Translocation of loops regulates transport activity of mitochondrial ADP/ATP carrier deduced from formation of a specific intermolecular disulfide bridge catalyzed by copper-o-phenanthroline

Role of critical thiol groups on the matrix surface of the adenine nucleotide translocase in the mechanism of the mitochondrial permeability transition pore

Opening of the mitochondrial permeability transition pore (MPTP) is sensitized to [Ca(2+)] by oxidative stress (diamide) and phenylarsine oxide (PAO). We have proposed that both agents cross-link two thiol groups on the adenine nucleotide translocase (ANT) involved in ADP and cyclophilin-D (CyP-D) binding. Here, we demonstrate that blocking Cys(160) with 80 microM eosin 5-maleimide (EMA) or 500 microM N-ethylmaleimide (NEM) greatly decreased ADP inhibition of the MPTP. The ability of diamide, but not PAO, to block ADP inhibition of the MPTP was antagonized by treatment of mitochondria with 50 microM NEM to alkylate matrix glutathione. Binding of detergent-solubilized ANT to a PAO-affinity matrix was prevented by pre-treatment of mitochondria with diamide, EMA or PAO, but not NEM. EMA binding to the ANT in submitochondrial particles (SMPs) was prevented by pre-treatment of mitochondria with either PAO or diamide, implying that both agents modify Cys(160). Diamide and PAO pre-treatments also inhibited binding of solubilized ANT to a glutathione S-transferase-CyP-D affinity column, both effects being blocked by 100 microM EMA. Intermolecular cross-linking of adjacent ANT molecules via Cys(57) by copper phenanthroline treatment of SMPs was abolished by pre-treatment of mitochondria with diamide and PAO, but not with EMA. Our data suggest that PAO and diamide cause intramolecular cross-linking between Cys(160) and Cys(257) directly (not antagonized by 50 microM NEM) or using glutathione (antagonized by 50 microM NEM) respectively. This cross-linking stabilizes the "c" conformation of the ANT, reducing the reactivity of Cys(57), while enhancing CyP-D binding to the ANT and antagonizing ADP binding. The two effects together greatly sensitize the MPTP to [Ca(2+)].

A novel adenine nucleotide translocase inhibitor, MT-21, induces cytochrome c release by a mitochondrial permeability transition-independent mechanism

The release of cytochrome c from mitochondria is a critical step during apoptosis. In order to study this process, we have used a synthetic compound, MT-21, that is able to initiate release of cytochrome c from isolated mitochondria. We demonstrate that MT-21 significantly inhibits ADP transport activity in mitochondria and reduces binding of the adenine nucleotide translocase (ANT) to a phenylarsine oxide affinity matrix. These results suggest that ANT, one of the components of the mitochondrial permeability transition (PT) pore, is the molecular target for MT-21. In agreement with this, the MT-21-induced cytochrome c release was effectively inhibited in the presence of ANT ligands, and MT-21 could dissociate ANT from a complex with a glutathione S-transferase-cyclophilin D fusion protein. Interestingly, we also found that specific inhibitors of ANT such as MT-21 and atractyloside could induce cytochrome c release without mitochondrial swelling and that this event was highly dependent on the presence of Mg(2+). These results suggest that although ANT resides in the mitochondrial inner membrane, specific ANT inhibitors can induce cytochrome c release without having an effect on inner membrane permeability. Therefore, MT-21 can be a powerful tool for studying the mechanism of PT-independent cytochrome c release from mitochondria.

Epitope mapping of mitochondrial adenine nucleotide translocase-1 in idiopathic dilated cardiomyopathy

Mitochondrial adenine nucleotide translocase (ANT) is a specific target for the autoantibody response in idiopathic dilated cardiomyopathy (IDCM). We have undertaken an epitope analysis of ANT in IDCM by immunoblot with recombinant GST-ANT fusion proteins and with cellulose-bound decapeptides of human ANT1. Forty-five patients with IDCM, 17 patients with ischemic left ventricle dysfunction (LVD) and 20 controls were analyzed for circulating antibodies against ANT (AAb-ANT). Sixteen of the 45 (36%) IDCM patients showed AAb-ANT above controls. In immunoblots, AAb-ANT detected purified bovine heart ANT and GST-ANT1 and GST-ANT2 isoforms and, less frequently, the GST-ANT3 isoform. A construct lacking the last 146 amino acids did not react with AAb-ANT, indicating that the main epitopes are in the C-terminal 146 amino acids. Immunodetection of decapeptides covering this region shows that AAb-ANT detects at least three epitopes, demonstrating that ANT is the primary target of AAb-ANT. The most significant epitopes belong to the M2 and M3 hydrophilic loops of ANT suggesting that apart from being essential for its activity, these loops are highly immunogenic.

