The hepatitis C virus core protein interacts with NS5A and activates its caspase-mediated proteolytic cleavage

Stress proteins: the biological functions in virus infection, present and challenges for target-based antiviral drug development

Stress proteins (SPs) including heat-shock proteins (HSPs), RNA chaperones, and ER associated stress proteins are molecular chaperones essential for cellular homeostasis. The major functions of HSPs include chaperoning misfolded or unfolded polypeptides, protecting cells from toxic stress, and presenting immune and inflammatory cytokines. Regarded as a double-edged sword, HSPs also cooperate with numerous viruses and cancer cells to promote their survival. RNA chaperones are a group of heterogeneous nuclear ribonucleoproteins (hnRNPs), which are essential factors for manipulating both the functions and metabolisms of pre-mRNAs/hnRNAs transcribed by RNA polymerase II. hnRNPs involve in a large number of cellular processes, including chromatin remodelling, transcription regulation, RNP assembly and stabilization, RNA export, virus replication, histone-like nucleoid structuring, and even intracellular immunity. Dysregulation of stress proteins is associated with many human diseases including human cancer, cardiovascular diseases, neurodegenerative diseases (e.g., Parkinson’s diseases, Alzheimer disease), stroke and infectious diseases. In this review, we summarized the biologic function of stress proteins, and current progress on their mechanisms related to virus reproduction and diseases caused by virus infections. As SPs also attract a great interest as potential antiviral targets (e.g., COVID-19), we also discuss the present progress and challenges in this area of HSP-based drug development, as well as with compounds already under clinical evaluation.

Identification of cleavage of NS5A of C-strain classical swine fever virus

NS5A is a multifunctional non-structural protein of classical swine fever virus (CSFV) that plays an important role in viral replication, but how it exerts its functions is unknown. Here, we report the cleavage of NS5A of the vaccine C-strain, resulting in two truncated forms (b and c). Further experiments using calpain- and caspase-family-specific inhibitors, followed by a caspase-6-specific shRNAs and inhibitor, showed that the cleavage of C-strain NS5A to produce truncated form c is mediated by caspase-6, mapping to ²⁷²DTTD²⁷⁵, while the cleavage producing truncated form b is probably mediated by another unknown protease. shRNA-mediated downregulation of caspase-6 and blocking of enzyme activity in ST cells significantly impaired genome replication and virus production, indicating that NS5A cleavage is required for CSFV replication.

Understanding the interaction determinants of CAPN1 inhibition by CAST4 from bovines using molecular modeling techniques

HCV-induced CAPN activation and its effects on virus-infected cells in a host-immune system have been studied recently. It has been shown that the HCV-nonstructural 5A protein acts as both an inducer and a substrate for host CAPN protease; it participates in suppressing the TNF-α-induced apoptosis response and downstream IFN-induced antiviral processes. However, little is known regarding the disturbance of antiviral responses generated by bovine CAPN activation by BVDV, which is a surrogate model of HCV and is one of the most destructive diseases leading to great economic losses in cattle herds worldwide. This is also thought to be associated with the effects of either small CAPN inhibitors or the natural inhibitor CAST. They mainly bind to the binding site of CAPN substrate proteins and competitively inhibit the binding of the enzyme substrates to possibly defend against the two viruses (HCV and BVDV) for anti-viral immunity. To devise a new stratagem to discover lead candidates for an anti-BVDV drug, we first attempted to understand the bovine CAPN-CAST interaction sites and the interaction constraints of local binding architectures, were well reflected in the geometry between the pharmacophore features and its shape constraints identified using our modeled bovine CAPN1/CAST4 complex structures. We propose a computer-aided molecular design of an anti-BVDV drug as a mimetic CAST inhibitor to develop a rule-based screening function for adjusting the puzzle of relationship between bovine CAPN1 and the BVDV nonstructural proteins from all of the data obtained in the study.

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.

The C terminus of NS5A domain II is a key determinant of hepatitis C virus genome replication, but is not required for virion assembly and release

The NS5A protein of hepatitis C virus (HCV) plays roles in both virus genome replication and the assembly of infectious virus particles. NS5A comprises three domains, separated by low-complexity sequences. Whilst the function of domain I appears to be predominantly involved with genome replication, the roles of domains II and III are less well defined. It has been reported previously that a deletion spanning the majority of domain II but retaining the C-terminal 35 residues had no effect on virus production; however, deletion of the entire domain II eliminated genome replication, pointing to a key role for the C terminus of this domain. Recent work has also highlighted this region as the potential binding site of the host factor cyclophilin A (CypA). To define this requirement for replication in more detail, and to investigate the involvement of CypA, we conducted a mutagenic study of the C-terminal 30 residues of domain II within the context of both the infectious JFH-1 virus and a JFH-1-derived subgenomic replicon. We showed that 12 of these residues were absolutely required for virus genome replication, whilst mutations of the remainder either had no phenotype or exhibited a partial reduction in genome replication. There was an absolute correlation between the datasets for virus and subgenomic replicon, indicating that this region is involved solely in the process of genome replication. Comparison of our data with a previously published analysis of the same region in genotype 1b revealed some important differences between the two genotypes of HCV.

Hepatitis C virus-induced mitochondrial dysfunctions

Chronic hepatitis C is characterized by metabolic disorders and a microenvironment in the liver dominated by oxidative stress, inflammation and regeneration processes that lead in the long term to hepatocellular carcinoma. Many lines of evidence suggest that mitochondrial dysfunctions, including modification of metabolic fluxes, generation and elimination of oxidative stress, Ca²⁺ signaling and apoptosis, play a central role in these processes. However, how these dysfunctions are induced by the virus and whether they play a role in disease progression and neoplastic transformation remains to be determined. Most in vitro studies performed so far have shown that several of the hepatitis C virus (HCV) proteins localize to mitochondria, but the consequences of these interactions on mitochondrial functions remain contradictory, probably due to the use of artificial expression and replication systems. In vivo studies are hampered by the fact that innate and adaptive immune responses will overlay mitochondrial dysfunctions induced directly in the hepatocyte by HCV. Thus, the molecular aspects underlying HCV-induced mitochondrial dysfunctions and their roles in viral replication and the associated pathology need yet to be confirmed in the context of productively replicating virus and physiologically relevant in vitro and in vivo model systems.

Regulation of hepatitis C virus replication by nuclear translocation of nonstructural 5A protein and transcriptional activation of host genes

Hepatitis C virus (HCV) nonstructural protein 5A (NS5A) is involved in regulating viral replication through its direct interaction with the HCV RNA-dependent RNA polymerase. NS5A also alters infected cell metabolism through complex interactions with numerous host cell proteins. NS5A has furthermore been suggested to act as a transcriptional activator, although the impact on viral replication is unclear. To study this, HCV NS5A variants were amplified from hepatic tissue from an HCV-infected patient, and their abilities to activate gene transcription were analyzed in a single-hybrid yeast (Saccharomyces cerevisiae) model. Different variants isolated from the same patient displayed different transactivational activities. When these variants were inserted into the HCV subgenomic replicon system, they demonstrated various levels of RNA replication, which correlated with their transactivational activities. We showed that the C-terminal fragment of NS5A was localized to the nucleus and that a functional NS5A nuclear localization signal and cellular caspase activity were required for this process. Furthermore, nuclear localization of NS5A was necessary for viral replication. Finally, we demonstrate that nuclear NS5A binds to host cell promoters of several genes previously identified as important for efficient HCV RNA replication, inducing their transcription. Taken together, these results demonstrate a new mechanism by which HCV modulates its cellular environment, thereby enhancing viral replication.

