Purification of a reconstitutively active mitochondrial phosphate transport protein

The mitochondrial phosphate carrier: Role in oxidative metabolism, calcium handling and mitochondrial disease

The mitochondrial phosphate carrier (PiC) is a mitochondrial solute carrier protein, which is encoded by SLC25A3 in humans. PiC delivers phosphate, a key substrate of oxidative phosphorylation, across the inner mitochondrial membrane. This transport activity is also relevant for allowing effective mitochondrial calcium handling. Furthermore, PiC has also been described to affect cell survival mechanisms via interactions with cyclophilin D and the viral mitochondrial-localized inhibitor of apoptosis (vMIA). The significance of PiC has been supported by the recent discovery of a fatal human condition associated with PiC mutations. Here, we present first the early studies that lead to the discovery and molecular characterization of the PiC, then discuss the very recently developed mouse models for PiC and pathological mutations in the human SLC25A3 gene.

The phosphate carrier from yeast mitochondria. Dimerization is a prerequisite for function

Wild type phosphate carrier (PIC) from Saccharomyces cerevisiae and recombinant PIC proteins with different C-terminal extensions were expressed in Escherichia coli as inclusion bodies. From these, PIC was isolated with the detergent sodium lauroyl sarcosinate in a form, partially monomeric and unfolded. This PIC associates to stable dimers after exchanging the detergent to the polyoxyethylene detergent C12E8 and dialysis. Combining two differently tagged monomers of PIC and following this with affinity chromatography yields defined homo- and heterodimeric forms of PIC, which are all fully active after reconstitution. As a member of the mitochondrial carrier family PIC is supposed to function as a homodimer. We investigated its dimeric nature in the functionally active state after reconstitution. When reconstituting PIC monomers a sigmoidal dependence of transport activity on the amount of inserted protein is observed, whereas insertion of PIC dimers leads to a linear dependence. Heterodimeric PIC constructs consisting of both an active and an inactivated subunit do not catalyze phosphate transport. In contrast, reconstitution of a mixture of active and inactive monomeric subunits led to partially active carrier. These experiments prove (i) that PIC does not function in monomeric form, (ii) that PIC dimers are stable both in the solubilized state and after membrane insertion, and (iii) that transport catalyzed by PIC dimers involves functional cross-talk between the two monomers.

The reversible antiport-uniport conversion of the phosphate carrier from yeast mitochondria depends on the presence of a single cysteine

Wild type and mutant phosphate carriers (PIC) from Saccharomyces cerevisiae mitochondria were expressed in Escherichia coli as inclusion bodies, solubilized, purified, and optimally reconstituted into liposomal membranes. This PIC can function as coupled antiport (Pi-/Pi- antiport and Pi- net transport, i.e. Pi-/OH- antiport) and uncoupled uniport (mercuric chloride-induced Pi- efflux). The basic kinetic properties of these three transport modes were analyzed. The kinetic properties closely resemble those of the reconstituted PIC from beef heart mitochondria. A competitive inhibitor of phosphate transport by the PIC, phosphonoformic acid, was used to establish functional overlap between the the physiological transport modes and the induced efflux mode. Replacement mutants were used to relate the reversible switch from antiport to uniport to a specific residue of the carrier. There are only three cysteines in the yeast PIC. They are at positions 28, 134, and 300 and were replaced by serine, both individually and in combinations. Cysteine 300 near the C-terminal loop and cysteine 134 located within the third transmembrane segment are accessible to bulky hydrophilic reagents from the cytosolic side, whereas cysteine 28 within the first transmembrane segment is not. None of the three cysteines is relevant to the two antiport modes. Cysteine 134 was identified to be the major target of bulky SH reagents, that lead to complete inactivation of the physiological transport modes. The reversible conversion between coupled antiport and uncoupled uniport of the PIC depends on the presence of one single cysteine (cysteine 28) in the PIC monomer, i.e. two cysteines in the functionally active dimer. The consequences of this result with respect to a functional model of the carrier protein are discussed.

