101
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Nishizawa M, Nishizawa K. Coupling of S4 helix translocation and S6 gating analyzed by molecular-dynamics simulations of mutated Kv channels. Biophys J 2009; 97:90-100. [PMID: 19580747 DOI: 10.1016/j.bpj.2009.02.074] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2008] [Revised: 02/05/2009] [Accepted: 02/06/2009] [Indexed: 11/24/2022] Open
Abstract
The recently determined crystal structure of a chimeric Kv1.2-Kv2.1 Kv channel at 2.4 A resolution motivated this molecular-dynamics simulation study of the chimeric channel and its mutants embedded in a DPPC membrane. For the channel protein, we used two types of C-terminus: E+ and Eo. E+ contains, and Eo lacks, the EGEE residue quartet located distal to the S6 helix. For both E+ and Eo, the following trend was observed: When S4 helices were restrained at the same position as in the x-ray structure (S4high), the S6 gate remained open for 12 ns. The results were similar when the S4 helices were pulled downward 7 A (S4low). However, S4middle (or S4low) facilitated the S6 gate-narrowing for the following mutated channels (shown in order of increasing effect): 1), E395W; 2), E395W-F401A-F402A; and 3), E395W-F401A-F402A-V478W. The amino acid numbering system is that used for the Shaker channel. Even though all four subunits were set at S4low, S6 gate-narrowing was often brought about by movements of only two opposing S6 helices toward the central axis of the pore, resulting in a twofold symmetry-like structure. A free-energy profile analysis over the ion conduction pathway shows that the two opposing S6 helices whose peptide backbones are approximately 10.4 A distant from each other lead to an energetic barrier of approximately 25 kJ/mol. S6 movement was coupled with translocation of the S4-S5 linker toward the central axis of the same subunit, and the coupling was mediated by salt bridges formed between the inner (intracellular side) end of S4 and that of S6. Simulations in which S4 of only one subunit was pulled down to S4low showed that a weak intersubunit coordination is present for S5 movement, whereas the coupling between the S4-S5 linker and S6 is largely an intrasubunit one. In general, whereas subunit-based behavior appears to be dominant and to permit heteromeric conformations of the pore domain, direct intersubunit coupling of S5 or S6 is weak. Therefore, the "concerted transition" of the pore domain that has been predicted based on electrophysiological analyses is likely to be mediated mainly by the dual effects of S4 and the S4-S5 linker; these segments of one subunit can interact with both S5 of the same subunit and that of the adjacent subunit.
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Affiliation(s)
- Manami Nishizawa
- Department of Laboratory Medicine, Teikyo University School of Medical Technology, Tokyo 173-8605, Japan
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102
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Interactions between lipids and voltage sensor paddles detected with tarantula toxins. Nat Struct Mol Biol 2009; 16:1080-5. [PMID: 19783984 PMCID: PMC2782670 DOI: 10.1038/nsmb.1679] [Citation(s) in RCA: 124] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/20/2009] [Accepted: 08/12/2009] [Indexed: 02/06/2023]
Abstract
Voltage-activated ion channels open and close in response to changes in voltage, a property that is essential for generating nerve impulses. Studies on voltage-activated potassium (Kv) channels show that voltage-sensor activation is sensitive to the composition of lipids in the surrounding membrane. Here we explore the interaction of lipids with S1-S4 voltage-sensing domains and find that the conversion of the membrane lipid sphingomyelin to ceramide-1-phosphate alters voltage-sensor activation in an S1-S4 voltage-sensing protein lacking an associated pore domain, and that the S3b-S4 paddle motif determines the effects of lipid modification on Kv channels. Using tarantula toxins that bind to paddle motifs within the membrane, we identify mutations in the paddle motif that weaken toxin binding by disrupting lipid-paddle interactions. Our results suggest that lipids bind to voltage-sensing domains and demonstrate that the pharmacological sensitivities of voltage-activated ion channels are influenced by the surrounding lipid membrane.
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103
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Koag MC, Papazian DM. Voltage-dependent conformational changes of KVAP S4 segment in bacterial membrane environment. Channels (Austin) 2009; 3:356-65. [PMID: 19713752 DOI: 10.4161/chan.3.5.9697] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
Abstract
The nature and magnitude of voltage sensor conformational changes during ion channel activation are controversial. We have analyzed the topology of the K(V)AP voltage sensor domain in the absence and presence of a hyperpolarized voltage using native, right-side out membrane vesicles from E. coli. This approach does not disrupt the normal membrane environment of the channel protein and does not involve detergent solubilization. We found that voltage-dependent conformational changes are focused in the N-terminal half of the K(V)AP S4 segment, in excellent agreement with results obtained with Shaker. Homologous residues in the K(V)AP and Shaker S4 segments are transferred from the extracellular to the intracellular compartment upon hyperpolarization. Taken together with X-ray structures indicating that the K(V)AP S4 segment is outwardly displaced at 0 mV compared to S4 in a mammalian Shaker channel, our results are consistent with the idea that S4 moves further during voltage-dependent activation in K(V)AP than in Shaker.
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Affiliation(s)
- Myong-Chul Koag
- Department of Physiology and Molecular Biology Institute, David Geffen School of Medicine, University of California at Los Angeles, Los Angeles, CA, USA
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104
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Deng M, Kuang F, Sun Z, Tao H, Cai T, Zhong L, Chen Z, Xiao Y, Liang S. Jingzhaotoxin-IX, a novel gating modifier of both sodium and potassium channels from Chinese tarantula Chilobrachys jingzhao. Neuropharmacology 2009; 57:77-87. [DOI: 10.1016/j.neuropharm.2009.04.009] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/23/2008] [Revised: 04/19/2009] [Accepted: 04/20/2009] [Indexed: 12/19/2022]
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105
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Jarecki BW, Sheets PL, Xiao Y, Jackson JO, Cummins TR. Alternative splicing of Na(V)1.7 exon 5 increases the impact of the painful PEPD mutant channel I1461T. Channels (Austin) 2009; 3:259-67. [PMID: 19633428 DOI: 10.4161/chan.3.4.9341] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
Alternative splicing is known to alter pharmacological sensitivities, kinetics, channel distribution under pathological conditions, and developmental regulation of VGSCs. Mutations that alter channel properties in Na(V)1.7 have been genetically implicated in patients with bouts of extreme pain classified as inherited erythromelalgia (IEM) or paroxysmal extreme pain disorder (PEPD). Furthermore, patients with IEM or PEPD report differential age onsets. A recent study reported that alternative splicing of Na(V)1.7 exon 5 affects ramp current properties. Since IEM and PEPD mutations also alter Na(V)1.7 ramp current properties we speculated that alternative splicing might impact the functional consequences of IEM or PEPD mutations. We compared the effects alternative splicing has on the biophysical properties of Na(V)1.7 wild-type, IEM (I136V) and PEPD (I1461T) channels. Our major findings demonstrate that although the 5A splice variant of the IEM channel had no functional impact, the 5A splice variant of the PEPD channel significantly hyperpolarized the activation curve, slowed deactivation and closed-state inactivation, shifted the ramp current activation to more hyperpolarized potentials, and increased ramp current amplitude. We hypothesize a D1/S3-S4 charged residue difference between the 5N (Asn) and the 5A (Asp) variants within the coding region of exon 5 may contribute to shifts in channel activation and deactivation. Taken together, the additive effects observed on ramp currents from exon 5 splicing and the PEPD mutation (I1461T) are likely to impact the disease phenotype and may offer insight into how alternative splicing may affect specific intramolecular interactions critical for voltage-dependent gating.
