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

UNLABELLED: 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.

Neutralizing antibody-resistant hepatitis C virus cell-to-cell transmission.

Hepatitis C virus (HCV) can initiate infection by cell-free particle and cell-cell contact-dependent transmission. In this study we use a novel infectious coculture system to examine these alternative modes of infection. Cell-to-cell transmission is relatively resistant to anti-HCV glycoprotein monoclonal antibodies and polyclonal immunoglobulin isolated from infected individuals, providing an effective strategy for escaping host humoral immune responses. Chimeric viruses expressing the structural proteins representing the seven major HCV genotypes demonstrate neutralizing antibody-resistant cell-to-cell transmission. HCV entry is a multistep process involving numerous receptors. In this study we demonstrate that, in contrast to earlier reports, CD81 and the tight-junction components claudin-1 and occludin are all essential for both cell-free and cell-to-cell viral transmission. However, scavenger receptor BI (SR-BI) has a more prominent role in cell-to-cell transmission of the virus, with SR-BI-specific antibodies and small-molecule inhibitors showing preferential inhibition of this infection route. These observations highlight the importance of targeting host cell receptors, in particular SR-BI, to control viral infection and spread in the liver.

Structure of the zinc-binding domain of an essential component of the hepatitis C virus replicase.

Hepatitis C virus (HCV) is a human pathogen affecting nearly 3% of the world's population. Chronic infections can lead to cirrhosis and liver cancer. The RNA replication machine of HCV is a multi-subunit membrane-associated complex. The non-structural protein NS5A is an active component of HCV replicase, as well as a pivotal regulator of replication and a modulator of cellular processes ranging from innate immunity to dysregulated cell growth. NS5A is a large phosphoprotein (56-58 kDa) with an amphipathic alpha-helix at its amino terminus that promotes membrane association. After this helix region, NS5A is organized into three domains. The N-terminal domain (domain I) coordinates a single zinc atom per protein molecule. Mutations disrupting either the membrane anchor or zinc binding of NS5A are lethal for RNA replication. However, probing the role of NS5A in replication has been hampered by a lack of structural information about this multifunctional protein. Here we report the structure of NS5A domain I at 2.5-A resolution, which contains a novel fold, a new zinc-coordination motif and a disulphide bond. We use molecular surface analysis to suggest the location of protein-, RNA- and membrane-interaction sites.

Regulation of hepatitis C virion production via phosphorylation of the NS5A protein.

Hepatitis C virus (HCV) is a significant pathogen, infecting some 170 million people worldwide. Persistent virus infection often leads to cirrhosis and liver cancer. In the infected cell many RNA directed processes must occur to maintain and spread infection. Viral genomic RNA is constantly replicating, serving as template for translation, and being packaged into new virus particles; processes that cannot occur simultaneously. Little is known about the regulation of these events. The viral NS5A phosphoprotein has been proposed as a regulator of events in the HCV life cycle for years, but the details have remained enigmatic. NS5A is a three-domain protein and the requirement of domains I and II for RNA replication is well documented. NS5A domain III is not required for RNA replication, and the function of this region in the HCV lifecycle is unknown. We have identified a small deletion in domain III that disrupts the production of infectious virus particles without altering the efficiency of HCV RNA replication. This deletion disrupts virus production at an early stage of assembly, as no intracellular virus is generated and no viral RNA and nucleocapsid protein are released from cells. Genetic mapping has indicated a single serine residue within the deletion is responsible for the observed phenotype. This serine residue lies within a casein kinase II consensus motif, and mutations that mimic phosphorylation suggest that phosphorylation at this position regulates the production of infectious virus. We have shown by genetic silencing and chemical inhibition experiments that NS5A requires casein kinase II phosphorylation at this position for virion production. A mutation that mimics phosphorylation at this position is insensitive to these manipulations of casein kinase II activity. These data provide the first evidence for a function of the domain III of NS5A and implicate NS5A as an important regulator of the RNA replication and virion assembly of HCV. The ability to uncouple virus production from RNA replication, as described herein, may be useful in understanding HCV assembly and may be therapeutically important.

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.

Reverse transcription PCR-based sequence analysis of hepatitis C virus replicon RNA.