FLX1 codes for a carrier protein involved in maintaining a proper balance of flavin nucleotides in yeast mitochondria

Respiratory defective mutants of Saccharomyces cerevisiae previously assigned to complementation group G178 are characterized by an abnormally low ratio of FAD/FMN in mitochondria. A nuclear gene, designated FLX1, was selected from a yeast genomic library, based on its ability to confer wild-type growth properties to a representative G178 mutant. Genetic evidence has confirmed that the flavin nucleotide imbalance of G178 mutants is caused by mutations in FLX1. The sequence of FLX1 is identical to a reading frame recently reported to be present on yeast chromosome IX (GenBank Z47047). The sequence and tripartite repeat structure of the FLX1 product (Flx1p) indicate it is a member of a protein family consisting of mitochondrial substrate and nucleotide carriers. In yeast, FAD synthetase is present in the soluble cytoplasmic protein fraction but not in mitochondria. Riboflavin kinase, the preceding enzyme in flavin biosynthesis, is present in both subcellular fractions. The absence of FAD synthetase in mitochondria implies that FAD is imported from the cytoplasm. The lower concentration of mitochondrial FAD in flx1 mutants suggests that Flx1p is involved in flavin transport, a role that is also supported by biochemical evidence indicating more efficient flux of FAD across mitochondrial membrane vesicles prepared from wild-type strains than membrane vesicles from flx1 mutants.

Phosphate transport in mitochondria: past accomplishments, present problems, and future challenges

The requirement of inorganic phosphate (Pi) for oxidative phosphorylation in eukaryotic cells is fulfilled through specific Pi transport systems. The mitochondrial proton/phosphate symporter (Pic) is a membrane-embedded protein which translocates Pi from the cytosol into the mitochondrial matrix. Pic is responsible for the very rapid transport of most of the Pi used in ATP synthesis. During the past five years there have been advances on several fronts. Genomic and cDNA clones for yeast, bovine, rat, and human Pic have been isolated and sequenced. Functional expression of yeast Pic in yeast strains deficient in Pi transport and expression in Escherichia coli of a chimera protein involving Pic and ATP synthase alpha subunit have been accomplished. Pic, in contrast to other members of the family of transporters involved in energy metabolism, was demonstrated to have a presequence, which optimizes the import of the precursor protein into mitochondria. Six transmembrane segments appear to be a structural feature shared between Pic and other mitochondrial anion carriers, and recent-site directed mutagenesis studies implicate structure-functional relationships to bacteriorhodopsin. These recent advances on Pic will be assessed in light of a more global interpretation of transport mechanism across the inner mitochondrial membrane.

Functional properties of purified and reconstituted mitochondrial metabolite carriers

Eight mitochondrial carrier proteins were solubilized and purified in the authors' laboratories using variations of a general procedure based on hydroxyapatite and Celite chromatography. The molecular mass of all the carriers ranges between 28 and 34 kDa on SDS-PAGE. The purified carrier proteins were reconstituted into liposomes mainly by using a method of detergent removal by hydrophobic chromatography on polystyrene beads. The various carriers were identified in the reconstituted state by their kinetic properties . A complete set of basic kinetic data including substrate specificity, affinity, interaction with inhibitors, and activation energy was obtained. These data closely resemble those of intact mitochondria, as far as they are available from the intact organelle. Mainly on the basis of kinetic data, the asymmetric orientation of most of the reconstituted carrier proteins were established. Several of their functional properties are significantly affected by the type of phospholipids used for reconstitution. All carriers which have been investigated in proteoliposomes function according to a simultaneous (sequential) mechanism of transport; i.e., a ternary complex, made up of two substrates and the carrier protein, is involved in the catalytic cycle. The only exception was the carnitine carrier, where a ping-pong mechanism of transport was found. By reaction of particular cysteine residues with mercurial reagents, several carriers could be reversibly converted to a functional state different from the various physiological transport modes. This "unphysiological" transport mode is characterized by a combination of channel-type and carrier-type properties.

Functional properties of the reconstituted phosphate carrier from bovine heart mitochondria: evidence for asymmetric orientation and characterization of three different transport modes

The phosphate carrier from bovine heart mitochondria was reconstituted into liposomes by the removal of detergent using hydrophobic ion-exchange columns. Reversible blocking of the carrier function during chromatographic steps was possible by the application of the inhibitor p-(chloromercuri)benzenesulfonate at low temperature. Thus, both forward and backward exchange experiments for kinetic characterization of Pi/Pi-antiport as well as the Pi/H(+)-symport could be performed. The maximum rate of Pi/Pi-antiport was 90 mumol min-1 (mg protein)-1. Only one single half-saturation constant (Km) for phosphate was observed at each side of the membrane under antiport conditions, 1.8 mM at the external and 9.4 mM at the internal side. By comparing the Km values at both sides of the membrane with the values found in intact mitochondria, a right-side-out orientation of the reconstituted phosphate carrier was concluded. Furthermore, the influence of various sulfhydryl reagents on the carrier was investigated. After modification with HgCl2, the phosphate carrier reveals a third (nonphysiological) unidirectional transport mode. This was characterized by a significantly reduced substrate specificity. In view of similar observations with several other mitochondrial carriers, these results again indicate that the phosphate carrier is a member of the postulated functional family of mitochondrial carrier proteins.

