Correlated movement of K⁺ ions and polyamine in the MthK pore. (A) Simulation trajectory showing the positions of the N_L of SPD³⁺ (blue) and K⁺ ions in the SF (shades of purple), relative to oxygen atoms of SF residues (red, as indicated to the right) with 0 mV transmembrane voltage. (B) Simulation trajectory for SPD³⁺ with 500 mV transmembrane voltage. (C) Simulation trajectory for SPM⁴⁺ with 0 mV transmembrane voltage. (D) Simulation trajectory for SPM⁴⁺ with 500 mV transmembrane voltage. These data illustrate that movement of N_L to positions within hydrogen-bonding distance of T59 oxygen atoms depends on outward displacement of K⁺ ions.