A4GNT is a type-II single-pass Golgi membrane glycosyltransferase that transfers N-acetylglucosamine from UDP-GlcNAc in an alpha-1,4 linkage to terminal beta-linked galactose residues on mucin-type O-glycans. It preferentially modifies core-2-branched O-GalNAc glycans, generating the terminal GlcNAc-alpha-1,4-Gal-beta-R structure characteristic of gastric gland (class III) mucin. In human gastrointestinal gland mucous cells, A4GNT is concentrated in the Golgi region, and its glycan product occurs on mucins including MUC5AC and MUC6. A4GNT transcripts are detected in stomach and pancreas, consistent with its specialized role in glandular mucin glycan elaboration. The terminal alpha-GlcNAc cap can inhibit Helicobacter pylori growth by blocking bacterial cholesteryl-glucoside biosynthesis. In mice, A4gnt loss eliminates gastric alpha-GlcNAc and causes inflammation-associated gastric adenocarcinoma even without H. pylori, supporting a protective role for the glycan in gastric mucosal homeostasis.
Definition: Catalysis of the reaction: a beta-D-galactoside + UDP-N-acetyl-alpha-D-glucosamine = an N-acetyl-alpha-D-glucosaminyl-(1->4)-beta-D-galactosyl derivative + UDP + H(+).
Justification: GO:0008375 captures the transferred N-acetylglucosamine class but not the terminal beta-D-galactoside acceptor or alpha-1,4 linkage that define A4GNT. RHEA:85983 represents the reaction explicitly, and the human biochemical study demonstrates the linkage and preference for mucin O-glycan acceptors. RHEA:86035, RHEA:86039, RHEA:86043, and RHEA:86059 provide protein-linked core-2 O-glycan instances of this generic reaction.
Parent term: acetylglucosaminyltransferase activity
Mappings:
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0006493 protein O-linked glycosylation | IBA GO_REF:0000033 | MODIFY | Summary: The phylogenetic inference correctly places A4GNT in protein O-glycan elaboration, but GO:0006493 covers all O-linked glycosylation routes. Human A4GNT preferentially modifies core 2 branched mucin O-glycans initiated with GalNAc. Reason: GO:0016266 is the evidence-supported, more informative child process because it includes elongation of GalNAc-initiated O-glycans; A4GNT does not act across the full breadth of protein O-linked glycosylation. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH Sources checked: MGI:MGI:2143261 Β· mouse A4gnt SUPPORTS TRANSFER The ortholog supports transfer of an O-glycosylation role, but the imported term is broader than the characterized mucin O-GalNAc pathway. PANTHER:PTN000247933 Β· A4GNT family tree node SUPPORTS TRANSFER The family node supports the pathway assignment; the issue is term granularity rather than family divergence. Proposed replacements: protein O-linked glycosylation via N-acetylgalactosamine Supporting Evidence: PMID:10430883 The Ξ±4GnT was found to act more efficiently on core 2 branched O -glycans and less efficiently on core 1 oligosaccharide. |
| GO:0008375 acetylglucosaminyltransferase activity | IBA GO_REF:0000033 | ACCEPT | Summary: A4GNT transfers N-acetylglucosamine from UDP-GlcNAc to terminal galactose residues on O-glycans. GO:0008375 correctly captures the transferred sugar class, although it does not encode the alpha-1,4 linkage specificity. Reason: This is the best available GO molecular-function term for the directly demonstrated core catalytic activity. Propagation Review Root cause: NO FAILURE CORE Sources checked: MGI:MGI:2143261 Β· mouse A4gnt SUPPORTS TRANSFER The mouse ortholog supports transfer of the conserved catalytic function. PANTHER:PTN000247933 Β· A4GNT family tree node SUPPORTS TRANSFER The family node assigns the same conserved transferase activity. UniProtKB:Q9UNA3 Β· human A4GNT recipient SUPPORTS TRANSFER Self-reference: the target is its own IBD seed, which is expected rather than circular -- its own IDA/TAS annotation to this term is one of the descendant evidences behind the IBD. The IBA then asserts the additional claim that the function is inherited rather than lineage-specific. Supporting Evidence: PMID:10430883 These results indicate that the Ξ±4GnT is a glycosyltransferase that forms Ξ±1,4-linked GlcNAc residues, preferentially in O -glycans. |
