An analysis of lactose permease “sugar specificity” mutations which also affect the coupling between proton and lactose transport

Equilibrium properties of E. coli lactose permease symport—A random-walk model approach

The symport of lactose and H⁺ is an important physiological process in E. coli, for it is closely related to cellular energy supply. In this paper, we review, extend and analyse a newly proposed cotransport model that takes the “leakage” phenomenon (uncoupled particle translocation) into account and also satisfies the static head equilibrium condition. Then, we use the model to study the equilibrium properties, including equilibrium solution and the time required to reach equilibrium, of the symport process of E. coli LacY protein, when varying the parameters of the initial state of cotransport system. It can be found that in our extended model, H⁺ and lactose will reach their equilibrium state separately, and when “leakage” exists, it linearly affects the equilibrium solution, which is a useful property that the original model does not have. We later investigated the effect of the volume of periplasm and cytoplasm on the equilibrium properties. For a certain E. coli cell, as it continues to lose water and contract, the time for cytoplasm pH to be stabilized by symport increases monotonically when the cell survives. Finally, we reproduce the experimental data from a literature to verify the validity of the extension in this symport process. The above phenomena and other findings in this paper may help us to not only further validate or improve the model, but also deepen our understanding of the cotransport process of E. coli LacY protein.

Structures and General Transport Mechanisms by the Major Facilitator Superfamily (MFS)

The major facilitator superfamily (MFS) is the largest known superfamily of secondary active transporters. MFS transporters are responsible for transporting a broad spectrum of substrates, either down their concentration gradient or uphill using the energy stored in the electrochemical gradients. Over the last 10 years, more than a hundred different MFS transporter structures covering close to 40 members have provided an atomic framework for piecing together the molecular basis of their transport cycles. Here, we summarize the remarkable promiscuity of MFS members in terms of substrate recognition and proton coupling as well as the intricate gating mechanisms undergone in achieving substrate translocation. We outline studies that show how residues far from the substrate binding site can be just as important for fine-tuning substrate recognition and specificity as those residues directly coordinating the substrate, and how a number of MFS transporters have evolved to form unique complexes with chaperone and signaling functions. Through a deeper mechanistic description of glucose (GLUT) transporters and multidrug resistance (MDR) antiporters, we outline novel refinements to the rocker-switch alternating-access model, such as a latch mechanism for proton-coupled monosaccharide transport. We emphasize that a full understanding of transport requires an elucidation of MFS transporter dynamics, energy landscapes, and the determination of how rate transitions are modulated by lipids.

Coupling efficiency of secondary active transporters

Secondary active transporters are fundamental to a myriad of biological processes. They use the electrochemical gradient of one solute to drive transport of another solute against its concentration gradient. Central to this mechanism is that the transport of one does not occur in the absence of the other. However, like in most of biology, imperfections in the coupling mechanism exist and we argue that these are innocuous and may even be beneficial for the cell. We discuss the energetics and kinetics of alternating-access in secondary transport and focus on the mechanistic aspects of imperfect coupling that give rise to leak pathways. Additionally, inspection of available transporter structures gives valuable insight into coupling mechanics, and we review literature where proteins have been altered to change their coupling efficiency.

Overflow metabolism in Escherichia coli results from efficient proteome allocation

Overflow metabolism refers to the seemingly wasteful strategy in which cells use fermentation instead of the more efficient respiration to generate energy, despite the availability of oxygen. Known as Warburg effect in the context of cancer growth, this phenomenon occurs ubiquitously for fast growing cells, including bacteria, fungi, and mammalian cells, but its origin has remained mysterious despite decades of research. Here we study metabolic overflow in E. coli and show that it is a global physiological response used to cope with changing proteomic demands of energy biogenesis and biomass synthesis under different growth conditions. A simple model of proteomic resource allocation can quantitatively account for all of the observed behaviors and accurately predict responses to novel perturbations. The key hypothesis of the model, that the proteome cost of energy biogenesis by respiration exceeds that by fermentation, is quantitatively confirmed by direct measurement of protein abundances via quantitative mass spectrometry.

