170 million chronic infections worldwide. Although highly potent direct-acting antiviral drugs to treat chronic hepatitis C have been approved recently, owing to their high costs and limited availability and a large number of undiagnosed infections, the burden of disease is expected to rise in the next few years. In addition, HCV is an excellent paradigm for understanding the tight link between a pathogen and host cell pathways, most notably lipid metabolism. HCV extensively remodels intracellular membranes to establish its cytoplasmic replication factory and also usurps components of the intercellular lipid transport system for production of infectious virus particles. Here, we review the molecular mechanisms of viral replicase function, cellular pathways employed during HCV replication factory biogenesis, and viral, as well as cellular, determinants of progeny virus production.

The intraviral protein interaction network of hepatitis C virus

Hepatitis C virus (HCV) is a global health problem and one of the main reasons for chronic liver diseases such as cirrhosis and hepatocellular carcinoma. The HCV genome is translated into a polyprotein which is proteolytically processed into 10 viral proteins. The interactome of the HCV proteins with the host cell has been worked out; however, it remains unclear how viral proteins interact with each other. We aimed to generate the interaction network of these 10 HCV proteins using a flow-cytometry-based FRET assay established in our laboratory (Banning, C., Votteler, J., Hoffmann, D., Koppensteiner, H., Warmer, M., Reimer, R., Kirchhoff, F., Schubert, U., Hauber, J., and Schindler, M. (2010) A flow cytometry-based FRET assay to identify and analyse protein-protein interactions in living cells. PLoS One 5, e9344). HCV proteins were constructed as fusions with the chromophores CFP and YFP. All HCV fusions were expressed and localized to specific subcellular compartments, indicating that they were functional. FACS-FRET measurements identified a total of 20 interactions; 13 of these were previously described and have now been confirmed in living cells via our method. Among the seven novel protein binding pairs, HCV p7 plays a pivotal role. It binds to the HCV capsid protein Core and the two glycoproteins E1 and E2. These interplays were further demonstrated in the relevant context of Huh7.5 liver cells expressing infectious HCV. Our work demonstrates the feasibility of rapidly generating small interaction networks via FACS-FRET and defines the network of intra-HCV protein interactions. Furthermore, our data support an important role of p7 in HCV assembly.

Hepatitis C virus proteins: from structure to function

Great progress has been made over the past years in elucidating the structure and function of the hepatitis C virus (HCV) proteins, most of which are now actively being pursued as antiviral targets. The structural proteins, which form the viral particle, include the core protein and the envelope glycoproteins E1 and E2. The nonstructural proteins include the p7 viroporin, the NS2 protease, the NS3-4A complex harboring protease and NTPase/RNA helicase activities, the NS4B and NS5A proteins, and the NS5B RNA-dependent RNA polymerase. NS4B is a master organizer of replication complex formation while NS5A is a zinc-containing phosphoprotein involved in the regulation of HCV RNA replication versus particle production. Core to NS2 make up the assembly module while NS3 to NS5B represent the replication module (replicase). However, HCV proteins exert multiple functions during the viral life cycle, and these may be governed by different structural conformations and/or interactions with viral and/or cellular partners. Remarkably, each viral protein is anchored to intracellular membranes via specific determinants that are essential to protein function in the cell. This review summarizes current knowledge of the structure and function of the HCV proteins and highlights recent advances in the field.

Morphological and biochemical characterization of the membranous hepatitis C virus replication compartment

Like all other positive-strand RNA viruses, hepatitis C virus (HCV) induces rearrangements of intracellular membranes that are thought to serve as a scaffold for the assembly of the viral replicase machinery. The most prominent membranous structures present in HCV-infected cells are double-membrane vesicles (DMVs). However, their composition and role in the HCV replication cycle are poorly understood. To gain further insights into the biochemcial properties of HCV-induced membrane alterations, we generated a functional replicon containing a hemagglutinin (HA) affinity tag in nonstructural protein 4B (NS4B), the supposed scaffold protein of the viral replication complex. By using HA-specific affinity purification we isolated NS4B-containing membranes from stable replicon cells. Complementing biochemical and electron microscopy analyses of purified membranes revealed predominantly DMVs, which contained viral proteins NS3 and NS5A as well as enzymatically active viral replicase capable of de novo synthesis of HCV RNA. In addition to viral factors, co-opted cellular proteins, such as vesicle-associated membrane protein-associated protein A (VAP-A) and VAP-B, that are crucial for viral RNA replication, as well as cholesterol, a major structural lipid of detergent-resistant membranes, are highly enriched in DMVs. Here we describe the first isolation and biochemical characterization of HCV-induced DMVs. The results obtained underline their central role in the HCV replication cycle and suggest that DMVs are sites of viral RNA replication. The experimental approach described here is a powerful tool to more precisely define the molecular composition of membranous replication factories induced by other positive-strand RNA viruses, such as picorna-, arteri- and coronaviruses.

