Publications

Publications

  • Fluorescent Visualization of Cellular Proton Fluxes.

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    Fluorescent Visualization of Cellular Proton Fluxes.

    Cell Chem Biol. 2016 Nov 14;:

    Authors: Zhang L, Bellve K, Fogarty K, Kobertz WR

    Abstract
    Cells use plasma membrane proton fluxes to maintain cytoplasmic and extracellular pH and to mediate the co-transport of metabolites and ions. Because proton-coupled transport often involves movement of multiple substrates, traditional electrical measurements provide limited information about proton transport at the cell surface. Here we visualize voltage-dependent proton fluxes over the entire landscape of a cell by covalently attaching small-molecule fluorescent pH sensors to the cell's glycocalyx. We found that the extracellularly facing sensors enable real-time detection of proton accumulation and depletion at the plasma membrane, providing an indirect readout of channel and transporter activity that correlated with whole-cell proton current. Moreover, the proton wavefront emanating from one cell was readily visible as it crossed over nearby cells. Given that any small-molecule fluorescent sensor can be covalently attached to a cell's glycocalyx, our approach is readily adaptable to visualize most electrogenic and non-electrogenic transport events at the plasma membrane.

    PMID: 27916567 [PubMed - as supplied by publisher]

  • Mutant SOD1 protein increases Nav1.3 channel excitability.

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    Mutant SOD1 protein increases Nav1.3 channel excitability.

    J Biol Phys. 2016 Jun;42(3):351-70

    Authors: Kubat Öktem E, Mruk K, Chang J, Akin A, Kobertz WR, Brown RH

    Abstract
    Amyotrophic lateral sclerosis (ALS) is a lethal paralytic disease caused by the degeneration of motor neurons in the spinal cord, brain stem, and motor cortex. Mutations in the gene encoding copper/zinc superoxide dismutase (SOD1) are present in ~20% of familial ALS and ~2% of all ALS cases. The most common SOD1 gene mutation in North America is a missense mutation substituting valine for alanine (A4V). In this study, we analyze sodium channel currents in oocytes expressing either wild-type or mutant (A4V) SOD1 protein. We demonstrate that the A4V mutation confers a propensity to hyperexcitability on a voltage-dependent sodium channel (Nav1.3) mediated by heightened total Na(+) conductance and a hyperpolarizing shift in the voltage dependence of Nav1.3 activation. To estimate the impact of these channel effects on excitability in an intact neuron, we simulated these changes in the program NEURON; this shows that the changes induced by mutant SOD1 increase the spontaneous firing frequency of the simulated neuron. These findings are consistent with the view that excessive excitability of neurons is one component in the pathogenesis of this disease.

    PMID: 27072680 [PubMed - in process]

  • Bioreactive Tethers.

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    Bioreactive Tethers.

    Adv Exp Med Biol. 2015;869:77-100

    Authors: Mruk K, Kobertz WR

    Abstract
    Ion channel complexes are challenging to study by traditional biochemical methods due to their membranous lipid environment and large size. Bioreactive tethers are specialized chemical probes that have been used in electrophysiological experiments to provide unique insight into ion channel structure and function. Because bioreactive tethers are small molecular probes, they can be used to manipulate ion channel function in heterologous expression systems, native cells and animal models. This chapter covers three classes of tethers: photoswitchable, molecular rulers, and chemically reactive. The modular nature of bioreactive tethers enables the facile synthesis of next generation reagents with enhanced functionalities to interrogate and control ion channels in novel and multifarious ways.

    PMID: 26381941 [PubMed - indexed for MEDLINE]

  • LQT1 mutations in KCNQ1 C-terminus assembly domain suppress IKs using different mechanisms.

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    LQT1 mutations in KCNQ1 C-terminus assembly domain suppress IKs using different mechanisms.

