Isolation and expression of the gene encoding mitochondrial ADP/ATP carrier (AAC) from the pathogenic yeast Candida parapsilosis

A gene homologous to Saccharomyces cerevisiae AAC genes coding for mitochondrial ADP/ATP carriers has been cloned from the pathogenic yeast Candida parapsilosis. A probe obtained by PCR amplification from C. parapsilosis DNA, using primers derived from the conserved transmembrane region of yeast ADP/ATP carriers, was used for screening of the C. parapsilosis genomic library. The cloned gene was sequenced and found to encode a polypeptide of 303 amino acids that shows homology with other yeast and fungal mitochondrial ADP/ATP carriers. The gene was designated CpAAC1 and was able to complement the growth phenotypes of S. cerevisiae double deletion mutant (Deltaaac2; Deltaaac3). The expression of the CpAAC1 gene was reduced under semi-anaerobic conditions and it was affected at normal aerobic conditions by the nature of carbon sources used for growth. Hybridization experiments indicate that C. parapsilosis possesses a single gene encoding a mitochondrial ADP/ATP carrier.

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.

Kinetics of electrogenic transport by the ADP/ATP carrier

The electrogenic transport of ATP and ADP by the mitochondrial ADP/ATP carrier (AAC) was investigated by recording transient currents with two different techniques for performing concentration jump experiments: 1) the fast fluid injection method: AAC-containing proteoliposomes were adsorbed to a solid supported membrane (SSM), and the carrier was activated via ATP or ADP concentration jumps. 2) BLM (black lipid membrane) technique: proteoliposomes were adsorbed to a planar lipid bilayer, while the carrier was activated via the photolysis of caged ATP or caged ADP with a UV laser pulse. Two transport modes of the AAC were investigated, ATP(ex)-0(in) and ADP(ex)-0(in). Liposomes not loaded with nucleotides allowed half-cycles of the ADP/ATP exchange to be studied. Under these conditions the AAC transports ADP and ATP electrogenically. Mg(2+) inhibits the nucleotide transport, and the specific inhibitors carboxyatractylate (CAT) and bongkrekate (BKA) prevent the binding of the substrate. The evaluation of the transient currents yielded rate constants of 160 s(-1) for ATP and >/=400 s(-1) for ADP translocation. The function of the carrier is approximately symmetrical, i.e., the kinetic properties are similar in the inside-out and right-side-out orientations. The assumption from previous investigations, that the deprotonated nucleotides are exclusively transported by the AAC, is supported by further experimental evidence. In addition, caged ATP and caged ADP bind to the carrier with similar affinities as the free nucleotides. An inhibitory effect of anions (200-300 mM) was observed, which can be explained as a competitive effect at the binding site. The results are summarized in a transport model.

Transport of the ADP/ATP carrier of mitochondria from the TOM complex to the TIM22.54 complex

Members of the mitochondrial carrier family such as the ADP/ATP carrier (AAC) are composed of three structurally related modules. Here we show that each of the modules contains a mitochondrial import signal recognized by Tim10 and Tim12 in the intermembrane space. The first and the second module are translocated across the outer membrane independently of the membrane potential, DeltaDeltapsipsi, but they are not inserted into the inner membrane. The third module interacts tightly with the TOM complex and thereby prevents complete translocation of the precursor across the outer membrane. At this stage, binding of a TIM9.10 complex confers a topology to the translocation intermediate which reflects the modular structure of the AAC. The precursor is then transferred to the TIM9.10.12 complex, still interacting with the TOM complex. Release of the precursor from the TOM complex and insertion into the inner membrane by the TIM22.54 complex requires a DeltaDeltapsipsi-responsive signal in the third module.

Genetic and structural characterization of the human mitochondrial inner membrane translocase

Translocation of nuclear-encoded mitochondrial preproteins is mediated by translocases in the outer and inner membranes. In the yeast Saccharomyces cerevisiae, translocation of preproteins into the matrix requires the membrane proteins Tim23, Tim17 and Tim44, which drive translocation in cooperation with mtHsp70 and its co-chaperone Mge1p. We have cloned and functionally analyzed the human homologues of Tim17, Tim23 and Tim44. In contrast to yeast, two TIM17 genes were found to be expressed in humans. TIM44, TIM23 and TIM17a genes were mapped to chromosomes 19p13.2-p13.3, 10q11. 21-q11.23 and 1q32. The TIM17b gene mapped to Xp11.23, near the fusion point where an autosomal region was proposed to have been added to the "ancient" part of the X chromosome about 80-130 MY ago. The primary sequences of the two proteins, hTim17a and hTim17b, are essentially identical, significant differences being restricted to their C termini. They are ubiquitously expressed in fetal and adult tissues, and both show expression levels comparable to that of hTim23. Biochemical characterization of the human Tim components revealed that hTim44 is localized in the matrix and, in contrast to yeast, only loosely associated with the inner membrane. hTim23 is organized into two distinct complexes in the inner membrane, one containing hTim17a and one containing hTim17b. Both TIM complexes display a native molecular mass of 110 kDa. We suggest that the structural organization of TIM23.17 preprotein translocases is conserved from low to high eukaryotes.

The mitochondrial adenine nucleotide translocator from Dictyostelium discoideum. Functional characterization and DNA sequencing

The mitochondrial adenine nucleotide translocator (ANT) catalyses the exchange of ATP and ADP between the mitochondria and the cytosol. We have cloned and sequenced the gene encoding the Dictyostelium discoideum ANT (DdANT) and analysed its transcriptional regulation. The single copy D. discoideum ant gene encodes a protein of 309 amino acid residues with a predicted molecular mass of 33,469 Da and a pI of 9.85. These values are comparable to those of ANTs from mammals, insects and fungi. The long N-terminal extension characteristic of plant ANT is absent in DdANT. The protein coding region of the D. discoideum ant gene is interrupted by three introns. Polyclonal antibodies directed against the beef heart mitochondrial ANT or its C-terminal peptide recognized the D. discoideum protein. Northern blot analysis revealed that the expression of the D. discoideum ant gene decreased rapidly during the first hours of multicellular development but the amount of protein remained stable throughout differentiation.

Membrane topology of the Rickettsia prowazekii ATP/ADP translocase revealed by novel dual pho-lac reporters

Here, we report the construction and characterization of dual reporters, consisting of both an Escherichia coli alkaline phosphatase (AP) gene and an alpha-fragment of the beta-galactosidase (BG) gene, for studying membrane protein topology by the gene fusion approach. Each of the reporters, when fused to periplasmic domains of polytopic proteins, produces fusions with high AP activity and, when fused to cytoplasmic domains, produces fusions with high BG activity in E. coli strains capable of alpha-complementation. The dual nature of these reporters simplifies interpretation of data obtained with poorly expressed fusions and allows one to evaluate the reliability of topological data. Deleterious effects resulting from the cell's attempt to export the full-length BG are eliminated in this approach. We describe dual indicator plates that allow for discrimination between colonies bearing cytoplasmic fusions, periplasmic fusions, and no fusions. We have generated a set of fusions to the topologically well-studied lactose permease of E. coli and demonstrated that topological information generated by these new reporters is in good agreement with the existing model. We used this new methodology for the determination of membrane topology of the Rickettsia prowazekii ATP/ADP translocase (Tlc). Our results were in agreement with the proposed in silico topological model in which Tlc traverses the cytoplasmic membrane of E. coli 12 times with its N and C termini facing the cytoplasm.

