Coupled Tertiary Folding and Oligomerization of the T1 Domain of Kv Channels

Functional analysis of ctenophore Shaker K+ channels: N-type inactivation in the animal roots

Here we explore the evolutionary origins of fast N-type ball-and-chain inactivation in Shaker (Kv1) K+ channels by functionally characterizing Shaker channels from the ctenophore (comb jelly) Mnemiopsis leidyi. Ctenophores are the sister lineage to other animals and Mnemiopsis has >40 Shaker-like K+ channels, but they have not been functionally characterized. We identified three Mnemiopsis channels (MlShak3-5) with N-type inactivation ball-like sequences at their N termini and functionally expressed them in Xenopus oocytes. Two of the channels, MlShak4 and MlShak5, showed rapid inactivation similar to cnidarian and bilaterian Shakers with rapid N-type inactivation, whereas MlShak3 inactivated ∼100-fold more slowly. Fast inactivation in MlShak4 and MlShak5 required the putative N-terminal inactivation ball sequences. Furthermore, the rate of fast inactivation in these channels depended on the number of inactivation balls/channel, but the rate of recovery from inactivation did not. These findings closely match the mechanism of N-type inactivation first described for Drosophila Shaker in which 1) inactivation balls on the N termini of each subunit can independently block the pore, and 2) only one inactivation ball occupies the pore binding site at a time. These findings suggest classical N-type activation evolved in Shaker channels at the very base of the animal phylogeny in a common ancestor of ctenophores, cnidarians, and bilaterians and that fast-inactivating Shakers are therefore a fundamental type of animal K+ channel. Interestingly, we find evidence from functional co-expression experiments and molecular dynamics that MlShak4 and MlShak5 do not co-assemble, suggesting that Mnemiopsis has at least two functionally independent N-type Shaker channels.

Pairwise biosynthesis of ion channels stabilizes excitability and mitigates arrhythmias

Coordinated expression of ion channels is crucial for cardiac rhythms, neural signaling, and cell cycle progression. Perturbation of this balance results in many disorders including cardiac arrhythmias. Prior work revealed association of mRNAs encoding cardiac NaV1.5 (SCN5A) and hERG1 (KCNH2), but the functional significance of this association was not established. Here, we provide a more comprehensive picture of KCNH2, SCN5A, CACNA1C, and KCNQ1 transcripts collectively copurifying with nascent hERG1, NaV1.5, CaV1.2, or KCNQ1 channel proteins. Single-molecule fluorescence in situ hybridization (smFISH) combined with immunofluorescence reveals that the channel proteins are synthesized predominantly as heterotypic pairs from discrete molecules of mRNA, not as larger cotranslational complexes. Puromycin disrupted colocalization of mRNA with its encoded protein, as expected, but remarkably also pairwise mRNA association, suggesting that transcript association relies on intact translational machinery or the presence of the nascent protein. Targeted depletion of KCHN2 by specific shRNA resulted in concomitant reduction of all associated mRNAs, with a corresponding reduction in the encoded channel currents. This co-knockdown effect, originally described for KCNH2 and SCN5A, thus appears to be a general phenomenon among transcripts encoding functionally related proteins. In multielectrode array recordings, proarrhythmic behavior arose when IKr was reduced by the selective blocker dofetilide at IC50 concentrations, but not when equivalent reductions were mediated by shRNA, suggesting that co-knockdown mitigates proarrhythmic behavior expected from the selective reduction of a single channel species. We propose that coordinated, cotranslational association of functionally related ion channel mRNAs confers electrical stability by co-regulating complementary ion channels in macromolecular complexes.

A CUG-initiated CATSPERθ functions in the CatSper channel assembly and serves as a checkpoint for flagellar trafficking

Calcium signaling is critical for successful fertilization. In spermatozoa, calcium influx into the sperm flagella mediated by the sperm-specific CatSper calcium channel is necessary for hyperactivated motility and male fertility. CatSper is a macromolecular complex and is repeatedly arranged in zigzag rows within four linear nanodomains along the sperm flagella. Here, we report that the Tmem249-encoded transmembrane (TM) domain-containing protein, CATSPERθ is essential for the CatSper channel assembly during sperm tail formation. CATSPERθ facilitates the channel assembly by serving as a scaffold for a pore-forming subunit CATSPER4. CATSPERθ is specifically localized at the interface of a CatSper dimer and can self-interact, suggesting its potential role in CatSper dimer formation. Male mice lacking CATSPERθ are infertile because the sperm lack the entire CatSper channel from sperm flagella, rendering sperm unable to hyperactivate, regardless of their normal expression in the testis. In contrast, genetic abrogation of any of the other CatSper TM subunits results in loss of CATSPERθ protein in the spermatid cells during spermatogenesis. CATSPERθ might act as a checkpoint for the properly assembled CatSper channel complex to traffic to sperm flagella. This study provides insights into the CatSper channel assembly and elucidates the physiological role of CATSPERθ in sperm motility and male fertility.

A CUG-initiated CATSPERθ functions in the CatSper channel assembly and serves as a checkpoint for flagellar trafficking

Calcium signaling is critical for successful fertilization. In spermatozoa, calcium influx into the sperm flagella mediated by the sperm specific CatSper calcium channel is necessary for hyperactivated motility and male fertility. CatSper is a macromolecular complex and is repeatedly arranged in zigzag rows within four linear nanodomains along the sperm flagella. Here, we report that the Tmem249 -encoded transmembrane domain containing protein, CATSPERθ, is essential for the CatSper channel assembly during sperm tail formation. CATSPERθ facilitates the channel assembly by serving as a scaffold for a pore forming subunit CATSPER4. CATSPERθ is specifically localized at the interface of a CatSper dimer and can self-interact, suggesting its potential role in CatSper dimer formation. Male mice lacking CATSPERθ are infertile because the sperm lack the entire CatSper channel from sperm flagella, rendering sperm unable to hyperactivate, regardless of their normal expression in the testis. In contrast, genetic abrogation of any of the other CatSper transmembrane subunits results in loss of CATSPERθ protein in the spermatid cells during spermatogenesis. CATSPERθ might acts as a checkpoint for the properly assembled CatSper channel complex to traffic to sperm flagella. This study provides insights into the CatSper channel assembly and elucidates the physiological role of CATSPERθ in sperm motility and male fertility.

