PLGG1 (plastidal glycolate/glycerate translocator 1; also AtLrgB; At1g32080) is a polytopic membrane protein of the chloroplast envelope, with twelve predicted transmembrane helices, belonging to the LrgB/CidB family that is shared with bacteria, archaea and fungi. It moves the photorespiratory intermediates glycolate and glycerate across the envelope, exporting glycolate made from 2-phosphoglycolate in the stroma and importing glycerate returning from the peroxisome for phosphorylation by glycerate kinase. It behaves as a glycolate/glycerate exchanger and shares glycolate export with the bile acid sodium symporter BASS6, whereas glycerate import depends on PLGG1 alone. Loss of PLGG1 causes accumulation of glycolate and glycerate, interveinal leaf chlorosis and cell death, and reduced photosynthesis in normal air, all suppressed by elevated CO2. The leaf phenotypes first led to proposals that it controls chloroplast development or cell death. Mutants are also reported to respond differently to abscisic acid in lateral root growth, stomatal movement and drought tolerance. PLGG1 is expressed in leaves, stems and flowers but not roots.
Definition: Enables the transfer of a solute or solutes from one side of a membrane to the other according to the reaction: glycolate(in) + D-glycerate(out) = glycolate(out) + D-glycerate(in).
Justification: The chloroplast glycolate/glycerate carrier was characterized biochemically as an exchanger before its gene was known, and PLGG1 is described as a glycolate/glycerate antiporter, but GO only has separate glycolate and glycerate transporter activity terms (OLS search found no glycolate or glycerate antiporter term). A coupled exchange term would capture the stoichiometric link between the export and import legs of the C2 cycle. Besides the asserted parent GO:0043879, the term belongs under GO:0015297 antiporter activity, where GO places its other antiporter terms; proposed_parent holds only one term.
Parent term: glycolate transmembrane transporter activity
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003674 molecular_function | ND GO_REF:0000015 | MODIFY | Summary: Root-term placeholder stating that no molecular function data were available. Reason: Out of date. Transport assays, chloroplast physiology, 18O2 flux analysis and yeast complementation establish PLGG1 as a glycolate and glycerate transmembrane transporter; both specific terms are already present in GOA with IMP evidence. Proposed replacements: glycolate transmembrane transporter activity glycerate transmembrane transporter activity Supporting Evidence: PMID:23382251 Our results indicate that PLGG1 is the chloroplastidic glycolate/glycerate transporter, which is required for the function of the photorespiratory cycle. |
| GO:0009507 chloroplast | HDA PMID:18431481 Sorting signals, N-terminal modifications and abundance of t... | ACCEPT | Summary: Detected in a large-scale MS analysis of purified Arabidopsis chloroplasts. Reason: Correct. PLGG1 has a chloroplast transit peptide and is in the envelope; the chloroplast envelope and inner membrane rows give the precise compartment, but a chloroplast assignment from an organelle proteome is still informative. Supporting Evidence: PMID:18431481 we present a large scale analysis by nanoLC-Q-TOF and nanoLC-LTQ-Orbitrap mass spectrometry (MS) of ten independent chloroplast preparations from Arabidopsis thaliana which unambiguously identified 1325 proteins |
| GO:0009507 chloroplast | ISM GO_REF:0000122 | ACCEPT | Summary: AtSubP sequence-based prediction of chloroplast location. Reason: The prediction is consistent with experimental envelope localization. Supporting Evidence: PMID:23382251 The localization of PLGG1 in the chloroplast envelope ( Fig. S3 ), together with the metabolite accumulation patterns ( Figs. 2 and 3 ), identify PLGG1 as the chloroplastidic glycolate/glycerate transporter |
| GO:0009658 chloroplast organization | IMP PMID:21916894 A chloroplast envelope membrane protein containing a putativ... | MARK AS OVER ANNOTATED | Summary: Based on the interveinal chlorosis, premature necrosis and delayed greening of AtLrgB T-DNA mutants and overexpressors, which led the authors to propose a role in chloroplast development. Reason: The phenotype is real, but its interpretation has been superseded. The authors themselves state that the molecular mechanism was unknown. The later study that identified the transport activity showed that the chlorosis is suppressed by elevated CO2, that photorespiratory metabolites accumulate before visible symptoms, and that chloroplasts isolated from