Diacylglycerol acyltransferase-1 localizes hepatitis C virus NS5A protein to lipid droplets and enhances NS5A interaction with the viral capsid core

The triglyceride-synthesizing enzyme acyl CoA:diacylglycerol acyltransferase 1 (DGAT1) plays a critical role in hepatitis C virus (HCV) infection by recruiting the HCV capsid protein core onto the surface of cellular lipid droplets (LDs). Here we find a new interaction between the non-structural protein NS5A and DGAT1 and show that the trafficking of NS5A to LDs depends on DGAT1 activity. DGAT1 forms a complex with NS5A and core and facilitates the interaction between both viral proteins. A catalytically inactive mutant of DGAT1 (H426A) blocks the localization of NS5A, but not core, to LDs in a dominant-negative manner and impairs the release of infectious viral particles, underscoring the importance of DGAT1-mediated translocation of NS5A to LDs in viral particle production. We propose a model whereby DGAT1 serves as a cellular hub for HCV core and NS5A proteins, guiding both onto the surface of the same subset of LDs, those generated by DGAT1. These results highlight the critical role of DGAT1 as a host factor for HCV infection and as a potential drug target for antiviral therapy.

Cell culture-adaptive mutations promote viral protein-protein interactions and morphogenesis of infectious hepatitis C virus

Recent genetic studies suggested that viral nonstructural (NS) proteins play important roles in morphogenesis of flaviviruses, particularly hepatitis C virus (HCV). Adaptive and compensatory mutations occurring in different NS proteins were demonstrated to promote HCV production in cell culture. However, the underlying molecular mechanism of NS proteins in HCV morphogenesis is poorly understood. We have isolated a cell culture-adapted HCV of genotype 2a (JFH1) which grew to an infectious titer 3 orders of magnitude higher than that of wild-type virus. Sequence analysis identified a total of 16 amino acid mutations in core (C), E1, NS2, NS3, NS5A, and NS5B, with the majority of mutations clustered in NS5A. Reverse genetic analysis of these mutations individually or in different combinations demonstrated that amino acid mutations in NS2 and NS5A markedly enhanced HCV production. Additionally, mutations in C, E1, NS3, and NS5B synergistically promoted HCV production in the background of NS2 and NS5A mutations. Adaptive mutations in NS5A domains I, II, and III independently enhanced HCV production, suggesting that all three domains of NS5A are important for HCV morphogenesis. More importantly, adaptive mutations greatly enhanced physical interactions among HCV structural and NS proteins, as determined by studies with coimmunoprecipitation and mammalian two-hybrid assays. Collectively, these findings demonstrate that adaptive mutations can enhance specific protein-protein interactions among viral structural and NS proteins and therefore promote the assembly of infectious HCV particles.

Transient structure and SH3 interaction sites in an intrinsically disordered fragment of the hepatitis C virus protein NS5A

Understanding the molecular mechanisms involved in virus replication and particle assembly is of primary fundamental and biomedical importance. Intrinsic conformational disorder plays a prominent role in viral proteins and their interaction with other viral and host cell proteins via transiently populated structural elements. Here, we report on the results of an investigation of an intrinsically disordered 188-residue fragment of the hepatitis C virus non-structural protein 5A (NS5A), which contains a classical poly-proline Src homology 3 (SH3) binding motif, using sensitivity- and resolution-optimized multidimensional NMR methods, complemented by small-angle X-ray scattering data. Our study provides detailed atomic-resolution information on transient local and long-range structure, as well as fast time scale dynamics in this NS5A fragment. In addition, we could characterize two distinct interaction modes with the SH3 domain of Bin1 (bridging integrator protein 1), a pro-apoptotic tumor suppressor. Despite being largely disordered, the protein contains three regions that transiently adopt α-helical structures, partly stabilized by long-range tertiary interactions. Two of these transient α-helices form a noncanonical SH3-binding motif, which allows low-affinity SH3 binding. Our results contribute to a better understanding of the role of the NS5A protein during hepatitis C virus infection. The present work also highlights the power of NMR spectroscopy to characterize multiple binding events including short-lived transient interactions between globular and highly disordered proteins.

Caspase cleavage of viral proteins, another way for viruses to make the best of apoptosis

Viral infection constitutes an unwanted intrusion that needs to be eradicated by host cells. On one hand, one of the first protective barriers set up to prevent viral replication, spread or persistence involves the induction of apoptotic cell death that aims to limit the availability of the cellular components for viral amplification. On the other hand, while they completely depend on the host molecular machinery, viruses also need to evade the cellular responses that are meant to destroy them. The existence of numerous antiapoptotic products within the viral kingdom proves that apoptosis constitutes a major threat that should better be bypassed. Among the different strategies developed to deal with apoptosis, one is based on what viruses do best: backfiring the cell on itself. Several unrelated viruses have been described to take advantage of apoptosis induction by expressing proteins targeted by caspases, the key effectors of apoptotic cell death. Caspase cleavage of these proteins results in various consequences, from logical apoptosis inhibition to more surprising enhancement or attenuation of viral replication. The present review aims at discussing the characterization and relevance of this post-translational modification that adds a new complexity in the already intricate host-apoptosis-virus triangle.

Interaction between Core protein of classical swine fever virus with cellular IQGAP1 protein appears essential for virulence in swine

Here we show that IQGAP1, a cellular protein that plays a pivotal role as a regulator of the cytoskeleton interacts with Classical Swine Fever Virus (CSFV) Core protein. Sequence analyses identified residues within CSFV Core protein (designated as areas I, II, III and IV) that maintain homology to regions within the matrix protein of Moloney Murine Leukemia Virus (MMLV) that mediate binding to IQGAP1 [EMBO J, 2006 25:2155]. Alanine-substitution within Core regions I, II, III and IV identified residues that specifically mediate the Core-IQGAP1 interaction. Recombinant CSFV viruses harboring alanine substitutions at residues (207)ATI(209) (I), (210)VVE(212) (II), (213)GVK(215) (III), or (232)GLYHN(236) (IV) have defective growth in primary swine macrophage cultures. In vivo, substitutions of residues in areas I and III yielded viruses that were completely attenuated in swine. These data shows that the interaction of Core with an integral component of cytoskeletal regulation plays a role in the CSFV cycle.

Effects of the interactions of classical swine fever virus Core protein with proteins of the SUMOylation pathway on virulence in swine

Here we have identified host cell proteins involved with the cellular SUMOylation pathway, SUMO-1 (small ubiquitin-like modifier) and UBC9, a SUMO-1 conjugating enzyme that interact with classical swine fever virus (CSFV) Core protein. Five highly conserved lysine residues (K179, K180, K220, K221, and K246) within the CSFV Core were identified as putative SUMOylation sites. Analysis of these interactions showed that K179A, K180A, and K221A substitutions disrupt Core-SUMO-1 binding, while K220A substitution precludes Core-UBC9 binding. In vivo, Core mutant viruses (K179A, K180A, K220A, K221A) and (K220A, K221A) harboring those substitutions were attenuated in swine. These data shows a clear correlation between the disruption of Core protein binding to SUMO-1 and UBC9 and CSFV attenuation. Overall, these data suggest that the interaction of Core with the cellular SUMOylation pathway plays a significant role in the CSFV growth cycle in vivo.

