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Tronin AY, Maciunas LJ, Grasty KC, Loll PJ, Ambaye HA, Parizzi AA, Lauter V, Geragotelis AD, Freites JA, Tobias DJ, Blasie JK. Voltage-Dependent Profile Structures of a Kv-Channel via Time-Resolved Neutron Interferometry. Biophys J 2019; 117:751-766. [PMID: 31378315 PMCID: PMC6712512 DOI: 10.1016/j.bpj.2019.07.011] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/11/2019] [Revised: 06/27/2019] [Accepted: 07/09/2019] [Indexed: 10/26/2022] Open
Abstract
Available experimental techniques cannot determine high-resolution three-dimensional structures of membrane proteins under a transmembrane voltage. Hence, the mechanism by which voltage-gated cation channels couple conformational changes within the four voltage sensor domains, in response to either depolarizing or polarizing transmembrane voltages, to opening or closing of the pore domain's ion channel remains unresolved. Single-membrane specimens, composed of a phospholipid bilayer containing a vectorially oriented voltage-gated K+ channel protein at high in-plane density tethered to the surface of an inorganic multilayer substrate, were developed to allow the application of transmembrane voltages in an electrochemical cell. Time-resolved neutron reflectivity experiments, enhanced by interferometry enabled by the multilayer substrate, were employed to provide directly the low-resolution profile structures of the membrane containing the vectorially oriented voltage-gated K+ channel for the activated, open and deactivated, closed states of the channel under depolarizing and hyperpolarizing transmembrane voltages applied cyclically. The profile structures of these single membranes were dominated by the voltage-gated K+ channel protein because of the high in-plane density. Importantly, the use of neutrons allowed the determination of the voltage-dependent changes in both the profile structure of the membrane and the distribution of water within the profile structure. These two key experimental results were then compared to those predicted by three computational modeling approaches for the activated, open and deactivated, closed states of three different voltage-gated K+ channels in hydrated phospholipid bilayer membrane environments. Of the three modeling approaches investigated, only one state-of-the-art molecular dynamics simulation that directly predicted the response of a voltage-gated K+ channel within a phospholipid bilayer membrane to applied transmembrane voltages by utilizing very long trajectories was found to be in agreement with the two key experimental results provided by the time-resolved neutron interferometry experiments.
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Affiliation(s)
- Andrey Y Tronin
- Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania
| | - Lina J Maciunas
- Department of Biochemistry and Molecular Biology, Drexel University College of Medicine, Philadelphia, Pennsylvania
| | - Kimberly C Grasty
- Department of Biochemistry and Molecular Biology, Drexel University College of Medicine, Philadelphia, Pennsylvania
| | - Patrick J Loll
- Department of Biochemistry and Molecular Biology, Drexel University College of Medicine, Philadelphia, Pennsylvania
| | - Haile A Ambaye
- Neutron Scattering Division, Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, Tennessee
| | - Andre A Parizzi
- Neutron Scattering Division, Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, Tennessee
| | - Valeria Lauter
- Neutron Scattering Division, Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, Tennessee
| | | | - J Alfredo Freites
- Department of Chemistry, University of California Irvine, Irvine, California
| | - Douglas J Tobias
- Department of Chemistry, University of California Irvine, Irvine, California
| | - J Kent Blasie
- Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania.