Characterization of SH groups in porin of bovine heart mitochondria. Porin cysteines are localized in the channel walls

Porin from bovine heart mitochondria contains probably two cysteines (Cys126 and Cys230 in human porin, Kayser, H., Kratzin, H. D., Thinnes, F. P., Götz, H., Schmidt, W. E., Eckart, K. & Hilschmann, N. (1989) Biol. Chem. Hoppe-Seyler 370, 1265-1278). Reduced and oxidized forms of these cysteines were investigated in purified protein and in intact mitochondria using the agents dithioerythritol, cuprous(II) phenantroline, diamide and performic acid. Furthermore, intact mitochondria were labelled with the sulfhydryl-alkylating agents N-[14C]ethylmaleimide, eosin-5-maleimide and N-(1-pyrenyl)-maleimide. Affinity chromatography of bovine heart porin was performed with cysteine-specific material. The results can be summarized as follows: (1) Porin has one reduced and two oxidized forms of apparent molecular masses between 30 and 35 kDa. The native form of porin is the reduced 33 kDa form. The oxidized forms only appear after denaturation with SDS. (2) The 35-kDa reduced and the 33.5-kDa oxidized forms of porin show the same pore-forming properties after reconstitution of the protein into lipid bilayer membranes. (3) Labelling of cysteines by eosin-5-maleimide and N-(1-pyrenyl)-maleimide suggested their location at a boundary between the water-phase and the lipid-phase. Incubation of intact mitochondria with N-ethylmaleimide prior to eosin-5-maleimide and N-(1-pyrenyl)maleimide treatment resulted in the inhibition of the fluorescent labelling. Among the cysteines present in the primary structure, Cys126 is the most sensitive to N-ethylmaleimide binding. (4) Bovine heart mitochondrial porin covalently bound to Affi-Gel 501 (with a 1.75 nm long spacer), but not to Thiopropyl-Sepharose 6B (with a 0.51 nm spacer). This suggests that at least one of the cysteines is localized between 0.51 nm and 1.75 nm deep in the protein micelle.

The rate of phosphate transport during recovery from muscular exercise depends on cytosolic [H+]. A 31P-MR spectroscopy study in humans

31-Phosphorus magnetic resonance spectroscopy was used to study in vivo the effect of cytosolic [H+] on the kinetics of initial post-exercise recovery of inorganic phosphate (Pi) in human gastrocnemius muscle. Linear correlations were found between the rate of initial phosphate recovery and: a) the minimum value of cytosolic pH reached during recovery, and b) the minimum percentage of divalent anion present. These linear relationships are consistent with the current knowledge of Pi transport, and represent new invariant parameters for the study of muscle pathologies that may involve Pi and/or H+ transport.

The effect of inorganic phosphate on chick (Gallus domesticus) heart mitochondrial volume and cation content

1. In the absence of exogenous Ca(II), Pi induces a swelling change that is kinetically first order with k = 1.08 +/- 0.1 min-1. The first-order rate constant is independent of [Pi] over the range of 0.5-45 mM. 2. In the presence of exogenous substrate, the volume change induced by Pi is monophasic and can be reversed by ADP. 3. The swelling process and the approach to steady state is accompanied by controlled losses of both K+ and Mg(II) from within the mitochondria. 4. The loss of K+ is biphasic as a function of time with ki = 14.1 +/- 1.6 and k2 = 4.4 +/- 0.34 nmol min-1 mg mitochondria-1. 5. The loss of Mg(II) is monophasic and the rate at which this cation is released decreases as a function of time. Ca(II) fluxes are not involved in the volume occurring secondary to Pi uptake. 6. In the absence of exogenous substrate, Pi induces a triphasic change in mitochondrial volume. 7. The sequence of volume changes corresponds to an initial first-order swelling secondary to the addition of Pi, a contraction apparently triggered by the loss of approximately 85% of total intra-mitochondrial Mg(II), and a second larger swelling phase that cannot be reversed with ADP. 8. The Pi-induced swelling of chick heart mitochondria is not inhibited by EGTA and does not depend on the provision of exogenous Ca(II). 9. The Ca(II) and Mg(II) ions released from within the mitochondria are responsible for activating divalent cation-dependent ATPases which cosediment with isolated chick heart mitochondria.

