ADP/ATP carrier and uncoupling protein

Pathophysiological implications of mitochondrial oxidative stress mediated by mitochondriotropic agents and polyamines: the role of tyrosine phosphorylation

Mitochondria, once merely considered as the "powerhouse" of cells, as they generate more than 90 % of cellular ATP, are now known to play a central role in many metabolic processes, including oxidative stress and apoptosis. More than 40 known human diseases are the result of excessive production of reactive oxygen species (ROS), bioenergetic collapse and dysregulated apoptosis. Mitochondria are the main source of ROS in cells, due to the activity of the respiratory chain. In normal physiological conditions, ROS generation is limited by the anti-oxidant enzymatic systems in mitochondria. However, disregulation of the activity of these enzymes or interaction of respiratory complexes with mitochondriotropic agents may lead to a rise in ROS concentrations, resulting in oxidative stress, mitochondrial permeability transition (MPT) induction and triggering of the apoptotic pathway. ROS concentration is also increased by the activity of amine oxidases located inside and outside mitochondria, with oxidation of biogenic amines and polyamines. However, it should also be recalled that, depending on its concentration, the polyamine spermine can also protect against stress caused by ROS scavenging. In higher organisms, cell signaling pathways are the main regulators in energy production, since they act at the level of mitochondrial oxidative phosphorylation and participate in the induction of the MPT. Thus, respiratory complexes, ATP synthase and transition pore components are the targets of tyrosine kinases and phosphatases. Increased ROS may also regulate the tyrosine phosphorylation of target proteins by activating Src kinases or phosphatases, preventing or inducing a number of pathological states.

Physiological and pathological roles of mitochondrial SLC25 carriers

The mitochondrion relies on compartmentalization of certain enzymes, ions and metabolites for the sake of efficient metabolism. In order to fulfil its activities, a myriad of carriers are properly expressed, targeted and folded in the inner mitochondrial membrane. Among these carriers, the six-transmembrane-helix mitochondrial SLC25 (solute carrier family 25) proteins facilitate transport of solutes with disparate chemical identities across the inner mitochondrial membrane. Although their proper function replenishes building blocks needed for metabolic reactions, dysfunctional SLC25 proteins are involved in pathological states. It is the purpose of the present review to cover the current knowledge on the role of SLC25 transporters in health and disease.

Biogenesis of yeast dicarboxylate carrier: the carrier signature facilitates translocation across the mitochondrial outer membrane

A family of related carrier proteins mediates the exchange of metabolites across the mitochondrial inner membrane. The carrier signature Px[D/E]xx[K/R] is a highly conserved sequence motif in all members of this family. To determine its function in the biogenesis of carrier proteins, we used the dicarboxylate carrier (DIC) of yeast as a model protein. We found that the carrier signature was dispensable in binding of the newly synthesized protein to the import receptor Tom70, but that it was specifically required for efficient translocation across the mitochondrial outer membrane. To determine the relevance of individual amino acid residues of the carrier signature in the transport activity of the protein, we exchanged defined residues with alanine and reconstituted the mutant proteins in vitro. Substitution of the carrier signature in helix H1 reduced the transport activity for [33P]-phosphate by approximately 90% and an additional substitution of the carrier signature in helix H5 blocked the transport activity completely. We conclude that the carrier signature of the dicarboxylate carrier is involved both in the biogenesis and in the transport activity of the functional protein.

Molecular properties of purified human uncoupling protein 2 refolded from bacterial inclusion bodies

One way to study low-abundance mammalian mitochondrial carriers is by ectopically expressing them as bacterial inclusion bodies. Problems encountered with this approach include protein refolding, homogeneity, and stability. In this study, we investigated protein refolding and homogeneity properties of inclusion body human uncoupling protein 2 (UCP2). N-methylanthraniloyl-tagged ATP (Mant-ATP) experiments indicated two independent inclusion body UCP2 binding sites with dissociation constants (Kd) of 0.3-0.5 and 23-92 microM. Dimethylanthranilate, the fluorescent tag without nucleotide, bound with a Kd of greater than 100 microM, suggesting that the low affinity site reflected binding of the tag. By direct titration, UCP2 bound [8-(14)C] ATP and [8-(14)C] ADP with Kds of 4-5 and 16-18 microM, respectively. Mg2+ (2 mM) reduced the apparent ATP affinity to 53 microM, an effect entirely explained by chelation of ATP; with Mg2+, Kd using calculated free ATP was 3 microM. A combination of gel filtration, Cu2+-phenanthroline cross-linking, and ultracentrifugation indicated that 75-80% of UCP2 was in a monodisperse, 197 kDa form while the remainder was aggregated. We conclude that (a) Mant-tagged nucleotides are useful fluorescent probes with isolated UCP2 when used with dimethylanthranilate controls; (b) UCP2 binds Mg2+-free nucleotides: the Kd for ATP is about 3-5 microM and for Mant-ATP it is about 10 times lower; and (c) in C12E9 detergent, the monodisperse protein may be in dimeric form.

