Chronic hepatitis C virus (HCV) infection develops in 70-80% of all exposed individuals and affects 3% of the world's population. There is a need for more effective and better-tolerated HCV therapies. Our long-term goals are to define host-HCV molecular interactions and in so doing, to identify novel therapeutic agents for HCV therapy. We have completed a functional genomic screen for host cofactors for HCV replication using a whole- genome siRNA library. Among the many novel host cofactors identified is PI4KA, a phosphatidylinositol 4- kinase (PI 4-kinase). We hypothesize that PI4KA is necessary for formation of the altered host membranes required for HCV replication. This proposal describes a 5 year mentored training program to acquire further technical skills in the fields of molecular virology, phosphoinositides and cell trafficking, and cell imaging. These skills will be used to develop novel model systems to study the formation and composition of the HCV replication complex and its associated host membranes and proteins. The specific aims of this project are (1) to characterize the specificity of PI4KA inhibition for suppression of HCV replication, (2) to characterize the role of PI4KA through the viral replication cycle, and (3) to develop systems to study HCV replication complex assembly and composition. Although the specific aims are directed towards PI4KA, the systems and techniques to be developed during the award period will be applied to the study of other host cofactors of HCV replication. The proposed research will have direct public health benefits as a better understanding of the mechanisms that underline HCV replication complex assembly may lead to novel targets for HCV therapies. A better understanding of the HCV-host relationship is a stated goal of the NIH Action Plan for Liver Disease Research (Goal B2a). We have recently performed a whole-genome screen for human (host) proteins that are required by the hepatitis C virus for its replication. Such proteins are potential targets for new therapies for HCV. This proposal describes a 5-year research plan focusing on developing new systems to study how host cofactors direct the assembly of HCV replication complexes.