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a, The ion conduction pore comprised of IS5–6 (left, green) and IIS5–6 (right, red). Ba2+ ions are shown as blue spheres. b, Structures of the selectivity filter formed by Filter I (left) and Filter II (right). c, Side view of the bundle crossing formed by IS6 pair (left) and IIS6 pair (right). Numbers are diagonal distances (in Å) of the constriction points.
