Identification, purification, and reconstitution of OxlT, the oxalate: formate antiport protein of Oxalobacter formigenes

Purification of UhpT, the sugar phosphate transporter of Escherichia coli

To purify UhpT, the sugar phosphate carrier of Escherichia coli, we constructed a variant (HisUhpT) in which 10 tandem histidine residues were placed at the UhpT N terminus and then used Ni(2+)-agarose affinity chromatography of detergent-solubilized proteins. Membrane vesicles from a strain overexpressing His-UhpT were extracted at pH 7.4 with either 1.5% n-octyl-beta-D-glucopyranoside (octylglucoside) or 1.5% n-dodecyl-beta-D-maltoside (dodecylmaltoside) in 200 mM sodium chloride, 100 mM potassium phosphate, 50 mM glucose 6-phosphate, 10-20% glycerol, 0.2% E. coli phospholipid, and 5 mM beta-mercaptoethanol. After the detergent extract was applied to a Ni(2+)-agarose column, nonspecifically bound material was removed by washing at pH 7 with the same buffer also containing 50 mM imidazole. Purified HisUhpT was released subsequently, when sodium chloride was replaced with 300 mM imidazole or 100 mM EDTA, giving an overall yield of about 25 micrograms HisUhpT/mg vesicle protein. Whether eluted by imidazole or EDTA in either octylglucoside or dodecylmaltoside, purified HisUhpT showed a specific activity of 2.5-3 mumol/min per milligram of protein as monitored by [14C]glucose 6-phosphate transport by proteoliposomes loaded with 100 mM potassium phosphate. This corresponded to a calculated turnover number near 20 s-1 for the heterologous exchange of external sugar phosphate with internal phosphate. At low temperature (4 degrees C) HisUhpT retained full activity in either octylglucoside or dodecylmaltoside; however, at elevated temperature (> or = 23 degrees C), the protein displayed a marked lability in octylglucoside (t1/2 = 11 min), but not in dodecylmaltoside (t1/2 > or = 200-300 min).

Exchange of glutamate and gamma-aminobutyrate in a Lactobacillus strain

Lactobacillus sp. strain E1 catalyzed the decarboxylation of glutamate (Glu), resulting in a nearly stoichiometric release of the products gamma-aminobutyrate (GABA) and CO2. This decarboxylation was associated with the net synthesis of ATP. ATP synthesis was inhibited almost completely by nigericin and about 70% by N,N'-dicyclohexylcarbodiimide (DCCD), without inhibition of the decarboxylation. These findings are consistent with the possibility that a proton motive force arises from the cytoplasmic proton consumption that accompanies glutamate decarboxylation and the electrogenic Glu/GABA antiporter and the possibility that this proton motive force is coupled with ATP synthesis by DCCD-sensitive ATPase.

DNA sequencing and expression of the formyl coenzyme A transferase gene, frc, from Oxalobacter formigenes

Oxalic acid, a highly toxic by-product of metabolism, is catabolized by a limited number of bacterial species utilizing an activation-decarboxylation reaction which yields formate and CO2. frc, the gene encoding formyl coenzyme A transferase, an enzyme which transfers a coenzyme A moiety to activate oxalic acid, was cloned from the bacterium Oxalobacter formigenes. DNA sequencing revealed a single open reading frame of 1,284 bp capable of encoding a 428-amino-acid protein. A presumed promoter region and a rho-independent termination sequence suggest that this gene is part of a monocistronic operon. A PCR fragment containing the open reading frame, when overexpressed in Escherichia coli, produced a product exhibiting enzymatic activity similar to the purified native enzyme. With this, the two genes necessary for bacterial catabolism of oxalate, frc and oxc, have now been cloned, sequenced, and expressed.

