201
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Dimeric subunit stoichiometry of the human voltage-dependent proton channel Hv1. Proc Natl Acad Sci U S A 2008; 105:7692-5. [PMID: 18509058 DOI: 10.1073/pnas.0803277105] [Citation(s) in RCA: 152] [Impact Index Per Article: 9.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
In voltage-gated Na(+), K(+), and Ca(2+) channels, four voltage-sensor domains operate on a central pore domain in response to membrane voltage. In contrast, the voltage-gated proton channel (Hv) contains only a voltage-sensor domain, lacking a separate pore domain. The subunit stoichiometry and organization of Hv has been unknown. Here, we show that human Hv1 forms a dimer in the membrane and define regions that are close to the dimer interface by using cysteine cross-linking. Two dimeric interfaces appear to exist in Hv1, one mediated by S1 and the adjacent extracellular loop, and the other mediated by a putative intracellular coiled-coil domain. It may be significant that Hv1 uses for its dimer interface a surface that corresponds to the interface between the voltage sensor and pore in Kv channels.
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202
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Vamvouka M, Cieslak J, Van Eps N, Hubbell W, Gross A. The structure of the lipid-embedded potassium channel voltage sensor determined by double-electron-electron resonance spectroscopy. Protein Sci 2008; 17:506-17. [PMID: 18287283 DOI: 10.1110/ps.073310008] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
Abstract
A four-pulse electron paramagnetic resonance experiment was used to measure long-range inter-subunit distances in reconstituted KvAP, a voltage-dependent potassium (Kv) channel. The measurements have allowed us to reach the following five conclusions about the native structure of the voltage sensor of KvAP. First, the S1 helix of the voltage sensor engages in a helix packing interaction with the pore domain. Second, the crystallographically observed antiparallel helix-turn-helix motif of the voltage-sensing paddle is retained in the membrane-embedded voltage sensor. Third, the paddle is oriented in such a way as to expose one face to the pore domain and the opposite face to the membrane. Fourth, the paddle and the pore domain appear to be separated by a gap that is sufficiently wide for lipids to penetrate between the two domains. Fifth, the critical voltage-sensing arginine residues on the paddle appear to be lipid exposed. These results demonstrate the importance of the membrane for the native structure of Kv channels, suggest that lipids are an integral part of their native structure, and place the voltage-sensing machinery into a complex lipid environment near the pore domain.
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Affiliation(s)
- Magdalini Vamvouka
- Department of Molecular Pharmacology and Biological Chemistry, Northwestern University Medical School, Chicago, Illinois 60611, USA
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203
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Gao Y, Chotoo CK, Balut CM, Sun F, Bailey MA, Devor DC. Role of S3 and S4 transmembrane domain charged amino acids in channel biogenesis and gating of KCa2.3 and KCa3.1. J Biol Chem 2008; 283:9049-59. [PMID: 18227067 DOI: 10.1074/jbc.m708022200] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The role of positively charged arginines in the fourth transmembrane domain (S4) and a single negatively charged amino acid in the third transmembrane domain (S3) on channel biogenesis and gating of voltage-gated K(+) channels (Kv) has been well established. Both intermediate (KCa3.1) and small (KCa2.x) conductance, Ca(2+)-activated K(+) channels have two conserved arginines in S4 and a single conserved glutamic acid in S3, although these channels are voltage-independent. We demonstrate that mutation of any of these charged amino acids in KCa3.1 or KCa2.3 to alanine, glutamine, or charge reversal mutations results in a rapid degradation (<30 min) of total protein, confirming the critical role of these amino acids in channel biogenesis. Mutation of the S4 arginine closest to the cytosolic side of KCa3.1 to histidine resulted in expression at the cell surface. Excised patch clamp experiments revealed that this Arg/His mutation had a dramatically reduced open probability (P(o)), relative to wild type channels. Additionally, we demonstrate, using a combination of short hairpin RNA, dominant negative, and co-immunoprecipitation studies, that both KCa3.1 and KCa2.3 are translocated out of the endoplasmic reticulum associated with Derlin-1. These misfolded channels are poly-ubiquitylated, recognized by p97, and targeted for proteasomal degradation. Our results suggest that S3 and S4 charged amino acids play an evolutionarily conserved role in the biogenesis and gating of KCa channels. Furthermore, these improperly folded K(+) channels are translocated out of the endoplasmic reticulum in a Derlin-1- and p97-dependent fashion, poly-ubiquitylated, and targeted for proteasomal degradation.
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Affiliation(s)
- Yajuan Gao
- Department of Cell Biology and Physiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261,USA
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204
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Pathak MM, Yarov-Yarovoy V, Agarwal G, Roux B, Barth P, Kohout S, Tombola F, Isacoff EY. Closing in on the resting state of the Shaker K(+) channel. Neuron 2008; 56:124-40. [PMID: 17920020 DOI: 10.1016/j.neuron.2007.09.023] [Citation(s) in RCA: 236] [Impact Index Per Article: 14.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2007] [Revised: 08/14/2007] [Accepted: 09/18/2007] [Indexed: 10/22/2022]
Abstract
Membrane depolarization causes voltage-gated ion channels to transition from a resting/closed conformation to an activated/open conformation. We used voltage-clamp fluorometry to measure protein motion at specific regions of the Shaker Kv channel. This enabled us to construct new structural models of the resting/closed and activated/open states based on the Kv1.2 crystal structure using the Rosetta-Membrane method and molecular dynamics simulations. Our models account for the measured gating charge displacement and suggest a molecular mechanism of activation in which the primary voltage sensors, S4s, rotate by approximately 180 degrees as they move "outward" by 6-8 A. A subsequent tilting motion of the S4s and the pore domain helices, S5s, of all four subunits induces a concerted movement of the channel's S4-S5 linkers and S6 helices, allowing ion conduction. Our models are compatible with a wide body of data and resolve apparent contradictions that previously led to several distinct models of voltage sensing.
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Affiliation(s)
- Medha M Pathak
- Biophysics Graduate Program, University of California Berkeley, Berkeley, CA 94720, USA
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205
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Papaloukas C, Granseth E, Viklund H, Elofsson A. Estimating the length of transmembrane helices using Z-coordinate predictions. Protein Sci 2007; 17:271-8. [PMID: 18096645 DOI: 10.1110/ps.073036108] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
Abstract
Zpred2 is an improved version of ZPRED, a predictor for the Z-coordinates of alpha-helical membrane proteins, that is, the distance of the residues from the center of the membrane. Using principal component analysis and a set of neural networks, Zpred2 analyzes data extracted from the amino acid sequence, the predicted topology, and evolutionary profiles. Zpred2 achieves an average accuracy error of 2.18 A (2.17 A when an independent test set is used), an improvement by 15% compared to the previous version. We show that this accuracy is sufficient to enable the predictions of helix lengths with a correlation coefficient of 0.41. As a comparison, two state-of-the-art HMM-based topology prediction methods manage to predict the helix lengths with a correlation coefficient of less than 0.1. In addition, we applied Zpred2 to two other problems, the re-entrant region identification and model validation. Re-entrants were able to be detected with a certain consistency, but not better than with previous approaches, while incorrect models as well as mispredicted helices of transmembrane proteins could be distinguished based on the Z-coordinate predictions.
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Affiliation(s)
- Costas Papaloukas
- Stockholm Bioinformatics Center, Center for Biomembrane Research, Department of Biochemistry and Biophysics, Stockholm University, SE-106 91 Stockholm, Sweden
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206
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Bett GCL, Rasmusson RL. Modification of K+ channel-drug interactions by ancillary subunits. J Physiol 2007; 586:929-50. [PMID: 18096604 DOI: 10.1113/jphysiol.2007.139279] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
Reconciling ion channel alpha-subunit expression with native ionic currents and their pharmacological sensitivity in target organs has proved difficult. In native tissue, many K(+) channel alpha-subunits co-assemble with ancillary subunits, which can profoundly affect physiological parameters including gating kinetics and pharmacological interactions. In this review, we examine the link between voltage-gated potassium ion channel pharmacology and the biophysics of ancillary subunits. We propose that ancillary subunits can modify the interaction between pore blockers and ion channels by three distinct mechanisms: changes in (1) binding site accessibility; (2) orientation of pore-lining residues; (3) the ability of the channel to undergo post-binding conformational changes. Each of these subunit-induced changes has implications for gating, drug affinity and use dependence of their respective channel complexes. A single subunit may modulate its associated alpha-subunit by more than one of these mechanisms. Voltage-gated potassium channels are the site of action of many therapeutic drugs. In addition, potassium channels interact with drugs whose primary target is another channel, e.g. the calcium channel blocker nifedipine, the sodium channel blocker quinidine, etc. Even when K(+) channel block is the intended mode of action, block of related channels in non-target organs, e.g. the heart, can result in major and potentially lethal side-effects. Understanding factors that determine specificity, use dependence and other properties of K(+) channel drug binding are therefore of vital clinical importance. Ancillary subunits play a key role in determining these properties in native tissue, and so understanding channel-subunit interactions is vital to understanding clinical pharmacology.