Streptozotocin-induced alterations in the levels of functional mitochondrial anion transport proteins

The effect of streptozotocin-induced diabetes on the levels of functional mitochondrial anion transport proteins has been determined. The experimental approach utilized for these studies consisted of the extraction of each of four mitochondrial anion transport proteins from rat liver mitoplasts (isolated from diabetic and control animals) with the nonionic detergent Triton X-114, followed by the functional reconstitution of each transporter in a liposomal system via the freeze-thaw-sonication technique. This approach permitted the quantification of transporter function without the complications that occur when such measurements are carried out with intact mitochondria (or mitoplasts). We found that experimental diabetes caused an increase in the extractable and reconstitutable specific (and total) transport activities of the pyruvate and dicarboxylate transporters, a decrease in the activity of the citrate transporter, and no significant change in the activity of the phosphate transporter relative to control values. An examination of the time course of the appearance of changes in the reconstitutable activities of the pyruvate and citrate transporters following the injection of streptozotocin revealed differences. Thus, whereas the activity of the pyruvate transporter displayed the most pronounced increase (193%) 1 week following streptozotocin injection and then subsequently declined from this peak and plateaued at later times (99% and 96% increases at 3 and 8 weeks, respectively), the activity of the citrate transporter progressively decreased with time (31-51% decreases at 1-8 weeks). We suggest that the observed diabetes-induced changes in mitochondrial anion transporter function are predictable on the basis of diabetes-induced alterations in the activities of enzymes that constitute metabolic pathways to which these transporters either supply substrate or remove product. Furthermore, we speculate that mitochondrial anion transport proteins may be regulated in coordination with the enzymes of such associated metabolic pathways.

Interaction of phenylisothiocyanates with the mitochondrial phosphate carrier. I. Covalent modification and inhibition of phosphate transport

The effects of phenylisothiocyanate (PITC) and of the polar analogue p-sulfophenylisothiocyanate (p-sulfoPITC) on the phosphate carrier of bovine heart mitochondria have been investigated. Incubation of mitochondria with the two phenylisothiocyanates leads to inhibition of the phosphate carrier protein. The inhibition of phosphate transport by PITC is unaffected by the addition of dithioerythritol (DTE) or by variation of the pH. The inhibition by p-sulfoPITC is in part removed by DTE; the remaining inactivation of the phosphate carrier, which can be attributed to the reaction with NH2 groups, is temperature and pH-dependent. Inhibition of phosphate transport by both p-sulfoPITC and PITC depends on the time of incubation and the concentration of the inhibitor. Preincubation with mersalyl protects the carrier protein against the inactivation by p-sulfoPITC but not against PITC. Other SH reagents tested do not show any protective effect. It can thus be concluded that two types of lysine residues are essential for the activity of the phosphate carrier. Lysine(s) of the former type are located at the surface of the membrane and are topologically related to the functional SH groups of the protein. Lysine residue(s) of the latter type are buried in the hydrophobic phase of the membrane.

The mitochondrial high-capacity low-affinity (+/-)-[3H]nitrendipine binding site is regulated by nucleotides

The high-capacity, low-affinity (+/-)-[3H]nitrendipine binding site in the inner mitochondrial membrane from guinea-pig heart is regulated by purine and pyrimidine nucleotides. The rank order in (+/-)-[3H]nitrendipine binding inhibition assays (with decreasing potency) was: ATP (IC50 11.8 microM) = adenosine 5'-O-(2-thiotriphosphate (ATP gamma S) greater than 5'-adenylylimidodiphosphate (AppNHp) greater than ADP much greater than GTP = ITP = CTP greater than UTP greater than guanosine 5'-tetraphosphate (GT4P) greater than guanosine 5'-O-(2-thiotriphosphate) (GTP gamma S) greater than 5'-guanylylimidodiphosphate (GppNHp) greater than IDP greater than CDP greater than GDP. There was no effect of AMP, adenosine 3':5'-cyclic monophosphate (cAMP), adenosine, UDP, NAD, and NADP. The ATP effect was fully reversible upon wash-out. Adenine nucleotides and analogs had a (+/-)-[3H]nitrendipine binding inhibition profile in mitochondrial membranes from guinea-pig liver or kidney similar to that obtained in heart mitochondrial membranes. In heart mitochondria, 0.3 mM ATP decreased the Bmax from 1.69 +/- 0.04 nmol/mg protein to 0.73 +/- 0.24 nmol/mg protein whilst it decreased the KD only moderately, from 521 +/- 50 to 352 +/- 43 nM, in equilibrium saturation studies. In kinetic studies, ATP slowed down the dissociation rate of the (+/-)-[3H]nitrendipine binding site complex from 0.016 +/- 0.004 to 0.0042 +/- 0.0002 min-1 but it also decreased the association rate constant from 1.52 +/- 0.15 to 0.41 +/- 0.28 10(4).M-1.min-1, yielding a kinetically determined KD (1024 nM) identical to the control KD (1053 nM).(ABSTRACT TRUNCATED AT 250 WORDS)