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.

Identification of 30 kDa calsequestrin-binding protein, which regulates calcium release from sarcoplasmic reticulum of rabbit skeletal muscle

In a previous study [Yamaguchi, Kawasaki and Kasai (1995) Biochem. Biophys. Res. Commun. 210, 648-653], we showed that the stilbene derivative 4,4'-di-isothiocyanostilbene-2,2'-disulphonic acid activates the Ca2+ channel in the sarcoplasmic reticulum (SR) in rabbit skeletal muscle, and it does not bind to the channel protein itself but to the SR 30 kDa protein. Furthermore, the 30 kDa protein was shown to bind to calsequestrin (CSQ), which is one of the regulators of the Ca2+ release channel in the SR. In the present study, we determined the partial amino acid sequence of the CSQ-binding 30 kDa protein and, consequently, this protein was proved to be highly similar to ADP/ATP translocase (AAT) expressed in the mitochondria in a variety of cells. By Western-blotting analysis, the CSQ-binding 30 kDa protein was recognized by the antibody raised against bovine cardiac AAT and, furthermore, depolarization-induced Ca2+ release monitored in the rabbit skeletal muscle triads was significantly activated by the antibody. As a result of cloning and sequencing of the cDNA encoding AAT of the rabbit skeletal muscle, the amino acid sequence was found to be the same as that of the CSQ-binding 30 kDa protein determined above. Furthermore, the expressed product of the cDNA encoding AAT in Escherichia coli was proved to bind to CSQ. These results suggest that AAT itself is expressed in the rabbit skeletal muscle SR and regulates the Ca2+ release from the SR; that is, excitation-contraction coupling of the skeletal muscle cell.

Mitochondrial adenine nucleotide translocase is modified oxidatively during aging

The purpose of this study was to test the hypothesis that elevation in protein oxidative damage during the aging process is a targeted rather than a stochastic phenomenon. Oxidative damage to proteins in mitochondrial membranes in the flight muscles of the housefly, manifested as carbonyl modifications, was detected immunochemically with anti-dinitrophenyl antibodies. Adenine nucleotide translocase (ANT) was found to be the only protein in the mitochondrial membranes exhibiting a detectable age-associated increase in carbonyls. The age-related elevation in ANT carbonyl content was correlated with a corresponding loss in its functional activity. Senescent flies that had lost the ability to fly exhibited a relatively higher degree of ANT oxidation and a greater loss of functional activity than their cohorts of the same age that were still able to fly. Exposure of flies to 100% oxygen resulted in an increase in the level of ANT carbonyl content and a loss in its activity. In vitro treatment of mitochondria with a system that generated hydroxyl free radicals caused an increase in ANT carbonyl level and a decrease in ANT exchange activity. ANT was also the only mitochondrial membrane protein exhibiting adducts of the lipid peroxidation product 4-hydroxynonenal. Results of this study indicate that proteins in mitochondrial membranes are modified selectively during aging.

Bax and adenine nucleotide translocator cooperate in the mitochondrial control of apoptosis

The proapoptotic Bax protein induces cell death by acting on mitochondria. Bax binds to the permeability transition pore complex (PTPC), a composite proteaceous channel that is involved in the regulation of mitochondrial membrane permeability. Immunodepletion of Bax from PTPC or purification of PTPC from Bax-deficient mice yielded a PTPC that could not permeabilize membranes in response to atractyloside, a proapoptotic ligand of the adenine nucleotide translocator (ANT). Bax and ANT coimmunoprecipitated and interacted in the yeast two-hybrid system. Ectopic expression of Bax induced cell death in wild-type but not in ANT-deficient yeast. Recombinant Bax and purified ANT, but neither of them alone, efficiently formed atractyloside-responsive channels in artificial membranes. Hence, the proapoptotic molecule Bax and the constitutive mitochondrial protein ANT cooperate within the PTPC to increase mitochondrial membrane permeability and to trigger cell death.

Brittle-1, an adenylate translocator, facilitates transfer of extraplastidial synthesized ADP--glucose into amyloplasts of maize endosperms

Amyloplasts of starchy tissues such as those of maize (Zea mays L.) function in the synthesis and accumulation of starch during kernel development. ADP-glucose pyrophosphorylase (AGPase) is known to be located in chloroplasts, and for many years it was generally accepted that AGPase was also localized in amyloplasts of starchy tissues. Recent aqueous fractionation of young maize endosperm led to the conclusion that 95% of the cellular AGPase was extraplastidial, but immunolocalization studies at the electron- and light-microscopic levels supported the conclusion that maize endosperm AGPase was localized in the amyloplasts. We report the results of two nonaqueous procedures that provide evidence that in maize endosperms in the linear phase of starch accumulation, 90% or more of the cellular AGPase is extraplastidial. We also provide evidence that the brittle-1 protein (BT1), an adenylate translocator with a KTGGL motif common to the ADP-glucose-binding site of starch synthases and bacterial glycogen synthases, functions in the transfer of ADP-glucose into the amyloplast stroma. The importance of the BT1 translocator in starch accumulation in maize endosperms is demonstrated by the severely reduced starch content in bt1 mutant kernels.

Reconstituted adenine nucleotide translocase forms a channel for small molecules comparable to the mitochondrial permeability transition pore

Highly purified adenylate translocase (ANT) from rat heart mitochondria was functionally reconstituted as ATP/ADP exchange carrier in asolectin/cardiolipin vesicles. The ANT preparations used were free of porin, cyclophilin D, and Bax as analysed immunologically and by activity measurements. After pre-loading the ANT-containing proteoliposomes with ATP, malate or AMP, a gradual release of the trapped compounds by increasing the external Ca2+ concentrations could be demonstrated. N-Methyl-Val-4-cyclosporin did not inhibit the Ca2+ dependent release of internal substances from ANT liposomes. This inhibitor was found to be specific for the mitochondrial permeability transition pore (MTP) in intact mitochondria or reconstituted MTP-like protein complexes (e.g. hexokinase, porin, ANT complex). However, ADP in concentrations > 20 microM inhibited the liberation of internal compounds, while in contrast, atractyloside (30 microM) and HgCl2 (5 microM) both induced permeability of the ANT-containing liposomes resulting in a release of trapped substances. These results strongly suggest that ANT itself is capable to adopt a pore-like structure under conditions known to induce the permeability transition in mitochondria.