A microtranslatome coordinately regulates sodium and potassium currents in the human heart

Catastrophic arrhythmias and sudden cardiac death can occur with even a small imbalance between inward sodium currents and outward potassium currents, but mechanisms establishing this critical balance are not understood. Here, we show that mRNA transcripts encoding I_Na and I_Kr channels (SCN5A and hERG, respectively) are associated in defined complexes during protein translation. Using biochemical, electrophysiological and single-molecule fluorescence localization approaches, we find that roughly half the hERG translational complexes contain SCN5A transcripts. Moreover, the transcripts are regulated in a way that alters functional expression of both channels at the membrane. Association and coordinate regulation of transcripts in discrete ‘microtranslatomes’ represents a new paradigm controlling electrical activity in heart and other excitable tissues.

Multimerization of Homo sapiens TRPA1 ion channel cytoplasmic domains

The transient receptor potential Ankyrin-1 (TRPA1) ion channel is modulated by myriad noxious stimuli that interact with multiple regions of the channel, including cysteine-reactive natural extracts from onion and garlic which modify residues in the cytoplasmic domains. The way in which TRPA1 cytoplasmic domain modification is coupled to opening of the ion-conducting pore has yet to be elucidated. The cryo-EM structure of TRPA1 revealed a tetrameric C-terminal coiled-coil surrounded by N-terminal ankyrin repeat domains (ARDs), an architecture shared with the canonical transient receptor potential (TRPC) ion channel family. Similarly, structures of the TRP melastatin (TRPM) ion channel family also showed a C-terminal coiled-coil surrounded by N-terminal cytoplasmic domains. This conserved architecture may indicate a common gating mechanism by which modification of cytoplasmic domains can transduce conformational changes to open the ion-conducting pore. We developed an in vitro system in which N-terminal ARDs and C-terminal coiled-coil domains can be expressed in bacteria and maintain the ability to interact. We tested three gating regulators: temperature; the polyphosphate compound IP6; and the covalent modifier allyl isothiocyanate to determine whether they alter N- and C-terminal interactions. We found that none of the modifiers tested abolished ARD-coiled-coil interactions, though there was a significant reduction at 37˚C. We found that coiled-coils tetramerize in a concentration dependent manner, with monomers and trimers observed at lower concentrations. Our system provides a method for examining the mechanism of oligomerization of TRPA1 cytoplasmic domains as well as a system to study the transmission of conformational changes resulting from covalent modification.

The S6 gate in regulatory Kv6 subunits restricts heteromeric K+ channel stoichiometry

Atypical substitutions in the S6 activation gate sequence distinguish “regulatory” Kv subunits, which cannot homotetramerize due to T1 self-incompatibility. Pisupati et al. show that such substitutions in Kv6 work together with self-incompatibility to restrict Kv2:Kv6 heteromeric stoichiometry to 3:1.

The Shaker-like family of voltage-gated K⁺ channels comprises four functionally independent gene subfamilies, Shaker (Kv1), Shab (Kv2), Shaw (Kv3), and Shal (Kv4), each of which regulates distinct aspects of neuronal excitability. Subfamily-specific assembly of tetrameric channels is mediated by the N-terminal T1 domain and segregates Kv1–4, allowing multiple channel types to function independently in the same cell. Typical Shaker-like Kv subunits can form functional channels as homotetramers, but a group of mammalian Kv2-related genes (Kv5.1, Kv6s, Kv8s, and Kv9s) encodes subunits that have a “silent” or “regulatory” phenotype characterized by T1 self-incompatibility. These channels are unable to form homotetramers, but instead heteromerize with Kv2.1 or Kv2.2 to diversify the functional properties of these delayed rectifiers. While T1 self-incompatibility predicts that these heterotetramers could contain up to two regulatory (R) subunits, experiments show a predominance of 3:1R stoichiometry in which heteromeric channels contain a single regulatory subunit. Substitution of the self-compatible Kv2.1 T1 domain into the regulatory subunit Kv6.4 does not alter the stoichiometry of Kv2.1:Kv6.4 heteromers. Here, to identify other channel structures that might be responsible for favoring the 3:1R stoichiometry, we compare the sequences of mammalian regulatory subunits to independently evolved regulatory subunits from cnidarians. The most widespread feature of regulatory subunits is the presence of atypical substitutions in the highly conserved consensus sequence of the intracellular S6 activation gate of the pore. We show that two amino acid substitutions in the S6 gate of the regulatory subunit Kv6.4 restrict the functional stoichiometry of Kv2.1:Kv6.4 to 3:1R by limiting the formation and function of 2:2R heteromers. We propose a two-step model for the evolution of the asymmetric 3:1R stoichiometry, which begins with evolution of self-incompatibility to establish the regulatory phenotype, followed by drift of the activation gate consensus sequence under relaxed selection to limit stoichiometry to 3:1R.

Transmural gradients in ion channel and auxiliary subunit expression

Evolution has acted to shape the action potential in different regions of the heart in order to produce a maximally stable and efficient pump. This has been achieved by creating regional differences in ion channel expression levels within the heart as well as differences between equivalent cardiac tissues in different species. These region- and species-dependent differences in channel expression are established by regulatory evolution, evolution of the regulatory mechanisms that control channel expression levels. Ion channel auxiliary subunits are obvious targets for regulatory evolution, in order to change channel expression levels and/or modify channel function. This review focuses on the transmural gradients of ion channel expression in the heart and the role that regulation of auxiliary subunit expression plays in generating and shaping these gradients.

