The KCNH voltage-dependent potassium channels (ether-á-go-go, EAG; EAG-related gene, ERG; EAG-like channels, ELK) are important regulators of cellular excitability1-3 and have key roles in diseases such as cardiac long QT syndrome type 2 (LQT2)4, epilepsy5, schizophrenia6 and cancer7. The intracellular domains of KCNH channels are structurally distinct from other voltage-gated channels. The amino-terminal region contains an eag domain, which is comprised of a Per-Arnt-Sim (PAS) domain and a PAS-cap domain8, while the carboxy-terminal region contains a cyclic nucleotide-binding homology domain (CNBHD) which is connected to the pore through a C-linker domain. Many disease-causing mutations localize to these specialized intracellular domains, which underlie the unique gating and regulation of KCNH channels9. It has been suggested that the eag domain may regulate the channel by interacting with either the S4-S5 linker or the CNBHD8,10. Here we present a 2-Å resolution crystal structure of the eag domain-CNBHD complex of the mouse EAG1 (mEAG1) channel. It displays extensive interactions between the eag domain and the CNBHD, indicating that the regulatory mechanism of the eag domain involves primarily the CNBHD. Surprisingly, the structure reveals that a number of LQT2 mutations at homologous positions in hERG, and cancer-associated mutations in EAG channels, localize to the eag domain-CNBHD interface. Furthermore, mutations at the interface produced dramatic effects on channel gating demonstrating the important physiological role of the eag domain-CNBHD interaction. Our structure of the eag domain-CNBHD complex of mEAG1 provides unique insights into the physiological and pathophysiological mechanisms of KCNH channels.

The Kv7 subfamily of voltage-dependent potassium channels, distinct from other subfamilies by dint of its large intracellular COOH terminus, acts to regulate excitability in cardiac and neuronal tissues. KCNQ1 (Kv7.1), the founding subfamily member, encodes a channel subunit directly implicated in genetic disorders, such as the long QT syndrome, a cardiac pathology responsible for arrhythmias. We have used a recombinant protein preparation of the COOH terminus to probe the structure and function of this domain and its individual modules. The COOHterminal proximal half associates with one calmodulin constitutively bound to each subunit where calmodulin is critical for proper folding of the whole intracellular domain. The distal half directs tetramerization, employing tandem coiled-coils. The firstcoiled-coilcomplexisdimericandundergoesconcentrationdependent self-association to form a dimer of dimers. The outer coiled-coil is parallel tetrameric, the details of which have been elucidated based on 2.0 Å crystallographic data. Both coiledcoils act in a coordinate fashion to mediate the formation and stabilization of the tetrameric distal half. Functional studies, including characterization of structure-based and long QT mutants, prove the requirement for both modules and point to complex roles for these modules, including folding, assembly, trafficking, and regulation.

Abstract-The slow I KS K ϩ channel plays a major role in repolarizing the cardiac action potential and consists of the assembly of KCNQ1 and KCNE1 subunits. Mutations in either KCNQ1 or KCNE1 genes produce the long-QT syndrome, a life-threatening ventricular arrhythmia. Here, we show that long-QT mutations located in the KCNQ1 C terminus impair calmodulin (CaM) binding, which affects both channel gating and assembly. The mutations produce a voltage-dependent macroscopic inactivation and dramatically alter channel assembly. KCNE1 forms a ternary complex with wild-type KCNQ1 and Ca 2ϩ -CaM that prevents inactivation, facilitates channel assembly, and mediates a Ca 2ϩ -sensitive increase of I KS-current, with a considerable Ca 2ϩ -dependent left-shift of the voltage-dependence of activation. Coexpression of KCNQ1 or I KS channels with a Ca 2ϩ -insensitive CaM mutant markedly suppresses the currents and produces a right shift in the voltage-dependence of channel activation. KCNE1 association to KCNQ1 long-QT mutants significantly improves mutant channel expression and prevents macroscopic inactivation. However, the marked right shift in channel activation and the subsequent decrease in current amplitude cannot restore normal levels of I KS channel activity. Our data indicate that in healthy individuals, CaM binding to KCNQ1 is essential for correct channel folding and assembly and for conferring Ca 2ϩ -sensitive I KS -current stimulation, which increases the cardiac repolarization reserve and hence prevents the risk of ventricular arrhythmias. Key Words: KCNQ Ⅲ potassium channels Ⅲ Kv7 Ⅲ calmodulin Ⅲ KCNE Ⅲ long QT K CNQ channels represent a family of voltage-gated K ϩ channels (Kv7) that plays a major role in brain and cardiac excitability. 1,2 Mutations of human KCNQ genes lead to severe cardiovascular and neurological disorders such as the cardiac long-QT syndrome (LQT) and neonatal epilepsy. Coassembly of KCNQ1 with KCNE1 ␤ subunits produces the I KS -current that is crucial for repolarization of the cardiac action potential. [3][4][5] The cytoplasmic KCNQ C-termini were shown to feature 4 ␣ helices. 6 We previously identified the last ␣ helix of the C terminus (helix D, aa.589 -620) as a region important for the tetrameric assembly of KCNQ1 ␣ subunits. 7 This region also binds Yotiao, an A-kinase-anchoring protein that targets PKA on the I KS channel complex. 8 The first 2␣ helices of KCNQ1-5 form a calmodulin-binding domain (CBD), including an IQ motif that mediates Ca 2ϩ -free calmodulin (apoCaM) binding. 6,9 Although KCNQ channels bind calmodulin (CaM), the role of CaM in channel function remains controversial. Recent studies found a role for CaM as a Ca 2ϩ -sensor of KCNQ2/4/5 channels, 10,11 whereas others suggested a role in channel assembly. 9 So far, no information has been available about the interaction of calmodulin with cardiac I KS channels and its pathophysiological impact to KCNQ1-related LQT channnelopathies. Here, we show that LQT mutations located near the IQ motif of KCNQ1 C terminus impair ...

