Hydrogen-Bonding Partner of the Proton-Conducting Histidine in the Influenza M2 Proton Channel Revealed From ¹H Chemical Shifts

Abstract

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 ¹H^N chemical shifts of His37 at different temperatures and pH using 2D ¹⁵N–¹H 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