Mechanism and energetics of the secondary phosphate transport system of Acinetobacter johnsonii 210A

The impact of pH on the anaerobic and aerobic metabolism of Tetrasphaera-enriched polyphosphate accumulating organisms

Members of the genus Tetrasphaera are putative polyphosphate accumulating organisms (PAOs) that have been found in greater abundance than Accumulibacter in many full-scale enhanced biological phosphorus removal (EBPR) wastewater treatment plants worldwide. Nevertheless, previous studies on the effect of environmental conditions, such as pH, on the performance of EBPR have focused mainly on the response of Accumulibacter to pH changes. This study examines the impact of pH on a Tetrasphaera PAO enriched culture, over a pH range from 6.0 to 8.0 under both anaerobic and aerobic conditions, to assess its impact on the stoichiometry and kinetics of Tetrasphaera metabolism. It was discovered that the rates of phosphorus (P) uptake and P release increased with an increase of pH within the tested range, while PHA production, glycogen consumption and substrate uptake rate were less sensitive to pH changes. The results suggest that Tetrasphaera PAOs display kinetic advantages at high pH levels, which is consistent with what has been observed previously for Accumulibacter PAOs. The results of this study show that pH has a substantial impact on the P release and uptake kinetics of PAOs, where the P release rate was >3 times higher and the P uptake rate was >2 times higher at pH 8.0 vs pH 6.0, respectively. Process operational strategies promoting both Tetrasphaera and Accumulibacter activity at high pH do not conflict with each other, but lead to a potentially synergistic impact that can benefit EBPR performance.

Characterization of a group of peptides for potential applications in hydrogen phosphate and heavy metals accumulation

Phosphorus and heavy metals are discarded to the domestic sewage in our daily life, it is necessary to find easy methods for phosphorus and heavy metals accumulation. Here, a group of short peptides (ChBpHs) were found to react with hydrogen phosphate forming insoluble substances. ChBpHs are composed by a choline binding peptides (ChBp) and a C-terminal histidine rich tail. The reaction region to hydrogen phosphate was determined at 1-18th amino acid in ChBp. The affinities of ChBpHs are different, with minimum react concentrations of Na₂HPO₄ ranging from 2 to 12 mM. In addition, the C-terminal histidine tail enables ChBpHs with affinities to metal ions in vitro. Prokaryotic expression of ChBpH1 in Escherichia coli resulted in the reduction of soluble hydrogen phosphate in the culture medium. The accumulation of phosphate is time and concentration dependent, maximum reduction was detected at 24 h post induction (23% in phosphate rich medium and 14% in normal medium). The reduction of nickel ions (about 20%) was only detected after cells were broken. In conclusion, this preliminary investigation of ChBpHs indicates the potential applications for bioconcentration of soluble phosphate in the future.

Inorganic polyphosphate in cardiac myocytes: from bioenergetics to the permeability transition pore and cell survival

Inorganic polyphosphate (polyP) is a linear polymer of Pi residues linked together by high-energy phosphoanhydride bonds as in ATP. PolyP is present in all living organisms ranging from bacteria to human and possibly even predating life of this planet. The length of polyP chain can vary from just a few phosphates to several thousand phosphate units long, depending on the organism and the tissue in which it is synthesized. PolyP was extensively studied in prokaryotes and unicellular eukaryotes by Kulaev's group in the Russian Academy of Sciences and by the Nobel Prize Laureate Arthur Kornberg at Stanford University. Recently, we reported that mitochondria of cardiac ventricular myocytes contain significant amounts (280±60 pmol/mg of protein) of polyP with an average length of 25 Pi and that polyP is involved in Ca(2+)-dependent activation of the mitochondrial permeability transition pore (mPTP). Enzymatic polyP depletion prevented Ca(2+)-induced mPTP opening during ischaemia; however, it did not affect reactive oxygen species (ROS)-mediated mPTP opening during reperfusion and even enhanced cell death in cardiac myocytes. We found that ROS generation was actually enhanced in polyP-depleted cells demonstrating that polyP protects cardiac myocytes against enhanced ROS formation. Furthermore, polyP concentration was dynamically changed during activation of the mitochondrial respiratory chain and stress conditions such as ischaemia/reperfusion (I/R) and heart failure (HF) indicating that polyP is required for the normal heart metabolism. This review discusses the current literature on the roles of polyP in cardiovascular health and disease.

