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.     

Proline transport in Salmonella typhimurium: putP permease mutants with altered substrate specificity.

The putP gene encodes a proline permease required for Salmonella typhimurium LT2 to grow on proline as the sole source of nitrogen. The wild-type strain is sensitive to two toxic proline analogs (azetidine-2-carboxylic acid and 3,4-dehydroproline) also transported by the putP permease. Most mutations in putP prevent transport of all three substrates. Such mutants are unable to grow on proline and are resistant to both of the analogs. To define domains of the putP gene that specify the substrate binding site, we used localized mutagenesis to isolate rare mutants with altered substrate specificity. The position of the mutations in the putP gene was determined by deletion mapping. Most of the mutations are located in three small (approximately 100-base-pair) deletion intervals of the putP gene. The sensitivity of the mutants to the proline analogs was quantitated by radial streaking to determine the affinity of the mutant permeases for the substrates. Some of the mutants showed apparent changes in the kinetics of the substrates transported. These results indicate that the substrate specificity mutations are probably due to amino acid substitutions at or near the active site of proline permease.     

Mechanism of proline transport in Escherichia coli K12. II. Effect of alkaline cations on binding of proline to a H+/proline symport carrier in cytoplasmic membrane vesicles

The substrate binding reaction of the proline carrier was investigated in nonenergized conditions using cytoplasmic membrane vesicles prepared from the proline carrier-overproducing strain MinS/ pLC4 -45 of Escherichia coli K12. The binding activity specifically required both alkaline cations (X+), Na+ and Li+, and protons. The Na+-dependent binding activity was dependent on the proline carrier, which is the product of the putP gene, and was not affected by ionophores and energy transduction inhibitors. The parameters of proline binding were determined by double reciprocal plots in reaction media with various combinations of Na+ and H+ concentrations. The apparent dissociation constant was greatly affected by the Na+ and H+ concentrations of the medium and could be expressed as a combination of the reciprocals of the Na+ and H+ concentrations, while the maximum number of binding sites remained constant. The characteristics of proline binding to the carrier can be explained by a mechanism in which the unloaded carrier forms a carrier/H+/X+ (CH+X+) complex by a random equilibrium and only the CH+X+ complex binds substrate in nonenergized conditions, as proposed for the Na+/H+/glutamate symport carrier of E. coli B ( Fujimura , T., Yamato , I., and Anraku , Y. (1983) Biochemistry 22, 1954-1959).     

Activation of a new proline transport system in Salmonella typhimurium.

Proline uptake can be mediated by three different transport systems in wild-type Salmonella typhimurium: a high-affinity proline transport system encoded by the putP gene and two glycine-betaine transport systems with a low affinity for proline encoded by the proP and proU genes. However, only the PutP permease transports proline well enough t allow growth on proline as a sole carbon or nitrogen source. By selecting for mutations that allow a putP mutant to grow on proline as a sole nitrogen source, we isolated mutants (designated proZ) that appeared to activate a cryptic proline transport system. These mutants enhanced the transport of proline and proline analogs but did not require the function of any of the known proline transport genes. The mutations mapped between 75 and 77.5 min on the S. typhimurium linkage map. Proline transport by the proZ mutants was competitively inhibited by isoleucine and leucine, which suggests that the ProZ phenotype may be due to unusual mutations that alter the substrate specificity of the branched-chain amino acid transport system encoded by the liv genes.     

Solubilization and functional reconstitution of the proline transport system of Escherichia coli

The membrane carrier for L-proline (product of the putP gene) of Escherichia coli K12 was solubilized and functionally reconstituted with E. coli phospholipid by the cholate dilution method. The counterflow activity of the reconstituted system was studied by preloading the proteoliposomes with either L-proline or the proline analogues: L-azetidine-2-carboxylate or 3,4-dehydro-L-proline. The dilution of such preloaded proteoliposomes into a buffer containing [3H]proline resulted in the accumulation of this amino acid against a considerable concentration gradient. A second driving force for proline accumulation was an electrochemical potential difference for Na+ across the membrane. More than a 10-fold accumulation was seen with a sodium electrochemical gradient while no accumulation was found with proton motive force alone. The optimal pH for the L-proline carrier activities for both counterflow and sodium gradient-driven uptake was between pH 6.0 and 7.0. The stoichiometry of the co-transport system was approximately one Na+ for one proline. The effect of different phospholipids on the proline transport activity of the reconstituted carrier was also studied. Both phosphatidylethanolamine and phosphatidylglycerol stimulate the carrier activity while phosphatidylcholine and cardiolipin were almost inactive.     

