Instantaneous ion configurations in the K + ion channel selectivity filter revealed by 2D IR spectroscopy

• DOI: 10.1126/science.aag1447
• Publication Date: 2016-09-01T18:54:29Z
• Authors: Huong T. Kratochvil; Joshua K. Carr; Kimberly Matulef; Alvin W. Annen; Hui Li; Michał Maj; Jared Ostmeyer; Arnaldo L. Serrano; H. Raghuraman; Sean D. Moran; J. L. Skinner; Eduardo Perozo; Benoît Roux; Francis I. Valiya...; Martin T. Zanni
• Keywords: 0301 basic medicine; Carbon Isotopes; Binding Sites; Potassium Channels; Spectrophotometry, Infrared; Protein Conformation; Sodium; Water; Molecular Dynamics Simulation; Oxygen Isotopes; Crystallography, X-Ray; 03 medical and health sciences; Bacterial Proteins; Models, Chemical; Isotope Labeling

Potassium channels are responsible for the selective permeation of K + ions across cell membranes. K + ions permeate in single file through the selectivity filter, a narrow pore lined by backbone carbonyls that compose four K + binding sites. Here, we report on the two-dimensional infrared (2D IR) spectra of a semisynthetic KcsA channel with site-specific heavy ( 13 C 18 O) isotope labels in the selectivity filter. The ultrafast time resolution of 2D IR spectroscopy provides an instantaneous snapshot of the multi-ion configurations and structural distributions that occur spontaneously in the filter. Two elongated features are resolved, revealing the statistical weighting of two structural conformations. The spectra are reproduced by molecular dynamics simulations of structures with water separating two K + ions in the binding sites, ruling out configurations with ions occupying adjacent sites.