Role of β-hydroxybutyrate, its polymer poly-β-hydroxybutyrate and inorganic polyphosphate in mammalian health and disease

We provide a comprehensive review of the role of β-hydroxybutyrate (β-OHB), its linear polymer poly-β-hydroxybutyrate (PHB), and inorganic polyphosphate (polyP) in mammalian health and disease. β-OHB is a metabolic intermediate that constitutes 70% of ketone bodies produced during ketosis. Although ketosis has been generally considered as an unfavorable pathological state (e.g., diabetic ketoacidosis in type-1 diabetes mellitus), it has been suggested that induction of mild hyperketonemia may have certain therapeutic benefits. β-OHB is synthesized in the liver from acetyl-CoA by β-OHB dehydrogenase and can be used as alternative energy source. Elevated levels of PHB are associated with pathological states. In humans, short-chain, complexed PHB (cPHB) is found in a wide variety of tissues and in atherosclerotic plaques. Plasma cPHB concentrations correlate strongly with atherogenic lipid profiles, and PHB tissue levels are elevated in type-1 diabetic animals. However, little is known about mechanisms of PHB action especially in the heart. In contrast to β-OHB, PHB is a water-insoluble, amphiphilic polymer that has high intrinsic viscosity and salt-solvating properties. cPHB can form non-specific ion channels in planar lipid bilayers and liposomes. PHB can form complexes with polyP and Ca(2+) which increases membrane permeability. The biological roles played by polyP, a ubiquitous phosphate polymer with ATP-like bonds, have been most extensively studied in prokaryotes, however polyP has recently been linked to a variety of functions in mammalian cells, including blood coagulation, regulation of enzyme activity in cancer cells, cell proliferation, apoptosis and mitochondrial ion transport and energy metabolism. Recent evidence suggests that polyP is a potent activator of the mitochondrial permeability transition pore in cardiomyocytes and may represent a hitherto unrecognized key structural and functional component of the mitochondrial membrane system.

Inorganic polyphosphate is a potent activator of the mitochondrial permeability transition pore in cardiac myocytes

Mitochondrial dysfunction caused by excessive Ca²⁺ accumulation is a major contributor to cardiac cell and tissue damage during myocardial infarction and ischemia–reperfusion injury (IRI). At the molecular level, mitochondrial dysfunction is induced by Ca²⁺-dependent opening of the mitochondrial permeability transition pore (mPTP) in the inner mitochondrial membrane, which leads to the dissipation of mitochondrial membrane potential (ΔΨ_m), disruption of adenosine triphosphate production, and ultimately cell death. Although the role of Ca²⁺ for induction of mPTP opening is established, the exact molecular mechanism of this process is not understood. The aim of the present study was to test the hypothesis that the adverse effect of mitochondrial Ca²⁺ accumulation is mediated by its interaction with inorganic polyphosphate (polyP), a polymer of orthophosphates linked by phosphoanhydride bonds. We found that cardiac mitochondria contained significant amounts (280 ± 60 pmol/mg of protein) of short-chain polyP with an average length of 25 orthophosphates. To test the role of polyP for mPTP activity, we investigated kinetics of Ca²⁺ uptake and release, ΔΨ_m and Ca²⁺-induced mPTP opening in polyP-depleted mitochondria. polyP depletion was achieved by mitochondria-targeted expression of a polyP-hydrolyzing enzyme. Depletion of polyP in mitochondria of rabbit ventricular myocytes led to significant inhibition of mPTP opening without affecting mitochondrial Ca²⁺ concentration by itself. This effect was observed when mitochondrial Ca²⁺ uptake was stimulated by increasing cytosolic [Ca²⁺] in permeabilized myocytes mimicking mitochondrial Ca²⁺ overload observed during IRI. Our findings suggest that inorganic polyP is a previously unrecognized major activator of mPTP. We propose that the adverse effect of polyphosphate might be caused by its ability to form stable complexes with Ca²⁺ and directly contribute to inner mitochondrial membrane permeabilization.

