Molecular origins of asymmetric proton conduction in the influenza M2 channel

The M2 proton channel of influenza A is embedded into the viral envelope and allows acidification of the virion when the external pH is lowered. In contrast, no outward proton conductance is observed when the internal pH is lowered, although outward current is observed at positive voltage. Residues Trp41 and Asp44 are known to play a role in preventing pH-driven outward conductance, but the mechanism for this is unclear. We investigate this issue using classical molecular dynamics simulations with periodic proton hops. When all key His37 residues are neutral, inward proton movement is much more facile than outward movement if the His are allowed to shuttle the proton. The preference for inward movement increases further as the charge on the His37 increases. Analysis of the trajectories reveals three factors accounting for this asymmetry. First, in the outward direction, Asp44 traps the hydronium by strong electrostatic interactions. Secondly, Asp44 and Trp41 orient the hydronium with the protons pointing inward, hampering outward Grotthus hopping. As a result, the effective barrier is lower in the inward direction. Trp41 adds to the barrier by weakly H-bonding to potential H thorn acceptors. Finally, for charged His, the H3O thorn in the inner vestibule tends to get trapped at lipid-lined fenestrations of the cone-shaped channel. Simulations qualitatively reproduce the experimentally observed higher outward conductance of mutants. The ability of positive voltage, unlike proton gradient, to induce an outward current appears to arise from its ability to bias H3O thorn and the waters around it toward more H-outward orientations.

Automated summary

The M2 proton channel in influenza A virus allows acidification of the virion when external pH is lowered, but does not conduct protons outward when internal pH is lowered. Trp41 and Asp44 residues play a role in preventing pH-driven outward conductance, but the mechanism is unclear. Molecular dynamics simulations reveal that inward proton movement is easier than outward movement when key His37 residues are neutral, and this preference for inward movement increases with increasing charge on His37. Several factors contribute to this asymmetry, including electrostatic interactions, orientation of hydronium ions, and trapping of ions at specific sites in the channel.