GGAT1 (glutamate:glyoxylate aminotransferase 1; also AOAT1/GGT1; At1g23310) is a peroxisomal, pyridoxal 5'-phosphate-dependent class-I aminotransferase of the alanine aminotransferase subfamily. It is the major leaf glutamate:glyoxylate aminotransferase of the photorespiratory C2 cycle, transaminating glyoxylate (produced by glycolate oxidase) to glycine with glutamate as amino donor and releasing 2-oxoglutarate. The recombinant enzyme also catalyses alanine:glyoxylate, alanine:2-oxoglutarate and glutamate:pyruvate transaminations, but not serine:glyoxylate or aspartate transamination. GGAT1 carries a C-terminal type 1 peroxisomal targeting signal, and its activity is confined to peroxisomes. Knockout plants lose most leaf GGAT activity and show a mild, high-CO2-rescuable photorespiratory growth defect with altered glutamate and serine levels; the closely related GGAT2 (At1g70580) supplies residual activity. GGAT1 is also expressed in roots, and overexpression raises leaf serine, glycine and citrulline, indicating a wider role in amino acid metabolism.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000325 plant-type vacuole | HDA PMID:15539469 The vegetative vacuole proteome of Arabidopsis thaliana reve... | MARK AS OVER ANNOTATED | Summary: Identification of GGAT1 peptides in a proteome of vacuoles isolated from leaf protoplasts. Reason: GGAT1 is an abundant peroxisomal matrix enzyme with a functional PTS1 and no vacuolar sorting signal; GGT activity fractionates exclusively with peroxisomes. Detection in an isolated vacuole preparation is most likely contamination and does not indicate a vacuolar pool. Following projects/ARATH/organelle-audit.md rule 3, this single proteomics hit for a protein localized elsewhere is marked over-annotated rather than removed. Supporting Evidence: PMID:15539469 Vacuoles were isolated from protoplasts derived from rosette leaf tissue. PMID:12631323 When AOAT1 fused to green fluorescent protein (GFP) was expressed in BY-2 cells, it was found to be localized to peroxisomes depending on PTS1. PMID:12529529 GGT activity was localized exclusively to Arabidopsis peroxisomes |
| GO:0001666 response to hypoxia | IEP PMID:17319845 Alanine aminotransferase catalyses the breakdown of alanine ... | KEEP AS NON CORE | Summary: Expression-based annotation; the study of alanine aminotransferases under low oxygen reported slight hypoxic induction of GGAT1 transcripts in shoots (as summarized by UniProt). Reason: The cached paper is abstract-only and focuses on AlaAT1/AlaAT2; the GGAT1 induction is reported in the full text and is captured in the curated UniProt record. A transcriptional response is a valid IEP claim but is peripheral to the photorespiratory function of GGAT1, and the alanine-breakdown role during hypoxia recovery is attributed to AlaAT1, not GGAT1. Supporting Evidence: file:ARATH/GGAT1/GGAT1-uniprot.txt Slightly induced upon low-oxygen stress in shoots. PMID:17319845 suggesting that AlaAT1 is the major AlaAT isozyme in Arabidopsis |
| GO:0003729 mRNA binding | IDA PMID:32344669 Discovering the RNA-Binding Proteome of Plant Leaves with an... | MARK AS OVER ANNOTATED | Summary: GGAT1 was captured in a leaf RNA interactome capture (RIC) screen after UV crosslinking. Reason: The authors list glutamate-glyoxylate aminotransferase among metabolic enzymes newly found in the RBPome with no prior link to RNA biology. There is no follow-up showing specific mRNA binding or a function for it, and GGAT1 acts in the peroxisomal matrix, away from mRNA. The screen hit is retained as data but not as a function of the gene. Supporting Evidence: PMID:32344669 Additional moonlighting enzymes were identified in the leaf RBPome, such as COBALAMIN-INDEPENDENT METHIONINE SYNTHASE, (S)-2-HYDROXY-ACID OXIDASE, GLYCINE DEHYDROGENASE and GLUTAMATE-GLYOXYLATE AMINOTRANSFERASE, which, to our knowledge, have not previously been linked to RNA biology |
