101
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Identification of a novel bacterial K(+) channel. J Membr Biol 2011; 242:153-64. [PMID: 21744086 DOI: 10.1007/s00232-011-9386-2] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/04/2011] [Accepted: 06/27/2011] [Indexed: 10/18/2022]
Abstract
In an attempt to explore unknown K(+) channels in mammalian cells, especially ATP-sensitive K(+) (K(ATP)) channels, we compared the sequence homology of Kir6.1 and Kir6.2, two pore-forming subunits of mammalian K(ATP) channel genes, with bacterial genes that code for selective proteins with confirmed or putative ion transport properties. BLAST analysis revealed that a prokaryotic gene (ydfJ) expressed in Escherichia coli K12 strain shared 8.6% homology with Kir6.1 and 8.3% with Kir6.2 genes. Subsequently, we cloned and sequenced ydfJ gene from E. coli K12 and heterologously expressed it in mammalian HEK-293 cells. The whole-cell patch-clamp technique was used to record ion channel currents generated by ydfJ-encoded protein. Heterologous expression of ydfJ gene in HEK-293 cells yielded a novel K(+) channel current that was inwardly rectified and had a reversal potential close to K(+) equilibrium potential. The expressed ydfJ channel was blocked reversibly by low concentration of barium in a dose-dependent fashion. Specific K(ATP) channel openers or blockers did not alter the K(+) current generated by ydfJ expression alone or ydfJ coexpressed with rvSUR1 or rvSUR2B subunits of K(ATP) channel complex. Furthermore, this coexpressed ydfJ/rvSUR1 channels were not inhibited by ATP dialysis. On the other hand, ydfJ K(+) currents were inhibited by protopine (a nonspecific K(+) channel blocker) but not by dofetilide (a HERG channel blocker). In summary, heterologously expressed prokaryotic ydfJ gene formed a novel functional K(+) channel in mammalian cells.
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102
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Action potentials in primary osteoblasts and in the MG-63 osteoblast-like cell line. J Bioenerg Biomembr 2011; 43:311-22. [PMID: 21523406 DOI: 10.1007/s10863-011-9354-7] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/02/2011] [Accepted: 04/03/2011] [Indexed: 01/25/2023]
Abstract
Whole-cell patch-clamp analysis revealed a resting membrane potential of -60 mV in primary osteoblasts and in the MG-63 osteoblast-like cells. Depolarization-induced action potentials were characterized by duration of 60 ms, a minimal peak-to-peak distance of 180 ms, a threshold value of -20 mV and a repolarization between the spikes to -45 mV. Expressed channels were characterized by application of voltage pulses between -150 mV and 90 mV in 10 mV steps, from a holding potential of -40 mV. Voltages below -60 mV induced an inward current. Depolarizing voltages above -30 mV evoked two currents: (a) a fast activated and inactivated inward current at voltages between -30 and 30 mV, and (b) a delayed-activated outward current that was induced by voltages above -30 mV. Electrophysiological and pharmacological parameters indicated that hyperpolarization activated strongly rectifying K(+) (K(ir)) channels, whereas depolarization activated tetrodotoxin sensitive voltage gated Na(+) (Na(v)) channels as well as delayed, slowly activated, non-inactivating, and tetraethylammonium sensitive voltage gated K(+) (K(v)) channels. In addition, RT-PCR showed expression of Na(v)1.3, Na(v)1.4, Na(v)1.5, Na(v)1.6, Na(v)1.7, and K(ir)2.1, K(ir)2.3, and K(ir)2.4 as well as K(v)2.1. We conclude that osteoblasts express channels that allow firing of action potentials.
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103
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Rosenhouse-Dantsker A, Logothetis DE, Levitan I. Cholesterol sensitivity of KIR2.1 is controlled by a belt of residues around the cytosolic pore. Biophys J 2011; 100:381-9. [PMID: 21244834 DOI: 10.1016/j.bpj.2010.11.086] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/08/2010] [Revised: 11/15/2010] [Accepted: 11/23/2010] [Indexed: 11/18/2022] Open
Abstract
Kir channels play an important role in setting the resting membrane potential and modulating membrane excitability. A common feature of several Kir channels is that they are regulated by cholesterol. Yet, the mechanism by which cholesterol affects channel function is unclear. We recently showed that the cholesterol sensitivity of Kir2 channels depends on several CD-loop residues. Here we show that this cytosolic loop is part of a regulatory site that also includes residues in the G-loop, the N-terminus, and the connecting segment between the C-terminus and the inner transmembrane helix. Together, these residues form a cytosolic belt that surrounds the pore of the channel close to its interface with the transmembrane domain, and modulate the cholesterol sensitivity of the channel. Furthermore, we show that residues in this cluster are correlated with residues located in the most flexible region of the G-loop, the major cytosolic gate of Kir2.1, implying that the importance of these residues extends beyond their effect on the channel's cholesterol sensitivity. We suggest that the residues of the cholesterol sensitivity belt are critical for channel gating.
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104
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Gu C, Barry J. Function and mechanism of axonal targeting of voltage-sensitive potassium channels. Prog Neurobiol 2011; 94:115-32. [PMID: 21530607 DOI: 10.1016/j.pneurobio.2011.04.009] [Citation(s) in RCA: 41] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/24/2010] [Revised: 03/22/2011] [Accepted: 04/01/2011] [Indexed: 12/20/2022]
Abstract
Precise localization of various ion channels into proper subcellular compartments is crucial for neuronal excitability and synaptic transmission. Axonal K(+) channels that are activated by depolarization of the membrane potential participate in the repolarizing phase of the action potential, and hence regulate action potential firing patterns, which encode output signals. Moreover, some of these channels can directly control neurotransmitter release at axonal terminals by constraining local membrane excitability and limiting Ca(2+) influx. K(+) channels differ not only in biophysical and pharmacological properties, but in expression and subcellular distribution as well. Importantly, proper targeting of channel proteins is a prerequisite for electrical and chemical functions of axons. In this review, we first highlight recent studies that demonstrate different roles of axonal K(+) channels in the local regulation of axonal excitability. Next, we focus on research progress in identifying axonal targeting motifs and machinery of several different types of K(+) channels present in axons. Regulation of K(+) channel targeting and activity may underlie a novel form of neuronal plasticity. This research field can contribute to generating novel therapeutic strategies through manipulating neuronal excitability in treating neurological diseases, such as multiple sclerosis, neuropathic pain, and Alzheimer's disease.
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Affiliation(s)
- Chen Gu
- Department of Neuroscience and Center for Molecular Neurobiology, The Ohio State University, Columbus, USA.
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105
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Inanobe A, Matsuura T, Nakagawa A, Kurachi Y. Inverse agonist-like action of cadmium on G-protein-gated inward-rectifier K+ channels. Biochem Biophys Res Commun 2011; 407:366-71. [DOI: 10.1016/j.bbrc.2011.03.025] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/22/2011] [Accepted: 03/03/2011] [Indexed: 11/28/2022]
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106
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The salt-wasting phenotype of EAST syndrome, a disease with multifaceted symptoms linked to the KCNJ10 K+ channel. Pflugers Arch 2011; 461:423-35. [DOI: 10.1007/s00424-010-0915-0] [Citation(s) in RCA: 53] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/21/2010] [Revised: 12/10/2010] [Accepted: 12/17/2010] [Indexed: 11/25/2022]
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107
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Yang Y, Shi W, Chen X, Cui N, Konduru AS, Shi Y, Trower TC, Zhang S, Jiang C. Molecular basis and structural insight of vascular K(ATP) channel gating by S-glutathionylation. J Biol Chem 2011; 286:9298-307. [PMID: 21216949 DOI: 10.1074/jbc.m110.195123] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The vascular ATP-sensitive K(+) (K(ATP)) channel is targeted by a variety of vasoactive substances, playing an important role in vascular tone regulation. Our recent studies indicate that the vascular K(ATP) channel is inhibited in oxidative stress via S-glutathionylation. Here we show evidence for the molecular basis of the S-glutathionylation and its structural impact on channel gating. By comparing the oxidant responses of the Kir6.1/SUR2B channel with the Kir6.2/SUR2B channel, we found that the Kir6.1 subunit was responsible for oxidant sensitivity. Oxidant screening of Kir6.1-Kir6.2 chimeras demonstrated that the N terminus and transmembrane domains of Kir6.1 were crucial. Systematic mutational analysis revealed three cysteine residues in these domains: Cys(43), Cys(120), and Cys(176). Among them, Cys(176) was prominent, contributing to >80% of the oxidant sensitivity. The Kir6.1-C176A/SUR2B mutant channel, however, remained sensitive to both channel opener and inhibitor, which indicated that Cys(176) is not a general gating site in Kir6.1, in contrast to its counterpart (Cys(166)) in Kir6.2. A protein pull-down assay with biotinylated glutathione ethyl ester showed that mutation of Cys(176) impaired oxidant-induced incorporation of glutathione (GSH) into the Kir6.1 subunit. In contrast to Cys(176), Cys(43) had only a modest contribution to S-glutathionylation, and Cys(120) was modulated by extracellular oxidants but not intracellular GSSG. Simulation modeling of Kir6.1 S-glutathionylation suggested that after incorporation to residue 176, the GSH moiety occupied a space between the slide helix and two transmembrane helices. This prevented the inner transmembrane helix from undergoing conformational changes necessary for channel gating, retaining the channel in its closed state.
