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Abstract
The topologies of α-helical membrane proteins are generally thought to be determined during their cotranslational insertion into the membrane. It is typically assumed that membrane topologies remain static after this process has ended. Recent findings, however, question this static view by suggesting that some parts of, or even the whole protein, can reorient in the membrane on a biologically relevant time scale. Here, we focus on antiparallel homo- or heterodimeric small multidrug resistance proteins and examine whether the individual monomers can undergo reversible topological inversion (flip flop) in the membrane until they are trapped in a fixed orientation by dimerization. By perturbing dimerization using various means, we show that the membrane orientation of a monomer is unaffected by the presence or absence of its dimerization partner. Thus, membrane-inserted monomers attain their final orientations independently of dimerization, suggesting that wholesale topological inversion is an unlikely event in vivo.
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2
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Woodall NB, Hadley S, Yin Y, Bowie JU. Complete topology inversion can be part of normal membrane protein biogenesis. Protein Sci 2017; 26:824-833. [PMID: 28168866 DOI: 10.1002/pro.3131] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/17/2017] [Revised: 02/01/2017] [Accepted: 02/02/2017] [Indexed: 01/14/2023]
Abstract
The topology of helical membrane proteins is generally defined during insertion of the transmembrane helices, yet it is now clear that it is possible for topology to change under unusual circumstances. It remains unclear, however, if topology reorientation is part of normal biogenesis. For dual topology dimer proteins such as the multidrug transporter EmrE, there may be evolutionary pressure to allow topology flipping so that the populations of both orientations can be equalized. We previously demonstrated that when EmrE is forced to insert in a distorted topology, topology flipping of the first transmembrane helix can occur during translation. Here, we show that topological malleability also extends to the C-terminal helix and that even complete topology inversion of the entire EmrE protein can occur after the full protein is translated and inserted. Thus, topology rearrangements are possible during normal biogenesis. Wholesale topology flipping is remarkable given the physical constraints of the membrane and expands the range of possible membrane protein folding pathways, both productive and detrimental.
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Affiliation(s)
- Nicholas B Woodall
- Department of Chemistry and Biochemistry, UCLA-DOE Institute, Molecular Biology Institute, University of California, Los Angeles, California
| | - Sarah Hadley
- Department of Chemistry and Biochemistry, UCLA-DOE Institute, Molecular Biology Institute, University of California, Los Angeles, California
| | - Ying Yin
- Department of Chemistry and Biochemistry, UCLA-DOE Institute, Molecular Biology Institute, University of California, Los Angeles, California
| | - James U Bowie
- Department of Chemistry and Biochemistry, UCLA-DOE Institute, Molecular Biology Institute, University of California, Los Angeles, California
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3
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Protonation-dependent conformational dynamics of the multidrug transporter EmrE. Proc Natl Acad Sci U S A 2016; 113:1220-5. [PMID: 26787875 DOI: 10.1073/pnas.1520431113] [Citation(s) in RCA: 49] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The small multidrug transporter from Escherichia coli, EmrE, couples the energetically uphill extrusion of hydrophobic cations out of the cell to the transport of two protons down their electrochemical gradient. Although principal mechanistic elements of proton/substrate antiport have been described, the structural record is limited to the conformation of the substrate-bound state, which has been shown to undergo isoenergetic alternating access. A central but missing link in the structure/mechanism relationship is a description of the proton-bound state, which is an obligatory intermediate in the transport cycle. Here we report a systematic spin labeling and double electron electron resonance (DEER) study that uncovers the conformational changes of EmrE subsequent to protonation of critical acidic residues in the context of a global description of ligand-induced structural rearrangements. We find that protonation of E14 leads to extensive rotation and tilt of transmembrane helices 1-3 in conjunction with repacking of loops, conformational changes that alter the coordination of the bound substrate and modulate its access to the binding site from the lipid bilayer. The transport model that emerges from our data posits a proton-bound, but occluded, resting state. Substrate binding from the inner leaflet of the bilayer releases the protons and triggers alternating access between inward- and outward-facing conformations of the substrate-loaded transporter, thus enabling antiport without dissipation of the proton gradient.
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4
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Lloris-Garcerá P, Slusky JSG, Seppälä S, Prieß M, Schäfer LV, von Heijne G. In vivo trp scanning of the small multidrug resistance protein EmrE confirms 3D structure models'. J Mol Biol 2013; 425:4642-51. [PMID: 23920359 DOI: 10.1016/j.jmb.2013.07.039] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/21/2013] [Revised: 07/10/2013] [Accepted: 07/28/2013] [Indexed: 11/28/2022]
Abstract
The quaternary structure of the homodimeric small multidrug resistance protein EmrE has been studied intensely over the past decade. Structural models derived from both two- and three-dimensional crystals show EmrE as an anti-parallel homodimer. However, the resolution of the structures is rather low and their relevance for the in vivo situation has been questioned. Here, we have challenged the available structural models by a comprehensive in vivo Trp scanning of all four transmembrane helices in EmrE. The results are in close agreement with the degree of lipid exposure of individual residues predicted from coarse-grained molecular dynamics simulations of the anti-parallel dimeric structure obtained by X-ray crystallography, strongly suggesting that the X-ray structure provides a good representation of the active in vivo form of EmrE.
