Uncoupling proteins 1 and 2 (UCP1 and UCP2) from Arabidopsis thaliana are mitochondrial transporters of aspartate, glutamate and dicarboxylates

TheArabidopsis thalianagenome contains 58 members of the solute carrier family SLC25, also called the mitochondrial carrier family, many of which have been shown to transport specific metabolites, nucleotides and cofactors across the mitochondrial membrane. Here two Arabidopsis members of this family, AtUCP1 and AtUCP2, which were previously thought to be uncoupling proteins and hence named UCP1/PUMP1 and UCP2/PUMP2, respectively, are assigned with a novel function. They were expressed in bacteria, purified and reconstituted in phospholipid vesicles. Their transport properties demonstrate that they transport amino acids (aspartate, glutamate, cysteinesulfinate and cysteate), dicarboxylates (malate, oxaloacetate and 2-oxoglutarate), phosphate, sulfate and thiosulfate. Transport was saturable and inhibited by mercurials and other mitochondrial carrier inhibitors at various degrees. AtUCP1 and AtUCP2 catalyzed a fast counter-exchange transport as well as a low uniport of substrates with transport rates of AtUCP1 being much higher than those of AtUCP2 in both cases. The aspartate/glutamate hetero-exchange mediated by AtUCP1 and AtUCP2 is electroneutral, in contrast to that mediated by the mammalian mitochondrial aspartate glutamate carrier. Furthermore, both carriers were found to be targeted to mitochondria. Metabolite profiling of single and double knockouts show changes in organic acid and amino acid levels. Notably, AtUCP1 and AtUCP2 are the first reported mitochondrial carriers in Arabidopsis to transport aspartate and glutamate. It is proposed that the primary function of AtUCP1 and AtUCP2 is to catalyze an aspartateout/glutamateinexchange across the mitochondrial membrane and thereby contribute to the export of reducing equivalents from the mitochondria in photorespiration.