A CaV2.1 N-terminal fragment relieves the dominant-negative inhibition by an Episodic ataxia 2 mutant

Episodic ataxia 2 (EA2) is an autosomal dominant disorder caused by mutations in the gene CACNA1A that encodes the pore-forming Ca_V2.1 calcium channel subunit. The majority of EA2 mutations reported so far are nonsense or deletion/insertion mutations predicted to form truncated proteins. Heterologous expression of wild-type Ca_V2.1, together with truncated constructs that mimic EA2 mutants, significantly suppressed wild-type calcium channel function, indicating that the truncated protein produces a dominant-negative effect (Jouvenceau et al., 2001; Page et al., 2004). A similar finding has been shown for Ca_V2.2 (Raghib et al., 2001). We show here that a highly conserved sequence in the cytoplasmic N-terminus is involved in this process, for both Ca_V2.1 and Ca_V2.2 channels. Additionally, we were able to interfere with the suppressive effect of an EA2 construct by mutating key N-terminal residues within it. We postulate that the N-terminus of the truncated channel plays an essential part in its interaction with the full-length Ca_V2.1, which prevents the correct folding of the wild-type channel. In agreement with this, we were able to disrupt the interaction between EA2 and the full length channel by co-expressing a free N-terminal peptide.

Cotranslational association of mRNA encoding subunits of heteromeric ion channels

Oligomers of homomeric voltage-gated potassium channels associate early in biogenesis as the nascent proteins emerge from the polysome. Less is known about how proteins emerging from different polysomes associate to form hetero-oligomeric channels. Here, we report that alternate mRNA transcripts encoding human ether-à-go-go-related gene (hERG) 1a and 1b subunits, which assemble to produce ion channels mediating cardiac repolarization, are physically associated during translation. We show that shRNA specifically targeting either hERG 1a or 1b transcripts reduced levels of both transcripts, but only when they were coexpressed heterologously. Both transcripts could be copurified with an Ab against the nascent hERG 1a N terminus. This interaction occurred even when translation of 1b was prevented, indicating the transcripts associate independent of their encoded proteins. The association was also demonstrated in cardiomyocytes, where levels of both hERG transcripts were reduced by either 1a or 1b shRNA, but native KCNE1 and ryanodine receptor 2 (RYR2) transcripts were unaffected. Changes in protein levels and membrane currents mirrored changes in transcript levels, indicating the targeted transcripts were undergoing translation. The physical association of transcripts encoding different subunits provides the spatial proximity required for nascent proteins to interact during biogenesis, and may represent a general mechanism facilitating assembly of heteromeric protein complexes involved in a range of biological processes.

Positive Allosteric Modulation of Kv Channels by Sevoflurane: Insights into the Structural Basis of Inhaled Anesthetic Action

Inhalational general anesthesia results from the poorly understood interactions of haloethers with multiple protein targets, which prominently includes ion channels in the nervous system. Previously, we reported that the commonly used inhaled anesthetic sevoflurane potentiates the activity of voltage-gated K⁺ (Kv) channels, specifically, several mammalian Kv1 channels and the Drosophila K-Shaw2 channel. Also, previous work suggested that the S4-S5 linker of K-Shaw2 plays a role in the inhibition of this Kv channel by n-alcohols and inhaled anesthetics. Here, we hypothesized that the S4-S5 linker is also a determinant of the potentiation of Kv1.2 and K-Shaw2 by sevoflurane. Following functional expression of these Kv channels in Xenopus oocytes, we found that converse mutations in Kv1.2 (G329T) and K-Shaw2 (T330G) dramatically enhance and inhibit the potentiation of the corresponding conductances by sevoflurane, respectively. Additionally, Kv1.2-G329T impairs voltage-dependent gating, which suggests that Kv1.2 modulation by sevoflurane is tied to gating in a state-dependent manner. Toward creating a minimal Kv1.2 structural model displaying the putative sevoflurane binding sites, we also found that the positive modulations of Kv1.2 and Kv1.2-G329T by sevoflurane and other general anesthetics are T1-independent. In contrast, the positive sevoflurane modulation of K-Shaw2 is T1-dependent. In silico docking and molecular dynamics-based free-energy calculations suggest that sevoflurane occupies distinct sites near the S4-S5 linker, the pore domain and around the external selectivity filter. We conclude that the positive allosteric modulation of the Kv channels by sevoflurane involves separable processes and multiple sites within regions intimately involved in channel gating.

Determinants of pore folding in potassium channel biogenesis

Many ion channels, both selective and nonselective, have reentrant pore loops that contribute to the architecture of the permeation pathway. It is a fundamental feature of these diverse channels, regardless of whether they are gated by changes of membrane potential or by neurotransmitters, and is critical to function of the channel. Misfolding of the pore loop leads to loss of trafficking and expression of these channels on the cell surface. Mature tetrameric potassium channels contain an α-helix within the pore loop. We systematically mutated the "pore helix" residues of the channel Kv1.3 and assessed the ability of the monomer to fold into a tertiary reentrant loop. Our results show that pore loop residues form a canonical α-helix in the monomer early in biogenesis and that disruption of tertiary folding is caused by hydrophilic substitutions only along one face of this α-helix. These results provide insight into the determinants of the reentrant pore conformation, which is essential for ion channel function.

Social networking among voltage-activated potassium channels

Voltage-activated potassium channels (Kv channels) are ubiquitously expressed proteins that subserve a wide range of cellular functions. From their birth in the endoplasmic reticulum, Kv channels assemble from multiple subunits in complex ways that determine where they live in the cell, their biophysical characteristics, and their role in enabling different kinds of cells to respond to specific environmental signals to generate appropriate functional responses. This chapter describes the types of protein-protein interactions among pore-forming channel subunits and their auxiliary protein partners, as well as posttranslational protein modifications that occur in various cell types. This complex oligomerization of channel subunits establishes precise cell type-specific Kv channel localization and function, which in turn drives a diverse range of cellular signal transduction mechanisms uniquely suited to the physiological contexts in which they are found.

Dynamic of Ion Channel Expression at the Plasma Membrane of Cardiomyocytes

Cardiac myocytes are characterized by distinct structural and functional entities involved in the generation and transmission of the action potential and the excitation-contraction coupling process. Key to their function is the specific organization of ion channels and transporters to and within distinct membrane domains, which supports the anisotropic propagation of the depolarization wave. This review addresses the current knowledge on the molecular actors regulating the distinct trafficking and targeting mechanisms of ion channels in the highly polarized cardiac myocyte. In addition to ubiquitous mechanisms shared by other excitable cells, cardiac myocytes show unique specialization, illustrated by the molecular organization of myocyte-myocyte contacts, e.g., the intercalated disc and the gap junction. Many factors contribute to the specialization of the cardiac sarcolemma and the functional expression of cardiac ion channels, including various anchoring proteins, motors, small GTPases, membrane lipids, and cholesterol. The discovery of genetic defects in some of these actors, leading to complex cardiac disorders, emphasizes the importance of trafficking and targeting of ion channels to cardiac function. A major challenge in the field is to understand how these and other actors work together in intact myocytes to fine-tune ion channel expression and control cardiac excitability.