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Affiliation(s)
- Pilar Lloris-Garcerá
- Center for Biomembrane Research, Department of Biochemistry and Biophysics, Stockholm University, SE-10691 Stockholm, Sweden
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5
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Kolbusz MA, Slotboom DJ, Lolkema JS. Role of individual positive charges in the membrane orientation and activity of transporters of the small multidrug resistance family. Biochemistry 2012; 51:8867-76. [PMID: 23043311 DOI: 10.1021/bi300854c] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The effect of individual positively charged residues on the orientation in the membrane was analyzed in three dual-topology transporters of the small multidrug resistance (SMR) family: AAVE4701aave of Acidovorax avenae, EMREecol of Escherichia coli, and RRUA0272rrub of Rhodospirillum rubrum. It is shown that (i) individual positive charges have different impacts on the orientation, (ii) positive charges that are conserved in the three different proteins do not have the same impact on the orientation, (iii) positive charges in odd- and even-numbered loops have different impacts, (iv) for some, but not all, the impact depends on the presence of other positive charges, and (v) proteins from which all positive charges are removed in some cases are dual-topology proteins and in other cases have a single orientation. A small number of positive charges placed in the loops of the latter proteins results in the violation of the so-called positive-inside rule that has been reported previously [Kolbusz, M. A., et al. (2010) J. Mol. Biol. 402, 127-138]. We conclude that each positive charge shifts the distribution between the two orientations toward the state that has the positive charge in the cytoplasm but that intrinsic factors other than positive charges determine the orientation as well. The ability of the mutants of AAVE4701aave and EMREecol to confer resistance against ethidium bromide revealed an essential role in catalysis for a conserved pair of positive charges in the second loop. No significant relation between activity and the relative orientation of the monomeric subunits in the dimer could be demonstrated.
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Affiliation(s)
- Magdalena A Kolbusz
- Molecular Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Groningen, The Netherlands
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6
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Bay DC, Turner RJ. Membrane composition influences the topology bias of bacterial integral membrane proteins. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2012; 1828:260-70. [PMID: 22982496 DOI: 10.1016/j.bbamem.2012.09.003] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/23/2012] [Revised: 08/31/2012] [Accepted: 09/07/2012] [Indexed: 11/19/2022]
Abstract
Small multidrug resistance (SMR) protein family members confer bacterial resistance to toxic antiseptics and are believed to function as dual topology oligomers. If dual topology is essential for SMR activity, then the topology bias should change as bacterial membrane lipid compositions alter to maintain a "neutral" topology bias. To test this hypothesis, a bioinformatic analysis of bacterial SMR protein sequences was performed to determine a membrane protein topology based on charged amino acid residues within loops, and termini regions according to the positive inside rule. Three bacterial lipid membrane parameters were examined, providing the proportion of polar lipid head group charges at the membrane surface (PLH), the relative hydrophobic fatty acid length (FAL), and the proportion of fatty acid unsaturation (FAU). Our analysis indicates that individual SMR pairs, and to a lesser extent SMR singleton topology biases, are significantly correlated to increasing PLH, FAL and FAU differences validating the hypothesis. Correlations between the topology biases of SMR proteins identified in Gram+ compared to Gram- species and each lipid parameter demonstrated a linear inverse relationship.
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Affiliation(s)
- Denice C Bay
- Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada T2N 1N4
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7
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Lloris-Garcerá P, Bianchi F, Slusky JSG, Seppälä S, Daley DO, von Heijne G. Antiparallel dimers of the small multidrug resistance protein EmrE are more stable than parallel dimers. J Biol Chem 2012; 287:26052-9. [PMID: 22700980 DOI: 10.1074/jbc.m112.357590] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The bacterial multidrug transporter EmrE is a dual-topology membrane protein and as such is able to insert into the membrane in two opposite orientations. The functional form of EmrE is a homodimer; however, the relative orientation of the subunits in the dimer is under debate. Using EmrE variants with fixed, opposite orientations in the membrane, we now show that, although the proteins are able to form parallel dimers, an antiparallel organization of the subunits in the dimer is preferred. Blue-native PAGE analyses of intact oligomers and disulfide cross-linking demonstrate that in membranes, the proteins form parallel dimers only if no oppositely orientated partner is present. Co-expression of oppositely orientated proteins almost exclusively yields antiparallel dimers. Finally, parallel dimers can be disrupted and converted into antiparallel dimers by heating of detergent-solubilized protein. Importantly, in vivo function is correlated clearly to the presence of antiparallel dimers. Our results suggest that an antiparallel arrangement of the subunits in the dimer is more stable than a parallel organization and likely corresponds to the functional form of the protein.
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Affiliation(s)
- Pilar Lloris-Garcerá
- Center for Biomembrane Research, Department of Biochemistry and Biophysics, Stockholm University SE-106 91 Stockholm, Sweden
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8
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Schuldiner S. Undecided membrane proteins insert in random topologies. Up, down and sideways: it does not really matter. Trends Biochem Sci 2012; 37:215-9. [PMID: 22444835 DOI: 10.1016/j.tibs.2012.02.006] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/27/2011] [Revised: 02/08/2012] [Accepted: 02/10/2012] [Indexed: 10/28/2022]
Abstract
It is usually assumed that to ensure proper function, membrane proteins must be inserted in a unique topology. However, a number of dimeric small multidrug transporters can function in the membrane in various topologies. Thus, the dimers can be a random mixture of NiCi (N and C termini facing the cell cytoplasm) and NoCo (N and C termini facing the outside) orientation. In addition, the dimer functions whether the two protomers are parallel (N and C termini of both protomers on the same side of the membrane) or antiparallel (N and C termini of each protomer on opposite sides of the membrane). This unique phenomenon provides strong support for a simple mechanism of transport where the directionality is determined solely by the driving force.
