Gene Ontology annotation through association of InterPro records with GO terms
Manual transfer of experimentally-verified manual GO annotation data to orthologs by curator judgment of sequence similarity
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Curator sequence-similarity transfer from experimentally characterized orthologs supplies the ISS annotations of DDOST to endoplasmic reticulum membrane (GO:0005789), oligosaccharyltransferase complex (GO:0008250) and glycoprotein biosynthetic process (GO:0009101); the first two are well corroborated by direct human evidence, the third is a broad parent of protein N-linked glycosylation.
Annotation inferences using phylogenetic trees
Gene Ontology annotation based on curation of immunofluorescence data
Electronic Gene Ontology annotations created by ARBA machine learning models
Combined Automated Annotation using Multiple IEA Methods
Defining the membrane proteome of NK cells.
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DDOST was detected in a high-throughput membrane-proteome survey of the NK-like YTS cell line; the study assigns no function and supports only generic membrane association.
"Mass spectrometric analysis identified 1843 proteins with high confidence scores. On the basis of the presence of transmembrane regions or evidence of posttranslational modifications and prediction algorithms, approximately 40% of the identified proteins were predicted as plausible membrane proteins."
Mapping a dynamic innate immunity protein interaction network regulating type I interferon production.
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DDOST was identified in a large-scale affinity-purification map of innate-immunity protein complexes (the HI5 interactome), the source of its bare protein-binding IPI annotation.
"Fifty-eight baits were associated with 260 interacting proteins forming a human innate immunity interactome for type I interferon (HI5) of 401 unique interactions"
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The shared STING1/UNC93B1 binding partners recovered in this screen are predominantly ER membrane proteins of the protein-translocation machinery; the paper does not name DDOST among them, but the DDOST hit is consistent with ER co-residence rather than with a role in interferon signalling.
"STING and UNC93B1 interacted with 10 common binding partners and most of them are ER membrane proteins involved in protein translocation."
The oligosaccharyltransferase subunits OST48, DAD1 and KCP2 function as ubiquitous and selective modulators of mammalian N-glycosylation.
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siRNA depletion shows that OST48 (DDOST) is required for assembly of both the STT3A-containing and the STT3B-containing OST complexes, establishing it as a shared core subunit rather than a complex-specific accessory.
"Our results demonstrate that OST48 and DAD1 are required for the assembly of both STT3A- and STT3B-containing OST complexes."
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Loss of OST48 structurally perturbs the OST complex and produces a pronounced global hypoglycosylation phenotype, so DDOST acts as a ubiquitous modulator of OST stability and hence of N-glycosylation.
"The structural perturbations of these complexes we observe in OST48- and DAD1-depleted cells underlie their pronounced hypoglycosylation phenotypes. Thus, OST48 and DAD1 are global modulators of OST stability and hence N-glycosylation."
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Catalysis itself is supplied by the STT3 subunit; the non-catalytic subunits such as OST48 play structural roles or modulate efficiency for particular precursors, which is why DDOST is best modelled as contributing to, rather than carrying, the glycotransferase activity.
"Comparative studies have clearly shown that eukaryotic STT3 proteins alone can fulfil the enzymatic requirements for N-glycosylation, yet in many cases STT3 homologues form stable complexes with a variety of non-catalytic OST subunits. Whereas some of these additional components might play a structural role, others appear to increase or modulate N-glycosylation efficiency for certain precursors."
The cancer-associated microprotein CASIMO1 controls cell proliferation and interacts with squalene epoxidase modulating lipid droplet formation.
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This paper characterizes the CASIMO1/SMIM22 microprotein and its effect on squalene epoxidase and lipid droplets; the DDOST-SMIM22 interaction it sources is incidental to the study's conclusions and does not define a DDOST function.
"Here, we report the identification and characterization of a novel microprotein of 10 kDa, which we named Cancer-Associated Small Integral Membrane Open reading frame 1 (CASIMO1)."
Cryo-electron microscopy structures of human oligosaccharyltransferase complexes OST-A and OST-B.