Since the advent of efficient cell-culture methods for HCV replication and, more recently, infection, there has been a need to efficiently sequence the viral RNA in these systems. This need is especially urgent in light of the error-prone nature of HCV RNA replication, which leads to a variety of interesting mutations. The adaptation of hepatitis C replicons to cell culture, which greatly increased their replication capacity, and the subsequent identification of viral point mutations responsible for this adaptation are prime examples of the type of phenotype-genotype connection that viral RNA sequencing methods can provide. More recently, researchers have used similar sequencing methods to identify changes in replicons that represent viral adaptation to engineered mutations, adaptation to a variety of host cells, and viral evasion of antiviral compound susceptibility. Here, we describe the cloning and isolation of HCV replicon-bearing cells, the extraction of total RNA, the generation of cDNA, and the amplification of specific HCV replicon sequences for sequence analysis. The methods we describe permit rapid and robust determination of HCV RNA sequences from cell culture.

Transmission genetics of drug-resistant hepatitis C virus.

Antiviral development is plagued by drug resistance and genetic barriers to resistance are needed. For HIV and hepatitis C virus (HCV), combination therapy has proved life-saving. The targets of direct-acting antivirals for HCV infection are NS3/4A protease, NS5A phosphoprotein and NS5B polymerase. Differential visualization of drug-resistant and -susceptible RNA genomes within cells revealed that resistant variants of NS3/4A protease and NS5A phosphoprotein are cis-dominant, ensuring their direct selection from complex environments. Confocal microscopy revealed that RNA replication complexes are genome-specific, rationalizing the non-interaction of wild-type and variant products. No HCV antivirals yet display the dominance of drug susceptibility shown for capsid proteins of other viruses. However, effective inhibitors of HCV polymerase exact such high fitness costs for drug resistance that stable genome selection is not observed. Barriers to drug resistance vary with target biochemistry and detailed analysis of these barriers should lead to the use of fewer drugs.

A Massively Parallel Selection of Small Molecule-RNA Motif Binding Partners Informs Design of an Antiviral from Sequence.

Many RNAs cause disease; however, RNA is rarely exploited as a small-molecule drug target. Our programmatic focus is to define privileged RNA motif small-molecule interactions to enable the rational design of compounds that modulate RNA biology starting from only sequence. We completed a massive, library-versus-library screen that probed over 50 million binding events between RNA motifs and small molecules. The resulting data provide a rich encyclopedia of small-molecule RNA recognition patterns, defining chemotypes and RNA motifs that confer selective, avid binding. The resulting interaction maps were mined against the entire viral genome of hepatitis C virus (HCV). A small molecule was identified that avidly bound RNA motifs present in the HCV 30 UTR and inhibited viral replication while having no effect on host cells. Collectively, this study represents the first whole-genome pattern recognition between small molecules and RNA folds.

Interaction between hepatitis C virus proteins and host cell factors.

Since the discovery of the hepatitis C virus (HCV) as the causative agent of non-A, non-B hepatitis, significant effort has been devoted to understanding this important pathogen. Despite the difficulty in culturing this virus efficiently, much is known about the organization of the viral genome and the functions of many of the viral proteins. Through the use of surrogate expression systems combined with cellular fractionation, pull-down experiments and yeast two-hybrid screens, numerous interactions between hepatitis C virus proteins and cellular components have been identified. The relevance of many of these interactions to hepatitis C biology remains to be demonstrated. This review discusses recent developments in this area of HCV research.

Development of novel therapies for hepatitis C.

The current standard of care for the treatment of hepatitis C virus (HCV) infection is a combination of pegylated IFN and ribavirin (Peg-IFN/RBV). Because of the adverse effects associated with both IFN and ribavirin and because Peg-IFN/RBV provides only about a 45-50% sustained virological response (SVR, undetectable HCV RNA for greater than 24 weeks after cessation of therapy) in genotype 1-infected individuals, there is a need for more potent anti-HCV compounds with fewer adverse effects. The twenty-first International Conference on Antiviral Research held in May 2009 in Miami Beach, Florida, featured a special session focused on novel targets for HCV therapy. The session included presentations by world-renowned experts in HCV virology and covered a diverse array of potential targets for the development of new classes of HCV therapies. This review contains concise summaries of discussed topics that included the innate immune response, virus entry, the NS2 protease, the NS3 helicase, NS4B, and NS5A. Each presenter discussed the current knowledge of these targets and provided examples of recent scientific breakthroughs that are enhancing our understanding of these targets. As our understanding of the role of these novel anti-HCV targets increases so will our ability to discover new, more safe and effective anti-HCV therapies.

