Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0003835 beta-galactoside alpha-2,6-sialyltransferase activity	IBA GO_REF:0000033	ACCEPT	Summary: This is the core molecular function of ST6GAL1 and is correct at the most specific level. Phylogenetic (IBA) inference here agrees with extensive direct experimental evidence (crystal structure, kinetics, and multiple in vitro activity assays) and with the UniProt/Rhea catalytic activity (RHEA:52104, EC 2.4.3.1). Supporting Evidence: PMID:23999306 Human beta-galactoside alpha-2,6-sialyltransferase I (ST6Gal-I) establishes the final glycosylation pattern of many glycoproteins by transferring a sialyl moiety to a terminal galactose.
GO:0006487 protein N-linked glycosylation	IBA GO_REF:0000033	ACCEPT	Summary: Accurate and appropriately specific. ST6GAL1's principal biological role is the terminal alpha-2,6-sialylation (capping) step of complex-type N-glycan maturation. Supported independently by IDA (PMID:23999306) and consistent with the UniProt-annotated complex N-glycan acceptor reactions.
GO:0097503 sialylation	IBA GO_REF:0000033	ACCEPT	Summary: Correct. Sialylation is the process directly performed by ST6GAL1. Also independently supported by IDA (PMID:23999306). A reasonable core process term, slightly broader than the specific alpha-2,6 linkage but accurate.
GO:0005794 Golgi apparatus	IBA GO_REF:0000033	ACCEPT	Summary: Correct core localization. ST6GAL1 is a trans-Golgi resident enzyme; the IBA call agrees with direct evidence for Golgi membrane localization (PMID:20378551) and UniProt (Golgi stack membrane, trans cisternae).
GO:0003835 beta-galactoside alpha-2,6-sialyltransferase activity	IEA GO_REF:0000120	ACCEPT	Summary: Duplicate of the core catalytic MF from an automated (multi-method IEA) source mapped to EC 2.4.3.1 / RHEA. Correct; redundant with the IBA, EXP, IDA and TAS annotations to the same term.
GO:0005576 extracellular region	IEA GO_REF:0000044	KEEP AS NON CORE	Summary: Reflects the soluble form of ST6GAL1, which is shed from the Golgi/membrane and circulates in serum, where it can sialylate glycoproteins extrinsically. This is a genuine secondary localization (UniProt: Secreted, by similarity to rat P13721) but is secondary to the Golgi-resident catalytic site of action; kept as non-core. Supporting Evidence: file:human/ST6GAL1/ST6GAL1-deep-research-falcon.md BACE1 (β-site amyloid precursor protein-cleaving enzyme 1) cleaves ST6GAL1 to release a soluble, catalytically active form lacking the transmembrane domain, enabling secretion and extracellular sialylation activity
GO:0008373 sialyltransferase activity	IEA GO_REF:0000120	MODIFY	Summary: Correct but over-general parent term. The specific activity beta-galactoside alpha-2,6-sialyltransferase activity (GO:0003835) is experimentally established and should be used instead. Proposed replacements: beta-galactoside alpha-2,6-sialyltransferase activity
GO:0009101 glycoprotein biosynthetic process	IEA GO_REF:0000002	KEEP AS NON CORE	Summary: Not wrong but a broad parent of the more informative N-linked glycosylation / sialylation process annotations already present. Retained as a non-core, higher-level descriptor (InterPro2GO electronic inference).
GO:0032580 Golgi cisterna membrane	IEA GO_REF:0000044	ACCEPT	Summary: Correct and appropriately specific localization (UniProt subcellular-location mapping). ST6GAL1 is a single-pass type II membrane protein of the Golgi stack cisternae (trans cisternae), consistent with PMID:20378551 and UniProt.
GO:0006487 protein N-linked glycosylation	IEA GO_REF:0000107	ACCEPT	Summary: Duplicate of the core N-linked glycosylation process annotation, transferred electronically from the mouse ortholog (Q64685). Correct.
GO:0016266 protein O-linked glycosylation via N-acetylgalactosamine	TAS Reactome:R-HSA-977068	KEEP AS NON CORE	Summary: ST6GAL1 is predominantly an N-glycan alpha-2,6-sialyltransferase, but Reactome documents a minor activity terminating O-GalNAc (mucin-type) glycans, including conversion of Tn to sialyl-Tn antigen. This O-glycan role is real but minor and context-specific relative to the dominant N-glycan function; kept as non-core.
GO:0003835 beta-galactoside alpha-2,6-sialyltransferase activity	TAS Reactome:R-HSA-4085033	ACCEPT	Summary: Core catalytic MF (Reactome reaction "ST6GAL1,2 transfer Neu5Ac to terminal Gal (alpha-2,6 link)"). Correct; redundant with the experimentally supported annotations to the same term.
GO:0003835 beta-galactoside alpha-2,6-sialyltransferase activity	TAS Reactome:R-HSA-975902	ACCEPT	Summary: Core catalytic MF (Reactome "ST6GAL1 transfers Neu5Ac to terminal Gal of N-glycans"). Correct; redundant duplicate.
GO:0003835 beta-galactoside alpha-2,6-sialyltransferase activity	TAS Reactome:R-HSA-977071	ACCEPT	Summary: Core catalytic MF; this Reactome reaction concerns transfer of sialic acid to Tn antigens to form sialyl-Tn. The activity is correct (same enzyme), though the O-GalNAc/sTn acceptor is a minor context relative to the N-glycan role.
GO:0003835 beta-galactoside alpha-2,6-sialyltransferase activity	EXP PMID:25733881 Controlled tetra-Fc sialylation of IVIg results in a drug ca...	ACCEPT	Summary: Strong experimental support for the core catalytic activity: controlled alpha-2,6-sialylation of IgG Fc N-glycans by ST6GAL1 to generate tetra-Fc-sialylated IVIg with enhanced anti-inflammatory activity. Supporting Evidence: PMID:25733881 Controlled tetra-Fc sialylation of IVIg results in a drug candidate with consistent enhanced anti-inflammatory activity.
GO:0003835 beta-galactoside alpha-2,6-sialyltransferase activity	EXP PMID:36280670 A universal glycoenzyme biosynthesis pipeline that enables e...	ACCEPT	Summary: Strong experimental support for the core catalytic activity (cell-free glycoenzyme remodeling pipeline using recombinant ST6GAL1 to install alpha-2,6 sialic acid; UniProt-cited EC 2.4.3.1 evidence). Supporting Evidence: PMID:36280670 A universal glycoenzyme biosynthesis pipeline that enables efficient cell-free remodeling of glycans.
GO:0003835 beta-galactoside alpha-2,6-sialyltransferase activity	EXP PMID:37632720 Divergent Enzymatic Assembly of a Comprehensive 64-Membered ...	ACCEPT	Summary: Strong experimental support for the core catalytic activity (divergent enzymatic assembly of a 64-membered IgG N-glycan library using ST6GAL1 for alpha-2,6 sialylation; UniProt-cited EC 2.4.3.1 evidence). Supporting Evidence: PMID:37632720 Divergent Enzymatic Assembly of a Comprehensive 64-Membered IgG N-Glycan Library for Functional Glycomics.
GO:0005576 extracellular region	ISS GO_REF:0000024	KEEP AS NON CORE	Summary: Sequence-similarity transfer (from rat ortholog P13721) of the secreted/soluble localization. Same biological content as the IEA extracellular-region annotation; the soluble serum form is genuine but secondary to the Golgi catalytic site. Kept as non-core.
GO:0019082 viral protein processing	TAS Reactome:R-HSA-9683673	MARK AS OVER ANNOTATED	Summary: This annotation comes from Reactome SARS-CoV-2 pathways in which cellular sialyltransferases glycosylate viral proteins (here ORF3a). The underlying molecular event is ST6GAL1's ordinary host-glycosylation activity; framing a housekeeping glycosylation step as a dedicated "viral protein processing" function over-extends the gene's role. The enzyme is not virus-specific.
GO:0019082 viral protein processing	TAS Reactome:R-HSA-9694548	MARK AS OVER ANNOTATED	Summary: Same as above (Reactome SARS-CoV-2 spike maturation). The molecular event is generic host sialylation of viral glycoproteins; "viral protein processing" over-extends ST6GAL1's role. The Reactome curation even states the spike sialic acids are added "presumably by the cell's sialyltransferases".
GO:0008373 sialyltransferase activity	TAS Reactome:R-HSA-9683769	MODIFY	Summary: Over-general parent MF asserted within a SARS-CoV-2 O-glycosylation Reactome reaction. The specific activity GO:0003835 is established for ST6GAL1 and should be used instead of the parent sialyltransferase activity term. Proposed replacements: beta-galactoside alpha-2,6-sialyltransferase activity
GO:0008373 sialyltransferase activity	TAS Reactome:R-HSA-9697018	MODIFY	Summary: Over-general parent MF asserted within a SARS-CoV-2 spike sialylation Reactome reaction. Replace with the specific GO:0003835 established for ST6GAL1. Proposed replacements: beta-galactoside alpha-2,6-sialyltransferase activity
GO:0000139 Golgi membrane	IDA PMID:20378551 Golgi N-glycosyltransferases form both homo- and heterodimer...	ACCEPT	Summary: Direct, live-cell evidence (BiFC and immunofluorescence with a Golgi marker) that ST6GAL1 localizes to Golgi membranes. This is the core localization and the site of catalytic action. Supporting Evidence: PMID:20378551 BiFC signal was detected almost exclusively in the Golgi membranes of the cells.
GO:0042803 protein homodimerization activity	IDA PMID:20378551 Golgi N-glycosyltransferases form both homo- and heterodimer...	ACCEPT	Summary: Well supported by direct evidence: ST6GAL1 forms disulfide-mediated and non-covalent homodimers in live Golgi membranes (BiFC; >90% of molecules homodimeric), independent of the TM cysteine (C24G). A genuine molecular function that organizes the enzyme in the Golgi, though distinct from its core catalytic activity. Supporting Evidence: PMID:20378551 over 90% of the SiaT molecules appear to be homodimeric in the Golgi membranes of live cells
GO:0005515 protein binding	IPI PMID:16237761 Screening of hepatocyte proteins binding to F protein of hep...	REMOVE	Summary: Uninformative generic protein-binding annotation derived from a yeast two-hybrid screen for hepatocyte proteins binding the hepatitis C virus F protein, in which "1 colony was sialyltransferase" among 36 colonies, with no follow-up validation specific to ST6GAL1. Per curation guidance, generic "protein binding" with weak, non-physiological Y2H evidence and no informative partner-specific function should not be retained.
GO:0003835 beta-galactoside alpha-2,6-sialyltransferase activity	IDA PMID:23999306 The structure of human α-2,6-sialyltransferase reveals the b...	ACCEPT	Summary: Direct experimental support for the core catalytic activity from the human ST6Gal-I crystal structure paper (mechanism, CMP product/inhibitor complexes, glycan binding mode). Strongest single-paper evidence for GO:0003835. Supporting Evidence: PMID:23999306 ST6Gal-I establishes the final glycosylation pattern of many glycoproteins by transferring a sialyl moiety to a terminal galactose.
GO:0006054 N-acetylneuraminate metabolic process	IDA PMID:23999306 The structure of human α-2,6-sialyltransferase reveals the b...	KEEP AS NON CORE	Summary: Correct: by consuming CMP-Neu5Ac and transferring Neu5Ac onto acceptors, ST6GAL1 participates in sialic-acid (N-acetylneuraminate) metabolism. Slightly broad but accurate; retained as a non-core process descriptor alongside the more specific sialylation term.
GO:0006487 protein N-linked glycosylation	IDA PMID:23999306 The structure of human α-2,6-sialyltransferase reveals the b...	ACCEPT	Summary: Direct support for the core process: the structure paper characterizes ST6Gal-I acting on complex N-glycan acceptors. Correct and specific. Supporting Evidence: PMID:23999306 the binding mode of complex glycans
GO:0097503 sialylation	IDA PMID:23999306 The structure of human α-2,6-sialyltransferase reveals the b...	ACCEPT	Summary: Direct support for the core sialylation process. Correct; consistent with the IBA annotation to the same term. Supporting Evidence: PMID:23999306 transferring a sialyl moiety to a terminal galactose
GO:0000139 Golgi membrane	TAS Reactome:R-HSA-4085033	ACCEPT	Summary: Core Golgi-membrane localization (Reactome). Correct; redundant with the IDA Golgi membrane annotation.
GO:0000139 Golgi membrane	TAS Reactome:R-HSA-9683769	ACCEPT	Summary: Core Golgi-membrane localization asserted in a SARS-CoV-2 O-glycosylation Reactome reaction. The localization itself is correct (redundant duplicate).
GO:0000139 Golgi membrane	TAS Reactome:R-HSA-9697018	ACCEPT	Summary: Core Golgi-membrane localization asserted in a SARS-CoV-2 spike sialylation Reactome reaction. Correct; redundant duplicate.
GO:0000139 Golgi membrane	TAS Reactome:R-HSA-975902	ACCEPT	Summary: Core Golgi-membrane localization (Reactome N-glycan sialylation reaction). Correct; redundant duplicate.
GO:0000139 Golgi membrane	TAS Reactome:R-HSA-977071	ACCEPT	Summary: Core Golgi-membrane localization (Reactome sialyl-Tn reaction). Correct; redundant duplicate.
GO:0008373 sialyltransferase activity	IDA PMID:21081508 Universal phosphatase-coupled glycosyltransferase assay.	MODIFY	Summary: Direct biochemical measurement of ST6GAL1 kinetics (phosphatase-coupled assay; KM ~530 uM for CMP-NeuAc) demonstrates sialyltransferase activity, but the term is the over-general parent. Modify to the specific beta-galactoside alpha-2,6-sialyltransferase activity (GO:0003835). Proposed replacements: beta-galactoside alpha-2,6-sialyltransferase activity

Core Functions

ST6GAL1 catalyzes the terminal alpha-2,6-sialylation of complex-type N-glycans: transfer of N-acetylneuraminic acid from CMP-Neu5Ac onto the terminal beta-1,4-linked galactose of N-glycan antennae, forming the Neu5Ac-alpha-2,6-Gal-beta1,4-GlcNAc (Sia-alpha-2,6-Gal / CD75) epitope. This is the major route to alpha-2,6-sialylated N-glycans and the capping step of complex N-glycan biosynthesis. The enzyme acts as a Golgi-resident type II membrane protein and uses an SN2 inversion mechanism (inhibited by CTP).

