{"gene":"STT3B","run_date":"2026-06-10T10:51:54","timeline":{"discoveries":[{"year":2013,"finding":"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.","method":"Biosynthetic pulse labeling of five human glycoproteins, STT3A/STT3B-deficient cell lines, bioinformatics analysis of glycopeptide databases","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (pulse labeling, defined KO/KD cell lines across multiple substrates), replicated across multiple glycoproteins","pmids":["23530066"],"is_preprint":false},{"year":2012,"finding":"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.","method":"Folding and ERAD perturbation analyses, STT3B knockdown, detergent solubility assays, cell proliferation assays in mutant TTR-expressing cells","journal":"Molecular cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple functional assays in a single lab with clear pathway placement via epistasis-style perturbation","pmids":["22607976"],"is_preprint":false},{"year":2013,"finding":"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.","method":"Patient fibroblast glycosylation assays, GFP biomarker glycosylation rescue experiments with corresponding cDNA, transferrin glycosylation analysis","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — human genetic disease causation confirmed by rescue with wild-type cDNA and substrate-specific glycosylation analysis in patient cells","pmids":["23842455"],"is_preprint":false},{"year":2019,"finding":"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.","method":"Quantitative glycoproteomics comparing ~1,000 acceptor sites in wild-type vs. STT3A/STT3B mutant cells; ER stress induction with thapsigargin, DTT, and NGI-1","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — large-scale quantitative glycoproteomics with defined KO cell lines, multiple ER stress conditions, single rigorous study with broad coverage","pmids":["31296534"],"is_preprint":false},{"year":2019,"finding":"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.","method":"CRISPR-Cas9 knockout of STT3A and STT3B, affinity purification-mass spectrometry (AP-MS), site-directed mutagenesis of CXXC motifs, recombinant arenavirus replication assays","journal":"Journal of virology","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — CRISPR KO, AP-MS, and active-site mutagenesis in a single study with multiple orthogonal methods","pmids":["31511384"],"is_preprint":false},{"year":2005,"finding":"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.","method":"Subcellular fractionation, reporter fusion experiments (ovalbumin SIINFEKL coupled to SIMP), MHC I peptide presentation assays","journal":"International immunology","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — direct localization and functional presentation assay with mechanistic follow-up, single lab","pmids":["16263756"],"is_preprint":false},{"year":2005,"finding":"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.","method":"Reporter protein fusion constructs, cellular localization assays, structural comparison of STT3A and STT3B C-terminal domains","journal":"Archives of biochemistry and biophysics","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — direct experimental localization using reporter constructs with defined sequence elements, single lab","pmids":["16297371"],"is_preprint":false},{"year":2021,"finding":"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.","method":"In vitro translation system with defined redox conditions, NADPH-dependent pathway inhibition, membrane-impermeable reducing agent addition, STT3A/STT3B-specific analysis","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro reconstitution system with defined conditions, but single lab and single study","pmids":["34734627"],"is_preprint":false},{"year":2018,"finding":"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.","method":"STT3A/STT3B knockout cell lines, fluorescence assays with mEGFP reporter constructs containing C-terminal glycosylation sequons","journal":"The FEBS journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO cell lines with engineered reporter substrates and fluorescence readout, two orthogonal confirmatory approaches, single lab","pmids":["29282902"],"is_preprint":false},{"year":2022,"finding":"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.","method":"Proteomics and glycoproteomics in STT3A-KO and STT3B-KO HEK293 cells, identification of 4265 unique N-linked intact glycopeptides from 629 glycosites","journal":"Cells","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — quantitative glycoproteomics in KO cells, single lab with broad substrate