GALNT1 (polypeptide N-acetylgalactosaminyltransferase 1, GalNAc-T1; EC 2.4.1.41) is a Golgi-resident type II membrane glycosyltransferase that catalyses the first and committed step of mucin-type O-glycosylation. It transfers N-acetylgalactosamine from UDP-GalNAc in an alpha linkage onto the hydroxyl group of serine or threonine residues in acceptor polypeptides, creating the Tn antigen (GalNAc-alpha1-O-Ser/Thr) from which all mucin-type O-glycans are subsequently built. Catalysis requires a divalent manganese ion coordinated by the GT-A fold, and the enzyme carries a C-terminal ricin-type lectin domain that recognises already-glycosylated substrate and contributes to the dense, ordered glycan arrays characteristic of mucins. GALNT1 is one of twenty human polypeptide GalNAc-transferases, an unusually large family whose members differ in tissue expression and in peptide acceptor preference; which sites on a given protein become glycosylated is determined by the particular repertoire of isoforms a cell expresses rather than by any one enzyme. GALNT1 itself is widely expressed and is strict for UDP-GalNAc as donor. The enzyme resides in the Golgi stack membrane and undergoes COPI-independent retrograde traffic to the endoplasmic reticulum. UniProt additionally records a soluble form as a separate chain, and Golgi glycosyltransferases as a class are known to be shed by SPPL3-mediated intramembrane proteolysis, but release of a soluble GALNT1 ectodomain into the extracellular space is not established for this enzyme specifically: the validated GALNT-family substrate of that mechanism is GALNT2.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-8849348 | ACCEPT | Summary: Golgi membrane is the functional location of GALNT1. UniProt records it specifically as a Golgi stack membrane, single-pass type II membrane protein, and the enzyme initiates O-glycosylation throughout the Golgi apparatus. Reason: Correct compartment at the right specificity for a type II Golgi glycosyltransferase. Three of these four rows come from RAB6/dynein trafficking reactions in which GALNT1 features as Golgi cargo, and one from the mucin GalNAc-transfer reaction itself; all place the protein in the same, correct compartment. Supporting Evidence: PMID:9394011 In this study, we have localized endogenous and epitope-tagged human GalNAc-T1, -T2 and -T3 to the Golgi apparatus in HeLa cells by subcellular fractionation, immunofluorescence and immunoelectron microscopy. PMID:12506059 Five monoclonal antibodies raised against the GalNAc-T1, -T2, -T3, -T4, and -T6 isoenzymes, were used for immunofluorescence microscopic localization according to standard protocols |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-8849350 | ACCEPT | Summary: Golgi membrane is the functional location of GALNT1. UniProt records it specifically as a Golgi stack membrane, single-pass type II membrane protein, and the enzyme initiates O-glycosylation throughout the Golgi apparatus. Reason: Correct compartment at the right specificity for a type II Golgi glycosyltransferase. Three of these four rows come from RAB6/dynein trafficking reactions in which GALNT1 features as Golgi cargo, and one from the mucin GalNAc-transfer reaction itself; all place the protein in the same, correct compartment. Supporting Evidence: PMID:9394011 In this study, we have localized endogenous and epitope-tagged human GalNAc-T1, -T2 and -T3 to the Golgi apparatus in HeLa cells by subcellular fractionation, immunofluorescence and immunoelectron microscopy. PMID:12506059 Five monoclonal antibodies raised against the GalNAc-T1, -T2, -T3, -T4, and -T6 isoenzymes, were used for immunofluorescence microscopic localization according to standard protocols |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-8849353 | ACCEPT | Summary: Golgi membrane is the functional location of GALNT1. UniProt records it specifically as a Golgi stack membrane, single-pass type II membrane protein, and the enzyme initiates O-glycosylation throughout the Golgi apparatus. Reason: Correct compartment at the right specificity for a type II Golgi glycosyltransferase. Three of these four rows come from RAB6/dynein trafficking reactions in which GALNT1 features as Golgi cargo, and one from the mucin GalNAc-transfer reaction itself; all place the protein in the same, correct compartment. Supporting Evidence: PMID:9394011 In this study, we have localized endogenous and epitope-tagged