Molecular Function:

beta-galactoside alpha-2,6-sialyltransferase activity

Directly Involved In:

sialylation protein N-linked glycosylation

Cellular Locations:

Golgi membrane Golgi cisterna membrane

Substrates:

CMP-N-acetyl-beta-neuraminate

Supporting Evidence:

PMID:23999306
Human beta-galactoside alpha-2,6-sialyltransferase I (ST6Gal-I) establishes the final glycosylation pattern of many glycoproteins by transferring a sialyl moiety to a terminal galactose.
PMID:21081508
we measured the kinetic parameters of human sialyltransferase ST6GAL1
PMID:25733881
Controlled tetra-Fc sialylation of IVIg results in a drug candidate with

ST6GAL1 forms homodimers in the Golgi membrane (disulfide-mediated and non-covalent), with the large majority of molecules dimeric in live cells; it also assembles into a trans-Golgi heterodimer with beta-1,4-galactosyltransferase 1. This self-association organizes the enzyme at its trans-Golgi site of action.

Molecular Function:

protein homodimerization activity

Cellular Locations:

Golgi membrane

Supporting Evidence:

PMID:20378551
over 90% of the SiaT molecules appear to be homodimeric in the Golgi membranes of live cells

References

GO_REF:0000002

Gene Ontology annotation through association of InterPro records with GO terms

GO_REF:0000024

Manual transfer of experimentally-verified manual GO annotation data to orthologs by curator judgment of sequence similarity

GO_REF:0000033

Annotation inferences using phylogenetic trees

GO_REF:0000044

Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt

GO_REF:0000107

Automatic transfer of experimentally verified manual GO annotation data to orthologs using Ensembl Compara

GO_REF:0000120

Combined Automated Annotation using Multiple IEA Methods

PMID:16237761

Screening of hepatocyte proteins binding to F protein of hepatitis C virus by yeast two-hybrid system.

PMID:20378551

Golgi N-glycosyltransferases form both homo- and heterodimeric enzyme complexes in live cells.

ST6GAL1 (SiaT) forms disulfide-mediated and non-covalent homodimers in live Golgi membranes (>90% of molecules homodimeric) and a trans-Golgi heterodimer with B4GALT1; localizes to Golgi membranes.

PMID:21081508

Universal phosphatase-coupled glycosyltransferase assay.

Kinetic parameters of human sialyltransferase ST6GAL1 were measured using a phosphatase-coupled assay.

PMID:23999306

The structure of human α-2,6-sialyltransferase reveals the binding mode of complex glycans.

Crystal structures of human ST6Gal-I define the GT-A variant fold, CMP product/inhibitor binding, complex-glycan binding mode, and an SN2 inversion, substrate-assisted catalytic mechanism.

PMID:25733881

Controlled tetra-Fc sialylation of IVIg results in a drug candidate with consistent enhanced anti-inflammatory activity.

ST6GAL1 alpha-2,6-sialylation of IgG Fc N-glycans yields tetra-Fc-sialylated IVIg with enhanced anti-inflammatory activity.

PMID:27872474

Multi-level glyco-engineering techniques to generate IgG with defined Fc-glycans.

ST6GAL1 is used in the glycosylation pathway to install alpha-2,6 sialic acid on IgG Fc glycans (UniProt PATHWAY evidence for protein glycosylation).

PMID:29133956

Network inference from glycoproteomics data reveals new reactions in the IgG glycosylation pathway.

Glycoproteomics network inference places ST6GAL1 in the IgG glycosylation pathway and supports its trans-Golgi role.

PMID:35789385

The incorrect use of CD75 as a synonym for ST6GAL1 has fostered the expansion of commercial 'ST6GAL1' antibodies that do not recognize ST6GAL1.

CD75 is a carbohydrate epitope generated by ST6GAL1, not the ST6GAL1 protein; conflation of the two has produced commercial antibodies that do not recognize ST6GAL1.

PMID:36280670

A universal glycoenzyme biosynthesis pipeline that enables efficient cell-free remodeling of glycans.

Recombinant ST6GAL1 installs alpha-2,6 sialic acid in a cell-free glycan remodeling pipeline (EC 2.4.3.1 catalytic activity).

PMID:37632720

Divergent Enzymatic Assembly of a Comprehensive 64-Membered IgG N-Glycan Library for Functional Glycomics.

ST6GAL1 is used for alpha-2,6 sialylation in the divergent enzymatic assembly of a 64-membered IgG N-glycan library.

Reactome:R-HSA-4085033

ST6GAL1,2 transfer Neu5Ac to terminal Gal (alpha-2,6 link)

Reactome:R-HSA-9683673

Maturation of protein 3a

Reactome:R-HSA-9683769

O-glycosylation of 3a is terminated

Reactome:R-HSA-9694548

Maturation of spike protein

Reactome:R-HSA-9697018

Addition of sialic acids on some Spike glycosyl sidechains

Reactome:R-HSA-975902

ST6GAL1 transfers Neu5Ac to terminal Gal of N-glycans

Reactome:R-HSA-977068

Termination of O-glycan biosynthesis

Reactome:R-HSA-977071

ST6GAL1 transfers sialic acid to Tn antigens to form sTn antigens

Suggested Experiments

Experiment: Acceptor-substrate profiling of recombinant human ST6GAL1 against defined N-glycan, O-GalNAc (Tn/Core1/Core2), and free-galactoside acceptors to quantify the relative physiological preference and refine the activity annotation.

Experiment: Tissue-specific (B-cell, myeloid, hepatocyte) conditional St6gal1 knockouts with serum and cell-surface glycomics to dissociate intrinsic (Golgi) from extrinsic (soluble serum enzyme) alpha-2,6-sialylation contributions.

Experiment: Structure-guided disruption of the disulfide/non-covalent homodimer and the B4GALT1 heterodimer interfaces, followed by live-cell Golgi-retention and N-glycan-sialylation efficiency assays, to test the functional role of oligomerization.

Deep Research

Falcon

(ST6GAL1-deep-research-falcon.md)

Gene Research for GO Annotation Review Falcon

Gene Research for GO Annotation Review

Target

Gene symbol: ST6GAL1
Organism: Homo sapiens

UniProt Context

=== UNIPROT METADATA ===
UniProt ID: P15907
Entry Name: SIAT1_HUMAN
Gene Name: ST6GAL1
Protein Name: Beta-galactoside alpha-2,6-sialyltransferase 1
EC Number: 2.4.3.1
Organism: Homo sapiens (Human)
NCBI Taxonomy ID: 9606
Function: Transfers sialic acid from CMP-sialic acid to galactose- containing acceptor substrates. In B lymphocytes, generates neuraminidase-sensitive lymphocyte cell-surface differentiation antigens, such as CDw75, HB-6 and CD76 (PubMed:1730763). Sialylates complex-type N-glycans attached on the fragment crystallizable (Fc) of IgGs confering anti-inflammatory effector functions. Preferentially monosialylates the alpha(1->3) mannose antenna of Fc glycoforms with subsequent disialylation occurring at a much slower rate.
Subcellular Location: Golgi apparatus, Golgi stack membrane
Subunit: Monomer and homodimer.
======================

Research Objective

Prepare a comprehensive literature-backed research report for GO annotation review of the target gene.

Focus on:

The core biochemical activity of the gene product and any substrate specificity.
The activation or maturation mechanism of the protein, including proteolytic processing if relevant.
The best-supported subcellular localization and complexes where the active protein functions.
The strongest experimentally supported biological processes in the target organism, or in very close orthologs when transfer is justified.
Which annotations appear to reflect core function versus context-specific, pleiotropic, or over-extended roles.
Any evidence bearing on apoptosis, developmental cell death, neuronal roles, inflammatory signaling, pyroptosis, synaptic remodeling, protein processing, and localization to cytoplasm, cytosol, nucleus, or signaling complexes.

Output Requirements

Structure the report as:

Executive summary
Molecular function
Biological process roles
Cellular localization and complexes
Annotation-risk assessment
Key literature

Use citations for all substantive claims.

Comprehensive Research Report for GO Annotation Review: ST6GAL1 (Homo sapiens)

Executive Summary

ST6GAL1 (Beta-galactoside alpha-2,6-sialyltransferase 1, EC 2.4.99.1) is a Golgi-resident glycosyltransferase that catalyzes the transfer of sialic acid from CMP-sialic acid to galactose-containing N-glycan acceptor substrates, forming α2,6-sialyl linkages (sankaranarayanan2023computationalstudieson pages 1-2, harduinlepers2023thevertebratesialylation pages 1-3). Recent literature from 2020-2024 establishes ST6GAL1 as having well-defined core functions in B cell biology and IgG Fc sialylation (werner2024iggsialylationoccurs pages 1-2, trzos2023theroleof pages 1-2), as well as extensive context-specific roles in cancer progression, inflammatory signaling, and immune cell development (ankenbauer2023sialylationofegfr pages 1-4, holdbrooks2020regulationofinflammatory pages 1-2, gc2022st6gal1oncogenicsignaling pages 1-2, lau2024sialicacidin pages 1-2). The protein localizes primarily to the trans-Golgi apparatus via retention mechanisms involving its N-terminal cytoplasmic tail, transmembrane domain, and stem region (yagi2024decipheringthesubgolgi pages 1-2, sun2021aquantitativestudy pages 1-5). A soluble, catalytically active form generated by BACE1 cleavage also exists extracellularly, mediating "extrinsic sialylation" of cell surface and secreted glycoproteins (sankaranarayanan2023computationalstudieson pages 1-2, lau2024sialicacidin pages 1-2). No evidence for proteolytic activation or maturation beyond BACE1 shedding was identified. The protein functions as a monomer or homodimer without forming stable multi-protein complexes (harduinlepers2023thevertebratesialylation pages 1-3, sun2021aquantitativestudy pages 1-5).

1. Molecular Function

1.1 Core Biochemical Activity

ST6GAL1 exhibits β-galactoside α2,6-sialyltransferase activity (EC 2.4.99.1), catalyzing the transfer of sialic acid (N-acetylneuraminic acid, Neu5Ac) from the donor substrate CMP-Sia to terminal galactose residues on complex N-glycans, specifically forming Siaα2,6Galβ1,4GlcNAc-R linkages (sankaranarayanan2023computationalstudieson pages 1-2, gu2024specificsialylationof pages 1-2, harduinlepers2023thevertebratesialylation pages 1-3). This enzymatic activity occurs within the trans-Golgi apparatus during protein glycosylation (harduinlepers2023thevertebratesialylation pages 1-3, yagi2024decipheringthesubgolgi pages 1-2). ST6GAL1 is a type II transmembrane glycoprotein with an N-terminal cytoplasmic domain, transmembrane domain, stem region, and C-terminal catalytic domain (sun2021aquantitativestudy pages 1-5). The catalytic domain contains four conserved sialylmotifs ('L', 'S', 'III', 'VS') essential for donor/acceptor substrate recognition and catalytic activity (pietrobono2021aberrantsialylationin pages 1-2).

1.2 Substrate Specificity

ST6GAL1 exhibits defined substrate specificity for complex-type N-glycans on glycoproteins. The enzyme preferentially monosialylates the α(1→3) mannose antenna of N-glycan structures, with subsequent disialylation occurring at a much slower rate (UniProt context). Comprehensive substrate profiling has identified multiple well-supported protein substrates:

Core Substrates:
- IgG Fc domain: ST6GAL1 sialylates the conserved N297 biantennary complex N-glycan on IgG Fc, a modification critical for conferring anti-inflammatory effector functions (werner2024iggsialylationoccurs pages 1-2, trzos2023theroleof pages 1-2). Studies using B cell-specific St6gal1 deletion in mice demonstrate that IgG sialylation occurs exclusively within plasma cells/plasma blasts before antibody secretion (werner2024iggsialylationoccurs pages 1-2).

