PH-dependent conformation, dynamics, and aromatic interaction of the gating tryptophan residue of the influenza M2 proton channel from solid-state NMR.

ABSTRACT: The M2 protein of the influenza virus conducts protons into the virion under external acidic pH. The proton selectivity of the tetrameric channel is controlled by a single histidine (His(37)), whereas channel gating is accomplished by a single tryptophan (Trp(41)) in the transmembrane domain of the protein. Aromatic interaction between these two functional residues has been previously observed in Raman spectra, but atomic-resolution evidence for this interaction remains scarce. Here we use high-resolution solid-state NMR spectroscopy to determine the side-chain conformation and dynamics of Trp(41) in the M2 transmembrane peptide by measuring the Trp chemical shifts, His(37)-Trp(41) distances, and indole dynamics at high and low pH. The interatomic distances constrain the Trp41 side-chain conformation to trans for ?1 and 120-135° for ?2. This t90 rotamer points the N?1-C?2-C?2 side of the indole toward the aqueous pore. The precise ?1 and ?2 angles differ by ?20° between high and low pH. These differences, together with the known changes in the helix tilt angle between high and low pH, push the imidazole and indole rings closer together at low pH. Moreover, the measured order parameters indicate that the indole rings undergo simultaneous ?1 and ?2 torsional fluctuations at acidic pH, but only restricted ?1 fluctuations at high pH. As a result, the Trp(41) side chain periodically experiences strong cation-? interactions with His(37) at low pH as the indole sweeps through its trajectory, whereas at high pH the indole ring is further away from the imidazole. These results provide the structural basis for understanding how the His(37)-water proton exchange rate measured by NMR is reduced to the small proton flux measured in biochemical experiments. The indole dynamics, together with the known motion of the imidazolium, indicate that this compact ion channel uses economical side-chain dynamics to regulate proton conduction and gating.

SUBMITTER: Williams JK 

PROVIDER: S-EPMC3627873 | biostudies-literature | 2013 Apr

REPOSITORIES: biostudies-literature