st6gal1

ST6GAL1 (beta-galactoside alpha-2,6-sialyltransferase 1)

ST6GAL1 is the enzyme responsible for adding α2,6-linked sialic acid (Neu5Ac) to terminal galactose residues on N-glycans. In the IgG Fc N-glycosylation cascade, it is the final enzymatic step, adding sialic acid to galactosylated (G1 or G2) glycoforms to produce the sialylated species (G1S, G2S, G2FS1, G2FS2) that are the dominant anti-inflammatory glycoforms of IgG. Sialylated IgG Fc is the mechanistic basis of the anti-inflammatory action of intravenous immunoglobulin (IVIG) ¹². ST6GAL1 and mgat3 carry the two strongest GWAS signals for IgG glycan variation in humans ³.

Identity

• UniProt: P15907 (SIAT1_HUMAN) — confirmed via UniProt REST API 2026-05-20
• NCBI Gene: 6480
• HGNC: 10860
• Ensembl: ENSG00000073849
• Mouse ortholog: St6gal1
• Length: 406 amino acids (canonical isoform)
• EC number: 2.4.3.1 (current IUBMB/ExPASy designation; formerly catalogued as EC 2.4.99.1 — ExPASy ENZYME database now lists 2.4.99.1 as a “Transferred entry: 2.4.3.1” following a ~2018 sialyltransferase class renumbering. UniProt P15907 correctly reflects 2.4.3.1.)

Enzymology

ST6GAL1 is a type II single-pass transmembrane glycoprotein localized primarily to the Golgi apparatus (trans cisternae), with a soluble form (sST6GAL1) released into the circulation by BACE1 (β-site APP-cleaving enzyme 1) proteolytic cleavage of the stem region. The enzymatic reaction is:

CMP-Neu5Ac + terminal-Gal-GlcNAc-Asn → Neu5Ac(α2,6)-Gal-GlcNAc-Asn + CMP

Key specificities:

• Linkage specificity: α2,6 — not α2,3. Humans (unlike mice) do not produce α2,3-linked sialic acid on IgG Fc N-glycans. All sialic acid on human serum IgG Fc is α2,6-linked, produced exclusively by ST6GAL1.
• Substrate dependency: ST6GAL1 activity is substrate-limited by prior B4GALT1-mediated galactosylation. Agalactosylated glycoforms (G0, G0F) cannot be sialylated. This means that age-related decline in b4galt1 activity indirectly suppresses sialylation even if ST6GAL1 activity is maintained.
• ST6GAL1 vs ST6GAL2: ST6GAL2 is a closely related paralog (74% catalytic domain sequence identity) with different tissue distribution (predominantly expressed in brain/neural tissues); ST6GAL2 does not contribute substantially to IgG Fc sialylation in B cells.

The soluble serum form (sST6GAL1, released by BACE1 cleavage) has been detected in plasma and proposed as a biomarker of systemic inflammation / liver function, though its activity on extracellular substrates is debated ⁴. The IgG Fc sialylation occurs intracellularly within B cells, as demonstrated by bone marrow chimeric experiments showing that B-cell-specific ST6Gal1 deletion abrogates IgG sialylation, confirming that circulating sialidases or extracellular ST6GAL1 do not contribute meaningfully ⁵.

Position in the IgG Fc glycosylation cascade

ST6GAL1 is the final step (Step 5) in the Golgi processing cascade for IgG:

Step	Enzyme	Product
1–2	MGAT1, MGAT2	Biantennary complex-type scaffold
3	fut8 / mgat3	Core fucosylation or bisecting GlcNAc
4	b4galt1	G0 → G1/G2 (galactosylation)
5	ST6GAL1	G1/G2 → G1S/G2S (sialylation)

Aging-relevant biology

Sialylated IgG as the anti-inflammatory glycoform

Approximately 5% of serum IgG is sialylated in healthy adults ² (note: full-text verification unavailable; abstract-level only no-fulltext-access). This fraction is functionally disproportionate: it mediates the anti-inflammatory mechanism of IVIG ²¹. The mechanistic pathway:

1. Sialylated IgG Fc engages DC-SIGN (CD209) on regulatory macrophages / dendritic cells
2. DC-SIGN engagement → upregulates FcγRIIb (inhibitory FcγR, CD32b) on effector cells
3. FcγRIIb upregulation → global dampening of FcγRIII-mediated ADCC and cytokine release

Recombinant α2,6-sialylated IgG1 Fc fragments recapitulate IVIG anti-inflammatory activity in mouse models at ~30 mg/kg, compared to 1,000–2,000 mg/kg for native IVIG — a 30- to 67-fold dose reduction ¹.

