1
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Licht JA, Berry SP, Gutierrez MA, Gaudet R. They all rock: A systematic comparison of conformational movements in LeuT-fold transporters. bioRxiv 2024:2024.01.24.577062. [PMID: 38352416 PMCID: PMC10862720 DOI: 10.1101/2024.01.24.577062] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Indexed: 02/19/2024]
Abstract
Many membrane transporters share the LeuT fold-two five-helix repeats inverted across the membrane plane. Despite hundreds of structures, whether distinct conformational mechanisms are supported by the LeuT fold has not been systematically determined. After annotating published LeuT-fold structures, we analyzed distance difference matrices (DDMs) for nine proteins with multiple available conformations. We identified rigid bodies and relative movements of transmembrane helices (TMs) during distinct steps of the transport cycle. In all transporters the bundle (first two TMs of each repeat) rotates relative to the hash (third and fourth TMs). Motions of the arms (fifth TM) to close or open the intracellular and outer vestibules are common, as is a TM1a swing, with notable variations in the opening-closing motions of the outer vestibule. Our analyses suggest that LeuT-fold transporters layer distinct motions on a common bundle-hash rock and demonstrate that systematic analyses can provide new insights into large structural datasets.
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Affiliation(s)
- Jacob A. Licht
- Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA, USA
| | - Samuel P. Berry
- Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA, USA
| | - Michael A. Gutierrez
- Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA, USA
- Present address: Novartis Biomedical Research, Cambridge, MA, USA
| | - Rachelle Gaudet
- Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA, USA
- Lead contact
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2
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Kumar S, Mollo A, Rubino FA, Kahne D, Ruiz N. Chloride Ions Are Required for Thermosipho africanus MurJ Function. mBio 2023; 14:e0008923. [PMID: 36752629 PMCID: PMC9973255 DOI: 10.1128/mbio.00089-23] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/12/2023] [Accepted: 01/17/2023] [Indexed: 02/09/2023] Open
Abstract
Most bacteria have a peptidoglycan cell wall that determines their cell shape and helps them resist osmotic lysis. Peptidoglycan synthesis depends on the translocation of the lipid-linked precursor lipid II across the cytoplasmic membrane by the MurJ flippase. Structure-function analyses of MurJ from Thermosipho africanus (MurJTa) and Escherichia coli (MurJEc) have revealed that MurJ adopts multiple conformations and utilizes an alternating-access mechanism to flip lipid II. MurJEc activity relies on membrane potential, but the specific counterion has not been identified. Crystal structures of MurJTa revealed a chloride ion bound to the N-lobe of the flippase and a sodium ion in its C-lobe, but the role of these ions in transport is unknown. Here, we investigated the effect of various ions on the function of MurJTa and MurJEc in vivo. We found that chloride, and not sodium, ions are necessary for MurJTa function, but neither ion is required for MurJEc function. We also showed that murJTa alleles encoding changes at the crystallographically identified sodium-binding site still complement the loss of native murJEc, although they decreased protein stability and/or function. Based on our data and previous work, we propose that chloride ions are necessary for the conformational change that resets MurJTa after lipid II translocation and suggest that MurJ orthologs may function similarly but differ in their requirements for counterions. IMPORTANCE The biosynthetic pathway of the peptidoglycan cell wall is one of the most favorable targets for antibiotic development. Lipid II, the lipid-linked PG precursor, is made in the inner leaflet of the cytoplasmic membrane and then transported by the MurJ flippase so that it can be used to build the peptidoglycan cell wall. MurJ functions using an alternating-access mechanism thought to depend on a yet-to-be-identified counterion. This study fills a gap in our understanding of MurJ's energy-coupling mechanism by showing that chloride ions are required for MurJ in some, but not all, organisms. Based on our data and prior studies, we propose that, while the general transport mechanism of MurJ may be conserved, its specific mechanistic details may differ across bacteria, as is common in transporters. These findings are important to understand MurJ function and its development as an antibiotic target.
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Affiliation(s)
- Sujeet Kumar
- Department of Microbiology, The Ohio State University, Columbus, Ohio, USA
| | - Aurelio Mollo
- Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, USA
| | - Frederick A. Rubino
- Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, USA
| | - Daniel Kahne
- Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, USA
- Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts, USA
- Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts, USA
| | - Natividad Ruiz
- Department of Microbiology, The Ohio State University, Columbus, Ohio, USA
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3
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Abstract
The γ-aminobutyric acid transporter 1 (GAT1) is a transporter which clears the inhibitory neurotransmitter γ-aminobutyric acid (GABA) from the synaptic cleft. The paper by Motiwala et al. documents a structure of GAT1 in complex with the antiepileptic drug tiagabine. This study will enable structure-based docking of large chemical libraries for the discovery of novel antiepileptics.
