B3GALT6 is a Golgi type II membrane glycosyltransferase, also called galactosyltransferase II. It transfers galactose from UDP-galactose to galactose-beta-1,4-xylose attached to a proteoglycan core protein, forming the second galactose and third sugar of the common glycosaminoglycan linker. This reaction supports efficient synthesis of heparan sulfate, chondroitin sulfate and dermatan sulfate proteoglycans. Its catalytic domain faces the Golgi lumen, with enrichment in medial Golgi cisternae. Biallelic loss-of-function variants disrupt proteoglycan glycanation and cause skeletal and connective-tissue disorders, including spondylodysplastic Ehlers-Danlos syndrome and spondyloepimetaphyseal dysplasia with joint laxity.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000139 Golgi membrane | IBA GO_REF:0000033 | ACCEPT | Summary: Golgi membrane localization agrees with the established enzyme topology. Reason: The ancestral localization inference is consistent with human medial-Golgi localization and the type II membrane topology in the normal UniProt record. The phylogenetic tree and ancestral node were not independently reconstructed; no claim about a particular donor's biochemical specificity is needed for this localization. |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-1889978 | ACCEPT | Summary: The normal linker-synthesis reaction occurs at the Golgi membrane. Reason: Reactome places the B3GALT6 catalytic step in the Golgi. Human localization and membrane topology independently support the compartment. |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-4420365 | ACCEPT | Summary: The disease reaction preserves the normal enzyme's Golgi location. Reason: The Reactome disease event describes disruption of the normal B3GALT6 linker step. Its location is consistent with wild-type Golgi localization; this does not mean every pathogenic variant reaches the Golgi. |
| GO:0005794 Golgi apparatus | IEA GO_REF:0000107 | ACCEPT | Summary: The mouse-ortholog Golgi inference agrees with direct human evidence. Reason: Human B3GALT6 localization supports the transferred Golgi annotation. Retain the source assertion without treating orthology as proof of identical behavior for each mutant protein. |
| GO:0005794 Golgi apparatus | IMP PMID:29443383 A B3GALT6 variant in patient originally described as Al-Gaza... | ACCEPT | Summary: Wild-type Golgi localization is consistent with the mutant-trafficking study. Reason: The available abstract distinguishes ER-retained variants from variants that traffic normally; it does not supply all localization images. Retain the curator's Golgi assignment with independent human localization support. Mutant ER retention does not establish a wild-type ER function. |
| GO:0005797 Golgi medial cisterna | IDA PMID:11551958 Biosynthesis of the linkage region of glycosaminoglycans: cl... | ACCEPT | Summary: B3GALT6 localizes to the medial Golgi. Reason: The original characterization explicitly reports medial-Golgi localization, fitting assembly of the proteoglycan linker in this compartment. |
| GO:0005797 Golgi medial cisterna | IEA GO_REF:0000120 | ACCEPT | Summary: The combined automated medial-Golgi assignment agrees with human evidence. Reason: The location is independently supported by the original human characterization. No new assertion about the ARBA rule or the mouse experimental assay is made. |
| GO:0006024 glycosaminoglycan biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: Linker galactosylation directly contributes to GAG biosynthesis. Reason: B3GALT6 performs a sugar-transfer step of the pathway. Human functional evidence agrees with the ancestral inference. The target's own appearance in WITH/FROM is legitimate descendant evidence, not circular support; the IBD placement was not re-audited. |
| GO:0006024 glycosaminoglycan biosynthetic process | IEA GO_REF:0000117 | ACCEPT | Summary: The automated GAG-biosynthesis assignment matches the enzyme reaction. Reason: Direct characterization establishes a linker-building reaction required for efficient GAG assembly. The statement is supported independently of the ARBA prediction. |
| GO:0006024 glycosaminoglycan biosynthetic process | IMP PMID:11551958 Biosynthesis of the linkage region of glycosaminoglycans: cl... | ACCEPT | Summary: B3GALT6 perturbation impairs GAG synthesis. Reason: The original paper reports reduced GAG synthesis after siRNA treatment of HeLa S3 cells. Together with the enzyme assay, this establishes pathway participation rather than only a downstream phenotype. |