    Cardiovasc Res. 2014 Dec 01;104(3):501-11

    Authors: Aromolaran AS, Subramanyam P, Chang DD, Kobertz WR, Colecraft HM

    Abstract
    AIMS: Long QT syndrome 1 (LQT1) mutations in KCNQ1 that decrease cardiac IKs (slowly activating delayed rectifier K(+) current) underlie ventricular arrhythmias and sudden death. LQT1 mutations may suppress IKs by preventing KCNQ1 assembly, disrupting surface trafficking, or inhibiting gating. We investigated mechanisms underlying how three LQT1 mutations in KCNQ1 C-terminus assembly domain (R555H/G589D/L619M) decrease IKs in heterologous cells and cardiomyocytes.
    METHODS AND RESULTS: In Chinese hamster ovary (CHO) cells, mutant KCNQ1 + KCNE1 channels either produced no currents (G589D/L619M) or displayed markedly reduced IKs with a right-shifted voltage-dependence of activation (R555H). When co-expressed with wild-type (wt) KCNQ1, the mutant KCNQ1s displayed varying intrinsic dominant-negative capacities that were affected by auxiliary KCNE1. All three mutant KCNQ1s assembled with wt KCNQ1 as determined by fluorescence resonance energy transfer (FRET). We developed an optical quantum dot labelling assay to measure channel surface density. G589D/R555H displayed substantial reductions in surface density, which were either partially (G589D) or fully (R555H) rescued by wt KCNQ1. Unexpectedly, L619M showed no trafficking defect. In adult rat cardiomyocytes, adenovirus-expressed homotetrameric G589D/L619M + KCNE1 channels yielded no currents, whereas R555H + KCNE1 produced diminished IKs with a right-shifted voltage-dependence of activation, mimicking observations in CHO cells. In contrast to heterologous cells, homotetrameric R555H channels showed no trafficking defect in cardiomyocytes.
    CONCLUSION: Distinct LQT1 mutations in KCNQ1 assembly domain decrease IKs using unique combinations of biophysical and trafficking mechanisms. Functional deficits in IKs observed in heterologous cells are mostly, but not completely, recapitulated in adult rat cardiomyocytes. A 'methodological chain' combining approaches in heterologous cells and cardiomyocytes provides mechanistic insights that may help advance personalized therapy for LQT1 mutations.

    PMID: 25344363 [PubMed - indexed for MEDLINE]

  • The middle X residue influences cotranslational N-glycosylation consensus site skipping.

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    The middle X residue influences cotranslational N-glycosylation consensus site skipping.

    Biochemistry. 2014 Aug 05;53(30):4884-93

    Authors: Malaby HL, Kobertz WR

    Abstract
    Asparagine (N)-linked glycosylation is essential for efficient protein folding in the endoplasmic reticulum (ER) and anterograde trafficking through the secretory pathway. N-Glycans are attached to nascent polypeptides at consensus sites, N-X-T/S (X ≠ P), by one of two enzymatic isoforms of the oligosaccharyltransferase (OST), STT3A or STT3B. Here, we examined the effect of the consensus site X and hydroxyl residue on the distributions of co- and post-translational N-glycosylation of a type I transmembrane glycopeptide scaffold. Using rapid radioactive pulse-chase experiments to resolve co-translational (STT3A) and post-translational (STT3B) events, we determined that NXS consensus sites containing large hydrophobic and negatively charged middle residues are frequently skipped by STT3A during protein translation. Post-translational modification of the cotranslationally skipped sites by STT3B was similarly hindered by the middle X residue, resulting in hypoglycosylation of NXS sites containing large hydrophobic and negatively charged side chains. In contrast, NXT consensus sites (barring NWT) were efficiently modified by the cotranslational machinery, reducing STT3B's role in modifying consensus sites skipped during protein translation. A strong correlation between cotranslational N-glycosylation efficiency and the rate of post-translational N-glycosylation was determined, showing that the OST STT3A and STT3B isoforms are similarly influenced by the hydroxyl and middle X consensus site residues. Substituting various middle X residues into an OST eubacterial homologous structure revealed that small and polar consensus site X residues fit well in the peptide binding site whereas large hydrophobic and negatively charged residues were harder to accommodate, indicating conserved enzymatic mechanisms for the mammalian OST isoforms.

