Figure 5: Outer and inner diameters of the selectivity filter pore in the NaKTs channel.

(a) The outer diameter D_outer taken as the Cα–Cα distance between the diagonally placed Pro68 is shown. (b) The inner diameter D_inner taken as the Cα–Cα distance between the diagonally placed Pro63 is shown. Supplementary Fig. 4 shows the relative positions of the proline residues. The diameters from the NaKTs channel crystal structures are presented by black symbols. They are grouped by the number of Ca²⁺ at the selectivity filter S_Ca site. States I (○ and ▵) and II (□) have no Ca²⁺ at the S_Ca site. Two structures are used for State I. The difference between the two lies in that the second structure (▵) has a Ca²⁺ in the centre of the intracellular vestibule. State III (⋄ and ♦) has two Ca²⁺ at the diagonally place S_Ca sites; State IV ( has all four S_Ca sites occupied. The averaged D_outer and D_inner in the MD simulations (−500 mV) are plotted with red symbols. The error bars indicate the standard deviations of the distances calculated from the trajectories. The distances are calculated from two pairs of diagonally placed subunits. In all cases, filled symbols indicate that the distance is taken from subunits with the S_Ca sites occupied by Ca²⁺. The D_outer and D_inner in the NaK channel (2AHY, solid line) and KcsA (1K4C, dotted line) are shown as references.