Binding and Proton Blockage by Amantadine Variants of the Influenza M2_WT and M2_S31N Explained

While aminoadamantanes are well-established inhibitors of the influenza A M2 proton channel, the mechanisms by which they are rendered ineffective against M2_S31N are unclear. Solid state NMR, isothermal titration calorimetry, electrophysiology, antiviral assays, and molecular dynamics simulations suggest stronger binding interactions for aminoadamantanes to M2_WT compared to negligible or weak binding to M2_S31N. This is due to reshaping of the M2 pore when N31 is present, which, in contrast to wild-type (WT), leads (A) to the loss of the V27 pocket for the adamantyl cage and to a predominant orientation of the ligand's ammonium group toward the N-terminus and (B) to the lack of a helical kink upon ligand binding. The kink, which reduces the tilt of the C-terminal helical domain relative to the bilayer normal, includes the W41 primary gate for proton conductance and may prevent the gate from opening, representing an alternative view for how these drugs prevent proton conductance.