2.A.7.9.5
The plastidic phosphate/triosephosphate transporter, TPT (Linka et al., 2008). TPT catalyses the strict 1:1 exchange of triose-phosphate, 3-phosphoglycerate and inorganic phosphate across the chloroplast envelope Lee et al. 2017 reported crystal structures of TPT bound to two different substrates, 3-phosphoglycerate and inorganic phosphate, in occluded conformations. The structures reveal that TPT adopts a 10-transmembrane drug/metabolite transporter fold. Both substrates are bound within the same central pocket, where conserved lysine, arginine and tyrosine residues recognize the shared phosphate group. A structural comparison with the outward-open conformation of the bacterial drug/metabolite transporter suggests a rocker-switch motion of helix bundles, and molecular dynamics simulations support a model in which this rocker-switch motion is tightly coupled to substrate binding to ensure strict 1:1 exchange. The results reveal the mechanism of sugar phosphate/phosphate exchange by TPT. TPTexports  Calvin cycle intermediates from chloroplasts and plays fundamental roles in nearly all photosynthetic eukaryotes (Lee et al. 2017).