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Affiliation(s)
- Glenna C L Bett
- Center for Cellular and Systems Electrophysiology, Department of Physiology and Biophysics, School of Medicine and Biomedical Sciences, 124 Sherman Hall, State University of New York at Buffalo, Buffalo, NY 14214-3005, USA
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207
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Portability of paddle motif function and pharmacology in voltage sensors. Nature 2007; 450:370-5. [PMID: 18004375 DOI: 10.1038/nature06266] [Citation(s) in RCA: 185] [Impact Index Per Article: 10.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2007] [Accepted: 09/17/2007] [Indexed: 01/28/2023]
Abstract
Voltage-sensing domains enable membrane proteins to sense and react to changes in membrane voltage. Although identifiable S1-S4 voltage-sensing domains are found in an array of conventional ion channels and in other membrane proteins that lack pore domains, the extent to which their voltage-sensing mechanisms are conserved is unknown. Here we show that the voltage-sensor paddle, a motif composed of S3b and S4 helices, can drive channel opening with membrane depolarization when transplanted from an archaebacterial voltage-activated potassium channel (KvAP) or voltage-sensing domain proteins (Hv1 and Ci-VSP) into eukaryotic voltage-activated potassium channels. Tarantula toxins that partition into membranes can interact with these paddle motifs at the protein-lipid interface and similarly perturb voltage-sensor activation in both ion channels and proteins with a voltage-sensing domain. Our results show that paddle motifs are modular, that their functions are conserved in voltage sensors, and that they move in the relatively unconstrained environment of the lipid membrane. The widespread targeting of voltage-sensor paddles by toxins demonstrates that this modular structural motif is an important pharmacological target.
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208
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Borbat PP, Surendhran K, Bortolus M, Zou P, Freed JH, Mchaourab HS. Conformational motion of the ABC transporter MsbA induced by ATP hydrolysis. PLoS Biol 2007; 5:e271. [PMID: 17927448 PMCID: PMC2001213 DOI: 10.1371/journal.pbio.0050271] [Citation(s) in RCA: 124] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2007] [Accepted: 08/15/2007] [Indexed: 11/24/2022] Open
Abstract
We measured the amplitude of conformational motion in the ATP-binding cassette (ABC) transporter MsbA upon lipopolysaccharide (LPS) binding and following ATP turnover by pulse double electron-electron resonance and fluorescence homotransfer. The distance constraints from both methods reveal large-scale movement of opposite signs in the periplasmic and cytoplasmic part of the transporter upon ATP hydrolysis. LPS induces distinct structural changes that are inhibited by trapping of the transporter in an ATP post-hydrolysis intermediate. The formation of this intermediate involves a 33-Å distance change between the two ABCs, which is consistent with a dimerization-dissociation cycle during transport that leads to their substantial separation in the absence of nucleotides. Our results suggest that ATP-powered transport entails LPS sequestering into the open cytoplasmic chamber prior to its translocation by alternating access of the chamber, made possible by 10–20-Å conformational changes. Clinical multidrug resistance in the treatment of bacterial and fungal infections and cancer chemotherapy can result from the expression of pumps that extrude toxic molecules from the cell. A subclass of these pumps—ATP-binding cassette (ABC) transporters—use energy from ATP to remove a wide range of molecules. MsbA is a conserved ABC transporter from Gram-negative bacteria with sequence similarity to human multi-drug ABC transporters. MsbA flips the building block of the outer membrane, lipid A, across the inner membrane. The input of ATP energy occurs in two dedicated nucleotide-binding domains (NBDs), whose configuration in intact transporters is controversial. We determined the amplitude of MsbA conformational motion that couples energy expenditure to substrate movement across the membrane. Using molecular probes introduced into the protein sequence, we found that ATP hydrolysis fuels a relative motion of the NBDs close to 30 Å. The movement of the NBDs is coupled to reorientation of the chamber, which binds the lipid substrate from cytoplasmic-facing to extracellular-facing through large amplitude motion on either side of the transporters. In addition to revealing the structural mechanics of transport, these results challenge current models deduced from studies of substrate-specific ABC importers that envisions the two NBDs in contact throughout the ATP hydrolysis cycle. Analysis of the conformational changes that occur in a conserved ATP-binding cassette (ABC) transporter challenges current models of this clinically important class of molecules.
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Affiliation(s)
- Peter P Borbat
- Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York, United States of America
| | - Kavitha Surendhran
- Department of Molecular Physiology and Biophysics, Vanderbilt University Medical Center, Nashville, Tennessee, United States of America
| | - Marco Bortolus
- Department of Molecular Physiology and Biophysics, Vanderbilt University Medical Center, Nashville, Tennessee, United States of America
| | - Ping Zou
- Department of Molecular Physiology and Biophysics, Vanderbilt University Medical Center, Nashville, Tennessee, United States of America
| | - Jack H Freed
- Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York, United States of America
| | - Hassane S Mchaourab
- Department of Molecular Physiology and Biophysics, Vanderbilt University Medical Center, Nashville, Tennessee, United States of America
- * To whom correspondence should be addressed. E-mail:
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209
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Long SB, Tao X, Campbell EB, MacKinnon R. Atomic structure of a voltage-dependent K+ channel in a lipid membrane-like environment. Nature 2007; 450:376-82. [PMID: 18004376 DOI: 10.1038/nature06265] [Citation(s) in RCA: 1139] [Impact Index Per Article: 67.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/08/2007] [Accepted: 09/17/2007] [Indexed: 11/09/2022]
Abstract
Voltage-dependent K+ (Kv) channels repolarize the action potential in neurons and muscle. This type of channel is gated directly by membrane voltage through protein domains known as voltage sensors, which are molecular voltmeters that read the membrane voltage and regulate the pore. Here we describe the structure of a chimaeric voltage-dependent K+ channel, which we call the 'paddle-chimaera channel', in which the voltage-sensor paddle has been transferred from Kv2.1 to Kv1.2. Crystallized in complex with lipids, the complete structure at 2.4 ångström resolution reveals the pore and voltage sensors embedded in a membrane-like arrangement of lipid molecules. The detailed structure, which can be compared directly to a large body of functional data, explains charge stabilization within the membrane and suggests a mechanism for voltage-sensor movements and pore gating.
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Affiliation(s)
- Stephen B Long
- Howard Hughes Medical Institute, Laboratory of Molecular Neurobiology and Biophysics, Rockefeller University, 1230 York Avenue, New York, New York 10065, USA
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210
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Chiu PL, Pagel MD, Evans J, Chou HT, Zeng X, Gipson B, Stahlberg H, Nimigean CM. The structure of the prokaryotic cyclic nucleotide-modulated potassium channel MloK1 at 16 A resolution. Structure 2007; 15:1053-64. [PMID: 17850745 PMCID: PMC2000844 DOI: 10.1016/j.str.2007.06.020] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2007] [Revised: 06/11/2007] [Accepted: 06/14/2007] [Indexed: 11/24/2022]
Abstract
The gating ring of cyclic nucleotide-modulated channels is proposed to be either a two-fold symmetric dimer of dimers or a four-fold symmetric tetramer based on high-resolution structure data of soluble cyclic nucleotide-binding domains and functional data on intact channels. We addressed this controversy by obtaining structural data on an intact, full-length, cyclic nucleotide-modulated potassium channel, MloK1, from Mesorhizobium loti, which also features a putative voltage-sensor. We present here the 3D single-particle structure by transmission electron microscopy and the projection map of membrane-reconstituted 2D crystals of MloK1 in the presence of cAMP. Our data show a four-fold symmetric arrangement of the CNBDs, separated by discrete gaps. A homology model for full-length MloK1 suggests a vertical orientation for the CNBDs. The 2D crystal packing in the membrane-embedded state is compatible with the S1-S4 domains in the vertical "up" state.
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Affiliation(s)
- Po-Lin Chiu
- Molecular and Cellular Biology, College of Biological Sciences, University of California-Davis, 1 Shields Avenue, Davis, CA 95616, USA
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211
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Nauli S, Farr S, Lee YJ, Kim HY, Faham S, Bowie JU. Polymer-driven crystallization. Protein Sci 2007; 16:2542-51. [PMID: 17962407 PMCID: PMC2211692 DOI: 10.1110/ps.073074207] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2007] [Revised: 07/31/2007] [Accepted: 08/01/2007] [Indexed: 10/22/2022]
Abstract
Obtaining well-diffracting crystals of macromolecules remains a significant barrier to structure determination. Here we propose and test a new approach to crystallization, in which the crystallization target is fused to a polymerizing protein module, so that polymer formation drives crystallization of the target. We test the approach using a polymerization module called 2TEL, which consists of two tandem sterile alpha motif (SAM) domains from the protein translocation Ets leukemia (TEL). The 2TEL module is engineered to polymerize as the pH is lowered, which allows the subtle modulation of polymerization needed for crystal formation. We show that the 2TEL module can drive the crystallization of 11 soluble proteins, including three that resisted prior crystallization attempts. In addition, the 2TEL module crystallizes in the presence of various detergents, suggesting that it might facilitate membrane protein crystallization. The crystal structures of two fusion proteins show that the TELSAM polymer is responsible for the majority of contacts in the crystal lattice. The results suggest that biological polymers could be designed as crystallization modules.