| GO:0000139 Golgi membrane | IEA GO_REF:0000044 | ACCEPT | Summary: The UniProt subcellular-location mapping places this type II membrane glycosyltransferase at the Golgi membrane, consistent with direct human-tissue immunolocalization to the Golgi region. Reason: Golgi membrane is the biologically informative and experimentally corroborated site of A4GNT-mediated mucin glycan elaboration. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB-SubCell:SL-0134 Β· Golgi apparatus membrane SUPPORTS TRANSFER The controlled-vocabulary mapping agrees with direct Golgi-region immunolocalization. Supporting Evidence: PMID:11304796 Expression of alpha4GnT was largely associated with the Golgi |
| GO:0008194 UDP-glycosyltransferase activity | IEA GO_REF:0000117 | MODIFY | Summary: The ARBA term recognizes A4GNT as a UDP-sugar-dependent glycosyltransferase, but GO:0008194 defines transfer to a small hydrophobic acceptor whereas A4GNT modifies galactose residues on mucin O-glycans. The GO graph nevertheless places GO:0008375 beneath GO:0008194, so this is an ontology-parent/scoping inconsistency rather than evidence that the reaction is wholly false. Reason: Replace the definitionally mismatched broad parent with GO:0008375, which correctly specifies transfer of an N-acetylglucosaminyl residue from UDP-GlcNAc to a sugar acceptor and matches the demonstrated A4GNT chemistry. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH Sources checked: ARBA:ARBA00028346 Β· ARBA UDP-glycosyltransferase rule SUPPORTS TRANSFER The family-level inference correctly recognizes related UDP-sugar transfer chemistry, but lands on a definitionally mismatched parent term. Proposed replacements: acetylglucosaminyltransferase activity Supporting Evidence: PMID:10430883 These results indicate that the Ξ±4GnT is a glycosyltransferase that forms Ξ±1,4-linked GlcNAc residues, preferentially in O -glycans. |
| GO:0016758 hexosyltransferase activity | IEA GO_REF:0000117 | MODIFY | Summary: Hexosyltransferase activity captures only a very broad glycosyl-transfer category. A4GNT is experimentally characterized specifically as an N-acetylglucosaminyltransferase. Reason: GO:0008375 records the transferred GlcNAc residue and UDP-GlcNAc donor class and is therefore substantially more informative than GO:0016758. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH Sources checked: ARBA:ARBA00026306 Β· ARBA hexosyltransferase rule SUPPORTS TRANSFER The inference captures the enzyme class but yields a generic parent instead of the supported GlcNAc-transferase function. Proposed replacements: acetylglucosaminyltransferase activity Supporting Evidence: PMID:10430883 These results indicate that the Ξ±4GnT is a glycosyltransferase that forms Ξ±1,4-linked GlcNAc residues, preferentially in O -glycans. |
| GO:0008375 acetylglucosaminyltransferase activity | IEA GO_REF:0000107 | ACCEPT | Summary: Ensembl Compara transfers acetylglucosaminyltransferase activity from the mouse A4gnt ortholog. The inferred activity is independently demonstrated for human A4GNT. Reason: The orthology transfer is biologically sound and matches direct human expression-cloning and glycosyltransferase-assay evidence. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:Q14BT6 Β· mouse A4gnt SUPPORTS TRANSFER The mouse ortholog supports transfer of the conserved catalytic activity. ensembl:ENSMUSP00000045629 Β· mouse A4gnt Ensembl protein CIRCULAR OR REDUNDANT This is a second identifier for the same mouse ortholog, not an independent donor. Supporting Evidence: PMID:10430883 These results indicate that the Ξ±4GnT is a glycosyltransferase that forms Ξ±1,4-linked GlcNAc residues, preferentially in O -glycans. |
