1
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Nitric oxide signaling in yeast. Appl Microbiol Biotechnol 2016; 100:9483-9497. [DOI: 10.1007/s00253-016-7827-7] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/28/2016] [Revised: 08/17/2016] [Accepted: 08/22/2016] [Indexed: 12/11/2022]
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2
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Lewinska A, Bartosz G. Yeast flavohemoglobin protects against nitrosative stress and controls ferric reductase activity. Redox Rep 2013; 11:231-9. [PMID: 17132272 DOI: 10.1179/135100006x154987] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/31/2022] Open
Abstract
The role of Saccharomyces cerevisiae flavohemoglobin (Yhb1) is controversial and far from understood. This study compares the effects of nitrosative and oxidative challenge on the yeast mutant lacking the YHB1 gene. Growth of the mutant was impaired by nitrosoglutathione and peroxynitrite, whereas increased sensitivity to reactive oxygen species was not observed. Increased levels of intracellular NO(*) after incubation with NO(*) donors were found in the mutants cells as compared to the wild-type cells. Deletion of the YHB1 gene was found to augment the reduction of Fe(3+) by yeast cells which suggests that flavohemoglobin participates in regulation of the activity of plasma membrane ferric reductase(s).
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Affiliation(s)
- Anna Lewinska
- Department of Biochemistry and Cell Biology, University of Rzeszow, Rzeszow, Poland.
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3
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Horie T, Brodsky DE, Costa A, Kaneko T, Lo Schiavo F, Katsuhara M, Schroeder JI. K+ transport by the OsHKT2;4 transporter from rice with atypical Na+ transport properties and competition in permeation of K+ over Mg2+ and Ca2+ ions. PLANT PHYSIOLOGY 2011; 156:1493-507. [PMID: 21610181 PMCID: PMC3135959 DOI: 10.1104/pp.110.168047] [Citation(s) in RCA: 90] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/25/2010] [Accepted: 05/20/2011] [Indexed: 05/18/2023]
Abstract
Members of class II of the HKT transporters, which have thus far only been isolated from grasses, were found to mediate Na(+)-K(+) cotransport and at high Na(+) concentrations preferred Na(+)-selective transport, depending on the ionic conditions. But the physiological functions of this K(+)-transporting class II of HKT transporters remain unknown in plants, with the exception of the unique class II Na(+) transporter OsHKT2;1. The genetically tractable rice (Oryza sativa; background Nipponbare) possesses two predicted K(+)-transporting class II HKT transporter genes, OsHKT2;3 and OsHKT2;4. In this study, we have characterized the ion selectivity of the class II rice HKT transporter OsHKT2;4 in yeast and Xenopus laevis oocytes. OsHKT2;4 rescued the growth defect of a K(+) uptake-deficient yeast mutant. Green fluorescent protein-OsHKT2;4 is targeted to the plasma membrane in transgenic plant cells. OsHKT2;4-expressing oocytes exhibited strong K(+) permeability. Interestingly, however, K(+) influx in OsHKT2;4-expressing oocytes did not require stimulation by extracellular Na(+), in contrast to other class II HKT transporters. Furthermore, OsHKT2;4-mediated currents exhibited permeabilities to both Mg(2+) and Ca(2+) in the absence of competing K(+) ions. Comparative analyses of Ca(2+) and Mg(2+) permeabilities in several HKT transporters, including Arabidopsis thaliana HKT1;1 (AtHKT1;1), Triticum aestivum HKT2;1 (TaHKT2;1), OsHKT2;1, OsHKT2;2, and OsHKT2;4, revealed that only OsHKT2;4 and to a lesser degree TaHKT2;1 mediate Mg(2+) transport. Interestingly, cation competition analyses demonstrate that the selectivity of both of these class II HKT transporters for K(+) is dominant over divalent cations, suggesting that Mg(2+) and Ca(2+) transport via OsHKT2;4 may be small and would depend on competing K(+) concentrations in plants.
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4
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Yuchi Z, Pau VPT, Lu BX, Junop M, Yang DSC. An engineered right-handed coiled coil domain imparts extreme thermostability to the KcsA channel. FEBS J 2009; 276:6236-46. [PMID: 19780836 DOI: 10.1111/j.1742-4658.2009.07327.x] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
KcsA, a potassium channel from Streptomyces lividans, was the first ion channel to have its transmembrane domain structure determined by crystallography. Previously we have shown that its C-terminal cytoplasmic domain is crucial for the thermostability and the expression of the channel. Expression was almost abolished in its absence, but could be rescued by the presence of an artificial left-handed coiled coil tetramerization domain GCN4. In this study, we noticed that the handedness of GCN4 is not the same as the bundle crossing of KcsA. Therefore, a compatible right-handed coiled coil structure was identified from the Protein Data Bank and used to replace the C-terminal domain of KcsA. The hybrid channel exhibited a higher expression level than the wild-type and is extremely thermostable. Surprisingly, this stable hybrid channel is equally active as the wild-type channel in conducting potassium ions through a lipid bilayer at an acidic pH. We suggest that a similar engineering strategy could be applied to other ion channels for both functional and structural studies.
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Affiliation(s)
- Zhiguang Yuchi
- Department of Biochemistry and Biomedical Sciences, Faculty of Health Sciences, McMaster University, Hamilton, Canada
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5
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Dreyer I, Blatt MR. What makes a gate? The ins and outs of Kv-like K+ channels in plants. TRENDS IN PLANT SCIENCE 2009; 14:383-90. [PMID: 19540150 DOI: 10.1016/j.tplants.2009.04.001] [Citation(s) in RCA: 45] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/16/2009] [Revised: 04/06/2009] [Accepted: 04/07/2009] [Indexed: 05/18/2023]
Abstract
Gating of K(+) and other ion channels is 'hard-wired' within the channel protein. So it remains a puzzle how closely related channels in plants can show an unusually diverse range of biophysical properties. Gating of these channels lies at the heart of K(+) mineral nutrition, signalling, abiotic and biotic stress responses in plants. Thus, our knowledge of the molecular mechanics underpinning K(+) channel gating will be important for rational engineering of related traits in agricultural crops. Several key studies have added significantly to our understanding of channel gating in plants and have challenged current thinking about analogous processes found in animal K(+) channels. Such studies highlight how much of K(+) channel gating remains to be explored in plants.
