Figure 2: X-ray crystal structure of the NaKTs ion channel determined at 3.15 Å resolution.

(a) Ribbon representation of top view of the NaKTs tetramer (chain ABDE—dataset 2), along with bound Ca²⁺ ions. The M0 helix (residues 5–19) is shown in blue, the M1 outer helix (residues 25–45) is shown in magenta, the pore helix (residues 46–61) is shown in green, the selectivity filter (residues 62–69) is shown in yellow, the M2 inner helix is shown in cyan and the loop regions are shown in green. Calcium ions are shown as purple spheres, with the 2F_o−F_c map (grey) at 1σ contour and F_o−F_c map (green) at 4.5σ contour. (b) The TM region of two opposing monomers of the NaKTs ion channel protein (chain A, chain D) shown with the 2F_o−F_c density map at 1σ contour for the selectivity filter region (yellow sticks). (c) F_o−F_c map (green) at 4.5σ contour and 2F_o−F_c map (grey) at 1σ contour showing the location of Ca²⁺ ion (purple sphere) at site S_Ca. The pore helix (green cartoon), the selectivity filter residues (yellow sticks) and Ca²⁺ coordinating AA’s are labelled accordingly. (d) Close-up view of two opposing monomers of the NaKTs selectivity filter (chain A, chain D) shown in yellow sticks with bound Ca²⁺ ions at sites S2 and S_Ca.