GCNT1 (core 2 beta-1,6-N-acetylglucosaminyltransferase 1, C2GnT1; EC 2.4.1.102) is a Golgi-resident type II membrane glycosyltransferase that creates the branch point of mucin-type O-glycans. It transfers N-acetylglucosamine from UDP-GlcNAc in a beta-1,6 linkage onto the GalNAc of the core 1 disaccharide Gal-beta1-3-GalNAc-alpha1-O-Ser/Thr, producing the branched core 2 structure Gal-beta1-3(GlcNAc-beta1-6)GalNAc. The enzyme is specific for this acceptor and shows no activity toward the acceptors used by other beta-1,6-GlcNAc transferases. Branching to core 2 is what allows an O-glycan to be extended into a poly-N-acetyllactosamine chain and capped with sialyl Lewis x, so GCNT1 determines whether mucin-type O-glycans can become functional selectin ligands. Its best-characterised product in this regard is the core 2-associated sialyl Lewis x carried by P-selectin glycoprotein ligand-1 (PSGL-1) on leukocytes. Correct Golgi retention of the enzyme, which depends on an LLRRR motif in its cytoplasmic tail that binds GOLPH3, is required for this biosynthetic output. Core 2 branching activity rises during T-cell activation and is altered in immunodeficiency and in cancer metastasis.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000139 Golgi membrane | EXP PMID:23027862 Golgi phosphoprotein 3 determines cell binding properties un... | ACCEPT | Summary: Confocal microscopy in the cited study shows substantial Golgi localisation of C2GnT1, assessed against cis-, medial- and trans-Golgi markers. This is the compartment in which the enzyme must reside to make its product. Reason: Directly observed, at the right level of specificity for a type II Golgi membrane glycosyltransferase. The same study demonstrates that this localisation is functionally required, since disrupting it abolishes product synthesis. Supporting Evidence: PMID:23027862 Confocal fluorescence microscopic analysis shows substantial Golgi co-localization of C2GnT1 and GOLPH3 PMID:23027862 The results clearly show the requirement of Golgi localization of C2GnT1 for the synthesis of C2-O-sLe x |
| GO:0000139 Golgi membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Automated mapping from the UniProt Golgi membrane subcellular-location keyword, agreeing with the experimental localisation. Reason: The electronic assignment converges on the same compartment as the direct microscopy, at the same specificity. No correction needed. Supporting Evidence: PMID:23027862 Confocal fluorescence microscopic analysis shows substantial Golgi co-localization of C2GnT1 and GOLPH3 |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-914012 | ACCEPT | Summary: Reactome places the core 2 branching reaction in the Golgi membrane, consistent with the experimental evidence. Reason: Correct compartment; duplication across evidence types and sources is expected and unproblematic. Supporting Evidence: PMID:23027862 Confocal fluorescence microscopic analysis shows substantial Golgi co-localization of C2GnT1 and GOLPH3 |
| GO:0003829 beta-1,3-galactosyl-O-glycosyl-glycoprotein beta-1,6-N-acetylglucosaminyltransferase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic propagation from an ancestral node supported by the mouse and rat orthologues alongside the human gene. The core 2 branching activity is a conserved vertebrate function and the node placement is consistent with the experimental annotations on all three members. Reason: The term is at exactly the right specificity and the node placement is unproblematic. The target's own accession (Q02742) in the WITH/FROM field is expected: this gene's direct expression-cloning evidence is one of the descendant experimental annotations the PAINT curator used to place the node, so its reappearance marks experimental grounding on the target rather than circular support. Supporting Evidence: PMID:1329093 enabled us to demonstrate unequivocally that the cDNA encodes the core 2 beta-1,6-N-acetylglucosaminyltransferase, the enzyme responsible for the formation of Gal beta 1-3(GlcNAc beta 1-6)GalNAc structures |