The structural basis of secondary active transport mechanisms

Secondary active transporters couple the free energy of the electrochemical potential of one solute to the transmembrane movement of another. As a basic mechanistic explanation for their transport function the model of alternating access was put forward more than 40 years ago, and has been supported by numerous kinetic, biochemical and biophysical studies. According to this model, the transporter exposes its substrate binding site(s) to one side of the membrane or the other during transport catalysis, requiring a substantial conformational change of the carrier protein. In the light of recent structural data for a number of secondary transport proteins, we analyze the model of alternating access in more detail, and correlate it with specific structural and chemical properties of the transporters, such as their assignment to different functional states in the catalytic cycle of the respective transporter, the definition of substrate binding sites, the type of movement of the central part of the carrier harboring the substrate binding site, as well as the impact of symmetry on fold-specific conformational changes. Besides mediating the transmembrane movement of solutes, the mechanism of secondary carriers inherently involves a mechanistic coupling of substrate flux to the electrochemical potential of co-substrate ions or solutes. Mainly because of limitations in resolution of available transporter structures, this important aspect of secondary transport cannot yet be substantiated by structural data to the same extent as the conformational change aspect. We summarize the concepts of coupling in secondary transport and discuss them in the context of the available evidence for ion binding to specific sites and the impact of the ions on the conformational state of the carrier protein, which together lead to mechanistic models for coupling.

Amino acids that confer transport of raffinose and maltose sugars in the raffinose permease (RafB) of Escherichia coli as implicated by spontaneous mutations at Val-35, Ser-138, Ser-139, Gly-389 and Ile-391

In order to identify amino acid residues in the Escherichia coli raffinose-H(+) permease (RafB) that play a role in sugar selection and transport, we first incubated E. coli HS4006 containing plasmid pRU600 (expresses inducible raffinose permease and alpha-galactosidase) on maltose MacConkey indicator plates overnight. Initially, all colonies were white, indicating no fermentation of maltose. Upon further incubation, 100 mutants appeared red. pRU600 DNA was prepared from 55 mutants. Five mutants transferred the phenotype for fermentation of maltose (red). Plasmid DNA from five maltose-positive phenotype transformants was prepared and sequenced, revealing three distinct types of mutations. Two mutants exhibited Val-35-->Ala (MT1); one mutant had Ile-391-->Ser (MT2); and two mutants had Ser-138-->Asp, Ser-139-->Leu and Gly-389-->Ala (MT3). Transport studies of [(3)H]-maltose showed that cells harboring MT1, MT2 and MT3 had greater uptake (P <or= 0.05) than cells harboring wild-type RafB. However, [(14)C]-raffinose uptake was reduced in all mutant cells (P <or= 0.05) with MT1, MT2 and MT3 mutants compared to cells harboring wild-type RafB. Kinetic analysis showed enhanced apparent K (m) values for maltose and reduced V (max)/ K (m) ratios for raffinose compared to wild-type values. The apparent K (i) value of maltose for RafB indicates a competitive relationship between maltose and raffinose. Maltose "uphill" accumulation was greater for mutants (P <or= 0.05) than for cells with wild-type RafB. Thus, we implicate residues in RafB that are responsible for raffinose transport and suggest that the substituted residues in RafB dictate structures that enhance transport of maltose.

Lactose permease H+-lactose symporter: mechanical switch or Brownian ratchet?