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Affiliation(s)
- Ingo Dreyer
- Heisenberg-Group BPMPB, Institut für Biochemie und Biologie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, Potsdam-Golm, Germany
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6
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Santos JS, Grigoriev SM, Montal M. Molecular template for a voltage sensor in a novel K+ channel. III. Functional reconstitution of a sensorless pore module from a prokaryotic Kv channel. ACTA ACUST UNITED AC 2009; 132:651-66. [PMID: 19029373 PMCID: PMC2585861 DOI: 10.1085/jgp.200810077] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
KvLm is a prokaryotic voltage-gated K+ (Kv) channel from Listeria monocytogenes. The sequence of the voltage-sensing module (transmembrane segments S1-S4) of KvLm is atypical in that it contains only three of the eight conserved charged residues known to be deterministic for voltage sensing in eukaryotic Kv's. In contrast, the pore module (PM), including the S4-S5 linker and cytoplasmic tail (linker-S5-P-S6-C-terminus) of KvLm, is highly conserved. Here, the full-length (FL)-KvLm and the KvLm-PM only proteins were expressed, purified, and reconstituted into giant liposomes. The properties of the reconstituted FL-KvLm mirror well the characteristics of the heterologously expressed channel in Escherichia coli spheroplasts: a right-shifted voltage of activation, micromolar tetrabutylammonium-blocking affinity, and a single-channel conductance comparable to that of eukaryotic Kv's. Conversely, ionic currents through the PM recapitulate both the conductance and blocking properties of the FL-KvLm, yet the KvLm-PM exhibits only rudimentary voltage dependence. Given that the KvLm-PM displays many of the conduction properties of FL-KvLm and of other eukaryotic Kv's, including strict ion selectivity, we conclude that self-assembly of the PM subunits in lipid bilayers, in the absence of the voltage-sensing module, generates a conductive oligomer akin to that of the native KvLm, and that the structural independence of voltage sensing and PMs observed in eukaryotic Kv channels was initially implemented by nature in the design of prokaryotic Kv channels. Collectively, the results indicate that this robust functional module will prove valuable as a molecular template for coupling new sensors and to elucidate PM residue–specific contributions to Kv conduction properties.
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Affiliation(s)
- Jose S Santos
- Section of Neurobiology, Division of Biological Sciences, University of California, San Diego, La Jolla, CA 92093, USA
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7
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Gajdanowicz P, Garcia-Mata C, Gonzalez W, Morales-Navarro SE, Sharma T, González-Nilo FD, Gutowicz J, Mueller-Roeber B, Blatt MR, Dreyer I. Distinct roles of the last transmembrane domain in controlling Arabidopsis K+ channel activity. THE NEW PHYTOLOGIST 2009; 182:380-391. [PMID: 19192193 DOI: 10.1111/j.1469-8137.2008.02749.x] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
The family of voltage-gated potassium channels in plants presumably evolved from a common ancestor and includes both inward-rectifying (K(in)) channels that allow plant cells to accumulate K(+) and outward-rectifying (K(out)) channels that mediate K(+) efflux. Despite their close structural similarities, the activity of K(in) channels is largely independent of K(+) and depends only on the transmembrane voltage, whereas that of K(out) channels responds to the membrane voltage and the prevailing extracellular K(+) concentration. Gating of potassium channels is achieved by structural rearrangements within the last transmembrane domain (S6). Here we investigated the functional equivalence of the S6 helices of the K(in) channel KAT1 and the K(out) channel SKOR by domain-swapping and site-directed mutagenesis. Channel mutants and chimeras were analyzed after expression in Xenopus oocytes. We identified two discrete regions that influence gating differently in both channels, demonstrating a lack of functional complementarity between KAT1 and SKOR. Our findings are supported by molecular models of KAT1 and SKOR in the open and closed states. The role of the S6 segment in gating evolved differently during specialization of the two channel subclasses, posing an obstacle for the transfer of the K(+)-sensor from K(out) to K(in) channels.
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Affiliation(s)
- Pawel Gajdanowicz
- Universität Potsdam, Institut für Biochemie und Biologie, Heisenberg-Gruppe Biophysik und Molekulare Pflanzenbiologie, 14476 Potsdam/Golm, Germany
| | - Carlos Garcia-Mata
- Laboratory of Plant Physiology and Biophysics, IBLS Plant Sciences, Bower Building, University of Glasgow, Glasgow G12 8QQ, UK
- Laboratorio de Fisiologia Molecular e Integrativa, Institutos de Investigaciones Biologicas, Universidad Nacional de Mar del Plata, 7600 Mar del Plata, Buenos Aires, Argentina
| | - Wendy Gonzalez
- Universität Potsdam, Institut für Biochemie und Biologie, Heisenberg-Gruppe Biophysik und Molekulare Pflanzenbiologie, 14476 Potsdam/Golm, Germany
- Centro de Bioinformática y Simulación Molecular, Universidad de Talca, Casilla 721, Talca, Chile
| | | | - Tripti Sharma
- Universität Potsdam, Institut für Biochemie und Biologie, Heisenberg-Gruppe Biophysik und Molekulare Pflanzenbiologie, 14476 Potsdam/Golm, Germany
- Max-Planck Institute of Molecular Plant Physiology, 14476 Potsdam/Golm, Germany
| | | | - Jan Gutowicz
- Department of Physical Chemistry of Microorganisms, Institute of Genetics and Microbiology, University of Wrocław, 51148 Wrocław, Poland
| | - Bernd Mueller-Roeber
- Max-Planck Institute of Molecular Plant Physiology, 14476 Potsdam/Golm, Germany
- Universität Potsdam, Institut für Biochemie und Biologie, Abteilung Molekularbiologie, 14476 Potsdam/Golm, Germany
| | - Michael R Blatt
- Laboratory of Plant Physiology and Biophysics, IBLS Plant Sciences, Bower Building, University of Glasgow, Glasgow G12 8QQ, UK
| | - Ingo Dreyer
- Universität Potsdam, Institut für Biochemie und Biologie, Heisenberg-Gruppe Biophysik und Molekulare Pflanzenbiologie, 14476 Potsdam/Golm, Germany
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8
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Abstract
Distinct potassium, anion, and calcium channels in the plasma membrane and vacuolar membrane of plant cells have been identified and characterized by patch clamping. Primarily owing to advances in Arabidopsis genetics and genomics, and yeast functional complementation, many of the corresponding genes have been identified. Recent advances in our understanding of ion channel genes that mediate signal transduction and ion transport are discussed here. Some plant ion channels, for example, ALMT and SLAC anion channel subunits, are unique. The majority of plant ion channel families exhibit homology to animal genes; such families include both hyperpolarization- and depolarization-activated Shaker-type potassium channels, CLC chloride transporters/channels, cyclic nucleotide-gated channels, and ionotropic glutamate receptor homologs. These plant ion channels offer unique opportunities to analyze the structural mechanisms and functions of ion channels. Here we review gene families of selected plant ion channel classes and discuss unique structure-function aspects and their physiological roles in plant cell signaling and transport.