A hepatitis C virus NS4B inhibitor suppresses viral genome replication by disrupting NS4B's dimerization/multimerization as well as its interaction with NS5A

Chronic hepatitis C virus (HCV) infection is responsible for severe liver diseases including liver cirrhosis and hepatocellular carcinoma. An HCV non-structural protein 4B (NS4B) plays an essential role in viral RNA genome replication by building multi-vesicular structures around endoplasmic reticulum membranes. Especially, the second amphipathic helix of NS4B (NS4B-AH2) was shown to be essential for this process. By screening compounds against a membrane-aggregating activity of NS4B-AH2, several anti-HCV replication small molecules targeting NS4B-AH2 were discovered. However, little is known about detailed molecular mechanism of action for these NS4B-AH2 inhibitors. In this report, we provide evidences that NS4B-AH2 is required for NS4B's dimerization/multimerization, its proper subcellular localization, as well as its interaction with NS5A. More importantly, one of NS4B-AH2 inhibitors called "anguizole" was found to be able to disrupt all of these NS4B-AH2-mediated biological functions of NS4B. This newly elucidated mechanism of action will enable us not only to better understand a central role of NS4B-AH2 in HCV life cycle but also to develop a more safe and effective new class of NS4B-AH2 inhibitors of HCV replication in the future.

HCV Replication Inhibitors That Interact with NS4B

We describe the discovery, development and in vivo activity of small molecules that inhibits HCV replication via direct interaction with the viral NS4B protein. The inhibitors were identified through a phenotypic, cell based, high throughput screen using the HCV subgenomic replicon. Compounds were then optimized to extremely high potency and pharmacokinetics. Mechanistic data generated suggests a hypothesis wherein the compounds described function by binding to NS4B, preventing the formation of the characteristic HCV induced sub‐cellular membranous web required for viral replication. Finally, in vivo proof of mechanism was established by employing a chimeric “humanized” mouse model of HCV infection to demonstrate for the first time that a small molecule with high in vitro affinity for NS4B can inhibit viral replication in vivo.

The molecular and structural basis of advanced antiviral therapy for hepatitis C virus infection

The availability of the first molecular clone of the hepatitis C virus (HCV) genome allowed the identification and biochemical characterization of two viral enzymes that are targets for antiviral therapy: the protease NS3-4A and the RNA-dependent RNA polymerase NS5B. With the advent of cell culture systems that can recapitulate either the intracellular steps of the viral replication cycle or the complete cycle, additional drug targets have been identified, most notably the phosphoprotein NS5A, but also host cell factors that promote viral replication, such as cyclophilin A. Here, we review insights into the structures of these proteins and the mechanisms by which they contribute to the HCV replication cycle, and discuss how these insights have facilitated the development of new, directly acting antiviral compounds that have started to enter the clinic.

N-terminal AH2 segment of protein NS4B from hepatitis C virus. Binding to and interaction with model biomembranes

HCV NS4B, a highly hydrophobic protein involved in the alteration of the intracellular host membranes forming the replication complex, plays a critical role in the HCV life cycle. NS4B is a multifunctional membrane protein that possesses different regions where diverse and significant functions are located. One of these important regions is the AH2 segment, which besides being highly conserved has been shown to play a significant role in NS4B functioning. We have carried out an in-depth biophysical study aimed at the elucidation of the capacity of this region to interact, modulate and disrupt membranes, as well as to study the structural and dynamic features relevant for that disruption. We show that a peptide derived from this region, NS4BAH2, is capable of specifically binding phosphatidyl inositol phosphates with high affinity, and its interfacial properties suggest that this segment could behave similarly to a pre-transmembrane domain partitioning into and interacting with the membrane depending on the membrane composition and/or other proteins. Moreover, NS4BAH2 is capable of rupturing membranes even at very low peptide-to-lipid ratios and its membrane-activity is modulated by lipid composition. NS4BAH2 is located in a shallow position in the membrane but it is able to affect the lipid environment from the membrane surface down to the hydrophobic core. The NS4B region where peptide NS4BAH2 resides might have an essential role in the membrane replication and/or assembly of the viral particle through the modulation of the membrane structure and hence the replication complex.