coverage","pmids":["36139350"],"is_preprint":false},{"year":2023,"finding":"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.","method":"Genome-wide CRISPR screen, in silico drug screening, in vivo mouse model validation, liver organoid assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — genome-wide CRISPR screen combined with in vivo validation and mechanistic inhibitor characterization across multiple model systems","pmids":["37193694"],"is_preprint":false},{"year":2024,"finding":"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.","method":"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","journal":"International journal of oral science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mutagenesis of specific glycosylation site combined with KD and pharmacological inhibition, in vivo validation, single lab","pmids":["38945975"],"is_preprint":false},{"year":2025,"finding":"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.","method":"CRISPR-Cas9 knockout of STT3A and STT3B, pharmacological N-glycosylation inhibitors, viral replication assays","journal":"Journal of virology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — CRISPR KO combined with pharmacological inhibition and viral replication assay, single lab","pmids":["39945486"],"is_preprint":false},{"year":2025,"finding":"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.","method":"Immunoprecipitation/mass spectrometry (IP/MS), anti-STT3B blocking assay, western blotting, immunofluorescence, ZDOCK and AlphaFold3 structural modeling","journal":"Molecular medicine reports","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single Co-IP/MS identification with blocking assay confirmation, computational modeling; single lab, no in vitro reconstitution","pmids":["41574665"],"is_preprint":false},{"year":2025,"finding":"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.","method":"STT3A/STT3B CRISPR knockout cells, site-specific glycan analysis of recombinant Env proteins, HIV-1 neutralization assays with broadly neutralizing antibodies","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 2 / Weak — KO cells and glycan analysis but preprint, single lab, not yet peer-reviewed","pmids":["bio_10.1101_2025.09.03.674041"],"is_preprint":true}],"current_model":"STT3B is the catalytic subunit of the STT3B-OST complex that primarily mediates posttranslocational N-glycosylation in the ER lumen, rescuing acceptor sites skipped by the co-translational STT3A-OST complex—particularly C-terminal sites (within ~50-55 residues of the protein C-terminus), sites in short loops of multi-spanning membrane proteins, and cryptic sites exposed on misfolded proteins—with its unique subunits MAGT1/TUSC3 (bearing a CXXC oxidoreductase motif) contributing to substrate recognition, and cytosolic NADPH-dependent redox conditions modulating efficiency at disulfide-adjacent sites; this activity is essential for quality control of secretory proteins via EDEM3-mediated ERAD, for glycosylation of specific viral glycoproteins, and for stability of glycoproteins such as EREG, while loss-of-function mutations cause a congenital disorder of glycosylation (STT3B-CDG)."},"narrative":{"mechanistic_narrative":"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].","teleology":[{"year":2005,"claim":"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","pmids":["16263756","16297371"],"confidence":"Medium","gaps":["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"]},{"year":2012,"claim":"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","pmids":["22607976"],"confidence":"Medium","gaps":["Generality beyond transthyretin not addressed here","Mechanism by which cryptic sites are recognized unresolved","Single-lab epistasis-style placement"]},{"year":2013,"claim":"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","pmids":["23530066","23842455"],"confidence":"High","gaps":["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"]},{"year":2019,"claim":"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","pmids":["31296534","31511384"],"confidence":"High","gaps":["How loop geometry dictates STT3B preference is unresolved","Division of labor between MAGT1 and TUSC3 not separated","Structural model of substrate engagement absent"]},{"year":2021,"claim":"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","pmids":["34734627"],"confidence":"Medium","gaps":["Identity of the trans-membrane redox relay not defined","In vitro reconstitution from a single lab","Physiological scope of redox modulation unquantified"]},{"year":2022,"claim":"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","pmids":["36139350"],"confidence":"Medium","gaps":["Substrate-specific