Structural basis for lipid-mediated interactions between mitochondrial ADP/ATP carrier monomers

The oligomerization state of the ADP/ATP carrier is an important issue in understanding the mechanism underlying nucleotide exchange across the inner mitochondrial membrane. The first high resolution structure obtained in the presence of carboxyatractyloside revealed a large cavity formed within a monomer in which the inhibitor is strongly bound. Whereas the protein-protein interactions implicated in the first crystal form are not biologically relevant, the new crystal form described herein, highlights favorable protein-protein interactions. The interactions are mediated by endogenous cardiolipins, which are tightly bound to the protein, two cardiolipins being sandwiched between the monomers on the matrix side. The putative dimerization interface evidenced here is consistent with other structural, biochemical or functional data published so far.     

The delivery of ADP/ATP carrier protein to mitochondria probed by fusions with green fluorescent protein and beta-galactosidase

The import of proteins into mitochondria is an essential process, largely investigated in vitro with isolated mitochondria and radioactively labeled precursors. In this study, we used intact cells and fusions with genes encoding two reporter proteins, green fluorescent protein (GFP) and beta-galactosidase (lacZ), to probe the import of the ADP/ATP carrier (AAC). Typical mitochondrial fluorescence was observed with AAC-GFP fusions containing at least one complete transmembrane loop. This confirms the results of in vitro analysis demonstrating that an internal targeting signal was present in each one of the three transmembrane loops of the carrier. The fusions of AAC fragments to beta-galactosidase demonstrated that the targeting signal was capable of delivering the reporter molecule to the mitochondrial surface, but not to internalize it to a protease-inaccessible location. The delivery to a protease-inaccessible location required the presence of more distal sequences present within the third (C-terminal) transmembrane loop of the carrier molecule. The results of our study provide an alternative for investigation in a natural context of mitochondrial protein import in cells when the isolation of intact, functional mitochondria is not achievable.     

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 ADP/ATP carrier: functional and structural studies in the route of elucidating pathophysiological aspects

The mitochondrial ADP/ATP carrier plays a central role in aerobic cell energetics by providing to the cytosol the ATP generated by oxidative phosphorylation. Though discovered around 40 years ago owing to the existence of unique inhibitors and in spite of numerous experimental approaches, this carrier, which stands as a model of the mitochondrial solute carriers keeps some long-standing mystery. There are still open challenging questions among them the precise ADP/ATP transport mechanism, the functional oligomeric state of the carrier and relationships between human ADP/ATP carrier dysfunctioning and pathologies. Deciphering the 3D structure of this carrier afforded a considerable progress of the knowledge but requires now additional data focused on molecular dynamics from this static picture. State of the art in this topic is reviewed and debated in this paper in view of better comprehending origin of the discrepancies in these questions and, finally, the multiple physiological roles of this carrier in eukaryotic cell economy.     

Studies of the ADP/ATP carrier of mitochondria with fluorescent ADP analogue formycin diphosphate

The ADP/ATP carrier was studied by a fluorescent substrate, formycin diphosphate which is the only fluorescent ADP analogue to bind. Its low quantum yield, short decay time and spectral overlap with tryptophan has as yet prevented its wider use.By incorporating fluorescent acceptors of formycin diphosphate fluorescence, anthracene-maleimide and vinylanthracene, into the membrane, these difficulties were circumvented. Only bound formycin diphosphate transfers energy to the probes so that the secondary emission of these probes is a measure for membrane-bound formycin diphosphate.The fluorescent transfer is inhibited by ADP, bongkrekate and carboxy-atractylate whether added before or after incubation of formycin diphosphate showing that only binding to the adenine nucleotide carrier is measured. It also shows directly that the earlier demonstrated ADP fixation by bongkrekate is indeed a displacement into the matrix.The fluorescence decay time of the bound formycin diphosphate is measured as 1.95 ns compared to 0.95 ns of the free formycin diphosphate, indicating that formycin diphosphate is bound at the carrier in a non-polar environment.The depolarization decay time was found to be larger than 15 ns, indicating that carrier-bound formycin diphosphate is immobile within this time period.     