Lactose permease structure is deemed consistent with a mechanical switch device for H(+)-coupled symport. Because the crystallography-assigned docking position of thiodigalactoside (TDG) does not make close contact with several amino acids essential for symport; the switch model requires allosteric interactions between the proton and sugar binding sites. The docking program, Autodock 3 reveals other lactose-docking sites. An alternative cotransport mechanism is proposed where His-322 imidazolium, positioned in the central pore equidistant (5-7 A) between six charged amino acids, Arg-302 and Lys-319 opposing Glu-269, Glu-325, Asp-237, and Asp-240, transfers a proton transiently to an H-bonded lactose hydroxyl group. Protonated lactose and its dissociation product H(3)O+ are repelled by reprotonated His-322 and drift in the electrostatic field toward the cytosol. This Brownian ratchet model, unlike the conventional carrier model, accounts for diminished symport by H322N mutant; how H322 mutants become uniporters; why exchanging Lys-319 with Asp-240 paradoxically inactivates symport; how some multiple mutants become revertant transporters; the raised export rate and affinity toward lactose of uncoupled mutants; the altered specificity toward lactose, melibiose, and galactose of some mutants, and the proton dissociation rate of H322 being 100-fold faster than the symport turnover rate.

A suppressor analysis of residues involved in cation transport in the lactose permease: identification of a coupling sensor

Four amino acids critical for lactose permease function were altered using site-directed mutagenesis. The resulting Quad mutant (E269Q/R302L/H322Q/E325Q) was expressed at 60% of wild-type levels but found to have negligible transport activity. The Quad mutant was used as a parental strain to isolate suppressors that regained the ability to ferment the alpha-galactoside melibiose. Six different suppressors were identified involving five discrete amino acid changes and one amino acid deletion (Q60L, V229G, Y236D, S306L, K319N and DeltaI298). All of the suppressors transported alpha-galactosides at substantial rates. In addition, the Q60L, DeltaI298 and K319N suppressors regained a small but detectable amount of lactose transport. Assays of sugar-driven cation transport showed that both the Q60L and K319N suppressors couple the influx of melibiose with cations (H(+) or H(3)O(+)). Taken together, the data show that the cation-binding domain in the lactose permease is not a fixed structure as proposed in previous models. Rather, the data are consistent with a model in which several ionizable residues form a dynamic coupling sensor that also may interact directly with the cation and lactose.

Control of H+/Lactose Coupling by Ionic Interactions in the Lactose Permease ofEscherichia coli

A combinatorial approach was used to study putative interactions among six ionizable residues (Asp-240, Glu-269, Arg-302, Lys-319, His-322, and Glu-325) in the lactose permease. Neutral mutations were made involving five ion pairs that had not been previously studied. Double mutants, R302L/E325Q and D240N/H322Q, had moderate levels of downhill [(14)C]-lactose transport. Mutants in which only one of these six residues was left unchanged (pentuple mutants) were also made. A Pent269(-) mutant (in which only Glu-269 remains) catalyzed a moderate level of downhill lactose transport. Pent240(-) and Pent 322(+) also showed low levels of downhill lactose transport. Additionally, a Pent240(-) mutant exhibited proton transport upon addition of melibiose, but not lactose. This striking result demonstrates that neutralization of up to five residues of the lactose permease does not abolish proton transport. A mutant with neutral replacements at six ionic residues (hextuple mutant) had low levels of downhill lactose transport, but no uphill accumulation or proton transport. Since none of the mutants in this study catalyzes active accumulation of lactose, this is consistent with other reports that have shown that each residue is essential for proper coupling. Nevertheless, none of the six ionizable residues is individually required for substrate-induced proton cotransport. These results suggest that the H(+) binding domain may be elsewhere in the permease or that cation binding may involve a flexible network of charged residues.