Evaluation of secondary structure of OxlT, the oxalate transporter of Oxalobacter formigenes, by circular dichroism spectroscopy

OxlT, the oxalate/formate exchange transporter of Oxalobacter formigenes, was purified as a histidine-tagged variant, OxlTHis, using Ni2+-linked affinity chromatography. OxlTHis was readily obtained in high purity (>/=95%) and reasonable yield (>/=60%), and showed kinetic and biochemical features characteristic of its parent, OxlT, including an unusually high maximal velocity (60 micromol/min per mg of protein at 4 degrees C). Circular dichroism spectroscopy of purified OxlTHis identified the alpha-helix as its dominant secondary structural unit, encompassing 60-70% of OxlTHis residues and consistent with a model suggesting 60% of OxlT (OxlTHis) residues are involved in the construction of 12 transmembrane alpha-helices (Abe, K., Ruan, Z.-S., and Maloney, P. C. (1996) J. Biol. Chem. 271, 6789-6793). In either octyl glucoside/lipid or dodecylmaltoside/lipid micelles, solubilized OxlTHis showed a striking substrate-induced stabilization of function, and at saturating levels of substrate (1000 x KD) activity recoverable by reconstitution disappeared with a half-life of 7 days at 23 degrees C. Measurement of changes of ellipticity at 222 nm as a function of time and substrate concentration showed that maintenance of function was attributable to a substrate-induced stabilization of the alpha-helical ensemble with a KD of 10 microM for the 1:1 binding of oxalate to OxlTHis.

Enzymatic and biochemical probes of residues external to the translocation pathway of UhpT, the sugar phosphate carrier of Escherichia coli

Part of the substrate translocation pathway through UhpT, the Escherichia coli sugar phosphate carrier, has been assigned to a transmembrane helix extending between residues 260 and 282. To set limits on the external portion of the pathway, we identified nearby residues fully exposed to the periplasm. In one case, we used Western blots to evaluate cleavage by extracellular trypsin. The protease cleaved UhpT variants retaining lysine 294, but not those lacking lysine 294, indicating that trypsin acts at a single extracellular site, lysine 294. In other work we labeled single-cysteine variants with 3-(N-maleimidylpropionyl)biocytin and scored accessibility to extracellular streptavidin by shifts of SDS-polyacrylamide gel electrophoresis mobility. Positions 283 and 284 were fully exposed to the periplasm, since the modified residue was bound by streptavidin in the native protein; by contrast, although the biotin-linked probe modified position 276, streptavidin decoration was not achieved without protein denaturation. We conclude that a 12-residue stretch(283-294) of UhpT is sufficiently exposed to be accessible to large probes (trypsin, streptavidin), while position 276 and more proximal residues are more deeply buried or otherwise shielded from the external phase.

Generation of a proton motive force by the anaerobic oxalate-degrading bacterium Oxalobacter formigenes

The generation of transmembrane ion gradients by Oxalobacter formigenes cells metabolizing oxalate was studied. The magnitudes of both the transmembrane electrical potential (delta psi) and the pH gradient (internal alkaline) decreased with increasing external pH; quantitatively, the delta psi was the most important component of the proton motive force. As the extracellular pH of metabolizing cells was increased, intracellular pH increased and remained alkaline relative to the external pH, indicating that O. formigenes possesses a limited capacity to regulate internal pH. The generation of a delta psi by concentrated suspensions of O. formigenes cells was inhibited by the K+ ionophore valinomycin and the protonophore carbonyl cyanide-m-chlorophenylhydrazone, but not by the Na+ ionophore monensin. The H+ ATPase inhibitor N,N'-dicyclohexyl-carbodiimide inhibited oxalate catabolism but did not dissipate the delta psi. The results support the concept that energy from oxalate metabolism by O. formigenes is conserved not as a sodium ion gradient but rather, at least partially, as a transmembrane hydrogen ion gradient produced during the electrogenic exchange of substrate (oxalate) and product (formate) and from internal proton consumption during oxalate decarboxylation.

Cloning, sequencing, and expression in escherichia coli of OxlT, the oxalate:formate exchange protein of Oxalobacter formigenes

OxlT is the oxalate/formate exchange protein that represents the vectorial component of a proton-motive metabolic cycle in Oxalobacter formigenes. Here we report the cloning and sequencing of OxlT and describe its expression in Escherichia coli. The OxlT amino acid sequence specifies a polytopic hydrophobic protein of 418 residues with a mass of 44,128 daltons. Analysis of hydropathy and consideration of the distribution of charged residues suggests an OxlT secondary structure having 12 transmembrane segments, oriented so that the N and C termini face the cytoplasm. Expression of OxlT in E. coli coincides with appearance of a capacity to carry out the self-exchange of oxalate and the heterologous, electrogenic exchange of oxalate with formate. The unusually high velocity of OxlT-mediated transport is also preserved in E. coli. We conclude that the essential features of OxlT are retained on its expression in E. coli.