| GO:0003729 mRNA binding | IEA GO_REF:0000117 | MARK AS OVER ANNOTATED | Summary: ARBA machine-learning prediction of mRNA binding. Reason: The ARBA rule most likely reflects the same class of RNA interactome screen hits; there is no evidence that mRNA binding is a function of this peroxisomal aminotransferase. Consistent with the IDA row for the same term. Supporting Evidence: PMID:32344669 Additional moonlighting enzymes were identified in the leaf RBPome, such as COBALAMIN-INDEPENDENT METHIONINE SYNTHASE, (S)-2-HYDROXY-ACID OXIDASE, GLYCINE DEHYDROGENASE and GLUTAMATE-GLYOXYLATE AMINOTRANSFERASE, which, to our knowledge, have not previously been linked to RNA biology |
| GO:0004021 L-alanine:2-oxoglutarate transaminase activity | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: PAINT inference of alanine:2-oxoglutarate (AlaAT) activity for the alanine aminotransferase clade; GGAT1 itself is among the experimentally characterized descendants. Reason: Purified recombinant GGAT1 catalyses the reversible Ala:2-oxoglutarate / Glu:pyruvate transamination characteristic of AlaATs, so the activity is real and ancestral to the clade. In GGAT1 it is secondary to the glutamate:glyoxylate reaction that defines its photorespiratory role, so it is kept as non-core. Supporting Evidence: PMID:12529529 Both purified recombinant proteins catalyzed GGT, GPT, and AKT reactions |
| GO:0004021 L-alanine:2-oxoglutarate transaminase activity | IDA PMID:12529529 Alanine aminotransferase homologs catalyze the glutamate:gly... | KEEP AS NON CORE | Summary: Purified recombinant GGAT1 (and leaf-purified enzyme) catalyses the Ala:2-oxoglutarate (AKT) reaction with measured kinetics. Reason: Directly demonstrated activity with reasonable Km values, but a secondary activity relative to the photorespiratory glutamate:glyoxylate reaction. Supporting Evidence: PMID:12529529 Four aminotransferase activities were specifically associated with GGT1 and GGT2, using the substrate pairs glutamate (Glu):glyoxylate, Ala:glyoxylate, Glu:pyruvate, and Ala:2-oxoglutarate. |
| GO:0004021 L-alanine:2-oxoglutarate transaminase activity | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: Electronic mapping from EC 2.6.1.2 / RHEA:19453. Reason: Consistent with the experimentally demonstrated AKT/GPT activity of GGAT1; non-core relative to the glutamate:glyoxylate activity. Supporting Evidence: PMID:12529529 Both purified recombinant proteins catalyzed GGT, GPT, and AKT reactions |
| GO:0004021 L-alanine:2-oxoglutarate transaminase activity | IMP PMID:17318317 Altered ABA, proline and hydrogen peroxide in an Arabidopsis... | KEEP AS NON CORE | Summary: ggt1 mutant plants isolated in an ABA-response screen; UniProt records loss of alanine aminotransferase activity in ggat1 disruption lines. Reason: The cached paper is abstract-only, so the activity measurement behind this IMP cannot be read, but the claim agrees with the biochemistry of the recombinant enzyme and with the reduced AOAT activity reported in the aoat1-1 knockout. Deferring to the curator; non-core activity. Supporting Evidence: PMID:12631323 The activity of GGAT and alanine:glyoxylate aminotransferase (AGAT) in the above-ground tissues of aoat1-1 was reduced drastically and, AOAT and glutamate:pyruvate aminotransferase (GPAT) activity also decreased. |
| GO:0005634 nucleus | HDA PMID:28887381 Global Analysis of Membrane-associated Protein Oligomerizati... | MARK AS OVER ANNOTATED | Summary: Detection in a cell-fractionation / size-exclusion profiling study of membrane-associated protein complexes. Reason: No evidence for a nuclear pool of GGAT1; it is a PTS1-targeted peroxisomal matrix enzyme whose activity is exclusively peroxisomal. Detection in crude organelle/membrane fractions of a highly abundant leaf enzyme is best regarded as contamination. Following projects/ARATH/organelle-audit.md rule 3, this single proteomics hit for a protein localized elsewhere is marked over-annotated rather than removed. Supporting Evidence: PMID:28887381 Here we combined cell fractionation of Arabidopsis leaves with nondenaturing detergent solubilization and LC/MS-based profiling of size exclusion chromatography fractions PMID:12529529 GGT activity was localized exclusively to Arabidopsis peroxisomes |