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Affiliation(s)
- Yang Yang
- Department of Biology, Georgia State University, Atlanta, Georgia 30302-4010, USA
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108
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Dassau L, Conti LR, Radeke CM, Ptáček LJ, Vandenberg CA. Kir2.6 regulates the surface expression of Kir2.x inward rectifier potassium channels. J Biol Chem 2011; 286:9526-41. [PMID: 21209095 DOI: 10.1074/jbc.m110.170597] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Precise trafficking, localization, and activity of inward rectifier potassium Kir2 channels are important for shaping the electrical response of skeletal muscle. However, how coordinated trafficking occurs to target sites remains unclear. Kir2 channels are tetrameric assemblies of Kir2.x subunits. By immunocytochemistry we show that endogenous Kir2.1 and Kir2.2 are localized at the plasma membrane and T-tubules in rodent skeletal muscle. Recently, a new subunit, Kir2.6, present in human skeletal muscle, was identified as a gene in which mutations confer susceptibility to thyrotoxic hypokalemic periodic paralysis. Here we characterize the trafficking and interaction of wild type Kir2.6 with other Kir2.x in COS-1 cells and skeletal muscle in vivo. Immunocytochemical and electrophysiological data demonstrate that Kir2.6 is largely retained in the endoplasmic reticulum, despite high sequence identity with Kir2.2 and conserved endoplasmic reticulum and Golgi trafficking motifs shared with Kir2.1 and Kir2.2. We identify amino acids responsible for the trafficking differences of Kir2.6. Significantly, we show that Kir2.6 subunits can coassemble with Kir2.1 and Kir2.2 in vitro and in vivo. Notably, this interaction limits the surface expression of both Kir2.1 and Kir2.2. We provide evidence that Kir2.6 functions as a dominant negative, in which incorporation of Kir2.6 as a subunit in a Kir2 channel heterotetramer reduces the abundance of Kir2 channels on the plasma membrane.
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Affiliation(s)
- Lior Dassau
- Department of Molecular, Cellular, and Developmental Biology and the Neuroscience Research Institute, University of California, Santa Barbara, California 93106, USA
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109
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Paynter JJ, Andres-Enguix I, Fowler PW, Tottey S, Cheng W, Enkvetchakul D, Bavro VN, Kusakabe Y, Sansom MSP, Robinson NJ, Nichols CG, Tucker SJ. Functional complementation and genetic deletion studies of KirBac channels: activatory mutations highlight gating-sensitive domains. J Biol Chem 2010; 285:40754-61. [PMID: 20876570 PMCID: PMC3003375 DOI: 10.1074/jbc.m110.175687] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2010] [Revised: 09/27/2010] [Indexed: 12/16/2022] Open
Abstract
The superfamily of prokaryotic inwardly rectifying (KirBac) potassium channels is homologous to mammalian Kir channels. However, relatively little is known about their regulation or about their physiological role in vivo. In this study, we have used random mutagenesis and genetic complementation in K(+)-auxotrophic Escherichia coli and Saccharomyces cerevisiae to identify activatory mutations in a range of different KirBac channels. We also show that the KirBac6.1 gene (slr5078) is necessary for normal growth of the cyanobacterium Synechocystis PCC6803. Functional analysis and molecular dynamics simulations of selected activatory mutations identified regions within the slide helix, transmembrane helices, and C terminus that function as important regulators of KirBac channel activity, as well as a region close to the selectivity filter of KirBac3.1 that may have an effect on gating. In particular, the mutations identified in TM2 favor a model of KirBac channel gating in which opening of the pore at the helix-bundle crossing plays a far more important role than has recently been proposed.
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Affiliation(s)
| | | | - Philip W. Fowler
- the Structural Bioinformatics and Computational Biochemistry Unit, Department of Biochemistry
| | - Stephen Tottey
- the Institute for Cell and Molecular Biosciences, Medical School, Newcastle University, Newcastle NE1 7RU, United Kingdom
| | - Wayland Cheng
- the OXION Initiative, University of Oxford, Oxford OX1 3PU, United Kingdom
| | - Decha Enkvetchakul
- the Department of Pharmacological and Physiological Science, Saint Louis University School of Medicine, St. Louis, Missouri 63104, and
| | | | | | - Mark S. P. Sansom
- the Structural Bioinformatics and Computational Biochemistry Unit, Department of Biochemistry
- the OXION Initiative, University of Oxford, Oxford OX1 3PU, United Kingdom
| | - Nigel J. Robinson
- the Institute for Cell and Molecular Biosciences, Medical School, Newcastle University, Newcastle NE1 7RU, United Kingdom
| | - Colin G. Nichols
- the Department of Cell Biology and Physiology, and Center for Investigation of Membrane Excitability Disorders, Washington University School of Medicine, St. Louis, Missouri 63110
| | - Stephen J. Tucker
- the Department of Physics, Clarendon Laboratory, and
- the OXION Initiative, University of Oxford, Oxford OX1 3PU, United Kingdom
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110
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Inanobe A, Nakagawa A, Matsuura T, Kurachi Y. A structural determinant for the control of PIP2 sensitivity in G protein-gated inward rectifier K+ channels. J Biol Chem 2010; 285:38517-23. [PMID: 20880843 PMCID: PMC2992284 DOI: 10.1074/jbc.m110.161703] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/06/2010] [Revised: 09/01/2010] [Indexed: 01/04/2023] Open
Abstract
Inward rectifier K(+) (Kir) channels are activated by phosphatidylinositol-(4,5)-bisphosphate (PIP(2)), but G protein-gated Kir (K(G)) channels further require either G protein βγ subunits (Gβγ) or intracellular Na(+) for their activation. To reveal the mechanism(s) underlying this regulation, we compared the crystal structures of the cytoplasmic domain of K(G) channel subunit Kir3.2 obtained in the presence and the absence of Na(+). The Na(+)-free Kir3.2, but not the Na(+)-plus Kir3.2, possessed an ionic bond connecting the N terminus and the CD loop of the C terminus. Functional analyses revealed that the ionic bond between His-69 on the N terminus and Asp-228 on the CD loop, which are known to be critically involved in Gβγ- and Na(+)-dependent activation, lowered PIP(2) sensitivity. The conservation of these residues within the K(G) channel family indicates that the ionic bond is a character that maintains the channels in a closed state by controlling the PIP(2) sensitivity.
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Affiliation(s)
- Atsushi Inanobe
- From the Department of Pharmacology, Graduate School of Medicine
- Center for Advanced Medical Engineering and Informatics
| | | | - Takanori Matsuura
- Laboratory of Protein Informatics, Institute for Protein Research, Osaka University, Osaka 565-0871, Japan
| | - Yoshihisa Kurachi
- From the Department of Pharmacology, Graduate School of Medicine
- Center for Advanced Medical Engineering and Informatics
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111
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Yokogawa M, Osawa M, Takeuchi K, Mase Y, Shimada I. NMR analyses of the Gbetagamma binding and conformational rearrangements of the cytoplasmic pore of G protein-activated inwardly rectifying potassium channel 1 (GIRK1). J Biol Chem 2010; 286:2215-23. [PMID: 21075842 DOI: 10.1074/jbc.m110.160754] [Citation(s) in RCA: 43] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
G protein-activated inwardly rectifying potassium channel (GIRK) plays crucial roles in regulating heart rate and neuronal excitability in eukaryotic cells. GIRK is activated by the direct binding of heterotrimeric G protein βγ subunits (Gβγ) upon stimulation of G protein-coupled receptors, such as M2 acetylcholine receptor. The binding of Gβγ to the cytoplasmic pore (CP) region of GIRK causes structural rearrangements, which are assumed to open the transmembrane ion gate. However, the crucial residues involved in the Gβγ binding and the structural mechanism of GIRK gating have not been fully elucidated. Here, we have characterized the interaction between the CP region of GIRK and Gβγ, by ITC and NMR. The ITC analyses indicated that four Gβγ molecules bind to a tetramer of the CP region of GIRK with a dissociation constant of 250 μM. The NMR analyses revealed that the Gβγ binding site spans two neighboring subunits of the GIRK tetramer, which causes conformational rearrangements between subunits. A possible binding mode and mechanism of GIRK gating are proposed.
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Affiliation(s)
- Mariko Yokogawa
- Division of Physical Chemistry, Graduate School of Pharmaceutical Sciences, The University of Tokyo, Tokyo 113-0033, Japan
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112
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Paynter JJ, Shang L, Bollepalli MK, Baukrowitz T, Tucker SJ. Random mutagenesis screening indicates the absence of a separate H(+)-sensor in the pH-sensitive Kir channels. Channels (Austin) 2010; 4:390-7. [PMID: 20699659 DOI: 10.4161/chan.4.5.13006] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
Abstract
Several inwardly-rectifying (Kir) potassium channels (Kir1.1, Kir4.1 and Kir4.2) are characterised by their sensitivity to inhibition by intracellular H(+) within the physiological range. The mechanism by which these channels are regulated by intracellular pH has been the subject of intense scrutiny for over a decade, yet the molecular identity of the titratable pH-sensor remains elusive. In this study we have taken advantage of the acidic intracellular environment of S. cerevisiae and used a K(+) -auxotrophic strain to screen for mutants of Kir1.1 with impaired pH-sensitivity. In addition to the previously identified K80M mutation, this unbiased screening approach identified a novel mutation (S172T) in the second transmembrane domain (TM2) that also produces a marked reduction in pH-sensitivity through destabilization of the closed-state. However, despite this extensive mutagenic approach, no mutations could be identified which removed channel pH-sensitivity or which were likely to act as a separate H(+) -sensor unique to the pH-sensitive Kir channels. In order to explain these results we propose a model in which the pH-sensing mechanism is part of an intrinsic gating mechanism common to all Kir channels, not just the pH-sensitive Kir channels. In this model, mutations which disrupt this pH-sensor would result in an increase, not reduction, in pH-sensitivity. This has major implications for any future studies of Kir channel pH-sensitivity and explains why formal identification of these pH-sensing residues still represents a major challenge.
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Affiliation(s)
- Jennifer J Paynter
- Department of Physiology, Anatomy and Genetics, University of Oxford, UK
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113
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Zylbergold P, Ramakrishnan N, Hebert T. The role of G proteins in assembly and function of Kir3 inwardly rectifying potassium channels. Channels (Austin) 2010; 4:411-21. [PMID: 20855978 DOI: 10.4161/chan.4.5.13327] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023] Open
Abstract
Kir3 channels (also known as GIRK channels) are important regulators of electrical excitability in both cardiomyocytes and neurons. Much is known regarding the assembly and function of these channels and the roles that their interacting proteins play in controlling these events. Further, they are one of the best studied effectors of heterotrimeric G proteins in general and Gβγ subunits in particular. However, our understanding of the roles of multiple Gβγ binding sites on Kir3 channels is still rudimentary. We discuss potential roles for Gβγ in channel assembly and trafficking in addition to their known role in cellular signaling.