| GO:0009101 glycoprotein biosynthetic process | IEA GO_REF:0000002 | MODIFY | Summary: Specify the proteoglycan linker step within glycoprotein biosynthesis. Reason: The InterPro transfer is biologically consistent but does not identify the particular glycan. Human B3GALT6 synthesizes the common GAG linker; use the specific linker-biosynthesis term already supported by the Reactome pathway. This refines this broad source assertion and adds no new pathway claim. Proposed replacements: glycosaminoglycan-protein linkage region biosynthetic process |
| GO:0015012 heparan sulfate proteoglycan biosynthetic process | IEA GO_REF:0000041 | ACCEPT | Summary: The shared linker reaction contributes to heparan sulfate proteoglycan synthesis. Reason: This enzyme performs the third sugar addition before heparan sulfate chain assembly. Patient fibroblasts independently show reduced heparan sulfate after B3GALT6 disruption; B3GALT6 is not a heparan-chain polymerase. |
| GO:0015012 heparan sulfate proteoglycan biosynthetic process | IMP PMID:23664118 Defective initiation of glycosaminoglycan synthesis due to B... | ACCEPT | Summary: Patient fibroblast evidence supports the heparan sulfate pathway contribution. Reason: The reported reduction in heparan sulfate is consistent with impaired synthesis of the common linker. The enzyme's direct chemical step, rather than the disease phenotype alone, supports participation. |
| GO:0016020 membrane | HDA PMID:19946888 Defining the membrane proteome of NK cells. | KEEP AS NON CORE | Summary: Retain the broad membrane-proteomics observation. Reason: The study profiled membrane fractions of YTS NK-like cells. The available abstract does not identify individual B3GALT6 peptides, so defer to the curator for the detection. Membrane association is biologically consistent, but this broad fraction assignment does not establish a specific Golgi subcompartment; that is represented separately. |
| GO:0016020 membrane | IEA GO_REF:0000002 | MODIFY | Summary: Specify the Golgi membrane for the broad membrane mapping. Reason: The generic InterPro membrane mapping is consistent with the type II topology but omits the established Golgi compartment. Independent human evidence supports refinement to Golgi membrane, without inferring plasma-membrane localization. Proposed replacements: Golgi membrane |
| GO:0016758 hexosyltransferase activity | IEA GO_REF:0000002 | MODIFY | Summary: Specify the galactosylxylosylprotein acceptor and linkage. Reason: Hexosyltransferase activity is too general for this experimentally characterized enzyme. B3GALT6 adds galactose in a beta-1,3 linkage to the galactosylxylose linker acceptor; it does not share every acceptor specificity of the broader GT31 family. Proposed replacements: galactosylxylosylprotein 3-beta-galactosyltransferase activity |
| GO:0030166 proteoglycan biosynthetic process | IMP PMID:29443383 A B3GALT6 variant in patient originally described as Al-Gaza... | ACCEPT | Summary: B3GALT6 catalysis contributes directly to proteoglycan biosynthesis. Reason: The variant study addresses proteoglycan maturation defects and mutant trafficking. Retain the curated process assignment with the independent linker-enzyme characterization. ER retention of some variants is a pathogenic mechanism, not the normal biosynthetic activity. |
| GO:0032580 Golgi cisterna membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Golgi cisterna membrane localization fits the enzyme topology. Reason: The UniProt subcellular-location mapping agrees with the N-terminal membrane anchor and lumen-facing catalytic region. Medial-Golgi localization provides the more specific cisternal context. |
| GO:0035250 UDP-galactosyltransferase activity | IDA PMID:11551958 Biosynthesis of the linkage region of glycosaminoglycans: cl... | ACCEPT | Summary: UDP-galactose is the demonstrated sugar donor. Reason: The original biochemical characterization demonstrates transfer from UDP-galactose. This donor-level activity is correct; the substrate-specific enzyme activity is also represented by existing annotations and the core function. |
| GO:0047220 galactosylxylosylprotein 3-beta-galactosyltransferase activity | EXP PMID:11551958 Biosynthesis of the linkage region of glycosaminoglycans: cl... | ACCEPT | Summary: The experimentally established activity is galactosyltransferase II. Reason: The original study identifies galactose transfer to the Gal-Xyl linker acceptor and distinguishes it from related enzymes acting on terminal glucosamine or galactosamine. This is the defining molecular activity of B3GALT6. |
| GO:0047220 galactosylxylosylprotein 3-beta-galactosyltransferase activity | EXP PMID:23664117 Mutations in B3GALT6, which encodes a glycosaminoglycan link... | ACCEPT | Summary: The human disease study supports loss of linker-galactosyltransferase activity. Reason: The normal cache contains the disease-study abstract, and the official PubMed figure legend describes decreased GalT-II activity for tested missense variants. Retain the curated experimental assignment, independently supported by the original enzyme characterization; not every listed clinical variant was assayed identically. |