    PMID: 25029371 [PubMed - indexed for MEDLINE]

  • Calmodulation meta-analysis: predicting calmodulin binding via canonical motif clustering.

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    Calmodulation meta-analysis: predicting calmodulin binding via canonical motif clustering.

    J Gen Physiol. 2014 Jul;144(1):105-14

    Authors: Mruk K, Farley BM, Ritacco AW, Kobertz WR

    Abstract
    The calcium-binding protein calmodulin (CaM) directly binds to membrane transport proteins to modulate their function in response to changes in intracellular calcium concentrations. Because CaM recognizes and binds to a wide variety of target sequences, identifying CaM-binding sites is difficult, requiring intensive sequence gazing and extensive biochemical analysis. Here, we describe a straightforward computational script that rapidly identifies canonical CaM-binding motifs within an amino acid sequence. Analysis of the target sequences from high resolution CaM-peptide structures using this script revealed that CaM often binds to sequences that have multiple overlapping canonical CaM-binding motifs. The addition of a positive charge discriminator to this meta-analysis resulted in a tool that identifies potential CaM-binding domains within a given sequence. To allow users to search for CaM-binding motifs within a protein of interest, perform the meta-analysis, and then compare the results to target peptide-CaM structures deposited in the Protein Data Bank, we created a website and online database. The availability of these tools and analyses will facilitate the design of CaM-related studies of ion channels and membrane transport proteins.

    PMID: 24935744 [PubMed - indexed for MEDLINE]

  • Stoichiometry of the cardiac IKs complex.

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    Stoichiometry of the cardiac IKs complex.

    Proc Natl Acad Sci U S A. 2014 Apr 08;111(14):5065-6

    Authors: Kobertz WR

    PMID: 24706929 [PubMed - indexed for MEDLINE]

  • Molecular determinants of co- and post-translational N-glycosylation of type I transmembrane peptides.

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    Molecular determinants of co- and post-translational N-glycosylation of type I transmembrane peptides.

    Biochem J. 2013 Aug 01;453(3):427-34

    Authors: Malaby HL, Kobertz WR

    Abstract
    Type I transmembrane peptides acquire N-linked glycans during and after protein synthesis to facilitate anterograde trafficking through the secretory pathway. Mutations in N-glycosylation consensus sites (NXT and NXS, where X≠P) that alter the kinetics of the initial N-glycan attachment have been associated with cardiac arrhythmias; however, the molecular determinants that define co- and post-translational consensus sites in proteins are not known. In the present study, we identified co- and post-translational consensus sites in the KCNE family of K+ channel regulatory subunits to uncover three determinants that favour co-translational N-glycosylation kinetics of type I transmembrane peptides which lack a cleavable signal sequence: threonine-containing consensus sites (NXT), multiple N-terminal consensus sites and long C-termini. The identification of these three molecular determinants now makes it possible to predict co- and post-translational consensus sites in type I transmembrane peptides.

    PMID: 23718681 [PubMed - indexed for MEDLINE]

  • Chemical derivatization and purification of peptide-toxins for probing ion channel complexes.

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    Chemical derivatization and purification of peptide-toxins for probing ion channel complexes.

    Methods Mol Biol. 2013;995:19-30

    Authors: Hua Z, Kobertz WR

    Abstract
    Ion channels function as multi-protein complexes made up of ion-conducting α-subunits and regulatory β-subunits. To detect, identify, and quantitate the regulatory β-subunits in functioning K(+) channel complexes, we have chemically derivatized peptide-toxins that specifically react with strategically placed cysteine residues in the channel complex. Two protein labeling approaches have been developed to derivatize the peptide-toxin, charybdotoxin, with hydrophilic and hydrophobic bismaleimides, and other molecular probes. Using these cysteine-reactive peptide-toxins, we have specifically targeted KCNQ1-KCNE1 K(+) channel complexes expressed in both Xenopus oocytes and mammalian cells. The modular design of the reagents should permit this approach to be applied to the many ion channel complexes involved in electrical excitability as well as salt and water homoeostasis.

