B4GALNT1 is a Golgi type II membrane glycosyltransferase, also called GM2/GD2 synthase (EC 2.4.1.92; CAZy family GT12). It transfers N-acetylgalactosamine from UDP-GalNAc in beta-1,4 linkage to glycolipid acceptors, generating the gangliosides GM2 from GM3 and GD2 from GD3. It also supports GT2 synthesis from GT3 and the neutral GA2 branch from lactosylceramide. Its lumen-facing catalytic domain acts as a homodimer. These reactions generate gangliosides and precursors for further glycosphingolipid synthesis in the nervous system. Biallelic loss-of-function variants cause hereditary spastic paraplegia type 26, linking impaired ganglioside synthesis to neurological dysfunction.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0008376 acetylgalactosaminyltransferase activity | IBA GO_REF:0000033 | ACCEPT | Summary: The inherited acetylgalactosaminyltransferase activity is part of the core catalytic function. Reason: Retain the PAINT assertion as a curator-assessed inherited GalNAc-transferase function. Its WITH/FROM includes B4GALNT1 and Q8NHY0 (B4GALNT2), consistent with a shared catalytic class across paralogs with different acceptor specificity. That donor list alone does not establish the exact ancestral node placement, which has not been independently reconstructed. In contrast, the ARBA IEA is refined using B4GALNT1-specific GM3/GD3 assays; those assays do not justify narrowing the ancestral assertion for the whole clade. Donor count and target self-membership are not weaknesses. |
| GO:0001574 ganglioside biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: B4GALNT1 directly performs a glycosyltransfer step in ganglioside biosynthesis. Reason: This enzyme catalyzes a biosynthetic reaction, rather than merely being necessary for a downstream outcome. The IBA agrees with experimental rows. The recorded target self-source is valid PAINT provenance. |
| GO:0000139 Golgi membrane | IEA GO_REF:0000120 | ACCEPT | Summary: Golgi membrane localization agrees with the enzyme topology and independent localization evidence. Reason: The normal UniProt record describes a single-pass type II Golgi protein; rat-ortholog ISS and the human Reactome reaction corroborate the compartment. The electronic mapping itself is not a new human localization experiment. |
| GO:0003947 (N-acetylneuraminyl)-galactosylglucosylceramide N-acetylgalactosaminyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: The specific GM3/GD3 GalNAc-transferase activity is supported by direct biochemical evidence. Reason: The combined mapping agrees with the specific activity established in the original expression and substrate assays. It is retained as a correct core function. |
| GO:0008376 acetylgalactosaminyltransferase activity | IEA GO_REF:0000117 | MODIFY | Summary: The broad electronic catalytic assertion can be refined to the established ganglioside GalNAc-transferase activity. Reason: Refine this gene-level ARBA assertion to GO:0003947 using the established B4GALNT1 GM3/GD3 transfer reaction. This differs from the IBA row, which records the broader function inherited from a PAINT ancestral node; no revised ancestral substrate specificity is inferred. The specific replacement is already represented by independent evidence rows and adds no new assertion. Proposed replacements: (N-acetylneuraminyl)-galactosylglucosylceramide N-acetylgalactosaminyltransferase activity |
| GO:0016758 hexosyltransferase activity | IEA GO_REF:0000002 | MODIFY | Summary: The hexosyltransferase assertion is accurate but can be made more specific. Reason: A general parent is not an unsupported over-annotation. The experimentally established activity supports GO:0003947; this refinement does not assign other reactions merely from GT12 family membership. Proposed replacements: (N-acetylneuraminyl)-galactosylglucosylceramide N-acetylgalactosaminyltransferase activity |
| GO:1901135 carbohydrate derivative metabolic process | IEA GO_REF:0000117 | MODIFY | Summary: The broad metabolic assertion can be refined to ganglioside biosynthesis. Reason: B4GALNT1 itself transfers GalNAc to ganglioside precursors. GO:0001574 specifies this direct contribution more precisely than the broad ARBA metabolic term and is already present. Proposed replacements: ganglioside biosynthetic process |
