Characterization of the Hepatitis C Virus Life Cycle and Regulation of the Capsid Protein

With more than 170 million people infected, hepatitis C virus (HCV) is a global health burden causing cirrhosis, liver steatosis, and hepatocellular carcinoma. As no prophylactic vaccine is available and the current treatments are only efficacious in a subset of patients, the identification of novel drug targets and development of better therapies are urgently needed. Because an infectious cell culture system was not developed until 2005, HCV research previously focused on particular isolated aspects of the viral life cycle such as RNA replication and entry recapitulated by replicons and HCV pseudoparticles, respectively. Hence, we utilized the relatively new infectious cell culture system to perform studies characterizing the kinetics of the entire life cycle. Specifically, we have defined the dynamics of HCV RNA amplification, HCV protein accumulation, and release of infectious virions. Although all ten viral proteins are translated as one polyprotein which is then processed into individual proteins, our data indicates that the core protein, which binds and encapsidates the viral genome, is differentially regulated throughout infection relative to the other viral proteins NS3 and NS5B. While NS3 accumulates gradually over time and parallels the amplification HCV RNA, core accumulation remains undetectable prior to HCV RNA replication and then rapidly increase later in the life cycle. Investigation of the molecular mechanism effecting differential core accumulation revealed that core is unstable early in the life cycle before RNA amplification, but becomes stable late in the life cycle after RNA replication. Functional RNAi knockdown studies indicated that the cellular factors E6AP and PA28γ are involved in degrading core early in the life cycle. Additionally, we show in transient transfection experiments that HCV RNA is sufficient to induce the intracellular accumulation and stabilization of core. As such we speculate HCV has evolved a strategy for ensuring that when HCV RNA levels are low that it is first used to generate viral proteins and replicate the viral genome to adequate levels before excess viral RNA triggers core stabilization and subsequent assembly. Hence, this study characterizes the kinetics of the HCV life cycle and identifies a novel regulation of core.