Two Distinct Molecular Mechanisms Underlie pH Sensitivity of the Human Potassium Leak Channel K2P2.1

Abstract

The mammalian K2P2.1 potassium channel (TREK-1, KCNK2) is highly expressed in excitable tissues, where it plays a key role in the cellular mechanisms of neuroprotection, anaesthesia, pain perception and depression. Here, we report that external acidification, within the physiological range, strongly inhibits the human K2P2.1 channel in two distinct time scales. We have identified two histidine residues (i.e., H87 and H141), located in the first external loop of the channel, which govern the fast response of the channel to external pH (in the time scale of seconds). We demonstrate that these residues are within physical proximity to glutamate 84, homologous to Shaker E418, KcsA E51 and KCNK0 E28 residues, all previously reported to stabilize the outer pore gate in the open conformation by forming hydrogen bonds with pore-adjacent residues. We thus propose a novel mechanism for pH sensing in which protonation of H141 and H87 generates a local positive charge that serves to draw E84 away from its natural interactions, facilitating the collapse of the selectivity filter region, a mechanism which resembles C-type gating of voltage dependent potassium channels. In accordance with this proposed mechanism, the proton-mediated effect was inhibited by external potassium ions, modified the channel's ion selectivity and was enhanced by a mutation, S164Y, known to accelerate C-type gating. In addition, we show that the slow regulatory effect (in the time scale of minutes) is mediated by proton-sensitive G-protein coupled receptors (GPCRs), which activated phospholipase C via the Gq pathway. We demonstrate that three residues within the C-terminal of K2P2.1 mediate the channel's response to the observed GPCRs activation by acidic pH.Taken together, our results highlight the physiological importance of human K2P2.1 channels as sensors of extracellular pH in the central nervous system.