| Comment | The 6-8 TMS Triose-phosphate Transporter (TPT) Family (TC 2.A.7.9)
Functionally characterized members of the TPT family are derived from the inner envelope membranes of chloroplasts and nongreen plastids of plants. However,
homologues are also present in yeast. Saccharomyces cerevisiae has three functionally uncharacterized TPT paralogues encoded within its genome. Under normal physiological
conditions, chloroplast TPTs mediate a strict antiport of substrates, frequently exchanging an organic three carbon compound phosphate ester for inorganic phosphate (Pi).
Normally, a triose-phosphate, 3-phosphoglycerate, or another phosphorylated C3 compound made in the chloroplast during photosynthesis, exits the organelle into the
cytoplasm of the plant cell in exchange for Pi. However, experiments with reconstituted translocator in artificial membranes indicate that transport can also occur by a
channel-like uniport mechanism with up to 10-fold higher transport rates. Channel opening may be induced by a membrane potential of large magnitude and/or by high substrate
concentrations. Nongreen plastid and chloroplast carriers, such as those from maize endosperm and root membranes, mediate transport of C3 compounds phosphorylated at
carbon atom 2, particularly phosphenolpyruvate, in exchange for Pi. These are the phosphoenolpyruvate:Pi antiporters (PPT). Glucose-6-P has also been shown to be a
substrate of some plastid translocators (GPT). The three types of proteins (TPT, PPT and GPT) are divergent in sequence as well as substrate specificity, but their substrate
specificities overlap.
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