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Time-Resolved Neutron Interferometry and the Mechanism of Electromechanical Coupling in Voltage-Gated Ion Channels. Methods Enzymol 2018. [PMID: 29673535 DOI: 10.1016/bs.mie.2018.01.019] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register]
Abstract
The mechanism of electromechanical coupling for voltage-gated ion channels (VGICs) involved in neurological signal transmission, primarily Nav- and Kv-channels, remains unresolved. Anesthetics have been shown to directly impact this mechanism, at least for Kv-channels. Molecular dynamics computer simulations can now predict the structures of VGICs embedded within a hydrated phospholipid bilayer membrane as a function of the applied transmembrane voltage, but significant assumptions are still necessary. Nevertheless, these simulations are providing new insights into the mechanism of electromechanical coupling at the atomic level in 3-D. We show that time-resolved neutron interferometry can be used to investigate directly the profile structure of a VGIC, vectorially oriented within a single hydrated phospholipid bilayer membrane at the solid-liquid interface, as a function of the applied transmembrane voltage in the absence of any assumptions or potentially perturbing modifications of the VGIC protein and/or the host membrane. The profile structure is a projection of the membrane's 3-D structure onto the membrane normal and, in the absence of site-directed deuterium labeling, is provided at substantially lower spatial resolution than the atomic level. Nevertheless, this novel approach can be used to directly test the validity of the predictions from molecular dynamics simulations. We describe the key elements of our novel experimental approach, including why each is necessary and important to providing the essential information required for this critical comparison of "simulation" vs "experiment." In principle, the approach could be extended to higher spatial resolution and to include the effects of anesthetics on the electromechanical coupling mechanism in VGICs.
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Voltage Sensing in Membranes: From Macroscopic Currents to Molecular Motions. J Membr Biol 2015; 248:419-30. [PMID: 25972106 DOI: 10.1007/s00232-015-9805-x] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/09/2015] [Accepted: 04/24/2015] [Indexed: 01/06/2023]
Abstract
Voltage-sensing domains (VSDs) are integral membrane protein units that sense changes in membrane electric potential, and through the resulting conformational changes, regulate a specific function. VSDs confer voltage-sensitivity to a large superfamily of membrane proteins that includes voltage-gated Na[Formula: see text], K[Formula: see text], Ca[Formula: see text] ,and H[Formula: see text] selective channels, hyperpolarization-activated cyclic nucleotide-gated channels, and voltage-sensing phosphatases. VSDs consist of four transmembrane segments (termed S1 through S4). Their most salient structural feature is the highly conserved positions for charged residues in their sequences. S4 exhibits at least three conserved triplet repeats composed of one basic residue (mostly arginine) followed by two hydrophobic residues. These S4 basic side chains participate in a state-dependent internal salt-bridge network with at least four acidic residues in S1-S3. The signature of voltage-dependent activation in electrophysiology experiments is a transient current (termed gating or sensing current) upon a change in applied membrane potential as the basic side chains in S4 move across the membrane electric field. Thus, the unique structural features of the VSD architecture allow for competing requirements: maintaining a series of stable transmembrane conformations, while allowing charge motion, as briefly reviewed here.
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Tronin A, Nordgren CE, Strzalka JW, Kuzmenko I, Worcester DL, Lauter V, Freites JA, Tobias DJ, Blasie JK. Direct evidence of conformational changes associated with voltage gating in a voltage sensor protein by time-resolved X-ray/neutron interferometry. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2014; 30:4784-4796. [PMID: 24697545 PMCID: PMC4007984 DOI: 10.1021/la500560w] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 02/13/2014] [Indexed: 06/03/2023]
Abstract
The voltage sensor domain (VSD) of voltage-gated cation (e.g., Na(+), K(+)) channels central to neurological signal transmission can function as a distinct module. When linked to an otherwise voltage-insensitive, ion-selective membrane pore, the VSD imparts voltage sensitivity to the channel. Proteins homologous with the VSD have recently been found to function themselves as voltage-gated proton channels or to impart voltage sensitivity to enzymes. Determining the conformational changes associated with voltage gating in the VSD itself in the absence of a pore domain thereby gains importance. We report the direct measurement of changes in the scattering-length density (SLD) profile of the VSD protein, vectorially oriented within a reconstituted phospholipid bilayer membrane, as a function of the transmembrane electric potential by time-resolved X-ray and neutron interferometry. The changes in the experimental SLD profiles for both polarizing and depolarizing potentials with respect to zero potential were found to extend over the entire length of the isolated VSD's profile structure. The characteristics of the changes observed were in qualitative agreement with molecular dynamics simulations of a related membrane system, suggesting an initial interpretation of these changes in terms of the VSD's atomic-level 3-D structure.