human GalNAc-T1, -T2 and -T3 to the Golgi apparatus in HeLa cells by subcellular fractionation, immunofluorescence and immunoelectron microscopy. PMID:12506059 Five monoclonal antibodies raised against the GalNAc-T1, -T2, -T3, -T4, and -T6 isoenzymes, were used for immunofluorescence microscopic localization according to standard protocols |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-913675 | ACCEPT | Summary: Golgi membrane is the functional location of GALNT1. UniProt records it specifically as a Golgi stack membrane, single-pass type II membrane protein, and the enzyme initiates O-glycosylation throughout the Golgi apparatus. Reason: Correct compartment at the right specificity for a type II Golgi glycosyltransferase. Three of these four rows come from RAB6/dynein trafficking reactions in which GALNT1 features as Golgi cargo, and one from the mucin GalNAc-transfer reaction itself; all place the protein in the same, correct compartment. Supporting Evidence: PMID:9394011 In this study, we have localized endogenous and epitope-tagged human GalNAc-T1, -T2 and -T3 to the Golgi apparatus in HeLa cells by subcellular fractionation, immunofluorescence and immunoelectron microscopy. PMID:12506059 Five monoclonal antibodies raised against the GalNAc-T1, -T2, -T3, -T4, and -T6 isoenzymes, were used for immunofluorescence microscopic localization according to standard protocols |
| GO:0004653 polypeptide N-acetylgalactosaminyltransferase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic propagation across a very large, well-sampled node: the WITH/FROM field spans ten Drosophila genes, six mouse genes, three C. elegans genes, Xenopus, rat and fourteen human GALNT paralogues. The polypeptide GalNAc-transferase family is ancient and its defining activity is shared across it. Reason: The node placement is exceptionally well supported and the term is at the correct specificity for the family-level activity. The target's own accession appearing among the sources is expected: GALNT1's direct assays are among the descendant evidences the PAINT curator used, so its presence marks experimental grounding on the target rather than circularity. Note that this IBA asserts the shared family activity, not GALNT1's individual acceptor preference, which is what distinguishes the paralogues from one another. Supporting Evidence: PMID:22186971 The formation of mucin-type O-glycans is initiated by an evolutionarily conserved family of enzymes, the UDP-N-acetyl-Ξ±-D-galactosamine:polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts) |
| GO:0004653 polypeptide N-acetylgalactosaminyltransferase activity | IDA PMID:16638743 Polypeptide GalNAc-transferase T3 and familial tumoral calci... | ACCEPT | Summary: Direct-assay evidence from a paper whose title and abstract foreground GalNAc-T3 and its role in FGF23 secretion. The cached record is abstract-only, so the GALNT1 measurement is not visible here; studies from this group routinely assay panels of GalNAc-transferases to establish that an effect is isoform-selective, and a GalNAc-T1 comparator is the expected control for the claim that GalNAc-T3 'selectively directs' the modification. Reason: The curator making this annotation read the full text, which is not in our cache. The selectivity claim in the abstract only makes sense against other isoforms, so a GALNT1 assay is very likely present. In any case the asserted activity is correct for this gene beyond doubt, being independently established by three other direct assays, by EC mapping and by phylogeny. Deferring to the curator is the right call; there is no basis here for alleging mis-attribution. Supporting Evidence: PMID:16638743 GalNAc-T3 selectively directs O-glycosylation in a subtilisin-like proprotein convertase recognition sequence motif, which blocks processing of FGF23 |
| GO:0004653 polypeptide N-acetylgalactosaminyltransferase activity | IDA PMID:22186971 UDP-N-acetyl-Ξ±-D-galactosamine:polypeptide N-acetylgalactosa... | ACCEPT | Summary: Direct assay of polypeptide GalNAc-transferase activity, from the family-completion study, which assayed the previously uncharacterised isoforms alongside established ones including GalNAc-T1. Reason: This is the gene's core molecular function, established by direct enzymatic assay. The term matches the assayed reaction exactly: transfer of GalNAc from UDP-GalNAc to serine and threonine residues of peptide acceptors. Supporting Evidence: PMID:22186971 The formation of mucin-type O-glycans is initiated by an evolutionarily conserved family of enzymes, the UDP-N-acetyl-Ξ±-D-galactosamine:polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts) |