determinants of STT3B dependence not mechanistically dissected","Single cell-line context","Functional consequences of reduced site occupancy not assessed"]},{"year":2024,"claim":"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","pmids":["38945975"],"confidence":"Medium","gaps":["Whether EREG is a direct posttranslocational substrate not formally separated from STT3A","Single tumor context","Mechanism connecting EREG glycosylation to PD-L1 incompletely defined"]},{"year":2023,"claim":"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","pmids":["37193694"],"confidence":"High","gaps":["The glycoprotein substrate mediating α-amanitin toxicity not identified","ICG specificity for STT3B versus other OST functions not fully delineated"]},{"year":2025,"claim":"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","pmids":["39945486","41574665"],"confidence":"Low","gaps":["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"]},{"year":null,"claim":"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.","evidence":"","pmids":[],"confidence":"Medium","gaps":["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":{"molecular_activity":[{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[0,3,4]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,2,11]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[1,5,6]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,3]},{"term_id":"R-HSA-8953897","term_label":"Cellular responses to stimuli","supporting_discovery_ids":[1,3]}],"complexes":["STT3B-OST complex"],"partners":["MAGT1","TUSC3"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q8TCJ2","full_name":"Dolichyl-diphosphooligosaccharide--protein glycosyltransferase subunit STT3B","aliases":["Source of immunodominant MHC-associated peptides homolog"],"length_aa":826,"mass_kda":93.7,"function":"Catalytic subunit of the oligosaccharyl transferase (OST) complex that catalyzes the initial transfer of a defined glycan (Glc(3)Man(9)GlcNAc(2) in eukaryotes) from the lipid carrier dolichol-pyrophosphate to an asparagine residue within an Asn-X-Ser/Thr consensus motif in nascent polypeptide chains, the first step in protein N-glycosylation (PubMed:19167329, PubMed:31296534, PubMed:31831667, PubMed:39509507). N-glycosylation occurs cotranslationally and the complex associates with the Sec61 complex at the channel-forming translocon complex that mediates protein translocation across the endoplasmic reticulum (ER) (PubMed:19167329, PubMed:31296534, PubMed:31831667, PubMed:39509507). All subunits are required for a maximal enzyme activity. This subunit contains the active site and the acceptor peptide and donor lipid-linked oligosaccharide (LLO) binding pockets (PubMed:19167329, PubMed:31296534, PubMed:31831667, PubMed:39509507). STT3B is present in a small subset of OST complexes (OST-B) and mediates both cotranslational and post-translational N-glycosylation of target proteins: STT3B-containing complexes are required for efficient post-translational glycosylation and while they are less competent than STT3A-containing complexes for cotranslational glycosylation, they have the ability to mediate glycosylation of some nascent sites that are not accessible for STT3A (PubMed:19167329, PubMed:22607976, PubMed:31296534, PubMed:39509507). STT3B-containing complexes also act post-translationally and mediate modification of skipped glycosylation sites in unfolded proteins (PubMed:19167329, PubMed:22607976, PubMed:39509507). Plays a role in ER-associated degradation (ERAD) pathway that mediates ubiquitin-dependent degradation of misfolded endoplasmic reticulum proteins by mediating N-glycosylation of unfolded proteins, which are then recognized by the ERAD pathway and targeted for degradation (PubMed:19167329, PubMed:22607976). Mediates glycosylation of the disease variant AMYL-TTR 'Asp-38' of TTR at 'Asn-118', leading to its degradation (PubMed:19167329, PubMed:22607976)","subcellular_location":"Endoplasmic reticulum; Endoplasmic reticulum membrane","url":"https://www.uniprot.org/uniprotkb/Q8TCJ2/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/STT3B","classification":"Not Classified","n_dependent_lines":131,"n_total_lines":1208,"dependency_fraction":0.10844370860927152},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000163527","cell_line_id":"CID000182","localizations":[{"compartment":"er","grade":3}],"interactors":[{"gene":"DAD1","stoichiometry":10.0},{"gene":"RPN2","stoichiometry":10.0},{"gene":"MLEC","stoichiometry":10.0},{"gene":"MAGT1","stoichiometry":10.0},{"gene":"POR","stoichiometry":10.0},{"gene":"PGRMC1","stoichiometry":10.0},{"gene":"SEC24B","stoichiometry":10.0},{"gene":"FKBP8","stoichiometry":4.0},{"gene":"RAB2A","stoichiometry":4.0},{"gene":"TUSC3","stoichiometry":4.0}],"url":"https://opencell.sf.czbiohub.org/target/CID000182","total_profiled":1310},"omim":[{"mim_id":"619029","title":"KERATINOCYTE-ASSOCIATED