The transmembrane prolines of the mitochondrial ADP/ATP carrier are involved in nucleotide binding and transport and its biogenesis

The mitochondrial ADP/ATP carrier (Ancp) is a paradigm of the mitochondrial carrier family, which allows cross-talk between mitochondria, where cell energy is mainly produced, and cytosol, where cell energy is mainly consumed. The members of this family share numerous structural and functional characteristics. Resolution of the atomic structure of the bovine Ancp, in a complex with one of its specific inhibitors, revealed interesting features and suggested the involvement of some particular residues in the movements of the protein to perform translocation of nucleotides from one side of the membrane to the other. They correspond to three prolines located in the odd-numbered transmembrane helices (TMH), Pro-27, Pro-132, and Pro-229. The corresponding residues of the yeast Ancp (Pro-43, Ser-147, and Pro-247) were mutated into alanine or leucine, one at a time and analysis of the various mutants evidenced a crucial role of Pro-43 and Pro-247 during nucleotide transport. Beside, replacement of Ser-147 with proline does not inactivate Ancp and this is discussed in view of the conservation of the three prolines at equivalent positions in the Ancp sequences. These prolines belong to the signature sequences of the mitochondrial carriers and we propose they play a dual role in the mitochondrial ADP/ATP carrier function and biogenesis. Unexpectedly their mutations cause more general effects on mitochondrial biogenesis and morphology, as evidenced by measurements of respiratory rates, cytochrome contents, and also clearly highlighted by fluorescence microscopy.     

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 ADP^3– against mitochondrial ATP^4– 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 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 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.     

Four mutations in transmembrane domains of the mitochondrial ADP/ATP carrier increase resistance to bongkrekic acid

Two distinct conformations of the mitochondrial ADP/ATP carrier involved in the adenine nucleotide transport are called BA and CATR conformations, as they were distinguished by binding of specific inhibitors bongkrekic acid (BA) and carboxyatractyloside (CATR), respectively. To find out which amino acids are implicated in the transition between these two conformations, which occurs during transport, mutants of the Saccharomyces cerevisiae ADP/ATP carrier Anc2p responsible for resistance of yeast cells to BA were identified and characterized after in vivo chemical or UV mutagenesis. Only four different mutations could be identified in spite of a large number of mutants analyzed. They are located in the Anc2p transmembrane segments I (G30S), II (Y97C), III (L142S), and VI (G298S), and are independently enabling growth of cells in the presence of BA. The variant and wild-type Anc2p were produced practically to the same level in mitochondria, as evidenced by immunochemical analysis and by atractyloside binding experiments. ADP/ATP exchange mediated by Anc2p variants in isolated mitochondria was more efficient than that of the wild-type Anc2p in the presence of BA, confirming that BA resistance of the mutant cells was linked to the functional properties of the modified ADP/ATP carrier. These results suggest that resistance to BA is caused by alternate conformation of Anc2p due to appearance of Ser or Cys at specific positions. Different interactions of these residues with other amino acids and/or BA could prevent formation of stable inactive Anc2p BA complex.     

Fatty acid-promoted mitochondrial permeability transition by membrane depolarization and binding to the ADP/ATP carrier

The mechanism by which non-esterified long-chain fatty acids (FFA) promote mitochondrial permeability transition (MPT) is not clear. We examined with energized rat liver mitochondria the role of two possible actions of FFA in MPT, (i) the reduction of the transmembrane potential (Δψ) and (ii) the increase of the negative surface charge of the inner mitochondrial membrane [Broekemeier, K.M. and Pfeiffer, D.G., Biochemistry 43, (1995) 16440–16449]. It was found that the ability of FFA to stimulate large amplitude swelling is clearly related to their uncoupling activity. Moreover, compared with classical protonophores (FCCP) FFA increase the sensitivity of the pore opening process to Δψ changes. In addition, FFA interact like their thioester derivatives in a structure-dependent manner with the ADP/ATP carrier (measured as inhibition of [³H]atractyloside binding to the AAC protein). It is suggested that not only the protonophoric action of FFA, but also a presumable stabilization of the ‘cytosolic' conformation of AAC contribute to the FFA-promoted MPT.     

Choline head groups stabilize the matrix loop regions of the ATP/ADP carrier ScAAC2

ATP/ADP carriers (AACs) are essential to the cell as they exchange ATP produced in mitochondria for cytosolic ADP. Monoclonal antibodies against the isoform 2 of Saccharomyces cerevisiae AAC (ScAAC2) were used to probe the accessibility of the matrix loops 1 and 3 depending on the environment of the carrier. In mitochondrial membranes ScAAC2 was not recognized, whereas in dodecylmaltoside the antibodies bound to the carrier, suggesting that the epitopes are hidden in the native environment. Exposure of the epitopes by detergents was reversed by reconstitution of the carrier in phospholipids or by exchanging with detergents having a choline or a trimethylammonium head group. Circular dichroism spectroscopy on peptides representing the C-terminal regions of all three matrix loops showed that only phosphocholine detergents induced a structural reorganization. Since in addition phosphatidylcholine was found to be tightly associated with the purified carrier, the matrix loop regions are likely to be associated to the membrane by phosphatidylcholine.     

