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

Natural and azido fatty acids inhibit phosphate transport and activate fatty acid anion uniport mediated by the mitochondrial phosphate carrier

The electroneutral P(i) uptake via the phosphate carrier (PIC) in rat liver and heart mitochondria is inhibited by fatty acids (FAs), by 12-(4-azido-2-nitrophenylamino)dodecanoic acid (AzDA) and heptylbenzoic acid ( approximately 1 microm doses) and by lauric, palmitic, or 12-azidododecanoic acids ( approximately 0.1 mm doses). In turn, reconstituted E. coli-expressed yeast PIC mediated anionic FA uniport with a similar pattern leading to FA cycling and H(+) uniport. The kinetics of P(i)/P(i) exchange on recombinant PIC in the presence of AzDA better corresponded to a competitive inhibition mechanism. Methanephosphonate was identified as a new PIC substrate. Decanephosphonate, butanephosphonate, 4-nitrophenylphosphate, and other P(i) analogs were not translocated and did not inhibit P(i) transport. However, methylenediphosphonate and iminodi(methylenephosphonate) inhibited both electroneutral P(i) uptake and FA cycling via PIC. AzDA analog 16-(4-azido-2-nitrophenylamino)-[(3)H(4)]-hexadecanoic acid ((3)H-AzHA) bound upon photoactivation to several mitochondrial proteins, including the 30- and 34-kDa bands. The latter was ascribed to PIC due to its specific elution pattern on Blue Sepharose and Affi-Gel. (3)H-AzHA photolabeling of recombinant PIC was prevented by methanephosphonate and diphosphonates and after premodification with 4-azido-2-nitrophenylphosphate. Hence, the demonstrated PIC interaction with monovalent long-chain FA anions, but with divalent phosphonates of short chain only, indicates a pattern distinct from that valid for the mitochondrial uncoupling protein-1.

Structure-function relationships in UCP1, UCP2 and chimeras: EPR analysis and retinoic acid activation of UCP2

Uncoupling proteins (UCPs) are composed of three repeated domains of approximately 100 amino acids each. We have used chimeras of UCP1 and UCP2, and electron paramagnetic resonance (EPR), to investigate domain specific properties of these UCPs. Questions include: are the effects of nucleotide binding on proton transport solely mediated by amino acids in the third C-terminal domain, and are the amino acids in the first two domains involved in retinoic or fatty acid activation? We first confirmed that our reconstitution system produced UCP1 that exhibited known properties, such as activation by fatty acids and inhibition of proton transport by purine nucleotides. Our results confirm the observations reported for recombinant yeast that retinoic acid, but not fatty acids known to activate UCP1, activates proton transport by UCP2 and that this activation is insensitive to nucleotide inhibition. We constructed chimeras in which the last domains of UCP1 or UCP2 were switched and tested for activation by fatty acids or retinoic acid and inhibition by nucleotides. U1U2 is composed of mUCP1 (amino acids 1-198) and hUCP2 (amino acids 211-309). Fatty acids activated proton transport of U1U2 and GTP mediated inhibition. In the other chimeric construct U2U1, hUCP2 (amino acids 1-210) and mUCP1 (amino acids 199-307), retinoic acid still acted as an activator, but no inhibition was observed with GTP. Using EPR, a method well suited to the analysis of the structure of membrane proteins such as UCPs, we confirmed that UCP2 binds nucleotides. The EPR data show large structural changes in UCP1 and UCP2 on exposure to ATP, implying that a putative nucleotide-binding site is present on UCP2. EPR analysis also demonstrated changes in conformation of UCP1/UCP2 chimeras following exposure to purine nucleotides. These data demonstrate that a nucleotide-binding site is present in the C-terminal domain of UCP2. This domain was able to inhibit proton transport only when fused to the N-terminal part of UCP1 (chimera U1U2). Thus, residues involved in nucleotide inhibition of proton transport are located in the two first carrier motifs of UCP1. While these results are consistent with previously reported effects of the C-terminal domain on nucleotide binding, they also demonstrate that interactions with the N-terminal domains are necessary to inhibit proton transport. Finally, the results suggest that proteins such as UCP2 may transport protons even though they are not responsible for basal or cold-induced thermogenesis.