| GO:0016266 protein O-linked glycosylation via N-acetylgalactosamine | TAS Reactome:R-HSA-913709 | ACCEPT | Summary: A4GNT elaborates mucin O-glycans after the initiating GalNAc linkage, with the strongest activity on core 2 branched acceptors. This is within mucin-type O-GalNAc glycosylation. Reason: The Reactome pathway term correctly includes sequential elongation of GalNAc-initiated O-glycans and captures the core biological process. Supporting Evidence: PMID:10430883 The Ξ±4GnT was found to act more efficiently on core 2 branched O -glycans and less efficiently on core 1 oligosaccharide. |
| GO:0008375 acetylglucosaminyltransferase activity | IDA PMID:10430883 Expression cloning of a human alpha1, 4-N-acetylglucosaminyl... | ACCEPT | Summary: Expression cloning, gain-of-function glycan production, and soluble-enzyme assays directly establish human A4GNT as an acetylglucosaminyltransferase. Reason: This experimental molecular-function annotation is correct and represents the core catalytic activity at the best available GO specificity. Supporting Evidence: PMID:10430883 These results indicate that the Ξ±4GnT is a glycosyltransferase that forms Ξ±1,4-linked GlcNAc residues, preferentially in O -glycans. |
| GO:0009101 glycoprotein biosynthetic process | IDA PMID:10430883 Expression cloning of a human alpha1, 4-N-acetylglucosaminyl... | MODIFY | Summary: A4GNT synthesizes a terminal alpha-1,4-GlcNAc structure on glycoprotein O-glycans, so glycoprotein biosynthesis is correct but much broader than the experimentally established mucin O-GalNAc pathway. Reason: GO:0016266 more precisely captures the GalNAc-initiated O-glycan elongation demonstrated with core 2 and core 1 O-glycan acceptors. The replacement should use the involved_in relation appropriate to participation in this process, rather than carrying forward the source row's broader acts_upstream_of_or_within relation. Proposed replacements: protein O-linked glycosylation via N-acetylgalactosamine Supporting Evidence: PMID:10430883 The Ξ±4GnT was found to act more efficiently on core 2 branched O -glycans and less efficiently on core 1 oligosaccharide. |
| GO:0008375 acetylglucosaminyltransferase activity | TAS Reactome:R-HSA-5694487 | ACCEPT | Summary: Reactome assigns A4GNT the transfer of GlcNAc to core 2 mucins, creating an alpha-1,4 linkage to beta-galactose residues. Reason: The reaction is a direct instance of acetylglucosaminyltransferase activity and agrees with the primary biochemical characterization. Supporting Evidence: Reactome:R-HSA-5694487 Alpha-1,4-N-acetylglucosaminyltransferase (A4GNT) can catalyse the transfer of N-acetylglucosamine (GlcNAc) to core 2 branched mucins, creating an alpha1,4-linkage with beta-Gal residues |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-5694487 | ACCEPT | Summary: The Reactome reaction assigns A4GNT to the Golgi membrane, consistent with human-tissue immunohistochemistry that localized the enzyme to the Golgi region of mucous cells. Reason: Golgi membrane is the appropriate core compartment for this type II membrane glycosyltransferase and is supported independently of the Reactome assertion. Supporting Evidence: PMID:11304796 Expression of alpha4GnT was largely associated with the Golgi |
| GO:0005975 carbohydrate metabolic process | TAS PMID:10430883 Expression cloning of a human alpha1, 4-N-acetylglucosaminyl... | MODIFY | Summary: Carbohydrate metabolism is technically true because A4GNT transfers GlcNAc during glycan synthesis, but it obscures the specific protein O-glycosylation pathway established in the cited study. Reason: Replace this root-level metabolic process with GO:0016266, which captures elongation of GalNAc-initiated mucin O-glycans. Proposed replacements: protein O-linked glycosylation via N-acetylgalactosamine Supporting Evidence: PMID:10430883 The Ξ±4GnT was found to act more efficiently on core 2 branched O -glycans and less efficiently on core 1 oligosaccharide. |
