Purified reconstituted lac carrier protein from Escherichia coli is fully functional

The Complex GH32 Enzyme Orchestra from Priestia megaterium Holds the Key to Better Discriminate Sucrose-6-phosphate Hydrolases from Other β-Fructofuranosidases in Bacteria

Inulin is widely used as a prebiotic and emerging as a priming compound to counteract plant diseases. We isolated inulin-degrading strains from the lettuce phyllosphere, identified as Bacillus subtilis and Priestia megaterium, species hosting well-known biocontrol organisms. To better understand their varying inulin degradation strategies, three intracellular β-fructofuranosidases from P. megaterium NBRC15308 were characterized after expression in Escherichia coli: a predicted sucrose-6-phosphate (Suc6P) hydrolase (SacAP1, supported by molecular docking), an exofructanase (SacAP2), and an invertase (SacAP3). Based on protein multiple sequence and structure alignments of bacterial glycoside hydrolase family 32 enzymes, we identified conserved residues predicted to be involved in binding phosphorylated (Suc6P hydrolases) or nonphosphorylated substrates (invertases and fructanases). Suc6P hydrolases feature positively charged residues near the structural catalytic pocket (histidine, arginine, or lysine), whereas other β-fructofuranosidases contain tryptophans. This correlates with our phylogenetic tree, grouping all predicted Suc6P hydrolases in a clan associated with genomic regions coding for transporters involved in substrate phosphorylation. These results will help to discriminate between Suc6P hydrolases and other β-fructofuranosidases in future studies and to better understand the interaction of B. subtilis and P. megaterium endophytes with sucrose and/or fructans, sugars naturally present in plants or exogenously applied in the context of defense priming.

Establishing mammalian GLUT kinetics and lipid composition influences in a reconstituted-liposome system

Glucose transporters (GLUTs) are essential for organism-wide glucose homeostasis in mammals, and their dysfunction is associated with numerous diseases, such as diabetes and cancer. Despite structural advances, transport assays using purified GLUTs have proven to be difficult to implement, hampering deeper mechanistic insights. Here, we have optimized a transport assay in liposomes for the fructose-specific isoform GLUT5. By combining lipidomic analysis with native MS and thermal-shift assays, we replicate the GLUT5 transport activities seen in crude lipids using a small number of synthetic lipids. We conclude that GLUT5 is only active under a specific range of membrane fluidity, and that human GLUT1-4 prefers a similar lipid composition to GLUT5. Although GLUT3 is designated as the high-affinity glucose transporter, in vitro D-glucose kinetics demonstrates that GLUT1 and GLUT3 actually have a similar K_M, but GLUT3 has a higher turnover. Interestingly, GLUT4 has a high K_M for D-glucose and yet a very slow turnover, which may have evolved to ensure uptake regulation by insulin-dependent trafficking. Overall, we outline a much-needed transport assay for measuring GLUT kinetics and our analysis implies that high-levels of free fatty acid in membranes, as found in those suffering from metabolic disorders, could directly impair glucose uptake.

Mechanistic aspects of IPTG (isopropylthio-β-galactoside) transport across the cytoplasmic membrane of Escherichia coli-a rate limiting step in the induction of recombinant protein expression

Coupling transcription of a cloned gene to the lac operon with induction by isopropylthio-β-galactoside (IPTG) has been a favoured approach for recombinant protein expression using Escherichia coli as a heterologous host for more than six decades. Despite a wealth of experimental data gleaned over this period, a quantitative relationship between extracellular IPTG concentration and consequent levels of recombinant protein expression remains surprisingly elusive across a broad spectrum of experimental conditions. This is because gene expression under lac operon regulation is tightly correlated with intracellular IPTG concentration due to allosteric regulation of the lac repressor protein (lacY). An in-silico mathematical model established that uptake of IPTG across the cytoplasmic membrane of E. coli by simple diffusion was negligible. Conversely, lacY mediated active transport was a rapid process, taking only some seconds for internal and external IPTG concentrations to equalize. Optimizing kcat and KM parameters by targeted mutation of the galactoside binding site in lacY could be a future strategy to improve the performance of recombinant protein expression. For example, if kcat were reduced whilst KM was increased, active transport of IPTG across the cytoplasmic membrane would be reduced, thereby lessening the metabolic burden on the cell and expediating accumulation of recombinant protein. The computational model described herein is made freely available and is amenable to optimize recombinant protein expression in other heterologous hosts.

ONE-SENTENCE SUMMARY

A computational model made freely available to optimize recombinant protein expression in Escherichia coli other heterologous hosts.

Investigation of sugar binding kinetics of the E. coli sugar/H+ symporter XylE using solid supported membrane-based electrophysiology

Bacterial transporters are difficult to study using conventional electrophysiology because of their low transport rates and the small size of bacterial cells. Here, we applied solid-supported membrane–based electrophysiology to derive kinetic parameters of sugar translocation by the Escherichia coli xylose permease (XylE), including functionally relevant mutants. Many aspects of the fucose permease (FucP) and lactose permease (LacY) have also been investigated, which allow for more comprehensive conclusions regarding the mechanism of sugar translocation by transporters of the major facilitator superfamily. In all three of these symporters, we observed sugar binding and transport in real time to determine K_M, V_max, K_D, and k_obs values for different sugar substrates. K_D and k_obs values were attainable because of a conserved sugar-induced electrogenic conformational transition within these transporters. We also analyzed interactions between the residues in the available X-ray sugar/H⁺ symporter structures obtained with different bound sugars. We found that different sugars induce different conformational states, possibly correlating with different charge displacements in the electrophysiological assay upon sugar binding. Finally, we found that mutations in XylE altered the kinetics of glucose binding and transport, as Q175 and L297 are necessary for uncoupling H⁺ and d-glucose translocation. Based on the rates for the electrogenic conformational transition upon sugar binding (>300 s⁻¹) and for sugar translocation (2 s⁻¹ − 30 s⁻¹ for different substrates), we propose a multiple-step mechanism and postulate an energy profile for sugar translocation. We also suggest a mechanism by which d-glucose can act as an inhibitor for XylE.

It's Better To Be Lucky Than Smart

Bacterial cytoplasmic membrane vesicles provide a unique experimental system for studying active transport. Vesicles are prepared by lysis of osmotically sensitized cells (i.e., protoplasts or spheroplasts) and comprise osmotically intact, unit-membrane-bound sacs that are approximately 0.5-1.0 μm in diameter and devoid of internal structure. Their metabolic activities are restricted to those provided by the enzymes of the membrane itself, and each vesicle is functional. The energy source for accumulation of a particular substrate can be determined by studying which compounds or experimental conditions drive solute accumulation, and metabolic conversion of the transported substrate or the energy source is minimal. These properties of the vesicle system constitute a considerable advantage over intact cells, as the system provides clear definition of the reactions involved in the transport process. This discussion is not intended as a general review but is concerned with respiration-dependent active transport in membrane vesicles from Escherichia coli. Emphasis is placed on experimental observations demonstrating that respiratory energy is converted primarily into work in the form of a solute concentration gradient that is driven by a proton electrochemical gradient, as postulated by the chemiosmotic theory of Peter Mitchell.