Highly conserved charge-pair networks in the mitochondrial carrier family

Selection for regain-of-function mutations in the yeast ADP/ATP carrier AAC2 has revealed an unexpected series of charge-pairs. Four of the six amino acids involved are found in the mitochondrial energy transfer motifs used to define this family of proteins. As such, the results found with the ADP/ATP carrier may apply to the family as a whole. Mitochondrial carriers are built from three homologous domains, each with the conserved motif PX(D,E)XX(K,R). Neutralization of the conserved positive charges at K48, R152 or R252 in these motifs results in respiration defective yeast. Neutralization of the negative charges at D149 and D249 also make respiration defective yeast, though E45G or E45Q mutants are able to grow on glycerol. Regain of function occurs when a complementary charge is lost from another site in the molecule. This phenomenon has been observed independently eight times and thus is strong evidence for charge-pairs existing between the affected residues. Five different charge-pairs have been detected in the yeast AAC2 by this method and three more can be predicted based on homology between the domains. The highly conserved charge-pairs occurring within or between the three mitochondrial energy transfer signatures seem to be a critical feature of mitochondrial carrier structure, independent of the substrates transported. Conformational switching between alternative charge-pairs may constitute part of the basis for transport.

Occurrence of two plastidic ATP/ADP transporters in Arabidopsis thaliana L.--molecular characterisation and comparative structural analysis of similar ATP/ADP translocators from plastids and Rickettsia prowazekii

Recently, we sequenced a cDNA clone from Arabidopsis thaliana L. encoding an ATP/ADP transporter protein (AATP1) located in the plastid envelope membrane. The deduced amino acid sequence of AATP1 exhibits a high degree of similarity (> 66%) to the ATP/ADP transporter from the obligate intracellular gram-negative bacterium Rickettsia prowazekii. Here we report a second plastidic ATP/ADP carrier from A. thaliana (AATP2). As deduced from the amino acid sequence, AATP2 exhibits 77.6% identity to AATP1 and 36% to the rickettsial protein. Hydropathy analysis indicates that all three translocators are highly hydrophobic membrane proteins, which exhibit marked similarities and differences. The AATP1 translocator lacks the sixth transmembrane domain that is present in AATP2 and the bacterial transporter in R. prowazekii. In contrast to AATP1 and the bacterial transport protein, only AATP2 exhibits a truncated C-terminal end. To compare the general biochemical properties of AATP2 with the known transport properties of AATP1 we cloned the entire AATP2 cDNA into plasmid pJT118, leading to the presence of an additional N-terminal histidine tag of 10 amino acids. For heterologous expression of His10-AATP2 we chose the Escherichia coli strain C43, which was reported recently to allow overproduction of eucaryotic membrane transport proteins. After transformation and subsequent induction by isopropylthio-2-D-galactopyranoside intact E. coli cells harbouring plasmid pJT118 showed import of radioactively labelled ATP and ADP. As deduced from a Lineweaver-Burk analysis His10-AATP2 exhibited apparent Km values for ATP and ADP of 22 microM and 20 microM, respectively. Import of ADP into His10-AATP2-expressing E. coli cells occurred at a rate of 24 nmol x mg protein(-1) x h(-1), which was about threefold faster than import of ATP. These biochemical characteristics are similar to transport properties of the heterologously expressed His10-AATP1 protein.

The mitochondrial ADP/ATP carrier: structural, physiological and pathological aspects

Under the conditions of oxidative phosphorylation, the mitochondrial ADP/ATP carrier catalyses the one to one exchange of cytosolic ADP against matrix ATP across the inner mitochondrial membrane. The ADP/ATP transport system can be blocked very specifically by two families of inhibitors: atractyloside (ATR) and carboxyatractyloside (CATR) on one hand, and bongkrekic acid (BA) and isobongkrekic acid (isoBA) on the other hand. It is well established that these inhibitors recognise two different conformations of the carrier protein, the CATR- and BA-conformations, which exhibit different chemical, immunochemical and enzymatic reactivities. The reversible transition of the ADP/ATP carrier between the two conformations was studied by fluorometric techniques. This transconversion, which is only triggered by transportable nucleotides, is probably the same as that which occurs during the functioning of ADP/ATP transport system. The fluorometric approach, using the tryptophanyl residues of the yeast carrier as intrinsic fluorescence probes, was combined to a mutagenesis approach to elucidate the ADP/ATP transport mechanism at the molecular level. Finally, recent reports that myopathies might result from defect in ADP/ATP transport led us to develop a method to quantify the carrier protein in muscular biopsies.

Mutagenesis of some positive and negative residues occurring in repeat triad residues in the ADP/ATP carrier from yeast

In AAC2 from Saccharomyces cerevisiae, nine additional charged residues (six positive, three negative) were neutralized by mutagenesis following the previous mutation of six arginines. Oxidative phosphorylation (OxPhos) in cells and mitochondria, the expression level of AAC protein, and the various transport modes of AAC in the reconstituted system were measured. Mutations are: within the first helix at K38A which is exclusive for AAC; K48A, and R152A, part of a positive triad occurring in the matrix portion of each repeat; two matrix lysines, K179M and K182I, and the negative triad helix-terminating residues, E45G, D149S, D249S. Cellular ATP synthesis (OxPhos) is nearly completely inhibited in K48A, R152A, D149S, and D249S, but still amounts to 10% in K38A and between 30% and 90% in the gly+ mutants K179M, K179I + K182I, and E45G. Comparison of the AAC content measured by ELISA and the binding of [3H]CAT and [3H]BKA reveals discrepancies in K48A, D149S, and D249S mitochondria, which provide evidence that these mutations largely abolish inhibitor binding. Also these mitochondria have undetectable OxPhos. Differently in K38A, CAT and BKA binding are retained at high AAC levels but OxPhos is very low. This reveals a special functional role of K38, different from the more structural role of R152, K48, D149, and D249. Transport activity was measured with reconstituted AAC. The electroneutral ADP/ADP exchange of gly- mutants is largely or fully suppressed in K48A, D149S, and D249S. K38A and R152A are still active at 18% and 30% of wt. The other three exchange modes, ATP/ADP, ADP/ATP, and ATP/ATP, are nearly suppressed in all gly- mutants but remain high in gly+ mutants. ATP-linked modes are higher than the ADP/ADP mode in gly+ but lower in gly- mutants, resulting in an exchange mode inversion (EMI). In the competition for AAC2 transport capacity, the weak ATP exporting modes are suppressed by the much stronger unproductive ADP/ADP mode causing inhibition of OxPhos. Together with previous results all members of three charge triads are now mutagenized, revealing drastic functional rotatory asymmetries within the three repeat domains. In the intrahelical arginine triad the third (R294A), in the positive matrix triad the second (R152A), and in the helix-terminating negative triad the first (E45G) still show high activity.

