Hydrogen-bonding partner of the proton-conducting histidine in the influenza M2 proton channel revealed from 1H chemical shifts.

The influenza M2 protein conducts protons through a critical histidine (His) residue, His37. Whether His37 only interacts with water to relay protons into the virion or whether a low-barrier hydrogen bond (LBHB) also exists between the histidines to stabilize charges before proton conduction is actively debated. To address this question, we have measured the imidazole (1)H(N) chemical shifts of His37 at different temperatures and pH using 2D (15)N-(1)H correlation solid-state NMR. At low temperature, the H(N) chemical shifts are 8-15 ppm at all pH values, indicating that the His37 side chain forms conventional hydrogen bonds (H-bonds) instead of LBHBs. At ambient temperature, the dynamically averaged H(N) chemical shifts are 4.8 ppm, indicating that the H-bonding partner of the imidazole is water instead of another histidine in the tetrameric channel. These data show that His37 forms H-bonds only to water, with regular strength, thus supporting the His-water proton exchange model and ruling out the low-barrier H-bonded dimer model.