Pivoting between calmodulin lobes triggered by calcium in the Kv7.2/calmodulin complex

The paracetamol metabolite N-acetyl-4-benzoquinoneimine (NAPQI) prevents modulation of KV7 channels via G-protein coupled receptors by interference with PIP2 and Ca2+ sensitivity

BACKGROUND AND PURPOSE

Paracetamol has been found to alleviate inflammatory pain by modulating KV7 channels. Its metabolite N-acetyl-4-benzoquinoneimine (NAPQI) increases currents through these channels via a stretch of three cysteine residues in the channel S2-S3 linker. Through this effect, the excitability of neurons in the pain pathway is dampened. Inflammatory mediators, in turn, enhance the excitability of sensory neurons by inhibiting KV7 channels. Here, a specific interaction between NAPQI and the so-called inflammatory soup was investigated.

EXPERIMENTAL APPROACH

Currents through KV7 channels were measured in sensory neurons and after heterologous expression in tsA201 cells. In addition, changes in cytosolic Ca2+ and in the distribution of PIP2 (PI(4,5)P2) between membrane and cytosol were determined by fluorescence microscopy.

KEY RESULTS

NAPQI abolished Ca2+-mediated inhibitory effects of an 'inflammatory soup' containing ADP, ATP, bradykinin, histamine, 5-hydroxytryptamine, prostaglandin E2, substance P and a PAR2 agonist on KV7 channel currents in sensory neurons. Moreover, the increase of KV7.2 channel currents by quenching of cytosolic Ca2+ as well as the current decrease by depletion of membrane PIP2 was impaired by NAPQI. These effects were lost in mutant channels lacking the three cysteines in the S2-S3 linker.

CONCLUSION AND IMPLICATION

NAPQI targets the three-cysteine motif in the S2-S3 linker of KV7.2 channels to counteract the signalling cascades employed by inflammatory mediators that inhibit these channels. In sensory neurons, this abolishes the closure of KV7 channels by the inflammatory soup. This mechanism is likely involved in the alleviation of inflammatory pain by paracetamol.

Fluorometric Measurement of Calmodulin-Dependent Peptide-Protein Interactions Using Dansylated Calmodulin

The assessment of peptide-protein interactions is a pivotal aspect of studying the functionality and mechanisms of various bioactive peptides. In this context, it is essential to employ methods that meet specific criteria, including sensitivity, biocompatibility, versatility, simplicity, and the ability to offer real-time monitoring. In cellular contexts, only a few proteins naturally possess inherent fluorescence, specifically those containing aromatic amino acids, particularly tryptophan. Nonetheless, by covalently attaching fluorescent markers, almost all proteins can be modified for monitoring purposes. Among the early extrinsic fluorescent probes designed for this task, dansyl chloride (DNSC) is a notable option due to its versatile nature and reliable performance. DNSC has been the primary choice as a fluorogenic derivatizing reagent for analyzing amino acids in proteins and peptides for an extended period of time. In our work, we have effectively utilized the distinctive properties of dansylated-calmodulin (D-CaM) for monitoring the interaction dynamics between proteins and peptides, particularly in the context of their association with calmodulin (CaM), a calcium-dependent regulatory protein. This technique not only enables us to scrutinize the affinity of diverse ligands but also sheds light on the intricate role played by calcium in these interactions. Key features • Dynamic fluorescence and real-time monitoring: dansyl-modified CaM enables sensitive, real-time fluorescence, providing valuable insights into the dynamics of molecular interactions and ligand binding. • Selective interaction and stable fluorescent adducts: DNSC selectively interacts with primary amino groups, ensuring specific detection and forming stable fluorescent sulfonamide adducts. • Versatility in research and ease of identification: D-CaM is a versatile tool in biological research, facilitating identification, precise quantification, and drug assessment for therapeutic development. • Sensitivity to surrounding alterations: D-CaM exhibits sensitivity to its surroundings, particularly ligand-induced changes, offering subtle insights into molecular interactions and environmental influences.