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Affiliation(s)
- Andrey
Y. Tronin
- Department
of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
| | - C. Erik Nordgren
- Department
of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
| | - Joseph W. Strzalka
- X-ray
Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States
| | - Ivan Kuzmenko
- X-ray
Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States
| | - David L. Worcester
- Department
of Physiology & Biophysics, University
of California Irvine, Irvine, California 92697, United States
| | - Valeria Lauter
- Spallation
Neutron Source, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
| | - J. Alfredo Freites
- Department
of Chemistry, University of California Irvine, Irvine, California 92697, United States
| | - Douglas J. Tobias
- Department
of Chemistry, University of California Irvine, Irvine, California 92697, United States
| | - J. Kent Blasie
- Department
of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
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Tronin A, Chen CH, Gupta S, Worcester D, Lauter V, Strzalka J, Kuzmenko I, Blasie JK. Structural changes in single membranes in response to an applied transmembrane electric potential revealed by time-resolved neutron/X-ray interferometry. Chem Phys 2013; 422. [PMID: 24222930 DOI: 10.1016/j.chemphys.2013.01.016] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The profile structure of a hybrid lipid bilayer, tethered to the surface of an inorganic substrate and fully hydrated with a bulk aqueous medium in an electrochemical cell, was investigated as a function of the applied transbilayer electric potential via time-resolved neutron reflectivity, enhanced by interferometry. Significant, and fully reversible structural changes were observed in the distal half (with respect to the substrate surface) of the hybrid bilayer comprised of a zwitterionic phospholipid in response to a +100mV potential with respect to 0mV. These arise presumably due to reorientation of the electric dipole present in the polar headgroup of the phospholipid and its resulting effect on the thickness of the phospholipid's hydrocarbon chain layer within the hybrid bilayer's profile structure. The profile structure of the voltage-sensor domain from a voltage-gated ion channel protein within a phospholipid bilayer membrane, tethered to the surface of an inorganic substrate and fully hydrated with a bulk aqueous medium in an electrochemical cell, was also investigated as a function of the applied transmembrane electric potential via time-resolved X-ray reflectivity, enhanced by interferometry. Significant, fully-reversible, and different structural changes in the protein were detected in response to ±100mV potentials with respect to 0mV. The approach employed is that typical of transient spectroscopy, shown here to be applicable to both neutron and X-ray reflectivity of thin films.
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Affiliation(s)
- A Tronin
- Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104
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Gupta S, Dura J, Freites J, Tobias D, Blasie JK. Structural characterization of the voltage-sensor domain and voltage-gated K+-channel proteins vectorially oriented within a single bilayer membrane at the solid/vapor and solid/liquid interfaces via neutron interferometry. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2012; 28:10504-20. [PMID: 22686684 PMCID: PMC3406608 DOI: 10.1021/la301219z] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
The voltage-sensor domain (VSD) is a modular four-helix bundle component that confers voltage sensitivity to voltage-gated cation channels in biological membranes. Despite extensive biophysical studies and the recent availability of X-ray crystal structures for a few voltage-gated potassium (Kv) channels and a voltage-gate sodium (Nav) channel, a complete understanding of the cooperative mechanism of electromechanical coupling, interconverting the closed-to-open states (i.e., nonconducting to cation conducting) remains undetermined. Moreover, the function of these domains is highly dependent on the physical-chemical properties of the surrounding lipid membrane environment. The basis for this work was provided by a recent structural study of the VSD from a prokaryotic Kv-channel vectorially oriented within a single phospholipid (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)) membrane investigated by X-ray interferometry at the solid/moist He (or solid/vapor) and solid/liquid interfaces, thus achieving partial to full hydration, respectively (Gupta et