How does the mitochondrial ADP/ATP carrier distinguish transportable ATP and ADP from untransportable AMP and GTP?Dynamic modeling of the recognition/translocation process in the major substrate binding region

To understand the transport mechanism of the bovine heart mitochondrial ADP/ATP carrier at the atomic level, we studied the four-dimensional features of the interaction of various purine nucleotides with the adenine nucleotide binding region (ABR) consisting of Arg(151)-Asp(167) in the second loop facing the matrix side. After three-dimensional modeling of ABR based on the experimental results, its structural changes on interaction with purine nucleotides were examined by molecular dynamics computation at 300 K. ATP/ADP were translocated to a considerable degree from the matrix side to the inner membrane region accompanied by significant backbone conformational changes, whereas neither appreciable translocation nor a significant conformational change was observed with the untransportable nucleotides AMP/GTP. The results suggested that binding of the terminal phosphate group and the adenine ring of ATP/ADP with Arg(151) and Lys(162), respectively, and subsequent interaction of a phosphate group(s) other than the terminal phosphate with Lys(162) triggered the expansion and subsequent contraction of the backbone conformation of ABR, leading to the translocation of ATP/ADP. Based on a simplified molecular dynamic simulation, we propose a dynamic model for the initial recognition process of ATP/ADP with the carrier.

Structure and association of ATP-binding cassette transporter nucleotide-binding domains

ATP-binding cassette transporters are responsible for the uptake and efflux of a multitude of substances across both eukaryotic and prokaryotic membranes. Members of this family of proteins are involved in diverse physiological processes including antigen presentation, drug efflux from cancer cells, bacterial nutrient uptake and cystic fibrosis. In order to understand more completely the role of these multidomain transporters an integrated approach combining structural, pharmacological and biochemical methods is being adopted. Recent structural data have been obtained on the cytoplasmic, nucleotide-binding domains of prokaryotic ABC transporters. This review evaluates both these data and the conflicting implications they have for domain communication in ABC transporters. Areas of biochemical research that attempt to resolve these conflicts will be discussed.

The permeability transition pore complex: another view

Mitochondria play a critical role in initiating both apoptotic and necrotic cell death. A major player in this process is the mitochondrial permeability transition pore (MPTP), a non-specific pore, permeant to any molecule of < 1.5 kDa, that opens in the inner mitochondrial membrane under conditions of elevated matrix [Ca(2+)], especially when this is accompanied by oxidative stress and depleted adenine nucleotides. Opening of the MPTP causes massive swelling of mitochondria, rupture of the outer membrane and release of intermembrane components that induce apoptosis. In addition mitochondria become depolarised causing inhibition of oxidative phosphorylation and stimulation of ATP hydrolysis. Pore opening is inhibited by cyclosporin A analogues with the same affinity as they inhibit the peptidyl-prolyl cis-trans isomerase activity of mitochondrial cyclophilin (CyP-D). These data and the observation that different ligands of the adenine nucleotide translocase (ANT) can either stimulate or inhibit pore opening led to the proposal that the MPTP is formed by a Ca-triggered conformational change of the ANT that is facilitated by the binding of CyP-D. Our model is able to explain the mode of action of a wide range of known modulators of the MPTP that exert their effects by changing the binding affinity of the ANT for CyP-D, Ca(2+) or adenine nucleotides. The extensive evidence for this model from our own and other laboratories is presented, including reconstitution studies that demonstrate the minimum configuration of the MPTP to require neither the voltage activated anion channel (VDAC or porin) nor any other outer membrane protein. However, other proteins including Bcl-2, BAX and virus-derived proteins may interact with the ANT to regulate the MPTP. Recent data suggest that oxidative cross-linking of two matrix facing cysteine residues on the ANT (Cys(56) and Cys(159)) plays a key role in regulating the MPTP. Adenine nucleotide binding to the ANT is inhibited by Cys(159) modification whilst oxidation of Cys(56) increases CyP-D binding to the ANT, probably at Pro(61).

Chimers of two fused ADP/ATP carrier monomers indicate a single channel for ADP/ATP transport

The mitochondrial ADP/ATP carrier (AAC) is generally believed to function as a homodimer (Wt. Wt). It remains unknown whether the two monomers possess two independent but fully anticooperative channels or they form a single central channel for nucleotide transport. Here we generated fusion proteins consisting of two tandem covalent-linked AAC monomers and studied the kinetics of ADP/ATP transport in reconstituted proteoliposomes. Functional 64-kDa fusion proteins Wt-Wt and Wt-R294A (wild-type AAC linked to a mutant having low ATP transport activity) were expressed in mitochondria of yeast transformants. Compared to homodimer Wt. Wt, the fusion protein Wt-Wt retained the transport activity and selectivity of ADP versus ATP. The strongly divergent selectivities of Wt and R294A were partially propagated in the Wt-R294A fusion protein, suggesting a limited cooperativity during solute translocation. The rates of ADP or ATP transport were significantly higher than those predicted by the two-channel model. Fusion proteins for Wt-R204L (Wt linked to an inactive mutant) and R204L-Wt were not expressed in aerobically grown yeast cells, which contained plasmid rearrangements that regenerated the fully active 32-kDa homodimer Wt. Wt, suggesting that these fusion proteins are inactive in ADP/ATP transport. These results favor a single binding center gated pore model [Klingenberg, M. (1991) in A Study of Enzymes, Vol. 2: pp. 367-388] in which two AAC subunits cooperate for a coordinated ADP/ATP exchange through a single channel.