The yeast mitochondrial carrier Leu5p and its human homologue Graves' disease protein are required for accumulation of coenzyme A in the matrix

The transport of metabolites, coenzymes, and ions across the mitochondrial inner membrane is still poorly understood. In most cases, membrane transport is facilitated by the so-called mitochondrial carrier proteins. The yeast Saccharomyces cerevisiae contains 35 members of the carrier family, but a function has been identified for only 13 proteins. Here, we investigated the yeast carrier Leu5p (encoded by the gene YHR002w) and its close human homologue Graves' disease protein. Leu5p is inserted into the mitochondrial inner membrane along the specialized import pathway used by carrier proteins. Deletion of LEU5 (strain Deltaleu5) was accompanied by a 15-fold reduction of mitochondrial coenzyme A (CoA) levels but did not affect the cytosolic CoA content. As a consequence, the activities of several mitochondrial CoA-dependent enzymes were strongly decreased in Deltaleu5 cells. Our in vitro and in vivo analyses assign a function to Leu5p in the accumulation of CoA in mitochondria, presumably by serving as a transporter of CoA or a precursor thereof. Expression of the Graves' disease protein in Deltaleu5 cells can replace the function of Leu5p, demonstrating that the human protein represents the orthologue of yeast Leu5p. The function of the human protein might not be directly linked to the disease, as antisera derived from patients with active Graves' disease do not recognize the protein after expression in yeast, suggesting that it does not represent a major autoantigen. The two carrier proteins characterized herein are the first components for which a role in the subcellular distribution of CoA has been identified.

Coenzyme Q is an obligatory cofactor for uncoupling protein function

Uncoupling proteins (UCPs) are thought to be intricately controlled uncouplers that are responsible for the futile dissipation of mitochondrial chemiosmotic gradients, producing heat rather than ATP. They occur in many animal and plant cells and form a subfamily of the mitochondrial carrier family. Physiological uncoupling of oxidative phosphorylation must be strongly regulated to avoid deterioration of the energy supply and cell death, which is caused by toxic uncouplers. However, an H+ transporting uncoupling function is well established only for UCP1 from brown adipose tissue, and the regulation of UCP1 by fatty acids, nucleotides and pH remains controversial. The failure of UCP1 expressed in Escherichia coli inclusion bodies to carry out fatty-acid-dependent H+ transport activity inclusion bodies made us seek a native UCP cofactor. Here we report the identification of coenzyme Q (ubiquinone) as such a cofactor. On addition of CoQ10 to reconstituted UCP1 from inclusion bodies, fatty-acid-dependent H+ transport reached the same rate as with native UCP1. The H+ transport was highly sensitive to purine nucleotides, and activated only by oxidized but not reduced CoQ. H+ transport of native UCP1 correlated with the endogenous CoQ content.

Use of sulfhydryl reagents to investigate branched chain alpha-keto acid transport in mitochondria

The goal of this paper was to determine the contribution of the mitochondrial branched chain aminotransferase (BCATm) to branched chain alpha-keto acid transport within rat heart mitochondria. Isolated heart mitochondria were treated with sulfhydryl reagents of varying permeability, and the data suggest that essential cysteine residues in BCATm are accessible from the cytosolic face of the inner membrane. Treatment with 15 nmol/mg N-ethylmaleimide (NEM) inhibited initial rates of alpha-ketoisocaproate (KIC) uptake in reconstituted mitochondrial detergent extracts by 70% and in the intact organelle by 50%. KIC protected against inhibition suggesting that NEM labeled a cysteine residue that is inaccessible when substrate is bound to the enzyme. Additionally, the apparent mitochondrial equilibrium KIC concentration was decreased 50-60% after NEM labeling, and this difference could not be attributed to effects of NEM on matrix pH or KIC oxidation. In fact, NEM was a better inhibitor of KIC oxidation than rotenone. Measuring matrix aspartate and glutamate levels revealed that the effects of NEM on the steady-state KIC concentration resulted from inhibition of BCATm catalyzed transamination of KIC with matrix glutamate to form leucine. Furthermore, circular dichroism spectra of recombinant human BCATm with liposomes showed that the commercial lipids used in the reconstituted transport assay contain BCAT amino acid substrates. Thus BCATm is distinct from the branched chain alpha-keto acid carrier but may interact with the inner mitochondrial membrane, and it is necessary to inhibit or remove transaminase activity in both intact and reconstituted systems prior to quantifying transport of alpha-keto acids which are transaminase substrates.

