Ion–Ion Interactions at the Selectivity Filter: Evidence from K⁺-Dependent Modulation of Tetraethylammonium Efficacy in Kv2.1 Potassium Channels

In the Kv2.1 potassium channel, binding of K⁺ to a high-affinity site associated with the selectivity filter modulates channel sensitivity to external TEA. In channels carrying Na⁺ current, K⁺ interacts with the TEA modulation site at concentrations ≤30 μM. In this paper, we further characterized the TEA modulation site and examined how varying K⁺ occupancy of the pore influenced the interaction of K⁺ with this site. In the presence of high internal and external [K⁺], TEA blocked 100% of current with an IC₅₀ of 1.9 ± 0.2 mM. In the absence of a substitute permeating ion, such as Na⁺, reducing access of K⁺ to the pore resulted in a reduction of TEA efficacy, but produced little or no change in TEA potency (under conditions in which maximal block by TEA was just 32%, the IC₅₀ for block was 2.0 ± 0.6 mM). The all-or-none nature of TEA block (channels were either completely sensitive or completely insensitive), indicated that one selectivity filter binding site must be occupied for TEA sensitivity, and that one selectivity filter binding site is not involved in modulating TEA sensitivity. At three different levels of K⁺ occupancy, achieved by manipulating access of internal K⁺ to the pore, elevation of external [K⁺] shifted channels from a TEA-insensitive to -sensitive state with an EC₅₀ of ∼10 mM. Combined with previous results, these data demonstrate that the TEA modulation site has a high affinity for K⁺ when only one K⁺ is in the pore and a low affinity for K⁺ when the pore is already occupied by K⁺. These results indicate that ion–ion interactions occur at the selectivity filter. These results also suggest that the selectivity filter is the site of at least one low affinity modulatory effect of external K⁺, and that the selectivity filter K⁺ binding sites are not functionally interchangeable.