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Affiliation(s)
- Brian W Jarecki
- Department of Pharmacology and Toxicology, Stark Neurosciences Research Institute, Indiana University School of Medicine, Indianapolis, IN, USA
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106
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Unnerståle S, Lind J, Papadopoulos E, Mäler L. Solution structure of the HsapBK K+ channel voltage-sensor paddle sequence. Biochemistry 2009; 48:5813-21. [PMID: 19456106 DOI: 10.1021/bi9004599] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
Voltage-gated potassium channels open and close in response to changes in the membrane potential. In this study, we have determined the NMR solution structure of the putative S3b-S4 voltage-sensor paddle fragment, the part that moves to mediate voltage gating, of the HsapBK potassium channel in dodecylphosphocholine (DPC) micelles. This paper presents the first structure of the S3b-S4 fragment from a BK channel. Diffusion coefficients as determined from PFG NMR experiments showed that a well-defined complex between the peptide and DPC molecules was formed. The structure reveals a helix-turn-helix motif, which is in agreement with crystal structures of other voltage-gated potassium channels, thus indicating that it is feasible to study the isolated fragment. The paddle motifs generally contain several basic residues, implicated in the gating. The critical Arg residues in this structure all reside on the surface, which is in agreement with crystal structures of K(v) channels. Similarities in the structure of the S3b-S4 fragment in BK and K(v) channels as well as important differences are seen, which may be important for explaining the details in paddle movement within a bilayer.
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Affiliation(s)
- Sofia Unnerståle
- Department of Biochemistry and Biophysics, Center for Biomembrane Research, The Arrhenius Laboratories for Natural Sciences, Stockholm University, SE-106 91 Stockholm, Sweden
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107
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Bell DC, Turbendian HK, Valley MT, Zhou L, Riley JH, Siegelbaum SA, Tibbs GR. Probing S4 and S5 segment proximity in mammalian hyperpolarization-activated HCN channels by disulfide bridging and Cd2+ coordination. Pflugers Arch 2009; 458:259-72. [PMID: 19034494 PMCID: PMC2748781 DOI: 10.1007/s00424-008-0613-3] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/06/2008] [Accepted: 10/30/2008] [Indexed: 11/26/2022]
Abstract
We explored the structural basis of voltage sensing in the HCN1 hyperpolarization-activated cyclic nucleotide-gated cation channel by examining the relative orientation of the voltage sensor and pore domains. The opening of channels engineered to contain single cysteine residues at the extracellular ends of the voltage-sensing S4 (V246C) and pore-forming S5 (C303) domains is inhibited by formation of disulfide or cysteine:Cd(2+) bonds. As Cd(2+) coordination is promoted by depolarization, the S4-S5 interaction occurs preferentially in the closed state. The failure of oxidation to catalyze dimer formation, as assayed by Western blotting, indicates the V246C:C303 interaction occurs within a subunit. Intriguingly, a similar interaction has been observed in depolarization-activated Shaker voltage-dependent potassium (Kv) channels at depolarized potentials but such an intrasubunit interaction is inconsistent with the X-ray crystal structure of Kv1.2, wherein S4 approaches S5 of an adjacent subunit. These findings suggest channels of opposite voltage-sensing polarity adopt a conserved S4-S5 orientation in the depolarized state that is distinct from that trapped upon crystallization.
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Affiliation(s)
- Damian C. Bell
- Department of Neuroscience, College of Surgeons & Physicians, Columbia University New York, NY 10032, U.S.A
| | - Harma K. Turbendian
- Department of Anesthesiology, College of Surgeons & Physicians, Columbia University New York, NY 10032, U.S.A
| | - Matthew T. Valley
- Department of Neuroscience, College of Surgeons & Physicians, Columbia University New York, NY 10032, U.S.A
| | - Lei Zhou
- Department of Neuroscience, College of Surgeons & Physicians, Columbia University New York, NY 10032, U.S.A
| | - John H. Riley
- Department of Neuroscience, College of Surgeons & Physicians, Columbia University New York, NY 10032, U.S.A
| | - Steven A. Siegelbaum
- Department of Neuroscience, College of Surgeons & Physicians, Columbia University New York, NY 10032, U.S.A
- Department of Pharmacology, College of Surgeons & Physicians, Columbia University New York, NY 10032, U.S.A
- The Howard Hughes Medical Institute, College of Surgeons & Physicians, Columbia University New York, NY 10032, U.S.A
| | - Gareth R. Tibbs
- Department of Anesthesiology, College of Surgeons & Physicians, Columbia University New York, NY 10032, U.S.A
- Department of Pharmacology, College of Surgeons & Physicians, Columbia University New York, NY 10032, U.S.A
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108
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Lee SY, Banerjee A, MacKinnon R. Two separate interfaces between the voltage sensor and pore are required for the function of voltage-dependent K(+) channels. PLoS Biol 2009; 7:e47. [PMID: 19260762 PMCID: PMC2650729 DOI: 10.1371/journal.pbio.1000047] [Citation(s) in RCA: 122] [Impact Index Per Article: 8.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2008] [Accepted: 01/13/2009] [Indexed: 01/17/2023] Open
Abstract
Voltage-dependent K+ (Kv) channels gate open in response to the membrane voltage. To further our understanding of how cell membrane voltage regulates the opening of a Kv channel, we have studied the protein interfaces that attach the voltage-sensor domains to the pore. In the crystal structure, three physical interfaces exist. Only two of these consist of amino acids that are co-evolved across the interface between voltage sensor and pore according to statistical coupling analysis of 360 Kv channel sequences. A first co-evolved interface is formed by the S4-S5 linkers (one from each of four voltage sensors), which form a cuff surrounding the S6-lined pore opening at the intracellular surface. The crystal structure and published mutational studies support the hypothesis that the S4-S5 linkers convert voltage-sensor motions directly into gate opening and closing. A second co-evolved interface forms a small contact surface between S1 of the voltage sensor and the pore helix near the extracellular surface. We demonstrate through mutagenesis that this interface is necessary for the function and/or structure of two different Kv channels. This second interface is well positioned to act as a second anchor point between the voltage sensor and the pore, thus allowing efficient transmission of conformational changes to the pore's gate. Voltage-dependent ion channels open with a voltage dependence that is remarkably steep. This steep voltage dependence, which is essential to the propagation of nerve impulses, originates in the interaction between voltage-sensor domains of the ion channel and its pore. The voltage-sensor domains transmit voltage-driven conformational changes to the pore. To understand how this “electromechanical coupling” mechanism works, we have studied the protein–protein interfaces that connect the voltage sensors to the pore using bioinformatics, electrophysiological recordings, site-directed mutagenesis, and chemical cross-linking. We identify two functionally important interfaces: one links the mobile “voltage-sensor paddle” to the pore's gate near the intracellular membrane surface, while the other links an immobile region of the voltage sensor to the pore near the extracellular membrane surface. The two interfaces encompass only a small fraction of the voltage-sensor surface area, but appear to operate in unison to enable voltage-driven conformational changes within the voltage sensor so as to efficiently regulate the pore's gate. Voltage-sensor domains contact the pore in potassium channels through two surfaces: one attaches the voltage sensor to the pore, while the other imparts force on the gate, in a process that is central to the generation of action potentials in nerve cells.
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Affiliation(s)
- Seok-Yong Lee
- Howard Hughes Medical Institute, Rockefeller University, New York, New York, United States of America
- Laboratory of Molecular Neurobiology and Biophysics, Rockefeller University, New York, New York, United States of America
| | - Anirban Banerjee
- Howard Hughes Medical Institute, Rockefeller University, New York, New York, United States of America
- Laboratory of Molecular Neurobiology and Biophysics, Rockefeller University, New York, New York, United States of America
| | - Roderick MacKinnon
- Howard Hughes Medical Institute, Rockefeller University, New York, New York, United States of America
- Laboratory of Molecular Neurobiology and Biophysics, Rockefeller University, New York, New York, United States of America
- * To whom correspondence should be addressed. E-mail:
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109
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Electromechanical coupling in the membranes of Shaker-transfected HEK cells. Proc Natl Acad Sci U S A 2009; 106:6626-31. [PMID: 19366664 DOI: 10.1073/pnas.0808045106] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023] Open
Abstract
Membranes flex with changes in transmembrane potential as a result of changes in interfacial tension, the Lippman effect. We studied the membrane electromotility of Shaker K(+)-transfected HEK-293 cells in real time by using combined patch-clamp atomic force microscopy. In the voltage range where the channels were closed, Shaker expression had little effect on electromotility relative to wild-type cells. Depolarization between -120 and -40 mV resulted in a linear upward cantilever deflection equivalent to an increase in membrane tension. However, when depolarized sufficiently for channel opening, the electromotility saturated and only recovered over 10 s of milliseconds. This remarkable loss of motility was associated with channel opening, not ionic flux or movement of the voltage sensors. The IL mutant of Shaker, in which the voltage dependence of channel opening but not sensor movement is shifted to more positive potentials, caused the loss of electromotility saturation also to shift to more positive potentials. The temporary loss of electromotility associated with channel opening is probably caused by local buckling of the bilayer as the inner half of the channel expands as expected from X-ray structural data.
