The hepatitis C virus (HCV) is the leading cause of liver cancer in the Western Hemisphere. Over 170 million people are infected with this virus and thus at increased risk of liver damage and cancer. HCV infection is curable, and thus such health consequences are completely avoidable. However the current HCV therapy is inadequate, as it is associated with severe side effects and only effective in about half of treate individuals. Effective HCV treatment will require antivirals targeting multiple HCV replication steps that can be used in combination to avoid the derivation of resistant viruses. The HCV cell entry process is an attractive target for antiviral development. Inhibiting this stage of the virallife cycle would curtail the persistence of HCV in vivo, which requires constant infection of new cells, as well as the spread of viral mutants that have developed resistance to other HCV inhibitors. The research proposed in this application will advance our understanding of HCV cell entry mechanisms, with the ultimate goal of uncovering novel antiviral targets. Although we now know that numerous host factors are required for HCV cell entry, we do not understand how they mediate HCV infection. The experiments described in this application are based on the hypothesis that the incoming HCV virion utilizes each of these cellular factors in a sequential manner to mediate cell entry. As two of the host proteins that are required for HCV cell entry are tight junction proteins, it is likely that cell polarity, which regulates the localization and funcions of these proteins, impacts the HCV cell entry process. Although the majority of HCV cell entry studies have been conducted in non-polarized cells, we have recently developed the first polarized cell system that permits efficient infection of authentic HCV grown in cell culture. This system will serve as the foundation for the studies proposed in this application. Specifically, we propose experiments to compare HCV cell entry pathways in non-polarized and polarized using a variety of HCV cell entry factor specific inhibitors. Furthermore, we will test how modulating cell polarity influences HCV cell entry, and examine the host factor requirements with entry factor specific inhibitors, silencing, and complementation with a variety of mutants. We will also analyze when and where each entry factor is used by analyzing the kinetics of action of entry factor specific inhibitors and visualizing the HCV cell entry process in non-polarized and polarized cells. Finally, we will identify physical and genetic interactions between host factors and the incoming HCV virion, and explore how such interactions are affected by cell polarity.