Redox regulation of KV7 channels through EF3 hand of calmodulin

Neuronal KV7 channels, important regulators of cell excitability, are among the most sensitive proteins to reactive oxygen species. The S2S3 linker of the voltage sensor was reported as a site-mediating redox modulation of the channels. Recent structural insights reveal potential interactions between this linker and the Ca2+-binding loop of the third EF-hand of calmodulin (CaM), which embraces an antiparallel fork formed by the C-terminal helices A and B, constituting the calcium responsive domain (CRD). We found that precluding Ca2+ binding to the EF3 hand, but not to EF1, EF2, or EF4 hands, abolishes oxidation-induced enhancement of KV7.4 currents. Monitoring FRET (Fluorescence Resonance Energy Transfer) between helices A and B using purified CRDs tagged with fluorescent proteins, we observed that S2S3 peptides cause a reversal of the signal in the presence of Ca2+ but have no effect in the absence of this cation or if the peptide is oxidized. The capacity of loading EF3 with Ca2+ is essential for this reversal of the FRET signal, whereas the consequences of obliterating Ca2+ binding to EF1, EF2, or EF4 are negligible. Furthermore, we show that EF3 is critical for translating Ca2+ signals to reorient the AB fork. Our data are consistent with the proposal that oxidation of cysteine residues in the S2S3 loop relieves KV7 channels from a constitutive inhibition imposed by interactions between the EF3 hand of CaM which is crucial for this signaling.

The use of pharmacological chaperones in rare diseases caused by reduced protein stability

Rare diseases are most often caused by inherited genetic disorders that, after translation, will result in a protein with altered function. Decreased protein stability is the most frequent mechanism associated with a congenital pathogenic missense mutation and it implies the destabilization of the folded conformation in favour of unfolded or misfolded states. In the cellular context and when experimental data is available, a mutant protein with altered thermodynamic stability often also results in impaired homeostasis, with the deleterious accumulation of protein aggregates, metabolites and/or metabolic by-products. In the last decades, a significant effort has enabled the characterization of rare diseases associated to protein stability defects and triggered the development of innovative therapeutic intervention lines, say, the use of pharmacological chaperones to correct the intracellular impaired homeostasis. Here, we review the current knowledge on rare diseases caused by reduced protein stability, paying special attention to the thermodynamic aspects of the protein destabilization, also focusing on some examples where pharmacological chaperones are being tested.

An epilepsy-causing mutation leads to co-translational misfolding of the Kv7.2 channel

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.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01040-1.

Two KCNQ2 Encephalopathy Variants in the Calmodulin-Binding Helix A Exhibit Dominant-Negative Effects and Altered PIP2 Interaction

Heterozygous missense variants in KCNQ2, which encodes the potassium channel subunit Kv7.2, are among the most common genetic causes of severe neonatal-onset epileptic encephalopathy. Because about 20% of known severe Kv7.2 missense changes lie within the intracellular C-terminal region, improving understanding of the underlying pathogenic mechanisms is important. We analyzed the basis for the severe phenotypes of Kv7.2 A337T and A337G, variants in the C-terminal’s calmodulin (CaM)-binding Helix A. When expressed heterologously in mammalian cells, alone or in combination with wild type Kv7.2 or with wild type Kv7.2 and Kv7.3, both variants strongly suppressed channel currents. A337T channels expressed alone exhibited significantly reduced protein half-life and surface trafficking and co-immunoprecipitated less CaM. For both variants, increasing cellular phosphatidylinositol 4,5-bisphosphate (PIP₂) by overexpression of PI(4)P5-kinase restored current densities. For both variants, the fraction of current suppressed by activation of M1 muscarinic receptors with 10 μM oxotremorine methiodide, which depletes PIP₂, was less than for controls. During voltage-sensitive phosphatase-induced transient PIP₂ depletion and resynthesize, potassium current inhibition and recovery kinetics were both markedly slowed. These results suggest that these variants may reduce currents by a mechanism not previously described: slowing of PIP₂ migration between the bulk membrane and binding sites mediating channel electromechanical coupling. A novel Kv7.2/3-selective opener, SF0034, rescued current amplitudes. Our findings show that these two Helix A variants suppress channel current density strongly, consistent with their severe heterozygous phenotypes, implicate impairment of CaM and PIP₂ regulation in KCNQ2 encephalopathy pathogenesis, and highlight the potential usefulness of selective Kv7 openers for this distinctive pathogenic mechanism and patient subgroup.

