Fig. 9.

Working hypothesis of initial steps in MthK inactivation. Schematic diagram of the MthK selectivity filter in a series of different ion occupancy states. From the state (far left) with K⁺ ions occupying the S0/S2/S4 positions (purple spheres), one Ca²⁺ (cyan sphere) occupies the S_cav position. Depolarization drives the K⁺ ions outward in register as Ca²⁺ moves to the S5 position. Lowered extracellular [K⁺] drives the filter from the three-ion states to the two-ion states by lowering the probability of K⁺ rebinding after its dissociation from S0. In the final step (far right), carbonyl rotation occurs (indicated by the green arrow), leading to inactivation.