Functional Characterization of TbMCP5, a Conserved and Essential ADP/ATP Carrier Present in the Mitochondrion of the Human Pathogen Trypanosoma brucei

Trypanosoma brucei is a kinetoplastid parasite of medical and veterinary importance. Its digenetic life cycle alternates between the bloodstream form in the mammalian host and the procyclic form (PCF) in the bloodsucking insect vector, the tsetse fly. PCF trypanosomes rely in the glucose-depleted environment of the insect vector primarily on the mitochondrial oxidative phosphorylation of proline for their cellular ATP provision. We previously identified two T. brucei mitochondrial carrier family proteins, TbMCP5 and TbMCP15, with significant sequence similarity to functionally characterized ADP/ATP carriers from other eukaryotes. Comprehensive sequence analysis confirmed that TbMCP5 contains canonical ADP/ATP carrier sequence features, whereas they are not conserved in TbMCP15. Heterologous expression in the ANC-deficient yeast strain JL1Δ2Δ3u⁻ revealed that only TbMCP5 was able to restore its growth on the non-fermentable carbon source lactate. Transport studies in yeast mitochondria showed that TbMCP5 has biochemical properties and ADP/ATP exchange kinetics similar to those of Anc2p, the prototypical ADP/ATP carrier of S. cerevisiae. Immunofluorescence microscopy and Western blot analysis confirmed that TbMCP5 is exclusively mitochondrial and is differentially expressed with 4.5-fold more TbMCP5 in the procyclic form of the parasite. Silencing of TbMCP5 expression in PCF T. brucei revealed that this ADP/ATP carrier is essential for parasite growth, particularly when depending on proline for energy generation. Moreover, ADP/ATP exchange in isolated T. brucei mitochondria was eliminated upon TbMCP5 depletion. These results confirmed that TbMCP5 functions as the main ADP/ATP carrier in the trypanosome mitochondrion. The important role of TbMCP5 in the T. brucei energy metabolism is further discussed.     

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

Peptide maps were generated of the CNBr-digested mitochondrial phosphate-transport protein and ADP/ATP carrier from bovine and rat heart, rat liver and blowfly flight muscle. Total mitochondrial proteins from the same sources plus pig heart were separated by SDS-polyacrylamide gel electrophoresis. The peptide maps and the total mitochondrial proteins were electroblotted onto nitrocellulose membranes and reacted with rabbit antisera raised against the purified bovine heart phosphate-transport protein and the ADP/ATP carrier. On the basis of antibody specificity, mobility in SDS-polyacrylamide gel electrophoresis, and peptide maps the following was concluded. (1) Phosphate-transport protein and phosphate-transport protein β (pig and bovine heart) react equally with the first and also with the second of two independent phosphate-transport protein-antisera. (2) Tissue-specific structural domains exist for both the phosphate-transport protein and the ADP/ATP carrier, i.e., one phosphate-transport protein-antiserum reacts with the phosphate-transport protein from all assayed sources, the other only with the cardiac phosphate-transport protein. These differences may reflect tissue-specific regulation of phosphate and adenine nucleotide transport. (3) Homologies among the different species are found for the phosphate transport protein and the ADP/ATP carrier, except for the flight muscle ADP/ATP carrier. These conserved structural domains of the phosphate-transport protein may relate directly to catalytic activity. (4) Alkylation of the purified phosphate-transport proteins and the ADP/ATP carriers by the transport inhibitor N-ethylmaleimide affects electrophoretic mobilities but not the antibody binding. (5) Neither of the two phosphate-transport protein-antisera nor the ADP/ATP-carrier antiserum react with both phosphate transport protein and ADP/ATP carrier, even though these two proteins possess similarities in primary structure and function. Possible mechanisms for generating tissue-specific structural differences in the proteins are discussed.     

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.     