Altered chromatographic behaviour of mitochondrial ADP/ATP translocase induced by stabilization of the protein by binding of 6'-O-fluorescein-atractyloside

Atractyloside (ATR) is a high-affinity specific inhibitor of the mitochondrial ADP/ATP translocase (AAT). The binding of a fluorescent derivative, 6'- O -fluorescein-ATR (FATR), to mitochondria has been characterized. The binding constants obtained are in agreement with previously published values for ATR, demonstrating that FATR is a suitable probe of the AAT. AAT inhibited by FATR (FATR-AAT) was solubilized in dodecyl maltoside and purified by two separate ion-exchange chromatography steps at different pHs, which allowed FATR-AAT to be purified to homogeneity. The presence of the bound fluorescent probe enabled the inhibited AAT to be distinguished from the unliganded protein during chromatography, as they were markedly different in their chromatographic behaviour. The purified FATR-AAT was dimeric and in a single major conformation containing 1 mole FATR per mole of AAT dimer. In contrast, uninhibited AAT was monomeric and conformationally unstable. Use of the fluorescent ATR derivative in the development of the protocol enabled the stable dimeric AAT to be monitored directly and purified more effectively. The purification protocol was repeated using non-derivatized ATR, and highly pure AAT was obtained that was devoid of other members of the mitochondrial carrier family.

Reconstituted TOM core complex and Tim9/Tim10 complex of mitochondria are sufficient for translocation of the ADP/ATP carrier across membranes

Precursor proteins of the solute carrier family and of channel forming Tim components are imported into mitochondria in two main steps. First, they are translocated through the TOM complex in the outer membrane, a process assisted by the Tim9/Tim10 complex. They are passed on to the TIM22 complex, which facilitates their insertion into the inner membrane. In the present study, we have analyzed the function of the Tim9/Tim10 complex in the translocation of substrates across the outer membrane of mitochondria. The purified TOM core complex was reconstituted into lipid vesicles in which purified Tim9/Tim10 complex was entrapped. The precursor of the ADP/ATP carrier (AAC) was found to be translocated across the membrane of such lipid vesicles. Thus, these components are sufficient for translocation of AAC precursor across the outer membrane. Peptide libraries covering various substrate proteins were used to identify segments that are bound by Tim9/Tim10 complex upon translocation through the TOM complex. The patterns of binding sites on the substrate proteins suggest a mechanism by which portions of membrane-spanning segments together with flanking hydrophilic segments are recognized and bound by the Tim9/Tim10 complex as they emerge from the TOM complex into the intermembrane space.

Structural and functional implications of the instability of the ADP/ATP transporter purified from mitochondria as revealed by FTIR spectroscopy

The ADP/ATP transporter shows a high instability when solubilized, making it difficult to obtain functional protein with sufficient purity for long-term spectroscopic studies. When solubilized in the detergent dodecyl maltoside the protein is in equilibrium between the so-called CATR and BA conformations and in a few hours it becomes nonfunctional, unable to bind either its inhibitors or its substrates. By Fourier transform infrared spectroscopy, we studied the structural changes involved in this denaturation process. To do so, the carboxyatractyloside-inhibited protein was used as a structural model for the protein in the CATR conformation and its spectrum was compared with that of the unliganded time-inactivated protein. From the difference spectra of the amide I, amide II, and amide A bands combined with dichroism spectra of the carboxyatractyloside-inhibited protein, we concluded that few structural differences exist between both states, affecting as few as 11 amino acids (3.5% of the protein); the structural changes consisted in the disappearance of large loop structure and the appearance of aggregated strands. We hypothesize that some mitochondrial loop (tentatively loop M1) shows a high tendency to aggregate, being responsible for the observed features. The functional consequences of this hypothesis are discussed.

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.

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

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

The permeability transition pore complex: another view

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

Mitochondria make a come back

This review attempts to summarize our present state of knowledge of mitochondria in relation to a number of areas of biology, and to indicate where future research might be directed. In the evolution of eukaryotic cells mitochondria have for a long time played a prominent role. Nowadays their integration into many activities of a cell, and their dynamic behavior as subcellular organelles within a cell and during cell division are a major focus of attention. The crystal structures of the major complexes of the electron transport chain (except complex I) have been established, permitting increasingly detailed analyses of the important mechanism of proton pumping coupled to electron transport. The mitochondrial genome and its replication and expression are beginning to be understood in considerable detail, but more questions remain with regard to mutations and their repair, and the segregation of the mtDNA in oogenesis and development. Much emphasis and a large effort have recently been devoted to understand the role of mitochondria in programmed cell death (apoptosis). The understanding of their central role in mitochondrial diseases is a major achievement of the past decade. Finally, various drugs have traditionally played a part in understanding biochemical mechanisms within mitochondria; the repertoire of drugs with novel and interesting targets is expanding.

