Structure and Mechanism of the M2 Proton Channel of Influenza A Virus

摘要

The integral membrane protein, M2, of influenza virus forms pH-gated proton channels in the viral lipid envelope. The low pH of an endosome activates the M2 channel prior to hemagglutinin-mediated fusion. Conductance of protons acidifies the viral interior and thereby facilitates dissociation of the matrix protein from the viral nucleoproteins – a required process for unpacking of the viral genome. In addition to its role in release of viral nucleoproteins, M2 in the trans-Golgi network (TGN) membrane prevents premature conformational rearrangement of newly synthesized hemagglutinin during transport to the cell surface by equilibrating the pH of the TGN with that of the host cell cytoplasm. Inhibitng the proton conductance of M2 with the anti-viral drug amantadine or rimantadine inhibits viral replication^–. We have determined by NMR the structure of the tetrameric M2 channel in complex with rimantadine. In the closed state, four tightly packed transmembrane (TM) helices define a narrow channel, in which a “tryptophan gate” is locked by inter-molecular interactions with aspartic acid. A C-terminal, amphipathic (AP) helix oriented nearly perpendicular to the TM helix, forms an inward facing base. Lowering the pH destabilizes the TM helical packing and unlocks the gate, admitting water to conduct protons, while the C-terminal base remains intact, preventing dissociation of the tetramer. Rimantadine binds at four equivalent sites near the gate on the lipid facing side of the channel and stabilizes the closed conformation of the pore. Drug-resistance mutations are predicted to counter the effect of drug binding by either increasing the hydrophilicity of the pore or weakening helix-helix packing, thus facilitating channel opening.