A hybrid model to study pathological mutations of the human ADP/ATP carriers

The adenine nucleotide carrier (Ancp) plays an essential role in the metabolism of cellular energy by catalyzing the transport of ADP and ATP across the inner mitochondrial membrane. Previous reports have indicated that mutations in the HANC1 gene, encoding the muscle isoform of human Ancp (HAnc1p), are directly involved in several diseases, including autosomal dominant progressive external ophthalmoplegia and cardiomyopathies. In this work, we studied three pathogenic HANC1 mutations at the biochemical level. To do so, we expressed the DdANCA gene, encoding the unique Ancp carrier of Dictyostelium discoideum (DdAncAp), in a yeast strain lacking all endogenous ANC genes. Our results indicate that DdAncAp is a good model for the human protein. It allows the carrier to be studied in yeast, and provides information on how the HANC1 mutations impair ADP/ATP transport in humans. A94D, A126D and V291M mutations, corresponding to A90D, A123D and V289M in HAnc1p, respectively, did not affect levels of DdAncAp in yeast mitochondria. However, while the wild-type DdAncAp fully restored growth of the ANC-null yeast strain on a non-fermentable carbon source, the carriers encompassing either the A94D or the A126D mutation failed to complement the null strain. The effect of the V291M mutation was not as pronounced, but led to impairment mainly of the nucleotide translocation process per se. These findings provide new insights into the mechanisms responsible for the diseases induced by HAnc1p mutations.     

The Saccharomyces cerevisiae ADP/ATP carrier-iso 1 cytochrome c fusion protein: one-step purification and functional analysis in vitro

Genetic expression versus plasmidic overexpression of a functional recombinant fusion protein combining the yeast Saccharomyces cerevisiae mitochondrial ADP/ATP carrier (Anc2p) and the iso-1-cytochrome c (Cyc1p) has been investigated, with the main aim of increasing the polar surface of the carrier to improve its crystallization properties. The gene encoding the his6-tagged fusion protein was expressed in yeast under the control of the regulatory sequences of ScANC2 or under the control of the strong yeast PMA1 promoter. In both cases, the chimeric carrier, Anc2-Cyc1(His6)p, was able to restore growth on a non-fermentable carbon source of a yeast strain devoid of functional ADP/ATP carrier, demonstrating its transport activity. Nevertheless, when the expression vector was used, the level of expression of Anc2-Cyc1(His6)p was no greater than that of the chimeric carrier obtained in yeast mitochondria after homologous recombination. Optimal conditions to extract and to purify Anc2-Cyc1(His6)p were determined. A series of detergents was screened for their ability to extract and to preserve in vitro the chimeric carrier. A rapid, single step purification of Anc2-Cyc1(His6)p was developed, using n-dodecyl-beta-d-maltoside (DoDM) as the best detergent to solubilize the chimeric protein. Carboxyatractyloside- (CATR-) and nucleotide-binding sites were preserved in the purified protein. Moreover, the Cyc1p moiety of Anc2-Cyc1(His6)p-CATR complex solubilized in DoDM was still able to interact in vitro with the cytochrome c oxidase (COX), with the same affinity as yeast Cyc1p. Improved production and purification of Anc2-Cyc1(His6)p-CATR complex opens up new possibilities for the use of this protein in crystallographic approaches to the yeast ADP/ATP carrier. Furthermore, Anc2-Cyc1(His6)p may be an useful molecular tool to investigate in vivo interactions between components of the respiratory chain complexes such as COX and the proteins implicated in ATP biogenesis, such as the ATP/ADP carrier.     

Isolation and characterization of a rice cDNA clone encoding ATP/ADP translocator

We isolated a rice cDNA clone which encodes an open reading frame of 382 amino acids. Its deduced amino acid sequence corresponds to an ATP/ADP translocator protein. Its homology with a maize ATP/ADP translocator was 83.9% in nucleotide sequence, and 90.2% of the amino acid level. Expression of this gene is regulated by such external stresses as salinity and low temperature.     

Heterologous complementation of the Klaac null mutation of Kluyveromyces lactis by the Saccharomyces cerevisiae AAC3 gene encoding the ADP/ATP carrier

The KlAAC gene, encoding the ADP/ATP carrier, has been assumed to be a single gene in Kluyveromyces lactis, an aerobic, petite-negative yeast species. The Klaac null mutation, which causes a respiratory-deficient phenotype, was fully complemented by AAC2, the Saccharomyces cerevisiae major gene for the ADP/ATP carrier and also by AAC1, a gene that is poorly expressed in S. cerevisiae. In this study, we demonstrate that the Klaac null mutation is partially complemented by the ScAAC3 gene, encoding the hypoxic ADP/ATP carrier isoform, whose expression in S. cerevisiae is prevented by oxygen. Once introduced into K. lactis, the AAC3 gene was expressed both under aerobic and under partial anaerobic conditions but did not support the growth of K. lactis under strict anaerobic conditions.     

