AGT1 (At2g13360), also called serine:glyoxylate aminotransferase (SGAT) or alanine:glyoxylate aminotransferase 1, is a peroxisomal, pyridoxal 5'-phosphate-dependent class-V aminotransferase related to animal AGT1/AGXT. Encoded by a single gene, it catalyses the photorespiratory serine:glyoxylate transamination. Serine made in the mitochondria is converted to hydroxypyruvate, and its amino group aminates a second glyoxylate to glycine, which closes the nitrogen balance of the C2 cycle. The enzyme is a homodimer with a C-terminal type 1 peroxisomal targeting signal, and serine-binding residues from the partner subunit explain its preference for serine over alanine. It also transaminates alanine:glyoxylate, serine:pyruvate and asparagine with glyoxylate, pyruvate or hydroxypyruvate, and asparagine is a high-capacity donor. The sat mutants, which carry a P251L substitution at the dimer interface, completely lack serine:glyoxylate aminotransferase activity, accumulate serine and glycine, and are inviable in air but normal at elevated CO2. AGT1 is most highly expressed in green tissues, but it is also expressed in roots, where it is induced by asparagine.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003729 mRNA binding | IDA PMID:32344669 Discovering the RNA-Binding Proteome of Plant Leaves with an... | MARK AS OVER ANNOTATED | Summary: AGT1 was recovered in a leaf RNA interactome capture (RIC) screen; the paper discusses metabolic enzymes in the leaf RBPome as putative moonlighting RNA binders. Reason: A proteome-wide crosslinking screen hit without follow-up on binding specificity or function; AGT1 is a peroxisomal matrix enzyme. Not a function of the gene in any established sense. Supporting Evidence: PMID:32344669 Moonlighting functions have been described for a number of metabolic enzymes across eukaryotes, including plants |
| GO:0004760 L-serine:pyruvate transaminase activity | EXP PMID:23098902 Characterization of Arabidopsis serine:glyoxylate aminotrans... | KEEP AS NON CORE | Summary: Recombinant His-tagged AGT1 characterized with Ser, Ala and Asn as donors and glyoxylate, pyruvate and hydroxypyruvate as acceptors, including Ser:pyruvate. Reason: The serine:pyruvate activity is demonstrated in vitro, but the preferred and physiologically essential reaction is serine:glyoxylate. Kept as a genuine secondary activity. Supporting Evidence: PMID:23098902 Recombinant N-terminal His-tagged AGT1 purified from Escherichia coli was characterized with Ser, alanine (Ala) and Asn as amino acid donors and glyoxylate, pyruvate and hydroxypyruvate as organic acid acceptors. |
| GO:0004760 L-serine:pyruvate transaminase activity | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: PAINT inference of serine:pyruvate transaminase activity for the AGT1/AGXT clade, with AGT1 itself among the experimentally characterized descendants. Reason: Consistent with the demonstrated Ser:pyruvate activity of recombinant AGT1; the activity is ancestral to the clade but secondary in AGT1, whose physiological role is the serine:glyoxylate step of photorespiration. Supporting Evidence: PMID:11309139 Purified, recombinant Arabidopsis AGT1 expressed in Escherichia coli catalyzed three transamination reactions using the following amino donor : acceptor combinations: alanine : glyoxylate, serine : glyoxylate, and serine : pyruvate. |
| GO:0004760 L-serine:pyruvate transaminase activity | IDA PMID:11309139 Peroxisomal alanine : glyoxylate aminotransferase (AGT1) is ... | KEEP AS NON CORE | Summary: Purified recombinant AGT1 catalyses the serine:pyruvate transamination. Reason: Directly demonstrated, but the same study concludes that serine:glyoxylate is the preferred in vivo reaction. Supporting Evidence: PMID:11309139 Purified, recombinant Arabidopsis AGT1 expressed in Escherichia coli catalyzed three transamination reactions using the following amino donor : acceptor combinations: alanine : glyoxylate, serine : glyoxylate, and serine : pyruvate. |