Fatty acids induced uncoupling of Saccharomyces cerevisiae mitochondria requires an intact ADP/ATP carrier

Fatty acids stimulate the oxidation rate of mitochondria isolated from the wild-type Saccharomyces cerevisiae, but do not affect significantly the respiration of mitochondria isolated from mutants, in which the ADP/ATP carrier (AAC) was either modified (R96H) or deleted (delta aac2). Similarly as in mammalian mitochondria, the transmembrane electrical potential difference (delta psi) in the wild-type yeast mitochondria was dissipated by low concentrations of free fatty acids, and this was partially inhibited by bongkrecate. In contrast to the wild-type mitochondria, the addition of increasing concentrations of fatty acids to the op1 (R96H) mutant mitochondria abolished only a small portion of delta psi, as compared to the change induced by classical uncouplers. The different effects of fatty acids on both, the respiration and the delta psi of mitochondria isolated from the wild-type and the aac mutants, respectively, demonstrates that the intact AAC is essential for the fatty acids induced H+ permeability of mitochondrial membrane.

Probing the role of positive residues in the ADP/ATP carrier from yeast. The effect of six arginine mutations of oxidative phosphorylation and AAC expression

ADP/ATP transport is the terminal step of oxidative phosphorylation in mitochondria. In this paper seven mutants of AAC2 from Saccharomyces cerevisiae are studied on the cellular and mitochondrial level. Six conspicuously located arginines were mutated into mostly neutral residues [Nelson, D. R., Lawson, J. E., Klingenberg, M., & Douglas, M. G. (1993) J. Mol. Biol. 230, 1159-1170]. R96A, R96H, R204L, and R294A are located in the second transmembrane helix of each repeat while R252I, R253I, and R254I are in the arginine triplet of the last domain. All six arginine residues are conserved in all known ADP/ATP carrier sequences. At the cellular level, oxidative phosphorylation in R96H and R294A retains 8% of the wild-type rate, but it is virtually zero in the other mutants. However, cytochrome c, a parameter of oxidative capacity, remains at 4-42% of wt. The weak coordination of respiratory chain and AAC expression indicates that respiration is needed also for other purposes. In mitochondria the AAC-linked ATP synthesis is measured and segregated by using the AAC inhibitor bongkrekate (BKA). Only the R96H and R294A mutants express a significant rate of AAC-dependent ATP synthesis amounting to 2-18% of the plasmid-borne wild-type AAC2 mitochondria. In all other mutants it is virtually zero. However, respiratory capacity and cytochrome c content are reduced only by 20-70%. Whereas in immunoblots the presence of AAC is detected in all mutant mitochondria, by quantitative ELISA no AAC can be measured down to 0.05 mumol of AAC dimer/g of protein in R96A and R204L, whereas in R96H, R2521, R2531, and R254I the content is around 0.2 and in R294A the content is 0.46 as compared to 0.6 in the plasmid wild type. Also the [3H]CAT and [3H]BKA binding is virtually zero in some mutants and closely parallels the ELISA-determined AAC content, indicating that the mutations did not affect the inhibitor binding site. The turnover of AAC [V(ATP)/AAC content] in oxidative phosphorylation is reduced to 10% or 20% except for the two intrahelical mutants R96H and R294A. In the three Arg triplet mutants, it is nearly zero. In conclusion, the first two intrahelical arginines R96 and R204, are essential for expression but probably also for the activity of AAC. R294A still retains good transport activity and remarkably high expression of AAC. All arginines in the triplet 252, 253, 254 are essential. Extrapolation of the in vitro phosphorylation rates to the cellular level by the cytochrome c factor reveals a large discrepancy to the in vivo rates in particular for R294A. This indicates that these mutations render the AAC more sensitive to the regulatory intracellular ATP/ADP ratio than the wt AAC.

Probing the role of positive residues in the ADP/ATP carrier from yeast. The effect of six arginine mutations on transport and the four ATP versus ADP exchange modes

Mutagenesis of three intrahelical arginines, R96, R204, or R294, and of each member of the arginine triplet R252, R253, R254 into neutral residues had resulted in a strong suppression of oxidative phosphorylation in cells and isolated mitochondria [Müller, V., Basset, G., Nelson, D. R., & Klingenberg, M. (1996) Biochemistry 35, 16132-16143]. Here we determine the transport activity of wild-type and mutant AAC in reconstituted proteoliposomes using a new rapid removal-stop method without relying on the inhibitor stop which can be compromised by mutations. The basic electroneutral ADP/ADP exchange activity is strongly or totally suppressed in six out of seven of these mutations, with the exception of R294A, which retains nearly wild-type activity. Carboxyatractylate (CAT) inhibits the ADP/ATP exchange rate only to 3-10% in wild type and R294A and up to 40% in other mutants, whereas bongkrekic acid (BKA) inhibits 50% (wild type and R294A) and 90% (other mutants). Consequently, AAC is preferentially reconstituted with the matrix surface outside. All these mutations drastically change activity distribution among the four exchange modes ADP/ADP, ADP/ATP, ATP/ADP, and ATP/ATP. Whereas in wild-type AAC the homo ATP/ATP exchange is twice as high as the ADP/ADP exchange, in mutants it is 10 to 15 times lower. Similarly, the hetero ATP/ADP exchange in wild-type AAC is higher than the ADP/ ATP exchange, but in mutants it is several times lower. Thus, these mutations afflict the ATP-linked modes, in particular those linked to external ATP. The inhibition of oxidative phosphorylation is thus explained by the suppression of ATP export versus ADP import mode. The "extra"-inhibition of oxidative phosphorylation in mutant cells is explained by the extreme shift in mutants in favour of ATP import versus ADP export. Besides structural changes, the mutant effects indicate electrostatic interactions of these arginines with the anionic substrates. The loss of one positive charge raises the translocation barrier the more negative the substrate, i.e. more for ATP4- than for ADP3-. In none of these arginine mutants was the binding of CAT or BKA abolished.

Conformational changes of the yeast mitochondrial adenosine diphosphate/adenosine triphosphate carrier studied through its intrinsic fluorescence. 2. Assignment of tryptophanyl residues of the carrier to the responses to specific ligands

Tryptophanyl substitution of the Saccharomyces cerevisiae adenine nucleotide carrier (Anc2p isoform) was not deleterious for the transport activity or the folding of the carrier [preceding paper by Le Saux et al. (1996) Biochemistry 35, 16116-16124]. Conformational changes of the isolated wild-type and Trp-substituted Anc2p variants, induced upon binding of specific substrates [adenosine triphosphate (ATP) or diphosphate (ADP)] or inhibitors [carboxyatractyloside (CATR) or bongkrekic acid (BA)], were studied by measurement of intrinsic fluorescence. Titration of CATR and BA binding sites ended in the same number of sites, namely, 6-7 nmol/mg of wild-type and variant Anc2p. Isolated Anc2p in detergent presented similar emission spectra, suggesting that all tryptophanyl residues were in environments of similar hydrophobicity. Trp87 and Trp126 contributed largely and to a similar extent to the fluorescence enhancement observed in response to ATP binding, while Trp235 contributed negatively and to a small extent to the fluorescence change. Both Trp126 and Trp235, and to a lower extent Trp87, participate in the CATR-induced fluorescence decrease of Anc2p. Responses to BA binding were observed only in the presence of ATP; they consisted of a further fluorescence increase of the Anc2p.ATP complex, which was mainly due to Trp87 and Trp126, Trp235 being much less responsive. The different fluorescence responses of the three Trp residues of Anc2 variants to ATP, CATR, and BA are in agreement with distinct binding sites for these ligands and distinct conformations of the carrier protein recognizing specifically CATR or BA. A mechanistic model is proposed to interpret the transitions between the different conformational states of Anc2p.