Conformational Dynamics of the Bovine Mitochondrial ADP/ATP Carrier Isoform 1 Revealed by Hydrogen/Deuterium Exchange Coupled to Mass Spectrometry

The mitochondrial adenine nucleotide carrier (Ancp) catalyzes the transport of ADP and ATP across the mitochondrial inner membrane, thus playing an essential role in cellular energy metabolism. During the transport mechanism the carrier switches between two different conformations that can be blocked by two toxins: carboxyatractyloside (CATR) and bongkrekic acid. Therefore, our understanding of the nucleotide transport mechanism can be improved by analyzing structural differences of the individual inhibited states. We have solved the three-dimensional structure of bovine carrier isoform 1 (bAnc1p) in a complex with CATR, but the structure of the carrier-bongkrekic acid complex, and thus, the detailed mechanism of transport remains unknown. Improvements in sample processing in the hydrogen/deuterium exchange technique coupled to mass spectrometry (HDX-MS) have allowed us to gain novel insights into the conformational changes undergone by bAnc1p. This paper describes the first study of bAnc1p using HDX-MS. Results obtained with the CATR-bAnc1p complex were fully in agreement with published results, thus, validating our approach. On the other hand, the HDX kinetics of the two complexes displays marked differences. The bongkrekic acid-bAnc1p complex exhibits greater accessibility to the solvent on the matrix side, whereas the CATR-bAnc1p complex is more accessible on the intermembrane side. These results are discussed with respect to the structural and biochemical data available on Ancp.     

Structure, evolution and expression of the mitochondrial ADP/ATP translocator gene from Chlamydomonas reinhardtii

Summary The first AUG in the Chlamydomonas reinhardtii ADP/ATP translocator (CRANT) mRNA initiates an open reading frame (ORF) which is very similar (51–79% amino acid identity) to other ANT proteins. In contrast to higher plants, no evidence for a long amino-terminal extension was obtained. The 5 non-transcribed region of the single-copy CRANT gene contains sequence motifs present in other C. reinhardtii nuclear genes. Four introns, whose positions are not conserved in other ANT genes, interrupt the protein coding region. A short heat shock specifically reduces CRANT mRNA levels. CRANT mRNA levels were unaffected by a mutation in photosynthesis. In a dark/light regime CRANT mRNA levels are high in the dark phase and low in the early light phase. Data on translation initiation sites, splice junctions and the codon preferences of C. reinhardtii nuclear genes were compiled. With the exception of two rare codons, ACA and GGA, the CRANT gene exhibits the biased codon usage of C. reinhardtii nuclear genes that are highly expressed during normal vegetative growth.     

An ADP/ATP-specific mitochondrial carrier protein in the microsporidian Antonospora locustae

The mitochondrion is one of the defining characteristics of eukaryotic cells, and to date, no eukaryotic lineage has been shown to have lost mitochondria entirely. In certain anaerobic or microaerophilic lineages, however, the mitochondrion has become severely reduced that it lacks a genome and no longer synthesizes ATP. One example of such a reduced organelle, called the mitosome, is found in microsporidian parasites. Only a handful of potential mitosomal proteins were found to be encoded in the complete genome of the microsporidian Encephalitozoon cuniculi, and significantly no proteins of the mitochondrial carrier family were identified. These carriers facilitate the transport of solutes across the inner mitochondrial membrane, are a means of communication between the mitochondrion and cytosol, and are abundant in organisms with aerobic mitochondria. Here, we report the characterization of a mitochondrial carrier protein in the microsporidian Antonospora locustae and demonstrate that the protein is heterologously targeted to mitochondria in Saccharomyces cerevisiae. The protein is phylogenetically allied to the NAD⁺ transporter of S. cerevisiae, but we show that it has high specificity for ATP and ADP when expressed in Escherichia coli. An ADP/ATP carrier may provide ATP for essential ATP-dependent mitosomal processes such as Hsp70-dependent protein import and export of iron-sulfur clusters to the cytosol.