Mentors: Ron Kaback

Carrasco reflects on her postdoctoral advisor, Ron Kaback—an exceptional scientist and inspiring mentor.

Interplay between the electrostatic membrane potential and conformational changes in membrane proteins

Transmembrane electrostatic membrane potential is a major energy source of the cell. Importantly, it determines the structure as well as function of charge-carrying membrane proteins. Here, we discuss the relationship between membrane potential and membrane proteins, in particular whether the conformation of these proteins is integrally connected to the membrane potential. Together, these concepts provide a framework for rationalizing the types of conformational changes that have been observed in membrane proteins and for better understanding the electrostatic effects of the membrane potential on both reversible as well as unidirectional dynamic processes of membrane proteins.

Engineered occluded apo-intermediate of LacY

The lactose permease of Escherichia coli (LacY) utilizes an alternating access symport mechanism with multiple conformational intermediates, but only inward (cytoplasmic)- or outward (periplasmic)-open structures have been characterized by X-ray crystallography. It is demonstrated here with sugar-binding studies that cross-linking paired-Cys replacements across the closed cytoplasmic cavity stabilize an occluded conformer with an inaccessible sugar-binding site. In addition, a nanobody (Nb) that stabilizes a periplasmic-open conformer with an easily accessible sugar-binding site in WT LacY fails to cause the cytoplasmic cross-linked mutants to become accessible to galactoside, showing that the periplasmic cavity is closed. These results are consistent with tight association of the periplasmic ends in two pairs of helices containing clusters of small residues in the packing interface between N- and C-terminal six-helix bundles of the symporter. However, after reduction of the disulfide bond, the Nb markedly increases the rate of galactoside binding, indicating unrestricted access to the Nb epitope and the galactoside-binding site from the periplasm. The findings indicate that the cross-linked cytoplasmic double-Cys mutants resemble an occluded apo-intermediate in the transport cycle.

Oversized galactosides as a probe for conformational dynamics in LacY

Binding kinetics of α-galactopyranoside homologs with fluorescent aglycones of different sizes and shapes were determined with the lactose permease (LacY) of Escherichia coli by FRET from Trp151 in the binding site of LacY to the fluorophores. Fast binding was observed with LacY stabilized in an outward-open conformation (k_on = 4-20 μM^-1·s^-1), indicating unobstructed access to the binding site even for ligands that are much larger than lactose. Dissociation rate constants (k_off) increase with the size of the aglycone so that K_d values also increase but remain in the micromolar range for each homolog. Phe27 (helix I) forms an apparent constriction in the pathway for sugar by protruding into the periplasmic cavity. However, replacement of Phe27 with a bulkier Trp does not create an obstacle in the pathway even for large ligands, since binding kinetics remain unchanged. High accessibility of the binding site is also observed in a LacY/nanobody complex with partially blocked periplasmic opening. Remarkably, E. coli expressing WT LacY catalyzes transport of α- or β-galactopyranosides with oversized aglycones such as bodipy or Aldol518, which may require an extra space within the occluded intermediate. The results confirm that LacY specificity is strictly directed toward the galactopyranoside ring and also clearly indicate that the opening on the periplasmic side is sufficiently wide to accommodate the large galactoside derivatives tested here. We conclude that the actual pathway for the substrate entering from the periplasmic side is wider than the pore diameter calculated in the periplasmic-open X-ray structures.

Thermodynamic secrets of multidrug resistance: A new take on transport mechanisms of secondary active antiporters

Multidrug resistance (MDR) presents a growing challenge to global public health. Drug extrusion transporters play a critical part in MDR; thus, their mechanisms of substrate recognition are being studied in great detail. In this work, we review common structural features of key transporters involved in MDR. Based on our membrane potential-driving hypothesis, we propose a general energy-coupling mechanism for secondary-active antiporters. This putative mechanism provides a common framework for understanding poly-specificity of most-if not all-MDR transporters.

An Asymmetric Conformational Change in LacY

Galactoside/H+ symport by the lactose permease of Escherichia coli (LacY) involves reciprocal opening and closing of periplasmic and cytoplasmic cavities so that sugar- and H+-binding sites become alternatively accessible to either side of the membrane. After reconstitution into proteoliposomes, LacY with the periplasmic cavity sealed by cross-linking paired-Cys residues does not bind sugar from the periplasmic side. However, reduction of the S-S bond restores opening of the periplasmic cavity and galactoside binding. Furthermore, nanobodies that stabilize the double-Cys mutant in a periplasmic-open conformation and allow free access of galactoside to the binding site do so only after reduction of the S-S bond. In contrast, when cross-linked LacY is solubilized in detergent, galactoside binding is observed, indicating that the cytoplasmic cavity is patent. Sugar binding from the cytoplasmic side exhibits nonlinear stopped-flow kinetics, and analysis reveals a two-step process in which a conformational change precedes binding. Because the cytoplasmic cavity is spontaneously closing and opening in the symporter with a sealed periplasmic cavity, it is apparent that an asymmetrical conformational transition controls access of sugar to the binding site.

Crystal structure of a LacY-nanobody complex in a periplasmic-open conformation

The lactose permease of Escherichia coli (LacY), a dynamic polytopic membrane protein, catalyzes galactoside-H⁺ symport and operates by an alternating access mechanism that exhibits multiple conformations, the distribution of which is altered by sugar binding. We have developed single-domain camelid nanobodies (Nbs) against a mutant in an outward (periplasmic)-open conformation to stabilize this state of the protein. Here we describe an X-ray crystal structure of a complex between a double-Trp mutant (Gly46→Trp/Gly262→Trp) and an Nb in which free access to the sugar-binding site from the periplasmic cavity is observed. The structure confirms biochemical data indicating that the Nb binds stoichiometrically with nanomolar affinity to the periplasmic face of LacY primarily to the C-terminal six-helix bundle. The structure is novel because the pathway to the sugar-binding site is constricted and the central cavity containing the galactoside-binding site is empty. Although Phe27 narrows the periplasmic cavity, sugar is freely accessible to the binding site. Remarkably, the side chains directly involved in binding galactosides remain in the same position in the absence or presence of bound sugar.

Energy coupling mechanisms of MFS transporters

Major facilitator superfamily (MFS) is a large class of secondary active transporters widely expressed across all life kingdoms. Although a common 12-transmembrane helix-bundle architecture is found in most MFS crystal structures available, a common mechanism of energy coupling remains to be elucidated. Here, we discuss several models for energy-coupling in the transport process of the transporters, largely based on currently available structures and the results of their biochemical analyses. Special attention is paid to the interaction between protonation and the negative-inside membrane potential. Also, functional roles of the conserved sequence motifs are discussed in the context of the 3D structures. We anticipate that in the near future, a unified picture of the functions of MFS transporters will emerge from the insights gained from studies of the common architectures and conserved motifs.