An isotope-edited FT-IR study of a symporter, the lactose permease

The lactose permease of Escherichia coli transports protons and lactose across the plasma membrane and uses a transmembrane ion gradient as the energy source to drive the uphill accumulation of lactose. In this report, the effect of the electrochemical gradient on the permease has been studied. Bacteriorhodopsin was co-reconstituted with the lactose permease to provide a light-triggered electrochemical gradient. Reaction-induced Fourier transform infrared spectra were acquired, and bacteriorhodopsin contributions were subtracted. In previous work, positive bands in the 1765-1730 cm(-1) region of the reaction-induced FT-IR spectrum were attributed to the perturbation of carboxylic acid residues in the permease [Patzlaff, J. S., Brooker, R. J., and Barry, B. A. (2000) J. Biol. Chem. 275, 28695-28700]. In this study, we have globally labeled the permease with (13)C or (15)N. Isotopic labeling demonstrates that features in the reaction-induced FT-IR spectrum arise from permease carboxylic acid, amide I, and amide II vibrational modes. In addition, isotope labeling leads to a tentative assignment of spectral features to lysine, arginine, histidine, glutamine, and/or asparagine in the permease. These results indicate that the electrochemical gradient causes changes in the environment or protonation state of carboxylic acid residues in the permease and suggest an interaction between these carboxylic acid side chains and nitrogen-containing amino acid side chains. Evidence for a change in secondary structure, corresponding to an interconversion of secondary structural elements, a change in the hydrogen-bonding strength, or coupling of peptide vibrational modes, is also presented. These experiments demonstrate the usefulness of reaction-induced spectroscopy in the study of transmembrane transport.

Xyloside transport by XylP, a member of the galactoside-pentoside-hexuronide family

This paper describes the functional characterization of the xyloside transporter, XylP, of Lactobacillus pentosus with the aid of a spectroscopy-based assay system. In order to monitor the transport reaction, the natural xyloside isoprimeverose, a building block of hemicellulose, and the analogue methyl-isoprimeverose were chemically synthesized by a new and efficient procedure. The XylP protein was purified by metal affinity chromatography, following high level expression in Lactococcus lactis from the nisin-inducible promoter. The purified XylP protein was incorporated into liposomes, in which the glucose dehydrogenase from Acinetobacter calcoaceticus (sGDH) was entrapped. sGDH can oxidize aldose sugars in the presence of dichlorophenol-indophenol as electron acceptor. The coupled assay thus involves XylP-mediated isoprimeverose uptake followed by internal oxidation of the sugar by sGDH, which can be monitored from the reduction of 2,6-dichlorophenol-indophenol at 600 nm. The uptake of isoprimeverose was stimulated by the presence of the non-oxidizable methyl-isoprimeverose on the trans-side of the membrane, indicating that exchange transport is faster than unidirectional downhill uptake. Unlike other members of the galactoside-pentoside-hexuronide family, XylP does not transport monosaccharides (xylose) but requires a glycosidic linkage at the anomeric carbon position. Consistent with a proton motive force-driven mechanism, the uptake was stimulated by a membrane potential (inside negative relative to outside) and inhibited by a pH gradient (inside acidic relative to outside). The advantages of the here-described transport assay for studies of carbohydrate transport are discussed.

A K319N/E325Q double mutant of the lactose permease cotransports H+ with lactose. Implications for a proposed mechanism of H+/lactose symport

In this study, we have examined the transport characteristics of the wild-type lactose permease, single mutants in which Lys-319 was changed to asparagine or alanine or Glu-325 was changed to glutamine or alanine, and the corresponding double mutant strains. The wild-type and Asn-319 mutant showed high levels of lactose uptake, with Km values of 0.42 and 1.30 mM and Vmax values of 102.6 and 48.3 nmol of lactose/min/mg of protein, respectively. The Asn-319/Gln-325 strain had a normal Km of 0.36 mM and a moderate Vmax of 18.5 nmol of lactose/min/mg of protein. By comparison, the single E325Q strain had a normal Km of 0.27 mM but a very defective Vmax of 1.3 nmol of lactose/min/mg of protein. A similar trend was observed among the alanine substitutions at these positions, although the Vmax values were lower for the Ala-319 mutations. When comparing the Vmax values between the single position 325 mutants with those of the double mutants, these results indicate that neutral 319 mutations substantially alleviate a defect in Vmax caused by neutral 325 mutations. With regard to H+/lactose coupling, the wild-type permease is normally coupled and can transport lactose against a gradient. The position 325 single mutants showed no evidence of H+ transport with lactose or thiodigalactoside (TDG) and were unable to facilitate uphill lactose transport. The single Asn-319 mutant and double Asn-319/Gln-325 mutant were able to transport H+ upon the addition of lactose or TDG. In addition, both of these strains catalyzed a sugar-dependent H+ leak that inhibited cell growth in the presence of TDG. These two strains were also defective in uphill transport, which may be related to their sugar-dependent leak pathway. Based on these and other results in the literature, a model is presented that describes how the interactions among several ionizable residues within the lactose permease act in a concerted manner to control H+/lactose coupling. In this model, Lys-319 and Glu-325 play a central role in governing the ability of the lactose permease to couple the transport of H+ and lactose.

