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

Substrate-bound outward-open structure of a Na+-coupled sialic acid symporter reveals a new Na+ site

Many pathogenic bacteria utilise sialic acids as an energy source or use them as an external coating to evade immune detection. As such, bacteria that colonise sialylated environments deploy specific transporters to mediate import of scavenged sialic acids. Here, we report a substrate-bound 1.95 Å resolution structure and subsequent characterisation of SiaT, a sialic acid transporter from Proteus mirabilis. SiaT is a secondary active transporter of the sodium solute symporter (SSS) family, which use Na⁺ gradients to drive the uptake of extracellular substrates. SiaT adopts the LeuT-fold and is in an outward-open conformation in complex with the sialic acid N-acetylneuraminic acid and two Na⁺ ions. One Na⁺ binds to the conserved Na2 site, while the second Na⁺ binds to a new position, termed Na3, which is conserved in many SSS family members. Functional and molecular dynamics studies validate the substrate-binding site and demonstrate that both Na⁺ sites regulate N-acetylneuraminic acid transport.

Functional characterization of a Na+-dependent dicarboxylate transporter from Vibrio cholerae

VcINDY, a bacterial homolog of transporters implicated in lifespan in fruit flies and insulin resistance in mammals, is a high affinity, electrogenic, Na⁺-dependent dicarboxylate transporter.

The SLC13 transporter family, whose members play key physiological roles in the regulation of fatty acid synthesis, adiposity, insulin resistance, and other processes, catalyzes the transport of Krebs cycle intermediates and sulfate across the plasma membrane of mammalian cells. SLC13 transporters are part of the divalent anion:Na⁺ symporter (DASS) family that includes several well-characterized bacterial members. Despite sharing significant sequence similarity, the functional characteristics of DASS family members differ with regard to their substrate and coupling ion dependence. The publication of a high resolution structure of dimer VcINDY, a bacterial DASS family member, provides crucial structural insight into this transporter family. However, marrying this structural insight to the current functional understanding of this family also demands a comprehensive analysis of the transporter’s functional properties. To this end, we purified VcINDY, reconstituted it into liposomes, and determined its basic functional characteristics. Our data demonstrate that VcINDY is a high affinity, Na⁺-dependent transporter with a preference for C₄- and C₅-dicarboxylates. Transport of the model substrate, succinate, is highly pH dependent, consistent with VcINDY strongly preferring the substrate’s dianionic form. VcINDY transport is electrogenic with succinate coupled to the transport of three or more Na⁺ ions. In contrast to succinate, citrate, bound in the VcINDY crystal structure (in an inward-facing conformation), seems to interact only weakly with the transporter in vitro. These transport properties together provide a functional framework for future experimental and computational examinations of the VcINDY transport mechanism.

Proper fatty acid composition rather than an ionizable lipid amine is required for full transport function of lactose permease from Escherichia coli

Energy-dependent uphill transport but not energy-independent downhill transport by lactose permease (LacY) is impaired when expressed in Escherichia coli cells or reconstituted in liposomes lacking phosphatidylethanolamine (PE) and containing only anionic phospholipids. The absence of PE results in inversion of the N-terminal half and misfolding of periplasmic domain P7, which are required for uphill transport of substrates. Replacement of PE in vitro by lipids with no net charge (phosphatidylcholine (PC), monoglucosyl diacylglycerol (GlcDAG), or diglucosyl diacylglycerol (GlcGlcDAG)) supported wild type transmembrane topology of the N-terminal half of LacY. The restoration of uphill transport in vitro was dependent on LacY native topology and proper folding of P7. Support of uphill transport by net neutral lipids in vitro (PE > PC ≫ GlcDAG ≠ GlcGlcDAG provided that PE or PC contained one saturated fatty acid) paralleled the results observed previously in vivo (PE = PC > GlcDAG ≠ GlcGlcDAG). Therefore, a free amino group is not required for uphill transport as previously concluded based on the lack of in vitro uphill transport when fully unsaturated PC replaced E. coli-derived PE. A close correlation was observed in vivo and in vitro between the ability of LacY to carry out uphill transport, the native conformation of P7, and the lipid headgroup and fatty acid composition. Therefore, the headgroup and the fatty acid composition of lipids are important for defining LacY topological organization and catalytically important structural features, further illustrating the direct role of lipids, independent of other cellular factors, in defining membrane protein structure/function.