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Affiliation(s)
- Shimon Schuldiner
- Department of Biological Chemistry, Alexander Silberman Institute of Life Sciences, Hebrew University of Jerusalem, 91904 Jerusalem, Israel.
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9
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Spectroscopic analysis of small multidrug resistance protein EmrE in the presence of various quaternary cation compounds. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2012; 1818:1318-31. [PMID: 22326892 DOI: 10.1016/j.bbamem.2012.01.022] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/28/2011] [Revised: 01/09/2012] [Accepted: 01/26/2012] [Indexed: 12/12/2022]
Abstract
Escherichia coli EmrE protein is the archetypical member of the small multidrug resistance protein family in bacteria and confers host resistance to a wide assortment of toxic quaternary cation compounds by secondary active efflux. This protein can form a variety of multimers under various membrane mimetic conditions, and the consensus of most biochemical and biophysical studies indicate that the active form is a dimer. The purpose of this study is to characterize the conformation of organically extracted detergent solubilized EmrE protein known to predominate as monomer yet demonstrates ligand binding ability. Active site EmrE-E14 replacements were also examined as functionally inactive controls for this study. EmrE was solubilized in detergents, sodium dodecyl sulfate (SDS) and dodecyl maltoside (DDM), and protein conformation was examined in the presence of four known quaternary cation compound (QCC) substrates, tetraphenyl phosphonium (TPP), methyl viologen, cetylpyridinium, and ethidium. SDS-Tricine PAGE analysis of both detergent solubilized proteins revealed that DDM-EmrE preparations enhanced the formation of dimer (and in some cases trimer) forms in the presence of all four QCC above 25 QCC:1 EmrE molar ratios. Examination of EmrE and its active site variant tertiary structures in DDM by circular dichroism spectropolarimetry, intrinsic Trp fluorescence quenching and second order derivative ultraviolet absorbance revealed that the variant fails to bind TPP but interacts with all other compounds. The results of this study show that monomeric detergent solubilized EmrE is capable of forming multimeric complexes that are enhanced by chemically diverse QCCs.
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Antiparallel EmrE exports drugs by exchanging between asymmetric structures. Nature 2011; 481:45-50. [PMID: 22178925 PMCID: PMC3253143 DOI: 10.1038/nature10703] [Citation(s) in RCA: 161] [Impact Index Per Article: 12.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/20/2011] [Accepted: 11/07/2011] [Indexed: 11/14/2022]
Abstract
Small multidrug resistance (SMR) transporters provide an ideal system to study the minimal requirements for active transport. EmrE is an E. coli SMR transporter that exports a broad class of polyaromatic cation substrates, thus conferring resistance to drug compounds matching this chemical description. However, a great deal of controversy has surrounded the topology of the EmrE homodimer. Here we show that asymmetric antiparallel EmrE exchanges between inward- and outward-facing states that are identical except that they have opposite orientation in the membrane. We quantitatively measure the global conformational exchange between these two states for substrate-bound EmrE in bicelles using solution NMR dynamics experiments. FRET reveals that the monomers within each dimer are antiparallel, and paramagnetic relaxation enhancement NMR experiments demonstrate differential water accessibility of the two monomers within each dimer. Our experiments reveal a “dynamic symmetry” that reconciles the asymmetric EmrE structure with the functional symmetry of residues in the active site.
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11
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Henzler-Wildman K. Analyzing conformational changes in the transport cycle of EmrE. Curr Opin Struct Biol 2011; 22:38-43. [PMID: 22100111 DOI: 10.1016/j.sbi.2011.10.004] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/15/2011] [Revised: 10/12/2011] [Accepted: 10/13/2011] [Indexed: 11/24/2022]
Abstract
The small multidrug resistance transporters represent a unique model system for studying the mechanism of secondary active transport and membrane protein evolution. However, this seemingly simple protein has been highly controversial. Recent studies have provided experimental evidence that EmrE exists as an asymmetric dimer that exchanges between identical inward-facing and outward-facing states. Re-examination of the published literature in light of these findings fills in many details of the microscopic steps in the transport cycle. Future work will need to examine how the symmetry observed in vitro affects EmrE function in the asymmetric environment of its native Escherichia coli membrane.
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Affiliation(s)
- Katherine Henzler-Wildman
- Department of Biochemistry and Molecular Biophysics Washington University School of Medicine, St. Louis, MO 63110, United States.
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12
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Global transcriptome analysis of the E. coli O157 response to Agrimonia pilosa extract. Mol Cell Toxicol 2011. [DOI: 10.1007/s13273-011-0036-7] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
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13
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Modulation of substrate efflux in bacterial small multidrug resistance proteins by mutations at the dimer interface. J Bacteriol 2011; 193:5929-35. [PMID: 21890698 DOI: 10.1128/jb.05846-11] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Bacteria evade the effects of cytotoxic compounds through the efflux activity of membrane-bound transporters such as the small multidrug resistance (SMR) proteins. Consisting typically of ca. 110 residues with four transmembrane (TM) α-helices, crystallographic studies have shown that TM helix 1 (TM1) through TM helix 3 (TM3) of each monomer create a substrate binding "pocket" within the membrane bilayer, while a TM4-TM4 interaction accounts for the primary dimer formation. Previous work from our lab has characterized a highly conserved small-residue heptad motif in the Halobacterium salinarum transporter Hsmr as (90)GLXLIXXGV(98) that lies along the TM4-TM4 dimer interface of SMR proteins as required for function. Focusing on conserved positions 91, 93, 94, and 98, we substituted the naturally occurring Hsmr residue for Ala, Phe, Ile, Leu, Met, and Val at each position in the Hsmr TM4-TM4 interface. Large-residue replacements were studied for their ability to dimerize on SDS-polyacrylamide gels, to bind the cytotoxic compound ethidium bromide, and to confer resistance by efflux. Although the relative activity of mutants did not correlate with dimer strength for all mutants, all functional mutants lay within 10% of dimerization relative to the wild type (WT), suggesting that the optimal dimer strength at TM4 is required for proper efflux. Furthermore, nonfunctional substitutions at the center of the dimerization interface that do not alter dimer strength suggest a dynamic TM4-TM4 "pivot point" that responds to the efflux requirements of different substrates. This functionally critical region represents a potential target for inhibiting the ability of bacteria to evade the effects of cytotoxic compounds.