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Affiliation(s)
- John M. Ward
- Department of Plant Biology, University of Minnesota, St. Paul, Minnesota 55108;
| | - Pascal Mäser
- Institute of Cell Biology, University of Bern, CH-3012 Bern, Switzerland
| | - Julian I. Schroeder
- Division of Biological Sciences, Cell and Developmental Biology Section, University of California, San Diego, La Jolla, California 92093;
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9
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Single mutations convert an outward K+ channel into an inward K+ channel. Proc Natl Acad Sci U S A 2008; 105:2871-6. [PMID: 18287042 DOI: 10.1073/pnas.0712349105] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Shaker-type K(+) channels in plants display distinct voltage-sensing properties despite sharing sequence and structural similarity. For example, an Arabidopsis K(+) channel (SKOR) and a tomato K(+) channel (LKT1) share high amino acid sequence similarity and identical domain structures; however, SKOR conducts outward K(+) current and is activated by positive membrane potentials (depolarization), whereas LKT1 conducts inward current and is activated by negative membrane potentials (hyperpolarization). The structural basis for the "opposite" voltage-sensing properties of SKOR and LKT1 remains unknown. Using a screening procedure combined with random mutagenesis, we identified in the SKOR channel single amino acid mutations that converted an outward-conducting channel into an inward-conducting channel. Further domain-swapping and random mutagenesis produced similar results, suggesting functional interactions between several regions of SKOR protein that lead to specific voltage-sensing properties. Dramatic changes in rectifying properties can be caused by single amino acid mutations, providing evidence that the inward and outward channels in the Shaker family from plants may derive from the same ancestor.
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10
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Michard E, Lacombe B, Porée F, Mueller-Roeber B, Sentenac H, Thibaud JB, Dreyer I. A unique voltage sensor sensitizes the potassium channel AKT2 to phosphoregulation. ACTA ACUST UNITED AC 2006; 126:605-17. [PMID: 16316977 PMCID: PMC2266593 DOI: 10.1085/jgp.200509413] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Among all voltage-gated K+ channels from the model plant Arabidopsis thaliana, the weakly rectifying K+ channel (Kweak channel) AKT2 displays unique gating properties. AKT2 is exceptionally regulated by phosphorylation: when nonphosphorylated AKT2 behaves as an inward-rectifying potassium channel; phosphorylation of AKT2 abolishes inward rectification by shifting its activation threshold far positive (>200 mV) so that it closes only at voltages positive of +100 mV. In its phosphorylated form, AKT2 is thus locked in the open state in the entire physiological voltage range. To understand the molecular grounds of this unique gating behavior, we generated chimeras between AKT2 and the conventional inward-rectifying channel KAT1. The transfer of the pore from KAT1 to AKT2 altered the permeation properties of the channel. However, the gating properties were unaffected, suggesting that the pore region of AKT2 is not responsible for the unique Kweak gating. Instead, a lysine residue in S4, highly conserved among all Kweak channels but absent from other plant K+ channels, was pinpointed in a site-directed mutagenesis approach. Substitution of the lysine by serine or aspartate abolished the “open-lock” characteristic and converted AKT2 into an inward-rectifying channel. Interestingly, phosphoregulation of the mutant AKT2-K197S appeared to be similar to that of the Kin channel KAT1: as suggested by mimicking the phosphorylated and dephosphorylated states, phosphorylation induced a shift of the activation threshold of AKT2-K197S by about +50 mV. We conclude that the lysine residue K197 sensitizes AKT2 to phosphoregulation. The phosphorylation-induced reduction of the activation energy in AKT2 is ∼6 kT larger than in the K197S mutant. It is discussed that this hypersensitive response of AKT2 to phosphorylation equips a cell with the versatility to establish a potassium gradient and to make efficient use of it.
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Affiliation(s)
- Erwan Michard
- Universität Potsdam, Institut für Biochemie und Biologie, Abteilung Molekularbiologie, D-14476 Potsdam-Golm, Germany
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11
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Lai HC, Grabe M, Jan YN, Jan LY. The S4 voltage sensor packs against the pore domain in the KAT1 voltage-gated potassium channel. Neuron 2005; 47:395-406. [PMID: 16055063 DOI: 10.1016/j.neuron.2005.06.019] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2004] [Revised: 03/09/2005] [Accepted: 06/09/2005] [Indexed: 10/25/2022]
Abstract
In voltage-gated ion channels, the S4 transmembrane segment responds to changes in membrane potential and controls channel opening. The local environment of S4 is still unknown, even regarding the basic question as to whether S4 is close to the pore domain. Relying on the ability of functional KAT1 channels to rescue potassium (K+) transport-deficient yeast, we have performed an unbiased mutagenesis screen aimed at determining whether S4 packs against S5 of the pore domain. Starting with semilethal mutations of surface-exposed S5 residues of the KAT1 pore domain, we have screened randomly mutagenized libraries of S4 or S1-S3 for second-site suppressors. Our study identifies two S4 residues that interact in a highly specific manner with two S5 residues in the middle of the membrane-spanning regions, supporting a model in which the S4 voltage sensor packs against the pore domain in the hyperpolarized, or "down," state of S4.