Exchange of aspartate and alanine. Mechanism for development of a proton-motive force in bacteria

We examined the idea that aspartate metabolism by Lactobacillus subsp. M3 is organized as a proton-motive metabolic cycle by using reconstitution to monitor the activity of the carrier, termed AspT, expected to carry out the electrogenic exchange of precursor (aspartate) and product (alanine). Membranes of Lactobacillus subsp. M3 were extracted with 1.25% octyl glucoside in the presence of 0. 4% Escherichia coli phospholipid and 20% glycerol. The extracts were then used to prepare proteoliposomes loaded with either aspartate or alanine. Aspartate-loaded proteoliposomes accumulated external [3H]aspartate by exchange with internal substrate; this homologous self-exchange (Kt = 0.4 mm) was insensitive to potassium or proton ionophores and was unaffected by the presence or absence of Na+, K+, or Mg2+. Alanine-loaded proteoliposomes also took up [3H]aspartate in a heterologous antiport reaction that was stimulated or inhibited by an inside-positive or inside-negative membrane potential, respectively. Several lines of evidence suggest that these homologous and heterologous exchange reactions were catalyzed by the same functional unit. Thus, [3H]aspartate taken up by AspT during self-exchange was released by a delayed addition of alanine. In addition, the spontaneous loss of AspT activity that occurs when a detergent extract is held at 37 degrees C prior to reconstitution was prevented by the presence of either aspartate (KD(aspartate) = 0.3 mm) or alanine (KD(alanine) > or = 10 mm), indicating that both substrates interact directly with AspT. These findings are consistent with operation of a proton-motive metabolic cycle during aspartate metabolism by Lactobacillus subsp. M3.

The molecular and cell biology of anion transport by bacteria

This article summarizes the study of anion exchange mechanisms in bacteria. Along with defining at least two different families of anion exchange, an examination of such carrier-mediated antiport reactions has led to techniques that considerably broaden the scope of biochemical methods for examining membrane proteins. Such advances have been exploited to show that anion exchange itself forms the mechanistic base of an entirely new kind of proton pump, one which may shed light on a variety of bacterial events, including methanogenesis. Perhaps most important, the study of exchange provided the final link in a chain of evidence pointing to a structural 'rhythm' that seems to characterize membrane carriers. These three issues--a biochemical tool, a new proton pump, and a common structural rhythm--are briefly examined in the context of their origins in the analysis of bacterial anion exchange.

Purification and reconstitution of functional human P-glycoprotein

The overexpression of the P-glycoprotein, the MDR1 gene product, has been linked to the development of resistance to multiple cytotoxic natural product anticancer drugs in certain cancers and cell lines derived from tumors. P-glycoprotein, a member of the ATP-binding cassette (ABC) superfamily of transporters, is believed to function as an ATP-dependent drug efflux pump with broad specificity for chemically unrelated hydrophobic compounds. We review here recent studies on the purification and reconstitution of P-glycoprotein to elucidate the mechanism of drug transport. P-glycoprotein from the human carcinoma multidrug resistant cell line, KB-V1, was purified by sequential chromatography on anion exchange followed by a lectin (wheat germ agglutinin) column. Proteoliposomes reconstituted with pure protein exhibited high levels of drug-stimulated ATPase activity as well as ATP-dependent [3H]vinblastine accumulation. Both the ATPase and vinblastine transport activities of the reconstituted P-glycoprotein were inhibited by vanadate. In addition, the vinblastine transport was inhibited by verapamil and daunorubicin. These studies provide strong evidence that the human P-glycoprotein functions as an ATP-dependent drug transporter. The development of the reconstitution system and the availability of recombinant protein in large amounts due to recent advances in overexpression of P-glycoprotein in a heterologous expression system should facilitate a better understanding of the function of this novel protein.