PROTEIN 2; KRTCAP2","url":"https://www.omim.org/entry/619029"},{"mim_id":"618932","title":"OLIGOSACCHARYLTRANSFERASE COMPLEX, SUBUNIT 4, NONCATALYTIC; OST4","url":"https://www.omim.org/entry/618932"},{"mim_id":"615597","title":"CONGENITAL DISORDER OF GLYCOSYLATION, TYPE Ix; CDG1X","url":"https://www.omim.org/entry/615597"},{"mim_id":"615596","title":"CONGENITAL DISORDER OF GLYCOSYLATION, TYPE Iw, AUTOSOMAL RECESSIVE; CDG1WAR","url":"https://www.omim.org/entry/615596"},{"mim_id":"608605","title":"OLIGOSACCHARYLTRANSFERASE COMPLEX, CATALYTIC SUBUNIT STT3B; STT3B","url":"https://www.omim.org/entry/608605"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Endoplasmic reticulum","reliability":"Supported"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/STT3B"},"hgnc":{"alias_symbol":["SIMP","FLJ90106","STT3-B"],"prev_symbol":[]},"alphafold":{"accession":"Q8TCJ2","domains":[{"cath_id":"3.40.50.12610","chopping":"578-771","consensus_level":"high","plddt":91.5216,"start":578,"end":771}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8TCJ2","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q8TCJ2-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q8TCJ2-F1-predicted_aligned_error_v6.png","plddt_mean":79.81},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=STT3B","jax_strain_url":"https://www.jax.org/strain/search?query=STT3B"},"sequence":{"accession":"Q8TCJ2","fasta_url":"https://rest.uniprot.org/uniprotkb/Q8TCJ2.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q8TCJ2/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8TCJ2"}},"corpus_meta":[{"pmid":"23530066","id":"PMC_23530066","title":"Extreme C-terminal sites are posttranslocationally glycosylated by the STT3B isoform of the OST.","date":"2013","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/23530066","citation_count":87,"is_preprint":false},{"pmid":"22607976","id":"PMC_22607976","title":"STT3B-dependent posttranslational N-glycosylation as a surveillance system for secretory protein.","date":"2012","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/22607976","citation_count":69,"is_preprint":false},{"pmid":"23842455","id":"PMC_23842455","title":"Mutations in STT3A and STT3B cause two congenital disorders of glycosylation.","date":"2013","source":"Human molecular genetics","url":"https://pubmed.ncbi.nlm.nih.gov/23842455","citation_count":68,"is_preprint":false},{"pmid":"31296534","id":"PMC_31296534","title":"Quantitative glycoproteomics reveals new classes of STT3A- and STT3B-dependent N-glycosylation sites.","date":"2019","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/31296534","citation_count":63,"is_preprint":false},{"pmid":"3841174","id":"PMC_3841174","title":"SIMP: a computer program in BASIC for nonlinear curve fitting.","date":"1985","source":"Journal of pharmacological methods","url":"https://pubmed.ncbi.nlm.nih.gov/3841174","citation_count":42,"is_preprint":false},{"pmid":"37193694","id":"PMC_37193694","title":"Identification of indocyanine green as a STT3B inhibitor against mushroom α-amanitin cytotoxicity.","date":"2023","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/37193694","citation_count":23,"is_preprint":false},{"pmid":"31511384","id":"PMC_31511384","title":"Comprehensive Interactome Analysis Reveals that STT3B Is Required for N-Glycosylation of Lassa Virus Glycoprotein.","date":"2019","source":"Journal of virology","url":"https://pubmed.ncbi.nlm.nih.gov/31511384","citation_count":23,"is_preprint":false},{"pmid":"36139350","id":"PMC_36139350","title":"Proteome and Glycoproteome Analyses Reveal the Protein N-Linked Glycosylation Specificity of STT3A and STT3B.","date":"2022","source":"Cells","url":"https://pubmed.ncbi.nlm.nih.gov/36139350","citation_count":19,"is_preprint":false},{"pmid":"16263756","id":"PMC_16263756","title":"The structure and location of SIMP/STT3B account for its prominent imprint on the MHC I immunopeptidome.","date":"2005","source":"International immunology","url":"https://pubmed.ncbi.nlm.nih.gov/16263756","citation_count":18,"is_preprint":false},{"pmid":"39192160","id":"PMC_39192160","title":"Exosomes from Hypoxic Pretreatment ADSCs Ameliorate Cardiac Damage Post-MI via Activated circ-Stt3b/miR-15a-5p/GPX4 Signaling and Decreased Ferroptosis.","date":"2024","source":"Cardiovascular toxicology","url":"https://pubmed.ncbi.nlm.nih.gov/39192160","citation_count":15,"is_preprint":false},{"pmid":"38945975","id":"PMC_38945975","title":"Stabilization