Conformation and Ion-Channeling Activity of a 27-Residue Peptide Modeled on the Single-Transmembrane Segment of the IsK (minK) Protein
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- 作者：Amalia Aggeli ; Mark L. Bannister ; Mark Bell ; Neville Boden ; John B. C. Findlay ; Malcolm Hunter ; Peter F. Knowles ; and Ji-Chun Yang
- 刊名：Biochemistry
- 出版年：1998
- 出版时间：June 2, 1998
- 年：1998
- 卷：37
- 期：22
- 页码：8121 - 8131
- 全文大小：153K
- 年卷期：v.37,no.22(June 2, 1998)
- ISSN：1520-4995
文摘
IsK (minK) protein, in concert with another channel proteinKVLQT1, mediates a distinct,slowly activating, voltage-gated potassium current across certainmammalian cell membranes. Site-directedmutational studies have led to the proposal that the singletransmembrane segment of IsK participates inthe pore of the potassium channel [Takumi, T. (1993) NewsPhysiol. Sci. 8, 175-178]. We presentfunctional and structural studies of a short peptide (K27) with primarystructure NH2-1KLEALYILMVLGFFGFFTLGIMLSYI27R-COOH, corresponding to thetransmembrane segment of IsK (residues42-68). When K27 was incorporated, at low concentrations, intophosphatidylethanolamine, black-lipidmembranes, single-channel activity was observed, with no strong ionselectivity. IR measurements revealthe peptide has a predominantly helical conformation in the membrane.The atomic resolution structureof the helix has been established by high-resolution 1H NMRspectroscopy studies. These studies werecarried out in a solvent comprising 86% v/v1,1,1,3,3,3-hexafluoro-isopropanol-14% v/v water, inwhichthe IR spectrum of the peptide was found to be very similar to thatobserved in the bilayer. The NMRstudies have established that residues 1-3 are disordered, whileresidues 4-27 have an 
hars/alpha.gif" BORDER=0>-helicalconformation, the helix being looser near the termini and more stablein 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 thehydrocarbon chains (2.3 ± 0.25 nm) of fully hydrated bilayers ofphosphatidylcholine lipid mixture fromegg yolk. The side chains on the helix surface are predominantlyhydrophobic, consistent with atransmembrane location of the helix. The ion-channeling activityis believed to stem from long-livedaggregates of these helices. The aggregation is mediated by thehars/pi.gif" BORDER=0 >-hars/pi.gif" BORDER=0 > stacking of phenylalanine aromaticrings of adjacent helices and favorable interactions of the opposingaliphatic-like side chains, such asleucine and methionine, with the lipid chains of the bilayer. Thismechanism is in keeping with site-directed mutational studies which suggest that the transmembrane segmentof IsK is an integral part ofthe pore of the potassium channel and has a similar disposition to thatin the peptide model system.
hars/alpha.gif" BORDER=0>-helicalconformation, the helix being looser near the termini and more stablein 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 thehydrocarbon chains (2.3 ± 0.25 nm) of fully hydrated bilayers ofphosphatidylcholine lipid mixture fromegg yolk. The side chains on the helix surface are predominantlyhydrophobic, consistent with atransmembrane location of the helix. The ion-channeling activityis believed to stem from long-livedaggregates of these helices. The aggregation is mediated by thehars/pi.gif" BORDER=0 >-hars/pi.gif" BORDER=0 > stacking of phenylalanine aromaticrings of adjacent helices and favorable interactions of the opposingaliphatic-like side chains, such asleucine and methionine, with the lipid chains of the bilayer. Thismechanism is in keeping with site-directed mutational studies which suggest that the transmembrane segmentof IsK is an integral part ofthe pore of the potassium channel and has a similar disposition to thatin the peptide model system.