| GO:0004653 polypeptide N-acetylgalactosaminyltransferase activity | IDA PMID:8690719 cDNA cloning, expression, and chromosomal localization of a ... | ACCEPT | Summary: Direct assay of polypeptide GalNAc-transferase activity, from the cDNA cloning and expression paper for this transferase. Reason: This is the gene's core molecular function, established by direct enzymatic assay. The term matches the assayed reaction exactly: transfer of GalNAc from UDP-GalNAc to serine and threonine residues of peptide acceptors. Supporting Evidence: PMID:22186971 The formation of mucin-type O-glycans is initiated by an evolutionarily conserved family of enzymes, the UDP-N-acetyl-Ξ±-D-galactosamine:polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts) |
| GO:0004653 polypeptide N-acetylgalactosaminyltransferase activity | IDA PMID:9295285 Substrate specificities of three members of the human UDP-N-... | ACCEPT | Summary: Direct assay of polypeptide GalNAc-transferase activity, from the definitive substrate-specificity study, in which GalNAc-T1, -T2 and -T3 were expressed as soluble proteins in insect cells, purified to near homogeneity and assayed on peptide acceptors including MUC1 tandem repeats. Reason: This is the gene's core molecular function, established by direct enzymatic assay. The term matches the assayed reaction exactly: transfer of GalNAc from UDP-GalNAc to serine and threonine residues of peptide acceptors. Supporting Evidence: PMID:9295285 The enzymes have distinct but partly overlapping specificities with short peptide acceptor substrates PMID:9295285 GalNAc-T1 and -T3 showed strict donor substrate specificities for UDP-GalNAc |
| GO:0004653 polypeptide N-acetylgalactosaminyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Automated assignment combining EC 2.4.1.41, an ARBA rule and orthology to the mouse and rat enzymes. EC 2.4.1.41 is the polypeptide N-acetylgalactosaminyltransferase activity, matching the assayed reaction. Reason: Reaction- and orthology-based mapping in agreement with four independent direct assays, at the correct specificity. Supporting Evidence: PMID:9295285 The enzymes have distinct but partly overlapping specificities with short peptide acceptor substrates PMID:9295285 GalNAc-T1 and -T3 showed strict donor substrate specificities for UDP-GalNAc |
| GO:0004653 polypeptide N-acetylgalactosaminyltransferase activity | TAS PMID:7592619 Purification and cDNA cloning of a human UDP-N-acetyl-alpha-... | ACCEPT | Summary: Author statement from the purification and cDNA cloning paper for this transferase. Reason: Consistent with the direct experimental evidence; a traceable author statement for the same, correct activity. Supporting Evidence: PMID:22186971 The formation of mucin-type O-glycans is initiated by an evolutionarily conserved family of enzymes, the UDP-N-acetyl-Ξ±-D-galactosamine:polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts) |
| GO:0004653 polypeptide N-acetylgalactosaminyltransferase activity | TAS Reactome:R-HSA-913675 | ACCEPT | Summary: Reactome's statement of GalNAc transfer, in the mucin Tn-antigen reaction and in the SARS-CoV-2 protein 3a glycosylation reaction respectively. Reason: Correct activity at the right specificity. The activity is the same regardless of which acceptor protein a given Reactome reaction happens to model. Supporting Evidence: PMID:9295285 The enzymes have distinct but partly overlapping specificities with short peptide acceptor substrates PMID:9295285 GalNAc-T1 and -T3 showed strict donor substrate specificities for UDP-GalNAc |
| GO:0004653 polypeptide N-acetylgalactosaminyltransferase activity | TAS Reactome:R-HSA-9683760 | ACCEPT | Summary: Reactome's statement of GalNAc transfer, in the mucin Tn-antigen reaction and in the SARS-CoV-2 protein 3a glycosylation reaction respectively. Reason: Correct activity at the right specificity. The activity is the same regardless of which acceptor protein a given Reactome reaction happens to model. Supporting Evidence: PMID:9295285 The enzymes have distinct but partly overlapping specificities with short peptide acceptor substrates PMID:9295285 GalNAc-T1 and -T3 showed strict donor substrate specificities for UDP-GalNAc |