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Affiliation(s)
- Sehat Nauli
- UCLA-DOE Institute of Genomics and Proteomics, University of California, Los Angeles 90095-1570, USA
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212
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Neale EJ, Rong H, Cockcroft CJ, Sivaprasadarao A. Mapping the membrane-aqueous border for the voltage-sensing domain of a potassium channel. J Biol Chem 2007; 282:37597-604. [PMID: 17951256 DOI: 10.1074/jbc.m706437200] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Voltage-sensing domains (VSDs) play diverse roles in biology. As integral components, they can detect changes in the membrane potential of a cell and couple these changes to activity of ion channels and enzymes. As independent proteins, homologues of the VSD can function as voltage-dependent proton channels. To sense voltage changes, the positively charged fourth transmembrane segment, S4, must move across the energetically unfavorable hydrophobic core of the bilayer, which presents a barrier to movement of both charged species and protons. To reduce the barrier to S4 movement, it has been suggested that aqueous crevices may penetrate the protein, reducing the extent of total movement. To investigate this hypothesis in a system containing fully functional channels in a native environment with an intact membrane potential, we have determined the contour of the membrane-aqueous border of the VSD of KvAP in Escherichia coli by examining the chemical accessibility of introduced cysteines. The results revealed the contour of the membrane-aqueous border of the VSD in its activated conformation. The water-inaccessible regions of S1 and S2 correspond to the standard width of the membrane bilayer (~28 A), but those of S3 and S4 are considerably shorter (> or = 40%), consistent with aqueous crevices pervading both the extracellular and intracellular ends. One face of S3b and the entire S3a were water-accessible, reducing the water-inaccessible region of S3 to just 10 residues, significantly shorter than for S4. The results suggest a key role for S3 in reducing the distance S4 needs to move to elicit gating.
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Affiliation(s)
- Edward J Neale
- Institute of Membrane and Systems Biology, Faculty of Biological Sciences, University of Leeds, Leeds, UK
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213
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Milescu M, Vobecky J, Roh SH, Kim SH, Jung HJ, Kim JI, Swartz KJ. Tarantula toxins interact with voltage sensors within lipid membranes. ACTA ACUST UNITED AC 2007; 130:497-511. [PMID: 17938232 PMCID: PMC2151668 DOI: 10.1085/jgp.200709869] [Citation(s) in RCA: 98] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Voltage-activated ion channels are essential for electrical signaling, yet the mechanism of voltage sensing remains under intense investigation. The voltage-sensor paddle is a crucial structural motif in voltage-activated potassium (Kv) channels that has been proposed to move at the protein–lipid interface in response to changes in membrane voltage. Here we explore whether tarantula toxins like hanatoxin and SGTx1 inhibit Kv channels by interacting with paddle motifs within the membrane. We find that these toxins can partition into membranes under physiologically relevant conditions, but that the toxin–membrane interaction is not sufficient to inhibit Kv channels. From mutagenesis studies we identify regions of the toxin involved in binding to the paddle motif, and those important for interacting with membranes. Modification of membranes with sphingomyelinase D dramatically alters the stability of the toxin–channel complex, suggesting that tarantula toxins interact with paddle motifs within the membrane and that they are sensitive detectors of lipid–channel interactions.
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Affiliation(s)
- Mirela Milescu
- Molecular Physiology and Biophysics Section, Porter Neuroscience Research Center, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892, USA.
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214
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Kuznetsova AY, Deth RC. A model for modulation of neuronal synchronization by D4 dopamine receptor-mediated phospholipid methylation. J Comput Neurosci 2007; 24:314-29. [PMID: 17929154 DOI: 10.1007/s10827-007-0057-3] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/10/2006] [Revised: 08/29/2007] [Accepted: 09/24/2007] [Indexed: 12/16/2022]
Abstract
We describe a new molecular mechanism of dopamine-induced membrane protein modulation that can tune neuronal oscillation frequency to attention-related gamma rhythm. This mechanism is based on the unique ability of D4 dopamine receptors (D4R) to carry out phospholipid methylation (PLM) that may affect the kinetics of ion channels. We show that by deceasing the inertia of the delayed rectifier potassium channel, a transition to 40 Hz oscillations can be achieved. Decreased potassium channel inertia shortens spike duration and decreases the interspike interval via its influence on the calcium-dependent potassium current. This mechanism leads to a transition to attention-related gamma oscillations in a pyramidal cell-interneuron network. The higher frequency and better synchronization is observed with PLM affecting pyramidal neurons only, and recurrent excitation between pyramidal neurons is important for synchronization. Thus dopamine-stimulated methylation of membrane phospholipids may be an important mechanism for modulating firing activity, while impaired methylation can contribute to disorders of attention.
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Affiliation(s)
- Anna Y Kuznetsova
- Neuroscience Center of Excellence, Louisiana State University Health Sciences Center, 2020 Gravier St., Suite D, New Orleans, LA 70112, USA
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215
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Poget SF, Girvin ME. Solution NMR of membrane proteins in bilayer mimics: small is beautiful, but sometimes bigger is better. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2007; 1768:3098-106. [PMID: 17961504 DOI: 10.1016/j.bbamem.2007.09.006] [Citation(s) in RCA: 86] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/11/2007] [Revised: 09/07/2007] [Accepted: 09/10/2007] [Indexed: 10/22/2022]
Abstract
Considerable progress has been made recently on solution NMR studies of multi-transmembrane helix membrane protein systems of increasing size. Careful correlation of structure with function has validated the physiological relevance of these studies in detergent micelles. However, larger micelle and bicelle systems are sometimes required to stabilize the active forms of dynamic membrane proteins, such as the bacterial small multidrug resistance transporters. Even in these systems with aggregate molecular weights well over 100 kDa, solution NMR structural studies are feasible-but challenging.
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Affiliation(s)
- Sébastien F Poget
- Biochemistry Department, Albert Einstein College of Medicine, 1300 Morris Park Ave. Bronx, NY 10461, USA
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216
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Minor DL. The neurobiologist's guide to structural biology: a primer on why macromolecular structure matters and how to evaluate structural data. Neuron 2007; 54:511-33. [PMID: 17521566 PMCID: PMC3011226 DOI: 10.1016/j.neuron.2007.04.026] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
Structural biology now plays a prominent role in addressing questions central to understanding how excitable cells function. Although interest in the insights gained from the definition and dissection of macromolecular anatomy is high, many neurobiologists remain unfamiliar with the methods employed. This primer aims to help neurobiologists understand approaches for probing macromolecular structure and where the limits and challenges remain. Using examples of macromolecules with neurobiological importance, the review covers X-ray crystallography, electron microscopy (EM), small-angle X-ray scattering (SAXS), and nuclear magnetic resonance (NMR) and biophysical methods with which these approaches are often paired: isothermal titration calorimetry (ITC), equilibrium analytical ultracentifugation, and molecular dynamics (MD).
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Affiliation(s)
- Daniel L Minor
- Cardiovascular Research Institute, Department of Biochemistry and Biophysics, California Institute for Quantitative Biomedical Research, University of California, San Francisco, San Francisco, CA 94158-2330, USA.
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217
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Schuldiner S. When biochemistry meets structural biology: the cautionary tale of EmrE. Trends Biochem Sci 2007; 32:252-8. [PMID: 17452106 DOI: 10.1016/j.tibs.2007.04.002] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/01/2007] [Revised: 03/19/2007] [Accepted: 04/13/2007] [Indexed: 11/23/2022]
Abstract
When biochemistry meets structural biology a more complete understanding of the mechanism of biological macromolecules is usually achieved. Several high-resolution structures of ion-coupled transporters have enriched the understanding of mechanisms of substrate recognition, translocation and coupling of substrate fluxes. However, two X-ray structures of EmrE, the smallest ion-coupled multi-drug transporter, raised questions over the veracity of the structural model and represented a cautionary tale about the difficulty of determining the 3D structures of membrane proteins and the dangers of ignoring biochemical results. The 3D structures of EmrE have since been retracted because of faulty software, but the suggestion that the protomers in the dimer are in an antiparallel topological orientation sparked controversy that is still ongoing.