Outward-facing conformers of LacY stabilized by nanobodies

The lactose permease of Escherichia coli (LacY), a highly dynamic polytopic membrane protein, catalyzes stoichiometric galactoside/H(+) symport by an alternating access mechanism and exhibits multiple conformations, the distribution of which is altered by sugar binding. We have developed single-domain camelid nanobodies (Nbs) against a LacY mutant in an outward (periplasmic)-open conformation to stabilize this state of the WT protein. Twelve purified Nbs inhibit lactose transport in right-side-out membrane vesicles, indicating that the Nbs recognize epitopes on the periplasmic side of LacY. Stopped-flow kinetics of sugar binding by WT LacY in detergent micelles or reconstituted into proteoliposomes reveals dramatic increases in galactoside-binding rates induced by interaction with the Nbs. Thus, WT LacY in complex with the great majority of the Nbs exhibits varied increases in access of sugar to the binding site with an increase in association rate constants (kon) of up to ∼ 50-fold (reaching 10(7) M(-1) ⋅ s(-1)). In contrast, with the double-Trp mutant, which is already open on the periplasmic side, the Nbs have little effect. The findings are clearly consistent with stabilization of WT conformers with an open periplasmic cavity. Remarkably, some Nbs drastically decrease the rate of dissociation of bound sugar leading to increased affinity (greater than 200-fold for lactose).

Function, Structure, and Evolution of the Major Facilitator Superfamily: The LacY Manifesto

The major facilitator superfamily (MFS) is a diverse group of secondary transporters with members found in all kingdoms of life. A paradigm for MFS is the lactose permease (LacY) of Escherichia coli, which couples the stoichiometric translocation of a galactopyranoside and an H+ across the cytoplasmic membrane. LacY has been the test bed for the development of many methods applied for the analysis of transport proteins. X-ray structures of an inward-facing conformation and the most recent structure of an almost occluded conformation confirm many conclusions from previous studies. Although structure models are critical, they are insufficient to explain the catalysis of transport. The clues to understanding transport are based on the principles of enzyme kinetics. Secondary transport is a dynamic process—static snapshots of X-ray crystallography describe it only partially. However, without structural information, the underlying chemistry is virtually impossible to conclude. A large body of biochemical/biophysical data derived from systematic studies of site-directed mutants in LacY suggests residues critically involved in the catalysis, and a working model for the symport mechanism that involves alternating access of the binding site is presented. The general concepts derived from the bacterial LacY are examined for their relevance to other MFS transporters.

Real-time conformational changes in LacY

Galactoside/H(+) symport across the cytoplasmic membrane of Escherichia coli is catalyzed by lactose permease (LacY), which uses an alternating access mechanism with opening and closing of deep cavities on the periplasmic and cytoplasmic sides. In this study, conformational changes in LacY initiated by galactoside binding were monitored in real time by Trp quenching/unquenching of bimane, a small fluorophore covalently attached to the protein. Rates of change in bimane fluorescence on either side of LacY were measured by stopped flow with LacY in detergent or in proteoliposomes and were compared with rates of galactoside binding. With LacY in proteoliposomes, the periplasmic cavity is tightly sealed and the substrate-binding rate is limited by the rate of opening of this cavity. Rates of opening, measured as unquenching of bimane fluorescence, are 20-30 s(-1), independent of sugar concentration and essentially the same in detergent or in proteoliposomes. On the cytoplasmic side of LacY in proteoliposomes, slow bimane quenching (i.e., closing of the cavity) is observed at a rate that is also independent of sugar concentration and similar to the rate of sugar binding from the periplasmic side. Therefore, opening of the periplasmic cavity not only limits access of sugar to the binding site of LacY but also controls the rate of closing of the cytoplasmic cavity.

Flexible gates generate occluded intermediates in the transport cycle of LacY

The major facilitator superfamily (MFS) transporter lactose permease (LacY) alternates between cytoplasmic and periplasmic open conformations to co-transport a sugar molecule together with a proton across the plasma membrane. Indirect experimental evidence suggested the existence of an occluded transition intermediate of LacY, which would prevent leaking of the proton gradient. As no experimental structure is known, the conformational transition is not fully understood in atomic detail. We simulated transition events from a cytoplasmic open conformation to a periplasmic open conformation with the dynamic importance sampling molecular dynamics method and observed occluded intermediates. Analysis of water permeation pathways and the electrostatic free-energy landscape of a solvated proton indicated that the occluded state contains a solvated central cavity inaccessible from either side of the membrane. We propose a pair of geometric order parameters that capture the state of the pathway through the MFS transporters as shown by a survey of available crystal structures and models. We present a model for the occluded state of apo-LacY, which is similar to the occluded crystal structures of the MFS transporters EmrD, PepT_So, NarU, PiPT and XylE. Our simulations are consistent with experimental double electron spin–spin distance measurements that have been interpreted to show occluded conformations. During the simulations, a salt bridge that has been postulated to be involved in driving the conformational transition formed. Our results argue against a simple rigid-body domain motion as implied by a strict “rocker-switch mechanism” and instead hint at an intricate coupling between two flexible gates.

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The transport mechanism of LacY is hypothesized to involve an intermediate “occluded” state.

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Such a state is observed in computer simulations of the conformational transitions.

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Simulation data are validated with experimental double electron–electron spin resonance measurements.

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The structural gating elements of LacY are identified.

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Occluded LacY is similar to known occluded structures of homologous proteins.

Proteoliposomes as tool for assaying membrane transporter functions and interactions with xenobiotics

Proteoliposomes represent a suitable and up to date tool for studying membrane transporters which physiologically mediate absorption, excretion, trafficking and reabsorption of nutrients and metabolites. Using recently developed reconstitution strategies, transporters can be inserted in artificial bilayers with the same orientation as in the cell membranes and in the absence of other interfering molecular systems. These methodologies are very suitable for studying kinetic parameters and molecular mechanisms. After the first applications on mitochondrial transporters, in the last decade, proteoliposomes obtained with optimized methodologies have been used for studying plasma membrane transporters and defining their functional and kinetic properties and structure/function relationships. A lot of information has been obtained which has clarified and completed the knowledge on several transporters among which the OCTN sub-family members, transporters for neutral amino acid, B0AT1 and ASCT2, and others. Transporters can mediate absorption of substrate-like derivatives or drugs, improving their bioavailability or can interact with these compounds or other xenobiotics, leading to side/toxic effects. Therefore, proteoliposomes have recently been used for studying the interaction of some plasma membrane and mitochondrial transporters with toxic compounds, such as mercurials, H₂O₂ and some drugs. Several mechanisms have been defined and in some cases the amino acid residues responsible for the interaction have been identified. The data obtained indicate proteoliposomes as a novel and potentially important tool in drug discovery.

