The chloroplastic 2-oxoglutarate/malate transporter has dual function as the malate valve and in carbon/nitrogen metabolism

Transport of dicarboxylates across the chloroplast envelope plays an important role in transferring carbon skeletons to the nitrogen assimilation pathway and exporting reducing equivalent to the cytosol to prevent photo-inhibition (the malate valve). It was previously shown that the Arabidopsis plastidic 2-oxoglutarate/malate transporter (AtpOMT1) and the general dicarboxylate transporter (AtpDCT1) play crucial roles at the interface between carbon and nitrogen metabolism. However, based on the in vitro transport properties of the recombinant transporters, it was hypothesized that AtpOMT1 might play a dual role, also functioning as an oxaloacetate/malate transporter, which is a crucial but currently unidentified component of the chloroplast malate valve. Here, we test this hypothesis using Arabidopsis T-DNA insertional mutants of AtpOMT1. Transport studies revealed a dramatically reduced rate of oxaloacetate uptake into chloroplasts isolated from the knockout plant. CO(2) -dependent O(2) evolution assays showed that cytosolic oxaloacetate is efficiently transported into chloroplasts mainly by AtpOMT1, and supported the absence of additional oxaloacetate transporters. These findings strongly indicate that the high-affinity oxaloacetate transporter in Arabidopsis chloroplasts is AtpOMT1. Further, the knockout plants showed enhanced photo-inhibition under high light due to greater accumulation of reducing equivalents in the stroma, indicating malfunction of the malate valve in the knockout plants. The knockout mutant showed a phenotype consistent with reductions in 2-oxoglutarate transport, glutamine synthetase/glutamate synthase activity, subsequent amino acid biosynthesis and photorespiration. Our results demonstrate that AtpOMT1 acts bi-functionally as an oxaloacetate/malate transporter in the malate valve and as a 2-oxoglutarate/malate transporter mediating carbon/nitrogen metabolism.

Differentiation of dicarboxylate transporters in mesophyll and bundle sheath chloroplasts of maize

In NADP-malic enzyme-type C(4) plants such as maize (Zea mays L.), efficient transport of oxaloacetate and malate across the inner envelope membranes of chloroplasts is indispensable. We isolated four maize cDNAs, ZmpOMT1 and ZmpDCT1 to 3, encoding orthologs of plastidic 2-oxoglutarate/malate and general dicarboxylate transporters, respectively. Their transcript levels were upregulated by light in a cell-specific manner; ZmpOMT1 and ZmpDCT1 were expressed in the mesophyll cell (MC) and ZmpDCT2 and 3 were expressed in the bundle sheath cell (BSC). The recombinant ZmpOMT1 protein expressed in yeast could transport malate and 2-oxoglutarate but not glutamate. By contrast, the recombinant ZmpDCT1 and 2 proteins transported 2-oxoglutarate and glutamate at similar affinities in exchange for malate. The recombinant proteins could also transport oxaloacetate at the same binding sites as those for the dicarboxylates. In particular, ZmpOMT1 transported oxaloacetate at a higher efficiency than malate or 2-oxoglutarate. We also compared the activities of oxaloacetate transport between MC and BSC chloroplasts from maize leaves. The K(m) value for oxaloacetate in MC chloroplasts was one order of magnitude lower than that in BSC chloroplasts, and was close to that determined with the recombinant ZmpOMT1 protein. Southern analysis revealed that maize has a single OMT gene. These findings suggest that ZmpOMT1 participates in the import of oxaloacetate into MC chloroplasts in exchange for stromal malate. In BSC chloroplasts, ZmpDCT2 and/or ZmpDCT3 were expected to import malate that is transported from MC.