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Affiliation(s)
- Peter Zylbergold
- Department of Pharmacology and Therapeutics, McGill University, Québec, Canada
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114
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Flecainide increases Kir2.1 currents by interacting with cysteine 311, decreasing the polyamine-induced rectification. Proc Natl Acad Sci U S A 2010; 107:15631-6. [PMID: 20713726 DOI: 10.1073/pnas.1004021107] [Citation(s) in RCA: 62] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Both increase and decrease of cardiac inward rectifier current (I(K1)) are associated with severe cardiac arrhythmias. Flecainide, a widely used antiarrhythmic drug, exhibits ventricular proarrhythmic effects while effectively controlling ventricular arrhythmias associated with mutations in the gene encoding Kir2.1 channels that decrease I(K1) (Andersen syndrome). Here we characterize the electrophysiological and molecular basis of the flecainide-induced increase of the current generated by Kir2.1 channels (I(Kir2.1)) and I(K1) recorded in ventricular myocytes. Flecainide increases outward I(Kir2.1) generated by homotetrameric Kir2.1 channels by decreasing their affinity for intracellular polyamines, which reduces the inward rectification of the current. Flecainide interacts with the HI loop of the cytoplasmic domain of the channel, Cys311 being critical for the effect. This explains why flecainide does not increase I(Kir2.2) and I(Kir2.3), because Kir2.2 and Kir2.3 channels do not exhibit a Cys residue at the equivalent position. We further show that incubation with flecainide increases expression of functional Kir2.1 channels in the membrane, an effect also determined by Cys311. Indeed, flecainide pharmacologically rescues R67W, but not R218W, channel mutations found in Andersen syndrome patients. Moreover, our findings provide noteworthy clues about the structural determinants of the C terminus cytoplasmic domain of Kir2.1 channels involved in the control of gating and rectification.
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115
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Tang X, Hang D, Sand A, Kofuji P. Variable loss of Kir4.1 channel function in SeSAME syndrome mutations. Biochem Biophys Res Commun 2010; 399:537-41. [PMID: 20678478 DOI: 10.1016/j.bbrc.2010.07.105] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2010] [Accepted: 07/27/2010] [Indexed: 11/28/2022]
Abstract
SeSAME syndrome is a complex disease characterized by seizures, sensorineural deafness, ataxia, mental retardation and electrolyte imbalance. Mutations in the inwardly rectifying potassium channel Kir4.1 (KCNJ10 gene) have been linked to this condition. Kir4.1 channels are weakly rectifying channels expressed in glia, kidney, cochlea and possibly other tissues. We determined the electrophysiological properties of SeSAME mutant channels after expression in transfected mammalian cells. We found that a majority of mutations (R297C, C140R, R199X, T164I) resulted in complete loss of Kir4.1 channel function while two mutations (R65P and A167V) produced partial loss of function. All mutant channels were rescued upon co-transfection of wild-type Kir4.1 but not Kir5.1 channels. Cell-surface biotinylation assays indicate significant plasma membrane expression of all mutant channels with exception of the non-sense mutant R199X. These results indicate the differential loss of Kir channel function among SeSAME syndrome mutations.
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Affiliation(s)
- Xiaofang Tang
- Department of Neuroscience, University of Minnesota, Minneapolis, MN 55455,USA
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116
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Gupta S, Bavro VN, D’Mello R, Tucker SJ, Vénien-Bryan C, Chance MR. Conformational changes during the gating of a potassium channel revealed by structural mass spectrometry. Structure 2010; 18:839-46. [PMID: 20637420 PMCID: PMC3124773 DOI: 10.1016/j.str.2010.04.012] [Citation(s) in RCA: 58] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/19/2010] [Revised: 03/21/2010] [Accepted: 04/01/2010] [Indexed: 10/19/2022]
Abstract
Potassium channels are dynamic proteins that undergo large conformational changes to regulate the flow of K(+) ions across the cell membrane. Understanding the gating mechanism of these channels therefore requires methods for probing channel structure in both their open and closed conformations. Radiolytic footprinting is used to study the gating mechanism of the inwardly-rectifying potassium channel KirBac3.1. The purified protein stabilized in either open or closed conformations was exposed to focused synchrotron X-ray beams on millisecond timescales to modify solvent accessible amino acid side chains. These modifications were identified and quantified using high-resolution mass spectrometry. The differences observed between the closed and open states were then used to reveal local conformational changes that occur during channel gating. The results provide support for a proposed gating mechanism of the Kir channel and demonstrate a method of probing the dynamic gating mechanism of other integral membrane proteins and ion channels.
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Affiliation(s)
- Sayan Gupta
- Center for Synchrotron Biosciences, Case Western Reserve University, Cleveland Ohio, 44022, USA
- Center for Proteomics and Bioinformatics, Case Western Reserve University, Cleveland Ohio, 44022, USA
| | - Vassiliy N. Bavro
- Clarendon Laboratory, Department of Physics, University of Oxford, Oxford, United Kingdom
- OXION Initiative, Sherrington Building, University of Oxford, Oxford, United Kingdom
| | - Rhijuta D’Mello
- Center for Synchrotron Biosciences, Case Western Reserve University, Cleveland Ohio, 44022, USA
- Center for Proteomics and Bioinformatics, Case Western Reserve University, Cleveland Ohio, 44022, USA
| | - Stephen J. Tucker
- Clarendon Laboratory, Department of Physics, University of Oxford, Oxford, United Kingdom
- OXION Initiative, Sherrington Building, University of Oxford, Oxford, United Kingdom
| | - Catherine Vénien-Bryan
- OXION Initiative, Sherrington Building, University of Oxford, Oxford, United Kingdom
- Department of Biochemistry, University of Oxford, Oxford, United Kingdom
| | - Mark R. Chance
- Center for Synchrotron Biosciences, Case Western Reserve University, Cleveland Ohio, 44022, USA
- Center for Proteomics and Bioinformatics, Case Western Reserve University, Cleveland Ohio, 44022, USA
- Department of Physiology & Biophysics, Case Western Reserve University, Cleveland Ohio, 44022, USA
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117
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de Boer TP, Houtman MJC, Compier M, van der Heyden MAG. The mammalian K(IR)2.x inward rectifier ion channel family: expression pattern and pathophysiology. Acta Physiol (Oxf) 2010; 199:243-56. [PMID: 20331539 DOI: 10.1111/j.1748-1716.2010.02108.x] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
Abstract
Inward rectifier currents based on K(IR)2.x subunits are regarded as essential components for establishing a stable and negative resting membrane potential in many excitable cell types. Pharmacological inhibition, null mutation in mice and dominant positive and negative mutations in patients reveal some of the important functions of these channels in their native tissues. Here we review the complex mammalian expression pattern of K(IR)2.x subunits and relate these to the outcomes of functional inhibition of the resultant channels. Correlations between expression and function in muscle and bone tissue are observed, while we recognize a discrepancy between neuronal expression and function.
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Affiliation(s)
- T P de Boer
- Department of Medical Physiology, UMCU, Utrecht, the Netherlands
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118
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Shah MM, Hammond RS, Hoffman DA. Dendritic ion channel trafficking and plasticity. Trends Neurosci 2010; 33:307-16. [PMID: 20363038 PMCID: PMC2902701 DOI: 10.1016/j.tins.2010.03.002] [Citation(s) in RCA: 110] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2009] [Revised: 02/22/2010] [Accepted: 03/04/2010] [Indexed: 11/29/2022]
Abstract
Dendritic ion channels are essential for the regulation of intrinsic excitability as well as modulating the shape and integration of synaptic signals. Changes in dendritic channel function have been associated with many forms of synaptic plasticity. Recent evidence suggests that dendritic ion channel modulation and trafficking could contribute to plasticity-induced alterations in neuronal function. In this review we discuss our current knowledge of dendritic ion channel modulation and trafficking and their relationship to cellular and synaptic plasticity. We also consider the implications for neuronal function. We argue that to gain an insight into neuronal information processing it is essential to understand the regulation of dendritic ion channel expression and properties.
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Affiliation(s)
- Mala M Shah
- Department of Pharmacology, The School of Pharmacy, University of London, London, WC1N 1AX, UK.
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119
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Identification and characterization of a novel bacterial ATP-sensitive K+ channel. J Microbiol 2010; 48:325-30. [PMID: 20571950 DOI: 10.1007/s12275-010-9231-9] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2009] [Accepted: 12/22/2009] [Indexed: 10/19/2022]
Abstract
Five bacterial species that are most likely to have putative prokaryotic inward rectifier K(+) (Kir) channels were selected by in silico sequence homology and membrane topology analyses with respect to the number of transmembrane domains (TMs) and the presence of K(+) selectivity filter and/or ATP binding sites in reference to rabbit heart inward rectifier K(+) channel (Kir6.2). A dot blot assay with genomic DNAs when probed with whole rabbit Kir6.2 cDNA further supported the in silico analysis by exhibiting a stronger hybridization in species with putative Kir's compared to one without a Kir. Among them, Chromobacterium violaceum gave rise to a putative Kir channel gene, which was PCR-cloned into the bacterial expression vector pET30b(+), and its expression was induced in Escherichia coli and confirmed by gel purification and immunoblotting. On the other hand, this putative bacterial Kir channel was functionally expressed in Xenopus oocytes and its channel activity was measured electrophysiologically by using two electrode voltage clamping (TEVC). Results revealed a K(+) current with characteristics similar to those of the ATP-sensitive K(+) (K-ATP) channel. Collectively, cloning and functional characterization of bacterial ion channels could be greatly facilitated by combining the in silico analysis and heterologous expression in Xenopus oocytes.