| GO:0006493 protein O-linked glycosylation | TAS PMID:10430883 Expression cloning of a human alpha1, 4-N-acetylglucosaminyl... | MODIFY | Summary: Protein O-linked glycosylation is correct, but it includes routes initiated by several different amino-acid-linked sugars. A4GNT specifically elongates mucin-type O-GalNAc glycans. Reason: GO:0016266 is the supported, more precise child process and explicitly covers sequential elongation after the initiating GalNAc linkage. Proposed replacements: protein O-linked glycosylation via N-acetylgalactosamine Supporting Evidence: PMID:10430883 The Ξ±4GnT was found to act more efficiently on core 2 branched O -glycans and less efficiently on core 1 oligosaccharide. |
| GO:0008375 acetylglucosaminyltransferase activity | TAS PMID:10430883 Expression cloning of a human alpha1, 4-N-acetylglucosaminyl... | ACCEPT | Summary: The cited expression-cloning and biochemical study directly demonstrates alpha-1,4-linked GlcNAc transfer by human A4GNT. Reason: GO:0008375 correctly captures the enzyme's core molecular function at the most informative currently available GO level. Supporting Evidence: PMID:10430883 These results indicate that the Ξ±4GnT is a glycosyltransferase that forms Ξ±1,4-linked GlcNAc residues, preferentially in O -glycans. |
| GO:0016020 membrane | TAS PMID:10430883 Expression cloning of a human alpha1, 4-N-acetylglucosaminyl... | MODIFY | Summary: The cloning study predicts a type II membrane protein, so membrane association is correct but uninformative. Subsequent human-tissue immunolocalization places A4GNT specifically in the Golgi region. Reason: GO:0000139 is the evidence-supported specific membrane compartment and should replace the generic membrane term. Proposed replacements: Golgi membrane Supporting Evidence: PMID:10430883 The deduced amino acid sequence predicts a type II membrane protein with 340 amino acids, showing no significant similarity with any other proteins. PMID:11304796 Expression of alpha4GnT was largely associated with the Golgi |
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Download this section (compressed HTML)Q: Which endogenous human mucin glycosites and acceptor structures are the principal A4GNT substrates, and are MUC5AC and MUC6 the dominant carriers in each gastrointestinal tissue where A4GNT is expressed?
Q: Which Golgi subcompartment and resident partners determine A4GNT access to core-2-branched rather than core-1 or core-3 O-glycan acceptors?
Q: By what epithelial and immune mechanisms does loss of the A4GNT-generated alpha-GlcNAc cap initiate sterile gastric inflammation and tumorigenesis, and how separable is that protection from its anti-H. pylori effect?
Experiment: Compare intact glycopeptide and released-glycan profiles from A4GNT-knockout human gastric organoids, wild-type rescue, and catalytic-dead rescue, with targeted analysis of MUC5AC and MUC6 glycosites and linkage-sensitive validation of the terminal alpha-GlcNAc product.
Hypothesis: Endogenous A4GNT preferentially installs the GlcNAc-alpha-1,4-Gal-beta-R cap at a restricted set of core-2 O-glycosites on MUC5AC and MUC6.
Type: genetic rescue and glycoproteomics
Experiment: Measure steady-state kinetics of purified human A4GNT against a structurally defined acceptor panel spanning core-1, core-2, and core-3 O-glycans with beta-1,3- or beta-1,4-linked terminal galactose, confirming product linkages by mass spectrometry or NMR.
Hypothesis: A4GNT's physiological specificity is determined by both terminal galactose linkage and the underlying mucin O-glycan core.
Type: enzyme kinetics and structural glycomics
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