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(+), 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 (86)Rb(+) fluxes, Na(+) gradients preferentially support (22)Na(+) fluxes. This suggests differential ion selectivity in constrained versus dilated states, potentially providing a structural basis for this anomalous mole fraction effect.