Analysis of hepatitis C virus core/NS5A protein co-localization using novel cell culture systems expressing core-NS2 and NS5A of genotypes 1-7

Identification of a negative-strand RNA virus with natural plant and fungal hosts

The presence of viruses that spread to both plant and fungal populations in nature has posed intriguingly scientific question. We found a negative-strand RNA virus related to members of the family Phenuiviridae, named Valsa mali negative-strand RNA virus 1 (VmNSRV1), which induced strong hypovirulence and was prevalent in a population of the phytopathogenic fungus of apple Valsa canker (Valsa mali) infecting apple orchards in the Shaanxi Province of China. Intriguingly, VmNSRV1 encodes a protein with a viral cell-to-cell movement function in plant tissue. Mechanical leaf inoculation showed that VmNSRV1 could systemically infect plants. Moreover, VmNSRV1 was detected in 24 out of 139 apple trees tested in orchards in Shaanxi Province. Fungal inoculation experiments showed that VmNSRV1 could be bidirectionally transmitted between apple plants and V. mali, and VmNSRV1 infection in plants reduced the development of fungal lesions on leaves. Additionally, the nucleocapsid protein encoded by VmNSRV1 is associated with and rearranged lipid droplets in both fungal and plant cells. VmNSRV1 represents a virus that has adapted and spread to both plant and fungal hosts and shuttles between these two organisms in nature (phyto-mycovirus) and is potential to be utilized for the biocontrol method against plant fungal diseases. This finding presents further insights into the virus evolution and adaptation encompassing both plant and fungal hosts.

Mechanisms of Lipid Droplet Accumulation in Steatotic Liver Diseases

The steatotic diseases of metabolic dysfunction-associated steatotic liver disease (MASLD), alcohol-associated liver disease (ALD), and chronic hepatitis C (HCV) account for the majority of liver disease prevalence, morbidity, and mortality worldwide. While these diseases have distinct pathogenic and clinical features, dysregulated lipid droplet (LD) organelle biology represents a convergence of pathogenesis in all three. With increasing understanding of hepatocyte LD biology, we now understand the roles of LD proteins involved in these diseases but also how genetics modulate LD biology to either exacerbate or protect against the phenotypes associated with steatotic liver diseases. Here, we review the history of the LD organelle and its biogenesis and catabolism. We also review how this organelle is critical not only for the steatotic phenotype of liver diseases but also for their advanced phenotypes. Finally, we summarize the latest attempts and challenges of leveraging LD biology for therapeutic gain in steatotic diseases. In conclusion, the study of dysregulated LD biology may lead to novel therapeutics for the prevention of disease progression in the highly prevalent steatotic liver diseases of MASLD, ALD, and HCV.

Interplay between Lipid Metabolism, Lipid Droplets, and DNA Virus Infections

Lipid droplets (LDs) are cellular organelles rich in neutral lipids such as triglycerides and cholesterol esters that are coated by a phospholipid monolayer and associated proteins. LDs are known to play important roles in the storage and availability of lipids in the cell and to serve as a source of energy reserve for the cell. However, these structures have also been related to oxidative stress, reticular stress responses, and reduced antigen presentation to T cells. Importantly, LDs are also known to modulate viral infection by participating in virus replication and assembly. Here, we review and discuss the interplay between neutral lipid metabolism and LDs in the replication cycle of different DNA viruses, identifying potentially new molecular targets for the treatment of viral infections.

Differentiated Cells in Prolonged Hypoxia Produce Highly Infectious Native-Like Hepatitis C Virus Particles

BACKGROUND AND AIMS

Standard hepatitis C virus (HCV) cell-culture models present an altered lipid metabolism and thus produce lipid-poor lipoviral particles (LVPs). These models are thereby weakly adapted to explore the complete natural viral life cycle.

