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Hariharan P, Bakhtiiari A, Liang R, Guan L. Distinct roles of the major binding residues in the cation-binding pocket of the melibiose transporter MelB. J Biol Chem 2024; 300:107427. [PMID: 38823641 PMCID: PMC11259710 DOI: 10.1016/j.jbc.2024.107427] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/09/2024] [Revised: 05/11/2024] [Accepted: 05/22/2024] [Indexed: 06/03/2024] Open
Abstract
Salmonella enterica serovar Typhimurium melibiose permease (MelBSt) is a prototype of the major facilitator superfamily (MFS) transporters, which play important roles in human health and diseases. MelBSt catalyzed the symport of galactosides with Na+, Li+, or H+ but prefers the coupling with Na+. Previously, we determined the structures of the inward- and outward-facing conformation of MelBSt and the molecular recognition for galactoside and Na+. However, the molecular mechanisms for H+- and Na+-coupled symport remain poorly understood. In this study, we solved two x-ray crystal structures of MelBSt, the cation-binding site mutants D59C at an unliganded apo-state and D55C at a ligand-bound state, and both structures display the outward-facing conformations virtually identical as published. We determined the energetic contributions of three major Na+-binding residues for the selection of Na+ and H+ by free energy simulations. Transport assays showed that the D55C mutant converted MelBSt to a solely H+-coupled symporter, and together with the free-energy perturbation calculation, Asp59 is affirmed to be the sole protonation site of MelBSt. Unexpectedly, the H+-coupled melibiose transport exhibited poor activities at greater bulky ΔpH and better activities at reversal ΔpH, supporting the novel theory of transmembrane-electrostatically localized protons and the associated membrane potential as the primary driving force for the H+-coupled symport mediated by MelBSt. This integrated study of crystal structure, bioenergetics, and free energy simulations, demonstrated the distinct roles of the major binding residues in the cation-binding pocket of MelBSt.
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Affiliation(s)
- Parameswaran Hariharan
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas, USA
| | | | - Ruibin Liang
- Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas, USA.
| | - Lan Guan
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas, USA.
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2
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Hariharan P, Guan L. Cooperative binding ensures the obligatory melibiose/Na+ cotransport in MelB. J Gen Physiol 2021; 153:212278. [PMID: 34110360 PMCID: PMC8200842 DOI: 10.1085/jgp.202012710] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/18/2020] [Revised: 04/07/2021] [Accepted: 05/14/2021] [Indexed: 11/20/2022] Open
Abstract
MelB catalyzes the obligatory cotransport of melibiose with Na+, Li+, or H+. Crystal structure determination of the Salmonella typhimurium MelB (MelBSt) has revealed a typical major facilitator superfamily (MFS) fold at a periplasmic open conformation. Cooperative binding of Na+ and melibiose has been previously established. To determine why cotranslocation of sugar solute and cation is obligatory, we analyzed each binding in the thermodynamic cycle using three independent methods, including the determination of melting temperature by circular dichroism spectroscopy, heat capacity change (ΔCp), and regulatory phosphotransferase EIIAGlc binding with isothermal titration calorimetry (ITC). We found that MelBSt thermostability is increased by either substrate (Na+ or melibiose) and observed a cooperative effect of both substrates. ITC measurements showed that either binary formation yields a positive sign in the ΔCp, suggesting MelBSt hydration and a likely widening of the periplasmic cavity. Conversely, formation of a ternary complex yields negative values in ΔCp, suggesting MelBSt dehydration and cavity closure. Lastly, we observed that EIIAGlc, which has been suggested to trap MelBSt at an outward-open state, readily binds to the MelBSt apo state at an affinity similar to MelBSt/Na+. However, it has a suboptimal binding to the ternary state, implying that MelBSt in the ternary complex may be conformationally distant from the EIIAGlc-preferred outward-facing conformation. Our results consistently support the notion that binding of one substrate (Na+ or melibiose) favors MelBSt at open states, whereas the cooperative binding of both substrates triggers the alternating-access process, thus suggesting this conformational regulation could ensure the obligatory cotransport.
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Affiliation(s)
- Parameswaran Hariharan
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, TX
| | - Lan Guan
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, TX
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3
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Drew D, North RA, Nagarathinam K, Tanabe M. Structures and General Transport Mechanisms by the Major Facilitator Superfamily (MFS). Chem Rev 2021; 121:5289-5335. [PMID: 33886296 PMCID: PMC8154325 DOI: 10.1021/acs.chemrev.0c00983] [Citation(s) in RCA: 168] [Impact Index Per Article: 56.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/09/2020] [Indexed: 12/12/2022]
Abstract
The major facilitator superfamily (MFS) is the largest known superfamily of secondary active transporters. MFS transporters are responsible for transporting a broad spectrum of substrates, either down their concentration gradient or uphill using the energy stored in the electrochemical gradients. Over the last 10 years, more than a hundred different MFS transporter structures covering close to 40 members have provided an atomic framework for piecing together the molecular basis of their transport cycles. Here, we summarize the remarkable promiscuity of MFS members in terms of substrate recognition and proton coupling as well as the intricate gating mechanisms undergone in achieving substrate translocation. We outline studies that show how residues far from the substrate binding site can be just as important for fine-tuning substrate recognition and specificity as those residues directly coordinating the substrate, and how a number of MFS transporters have evolved to form unique complexes with chaperone and signaling functions. Through a deeper mechanistic description of glucose (GLUT) transporters and multidrug resistance (MDR) antiporters, we outline novel refinements to the rocker-switch alternating-access model, such as a latch mechanism for proton-coupled monosaccharide transport. We emphasize that a full understanding of transport requires an elucidation of MFS transporter dynamics, energy landscapes, and the determination of how rate transitions are modulated by lipids.
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Affiliation(s)
- David Drew
- Department
of Biochemistry and Biophysics, Stockholm
University, SE 106 91 Stockholm, Sweden
| | - Rachel A. North
- Department
of Biochemistry and Biophysics, Stockholm
University, SE 106 91 Stockholm, Sweden
| | - Kumar Nagarathinam
- Center
of Structural and Cell Biology in Medicine, Institute of Biochemistry, University of Lübeck, D-23538, Lübeck, Germany
| | - Mikio Tanabe
- Structural
Biology Research Center, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Oho 1-1, Tsukuba, Ibaraki 305-0801, Japan
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4
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Wang LY, Ravi VM, Leblanc G, Padrós E, Cladera J, Perálvarez-Marín A. Helical unwinding and side-chain unlocking unravel the outward open conformation of the melibiose transporter. Sci Rep 2016; 6:33776. [PMID: 27658476 PMCID: PMC5034317 DOI: 10.1038/srep33776] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/17/2015] [Accepted: 08/30/2016] [Indexed: 11/09/2022] Open
Abstract
Molecular dynamics simulations have been used to study the alternate access mechanism of the melibiose transporter from Escherichia coli. Starting from the outward-facing partially occluded form, 2 out of 12 simulations produced an outward full open form and one partially open, whereas the rest yielded fully or partially occluded forms. The shape of the outward-open form resembles other outward-open conformations of secondary transporters. During the transporter opening, conformational changes in some loops are followed by changes in the periplasm region of transmembrane helix 7. Helical curvature relaxation and unlocking of hydrophobic and ionic locks promote the outward opening of the transporter making accessible the substrate binding site. In particular, FRET studies on mutants of conserved aromatic residues of extracellular loop 4 showed lack of substrate binding, emphasizing the importance of this loop for making crucial interactions that control the opening of the periplasmic side. This study indicates that the alternate access mechanism for the melibiose transporter fits better into a flexible gating mechanism rather than the archetypical helical rigid-body rocker-switch mechanism.
