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 Ba 2+ as a probe to determine the location of the ligand‐sensitive gate in an inwardly rectifying K + channel (Kir6.2). To a K + channel, Ba 2+ and K + are of similar sizes, but Ba 2+ blocks the pore by binding within the selectivity filter. We found that internal Ba 2+ could still access its binding site when the channel was shut, which indicates that the ligand‐sensitive gate lies above the Ba 2+ ‐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.