| GO:0004760 L-serine:pyruvate transaminase activity | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: Electronic mapping from EC 2.6.1.51 / RHEA:22852. Reason: Consistent with the experimental Ser:pyruvate activity; non-core as for the other rows. Supporting Evidence: PMID:11309139 Purified, recombinant Arabidopsis AGT1 expressed in Escherichia coli catalyzed three transamination reactions using the following amino donor : acceptor combinations: alanine : glyoxylate, serine : glyoxylate, and serine : pyruvate. |
| GO:0005739 mitochondrion | HDA PMID:28887381 Global Analysis of Membrane-associated Protein Oligomerizati... | MARK AS OVER ANNOTATED | Summary: Detection in a leaf cell-fractionation / size-exclusion profiling study of membrane-associated protein complexes. Reason: AGT1 is a PTS1-targeted peroxisomal enzyme imported into peroxisomes in vitro and recovered in peroxisomal fractions; there is no evidence for a mitochondrial pool in Arabidopsis. Detection of an abundant leaf enzyme in a crude membrane fraction is most likely 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:11309139 In vitro import experiments and subcellular fractionations localized AGT1 to peroxisomes. |
| GO:0005777 peroxisome | EXP PMID:24505423 RabGAP22 is required for defense to the vascular pathogen Ve... | ACCEPT | Summary: BiFC between AGT1 and RabGAP22 in N. benthamiana gave a peroxisomal signal that co-localized with a peroxisomal marker. Reason: Experimental localization consistent with the earlier import and fractionation data; core location. Supporting Evidence: PMID:24505423 A clear fluorescence signal was detected in the peroxisomes, demonstrating interaction of the two proteins (Figure 2D). |
| GO:0005777 peroxisome | HDA PMID:17951448 Proteome analysis of Arabidopsis leaf peroxisomes reveals no... | ACCEPT | Summary: Ser-glyoxylate aminotransferase is among the most abundant matrix enzymes of isolated leaf peroxisomes. Reason: High-throughput evidence agreeing with direct localization data. Supporting Evidence: PMID:17951448 This pattern was most obvious for highly abundant matrix enzymes, namely GGT1, Ser-glyoxylate aminotransferase, HPR, and pMDH2 |
| GO:0005777 peroxisome | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT inference of peroxisomal localization for the plant AGT1 clade, grounded in AGT1's own experimental data. Reason: Consistent with import, fractionation, proteomic and BiFC evidence. Supporting Evidence: PMID:11309139 In vitro import experiments and subcellular fractionations localized AGT1 to peroxisomes. |
| GO:0005777 peroxisome | IDA PMID:11309139 Peroxisomal alanine : glyoxylate aminotransferase (AGT1) is ... | ACCEPT | Summary: In vitro import into peroxisomes and subcellular fractionation localize AGT1 to peroxisomes. Reason: Direct localization evidence; core location of the SGAT step. Supporting Evidence: PMID:11309139 In vitro import experiments and subcellular fractionations localized AGT1 to peroxisomes. |
| GO:0005777 peroxisome | IEA GO_REF:0000044 | ACCEPT | Summary: Mapping of the UniProt subcellular location (Peroxisome). Reason: Consistent with experimental data. Supporting Evidence: PMID:11309139 In vitro import experiments and subcellular fractionations localized AGT1 to peroxisomes. |
| GO:0005829 cytosol | HDA PMID:25293756 A proteomic strategy for global analysis of plant protein co... | MARK AS OVER ANNOTATED | Summary: Detection in size-exclusion profiling of a crude cytosolic fraction of leaves. Reason: Peroxisomal matrix enzymes are released into crude cytosolic extracts when organelles break during homogenization. AGT1 is a peroxisomal PTS1 protein, and this 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:11309139 In vitro import experiments and subcellular fractionations localized AGT1 to peroxisomes. |
| GO:0005886 plasma membrane | HDA PMID:14506206 Large-scale analysis of in vivo phosphorylated membrane prot... | MARK AS OVER ANNOTATED | Summary: Identified in a large-scale IMAC phosphopeptide analysis of plasma membrane preparations. Reason: AGT1 is a soluble peroxisomal matrix enzyme with no transmembrane segment; identification in a plasma membrane phosphoproteome reflects contamination of the membrane preparation by an abundant leaf protein. 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:14506206 for the identification of plasma membrane phosphoproteins of Arabidopsis PMID:11309139 In vitro import experiments and subcellular fractionations localized AGT1 to peroxisomes. |