| GO:0003829 beta-1,3-galactosyl-O-glycosyl-glycoprotein beta-1,6-N-acetylglucosaminyltransferase activity | IDA PMID:1329093 Expression cloning of a cDNA encoding UDP-GlcNAc:Gal beta 1-... | ACCEPT | Summary: The defining experiment. Expression cloning in CHO cells, followed by expression of the catalytic domain as a protein A fusion, demonstrated unequivocally that the cDNA encodes the core 2 beta-1,6-N-acetylglucosaminyltransferase responsible for forming Gal-beta1-3(GlcNAc-beta1-6)GalNAc, with no activity toward the acceptors of other beta-1,6-GlcNAc transferases. Reason: Direct assay of the recombinant catalytic domain with the product structure identified and acceptor specificity explicitly tested against related enzymes. This is the gene's core molecular function, and the negative result against other acceptors rules out the paralog-specificity error that afflicts large glycosyltransferase families. Supporting Evidence: PMID:1329093 enabled us to demonstrate unequivocally that the cDNA encodes the core 2 beta-1,6-N-acetylglucosaminyltransferase, the enzyme responsible for the formation of Gal beta 1-3(GlcNAc beta 1-6)GalNAc structures PMID:1329093 No activity with this enzyme was detected toward the acceptors for other beta 1-6GlcNAc transferases |
| GO:0003829 beta-1,3-galactosyl-O-glycosyl-glycoprotein beta-1,6-N-acetylglucosaminyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Automated assignment combining orthology to mouse Gcnt1 (Q09324) with RHEA reaction mappings, converging on the same specific activity as the direct assay. Reason: Reaction-level mapping in agreement with the experimental evidence and at the correct specificity. Supporting Evidence: PMID:1329093 enabled us to demonstrate unequivocally that the cDNA encodes the core 2 beta-1,6-N-acetylglucosaminyltransferase, the enzyme responsible for the formation of Gal beta 1-3(GlcNAc beta 1-6)GalNAc structures |
| GO:0003829 beta-1,3-galactosyl-O-glycosyl-glycoprotein beta-1,6-N-acetylglucosaminyltransferase activity | TAS Reactome:R-HSA-914012 | ACCEPT | Summary: Reactome's statement of the core 2 branching reaction, matching the assayed activity. Reason: Correct and specific; consistent with the direct experimental evidence. Supporting Evidence: PMID:1329093 enabled us to demonstrate unequivocally that the cDNA encodes the core 2 beta-1,6-N-acetylglucosaminyltransferase, the enzyme responsible for the formation of Gal beta 1-3(GlcNAc beta 1-6)GalNAc structures |
| GO:0005515 protein binding | IPI PMID:23027862 Golgi phosphoprotein 3 determines cell binding properties un... | REMOVE | Summary: The partner is GOLPH3 (Q9H4A6), identified in a yeast two-hybrid screen using the C2GnT1 cytoplasmic tail as bait, with binding mapped to the tail's LLRRR motif. The interaction is real, well mapped and functionally consequential: it is what retains the enzyme in the Golgi. Reason: Generic protein binding carries no functional information, and unlike the chaperone-client cases elsewhere in this pathway there is no more informative molecular function term to move to. What the interaction accomplishes is Golgi retention, which is a localisation mechanism rather than a molecular function of GCNT1, and it is already captured by the gene's Golgi cellular-component annotations. Removing the uninformative term does not dispute the interaction, which is well documented in the cited work. Supporting Evidence: PMID:23027862 demonstrated that C2GnT1 binds to GOLPH3 via the LLRRR(9) sequence in the CT |