Purification and reconstitution of Escherichia coli proline carrier using a site specifically cleavable fusion protein

We previously constructed a bifunctionally active membrane-bound fusion protein, in which Escherichia coli proline carrier (the product of the putP gene) was linked with beta-galactosidase (the product of the lacZ gene) through a collagen linker (Hanada, K., Yamato, I., and Anraku, Y. (1987) J. Biol. Chem. 262, 14100-14104). The proline carrier was purified from this site specifically cleavable fusion protein. Cytoplasmic membranes overproducing the fusion protein were solubilized with dodecylmaltoside, and the solubilized fraction was subjected to anti-beta-galactosidase IgG-Sepharose chromatography. The fusion protein was specifically adsorbed to the immunoaffinity resin and then treated with collagenase for splitting the proline carrier moiety of the fusion protein from the beta-galactosidase moiety. The collagenase used for the collagenolysis was then removed by anti-collagenase IgG-Sepharose chromatography. In this way, the proline carrier was purified to more than 95% homogeneity of the protein. Proline transport in proteoliposomes reconstituted with the purified carrier was dependent on the membrane potential and the chemical gradient of Na+ across the membrane with apparent Michaelis constants for proline and for Na+ stimulation of 3.6 microM and 31 microM, respectively. These results indicated that the proline carrier mediates electrogenic Na+/proline symport.     

Mutations of putP that alter the lithium sensitivity of Salmonella typhimurium

The putP gene encodes the major proline permease in Salmonella typhimurium that couples transport of proline to the sodium electrochemical gradient. To identify residues involved in the cation binding site, we have isolated putP mutants that confer resistance to lithium during growth on proline. Wild-type S. typhimurium can grow well on proline as the sole carbon source in media supplemented with NaCl, but grows poorly when LiCl is substituted for NaCl. In contrast to the growth phenotype, proline permease is capable of transporting proline via Na+/proline or Li+/proline symport. Therefore, we selected mutants that grow well on media containing proline as the sole carbon source in the presence of lithium ions. All of the mutants assayed exhibit decreased rates of Li+/proline and Na+/proline cotransport relative to wild type. The location of each mutation was determined by deletion mapping: the mutations cluster in two small deletion intervals at the 5' and 3' termini of the putP gene. The map positions of these lithium resistance mutations are different from the locations of the previously isolated substrate specificity mutations. These results suggest that Lir mutations may define domains of the protein that fold to form the cation binding site of proline permease.     

Two proline porters in Escherichia coli K-12

Escherichia coli mutants defective at putP and putA lack proline transport via proline porter I and proline dehydrogenase activity, respectively. They retain a proline uptake system (proline porter II) that is induced during tryptophan-limited growth and are sensitive to the toxic L-proline analog, 3,4-dehydroproline. 3,4-Dehydroproline-resistant mutants derived from a putP putA mutant lack proline porter II. Auxotrophic derivatives derived from putP+ or putP bacteria can grow if provided with proline at low concentration (25 microM); those derived from the 3,4-dehydroproline-resistant mutants require high proline for growth (2.5 mM). We conclude that E. coli, like Salmonella typhimurium, possesses a second proline porter that is inactivated by mutations at the proP locus.     