Core as a novel viral target for hepatitis C drugs

Hepatitis C virus (HCV) infects over 130 million people worldwide and is a major cause of liver disease. No vaccine is available. Novel specific drugs for HCV are urgently required, since the standard-of-care treatment of pegylated interferon combined with ribavirin is poorly tolerated and cures less than half of the treated patients. Promising, effective direct-acting drugs currently in the clinic have been described for three of the ten potential HCV target proteins: NS3/NS4A protease, NS5B polymerase and NS5A, a regulatory phosphoprotein. We here present core, the viral capsid protein, as another attractive, non-enzymatic target, against which a new class of anti-HCV drugs can be raised. Core plays a major role in the virion's formation, and interacts with several cellular proteins, some of which are involved in host defense mechanisms against the virus. This most conserved of all HCV proteins requires oligomerization to function as the organizer of viral particle assembly. Using core dimerization as the basis of transfer-of-energy screening assays, peptides and small molecules were identified which not only inhibit core-core interaction, but also block viral production in cell culture. Initial chemical optimization resulted in compounds active in single digit micromolar concentrations. Core inhibitors could be used in combination with other HCV drugs in order to provide novel treatments of Hepatitis C.

Intracellular cleavage of sigmaA protein of avian reovirus

By Western blot analyzes of expression of avian reovirus proteins, one unknown fragment was detected by an anti-sigmaA monoclonal antibody in virus-infected cells lysate. It was interesting to note that RNA interference against sigmaA resulted in the suppression of the unknown fragment. Using various lengths of sigmaA constructs conjugated with different tags, we present evidences to demonstrate that the fragment comes from the cleavage of sigmaA and is the larger carboxyl-terminus, termed sigmaAC. Cleavage of sigmaA simultaneously produces a smaller amino-terminus, named sigmaAN. sigmaAC could be seen early in viral infection and accumulated with time and dose of infection, indicating that the derived products are not just transient intermediates of protein degradation. The same type of cleaved products were also observed in different genotypes and serotypes of ARV as well as in different cell lines, suggesting that this intracellular modification of sigmaA is common to all ARVs. Similar localization of sigmaAC in both cytosol and nucleus with sigmaA suggested that further modification of sigmaA may be important for its function. Our evidences suggest that besides the outer capsid protein muB, sigmaA may also have post-translational cleavage which has never been reported before even in related mammalian reovirus.

Hepatitis C virus core protein induces apoptosis-like caspase independent cell death

Background

Hepatitis C virus (HCV) associated liver diseases may be related to apoptotic processes. Thus, we investigated the role of different HCV proteins in apoptosis induction as well as their potency to interact with different apoptosis inducing agents.

Methods and Results

The use of a tightly adjustable tetracycline (Tet)-dependent HCV protein expression cell system with the founder osteosarcoma cell line U-2 OS allowed switch-off and on of the endogenous production of HCV proteins. Analyzed were cell lines expressing the HCV polyprotein, the core protein, protein complexes of the core, envelope proteins E1, E2 and p7, and non-structural proteins NS3 and NS4A, NS4B or NS5A and NS5B. Apoptosis was measured mainly by the detection of hypodiploid apoptotic nuclei in the absence or presence of mitomycin C, etoposide, TRAIL and an agonistic anti-CD95 antibody. To further characterize cell death induction, a variety of different methods like fluorescence microscopy, TUNEL (terminal deoxynucleotidyl transferase (TdT)-catalyzed deoxyuridinephosphate (dUTP)-nick end labeling) assay, Annexin V staining, Western blot and caspase activation assays were included into our analysis.

Two cell lines expressing the core protein but not the total polyprotein exerted a strong apoptotic effect, while the other cell lines did not induce any or only a slight effect by measuring the hypodiploid nuclei. Cell death induction was caspase-independent since it could not be blocked by zVAD-fmk. Moreover, caspase activity was absent in Western blot analysis and fluorometric assays while typical apoptosis-associated morphological features like the membrane blebbing and nuclei condensation and fragmentation could be clearly observed by microscopy. None of the HCV proteins influenced the apoptotic effect mediated via the mitochondrial apoptosis pathway while only the core protein enhanced death-receptor-mediated apoptosis.

Conclusion

Our data showed a caspase-independent apoptosis-like effect of the core protein, which seems to be inhibited in the presence of further HCV proteins like the non structural (NS) proteins. This observation could be of relevance for the viral spread since induction of an apoptosis-like cell death by the core protein may have some impact on the release of the HCV particles from the host cell.

Identification of in vivo interaction between Hepatitis C Virus core protein and 5' and 3' UTR RNA

Here, we investigated the ability of the Hepatitis C Virus (HCV) core protein to interact specifically with the 5' and 3' untranslated regions (UTRs) of HCV using an in vivo cell-based translation inhibition assay. HCV core protein interacts weakly but specifically with the SLIII stem loop in the 5' UTR in which the SLIIIb subdomain is the major determinant and the SL2 loop in the X region of the 3' UTR. These results revealed for the first time in vivo interaction of the core protein with 5' and 3' UTRs involved in the viral life cycle. This system provides a useful tool for further investigating interactions between the HCV core protein and 5' and 3' UTRs.

Architects of assembly: roles of Flaviviridae non-structural proteins in virion morphogenesis

Viruses of the Flaviviridae family, including hepatitis C, dengue and bovine viral diarrhoea, are responsible for considerable morbidity and mortality worldwide. Recent advances in our understanding of virion assembly have uncovered commonalities among distantly related members of this family. We discuss the emerging hypothesis that physical virion components are not alone in forming the infectious particle, but that non-structural proteins are intimately involved in orchestrating morphogenesis. Pinpointing the roles of Flaviviridae proteins in virion production could reveal new avenues for antiviral therapeutics.

Viral subversion of apoptotic enzymes: escape from death row

To prolong cell viability and facilitate replication, viruses have evolved multiple mechanisms to inhibit the host apoptotic response. Cellular proteases such as caspases and serine proteases are instrumental in promoting apoptosis. Thus, these enzymes are logical targets for virus-mediated modulation to suppress cell death. Four major classes of viral inhibitors antagonize caspase function: serpins, p35 family members, inhibitor of apoptosis proteins, and viral FLICE-inhibitory proteins. Viruses also subvert activity of the serine proteases, granzyme B and HtrA2/Omi, to avoid cell death. The combined efforts of viruses to suppress apoptosis suggest that this response should be avoided at all costs. However, some viruses utilize caspases during replication to aid virus protein maturation, progeny release, or both. Hence, a multifaceted relationship exists between viruses and the apoptotic response they induce. Examination of these interactions contributes to our understanding of both virus pathogenesis and the regulation of apoptotic enzymes in normal cellular functions.