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120
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Paz JT, Christian CA, Parada I, Prince DA, Huguenard JR. Focal cortical infarcts alter intrinsic excitability and synaptic excitation in the reticular thalamic nucleus. J Neurosci 2010; 30:5465-79. [PMID: 20392967 PMCID: PMC2861582 DOI: 10.1523/jneurosci.5083-09.2010] [Citation(s) in RCA: 56] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/12/2009] [Revised: 02/10/2010] [Accepted: 03/06/2010] [Indexed: 11/21/2022] Open
Abstract
Focal cortical injuries result in death of cortical neurons and their efferents and ultimately in death or damage of thalamocortical relay (TCR) neurons that project to the affected cortical area. Neurons of the inhibitory reticular thalamic nucleus (nRT) receive excitatory inputs from corticothalamic and thalamocortical axons and are thus denervated by such injuries, yet nRT cells generally survive these insults to a greater degree than TCR cells. nRT cells inhibit TCR cells, regulate thalamocortical transmission, and generate cerebral rhythms including those involved in thalamocortical epilepsies. The survival and reorganization of nRT after cortical injury would determine recovery of thalamocortical circuits after injury. However, the physiological properties and connectivity of the survivors remain unknown. To study possible alterations in nRT neurons, we used the rat photothrombosis model of cortical stroke. Using in vitro patch-clamp recordings at various times after the photothrombotic injury, we show that localized strokes in the somatosensory cortex induce long-term reductions in intrinsic excitability and evoked synaptic excitation of nRT cells by the end of the first week after the injury. We find that nRT neurons in injured rats show (1) decreased membrane input resistance, (2) reduced low-threshold calcium burst responses, and (3) weaker evoked excitatory synaptic responses. Such alterations in nRT cellular excitability could lead to loss of nRT-mediated inhibition in relay nuclei, increased output of surviving TCR cells, and enhanced thalamocortical excitation, which may facilitate recovery of thalamic and cortical sensory circuits. In addition, such changes could be maladaptive, leading to injury-induced epilepsy.
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Affiliation(s)
- Jeanne T. Paz
- Department of Neurology and Neurological Sciences, Stanford University School of Medicine, Stanford, California 94305
| | - Catherine A. Christian
- Department of Neurology and Neurological Sciences, Stanford University School of Medicine, Stanford, California 94305
| | - Isabel Parada
- Department of Neurology and Neurological Sciences, Stanford University School of Medicine, Stanford, California 94305
| | - David A. Prince
- Department of Neurology and Neurological Sciences, Stanford University School of Medicine, Stanford, California 94305
| | - John R. Huguenard
- Department of Neurology and Neurological Sciences, Stanford University School of Medicine, Stanford, California 94305
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121
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Luján R. Organisation of potassium channels on the neuronal surface. J Chem Neuroanat 2010; 40:1-20. [PMID: 20338235 DOI: 10.1016/j.jchemneu.2010.03.003] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/21/2010] [Revised: 03/10/2010] [Accepted: 03/11/2010] [Indexed: 11/30/2022]
Abstract
Potassium channels are a family of ion channels that govern the intrinsic electrical properties of neurons in the brain. Molecular cloning has revealed over 100 genes encoding the pore-forming alpha subunits of potassium channels in mammals, making them the most diverse subset of ion channels. Multiplicity in this ion channel family is further generated through alternative splicing. The precise location of potassium channels along the dendro-somato-axonic surface of the neurons is an important factor in determining its functional impact. Today, it is widely accepted that potassium channels can be located at any subcellular compartment on the neuronal surface, at synaptic and extrasynaptic sites, from somata to dendritic shafts, dendritic spines, axons or axon terminals. However, they are not evenly distributed on the neuronal surface and depending on the potassium channel subtype, are instead concentrated at different compartments. This selective localization of ion channels to specific neuronal compartments has many different functional implications. One factor necessary to understand the role of potassium channels in neuronal function is to unravel their specialized distribution and subcellular localization within a cell, and this can only be achieved by electron microscopy. In this review, I summarize anatomical findings, describing their distribution in the central nervous system. The distinct regional, cellular and subcellular distribution of potassium channels in the brain will be discussed in view of their possible functional implications.
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Affiliation(s)
- Rafael Luján
- Departamento de Ciencias Médicas, Instituto de Investigación en Discapacidades Neurológicas (IDINE), Facultad de Medicina, Universidad de Castilla-La Mancha, Campus Biosanitario, C/Almansa 14, 02006 Albacete, Spain.
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122
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Hibino H, Inanobe A, Furutani K, Murakami S, Findlay I, Kurachi Y. Inwardly rectifying potassium channels: their structure, function, and physiological roles. Physiol Rev 2010; 90:291-366. [PMID: 20086079 DOI: 10.1152/physrev.00021.2009] [Citation(s) in RCA: 1081] [Impact Index Per Article: 77.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.
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Affiliation(s)
- Hiroshi Hibino
- Department of Pharmacology, Graduate School of Medicine and The Center for Advanced Medical Engineering and Informatics, Osaka University, Osaka 565-0871, Japan
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123
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Abstract
A variety of ion channels, including members of all major ion channel families, have been shown to be regulated by changes in the level of membrane cholesterol and partition into cholesterol-rich membrane domains. In general, several types of cholesterol effects have been described. The most common effect is suppression of channel activity by an increase in membrane cholesterol, an effect that was described for several types of inwardly-rectifying K(+) channels, voltage-gated K(+) channels, Ca(+2) sensitive K(+) channels, voltage-gated Na(+) channels, N-type voltage-gated Ca(+2) channels and volume-regulated anion channels. In contrast, several types of ion channels, such as epithelial amiloride-sensitive Na(+) channels and Transient Receptor Potential channels, as well as some of the types of inwardly-rectifying and voltage-gated K(+) channels were shown to be inhibited by cholesterol depletion. Cholesterol was also shown to alter the kinetic properties and current-voltage dependence of several voltage-gated channels. Finally, maintaining membrane cholesterol level is required for coupling ion channels to signalling cascades. In terms of the mechanisms, three general mechanisms have been proposed: (i) specific interactions between cholesterol and the channel protein, (ii) changes in the physical properties of the membrane bilayer and (iii) maintaining the scaffolds for protein-protein interactions. The goal of this review is to describe systematically the role of cholesterol in regulation of the major types of ion channels and to discuss these effects in the context of the models proposed.
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Affiliation(s)
- Irena Levitan
- Department of Medicine, University of Illinois at Chicago, Chicago, IL, USA.
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124
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GABAB Receptor-Mediated Modulation of Metabotropic Glutamate Signaling and Synaptic Plasticity in Central Neurons. GABABRECEPTOR PHARMACOLOGY - A TRIBUTE TO NORMAN BOWERY 2010; 58:149-73. [DOI: 10.1016/s1054-3589(10)58007-4] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
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125
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Yao X, Horie T, Xue S, Leung HY, Katsuhara M, Brodsky DE, Wu Y, Schroeder JI. Differential sodium and potassium transport selectivities of the rice OsHKT2;1 and OsHKT2;2 transporters in plant cells. PLANT PHYSIOLOGY 2010; 152:341-55. [PMID: 19889878 PMCID: PMC2799368 DOI: 10.1104/pp.109.145722] [Citation(s) in RCA: 95] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/03/2009] [Accepted: 10/26/2009] [Indexed: 05/18/2023]
Abstract
Na(+) and K(+) homeostasis are crucial for plant growth and development. Two HKT transporter/channel classes have been characterized that mediate either Na(+) transport or Na(+) and K(+) transport when expressed in Xenopus laevis oocytes and yeast. However, the Na(+)/K(+) selectivities of the K(+)-permeable HKT transporters have not yet been studied in plant cells. One study expressing 5' untranslated region-modified HKT constructs in yeast has questioned the relevance of cation selectivities found in heterologous systems for selectivity predictions in plant cells. Therefore, here we analyze two highly homologous rice (Oryza sativa) HKT transporters in plant cells, OsHKT2;1 and OsHKT2;2, that show differential K(+) permeabilities in heterologous systems. Upon stable expression in cultured tobacco (Nicotiana tabacum) Bright-Yellow 2 cells, OsHKT2;1 mediated Na(+) uptake, but little Rb(+) uptake, consistent with earlier studies and new findings presented here in oocytes. In contrast, OsHKT2;2 mediated Na(+)-K(+) cotransport in plant cells such that extracellular K(+) stimulated OsHKT2;2-mediated Na(+) influx and vice versa. Furthermore, at millimolar Na(+) concentrations, OsHKT2;2 mediated Na(+) influx into plant cells without adding extracellular K(+). This study shows that the Na(+)/K(+) selectivities of these HKT transporters in plant cells coincide closely with the selectivities in oocytes and yeast. In addition, the presence of external K(+) and Ca(2+) down-regulated OsHKT2;1-mediated Na(+) influx in two plant systems, Bright-Yellow 2 cells and intact rice roots, and also in Xenopus oocytes. Moreover, OsHKT transporter selectivities in plant cells are shown to depend on the imposed cationic conditions, supporting the model that HKT transporters are multi-ion pores.
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Affiliation(s)
| | | | | | | | | | | | | | - Julian I. Schroeder
- Division of Biological Sciences, Cell and Developmental Biology Section, and Center for Molecular Genetics, University of California, San Diego, La Jolla, California 92093–0116 (X.Y., T.H., S.X., H.-Y.L., D.E.B., J.I.S.); Key Laboratory of Ministry of Education for Plant Developmental Biology, College of Life Sciences, Wuhan University, Wuhan, Hubei 430072, China (X.Y., Y.W.); and Group of Molecular and Functional Plant Biology, Research Institute for Bioresources, Okayama University, Kurashiki, Okayama 710–0046, Japan (T.H., M.K.)
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126
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Titov AV, Wang B, Sint K, Král P. Controllable Synthetic Molecular Channels: Biomimetic Ammonia Switch. J Phys Chem B 2009; 114:1174-9. [DOI: 10.1021/jp9103933] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Alexey V. Titov
- Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60607
| | - Boyang Wang
- Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60607
| | - Kyaw Sint
- Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60607
| | - Petr Král
- Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60607
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127
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Liu B, Yao J, Wang Y, Li H, Qin F. Proton inhibition of unitary currents of vanilloid receptors. ACTA ACUST UNITED AC 2009; 134:243-58. [PMID: 19720962 PMCID: PMC2737227 DOI: 10.1085/jgp.200910255] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
Protons, which are released during inflammation and injury, regulate many receptors and ion channels involved in pain transduction, including capsaicin channels (transient receptor potential vanilloid receptors 1). Whereas extracellular acidification both sensitizes and directly activates the channel, it also causes concomitant reduction of the unitary current amplitudes. Here, we investigate the mechanisms and molecular basis of this inhibitory effect of protons on channel conductance. Single-channel recordings showed that the unitary current amplitudes decreased with extracellular pH in a dose-dependent manner, consistent with a model in which protons bind to a site within the channel with an apparent pKa of ∼6. The inhibition was voltage dependent, ∼65% at −60 mV and 37% at +60 mV when pH was reduced from 7.4 to 5.5. The unitary current amplitudes reached saturation at [K+] ≥ 1 M, and notably the maximum amplitudes did not converge with different pHs, inconsistent with a blockade model based on surface charge screening or competitive inhibition of permeating ions. Mutagenesis experiments uncovered two acidic residues critical for proton inhibition, one located at the pore entrance and the other on the pore helix. Based on homology to the KcsA structure, the two acidic residues, along with another basic residue also on the pore helix, could form a triad interacting with each other through extensive hydrogen bonds and electrostatic contacts, suggesting that protons may mediate the interactions between the selectivity filter and pore helix, thereby altering the local structure in the filter region and consequently the conductance of the channel.
