Potassium channel selectivity filter dynamics revealed by single-molecule FRET.

Abstract

Potassium (K) channels exhibit exquisite selectivity for conduction of K+ ions over other cations, particularly Na+. High resolution structures reveal an archetypal selectivity filter (SF) conformation in which dehydrated K+, but not Na+, ions are perfectly coordinated. Using single molecule FRET (smFRET), we show that the SF-forming loop (SF-loop) in KirBac1.1 transitions between constrained and dilated conformations as a function of ion concentrations. The constrained conformation, essential for selective K+ permeability, is stabilized by K+ but not Na+ ions. Mutations that render channels non-selective result in dilated and dynamically unstable conformations, independent of the permeant ion. Further, while wild type KirBac1.1 channels are K+-selective in physiological conditions, Na+ permeates in the absence of K+. Moreover, while K+ gradients preferentially support 86Rb+ fluxes, Na+ gradients preferentially support 22Na+ fluxes. This suggests differential ion selectivity in constrained versus dilated states, potentially providing a structural basis for this anomalous mole fraction effect.