Specific interactions between alkali metal cations and the KcsA channel studied using ATR-FTIR spectroscopy

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Author(s)

    • Furutani Yuji
    • Department of Life and Coordination-Complex Molecular Science, Institute for Molecular Science|Department of Frontier Materials, Nagoya Institute of Technology
    • Shimizu Hirofumi
    • Department of Molecular Physiology and Biophysics, Faculty of Medical Sciences, University of Fukui
    • Asai Yusuke
    • Department of Frontier Materials, Nagoya Institute of Technology
    • Oiki Shigetoshi
    • Department of Molecular Physiology and Biophysics, Faculty of Medical Sciences, University of Fukui
    • Kandori Hideki
    • Department of Frontier Materials, Nagoya Institute of Technology

Abstract

The X-ray structure of KcsA, a eubacterial potassium channel, displays a selectivity filter composed of four parallel peptide strands. The backbone carbonyl oxygen atoms of these strands solvate multiple K<sup>+</sup> ions. KcsA structures show different distributions of ions within the selectivity filter in solutions containing different cations. To assess the interactions of cations with the selectivity filter, we used attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy. Ion-exchange-induced ATR-FTIR difference spectra were obtained by subtracting the spectrum of KcsA soaked in K<sup>+</sup> solution from that obtained in Li<sup>+</sup>, Na<sup>+</sup>, Rb<sup>+</sup>, and Cs<sup>+</sup> solutions. Large spectral changes in the amide-I and -II regions were observed upon replacing K<sup>+</sup> with smaller-sized cations Li<sup>+</sup> and Na<sup>+</sup> but not with larger-sized cations Rb<sup>+</sup> and Cs<sup>+</sup>. These results strongly suggest that the selectivity filter carbonyls coordinating Rb<sup>+</sup> or Cs<sup>+</sup> adopt a conformation similar to those coordinating K<sup>+</sup> (cage configuration), but those coordinating Li<sup>+</sup> or Na<sup>+</sup> adopt a conformation (plane configuration) considerably different from those coordinating K<sup>+</sup>. We have identified a cation-type sensitive amide-I band at 1681 cm<sup>−1</sup> and an insensitive amide-I band at 1659 cm<sup>–1</sup>. The bands at 1650, 1639, and 1627 cm<sup>–1</sup> observed for Na<sup>+</sup>-coordinating carbonyls were almost identical to those observed in Li<sup>+</sup> solution, suggesting that KcsA forms a similar filter structure in Li<sup>+</sup> and Na<sup>+</sup> solutions. Thus, we conclude that the filter structure adopts a collapsed conformation in Li<sup>+</sup> solution that is similar to that in Na<sup>+</sup> solution but is in clear contrast to the X-ray crystal structure of KcsA with Li<sup>+</sup>.

Journal

  • Biophysics and Physicobiology

    Biophysics and Physicobiology 12(0), 37-45, 2015

    The Biophysical Society of Japan

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