B3GALNT1 (UDP-GalNAc:beta-1,3-N-acetylgalactosaminyltransferase 1; also known as beta-1,3-galactosyltransferase 3/beta3Gal-T3/B3GALT3, or globoside synthase) is a Golgi-resident, single-pass type II membrane glycosyltransferase of the GT31 (glycosyltransferase 31) family. It catalyzes the beta-1,3 transfer of N-acetylgalactosamine from UDP-GalNAc onto globotriaosylceramide (Gb3Cer/CD77), producing globoside (Gb4Cer/globotetraosylceramide) in the globo-series branch of glycosphingolipid biosynthesis; the reaction requires Mg2+ (EC 2.4.1.79). Globoside (Gb4) is the P blood-group antigen and the most abundant neutral glycosphingolipid of the human erythrocyte membrane. B3GALNT1 is expressed most highly in heart and brain. Genetic variation in B3GALNT1 determines the P1PK and GLOB blood-group systems, and loss-of-function variants cause the rare P antigen-deficient (P-null/Pk) phenotypes.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000139 Golgi membrane | IBA GO_REF:0000033 | ACCEPT | Summary: B3GALNT1 is a Golgi apparatus membrane, single-pass type II membrane protein; the Golgi is where it acts on Gb3Cer to make globoside. Consistent across IBA, IEA, and TAS evidence and with UniProt subcellular location. Reason: Correct, well-supported core localization for this Golgi glycosyltransferase. |
| GO:0009312 oligosaccharide biosynthetic process | IBA GO_REF:0000033 | MODIFY | Summary: The product of B3GALNT1 is a membrane glycosphingolipid (globoside/Gb4Cer), not a free oligosaccharide. The specific and better-supported process is globoside (glycosphingolipid) biosynthesis. Reason: Over-general and imprecise; the biosynthesized species is a globo-series glycosphingolipid, so globoside biosynthetic process is the correct process term. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH FUNCTIONAL DIVERGENCE Sources checked: PANTHER:PTN002598865 SUPPORTS SOURCE BUT NOT TARGET Family-level oligosaccharide-biosynthesis term is too broad for this glycosphingolipid-branch member, which builds a membrane globoside rather than a free oligosaccharide. Proposed replacements: globoside biosynthetic process |
| GO:0008499 N-acetyl-beta-D-glucosaminide beta-(1,3)-galactosyltransferase activity | IBA GO_REF:0000033 | MODIFY | Summary: This is the family-level GT31 activity (transfer of galactose onto a beta-GlcNAc acceptor) catalyzed by paralogs such as B3GALT1/2/4/5. B3GALNT1 does not catalyze this reaction; it transfers GalNAc onto a galactose acceptor (Gb3Cer). The primary paper explicitly notes that members of this family do not necessarily use the same acceptor or sugar donor. Reason: IBA propagated the ancestral family activity, but the demonstrated activity of this member is Gb4 (globoside) synthase, so the specific MF should replace it. Propagation Review Root cause: PROPAGATION BAD Failure modes: WRONG ORTHOLOG OR PARALOG FUNCTIONAL DIVERGENCE Sources checked: UniProtKB:Q9Y5Z6 SUPPORTS SOURCE BUT NOT TARGET B3GALT5 and other GT31 paralogs genuinely transfer galactose onto beta-GlcNAc acceptors; B3GALNT1 diverged to transfer GalNAc onto a galactose (Gb3) acceptor, so the family MF should not propagate to it. PANTHER:PTN007495810 SUPPORTS SOURCE BUT NOT TARGET Ancestral GT31 galactosyltransferase activity; the specific member catalyzes the distinct Gb4 synthase reaction (GO:0047273). Proposed replacements: galactosylgalactosylglucosylceramide beta-D-acetylgalactosaminyltransferase activity |
| GO:0000139 Golgi membrane | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt SubCell mapping to Golgi apparatus membrane; agrees with the IBA and TAS Golgi annotations and UniProt subcellular location. Reason: Correct core localization. |
| GO:0009101 glycoprotein biosynthetic process | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: B3GALNT1 glycosylates a glycosphingolipid acceptor (ceramide-linked Gb3Cer), not a protein. This InterPro2GO mapping reflects the generic GT31 family (many members act on glycoproteins), but for this member the biosynthesized species is a glycolipid, not a glycoprotein. Reason: Generic family-level electronic mapping; the demonstrated product is a glycosphingolipid, so glycoprotein biosynthesis over-annotates this member. |
| GO:0016020 membrane | IEA GO_REF:0000002 | MODIFY | Summary: Correct but uninformative; the specific and well-supported localization is the Golgi membrane. Reason: Over-general cellular component; replace with the specific Golgi membrane localization. Proposed replacements: Golgi membrane |
| GO:0016758 hexosyltransferase activity | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: A correct but broad parent of the specific Gb4 synthase activity. Retained as a true but non-core generalization. Reason: Accurate high-level MF parent; the specific globoside synthase activity captures the core function. |
| GO:0047273 galactosylgalactosylglucosylceramide beta-D-acetylgalactosaminyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: This is the exact globoside (Gb4) synthase reaction (UDP-GalNAc + Gb3Cer -> Gb4Cer + UDP), matching UniProt EC 2.4.1.79 and RHEA:22252. This is the core molecular function. Reason: Correct, specific core MF; concordant with EXP and TAS evidence and the UniProt catalytic activity. |
| GO:1901135 carbohydrate derivative metabolic process | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: Correct but very general ARBA mapping; the specific globoside biosynthetic process better captures this enzyme's role. Reason: Accurate broad process parent, retained as non-core. |
| GO:0006688 glycosphingolipid biosynthetic process | TAS Reactome:R-HSA-9840309 | ACCEPT | Summary: B3GALNT1 acts in the glycosphingolipid biosynthesis pathway (globo-series branch), adding GalNAc to Gb3Cer. Correct process term; the more precise child globoside biosynthetic process is used in core_functions. Reason: Correct BP; well supported by Reactome and the primary literature. |
| GO:0047273 galactosylgalactosylglucosylceramide beta-D-acetylgalactosaminyltransferase activity | TAS Reactome:R-HSA-8878914 | ACCEPT | Summary: Reactome reaction "B3GALNT1 transfers GalNAc to Gb3Cer to form Gb4Cer" directly supports the globoside synthase MF. Reason: Correct core MF, concordant with EXP and IEA(EC/RHEA) evidence. |
| GO:0047273 galactosylgalactosylglucosylceramide beta-D-acetylgalactosaminyltransferase activity | EXP PMID:10993897 Expression cloning of human globoside synthase cDNAs. Identi... | ACCEPT | Summary: Experimental expression cloning and substrate-specificity analysis of extracts from cDNA-transfected cells demonstrated that B3GALNT1 (beta3Gal-T3) specifically transfers N-acetylgalactosamine onto globotriaosylceramide, and TLC immunostaining confirmed the product as globoside. This is the defining experimental evidence for the core MF. Reason: Direct experimental demonstration of globoside (Gb4) synthase activity. |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-8878914 | ACCEPT | Summary: Reactome describes B3GALNT1 as a Golgi membrane-associated protein performing the Gb3Cer -> Gb4Cer reaction; consistent with UniProt and the IBA/IEA Golgi annotations. Reason: Correct core localization. |
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