Interaction of hepatitis C virus nonstructural protein 5A with core protein is critical for the production of infectious virus particles

Nonstructural protein 5A (NS5A) of the hepatitis C virus (HCV) possesses multiple and diverse functions in RNA replication, interferon resistance, and viral pathogenesis. Recent studies suggest that NS5A is involved in the assembly and maturation of infectious viral particles; however, precisely how NS5A participates in virus production has not been fully elucidated. In the present study, we demonstrate that NS5A is a prerequisite for HCV particle production as a result of its interaction with the viral capsid protein (core protein). The efficiency of virus production correlated well with the levels of interaction between NS5A and the core protein. Alanine substitutions for the C-terminal serine cluster in domain III of NS5A (amino acids 2428, 2430, and 2433) impaired NS5A basal phosphorylation, leading to a marked decrease in NS5A-core interaction, disturbance of the subcellular localization of NS5A, and disruption of virion production. Replacing the same serine cluster with glutamic acid, which mimics the presence of phosphoserines, partially preserved the NS5A-core interaction and virion production, suggesting that phosphorylation of these serine residues is important for virion production. In addition, we found that the alanine substitutions in the serine cluster suppressed the association of the core protein with viral genome RNA, possibly resulting in the inhibition of nucleocapsid assembly. These results suggest that NS5A plays a key role in regulating the early phase of HCV particle formation by interacting with core protein and that its C-terminal serine cluster is a determinant of the NS5A-core interaction.

Interaction of hepatitis C virus nonstructural protein 5A with core protein is critical for the production of infectious virus particles

Nonstructural protein 5A (NS5A) of the hepatitis C virus (HCV) possesses multiple and diverse functions in RNA replication, interferon resistance, and viral pathogenesis. Recent studies suggest that NS5A is involved in the assembly and maturation of infectious viral particles; however, precisely how NS5A participates in virus production has not been fully elucidated. In the present study, we demonstrate that NS5A is a prerequisite for HCV particle production as a result of its interaction with the viral capsid protein (core protein). The efficiency of virus production correlated well with the levels of interaction between NS5A and the core protein. Alanine substitutions for the C-terminal serine cluster in domain III of NS5A (amino acids 2428, 2430, and 2433) impaired NS5A basal phosphorylation, leading to a marked decrease in NS5A-core interaction, disturbance of the subcellular localization of NS5A, and disruption of virion production. Replacing the same serine cluster with glutamic acid, which mimics the presence of phosphoserines, partially preserved the NS5A-core interaction and virion production, suggesting that phosphorylation of these serine residues is important for virion production. In addition, we found that the alanine substitutions in the serine cluster suppressed the association of the core protein with viral genome RNA, possibly resulting in the inhibition of nucleocapsid assembly. These results suggest that NS5A plays a key role in regulating the early phase of HCV particle formation by interacting with core protein and that its C-terminal serine cluster is a determinant of the NS5A-core interaction.

Essential role of domain III of nonstructural protein 5A for hepatitis C virus infectious particle assembly

Persistent infection with the hepatitis C virus (HCV) is a major risk factor for the development of liver cirrhosis and hepatocellular carcinoma. With an estimated about 3% of the world population infected with this virus, the lack of a prophylactic vaccine and a selective therapy, chronic hepatitis C currently is a main indication for liver transplantation. The establishment of cell-based replication and virus production systems has led to first insights into the functions of HCV proteins. However, the role of nonstructural protein 5A (NS5A) in the viral replication cycle is so far not known. NS5A is a membrane-associated RNA-binding protein assumed to be involved in HCV RNA replication. Its numerous interactions with the host cell suggest that NS5A is also an important determinant for pathogenesis and persistence. In this study we show that NS5A is a key factor for the assembly of infectious HCV particles. We specifically identify the C-terminal domain III as the primary determinant in NS5A for particle formation. We show that both core and NS5A colocalize on the surface of lipid droplets, a proposed site for HCV particle assembly. Deletions in domain III of NS5A disrupting this colocalization abrogate infectious particle formation and lead to an enhanced accumulation of core protein on the surface of lipid droplets. Finally, we show that mutations in NS5A causing an assembly defect can be rescued by trans-complementation. These data provide novel insights into the production of infectious HCV and identify NS5A as a major determinant for HCV assembly. Since domain III of NS5A is one of the most variable regions in the HCV genome, the results suggest that viral isolates may differ in their level of virion production and thus in their level of fitness and pathogenesis.

Essential role of domain III of nonstructural protein 5A for hepatitis C virus infectious particle assembly

Persistent infection with the hepatitis C virus (HCV) is a major risk factor for the development of liver cirrhosis and hepatocellular carcinoma. With an estimated about 3% of the world population infected with this virus, the lack of a prophylactic vaccine and a selective therapy, chronic hepatitis C currently is a main indication for liver transplantation. The establishment of cell-based replication and virus production systems has led to first insights into the functions of HCV proteins. However, the role of nonstructural protein 5A (NS5A) in the viral replication cycle is so far not known. NS5A is a membrane-associated RNA-binding protein assumed to be involved in HCV RNA replication. Its numerous interactions with the host cell suggest that NS5A is also an important determinant for pathogenesis and persistence. In this study we show that NS5A is a key factor for the assembly of infectious HCV particles. We specifically identify the C-terminal domain III as the primary determinant in NS5A for particle formation. We show that both core and NS5A colocalize on the surface of lipid droplets, a proposed site for HCV particle assembly. Deletions in domain III of NS5A disrupting this colocalization abrogate infectious particle formation and lead to an enhanced accumulation of core protein on the surface of lipid droplets. Finally, we show that mutations in NS5A causing an assembly defect can be rescued by trans-complementation. These data provide novel insights into the production of infectious HCV and identify NS5A as a major determinant for HCV assembly. Since domain III of NS5A is one of the most variable regions in the HCV genome, the results suggest that viral isolates may differ in their level of virion production and thus in their level of fitness and pathogenesis.

The hepatitis C virus (HCV) is a major cause of acute and chronic liver diseases worldwide. In spite of high medical need there is no selective antiviral therapy available and a prophylactic vaccine is not in sight. Their development requires cellular replication systems that have become available just recently. One of the most fascinating insights gained with these systems is the finding that infectious HCV particles assemble in close association with an intracellular lipid storage compartment termed lipid droplets. In this study we show that nonstructural protein 5A (NS5A), a component of the viral RNA replication machinery is a key factor for the formation of infectious HCV particles. We identify a distinct domain in NS5A as the primary “assembly determinant” and show that NS5A and the core protein, which is a major constituent of the virus particle, accumulate on the surface of lipid droplets. Deletions in NS5A disrupting the colocalization of core and NS5A on lipid droplets abrogate infectious HCV production. These studies unravel a unique pathway of infectious virus formation and identify NS5A as a factor modulating HCV replication and assembly and thus viral fitness.

Identification of residues required for RNA replication in domains II and III of the hepatitis C virus NS5A protein

The NS5A protein of hepatitis C virus (HCV) plays an important but undefined role in viral RNA replication. NS5A has been proposed to be a three-domain protein, and the crystal structure of the well-conserved amino-terminal domain I has been determined. The remaining two domains of NS5A, designated domains II and III, and their corresponding interdomain regions are poorly understood. We have conducted a detailed mutagenesis analysis of NS5A domains II and III using the genotype 1b HCV replicon system. The majority of the mutants containing 15 small (8- to 15-amino-acid) deletions analyzed were capable of efficient RNA replication. Only five deletion mutations yielded lethal phenotypes, and these were colinear, spanning a 56-amino-acid region within domain II. This region was further analyzed by combining triple and single alanine scanning mutagenesis to identify individual residues required for RNA replication. Based upon this analysis, 23 amino acids were identified that were found to be essential. In addition, two residues were identified that yielded a small colony phenotype while possessing only a moderate defect in RNA replication. These results indicate that the entire domain III region and large portions of domain II of the NS5A protein are not required for the function of NS5A in HCV RNA replication.