Age-related decline in sialylation

ST6GAL1 activity in B cells declines with age, contributing to the age-associated fall in sialylated IgG fractions observed in large population studies ⁶. The loss of sialylated IgG with age removes the DC-SIGN/FcγRIIb anti-inflammatory brake, contributing to the inflammaging phenotype. This is distinct from (but synergistic with) the B4GALT1-mediated G0 rise: both galactosylation loss and sialylation loss shift the IgG glycome toward a pro-inflammatory profile.

Inflammation-mediated suppression

Like B4GALT1, ST6GAL1 is downregulated by pro-inflammatory cytokines (IL-6, TNF-α, type I IFNs) in B cells, creating a bidirectional feedback loop ⁶. Type I interferons produced by senescent cells may contribute specifically to ST6GAL1 suppression via the SASP — a mechanistic link between cellular senescence, the IgG glycome, and inflammaging that remains to be directly tested in vivo. needs-replication

Sialylation occurs in B cells pre-secretion

A key mechanistic question was whether serum IgG sialylation reflects enzymatic activity occurring in B cells during secretion, or whether extracellular sialylation/desialylation events (by plasma sialidases or sST6GAL1) reshape the glycome post-secretion. Werner et al. 2024 resolved this using bone marrow chimeric mice with B-cell-specific ST6Gal1 deletion: IgG sialylation is lost when only B cells lack ST6Gal1, demonstrating that sialylation occurs within B cells before antibody secretion ⁵. This localizes the therapeutic target to B-cell intrinsic ST6GAL1 expression.

GWAS and genetic variation

ST6GAL1 is one of the two loci with the strongest GWAS signal for IgG glycan variation (alongside mgat3) in Landini et al. 2022 (n=2,020; CROATIA KORCULA + VIKING cohorts), as well as in the earlier Wahl et al. 2018 GWAS (n=~1,800; KORA + Rotterdam Study) ⁷³. The ST6GAL1 signal associates primarily with IgG sialylation peaks (not galactosylation or fucosylation). The mr-causal-evidence field is partial: GWAS instruments at ST6GAL1 have been described, but a dedicated Mendelian randomization study using ST6GAL1 instruments to test IgG sialylation → aging/inflammatory outcomes has not been published as of 2026-05-20. needs-replication

Knockout phenotype (mouse)

ST6GAL1 (ST6Gal-I) knockout mice lack α2,6-linked sialylation on B cell surface glycoproteins. Collins et al. 2002 demonstrated that:

• ST6Gal-I KO B cells have constitutively unmasked CD22 — the B cell inhibitory coreceptor CD22 (Siglec-2) binds ST6GAL1-generated α2,6-SA on neighboring glycoproteins (cis ligands) in normal B cells, keeping CD22 masked. Without ST6GAL1, CD22 is permanently unmasked, dysregulating B cell receptor signaling ⁸.
• The KO mice are described as “immunocompromised ligand-deficient,” suggesting impaired B cell activation thresholds.

The CD22-masking mechanism is separate from the IgG Fc sialylation story — it reflects ST6GAL1’s role in regulating B cell signaling via cell-surface glycan biology, not just antibody glycoform output.

Soluble serum ST6GAL1 as a circulating marker

Proteolytic shedding of the ST6GAL1 ectodomain (by BACE1) releases a soluble form into the circulation. Serum sST6GAL1 levels have been explored as a marker of liver function and inflammation ⁴. Whether sST6GAL1 reflects relevant aging biology (B-cell intrinsic ST6GAL1 decline) or simply constitutive liver shedding is not established. The circulating form is unlikely to contribute to IgG sialylation (Werner 2024 data). needs-replication

Therapeutic interest

• Recombinant sialylated IgG Fc / enriched sialylated IVIG (rIVIG): The Ravetch group demonstrated that sialylated Fc fragments recapitulate IVIG activity at dramatically lower doses ¹. Clinical development for autoimmune indications is ongoing; aging-context use is speculative but biologically motivated.
• No ST6GAL1-activating small molecule has reached clinical testing. Enhancing ST6GAL1 expression in B cells (e.g., via NF-κB pathway modulation or epigenetic approaches) is a proposed but unvalidated strategy.

Substrate breadth beyond IgG Fc

ST6GAL1 sialylates many substrates: IgA, IgM, transferrin, alpha-2-macroglobulin, cell-surface CD45 (modulating CD45 phosphatase activity), integrins, and serum glycoproteins broadly. The IgG Fc context is the most aging-relevant but not the only biologically important substrate.

