Affinage

STT3B

Dolichyl-diphosphooligosaccharide--protein glycosyltransferase subunit STT3B · UniProt Q8TCJ2

Length
826 aa
Mass
93.7 kDa
Annotated
2026-06-10
22 papers in source corpus 15 papers cited in narrative 15 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 6/6 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

STT3B is the catalytic subunit of an oligosaccharyltransferase (OST) complex that performs posttranslocational N-glycosylation in the ER lumen, transferring N-glycans onto acceptor sites that escape the co-translational STT3A-OST machinery (PMID:23530066). Its defining substrate specificity is for extreme C-terminal acceptor sites located within ~50-55 residues of the protein C-terminus, with C-terminal NXT sequons glycosylated more efficiently than NXS sites (PMID:23530066), and quantitative glycoproteomics extended this preference to acceptor sites in short loops of multi-spanning membrane proteins and to stress-induced hyperglycosylation of normally silent sites (PMID:31296534). STT3B functions in secretory-protein quality control: prolonged unfolding of proteins such as transthyretin exposes cryptic sequons that STT3B glycosylates, routing terminally misfolded substrates into an EDEM3-mediated, N-glycan-dependent ERAD pathway (PMID:22607976). Substrate selection by the STT3B-OST complex depends on its dedicated thioredoxin subunits MAGT1 and TUSC3, whose CXXC oxidoreductase active-site motifs are required for glycosylation of substrates such as Lassa virus glycoprotein (PMID:31511384), and efficiency at disulfide-adjacent acceptor sites is further tuned by a cytosolic NADPH-dependent reductive pathway (PMID:34734627). Through this activity STT3B controls glycosylation and stability of specific glycoproteins, including EREG (glycosylated at N47), where loss of STT3B causes EREG degradation and reduced PD-L1 upregulation (PMID:38945975), and is preferentially required for glycosylation of certain viral glycoproteins (PMID:31511384, PMID:39945486). Loss-of-function mutation in STT3B causes a congenital disorder of glycosylation (STT3B-CDG) with neurologic abnormalities, establishing its non-redundant catalytic role in vivo (PMID:23842455).

Mechanistic history

Synthesis pass · year-by-year structured walk · 9 steps
  1. 2005 Medium

    Before its OST function was resolved, work defined STT3B's membrane topology and ER localization, establishing the structural basis for its lumenal catalytic orientation and identifying sequence determinants distinguishing it from STT3A.

    Evidence Subcellular fractionation, reporter fusion constructs and topogenic/targeting-sequence mapping

    PMID:16263756 PMID:16297371

    Open questions at the time
    • Catalytic glycosyltransferase activity not yet demonstrated in these studies
    • Functional relevance of the C-terminal nucleolar targeting sequence to glycosylation unestablished
    • Single-lab reporter-based localization
  2. 2012 Medium

    Established that STT3B-mediated glycosylation is a surveillance mechanism, showing cryptic sequons exposed by unfolding are glycosylated and channeled into a distinct EDEM3-dependent ERAD route.

    Evidence Folding/ERAD perturbation and STT3B knockdown with mutant transthyretin substrates

    PMID:22607976

    Open questions at the time
    • Generality beyond transthyretin not addressed here
    • Mechanism by which cryptic sites are recognized unresolved
    • Single-lab epistasis-style placement
  3. 2013 High

    Defined the core substrate logic of STT3B—posttranslocational glycosylation of C-terminal sites missed by STT3A—and proved its catalytic role is non-redundant in humans via a disease-causing mutation.

    Evidence Biosynthetic pulse labeling across multiple glycoproteins in STT3A/STT3B-deficient cells; patient fibroblast glycosylation assays with cDNA rescue

    PMID:23530066 PMID:23842455

    Open questions at the time
    • Structural basis for C-terminal site preference not determined
    • Full spectrum of STT3B-specific substrates unknown at this stage
    • Genotype-phenotype relationship in STT3B-CDG limited to one mutation
  4. 2019 High

    Broadened the STT3B substrate map and mechanistic determinants, identifying short-loop membrane sites and stress-induced hyperglycosylation, and showing the MAGT1/TUSC3 CXXC oxidoreductase motifs drive recognition of redox-sensitive substrates.