KCNE Regulation of K(+) Channel Trafficking - a Sisyphean Task?

Voltage-gated potassium (Kv) channels shape the action potentials of excitable cells and regulate membrane potential and ion homeostasis in excitable and non-excitable cells. With 40 known members in the human genome and a variety of homomeric and heteromeric pore-forming α subunit interactions, post-translational modifications, cellular locations, and expression patterns, the functional repertoire of the Kv α subunit family is monumental. This versatility is amplified by a host of interacting proteins, including the single membrane-spanning KCNE ancillary subunits. Here, examining both the secretory and the endocytic pathways, we review recent findings illustrating the surprising virtuosity of the KCNE proteins in orchestrating not just the function, but also the composition, diaspora and retrieval of channels formed by their Kv α subunit partners.

Subunit-dependent axonal trafficking of distinct alpha heteromeric potassium channel complexes

Voltage-gated potassium (Kv) channels are critical for neuronal excitability and are targeted to specific subcellular compartments to carry out their unique functions. While it is widely believed that Kv channels exist as heteromeric complexes in neurons, direct tests of the hypothesis that specific heteromeric channel populations display divergent spatial and temporal dynamics are limited. Using a bimolecular fluorescence complementation approach, we monitored the assembly and localization of cell surface channel complexes in living cells. While PSD95-mediated clustering was subunit independent, selective visualization of heteromeric Kv complexes in rat hippocampal neurons revealed subunit-dependent localization that was not predicted by analyzing individual subunits. Assembly of Kv1.1 with Kv1.4 prevented axonal localization but not surface expression, while inclusion of Kv1.2 imparted clustering at presynaptic sites and decreased channel mobility within the axon. This mechanism by which specific Kv channel subunits can act in a dominant manner to impose unique trafficking properties to heteromeric complexes extended to Shab-related family of Kv channels. When coexpressed, Kv2.1 and Kv2.2 heteromultimers did not aggregate in somatodendritic clusters observed with expression of Kv2.1 alone. These studies demonstrate selective axonal trafficking and surface localization of distinct Kv channels based on their subunit composition.

Unravelling the folding and stability of an ABC (ATP-binding cassette) transporter

Prokaryotic importers from the large family of ABC (ATP-binding cassette) transporters comprise four separate subunits: two membrane-embedded and two cytoplasmic ATP-binding subunits. This modular construction makes them ideal candidates for studies of the intersubunit interactions of membrane protein complexes that contain both hydrophobic and hydrophilic subunits. In the present paper, we focus on the vitamin B12 importer of Escherichia coli, BtuCD, that contains two transmembrane BtuC subunits and two ATP-binding BtuD subunits. We have studied the factors that induce subunit dissociation and unfolding in vitro. The BtuCD complex remains intact in alcohol and mild detergents, but urea or SDS separate the BtuC and BtuD subunits, with 6 M urea causing 80% of BtuD to be removed from BtuCD. ATP is found to stabilize the complex as a result of its binding to the BtuD subunits. In the absence of ATP, low concentrations of urea (0.5-3 M) also induce some unfolding, with approximately 14% reduction in helicity in 3 M urea, whereas, in the presence of ATP, no changes are observed. Disassembly at the BtuD-BtuD dimeric interface in BtuCD can be achieved with smaller concentrations of urea (0.5-3 M) than that required to cause disassembly at the BtuC-BtuD transmission interface (3-8 M), suggesting a stronger interaction of the latter. The results also suggest that unfolding and disassociation of subunits appear to be coupled processes. Our work provides insights into the subunit interactions of an ABC transporter and lays the foundation for studies of the reassembly of BtuCD.

Random insertion of split-cans of the fluorescent protein venus into Shaker channels yields voltage sensitive probes with improved membrane localization in mammalian cells

FlaSh-YFP, a fluorescent protein (FP) voltage sensor that is a fusion of the Shaker potassium channel with yellow fluorescent protein (YFP), is primarily expressed in the endoplasmic reticulum (ER) of mammalian cells, possibly due to misfolded monomers. In an effort to improve plasma membrane expression, the FP was split into two non-fluorescent halves. Each half was randomly inserted into Shaker monomers via a transposon reaction. Shaker subunits containing the 5' half were co-expressed with Shaker subunits containing the 3' half. Tetramerization of Shaker subunits is required for re-conjugation of the FP. The misfolded monomers trapped in ER are unlikely to tetramerize and reconstitute the beta-can structure, and thus intracellular fluorescence might be reduced. This split-can transposon approach yielded 56 fluorescent probes, 30 (54%) of which were expressed at the plasma membrane and were capable of optically reporting changes in membrane potential. The largest signal from these novel FP-sensors was a -1.4% in ΔF/F for a 100 mV depolarization, with on time constants of about 15 ms and off time constants of about 200 ms. This split-can transposon approach has the potential to improve other multimeric probes.

Ion channel trafficking: a new therapeutic horizon for atrial fibrillation

Atrial fibrillation (AF) is a common cardiac arrhythmia with potentially life-threatening complications. Drug therapies for treatment of AF that seek long-term maintenance of normal sinus rhythm remain elusive due in large part to proarrhythmic ventricular actions. Kv1.5, which underlies the atrial specific I(Kur) current, is a major focus of research efforts seeking new therapeutic strategies and targets. Recent work has shown a novel effect of antiarrhythmic drugs where compounds that block Kv1.5 channel current also can alter ion channel trafficking. This work further suggests that the pleiotropic effects of antiarrhythmic drugs may be separable. Although this finding highlights the therapeutic potential for selective manipulation of ion channel surface density, it also reveals an uncertainty regarding the specificity of modulating trafficking pathways without risk of off-target effects. Future studies may show that specific alteration of Kv1.5 trafficking can overcome the proarrhythmic limitations of current pharmacotherapy and provide an effective method for long-term cardioversion in AF.