of EREG via STT3B-mediated N-glycosylation is critical for PDL1 upregulation and immune evasion in head and neck squamous cell carcinoma.","date":"2024","source":"International journal of oral science","url":"https://pubmed.ncbi.nlm.nih.gov/38945975","citation_count":12,"is_preprint":false},{"pmid":"29282902","id":"PMC_29282902","title":"Construction of green fluorescence protein mutant to monitor STT3B-dependent N-glycosylation.","date":"2018","source":"The FEBS journal","url":"https://pubmed.ncbi.nlm.nih.gov/29282902","citation_count":9,"is_preprint":false},{"pmid":"39945486","id":"PMC_39945486","title":"STT3B promotes porcine epidemic diarrhea virus replication by regulating N-glycosylation of PEDV S protein.","date":"2025","source":"Journal of virology","url":"https://pubmed.ncbi.nlm.nih.gov/39945486","citation_count":7,"is_preprint":false},{"pmid":"39830021","id":"PMC_39830021","title":"A novel allosteric driver mutation of β-glucuronidase promotes head and neck squamous cell carcinoma progression through STT3B-mediated PD-L1 N-glycosylation.","date":"2025","source":"MedComm","url":"https://pubmed.ncbi.nlm.nih.gov/39830021","citation_count":5,"is_preprint":false},{"pmid":"18407520","id":"PMC_18407520","title":"Cloning, characterization and expression analysis of SIMP (source of immunodominant MHC-associated peptides) in grass carp Ctenopharyngodon idella.","date":"2007","source":"Fish & shellfish immunology","url":"https://pubmed.ncbi.nlm.nih.gov/18407520","citation_count":5,"is_preprint":false},{"pmid":"34734627","id":"PMC_34734627","title":"A cytosolic reductase pathway is required for efficient N-glycosylation of an STT3B-dependent acceptor site.","date":"2021","source":"Journal of cell science","url":"https://pubmed.ncbi.nlm.nih.gov/34734627","citation_count":5,"is_preprint":false},{"pmid":"35301756","id":"PMC_35301756","title":"A conserved Plasmodium structural integrity maintenance protein (SIMP) is associated with sporozoite membrane and is essential for maintaining shape and infectivity.","date":"2022","source":"Molecular microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/35301756","citation_count":3,"is_preprint":false},{"pmid":"16297371","id":"PMC_16297371","title":"Identification of two distinct intracellular localization signals in STT3-B.","date":"2005","source":"Archives of biochemistry and biophysics","url":"https://pubmed.ncbi.nlm.nih.gov/16297371","citation_count":3,"is_preprint":false},{"pmid":"35152214","id":"PMC_35152214","title":"Discovery and Characterization of the Phospholemman/SIMP/Viroporin Superfamily.","date":"2022","source":"Microbial physiology","url":"https://pubmed.ncbi.nlm.nih.gov/35152214","citation_count":2,"is_preprint":false},{"pmid":"38723849","id":"PMC_38723849","title":"Proteomic profiling of FFPE specimens: Discovery of HNRNPA2/B1 and STT3B as biomarkers for determining formalin fixation durations.","date":"2024","source":"Journal of proteomics","url":"https://pubmed.ncbi.nlm.nih.gov/38723849","citation_count":2,"is_preprint":false},{"pmid":"40056505","id":"PMC_40056505","title":"Unveiling the Role of Oligosaccharyltransferase STT3B in Colorectal Cancer Tissues: Clinical significance and Molecular Mechanisms Driving the Formation of Tertiary Lymphoid Structures.","date":"2025","source":"Immunobiology","url":"https://pubmed.ncbi.nlm.nih.gov/40056505","citation_count":2,"is_preprint":false},{"pmid":"41574665","id":"PMC_41574665","title":"HMGN2 induces pyroptosis in tumour cells by modulating the STT3B/PD‑L1/caspase‑1/GSDMD axis.","date":"2026","source":"Molecular medicine reports","url":"https://pubmed.ncbi.nlm.nih.gov/41574665","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.09.03.674041","title":"Differential contributions of human oligosaccharyltransferase complexes OST-A and OST-B to HIV-1 envelope glycoprotein glycosylation","date":"2025-09-03","source":"bioRxiv","url":"https://doi.org/10.1101/2025.09.03.674041","citation_count":0,"is_preprint":true},{"pmid":null,"id":"bio_10.1101_2024.12.03.626593","title":"OST Catalytic Subunit Redundancy Enables Therapeutic Targeting of N-Glycosylation","date":"2024-12-05","source":"bioRxiv","url":"https://doi.org/10.1101/2024.12.03.626593","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":14030,"output_tokens":4066,"usd":0.05154,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":11795,"output_tokens":3588,"usd":0.074338,"stage2_stop_reason":"end_turn"},"total_usd":0.125878,"stage1_batch_id":"msgbatch_017qqVz33Ga63SCDwzb35XPb","stage2_batch_id":"msgbatch_01Scr2Ei9x4HJMiRGxwrmvLK","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2013,\n      \"finding\": \"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.