External TEA Block of Shaker K⁺ Channels Is Coupled to the Movement of K⁺ Ions within the Selectivity Filter

Recent molecular dynamic simulations and electrostatic calculations suggested that the external TEA binding site in K⁺ channels is outside the membrane electric field. However, it has been known for some time that external TEA block of Shaker K⁺ channels is voltage dependent. To reconcile these two results, we reexamined the voltage dependence of block of Shaker K⁺ channels by external TEA. We found that the voltage dependence of TEA block all but disappeared in solutions in which K⁺ ions were replaced by Rb⁺. These and other results with various concentrations of internal K⁺ and Rb⁺ ions suggest that the external TEA binding site is not within the membrane electric field and that the voltage dependence of TEA block in K⁺ solutions arises through a coupling with the movement of K⁺ ions through part of the membrane electric field. Our results suggest that external TEA block is coupled to two opposing voltage-dependent movements of K⁺ ions in the pore: (a) an inward shift of the average position of ions in the selectivity filter equivalent to a single ion moving ∼37% into the pore from the external surface; and (b) a movement of internal K⁺ ions into a vestibule binding site located ∼13% into the membrane electric field measured from the internal surface. The minimal voltage dependence of external TEA block in Rb⁺ solutions results from a minimal occupancy of the vestibule site by Rb⁺ ions and because the energy profile of the selectivity filter favors a more inward distribution of Rb⁺ occupancy.