APPROACH AND RESULTS

To overcome these limitations, we used an HCV cell-culture model based on both cellular differentiation and sustained hypoxia to better mimic the host-cell environment. The long-term exposure of Huh7.5 cells to DMSO and hypoxia (1% O₂ ) significantly enhanced the expression of major differentiation markers and the cellular hypoxia adaptive response by contrast with undifferentiated and normoxic (21% O₂ ) standard conditions. Because hepatocyte-like differentiation and hypoxia are key regulators of intracellular lipid metabolism, we characterized the distribution of lipid droplets (LDs) and demonstrated that experimental cells significantly accumulate larger and more numerous LDs relative to standard cell-culture conditions. An immunocapture (IC) and transmission electron microscopy (TEM) method showed that differentiated and hypoxic Huh7.5 cells produced lipoproteins significantly larger than those produced by standard Huh7.5 cell cultures. The experimental cell culture model is permissive to HCV-Japanese fulminant hepatitis (JFH1) infection and produces very-low-buoyant-density LVPs that are 6-fold more infectious than LVPs formed by standard JFH1-infected Huh7.5 cells. Finally, the IC-TEM approach and antibody-neutralization experiments revealed that LVPs were highly lipidated, had a global ultrastructure and a conformation of the envelope glycoprotein complex E1E2 close to that of the ones circulating in infected individuals.

CONCLUSIONS

This relevant HCV cell culture model thus mimics the complete native intracellular HCV life cycle and, by extension, can be proposed as a model of choice for studies of other hepatotropic viruses.

Cellular factors involved in the hepatitis C virus life cycle

The hepatitis C virus (HCV), an obligatory intracellular pathogen, highly depends on its host cells to propagate successfully. The HCV life cycle can be simply divided into several stages including viral entry, protein translation, RNA replication, viral assembly and release. Hundreds of cellular factors involved in the HCV life cycle have been identified over more than thirty years of research. Characterization of these cellular factors has provided extensive insight into HCV replication strategies. Some of these cellular factors are targets for anti-HCV therapies. In this review, we summarize the well-characterized and recently identified cellular factors functioning at each stage of the HCV life cycle.

Hepatitis C virus genotype 4: A poorly characterized endemic genotype

Globally, 13% of all hepatitis C virus (HCV) infections are caused by genotype 4 (GT4), which consists of 17 subtypes with various levels of susceptibility to anti-HCV therapy. This genotype is endemic in the Middle East and Africa and has considerably spread to Europe lately. The molecular features of HCV-GT4 infection, as well as its appropriate therapeutics, are poorly characterized as it has not been the subject of widespread basic research. As such, in this review, we aim to gather the current state of knowledge of this genotype with a particular emphasis on its heterogeneity, sequence signatures, resistance-associated substitutions, and available in vivo and in vitro models used for its study. We urge developing more cell-culture models based on different GT4 subtypes to better understand the virology and therapeutic response of this particular genotype. This review may raise more awareness about this genotype and trigger more basic research work to develop its research tools. This will be critical to design better therapeutics and help to provide adequate guidelines for physicians working with HCV-GT4 patients.

Lipid Droplets Accumulation during Hepatitis C Virus Infection in Cell-Culture Varies among Genotype 1-3 Strains and Does Not Correlate with Virus Replication

Liver steatosis is a common complication of chronic hepatitis C virus (HCV) infection, which can result in accelerated liver fibrosis development, especially in patients infected with genotype 3a. The precise mechanisms of HCV-induced liver steatosis remain unclear, but it is often posited that increased intracellular lipid accumulation is the underlying cause of steatosis. To study experimentally how HCV infection in human liver derived cells by different genotypes and subtypes might affect lipid accumulation, we performed detailed cytofluorimetric and microscopy analyses of intracellular lipid droplets (LDs) in relation to the viral Core and to cell endoplasmic reticulum proteins. Following culture infection with HCV genotype 1a, 2a, 2b, 2c, and 3a strains, we found variable levels of intracellular LDs accumulation, associated to the infecting strain rather than to the specific genotype. Although two genotype 3a strains showed high levels of lipid accumulation, as previously observed, some strains of other genotypes displayed a similar phenotype. Moreover, the analyses of LDs size, number, and shape indicated that the apparent increase in lipid accumulation is due to an increase in the overall number rather than in the size of droplets. Finally, differences in total lipid content across genotypes did not correlate to differences in Core distribution nor Core levels. In conclusion, our study provides a quantitative in-depth analysis of the effect of HCV infection on LDs accumulation in cell-culture.

HCV Assembly and Egress via Modifications in Host Lipid Metabolic Systems

Hepatitis C virus (HCV) proliferates by hijacking the host lipid machinery. In vitro replication systems revealed many aspects of the virus life cycle; in particular, viral utilization of host lipid metabolism during HCV proliferation. HCV interacts with lipid droplets (LDs) before starting the process of virus capsid formation at the lipid-rich endoplasmic reticulum (ER) membrane compartment. HCV buds into the ER via lipoprotein assembly and secretion. Exchangeable apolipoproteins, represented by apolipoprotein E (apoE), play pivotal roles in enhancing HCV-specific infectivity. HCV virions are likely to interact with other lipoproteins circulating in blood vessels and incorporate apolipoproteins as well as lipids. This review focuses on virus assembly and egress by briefly describing the recent advances in this area.

