Helical structure of the COOH terminus of S3 and its contribution to the gating modifier toxin receptor in voltage-gated ion channels.
The voltage-sensing domains in voltage-gated K(+) channels each contain four transmembrane (TM) segments, termed S1 to S4. Previous scanning mutagenesis studies suggest that S1 and S2 are amphipathic membrane spanning alpha-helices that interface directly with the lipid membrane. In contrast, the secondary structure of and/or the environments surrounding S3 and S4 are more complex. For S3, although the NH(2)-terminal part displays significant helical character in both tryptophan- and alanine-scanning mutagenesis studies, the structure of the COOH-terminal portion of this TM is less clear. The COOH terminus of S3 is particularly interesting because this is where gating modifier toxins like Hanatoxin interact with different voltage-gated ion channels. To further examine the secondary structure of the COOH terminus of S3, we lysine-scanned this region in the drk1 K(+) channel and examined the mutation-induced changes in channel gating and Hanatoxin binding affinity, looking for periodicity characteristic of an alpha-helix. Both the mutation-induced perturbation in the toxin-channel interaction and in gating support the presence of an alpha-helix of at least 10 residues in length in the COOH terminus of S3. Together with previous scanning mutagenesis studies, these results suggest that, in voltage-gated K(+) channels, the entire S3 segment is helical, but that it can be divided into two parts. The NH(2)-terminal part of S3 interfaces with both lipid and protein, whereas the COOH-terminal part interfaces with water (where Hanatoxin binds) and possibly protein. A conserved proline residue is located near the boundary between the two parts of S3, arguing for the presence of a kink in this region. Several lines of evidence suggest that these structural features of S3 probably exist in all voltage-gated ion channels.
Molecular Physiology and Biophysics Unit, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892, USA.
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Y
Li-Smerin
K J
KJ
Swartz
D001665:Binding Sites; D005583:Fourier Analysis; D015640:Ion Channel Gating / Q000502:physiology*; D008239:Lysine; D016296:Mutagenesis; D010455:Peptides / Q000494:pharmacology; D016133:Polymerase Chain Reaction; D015221:Potassium Channels / Q000502:physiology*; D017433:Protein Structure, Secondary
D010455:Peptides; D015221:Potassium Channels; C096469:hanatoxin; D008239:Lysine