Solute transport and energy transduction in bacteria

In bacteria two forms of metabolic energy are usually present, i.e. ATP and transmembrane ion-gradients, that can be used to drive the various endergonic reactions associated with cellular growth. ATP can be formed directly in substrate level phosphorylation reactions whereas primary transport processes can generate the ion-gradients across the cytoplasmic membrane. The two forms of metabolic energy can be interconverted by the action of ion-translocating ATPases. For fermentative organisms it has long been thought that ion-gradients could only be generated at the expense of ATP hydrolysis by the F0F1-ATPase. In the present article, an overview is given of the various secondary transport processes that form ion-gradients at the expense of precursor (substrate) and/or end-product concentration gradients. The metabolic energy formed by these chemiosmotic circuits contributes to the 'energy status' of the bacterial cell which is particularly important for anaerobic/fermentative organisms.

Bacterial transporters

Recent experiments in bacterial systems have established an extended database of sequences broadly relevant to all membrane transporters, allowing serious study of evolutionary relationships. The database will be especially useful in integrating conclusions derived from work with proteins in the major facilitator superfamily, because this kinship includes both eukaryotic and prokaryotic model systems. Even among carriers not linked by evolution, clear hints of functional homology have been note. Advances are also evident in the structural analysis of membrane carriers. Site-directed mutagenesis in a bacterial antiporter has shown how the translocation pathway might be identified; this should complement recent progress in preparing two-dimensional crystals of the eukaryotic anion-exchange protein, band 3. Together, these studies could soon verify or reject the idea that the transport pathway lies at the interface between the amino-terminal and carboxy-terminal helical bundles found in the hydrophobic core of most carrier proteins. If verified, the argument might allow construction of informed three-dimensional models in the absence of crystallographic evidence.

Oxalate- and Glyoxylate-Dependent Growth and Acetogenesis by Clostridium thermoaceticum

The acetogenic bacterium Clostridium thermoaceticum ATCC 39073 grew at the expense of the two-carbon substrates oxalate and glyoxylate. Other two-carbon substrates (acetaldehyde, acetate, ethanol, ethylene glycol, glycolaldehyde, glycolate, and glyoxal) were not growth supportive. Growth increased linearly with increasing substrate concentrations up to 45 mM oxalate and glyoxylate, and supplemental CO 2 was not required for growth. Oxalate and glyoxylate yielded 4.9 and 9.4 g, respectively, of cell biomass (dry weight) per mol of substrate utilized. Acetate was the major reduced end product recovered from oxalate and glyoxylate cultures. 14 C labeling studies showed that oxalate was subject to decarboxylation, and product analysis indicated that oxalate was utilized by the following reaction: 4 - OOC-COO - + 5H 2 O → CH 3 COO - + 6HCO 3 - + OH - . Oxalate- and glyoxylate-dependent growth produced lower acetate concentrations per unit of cell biomass synthesized than did H 2 -, CO-, methanol-, formate-, O -methyl-, or glucose-dependent growth. Protein profiles of oxalate-grown cells were dissimilar from protein profiles of glyoxylate-, CO-, or formate-grown cells, suggesting induction of new proteins for the utilization of oxalate. C. thermoaceticum DSM 2955 and Clostridium thermoautotrophicum JW 701/3 also grew at the expense of oxalate and glyoxylate. However, oxalate and glyoxylate did not support the growth of C. thermoaceticum OMD (a nonautotrophic strain) or six other species of acetogenic bacteria tested.

Generation of a proton motive force by histidine decarboxylation and electrogenic histidine/histamine antiport in Lactobacillus buchneri

Lactobacillus buchneri ST2A vigorously decarboxylates histidine to the biogenic amine histamine, which is excreted into the medium. Cells grown in the presence of histidine generate both a transmembrane pH gradient, inside alkaline, and an electrical potential (delta psi), inside negative, upon addition of histidine. Studies of the mechanism of histidine uptake and histamine excretion in membrane vesicles and proteoliposomes devoid of cytosolic histidine decarboxylase activity demonstrate that histidine uptake, histamine efflux, and histidine/histamine exchange are electrogenic processes. Histidine/histamine exchange is much faster than the unidirectional fluxes of these substrates, is inhibited by an inside-negative delta psi and is stimulated by an inside positive delta psi. These data suggest that the generation of metabolic energy from histidine decarboxylation results from an electrogenic histidine/histamine exchange and indirect proton extrusion due to the combined action of the decarboxylase and carrier-mediated exchange. The abundance of amino acid decarboxylation reactions among bacteria suggests that this mechanism of metabolic energy generation and/or pH regulation is widespread.