Transmembrane Helix Straightening and Buckling Underlies Activation of Mechanosensitive and Thermosensitive K2P Channels

Abstract

SUMMARY Mechanical and thermal activation of ion channels is central to touch, thermosensation, and pain. The TRAAK/TREK K2P potassium channel subfamily produces background currents that alter neuronal excitability in response to pressure, temperature, signaling lipids, and anesthetics. How such diverse stimuli control channel function is unclear. Here we report structures of K2P4.1 (TRAAK) bearing C-type gate-activating mutations that reveal a tilting and straightening of the M4 inner transmembrane helix and a buckling of the M2 transmembrane helix. These conformational changes move M4 in a direction opposite to that in classical potassium channel activation mechanisms and open a passage lateral to the pore that faces the lipid bilayer inner leaflet. Together, our findings uncover a unique aspect of K2P modulation, indicate a means for how the K2P C-terminal cytoplasmic domain affects the C-type gate which lies ~40Å away, and suggest how lipids and bilayer inner leaflet deformations may gate the channel.

Breathing Stimulant Compounds Inhibit TASK-3 Potassium Channel Function Likely by Binding at a Common Site in the Channel Pore

“…TASK potassium channel conductance is inhibited by extracellular acidic pH and is a homo-or heterodimeric channel composed of two TASK-1 protein subunits, two TASK-3 subunits, or one TASK-1 and one TASK-3 subunit. Crystal structures of three homologous members of the human family, TWIK-1, TRAAK, and TREK-2, were recently solved (Brohawn et al, 2012;Miller and Long, 2012;Lolicato et al, 2014;Dong et al, 2015). These structures have markedly advanced understanding of tandem pore structure and function and support homology model development to guide TASK studies (Fig.…”

Gating, Regulation, and Structure in K2P K+ Channels: In Varietate Concordia?

“…Two other papers have addressed the structural basis of gating in TREK-2 (Dong et al, 2015) and TRAAK (Lolicato et al, 2014). In both studies, the up and down conformations of TM4 determined whether the structure possessed closed or open side fenestrations, respectively, and TREK-2 structural information is consistent with a model in which the down conformation corresponds to the closed state (Dong et al, 2015).…”

“…No evidence for differences in the SF or the pore helices between these putative activation states has been found, however. In the TRAAK study (Lolicato et al, 2014), gain-offunction mutant channel structures that had previously been functionally characterized (Bagriantsev et al, 2012) were used to probe the C terminus-TM4-SF coupling hypothesis of gating. In contrast to what has been described here, the structures revealed a situation in which the down conformation was conductive, and the up conformation corresponded to the closed channel; this discordant finding of Lolicato et al (2014) remains unexplained.…”

Cryo-EM structure of the Mechanotransduction Channel NOMPC

“…How force gates mechanotransduction channels is a central question in the field, for which there are two major models. One is the membrane-tension model: force applied to the membrane generates a change in membrane tension that is sufficient to gate the channel, as in the case of bacterial MscL channel and certain eukaryotic potassium channels [2][3][4][5] . The other is the tether model: force is transmitted via a tether to gate the channel.…”

“…It appears that a motion of the AR domain greater than what we observed in the three classes would be required to drive channel opening, though we cannot exclude the possibility that some of these closed channel conformations correspond to a desensitized state or a captured intermediate state during gating transitions (Extended Discussion in Methods). While it is unlikely that membrane deformation alone is the driving force in NOMPC gating 6 , as in the case of other mechanosensitive channels such as Piezo and TRAAK channels [3][4][5]30 , the proximity of a lipid molecule interacting with the functionally important His1423 at the S4-S5 linker suggests that lipid protein interactions may play an important role in channel activity. It will be of interest to explore the conformational changes that underlie mechanogating in future studies.…”