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Eisenhauer K, Weber W, Kemp P, Gebhardt C, Kaufmann M, Tewes N, Zhdanova H, Tietze A, Rauh O, Stein V. Scaling the Functional Nanopore (FuN) Screen: Systematic Evaluation of Self-Assembling Membrane Peptides and Extension with a K +-Responsive Fluorescent Protein Sensor. ACS Synth Biol 2024; 13:1382-1392. [PMID: 38598783 DOI: 10.1021/acssynbio.3c00671] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 04/12/2024]
Abstract
The functional analysis of protein nanopores is typically conducted in planar lipid bilayers or liposomes exploiting high-resolution but low-throughput electrical and optical read-outs. Yet, the reconstitution of protein nanopores in vitro still constitutes an empiric and low-throughput process. Addressing these limitations, nanopores can now be analyzed using the functional nanopore (FuN) screen exploiting genetically encoded fluorescent protein sensors that resolve distinct nanopore-dependent Ca2+ in- and efflux patterns across the inner membrane of Escherichia coli. With a primary proof-of-concept established for the S2168 holin, and thereof based recombinant nanopore assemblies, the question arises to what extent alternative nanopores can be analyzed with the FuN screen and to what extent alternative fluorescent protein sensors can be adapted. Focusing on self-assembling membrane peptides, three sets of 13 different nanopores are assessed for their capacity to form nanopores in the context of the FuN screen. Nanopores tested comprise both natural and computationally designed nanopores. Further, the FuN screen is extended to K+-specific fluorescent protein sensors and now provides a capacity to assess the specificity of a nanopore or ion channel. Finally, a comparison to high-resolution biophysical and electrophysiological studies in planar lipid bilayers provides an experimental benchmark for future studies.
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Affiliation(s)
- Klara Eisenhauer
- Department of Biology, TU Darmstadt, 64287 Darmstadt, Germany
- Centre for Synthetic Biology, TU Darmstadt, 64283 Darmstadt, Germany
| | - Wadim Weber
- Department of Biology, TU Darmstadt, 64287 Darmstadt, Germany
- Centre for Synthetic Biology, TU Darmstadt, 64283 Darmstadt, Germany
| | - Philipp Kemp
- Department of Biology, TU Darmstadt, 64287 Darmstadt, Germany
- Centre for Synthetic Biology, TU Darmstadt, 64283 Darmstadt, Germany
| | - Carolin Gebhardt
- Department of Biology, TU Darmstadt, 64287 Darmstadt, Germany
- Centre for Synthetic Biology, TU Darmstadt, 64283 Darmstadt, Germany
| | - Marwan Kaufmann
- Department of Biology, TU Darmstadt, 64287 Darmstadt, Germany
- Centre for Synthetic Biology, TU Darmstadt, 64283 Darmstadt, Germany
| | - Noel Tewes
- Department of Biology, TU Darmstadt, 64287 Darmstadt, Germany
| | - Hanna Zhdanova
- Department of Chemistry and Molecular Biology, Wallenberg Centre for Molecular and Translational Medicine, University of Gothenburg, 412 96 Göteborg, Sweden
| | - Alesia Tietze
- Department of Chemistry and Molecular Biology, Wallenberg Centre for Molecular and Translational Medicine, University of Gothenburg, 412 96 Göteborg, Sweden
| | - Oliver Rauh
- Department of Biology, TU Darmstadt, 64287 Darmstadt, Germany
| | - Viktor Stein
- Department of Biology, TU Darmstadt, 64287 Darmstadt, Germany
- Centre for Synthetic Biology, TU Darmstadt, 64283 Darmstadt, Germany
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2
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Asrani P, Seebohm G, Stoll R. Potassium viroporins as model systems for understanding eukaryotic ion channel behaviour. Virus Res 2022; 320:198903. [PMID: 36037849 DOI: 10.1016/j.virusres.2022.198903] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/14/2022] [Revised: 08/24/2022] [Accepted: 08/25/2022] [Indexed: 11/29/2022]
Abstract
Ion channels are membrane proteins essential for a plethora of cellular functions including maintaining cell shape, ion homeostasis, cardiac rhythm and action potential in neurons. The complexity and often extensive structure of eukaryotic membrane proteins makes it difficult to understand their basic biological regulation. Therefore, this article suggests, viroporins - the miniature versions of eukaryotic protein homologs from viruses - might serve as model systems to provide insights into behaviour of eukaryotic ion channels in general. The structural requirements for correct assembly of the channel along with the basic functional properties of a K+ channel exist in the minimal design of the viral K+ channels from two viruses, Chlorella virus (Kcv) and Ectocarpus siliculosus virus (Kesv). These small viral proteins readily assemble into tetramers and they sort in cells to distinct target membranes. When these viruses-encoded channels are expressed into the mammalian cells, they utilise their protein machinery and hence can serve as excellent tools to study the cells protein sorting machinery. This combination of small size and robust function makes viral K+ channels a valuable model system for detection of basic structure-function correlations. It is believed that molecular and physiochemical analyses of these viroporins may serve as basis for the development of inhibitors or modulators to ion channel activity for targeting ion channel diseases - so called channelopathies. Therefore, it may provide a potential different scope for molecular pharmacology studies aiming at novel and innovative therapeutics associated with channel related diseases. This article reviews the structural and functional properties of Kcv and Kesv upon expression in mammalian cells and Xenopus oocytes. The mechanisms behind differential protein sorting in Kcv and Kesv are also thoroughly discussed.
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Affiliation(s)
- Purva Asrani
- Biomolecular Spectroscopy and RUBiospec|NMR, Faculty of Chemistry and Biochemistry, Ruhr University of Bochum, Bochum D-44780, Germany
| | - Guiscard Seebohm
- Institute for Genetics of Heart Diseases (IfGH), Department of Cardiovascular Medicine, University Hospital Münster, Münster D-48149, Germany
| | - Raphael Stoll
- Biomolecular Spectroscopy and RUBiospec|NMR, Faculty of Chemistry and Biochemistry, Ruhr University of Bochum, Bochum D-44780, Germany.
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3
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Weber W, Roeder M, Probanowski T, Yang J, Abujubara H, Koeppl H, Tietze A, Stein V. Functional Nanopore Screen: A Versatile High-Throughput Assay to Study and Engineer Protein Nanopores in Escherichia coli. ACS Synth Biol 2022; 11:2070-2079. [PMID: 35604782 DOI: 10.1021/acssynbio.1c00635] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Nanopores comprise a versatile class of membrane proteins that carry out a range of key physiological functions and are increasingly developed for different biotechnological applications. Yet, a capacity to study and engineer protein nanopores by combinatorial means has so far been hampered by a lack of suitable assays that combine sufficient experimental resolution with throughput. Addressing this technological gap, the functional nanopore (FuN) screen now provides a quantitative and dynamic readout of nanopore assembly and function in the context of the inner membrane of Escherichia coli. The assay is based on genetically encoded fluorescent protein sensors that resolve the nanopore-dependent influx of Ca2+ across the inner membrane of E. coli. Illustrating its versatile capacity, the FuN screen is first applied to dissect the molecular features that underlie the assembly and stability of nanopores formed by the S2168 holin. In a subsequent step, nanopores are engineered by recombining the transmembrane module of S2168 with different ring-shaped oligomeric protein structures that feature defined hexa-, hepta-, and octameric geometries. Library screening highlights substantial plasticity in the ability of the S2168 transmembrane module to oligomerize in alternative geometries, while the functional properties of the resultant nanopores can be fine-tuned through the identity of the connecting linkers. Overall, the FuN screen is anticipated to facilitate both fundamental studies and complex nanopore engineering endeavors with many potential applications in biomedicine, biotechnology, and synthetic biology.
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Affiliation(s)
- Wadim Weber
- Department of Biology, TU Darmstadt, 64287 Darmstadt, Germany
- Centre for Synthetic Biology, TU Darmstadt, 64283 Darmstadt, Germany
| | - Markus Roeder
- Department of Biology, TU Darmstadt, 64287 Darmstadt, Germany
| | - Tobias Probanowski
- Department of Biology, TU Darmstadt, 64287 Darmstadt, Germany
- Centre for Synthetic Biology, TU Darmstadt, 64283 Darmstadt, Germany
| | - Jie Yang
- Wallenberg Centre, University of Gothenburg, 41296 Gothenburg, Sweden
| | - Helal Abujubara
- Wallenberg Centre, University of Gothenburg, 41296 Gothenburg, Sweden
| | - Heinz Koeppl
- Centre for Synthetic Biology, TU Darmstadt, 64283 Darmstadt, Germany
- Department of Electrical Engineering and Information Technology, TU Darmstadt, 64283 Darmstadt, Germany
| | - Alesia Tietze
- Wallenberg Centre, University of Gothenburg, 41296 Gothenburg, Sweden
| | - Viktor Stein
- Department of Biology, TU Darmstadt, 64287 Darmstadt, Germany
- Centre for Synthetic Biology, TU Darmstadt, 64283 Darmstadt, Germany
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4
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Rauh O, Kukovetz K, Winterstein L, Introini B, Thiel G. Combining in vitro translation with nanodisc technology and functional reconstitution of channels in planar lipid bilayers. Methods Enzymol 2021; 652:293-318. [PMID: 34059286 DOI: 10.1016/bs.mie.2021.02.003] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
Experimental studies on membrane proteins have been recently enriched by two promising method developments: protocols for cell-free protein synthesis and the use of soluble nanoscale lipid bilayers, so called nanodiscs, as membrane mimics for keeping these proteins in a soluble form. Here, we show how the advantages of these techniques can be combined with the classical planar lipid bilayer method for a functional reconstitution of channel activity. The present data demonstrate that the combination of these methods offers a very rapid and reliable way of recording channel activity in different bilayer systems. This approach has additional advantages in that it strongly lowers the propensity of contamination from the expression system and allows the simultaneous reconstitution of thousands of channel proteins for macroscopic current measurements without compromising bilayer stability.
