MOLECULAR CHARACTERIZATION OF A CARBON TRANSPORTER IN PLASTIDS FROM HETEROTROPHIC TISSUES - THE GLUCOSE-6-PHOSPHATE PHOSPHATE ANTIPORTER

Plastids of nongreen tissues import carbon as a source of biosynthetic pathways and energy, Within plastids, carbon can be used in the biosy nthesis of starch or as a substrate for the oxidative pentose phosphat e pathway, for example, We have used maize endosperm to purify a plast idic glucose 6-phosphate/phosphate translocator (GPT), The correspondi ng cDNA was isolated from maize endosperm as well as from tissues of p ea roots and potato tubers, Analysis of the primary sequences of the c DNAs revealed that the GPT proteins have a high degree of identity wit h each other but share only similar to 38% identical amino acids with members of both the triose phosphate/phosphate translocator (TPT) and the phosphoenolpyruvate/phosphate translocator (PPT) families. Thus, t he GPTs represent a third group of plastidic phosphate antiporters. Al l three classes of phosphate translocator genes show differential patt erns of expression. Whereas the TPT gene is predominantly present in t issues that perform photosynthetic carbon metabolism and the PPT gene appears to be ubiquitously expressed, the expression of the GPT gene i s mainly restricted to heterotrophic tissues. Expression of the coding region of the GPT in transformed yeast cells and subsequent transport experiments with the purified protein demonstrated that the GPT prote in mediates a 1:1 exchange of glucose B-phosphate mainly with inorgani c phosphate and triose phosphates, Glucose B-phosphate imported via th e GPT can thus be used either for starch biosynthesis, during which pr ocess inorganic phosphate is released, or as a substrate for the oxida tive pentose phosphate pathway, yielding triose phosphates.