A protonmotive force as the source of energy for galactoside transport in energy depleted Escherichia coli

The Chlorella hexose/H(+)-symporters

The physiology, molecular biology, and biochemistry of the inducible hexose uptake protein of Chlorella kessleri is reviewed. The protein encoded by the HUP1 gene is the most intensively studied membrane transporter of plants. Responsible for substrate accumulation up to 1500-fold, it translocates one proton together with one hexose, and the cell invests 1 ATP per sugar transported. Kinetics suggest that substrate accumulation is mainly brought about by a large delta Km (Kminside >> Kmoutside). The HUP1 protein (534aa) consists of 12 transmembrane helices of which at least helices I, V, VII, and XI interact with the sugar during translocation and participate in lining the transport path through the membrane. The helix packing might very well be identical to the one suggested for the E. coli lac permease, although the mechanism for transport and proton coupling that has been suggested for lac permease (Kaback, 1997) certainly does not hold for the Chlorella symporter; both are distantly related members, however, of the MFS-family of transporters. HUP1 has been functionally expressed in Schizosaccharomyces pombe, Saccharomyces cerevisiae, Escherichia coli, Volvox carteri, and in Xenopus oocytes.

Isolation of lactose permease mutants which recognize arabinose

In the present study lactose permease mutants were isolated which recognize the monosaccharide, L-arabinose. Although the wild-type permease exhibits a poor recognition for L-arabinose, seven independent mutants were identified by their ability to grow on L-arabinose minimal plates. When subjected to DNA sequencing, it was found that all seven of these mutants were single-site mutations in which alanine 177 was changed to valine. The wild type and valine 177 mutant were then analyzed with regard to their abilities to recognize and transport monosaccharides and disaccharides. Free L-arabinose was shown to competitively inhibit [14C]-lactose transport yielding a Ki value of 121 mM for the Val177 mutant and a much higher value of 320 mM for the wild-type. Among several monosaccharides, D-glucose as well as L-arabinose inhibited lactose transport in the Val177 mutant to a significantly greater extent, while D-arabinose and D-xylose only caused a slight inhibition. On the other hand, kinetic studies with sugars which are normally recognized by the wild-type permease such as [14C]-galactose and [14C]-lactose revealed that the Val177 mutant and wild-type strains had similar transport characteristics for these two sugars. Overall, these results are consistent with the notion that the Val177 substitution causes an enhanced recognition for particular sugars (i.e. L-arabinose) but does not universally affect the recognition and unidirectional transport for all sugars. This idea is further supported by the observation that site-directed mutants containing isoleucine, leucine, phenylalanine, or proline at position 177 also were found to possess an enhanced recognition for L-arabinose.

Proton-linked L-fucose transport in Escherichia coli

1. Addition of L-fucose to energy-depleted anaerobic suspensions of Escherichia coli elicited an uncoupler-sensitive alkaline pH change diagnostic of L-fucose/H+ symport activity. 2. L-Galactose or D-arabinose were also substrates, but not inducers, for the L-fucose/H+ symporter. 3. L-Fucose transport into subcellular vesicles was dependent upon respiration, displayed a pH optimum of about 5.5, and was inhibited by protonophores and ionophores. 4. These results showed that L-fucose transport into E. coli was energized by the transmembrane electrochemical gradient of protons. 5. Neither steady state kinetic measurements nor assays of L-fucose binding to periplasmic proteins revealed the existence of a second L-fucose transport system.

Energetic studies of lactose active transport in Escherichia coli membrane vesicles

The energetics of D-lactate-driven active transport of lactose in right-side-out Escherichia coli membrane vesicles has been investigated with a microcalorimetric method. Changes of enthalpy (delta Hox), free energy (delta Gox), and entropy (delta Sox) during the D-lactate oxidation reaction in the presence of membrane vesicles are -39.9 kcal, -46.4 kcal, and 22 cal/deg per mole of D-lactate, respectively. The free energy released by this reaction is utilized to form a proton electrochemical potential (delta-microH+) across the membrane. The higher observed heat in the D-lactate oxidation reaction in the presence of carbonylcyanide m-chlorophenylhydrazone (a proton ionophore) supports the postulate that delta-microH+ is formed across the membrane vesicles. Thermodynamic quantities for the formation of delta-microH+ are delta Hm = 14.1 kcal, delta Gm = 0.6 kcal, and delta Sm = 45 cal/deg per mole of D-lactate. The efficiency in the free energy transfer from the oxidation reaction to the formation of delta-microH+ (defined by delta Gm/delta Gox) was 2%, as compared to that in the heat transfer (defined by delta Hm/delta Hox) of 35%. The energetics of the movement of lactose in symport with proton across the membrane as a consequence of the formation of delta-microH+ are delta H1 = -19 kcal, delta G1 = -0.5 kcal, and delta S1 = -62 cal/deg per mole of lactose. No heat of reaction is contributed by lactose movement across the membrane without symport with H+.