| GO:0005576 extracellular region | IEA GO_REF:0000044 | MARK AS OVER ANNOTATED | Summary: Mapped from the UniProt Secreted subcellular-location keyword via GO_REF:0000044. UniProt records a curated 'Polypeptide N-acetylgalactosaminyltransferase 1 soluble form' chain (residues 41-559, PRO_0000012257) cleaved in the stem just after the transmembrane anchor, and the Secreted location is attached to that chain. That location line carries no evidence code. The same is true of the competing Golgi stack membrane location line, so the absence of a code does not by itself separate the two; what does is that the Golgi location is independently established experimentally while the secreted one is not. Reason: An earlier draft of this review accepted this row, reasoning that UniProt models the soluble form as a separate chain rather than as an alternative location for the intact enzyme, and that this made it a deliberate curatorial act rather than an artefact. That reasoning treated the chain as evidence of shedding when it is really an assertion of it. Three things count against the annotation. First, the UniProt 'Secreted' line has no ECO evidence code, so nothing in the entry points to an experiment for it. (The Golgi location line is likewise uncoded, so this is not a contrast between the two lines; the difference is that the Golgi location has independent experimental support, in PMID:9394011, and the secreted form has none.) Second, no GALNT1-specific secretion or extracellular-localisation study could be found. Third, and most tellingly, the shedding mechanism that would justify the term has been characterised for this enzyme family, and GALNT1 is not among the enzymes it was shown for: SPPL3-mediated intramembrane proteolysis releases soluble Golgi enzymes into blood, and the GALNT family member validated by immunoblot as an SPPL3 substrate in that work is GALNT2. GALNT1 is not mentioned in it. The term is therefore not wrong in principle, since Golgi glycosyltransferases genuinely are shed, but as applied to this gene it runs ahead of the evidence. Retained rather than removed because the mechanism is real and a shed GALNT1 species remains plausible; marked over-annotated so it does not read as an established location. Supporting Evidence: PMID:35279766 Secretion of soluble Golgi enzymes that are released from their membrane anchor by endoprotease activity is a wide-spread yet largely unexplored phenomenon PMID:35279766 secretion of select novel substrates such as the key mucin-type O-glycosylation enzyme GALNT2 is dependent on endogenous SPPL3 protease activity file:human/GALNT1/GALNT1-hypotheses/function-hypothesis-go-0005576/openscientist.md UniProt attaches that "Secreted" keyword **without any PubMed evidence** of its own |
| GO:0005789 endoplasmic reticulum membrane | TAS Reactome:R-HSA-8849353 | KEEP AS NON CORE | Summary: This row comes from the Reactome reaction 'Dynein drives COPI-independent retrograde traffic from the Golgi to the ER', in which GALNT1 appears as retrograde cargo. It records a compartment the protein passes into during retrograde transport, not the compartment in which it does its work. Reason: Retrograde relocation of GalNAc-transferases to the ER is a real and biologically interesting phenomenon, so the annotation should not be discarded. But it describes a trafficking itinerary rather than the enzyme's functional site: UniProt places the active enzyme in the Golgi stack membrane, and every direct assay and localisation study concerns the Golgi pool. Kept as a genuine but non-core location. Supporting Evidence: PMID:9394011 In this study, we have localized endogenous and epitope-tagged human GalNAc-T1, -T2 and -T3 to the Golgi apparatus in HeLa cells by subcellular fractionation, immunofluorescence and immunoelectron microscopy. PMID:12506059 Five monoclonal antibodies raised against the GalNAc-T1, -T2, -T3, -T4, and -T6 isoenzymes, were used for immunofluorescence microscopic localization according to standard protocols |