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Affiliation(s)
- Helen C Lai
- Graduate Group in Biophysics, University of California, San Francisco, San Francisco, California 94143, USA
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12
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Porée F, Wulfetange K, Naso A, Carpaneto A, Roller A, Natura G, Bertl A, Sentenac H, Thibaud JB, Dreyer I. Plant K(in) and K(out) channels: approaching the trait of opposite rectification by analyzing more than 250 KAT1-SKOR chimeras. Biochem Biophys Res Commun 2005; 332:465-73. [PMID: 15894288 DOI: 10.1016/j.bbrc.2005.04.150] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2005] [Accepted: 04/29/2005] [Indexed: 10/25/2022]
Abstract
Members of the Shaker-like plant K(+) channel family share a common structure, but are highly diverse in their function: they behave as either hyperpolarization-activated inward-rectifying (K(in)) channels, or leak-like (K(weak)) channels, or depolarization-activated outward-rectifying (K(out)) channels. Here we created 256 chimeras between the K(in) channel KAT1 and the K(out) channel SKOR. The chimeras were screened in a potassium-uptake deficient yeast strain to identify those, which mediate potassium inward currents, i.e., which are functionally equivalent to KAT1. This strategy allowed us to identify three chimeras which differ from KAT1 in three parts of the polypeptide: the cytosolic N-terminus, the cytosolic C-terminus, and the putative voltage-sensor S4. Additionally, mutations in the K(out) channel SKOR were generated in order to localize molecular entities underlying its depolarization activation. The triple mutant SKOR-D312N-M313L-I314G, carrying amino-acid changes in the S6 segment, was identified as a channel which did not display any rectification in the tested voltage-range.
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Affiliation(s)
- Fabien Porée
- Biochimie et Physiologie Moléculaires des Plantes, UMR 5004, Agro.M-CNRS-INRA-UM2, Montpellier, France
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13
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Mura CV, Cosmelli D, Muñoz F, Delgado R. Orientation of Arabidopsis thaliana KAT1 channel in the plasma membrane. J Membr Biol 2005; 201:157-65. [PMID: 15711775 DOI: 10.1007/s00232-004-0713-8] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/04/2004] [Revised: 08/18/2004] [Indexed: 10/25/2022]
Abstract
The Arabidopsis thaliana KAT1, an inward-rectifying potassium channel, shares molecular features with the Shaker family of outward rectifier K(+) channels. The KAT1 amino-acid sequence reveals the presence of a positively charged S4 and a segment containing the TXGYGD signature sequence in the pore (P) region. To test whether the inward-rectifying properties of KAT1 are due to reverse orientation in the membrane, such that the voltage sensor is oriented in the opposite direction of the electric field compared with the Shaker K(+) channel, we have inserted a flag epitope in the NH(2) terminus or the S3-S4 loop. The KAT1 and tagged constructs expressed functional channels in whole cells, Xenopus oocytes and COS-7. The electrophysiological properties of both tagged constructs were similar to those of the wild type. Immunofluorescence with an antibody against the flag epitope and an anti-C terminal KAT1 determined the membrane localization of these epitopes and the orientation of the KAT1 channel in the membrane. Our data confirm that KAT1 in eukaryotic cells has an orientation similar to the Shaker K(+) channel.
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Affiliation(s)
- C V Mura
- Instituto Milenio de Estudios Avanzados en Biología Celular y Biotecnología, Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425, Nunoa, Santiago, Chile.
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14
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Latorre R, Olcese R, Basso C, Gonzalez C, Munoz F, Cosmelli D, Alvarez O. Molecular coupling between voltage sensor and pore opening in the Arabidopsis inward rectifier K+ channel KAT1. ACTA ACUST UNITED AC 2004; 122:459-69. [PMID: 14517271 PMCID: PMC2233774 DOI: 10.1085/jgp.200308818] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Abstract
Animal and plant voltage-gated ion channels share a common architecture. They are made up of four subunits and the positive charges on helical S4 segments of the protein in animal K+ channels are the main voltage-sensing elements. The KAT1 channel cloned from Arabidopsis thaliana, despite its structural similarity to animal outward rectifier K+ channels is, however, an inward rectifier. Here we detected KAT1-gating currents due to the existence of an intrinsic voltage sensor in this channel. The measured gating currents evoked in response to hyperpolarizing voltage steps consist of a very fast (τ = 318 ± 34 μs at −180 mV) and a slower component (4.5 ± 0.5 ms at −180 mV) representing charge moved when most channels are closed. The observed gating currents precede in time the ionic currents and they are measurable at voltages (less than or equal to −60) at which the channel open probability is negligible (≈10−4). These two observations, together with the fact that there is a delay in the onset of the ionic currents, indicate that gating charge transits between several closed states before the KAT1 channel opens. To gain insight into the molecular mechanisms that give rise to the gating currents and lead to channel opening, we probed external accessibility of S4 domain residues to methanethiosulfonate-ethyltrimethylammonium (MTSET) in both closed and open cysteine-substituted KAT1 channels. The results demonstrate that the putative voltage–sensing charges of S4 move inward when the KAT1 channels open.
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Affiliation(s)
- Ramon Latorre
- Laboratory of Biophysics and Molecular Physiology, Centro de Estudios Científicos, Valdivia, Chile.
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15
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Senn ME, Rubio F, Bañuelos MA, Rodríguez-Navarro A. Comparative functional features of plant potassium HvHAK1 and HvHAK2 transporters. J Biol Chem 2001; 276:44563-9. [PMID: 11562376 DOI: 10.1074/jbc.m108129200] [Citation(s) in RCA: 59] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Plant K(+) transporters of the HAK family belong to four rather divergent phylogenetic clusters, although most of the transporters belong to clusters I or II. A simple phylogenetic analysis of fungal and plant HAK transporters suggests that an original HAK gene duplicated even before fungi and plants diverged, generating transporters that at present fulfill different functions in the plant. The HvHAK1 transporter belongs to cluster I and mediates high-affinity K(+) uptake in barley roots, but no function is known for the cluster II transporter, HvHAK2, which is not functional in yeast. The function of HvHAK2 was investigated by constructing HvHAK1-HAK2 chimeric transporters, which were not functional even when they included only short fragments of HvHAK2. Then, amino acids characteristic of cluster II in the N terminus and in the first transmembrane domain were introduced into HvHAK1. All of these changes increased the Rb(+) K(m), introducing minimal changes in the Na(+) K(m), which suggested that HvHAK2 is a low-affinity, Na(+)-sensitive K(+) transporter. Using a K(+)-defective Escherichia coli mutant, we functionally expressed HvHAK2 and found that the predicted characteristics were correct, as well as discovering that the bacterial expression of HvHAK2 is functional at pH 5.5 but not at 7.5. We discuss whether HvHAK2 may be a tonoplast transporter effective for vacuolar K(+) depletion in K(+) starved plants.