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Affiliation(s)
- Baruch I Kanner
- Department of Biochemistry and Molecular Biology, Institute for Medical Research Israel-Canada, Hebrew University, Hadassah Medical School, Jerusalem 91120, Israel.
| | - Oshrat Dayan-Alon
- Department of Biochemistry and Molecular Biology, Institute for Medical Research Israel-Canada, Hebrew University, Hadassah Medical School, Jerusalem 91120, Israel
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4
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Jiang X, Yan N, Deng D, Yan C. Structural aspects of the glucose and monocarboxylate transporters involved in the Warburg effect. IUBMB Life 2022; 74:1180-1199. [PMID: 36082803 DOI: 10.1002/iub.2668] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/31/2022] [Accepted: 08/02/2022] [Indexed: 11/11/2022]
Abstract
Cancer cells shift their glucose catabolism from aerobic respiration to lactic fermentation even in the presence of oxygen, and this is known as the "Warburg effect". To accommodate the high glucose demands and to avoid lactate accumulation, the expression levels of human glucose transporters (GLUTs) and human monocarboxylate transporters (MCTs) are elevated to maintain metabolic homeostasis. Therefore, inhibition of GLUTs and/or MCTs provides potential therapeutic strategies for cancer treatment. Here, we summarize recent advances in the structural characterization of GLUTs and MCTs, providing a comprehensive understanding of their transport and inhibition mechanisms to facilitate further development of anticancer therapies.
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Affiliation(s)
- Xin Jiang
- School of Biotechnology and Biomolecular Sciences, The University of New South Wales, Sydney, Australia
| | - Nieng Yan
- Department of Molecular Biology, Princeton University, Princeton, New Jersey, USA
| | - Dong Deng
- Department of Obstetrics, Key Laboratory of Birth Defects and Related Disease of Women and Children of MOE, State Key Laboratory of Biotherapy, West China Second Hospital, Sichuan University, Chengdu, China
| | - Chuangye Yan
- Beijing Frontier Research Center for Biological Structure, Beijing Advanced Innovation Center for Structural Biology, Tsinghua-Peking Center for Life Sciences, School of Life Sciences, Tsinghua University, Beijing, China
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5
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Del Alamo D, Meiler J, Mchaourab HS. Principles of Alternating Access in LeuT-fold Transporters: Commonalities and Divergences. J Mol Biol 2022; 434:167746. [PMID: 35843285 DOI: 10.1016/j.jmb.2022.167746] [Citation(s) in RCA: 8] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/02/2022] [Revised: 07/08/2022] [Accepted: 07/09/2022] [Indexed: 11/15/2022]
Abstract
Found in all domains of life, transporters belonging to the LeuT-fold class mediate the import and exchange of hydrophilic and charged compounds such as amino acids, metals, and sugar molecules. Nearly two decades of investigations on the eponymous bacterial transporter LeuT have yielded a library of high-resolution snapshots of its conformational cycle linked by solution-state experimental data obtained from multiple techniques. In parallel, its topology has been observed in symporters and antiporters characterized by a spectrum of substrate specificities and coupled to gradients of distinct ions. Here we review and compare mechanistic models of transport for LeuT, its well-studied homologs, as well as functionally distant members of the fold, emphasizing the commonalities and divergences in alternating access and the corresponding energy landscapes. Our integrated summary illustrates how fold conservation, a hallmark of the LeuT fold, coincides with divergent choreographies of alternating access that nevertheless capitalize on recurrent structural motifs. In addition, it highlights the knowledge gap that hinders the leveraging of the current body of research into detailed mechanisms of transport for this important class of membrane proteins.
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Affiliation(s)
- Diego Del Alamo
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, TN, USA; Department of Chemistry, Vanderbilt University, Nashville, TN, USA. https://twitter.com/DdelAlamo
| | - Jens Meiler
- Department of Chemistry, Vanderbilt University, Nashville, TN, USA; Institute for Drug Discovery, Leipzig University, Leipzig, DE, USA. https://twitter.com/MeilerLab
| | - Hassane S Mchaourab
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, TN, USA.
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6
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Claxton DP, Jagessar KL, Mchaourab HS. Principles of Alternating Access in Multidrug and Toxin Extrusion (MATE) Transporters. J Mol Biol 2021; 433:166959. [PMID: 33774036 DOI: 10.1016/j.jmb.2021.166959] [Citation(s) in RCA: 10] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2021] [Revised: 03/15/2021] [Accepted: 03/18/2021] [Indexed: 12/18/2022]
Abstract
The multidrug and toxin extrusion (MATE) transporters catalyze active efflux of a broad range of chemically- and structurally-diverse compounds including antimicrobials and chemotherapeutics, thus contributing to multidrug resistance in pathogenic bacteria and cancers. Multiple methodological approaches have been taken to investigate the structural basis of energy transduction and substrate translocation in MATE transporters. Crystal structures representing members from all three MATE subfamilies have been interpreted within the context of an alternating access mechanism that postulates occupation of distinct structural intermediates in a conformational cycle powered by electrochemical ion gradients. Here we review the structural biology of MATE transporters, integrating the crystallographic models with biophysical and computational studies to define the molecular determinants that shape the transport energy landscape. This holistic analysis highlights both shared and disparate structural and functional features within the MATE family, which underpin an emerging theme of mechanistic diversity within the framework of a conserved structural scaffold.
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Affiliation(s)
- Derek P Claxton
- Department of Molecular Physiology and Biophysics, Vanderbilt University, 747 Light Hall, 2215 Garland Avenue, Nashville, TN 37232, USA
| | - Kevin L Jagessar
- Department of Molecular Physiology and Biophysics, Vanderbilt University, 747 Light Hall, 2215 Garland Avenue, Nashville, TN 37232, USA
| | - Hassane S Mchaourab
- Department of Molecular Physiology and Biophysics, Vanderbilt University, 747 Light Hall, 2215 Garland Avenue, Nashville, TN 37232, USA.