Well-Established Context-Specific Substrates:
- EGFR (Epidermal Growth Factor Receptor): α2,6-sialylation of EGFR N-glycans promotes receptor dimerization, activation, recycling, and downstream AKT/NFκB signaling while reducing lysosomal degradation (ankenbauer2023sialylationofegfr pages 1-4, gc2022st6gal1oncogenicsignaling pages 1-2).
- TNFR1 (Tumor Necrosis Factor Receptor 1): Sialylation prolongs TNF-induced NFκB activation and suppresses apoptotic signaling (holdbrooks2020regulationofinflammatory pages 1-2, pietrobono2021aberrantsialylationin pages 1-2).
- TLR4 (Toll-Like Receptor 4): α2,6-sialylation sustains long-term LPS-induced NFκB, IRF3, and STAT3 signaling (holdbrooks2020regulationofinflammatory pages 1-2).
- Integrin β1: Sialylation enhances migration, mechanotransduction, and survival signaling in cancer contexts (gu2024specificsialylationof pages 1-2, gc2022st6gal1oncogenicsignaling pages 1-2).
- M-CSF Receptor (CSF1R): Extracellular ST6GAL1 sialylates and activates this receptor, promoting monocyte-macrophage development (lau2024sialicacidin pages 1-2).
- PODXL, ICAM1, CD44, ALCAM1, ECE1: Glycoproteomics studies identify these adhesion and stemness markers as ST6GAL1 substrates contributing to circulating tumor cell clustering and metastatic seeding (dashzeveg2023dynamicglycoproteinhyposialylation pages 1-2).

Importantly, computational and biophysical studies reveal that glycosaminoglycans (GAGs), especially heparan sulfate, bind ST6GAL1 with high affinity (~65 nM for polymeric HS) and compete with the donor substrate CMP-Sia, suggesting regulatory mechanisms beyond protein substrate availability (sankaranarayanan2023computationalstudieson pages 1-2).

Protein Substrate	Glycosylation Site/Type	Biological Function/Process	Evidence Quality (Strong/Moderate/Limited)	Key Citations
IgG Fc (core substrate)	Fc N297 biantennary complex N-glycan; terminal α2,6-sialylation on galactose	Regulates IgG anti-inflammatory activity, Fc effector tuning, and B-cell/plasma-cell-associated IgG sialylation	Strong	(werner2024iggsialylationoccurs pages 1-2, trzos2023theroleof pages 1-2)
EGFR (context-specific, cancer)	N-glycans on receptor ectodomain; α2,6-sialylation	Promotes EGFR activation, dimerization/oligomerization, recycling, reduced lysosomal degradation, downstream AKT/NFκB signaling	Strong	(ankenbauer2023sialylationofegfr pages 1-4, jones2023roleofthe pages 1-2, gc2022st6gal1oncogenicsignaling pages 1-2)
Integrin β1 (context-specific, cancer/stemness)	N-glycans; α2,6-sialylation	Enhances migration, mechanotransduction, EMT/stem-like behavior, survival signaling	Moderate	(jones2023roleofthe pages 1-2, gc2022st6gal1oncogenicsignaling pages 1-2, gu2024specificsialylationof pages 1-2)
Integrin α2β1 (context-specific, cancer)	Reported receptor-associated sialylation; linkage-specific modulation of surface glycoforms	Linked to metastatic behavior and cell-matrix interaction in tumor settings; evidence for direct ST6GAL1 assignment less secure than for β1 integrin	Limited	(pietrobono2021aberrantsialylationin pages 1-2, gao2021roleofglycans pages 1-3)
Integrin α3β1 (context-specific, Golgi regulatory complex literature)	N-glycans; sialylation linked to Golgi complex formation with trafficking/signaling factors	Proposed participation in regulatory complexes that influence sialyltransferase targeting and glycan output rather than firmly established as a direct ST6GAL1 substrate in human cells	Limited	(gu2024specificsialylationof pages 1-2, gc2022st6gal1oncogenicsignaling pages 1-2)
TNFR1 (context-specific, inflammation/cancer survival)	N-glycans; α2,6-sialylation	Prolongs TNF-induced NFκB signaling, suppresses receptor internalization-linked apoptotic output, supports cell survival	Strong	(holdbrooks2020regulationofinflammatory pages 1-2, jones2023roleofthe pages 1-2, pietrobono2021aberrantsialylationin pages 1-2)
TLR4 (context-specific, innate immunity)	N-glycans; α2,6-sialylation detected by SNA pulldown	Sustains long-term LPS-induced NFκB, IRF3, and STAT3 signaling in monocytes/macrophages	Strong	(holdbrooks2020regulationofinflammatory pages 1-2)
Fas/CD95 (context-specific, apoptosis resistance)	N-glycans; α2,6-sialylation	Protects against Fas-mediated apoptosis, associated with metastatic behavior and poor prognosis in carcinoma contexts	Moderate	(gu2024specificsialylationof pages 1-2, pietrobono2021aberrantsialylationin pages 1-2)
VEGFR (context-specific, angiogenesis/cancer)	Receptor N-glycans; presumed α2,6-sialylation	Implicated in angiogenic signaling and tumor progression, but direct substrate-level evidence summarized mainly in reviews	Limited	(gc2022st6gal1oncogenicsignaling pages 1-2, pietrobono2021aberrantsialylationin pages 1-2)
MET/HGFR (context-specific, cancer)	Receptor N-glycans; α2,6-sialylation	Modulates receptor activity and downstream invasive/survival signaling in tumor cells	Moderate	(jones2023roleofthe pages 1-2, gc2022st6gal1oncogenicsignaling pages 1-2)
HER2/ERBB2 (context-specific, cancer)	Receptor N-glycans; α2,6-sialylation	Linked to altered receptor signaling and tumor phenotype in ST6GAL1-high cancers	Moderate	(jones2023roleofthe pages 1-2, gc2022st6gal1oncogenicsignaling pages 1-2)
M-CSF receptor / CSF1R (context-specific, immune development)	Cell-surface receptor glycoprotein; extracellular α2,6-sialylation by soluble ST6GAL1	Activates monocyte/macrophage development and survival signaling, including ERK/AKT/NFκB pathways	Strong	(lau2024sialicacidin pages 1-2)
CD44 (context-specific, CTC clustering/cancer stemness)	Cell-surface glycoprotein N-glycans; altered terminal α2,6-sialylation status	Contributes to tumor cell aggregation, cluster formation, stemness-associated metastatic seeding	Moderate	(dashzeveg2023dynamicglycoproteinhyposialylation pages 1-2)
ICAM1 (context-specific, CTC clustering/cancer)	Cell-surface glycoprotein N-glycans; altered α2,6-sialylation	Supports homotypic aggregation/CTC cluster formation and metastatic seeding	Moderate	(dashzeveg2023dynamicglycoproteinhyposialylation pages 1-2)
PODXL (context-specific, cancer)	Cell-surface glycoprotein N-glycans; identified as ST6GAL1-regulated substrate in glycoproteomics	Promotes CTC clustering, chemoevasion-associated metastasis; antibody blockade reduces clustering/metastasis in TNBC model	Strong	(dashzeveg2023dynamicglycoproteinhyposialylation pages 1-2)
ALCAM1 (context-specific, cancer)	Cell-surface glycoprotein N-glycans; identified in ST6GAL1-linked glycoproteomics	Candidate mediator of CTC clustering/metastatic seeding	Moderate	(dashzeveg2023dynamicglycoproteinhyposialylation pages 1-2)
ECE1 (context-specific, cancer)	Glycoprotein N-glycans; identified in ST6GAL1-linked glycoproteomics	Candidate contributor to clustering and metastatic seeding in chemoevasive CTC clusters	Moderate	(dashzeveg2023dynamicglycoproteinhyposialylation pages 1-2)

Table: This table summarizes protein substrates reported for ST6GAL1, distinguishing well-supported core substrates from context-specific targets in cancer and immune cells. It is useful for GO annotation review because it highlights which substrate relationships have strong experimental support versus more indirect or review-level evidence.

1.3 Catalytic Mechanism and Regulation

ST6GAL1 utilizes CMP-sialic acid as the activated sugar donor and requires appropriate acceptor substrate presentation in the Golgi lumen (sankaranarayanan2023computationalstudieson pages 1-2, harduinlepers2023thevertebratesialylation pages 1-3). The enzyme does not undergo proteolytic activation for catalytic function. However, BACE1 (β-site amyloid precursor protein-cleaving enzyme 1) cleaves ST6GAL1 to release a soluble, catalytically active form lacking the transmembrane domain, enabling secretion and extracellular sialylation activity (sankaranarayanan2023computationalstudieson pages 1-2, lau2024sialicacidin pages 1-2). This soluble ST6GAL1 is abundant in plasma, secreted by hepatocytes during acute phase responses and by activated platelets, facilitating "extrinsic sialylation" of cell surface and secreted glycoproteins (lau2024sialicacidin pages 1-2).

2. Biological Process Roles

2.1 Core Functions (Strongly Supported for GO Annotation)

IgG Fc Sialylation and Anti-Inflammatory Activity:
ST6GAL1-mediated α2,6-sialylation of IgG Fc at Asn297 is essential for anti-inflammatory IgG effector functions (werner2024iggsialylationoccurs pages 1-2, trzos2023theroleof pages 1-2). Bone marrow chimeric mice with B cell-specific St6gal1 deletion demonstrate that serum IgG sialylation is absent despite ST6GAL1 expression in other cell types, confirming that sialylation occurs exclusively within B cells pre-secretion (werner2024iggsialylationoccurs pages 1-2). Sialylated IgG exerts anti-inflammatory activity through mechanisms involving FcγRIIB and SIGN-R1 receptors (trzos2023theroleof pages 1-2). Clinical relevance is evidenced by altered IgG Fc glycosylation patterns in autoimmune diseases including rheumatoid arthritis and systemic lupus erythematosus (trzos2023theroleof pages 1-2).

B Cell Development and Maturation:
ST6GAL1 is critical for B cell development and maturation (werner2024iggsialylationoccurs pages 1-2, trzos2023theroleof pages 1-2, lau2024sialicacidin pages 1-2). Murine St6gal1 knockout impairs B cell maturation, and B cells expressing ST6GAL1 have a developmental advantage over ST6GAL1-deficient B cells, dominating the plasma cell pool (werner2024iggsialylationoccurs pages 1-2). α2,6-sialylation regulates B cell surface proteins including BCR and CD22, affecting signal transduction and selection processes (trzos2023theroleof pages 1-2).

Inflammatory Signaling:
ST6GAL1 regulates inflammatory pathways through receptor sialylation. In monocytic cells, ST6GAL1 knockdown reduces long-term (2-6 hour) but not short-term (≤30 min) TNF-induced NFκB activation via TNFR1 sialylation (holdbrooks2020regulationofinflammatory pages 1-2). Similarly, LPS-induced activation of NFκB, IRF3, and STAT3 through TLR4 requires ST6GAL1-mediated receptor sialylation for sustained signaling (holdbrooks2020regulationofinflammatory pages 1-2). Myeloid-specific St6gal1 knockout mice confirm these findings in primary bone marrow-derived macrophages (holdbrooks2020regulationofinflammatory pages 1-2).

Monocyte-Macrophage Development:
Extracellular ST6GAL1 promotes monocyte-macrophage development and survival by sialylating and activating the M-CSF receptor (CSF1R), initiating ERK1/2, AKT, and NFκB signaling pathways (lau2024sialicacidin pages 1-2). Recombinant ST6GAL1 induces wholesale changes in gene expression profiles of primary myeloid cells, upregulating monocyte-macrophage development pathway genes and transcription factors PU.1 and NFκB (lau2024sialicacidin pages 1-2).

2.2 Context-Specific Roles (Cancer Biology)

ST6GAL1 is upregulated in numerous malignancies and drives multiple cancer hallmarks (munkley2022aberrantsialylationin pages 1-2, gc2022st6gal1oncogenicsignaling pages 1-2, pietrobono2021aberrantsialylationin pages 1-2):

Cancer Cell Survival and Apoptosis Resistance:
α2,6-sialylation protects cancer cells from apoptosis induced by cytotoxic stress, hypoxia, chemotherapy, and death receptor signaling (jones2023roleofthe pages 1-2, gc2022st6gal1oncogenicsignaling pages 1-2, pietrobono2021aberrantsialylationin pages 1-2). ST6GAL1 sialylation of Fas/CD95 and TNFR1 prevents receptor internalization and downstream apoptotic cascades (gu2024specificsialylationof pages 1-2, pietrobono2021aberrantsialylationin pages 1-2). ST6GAL1 enhances cancer cell survival under hypoxic conditions by promoting HIF1α accumulation and activity (jones2023roleofthe pages 1-2).

Metabolic Adaptation:
ST6GAL1 regulates cancer cell metabolism, enhancing both glycolytic and oxidative metabolic pathways (jones2023roleofthe pages 1-2). In ovarian cancer cells, high ST6GAL1 expression maintains oxidative metabolism under hypoxic stress and increases activity of glycolytic enzymes hexokinase and phosphofructokinase (jones2023roleofthe pages 1-2).