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Affiliation(s)
- Shimon Schuldiner
- Department of Biological Chemistry, Alexander Silberman Institute of Life Sciences, Hebrew University of Jerusalem, 91904 Jerusalem, Israel.
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218
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Zhang M, Liu XS, Diochot S, Lazdunski M, Tseng GN. APETx1 from Sea Anemone Anthopleura elegantissima Is a Gating Modifier Peptide Toxin of the Human Ether-a-go-go- Related Potassium Channel. Mol Pharmacol 2007; 72:259-68. [PMID: 17473056 DOI: 10.1124/mol.107.035840] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
We studied the mechanism of action and the binding site of APETx1, a peptide toxin purified from sea anemone, on the human ether-a-go-go-related gene (hERG) channel. Similar to the effects of gating modifier toxins (hanatoxin and SGTx) on the voltage-gated potassium (Kv) 2.1 channel, APETx1 shifts the voltage-dependence of hERG activation in the positive direction and suppresses its current amplitudes elicited by strong depolarizing pulses that maximally activate the channels. The APETx1 binding site is distinctly different from that of a pore-blocking peptide toxin, BeKm-1. Mutations in the S3b region of hERG have dramatic impact on the responsiveness to APETx1: G514C potentiates whereas E518C abolishes the APETx1 effect. Restoring the negative charge at position 518 (methanethiosulfonate ethylsulfonate modification of 518C) partially restores APETx1 responsiveness, supporting an electrostatic interaction between E518 and APETx1. Among the three hERG isoforms, hERG1 and hERG3 are equally responsive to APETx1, whereas hERG2 is insensitive. The key feature seems to be an arginine residue uniquely present at the 514-equivalent position in hERG2, where the other two isoforms possess a glycine. Our data show that APETx1 is a gating modifier toxin of the hERG channel, and its binding site shares characteristics with those of gating modifier toxin binding sites on other Kv channels.
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Affiliation(s)
- M Zhang
- Department of Physiology, Medical College of Virginia, Virginia Commonwealth University, 1101 E. Marshall Street, Richmond, VA 23298, USA
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219
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Bonnet S, Archer SL. Potassium channel diversity in the pulmonary arteries and pulmonary veins: implications for regulation of the pulmonary vasculature in health and during pulmonary hypertension. Pharmacol Ther 2007; 115:56-69. [PMID: 17583356 DOI: 10.1016/j.pharmthera.2007.03.014] [Citation(s) in RCA: 70] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/27/2007] [Accepted: 03/28/2007] [Indexed: 12/15/2022]
Abstract
This review describes the ionic heterogeneity manifest in the pulmonary circulation, particularly as it pertains to hypoxic pulmonary vasoconstriction (HPV) and pulmonary arterial hypertension (PAH). Heterogeneity in potassium (K(+)) channels, key regulators of vascular tone, cell proliferation, and apoptosis rates, contribute to the diverse response of vascular segments to hypoxia and to the localization of pathological changes in PAH. Pulmonary artery (PA) and pulmonary vein (PV) smooth muscle cells (SMC) express several K(+) channel families, including calcium-sensitive (KCa), voltage-gated (K(v)), inward rectifier (Kir), and 2-pore channels. Diversity is created by heterogeneous occurrence of alternatively spliced, mRNA species, assembly of heterotetrameric channels from diverse alpha-subunits, and association of channels with regulatory beta-subunits. Local heterogeneity in transcription factor activity may underlie differences in channel expression. Enrichment of resistance PASMCs with O(2)-sensitive K(+) channels, such as K(v)1.5, partially explains the greater HPV in resistance versus conduit PAs. In addition, resistance PAs are unique in having mitochondria which dynamically alter production of reactive O(2) species (ROS) in proportion to PO(2), thereby regulating K(+) channel activity and controlling expression through transcription factors, such as HIF-1alpha. In intraparenchymal PVs, a coaxial layer of cardiomyocytes encompasses a media of typical vascular SMCs. PV cardiomyocytes have rhythmic contraction and their Kir-enriched channels may be relevant to genesis of atrial arrhythmias and pulmonary edema. K(v) channel expression is decreased in PAH, leading to elevations of cytosolic K(+) and Ca(2+) that impair apoptosis and increase proliferation. Understanding ionic diversity may allow development of therapies that locally increase K(+) channel current and expression to treat PHT.
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Affiliation(s)
- Sébastien Bonnet
- Department of Medicine (Cardiology), University of Alberta, Edmonton, Canada
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220
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Morris CE, Juranka PF. Lipid stress at play: mechanosensitivity of voltage-gated channels. CURRENT TOPICS IN MEMBRANES 2007; 59:297-338. [PMID: 25168141 DOI: 10.1016/s1063-5823(06)59011-8] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/10/2022]
Abstract
Membrane stretch modulates the activity of voltage-gated channels (VGCs). These channels are nearly ubiquitous among eukaryotes and they are present, too, in prokaryotes, so the potential ramifications of VGC mechanosensitivity are diverse. In situ traumatic stretch can irreversibly alter VGC activity with lethal results but that is pathology. This chapter discusses the reversible responses of VGCs to stretch, with the general relation of stretch stimuli to other forms of lipid stress, and briefly, with some irreversible stretch effects (=stretch trauma). A working assumption throughout is that mechanosensitive (MS) VGC motions-that is, motions that respond reversibly to bilayer stretch-are susceptible to other forms of lipid stress, such as the stresses produced when amphiphilic molecules (anesthetics, lipids, alcohols, and lipophilic drugs) are inserted into the bilayer. Insofar as these molecules change the bilayer's lateral pressure profile, they can be termed bilayer mechanical reagents (BMRs). The chapter also discusses the MS VGC behavior against the backdrop of eukaryotic channels more widely accepted as "MS channels"--namely, the transient receptor potential (TRP)-based MS cation channels.
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Affiliation(s)
- Catherine E Morris
- Neuroscience, Ottawa Health Research Institute, Ottawa Hospital, Ottawa, Ontario K1Y 4E9, Canada
| | - Peter F Juranka
- Neuroscience, Ottawa Health Research Institute, Ottawa Hospital, Ottawa, Ontario K1Y 4E9, Canada
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221
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Sands ZA, Sansom MS. How does a voltage sensor interact with a lipid bilayer? Simulations of a potassium channel domain. Structure 2007; 15:235-44. [PMID: 17292841 PMCID: PMC1885962 DOI: 10.1016/j.str.2007.01.004] [Citation(s) in RCA: 87] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/30/2006] [Revised: 12/14/2006] [Accepted: 01/04/2007] [Indexed: 11/17/2022]
Abstract
The nature of voltage sensing by voltage-activated ion channels is a key problem in membrane protein structural biology. The way in which the voltage-sensor (VS) domain interacts with its membrane environment remains unclear. In particular, the known structures of Kv channels do not readily explain how a positively charged S4 helix is able to stably span a lipid bilayer. Extended (2 × 50 ns) molecular dynamics simulations of the high-resolution structure of the isolated VS domain from the archaebacterial potassium channel KvAP, embedded in zwitterionic and in anionic lipid bilayers, have been used to explore VS/lipid interactions at atomic resolution. The simulations reveal penetration of water into the center of the VS and bilayer. Furthermore, there is significant local deformation of the lipid bilayer by interactions between lipid phosphate groups and arginine side chains of S4. As a consequence of this, the electrostatic field is “focused” across the center of the bilayer.
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Affiliation(s)
- Zara A. Sands
- Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, United Kingdom
| | - Mark S.P. Sansom
- Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, United Kingdom
- Corresponding author
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222
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McNulty MM, Edgerton GB, Shah RD, Hanck DA, Fozzard HA, Lipkind GM. Charge at the lidocaine binding site residue Phe-1759 affects permeation in human cardiac voltage-gated sodium channels. J Physiol 2007; 581:741-55. [PMID: 17363383 PMCID: PMC2075178 DOI: 10.1113/jphysiol.2007.130161] [Citation(s) in RCA: 67] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Abstract
Our homology molecular model of the open/inactivated state of the Na(+) channel pore predicts, based on extensive mutagenesis data, that the local anaesthetic lidocaine docks eccentrically below the selectivity filter, such that physical occlusion is incomplete. Electrostatic field calculations suggest that the drug's positively charged amine produces an electrostatic barrier to permeation. To test the effect of charge at this pore level on permeation in hNa(V)1.5 we replaced Phe-1759 of domain IVS6, the putative binding site for lidocaine's alkylamino end, with positively and negatively charged residues as well as the neutral cysteine and alanine. These mutations eliminated use-dependent lidocaine block with no effect on tonic/rested state block. Mutant whole cell currents were kinetically similar to wild type (WT). Single channel conductance (gamma) was reduced from WT in both F1759K (by 38%) and F1759R (by 18%). The negatively charged mutant F1759E increased gamma by 14%, as expected if the charge effect were electrostatic, although F1759D was like WT. None of the charged mutations affected Na(+)/K(+) selectivity. Calculation of difference electrostatic fields in the pore model predicted that lidocaine produced the largest positive electrostatic barrier, followed by lysine and arginine, respectively. Negatively charged glutamate and aspartate both lowered the barrier, with glutamate being more effective. Experimental data were in rank order agreement with the predicted changes in the energy profile. These results demonstrate that permeation rate is sensitive to the inner pore electrostatic field, and they are consistent with creation of an electrostatic barrier to ion permeation by lidocaine's charge.