Photoaffinity labeling of the mitochondrial phosphate carrier by 4-azido-2-nitrophenyl phosphate

The effect of 4-azido-2-nitrophenyl phosphate (ANPP), a photoreactive analogue of phosphate, on the phosphate carrier of pig-heart mitochondria has been investigated. In the dark, ANPP inhibits the transport of phosphate in a competitive manner with a Ki of 3.2 mM. Upon photoirradiation with visible light, [32P]ANPP binds covalently to the phosphate carrier and the inhibition becomes irreversible. Both the inhibition of phosphate transport and the incorporation of [32P]ANPP into the phosphate carrier depend on the concentration of the inhibitor and the pH of the medium. Incubation of the mitochondria with phosphate during illumination in the presence of ANPP protects the carrier against inactivation and decreases the amount of radioactivity which is found to be associated with the purified protein. By extrapolation it is calculated that at 100% inactivation of the phosphate carrier 0.35 mol of reagent are bound per mol of 33 kDa carrier protein. It is concluded that ANPP can be used for photoaffinity labeling of the mitochondrial phosphate carrier at the substrate-binding site.

Conserved structural domains among species and tissues-specific differences in the mitochondrial phosphate-transport protein and the ADP/ATP carrier

Peptide maps were generated of the CNBr-digested mitochondrial phosphate-transport protein and ADP/ATP carrier from bovine and rat heart, rat liver and blowfly flight muscle. Total mitochondrial proteins from the same sources plus pig heart were separated by SDS-polyacrylamide gel electrophoresis. The peptide maps and the total mitochondrial proteins were electroblotted onto nitrocellulose membranes and reacted with rabbit antisera raised against the purified bovine heart phosphate-transport protein and the ADP/ATP carrier. On the basis of antibody specificity, mobility in SDS-polyacrylamide gel electrophoresis, and peptide maps the following was concluded. Phosphate-transport protein alpha and phosphate-transport protein beta (pig and bovine heart) react equally with the first and also with the second of two independent phosphate-transport protein-antisera. Tissue-specific structural domains exist for both the phosphate-transport protein and the ADP/ATP carrier, i.e., one phosphate-transport protein-antiserum reacts with the phosphate-transport protein from all assayed sources, the other only with the cardiac phosphate-transport protein. These differences may reflect tissue-specific regulation of phosphate and adenine nucleotide transport. Homologies among the different species are found for the phosphate transport protein and the ADP/ATP carrier, except for the flight muscle ADP/ATP carrier. These conserved structural domains of the phosphate-transport protein may relate directly to catalytic activity. Alkylation of the purified phosphate-transport proteins and the ADP/ATP carriers by the transport inhibitor N-ethylmaleimide affects electrophoretic mobilities but not the antibody binding. Neither of the two phosphate-transport protein-antisera nor the ADP/ATP-carrier antiserum react with both phosphate transport protein and ADP/ATP carrier, even though these two proteins possess similarities in primary structure and function. Possible mechanisms for generating tissue-specific structural differences in the proteins are discussed.

Isolation and functional reconstitution of the aspartate/glutamate carrier from mitochondria

The aspartate/glutamate carrier from beef heart mitochondria was solubilized by the detergent dodecyloctaoxyethylene ether (C12E8) in the presence of high concentrations of ammonium acetate. After separating the bulk amount of contaminating proteins by differential solubilization and by hydroxyapatite centrifugation chromatography, the aspartate/glutamate carrier was purified by high-performance liquid chromatography on hydroxyapatite. During the purification process, the aspartate/glutamate carrier as well as other transport proteins was identified by functional reconstitution. In sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis the purified aspartate/glutamate carrier protein appears as a protein band with an apparent molecular mass of 68 kDa. Small amounts of some contaminating proteins mainly at 31 kDa were also found. Since the ADP/ATP carrier has an apparent molecular mass of 31 kDa in SDS-gel electrophoresis, possible contamination by the nucleotide carrier was analyzed by immunological methods. The enrichment of the aspartate/glutamate carrier--based on functional reconstitution--was about 570-fold, the protein yield was 0.1%.

ATP synthase complex from beef heart mitochondria. Separation of protein bands in the region of 28-31 kDa

ATPase/ATP synthase preparations originally contain protein bands in the 28-31-kDa region. The present study demonstrates separation of the band at 29 kDa (adenine nucleotide translocator) from a band at approximately 31 kDa. In cholate/ammonium sulfate-extracted ATP synthases removal of the 31-kDa band results in decrease of ATP-Pi exchange and oligomycin sensitivity of the ATPase activity. It is suggested that the protein band at 31 kDa is heterogeneous, containing diverse activities, the identities of which are yet to be determined.