    PMID: 23494369 [PubMed - indexed for MEDLINE]

  • Structural insights into neuronal K+ channel-calmodulin complexes.

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    Structural insights into neuronal K+ channel-calmodulin complexes.

    Proc Natl Acad Sci U S A. 2012 Aug 21;109(34):13579-83

    Authors: Mruk K, Shandilya SM, Blaustein RO, Schiffer CA, Kobertz WR

    Abstract
    Calmodulin (CaM) is a ubiquitous intracellular calcium sensor that directly binds to and modulates a wide variety of ion channels. Despite the large repository of high-resolution structures of CaM bound to peptide fragments derived from ion channels, there is no structural information about CaM bound to a fully folded ion channel at the plasma membrane. To determine the location of CaM docked to a functioning KCNQ K(+) channel, we developed an intracellular tethered blocker approach to measure distances between CaM residues and the ion-conducting pathway. Combining these distance restraints with structural bioinformatics, we generated an archetypal quaternary structural model of an ion channel-CaM complex in the open state. These models place CaM close to the cytoplasmic gate, where it is well positioned to modulate channel function.

    PMID: 22869708 [PubMed - indexed for MEDLINE]

  • The acid-sensitive, anesthetic-activated potassium leak channel, KCNK3, is regulated by 14-3-3β-dependent, protein kinase C (PKC)-mediated endocytic trafficking.

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    The acid-sensitive, anesthetic-activated potassium leak channel, KCNK3, is regulated by 14-3-3β-dependent, protein kinase C (PKC)-mediated endocytic trafficking.

    J Biol Chem. 2012 Sep 21;287(39):32354-66

    Authors: Gabriel L, Lvov A, Orthodoxou D, Rittenhouse AR, Kobertz WR, Melikian HE

    Abstract
    The acid-sensitive neuronal potassium leak channel, KCNK3, is vital for setting the resting membrane potential and is the primary target for volatile anesthetics. Recent reports demonstrate that KCNK3 activity is down-regulated by PKC; however, the mechanisms responsible for PKC-induced KCNK3 down-regulation are undefined. Here, we report that endocytic trafficking dynamically regulates KCNK3 activity. Phorbol esters and Group I metabotropic glutamate receptor (mGluR) activation acutely decreased both native and recombinant KCNK3 currents with concomitant KCNK3 surface losses in cerebellar granule neurons and cell lines. PKC-mediated KCNK3 internalization required the presence of both 14-3-3β and a novel potassium channel endocytic motif, because depleting either 14-3-3β protein levels or ablating the endocytic motif completely abrogated PKC-regulated KCNK3 trafficking. These results demonstrate that neuronal potassium leak channels are not static membrane residents but are subject to 14-3-3β-dependent regulated trafficking, providing a straightforward mechanism to modulate neuronal excitability and synaptic plasticity by Group I mGluRs.

    PMID: 22846993 [PubMed - indexed for MEDLINE]

  • Xenopus laevis oocytes infected with multi-drug-resistant bacteria: implications for electrical recordings.

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    Xenopus laevis oocytes infected with multi-drug-resistant bacteria: implications for electrical recordings.

    J Gen Physiol. 2011 Aug;138(2):271-7

    Authors: O'Connell D, Mruk K, Rocheleau JM, Kobertz WR

    Abstract
    The Xenopus laevis oocyte has been the workhorse for the investigation of ion transport proteins. These large cells have spawned a multitude of novel techniques that are unfathomable in mammalian cells, yet the fickleness of the oocyte has driven many researchers to use other membrane protein expression systems. Here, we show that some colonies of Xenopus laevis are infected with three multi-drug-resistant bacteria: Pseudomonas fluorescens, Pseudomonas putida, and Stenotrophomonas maltophilia. Oocytes extracted from infected frogs quickly (3-4 d) develop multiple black foci on the animal pole, similar to microinjection scars, which render the extracted eggs useless for electrical recordings. Although multi-drug resistant, the bacteria were susceptible to amikacin and ciprofloxacin in growth assays. Supplementing the oocyte storage media with these two antibiotics prevented the appearance of the black foci and afforded oocytes suitable for whole-cell recordings. Given that P. fluorescens associated with X. laevis has become rapidly drug resistant, it is imperative that researchers store the extracted oocytes in the antibiotic cocktail and not treat the animals harboring the multi-drug-resistant bacteria.

