The metabolite transporters of the plastid envelope: an update

Abstract

The engulfment of a photoautotrophic cyanobacterium by a primitive mitochondria-bearing eukaryote traces back to more than 1.2 billion years ago. This single endosymbiotic event not only provided the early petroalgae with the metabolic capacity to perform oxygenic photosynthesis, but also introduced a plethora of other metabolic routes ranging from fatty acids and amino acids biosynthesis, nitrogen and sulfur assimilation to secondary compounds synthesis. This implicated the integration and coordination of the newly acquired metabolic entity with the host metabolism. The interface between the host cytosol and the plastidic stroma became of crucial importance in sorting precursors and products between the plastid and other cellular compartments. The plastid envelope membranes fulfill different tasks: they perform important metabolic functions, as they are involved in the synthesis of carotenoids, chlorophylls, and galactolipids. In addition, since most genes of cyanobacterial origin have been transferred to the nucleus, plastidial proteins encoded by nuclear genes are post-translationally transported across the envelopes through the TIC-TOC import machinery. Most importantly, chloroplasts supply the photoautotrophic cell with photosynthates in form of reduced carbon. The innermost bilayer of the plastidic envelope represents the permeability barrier for the metabolites involved in the carbon cycle and is literally stuffed with transporter proteins facilitating their transfer. The intracellular metabolite transporters consist of polytopic proteins containing membrane spans usually in the number of four or more α-helices. Phylogenetic analyses revealed that connecting the plastid with the host metabolism was mainly a process driven by the host cell. In Arabidopsis, 58% of the metabolite transporters are of host origin, whereas only 12% are attributable to the cyanobacterial endosymbiont. This review focuses on the metabolite transporters of the inner envelope membrane of plastids, in particular the electrochemical potential-driven class of transporters. Recent advances in elucidating the plastidial complement of metabolite transporters are provided, with an update on phylogenetic relationship of selected proteins.

Keywords: endosymbiosis; envelope membrane; translocator.

Figure 1

Plastid evolution in photosynthetic eukaryotes. The uptake of a cyanobacterium resulted in a photosynthetic plantae ancestor which subsequently diverged in the three lineages containing primary plastids: the chlorophytes (including the land plants), the rhodophytes, and the glaucophytes. The subsequent secondary endosymbioses of green and red algae engulfed by different hosts resulted in the euglenophyta and chlorarachniophyta (greens) and in the possibly monophyletic chromalveolates (reds). Paulinella chromatophora represents an independent primary endosymbiosis in progress.

Figure 2

Overview of the characterized chloroplast envelope metabolite transporters. Transport processes of carbon, nitrogen compounds and energy across the envelope membrane of photosynthetic plastids in green plants are depicted. Abbreviations: 2-OG, 2-oxoglutarate; ADP-Glc, ADP-glucose; BT1L, BT1-like transporter; CLT, CRT-like transporter; Cys, cysteine; Ery 4-P, Erythrose 4-phosphate; ETC, electron transport chain; Fru1,6P₂, fructose-1,6-bisphosphate; Γ-EC, Γ-glutamylcysteine; Glc, glucose; Glc 1-P, glucose 1-phosphate; Glc 6-P, glucose 6-phosphate; Glu, glutamine; GPT, glucose 6-phosphate/phosphate translocator; Mal, maltose; MEX, maltose exporter; NTT, nucleoside triphosphate transporter; OPPP, oxidative pentose phosphate pathway; 3-PGA, 3-phosphoglyceric acid; PEP, phosphoenolpyruvate; pGlcT, plastidic glucose transporter; Pi, inorganic phosphate; PPT, PEP/phosphate translocator; TP, triose phosphate; TPT, triose phosphate/phosphate translocator; XPT, xylulose 5-phosphate/phosphate translocator; Xul 5-P, xylulose 5-phosphate.

Figure 3

Characterized and putative metabolite transporters of the glaucophaye Cyanophora paradoxa (A) and the rhodophyte Galdieria sulphuraria (B). Proposed model of carbon export from C. paradoxa based on uptake experiments with isolated cyanelles. The identity of the transporter responsible for the TPT activity is not elucidated yet (A). Characterized metabolite transporters of G. sulphuraria and their substrate specificities. The GsTPT, unlike for the green plant chloroplasts, is unable to transport 3-PGA pointing to the presence of an alternate reduction shuttle in the red algae (B). Abbreviations: ADP-Glc, ADP-glucose; ETC, electron transport chain; Glc 1-P, glucose 1-phosphate; Glc 6-P, glucose 6-phosphate; GPT, glucose 6-phosphate/phosphate translocator; NTT, nucleoside triphosphate transporter; OPPP, oxidative pentose phosphate pathway; 3-PGA, 3-phosphoglyceric acid; PEP, phosphoenolpyruvate; Pi, inorganic phosphate; PPT, PEP/phosphate translocator; TP, triose phosphate; TPT, triose phosphate/phosphate translocator; UDP-Glc, UDP-glucose; UDP-Gal, UDP-galactose.

