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Most calmodulin (CaM) targets are α-helices. It is not clear if CaM induces the adoption of an α-helix configuration to its targets or if those targets are selected as they spontaneously adopt an α-helical conformation. Other than an α-helix propensity, there is a great variety of CaM targets with little more in common. One exception to this rule is the IQ site that can be recognized in a number of targets, such as those ion channels belonging to the KCNQ family. Although there is negligible sequence similarity between the IQ motif and the docking site on SK2 channels, both adopt a similar three-dimensional disposition. The isolated SK2 target presents a pre-folded core region that becomes fully α-helical upon binding to CaM. The existence of this pre-folded state suggests the occurrence of capping within CaM targets. In this review, we examine the capping properties within the residues flanking this core domain, and relate known IQ motifs and capping., The Government of the Autonomous Community of the Basque Country (IT1165-19 and KK-2020/00110) and the Spanish Ministry of Science and Innovation (RTI2018-097839-B-100 to A.V. and PID2019-105488GB-I00 to A.B., A.L., and O.R.B.) and FEDER funds provided financial support for this work. A.M-M. is supported by predoctoral contracts from the Basque Government administered by University of the Basque Country.

[Background]: The amino acid sequence of proteins generally carries all the necessary information for acquisition of native conformations, but the vectorial nature of translation can additionally determine the folding outcome. Such consideration is particularly relevant in human diseases associated to inherited mutations leading to structural instability, aggregation, and degradation. Mutations in the KCNQ2 gene associated with human epilepsy have been suggested to cause misfolding of the encoded Kv7.2 channel. Although the effect on folding of mutations in some domains has been studied, little is known of the way pathogenic variants located in the calcium responsive domain (CRD) affect folding. Here, we explore how a Kv7.2 mutation (W344R) located in helix A of the CRD and associated with hereditary epilepsy interferes with channel function., [Results]: We report that the epilepsy W344R mutation within the IQ motif of CRD decreases channel function, but contrary to other mutations at this site, it does not impair the interaction with Calmodulin (CaM) in vitro, as monitored by multiple in vitro binding assays. We find negligible impact of the mutation on the structure of the complex by molecular dynamic computations. In silico studies revealed two orientations of the side chain, which are differentially populated by WT and W344R variants. Binding to CaM is impaired when the mutated protein is produced in cellulo but not in vitro, suggesting that this mutation impedes proper folding during translation within the cell by forcing the nascent chain to follow a folding route that leads to a non-native configuration, and thereby generating non-functional ion channels that fail to traffic to proper neuronal compartments., [Conclusions]: Our data suggest that the key pathogenic mechanism of Kv7.2 W344R mutation involves the failure to adopt a configuration that can be recognized by CaM in vivo but not in vitro., The Government of the Autonomous Community of the Basque Country (IT1165-19 and KK-2020/00110) and the Spanish Ministry of Science and Innovation (RTI2018-097839-B-100 to A.V. and FIS2016-76617-P to A.B.) and FEDER funds and the US National Institute of Neurological Disorders (NINDS) and Stroke Research Project Grant (R01NS083402 to H.J.C.) provided financial support for this work. E.N. and A.M-M. are supported by predoctoral contracts from the Basque Government administered by University of the Basque Country. C.M. was supported by the Basque Government through a Basque Excellence Research Centre (BERC) grant administered by Fundación Biofisika Bizkaia (FBB). J.U. was partially supported by BERC funds. O.R.B. was supported by the Basque Government through a BERC grant administered by Donostia International Physics Center. J.Z. and H.J.C. was supported by the NINDS Research Project Grant #R01NS083402 (PI: H.J.C.).