Identification of PTC725, an orally bioavailable small molecule that selectively targets the hepatitis C Virus NS4B protein

While new direct-acting antiviral agents for the treatment of chronic hepatitis C virus (HCV) infection have been approved, there is a continued need for novel antiviral agents that act on new targets and can be used in combination with current therapies to enhance efficacy and to restrict the emergence of drug-resistant viral variants. To this end, we have identified a novel class of small molecules, exemplified by PTC725, that target the nonstructural protein 4B (NS4B). PTC725 inhibited HCV 1b (Con1) replicons with a 50% effective concentration (EC50) of 1.7 nM and an EC90 of 9.6 nM and demonstrated a >1,000-fold selectivity window with respect to cytotoxicity. The compounds were fully active against HCV replicon mutants that are resistant to inhibitors of NS3 protease and NS5B polymerase. Replicons selected for resistance to PTC725 harbored amino acid substitutions F98L/C and V105M in NS4B. Anti-replicon activity of PTC725 was additive to synergistic in combination with alpha interferon or with inhibitors of HCV protease and polymerase. Immunofluorescence microscopy demonstrated that neither the HCV inhibitors nor the F98C substitution altered the subcellular localization of NS4B or NS5A in replicon cells. Oral dosing of PTC725 showed a favorable pharmacokinetic profile with high liver and plasma exposure in mice and rats. Modeling of dosing regimens in humans indicates that a once-per-day or twice-per-day oral dosing regimen is feasible. Overall, the preclinical data support the development of PTC725 for use in the treatment of chronic HCV infection.

Modulation of hepatitis C virus genome encapsidation by nonstructural protein 4B

Hepatitis C Virus (HCV) NS4B protein has many roles in HCV genome replication. Recently, our laboratory (Q. Han, J. Aligo, D. Manna, K. Belton, S. V. Chintapalli, Y. Hong, R. L. Patterson, D. B. van Rossum, and K. V. Konan, J. Virol. 85:6464-6479, 2011) and others (D. M. Jones, A. H. Patel, P. Targett-Adams, and J. McLauchlan, J. Virol. 83:2163-2177, 2009; D. Paul, I. Romero-Brey, J. Gouttenoire, S. Stoitsova, J. Krijnse-Locker, D. Moradpour, and R. Bartenschlager, J. Virol. 85:6963-6976, 2011) have also reported NS4B's function in postreplication steps. Indeed, replacement of the NS4B C-terminal domain (CTD) in the HCV JFH1 (genotype 2a [G2a]) genome with sequences from Con1 (G1b) or H77 (G1a) had a negligible impact on JFH1 genome replication but attenuated virus production. Since NS4B interacts weakly with the HCV genome, we postulated that NS4B regulates the function of host or virus proteins directly involved in HCV production. In this study, we demonstrate that the integrity of the JFH1 NS4B CTD is crucial for efficient JFH1 genome encapsidation. Further, two adaptive mutations (NS4B N216S and NS5A C465S) were identified, and introduction of these mutations into the chimera rescued virus production to various levels, suggesting a genetic interaction between the NS4B and NS5A proteins. Interestingly, cells infected with chimeric viruses displayed a markedly decreased NS5A hyperphosphorylation state (NS5A p58) relative to JFH1, and the adaptive mutations differentially rescued NS5A p58 formation. However, immunofluorescence staining indicated that the decrease in NS5A p58 did not alter NS5A colocalization with the core around lipid droplets (LDs), the site of JFH1 assembly, suggesting that NS5A fails to facilitate the transfer of HCV RNA to the capsid protein on LDs. Alternatively, NS4B's function in HCV genome encapsidation may entail more than its regulation of the NS5A phosphorylation state.