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Affiliation(s)
- Li-Ying Wang
- Unitat de Biofísica, Departament de Bioquímica i de Biologia Molecular, Facultat de Medicina, and Centre d'Estudis en Biofísica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain
| | - Vidhya M Ravi
- Unitat de Biofísica, Departament de Bioquímica i de Biologia Molecular, Facultat de Medicina, and Centre d'Estudis en Biofísica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain
| | - Gérard Leblanc
- Direction des Sciences du Vivant, Direction des programmes et valorization, CEA Fontenay-aux-Roses, 92265 Fontenay-aux-Roses CEDEX France
| | - Esteve Padrós
- Unitat de Biofísica, Departament de Bioquímica i de Biologia Molecular, Facultat de Medicina, and Centre d'Estudis en Biofísica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain
| | - Josep Cladera
- Unitat de Biofísica, Departament de Bioquímica i de Biologia Molecular, Facultat de Medicina, and Centre d'Estudis en Biofísica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain
| | - Alex Perálvarez-Marín
- Unitat de Biofísica, Departament de Bioquímica i de Biologia Molecular, Facultat de Medicina, and Centre d'Estudis en Biofísica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain
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5
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Fuerst O, Lin Y, Granell M, Leblanc G, Padrós E, Lórenz-Fonfría VA, Cladera J. The Melibiose Transporter of Escherichia coli: CRITICAL CONTRIBUTION OF LYS-377 TO THE STRUCTURAL ORGANIZATION OF THE INTERACTING SUBSTRATE BINDING SITES. J Biol Chem 2015; 290:16261-71. [PMID: 25971963 PMCID: PMC4481225 DOI: 10.1074/jbc.m115.642678] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2015] [Revised: 05/12/2015] [Indexed: 01/27/2023] Open
Abstract
We examine the role of Lys-377, the only charged residue in helix XI, on the functional mechanism of the Na(+)-sugar melibiose symporter from Escherichia coli. Intrinsic fluorescence, FRET, and Fourier transform infrared difference spectroscopy reveal that replacement of Lys-377 with either Cys, Val, Arg, or Asp disables both Na(+) and melibiose binding. On the other hand, molecular dynamics simulations extending up to 200-330 ns reveal that Lys-377 (helix XI) interacts with the anionic side chains of two of the three putative ligands for cation binding (Asp-55 and Asp-59 in helix II). When Asp-59 is protonated during the simulations, Lys-377 preferentially interacts with Asp-55. Interestingly, when a Na(+) ion is positioned in the Asp-55-Asp-59 environment, Asp-124 in helix IV (a residue essential for melibiose binding) reorients and approximates the Asp-55-Asp-59 pair, and all three acidic side chains act as Na(+) ligands. Under these conditions, the side chain of Lys-377 interacts with the carboxylic moiety of these three Asp residues. These data highlight the crucial role of the Lys-377 residue in the spatial organization of the Na(+) binding site. Finally, the analysis of the second-site revertants of K377C reveals that mutation of Ile-22 (in helix I) preserves Na(+) binding, whereas that of melibiose is largely abolished according to spectroscopic measurements. This amino acid is located in the border of the sugar-binding site and might participate in sugar binding through apolar interactions.
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Affiliation(s)
- Oliver Fuerst
- From the Unitat de Biofísica, Departament de Bioquímica i de Biologia Molecular, Facultat de Medicina, and Centre d'Estudis en Biofísica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain
| | - Yibin Lin
- From the Unitat de Biofísica, Departament de Bioquímica i de Biologia Molecular, Facultat de Medicina, and Centre d'Estudis en Biofísica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain
| | - Meritxell Granell
- From the Unitat de Biofísica, Departament de Bioquímica i de Biologia Molecular, Facultat de Medicina, and Centre d'Estudis en Biofísica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain
| | - Gérard Leblanc
- the Direction des Sciences du Vivant, Direction des progammes et valorization, CEA Fontenay-aux-Roses, 92265 Fontenay-aux-Roses Cedex, France, and
| | - Esteve Padrós
- From the Unitat de Biofísica, Departament de Bioquímica i de Biologia Molecular, Facultat de Medicina, and Centre d'Estudis en Biofísica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain
| | - Víctor A Lórenz-Fonfría
- From the Unitat de Biofísica, Departament de Bioquímica i de Biologia Molecular, Facultat de Medicina, and Centre d'Estudis en Biofísica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain, Experimental Molecular Biophysics, Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany
| | - Josep Cladera
- From the Unitat de Biofísica, Departament de Bioquímica i de Biologia Molecular, Facultat de Medicina, and Centre d'Estudis en Biofísica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain,
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6
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Saidijam M, Patching SG. Amino acid composition analysis of secondary transport proteins from Escherichia coli with relation to functional classification, ligand specificity and structure. J Biomol Struct Dyn 2015; 33:2205-20. [DOI: 10.1080/07391102.2014.998283] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Affiliation(s)
- Massoud Saidijam
- Department of Molecular Medicine and Genetics, Research Centre for Molecular Medicine, School of Medicine, Hamadan University of Medical Sciences , Hamadan, Iran
| | - Simon G. Patching
- Department of Molecular Medicine and Genetics, Research Centre for Molecular Medicine, School of Medicine, Hamadan University of Medical Sciences , Hamadan, Iran
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7
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Ethayathulla AS, Yousef MS, Amin A, Leblanc G, Kaback HR, Guan L. Structure-based mechanism for Na(+)/melibiose symport by MelB. Nat Commun 2014; 5:3009. [PMID: 24389923 PMCID: PMC4026327 DOI: 10.1038/ncomms4009] [Citation(s) in RCA: 116] [Impact Index Per Article: 11.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/17/2013] [Accepted: 11/22/2013] [Indexed: 12/12/2022] Open
Abstract
The bacterial melibiose permease (MelB) belongs to the glycoside-pentoside-hexuronide:cation symporter family, a part of the major facilitator superfamily (MFS). Structural information regarding glycoside-pentoside-hexuronide:cation symporter family transporters and other Na(+)-coupled permeases within MFS has been lacking, although a wealth of biochemical and biophysical data are available. Here we present the three-dimensional crystal structures of Salmonella typhimurium MelBSt in two conformations, representing an outward partially occluded and an outward inactive state of MelBSt. MelB adopts a typical MFS fold and contains a previously unidentified cation-binding motif. Three conserved acidic residues form a pyramidal-shaped cation-binding site for Na(+), Li(+) or H(+), which is in close proximity to the sugar-binding site. Both cosubstrate-binding sites are mainly contributed by the residues from the amino-terminal domain. These two structures and the functional data presented here provide mechanistic insights into Na(+)/melibiose symport. We also postulate a structural foundation for the conformational cycling necessary for transport catalysed by MFS permeases in general.
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Affiliation(s)
- Abdul S. Ethayathulla
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, USA
| | - Mohammad S. Yousef
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, USA
- Present address: Department of Physics, College of Arts and Sciences, Southern Illinois University, Edwardsville, Illinois 62026-1654, USA (on leave from: Biophysics Department, Faculty of Science, Cairo University, Egypt)
| | - Anowarul Amin
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, USA
| | - Gérard Leblanc
- Department of Physiology, University of California, Los Angeles, California 90095, USA
- Present address: CEA-DSV-Fontenay aux Roses, Cross Division of Toxicology, 92 265 Fontenay aux Roses BP 6, France
| | - H. Ronald Kaback
- Department of Physiology, University of California, Los Angeles, California 90095, USA
| | - Lan Guan
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, USA
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8
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The substitution of Arg149 with Cys fixes the melibiose transporter in an inward-open conformation. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2013; 1828:1690-9. [PMID: 23500619 DOI: 10.1016/j.bbamem.2013.03.003] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/01/2012] [Revised: 02/15/2013] [Accepted: 03/01/2013] [Indexed: 11/22/2022]
Abstract
The melibiose transporter from Escherichia coli (MelB) can use the electrochemical energy of either H(+), Na(+) or Li(+) to transport the disaccharide melibiose to the cell interior. By using spectroscopic and biochemical methods, we have analyzed the role of Arg149 by mutagenesis. According to Fourier transform infrared difference and fluorescence spectroscopy studies, R149C, R149Q and R149K all bind substrates in proteoliposomes, where the protein is disposed inside-out. Analysis of right-side-out (RSO) and inside-out (ISO) membrane vesicles showed that the functionally active R149Q and R149K mutants could bind externally added fluorescent sugar analog in both types of vesicles. In contrast, the non-transporting R149C mutant does bind the fluorescent sugar analog as well as melibiose and Na(+) in ISO, but not in RSO vesicles. Therefore, the mutation of Arg149 into cysteine restrains the orientation of transporter to an inward-open conformation, with the inherent consequences of a) reducing the frequency of access of outer substrates to the binding sites, and b) impairing active transport. It is concluded that Arg149, most likely located in the inner (cytoplasmic) half of transmembrane helix 5, is critically involved in the reorientation mechanism of the substrate-binding site accessibility in MelB.