Structural basis of the transcriptional regulation of the proline utilization regulon by multifunctional PutA

The multifunctional Escherichia coli proline utilization A (PutA) flavoprotein functions both as a membrane-associated proline catabolic enzyme and as a transcriptional repressor of the proline utilization genes putA and putP. To better understand the mechanism of transcriptional regulation by PutA, we have mapped the put-regulatory region, determined a crystal structure of the PutA ribbon-helix-helix domain (PutA52, a polypeptide corresponding to residues 1-52 of E. coli PutA) complexed with DNA, and examined the thermodynamics of DNA binding to PutA52. Five operator sites, each containing the sequence motif 5'-GTTGCA-3', were identified using gel-shift analysis. Three of the sites are shown to be critical for repression of putA, whereas the two other sites are important for repression of putP. The 2.25-A-resolution crystal structure of PutA52 bound to one of the operators (operator 2; 21 bp) shows that the protein contacts a 9-bp fragment corresponding to the GTTGCA consensus motif plus three flanking base pairs. Since the operator sequences differ in flanking bases, the structure implies that PutA may have different affinities for the five operators. This hypothesis was explored using isothermal titration calorimetry. The binding of PutA52 to operator 2 is exothermic, with an enthalpy of -1.8 kcal/mol and a dissociation constant of 210 nM. Substitution of the flanking bases of operator 4 into operator 2 results in an unfavorable enthalpy of 0.2 kcal/mol and a 15-fold-lower affinity, showing that base pairs outside of the consensus motif impact binding. Structural and thermodynamic data suggest that hydrogen bonds between Lys9 and bases adjacent to the GTTGCA motif contribute to transcriptional regulation by fine-tuning the affinity of PutA for put control operators.

Direct linking of metabolism and gene expression in the proline utilization A protein from Escherichia coli

The control of gene expression by enzymes provides a direct pathway for cells to respond to fluctuations in metabolites and nutrients. One example is the proline utilization A (PutA) protein from Escherichia coli. PutA is a membrane-associated enzyme that catalyzes the oxidation of L: -proline to glutamate using a flavin containing proline dehydrogenase domain and a NAD(+) dependent Delta(1)-pyrroline-5-carboxylate dehydrogenase domain. In some Gram-negative bacteria such as E. coli, PutA is also endowed with a ribbon-helix-helix DNA-binding domain and acts as a transcriptional repressor of the proline utilization genes. PutA switches between transcriptional repressor and enzymatic functions in response to proline availability. Molecular insights into the redox-based mechanism of PutA functional switching from recent studies are reviewed. In addition, new results from cell-based transcription assays are presented which correlate PutA membrane localization with put gene expression levels. General membrane localization of PutA, however, is not sufficient to activate the put genes.

Structure and kinetics of monofunctional proline dehydrogenase from Thermus thermophilus

Proline dehydrogenase (PRODH) and Delta(1)-pyrroline-5-carboxylate dehydrogenase (P5CDH) catalyze the two-step oxidation of proline to glutamate. They are distinct monofunctional enzymes in all eukaryotes and some bacteria but are fused into bifunctional enzymes known as proline utilization A (PutA) in other bacteria. Here we report the first structure and biochemical data for a monofunctional PRODH. The 2.0-A resolution structure of Thermus thermophilus PRODH reveals a distorted (betaalpha)(8) barrel catalytic core domain and a hydrophobic alpha-helical domain located above the carboxyl-terminal ends of the strands of the barrel. Although the catalytic core is similar to that of the PutA PRODH domain, the FAD conformation of T. thermophilus PRODH is remarkably different and likely reflects unique requirements for membrane association and communication with P5CDH. Also, the FAD of T. thermophilus PRODH is highly solvent-exposed compared with PutA due to a 4-A shift of helix 8. Structure-based sequence analysis of the PutA/PRODH family led us to identify nine conserved motifs involved in cofactor and substrate recognition. Biochemical studies show that the midpoint potential of the FAD is -75 mV and the kinetic parameters for proline are K(m) = 27 mm and k(cat) = 13 s(-1). 3,4-Dehydro-l-proline was found to be an efficient substrate, and l-tetrahydro-2-furoic acid is a competitive inhibitor (K(I) = 1.0 mm). Finally, we demonstrate that T. thermophilus PRODH reacts with O(2) producing superoxide. This is significant because superoxide production underlies the role of human PRODH in p53-mediated apoptosis, implying commonalities between eukaryotic and bacterial monofunctional PRODHs.