| GO:0006545 glycine biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT inference of glycine biosynthesis (from glyoxylate) for the AGT1/AGXT clade. Reason: AGT1 forms glycine directly by aminating glyoxylate with the amino group of serine, so it catalyses glycine formation within photorespiration. Supporting Evidence: PMID:22303256 The amino group is used for the concomitant amination of glyoxylate to glycine. |
| GO:0008453 L-alanine:glyoxylate transaminase activity | EXP PMID:18235971 Properties of serine:glyoxylate aminotransferase purified fr... | KEEP AS NON CORE | Summary: Native SGAT purified from leaves transaminates L-alanine with glyoxylate at about 55% of the serine rate, with a low Km for alanine. Reason: Real activity of the native enzyme, and the authors argue that Ala may be used as amino donor during photorespiration. However, AGT1 is distinguished from its alanine-preferring animal orthologs by serine-binding active-site residues, and the essential, non-redundant function revealed by the sat mutants is serine:glyoxylate transamination. Kept as non-core. Supporting Evidence: PMID:18235971 The enzyme's transaminating activity with L-alanine and glyoxylate as substrates was approximately 55% of that observed with L-serine and glyoxylate. PMID:31681359 In comparison, AGT1 from humans and AGT1 from Anabaena lack these two residues and instead position a tyrosine ring into the binding site, which accounts for their preference for l-alanine instead of l-serine. |
| GO:0008453 L-alanine:glyoxylate transaminase activity | EXP PMID:23098902 Characterization of Arabidopsis serine:glyoxylate aminotrans... | KEEP AS NON CORE | Summary: Recombinant AGT1 characterized with Ala as donor and glyoxylate as acceptor. Reason: Demonstrated secondary activity; see the other rows for this term. Supporting Evidence: PMID:23098902 Recombinant N-terminal His-tagged AGT1 purified from Escherichia coli was characterized with Ser, alanine (Ala) and Asn as amino acid donors and glyoxylate, pyruvate and hydroxypyruvate as organic acid acceptors. |
| GO:0008453 L-alanine:glyoxylate transaminase activity | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: PAINT inference of alanine:glyoxylate transaminase, the ancestral activity of the AGT1/AGXT clade (human AGXT, yeast AGX1 and others), including AGT1's own experimental data. Reason: The inference is sound (AGT1 has demonstrable AGT activity), but in Arabidopsis AGT1 the activity has become secondary to serine:glyoxylate transamination. Supporting Evidence: PMID:11309139 AGT1 had the highest specific activity with the serine : glyoxylate transamination, and apparent Km measurements indicate that this is the preferred in vivo reaction. |
| GO:0008453 L-alanine:glyoxylate transaminase activity | IDA PMID:11309139 Peroxisomal alanine : glyoxylate aminotransferase (AGT1) is ... | KEEP AS NON CORE | Summary: Purified recombinant AGT1 catalyses the alanine:glyoxylate transamination. Reason: Directly demonstrated (and abolished by the sat P251L mutation), but serine:glyoxylate is the preferred reaction. Supporting Evidence: PMID:11309139 Purified, recombinant Arabidopsis AGT1 expressed in Escherichia coli catalyzed three transamination reactions using the following amino donor : acceptor combinations: alanine : glyoxylate, serine : glyoxylate, and serine : pyruvate. |
| 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 experimental AGT activity; non-core as for the other rows of this term. Supporting Evidence: PMID:18235971 The enzyme's transaminating activity with L-alanine and glyoxylate as substrates was approximately 55% of that observed with L-serine and glyoxylate. |