Suppressor scanning at positions 177 and 236 in the Escherichia coli lactose/H+ cotransporter and stereotypical effects of acidic substituents that suggest a favored orientation of transmembrane segments relative to the lipid bilayer

Acidic substituents for Ala-177 (helix 6) or Tyr-236 (helix 7) in LacY cause effects on sugar recognition and cosubstrate coupling that are stereotypical of neutral substituents. Thus, helices 6 and 7 are probably oriented to produce little side-chain contact with the low dielectric lipid bilayer at positions 177 and 236.

Cysteine-scanning mutagenesis of helix VI and the flanking hydrophilic domains on the lactose permease of Escherichia coli

Using a functional lactose permease mutant devoid of Cys residues (C-less permease), each amino acid residue in putative transmembrane helix VI and the flanking hydrophilic loops (residues 164- 211) was replaced individually with Cys. Of the 48 mutants, 43 accumulate lactose at highly significant rates to > 80% of the steady state observed with C-less permease. Three mutants (Phe185--> Cys, Ala187--> Cys, and Phe208--> Cys) exhibit lower but significant levels of accumulation (30-60% of C-less). Cys replacement for Ala177 or Leu184 results in low transport activity (ca. 20%) in the C-less background but much higher activity (60-70%) in the wild type. Immunoblot analysis reveals that all of the mutants are inserted into the membrane at concentrations comparable to that of C-less perrmease. The transport activity of the great majority of the mutants is unaffected by treatment with N-ethylmaleimide (NEM). Relatively modest but significant inactivation (ca. 50%) is observed with mutants Phe170--> Cys, Gly173--> Cys, and Ala187--> Cys, and these positions cluster on the same face of the helix VI. Moreover, the two positions where single Cys replacements result in low activity (Ala177 and Lcu184) are on the same face of helix VI. The results demonstrate the following. (i) Permease function is not disrupted by replacement of most residues with Cys, but function is disrupted when some of the residues are further altered by addition of the NeM moiety. (ii) The latter residues lie on a stripe down one face of an alpha-helix, and within the same stripe are residues where Cys substitution itself leads ti inhibition of function.

Friction analysis of kinetic schemes: the friction coefficient

Friction analysis is proposed as the application of general control analysis to single enzymes to describe the control of elementary kinetic steps on the overall catalytic rate. For each transition, a friction coefficient is defined that measures the sensitivity of the turnover rate to the free energy of the transition state complex of the transition. The latter is captured in a single property of the transition, termed friction, as the geometrical mean of the inverse of the forward and backward rate constants. By definition, the friction coefficient measures the relative change in the turnover rate in response to a small change in the friction. The friction coefficient is the sum of the flux control coefficients of the forward and backward rate constants from general control theory and measures the extent to which an elementary step is rate determining. Two basic rules apply to the friction coefficients: (i) the summation theorem states that summation of the friction coefficients over all the steps in a scheme results in a value of 1, and (ii) the group rule states that grouping of rate constants of similar transitions results in a friction coefficient for the group that is the sum of the friction coefficients of the individual steps in the group. The friction coefficients are derived for a number a kinetic schemes taking the rate equations as the starting point and both rules are demonstrated. In fully coupled systems the friction coefficients of individual steps lie between 0 and 1. In partially uncoupled systems the summation theorem applies to all the rates in the system, however, the summation of subsets of friction coefficients may exceed the value of one, implying negative values for other steps in the scheme. The values of individual friction coefficients lie between -1 and 1. The friction coefficient is redefined in a numerical treatment of the steady state of more complex enzymatic schemes.