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Affiliation(s)
- Oliver Rauh
- Membrane Biophysics and Center for Synthetic Biology, Technische Universität Darmstadt, Darmstadt, Germany
| | - Kerri Kukovetz
- Membrane Biophysics and Center for Synthetic Biology, Technische Universität Darmstadt, Darmstadt, Germany
| | - Laura Winterstein
- Membrane Biophysics and Center for Synthetic Biology, Technische Universität Darmstadt, Darmstadt, Germany
| | - Bianca Introini
- Department of Biosciences and CNR IBF-Mi, Università degli Studi di Milano, Milano, Italy
| | - Gerhard Thiel
- Membrane Biophysics and Center for Synthetic Biology, Technische Universität Darmstadt, Darmstadt, Germany.
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Krajewska M, Koprowski P. Solubilization, purification, and functional reconstitution of human ROMK potassium channel in copolymer styrene-maleic acid (SMA) nanodiscs. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2021; 1863:183555. [PMID: 33444624 DOI: 10.1016/j.bbamem.2021.183555] [Citation(s) in RCA: 13] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Subscribe] [Scholar Register] [Received: 09/21/2020] [Revised: 12/13/2020] [Accepted: 01/04/2021] [Indexed: 12/18/2022]
Abstract
Expression, purification, and functional reconstitution of mammalian ion channels are often challenging. Heterologous expression of mammalian channels in bacteria can be advantageous due to unrelated protein environment and the lack of risk of copurification of endogenous proteins, e.g., accessory channel subunits that can influence the channel activity. Also, direct recording of channel activity could be challenging due to their intracellular localization like in the case of mitochondrial channels. The activity of purified channels can be characterized at the single-molecule level by electrophysiological techniques, such as planar lipid bilayers (PLB). In this work, we describe a simple approach to accomplish PLB recording of the activity of single renal outer medullary potassium channels ROMK expressed in E. coli. We focused on the ROMK2 isoform that is present at low levels in the mitochondria and can be responsible for mitoKATP activity. We screened for the best construct to express the codon-optimized ROMK proteins with a 6xHis tag for protein purification. The strategy involved the use of optimal styrene-maleic acid (SMA) copolymer, which forms so-called polymer nanodiscs, to solubilize and purify ROMK-containing SMA lipid particles (SMALPs), which were amenable for fusion with PLB. Reconstituted ROMK channels exhibited ion selectivity, rectification, and pharmacological properties, which are in agreement with previous work on ROMK channels.
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Affiliation(s)
- Milena Krajewska
- Laboratory of Intracellular Ion Channels, Nencki Institute of Experimental Biology PAS, Pasteur str. 3, Warsaw 02-093, Poland
| | - Piotr Koprowski
- Laboratory of Intracellular Ion Channels, Nencki Institute of Experimental Biology PAS, Pasteur str. 3, Warsaw 02-093, Poland.
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6
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Van Etten JL, Agarkova IV, Dunigan DD. Chloroviruses. Viruses 2019; 12:E20. [PMID: 31878033 PMCID: PMC7019647 DOI: 10.3390/v12010020] [Citation(s) in RCA: 39] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/30/2019] [Revised: 12/13/2019] [Accepted: 12/15/2019] [Indexed: 12/20/2022] Open
Abstract
Chloroviruses are large dsDNA, plaque-forming viruses that infect certain chlorella-like green algae; the algae are normally mutualistic endosymbionts of protists and metazoans and are often referred to as zoochlorellae. The viruses are ubiquitous in inland aqueous environments throughout the world and occasionally single types reach titers of thousands of plaque-forming units per ml of native water. The viruses are icosahedral in shape with a spike structure located at one of the vertices. They contain an internal membrane that is required for infectivity. The viral genomes are 290 to 370 kb in size, which encode up to 16 tRNAs and 330 to ~415 proteins, including many not previously seen in viruses. Examples include genes encoding DNA restriction and modification enzymes, hyaluronan and chitin biosynthetic enzymes, polyamine biosynthetic enzymes, ion channel and transport proteins, and enzymes involved in the glycan synthesis of the virus major capsid glycoproteins. The proteins encoded by many of these viruses are often the smallest or among the smallest proteins of their class. Consequently, some of the viral proteins are the subject of intensive biochemical and structural investigation.
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Affiliation(s)
- James L. Van Etten
- Department of Plant Pathology, Nebraska Center for Virology, University of Nebraska-Lincoln, Lincoln, NE 68583-0900, USA; (I.V.A.); (D.D.D.)
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7
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A light-gated potassium channel for sustained neuronal inhibition. Nat Methods 2018; 15:969-976. [PMID: 30377377 DOI: 10.1038/s41592-018-0186-9] [Citation(s) in RCA: 36] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2018] [Accepted: 09/26/2018] [Indexed: 12/14/2022]
Abstract
Currently available inhibitory optogenetic tools provide short and transient silencing of neurons, but they cannot provide long-lasting inhibition because of the requirement for high light intensities. Here we present an optimized blue-light-sensitive synthetic potassium channel, BLINK2, which showed good expression in neurons in three species. The channel is activated by illumination with low doses of blue light, and in our experiments it remained active over (tens of) minutes in the dark after the illumination was stopped. This activation caused long periods of inhibition of neuronal firing in ex vivo recordings of mouse neurons and impaired motor neuron response in zebrafish in vivo. As a proof-of-concept application, we demonstrated that in a freely moving rat model of neuropathic pain, the activation of a small number of BLINK2 channels caused a long-lasting (>30 min) reduction in pain sensation.
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8
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Rondelli V, Del Favero E, Brocca P, Fragneto G, Trapp M, Mauri L, Ciampa M, Romani G, Braun C, Winterstein L, Schroeder I, Thiel G, Moroni A, Cantu' L. Directional K+ channel insertion in a single phospholipid bilayer: Neutron reflectometry and electrophysiology in the joint exploration of a model membrane functional platform. Biochim Biophys Acta Gen Subj 2018; 1862:1742-1750. [DOI: 10.1016/j.bbagen.2018.05.007] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/28/2018] [Revised: 04/27/2018] [Accepted: 05/07/2018] [Indexed: 01/05/2023]
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9
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Winterstein LM, Kukovetz K, Rauh O, Turman DL, Braun C, Moroni A, Schroeder I, Thiel G. Reconstitution and functional characterization of ion channels from nanodiscs in lipid bilayers. J Gen Physiol 2018; 150:637-646. [PMID: 29487088 PMCID: PMC5881443 DOI: 10.1085/jgp.201711904] [Citation(s) in RCA: 30] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/15/2017] [Revised: 12/20/2017] [Accepted: 01/30/2018] [Indexed: 11/20/2022] Open
Abstract
Recent studies have shown that membrane proteins can be efficiently synthesized in vitro before spontaneously inserting into soluble nanoscale lipid bilayers called nanodiscs (NDs). In this paper, we present experimental details that allow a combination of in vitro translation of ion channels into commercially available NDs followed by their direct reconstitution from these nanobilayers into standard bilayer setups for electrophysiological characterization. We present data showing that two model K+ channels, Kcv and KcsA, as well as a recently discovered dual-topology F- channel, Fluc, can be reliably reconstituted from different types of NDs into bilayers without contamination from the in vitro translation cocktail. The functional properties of Kcv and KcsA were characterized electrophysiologically and exhibited sensitivity to the lipid composition of the target DPhPC bilayer, suggesting that the channel proteins were fully exposed to the target membrane and were no longer surrounded by the lipid/protein scaffold. The single-channel properties of the three tested channels are compatible with studies from recordings of the same proteins in other expression systems. Altogether, the data show that synthesis of ion channels into NDs and their subsequent reconstitution into conventional bilayers provide a fast and reliable method for functional analysis of ion channels.