Identification and conformational changes of the intestinal proline carrier

Fluorescein isothiocyanate (FITC) was used to selectively label the rabbit intestinal brush-border imino carrier, identify the binding protein on SDS-polyacrylamide gel electrophoresis, and monitor the effect of ions on fluorescein quenching. FITC inhibits Na+-dependent L-proline transport irreversibly, but transport is protected by physiological concentrations of Na+ and L-proline. About 1 nmol of FITC/mg of protein binds specifically to the transporter, which was identified by SDS-polyacrylamide gel electrophoresis as a 100 +/- 5-kDa peptide. Na+ produced a specific, saturable quench in the fluorescence of FITC bound to the proline carrier. Both transport and FITC quenching are inhibited by n-acetylimidazole, and membranes are protected from acetylation by Na+. We conclude that Na+ binds to the proline carrier (100-kDa peptide) to produce a change in conformation that results in an increase in the affinity of the carrier for proline.     

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.     

The γ-Aminobutyrate Permease GabP Serves as the Third Proline Transporter of Bacillus subtilis

PutP and OpuE serve as proline transporters when this imino acid is used by Bacillus subtilis as a nutrient or as an osmostress protectant, respectively. The simultaneous inactivation of the PutP and OpuE systems still allows the utilization of proline as a nutrient. This growth phenotype pointed to the presence of a third proline transport system in B. subtilis. We took advantage of the sensitivity of a putP opuE double mutant to the toxic proline analog 3,4-dehydro-dl-proline (DHP) to identify this additional proline uptake system. DHP-resistant mutants were selected and found to be defective in the use of proline as a nutrient. Whole-genome resequencing of one of these strains provided the lead that the inactivation of the γ-aminobutyrate (GABA) transporter GabP was responsible for these phenotypes. DNA sequencing of the gabP gene in 14 additionally analyzed DHP-resistant strains confirmed this finding. Consistently, each of the DHP-resistant mutants was defective not only in the use of proline as a nutrient but also in the use of GABA as a nitrogen source. The same phenotype resulted from the targeted deletion of the gabP gene in a putP opuE mutant strain. Hence, the GabP carrier not only serves as an uptake system for GABA but also functions as the third proline transporter of B. subtilis. Uptake studies with radiolabeled GABA and proline confirmed this conclusion and provided information on the kinetic parameters of the GabP carrier for both of these substrates.     

Specificity of intestinal brush-border proline transport: cyanine dye studies

The ability of rabbit jejunal brush borders to transport inhibitors of the imino carrier was investigated in membrane vesicles by measuring their ability to depolarize the membrane potential. Membrane potentials were monitored using a voltage-sensitive cyanine dye. Piperidine and pyrrolidine carboxylic acids, which are potent inhibitors of Na+-dependent proline transport (Ki less than 0.5 mM) depolarize the potential in a Na+-dependent, saturable manner indicating transport. On the other hand, N-methylated amino acids, which are fair inhibitors (Ki 2-10 mM), do not depolarize the membrane to any significant extent, but they competitively inhibit the L-proline transport signal. This indicates that these analogs are nontransported inhibitors of the imino carrier. The poor inhibitors niacin and pipolinic acid (Ki greater than 60 mM) depolarize the membrane about twice as much as proline and with low Kf values. This suggests separate carriers for these substrates.     

Evidence for tyrosyl residues at the Na+ site on the intestinal Na+/glucose cotransporter

A tyrosine group has been identified at, or near, the Na+-binding site of the Na+/glucose and Na+/proline cotransporters of rabbit intestinal brush-borders. Three tyrosine group-specific reagents, n-acetylimidazole, tetranitromethane, and p-nitrobenzene sulfonyl fluoride, were used to evaluate the role of tyrosyl groups in Na+-dependent glucose transport, Na+-dependent phlorizin binding, and the Na+-induced fluorescence quenching of fluorescein isothiocyanate bound to the glucose site of the carrier. All three reagents inhibited glucose transport, phlorizin binding, and fluorescein isothiocyanate quenching by 50-85% with Ki values in the range 7-50 microM. The presence of Na+ during the exposure of membranes to the reagents completely protected against inhibition, the Na+ concentration required to produce 50% protection was 14-36 mM. Fluorescent derivatives of n-acetylimidazole were synthesized to identify the tyrosyl residues on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A total of five polypeptide bands were labeled with eosin or fluorescein n-acetylimidazole in a Na+-sensitive manner. Two of these bands, previously identified as the glucose (75,000-dalton) and proline (100,000-dalton) binding sites of the glucose and proline carriers, account for 50% of the Na+-sensitive tyrosyl residues. On the basis of these studies, we believe that the Na+/glucose cotransporter contains both the Na+ and glucose active sites on the same polypeptide or that the cotransporter consists of two similar polypeptides, each containing one substrate binding site.     