| GO:0005794 Golgi apparatus | IDA PMID:12506059 The cell-layer- and cell-type-specific distribution of GalNA... | ACCEPT | Summary: Immunofluorescence localisation using a monoclonal antibody panel raised against the GalNAc-T1, -T2, -T3, -T4 and -T6 isoenzymes in ocular surface epithelia. The colocalizes_with relationship is appropriately hedged for an immunofluorescence overlap. Reason: Correct compartment, supported by isoform-specific antibodies, and consistent with UniProt's Golgi stack membrane assignment and with the enzyme's function. The hedged qualifier suits the evidence type. Supporting Evidence: PMID:9394011 In this study, we have localized endogenous and epitope-tagged human GalNAc-T1, -T2 and -T3 to the Golgi apparatus in HeLa cells by subcellular fractionation, immunofluorescence and immunoelectron microscopy. PMID:12506059 Five monoclonal antibodies raised against the GalNAc-T1, -T2, -T3, -T4, and -T6 isoenzymes, were used for immunofluorescence microscopic localization according to standard protocols |
| GO:0006493 protein O-linked glycosylation | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic propagation of the O-linked glycosylation process across the same broad GalNAc-transferase node used for the molecular function. The family's defining role is to initiate mucin-type O-glycosylation. Reason: Correct process, well-placed node, and GALNT1 passes the participation test directly: it catalyses the first committed step of the pathway rather than merely being required for it. Supporting Evidence: PMID:12506059 The initial step in O-glycosylation is the enzymatic addition of N-acetyl galactosamine (GalNAc) to serine and threonine residues by a large family of polypeptide GalNAc-transferases (GalNAc-Ts) |
| GO:0006493 protein O-linked glycosylation | ISS GO_REF:0000024 | ACCEPT | Summary: Curator-judged sequence-similarity transfer from UniProtKB:Q07537, a characterised GalNAc-transferase family member with the same process annotation. Reason: Sound similarity transfer of a process that is independently established for this gene by two direct assays and by phylogeny. The donor is a functionally characterised member of the same family. Supporting Evidence: PMID:12506059 The initial step in O-glycosylation is the enzymatic addition of N-acetyl galactosamine (GalNAc) to serine and threonine residues by a large family of polypeptide GalNAc-transferases (GalNAc-Ts) |
| GO:0006493 protein O-linked glycosylation | TAS PMID:9592121 Genomic organization and chromosomal localization of three m... | ACCEPT | Summary: Author statement from the study of genomic organisation and chromosomal localisation of GalNAc-transferase family members. Reason: Correct process assignment; redundant with stronger evidence on the same gene but not incorrect. Supporting Evidence: PMID:12506059 The initial step in O-glycosylation is the enzymatic addition of N-acetyl galactosamine (GalNAc) to serine and threonine residues by a large family of polypeptide GalNAc-transferases (GalNAc-Ts) |
| GO:0016020 membrane | HDA PMID:19946888 Defining the membrane proteome of NK cells. | MODIFY | Summary: From a high-throughput proteomic definition of the NK cell membrane proteome. The detection is credible for a type II membrane protein, but the term names no organelle and the study was not designed to resolve one. Reason: The bare membrane term is among the least informative in the ontology, and this gene's compartment is established specifically: UniProt assigns the Golgi stack membrane, immunofluorescence with isoform-specific antibodies places it in the Golgi, and four Reactome rows agree. A membrane-proteome catalogue confirms membrane association without adding compartment information, so the row should be collapsed to the specific term. Proposed replacements: Golgi membrane Supporting Evidence: PMID:9394011 In this study, we have localized endogenous and epitope-tagged human GalNAc-T1, -T2 and -T3 to the Golgi apparatus in HeLa cells by subcellular fractionation, immunofluorescence and immunoelectron microscopy. PMID:12506059 Five monoclonal antibodies raised against the GalNAc-T1, -T2, -T3, -T4, and -T6 isoenzymes, were used for immunofluorescence microscopic localization according to standard protocols |
| GO:0016266 protein O-linked glycosylation via N-acetylgalactosamine | IDA PMID:22186971 UDP-N-acetyl-Ξ±-D-galactosamine:polypeptide N-acetylgalactosa... | ACCEPT | Summary: Direct assay of transfer of GalNAc onto serine and threonine residues of peptide acceptors, which is by definition the initiating step of GalNAc-linked mucin-type O-glycosylation. Reason: The process term matches the assayed chemistry precisely, and it is more informative than the parent GO:0006493 because the linking sugar is known to be GalNAc. GALNT1 performs this step itself. Supporting Evidence: PMID:22186971 The formation of mucin-type O-glycans is initiated by an evolutionarily conserved family of enzymes, the UDP-N-acetyl-Ξ±-D-galactosamine:polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts) |