Conformational changes of the yeast mitochondrial adenosine diphosphate/adenosine triphosphate carrier studied through its intrinsic fluorescence. 1. Tryptophanyl residues of the carrier can be mutated without impairing protein activity

During the transport process the mitochondrial adenine nucleotide carrier (Ancp) undergoes conformational changes which result in modifications of the intrinsic fluorescence of the carrier. To further study these changes by a fluorometric approach, the three tryptophanyl residues (Trp87, Trp126, and Trp235) of the Saccharomyces cerevisiae Anc2p were individually mutated to their tyrosine counterparts. The resulting mutated genes (two-Trp, one-Trp or Trp-less variants) were integrated at the ANC2 locus. A prerequisite for such studies is that all the engineered carrier molecules are still able to catalyze ADP/ATP exchange. The cellular characteristics of the strains expressing the mutated Anc2p and the biochemical properties of the variant Anc2p in mitochondria were examined. Although Trp87 is absolutely conserved in all 30 available Ancp sequences, none of the tryptophanyl residues is essential to the carrier protein folding and the transport activity. The mutated and wild-type Anc2p were expressed to the same level, as evidenced by both ligand binding and immunochemical analyses. When isolated in the presence of detergent, all the variant Anc2p preparations contained ergosterol in similar amounts (9 mol/mol of 35 kDa Anc2p) but no specific interaction was revealed. Our results show that the tryptophanmutated Anc2p are suitable for fluorescence studies, which are reported in the accompanying paper by Roux et al. [(1996) Biochemistry 35, 16125-16131].

Peroxidative modification of a membrane protein. Conformation-dependent chemical modification of adenine nucleotide translocase in Cu2+/tert-butyl hydroperoxide treated mitochondria

Peroxidative treatment of rat heart mitochondria results in a gradual increase of the apparent molecular weight of the adenine nucleotide translocase (ANT) by up to 1.2 kDa. ANT isolated from mitochondria treated with 1 mM tert-butyl hydroperoxide and 5-40 microM Cu2+ for 1 h at 37 degrees C exhibited a progressive loss of lysine, cysteine, arginine, and valine residues compared to native ANT. N-Ethylmaleimide, dithiothreitol, and the specific inhibitor of ANT, carboxyatractyloside (CAT), inhibited the peroxidation-induced molecular weight shift without inhibiting lipid peroxidation, which is believed to be the primary cause of the observed ANT modification. Bongkrekic acid, which stabilizes ANT in a conformation different from that brought about by CAT, did not inhibit the ANT molecular weight shift. Dithiothreitol, as well as CAT, was found to protect ANT against most of the losses of amino acid residues, indicating that alteration of sulfhydryl residues is required for chemical modification of, not only cysteine, but also lysine, arginine, and valine. We conclude that the peroxidative modification of ANT is conformation-dependent and involves chemical modification of cysteine as a critical step.

Purification and characterization of the reconstitutively active adenine nucleotide carrier from maize mitochondria

The adenine nucleotide carrier from maize (Zea mays L. cv B 73) shoot mitochondria was solubilized with Triton X-100 and purified by sequential chromatography on hydroxyapatite and Matrex Gel Blue B in the presence of cardiolipin and asolectin. Sodium dodecyl sulfate-gel electrophoresis of the purified fraction showed a single polypeptide band with an apparent molecular mass of 32 kD. When reconstituted in liposomes, the adenine nucleotide carrier catalyzed a pyridoxal 5'-phosphate-sensitive ATP/ATP exchange. It was purified 168-fold with a recovery of 60% and a protein yield of 0.25% with respect to the mitochondrial extract. Among the various substrates and inhibitors tested, the reconstituted protein transported only ADP, ATP, GDP, and GTP, and was inhibited by atractyloside, bongkrekate, phenylisothiocianate, pyridoxal 5'-phosphate, and mersalyl (but not N-ethylmaleimide). Maximum initial velocity of the reconstituted ATP/ATP exchange was determined to be 2.2 mumol min-1 mg-1 protein at 25 degrees C. The half-saturation constants and the corresponding inhibition constants were 17 microM for ATP, 26 microM for ADP, 59 microM for GTP, and 125 microM for GDP. The activation energy of the ATP/ATP exchange was 48 kilojoule/mol between 0 and 15 degrees C, and 22 kilojoule/mol between 15 and 35 degrees C. Partial amino acid sequences showed that the purified protein was the product of the ANT-G1 gene sequenced previously (B. Bathgate, A. Baker, C.J. Leaver [1989] Eur J Biochem 183: 303-310).

Direct thyroid hormone signalling via ADP-ribosylation controls mitochondrial nucleotide transport and membrane leakiness by changing the conformation of the adenine nucleotide transporter

Addition of triiodothyronine at 10 pM in vitro to hypothyroid rat liver mitochondria doubles the rate of the adenine nucleotide transporter at low ADP concentrations. Nicotinamide abolishes this effect in parallel with its inhibition of the ADP-ribosylation of an inner membrane protein identical in size to the transporter. Nicotinamide also renders euthyroid preparations indistinguishable from hypothyroid ones. A mechanism is offered to explain these findings in which it is proposed that the adenine nucleotide transporter is a true allosteric protein and that its covalent modification by ADP-ribosylation increases the stability of the less favoured externally-facing C-conformation and thus increases the proportion of transporters in this orientation: although the C-conformation is significantly more leaky to cations than the tight matrix-facing M-conformation, this enhances ADP import. This model is shown to offer an explanation not only for the transport effects of T3 but also for those of oxidative stress and ADP-ribosylation inhibitors on Ca2+, H+ and K+ transfer across the mitochondrial inner membrane. Ca2+ at 30 nM appears to stabilize the M-conformation of the transporter by a mechanism other than ADP-ribosylation.

The yeast ADP/ATP carrier. Mutagenesis and second-site revertants

Results of mutagenesis and selection of spontaneous second-site revertants of the yeast ADP/ATP carrier AAC2 is described. Currently, 50 mutants have been made in AAC2 at 35 locations. Yeast carrying mutations at K38, K48, R96, D149, R152, R204, D249, R252, R253, R254 and R294 are all unable to grow on glycerol. Seven of these mutants have yielded second-site revertants when plated on rich yeast media containing glycerol and ethanol. The R96 mutants and the R254 and R253 mutants produce similar changes in the AAC2 molecule because the same sites are affected by their revertant mutations. This system of mutations and revertants is now poised to yield insights into the dynamics of ADP and ATP transport, and mitochondrial carrier structure in general.