Targeting the non-structural proteins of hepatitis C virus: beyond hepatitis C virus protease and polymerase

BACKGROUND

Chronic hepatitis C virus (HCV) infection is a main cause of cirrhosis of the liver and hepatocellular carcinoma. The standard of care is a combination of pegylated interferon with ribavirin, a regimen that has undesirable side effects and is frequently ineffective. Compounds targeting HCV protease and polymerase are in late-stage clinical trials and have been extensively reviewed elsewhere.

OBJECTIVE

To review and evaluate the progress towards finding novel HCV antivirals targeting HCV proteins beyond the already precedented NS3 protease and NS5B polymerase.

METHODS

Searches of CAplus and Medline databases were combined with information from key conferences. This review focuses on NS2/3 serine protease, NS3 helicase activity and the non-structural proteins 4A, 4B and 5A.

CONCLUSIONS

Use of the replicon model of HCV replication and biochemical assays of specific targets has allowed screening of vast libraries of compounds, but resulted in clinical candidates from only NS4A and NS5A. The field is hindered by a lack of good chemical matter that inhibits the remaining enzymes from HCV, and a lack of understanding of the functions of non-structural proteins 4A, 4B and 5A in the replication of HCV.

Hepatitis C virus NS4B targets lipid droplets through hydrophobic residues in the amphipathic helices

Lipid droplets (LD) are dynamic storage organelles that are involved in lipid homeostasis. Hepatitis C virus (HCV) is closely associated with LDs. HCV Core and nonstructural (NS) proteins colocalize with LDs and presumably are involved in virion formation at that site. We demonstrated that HCV NS4B, an integral membrane protein in endoplasmic reticulum (ER), strongly targeted LDs. Confocal imaging studies showed that NS4B localized at the margins of LDs. Biochemical fractionation of HCV-replicating cells suggested that NS4B existed in membranes associated with LDs rather than on the LD surface membrane itself. The N- and C-terminal cytosolic domains of NS4B showed targeting of LDs, with the former being much stronger. In both domains, activity was present in the region containing an amphipathic α-helix, in which 10 hydrophobic residues were identified as putative determinants for targeting LDs. JFH1 mutants with alanine substitutions for the hydrophobic residues were defective for virus replication. W43A mutant with a single alanine substitution showed loss of association of NS4B with LDs and severely reduced release of infectious virions compared with wild-type JFH1. NS4B plays a crucial role in virus replication at the site of virion formation, namely, the microenvironment associated with LDs.

Modulation of replication efficacy of the hepatitis C virus replicon Con1 by site-directed mutagenesis of an NS4B aminoterminal basic leucine zipper

The hepatitis C virus (HCV) nonstructural protein 4B (NS4B) is assumed to function as a membrane anchor and protein hub for the viral replication complex. The aim of the current work was to modulate HCV replication efficacy in the subgenomic Con1 replicon by mutations of specific sites within the aminoterminal-located basic leucine zipper (bZIP), a candidate motif for protein-protein interactions involving NS4B. Mutational sites and amino acid substitutes were determined by in-silico sequence analyses of the NS4B-bZIP motif in 357 isolates of HCV genotype 1b from the euHCVdB and LosAlamos database and consecutive analysis of conserved physico-chemical properties at bZIP specific positions. Mutants with predicted minor, medium or major reduction of replication efficacy were tested in the pFKI389neo/NS3-3'/ET plasmid replicon model. Four sites (L25, T29, V39 and W43) of crucial importance for bZIP-mediated protein interaction with predicted apolarity of respective amino acid positions were selected for mutational studies. Substitutes with physico-chemical properties matching the predicted requirements either well (T29A), moderately (L25W, V39W), or insufficiently (T29E, W43E) were associated with slightly improved, moderate and marked decreased replication efficacy, respectively. Spontaneous (T29G) and adaptive (A28G, E40G) mutations occurred in the T29E mutation isolate only and were associated with marked reduction of replication efficacy. The bZIP motif region of NS4B is crucial for RNA replication in the subgenomic Con1 replicon system. RNA replication efficacy can be modulated by site-directed mutagenesis at specific bZIP functional sites. New adaptive amino acid mutations were identified within the HCV NS4B protein.