Isolation and nucleotide sequencing of lactose carrier mutants that transport maltose

The wild-type lactose carrier of Escherichia coli has a poor ability to transport the disaccharide maltose. However, it is possible to select lactose carrier mutants that have an enhanced ability to transport maltose by growing E. coli cells on maltose minimal plates in the presence of isopropyl thiogalactoside (an inducer of the lac operon). We have utilized this approach to isolate 18 independent lactose permease mutants that transport maltose. The relevant DNA sequences have been determined, and all of the mutations were found to be single base pair changes either at triplet 177 or at triplet 236. The nucleotide changes replace alanine-177 with valine or threonine, or tyrosine-236 with phenylalanine, asparagine, serine, or histidine. Transport experiments indicate that all of the mutants have faster maltose transport compared with the wild-type strain. Position 177 mutants retain the ability to transport galactosides, such as lactose and melibiose, at rates similar to the rate of the wild-type strain. In contrast, the position 236 mutants are markedly defective in the ability to transport galactosides. With regard to secondary structure, alanine-177 and tyrosine-236 are located on adjacent hydrophobic segments of the lactose carrier that are predicted to span the membrane. Thus, the results of this study indicate that the substrate recognition site of the lactose carrier is located within the plane of the lipid bilayer. In addition, a tertiary structure model is proposed that suggests how certain transmembrane segments might be localized relative to one another.

A sodium-independent mechanism for L-arginine uptake by rat renal brush border membrane vesicles

L-arginine accumulation in renal cortex slices or in brush border membrane vesicles differs from that of neutral amino acids and sugars, since it can occur despite a lack of sodium in the incubation medium. L-arginine uptake was measured under different conditions in order to determine the relative importance of the Na+-independent component of this transport. In contrast to D-glucose, which was accumulated into the vesicles only in the presence of a NaCl gradient, the accumulation of L-arginine was induced by NaCl, LiCl, KCl, or choline Cl gradients. The uptakes of L-arginine and D-glucose wer proportional to the membrane potential (inside less than 0), but the presence of Na+ was not required for the L-arginine uptake. No difference was observed in Kj values of the L-arginine uptake measured with low or high concentration of salt (Li+ or Na+). Jmax were also in the same range of magnitude. An influx of D-glucose or L-alanine (Na+-cotransported molecules) induced an increase of the sodium content in vesicles equilibrated with 22Na; the L-arginine uptake did not induce it. These different results suggest that the electrical potential and not the chemical gradient was the major driving force for the L-arginine uptake.

Lactose carrier protein of Escherichia coli. Reconstitution of galactoside binding and countertransport

A procedure for the reconstitution of the lactose carrier protein, a galactoside:proton symporter in Escherichia coli, is described. Starting from cytoplasmic membranes derived from carrier-overproducing strains, essentially all proteins including 89% of the carrier are solubilized by a mixture of dodecyl/tetradecyl polyoxyethylene (n = 9.5) ether and dodecyl O-beta-D-maltoside. In the micellar state the carrier binds substrates with reduced affinity. Addition of E. coli phospholipids and removal of detergents by a hydrophobic column yields small vesicles (50-60-nm diameter). In these vesicles, about 70% of the carrier is recovered and reconstituted carrier is identical to native carrier in terms of substrate binding. After fusion of the small vesicles into larger vesicles (1-5 micrometers), rapid countertransport of galactosides is demonstrated. Attempts to show active galactoside transport by the imposition of artificial electrical potential or pH gradients were unsuccessful, most likely because the reconstituted vesicles are in fact highly permeable to protons.