mutants grown in high CO2 are biochemically active, so the defect in chloroplasts is a secondary consequence of glycolate and glycerate toxicity rather than a role in chloroplast organization. UniProt carries a corresponding CAUTION. The acts_upstream_of_or_within phrasing does not make the claim false, but it is an indirect consequence of the transport function. Supporting Evidence: PMID:21916894 Our work indicates that AtLrgB is a new player in chloroplast development, carbon partitioning and leaf senescence, although its molecular mechanism remains to be established. PMID:23382251 Because the phenotype can be suppressed with high CO 2 ( Fig. 1 ) and biochemically active chloroplasts can be isolated from plgg1-1 mutant plants grown in high CO 2 ( Table 2 ), we posit that the visible symptoms are due to the accumulation of toxic concentrations of glycolate and glycerate. |
| GO:0009706 chloroplast inner membrane | HDA PMID:12766230 Proteomics of the chloroplast envelope membranes from Arabid... | ACCEPT | Summary: Identified in proteomics of purified Arabidopsis chloroplast envelope membranes; this study also defined the N-terminus of the mature protein (N-acetyl-Ala-77). Reason: A multispanning metabolite translocator of the envelope is expected in the inner envelope membrane, which is the permeability barrier for metabolites. The cached record is abstract-only, so the subfraction data cannot be checked here; consistent with the IDA row from PMID:21916894, and accepted as the core location. Supporting Evidence: PMID:12766230 These proteins are a) involved in ion and metabolite transport, b) components of the protein import machinery, and c) involved in chloroplast lipid metabolism. |
| GO:0009706 chloroplast inner membrane | IDA PMID:21916894 A chloroplast envelope membrane protein containing a putativ... | ACCEPT | Summary: Localization of AtLrgB to the chloroplast envelope membrane by cell biological and biochemical analysis. Reason: The cached abstract only states that AtLrgB is a chloroplast envelope membrane protein; the inner-membrane assignment must come from the full text, which I cannot read. The curator read the full text, and the assignment fits a multispanning metabolite carrier, so the annotation is accepted as the core location. Supporting Evidence: PMID:21916894 A protein encoded by At1g32080 was consistently identified in proteomic studies of Arabidopsis chloroplast envelope membranes, but its function remained unclear. |
| GO:0009737 response to abscisic acid | IMP PMID:29097201 Chloroplast protein PLGG1 is involved in abscisic acid-regul... | KEEP AS NON CORE | Summary: plgg1 alleles show ABA-hypersensitive lateral root growth and altered ABA-inhibited germination, and complementation restores the root response. Reason: Experimental mutant phenotype with complementation, reported in an abstract-only cached paper. No mechanism links a plastid glycolate/glycerate carrier to ABA signalling, and the effect is likely indirect (photorespiratory metabolite or carbon status), which the acts_upstream_of_or_within qualifier allows. Not part of the core transport function. Supporting Evidence: PMID:29097201 The length and number of LRs at the root-hypocotyl junction of plgg1-1 and plgg1-2 were significantly impaired under exogenous ABA treatment, and the transgenic plant complementary lines of plgg1-2 restored LR growth in response to ABA. |
| GO:0009853 photorespiration | IMP PMID:23382251 PLGG1, a plastidic glycolate glycerate transporter, is requi... | ACCEPT | Summary: plgg1-1 grows like wild type only in elevated CO2, accumulates glycolate, glycerate and downstream photorespiratory intermediates, and is complemented by the genomic PLGG1 gene. Reason: PLGG1 carries out the two chloroplast envelope transport steps of the C2 cycle (glycolate export and glycerate import), so it does part of the work of the process. Core process. Supporting Evidence: PMID:23382251 A mutant deficient in PLGG1 shows WT-like growth only in an elevated carbon dioxide atmosphere. PMID:23382251 The phenotype was complemented when PLGG1 was expressed under its own promoter in plgg1-1 plants ( Fig. 1C ). PMID:28351992 BASS6 and PLGG1 therefore balance the export of two glycolate molecules with the import of one glycerate molecule during photorespiration. |
| GO:0009941 chloroplast envelope | HDA PMID:12766230 Proteomics of the chloroplast envelope membranes from Arabid... | ACCEPT | Summary: Envelope membrane proteomics of Arabidopsis chloroplasts. Reason: Correct; the envelope is where PLGG1 acts. The inner membrane rows give the more precise compartment. Supporting Evidence: PMID:12766230 To provide the most exhaustive view of the protein repertoire of chloroplast envelope membranes, we analyzed this membrane system using proteomics. |