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Affiliation(s)
| | - Kerri Kukovetz
- Plant Membrane Biophysics, Technische Universität Darmstadt, Darmstadt, Germany
| | - Oliver Rauh
- Plant Membrane Biophysics, Technische Universität Darmstadt, Darmstadt, Germany
| | - Daniel L Turman
- Department of Biochemistry and Howard Hughes Medical Institute, Brandeis University, Waltham, MA
| | - Christian Braun
- Plant Membrane Biophysics, Technische Universität Darmstadt, Darmstadt, Germany
| | - Anna Moroni
- Department of Biosciences and Consiglio Nazionale delle Ricerche - Istituto di Biofisica, Università degli Studi di Milano, Milano, Italy
| | - Indra Schroeder
- Plant Membrane Biophysics, Technische Universität Darmstadt, Darmstadt, Germany
| | - Gerhard Thiel
- Plant Membrane Biophysics, Technische Universität Darmstadt, Darmstadt, Germany
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10
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Rauh O, Hansen U, Mach S, Hartel AJ, Shepard KL, Thiel G, Schroeder I. Extended beta distributions open the access to fast gating in bilayer experiments-assigning the voltage-dependent gating to the selectivity filter. FEBS Lett 2017; 591:3850-3860. [PMID: 29106736 PMCID: PMC5747313 DOI: 10.1002/1873-3468.12898] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/06/2017] [Revised: 10/17/2017] [Accepted: 10/27/2017] [Indexed: 01/02/2023]
Abstract
Lipid bilayers provide many benefits for ion channel recordings, such as control of membrane composition and transport molecules. However, they suffer from high membrane capacitance limiting the bandwidth and impeding analysis of fast gating. This can be overcome by fitting the deviations of the open-channel noise from the baseline noise by extended beta distributions. We demonstrate this analysis step-by-step by applying it to the example of viral K+ channels (Kcv), from the choice of the gating model through the fitting process, validation of the results, and what kinds of results can be obtained. These voltage sensor-less channels show profoundly voltage-dependent gating with dwell times in the closed state of about 50 μs. Mutations assign it to the selectivity filter.
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Affiliation(s)
- Oliver Rauh
- Plant Membrane BiophysicsTechnische Universität DarmstadtGermany
| | - Ulf‐Peter Hansen
- Department of Structural BiologyChristian‐Albrechts‐University of KielGermany
| | - Sebastian Mach
- Plant Membrane BiophysicsTechnische Universität DarmstadtGermany
| | | | | | - Gerhard Thiel
- Plant Membrane BiophysicsTechnische Universität DarmstadtGermany
| | - Indra Schroeder
- Plant Membrane BiophysicsTechnische Universität DarmstadtGermany
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11
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Lee SW, Lee EH, Thiel G, Van Etten JL, Saraf RF. Noninvasive Measurement of Electrical Events Associated with a Single Chlorovirus Infection of a Microalgal Cell. ACS NANO 2016; 10:5123-30. [PMID: 27139597 DOI: 10.1021/acsnano.6b00299] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/21/2023]
Abstract
Chlorovirus Paramecium bursaria chlorella virus 1 (PBCV-1) contains a viral-encoded K(+) channel imbedded in its internal membrane, which triggers host plasma membrane depolarization during virus infection. This early stage of infection was monitored at high resolution by recording the cell membrane depolarization of a single Chlorella cell during infection by a single PBCV-1 particle. The measurement was achieved by depositing the cells onto a network of one-dimensional necklaces of Au nanoparticles, which spanned two electrodes 70 μm apart. The nanoparticle necklace array has been shown to behave as a single-electron device at room temperature. The resulting electrochemical field-effect transistor (eFET) was gated by the cell membrane potential, which allowed a quantitative measurement of the electrophysiological changes across the rigid cell wall of the microalgae due to a single viral attack at high sensitivity. The single viral infection signature was quantitatively confirmed by coupling the eFET measurement with a method in which a single viral particle was delivered for infection by a scanning probe microscope cantilever.
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Affiliation(s)
- Seung-Woo Lee
- Department of Chemical and Biomolecular Engineering, University of Nebraska-Lincoln , Lincoln, Nebraska 68588, United States
- Post-Silicon Semiconductor Institute, Korea Institute of Science and Technology (KIST) , Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul 02792, Republic of Korea
| | - Eun-Hee Lee
- Department of Chemical and Biomolecular Engineering, University of Nebraska-Lincoln , Lincoln, Nebraska 68588, United States
- Department of Environmental Science and Engineering, Ewha Womans University , Ewhayeodae-gil 52, Seodaemun-gu, Seoul 03760, Republic of Korea
| | - Gerhard Thiel
- Department of Biology, Technische Universität-Darmstadt , Schnittspahnstrasse 3, Darmstadt 64287, Germany
| | - James L Van Etten
- Department of Plant Pathology and Nebraska Center for Virology, University of Nebraska-Lincoln , Lincoln, Nebraska 68583, United States
| | - Ravi F Saraf
- Department of Chemical and Biomolecular Engineering, University of Nebraska-Lincoln , Lincoln, Nebraska 68588, United States
- Nebraska Center for Materials and Nanosciences, University of Nebraska-Lincoln , Lincoln, Nebraska 68588, United States
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12
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Engineering a Ca⁺⁺⁺-sensitive (bio)sensor from the pore-module of a potassium channel. SENSORS 2015; 15:4913-24. [PMID: 25734643 PMCID: PMC4435187 DOI: 10.3390/s150304913] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 11/17/2014] [Revised: 12/19/2014] [Accepted: 02/02/2015] [Indexed: 11/30/2022]
Abstract
Signals recorded at the cell membrane are meaningful indicators of the physiological vs. pathological state of a cell and will become useful diagnostic elements in nanomedicine. In this project we present a coherent strategy for the design and fabrication of a bio-nano-sensor that monitors changes in intracellular cell calcium concentration and allows an easy read out by converting the calcium signal into an electrical current in the range of microampere that can be easily measured by conventional cell electrophysiology apparatus.
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13
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Siotto F, Martin C, Rauh O, Van Etten JL, Schroeder I, Moroni A, Thiel G. Viruses infecting marine picoplancton encode functional potassium ion channels. Virology 2014; 466-467:103-11. [PMID: 25441713 DOI: 10.1016/j.virol.2014.05.002] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/27/2014] [Revised: 04/29/2014] [Accepted: 05/03/2014] [Indexed: 01/19/2023]
Abstract
Phycodnaviruses are dsDNA viruses, which infect algae. Their large genomes encode many gene products, like small K(+) channels, with homologs in prokaryotes and eukaryotes. Screening for K(+) channels revealed their abundance in viruses from fresh-water habitats. Recent sequencing of viruses from marine algae or from salt water in Antarctica revealed sequences with the predicted characteristics of K(+) channels but with some unexpected features. Two genes encode either 78 or 79 amino acid proteins, which are the smallest known K(+) channels. Also of interest is an unusual sequence in the canonical α-helixes in K(+) channels. Structural prediction algorithms indicate that the new channels have the conserved α-helix folds but the algorithms failed to identify the expected transmembrane domains flanking the K(+) channel pores. In spite of these unexpected properties electophysiological studies confirmed that the new proteins are functional K(+) channels.
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Affiliation(s)
- Fenja Siotto
- Membrane Biophysics Group, Dept. of Biology, Technical University Darmstadt, Germany
| | - Corinna Martin
- Membrane Biophysics Group, Dept. of Biology, Technical University Darmstadt, Germany
| | - Oliver Rauh
- Membrane Biophysics Group, Dept. of Biology, Technical University Darmstadt, Germany
| | - James L Van Etten
- Department of Plant Pathology and Nebraska Center for Virology, University of Nebraska, Lincoln, NE 68583-0900, USA
| | - Indra Schroeder
- Membrane Biophysics Group, Dept. of Biology, Technical University Darmstadt, Germany
| | - Anna Moroni
- Dipartimento di Biologia Università degli Studi di Milano e Istituto di Biofisica, CNR, Milano, Italy
| | - Gerhard Thiel
- Membrane Biophysics Group, Dept. of Biology, Technical University Darmstadt, Germany.
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14
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von Charpuis C, Meckel T, Moroni A, Thiel G. The sorting of a small potassium channel in mammalian cells can be shifted between mitochondria and plasma membrane. Cell Calcium 2014; 58:114-21. [PMID: 25449299 DOI: 10.1016/j.ceca.2014.09.009] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/26/2014] [Revised: 09/24/2014] [Accepted: 09/24/2014] [Indexed: 10/24/2022]
Abstract
The two small and similar viral K(+) channels Kcv and Kesv are sorted in mammalian cells and yeast to different destinations. Analysis of the sorting pathways shows that Kcv is trafficking via the secretory pathway to the plasma membrane, while Kesv is inserted via the TIM/TOM complex to the inner membrane of mitochondria. Studies with Kesv mutants show that an N-terminal mitochondrial targeting sequence in this channel is neither necessary nor sufficient for sorting of Kesv the mitochondria. Instead the sorting of Kesv can be redirected from the mitochondria to the plasma membrane by an insertion of ≥2 amino acids in a position sensitive manner into the C-terminal transmembrane domain (TMD2) of this channel. The available data advocate the presence of a C-terminal sorting signal in TMD2 of Kesv channel, which is presumably not determined by the length of this domain.
