Insights into the Structure of the Chloroplast Phosphate Translocator Protein

Abstract

The triose phosphate-3-phosphoglycerate-phosphate translocator (in short, phosphate translocator) can be regarded as the main transport function of the inner envelope membrane of C3-chloroplasts (for a recent review see Flugge and Heldt, 1991). It mediates a strict counter-exchange of the above mentioned metabolites thereby exporting the fixed carbon in the form of triose phosphate and 3-phosphoglycerate from the chloroplasts to the cytosol. Triose phosphate is then used for the synthesis of sucrose and 3-phosphoglycerate for that of amino acids. Sucrose and amino acids are the main products translocated via the sieve tubes to other parts of the plants. In C3-type chloroplasts the phosphate translocator accepts at its substrate binding site only phosphate and C3 compounds with the phosphate attached to the C3 atom. However, that from C4-mesophyll chloroplasts also transports C3 compounds with the phosphate attached to the C2 atom i.e. phosphoenolpyruvate which is formed inside the chloroplasts and transferred via the C4-phosphate translocator to the cytosol to be carboxylated to oxaloacetate (Gross, Bruckner Heldt, and Flugge,1990). In amyloplasts, the phosphate translocator accepts at its binding site all the mentioned substrates and, in addition, glucose 6-phosphate as the substrate for starch biosynthesis and for the oxidative pentose phosphate pathway (Borchert, Grose, and Heldt, 1989). It becomes obvious that in different plastids the phosphate translocators of the inner envelope membrane can be rather different with respect to their transport specificities.