Novel Dominant KCNQ2 Exon 7 Partial In-Frame Duplication in a Complex Epileptic and Neurodevelopmental Delay Syndrome

Complex neurodevelopmental syndromes frequently have an unknown etiology, in which genetic factors play a pathogenic role. This study utilizes whole-exome sequencing (WES) to examine four members of a family with a son presenting, since birth, with epileptic-like crises, combined with cerebral palsy, severe neuromotor and developmental delay, dystonic tetraparexia, axonal motor affectation, and hyper-excitability of unknown origin. The WES study detected within the patient a de novo heterozygous in-frame duplication of thirty-six nucleotides within exon 7 of the human KCNQ2 gene. This insertion duplicates the first twelve amino acids of the calmodulin binding site I. Molecular dynamics simulations of this KCNQ2 peptide duplication, modelled on the 3D structure of the KCNQ2 protein, suggest that the duplication may lead to the dysregulation of calcium inhibition of this protein function.

Atomistic Insights of Calmodulin Gating of Complete Ion Channels

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.

Genetic intolerance analysis as a tool for protein science

Recent advances in whole genome and exome sequencing have dramatically increased the database of human gene variations. There are now enough sequenced human exomes and genomes to begin to identify gene variations that are notable because they are NOT observed in sequenced human genomes, apparently because they are subject to "purifying selection", exemplifying genetic intolerance. Such "dysprocreative" gene variations are embryonic lethal or prevent reproduction through any one of a number of possible mechanisms. Here we review an emerging quantitative approach, "Missense Tolerance Ratio" (MTR) analysis, that is used to assess protein-encoding gene (cDNA) sequence intolerance to missense mutations based on analysis of the >100 K and growing number of currently available human genome and exome sequences. This approach is already useful for analyzing intolerance to mutations in cDNA segments with a resolution on the order of 90 bases. Moreover, as the number of sequenced genomes/exomes increases by orders of magnitude it may eventually be possible to assess mutational tolerance in a statistically robust manner at or near single site resolution. Here we focus on how cDNA intolerance analysis complements other bioinformatic methods to illuminate structure-folding-function relationships for the encoded proteins. A set of disease-linked membrane proteins is employed to provide examples.

Lack of correlation between surface expression and currents in epileptogenic AB-calmodulin binding domain Kv7.2 potassium channel mutants

Heteromers of Kv7.2/Kv7.3 subunits constitute the main substrate of the neuronal M-current that limits neuronal hyper-excitability and firing frequency. Calmodulin (CaM) binding is essential for surface expression of Kv7 channels, and disruption of this interaction leads to diseases ranging from mild epilepsy to early onset encephalopathy. In this study, we addressed the impact of a charge neutralizing mutation located at the periphery of helix B (K526N). We found that, CaM binding and surface expression was impaired, although current amplitude was not altered. Currents were reduced at a faster rate after activation of a voltage-dependent phosphatase, suggesting that phosphatidylinositol-4,5-bisphosphate (PIP₂) binding was weaker. In contrast, a charge neutralizing mutation located at the periphery of helix A (R333Q) did not affect CaM binding, but impaired trafficking and led to a reduction in current amplitude. Taken together, these results suggest that disruption of CaM-dependent or CaM-independent trafficking of Kv7.2/Kv7.3 channels can lead to pathology regardless of the consequences on the macroscopic ionic flow through the channel.

