Abstract
Plastids contain various transport proteins that mediate the exchange of metabolites between the plastids and the cytosol. These metabolite translocators reside in the inner envelope membrane and are involved in the translocation of photoassimilates in both photosynthetic and heterotrophic tissues. The characteristics of three of these translocators, all functioning as antiporters, are described in more detail. First, the chloroplast triose phosphate/phosphate translocator (cTPT) that exports the fixed carbon in form of triose phosphates and 3-phosphoglycerate from the chloroplasts in exchange for inorganic phosphate is described. Secondly, a phosphoenolpyruvate/phosphate translocator that is present in both photosynthetic and non-green tissues is considered. The main purpose of this transporter is presumably to supply the plastids with phosphoenolpyruvate as a substrate for the shikimate pathway. Finally, a dicarboxylate translocator that imports carbon skeletons into chloroplasts in exchange with malate for ammonia assimilation is discussed. This nitrogen source is used for the formation of amino acids that are subsequently exported.
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