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

The cross-linking reagent copper-o-phenanthroline complex (Cu(OP)2) specifically caused a decrease in the amount of the 30-kDa ADP/ATP carrier in bovine submitochondrial particles associated predominantly with formation of a 60-kDa protein consisting of a cross-linked dimer of the carrier. However, Cu(OP)2 had no effect on mitochondria. The transport of ADP via the carrier through submitochondrial particle membranes was found to be inhibited in parallel with the progress of intermolecular cross-linking. Analysis of the cross-linked site showed that a disulfide bridge was formed only between two Cys56 residues in a pair of the first loops facing the matrix space. The transport inhibitor bongkrekic acid, which locks the m-state conformation of the carrier, had no effect on disulfide bridge formation catalyzed by Cu(OP)2, but carboxyatractyloside, which locks the c-state conformation by acting from the cytosolic side, completely inhibited the cross-linking. These results show that the ADP/ATP carrier functions as a dimer form, and a pair of the first loops protrudes into the matrix space in the m-state, but possibly intrudes into the membrane in the c-state. Thus, it is suggested that a pair of the first loops acts as a gate and that its opening and closing are regulated by their translocation.

Molecular characterization of an Arabidopsis thaliana cDNA encoding a novel putative adenylate translocator of higher plants

We have isolated an Arabidopsis thaliana cDNA encoding a highly hydrophobic membrane protein of 589 amino acids which contains 12 potential transmembrane helices and shows a high degree of similarity (43.5% identity, 66.2% similarity) to the ATP/ADP translocase of the Gram-negative bacterium Rickettsia prowazekii, an obligate intracellular parasite responsible for the epidemic typhus. This rickettsial translocator resides in the cytoplasmic membrane and allows the bacterium to exploit the host cytoplasmic ATP pool. We hypothesize that the A. thaliana homolog of the R. prowazekii ATP/ADP translocase is the functional eukaryotic equivalent and resides in the plastid inner envelope membrane where it functions as an ATP importer.

Inhibition of adenine nucleotide translocator by lipid peroxidation products

Our previous data showed that aldehydic lipid peroxidation products, interacting with mitochondrial membrane lipids, could alter the physicochemical status of the membrane. This study was initiated to examine the interaction of these aldehydes with a major mitochondrial protein, the adenine nucleotide translocator (ANT). Our findings showed that the transporting activity of ANT in intact mitochondria was inhibited by two unsaturated aldehydes, 4-hydroxynonenal (HNE) and 4-hydroxyhexenal (HHE). To probe further into the underlying mechanism of this inhibition, a reconstituted ANT model was developed by incorporating isolated ANT into liposomes. Pretreatment of ANT with HNE prior to reconstitution resulted in decreased activity in the reconstituted ANT. Further investigation revealed that this decreased activity was probably due to loss of sulfhydryl groups, which are essential for ANT activity. Interestingly, pretreatment of the liposomes with HNE also caused a decrease in the reconstituted ANT activity by indirectly altering the physiochemical status of the lipid environment in which ANT was embedded. These results demonstrate that the reactive aldehydes derived from mitochondrial lipid peroxidation can impair the membrane function by interacting with both the protein and the lipid moieties in the membrane. Thus, the varied damaging effects associated with lipid peroxidation may be mediated by their secondary aldehydic byproducts.

Molecular characterisation of the ADP/ATP-transporter cDNA from the human malaria parasite Plasmodium falciparum

We have isolated a cDNA sequence encoding the ADP/ATP transporter in Plasmodium falciparum. The sequence analysis revealed an open reading frame encoding 301 amino acids and showed significant similarities to known eukaryotic translocases such as that of Chlorella (up to 67.2% identity) and the human transporter (61.2%). RNA blot analysis showed the presence of mRNA encoding for a 33.7-kDa ADP/ATP transporter. During the cell cycle of the parasite the expression levels of the transcripts fluctuate. The mitochondrial ADP/ATP transporter could play a role in energy metabolism of P. falciparum and makes this transporter an excellent target for chemotherapy.

[3H]7-azido-4-isopropylacridone labels Cys159 of the bovine mitochondrial ADP/ATP-carrier protein

Acridones (9-azaanthracen-10-ones), especially halogen-substituted acridone-carboxylic acids are efficient inhibitors of [14C]ADP binding to the bovine mitochondrial ADP/ATP-carrier protein. 7-Iodoacridone-4-carboxylic acid displaces [14C]ADP in a competitive manner, which indicates that both compounds share an identical binding site. Upon ultraviolet illumination, 7-azido-4-[1',2'-3H]isopropyl-acridone binds covalently to the ADP/ATP-carrier protein. By sequencing of its proteolytic fragments, radioactivity was detected in Cys159. This amino acid is located close to Lys162, a target of 2-azido-[alpha-32P]ADP [Dalbon, P., Brandolin, G., Boulay, F., Hoppe, J. & Vignais, P. V. (1988) Biochemistry 27, 5141-5149].

The reconstituted mitochondrial adenine nucleotide translocator: effects of lipid polymorphism

This study investigates the role of polymorphic or nonbilayer lipids in the function of an integral membrane protein which is a key component of the mitochondrial energy transduction apparatus. The adenine nucleotide translocator (AdNT) has been isolated from rat heart mitochondria and reconstituted into ATP-containing liposomes composed of dioleoylphosphatidylcholine (DOPC), dioleoylphosphatidylethanolamine (DOPE), and cardiolipin (CL). CL content was held constant at 11.1 mol%; the ratio of DOPC:DOPE was varied to manipulate R0, the intrinsic radius of curvature of the bilayer [S. M. Gruner (1985) Proc. Natl. Acad. Sci. USA 82, 3665-3669]. Translocator activity was determined fluorometrically, using a coupled enzyme system to measure ADP-induced efflux of ATP. Specific activity was calculated based on the number of functional translocators in each preparation, quantified using the tight-binding inhibitor carboxyatractylate (CAT). AdNT specific activity was a smooth function of R0, with a maximum at a lipid composition similar to that of the inner mitochondrial membrane. Protein incorporation was constant at DOPC:DOPE ratios > 1, but appeared to increase at ratios < or = 1. The fraction of reconstituted AdNT incorporated in the native mitochondrial orientation, estimated from inhibition by 10 microM CAT, was independent of lipid composition and > 85%. Leakage of encapsulated ATP increased at low R0 values both in the presence and absence of protein.