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9
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Hahn A, Stevanovic M, Mirus O, Schleiff E. The TolC-like protein HgdD of the cyanobacterium Anabaena sp. PCC 7120 is involved in secondary metabolite export and antibiotic resistance. J Biol Chem 2012; 287:41126-38. [PMID: 23071120 DOI: 10.1074/jbc.m112.396010] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022] Open
Abstract
The role of TolC has largely been explored in proteobacteria, where it functions as a metabolite and protein exporter. In contrast, little research has been carried out on the function of cyanobacterial homologues, and as a consequence, not much is known about the mechanism of cyanobacterial antibiotic uptake and metabolite secretion in general. It has been suggested that the TolC-like homologue of the filamentous, heterocyst-forming cyanobacterium Anabaena sp. PCC 7120, termed heterocyst glycolipid deposition protein D (HgdD), is involved in both protein and lipid secretion. To describe its function in secondary metabolite secretion, we established a system to measure the uptake of antibiotics based on the fluorescent molecule ethidium bromide. We analyzed the rate of porin-dependent metabolite uptake and confirmed the functional relation between detoxification and the action of HgdD. Moreover, we identified two major facilitator superfamily proteins that are involved in this process. It appears that anaOmp85 (Alr2269) is not required for insertion or assembly of HgdD, because an alr2269 mutant does not exhibit a phenotype similar to the hgdD mutant. Thus, we could assign components of the metabolite efflux system and describe parameters of detoxification by Anabaena sp. PCC 7120.
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Affiliation(s)
- Alexander Hahn
- Department of Biosciences, Center of Membrane Proteomics, Cluster of Excellence Frankfurt, Goethe University, 60438 Frankfurt, Germany
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10
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Reduced Na+ affinity increases turnover of Salmonella enterica serovar Typhimurium MelB. J Bacteriol 2012; 194:5538-44. [PMID: 22865849 DOI: 10.1128/jb.01206-12] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The melibiose permease of Salmonella enterica serovar Typhimurium (MelB(St)) catalyzes symport of melibiose with Na(+), Li(+), or H(+). Bioinformatics and mutational analyses indicate that a conserved Gly117 (helix IV) is a component of the Na(+)-binding site. In this study, Gly117 was mutated to Ser, Asn, or Cys. All three mutations increase the maximum rate (V(max)) for melibiose transport in Escherichia coli DW2 and greatly decrease Na(+) affinity, indicating that intracellular release of Na(+) is facilitated. Rapid melibiose transport, particularly by the G117N mutant, triggers osmotic lysis in the lag phase of growth. The findings support the previous conclusion that Gly117 plays an important role in cation binding and translocation. Furthermore, a spontaneous second-site mutation (P148L between loop(4-5) and helix V) in the G117C mutant prevents cell lysis. This mutation significantly decreases V(max) with little effect on cosubstrate binding in G117C, G117S, and G117N mutants. Thus, the P148L mutation specifically inhibits transport velocity and thereby blocks the lethal effect of elevated melibiose transport in the Gly117 mutants.
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11
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Lórenz-Fonfría VA, León X, Padrós E. Studying substrate binding to reconstituted secondary transporters by attenuated total reflection infrared difference spectroscopy. Methods Mol Biol 2012; 914:107-126. [PMID: 22976025 DOI: 10.1007/978-1-62703-023-6_7] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
The determination of protein conformational changes induced by the interaction of substrates with secondary transporters is an important step toward the elucidation of their transport mechanism. Since conformational changes in a protein alter its vibrational patterns, they can be detected with high sensitivity by infrared difference (IR(diff)) spectroscopy without the need for external probes. We describe a general procedure to obtain substrate-induced IR(diff) spectra by alternating perfusion of buffers over an attenuated total reflection (ATR) crystal containing an adhered film of a membrane protein reconstituted in lipids. As an example, we provide specific protocols to obtain melibiose and Na(+)-induced ATR-IR(diff) spectra of reconstituted melibiose permease, a sodium/melibiose co-transporter from E. coli. The presented methodology is applicable in principle to any membrane protein, provided that it can be purified and reconstituted in functional form, and appropriate substrates are available.
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Affiliation(s)
- Víctor A Lórenz-Fonfría
- Departament de Bioquímica i de Biologia Molecular, Facultat de Medicina, and Centre d'Estudis en Biofísica, Universitat Autònoma de Barcelona, Barcelona, Spain.
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12
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G117C MelB, a mutant melibiose permease with a changed conformational equilibrium. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2011; 1808:2508-16. [PMID: 21801712 DOI: 10.1016/j.bbamem.2011.07.017] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/02/2011] [Revised: 06/17/2011] [Accepted: 07/05/2011] [Indexed: 11/22/2022]
Abstract
Replacement of the glycine at position 117 by a cysteine in the melibiose permease creates an interesting phenotype: while the mutant transporter shows still transport activity comparable to the wild type its pre steady-state kinetic properties are drastically altered. The transient charge displacements after substrate concentration jumps are strongly reduced and the fluorescence changes disappear. Together with its maintained transport activity this indicates that substrate translocation in G117C melibiose permease is not impaired but that the initial conformation of the mutant transporter differs from that of the wild type permease. A kinetic model for the G117C melibiose permease based on a rapid dynamic equilibrium of the substrate free transporter is proposed. Implications of the kinetic model for the transport mechanism of the wild type permease are discussed.
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13
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Structural insights into the activation mechanism of melibiose permease by sodium binding. Proc Natl Acad Sci U S A 2010; 107:22078-83. [PMID: 21135207 DOI: 10.1073/pnas.1008649107] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The melibiose carrier from Escherichia coli (MelB) couples the accumulation of the disaccharide melibiose to the downhill entry of H(+), Na(+), or Li(+). In this work, substrate-induced FTIR difference spectroscopy was used in combination with fluorescence spectroscopy to quantitatively compare the conformational properties of MelB mutants, implicated previously in sodium binding, with those of a fully functional Cys-less MelB permease. The results first suggest that Asp55 and Asp59 are essential ligands for Na(+) binding. Secondly, though Asp124 is not essential for Na(+) binding, this acidic residue may play a critical role, possibly by its interaction with the bound cation, in the full Na(+)-induced conformational changes required for efficient coupling between the ion- and sugar-binding sites; this residue may also be a sugar ligand. Thirdly, Asp19 does not participate in Na(+) binding but it is a melibiose ligand. The location of these residues in two independent threading models of MelB is consistent with their proposed role.
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14
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Lórenz-Fonfría VA, Granell M, León X, Leblanc G, Padrós E. In-plane and out-of-plane infrared difference spectroscopy unravels tilting of helices and structural changes in a membrane protein upon substrate binding. J Am Chem Soc 2010; 131:15094-5. [PMID: 19803513 DOI: 10.1021/ja906324z] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Attenuated total reflection infrared (ATR-IR) difference spectroscopy stands out because of its ability to provide information on the interaction of substrates with membrane proteins in their native lipid bilayer environment. We show how the study and interpretation of the structural changes in membrane proteins upon substrate binding is simplified by obtaining ATR-IR difference spectra with polarized light and then computing the difference spectra in the z and x,y directions, where structural and orientation changes give specific difference absorbance patterns. In combination with a maximum-entropy band-narrowing method and some simple spectroscopic rules, the present approach allows us to unambiguously identify changes in the tilt of some helices in the secondary transporter melibiose permease following melibiose binding in the presence of sodium, suggesting the formation of an occluded state during the transport mechanism of the substrates.
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Affiliation(s)
- Víctor A Lórenz-Fonfría
- Unitat de Biofísica, Departament de Bioquímica i de Biologia Molecular, and Centre d'Estudis en Biofísica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain.