The Crossroad of Ion Channels and Calmodulin in Disease

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.

Calmodulin: A Multitasking Protein in Kv7.2 Potassium Channel Functions

The ubiquitous calcium transducer calmodulin (CaM) plays a pivotal role in many cellular processes, regulating a myriad of structurally different target proteins. Indeed, it is unquestionable that CaM is the most relevant transductor of calcium signals in eukaryotic cells. During the last two decades, different studies have demonstrated that CaM mediates the modulation of several ion channels. Among others, it has been indicated that Kv7.2 channels, one of the members of the voltage gated potassium channel family that plays a critical role in brain excitability, requires CaM binding to regulate the different mechanisms that govern its functions. The purpose of this review is to provide an overview of the most recent advances in structure–function studies on the role of CaM regulation of Kv7.2 and the other members of the Kv7 family.

Structural basis and energy landscape for the Ca2+ gating and calmodulation of the Kv7.2 K+ channel

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.

A Calmodulin C-Lobe Ca2+-Dependent Switch Governs Kv7 Channel Function

Kv7 (KCNQ) voltage-gated potassium channels control excitability in the brain, heart, and ear. Calmodulin (CaM) is crucial for Kv7 function, but how this calcium sensor affects activity has remained unclear. Here, we present X-ray crystallographic analysis of CaM:Kv7.4 and CaM:Kv7.5 AB domain complexes that reveal an Apo/CaM clamp conformation and calcium binding preferences. These structures, combined with small-angle X-ray scattering, biochemical, and functional studies, establish a regulatory mechanism for Kv7 CaM modulation based on a common architecture in which a CaM C-lobe calcium-dependent switch releases a shared Apo/CaM clamp conformation. This C-lobe switch inhibits voltage-dependent activation of Kv7.4 and Kv7.5 but facilitates Kv7.1, demonstrating that mechanism is shared by Kv7 isoforms despite the different directions of CaM modulation. Our findings provide a unified framework for understanding how CaM controls different Kv7 isoforms and highlight the role of membrane proximal domains for controlling voltage-gated channel function. VIDEO ABSTRACT.

Calmodulin confers calcium sensitivity to the stability of the distal intracellular assembly domain of Kv7.2 channels

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.

Competition of calcified calmodulin N lobe and PIP2 to an LQT mutation site in Kv7.1 channel

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 (PIP₂), but the role of CaM in channel function is still unclear, and its possible interaction with PIP₂ 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 PIP₂ 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 helix B form a critical site where CaM competes with PIP₂ to stabilize the channel open state. Data indicate that both PIP₂ and Ca²⁺-CaM perform the same function on I_KS channel gating by producing a left shift in the voltage dependence of activation. The LQT mutant K526E revealed a severely impaired channel function with a right shift in the voltage dependence of activation, a reduced current density, and insensitivity to gating modulation by Ca²⁺-CaM. The results suggest that, after receptor-mediated PIP₂ depletion and increased cytosolic Ca²⁺, calcified CaM N lobe interacts with helix B in place of PIP₂ to limit excessive I_KS current inhibition.

Modulation of Kv7 channels and excitability in the brain

Neuronal Kv7 channels underlie a voltage-gated non-inactivating potassium current known as the M-current. Due to its particular characteristics, Kv7 channels show pronounced control over the excitability of neurons. We will discuss various factors that have been shown to drastically alter the activity of this channel such as protein and phospholipid interactions, phosphorylation, calcium, and numerous neurotransmitters. Kv7 channels locate to key areas for the control of action potential initiation and propagation. Moreover, we will explore the dynamic surface expression of the channel modulated by neurotransmitters and neural activity. We will also focus on known principle functions of neural Kv7 channels: control of resting membrane potential and spiking threshold, setting the firing frequency, afterhyperpolarization after burst firing, theta resonance, and transient hyperexcitability from neurotransmitter-induced suppression of the M-current. Finally, we will discuss the contribution of altered Kv7 activity to pathologies such as epilepsy and cognitive deficits.