Characterization of loops of the yeast mitochondrial ADP/ATP carrier facing the cytosol by site-directed mutagenesis

To characterize structural features of the regions of the yeast type 2 ADP/ATP carrier (yAAC2) facing the cytosol, we prepared its Cys-less mutant, in which all four cysteine residues were replaced by alanine residues. The Cys-less mutant functioned like native yAAC2, showing that the cysteine residues are not essential. We then prepared cysteine mutants by substituting Ser(21) in the putative N-terminal region, Ala(124) and Ser(222) in the first and second loops facing cytosol, respectively, and Leu(312) in the C-terminal region of the Cys-less mutant for cysteine and examined the labeling of the substituted cysteine residues of the mutants with the membrane-impermeable SH reagent eosin-5-maleimide (EMA) from the cytosol. EMA labeled all the mutants, showing that all regions containing mutated residues faced the cytosolic side. The effects of transport inhibitors on EMA labeling were also examined. From the results, the location and conformation of the region around mutated residues were discussed.

Tributyltin interacts with mitochondria and induces cytochrome c release

Although triorganotins are potent inducers of apoptosis in various cell types, the critical targets of these compounds and the mechanisms by which they lead to cell death remain to be elucidated. There are two major pathways by which apoptotic cell death occurs: one is triggered by a cytokine mediator and the other is by a mitochondrion-dependent mechanism. To elucidate the mechanism of triorganotin-induced apoptosis, we studied the effect of tributyltin on mitochondrial function. We found that moderately low doses of tributyltin decrease mitochondrial membrane potential and induce cytochrome c release by a mechanism inhibited by cyclosporine A and bongkrekic acid. Tributyltin-induced cytochrome c release is also prevented by dithiols such as dithiothreitol and 2,3-dimercaptopropanol but not by monothiols such as GSH, N-acetyl-L-cysteine, L-cysteine and 2-mercaptoethanol. Further studies with phenylarsine oxide agarose revealed that tributyltin interacts with the adenine nucleotide translocator, a functional constituent of the mitochondrial permeability transition pore, which is selectively inhibited by dithiothreitol. These results suggest that, at low doses, tributyltin interacts selectively with critical thiol residues in the adenine nucleotide translocator and opens the permeability transition pore, thereby decreasing membrane potential and releasing cytochrome c from mitochondria, a series of events consistent with established mechanistic models of apoptosis.

Specific labeling of the bovine heart mitochondrial phosphate carrier with fluorescein 5-isothiocyanate: roles of Lys185 and putative adenine nucleotide recognition site in phosphate transport

The amine/SH-modifying fluorescein 5-isothiocyanate (FITC) specifically labeled Lys(185) in the putative membrane-spanning region of the phosphate carrier from both the cytosolic and matrix sides of bovine heart mitochondria at 0 degrees C and pH 7.2, and the labeling inhibited the phosphate transport. Nonmodifying fluorescein derivatives having similar structural features to those of ADP and ATP (Majima, E., Yamaguchi, N., Chuman, H., Shinohara, Y., Ishida, M., Goto, S., and Terada, H. (1998) Biochemistry 37, 424-432) inhibited the specific FITC labeling and phosphate transport, but the nonfluorescein phenylisothiocyanate did not inhibit FITC labeling, suggesting that there is a region recognizing the adenine nucleotides in the phosphate carrier and that this region is closely associated with the transport activity. The phosphate transport inhibitor pyridoxal 5'-phosphate inhibited the specific FITC labeling, possibly due to competitive modification of Lys(185). In addition, FITC inhibited the ADP transport and specific labeling of the ADP/ATP carrier with the fluorescein SH reagent eosin 5-maleimide. Based on these results, we discuss the structural features of the phosphate carrier in relation to its transport activity.

Possible involvement of the adenine nucleotide translocase in the activation of the permeability transition pore induced by cadmium

Low levels of cadmium induce a rapid calcium efflux in energized rat kidney mitochondria. This is accompanied by the collapse of the transmembrane gradient in a partial CSA-sensitive fashion. The binding of 109Cd2+ to mitochondria is a saturable function; in the presence of NEM, the binding of 2.5 nmol 109Cd2+/mg of protein suffices to induce the opening of the permeability transition pore. It was found that cadmium bound mainly to proteins of molecular weight between 30 and 50 kDa. In the presence of the monothiol reagent NEM, the label is concentrated in the 30 kDa protein. Following the addition of the reducing agent dithiothreitol, calcium is reaccumulated and the membrane potential restored. This correlates with a significant loss of label in the 30 kDa protein region. The 30 kDa protein was identified as the adenine nucleotide translocase by labelling experiments with eosin 5-maleimide and experiments of reconstitution.