    PMID: 21788613 [PubMed - indexed for MEDLINE]

  • Post-translational N-glycosylation of type I transmembrane KCNE1 peptides: implications for membrane protein biogenesis and disease.

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    Post-translational N-glycosylation of type I transmembrane KCNE1 peptides: implications for membrane protein biogenesis and disease.

    J Biol Chem. 2011 Aug 12;286(32):28150-9

    Authors: Bas T, Gao GY, Lvov A, Chandrasekhar KD, Gilmore R, Kobertz WR

    Abstract
    N-Glycosylation of membrane proteins is critical for their proper folding, co-assembly and subsequent matriculation through the secretory pathway. Here, we examine the kinetics of N-glycan addition to type I transmembrane KCNE1 K(+) channel β-subunits, where point mutations that prevent N-glycosylation at one consensus site give rise to disorders of the cardiac rhythm and congenital deafness. We show that KCNE1 has two distinct N-glycosylation sites: a typical co-translational site and a consensus site ∼20 residues away that unexpectedly acquires N-glycans after protein synthesis (post-translational). Mutations that ablate the co-translational site concomitantly reduce glycosylation at the post-translational site, resulting in unglycosylated KCNE1 subunits that cannot reach the cell surface with their cognate K(+) channel. This long range inhibition is highly specific for post-translational N-glycosylation because mutagenic conversion of the KCNE1 post-translational site into a co-translational site restored both monoglycosylation and anterograde trafficking. These results directly explain how a single point mutation can prevent N-glycan attachment at multiple sites, providing a new biogenic mechanism for human disease.

    PMID: 21676880 [PubMed - indexed for MEDLINE]

  • O-glycosylation of the cardiac I(Ks) complex.

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    O-glycosylation of the cardiac I(Ks) complex.

    J Physiol. 2011 Aug 01;589(Pt 15):3721-30

    Authors: Chandrasekhar KD, Lvov A, Terrenoire C, Gao GY, Kass RS, Kobertz WR

    Abstract
    Post-translational modifications of the KCNQ1–KCNE1 (Kv7) K+ channel complex are vital for regulation of the cardiac IKs current and action potential duration. Here, we show the KCNE1 regulatory subunit is O-glycosylated with mucin-type glycans in vivo. As O-linked glycosylation sites are not recognizable by sequence gazing, we designed a novel set of glycosylation mutants and KCNE chimeras and analysed their glycan content using deglycosylation enzymes. Our results show that KCNE1 is exclusively O-glycosylated at Thr-7, which is also required for N-glycosylation at Asn-5. For wild type KCNE1, the overlapping N- and O-glycosylation sites are innocuous for subunit biogenesis; however, mutation of Thr-7 to a non-hydroxylated residue yielded mostly unglycosylated protein and a small fraction of mono-N-glycosylated protein. The compounded hypoglycosylation was equally deleterious for KCNQ1–KCNE1 cell surface expression, demonstrating that KCNE1 O-glycosylation is a post-translational modification that is integral for the proper biogenesis and anterograde trafficking of the cardiac IKs complex. The enzymatic assays and panel of glycosylation mutants used here will be valuable for identifying the different KCNE1 glycoforms in native cells and determining the roles N- and O-glycosylation play in KCNQ1–KCNE1 function and localization in cardiomyocytes,

    PMID: 21669976 [PubMed - indexed for MEDLINE]

  • Chemical control of metabolically-engineered voltage-gated K+ channels.

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    Chemical control of metabolically-engineered voltage-gated K+ channels.

