Two residues of a conserved aromatic ladder of the mitochondrial ADP/ATP carrier are crucial to nucleotide transport.
ABSTRACT The mitochondrial ADP/ATP carrier is the paradigm of the mitochondrial carrier family (MCF), whose members are crucial for cross-talks between mitochondria, where cell energy is mainly produced, and the cytosol, where cell energy is mainly consumed. These carriers share structural and functional characteristics. Resolution of the 3D structure of the beef mitochondrial ADP/ATP carrier, in a complex with one of its specific inhibitors, revealed interesting features and suggested the involvement of some particular residues in substrate binding and transfer from the outside to the inside of mitochondria. To ascertain the role of these residues, namely, Y186, Y190, F191, and Y194, they were mutated into alanine in the yeast mitochondrial ADP/ATP carrier at equivalent positions (Y203, Y207, F208, and Y211). Two residues, Y203 and F208, appeared to be crucial for transport activity but not for substrate binding per se, indicating their involvement in the substrate transfer process through the carrier. Furthermore, it was possible to show that these mutations precluded conformational changes of the matrix loop m2, whose movements were demonstrated to participate in substrate transport by the wild-type carrier. Therefore, these aromatic residues may be involved in substrate gliding, and they may also confer specificity toward adenine nucleotides for the ADP/ATP carrier as compared with the MCF members.
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ABSTRACT: Trypanosoma brucei is a kinetoplastid parasite of medical and veterinary importance. Its digenetic lifecycle alternates between the bloodstream form (BSF) 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 characterised 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 similar biochemical properties and ADP/ATP-exchange kinetics as 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 suggested 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.Journal of Biological Chemistry 10/2012; · 4.65 Impact Factor
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ABSTRACT: Abstract Mitochondrial carriers transport inorganic ions, nucleotides, amino acids, keto acids and cofactors across the mitochondrial inner membrane. Structurally they consist of three domains, each containing two transmembrane α-helices linked by a short α-helix and loop. The substrate binds to three major contact points in the central cavity. The class of substrate (e.g., adenine nucleotides) is determined by contact point II on transmembrane α-helix H4 and the type of substrate within the class (e.g., ADP, coenzyme A) by contact point I in H2, whereas contact point III on H6 is most usually a positively charged residue, irrespective of the type or class. Two salt bridge networks, consisting of conserved and symmetric residues, are located on the matrix and cytoplasmic side of the cavity. These residues are part of the gates that are involved in opening and closing of the carrier during the transport cycle, exposing the central substrate binding site to either side of the membrane in an alternating way. Here we revisit the plethora of mutagenesis data that have been collected over the last two decades to see if the residues in the proposed binding site and salt bridge networks are indeed important for function. The analysis shows that the major contact points of the substrate binding site are indeed crucial for function and in defining the specificity. The matrix salt bridge network is more critical for function than the cytoplasmic salt bridge network in agreement with its central position, but neither is likely to be involved in substrate recognition directly.Molecular Membrane Biology 03/2013; 30(2):149-159. · 3.13 Impact Factor
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ABSTRACT: Iron is an essential element for almost all organisms. In eukaryotes, it is mainly used in mitochondria for the biosynthesis of iron-sulphur clusters and heme-group maturation. Iron is delivered into the mitochondrion by mitoferrins, members of the mitochondrial carrier family (MCF), through an unknown mechanism. In this article, the yeast homologs of these proteins, Mrs3p and Mrs4p, were studied by inserting them into liposomes. In this context, they could transport iron (II) across the proteoliposomes membrane, as revealed using the iron-chelator bathophenanthroline. A series of amino acid-modifying reagents were screened for their effects on Mrs3p-mediated iron transport. Results suggested that carboxylic and imidazole groups are essential for iron transport. This was confirmed by in vivo complementation assays, which demonstrated that three highly conserved His residues are important for Mrs3p function. These His residues are not conserved in other MCF members, thus they are likely to play a specific role in iron transport. A model describing how these residues help iron to transit smoothly across the carrier cavity is proposed and compared to structural and biochemical data available for other carriers in this family.Biochemical Journal 03/2014; · 4.65 Impact Factor