Cotransport of proline and Li+ in Escherichia coli

Secondary transport of amino acids in prokaryotes

Amino acid transport is a ubiquitous phenomenon and serves a variety of functions in prokaryotes, including supply of carbon and nitrogen for catabolic and anabolic processes, pH homeostasis, osmoprotection, virulence, detoxification, signal transduction and generation of electrochemical ion gradients. Many of the participating proteins have eukaryotic relatives and are successfully used as model systems for exploration of transporter structure and function. Distribution, physiological roles, functional properties, and structure-function relationships of prokaryotic alpha-amino acid transporters are discussed.

Charge translocation during cosubstrate binding in the Na+/proline transporter of E.coli

Charge translocation associated with the activity of the Na(+)/proline cotransporter PutP of Escherichia coli was analyzed for the first time. Using a rapid solution exchange technique combined with a solid-supported membrane (SSM), it was demonstrated that Na(+)and/or proline individually or together induce a displacement of charge. This was assigned to an electrogenic Na(+)and/or proline binding process at the cytoplasmic face of the enzyme with a rate constant of k>50s(-1) which preceeds the rate-limiting step. Based on the kinetic analysis of our electrical signals, the following characteristics are proposed for substrate binding in PutP. (1) Substrate binding is electrogenic not only for Na(+), but also for the uncharged cosubstrate proline. The charge displacement associated with the binding of both substrates is of comparable size and independent of the presence of the respective cosubstrate. (2) Both substrates can bind individually to the transporter. Under physiological conditions, an ordered binding mechanism prevails, while at sufficiently high concentrations, each substrate can bind in the absence of the other. (3) Both substrate binding sites interact cooperatively with each other by increasing the affinity and/or the speed of binding of the respective cosubstrate. (4) Proline binding proceeds in a two-step process: low affinity (approximately 1mM) electroneutral substrate binding followed by a nearly irreversible electrogenic conformational transition.

Towards the molecular mechanism of Na(+)/solute symport in prokaryotes

The Na(+)/solute symporter family (SSF, TC No. 2.A.21) contains more than 40 members of pro- and eukaryotic origin. Besides their sequence similarity, the transporters share the capability to utilize the free energy stored in electrochemical Na(+) gradients for the accumulation of solutes. As part of catabolic pathways most of the transporters are most probably involved in the acquisition of nutrients. Some transporters play a role in osmoadaptation. With a high resolution structure still missing, a combination of genetic, protein chemical and spectroscopic methods has been used to gain new insights into the structure and molecular mechanism of action of the transport proteins. The studies suggest a common 13-helix motif for all members of the SSF according to which the N-terminus is located in the periplasm and the C-terminus is directed into the cytoplasm (except for proteins containing a N- or C-terminal extension). Furthermore, an amino acid substitution analysis of the Na(+)/proline transporter (PutP) of Escherichia coli, a member of the SSF, has identified regions of particular functional importance. For example, amino acids of TM II of PutP proved to be critical for high affinity binding of Na(+) and proline. In addition, it was shown that ligand binding induces widespread conformational alterations in the transport protein. Taken together, the studies substantiate the common idea that Na(+)/solute symport is the result of a series of ligand-induced structural changes.

Sodium-substrate cotransport in bacteria

A variety of sodium-substrate cotransport systems are known in bacteria. Sodium enters the cell down an electrochemical concentration gradient. There is obligatory coupling between the entry of the ion and the entry of substrate with a stoichiometry (in the cases studied) of 1:1. Thus, the downhill movement of sodium ion into the cell leads to the accumulation of substrate within the cell. The melibiose carrier of Escherichia coli is perhaps the most carefully studied of the sodium cotransport systems in bacteria. This carrier is of special interest because it can also use protons or lithium ions for cotransport. Other sodium cotransport carriers that have been studied recently are for proline, glutamate, serine-threonine, citrate and branched chain amino acids.

Aspartate 55 in the Na+/proline permease of Escherichia coli is essential for Na+-coupled proline uptake