Current concepts in membrane protein reconstitution

The reconstitution of integral proteins into artificial lipid vesicles is largely prompted by the complexity of most biological membranes and the inherent difficulty of studying individual components in situ. Ideally, therefore, the reconstituted system should consist of a single protein in a lipid matrix which mimics the native membrane in all but its diversity. While such an approach allows individual components of a complex system to be studied in isolation it should also be sufficiently versatile to permit the generation of increasingly sophisticated multicomponent models. From the considerable number of reconstitution techniques which have been developed I have tried in this review to identify those characteristics of a particular system which maximise both the information it can provide and its versatility.

Comparative partial purification of the active dicarboxylate transport system of rat liver, kidney and heart mitochondria

Hydroxylapatite chromatography of Triton-extracted inner-membrane proteins from rat liver mitochondria allowed a ten-fold purification of the dicarboxylate carrier. The purified system, reconstituted into liposomes, displayed all the properties of the dicarboxylate carrier and mediated malonate-malate and malonate-phosphate exchanges. Six protein bands of Mr ranging from 27,000 to 34,000 could be resolved by sodium dodecylsulfate-polyacrylamide gel electrophoresis. The purification of the dicarboxylate carriers of liver, kidney and heart mitochondria were carried out by this method and their properties were compared with respect to transport activity and electrophoresis patterns. Our results demonstrate that the dicarboxylate carrier of rat mitochondria can be obtained in an advanced state of purification and with a high specific activity.

Purification of reconstitutively active alpha-oxoglutarate carrier from pig heart mitochondria

The alpha-oxoglutarate carrier from pig heart mitochondria has been solubilized with Triton X-114 and purified by chromatography on hydroxyapatite and celite in the presence of cardiolipin. When applied to SDS gel electrophoresis, the purified protein consists of only a single protein band with an apparent Mr of 31.5 kDa. It corresponds to band 4 of the five protein bands previously identified in the hydroxyapatite pass-through of Triton X-114 solubilized heart mitochondria (Bisaccia, F. and Palmieri, F. (1984) Biochim. Biophys. Acta 766, 386-394). When reconstituted into liposomes the alpha-oxoglutarate transport protein catalyzes a phthalonate-sensitive alpha-oxoglutarate/alpha-oxoglutarate exchange. It is purified 250-fold with a recovery of 62% and a protein yield of 0.1% with respect to the mitochondrial extract. The properties of the reconstituted carrier, i.e., the requirements for a counteranion, the substrate specificity and the inhibitor sensitivity, are similar to those described for alpha-oxoglutarate transport in mitochondria.

Solubilization and reconstitution of the renal phosphate transporter

Proteins from brush-border membrane vesicles of rabbit kidney cortex were solubilized with 1% octylglucoside (protein to detergent ratio, 1:4 (w/w). The solubilized proteins (80.2 +/- 2.3% of the original brush-border proteins, n = 10, mean +/- S.E.) were reconstituted into artificial lipid vesicles or liposomes prepared from purified egg yolk phosphatidylcholine (80%) and cholesterol (20%). Transport of Pi into the proteoliposomes was measured by rapid filtration in the presence of a Na+ or a K+ gradient (out greater than in). In the presence of a Na+ gradient, the uptake of Pi was significantly faster than in the presence of a K+ gradient. Na+ dependency of Pi uptake was not observed when the liposomes were reconstituted with proteins extracted from brush-border membrane vesicles which had been previously treated with papain, a procedure that destroys Pi transport activity. Measurement of Pi uptake in media containing increasing amounts of sucrose indicated that Pi was transported into an intravesicular (osmotically sensitive) space, although about 70% of the Pi uptake appeared to be the result of adsorption or binding of Pi. However, this binding of Pi was not dependent upon the presence of Na+. Both Na+-dependent transport and the Na+-independent binding of Pi were inhibited by arsenate. The initial Na+-dependent Pi transport rate in control liposomes of 0.354 nmol Pi/mg protein per min was reduced to 0.108 and 0 nmol Pi/mg protein per min in the presence of 1 and 10 mM arsenate, respectively. Future studies on reconstitution of Pi transport systems must analyze and correct for the binding of Pi by the lipids used in the formation of the proteoliposomes.

Functional reconstitution of the partially purified aspartate-glutamate carrier from mitochondria

The aspartate/glutamate carrier from beef heart mitochondria has been solubilized with detergent. The transport protein was partially purified by chromatography on hydroxyapatite in the presence of dodecyl octaoxyethylene ether and high concentrations of ammonium acetate. During purification, the aspartate/glutamate carrier was identified by functional reconstitution into egg yolk phospholipid liposomes. After hydroxyapatite chromatography the protein is 30 fold enriched in aspartate/glutamate transport activity but still contains ADP/ATP-carrier and phosphate carrier. The reconstituted activity is specific for exchange of L-aspartate and L-glutamate and is similar to intact mitochondria with respect to substrate affinity and inhibitor sensitivity.

