Ion-Protein Interactions between a Potassium Channel and Alkali Metal Cations Studied by ATR-FTIR Spectroscopy

Abstract

Potassium channels are essential for selective permeation of potassium ions in cell membrane. The selectivity is achieved by the specific part called “selectivity filter”, which is composed of several carbonyl groups centripetally toward the center of the pore. X-ray crystallography clearly showed that each potassium ion is favorably coordinated by eight carbonyl oxygens in the selectivity filters of various kinds of potassium channels. Further analysis was performed on a well-known potassium channel, KcsA, where the coordination manners of alkali metal cations in the selectivity filter are somewhat different from each other. In particular, sodium ion is coordinated by four carbonyl groups and the structure of the filter is deformed in the center of the filter. To understand the ion-protein interactions of potassium channels in more detail, the structure around the selectivity filter should be analyzed by other physicochemical methods, such as infrared difference spectroscopy.The infrared difference spectroscopy on KcsA with ion-exchange reaction was firstly reported in 2012 [1]. The significant spectral change was observed in the amide I mode of the peptide carbonyl groups, which were assigned to the selectivity filter and nearby pore helices by mutation [1] and a computational method [2]. The interactions with alkali metal cations were further investigated [3], revealing that the selectivity filter carbonyls coordinating Rb+ or Cs+ adopt a conformation similar to those coordinating K+, while those coordinating Li+ or Na+ considerably differ from those coordinating K+. Similar approach applied on a mammalian potassium channel is now in progress.[1] Y. Furutani et al. J. Phys. Chem. Lett. 3, 3806-3810, 2012[2] P. Stevenson et al. J. Phys. Chem. B 119, 5824-5831, 2015[3] Y. Furutani et al. Biophysics and Physicobiology 12, 37-45, 2015