Close location of the first loop to the third loop of the mitochondrial ADP/ATP carrier deduced from cross-linking catalyzed by copper-o-phenanthroline of the solubilized carrier with Triton X-100

On the Detection and Functional Significance of the Protein-Protein Interactions of Mitochondrial Transport Proteins

Protein-protein assemblies are highly prevalent in all living cells. Considerable evidence has recently accumulated suggesting that particularly transient association/dissociation of proteins represent an important means of regulation of metabolism. This is true not only in the cytosol and organelle matrices, but also at membrane surfaces where, for example, receptor complexes, as well as those of key metabolic pathways, are common. Transporters also frequently come up in lists of interacting proteins, for example, binding proteins that catalyze the production of their substrates or that act as relays within signal transduction cascades. In this review, we provide an update of technologies that are used in the study of such interactions with mitochondrial transport proteins, highlighting the difficulties that arise in their use for membrane proteins and discussing our current understanding of the biological function of such interactions.

Tamoxifen inhibits mitochondrial oxidative stress damage induced by copper orthophenanthroline

In this work, we studied the effect of tamoxifen and cyclosporin A on mitochondrial permeability transition caused by addition of the thiol-oxidizing pair Cu²⁺ -orthophenanthroline. The findings indicate that tamoxifen and cyclosporin A circumvent the oxidative membrane damage manifested by matrix Ca²⁺ release, mitochondrial swelling, and transmembrane electrical gradient collapse. Furthermore, it was found that tamoxifen and cyclosporin A prevent the generation of TBARs promoted by Cu²⁺ -orthophenanthroline, as well as the inactivation of the mitochondrial enzyme aconitase and disruption of mDNA. Electrophoretic analysis was unable to demonstrate a cross-linking reaction between membrane proteins. Yet, it was found that Cu²⁺ -orthophenanthroline induced the generation of reactive oxygen species. It is thus plausible that membrane leakiness is due to an oxidative stress injury.

The transport mechanism of the mitochondrial ADP/ATP carrier

The mitochondrial ADP/ATP carrier imports ADP from the cytosol and exports ATP from the mitochondrial matrix, which are key transport steps for oxidative phosphorylation in eukaryotic organisms. The transport protein belongs to the mitochondrial carrier family, a large transporter family in the inner membrane of mitochondria. It is one of the best studied members of the family and serves as a paradigm for the molecular mechanism of mitochondrial carriers. Structurally, the carrier consists of three homologous domains, each composed of two transmembrane α-helices linked with a loop and short α-helix on the matrix side. The transporter cycles between a cytoplasmic and matrix state in which a central substrate binding site is alternately accessible to these compartments for binding of ADP or ATP. On both the cytoplasmic and matrix side of the carrier are networks consisting of three salt bridges each. In the cytoplasmic state, the matrix salt bridge network is formed and the cytoplasmic network is disrupted, opening the central substrate binding site to the intermembrane space and cytosol, whereas the converse occurs in the matrix state. In the transport cycle, tighter substrate binding in the intermediate states allows the interconversion of conformations by lowering the energy barrier for disruption and formation of these networks, opening and closing the carrier to either side of the membrane in an alternating way. Conversion between cytoplasmic and matrix states might require the simultaneous rotation of three domains around a central translocation pathway, constituting a unique mechanism among transport proteins. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

Mitochondrial carriers function as monomers

Mitochondrial carriers link biochemical pathways in the mitochondrial matrix and cytosol by transporting metabolites, inorganic ions, nucleotides and cofactors across the mitochondrial inner membrane. Uncoupling proteins that dissipate the proton electrochemical gradient also belong to this protein family. For almost 35 years the general consensus has been that mitochondrial carriers are dimeric in structure and function. This view was based on data from inhibitor binding studies, small-angle neutron scattering, electron microscopy, differential tagging/affinity chromatography, size-exclusion chromatography, analytical ultracentrifugation, native gel electrophoresis, cross-linking experiments, tandem-fusions, negative dominance studies and mutagenesis. However, the structural folds of the ADP/ATP carriers were found to be monomeric, lacking obvious dimerisation interfaces. Subsequently, the yeast ADP/ATP carrier was demonstrated to function as a monomer. Here, we revisit the data that have been published in support of a dimeric state of mitochondrial carriers. Our analysis shows that when critical factors are taken into account, the monomer is the only plausible functional form of mitochondrial carriers. We propose a transport model based on the monomer, in which access to a single substrate binding site is controlled by two flanking salt bridge networks, explaining uniport and strict exchange of substrates.

Roles of adjoining Asp and Cys residues of first matrix-facing loop in transport activity of yeast and bovine mitochondrial ADP/ATP carriers

The mitochondrial ADP/ATP carrier (AAC) transports substrate by interconversion of its conformation between m- and c-states. The 1st loop facing the matrix (LM1) is extruded into the matrix in the m-state and is suggested to intrude into the mitochondrial membrane on conversion to the c-state conformation [Hashimoto, M., Majima, E., Goto, S., Shinohara, Y., and Terada, H. (1999) Biochemistry 38, 1050-1056]. To elucidate the mechanism of the translocation of LM1, we examined the effects of site-directed mutagenesis of two adjoining residues, Cys56 and Asp55 in the bovine type 1 AAC and Cys73 and Asp72 in the yeast type 2 AAC, on the substrate transport activity. We found that (i) replacement of the Cys by bulky and hydrophilic residues was unfavorable for efficient transport activity, (ii) the carboxyl groups of the Asp residues of the bovine and yeast AACs were essential and strictly position-specific, and (iii) hence, the mutation to Glu showed transport activity comparable to that of the native AACs. Based on these results, we discussed the functional role of LM1 in the transport activity of AAC.