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Affiliation(s)
- Megan M McNulty
- Cardiac Electrophysiology Laboratory, Department of Medicine, University of Chicago, Chicago, IL 60637, USA
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223
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Soler-Llavina GJ, Chang TH, Swartz KJ. Functional interactions at the interface between voltage-sensing and pore domains in the Shaker K(v) channel. Neuron 2007; 52:623-34. [PMID: 17114047 DOI: 10.1016/j.neuron.2006.10.005] [Citation(s) in RCA: 82] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/27/2006] [Revised: 10/04/2006] [Accepted: 10/09/2006] [Indexed: 01/21/2023]
Abstract
Voltage-activated potassium (K(v)) channels contain a central pore domain that is partially surrounded by four voltage-sensing domains. Recent X-ray structures suggest that the two domains lack extensive protein-protein contacts within presumed transmembrane regions, but whether this is the case for functional channels embedded in lipid membranes remains to be tested. We investigated domain interactions in the Shaker K(v) channel by systematically mutating the pore domain and assessing tolerance by examining channel maturation, S4 gating charge movement, and channel opening. When mapped onto the X-ray structure of the K(v)1.2 channel the large number of permissive mutations support the notion of relatively independent domains, consistent with crystallographic studies. Inspection of the maps also identifies portions of the interface where residues are sensitive to mutation, an external cluster where mutations hinder voltage sensor activation, and an internal cluster where domain interactions between S4 and S5 helices from adjacent subunits appear crucial for the concerted opening transition.
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Affiliation(s)
- Gilberto J Soler-Llavina
- Molecular Physiology and Biophysics Section, Porter Neuroscience Research Center, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892, USA
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224
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Bond PJ, Sansom MSP. Bilayer deformation by the Kv channel voltage sensor domain revealed by self-assembly simulations. Proc Natl Acad Sci U S A 2007; 104:2631-6. [PMID: 17301243 PMCID: PMC1797625 DOI: 10.1073/pnas.0606822104] [Citation(s) in RCA: 89] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022] Open
Abstract
Coarse-grained molecular dynamics simulations are used to explore the interaction with a phospholipid bilayer of the voltage sensor (VS) domain and the S4 helix from the archaebacterial voltage-gated potassium (Kv) channel KvAP. Multiple 2-mus self-assembly simulations reveal that the isolated S4 helix may adopt either interfacial or transmembrane (TM) locations with approximately equal probability. In the TM state, the insertion of the voltage-sensing region of S4 is facilitated via local bilayer deformation that, combined with side chain "snorkeling," enables its Arg side chains to interact with lipid headgroups and water. Multiple 0.2-mus self-assembly simulations of the VS domain are also performed, along with simulations of MscL and KcsA, to permit comparison with more "canonical" integral membrane protein structures. All three stably adopt a TM orientation within a bilayer. For MscL and KcsA, there is no significant bilayer deformation. In contrast, for the VS, there is considerable local deformation, which is again primarily due to the lipid-exposed S4. It is shown that for both the VS and isolated S4 helix, the positively charged side chains of S4 are accommodated within the membrane through a combination of stabilizing interactions with lipid glycerol and headgroup regions, water, and anionic side chains. Our results support the possibility that bilayer deformation around key gating charge residues in Kv channels may result in "focusing" of the electrostatic field, and indicate that, when considering competing models of voltage-sensing, it is essential to consider the dynamics and structure of not only the protein but also of the local lipid environment.
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Affiliation(s)
- Peter J. Bond
- Department of Biochemistry, University of Oxford, South Parks Road, Oxford 1 3QU, United Kingdom
| | - Mark S. P. Sansom
- Department of Biochemistry, University of Oxford, South Parks Road, Oxford 1 3QU, United Kingdom
- *To whom correspondence should be addressed. E-mail:
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225
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Koval OM, Fan Y, Rothberg BS. A role for the S0 transmembrane segment in voltage-dependent gating of BK channels. ACTA ACUST UNITED AC 2007; 129:209-20. [PMID: 17296928 PMCID: PMC2151615 DOI: 10.1085/jgp.200609662] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/14/2023]
Abstract
BK (Maxi-K) channel activity is allosterically regulated by a Ca2+ sensor, formed primarily by the channel's large cytoplasmic carboxyl tail segment, and a voltage sensor, formed by its transmembrane helices. As with other voltage-gated K channels, voltage sensing in the BK channel is accomplished through interactions of the S1–S4 transmembrane segments with the electric field. However, the BK channel is unique in that it contains an additional amino-terminal transmembrane segment, S0, which is important in the functional interaction between BK channel α and β subunits. In this study, we used perturbation mutagenesis to analyze the role of S0 in channel gating. Single residues in the S0 region of the BK channel were substituted with tryptophan to give a large change in side chain volume; native tryptophans in S0 were substituted with alanine. The effects of the mutations on voltage- and Ca2+-dependent gating were quantified using patch-clamp electrophysiology. Three of the S0 mutants (F25W, L26W, and S29W) showed especially large shifts in their conductance–voltage (G-V) relations along the voltage axis compared to wild type. The G-V shifts for these mutants persisted at nominally 0 Ca2+, suggesting that these effects cannot arise simply from altered Ca2+ sensitivity. The basal open probabilities for these mutants at hyperpolarized voltages (where voltage sensor activation is minimal) were similar to wild type, suggesting that these mutations may primarily perturb voltage sensor function. Further analysis using the dual allosteric model for BK channel gating showed that the major effects of the F25W, L26W, and S29W mutations could be accounted for primarily by decreasing the equilibrium constant for voltage sensor movement. We conclude that S0 may make functional contact with other transmembrane regions of the BK channel to modulate the equilibrium between resting and active states of the channel's voltage sensor.
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Affiliation(s)
- Olga M Koval
- Department of Physiology, University of Texas Health Science Center at San Antonio, TX 78229, USA
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226
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de la Vega RCR, Possani LD. Novel paradigms on scorpion toxins that affects the activating mechanism of sodium channels. Toxicon 2007; 49:171-80. [PMID: 17081580 DOI: 10.1016/j.toxicon.2006.09.016] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Scorpion toxins classified as beta-class are reviewed using a new paradigm. Four distinct sub types are recognized: "classical", "Tsgamma-like", "excitatory" and "depressant"beta-scorpion toxins. Recent experimental data have made possible to identify the interacting interfaces of the Na(+) channel-receptor site 4 with some of these toxins. The voltage-sensor trapping mechanism proposed for the action of these toxic peptides is analyzed in the context of what causes a modification of the activating mechanism of Na(+) channels. A cartoon model is presented with the purpose of summarizing the most current knowledge on the field. Finally, the recent advances on the knowledge of the specific interactions of beta-toxins and different sub types of Na(+) channels are also reviewed.
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Affiliation(s)
- Ricardo C Rodríguez de la Vega
- Department of Molecular Medicine and Bioprocesses, Institute of Biotechnology, National Autonomous University of Mexico, Av. Universidad 2001, Cuernavaca Morelos 62210, México.
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227
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Wang SY, Tikhonov DB, Zhorov BS, Mitchell J, Wang GK. Serine-401 as a batrachotoxin- and local anesthetic-sensing residue in the human cardiac Na+ channel. Pflugers Arch 2007; 454:277-87. [PMID: 17205354 DOI: 10.1007/s00424-006-0202-2] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2006] [Accepted: 12/12/2006] [Indexed: 11/26/2022]
Abstract
Sequence alignment of four S6 segments in the human cardiac Na+ channel suggests that serine-401 (hNav1.5-S401) at D1S6 along with asparagine-927 (N927) at D2S6, serine-1458 (S1458) at D3S6, and phenylalanine-1760 (F1760) at D4S6 may jointly form a pore-facing S(401)N(927)S(1458)F(1760) ring. Importantly, this pore-facing structure is adjacent to the putative gating-hinge (G(400)G(926)G(1457)S(1759)) and close to the selectivity filter. Within this SNSF ring, only S401 has not yet been identified as a batrachotoxin (BTX) sensing residue. We therefore created S401 mutants with 12 substitutions (S401C,W,P,A,K,F,R,E,L,N,D,G) and assayed their BTX sensitivity. All S401 mutants expressed Na+ currents but often with altered gating characteristics. Ten mutants were found sensitive to 5 muM BTX, which eliminated Na+ channel fast inactivation after repetitive pulses. However, S401K and S401R became BTX resistant. In addition, the block of open and inactivated hNav1.5-S401K Na+ channels by local anesthetic bupivacaine was reduced by approximately 8-10-fold, but not the block of resting Na+ channels. Qualitatively, these ligand-sensing phenotypes of hNav1.5-S401K channels resemble those of S1458K and F1760K channels reported earlier. Together, our results support that residue hNav1.5-S401 at D1S6 is facing the inner cavity and is in close proximity to the receptor sites for BTX and for local anesthetics.
