Separate Gating Mechanisms Mediate the Regulation of K 2P Potassium Channel TASK-2 by Intra- and Extracellular pH *

TASK-2 (KCNK5 or K 2P 5.1) is a background K + channel that is opened by extracellular alkalinization and plays a role in renal bicarbonate reabsorption and central chemoreception. Here, we demonstrate that in addition to its regulation by extracellular protons (pH o ) TASK-2 is gated open by intracellular alkalinization. The following pieces of evidence suggest that the gating process controlled by intracellular pH (pH i ) is independent from that under the command of pH o. It was not possible to overcome closure by extracellular acidification by means of intracellular alkalinization. The mutant TASK-2-R224A that lacks sensitivity to pH o had normal pH i -dependent gating. Increasing extracellular K + concentration acid shifts pH o activity curve of TASK-2 yet did not affect pH i gating of TASK-2. pH o modulation of TASK-2 is voltage-dependent, whereas pH i gating was not altered by membrane potential. These results suggest that pH o, which controls a selectivity filter external gate, and pH i act at different gating processes to open and close TASK-2 channels. We speculate that pH i regulates an inner gate. We demonstrate that neutralization of a lysine residue (Lys 245 ) located at the C-terminal end of transmembrane domain 4 by mutation to alanine abolishes gating by pH i. We postulate that this lysine acts as an intracellular pH sensor as its mutation to histidine acid-shifts the pH i -dependence curve of TASK-2 as expected from its lower p Ka. We conclude that intracellular pH, together with pH o, is a critical determinant of TASK-2 activity and therefore of its physiological function.