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7
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Lu PH, Li CC, Chiang YW, Liu JH, Chiang WT, Chao YH, Li GS, Weng SE, Lin SY, Hu NJ. Dissecting the Conformational Dynamics of the Bile Acid Transporter Homologue ASBT NM. J Mol Biol 2021; 433:166764. [PMID: 33359100 DOI: 10.1016/j.jmb.2020.166764] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/17/2020] [Revised: 12/11/2020] [Accepted: 12/11/2020] [Indexed: 11/21/2022]
Abstract
Apical sodium-dependent bile acid transporter (ASBT) catalyses uphill transport of bile acids using the electrochemical gradient of Na+ as the driving force. The crystal structures of two bacterial homologues ASBTNM and ASBTYf have previously been determined, with the former showing an inward-facing conformation, and the latter adopting an outward-facing conformation accomplished by the substitution of the critical Na+-binding residue glutamate-254 with an alanine residue. While the two crystal structures suggested an elevator-like movement to afford alternating access to the substrate binding site, the mechanistic role of Na+ and substrate in the conformational isomerization remains unclear. In this study, we utilized site-directed alkylation monitored by in-gel fluorescence (SDAF) to probe the solvent accessibility of the residues lining the substrate permeation pathway of ASBTNM under different Na+ and substrate conditions, and interpreted the conformational states inferred from the crystal structures. Unexpectedly, the crosslinking experiments demonstrated that ASBTNM is a monomer protein, unlike the other elevator-type transporters, usually forming a homodimer or a homotrimer. The conformational dynamics observed by the biochemical experiments were further validated using DEER measuring the distance between the spin-labelled pairs. Our results revealed that Na+ ions shift the conformational equilibrium of ASBTNM toward the inward-facing state thereby facilitating cytoplasmic uptake of substrate. The current findings provide a novel perspective on the conformational equilibrium of secondary active transporters.
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8
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Wang N, Jiang X, Zhang S, Zhu A, Yuan Y, Xu H, Lei J, Yan C. Structural basis of human monocarboxylate transporter 1 inhibition by anti-cancer drug candidates. Cell 2020; 184:370-383.e13. [PMID: 33333023 DOI: 10.1016/j.cell.2020.11.043] [Citation(s) in RCA: 117] [Impact Index Per Article: 29.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2020] [Revised: 10/16/2020] [Accepted: 11/23/2020] [Indexed: 12/20/2022]
Abstract
Proton-coupled monocarboxylate transporters MCT1-4 catalyze the transmembrane movement of metabolically essential monocarboxylates and have been targeted for cancer treatment because of their enhanced expression in various tumors. Here, we report five cryo-EM structures, at resolutions of 3.0-3.3 Å, of human MCT1 bound to lactate or inhibitors in the presence of Basigin-2, a single transmembrane segment (TM)-containing chaperon. MCT1 exhibits similar outward-open conformations when complexed with lactate or the inhibitors BAY-8002 and AZD3965. In the presence of the inhibitor 7ACC2 or with the neutralization of the proton-coupling residue Asp309 by Asn, similar inward-open structures were captured. Complemented by structural-guided biochemical analyses, our studies reveal the substrate binding and transport mechanism of MCTs, elucidate the mode of action of three anti-cancer drug candidates, and identify the determinants for subtype-specific sensitivities to AZD3965 by MCT1 and MCT4. These findings lay out an important framework for structure-guided drug discovery targeting MCTs.
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Affiliation(s)
- Nan Wang
- State Key Laboratory of Membrane Biology, Beijing Advanced Innovation Center for Structural Biology, Tsinghua-Peking Center for Life Sciences, School of Life Sciences, Tsinghua University, Beijing 100084, China
| | - Xin Jiang
- School of Biotechnology and Biomolecular Sciences, the University of New South Wales, Sydney, NSW 2052, Australia; Department of Molecular Biology, Princeton University, Princeton, NJ 08544, USA.
| | - Shuo Zhang
- State Key Laboratory of Membrane Biology, Beijing Advanced Innovation Center for Structural Biology, Tsinghua-Peking Center for Life Sciences, School of Life Sciences, Tsinghua University, Beijing 100084, China
| | - Angqi Zhu
- State Key Laboratory of Membrane Biology, Beijing Advanced Innovation Center for Structural Biology, Tsinghua-Peking Center for Life Sciences, School of Life Sciences, Tsinghua University, Beijing 100084, China
| | - Yafei Yuan
- State Key Laboratory of Membrane Biology, Beijing Advanced Innovation Center for Structural Biology, Tsinghua-Peking Center for Life Sciences, School of Life Sciences, Tsinghua University, Beijing 100084, China
| | - Hanwen Xu
- State Key Laboratory of Membrane Biology, Beijing Advanced Innovation Center for Structural Biology, Tsinghua-Peking Center for Life Sciences, School of Life Sciences, Tsinghua University, Beijing 100084, China
| | - Jianlin Lei
- Technology Center for Protein Sciences, Ministry of Education Key Laboratory of Protein Sciences, School of Life Sciences, Tsinghua University, Beijing 100084, China
| | - Chuangye Yan
- State Key Laboratory of Membrane Biology, Beijing Advanced Innovation Center for Structural Biology, Tsinghua-Peking Center for Life Sciences, School of Life Sciences, Tsinghua University, Beijing 100084, China.