Saccharomyces cerevisiae: a versatile eukaryotic system in virology

The yeast Saccharomyces cerevisiae is a well-established model system for understanding fundamental cellular processes relevant to higher eukaryotic organisms. Less known is its value for virus research, an area in which Saccharomyces cerevisiae has proven to be very fruitful as well. The present review will discuss the main achievements of yeast-based studies in basic and applied virus research. These include the analysis of the function of individual proteins from important pathogenic viruses, the elucidation of key processes in viral replication through the development of systems that allow the replication of higher eukayotic viruses in yeast, and the use of yeast in antiviral drug development and vaccine production.

Phosphorylation of hepatitis C virus NS5A nonstructural protein: a new paradigm for phosphorylation-dependent viral RNA replication?

The hepatitis C virus (HCV) nonstructural 5A (NS5A) phosphoprotein has been intensely studied due to its ability to subvert the host interferon-induced antiviral response. However, more recent studies suggest that it may also play an important regulatory role in HCV RNA replication as well as modulate host intracellular signaling pathways. Phosphorylation of NS5A appears to be a highly regulated process and several cellular protein kinases responsible for NS5A phosphorylation have been identified in vitro. Studies utilizing the HCV replicon cell culture system have suggested a provocative role for the differential phosphorylation of NS5A in the regulation of viral RNA replication through its association with the viral replication complex, including several host cell factors. Importantly, recent in vivo data linking loss of NS5A hyperphosphorylation to non-productive HCV replication in the chimpanzee model have provided high validation for targeting the cellular kinases involved, particularly the kinases responsible for NS5A phosphorylation, for antiviral therapeutic intervention. Understanding the process of NS5A phosphorylation and the definite identification of the culprit cellular protein kinase(s) will shed light on the mechanisms of HCV RNA replication and/or pathogenesis.

Hepatitis C virus polyprotein vaccine formulations capable of inducing broad antibody and cellular immune responses

Although approximately 3 % of the world's population is infected with Hepatitis C virus (HCV), there is no prophylactic vaccine available. This study reports the design, cloning and purification of a single polyprotein comprising the HCV core protein and non-structural proteins NS3, NS4a, NS4b, NS5a and NS5b. The immunogenicity of this polyprotein, which was formulated in alum, oil-in-water emulsion MF59 or poly(dl-lactide co-glycolide) in the presence or absence of CpG adjuvant, was then determined in a murine model for induction of B- and T-cell responses. The addition of adjuvants or a delivery system to the HCV polyprotein enhanced serum antibody and T-cell proliferative responses, as well as IFN-gamma responses, by CD4+ T cells. The antibody responses were mainly against the NS3 and NS5 components of the polyprotein and relatively poor responses were elicited against NS4 and the core components. IFN-gamma responses, however, were induced against all of the individual components of the polyprotein. These data suggest that the HCV polyprotein delivered with adjuvants induces broad B- and T-cell responses and could be a vaccine candidate against HCV.

Intrahepatic activation of caspases in human fulminant hepatic failure

BACKGROUND/AIMS

Apoptosis has been implicated in the pathogenesis of fulminant hepatic failure (FHF) potentially involving caspases. Thus far, apoptosis in FHF has mainly been studied in animal models while human data are sparse.

METHODS

Caspases-3, -8 and -9 activities and Fas expression were analyzed in correlation to TdT-mediated dUTP nick end labelling (TUNEL) positive apoptotic cells in livers of patients with FHF (n=26), chronic liver disease (CLD) (n=60) and normal controls (NC) (n=10).

RESULTS

Numbers of TUNEL-positive cells were higher in FHF than in CLD and NC (P<0.001) correlating to the intrahepatic activities of caspase-3. The highest caspase-3 activities were found in fulminant hepatitis B, significantly surpassing those in FHF of any other etiology. In fulminant hepatitis B, caspase-9 activity was also higher than in controls, while caspase-8 activation was not higher than in NC. Unlike caspase-3, caspases -8 and -9 activities were not correlated to the numbers of TUNEL positive cells. Fas expression was also the highest in FHF but did not differ between hepatitis B virus-FHF and other FHF.

CONCLUSIONS

Our data indicate differential activation of intrahepatic caspases in FHF depending on the underlying etiology. Massive activation of caspases in fulminant hepatitis B confirms a pivotal role of apoptotic pathways in the pathogenesis of human fulminant hepatitis B.

Viral hepatitis and liver cancer: the case of hepatitis C

Chronic infection with the hepatitis C virus (HCV) is a major risk factor for the development of hepatocellular carcinoma (HCC) worldwide. The pathogenesis of HCC in HCV infection has extensively been analysed. Hepatitis C virus-induced chronic inflammation and the effects of cytokines in the development of fibrosis and liver cell proliferation are considered as one of the major pathogenic mechanisms. Increasing experimental evidence suggests that HCV contributes to HCC by directly modulating pathways that promote the malignant transformation of hepatocytes. Hepatitis C virus is an RNA virus that does not integrate into the host genome but HCV proteins interact with many host-cell factors well beyond their roles in the viral life cycle and are involved in a wide range of activities, including cell signaling, transcription, cell proliferation, apoptosis, membrane rearrangements, vesicular trafficking and translational regulation. At least four of the HCV gene products, namely HCV core, NS3, NS4B and NS5A, have been shown to exhibit transformation potential in tissue culture and several potentially oncogenic pathways have been shown to be altered by the expression of HCV proteins. Both HCV core and NS5A induce the accumulation of wild-type beta-catenin and the Wnt-beta-catenin pathway emerges as a common target for HCV (and HBV) in human HCCs, also independently from axin/beta-catenin gene mutations. Induction of both endoplasmic reticulum stress and oxidative stress by HCV proteins might also contribute to HCV transformation. Most of the putative transforming functions of the HCV proteins have been defined in artificial cellular systems, which may not be applicable to HCV infection in vivo, and still need to be established in relevant infection and disease models.

E2 of hepatitis C virus inhibits apoptosis

Hepatitis C virus (HCV) is the major causative agent of chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma, and can be involved in very long chronic infections up to 30 years or more. Therefore, it has been speculated that HCV possesses mechanisms capable of modulating host defense systems such as innate and adaptive immunity. To investigate this virus-host interaction, we generated HCV replicons containing various HCV structural proteins and then analyzed the sensitivity of replicon-containing cells to the apoptosis-inducing agent, TRAIL. TRAIL-induced apoptosis was monitored by cleavage of procaspase-3 and procaspase-9 as well as that of their substrate poly(ADP-ribose) polymerase. TRAIL-induced apoptosis was inhibited in cells expressing HCV E2. Moreover, expression of HCV E2 enhanced the colony forming efficiency of replicon-containing cells by 25-fold. Blockage of apoptosis by E2 seems to be related to inhibition of TRAIL-induced cytochrome c release from the mitochondria. Based on these results, we propose that E2 augments persistent HCV infection by blocking host-induced apoptosis of infected cells.

