MGAT1 is alpha-1,3-mannosyl-glycoprotein 2-beta-N-acetylglucosaminyltransferase (GlcNAc-transferase I, GnT-I; EC 2.4.1.101), a Mn2+-dependent, UDP-GlcNAc-using glycosyltransferase of CAZy family GT13. It is a type II single-pass membrane protein of the medial Golgi apparatus, with a short N-terminal cytoplasmic tail, a single transmembrane signal-anchor, and a large lumenal catalytic domain. MGAT1 catalyzes the first committed (gatekeeper) step of complex and hybrid N-glycan biosynthesis: it transfers a single N-acetylglucosamine in beta-1,2 linkage onto the alpha-1,3-linked mannose arm of the Man5GlcNAc2 N-glycan that is attached to asparagine residues of glycoproteins traversing the secretory pathway. The product (GlcNAc-beta1,2-Man-alpha1,3-arm) is the obligate substrate for subsequent trimming by Golgi alpha-mannosidase II (MAN2A1/MAN2A2) and for further branching by MGAT2, MGAT4 and MGAT5, so loss of MGAT1 activity blocks all hybrid and complex N-glycan synthesis and leaves only oligomannose structures (as in the Chinese hamster ovary Lec1 mutant from which the gene was cloned by complementation). Because complex N-glycans decorate hundreds of secreted and cell-surface glycoproteins, MGAT1 is essential for normal mammalian development and broadly influences cell-surface receptor biology, but its own molecular activity is a single, highly conserved glycosyltransfer reaction carried out in the Golgi.
Definition: The committed step of N-glycan maturation in which a single N-acetylglucosamine is added in beta-1,2 linkage to the alpha-1,3-mannose arm of an oligomannose (Man5GlcNAc2) protein N-glycan, producing the GlcNAcMan5GlcNAc2 intermediate that is the obligate substrate for alpha-mannosidase II trimming and subsequent branching, thereby initiating hybrid and complex N-glycan biosynthesis.
Justification: MGAT1 occupies a uniquely defined branch point in N-glycan processing (GO:0006491), and existing process terms are either too general (protein N-linked glycosylation, N-glycan processing) or describe the whole complex-N-glycan biosynthetic pathway rather than the specific committed initiating step. A child of GO:0006491 (N-glycan processing) grounded on the RHEA:11456 reaction would let MGAT1 and its orthologs be annotated to the precise gatekeeper step. This is a candidate term; the existing GO:0006487 annotation is retained as the best current process term.
Parent term: N-glycan processing
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003827 alpha-1,3-mannosylglycoprotein 2-beta-N-acetylglucosaminyltransferase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetically inferred core molecular function and exactly the experimentally established activity of MGAT1/GnT-I (EC 2.4.1.101). This is the specific, informative GlcNAc-TI term and is the core function of the gene. |
| GO:0006487 protein N-linked glycosylation | IBA GO_REF:0000033 | ACCEPT | Summary: Correct core biological process. MGAT1 performs the committed step that converts oligomannose to hybrid/complex N-glycans during protein N-linked glycosylation. Concordant with the IDA annotation to the same term. Accept. |
| GO:0005794 Golgi apparatus | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic inference of Golgi localization, consistent with the experimental IDA Golgi membrane annotation and with MGAT1 being a medial-Golgi glycosyltransferase. The more specific Golgi membrane term is preferable, but this organelle-level call is correct. Accept. |
| GO:0000139 Golgi membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic mapping from the UniProt subcellular-location keyword (Golgi apparatus membrane). Same content as the experimental IDA Golgi membrane annotation; this is the core, correct subcellular location. Accept. |
