Ion channels

Abstract:

Ion channels are proteins which form gated nanopores in biological membranes. Many channels exhibit hydrophobic gating, whereby functional closure of a pore occurs by local dewetting. The pentameric ligand gated ion channels (pLGICs) provide a biologically important example of hydrophobic gating. Molecular simulation studies comparing additive vs polarizable models indicate predictions of hydrophobic gating are robust to the model employed. However, polarizable models suggest favorable interactions of hydrophobic pore-lining regions with chloride ions, of relevance to both synthetic carriers and channel proteins. Electrowetting of a closed pLGIC hydrophobic gate requires too high a voltage to occur physiologically but may inform designs for switchable nanopores. Global analysis of ∼200 channels yields a simple heuristic for structure-based prediction of (closed) hydrophobic gates. Simulation-based analysis is shown to provide an aid to interpretation of functional states of new channel structures. These studies indicate the importance of understanding the behavior of water and ions within the nanoconfined environment presented by ion channels.

Abstract:

In this study we examined the influence of a transmembrane voltage on the hydrophobic gating of nanopores using molecular dynamics simulations. We observed electric field induced wetting of a hydrophobic gate in a biologically inspired model nanopore based on the 5-HT3 receptor in its closed state, with a field of at least ∼100 mV nm–1 (corresponding to a supra-physiological potential difference of ∼0.85 V across the membrane) required to hydrate the pore. We also found an unequal distribution of charged residues can generate an electric field intrinsic to the nanopore which, depending on its orientation, can alter the effect of the external field, thus making the wetting response asymmetric. This wetting response could be described by a simple model based on water surface tension, the volumetric energy contribution of the electric field, and the influence of charged amino acids lining the pore. Finally, the electric field response was used to determine time constants characterizing the phase transitions of water confined within the nanopore, revealing liquid–vapor oscillations on a time scale of ∼5 ns. This time scale was largely independent of the water model employed and was similar for different sized pores representative of the open and closed states of the pore. Furthermore, our finding that the threshold voltage required for hydrating a hydrophobic gate depends on the orientation of the electric field provides an attractive perspective for the design of rectifying artificial nanopores.

Abstract:

TWIK-related acid-sensitive potassium (TASK) channels—members of the two pore domain potassium (K2P) channel family—are found in neurons1, cardiomyocytes2–4 and vascular smooth muscle cells5, where they are involved in the regulation of heart rate6, pulmonary artery tone5,7, sleep/wake cycles8 and responses to volatile anaesthetics8–11. K2P channels regulate the resting membrane potential, providing background K+ currents controlled by numerous physiological stimuli12–15. Unlike other K2P channels, TASK channels are able to bind inhibitors with high affinity, exceptional selectivity and very slow compound washout rates. As such, these channels are attractive drug targets, and TASK-1 inhibitors are currently in clinical trials for obstructive sleep apnoea and atrial fibrillation16. In general, potassium channels have an intramembrane vestibule with a selectivity filter situated above and a gate with four parallel helices located below; however, the K2P channels studied so far all lack a lower gate. Here we present the X-ray crystal structure of TASK-1, and show that it contains a lower gate—which we designate as an ‘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 (243VLRFMT248) that are essential for responses to volatile anaesthetics10, neurotransmitters13 and G-protein-coupled receptors13. Mutations within the X-gate and the surrounding regions markedly affect both the channel-open probability and the activation of the channel by anaesthetics. Structures of TASK-1 bound to two high-affinity inhibitors show 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. The presence of the X-gate in TASK channels explains many aspects of their physiological and pharmacological behaviour, which will be beneficial for the future development and optimization of TASK modulators for the treatment of heart, lung and sleep disorders.