| GO:0005737 cytoplasm | ISM GO_REF:0000122 | MODIFY | Summary: AtSubP sequence-based prediction of cytoplasmic localization. Reason: Peroxisomes are part of the cytoplasm, so the term is not wrong, but it is far less specific than the experimental evidence: GFP targeting via PTS1, fractionation and peroxisome proteomics all place GGAT1 in the peroxisome, a term the gene already carries. Proposed replacements: peroxisome Supporting Evidence: PMID:12631323 When AOAT1 fused to green fluorescent protein (GFP) was expressed in BY-2 cells, it was found to be localized to peroxisomes depending on PTS1. |
| GO:0005777 peroxisome | HDA PMID:17951448 Proteome analysis of Arabidopsis leaf peroxisomes reveals no... | ACCEPT | Summary: GGAT1 is one of the dominant proteins of the isolated mature leaf peroxisome proteome and was distinguished from GGAT2 by isoform-specific peptides. Reason: High-throughput evidence agreeing with GFP targeting and subcellular fractionation of activity. Supporting Evidence: PMID:17951448 Of the two isoforms each of Glu-glyoxylate aminotransferase (GGT) and peroxisomal malate dehydrogenase (pMDH), GGT1 (At1g23310; SKM>) and pMDH2 (At5g09660; RIx 5 HL) dominated over the second isoform |
| GO:0005777 peroxisome | IDA PMID:12154131 Proteomic analysis of leaf peroxisomal proteins in greening ... | ACCEPT | Summary: Leaf peroxisomes isolated from greening cotyledons were analysed by MALDI-TOF MS; glycolate pathway enzymes were among the identified proteins. Reason: The cached record is abstract-only and the GGAT1 identification is in the full text; the localization is corroborated by GFP, fractionation and later proteomic studies, so the curator's annotation is accepted. Supporting Evidence: PMID:12154131 Among them, five proteins are related to the glycolate pathway PMID:12529529 GGT activity was localized exclusively to Arabidopsis peroxisomes |
| GO:0005777 peroxisome | IDA PMID:12529529 Alanine aminotransferase homologs catalyze the glutamate:gly... | ACCEPT | Summary: Sucrose-gradient fractionation of seedling extracts: GGT activity co-fractionates with the peroxisomal marker catalase. Reason: Direct biochemical localization; core location of the photorespiratory GGAT step. Supporting Evidence: PMID:12529529 These subcellular fractionation data are consistent with a solely peroxisomal localization of GGT activity in Arabidopsis. |
| GO:0005777 peroxisome | IDA PMID:12631323 Identification of photorespiratory glutamate:glyoxylate amin... | ACCEPT | Summary: AOAT1-GFP is targeted to peroxisomes in BY-2 cells in a PTS1-dependent manner; peroxisomal GGAT activity is lost in aoat1-1. Reason: Direct localization evidence; core location. Supporting Evidence: PMID:12631323 When AOAT1 fused to green fluorescent protein (GFP) was expressed in BY-2 cells, it was found to be localized to peroxisomes depending on PTS1. PMID:12631323 Peroxisomal GGAT was detected in the wild type but not in aoat1-1. |
| GO:0005777 peroxisome | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic mapping of the UniProt subcellular location (Peroxisome). Reason: Consistent with experimental peroxisomal localization. Supporting Evidence: PMID:12529529 GGT activity was localized exclusively to Arabidopsis peroxisomes |
