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 punch-through 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.