| GO:0047220 galactosylxylosylprotein 3-beta-galactosyltransferase activity | IBA GO_REF:0000033 | ACCEPT | Summary: The ancestral enzyme activity agrees with direct human characterization. Reason: The transfer is supported by known human B3GALT6 catalysis. Human Q96L58 in WITH/FROM represents legitimate experimental grounding used for ancestral inference. The PAINT node placement was not independently re-examined, and the broad family name does not override target-specific chemistry. |
| GO:0047220 galactosylxylosylprotein 3-beta-galactosyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: The combined enzyme mapping has the correct donor and acceptor specificity. Reason: Rhea:11780 and EC:2.4.1.134 describe the same linker-galactosylation reaction as the human characterization. The ortholog and automated-rule components are concordant, without substituting family similarity for the direct evidence. |
| GO:0047220 galactosylxylosylprotein 3-beta-galactosyltransferase activity | IMP PMID:29443383 A B3GALT6 variant in patient originally described as Al-Gaza... | ACCEPT | Summary: Retain the variant study's curated GalT-II activity assignment. Reason: The abstract describes pathogenic B3GALT6 variants and trafficking differences but not every catalytic assay. Defer to the experimental curator with independent target-specific enzyme evidence; the abstract's emphasis on ER quality control is not evidence of misattribution. |
| GO:0047220 galactosylxylosylprotein 3-beta-galactosyltransferase activity | TAS Reactome:R-HSA-1889978 | ACCEPT | Summary: The normal Reactome reaction matches GalT-II catalysis. Reason: B3GALT6 supplies the second galactose of the common linker. The reaction identity is supported; its separate historical manganese citation is not needed to establish the sugar-transfer activity. |
| GO:0047220 galactosylxylosylprotein 3-beta-galactosyltransferase activity | TAS Reactome:R-HSA-4420365 | ACCEPT | Summary: The disease event identifies disruption of the normal GalT-II reaction. Reason: Reactome explicitly describes the normal second-galactose transfer before discussing deficient variants. Retain the normal gene-product activity; this does not assign catalytic competence to loss-of-function variants. |
| GO:0050650 chondroitin sulfate proteoglycan biosynthetic process | IMP PMID:23664118 Defective initiation of glycosaminoglycan synthesis due to B... | ACCEPT | Summary: The common linker step contributes to chondroitin sulfate proteoglycan synthesis. Reason: The patient fibroblast study reports impaired decorin glycanation. B3GALT6 directly builds the linker before chondroitin chain extension; it is not itself the repeating-disaccharide polymerase. |
| GO:0050651 dermatan sulfate proteoglycan biosynthetic process | IMP PMID:23664118 Defective initiation of glycosaminoglycan synthesis due to B... | ACCEPT | Summary: The common linker step contributes to dermatan sulfate proteoglycan synthesis. Reason: The study connects B3GALT6 deficiency to impaired glycanation of the chondroitin/dermatan sulfate proteoglycan decorin. The shared linker reaction supports this pathway participation, without assigning the later glucuronate-to-iduronate epimerization to B3GALT6. |
| GO:0120532 glycosaminoglycan-protein linkage region biosynthetic process | TAS Reactome:R-HSA-1971475 | ACCEPT | Summary: Linker-region biosynthesis is the precise core process. Reason: B3GALT6 catalyzes addition of the third sugar to the core-protein-linked oligosaccharide. This directly performs a step of the defined process. No new ancestor or parallel polymerization term is needed to summarize its role. |
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Download this section (compressed HTML)Q: How broadly does the B3GAT3 bypass observed in tested human cells operate across cell types and proteoglycan substrates, and how much residual GAG synthesis reflects variant activity versus compensation?
Q: How do loss of catalytic activity and failure of Golgi delivery separately determine the effects of individual B3GALT6 variants?
Q: Which direct B3GALT6 experiments establish metal requirements independently of the historical switched-cDNA citation flagged in UniProt?
Experiment: Compare purified wild-type and pathogenic B3GALT6 variants on defined phosphorylated and unphosphorylated Gal-Xyl acceptors, measuring protein integrity and product linkage separately from trafficking.
Experiment: Measure linker composition and GAG output in matched human B3GALT6-deficient cells with controlled B3GAT3 abundance, alongside wild-type rescue and enzyme-dead controls, to distinguish residual synthesis from compensation.
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