Compensatory Mutations in E1, p7, NS2, and NS3 Enhance Yields of Cell Culture-Infectious Intergenotypic Chimeric Hepatitis C Virus

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Abstract
There is little understanding of mechanisms underlying the assembly and release of infectious hepatitis C virus (HCV) from cultured cells. Cells transfected with synthetic genomic RNA from a unique genotype 2a virus (JFH1) produce high titers of virus, while virus yields are much lower with a prototype genotype 1a RNA containing multiple cell culture-adaptive mutations (H77S). To characterize the basis for this difference in infectious particle production, we constructed chimeric genomes encoding the structural proteins of H77S within the background of JFH1. RNAs encoding polyproteins fused at the NS2/NS3 junction (“H-NS2/NS3-J”) and at a site of natural, intergenotypic recombination within NS2 [“H-(NS2)-J”] produced infectious virus. In contrast, no virus was produced by a chimera fused at the p7-NS2 junction. Chimera H-NS2/NS3-J virus (vH-NS2/NS3-J) recovered from transfected cultures contained compensatory mutations in E1 and NS3 that were essential for the production of infectious virus, while yields of infectious vH-(NS2)-J were enhanced by mutations within p7 and NS2. These compensatory mutations were chimera specific and did not enhance viral RNA replication or polyprotein processing; thus, they likely compensate for incompatibilities between proteins of different genotypes at sites of interactions essential for virus assembly and/or release. Mutations in p7 and NS2 acted additively and increased the specific infectivity of vH-(NS2)-J particles, while having less impact on the numbers of particles released. We conclude that interactions between NS2 and E1 and p7 as well as between NS2 and NS3 are essential for virus assembly and/or release and that each of these viral proteins plays an important role in this process.