Research advances and challenges in the microbiology of enhanced biological phosphorus removal--a critical review

Enhanced biological phosphorus removal (EBPR) is a well-established technology for removing phosphorus from wastewater. However, the process remains operationally unstable in many systems, primarily because there is a lack of understanding regarding the microbiology of EBPR. This paper presents a review of advances made in the study of EBPR microbiology and focuses on the identification, enrichment, classification, morphology, and metabolic capacity of polyphosphate- and glycogen-accumulating organisms. The paper also highlights knowledge gaps and research challenges in the field of EBPR microbiology. Based on the review, the following recommendations regarding the future direction of EBPR microbial research were developed: (1) shifting from a reductionist approach to a more holistic system-based approach, (2) using a combination of culture-dependent and culture-independent techniques in characterizing microbial composition, (3) integrating ecological principles into system design to enhance stability, and (4) reexamining current theoretical explanations of why and how EBPR occurs.

Inactivation of the Lactococcus lactis high-affinity phosphate transporter confers oxygen and thiol resistance and alters metal homeostasis

Numerous strategies allowing bacteria to detect and respond to oxidative conditions depend on the cell redox state. Here we examined the ability of Lactococcus lactis to survive aerobically in the presence of the reducing agent dithiothreitol (DTT), which would be expected to modify the cell redox state and disable the oxidative stress response. DTT inhibited L. lactis growth at 37 degrees C in aerobic conditions, but not in anaerobiosis. Mutants selected as DTT resistant all mapped to the pstFEDCBA locus, encoding a high-affinity phosphate transporter. Transcription of pstFEDCBA and a downstream putative regulator of stress response, phoU, was deregulated in a pstA strain, but amounts of major oxidative stress proteins were unchanged. As metals participate in oxygen radical formation, we compared metal sensitivity of wild-type and pstA strains. The pstA mutant showed approximately 100-fold increased resistance to copper and zinc. Furthermore, copper or zinc addition exacerbated the sensitivity of a wild-type L. lactis strain to DTT. Inactivation of pstA conferred a more general resistance to oxidative stress, alleviating the oxygen- and thermo-sensitivity of a clpP mutant. This study establishes a role for the pst locus in metal homeostasis, suggesting that pst inactivation lowers intracellular reactivity of copper and zinc, which would limit bacterial sensitivity to oxygen.

The Phn system of Mycobacterium smegmatis: a second high-affinity ABC-transporter for phosphate

Uptake of inorganic phosphate, an essential but often limiting nutrient, in bacteria is usually accomplished by the high-affinity ABC-transport system Pst. Pathogenic species of mycobacteria contain several copies of the genes encoding the Pst system (pstSCAB), and two of the encoded proteins, PstS1 and PstS2, have been shown to be virulence factors in Mycobacterium tuberculosis. The fast-growing Mycobacterium smegmatis contains only a single copy of the pst operon. This study reports the biochemical and molecular characterization of a second high-affinity phosphate transport system, designated Phn. The Phn system is encoded by a three-gene operon that constitutes the components of a putative ABC-type phosphonate/phosphate transport system. Expression studies using phnD- and pstS-lacZ transcriptional fusions showed that both operons were induced when the culture entered phosphate limitation, indicating a role for both systems in phosphate uptake at low extracellular concentrations. Deletion mutants in either phnD or pstS failed to grow in minimal medium with a 10 mM phosphate concentration, while the isogenic wild-type strain mc(2)155 grew at micromolar phosphate concentrations. Analysis of the kinetics of phosphate transport in the wild-type and mutant strains led to the proposal that the Phn and Pst systems are both high-affinity phosphate transporters with similar affinities for phosphate (i.e. apparent K(m) values between 40 and 90 muM P(i)). The Phn system of M. smegmatis appears to be unique in that, unlike previously identified Phn systems, it does not recognize phosphonates or phosphite as substrates.

