Domain 3 of NS5A protein from the hepatitis C virus has intrinsic {alpha}-helical propensity and is a substrate of Cyclophilin A

Nonstructural protein 5A (NS5A) is essential for Hepatitis C Virus (HCV) replication and constitutes an attractive target for antiviral drug development. While structural data for its in-plane membrane anchor and domain D1 are available, the structure of domains 2 (D2) and 3 (D3) remain poorly defined. We report here a comparative molecular characterization of the NS5A-D3 domains of the HCV JFH-1 (genotype 2a) and Con1 (genotype 1b) strains. Combining gel filtration, circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopy analyses, we show that NS5A-D3 is natively unfolded. However, NS5A-D3 domains from both JFH-1 and Con1 strains exhibit propensity to partially fold into an alpha-helix. NMR analysis identifies two putative alpha-helices, for which a molecular model could be obtained. The amphipathic nature of the first helix and its conservation in all genotypes suggest that it might correspond to a molecular recognition element (MoRE), and as such promote the interaction with relevant biological partner(s). As mutations conferring resistance to cyclophilin inhibitors have been mapped into NS5A-D3, we also investigated the functional interaction between NS5A-D3 and Cyclophilin A (CypA). CypA indeed interacts with NS5A-D3 and this interaction is completely abolished by Cyclosporin A (CsA). NMR heteronuclear exchange experiments demonstrate that CypA has in vitro PPIase activity toward some, but not all, of the peptidyl-prolyl bonds in NS5A-D3. These studies lead to novel insights into the structural features of NS5A-D3 and its relationships with CypA.