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14
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Amadi ST, Koteiche HA, Mishra S, Mchaourab HS. Structure, dynamics, and substrate-induced conformational changes of the multidrug transporter EmrE in liposomes. J Biol Chem 2010; 285:26710-8. [PMID: 20551331 PMCID: PMC2924113 DOI: 10.1074/jbc.m110.132621] [Citation(s) in RCA: 45] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/09/2010] [Revised: 06/14/2010] [Indexed: 11/06/2022] Open
Abstract
EmrE, a member of the small multidrug transporters superfamily, extrudes positively charged hydrophobic compounds out of Escherichia coli cytoplasm in exchange for inward movement of protons down their electrochemical gradient. Although its transport mechanism has been thoroughly characterized, the structural basis of energy coupling and the conformational cycle mediating transport have yet to be elucidated. In this study, EmrE structure in liposomes and the substrate-induced conformational changes were investigated by systematic spin labeling and EPR analysis. Spin label mobilities and accessibilities describe a highly dynamic ligand-free (apo) conformation. Dipolar coupling between spin labels across the dimer reveals at least two spin label populations arising from different packing interfaces of the EmrE dimer. One population is consistent with antiparallel arrangement of the monomers, although the EPR parameters suggest deviations from the crystal structure of substrate-bound EmrE. Resolving these discrepancies requires an unusual disposition of TM3 relative to the membrane-water interface and a kink in its backbone that enables bending of its C-terminal part. Binding of the substrate tetraphenylphosphonium changes the environment of spin labels and their proximity in three transmembrane helices. The underlying conformational transition involves repacking of TM1, tilting of TM2, and changes in the backbone configurations of TM3 and the adjacent loop connecting it to TM4. A dynamic apo conformation is necessary for the polyspecificity of EmrE allowing the binding of structurally diverse substrates. The flexibility of TM3 may play a critical role in movement of substrates across the membrane.
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Affiliation(s)
- Sepan T. Amadi
- From the Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee 37232
| | - Hanane A. Koteiche
- From the Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee 37232
| | - Sanjay Mishra
- From the Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee 37232
| | - Hassane S. Mchaourab
- From the Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee 37232
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15
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Nasie I, Steiner-Mordoch S, Gold A, Schuldiner S. Topologically random insertion of EmrE supports a pathway for evolution of inverted repeats in ion-coupled transporters. J Biol Chem 2010; 285:15234-15244. [PMID: 20308069 DOI: 10.1074/jbc.m110.108746] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Inverted repeats in ion-coupled transporters have evolved independently in many unrelated families. It has been suggested that this inverted symmetry is an essential element of the mechanism that allows for the conformational transitions in transporters. We show here that small multidrug transporters offer a model for the evolution of such repeats. This family includes both homodimers and closely related heterodimers. In the former, the topology determinants, evidently identical in each protomer, are weak, and we show that for EmrE, an homodimer from Escherichia coli, the insertion into the membrane is random, and dimers are functional whether they insert into the cytoplasmic membrane with the N- and C-terminal domains facing the inside or the outside of the cell. Also, mutants designed to insert with biased topology are functional regardless of the topology. In the case of EbrAB, a heterodimer homologue supposed to interact antiparallel, we show that one of the subunits, EbrB, can also function as a homodimer, most likely in a parallel mode. In addition, the EmrE homodimer can be forced to an antiparallel topology by fusion of an additional transmembrane segment. The simplicity of the mechanism of coupling ion and substrate transport and the few requirements for substrate recognition provide the robustness necessary to tolerate such a unique and unprecedented ambiguity in the interaction of the subunits and in the dimer topology relative to the membrane. The results suggest that the small multidrug transporters are at an evolutionary junction and provide a model for the evolution of structure of transport proteins.
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Affiliation(s)
- Iris Nasie
- Department of Biological Chemistry, Alexander A. Silberman Institute of Life Sciences, Hebrew University of Jerusalem, 91904 Jerusalem, Israel
| | - Sonia Steiner-Mordoch
- Department of Biological Chemistry, Alexander A. Silberman Institute of Life Sciences, Hebrew University of Jerusalem, 91904 Jerusalem, Israel
| | - Ayala Gold
- Department of Biological Chemistry, Alexander A. Silberman Institute of Life Sciences, Hebrew University of Jerusalem, 91904 Jerusalem, Israel
| | - Shimon Schuldiner
- Department of Biological Chemistry, Alexander A. Silberman Institute of Life Sciences, Hebrew University of Jerusalem, 91904 Jerusalem, Israel.