Monoglucosyldiacylglycerol, a Foreign Lipid, Can Substitute for Phosphatidylethanolamine in Essential Membrane-associated Functions in Escherichia coli

The mechanisms by which lipid bilayer properties govern or influence membrane protein functions are little understood, but a liquid-crystalline state and the presence of anionic and nonbilayer (NB)-prone lipids seem important. An Escherichia coli mutant lacking the major membrane lipid phosphatidylethanolamine (NB-prone) requires divalent cations for viability and cell integrity and is impaired in several membrane functions that are corrected by introduction of the "foreign" NB-prone neutral glycolipid alpha-monoglucosyldiacylglycerol (MGlcDAG) synthesized by the MGlcDAG synthase from Acholeplasma laidlawii. Dependence on Mg(2+) was reduced, and cellular yields and division malfunction were greatly improved. The increased passive membrane permeability of the mutant was not abolished, but protein-mediated osmotic stress adaptation to salts and sucrose was recovered by the presence of MGlcDAG. MGlcDAG also restored tryptophan prototrophy and active transport function of lactose permease, both critically dependent on phosphatidylethanolamine. Three mechanisms can explain the observed effects: NB-prone MGlcDAG improves the quenched lateral pressure profile across the bilayer; neutral MGlcDAG dilutes the high anionic lipid surface charge; MGlcDAG provides a neutral lipid that can hydrogen bond and/or partially ionize. The reduced dependence on Mg(2+) and lack of correction by high monovalent salts strongly support the essential nature of the NB properties of MGlcDAG.

An ABC transporter with a secondary-active multidrug translocator domain

Multidrug resistance, by which cells become resistant to multiple unrelated pharmaceuticals, is due to the extrusion of drugs from the cell's interior by active transporters such as the human multidrug resistance P-glycoprotein. Two major classes of transporters mediate this extrusion. Primary-active transporters are dependent on ATP hydrolysis, whereas secondary-active transporters are driven by electrochemical ion gradients that exist across the plasma membrane. The ATP-binding cassette (ABC) transporter LmrA is a primary drug transporter in Lactococcus lactis that can functionally substitute for P-glycoprotein in lung fibroblast cells. Here we have engineered a truncated LmrA protein that lacks the ATP-binding domain. Surprisingly, this truncated protein mediates a proton-ethidium symport reaction without the requirement for ATP. In other words, it functions as a secondary-active multidrug uptake system. These findings suggest that the evolutionary precursor of LmrA was a secondary-active substrate translocator that acquired an ATP-binding domain to enable primary-active multidrug efflux in L. lactis.

The Saccharomyces cerevisiae high affinity phosphate transporter encoded by PHO84 also functions in manganese homeostasis

In the bakers' yeast Saccharomyces cerevisiae, high affinity manganese uptake and intracellular distribution involve two members of the Nramp family of genes, SMF1 and SMF2. In a search for other genes involved in manganese homeostasis, PHO84 was identified. The PHO84 gene encodes a high affinity inorganic phosphate transporter, and we find that its disruption results in a manganese-resistant phenotype. Resistance to zinc, cobalt, and copper ions was also demonstrated for pho84Delta yeast. When challenged with high concentrations of metals, pho84Delta yeast have reduced metal ion accumulation, suggesting that resistance is due to reduced uptake of metal ions. Pho84p accounted for virtually all the manganese accumulated under metal surplus conditions, demonstrating that this transporter is the major source of excess manganese accumulation. The manganese taken in via Pho84p is indeed biologically active and can not only cause toxicity but can also be incorporated into manganese-requiring enzymes. Pho84p is essential for activating manganese enzymes in smf2Delta mutants that rely on low affinity manganese transport systems. A role for Pho84p in manganese accumulation was also identified in a standard laboratory growth medium when high affinity manganese uptake is active. Under these conditions, cells lacking both Pho84p and the high affinity Smf1p transporter accumulated low levels of manganese, although there was no major effect on activity of manganese-requiring enzymes. We conclude that Pho84p plays a role in manganese homeostasis predominantly under manganese surplus conditions and appears to be functioning as a low affinity metal transporter.

