Crystal structures of an activated two-pore potassium channel reveal a cryptic binding pocket that binds small-molecule activators that restrict the mobility of the selectivity filter and surrounding structure, stabilizing an active ‘leak-mode’ conformation. The two-pore domain potassium channels (K2P) are dimeric channels of the voltage-gated ion channel superfamily and are responsive to temperature, mechanical stimuli and small molecules. Despite their physiological importance in pain and temperature perception as well as mood regulation, their pharmacology (how small molecules modulate their activity) is not well understood. In this work, crystal structures of an activated K2P channel are reported, revealing a cryptic binding pocket behind the selectivity filter of the chanel. The small-molecule activators bind to this pocket and restrict the mobility of the selectivity filter and the surrounding structure. This stabilizes a 'leak-mode' conformation, activating the channel and allowing current to leak through. Polymodal thermo- and mechanosensitive two-pore domain potassium (K2P) channels of the TREK1 subfamily generate ‘leak’ currents that regulate neuronal excitability, respond to lipids, temperature and mechanical stretch, and influence pain, temperature perception and anaesthetic responses1,2,3. These dimeric voltage-gated ion channel (VGIC) superfamily members have a unique topology comprising two pore-forming regions per subunit4,5,6. In contrast to other potassium channels, K2P channels use a selectivity filter ‘C-type’ gate7,8,9,10 as the principal gating site. Despite recent advances3,11,12, poor pharmacological profiles of K2P channels limit mechanistic and biological studies. Here we describe a class of small-molecule TREK activators that directly stimulate the C-type gate by acting as molecular wedges that restrict interdomain interface movement behind the selectivity filter. Structures of K2P2.1 (also known as TREK-1) alone and with two selective K2P2.1 (TREK-1) and K2P10.1 (TREK-2) activators—an N-aryl-sulfonamide, ML335, and a thiophene-carboxamide, ML402—define a cryptic binding pocket unlike other ion channel small-molecule binding sites and, together with functional studies, identify a cation–π interaction that controls selectivity. Together, our data reveal a druggable K2P site that stabilizes the C-type gate ‘leak mode’ and provide direct evidence for K2P selectivity filter gating.

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are dually activated by hyperpolarization and binding of cAMP to their cyclic nucleotide binding domain (CNBD). HCN isoforms respond differently to cAMP; binding of cAMP shifts activation of HCN2 and HCN4 by 17 mV but shifts that of HCN1 by only 2–4 mV. To explain the peculiarity of HCN1, we solved the crystal structures and performed a biochemical-biophysical characterization of the C-terminal domain (C-linker plus CNBD) of the three isoforms. Our main finding is that tetramerization of the C-terminal domain of HCN1 occurs at basal cAMP concentrations, whereas those of HCN2 and HCN4 require cAMP saturating levels. Therefore, HCN1 responds less markedly than HCN2 and HCN4 to cAMP increase because its CNBD is already partly tetrameric. This is confirmed by voltage clamp experiments showing that the right-shifted position of V_½ in HCN1 is correlated with its propensity to tetramerize in vitro. These data underscore that ligand-induced CNBD tetramerization removes tonic inhibition from the pore of HCN channels.

The innately heterodimeric K _2P filter permits two C-type gating mechanisms, pinching and dilation, to operate in one channel.