Helical structure and packing orientation of the S2 segment in the Shaker K+ channel

Six transmembrane segments, S1-S6, cluster around the central pore-forming region in voltage gated K+ channels. To investigate the structural characte ristics of the S2 segment in the Shaker K+ channel, we replaced each residu e in S2 singly with tryptophan (or with alanine for the native tryptophan). All but one of the 23 Trp mutants expressed voltage-dependent K+ currents in Xenopus oocytes. The effects of the mutations were classified as being o f low or high impact on channel gating properties. The periodicity evident in the effects of these mutations supports an alpha-helical structure for t he S2 segment. The high- and low-impact residues cluster onto opposite face s of a helical wheel projection of the S2 segment. The low-impact face is a lso tolerant of single mutations to asparagine. All results are consistent with the idea that the low-impact face projects toward membrane lipids and that changes in S2 packing occur upon channel opening. We conclude that the S2 segment is a transmembrane or helix and that the high-impact face packs against other transmembrane segments in the functional channel.