(31)P NMR of apicomplexans and the effects of risedronate on Cryptosporidium parvum growth

High-resolution 303.6 MHz (31)P NMR spectra have been obtained of perchloric acid extracts of Plasmodium berghei trophozoites, Toxoplasma gondii tachyzoites, and Cryptosporidium parvum oocysts. Essentially complete resonance assignments have been made based on chemical shifts and by coaddition of authentic reference compounds. Signals corresponding to inorganic pyrophosphate were detected in all three species. In T. gondii and C. parvum, additional resonances were observed corresponding to linear triphosphate as well as longer chain polyphosphates. Spectra of P. berghei and T. gondii also indicated the presence of phosphomonoesters and nucleotide phosphates. We also report that the pyrophosphate analog drug, risedronate (used in bone resorption therapy), inhibits the growth of C. parvum in a mouse xenograft model. When taken together, our results indicate that all the major disease-causing apicomplexan parasites contain extensive stores of condensed phosphates and that as with Plasmodium falciparum and T. gondii, the pyrophosphate analog drug risedronate is an inhibitor of C. parvum cell growth.

Evidence for the transport of zinc(II) ions via the pit inorganic phosphate transport system in Escherichia coli

A locus involved in zinc(II) uptake in Escherichia coli K-12 was identified through the generation of a zinc(II)-resistant mutant by transposon (Tn10dCam) mutagenesis. The mutation was located within the pitA gene, which encodes the low-affinity inorganic phosphate transport system (Pit). The pitA mutant accumulated reduced amounts of zinc(II) when exposed to 0.5-2.0 mM ZnSO(4) during growth in Luria-Bertani medium.

Studies of cytochrome c oxidase-driven H(+)-coupled phosphate transport catalyzed by the Saccharomyces cerevisiae Pho84 permease in coreconstituted vesicles

The proton-coupled Pho84 phosphate permease of Saccharomyces cerevisiae, overexpressed as a histidine-tagged chimera in Escherichia coli, was detergent-solubilized, purified, and reconstituted into proteoliposomes. Proteoliposomes containing the Pho84 protein were fused with proteoliposomes containing purified cytochrome c oxidase from beef heart mitochondria. Both components of the coreconstituted system were functionally incorporated in tightly sealed membrane vesicles in which the cytochrome c oxidase-generated electrochemical proton gradient could drive phosphate transport via the proton-coupled Pho84 permease. The metal dependency of transport indicates that a metal-phosphate complex is the translocated substrate.

Functional characteristics of pyruvate transport in Phycomyces blakesleeanus

A saturable and accumulative transport system for pyruvate has been detected in Phycomyces blakesleeanus NRRL 1555(-) mycelium. It was strongly inhibited by alpha-cyano-4-hydroxycinnamate. l-Lactate and acetate were competitive inhibitors of pyruvate transport. The initial pyruvate uptake velocity and accumulation ratio was dependent on the external pH. The Vmax of transport greatly decreased with increasing pH, whereas the affinity of the carrier for pyruvate was not affected. The pyruvate transport system mediated its homologous exchange, which was essentially pH independent, and efflux, which increased with increasing external pH. The uptake of pyruvate was energy dependent and was strongly inhibited by inhibitors of oxidative phosphorylation and of the formation of proton gradients. Glucose counteracted the inhibitory effect of the pyruvate transport produced by inhibitors of mitochondrial ATP synthesis. Our results are consistent with a pyruvate/proton cotransport in P. blakesleeanus probably driven by an electrochemical gradient of H+ generated by a plasma membrane H+-ATPase.