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15
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Alteration of sugar-induced conformational changes of the melibiose permease by mutating Arg141 in loop 4-5. Biophys J 2009; 96:4877-86. [PMID: 19527646 DOI: 10.1016/j.bpj.2009.03.025] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2008] [Revised: 03/16/2009] [Accepted: 03/20/2009] [Indexed: 11/22/2022] Open
Abstract
The melibiose permease (MelB) from Escherichia coli couples the uptake of melibiose to that of Na+, Li+, or H+. In this work, we applied attenuated total reflection Fourier transform infrared (ATR-FTIR) difference spectroscopy to obtain information about the structural changes involved in substrate interaction with the R141C mutant and with the wild-type MelB reacted with N-ethylmaleimide (NEM). These modified permeases have the ability to bind the substrates but fail to transport them. It is shown that the sugar-induced ATR-FTIR difference spectra of the R141C mutant are different from those corresponding to the Cys-less permease from which it is derived. There are alterations of peaks assigned to turns and beta-structures located most likely in loop 4-5. In addition, and quite notably, a peak at 1659 cm(-1), assigned to changes at the level of one alpha-helix subpopulation, disappears in the melibiose-induced difference spectrum in the presence of Na+, suggesting a reduction of the conformational change capacity of the mutated MelB. These helices may involve structural components that couple the cation- and sugar-binding sites. On the other hand, MelB-NEM difference spectra are proportionally less disrupted than the R141C ones. Hence, the transport cycle of these two permeases, modified at two different loops, is most likely impaired at a different stage. It is proposed that the R141C mutant leads to the generation of a partially defective ternary complex that is unable to catalyze the subsequent conformational change necessary for substrate translocation.
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Smith KM, Slugoski MD, Cass CE, Baldwin SA, Karpinski E, Young JD. Cation coupling properties of human concentrative nucleoside transporters hCNT1, hCNT2 and hCNT3. Mol Membr Biol 2009; 24:53-64. [PMID: 17453413 DOI: 10.1080/09687860600942534] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
Abstract
The SLC28 family of concentrative nucleoside transporter (CNT) proteins in mammalian cells contains members of two distinct phylogenic subfamilies. In humans, hCNT1 and hCNT2 belong to one subfamily, and hCNT3 to the other. All three CNTs mediate inwardly-directed Na(+)/nucleoside cotransport, and are either pyrimidine nucleoside-selective (hCNT1), purine nucleoside-selective (hCNT2), or broadly selective for both pyrimidine and purine nucleosides (hCNT3). While previous studies have characterized cation interactions with both hCNT1 and hCNT3, little is known about the corresponding properties of hCNT2. In the present study, heterologous expression in Xenopus oocytes in combination with radioisotope flux and electrophysiological techniques has allowed us to undertake a side-by-side comparison of hCNT2 with other hCNT family members. Apparent K (50) values for Na(+) activation were voltage-dependent, and similar in magnitude for all three transporters. Only hCNT3 was also able to couple transport of uridine to uptake of H(+). The Na(+)/nucleoside stoichiometry of hCNT2, as determined from both Hill coefficients and direct charge/flux measurements, was 1:1. This result was the same as for hCNT1, but different from that of hCNT3 (2:1). The charge-to-(22)Na(+) uptake stoichiometry was 1:1 for all three hCNTs. In parallel with their division into two separate CNT subfamilies, hCNT2 shares common cation specificity and coupling characteristics with hCNT1, which differ markedly from those of hCNT3.
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Affiliation(s)
- Kyla M Smith
- The Membrane Protein Research Group, Department of Physiology, University of Alberta, Alberta, Canada
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17
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Ganea C, Fendler K. Bacterial transporters: Charge translocation and mechanism. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2009; 1787:706-13. [DOI: 10.1016/j.bbabio.2009.02.002] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/19/2008] [Revised: 02/02/2009] [Accepted: 02/02/2009] [Indexed: 12/01/2022]
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18
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Meyer-Lipp K, Séry N, Ganea C, Basquin C, Fendler K, Leblanc G. The Inner Interhelix Loop 4–5 of the Melibiose Permease from Escherichia coli Takes Part in Conformational Changes after Sugar Binding. J Biol Chem 2006; 281:25882-92. [PMID: 16822867 DOI: 10.1074/jbc.m601259200] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Cytoplasmic loop 4-5 of the melibiose permease from Escherichia coli is essential for the process of Na+-sugar translocation (Abdel-Dayem, M., Basquin, C., Pourcher, T., Cordat, E., and Leblanc, G. (2003) J. Biol. Chem. 278, 1518-1524). In the present report, we analyze functional consequences of mutating each of the three acidic amino acids in this loop into cysteines. Among the mutants, only the E142C substitution impairs selectively Na+-sugar translocation. Because R141C has a similar defect, we investigated these two mutants in more detail. Liposomes containing purified mutated melibiose permease were adsorbed onto a solid supported lipid membrane, and transient electrical currents resulting from different substrate concentration jumps were recorded. The currents evoked by a melibiose concentration jump in the presence of Na+, previously assigned to an electrogenic conformational transition (Meyer-Lipp, K., Ganea, C., Pourcher, T., Leblanc, G., and Fendler, K. (2004) Biochemistry 43, 12606-12613), were much smaller for the two mutants than the corresponding signals in cysteineless MelB. Furthermore, in R141C the stimulating effect of melibiose on Na+ affinity was lost. Finally, whereas tryptophan fluorescence spectroscopy revealed impaired conformational changes upon melibiose binding in the mutants, fluorescence resonance energy transfer measurements indicated that the mutants still show cooperative modification of their sugar binding sites by Na+. These data suggest that: 1) loop 4-5 contributes to the coordinated interactions between the ion and sugar binding sites; 2) it participates in an electrogenic conformational transition after melibiose binding that is essential for the subsequent obligatory coupled translocation of substrates. A two-step mechanism for substrate translocation in the melibiose permease is suggested.
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Affiliation(s)
- Kerstin Meyer-Lipp
- Max Planck Institute of Biophysics, Max-von-Laue-Strasse 3, 60438 Frankfurt/M, Germany
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19
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Smith KM, Slugoski MD, Loewen SK, Ng AML, Yao SYM, Chen XZ, Karpinski E, Cass CE, Baldwin SA, Young JD. The Broadly Selective Human Na+/Nucleoside Cotransporter(hCNT3) Exhibits Novel Cation-coupled Nucleoside TransportCharacteristics. J Biol Chem 2005; 280:25436-49. [PMID: 15870078 DOI: 10.1074/jbc.m409454200] [Citation(s) in RCA: 66] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The concentrative nucleoside transporter (CNT) protein family in humans is represented by three members, hCNT1, hCNT2, and hCNT3. hCNT3, a Na+/nucleoside symporter, transports a broad range of physiological purine and pyrimidine nucleosides as well as anticancer and antiviral nucleoside drugs, and belongs to a different CNT subfamily than hCNT1/2. H+-dependent Escherichia coli NupC and Candida albicans CaCNT are also CNT family members. The present study utilized heterologous expression in Xenopus oocytes to investigate the specificity, mechanism, energetics, and structural basis of hCNT3 cation coupling. hCNT3 exhibited uniquely broad cation interactions with Na+, H+, and Li+ not shared by Na+-coupled hCNT1/2 or H+-coupled NupC/CaCNT. Na+ and H+ activated hCNT3 through mechanisms to increase nucleoside apparent binding affinity. Direct and indirect methods demonstrated cation/nucleoside coupling stoichiometries of 2:1 in the presence of Na+ and both Na+ plus H+, but only 1:1 in the presence of H+ alone, suggesting that hCNT3 possesses two Na+-binding sites, only one of which is shared by H+. The H+-coupled hCNT3 did not transport guanosine or 3'-azido-3'-deoxythymidine and 2',3'-dideoxycytidine, demonstrating that Na+- and H+-bound versions of hCNT3 have significantly different conformations of the nucleoside binding pocket and/or translocation channel. Chimeric studies between hCNT1 and hCNT3 located hCNT3-specific cation interactions to the C-terminal half of hCNT3, setting the stage for site-directed mutagenesis experiments to identify the residues involved.
