Conformation and Ion-Channeling Activity of a 27-Residue Peptide Modeled on the Single-Transmembrane Segment of the IsK (minK) Protein,

Abstract

IsK (minK) protein, in concert with another channel protein KVLQT1, mediates a distinct, slowly activating, voltage-gated potassium current across certain mammalian cell membranes. Site-directed mutational studies have led to the proposal that the single transmembrane segment of IsK participates in the pore of the potassium channel [Takumi, T. (1993) News Physiol. Sci. 8, 175−178]. We present functional and structural studies of a short peptide (K27) with primary structure NH₂-¹KLEALYILMVLGFFGFFTLGIMLSYI²⁷R-COOH, corresponding to the transmembrane segment of IsK (residues 42−68). When K27 was incorporated, at low concentrations, into phosphatidylethanolamine, black-lipid membranes, single-channel activity was observed, with no strong ion selectivity. IR measurements reveal the peptide has a predominantly helical conformation in the membrane. The atomic resolution structure of the helix has been established by high-resolution ¹H NMR spectroscopy studies. These studies were carried out in a solvent comprising 86% v/v 1,1,1,3,3,3-hexafluoro-isopropanol−14% v/v water, in which the IR spectrum of the peptide was found to be very similar to that observed in the bilayer. The NMR studies have established that residues 1−3 are disordered, while residues 4−27 have an α-helical conformation, the helix being looser near the termini and more stable in the central region of the molecule. The length (2.6 nm) of the hydrophobic segment of the helix, residues 7−23, matches the span of the hydrocarbon chains (2.3 ± 0.25 nm) of fully hydrated bilayers of phosphatidylcholine lipid mixture from egg yolk. The side chains on the helix surface are predominantly hydrophobic, consistent with a transmembrane location of the helix. The ion-channeling activity is believed to stem from long-lived aggregates of these helices. The aggregation is mediated by the π-π stacking of phenylalanine aromatic rings of adjacent helices and favorable interactions of the opposing aliphatic-like side chains, such as leucine and methionine, with the lipid chains of the bilayer. This mechanism is in keeping with site-directed mutational studies which suggest that the transmembrane segment of IsK is an integral part of the pore of the potassium channel and has a similar disposition to that in the peptide model system.