A unique lower X-gate in TASK channels traps inhibitors within the vestibule

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Abstract

TASK channels are unusual members of the two-pore domain potassium (K 2P ) channel family, with unique and unexplained physiological and pharmacological characteristics. TASKs are found in neurons 1,2 , cardiomyocytes 3–5 and vascular smooth muscle cells 6 where they are involved in regulation of heart rate 7 , pulmonary artery tone 6,8 , sleep/wake cycles 9 and responses to volatile anaesthetics 9–12 . K 2P channels regulate the resting membrane potential, providing background K + currents controlled by numerous physiological stimuli 13,14 . Unlike other K 2P channels, TASK channels have the capacity to bind inhibitors with high affinity, exceptional selectivity and very slow compound washout rates. These characteristics make the TASK channels some of the the most easily druggable potassium channels, and indeed TASK-1 inhibitors are currently in clinical trials for obstructive sleep apnea (OSA) and atrial fibrillation (Afib) 15 (The DOCTOS and SANDMAN Trials). Generally, potassium channels have an intramembrane vestibule with a selectivity filter above and a gate with four parallel helices below. However, K 2P channels studied to date all lack a lower gate. Here we present the structure of TASK-1, revealing a unique lower gate created by interaction of the two crossed C-terminal M4 transmembrane helices at the vestibule entrance, which we designate as an ‟X-gate”. This structure is formed by six residues (V 243 LRFMT 248 ) that are essential for responses to volatile anaesthetics 11 , neuro-transmitters 16 and G-protein coupled receptors 16 . Interestingly, mutations within the X-gate and surrounding regions drastically affect both open probability and activation by anaesthetics. Structures of TASK-1 with two novel, high-affinity blockers, shows both inhibitors bound below the selectivity filter, trapped in the vestibule by the X-gate, thus explaining their exceptionally low wash-out rates. Thus, the presence of the X-gate in TASK channels explains many aspects of their unusual physiological and pharmacological behaviour, which is invaluable for future development and optimization of TASK modulators for treatment of heart, lung and sleep disorders.

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