The domain 2 of the HCV NS5A protein is intrinsically unstructured.

We present here our current understanding of the NS5A-D2 domain of the hepatitis C virus. Whereas this protein domain is globally unstructured as assessed by macroscopic techniques such as size exclusion chromatography, circular dichroism and homonuclear NMR spectroscopy, high resolution triple resonance spectroscopy allows the identification of a small region of residual structure. This region corresponds moreover to the most conserved sequence over the different genotypes of the virus, underscoring its functional importance. We show that it forms an anchoring point for the host cell cyclophilin prolyl cis/trans isomerase, providing a molecular basis for the use of cyclophilin inhibitors in an antiviral strategy.