[Closure of Ca2+-dependent pores by cyclosporin A: the role of magnesium ions, adenine nucleotides, and conformation status of the ADP/ATP antiporter]

Effects of ADP and Mg2+ on the ability of cyclosporin A to "reseal" mitochondria permeabilized by Ca2+ and P(i) have been studied. Cyclosporin A was completely ineffective, when ADP and Mg2+ were not included into the incubation medium. Both ADP and Mg2+ used at high concentrations potentiated the effect of cyclosporin A and prevented it reversal by carboxyatractylate. Data on the influence of different concentrations of ADP and Mg2+ on the resealing efficiency of cyclosporin A suggest that the true effector modulating the state of the Ca(2+)-dependent pore is the ADP-Mg2+ complex, but not ADP or Mg2+ used separately. The ability of non-hydrolyzable analogs of adenine nucleotides, ADP-S and ATP-S, to potentiate the resealing action of cyclosporin on mitochondria permeabilized by loading of different Ca2+ concentrations to that of ADP was compared. ATP-S was ineffective when the pore was induced by high concentrations of Ca2+. The results obtained are discussed in terms of hypothesis on the direct involvement of the ADP/ATP antiporter in regulation of the inner mitochondrial membrane Ca(2+)-dependent pore state.

Isolation and sequence analysis of a cDNA encoding an adenine nucleotide translocator from Plasmodium falciparum

A cDNA clone encoding the polypeptide for Plasmodium falciparum adenine nucleotide translocator (ANT) was isolated by screening a cDNA library with a 150 base pair fragment of genomic DNA which had been enzymatically amplified using two oligonucleotide primers designed from conserved regions of ANT's from other sources. The deduced amino acid sequence of the P. falciparum cloned insert was highly homologous to ANT of other organisms. Features of the sequence are discussed with reference to the targeting and membrane insertion of ANT. The protein has a molecular mass of 35 kDa as predicted from the 303 amino acids encoded in the open reading frame.

Isolation and characterization of cDNA clones and a genomic clone encoding rat mitochondrial adenine nucleotide translocator

Two cDNA clones encoding rat mitochondrial adenine nucleotide translocator were isolated from libraries constructed from mRNAs of heart and liver. These two clones corresponded to the heart-skeletal muscle type (ANT1) and fibroblast type (ANT2), respectively. A genomic clone encoding rat ANT1 was also isolated and characterized.

Transmembrane topology, genes, and biogenesis of the mitochondrial phosphate and oxoglutarate carriers

Phosphate and oxoglutarate carriers transport phosphate and oxoglutarate across the inner membranes of mitochondria in exchange for OH- and malate, respectively. Both carriers belong to the mitochondrial carrier protein family, characterized by a tripartite structure made up of related sequences about 100 amino acids in length. The results obtained on the topology of the phosphate and oxoglutarate carriers are consistent with the six alpha-helix model proposed by Saraste and Walker. In both carriers the N- and C-terminal regions are exposed toward the cytosol. In addition, the oxoglutarate carrier has been shown to be a dimer by means of crosslinking studies. The bovine and human genes coding for the oxoglutarate carrier are split into eight and six exons, respectively, and five introns are found to the same position in both genes. The bovine and human phosphate carrier genes have the same organization with nine exons separated by eight introns at exactly the same positions. The phosphate carrier of mammalian mitochondria is synthesized with a cleavable presequence, in contrast to the oxoglutarate carrier and the other members of the mitochondrial carrier family. The precursor of the phosphate carrier is efficiently imported, proteolytically processed, and correctly assembled in isolated mitochondria. The presequence-deficient phosphate carrier is imported with an efficiency of about 50% as compared with the precursor of the phosphate carrier and is correctly assembled, demonstrating that the mature portion of the phosphate carrier contains sufficient information for import and assembly into mitochondria.

Dialectics in carrier research: the ADP/ATP carrier and the uncoupling protein

A concise review is given of the research in our laboratory on the ADP/ATP carrier (AAC) and the uncoupling protein (UCP). Although homologous proteins, their widely different functions and contrasts are stressed. The pioneer role of research on the AAC, not only for the mitochondrial but also for other carriers, and the present state of their structure-function relationship is reviewed. The function of UCP as a highly regulated H+ carrier is described in contrast to the largely unregulated ADP/ATP exchange in AAC. General principles of carrier catalysis as derived from studies on the AAC and UCP are elucidated.

The mitochondrial transport protein superfamily

The ADP/ATP, phosphate, and oxoglutarate/malate carrier proteins found in the inner membranes of mitochondria, and the uncoupling protein from mitochondria in mammalian brown adipose tissue, belong to the same protein superfamily. Established members of this superfamily have polypeptide chains approximately 300 amino acids long that consist of three tandem related sequences of about 100 amino acids. The tandem repeats from the different proteins are interrelated, and probably have similar secondary structures. The common features of this superfamily are also present in nine proteins of unknown functions characterized by DNA sequencing in various species, most notably in Caenorhabditis elegans and Saccharomyces cerevisiae. The high level expression in Escherichia coli of the bovine oxoglutarate/malate carrier, and the reconstitution of active carrier from the expressed protein, offers encouragement that the identity of superfamily members of known sequence but unknown function may be uncovered by a similar route.

Chemical, immunological, enzymatic, and genetic approaches to studying the arrangement of the peptide chain of the ADP/ATP carrier in the mitochondrial membrane

In the process of oxidative phosphorylation, the exchange of cytosolic ADP3- against mitochondrial ATP4- across the inner mitochondrial membrane is mediated by a specific carrier protein. Two different conformations for this carrier have been demonstrated on the basis of interactions with specific inhibitors, namely carboxyatractyloside (CATR) and bongkrekic acid (BA). The two conformations, referred to as CATR and BA conformations, are interconvertible, provided that ADP or ATP are present. The functional ADP/ATP carrier is probably organized as a tetramer. In the presence of CATR or BA the tetramer is split into two dimers combined with either of the two inhibitors. The amino acid sequence of the beef heart carrier monomer (297 residues) contains three repeats of about 100 residues each. Experimental results obtained through different approaches, including photolabeling, immunochemistry, and limited proteolysis, can be interpreted on the basis of a model with five or six transmembrane alpha helices per carrier monomer. Two mobile regions involved in the binding of nucleotides and accessible to proteolytic enzymes have been identified. Each of them may be visualized as consisting of two pairs of short amphipathic alpha helices, which can be juxtaposed to form hydrophilic channels facilitating the nucleotide transport. Mutagenesis in yeast is currently being used to detect strategic amino acids in ADP/ATP transport.

Abnormalities of ADP/ATP carrier protein in J-2-N cardiomyopathic hamsters

ADP/ATP carrier protein (AAC) is located in the mitochondrial inner membrane and has an important function in mitochondrial energy supply. This protein transports ATP to the cytoplasm and counter transports ADP into the mitochondria. J-2-N cardiomyopathic hamsters were investigated to determine the AAC content in cardiac mitochondria. After recording an electrocardiogram and collecting blood, the cardiac mitochondria were isolated. The mitochondrial membranes were labelled with eosin-5-maleimide (EMA) and separated on SDS polyacrylamide gels. The position of the AAC component was identified by exposing the gel under UV light, and the AAC content was determined by densitometry after staining with Coomassie blue. The AAC content ratio was significantly decreased in both 10-week-old and 1-year survived J-2-N hamsters when compared to control Golden hamster. Among 10-week-old J-2-N hamsters, the decrease in the AAC content ratio was more marked for the animals with more severe myocardial damage. The H(+)-ATPase activities of mitochondrial membrane were higher in 10-week-old J-2-N hamsters than in control hamsters. These results suggest that the decrease of AAC in J-2-N hamster plays an important role in the pathogenesis of cardiomyopathy in J-2-N hamsters.