Structural Insights into the M-Channel Proximal C-Terminus/Calmodulin Complex

The Kv7 (KCNQ) channel family, comprising voltage-gated potassium channels, plays major roles in fine-tuning cellular excitability by reducing firing frequency and controlling repolarization. Kv7 channels have a unique intracellular C-terminal (CT) domain bound constitutively by calmodulin (CaM). This domain plays key functions in channel tetramerization, trafficking, and gating. CaM binds to the proximal CT, comprising helices A and B. Kv7.2 and Kv7.3 are expressed in neural tissues. Together, they form the heterotetrameric M channel. We characterized Kv7.2, Kv7.3, and chimeric Kv7.3 helix A-Kv7.2 helix B (Q3A-Q2B) proximal CT/CaM complexes by solution methods at various Ca(2+)concentrations and determined them all to have a 1:1 stoichiometry. We then determined the crystal structure of the Q3A-Q2B/CaM complex at high Ca(2+) concentration to 2.0 Å resolution. CaM hugs the antiparallel coiled coil of helices A and B, braced together by an additional helix. The structure displays a hybrid apo-Ca(2+) CaM conformation even though four Ca(2+) ions are bound. Our results pinpoint unique interactions enabling the possible intersubunit pairing of Kv7.3 helix A and Kv7.2 helix B while underlining the potential importance of Kv7.3 helix A's role in stabilizing channel oligomerization. Also, the structure can be used to rationalize various channelopathic mutants. Functional testing of the chimeric channel found it to have a voltage-dependence similar to the M channel, thereby demonstrating helix A's importance in imparting gating properties.

Protein Conformational Changes Are Detected and Resolved Site Specifically by Second-Harmonic Generation

We present here a straightforward, broadly applicable technique for real-time detection and measurement of protein conformational changes in solution. This method is based on tethering proteins labeled with a second-harmonic generation (SHG) active dye to supported lipid bilayers. We demonstrate our method by measuring the conformational changes that occur upon ligand binding with three well-characterized proteins labeled at lysine residues: calmodulin (CaM), maltose-binding protein (MBP), and dihydrofolate reductase (DHFR). We also create a single-site cysteine mutant of DHFR engineered within the Met20 catalytic loop region and study the protein’s structural motion at this site. Using published x-ray crystal structures, we show that the changes in the SHG signals upon ligand binding are the result of structural motions that occur at the labeled sites between the apo and ligand-bound forms of the proteins, which are easily distinguished from each other. In addition, we demonstrate that different magnitudes of the SHG signal changes are due to different and specific ligand-induced conformational changes. Taken together, these data illustrate the potential of the SHG approach for detecting and measuring protein conformational changes for a wide range of biological applications.

Uncoupling PIP2-calmodulin regulation of Kv7.2 channels by an assembly destabilizing epileptogenic mutation

We show that the combination of an intracellular bi-partite calmodulin (CaM)-binding site and a distant assembly region affect how an ion channel is regulated by a membrane lipid. Our data reveal that regulation by phosphatidylinositol(4,5)bisphosphate (PIP2) and stabilization of assembled Kv7.2 subunits by intracellular coiled-coil regions far from the membrane are coupled molecular processes. Live-cell fluorescence energy transfer measurements and direct binding studies indicate that remote coiled-coil formation creates conditions for different CaM interaction modes, each conferring different PIP2 dependency to Kv7.2 channels. Disruption of coiled-coil formation by epilepsy-causing mutation decreases apparent CaM-binding affinity and interrupts CaM influence on PIP2 sensitivity.