    Evidence Quantitative glycoproteomics across ~1,000 sites in KO cells under ER stress; CRISPR KO, AP-MS and CXXC active-site mutagenesis with Lassa virus glycoprotein

    PMID:31296534 PMID:31511384

    Open questions at the time
    • How loop geometry dictates STT3B preference is unresolved
    • Division of labor between MAGT1 and TUSC3 not separated
    • Structural model of substrate engagement absent
  5. 2021 Medium

    Connected cytosolic redox state to lumenal glycosylation efficiency, showing a NADPH-dependent reductive pathway governs occupancy at disulfide-adjacent STT3B-dependent sites.

    Evidence In vitro translation with defined redox conditions, NADPH-pathway inhibition and membrane-impermeable reductant in STT3B-specific assays

    PMID:34734627

    Open questions at the time
    • Identity of the trans-membrane redox relay not defined
    • In vitro reconstitution from a single lab
    • Physiological scope of redox modulation unquantified
  6. 2022 Medium

    Quantified the relative cellular impact of STT3B versus STT3A, confirming STT3B handles a smaller, specific posttranslocational substrate set while STT3A loss drives compensatory UPR-linked hyperglycosylation.

    Evidence Comparative proteomics/glycoproteomics in STT3A-KO and STT3B-KO HEK293 cells

    PMID:36139350

    Open questions at the time
    • Substrate-specific determinants of STT3B dependence not mechanistically dissected
    • Single cell-line context
    • Functional consequences of reduced site occupancy not assessed
  7. 2024 Medium

    Linked STT3B activity to a defined disease-relevant glycoprotein, showing N47 glycosylation of EREG is required for its stability and downstream PD-L1 upregulation, nominating STT3B/OST inhibition as a therapeutic axis.

    Evidence STT3B knockdown, N47 site-directed mutagenesis, NGI-1 inhibition, and xenograft combination with anti-PD-L1 in head and neck carcinoma cells

    PMID:38945975

    Open questions at the time
    • Whether EREG is a direct posttranslocational substrate not formally separated from STT3A
    • Single tumor context
    • Mechanism connecting EREG glycosylation to PD-L1 incompletely defined
  8. 2023 High

    Identified STT3B as a host determinant of a small-molecule toxin's cytotoxicity, providing a tractable pharmacological handle (ICG inhibitor) and validating STT3B function in vivo.

    Evidence Genome-wide CRISPR screen, in silico inhibitor discovery, liver organoid and mouse validation

    PMID:37193694

    Open questions at the time
    • The glycoprotein substrate mediating α-amanitin toxicity not identified
    • ICG specificity for STT3B versus other OST functions not fully delineated
  9. 2025 Low

    Reinforced STT3B's selective role in viral glycoprotein maturation and raised candidate membrane-surface interactions, while leaving the latter mechanistically immature.

    Evidence CRISPR KO and viral replication assays (PEDV spike); IP/MS, blocking assay and structural modeling for an HMGN2 interaction

    PMID:39945486 PMID:41574665

    Open questions at the time
    • HMGN2–STT3B interaction rests on a single Co-IP/MS without reciprocal validation or reconstitution
    • A surface-membrane pool of STT3B is hard to reconcile with its lumenal OST role and is unconfirmed
    • PEDV substrate specificity not extended to other coronaviral spikes

Open questions

Synthesis pass · forward-looking unresolved questions
  • A structural model explaining how the STT3B-OST complex selects C-terminal, short-loop and redox-sensitive acceptor sites—and the full physiological substrate repertoire and tissue phenotypes of STT3B loss—remains to be defined.
  • No experimental structure of the STT3B-OST complex engaging substrate in the timeline
  • Comprehensive in vivo substrate atlas and STT3B-CDG genotype-phenotype map incomplete
  • Mechanistic separation of MAGT1 vs TUSC3 contributions unresolved