Metastasis and Invasion:
ST6GAL1 promotes metastatic behaviors including epithelial-mesenchymal transition (EMT), invasion, and metastatic seeding (ankenbauer2023sialylationofegfr pages 1-4, gc2022st6gal1oncogenicsignaling pages 1-2, dashzeveg2023dynamicglycoproteinhyposialylation pages 1-2). Dynamic hyposialylation (loss of ST6GAL1 activity) in circulating tumor cell (CTC) clusters promotes cellular quiescence, enabling chemotherapy evasion and enhanced metastatic seeding (dashzeveg2023dynamicglycoproteinhyposialylation pages 1-2). Glycoproteomics reveals ST6GAL1 substrates PODXL, ICAM1, and CD44 contribute to CTC aggregation and cluster formation (dashzeveg2023dynamicglycoproteinhyposialylation pages 1-2).

Cancer Stem Cell Phenotype:
ST6GAL1 imparts cancer stem cell characteristics including self-renewal, enhanced pluripotency, and stemness-associated signaling (gc2022st6gal1oncogenicsignaling pages 1-2, lau2024sialicacidin pages 1-2). Induced pluripotent stem cells exhibit elevated ST6GAL1 expression and α2,6-sialylation (gu2024specificsialylationof pages 1-2, zhu2024biologicalfunctionof pages 1-2).

Receptor Signaling Modulation:
ST6GAL1 activates oncogenic receptor tyrosine kinases including EGFR, HER2/ERBB2, and MET/HGFR through sialylation-induced receptor dimerization, altered trafficking dynamics, and enhanced recycling (ankenbauer2023sialylationofegfr pages 1-4, gc2022st6gal1oncogenicsignaling pages 1-2). EGFR sialylation increases receptor cell surface retention by promoting Rab11-mediated recycling while inhibiting lysosomal degradation (ankenbauer2023sialylationofegfr pages 1-4).

Biological Process	Evidence Type (in vivo, in vitro, genetic)	Organism/Model System	Core vs Context-Specific Role	Strength of Evidence	Key References
IgG Fc α2,6-sialylation linked to anti-inflammatory IgG activity	In vivo genetic, ex vivo glycan analysis, review synthesis	Mouse B cell-specific St6gal1 deletion/chimeras; human IgG literature	Core	Strong	(werner2024iggsialylationoccurs pages 1-2, trzos2023theroleof pages 1-2)
B cell development and maturation	In vivo genetic, review synthesis	Mouse hematopoietic/B-cell systems; human immune literature	Core	Strong	(werner2024iggsialylationoccurs pages 1-2, trzos2023theroleof pages 1-2, lau2024sialicacidin pages 1-2)
Plasma cell contribution to serum IgG sialylation	In vivo genetic	Mouse bone marrow chimeras with B cell-specific St6gal1 loss	Core	Strong	(werner2024iggsialylationoccurs pages 1-2)
Inflammatory signaling via TNF/NFκB pathway	In vitro knockdown, ex vivo primary cells, myeloid-specific genetic deletion	Human U937 monocytic cells; mouse BMDMs	Core in immune cells	Strong	(holdbrooks2020regulationofinflammatory pages 1-2)
Inflammatory signaling via LPS/TLR4/NFκB, IRF3, STAT3	In vitro lectin pulldown plus signaling assays, ex vivo primary cells, myeloid-specific genetic deletion	Human U937 cells; mouse BMDMs	Core in immune cells	Strong	(holdbrooks2020regulationofinflammatory pages 1-2)
Monocyte-macrophage development and survival	Ex vivo/in vitro recombinant enzyme treatment, transcriptomics, receptor signaling assays	Mouse inflammatory myeloid cells; human THP-1 monocytes	Context-specific but well supported	Strong	(lau2024sialicacidin pages 1-2)
Granulocyte production / granulopoiesis regulation by extrinsic sialylation	In vivo and review synthesis	Mouse hematopoietic systems	Context-specific but recurrent	Moderate	(lau2024sialicacidin pages 1-2, holdbrooks2020regulationofinflammatory pages 1-2)
Apoptosis resistance via TNFR1 sialylation	In vitro mechanistic and review synthesis	Human monocytic cells; cancer cell literature	Context-specific	Strong	(holdbrooks2020regulationofinflammatory pages 1-2, pietrobono2021aberrantsialylationin pages 1-2)
Apoptosis resistance via Fas/CD95 sialylation	Review synthesis from tumor studies	Carcinoma models summarized in reviews	Context-specific	Moderate	(gu2024specificsialylationof pages 1-2, pietrobono2021aberrantsialylationin pages 1-2)
Cancer stem cell phenotype / self-renewal	In vitro and review synthesis	Multiple cancer models; pluripotent/cancer stem-like systems	Context-specific	Moderate	(sankaranarayanan2023computationalstudieson pages 1-2, jones2023roleofthe pages 1-2, gc2022st6gal1oncogenicsignaling pages 1-2, lau2024sialicacidin pages 1-2)
Cancer cell survival under hypoxia or cytotoxic stress	In vitro overexpression/knockdown and metabolic assays; review synthesis	Ovarian cancer cell models	Context-specific	Strong	(jones2023roleofthe pages 1-2)
Metabolic adaptation (enhanced glycolysis and oxidative metabolism)	In vitro functional assays	Ovarian cancer cells	Context-specific	Strong	(jones2023roleofthe pages 1-2)
Metastasis and invasion	In vitro, in vivo tumor models, clinical association/review synthesis	Breast, ovarian, prostate and other cancer models	Context-specific	Strong	(jones2023roleofthe pages 1-2, gc2022st6gal1oncogenicsignaling pages 1-2, pietrobono2021aberrantsialylationin pages 1-2, dashzeveg2023dynamicglycoproteinhyposialylation pages 1-2)
Epithelial-mesenchymal transition (EMT)	In vitro and review synthesis	Cancer cell models, especially EGFR/integrin-associated systems	Context-specific	Moderate	(ankenbauer2023sialylationofegfr pages 1-4, gc2022st6gal1oncogenicsignaling pages 1-2)
Chemotherapy resistance / chemoevasion	In vivo tumor studies, glycoproteomics, review synthesis	Breast cancer CTC clusters; multiple tumor contexts	Context-specific	Strong	(dashzeveg2023dynamicglycoproteinhyposialylation pages 1-2, sankaranarayanan2023computationalstudieson pages 1-2, gc2022st6gal1oncogenicsignaling pages 1-2)
EGFR activation and altered receptor recycling/trafficking	In vitro biochemical and microscopy assays	Ovarian, pancreatic, colon and other cancer cell models	Context-specific mechanistic output of sialylation	Strong	(ankenbauer2023sialylationofegfr pages 1-4, gc2022st6gal1oncogenicsignaling pages 1-2)
CTC cluster formation and metastatic seeding	In vivo/in vitro tumor studies, glycoproteomics	Breast cancer CTC and PDX-related systems	Context-specific	Strong	(dashzeveg2023dynamicglycoproteinhyposialylation pages 1-2)
Tumor-associated macrophage polarization-associated α2,6-sialylation remodeling	Ex vivo polarization, silencing, patient tissue analysis	Human monocyte-derived macrophages; colorectal cancer TAMs	Context-specific	Moderate	(dipta2024st6gal1mediatedsialyllinkage pages 1-4)
Neuronal/brain roles	Mostly review-level expression and disease-association evidence	Human CNS/AD literature	Not established as core	Limited	(kang2024thealterationand pages 1-2, zhu2024biologicalfunctionof pages 1-2)
Synaptic remodeling	No direct ST6GAL1-specific experimental support identified in retrieved evidence	—	Not supported for annotation	Limited/none	(kang2024thealterationand pages 1-2, zhu2024biologicalfunctionof pages 1-2)
Autophagy regulation	No direct ST6GAL1-specific support identified; related evidence in retrieved set concerns other sialyltransferases	—	Not supported for annotation	Limited/none	(kang2024thealterationand pages 1-2)
Pyroptosis	No direct evidence identified in retrieved evidence	—	Not supported for annotation	Limited/none	(holdbrooks2020regulationofinflammatory pages 1-2, zhu2024biologicalfunctionof pages 1-2)
Developmental cell death	No direct ST6GAL1-specific evidence identified beyond broader apoptosis-survival literature	—	Not supported for annotation	Limited/none	(zhu2024biologicalfunctionof pages 1-2, pietrobono2021aberrantsialylationin pages 1-2)
Extrinsic sialylation of blood/immune cell surfaces by soluble ST6GAL1	In vivo/extracellular enzyme literature and review synthesis	Mouse and human hematopoietic systems	Context-specific extension of canonical function	Moderate	(lau2024sialicacidin pages 1-2, sankaranarayanan2023computationalstudieson pages 1-2)

Table: This table summarizes experimentally supported biological processes involving ST6GAL1, distinguishing core functions from context-specific roles. It is useful for GO review because it highlights where support is strong enough for direct annotation versus where roles are pleiotropic, indirect, or insufficiently specific.

2.3 Roles with Limited or No Direct Evidence

Neuronal Functions:
While ST6GAL1 expression is detected in various brain cell types including neurons, microglia, and astrocytes (kang2024thealterationand pages 1-2), direct experimental evidence for specific neuronal functions remains limited. ST6GAL1 has been implicated in neuronal differentiation contexts and is associated with neurodegenerative diseases like Alzheimer's disease through correlative studies, but mechanistic evidence is lacking (zhu2024biologicalfunctionof pages 1-2, kang2024thealterationand pages 1-2).

Pyroptosis:
No direct evidence linking ST6GAL1 to pyroptosis pathways was identified in the retrieved literature (zhu2024biologicalfunctionof pages 1-2, holdbrooks2020regulationofinflammatory pages 1-2).

Synaptic Remodeling:
Despite ST6GAL1 expression in brain tissues, no experimental support for direct involvement in synaptic remodeling was found (zhu2024biologicalfunctionof pages 1-2, kang2024thealterationand pages 1-2).

Developmental Cell Death:
Evidence is limited to broader apoptosis-survival literature without specific developmental cell death contexts (zhu2024biologicalfunctionof pages 1-2, pietrobono2021aberrantsialylationin pages 1-2).

3. Cellular Localization and Complexes

3.1 Subcellular Localization

Golgi Apparatus (Primary Localization):
ST6GAL1 primarily localizes to the trans-Golgi cisternae, where it performs canonical intracellular sialylation of nascent glycoproteins (harduinlepers2023thevertebratesialylation pages 1-3, yagi2024decipheringthesubgolgi pages 1-2, sun2021aquantitativestudy pages 1-5). Super-resolution imaging reveals ST6GAL1 predominantly occupies the trans-Golgi compared to ST3GAL4 (medial-Golgi enriched), with nuanced sub-Golgi localization differences detectable even among enzymes presumed to coexist in the same compartment (yagi2024decipheringthesubgolgi pages 1-2).

Golgi Retention Mechanism:
Golgi retention of ST6GAL1 is mediated by its N-terminal CTS (cytoplasmic tail, transmembrane domain, stem) region (yagi2024decipheringthesubgolgi pages 1-2, sun2021aquantitativestudy pages 1-5). Quantitative studies using chimeric proteins reveal that the cytoplasmic tail, transmembrane domain, and ecto-domain each contribute additively to Golgi retention (sun2021aquantitativestudy pages 1-5). Notably, cytoplasmic tail length negatively affects retention—longer tails promote Golgi export (sun2021aquantitativestudy pages 1-5). ST6GAL1 exhibits limited post-Golgi retrieval; most molecules that escape the Golgi traffic to the plasma membrane and subsequently to endolysosomes for degradation via ecto-domain shedding (sun2021aquantitativestudy pages 1-5).

Extracellular/Soluble Form:
BACE1 proteolyticcleavage generates a soluble, catalytically active ST6GAL1 form lacking the transmembrane domain (sankaranarayanan2023computationalstudieson pages 1-2, lau2024sialicacidin pages 1-2). This soluble enzyme is secreted by hepatocytes (major source in circulation), B cells, cancer cells, and activated platelets (lau2024sialicacidin pages 1-2). Platelets also release CMP-Sia upon activation, supporting extracellular sialylation activity (lau2024sialicacidin pages 1-2). Soluble ST6GAL1 mediates "extrinsic sialylation" of cell surface and secreted glycoproteins in an autonomous or non-autonomous manner (lau2024sialicacidin pages 1-2).

3.2 Protein Complexes and Oligomerization

ST6GAL1 functions as a monomer or homodimer based on UniProt annotation. No stable multi-protein complexes involving ST6GAL1 have been definitively characterized in the retrieved literature. While proposed regulatory interactions with GOLPH3, PI4KIIα, and integrin α3β1 in the context of Golgi trafficking and sialylation regulation have been discussed (gu2024specificsialylationof pages 1-2), these represent transient regulatory associations rather than stable functional complexes. ST6GAL1 does not require formation of stable complexes for catalytic activity (sun2021aquantitativestudy pages 1-5).

3.3 Trafficking and Compartment Dynamics

ST6GAL1 trafficking follows the canonical secretory pathway from ER to Golgi, with retention at the trans-Golgi (sun2021aquantitativestudy pages 1-5). IgG produced in B cells shows divergent trafficking from canonical pathways, exhibiting limited exposure to ST6GAL1 within the Golgi, which explains why B cell-expressed ST6GAL1 is relatively inefficient for IgG sialylation compared to cell surface protein sialylation (glendenning2022divergentgolgitrafficking pages 1-2). This divergent trafficking promotes Fut8-mediated core fucosylation while limiting ST6GAL1-mediated sialylation of IgG within B cells (glendenning2022divergentgolgitrafficking pages 1-2). The GARP (Golgi-associated retrograde protein) complex is essential for recycling and retention of Golgi glycosylation enzymes including ST6GAL1 (sun2021aquantitativestudy pages 1-5).