Copper sensitizes the mitochondrial permeability transition to carboxytractyloside and oleate

Addition of 5 microM copper to rat kidney mitochondria enhances the effect of carboxyatractyloside and oleate on pore opening, in a cyclosporin A-sensitive fashion. The effects of the pair copper-carboxyatractyloside were observed on matrix Ca2+ efflux, mitochondrial swelling and on the transmembrane electric gradient. The effect of Cu2+ emphasizes the importance of membrane thiol groups located, probably, in the ADP/ATP translocase (ANT), on permeability transition. It was also found that Cu2+ does not block the fluorescent label of ANT by eosin 5-maleimide, but abolishes the inhibition by CAT on the labeling. This suggests that the binding of Cu2+ to cysteine residues of ANT promotes a conformational change in the carrier, strengthening the effect of CAT and oleate on membrane leakage.

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.

Mitochondrial permeability transition induced by 1-hydroxyethyl radical

Impairment of mitochondrial functions has been found in ethanol-induced liver injury. Ethanol can be oxidized to the 1-hydroxyethyl radical (HER) by rat liver microsomal systems. Experiments were carried out to evaluate the ability of HER to cause mitochondrial swelling as an indicator of the mitochondrial permeability transition (MPT). Electron spin resonance (ESR) spectroscopy was used to detect HER and to study its interaction with mitochondria. The ESR signal intensity of the spin adduct formed from alpha-(4-pyridyl-1-oxide) N-tert-butylnitrone (POBN) and HER generated from either a thermic decomposition of 1,1'-dihydroxyazoethane (DHAE) or a Fenton reaction system containing ethanol was markedly diminished by the addition of mitochondria, indicating an interaction between HER and mitochondria. Exposure of rat liver mitochondria to HER generated from either system caused swelling, as reflected by a decrease in absorbance at 540 nm, in a HER concentration-dependent and a cyclosporin A-sensitive manner. Mitochondrial swelling was also induced in the Fenton reaction system without ethanol. The DHAE-dependent generation of HER in mitochondrial suspension resulted in a decrease of membrane protein thiols and collapse of the membrane potential (delta psi). The swelling induced by HER was prevented by glutathione and vitamin E, but not by superoxide dismutase. Catalase did not prevent the swelling caused by the acetaldehyde/hydroxylamine O-sulfonate (HOS) system, but was inhibitory in the Fenton reaction system with or without ethanol. These results indicate that HER, as well as hydroxyl radical, can induce the MPT, and suggest the possibility that the collapse of delta psi caused by HER may, at least in part, contribute to impairment of mitochondrial function caused by ethanol and in ethanol-induced liver injury.

Oxidation of a critical thiol residue of the adenine nucleotide translocator enforces Bcl-2-independent permeability transition pore opening and apoptosis

Mitochondrial membrane permeabilization is a critical event in the process leading to physiological or chemotherapy-induced apoptosis. This permeabilization event is at least in part under the control of the permeability transition pore complex (PTPC), which interacts with oncoproteins from the Bcl-2 family as well as with tumor suppressor proteins from the Bax family, which inhibit or facilitate membrane permeabilization, respectively. Here we show that thiol crosslinking agents including diazenedicarboxylic acid bis 5N, N-dimethylamide (diamide), dithiodipyridine (DTDP), or bis-maleimido-hexane (BMH) can act on the adenine nucleotide translocator (ANT), one of the proteins within the PTPC. ANT alone reconstituted into artificial lipid bilayers suffices to confer a membrane permeabilization response to thiol crosslinking agents. Diamide, DTDP, and BMH but not tert-butylhydroperoxide or arsenite cause the oxidation of a critical cysteine residue (Cys 56) of ANT. Thiol modification within ANT is observed in intact cells, isolated mitochondria, and purified ANT. Recombinant Bcl-2 fails to prevent thiol modification of ANT. Concomitantly, a series of different thiol crosslinking agents (diamide, DTDP, and BMH, phenylarsine oxide) but not tert-butylhydroperoxide or arsenite induce mitochondrial membrane permeabilization and cell death irrespective of the expression level of Bcl-2. These data indicate that thiol crosslinkers cause a covalent modification of ANT which, beyond any control by Bcl-2, leads to mitochondrial membrane permeabilization and cell death.

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.