| GO:0001574 ganglioside biosynthetic process | IEA GO_REF:0000107 | ACCEPT | Summary: The orthology-based ganglioside synthesis assertion agrees with direct functional evidence. Reason: The recorded donor Q09200 is the mouse ortholog. Direct human-enzyme evidence supports the same biosynthetic role; the transfer is not treated as an independent human assay. |
| GO:0000139 Golgi membrane | ISS GO_REF:0000024 | ACCEPT | Summary: Golgi membrane localization is transferred from the rat ortholog Q10468. Reason: The GOA donor is UniProtKB:Q10468, the rat B4galnt1 protein, not mouse. Its curator-transferred location agrees with the human protein topology and Reactome location. The duplicated raw ISS tuple is one distinct assertion. |
| GO:0001574 ganglioside biosynthetic process | IDA PMID:7487055 Analysis of melanoma cells stably transfected with beta 1,4G... | ACCEPT | Summary: Transfectant membranes synthesize GM2 and GD2 from GM3 and GD3. Reason: The cached abstract explicitly reports synthesis of both products by membrane preparations from transfected cells. Mouse B16 and human MeWo identify recipient lines; they do not by themselves establish the species of the cloned cDNA. Supporting Evidence: PMID:7487055 synthesis of GM2 or GD2 |
| GO:0001574 ganglioside biosynthetic process | IDA PMID:7890749 Substrate specificity of beta 1,4-N-acetylgalactosaminyltran... | ACCEPT | Summary: The tested enzyme preparations produce GM2 and GD2; use of additional acceptors depends on context. Reason: Both enzyme preparations generated GM2/GD2. Other acceptors had lower efficiency in vitro, yet neutral asialo-GM2 could accumulate in particular cellular contexts. Asialo-GM2 is not a sialylated ganglioside; the GM2/GD2 products support this process annotation. Supporting Evidence: PMID:7890749 Both enzyme sources were capable of producing not only GM2 and GD2 |
| GO:0003947 (N-acetylneuraminyl)-galactosylglucosylceramide N-acetylgalactosaminyltransferase activity | IDA PMID:7487055 Analysis of melanoma cells stably transfected with beta 1,4G... | ACCEPT | Summary: Direct assays support GalNAc transfer to the ganglioside precursors GM3 and GD3. Reason: The membrane assays described in the source abstract use GM3 and GD3 acceptors and detect the respective products. The original IDA assignment is preserved; complete experimental Methods were not available in the cache. Supporting Evidence: PMID:7487055 synthesis of GM2 or GD2 in in vitro assays using GM3 or GD3, respectively, as ganglioside acceptors. |
| GO:0003947 (N-acetylneuraminyl)-galactosylglucosylceramide N-acetylgalactosaminyltransferase activity | IDA PMID:7890749 Substrate specificity of beta 1,4-N-acetylgalactosaminyltran... | ACCEPT | Summary: The catalytic-domain fusion and transfectant extracts support the specific transferase activity. Reason: The source compares a catalytic-domain/protein-A fusion with cDNA-transfectant extracts. Both make GM2/GD2. Additional substrate use does not negate this activity, and low efficiency alone does not establish that alternate products are artifacts. Supporting Evidence: PMID:7890749 Both enzyme sources were capable of producing not only GM2 and GD2 |
| GO:0005794 Golgi apparatus | ISS GO_REF:0000024 | MODIFY | Summary: The Golgi apparatus location can be refined to the Golgi membrane. Reason: The same rat Q10468 donor already supports the more specific Golgi-membrane ISS assertion, and the human type II topology and Reactome reaction agree. Refine this true but general location to the existing GO:0000139, consistently with the generic membrane TAS row. Generality alone does not make this core location non-core or incorrect. Proposed replacements: Golgi membrane |
| GO:0001574 ganglioside biosynthetic process | IMP PMID:1601877 Expression cloning of beta 1,4 N-acetylgalactosaminyltransfe... | ACCEPT | Summary: Expression cloning links the gene to cellular GM2/GD2 synthesis. Reason: The cloned cDNA drives GM2/GD2 expression in recipient melanoma cells. Preserve the curator’s IMP code. The linked 1994 erratum concerns sequence revision according to UniProt; its correction text was not directly read. Supporting Evidence: PMID:1601877 resulting in the synthesis of GM2 and GD2 |
