Structure of the amantadine binding site of influenza M2 proton channels in lipid bilayers

The current H1N1 strain pandemic virus is resistant to the established antiviral agents amantadine and rimantadine, which target the M2 protein, a multifunctional membrane-spanning proton channel. The structure of this channel has been a subject of some controversy, since an X-ray crystal structure of part of the M2 channel showed electron density that corresponded to a single molecule of amantadine in the N-terminal half of the pore, whereas a solution NMR structure of a larger portion of the channel showed four rimantadine molecules bound to the C-terminal lipid-facing surface of the helices. The matter now appears resolved with the publication of the high-resolution structure of the M2 channel in a phospholipid bilayer, determined using solid-state NMR spectroscopy. This reveals two amantadine-binding sites: a high-affinity site in the N-terminal channel lumen and a low-affinity site on the C-terminal protein surface. This work could be of value for the development of new anti-influenza drugs, an important goal since the 2009 seasonal virus is amantadine-sensitive but resistant to Tamiflu, raising the possibility that multiply resistant virus types might emerge in future. The antiviral drugs amantadine and rimantadine target the M2 protein of influenza A virus, making an understanding of its structure important for the study of drug resistance. The results of a recent crystal structure of M2 differ from those of a solution NMR structure with regards to binding of these drugs, indicating a different mechanism of inhibition in each case. Here, using solid-state NMR spectroscopy, two different amantadine-binding sites are shown to exist in the phospholipid bilayers of M2. The M2 protein of influenza A virus is a membrane-spanning tetrameric proton channel targeted by the antiviral drugs amantadine and rimantadine1. Resistance to these drugs has compromised their effectiveness against many influenza strains, including pandemic H1N1. A recent crystal structure of M2(22–46) showed electron densities attributed to a single amantadine in the amino-terminal half of the pore2, indicating a physical occlusion mechanism for inhibition. However, a solution NMR structure of M2(18–60) showed four rimantadines bound to the carboxy-terminal lipid-facing surface of the helices3, suggesting an allosteric mechanism. Here we show by solid-state NMR spectroscopy that two amantadine-binding sites exist in M2 in phospholipid bilayers. The high-affinity site, occupied by a single amantadine, is located in the N-terminal channel lumen, surrounded by residues mutated in amantadine-resistant viruses. Quantification of the protein–amantadine distances resulted in a 0.3 A-resolution structure of the high-affinity binding site. The second, low-affinity, site was observed on the C-terminal protein surface, but only when the drug reaches high concentrations in the bilayer. The orientation and dynamics of the drug are distinct in the two sites, as shown by 2H NMR. These results indicate that amantadine physically occludes the M2 channel, thus paving the way for developing new antiviral drugs against influenza viruses. The study demonstrates the ability of solid-state NMR to elucidate small-molecule interactions with membrane proteins and determine high-resolution structures of their complexes.