Figure 2.

Overview of characterized and uncharacterized major fixed carbon and reductant transporters on inner chloroplast and mitochondrial envelope membranes. (A) Transporters on the chloroplast envelope in Chlamydomonas are divided into two major groups: sugar-P/phosphate transporters, including GPTs, TPTs, and PPTs, which transport both fixed carbon and reductants, and dicarboxylate transporters, which mainly shuttle reductants. Based on Doebbe et al. [19], there might also be a hexose transporter on the chloroplast envelope, although the specificity and the mechanism of transport have not been elucidated. While the substrates transported are indicated, many of these bidirectional sugar-P transporters can use multiple substrates (and counter-substrate); Pi is considered a preferred counter-substrate. (B) Transporters on the mitochondrial inner membrane of plants, with analogous proteins in Chlamydomonas), include di- and tricarboxylate transporters. Most of the metabolites translocated across the mitochondria membrane are associated with the TCA cycle. Reductant imported into the mitochondria can drive oxidative phosphorylation. CCPs (two in Chlamydomonas) are mitochondrial transporters that increase in abundance (transcript level) under low CO₂ conditions, but their functions are yet to be determined. Dark gray indicates that some transporters from this category have been characterized in Chlamydomonas, while light gray indicates that the transporter has not yet been characterized; the presence of these transporters on the mitochondrial envelope of Chlamydomonas and their contribution to its metabolism are predicted from information derived from plant system. DIC, dicarboxylate translocator on the mitochondrial envelope; DiT, dicarboxylate translocator on the chloroplast envelope; DTC, tricarboxylate translocator; GPT, glucose 6-phosphate/phosphate transporter; HK, hexokinase; HXT, hexose transporter; MDH, malate dehydrogenase; PPT, phosphoenol pyruvate/phosphate transporter; TPT, triose-phosphate/phosphate transporter; DOXP/MEP, 1-deoxyxylulose 5-phosphate/2-C-methylerythritol 4-phosphate pathway; TCA, tricarboxylic acid cycle; AA, amino acid; ATP, adenosine triphosphate; Asp, aspartate; CO₂, carbon dioxide; DHAP, dihydroxyacetone phosphate; E4P, D-erythrose 4-phosphate; Glc, glucose; Glc1P, glucose 1-phosphate; Glc6P, glucose 6-phosphate; G3P, glyceraldehyde-3-phosphate; Glu, glutamate; 3-PGA, 3-phosphoglyceric acid; NAD(P)(H), nicotinamide adenine dinucleotide (phosphate); OAA, oxaloacetate; 2-OG, 2-oxoglutarate; PEP, phosphoenol pyruvate; Pi, inorganic phosphate; RuBP, ribulose 1,5-bisphosphate; Ru5P, ribose-5-phosphate; ICEM, inner chloroplast envelope membrane; OCEM, outer chloroplast envelope membrane; IMM, inner mitochondrion membrane; OMM, outer mitochondrion membrane. Created with BioRender.com.