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Affiliation(s)
- M E Senn
- Departamento de Biotecnologia, Escuela Técnica Superior de Ingenieros Agrónomos, Universidad Politécnica de Madrid, 28040 Madrid, Spain
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16
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Uozumi N. Escherichia coli as an expression system for K(+) transport systems from plants. Am J Physiol Cell Physiol 2001; 281:C733-9. [PMID: 11502550 DOI: 10.1152/ajpcell.2001.281.3.c733] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The value of the Escherichia coli expression system has long been established because of its effectiveness in characterizing the structure and function of exogenously expressed proteins. When eukaryotic membrane proteins are functionally expressed in E. coli, this organism can serve as an alternative to eukaryotic host cells. A few examples have been reported of functional expression of animal and plant membrane proteins in E. coli. This mini-review describes the following findings: 1) homologous K(+) transporters exist in prokaryotic cells and in eukaryotic cells; 2) plant K(+) transporters can functionally complement mutant K(+) transporter genes in E. coli; and 3) membrane structures of plant K(+) transporters can be elucidated in an E. coli system. These experimental findings suggest the possibility of utilizing the E. coli bacterium as an expression system for other eukaryotic membrane transport proteins.
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Affiliation(s)
- N Uozumi
- Bioscience Center, Nagoya University, Nagoya 464-8601, Japan.
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17
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Urbach S, Chérel I, Sentenac H, Gaymard F. Biochemical characterization of the Arabidopsis K+ channels KAT1 and AKT1 expressed or co-expressed in insect cells. THE PLANT JOURNAL : FOR CELL AND MOLECULAR BIOLOGY 2000; 23:527-38. [PMID: 10972879 DOI: 10.1046/j.1365-313x.2000.00828.x] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
KAT1 and AKT1 belong to the multigenic family of the inwardly rectifying Shaker-like plant K+ channels. They were biochemically characterized after expression in insect cells using recombinant baculoviruses. The channels were solubilized from microsomal fractions prepared from infected cells (among eight different detergents only one, L-alpha-lysophosphatidylcholine, was efficient for solubilization), and purified to homogeneity using immunoaffinity (KAT1) or ion-exchange and size exclusion (AKT1) techniques. The following results were obtained with the purified polypeptides: (i) neither KAT1 nor AKT1 was found to be glycosylated; (ii) both polypeptides were mainly present as homotetrameric structures, supporting the hypothesis of a tetrameric structure for the functional channels; (iii) no heteromeric KAT1/AKT1 assembly was detected when the two polypeptides were co-expressed in insect cells. The use of the two-hybrid system in yeast also failed to detect any interaction between KAT1 and AKT1 polypeptides. Because of these negative results, the hypothesis that plant K+-channel subunits are able to co-assemble without any discrimination, previously put forward based on co-expression in Xenopus oocytes of various K+-channel subunits (including KAT1 and AKT1), has still to be supported by independent approaches. Co-localization of channel subunits within the same plant tissue/cell does not allow us to conclude that the subunits form heteromultimeric channels.
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Affiliation(s)
- S Urbach
- Laboratoire de Biochimie et Physiologie Moléculaire des Plantes, UMR 5004, Agro-M/CNRS/INRA/UMII, 34060 Montpellier Cedex 1, France
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18
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Miller AJ, Zhou JJ. Xenopus oocytes as an expression system for plant transporters. BIOCHIMICA ET BIOPHYSICA ACTA 2000; 1465:343-58. [PMID: 10748264 DOI: 10.1016/s0005-2736(00)00148-6] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
Abstract
The Xenopus oocyte provides a powerful system for the expression and characterisation of plant membrane proteins. Many different types of plant membrane proteins have been expressed and characterised using this system. As there are already several general reviews on the methodology for oocyte expression of channel proteins, we have summarised the particular advantages and disadvantages of using the system for the characterisation of plant cotransporter proteins. As an example of how the system can be used to identify transporters, we describe evidence for a low affinity nitrate transporter in oocytes injected with poly(A) RNA extracted from nitrate-induced barley roots. Furthermore, we describe evidence that the expression of some transporters in oocytes can modify the properties of endogenous membrane proteins. We conclude that although care must be taken in the interpretation of results and in choosing appropriate controls for experiments, oocyte expression is an excellent tool which will have an important role in characterising plant membrane proteins.
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Affiliation(s)
- A J Miller
- Biochemistry and Physiology Department, IACR-Rothamsted, Harpenden, UK.
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19
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Schachtman DP. Molecular insights into the structure and function of plant K(+) transport mechanisms. BIOCHIMICA ET BIOPHYSICA ACTA 2000; 1465:127-39. [PMID: 10748250 DOI: 10.1016/s0005-2736(00)00134-6] [Citation(s) in RCA: 84] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
Our understanding of plant potassium transport has increased in the past decade through the application of molecular biological techniques. In this review, recent work on inward and outward rectifying K(+) channels as well as high affinity K(+) transporters is described. Through the work on inward rectifying K(+) channels, we now have precise details on how the structure of these proteins determines functional characteristics such as ion conduction, pH sensitivity, selectivity and voltage sensing. The physiological function of inward rectifying K(+) channels in plants has been clarified through the analysis of expression patterns and mutational analysis. Two classes of outward rectifying K(+) channels have now been cloned from plants and their initial characterisation is reviewed. The physiological role of one class of outward rectifying K(+) channel has been demonstrated to be involved in long distance transport of K(+) from roots to shoots. The molecular structure and function of two classes of energised K(+) transporters are also reviewed. The first class is energised by Na(+) and shares structural similarities with K(+) transport mechanisms in bacteria and fungi. Structure-function studies suggest that it should be possible to increase the K(+) and Na(+) selectivity of these transporters, which will enhance the salt tolerance of higher plants. The second class of K(+) transporter is comprised of a large gene family and appears to have a dual affinity for K(+). A suite of molecular techniques, including gene cloning, oocyte expression, RNA localisation and gene inactivation, is now being used to fully characterise the biophysical and physiological function of plants K(+) transport mechanisms.
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Affiliation(s)
- D P Schachtman
- CSIRO Plant Industry, Horticulture Unit, GPO Box 350, Glen Osmond, Australia.
