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

The fast inactivation of the human ether-a-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+.