Interaction networks of hepatitis C virus NS4B: implications for antiviral therapy

Hepatitis C virus (HCV) is an important human pathogen infecting more than 170 million people worldwide with approximately three million new cases each year. HCV depends heavily on interactions between viral proteins and host factors for its survival and propagation. Among HCV viral proteins, the HCV non-structural protein 4B (NS4B) has been shown to mediate virus-host interactions that are essential for HCV replication and pathogenesis and emerged as the target for anti-HCV therapy. Here, we reviewed recent knowledge about the NS4B interaction networks with host factors and its possible regulatory mechanisms, which will both advance our understanding of the role of NS4B in HCV life cycle and illuminate potential viral and host therapeutic targets.

Persistent expression of hepatitis C virus non-structural proteins leads to increased autophagy and mitochondrial injury in human hepatoma cells

HCV infection is a major cause of chronic liver disease and liver cancer in the United States. To address the pathogenesis caused by HCV infection, recent studies have focused on the direct cytopathic effects of individual HCV proteins, with the objective of identifying their specific roles in the overall pathogenesis. However, this approach precludes examination of the possible interactions between different HCV proteins and organelles. To obtain a better understanding of the various cytopathic effects of and cellular responses to HCV proteins, we used human hepatoma cells constitutively replicating HCV RNA encoding either the full-length polyprotein or the non-structural proteins, or cells constitutively expressing the structural protein core, to model the state of persistent HCV infection and examined the combination of various HCV proteins in cellular pathogenesis. Increased reactive oxygen species (ROS) generation in the mitochondria, mitochondrial injury and degeneration, and increased lipid accumulation were common among all HCV protein-expressing cells regardless of whether they expressed the structural or non-structural proteins. Expression of the non-structural proteins also led to increased oxidative stress in the cytosol, membrane blebbing in the endoplasmic reticulum, and accumulation of autophagocytic vacuoles. Alterations of cellular redox state, on the other hand, significantly changed the level of autophagy, suggesting a direct link between oxidative stress and HCV-mediated activation of autophagy. With the wide-spread cytopathic effects, cells with the full-length HCV polyprotein showed a modest antioxidant response and exhibited a significant increase in population doubling time and a concomitant decrease in cyclin D1. In contrast, cells expressing the non-structural proteins were able to launch a vigorous antioxidant response with up-regulation of antioxidant enzymes. The population doubling time and cyclin D1 level were also comparable to that of control cells. Finally, the cytopathic effects of core protein appeared to focus on the mitochondria without remarkable disturbances in the cytosol.

NS4B self-interaction through conserved C-terminal elements is required for the establishment of functional hepatitis C virus replication complexes

Hepatitis C virus (HCV) is an important human pathogen, persistently infecting more than 170 million individuals worldwide. Studies of the HCV life cycle have become possible with the development of cell culture systems supporting the replication of viral RNA and the production of infectious virus. However, the exact functions of individual proteins, especially of nonstructural protein 4B (NS4B), remain poorly understood. NS4B triggers the formation of specific, vesicular membrane rearrangements, referred to as membranous webs, which have been reported to represent sites of HCV RNA replication. However, the mechanism of vesicle induction is not known. In this study, a panel of 15 mutants carrying substitutions in the highly conserved NS4B C-terminal domain was generated. Five mutations had only a minor effect on replication, but two of them enhanced assembly and release of infectious virus. Ten mutants were replication defective and used for selection of pseudoreversions. Most of the pseudoreversions also localized to the highly conserved NS4B C-terminal domain and were found to restore replication competence upon insertion into the corresponding primary mutant. Importantly, pseudoreversions restoring replication competence also restored heterotypic NS4B self-interaction, which was disrupted by the primary mutation. Finally, electron microscopy analyses of membrane alterations induced by NS4B mutants revealed striking morphological abnormalities, which were restored to wild-type morphology by the corresponding pseudoreversion. These findings demonstrate the important role of the C-terminal domain in NS4B self-interaction and the formation of functional HCV replication complexes.