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Affiliation(s)
- Beiying Liu
- Department of Physiology and Biophysical Sciences, State University of New York at Buffalo, Buffalo, NY 14214, USA
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128
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Abstract
To date, most of the major types of Kir channels, Kir2s, Kir3s, Kir4s, and Kir6s, have been found to partition into cholesterol-rich membrane domains and/or to be regulated by changes in the level of membrane cholesterol. Surprisingly, however, in spite of the structural similarities between different Kirs, effects of cholesterol on different types of Kir channels vary from cholesterol-induced decrease in the current density (Kir2 channels) to the loss of channel activity by cholesterol depletion (Kir4 channels) and loss of channel coupling by different mediators (Kir3 and Kir6 channels). Recently, we have gained initial insights into the mechanisms responsible for cholesterol-induced suppression Kir2 channels, but mechanisms underlying cholesterol sensitivity of other Kir channels are mostly unknown. The goal of this review is to present a summary of the current knowledge of the distinct effects of cholesterol on different types of Kir channels in vitro and in vivo.
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Affiliation(s)
- Irena Levitan
- Department of Medicine, Pulmonary Section, University of Illinois at Chicago, Chicago, IL 60612, USA.
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129
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Shang L, Ranson SV, Tucker SJ. Kir5.1 underlies long-lived subconductance levels in heteromeric Kir4.1/Kir5.1 channels from Xenopus tropicalis. Biochem Biophys Res Commun 2009; 388:501-5. [PMID: 19665991 PMCID: PMC2764340 DOI: 10.1016/j.bbrc.2009.08.032] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/28/2009] [Accepted: 08/02/2009] [Indexed: 11/28/2022]
Abstract
The inwardly-rectifying potassium channel subunit Kir5.1 selectively co-assembles with members of the Kir4.0 subfamily to form novel pH-sensitive heteromeric channels with unique single channel properties. In this study, we have cloned orthologs of Kir4.1 and Kir5.1 from the genome of the amphibian, Xenopus tropicalis (Xt). Heteromeric XtKir4.1/XtKir5.1 channels exhibit similar macroscopic current properties to rat Kir4.1/Kir5.1 with a faster time-dependent rate of activation. However, single channel analysis of heteromeric XtKir4.1/XtKir5.1 channels reveals that they have markedly different long-lived, multi-level subconductance states. Furthermore, we demonstrate that the XtKir5.1 subunit is responsible for these prominent subconductance levels. These results are consistent with a model in which the slow transitions between sublevel states represent the movement of individual subunits. These novel channels now provide an excellent model system to determine the structural basis of subconductance levels and contribution of heteromeric pore architecture to this process.
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Affiliation(s)
| | | | - Stephen J. Tucker
- Clarendon Laboratory, Department of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom
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130
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Ashmole I, Vavoulis DV, Stansfeld PJ, Mehta PR, Feng JF, Sutcliffe MJ, Stanfield PR. The response of the tandem pore potassium channel TASK-3 (K(2P)9.1) to voltage: gating at the cytoplasmic mouth. J Physiol 2009; 587:4769-83. [PMID: 19703964 PMCID: PMC2770146 DOI: 10.1113/jphysiol.2009.175430] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2009] [Accepted: 08/24/2009] [Indexed: 02/04/2023] Open
Abstract
Although the tandem pore potassium channel TASK-3 is thought to open and shut at its selectivity filter in response to changes of extracellular pH, it is currently unknown whether the channel also shows gating at its inner, cytoplasmic mouth through movements of membrane helices M2 and M4. We used two electrode voltage clamp and single channel recording to show that TASK-3 responds to voltage in a way that reveals such gating. In wild-type channels, P(open) was very low at negative voltages, but increased with depolarisation. The effect of voltage was relatively weak and the gating charge small, 0.17. Mutants A237T (in M4) and N133A (in M2) increased P(open) at a given voltage, increasing mean open time and the number of openings per burst. In addition, the relationship between P(open) and voltage was shifted to less positive voltages. Mutation of putative hinge glycines (G117A, G231A), residues that are conserved throughout the tandem pore channel family, reduced P(open) at a given voltage, shifting the relationship with voltage to a more positive potential range. None of these mutants substantially affected the response of the channel to extracellular acidification. We have used the results from single channel recording to develop a simple kinetic model to show how gating occurs through two classes of conformation change, with two routes out of the open state, as expected if gating occurs both at the selectivity filter and at its cytoplasmic mouth.
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Affiliation(s)
- I Ashmole
- Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, UK
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131
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Raveh A, Riven I, Reuveny E. Elucidation of the gating of the GIRK channel using a spectroscopic approach. J Physiol 2009; 587:5331-5. [PMID: 19752111 DOI: 10.1113/jphysiol.2009.180158] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
The traditional view of G protein-coupled receptor (GPCR)-mediated signalling puts the players in this signalling cascade, namely the GPCR, the G protein and its effector, as individual components in space, where the signalling specificity is obtained mainly by the interaction of the GPCR and the Galpha subunits of the G protein. A question is then raised as to how fidelity in receptor signalling is achieved, given that many systems use the same components of the G protein signalling machinery. One possible mechanism for obtaining the specific flow of the downstream signals, from the activated G protein to its specific effector target, in a timely manner, is compartmentalization, a spatial arrangement of the complex in a rather restricted space. Here we review our recent findings related to these issues, using the G protein-coupled potassium channel (GIRK) as a model effector and fluorescence-based approaches to reveal how the signalling complex is arranged and how the G protein exerts its action to activate the GIRK channel in intact cells.
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Affiliation(s)
- Adi Raveh
- Department of Biological Chemistry, Weizmann Institute of Science, Rehovot, Israel
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132
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Singh DK, Rosenhouse-Dantsker A, Nichols CG, Enkvetchakul D, Levitan I. Direct regulation of prokaryotic Kir channel by cholesterol. J Biol Chem 2009; 284:30727-36. [PMID: 19740741 DOI: 10.1074/jbc.m109.011221] [Citation(s) in RCA: 62] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
Our earlier studies have shown that channel activity of Kir2 subfamily of inward rectifiers is strongly suppressed by the elevation of cellular cholesterol. The goal of this study is to determine whether cholesterol suppresses Kir channels directly. To achieve this goal, purified prokaryotic Kir (KirBac1.1) channels were incorporated into liposomes of defined lipid composition, and channel activity was assayed by (86)Rb(+) uptake. Our results show that (86)Rb(+) flux through KirBac1.1 is strongly inhibited by cholesterol. Incorporation of 5% (mass cholesterol/phospholipid) cholesterol into the liposome suppresses (86)Rb(+) flux by >50%, and activity is completely inhibited at 12-15%. However, epicholesterol, a stereoisomer of cholesterol with similar physical properties, has significantly less effect on KirBac-mediated (86)Rb(+) uptake than cholesterol. Furthermore, analysis of multiple sterols suggests that cholesterol-induced inhibition of KirBac1.1 channels is mediated by specific interactions rather than by changes in the physical properties of the lipid bilayer. In contrast to the inhibition of KirBac1.1 activity, cholesterol had no effect on the activity of reconstituted KscA channels (at up to 250 microg/mg of phospholipid). Taken together, these observations demonstrate that cholesterol suppresses Kir channels in a pure protein-lipid environment and suggest that the interaction is direct and specific.
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Affiliation(s)
- Dev K Singh
- Department of Medicine, University of Illinois, Chicago, Illinois 60612, USA.
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133
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Osawa M, Yokogawa M, Muramatsu T, Kimura T, Mase Y, Shimada I. Evidence for the direct interaction of spermine with the inwardly rectifying potassium channel. J Biol Chem 2009; 284:26117-26. [PMID: 19620244 DOI: 10.1074/jbc.m109.029355] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
The inwardly rectifying potassium channel (Kir) regulates resting membrane potential, K+ homeostasis, heart rate, and hormone secretion. The outward current is blocked in a voltage-dependent manner, upon the binding of intracellular polyamines or Mg2+ to the transmembrane pore domain. Meanwhile, electrophysiological studies have shown that mutations of several acidic residues in the intracellular regions affected the inward rectification. Although these acidic residues are assumed to bind polyamines, the functional role of the binding of polyamines and Mg2+ to the intracellular regions of Kirs remains unclear. Here, we report thermodynamic and structural studies of the interaction between polyamines and the cytoplasmic pore of mouse Kir3.1/GIRK1, which is gated by binding of G-protein betagamma-subunit (Gbetagamma). ITC analyses showed that two spermine molecules bind to a tetramer of Kir3.1/GIRK1 with a dissociation constant of 26 microM, which is lower than other blockers. NMR analyses revealed that the spermine binding site is Asp-260 and its surrounding area. Small but significant chemical shift perturbations upon spermine binding were observed in the subunit-subunit interface of the tetramer, suggesting that spermine binding alters the relative orientations of the four subunits. Our ITC and NMR results postulated a spermine binding mode, where one spermine molecule bridges two Asp-260 side chains from adjacent subunits, with rearrangement of the subunit orientations. This suggests the functional roles of spermine binding to the cytoplasmic pore: stabilization of the resting state conformation of the channel, and instant translocation to the transmembrane pore upon activation through the Gbetagamma-induced conformational rearrangement.