Specific elution from hydroxylapatite of the mitochondrial phosphate carrier by cardiolipin

The role of cardiolipin in the purification of the mitochondrial phosphate carrier by hydroxylapatite has been investigated. Without added cardiolipin, the reconstituted phosphate-transport activity in the hydroxylapatite eluate is small and only confined to the first fraction. With cardiolipin added to the extract, the eluted activity is much higher and present until fraction 6. The activity retained by hydroxylapatite in the absence of cardiolipin is eluted after addition of this phospholipid to the column. The requirement of added cardiolipin diminishes on increasing the concentration of solubilized mitochondria. The hydroxylapatite eluate contains five protein bands in the Mr-region of 30 000-35 000, which are differently distributed in the various fractions. Among these, only the presence and the relative amount of band 3 of Mr 33 000 corresponds to the phosphate transport activity. Cardiolipin is the only phospholipid tested which causes elution of band 3 from hydroxylapatite; on the other hand, it prevents the elution of band 2 and retards that of band 5 (the ADP/ATP carrier). Band 1 starts to appear in the second fraction even without cardiolipin. On increasing the concentration of cardiolipin, in the first fraction of the hydroxylapatite eluate band 3 increases and the contamination of band 4 decreases. Under optimal conditions a preparation of band 3 about 90% pure and with high reconstituted phosphate transport activity is obtained. It is concluded that the elution of the phosphate carrier from hydroxylapatite requires cardiolipin and that the phosphate carrier is identical with (or with part of) band 3 of the hydroxylapatite eluate.

Reactivity of the -SH groups of the mitochondrial phosphate carrier under native, solubilized and reconstituted conditions

The isolated and liposome-reconstituted mitochondrial phosphate carrier exhibits a sigmoidal inhibition curve by mersalyl, similar to that found with intact mitochondria. In contrast a hyperbolic inhibition curve is found (a) by titration of the soluble carrier with mersalyl before reconstitution in liposomes and (b) by titration of the reconstituted carrier with mersalyl after successively pretreatment of the mitochondria with low, non-inhibitory concentrations of mersalyl, excess N-ethylmaleimide and dithiothreitol. The inhibition of the reconstituted, but not of the soluble, phosphate carrier by mersalyl can be reversed by dithiothreitol. Cupric di(1,10-phenanthroline) inhibits the soluble but not the reconstituted phosphate carrier. The inhibited phosphate carrier can be reactivated by dithiothreitol in the soluble state but not after reconstitution in liposomes. The data support the previously suggested model of the phosphate carrier, assuming a dimer of two identical subunits for the active unit.

Enzymatic removal of alkaline phosphatase from renal brush-border membranes. Effect on phosphate transport and on phosphate binding

Brush-border membrane vesicles prepared from rabbit kidney cortex were incubated at 37 degrees C for 30 min with phosphatidylinositol-specific phospholipase C. This maneuver resulted in a release of approx. 85% of the brush-border membrane-linked enzyme alkaline phosphatase as determined by its enzymatic activity. Transport of inorganic [32P]phosphate (100 microM) by the PI-specific phospholipase C-treated brush-border membrane vesicles was measured at 20-22 degrees C in the presence of an inwardly directed 100 mM Na+ gradient. Neither initial uptake rates, as estimated from 10-s uptake values (103.5 +/- 6.8%, n = 7 experiments), nor equilibrium uptake values, measured after 2 h (102 +/- 3.4%) were different from controls (100%). Control and PI-specific phospholipase C-treated brush-border membrane vesicles were extracted with chloroform/methanol to obtain a proteolipid fraction which has been shown to bind Pi with high affinity and specificity (Kessler, R.J., Vaughn, D.A. and Fanestil, D.D. (1982) J. Biol. Chem. 257, 14311-14317). Phosphate binding (at 10 microM Pi) by the extracted proteolipid was measured. No significant difference in binding was observed between the two types of preparations: 31.0 +/- 9.37 in controls and 29.8 +/- 8.3 nmol/mg protein in the proteolipid extracted from PI-specific phospholipase C-treated brush-border membrane vesicles. It appears therefore that alkaline phosphatase activity is essential neither for Pi transport by brush-border membrane vesicles nor for Pi binding by proteolipid extracted from brush-border membrane. These results dissociate alkaline phosphatase activity, but not brush-border membrane vesicle transport of phosphate, from phosphate binding by proteolipid.