An overview of HCV molecular biology, replication and immune responses

Hepatitis C virus (HCV) causes acute and chronic hepatitis which can eventually lead to permanent liver damage, hepatocellular carcinoma and death. Currently, there is no vaccine available for prevention of HCV infection due to high degree of strain variation. The current treatment of care, Pegylated interferon α in combination with ribavirin is costly, has significant side effects and fails to cure about half of all infections. In this review, we summarize molecular virology, replication and immune responses against HCV and discussed how HCV escape from adaptive and humoral immune responses. This advance knowledge will be helpful for development of vaccine against HCV and discovery of new medicines both from synthetic chemistry and natural sources.

The Future of HCV Therapy: NS4B as an Antiviral Target

Chronic hepatitis C virus (HCV) infection is a major worldwide cause of liver disease, including cirrhosis and hepatocellular carcinoma. It is estimated that more than 170 million individuals are infected with HCV, with three to four million new cases each year. The current standard of care, combination treatment with interferon and ribavirin, eradicates the virus in only about 50% of chronically infected patients. Notably, neither of these drugs directly target HCV. Many new antiviral therapies that specifically target hepatitis C (e.g. NS3 protease or NS5B polymerase inhibitors) are therefore in development, with a significant number having advanced into clinical trials. The nonstructural 4B (NS4B) protein, is among the least characterized of the HCV structural and nonstructural proteins and has been subjected to few pharmacological studies. NS4B is an integral membrane protein with at least four predicted transmembrane (TM) domains. A variety of functions have been postulated for NS4B, such as the ability to induce the membranous web replication platform, RNA binding and NTPase activity. This review summarizes potential targets within the nonstructural protein NS4B, with a focus on novel classes of NS4B inhibitors.

The acidic domain of hepatitis C virus NS4A contributes to RNA replication and virus particle assembly

Hepatitis C virus NS3-4A is a membrane-bound enzyme complex that exhibits serine protease, RNA helicase, and RNA-stimulated ATPase activities. This enzyme complex is essential for viral genome replication and has been recently implicated in virus particle assembly. To help clarify the role of NS4A in these processes, we conducted alanine scanning mutagenesis on the C-terminal acidic domain of NS4A in the context of a chimeric genotype 2a reporter virus. Of 13 mutants tested, two (Y45A and F48A) had severe defects in replication, while seven (K41A, L44A, D49A, E50A, M51A, E52A, and E53A) efficiently replicated but had severe defects in virus particle assembly. Multiple strategies were used to identify second-site mutations that suppressed these NS4A defects. The replication defect of NS4A F48A was partially suppressed by mutation of NS4B I7F, indicating that a genetic interaction between NS4A and NS4B contributes to RNA replication. Furthermore, the virus assembly defect of NS4A K41A was suppressed by NS3 Q221L, a mutation previously implicated in overcoming other virus assembly defects. We therefore examined the known enzymatic activities of wild-type or mutant forms of NS3-4A but did not detect specific defects in the mutants. Taken together, our data reveal interactions between NS4A and NS4B that control genome replication and between NS3 and NS4A that control virus assembly.

Amphipathic alpha-helix AH2 is a major determinant for the oligomerization of hepatitis C virus nonstructural protein 4B

Nonstructural protein 4B (NS4B) is a key organizer of hepatitis C virus (HCV) replication complex formation. It induces a specific membrane rearrangement, designated membranous web, that serves as a scaffold for the HCV replication complex. However, the mechanisms underlying membranous web formation are poorly understood. Based on fluorescence resonance energy transfer (FRET) and confirmatory coimmunoprecipitation analyses, we provide evidence for an oligomerization of NS4B in the membrane environment of intact cells. Several conserved determinants were found to be involved in NS4B oligomerization, through homotypic and heterotypic interactions. N-terminal amphipathic α-helix AH2, comprising amino acids 42 to 66, was identified as a major determinant for NS4B oligomerization. Mutations that affected the oligomerization of NS4B disrupted membranous web formation and HCV RNA replication, implying that oligomerization of NS4B is required for the creation of a functional replication complex. These findings enhance our understanding of the functional architecture of the HCV replication complex and may provide new angles for therapeutic intervention. At the same time, they expand the list of positive-strand RNA virus replicase components acting as oligomers.