| GO:0005515 protein binding | IPI PMID:23027862 Golgi phosphoprotein 3 determines cell binding properties un... | REMOVE | Summary: Second GOA row for the same GOLPH3 interaction, assigned by UniProt where the other is assigned by IntAct. Exact-duplicate rows from different assigning sources are normal in GOA and each needs its own entry. Reason: Generic protein binding carries no functional information, and unlike the chaperone-client cases elsewhere in this pathway there is no more informative molecular function term to move to. What the interaction accomplishes is Golgi retention, which is a localisation mechanism rather than a molecular function of GCNT1, and it is already captured by the gene's Golgi cellular-component annotations. Removing the uninformative term does not dispute the interaction, which is well documented in the cited work. This row is the UniProt-assigned duplicate of the IntAct row above and receives the same verdict for the same reasons. Supporting Evidence: PMID:23027862 demonstrated that C2GnT1 binds to GOLPH3 via the LLRRR(9) sequence in the CT |
| GO:0005802 trans-Golgi network | IDA PMID:23027862 Golgi phosphoprotein 3 determines cell binding properties un... | MARK AS OVER ANNOTATED | Summary: The cited study compared C2GnT1 localisation against cis-, medial- and trans-Golgi markers. It reports only partial co-localisation with the trans-Golgi marker TGN46, and explicitly notes a higher degree of co-localisation of C2GnT1 with the cis-Golgi marker GM130 than GOLPH3 shows. The enzyme is distributed across the stack rather than concentrated in the trans-Golgi network. Reason: The paper's own comparison places C2GnT1 across the Golgi stack with a cis-leaning distribution relative to GOLPH3, so singling out the trans-Golgi network overstates what was observed. This is not a wrong compartment, and the partial TGN46 overlap is real, but as a located_in claim it is more specific than the data support. The general Golgi membrane annotations already carry the well-supported localisation. Supporting Evidence: PMID:23027862 Our data also indicate partial co-localization of C2GnT1 with a trans- Golgi marker TGN46 PMID:23027862 We found a higher degree of colocalization of a cis- Golgi protein GM130 |
| GO:0009101 glycoprotein biosynthetic process | IMP PMID:23027862 Golgi phosphoprotein 3 determines cell binding properties un... | MODIFY | Summary: Supported by direct knockdown of the gene itself: C2GnT1 siRNA produced a 74.6% decrease in PSGL-1-associated core 2 sialyl Lewis x, without reducing PSGL-1 protein levels. The evidence is solid, but the term names the broadest possible class of glycoprotein biosynthesis. Reason: The experiment measures loss of a specific mucin-type O-glycan structure on a specific carrier protein, so the process is known precisely. GO:0016266 protein O-linked glycosylation via N-acetylgalactosamine is the term that matches what was assayed, and it is already annotated to this gene from other sources. Leaving the generic parent in place adds nothing and would let the gene surface in queries about N-glycan and other glycoprotein biosynthesis that it has nothing to do with. Proposed replacements: protein O-linked glycosylation via N-acetylgalactosamine Supporting Evidence: PMID:23027862 following the treatment with C2GnT1 siRNA, there was a 74.6% decrease in PSGL-1-associated C2-O-sLe x |
| GO:0016020 membrane | IEA GO_REF:0000002 | MODIFY | Summary: InterPro2GO mapping from the glycosyltransferase family 14 signature to the bare membrane term. Consistent with the type II topology but names no organelle. Reason: The Golgi membrane is established here by direct microscopy, by UniProt keyword and by Reactome. A domain signature mapping to the generic membrane parent is defensible only when the compartment is unknown, which is not the case for this gene. Proposed replacements: Golgi membrane Supporting Evidence: PMID:23027862 Confocal fluorescence microscopic analysis shows substantial Golgi co-localization of C2GnT1 and GOLPH3 |
| GO:0016020 membrane | TAS PMID:1329093 Expression cloning of a cDNA encoding UDP-GlcNAc:Gal beta 1-... | MODIFY | Summary: From the expression-cloning paper's statement that the cDNA sequence predicts a protein with type II membrane topology. The topology is correct; the term is uninformative about compartment. Reason: Same collapse to the specific compartment as the InterPro-derived membrane row. In 1992 the Golgi residence had not yet been shown, so the general term was appropriate then; it is not now. Proposed replacements: Golgi membrane Supporting Evidence: PMID:1329093 The cDNA sequence predicts a protein with type II membrane topology PMID:23027862 Confocal fluorescence microscopic analysis shows substantial Golgi co-localization of C2GnT1 and GOLPH3 |