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Affiliation(s)
- Sho-Ya Wang
- Department of Biology, State University of New York at Albany, Albany, NY 12222, USA.
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228
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Structures of the Prokaryotic Mechanosensitive Channels MscL and MscS. CURRENT TOPICS IN MEMBRANES 2007. [DOI: 10.1016/s1063-5823(06)58001-9] [Citation(s) in RCA: 154] [Impact Index Per Article: 9.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/04/2022]
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229
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Li L, Mustafi D, Fu Q, Tereshko V, Chen DL, Tice JD, Ismagilov RF. Nanoliter microfluidic hybrid method for simultaneous screening and optimization validated with crystallization of membrane proteins. Proc Natl Acad Sci U S A 2006; 103:19243-8. [PMID: 17159147 PMCID: PMC1748211 DOI: 10.1073/pnas.0607502103] [Citation(s) in RCA: 210] [Impact Index Per Article: 11.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
High-throughput screening and optimization experiments are critical to a number of fields, including chemistry and structural and molecular biology. The separation of these two steps may introduce false negatives and a time delay between initial screening and subsequent optimization. Although a hybrid method combining both steps may address these problems, miniaturization is required to minimize sample consumption. This article reports a "hybrid" droplet-based microfluidic approach that combines the steps of screening and optimization into one simple experiment and uses nanoliter-sized plugs to minimize sample consumption. Many distinct reagents were sequentially introduced as approximately 140-nl plugs into a microfluidic device and combined with a substrate and a diluting buffer. Tests were conducted in approximately 10-nl plugs containing different concentrations of a reagent. Methods were developed to form plugs of controlled concentrations, index concentrations, and incubate thousands of plugs inexpensively and without evaporation. To validate the hybrid method and demonstrate its applicability to challenging problems, crystallization of model membrane proteins and handling of solutions of detergents and viscous precipitants were demonstrated. By using 10 microl of protein solution, approximately 1,300 crystallization trials were set up within 20 min by one researcher. This method was compatible with growth, manipulation, and extraction of high-quality crystals of membrane proteins, demonstrated by obtaining high-resolution diffraction images and solving a crystal structure. This robust method requires inexpensive equipment and supplies, should be especially suitable for use in individual laboratories, and could find applications in a number of areas that require chemical, biochemical, and biological screening and optimization.
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Affiliation(s)
- Liang Li
- *Department of Chemistry and Institute for Biophysical Dynamics and
| | - Debarshi Mustafi
- *Department of Chemistry and Institute for Biophysical Dynamics and
| | - Qiang Fu
- *Department of Chemistry and Institute for Biophysical Dynamics and
| | - Valentina Tereshko
- Department of Biochemistry and Molecular Biology, University of Chicago, 929 East 57th Street, Chicago, IL 60637
| | - Delai L. Chen
- *Department of Chemistry and Institute for Biophysical Dynamics and
| | - Joshua D. Tice
- *Department of Chemistry and Institute for Biophysical Dynamics and
| | - Rustem F. Ismagilov
- *Department of Chemistry and Institute for Biophysical Dynamics and
- To whom correspondence should be addressed. E-mail:
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230
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Pinkett HW, Lee AT, Lum P, Locher KP, Rees DC. An inward-facing conformation of a putative metal-chelate-type ABC transporter. Science 2006; 315:373-7. [PMID: 17158291 DOI: 10.1126/science.1133488] [Citation(s) in RCA: 233] [Impact Index Per Article: 12.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
The crystal structure of a putative metal-chelate-type adenosine triphosphate (ATP)-binding cassette (ABC) transporter encoded by genes HI1470 and HI1471 of Haemophilus influenzae has been solved at 2.4 angstrom resolution. The permeation pathway exhibits an inward-facing conformation, in contrast to the outward-facing state previously observed for the homologous vitamin B12 importer BtuCD. Although the structures of both HI1470/1 and BtuCD have been solved in nucleotide-free states, the pairs of ABC subunits in these two structures differ by a translational shift in the plane of the membrane that coincides with a repositioning of the membrane-spanning subunits. The differences observed between these ABC transporters involve relatively modest rearrangements and may serve as structural models for inward- and outward-facing conformations relevant to the alternating access mechanism of substrate translocation.
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Affiliation(s)
- H W Pinkett
- Division of Chemistry and Chemical Engineering, Howard Hughes Medical Institute, MC 114-96, California Institute of Technology (Caltech), Pasadena, CA 91125, USA
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231
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Schmidt D, Jiang QX, MacKinnon R. Phospholipids and the origin of cationic gating charges in voltage sensors. Nature 2006; 444:775-9. [PMID: 17136096 DOI: 10.1038/nature05416] [Citation(s) in RCA: 306] [Impact Index Per Article: 17.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/05/2006] [Accepted: 11/03/2006] [Indexed: 11/08/2022]
Abstract
Cells communicate with their external environment through physical and chemical processes that take place in the cell-surrounding membrane. The membrane serves as a barrier as well as a special environment in which membrane proteins are able to carry out important processes. Certain membrane proteins have the ability to detect the membrane voltage and regulate ion conduction or enzyme activity. Such voltage-dependent processes rely on the action of protein domains known as voltage sensors, which are embedded inside the cell membrane and contain an excess of positively charged amino acids, which react to an electric field. How does the membrane create an environment suitable for voltage sensors? Here we show under a variety of conditions that the function of a voltage-dependent K+ channel is dependent on the negatively charged phosphodiester of phospholipid molecules. A non-voltage-dependent K+ channel does not exhibit the same dependence. The data lead us to propose that the phospholipid membrane, by providing stabilizing interactions between positively charged voltage-sensor arginine residues and negatively charged lipid phosphodiester groups, provides an appropriate environment for the energetic stability and operation of the voltage-sensing machinery. We suggest that the usage of arginine residues in voltage sensors is an adaptation to the phospholipid composition of cell membranes.
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Affiliation(s)
- Daniel Schmidt
- Howard Hughes Medical Institute, Laboratory of Molecular Neurobiology and Biophysics, Rockefeller University, 1230 York Avenue, New York, New York 10021, USA
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232
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Darman RB, Ivy AA, Ketty V, Blaustein RO. Constraints on voltage sensor movement in the shaker K+ channel. ACTA ACUST UNITED AC 2006; 128:687-99. [PMID: 17101817 PMCID: PMC2151604 DOI: 10.1085/jgp.200609624] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
In nerve and muscle cells, the voltage-gated opening and closing of cation-selective ion channels is accompanied by the translocation of 12-14 elementary charges across the membrane's electric field. Although most of these charges are carried by residues in the S4 helix of the gating module of these channels, the precise nature of their physical movement is currently the topic of spirited debate. Broadly speaking, two classes of models have emerged: those that suggest that small-scale motions can account for the extensive charge displacement, and those that invoke a much larger physical movement. In the most recent incarnation of the latter type of model, which is based on structural and functional data from the archaebacterial K(+) channel KvAP, a "voltage-sensor paddle" comprising a helix-turn-helix of S3-S4 translocates approximately 20 A through the bilayer during the gating cycle (Jiang, Y., A. Lee, J. Chen, V. Ruta, M. Cadene, B.T. Chait, and R. MacKinnon. 2003. Nature. 423:33-41; Jiang, Y., V. Ruta, J. Chen, A. Lee, and R. MacKinnon. 2003. Nature. 423:42-48.; Ruta, V., J. Chen, and R. MacKinnon. 2005. Cell. 123:463-475). We used two methods to test for analogous motions in the Shaker K(+) channel, each examining the aqueous exposure of residues near S3. In the first, we employed a pore-blocking maleimide reagent (Blaustein, R.O., P.A. Cole, C. Williams, and C. Miller. 2000. Nat. Struct. Biol. 7:309-311) to probe for state-dependent changes in the chemical reactivity of substituted cysteines; in the second, we tested the state-dependent accessibility of a tethered biotin to external streptavidin (Qiu, X.Q., K.S. Jakes, A. Finkelstein, and S.L. Slatin. 1994. J. Biol. Chem. 269:7483-7488; Slatin, S.L., X.Q. Qiu, K.S. Jakes, and A. Finkelstein. 1994. Nature. 371:158-161). In both types of experiments, residues predicted to lie near the top of S3 did not exhibit any change in aqueous exposure during the gating cycle. This lack of state dependence argues against large-scale movements, either axially or radially, of Shaker's S3-S4 voltage-sensor paddle.