Topography of the membrane-bound ADP/ATP carrier assessed by enzymatic proteolysis

The folding of the peptide chain of the beef heart ADP/ATP carrier in the inner mitochondrial membrane was investigated by enzymatic and immunochemical approaches, using specific proteases and polyclonal antibodies directed against the whole protein and specific regions of the carrier. The accessibility of the membrane-bound ADP/ATP carrier to proteases was followed by immunodetection of the cleavage products, using mitochondria devoid of outer membrane (mitoplasts) and inside-out submitochondrial particles (SMP) in the presence of either carboxyatractyloside (CATR) or bongkrekic acid (BA), two specific inhibitors which are able to bind to the outer face or the inner face of the carrier, respectively. Four types of particles were investigated, namely, mitoplasts-CATR, mitoplasts-BA, SMP-CATR, and SMP-BA. Only the ADP/ATP carrier in SMP-BA was cleaved by two specific proteases, namely, trypsin and lysine C endoprotease, at low doses for short periods of time. Two initial cleavage sites were found between Lys-42 and Glu-43, and between Lys-244 and Gly-245. After a longer period of incubation, an additional cleavage site between Lys-146 and Gly-147 could be demonstrated. Despite cleavage of the membrane-embedded carrier, the binding capacity and affinity of SMP for BA were not altered. A number of other proteases tested, including V8 protease, proline C endoprotease, thrombin, alpha-chymotrypsin, and thermolysin had virtually no effect. These results are explained by a dynamic model of the arrangement of the peptide chain of the ADP/ATP carrier.(ABSTRACT TRUNCATED AT 250 WORDS)

Peroxidative damage to cardiac mitochondria: identification and purification of modified adenine nucleotide translocase

Rat myocardial membranes exposed to free radical-generating systems exhibit both lipid peroxidation and protein alterations. The most sensitive protein, a 28-kDa polypeptide, was previously shown to increase slightly in apparent molecular weight before disappearing completely from the protein profile [N. L. Parinandi, C. W. Zwizinski, and H. H. O. Schmid (1991) Arch. Biochem. Biophys. 289, 118-123]. We now report that isolated cardiac mitochondria contain a 28-kDa protein which responds in the same manner to treatment with Cu2+/t-butylhydroperoxide. The protein exhibits several characteristic properties of the mitochondrial adenine nucleotide translocase. This assignment is supported by the finding that carboxyatractyloside, a specific inhibitor of the adenine nucleotide translocase, can prevent the oxidant-induced changes in the 28-kDa protein. Efficient purification schemes for the isolation of milligram quantities of unmodified and oxidatively altered adenine nucleotide translocase from rat heart mitochondria are described.

Calcium-dependent opening of a non-specific pore in the mitochondrial inner membrane is inhibited at pH values below 7. Implications for the protective effect of low pH against chemical and hypoxic cell damage

1. The rate of opening of the Ca(2+)-induced non-specific, cyclosporin A-inhibited, pore of the mitochondrial inner membrane of rat heart and liver mitochondria at pH 6.0 was less than 10% of that at pH 7.4. 2. The effect could not be explained by inhibition of Ca2+ uptake into the mitochondria, or of the matrix peptidyl-prolyl cis-trans isomerase (PPIase), or of the Ca(2+)-induced conformational change of the adenine nucleotide translocase. 3. It is suggested that the proposed interaction of matrix PPIase with the 'c' conformation of the adenine nucleotide carrier in the presence of Ca2+ [Griffiths & Halestrap (1991) Biochem. J. 274, 611-614] is inhibited by low pH. 4. The relevance of this to the protective effect of low pH on hypoxic and chemical-induced cell damage is discussed.

Mitochondrial nucleotide translocase from skeletal muscle of halothane sensitive pigs: an electrophoretic study

ATP translocation into mitochondria isolated from halothane-sensitive pig (HP) muscle was dramatically reduced compared with normal pigs (NP). To determine if this was due to a decreased amount of ATP translocase in the mitochondrial membranes, or a structural modification of this protein, an electrophoretic study was undertaken. Total proteins and purified translocase preparations from (NP) and (HP) mitochondria were analyzed by SDS gel electrophoresis. In the two types of mitochondria no significant differences were observed either in the amount of ATP translocase or in the molecular weight. Also, neither nonequilibrium pH gradient gel electrophoresis nor the analysis of peptides produced by limited proteolysis revealed any structural difference between the two types of protein. On the basis of these results, the depressed translocase activity observed in (HP) mitochondria cannot be explained by a reduced amount of the nucleotide translocase, nor a structural alteration of this protein. Possible inhibition of (HP) translocase activity by Ca2+ accumulation or by other mechanisms is discussed.

Lipid-protein interactions in ADP-ATP carrier/egg phosphatidylcholine recombinants studied by spin-label ESR spectroscopy

The stoichiometry and specificity of lipid-protein interaction, as well as the lipid exchange rates at the protein interface, have been determined from the electron spin resonance spectra of spin-labeled lipids in reconstituted complexes of the mitochondrial ADP-ATP carrier with egg phosphatidylcholine. With the exception of cardiolipin and phosphatidic acid, the lipids studied are found to compete for approximately 50 sites at the intramembranous surface of the protein dimer. This number of first-shell lipid sites is unusually large for a protein of this size. The specificity for the protein is in the order stearic acid approximately phosphatidic acid approximately cardiolipin greater than phosphatidylserine greater than phosphatidylglycerol approximately phosphatidylcholine, with the maximum association constant relative to phosphatidylcholine being approximately 4. The selectivity for anionic lipids was partially screened with increasing ionic strength, but to a lesser extent for cardiolipin and phosphatidic acid than for stearic acid. Only in the case of phosphatidylserine was the selectivity reduced at high ionic strength to a level close to that for phosphatidylcholine. The off rates for lipid exchange at the protein surface were independent of lipid/protein ratio and correlated in a reciprocal fashion with the different lipid selectivities, varying from 5 x 10(6) s-1 for stearic acid at low ionic strength to 2 x 10(7) s-1 for phosphatidylcholine and phosphatidylglycerol. The off rates for cardiolipin were unusually low in comparison with the observed selectivity, and indicated the existence of a special population of sites (ca. 30% of the total) for cardiolipin, at which the exchange rate was very low.(ABSTRACT TRUNCATED AT 250 WORDS)