Fast inactivation in potassium channels: an interplay of cytoplasmic domains

Fast inactivation in voltage-gated potassium channels has traditionally been associated exclusively with the N-terminus. Here, we explore the role of the T1 domain using a series of chimeric channels. A chimeric channel, 4N/2, (N-terminus from the rapidly inactivating hKv1.4, and the channel body from the non-inactivating hKv1.2), exhibited slower and incomplete inactivation as compared to the wild-type hKv1.4. Replacing the T1 domain of 4N2 with that from hKv1.2 (4N/2T1/2), restored inactivation, while that from hKv1.1 (4N/1T1/2) completely abolished inactivation. Based on these observations, we hypothesize a correlation between the tetramerization domain and the putative inactivation domain receptor in the process of rapid inactivation of hKv1 channels.

Novel mechanisms of trafficking defect caused by KCNQ1 mutations found in long QT syndrome

Long QT syndrome (LQTS) is a hereditary arrhythmia caused by mutations in genes for cardiac ion channels, including a potassium channel, KvLQT1. Inheritance of LQTS is usually autosomal-dominant, but autosomal-recessive inheritance can be observed in patients with LQTS accompanied by hearing loss. In this study, we investigated the functional alterations caused by KCNQ1 mutations, a deletion (delV595) and a frameshift (P631fs/19), which were identified in compound heterozygous state in two patients with autosomal-recessive LQTS not accompanied by hearing loss. Functional analyses showed that both mutations impaired cell surface expression due to trafficking defects. The mutations severely affected outward potassium currents without apparent dominant negative effects. It was found that delV595 impaired subunit binding, whereas P631fs/19 was retained in endoplasmic reticulum due to the newly added 19-amino acid sequence containing two retention motifs (R(633)GR and R(646)LR). This is the first report of novel mechanisms for trafficking abnormality of cardiac ion channels, providing us new insights into the molecular mechanisms of LQTS.

Tertiary interactions within the ribosomal exit tunnel

Although tertiary folding of whole protein domains is prohibited by the cramped dimensions of the ribosomal tunnel, dynamic tertiary interactions may permit folding of small elementary units within the tunnel. To probe this possibility, we used a β-hairpin as well as an α-helical hairpin from the cytosolic N-terminus of a voltage-gated potassium channel and determined a probability of folding for each at defined locations inside and outside the tunnel. Minimalist tertiary structures can form near the exit port of the tunnel, a region that provides an entropic window for initial exploration of local peptide conformations. Tertiary subdomains of the nascent peptide fold sequentially, but not independently, during translation. These studies offer an approach for diagnosing the molecular basis for folding defects that lead to protein malfunction and provide insight into the role of the ribosome during early potassium channel biogenesis.

PPTX, a pentraxin domain-containing protein, interacts with the T1 domain of K v 4

Voltage-gated K(+) (K(v)) channels reside as tetramers in the membrane. The events that coordinate folding, trafficking, and tetramerization are mediated by an array of associated proteins and phospholipids whose identification is vital to understanding the dynamic nature of channel expression and activity. An interaction between an A-type K(+) channel, K(v)4.2, and a protein containing a pentraxin domain (PPTX) was demonstrated in the cochlea (Duzhyy et al. [ 2005] J. Biol. Chem. 280:15165-15172). Here, we present results based on fold recognition and homology modeling that revealed the tetramerization (T1) domain of K(v)4.2 as a potential docking site for interacting proteins such as PPTX. By using this model, putative sites were experimentally tested with the yeast two-hybrid system to assay interactions between PPTX and the T1 domain of K(v)4.2 wild type (wt) and mutants (mut). Results showed that amino acid residues 86 and 118 in the T1 domain are essential for interaction, because replacing these negatively charged with neutrally charged amino acids inhibits interactions. Cotransfections of Chinese hamster ovary cells with PPTX and K(v)4.2wt further revealed that PPTX increases K(v)4.2 wt expression in vitro when analyzing total lysates, whereas interactions with K(v)4.2 microt resulted in a decrease. These studies suggest that portions of the T1 domain can act as docking sites for proteins such as PPTX, further underscoring the significance of this domain.

Mutation of histidine 105 in the T1 domain of the potassium channel Kv2.1 disrupts heteromerization with Kv6.3 and Kv6.4

The voltage-activated K(+) channel subunit Kv2.1 can form heterotetramers with members of the Kv6 subfamily, generating channels with biophysical properties different from homomeric Kv2.1 channels. The N-terminal tetramerization domain (T1) has been shown previously to play a role in Kv channel assembly, but the mechanisms controlling specific heteromeric assembly are still unclear. In Kv6.x channels the histidine residue of the zinc ion-coordinating C3H1 motif of Kv2.1 is replaced by arginine or valine. Using a yeast two-hybrid assay, we found that substitution of the corresponding histidine 105 in Kv2.1 by valine (H105V) or arginine (H105R) disrupted the interaction of the T1 domain of Kv2.1 with the T1 domains of both Kv6.3 and Kv6.4, whereas interaction of the T1 domain of Kv2.1 with itself was unaffected by this mutation. Using fluorescence resonance energy transfer (FRET), interaction could be detected between the subunits Kv2.1/Kv2.1, Kv2.1/Kv6.3, and Kv2.1/Kv6.4. Reduced FRET signals were obtained after co-expression of Kv2.1(H105V) or Kv2.1(H105R) with Kv6.3 or Kv6.4. Wild-type Kv2.1 but not Kv2.1(H105V) could be co-immunoprecipitated with Kv6.4. Co-expression of dominant-negative mutants of Kv6.3 reduced the current produced Kv2.1, but not of Kv2.1(H105R) mutants. Co-expression of Kv6.3 or Kv6.4 with wt Kv2.1 but not with Kv2.1(H105V) or Kv2.1(H105R) changed the voltage dependence of activation of the channels. Our results suggest that His-105 in the T1 domain of Kv2.1 is required for functional heteromerization with members of the Kv6 subfamily. We conclude from our findings that Kv2.1 and Kv6.x subunits have complementary T1 domains that control selective heteromerization.