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20
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Trajkovic V, Stepanovic S, Samardzic T, Jankovic V, Badovinac V, Mostarica Stojkovic M. Cryptococcus neoformans neutralizes macrophage and astrocyte derived nitric oxide without interfering with inducible nitric oxide synthase induction or catalytic activity - possible involvement of nitric oxide consumption. Scand J Immunol 2000; 51:384-91. [PMID: 10736111 DOI: 10.1046/j.1365-3083.2000.00683.x] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The effect of Cryptococcus neoformans on the accumulation of nitrite, an indicator of nitric oxide (NO) synthesis, was investigated in cytokine (interferon-gamma [IFN-gamma] and interleukin [IL]-1)-stimulated cultures of rat peritoneal macrophages and C6 astrocytoma cells. Cytokine-induced nitrite generation in cultures of both cell types was inhibited in a dose-dependent manner by live C. neoformans, but not by heat-killed cryptococcal cells or conditioned medium from yeast cultures. C. neoformans-mediated reduction of nitrite formation coincided with impairment of NO-dependent macrophage tumoricidal activity. Cytokine-triggered induction of inducible NO synthase (iNOS) was unaffected in C6 cells, and only marginally reduced in macrophages. When cells were pretreated with cytokines for 24 h to induce iNOS, and any further induction was prevented by inhibition of protein synthesis, C. neoformans was still able to reduce nitrite accumulation in cultures of both cell types. Finally, live C. neoformans, but not heat-killed yeast cells or yeast culture supernatant, significantly reduced nitrite production in a culture solution of NO-releasing compound S-nitrosoglutathione (GSNO). Thus, it appears that cryptococcal reduction of nitrite formation in macrophage and C6 cultures was caused by the consumption of NO by some yeast molecule, rather than by the inhibition of cellular NO synthesis.
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Affiliation(s)
- V Trajkovic
- Institute of Microbiology, School of Medicine, University of Belgrade; Institute for Biological Research 'Sinisa Stankovic', Belgrade, Yugoslavia
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21
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Patten CD, Caprini M, Planells-Cases R, Montal M. Structural and functional modularity of voltage-gated potassium channels. FEBS Lett 1999; 463:375-81. [PMID: 10606757 DOI: 10.1016/s0014-5793(99)01659-2] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
Sequence similarity among known potassium channels indicates the voltage-gated potassium channels consist of two modules: the N-terminal portion of the channel up to and including transmembrane segment S4, called in this paper the 'sensor' module, and the C-terminal portion from transmembrane segment S5 onwards, called the 'pore' module. We investigated the functional role of these modules by constructing chimeric channels which combine the 'sensor' from one native voltage-gated channel, mKv1.1, with the 'pore' from another, Shaker H4, and vice versa. Functional studies of the wild type and chimeric channels show that these modules can operate outside their native context. Each channel has a unique conductance-voltage relation. Channels incorporating the mKv1.1 sensor module have similar rates of activation while channels having the Shaker pore module show similar rates of deactivation. This observation suggests the mKv1.1 sensor module limits activation and the Shaker pore module determines deactivation. We propose a model that explains the observed equilibrium and kinetic properties of the chimeric constructs in terms of the characteristics of the native modules and a novel type of intrasubunit cooperativity. The properties ascribed to the modules are the same whether the modules function in their native context or have been assembled into a chimera.
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Affiliation(s)
- C D Patten
- Department of Physics, University of California at San Diego, La Jolla, CA, USA.
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22
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Zhang X, Ma J, Berkowitz GA. Evaluation of functional interaction between K(+) channel alpha- and beta-subunits and putative inactivation gating by Co-expression in Xenopus laevis oocytes. PLANT PHYSIOLOGY 1999; 121:995-1002. [PMID: 10557249 PMCID: PMC59464 DOI: 10.1104/pp.121.3.995] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/29/1999] [Accepted: 07/22/1999] [Indexed: 05/18/2023]
Abstract
Animal K(+) channel alpha- (pore-forming) subunits form native proteins by association with beta-subunits, which are thought to affect channel function by modifying electrophysiological parameters of currents (often by inducing fast inactivation) or by stabilizing the protein complex. We evaluated the functional association of KAT1, a plant K(+) channel alpha-subunit, and KAB1 (a putative homolog of animal K(+) channel beta-subunits) by co-expression in Xenopus laevis oocytes. Oocytes expressing KAT1 displayed inward-rectifying, non-inactivating K(+) currents that were similar in magnitude to those reported in prior studies. K(+) currents recorded from oocytes expressing both KAT1 and KAB1 had similar gating kinetics. However, co-expression resulted in greater total current, consistent with the possibility that KAB1 is a beta-subunit that stabilizes and therefore enhances surface expression of K(+) channel protein complexes formed by alpha-subunits such as KAT1. K(+) channel protein complexes formed by alpha-subunits such as KAT1 that undergo (voltage-dependent) inactivation do so by means of a "ball and chain" mechanism; the ball portion of the protein complex (which can be formed by the N terminus of either an alpha- or beta-subunit) occludes the channel pore. KAT1 was co-expressed in oocytes with an animal K(+) channel alpha-subunit (hKv1.4) known to contain the N-terminal ball and chain. Inward currents through heteromeric hKv1. 4:KAT1 channels did undergo typical voltage-dependent inactivation. These results suggest that inward currents through K(+) channel proteins formed at least in part by KAT1 polypeptides are capable of inactivation, but the structural component facilitating inactivation is not present when channel complexes are formed by either KAT1 or KAB1 in the absence of additional subunits.
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Affiliation(s)
- X Zhang
- Department of Plant Science, University of Connecticut, Storrs, Connecticut 06269-4067, USA
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23
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Uozumi N, Nakamura T, Schroeder JI, Muto S. Determination of transmembrane topology of an inward-rectifying potassium channel from Arabidopsis thaliana based on functional expression in Escherichia coli. Proc Natl Acad Sci U S A 1998; 95:9773-8. [PMID: 9707551 PMCID: PMC21412 DOI: 10.1073/pnas.95.17.9773] [Citation(s) in RCA: 69] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
We report here that the inward-rectifying potassium channels KAT1 and AKT2 were functionally expressed in K+ uptake-deficient Escherichia coli. Immunological assays showed that KAT1 was translocated into the cell membrane of E. coli. Functional assays suggested that KAT1 was inserted topologically correctly into the cell membrane. In control experiments, the inactive point mutation in KAT1, T256R, did not complement for K+ uptake in E. coli. The inward-rectifying K+ channels of plants share a common hydrophobic domain comprising at least six membrane-spanning segments (S1-S6). The finding that a K+ channel can be expressed in bacteria was further exploited to determine the KAT1 membrane topology by a gene fusion approach using the bacterial reporter enzymes, alkaline phosphatase, which is active only in the periplasm, and beta-galactosidase. The enzyme activity from the alkaline phosphatase and beta-galactosidase fusion plasmid showed that the widely predicted S1, S2, S5, and S6 segments were inserted into the membrane. Although the S3 segment in the alkaline phosphatase fusion protein could not function as an export signal, the replacement of a negatively charged residue inside S3 with a neutral amino acid resulted in an increase in alkaline phosphatase activity, which indicates that the alkaline phosphatase was translocated into the periplasm. For membrane translocation of S3, the neutralization of a negatively charged residue in S3 may be required presumably because of pairing with a positively charged residue of S4. These results revealed that KAT1 has the common six transmembrane-spanning membrane topology that has been predicted for the Shaker superfamily of voltage-dependent K+ channels. Furthermore, the functional complementation of a bacterial K+ uptake mutant in this study is shown to be an alternative expression system for plant K+ channel proteins and a potent tool for their topological analysis.