Phosphate assimilation in Rhizobium (Sinorhizobium) meliloti: identification of a pit-like gene

Rhizobium meliloti mutants defective in the phoCDET-encoded phosphate transport system form root nodules on alfalfa plants that fail to fix nitrogen (Fix-). We have previously reported that two classes of second-site mutations can suppress the Fix- phenotype of phoCDET mutants to Fix+. Here we show that one of these suppressor loci (sfx1) contains two genes, orfA and pit, which appear to form an operon transcribed in the order orfA-pit. The Pit protein is homologous to various phosphate transporters, and we present evidence that three suppressor mutations arose from a single thymidine deletion in a hepta-thymidine sequence centered 54 nucleotides upstream of the orfA transcription start site. This mutation increased the level of orfA-pit transcription. These data, together with previous biochemical evidence, show that the orfA-pit genes encode a Pi transport system that is expressed in wild-type cells grown with excess Pi but repressed in cells under conditions of Pi limitation. In phoCDET mutant cells, orfA-pit expression is repressed, but this repression is alleviated by the second-site suppressor mutations. Suppression increases orfA-pit expression compensating for the deficiencies in phosphate assimilation and symbiosis of the phoCDET mutants.

Regulation of phosphate assimilation in Rhizobium (Sinorhizobium) meliloti

We report the isolation of phoB and phoU mutants of the bacterium Rhizobium (Sinorhizobium) meliloti. These mutants form N2-fixing nodules on the roots of alfalfa plants. R. meliloti mutants defective in the phoCDET (ndvF) encoded phosphate transport system grow slowly in media containing 2 mM Pi, and form nodules which fail to fix nitrogen (Fix-). We show that the transfer of phoB or phoU insertion mutations into phoC mutant strains restores the ability of these mutants to: (i) form normal N2-fixing root-nodules, and (ii) grow like the wild type in media containing 2 mM Pi. We also show that expression of the alternate orfA pit encoded Pi transport system is negatively regulated by the phoB gene product, whereas phoB is required for phoCDET expression. We suggest that in R. meliloti cells growing under Pi limiting conditions, PhoB protein activates phoCDET transcription and represses orfA pit transcription. Our results suggest that there are major differences between the Escherichia coli and R. meliloti phosphate regulatory systems.

Inorganic polyphosphate and the induction of rpoS expression

Inorganic polyphosphate [poly(P)] levels in Escherichia coli were reduced to barely detectable concentrations by expression of the plasmid-borne gene for a potent yeast exopolyphosphatase [poly(P)ase]. As a consequence, resistance to H2O2 was greatly diminished, particularly in katG (catalase HPI) mutants, implying a major role for the other catalase, the stationary-phase KatE (HPII), which is rpoS dependent. Resistance was restored to wild-type levels by complementation with plasmids expressing ppk, the gene for PPK [the polyphosphate kinase that generates poly(P)]. Induction of expression of both katE and rpoS (the stationary-phase sigma factor) was prevented in cells in which the poly(P)ase was overproduced. Inasmuch as this inhibition by poly(P)ase did not affect the levels of the stringent-response guanosine nucleotides (pppGpp and ppGpp) and in view of the capacity of additional rpoS expression to suppress the poly(P)ase inhibition of katE expression, a role is proposed for poly(P) in inducing the expression of rpoS.