Four acidic residues in the N-terminal domain of Na+/proline permease of Escherichia coli (Asp33, Asp34, and Asp55 in putative loop 2, Glu75 in putative transmembrane domain II) were individually replaced with neutral or charged amino acid residues. Replacement of Glu75, the only residue in the permease presumed to be in the middle of a transmembrane domain, Asp33, or Asp34 had little or no influence on the kinetics of Na+-coupled proline transport. In contrast, removal of the carboxylate at position 55 (Asp55 --> Asn or Asp55 --> Cys permease) impaired proline uptake completely while lengthening of the side chain at this position by one methylene group (Asp55 --> Glu permease) allowed transport at a reduced initial rate. Importantly, all permease molecules were present in the membrane at concentrations comparable to the wild-type protein. Kinetic analysis of Na+-coupled proline transport catalyzed by Asp55 --> Glu permease revealed a 5-fold increase of the K(m) for proline and a 30-fold decrease of the V(max) compared to wild-type. Remarkably, replacement of Asp55 by Glu led to a 50-fold decrease of the apparent affinity of the permease for Na+. Furthermore, replacement of Asp55 with Cys or Asn blocked proline-induced Na+ uptake whereas significant Na+ transport was observed with Asp55 --> Glu permease. In addition, transport of proline down its concentration gradient was not detectable with deenergized cells containing Asp55 --> Glu permease at low Na+ concentrations. However, downhill transport activity was observed in the presence of high Na+ concentrations. Replacement of Asp55 with Asn or Cys impaired downhill transport under all conditions tested. The observations demonstrate that a carboxylate at position 55 of proline permease is essential for Na+-coupled proline transport. It is suggested that Asp55 may be involved in binding of the coupling ion.

Ser57 in the Na+/proline permease of Escherichia coli is critical for high-affinity proline uptake

Ser57 in the Na+/proline permease of Escherichia coli has been replaced with alanine, cysteine, glycine, or threonine, and properties of the corresponding putP mutants have been analyzed. Although Ser57 is not essential for activity, the amino acid side chain at this position is critical for proline uptake. Thus, alanine, cysteine, glycine, or threonine in place of Ser57 reduces the initial rate of proline transport under standard conditions to less than 10% of the wild-type value. In addition, substitution of Ser57 in the Na+/proline permease reduces the sensitivity of E. coli cells to the toxic proline analogs L-azetidine-2-carboxylate and 3.4-dehydro-D.L-proline. Replacement of Ser57 with alanine or cysteine results in apparent affinities for proline that are reduced by more than two orders of magnitude, and permeases with threonine and glycine in place of Ser57 yield apparent affinities reduced by a factor of 60 and 18 respectively, relative to wild-type. In contrast, all of the Ser57 replacements analyzed cause only small changes in Vmax values. All permease molecules containing Ser57 substitutions are inserted into the membrane in amounts comparable to the wild-type protein as shown by immunoblot analysis. These results indicate that alterations of proline transport and sensitivity to toxic proline analogs have to be attributed primarily to defects in substrate binding. It is suggested that the serine residue at position 57 of the permease is located within the substrate-binding domain of the protein.

Sodium ion and proline binding sites in the Na+/proline symport carrier of Escherichia coli

Proline binding activity of the Escherichia coli Na+/proline symport carrier is inhibited by a sulfhydryl reagent, N-ethylmaleimide (NEM). Proline and its analogs protected the carrier against the NEM-inactivation in a Na+ (or Li+)-dependent manner. Na+ alone, even in the absence of proline, partially protected it from the NEM-inactivation. Mutant proline carriers, CS281, CS344 and CS349, which have a serine residue in place of Cys-281, Cys-344 and Cys-349, respectively (Yamato, I. and Anraku, Y. (1988) J. Biol. Chem. 263, 16055-16057) were also analyzed for cation-dependent proline binding and NEM-sensitivity. Proline binding activities of CS281 and CS344 were almost completely resistant to NEM, whereas that of CS349 was not. Furthermore, the proline binding activity of CS344 was remarkably lower than those of the wild-type, CS281 and CS349 carriers. These results indicate that Cys-344, which is located in the putative eighth membrane-spanning domain in the carrier, is a cysteine residue functionally involved in the high-affinity binding for sodium ion and proline.

Mechanism of Na+/proline symport in Escherichia coli: reappraisal of the effect of cation binding to the Na+/proline symport carrier

The proton and sodium ion dependences of the proline binding and transport activities of the proline carrier in Escherichia coli were investigated in detail. The binding activity in cytoplasmic membrane vesicles from a carrier over-producing strain (PT21/pTMP5) was absolutely dependent on the presence of Na+, but did not necessarily require protonation of the carrier, in contrast to the model previously reported (Mogi, T., Anraku, Y. 1984. J. Biol. Chem. 259:7797-7801). Based on this and previous observations, we propose a modified model of the proline binding reaction of the proline carrier, in which a proton is supposed to be a regulatory factor for the binding activity. The apparent Michaelis constant of proline (Kt) of the transport activity of cytoplasmic membrane vesicles from the wild type E. coli strain driven by a respiratory substrate, ascorbate, showed dependence on a low concentration of sodium ion. The Michaelis constant of sodium ion for transport (KtNa) was estimated to be 25 microM. The proline transport activities in membrane vesicles and intact cells were modulated by H+ concentration, the inhibitory effect of protons (pKa approximately equal to 6) being similar to that observed previously (Mogi, T., Anraku, Y. 1984. J. Biol. Chem. 259:7802-7806). Based on these observations and the modified model of substrate binding to the proline carrier, a model of the proline/Na+ symport mechanism is proposed, in which a proton is postulated to be a regulatory factor of the transport activity.