Topography of the active site of the Saccharomyces cerevisiae plasmalemmal dicarboxylate transporter studied using lipophilic derivatives of its substrates

2-Alkylmalonates and O-acyl-L-malates have been found to competitively inhibit the dicarboxylate transporter of Saccharomyces cerevisiae cells, and the substrate derivatives chosen did not penetrate across the plasmalemma under the experiment conditions. Probing of the active site of this transporter has revealed a large lipophilic area stretching between the 0.72 to 2.5 nm from the substrate-binding site. Itaconate inhibited the transport fivefold more effectively than L-malate. This suggests the existence of a hydrophobic region immediately near the dicarboxylate-binding site (to 0.72 nm). The yeast plasmalemmal transporter was different from the rat liver mitochondrial dicarboxylate transporter. An area with variable lipophilicity adjoining the substrate-binding site has been revealed in the latter by a similar method. This area is mainly hydrophobic at distances up to 1.76 nm from the binding site and is separated by a hydrophilic region from 0.38 to 0.88 nm. Fumarate but not maleate competitively inhibited succinate transport into the S. cerevisiae cells. It is suggested that the plasmalemmal transporter binds the substrate in the trans-conformation. The prospects of the proposed approach for scanning lipophilic profiles of channels of different transporters are discussed.

Difference between Yeast and Bovine Mitochondrial ADP/ATP Carriers in Terms of Conformational Properties of the First Matrix Loop as Deduced by Use of Copper-o-phenanthroline

The mitochondrial ADP/ATP carrier (AAC) has 6 transmembrane regions and 3 matrix loops. Our previous mutational study on the Cys residue in the LM1s of chimeric bovine type 1 AAC (yN-bAAC1), in which the N-terminal 11 amino acids of bovine type 1 AAC are substituted with the corresponding 26 amino acids of yeast type 2 AAC (yAAC2), and yAAC2 in the yeast expression system suggested the possibility of a different structural feature between their LM1s. In the present study, we compared the effects of the SH cross-linking reagent copper-o-phenanthroline (Cu(OP)(2)) on yN-bAAC1 and yAAC2 in order to study the difference between these LM1s of the 2 carriers. Cu(OP)(2) is known to cross-link 2 AAC molecules in a functional dimer via a Cys residue in each first matrix loop (LM1). yN-bAAC1 exhibited intra- and inter-molecular cross-linking, in agreement with the results of a previous study on the native bovine carrier and suggesting that yN-bAAC1 had the same structure as the native carrier. yAAC2 also showed intra- and inter-molecular cross-linking. However, the speed of formation of the inter-molecular cross-linking of yN-bAAC1 was faster than that of yAAC2, suggesting that the conformational state of the LM1 was different between the 2 carriers. In addition, we also studied the effects of AAC-specific inhibitors and solubilization with Triton X-100 on the cross-linking.

Properties of yeast Saccharomyces cerevisiae plasma membrane dicarboxylate transporter

Transport of succinate into Saccharomyces cerevisiae cells was determined using the endogenous coupled mitochondrial succinate oxidase system. The dependence of succinate oxidation rate on the substrate concentration was a curve with saturation. At neutral pH the K(m) value of the mitochondrial "succinate oxidase" was fivefold less than that of the cellular "succinate oxidase". O-Palmitoyl-L-malate, not penetrating across the plasma membrane, completely inhibited cell respiration in the presence of succinate but not glucose or pyruvate. The linear inhibition in Dixon plots indicates that the rate of succinate oxidation is limited by its transport across the plasmalemma. O-Palmitoyl-L-malate and L-malate were competitive inhibitors (the K(i) values were 6.6 +/- 1.3 microM and 17.5 +/- 1.1 mM, respectively). The rate of succinate transport was also competitively inhibited by the malonate derivative 2-undecyl malonate (K(i) = 7.8 +/- 1.2 microM) but not phosphate. Succinate transport across the plasma membrane of S. cerevisiae is not coupled with proton transport, but sodium ions are necessary. The plasma membrane of S. cerevisiae is established to have a carrier catalyzing the transport of dicarboxylates (succinate and possibly L-malate and malonate).

Study of active site topography of rat liver mitochondrial dicarboxylate transporter using lipophilic substrate derivatives

Earlier it has been demonstrated that the active site (substrate-binding site + active site channel) of rat liver mitochondrial dicarboxylate transporter is characterized by rather complex topography. Probing the active site with 2-monoalkylmalonates revealed the existence of internal and external lipophilic areas separated by a polar region. A two substrate-binding site model of the transporter has been supposed. The correctness of this model has been evaluated by probing the active site with O-acyl-L-malates differing from 2-monoalkylmalonates by 0.23 nm longer distance from the anion groups to the aliphatic chain. Changes in the polar group of the probe did not prevent its binding and showed the same variable lipophilicity pattern for the transporter channel. Probing with alpha,omega-alkylene dimalonates did not reveal the second substrate-binding site at the active site. The substrate-binding site did not show any differences in affinity to O-acyl-derivatives of L-malate and D-malate, except L-malate binds more effectively than D-malate. This suggests involvement of the L-malate hydroxyl group in substrate binding and stereospecific behavior of the transporter substrate-binding site. A modified one substrate-binding site model of the dicarboxylate transporter is discussed.

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+)].