The ligand-sensitive gate of a potassium channel lies close to the selectivity filter

Potassium channels selectively conduct K⁺ ions across cell membranes and have key roles in cell excitability. Their opening and closing can be spontaneous or controlled by membrane voltage or ligand binding. We used Ba²⁺ as a probe to determine the location of the ligand-sensitive gate in an inwardly rectifying K⁺ channel (Kir6.2). To a K⁺ channel, Ba²⁺ and K⁺ are of similar sizes, but Ba²⁺ blocks the pore by binding within the selectivity filter. We found that internal Ba²⁺ could still access its binding site when the channel was shut, which indicates that the ligand-sensitive gate lies above the Ba²⁺-block site, and thus within or above the selectivity filter. This is in marked contrast to the voltage-dependent gate of K_V channels, which is located at the intracellular mouth of the pore.