Molecular and contextual markers of hepatitis C virus and drug abuse.

The spread of hepatitis C virus (HCV) infection involves a complex interplay of social risks, and molecular factors of both virus and host. Injection drug abuse is the most powerful risk factor for HCV infection, followed by sexual transmission and additional non-injection drug abuse factors such as co-infection with other viruses and barriers to treatment. It is clearly important to understand the wider context in which the factors related to HCV infection occur. This understanding is required for a comprehensive approach leading to the successful prevention, diagnosis, and treatment of HCV. An additional consideration is that current treatments and advanced molecular methods are generally unavailable to socially disadvantaged patients. Thus, the recognition of behavioral/social, viral, and host factors as components of an integrated approach to HCV is important to help this vulnerable group. Equally important, this approach is key to the development of personalized patient treatment - a significant goal in global healthcare. In this review, we discuss recent findings concerning the impact of drug abuse, epidemiology, social behavior, virology, immunopathology, and genetics on HCV infection and the course of disease.

Cellular cofactors affecting hepatitis C virus infection and replication.

Recently identified hepatitis C virus (HCV) isolates that are infectious in cell culture provide a genetic system to evaluate the significance of virus-host interactions for HCV replication. We have completed a systematic RNAi screen wherein siRNAs were designed that target 62 host genes encoding proteins that physically interact with HCV RNA or proteins or belong to cellular pathways thought to modulate HCV infection. This includes 10 host proteins that we identify in this study to bind HCV NS5A. siRNAs that target 26 of these host genes alter infectious HCV production >3-fold. Included in this set of 26 were siRNAs that target Dicer, a principal component of the RNAi silencing pathway. Contrary to the hypothesis that RNAi is an antiviral pathway in mammals, as has been reported for subgenomic HCV replicons, siRNAs that target Dicer inhibited HCV replication. Furthermore, siRNAs that target several other components of the RNAi pathway also inhibit HCV replication. MicroRNA profiling of human liver, human hepatoma Huh-7.5 cells, and Huh-7.5 cells that harbor replicating HCV demonstrated that miR-122 is the predominant microRNA in each environment. miR-122 has been previously implicated in positively regulating the replication of HCV genotype 1 replicons. We find that 2'-O-methyl antisense oligonucleotide depletion of miR-122 also inhibits HCV genotype 2a replication and infectious virus production. Our data define 26 host genes that modulate HCV infection and indicate that the requirement for functional RNAi for HCV replication is dominant over any antiviral activity this pathway may exert against HCV.

The NS5A protein of bovine viral diarrhea virus contains an essential zinc-binding site similar to that of the hepatitis C virus NS5A protein.

The recent demonstration that the NS5A protein of hepatitis C virus (HCV) contains an unconventional zinc-binding site with the format Cx(17)CxCx(20)C and the presence of a similar sequence element in the NS5A proteins of members of the Pestivirus genus has led to the hypothesis that the NS5A protein of the pestivirus bovine viral diarrhea virus (BVDV) is a zinc-binding protein. A method for the expression and partial purification of BVDV NS5A was developed, and the partially purified protein was analyzed for zinc content by atomic absorption spectroscopy. BVDV NS5A was found to coordinate a single zinc atom per protein molecule. Mutation of any of the four cysteines of the predicted zinc-binding motif eliminated zinc coordination. Furthermore, analysis of mutations at these cysteine residues in the context of a BVDV replicon system indicated that these residues were absolutely essential for RNA replication. The recently determined crystal structure of the N-terminal zinc-binding domain of the HCV NS5A protein, combined with secondary structure predictions of the region surrounding the mapped BVDV zinc-binding region, indicates that the BVDV zinc-binding motif fits the general template Cx(22)CxCx(24)C and likely comprises a three-stranded antiparallel beta-sheet fold. These data highlight the similarities between the Hepacivirus and Pestivirus NS5A proteins and suggest that both proteins perform a not-yet-defined function in RNA replication that requires coordination of a single zinc atom.