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110
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Santos JS, Grigoriev SM, Montal M. Molecular template for a voltage sensor in a novel K+ channel. III. Functional reconstitution of a sensorless pore module from a prokaryotic Kv channel. ACTA ACUST UNITED AC 2009; 132:651-66. [PMID: 19029373 PMCID: PMC2585861 DOI: 10.1085/jgp.200810077] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
KvLm is a prokaryotic voltage-gated K+ (Kv) channel from Listeria monocytogenes. The sequence of the voltage-sensing module (transmembrane segments S1-S4) of KvLm is atypical in that it contains only three of the eight conserved charged residues known to be deterministic for voltage sensing in eukaryotic Kv's. In contrast, the pore module (PM), including the S4-S5 linker and cytoplasmic tail (linker-S5-P-S6-C-terminus) of KvLm, is highly conserved. Here, the full-length (FL)-KvLm and the KvLm-PM only proteins were expressed, purified, and reconstituted into giant liposomes. The properties of the reconstituted FL-KvLm mirror well the characteristics of the heterologously expressed channel in Escherichia coli spheroplasts: a right-shifted voltage of activation, micromolar tetrabutylammonium-blocking affinity, and a single-channel conductance comparable to that of eukaryotic Kv's. Conversely, ionic currents through the PM recapitulate both the conductance and blocking properties of the FL-KvLm, yet the KvLm-PM exhibits only rudimentary voltage dependence. Given that the KvLm-PM displays many of the conduction properties of FL-KvLm and of other eukaryotic Kv's, including strict ion selectivity, we conclude that self-assembly of the PM subunits in lipid bilayers, in the absence of the voltage-sensing module, generates a conductive oligomer akin to that of the native KvLm, and that the structural independence of voltage sensing and PMs observed in eukaryotic Kv channels was initially implemented by nature in the design of prokaryotic Kv channels. Collectively, the results indicate that this robust functional module will prove valuable as a molecular template for coupling new sensors and to elucidate PM residue–specific contributions to Kv conduction properties.
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Affiliation(s)
- Jose S Santos
- Section of Neurobiology, Division of Biological Sciences, University of California, San Diego, La Jolla, CA 92093, USA
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111
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Abstract
K(+) channels are revered for their universal action of suppressing electrical activity in nerve and muscle, as well as regulating salt and water transport in epithelial tissues involved in metabolism and digestion. These multisubunit membrane-embedded proteins carry out their physiological chore, selectively allowing the passage of potassium across the membrane, in response to changes in membrane voltage and ligand concentration. Elucidating the diverse gating properties of K(+) channels is of great biological interest since their molecular motions provide insight into how these structurally similar proteins function in a wide variety of tissues. Armed with patch clamps, chart recorders, and now high-resolution structures, electrophysiologists have been dipping into the top tray of the chemist's tool box: synthesizing cysteine-modifying agents and organic cations and grinding up insects, spiders, and other vermin to isolate natural products to poke, probe, and prod K(+) channels. Recently, there has been further cross-fertilization between chemists and K(+) channelologists, resulting in greater accessibility to more elaborate synthetic methodologies and screening approaches. In this review, we catalogue the evolution of chemical tools and approaches that have been utilized to elucidate the mechanistic underpinnings of K(+) channel biology.
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Affiliation(s)
- Christopher A Ahern
- Department of Anesthesiology, University of British Columbia, 2350 Health Science Mall, Vancouver, BC V6T 1Z3, Canada.
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112
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Abstract
The detection of electrical potentials across lipid bilayers by specialized membrane proteins is required for many fundamental cellular processes such as the generation and propagation of nerve impulses. These membrane proteins possess modular voltage-sensing domains, a notable example being the S1-S4 domains of voltage-activated ion channels. Ground-breaking structural studies on these domains explain how voltage sensors are designed and reveal important interactions with the surrounding lipid membrane. Although further structures are needed to understand the conformational changes that occur during voltage sensing, the available data help to frame several key concepts that are fundamental to the mechanism of voltage sensing.
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113
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Struyk AF, Markin VS, Francis D, Cannon SC. Gating pore currents in DIIS4 mutations of NaV1.4 associated with periodic paralysis: saturation of ion flux and implications for disease pathogenesis. ACTA ACUST UNITED AC 2008; 132:447-64. [PMID: 18824591 PMCID: PMC2553391 DOI: 10.1085/jgp.200809967] [Citation(s) in RCA: 63] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
S4 voltage–sensor mutations in CaV1.1 and NaV1.4 channels cause the human muscle disorder hypokalemic periodic paralysis (HypoPP). The mechanism whereby these mutations predispose affected sarcolemma to attacks of sustained depolarization and loss of excitability is poorly understood. Recently, three HypoPP mutations in the domain II S4 segment of NaV1.4 were shown to create accessory ionic permeation pathways, presumably extending through the aqueous gating pore in which the S4 segment resides. However, there are several disparities between reported gating pore currents from different investigators, including differences in ionic selectivity and estimates of current amplitude, which in turn have important implications for the pathological relevance of these aberrant currents. To clarify the features of gating pore currents arising from different DIIS4 mutants, we recorded gating pore currents created by HypoPP missense mutations at position R666 in the rat isoform of Nav1.4 (the second arginine from the outside, at R672 in human NaV1.4). Extensive measurements were made for the index mutation, R666G, which created a gating pore that was permeable to K+ and Na+. This current had a markedly shallow slope conductance at hyperpolarized voltages and robust inward rectification, even when the ionic gradient strongly favored outward ionic flow. These characteristics were accounted for by a barrier model incorporating a voltage-gated permeation pathway with a single cation binding site oriented near the external surface of the electrical field. The amplitude of the R666G gating pore current was similar to the amplitude of a previously described proton-selective current flowing through the gating pore in rNaV1.4-R663H mutant channels. Currents with similar amplitude and cation selectivity were also observed in R666S and R666C mutant channels, while a proton-selective current was observed in R666H mutant channels. These results add support to the notion that HypoPP mutations share a common biophysical profile comprised of a low-amplitude inward current at the resting potential that may contribute to the pathological depolarization during attacks of weakness.
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Affiliation(s)
- Arie F Struyk
- Department of Neurology, University of Texas-Southwestern Medical Center, Dallas, TX 75390, USA.
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114
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Han M, Zhang JZH. Molecular Dynamic Simulation of the Kv1.2 Voltage-Gated Potassium Channel in Open and Closed State Conformations. J Phys Chem B 2008; 112:16966-74. [DOI: 10.1021/jp807905p] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Ming Han
- Institute of Theoretical and Computational Chemistry, Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China, State Key Laboratory of Precision Spectroscopy, Department of Physics, East China Normal University, Shanghai 200062, China, and Department of Chemistry, New York University, New York, New York 10003
| | - John Z. H. Zhang
- Institute of Theoretical and Computational Chemistry, Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China, State Key Laboratory of Precision Spectroscopy, Department of Physics, East China Normal University, Shanghai 200062, China, and Department of Chemistry, New York University, New York, New York 10003
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115
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Deconstructing voltage sensor function and pharmacology in sodium channels. Nature 2008; 456:202-8. [PMID: 19005548 PMCID: PMC2587061 DOI: 10.1038/nature07473] [Citation(s) in RCA: 227] [Impact Index Per Article: 14.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/22/2008] [Accepted: 09/30/2008] [Indexed: 12/04/2022]
Abstract
Voltage-activated sodium (Nav) channels are crucial for the generation and propagation of nerve impulses, and as such are amongst the most widely targeted ion channels by toxins and drugs. The four voltage sensors in Nav channels have distinct amino acid sequences, raising fundamental questions about their relative contributions to the function and pharmacology of the channel. Here we use four-fold symmetric voltage-activated potassium (Kv) channels as reporters to examine the contributions of individual Nav channel S3b-S4 paddle motifs to the kinetics of voltage sensor activation and to forming toxin receptors. Our results uncover binding sites for toxins from tarantula and scorpion venom on each of the four paddle motifs in Nav channels and reveal how paddle-specific interactions can be used to reshape Nav channel activity. One paddle motif is unique in that it slows voltage sensor activation and toxins selectively targeting this motif impede Nav channel inactivation. This reporter approach and the principles that emerge will be useful in developing new drugs for treating pain and Nav channelopathies.