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9
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Ponzoni L, Zhang S, Cheng MH, Bahar I. Shared dynamics of LeuT superfamily members and allosteric differentiation by structural irregularities and multimerization. Philos Trans R Soc Lond B Biol Sci 2019; 373:rstb.2017.0177. [PMID: 29735731 PMCID: PMC5941172 DOI: 10.1098/rstb.2017.0177] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 12/19/2017] [Indexed: 12/14/2022] Open
Abstract
The LeuT-fold superfamily includes secondary active transporters from different functional families, which share a common tertiary structure, despite having a remarkably low sequence similarity. By identifying the common structural and dynamical features upon principal component analysis of a comprehensive ensemble of 90 experimentally resolved structures and anisotropic network model evaluation of collective motions, we provide a unified point of view for understanding the reasons why this particular fold has been selected by evolution to accomplish such a broad spectrum of functions. The parallel identification of conserved sequence features, localized at specific sites of transmembrane helices, sheds light on the role of broken helices (TM1 and TM6 in LeuT) in promoting ion/substrate binding and allosteric interconversion between the outward- and inward-facing conformations of transporters. Finally, the determination of the dynamics landscape for the structural ensemble provides a promising framework for the classification of transporters based on their dynamics, and the characterization of the collective movements that favour multimerization.This article is part of a discussion meeting issue 'Allostery and molecular machines'.
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Affiliation(s)
- Luca Ponzoni
- Department of Computational and Systems Biology, School of Medicine, University of Pittsburgh, Pittsburgh, PA 15213, USA
| | - She Zhang
- Department of Computational and Systems Biology, School of Medicine, University of Pittsburgh, Pittsburgh, PA 15213, USA
| | - Mary Hongying Cheng
- Department of Computational and Systems Biology, School of Medicine, University of Pittsburgh, Pittsburgh, PA 15213, USA
| | - Ivet Bahar
- Department of Computational and Systems Biology, School of Medicine, University of Pittsburgh, Pittsburgh, PA 15213, USA
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10
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Giladi M, Shor R, Lisnyansky M, Khananshvili D. Structure-Functional Basis of Ion Transport in Sodium-Calcium Exchanger (NCX) Proteins. Int J Mol Sci 2016; 17:E1949. [PMID: 27879668 DOI: 10.3390/ijms17111949] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2016] [Revised: 11/13/2016] [Accepted: 11/14/2016] [Indexed: 01/19/2023] Open
Abstract
The membrane-bound sodium–calcium exchanger (NCX) proteins shape Ca2+ homeostasis in many cell types, thus participating in a wide range of physiological and pathological processes. Determination of the crystal structure of an archaeal NCX (NCX_Mj) paved the way for a thorough and systematic investigation of ion transport mechanisms in NCX proteins. Here, we review the data gathered from the X-ray crystallography, molecular dynamics simulations, hydrogen–deuterium exchange mass-spectrometry (HDX-MS), and ion-flux analyses of mutants. Strikingly, the apo NCX_Mj protein exhibits characteristic patterns in the local backbone dynamics at particular helix segments, thereby possessing characteristic HDX profiles, suggesting structure-dynamic preorganization (geometric arrangements of catalytic residues before the transition state) of conserved α1 and α2 repeats at ion-coordinating residues involved in transport activities. Moreover, dynamic preorganization of local structural entities in the apo protein predefines the status of ion-occlusion and transition states, even though Na+ or Ca2+ binding modifies the preceding backbone dynamics nearby functionally important residues. Future challenges include resolving the structural-dynamic determinants governing the ion selectivity, functional asymmetry and ion-induced alternating access. Taking into account the structural similarities of NCX_Mj with the other proteins belonging to the Ca2+/cation exchanger superfamily, the recent findings can significantly improve our understanding of ion transport mechanisms in NCX and similar proteins.
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11
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Abstract
The Major Facilitator Superfamily (MFS) is one of the largest classes of secondary active transporters and is widely expressed in many domains of life. It is characterized by a common 12-transmembrane helix motif that allows the selective transport of a vast range of diverse substrates across the membrane. MFS transporters play a central role in many physiological processes and are increasingly recognized as potential drug targets. Despite intensive efforts, there are still only a handful of crystal structures and therefore homology modeling is likely to be a necessary process for providing models to interpret experiments for many years to come. However, the diversity of sequences and the multiple conformational states these proteins can exist in makes the process significantly more complicated, especially for sequences for which there is very little sequence identity to known templates. Inspired by the approach adopted many years ago for GPCRs, we have analyzed the large number of MFS sequences now available alongside the current structural information to propose a series of conserved contact points that can provide additional guidance for the homology modeling process. To enable cross-comparison across MFS models we also present a numbering scheme that can be used to provide a point of reference within each of the 12 transmembrane regions.
