Hepatitis C virus (HCV) is an enveloped virus with a positive-sense, single-stranded RNA of approximately 9.6 kb, a member of the genus Hepaci virus within the family Flaviviridae. The genome contains a single large open reading frame encoding a 3000 residue polyprotein. The non-structural 5A protein (NS5A) is a highly phosphorylated protein, whichis comprised of three domains (I, II and III). Previously, we demonstrated that two residues within NS5A domain I (V67 and P145) play critical roles in HCV assembly challenging the dogma that NS5A domain I exclusively participated in genome replication. In this study, we identified 8 surface exposed residues of domain Iwhich were located in close proximity to V67 and P145. The mutants were cloned into a JFH-1 derived subgenomic replicons (mSGR-luc-JFH1) to confirm whether they are required in genome replication. The results of luciferase assay suggested that I52A exhibited the same phenotype as V67A and P145A and is a further candidate for regulating assembly of HCV. In parallel we sought to investigate whether domain I was involved in the interaction of NS5A with cyclophilin A (CypA), a cellular peptidyl-prolyl isomerase required for HCV replication. CypA can be inhibited by cyclosporin A (CsA), which also inhibits HCV genome replication. Surprisingly, all three mutant replicons (I52A, V67A and P145A) were more sensitive to CsA treatment than wildtype, suggesting that domain I does indeed interact with CypA. Ongoing studies will therefore investigate the roles of both domain I and CypA in genome replication and assembly of infectious HCV particles.

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