| GO:0003947 (N-acetylneuraminyl)-galactosylglucosylceramide N-acetylgalactosaminyltransferase activity | IMP PMID:1601877 Expression cloning of beta 1,4 N-acetylgalactosaminyltransfe... | ACCEPT | Summary: Expression cloning supports the GM3/GD3 GalNAc-transferase identity. Reason: The original abstract identifies transfer to GM3/GD3 and the corresponding products. Preserve IMP and the original reference. Later biochemical evidence independently establishes the activity; recipient species are not used to infer cDNA donor species. Supporting Evidence: PMID:1601877 catalyze the transfer of GalNAc onto GM3 and GD3 by a beta 1,4 linkage |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-8856223 | ACCEPT | Summary: The Reactome reaction places the catalytic enzyme at the Golgi membrane. Reason: The normal Reactome record identifies the B4GALNT1 dimer as catalyst and places the reaction at the Golgi membrane. Its catalytic role is distinct from the glycosphingolipid substrates. |
| GO:0003947 (N-acetylneuraminyl)-galactosylglucosylceramide N-acetylgalactosaminyltransferase activity | TAS PMID:8702839 Genomic organization and chromosomal assignment of the human... | ACCEPT | Summary: The genomic study provides a traceable statement about the enzyme whose activity is established independently. Reason: PMID:8702839 primarily describes genomic organization and transcription units. It is not recast as a direct catalytic assay; the traceable enzyme statement agrees with the independent activity studies. |
| GO:0005975 carbohydrate metabolic process | TAS PMID:1601877 Expression cloning of beta 1,4 N-acetylgalactosaminyltransfe... | MODIFY | Summary: The broad metabolic assertion can be refined to the directly executed ganglioside biosynthetic process. Reason: The cited cloning result supports ganglioside synthesis directly. GO:0001574 is an existing, evidence-supported refinement of the true but broad legacy TAS assertion. Generality does not justify MARK_AS_OVER_ANNOTATED. Proposed replacements: ganglioside biosynthetic process |
| GO:0006687 glycosphingolipid metabolic process | TAS PMID:1601877 Expression cloning of beta 1,4 N-acetylgalactosaminyltransfe... | ACCEPT | Summary: Glycosphingolipid metabolism includes the enzyme’s ganglioside and neutral glycolipid reactions. Reason: The enzyme directly contributes to glycosphingolipid metabolism, including synthesis of neutral GA2 from lactosylceramide. This core process is not peripheral merely because a more specific ganglioside annotation also exists. |
| GO:0016020 membrane | TAS PMID:1601877 Expression cloning of beta 1,4 N-acetylgalactosaminyltransfe... | MODIFY | Summary: The membrane assertion is accurate and can be refined to the Golgi membrane. Reason: The original cloning abstract infers type II membrane topology. Independent UniProt, ortholog localization and Reactome evidence supply the Golgi compartment. Refine to the already present GO:0000139 without claiming the cloning abstract directly demonstrated that compartment. Proposed replacements: Golgi membrane |
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Download this section (compressed HTML)Q: The 2025 structure predicts misfolding for R300C and dimer destabilization for D433A. How do these defects affect full-length cellular stability, Golgi retention and ganglioside versus neutral GA2 synthesis?
Q: How do acceptor availability and competing sialyltransferases determine GM3-to-GM2, GD3-to-GD2 and LacCer-to-GA2 flux in neurons, and their downstream complex glycosphingolipid products?
Experiment: Test the predicted R300C folding defect and D433A dimer defect with full-length proteins in B4GALNT1-null cells. Compare abundance, folding, oligomerization and Golgi retention alongside lipidomic GM2, GD2 and neutral GA2 output, with wild-type rescue and a catalytic-site control.
Experiment: Measure acceptor competition across GM3, GD3, GT3 and LacCer using membrane-associated B4GALNT1 and defined liposomes. Vary lipid composition and quantify products over time; account for membrane tethering before inferring kinetic preferences from endpoint activities.
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