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Cryo-EM structures of both human OST complexes resolve DDOST/OST48 as a subunit of the co-translational OST-A and the post-translocational OST-B.
"Mammals express two distinct OST complexes that act in a cotranslational (OST-A) or posttranslocational (OST-B) manner. Here, we present high-resolution cryo-electron microscopy structures of human OST-A and OST-B."
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The two complexes share an overall architecture and differ mainly in the catalytic subunit and its complex-specific partner, so the core subunits including DDOST are common to both.
"Although they have similar overall architectures, structural differences in the catalytic subunits STT3A and STT3B facilitate contacts to distinct OST subunits, DC2 in OST-A and MAGT1 in OST-B."
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The activity of the DDOST-containing complexes is transfer of a high-mannose glycan onto secretory proteins in the endoplasmic reticulum.
"Oligosaccharyltransferase (OST) catalyzes the transfer of a high-mannose glycan onto secretory proteins in the endoplasmic reticulum."
Visualization of translation and protein biogenesis at the ER membrane.
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Cryo-electron tomography of native ER membranes yields an atomic model of the most abundant translocon variant, comprising SEC61, TRAP and the DDOST-containing oligosaccharyltransferase complex A.
"The near-complete atomic model of the most abundant ER translocon variant comprising the protein-conducting channel SEC61, TRAP and the oligosaccharyltransferase complex A (OSTA) reveals specific interactions of TRAP with other translocon components."
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The ribosome-translocon complex containing OST-A carries out N-glycosylation of nascent chains together with their synthesis, translocation, membrane insertion and folding.
"It governs the synthesis, translocation, membrane insertion, N-glycosylation, folding and disulfide-bond formation of nascent proteins."
Positive selection CRISPR screens reveal a druggable pocket in an oligosaccharyltransferase required for inflammatory signaling to NF-κB.
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Genome-wide CRISPR screening shows the DDOST-containing OST-A complex is specifically required for N-glycosylation and cell-surface delivery of TLR4, evidence that the two OST complexes differ in substrate range.
"we discovered that the LPS receptor Toll-like receptor 4 (TLR4) is specifically dependent on the oligosaccharyltransferase complex OST-A for N-glycosylation and cell-surface localization"
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Cryo-EM of the OST-A complex localizes the NGI-1 inhibitor to the STT3A catalytic site, confirming that catalysis resides in STT3A and not in accessory subunits such as DDOST.
"These variants, in conjunction with cryoelectron microscopy studies, revealed that NGI-1 binds the catalytic site of STT3A, where it traps a molecule of the donor substrate dolichyl-PP-GlcNAc2-Man9-Glc3, suggesting an uncompetitive inhibition mechanism."
Interleukin-2 induces N-glycosylation in T-cells: characterization of human lymphocyte oligosaccharyltransferase.
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DDOST/OST48 was identified as the 50-kDa Wbp1 homologue among the four predominant proteins of a purified human lymphocyte oligosaccharyltransferase preparation, giving direct biochemical evidence of OST-complex membership.
"After solubilization and 85-fold purification from salt-washed microsomes, the enzyme preparation contained four predominant proteins. N-terminal sequence analysis identified the proteins as ribophorin I, ribophorin II (doublet), and a 50-kDa homologue of Wbp1, a yeast protein essential for N-glycosylation."
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N-glycosylation activity in peripheral blood lymphocytes rises about ten-fold on mitogen activation and stays elevated under IL-2 support; this is a property of the T-cell system in which the OST enzyme was assayed, not a DDOST-intrinsic role in cytokine response or T-cell activation.
"N-glycosylation activity increased 10-fold after mitogen activation of PBLs. N-glycosylation activity remained elevated during long-term culture and expansion of human lymphocytes when growth was supported by interleukin-2."
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The DDOST-containing enzyme fractionates predominantly with microsomes, consistent with the ER-membrane localization of the OST complex.
"The enzyme mediating N-glycosylation in lymphocytes was localized predominantly but not entirely to a microsomal organelle by subcellular fractionation."