Functional reconstitution of SdcS, a Na+-coupled dicarboxylate carrier protein from Staphylococcus aureus

In Staphylococcus aureus, the transport of dicarboxylates is mediated in part by the Na+-linked carrier protein SdcS. This transporter is a member of the divalent-anion/Na+ symporter (DASS) family, a group that includes the mammalian Na+/dicarboxylate cotransporters NaDC1 and NaDC3. In earlier work, we cloned and expressed SdcS in Escherichia coli and found it to have transport properties similar to those of its eukaryotic counterparts (J. A. Hall and A. M. Pajor, J. Bacteriol. 187:5189-5194, 2005). Here, we report the partial purification and subsequent reconstitution of functional SdcS into liposomes. These proteoliposomes exhibited succinate counterflow activity, as well as Na+ electrochemical-gradient-driven transport. Examination of substrate specificity indicated that the minimal requirement necessary for transport was a four-carbon terminal dicarboxylate backbone and that productive substrate-transporter interaction was sensitive to substitutions at the substrate C-2 and C-3 positions. Further analysis established that SdcS facilitates an electroneutral symport reaction having a 2:1 cation/dicarboxylate ratio. This study represents the first characterization of a reconstituted Na+-coupled DASS family member, thus providing an effective method to evaluate functional, as well as structural, aspects of DASS transporters in a system free of the complexities and constraints associated with native membrane environments.

Functional characterization of a Na(+)-coupled dicarboxylate carrier protein from Staphylococcus aureus

We have cloned and functionally characterized a Na(+)-coupled dicarboxylate transporter, SdcS, from Staphylococcus aureus. This carrier protein is a member of the divalent anion/Na(+) symporter (DASS) family and shares significant sequence homology with the mammalian Na(+)/dicarboxylate cotransporters NaDC-1 and NaDC-3. Analysis of SdcS function indicates transport properties consistent with those of its eukaryotic counterparts. Thus, SdcS facilitates the transport of the dicarboxylates fumarate, malate, and succinate across the cytoplasmic membrane in a Na(+)-dependent manner. Furthermore, kinetic work predicts an ordered reaction sequence with Na(+) (K(0.5) of 2.7 mM) binding before dicarboxylate (K(m) of 4.5 microM). Because this transporter and its mammalian homologs are functionally similar, we suggest that SdcS may serve as a useful model for DASS family structural analysis.

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.

Phospholipids stabilize the secondary structure of the sodium-coupled branched-chain amino acid carrier of Pseudomonas aeruginosa