| GO:0009436 glyoxylate catabolic process | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: PAINT inference of glyoxylate catabolism, seeded by animal AGT1/AGXT orthologs that detoxify glyoxylate. Reason: AGT1 does consume glyoxylate (converting it to glycine), so the inference is not wrong. In plants, however, this transamination is a step of the photorespiratory C2 cycle rather than a glyoxylate disposal pathway, so the term is kept but is not the core process. Supporting Evidence: PMID:22303256 The amino group is used for the concomitant amination of glyoxylate to glycine. |
| 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: AGT1 has no chloroplast transit peptide and is a peroxisomal enzyme. Peroxisomes adhere to and co-purify with chloroplasts, and the study discusses contaminants in its preparations. 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:11309139 In vitro import experiments and subcellular fractionations localized AGT1 to peroxisomes. |
| GO:0009570 chloroplast stroma | HDA PMID:18633119 Quantitative proteomics of a chloroplast SRP54 sorting mutan... | MARK AS OVER ANNOTATED | Summary: Detection in quantitative chloroplast proteomics of a cpSRP54 mutant. Reason: The same study reports up-regulation of the photorespiratory pathway in peroxisomes and mitochondria, i.e. it recognizes these as non-plastid enzymes. Detection of AGT1 in chloroplast preparations is contamination by co-purifying peroxisomes. 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:18633119 with an up-regulation of the Calvin cycle and the photorespiratory pathway in peroxisomes and mitochondria in young leaves but not in old leaves PMID:11309139 In vitro import experiments and subcellular fractionations localized AGT1 to peroxisomes. |
| GO:0009570 chloroplast stroma | HDA PMID:20061580 AT_CHLORO, a comprehensive chloroplast proteome database wit... | MARK AS OVER ANNOTATED | Summary: Listed in the AT_CHLORO chloroplast proteome database with a stromal assignment. Reason: Peroxisomal matrix enzymes are frequent contaminants of chloroplast stroma fractions; AGT1 is experimentally peroxisomal, and no plastid targeting has been shown. 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:20061580 present in highly purified chloroplasts and their subfractions prepared from Arabidopsis thaliana leaves PMID:11309139 In vitro import experiments and subcellular fractionations localized AGT1 to peroxisomes. |
| GO:0009853 photorespiration | NAS PMID:11309139 Peroxisomal alanine : glyoxylate aminotransferase (AGT1) is ... | ACCEPT | Summary: AGT1 is the photorespiratory serine:glyoxylate aminotransferase; the sat photorespiratory mutants carry a point mutation in AGT1 that abolishes activity. Reason: Core process, supported by biochemistry, peroxisomal localization and genetics. sat mutants lacking SGAT activity are inviable in air but normal at high CO2 and accumulate serine and glycine; the sat lesion maps to AGT1. Supporting Evidence: PMID:11309139 We conclude that Arabidopsis AGT1 is a peroxisomal photorespiratory enzyme that catalyzes transamination reactions with multiple substrates. PMID:11309139 Sequence analysis of the photorespiratory sat mutants revealed a single nucleotide substitution in the AGT1 gene from these plants. PMID:16592821 These mutants were viable and exhibited normal photosynthesis under conditions that suppressed photorespiration, but they were inviable and photosynthesized at greatly reduced rates under conditions that promoted photorespiration. |
| GO:0047297 L-asparagine:oxo-acid transaminase activity | IEA GO_REF:0000116 | KEEP AS NON CORE | Summary: Rhea mapping from the asparagine:glyoxylate and asparagine:hydroxypyruvate reactions that UniProt records for AGT1. Reason: Recombinant AGT1 transaminates asparagine with high Vmax, and AGT1 transcripts are induced by Asn in roots, so a role in asparagine catabolism is plausible. This is outside the core photorespiratory function. Supporting Evidence: PMID:23098902 The V(max) of AGT1 with Asn was higher than with Ser or Ala by ca. 5- to 20-fold. PMID:23098902 These results suggest that AGT1 is involved in Asn metabolism in Arabidopsis. |