Mitochondrial transport of cations: channels, exchangers, and permeability transition

This review provides a selective history of how studies of mitochondrial cation transport (K+, Na+, Ca2+) developed in relation to the major themes of research in bioenergetics. It then covers in some detail specific transport pathways for these cations, and it introduces and discusses open problems about their nature and physiological function, particularly in relation to volume regulation and Ca2+ homeostasis. The review should provide the basic elements needed to understand both earlier mitochondrial literature and current problems associated with mitochondrial transport of cations and hopefully will foster new interest in the molecular definition of mitochondrial cation channels and exchangers as well as their roles in cell physiology.

Structural and functional study of a conserved region in the uncoupling protein UCP1: the three matrix loops are involved in the control of transport

It has been reported that the region 261-269 of the uncoupling protein from brown adipose tissue mitochondria, UCP1, has an important role in the control of its proton translocating activity. Thus the deletion of residues Phe267-Lys268-Gly269 leads to the loss of the nucleotide regulation of the protein, while the complete deletion of the segment leads to the formation of a pore. The region displays sequence homology with the DNA-binding domain of the estrogen receptor. The present report analyzes the structure, by NMR and circular dichroism, of a 20 amino acid residue peptide containing the residues of interest. We demonstrate that residues 263-268 adopt an alpha-helical structure. The helix is at the N-terminal end of the sixth transmembrane domain. The functional significance of this helix has been examined by site-directed mutagenesis of the protein expressed recombinantly in yeasts. Alterations in the structure or orientation of the region leads to an impairment of the regulation, by nucleotides and fatty acids, of the transport activity. UCP1 is one member of the family formed by the carriers of the mitochondrial inner membrane. The family is characterized by a tripartite structure with three repeated segments of about 100 amino acid residues. Two of the mutations have also been performed in the first and second matrix loops and the effect on UCP1 function is very similar. We conclude that the three matrix loops contribute to the formation of the gating domain in UCP1 and propose that they form a hydrophobic pocket that accommodates the purine moiety of the bound nucleotide.

Functional expression of the tandem-repeated homodimer of the mitochondrial ADP/ATP carrier in Saccharomyces cerevisiae

The mitochondrial ADP/ATP carrier (AAC) is believed to function as a dimer. To characterize the oligomeric state of the yeast type 2 AAC (yAAC2), we tried to express its tandem-repeated homodimer, in which the C-terminus of the first repeat was fused to the N-terminus of the second repeat, in yeast mitochondria. The tandem dimer was expressed in the mitochondrial membrane at the same level as that of yAAC2, being inserted into the mitochondrial membrane as in yAAC2, and it showed very similar transport activity to that of yAAC2. It was suggested that the two carrier molecules in a dimeric form are located in the membrane facing each other in the same orientation.

The isocitrate dehydrogenase kinase/phosphatase from Escherichia coli is highly sensitive to in-vitro oxidative conditions role of cysteine67 and cysteine108 in the formation of a disulfide-bonded homodimer

Isocitrate dehydrogenase kinase/phosphatase (IDHK/P) is a homodimeric enzyme which controls the oxidative metabolism of Escherichia coli, and exibits a high intrinsic ATPase activity. When subjected to electrophoresis under nonreducing conditions, the purified enzyme migrates partially as a dimer. The proportion of the dimer over the monomer is greatly increased by treatment with cupric 1,10 phenanthrolinate or 5,5'-dithio-bis(2-nitrobenzoic acid), and fully reversed by dithiothreitol, indicating that covalent dimerization is produced by a disulfide bond. To identify the residue(s) involved in this intermolecular disulfide-bond, each of the eight cysteines of the enzyme was individually mutated into a serine. It was found that, under nonreducing conditions, the electrophoretic patterns of all corresponding mutants are identical to that of the wild-type, except for the Cys67-->Ser which migrates exclusively as a monomer and for the Cys108-->Ser which migrates preferentially as a dimer. Furthermore, in contrast to the wild-type enzyme and all the other mutants, the Cys67-->Ser mutant still migrates as a monomer after treatment with cupric 1,10 phenanthrolinate. This result indicates that the intermolecular disulfide bond involves only Cys67 in each IDHK/P wild-type monomer. This was further supported by mass spectrum analysis of the tryptic peptides derived from either the cupric 1,10 phenanthrolinate-treated wild-type enzyme or the native Cys108-->Ser mutant, which show that they both contain a Cys67-Cys67 disulfide bond. Moreover, both the cupric 1,10 phenanthrolinate-treated wild-type enzyme and the native Cys108-->Ser mutant contain another disulfide bond between Cys356 and Cys480. Previous results have shown that this additional Cys356-Cys480 disulfide bond is intramolecular [Oudot, C., Jault, J.-M., Jaquinod, M., Negre, D., Prost, J.-F., Cozzone, A.J. & Cortay, J.-C. (1998) Eur. J. Biochem. 258, 579-585].