    Bioorg Med Chem Lett. 2011 Sep 01;21(17):5021-4

    Authors: Hua Z, Lvov A, Morin TJ, Kobertz WR

    Abstract
    Metabolic oligosaccharide engineering is a powerful approach for installing unnatural glycans with unique functional groups into the glycocalyx of living cells and animals. Using this approach, we showed that K(+) channel complexes decorated with thiol-containing sialic acids were irreversibly inhibited with scorpion toxins bearing a pendant maleimide group. Irreversible inhibition required a glycosylated K(+) channel subunit and was completely reversible with mild reductant when the tether connecting the toxin to the maleimide contained a disulfide bond. Cleavage of the disulfide bond not only restored function, but delivered a biotin moiety to the modified K(+) channel subunit, providing a novel approach for preferentially labeling wild type K(+) channel complexes functioning in cells.

    PMID: 21576020 [PubMed - indexed for MEDLINE]

  • Identification of a protein-protein interaction between KCNE1 and the activation gate machinery of KCNQ1.

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    Identification of a protein-protein interaction between KCNE1 and the activation gate machinery of KCNQ1.

    J Gen Physiol. 2010 Jun;135(6):607-18

    Authors: Lvov A, Gage SD, Berrios VM, Kobertz WR

    Abstract
    KCNQ1 channels assemble with KCNE1 transmembrane (TM) peptides to form voltage-gated K(+) channel complexes with slow activation gate opening. The cytoplasmic C-terminal domain that abuts the KCNE1 TM segment has been implicated in regulating KCNQ1 gating, yet its interaction with KCNQ1 has not been described. Here, we identified a protein-protein interaction between the KCNE1 C-terminal domain and the KCNQ1 S6 activation gate and S4-S5 linker. Using cysteine cross-linking, we biochemically screened over 300 cysteine pairs in the KCNQ1-KCNE1 complex and identified three residues in KCNQ1 (H363C, P369C, and I257C) that formed disulfide bonds with cysteine residues in the KCNE1 C-terminal domain. Statistical analysis of cross-link efficiency showed that H363C preferentially reacted with KCNE1 residues H73C, S74C, and D76C, whereas P369C showed preference for only D76C. Electrophysiological investigation of the mutant K(+) channel complexes revealed that the KCNQ1 residue, H363C, formed cross-links not only with KCNE1 subunits, but also with neighboring KCNQ1 subunits in the complex. Cross-link formation involving the H363C residue was state dependent, primarily occurring when the KCNQ1-KCNE1 complex was closed. Based on these biochemical and electrophysiological data, we generated a closed-state model of the KCNQ1-KCNE1 cytoplasmic region where these protein-protein interactions are poised to slow activation gate opening.

    PMID: 20479109 [PubMed - indexed for MEDLINE]

  • Discovery of a novel activator of KCNQ1-KCNE1 K channel complexes.

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    Discovery of a novel activator of KCNQ1-KCNE1 K channel complexes.

    PLoS One. 2009;4(1):e4236

    Authors: Mruk K, Kobertz WR

    Abstract
    KCNQ1 voltage-gated K(+) channels (Kv7.1) associate with the family of five KCNE peptides to form complexes with diverse gating properties and pharmacological sensitivities. The varied gating properties of the different KCNQ1-KCNE complexes enables the same K(+) channel to function in both excitable and non excitable tissues. Small molecule activators would be valuable tools for dissecting the gating mechanisms of KCNQ1-KCNE complexes; however, there are very few known activators of KCNQ1 channels and most are ineffective on the physiologically relevant KCNQ1-KCNE complexes. Here we show that a simple boronic acid, phenylboronic acid (PBA), activates KCNQ1/KCNE1 complexes co-expressed in Xenopus oocytes at millimolar concentrations. PBA shifts the voltage sensitivity of KCNQ1 channel complexes to favor the open state at negative potentials. Analysis of different-sized charge carriers revealed that PBA also targets the permeation pathway of KCNQ1 channels. Activation by the boronic acid moiety has some specificity for the Kv7 family members (KCNQ1, KCNQ2/3, and KCNQ4) since PBA does not activate Shaker or hERG channels. Furthermore, the commercial availability of numerous PBA derivatives provides a large class of compounds to investigate the gating mechanisms of KCNQ1-KCNE complexes.