Uncoupling in secondary transport proteins. A mechanistic explanation for mutants of lac permease with an uncoupled phenotype

The kinetic behavior of a H(+)-substrate symporter has been studied in which in addition to the unloaded (E) and fully loaded states (E.S.H) of the carrier also one of the binary complexes (E.S or E.H) may reorient its binding sites. This results in two types of uncoupled mutants, the ES leak and the EH leak type. The effects of pH and substrate concentration (pS) on the coupling of transport have been analyzed. In the enzyme with the ES leak, the proton:substrate stoichiometry (v(H+)/v(S)) and the substrate accumulation levels decrease sigmoidally from fully coupled at low pH to completely uncoupled at high pH. Importantly, the coupling inferred from initial rate measurements is higher than from steady state accumulation levels. In the enzyme with the EH leak, the coupling inferred from the accumulation levels increases from no coupling at low pH to full coupling at high pH and saturating substrate concentration. The v(H+)/v(S) increases sigmoidally with pH from < 1 to > 1 and is highly dependent on pS. At each pH value a substrate concentration can be found that results in apparent complete coupling between the two fluxes. The ES leak and the EH leak mutants provide a mechanism for substrate-induced and substrate-inhibited proton leakage, respectively. Furthermore, substrate efflux down a concentration gradient is inhibited by a membrane potential (inside negative) under uncoupled conditions in the case of an ES leak but not in the case of an EH leak. The properties of the mutants mimic those of various transport mutants that have been described, in particular mutants of the lactose transport protein of Escherichia coli. The analysis offers general means for targeted experimentation, which allows discrimination between various types of transport mutants.

The role of transmembrane domain III in the lactose permease of Escherichia coli

Deletion of putative transmembrane helix III from the lactose permease of Escherichia coli results in complete loss of transport activity. Similarly, replacement of this region en bloc with 23 contiguous Ala, Leu, or Phe residues abolishes active lactose transport. The observations suggest that helix III may contain functionally important residues; therefore, this region was subjected to Cys-scanning mutagenesis. Using a functional mutant devoid of Cys residues (C-less permease) each residue from Tyr 75 to Leu 99 was individually replaced with Cys. Twenty-one of the 25 mutants accumulate lactose to > 70% of the steady-state exhibited by C-less permease, and an additional 3 mutants transport to lower, but significant levels (40-60% of C-less). Cys replacement for Leu 76 results in low transport activity (18% of C-less). However, when placed in the wild-type background, mutant Leu 76-->Cys exhibits highly significant rates of transport (55% of wild type) and steady-state levels of lactose accumulation (65% of wild type). Immunoblots reveal that the mutants are inserted into the membrane at concentrations comparable to wild type. Studies with N-ethylmaleimide show that mutant Gly 96-->Cys is rapidly inactivated, whereas the other single-Cys mutants are not altered significantly by the alkylating agent. Moreover, the rate of inactivation of Gly 96-->Cys permease is enhanced at least 2-fold in the presence of beta-galactopyranosyl 1-thio-beta, D-galactopyranoside. The observations demonstrate that although no residue per se appears to be essential, structural properties of helix III are important for active lactose transport.