Orientation of the carboxyl terminus of the Na+/proline symport carrier in Escherichia coli

The orientation of the carboxyl terminal region of the Escherichia coli proline carrier in the cytoplasmic membrane was studied. The beta-galactosidase moiety of the PutP-LacZ fusion protein [(1987) J. Biol. Chem. 262, 14100-14104] was exposed outside the inside-out vesicles and inside the right-side-out vesicles. A site-directed antibody raised against a synthetic peptide corresponding to the putative carboxyl terminal region of the carrier reacted preferentially with the inside-out vesicles prepared from a wild-type proline carrier overproducing strain and less with the right-side-out vesicles. These results indicate that the carboxyl terminus of the proline carrier is exposed to the cytoplasmic side of the membrane.

Sodium-dependent transport of branched-chain amino acids by a monensin-sensitive ruminal peptostreptococcus

A recently isolated ruminal peptostreptococcus which produced large amounts of branched-chain volatile fatty acids grew rapidly with leucine as an energy source in the presence but not the absence of Na. Leucine transport could be driven by an artificial membrane potential (delta psi) only when Na was available, and a chemical gradient of Na+ (delta uNa+) also drove uptake. Because Na+ was taken up with leucine and a Z delta pH could not serve as a driving force (with or without Na), it appeared that leucine was transported in symport with Na+. The leucine carrier could use Li as well as Na and had a single binding site for Na+. The Km for Na was 5.2 mM, and the Km and Vmax for leucine were 77 microM and 328 nmol/mg of protein per min, respectively. Since valine and isoleucine competitively inhibited (Kis of 90 and 49 microM, respectively) leucine transport, it appeared that the peptostreptococcus used a common carrier for branched-chain amino acids. Valine or isoleucine was taken up rapidly, but little ammonia was produced if they were provided individually. The lack of ammonia could be explained by an accumulation of reducing equivalents. The ionophore, monensin, inhibited growth, but leucine was taken up and deaminated at a slow rate. Monensin caused a loss of K, an increase in Na, a slight increase in delta psi, and a decrease in intracellular pH. The inhibition of growth was consistent with a large decrease in ATP.(ABSTRACT TRUNCATED AT 250 WORDS)

Na+(Li+)/branched-chain amino acid cotransport in Pseudomonas aeruginosa

A transport system for branched-chain amino acids (designated as LIV-II system) in Pseudomonas aeruginosa requires Na+ for its operation. Coupling cation for this system was identified by measuring cation movement during substrate entry using cation-selective electrodes. Uptakes of Na+ and Li+ were induced by the imposition of an inwardly-directed concentration gradient of leucine, isoleucine, or valine. No uptake of H+ was found, however, under the same conditions. In addition, effects of Na+ and Li+ on the kinetic property of the system were examined. At chloride salt concentration of 2.5 mM, values of apparent Km and Vmax for leucine uptake were larger in the presence of Na+ than Li+. These results indicate that the LIV-II transport system is a Na+(Li+)/substrate cotransport system, although effects of Na+ and Li+ on kinetics of the system are different.

Proline porters effect the utilization of proline as nutrient or osmoprotectant for bacteria

Proline is utilized by all organisms as a protein constituent. It may also serve as a source of carbon, energy and nitrogen for growth or as an osmoprotectant. The molecular characteristics of the proline transport systems which mediate the multiple functions of proline in the Gram negative enteric bacteria, Escherichia coli and Salmonella typhimurium, are now becoming apparent. Recent research on those organisms has provided both protocols for the genetic and biochemical characterization of the enzymes mediating proline transport and molecular probes with which the degree of homology among the proline transport systems of archaebacteria, eubacteria and eukaryotes can be assessed. This review has provided a detailed summary of recent research on proline transport in E. coli and S. typhimurium; the properties of other organisms are cited primarily to illustrate the generality of those observations and to show where homologous proline transport systems might be expected to occur. The characteristics of proline transport in eukaryotic microorganisms have recently been reviewed (Horak, 1986).