    PMID: 19156197 [PubMed - indexed for MEDLINE]

  • Tethering chemistry and K+ channels.

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    Tethering chemistry and K+ channels.

    J Biol Chem. 2008 Sep 12;283(37):25105-9

    Authors: Morin TJ, Kobertz WR

    Abstract
    Voltage-gated K+ channels are dynamic macromolecular machines that open and close in response to changes in membrane potential. These multisubunit membrane-embedded proteins are responsible for governing neuronal excitability, maintaining cardiac rhythmicity, and regulating epithelial electrolyte homeostasis. High resolution crystal structures have provided snapshots of K+ channels caught in different states with incriminating molecular detail. Nonetheless, the connection between these static images and the specific trajectories of K+ channel movements is still being resolved by biochemical experimentation. Electrophysiological recordings in the presence of chemical modifying reagents have been a staple in ion channel structure/function studies during both the pre- and post-crystal structure eras. Small molecule tethering agents (chemoselective electrophiles linked to ligands) have proven to be particularly useful tools for defining the architecture and motions of K+ channels. This Minireview examines the synthesis and utilization of chemical tethering agents to probe and manipulate the assembly, structure, function, and molecular movements of voltage-gated K+ channel protein complexes.

    PMID: 18541528 [PubMed - indexed for MEDLINE]

  • Counting membrane-embedded KCNE beta-subunits in functioning K+ channel complexes.

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    Counting membrane-embedded KCNE beta-subunits in functioning K+ channel complexes.

    Proc Natl Acad Sci U S A. 2008 Feb 05;105(5):1478-82

    Authors: Morin TJ, Kobertz WR

    Abstract
    Ion channels are multisubunit proteins responsible for the generation and propagation of action potentials in nerve, skeletal muscle, and heart as well as maintaining salt and water homeostasis in epithelium. The subunit composition and stoichiometry of these membrane protein complexes underlies their physiological function, as different cells pair ion-conducting alpha-subunits with specific regulatory beta-subunits to produce complexes with diverse ion-conducting and gating properties. However, determining the number of alpha- and beta-subunits in functioning ion channel complexes is challenging and often fraught with contradictory results. Here we describe the synthesis of a chemically releasable, irreversible K(+) channel inhibitor and its iterative application to tally the number of beta-subunits in a KCNQ1/KCNE1 K(+) channel complex. Using this inhibitor in electrical recordings, we definitively show that there are two KCNE subunits in a functioning tetrameric K(+) channel, breaking the apparent fourfold arrangement of the ion-conducting subunits. This digital determination rules out any measurable contribution from supra, sub, and multiple stoichiometries, providing a uniform structural picture to interpret KCNE beta-subunit modulation of voltage-gated K(+) channels and the inherited mutations that cause dysfunction. Moreover, the architectural asymmetry of the K(+) channel complex affords a unique opportunity to therapeutically target ion channels that coassemble with KCNE beta-subunits.

    PMID: 18223154 [PubMed - indexed for MEDLINE]

  • Secondary structure of a KCNE cytoplasmic domain.

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    Secondary structure of a KCNE cytoplasmic domain.

    J Gen Physiol. 2006 Dec;128(6):721-9

    Authors: Rocheleau JM, Gage SD, Kobertz WR

    Abstract
    Type I transmembrane KCNE peptides contain a conserved C-terminal cytoplasmic domain that abuts the transmembrane segment. In KCNE1, this region is required for modulation of KCNQ1 K(+) channels to afford the slowly activating cardiac I(Ks) current. We utilized alanine/leucine scanning to determine whether this region possesses any secondary structure and to identify the KCNE1 residues that face the KCNQ1 channel complex. Helical periodicity analysis of the mutation-induced perturbations in voltage activation and deactivation kinetics of KCNQ1-KCNE1 complexes defined that the KCNE1 C terminus is alpha-helical when split in half at a conserved proline residue. This helical rendering assigns all known long QT mutations in the KCNE1 C-terminal domain as protein facing. The identification of a secondary structure within the KCNE1 C-terminal domain provides a structural scaffold to map protein-protein interactions with the pore-forming KCNQ1 subunit as well as the cytoplasmic regulatory proteins anchored to KCNQ1-KCNE complexes.

