The ADP and ATP transport in mitochondria and its carrier

Nonmitochondrial ATP/ADP transporters accept phosphate as third substrate

Chlamydiales and Rickettsiales as metabolically impaired, intracellular pathogenic bacteria essentially rely on "energy parasitism" by the help of nucleotide transporters (NTTs). Also in plant plastids NTT-type carriers catalyze ATP/ADP exchange to fuel metabolic processes. The uptake of ATP4-, followed by energy consumption and the release of ADP3-, would lead to a metabolically disadvantageous accumulation of negative charges in form of inorganic phosphate (Pi) in the bacterium or organelle if no interacting Pi export system exists. We identified that Pi is a third substrate of several NTT-type ATP/ADP transporters. During adenine nucleotide hetero-exchange, Pi is cotransported with ADP in a one-to-one stoichiometry. Additionally, Pi can be transported in exchange with solely Pi. This Pi homo-exchange depends on the presence of ADP and provides a first indication for only one binding center involved in import and export. Furthermore, analyses of mutant proteins revealed that Pi interacts with the same amino acid residue as the gamma-phosphate of ATP. Import of ATP in exchange with ADP plus Pi is obviously an efficient way to couple energy provision with the export of the two metabolic products (ADP plus Pi) and to maintain cellular phosphate homeostasis in intracellular living "energy parasites" and plant plastids. The additional Pi transport capacity of NTT-type ATP/ADP transporters makes the existence of an interacting Pi exporter dispensable and might explain why a corresponding protein so far has not been identified.

Dominant membrane uncoupling by mutant adenine nucleotide translocase in mitochondrial diseases

Adenine nucleotide translocase (Ant) is the most abundant protein on the mitochondrial inner membrane (MIM) primarily involved in ADP/ATP exchange. Ant also possesses a discrete membrane uncoupling activity. Specific mis-sense mutations in the human Ant1 cause autosomal dominant Progressive External Ophthalmoplegia (adPEO), mitochondrial myopathy and cardiomyopathy, which are commonly manifested by fractional mitochondrial DNA (mtDNA) deletions. It is currently thought that the pathogenic mutations alter substrate preference (e.g. ATP versus ADP) thereby dominantly disturbing adenine nucleotide homeostasis in mitochondria. This may interfere with mtDNA replication, consequently affecting mtDNA stability and oxidative phosphorylation. Here, we showed that the adPEO-type A128P, A106D and M114P mutations in the yeast Aac2p share the following common dominant phenotypes: electron transport chain damage, intolerance to moderate over-expression, synthetic lethality with low Deltapsi(m) conditions, hypersensitivity to the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP) and mtDNA instability. More interestingly, the aac2(A137D) allele mimicking ant1(A123D) in mitochondrial myopathy and cardiomyopathy exhibits similar dominant phenotypes. Because Aac2(A137D) is known to completely lack transport activity, it is strongly argued that the dominant mitochondrial damages are not caused by aberrant nucleotide transport. The four pathogenic mutations occur in a structurally dynamic gating region on the cytosolic side. We provided direct evidence that the mutant alleles uncouple mitochondrial respiration. The pathogenic mutations likely enhance the intrinsic proton-conducting activity of Ant, which excessively uncouples the MIM thereby affecting energy transduction and mitochondrial biogenesis. mtDNA disintegration is a phenotype co-lateral to mitochondrial damages. These findings provide mechanistic insights into the pathogenesis of the Ant1-induced diseases.