Potassium channel selectivity filter dynamics revealed by single-molecule FRET

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⁺ ions, 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 concentration. The constrained conformation, essential for selective K⁺ permeability, is stabilized by K⁺ but not Na⁺ ions. Mutations that render channels nonselective 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, whereas 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.