| GO:0009941 chloroplast envelope | HDA PMID:12938931 Proteomic study of the Arabidopsis thaliana chloroplastic en... | ACCEPT | Summary: Identified in a MudPIT/GeLC-MS proteome of Arabidopsis chloroplast envelope membranes. Reason: Correct and consistent with the other envelope proteomics and GFP data. Supporting Evidence: PMID:12938931 We have pursued a high-throughput mass spectrometry-based proteomics approach to identify and characterize membrane proteins localized to the Arabidopsis thaliana chloroplastic envelope membrane. |
| GO:0009941 chloroplast envelope | HDA PMID:20061580 AT_CHLORO, a comprehensive chloroplast proteome database wit... | ACCEPT | Summary: Envelope assignment in the AT_CHLORO subplastidial proteome database. Reason: Correct and consistent with the other envelope proteomics and GFP data. Supporting Evidence: PMID:20061580 We assessed the partitioning of each protein in the three chloroplast compartments by using a semiquantitative proteomics approach (spectral count). |
| GO:0009941 chloroplast envelope | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic inference placed at a plant PLGG1 node (PTN007824690), seeded by the experimental envelope localization of At1g32080 itself. Reason: The node is grounded in PLGG1's own experimental annotation, which is expected for an IBA, and the IBA adds the claim that envelope localization is inherited by plant PLGG1 orthologs, which fits the plastid-targeted rice and moss homologs reported in the literature. Correct. Supporting Evidence: PMID:23382251 The localization of PLGG1 in the chloroplast envelope ( Fig. S3 ), together with the metabolite accumulation patterns ( Figs. 2 and 3 ), identify PLGG1 as the chloroplastidic glycolate/glycerate transporter |
| GO:0009941 chloroplast envelope | IDA PMID:23382251 PLGG1, a plastidic glycolate glycerate transporter, is requi... | ACCEPT | Summary: PLGG1-GFP expressed transiently in N. benthamiana was imaged in protoplasts at the chloroplast envelope. Reason: Direct localization evidence; the envelope is where the transport function is exerted. Supporting Evidence: PMID:23382251 The pronounced accumulation of glycolate and glycerate and the localization of this transporter at the chloroplast envelope membrane (ref. 25 ; Fig. S2 ) point to PLGG1 as a glycolate/glycerate transporter. |
| GO:0010118 stomatal movement | IMP PMID:29097201 Chloroplast protein PLGG1 is involved in abscisic acid-regul... | KEEP AS NON CORE | Summary: plgg1 mutants are reported to have altered ABA-mediated stomatal movement. Reason: Mutant phenotype from an abstract-only paper; the effect is plausibly indirect, through altered photorespiratory metabolism in guard cells or mesophyll, and is covered by the weak acts_upstream_of_or_within qualifier. Not a core function. Supporting Evidence: PMID:29097201 we found that PLGG1 is involved in other major ABA responses, including ABA-inhibited seed germination, ABA-mediated stomatal movement, and drought tolerance |
| GO:0031969 chloroplast membrane | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping (Plastid, chloroplast membrane; multi-pass membrane protein). Reason: Correct. It is less precise than chloroplast inner membrane, but it is the UniProt-curated location backed by envelope proteomics and the GFP data, and it is not misleading. Supporting Evidence: PMID:23382251 The localization of PLGG1 in the chloroplast envelope ( Fig. S3 ), together with the metabolite accumulation patterns ( Figs. 2 and 3 ), identify PLGG1 as the chloroplastidic glycolate/glycerate transporter |
| GO:0042631 cellular response to water deprivation | IMP PMID:29097201 Chloroplast protein PLGG1 is involved in abscisic acid-regul... | KEEP AS NON CORE | Summary: plgg1 mutants are reported to have altered drought tolerance. Reason: Whole-plant drought phenotype reported in an abstract-only paper; the term says "cellular" response, which the abstract does not specifically address, and the effect is probably indirect. Kept as a non-core upstream phenotype; I cannot read the full text to check whether a cellular assay was used. Supporting Evidence: PMID:29097201 we found that PLGG1 is involved in other major ABA responses, including ABA-inhibited seed germination, ABA-mediated stomatal movement, and drought tolerance |