al. Phys. Rev. E2011, 84, 031911-1-15). Here, we utilize neutron interferometry to characterize this system in substantially greater structural detail at the submolecular level, due to its inherent advantages arising from solvent contrast variation coupled with the deuteration of selected submolecular membrane components, especially important for the membrane at the solid/liquid interface. We demonstrate the unique vectorial orientation of the VSD and the retention of its molecular conformation manifest in the asymmetric profile structure of the protein within the profile structure of this single bilayer membrane system. We definitively characterize the asymmetric phospholipid bilayer solvating the lateral surfaces of the VSD protein within the membrane. The profile structures of both the VSD protein and phospholipid bilayer depend upon the hydration state of the membrane. We also determine the distribution of water and exchangeable hydrogen throughout the profile structure of both the VSD itself and the VSD:POPC membrane. These two experimentally determined water and exchangeable hydrogen distribution profiles are in good agreement with molecular dynamics simulations of the VSD protein vectorially oriented within a fully hydrated POPC bilayer membrane, supporting the existence of the VSD's water pore. This approach was extended to the full-length Kv-channel (KvAP) at a solid/liquid interface, providing the separate profile structures of the KvAP protein and the POPC bilayer within the reconstituted KvAP:POPC membrane.
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Affiliation(s)
- S. Gupta
- Department of Chemistry, University of Pennsylvania, 231 S. 34St., Philadelphia, PA 19104
| | - J.A. Dura
- NIST Center for Neutron Research, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899
| | - J.A. Freites
- Department of Chemistry, University of California, Irvine, CA 92697
| | - D.J. Tobias
- Department of Chemistry, University of California, Irvine, CA 92697
| | - J. K. Blasie
- Department of Chemistry, University of Pennsylvania, 231 S. 34St., Philadelphia, PA 19104
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Koo J, Park J, Tronin A, Zhang R, Krishnan V, Strzalka J, Kuzmenko I, Fry HC, Therien MJ, Blasie JK. Acentric 2-D ensembles of D-br-A electron-transfer chromophores via vectorial orientation within amphiphilic n-helix bundle peptides for photovoltaic device applications. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2012; 28:3227-38. [PMID: 22242787 PMCID: PMC3391659 DOI: 10.1021/la205002f] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
We show that simply designed amphiphilic 4-helix bundle peptides can be utilized to vectorially orient a linearly extended donor-bridge-acceptor (D-br-A) electron transfer (ET) chromophore within its core. The bundle's interior is shown to provide a unique solvation environment for the D-br-A assembly not accessible in conventional solvents and thereby control the magnitudes of both light-induced ET and thermal charge recombination rate constants. The amphiphilicity of the bundle's exterior was employed to vectorially orient the peptide-chromophore complex at a liquid-gas interface, and its ends were tailored for subsequent covalent attachment to an inorganic surface, via a "directed assembly" approach. Structural data, combined with evaluation of the excited state dynamics exhibited by these peptide-chromophore complexes, demonstrate that densely packed, acentrically ordered 2-D monolayer ensembles of such complexes at high in-plane chromophore densities approaching 1/200 Å(2) offer unique potential as active layers in binary heterojunction photovoltaic devices.
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Affiliation(s)
- Jaseung Koo
- Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, U.S.A
| | - Jaehong Park
- Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, U.S.A
- Department of Chemistry, Duke University, Durham, NC 27708, U.S.A
| | - Andrey Tronin
- Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, U.S.A
| | - Ruili Zhang
- Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, U.S.A
| | - Venkata Krishnan
- Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, U.S.A
| | - Joseph Strzalka
- X-ray Science Division, Argonne National Laboratory, Argonne, IL 60439, U.S.A
| | - Ivan Kuzmenko
- X-ray Science Division, Argonne National Laboratory, Argonne, IL 60439, U.S.A
| | - H. Christopher Fry
- Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, U.S.A
| | | | - J. Kent Blasie
- Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, U.S.A
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