Uncoupling proteins 2 and 3 interact with members of the 14.3.3 family

Uncoupling proteins (UCPs) are members of the superfamily of the mitochondrial anion carrier proteins (MATP). Localized in the inner membrane of the organelle, they are postulated to be regulators of mitochondrial uncoupling. UCP2 and 3 may play an important role in the regulation of thermogenesis and, thus, on the resting metabolic rate in humans. To identify interacting proteins that may be involved in the regulation of the activity of UCPs, the yeast two-hybrid system was applied. Segments of hUCP2 containing the hydrophilic loops facing the intermembrane space, or combinations of these, were used to screen an adipocyte activation domain (AD) fusion library. The 14.3.3 protein isoforms theta, beta, zeta were identified as possible interacting partners of hUCP2. Screening of a human skeletal muscle AD fusion library, on the other hand, yielded several clones all of them encoding the gamma isoform of the 14.3.3 family. Mapping experiments further revealed that all these 14.3.3 proteins interact specifically with the C-terminal intermembrane space domain of both hUCP2 and hUCP3 whereas no interactions could be detected with the C-terminal part of hUCP1. Direct interaction between UCP3 and 14.3.3 theta could be demonstrated after in vitro translation by coimmunoprecipitation. When coexpressed in a heterologous yeast system, 14.3.3 proteins potentiated the inhibitory effect of UCP3 overexpression on cell growth. These findings suggest that 14.3.3 proteins could be involved in the targeting of UCPs to the mitochondria.

Oligomerization of serotonin transporter and its functional consequences

Two forms of serotonin transporter (SERT) were prepared with different epitope tags. When co-expressed in HeLa cells, the form containing a FLAG tag (Res-FLAG) was associated with the form containing a c-myc tag (Sens-myc). Antibody against c-myc precipitated Res-FLAG from detergent extracts of cells expressing both forms, but not when Res-FLAG was expressed alone. The specificity of the interaction was demonstrated by the observation that anti-myc antibodies did not precipitate the unrelated vesicular stomatitis virus coat glycoprotein when it was co-expressed with Sens-myc. Sens-myc contained a reactive cysteine at position 172, which reacted with both (2-aminoethyl)methanethiosulfonate and N-biotinylaminoethyl methanethiosulfonate on the surface of intact cells. Sens-myc, but not Res-FLAG, was inactivated by these reagents. When co-expressed with Sens-myc, functionally active Res-FLAG was precipitated by immobilized streptavidin from digitonin-solubilized cells that had been treated with N-biotinylaminoethyl methanethiosulfonate. In cells co-expressing mixtures of Sens-myc and Res-FLAG, the amount of inactivation by (2-aminoethyl)methanethiosulfonate was less than expected if the two forms were independent. The results are consistent with a dimeric form of SERT with functional interactions between subunits, and with association of dimers into a higher order complex, possibly a tetramer.

Oligomerization of serotonin transporter and its functional consequences

Two forms of serotonin transporter (SERT) were prepared with different epitope tags. When co-expressed in HeLa cells, the form containing a FLAG tag (Res-FLAG) was associated with the form containing a c-myc tag (Sens-myc). Antibody against c-myc precipitated Res-FLAG from detergent extracts of cells expressing both forms, but not when Res-FLAG was expressed alone. The specificity of the interaction was demonstrated by the observation that anti-myc antibodies did not precipitate the unrelated vesicular stomatitis virus coat glycoprotein when it was co-expressed with Sens-myc. Sens-myc contained a reactive cysteine at position 172, which reacted with both (2-aminoethyl)methanethiosulfonate and N-biotinylaminoethyl methanethiosulfonate on the surface of intact cells. Sens-myc, but not Res-FLAG, was inactivated by these reagents. When co-expressed with Sens-myc, functionally active Res-FLAG was precipitated by immobilized streptavidin from digitonin-solubilized cells that had been treated with N-biotinylaminoethyl methanethiosulfonate. In cells co-expressing mixtures of Sens-myc and Res-FLAG, the amount of inactivation by (2-aminoethyl)methanethiosulfonate was less than expected if the two forms were independent. The results are consistent with a dimeric form of SERT with functional interactions between subunits, and with association of dimers into a higher order complex, possibly a tetramer.