Expression of the mitochondrial ADP/ATP carrier in Escherichia coli. Renaturation, reconstitution, and the effect of mutations on 10 positive residues

Previously, the role of residues in the ADP/ATP carrier (AAC) from Saccharomyces cerevisiae has been studied by mutagenesis, but the dependence of mitochondrial biogenesis on functional AAC impedes segregation of the mutational effects on transport and biogenesis. Unlike other mitochondrial carriers, expression of the AAC from yeast or mammalians in Escherichia coli encountered difficulties because of disparate codon usage. Here we introduce the AAC from Neurospora crassa in E. coli, where it is accumulated in inclusion bodies and establish the reconstitution conditions. AAC expressed with heat shock vector gave higher activity than with pET-3a. Transport activity was absolutely dependent on cardiolipin. The 10 single mutations of intrahelical positive residues and of the matrix repeat (+X+) motif resulted in lower activity, except of R245A. R143A had decreased sensitivity toward carboxyatractylate. The ATP-linked exchange is generally more affected than ADP exchange. This reflects a charge network that propagates positive charge defects to ATP(4-) more strongly than to ADP(3-) transport. Comparison to the homologous mutants of yeast AAC2 permits attribution of the roles of these residues more to ADP/ATP transport or to AAC import into mitochondria.

In the uncoupling protein (UCP-1) His-214 is involved in the regulation of purine nucleoside triphosphate but not diphosphate binding

The nucleotide binding to uncoupling protein (UCP-1) of brown adipose tissue is regulated by pH. The binding pocket of the nucleotide phosphate moiety has been proposed to be controlled by the protonization of a carboxyl group (pK approximately 4.5) for both nucleoside diphosphates (NDP) and nucleoside triphosphates (NTP) (identified as Glu-190) and of a histidine (pK approximately 7. 2) for NTP only. Here we identify His-214 as a pH sensor specific for NTP binding only. In reconstituted UCP-1 from hamster, DEPC diminishes binding of NTP but not of NDP. It also prevents inhibition of H+ transport by NTP but not by NDP. Hamster UCP-1 expressed in Saccharomyces cerevisiae was mutated to H214N resulting in only moderate change of the binding affinity for NTP (GTP) but a 10-fold affinity decrease with the bulkier substituent in H214W, whereas the affinity for NDP (ADP) was largely unchanged. The steep decrease with pH of the binding affinity for NTP in wild type (from pH 6.0 to 7.5) was much flatter in the mutants. Also, the pH dependence of binding and dissociation rates was diminished in these mutants. The transport of H+ and Cl- was not affected. Thus, His-214 is only involved in nucleotide binding, whereas, as previously shown, His-145 and His-147 are involved only in H+ transport. The results validate the earlier proposal of a histidine regulating the NTP binding in addition to a carboxyl group controlling both NTP and NDP binding. It is proposed that His-214 protrudes into the binding pocket for the gamma-phosphate thus inhibiting NTP binding and that His214H+ is retracted by a background -CO2- group to give way for the gamma-phosphate.

Mutagenesis of the uncoupling protein of brown adipose tissue. Neutralization Of E190 largely abolishes pH control of nucleotide binding

For expression in Saccharomyces cerevisiae the cDNA of the uncoupling protein (UCP) of brown adipose tissue from hamster has been isolated and used to transform yeast cells. Optimized expression conditions yielded 2% of mitochondrial protein as UCP. UCP was isolated, avoiding copurification of ADP/ATP carrier and porin. Intrahelical E190, previously suggested to be the pH sensor for nucleotide binding, was neutralized to glutamine by mutagenesis. In binding titrations with [14C]guanosine 5'-triphosphate (GTP) and with fluorescent dansyl-GTP, near equal binding capacity for GTP was measured in wild-type (wt) and E190Q. The KD for GTP binding to UCP from yeast has the same strong pH dependence as the original UCP from hamster. With both [14C]GTP and dansyl-GTP, the KD in wt increased 16-19-fold from pH 6.0 to 7.5, while in E190Q this increase was only 2.5-2.9-fold. As a result, at pH 7.5, both [14C]GTP and dansyl-GTP bind 6-fold tighter to E190Q than to wt. The binding rate of GTP decreased 10-fold from pH 6.0 to 7.5 in wt and only 4-fold in E190Q. Woodward reagent K (WRK) known to interact specifically with E190 [Winkler, E., Wachter, E., and Klingenberg, M. (1997) Biochemistry 36, 148-155] abolished [14C]GTP and dansyl-GTP binding to wt UCP, whereas binding to E190Q was fully resistant to WRK. H+ and Cl- transport activity in reconstituted vesicles were the same with wt and E190Q. At pH 7.5, 5 microM GTP is unable to inhibit H+ and Cl- transport in wt but inhibits in E190Q to maximum level. The different sensitivity toward GTP versus GDP found in wt is absent in E190Q. Thus, the mutation E190Q results in the predicted gain of function in binding and proves the role of the intrahelical E190 as a pH sensor for nucleotide binding but excludes a role in transport.