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Affiliation(s)
- Charlotte von Charpuis
- Plant Membrane Biophysics, TU Darmstadt, Schnittspahnstrasse 3, 64287 Darmstadt, Germany
| | - Tobias Meckel
- Plant Membrane Biophysics, TU Darmstadt, Schnittspahnstrasse 3, 64287 Darmstadt, Germany
| | - Anna Moroni
- Department of Biology and CNR IBF-Mi, and Istituto Nazionale di Fisica della Materia, Università degli Studi di Milano, Via Celoria 26, 20133 Milano, Italy
| | - Gerhard Thiel
- Plant Membrane Biophysics, TU Darmstadt, Schnittspahnstrasse 3, 64287 Darmstadt, Germany.
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15
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Arrigoni C, Schroeder I, Romani G, Van Etten JL, Thiel G, Moroni A. The voltage-sensing domain of a phosphatase gates the pore of a potassium channel. ACTA ACUST UNITED AC 2013; 141:389-95. [PMID: 23440279 PMCID: PMC3581695 DOI: 10.1085/jgp.201210940] [Citation(s) in RCA: 47] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The modular architecture of voltage-gated K(+) (Kv) channels suggests that they resulted from the fusion of a voltage-sensing domain (VSD) to a pore module. Here, we show that the VSD of Ciona intestinalis phosphatase (Ci-VSP) fused to the viral channel Kcv creates Kv(Synth1), a functional voltage-gated, outwardly rectifying K(+) channel. Kv(Synth1) displays the summed features of its individual components: pore properties of Kcv (selectivity and filter gating) and voltage dependence of Ci-VSP (V(1/2) = +56 mV; z of ~1), including the depolarization-induced mode shift. The degree of outward rectification of the channel is critically dependent on the length of the linker more than on its amino acid composition. This highlights a mechanistic role of the linker in transmitting the movement of the sensor to the pore and shows that electromechanical coupling can occur without coevolution of the two domains.
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Affiliation(s)
- Cristina Arrigoni
- Department of Biosciences, Consiglio Nazionale delle Ricerche, Università degli Studi di Milano, 20133 Milano, Italy
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16
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Chowdhury S, Chanda B. Perspectives on: conformational coupling in ion channels: thermodynamics of electromechanical coupling in voltage-gated ion channels. ACTA ACUST UNITED AC 2013. [PMID: 23183697 PMCID: PMC3514737 DOI: 10.1085/jgp.201210840] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Affiliation(s)
- Sandipan Chowdhury
- Graduate Program in Biophysics, University of Wisconsin, Madison, WI 53706, USA
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17
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Romani G, Piotrowski A, Hillmer S, Gurnon J, Van Etten JL, Moroni A, Thiel G, Hertel B. A virus-encoded potassium ion channel is a structural protein in the chlorovirus Paramecium bursaria chlorella virus 1 virion. J Gen Virol 2013; 94:2549-2556. [PMID: 23918407 DOI: 10.1099/vir.0.055251-0] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Most chloroviruses encode small K(+) channels, which are functional in electrophysiological assays. The experimental finding that initial steps in viral infection exhibit the same sensitivity to channel inhibitors as the viral K(+) channels has led to the hypothesis that the channels are structural proteins located in the internal membrane of the virus particles. This hypothesis was questioned recently because proteomic studies failed to detect the channel protein in virions of the prototype chlorovirus Paramecium bursaria chlorella virus 1 (PBCV-1). Here, we used a mAb raised against the functional K(+) channel from chlorovirus MA-1D to search for the viral K(+) channel in the virus particle. The results showed that the antibody was specific and bound to the tetrameric channel on the extracellular side. The antibody reacted in a virus-specific manner with protein extracts from chloroviruses that encoded channels similar to that from MA-1D. There was no cross-reactivity with chloroviruses that encoded more diverse channels or with a chlorovirus that lacked a K(+) channel gene. Together with electron microscopic imaging, which revealed labelling of individual virus particles with the channel antibody, these results establish that the viral particles contain an active K(+) channel, presumably located in the lipid membrane that surrounds the DNA in the mature virions.
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Affiliation(s)
- Giulia Romani
- Dipartimento di Bioscienze, Università degli Studi di Milano e Istituto di Biofisica, CNR, Milano, Italy
| | - Adrianna Piotrowski
- Membrane Biophysics Group, Department of Biology, Technical University Darmstadt, Germany
| | - Stefan Hillmer
- COS - Entwicklungsbiologie der Pflanzen, University of Heidelberg, Germany
| | - James Gurnon
- Department of Plant Pathology and Nebraska Center for Virology, University of Nebraska, Lincoln, NE 68583-0900, USA
| | - James L Van Etten
- Department of Plant Pathology and Nebraska Center for Virology, University of Nebraska, Lincoln, NE 68583-0900, USA
| | - Anna Moroni
- Dipartimento di Bioscienze, Università degli Studi di Milano e Istituto di Biofisica, CNR, Milano, Italy
| | - Gerhard Thiel
- Membrane Biophysics Group, Department of Biology, Technical University Darmstadt, Germany
| | - Brigitte Hertel
- Membrane Biophysics Group, Department of Biology, Technical University Darmstadt, Germany
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18
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Braun CJ, Lachnit C, Becker P, Henkes LM, Arrigoni C, Kast SM, Moroni A, Thiel G, Schroeder I. Viral potassium channels as a robust model system for studies of membrane-protein interaction. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2013; 1838:1096-103. [PMID: 23791706 DOI: 10.1016/j.bbamem.2013.06.010] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/25/2013] [Revised: 05/31/2013] [Accepted: 06/08/2013] [Indexed: 11/15/2022]
Abstract
The viral channel KcvNTS belongs to the smallest K(+) channels known so far. A monomer of a functional homotetramer contains only 82 amino acids. As a consequence of the small size the protein is almost fully submerged into the membrane. This suggests that the channel is presumably sensitive to its lipid environment. Here we perform a comparative analysis for the function of the channel protein embedded in three different membrane environments. 1. Single-channel currents of KcvNTS were recorded with the patch clamp method on the plasma membrane of HEK293 cells. 2. They were also measured after reconstitution of recombinant channel protein into classical planar lipid bilayers and 3. into horizontal bilayers derived from giant unilamellar vesicles (GUVs). The recombinant channel protein was either expressed and purified from Pichia pastoris or from a cell-free expression system; for the latter a new approach with nanolipoprotein particles was used. The data show that single-channel activity can be recorded under all experimental conditions. The main functional features of the channel like a large single-channel conductance (80pS), high open-probability (>50%) and the approximate duration of open and closed dwell times are maintained in all experimental systems. An apparent difference between the approaches was only observed with respect to the unitary conductance, which was ca. 35% lower in HEK293 cells than in the other systems. The reason for this might be explained by the fact that the channel is tagged by GFP when expressed in HEK293 cells. Collectively the data demonstrate that the small viral channel exhibits a robust function in different experimental systems. This justifies an extrapolation of functional data from these systems to the potential performance of the channel in the virus/host interaction. This article is part of a Special Issue entitled: Viral Membrane Proteins-Channels for Cellular Networking.
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Affiliation(s)
- Christian J Braun
- Membrane Biophysics, Technical University of Darmstadt, Schnittspahnstrasse 3, 64287 Darmstadt, Germany
| | - Christine Lachnit
- Membrane Biophysics, Technical University of Darmstadt, Schnittspahnstrasse 3, 64287 Darmstadt, Germany
| | - Patrick Becker
- Membrane Biophysics, Technical University of Darmstadt, Schnittspahnstrasse 3, 64287 Darmstadt, Germany
| | - Leonhard M Henkes
- Physikalische Chemie III, Technische Universität Dortmund, Otto-Hahn-Str. 6, 44227 Dortmund, Germany
| | - Cristina Arrigoni
- Department of Biosciences, University of Milan, Via Celoria 26, 20133 Milano, Italy
| | - Stefan M Kast
- Physikalische Chemie III, Technische Universität Dortmund, Otto-Hahn-Str. 6, 44227 Dortmund, Germany
| | - Anna Moroni
- Department of Biosciences, University of Milan, Via Celoria 26, 20133 Milano, Italy; CNR-IBF, Via Celoria 26, 20133 Milano, Italy
| | - Gerhard Thiel
- Membrane Biophysics, Technical University of Darmstadt, Schnittspahnstrasse 3, 64287 Darmstadt, Germany
| | - Indra Schroeder
- Membrane Biophysics, Technical University of Darmstadt, Schnittspahnstrasse 3, 64287 Darmstadt, Germany.