Structure of the YajR transporter suggests a transport mechanism based on the conserved motif A

The major facilitator superfamily (MFS) is the largest family of secondary active transporters and is present in all life kingdoms. Detailed structural basis of the substrate transport and energy-coupling mechanisms of these proteins remain to be elucidated. YajR is a putative proton-driven MFS transporter found in many Gram-negative bacteria. Here we report the crystal structure of Escherichia coli YajR at 3.15 Å resolution in an outward-facing conformation. In addition to having the 12 canonical transmembrane helices, the YajR structure includes a unique 65-residue C-terminal domain which is independently stable. The structure is unique in illustrating the functional role of "sequence motif A." This highly conserved element is seen to stabilize the outward conformation of YajR and suggests a general mechanism for the conformational change between the inward and outward states of the MFS transporters.

Functional characterization of a ClC transporter by solid-supported membrane electrophysiology

EcClC, a prokaryotic member of the ClC family of chloride channels and transporters, works as coupled H⁺/Cl⁻ exchanger. With a known structure and the possibility of investigating its behavior with different biochemical and biophysical techniques, the protein has become an important model system for the family. Although many aspects of its function have been previously characterized, it was difficult to measure transport on the same sample under different environmental conditions. To overcome this experimental limitation, we have studied EcClC by solid-supported membrane electrophysiology. The large transport-related transient currents and a simple way of relating transport rates to the measured signal have allowed a thorough investigation of ion selectivity, inhibition, and the dependence of transport on changes in ion concentration and pH. Our results confirm that the protein transports larger anions with about similar rates, whereas the smaller fluoride is not a substrate. We also show that 4,4′-diisothiocyano-2,2’-stilbenedisulfonic acid (DIDS), a known inhibitor of other anion transport protein, irreversibly inhibits EcClC from the intracellular side. The chloride dependence shows an apparent saturation at millimolar concentrations that resembles a similar behavior in eukaryotic ClC channels. Our experiments have also allowed us to quantify the pH dependence of transport. EcClC shows a strong activation at low pH with an apparent pKa of 4.6. The pronounced pH dependence is lost by the mutation of a conserved glutamate facing the extracellular solution that was previously shown to be an acceptor for transported protons, whereas it is largely retained by the mutation of an equivalent residue at the intracellular side. Our results have provided a quantitative basis for the transport behavior of EcClC, and they will serve as a reference for future investigations of novel electrogenic transporters with still-uncharacterized properties.

Phosphatidylethanolamine-lactose permease interaction: a comparative study based on FRET

In this work we have investigated the selectivity of lactose permease (LacY) of Escherichia coli (E. coli) for its surrounding phospholipids when reconstituted in binary mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), 1,2-Palmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), or 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) with 1-palmitoyl-2-oleoyl-sn-glycero-3-(phospho-rac-(1-glycerol)) (POPG). Förster resonance energy transfer (FRET) measurements have been performed to investigate the selectivity between a single tryptophan mutant of LacY used as donor (D), and two analogues of POPE and POPG labeled with pyrene in the acyl chains (Pyr-PE and Pyr-PG) used as acceptors. As a difference from previous works, now the donor has been single-W151/C154G/D68C LacY. It has been reported that the replacement of the aspartic acid in position 68 by cysteine inhibits active transport in LacY. The objectives of this work were to elucidate the phospholipid composition of the annular region of this mutant and to determine whether the mutation performed, D68C, induced changes in the protein-lipid selectivity. FRET efficiencies for Pyr-PE were always higher than for Pyr-PG. The values of the probability of each site in the annular ring being occupied by a label (μ) were similar at the studied temperatures (24 °C and 37 °C), suggesting that the lipid environment is not significantly affected when increasing the temperature. By comparing the results with those obtained for single-W151/C154G LacY, we observe that the mutation in the 68 residue indeed changes the selectivity of the protein for the phospholipids. This might be probably due to a change in the conformational dynamics of LacY.

Apo-intermediate in the transport cycle of lactose permease (LacY)

The lactose permease (LacY) catalyzes coupled stoichiometric symport of a galactoside and an H(+). Crystal structures reveal 12, mostly irregular, transmembrane α-helices surrounding a cavity with sugar- and H(+)- binding sites at the apex, which is accessible from the cytoplasm and sealed on the periplasmic side (an inward-facing conformer). An outward-facing model has also been proposed based on biochemical and spectroscopic measurements, as well as the X-ray structure of a related symporter. Converging lines of evidence demonstrate that LacY functions by an alternating access mechanism. Here, we generate a model for an apo-intermediate of LacY based on crystallographic coordinates of LacY and the oligopeptide/H(+) symporter. The model exhibits a conformation with an occluded cavity inaccessible from either side of the membrane. Furthermore, kinetic considerations and double electron-electron resonance measurements suggest that another occluded conformer with bound sugar exists during turnover. An energy profile for symport is also presented.

Lactose permease and the alternating access mechanism

Crystal structures of the lactose permease of Escherichia coli (LacY) reveal 12, mostly irregular transmembrane α-helices surrounding a large cavity open to the cytoplasm and a tightly sealed periplasmic side (inward-facing conformation) with the sugar-binding site at the apex of the cavity and inaccessible from the periplasm. However, LacY is highly dynamic, and binding of a galactopyranoside causes closing of the inward-facing cavity with opening of a complementary outward-facing cavity. Therefore, the coupled, electrogenic translocation of a sugar and a proton across the cytoplasmic membrane via LacY very likely involves a global conformational change that allows alternating access of sugar- and H(+)-binding sites to either side of the membrane. Here the various biochemical and biophysical approaches that provide strong support for the alternating access mechanism are reviewed. Evidence is also presented indicating that opening of the periplasmic cavity is probably the limiting step for binding and perhaps transport.

Opening the periplasmic cavity in lactose permease is the limiting step for sugar binding

The lactose permease (LacY) catalyzes galactoside/H(+) symport via an alternating access mechanism in which sugar- and H(+)-binding sites in the middle of the molecule are alternatively exposed to either side of the membrane by opening and closing of inward- and outward-facing cavities. The crystal structures of wild-type LacY, as well as accessibility data for the protein in the membrane, provide strong support for a conformation with a tightly closed periplasmic side and an open cytoplasmic side (an inward-facing conformation). In this study, rates of substrate binding were measured by stopped-flow with purified LacY either in detergent or in reconstituted proteoliposomes. Binding rates are compared with rates of sugar-induced opening of the periplasmic pathway obtained by using a recently developed method based on unquenching of Trp fluorescence. A linear dependence of galactoside-binding rates on sugar concentration is observed in detergent, whereas reconstituted LacY binds substrate at a slower rate that is independent of sugar concentration. Rates of opening of the periplasmic cavity with LacY in detergent are independent of substrate concentration and are essentially the same for different galactosidic sugars. The findings demonstrate clearly that reconstituted LacY is oriented physiologically with a closed periplasmic side that limits access of sugar to the binding site. Moreover, opening of the periplasmic cavity is the limiting factor for sugar binding with reconstituted LacY and may be the limiting step in the overall transport reaction.

