Interaction between tetraethylammonium and permeant cations at the inactivation gate of the HERG potassium channel

The fast inactivation of the human ether-à-go-go related gene product (HERG) channel is a form of C-type inactivation and is decelerated by external tetraethylammonium (TEA) and potassium. From the time constant of inactivation, the dissociation constants of TEA (K(TEA)) and potassium (K(K)) to the inactivation-impeding site were evaluated. K(TEA) was found to exhibit unexpected voltage dependence: K(TEA) decreased with depolarization. This was opposite the voltage dependence of K(K) on inactivation, in which permeating potassium impeded closure of the inactivation gate upon binding to a site in the pore (a "foot-in-the-door" mechanism). Further experiments on inactivation revealed anomalous mole fraction effects between permeating alkali cations and TEA, while no anomalous effects were seen between permeating ion species (K+, Rb+, Cs+). The results indicate that TEA and permeating ions impede inactivation through binding to different but closely interacting sites. K(TEA) was influenced by permeating ions through their bindings in the pore. As the size of the occupied ion was increased the dissociation constant of TEA to the ion-occupied pore decreased. Thus, we conclude that an ion bound to the inactivation-impeding site in the selectivity filter is located in close proximity to TEA bound at the entrance of the filter. The order of affinity of alkali cations for the inactivation-impeding site, Rb+ > Cs+ > K+, indicated that the selectivity of the site differed significantly from permeation selectivity, K+ > Rb+ > Cs+.