Structure, function, and modification of the voltage sensor in voltage-gated ion channels

Voltage-gated ion channels are crucial for both neuronal and cardiac excitability. Decades of research have begun to unravel the intriguing machinery behind voltage sensitivity. Although the details regarding the arrangement and movement in the voltage-sensor domain are still debated, consensus is slowly emerging. There are three competing conceptual models: the helical-screw, the transporter, and the paddle model. In this review we explore the structure of the activated voltage-sensor domain based on the recent X-ray structure of a chimera between Kv1.2 and Kv2.1. We also present a model for the closed state. From this we conclude that upon depolarization the voltage sensor S4 moves approximately 13 A outwards and rotates approximately 180 degrees, thus consistent with the helical-screw model. S4 also moves relative to S3b which is not consistent with the paddle model. One interesting feature of the voltage sensor is that it partially faces the lipid bilayer and therefore can interact both with the membrane itself and with physiological and pharmacological molecules reaching the channel from the membrane. This type of channel modulation is discussed together with other mechanisms for how voltage-sensitivity is modified. Small effects on voltage-sensitivity can have profound effects on excitability. Therefore, medical drugs designed to alter the voltage dependence offer an interesting way to regulate excitability.

Localization of Kv2.2 protein in Xenopus laevis embryos and tadpoles

Voltage-gated potassium (Kv) channels sculpt neuronal excitability and play important developmental roles. Kv channels consist of pore-forming alpha- and auxiliary subunits. For many Kv alpha-subunits, existing mRNA probes and antibodies have allowed analysis of expression patterns, typically during adult stages. Here, we focus on the Kv2.2 alpha-subunit, for which the mRNA shows broad expression in the embryo and adult. A lack of suitable antibodies, however, has hindered detailed analysis of Kv2.2 protein localization, especially during development. We developed an antibody that specifically recognizes Kv2.2 protein in Xenopus laevis, a vertebrate well suited for study of early developmental stages. The Kv2.2 antibody recognized heterologously expressed Kv2.2 but not the closely related Kv2.1 protein. Immunodetection of the protein showed its presence at St 32 in ventrolateral regions of the hindbrain and spinal cord. At later stages, several sensory tissues (retina, otic, and olfactory epithelia) also expressed Kv2.2 protein. As development progressed in the central nervous system, Kv2.2 protein distribution expanded in close association with the cytoskeletal marker alpha-tubulin, consistent with growth of neuronal tracts. We analyzed the subcellular distribution of Kv2.2 protein within single cultured neurons. In addition to a surface membrane presence, Kv2.2 protein also resided intracellularly closely associated with alpha-tubulin, as in vivo. Furthermore, in contrast to Kv2.1, Kv2.2 protein localized to long, axonal-like processes, consistent with its in vivo location in tracts. Despite their primary sequence similarity, the contrasting localizations of Kv2.1 and Kv2.2 support different roles for the two during development and neuronal signaling.

An overview of trafficking and assembly of neurotransmitter receptors and ion channels (Review)

Ionotropic neurotransmitter receptors and voltage-gated ion channels assemble from several homologous and non-homologous subunits. Assembly of these multimeric membrane proteins is a tightly controlled process subject to primary and secondary quality control mechanisms. An assembly pathway involving a dimerization of dimers has been demonstrated for a voltage-gated potassium channel and for different types of glutamate receptors. While many novel C-terminal assembly domains have been identified in various members of the voltage-gated cation channel superfamily, the assembly pathways followed by these proteins remain largely elusive. Recent progress on the recognition of polar residues in the transmembrane segments of membrane proteins by the retrieval factor Rer1 is likely to be relevant for the further investigation of trafficking defects in channelopathies. This mechanism might also contribute to controlling the assembly of ion channels by retrieving unassembled subunits to the endoplasmic reticulum. The endoplasmic reticulum is a metabolic compartment studded with small molecule transporters. This environment provides ligands that have recently been shown to act as pharmacological chaperones in the biogenesis of ligand-gated ion channels. Future progress depends on the improvement of tools, in particular the antibodies used by the field, and the continued exploitation of genetically tractable model organisms in screens and physiological experiments.

The neuronal Kv4 channel complex

Kv4 channel complexes mediate the neuronal somatodendritic A-type K(+) current (I(SA)), which plays pivotal roles in dendritic signal integration. These complexes are composed of pore-forming voltage-gated alpha-subunits (Shal/Kv4) and at least two classes of auxiliary beta-subunits: KChIPs (K(+)-Channel-Interacting-Proteins) and DPLPs (Dipeptidyl-Peptidase-Like-Proteins). Here, we review our investigations of Kv4 gating mechanisms and functional remodeling by specific auxiliary beta-subunits. Namely, we have concluded that: (1) the Kv4 channel complex employs novel alternative mechanisms of closed-state inactivation; (2) the intracellular Zn(2+) site in the T1 domain undergoes a conformational change tightly coupled to voltage-dependent gating and is targeted by nitrosative modulation; and (3) discrete and specific interactions mediate the effects of KChIPs and DPLPs on activation, inactivation and permeation of Kv4 channels. These studies are shedding new light on the molecular bases of I(SA) function and regulation.

Localization and trafficking of cardiac voltage-gated potassium channels

The proper trafficking and localization of cardiac potassium channels is profoundly important to the regulation of the regionally distinct action potentials across the myocardium. These processes are only beginning to be unravelled and involve modulators of channel synthesis and assembly, post-translational processing, various molecular motors and an increasing number of modifying enzymes and molecular anchors. The roles of anchoring proteins, molecular motors and kinases are explored and recent findings on channel internalization and trafficking are presented.

Mg2+ mediates interaction between the voltage sensor and cytosolic domain to activate BK channels

The voltage-sensor domain (VSD) of voltage-dependent ion channels and enzymes is critical for cellular responses to membrane potential. The VSD can also be regulated by interaction with intracellular proteins and ligands, but how this occurs is poorly understood. Here, we show that the VSD of the BK-type K(+) channel is regulated by a state-dependent interaction with its own tethered cytosolic domain that depends on both intracellular Mg(2+) and the open state of the channel pore. Mg(2+) bound to the cytosolic RCK1 domain enhances VSD activation by electrostatic interaction with Arg-213 in transmembrane segment S4. Our results demonstrate that a cytosolic domain can come close enough to the VSD to regulate its activity electrostatically, thereby elucidating a mechanism of Mg(2+)-dependent activation in BK channels and suggesting a general pathway by which intracellular factors can modulate the function of voltage-dependent proteins.