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19
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Euden J, Mason SA, Williams AJ. Functional characterization of the cardiac ryanodine receptor pore-forming region. PLoS One 2013; 8:e66542. [PMID: 23776685 PMCID: PMC3680380 DOI: 10.1371/journal.pone.0066542] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/27/2013] [Accepted: 05/07/2013] [Indexed: 01/16/2023] Open
Abstract
Ryanodine receptors are homotetrameric intracellular calcium release channels. The efficiency of these channels is underpinned by exceptional rates of cation translocation through the open channel and this is achieved at the expense of the high degree of selectivity characteristic of many other types of channel. Crystallization of prokaryotic potassium channels has provided insights into the structures and mechanisms responsible for ion selection and movement in these channels, however no equivalent structural detail is currently available for ryanodine receptors. Nevertheless both molecular modeling and cryo-electron microscopy have identified the probable pore-forming region (PFR) of the ryanodine receptor (RyR) and suggest that this region contains structural elements equivalent to those of the PFRs of potassium-selective channels. The aim of the current study was to establish if the isolated putative cardiac RyR (RyR2) PFR could form a functional ion channel. We have expressed and purified the RyR2 PFR and shown that function is retained following reconstitution into planar phospholipid bilayers. Our data provide the first direct experimental evidence to support the proposal that the conduction pathway of RyR2 is formed by structural elements equivalent to those of the potassium channel PFR.
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Affiliation(s)
- Joanne Euden
- Institute of Molecular and Experimental Medicine, Cardiff University, Cardiff, United Kingdom.
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20
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Schroeder I, Gazzarrini S, Ferrara G, Thiel G, Hansen UP, Moroni A. Creation of a reactive oxygen species-insensitive Kcv channel. Biochemistry 2013; 52:3130-7. [PMID: 23578303 DOI: 10.1021/bi3016197] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
The current of the minimal viral K(+) channel Kcv(PCBV-1) heterologously expressed in Xenopus oocytes is strongly inhibited by reactive oxygen species (ROS) like H(2)O(2). Possible targets for the ROS effect are two cysteines (C53 and C79) and four methionines (M1, M15, M23, and M26). The C53A/C79A and M23L/M26L double mutations maintained the same ROS sensitivity as the wild type. However, M15L as a single mutant or in combination with C53A/C79A and/or M23L/M26L caused a complete loss of sensitivity to H(2)O(2). These results indicate a prominent role of M15 at the cytosolic end of the outer transmembrane helix for gating of Kcv(PCBV-1). Furthermore, even though the channel lacks a canonical voltage sensor, it exhibits a weak voltage sensitivity, resulting in a slight activation in the millisecond range after a voltage step to negative potentials. The M15L mutation inverts this kinetics into an inactivation, underlining the critical role of this residue for gating. The negative slope of the I-V curves of M15L is the same as in the wild type, indicating that the selectivity filter is not involved.
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Affiliation(s)
- Indra Schroeder
- Department of Biosciences, University of Milan, Via Celoria 26, 20133 Milano, Italy.
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21
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Gray NW, Zhorov BS, Moczydlowski EG. Interaction of local anesthetics with the K (+) channel pore domain: KcsA as a model for drug-dependent tetramer stability. Channels (Austin) 2013; 7:182-93. [PMID: 23545989 DOI: 10.4161/chan.24455] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023] Open
Abstract
Local anesthetics and related drugs block ionic currents of Na (+) , K (+) and Ca ( 2+) conducted across the cell membrane by voltage-dependent ion channels. Many of these drugs bind in the permeation pathway, occlude the pore and stop ion movement. However channel-blocking drugs have also been associated with decreased membrane stability of certain tetrameric K (+) channels, similar to the destabilization of channel function observed at low extracellular K (+) concentration. Such drug-dependent stability may result from electrostatic repulsion of K (+) from the selectivity filter by a cationic drug molecule bound in the central cavity of the channel. In this study we used the pore domain of the KcsA K (+) channel protein to test this hypothesis experimentally with a biochemical assay of tetramer stability and theoretically by computational simulation of local anesthetic docking to the central cavity. We find that two common local anesthetics, lidocaine and tetracaine, promote thermal dissociation of the KcsA tetramer in a K (+) -dependent fashion. Docking simulations of these drugs with open, open-inactivated and closed crystal structures of KcsA yield many energetically favorable drug-channel complexes characterized by nonbonded attraction to pore-lining residues and electrostatic repulsion of K (+) . The results suggest that binding of cationic drugs to the inner cavity can reduce tetramer stability of K (+) channels.
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Affiliation(s)
- Noel W Gray
- Neuroscience & Physiology, SUNY Upstate Medical University, Syracuse, NY, USA
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22
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Tan Q, Ritzo B, Tian K, Gu LQ. Tuning the tetraethylammonium sensitivity of potassium channel Kcv by subunit combination. ACTA ACUST UNITED AC 2012; 139:295-304. [PMID: 22450486 PMCID: PMC3315146 DOI: 10.1085/jgp.201110725] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
Tetraethylammonium (TEA) is a potassium (K+) channel inhibitor that has been extensively used as a molecular probe to explore the structure of channels’ ion pathway. In this study, we identified that Leu70 of the virus-encoded potassium channel Kcv is a key amino acid that plays an important role in regulating the channel’s TEA sensitivity. Site-directed mutagenesis of Leu70 can change the TEA sensitivity by 1,000-fold from ∼100 µM to ∼100 mM. Because no compelling trends exist to explain this amino acid’s specific interaction with TEA, the role of Leu70 at the binding site is likely to ensure an optimal conformation of the extracellular mouth that confers high TEA affinity. We further assembled the subunits of mutant and wt-Kcv into a series of heterotetramers. The differences in these heterochannels suggest that all of the four subunits in a Kcv channel additively participate in the TEA binding, and each of the four residues at the binding site independently contributes an equal binding energy. We therefore can present a series of mutant/wild-type tetramer combinations that can probe TEA over three orders of magnitude in concentration. This study may give insight into the mechanism for the interaction between the potassium channel and its inhibitor.
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Affiliation(s)
- Qiulin Tan
- Department of Biological Engineering, University of Missouri, Columbia, MO 65211, USA
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23
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Hamacher K, Greiner T, Ogata H, Van Etten JL, Gebhardt M, Villarreal LP, Cosentino C, Moroni A, Thiel G. Phycodnavirus potassium ion channel proteins question the virus molecular piracy hypothesis. PLoS One 2012; 7:e38826. [PMID: 22685610 PMCID: PMC3369850 DOI: 10.1371/journal.pone.0038826] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2012] [Accepted: 05/11/2012] [Indexed: 11/26/2022] Open
Abstract
Phycodnaviruses are large dsDNA, algal-infecting viruses that encode many genes with homologs in prokaryotes and eukaryotes. Among the viral gene products are the smallest proteins known to form functional K(+) channels. To determine if these viral K(+) channels are the product of molecular piracy from their hosts, we compared the sequences of the K(+) channel pore modules from seven phycodnaviruses to the K(+) channels from Chlorella variabilis and Ectocarpus siliculosus, whose genomes have recently been sequenced. C. variabilis is the host for two of the viruses PBCV-1 and NY-2A and E. siliculosus is the host for the virus EsV-1. Systematic phylogenetic analyses consistently indicate that the viral K(+) channels are not related to any lineage of the host channel homologs and that they are more closely related to each other than to their host homologs. A consensus sequence of the viral channels resembles a protein of unknown function from a proteobacterium. However, the bacterial protein lacks the consensus motif of all K(+) channels and it does not form a functional channel in yeast, suggesting that the viral channels did not come from a proteobacterium. Collectively, our results indicate that the viruses did not acquire their K(+) channel-encoding genes from their current algal hosts by gene transfer; thus alternative explanations are required. One possibility is that the viral genes arose from ancient organisms, which served as their hosts before the viruses developed their current host specificity. Alternatively the viral proteins could be the origin of K(+) channels in algae and perhaps even all cellular organisms.