Hepatitis C Virus: 30 Years after Its Discovery

Evidence for the existence of another hepatitis-causing pathogen, other than the known hepatitis A and B viruses, emerged in the mid-1970s. A frustrating search of 15 years was ended by the identification of the hepatitis C virus in 1989 using a recombinant DNA immunoscreening method. This discovery quickly led to blood tests that eliminated posttransfusion hepatitis C and could show the partial efficacy of type 1 interferon-based therapies. Subsequent knowledge of the viral replication cycle then led to the development of effective direct-acting antivirals targeting its serine protease, polymerase, and nonstructural protein 5A that resulted in the approval of orally available drug combinations that can cure patients within a few months with few side effects. Meanwhile, vaccine strategies have been shown to be feasible, and they are still required to effectively control this global epidemic.

Current status and future development of infectious cell-culture models for the major genotypes of hepatitis C virus: Essential tools in testing of antivirals and emerging vaccine strategies

In this review, we summarize the relevant scientific advances that led to the development of infectious cell culture systems for hepatitis C virus (HCV) with the corresponding challenges and successes. We also provide an overview of how these systems have contributed to the study of antiviral compounds and their relevance for the development of a much-needed vaccine against this major human pathogen. An efficient infectious system to study HCV in vitro, using human hepatoma derived cells, has only been available since 2005, and was limited to a single isolate, named JFH1, until 2012. Successive developments have been slow and cumbersome, as each available system has been the result of a systematic effort for discovering adaptive mutations conferring culture replication and propagation to patient consensus clones that are inherently non-viable in vitro. High genetic heterogeneity is a paramount characteristic of this virus, and as such, it should preferably be reflected in basic, translational, and clinical studies. The limited number of efficient viral culture systems, in the context of the vast genetic diversity of HCV, continues to represent a major hindrance for the study of this virus, posing a significant barrier towards studies of antivirals (particularly of resistance) and for advancing vaccine development. Intensive research efforts, driven by isolate-specific culture adaptation, have only led to efficient full-length infectious culture systems for a few strains of HCV genotypes 1, 2, 3, and 6. Hence research aimed at identifying novel strategies that will permit universal culture of HCV will be needed to further our understanding of this unique virus causing 400 thousand deaths annually.

Recombinant hepatitis C virus genotype 5a infectious cell culture systems expressing minimal JFH1 NS5B sequences permit polymerase inhibitor studies

The six major epidemiologically important hepatitis C virus (HCV) genotypes differ in global distribution and antiviral responses. Full-length infectious cell-culture adapted clones, the gold standard for HCV studies in vitro, are missing for genotypes 4 and 5. To address this challenge for genotype 5, we constructed a consensus full-length clone of strain SA13 (SA13fl), which was found non-viable in Huh7.5 cells. Step-wise adaptation of SA13fl-based recombinants, beginning with a virus encoding the NS5B-thumb domain and 3´UTR of JFH1 (SA13/JF372-X), resulted in a high-titer SA13 virus with only 41 JFH1-encoded NS5B-thumb residues (SA13/JF470-510cc); this required sixteen cell-culture adaptive substitutions within the SA13fl polyprotein and two 3´UTR-changes. SA13/JF372-X and SA13/JF470-510cc were equally sensitive to nucleoside polymerase inhibitors, including sofosbuvir, but showed differential sensitivity to inhibitors targeting the NS5B palm or thumb. SA13/JF470-510cc represents a model to elucidate the influence of HCV RNA elements on viral replication and map determinants of sensitivity to polymerase inhibitors.