| GO:0016266 protein O-linked glycosylation via N-acetylgalactosamine | IDA PMID:1329093 Expression cloning of a cDNA encoding UDP-GlcNAc:Gal beta 1-... | ACCEPT | Summary: The core 2 branch is added to the GalNAc of the core 1 structure on Ser/Thr-linked mucin-type O-glycans, which places this enzyme squarely within the GalNAc-initiated O-glycosylation process. Reason: Correct process at the right specificity, and the gene passes the participation test directly: it catalyses a step of the pathway rather than merely being required for it. Supporting Evidence: PMID:1329093 enabled us to demonstrate unequivocally that the cDNA encodes the core 2 beta-1,6-N-acetylglucosaminyltransferase, the enzyme responsible for the formation of Gal beta 1-3(GlcNAc beta 1-6)GalNAc structures |
| GO:0016266 protein O-linked glycosylation via N-acetylgalactosamine | IEA GO_REF:0000107 | ACCEPT | Summary: Automatic orthology transfer from mouse Gcnt1, which carries the same process annotation experimentally. Mouse and human core 2 branching enzymes are functional orthologues. Reason: Sound orthology transfer of a term that is independently supported by direct evidence on the human gene. Supporting Evidence: PMID:1329093 enabled us to demonstrate unequivocally that the cDNA encodes the core 2 beta-1,6-N-acetylglucosaminyltransferase, the enzyme responsible for the formation of Gal beta 1-3(GlcNAc beta 1-6)GalNAc structures |
| GO:0016266 protein O-linked glycosylation via N-acetylgalactosamine | TAS Reactome:R-HSA-913709 | ACCEPT | Summary: Reactome's placement of core 2 branching within mucin-type O-glycan biosynthesis. Reason: Correct process assignment, consistent with the direct evidence. Supporting Evidence: PMID:1329093 enabled us to demonstrate unequivocally that the cDNA encodes the core 2 beta-1,6-N-acetylglucosaminyltransferase, the enzyme responsible for the formation of Gal beta 1-3(GlcNAc beta 1-6)GalNAc structures |
| GO:0016607 nuclear speck | IDA GO_REF:0000052 | REMOVE | Summary: An immunofluorescence-curation annotation placing this enzyme in nuclear speckles. Nuclear speckles are membraneless nuclear bodies involved in pre-mRNA splicing factor storage. GCNT1 is a single-pass type II membrane glycosyltransferase whose catalytic domain faces the Golgi lumen and which has been localised to the Golgi by confocal microscopy against three independent Golgi markers in the primary literature. Reason: A membrane-anchored, lumenal-facing enzyme cannot reside in a membraneless nuclear body, and no mechanism has been proposed by which it would: the catalytic domain occupies residues 33-428 on the lumenal face, leaving a nine-residue cytoplasmic tail. This is not an experimental annotation whose full text might contain supporting data the cache omits; it comes from an immunofluorescence curation pipeline, where a single antibody staining pattern is the whole of the evidence. A comparator check across the Human Protein Atlas records of Golgi type II glycosyltransferases makes the artefact reading concrete, and the decisive contrast is between replacing and supplementing the Golgi signal. GCNT4, ST6GALNAC1 and FUT8 each carry a nucleoplasm background call alongside a retained Golgi apparatus call, which is what genuine dual localisation or ordinary nuclear background looks like. GCNT1 instead has 'Nuclear speckles' as its sole location with no Golgi call at all, i.e. the antibody misses the compartment where this enzyme's biochemistry demonstrably occurs. An antibody that fails to detect the true target while producing a punctate nuclear pattern is the signature of off-target binding. Retaining the term would corrupt any compartment-based analysis of the glycosylation machinery. Supporting Evidence: PMID:23027862 Confocal fluorescence microscopic analysis shows substantial Golgi co-localization of C2GnT1 and GOLPH3 PMID:1329093 The cDNA sequence predicts a protein with type II membrane topology file:human/GCNT1/GCNT1-hypotheses/function-hypothesis-go-0016607/openscientist.md GCNT1's speckle call **replaces** rather than **supplements** the expected Golgi signal file:human/GCNT1/GCNT1-hypotheses/function-hypothesis-go-0016607/openscientist.md Punctate nuclear staining is a frequent HPA false-positive class |