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Affiliation(s)
- Rachel B Darman
- Molecular Cardiology Research Institute, Tufts-New England Medical Center, Boston, MA 02111, USA
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233
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Abstract
Neurons transmit information through electrical signals generated by voltage-gated ion channels. These channels consist of a large superfamily of proteins that form channels selective for potassium, sodium, or calcium ions. In this review we focus on the molecular mechanisms by which these channels convert changes in membrane voltage into the opening and closing of "gates" that turn ion conductance on and off. An explosion of new studies in the last year, including the first X-ray crystal structure of a mammalian voltage-gated potassium channel, has led to radically different interpretations of the structure and molecular motion of the voltage sensor. The interpretations are as distinct as the techniques employed for the studies: crystallography, fluorescence, accessibility analysis, and electrophysiology. We discuss the likely causes of the discrepant results in an attempt to identify the missing information that will help resolve the controversy and reveal the mechanism by which a voltage sensor controls the channel's gates.
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Affiliation(s)
- Francesco Tombola
- Department of Molecular and Cell Biology, University of California, Berkeley, California 94720, USA.
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234
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Richardson J, Blunck R, Ge P, Selvin PR, Bezanilla F, Papazian DM, Correa AM. Distance measurements reveal a common topology of prokaryotic voltage-gated ion channels in the lipid bilayer. Proc Natl Acad Sci U S A 2006; 103:15865-70. [PMID: 17043236 PMCID: PMC1635094 DOI: 10.1073/pnas.0607532103] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Voltage-dependent ion channels are fundamental to the physiology of excitable cells because they underlie the generation and propagation of the action potential and excitation-contraction coupling. To understand how ion channels work, it is important to determine their structures in different conformations in a membrane environment. The validity of the crystal structure for the prokaryotic K(+) channel, K(V)AP, has been questioned based on discrepancies with biophysical data from functional eukaryotic channels, underlining the need for independent structural data under native conditions. We investigated the structural organization of two prokaryotic voltage-gated channels, NaChBac and K(V)AP, in liposomes by using luminescence resonance energy transfer. We describe here a transmembrane packing representation of the voltage sensor and pore domains of the prokaryotic Na channel, NaChBac. We find that NaChBac and K(V)AP share a common arrangement in which the structures of the Na and K selective pores and voltage-sensor domains are conserved. The packing arrangement of the voltage-sensing region as determined by luminescence resonance energy transfer differs significantly from that of the K(V)AP crystal structure, but resembles that of the eukaryotic K(V)1.2 crystal structure. However, the voltage-sensor domain in prokaryotic channels is closer to the pore domain than in the K(V)1.2 structure. Our results indicate that prokaryotic and eukaryotic channels that share similar functional properties have similar helix arrangements, with differences arising likely from the later introduction of additional structural elements.
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Affiliation(s)
- Jessica Richardson
- *Physiology, David Geffen School of Medicine, University of California, Los Angeles, CA 90095; and
| | - Rikard Blunck
- *Physiology, David Geffen School of Medicine, University of California, Los Angeles, CA 90095; and
| | - Pinghua Ge
- Department of Physics and Biophysics Center, University of Illinois at Urbana-Champaign, Urbana, IL 61801
| | - Paul R. Selvin
- Department of Physics and Biophysics Center, University of Illinois at Urbana-Champaign, Urbana, IL 61801
| | - Francisco Bezanilla
- Departments of Anesthesiology and
- *Physiology, David Geffen School of Medicine, University of California, Los Angeles, CA 90095; and
- To whom correspondence may be sent at the present address:
Institute for Molecular Pediatric Sciences, University of Chicago, Chicago, IL 60637 E-mail:
or
| | - Diane M. Papazian
- *Physiology, David Geffen School of Medicine, University of California, Los Angeles, CA 90095; and
| | - Ana M. Correa
- Departments of Anesthesiology and
- To whom correspondence may be sent at the present address:
Institute for Molecular Pediatric Sciences, University of Chicago, Chicago, IL 60637 E-mail:
or
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235
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Abstract
Voltage-activated ion channels open and close in response to changes in membrane voltage, a process that is crucial for electrical signaling in the nervous system. The venom from many poisonous creatures contains a diverse array of small protein toxins that bind to voltage-activated channels and modify the gating mechanism. Hanatoxin and a growing number of related tarantula toxins have been shown to inhibit activation of voltage-activated potassium (K(v)) channels by interacting with their voltage-sensing domains. This review summarizes our current understanding of the mechanism by which these toxins alter gating, the location of the toxin receptor within K(v) channels and the disposition of this receptor with respect to the lipid membrane. The conservation of tarantula toxin receptors among voltage-activated ion channels will also be discussed.
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Affiliation(s)
- Kenton J Swartz
- Molecular Physiology and Biophysics Section, Porter Neuroscience Research Center, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892, USA.
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236
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Abstract
Voltage-sensor (VS) domains cause the pore of voltage-gated ion channels to open and close in response to changes in transmembrane potential. Recent experimental studies suggest that VS domains are independent structural units. This independence is revealed dramatically by a voltage-dependent proton-selective channel (Hv), which has a sequence homologous to the VS domains of voltage-gated potassium channels (Kv). Here we show by means of molecular dynamics simulations that the isolated open-state VS domain of the KvAP channel in a lipid membrane has a configuration consistent with a water channel, which we propose as a common feature underlying the conductance of protons, and perhaps other cations, through VS domains.
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Affiliation(s)
- J Alfredo Freites
- Institute for Genomics and Bioinformatics, University of California, Irvine, CA 92697, USA
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237
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Abstract
Site-3 toxins are small polypeptide venoms from scorpions, sea anemones, and spiders that bind with a high specificity to the extracellular surface of voltage-gated Na channels. After binding to a site near the S4 segment in domain IV the toxin causes disruption of the normal fast inactivation transition resulting in a marked prolongation of the action potentials of excitable tissues including those of cardiac and skeletal muscle and nerve. In this review we discuss the specific binding interactions between residues of the toxin and those of the Na channel, and the specific modification of Na channel kinetic behavior leading to a change in fast inactivation focusing on interactions deduced primarily from the study of sea anemone toxins and the cardiac Na channel (Na(V)1.5). We also illustrate the usefulness of site-3 toxins in the study of altered Na channel behavior by drug-modification.
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Affiliation(s)
- Dorothy A Hanck
- Department of Medicine, The University of Chicago, Chicago, IL 60637, USA.
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238
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Fleishman SJ, Ben-Tal N. Progress in structure prediction of α-helical membrane proteins. Curr Opin Struct Biol 2006; 16:496-504. [PMID: 16822664 DOI: 10.1016/j.sbi.2006.06.003] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/23/2006] [Revised: 06/05/2006] [Accepted: 06/16/2006] [Indexed: 10/24/2022]
Abstract
Transmembrane (TM) proteins comprise 20-30% of the genome but, because of experimental difficulties, they represent less than 1% of the Protein Data Bank. The dearth of membrane protein structures makes computational prediction a potentially important means of obtaining novel structures. Recent advances in computational methods have been combined with experimental data to constrain the modeling of three-dimensional structures. Furthermore, threading and ab initio modeling approaches that were effective for soluble proteins have been applied to TM domains. Surprisingly, experimental structures, proteomic analyses and bioinformatics have revealed unexpected architectures that counter long-held views on TM protein structure and stability. Future computational and experimental studies aimed at understanding the thermodynamic and evolutionary bases of these architectural details will greatly enhance predictive capabilities.
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Affiliation(s)
- Sarel J Fleishman
- Department of Biochemistry, George S. Wise Faculty of Life Sciences, Tel-Aviv University Ramat Aviv 69978, Israel
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239
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Tate CG. Comparison of three structures of the multidrug transporter EmrE. Curr Opin Struct Biol 2006; 16:457-64. [PMID: 16828280 DOI: 10.1016/j.sbi.2006.06.005] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2006] [Revised: 05/15/2006] [Accepted: 06/22/2006] [Indexed: 11/30/2022]
Abstract
The small multidrug resistance proteins constitute a family of bacterial antiporters that confer multidrug resistance by H(+)-linked drug efflux across the bacterial cytoplasmic membrane. The structure of EmrE, the family archetype, has been determined by electron crystallography and shows that EmrE in the membrane is an asymmetric homodimer composed of a tightly packed bundle of eight alpha helices, six of which form the substrate-binding site, which has a single molecule of tetraphenylphosphonium at its centre. Two X-ray structures of EmrE have been determined; the first structure was of a non-native conformation of EmrE that formed a crystallographic tetramer, whereas EmrE in the second structure was an asymmetric dimer containing a single molecule of bound tetraphenylphosphonium. This recent EmrE structure bears a superficial resemblance to the electron crystallographic structure and the differences were ascribed to conformational changes. However, the biological relevance of these conformational differences is questionable.