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Affiliation(s)
- Joanna Lee
- Department of Biochemistry, University of Oxford Oxford, UK
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12
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Abstract
The ancient and ubiquitous major facilitator superfamily (MFS) represents the largest secondary transporter family and plays a crucial role in a multitude of physiological processes. MFS proteins transport a broad spectrum of ions and solutes across membranes via facilitated diffusion, symport, or antiport. In recent years, remarkable advances in understanding the structural biology of the MFS transporters have been made. This article reviews the history, classification, and general features of the MFS proteins; summarizes recent structural progress with a focus on the sugar porter family transporters exemplified by GLUT1; and discusses the molecular mechanisms of substrate binding, alternating access, and cotransport coupling.
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Affiliation(s)
- Nieng Yan
- State Key Laboratory of Bio-membrane and Membrane Biotechnology, Center for Structural Biology, School of Medicine, Tsinghua-Peking Joint Center for Life Sciences, Tsinghua University, Beijing 100084, China;
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13
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Abstract
Symmetry is a common feature among natural systems, including protein structures. A strong propensity toward symmetric architectures has long been recognized for water-soluble proteins, and this propensity has been rationalized from an evolutionary standpoint. Proteins residing in cellular membranes, however, have traditionally been less amenable to structural studies, and thus the prevalence and significance of symmetry in this important class of molecules is not as well understood. In the past two decades, researchers have made great strides in this area, and these advances have provided exciting insights into the range of architectures adopted by membrane proteins. These structural studies have revealed a similarly strong bias toward symmetric arrangements, which were often unexpected and which occurred despite the restrictions imposed by the membrane environment on the possible symmetry groups. Moreover, membrane proteins disproportionately contain internal structural repeats resulting from duplication and fusion of smaller segments. This article discusses the types and origins of symmetry in membrane proteins and the implications of symmetry for protein function.
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Affiliation(s)
- Lucy R Forrest
- Computational Structural Biology Group, Porter Neuroscience Center, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20852;
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14
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Deng D, Yan N. GLUT, SGLT, and SWEET: Structural and mechanistic investigations of the glucose transporters. Protein Sci 2016; 25:546-58. [PMID: 26650681 DOI: 10.1002/pro.2858] [Citation(s) in RCA: 172] [Impact Index Per Article: 21.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/15/2015] [Revised: 11/20/2015] [Accepted: 12/01/2015] [Indexed: 12/12/2022]
Abstract
Glucose is the primary fuel to life on earth. Cellular uptake of glucose is a fundamental process for metabolism, growth, and homeostasis. Three families of secondary glucose transporters have been identified in human, including the major facilitator superfamily glucose facilitators GLUTs, the sodium-driven glucose symporters SGLTs, and the recently identified SWEETs. Structures of representative members or their prokaryotic homologs of all three families were obtained. This review focuses on the recent advances in the structural elucidation of the glucose transporters and the mechanistic insights derived from these structures, including the molecular basis for substrate recognition, alternating access, and stoichiometric coupling of co-transport.
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Affiliation(s)
- Dong Deng
- State Key Laboratory of Bio-Membrane and Membrane Biotechnology, Center for Structural Biology, Tsinghua-Peking Center for Life Sciences, School of Life Sciences and School of Medicine, Tsinghua University, Beijing, 100084, China
| | - Nieng Yan
- State Key Laboratory of Bio-Membrane and Membrane Biotechnology, Center for Structural Biology, Tsinghua-Peking Center for Life Sciences, School of Life Sciences and School of Medicine, Tsinghua University, Beijing, 100084, China
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15
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Tavoulari S, Margheritis E, Nagarajan A, DeWitt DC, Zhang YW, Rosado E, Ravera S, Rhoades E, Forrest LR, Rudnick G. Two Na+ Sites Control Conformational Change in a Neurotransmitter Transporter Homolog. J Biol Chem 2015; 291:1456-71. [PMID: 26582198 DOI: 10.1074/jbc.m115.692012] [Citation(s) in RCA: 50] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/11/2015] [Indexed: 12/13/2022] Open
Abstract
In LeuT, a prokaryotic homolog of neurotransmitter transporters, Na(+) stabilizes outward-open conformational states. We examined how each of the two LeuT Na(+) binding sites contributes to Na(+)-dependent closure of the cytoplasmic pathway using biochemical and biophysical assays of conformation. Mutating either of two residues that contribute to the Na2 site completely prevented cytoplasmic closure in response to Na(+), suggesting that Na2 is essential for this conformational change, whereas Na1 mutants retained Na(+) responsiveness. However, mutation of Na1 residues also influenced the Na(+)-dependent conformational change in ways that varied depending on the position mutated. Computational analyses suggest those mutants influence the ability of Na1 binding to hydrate the substrate pathway and perturb an interaction network leading to the extracellular gate. Overall, the results demonstrate that occupation of Na2 stabilizes outward-facing conformations presumably through a direct interaction between Na(+) and transmembrane helices 1 and 8, whereas Na(+) binding at Na1 influences conformational change through a network of intermediary interactions. The results also provide evidence that N-terminal release and helix motions represent distinct steps in cytoplasmic pathway opening.
