Supplementary Box 1 | Detailed structure of K+ channels

The voltage-gated (Kv) and calcium-activated (KCa) channels constitute the six/seven transmembrane-one Pregion class of K+ channels. While KCa channels are largely limited to neural, smooth muscle, and endothelial cells,1 members of the Shaker-related (Kv1), Shal-related (Kv4), KCNQ (Kv7), and human ether-a-go-go-related (hERG or Kv11) Kv channel subfamilies conduct the IKur (Kv1.5), Ito (Kv4.3), IKs (Kv7.1), and IKr (Kv11.1) repolarizing currents in the heart, respectively. Structurally, Kvαsubunits are composed of six transmembrane-spanning segments (S1–S6), including the positively charged S4 segment that serves as the voltage sensor, flanked by cytoplasmic amino (N)terminal and carboxyl (C)terminal domains.2–4 Collectively, the extracellular S5–S6 pore loop linker that contains the characteristic T–X–G–Y–G or T–X–G–F–G K+ selectivity filter and the S5 and S6 transmembrane segments form the pore that controls the selectivity and permeation of K+ ions.2,5 The assembly of four identical αsubunits (homotetramer) or a combination of different αsubunits within the same or rarely different Kv subfamily (heterotetramer), such that the S5–pore–S6 regions form a central pore, is required to form functional Kv channels. In Kv1 through Kv4 subfamilies, the tetrameric assembly of αsubunits seems to be largely dependent on the interaction of Nterminal T1 domains that reside proximal to the S1 segment,6,7 whereas in the hERG/KCNH (Kv11) and KCNQ (Kv7) subfamilies, their tetrameric assembly primarily involves a Cterminal subunit-assembly domain.8,9

The inward-rectifying K+ channels (Kir) constitute the second major class of K+ channels: the two transmembrane segment-one Pregion K+ channels. In the human heart, IK1 (Kir2.1 and Kir2.2), IKAch (Kir3.1 and Kir3.4) and IKATP (Kir6.2) are conducted by members of the seven 2TM1P subfamilies. Structurally, all Kir channel αsubunits are composed of two transmembrane-spanning segments (TM1 and TM2) connected by an extracellular pore loop that contains a T–X–G–Y(F)–G K+ selectivity sequence and flanked by intracellular Nterminal and Cterminal domains.2–4 Similar to Kv channels, the assembly of four Kirαsubunits into homotetramers and heterotetramers composed of subunits from the same subfamily (for example, association of Kir2.1 with other Kir2.x subfamily members) is required to form functional Kir channels.10,11 Kir channels lack a voltage-sensing region and thus do not respond to changes in membrane voltage and would conduct at virtually all membrane potentials in the absence of specific mechanisms that regulate channel activity (such as ATP-blocking KATP channels).11 Interestingly, the inward rectification of Kir channels is not an inherent property, but rather occurs because of the block of K+ permeation that results from the interaction between intracellular Mg2+ and or polyamines and residues within the pore of the channel.12,13

The six two-pore-domain K+ channel (K2P) subfamilies (TASK, TWIK, TREK, THIK, TALK, and TRESK) comprise the third and final major class of K+ channels, the four transmembrane two Pregion K+ channels. In general, the K2P family members are expressed ubiquitously and conduct background or ‘leak’ K+ currents that maintain resting membrane potential and cell excitability in a variety of cell types.14 While the maintenance of resting membrane potential in the human heart is attributed mostly to IK1, several K2P channels, namely KCNK2-encoded K2P2.1 (TREK1) and KCNK3-encoded K2P3.1 (TASK1), are expressed at appreciable levels. Still, their precise contribution to the human cardiac action potential remains unclear.14,15 Structurally, K2P channels assemble into dimers with an overall topology reminiscent of the inward rectifiers. Although K2P channels are noninactivating, show little-to-no voltage-dependence, and seem to be insensitive to traditional K+ channel blockers, they do display differential sensitivity to a spectrum of physical and chemical factors including membrane stretch, pH changes, anesthetic agents (both inhalation and local), and fatty acids and phospholipids, which suggests a potential role in cardiac physiology that goes beyond functioning as passive leak currents.4,16

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8.Kupershmidt, S., Snyders, D.J., Raes, A. & Roden, D.M. A K+ channel splice variant common in human heart lacks a Cterminal domain required for expression of rapidly activating delayed rectifier current. J. Biol. Chem.273, 27, 231–27, 235 (1998).

9.Haitin, Y. etal. Intracellular domains interactions and gated motions of IKs potassium channel subunits. EMBO J.28, 1994–2005 (2009).

10.Glowatzki, E. etal. Subunit-dependent assembly of inward-rectifier K+ channels. Proc. Biol. Sci.261, 251–261 (1995).

11.Hibino, H. etal. Inwardly rectifying potassium channels: their structure, function, and physiological roles. Physiol. Rev.90, 291–366 (2010).

12.Lopatin, A.N., Makhina, E.N. & Nichols, C.G. Potassium channel block by cytoplasmic polyamines as the mechanism of intrinsic rectification. Nature372, 366–369 (1994).

13.Matsuda, H., Saigusa, A. & Irisawa, H. Ohmic conductance through the inwardly rectifying K channel and blocking by internal Mg2+. Nature325, 156–159 (1987).

14.Gurney, A. & Manoury, B. Two-pore potassium channels in the cardiovascular system. Eur. Biophys. J.38, 305–318 (2009).

15.Lesage, F. & Lazdunski, M. Molecular and functional properties of twoporedomain potassium channels. Am. J. Physiol. Renal Physiol.279, F793–F801 (2000).

16.Goldstein, S.A., Bockenhauer, D., O’Kelly, I. & Zilberberg, N. Potassium leak channels and the KCNK family of two-Pdomain subunits. Nat. Rev. Neurosci.2, 175–184 (2001).

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