| GO:0005829 cytosol | HDA PMID:25293756 A proteomic strategy for global analysis of plant protein co... | MARK AS OVER ANNOTATED | Summary: Detection of GGAT1 in size-exclusion profiling of a crude cytosolic fraction of leaves. Reason: The study analysed a crude cytosolic fraction, into which matrix enzymes leak from peroxisomes broken during homogenization. GGT activity fractionates exclusively with peroxisomes, and the NOT cytosol annotation from the cytosolic proteome study (accepted below) agrees. The observation does not establish a cytosolic pool. Following projects/ARATH/organelle-audit.md rule 3, this single proteomics hit for a protein localized elsewhere is marked over-annotated rather than removed. Supporting Evidence: PMID:25293756 Applying this approach to the crude cytosolic fraction of Arabidopsis thaliana leaves using size exclusion chromatography PMID:12529529 GGT activity was localized exclusively to Arabidopsis peroxisomes |
| GO:0005829 cytosol | RCA NOT PMID:21166475 Analysis of the Arabidopsis cytosolic proteome highlights su... | ACCEPT | Summary: NOT annotation: GGAT1 was not part of the experimentally defined cytosolic proteome of Arabidopsis cell cultures. Reason: The negative assertion agrees with the PTS1-dependent peroxisomal targeting and the exclusively peroxisomal GGT activity of GGAT1. Supporting Evidence: PMID:21166475 defines an experimental set of plant protein sequences that are not targeted to subcellular locations following translation and folding in the cytosol PMID:12529529 GGT activity was localized exclusively to Arabidopsis peroxisomes |
| GO:0006545 glycine biosynthetic process | IEA GO_REF:0000041 | ACCEPT | Summary: UniPathway mapping (glycine biosynthesis; glycine from glyoxylate). GGAT1 forms glycine by transamination of glyoxylate. Reason: GGAT1 catalyses the step itself (glyoxylate + glutamate to glycine + 2-oxoglutarate), so it is a direct participant in glycine formation, here within the photorespiratory cycle. Supporting Evidence: PMID:12631323 In the photorespiratory process, peroxisomal glutamate:glyoxylate aminotransferase (GGAT) catalyzes the reaction of glutamate and glyoxylate to 2-oxoglutarate and glycine. PMID:12631323 Here, we report that an alanine:2-oxoglutarate aminotransferase (AOAT)-like protein functions as GGAT in peroxisomes. |
| GO:0008453 L-alanine:glyoxylate transaminase activity | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: PAINT inference for the plant peroxisomal GGAT subclade (GGAT1/GGAT2), seeded by their own experimental data. Reason: Alanine:glyoxylate (AGT) activity of GGAT1 has been detected with both recombinant and leaf-purified enzyme and is reduced in the knockout, so the inference is sound. Its importance for photorespiratory glycine formation, compared with glutamate as amino donor, is disputed, so it is kept as non-core. Supporting Evidence: PMID:12529529 Both recombinant proteins and the purified plant protein displayed AGT activity (data not shown), but this activity was not characterized kinetically because of technical limitations of the AGT assay. PMID:12529529 but the physiological significance of the AGT reaction for photorespiration has been disputed |
| GO:0008453 L-alanine:glyoxylate transaminase activity | IDA PMID:12529529 Alanine aminotransferase homologs catalyze the glutamate:gly... | KEEP AS NON CORE | Summary: AGT activity detected with recombinant GGAT1 and with the leaf-purified enzyme. Reason: Real activity (also reduced in aoat1-1 leaves), but not kinetically characterized and of disputed physiological weight relative to the glutamate:glyoxylate reaction. Supporting Evidence: PMID:12529529 Both recombinant proteins and the purified plant protein displayed AGT activity (data not shown), but this activity was not characterized kinetically because of technical limitations of the AGT assay. PMID:12631323 The activity of GGAT and alanine:glyoxylate aminotransferase (AGAT) in the above-ground tissues of aoat1-1 was reduced drastically |
| GO:0008453 L-alanine:glyoxylate transaminase activity | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: Electronic mapping from EC 2.6.1.44 / RHEA:24248. Reason: Consistent with the experimental AGT activity; non-core as for the other rows of this term. Supporting Evidence: PMID:12529529 Four aminotransferase activities were specifically associated with GGT1 and GGT2, using the substrate pairs glutamate (Glu):glyoxylate, Ala:glyoxylate, Glu:pyruvate, and Ala:2-oxoglutarate. |