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Affiliation(s)
- Kyla M Smith
- Membrane Protein Research Group, Departments of Physiology and Oncology, University of Alberta Cross Cancer Institute, Edmonton, Alberta T6G 2H7, Canada
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20
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Transport Proteins on Solid-Supported Membranes: From Basic Research to Drug Discovery. ACTA ACUST UNITED AC 2004. [DOI: 10.1007/978-3-662-05204-4_13] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 03/11/2023]
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21
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Abdel-Dayem M, Basquin C, Pourcher T, Cordat E, Leblanc G. Cytoplasmic loop connecting helices IV and V of the melibiose permease from Escherichia coli is involved in the process of Na+-coupled sugar translocation. J Biol Chem 2003; 278:1518-24. [PMID: 12421811 DOI: 10.1074/jbc.m210053200] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Previous photolabeling and limited proteolysis studies suggested that one of the four basic residues (Arg-141) of the N-terminal cytoplasmic loop connecting helices IV and V (loop 4-5) of the melibiose permease (MelB) from Escherichia coli has a potential role in its symport function (Ambroise, Y., Leblanc, G., and Rousseau, B. (2000) Biochemistry 39, 1338-1345). A mutagenesis study of Arg-141 and of the other three basic residues of loop 4-5 was undertaken to further examine this hypothesis. Cys replacement analysis indicated that Arg-141 and Arg-149, but not Lys-138 and Arg-139, are essential for MelB transport activity. Replacement of Arg-141 by neutral residues (Cys or Gln) inactivated transport and energy-independent carrier-mediated flows of substrates (counterflow, efflux), whereas it had a limited effect on co-substrate binding. R141C sugar transport was partially rescued on reintroducing a positive charge with a charged and permeant thiol reagent. Whereas R149C was completely inactive, R149K and R149Q remained functional. Strikingly, introduction of an additional mutation in the C-terminal helix X (Gly for Val-343) of R149C restored sugar transport. Impermeant thiol reagents inhibited R149C/V343G transport activity in right-side-out membrane vesicles and prevented sugar binding in a sugar-protected manner. All these data suggest that MelB loop 4-5 is close to the sugar binding site and that the charged residue Arg-141 is involved in the reaction of co-substrate translocation or substrate release in the inner compartment.
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Affiliation(s)
- Manal Abdel-Dayem
- Laboratoire de Physiologie des Membranes Cellulaires, Université de Nice Sophia-Antipolis and CNRS UMR 6078, Commissariat à l'Energie Atomique (LRC-CEA 16V), Villefranche sur mer, 06230 France
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22
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Hacksell I, Rigaud JL, Purhonen P, Pourcher T, Hebert H, Leblanc G. Projection structure at 8 A resolution of the melibiose permease, an Na-sugar co-transporter from Escherichia coli. EMBO J 2002; 21:3569-74. [PMID: 12110569 PMCID: PMC126123 DOI: 10.1093/emboj/cdf378] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Abstract
The ion-coupled sugar membrane symporter or co-transporter melibiose permease (MelB), responsible for alpha-galactoside accumulation in Escherichia coli, is a representative member of the glycoside-pentoside- hexuronide family of the vast class of electrochemical potential-driven porters. Pure solubilized preparations of a MelB recombinant protein were subjected to two-dimensional crystallization trials and several crystal forms were observed. Two of these appeared as large wide tubes suitable for analysis by electron crystallography. Flattened tubes on carbon support film, embedded in amorphous ice prior to electron cryomicroscopy, showed two-sided plane group symmetries P12(1) or P222(1), with unit cell dimensions a = 89.9 A, b = 51.6 A, gamma = 91.9 degrees and a = 188.9 A, b = 48.8 A, gamma = 90 degrees, respectively. The projection map from the P222(1 )crystals at 8 A resolution displayed an asymmetric protein unit consisting of two domains lining a central and curve-shaped cleft. Together, the MelB monomer could host the 12 predicted transmembrane alpha-helices. Overall, the MelB helix packing arrangement compared more favorably with that of the Na(+)/H(+) antiporter NhaA than that of the oxalate antiporter.
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Affiliation(s)
| | - Jean-Louis Rigaud
- Karolinska Institutet, Department of Biosciences, S-141 57 Huddinge, Sweden,
Institut Curie, UMR-CNRS 168 and LRC-CEA 8, 11 rue Pierre et Marie Curie, 75231 Paris cedex 05 and UMR 6078 CNRS-Université de Nice and LRC CEA 16, 06230 Villefranche-sur-mer, France Corresponding author e-mail:
| | | | - Thierry Pourcher
- Karolinska Institutet, Department of Biosciences, S-141 57 Huddinge, Sweden,
Institut Curie, UMR-CNRS 168 and LRC-CEA 8, 11 rue Pierre et Marie Curie, 75231 Paris cedex 05 and UMR 6078 CNRS-Université de Nice and LRC CEA 16, 06230 Villefranche-sur-mer, France Corresponding author e-mail:
| | - Hans Hebert
- Karolinska Institutet, Department of Biosciences, S-141 57 Huddinge, Sweden,
Institut Curie, UMR-CNRS 168 and LRC-CEA 8, 11 rue Pierre et Marie Curie, 75231 Paris cedex 05 and UMR 6078 CNRS-Université de Nice and LRC CEA 16, 06230 Villefranche-sur-mer, France Corresponding author e-mail:
| | - Gérard Leblanc
- Karolinska Institutet, Department of Biosciences, S-141 57 Huddinge, Sweden,
Institut Curie, UMR-CNRS 168 and LRC-CEA 8, 11 rue Pierre et Marie Curie, 75231 Paris cedex 05 and UMR 6078 CNRS-Université de Nice and LRC CEA 16, 06230 Villefranche-sur-mer, France Corresponding author e-mail:
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23
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Dave N, Lórenz-Fonfría VA, Villaverde J, Lemonnier R, Leblanc G, Padrós E. Study of amide-proton exchange of Escherichia coli melibiose permease by attenuated total reflection-Fourier transform infrared spectroscopy: evidence of structure modulation by substrate binding. J Biol Chem 2002; 277:3380-7. [PMID: 11729178 DOI: 10.1074/jbc.m105466200] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The accessibility of Escherichia coli melibiose permease to aqueous solvent was studied following hydrogen-deuterium exchange kinetics monitored by attenuated total reflection-Fourier transform infrared spectroscopy under four distinct conditions where MelB forms different complexes with its substrates (H(+), Na(+), melibiose). Analysis of the amide II band upon (2)H(2)O exposure discloses a significant sugar protection of the protein against aqueous solvent, resulting in an 8% less exchange of the corresponding H(+)*melibiose*MelB complex compared with the protein in the absence of sugar. Investigation of the amide I exchange reveals clear substrate effects on beta-sheet accessibility, with the complex H(+)*melibiose*MelB being the most protected state against exchange, followed by Na(+)*melibiose*MelB. Although of smaller magnitude, similar changes in alpha-helices plus non-ordered structures are detected. Finally, no differences are observed when analyzing reverse turn structures. The results suggest that sugar binding induces a remarkable compactness of the carrier's structure, affecting mainly beta-sheet domains of the transporter, which, according to secondary structure predictions, may include cytoplasmic loops 4-5 and 10-11. A possible catalytic role of these two loops in the functioning of MelB is hypothesized.
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Affiliation(s)
- Natàlia Dave
- Unitat de Biofisica, Departament de Bioquimica i de Biologia Molecular, Facultat de Medicina, Universitat Autònoma de Barcelona, Bellaterra 08193, Barcelona, Spain
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24
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Häse CC, Fedorova ND, Galperin MY, Dibrov PA. Sodium ion cycle in bacterial pathogens: evidence from cross-genome comparisons. Microbiol Mol Biol Rev 2001; 65:353-70, table of contents. [PMID: 11528000 PMCID: PMC99031 DOI: 10.1128/mmbr.65.3.353-370.2001] [Citation(s) in RCA: 189] [Impact Index Per Article: 8.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
Analysis of the bacterial genome sequences shows that many human and animal pathogens encode primary membrane Na+ pumps, Na+-transporting dicarboxylate decarboxylases or Na+ translocating NADH:ubiquinone oxidoreductase, and a number of Na+ -dependent permeases. This indicates that these bacteria can utilize Na+ as a coupling ion instead of or in addition to the H+ cycle. This capability to use a Na+ cycle might be an important virulence factor for such pathogens as Vibrio cholerae, Neisseria meningitidis, Salmonella enterica serovar Typhi, and Yersinia pestis. In Treponema pallidum, Chlamydia trachomatis, and Chlamydia pneumoniae, the Na+ gradient may well be the only energy source for secondary transport. A survey of preliminary genome sequences of Porphyromonas gingivalis, Actinobacillus actinomycetemcomitans, and Treponema denticola indicates that these oral pathogens also rely on the Na+ cycle for at least part of their energy metabolism. The possible roles of the Na+ cycling in the energy metabolism and pathogenicity of these organisms are reviewed. The recent discovery of an effective natural antibiotic, korormicin, targeted against the Na+ -translocating NADH:ubiquinone oxidoreductase, suggests a potential use of Na+ pumps as drug targets and/or vaccine candidates. The antimicrobial potential of other inhibitors of the Na+ cycle, such as monensin, Li+ and Ag+ ions, and amiloride derivatives, is discussed.