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Affiliation(s)
- Kay Hamacher
- Computational Biology Group, Technische Universität Darmstadt, Darmstadt, Germany
| | - Timo Greiner
- Membrane Biophysics Group, Technische Universität Darmstadt, Darmstadt, Germany
| | - Hiroyuki Ogata
- Structural and Genomic Information Laboratory, Aix-Marseille University, Marseille, France
| | - James L. Van Etten
- Department of Plant Pathology and Nebraska Center for Virology, University of Nebraska, Lincoln, Nebraska, United States of America
| | - Manuela Gebhardt
- Membrane Biophysics Group, Technische Universität Darmstadt, Darmstadt, Germany
| | - Luis P. Villarreal
- Center of Virus Research, University of California Irvine, Irvine, California, United States of America
| | | | - Anna Moroni
- Department of Biology, Università degli Studi di Milano, Milan, Italy
| | - Gerhard Thiel
- Membrane Biophysics Group, Technische Universität Darmstadt, Darmstadt, Germany
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24
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Van Etten JL, Dunigan DD. Chloroviruses: not your everyday plant virus. TRENDS IN PLANT SCIENCE 2012; 17:1-8. [PMID: 22100667 PMCID: PMC3259250 DOI: 10.1016/j.tplants.2011.10.005] [Citation(s) in RCA: 67] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/25/2011] [Revised: 10/20/2011] [Accepted: 10/25/2011] [Indexed: 05/29/2023]
Abstract
Viruses infecting higher plants are among the smallest viruses known and typically have four to ten protein-encoding genes. By contrast, many viruses that infect algae (classified in the virus family Phycodnaviridae) are among the largest viruses found to date and have up to 600 protein-encoding genes. This brief review focuses on one group of plaque-forming phycodnaviruses that infect unicellular chlorella-like green algae. The prototype chlorovirus PBCV-1 has more than 400 protein-encoding genes and 11 tRNA genes. About 40% of the PBCV-1 encoded proteins resemble proteins of known function including many that are completely unexpected for a virus. In many respects, chlorovirus infection resembles bacterial infection by tailed bacteriophages.
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Affiliation(s)
- James L Van Etten
- Department of Plant Pathology and Nebraska Center for Virology, University of Nebraska, Lincoln, NE 68583-0900, USA.
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25
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Nelson PH. A permeation theory for single-file ion channels: one- and two-step models. J Chem Phys 2011; 134:165102. [PMID: 21528981 DOI: 10.1063/1.3580562] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
How many steps are required to model permeation through ion channels? This question is investigated by comparing one- and two-step models of permeation with experiment and MD simulation for the first time. In recent MD simulations, the observed permeation mechanism was identified as resembling a Hodgkin and Keynes knock-on mechanism with one voltage-dependent rate-determining step [Jensen et al., PNAS 107, 5833 (2010)]. These previously published simulation data are fitted to a one-step knock-on model that successfully explains the highly non-Ohmic current-voltage curve observed in the simulation. However, these predictions (and the simulations upon which they are based) are not representative of real channel behavior, which is typically Ohmic at low voltages. A two-step association/dissociation (A/D) model is then compared with experiment for the first time. This two-parameter model is shown to be remarkably consistent with previously published permeation experiments through the MaxiK potassium channel over a wide range of concentrations and positive voltages. The A/D model also provides a first-order explanation of permeation through the Shaker potassium channel, but it does not explain the asymmetry observed experimentally. To address this, a new asymmetric variant of the A/D model is developed using the present theoretical framework. It includes a third parameter that represents the value of the "permeation coordinate" (fractional electric potential energy) corresponding to the triply occupied state n of the channel. This asymmetric A/D model is fitted to published permeation data through the Shaker potassium channel at physiological concentrations, and it successfully predicts qualitative changes in the negative current-voltage data (including a transition to super-Ohmic behavior) based solely on a fit to positive-voltage data (that appear linear). The A/D model appears to be qualitatively consistent with a large group of published MD simulations, but no quantitative comparison has yet been made. The A/D model makes a network of predictions for how the elementary steps and the channel occupancy vary with both concentration and voltage. In addition, the proposed theoretical framework suggests a new way of plotting the energetics of the simulated system using a one-dimensional permeation coordinate that uses electric potential energy as a metric for the net fractional progress through the permeation mechanism. This approach has the potential to provide a quantitative connection between atomistic simulations and permeation experiments for the first time.
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Affiliation(s)
- Peter Hugo Nelson
- Department of Physics, Benedictine University, Lisle, Illinois 60532, USA.
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26
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Maîtrejean M, Wudick MM, Voelker C, Prinsi B, Mueller-Roeber B, Czempinski K, Pedrazzini E, Vitale A. Assembly and sorting of the tonoplast potassium channel AtTPK1 and its turnover by internalization into the vacuole. PLANT PHYSIOLOGY 2011; 156:1783-96. [PMID: 21697507 PMCID: PMC3149923 DOI: 10.1104/pp.111.177816] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/08/2011] [Accepted: 06/18/2010] [Indexed: 05/18/2023]
Abstract
The assembly, sorting signals, and turnover of the tonoplast potassium channel AtTPK1 of Arabidopsis (Arabidopsis thaliana) were studied. We used transgenic Arabidopsis expressing a TPK1-green fluorescent protein (GFP) fusion or protoplasts transiently transformed with chimeric constructs based on domain exchange between TPK1 and TPK4, the only TPK family member not located at the tonoplast. The results show that TPK1-GFP is a dimer and that the newly synthesized polypeptides transiently interact with a thus-far unidentified 20-kD polypeptide. A subset of the TPK1-TPK4 chimeras were unable to assemble correctly and these remained located in the endoplasmic reticulum where they interacted with the binding protein chaperone. Therefore, TPK1 must assemble correctly to pass endoplasmic reticulum quality control. Substitution of the cytosolic C terminus of TPK4 with the corresponding domain of TPK1 was sufficient to allow tonoplast delivery, indicating that this domain contains tonoplast sorting information. Pulse-chase labeling indicated that TPK1-GFP has a half-life of at least 24 h. Turnover of the fusion protein involves internalization into the vacuole where the GFP domain is released. This indicates a possible mechanism for the turnover of tonoplast proteins.
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27
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Raychaudhuri P, Li Q, Mason A, Mikhailova E, Heron AJ, Bayley H. Fluorinated Amphiphiles Control the Insertion of α-Hemolysin Pores into Lipid Bilayers. Biochemistry 2011; 50:1599-606. [DOI: 10.1021/bi1012386] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Affiliation(s)
- Pinky Raychaudhuri
- Department of Chemistry, University of Oxford, Oxford OX1 3TA, United Kingdom
| | - Qiuhong Li
- Department of Chemistry, University of Oxford, Oxford OX1 3TA, United Kingdom
| | - Amy Mason
- Department of Chemistry, University of Oxford, Oxford OX1 3TA, United Kingdom
| | - Ellina Mikhailova
- Department of Chemistry, University of Oxford, Oxford OX1 3TA, United Kingdom
| | - Andrew J. Heron
- Department of Chemistry, University of Oxford, Oxford OX1 3TA, United Kingdom
| | - Hagan Bayley
- Department of Chemistry, University of Oxford, Oxford OX1 3TA, United Kingdom
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28
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Thiel G, Baumeister D, Schroeder I, Kast SM, Van Etten JL, Moroni A. Minimal art: or why small viral K(+) channels are good tools for understanding basic structure and function relations. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2010; 1808:580-8. [PMID: 20417613 DOI: 10.1016/j.bbamem.2010.04.008] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/11/2010] [Revised: 04/09/2010] [Accepted: 04/13/2010] [Indexed: 11/17/2022]
Abstract
Some algal viruses contain genes that encode proteins with the hallmarks of K(+) channels. One feature of these proteins is that they are less than 100 amino acids in size, which make them truly minimal for a K(+) channel protein. That is, they consist of only the pore module present in more complex K(+) channels. The combination of miniature size and the functional robustness of the viral K(+) channels make them ideal model systems for studying how K(+) channels work. Here we summarize recent structure/function correlates from these channels, which provide insight into functional properties such as gating, pharmacology and sorting in cells.
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Affiliation(s)
- Gerhard Thiel
- Institute of Botany, Technische Universität Darmstadt, Schnittspahnstrasse 3, 64287 Darmstadt, Germany.
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29
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Tan Q, Shim JW, Gu LQ. Separation of heteromeric potassium channel Kcv towards probing subunit composition-regulated ion permeation and gating. FEBS Lett 2010; 584:1602-8. [PMID: 20303961 DOI: 10.1016/j.febslet.2010.03.023] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/11/2010] [Revised: 03/12/2010] [Accepted: 03/15/2010] [Indexed: 11/19/2022]
Abstract
The chlorella virus-encoded Kcv can form a homo-tetrameric potassium channel in lipid membranes. This miniature peptide can be synthesized in vitro, and the tetramer purified from the SDS-polyacrylamide gel retains the K(+) channel functionality. Combining this capability with the mass-tagging method, we propose a simple, straightforward approach that can generically manipulate individual subunits in the tetramer, thereby enabling the detection of contribution from individual subunits to the channel functions. Using this approach, we showed that the structural change in the selectivity filter from only one subunit is sufficient to cause permanent channel inactivation ("all-or-none" mechanism), whereas the mutation near the extracellular entrance additively modifies the ion permeation with the number of mutant subunits in the tetramer ("additive" mechanism).