Transfer of N-glycan to the protein
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Reactome lists DDOST (OST48) among the subunits of the ER-membrane OST complex that transfers the 14-sugar lipid-linked oligosaccharide en bloc onto nascent protein, with STT3A/STT3B carrying the catalytic domain.
"This reaction is catalyzed by the oligosaccharyltransferase (OST) complex, comprising at least seven proteins; DAD1 (Dolichyl-diphosphooligosaccharide--protein glycosyltransferase subunit DAD1), DDOST (OST48 in yeast), RPN1 (ribophorin 1), RPN2 (ribophorin 2), OST4, TUSC3 (N33), MAGT1 (magnesium transporter protein 1) and either STT3A or STT3B"
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Transfer occurs co-translationally in the ER lumen as the nascent chain emerges from the ribosome, which is the basis of the ER-membrane localization annotation.
"The reaction occurs cotranslationally as the growing peptide chain leaves a ribosome associated with the ER membrane and enters the ER lumen."
Exocytosis of azurophil granule membrane proteins
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DDOST is included in this neutrophil azurophil-granule membrane-proteome record, which is the source of the plasma membrane (GO:0005886) and azurophil granule membrane (GO:0035577) TAS annotations; the record describes granule biology and does not implicate DDOST's oligosaccharyltransferase function at those sites.
"Azurophil granules undergo limited exocytosis in response to stimulation (Sengelov et al. 1993, Faurschou et al. 2002), their primary role is believed to be killing and degradation of engulfed microbes in the phagolysosome (Joiner et al. 1989)."
Spike protein gets N-glycosylated
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DDOST is listed as an OST-complex participant in the ER N-glycosylation of the SARS-CoV-2 spike protein; a substrate-specific reaction record rather than evidence for a distinct DDOST function.
"Glycosyltransferases in the endoplasmatic reticulum are responsible for the attachment of numerous high-mannose N-glycans on the SARS-CoV-2 spike protein."
CDH1 is N-glycosylated on asparagine residues in endoplasmic reticulum
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DDOST participates as an OST subunit in ER N-glycosylation of E-cadherin; a substrate-level reaction record supporting the ER-membrane localization rather than a CDH1-specific DDOST role.
"All four glycosylated asparagines in CDH1 conform with the glycosylation sequon Asn-X-Thr/Ser which is glycosylated in the endoplasmic reticulum (ER) through transfer of the preassembled, high-mannose oligosaccharide"
E is N-glycosylated
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DDOST appears as an OST-complex participant in ER N-glycosylation of the dengue virus E glycoprotein; a host-pathway reaction record, not evidence of a virus-specific DDOST function.
"DENV-2 E protein is known to be N-glycosylated at Asn-67 and Asn-153 (Asn-347 and Asn-433 of the polyprotein, respectively), a process requiring, among others, the oligosaccharyl transferase activity of the host OST complex"
pre-M is N-glycosylated
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DDOST appears as an OST-complex participant in ER N-glycosylation of the dengue virus prM protein; a substrate-level record supporting only ER-membrane localization.
"Dengue virus pre-prM is N-glycosylated at Asn-7, Asn-31, Asn-52, and Asn-69 (Asn-121, Asn-145, Asn-166 and Asn-183 of the polyprotein), requiring, among others, the oligosaccharyl transferase activity of the OST complex"
Pre-NS1 folds and is N-glycosylated
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DDOST appears as an OST-complex participant in the co-folding N-glycosylation of dengue virus NS1; a substrate-level record supporting ER-membrane localization only.
"Pre-NS1 protein, while being folded, is glycosylated at Asn-130 and Asn-207 (Asn-905 and Asn-982 in the polyprotein, respectively)."
CD274 N-linked glycosylation in ER
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DDOST appears as an OST-complex participant in ER N-glycosylation of PD-L1 (CD274); the record attributes catalysis to STT3A/STT3B, consistent with DDOST being a non-catalytic subunit.
"PD-L1 is N-glycosylated at N192, N200 and N219 by the OST complex (STT3A and STT3A) in the ER lumen."