Figure 4

Substrates of the triose phosphate/phosphate translocators (TPTs) and nucleoside triphosphate transporters (NTTs) of the apicoplast of Plasmodium falciparum (A), of the diatom plastids from Thalassiosira pseudonana and Phaedactylum tricornutum (B), and of the crytomonad plastid from Guillardia theta (C). Overview of the metabolite transport processes for the characterized transporters of the chromalveolates. The presence of four envelope membranes reflects their secondary endosymbiotic origin. The TPT of the apicoplast of P. falciparum has a broader substrate specificity compared to the green counterpart, accepting also PEP (A). Diatom plastids are not able to synthesize nucleotides and therefore they depend on the import of nucleotides from the cytosol. They possess NTTs which catalyze a uniport mode of transport as in the chlamydial intracellular parasites from where these transporters were acquired (B). The cryptomonads contain a less reduced secondary endosymbiont, still harboring a vestigial nucleus in the periplasmic compartment, the former algal cytoplasm, where the starch is synthesized. The day and night path of carbon metabolism are regulated by differential expression of TPT genes whose products localize in different envelopes (C). Abbreviations: (d)NTP, (deoxy)nucleotide triphosphate; G3P, glycerol 3-phosphate; PEP, phosphoenolpyruvate; PfiTPT, P. falciparum innermost envelope TPT; PfoTPT, P. falciparum outermost envelope TPT; 3-PGA, 3-phosphoglycerate; Pi, inorganic phosphate; TP, triose phosphate; UDP-Glc, UDP-glucose.

Figure 5

Overview of the characterized metabolite transporters of the heterotrophic plastid. Non-photosynthetic plastids rely on the import of photosynthates to drive the metabolic reactions in the sink tissues. Glucose 6-phosphate is transported by GPT to provide heterotrophic plastids with substrates further used to produce reducing equivalents and storage compounds. In maize, the substrate for starch synthesis, ADP-glucose, is produced in the cytoplasm and shuttled to the seeds by the brittle-1 transporter. Abbreviations: ADP-Glc, ADP-glucose; BT1, Brittle-1 ADP-glucose carrier; Glc 1-P, glucose 1-phosphate; Glc 6-P, glucose 6-phosphate; GPT, glucose 6-phosphate/phosphate translocator; Mal, maltose; NTT, nucleoside triphosphate transporter; OPPP, oxidative pentose phosphate pathway; 3-PGA, 3-phosphoglyceric acid; PEP, phosphoenolpyruvate; Pi, inorganic phosphate; PPi, pyrophosphate; PPT, PEP/phosphate translocator; XPT, xylulose 5-phosphate/phosphate translocator; Xul 5-P, xylulose 5-phosphate.

Figure 6

Molecular phylogenetic analysis of the MEX1 proteins. The evolutionary history was inferred by using the maximum likelihood method based on the WAG model (Whelan and Goldman, 2001). Sequence alignment was performed with MUSCLE included in MEGA5 (Tamura et al., 2011). Alignment quality was assessed by GUIDANCE giving an alignment score of 0.783. The bootstrap consensus tree inferred from 500 replicates is taken to represent the evolutionary history of the taxa analyzed. The percentage of replicate trees in which the associated taxa clustered together in the bootstrap test (500 replicates) are shown next to the branches. A discrete gamma distribution was used to model evolutionary rate differences among sites (five categories, +G, parameter = 3.3094). The rate variation model allowed for some sites to be evolutionarily invariable ([+I], 5.4532% sites). The tree is drawn to scale, with branch lengths measured in the number of substitutions per site. The analysis involved 14 amino acid sequences. All positions containing gaps and missing data were eliminated. There were a total of 173 positions in the final dataset. Evolutionary analyses were conducted in MEGA5 (Tamura et al., 2011).

Figure 7

Model for the pyruvate uptake into the plastid. The two-translocator system responsible for pyruvate uptake into the plastid of certain C₄ species is based on a sodium:pyruvate cotransporter (BASS2) and a sodium:proton antiporter (NHD1). NHD1 is able to establish a sodium gradient across the envelope membrane, which in turn drives pyruvate import by the cotransporter BASS2. Abbreviations: BASS2, BILE ACID:SODIUM SYMPORTER FAMILY PROTEIN 2; NHD1, sodium:proton antiporter; OAA, oxaloacetate; PEP, phosphoenolpyruvate; PPT, PEP/phosphate translocator.