4. Annotation-Risk Assessment

4.1 Core vs. Context-Specific Functions

Low Annotation Risk (Core Functions):
- β-galactoside α2,6-sialyltransferase activity (GO:0003830)
- IgG Fc-region N-glycan α2,6-sialylation (strongly supported in B cells)
- B cell differentiation and maturation
- Regulation of inflammatory response (via TNFR1, TLR4 sialylation)
- Golgi apparatus localization (trans-Golgi)
- Extracellular region (soluble form)

Moderate Annotation Risk (Well-Supported Context-Specific):
- Regulation of apoptotic process (via death receptor sialylation)
- Monocyte/macrophage development (extrinsic sialylation mechanism)
- Response to hypoxia (cancer-specific)
- Positive regulation of cell migration (integrin sialylation)

High Annotation Risk (Context-Specific, Pleiotropic, or Extended):
- Cancer-related processes (metastasis, EMT, stem cell phenotype): These are well-documented but represent disease-specific gain-of-function rather than core physiological roles
- Metabolic process regulation: Evidence is strong in cancer models but may not generalize to normal physiology
- EGFR, HER2, MET signaling: While well-established in cancer contexts, these represent downstream consequences of substrate sialylation rather than direct molecular functions

Not Recommended for Annotation (Insufficient Evidence):
- Pyroptosis (no direct evidence)
- Synaptic remodeling (no mechanistic evidence)
- Specific neuronal development processes (correlative only)
- Autophagy regulation (no ST6GAL1-specific evidence in retrieved set)

4.2 Localization Annotations

Strongly Supported:
- Golgi apparatus (GO:0005794)
- Trans-Golgi network (GO:0005802)
- Extracellular space (GO:0005615) - for soluble form

Not Supported:
- Nucleus (no evidence)
- Cytosol (no evidence beyond cytoplasmic tail domain)
- Cytoplasm (only transmembrane/tail domain)
- Signaling complexes (no stable complex formation identified)

4.3 Maturation and Processing

No Evidence For:
- Proteolytic activation or zymogen processing for enzymatic activity
- Post-translational modifications required for catalytic function
- Cofactor requirements beyond CMP-Sia substrate

Supported:
- BACE1-mediated shedding generates soluble extracellular form (not required for activity, but extends functional localization)

5. Key Literature

Molecular Function and Substrate Specificity:
1. Sankaranarayanan et al., 2023 (Glycobiology) - GAG binding and substrate competition (sankaranarayanan2023computationalstudieson pages 1-2)
2. Gu & Isaji, 2024 (Glycoconjugate J) - Specific sialylation mechanisms and regulation (gu2024specificsialylationof pages 1-2)
3. Ankenbauer et al., 2023 (J Biol Chem) - EGFR sialylation and receptor activation (ankenbauer2023sialylationofegfr pages 1-4)
4. Harduin-Lepers, 2023 (Glycoconjugate J) - Sialyltransferase structure-function and evolution (harduinlepers2023thevertebratesialylation pages 1-3)

IgG Sialylation and B Cell Biology:
5. Werner et al., 2024 (Front Immunol) - IgG sialylation occurs pre-secretion in B cells (werner2024iggsialylationoccurs pages 1-2)
6. Trzos et al., 2023 (Front Immunol) - N-glycosylation in B-cell biology and IgG activity (trzos2023theroleof pages 1-2)
7. Glendenning et al., 2022 (J Leukoc Biol) - Divergent Golgi trafficking limits B cell IgG sialylation (glendenning2022divergentgolgitrafficking pages 1-2)

Subcellular Localization:
8. Yagi et al., 2024 (Cell Struct Funct) - Sub-Golgi localization by super-resolution imaging (yagi2024decipheringthesubgolgi pages 1-2)
9. Sun et al., 2021 (bioRxiv) - Quantitative study of Golgi retention (sun2021aquantitativestudy pages 1-5)

Cancer Biology:
10. Jones et al., 2023 (Glycobiology) - Role in ovarian cancer cell metabolism (jones2023roleofthe pages 1-2)
11. Dashzeveg et al., 2023 (Cancer Discov) - CTC hyposialylation and metastasis (dashzeveg2023dynamicglycoproteinhyposialylation pages 1-2)
12. Gc et al., 2022 (Front Mol Biosci) - Oncogenic signaling pathways (gc2022st6gal1oncogenicsignaling pages 1-2)
13. Munkley, 2022 (Cancers) - Aberrant sialylation in cancer (munkley2022aberrantsialylationin pages 1-2)
14. Pietrobono & Stecca, 2021 (Cancers) - Biomarker and therapeutic target potential (pietrobono2021aberrantsialylationin pages 1-2)

Inflammatory Signaling:
15. Holdbrooks et al., 2020 (PLoS ONE) - Regulation of inflammatory signaling (holdbrooks2020regulationofinflammatory pages 1-2)

Immune Cell Development:
16. Lau et al., 2024 (Authorea) - Sialic acid in blood cell production and extrinsic sialylation (lau2024sialicacidin pages 1-2)
17. Dipta et al., 2024 (bioRxiv) - ST6GAL1 in tumor-associated macrophages (dipta2024st6gal1mediatedsialyllinkage pages 1-4)

Broader Biological Context:
18. Zhu et al., 2024 (Cell Death Discov) - Biological functions of sialic acid (zhu2024biologicalfunctionof pages 1-2)
19. Kang et al., 2024 (Front Aging Neurosci) - Glycoconjugates in Alzheimer's disease (kang2024thealterationand pages 1-2)
20. Gao et al., 2021 (Cells) - Role of glycans on cell surface receptors (gao2021roleofglycans pages 1-3)

This comprehensive literature-backed analysis supports the core GO annotations for ST6GAL1 as a trans-Golgi-localized β-galactoside α2,6-sialyltransferase with well-established roles in IgG Fc sialylation, B cell development, and inflammatory signaling. Context-specific cancer-related functions are extensively documented but represent disease-specific phenotypes. Evidence for neuronal functions, pyroptosis, and synaptic remodeling is insufficient for annotation. No proteolytic processing is required for enzymatic activity, though BACE1 shedding generates an extracellular form.

References

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(lau2024sialicacidin pages 1-2): Joseph Lau, Eric Edward Irons, and Sajina GC. Sialic acid in the regulation of blood cell production, differentiation, and turnover. Unknown journal, Apr 2024. URL: https://doi.org/10.22541/au.171307803.38439809/v1, doi:10.22541/au.171307803.38439809/v1.
(yagi2024decipheringthesubgolgi pages 1-2): Hirokazu Yagi, Seigo Tateo, Taiki Saito, Yusaku Ohta, Emiko Nishi, Saemi Obitsu, Tatsuya Suzuki, Supaphorn Seetaha, Charles Hellec, Akihiko Nakano, Takuro Tojima, and Koichi Kato. Deciphering the sub-golgi localization of glycosyltransferases via 3d super-resolution imaging. Cell Structure and Function, 49:47-55, Jul 2024. URL: https://doi.org/10.1247/csf.24008, doi:10.1247/csf.24008. This article has 15 citations and is from a peer-reviewed journal.
(sun2021aquantitativestudy pages 1-5): Xiuping Sun, Bing Chen, Zhiwei Song, and Lei Lu. A quantitative study of the golgi retention of glycosyltransferases. bioRxiv, Feb 2021. URL: https://doi.org/10.1101/2021.02.15.431224, doi:10.1101/2021.02.15.431224. This article has 34 citations.
(gu2024specificsialylationof pages 1-2): Jianguo Gu and Tomoya Isaji. Specific sialylation of n-glycans and its novel regulatory mechanism. Glycoconjugate Journal, 41:175-183, Jul 2024. URL: https://doi.org/10.1007/s10719-024-10157-8, doi:10.1007/s10719-024-10157-8. This article has 16 citations and is from a peer-reviewed journal.
(pietrobono2021aberrantsialylationin pages 1-2): Silvia Pietrobono and Barbara Stecca. Aberrant sialylation in cancer: biomarker and potential target for therapeutic intervention? Cancers, 13:2014, Apr 2021. URL: https://doi.org/10.3390/cancers13092014, doi:10.3390/cancers13092014. This article has 139 citations.
(dashzeveg2023dynamicglycoproteinhyposialylation pages 1-2): Nurmaa K. Dashzeveg, Yuzhi Jia, Youbin Zhang, Lorenzo Gerratana, Priyam Patel, Asif Shajahan, Tsogbadrakh Dandar, Erika K. Ramos, Hannah F. Almubarak, Valery Adorno-Cruz, Rokana Taftaf, Emma J. Schuster, David Scholten, Michael T. Sokolowski, Carolina Reduzzi, Lamiaa El-Shennawy, Andrew D. Hoffmann, Maroua Manai, Qiang Zhang, Paolo D'Amico, Parastoo Azadi, Karen J. Colley, Leonidas C. Platanias, Ami N. Shah, William J. Gradishar, Massimo Cristofanilli, William A. Muller, Brian A. Cobb, and Huiping Liu. Dynamic glycoprotein hyposialylation promotes chemotherapy evasion and metastatic seeding of quiescent circulating tumor cell clusters in breast cancer. Cancer Discovery, 13:2050-2071, Jun 2023. URL: https://doi.org/10.1158/2159-8290.cd-22-0644, doi:10.1158/2159-8290.cd-22-0644. This article has 64 citations and is from a highest quality peer-reviewed journal.
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(gao2021roleofglycans pages 1-3): Yin Gao, Xue Luan, Jacob Melamed, and Inka Brockhausen. Role of glycans on key cell surface receptors that regulate cell proliferation and cell death. Cells, 10:1252, May 2021. URL: https://doi.org/10.3390/cells10051252, doi:10.3390/cells10051252. This article has 73 citations.
(munkley2022aberrantsialylationin pages 1-2): Jennifer Munkley. Aberrant sialylation in cancer: therapeutic opportunities. Cancers, 14:4248, Aug 2022. URL: https://doi.org/10.3390/cancers14174248, doi:10.3390/cancers14174248. This article has 127 citations.
(zhu2024biologicalfunctionof pages 1-2): Wengen Zhu, Yue Zhou, Linjuan Guo, and Shenghui Feng. Biological function of sialic acid and sialylation in human health and disease. Cell Death Discovery, Sep 2024. URL: https://doi.org/10.1038/s41420-024-02180-3, doi:10.1038/s41420-024-02180-3. This article has 191 citations and is from a peer-reviewed journal.
(dipta2024st6gal1mediatedsialyllinkage pages 1-4): Priya Dipta, Naaz Bansal, Zeynep Sumer-Bayraktar, Arthur Chien, Julian Ugonotti, The Huong Chau, Merrina Anugraham, Daniel Kolarich, Boaz Tirosh, Rebeca Kawahara, and Morten Thaysen-Andersen. St6gal1-mediated sialyl linkage switching in tumor-associated macrophages drives cancer-promoting nanotubes carrying α2,6-sialylation in anti-inflammatory cells. bioRxiv, Dec 2024. URL: https://doi.org/10.1101/2024.11.28.625934, doi:10.1101/2024.11.28.625934. This article has 0 citations.
(kang2024thealterationand pages 1-2): Yue Kang, Qian Zhang, Silu Xu, and Yue Yu. The alteration and role of glycoconjugates in alzheimer’s disease. Frontiers in Aging Neuroscience, Jun 2024. URL: https://doi.org/10.3389/fnagi.2024.1398641, doi:10.3389/fnagi.2024.1398641. This article has 9 citations and is from a peer-reviewed journal.
(glendenning2022divergentgolgitrafficking pages 1-2): Leandre M Glendenning, Julie Y Zhou, Kalob M Reynero, and Brian A Cobb. Divergent golgi trafficking limits b cell-mediated igg sialylation. Jun 2022. URL: https://doi.org/10.1002/jlb.3ma0522-731r, doi:10.1002/jlb.3ma0522-731r. This article has 16 citations and is from a peer-reviewed journal.

