Fig. 6: Proposed model for transition from conducting to non-conducting selectivity filter.

Cryo-EM maps of the selectivity filter obtained from the high-K structure (panels A–C) and low-K structure (panels D, E) with superimposed protein structures obtained from snapshots of the MDFF fits to the high-K (blue) or low-K (orange) cryo-EM maps. Stylized free energy diagrams for the transition from the conductive (C) to non-conductive (NC) state, via a transition state (T) are shown below each panel. Interactions stabilizing each state are highlighted as dashed lines in each panel (A, B: S620 – F627/G626, C, D: S620 – Y616; E: S620-V625). The transition from conducting to non-conducting states is initiated by K⁺ ions leaving the upper filter. B The energy barrier for movement of ions between S3 and S2 (faint pink line, panels A–C) is increased when all four V625 backbone carbonyls are flipped (red line in panels D, E). The gradient for [K⁺]_o indicates that the conductive state is favored at high [K⁺]_o and the non-conducting state is favored at low [K⁺]_o. A movie depicting the transition from the conductive to non-conductive states is shown in Supplementary Movie 1.