| GO:0008483 transaminase activity | IEA GO_REF:0000002 | MODIFY | Summary: InterPro2GO mapping from the ALAT1/2-like family signature. Reason: Correct but general. The specific, experimentally supported core activity is glutamate:glyoxylate transamination (GO:0047958 glycine:2-oxoglutarate transaminase activity), already carried by the gene. Proposed replacements: glycine:2-oxoglutarate transaminase activity Supporting Evidence: file:ARATH/GGAT1/GGAT1-uniprot.txt IPR045088; ALAT1/2-like PMID:12529529 Four aminotransferase activities were specifically associated with GGT1 and GGT2, using the substrate pairs glutamate (Glu):glyoxylate, Ala:glyoxylate, Glu:pyruvate, and Ala:2-oxoglutarate. |
| GO:0009507 chloroplast | HDA PMID:18431481 Sorting signals, N-terminal modifications and abundance of t... | MARK AS OVER ANNOTATED | Summary: Identification in a large-scale MS analysis of isolated chloroplast preparations. Reason: GGAT1 has no chloroplast transit peptide and is a peroxisomal PTS1 protein. Peroxisomes co-purify with chloroplasts, and the study itself considers the problem of contaminants in chloroplast preparations. The detection does not show a plastid pool. Following projects/ARATH/organelle-audit.md rule 3, this single proteomics hit for a protein localized elsewhere is marked over-annotated rather than removed. Supporting Evidence: PMID:18431481 This quantitative information was used to determine possible biases for protein targeting prediction by TargetP and also to understand the significance of protein contaminants. PMID:12529529 GGT activity was localized exclusively to Arabidopsis peroxisomes |
| GO:0009853 photorespiration | TAS PMID:12529529 Alanine aminotransferase homologs catalyze the glutamate:gly... | ACCEPT | Summary: GGAT1 catalyses the peroxisomal glutamate:glyoxylate transamination of the photorespiratory C2 cycle. Reason: Core process. Supported by biochemistry (recombinant and leaf-purified enzyme), peroxisomal localization, and the conditional, high-CO2-rescued growth defect and loss of leaf GGAT activity in the aoat1-1 knockout. The process term is the one used for this step in the photorespiration module. Supporting Evidence: PMID:12529529 Two peroxisomal glyoxylate aminotransferases are central to the photorespiratory pathway; one catalyzes the SGT transamination, the other catalyzes GGT and AGT transaminations PMID:12631323 Based on these results, it was concluded that AOAT1 is targeted to peroxisomes, functions as a photorespiratory GGAT, plays a markedly important role for plant growth and the metabolism of amino acids. PMID:22303256 Ggat 1 knock-outs show a weak photorespiratory phenotype and survive in air at moderate light intensities. |
| GO:0030170 pyridoxal phosphate binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the class I/II aminotransferase domain. Reason: Class I aminotransferases are PLP-dependent; PLP is the cofactor of the transamination mechanism. Retained as a cofactor-binding function supporting the core activity. Supporting Evidence: file:ARATH/GGAT1/GGAT1-uniprot.txt IPR004839; Aminotransferase_I/II_large file:ARATH/GGAT1/GGAT1-uniprot.txt Belongs to the class-I pyridoxal-phosphate-dependent aminotransferase family. Alanine aminotransferase subfamily. |
| GO:0042853 L-alanine catabolic process | IEA GO_REF:0000041 | MARK AS OVER ANNOTATED | Summary: UniPathway mapping (L-alanine degradation via transaminase pathway) based on the generic alanine aminotransferase reaction. Reason: GGAT1 can use alanine as an amino donor in vitro, but there is no evidence that it degrades alanine in vivo. Alanine breakdown, for example after hypoxia, is attributed to the mitochondrial AlaAT1 isozyme. In GGAT1 the alanine-using reactions are secondary to its photorespiratory glutamate:glyoxylate role. Supporting Evidence: PMID:17319845 These results indicate that the primary role of AlaAT1 is to break down alanine when it is in excess. PMID:12529529 but the physiological significance of the AGT reaction for photorespiration has been disputed |