The NS5A protein of the hepatitis C virus genotype 1a is cleaved by caspases to produce C-terminal-truncated forms of the protein that reside mainly in the cytosol

The nonstructural 5A (NS5A) protein of the hepatitis C virus (HCV) is a multifunctional protein that is implicated in viral replication and pathogenesis. We report here that NS5A of HCV-1a is cleaved at multiple sites by caspase proteases in transfected cells. Two cleavage sites at positions Asp154 and 248DXXD251 were mapped. Cleavage at Asp154 has been previously recognized as one of the caspase cleavage sites for the NS5A protein of HCV genotype 1b (1, 2) and results in the production of a 17-kDa fragment. The sequence 248DXXD251 is a novel caspase recognition motif for NS5A and is responsible for the production of a 31-kDa fragment. Furthermore, we show that Arg217 is implicated in the production of the previously described 24-kDa product, whose accumulation is affected by both calpain and caspase inhibitors. We also showed that caspase-mediated cleavage occurs in the absence of exogenous proapoptotic stimuli and is not related to the accumulation of the protein in the endoplasmic reticulum. Interestingly, our data indicate that NS5A is targeted by at least two different caspases and suggest that caspase 6 is implicated in the production of the 17-kDa fragment. Most importantly, we report that, all the detectable NS5A fragments following caspase-mediated cleavage are C-terminal-truncated forms of NS5A and are mainly localized in the cytosol. Thus, in sharp contrast to the current view we found no evidence supporting a role for caspase-mediated cleavage in the transport of the NS5A protein to the nucleus, which could lead to transcriptional activation.

Hepatitis C virus core protein: an update on its molecular biology, cellular functions and clinical implications

The present review article is an update on various features of hepatitis C virus (HCV) core protein including its molecular biology, role in HCV replication, involvement in HCV pathogenesis, etiological role in hepatocellular carcinogenesis, significance in diagnosis and vaccination against HCV infection. Core protein is a structural protein of HCV virus and has only recently been characterized. It was found to play a major role in HCV-induced viral hepatitis. Although published information shows a lot about the clinical significance of HCV core protein, several studies are still needed to demonstrate its exact significance in viral biology and underlying HCV pathogenesis.

Hepatitis C virus non-structural 5A protein can enhance full-length core protein-induced nuclear factor-kappaB activation

AIM

To study the effects of hepatitis C virus (HCV) core and non-structural 5A (NS5A) proteins on nuclear factor-kappaB (NF-kappaB) activity for understanding their biological function on chronic hepatitis caused by HCV infection.

METHODS

Luciferase assay was used to measure the activity of NF-kappaB in three different cell lines cotransfected with a series of deletion mutants of core protein alone or together with NS5A protein using pNF-kappaB-Luc as a reporter plasmid. Western blot and indirect immunofluorescence assays were used to confirm the expression of proteins and to detect their subcellular localization, respectively. Furthermore, Western blot was also used to detect the expression levels of NF-kappaB/p65, NF-kappaB/p50, and inhibitor kappaB-a (IkappaB-a).

RESULTS

The wild-type core protein (C191) and its mutant segments (C173 and C158) could activate NF-kappaB in Huh7 cells only and activation caused by (C191) could be enhanced by NS5A protein. Moreover, the full-length core protein and its different deletion mutants alone or together with NS5A protein did not enhance the expression level of NF-kappaB. The NF-kappaB activity was augmented due to the dissociation of NF-kappaB-IkappaB complex and the degradation of IkappaB-a.

CONCLUSION

NF-kappaB is the key transcription factor that can activate many genes that are involved in the cellular immune response and inflammation. Coexpression of the full-length core protein along with NS5A can enhance the NF-kappaB activation, and this activation may play a significant role in chronic liver diseases including hepatocellular carcinoma associated with HCV infection.

Use of an in vitro Model and Yeast Two-Hybrid System to Investigate the Pathogenesis of Hepatitis C

Fifteen years after the discovery of hepatitis C virus (HCV) in 1989, much remains to be learnt about the cell biology of this virus. Using the serum from a patient containing HCV RNA in high titer as a source, a Singapore strain of genotype 1b was recovered and characterized. This full-length HCV genome was then constructed into a tetracycline-inducible vector using the pSTAR plasmid. Transfection of hepatoma cell lines with this HCV genome under tetracycline induction indicated that chemokines (RANTES and monocyte chemoattractant protein-1) were upregulated, possibly contributing to the induction of immune responses. Using the yeast two-hybrid system to discover protein-protein interactions, nonstructural region NS3 was found to interact with itself, forming a dimer that increased helicase activity but was not essential for its activity, thereby disqualifying it as a suitable target of drug actions. The significance of the interaction between core and NS5A is unclear, and the cleavage of NS5A is related to the development of apoptosis. However, the interaction of p68 and NS56B appears to be important because the knockdown of p68 reduced the viral replication. Finally, a new cell model using chimeric CD81 linked to the cytoplasmic domain of either a low-density lipoprotein receptor or a transferrin receptor led to productive infection of HCV that had been recovered from infected serum. These studies allow us to examine the pathogenesis of HCV infection in more detail.

Systematic identification of hepatocellular proteins interacting with NS5A of the hepatitis C virus

The hepatitis C virus is associated with the development of liver cirrhosis and hepatocellular carcinomas. Among the 10 polyproteins produced by the virus, no function has been clearly assigned to the non-structural 5A (NS5A) protein. This study was designed to identify the hepatocellular proteins that interact with NS5A of the HCV. Yeast two-hybrid experiments were performed with a human liver cDNA prey-library, using five different NS5A derivatives as baits, the full-length NS5A (NS5A-F, amino acid (aa) 1 approximately 447) and its four different derivatives, denoted as NS5A-A (aa 1 approximately 150), -B (aa 1 approximately 300), -C (aa 300 approximately 447) and D (aa 150 approximately 447). NS5A-F, NS5A-B and NS5A-C gave two, two and 10 candidate clones, respectively, including an AHNAK-related protein, the secreted frizzled-related protein 4 (SFRP4), the N-myc downstream regulated gene 1 (NDRG1), the cellular retinoic acid binding protein 1 (CRABP-1), ferritin heavy chain (FTH1), translokin, tumor-associated calcium signal transducer 2 (TACSTD2), phosphatidylinositol 4-kinase (PI4K) and centaurindelta 2 (CENTdelta2). However, NS5A-A produced no candidates and NS5A-D was not suitable as bait due to transcriptional activity. Based on an in vitro binding assay, CRABP-1, PI4K, CENTdelta2 and two unknown fusion proteins with maltose binding protein (MBP), were confirmed to interact with the glutathione S-transferase (GST)/NS5A fusion protein. Furthermore, the interactions of CRABP-1, PI4K and CENTdelta2 were not related to the PXXP motif (class II), as judged by a domain analysis. While their biological relevance is under investigation, the results contribute to a better understanding of the possible role of NS5A in hepatocellular signaling pathways.

Ribavirin resistance in hepatitis C virus replicon-containing cell lines conferred by changes in the cell line or mutations in the replicon RNA

Ribavirin (RBV), used in combination with alpha interferon to treat hepatitis C virus (HCV) infections, is a guanosine nucleotide analog that can increase the error rate of viral RNA-dependent RNA polymerases, imbalance intracellular nucleotide pools, and cause toxicity in many cell types. To determine potential mechanisms of RBV resistance during HCV RNA replication, we passaged HCV replicon-containing cell lines in the presence of increasing concentrations of RBV. RBV-resistant, HCV replicon-containing cell lines were generated, and the majority of RBV resistance was found to be conferred by changes in the cell lines. The resistant cell lines were defective in RBV import, as measured by [(3)H]RBV uptake experiments. These cell lines displayed reduced RBV toxicity and reduced error accumulation during infection with poliovirus, whose replication is known to be sensitive to RBV-induced error. For one RBV-resistant isolate, two mutations in the replicon RNA contributed to the observed phenotype. Two responsible mutations resided in the C-terminal region of NS5A, G404S, and E442G and were each sufficient for low-level RBV resistance. Therefore, RBV resistance in HCV replicon cell lines can be conferred by changes in the cell line or by mutations in the HCV replicon.