Fluctuation of the first loop facing the matrix of the mitochondrial ADP/ATP carrier deduced from intermolecular cross-linking of Cys56 residues by bifunctional dimaleimides

The effects of six thiol-specific cross-linker dimaleimides, in which the distance of the two maleimide groups ranged from 7.7 to 16. 8 A, on bovine heart mitochondria were studied at pH 7.2 and 0 degrees C. None of the dimaleimides affected mitochondrial proteins, but they caused significantly specific intermolecular cross-linking of the 30 kDa ADP/ATP carrier in submitochondrial particles. All the cross-links were found to be formed specifically between two Cys56 residues in the first loop facing the matrix, as we observed previously in intermolecular disulfide bridge formation catalyzed by copper o-phenanthroline [Majima, E., Ikawa, K., Takeda, M., Hashimoto, M., Shinohara, Y., and Terada, H. (1995) J.Biol. Chem. 270, 29548-29554]. The dimerization was dependent on the cross-linking span of the dimaleimides, being maximum with the dimaleimide having a span of about 12 A. Cross-linking took place in the m-state carrier, but not in the c-state carrier, and inhibited ADP transport via the ADP/ATP carrier. We suggest that a pair of first loops with Cys56 residues in the dimer form of the m-state carrier fluctuates widely with a most probable distance between them of about 12 A, and that this fluctuation modulates the transport activity of the ADP/ATP carrier.

Expression of the bovine heart mitochondrial ADP/ATP carrier in yeast mitochondria: significantly enhanced expression by replacement of the N-terminal region of the bovine carrier by the corresponding regions of the yeast carriers

To characterize the transport mechanism mediated by the mammalian mitochondrial ADP/ATP carrier (AAC), we tried to express bovine heart mitochondrial AAC (bhAAC) in Saccharomyces cerevisiae. The open reading frame of the bhAAC was introduced into the haploid strain WB-12, in which intrinsic AAC genes were disrupted. Growth of the transformant was very low in glycerol medium, and a little amount of bhAAC was detected in the mitochondrial membrane. For improvement of bhAAC expression in WB-12, we introduced DNA fragments encoding chimeric bhAACs, in which the N-terminal region of the bhAAC extending into the cytosol was replaced by the corresponding regions of the type 1 and type 2 yeast AAC isoforms (yAAC1 and yAAC2). These transformants grew well, and the amounts of the chimeric bhAACs in their mitochondria were as high as that of yAAC2. The carriers expressed showed essentially the same ADP transport activities as that of AAC in bovine heart mitochondria.

Elucidating the molecular mechanism of the permeability transition pore and its role in reperfusion injury of the heart

First, we present a summary of the evidence for our model of the molecular mechanism of the permeability transition (MPT). Our proposal is that the MPT occurs as a result of the binding of mitochondrial cyclophilin (CyP-D) to the adenine nucleotide translocase (ANT) in the inner mitochondrial membrane. This binding is enhanced by thiol modification of the ANT caused by oxidative stress or other thiol reagents. CyP-D binding enhances the ability of the ANT to undergo a conformational change triggered by Ca2+. Binding of ADP or ATP to a matrix site of the ANT antagonises this effect of Ca2+; modification of other ANT thiol groups inhibits ADP binding and sensitises the MPT to [Ca2+]. Increased membrane potential changes the ANT conformation to enhance ATP binding and hence inhibit the MPT. Our most recent data shows that a fusion protein of CyP-D and glutathione-S-transferase immobilised to Sepharose specifically binds the ANT from Triton-solubilised inner mitochondrial membranes in a cyclosporin A (CsA) sensitive manner. Second we summarise the evidence for the MPT being a major factor in the transition from reversible to irreversible injury during reperfusion of a heart following a period of ischaemia. We describe how in the perfused heart [3H]deoxyglucose entrapment within mitochondria can be used to measure the opening of MPT pore in situ. During ischaemia pore opening does not occur, but significant opening does occur during reperfusion, and recovery of the heart is dependent on subsequent pore closure. Pore opening is inhibited by the presence in the perfusion medium of pyruvate and the anaesthetic propofol which both protect the heart from reperfusion injury. Third we discuss how the MPT may be involved in determining whether cell death occurs by necrosis (extensive pore opening and ATP depletion) or apoptosis (transient pore opening with maintenance of ATP).

Induction of the mitochondrial permeability transition by N-ethylmaleimide depends on secondary oxidation of critical thiol groups. Potentiation by copper-ortho-phenanthroline without dimerization of the adenine nucleotide translocase

Addition to energized rat liver mitochondria of low micromolar concentrations of the thiol oxidant, copper-o-phenanthroline [Cu(OP)2], causes opening of the permeability transition pore, a cyclosporin A-sensitive channel. The effects of Cu(OP)2 can be reversed by reduction with dithiothreitol (DTT), suggesting that a dithiol-disulfide interconversion is involved. However, at variance with all pore inducers known to act through dithiol oxidation, the effects of Cu(OP)2 are not prevented by treatment of mitochondria with low (10-20 microM) concentrations of N-ethylmaleimide (NEM). Rather, these concentrations of NEM potentiate the inducing effects of Cu(OP)2. We show that this enhancing effect of NEM is blocked by the subsequent addition of DTT, indicating that potentiation by NEM is mediated by an oxidative event rather than by substitution as such. We find that also pore induction by high (0.5-1.0 mM) concentrations of NEM in the absence of oxidants is completely blocked by reduction with DTT or beta-mercaptoethanol. These results underscore the unexpected importance of oxidative events in pore opening by substituting agents. Since we find that pore opening by Cu(OP)2 or by high concentrations of NEM is not accompanied by dimerization of the adenine nucleotide translocase, we conclude that the translocase itself is not the target of the pore-inducing oxidative events triggered by Cu(OP)2 and NEM.

