The lipid droplet binding domain of hepatitis C virus core protein is a major determinant for efficient virus assembly

Hepatitis C virus budding at lipid droplet-associated ER membrane visualized by 3D electron microscopy

The mechanisms underlying hepatitis C virus (HCV) morphogenesis remain elusive, but lipid droplets have recently been shown to be important organelles for virus production. We investigated the interaction between HCV-like particles and lipid droplets by three-dimensional reconstructions of serial ultrathin electron microscopy sections of cells producing the HCV core protein. The budding of HCV-like particles was mostly initiated at membranes close to the lipid droplets rather than at membranes directly apposed to the lipid droplets. This may have important implications for our understanding of the complex relationship between HCV and lipids and may make easier to dissect out the HCV life cycle.

Efficient trans-encapsidation of hepatitis C virus RNAs into infectious virus-like particles

Recently, complete replication of hepatitis C virus (HCV) in tissue culture was established using the JFH1 isolate. To analyze determinants of HCV genome packaging and virion assembly, we developed a system that supports particle production based on trans-packaging of subgenomic viral RNAs. Using JFH1 helper viruses, we show that subgenomic JFH1 replicons lacking the entire core to NS2 coding region are efficiently encapsidated into infectious virus-like particles. Similarly, chimeric helper viruses with heterologous structural proteins trans-package subgenomic JFH1 replicons. Like authentic cell culture-produced HCV (HCVcc) particles, these trans-complemented HCV particles (HCV(TCP)) penetrate target cells in a CD81 receptor-dependent fashion. Since HCV(TCP) production was limited by competition between the helper and subgenomic RNA and to avoid contamination of HCV(TCP) stocks with helper viruses, we created HCV packaging cells. These cells encapsidate various HCV replicons with high efficiency, reaching infectivity titers up to 10(6) tissue culture infectious doses 50 per milliliter. The produced particles display a buoyant density comparable to HCVcc particles and can be propagated in the packaging cell line but support only a single-round infection in naïve cells. Together, this work demonstrates that subgenomic HCV replicons are assembly competent, thus excluding cis-acting RNA elements in the core-to-NS2 genomic region essential for RNA packaging. The experimental system described here should be helpful to decipher the mechanisms of HCV assembly and to identify RNA elements and viral proteins involved in particle formation. Similar to other vector systems of plus-strand RNA viruses, HCV(TCP) may prove valuable for gene delivery or vaccination approaches.

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.