Efficient Hepatitis C Virus Genotype 1b Core-NS5A Recombinants Permit Efficacy Testing of Protease and NS5A Inhibitors

Hepatitis C virus (HCV) strains belong to seven genotypes with numerous subtypes that respond differently to antiviral therapies. Genotype 1, and primarily subtype 1b, is the most prevalent genotype worldwide. The development of recombinant HCV infectious cell culture systems for different variants, permitted by the high replication capacity of strain JFH1 (genotype 2a), has advanced efficacy and resistance testing of antivirals. However, efficient infectious JFH1-based cell cultures of subtype 1b are limited and comprise only the 5' untranslated region (5'UTR)-NS2, NS4A, or NS5A regions. Importantly, it has not been possible to develop efficient 1b infectious systems expressing the NS3/4A protease, an important target of direct-acting antivirals. We developed efficient infectious JFH1-based cultures with genotype 1b core-NS5A sequences of strains DH1, Con1, and J4 by using previously identified HCV cell culture adaptive substitutions A1226G, R1496L, and Q1773H. These viruses spread efficiently in Huh7.5 cells by acquiring additional adaptive substitutions, and final recombinants yielded peak supernatant infectivity titers of 4 to 5 log10 focus-forming units (FFU)/ml. We subsequently succeeded in adapting a JFH1-based 5'UTR-NS5A DH1 recombinant to efficient growth in cell culture. We evaluated the efficacy of clinically relevant NS3/4A protease and NS5A inhibitors against the novel genotype 1b viruses, as well as against previously developed 1a viruses. The inhibitors were efficient against all tested genotype 1 viruses, with NS5A inhibitors showing half-maximal effective concentrations several orders of magnitude lower than NS3/4A protease inhibitors. In summary, the developed HCV genotype 1b culture systems represent valuable tools for assessing the efficacy of various classes of antivirals and for other virological studies requiring genotype 1b infectious viruses.

Ultrastructural and biochemical basis for hepatitis C virus morphogenesis

Chronic infection with HCV is a leading cause of cirrhosis, hepatocellular carcinoma and liver failure. One of the least understood steps in the HCV life cycle is the morphogenesis of new viral particles. HCV infection alters the lipid metabolism and generates a variety of microenvironments in the cell cytoplasm that protect viral proteins and RNA promoting viral replication and assembly. Lipid droplets (LDs) have been proposed to link viral RNA synthesis and virion assembly by physically associating these viral processes. HCV assembly, envelopment, and maturation have been shown to take place at specialized detergent-resistant membranes in the ER, rich in cholesterol and sphingolipids, supporting the synthesis of luminal LDs-containing ApoE. HCV assembly involves a regulated allocation of viral and host factors to viral assembly sites. Then, virus budding takes place through encapsidation of the HCV genome and viral envelopment in the ER. Interaction of ApoE with envelope proteins supports the viral particle acquisition of lipids and maturation. HCV secretion has been suggested to entail the ion channel activity of viral p7, several components of the classical trafficking and autophagy pathways, ESCRT, and exosome-mediated export of viral RNA. Here, we review the most recent advances in virus morphogenesis and the interplay between viral and host factors required for the formation of HCV virions.

The history of hepatitis C virus (HCV): Basic research reveals unique features in phylogeny, evolution and the viral life cycle with new perspectives for epidemic control

The discovery of hepatitis C virus (HCV) in 1989 permitted basic research to unravel critical components of a complex life cycle for this important human pathogen. HCV is a highly divergent group of viruses classified in 7 major genotypes and a great number of subtypes, and circulating in infected individuals as a continuously evolving quasispecies destined to escape host immune responses and applied antivirals. Despite the inability to culture patient viruses directly in the laboratory, efforts to define the infectious genome of HCV resulted in development of experimental recombinant in vivo and in vitro systems, including replicons and infectious cultures in human hepatoma cell lines. And HCV has become a model virus defining new paradigms in virology, immunology and biology. For example, HCV research discovered that a virus could be completely dependent on microRNA for its replication since microRNA-122 is critical for the HCV life cycle. A number of other host molecules critical for HCV entry and replication have been identified. Thus, basic HCV research revealed important molecules for development of host targeting agents (HTA). The identification and characterization of HCV encoded proteins and their functional units contributed to the development of highly effective direct acting antivirals (DAA) against the NS3 protease, NS5A and the NS5B polymerase. In combination, these inhibitors have since 2014 permitted interferon-free therapy with cure rates above 90% among patients with chronic HCV infection; however, viral resistance represents a challenge. Worldwide control of HCV will most likely require the development of a prophylactic vaccine, and numerous candidates have been pursued. Research characterizing features critical for antibody-based virus neutralization and T cell based virus elimination from infected cells is essential for this effort. If the world community promotes an ambitious approach by applying current DAA broadly, continues to develop alternative viral- and host- targeted antivirals to combat resistant variants, and invests in the development of a vaccine, it would be possible to eradicate HCV. This would prevent about 500 thousand deaths annually. However, given the nature of HCV, the millions of new infections annually, a high chronicity rate, and with over 150 million individuals with chronic infection (which are frequently unidentified), this effort remains a major challenge for basic researchers, clinicians and communities.