Sodium gradient dependence of proline and glycine uptake in rat renal brush-border membrane vesicles.

The sodium-dependent entry of proline and glycine into rat renal brush-border membrane vesicles was examined. The high Km system for proline shows no sodium dependence. The low Km system for glycine entry is strictly dependent on a Na+ gradient but shows no evidence of the carrier system having any affinity for Na+. The low Km system for proline and high Km system for glycine transport appear to be shared. Both systems are stimulated by a Na+ gradient and appear to have an affinity for the Na+. The effect of decreasing the Na+ concentration in the ionic gradient is to alter the Km for amino acid entry and, at low Na+ concentrations, to inhibit the V for glycine entry.     

L-threonine transport in pig jejunal brush border membrane vesicles. Functional characterization of the unique system B in the intestinal epithelium.

Uptake and inhibitory kinetics of [3H]L-threonine were evaluated in preparations of pig jejunal brush border membrane vesicles. Uptake of [3H]L-threonine under O-trans, Na+ gradient, and O-trans, Na(+)-free conditions was best described by high affinity transport (Km < 0.01 mM) plus a nonsaturable component. The maximal velocity of transport was 3-fold greater under Na+ gradient conditions. 100 mM concentrations of all of the dipolar amino acids and 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid caused complete inhibition of [3H]L-threonine transport under Na+ gradient and Na(+)-free conditions. Imino acids, anionic amino acids, cationic amino acids, and methylamino-isobutyric acid caused significant partial inhibition of L-threonine uptake. Inhibitor concentration profiles for proline and lysine were consistent with low affinity competitive inhibition. The Ki values of alanine and phenylalanine approximated 0.2 and 0.5 mM, respectively, under both Na+ gradient and Na(+)-free conditions. These data indicate that the transport system available for L-threonine in the intestinal brush border membrane (system B) is functionally distinct from other amino acid transport systems. Comparison of kinetics parameters in the presence and absence of a Na+ gradient suggests that both partially and fully loaded forms of the carrier can function to translocate substrate and that Na+ serves to accelerate L-threonine transport by a mechanism that does not involve enhanced substrate binding.     

Role of Asp187 and Gln190 in the Na+/proline symporter (PutP) of Escherichia coli

Asp187 and Gln190 were predicted as conserved and closely located at the Na(+) binding site in a topology and homology model structure of Na(+)/proline symporter (PutP) of Escherichia coli. The replacement of Asp187 with Ala or Leu did not affect proline transport activity; whereas, change to Gln abolished the active transport. The binding affinity for Na(+) or proline of these mutants was similar to that of wild-type (WT) PutP. This result indicates Asp187 to be responsible for active transport of proline without affecting the binding. Replacement of Gln190 with Ala, Asn, Asp, Leu and Glu had no effect on transport or binding, suggesting that it may not have a role in the transport. However, in the negative D187Q mutant, a second mutation, of Gln190 to Glu or Leu, restored 46 or 7% of the transport activity of WT, respectively, while mutation to Ala, Asn or Asp had no effect. Thus, side chain at position 190 has a crucial role in suppressing the functional defect of the D187Q mutant. We conclude that Asp187 is responsible for transport activity instead of coupling-ion binding by constituting the translocation pathway of the ion and Gln190 provides a suppressing mutation site to regain PutP functional activity.     