    PMID: 17130521 [PubMed - indexed for MEDLINE]

  • KCNE1 subunits require co-assembly with K+ channels for efficient trafficking and cell surface expression.

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    KCNE1 subunits require co-assembly with K+ channels for efficient trafficking and cell surface expression.

    J Biol Chem. 2006 Dec 29;281(52):40015-23

    Authors: Chandrasekhar KD, Bas T, Kobertz WR

    Abstract
    KCNE peptides are a class of type I transmembrane beta subunits that assemble with and modulate the gating and ion conducting properties of a variety of voltage-gated K(+) channels. Accordingly, mutations that disrupt the assembly and trafficking of KCNE-K(+) channel complexes give rise to disease. The cellular mechanisms responsible for ensuring that KCNE peptides assemble with voltage-gated K(+) channels have yet to be elucidated. Using enzymatic deglycosylation, immunofluorescence, and quantitative cell surface labeling experiments, we show that KCNE1 peptides are retained in the early stages of the secretory pathway until they co-assemble with specific K(+) channel subunits; co-assembly mediates KCNE1 progression through the secretory pathway and results in cell surface expression. We also address an apparent discrepancy between our results and a previous study in human embryonic kidney cells, which showed wild type KCNE1 peptides can reach the plasma membrane without exogenously expressed K(+) channel subunits. By comparing KCNE1 trafficking in three cell lines, our data suggest that the errant KCNE1 trafficking observed in human embryonic kidney cells may be due, in part, to the presence of endogenous voltage-gated K(+) channels in these cells.

    PMID: 17065152 [PubMed - indexed for MEDLINE]

  • KCNE3 truncation mutants reveal a bipartite modulation of KCNQ1 K+ channels.

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    KCNE3 truncation mutants reveal a bipartite modulation of KCNQ1 K+ channels.

    J Gen Physiol. 2004 Dec;124(6):759-71

    Authors: Gage SD, Kobertz WR

    Abstract
    The five KCNE genes encode a family of type I transmembrane peptides that assemble with KCNQ1 and other voltage-gated K(+) channels, resulting in potassium conducting complexes with varied channel-gating properties. It has been recently proposed that a triplet of amino acids within the transmembrane domain of KCNE1 and KCNE3 confers modulation specificity to the peptide, since swapping of these three residues essentially converts the recipient KCNE into the donor (Melman, Y.F., A. Domenech, S. de la Luna, and T.V. McDonald. 2001. J. Biol. Chem. 276:6439-6444). However, these results are in stark contrast with earlier KCNE1 deletion studies, which demonstrated that a COOH-terminal region, highly conserved between KCNE1 and KCNE3, was responsible for KCNE1 modulation of KCNQ1 (Tapper, A.R., and A.L. George. 2000 J. Gen. Physiol. 116:379-389.). To ascertain whether KCNE3 peptides behave similarly to KCNE1, we examined a panel of NH(2)- and COOH-terminal KCNE3 truncation mutants to directly determine the regions required for assembly with and modulation of KCNQ1 channels. Truncations lacking the majority of their NH(2) terminus, COOH terminus, or mutants harboring both truncations gave rise to KCNQ1 channel complexes with basal activation, a hallmark of KCNE3 modulation. These results demonstrate that the KCNE3 transmembrane domain is sufficient for assembly with and modulation of KCNQ1 channels and suggests a bipartite model for KCNQ1 modulation by KCNE1 and KCNE3 subunits. In this model, the KCNE3 transmembrane domain is active in modulation and overrides the COOH terminus' contribution, whereas the KCNE1 transmembrane domain is passive and reveals COOH-terminal modulation of KCNQ1 channels. We furthermore test the validity of this model by using the active KCNE3 transmembrane domain to functionally rescue a nonconducting, yet assembly and trafficking competent, long QT mutation located in the conserved COOH-terminal region of KCNE1.

    PMID: 15572349 [PubMed - indexed for MEDLINE]

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