For functional reconstitution of bacterial cotransporters (carriers or permeases) including the sodium-coupled branched-chain amino acid carrier (LIV-II carrier) of Pseudomonas aeruginosa, the presence of phospholipid is required through the process of solubilization and purification of the transporters from the bacterial membranes, suggesting the possibility that phospholipid may stabilize the structure of the cotransporter proteins to be in a functional form. In this study, this possibility was examined by studying the effect of denaturant on the secondary structure of the LIV-II carrier purified in the absence and presence of phospholipid using circular dichroism (CD) spectroscopy. CD spectra of the purified LIV-II carrier solubilized in n-octyl-beta-D-glucopyranoside (OG), OG/dioleoylphosphatidylethanolamine (DOPE)/dioleoylphosphatidylglycerol (DOPG) mixture, and dispersed into DOPE/DOPG small unilamellar vesicles were measured in the absence of denaturant. The three spectra were very similar and had a trough at 222 nm with mean residue molar ellipticity of -23000 deg.cm(2)/dmol and a shoulder at 208 nm. CD spectral analyses with three different methods (S.W. Provencher, J. Glöckner, Estimation of globular protein secondary structure from circular dichroism, Biochemistry 20 (1981) 33-37; J.Y. Yang, C.-S.C. Wu, H.Z. Martinez, Calculation of protein conformation from circular dichroism, Methods Enzymol. 130 (1986) 208-269; N. Sreerama, R.W. Woody, A self-consistent method for the analysis of protein secondary structure from circular dichroism, Anal. Biochem. 209 (1993) 32-44) revealed that the LIV-II carrier solubilized in OG/DOPE/DOPG mixture contained 69-75% alpha-helix and 0-9% beta-sheet. Addition of 6 M guanidine hydrochloride decreased 48% of the amplitude at 222 nm of the CD spectrum of the carrier solubilized in OG alone and 9-14% of the CD amplitude of the carrier solubilized in OG/DOPE/DOPG or OG/dioleoylphosphatidylcholine mixture and dispersed in liposomes composed of DOPE/DOPG. These results show that the ordered secondary structure of the LIV-II carrier is partially unfolded in OG without phospholipid by denaturant but is greatly stabilized with phospholipids with oleoyl chains independently of their polar head group composition and suggest that the alpha-helical structure of the carrier is mainly embedded in the lipid environment.

Functional symmetry of UhpT, the sugar phosphate transporter of Escherichia coli

UhpT, the sugar phosphate transporter of Escherichia coli, acts to exchange internal inorganic phosphate for external hexose 6-phosphate. Because of this operational asymmetry, we studied variants in which right-side-out (RSO) or inside-out (ISO) orientations could be analyzed independently to ask whether the inward- and outward-facing UhpT surfaces have different substrate specificities. To study the RSO orientation, we constructed a histidine-tagged derivative, His10K291C/K294N, in which the sole external tryptic cleavage site (Lys294) had been removed. Functional assay as well as immunoblot analysis showed that trypsin treatment of proteoliposomes containing His10K291C/K294N led to loss of about 50% of the original population, reflecting retention of only the RSO population. To study the ISO orientation, we used a His10V284C derivative, in which a newly inserted external cysteine (Cys284) conferred sensitivity to the thiol-reactive agent, 3-(N-maleimidylpropionyl)biocytin. In this case, 3-(N-maleimidylpropionyl)biocytin treatment of proteoliposomes containing His10V284C gave about a 60% loss of activity, and immunodetection of biotin showed parallel modification of an equivalent fraction of the original population. Together, such findings indicate that the UhpT RSO and ISO orientations are in about equal proportion in proteoliposomes and that a single population can be generated by exposure of these derivatives to the appropriate agent. This allowed us to study proteoliposomes with UhpT functioning in RSO orientation (His10K291C/K294N) or ISO orientation (His10V284C) with respect to the kinetics of glucose 6-phosphate transport by phosphate-loaded proteoliposomes and also the inhibitions found with 2-deoxy-glucose 6-phosphate, mannose 6-phosphate, galactose 6-phosphate, fructose 6-phosphate, and inorganic phosphate. We found no significant differences in the behavior of UhpT in its different orientations, indicating that the transporter possesses an overall functional symmetry.

Rapid transmembrane movement of newly synthesized phosphatidylethanolamine across the inner membrane of Escherichia coli

For the first time the transmembrane movement of an endogenously synthesized phospholipid across the inner membrane of E. coli is reported. [14C]phosphatidylethanolamine (PE) was biosynthetically introduced into inner membrane vesicles from the PE-deficient strain AD93, by reconstitution with the enzyme phosphatidylserine (PS) synthetase. Upon addition of wild type cell lysate containing PS synthetase, and the metabolic substrates CTP and [14C]serine to inside-out vesicles from AD93, [14C]PS was synthesized, which was for the most part converted into [14C]PE. [14C]PE was introduced in right-side out vesicles by enclosing PS synthetase and CTP in the vesicle lumen and adding [14C]serine. The newly synthesized [14C]PE immediately equilibrated over both membrane leaflets (t1/2 less than one min), as determined by its accessibility toward the amino-reactive chemical fluorescamine. In both inside- out and right-side out vesicles, a 35-65% distribution was found of the newly synthesized PE over the cytoplasmic and periplasmic leaflet, respectively. The transport process of PE was not influenced by the presence of ATP or the proton motive force in inside out vesicles. Pretreatment of both types of vesicles with sulfhydryl reagents, or of right-side out vesicles with proteinase K, did not affect the rate and extent of the transmembrane distribution of the newly synthesized PE.

