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 assoc iated with the selectivity filter modulates channel sensitivity to external TEA. In channels carrying Na+ current, K+ interacts with the TEA modulatio n site at concentrations less than or equal to 30 mu M. In this paper, we f urther characterized the TEA modulation site and examined how varying K+ oc cupancy 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 w ith an IC50 of 1.9 +/- 0.2 mM. In the absence of a substitute permeating io n, such as Na+, reducing access of K+ to the pore resulted in a reduction o f TEA efficacy, but produced little or no change in TEE potency (under cond itions 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 c ompletely sensitive or completely insensitive), indicated that one selectiv ity filter binding site must be occupied for TEA sensitivity, and that one selectivity filter binding site is not involved in modulating TEA sensitivi ty. At three different levels of K+ occupancy, achieved by manipulating acc ess of internal K+ to the pore, elevation of external [K+] shifted channels from a TEA-insensitive to -sensitive state with an EC50 of similar to 10 m M. Combined with previous results, these data demonstrate that the TEA modu lation 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 result s indicate that ion-ion interactions occur at the selectivity filter. These results also suggest that the selectivity filter is the site of at least o ne low affinity modulatory effect of external K+, and that the selectivity filter K+ binding sites are not functionally interchangeable.