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.

Mitochondrial permeability transition as induced by cross-linking of the adenine nucleotide translocase

Mitochondrial permeability transition is caused by the opening of a transmembrane pore whose chemical nature has not been well established yet. The present work was aimed to further contribute to the knowledge of the membrane entity comprised in the formation of the non-specific channel. The increased permeability was established by analyzing the inability of rat kidney mitochondria to take up and accumulate Ca2+, as well as their failure to build up a transmembrane potential, after the cross-linking of membrane proteins by copper plus ortho-phenanthroline. To identify the cross-linked proteins, polyacrylamide gel electrophoresis was performed. The results are representative of at least three separate experiments. It is indicated that 30 microM Cu2+ induced the release of 4.3 nmol Ca2+ per mg protein. However, in the presence of 100 microM ortho-phenanthroline only 2 microM Cu2+ was required to attain the total release of the accumulated Ca2+; it should be noted that such a reaction is not inhibited by cyclosporin. The increased permeability corresponds to cross-linking of membrane proteins in which approximately 4 nmol thiol groups per mg protein appear to be involved. Such a linking process is inhibited by carboxyatractyloside. By using the fluorescent probe eosin-5-maleimide the label was found in a cross-linking 60 kDa dimer of two 30 kDa monomers. From the data presented it is concluded that copper-o-phenanthroline induces the intermolecular cross-linking of the adenine nucleotide translocase which in turn is converted to non-specific pore.

Binding of the fluorescein derivative eosin Y to the mitochondrial ADP/ATP carrier: characterization of the adenine nucleotide binding site

As the SH-reactive fluorescein derivative eosin-5-maleimide (EMA) specifically labels Cys159 in the second loop facing the matrix space (loop M2) of the ADP/ATP carrier in bovine heart submitochondrial particles [Majima, E., Koike, H., Hong, Y.-M., Shinohara, Y., and Terada, H. (1993) J. Biol. Chem. 268, 22181-22187], we studied the interaction of non-SH-reactive eosin Y, an analog of EMA, with the carrier under various conditions to characterize its binding. Eosin Y was found to inhibit ADP transport by binding to loop M2 in submitochondrial particles, but not in mitochondria. Its Ki for transport (0.33 microM) was found to be very similar to its Kd (0.53 microM) for specific binding to the carrier. Bound eosin Y was displaced by the transport substrates ADP and ATP, but not by untransportable GTP, suggesting that eosin Y bound to the specific binding site of ADP and ATP. The three-dimensional structure and electrostatic features of eosin Y were very similar to those of ADP, and the hydrophobic property and divalent charge of eosin Y were very important for its binding to the carrier. Based on these results, the features of the binding site of the transport substrates are considered.

The structure and function of the brown fat uncoupling protein UCP1: current status

The uncoupling protein of brown adipose tissue (UCP1) is a transporter that allows the dissipation as heat of the proton gradient generated by the respiratory chain. The discovery of new UCPs in other mammalian tissues and even in plants suggests that the proton permeability of the mitochondrial inner membrane can be regulated and its control is exerted by specialised proteins. The UCP1 is regulated both at the gene and the mitochondrial level to ensure a high thermogenic capacity to the tissue. The members of the mitochondrial transporter family, which includes the UCPs, present two behaviours with carrier and channel transport modes. It has been proposed that this property reflects a functional organization in two domains: a channel and a gating domain. Mounting evidence suggest that the matrix loops contribute to the formation of the gating domain and thus they are determinants to the control of transport activity.