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Affiliation(s)
| | | | - Anu Nagarajan
- the Computational Structural Biology Section, NINDS, National Institutes of Health, Rockville, Maryland 20852
| | - David C DeWitt
- Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520 and
| | | | | | | | - Elizabeth Rhoades
- Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520 and
| | - Lucy R Forrest
- the Computational Structural Biology Section, NINDS, National Institutes of Health, Rockville, Maryland 20852
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16
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Khelashvili G, Stanley N, Sahai MA, Medina J, LeVine MV, Shi L, De Fabritiis G, Weinstein H. Spontaneous inward opening of the dopamine transporter is triggered by PIP2-regulated dynamics of the N-terminus. ACS Chem Neurosci 2015; 6:1825-37. [PMID: 26255829 PMCID: PMC4653762 DOI: 10.1021/acschemneuro.5b00179] [Citation(s) in RCA: 81] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022] Open
Abstract
![]()
We
present the dynamic mechanism of concerted motions in a full-length
molecular model of the human dopamine transporter (hDAT), a member
of the neurotransmitter/sodium symporter (NSS) family, involved in
state-to-state transitions underlying function. The findings result
from an analysis of unbiased atomistic molecular dynamics simulation
trajectories (totaling >14 μs) of the hDAT molecule immersed
in lipid membrane environments with or without phosphatidylinositol
4,5-biphosphate (PIP2) lipids. The N-terminal region of
hDAT (N-term) is shown to have an essential mechanistic role in correlated
rearrangements of specific structural motifs relevant to state-to-state
transitions in the hDAT. The mechanism involves PIP2-mediated
electrostatic interactions between the N-term and the intracellular
loops of the transporter molecule. Quantitative analyses of collective
motions in the trajectories reveal that these interactions correlate
with the inward-opening dynamics of hDAT and are allosterically coupled
to the known functional sites of the transporter. The observed large-scale
motions are enabled by specific reconfiguration of the network of
ionic interactions at the intracellular end of the protein. The isomerization
to the inward-facing state in hDAT is accompanied by concomitant movements
in the extracellular vestibule and results in the release of an Na+ ion from the Na2 site and destabilization of the substrate
dopamine in the primary substrate binding S1 site. The dynamic mechanism
emerging from the findings highlights the involvement of the PIP2-regulated interactions between the N-term and the intracellular
loop 4 in the functionally relevant conformational transitions that
are also similar to those found to underlie state-to-state transitions
in the leucine transporter (LeuT), a prototypical bacterial homologue
of the NSS.
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Affiliation(s)
- George Khelashvili
- Department
of Physiology and Biophysics, Weill Cornell Medical College of Cornell University (WCMC), New York, New York 10065, United States
| | - Nathaniel Stanley
- Computational
Biophysics Laboratory (GRIB-IMIM), Universitat Pompeu Fabra, Barcelona
Biomedical Research Park (PRBB), C/Doctor Aiguader 88, 08003 Barcelona, Spain
| | - Michelle A. Sahai
- Department
of Physiology and Biophysics, Weill Cornell Medical College of Cornell University (WCMC), New York, New York 10065, United States
| | - Jaime Medina
- Department
of Physiology and Biophysics, Weill Cornell Medical College of Cornell University (WCMC), New York, New York 10065, United States
| | - Michael V. LeVine
- Department
of Physiology and Biophysics, Weill Cornell Medical College of Cornell University (WCMC), New York, New York 10065, United States
| | - Lei Shi
- Department
of Physiology and Biophysics, Weill Cornell Medical College of Cornell University (WCMC), New York, New York 10065, United States
- HRH
Prince Alwaleed Bin Talal Bin Abdulaziz Alsaud Institute of Computational
Biomedicine, Weill Cornell Medical College of Cornell University, New York, New York 10065, United States
| | - Gianni De Fabritiis
- Computational
Biophysics Laboratory (GRIB-IMIM), Universitat Pompeu Fabra, Barcelona
Biomedical Research Park (PRBB), C/Doctor Aiguader 88, 08003 Barcelona, Spain
- Institució Catalana de Recerca i Estudis Avançats, Passeig Lluis Companys 23, 08010 Barcelona, Spain
| | - Harel Weinstein
- Department
of Physiology and Biophysics, Weill Cornell Medical College of Cornell University (WCMC), New York, New York 10065, United States
- HRH
Prince Alwaleed Bin Talal Bin Abdulaziz Alsaud Institute of Computational
Biomedicine, Weill Cornell Medical College of Cornell University, New York, New York 10065, United States
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17
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Smirnova I, Kasho V, Jiang X, Pardon E, Steyaert J, Kaback HR. Transient conformers of LacY are trapped by nanobodies. Proc Natl Acad Sci U S A 2015; 112:13839-44. [PMID: 26512108 DOI: 10.1073/pnas.1519485112] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The lactose permease of Escherichia coli (LacY), a highly dynamic membrane protein, catalyzes symport of a galactopyranoside and an H(+) by using an alternating access mechanism, and the transport cycle involves multiple conformational states. Single-domain camelid nanobodies (Nbs) developed against a LacY mutant immobilized in an outward (periplasmic)-open conformation bind to the flexible WT protein and stabilize the open-outward conformation(s). Here, we use site-directed, distance-dependent Trp quenching/unquenching of fluorescent probes inserted on opposite surfaces of LacY to assess the conformational states of the protein complexed with each of eight unique Nbs that bind exclusively to the periplasmic side and block transport, but increase the accessibility of the sugar-binding site. Nb binding involves conformational selection of LacY molecules with exposed binding epitopes. Each of eight Nbs induces quenching with three pairs of cytoplasmic Trp/fluorophore probes, indicating closing of cytoplasmic cavity. In reciprocal fashion, the same Nbs induce unquenching of fluorescence in three pairs of periplasmic probes due to opening of the periplasmic cavity. Because the extent of fluorescence change with various Nbs differs and the differences correlate with changes in the rate of sugar binding, it is also concluded that the Nbs stabilize several different outward-open conformations of LacY.