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0016740 transferase activity 3 GO:0140096 catalytic activity, acting on a protein 3
Localization
GO:0005783 endoplasmic reticulum 3
Pathway
R-HSA-392499 Metabolism of proteins 2 R-HSA-8953897 Cellular responses to stimuli 2
Partners
MAGT1TUSC3
Complex memberships
STT3B-OST complex

Evidence

Reading pass · 15 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2013 STT3B isoform of the OST is responsible for posttranslocational glycosylation of extreme C-terminal acceptor sites (within 50-55 residues of the C-terminus) that are not reached by the translocation channel-associated STT3A isoform during co-translational glycosylation. C-terminal NXT sites were glycosylated more rapidly and efficiently than NXS sites via this STT3B-dependent mechanism. Biosynthetic pulse labeling of five human glycoproteins, STT3A/STT3B-deficient cell lines, bioinformatics analysis of glycopeptide databases The Journal of cell biology High 23530066
2012 STT3B-dependent posttranslational N-glycosylation acts as a surveillance mechanism for unfolded secretory proteins: prolonged unfolding of transthyretin (TTR) exposes cryptic N-glycosylation sites, which are then glycosylated by STT3B, providing an alternative EDEM3-mediated N-glycan-dependent ERAD pathway distinct from the Herp-mediated N-glycan-independent ERAD pathway. Folding and ERAD perturbation analyses, STT3B knockdown, detergent solubility assays, cell proliferation assays in mutant TTR-expressing cells Molecular cell Medium 22607976
2013 A homozygous intronic mutation (c.1539+20G>T) in STT3B causes a congenital disorder of glycosylation (STT3B-CDG) with neurologic abnormalities; the mutation impairs glycosylation of STT3B-specific acceptor substrates in patient fibroblasts, demonstrating STT3B's non-redundant catalytic role in N-glycosylation of specific substrates in vivo. Patient fibroblast glycosylation assays, GFP biomarker glycosylation rescue experiments with corresponding cDNA, transferrin glycosylation analysis Human molecular genetics High 23842455
2019 Quantitative glycoproteomics identified new classes of STT3B-dependent N-glycosylation sites: acceptor sites located in short loops of multi-spanning membrane proteins are preferentially dependent on STT3B. Additionally, GRP94 is hyperglycosylated on five silent sites in STT3A-deficient cells and in wild-type cells under ER stress, suggesting STT3B mediates stress-induced hyperglycosylation. Quantitative glycoproteomics comparing ~1,000 acceptor sites in wild-type vs. STT3A/STT3B mutant cells; ER stress induction with thapsigargin, DTT, and NGI-1 The Journal of cell biology High 31296534
2019 STT3B (but not STT3A) is required for N-glycosylation of Lassa virus glycoprotein (LASV GP); the two specific thioredoxin subunits of STT3B-OST, MAGT1 and TUSC3, are essential for this glycosylation, and the CXXC oxidoreductase active-site motif of MAGT1 or TUSC3 is required for LASV GP N-glycosylation. CRISPR-Cas9 knockout of STT3A and STT3B, affinity purification-mass spectrometry (AP-MS), site-directed mutagenesis of CXXC motifs, recombinant arenavirus replication assays Journal of virology High 31511384
2005 STT3B (SIMP) is located in the ER membrane in close proximity to the immunoproteasome; its ER-associated degradation pathway substrates contribute prominently to the MHC I immunopeptidome due to its lysine-rich region, propensity to misfold, and ER membrane localization. Coupling a peptide to STT3B/SIMP enhances its MHC I presentation. Subcellular fractionation, reporter fusion experiments (ovalbumin SIINFEKL coupled to SIMP), MHC I peptide presentation assays International immunology Medium 16263756
2005 The last transmembrane segment of STT3B functions as a topogenic determinant sufficient for proper integration and orientation of the STT3B C-terminal domain; additionally, a bipartite nuclear targeting sequence in the STT3B C-terminal tail (absent in STT3A) is sufficient to induce nucleolar localization of a reporter protein. Reporter protein fusion constructs, cellular localization assays, structural comparison of STT3A and STT3B C-terminal domains Archives of biochemistry and biophysics Medium 16297371