\",\n      \"method\": \"Biosynthetic pulse labeling of five human glycoproteins, STT3A/STT3B-deficient cell lines, bioinformatics analysis of glycopeptide databases\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (pulse labeling, defined KO/KD cell lines across multiple substrates), replicated across multiple glycoproteins\",\n      \"pmids\": [\"23530066\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"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.\",\n      \"method\": \"Folding and ERAD perturbation analyses, STT3B knockdown, detergent solubility assays, cell proliferation assays in mutant TTR-expressing cells\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple functional assays in a single lab with clear pathway placement via epistasis-style perturbation\",\n      \"pmids\": [\"22607976\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"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.\",\n      \"method\": \"Patient fibroblast glycosylation assays, GFP biomarker glycosylation rescue experiments with corresponding cDNA, transferrin glycosylation analysis\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — human genetic disease causation confirmed by rescue with wild-type cDNA and substrate-specific glycosylation analysis in patient cells\",\n      \"pmids\": [\"23842455\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"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.\",\n      \"method\": \"Quantitative glycoproteomics comparing ~1,000 acceptor sites in wild-type vs. STT3A/STT3B mutant cells; ER stress induction with thapsigargin, DTT, and NGI-1\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — large-scale quantitative glycoproteomics with defined KO cell lines, multiple ER stress conditions, single rigorous study with broad coverage\",\n      \"pmids\": [\"31296534\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"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.\",\n      \"method\": \"CRISPR-Cas9 knockout of STT3A and STT3B, affinity purification-mass spectrometry (AP-MS), site-directed mutagenesis of CXXC motifs, recombinant arenavirus replication assays\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — CRISPR KO, AP-MS, and active-site mutagenesis in a single study with multiple orthogonal methods\",\n      \"pmids\": [\"31511384\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"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.\",\n      \"method\": \"Subcellular fractionation, reporter fusion experiments (ovalbumin SIINFEKL coupled to SIMP), MHC I peptide presentation assays\",\n      \"journal\": \"International immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — direct localization and functional presentation assay with mechanistic follow-up, single lab\",\n      \"pmids\": [\"16263756\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"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.\",\n      \"method\": \"Reporter protein fusion constructs, cellular localization assays, structural comparison of STT3A and STT3B C-terminal domains\",\n      \"journal\": \"Archives of biochemistry and biophysics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — direct experimental localization using reporter constructs with defined sequence elements, single lab\",\n      \"pmids\": [\"16297371\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"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.\",\n      \"method\": \"In vitro translation system with defined redox conditions, NADPH-dependent pathway inhibition, membrane-impermeable reducing agent addition, STT3A/STT3B-specific analysis\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro reconstitution system with defined conditions, but single lab and single study\",\n      \"pmids\": [\"34734627\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"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.\",\n      \"method\": \"STT3A/STT3B knockout cell lines, fluorescence assays with mEGFP reporter constructs containing C-terminal glycosylation sequons\",\n      \"journal\": \"The FEBS journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO cell lines with engineered reporter substrates and fluorescence readout, two orthogonal confirmatory approaches, single lab\",\n      \"pmids\": [\"29282902\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"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.