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Affiliation(s)
- Masanori Osawa
- Division of Physical Chemistry, Graduate School of Pharmaceutical Sciences, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
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134
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Dai S, Hall DD, Hell JW. Supramolecular assemblies and localized regulation of voltage-gated ion channels. Physiol Rev 2009; 89:411-52. [PMID: 19342611 DOI: 10.1152/physrev.00029.2007] [Citation(s) in RCA: 264] [Impact Index Per Article: 17.6] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
This review addresses the localized regulation of voltage-gated ion channels by phosphorylation. Comprehensive data on channel regulation by associated protein kinases, phosphatases, and related regulatory proteins are mainly available for voltage-gated Ca2+ channels, which form the main focus of this review. Other voltage-gated ion channels and especially Kv7.1-3 (KCNQ1-3), the large- and small-conductance Ca2+-activated K+ channels BK and SK2, and the inward-rectifying K+ channels Kir3 have also been studied to quite some extent and will be included. Regulation of the L-type Ca2+ channel Cav1.2 by PKA has been studied most thoroughly as it underlies the cardiac fight-or-flight response. A prototypical Cav1.2 signaling complex containing the beta2 adrenergic receptor, the heterotrimeric G protein Gs, adenylyl cyclase, and PKA has been identified that supports highly localized via cAMP. The type 2 ryanodine receptor as well as AMPA- and NMDA-type glutamate receptors are in close proximity to Cav1.2 in cardiomyocytes and neurons, respectively, yet independently anchor PKA, CaMKII, and the serine/threonine phosphatases PP1, PP2A, and PP2B, as is discussed in detail. Descriptions of the structural and functional aspects of the interactions of PKA, PKC, CaMKII, Src, and various phosphatases with Cav1.2 will include comparisons with analogous interactions with other channels such as the ryanodine receptor or ionotropic glutamate receptors. Regulation of Na+ and K+ channel phosphorylation complexes will be discussed in separate papers. This review is thus intended for readers interested in ion channel regulation or in localization of kinases, phosphatases, and their upstream regulators.
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Affiliation(s)
- Shuiping Dai
- Department of Pharmacology, University of Iowa, Iowa City, IA 52242-1109, USA
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135
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Reliable activation of immature neurons in the adult hippocampus. PLoS One 2009; 4:e5320. [PMID: 19399173 PMCID: PMC2670498 DOI: 10.1371/journal.pone.0005320] [Citation(s) in RCA: 188] [Impact Index Per Article: 12.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2009] [Accepted: 03/25/2009] [Indexed: 01/25/2023] Open
Abstract
Neurons born in the adult dentate gyrus develop, mature, and connect over a long interval that can last from six to eight weeks. It has been proposed that, during this period, developing neurons play a relevant role in hippocampal signal processing owing to their distinctive electrical properties. However, it has remained unknown whether immature neurons can be recruited into a network before synaptic and functional maturity have been achieved. To address this question, we used retroviral expression of green fluorescent protein to identify developing granule cells of the adult mouse hippocampus and investigate the balance of afferent excitation, intrinsic excitability, and firing behavior by patch clamp recordings in acute slices. We found that glutamatergic inputs onto young neurons are significantly weaker than those of mature cells, yet stimulation of cortical excitatory axons elicits a similar spiking probability in neurons at either developmental stage. Young neurons are highly efficient in transducing ionic currents into membrane depolarization due to their high input resistance, which decreases substantially in mature neurons as the inward rectifier potassium (Kir) conductance increases. Pharmacological blockade of Kir channels in mature neurons mimics the high excitability characteristic of young neurons. Conversely, Kir overexpression induces mature-like firing properties in young neurons. Therefore, the differences in excitatory drive of young and mature neurons are compensated by changes in membrane excitability that render an equalized firing activity. These observations demonstrate that the adult hippocampus continuously generates a population of highly excitable young neurons capable of information processing.
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136
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Furutani K, Ohno Y, Inanobe A, Hibino H, Kurachi Y. Mutational and In Silico Analyses for Antidepressant Block of Astroglial Inward-Rectifier Kir4.1 Channel. Mol Pharmacol 2009; 75:1287-95. [DOI: 10.1124/mol.108.052936] [Citation(s) in RCA: 64] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
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137
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Wang S, Alimi Y, Tong A, Nichols CG, Enkvetchakul D. Differential roles of blocking ions in KirBac1.1 tetramer stability. J Biol Chem 2008; 284:2854-2860. [PMID: 19033439 DOI: 10.1074/jbc.m807474200] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Potassium channels are tetrameric proteins that mediate K(+)-selective transmembrane diffusion. For KcsA, tetramer stability depends on interactions between permeant ions and the channel pore. We have examined the role of pore blockers on the tetramer stability of KirBac1.1. In 150 mm KCl, purified KirBac1.1 protein migrates as a monomer (approximately 40 kDa) on SDS-PAGE. Addition of Ba(2+) (K(1/2) approximately 50 microm) prior to loading results in an additional tetramer band (approximately 160 kDa). Mutation A109C, at a residue located near the expected Ba(2+)-binding site, decreased tetramer stabilization by Ba(2+) (K(1/2) approximately 300 microm), whereas I131C, located nearby, stabilized tetramers in the absence of Ba(2+). Neither mutation affected Ba(2+) block of channel activity (using (86)Rb(+) flux assay). In contrast to Ba(2+), Mg(2+) had no effect on tetramer stability (even though Mg(2+) was a potent blocker). Many studies have shown Cd(2+) block of K(+) channels as a result of cysteine substitution of cavity-lining M2 (S6) residues, with the implicit interpretation that coordination of a single ion by cysteine side chains along the central axis effectively blocks the pore. We examined blocking and tetramer-stabilizing effects of Cd(2+) on KirBac1.1 with cysteine substitutions in M2. Cd(2+) block potency followed an alpha-helical pattern consistent with the crystal structure. Significantly, Cd(2+) strongly stabilized tetramers of I138C, located in the center of the inner cavity. This stabilization was additive with the effect of Ba(2+), consistent with both ions simultaneously occupying the channel: Ba(2+) at the selectivity filter entrance and Cd(2+) coordinated by I138C side chains in the inner cavity.
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Affiliation(s)
- Shizhen Wang
- Department of Pharmacological and Physiological Science, Saint Louis University School of Medicine, St. Louis, Missouri 63104; Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 63110
| | - Yewande Alimi
- Department of Pharmacological and Physiological Science, Saint Louis University School of Medicine, St. Louis, Missouri 63104
| | - Ailing Tong
- Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 63110
| | - Colin G Nichols
- Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 63110
| | - Decha Enkvetchakul
- Department of Pharmacological and Physiological Science, Saint Louis University School of Medicine, St. Louis, Missouri 63104.
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138
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Robertson JL, Palmer LG, Roux B. Long-pore electrostatics in inward-rectifier potassium channels. ACTA ACUST UNITED AC 2008; 132:613-32. [PMID: 19001143 PMCID: PMC2585864 DOI: 10.1085/jgp.200810068] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
Inward-rectifier potassium (Kir) channels differ from the canonical K+ channel structure in that they possess a long extended pore (∼85 Å) for ion conduction that reaches deeply into the cytoplasm. This unique structural feature is presumably involved in regulating functional properties specific to Kir channels, such as conductance, rectification block, and ligand-dependent gating. To elucidate the underpinnings of these functional roles, we examine the electrostatics of an ion along this extended pore. Homology models are constructed based on the open-state model of KirBac1.1 for four mammalian Kir channels: Kir1.1/ROMK, Kir2.1/IRK, Kir3.1/GIRK, and Kir6.2/KATP. By solving the Poisson-Boltzmann equation, the electrostatic free energy of a K+ ion is determined along each pore, revealing that mammalian Kir channels provide a favorable environment for cations and suggesting the existence of high-density regions in the cytoplasmic domain and cavity. The contribution from the reaction field (the self-energy arising from the dielectric polarization induced by the ion's charge in the complex geometry of the pore) is unfavorable inside the long pore. However, this is well compensated by the electrostatic interaction with the static field arising from the protein charges and shielded by the dielectric surrounding. Decomposition of the static field provides a list of residues that display remarkable correspondence with existing mutagenesis data identifying amino acids that affect conduction and rectification. Many of these residues demonstrate interactions with the ion over long distances, up to 40 Å, suggesting that mutations potentially affect ion or blocker energetics over the entire pore. These results provide a foundation for understanding ion interactions in Kir channels and extend to the study of ion permeation, block, and gating in long, cation-specific pores.
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Affiliation(s)
- Janice L Robertson
- Program in Physiology, Biophysics and Systems Biology, Weill Graduate School of Medical Sciences, Cornell University, New York, NY 10065, USA
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139
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Qian X, Li J, Ding J, Wang Z, Duan L, Hu G. Glibenclamide exerts an antitumor activity through reactive oxygen species-c-jun NH2-terminal kinase pathway in human gastric cancer cell line MGC-803. Biochem Pharmacol 2008; 76:1705-15. [PMID: 18840412 DOI: 10.1016/j.bcp.2008.09.009] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2008] [Revised: 08/28/2008] [Accepted: 09/05/2008] [Indexed: 02/08/2023]
Abstract
Glibenclamide, a blocker of ATP-sensitive potassium (K(ATP)) channels, can suppress progression of many cancers, but the involved mechanism is unclear. Herein we reported that MGC-803 cells expressed the K(ATP) channels composed of Kir6.2 and SUR1 subunits. Glibenclamide induced cellular viability decline, coupled with cell apoptosis and reactive oxygen species (ROS) generation in MGC-803 cells. Meanwhile, glibenclamide increased NADPH oxidase catalytic subunit gp91(phox) expression and superoxide anion (O2-) generation, and caused mitochondrial respiration dysfunction in MGC-803 cells, suggesting that glibenclamide induced an increase of ROS derived from NADPH oxidase and mitochondria. Glibenclamide could also lead to loss of mitochondrial membrane potential, release of cytochrome c and apoptosis-inducing factor (AIF), and activation of c-jun NH2-terminal kinase (JNK) in MGC-803 cells. Pretreatment with antioxidant N-acetyl-l-cysteine (NAC) prevented glibenclamide-induced JNK activation, apoptosis and cellular viability decline. Furthermore, glibenclamide greatly decreased the cellular viability, induced apoptosis and inhibited Akt activation in wild-type mouse embryonic fibroblast (MEF) cells but not in JNK1-/- or JNK2-/- MEF cells. Taken together, our study reveals that glibenclamide exerts an antitumor activity in MGC-803 cells by activating ROS-dependent, JNK-driven cell apoptosis. These findings provide insights into the use of glibenclamide in the treatment of human gastric cancer.