| GO:0047958 glycine:2-oxoglutarate transaminase activity | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT inference of glutamate:glyoxylate (GGT) activity for the plant peroxisomal GGAT subclade; GGAT1 appears among its own sources because it carries experimental annotations for this term. Reason: Core molecular function. The GO term is written in the reverse (glycine + 2-oxoglutarate) direction of the same reversible reaction (EC 2.6.1.4) that operates toward glycine in photorespiration. Supporting Evidence: PMID:12529529 Both purified recombinant proteins catalyzed GGT, GPT, and AKT reactions |
| GO:0047958 glycine:2-oxoglutarate transaminase activity | IDA PMID:12529529 Alanine aminotransferase homologs catalyze the glutamate:gly... | ACCEPT | Summary: Purified recombinant GGAT1 catalyses the glutamate:glyoxylate (GGT) reaction with kinetics similar to the enzyme purified from leaves (Km for glyoxylate 0.27 mM). Reason: Direct biochemical demonstration of the core activity. Supporting Evidence: PMID:12529529 Four aminotransferase activities were specifically associated with GGT1 and GGT2, using the substrate pairs glutamate (Glu):glyoxylate, Ala:glyoxylate, Glu:pyruvate, and Ala:2-oxoglutarate. PMID:12529529 The purified recombinant aminotransferases also had similar kinetic characteristics compared with the protein purified from Arabidopsis leaves |
| GO:0047958 glycine:2-oxoglutarate transaminase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic mapping from EC 2.6.1.4 / RHEA:14089. Reason: Consistent with the experimentally demonstrated core activity. Supporting Evidence: PMID:12529529 Both purified recombinant proteins catalyzed GGT, GPT, and AKT reactions |
| GO:0047958 glycine:2-oxoglutarate transaminase activity | IMP PMID:12631323 Identification of photorespiratory glutamate:glyoxylate amin... | ACCEPT | Summary: Leaf GGAT activity is drastically reduced in the aoat1-1 T-DNA knockout, and peroxisomal GGAT is absent from the mutant. Reason: Genetic evidence that GGAT1 provides most of the leaf GGAT activity; complements the biochemical IDA. Supporting Evidence: PMID:12631323 The activity of GGAT and alanine:glyoxylate aminotransferase (AGAT) in the above-ground tissues of aoat1-1 was reduced drastically PMID:12631323 Peroxisomal GGAT was detected in the wild type but not in aoat1-1. |
| GO:0048046 apoplast | HDA PMID:18538804 Hydroponic isotope labelling of entire plants (HILEP) for qu... | MARK AS OVER ANNOTATED | Summary: Detection in an apoplast-focused quantitative proteomics study under oxidative stress. Reason: GGAT1 has no secretory signal peptide and is a PTS1-targeted peroxisomal enzyme; detection of abundant intracellular enzymes in apoplastic extracts reflects cell leakage, particularly under stress. Following projects/ARATH/organelle-audit.md rule 3, this single proteomics hit for a protein localized elsewhere is marked over-annotated rather than removed. Supporting Evidence: PMID:18538804 it was possible to show differential protein expression in the apoplast of plants submitted to oxidative stress PMID:12529529 GGT activity was localized exclusively to Arabidopsis peroxisomes |
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Download this section (compressed HTML)Q: What fraction of photorespiratory glycine in leaves is formed with alanine rather than glutamate as amino donor by GGAT1/GGAT2, given that the enzyme catalyses both reactions in vitro?
Q: What is the metabolic role of GGAT1 in roots and in darkness, where photorespiratory flux is absent?
Experiment: Construct ggat1 ggat2 double mutants and measure air-sensitivity, leaf GGAT/AGT activities and 15N-labelling of glycine from glutamate versus alanine, to quantify the contribution of each amino donor and test for further glyoxylate aminotransferases.
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