Manipulation of independent synthesis and degradation of polyphosphate in Escherichia coli for investigation of phosphate secretion from the cell

The genes involved in polyphosphate metabolism in Escherichia coli were cloned behind different inducible promoters on separate plasmids. The gene coding for polyphosphate kinase (PPK), the enzyme responsible for polyphosphate synthesis, was placed behind the Ptac promoter. Polyphosphatase, a polyphosphate depolymerase, was similarly expressed by using the arabinose-inducible PBAD promoter. The ability of cells containing these constructs to produce active enzymes only when induced was confirmed by polyphosphate extraction, enzyme assays, and RNA analysis. The inducer concentrations giving optimal expression of each enzyme were determined. Experiments were performed in which ppk was induced early in growth, overproducing PPK and allowing large amounts of polyphosphate to accumulate (80 mumol in phosphate monomer units per g of dry cell weight). The ppx gene was subsequently induced, and polyphosphate was degraded to inorganic phosphate. Approximately half of this polyphosphate was depleted in 210 min. The phosphate released from polyphosphate allowed the growth of phosphate-starved cells and was secreted into the medium, leading to a down-regulation of the phosphate-starvation response. In addition, the steady-state polyphosphate level was precisely controlled by manipulating the degree of ppx induction. The polyphosphate content varied from 98 to 12 mumol in phosphate monomer units per g of dry cell weight as the arabinose concentration was increased from 0 to 0.02% by weight.

A phosphate transport system is required for symbiotic nitrogen fixation by Rhizobium meliloti

The bacterium Rhizobium meliloti forms N2-fixing root nodules on alfalfa plants. The ndvF locus, located on the 1,700-kb pEXO megaplasmid of R. meliloti, is required for nodule invasion and N2 fixation. Here we report that ndvF contains four genes, phoCDET, which encode an ABC-type transport system for the uptake of Pi into the bacteria. The PhoC and PhoD proteins are homologous to the Escherichia coli phosphonate transport proteins PhnC and PhnD. The PhoT and PhoE proteins are homologous to each other and to the E. coli phosphonate transport protein PhnE. We show that the R. meliloti phoD and phoE genes are induced in response to phosphate starvation and that the phoC promoter contains two elements which are similar in sequence to the PHO boxes present in E. coli phosphate-regulated promoters. The R. meliloti ndvF mutants grow poorly at a phosphate concentration of 2 mM, and we hypothesize that their symbiotic phenotype results from their failure to grow during the nodule infection process. Presumably, the PhoCDET transport system is employed by the bacteria in the soil environment, where the concentration of available phosphate is normally 0.1 to 1 microM.

A phospholipid acts as a chaperone in assembly of a membrane transport protein

A mutant of Escherichia coli lacking phosphatidylethanolamine (PE) and a monoclonal antibody (mAb 4B1) directed against a conformationally sensitive epitope (4B1) of lactose permease were used to establish a novel role for a phospholipid in the assembly of a membrane protein. Epitope 4B1 is readily detectable in spheroplasts and right-side-out membrane vesicles from PE-containing but not from PE-deficient cells expressing lactose permease. Lactose permease from PE-containing membranes, but not from PE-deficient membranes, subjected to sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and Western blot analysis is also recognized by mAb 4B1. If total E. coli phospholipids or PE (but not phosphatidylcholine, phosphatidylglycerol, or cardiolipin) are blotted on nitrocellulose sheets (Eastern blot) prior to transfer of proteins from SDS-polyacrylamide gels, the permease from PE-deficient cells regains its recognition by mAb 4B1. Therefore, PE is required during assembly to form epitope 4B1, but, once formed, sufficient "conformational memory" is retained in the permease to either retain or reform this epitope in the absence of PE. Lactose permease lacking epitope 4B1 can be induced to form the epitope if partially denatured and then renatured in the presence of PE specifically. These results establish for the first time a role for PE as a molecular chaperone in the assembly of the lactose permease.

