1.A.1.9.2
KCNK3 K⁺ channel (TASK1, OAT1, TBAK1) (the K⁺ leak conductance). TASK1 and 3 may play roles in nontumorigenic primary hyperaldosteronism (Davies et al., 2008).  KCNK3/9/15 expression limits membrane depolarization and depolarization-induced secretion at least in part by maintaining intracellular K⁺ (Huang et al. 2011). TWIK-related acid-sensitive potassium (TASK) channels, members of the two pore domain potassium (K2P) channel family, are found in neurons, cardiomyocytes and vascular smooth muscle cells, where they are involved in the regulation of heart rate, pulmonary artery tone, sleep/wake cycles and responses to volatile anaesthetics (Rödström et al. 2020). K2P channels regulate the resting membrane potential, providing background K⁺ currents controlled by numerous physiological stimuli. Unlike other K2P channels, TASK channels are able to bind inhibitors with high affinity, exceptional selectivity and very slow compound washout rates. In general, potassium channels have an intramembrane vestibule with a selectivity filter situated above and a gate with four parallel helices located below, but the K2P channels studied so far all lack a lower gate. Rödström et al. 2020 presented the X-ray crystal structure of TASK-1, and showed that it contains a lower gate designated 'X-gate', created by interaction of the two crossed C-terminal M4 transmembrane helices at the vestibule entrance. This structure is formed by six residues ((243)VLRFMT(248)) that are essential for responses to volatile anaesthetics, neurotransmitters and G-protein-coupled receptors. Mutations within the X-gate and the surrounding regions affect both the channel-open probability and the activation of the channel by anaesthetics. Structures of TASK-1 bound to two high-affinity inhibitors showed that both compounds bind below the selectivity filter and are trapped in the vestibule by the X-gate, which explains their exceptionally low washout rates (Rödström et al. 2020).