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16
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Miller D, Charalambous K, Rotem D, Schuldiner S, Curnow P, Booth PJ. In vitro Unfolding and Refolding of the Small Multidrug Transporter EmrE. J Mol Biol 2009; 393:815-32. [DOI: 10.1016/j.jmb.2009.08.039] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/19/2009] [Revised: 08/12/2009] [Accepted: 08/14/2009] [Indexed: 11/28/2022]
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17
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Abstract
AbstractIn order to fulfill their function, membrane transport proteins have to cycle through a number of conformational and/or energetic states. Thus, understanding the role of conformational dynamics seems to be the key for elucidation of the functional mechanism of these proteins. However, membrane proteins in general are often difficult to express heterologously and in sufficient amounts for structural studies. It is especially challenging to trap a stable energy minimum, e.g., for crystallographic analysis. Furthermore, crystallization is often only possible by subjecting the protein to conditions that do not resemble its native environment and crystals can only be snapshots of selected conformational states. Nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy are complementary methods that offer unique possibilities for studying membrane proteins in their natural membrane environment and for investigating functional conformational changes, lipid interactions, substrate-lipid and substrate-protein interactions, oligomerization states and overall dynamics of membrane transporters. Here, we review recent progress in the field including studies from primary and secondary active transporters.
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18
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EmrE, a model for studying evolution and mechanism of ion-coupled transporters. BIOCHIMICA ET BIOPHYSICA ACTA-PROTEINS AND PROTEOMICS 2009; 1794:748-62. [DOI: 10.1016/j.bbapap.2008.12.018] [Citation(s) in RCA: 130] [Impact Index Per Article: 8.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/27/2008] [Revised: 12/16/2008] [Accepted: 12/17/2008] [Indexed: 11/23/2022]
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19
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Manor J, Mukherjee P, Lin YS, Leonov H, Skinner JL, Zanni MT, Arkin IT. Gating mechanism of the influenza A M2 channel revealed by 1D and 2D IR spectroscopies. Structure 2009; 17:247-54. [PMID: 19217395 DOI: 10.1016/j.str.2008.12.015] [Citation(s) in RCA: 104] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/05/2008] [Revised: 12/04/2008] [Accepted: 12/07/2008] [Indexed: 11/19/2022]
Abstract
The pH-controlled M2 protein from influenza A is a critical component of the virus and serves as a target for the aminoadamantane antiflu agents that block its H+ channel activity. To better understand its H+ gating mechanism, we investigated M2 in lipid bilayers with a new combination of IR spectroscopies and theory. Linear Fourier transform infrared (FTIR) spectroscopy was used to measure the precise orientation of the backbone carbonyl groups, and 2D infrared (IR) spectroscopy was used to identify channel-lining residues. At low pH (open state), our results match previously published solid-state NMR and X-ray structures remarkably well. However, at neutral pH when the channel is closed, our measurements indicate that a large conformational change occurs that is consistent with the transmembrane alpha-helices rotating by one amino acid register--a structural rearrangement not previously observed. The combination of simulations and isotope-labeled FTIR and 2D IR spectroscopies provides a noninvasive means of interrogating the structures of membrane proteins in general and ion channels in particular.
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Affiliation(s)
- Joshua Manor
- The Alexander Silberman Institute of Life Sciences, Department of Biological Chemistry, The Hebrew University of Jerusalem, Edmund J. Safra Campus, Givat-Ram, Jerusalem 91904, Israel
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20
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Poulsen BE, Rath A, Deber CM. The assembly motif of a bacterial small multidrug resistance protein. J Biol Chem 2009; 284:9870-5. [PMID: 19224913 DOI: 10.1074/jbc.m900182200] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Multidrug transporters such as the small multidrug resistance (SMR) family of bacterial integral membrane proteins are capable of conferring clinically significant resistance to a variety of common therapeutics. As antiporter proteins of approximately 100 amino acids, SMRs must self-assemble into homo-oligomeric structures for efflux of drug molecules. Oligomerization centered at transmembrane helix four (TM4) has been implicated in SMR assembly, but the full complement of residues required to mediate its self-interaction remains to be characterized. Here, we use Hsmr, the 110-residue SMR family member of the archaebacterium Halobacterium salinarum, to determine the TM4 residue motif required to mediate drug resistance and SMR self-association. Twelve single point mutants that scan the central portion of the TM4 helix (residues 85-104) were constructed and were tested for their ability to confer resistance to the cytotoxic compound ethidium bromide. Six residues were found to be individually essential for drug resistance activity (Gly(90), Leu(91), Leu(93), Ile(94), Gly(97), and Val(98)), defining a minimum activity motif of (90)GLXLIXXGV(98) within TM4. When the propensity of these mutants to dimerize on SDS-PAGE was examined, replacements of all but Ile resulted in approximately 2-fold reduction of dimerization versus the wild-type antiporter. Our work defines a minimum activity motif of (90)GLXLIXXGV(98) within TM4 and suggests that this sequence mediates TM4-based SMR dimerization along a single helix surface, stabilized by a small residue heptad repeat sequence. These TM4-TM4 interactions likely constitute the highest affinity locus for disruption of SMR function by directly targeting its self-assembly mechanism.