A covalent tandem dimer of the mitochondrial ADP/ATP carrier is functional in vivo

The adenine nucleotide carrier, or Ancp, is an integral protein of the inner mitochondrial membrane. It is established that the inactive Ancp bound to one of its inhibitors (CATR or BA) is a dimer, but different contradictory models were proposed over the past years to describe the organization of the active Ancp. In order to decide in favor of a single model, it is necessary to establish the orientations of the N- and C-termini and thus the parity of the Ancp transmembrane segments (TMS). According to this, we have constructed a gene encoding a covalent tandem dimer of the Saccharomyces cerevisiae Anc2p and we demonstrate that it is stable and active in vivo as well as in vitro. The properties of the isolated dimer are strongly similar to those of the native Anc2p, as seen from nucleotide exchange and inhibitor binding experiments. We can therefore conclude that the native Anc2p has an even number of TMS and that the N- and C-terminal regions are exposed to the same cellular compartment. Furthermore, our results support the idea of a minimal dimeric functional organization of the Ancp in the mitochondrial membrane and we can suggest that TMS 1 of one monomer and TMS 6 of the other monomer in the native dimer are very close to each other.

Oligomeric state of wild-type and cysteine-less yeast mitochondrial citrate transport proteins

Experiments have been conducted to determine the oligomeric state of the mitochondrial citrate transport protein (CTP) from the yeast Saccharomyces cerevisiae. Both wild-type and cysteine-less (Cys-less) CTPs were overexpressed in E. coli and solubilized with sarkosyl. The purity of the solubilized material is approximately 75%. Upon incorporation into phospholipid vesicles, a high specific transport activity is obtained with both the wild-type and Cys-less CTPs, thereby demonstrating the structural and functional integrity of the preparations. Two independent approaches were utilized to determine native molecular weight. First, CTP molecular weight was determined via nondenaturing size-exclusion chromatography. With this methodology we obtained molecular weight values of 70,961 and 70,118 for the wild-type and Cys-less CTPs, respectively. Second, charge-shift native gel electrophoresis was carried out utilizing a low concentration of the negatively charged detergent sarkosyl, which served to both impart a charge shift to the CTP and the protein standards, as well as to promote protein solubility. Via the second method, we obtained molecular weight values of 69,122 and 74,911 for the wild-type and Cys-less CTPs, respectively. Both methods clearly indicate that following solubilization, the wild-type and the Cys-less CTPs exist exclusively as dimers. Furthermore, disulfide bonds are not required for either dimer formation or stabilization. The dimeric state of the CTP has important implications for the structural basis underlying the CTP translocation mechanism.

Transport function and regulation of mitochondrial uncoupling proteins 2 and 3

Uncoupling protein 1 (UCP1) dissipates energy and generates heat by catalyzing back-flux of protons into the mitochondrial matrix, probably by a fatty acid cycling mechanism. If the newly discovered UCP2 and UCP3 function similarly, they will enhance peripheral energy expenditure and are potential molecular targets for the treatment of obesity. We expressed UCP2 and UCP3 in Escherichia coli and reconstituted the detergent-extracted proteins into liposomes. Ion flux studies show that purified UCP2 and UCP3 behave identically to UCP1. They catalyze electrophoretic flux of protons and alkylsulfonates, and proton flux exhibits an obligatory requirement for fatty acids. Proton flux is inhibited by purine nucleotides but with much lower affinity than observed with UCP1. These findings are consistent with the hypothesis that UCP2 and UCP3 behave as uncoupling proteins in the cell.

Hyperornithinaemia-hyperammonaemia-homocitrullinuria syndrome is caused by mutations in a gene encoding a mitochondrial ornithine transporter

Neurospora crassa ARG13 and Saccharomyces cerevisiae ARG11 encode mitochondrial carrier family (MCF) proteins that transport ornithine across the mitochondrial inner membrane. We used their sequences to identify EST candidates that partially encode orthologous mammalian transporters. We thereby identified such a gene (ORNT1) that maps to 13q14 and whose expression, similar to that of other urea cycle (UC) components, was high in liver and varied with changes in dietary protein. ORNT1 expression restores ornithine metabolism in fibroblasts from patients with hyperammonaemia-hyperornithinaemia-homocitrullinuria (HHH) syndrome. In a survey of 11 HHH probands, we identified 3 ORNT1 mutant alleles that account for 21 of 22 possible mutant ORNT1 genes in our patients: F188delta, which is common in French-Canadian HHH patients and encodes an unstable protein; E180K, which encodes a stable, properly targeted protein that is inactive; and a 13q14 microdeletion. Our results show that ORNT1 encodes the mitochondrial ornithine transporter involved in UC function and is defective in HHH syndrome.

