Similarity of the ion occupations of K ⁺ and Tl ⁺ in the selectivity filter of a K ⁺ channel, as verified by single-channel currents

Voltage-dependent gating in the selectivity filter of potassium channels is modulated by the permeating ions binding within the filter. In the viral Kcv channel, filter gating occurs on the sub-millisecond time scale. Crystallographic data from KcsA in the literature imply that ion occupation of the binding sites in the selectivity filter is equal for K⁺ and Tl⁺. Here, we verify this equivalence by electrophysiological experiments in Kcv_NTS and extend the finding to negative voltages up to -160 mV. The analysis is based on our previous work, which correlated the voltage dependence of ion occupation with the voltage dependence of the rate constant of channel closure. This equivalence of K⁺ and Tl⁺ is further supported by experiments in the mutant Kcv_NTS S42T. Additionally, measurements with Tl⁺ and K⁺ at opposite sides of the membrane show that the ion in the selectivity filter determines the rate constant of channel closure. Tl⁺ on the external side increases this rate constant by a constant factor but does not change voltage dependence. A similar influence of the ion species is found for the only weakly voltage-dependent rate constant of channel opening. Here, Tl⁺ on the external side increases the voltage-independent scaling factor whereas Tl⁺ on the cytosolic side decreases it.