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Affiliation(s)
- Bradley E Poulsen
- Division of Molecular Structure and Function, Research Institute, Hospital for Sick Children, Toronto, Ontario M5G 1X8
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21
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Korkhov VM, Tate CG. An emerging consensus for the structure of EmrE. ACTA CRYSTALLOGRAPHICA. SECTION D, BIOLOGICAL CRYSTALLOGRAPHY 2009; 65:186-92. [PMID: 19171974 PMCID: PMC2631640 DOI: 10.1107/s0907444908036640] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 09/17/2008] [Accepted: 11/07/2008] [Indexed: 11/17/2022]
Abstract
The archetypical member of the small multidrug-resistance family is EmrE, a multidrug transporter that extrudes toxic polyaromatic cations from the cell coupled to the inward movement of protons down a concentration gradient. The architecture of EmrE was first defined from the analysis of two-dimensional crystals by cryoelectron microscopy (cryo-EM), which showed that EmrE was an unusual asymmetric dimer formed from a bundle of eight alpha-helices. The most favoured interpretation of the structure was that the monomers were oriented in opposite orientations in the membrane in an antiparallel orientation. A model was subsequently built based upon the cryo-EM data and evolutionary constraints and this model was consistent with mutagenic data indicating which amino-acid residues were important for substrate binding and transport. Two X-ray structures that differed significantly from the cryo-EM structure were subsequently retracted owing to a data-analysis error. However, the revised X-ray structure with substrate bound is extremely similar to the model built from the cryo-EM structure (r.m.s.d. of 1.4 A), suggesting that the proposed antiparallel orientation of the monomers is indeed correct; this represents a new structural paradigm in membrane-protein structures. The vast majority of mutagenic and biochemical data corroborate this structure, although cross-linking studies and recent EPR data apparently support a model of EmrE that contains parallel dimers.
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Charalambous K, Miller D, Curnow P, Booth PJ. Lipid bilayer composition influences small multidrug transporters. BMC BIOCHEMISTRY 2008; 9:31. [PMID: 19032749 PMCID: PMC2605743 DOI: 10.1186/1471-2091-9-31] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 08/01/2008] [Accepted: 11/25/2008] [Indexed: 11/10/2022]
Abstract
BACKGROUND Membrane proteins are influenced by their surrounding lipids. We investigate the effect of bilayer composition on the membrane transport activity of two members of the small multidrug resistance family; the Escherichia coli transporter, EmrE and the Mycobacterium tuberculosis, TBsmr. In particular we address the influence of phosphatidylethanolamine and anionic lipids on the activity of these multidrug transporters. Phosphatidylethanolamine lipids are native to the membranes of both transporters and also alter the lateral pressure profile of a lipid bilayer. Lipid bilayer lateral pressures affect membrane protein insertion, folding and activity and have been shown to influence reconstitution, topology and activity of membrane transport proteins. RESULTS Both EmrE and TBsmr are found to exhibit a similar dependence on lipid composition, with phosphatidylethanolamine increasing methyl viologen transport. Anionic lipids also increase transport for both EmrE and TBsmr, with the proteins showing a preference for their most prevalent native anionic lipid headgroup; phosphatidylglycerol for EmrE and phosphatidylinositol for TBsmr. CONCLUSION These findings show that the physical state of the membrane modifies drug transport and that substrate translocation is dependent on in vitro lipid composition. Multidrug transport activity seems to respond to alterations in the lateral forces exerted upon the transport proteins by the bilayer.
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Bay DC, Rommens KL, Turner RJ. Small multidrug resistance proteins: A multidrug transporter family that continues to grow. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2008; 1778:1814-38. [DOI: 10.1016/j.bbamem.2007.08.015] [Citation(s) in RCA: 183] [Impact Index Per Article: 11.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/06/2007] [Revised: 08/07/2007] [Accepted: 08/14/2007] [Indexed: 11/17/2022]
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Katzen F, Fletcher JE, Yang JP, Kang D, Peterson TC, Cappuccio JA, Blanchette CD, Sulchek T, Chromy BA, Hoeprich PD, Coleman MA, Kudlicki W. Insertion of Membrane Proteins into Discoidal Membranes Using a Cell-Free Protein Expression Approach. J Proteome Res 2008; 7:3535-42. [DOI: 10.1021/pr800265f] [Citation(s) in RCA: 111] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Federico Katzen
- Invitrogen Corporation, 5791 Van Allen Way, Carlsbad, California 92008, and Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94551
| | - Julia E. Fletcher
- Invitrogen Corporation, 5791 Van Allen Way, Carlsbad, California 92008, and Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94551
| | - Jian-Ping Yang
- Invitrogen Corporation, 5791 Van Allen Way, Carlsbad, California 92008, and Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94551
| | - Douglas Kang
- Invitrogen Corporation, 5791 Van Allen Way, Carlsbad, California 92008, and Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94551
| | - Todd C. Peterson
- Invitrogen Corporation, 5791 Van Allen Way, Carlsbad, California 92008, and Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94551
| | - Jenny A. Cappuccio
- Invitrogen Corporation, 5791 Van Allen Way, Carlsbad, California 92008, and Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94551
| | - Craig D. Blanchette
- Invitrogen Corporation, 5791 Van Allen Way, Carlsbad, California 92008, and Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94551
| | - Todd Sulchek
- Invitrogen Corporation, 5791 Van Allen Way, Carlsbad, California 92008, and Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94551
| | - Brett A. Chromy
- Invitrogen Corporation, 5791 Van Allen Way, Carlsbad, California 92008, and Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94551
| | - Paul D. Hoeprich
- Invitrogen Corporation, 5791 Van Allen Way, Carlsbad, California 92008, and Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94551
| | - Matthew A. Coleman
- Invitrogen Corporation, 5791 Van Allen Way, Carlsbad, California 92008, and Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94551
| | - Wieslaw Kudlicki
- Invitrogen Corporation, 5791 Van Allen Way, Carlsbad, California 92008, and Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94551
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25
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Elbaz Y, Salomon T, Schuldiner S. Identification of a glycine motif required for packing in EmrE, a multidrug transporter from Escherichia coli. J Biol Chem 2008; 283:12276-83. [PMID: 18321856 PMCID: PMC2431008 DOI: 10.1074/jbc.m710338200] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/09/2007] [Revised: 02/25/2008] [Indexed: 01/02/2023] Open
Abstract
Glycine residues may play functional and structural roles in membrane proteins. In this work we studied the role of glycine residues in EmrE, a small multidrug transporter from Escherichia coli. EmrE extrudes various drugs across the plasma membrane in exchange with protons and, as a result, confers resistance against their toxic effects. Each of 12 glycine residues was replaced by site-directed mutagenesis. Four of the 12 glycine residues in EmrE are evolutionary conserved within the small multidrug resistance family of multidrug transporters. Our analysis reveals that only two (Gly-67 and Gly-97) of these four highly conserved residues are essential for transporter activity. Moreover, two glycine positions that are less conserved, Gly-17 and Gly-90, demonstrate also a nil phenotype when substituted. Our present results identifying Gly-17 and Gly-67 as irreplaceable reinforce the importance of previously defined functional clusters. Two essential glycine residues, Gly-90 and Gly-97, form a protein motif in which glycine residues are separated by six other residues (GG7). Upon substitution of glycine in these positions, the protein ability to form dimers is impaired as evaluated by cross-linking and pull-down experiments.