Kv7 channels (KCNQ) represent a family of voltage-gated K + channels which plays a prominent role in brain and cardiac excitability. Their physiological importance is underscored by the existence of mutations in human Kv7 genes, leading to severe cardiovascular and neurological disorders such as the cardiac long QT syndrome and neonatal epilepsy. Kv7 channels exhibit some structural and functional features that are distinct from other Kv channels. Notably, the Kv7 C-terminus is long compared to other K + channels and is endowed with characteristic structural domains, including coiled-coils, amphipatic α helices containing calmodulin-binding motifs and basic amino acid clusters. Here we provide a brief overview of current insights and as yet unsettled issues about the structural and functional attributes of the C-terminus of Kv7 channels. Recent data indicate that the proximal half of the Kv7 C-terminus associates with one calmodulin constitutively bound to each subunit. Epilepsy and long QT mutations located in this proximal region impair calmodulin binding and can affect channel gating, folding and trafficking. The distal half of the Kv7 C-terminus directs tetramerization, employing tandem coiled-coils. Together, the data indicate that the Kv7 C-terminal domain is a multimodular structure playing a crucial role in channel gating, assembly and trafficking as well as in scaffolding the channel complex with signalling proteins.

The pore and gate regions of voltage-gated cation channels have been often targeted with drugs acting as channel modulators. In contrast, the voltage-sensing domain (VSD) was practically not exploited for therapeutic purposes, although it is the target of various toxins. We recently designed unique diphenylamine carboxylates that are powerful Kv7.2 voltage-gated K + channel openers or blockers. Here we show that a unique Kv7.2 channel opener, NH29, acts as a nontoxin gating modifier. NH29 increases Kv7.2 currents, thereby producing a hyperpolarizing shift of the activation curve and slowing both activation and deactivation kinetics. In neurons, the opener depresses evoked spike discharges. NH29 dampens hippocampal glutamate and GABA release, thereby inhibiting excitatory and inhibitory postsynaptic currents. Mutagenesis and modeling data suggest that in Kv7.2, NH29 docks to the external groove formed by the interface of helices S1, S2, and S4 in a way that stabilizes the interaction between two conserved charged residues in S2 and S4, known to interact electrostatically, in the open state of Kv channels. Results indicate that NH29 may operate via a voltagesensor trapping mechanism similar to that suggested for scorpion and sea-anemone toxins. Reflecting the promiscuous nature of the VSD, NH29 is also a potent blocker of TRPV1 channels, a feature similar to that of tarantula toxins. Our data provide a structural framework for designing unique gating-modifiers targeted to the VSD of voltage-gated cation channels and used for the treatment of hyperexcitability disorders.oltage-sensitive cation channels play crucial roles in brain and cardiac excitability. These channels are endowed with two main transmembrane modules, a voltage-sensing domain (VSD) and a pore domain. Mutations of ion channel genes in humans lead to severe inherited neurological, cardiovascular, or metabolic disorders, called "channelopathies" (1). So far, the medicinal toolbox has focused on the pore domain and its gate in an attempt to cure ion channel-related dysfunctions by channel blockers or openers (2).In contrast, the VSD of voltage-gated cation channels was virtually not exploited for therapeutic purposes. VSDs are found in voltage-dependent cation channels and other voltage-regulated proteins (3). In voltage-gated cation channels, the linker S4-S5 of the VSD serves as an electromechanical coupling device, which opens the channel pore. VSDs have also been recently characterized in voltage-regulated proteins that lack associated ion channel pores (4-6). A voltage-sensitive phosphatase, Ci-VSP, has a VSD that is coupled to a phosphatase domain (4). In the human voltage-activated proton channel (Hv1), the VSD itself functions as a proton channel (5-7).Crystallographic studies of voltage-gated K + channels (Kv) have described the VSD architecture in its open-state conformation. It forms a module of four membrane-spanning segments (S1-S4) with the S3b helix and the charge-bearing S4 helix forming a helix-turn-helix structure, termed the "paddle ...