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116
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Structure, function, and modification of the voltage sensor in voltage-gated ion channels. Cell Biochem Biophys 2008; 52:149-74. [PMID: 18989792 DOI: 10.1007/s12013-008-9032-5] [Citation(s) in RCA: 93] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 10/01/2008] [Indexed: 01/12/2023]
Abstract
Voltage-gated ion channels are crucial for both neuronal and cardiac excitability. Decades of research have begun to unravel the intriguing machinery behind voltage sensitivity. Although the details regarding the arrangement and movement in the voltage-sensor domain are still debated, consensus is slowly emerging. There are three competing conceptual models: the helical-screw, the transporter, and the paddle model. In this review we explore the structure of the activated voltage-sensor domain based on the recent X-ray structure of a chimera between Kv1.2 and Kv2.1. We also present a model for the closed state. From this we conclude that upon depolarization the voltage sensor S4 moves approximately 13 A outwards and rotates approximately 180 degrees, thus consistent with the helical-screw model. S4 also moves relative to S3b which is not consistent with the paddle model. One interesting feature of the voltage sensor is that it partially faces the lipid bilayer and therefore can interact both with the membrane itself and with physiological and pharmacological molecules reaching the channel from the membrane. This type of channel modulation is discussed together with other mechanisms for how voltage-sensitivity is modified. Small effects on voltage-sensitivity can have profound effects on excitability. Therefore, medical drugs designed to alter the voltage dependence offer an interesting way to regulate excitability.
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117
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Large-scale movement within the voltage-sensor paddle of a potassium channel-support for a helical-screw motion. Neuron 2008; 59:770-7. [PMID: 18786360 DOI: 10.1016/j.neuron.2008.07.008] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/07/2008] [Revised: 05/20/2008] [Accepted: 07/14/2008] [Indexed: 11/21/2022]
Abstract
The size of the movement and the molecular identity of the moving parts of the voltage sensor of a voltage-gated ion channel are debated. In the helical-screw model, the positively charged fourth transmembrane segment S4 slides and rotates along negative counter charges in S2 and S3, while in the paddle model, S4 carries the extracellular part of S3 (S3b) as a cargo. Here, we show that S4 slides 16-26 A along S3b. We introduced pairs of cysteines in S4 and S3b of the Shaker K channel to make disulfide bonds. Residue 325 in S3b makes close and state-dependent contacts with a long stretch of residues in S4. A disulfide bond between 325 and 360 was formed in the closed state, while a bond between 325 and 366 was formed in the open state. These data are not compatible with the voltage-sensor paddle model, but support the helical-screw model.
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118
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Posson DJ, Selvin PR. Extent of voltage sensor movement during gating of shaker K+ channels. Neuron 2008; 59:98-109. [PMID: 18614032 DOI: 10.1016/j.neuron.2008.05.006] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/08/2008] [Revised: 04/06/2008] [Accepted: 05/08/2008] [Indexed: 11/26/2022]
Abstract
Voltage-driven activation of Kv channels results from conformational changes of four voltage sensor domains (VSDs) that surround the K(+) selective pore domain. How the VSD helices rearrange during gating is an area of active research. Luminescence resonance energy transfer (LRET) is a powerful spectroscopic ruler uniquely suitable for addressing the conformational trajectory of these helices. Using a geometric analysis of numerous LRET measurements, we were able to estimate LRET probe positions relative to existing structural models. The experimental movement of helix S4 does not support a large 15-20 A transmembrane "paddle-type" movement or a near-zero A vertical "transporter-type" model. Rather, our measurements demonstrate a moderate S4 displacement of 10 +/- 5 A, with a vertical component of 5 +/- 2 A. The S3 segment moves 2 +/- 1 A in the opposite direction and is therefore not moving as an S3-S4 rigid body.
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Affiliation(s)
- David J Posson
- Department of Physics and Biophysics, University of Illinois Urbana-Champaign, Urbana IL 61801, USA
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119
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Lewis A, Jogini V, Blachowicz L, Lainé M, Roux B. Atomic constraints between the voltage sensor and the pore domain in a voltage-gated K+ channel of known structure. ACTA ACUST UNITED AC 2008; 131:549-61. [PMID: 18504314 PMCID: PMC2391244 DOI: 10.1085/jgp.200809962] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/05/2022]
Abstract
In voltage-gated K+ channels (Kv), membrane depolarization promotes a structural reorganization of each of the four voltage sensor domains surrounding the conducting pore, inducing its opening. Although the crystal structure of Kv1.2 provided the first atomic resolution view of a eukaryotic Kv channel, several components of the voltage sensors remain poorly resolved. In particular, the position and orientation of the charged arginine side chains in the S4 transmembrane segments remain controversial. Here we investigate the proximity of S4 and the pore domain in functional Kv1.2 channels in a native membrane environment using electrophysiological analysis of intersubunit histidine metallic bridges formed between the first arginine of S4 (R294) and residues A351 or D352 of the pore domain. We show that histidine pairs are able to bind Zn2+ or Cd2+ with high affinity, demonstrating their close physical proximity. The results of molecular dynamics simulations, consistent with electrophysiological data, indicate that the position of the S4 helix in the functional open-activated state could be shifted by ∼7–8 Å and rotated counterclockwise by 37° along its main axis relative to its position observed in the Kv1.2 x-ray structure. A structural model is provided for this conformation. The results further highlight the dynamic and flexible nature of the voltage sensor.
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Affiliation(s)
- Anthony Lewis
- Department of Pediatrics, University of Chicago, Chicago, IL 60637, USA
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120
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Abstract
Models of the transmembrane region of the NaChBac channel were developed in two open/inactivated and several closed conformations. Homology models of NaChBac were developed using crystal structures of Kv1.2 and a Kv1.2/2.1 chimera as templates for open conformations, and MlotiK and KcsA channels as templates for closed conformations. Multiple molecular-dynamic simulations were performed to refine and evaluate these models. A striking difference between the S4 structures of the Kv1.2-like open models and MlotiK-like closed models is the secondary structure. In the open model, the first part of S4 forms an alpha-helix, and the last part forms a 3(10) helix, whereas in the closed model, the first part of S4 forms a 3(10) helix, and the last part forms an alpha-helix. A conformational change that involves this type of transition in secondary structure should be voltage-dependent. However, this transition alone is not sufficient to account for the large gating charge movement reported for NaChBac channels and for experimental results in other voltage-gated channels. To increase the magnitude of the motion of S4, we developed another model of an open/inactivated conformation, in which S4 is displaced farther outward, and a number of closed models in which S4 is displaced farther inward. A helical screw motion for the alpha-helical part of S4 and a simple axial translation for the 3(10) portion were used to develop models of these additional conformations. In our models, four positively charged residues of S4 moved outwardly during activation, across a transition barrier formed by highly conserved hydrophobic residues on S1, S2, and S3. The S4 movement was coupled to an opening of the activation gate formed by S6 through interactions with the segment linking S4 to S5. Consistencies of our models with experimental studies of NaChBac and K(v) channels are discussed.
