Featured Article - August 2013
Short description: The NMR structure of HCV p7 channel reveals a funnel-like architecture, providing insight into cation conductance and drug-mediated inhibition.
The integral membrane protein p7 from hepatitis C virus (HCV) can self-assemble into a large cation channel, or viroporin, which is required for viral replication and therefore a potential anti-HCV drug target. Chou and colleagues (PSI MPSbyNMR) have now determined the NMR structure of the large hexameric p7 channel reconstituted in micelles (PDB 2M6X).
The authors first characterized the secondary structures of the monomers and then assembled the oligomer using intermonomer distance restraints and orientation constraints. Each monomer consists of three helices (H1–H3), and the monomers are intertwined to form a tightly packed channel, where H1 and H2 form the channel interior and H3 is lipid-facing. Each monomer interacts not only with its direct neighboring monomers, but also with the +2 and +3 monomers, thereby forming an unusual, funnel-like structure.
To define the elements involved in cation conductance and gating, the authors identified highly conserved polar residues in the channel interior. Residue Asn9, which has affinity for monovalent and divalent cations, forms a ring that was proposed to serve as a broad selectivity filter at the narrow end of the funnel. Indeed, replacing His9 with Ala caused a large reduction in channel conductance, as measured in whole-cell channel recordings. The Ile 6 ring at the funnel's narrowest point forms a hydrophobic gate that is thought to prevent water from passing through. Residue Arg35 forms a positively charged ring at the wider, C-terminal end of the channel, and was proposed to bind and obstruct anions at the pore entrance while allowing cations to diffuse into the pore unidirectionally. As expected, mutating Arg35 to a negatively charged residue hindered the diffusion of cations into the pore and reduced conductance.
The relatively poor stability of the p7 viroporin in complex with the drugs adamantine or rimantadine prevented full-scale structure determination of the protein-drug complex. Nevertheless, the available NMR data suggest that either drug binds to six equivalent hydrophobic pockets between the pore-forming and peripheral helices. The authors propose that channel activation may involve structural rearrangements, and that the binding of adamantine derivatives might allosterically inhibit cation conduction by restricting movements of the three helical segments, thereby preventing the channel from opening. More rigorous testing will be required to validate this attractive model.
B. OuYang et al. Unusual architecture of the p7 channel from hepatitis C virus.
Nature. 498, 521-525 (2013). doi:10.1038/nature12283