A novel tripartite motif involved in aquaporin topogenesis, monomer folding and tetramerization

Aquaporin (AQP) folding in the endoplasmic reticulum is characterized by two distinct pathways of membrane insertion that arise from divergent residues within the second transmembrane segment. We now show that in AQP1 these residues (Asn49 and Lys51) interact with Asp185 at the C terminus of TM5 to form a polar, quaternary structural motif that influences multiple stages of folding. Asn49 and Asp185 form an intramolecular hydrogen bond needed for proper helical packing, monomer formation and function. In contrast, Lys51 interacts with Asp185 on an adjacent monomer to stabilize the AQP1 tetramer. Although these residues are unique to AQP1, they share a highly conserved architecture whose functional properties can be transferred to other family members. These findings suggest a general mechanism by which evolutionary divergence of membrane proteins can confer new functional properties via alternative folding pathways that give rise to a common final structure.

Mapping the electrostatic potential within the ribosomal exit tunnel

Electrostatic potentials influence interactions among proteins and nucleic acids, the orientation of dipoles and quadrupoles, and the distribution of mobile charges. Consequently, electrostatic potentials can modulate macromolecular folding and conformational stability, as well as rates of catalysis and substrate binding. The ribosomal exit tunnel, along with its resident nascent peptide, is no less susceptible to these consequences. Yet, the electrostatics inside the tunnel have never been measured. Here we map both the electrostatic potential and accessibilities along the length of the tunnel and determine the electrostatic consequences of introducing a charged amino acid into the nascent peptide. To do this we developed novel probes and strategies. Our findings provide new insights regarding the dielectric of the tunnel and the dynamics of its local electric fields.

Engineering and characterization of an enhanced fluorescent protein voltage sensor

Background

Fluorescent proteins have been used to generate a variety of biosensors to optically monitor biological phenomena in living cells. Among this class of genetically encoded biosensors, reporters for membrane potential have been a particular challenge. The use of presently known voltage sensor proteins is limited by incorrect subcellular localization and small or absent voltage responses in mammalian cells.

Results

Here we report on a fluorescent protein voltage sensor with superior targeting to the mammalian plasma membrane and high responsiveness to membrane potential signaling in excitable cells.

Conclusions and Significance

This biosensor, which we termed VSFP2.1, is likely to lead to new methods of monitoring electrically active cells with cell type specificity, non-invasively and in large numbers, simultaneously.

AMPA receptor biogenesis and trafficking

AMPA-type glutamate receptors mediate the majority of fast excitatory transmission in the central nervous system. The trafficking of AMPA receptors to and from synapses alters synaptic strength and has been recognized as a central mechanism underlying various forms of synaptic plasticity. Both secretory and endocytic trafficking events seem to be driven by the subunit composition of AMPA receptor tetramers. Moreover, recent work suggests that synapses employ different tetramer combinations in response to altered synaptic input, suggesting the existence of signalling pathways that mediate remodelling of AMPA receptors. These latest developments and recent progress in elucidating the mechanisms that underlie channel assembly and trafficking are the subject of this review.

Mechanisms of cardiac potassium channel trafficking

The regulation of ion channels involves more than just modulation of their synthesis and kinetics, as controls on their trafficking and localization are also important. Although the body of knowledge is fairly large, the entire trafficking pathway is not known for any one channel. This review summarizes current knowledge on the trafficking of potassium channels that are expressed in the heart. Our knowledge of channel assembly, trafficking through the Golgi apparatus and on to the surface is covered, as are controls on channel surface retention and endocytosis.

Sulfonylureas correct trafficking defects of disease-causing ATP-sensitive potassium channels by binding to the channel complex

ATP-sensitive potassium (K(ATP)) channels mediate glucose-induced insulin secretion by coupling metabolic signals to beta-cell membrane potential and the secretory machinery. Reduced K(ATP) channel expression caused by mutations in the channel proteins: sulfonylurea receptor 1 (SUR1) and Kir6.2, results in loss of channel function as seen in congenital hyperinsulinism. Previously, we reported that sulfonylureas, oral hypoglycemic drugs widely used to treat type II diabetes, correct the endoplasmic reticulum to the plasma membrane trafficking defect caused by two SUR1 mutations, A116P and V187D. In this study, we investigated the mechanism by which sulfonylureas rescue these mutants. We found that glinides, another class of SUR-binding hypoglycemic drugs, also markedly increased surface expression of the trafficking mutants. Attenuating or abolishing the ability of mutant SUR1 to bind sulfonylureas or glinides by the following mutations: Y230A, S1238Y, or both, accordingly diminished the rescuing effects of the drugs. Interestingly, rescue of the trafficking defects requires mutant SUR1 to be co-expressed with Kir6.2, suggesting that the channel complex, rather than SUR1 alone, is the drug target. Observations that sulfonylureas also reverse trafficking defects caused by neonatal diabetes-associated Kir6.2 mutations in a way that is dependent on intact sulfonylurea binding sites in SUR1 further support this notion. Our results provide insight into the mechanistic and structural basis on which sulfonylureas rescue K(ATP) channel surface expression defects caused by channel mutations.

Coiled coils direct assembly of a cold-activated TRP channel

Transient receptor potential (TRP) channels mediate numerous sensory transduction processes and are thought to function as tetramers. TRP channel physiology is well studied; however, comparatively little is understood regarding TRP channel assembly. Here, we identify an autonomously folded assembly domain from the cold- and menthol-gated channel TRPM8. We show that the TRPM8 cytoplasmic C-terminal domain contains a coiled coil that is necessary for channel assembly and sufficient for tetramer formation. Cell biological experiments indicate that coiled-coil formation is required for proper channel maturation and trafficking and that the coiled-coil domain alone can act as a dominant-negative inhibitor of functional channel expression. Our data define an authentic TRP modular assembly domain, establish a clear role for coiled coils in ion channel assembly, demonstrate that coiled-coil assembly domains are a general feature of TRPM channels, and delineate a new tool that should be of general use in dissecting TRPM channel function.