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Affiliation(s)
- Xia Qian
- Department of Pharmacology, Nanjing Medical University, 140 Hanzhong Road, Nanjing, Jiangsu 210029, PR China
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140
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Solly K, Cassaday J, Felix JP, Garcia ML, Ferrer M, Strulovici B, Kiss L. Miniaturization and HTS of a FRET-based membrane potential assay for K(ir) channel inhibitors. Assay Drug Dev Technol 2008; 6:225-34. [PMID: 18471076 DOI: 10.1089/adt.2008.123] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
The K(ir) family of potassium-selective ion channels is characterized by their inward (anomalous) rectifying current-voltage relationship. K(ir) channels are widely expressed in mammalian cells and through their role in regulation of the cell membrane potential have been implicated in diverse physiological functions. To enable the identification of novel K(ir) channel inhibitors, a fluorescence resonance energy transfer (FRET)-based membrane potential assay was developed using a Chinese hamster ovary cell line stably expressing a human K(ir) channel. The FRET-based assay incorporates the use of two dyes {N-(6-chloro-7-hydroxycoumarin-3-carbonyl)-dimyristoylphosphatidylethanolamine (CC2-DMPE) and bis(1,3-diethylthiobarbiturate)trimethine oxonol [DiSBAC(2)(3)]} to track changes in membrane potential, thus enabling all of the advantages of ratiometric readout: reduced inaccuracies arising from well-to-well variation in cell number, dye loading, signal intensities, and plate inconsistencies. The assay was miniaturized to a 1,536-well microtiter plate format and read on a fluorometric imaging plate reader (FLIPR(Tetra), Molecular Devices, Sunnyvale, CA). The assay was automated and utilized to perform a primary high-throughput screening campaign to identify novel inhibitors of the K(ir) channel.
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Affiliation(s)
- Kelli Solly
- Automated Biotechnology, Merck Research Laboratories, North Wales, PA 19454, USA
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141
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Zampini V, Masetto S, Correia MJ. Elementary properties of Kir2.1, a strong inwardly rectifying K(+) channel expressed by pigeon vestibular type II hair cells. Neuroscience 2008; 155:1250-61. [PMID: 18652879 DOI: 10.1016/j.neuroscience.2008.06.048] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/02/2008] [Revised: 06/17/2008] [Accepted: 06/23/2008] [Indexed: 10/21/2022]
Abstract
By using the patch-clamp technique in the cell-attached configuration, we have investigated the single-channel properties of an inward rectifier potassium channel (Kir) expressed by pigeon vestibular type II hair cells in situ. In high-K(+) external solution with 2 mM Mg(2+), Kir inward current showed openings to at least four amplitude levels. The two most frequent open states (L2 and L3) had a mean slope conductance of 13 and 28 pS, respectively. L1 (7 pS) and L4 (36 pS) together accounted for less than 6% of the conductive state. Closed time distributions were fitted well using four exponential functions, of which the slowest time constant (tau(C4)) was clearly voltage-dependent. Open time distributions were fitted well with two or three exponential functions depending on voltage. The mean open probability (P(O)) decreased with hyperpolarization (0.13 at -50 mV and 0.03 at -120 mV). During pulse-voltage protocols, the Kir current-decay process (inactivation) accelerated and increased in extent with hyperpolarization. This phenomenon was associated with a progressive increase of the relative importance of tau(C4). Kir inactivation almost disappeared when Mg(2+) was omitted from the pipette solution. At the same time, P(O) increased at all membrane voltages and the relative importance of L4 increased to a mean value of 47%. The relative importance of tau(C4) decreased for all open states, while L4 only showed a significantly longer open time constant. The present work provides the first detailed quantitative description of the elementary properties of the Kir inward rectifier in pigeon vestibular type II hair cells and specifically describes the Kir gating properties and the molecule's sensitivity to extracellular Mg(2+) for all subconductance levels. The present results are consistent with the Kir2.1 protein sustaining a strong inwardly rectifying K(+) current in native hair cells, characterized by rapid activation time course and slow partial inactivation. The longest closed state (tau(C4)) appears as the main parameter involved in time- and Mg(2+)-dependent decay. Finally, in contrast to Kir2.1 results described so far for mammalian cells, external Mg(2+) had no effect on channel conductance.
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Affiliation(s)
- V Zampini
- Farmacologiche Cellulari-Molecolari Sez. Fisiologia Generale, Università di Pavia, Pavia, Italy
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142
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The alpha9beta1 integrin enhances cell migration by polyamine-mediated modulation of an inward-rectifier potassium channel. Proc Natl Acad Sci U S A 2008; 105:7188-93. [PMID: 18480266 DOI: 10.1073/pnas.0708044105] [Citation(s) in RCA: 75] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The alpha9beta1 integrin accelerates cell migration through binding of spermidine/spermine acetyltransferase (SSAT) to the alpha9 cytoplasmic domain. We now show that SSAT enhances alpha9-mediated migration specifically through catabolism of spermidine and/or spermine. Because spermine and spermidine are effective blockers of K(+) ion efflux through inward-rectifier K(+) (Kir) channels, we examined the involvement of Kir channels in this pathway. The Kir channel inhibitor, barium, or knockdown of a single subunit, Kir4.2, specifically inhibited alpha9-dependent cell migration. alpha9beta1 and Kir4.2 colocalized in focal adhesions at the leading edge of migrating cells and inhibition or knockdown of Kir4.2 caused reduced persistence and an increased number of lamellipodial extensions in cells migrating on an alpha9beta1 ligand. These results identify a pathway through which the alpha9 integrin subunit stimulates cell migration by localized polyamine catabolism and modulation of Kir channel function.
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143
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Ionic channel function in action potential generation: current perspective. Mol Neurobiol 2008; 35:129-50. [PMID: 17917103 DOI: 10.1007/s12035-007-8001-0] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/14/2006] [Revised: 11/30/1999] [Accepted: 11/10/2006] [Indexed: 10/23/2022]
Abstract
Over 50 years ago, Hodgkin and Huxley laid down the foundations of our current understanding of ionic channels. An impressive progress has been made during the following years that culminated in the revelation of the details of potassium channel structure. Nevertheless, even today, we cannot separate well currents recorded in central mammalian neurons. Many modern concepts about the function of sodium and potassium currents are based on experiments performed in nonmammalian cells. The recent recognition of the fast delayed rectifier current indicates that we need to reevaluate the biophysical role of sodium and potassium currents. This review will consider high quality voltage clamp data obtained from the soma of central mammalian neurons in the view of our current knowledge about proteins forming ionic channels. Fast sodium currents and three types of outward potassium currents, the delayed rectifier, the subthreshold A-type, and the D-type potassium currents, are discussed here. An updated current classification with biophysical role of each current subtype is provided. This review shows that details of kinetics of both sodium and outward potassium currents differ significantly from the classical descriptions and these differences may be of functional significance.
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144
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Lyashchenko AK, Tibbs GR. Ion binding in the open HCN pacemaker channel pore: fast mechanisms to shape "slow" channels. ACTA ACUST UNITED AC 2008; 131:227-43. [PMID: 18270171 PMCID: PMC2248720 DOI: 10.1085/jgp.200709868] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
IH pacemaker channels carry a mixed monovalent cation current that, under physiological ion gradients, reverses at ∼−34 mV, reflecting a 4:1 selectivity for K over Na. However, IH channels display anomalous behavior with respect to permeant ions such that (a) open channels do not exhibit the outward rectification anticipated assuming independence; (b) gating and selectivity are sensitive to the identity and concentrations of externally presented permeant ions; (c) the channels' ability to carry an inward Na current requires the presence of external K even though K is a minor charge carrier at negative voltages. Here we show that open HCN channels (the hyperpolarization-activated, cyclic nucleotide sensitive pore forming subunits of IH) undergo a fast, voltage-dependent block by intracellular Mg in a manner that suggests the ion binds close to, or within, the selectivity filter. Eliminating internal divalent ion block reveals that (a) the K dependence of conduction is mediated via K occupancy of site(s) within the pore and that asymmetrical occupancy and/or coupling of these sites to flux further shapes ion flow, and (b) the kinetics of equilibration between K-vacant and K-occupied states of the pore (10–20 μs or faster) is close to the ion transit time when the pore is occupied by K alone (∼0.5–3 μs), a finding that indicates that either ion:ion repulsion involving Na is adequate to support flux (albeit at a rate below our detection threshold) and/or the pore undergoes rapid, permeant ion-sensitive equilibration between nonconducting and conducting configurations. Biophysically, further exploration of the Mg site and of interactions of Na and K within the pore will tell us much about the architecture and operation of this unusual pore. Physiologically, these results suggest ways in which “slow” pacemaker channels may contribute dynamically to the shaping of fast processes such as Na-K or Ca action potentials.
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Affiliation(s)
- Alex K Lyashchenko
- Department of Anesthesiology, Columbia University, New York, NY 10032, USA
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145
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Mason AK, Jacobs BE, Welling PA. AP-2-dependent internalization of potassium channel Kir2.3 is driven by a novel di-hydrophobic signal. J Biol Chem 2008; 283:5973-84. [PMID: 18180291 DOI: 10.1074/jbc.m709756200] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023] Open
Abstract
The localization and density of Kir2.3 channels are influenced by the balance between PDZ protein interaction at the cell surface and routing into the endocytic pathway. Here, we explore mechanisms by which the Kir2.3 channel is directed into the endocytic pathway. We found that Kir2.3 channels are constitutively internalized from the cell surface in a dynamin-dependent manner, indicative of vesicle-mediated endocytosis. The rate of Kir2.3 endocytosis was dramatically attenuated following RNA interference-mediated knockdown of either alpha adaptin (AP-2 clathrin adaptor) or clathrin heavy chain, revealing that Kir2.3 is internalized by an AP-2 clathrin-dependent mechanism. Structure-rationalized mutagenesis studies of a number of different potential AP-2 interaction motifs indicate that internalization of Kir2.3 is largely dependent on a non-canonical di-isoleucine motif (II413) embedded within the C terminus. Internalization assays using CD4-Kir2.3 chimeras demonstrate that the di-isoleucine signal acts in an autonomous and transplantable manner. Kir2.3 co-immunoprecipitates with alpha adaptin, and disruption of the di-isoleucine motif decreased interaction of the channel with AP-2. Replacement of the di-isoleucine motif with a canonical di-leucine internalization signal actually blocked Kir2.3 endocytosis. Moreover, in yeast three-hybrid studies, the Kir2.3 di-isoleucine motif does not bind the AP-2 alphaC-sigma2 hemicomplex in the way that has been recently observed for canonical di-leucine signals. Altogether, the results indicate that Kir2.3 channels are marked for clathrin-dependent internalization from the plasma membrane by a novel AP-2-dependent signal.