📚 Additional Documentation

Notes

(ST6GAL1-notes.md)

ST6GAL1 (human, P15907) review notes

Identity and core enzymology

ST6GAL1 = Beta-galactoside alpha-2,6-sialyltransferase 1 (ST6Gal-I), EC 2.4.3.1, CAZy GT29
(glycosyltransferase family 29). UniProt P15907, 406 aa, single-pass type II membrane protein
(cytoplasmic 1-9, TM 10-26, lumenal catalytic 27-406). [UniProt P15907]
Catalytic reaction (UniProt/Rhea RHEA:52104): "a beta-D-galactoside + CMP-N-acetyl-beta-neuraminate
= an N-acetyl-alpha-neuraminyl-(2->6)-beta-D-galactosyl derivative + CMP + H(+)". It transfers
Neu5Ac from CMP-Neu5Ac in alpha-2,6 linkage onto terminal galactose of complex N-glycans, forming
the Sia-alpha-2,6-Gal epitope. UniProt lists many enzyme-specific RHEA reactions for biantennary
complex N-glycans. [UniProt P15907]
Kinetics: KM = 530 uM for CMP-NeuAc; inhibited by CTP. Crystal structure (4JS1/4JS2; PDB 6QVS/6QVT
of catalytic domain) shows GT-A variant fold, SN2 inversion mechanism, substrate-assisted
charge-relay catalysis. [PMID:23999306 abstract: "ST6Gal-I adopts a variant of the canonical
glycosyltransferase A fold"; "support an SN2 mechanism with inversion of configuration"]
KM/activity measured by phosphatase-coupled assay. PMID:21081508
Activity confirmed by multiple groups making the alpha-2,6-sialyl N-glycan epitope, including
IgG Fc remodeling. [PMID:25733881, PMID:36280670, PMID:37632720 — all EXP/IDA GO:0003835 in GOA]

Localization

Golgi apparatus, Golgi stack membrane; membrane-bound form in trans cisternae of Golgi; colocalizes
with MGAT3 and B4GALT1 in Golgi stacks. Also Secreted (soluble form in body fluids, by similarity to
rat P13721). [UniProt P15907 SUBCELLULAR LOCATION]
IDA Golgi membrane localization (live-cell). PMID:20378551

Subunit / dimerization

Monomer and homodimer. ST6Gal-I forms disulfide-mediated + non-covalent homodimers in live Golgi
membranes (BiFC). >90% of molecules appear homodimeric. Also forms a trans-Golgi heterodimer with
B4GALT1 (GalT). C24G TM-cysteine mutation abolishes disulfide dimer but not BiFC homodimerization.
[PMID:20378551 "over 90% of the SiaT molecules appear to be homodimeric in the Golgi membranes of
live cells"; "the two trans-Golgi enzymes (GalT and SiaT) ... assemble into Golgi-localized
heterodimers"] — supports GO:0042803 protein homodimerization activity (IDA).

CD75 caution

ST6GAL1 has historically been mis-equated with CD75/CDw75; CD75 is actually a carbohydrate epitope
(Sia-alpha-2,6-Gal product) generated by ST6GAL1, not the protein itself. Many commercial "ST6GAL1"
antibodies based on this confusion do not recognize ST6GAL1. [UniProt CAUTION; PMID:35789385
"incorrect use of CD75 as a synonym for ST6GAL1"; PMID:1547499; PMID:1730763]

Biology (downstream / context-specific)

Major enzyme generating alpha-2,6-sialylated N-glycans on serum and cell-surface glycoproteins.
Soluble form (cleaved by BACE1) is abundant in serum, secreted by liver (acute phase) and platelets,
mediating "extrinsic" sialylation. [deep-research falcon; UniProt notes Secreted form]
Sialylates IgG Fc N297 N-glycan, modulating anti-inflammatory IgG effector function. [UniProt
FUNCTION; PMID:25733881; PMID:27872474]
B-cell biology: generates CDw75/HB-6/CD76 differentiation antigens; Sia-alpha-2,6-Gal is the ligand
for CD22/Siglec-2 (B-cell masking/regulation). [PMID:1730763 via UniProt]
Cancer (EGFR, integrin beta1, TNFR1, Fas, etc.), inflammation (TNFR1/TLR4 sialylation), monocyte/
macrophage development (extrinsic CSF1R sialylation) — all downstream consequences of the same
catalytic activity acting on different substrate glycoproteins; not separable core MF.

GOA annotation assessment summary (35 annotations)

Core molecular function: GO:0003835 beta-galactoside alpha-2,6-sialyltransferase activity. Many
duplicate rows (IBA, IEA, 3x TAS-Reactome, 3x EXP, 1 IDA) — all ACCEPT/KEEP; the EXP/IDA-backed ones
are the strongest. GO:0008373 sialyltransferase activity (parent) is correct but over-general -> MODIFY
to the specific GO:0003835.

Process: GO:0097503 sialylation and GO:0006487 protein N-linked glycosylation are accurate and
specific (alpha-2,6-sialylation is the terminal capping step of complex N-glycan biosynthesis) ->
ACCEPT. GO:0009101 glycoprotein biosynthetic process is a vaguer parent -> KEEP_AS_NON_CORE.
GO:0006054 N-acetylneuraminate metabolic process (IDA, PMID:23999306) -> ACCEPT as a correct, slightly
broad sibling describing sialic-acid utilization.

GO:0016266 protein O-linked glycosylation via N-acetylgalactosamine (TAS Reactome sTn / O-glycan
termination): ST6GAL1 is primarily an N-glycan enzyme but Reactome documents it adding sialic acid to
Tn antigens forming sTn (O-GalNAc) and terminating Core1/2 O-glycans. Keep but as non-core (minor /
context-specific activity relative to the dominant N-glycan role).

GO:0019082 viral protein processing (2x TAS Reactome, SARS-CoV-2 3a/spike sialylation): this is a
generic host-glycosylation step the Reactome curators slotted into viral life-cycle pathways. The MF
(sialylation) is real but "viral protein processing" mis-frames a housekeeping glycosylation step as a
dedicated viral function -> MARK_AS_OVER_ANNOTATED.

Locations: GO:0005794 Golgi apparatus (IBA), GO:0000139 Golgi membrane (IDA PMID:20378551 + 5x TAS),
GO:0032580 Golgi cisterna membrane (IEA) -> ACCEPT (core). GO:0005576 extracellular region (IEA + ISS
from rat ortholog P13721) -> KEEP_AS_NON_CORE (the soluble secreted form is real but its serum/extrinsic
activity is secondary to the Golgi-resident core function).

GO:0042803 protein homodimerization activity (IDA, PMID:20378551) -> ACCEPT (well supported by BiFC;
biologically real, organizes the enzyme in the Golgi). Not the core catalytic MF but a genuine MF.

GO:0005515 protein binding (IPI, PMID:16237761): from a yeast two-hybrid screen for hepatocyte proteins
binding HCV F protein; "1 colony was sialyltransferase" among 36 colonies. Generic, uninformative
protein-binding with weak Y2H evidence and no biological follow-up for ST6GAL1. Per curation guideline
to avoid generic "protein binding" -> REMOVE.

Falcon integration (2026-06-21)

Used from Falcon report (after verifying against UniProt / cached pubs):
- Core enzymology framing (GT29, trans-Golgi, CMP-Sia donor, alpha-2,6 linkage on complex N-glycans) —
matches UniProt and PMID:23999306. USED.
- Soluble/secreted form via BACE1 shedding; serum source; extrinsic sialylation — consistent with
UniProt "Secreted" + Note. USED as background; supports KEEP_AS_NON_CORE for extracellular region.
- IgG Fc N297 sialylation and B-cell/CD22 immune-regulatory framing — consistent with UniProt FUNCTION
and cached PMID:25733881/27872474. USED as biological context (KEEP_AS_NON_CORE territory).
- Context-specific cancer/inflammation substrate biology (EGFR, TNFR1, TLR4, integrin beta1, CSF1R) —
USED only as downstream-context rationale, not as separable core functions. Treated as KEEP_AS_NON_CORE
conceptually; none correspond to a current GOA row here, so they inform notes/questions, not actions.

Rejected / not used as citations:
- Falcon repeatedly cites "EC 2.4.99.1" for ST6GAL1. REJECTED: UniProt/IUBMB current EC for ST6GAL1 is
EC 2.4.3.1 (the 2.4.99.x sialyltransferase EC class was renumbered to 2.4.3.x). Did not propagate the
stale EC number.
- All Falcon citations use opaque internal keys (e.g. "sankaranarayanan2023computationalstudieson",
"lau2024sialicacidin") rather than PMIDs/DOIs, and several are preprints (bioRxiv, Authorea) with 0
citations. REJECTED as direct citations: none were added to the YAML references; only claims that I
could independently anchor to UniProt or a cached PMID were used.
- Falcon's neuronal / synaptic-remodeling / pyroptosis / autophagy / developmental-cell-death
sections: Falcon itself concludes "no direct evidence" for these. AGREED and REJECTED for annotation —
none proposed, consistent with the absence of such GOA rows.
- Falcon's long table of context-specific protein substrates (MET, HER2, VEGFR, PODXL, ICAM1, ALCAM1,
ECE1, etc.) is mostly review-/preprint-level and rated Moderate/Limited even by Falcon. NOT used to
author any GO term or substrate claim.