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Affiliation(s)
- C C Häse
- Department of Infectious Diseases, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
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25
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Abstract
A variety of sodium-substrate cotransport systems are known in bacteria. Sodium enters the cell down an electrochemical concentration gradient. There is obligatory coupling between the entry of the ion and the entry of substrate with a stoichiometry (in the cases studied) of 1:1. Thus, the downhill movement of sodium ion into the cell leads to the accumulation of substrate within the cell. The melibiose carrier of Escherichia coli is perhaps the most carefully studied of the sodium cotransport systems in bacteria. This carrier is of special interest because it can also use protons or lithium ions for cotransport. Other sodium cotransport carriers that have been studied recently are for proline, glutamate, serine-threonine, citrate and branched chain amino acids.
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Affiliation(s)
- T H Wilson
- Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, MA 02115, USA.
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26
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Franco PJ, Jena AB, Wilson TH. Physiological evidence for an interaction between helices II and XI in the melibiose carrier of Escherichia coli. BIOCHIMICA ET BIOPHYSICA ACTA 2001; 1510:231-42. [PMID: 11342161 DOI: 10.1016/s0005-2736(00)00353-9] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
The melibiose carrier from Escherichia coli is a cation-substrate cotransporter that catalyzes the accumulation of galactosides at the expense of H(+), Na(+), or Li(+) electrochemical gradients. Charged residues on transmembrane domains in the amino-terminal portion of this carrier play an important role in the recognition of cations, while the carboxyl portion of the protein seems to be important for sugar recognition. In the present study, we substituted Lys-377 on helix XI with Val. This mutant carrier, K377V, had reduced melibiose transport activity. We subsequently used this mutant for the isolation of functional second-site revertants. Revertant strains showed the additional substitutions of Val or Asn for Asp-59 (helix II), or Leu for Phe-20 (helix I). Isolation of revertant strains where both Lys-377 and Asp-59 are substituted with neutral residues suggested the possibility that a salt bridge exists between helix II and helix XI. To further test this idea, we constructed three additional site-directed mutants: Asp-59-->Lys (D59K), Lys-377-->Asp (K377D), and a double mutant, Asp-59-->Lys/Lys-377-->Asp (D59K/K377D), in which the position of these charges was exchanged. K377D accumulated melibiose only marginally while D59K could not accumulate. However, the D59K/K377D double mutant accumulated melibiose to a modest level although this activity was no longer stimulated by Na(+). We suggest that Asp-59 and Lys-377 interact via a salt bridge that brings helix II and helix XI close to one another in the three-dimensional structure of the carrier.
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Affiliation(s)
- P J Franco
- Department of Cell Biology, Harvard Medical School, 240 Longwood Ave, Boston, MA 02115, USA
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27
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Dave N, Troullier A, Mus-Veteau I, Duñach M, Leblanc G, Padrós E. Secondary structure components and properties of the melibiose permease from Escherichia coli: a fourier transform infrared spectroscopy analysis. Biophys J 2000; 79:747-55. [PMID: 10920008 PMCID: PMC1300974 DOI: 10.1016/s0006-3495(00)76332-6] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
Abstract
The structure of the melibiose permease from Escherichia coli has been investigated by Fourier transform infrared spectroscopy, using the purified transporter either in the solubilized state or reconstituted in E. coli lipids. In both instances, the spectra suggest that the permease secondary structure is dominated by alpha-helical components (up to 50%) and contains beta-structure (20%) and additional components assigned to turns, 3(10) helix, and nonordered structures (30%). Two distinct and strong absorption bands are recorded at 1660 and 1653 cm(-1), i.e., in the usual range of absorption of helices of membrane proteins. Moreover, conditions that preserve the transporter functionality (reconstitution in liposomes or solubilization with dodecyl maltoside) make possible the detection of two separate alpha-helical bands of comparable intensity. In contrast, a single intense band, centered at approximately 1656 cm(-1), is recorded from the inactive permease in Triton X-100, or a merged and broader signal is recorded after the solubilized protein is heated in dodecyl maltoside. It is suggested that in the functional permease, distinct signals at 1660 and 1653 cm(-1) arise from two different populations of alpha-helical domains. Furthermore, the sodium- and/or melibiose-induced changes in amide I line shape, and in particular, in the relative amplitudes of the 1660 and 1653 cm(-1) bands, indicate that the secondary structure is modified during the early step of sugar transport. Finally, the observation that approximately 80% of the backbone amide protons can be exchanged suggests high conformational flexibility and/or a large accessibility of the membrane domains to the aqueous solvent.
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Affiliation(s)
- N Dave
- Unitat de Biofísica, Departament de Bioquímica i de Biologia Molecular, Facultat de Medicina, Universitat Autònoma de Barcelona, Bellaterra, Barcelona 08193, Spain
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28
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Ambroise Y, Mioskowski C, Leblanc G, Rousseau B. Syntheses and properties of photoactivatable sugar derivatives designed to probe the sugar-binding site of melibiose permease. Bioorg Med Chem Lett 2000; 10:1125-7. [PMID: 10843233 DOI: 10.1016/s0960-894x(00)00180-3] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
Three new photoreactive sugar analogues bearing an azido, a diazonium salt or a diazirine group as the photophore as well as a tritium atom were developed. Two of these new photoactivatable compounds gave excellent preliminary results, with a high affinity for the melibiose permease of Escherichia coli.
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Affiliation(s)
- Y Ambroise
- Service des Molécules Marquées, Département de Biologie Cellulaire et Moléculaire, CEA/Saclay, Gif sur Yvette, France
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29
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Matsuzaki S, Weissborn AC, Tamai E, Tsuchiya T, Wilson TH. Melibiose carrier of Escherichia coli: use of cysteine mutagenesis to identify the amino acids on the hydrophilic face of transmembrane helix 2. BIOCHIMICA ET BIOPHYSICA ACTA 1999; 1420:63-72. [PMID: 10446291 DOI: 10.1016/s0005-2736(99)00087-5] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The melibiose carrier from Escherichia coli is a galactoside-cation symporter. Based on both experimental evidence and hydropathy analysis, 12 transmembrane helices have been assigned to this integral membrane protein. Transmembrane helix 2 contains several charged and polar amino acids that have been shown to be essential for the cation-coupled transport of melibiose. Starting with the cysteine-less melibiose carrier, we have individually substituted cysteine for amino acids 39-66, which includes the proposed transmembrane helix 2. In the resulting derivative carriers, we measured the transport of melibiose, determined the effect of the hydrophilic sulfhydryl reagent, p-chloromercuribenzenesulfonic acid (PCMBS), on transport in intact cells and inside out vesicles, and examined the ability of melibiose to protect the carrier from inactivation by the sulfhydryl reagent. We found a set of seven positions in which the reaction with the sulfhydryl reagent caused partial or complete loss of carrier function measured in intact cells or inside-out vesicles. The presence of melibiose protected five of these positions from reaction with PCMBS. The reaction of two additional positions with PCMBS resulted in the partial loss of transport function only in inside-out vesicles. Melibiose protected these two positions from reaction with the reagent. Together, the PCMBS-sensitive sites and charged residues assigned to helix 2 form a cluster of amino acids that map in three rows with each row comprised of every fourth residue. This is the pattern expected of residues that are part of an alpha-helical structure and thus the rows are tilted at an angle of 25 degrees to the helical axis. We suggest that these residues line the path of melibiose and its associated cation through the carrier.