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Affiliation(s)
- N Uozumi
- Bioscience Center, Nagoya University, Nagoya, 464-8601, Japan.
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24
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Pennefather PS, Zhou W, DeCoursey TE. Idiosyncratic gating of HERG-like K+ channels in microglia. J Gen Physiol 1998; 111:795-805. [PMID: 9607937 PMCID: PMC2217153 DOI: 10.1085/jgp.111.6.795] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/1997] [Accepted: 03/18/1998] [Indexed: 11/20/2022] Open
Abstract
A simple kinetic model is presented to explain the gating of a HERG-like voltage-gated K+ conductance described in the accompanying paper (Zhou, W., F.S. Cayabyab, P.S. Pennefather, L.C. Schlichter, and T.E. DeCoursey. 1998. J. Gen. Physiol. 111:781-794). The model proposes two kinetically distinct closing pathways, a rapid one favored by depolarization (deactivation) and a slow one favored by hyperpolarization (inactivation). The overlap of these two processes leads to a window current between -50 and +20 mV with a peak at -36 mV of approximately 12% maximal conductance. The near absence of depolarization-activated outward current in microglia, compared with HERG channels expressed in oocytes or cardiac myocytes, can be explained if activation is shifted negatively in microglia. As seen with experimental data, availability predicted by the model was more steeply voltage dependent, and the midpoint more positive when determined by making the holding potential progressively more positive at intervals of 20 s (starting at -120 mV), rather than progressively more negative (starting at 40 mV). In the model, this hysteresis was generated by postulating slow and ultra-slow components of inactivation. The ultra-slow component takes minutes to equilibrate at -40 mV but is steeply voltage dependent, leading to protocol-dependent modulation of the HERG-like current. The data suggest that "deactivation" and "inactivation" are coupled through the open state. This is particularly evident in isotonic Cs+, where a delayed and transient outward current develops on depolarization with a decay time constant more voltage dependent and slower than the deactivation process observed at the same potential after a brief hyperpolarization.
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Affiliation(s)
- P S Pennefather
- Faculty of Pharmacy, University of Toronto, Toronto, Ontario M5S 2S2, Canada.
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25
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Abstract
This review summarizes current knowledge about genes whose products function in the transport of various cationic macronutrients (K, Ca) and micronutrients (Cu, Fe, Mn, and Zn) in plants. Such genes have been identified on the basis of function, via complementation of yeast mutants, or on the basis of sequence similarity, via database analysis, degenerate PCR, or low stringency hybridization. Not surprisingly, many of these genes belong to previously described transporter families, including those encoding Shaker-type K+ channels, P-type ATPases, and Nramp proteins. ZIP, a novel cation transporter family first identified in plants, also seems to be ubiquitous; members of this family are found in protozoa, yeast, nematodes, and humans. Emerging information on where in the plant each transporter functions and how each is controlled in response to nutrient availability may allow creation of food crops with enhanced mineral content as well as crops that bioaccumulate or exclude toxic metals.
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Affiliation(s)
- Tama Christine Fox
- Department of Biological Sciences, Dartmouth College, 6044 Gilman, Hanover, New Hampshire 03755; e-mail:
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26
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Bei Q, Luan S. Functional expression and characterization of a plant K+ channel gene in a plant cell model. THE PLANT JOURNAL : FOR CELL AND MOLECULAR BIOLOGY 1998; 13:857-65. [PMID: 9681022 DOI: 10.1046/j.1365-313x.1998.00084.x] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
Abstract
To express and characterize the function of a plant ion channel gene in plant cells, it is necessary to establish a model system that lacks the endogenous channel activity and can be genetically transformed. Patch-clamp techniques were used to survey voltage-dependent K+ channel activities in different cell types of tobacco plants. Interestingly, mesophyll cells lacked the inward K+ current found in guard cells. A transgene containing the inward K+ channel gene KAT1 from Arabidopsis was constructed and expressed in the mesophyll cells of transgenic tobacco plants. Expression of the KAT1 gene produced a large voltage-dependent inward current across the plasma membrane of mesophyll protoplasts. The KAT1 current was carried by K+ and activated at voltage more negative than -100 mV. This K+ current had a single-channel conductance of 6-10 pS and was highly sensitive to TEA, Cs+ and Ba2+. This study represents the first example in which a plant ion channel gene is functionally expressed and studied in plant cells. Tobacco mesophyll cells will provide a useful model for functional characterization of inward K+ channel genes from higher plants.
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Affiliation(s)
- Q Bei
- Department of Plant and Microbial Biology, University of California, Berkeley 94720, USA
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27
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Dikshit KL, Orii Y, Navani N, Patel S, Huang HY, Stark BC, Webster DA. Site-directed mutagenesis of bacterial hemoglobin: the role of glutamine (E7) in oxygen-binding in the distal heme pocket. Arch Biochem Biophys 1998; 349:161-6. [PMID: 9439594 DOI: 10.1006/abbi.1997.0432] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
The bacterial and yeast hemoglobins have a glutamine instead of histidine in the E7 position of the distal heme pocket. The recently determined crystal structure of Vitreoscilla hemoglobin (VHb) indicates that this residue is oriented out of the heme pocket and may not ligand the bound oxygen. This is in contrast to elephant myoglobin which also has a Gln(E7) but which does ligand the bound oxygen. This residue was changed in VHb using site-directed mutagenesis to leucine (VHbL) or to histidine (VHbH). Spectral and kinetic studies of the binding of oxygen and CO to VHbL showed that this substitution had little effect on the ligand-binding properties of this protein, evidence that Gln(E7) does not H-bond the bound ligand, in agreement with the findings of the crystallographic study of VHb. In contrast, the functional properties of VHbH were drastically altered in a way suggesting that the E7His may itself be liganded to the heme iron. These studies are further evidence that the distal heme pocket in VHb and related microbial hemoglobins differs from that in mammalian hemoglobins and may resemble in some ways the heme pocket in cytochrome b5.