Topology and function of the Na+/proline transporter of Escherichia coli, a member of the Na+/solute cotransporter family

The Na+/proline transporter of Escherichia coli (PutP) is a member of the Na+/solute contransporter family (SCF) and catalyzes the uptake of proline by a Na+ dependent transport mechanism. Hydropathy profile analysis suggests that the protein consists of 12 transmembrane domains (TMs) that traverse the membrane in zigzag fashion connected by hydrophilic loops. However, analysis of a series of putP-phoA (PutP-alkaline phosphatase) and putP-lacZ (PutP-beta-galactosidase) fusions and site-directed labeling of the transporter indicate a 13-helix motif with the N-terminus on the outside and the C-terminus facing the cytoplasm. The findings are discussed with respect to a common topological motif for all members of the SCF. Furthermore, amino acid substitution analysis indicates that the N-terminal part of PutP is important for ion binding. Thus, Asp55 (putative TM II) is essential for transport and proposed to interact directly with Na+. The functional importance of TM II is further confirmed by the observation that replacement of Arg40, Ser50, Ala53, or Ser57 alters transport kinetics dramatically.

Molecular basis for membrane phospholipid diversity: why are there so many lipids?

Phospholipids play multiple roles in cells by establishing the permeability barrier for cells and cell organelles, by providing the matrix for the assembly and function of a wide variety of catalytic processes, by acting as donors in the synthesis of macromolecules, and by actively influencing the functional properties of membrane-associated processes. The function, at the molecular level, of phosphatidylethanolamine, phosphatidylglycerol, and cardiolipin in specific cellular processes is reviewed, with a focus on the results of combined molecular genetic and biochemical studies in Escherichia coli. These results are compared with primarily biochemical data supporting similar functions for these phospholipids in eukaryotic organisms. The wide range of processes in which specific involvement of phospholipids has been documented explains the need for diversity in phospholipid structure and why there are so many membrane lipids.

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.

In vivo and in vitro studies of the purine-cytosine permease of Saccharomyces cerevisiae. Functional analysis of a mutant with an altered apparent transport constant of uptake

The FCY2 gene of the purine-cytosine permease (PCP) of Saccharomyces cerevisiae and the allele fcy2-21 have been cloned on the yeast multicopy plasmid pJDB207. The corresponding plasmids were introduced into a S. cerevisiae strain carrying a chromosomal deletion at the FCY2 locus. The resulting strains were designated pAB4 and pAB25 respectively. The pAB25 strain, which carries the fcy2-21 allele, contains four amino acid changes in the open reading frame of the PCP (Weber et al., 1989). The influence of these mutations was studied on cells by determination of the uptake constants of purine bases and cytosine [apparent Michaelis constant of transport (Ktapp) and Vmax] and on plasma-membrane preparations, by measurements of binding parameters at equilibrium [(Kd and maximum amount of binding sites/Bmax)]. For strain pAB4, the Ktapp and Vmax of uptake were almost similar for all solutes considered [1.8-2.6 microM and 8.5-10.2 nmol.min-1.(10(7) cells)-1]. The main effect of the mutations in strain pAB25 was based on a large increase in Ktapp for all ligands except adenine. Plasma membranes of each strain displayed one class of specific binding sites. Variations in Kd of 0.4-1 microM were observed for pAB4. These slight variations had no effect on the Ktapp of uptake measured for the corresponding solutes. In contrast, using pAB25 membranes, Kd increased dramatically; 2.6 microM, 40 microM and 96 microM for adenine, cytosine and hypoxanthine, respectively. These increments were correlated to variations in Ktapp of the uptake for cytosine and hypoxanthine. Therefore, we conclude that modification in the Ktapp of uptake in the strain carrying fcy2-21 allele is merely due to a modification of the binding ability of the permease for its ligands.

