Identification of a novel transporter for dicarboxylates and tricarboxylates in plant mitochondria - Bacterial expression, reconstitution, functional characterization, and tissue distribution

A cDNA from Arabidopsis thaliana and four related cDNAs from Nicotiana tabacum that we have isolated encode hitherto unidentified members of the mitochondrial carrier family. These proteins have been overexpressed in bacteria and reconstituted into phospholipid vesicles. Their transport properties demonstrate that they are orthologs/isoforms of a novel mitochondrial carrier capable of transporting both dicarboxylates (such as malate, oxaloacetate, oxoglutarate, and maleate) and tricarboxylates (such as citrate, isocitrate, cisaconitate, and trans-aconitate). The newly identified dicarboxylate-tricarboxylate carrier accepts only the single protonated form of citrate (H-citrate(2-)) and the unprotonated form of malate (malate(2-)) and catalyzes obligatory, electroneutral exchanges. Oxoglutarate, citrate, and malate are mutually competitive inhibitors, showing K-i close to the respective K-m. The carrier is expressed in all plant tissues examined and is largely spread in the plant kingdom. Furthermore, nitrate supply to nitrogen-starved tobacco plants leads to an increase in its mRNA in roots and leaves. The dicarboxylate-tricarboxylate carrier may play a role in important plant metabolic functions requiring organic acid flux to or from the mitochondria, such as nitrogen assimilation, export of reducing equivalents from the mitochondria, and fatty acid elongation.