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18
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Vergara-Jaque A, Fenollar-Ferrer C, Kaufmann D, Forrest LR. Repeat-swap homology modeling of secondary active transporters: updated protocol and prediction of elevator-type mechanisms. Front Pharmacol 2015; 6:183. [PMID: 26388773 PMCID: PMC4560100 DOI: 10.3389/fphar.2015.00183] [Citation(s) in RCA: 38] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/03/2015] [Accepted: 08/13/2015] [Indexed: 12/04/2022] Open
Abstract
Secondary active transporters are critical for neurotransmitter clearance and recycling during synaptic transmission and uptake of nutrients. These proteins mediate the movement of solutes against their concentration gradients, by using the energy released in the movement of ions down pre-existing concentration gradients. To achieve this, transporters conform to the so-called alternating-access hypothesis, whereby the protein adopts at least two conformations in which the substrate binding sites are exposed to one or other side of the membrane, but not both simultaneously. Structures of a bacterial homolog of neuronal glutamate transporters, GltPh, in several different conformational states have revealed that the protein structure is asymmetric in the outward- and inward-open states, and that the conformational change connecting them involves a elevator-like movement of a substrate binding domain across the membrane. The structural asymmetry is created by inverted-topology repeats, i.e., structural repeats with similar overall folds whose transmembrane topologies are related to each other by two-fold pseudo-symmetry around an axis parallel to the membrane plane. Inverted repeats have been found in around three-quarters of secondary transporter folds. Moreover, the (a)symmetry of these systems has been successfully used as a bioinformatic tool, called “repeat-swap modeling” to predict structural models of a transporter in one conformation using the known structure of the transporter in the complementary conformation as a template. Here, we describe an updated repeat-swap homology modeling protocol, and calibrate the accuracy of the method using GltPh, for which both inward- and outward-facing conformations are known. We then apply this repeat-swap homology modeling procedure to a concentrative nucleoside transporter, VcCNT, which has a three-dimensional arrangement related to that of GltPh. The repeat-swapped model of VcCNT predicts that nucleoside transport also occurs via an elevator-like mechanism.
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Affiliation(s)
- Ariela Vergara-Jaque
- Computational Structural Biology Section, Porter Neuroscience Research Center, National Institute of Neurological Disorders and Stroke - National Institutes of Health, Bethesda, MD USA
| | - Cristina Fenollar-Ferrer
- Computational Structural Biology Section, Porter Neuroscience Research Center, National Institute of Neurological Disorders and Stroke - National Institutes of Health, Bethesda, MD USA
| | - Desirée Kaufmann
- Computational Structural Biology Section, Porter Neuroscience Research Center, National Institute of Neurological Disorders and Stroke - National Institutes of Health, Bethesda, MD USA
| | - Lucy R Forrest
- Computational Structural Biology Section, Porter Neuroscience Research Center, National Institute of Neurological Disorders and Stroke - National Institutes of Health, Bethesda, MD USA
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19
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Yan N. Structural Biology of the Major Facilitator Superfamily Transporters. Annu Rev Biophys 2015. [PMID: 26098515 DOI: 10.1146/annurev-biophys-060414-0-33901] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/02/2023]
Abstract
The ancient and ubiquitous major facilitator superfamily (MFS) represents the largest secondary transporter family and plays a crucial role in a multitude of physiological processes. MFS proteins transport a broad spectrum of ions and solutes across membranes via facilitated diffusion, symport, or antiport. In recent years, remarkable advances in understanding the structural biology of the MFS transporters have been made. This article reviews the history, classification, and general features of the MFS proteins; summarizes recent structural progress with a focus on the sugar porter family transporters exemplified by GLUT1; and discusses the molecular mechanisms of substrate binding, alternating access, and cotransport coupling.
