Hepatitis C virus (HCV) belongs to the Flaviviridae family of viruses including important human pathogens such as West Nile virus and Dengue virus. All these viruses share a similar genome organization. However, HCV is the sole positive strand RNA virus that elicits both persistent infection and cancer in patients. It is estimated that 130-170 million people worldwide are chronically infected with HCV, a major agent of chronic liver disease leading to cirrhosis, liver cancer and approximately 10,000 annual deaths int he US. There is currently no vaccine and the available treatments are either toxic to patients or eliciting resistant virus strains. Thus, defining the molecular mechanism of HCV genome replication and particle assembly will likely increase the repertoire of drugs targeting virus and host factors which are essential for HCV replication and pathogenicity.
Like many positive strand RNA viruses, the entire life cycle of HCV takes place in the cytosol of infected cells. That creates a challenge for the virus. Thus, HCV relies heavily on host membranes and proteins to successfully replicate its genome, package it and release new virus particles. To understand the mechanisms of virus replication, assembly and release, my lab uses several approaches including genetics, biochemistry, proteomics, microscopy and homology structural modeling.
Our laboratory and others have identified nonstructural 4B (NS4B) protein as a key organizer of HCV replication complex (RC). Specifically, NS4B protein rearranges intracellular membranes to form the so-called membranous web, the site of HCV RC assembly. These intracellular membrane alterations require NS4B interaction with the membrane trafficking and fusion Rab proteins. These findings suggest that NS4B mimics SNARE proteins or interacts with these factors to facilitate HCV RC formation. Hence, insight into the virus and host determinants of HCV RC formation could lead to novel approaches for preventing HCV genome replication in infected cells.
As an integral membrane protein, NS4B can diffuse freely in the plane of the membrane bilayer unless it is engaged in multiple protein-protein interactions in the context of the membrane. Indeed, NS4B has several membrane-spanning domains (MSDs). These domains represent more than one-third of the entire protein. Our current findings suggest that the MSDs are engaged in intra- and intermolecular interactions that may be crucial for both HCV genome replication and virion production. We have identified two protein-protein interaction motifs in these MSDs. These results raise the interesting possibility that many NS4B protein-protein interactions could occur int he membrane bilayer and might be ideal targets for pharmacological intervention. This is significant given that 7 out of 10 virus proteins are integral membrane proteins and at least 4 proteins harbor such helix-helix interaction motifs.
How positive strand RNA viruses switch from replicating to packaging their genome into new virus particles is currently unknown. However, these processes require tight regulation to prevent conflict leading to abortive infection. Recently, we have identified NS4B as a player in the switch from replication to virus assembly mode. Using chimeric viruses, we have identified putative genetic interactions between NS4B and two virus proteins previously associated with HCV particle assembly. Additionally, we have found that NS4B regulates the stability and/or phosphorylation state of NS5A, a prerequisite for HCV particle production. Uncovering the host kinases/phosphatases/proteases as well as viral factors involved in HCV assembly and release will undoubtedly lead to the development of novel antiviral targets for the eradication of HCV and related Flaviviridae family of viruses.