Arabidopsis thaliana Mitochondrial ATP-Mg/Phosphate Carriers: Biochemical Characterization and Organ Expression

Members of the mitochondrial carrier family transport metabolites, nucleotides and cofactors across the inner membrane of mitochondria. Substrates for about half of the 58 family members in Arabidopsis thaliana have been identified (1). Here, we report the functional characterization and organ distribution for three of them, named AtAPC1, AtAPC2 and AtAPC3, that exhibit high sequence similarities with the human mitochondrial ATP-Mg/phosphate carriers (APC) (2). The expression levels of the AtAPCs in various tissues were analyzed by quantitative real-time PCR and the beta-glucuronidase (GUS) reporter system. Recombinant AtAPCs were expressed in Escherichia coli, purified and reconstituted into liposomes that were used in transport assays (3). Under normal physiological conditions AtAPC1 was expressed at least five times more than the other two isoforms in flower, leaf, stem, root and seedlings. However, in stress conditions the expression levels of AtAPC1 and AtAPC3 change. The transport assays showed that all three AtAPCs transport phosphate, AMP, ADP, ATP, adenosine 5'-phosphosulfate and, to a lesser extent, other nucleotides. AtAPC2 and AtAPC3 also have the ability to transport sulfate and thiosulfate. The three AtAPCs catalyzed a counter-exchange transport that was saturable and inhibited by pyridoxal-5'-phosphate. The transport activities of the AtAPCs were also inhibited by the addition of EDTA or EGTA and stimulated by the addition of Ca2+. Given that phosphate and sulfate can be recycled via their own specific carriers, these findings indicate that AtAPCs can catalyze net transfer of adenine nucleotides across the inner mitochondrial membrane in exchange for phosphate (or sulfate), and that this transport is regulated both at the transcriptional level and by Ca2+. (1) Palmieri, F., et al. 2011 Plant J. 88, 161-181 (2) Fiermonte, G., et al. 2004 J. Biol. Chem. 279, 30722-30730 (3) Palmieri, F., et al. 1995 Methods Enzymol. 260, 349-369