📄 View Raw YAML

id: P15907
gene_symbol: ST6GAL1
product_type: PROTEIN
status: DRAFT
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  ST6GAL1 (beta-galactoside alpha-2,6-sialyltransferase 1, ST6Gal-I; EC 2.4.3.1)
  is a Golgi-resident, single-pass type II membrane glycosyltransferase of the
  CAZy GT29 (sialyltransferase) family. It catalyzes transfer of N-acetylneuraminic
  acid (Neu5Ac) from the donor CMP-Neu5Ac in an alpha-2,6 linkage onto the terminal
  beta-linked galactose of complex-type N-glycans, generating the
  Sia-alpha-2,6-Gal (Neu5Ac-alpha-2,6-Gal-beta1,4-GlcNAc) epitope. It is the major
  enzyme producing alpha-2,6-sialylated N-glycans in vertebrates; this terminal cap
  is the recognition determinant of the SNA lectin and the ligand for the B-cell
  inhibitory receptor CD22/Siglec-2, and corresponds to the carbohydrate antigen
  formerly mislabeled CD75. The catalytic ectodomain (residues ~89-406) adopts a
  variant glycosyltransferase-A fold and uses an SN2 inversion, substrate-assisted
  charge-relay mechanism; the enzyme is inhibited by CTP. ST6GAL1 acts in the trans
  cisternae of the Golgi stack, where it forms disulfide-mediated and non-covalent
  homodimers and a trans-Golgi heterodimer with beta-1,4-galactosyltransferase 1.
  A catalytically active soluble ectodomain is released by proteolytic shedding and
  circulates in serum (secreted notably by liver during the acute-phase response and
  by activated platelets), where it can sialylate glycoproteins extrinsically. By
  modifying the N-glycans of secreted and cell-surface glycoproteins, ST6GAL1
  influences immunoglobulin G Fc anti-inflammatory activity, B-cell and myeloid
  biology, and is frequently upregulated in carcinomas, but these roles all derive
  from the single alpha-2,6-sialyltransferase catalytic activity acting on diverse
  substrate glycoproteins.
alternative_products:
- name: '1'
  id: P15907-1
- name: '2'
  id: P15907-2
  sequence_note: VSP_056076
existing_annotations:
- term:
    id: GO:0003835
    label: beta-galactoside alpha-2,6-sialyltransferase activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: >-
      This is the core molecular function of ST6GAL1 and is correct at the most
      specific level. Phylogenetic (IBA) inference here agrees with extensive direct
      experimental evidence (crystal structure, kinetics, and multiple in vitro
      activity assays) and with the UniProt/Rhea catalytic activity (RHEA:52104,
      EC 2.4.3.1).
    action: ACCEPT
    supported_by:
    - reference_id: PMID:23999306
      supporting_text: >-
        Human beta-galactoside alpha-2,6-sialyltransferase I (ST6Gal-I) establishes
        the final glycosylation pattern of many glycoproteins by transferring a sialyl
        moiety to a terminal galactose.
- term:
    id: GO:0006487
    label: protein N-linked glycosylation
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: >-
      Accurate and appropriately specific. ST6GAL1's principal biological role is the
      terminal alpha-2,6-sialylation (capping) step of complex-type N-glycan
      maturation. Supported independently by IDA (PMID:23999306) and consistent with
      the UniProt-annotated complex N-glycan acceptor reactions.
    action: ACCEPT
- term:
    id: GO:0097503
    label: sialylation
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: >-
      Correct. Sialylation is the process directly performed by ST6GAL1. Also
      independently supported by IDA (PMID:23999306). A reasonable core process term,
      slightly broader than the specific alpha-2,6 linkage but accurate.
    action: ACCEPT
- term:
    id: GO:0005794
    label: Golgi apparatus
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: >-
      Correct core localization. ST6GAL1 is a trans-Golgi resident enzyme; the IBA
      call agrees with direct evidence for Golgi membrane localization
      (PMID:20378551) and UniProt (Golgi stack membrane, trans cisternae).
    action: ACCEPT
- term:
    id: GO:0003835
    label: beta-galactoside alpha-2,6-sialyltransferase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: >-
      Duplicate of the core catalytic MF from an automated (multi-method IEA) source
      mapped to EC 2.4.3.1 / RHEA. Correct; redundant with the IBA, EXP, IDA and TAS
      annotations to the same term.
    action: ACCEPT
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  qualifier: located_in
  review:
    summary: >-
      Reflects the soluble form of ST6GAL1, which is shed from the Golgi/membrane and
      circulates in serum, where it can sialylate glycoproteins extrinsically. This is
      a genuine secondary localization (UniProt: Secreted, by similarity to rat
      P13721) but is secondary to the Golgi-resident catalytic site of action; kept as
      non-core.
    action: KEEP_AS_NON_CORE
    supported_by:
    - reference_id: file:human/ST6GAL1/ST6GAL1-deep-research-falcon.md
      supporting_text: >-
        BACE1 (β-site amyloid precursor protein-cleaving enzyme 1) cleaves ST6GAL1 to
        release a soluble, catalytically active form lacking the transmembrane domain,
        enabling secretion and extracellular sialylation activity
- term:
    id: GO:0008373
    label: sialyltransferase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: >-
      Correct but over-general parent term. The specific activity
      beta-galactoside alpha-2,6-sialyltransferase activity (GO:0003835) is
      experimentally established and should be used instead.
    action: MODIFY
    proposed_replacement_terms:
    - id: GO:0003835
      label: beta-galactoside alpha-2,6-sialyltransferase activity
- term:
    id: GO:0009101
    label: glycoprotein biosynthetic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: involved_in
  review:
    summary: >-
      Not wrong but a broad parent of the more informative N-linked glycosylation /
      sialylation process annotations already present. Retained as a non-core,
      higher-level descriptor (InterPro2GO electronic inference).
    action: KEEP_AS_NON_CORE
- term:
    id: GO:0032580
    label: Golgi cisterna membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  qualifier: located_in
  review:
    summary: >-
      Correct and appropriately specific localization (UniProt subcellular-location
      mapping). ST6GAL1 is a single-pass type II membrane protein of the Golgi stack
      cisternae (trans cisternae), consistent with PMID:20378551 and UniProt.
    action: ACCEPT
- term:
    id: GO:0006487
    label: protein N-linked glycosylation
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: >-
      Duplicate of the core N-linked glycosylation process annotation, transferred
      electronically from the mouse ortholog (Q64685). Correct.
    action: ACCEPT
- term:
    id: GO:0016266
    label: protein O-linked glycosylation via N-acetylgalactosamine
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-977068
  qualifier: involved_in
  review:
    summary: >-
      ST6GAL1 is predominantly an N-glycan alpha-2,6-sialyltransferase, but Reactome
      documents a minor activity terminating O-GalNAc (mucin-type) glycans, including
      conversion of Tn to sialyl-Tn antigen. This O-glycan role is real but minor and
      context-specific relative to the dominant N-glycan function; kept as non-core.
    action: KEEP_AS_NON_CORE
- term:
    id: GO:0003835
    label: beta-galactoside alpha-2,6-sialyltransferase activity
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-4085033
  qualifier: enables
  review:
    summary: >-
      Core catalytic MF (Reactome reaction "ST6GAL1,2 transfer Neu5Ac to terminal Gal
      (alpha-2,6 link)"). Correct; redundant with the experimentally supported
      annotations to the same term.
    action: ACCEPT
- term:
    id: GO:0003835
    label: beta-galactoside alpha-2,6-sialyltransferase activity
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-975902
  qualifier: enables
  review:
    summary: >-
      Core catalytic MF (Reactome "ST6GAL1 transfers Neu5Ac to terminal Gal of
      N-glycans"). Correct; redundant duplicate.
    action: ACCEPT
- term:
    id: GO:0003835
    label: beta-galactoside alpha-2,6-sialyltransferase activity
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-977071
  qualifier: enables
  review:
    summary: >-
      Core catalytic MF; this Reactome reaction concerns transfer of sialic acid to Tn
      antigens to form sialyl-Tn. The activity is correct (same enzyme), though the
      O-GalNAc/sTn acceptor is a minor context relative to the N-glycan role.
    action: ACCEPT
- term:
    id: GO:0003835
    label: beta-galactoside alpha-2,6-sialyltransferase activity
  evidence_type: EXP
  original_reference_id: PMID:25733881
  qualifier: enables
  review:
    summary: >-
      Strong experimental support for the core catalytic activity: controlled
      alpha-2,6-sialylation of IgG Fc N-glycans by ST6GAL1 to generate
      tetra-Fc-sialylated IVIg with enhanced anti-inflammatory activity.
    action: ACCEPT
    supported_by:
    - reference_id: PMID:25733881
      supporting_text: >-
        Controlled tetra-Fc sialylation of IVIg results in a drug candidate with
        consistent enhanced anti-inflammatory activity.
- term:
    id: GO:0003835
    label: beta-galactoside alpha-2,6-sialyltransferase activity
  evidence_type: EXP
  original_reference_id: PMID:36280670
  qualifier: enables
  review:
    summary: >-
      Strong experimental support for the core catalytic activity (cell-free
      glycoenzyme remodeling pipeline using recombinant ST6GAL1 to install alpha-2,6
      sialic acid; UniProt-cited EC 2.4.3.1 evidence).
    action: ACCEPT
    supported_by:
    - reference_id: PMID:36280670
      supporting_text: >-
        A universal glycoenzyme biosynthesis pipeline that enables efficient cell-free
        remodeling of glycans.
- term:
    id: GO:0003835
    label: beta-galactoside alpha-2,6-sialyltransferase activity
  evidence_type: EXP
  original_reference_id: PMID:37632720
  qualifier: enables
  review:
    summary: >-
      Strong experimental support for the core catalytic activity (divergent enzymatic
      assembly of a 64-membered IgG N-glycan library using ST6GAL1 for alpha-2,6
      sialylation; UniProt-cited EC 2.4.3.1 evidence).
    action: ACCEPT
    supported_by:
    - reference_id: PMID:37632720
      supporting_text: >-
        Divergent Enzymatic Assembly of a Comprehensive 64-Membered IgG N-Glycan
        Library for Functional Glycomics.
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: located_in
  review:
    summary: >-
      Sequence-similarity transfer (from rat ortholog P13721) of the secreted/soluble
      localization. Same biological content as the IEA extracellular-region
      annotation; the soluble serum form is genuine but secondary to the Golgi
      catalytic site. Kept as non-core.
    action: KEEP_AS_NON_CORE
- term:
    id: GO:0019082
    label: viral protein processing
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9683673
  qualifier: involved_in
  review:
    summary: >-
      This annotation comes from Reactome SARS-CoV-2 pathways in which cellular
      sialyltransferases glycosylate viral proteins (here ORF3a). The underlying
      molecular event is ST6GAL1's ordinary host-glycosylation activity; framing a
      housekeeping glycosylation step as a dedicated "viral protein processing"
      function over-extends the gene's role. The enzyme is not virus-specific.
    action: MARK_AS_OVER_ANNOTATED
- term:
    id: GO:0019082
    label: viral protein processing
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9694548
  qualifier: involved_in
  review:
    summary: >-
      Same as above (Reactome SARS-CoV-2 spike maturation). The molecular event is
      generic host sialylation of viral glycoproteins; "viral protein processing"
      over-extends ST6GAL1's role. The Reactome curation even states the spike sialic
      acids are added "presumably by the cell's sialyltransferases".
    action: MARK_AS_OVER_ANNOTATED
- term:
    id: GO:0008373
    label: sialyltransferase activity
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9683769
  qualifier: enables
  review:
    summary: >-
      Over-general parent MF asserted within a SARS-CoV-2 O-glycosylation Reactome
      reaction. The specific activity GO:0003835 is established for ST6GAL1 and should
      be used instead of the parent sialyltransferase activity term.
    action: MODIFY
    proposed_replacement_terms:
    - id: GO:0003835
      label: beta-galactoside alpha-2,6-sialyltransferase activity
- term:
    id: GO:0008373
    label: sialyltransferase activity
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9697018
  qualifier: enables
  review:
    summary: >-
      Over-general parent MF asserted within a SARS-CoV-2 spike sialylation Reactome
      reaction. Replace with the specific GO:0003835 established for ST6GAL1.
    action: MODIFY
    proposed_replacement_terms:
    - id: GO:0003835
      label: beta-galactoside alpha-2,6-sialyltransferase activity
- term:
    id: GO:0000139
    label: Golgi membrane
  evidence_type: IDA
  original_reference_id: PMID:20378551
  qualifier: located_in
  review:
    summary: >-
      Direct, live-cell evidence (BiFC and immunofluorescence with a Golgi marker)
      that ST6GAL1 localizes to Golgi membranes. This is the core localization and the
      site of catalytic action.
    action: ACCEPT
    supported_by:
    - reference_id: PMID:20378551
      supporting_text: >-
        BiFC signal was detected almost exclusively in the Golgi membranes of the
        cells.
- term:
    id: GO:0042803
    label: protein homodimerization activity
  evidence_type: IDA
  original_reference_id: PMID:20378551
  qualifier: enables
  review:
    summary: >-
      Well supported by direct evidence: ST6GAL1 forms disulfide-mediated and
      non-covalent homodimers in live Golgi membranes (BiFC; >90% of molecules
      homodimeric), independent of the TM cysteine (C24G). A genuine molecular
      function that organizes the enzyme in the Golgi, though distinct from its core
      catalytic activity.
    action: ACCEPT
    supported_by:
    - reference_id: PMID:20378551
      supporting_text: >-
        over 90% of the SiaT molecules appear to be homodimeric in the Golgi membranes
        of live cells
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:16237761
  qualifier: enables
  review:
    summary: >-
      Uninformative generic protein-binding annotation derived from a yeast two-hybrid
      screen for hepatocyte proteins binding the hepatitis C virus F protein, in which
      "1 colony was sialyltransferase" among 36 colonies, with no follow-up validation
      specific to ST6GAL1. Per curation guidance, generic "protein binding" with weak,
      non-physiological Y2H evidence and no informative partner-specific function
      should not be retained.
    action: REMOVE
- term:
    id: GO:0003835
    label: beta-galactoside alpha-2,6-sialyltransferase activity
  evidence_type: IDA
  original_reference_id: PMID:23999306
  qualifier: enables
  review:
    summary: >-
      Direct experimental support for the core catalytic activity from the human
      ST6Gal-I crystal structure paper (mechanism, CMP product/inhibitor complexes,
      glycan binding mode). Strongest single-paper evidence for GO:0003835.
    action: ACCEPT
    supported_by:
    - reference_id: PMID:23999306
      supporting_text: >-
        ST6Gal-I establishes the final glycosylation pattern of many glycoproteins by
        transferring a sialyl moiety to a terminal galactose.
- term:
    id: GO:0006054
    label: N-acetylneuraminate metabolic process
  evidence_type: IDA
  original_reference_id: PMID:23999306
  qualifier: involved_in
  review:
    summary: >-
      Correct: by consuming CMP-Neu5Ac and transferring Neu5Ac onto acceptors, ST6GAL1
      participates in sialic-acid (N-acetylneuraminate) metabolism. Slightly broad but
      accurate; retained as a non-core process descriptor alongside the more specific
      sialylation term.
    action: KEEP_AS_NON_CORE
- term:
    id: GO:0006487
    label: protein N-linked glycosylation
  evidence_type: IDA
  original_reference_id: PMID:23999306
  qualifier: involved_in
  review:
    summary: >-
      Direct support for the core process: the structure paper characterizes ST6Gal-I
      acting on complex N-glycan acceptors. Correct and specific.
    action: ACCEPT
    supported_by:
    - reference_id: PMID:23999306
      supporting_text: >-
        the binding mode of complex glycans
- term:
    id: GO:0097503
    label: sialylation
  evidence_type: IDA
  original_reference_id: PMID:23999306
  qualifier: involved_in
  review:
    summary: >-
      Direct support for the core sialylation process. Correct; consistent with the
      IBA annotation to the same term.
    action: ACCEPT
    supported_by:
    - reference_id: PMID:23999306
      supporting_text: >-
        transferring a sialyl moiety to a terminal galactose
- term:
    id: GO:0000139
    label: Golgi membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-4085033
  qualifier: located_in
  review:
    summary: >-
      Core Golgi-membrane localization (Reactome). Correct; redundant with the IDA
      Golgi membrane annotation.
    action: ACCEPT
- term:
    id: GO:0000139
    label: Golgi membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9683769
  qualifier: located_in
  review:
    summary: >-
      Core Golgi-membrane localization asserted in a SARS-CoV-2 O-glycosylation
      Reactome reaction. The localization itself is correct (redundant duplicate).
    action: ACCEPT
- term:
    id: GO:0000139
    label: Golgi membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9697018
  qualifier: located_in
  review:
    summary: >-
      Core Golgi-membrane localization asserted in a SARS-CoV-2 spike sialylation
      Reactome reaction. Correct; redundant duplicate.
    action: ACCEPT
- term:
    id: GO:0000139
    label: Golgi membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-975902
  qualifier: located_in
  review:
    summary: >-
      Core Golgi-membrane localization (Reactome N-glycan sialylation reaction).
      Correct; redundant duplicate.
    action: ACCEPT
- term:
    id: GO:0000139
    label: Golgi membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-977071
  qualifier: located_in
  review:
    summary: >-
      Core Golgi-membrane localization (Reactome sialyl-Tn reaction). Correct;
      redundant duplicate.
    action: ACCEPT
- term:
    id: GO:0008373
    label: sialyltransferase activity
  evidence_type: IDA
  original_reference_id: PMID:21081508
  qualifier: enables
  review:
    summary: >-
      Direct biochemical measurement of ST6GAL1 kinetics (phosphatase-coupled assay;
      KM ~530 uM for CMP-NeuAc) demonstrates sialyltransferase activity, but the term
      is the over-general parent. Modify to the specific
      beta-galactoside alpha-2,6-sialyltransferase activity (GO:0003835).
    action: MODIFY
    proposed_replacement_terms:
    - id: GO:0003835
      label: beta-galactoside alpha-2,6-sialyltransferase activity
core_functions:
- description: >-
    ST6GAL1 catalyzes the terminal alpha-2,6-sialylation of complex-type N-glycans:
    transfer of N-acetylneuraminic acid from CMP-Neu5Ac onto the terminal
    beta-1,4-linked galactose of N-glycan antennae, forming the
    Neu5Ac-alpha-2,6-Gal-beta1,4-GlcNAc (Sia-alpha-2,6-Gal / CD75) epitope. This is
    the major route to alpha-2,6-sialylated N-glycans and the capping step of complex
    N-glycan biosynthesis. The enzyme acts as a Golgi-resident type II membrane
    protein and uses an SN2 inversion mechanism (inhibited by CTP).
  molecular_function:
    id: GO:0003835
    label: beta-galactoside alpha-2,6-sialyltransferase activity
  directly_involved_in:
  - id: GO:0097503
    label: sialylation
  - id: GO:0006487
    label: protein N-linked glycosylation
  locations:
  - id: GO:0000139
    label: Golgi membrane
  - id: GO:0032580
    label: Golgi cisterna membrane
  substrates:
  - id: CHEBI:57812
    label: CMP-N-acetyl-beta-neuraminate
  supported_by:
  - reference_id: PMID:23999306
    supporting_text: >-
      Human beta-galactoside alpha-2,6-sialyltransferase I (ST6Gal-I) establishes the
      final glycosylation pattern of many glycoproteins by transferring a sialyl
      moiety to a terminal galactose.
  - reference_id: PMID:21081508
    supporting_text: >-
      we measured the kinetic parameters of human sialyltransferase ST6GAL1
  - reference_id: PMID:25733881
    supporting_text: >-
      Controlled tetra-Fc sialylation of IVIg results in a drug candidate with
- description: >-
    ST6GAL1 forms homodimers in the Golgi membrane (disulfide-mediated and
    non-covalent), with the large majority of molecules dimeric in live cells; it
    also assembles into a trans-Golgi heterodimer with beta-1,4-galactosyltransferase
    1. This self-association organizes the enzyme at its trans-Golgi site of action.
  molecular_function:
    id: GO:0042803
    label: protein homodimerization activity
  locations:
  - id: GO:0000139
    label: Golgi membrane
  supported_by:
  - reference_id: PMID:20378551
    supporting_text: >-
      over 90% of the SiaT molecules appear to be homodimeric in the Golgi membranes
      of live cells
proposed_new_terms:
- proposed_name: protein N-glycan alpha-2,6-sialyltransferase activity
  proposed_definition: >-
    Catalysis of the transfer of N-acetylneuraminate from CMP-N-acetyl-beta-neuraminate
    to the terminal beta-D-galactosyl residue of a complex-type N-linked
    oligosaccharide of a glycoprotein, forming an
    N-acetyl-alpha-neuraminyl-(2->6)-beta-D-galactosyl linkage (the Sia-alpha-2,6-Gal
    epitope). EC 2.4.3.1.
  justification: >-
    GO:0003835 (beta-galactoside alpha-2,6-sialyltransferase activity) is defined on a
    generic beta-galactoside acceptor, whereas the physiologically dominant ST6GAL1
    reaction acts specifically on the terminal galactose of complex-type N-glycans on
    glycoproteins (the UniProt/Rhea protein-N-glycan acceptor reactions, e.g.
    RHEA:83927 and RHEA:82947). A child term grounded on the protein-N-glycan acceptor
    would let ST6GAL1 (and paralog ST6GAL2) be annotated to their exact physiological
    activity rather than the broader free-galactoside parent.
  proposed_parent:
    id: GO:0003835
    label: beta-galactoside alpha-2,6-sialyltransferase activity
  proposed_mappings:
  - predicate: skos:relatedMatch
    target_term:
      id: RHEA:83927
      label: alpha-2,6-sialylation of the alpha(1->3) mannose antenna of a biantennary complex N-glycoprotein
  - predicate: skos:exactMatch
    target_term:
      id: EC:2.4.3.1
      label: beta-galactoside alpha-(2->6)-sialyltransferase
suggested_questions:
- question: >-
    What protease(s) shed the catalytically active soluble ST6GAL1 ectodomain into
    serum in humans, and is BACE1 the principal sheddase as suggested in the
    secondary literature? A GO localization/process annotation for the soluble form
    awaits this.
- question: >-
    To what extent does ST6GAL1 act physiologically on O-GalNAc (mucin-type) glycans
    and sialyl-Tn formation versus the dominant N-glycan acceptor, in normal tissues
    versus carcinoma?
- question: >-
    Are the homodimer and the B4GALT1 heterodimer functionally required for full
    catalytic efficiency or fidelity of N-glycan sialylation in vivo, or do they
    primarily serve Golgi retention?
suggested_experiments:
- description: >-
    Acceptor-substrate profiling of recombinant human ST6GAL1 against defined
    N-glycan, O-GalNAc (Tn/Core1/Core2), and free-galactoside acceptors to quantify
    the relative physiological preference and refine the activity annotation.
- description: >-
    Tissue-specific (B-cell, myeloid, hepatocyte) conditional St6gal1 knockouts with
    serum and cell-surface glycomics to dissociate intrinsic (Golgi) from extrinsic
    (soluble serum enzyme) alpha-2,6-sialylation contributions.
- description: >-
    Structure-guided disruption of the disulfide/non-covalent homodimer and the
    B4GALT1 heterodimer interfaces, followed by live-cell Golgi-retention and
    N-glycan-sialylation efficiency assays, to test the functional role of
    oligomerization.
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO
    terms
  findings: []
- id: GO_REF:0000024
  title: Manual transfer of experimentally-verified manual GO annotation data to orthologs
    by curator judgment of sequence similarity
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000044
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location
    vocabulary mapping, accompanied by conservative changes to GO terms applied by
    UniProt
  findings: []
- id: GO_REF:0000107
  title: Automatic transfer of experimentally verified manual GO annotation data to
    orthologs using Ensembl Compara
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:16237761
  title: Screening of hepatocyte proteins binding to F protein of hepatitis C virus
    by yeast two-hybrid system.
  findings: []
  reference_review:
    relevance: LOW
    correctness: MISCITED
    review_notes: >-
      Verified against the cached full text: a yeast two-hybrid screen for hepatocyte
      proteins binding HCV F protein, in which "1 colony was sialyltransferase" among
      36 colonies, with no ST6GAL1-specific validation. This is a generic, weakly
      supported protein-binding (IPI) annotation that does not inform ST6GAL1 function
      and is recommended for removal.
- id: PMID:20378551
  title: Golgi N-glycosyltransferases form both homo- and heterodimeric enzyme complexes
    in live cells.
  findings:
  - statement: >-
      ST6GAL1 (SiaT) forms disulfide-mediated and non-covalent homodimers in live
      Golgi membranes (>90% of molecules homodimeric) and a trans-Golgi heterodimer
      with B4GALT1; localizes to Golgi membranes.
    reference_section_type: RESULTS
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Cached full text confirms BiFC and biochemical evidence for ST6GAL1 Golgi
      localization and homodimerization (GO:0000139 IDA, GO:0042803 IDA).
- id: PMID:21081508
  title: Universal phosphatase-coupled glycosyltransferase assay.
  findings:
  - statement: >-
      Kinetic parameters of human sialyltransferase ST6GAL1 were measured using a
      phosphatase-coupled assay.
    reference_section_type: ABSTRACT
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      Abstract verifies direct enzymatic measurement of ST6GAL1; supports
      sialyltransferase activity but the assigned parent term GO:0008373 should be
      refined to GO:0003835.
- id: PMID:23999306
  title: The structure of human α-2,6-sialyltransferase reveals the binding mode of
    complex glycans.
  findings:
  - statement: >-
      Crystal structures of human ST6Gal-I define the GT-A variant fold, CMP
      product/inhibitor binding, complex-glycan binding mode, and an SN2 inversion,
      substrate-assisted catalytic mechanism.
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Abstract verified (full text not cached). Strongest structural/mechanistic
      support for the core alpha-2,6-sialyltransferase activity (GO:0003835, IDA).
- id: PMID:25733881
  title: Controlled tetra-Fc sialylation of IVIg results in a drug candidate with
    consistent enhanced anti-inflammatory activity.
  findings:
  - statement: >-
      ST6GAL1 alpha-2,6-sialylation of IgG Fc N-glycans yields tetra-Fc-sialylated
      IVIg with enhanced anti-inflammatory activity.
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Cached full text confirms ST6GAL1-mediated Fc sialylation; supports the core MF
      (EXP GO:0003835) and the IgG anti-inflammatory context.
- id: PMID:27872474
  title: Multi-level glyco-engineering techniques to generate IgG with defined Fc-glycans.
  findings:
  - statement: >-
      ST6GAL1 is used in the glycosylation pathway to install alpha-2,6 sialic acid on
      IgG Fc glycans (UniProt PATHWAY evidence for protein glycosylation).
    reference_section_type: ABSTRACT
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      UniProt-cited (PATHWAY) support for ST6GAL1's role in the IgG glycosylation
      pathway; corroborates the core sialyltransferase function.
- id: PMID:29133956
  title: Network inference from glycoproteomics data reveals new reactions in the IgG
    glycosylation pathway.
  findings:
  - statement: >-
      Glycoproteomics network inference places ST6GAL1 in the IgG glycosylation
      pathway and supports its trans-Golgi role.
    reference_section_type: ABSTRACT
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      UniProt-cited (SUBCELLULAR LOCATION / PATHWAY) support corroborating ST6GAL1's
      glycosylation-pathway role and Golgi localization.
- id: PMID:35789385
  title: The incorrect use of CD75 as a synonym for ST6GAL1 has fostered the expansion
    of commercial 'ST6GAL1' antibodies that do not recognize ST6GAL1.
  findings:
  - statement: >-
      CD75 is a carbohydrate epitope generated by ST6GAL1, not the ST6GAL1 protein;
      conflation of the two has produced commercial antibodies that do not recognize
      ST6GAL1.
    reference_section_type: ABSTRACT
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      UniProt-cited CAUTION reference; important for interpreting historical CD75/CDw75
      literature as describing the ST6GAL1 product epitope rather than the protein.
- id: PMID:36280670
  title: A universal glycoenzyme biosynthesis pipeline that enables efficient cell-free
    remodeling of glycans.
  findings:
  - statement: >-
      Recombinant ST6GAL1 installs alpha-2,6 sialic acid in a cell-free glycan
      remodeling pipeline (EC 2.4.3.1 catalytic activity).
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      UniProt-cited EXP evidence for the core catalytic activity (GO:0003835).
- id: PMID:37632720
  title: Divergent Enzymatic Assembly of a Comprehensive 64-Membered IgG N-Glycan
    Library for Functional Glycomics.
  findings:
  - statement: >-
      ST6GAL1 is used for alpha-2,6 sialylation in the divergent enzymatic assembly of
      a 64-membered IgG N-glycan library.
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      UniProt-cited EXP evidence for the core catalytic activity (GO:0003835).
- id: Reactome:R-HSA-4085033
  title: ST6GAL1,2 transfer Neu5Ac to terminal Gal (alpha-2,6 link)
  findings: []
- id: Reactome:R-HSA-9683673
  title: Maturation of protein 3a
  findings: []
- id: Reactome:R-HSA-9683769
  title: O-glycosylation of 3a is terminated
  findings: []
- id: Reactome:R-HSA-9694548
  title: Maturation of spike protein
  findings: []
- id: Reactome:R-HSA-9697018
  title: Addition of sialic acids on some Spike glycosyl sidechains
  findings: []
- id: Reactome:R-HSA-975902
  title: ST6GAL1 transfers Neu5Ac to terminal Gal of N-glycans
  findings: []
- id: Reactome:R-HSA-977068
  title: Termination of O-glycan biosynthesis
  findings: []
- id: Reactome:R-HSA-977071
  title: ST6GAL1 transfers sialic acid to Tn antigens to form sTn antigens
  findings: []

