Chemical rescue of hisitdine selectivity filter mutants of the M2 ion channel of influenza A virus.

Venkataraman P, Lamb RA, Pinto LH.

Neurobiology and Physiology, Northwestern University, Evanston, IL 60208.

The influenza virus M2 proton-selective ion channel activity facilitates virus uncoating, a process that occurs in the acidic environment of the endosome. The M2 channel causes acidification of the interior of the virus particle, which results in viral protein-protein dissociation. The M2 protein is a homotetramer that contains in its aqueous pore a histidine residue (H37) that acts as a selectivity filter and a tryptophan residue (W41) that acts as a channel gate. Substitution of H37 modifies M2 ion channel properties drastically. However, the results of such experiments are difficult to interpret because substitution of H37 could cause gross structural changes to the channel pore. We describe here experiments in which partial or, in some cases, full rescue of specific M2 ion channel properties of H37 substitution mutants was achieved by addition of imidazole to the bathing medium. Chemical rescue was demonstrated for three histidine substitution mutant ion channels: M2-H37G, M2-H37S and M2-H37T and for two double mutants in which the Trp41 channel gate was also mutated (H37GW41Y and H37GW41A). Currents of the M2-H37G mutant ion channel were inhibited by Cu(II), which has been shown to coordinate with His37 in the wt channel. Chemical rescue was very specific for imidazole. Buffer molecules that are neutral when protonated (MES and MOPSO) did not rescue ion channel activity of the M2-H37G mutant ion channel, but 1-methylimidazole did provide partial rescue of function. These results are consistent with a model for proton transport through the pore of the wt channel in which the imidazole side-chain of His37 acts as an intermediate proton acceptor/donor group.