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Affiliation(s)
- Yael Elbaz
- Department of Biological Chemistry, Alexander A. Silberman Institute of Life Sciences, Hebrew University of Jerusalem, 91904 Jerusalem, Israel
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26
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Lehner I, Basting D, Meyer B, Haase W, Manolikas T, Kaiser C, Karas M, Glaubitz C. The Key Residue for Substrate Transport (Glu14) in the EmrE Dimer Is Asymmetric. J Biol Chem 2008; 283:3281-3288. [DOI: 10.1074/jbc.m707899200] [Citation(s) in RCA: 49] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
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27
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Higashi K, Ishigure H, Demizu R, Uemura T, Nishino K, Yamaguchi A, Kashiwagi K, Igarashi K. Identification of a spermidine excretion protein complex (MdtJI) in Escherichia coli. J Bacteriol 2008; 190:872-8. [PMID: 18039771 PMCID: PMC2223573 DOI: 10.1128/jb.01505-07] [Citation(s) in RCA: 74] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/19/2007] [Accepted: 11/02/2007] [Indexed: 11/20/2022] Open
Abstract
A spermidine excretion protein in Escherichia coli was looked for among 33 putative drug exporters thus far identified. Cell toxicity and inhibition of growth due to overaccumulation of spermidine were examined in an E. coli strain deficient in spermidine acetyltransferase, an enzyme that metabolizes spermidine. Toxicity and inhibition of cell growth by spermidine were recovered in cells transformed with pUCmdtJI or pMWmdtJI, encoding MdtJ and MdtI, which belong to the small multidrug resistance family of drug exporters. Both mdtJ and mdtI are necessary for recovery from the toxicity of overaccumulated spermidine. It was also found that the level of mdtJI mRNA was increased by spermidine. The spermidine content in cells cultured in the presence of 2 mM spermidine was decreased, and excretion of spermidine from cells was enhanced by MdtJI, indicating that the MdtJI complex can catalyze excretion of spermidine from cells. It was found that Tyr4, Trp5, Glu15, Tyr45, Tyr61, and Glu82 in MdtJ and Glu5, Glu19, Asp60, Trp68, and Trp81 in MdtI are involved in the excretion activity of MdtJI.
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Affiliation(s)
- Kyohei Higashi
- Graduate School of Pharmaceutical Sciences, Chiba University, 1-8-1 Inohana, Chuo-ku, Chiba 260-8675, Japan
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Steiner-Mordoch S, Soskine M, Solomon D, Rotem D, Gold A, Yechieli M, Adam Y, Schuldiner S. Parallel topology of genetically fused EmrE homodimers. EMBO J 2007; 27:17-26. [PMID: 18059473 DOI: 10.1038/sj.emboj.7601951] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2007] [Accepted: 11/14/2007] [Indexed: 11/09/2022] Open
Abstract
EmrE is a small H+-coupled multidrug transporter in Escherichia coli. Claims have been made for an antiparallel topology of this homodimeric protein. However, our own biochemical studies performed with detergent-solubilized purified protein support a parallel topology of the protomers. We developed an alternative approach to constrain the relative topology of the protomers within the dimer so that their activity can be assayed also in vivo before biochemical handling. Tandem EmrE was built with two identical monomers genetically fused tail to head (C-terminus of the first to N-terminus of the second monomer) with hydrophilic linkers of varying length. All the constructs conferred resistance to ethidium by actively removing it from the cytoplasm. The purified proteins bound substrate and transported methyl viologen into proteoliposomes by a proton-dependent mechanism. A tandem where one of the essential glutamates was replaced with glutamine transported only monovalent substrates and displayed a modified stoichiometry. The results support a parallel topology of the protomers in the functional dimer. The implications regarding insertion and evolution of membrane proteins are discussed.