Oxidative stress, thiol reagents, and membrane potential modulate the mitochondrial permeability transition by affecting nucleotide binding to the adenine nucleotide translocase

Stimulation of the mitochondrial permeability transition (MPT) in de-energized mitochondria by phenylarsine oxide (PheArs) is greater than that by diamide and t-butylhydroperoxide (TBH), yet the increase in CyP binding to the inner mitochondrial membrane (Connern, C. P. and Halestrap, A. P. (1994) Biochem. J. 302, 321-324) is less. From a range of nucleotides tested only ADP, deoxy-ADP, and ATP inhibited the MPT. ADP inhibition involved two sites with Ki values of about 1 and 25 microM which were independent of [Ca2+] and CyP binding. Carboxyatractyloside (CAT) abolished the high affinity site. Following pretreatment of mitochondria with TBH or diamide, the Ki for ADP increased to 50-100 microM, whereas pretreatment with PheArs or eosin maleimide increased the Ki to >500 microM; only one inhibitory site was observed in both cases. Eosin maleimide is known to attack Cys159 of the adenine nucleotide translocase (ANT) in a CAT-sensitive manner (Majima, E., Shinohara, Y., Yamaguchi, N., Hong, Y. M., and Terada, H. (1994) Biochemistry 33, 9530-9536), and here we demonstrate CAT-sensitive binding of the ANT to a PheArs affinity column. In adenine nucleotide-depleted mitochondria, no stimulation of the MPT by uncoupler was observed in the presence or absence of thiol reagents, suggesting that membrane potential may inhibit the MPT by increasing adenine nucleotide binding through an effect on the ANT conformation. We conclude that CsA and ADP inhibit pore opening in distinct ways, CsA by displacing bound CyP and ADP by binding to the ANT. Both mechanisms act to decrease the Ca2+ sensitivity of the pore. Thiol reagents and oxidative stress may modify two thiol groups on the ANT and thus stimulate pore opening by both means.

Specific cardiolipin binding interferes with labeling of sulfhydryl residues in the adenosine diphosphate/adenosine triphosphate carrier protein from beef heart mitochondria

The interaction of cardiolipin with the isolated ADP/ATP carrier protein from beef heart mitochondria has been studied by means of the unmasking of a single cysteinyl residue, Cys56, which accompanies the conformational transition of the protein [Leblanc, P., & Clauser, H, (1972) FEBS Lett. 23, 107-113]. The unmasking was monitored by using the static fluorescence of the sulfhydryl reagent N-(1-pyrenyl)maleimide (PYM). The rate of PYM binding that was observed after initiation of the conformational transition by ADP was drastically reduced in the presence of cardiolipin (CL). Phospholipids other than CL were much less effective. It can be shown that the conformational transition and the binding reaction are both affected by CL, although to varying extents. An enhancement of the rate of the ADP-dependent PYM binding was observed upon digestion of the protein bound phospholipid by phospholipase A2. The phospholipase treatment also led to an increased ADP-independent PYM binding, thus indicating that the ADP control of the carrier transition was gradually lost. The ADP control could be fully restored through the addition of CL, provided that the phospholipase incubation had been terminated after approximately 1 h. These results will be discussed in relation to an earlier report of tight cardiolipin binding [Beyer, K., & Klingenberg, M. (1985) Biochemistry 24, 3821-3826] and to current structural models of the ADP/ATP carrier protein.

The signal transduction function for oxidative phosphorylation is at least second order in ADP

To maintain ATP constant in the cell, mitochondria must sense cellular ATP utilization and transduce this demand to F0-F1-ATPase. In spite of a considerable research effort over the past three decades, no combination of signal(s) and kinetic function has emerged with the power to explain ATP homeostasis in all mammalian cells. We studied this signal transduction problem in intact human muscle using 31P NMR spectroscopy. We find that the apparent kinetic order of the transduction function of the signal cytosolic ADP concentration ([ADP]) is at least second order and not first order as has been assumed. We show that amplified mitochondrial sensitivity to cytosolic [ADP] harmonizes with in vitro kinetics of [ADP] stimulation of respiration and explains ATP homeostasis also in mouse liver and canine heart. This result may well be generalizable to all mammalian cells.

The mitochondrial oxoglutarate carrier: sulfhydryl reagents bind to cysteine-184, and this interaction is enhanced by substrate binding

The interaction of sulfhydryl reagents with the oxoglutarate carrier (OGC) of bovine heart mitochondria was investigated in proteoliposomes reconstituted from purified carrier and lipids. Incubation of the proteoliposomes with maleimides or mercurials led to inhibition of the oxoglutarate carrier protein. The inhibition of oxoglutarate transport by mercurials was removed by dithioerythritol (DTE), whereas inhibition by maleimides was not. Preincubation of the proteoliposomes with mercurials protected the carrier protein against inactivation by the fluorescent sulfhydryl reagent N-(1-pyrenyl)maleimide (PM) and decreased the fluorescence associated with the carrier, indicating that mercurials bind to the same cysteine which is modified by PM. The presence of the substrates oxoglutarate and malate increased the binding of PM to the reconstituted carrier as well as the degree of inhibition of the reconstituted transport activity caused by PM, other maleimides, and mercurials. This result is consistent with the assumption that substrate binding causes a change in the tertiary structure of the carrier protein. The primary sequence of the oxoglutarate carrier contains three cysteines (Cys-184, Cys-221, and Cys-224). We provide evidence that PM labels only Cys-184, whereas Cys-221 and Cys-224 are linked by a disulfide bridge.