Probing of the rates of alternating access in LacY with Trp fluorescence

Sugar/H(+) symport by lactose permease (LacY) utilizes an alternating access mechanism in which sugar and H(+) binding sites in the middle of the molecule are alternatively exposed to either side of the membrane by sequential opening and closing of inward- and outward-facing hydrophilic cavities. Here, we introduce Trp residues on either side of LacY where they are predicted to be in close proximity to side chains of natural Trp quenchers in either the inward- or outward-facing conformers. In the inward-facing conformer, LacY is tightly packed on the periplasmic side, and Trp residues placed at positions 245 (helix VII) or 378 (helix XII) are in close contact with His-35 (helix I) or Lys-42 (helix II), respectively. Sugar binding leads to unquenching of Trp fluorescence in both mutants, a finding clearly consistent with opening of the periplasmic cavity. The pH dependence of Trp-245 unquenching exhibits a pK(a) of 8, typical for a His side chain interacting with an aromatic group. As estimated from stopped-flow studies, the rate of sugar-induced opening is approximately 100 s(-1). On the cytoplasmic side, Phe-140 (helix V) and Phe-334 (helix X) are located on opposite sides of a wide-open hydrophilic cavity. In precisely the opposite fashion from the periplasmic side, mutant Phe-140-->Trp/Phe-334-->His exhibits sugar-induced Trp quenching. Again, quenching is pH dependent (pK(a) = 8), but remarkably, the rate of sugar-induced quenching is only approximately 0.4 s(-1). The results provide yet another strong, independent line of evidence for the alternating access mechanism and demonstrate that the methodology described provides a sensitive probe to measure rates of conformational change in membrane transport proteins.

Residues gating the periplasmic pathway of LacY

X-ray crystal structures of LacY (lactose permease of Escherichia coli) exhibit a large cytoplasmic cavity containing the residues involved in sugar binding and H(+) translocation at the apex and a tightly packed side facing the periplasm. However, biochemical and biophysical evidence provide a strong indication that a hydrophilic pathway opens on the external surface of LacY with closing of the cytoplasmic side upon sugar binding. Thus, an alternating-access mechanism in which sugar- and H(+)-binding sites at the approximate middle of the molecule are alternatively exposed to either side of the membrane is likely to underlie LacY-catalyzed sugar/H(+) symport. To further investigate periplasmic opening, we replaced paired residues on the tightly packed periplasmic side of LacY with Cys, and the effect of cross-linking was studied by testing the accessibility/reactivity of Cys148 with the elongated ( approximately 29 A), impermeant hydrophilic reagent maleimide-PEG2-biotin. When the paired-Cys mutant Ile40-->Cys/Asn245-->Cys containing native Cys148 is oxidized to form a disulfide bond, the reactivity of Cys148 is markedly inhibited. Moreover, the reactivity of Cys148 in this mutant increases with the length of the cross-linking agent. In contrast, maleimide-PEG2-biotin reactivity of Cys148 is unaffected by oxidation of two other paired-Cys mutants at the mouth of the periplasmic cavity. The data indicate that residues Ile40 and Asn245 play a primary role in gating the periplasmic cavity and provide further support for the alternating-access model.

Electrophysiological characterization of LacY

Electrogenic events due to the activity of wild-type lactose permease from Escherichia coli (LacY) were investigated with proteoliposomes containing purified LacY adsorbed on a solid-supported membrane electrode. Downhill sugar/H(+) symport into the proteoliposomes generates transient currents. Studies at different lipid-to-protein ratios and at different pH values, as well as inactivation by N-ethylmaleimide, show that the currents are due specifically to the activity of LacY. From analysis of the currents under different conditions and comparison with biochemical data, it is suggested that the predominant electrogenic event in downhill sugar/H(+) symport is H(+) release. In contrast, LacY mutants Glu-325-->Ala and Cys-154-->Gly, which bind ligand normally, but are severely defective with respect to lactose/H(+) symport, exhibit only a small electrogenic event on addition of LacY-specific substrates, representing 6% of the total charge displacement of the wild-type. This activity is due either to substrate binding per se or to a conformational transition after substrate binding, and is not due to sugar/H(+) symport. We propose that turnover of LacY involves at least 2 electrogenic reactions: (i) a minor electrogenic step that occurs on sugar binding and is due to a conformational transition in LacY; and (ii) a major electrogenic step probably due to cytoplasmic release of H(+) during downhill sugar/H(+) symport, which is the limiting step for this mode of transport.

Determinants of bistability in induction of the Escherichia coli lac operon

The authors have developed a mathematical model of regulation of expression of the Escherichia coli lac operon, and have investigated bistability in its steady-state induction behaviour in the absence of external glucose. Numerical analysis of equations describing regulation by artificial inducers revealed two natural bistability parameters that can be used to control the range of inducer concentrations over which the model exhibits bistability. By tuning these bistability parameters, the authors found a family of biophysically reasonable systems that are consistent with an experimentally determined bistable region for induction by thio-methylgalactoside (TMG) (in Ozbudak et al. Nature, 2004, 427; p. 737). To model regulation by lactose, the authors developed similar equations in which allolactose, a metabolic intermediate in lactose metabolism and a natural inducer of lac, is the inducer. For biophysically reasonable parameter values, these equations yield no bistability in response to induction by lactose - only systems with an unphysically small permease-dependent export effect can exhibit small amounts of bistability for limited ranges of parameter values. These results cast doubt on the relevance of bistability in the lac operon within the natural context of E. coli, and help shed light on the controversy among existing theoretical studies that address this issue. The results also motivate a deeper experimental characterisation of permease-independent transport of lac inducers, and suggest an experimental approach to address the relevance of bistability in the lac operon within the natural context of E. coli. The sensitivity of lac bistability to the type of inducer emphasises the importance of metabolism in determining the functions of genetic regulatory networks.