| GO:0016757 glycosyltransferase activity | IEA GO_REF:0000120 | MODIFY | Summary: The broad glycosyltransferase parent, assigned from a family signature and an ARBA rule. The gene already carries its specific activity four times over, from direct assay, phylogeny, reaction mapping and Reactome. Reason: Collapsing the generic parent to the specific child is the dominant correction this project has found across glycosyltransferases, and the case here is unusually clear because the defining paper explicitly tested and excluded the activities of the related beta-1,6-GlcNAc transferases. Proposed replacements: beta-1,3-galactosyl-O-glycosyl-glycoprotein beta-1,6-N-acetylglucosaminyltransferase activity Supporting Evidence: PMID:1329093 No activity with this enzyme was detected toward the acceptors for other beta 1-6GlcNAc transferases |
| GO:0031985 Golgi cisterna | IDA PMID:23027862 Golgi phosphoprotein 3 determines cell binding properties un... | ACCEPT | Summary: Supported by the same marker-controlled confocal analysis, which localises C2GnT1 within the Golgi stack rather than to a single sub-compartment. Reason: This term fits the observation better than the trans-Golgi network row does: the study's finding is distribution across cisternae, with relatively more cis-Golgi overlap than its interaction partner shows. Supporting Evidence: PMID:23027862 We found a higher degree of colocalization of a cis- Golgi protein GM130 PMID:23027862 Confocal fluorescence microscopic analysis shows substantial Golgi co-localization of C2GnT1 and GOLPH3 |
| GO:0050901 leukocyte tethering or rolling | IMP PMID:23027862 Golgi phosphoprotein 3 determines cell binding properties un... | KEEP AS NON CORE | Summary: The tethering and rolling assays in this paper were performed on GOLPH3-depleted cells, in which C2GnT1 is mislocalised, rather than on cells depleted of C2GnT1 itself. The biology is nonetheless real: core 2 branching is required to build the sialyl Lewis x on PSGL-1 that selectins bind, and direct C2GnT1 knockdown does reduce that structure by 74.6%. Reason: Leukocyte tethering and rolling is a cell-level behaviour executed by the leukocyte through selectin-ligand engagement at the cell surface. GCNT1 is a Golgi enzyme that builds the ligand; it is upstream of the adhesion event and does no part of the work of tethering or rolling. This is the guilt-by-substrate pattern in which a biosynthetic enzyme inherits the physiology of the molecule it makes. Worth keeping, because loss of the enzyme genuinely compromises the process, but not as a core function. Supporting Evidence: PMID:23027862 GOLPH3 knockdown results in reduced synthesis of C2-O-sLe(x) associated with P-selectin glycoprotein ligand-1, reduced cell tethering to and rolling on immobilized P- or E-selectin PMID:23027862 Core 2 N-acetylglucosaminyltransferase 1 (C2GnT1) is a key enzyme participating in the synthesis of core 2-associated sialyl Lewis x (C2-O-sLe(x)), a ligand involved in selectin-mediated leukocyte trafficking and cancer metastasis |