Difference in sodium requirement of branched chain amino acid carrier between Pseudomonas aeruginosa PAO and PML strains is due to substitution of an amino acid at position 292

Sodium dependence of leucine transport, a measure of the Na+-coupled leucine-isoleucine-valine II (LIV-II) transport system in Pseudomonas aeruginosa, was compared between two wild-type strains, PAO and PML. The leucine transport activity was saturated at 0.1 mM NaCl for PAO and at 5.0 mM for PML. From kinetics experiments, the apparent Km value for Na+ with respect to leucine transport was estimated to be 3 microM for PAO and 95 microM for PML. The Km value for leucine was 6 microM for PAO and 13 microM for PML. The LIV-II carrier gene (braB) of PML was isolated for comparison of its amino acid sequence with that of the PAO carrier cloned previously. The Km values for Na+ and leucine of the cloned LIV-II carriers of PAO and PML expressed in LIV-II defective mutants were similar to those in wild-type strains. Determination of the nucleotide and deduced amino acid sequences of the LIV-II carrier gene of PML showed an amino acid difference at position 292 between the PAO and PML carriers. The amino acid was threonine for PAO and alanine for PML. These results indicate that the substitution of the amino acid at position 292 of the LIV-II carrier causes a difference in the sodium requirement of the carriers of the PAO and PML strains.     

Sodium-Dependent Proline Uptake in the Rat Hippocampal Formation: Association with Ipsilateral-Commissural Projections of CA3 Pyramidal

Na+-dependent uptake of L-[3H]proline was measured in a crude synaptosomal preparation from the entire rat hippocampal formation or from isolated hippocampal regions. Among hippocampal regions, Na+-dependent proline uptake was significantly greater in areas CA1 and CA2-CA3-CA4 than in the fascia dentata, whereas there was no marked regional difference in the distribution of Na+-dependent gamma-[14C]aminobutyric acid ([14C]GABA) uptake. A bilateral kainic acid lesion, which destroyed most of the CA3 hippocampal pyramidal cells, reduced Na+-dependent proline uptake by an average of 41% in area CA1 and 52% in area CA2-CA3-CA4, without affecting the Na+-dependent uptake of GABA. In the fascia dentata, neither proline nor GABA uptake was significantly altered. Kinetic studies suggested that hippocampal synaptosomes take up proline by both a high-affinity (KT = 6.7 microM) and a low-affinity (KT = 290 microM) Na+-dependent process, whereas L-[14C]glutamate is taken up predominantly by a high-affinity (KT = 6.1 microM) process. A bilateral kainic acid lesion reduced the Vmax of high-affinity proline uptake by an average of 72%, the Vmax of low-affinity proline uptake by 44%, and the Vmax of high affinity glutamate uptake by 43%, without significantly changing the affinity of the transport carriers for substrate. Ipsilateral-commissural projections of CA3 hippocampal pyramidal cells appear to possess nearly as great a capacity for taking up proline as for taking up glutamate, a probable transmitter of these pathways. Therefore proline may play an important role in transmission at synapses made by the CA3-derived Schaffer collateral, commissural, and ipsilateral associational fibers.     

Kinetics of the intestinal brush border proline (Imino) carrier

The kinetics of L-proline transport across intestinal brush borders via the Imino carrier were studied using membrane vesicles. The Imino carrier is defined as the agent responsible for L-alanine insensitive. Na+-dependent uptake of L-proline. Initial rate measurements were made under voltage clamped conditions (pD = 0) to investigate L-proline transport as a function of cis and trans Na+ and proline concentrations. Under zero-trans conditions, increasing cis Na+ activated proline uptake with a Hill coefficient of 1.7 and decreased the apparent Kt with no change in Jimax. The Jimax was approximately 60 pmol mg-1 s-1 and the apparent Kt ranged from 0.25 mM at cis Na = 100 to 1.0 mM at cis Na+ = 30 mM. Trans Na inhibited proline uptake via a reduction in Jimax. Trans proline had no significant effect in the absence of trans Na+, but it relieved the trans Na+ inhibition. Under equilibrium exchange conditions, the Jimax was twice that observed under zero-trans conditions. These kinetics of L-proline transport suggest a model in which uptake occurs by a rapid equilibrium iso-ordered ter ter system. Two Na+ ions bind first to the carrier on the cis face of the membrane to increase the affinity of the carrier for proline. The fully loaded complex then isomerizes to release the substrates to the trans side. The partially loaded Na+-only forms are unable to translocate across the membrane. A rate-limiting step appears to be the isomerization of unloaded carrier from the trans to the cis side of the membrane.