Direct sugar binding to LacY measured by resonance energy transfer

Trp151 in the lactose permease of Escherichia coli (LacY) is an important component of the sugar-binding site and the only Trp residue out of six that is in close proximity to the galactopyranoside in the structure (1PV7). The short distance between Trp151 and the sugar is favorable for Förster resonance energy transfer (FRET) to nitrophenyl or dansyl derivatives with the fluorophore at the anomeric position of galactose. Modeling of 4-nitrophenyl-alpha-d-galactopyranoside (alpha-NPG) in the binding-site of LacY places the nitrophenyl moiety about 12 A away from Trp151, a distance commensurate with the Förster distance for a Trp-nitrobenzoyl pair. We demonstrate here that alpha-NPG binding to LacY containing all six native Trp residues causes galactopyranoside-specific FRET from Trp151. Moreover, binding of alpha-NPG is sufficiently slow to resolve time-dependent fluorescence changes by stopped-flow. The rate of change in Trp --> alpha-NPG FRET is linearly dependent upon sugar concentration, which allows estimation of kinetic parameters for binding. Furthermore, 2-(4'-maleimidylanilino)naphthalene-6-sulfonic acid (MIANS) covalently attached to the cytoplasmic end of helix X is sensitive to sugar binding, reflecting a ligand-induced conformational change. Stopped-flow kinetics of Trp --> alpha-NPG FRET and sugar-induced changes in MIANS fluorescence in the same protein reveal a two-step process: a relatively rapid binding step detected by Trp151 --> alpha-NPG FRET followed by a slower conformational change detected by a change in MIANS fluorescence.

Lessons from lactose permease

An X-ray structure of the lactose permease of Escherichia coli (LacY) in an inward-facing conformation has been solved. LacY contains N- and C-terminal domains, each with six transmembrane helices, positioned pseudosymmetrically. Ligand is bound at the apex of a hydrophilic cavity in the approximate middle of the molecule. Residues involved in substrate binding and H+ translocation are aligned parallel to the membrane at the same level and may be exposed to a water-filled cavity in both the inward- and outward-facing conformations, thereby allowing both sugar and H+ release directly into either cavity. These structural features may explain why LacY catalyzes galactoside/H+ symport in both directions utilizing the same residues. A working model for the mechanism is presented that involves alternating access of both the sugar- and H+-binding sites to either side of the membrane.

Sequence alignment and homology threading reveals prokaryotic and eukaryotic proteins similar to lactose permease

Certain prokaryotic transport proteins similar to the lactose permease of Escherichia coli (LacY) have been identified by BLAST searches from available genomic databanks. These proteins exhibit conservation of amino acid residues that participate in sugar binding and H(+) translocation in LacY. Homology threading of prokaryotic transporters based on the X-ray structure of LacY (PDB ID: 1PV7) and sequence similarities reveals a common overall fold for sugar transporters belonging to the Major Facilitator Superfamily (MFS) and suggest new targets for study. Evolution-based searches for sequence similarities also identify eukaryotic proteins bearing striking resemblance to MFS sugar transporters. Like LacY, the eukaryotic proteins are predicted to have 12 transmembrane domains (TMDs), and many of the irreplaceable residues for sugar binding and H(+) translocation in LacY appear to be largely conserved. The overall size of the eukaryotic homologs is about twice that of prokaryotic permeases with longer N and C termini and loops between TMDs III-IV and VI-VII. The human gene encoding protein FLJ20160 consists of six exons located on more than 60,000 bp of DNA sequences and requires splicing to produce mature mRNA. Cellular localization predictions suggest membrane insertion with possible proteolysis at the N terminus, and expression studies with the human protein FJL20160 demonstrate membrane insertion in both E.coli and Pichia pastoris. Widespread expression of the eukaryotic sugar transport candidates suggests an important role in cellular metabolism, particularly in brain and tumors. Homology is observed in the TMDs of both the eukaryotic and prokaryotic proteins that contain residues involved in sugar binding and H(+) translocation in LacY.

Structure and mechanism of the lactose permease

More than 20% of the genes sequenced thus far appear to encode polytopic transmembrane proteins involved in a multitude of critical functions, particularly energy and signal transduction. Many are important with regard to human disease (e.g., depression, diabetes, drug resistance), and many drugs are targeted to membrane transport proteins (e.g., fluoxetine and omeprazole). However, the number of crystal structures of membrane proteins, especially ion-coupled transporters, is very limited. Recently, an inward-facing conformer of the Escherichia coli lactose permease (LacY), a paradigm for the Major Facilitator Superfamily, which contains almost 4000 members, was solved at about 3.5 A in collaboration with Jeff Abramson and So Iwata at Imperial College London. This intensively studied membrane transport protein is composed of two pseudo-symmetrical 6-helix bundles with a large internal cavity containing bound sugar and open to the cytoplasm only. Based on the structure and a large body of biochemical and biophysical evidence, a mechanism is proposed in which the binding site is alternatively accessible to either side of the membrane.

Structure of a Na+/H+ antiporter and insights into mechanism of action and regulation by pH

The control by Na+/H+ antiporters of sodium/proton concentration and cell volume is crucial for the viability of all cells. Adaptation to high salinity and/or extreme pH in plants and bacteria or in human heart muscles requires the action of Na+/H+ antiporters. Their activity is tightly controlled by pH. Here we present the crystal structure of pH-downregulated NhaA, the main antiporter of Escherichia coli and many enterobacteria. A negatively charged ion funnel opens to the cytoplasm and ends in the middle of the membrane at the putative ion-binding site. There, a unique assembly of two pairs of short helices connected by crossed, extended chains creates a balanced electrostatic environment. We propose that the binding of charged substrates causes an electric imbalance, inducing movements, that permit a rapid alternating-access mechanism. This ion-exchange machinery is regulated by a conformational change elicited by a pH signal perceived at the entry to the cytoplasmic funnel.

Intermolecular thiol cross-linking via loops in the lactose permease of Escherichia coli

Previous experiments using intermolecular thiol cross-linking to determine surface-exposed positions in the transmembrane helices of the lactose permease suggest that only positions accessible from the aqueous phase are susceptible to cross-linking. This approach is now extended to most of the remaining positions in the molecule. Of an additional 143 single-Cys mutants studied, homodimer formation is observed with both a 5-A- and a 21-A-long crosslinking agent containing bis-methane thiosulfonate reactive groups in 33 mutants and exclusively with the 21-A-long reagent in 43 mutants. Furthermore, intermolecular cross-linking has little or no effect on transport activity, thereby providing further support for the argument that lactose permease is functionally, as well as structurally, a monomer in the membrane. In addition, evidence is presented indicating that reentrance loops are unlikely in this polytopic membrane transport protein.

In vitro synthesis of lactose permease to probe the mechanism of membrane insertion and folding

Insertion and folding of polytopic membrane proteins is an important unsolved biological problem. To study this issue, lactose permease, a membrane transport protein from Escherichia coli, is transcribed, translated, and inserted into inside-out membrane vesicles in vitro. The protein is in a native conformation as judged by sensitivity to protease, binding of a monoclonal antibody directed against a conformational epitope, and importantly, by functional assays. By exploiting this system it is possible to express the N-terminal six helices of the permease (N(6)) and probe changes in conformation during insertion into the membrane. Specifically, when N(6) remains attached to the ribosome it is readily extracted from the membrane with urea, whereas after release from the ribosome or translation of additional helices, those polypeptides are not urea extractable. Furthermore, the accessibility of an engineered Factor Xa site to Xa protease is reduced significantly when N(6) is released from the ribosome or more helices are translated. Finally, spontaneous disulfide formation between Cys residues at positions 126 (Helix IV) and 144 (Helix V) is observed when N(6) is released from the ribosome and inserted into the membrane. Moreover, in contrast to full-length permease, N(6) is degraded by FtsH protease in vivo, and N(6) with a single Cys residue at position 148 does not react with N-ethylmaleimide. Taken together, the findings indicate that N(6) remains in a hydrophilic environment until it is released from the ribosome or additional helices are translated and continues to fold into a quasi-native conformation after insertion into the bilayer. Furthermore, there is synergism between N(6) and the C-terminal half of permease during assembly, as opposed to assembly of the two halves as independent domains.

