A Novel Mechanism of Voltage Sensing and Gating in K2P Potassium Channels

Two-pore domain (K2P) potassium channels are important regulators of cellular electrical excitability integrating a wide range of stimuli including phosphorylation, various lipids, temperature, intracellular/extracellular pH and membrane voltage. Here we investigated the mechanism of voltage sensing/gating in K2P channels. We show that TASK-3 and TRAAK are mostly closed at the resting membrane potential but become strongly activated by depolarisation similar to classical Kv channels. Moreover, strong voltage activation was also seen for other K2P channels such as TREK-1, TREK-2, TRESK and TALK-2, however, only when intracellular K+ was replaced by other permeant ions such as NH4+, Cs+ or Rb+. Strikingly, voltage activation was abolished upon intracellular Na+ or NMDG+ replacement suggesting that movements of permeant ions in the electric field focused in the selectivity filter power voltage gating and, thus, are likely to represent the gating charge in K2P channels. Indeed, mutations in the selectivity filter, experiments with high affinity pore blockers and cysteine modification establish the selectivity filter as the voltage gate. Furthermore, voltage activation in K2P channels exhibits distinct functional features compared to classical Kv channel gating as the V1/2 for voltage activation was strictly coupled to the K2P channel reversal potential and the kinetics of voltage gating were largely voltage-independent. The latter finding suggests a voltage-dependent fast multi ion binding step to a nonconductive but ion accessible selectivity filter and a second slower voltage-independent gating step leading to the conductive state of the selectivity filter. These findings demonstrate a novel concept of voltage sensing in K+ channels and point to a new role of K2P channels in cell excitability as the strong voltage dependence would enable K2P channel to contribute significantly to the repolarisation phase of neural (e.g. TRAAK) or cardiac (e.g. TASK-3) actions potentials.