Effect of tribenzylphosphate on the active phosphate transport and ATP synthesis in yeast mitochondria

Tribenzylphosphate (TBP), a specific inhibitor of the high affinity system for Pi transport in yeast mitochondria, inhibits the active Pi transport measured by the energy-linked swelling. The dependence of the rate of oligomycin sensitive ATP synthesis as a function of the external Pi concentration shows two kinetic systems. The high affinity system, corresponds to the range of the external Pi concentration which stimulates the respiratory rate. TBP inhibits both this system and the state 4 leads to state 3 transition.

Isolation and reconstitution of an N-ethylmaleimide-sensitive phosphate transport protein from rat liver mitochondria

An N-ethylmaleimide-sensitive phosphate transport protein has been isolated from rat liver mitochondria, substantially purified, and reconstituted into phospholipid vesicles. Purified inner mitochondrial membrane vesicles depleted of F1-ATPase by urea treatment proved to be the most satisfactory starting material. Treatment of these membrane vesicles with Triton X-100 resulted in solubilization of the phosphate transport protein. Further purification was achieved using hydroxylapatite powder. Polyacrylamide gel electrophoresis of the purified fraction in sodium dodecyl sulfate indicated the presence of two Coomassie blue-staining bands with apparent Mr's of 30,000 and 35,000. Labeling of the 35,000 Mr band by the Pi transport inhibitor diazobenzene sulfonate was reduced markedly by prior treatment of the mitochondria with the inhibitor N-ethylmaleimide. The purified fraction containing both proteins could be reconstituted into liposomes prepared from purified asolectin. Phosphate efflux from these vesicles was inhibited by N-ethylmaleimide, by the impermeant mercurial agent, p-chloromercuribenzoate, and by diazobenzene sulfonate. Treatment of the purified fraction with N-ethylmaleimide prior to incorporation into liposomes resulted in a reconstituted system incapable of catalyzing Pi efflux. These studies summarize the first detailed attempt to purify the Pi/H+ transport system from rat liver mitochondria and emphasize the need to commence the purification with purified inner membrane vesicles depleted of F1-ATPase. In addition, these studies show that the final fraction contains a reconstitutively active transport system which when incorporated into phospholipid vesicles has its essential sulfhydryl groups oriented outward. Finally, it is shown that the purified fraction also contains a 30,000 Mr component.

Reconstitution of the isolated phosphate-transport system of pig-heart mitochondria

The phosphate carrier of pig heart mitochondria has been isolated and reconstituted in liposomes. The highest specific activity for [32P]phosphate exchange was obtained with hydroxyapatite eluate from mitochondria, extracted with Triton X-114 in the presence of cardiolipin. This fraction, which is free from the ADP ATP-carrier, had a specific activity of 30 mumol 32Pi x min-1 x mg protein -1. The following conditions were found to inactivate the phosphate carrier irreversibly in the solubilized state: high ionic strength, high detergent concentrations and a high pH. The decrease of the activity by high detergent concentrations can be largely prevented by cardiolipin, present in the extraction buffer, suggesting a specific removal of this lipid by the detergent. After reconstitution in liposomes, the phosphate carrier is rather stable. The Arrhenius plot of the temperature-dependence of the reconstituted phosphate exchange showed different slopes above and below 27 degrees C. Between 0 degrees C and 27 degrees C the EA was 64 kJ . mol-1, between 27 degrees C and 42 degrees C 44 kJ . mol-1. The exchange of Pi followed a first-order kinetic.

Kinetic analysis of N-acylphosphatidylserine accumulation and implications for membrane assembly in Rhodopseudomonas sphaeroides

The accumulation of N-acylphosphatidylserine (NAPS) in response to the inclusion of Tris in the growth medium of Rhodopseudomonas sphaeroides strain M29-5 has been examined. In the accompanying paper (Donohue et al., J. Bacteriol. 152:000--000, 1982), we show that in response to Tris, NAPS accumulated to as much as 40% of the total cellular phospholipid content. NAPS accumulation began immediately upon addition of Tris and was reflected as an abrupt 12-fold increase in the apparent rate of NAPS accumulation. We suggest that Tris altered the flow of metabolites through a preexisting and previously unknown metabolic pathway. NAPS accumulation ceased immediately upon the removal of Tris; however, accumulated NAPS remained largely metabolically stable. Importantly, under conditions in which NAPS was not accumulated, the intracytoplasmic membrane was shown to be virtually devoid of newly synthesized NAPS. The significance of this observation is discussed in terms of its physiological implications on phospholipid transfer and membrane biogenesis in R. sphaeroides.