| GO:0003827 alpha-1,3-mannosylglycoprotein 2-beta-N-acetylglucosaminyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic prediction of the specific GlcNAc-TI activity. Identical in content to the IBA/IDA/EXP/TAS annotations to the same term and is the core function. Accept. |
| GO:0008375 acetylglucosaminyltransferase activity | IEA GO_REF:0000120 | MODIFY | Summary: Correct but over-general parent of the specific GlcNAc-TI activity. The precise child term GO:0003827 (alpha-1,3-mannosylglycoprotein 2-beta-N-acetylglucosaminyltransferase activity) is experimentally supported and should be used instead. Proposed replacements: alpha-1,3-mannosylglycoprotein 2-beta-N-acetylglucosaminyltransferase activity |
| GO:0009101 glycoprotein biosynthetic process | IEA GO_REF:0000002 | MODIFY | Summary: Over-general InterPro-derived process term. MGAT1 acts specifically in protein N-linked glycosylation (the oligomannose-to-complex committed step); the specific N-linked glycosylation term is more informative. Proposed replacements: protein N-linked glycosylation |
| GO:0048471 perinuclear region of cytoplasm | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Electronic mapping from the UniProt "Cytoplasm, perinuclear region" location, which derives from PMID:30983867 where MGAT1 co-localizes with BRI3 isoform 1 in the perinuclear area (consistent with the peri-Golgi position of a Golgi enzyme). This is a juxta-Golgi appearance rather than a distinct cytoplasmic compartment of MGAT1 function; keep as a plausible secondary location but not core. Supporting Evidence: PMID:30983867 intense colocalization of BRI3 with MGAT1, especially in the perinuclear area of Huh7 cells |
| GO:0005515 protein binding | IPI PMID:30983867 Identification of IFITM3 and MGAT1 as novel interaction part... | MARK AS OVER ANNOTATED | Summary: Generic "protein binding" from a yeast two-hybrid plus co-IP/colocalization interaction with BRI3 (ITM2C). Per curation guidelines this term is uninformative about molecular function, and the BRI3 interaction has no established link to MGAT1 catalytic activity or the N-glycosylation pathway (BRI3 is a poorly characterized lysosomal/TNF-pathway protein). Over-annotation; not a core function. Supporting Evidence: PMID:30983867 IFITM3 and MGAT1 proteins were confirmed as interaction partners by using cotransformation in yeast cells and coimmunoprecipitation from mammalian cell lines |
| GO:0003827 alpha-1,3-mannosylglycoprotein 2-beta-N-acetylglucosaminyltransferase activity | TAS Reactome:R-HSA-964768 | ACCEPT | Summary: Reactome traceable annotation for the reaction "Addition of GlcNAc to the glycan on the A arm", described as the first committed step in complex/hybrid N-glycan synthesis. Exactly the core GlcNAc-TI activity. Accept. Supporting Evidence: Reactome:R-HSA-964768 This is the first committed step in the synthesis of complex and hybrid N-glycans |
| GO:0003827 alpha-1,3-mannosylglycoprotein 2-beta-N-acetylglucosaminyltransferase activity | EXP PMID:36280670 A universal glycoenzyme biosynthesis pipeline that enables e... | ACCEPT | Summary: Experimental support for the GlcNAc-TI activity. PMID:36280670 (Jaroentomeechai et al.) is a cell-free glycoenzyme pipeline in which MGAT1/GnT-I was produced as a water-soluble enzyme and used in N-glycan remodeling; UniProt cites it as ECO:0000269 evidence for MGAT1 catalytic activity. The abstract describes the general 98-glycosyltransferase platform rather than MGAT1 by name, but the curated experimental evidence supports the specific activity, which is in any case the well-established core function. Accept. Supporting Evidence: PMID:36280670 facile production of 98 difficult-to-express GTs, predominantly of human origin |
| GO:0019082 viral protein processing | TAS Reactome:R-HSA-9683686 | KEEP AS NON CORE | Summary: Reactome annotation placing MGAT1 in SARS-CoV spike-protein maturation, where it performs its normal N-glycan branching on a viral glycoprotein substrate. This is the housekeeping GlcNAc-TI activity applied to a viral substrate rather than a dedicated antiviral/viral-processing function of MGAT1; keep but not core. |