\",\n      \"method\": \"Proteomics and glycoproteomics in STT3A-KO and STT3B-KO HEK293 cells, identification of 4265 unique N-linked intact glycopeptides from 629 glycosites\",\n      \"journal\": \"Cells\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — quantitative glycoproteomics in KO cells, single lab with broad substrate coverage\",\n      \"pmids\": [\"36139350\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"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.\",\n      \"method\": \"Genome-wide CRISPR screen, in silico drug screening, in vivo mouse model validation, liver organoid assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genome-wide CRISPR screen combined with in vivo validation and mechanistic inhibitor characterization across multiple model systems\",\n      \"pmids\": [\"37193694\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"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.\",\n      \"method\": \"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\",\n      \"journal\": \"International journal of oral science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mutagenesis of specific glycosylation site combined with KD and pharmacological inhibition, in vivo validation, single lab\",\n      \"pmids\": [\"38945975\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"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.\",\n      \"method\": \"CRISPR-Cas9 knockout of STT3A and STT3B, pharmacological N-glycosylation inhibitors, viral replication assays\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — CRISPR KO combined with pharmacological inhibition and viral replication assay, single lab\",\n      \"pmids\": [\"39945486\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"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.\",\n      \"method\": \"Immunoprecipitation/mass spectrometry (IP/MS), anti-STT3B blocking assay, western blotting, immunofluorescence, ZDOCK and AlphaFold3 structural modeling\",\n      \"journal\": \"Molecular medicine reports\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single Co-IP/MS identification with blocking assay confirmation, computational modeling; single lab, no in vitro reconstitution\",\n      \"pmids\": [\"41574665\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"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.\",\n      \"method\": \"STT3A/STT3B CRISPR knockout cells, site-specific glycan analysis of recombinant Env proteins, HIV-1 neutralization assays with broadly neutralizing antibodies\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 2 / Weak — KO cells and glycan analysis but preprint, single lab, not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.09.03.674041\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"STT3B is the catalytic subunit of the STT3B-OST complex that primarily mediates posttranslocational N-glycosylation in the ER lumen, rescuing acceptor sites skipped by the co-translational STT3A-OST complex—particularly C-terminal sites (within ~50-55 residues of the protein C-terminus), sites in short loops of multi-spanning membrane proteins, and cryptic sites exposed on misfolded proteins—with its unique subunits MAGT1/TUSC3 (bearing a CXXC oxidoreductase motif) contributing to substrate recognition, and cytosolic NADPH-dependent redox conditions modulating efficiency at disulfide-adjacent sites; this activity is essential for quality control of secretory proteins via EDEM3-mediated ERAD, for glycosylation of specific viral glycoproteins, and for stability of glycoproteins such as EREG, while loss-of-function mutations cause a congenital disorder of glycosylation (STT3B-CDG).\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"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 [#0]. 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 [#0], 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 [#3]. 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 [#1]. 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 [#4], and efficiency at disulfide-adjacent acceptor sites is further tuned by a cytosolic NADPH-dependent reductive pathway [#7]. 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 [#11], and is preferentially required for glycosylation of certain viral glycoproteins [#4, #12]. 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 [#2].\",\n  \"teleology\": [\n    {\n      \"year\": 2005,\n      \"claim\": \"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.\",\n      \"evidence\": \"Subcellular fractionation, reporter fusion constructs and topogenic/targeting-sequence mapping\",\n      \"pmids\": [\"16263756\", \"16297371\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"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\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"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.