Elucidation of substrate binding interactions in a membrane transport protein by mass spectrometry

Integration of biochemical and biophysical data on the lactose permease of Escherichia coli has culminated in a molecular model that predicts substrate-protein proximities which include interaction of a hydroxyl group in the galactopyranosyl ring with Glu269. In order to test this hypothesis, we studied covalent modification of carboxyl groups with carbodiimides using electrospray ionization mass spectrometry (ESI-MS) and demonstrate that substrate protects the permease against carbodiimide reactivity. Further more, a significant proportion of the decrease in carbodiimide reactivity occurs specifically in a nanopeptide containing Glu269. In contrast, carbodiimide reactivity of mutant Glu269-->Asp that exhibits lower affinity is unaffected by substrate. By monitoring the ability of different substrate analogs to protect against carbodiimide modification of Glu269, it is suggested that the C-3 OH group of the galactopyranosyl ring may play an important role in specificity, possibly by H-bonding with Glu269. The approach demonstrates that mass spectrometry can provide a powerful means of analyzing ligand interactions with integral membrane proteins.

The Escherichia coli multidrug transporter MdfA catalyzes both electrogenic and electroneutral transport reactions

The resistance of cells to many drugs simultaneously (multidrug resistance) often involves the expression of membrane transporters (Mdrs); each recognizes and expels a broad spectrum of chemically unrelated drugs from the cell. The Escherichia coli Mdr transporter MdfA is able to transport differentially charged substrates in exchange for protons. This includes neutral compounds, namely chloramphenicol and thiamphenicol, and lipophilic cations such as tetraphenylphosphonium and ethidium. Here we show that the chloramphenicol and thiamphenicol transport reactions are electrogenic, whereas the transport of several monovalent cationic substrates is electroneutral. Therefore, unlike with positively charged substrates, the transmembrane electrical potential (negative inside) constitutes a major part of the driving force for the transport of electroneutral substrates by MdfA. These results demonstrate an unprecedented ability of a single secondary transporter to catalyze discrete transport reactions that differ in their electrogenicity and are governed by different components of the proton motive force.

Changing the lactose permease of Escherichia coli into a galactose-specific symporter

N-ethylmaleimide (NEM) modification of a lactose permease mutant containing a single-Cys in place of Ala-122 (helix IV) abolishes active lactose transport. Moreover, lactose, melibiose, and beta,d-galactopyranosyl 1-thio-beta,D-galactopyranoside protect against NEM inactivation of lactose transport and/or alkylation of Cys-122 by [(14)C]NEM. Remarkably, however, D-galactose transport is relatively unaffected by NEM, and the monosaccharide affords no protection against NEM inactivation of lactose transport. Consistently, competitive inhibition of [(14)C]galactose transport by lactose, melibiose, or beta,D-galactopyranosyl 1-thio-beta,D-galactopyranoside is drastically reduced after NEM modification, whereas inhibition by unlabeled galactose is unaffected. The results indicate that alkylation of Cys-122 selectively inhibits binding and transport of disaccharides, whereas transport of the monosaccharide galactose remains largely unaffected. In addition, although the conservative mutation Ala-122 --> Ser causes only mild inhibition of lactose transport, the mutations Ala-122 --> Phe and Ala-122 --> Tyr lead to marked inhibition. In contradistinction, none of these replacements has a marked effect on galactose transport. The results demonstrate that Ala-122 is a component of the ligand-binding site and provide a strong indication that the side chain at position 122 abuts on the non-galactosyl moiety of D-galactopyranosides. This is in contrast to Cys-148, a neighboring residue in helix V, that interacts with the hydrophobic face of the galactosyl moiety of D-galactopyranosides.

Surface-exposed positions in the transmembrane helices of the lactose permease of Escherichia coli determined by intermolecular thiol cross-linking

Intermolecular thiol cross-linking was used to determine surface-exposed positions in 250 lactose permease mutants containing single-Cys replacements in each transmembrane helix. Significant cross-linking of monomers to produce homodimers is observed in nine mutants with a 5-A-long cross-linking agent containing bis-methane thiosulfonate reactive groups [position 78 (helix III); positions 185, 186, and 187 (helix VI); positions 263, 275, and 278 (helix VIII); and positions 308 (helix IX) and 398 (helix XII)]. The results are consistent with a current helix-packing model of the permease. Seven of the nine mutants that exhibit intermolecular cross-linking are located at or near the cytoplasmic ends of transmembrane helices; two are near periplasmic ends. The results suggest that only those Cys replacements accessible from the aqueous phase and not from the hydrophobic core of the membrane are susceptible to cross-linking because of the much higher reactivity of the thiolate anion relative to the thiol. Single-Cys mutants at positions 278 (helix VIII) and 398 (helix XII), which are located in opposite sides of the 12-helix bundle, exhibit similar rates of cross-linking with sigmoid kinetics. Furthermore, cross-linking is markedly decreased at 0 degrees C, suggesting that lateral diffusion of the permease within the plane of the membrane is important for intermolecular cross-linking. The findings confirm previous observations indicating that intermolecular cross-linking is a stochastic process resulting from random collisions and support a number of other lines of evidence that lactose permease is a monomer.

Helix packing in the lactose permease of Escherichia coli: localization of helix VI

Plasmids encoding "split" lactose permease constructs with discontinuities in either the periplasmic loop between helices V and VI (N(5)/C(7)) or between helices VI and VII (N(6)/C(6)) were used to localize helix VI within the tertiary structure by site-directed thiol cross-linking. A total of 57 double-Cys pairs, with one Cys residue in helix VI and another in helix V or VIII, were studied with homobifunctional cross-linking agents. Significant cross-linking is observed between the periplasmic ends of helices V (position 158 or 161) and VI (position 170) with rigid 6 or 10 A reagents. Furthermore, the Cys residue at position 170 (helix VI) also cross-links to a Cys residue at either position 264 or 265 (helix VIII) with a 21 A cross-linking agent. The data indicate that helices V, VI and VIII are in close proximity at the periplasmic face of the membrane, with helix VI significantly closer to helix V. In addition, beta,D-galactopyranosyl 1-thio-beta,D-galactopyranoside induces a significant increase in cross-linking efficiency between helices VI and VIII and between helices V and VIII, with no significant change in cross-linking between helices V and VI.