| GO:0019082 viral protein processing | TAS Reactome:R-HSA-9694548 | KEEP AS NON CORE | Summary: Duplicate Reactome "Maturation of spike protein" annotation (different stable ID). Same interpretation as the other viral protein processing annotation: substrate application of the normal Golgi N-glycan branching activity, not a core viral function. |
| GO:0008375 acetylglucosaminyltransferase activity | TAS Reactome:R-HSA-9683648 | MODIFY | Summary: Reactome MF annotation from "Spike trimer glycoside chains are extended", which states GlcNAc-TI (MGAT1) adds a GlcNAc residue to high-mannose chains. Correct direction but over-general; the specific GlcNAc-TI term GO:0003827 captures the activity. Proposed replacements: alpha-1,3-mannosylglycoprotein 2-beta-N-acetylglucosaminyltransferase activity Supporting Evidence: Reactome:R-HSA-9683648 The N-acetylglucosaminyltransferase called GlcNAc-TI (MGAT1) adds a GlcNAc residue in the core of some high-mannose chains |
| GO:0008375 acetylglucosaminyltransferase activity | TAS Reactome:R-HSA-9694656 | MODIFY | Summary: Duplicate Reactome "Spike trimer glycoside chains are extended" MF annotation. Same over-general issue; replace with the specific GlcNAc-TI activity term. Proposed replacements: alpha-1,3-mannosylglycoprotein 2-beta-N-acetylglucosaminyltransferase activity |
| GO:0033116 endoplasmic reticulum-Golgi intermediate compartment membrane | TAS Reactome:R-HSA-964768 | KEEP AS NON CORE | Summary: Reactome places the GlcNAc-TI reaction at the ERGIC membrane. The best-supported experimental location is the medial-Golgi membrane (IDA, PMID:20378551); an ERGIC pool is biologically plausible for an early secretory-pathway glycosyltransferase but is secondary to the Golgi location. Keep as non-core. |
| GO:0033116 endoplasmic reticulum-Golgi intermediate compartment membrane | TAS Reactome:R-HSA-9683648 | KEEP AS NON CORE | Summary: Duplicate ERGIC-membrane location from the viral spike Reactome reaction. Same interpretation: plausible secondary secretory-pathway location, secondary to the experimentally supported Golgi membrane. Keep as non-core. |
| GO:0033116 endoplasmic reticulum-Golgi intermediate compartment membrane | TAS Reactome:R-HSA-9694656 | KEEP AS NON CORE | Summary: Duplicate ERGIC-membrane location (third Reactome stable ID). Keep as a plausible secondary location; the core compartment is the Golgi membrane. |
| GO:0000139 Golgi membrane | IDA PMID:20378551 Golgi N-glycosyltransferases form both homo- and heterodimer... | ACCEPT | Summary: Direct experimental localization: live-cell bimolecular fluorescence complementation showed MGAT1 (GnTI) forms Golgi-localized homodimers and a medial-Golgi heterodimer with MGAT2 (GnTII), with signal detected only in the Golgi membranes. This is the core, experimentally supported subcellular location. Accept. Supporting Evidence: PMID:20378551 the BiFC signal with GnTI was detected only in the Golgi membranes of live cells |
| GO:0003827 alpha-1,3-mannosylglycoprotein 2-beta-N-acetylglucosaminyltransferase activity | IDA PMID:1702225 Cloning and expression of N-acetylglucosaminyltransferase I,... | ACCEPT | Summary: Foundational direct experimental support: the human gene was identified by complementation of the N-glycosylation defect of the Lec1 CHO mutant, and the cloned cDNA confers GlcNAc-TI (EC 2.4.1.101) activity. This is the defining core molecular function of MGAT1. Accept. Supporting Evidence: PMID:1702225 full-length cDNA encoding human GlcNAc-TI activity |