\",\n      \"evidence\": \"Folding/ERAD perturbation and STT3B knockdown with mutant transthyretin substrates\",\n      \"pmids\": [\"22607976\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Generality beyond transthyretin not addressed here\", \"Mechanism by which cryptic sites are recognized unresolved\", \"Single-lab epistasis-style placement\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"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.\",\n      \"evidence\": \"Biosynthetic pulse labeling across multiple glycoproteins in STT3A/STT3B-deficient cells; patient fibroblast glycosylation assays with cDNA rescue\",\n      \"pmids\": [\"23530066\", \"23842455\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"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\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"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.\",\n      \"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\",\n      \"pmids\": [\"31296534\", \"31511384\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How loop geometry dictates STT3B preference is unresolved\", \"Division of labor between MAGT1 and TUSC3 not separated\", \"Structural model of substrate engagement absent\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Connected cytosolic redox state to lumenal glycosylation efficiency, showing a NADPH-dependent reductive pathway governs occupancy at disulfide-adjacent STT3B-dependent sites.\",\n      \"evidence\": \"In vitro translation with defined redox conditions, NADPH-pathway inhibition and membrane-impermeable reductant in STT3B-specific assays\",\n      \"pmids\": [\"34734627\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Identity of the trans-membrane redox relay not defined\", \"In vitro reconstitution from a single lab\", \"Physiological scope of redox modulation unquantified\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"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.\",\n      \"evidence\": \"Comparative proteomics/glycoproteomics in STT3A-KO and STT3B-KO HEK293 cells\",\n      \"pmids\": [\"36139350\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Substrate-specific determinants of STT3B dependence not mechanistically dissected\", \"Single cell-line context\", \"Functional consequences of reduced site occupancy not assessed\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"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.\",\n      \"evidence\": \"STT3B knockdown, N47 site-directed mutagenesis, NGI-1 inhibition, and xenograft combination with anti-PD-L1 in head and neck carcinoma cells\",\n      \"pmids\": [\"38945975\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether EREG is a direct posttranslocational substrate not formally separated from STT3A\", \"Single tumor context\", \"Mechanism connecting EREG glycosylation to PD-L1 incompletely defined\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"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.\",\n      \"evidence\": \"Genome-wide CRISPR screen, in silico inhibitor discovery, liver organoid and mouse validation\",\n      \"pmids\": [\"37193694\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The glycoprotein substrate mediating α-amanitin toxicity not identified\", \"ICG specificity for STT3B versus other OST functions not fully delineated\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Reinforced STT3B's selective role in viral glycoprotein maturation and raised candidate membrane-surface interactions, while leaving the latter mechanistically immature.\",\n      \"evidence\": \"CRISPR KO and viral replication assays (PEDV spike); IP/MS, blocking assay and structural modeling for an HMGN2 interaction\",\n      \"pmids\": [\"39945486\", \"41574665\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"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\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"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.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"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\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [0, 3, 4]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 2, 11]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [1, 5, 6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 3]},\n      {\"term_id\": \"R-HSA-8953897\", \"supporting_discovery_ids\": [1, 3]}\n    ],\n    \"complexes\": [\"STT3B-OST complex\"],\n    \"partners\": [\"MAGT1\", \"TUSC3\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}