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Affiliation(s)
- Nieng Yan
- State Key Laboratory of Bio-membrane and Membrane Biotechnology, Center for Structural Biology, School of Medicine, Tsinghua-Peking Joint Center for Life Sciences, Tsinghua University, Beijing 100084, China;
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20
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Smirnova I, Kasho V, Sugihara J, Kaback HR. Trp replacements for tightly interacting Gly-Gly pairs in LacY stabilize an outward-facing conformation. Proc Natl Acad Sci U S A 2013; 110:8876-81. [PMID: 23671103 DOI: 10.1073/pnas.1306849110] [Citation(s) in RCA: 37] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Trp replacements for conserved Gly-Gly pairs between the N- and C-terminal six-helix bundles on the periplasmic side of lactose permease (LacY) cause complete loss of transport activity with little or no effect on sugar binding. Moreover, the detergent-solubilized mutants exhibit much greater thermal stability than WT LacY. A Cys replacement for Asn245, which is inaccessible/unreactive in WT LacY, alkylates readily in the Gly→Trp mutants, indicating that the periplasmic cavity is patent. Stopped-flow kinetic measurements of sugar binding with the Gly→Trp mutants in detergent reveal linear dependence of binding rates on sugar concentration, as observed with WT or the C154G mutant of LacY, and are compatible with free access to the sugar-binding site in the middle of the molecule. Remarkably, after reconstitution of the Gly→Trp mutants into proteoliposomes, the concentration dependence of sugar-binding rates increases sharply with even faster rates than measured in detergent. Such behavior is strikingly different from that observed for reconstituted WT LacY, in which sugar-binding rates are independent of sugar concentration because opening of the periplasmic cavity is limiting for sugar binding. The observations clearly indicate that Gly→Trp replacements, which introduce bulky residues into tight Gly-Gly interdomain interactions on the periplasmic side of LacY, prevent closure of the periplasmic cavity and, as a result, shift the distribution of LacY toward an outward-open conformation.
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21
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Weyand S, Shimamura T, Beckstein O, Sansom MSP, Iwata S, Henderson PJF, Cameron AD. The alternating access mechanism of transport as observed in the sodium-hydantoin transporter Mhp1. J Synchrotron Radiat 2011; 18:20-3. [PMID: 21169684 PMCID: PMC3004247 DOI: 10.1107/s0909049510032449] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/30/2010] [Accepted: 08/01/2010] [Indexed: 05/21/2023]
Abstract
Secondary active transporters move molecules across cell membranes by coupling this process to the energetically favourable downhill movement of ions or protons along an electrochemical gradient. They function by the alternating access model of transport in which, through conformational changes, the substrate binding site alternately faces either side of the membrane. Owing to the difficulties in obtaining the crystal structure of a single transporter in different conformational states, relatively little structural information is known to explain how this process occurs. Here, the structure of the sodium-benzylhydantoin transporter, Mhp1, from Microbacterium liquefaciens, has been determined in three conformational states; from this a mechanism is proposed for switching from the outward-facing open conformation through an occluded structure to the inward-facing open state.
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Affiliation(s)
- Simone Weyand
- Division of Molecular Biosciences, Membrane Protein Crystallography Group, Imperial College, London SW7 2AZ, UK
- Japan Science and Technology Agency, Exploratory Research for Advanced Technology, Human Receptor Crystallography Project, Yoshida-Konoe-cho, Sakyo-ku, Kyoto 606-8501, Japan
- Membrane Protein Laboratory, Diamond Light Source, Harwell Science and Innovation Campus, Chilton, Didcot, Oxfordshire OX11 0DE, UK
| | - Tatsuro Shimamura
- Division of Molecular Biosciences, Membrane Protein Crystallography Group, Imperial College, London SW7 2AZ, UK
- Japan Science and Technology Agency, Exploratory Research for Advanced Technology, Human Receptor Crystallography Project, Yoshida-Konoe-cho, Sakyo-ku, Kyoto 606-8501, Japan
- Department of Cell Biology, Graduate School of Medicine, Kyoto University, Yoshida-Konoe, Sakyo-Ku, Kyoto 606-8501, Japan
| | - Oliver Beckstein
- Department of Biochemistry, University of Oxford, Oxford OX1 3QU, UK
| | - Mark S. P. Sansom
- Department of Biochemistry, University of Oxford, Oxford OX1 3QU, UK
| | - So Iwata
- Division of Molecular Biosciences, Membrane Protein Crystallography Group, Imperial College, London SW7 2AZ, UK
- Japan Science and Technology Agency, Exploratory Research for Advanced Technology, Human Receptor Crystallography Project, Yoshida-Konoe-cho, Sakyo-ku, Kyoto 606-8501, Japan
- Membrane Protein Laboratory, Diamond Light Source, Harwell Science and Innovation Campus, Chilton, Didcot, Oxfordshire OX11 0DE, UK
- Department of Cell Biology, Graduate School of Medicine, Kyoto University, Yoshida-Konoe, Sakyo-Ku, Kyoto 606-8501, Japan
- Systems and Structural Biology Center, RIKEN, 1-7-22 Suehiro-cho, Tsurumi-ku, Yokohama 230-0045, Japan
| | - Peter J. F. Henderson
- Astbury Centre for Structural Molecular Biology, Institute for Membrane and Systems Biology, University of Leeds, Leeds LS2 9JT, UK
| | - Alexander D. Cameron
- Division of Molecular Biosciences, Membrane Protein Crystallography Group, Imperial College, London SW7 2AZ, UK
- Japan Science and Technology Agency, Exploratory Research for Advanced Technology, Human Receptor Crystallography Project, Yoshida-Konoe-cho, Sakyo-ku, Kyoto 606-8501, Japan
- Membrane Protein Laboratory, Diamond Light Source, Harwell Science and Innovation Campus, Chilton, Didcot, Oxfordshire OX11 0DE, UK
- Correspondence e-mail:
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