Active proton and urea transport by amphibian skin

Proton secretion in frog skin is mediated by an electrogenic H+ pump. Pharmacological and immunocytological approaches have identified this pump as belonging to the V-ATPase class. The key role of this V-ATPase in proton secretion (acid-base balance) and as a membrane energizer of other solute transport from very dilute solutions is outlined. It is shown that the frog skin constitutes a model of a V-ATPase-dependent Na+ transport mechanism applicable to other freshwater animals. On the other hand, attempts to implicate the V-ATPase in the active urea transport that develops through the skin of salt-adapted frogs have failed; the nature of the different urea transporters located on apical and basal epithelial cell membranes and those responsible for active urea reabsorption remain to be identified.

Fluorescence of native single-Trp mutants in the lactose permease from Escherichia coli: structural properties and evidence for a substrate-induced conformational change

Six single-Trp mutants were engineered by individually reintroducing each of the native Trp residues into a functional lactose permease mutant devoid of Trp (Trp-less permease; Menezes ME, Roepe PD, Kaback HR, 1990, Proc Natl Acad Sci USA 87:1638-1642), and fluorescent properties were studied with respect to solvent accessibility, as well as alterations produced by ligand binding. The emission of Trp 33, Trp 78, Trp 171, and Trp 233 is strongly quenched by both acrylamide and iodide, whereas Trp 151 and Trp 10 display a decrease in fluorescence in the presence of acrylamide only and no quenching by iodide. Of the six single-Trp mutants, only Trp 33 exhibits a significant change in fluorescence (ca. 30% enhancement) in the presence of the substrate analog beta,D-galactopyranosyl 1-thio-beta,D-galactopyranoside (TDG). This effect was further characterized by site-directed fluorescent studies with purified single-Cys W33-->C permease labeled with 2-(4'-maleimidylanilino)-naphthalene-6-sulfonic acid (MIANS). Titration of the change in the fluorescence spectrum reveals a 30% enhancement accompanied with a 5-nm blue shift in the emission maximum, and single exponential behavior with an apparent KD of 71 microM. The effect of substrate binding on the rate of MIANS labeling of single-Cys 33 permease was measured in addition to iodide and acrylamide quenching of the MIANS-labeled protein. Complete blockade of labeling is observed in the presence of TDG, as well as a 30% decrease in accessibility to iodide with no change in acrylamide quenching. Overall, the findings are consistent with the proposal (Wu J, Frillingos S, Kaback HR, 1995a, Biochemistry 34:8257-8263) that ligand binding induces a conformational change at the C-terminus of helix I such that Pro 28 and Pro 31, which are on one face, become more accessible to solvent, whereas Trp 33, which is on the opposite face, becomes less accessible to the aqueous phase. The findings regarding accessibility to collisional quenchers are also consistent with the predicted topology of the six native Trp residues in the permease.

A conformational change in the lactose permease of Escherichia coli is induced by ligand binding or membrane potential

Lactose transport in membrane vesicles containing lactose permease with a single Cys residue in place of Val 315 is inactivated by N-ethylmaleimide in a manner that is stimulated by substrate or by a H+ electrochemical gradient (delta microH+; Sahin-Tóth M, Kaback HR, 1993, Protein Sci 2:1024-1033). The findings are confirmed and extended in this communication. Purified, reconstituted Val 315-->Cys permease reacts with N-ethylmaleimide or hydrophobic fluorescent maleimides but not with a membrane impermeant thiol reagent, and beta-galactosides specifically stimulate the rate of labeling. Furthermore, the reactivity of purified Val 315-->Cys permease is enhanced by imposition of a membrane potential (delta psi, interior negative). The results indicate that either ligand binding or delta psi induces a conformational change in the permease that brings the N-terminus of helix X into an environment that is more accessible from the lipid phase.

What's new with lactose permease

The lactose permease of Escherichia coli is a paradigm for polytopic membrane transport proteins that transduce free energy stored in an electrochemical ion gradient into work in the form of a concentration gradient. Although the permease consists of 12 hydrophobic transmembrane domains in probable alpha-helical conformation that traverse the membrane in zigzag fashion connected by hydrophilic "loops", little information is available regarding the folded tertiary structure of the molecule. In a recent approach site-directed fluorescence labeling is being used to study proximity relationships in lactose permease. The experiments are based upon site-directed pyrene labeling of combinations of paired Cys replacements in a mutant devoid of Cys residues. Since pyrene exhibits excimer fluorescence if two molecules are within about 3.5A, the proximity between paired labeled residues can be determined. The results demonstrate that putative helices VIII and IX are close to helix X. Taken together with other findings indicating that helix VII is close to helices X and XI, the data lead to a model that describes the packing of helices VII to XI.

The N-terminal 22 amino acid residues in the lactose permease of Escherichia coli are not obligatory for membrane insertion or transport activity

When the lactose (lac) permease of Escherichia coli is expressed from the lac promoter at relatively low rates, deletion of amino acid residues 2-8 (delta 7) or 2-9 (delta 8) from the hydrophilic N terminus has a relatively minor effect on the ability of the permease to catalyze active lactose transport. Activity is essentially abolished, however, and the permease is hardly detected in the membrane when two additional amino acid residues are deleted (delta 10), and mutants deleted of residues 2-23 (delta 22) or 2-39 (delta 38) also exhibit no activity and are not inserted into the membrane. Dramatically, when the defective deletion mutants are overexpressed at high rates via the T7 promoter, delta 10 and delta 22 are inserted into the membrane in a stable form and catalyze active lactose transport in a highly significant manner, whereas delta 38 is hardly detected in the membrane and exhibits no activity. Interestingly, a fusion protein consisting of delta 38 and the ompA leader peptide is inserted into the membrane but exhibits no transport activity. The results indicate that the N-terminal hydrophilic domain of lac permease and the N-terminal half of the first putative transmembrane alpha-helix are not mandatory for either membrane insertion or transport activity.

Functional complementation of internal deletion mutants in the lactose permease of Escherichia coli

Using the lactose permease of Escherichia coli, a well-characterized membrane protein with 12 transmembrane domains, we demonstrated that certain paired in-frame deletion constructs complement each other functionally. Although cells expressing the deletion mutants individually are unable to catalyze active lactose accumulation, cells simultaneously expressing specific pairs of deletions catalyze transport up to 60% as do cells expressing wild-type permease. Moreover, complementation clearly does not occur at the level of DNA but probably occurs at the protein level. Remarkably, functional complementation is observed only with pairs of permease molecules containing large deletions and is not observed with missense mutations or point deletions. Although the mechanism of functional complementation is obscure, the findings indicate that certain pairs of permease molecules containing specific internal deletions can interact to form a functional complex in the same way phenomenologically as do independently expressed polypeptides corresponding to different N- and C-terminal portions of the permease.