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Affiliation(s)
- S Matsuzaki
- Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, MA 02115, USA
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30
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Maehrel C, Cordat E, Mus-Veteau I, Leblanc G. Structural studies of the melibiose permease of Escherichia coli by fluorescence resonance energy transfer. I. Evidence for ion-induced conformational change. J Biol Chem 1998; 273:33192-7. [PMID: 9837887 DOI: 10.1074/jbc.273.50.33192] [Citation(s) in RCA: 50] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Further insight into the cosubstrate-induced structural change of the melibiose permease (MelB) of Escherichia coli has been sought by investigating the binding and spectroscopic properties of the fluorescent sugar 2'-(N-5-dimethylaminonaphthalene-1-sulfonyl)aminoethyl 1-thio-beta-D-galactopyranoside (Dns2-S-Gal) and related analogs (Dns3-S-Gal or Dns6-S-Gal with a propyl or hexyl instead of an ethyl linker, respectively) interacting with MelB in membrane vesicles or in proteoliposomes. The three analogs efficiently inhibit melibiose transport and bind to MelB in a sodium-dependent fashion. Their dissociation constants (Kd) are in the micromolar range in the presence of NaCl and an order of magnitude higher in its absence. In the presence of NaCl and Dns2-S-Gal, sample excitation at 335 or 297 nm gives rise to a fluorescent signal at around 465 nm, whereas Dns3-S-Gal or Dns6-S-Gal emits a fluorescence light at 490 or 506 nm, respectively. Detailed study of the Dns2-S-Gal signal elicited by a 297 nm illumination indicates that a tryptophan-mediated fluorescence resonance energy transfer phenomenon is involved in the response. All fluorescence signals below 500 nm are prevented by addition of melibiose in excess, and the kinetic constants describing their dependence on the probe or NaCl concentrations closely correlate with the probe binding constants. Finally, the Dns2-S-Gal signal recorded in sodium-free medium is red shifted by up to 25 nm from that recorded in the presence of NaCl. Taken together, these results suggest (i) that the fluorescence signals below 500 nm arise from Dns-S-Gal molecules bound to MelB, (ii) the presence of a highly hydrophobic environment close to or at the sugar-binding site, the polarity of which increases on moving away from the sugar-binding site, and (iii) that the interaction of sodium ions with MelB enhances the hydrophobicity of this environment. These results are consistent with the induction of a cooperative change of the structure of the sugar-binding site or of its immediate vicinity by the ions.
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Affiliation(s)
- C Maehrel
- Laboratoire J. Maetz, Département de Biologie Cellulaire et Moléculaire du Commissariat à l'Energie Atomique and CNRS-ERS 1253, 06238 Villefranche sur Mer cedex, France
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Weissborn AC, Botfield MC, Kuroda M, Tsuchiya T, Wilson TH. The construction of a cysteine-less melibiose carrier from E. coli. BIOCHIMICA ET BIOPHYSICA ACTA 1997; 1329:237-44. [PMID: 9371415 DOI: 10.1016/s0005-2736(97)00116-8] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
The melibiose carrier of E. coli is a cation-sugar cotransport system. This membrane protein contains four cysteine residues and the transport function is inhibited by sulfhydryl reagents. In order to investigate the importance of the cysteines, we have constructed a set of four melibiose transporters each of which has one cysteine replaced with serine or valine. The sensitivity of this set of carriers to N-ethylmaleimide was tested and Cys364 was identified as the target of the reagent. In addition, we constructed a melibiose transporter in which all 4 cysteines were replaced with either serine (Cys110, Cys310, and Cys364) or valine (Cys235) and we found that, as expected, the resulting cysteine-less transporter was resistant to the action of N-ethylmaleimide. The cysteine-less melibiose carrier had no significant decrease in ability to accumulate melibiose with cotransported sodium ions or protons. Thus, none of the 4 cysteines are necessary for the function of the melibiose carrier.
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Affiliation(s)
- A C Weissborn
- Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA
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Zani M, Pourcher T, Leblanc G. Mutation of polar and charged residues in the hydrophobic NH2-terminal domains of the melibiose permease of Escherichia coli. J Biol Chem 1994. [DOI: 10.1016/s0021-9258(17)31473-4] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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33
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Reizer J, Reizer A, Saier MH. A functional superfamily of sodium/solute symporters. BIOCHIMICA ET BIOPHYSICA ACTA 1994; 1197:133-66. [PMID: 8031825 DOI: 10.1016/0304-4157(94)90003-5] [Citation(s) in RCA: 164] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
Eleven families of sodium/solute symporters are defined based on their degrees of sequence similarities, and the protein members of these families are characterized in terms of their solute and cation specificities, their sizes, their topological features, their evolutionary relationships, and their relative degrees and regions of sequence conservation. In some cases, particularly where site-specific mutagenesis analyses have provided functional information about specific proteins, multiple alignments of members of the relevant families are presented, and the degrees of conservation of the mutated residues are evaluated. Signature sequences for several of the eleven families are presented to facilitate identification of new members of these families as they become sequenced. Phylogenetic tree construction reveals the evolutionary relationships between members of each family. One of these families is shown to belong to the previously defined major facilitator superfamily. The other ten families do not show sufficient sequence similarity with each other or with other identified transport protein families to establish homology between them. This study serves to clarify structural, functional and evolutionary relationships among eleven distinct families of functionally related transport proteins.
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Affiliation(s)
- J Reizer
- Department of Biology, University of California at San Diego, La Jolla 92093-0116
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Krämer R. Functional principles of solute transport systems: concepts and perspectives. BIOCHIMICA ET BIOPHYSICA ACTA 1994; 1185:1-34. [PMID: 7511415 DOI: 10.1016/0005-2728(94)90189-9] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Affiliation(s)
- R Krämer
- Institut für Biotechnologie 1, Forschungszentrum Jülich, Germany
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35
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van Veen HW, Abee T, Kortstee GJ, Konings WN, Zehnder AJ. Translocation of metal phosphate via the phosphate inorganic transport system of Escherichia coli. Biochemistry 1994; 33:1766-70. [PMID: 8110778 DOI: 10.1021/bi00173a020] [Citation(s) in RCA: 97] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
Pi transport via the phosphate inorganic transport system (Pit) of Escherichia coli was studied in natural and artificial membranes. Pi uptake via Pit is dependent on the presence of divalent cations, like Mg2+, Ca2+, Co2+, or Mn2+, which form a soluble, neutral metal phosphate (MeHPO4) complex. Pi-dependent uptake of Mg2+ and Ca2+, equimolar cotransport of Pi and Ca2+, and inhibition by Mg2+ of Ca2+ uptake in the presence of Pi, but not of Pi uptake in the presence of Ca2+, indicate that a metal phosphate complex is the transported solute. Metal phosphate is transported in symport with H+ with a mechanistic stoichiometry of 1. Pit mediates efflux and homologous exchange of metal phosphate, but not heterologous metal phosphate exchange with Pi, glycerol-3P, or glucose-6P. The metal phosphate efflux rate increased with pH, whereas the rate of metal phosphate exchange was essentially pH independent. Metal phosphate uptake was inhibited at low internal pH. Efflux was inhibited by a proton motive force (interior negative and alkaline), whereas exchange was inhibited by the membrane potential only. These results have been evaluated in terms of ordered binding and dissociation of metal phosphate and proton on the outer and inner surface of the cytoplasmic membrane.
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Affiliation(s)
- H W van Veen
- Department of Microbiology, Agricultural University Wageningen, The Netherlands
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Jording D, Pühler A. The membrane topology of the Rhizobium meliloti C4-dicarboxylate permease (DctA) as derived from protein fusions with Escherichia coli K12 alkaline phosphatase (PhoA) and beta-galactosidase (LacZ). MOLECULAR & GENERAL GENETICS : MGG 1993; 241:106-14. [PMID: 8232193 DOI: 10.1007/bf00280207] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
The Rhizobium meliloti dctA gene encodes the C4-dicarboxylate permease which mediates uptake of C4-dicarboxylates, both in free-living and symbiotic cells. Based on the hydrophobicity of the DctA protein, 12 putative membrane spanning regions were predicted. The membrane topology was further analysed by isolating in vivo fusions of DctA to Escherichia coli alkaline phosphatase (PhoA) and E. coli beta-galactosidase (LacZ). Of 10 different fusions 7 indicated a periplasmic and 3 a cytoplasmic location of the corresponding region of the DctA protein. From these data a two-dimensional model of DctA was constructed which comprised twelve transmembrane alpha-helices with the amino-terminus and the carboxy-terminus located in the cytoplasm. In addition, four conserved amino acid motifs present in many eukaryotic and prokaryotic transport proteins were observed.