Inorganic polyphosphate in mammalian cells and tissues

Inorganic polyphosphate (polyP), a linear polymer of hundreds of orthophosphate (Pi) residues linked by high-energy, phosphoanhydride bonds, has been identified and measured in a variety of mammalian cell lines and tissues by unambiguous enzyme methods. Subpicomole amounts of polyP (0.5 pmol/100 micrograms of protein) were determined by its conversion to ATP by Escherichia coli polyphosphate kinase and, alternatively, to Pi by Saccharomyces cerevisiae exopolyphosphatase. Levels of 25 to 120 microM (in terms of Pi residues), in chains 50 to 800 residues long, were found in rodent tissues (brain, heart, kidneys, liver, and lungs) and in subcellular fractions (nuclei, mitochondria, plasma membranes, and microsomes). PolyP in brain was predominantly near 800 residues and found at similar levels pre- and postnatally. Conversion of Pi into polyP by cell lines of fibroblasts, T-cells, kidney, and adrenal cells attained levels in excess of 10 pmol per mg of cell protein per h. Synthesis of polyP from Pi in the medium bypasses intracellular Pi and ATP pools suggesting the direct involvement of membrane component(s). In confluent PC12 (adrenal pheochromocytoma) cells, polyP turnover was virtually complete in an hour, whereas in fibroblasts there was little turnover in four hours. The ubiquity of polyP and variations in its size, location, and metabolism are indicative of a multiplicity of functions for this polymer in mammalian systems.

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.

Energetics of alanine, lysine, and proline transport in cytoplasmic membranes of the polyphosphate-accumulating Acinetobacter johnsonii strain 210A

Amino acid transport in right-side-out membrane vesicles of Acinetobacter johnsonii 210A was studied. L-Alanine, L-lysine, and L-proline were actively transported when a proton motive force of -76 mV was generated by the oxidation of glucose via the membrane-bound glucose dehydrogenase. Kinetic analysis of amino acid uptake at concentrations of up to 80 microM revealed the presence of a single transport system for each of these amino acids with a Kt of less than 4 microM. The mode of energy coupling to solute uptake was analyzed by imposition of artificial ion diffusion gradients. The uptake of alanine and lysine was driven by a membrane potential and a transmembrane pH gradient. In contrast, the uptake of proline was driven by a membrane potential and a transmembrane chemical gradient of sodium ions. The mechanistic stoichiometry for the solute and the coupling ion was close to unity for all three amino acids. The Na+ dependence of the proline carrier was studied in greater detail. Membrane potential-driven uptake of proline was stimulated by Na+, with a half-maximal Na+ concentration of 26 microM. At Na+ concentrations above 250 microM, proline uptake was strongly inhibited. Generation of a sodium motive force and maintenance of a low internal Na+ concentration are most likely mediated by a sodium/proton antiporter, the presence of which was suggested by the Na(+)-dependent alkalinization of the intravesicular pH in inside-out membrane vesicles. The results show that both H+ and Na+ can function as coupling ions in amino acid transport in Acinetobacter spp.

Translocation of metal phosphate via the phosphate inorganic transport system of Escherichia coli

Pi transport via the phosphate inorganic transport system (Pit) of Escherichia coli was studied in natural and artificial membranes. Pi uptake via Pit is dependent on the presence of divalent cations, like Mg2+, Ca2+, Co2+, or Mn2+, which form a soluble, neutral metal phosphate (MeHPO4) complex. Pi-dependent uptake of Mg2+ and Ca2+, equimolar cotransport of Pi and Ca2+, and inhibition by Mg2+ of Ca2+ uptake in the presence of Pi, but not of Pi uptake in the presence of Ca2+, indicate that a metal phosphate complex is the transported solute. Metal phosphate is transported in symport with H+ with a mechanistic stoichiometry of 1. Pit mediates efflux and homologous exchange of metal phosphate, but not heterologous metal phosphate exchange with Pi, glycerol-3P, or glucose-6P. The metal phosphate efflux rate increased with pH, whereas the rate of metal phosphate exchange was essentially pH independent. Metal phosphate uptake was inhibited at low internal pH. Efflux was inhibited by a proton motive force (interior negative and alkaline), whereas exchange was inhibited by the membrane potential only. These results have been evaluated in terms of ordered binding and dissociation of metal phosphate and proton on the outer and inner surface of the cytoplasmic membrane.