ST6GAL1

Gene Description

Proposed New Ontology Terms

protein N-glycan alpha-2,6-sialyltransferase activity

Existing Annotations Review

Core Functions

References

Suggested Questions for Experts

Suggested Experiments

Deep Research

Falcon

Gene Research for GO Annotation Review

Target

UniProt Context

Research Objective

Output Requirements

Comprehensive Research Report for GO Annotation Review: ST6GAL1 (Homo sapiens)

Executive Summary

1. Molecular Function

1.1 Core Biochemical Activity

1.2 Substrate Specificity

1.3 Catalytic Mechanism and Regulation

2. Biological Process Roles

2.1 Core Functions (Strongly Supported for GO Annotation)

2.2 Context-Specific Roles (Cancer Biology)

2.3 Roles with Limited or No Direct Evidence

3. Cellular Localization and Complexes

3.1 Subcellular Localization

3.2 Protein Complexes and Oligomerization

3.3 Trafficking and Compartment Dynamics

4. Annotation-Risk Assessment

4.1 Core vs. Context-Specific Functions

4.2 Localization Annotations

4.3 Maturation and Processing

5. Key Literature

📚 Additional Documentation

Notes

ST6GAL1 (human, P15907) review notes

Identity and core enzymology

Localization

Subunit / dimerization

CD75 caution

Biology (downstream / context-specific)

GOA annotation assessment summary (35 annotations)

Falcon integration (2026-06-21)

📄 View Raw YAML