An unconventional calmodulin-anchoring site within the AB module of Kv7.2 channels

Calmodulin (CaM) binding to the AB module is crucial for multiple mechanisms governing the function of Kv7.2 (also known as KCNQ2) K(+) channel subunits, which mediate one of the main components of the non-inactivating K(+) M-current, a key controller of neuronal excitability. Structural analysis indicates that the CaM N-lobe engages with helix B, whereas the C-lobe anchors to the IQ site within helix A. Here, we report the identification of a new site between helices A and B that assists in CaM binding whose sequence is reminiscent of the TW helix within the CaM C-lobe anchoring site of SK2 K(+) channels (also known as KCNN2). Mutations that disrupt CaM binding within the TW site, helix B or helix A yield functional channels, whereas no function is observed when the TW site and helix A, or the TW site and helix B are mutated simultaneously. Our data indicate that the TW site is dispensable for function, contributes to the stabilization of the CaM-Kv7.2 complex and becomes essential when docking to either helix A or when helix B is perturbed.

Mechanisms and dynamics of AKAP79/150-orchestrated multi-protein signalling complexes in brain and peripheral nerve

A-kinase anchoring proteins (AKAPs) have emerged as a converging point of diverse signals to achieve spatiotemporal resolution of directed cellular regulation. With the extensive studies of AKAP79/150 in regulation of ion channel activity, the major questions to be posed centre on the mechanism and functional role of synergistic regulation of ion channels by such signalling proteins. In this review, we summarize recent discoveries of AKAP79/150-mediated modulation of voltage-gated neuronal M-type (KCNQ, Kv7) K(+) channels and L-type CaV 1 Ca(2+) channels, on both short- and longer-term time scales, highlighting the dynamics of the macromolecular signalling complexes in brain and peripheral nerve We also discuss several models for the possible mechanisms of these multi-protein assemblies and how they serve the agenda of the neurons in which they occur.

Channel-anchored protein kinase CK2 and protein phosphatase 1 reciprocally regulate KCNQ2-containing M-channels via phosphorylation of calmodulin

M-type potassium channels, encoded by the KCNQ family genes (KCNQ2-5), require calmodulin as an essential co-factor. Calmodulin bound to the KCNQ2 subunit regulates channel trafficking and stabilizes channel activity. We demonstrate that phosphorylation of calmodulin by protein kinase CK2 (casein kinase 2) rapidly and reversibly modulated KCNQ2 current. CK2-mediated phosphorylation of calmodulin strengthened its binding to KCNQ2 channel, caused resistance to phosphatidylinositol 4,5-bisphosphate depletion, and increased KCNQ2 current amplitude. Accordingly, application of CK2-selective inhibitors suppressed KCNQ2 current. This suppression was prevented by co-expression of CK2 phosphomimetic calmodulin mutants or pretreatment with a protein phosphatase inhibitor, calyculin A. We also demonstrated that functional CK2 and protein phosphatase 1 (PP1) were selectively tethered to the KCNQ2 subunit. We identified a functional KVXF consensus site for PP1 binding in the N-terminal tail of KCNQ2 subunit: mutation of this site augmented current density. CK2 inhibitor treatment suppressed M-current in rat superior cervical ganglion neurons, an effect negated by overexpression of phosphomimetic calmodulin or pretreatment with calyculin A Furthermore, CK2 inhibition diminished the medium after hyperpolarization by suppressing the M-current. These findings suggest that CK2-mediated phosphorylation of calmodulin regulates the M-current, which is tonically regulated by CK2 and PP1 anchored to the KCNQ2 channel complex.

Clicked bis-PEG-peptide conjugates for studying calmodulin-Kv7.2 channel binding

Small bis-conjugates helix A^329–350-PEG-triazole-PEG-helix B^508–526 (41 residues), prepared through click chemistry of PEGylated peptide derivatives, bind to CaM with nanomolar affinity, behaving as mimics of the Kv7.2 native fragment (239 residues).