[EU] Potasio-kanalak ia zelula guztien mintzean agertzen dira eta funtzio biologiko garrantzitsuak betetzen dituzte; besteak beste, korronte elektrikoak kontrolatzen dituzte zelula kitzikagarrietan. KV7 kanalen familia 5 kidez osatuta dago (KV7.1-KV7.5), eta horiek kodetzen dituzten geneak patologia esanguratsuekin erlazionatzen dira. KV7 kanalen estrukturak zelula-mintzean txertaturiko 6 segmentuz osaturiko ohiko estruktura partekatzen du; N- eta C-muturrak zelula barnekoak dira. Neuronetan, KV7.2 eta KV7.3 kanalak agertzen dira batik bat; M-korrontea sortuz, neuronen kitzikagarritasuna kontrolatzen duena. M-korrontearen erregulazioa konplexua da seinaleztapen-bidezidor desberdinen bidez erregula baitaiteke. Gq/11 proteinari akoplaturiko hartzaileen bidez erregulatzen da eta seinaleztapen-bidezidorra desberdina da aktibatutako hartzailearen arabera. Horrela, azetilkolinaren M1 hartzaile muskarinikoak KV7.2-aren korrontea inhibituko du PIP2-aren agorpenaren ondorioz. Bradikininaren hartzaileak, ordea, IP3-ak eragindako kaltzio kontzentrazioaren igoeraren bidez inhibituko du. Mekanismo horietan, hainbat proteinak hartzen dute parte, hala nola kalmodulinak, proteina kinasek eta ainguratze-proteinek. Berrikuspen honetan, KV7.2 kanalari erreparatuko diogu, hainbat gaixotasunen partaide izateagatik eta haren erregulazio konplexuagatik, ikuspuntu farmakologiko batetik itu interesgarria izan baitaiteke., [EN] Potassium channels are present in almost all cell membranes and perform important biological functions, including electrical currents control in excitable cells. The KV7 channels familiy consists of 5 members (KV7.1 KV7.5) and the genes that encode them are related to significant pathologies. The structure of KV7 channels shares the usual six transmembrane segment structure, with intracellular N- and C-termini. In neurons, KV7.2 and KV7.3 are the main channels, which generate the M-current that controls neuronal excitability. The M-current regulation is complex as it can be regulated by different signalling pathways. It is regulated by Gq/11 coupled receptors, and the signaling pathway depends on the activated receptor. Thus, the M1 muscarinic acetylcholine receptor inhibits KV7.2 current by PIP2 depletion. While the bradykinin receptor inhibits it through the calcium concentration increase driven by IP3. Among these mechanisms several proteins are involved, such as calmodulin, protein kinases and anchor proteins. In this review we will focus on KV7.2 channel, as it is involved in several diseases and for its complex regulation it can be an interesting target from a pharmacological point of view.

Protein folding to the native state is particularly relevant in human diseases where inherited mutations lead to structural instability, aggregation and degradation. In general, the amino acid sequence carries all the necessary information for the native conformation, but the vectorial nature of translation can determine the folding outcome. Calmodulin (CaM) recognizes the properly folded Calcium Responsive Domain (CRD) of Kv7.2 channels. Within the IQ motif (helix A), the W344R mutation found in epileptic patients has negligible consequences for the structure of the complex as monitored by multiple in vitro binding assays and molecular dynamic computations. In silico studies revealed two orientations of the side chain, which are differentially populated by WT and W344R variants. Binding to CaM is impaired when the mutated protein is produced in cellulo but not in vitro, suggesting that this mutation impedes proper folding during translation within the cell by forcing the nascent chain to follow a folding route that leads to a non-native configuration, and thereby generating non-functional ion channels that fail to traffic to proper neuronal compartments.

© 2020 by the authors, Intracellular calcium is essential for many physiological processes, from neuronal signaling and exocytosis to muscle contraction and bone formation. Ca2+ signaling from the extracellular medium depends both on membrane potential, especially controlled by ion channels selective to K+, and direct permeation of this cation through specialized channels. Calmodulin (CaM), through direct binding to these proteins, participates in setting the membrane potential and the overall permeability to Ca2+. Over the past years many structures of complete channels in complex with CaM at near atomic resolution have been resolved. In combination with mutagenesis-function, structural information of individual domains and functional studies, different mechanisms employed by CaM to control channel gating are starting to be understood at atomic detail. Here, new insights regarding four types of tetrameric channels with six transmembrane (6TM) architecture, Eag1, SK2/SK4, TRPV5/TRPV6 and KCNQ1–5, and its regulation by CaM are described structurally. Different CaM regions, N-lobe, C-lobe and EF3/EF4-linker play prominent signaling roles in different complexes, emerging the realization of crucial non-canonical interactions between CaM and its target that are only evidenced in the full-channel structure. Different mechanisms to control gating are used, including direct and indirect mechanical actuation over the pore, allosteric control, indirect effect through lipid binding, as well as direct plugging of the pore. Although each CaM lobe engages through apparently similar alpha-helices, they do so using different docking strategies. We discuss how this allows selective action of drugs with great therapeutic potential., The Government of the Autonomous Community of the Basque Country (IT1165–19) and the Spanish Ministry of Economy, Industry and Competitiveness (RTI2018–097839-B-100) provided financial support for this work. E.N. and A.M-M. were supported by predoctoral contracts of the Basque Government.

