Molecular Characterization of a Carbon Transporter in Plastids from Heterotrophic Tissues: The Glucose 6-Phosphate/Phosphate Antiporter

Abstract

Plastids of nongreen tissues import carbon as a source of biosynthetic pathways and energy. Within plastids, carbon can be used in the biosynthesis of starch or as a substrate for the oxidative pentose phosphate pathway, for example. We have used maize endosperm to purify a plastidic glucose 6-phosphate/phosphate translocator (GPT). The corresponding cDNA was isolated from maize endosperm as well as from tissues of pea roots and potato tubers. Analysis of the primary sequences of the cDNAs revealed that the GPT proteins have a high degree of identity with each other but share only ‫ف‬ 38% identical amino acids with members of both the triose phosphate/phosphate translocator (TPT) and the phosphoenolpyruvate/phosphate translocator (PPT) families. Thus, the GPTs represent a third group of plastidic phosphate antiporters. All three classes of phosphate translocator genes show differential patterns of expression. Whereas the TPT gene is predominantly present in tissues that perform photosynthetic carbon metabolism and the PPT gene appears to be ubiquitously expressed, the expression of the GPT gene is 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 protein mediates a 1:1 exchange of glucose 6-phosphate mainly with inorganic phosphate and triose phosphates. Glucose 6-phosphate imported via the GPT can thus be used either for starch biosynthesis, during which process inorganic phosphate is released, or as a substrate for the oxidative pentose phosphate pathway, yielding triose phosphates. INTRODUCTIONDuring C 3 photosynthesis, energy from solar radiation is used for the formation of phosphorylated C3 sugar phosphates, triose phosphates (trioseP), and 3-phosphoglycerate (3-PGA); these products are exported from the chloroplasts into the cytosol via the trioseP/3-PGA/phosphate translocator (TPT). In the mature leaves of most plants, the exported photosynthates are then used in the formation of sucrose, which is allocated via the phloem to the heterotrophic plant organs, such as young leaves, roots, seeds, fruits, or tubers. In these sink tissues, sucrose serves as a source of carbon and energy and is first cleaved by the action of invertases or sucrose synthase; the products of these reactions are converted into hexose phosphates.Nongreen plastids of heterotrophic tissues are carbohydrate-importing organelles and, in the case of amyloplasts of storage tissues, the site of starch synthesis. Because these plastids are normally not able to generate hexose phosphates from C3 compounds due to the absence of fructose 1,6-bisphosphatase activity (Entwistle and ap Rees, 1988), they rely on the import of cytosolically generated hexose phosphates as the source of carbon for starch biosynthesis and, in addition, for the oxidative pentose phosphate pathway. The results of transport measurements with intact organelles or reconstituted tissues from different plants suggest that thi...

Metabolite transport and associated sugar signalling systems underpinning source/sink interactions

“…The GPT group mediates the transport of glucose-6-phosphate, triose phosphates and phosphates into plastids [60]. The Arabidopsis genome contains two GPT genes, AtGPT1 and AtGPT2 [54].…”

Role of metabolite transporters in source-sink carbon allocation

“…Glucose 6-phosphate is imported into plastids by the Glc6P/phosphate translocator (GPT; “10” in Figure 2; Kammerer et al, 1998) in counter-exchange with inorganic phosphate or triose phosphate generated by the oxidative pentose phosphate pathway (OPPP). Plastids isolated from young oilseed rape embryos utilize Glc6P better than pyruvate for fatty acid synthesis whereas plastids isolated from older embryos prefer pyruvate over Glc6P as a substrate for fatty acid synthesis (Eastmond and Rawsthorne, 2000).…”

“…In cereal endosperm, starch synthesis differs from other starch-storing organs in that most of the AGPase activity is extraplastidic, i.e., cytosolic (Denyer et al, 1996; Thorbjörnsen et al, 1996; Beckles et al, 2001; James et al, 2003), requiring an additional uptake system for carbon skeletons into plastids supplementary to the GPT (“10” in Figure 2; Kammerer et al, 1998). Externally supplied ADP-glucose (ADP-G) was found to drive starch synthesis in isolated maize endosperm amyloplasts (Möhlmann et al, 1997) while Shannon et al (1998) found Brittle-1 (BT-1; “18” in Figure 2; Sullivan and Kaneko, 1995) to be responsible for the uptake of ADP-G into maize endosperm amyloplasts by including the bt-1 mutant in the analyses.…”

Recent advances in understanding photosynthesis

“…Molecular targets were mainly related to genes involved in sugar and starch metabolism. A promising approach was recently suggested comprising the simultaneous boosting of both source and sink capacities using the crop plant potato as an example 72 . Here, the moderate repression of the leaf ADP-glucose pyrophosphorylase, a key enzyme of transitory starch formation or, alternatively, the mesophyll-specific expression of a bacterial pyrophosphate to stimulate sucrose synthesis and to prevent sucrose breakdown were combined with plants that had an increased sink strength.…”

“…Both procedures resulted in an enhanced allocation of sucrose from source to sink tissues at the expense of leaf starch accumulation. The combined push-pull approach resulted in doubling the tuber starch yield 72 . This way of proceeding is currently being transferred to cassava, an important staple food in developing countries (Bill & Melinda Gates Foundation, CASS, OPP1113365).…”