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Affiliation(s)
- Qiulin Tan
- Department of Biological Engineering, Dalton Cardiovascular Research Center, University of Missouri, Columbia, MO 65211, USA
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30
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Thiel G, Moroni A, Dunigan D, Van Etten JL. Initial Events Associated with Virus PBCV-1 Infection of Chlorella NC64A. PROGRESS IN BOTANY. FORTSCHRITTE DER BOTANIK 2010; 71:169-183. [PMID: 21152366 PMCID: PMC2997699 DOI: 10.1007/978-3-642-02167-1_7] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Chlorella viruses (or chloroviruses) are very large, plaque-forming viruses. The viruses are multilayered structures containing a large double-stranded DNA genome, a lipid bilayered membrane, and an outer icosahedral capsid shell. The viruses replicate in certain isolates of the coccal green alga, Chlorella. Sequence analysis of the 330-kbp genome of Paramecium bursaria Chlorella virus 1 (PBCV-1), the prototype of the virus family Phycodnaviridae, reveals <365 protein-encoding genes and 11 tRNA genes. Products of about 40% of these genes resemble proteins of known function, including many that are unexpected for a virus. Among these is a virus-encoded protein, called Kcv, which forms a functional K(+) channel. This chapter focuses on the initial steps in virus infection and provides a plausible role for the function of the viral K(+) channel in lowering the turgor pressure of the host. This step appears to be a prerequisite for delivery of the viral genome into the host.
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Affiliation(s)
- Gerhard Thiel
- Institute of Botany, Technische Universitat Darmstadt, 64287, Darmstadt, Germany
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Abenavoli A, DiFrancesco ML, Schroeder I, Epimashko S, Gazzarrini S, Hansen UP, Thiel G, Moroni A. Fast and slow gating are inherent properties of the pore module of the K+ channel Kcv. ACTA ACUST UNITED AC 2009; 134:219-29. [PMID: 19720961 PMCID: PMC2737228 DOI: 10.1085/jgp.200910266] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
Kcv from the chlorella virus PBCV-1 is a viral protein that forms a tetrameric, functional K+ channel in heterologous systems. Kcv can serve as a model system to study and manipulate basic properties of the K+ channel pore because its minimalistic structure (94 amino acids) produces basic features of ion channels, such as selectivity, gating, and sensitivity to blockers. We present a characterization of Kcv properties at the single-channel level. In symmetric 100 mM K+, single-channel conductance is 114 ± 11 pS. Two different voltage-dependent mechanisms are responsible for the gating of Kcv. “Fast” gating, analyzed by β distributions, is responsible for the negative slope conductance in the single-channel current–voltage curve at extreme potentials, like in MaxiK potassium channels, and can be explained by depletion-aggravated instability of the filter region. The presence of a “slow” gating is revealed by the very low (in the order of 1–4%) mean open probability that is voltage dependent and underlies the time-dependent component of the macroscopic current.
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Affiliation(s)
- Alessandra Abenavoli
- Dipartimento di Biologia and Istituto di Biofisica-Consiglio Nazionale delle Ricerche, Università degli Studi di Milano, 20133 Milan, Italy
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32
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Chatelain FC, Gazzarrini S, Fujiwara Y, Arrigoni C, Domigan C, Ferrara G, Pantoja C, Thiel G, Moroni A, Minor DL. Selection of inhibitor-resistant viral potassium channels identifies a selectivity filter site that affects barium and amantadine block. PLoS One 2009; 4:e7496. [PMID: 19834614 PMCID: PMC2759520 DOI: 10.1371/journal.pone.0007496] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2009] [Accepted: 09/23/2009] [Indexed: 12/02/2022] Open
Abstract
Background Understanding the interactions between ion channels and blockers remains an important goal that has implications for delineating the basic mechanisms of ion channel function and for the discovery and development of ion channel directed drugs. Methodology/Principal Findings We used genetic selection methods to probe the interaction of two ion channel blockers, barium and amantadine, with the miniature viral potassium channel Kcv. Selection for Kcv mutants that were resistant to either blocker identified a mutant bearing multiple changes that was resistant to both. Implementation of a PCR shuffling and backcrossing procedure uncovered that the blocker resistance could be attributed to a single change, T63S, at a position that is likely to form the binding site for the inner ion in the selectivity filter (site 4). A combination of electrophysiological and biochemical assays revealed a distinct difference in the ability of the mutant channel to interact with the blockers. Studies of the analogous mutation in the mammalian inward rectifier Kir2.1 show that the T→S mutation affects barium block as well as the stability of the conductive state. Comparison of the effects of similar barium resistant mutations in Kcv and Kir2.1 shows that neighboring amino acids in the Kcv selectivity filter affect blocker binding. Conclusions/Significance The data support the idea that permeant ions have an integral role in stabilizing potassium channel structure, suggest that both barium and amantadine act at a similar site, and demonstrate how genetic selections can be used to map blocker binding sites and reveal mechanistic features.
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Affiliation(s)
- Franck C. Chatelain
- Cardiovascular Research Institute, University of California San Francisco, San Francisco, California, United States of America
| | - Sabrina Gazzarrini
- Dipartimento di Biologia e Istituto di Biofisica del Consiglio Nazionale delle Ricerche, Università degli Studi di Milano, Milan, Italy
| | - Yuichiro Fujiwara
- Cardiovascular Research Institute, University of California San Francisco, San Francisco, California, United States of America
| | - Cristina Arrigoni
- Dipartimento di Biologia e Istituto di Biofisica del Consiglio Nazionale delle Ricerche, Università degli Studi di Milano, Milan, Italy
| | - Courtney Domigan
- Cardiovascular Research Institute, University of California San Francisco, San Francisco, California, United States of America
| | - Giuseppina Ferrara
- Dipartimento di Biologia e Istituto di Biofisica del Consiglio Nazionale delle Ricerche, Università degli Studi di Milano, Milan, Italy
| | - Carlos Pantoja
- Department of Cellular and Molecular Pharmacology, University of California San Francisco, San Francisco, California, United States of America
| | - Gerhard Thiel
- Technische Universität Darmstadt, Institute für Botanik, Darmstadt, Germany
| | - Anna Moroni
- Dipartimento di Biologia e Istituto di Biofisica del Consiglio Nazionale delle Ricerche, Università degli Studi di Milano, Milan, Italy
| | - Daniel L. Minor
- Cardiovascular Research Institute, University of California San Francisco, San Francisco, California, United States of America
- Department of Biochemistry and Biophysics, University of California San Francisco, San Francisco, California, United States of America
- Department of Cellular and Molecular Pharmacology, University of California San Francisco, San Francisco, California, United States of America
- Department of California Institute for Quantitative Biomedical Research, University of California San Francisco, San Francisco, California, United States of America
- Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States of America
- * E-mail:
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Grunwald I, Rischka K, Kast SM, Scheibel T, Bargel H. Mimicking biopolymers on a molecular scale: nano(bio)technology based on engineered proteins. PHILOSOPHICAL TRANSACTIONS. SERIES A, MATHEMATICAL, PHYSICAL, AND ENGINEERING SCIENCES 2009; 367:1727-1747. [PMID: 19376768 DOI: 10.1098/rsta.2009.0012] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
Proteins are ubiquitous biopolymers that adopt distinct three-dimensional structures and fulfil a multitude of elementary functions in organisms. Recent systematic studies in molecular biology and biotechnology have improved the understanding of basic functional and architectural principles of proteins, making them attractive candidates as concept generators for technological development in material science, particularly in biomedicine and nano(bio)technology. This paper highlights the potential of molecular biomimetics in mimicking high-performance proteins and provides concepts for applications in four case studies, i.e. spider silk, antifreeze proteins, blue mussel adhesive proteins and viral ion channels.
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Affiliation(s)
- Ingo Grunwald
- Department of Adhesive Bonding Technology and Surfaces, Fraunhofer Institute for Manufacturing Technology and Applied Materials Research (IFAM)28359 Bremen, Germany
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Gazzarrini S, Kang M, Abenavoli A, Romani G, Olivari C, Gaslini D, Ferrara G, van Etten JL, Kreim M, Kast SM, Thiel G, Moroni A. Chlorella virus ATCV-1 encodes a functional potassium channel of 82 amino acids. Biochem J 2009; 420:295-303. [PMID: 19267691 PMCID: PMC2903877 DOI: 10.1042/bj20090095] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Chlorella virus PBCV-1 (Paramecium bursaria chlorella virus-1) encodes the smallest protein (94 amino acids, named Kcv) previously known to form a functional K+ channel in heterologous systems. In this paper, we characterize another chlorella virus encoded K+ channel protein (82 amino acids, named ATCV-1 Kcv) that forms a functional channel in Xenopus oocytes and rescues Saccharomyces cerevisiae mutants that lack endogenous K+ uptake systems. Compared with the larger PBCV-1 Kcv, ATCV-1 Kcv lacks a cytoplasmic N-terminus and has a reduced number of charged amino acids in its turret domain. Despite these deficiencies, ATCV-1 Kcv accomplishes all the major features of K+ channels: it assembles into a tetramer, is K+ selective and is inhibited by the canonical K+ channel blockers, barium and caesium. Single channel analyses reveal a stochastic gating behaviour and a voltage-dependent conductance that resembles the macroscopic I/V relationship. One difference between PBCV-1 and ATCV-1 Kcv is that the latter is more permeable to K+ than Rb+. This difference is partially explained by the presence of a tyrosine residue in the selective filter of ATCV-1 Kcv, whereas PBCV-1 Kcv has a phenylalanine. Hence, ATCV-1 Kcv is the smallest protein to form a K+ channel and it will serve as a model for studying structure-function correlations inside the potassium channel pore.