Hepatitis C virus (HCV) NS5A protein downregulates HCV IRES-dependent translation

Translation of the hepatitis C virus (HCV) polyprotein is mediated by an internal ribosome entry site (IRES) that is located mainly within the 5' non-translated region of the viral genome. In this study, the effect of the HCV non-structural 5A (NS5A) protein on the HCV IRES-dependent translation was investigated by using a transient transfection system. Three different cell lines (HepG2, WRL-68 and BHK-21) were co-transfected with a plasmid vector containing a bicistronic transcript carrying the chloramphenicol acetyltransferase (CAT) and the firefly luciferase genes separated by the HCV IRES sequences, and an expression vector producing the NS5A protein. Here, it was shown that the HCV NS5A protein inhibited HCV IRES-dependent translation in a dose-dependent manner. In contrast, NS5A had no detectable effect on cap-dependent translation of the upstream gene (CAT) nor on translation from another viral IRES. Further analysis using deleted forms of the NS5A protein revealed that a region of about 120 aa located just upstream of the nuclear localization signal of the protein is critical for this suppression. Overall, these results suggest that HCV NS5A protein negatively modulates the HCV IRES activity in a specific manner.

Calcium-dependent calpain proteases are implicated in processing of the hepatitis C virus NS5A protein

The nonstructural 5A (NS5A) protein of the hepatitis C virus (HCV) is a multifunctional phosphoprotein that is implicated in viral replication and HCV-mediated pathogenesis. We report here that the NS5A protein from the HCV genotype 1a is processed into shorter distinct forms when expressed in mammalian cells (Vero, HepG2, HuH-7, and WRL68) infected with an NS5A-expressing HSV-1-based amplicon vector or when transiently transfected with NS5A-expressing plasmids in the absence of exogenous apoptotic stimuli. Inhibitor studies combined with cell-free cleavage assays suggest that calcium-dependent calpain proteases, in addition to caspase-like proteases, are involved in NS5A processing. Interestingly, His-tagging experiments indicated that all the detectable NS5A-cleaved products are N-terminal forms of the protein. Additionally, immunofluorescence studies showed that, despite proteolytic cleavage, the NS5A protein exhibits a cytoplasm-perinuclear localization similar to that of the full-length protein. Thus, our results are consistent with recent data that demonstrated that NS5A is capable of perturbing intracellular calcium homeostasis and suggest that NS5A is both an inducer and a substrate of the calcium-dependent calpain protease(s). This may imply that cleavage of NS5A by calpain(s) could play a role in the modulation of NS5A function.

Characterization of a unique group-specific protein (U122) of the severe acute respiratory syndrome coronavirus

A novel coronavirus (CoV) has been identified as the etiological agent of severe acute respiratory syndrome (SARS). The SARS-CoV genome encodes the characteristic essential CoV replication and structural proteins. Additionally, the genome contains six group-specific open reading frames (ORFs) larger than 50 amino acids, with no known homologues. As with the group-specific genes of the other CoVs, little is known about the SARS-CoV group-specific genes. SARS-CoV ORF7a encodes a putative unique 122-amino-acid protein, designated U122 in this study. The deduced sequence contains a probable cleaved signal sequence and a C-terminal transmembrane helix, indicating that U122 is likely to be a type I membrane protein. The C-terminal tail also contains a typical endoplasmic reticulum (ER) retrieval motif, KRKTE. U122 was expressed in SARS-CoV-infected Vero E6 cells, as it could be detected by Western blot and immunofluorescence analyses. U122 is localized to the perinuclear region of both SARS-CoV-infected and transfected cells and colocalized with ER and intermediate compartment markers. Mutational analyses showed that both the signal peptide sequence and ER retrieval motif were functional.

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Recruitment of Hsp70 chaperones: a crucial part of viral survival strategies

Virus proliferation depends on the successful recruitment of host cellular components for their own replication, protein synthesis, and virion assembly. In the course of virus particle production a large number of proteins are synthesized in a relatively short time, whereby protein folding can become a limiting step. Most viruses therefore need cellular chaperones during their life cycle. In addition to their own protein folding problems viruses need to interfere with cellular processes such as signal transduction, cell cycle regulation and induction of apoptosis in order to create a favorable environment for their proliferation and to avoid premature cell death. Chaperones are involved in the control of these cellular processes and some viruses reprogram their host cell by interacting with them. Hsp70 chaperones, as central components of the cellular chaperone network, are frequently recruited by viruses. This review focuses on the function of Hsp70 chaperones at the different stages of the viral life cycle emphasizing mechanistic aspects.

Proteomic approach identifies HSP27 as an interacting partner of the hepatitis C virus NS5A protein

Chronic infection by HCV is closely correlated with liver diseases such as cirrhosis, steatosis, and hepatocellular carcinoma. To understand how long-term interaction between HCV and the host leads to pathogenesis, we identified cellular proteins that interact with NS5A and NS5B using a biochemical approach. Stable cell lines that express flag-NS5A or flag-NS5B under tetracycline induction were generated. The induced flag-tagged proteins were immunoprecipitated (IP'd) and associated proteins separated on 2D gels. Protein spots that specifically co-IP'd with NS5A or NS5B were identified by mass spectrometry. HSP27 was identified as a protein that specifically co-IP'd with NS5A but not with NS5B. The N-terminal regions of NS5A (a.a. 1-181) and HSP27 (a.a. 1-122) were defined to be the domains that interact with each other. HSP27 is generally distributed in the cytoplasm. When heat shocked, HSP27 is concentrated in the ER where NS5A is co-localized.

Cellular RNA helicase p68 relocalization and interaction with the hepatitis C virus (HCV) NS5B protein and the potential role of p68 in HCV RNA replication

Chronic infection by hepatitis C virus (HCV) can lead to severe hepatitis and cirrhosis and is closely associated with hepatocellular carcinoma. The replication cycle of HCV is poorly understood but is likely to involve interaction with host factors. In this report, we show that NS5B, the HCV RNA-dependent RNA polymerase (RdRp), interacts with a human RNA helicase, p68. Transient expression of NS5B alone, as well as the stable expression of all the nonstructural proteins in a HCV replicon-bearing cell line (V. Lohmann, F. Korner, J.-O. Koch, U. Herian, L. Theilmann, and R. Bartenschlager, Science 285:110-113), causes the redistribution of endogenous p68 from the nucleus to the cytoplasm. Deletion of the C-terminal two-thirds of NS5B (NS5BDeltaC) dramatically reduces its coimmunoprecipitation (co-IP) with endogenous p68, while the deletion of the N-terminal region (NS5BDeltaN1 and NS5BDeltaN2) does not affect its interaction with p68. In consistency with the co-IP results, NS5BDeltaC does not cause the relocalization of p68 whereas NS5BDeltaN1 does. With a replicon cell line, we were not able to detect a change in positive- and negative-strand synthesis when p68 levels were reduced using small interfering RNA (siRNA). In cells transiently transfected with a full-length HCV construct, however, the depletion (using specific p68 siRNA) of endogenous p68 correlated with a reduction in the transcription of negative-strand from positive-strand HCV RNA. Overexpression of NS5B and NS5BDeltaN1, but not that of NS5BDeltaC, causes a reduction in the negative-strand synthesis, indicating that overexpressed NS5B and NS5BDeltaN1 sequesters p68 from the replication complexes (thus reducing their replication activity levels). Identification of p68 as a cellular factor involved in HCV replication, at least for cells transiently transfected with a HCV expression construct, is a step towards understanding HCV replication.