2021 Efficient glycosylation of a hypoglycosylated STT3B-dependent acceptor site in hemopexin (adjacent to a cysteine in a short-range disulfide) requires a cytosolic NADPH-dependent reductive pathway; a membrane-impermeable reducing agent can substitute, demonstrating that cytosolic redox conditions influence STT3B-dependent glycosylation site occupancy. In vitro translation system with defined redox conditions, NADPH-dependent pathway inhibition, membrane-impermeable reducing agent addition, STT3A/STT3B-specific analysis Journal of cell science Medium 34734627
2018 An ER-localized mEGFP mutant containing an N-glycosylation sequon (NCT) at the C-terminus functions as a fluorescent reporter specifically for STT3B-dependent posttranslocational N-glycosylation; the N185-C186-T187 sequon variant showed the best glycosylation efficiency and fluorescence change in STT3B-dependent manner as confirmed by STT3A/STT3B knockout cell lines. STT3A/STT3B knockout cell lines, fluorescence assays with mEGFP reporter constructs containing C-terminal glycosylation sequons The FEBS journal Medium 29282902
2022 Proteome and glycoproteome analysis in STT3B-KO HEK293 cells showed that STT3B deletion has less impact on overall protein expression than STT3A deletion; STT3B deletion reduced glycosylation of specific posttranslocational substrates. Hyperglycosylation of ENPL was confirmed to result from ER stress caused specifically by STT3A deletion, mediated via ATF6 and PERK UPR pathways. Proteomics and glycoproteomics in STT3A-KO and STT3B-KO HEK293 cells, identification of 4265 unique N-linked intact glycopeptides from 629 glycosites Cells Medium 36139350
2023 STT3B is required for α-amanitin (mushroom toxin) cytotoxicity; indocyanine green (ICG) was identified as a STT3B inhibitor that blocks α-amanitin toxicity in cells, liver organoids, and mice. A genome-wide CRISPR screen identified the N-glycan biosynthesis pathway and STT3B as key mediators of α-amanitin toxicity. Genome-wide CRISPR screen, in silico drug screening, in vivo mouse model validation, liver organoid assays Nature communications High 37193694
2024 STT3B glycosylates EREG at N47; this N-glycosylation is essential for EREG protein stability, membrane localization, and biological function. Knockdown of STT3B suppresses glycosylated EREG and inhibits PDL1 upregulation in head and neck squamous cell carcinoma cells. The OST inhibitor NGI-1 blocks STT3B-mediated EREG glycosylation, causing EREG degradation. STT3B knockdown, site-directed mutagenesis of N47 glycosylation site, NGI-1 pharmacological inhibition, immunofluorescence for membrane localization, in vivo tumor xenograft with NGI-1 + anti-PDL1 combination International journal of oral science Medium 38945975
2025 STT3B-OST complex (but not STT3A) is preferentially required for N-glycosylation of porcine epidemic diarrhea virus (PEDV) spike protein; genetic ablation of STT3B reduces PEDV S protein glycosylation and impairs viral replication. CRISPR-Cas9 knockout of STT3A and STT3B, pharmacological N-glycosylation inhibitors, viral replication assays Journal of virology Medium 39945486
2025 HMGN2 binds to STT3B on the tumor cell membrane surface (identified by IP/MS); this interaction modulates the STT3B/PD-L1/caspase-1/GSDMD axis, triggering pyroptosis. After HMGN2 binding, PD-L1 expression increases and PD-L1 is translocated from the membrane to the nucleus. Immunoprecipitation/mass spectrometry (IP/MS), anti-STT3B blocking assay, western blotting, immunofluorescence, ZDOCK and AlphaFold3 structural modeling Molecular medicine reports Low 41574665
2025 STT3A knockout had a more pronounced negative effect on HIV-1 Env glycosylation and virus production/infectivity than STT3B knockout. STT3B knockout appeared to preferentially affect gp41 glycosylation and PNGS near the C-terminus of Env, consistent with STT3B's posttranslocational role for C-terminal sites. STT3A/STT3B CRISPR knockout cells, site-specific glycan analysis of recombinant Env proteins, HIV-1 neutralization assays with broadly neutralizing antibodies bioRxivpreprint Low bio_10.1101_2025.09.03.674041