Resistance of Hepatitis C Virus to Inhibitors: Complexity and Clinical Implications

Selection of inhibitor-resistant viral mutants is universal for viruses that display quasi-species dynamics, and hepatitis C virus (HCV) is no exception. Here we review recent results on drug resistance in HCV, with emphasis on resistance to the newly-developed, directly-acting antiviral agents, as they are increasingly employed in the clinic. We put the experimental observations in the context of quasi-species dynamics, in particular what the genetic and phenotypic barriers to resistance mean in terms of exploration of sequence space while HCV replicates in the liver of infected patients or in cell culture. Strategies to diminish the probability of viral breakthrough during treatment are briefly outlined.

The Replacement of 10 Non-Conserved Residues in the Core Protein of JFH-1 Hepatitis C Virus Improves Its Assembly and Secretion

Hepatitis C virus (HCV) assembly is still poorly understood. It is thought that trafficking of the HCV core protein to the lipid droplet (LD) surface is essential for its multimerization and association with newly synthesized HCV RNA to form the viral nucleocapsid. We carried out a mapping analysis of several complete HCV genomes of all genotypes, and found that the genotype 2 JFH-1 core protein contained 10 residues different from those of other genotypes. The replacement of these 10 residues of the JFH-1 strain sequence with the most conserved residues deduced from sequence alignments greatly increased virus production. Confocal microscopy of the modified JFH-1 strain in cell culture showed that the mutated JFH-1 core protein, C10M, was present mostly at the endoplasmic reticulum (ER) membrane, but not at the surface of the LDs, even though its trafficking to these organelles was possible. The non-structural 5A protein of HCV was also redirected to ER membranes and colocalized with the C10M core protein. Using a Semliki forest virus vector to overproduce core protein, we demonstrated that the C10M core protein was able to form HCV-like particles, unlike the native JFH-1 core protein. Thus, the substitution of a few selected residues in the JFH-1 core protein modified the subcellular distribution and assembly properties of the protein. These findings suggest that the early steps of HCV assembly occur at the ER membrane rather than at the LD surface. The C10M-JFH-1 strain will be a valuable tool for further studies of HCV morphogenesis.

Adaptive Mutations Enhance Assembly and Cell-to-Cell Transmission of a High-Titer Hepatitis C Virus Genotype 5a Core-NS2 JFH1-Based Recombinant

Recombinant hepatitis C virus (HCV) clones propagated in human hepatoma cell cultures yield relatively low infectivity titers. Here, we adapted the JFH1-based Core-NS2 recombinant SA13/JFH1C3405G,A3696G (termed SA13/JFH1orig), of the poorly characterized genotype 5a, to Huh7.5 cells, yielding a virus with greatly improved spread kinetics and an infectivity titer of 6.7 log10 focus-forming units (FFU)/ml. We identified several putative adaptive amino acid changes. In head-to-head infections at fixed multiplicities of infection, one SA13/JFH1orig mutant termed SA13/JFH1Core-NS5B, containing 13 amino acid changes (R114W and V187A [Core]; V235L [E1]; T385P [E2]; L782V [p7]; Y900C [NS2]; N2034D, E2238G, V2252A, L2266P, and I2340T [NS5A]; A2500S and V2841A [NS5B]), displayed fitness comparable to that of the polyclonal high-titer adapted virus. Single-cycle virus production assays in CD81-deficient Huh7-derived cells demonstrated that these changes did not affect replication but increased HCV assembly and specific infectivity as early as 24 h posttransfection. Infectious coculture assays in Huh7.5 cells showed a significant increase in cell-to-cell transmission for SA13/JFH1Core-NS5B viruses as well as viruses with only p7 and nonstructural protein mutations. Interestingly, the E2 hypervariable region 1 (HVR1) mutation T385P caused (i) increased sensitivity to neutralizing patient IgG and human monoclonal antibodies AR3A and AR4A and (ii) increased accessibility of the CD81 binding site without affecting the usage of CD81 and SR-BI. We finally demonstrated that SA13/JFH1orig and SA13/JFH1Core-NS5B, with and without the E2 mutation T385P, displayed similar biophysical properties following iodixanol gradient ultracentrifugation. This study has implications for investigations requiring high virus concentrations, such as studies of HCV particle composition and development of whole-virus vaccine antigens.