| GO:0016266 protein O-linked glycosylation via N-acetylgalactosamine | IDA PMID:9295285 Substrate specificities of three members of the human UDP-N-... | ACCEPT | Summary: Direct assay of transfer of GalNAc onto serine and threonine residues of peptide acceptors, which is by definition the initiating step of GalNAc-linked mucin-type O-glycosylation. Reason: The process term matches the assayed chemistry precisely, and it is more informative than the parent GO:0006493 because the linking sugar is known to be GalNAc. GALNT1 performs this step itself. Supporting Evidence: PMID:9295285 The enzymes have distinct but partly overlapping specificities with short peptide acceptor substrates PMID:9295285 GalNAc-T1 and -T3 showed strict donor substrate specificities for UDP-GalNAc |
| GO:0016266 protein O-linked glycosylation via N-acetylgalactosamine | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA rule-based assignment of the GalNAc-specific O-glycosylation process. Reason: The rule lands on the correct and appropriately specific process, matching the direct experimental evidence. Supporting Evidence: PMID:12506059 The initial step in O-glycosylation is the enzymatic addition of N-acetyl galactosamine (GalNAc) to serine and threonine residues by a large family of polypeptide GalNAc-transferases (GalNAc-Ts) |
| GO:0016266 protein O-linked glycosylation via N-acetylgalactosamine | TAS Reactome:R-HSA-913709 | ACCEPT | Summary: Reactome's 'O-linked glycosylation of mucins' pathway statement. Reason: Correct process at the right specificity, consistent with the direct evidence. Supporting Evidence: PMID:12506059 The initial step in O-glycosylation is the enzymatic addition of N-acetyl galactosamine (GalNAc) to serine and threonine residues by a large family of polypeptide GalNAc-transferases (GalNAc-Ts) |
| GO:0019082 viral protein processing | TAS Reactome:R-HSA-9683673 | MARK AS OVER ANNOTATED | Summary: This row derives from the Reactome reaction 'Maturation of protein 3a', part of the SARS-CoV-2 infection pathway, in which GALNT1 O-glycosylates the viral ORF3a protein. The underlying fact is that a host glycosyltransferase modifies a viral substrate. Reason: Viral protein processing is a process of the viral replication cycle. GALNT1 does not acquire a new activity when its acceptor happens to be virally encoded; it performs the same GalNAc transfer it performs on host mucins, and the O-glycosylation it carries out is already annotated. Attributing the viral process to the host enzyme is the guilt-by-substrate pattern applied to an infection pathway model, and it would make a constitutively expressed Golgi enzyme appear in queries for virus-specific functions. The modification event itself is not disputed. Supporting Evidence: PMID:9295285 The enzymes have distinct but partly overlapping specificities with short peptide acceptor substrates PMID:9295285 GalNAc-T1 and -T3 showed strict donor substrate specificities for UDP-GalNAc |
| GO:0030145 manganese ion binding | IDA PMID:9295285 Substrate specificities of three members of the human UDP-N-... | ACCEPT | Summary: Manganese ion binding assayed alongside the transferase activity. GalNAc-transferases have a GT-A fold with a DXD motif that coordinates a divalent metal, and UniProt records that the enzyme's N-terminal domain A (GT1 motif) is involved in manganese coordination and substrate binding. Reason: A genuine cofactor requirement rather than incidental metal binding: Mn2+ coordination is mechanistically necessary for nucleotide-sugar donor positioning in this enzyme class. Correct and informative. Supporting Evidence: PMID:9295285 The enzymes have distinct but partly overlapping specificities with short peptide acceptor substrates PMID:9295285 GalNAc-T1 and -T3 showed strict donor substrate specificities for UDP-GalNAc |
| GO:0032580 Golgi cisterna membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Mapped from the UniProt Golgi stack membrane subcellular-location keyword, which is the compartment UniProt assigns to the membrane-bound enzyme. Reason: More specific than the general Golgi membrane term and directly supported by the UniProt location for this chain. Retaining both is appropriate, since they describe the same location at different granularity. Supporting Evidence: PMID:9394011 In this study, we have localized endogenous and epitope-tagged human GalNAc-T1, -T2 and -T3 to the Golgi apparatus in HeLa cells by subcellular fractionation, immunofluorescence and immunoelectron microscopy. PMID:12506059 Five monoclonal antibodies raised against the GalNAc-T1, -T2, -T3, -T4, and -T6 isoenzymes, were used for immunofluorescence microscopic localization according to standard protocols |