Voltage-dependent gating rearrangements in the intracellular T1-T1 interface of a K+ channel

The intracellular tetramerization domain (T1) of most eukaryotic voltage-gated potassium channels (Kv channels) exists as a “hanging gondola” below the transmembrane regions that directly control activation gating via the electromechanical coupling between the S4 voltage sensor and the main S6 gate. However, much less is known about the putative contribution of the T1 domain to Kv channel gating. This possibility is mechanistically intriguing because the T1–S1 linker connects the T1 domain to the voltage-sensing domain. Previously, we demonstrated that thiol-specific reagents inhibit Kv4.1 channels by reacting in a state-dependent manner with native Zn²⁺ site thiolate groups in the T1–T1 interface; therefore, we concluded that the T1–T1 interface is functionally active and not protected by Zn²⁺ (Wang, G., M. Shahidullah, C.A. Rocha, C. Strang, P.J. Pfaffinger, and M. Covarrubias. 2005. J. Gen. Physiol. 126:55–69). Here, we co-expressed Kv4.1 channels and auxiliary subunits (KChIP-1 and DPPX-S) to investigate the state and voltage dependence of the accessibility of MTSET to the three interfacial cysteines in the T1 domain. The results showed that the average MTSET modification rate constant (k_MTSET) is dramatically enhanced in the activated state relative to the resting and inactivated states (∼260- and ∼47-fold, respectively). Crucially, under three separate conditions that produce distinct activation profiles, k_MTSET is steeply voltage dependent in a manner that is precisely correlated with the peak conductance–voltage relations. These observations strongly suggest that Kv4 channel gating is tightly coupled to voltage-dependent accessibility changes of native T1 cysteines in the intersubunit Zn²⁺ site. Furthermore, cross-linking of cysteine pairs across the T1–T1 interface induced substantial inhibition of the channel, which supports the functionally dynamic role of T1 in channel gating. Therefore, we conclude that the complex voltage-dependent gating rearrangements of eukaryotic Kv channels are not limited to the membrane-spanning core but must include the intracellular T1–T1 interface. Oxidative stress in excitable tissues may perturb this interface to modulate Kv4 channel function.

Electrophysiological characterization of three non-synonymous single nucleotide polymorphisms (R87Q, A251T, and P307S) found in hKv1.5

Non-synonymous single nucleotide polymorphisms (SNPs) in the KCNA5/hKv1.5 gene, which encodes for a voltage-gated K+ channel responsible for the I (Kur) current in the human atria, have been recently reported. To gain further knowledge on potential influence of hKv1.5 SNPs, we searched for their presence in a specific population of 96 French-Canadians and characterized electrophysiological properties of the variants in two cell lines. The presumed promoter (-83 bp) and coding regions were sequenced. We found three heterozygous SNPs: R87Q, A251T, and P307S. Functional analysis of SNPs transfected in Chinese hamster ovary (CHO) cells showed that both R87Q and P307S diminished the inactivation amplitude (e.g., at +60 mV, amplitudes were 89+/-26, 23+/-4, and 22+/-7 pA/pF for the wild type, R87Q and P307S, respectively; n=8, 6, and 8, respectively). Inactivation was slowed with these variants (e.g., tau (fast) at +50 mV were 270+/-48, 490+/-66, and 340+/-45 ms for the wild type, R87Q, and P307S, respectively) while R87Q additionally accelerated the rate of hKv1.5 channel opening. A dominant-negative effect was observed for R87Q but not for P307S. SNPs properties were not reproduced when expressed in the HEK293 cell line, suggesting that the regulatory beta-subunit present in CHO cells (and the human heart) is essential for the SNPs effects that we have observed.

Folding zones inside the ribosomal exit tunnel

Helicity of membrane proteins can be manifested inside the ribosome tunnel, but the determinants of compact structure formation inside the tunnel are largely unexplored. Using an extended nascent peptide as a molecular tape measure of the ribosomal tunnel, we have previously demonstrated helix formation inside the tunnel. Here, we introduce a series of consecutive polyalanines into different regions of the tape measure to monitor the formation of compact structure in the nascent peptide. We find that the formation of compact structure of the polyalanine sequence depends on its location. Calculation of free energies for the equilibria between folded and unfolded nascent peptides in different regions of the tunnel shows that there are zones of secondary structure formation inside the ribosomal exit tunnel. These zones may have an active role in nascent-chain compaction.

Molecular diversity and function of voltage-gated (Kv) potassium channels in epithelial cells

Voltage-gated K+ channels belonging to Kv1-9 subfamilies are widely expressed in excitable cells where they play an essential role in membrane hyperpolarization during an action potential and in the propagation of action potentials along the plasma membrane. Early patch clamp studies on epithelial cells revealed the presence of K+ currents with biophysical and pharmacologic properties characteristic of Kv channels expressed in excitable cells. More recently, molecular approaches including PCR and the availability of more selective antibodies directed against Kv alpha and auxiliary subunits, have demonstrated that epithelial cells from various organ systems, express a remarkable diversity Kv channel subunits. Unlike neurons and myocytes however, epithelial cells do not typically generate action potentials or exhibit dynamic changes in membrane potential necessary for activation of Kv alpha subunits. Moreover, the fact that many Kv channels expressed in epithelial cells exhibit inactivation suggest that their activities are relatively transient, making it difficult to ascribe a functional role for these channels in transepithelial electrolyte or nutrient transport. Other proposed functions have included (i) cell migration and wound healing, (ii) cell proliferation and cancer, (iii) apoptosis and (iv) O2 sensing. Certain Kv channels, particularly Kv1 and Kv2 subfamily members, have been shown to be involved in the proliferation of prostate, colon, lung and breast carcinomas. In some instances, a significant increase in Kv channel expression has been correlated with tumorogenesis suggesting the possibility of using these proteins as markers for transformation and perhaps reducing the rate of tumor growth by selectively inhibiting their functional activity.