| GO:0060352 cell adhesion molecule production | IMP PMID:23027862 Golgi phosphoprotein 3 determines cell binding properties un... | MARK AS OVER ANNOTATED | Summary: Derived from the same GOLPH3-knockdown experiments, reading the loss of functional PSGL-1 glycosylation as a defect in producing a cell adhesion molecule. PSGL-1 protein levels were explicitly shown to be unchanged; only its glycan was lost. Reason: The term denotes production of cell adhesion molecules, and the paper's own control shows that production of the adhesion molecule is unaffected: the decrease in core 2 sialyl Lewis x "was not due to reduced PSGL-1 protein expression". What changed is post-translational modification of an already-produced protein. Reading a production term out of a glycosylation defect stretches the term past its definition, and the underlying activity is captured precisely by the O-linked glycosylation process annotations. Supporting Evidence: PMID:23027862 following the treatment with C2GnT1 siRNA, there was a 74.6% decrease in PSGL-1-associated C2-O-sLe x |
| GO:1903238 positive regulation of leukocyte tethering or rolling | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Orthology transfer from mouse Gcnt1 of the regulatory form of the tethering and rolling term. The donor annotation rests on the same biology as the human IMP row: core 2 branching is needed for functional selectin ligands. Reason: The orthology transfer itself is sound, and the regulatory framing is arguably more defensible than the plain involved_in version, since a biosynthetic enzyme that determines ligand availability does modulate the downstream adhesion process rather than perform it. Still downstream physiology rather than a core function of a Golgi glycosyltransferase, so it is kept on the same footing as the human tethering annotation. Supporting Evidence: PMID:23027862 Core 2 N-acetylglucosaminyltransferase 1 (C2GnT1) is a key enzyme participating in the synthesis of core 2-associated sialyl Lewis x (C2-O-sLe(x)), a ligand involved in selectin-mediated leukocyte trafficking and cancer metastasis |
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Download this section (compressed HTML)Q: Where should the boundary sit between a glycosyltransferase and the physiology of the glycan it builds? GCNT1 carries leukocyte tethering or rolling, positive regulation of the same, and cell adhesion molecule production, all resting on the single fact that it makes the core 2 branch of a selectin ligand. Deciding this consistently matters well beyond this gene, since every glycosyltransferase is exposed to the same inheritance of its product's biology.
Q: Is the nuclear speck annotation reproducible with an independent antibody or a tagged construct? If not, it may be worth revisiting other immunofluorescence-curation localisations of Golgi glycosyltransferases as a class, rather than only this one.
Q: Is there an antibody-class artefact affecting immunofluorescence localisation of core-1 and core-2 O-glycan enzymes? Direct querying of the Human Protein Atlas shows GCNT1 called 'Nuclear speckles' and C1GALT1 called 'Nuclear bodies, Cytosol', both with no Golgi call, while GCNT2, GCNT3, GCNT4, ST6GALNAC1, B4GALT1 and FUT8 all recover the Golgi. Only GCNT1's nuclear call has been imported into GO. If this is a systematic pattern it would be worth auditing immunofluorescence-derived localisations across Golgi glycosyltransferases as a class rather than gene by gene.
Q: Is GCNT1 shed as a soluble ectodomain? PMID:35279766 reports unique peptides mapping to GCNT1 among Golgi glycosylation enzymes whose abundance tracks SPPL3 expression. This is recorded here as a lead only, not as an annotation: a single peptide observation in a study focused on SPPL3 is not sufficient to assert an extracellular location, and the same standard was applied in marking GALNT1's extracellular annotation over-annotated.
Experiment: Repeat the selectin tethering and rolling assays with direct GCNT1 knockdown rather than GOLPH3 knockdown, which would establish whether the cell-behaviour annotations rest on the enzyme itself rather than on a trafficking factor that also affects other cargo.
Experiment: Sub-Golgi fractionation or split-fluorophore imaging against cis, medial and trans markers to resolve quantitatively where in the stack GCNT1 acts, which would settle whether the trans-Golgi network annotation or the broader Golgi cisterna annotation is the better description.
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