Evolution of membrane bioenergetics

One of the first problems encountered by primitive cells was that of volume regulation; the continuous entry of ions, (eg, NaCl) and water in response to the internal colloid osmotic pressure threatening to destroy the cell by lysis. We propose that to meet this environmental challenge cells evolved an ATP-driven proton extrusion system plus a membrane carrier that would exchange external protons with internal Na+. With the appearance of the ability to generate proton gradients, additional mechanisms to harness this source of energy emerged. These would include proton-nutrient cotransport, K+ accumulation, nucleic acid entry, and motility. A more efficient system for the uptake of certain carbohydrates by vectorial phosphorylation via the PEP-phosphotransferase system probably appeared rather early in the evolution of anaerobic bacteria. The reversal of the proton-ATPase reaction to give net ATP synthesis became possible with the development of other types of efficient proton transporting machinery. Either light-driven bacterial rhodopsin or a redox system coupled to proton translocation would have served this function. Oxidation of one substrate coupled to the reduction of another substrate by membrane-bound enzymes evolved in such a manner that protons were extruded from the cell during the reaction. The progressive elaboration of this type of redox proton pump permitted the use of exogenous electron acceptors, such as fumarate, sulfate, and nitrate. The stepwise growth of these electron transport chains required the accretion of several flavoproteins, iron-sulfur proteins, quinones, and cytochromes. With modifications of these four basic components a chlorophyll-dependent photosynthetic system was subsequently evolved. The oxygen that was generated by this photosynthetic system from water would eventually accumulate in the atmosphere of the earth. With molecular oxygen present, the emergence of cytochrome oxidase would complete the respiratory chain. The proton economy of membrane energetics has been retained by most present-day microorganisms, mitochondria, chloroplasts, and cells of higher plants. A secondary use of the energy stored as an electrochemical difference of Na+ for powering membrane events probably also evolved in microorganisms. The exclusive age of the Na+ economy is distinctive of the plasma membrane of animal cells; the Na+-K+ ATPase sets up an electrochemical Na+ gradient that provides the energy for osmoregulation, Na+-nutrient co-transport, and the action potential of excitable cells.

Sucrose transport by the Escherichia coli lactose carrier

Several lines of evidence suggest that sucrose is transported by the lactose carrier of Escherichia coli. Entry of sucrose was monitored by an osmotic method which involves exposure of cells to a hyperosmotic solution of disaccharide (250 mM). Such cells shrink (optical density rises), and if the solute enters the cell, there is a return toward initial values (optical density falls). By this technique sucrose was found to enter cells at a rate approximately one third that of lactose. In addition, the entry of [14C]sucrose was followed by direct analysis of cell contents after separation of cells from the medium by centrifugation. Sucrose accumulated within the cell to a concentration 160% of that in the external medium. The addition of sucrose to an anaerobic suspension of cells resulted in a small alkalinization of the external medium. These data are consistent with the view that the lactose carrier can accumulate sucrose by a proton cotransport system. The carrier exhibits a very low affinity for the disaccharide (150 mM) but a moderately rapid Vmax.

The lactose system in Klebsiella aerogenes V9A. 6. Lactose transport

Klebsiella aerogenesV9A carrying alacplasmid in addition to its chromosomal operon showed strongly positive fermentation of lactose on MacConkey lactose agar plates, and was found to transport the lactose analogue thiomethyl-β-galactoside (TMG) at a rapid rate. The strain that had been freed of the plasmid showed moderate transport due to the chromosomallacoperon. When a plasmid bearing a mutation inlac Ywas inserted into a strain with a normalYgene, the resulting diploid became lactose-negative in phenotype. The presence ofE. coliF′lacfactors that carriedlac Ymutations, whether deletions or missense or nonsense mutations, also renderedlac Y+Klebsiellalactose-negative. Such diploids, after growth in 1% lactose, transported TMG at a much lower rate than the corresponding plasmid-freelac Y+Klebsiella. However, this interference bylac Y−plasmids with the expression of the chromosomallac Ygene was not seen when cells were induced with IPTG or when the chromosomal and plasmidlacoperons were both constitutive. It was found that this effect of the plasmids was dependent on their possessing an intactlacZgene.