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240
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Oz M. Receptor-independent actions of cannabinoids on cell membranes: Focus on endocannabinoids. Pharmacol Ther 2006; 111:114-44. [PMID: 16584786 DOI: 10.1016/j.pharmthera.2005.09.009] [Citation(s) in RCA: 132] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2005] [Accepted: 09/30/2005] [Indexed: 01/08/2023]
Abstract
Cannabinoids are a structurally diverse group of mostly lipophilic molecules that bind to cannabinoid receptors. In fact, endogenous cannabinoids (endocannabinoids) are a class of signaling lipids consisting of amides and esters of long-chain polyunsaturated fatty acids. They are synthesized from lipid precursors in plasma membranes via Ca(2+) or G-protein-dependent processes and exhibit cannabinoid-like actions by binding to cannabinoid receptors. However, endocannabinoids can produce effects that are not mediated by these receptors. In pharmacologically relevant concentrations, endocannabinoids modulate the functional properties of voltage-gated ion channels including Ca(2+) channels, Na(+) channels, various types of K(+) channels, and ligand-gated ion channels such as serotonin type 3, nicotinic acetylcholine, and glycine receptors. In addition, modulatory effects of endocannabinoids on other ion-transporting membrane proteins such as transient potential receptor-class channels, gap junctions and transporters for neurotransmitters have also been demonstrated. Furthermore, functional properties of G-protein-coupled receptors for different types of neurotransmitters and neuropeptides are altered by direct actions of endocannabinoids. Although the mechanisms of these effects are currently not clear, it is likely that these direct actions of endocannabinoids are due to their lipophilic structures. These findings indicate that additional molecular targets for endocannabinoids exist and that these targets may represent novel sites for cannabinoids to alter either the excitability of the neurons or the response of the neuronal systems. This review focuses on the results of recent studies indicating that beyond their receptor-mediated effects, endocannabinoids alter the functions of ion channels and other integral membrane proteins directly.
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Affiliation(s)
- Murat Oz
- National Institute on Drug Abuse, NIH/DHHS, Intramural Research Program, Cellular Neurobiology Branch, 5500 Nathan Shock Drive, Baltimore MD, 21224, USA.
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241
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Cohen L, Gilles N, Karbat I, Ilan N, Gordon D, Gurevitz M. Direct evidence that receptor site-4 of sodium channel gating modifiers is not dipped in the phospholipid bilayer of neuronal membranes. J Biol Chem 2006; 281:20673-20679. [PMID: 16720570 DOI: 10.1074/jbc.m603212200] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
In a recent note to Nature, R. MacKinnon has raised the possibility that potassium channel gating modifiers are able to partition in the phospholipid bilayer of neuronal membranes and that by increasing their partial concentration adjacent to their receptor, they affect channel function with apparent high affinity (Lee and MacKinnon (2004) Nature 430, 232-235). This suggestion was adopted by Smith et al. (Smith, J. J., Alphy, S., Seibert, A. L., and Blumenthal, K. M. (2005) J. Biol. Chem. 280, 11127-11133), who analyzed the partitioning of sodium channel modifiers in liposomes. They found that certain modifiers were able to partition in these artificial membranes, and on this basis, they have extrapolated that scorpion beta-toxins interact with their channel receptor in a similar mechanism as that proposed by MacKinnon. Since this hypothesis has actually raised a new conception, we examined it in binding assays using a number of pharmacologically distinct scorpion beta-toxins and insect and mammalian neuronal membrane preparations, as well as by analyzing the rate by which the toxin effect on gating of Drosophila DmNa(v)1 and rat brain rNa(v)1.2a develops. We show that in general, scorpion beta-toxins do not partition in neuronal membranes and that in the case in which a depressant beta-toxin partitions in insect neuronal membranes, this partitioning is unrelated to its interaction with the receptor site and the effect on the gating properties of the sodium channel. These results negate the hypothesis that the high affinity of beta-toxins for sodium channels is gained by their ability to partition in the phospholipid bilayer and clearly indicate that the receptor site for scorpion beta-toxins is accessible to the extracellular solvent.
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Affiliation(s)
- Lior Cohen
- Department of Plant Sciences, George S. Wise Faculty of Life Sciences, Tel-Aviv University, Ramat-Aviv 69978, Tel-Aviv, Israel
| | - Nicolas Gilles
- Commissariat à l'Energie Atomique, Department d'Ingenierie et d'Etudes des Proteines, C.E. Saclay, F-91191 Gif Sur Yvette Cedex, France
| | - Izhar Karbat
- Department of Plant Sciences, George S. Wise Faculty of Life Sciences, Tel-Aviv University, Ramat-Aviv 69978, Tel-Aviv, Israel
| | - Nitza Ilan
- Department of Plant Sciences, George S. Wise Faculty of Life Sciences, Tel-Aviv University, Ramat-Aviv 69978, Tel-Aviv, Israel
| | - Dalia Gordon
- Department of Plant Sciences, George S. Wise Faculty of Life Sciences, Tel-Aviv University, Ramat-Aviv 69978, Tel-Aviv, Israel.
| | - Michael Gurevitz
- Department of Plant Sciences, George S. Wise Faculty of Life Sciences, Tel-Aviv University, Ramat-Aviv 69978, Tel-Aviv, Israel.
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242
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Yarov-Yarovoy V, Baker D, Catterall WA. Voltage sensor conformations in the open and closed states in ROSETTA structural models of K(+) channels. Proc Natl Acad Sci U S A 2006; 103:7292-7. [PMID: 16648251 PMCID: PMC1464335 DOI: 10.1073/pnas.0602350103] [Citation(s) in RCA: 194] [Impact Index Per Article: 10.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Voltage-gated ion channels control generation and propagation of action potentials in excitable cells. Significant progress has been made in understanding structure and function of the voltage-gated ion channels, highlighted by the high-resolution open-state structure of the voltage-gated potassium channel, K(v)1.2. However, because the structure of the closed state is unknown, the gating mechanism remains controversial. We adapted the rosetta membrane method to model the structures of the K(v)1.2 and KvAP channels using homology, de novo, and domain assembly methods and selected the most plausible models using a limited number of experimental constraints. Our model of K(v)1.2 in the open state is very similar in overall topology to the x-ray structure of this channel. Modeling of KvAP in the open state suggests that orientation of the voltage-sensing domain relative to the pore-forming domain is considerably different from the orientation in the K(v)1.2 open state and that the magnitude of the vertical movement of S4 is significantly greater. Structural modeling of closed state of K(v)1.2 suggests gating movement that can be viewed as a sum of two previously suggested mechanisms: translation (2-4 A) plus rotation ( approximately 180 degrees ) of the S4 segment as proposed in the original "sliding helix" or "helical screw" models coupled with a rolling motion of the S1-S3 segments around S4, similar to recent "transporter" models of gating. We propose a unified mechanism of voltage-dependent gating for K(v)1.2 and KvAP in which this major conformational change moves the gating charge across the electric field in an analogous way for both channels.
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Affiliation(s)
| | - David Baker
- Biochemistry and
- Howard Hughes Medical Institute, University of Washington, Seattle, WA 98195
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243
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Ruta V, Chen J, MacKinnon R. Calibrated Measurement of Gating-Charge Arginine Displacement in the KvAP Voltage-Dependent K+ Channel. Cell 2005; 123:463-75. [PMID: 16269337 DOI: 10.1016/j.cell.2005.08.041] [Citation(s) in RCA: 177] [Impact Index Per Article: 9.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/18/2005] [Revised: 08/24/2005] [Accepted: 08/31/2005] [Indexed: 11/25/2022]
Abstract
Voltage-dependent ion channels open and conduct ions in response to changes in cell-membrane voltage. The voltage sensitivity of these channels arises from the motion of charged arginine residues located on the S4 helices of the channel's voltage sensors. In KvAP, a prokaryotic voltage-dependent K+ channel, the S4 helix forms part of a helical hairpin structure, the voltage-sensor paddle. We have measured the membrane depth of residues throughout the KvAP channel using avidin accessibility to different-length tethered biotin reagents. From these measurements, we have calibrated the tether lengths and derived the thickness of the membrane that forms a barrier to avidin penetration, allowing us to determine the magnitude of displacement of the voltage-sensor paddles during channel gating. Here we show that the voltage-sensor paddles are highly mobile compared to other regions of the channel and transfer the gating-charge arginines 15-20 A through the membrane to open the pore.
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Affiliation(s)
- Vanessa Ruta
- Howard Hughes Medical Institute, Laboratory of Molecular Neurobiology and Biophysics, New York 10021, USA
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