Identification and initial topological analysis of the Rickettsia prowazekii ATP/ADP translocase

The Rickettsia prowazekii ATP/ADP translocase was identified by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE) and immunoblot analysis using antibodies raised against a synthetic peptide corresponding in sequence to the carboxyl-terminal 17 amino acids of the carrier. Both the translocase of R. prowazekii and that expressed by Escherichia coli transformants containing the rickettsial gene had an apparent molecular mass of 36,500 Da by SDS-PAGE analysis, a mass considerably less than that deduced from the sequence of the gene. The SDS-solubilized translocase aggregated upon heating at 100 degrees C in the presence of disulfide bond-reducing agents. Similar concentrations of disulfide bond-reducing agents inhibited the exchange transport of adenine nucleotides by both R. prowazekii and translocase-expressing E. coli. These data suggested that an intramolecular disulfide bond in the translocase was essential for transport activity. The antipeptide antibodies used for identification of the translocase bound preferentially to inside-out membrane vesicles of translocase-expressing E. coli relative to right-side-out spheroplasts, thus indicating that the carboxyl terminus of the carrier is located on the cytoplasmic side of the bacterial inner membrane. Protease studies were unable to localize the carboxyl terminus because of the resistance of this region of the native translocase to proteolytic cleavage. These data in conjunction with hydrophobicity analysis were used to construct an initial topological model of the translocase within the cell membrane.

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.

Molecular cloning of gltS and gltP, which encode glutamate carriers of Escherichia coli B

Two genes encoding distinct glutamate carrier proteins of Escherichia coli B were cloned into an E. coli K-12 strain by using a cosmid vector, pHC79. One of them was the gltS gene coding for a glutamate carrier of an Na+-dependent, binding protein-independent, and glutamate-specific transport system. The content of the glutamate carrier was amplified about 25-fold in the cytoplasmic membranes from a gltS-amplified strain. The gltS gene was located in a 3.2-kilobase EcoRI-MluI fragment, and the gene product was identified as a membrane protein with an apparent Mr of 35,000 in a minicell system. A gene designated gltP was also cloned. The transport activity of the gltP system in cytoplasmic membrane vesicles from a gltP-amplified strain was driven by respiratory substrates and was independent of the concentrations of Na+, K+, and Li+. An uncoupler, carbonylcyanide m-chlorophenylhydrazone, completely inhibited the transport activities of both systems, whereas an ionophore, monensin, inhibited only that of the gltS system. The Kt value for glutamate was 11 microM in the gltP system and 3.5 microM in the gltS system. L-Aspartate inhibited the glutamate transport of the gltP system but not that of the gltS system. Aspartate was taken up actively by membrane vesicles from the gltP-amplified strain, although no aspartate uptake activity was detected in membrane vesicles from a wild-type E. coli strain. These results suggest that gltP is a structural gene for a carrier protein of an Na+-independent, binding protein-independent glutamate-aspartate transport system.

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.

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.

Solubilization and reconstitution of proline carrier in Escherichia coli; quantitative analysis and optimal conditions

Proline carrier of Escherichia coli was extracted from the carrier-overproducing membranes with dodecylmaltoside in the presence of phospholipid. The solubilized carrier showed the same proline binding activity as that in normal membranes. As judged from determinations of the binding activity in the micellar state as a marker of active carrier and the radioactivity of N-[ethyl-2-3H]ethylmaleimide-labeled carrier as a marker of carrier polypeptide, 80% of the carrier molecules in the membranes were extracted. Optimal conditions for reconstitution of the solubilized carrier were established. By a combination of freeze-thawing, sonication and dilution procedures, 70% of the solubilized carrier molecules were incorporated into proteoliposomes and the restored active transport of proline showed an apparent Kt of 1 microM and turnover number of 0.6 s-1. The transport of proline was driven by a membrane potential in a Na+ (or Li+)-dependent manner.