Replacements of basic and hydroxyl amino acids identify structurally and functionally sensitive regions of the mitochondrial phosphate transport protein

The mitochondrial phosphate transport protein (PTP) from the yeast Saccharomyces cerevisiae has been expressed in Escherichia coli, purified, and reconstituted. Basic and hydroxyl residues were replaced to identify structurally and functionally important regions in the protein. Physiologically relevant unidirectional transport from extraliposomal (cytosol) pH 6.8 to intraliposomal (matrix) pH 8.0 was assayed. Replacements that affect transport most dramatically are at Lys42 (matrix end of helix A), Thr79 (helix B), Lys90 (cytosol end of helix B), Arg140 and Arg142 (matrix end of helix C), Lys179 and Lys187 (helix D), Ser232 (helix E), and Arg276 (helix F). The deleterious nature of these mutations was confirmed by the observation that the yeast PTP null mutant transformed with any one of these mutant genes cannot grow or has difficulties growing with glycerol as the primary carbon source. More than 90% of transport activity can be blocked by various mutations without affecting growth on glycerol. Alterations in the structure of the transport protein caused by the mutations were characterized by determining the fraction of PTP incorporated into liposomes during reconstitution. The incorporation of all PTPs (wild type and mutant) into liposomes is 15.5 +/- 8.4 ng of PTP/25 microL and fairly independent of the amount of PTP in the initial reconstitution mix (49-212 ng of PTP/25 microL). Arg159Ala and Lys295Gln show the smallest incorporation of 2.3 +/- 1.6 ng of PTP/25 microL and 2.6 +/- 0.2 ng of PTP/25 microL, respectively. Ser145Ala shows the largest incorporation of 37.0 ng of PTP/25 microL. These three mutants show near wild-type reconstituted transport activity. Two of these three mutations are located in the loop connecting the matrix ends of helices C and D, Ser145 at its N-terminal (the matrix end of helix C) and Arg159 near its center. Lys295 is located at the C-terminal of PTP beyond helix F. These results, together with those from other mutations, suggest that like helix A, the protein segment consisting of the loop connecting helices C and D and helix D as well as the C-terminal of PTP beyond helix F faces the subunit interface of this homodimer. The role of the replacement-sensitive residues in the phosphate or in the coupled proton transport path is discussed.

Membrane topology of the mammalian CMP-sialic acid transporter

Nucleotide sugar transporters form a family of distantly related membrane proteins of the Golgi apparatus and the endoplasmic reticulum. The first transporter sequences have been identified within the last 2 years. However, information about the secondary and tertiary structure for these molecules has been limited to theoretical considerations. In the present study, an epitope-insertion approach was used to investigate the membrane topology of the CMP-sialic acid transporter. Immunofluorescence studies were carried out to analyze the orientation of the introduced epitopes in semipermeabilized cells. Both an amino-terminally introduced FLAG sequence and a carboxyl-terminal hemagglutinin tag were found to be oriented toward the cytosol. Results obtained with CMP-sialic acid transporter variants that contained the hemagglutinin epitope in potential intermembrane loop structures were in good correlation with the presence of 10 transmembrane regions. This building concept seems to be preserved also in other mammalian and nonmammalian nucleotide sugar transporters. Moreover, the functional analysis of the generated mutants demonstrated that insertions in or very close to membrane-spanning regions inactivate the transport process, whereas those in hydrophilic loop structures have no detectable effect on the activity. This study points the way toward understanding structure-function relationships of nucleotide sugar transporters.

The mitochondrial dicarboxylate carrier is essential for the growth of Saccharomyces cerevisiae on ethanol or acetate as the sole carbon source

The dicarboxylate carrier (DIC) is an integral membrane protein that catalyses a dicarboxylate-phosphate exchange across the inner mitochondrial membrane. We generated a yeast mutant lacking the gene for the DIC. The deletion mutant failed to grow on acetate or ethanol as sole carbon source but was viable on glucose, galactose, pyruvate, lactate and glycerol. The growth on ethanol or acetate was largely restored by the addition of low concentrations of aspartate, glutamate, fumarate, citrate, oxoglutarate, oxaloacetate and glucose, but not of succinate, leucine and lysine. The expression of the DIC gene in wild-type yeast was repressed in media containing ethanol or acetate with or without glycerol. These results indicate that the primary function of DIC is to transport cytoplasmic dicarboxylates into the mitochondrial matrix rather than to direct carbon flux to gluconeogenesis by exporting malate from the mitochondria. The delta DIC mutant may serve as a convenient host for overexpression of DIC and for the demonstration of its correct targeting and assembly.