| GO:0006487 protein N-linked glycosylation | IDA PMID:1702225 Cloning and expression of N-acetylglucosaminyltransferase I,... | ACCEPT | Summary: Direct experimental support for involvement in protein N-linked glycosylation: MGAT1 is "the medial Golgi transferase that initiates complex N-linked carbohydrate formation". Core biological process. Accept. Supporting Evidence: PMID:1702225 the medial Golgi transferase that initiates complex N-linked carbohydrate formation |
| GO:0030145 manganese ion binding | IDA PMID:1702225 Cloning and expression of N-acetylglucosaminyltransferase I,... | ACCEPT | Summary: MGAT1/GnT-I is a Mn2+-dependent glycosyltransferase (UniProt COFACTOR: Mn2+; GT-A-type metal coordination of the UDP-GlcNAc donor). Manganese ion binding is a genuine, experimentally supported cofactor-binding molecular function and a core enzymatic property. Accept. |
| GO:1903561 extracellular vesicle | HDA PMID:24769233 Proteomic analysis of cerebrospinal fluid extracellular vesi... | MARK AS OVER ANNOTATED | Summary: High-throughput proteomic detection in cerebrospinal-fluid extracellular vesicles. MGAT1 is a Golgi-resident type II membrane enzyme; its presence in a vesicle proteome reflects secretory-pathway/membrane carryover rather than a functional extracellular-vesicle localization. Over-annotation. |
| GO:0070062 extracellular exosome | HDA PMID:23533145 In-depth proteomic analyses of exosomes isolated from expres... | MARK AS OVER ANNOTATED | Summary: High-throughput exosome proteomics (expressed prostatic secretions). As with the other vesicle proteomics hits, this is bulk-MS detection in an exosome preparation, not evidence that MGAT1 functions in exosomes; it contradicts the well-established Golgi membrane location. Over-annotation. |
| GO:0016020 membrane | HDA PMID:19946888 Defining the membrane proteome of NK cells. | MARK AS OVER ANNOTATED | Summary: Trivial location from an NK-cell membrane-proteome MS dataset. MGAT1 is a single-pass membrane protein, so "membrane" is correct but uninformative; the specific Golgi membrane term supersedes it. |
| GO:0070062 extracellular exosome | HDA PMID:19199708 Proteomic analysis of human parotid gland exosomes by multid... | MARK AS OVER ANNOTATED | Summary: High-throughput parotid-gland exosome proteomics. Same interpretation as the other exosome HDA annotation: secretory-pathway carryover detected by bulk MS, not a functional exosomal location. Over-annotation. |
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Download this section (compressed HTML)Q: Beyond its medial-Golgi pool, does MGAT1 have a functional role at the ERGIC membrane (as modeled in Reactome), and does compartmentalization within the secretory pathway regulate the oligomannose-to-complex transition?
Q: Is the BRI3 (ITM2C) interaction functionally relevant to MGAT1 activity, Golgi retention, or turnover, or is it an incidental Y2H/co-IP association?
Q: The reported lactate-induced "mitochondrial MGAT1" role in regulatory T cells diverges from the canonical Golgi model; is there independent evidence for a non-Golgi, non-canonical localization or function of MGAT1?
Experiment: Quantitative glycomic/glycoproteomic comparison of MGAT1-knockout versus wild-type human cells (mass spectrometry of released N-glycans) to confirm the expected complete loss of hybrid/complex N-glycans and accumulation of Man5GlcNAc2, recapitulating the Lec1 phenotype in a human background.
Experiment: In vitro reconstitution of purified human MGAT1 with defined Man5GlcNAc2 glycopeptide acceptors and UDP-GlcNAc to measure kinetics, strict Mn2+ dependence (EDTA sensitivity), and arm specificity (alpha-1,3 vs alpha-1,6), establishing direct human enzymology rather than relying on ortholog data.
Experiment: Tagged-MGAT1 co-localization and proximity-labeling (e.g. APEX/BioID) in live cells to map its medial-Golgi multi-enzyme assemblies (with MGAT2, MAN2A2, and UDP-GlcNAc transporters) and to test whether the perinuclear/ERGIC pools are catalytically engaged.
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