Direct knock-on K ion movement in SF. (A) Pseudo–steady-state conformation of the SF with two resident K⁺ ions (black) in S2 and S3, immediately before permeation event initiated by entry of inner cavity K⁺ ion (yellow). For the SF, the backbone of residues IGYG (residues 144 to 147) and the full T143 are shown as sticks, as well as the rectification controller residue D173. For clarity, only two opposing domains are shown. (B) Snapshots of the SF during the ensuing conduction event. Surrounding water represented as sticks. Consecutive simulation snapshots show the complete permeation of the yellow K⁺ ion, followed by additional K⁺ ions (black) from the inner cavity, with no intervening water molecules.