Cysteine-scanning mutagenesis of putative helix VII in the lactose permease of Escherichia coli

Using a functional lactose permease mutant devoid of Cys residues (C-less permease), each amino acid residue in putative transmembrane helix VII and the flanking cytoplasmic and periplasmic regions (from Leu212 to Glu255) was replaced individually with Cys. Of the 44 single-Cys mutants, 40 exhibit high transport activity, accumulating lactose to > 50% of the steady-state observed with C-less permease. In contrast, permease with Cys in place of Ala213 or Tyr236 exhibits low but significant activity, and Cys substitution for Asp237 or Asp240 yields permease molecules with little or no activity due to disruption of charge-neutralizing interactions between Asp237 and Lys358 or Asp240 and Lys319, respectively. Immunological analysis reveals that membrane levels of the mutant proteins are comparable to that of C-less permease with the exception of Tyr228-->Cys, which exhibits reduced but significant levels of permease. Finally, the effect of N-ethylmaleimide (NEM) was tested on each mutant, and the results indicate that the transport activity of the great majority of the mutants is not affected by the alkylating agent. Remarkably, the six positions where Cys replacements render the permease highly sensitive to inactivation by NEM are confined to the C-terminal half of helix VII, a region that is strongly conserved among transport proteins homologous to lactose permease. The results demonstrate that although no residue per se in the region scanned is essential, structural features of the C terminus of helix VII may be important for transport activity.

Role of glutamate-269 in the lactose permease of Escherichia coli

Glu-269, which is located on the hydrophilic face of putative helix VIII in the lactose permease of Escherichia coli, has been replaced with Asp, Gln or Cys by oligonucleotide-directed, site specific mutagenesis. Cells expressing Asp-269 permease exhibit no lactose accumulation or lactose-induced H+ translocation, but retain some ability to mediate lactose influx down a concentration gradient at high substrate concentrations. Furthermore, right-side-out membrane vesicles containing Asp-269 permease do not catalyse active lactose transport, facilitated lactose efflux or equilibrium exchange. Remarkably, however, Asp-269 permease accumulates beta, D-galactopyranosyl 1-thio-beta,D-galactopyranoside in a partially uncoupled fashion, whereas no transport of methyl-beta,D-thiogalactopyranoside, sucrose or maltose is detectable. Mutant permeases containing neutral replacements (Gln or Cys) or Glu-269 are completely devoid of activity, although the proteins are present in the membrane at concentrations comparable with wild-type or Asp-269 permease. The observations demonstrate that a carboxylate at position 269 is essential for transport activity, and Glu-269 is important for substrate binding and/or recognition.

Use of site-directed fluorescence labeling to study proximity relationships in the lactose permease of Escherichia coli

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 this paper, we describe an approach to studying proximity relationships in lactose permease that is 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 3.5 A, 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-XI.

Coupling mechanisms in active transport

In primary and secondary active transport, the mobility and specificity of the carrier are controlled, over the course of the transport reaction, in accordance with a set of 'rules'. The rules are shown to depend on two mechanisms: a substrate--either the driving substrate (a transported ion or ATP) or the driven substrate--may shift a conformational equilibrium or accelerate a rate-limiting conformational change. From an analysis of coupling mechanisms the following conclusions emerge. (i) The ratio of coupled to uncoupled flux, which should be large, cannot be greater than the ratio of substrate dissociation constants in an initial complex and a conformationally altered state. A minimum value for the increased binding force can be estimated from steady-state constants. (ii) In an ordered mechanism, slippage is expected at high concentrations of the substrate adding to the carrier second, while slippage of the first substrate should remain low. (iii) Slippage in coupled transport is minimized if the driven substrate is last on in loading the carrier and last off in unloading, while the reverse order makes the affinity high in loading and low in unloading, as required for efficient transfer from one compartment to another; hence the preferred mechanism may depend on prevailing physiological conditions. (iv) A coupled transport system can be transformed into a facilitated system for one substrate or both if the control of carrier mobility is undermined through modification of the carrier.