Dimerization of the hepatitis C virus nonstructural protein 4B depends on the integrity of an aminoterminal basic leucine zipper

The hepatitis C virus (HCV) nonstructural (NS) protein 4B is known for protein-protein interactions with virus and host cell factors. Only little is known about the corresponding protein binding sites and underlying molecular mechanisms. Recently, we have predicted a putative basic leucine zipper (bZIP) motif within the aminoterminal part of NS4B. The aim of this study was to investigate the importance of this NS4B bZIP motif for specific protein-protein interactions. We applied in silico approaches for 3D-structure modeling of NS4B-homodimerization via the bZIP motif and identified crucial amino acid positions by multiple sequence analysis. The selected sites were used for site-directed mutagenesis within the NS4B bZIP motif and subsequent co-immunoprecipitation of wild-type and mutant NS4B molecules. Respective interaction energies were calculated for wild-type and mutant structural models. NS4B-homodimerization with a gradual alleviation of dimer interaction from wild-type towards the mutant-dimers was observed. The putative bZIP motif was confirmed by a co-immunoprecipitation assay and western blot analysis. NS4B-NS4B interaction depends on the integrity of the bZIP hydrophobic core and can be abolished due to changes of crucial residues within NS4B. In conclusion, our data indicate NS4B-homodimerization and that this interaction is facilitated by the aminoterminal part containing a bZIP motif.

Inhibition of hepatitis C virus replication by IFN-mediated ISGylation of HCV-NS5A

ISG15 is a ubiquitin-like molecule whose expression is induced by type I IFN (IFN-α/β) or in response to virus or bacterial infection. ISG15 or conjugation of ISG15 to target proteins was reported to play critical roles in the regulation of antiviral responses. IFN restricts replication of hepatitis C virus (HCV). However, molecular mechanism of IFN-α/β that inhibits HCV replication is not clear yet. In the current study, we demonstrated that replication of HCV was inhibited by overexpression of ISG15 and ISG15-conjugation enzymes in the HCV subgenomic replicon cells. Among various nonstructural proteins of HCV, NS5A was identified as the substrate for ISGylation. Furthermore, protein stability of NS5A was decreased by overexpression of ISG15 or ISG15-conjugating enzymes. The inhibitory effect of ISG15 or ISGylation on NS5A was efficiently blocked by substitution of lysine at 379 residue to arginine within the C-terminal region, suggesting that ISGylation directly controls protein stability of NS5A. Finally, the inhibitory effect of IFN-α/β on HCV replication was further enhanced by ISGylation, suggesting ISG15 as a therapeutic tool for combined therapy with IFN against HCV.

A small molecule inhibits HCV replication and alters NS4B's subcellular distribution

Hepatitis C Virus (HCV) is a leading cause of liver disease and represents a significant public health challenge. Treatments for this disease are inadequate and improved antiviral therapies are necessary. Several such antivirals are in development, most of which target the well-characterized NS3 protease or the NS5B polymerase. In contrast, the nonstructural 4B (NS4B) protein, though essential for HCV RNA replication, has been the subject of few pharmacological studies. One of the functions ascribed to this protein is the ability to form intracellular membrane-associated foci (MAF), which are believed to be related to the sites of viral replication. Here, we report the identification of a small molecule that inhibits HCV replication and disrupts the organization of these MAF. Genetic analysis links the compound's mode of action to the NS4B gene product, and transient transfections of NS4B-GFP demonstrate that treatment with this compound can lead to the formation of novel elongated assemblies of NS4B. Furthermore, an in vitro dynamic light scattering assay provides evidence that the second amphipathic helix of NS4B may be the target of the drug. Our results demonstrate that this molecule represents a new potential class of HCV inhibitors and also provides us with a useful tool for studying the HCV life cycle.

ER targeting and retention of the HCV NS4B protein relies on the concerted action of multiple structural features including its transmembrane domains

