Plugging the holes in hepatitis C virus antiviral therapy.

A growing number of RNA viruses are now known to depend on virus-encoded ion channels for efficient production of infectious virions and, in some cases, for the subsequent infection of naı̈ve cells. So-called viroporins are small hydrophobic proteins, usually less than 100 aa in length, and typically contain 1 or 2 transmembrane domains (TMDs); oligomerization is therefore necessary for the formation of ion channel complexes. By far the bestcharacterized viroporin is the M2 proton channel of influenza A virus, which is the target for the antiviral drugs amantadine and rimantadine (1). M2 sets a precedent for viroporins as therapeutic targets that has driven research into the ion channels of other clinically important viruses. In the light of rapid RNA virus evolution generating drug resistance, new compounds targeting viroporins could be a valuable addition to future combinatorial antiviral strategies. Difficulties associated with working with membrane proteins in high-throughput systems lend support to a rational approach for drug development based on the availability of high-resolution molecular structures. In this issue of PNAS, Luik et al. (2), describe the first structure of a complete viroporin complex, the p7 ion channel of hepatitis C virus (HCV), at 16-Å resolution by using single-particle electron microscopy. The hexameric p7 complex (42 kDa) is one of the smallest objects to be visualized by these methods to date which, combined with the hydrophobic nature of p7, renders this work an impressive technical achievement. Whereas high-resolution structural information is available for M2 (3, 4), where a short amino-terminal peptide is sufficient to form the tetrameric channel complex, p7 channels comprise both TMDs from each protomer and so have thus far proved elusive in crystallographic or NMR-based studies. This first p7 structure is therefore of great importance to the HCV field and provides vital information on channel size, stoichiometry, and the orientation of p7 protomers within the channel complex. The authors include an energy-minimized model for the channel complex that fits within the observed density and adopts a ‘‘f lower petal’’ conformation visible in the EM reconstructions. Combining molecular modeling with observed 3D structures not only provides clues to the gating of p7 channels, but could pave the way to future rational drug design, as a basis for deriving highresolution structures in the future, but also in the short term by validating these molecular models for use in virtual compound screening. Like other viroporins, p7 has been shown by several groups to display cation channel activity in vitro (5–7) and is now known to play a vital role during virion secretion in culture (8, 9). Critically, p7 is required for the virus to replicate in chimpanzees (10), and compounds that block its activity in vitro also inhibit particle production in culture, validating it as a therapeutic target (11, 12). The precise way in which p7 channel activity enhances particle production, however, is unknown. Growing evidence suggests that p7 behaves simi-