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121
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Vorobyov I, Li L, Allen TW. Assessing Atomistic and Coarse-Grained Force Fields for Protein−Lipid Interactions: the Formidable Challenge of an Ionizable Side Chain in a Membrane. J Phys Chem B 2008; 112:9588-602. [DOI: 10.1021/jp711492h] [Citation(s) in RCA: 98] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Affiliation(s)
- Igor Vorobyov
- Department of Chemistry, University of California, One Shields Avenue, Davis, California 95616
| | - Libo Li
- Department of Chemistry, University of California, One Shields Avenue, Davis, California 95616
| | - Toby W. Allen
- Department of Chemistry, University of California, One Shields Avenue, Davis, California 95616
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122
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Miki T, Zwingman TA, Wakamori M, Lutz CM, Cook SA, Hosford DA, Herrup K, Fletcher CF, Mori Y, Frankel WN, Letts VA. Two novel alleles of tottering with distinct Ca(v)2.1 calcium channel neuropathologies. Neuroscience 2008; 155:31-44. [PMID: 18597946 DOI: 10.1016/j.neuroscience.2008.05.028] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/10/2008] [Revised: 05/06/2008] [Accepted: 05/22/2008] [Indexed: 11/26/2022]
Abstract
The calcium channel CACNA1A gene encodes the pore-forming, voltage-sensitive subunit of the voltage-dependent calcium Ca(v)2.1 type channel. Mutations in this gene have been linked to several human disorders, including familial hemiplegic migraine, episodic ataxia 2 and spinocerebellar ataxia type 6. The mouse homologue, Cacna1a, is associated with the tottering, Cacna1a(tg), mutant series. Here we describe two new missense mutant alleles, Cacna1a(tg-4J) and Cacna1a(Tg-5J). The Cacna1a(tg-4J) mutation is a valine to alanine mutation at amino acid 581, in segment S5 of domain II. The recessive Cacna1a(tg-4J) mutant exhibited the ataxia, paroxysmal dyskinesia and absence seizures reminiscent of the original tottering mouse. The Cacna1a(tg-4J) mutant also showed altered activation and inactivation kinetics of the Ca(v)2.1 channel, not previously reported for other tottering alleles. The semi-dominant Cacna1a(Tg-5J) mutation changed a conserved arginine residue to glutamine at amino acid 1252 within segment S4 of domain III. The heterozygous mouse was ataxic and homozygotes rarely survived. The Cacna1a(Tg-5J) mutation caused a shift in both voltage activation and inactivation to lower voltages, showing that this arginine residue is critical for sensing Ca(v)2.1 voltage changes. These two tottering mouse models illustrate how novel allelic variants can contribute to functional studies of the Ca(v)2.1 calcium channel.
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Affiliation(s)
- T Miki
- Laboratory of Molecular Biology, Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Kyoto, Japan
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123
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Double bilayers and transmembrane gradients: a molecular dynamics study of a highly charged peptide. Biophys J 2008; 95:3161-73. [PMID: 18586841 DOI: 10.1529/biophysj.108.134049] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022] Open
Abstract
The position and extent of movement of a charged peptide within a membrane bilayer provides much controversy. In our study, we have examined the nature of the highly charged helix-turn-helix motif (S3b and S4) to address how a highly charged peptide is stabilized within a bilayer in the presence of various transmembrane electrical potentials. Our double-bilayer simulation results show how the variation of the salt concentrations between the inner and outer bath establishes a transmembrane potential. Our results also show that important features of the peptide affected by changes in electrical potential are the center of mass depth, the swivel/kink degrees of conformation, and the hydrogen-bonding patterns. As the voltage gradient across the bilayer increased, the center of mass of the peptide shifted in a direction toward the outer bath. The peptide also has a higher percent helical content and the swivel/kink conformation is more rigid for nonpolarized systems where no voltage drop occurred between salt baths. Our results also provide some suggestions for how this domain may be affected by environmental changes as part of the voltage sensor in a K-channel.
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124
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Banerjee A, MacKinnon R. Inferred motions of the S3a helix during voltage-dependent K+ channel gating. J Mol Biol 2008; 381:569-80. [PMID: 18632115 DOI: 10.1016/j.jmb.2008.06.010] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/07/2008] [Revised: 05/30/2008] [Accepted: 06/03/2008] [Indexed: 10/22/2022]
Abstract
The gating of voltage-dependent potassium channels is controlled by conformational changes in voltage sensor domains. Previous studies have shown that the S1 and the S2 helices of the voltage sensor are static with respect to motion across the membrane, while the voltage sensor paddle consisting of the C-terminal half of S3 (S3b) and the charge-bearing S4 is mobile. The mobile component is attached to S1 and S2 via the S2-S3 turn and the N-terminal half of S3 (S3a). In this study, we analyze KvAP, an archaebacterial voltage-dependent potassium channel, to study the mobility with respect to translation across the membrane of S3a. We utilize an assay based on attachment of tethered biotin and its site-specific accessibility to avidin. Our results reveal that the S3a helix does not move appreciably across the membrane in association with gating. The static behavior of S3a constrains the conformations available to the voltage sensor when it closes and suggests that a set of negative countercharges within the membrane's inner leaflet remains intact in the closed conformation.
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Affiliation(s)
- Anirban Banerjee
- Laboratory of Molecular Neurobiology and Biophysics, Howard Hughes Medical Institute, Rockefeller University, Box 47, 1230 York Avenue, New York, NY 10065, USA
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125
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Abstract
Voltage-gated K+ channels are dynamic macromolecular machines that open and close in response to changes in membrane potential. These multisubunit membrane-embedded proteins are responsible for governing neuronal excitability, maintaining cardiac rhythmicity, and regulating epithelial electrolyte homeostasis. High resolution crystal structures have provided snapshots of K+ channels caught in different states with incriminating molecular detail. Nonetheless, the connection between these static images and the specific trajectories of K+ channel movements is still being resolved by biochemical experimentation. Electrophysiological recordings in the presence of chemical modifying reagents have been a staple in ion channel structure/function studies during both the pre- and post-crystal structure eras. Small molecule tethering agents (chemoselective electrophiles linked to ligands) have proven to be particularly useful tools for defining the architecture and motions of K+ channels. This Minireview examines the synthesis and utilization of chemical tethering agents to probe and manipulate the assembly, structure, function, and molecular movements of voltage-gated K+ channel protein complexes.
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Affiliation(s)
- Trevor J Morin
- Department of Biochemistry and Molecular Pharmacology, Programs in Neuroscience and Chemical Biology, University of Massachusetts Medical School, Worcester, Massachusetts 01605-2324
| | - William R Kobertz
- Department of Biochemistry and Molecular Pharmacology, Programs in Neuroscience and Chemical Biology, University of Massachusetts Medical School, Worcester, Massachusetts 01605-2324.
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126
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Molecular dynamics simulation of Kv channel voltage sensor helix in a lipid membrane with applied electric field. Biophys J 2008; 95:1729-44. [PMID: 18487312 DOI: 10.1529/biophysj.108.130658] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023] Open
Abstract
In this article, we present the results of the molecular dynamics simulations of amphiphilic helix peptides of 13 amino-acid residues, placed at the lipid-water interface of dipalmitoylphosphatidylcholine bilayers. The peptides are identical with, or are derivatives of, the N-terminal segment of the S4 helix of voltage-dependent K channel KvAP, containing four voltage-sensing arginine residues (R1-R4). Upon changing the direction of the externally applied electric field, the tilt angle of the wild-type peptide changes relative to the lipid-water interface, with the N-terminus heading up with an outward electric field. These movements were not observed using an octane membrane in place of the dipalmitoylphosphatidylcholine membrane, and were markedly suppressed by 1), substituting Phe located one residue before the first arginine (R1) with a hydrophilic residue (Ser, Thr); or 2), changing the periodicity rule of Rs from at-every-third to at-every-fourth position; or 3), replacing R1 with a lysine residue (K). These and other findings suggest that the voltage-dependent movement requires deep positioning of Rs when the resting (inward) electric field is present. Later, we performed simulations of the voltage sensor domain (S1-S4) of Kv1.2 channel. In simulations with a strong electric field (0.1 V/nm or above) and positional restraints on the S1 and S2 helices, S4 movement was observed consisting of displacement along the S4 helix axis and a screwlike axial rotation. Gating-charge-carrying Rs were observed to make serial interactions with E183 in S1 and E226 in S2, in the outer water crevice. A 30-ns-backward simulation started from the open-state model gave rise to a structure similar to the recent resting-state model, with S4 moving vertically approximately 6.7 A. The energy landscape around the movement of S4 appears very ragged due to salt bridges formed between gating-charge-carrying residues and negatively charged residues of S1, S2, and S3 helices. Overall, features of S3 and S4 movements are consistent with the recent helical-screw model. Both forward and backward simulations show the presence of at least two stable intermediate structures in which R2 and R3 form salt bridges with E183 or E226, respectively. These structures are the candidates for the states postulated in previous gating kinetic models, such as the Zagotta-Hoshi-Aldrich model, to account for more than one transition step per subunit for activation.