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Affiliation(s)
- Sonia Steiner-Mordoch
- Department of Biological Chemistry, Alexander Silberman Institute of Life Sciences, The Hebrew University of Jerusalem, Jerusalem, Israel
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Abstract
EmrE, a multidrug transporter from Escherichia coli, functions as a homodimer of a small four-transmembrane protein. The membrane insertion topology of the two monomers is controversial. Although the EmrE protein was reported to have a unique orientation in the membrane, models based on electron microscopy and now defunct x-ray structures, as well as recent biochemical studies, posit an antiparallel dimer. We have now reanalyzed our x-ray data on EmrE. The corrected structures in complex with a transport substrate are highly similar to the electron microscopy structure. The first three transmembrane helices from each monomer surround the substrate binding chamber, whereas the fourth helices participate only in dimer formation. Selenomethionine markers clearly indicate an antiparallel orientation for the monomers, supporting a "dual topology" model.
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30
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31
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Kikukawa T, Miyauchi S, Araiso T, Kamo N, Nara T. Anti-parallel membrane topology of two components of EbrAB, a multidrug transporter. Biochem Biophys Res Commun 2007; 358:1071-5. [PMID: 17517373 DOI: 10.1016/j.bbrc.2007.05.032] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/01/2007] [Accepted: 05/07/2007] [Indexed: 11/26/2022]
Abstract
EbrAB is a multidrug-resistance transporter in Bacillus subtilis that belongs to the small multidrug resistance, and requires two polypeptides of both EbrA and EbrB, implying that it functions in the hetero-dimeric state. In this study, we investigated the transmembrane topologies of EbrA and EbrB. Various single-cysteine mutants were expressed in Escherichia coli cells, and the efflux activity was measured. Only mutants having a high activity were used for the topology experiments. The reactivity of a membrane impermeable NEM-fluorescein against the single cysteine of these fully functional mutants was examined when this reactive fluorophore was applied either from the outside or both sides of the cell membrane or in the denatured state. The results clearly showed that EbrA and EbrB have the opposite orientation within the membrane or an anti-parallel configuration.
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Affiliation(s)
- Takashi Kikukawa
- Laboratory of Biomolecular Systems, Creative Research Initiative Sosei (CRIS), Hokkaido University, Sapporo 001-0021, Japan.
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32
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Schuldiner S. When biochemistry meets structural biology: the cautionary tale of EmrE. Trends Biochem Sci 2007; 32:252-8. [PMID: 17452106 DOI: 10.1016/j.tibs.2007.04.002] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/01/2007] [Revised: 03/19/2007] [Accepted: 04/13/2007] [Indexed: 11/23/2022]
Abstract
When biochemistry meets structural biology a more complete understanding of the mechanism of biological macromolecules is usually achieved. Several high-resolution structures of ion-coupled transporters have enriched the understanding of mechanisms of substrate recognition, translocation and coupling of substrate fluxes. However, two X-ray structures of EmrE, the smallest ion-coupled multi-drug transporter, raised questions over the veracity of the structural model and represented a cautionary tale about the difficulty of determining the 3D structures of membrane proteins and the dangers of ignoring biochemical results. The 3D structures of EmrE have since been retracted because of faulty software, but the suggestion that the protomers in the dimer are in an antiparallel topological orientation sparked controversy that is still ongoing.
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Affiliation(s)
- Shimon Schuldiner
- Department of Biological Chemistry, Alexander Silberman Institute of Life Sciences, Hebrew University of Jerusalem, 91904 Jerusalem, Israel.
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33
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Abstract
The acquisition of multidrug resistance is a serious impediment to improved healthcare. Multidrug resistance is most frequently due to active transporters that pump a broad spectrum of chemically distinct, cytotoxic molecules out of cells, including antibiotics, antimalarials, herbicides and cancer chemotherapeutics in humans. The paradigm multidrug transporter, mammalian P-glycoprotein, was identified 30 years ago. Nonetheless, success in overcoming or circumventing multidrug resistance in a clinical setting has been modest. Recent structural and biochemical data for several multidrug transporters now provide mechanistic insights into how they work. Organisms have evolved several elegant solutions to ridding the cell of such cytotoxic compounds. Answers are emerging to questions such as how multispecificity for different drugs is achieved, why multidrug resistance arises so readily, and what chance there is of devising a clinical solution.
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Affiliation(s)
- Christopher F Higgins
- MRC Clinical Sciences Centre, Imperial College, Hammersmith Hospital Campus, Du Cane Road, London W12 0NN, UK.
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34
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Affiliation(s)
- Bert Poolman
- Department of Biochemistry, Groningen Biomolecular Sciences and Biotechnology Institute, and Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, Netherlands.
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35
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Rapp M, Seppälä S, Granseth E, von Heijne G. Emulating Membrane Protein Evolution by Rational Design. Science 2007; 315:1282-4. [PMID: 17255477 DOI: 10.1126/science.1135406] [Citation(s) in RCA: 111] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
How do integral membrane proteins evolve in size and complexity? Using the small multidrug-resistance protein EmrE from Escherichia coli as a model, we experimentally demonstrated that the evolution of membrane proteins composed of two homologous but oppositely oriented domains can occur in a small number of steps: An original dual-topology protein evolves, through a gene-duplication event, to a heterodimer formed by two oppositely oriented monomers. This simple evolutionary pathway can explain the frequent occurrence of membrane proteins with an internal pseudo-two-fold symmetry axis in the plane of the membrane.
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Affiliation(s)
- Mikaela Rapp
- Center for Biomembrane Research, Department of Biochemistry and Biophysics, Stockholm University, SE-106 91 Stockholm, Sweden
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