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Affiliation(s)
- Amanda K Mason
- Department of Physiology, University of Maryland School of Medicine, Baltimore, MD 21201, USA
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146
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Raveh A, Riven I, Reuveny E. The use of FRET microscopy to elucidate steady state channel conformational rearrangements and G protein interaction with the GIRK channels. Methods Mol Biol 2008; 491:199-212. [PMID: 18998095 DOI: 10.1007/978-1-59745-526-8_16] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
while X-ray crystallography provides extremely high-resolution snapshot of protein structure, it lacks the ability to provide dynamic information on the processes involving conformational rearrangements of the protein. Methods to record protein conformational dynamics are present, in particular those that are based on fluorescence measurements, and are now more and more utilized in studying proteins in their natural environment. Here we describe the use of fluorescence resonance energy transfer (FRET) technique to monitor the conformational rearrangements associated with the gating of the G protein-coupled potassium channel (GIRK/Kir3.x), and its relation with the G protein subunits. The FRET technique is combined with total internal fluorescence (TIRF) microscopy, and allows the dissection of the signal originating from channel proteins that reside exclusively in the plasma membrane. Since most of the components associated with GIRK channel gating are intracellular, that involve various biochemical steps, proteins were labeled with genetically encoded variants of the green fluorescence protein and signals were acquired from live cells in culture. Using these methodologies we were able to show that gating conformational rearrangements, i.e. the opening of the channel, involve the rotation and expansion of the channel subunits cytosolic termini, along the channel's central axis. In addition, the G proteins that trigger this process reside very close to the channel, to ensure high signaling specificity and to provide temporal precision of the gating process.
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Affiliation(s)
- Adi Raveh
- Department of Biological Chemistry, Weizmann Institute of Science, Rehovot, Israel
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147
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Smith PD, Brett SE, Luykenaar KD, Sandow SL, Marrelli SP, Vigmond EJ, Welsh DG. KIR channels function as electrical amplifiers in rat vascular smooth muscle. J Physiol 2007; 586:1147-60. [PMID: 18063660 DOI: 10.1113/jphysiol.2007.145474] [Citation(s) in RCA: 92] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022] Open
Abstract
Strong inward rectifying K(+) (K(IR)) channels have been observed in vascular smooth muscle and can display negative slope conductance. In principle, this biophysical characteristic could enable K(IR) channels to 'amplify' responses initiated by other K(+) conductances. To test this, we have characterized the diversity of smooth muscle K(IR) properties in resistance arteries, confirmed the presence of negative slope conductance and then determined whether K(IR) inhibition alters the responsiveness of middle cerebral, coronary septal and third-order mesenteric arteries to K(+) channel activators. Our initial characterization revealed that smooth muscle K(IR) channels were highly expressed in cerebral and coronary, but not mesenteric arteries. These channels comprised K(IR)2.1 and 2.2 subunits and electrophysiological recordings demonstrated that they display negative slope conductance. Computational modelling predicted that a K(IR)-like current could amplify the hyperpolarization and dilatation initiated by a vascular K(+) conductance. This prediction was consistent with experimental observations which showed that 30 mum Ba(2+) attenuated the ability of K(+) channel activators to dilate cerebral and coronary arteries. This attenuation was absent in mesenteric arteries where smooth muscle K(IR) channels were poorly expressed. In summary, smooth muscle K(IR) expression varies among resistance arteries and when channel are expressed, their negative slope conductance amplifies responses initiated by smooth muscle and endothelial K(+) conductances. These findings highlight the fact that the subtle biophysical properties of K(IR) have a substantive, albeit indirect, role in enabling agonists to alter the electrical state of a multilayered artery.
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Affiliation(s)
- Pamela D Smith
- Smooth Muscle Research Group and Department of Physiology & Biophysics, University of Calgary, Calgary, Alberta, Canada
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148
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Identification of yeast proteins necessary for cell-surface function of a potassium channel. Proc Natl Acad Sci U S A 2007; 104:18079-84. [PMID: 17989219 DOI: 10.1073/pnas.0708765104] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Inwardly rectifying potassium (Kir) channels form gates in the cell membrane that regulate the flow of K(+) ions into and out of the cell, thereby influencing the membrane potential and electrical signaling of many cell types, including neurons and cardiomyocytes. Kir-channel function depends on other cellular proteins that aid in the folding of channel subunits, assembly into tetrameric complexes, trafficking of quality-controlled channels to the plasma membrane, and regulation of channel activity at the cell surface. We used the yeast Saccharomyces cerevisiae as a model system to identify proteins necessary for the functional expression of a mammalian Kir channel at the cell surface. A screen of 376 yeast strains, each lacking one nonessential protein localized to the early secretory pathway, identified seven deletion strains in which functional expression of the Kir channel at the plasma membrane was impaired. Six deletions were of genes with known functions in trafficking and lipid biosynthesis (sur4Delta, csg2Delta, erv14Delta, emp24Delta, erv25Delta, and bst1Delta), and one deletion was of an uncharacterized gene (yil039wDelta). We provide genetic and functional evidence that Yil039wp, a conserved, phosphoesterase domain-containing protein, which we named "trafficking of Emp24p/Erv25p-dependent cargo disrupted 1" (Ted1p), acts together with Emp24p/Erv25p in cargo exit from the endoplasmic reticulum (ER). The seven yeast proteins identified in our screen likely impact Kir-channel functional expression at the level of vesicle budding from the ER and/or the local lipid environment at the plasma membrane.
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149
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Yuill KH, Stansfeld PJ, Ashmole I, Sutcliffe MJ, Stanfield PR. The selectivity, voltage-dependence and acid sensitivity of the tandem pore potassium channel TASK-1: contributions of the pore domains. Pflugers Arch 2007; 455:333-48. [PMID: 17541788 PMCID: PMC2492388 DOI: 10.1007/s00424-007-0282-7] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/02/2007] [Accepted: 04/25/2007] [Indexed: 10/23/2022]
Abstract
We have investigated the contribution to ionic selectivity of residues in the selectivity filter and pore helices of the P1 and P2 domains in the acid sensitive potassium channel TASK-1. We used site directed mutagenesis and electrophysiological studies, assisted by structural models built through computational methods. We have measured selectivity in channels expressed in Xenopus oocytes, using voltage clamp to measure shifts in reversal potential and current amplitudes when Rb+ or Na+ replaced extracellular K+. Both P1 and P2 contribute to selectivity, and most mutations, including mutation of residues in the triplets GYG and GFG in P1 and P2, made channels non-selective. We interpret the effects of these--and of other mutations--in terms of the way the pore is likely to be stabilised structurally. We show also that residues in the outer pore mouth contribute to selectivity in TASK-1. Mutations resulting in loss of selectivity (e.g. I94S, G95A) were associated with slowing of the response of channels to depolarisation. More important physiologically, pH sensitivity is also lost or altered by such mutations. Mutations that retained selectivity (e.g. I94L, I94V) also retained their response to acidification. It is likely that responses both to voltage and pH changes involve gating at the selectivity filter.
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Affiliation(s)
- KH Yuill
- Molecular Physiology Group, Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, UK
- Department of Pharmacy and Pharmacology, University of Bath, Bath BA2 7AY, UK
| | - PJ Stansfeld
- Department of Cell Physiology and Pharmacology, University of Leicester, PO Box 138, Leicester, LE1 9HN, UK
| | - I Ashmole
- Molecular Physiology Group, Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, UK
| | - MJ Sutcliffe
- Manchester Interdisciplinary Biocentre, University of Manchester, 131 Princess Street, Manchester, M1 7DN, UK
| | - PR Stanfield
- Molecular Physiology Group, Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, UK
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150
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Rapedius M, Fowler PW, Shang L, Sansom MS, Tucker SJ, Baukrowitz T. H bonding at the helix-bundle crossing controls gating in Kir potassium channels. Neuron 2007; 55:602-14. [PMID: 17698013 PMCID: PMC1950231 DOI: 10.1016/j.neuron.2007.07.026] [Citation(s) in RCA: 57] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/30/2007] [Revised: 06/12/2007] [Accepted: 07/19/2007] [Indexed: 11/21/2022]
Abstract
Specific stimuli such as intracellular H+ and phosphoinositides (e.g., PIP2) gate inwardly rectifying potassium (Kir) channels by controlling the reversible transition between the closed and open states. This gating mechanism underlies many aspects of Kir channel physiology and pathophysiology; however, its structural basis is not well understood. Here, we demonstrate that H+ and PIP2 use a conserved gating mechanism defined by similar structural changes in the transmembrane (TM) helices and the selectivity filter. Our data support a model in which the gating motion of the TM helices is controlled by an intrasubunit hydrogen bond between TM1 and TM2 at the helix-bundle crossing, and we show that this defines a common gating motif in the Kir channel superfamily. Furthermore, we show that this proposed H-bonding interaction determines Kir channel pH sensitivity, pH and PIP2 gating kinetics, as well as a K+-dependent inactivation process at the selectivity filter and therefore many of the key regulatory mechanisms of Kir channel physiology.
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Affiliation(s)
- Markus Rapedius
- Institute of Physiology II, Friedrich Schiller University, D-07743 Jena, Germany
| | - Philip W. Fowler
- Structural Bioinformatics and Computational Biochemistry Unit, Department of Biochemistry, University of Oxford, OX1 3QU Oxford, UK
| | - Lijun Shang
- Oxford Centre for Gene Function, Department of Physiology, Anatomy and Genetics, University of Oxford, OX1 3PT Oxford, UK
| | - Mark S.P. Sansom
- Structural Bioinformatics and Computational Biochemistry Unit, Department of Biochemistry, University of Oxford, OX1 3QU Oxford, UK
| | - Stephen J. Tucker
- Oxford Centre for Gene Function, Department of Physiology, Anatomy and Genetics, University of Oxford, OX1 3PT Oxford, UK
- Corresponding author
| | - Thomas Baukrowitz
- Institute of Physiology II, Friedrich Schiller University, D-07743 Jena, Germany
- Corresponding author
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