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127
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Tolia A, Horré K, De Strooper B. Transmembrane domain 9 of presenilin determines the dynamic conformation of the catalytic site of gamma-secretase. J Biol Chem 2008; 283:19793-803. [PMID: 18482978 DOI: 10.1074/jbc.m802461200] [Citation(s) in RCA: 61] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
One of the most prominent drug targets for the treatment of Alzheimer disease is gamma-secretase, a multi-protein complex responsible for the generation of the amyloid-beta peptide. The catalytic core of the complex lies on presenilin, a multi-spanning membrane protease, the activity of which depends on two aspartate residues located in transmembrane domains 6 and 7. We have recently shown by cysteine-scanning mutagenesis that these aspartates are facing a water-filled cavity in the lipid bilayer, demonstrating how proteolytic cleavage of the substrates can be taking place within the membrane. Here, we demonstrate that transmembrane domain 9 and hydrophobic domain VII in the large cytoplasmic loop of presenilin are dynamic structural parts of this cavity. Hydrophobic domain VII is associated with transmembrane domain 7 in the membrane, probably facilitating the entrance of water molecules in the catalytic site. Transmembrane domain 9, on the other hand, exhibits a highly flexible structure, potentially involved in the transport of substrates to the catalytic site, as well as in the binding of gamma-secretase inhibitors. The conserved proline-alanine-leucine motif at the cytoplasmic part of this domain is extremely close to the catalytic Asp257 and is crucial for conformational changes leading to the activation of the catalytic site. We, also, identify a unique mutant in this domain (I437C) that specifically blocks amyloid-beta peptide production without affecting the processing of the physiologically indispensable Notch substrate. Our data are finally combined to propose a model for the architectural organization and activation of the catalytic site of presenilin.
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Affiliation(s)
- Alexandra Tolia
- Department for Molecular and Developmental Genetics, VIB (Vlaams Instituut voor Biotechnologie), Belgium
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128
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S1 constrains S4 in the voltage sensor domain of Kv7.1 K+ channels. PLoS One 2008; 3:e1935. [PMID: 18398461 PMCID: PMC2275789 DOI: 10.1371/journal.pone.0001935] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/06/2007] [Accepted: 02/26/2008] [Indexed: 11/19/2022] Open
Abstract
Voltage-gated K+ channels comprise a central pore enclosed by four voltage-sensing domains (VSDs). While movement of the S4 helix is known to couple to channel gate opening and closing, the nature of S4 motion is unclear. Here, we substituted S4 residues of Kv7.1 channels by cysteine and recorded whole-cell mutant channel currents in Xenopus oocytes using the two-electrode voltage-clamp technique. In the closed state, disulfide and metal bridges constrain residue S225 (S4) nearby C136 (S1) within the same VSD. In the open state, two neighboring I227 (S4) are constrained at proximity while residue R228 (S4) is confined close to C136 (S1) of an adjacent VSD. Structural modeling predicts that in the closed to open transition, an axial rotation (∼190°) and outward translation of S4 (∼12 Å) is accompanied by VSD rocking. This large sensor motion changes the intra-VSD S1–S4 interaction to an inter-VSD S1–S4 interaction. These constraints provide a ground for cooperative subunit interactions and suggest a key role of the S1 segment in steering S4 motion during Kv7.1 gating.
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129
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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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130
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Abstract
The ionic gradients across cell membranes generate a transmembrane voltage that regulates the function of numerous membrane proteins such as ion channels, transporters, pumps and enzymes. The mechanisms by which proteins sense voltage is diverse: ion channels have a conserved, positively charged transmembrane region that moves in response to changes in membrane potential, some G-protein coupled receptors possess a specific voltage-sensing motif and some membrane pumps and transporters use the ions that they transport across membranes to sense membrane voltage. Characterizing the general features of voltage sensors might lead to the discovery of further membrane proteins that are voltage regulated.
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Affiliation(s)
- Francisco Bezanilla
- Department of Biochemistry and Molecular Biology, University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, USA.
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131
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Abstract
BK (Slo1) potassium channels are activated by millimolar intracellular Mg2+ as well as micromolar Ca2+ and membrane depolarization. Mg2+ and Ca2+ act in an approximately additive manner at different binding sites to shift the conductance–voltage (GK-V) relation, suggesting that these ligands might work through functionally similar but independent mechanisms. However, we find that the mechanism of Mg2+ action is highly dependent on voltage sensor activation and therefore differs fundamentally from that of Ca2+. Evidence that Ca2+ acts independently of voltage sensor activation includes an ability to increase open probability (PO) at extreme negative voltages where voltage sensors are in the resting state; 2 μM Ca2+ increases PO more than 15-fold at −120 mV. However 10 mM Mg2+, which has an effect on the GK-V relation similar to 2 μM Ca2+, has no detectable effect on PO when voltage sensors are in the resting state. Gating currents are only slightly altered by Mg2+ when channels are closed, indicating that Mg2+ does not act merely to promote voltage sensor activation. Indeed, channel opening is facilitated in a voltage-independent manner by Mg2+ in a mutant (R210C) whose voltage sensors are constitutively activated. Thus, 10 mM Mg2+ increases PO only when voltage sensors are activated, effectively strengthening the allosteric coupling of voltage sensor activation to channel opening. Increasing Mg2+ from 10 to 100 mM, to occupy very low affinity binding sites, has additional effects on gating that more closely resemble those of Ca2+. The effects of Mg2+ on steady-state activation and IK kinetics are discussed in terms of an allosteric gating scheme and the state-dependent interactions between Mg2+ and voltage sensor that may underlie this mechanism.
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Affiliation(s)
- Frank T Horrigan
- Department of Physiology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA.
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132
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Coarse-grained simulation: a high-throughput computational approach to membrane proteins. Biochem Soc Trans 2008; 36:27-32. [PMID: 18208379 DOI: 10.1042/bst0360027] [Citation(s) in RCA: 100] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
An understanding of the interactions of membrane proteins with a lipid bilayer environment is central to relating their structure to their function and stability. A high-throughput approach to prediction of membrane protein interactions with a lipid bilayer based on coarse-grained Molecular Dynamics simulations is described. This method has been used to develop a database of CG simulations (coarse-grained simulations) of membrane proteins (http://sbcb.bioch.ox.ac.uk/cgdb). Comparison of CG simulations and AT simulations (atomistic simulations) of lactose permease reveals good agreement between the two methods in terms of predicted lipid headgroup contacts. Both CG and AT simulations predict considerable local bilayer deformation by the voltage sensor domain of the potassium channel KvAP.
