Proton Association Constants of His 37 in the Influenza-A M2(18–60) Dimer-of-Dimers

[Image: see text] The membrane protein M2 from influenza-A forms a single-pass transmembrane helix that assembles in lipid membrane as homotetramers whose primary function is to act as a proton transporter for viral acidification. A single residue, histidine 37 (His 37), is known to be responsible for selectivity and plays an integral role in the protein’s function. We report pH-dependent (15)N MAS NMR spectra of His 37 within the influenza-A proton conduction domain of M2, M2(18–60), which has been previously shown to be a fully functional construct and was recently determined to adopt a dimer-of-dimers structure in lipids. By extracting the ratio of [His]/[HisH(+)] as a function of pH, we obtained two doubly degenerate proton disassociation constants, 7.63 ± 0.15 and 4.52 ± 0.15, despite a possible maximum of four. We also report the (1)H(Nε) chemical shifts at pH 6.5 recorded at 60 kHz MAS in a CP-based (1)H–(15)N spectrum. We were unable to detect resonances indicative of direct proton sharing among His 37 side chains when the tetramer is in the +2 state. In the neutral state, His 37 is exclusively in the τ tautomer, indicating that the δ nitrogen is protonated solely as a function of pH. We also found that the plot of [HisH(+)]/[His] as a function of pH is qualitatively similar to previously reported proton conduction rates, indicating that proton conduction rate is proportional to the level of histidine protonation within the channel. Two-dimensional (13)C–(13)C and (13)C–(15)N correlations suggest that at low pH multiple conformations are populated as the spectra broaden and eventually disappear as the acidity is increased. A second highly resolved state at low pH was not observed.