| GO:0047635 L-alanine:oxo-acid transaminase activity | IEA GO_REF:0000116 | TOO GENERAL | Summary: Rhea mapping (asparagine + pyruvate = 2-oxosuccinamate + L-alanine) to the generic alanine:oxo-acid transaminase term. Reason: True by reversibility, but uninformative: the more specific child GO:0008453 L-alanine:glyoxylate transaminase activity is retained (non-core) elsewhere in this review. Supporting Evidence: PMID:23098902 Recombinant N-terminal His-tagged AGT1 purified from Escherichia coli was characterized with Ser, alanine (Ala) and Asn as amino acid donors and glyoxylate, pyruvate and hydroxypyruvate as organic acid acceptors. |
| 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: AGT1 has no secretory signal peptide and is a peroxisomal enzyme; detection in apoplastic extracts reflects leakage of abundant intracellular proteins. 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:11309139 In vitro import experiments and subcellular fractionations localized AGT1 to peroxisomes. |
| GO:0050281 L-serine:glyoxylate transaminase activity | EXP PMID:18235971 Properties of serine:glyoxylate aminotransferase purified fr... | ACCEPT | Summary: Native SGAT purified from Arabidopsis leaves (identity confirmed by LC-MS/MS) was characterized kinetically. Reason: Core molecular function, measured with the native leaf enzyme. Supporting Evidence: PMID:18235971 The photorespiratory enzyme L-serine:glyoxylate aminotransferase (SGAT; EC 2.6.1.45) was purified from Arabidopsis thaliana leaves. |
| GO:0050281 L-serine:glyoxylate transaminase activity | EXP PMID:23098902 Characterization of Arabidopsis serine:glyoxylate aminotrans... | ACCEPT | Summary: Recombinant AGT1 characterized with serine as donor and glyoxylate as acceptor. Reason: Core molecular function; consistent with the other experimental rows. Supporting Evidence: PMID:23098902 Recombinant N-terminal His-tagged AGT1 purified from Escherichia coli was characterized with Ser, alanine (Ala) and Asn as amino acid donors and glyoxylate, pyruvate and hydroxypyruvate as organic acid acceptors. |
| GO:0050281 L-serine:glyoxylate transaminase activity | IDA PMID:11309139 Peroxisomal alanine : glyoxylate aminotransferase (AGT1) is ... | ACCEPT | Summary: Purified recombinant AGT1 has its highest specific activity and most favourable Km with serine:glyoxylate; the sat P251L mutation abolishes activity. Reason: Core molecular function. Supporting Evidence: PMID:11309139 AGT1 had the highest specific activity with the serine : glyoxylate transamination, and apparent Km measurements indicate that this is the preferred in vivo reaction. |
| GO:0050281 L-serine:glyoxylate transaminase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic mapping from EC 2.6.1.45 / RHEA:19125. Reason: Consistent with the experimentally demonstrated core activity. Supporting Evidence: PMID:11309139 AGT1 had the highest specific activity with the serine : glyoxylate transamination, and apparent Km measurements indicate that this is the preferred in vivo reaction. |
| GO:0140385 amino acid transaminase activity | IEA GO_REF:0000117 | MODIFY | Summary: ARBA prediction of generic amino acid transaminase activity. Reason: Correct but general; the specific, experimentally established core activity is L-serine:glyoxylate transaminase (GO:0050281), which the gene already carries. Proposed replacements: L-serine:glyoxylate transaminase activity Supporting Evidence: PMID:11309139 AGT1 had the highest specific activity with the serine : glyoxylate transamination, and apparent Km measurements indicate that this is the preferred in vivo reaction. |
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Download this section (compressed HTML)Q: How much of the in vivo SGAT flux uses alanine rather than serine as amino donor, given the divergent recombinant and native-enzyme kinetics?
Q: Does AGT1-mediated asparagine transamination in roots contribute measurably to nitrogen remobilization, independently of photorespiration?
Experiment: Feed 15N-asparagine and 15N-alanine to wild type and sat/agt1 knockout roots and leaves at ambient and high CO2 to quantify AGT1-dependent asparagine and alanine transamination in vivo.
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