Association of sindbis virus capsid protein with phospholipid membranes and the E2 glycoprotein: implications for alphavirus assembly.

A late stage in assembly of alphaviruses within infected cells is thought to be directed by interactions between the nucleocapsid and the cytoplasmic domain of the E2 protein, a component of the viral E1/E2 glycoprotein complex that is embedded in the plasma membrane. Recognition between the nucleocapsid protein and the E2 protein was explored in solution using NMR spectroscopy, as well as in binding assays using a model phospholipid membrane system that incorporated a variety of Sindbis virus E2 cytoplasmic domain (cdE2) and capsid protein constructs. In these binding assays, synthetic cdE2 peptides were reconstituted into phospholipid vesicles to simulate the presentation of cdE2 on the inner leaflet of the plasma membrane. Results from these binding assays showed a direct interaction between a peptide containing the C-terminal 16 amino acids of the cdE2 sequence and a Sindbis virus capsid protein construct containing amino acids 19-264. Additional experiments that probed the sequence specificity of this cdE2-capsid interaction are also described. Further binding assays demonstrated an interaction between the 19-264 capsid protein and artificial vesicles containing neutral or negatively charged phospholipids, while capsid protein constructs with N-terminal truncations displayed either little or no affinity for such vesicles. The membrane-binding property of the capsid protein suggests that the membrane may play an active role in alphavirus assembly. The results are consistent with an assembly process involving an initial membrane association, whereby an association with E2 glycoprotein further enhances capsid binding to facilitate membrane envelopment of the nucleocapsid for budding. Collectively, these experiments elucidate certain requirements for the binding of Sindbis virus capsid protein to the cytoplasmic domain of the E2 glycoprotein, a critical event in the alphavirus maturation pathway.

Complete replication of hepatitis C virus in cell culture.

Many aspects of the hepatitis C virus (HCV) life cycle have not been reproduced in cell culture, which has slowed research progress on this important human pathogen. Here, we describe a full-length HCV genome that replicates and produces virus particles that are infectious in cell culture (HCVcc). Replication of HCVcc was robust, producing nearly 10(5) infectious units per milliliter within 48 hours. Virus particles were filterable and neutralized with a monoclonal antibody against the viral glycoprotein E2. Viral entry was dependent on cellular expression of a putative HCV receptor, CD81. HCVcc replication was inhibited by interferon-alpha and by several HCV-specific antiviral compounds, suggesting that this in vitro system will aid in the search for improved antivirals.

The NS5A protein of hepatitis C virus is a zinc metalloprotein.

The NS5A protein of hepatitis C virus is believed to be an integral part of the viral replicase. Despite extensive investigation, the role of this protein remains elusive. Only limited biochemical characterization of NS5A has been performed, with most research to date involving the myriad of host proteins and signaling cascades that interact with NS5A. The need for better characterization of NS5A is paramount for elucidating the role of this protein in the virus life cycle. Examination of NS5A using bioinformatics tools suggested the protein consisted of three domains and contained an unconventional zinc binding motif within the N-terminal domain. We have developed a method to produce NS5A and performed limited proteolysis to confirm the domain organization model. The zinc content of purified NS5A and the N-terminal domain of NS5A was determined, and each of these proteins was found to coordinate one zinc atom per protein. The predicted zinc binding motif consists of four cysteine residues, conserved among the Hepacivirus and Pestivirus genera, fitting the formula of CX17CXCX20C. Mutation of any of the four cysteine components of this motif reduced NS5A zinc coordination and led to a lethal phenotype for HCV RNA replication, whereas mutation of other potential metal coordination residues in the N-terminal domain of NS5A, but outside the zinc binding motif, had little effect on zinc binding and, aside from one exception, were tolerated for replication. Collectively, these results indicate that NS5A is a zinc metalloprotein and that zinc coordination is likely required for NS5A function in the hepatitis C replicase.