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Affiliation(s)
- K L Dikshit
- Institute of Microbial Technology, Chandigarh, India
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28
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Nakamura RL, Anderson JA, Gaber RF. Determination of key structural requirements of a K+ channel pore. J Biol Chem 1997; 272:1011-8. [PMID: 8995396 DOI: 10.1074/jbc.272.2.1011] [Citation(s) in RCA: 87] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
Among the highly conserved sites in K+ channel pores, the tyrosine-glycine sequence is believed to play an important role in selectivity. Here we describe a novel approach in which comprehensive mutagenesis of the YG sites of the voltage-gated K+ channel, Kat1, is combined with phenotypic screening in Saccharomyces cerevisiae and electrophysiological analysis in Xenopus oocytes to determine the roles of these sites in K+ selectivity. We show that structural constraints necessitate a tyrosine or phenylalanine at the first position to confer full K+ selectivity. Substitution to arginine creates a channel titratable by external pH, suggesting that the side group at this position may line the channel pore. Permeation is abolished by any increase in bulk at the adjacent glycine position unless accompanied by a compensatory mutation at the tyrosine site. These results suggest a model in which the selectivity filter of the K+ channel requires an aromatic residue paired with glycine within the pore loop in order to maintain maximal K+ selectivity.
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Affiliation(s)
- R L Nakamura
- Department of Biochemistry, Molecular Biology, and Cell Biology, Northwestern University, Evanston, Illinois 60208, USA
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29
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Abstract
Potassium channels contribute to the excitability of neurons and signaling in the nervous system. They arise from multiple gene families including one for voltage-gated potassium channels and one for inwardly rectifying potassium channels. Features of potassium permeation, channel gating and regulation, and subunit interaction have been analyzed. Potassium channels of similar design have been found in animals ranging from jellyfish to humans, as well as in plants, yeast, and bacteria. Structural similarities are evident for the pore-forming alpha subunits and for the beta subunits, which could potentially regulate channel activity according to the level of energy and/or reducing power of the cell.
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Affiliation(s)
- L Y Jan
- Department of Physiology, University of California, San Francisco 94143-0724, USA
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30
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Abstract
Recently developed molecular and genetic approaches have enabled the identification and functional characterization of novel genes encoding ion channels, ion carriers, and water channels of the plant plasma membrane.
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Affiliation(s)
- S M Assmann
- Biology Department, Pennsylvania State University, 208 Mueller Laboratory, University Park, PA 16802, USA.
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31
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Smith PL, Baukrowitz T, Yellen G. The inward rectification mechanism of the HERG cardiac potassium channel. Nature 1996; 379:833-6. [PMID: 8587608 DOI: 10.1038/379833a0] [Citation(s) in RCA: 569] [Impact Index Per Article: 20.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
A human genetic defect associated with 'long Q-T syndrome', an abnormality of cardiac rhythm involving the repolarization of the action potential, was recently found to lie in the HERG gene, which codes for a potassium channel. The HERG K+ channel is unusual in that it seems to have the architectural plan of the depolarization-activated K+ channel family (six putative transmembrane segments), yet it exhibits rectification like that of the inward-rectifying K+ channels, a family with different molecular structure (two transmembrane segments). We have studied HERG channels expressed in mammalian cells and find that this inward rectification arises from a rapid and voltage-dependent inactivation process that reduces conductance at positive voltages. The inactivation gating mechanism resembles that of C-type inactivation, often considered to be the 'slow inactivation' mechanism of other K+ channels. The characteristics of this gating suggest a specific role for this channel in the normal suppression of arrhythmias.
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Affiliation(s)
- P L Smith
- Department of Neurobiology, Harvard Medical School, Boston, MA 02114-2698, USA
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32
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Uozumi N, Gassmann W, Cao Y, Schroeder JI. Identification of strong modifications in cation selectivity in an Arabidopsis inward rectifying potassium channel by mutant selection in yeast. J Biol Chem 1995; 270:24276-81. [PMID: 7592636 DOI: 10.1074/jbc.270.41.24276] [Citation(s) in RCA: 87] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
The Arabidopsis thaliana cDNA, KAT1, encodes a hyperpolarization-activated K+ channel. In the present study, we utilized a combination of random site-directed mutagenesis, genetic screening in a potassium uptake-deficient yeast strain, and electrophysiological analysis in Xenopus oocytes to identify strong modifications in cation selectivity of the inward rectifying K+ channel KAT1. Threonine at position 256 was replaced by 11 other amino acid residues. Six of these mutated KAT1 cDNAs complemented a K+ uptake-deficient yeast strain at low concentrations of potassium. Among these, two mutants (T256D and T256G) showed a sensitivity of yeast growth toward high ammonium concentrations and a dramatic increase in current amplitudes of rubidium and ammonium ions relative to K+ by 39-72-fold. These single site mutations gave rise to Rb+- and NH4(+)-selective channels with Rb+ and NH4+ currents that were approximately 10-13-fold greater in amplitude than K+ currents, whereas the NH4+ to K+ current amplitude ratio of wild type KAT1 was 0.28. This strong conversion in cation specificity without loss of general selectivity exceeds those reported for other mutations in the pore domain of voltage-dependent K+ channels. Yeast growth was greatly impaired by sodium in two other mutants at this site (T256E and T256Q), which were blocked by millimolar sodium (K1/2 = 1.1 mM for T256E), although the wild type channel was not blocked by 110 mM sodium. Interestingly, the ability of yeast to grow in the presence of toxic cations correlated to biophysical properties of KAT1 mutants, illustrating the potential for qualitative K+ channel mutant selection in yeast. These data suggest that the size of the side chain of the amino acid at position 256 in KAT1 is important for enabling cation permeation and that this site plays a crucial role in determining the cation selectivity of hyperpolarization-activated potassium channels.
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Affiliation(s)
- N Uozumi
- Department of Biology, University of California, San Diego, La Jolla 92093-0116, USA
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