Transmembrane four-helix bundle of influenza A M2 protein channel: structural implications from helix tilt and orientation.

The transmembrane portion of the M2 protein from the Influenza A virus has been studied in hydrated dimyristroylphosphotidylcholine lipid bilayers with solid-state NMR. Orientational constraints were obtained from isotopically labeled peptide samples mechanically aligned between thin glass plates. 15N chemical shifts from single site labeled samples constrain the molecular frame with respect to the magnetic field. When these constraints are applied to the peptide, modeled as a uniform alpha-helix, the tilt of the helix with respect to the bilayer normal was determined to be 33 degrees +/- 3 degrees. Furthermore, the orientation about the helix axis was also determined within an error of +/- 30 degrees. These results imply that the packing of this tetrameric protein is in a left-handed four-helix bundle. Only with such a large tilt angle are the hydrophilic residues aligned to the channel axis.