Proline carrier mutant of Escherichia coli K-12 with altered cation sensitivity of substrate-binding activity: cloning, biochemical characterization, and identification of the mutation

Two putP mutants of Escherichia coli K-12 that were defective in proline transport but retained the binding activities of the major proline carrier were isolated (T. Mogi, H. Yamamoto, T. Nakao, I. Yamato, and Y. Anraku, Mol. Gen. Genet. 202:35-41, 1986). One of these mutations and three null-type mutations (K. Motojima, I. Yamato, and Y. Anraku, J. Bacteriol. 136:5-9, 1978) were cloned into a pBR322 putP+ hybrid plasmid (pTMP5) by in vivo recombination. Cytoplasmic membrane vesicles were prepared from the mutant strains and strains harboring pTMP5 putP plasmids, and the properties of the proline-binding reaction of the mutant putP carriers in membranes were examined under nonenergized conditions. The putP19, putP21, and putP22 mutations, which were mapped in the same DNA segment of the putP gene (Mogi et al., Mol. Gen. Genet. 202:35-41, 1986), caused the complete loss of proline carrier activity. The proline carriers encoded by the mutant putP genes, putP9 and putP32, and putP32 in pTMP5-32, which was derived from in vivo recombination with the putP32 mutation, had altered sodium ion and proton dependence of binding affinities for proline and were resistant to N-ethylmaleimide inactivation without changes in the specificities for substrates and alkaline metal cations. The nucleotide sequence of the putP32 lesion located on the 0.35-megadalton RsaI-PvuII fragment in the putP gene in pTMP5-32 was determined; the mutation changed a cytosine at position 1001 to a thymine, causing the alteration of arginine to cysteine at amino acid position 257 in the primary structure of the proline carrier. It was shown that this one point mutation was enough to produce the phenotype of pTMP5-32 by in vitro DNA replacement of the AcyI-PvuII fragment of the wild-type putP gene with the DNA fragment containing the mutated nucleotide sequence.

Properties of a Na+-coupled serine-threonine transport system in Escherichia coli

Based on the following experimental results we conclude that the serine-threonine transport system in Escherichia coli is a Na+-coupled cotransport system. (1) Addition of serine to cell suspensions induced H+ efflux in the presence of Na+. (2) Addition of serine to cell suspensions induced Na+ uptake by cells. (3) Imposition of an artificial electrochemical potential of Na+ in starved cells induced serine uptake. Some of these phenomena were observed when threonine was added instead of serine or inhibited when cells were preincubated with threonine. The Na+/serine (threonine) cotransport system was considerably repressed when cells were grown on a mixture of amino acids. Serine transport in cells grown in the absence of amino acids mixture was stimulated by Na+. The half maximum concentration of Na+ was 21 microM. Sodium ion increased the Vmax of serine transport without affecting the Km.

Phenotypic properties of a unique rpoA mutation (phs) of Escherichia coli

The phs mutation of Escherichia coli has been suggested to affect the Na+/H+ antiport (D. Zilberstein, E. Padan, and S. Schuldiner, FEBS Lett. 168:327-330, 1980). We have recently shown that the mutation affects the rpoA gene and thus affects transcription. The extent of the pleiotropy of the phs mutation was investigated. In addition to the previously reported growth defect on L-glutamate and melibiose, the mutation also affects at least two other metabolic systems. The transport and metabolism of arabinose is impaired and the transport of sulfate is reduced. The extent to which the effects of the phs mutation on metabolism are due to a defect in the Na+/H+ antiport was investigated, and no causal role for this transport system in the metabolic defects was found.

Na+ (Li+)-proline cotransport in Escherichia coli

Na+ and Li+ were found to stimulate the transport of L-proline by cells of Escherichia coli induced for proline utilization. The gene product of the put P gene is involved in the expression of this transport activity since the put P+ strains CSH 4 and WG 148 show activity and the put P- strain RM 2 fails to show this cation coupled transport. The addition of proline was found to stimulate the uptake of Li+ and of Na+. Attempts to demonstrate proline stimulated H+ uptake were unsuccessful. It is concluded that the proline carrier (coded by the put P gene) is responsible for Na+ (or Li+)-proline cotransport.

Proline uptake through the major transport system of Salmonella typhimurium is coupled to sodium ions

Strains of Salmonella typhimurium deficient in one or more of the proline transport systems have been constructed and used to study the mechanism of energy coupling to transport. Proline uptake through the major proline permease (PP-I, putP) is shown to be absolutely coupled to Na+ ions and not to H+ ions as has previously been assumed. Transport through the minor proline permease (PP-II, proP), however, is unaffected by the presence or absence of Na+. The effect of Na+ on the kinetics of proline uptake shows that external Na+ increases the Vmax for transport. It seems probable that proline transport through PP-I is also coupled to Na+ ions in Escherichia coli.