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133
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Treptow W, Marrink SJ, Tarek M. Gating Motions in Voltage-Gated Potassium Channels Revealed by Coarse-Grained Molecular Dynamics Simulations. J Phys Chem B 2008; 112:3277-82. [DOI: 10.1021/jp709675e] [Citation(s) in RCA: 56] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Affiliation(s)
- Werner Treptow
- Center for Molecular Modeling, Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104-6323, Groningen Biomolecular Sciences and Biotechnology Institute, Department of Biophysical Chemistry, University of Groningen, The Netherlands, and UMR Structure et Réactivité des Systèmes Moléculaires Complexes, Nancy-University, CNRS, France
| | - Siewert-J Marrink
- Center for Molecular Modeling, Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104-6323, Groningen Biomolecular Sciences and Biotechnology Institute, Department of Biophysical Chemistry, University of Groningen, The Netherlands, and UMR Structure et Réactivité des Systèmes Moléculaires Complexes, Nancy-University, CNRS, France
| | - Mounir Tarek
- Center for Molecular Modeling, Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104-6323, Groningen Biomolecular Sciences and Biotechnology Institute, Department of Biophysical Chemistry, University of Groningen, The Netherlands, and UMR Structure et Réactivité des Systèmes Moléculaires Complexes, Nancy-University, CNRS, France
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134
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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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135
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Roux B. Chapter 13 A Brief Introduction to Voltage-Gated K+ Channels. CURRENT TOPICS IN MEMBRANES 2008. [DOI: 10.1016/s1063-5823(08)00013-6] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
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136
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Chapter 15 Charged Protein Side Chain Movement in Lipid Bilayers Explored with Free Energy Simulation. CURRENT TOPICS IN MEMBRANES 2008. [DOI: 10.1016/s1063-5823(08)00015-x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/08/2023]
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137
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Rocheleau JM, Kobertz WR. KCNE peptides differently affect voltage sensor equilibrium and equilibration rates in KCNQ1 K+ channels. ACTA ACUST UNITED AC 2007; 131:59-68. [PMID: 18079560 PMCID: PMC2174159 DOI: 10.1085/jgp.200709816] [Citation(s) in RCA: 53] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
KCNQ1 voltage-gated K+ channels assemble with the family of KCNE type I transmembrane peptides to afford membrane-embedded complexes with diverse channel gating properties. KCNQ1/KCNE1 complexes generate the very slowly activating cardiac IKs current, whereas assembly with KCNE3 produces a constitutively conducting complex involved in K+ recycling in epithelia. To determine whether these two KCNE peptides influence voltage sensing in KCNQ1 channels, we monitored the position of the S4 voltage sensor in KCNQ1/KCNE complexes using cysteine accessibility experiments. A panel of KCNQ1 S4 cysteine mutants was expressed in Xenopus oocytes, treated with the membrane-impermeant cysteine-specific reagent 2-(trimethylammonium) ethyl methanethiosulfonate (MTSET), and the voltage-dependent accessibility of each mutant was determined. Of these S4 cysteine mutants, three (R228C, G229C, I230C) were modified by MTSET only when KCNQ1 was depolarized. We then employed these state-dependent residues to determine how assembly with KCNE1 and KCNE3 affects KCNQ1 voltage sensor equilibrium and equilibration rates. In the presence of KCNE1, MTSET modification rates for the majority of the cysteine mutants were ∼10-fold slower, as was recently reported to indicate that the kinetics of the KCNQ1 voltage sensor are slowed by KCNE1 (Nakajo, K., and Y. Kubo. 2007 J. Gen. Physiol. 130:269–281). Since MTS modification rates reflect an amalgam of reagent accessibility, chemical reactivity, and protein conformational changes, we varied the depolarization pulse duration to determine whether KCNE1 slows the equilibration rate of the voltage sensors. Using the state-dependent cysteine mutants, we determined that MTSET modification rates were essentially independent of depolarization pulse duration. These results demonstrate that upon depolarization the voltage sensors reach equilibrium quickly in the presence of KCNE1 and the slow gating of the channel complex is not due to slowly moving voltage sensors. In contrast, all cysteine substitutions in the S4 of KCNQ1/KCNE3 complexes were freely accessible to MTSET independent of voltage, which is consistent with KCNE3 shifting the voltage sensor equilibrium to favor the active state at hyperpolarizing potentials. In total, these results suggest that KCNE peptides differently modulate the voltage sensor in KCNQ1 K+ channels.
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Affiliation(s)
- Jessica M Rocheleau
- Department of Biochemistry and Molecular Pharmacology, University of Massachusetts, Worcester, MA 01605, USA
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138
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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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139
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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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140
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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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141
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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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142
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Andér M, Luzhkov VB, Åqvist J. Ligand binding to the voltage-gated Kv1.5 potassium channel in the open state--docking and computer simulations of a homology model. Biophys J 2007; 94:820-31. [PMID: 17905851 PMCID: PMC2186239 DOI: 10.1529/biophysj.107.112045] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023] Open
Abstract
The binding of blockers to the human voltage-gated Kv1.5 potassium ion channel is investigated using a three-step procedure consisting of homology modeling, automated docking, and binding free energy calculations from molecular dynamics simulations, in combination with the linear interaction energy method. A reliable homology model of Kv1.5 is constructed using the recently published crystal structure of the Kv1.2 channel as a template. This model is expected to be significantly more accurate than earlier ones based on less similar templates. Using the three-dimensional homology model, a series of blockers with known affinities are docked into the cavity of the ion channel and their free energies of binding are calculated. The predicted binding free energies are in very good agreement with experimental data and the binding is predicted to be mainly achieved through nonpolar interactions, whereas the relatively small differences in the polar contribution determine the specificity. Apart from confirming the importance of residues V505, I508, V512, and V516 for ligand binding in the cavity, the results also show that A509 and P513 contribute significantly to the nonpolar binding interactions. Furthermore, we find that pharmacophore models based only on optimized free ligand conformations may not necessarily capture the geometric features of ligands bound to the channel cavity. The calculations herein give a detailed structural and energetic picture of blocker binding to Kv1.5 and this model should thus be useful for further ligand design efforts.
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Affiliation(s)
| | | | - Johan Åqvist
- Address reprint requests to Johan Åqvist, Tel.: 46-18-471-4109; Fax: 46-18-53-69-71.
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143
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Blanchet J, Chahine M. Accessibility of four arginine residues on the S4 segment of the Bacillus halodurans sodium channel. J Membr Biol 2007; 215:169-80. [PMID: 17568977 DOI: 10.1007/s00232-007-9016-1] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/12/2006] [Accepted: 02/28/2007] [Indexed: 01/22/2023]
Abstract
The voltage-gated Na(+) channel of Bacillus halodurans (NaChBac) is composed of six transmembrane segments (S1-S6), with a pore-forming region composed of segments S5 and S6 and a voltage-sensing domain composed of segments S1-S4. The S4 segment forms the core of the voltage sensor. We explored the accessibility of four arginine residues on the S4 segment of NaChBac, which are positioned at every third position from each other. These arginine residues on the S4 segment were replaced with cysteines using site-directed mutagenesis. Na(+) currents were recorded using the whole-cell configuration of the patch-clamp technique. We tested the effect of the sulfhydryl reagents applied from inside and outside the cellular space in the open and closed conformations. Structural models of the voltage sensor of NaChBac were constructed based on the recently crystallized KvAP and Kv1.2 K(+) channels to visualize arginine residue accessibility. Our results suggest that arginine accessibility did not change significantly between the open and closed conformations, supporting the idea of a small movement of the S4 segment during gating. Molecular modeling of the closed conformation also supported a small movement of S4, which is mainly characterized by a rotation and a tilt along the periphery of the pore. Interestingly, the second arginine residue of the S4 segment (R114) was accessible to sulfhydryl reagents from both sides of the membrane in the closed conformation and, based on our model, seemed to be at the junction of the intracellular and extracellular water crevices.
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Affiliation(s)
- Jonathan Blanchet
- Department of Medicine, Le Centre de recherche Université Laval Robert-Giffard, Québec, QC, Canada
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144
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Abstract
Voltage-gated ion channels are key players in fast neuronal signalling. Detailed knowledge about channel gating is essential for our understanding of channel function in general and of drug action of channels in particular. Despite a number of recent atomic channel structures, the opening of voltage-gated channels is the subject of heated debates. Here we will discuss two of the controversies: one concerning the mechanism of opening and closing the pore, and the other concerning the location and movement of the voltage sensor. The channels were originally suggested to open at a conserved proline rich sequence (PVP) at the intracellular end of the transmembrane segment 6 (S6). The crystallization of a channel in the open state instead suggested an opening involving a conserved glycine hinge located in the middle portion of S6. Based on pharmacological studies, autodocking and molecular dynamics simulations we have found support for the PVP-bend model. The voltage sensor, transmembrane segment 4 (S4), was originally suggested to be buried in the channel protein, undergoing a helical-screw-like motion to open the channel. A recent crystallographic study suggested that S4 is located in the periphery, facing lipid, and undergoing a paddle-like motion to open the channel. We have found experimental evidence for a novel helical-screw model; with the voltage sensor moving in a screw-like fashion but being located in the periphery of the channel. This model opens up for understanding how lipophilic drugs and toxins directly affect the voltage sensor.
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Affiliation(s)
- Fredrik Elinder
- Karolinska Institutet, Department of Neuroscience, The Nobel Institute for Neurophysiology, Stockholm, Sweden.
