Hot topics: A cryptic binding pocket in K2P2 exposes new avenues for drug development.

The TREK subfamily of K2P channels (K2P2, K2P4 and K2P10) pass background potassium currents that modulate the excitability of neuronal cells and cardiac myocytes. In recent years, these channels have received significant attention as potential drug targets. This is in part because of their proposed roles in the regulation of nociception, analgesia, anesthesia and depression and, also because they act as polymodal signal integrators for physiological influences as diverse as temperature, membrane tension, phosphorylation, and phospholipids [1-3]. However, despite accelerating progress, including atomic resolution structures of two TREK subfamily members [4,5] , and K2P1 [6], a deeper appreciation of how K2P channel structure-function relationships, including gating mechanics, relate to physiology and disease remains hampered by a paucity of specific blockers and activators. In an elegant new study, Lolicato and colleagues from the Minor lab highlight the synergistic power of combining structural and functional approaches to reveal new insights into the operation of membrane proteins and unveil a new avenue for the development of TREK-channel pharmacology [7].
Lolicato et al., describe crystal structures of mouse K2P2 (TREK1) in complex with a novel K2P2-specific activator (ML335), and a K2P10 activator (ML402) [7]. ML335 and ML402 occupy a previously unidentified binding site—the K2P modulator pocket. Like all K2P channels, K2Ps 2, 4 and 10 are composed of two subunits, each with two pore domains and can assemble as homomers or heterodimers. Given the bilateral nature of the K2P structure, each channel will have two modulator pockets. The modulator pocket is located between the P1 and M4 helical domains in each subunit where residues conserved among TREK subfamily channels interact with the ML335 and ML401 via cation-π and π-π interactions.

While the activity of most ion channels is controlled by multiple gates, experimental evidence has accumulated to support the idea that K2P channels use a single C-type gate at the outer pore which controls ionic flux by the mechanics of the selectivity filter for potassium ions [8-10]. Numerous studies suggest that the unique architecture of K2P channels routes diverse regulatory signals to the C-type gate to control channel activity [9,11-13]. Thus, operation of the C-type gate is directly sensitive to changes in the permeant ion [11,13] and indirectly influenced by various K2P channel regulators that interactions with domains that in-turn impact the C-type gate [12-16]. The modulator pocket described lies behind the selectivity filter. Functional studies show that ML335 holds the pocket in an open conformation and thereby, activates the channel by stabilizing the C-type gate of K2P2 [7]. Because modulator pocket activators appear to be sufficient to open K2P2 channels, the findings suggest that this previously unappreciated, druggable site can be leveraged for the development of novel channel gating-modulators with potential utility as analgesics, anesthetics or neuroprotective agents.

Comments by Leigh D. Plant, Ph. D. (Research Associate Professor, School of Pharmacy, Northeastern University)

References
[1] Goldstein, S. A. et al. (2001). Potassium leak channels and the KCNK family of two-P-domain subunits. Nat Rev Neurosci 2, 175-184. [PMID: 11256078</a].

[2] Enyedi, P. & Czirjak, G. (2010). Molecular background of leak K+ currents: two-pore domain potassium channels. Physiol Rev. 90, 559-605. [PMID: 20393194].

[3] Honore, E. (2007). The neuronal background K2P channels: focus on TREK1. Nat Rev Neurosci. 8, 251-261. [PMID: 17375039].

[4] Brohawn, S. G., del Marmol, J. & MacKinnon, R. (2012). Crystal structure of the human K2P TRAAK, a lipid- and mechano-sensitive K+ ion channel. Science, 335, 436-441. [PMID: 22282805].

[5] Dong, Y. Y. et al. K2P channel gating mechanisms revealed by structures of TREK-2 and a complex with Prozac. Science, 347, 1256-1259. [PMID: 25766236].

[6] Miller, A. N. & Long, S. B. (2012) Crystal structure of the human two-pore domain potassium channel K2P1. Science, 335, 432-436. [PMID: 22282804].

[7] Lolicato, M. et al. (2017). K2P2.1 (TREK-1)-activator complexes reveal a cryptic selectivity filter binding site. Nature, 547, 364-368. [PMID: 28693035].

[8] Zilberberg, N., Ilan, N. & Goldstein, S. A. (2001). KCNKØ: opening and closing the 2-P-domain potassium leak channel entails “C-type” gating of the outer pore. Neuron, 32, 635-648. [PMID: 11719204].

[9] Piechotta, P. L. et al. (2011). The pore structure and gating mechanism of K2P channels. EMBO J, 30, 3607-3619. [PMC: PMC3181484].

[10] Schewe, M. et al. (2016). A Non-canonical Voltage-Sensing Mechanism Controls Gating in K2P K(+) Channels. Cell 164, 937-949. [PMID: 26919430].

[11] Cohen, A., Ben-Abu, Y., Hen, S. & Zilberberg, N. (2008). A novel mechanism for human K2P2.1 channel gating. Facilitation of C-type gating by protonation of extracellular histidine residues. J Biol Chem, 283, 19448-19455. [PMID: 18474599].

[12] Bagriantsev, S. N., Clark, K. A. & Minor, D. L., Jr. (2012). Metabolic and thermal stimuli control K(2P)2.1 (TREK-1) through modular sensory and gating domains. EMBO J, 31, 3297-3308. [PMC: PMC3411076].

[13] Bagriantsev, S. N. et al. (2011). Multiple modalities converge on a common gate to control K2P channel function. EMBO J, 30, 3594-3606. [PMID: 21765396].

[14] Chemin, J. et al. (2005). A phospholipid sensor controls mechanogating of the K+ channel TREK-1. EMBO J, 24, 44-53. [PMID: 15577940].

[15] Murbartian, J., Lei, Q., Sando, J. J. & Bayliss, D. A. (2005). Sequential phosphorylation mediates receptor- and kinase-induced inhibition of TREK-1 background potassium channels. J Biol Chem, 280, 30175-30184. [PMID: 16006563].

[16] Honore, E., Maingret, F., Lazdunski, M. & Patel, A. J. (2002). An intracellular proton sensor commands lipid- and mechano-gating of the K(+) channel TREK-1. EMBO J, 21, 2968-2976. [PMID: 12065410].

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