Arabidopsis mitochondria have two basic amino acid transporters with partially overlapping specificities and differential expression in seedling development

To shed light on the metabolic role of two mitochondrial transporters for basic amino acids in Arabidopsis, we compared their functional properties in liposomes and expression during germination. Recombinant and purified BAC2, as previously reported for BAC1, transported various basic l-amino acids upon reconstitution in phospholipid vesicles. Both displayed highest affinity for arginine with similar Km and Vmax. However, BAC2 transported citrulline for which BAC1 had little or no affinity. Furthermore, BAC2 was less stereospecific than BAC1, transporting d-arginine and d-lysine at significant rates, and displayed a striking alkaline pH optimum (pH 8.0) whereas BAC1 activity was unaltered from pH 7.0 to 9.0. By semi-quantitative RT-PCR BAC1 transcript levels were found to be higher than those of BAC2 in germinated seeds. However, BAC2 expression transiently increased 2 days after germination. Disruption of the Arabidopsis arginase structural genes (ARGAH1 or ARGAH2) accentuated the increases of transcript levels of BAC1 at germination and of BAC2 2 days after germination and from 6 days on. Early expression of BAC1 and BAC2 is consistent with the delivery of arginine, released from seed reserves, to mitochondrial arginase and the export of ornithine. Increase of BAC2 transcript levels later in seedling development is consistent with roles in NO, polyamine or proline metabolism—processes involving arginine, citrulline and/or ornithine.