Up-regulating effect of hepatitis C virus NS5A protein on NS3TP6 gene promoter

AIM: To investigate the regulatory effects of non-structural protein 5A of hepatitis C virus (HCV NS) protein on NS3TP6 gene promoter.

METHODS: The sequence of NS3TP6 gene promoter was identified in GenBank by bioinformatics and amplified from HepG2 genome by polymerase chain reaction (PCR), which was cloned into pCAT3 reporter vector. The HepG2 cell line was transfected by pCAT3-NS3TP6-p, and co-tranfected by pCAT3-NS3TP6-p and pcDNA3.1(-)-NS5A, respectively. The chloramphenicol acetyltransferase (CAT) activity was detected by an enzyme-linked immunosorbent assay (ELISA) kit.

RESULTS: The recombinant vector of reporter gene expressive vector pCAT3-NS3TP6-p was approved correctly by restriction enzyme digestion and sequencing analysis. In the transfection experiment of HepG2, pCAT3-NS3TP6-p had higher activity of CAT expression than that of pCAT3-basic demonstrated by an ELISA kit. The expression level of CAT in co-transfection of pCAT3-NS3TP6-p and pcDNA3.1(-)-NS5A was 1.87 times as higher as that of pCAT3- NS3TP6-p plasmid alone.

CONCLUSION: Cell transfection and ELISA technology are successfully used to prove the results from microarray of HCV NS5A, which brings some new clues for studying the trans-regulated and immune regulation mechanism of NS5A.

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Hepatitis C virus NS5A protein interacts with 2′,5′-oligoadenylate synthetase and inhibits antiviral activity of IFN in an IFN sensitivity-determining region-independent manner

The non-structural protein 5A (NS5A) of hepatitis C virus (HCV) has been implicated in inhibition of antiviral activity of IFN. While previous studies have suggested an interaction between NS5A and the double-stranded RNA-dependent protein kinase (PKR), the possibility still remains that interaction with another molecule(s) is involved in the NS5A-mediated inhibition of IFN. In the present study, we investigated a possible interaction between NS5A and 2′,5′-oligoadenylate synthetase (2-5AS), another key molecule in antiviral activity. We observed that NS5A physically interacted with 2-5AS in cultured cells, with an N-terminal portion of NS5A [aa 1–148; NS5A(1–148)] and two separate portions of 2-5AS (aa 52–104 and 184–275) being involved in the interaction. Single point mutations at residue 37 of NS5A affected the degree of the interaction with 2-5AS, with a Phe-to-Leu mutation (F37L) augmenting and a Phe-to-Asn mutation (F37N) diminishing it. Virus rescue assay revealed that the full-length NS5A (NS5A-F) and NS5A(1–148), the latter of which contains neither the IFN sensitivity-determining region (ISDR) nor the PKR-binding domain, significantly counteracted the antiviral activity of IFN. Introduction of a F37N mutation into NS5A(1–148) impaired the otherwise more significant IFN-inhibitory activity of NS5A(1–148). It was also found that the F37N mutation was highly disadvantageous for the replication of an HCV RNA replicon. Taken together, our results suggest the possibility that NS5A interacts with 2-5AS and inhibits the antiviral activity of IFN in an ISDR-independent manner.

Inhibition of translation and induction of apoptosis by Bunyaviral nonstructural proteins bearing sequence similarity to reaper

Members of the California serogroup of bunyaviruses (family Bunyaviridae) are the leading cause of pediatric viral encephalitis in North America. Significant cell death is observed as part of the infection pathology. We now report that a Bunyaviral nonstructural protein termed NSs shows sequence similarity to Reaper, a proapoptotic protein from Drosophila. Although NSs proteins lack the Reaper N-terminal motif critical for IAP inhibition, they do retain other functions of Reaper that map to conserved C-terminal regions. Like Reaper, NSs proteins induce mitochondrial cytochrome c release and caspase activation in cell-free extracts and promote neuronal apoptosis and mortality in a mouse model. Independent of caspase activation, Bunyavirus NSs proteins also share with Reaper the ability to directly inhibit cellular protein translation. We have found that the shared capacity to inhibit translation and induce apoptosis resides in common sequence motifs present in both Reaper and NSs proteins. Data presented here suggest that NSs induce apoptosis through a mechanism similar to that used by Reaper, as both proteins bind to an apoptotic regulator called Scythe and can relieve Scythe inhibition of Hsp70. Thus, bunyavirus NSs proteins have multiple Reaper-like functions that likely contribute to viral pathogenesis by promoting cell death and/or inhibiting cellular translation.

Hepatitis C virus--cell interactions and their role in pathogenesis

In summary, HCV-cell interactions include those directly involved with the HCV life cycle such as virus attachment, entry, and replication. Included within this broad area of research are the interactions of HCV proteins with the IFN system, cytokine and chemokine pathways such as IL-8, and various other cellular proteins and pathways. The plethora of contradictory and sometimes confusing accessory HCV-host interactions defies precise predictions of their role in HCV biology. It is clear that these virus-cell interactions affect HCV replication, antiviral resistance, persistence, and pathogenesis. Because HCV-host interactions are initiated immediately on infection, they are operative during acute HCV infection, whereby HCV interacts with innate cellular antiviral and immune systems. The magnitude and duration of these HCV-host interactions therefore may influence the development of acquired immunity. Because HCV exists as a quasispecies in all infected individuals, heterogeneity in biological responses to HCV-host interactions is predicted, revealing opportunities for the development of various genotypic and phenotypic prognostic indicators. With the model systems in place, these hypotheses can be tested. The challenge for the future is to determine if there is a hierarchical importance to these interactions, to delineate how these virus-cell interactions affect the patient infected with HCV, and to determine whether any of these interactions represents a target for therapeutic intervention.

Rotavirus nonstructural protein NSP5 interacts with major core protein VP2

Rotavirus is a nonenveloped virus with a three-layered capsid. The inner layer, made of VP2, encloses the genomic RNA and two minor proteins, VP1 and VP3, with which it forms the viral core. Core assembly is coupled with RNA viral replication and takes place in definite cellular structures termed viroplasms. Replication and encapsidation mechanisms are still not fully understood, and little information is available about the intermolecular interactions that may exist among the viroplasmic proteins. NSP2 and NSP5 are two nonstructural viroplasmic proteins that have been shown to interact with each other. They have also been found to be associated with precore replication intermediates that are precursors of the viral core. In this study, we show that NSP5 interacts with VP2 in infected cells. This interaction was demonstrated with recombinant proteins expressed from baculovirus recombinants or in bacterial systems. NSP5-VP2 interaction also affects the stability of VP6 bound to VP2 assemblies. The data presented showed evidence, for the first time, of an interaction between VP2 and a nonstructural rotavirus protein. Published data and the interaction demonstrated here suggest a possible role for NSP5 as an adapter between NSP2 and the replication complex VP2-VP1-VP3 in core assembly and RNA encapsidation, modulating the role of NSP2 as a molecular motor involved in the packaging of viral mRNA.