| GO:0043879 glycolate transmembrane transporter activity | IMP PMID:23382251 PLGG1, a plastidic glycolate glycerate transporter, is requi... | ACCEPT | Summary: 18O2 metabolic flux profiling shows that glycolate is trapped in plgg1-1 chloroplasts, and liposome assays show glycerate uptake into glycolate-preloaded liposomes (counter-exchange). Reason: Core function. The mutant flux data and the reconstitution assay are supported by independent heterologous evidence that PLGG1 restores glycolate uptake in a yeast transport mutant. PLGG1 shares chloroplast glycolate export with BASS6. Supporting Evidence: PMID:23382251 Thus, plgg1-1 is impaired in the transport of both glycolate and glycerate. PMID:28351992 Our data indicate that both BASS6 and PLGG1 expressed in yeast facilitate transmembrane glycolate transport leading to rescue of the growth in glycolate uptake defective ady2Ξ mutant ( Figure 8B ). |
| GO:0048527 lateral root development | IMP PMID:29097201 Chloroplast protein PLGG1 is involved in abscisic acid-regul... | KEEP AS NON CORE | Summary: Lateral root number and length at the root-hypocotyl junction of plgg1 mutants are reduced under exogenous ABA; complementation restores the response. Reason: Experimental, complemented phenotype, but conditional on exogenous ABA and almost certainly indirect: PLGG1 is reported not to be expressed in roots, so a shoot metabolic effect on root growth is the likely explanation. The acts_upstream_of_or_within qualifier allows this. Not core; I cannot see the full text. Supporting Evidence: PMID:29097201 The length and number of LRs at the root-hypocotyl junction of plgg1-1 and plgg1-2 were significantly impaired under exogenous ABA treatment PMID:22180599 AtLrgB is expressed in leaves, stems and flowers, but not in roots |
| GO:0097339 glycolate transmembrane transport | IMP PMID:23382251 PLGG1, a plastidic glycolate glycerate transporter, is requi... | ACCEPT | Summary: Glycolate export from the chloroplast is impaired in plgg1-1, as shown by 18O2 labelling kinetics. Reason: Core process directly executed by the transporter. Supporting Evidence: PMID:23382251 This finding was consistent with a reduced export of glycolate from the chloroplast. |
| GO:1901974 glycerate transmembrane transporter activity | IMP PMID:23382251 PLGG1, a plastidic glycolate glycerate transporter, is requi... | ACCEPT | Summary: Isolated plgg1-1 chloroplasts lack glycerate-dependent O2 evolution while 3-PGA dependent O2 evolution is normal, and liposome assays show saturable glycerate uptake. Reason: Core function. PLGG1 is the only identified carrier for the glycerate import step; BASS6, which shares glycolate export, does not transport glycerate. Supporting Evidence: PMID:23382251 Hence, chloroplasts of the plgg1-1 mutant do not display glycerate-dependent oxygen evolution, which supports PLGG1βs function as a glycerate transporter. PMID:28351992 Our data indicate that BASS6 and PLGG1 partner in glycolate export from the chloroplast, whereas PLGG1 alone accounts for the import of glycerate. |
| GO:1901975 glycerate transmembrane transport | IMP PMID:23382251 PLGG1, a plastidic glycolate glycerate transporter, is requi... | ACCEPT | Summary: Glycerate uptake into chloroplasts is lost in plgg1-1. Reason: Core process directly executed by the transporter. Supporting Evidence: PMID:23382251 They were only unable to transport glycerate because no glycerate-dependent oxygen evolution was observed with plgg1-1 chloroplasts ( Table 2 ). |
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Download this section (compressed HTML)Q: Is PLGG1 a strict 1:1 glycolate/glycerate antiporter, or can it also catalyse unidirectional glycolate efflux (e.g. proton-coupled), given that two glycolate molecules leave the chloroplast for every glycerate that returns?
Q: Are the ABA-related phenotypes of plgg1 mutants (lateral roots, stomata, germination) caused by photorespiratory metabolite accumulation, and are they suppressed by elevated CO2?
Q: Does the bacterial LrgB/CidB homolog family also transport glycolate or related small organic acids, and does that underlie its reported role in bacterial cell lysis?
Experiment: Measure glycolate and glycerate transport by purified, reconstituted PLGG1 in proteoliposomes under defined counter-substrate and proton gradients, to determine stoichiometry, coupling and kinetics without the high diffusion background reported in earlier liposome assays.
Type: biochemical transport assay
Experiment: Test whether ABA-dependent lateral root, stomatal and germination phenotypes of plgg1 mutants persist when plants are grown in elevated CO2.
Type: genetic/physiological
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