IMPORTANCE

Hepatitis C virus (HCV) is a major global health care burden, affecting more than 150 million people worldwide. These individuals are at high risk of developing severe end-stage liver diseases. No vaccine exists. While it is possible to produce HCV particles resembling isolates of all HCV genotypes in human hepatoma cells (HCVcc), production efficacy varies. Thus, for several important studies, including vaccine development, in vitro systems enabling high-titer production of diverse HCV strains would be advantageous. Our study offers important functional data on how cell culture-adaptive mutations identified in genotype 5a JFH1-based HCVcc permit high-titer culture by affecting HCV genesis through increasing virus assembly and HCV fitness by enhancing the virus specific infectivity and cell-to-cell transmission ability, without influencing the biophysical particle properties. High-titer HCVcc like the one described in this study may be pivotal in future vaccine-related studies where large quantities of infectious HCV particles are necessary.

Advances in experimental systems to study hepatitis C virus in vitro and in vivo

Hepatitis C virus (HCV) represents a global health concern affecting over 185 million people worldwide. Chronic HCV infection causes liver fibrosis and cirrhosis and is the leading indication for liver transplantation. Recent advances in the field of direct-acting antiviral drugs (DAAs) promise a cure for HCV in over 90% of cases that will get access to these expensive treatments. Nevertheless, the lack of a protective vaccine and likely emergence of drug-resistant viral variants call for further studies of HCV biology. With chimpanzees being for a long time the only non-human in vivo model of HCV infection, strong efforts were put into establishing in vitro experimental systems. The initial models only enabled to study specific aspects of the HCV life cycle, such as viral replication with the subgenomic replicon and entry using HCV pseudotyped particles (HCVpp). Subsequent development of protocols to grow infectious HCV particles in cell-culture (HCVcc) ignited investigations on the full cycle of HCV infection and the virus-host interactions required for virus propagation. More recently, small animal models permissive to HCV were generated that allowed in vivo testing of novel antiviral therapies as well as vaccine candidates. This review provides an overview of the currently available in vitro and in vivo experimental systems to study HCV biology. Particular emphasis is given to how these model systems furthered our understanding of virus-host interactions, viral pathogenesis and immunological responses to HCV infection, as well as drug and vaccine development.

A comprehensive functional map of the hepatitis C virus genome provides a resource for probing viral proteins

Pairing high-throughput sequencing technologies with high-throughput mutagenesis enables genome-wide investigations of pathogenic organisms. Knowledge of the specific functions of protein domains encoded by the genome of the hepatitis C virus (HCV), a major human pathogen that contributes to liver disease worldwide, remains limited to insight from small-scale studies. To enhance the capabilities of HCV researchers, we have obtained a high-resolution functional map of the entire viral genome by combining transposon-based insertional mutagenesis with next-generation sequencing. We generated a library of 8,398 mutagenized HCV clones, each containing one 15-nucleotide sequence inserted at a unique genomic position. We passaged this library in hepatic cells, recovered virus pools, and simultaneously assayed the abundance of mutant viruses in each pool by next-generation sequencing. To illustrate the validity of the functional profile, we compared the genetic footprints of viral proteins with previously solved protein structures. Moreover, we show the utility of these genetic footprints in the identification of candidate regions for epitope tag insertion. In a second application, we screened the genetic footprints for phenotypes that reflected defects in later steps of the viral life cycle. We confirmed that viruses with insertions in a region of the nonstructural protein NS4B had a defect in infectivity while maintaining genome replication. Overall, our genome-wide HCV mutant library and the genetic footprints obtained by high-resolution profiling represent valuable new resources for the research community that can direct the attention of investigators toward unidentified roles of individual protein domains.

Our insertional mutagenesis library provides a resource that illustrates the effects of relatively small insertions on local protein structure and HCV viability. We have also generated complementary resources, including a website (http://hangfei.bol.ucla.edu) and a panel of epitope-tagged mutant viruses that should enhance the research capabilities of investigators studying HCV. Researchers can now detect epitope-tagged viral proteins by established antibodies, which will allow biochemical studies of HCV proteins for which antibodies are not readily available. Furthermore, researchers can now quickly look up genotype-phenotype relationships and base further mechanistic studies on the residue-by-residue information from the functional profile. More broadly, this approach offers a general strategy for the systematic functional characterization of viruses on the genome scale.