Na⁺ Block and Permeation in a K⁺ Channel of Known Structure

The effects of intracellular Na⁺ were studied on K⁺ and Rb⁺ currents through single KcsA channels. At low voltage, Na⁺ produces voltage-dependent block, which becomes relieved at high voltage by a “punchthrough” mechanism representing Na⁺ escaping from its blocking site through the selectivity filter. The Na⁺ blocking site is located in the wide, hydrated vestibule, and it displays unexpected selectivity for K⁺ and Rb⁺ against Na⁺. The voltage dependence of Na⁺ block reflects coordinated movements of the blocker with permeant ions in the selectivity filter.