The Hepatitis C virus (HCV) NS4B protein, a multispanning endoplasmic reticulum (ER) membrane protein, generates intracellular rearrangements of ER-derived membranes, essential for HCV replication. In this study, we characterized NS4B elements involved in the process of targeting, association and retention in the ER membrane. We investigated the localization and membrane association of a number of C- or N-terminal NS4B deletions expressed as GFP chimeras by biochemical and fluorescence microscopy techniques. A second set of GFP-NS4B chimeras containing the plasma membrane ecto-ATPase CD39 at the C-terminus of each NS4B deletion mutant was used to further examine the role of N-terminal NS4B sequences in ER retention. Several structural elements, besides the first two transmembrane domains (TMs), within the NS4B N-terminal half (residues 1-130) were found to mediate association of the NS4B-GFP chimeras with ER membranes. Both TM1 and TM2 are required for ER anchoring and retention but are not sufficient for ER retention. Sequences upstream of TM1 are also required. These include two putative amphipathic alpha-helices and a Leucine Rich Repeat-like motif, a sequence highly conserved in all HCV genotypes. The N-terminal 55peptidic sequence, containing the 1st amphipathic helix, mediates association of the 55N-GFP chimera with cellular membranes including the ER, but is dispensable for ER targeting of the entire NS4B molecule. Importantly, the C-terminal 70peptidic sequence can associate with membranes positive for ER markers in the absence of any predicted TMs. In conclusion, HCV NS4B targeting and retention in the ER results from the concerted action of several NS4B structural elements.

Identification of a class of HCV inhibitors directed against the nonstructural protein NS4B

New classes of drugs are needed to combat hepatitis C virus (HCV), an important worldwide cause of liver disease. We describe an activity of a key domain, an amphipathic helix we termed 4BAH2, within a specific HCV nonstructural protein, NS4B. In addition to its proposed role in viral replication, we validate 4BAH2 as essential for HCV genome replication and identify first-generation small-molecule inhibitors of 4BAH2 that specifically prevent HCV replication within cells. Mechanistic studies reveal that the inhibitors target 4BAH2 function by preventing either 4BAH2 oligomerization or 4BAH2 membrane association. 4BAH2 inhibitors represent an additional class of compounds with potential to effectively treat HCV.

Mutations in classical swine fever virus NS4B affect virulence in swine

NS4B is one of the nonstructural proteins of classical swine fever virus (CSFV), the etiological agent of a severe, highly lethal disease of swine. Protein domain analysis of the predicted amino acid sequence of the NS4B protein of highly pathogenic CSFV strain Brescia (BICv) identified a putative Toll/interleukin-1 receptor (TIR)-like domain. This TIR-like motif harbors two conserved domains, box 1 and box 2, also observed in other members of the TIR superfamily, including Toll-like receptors (TLRs). Mutations within the BICv NS4B box 2 domain (V2566A, G2567A, I2568A) produced recombinant virus NS4B.VGIv, with an altered phenotype displaying enhanced transcriptional activation of TLR-7-induced genes in swine macrophages, including a significant sustained accumulation of interleukin-6 (IL-6) mRNA. Transfection of swine macrophages with the wild-type NS4B gene partially blocked the TLR-7-activating effect of imiquimod (R837), while transfection with the NS4B gene harboring mutations in either of the putative boxes displayed decreased blocking activity. NS4B.VGIv showed an attenuated phenotype in swine, displaying reduced replication in the oronasal cavity and limited spread from the inoculation site to secondary target organs. Furthermore, the level and duration of IL-6 production in the tonsils of pigs intranasally inoculated with NS4B.VGIv were significantly higher than those for animals infected with BICv. The peak of IL-6 production in infected animals paralleled the ability of animals infected with NS4B.VGIv to resist challenge with virulent BICv. Interestingly, treatment of peripheral blood mononuclear cell cultures with recombinant porcine IL-6 results in a significant decrease in BICv replication.

Formation and function of hepatitis C virus replication complexes require residues in the carboxy-terminal domain of NS4B protein

During replication, hepatitis C virus (HCV) NS4B protein rearranges intracellular membranes to form foci, or the web, the putative site for HCV replication. To understand the role of the C-terminal domain (CTD) in NS4B function, mutations were introduced into NS4B alone or in the context of HCV polyprotein. First, we show that the CTD is required for NS4B-induced web structure, but it is not sufficient to form the web nor is it required for NS4B membrane association. Interestingly, all the mutations introduced into the CTD impeded HCV genome replication, but only two resulted in a disruption of NS4B foci. Further, we found that NS4B interacts with NS3 and NS5A, and that mutations causing NS4B mislocalization have a similar effect on these proteins. Finally, we show that the redistribution of Rab5 to NS4B foci requires an intact CTD, suggesting that Rab5 facilitates NS4B foci formation through interaction with the CTD.