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145
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Campos FV, Chanda B, Roux B, Bezanilla F. Two atomic constraints unambiguously position the S4 segment relative to S1 and S2 segments in the closed state of Shaker K channel. Proc Natl Acad Sci U S A 2007; 104:7904-9. [PMID: 17470814 PMCID: PMC1876545 DOI: 10.1073/pnas.0702638104] [Citation(s) in RCA: 144] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
It is now well established that the voltage-sensing S4 segment in voltage-dependent ion channels undergoes a conformational change in response to varying membrane potential. However, the magnitude of the movement of S4 relative to the membrane and the rest of the protein remains controversial. Here, by using histidine scanning mutagenesis in the Shaker K channel, we identified mutants I241H (S1 segment) and I287H (S2 segment) that generate inward currents at hyperpolarized potentials, suggesting that these residues are part of a hydrophobic plug that separates the water-accessible crevices. Additional experiments with substituted cysteine residues showed that, at hyperpolarized potentials, both I241C and I287C can spontaneously form disulphide and metal bridges with R362C, the position of the first charge-carrying residue in S4. These results constrain unambiguously the closed-state positions of the S4 segment with respect to the S1 and S2 segments, which are known to undergo little or no movement during gating. To satisfy these constraints, the S4 segment must undergo an axial rotation of approximately 180 degrees and a transmembrane (vertical) movement of approximately 6.5 A at the level of R362 in going from the open to the closed state of the channel, moving the gating charge across a focused electric field.
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Affiliation(s)
- Fabiana V. Campos
- *Institute for Molecular Pediatric Sciences, Department of Pediatrics and Department of Biochemistry and Molecular Biology, University of Chicago, 929 East 57th Street, Chicago, IL 60637; and
| | - Baron Chanda
- Department of Physiology, University of Wisconsin, Madison, WI 53706
| | - Benoît Roux
- *Institute for Molecular Pediatric Sciences, Department of Pediatrics and Department of Biochemistry and Molecular Biology, University of Chicago, 929 East 57th Street, Chicago, IL 60637; and
| | - Francisco Bezanilla
- *Institute for Molecular Pediatric Sciences, Department of Pediatrics and Department of Biochemistry and Molecular Biology, University of Chicago, 929 East 57th Street, Chicago, IL 60637; and
- To whom correspondence should be addressed. E-mail:
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146
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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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147
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Kumar A, Tyagi NK, Kinne RKH. Ligand-mediated conformational changes and positioning of tryptophans in reconstituted human sodium/d-glucose cotransporter1 (hSGLT1) probed by tryptophan fluorescence. Biophys Chem 2007; 127:69-77. [PMID: 17222499 DOI: 10.1016/j.bpc.2006.12.006] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/21/2006] [Revised: 11/23/2006] [Accepted: 12/12/2006] [Indexed: 10/23/2022]
Abstract
Recombinant purified human sodium/D-glucose cotransporter1 (hSGLT1) was reconstituted in a functional form into phospholipid vesicles and its conformational states in the absence and presence of ligands and inhibitors were probed by intrinsic tryptophan fluorescence. In the presence of sodium, sugars increase intrinsic fluorescence (maximum 17%) in a saturable manner in the following order alpha-MDG >D-Glu approximately D-Gal >> D-Man >D-All, with no effect of L-Glu. Apparent affinities ranging from 0.65 to 10.4 mM were observed. In addition, D-Glu increased the accessibility of the Trps to hydrophilic collisional quenchers. On the contrary, the transport inhibitor phlorizin decreased Trps fluorescence in a sodium-dependent manner by 50% with a red shift of 4-6 nm and decreased quencher accessibility, these effects were saturable with a high affinity of 5 microM. Furthermore, the positioning of the tryptophans in the reconstituted transporter was investigated. hSGLT1 Trps fluorescence was reduced by N-bromosuccinimide treatment maximally 25% in membranes and 65% in solution. The fluorescence was also significantly but differently quenched by the lipid-soluble spin labeled probes 5-Doxyl-phosphatidylcholine (40%) and 12-Doxyl-phosphatidylcholine (26%). Depth-calculation using the parallax method suggested a location of Trps at an average depth of 10 angstrom from the center of the bilayer. These studies demonstrate the existence of different conformational states of the membrane-embedded transporter in its glucose-free form, as sodium-glucose-carrier complex and as sodium-phlorizin-carrier complex. They further indicate that most of the Trp residues in hSGLT1 are located in hydrophobic regions of the protein or in contact with the lipid bilayer of the membrane. There, they are located close to the membrane-water interface contributing to the vectorial nature of the transporter.
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Affiliation(s)
- Azad Kumar
- Max Planck Institute of Molecular Physiology, Otto-Hahn-Str 11, Dortmund, 44227, Germany
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148
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Abstract
The ability to sense and respond to the environment is a hallmark of living systems. These processes occur at the levels of the organism, cells and individual molecules. Sensing of extracellular changes could result in a structural or chemical alteration in a molecule, which could in turn trigger a cascade of intracellular signals or regulated trafficking of molecules at the cell surface. These and other such processes allow cells to sense and respond to environmental changes. Often, these changes and the responses to them are spatially and/or temporally localized, and visualization of such events necessitates the use of high-resolution imaging approaches. Here we discuss optical imaging approaches and tools for imaging individual events at the cell surface with improved speed and resolution.
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Affiliation(s)
- Jyoti K Jaiswal
- The Rockefeller University, 1230 York Avenue, New York, New York 10021, USA
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149
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Dorairaj S, Allen TW. On the thermodynamic stability of a charged arginine side chain in a transmembrane helix. Proc Natl Acad Sci U S A 2007; 104:4943-8. [PMID: 17360368 PMCID: PMC1829244 DOI: 10.1073/pnas.0610470104] [Citation(s) in RCA: 231] [Impact Index Per Article: 13.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/26/2006] [Indexed: 11/18/2022] Open
Abstract
Biological membranes consist of bilayer arrangements of lipids forming a hydrophobic core that presents a physical barrier to all polar and charged molecules. This long-held notion has recently been challenged by biological translocon-based experiments that report small apparent free energies to insert charged side chains near the center of a transmembrane (TM) helix. We have carried out fully atomistic simulations to provide the free-energy profile for moving a TM helix containing a protonated Arg side chain across a lipid bilayer. Our results reveal the fundamental thermodynamics governing the stability of charged side chains in membranes and the microscopic interactions involved. Despite local membrane deformations, where large amounts of water and lipid head groups are pulled into the bilayer to interact with Arg, the free-energy barrier is 17 kcal/mol. We provide a rationale for the differences in our microscopic free energies and cell biological experiments using free-energy calculations that indicate that a protonated Arg at the central residue of a TM helix of the Leader peptidase might reside close to the interface and not at the membrane center. Our findings have implications for the gating mechanisms of voltage-gated ion channels, suggesting that movements of protonated Arg residues through the membrane will be prohibited.
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Affiliation(s)
- Sudha Dorairaj
- Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616
| | - Toby W. Allen
- Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616
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150
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Blanchet J, Pilote S, Chahine M. Acidic residues on the voltage-sensor domain determine the activation of the NaChBac sodium channel. Biophys J 2007; 92:3513-23. [PMID: 17325004 PMCID: PMC1853154 DOI: 10.1529/biophysj.106.090464] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The voltage-sensing domain of voltage-gated ion channels is characterized by specific, conserved, charged residues. Positively charged residues on segment S4 are the main contributors to voltage-sensing and negatively charged residues on the S2 and S3 segments are believed to participate to the process. However, their function in the voltage sensor is not well understood. To probe the role of three acidic residues in NaChBac (D-58 and E-68 in S2, and D-91 in S3), we employed site-directed mutagenesis to substitute native acidic residues with cysteine (neutral), lysine (positive charge), or either aspartate or glutamate (negative charge). We used a combination of the patch-clamp technique to record Na+ currents and molecular modeling to visualize interacting amino acid residues. We suggest that the acidic residues on the S2 and S3 segments form specific interactions with adjacent amino acids in the voltage-sensor domain. The main interactions in NaChBac are D-58 (S2) with A-97-G-98 (S3) and R-120 (S4), E-68 (S2) with R-129 (L4-5), and D-91 (S3) with R-72 (S2). Changing these acidic residues modified the interactions, which in turn altered the sensitivity of the voltage sensor.
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Affiliation(s)
- Jonathan Blanchet
- Research Centre and Department of Medicine, Hôpital Laval, Quebec City, Quebec, Canada G1V 4G5
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