Source papers

Stage 0 corpus · 22 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2013 Extreme C-terminal sites are posttranslocationally glycosylated by the STT3B isoform of the OST. The Journal of cell biology 87 23530066
2012 STT3B-dependent posttranslational N-glycosylation as a surveillance system for secretory protein. Molecular cell 69 22607976
2013 Mutations in STT3A and STT3B cause two congenital disorders of glycosylation. Human molecular genetics 68 23842455
2019 Quantitative glycoproteomics reveals new classes of STT3A- and STT3B-dependent N-glycosylation sites. The Journal of cell biology 63 31296534
1985 SIMP: a computer program in BASIC for nonlinear curve fitting. Journal of pharmacological methods 42 3841174
2023 Identification of indocyanine green as a STT3B inhibitor against mushroom α-amanitin cytotoxicity. Nature communications 23 37193694
2019 Comprehensive Interactome Analysis Reveals that STT3B Is Required for N-Glycosylation of Lassa Virus Glycoprotein. Journal of virology 23 31511384
2022 Proteome and Glycoproteome Analyses Reveal the Protein N-Linked Glycosylation Specificity of STT3A and STT3B. Cells 19 36139350
2005 The structure and location of SIMP/STT3B account for its prominent imprint on the MHC I immunopeptidome. International immunology 18 16263756
2024 Exosomes from Hypoxic Pretreatment ADSCs Ameliorate Cardiac Damage Post-MI via Activated circ-Stt3b/miR-15a-5p/GPX4 Signaling and Decreased Ferroptosis. Cardiovascular toxicology 15 39192160
2024 Stabilization of EREG via STT3B-mediated N-glycosylation is critical for PDL1 upregulation and immune evasion in head and neck squamous cell carcinoma. International journal of oral science 12 38945975
2018 Construction of green fluorescence protein mutant to monitor STT3B-dependent N-glycosylation. The FEBS journal 9 29282902
2025 STT3B promotes porcine epidemic diarrhea virus replication by regulating N-glycosylation of PEDV S protein. Journal of virology 7 39945486
2025 A novel allosteric driver mutation of β-glucuronidase promotes head and neck squamous cell carcinoma progression through STT3B-mediated PD-L1 N-glycosylation. MedComm 5 39830021
2021 A cytosolic reductase pathway is required for efficient N-glycosylation of an STT3B-dependent acceptor site. Journal of cell science 5 34734627
2007 Cloning, characterization and expression analysis of SIMP (source of immunodominant MHC-associated peptides) in grass carp Ctenopharyngodon idella. Fish & shellfish immunology 5 18407520
2022 A conserved Plasmodium structural integrity maintenance protein (SIMP) is associated with sporozoite membrane and is essential for maintaining shape and infectivity. Molecular microbiology 3 35301756
2005 Identification of two distinct intracellular localization signals in STT3-B. Archives of biochemistry and biophysics 3 16297371
2025 Unveiling the Role of Oligosaccharyltransferase STT3B in Colorectal Cancer Tissues: Clinical significance and Molecular Mechanisms Driving the Formation of Tertiary Lymphoid Structures. Immunobiology 2 40056505
2024 Proteomic profiling of FFPE specimens: Discovery of HNRNPA2/B1 and STT3B as biomarkers for determining formalin fixation durations. Journal of proteomics 2 38723849
2022 Discovery and Characterization of the Phospholemman/SIMP/Viroporin Superfamily. Microbial physiology 2 35152214
2026 HMGN2 induces pyroptosis in tumour cells by modulating the STT3B/PD‑L1/caspase‑1/GSDMD axis. Molecular medicine reports 0 41574665

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