Voltage-gated potassium 7.1 (Kv7.1) channel and KCNE1 protein coassembly forms the slow potassium current I KS that repolarizes the cardiac action potential. The physiological importance of the I KS channel is underscored by the existence of mutations in human Kv7.1 and KCNE1 genes, which cause cardiac arrhythmias, such as the long-QT syndrome (LQT) and atrial fibrillation. The proximal Kv7.1 C terminus (CT) binds calmodulin (CaM) and phosphatidylinositol-4,5-bisphosphate (PIP 2 ), but the role of CaM in channel function is still unclear, and its possible interaction with PIP 2 is unknown. Our recent crystallographic study showed that CaM embraces helices A and B with the apo C lobe and calcified N lobe, respectively. Here, we reveal the competition of PIP 2 and the calcified CaM N lobe to a previously unidentified site in Kv7.1 helix B, also known to harbor an LQT mutation. Protein pulldown, molecular docking, molecular dynamics simulations, and patch-clamp recordings indicate that residues K526 and K527 in Kv7.1

Molecular determinants of ion channel tetramerization are well characterized, but those involved in heteromeric channel assembly are less clearly understood. The heteromeric composition of native channels is often precisely controlled. Cyclic nucleotide-gated (CNG) channels from rod photoreceptors exhibit a 3:1 stoichiometry of CNGA1 and CNGB1 subunits that tunes the channels for their specialized role in phototransduction. Here we show, using electrophysiology, fluorescence, biochemistry, and X-ray crystallography, that the mechanism for this controlled assembly is the formation of a parallel 3-helix coiled-coil domain of the carboxy-terminal leucine zipper region of CNGA1 subunits, constraining the channel to contain three CNGA1 subunits, followed by preferential incorporation of a single CNGB1 subunit. Deletion of the carboxy-terminal leucine zipper domain relaxed the constraint and permitted multiple CNGB1 subunits in the channel. The X-ray crystal structures of the parallel 3-helix coiled-coil domains of CNGA1 and CNGA3 subunits were similar, suggesting that a similar mechanism controls the stoichiometry of cone CNG channels.

Cyclic nucleotide-gated (CNG) and hyperpolarization-activated cyclic nucleotide-regulated (HCN) ion channels play crucial physiological roles in phototransduction, olfaction, and cardiac pace making. These channels are characterized by the presence of a carboxylterminal cyclic nucleotide-binding domain (CNBD) that connects to the channel pore via a C-linker domain. Although cyclic nucleotide binding has been shown to promote CNG and HCN channel opening, the precise mechanism underlying gating remains poorly understood. Here we used cryoEM to determine the structure of the intact LliK CNG channel isolated from Leptospira licerasiae-which shares sequence similarity to eukaryotic CNG and HCN channels-in the presence of a saturating concentration of cAMP. A short S4-S5 linker connects nearby voltage-sensing and pore domains to produce a non-domain-swapped transmembrane architecture, which appears to be a hallmark of this channel family. We also observe major conformational changes of the LliK C-linkers and CNBDs relative to the crystal structures of isolated C-linker/CNBD fragments and the cryoEM structures of related CNG, HCN, and KCNH channels. The conformation of our LliK structure may represent a functional state of this channel family not captured in previous studies.yclic nucleotide-gated (CNG) and hyperpolarization-activated cyclic nucleotide-regulated (HCN) channels are cationpermeable ion channels regulated by the direct binding of cyclic nucleotides (cAMP or cGMP) (1). CNG channels are present in retinal photoreceptors and olfactory sensory neurons, where they perform chemoelectrical energy conversion in response to light or odor stimuli, respectively. Mutations in CNG channels have been associated with numerous inherited retinal degenerative disorders, achromatopsia, and anosmia (2). HCN channels are found in the cardiac sinoatrial node and throughout the nervous system, where they open in response to membrane hyperpolarization and generate a depolarizing current responsible for rhythmic firing (1). HCN channel mutations and mistrafficking have been associated with several disorders, including sinus bradycardia, epilepsy, and autism (3, 4).CNG and HCN channels possess a cyclic nucleotide-binding domain (CNBD) in their carboxyl-terminal region, and binding of cyclic nucleotide produces a large increase in the open probability of the channel pore. Cyclic nucleotide binding to HCN channels also shifts the voltage dependence of activation to more depolarized potentials, increasing the rate and extent of channel opening (5). CNG and HCN channels are part of a family that includes KCNH potassium channels (Fig. 1A and Fig. S1). KCNH channels, however, are distinct in that they possess a cyclic nucleotide-binding homology domain (CNBHD) occupied by an "intrinsic ligand" and are not directly regulated by cyclic nucleotides (6-8).CNG and HCN channels also harbor a C-linker domain situated between the pore and the CNBD. Based on its position, along with mutagenesis and cross-linking studies, this domain is thought t...