Requirement for membrane potential in active transport of glutamine by Escherichia coli

The effect of reducing the membrane potential on glutamine transport in cells of Escherichia coli has been investigated. Addition of valinomycin to tris(hydroxymethyl)aminomethane-ethylenediaminetetraacetic acid-treated E. coli cells in the presence of 20 mM exogenous potassium reduced the membrane potential, as measured by the uptake of the lipophilic cation triphenylmethylphosphonium, and caused a complete inhibition of glutamine transport. Valinomycin plus potassium also caused a rapid decrease in the intracellular levels of ATP of normal E. coli cells, but had little if any effect on the ATP levels of two mutants of E. coli carrying lesions in the energy-transducing ATP complex (unc mutants). Yet both the membrane potential and the capacity to transport glutamine were depressed in the unc mutants by valinomycin and potassium. These findings are consistent with the hypothesis that both ATP and a membrane potential are essential to the active transport of glutamine by E. coli cells.

The basic asymmetry of Na+-dependent glycine transport in Ehrlich cells

Influx and efflux of glycine have been examined as a function of external and internal Na+ concentrations, respectively, when : formula: (see text) = O. With : formula: (see text) = O it was found that at comparable external and cellular Na+ levels, the Km for efflux was larger by an order of magnitude than the value for influx and the V for efflux was several times greater than the V for influx. For both fluxes the major effect of Na+ was to decrease the Km value. The observations are consistent with the conclusion that the Na+-dependent transport system is asymmetric per se. Influx and efflux of glycine were increased in a near linear manner by increasing the Na+ concentration from 13 to 100 mM, the half-time for glycine equilibration being a funcion of the Na+ concentration in absence of an electrochemical potential difference for Na+. In Na+-free media ([Na+] less than 5 mM) equilibration of glycine between cells and medium was not achieved after 60 min at 25 degrees C. With : formula: (see text) = O, efflux (or uptake) of glycine was not affected by internal (or external) K+ between 20 and 120 mM suggesting that K+ plays no direct role in Na+-dependent transport of glycine in Ehrlich cells.

Metabolic control of lactose entry in Escherichia coli

A general method has been developed for determining the rate of entry of lactose into cells of Escherichia coli that contain beta-galactosidase. Lactose entry is measured by either the glucose or galactose released after lactose hydrolysis. Since lactose is hydrolyzed by beta-galactosidase as soon as it enters the cell, this assay measures the activity of the lactose transport system with respect to the translocation step. Using assays of glucose release, lactose entry was studied in strain GN2, which does not phosphorylate glucose. Lactose entry was stimulated 3-fold when cells were also presented with readily metabolizable substrates. Entry of omicron-nitrophenyl-beta-D-galactopyranoside (ONPG) was only slightly elevated (1.5-fold) under the same conditions. The effects of arsenate treatment and anaerobiosis suggest that lactose entry may be limited by the need for reextrusion of protons which enter during H+/sugar cotransport. Entry of omicron-nitrophenyl-beta-D-galactopyranoside is less dependent on the need for proton reextrusion, probably because the stoichiometry of H+/substrate cotransport is greater for lactose than for ONPG.

Similarity in effects of Na+ gradients and membrane potentials on D-glucose transport by, and phlorizin binding to, vesicles derived from brush borders of rattit intestinal mucosal cells

Both the presence of sodium and of an electrical potential difference across the membrane have been found to be necessary in order to achieve optimal D-glucose-protectable phlorizin binding to brush border membranes from rabbit small intestine. The effect of delta approximately muNa on phlorizin binding shows a close similarity to that on D-glucose transport, confirming that phlorizin is indeed bound to the D-glucose transporting protein. Possible modulations of binding by a transmembrane potential are discussed on the basis of some models.

Glutamate transport driven by an electrochemical gradient of sodium ions in Escherichia coli

The role of Na+ in glutamate transport was studied in Escherichia coli B, strain 29-78, which possesses a very high activity of glutamate transport (L. Frank and I. Hopkins, J. Bacteriol., 1969). Energy-depleted cells were exposed to radioactive glutamate in the presence of a sodium gradient, a membrane potential, or both. One hundred- to 200-fold accumulation of the amino acid was attained in the presence of both electrical and chemical driving forces for the sodium ion. Somewhat lower accumulation values were obtained when either chemical or electrical driving forces were applied separately. A chemical driving force was produced by the addition of external Na+ to Na+-free cells. A membrane potential was established by a diffusion potential either of H+ in the presence of carbonyl cyanide p-trifluoromethoxyphenylhydrazone or of SCN-. These results support the hypothesis of a Na+-glutamate cotransport. Na+-driven glutamate transport was also observed in wild-type E. coli B but not in a strain of K-12.