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Affiliation(s)
- D Jording
- Lehrstuhl für Genetik, Fakultät für Biologie, Universität Bielefeld, Germany
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37
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Pourcher T, Zani M, Leblanc G. Mutagenesis of acidic residues in putative membrane-spanning segments of the melibiose permease of Escherichia coli. I. Effect on Na(+)-dependent transport and binding properties. J Biol Chem 1993. [DOI: 10.1016/s0021-9258(18)53679-6] [Citation(s) in RCA: 52] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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38
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Mutagenesis of acidic residues in putative membrane-spanning segments of the melibiose permease of Escherichia coli. II. Effect on cationic selectivity and coupling properties. J Biol Chem 1993. [DOI: 10.1016/s0021-9258(18)53680-2] [Citation(s) in RCA: 57] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
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39
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Pourcher T, Bassilana M, Sarkar HK, Kaback HR, Leblanc G. Melibiose permease of Escherichia coli: mutation of histidine-94 alters expression and stability rather than catalytic activity. Biochemistry 1992; 31:5225-31. [PMID: 1606146 DOI: 10.1021/bi00137a018] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
Previous studies utilizing site-directed mutagenesis [Pourcher et al. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 468-472] indicate that out of seven histidinyl residues in the melibiose (mel) permease of Escherichia coli, only His94 is important. The role of His94 has now been investigated by replacing the residue with Asn, Gln, or Arg. Cells expressing mel permease with Asn94 or Gln94 retain 30% or 20% of wild-type activity, respectively, and surprisingly, immunological assays demonstrate that diminished transport activity is due to a proportional reduction in the amount of permease in the membrane. Moreover, kinetic analyses of transport and ligand binding studies with right-side-out membrane vesicles indicate that both substrate recognition and turnover (kcat) are comparable in the mutant permeases and the wild-type. Mel permease with Arg in place of His94 also binds ligand and catalyzes sugar accumulation, but only when the cells are grown at 30 degrees C, and evidence is presented that Arg94 permease is inactivated at 37 degrees C. Finally, labeling studies demonstrate that expression and/or insertion of the permease, but not degradation, is strongly dependent on the amino acid present at position 94 and temperature. The findings indicate that an imidazole group at position 94 is required for proper insertion and stability of mel permease, but not for transport activity per se. Since replacement of the other six histidinyl residues in mel permease with Arg has little or no effect on transport activity, it is concluded that histidinyl residues do not play a direct role in the mechanism of this secondary transport protein.
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Affiliation(s)
- T Pourcher
- Laboratoire J. Maetz, Département de Biologie Cellulaire et Moléculaire du Commissariat à l'Energie Atomique, Villefranche sur mer, France
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Wilson DM, Wilson TH. Asp-51 and Asp-120 are important for the transport function of the Escherichia coli melibiose carrier. J Bacteriol 1992; 174:3083-6. [PMID: 1569035 PMCID: PMC205965 DOI: 10.1128/jb.174.9.3083-3086.1992] [Citation(s) in RCA: 30] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022] Open
Abstract
Asp-51 and Asp-120 of the Escherichia coli melibiose carrier on plasmid pKKMB were separately replaced by amber codons and transformed into eight amber suppressor strains, producing eight amino acid substitutions for each site. Glu-51 and Glu-120 were the only replacements in the carrier that allowed the cells to ferment melibiose and that showed transport of melibiose against a concentration gradient. Revertants to Glu-51 and Glu-120 show less activity than the wild type. The Asp-51 position is more crucial for Na(+)-stimulated melibiose accumulation than is the Asp-120 site.
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Affiliation(s)
- D M Wilson
- Department of Cellular and Molecular Physiology, Harvard Medical School, Boston, Massachusetts 02115
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41
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Poolman B, Modderman R, Reizer J. Lactose transport system of Streptococcus thermophilus. The role of histidine residues. J Biol Chem 1992. [DOI: 10.1016/s0021-9258(19)50402-1] [Citation(s) in RCA: 24] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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42
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Sugar—Cation Symport Systems in Bacteria. ACTA ACUST UNITED AC 1992. [DOI: 10.1016/s0074-7696(08)62676-5] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 04/21/2023]
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43
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Pourcher T, Deckert M, Bassilana M, Leblanc G. Melibiose permease of Escherichia coli: mutation of aspartic acid 55 in putative helix II abolishes activation of sugar binding by Na+ ions. Biochem Biophys Res Commun 1991; 178:1176-81. [PMID: 1872836 DOI: 10.1016/0006-291x(91)91016-6] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
An aspartic residue (Asp55) located in the putative transmembrane alpha-helix II of the melibiose(mel) permease of Escherichia coli was replaced by Cys using oligonucleotide-directed, site-specific mutagenesis. Although D55C permease is expressed at 0.7 times the level of wild type permease, the mutated mel permease loses the ability to catalyse Na+ or H+ coupled melibiose transport against a concentration gradient. (3H) p-nitrophenyl-alpha-D-galactoside (NPG) binding studies demonstrated that D55C permease binds the sugar co-substrate but Na+ (or Li+) ions do no longer enhance the affinity of D55C permease for the co-transported sugar. In addition sugar binding on D55C permease but not on wild type permease is inactivated by sulfhydryl reagents and the inhibition protected by an excess of melibiose. These observations suggest 1) that the negatively-charged Asp55 residue, expected to be within the membrane embedded domain near the NH2 extremity of mel permease, is in or near the Na(+)-binding site and 2) that the cation and sugar binding sites may be overlapping.
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Affiliation(s)
- T Pourcher
- Laboratoire J. Maetz, Département de Biologie Cellulaire et Moléculaire du Commissariat à l'Energie Atomique, Villefranche sur mer, France
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44
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Abstract
The cell membranes of various bacteria contain proton-linked transport systems for D-xylose, L-arabinose, D-galactose, D-glucose, L-rhamnose, L-fucose, lactose, and melibiose. The melibiose transporter of E. coli is linked to both Na+ and H+ translocation. The substrate and inhibitor specificities of the monosaccharide transporters are described. By locating, cloning, and sequencing the genes encoding the sugar/H+ transporters in E. coli, the primary sequences of the transport proteins have been deduced. Those for xylose/H+, arabinose/H+, and galactose/H+ transport are homologous to each other. Furthermore, they are just as similar to the primary sequences of the following: glucose transport proteins found in a Cyanobacterium, yeast, alga, rat, mouse, and man; proteins for transport of galactose, lactose, or maltose in species of yeast; and to a developmentally regulated protein of Leishmania for which a function is not yet established. Some of these proteins catalyze facilitated diffusion of the sugar without cation transport. From the alignments of the homologous amino acid sequences, predictions of common structural features can be made: there are likely to be twelve membrane-spanning alpha-helices, possibly in two groups of six; there is a central hydrophilic region, probably comprised largely of alpha-helix; the highly conserved amino acid residues (40-50 out of 472-522 total) form discrete patterns or motifs throughout the proteins that are presumably critical for substrate recognition and the molecular mechanism of transport. Some of these features are found also in other transport proteins for citrate, tetracycline, lactose, or melibiose, the primary sequences of which are not similar to each other or to the homologous series of transporters. The glucose/Na+ transporter of rabbit and man is different in primary sequence to all the other sugar transporters characterized, but it is homologous to the proline/Na+ transporter of E. coli, and there is evidence for its structural similarity to glucose/H+ transporters in Plants. In vivo and in vitro mutagenesis of the lactose/H+ and melibiose/Na+ (H+) transporters of E. coli has identified individual amino acid residues alterations of which affect sugar and/or cation recognition and parameters of transport. Most of the bacterial transport proteins have been identified and the lactose/H+ transporter has been purified. The directions of future investigations are discussed.
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Affiliation(s)
- P J Henderson
- Department of Biochemistry, University of Cambridge, United Kingdom
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