| GO:0033116 endoplasmic reticulum-Golgi intermediate compartment membrane | TAS Reactome:R-HSA-9683760 | KEEP AS NON CORE | Summary: From the Reactome reaction 'GalNAc is transferred onto 3a', part of the SARS-CoV-2 pathway, which models GALNT1 acting on the viral ORF3a protein in the ER-Golgi intermediate compartment. Reason: Unlike the viral protein processing row, this is a location rather than a process claim, and GalNAc-transferases do cycle through the ERGIC. The compartment is plausible and specific to the modelled infection context, so it is worth keeping, but the enzyme's core functional location remains the Golgi stack membrane where its constitutive mucin-type substrates are glycosylated. Supporting Evidence: PMID:9394011 In this study, we have localized endogenous and epitope-tagged human GalNAc-T1, -T2 and -T3 to the Golgi apparatus in HeLa cells by subcellular fractionation, immunofluorescence and immunoelectron microscopy. PMID:12506059 Five monoclonal antibodies raised against the GalNAc-T1, -T2, -T3, -T4, and -T6 isoenzymes, were used for immunofluorescence microscopic localization according to standard protocols |
| GO:0048471 perinuclear region of cytoplasm | IDA PMID:12506059 The cell-layer- and cell-type-specific distribution of GalNA... | ACCEPT | Summary: Perinuclear cytoplasmic localisation observed by immunofluorescence with the isoform-specific antibody panel, consistent with the juxtanuclear position of the Golgi ribbon in the epithelial cells examined. Reason: Correct as an observation, and not in conflict with the Golgi annotations; the perinuclear region is where the Golgi sits in these cells. It is the least informative of this gene's location annotations, but it is accurate and reflects what was seen. Supporting Evidence: PMID:9394011 In this study, we have localized endogenous and epitope-tagged human GalNAc-T1, -T2 and -T3 to the Golgi apparatus in HeLa cells by subcellular fractionation, immunofluorescence and immunoelectron microscopy. PMID:12506059 Five monoclonal antibodies raised against the GalNAc-T1, -T2, -T3, -T4, and -T6 isoenzymes, were used for immunofluorescence microscopic localization according to standard protocols |
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Download this section (compressed HTML)Q: Should host glycosyltransferases acquire viral-process annotations when a pathway model includes them modifying a viral protein? GALNT1 carries viral protein processing solely because Reactome models it glycosylating SARS-CoV-2 ORF3a. The enzyme's activity is unchanged by the origin of its acceptor, and the same reasoning would give every constitutively expressed Golgi enzyme a virus-specific annotation.
Q: How should acceptor specificity be represented for the twenty human GalNAc-transferases? The family IBA correctly asserts the shared transferase activity, but what distinguishes these paralogues is which peptide sites each prefers, and that distinction is currently invisible in GO. This is the axis on which paralogue over-annotation in this family would occur.
Q: Should GO:0005576 be retained on GALNT1 at all? The annotation derives entirely from an evidence-free UniProt 'Secreted' keyword attached to a curated soluble-form chain, and the SPPL3 shedding work that characterises this phenomenon for Golgi enzymes validates GALNT2 rather than GALNT1. Only 2 of 10 human GALNT paralogues carry the term, so it is not a family-wide convention either.
Experiment: Isoform-resolved glycoproteomics in cells lacking GALNT1 alone versus combinations of GALNT paralogues, to establish which O-glycosylation sites depend specifically on this enzyme rather than on the family as a whole.
Experiment: Quantify the functional significance of the secreted soluble GalNAc-T1 ectodomain: whether it is catalytically active in the extracellular space on physiological acceptors, or whether the extracellular region annotation reflects shedding without a function there.
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