Limitations and gaps

• ST6GAL1 enzymatic activity in B cells is not directly measurable in peripheral blood; the IgG glycome sialylation fraction is the practical proxy.
• The molecular mechanism of ST6GAL1 transcriptional suppression by pro-inflammatory cytokines is not characterized at the promoter level. no-mechanism
• Whether restoring ST6GAL1 expression in aged B cells (via any intervention) is sufficient to reverse the inflammaging glycome phenotype has not been tested in vivo in humans. needs-replication
• Serum sST6GAL1 as an aging biomarker is unexplored. needs-replication

See also

• igg-fc-glycosylation — full IgG Fc N-glycosylation cascade and aging context
• glycanage-2017 — the GlycanAge clock; sialylation loss is one component
• mijakovac-2026-igg-glycome-mortality — 20,045-person mortality validation
• chronic-inflammation — inflammaging context; ST6GAL1 is a molecular mediator
• b4galt1 — galactosyltransferase acting upstream; substrate-limits ST6GAL1
• fut8 — core fucosyltransferase; acts at parallel step
• mgat3 — bisecting GlcNAc transferase; strongest co-GWAS locus with ST6GAL1

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Footnotes

Footnotes

1. doi:10.1126/science.1154315 · PMID 18420934 · in-vivo (mouse K/BxN arthritis) + in-vitro · Anthony RM, Nimmerjahn F et al., Ravetch JV · Science 320:373 (2008) · α2,6-sialylated IgG1 Fc active at 30 mg/kg vs 1,000–2,000 mg/kg native IVIG; DC-SIGN → FcγRIIb mechanism; α2,6-linkage specificity confirmed · archive: downloaded ↩ ↩² ↩³ ↩⁴

2. doi:10.1126/science.1129594 · PMID 16888140 · in-vivo (mouse arthritis models) + serum IgG fractionation · Kaneko Y, Nimmerjahn F, Ravetch JV · Science 313:670–673 (2006) · Fc sialylation confers anti-inflammatory IgG activity; ~5% sialylated fraction stated in reviews (full-text not available for percentage verification) · archive: not_oa no-fulltext-access ↩ ↩² ↩³

3. doi:10.1038/s41467-022-29189-5 · PMID 35332118 · GWAS (n=2,020) · Landini A, Trbojević-Akmačić I, Wilson JF, Klarić L et al. · Nat Commun 2022 · ST6GAL1 and MGAT3 among strongest IgG-specific GWAS loci; distinct from transferrin glycan loci · archive: downloaded ↩ ↩²

4. doi:10.1007/978-981-13-2835-0_17 · review · Dall’Olio F · Sub-Cell Biochem 90:505–526 (2018) · plasmatic forms of B4GALT1 and ST6GAL1 as aging-associated markers; review of glycobiology of aging · archive: not_oa ↩ ↩²

5. doi:10.3389/fimmu.2024.1402000 · PMID 38827747 · in-vivo (bone marrow chimeric mice; B cell-specific ST6Gal1 deletion) · Werner A, Nimmerjahn F et al. · Front Immunol 15:1402000 (2024) · IgG sialylation occurs exclusively within B cells before secretion; B cells expressing ST6Gal1 have developmental competitive advantage in mixed chimeras · archive: downloaded ↩ ↩²

6. doi:10.1016/j.cellimm.2018.07.009 · PMID 30107893 · review · Gudelj I, Lauc G, Pezer M · Cell Immunol 2018 · ST6GAL1 age-decline in B cells; bidirectional inflammation↔glycan loop · archive: download failed no-fulltext-access ↩ ↩²

7. doi:10.3389/fimmu.2018.00277 · PMID 29535710 · GWAS (~1,800 subjects) · Wahl A, van den Akker E, Klaric L et al., Gieger C · Front Immunol 9:277 (2018) · replicated GWAS signals at ST6GAL1, B4GALT1, FUT8, MGAT3 for IgG glycan variation · archive: status pending ↩

8. doi:10.1093/glycob/cwf067 · PMID 12213789 · in-vivo (ST6Gal-I KO mice) · Collins BE, Blixt O, DeSieno AR, Bovin N, Marth JD, Paulson JC · Glycobiology 12:563–571 (2002) · constitutively unmasked CD22 on ST6Gal-I KO B cells; dysregulated BCR signaling; “immunocompromised ligand-deficient” phenotype · archive: download failed (0 candidate URLs; permanently unverifiable via local archive) no-fulltext-access ↩