Significance Ion channels are sophisticated proteins that exert control over a plethora of body functions. Specifically, the members of the Kv7 family are prominent components of the nervous systems, responsible for the ion fluxes that regulate the electrical signaling in neurons and cardiac myocytes. Albeit its relevance, there are still several questions, including the Ca2+/calmodulin (CaM)-mediated gating mechanism. We found that Ca2+ binding to CaM triggers a segmental rotation that allosterically transmits the signal from the cytosol up to the transmembrane region. NMR-derived analysis of the dynamics demonstrates that it occurs through a conformational selection mechanism. Energetically, CaM association with the channel tunes the affinities of the CaM lobes (calmodulation) so that the channel can sense the specific changes in [Ca2+] resulting after an action potential., The Kv7.2 (KCNQ2) channel is the principal molecular component of the slow voltage-gated, noninactivating K+ M-current, a key controller of neuronal excitability. To investigate the calmodulin (CaM)-mediated Ca2+ gating of the channel, we used NMR spectroscopy to structurally and dynamically describe the association of helices hA and hB of Kv7.2 with CaM, as a function of Ca2+ concentration. The structures of the CaM/Kv7.2-hAB complex at two different calcification states are reported here. In the presence of a basal cytosolic Ca2+ concentration (10–100 nM), only the N-lobe of CaM is Ca2+-loaded and the complex (representative of the open channel) exhibits collective dynamics on the millisecond time scale toward a low-populated excited state (1.5%) that corresponds to the inactive state of the channel. In response to a chemical or electrical signal, intracellular Ca2+ levels rise up to 1–10 μM, triggering Ca2+ association with the C-lobe. The associated conformational rearrangement is the key biological signal that shifts populations to the closed/inactive channel. This reorientation affects the C-lobe of CaM and both helices in Kv7.2, allosterically transducing the information from the Ca2+-binding site to the transmembrane region of the channel.

Calmodulin (CaM) is the principal Ca2+ sensor in eukaryotic cells, orchestrating the activity of hundreds of proteins. Disease causing mutations at any of the three genes that encode identical CaM proteins lead to major cardiac dysfunction, revealing the importance in the regulation of excitability. In turn, some mutations at the CaM binding site of ion channels cause similar diseases. Here we provide a summary of the two sides of the partnership between CaM and ion channels, describing the diversity of consequences of mutations at the complementary CaM binding domains. The Department of Industry, Tourism and Trade of the Government of the Autonomous Community of the Basque Country (Elkartek 2017 bG17 kk-2017/000843M50.17.EK.C6) and the Spanish Ministry of Economy, Industry and Competitiveness (BFU2015-66910 and RTI2018-097839) provided financial support for this work. E.N. is supported by a predoctoral grant of the Basque Government.

Tetrameric coiled-coil structures are present in many ion channels, often adjacent to a calmodulin (CaM) binding site, although the relationship between the two is not completely understood. Here we examine the dynamic properties of the ABCD domain located in the intracellular C-terminus of tetrameric, voltage-dependent, potassium selective Kv7.2 channels. This domain encompasses the CaM binding site formed by helices A and B, followed by helix C, which is linked to the helix D coiled-coil. The data reveals that helix D stabilizes CaM binding, promoting trans-binding (CaM embracing neighboring subunits), and they suggest that the ABCD domain can be exchanged between subunits of the tetramer. Exchange is faster when mutations in AB weaken the CaM interaction. The exchange of ABCD domains is slower in the presence of Ca2+, indicating that CaM stabilization of the tetrameric assembly is enhanced when loaded with this cation. Our observations are consistent with a model that involves a dynamic mechanism of helix D assembly, which supports reciprocal allosteric coupling between the A-B module and the coiled-coil formed by the helix D. Thus, formation of the distal helix D tetramer influences CaM binding and CaM-dependent Kv7.2 properties, whereas reciprocally, CaM and Ca2+ influence the dynamic behavior of the helix D coiled-coil.