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Affiliation(s)
- Sabrina Gazzarrini
- Department of Biology and CNR - Istituto di Biofisica, Università degli Studi di Milano, Italy
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Salt bridges in the miniature viral channel Kcv are important for function. EUROPEAN BIOPHYSICS JOURNAL: EBJ 2009; 39:1057-68. [PMID: 19390850 DOI: 10.1007/s00249-009-0451-z] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/30/2009] [Revised: 03/14/2009] [Accepted: 03/31/2009] [Indexed: 10/20/2022]
Abstract
The viral potassium channel Kcv comprises only 94 amino acids, which represent the pore module of more complex K(+) channels. As for Kir-type channels, Kcv also has a short N-terminal helix exposed to the cytoplasm, upstream of the first transmembrane domain. Here we show that this helix is relevant for Kcv function. The presence of charged amino acids, which form dynamic inter- and intra-subunit salt bridges is crucial. Electrophysiological measurements, yeast rescue experiments and molecular dynamics simulations show that mutants in which the critical salt bridge formation is impaired have no or reduced channel activity. We conclude that these salt bridges destabilise the complexation of K(+) ions by negative charges on the inner transmembrane domain at the entrance into the cavity. This feature facilitates a continuous and coordinated transfer of ions between the cavity and the cytoplasm for channels without the canonical bundle crossing.
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36
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Syeda R, Holden MA, Hwang WL, Bayley H. Screening Blockers Against a Potassium Channel with a Droplet Interface Bilayer Array. J Am Chem Soc 2008; 130:15543-8. [DOI: 10.1021/ja804968g] [Citation(s) in RCA: 121] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Ruhma Syeda
- Department of Chemistry, University of Oxford, Mansfield Road, Oxford, OX1 3TA, U.K
| | - Matthew A. Holden
- Department of Chemistry, University of Oxford, Mansfield Road, Oxford, OX1 3TA, U.K
| | - William L. Hwang
- Department of Chemistry, University of Oxford, Mansfield Road, Oxford, OX1 3TA, U.K
| | - Hagan Bayley
- Department of Chemistry, University of Oxford, Mansfield Road, Oxford, OX1 3TA, U.K
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37
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Transmembrane domain length of viral K+ channels is a signal for mitochondria targeting. Proc Natl Acad Sci U S A 2008; 105:12313-8. [PMID: 18719119 DOI: 10.1073/pnas.0805709105] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
K(+) channels operate in the plasma membrane and in membranes of organelles including mitochondria. The mechanisms and topogenic information for their differential synthesis and targeting is unknown. This article describes 2 similar viral K(+) channels that are differentially sorted; one protein (Kesv) is imported by the Tom complex into the mitochondria, the other (Kcv) to the plasma membrane. By creating chimeras we discovered that mitochondrial sorting of Kesv depends on a hierarchical combination of N- and C-terminal signals. Crucial is the length of the second transmembrane domain; extending its C terminus by > or = 2 hydrophobic amino acids redirects Kesv from the mitochondrial to the plasma membrane. Activity of Kesv in the plasma membrane is detected electrically or by yeast rescue assays only after this shift in sorting. Hence only minor structural alterations in a transmembrane domain are sufficient to switch sorting of a K(+) channel between the plasma membrane and mitochondria.
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Krishnan MN, Trombley P, Moczydlowski EG. Thermal stability of the K+ channel tetramer: cation interactions and the conserved threonine residue at the innermost site (S4) of the KcsA selectivity filter. Biochemistry 2008; 47:5354-67. [PMID: 18419132 DOI: 10.1021/bi702281p] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
The selectivity filter of most K+ channels contains a highly conserved Thr residue that uniquely forms the S4 binding site for K+ by dual coordination with the backbone carbonyl oxygen and side chain hydroxyl of the same residue. This study examines the effect of mutations of Thr75 in the S4 site of theKcsA K+ channel on the cation dependence of the thermal stability of the tetramer, a phenomenon that reflects the structural role of cations in the filter. Conservative mutations of Thr75 destabilize the tetramer and alter its temperature dependence. Replacement of Thr with Ala or Cys lowers the apparent affinity ofK+, Rb+, and Cs+ for tetramer stabilization by factors ranging from 4- to 14-fold. These same mutations lower the apparent affinity of Ba2+ by approximately 10(3)- or approximately 10(4)-fold for Ala and Cys substitution, respectively,consistent with the known preference of the S4 site for Ba2+. In contrast, substitution of Ala or Cys at T75 anomalously enhances the ability of Na+ to stabilize the tetramer, suggesting that the native Thr residue at S4 is important for ultrahigh K+/Na+ selectivity of K+ channel pores. Elevated temperature orCu2+ cation catalyzes formation of covalent dimers of the T75C mutant of KcsA via formation of disulfide bonds between Cys residues of adjacent subunits. Thiophilic cations such as Hg2+ and Ag+ specifically protect the T75C tetramer against heat-induced dimer formation, demonstrating the contribution of cation interactions to tetramer stability in a channel with a non-native S4 site engineered to bind foreign cations.
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Affiliation(s)
- Manoj N Krishnan
- Department of Biology, Box 5805, Clarkson UniVersity, Potsdam, New York 13699, USA
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39
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Chlorella viruses evoke a rapid release of K+ from host cells during the early phase of infection. Virology 2008; 372:340-8. [DOI: 10.1016/j.virol.2007.10.024] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/09/2007] [Revised: 09/30/2007] [Accepted: 10/24/2007] [Indexed: 11/18/2022]
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40
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Schroeder I, Hansen UP. Saturation and microsecond gating of current indicate depletion-induced instability of the MaxiK selectivity filter. ACTA ACUST UNITED AC 2007; 130:83-97. [PMID: 17591987 PMCID: PMC2154363 DOI: 10.1085/jgp.200709802] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
Patch clamp experiments on single MaxiK channels expressed in HEK293 cells were performed with a high temporal resolution (50-kHz filter) in symmetrical solutions with 50, 150, or 400 mM KCl and 2.5 mM CaCl(2) and 2.5 mM MgCl(2). At membrane potentials >+100 mV, the single-channel current showed a negative slope resistance, concomitantly with a flickery block, which was not influenced by Ca(2+) or Mg(2+). The analysis of the amplitude histograms by beta distributions revealed that current in this voltage range was reduced by two effects: rate limitation at the cytosolic side of the pore and gating with rate constants 10-20-fold higher than the cutoff frequency of the filter (i.e., dwell times in the microsecond range). The data were analyzed in terms of a model that postulates a coupling between both effects; if the voltage over the selectivity filter withdraws ions from the cavity at a higher rate than that of refilling from the cytosol, the selectivity filter becomes instable because of ion depletion, and current is interrupted by the resulting flickering. The fit of the IV curves revealed a characteristic voltage of 35 mV. In contrast, the voltage dependence of the gating factor R, i.e., the ratio between true and apparent single-channel current, could be fitted by exponentials with a characteristic voltage of 60 mV, suggesting that only part of the transmembrane potential is felt by the flux through the selectivity filter.
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Affiliation(s)
- Indra Schroeder
- Department of Structural Biology, University of Kiel, 24098 Kiel, Germany
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Gazzarrini S, Abenavoli A, Gradmann D, Thiel G, Moroni A. Electrokinetics of miniature K+ channel: open-state V sensitivity and inhibition by K+ driving force. J Membr Biol 2007; 214:9-17. [PMID: 17568981 DOI: 10.1007/s00232-006-0024-3] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/23/2006] [Revised: 10/26/2006] [Indexed: 11/26/2022]
Abstract
Kcv, isolated from a Chlorella virus, is the smallest known K+ channel. When Kcv is expressed in Xenopus oocytes and exposed to 50 mM: [K+](o), its open-state current-voltage relationship (I-V) has the shape of a "tilted S" between -200 and +120 mV. Details of this shape depend on the conditioning voltage (V (c)) immediately before an I-V recording. Unexpectedly, the I-V relationships, recorded in different [K+](o), do intersect. These characteristics are numerically described here by fits of a kinetic model to the experimental data. In this model, the V (c) sensitivity of I-V is mainly assigned to an affinity increase of external K+ association at more positive voltages. The general, tilted-S shape as well as the unexpected intersections of the I-V relationships are kinetically described by a decrease of the cord conductance by the electrochemical driving force for K+ in either direction, like in fast V-dependent blocking by competing ions.
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Affiliation(s)
- Sabrina Gazzarrini
- Department of Biology and IBF-CNR, Università degli Studi di Milano, Via Celoria 26, 20133 Milan, Italy
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