Unusual architecture of the p7 channel from hepatitis C virus

The Hepatitis C virus (HCV) has developed a small membrane protein, p7, which remarkably can self-assemble into a large channel complex that selectively conducts cations(1-4). We are curious as to what structural solution has the viroporin adopted to afford selective cation conduction because p7 has no homology with any of the known prokaryotic or eukaryotic channel proteins. The p7 activity can be inhibited by amantadine and rimantadine(2,5), which also happen to be potent blockers of the influenza M2 channel(6) and licensed drugs against influenza infections(7). The adamantane derivatives were subjects of HCV clinical trials(8), but large variation in drug efficacy among the various HCV genotypes has been difficult to explain without detailed molecular structures. Here, we determined the structures of this HCV viroporin as well as its drug-binding site using the latest nuclear magnetic resonance (NMR) technologies. The structure exhibits an unusual mode of hexameric assembly, where the individual p7 monomers, i, not only interact with their immediate neighbors, but also reach farther to associate with the i+2 and i+3 monomers, forming a sophisticated, funnel-like architecture. The structure also alludes to a mechanism of cation selection: an asparagine/histidine ring that constricts the narrow end of the funnel serves as a broad cation selectivity filter while an arginine/lysine ring that defines the wide end of the funnel may selectively allow cation diffusion into the channel. Our functional investigation using whole-cell channel recording showed that these residues are indeed critical for channel activity. NMR measurements of the channel-drug complex revealed six equivalent hydrophobic pockets between the peripheral and pore-forming helices to which amantadine or rimantadine binds, and compound binding specifically to this position may allosterically inhibit cation conduction by preventing the channel from opening. Our data provide molecular explanation for p7-mediated cation conductance and its inhibition by adamantane derivatives.