Nonstructural protein 5A (NS5A) is essential for hepatitis C virus (HCV) replication and constitutes an attractive target for antiviral drug development. Although 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, CD, and NMR spectroscopy analyses, we show that NS5A-D3 is natively unfolded. However, NS5A-D3 domains from both JFH-1 and Con1 strains exhibit a propensity to partially fold into an ␣-helix. NMR analysis identifies two putative ␣-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 and, as such, promote the interaction with relevant biological partner(s). Because 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. NMR heteronuclear exchange experiments demonstrate that CypA has in vitro peptidyl-prolyl cis/trans-isomerase 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.
Hepatitis C virus (HCV),3 a small enveloped virus from the Flaviviridae family, is a major cause of chronic liver disease that may lead to steatosis, liver cirrhosis, and hepatocellular carcinoma. Given about 180 million chronically infected individuals worldwide, HCV is an important health challenge (1). Current therapy is based on a combination of pegylated interferon-␣ and ribavirin but is not fully satisfying because numerous patients are not responding or suffer from serious side effects caused by this treatment. The development of new drugs to treat HCV infections requires a better understanding of the structural and functional features of the viral proteins and their relationships with host cell factors. The HCV genome (ϳ9.6 kb) encodes for a single polyprotein precursor (ϳ3,000 aa) that is co-and post-translationally processed by cellular and viral proteases to yield 10 mature proteins (2, 3). They are classified into structural proteins (Core, E1, and E2), which constitute the viral particle, and nonstructural proteins, of which two, p7 and nonstructural protein 2 (NS2), are required for virus assembly. The remainder of the nonstructural proteins (NS3, NS4A, NS4B, NS5A, and NS5B) are involved in HCV RNA replication (reviewed in Refs. 2 and 3).Cyclophilins are host cell factors that in addition to viral proteins, are equally essential for HCV replication. The human genome encodes up to 16 different cyclophilins (4) that, despite differences in their tissue distributio...