Cysteine scanning mutagenesis of putative transmembrane helices IX and X in the lactose permease of Escherichia coli

Using a functional lactose permease mutant devoid of Cys residues (C-less permease), each amino-acid residue in putative transmembrane helices IX and X and the short intervening loop was systematically replaced with Cys (from Asn-290 to Lys-335). Thirty-four of 46 mutants accumulate lactose to high levels (70-100% or more of C-less), and an additional 7 mutants exhibit lower but highly significant lactose accumulation. As expected (see Kaback, H.R., 1992, Int. Rev. Cytol. 137A, 97-125), Cys substitution for Arg-302, His-322, or Glu-325 results in inactive permease molecules. Although Cys replacement for Lys-319 or Phe-334 also inactivates lactose accumulation, Lys-319 is not essential for active lactose transport (Sahin-Tóth, M., Dunten, R.L., Gonzalez, A., & Kaback, H.R., 1992, Proc. Natl. Acad. Sci. USA 89, 10547-10551), and replacement of Phe-334 with leucine yields permease with considerable activity. All single-Cys mutants except Gly-296 --> Cys are present in the membrane in amounts comparable to C-less permease, as judged by immunological techniques. In contrast, mutant Gly-296 --> Cys is hardly detectable when expressed at a relatively low rate from the lac promoter/operator but present in the membrane in stable form when expressed at a high rate from T7 promoter. Finally, studies with N-ethylmaleimide (NEM) show that only a few mutants are inactivated significantly. Remarkably, the rate of inactivation of Val-315 --> Cys permease is enhanced at least 10-fold in the presence of beta-galactopyranosyl 1-thio-beta-D-galactopyranoside (TDG) or an H+ electrochemical gradient (delta mu-H+). The results demonstrate that only three residues in this region of the permease -Arg-302, His-322, and Glu-325-are essential for active lactose transport. Furthermore, the enhanced reactivity of the Val-315 --> Cys mutant toward NEM in the presence of TDG or delta mu-H+ probably reflects a conformational alteration induced by either substrate binding or delta mu-H+.

Fluorocytosine causes uncoupled dissipation of the proton gradient and behaves as an imperfect substrate of the yeast cytosine permease

At pH 5-6 ATP-depleted washed cell preparations of strain NC233-10b[pII4-9], in which the cytosine permease was overexpressed, absorbed cytosine, hypoxanthine or fluorocytosine stoichiometrically with, respectively, about 1, 1.4 and 5 proton equivalents. The cellular pH fell proportionately. The membrane depolarization caused by each compound was assayed in the presence of glucose with a voltage-sensitive dye and increased in the same order. Fluorocytosine significantly lowered the growth yield that a 'petite' strain of the yeast formed at limiting glucose concentrations. At pH 5.6 with extracellular [K+] below 1 mM, each of the three substrates was accumulated about 200-fold from a dilute solution at the expense of the proton gradient. This concentration ratio corresponds to a solute gradient (delta mu(s)) of 13 kJ mol-1. Raising [K+]o systematically lowered the substrate accumulation ratio and delta muH. The mean ratio delta mu(s)/delta muH was 0.82 for all three substrates. It was concluded that whereas the behaviour of cytosine approximated to that expected for a symport of unit proton stoichiometry, the absorption of protons with fluorocytosine and, to a lesser extent, hypoxanthine, was only partly conserved as useful work. A possible mechanism of this novel phenomenon is outlined.

Transport proteins in bacteria: common themes in their design

Bacterial transport proteins mediate passive and active transport of small solutes across membranes. Comparison of amino acid sequences shows strong conservation not only among bacterial transporters, but also between them and many transporters of animal cells; thus the study of bacterial transporters is expected to contribute to our understanding of transporters in more complex cells. During the last few years, structures of three bacterial outer membrane transporters were solved by x-ray crystallography. Much progress has also occurred in the biochemical and molecular genetic studies of transporters in the cytoplasmic membranes of bacteria, and a unifying design among membrane transporters is gradually emerging. Common structural motives and evolutionary origins among transporters with diverse energy-coupling mechanisms suggest that many transporters contain a central module forming a transmembrane channel through which the solute may pass. Energy-coupling mechanisms can be viewed as secondary features added on to these fundamental translocation units.