DDOST (OST48) — gene review notes

Identity

• UniProt: P39656 (OST48_HUMAN); gene DDOST (HGNC:2728); synonyms KIAA0115, OST48.
• Full name: Dolichyl-diphosphooligosaccharide--protein glycosyltransferase 48 kDa subunit.
• 456 aa precursor; N-terminal signal (1..42), large lumenal domain (43..427), single TM helix (428..447), short cytoplasmic tail (448..456) → single-pass type I membrane glycoprotein of the ER.
• [file:human/DDOST/DDOST-uniprot.txt "Single-pass type I membrane protein"]
• Belongs to the "DDOST 48 kDa subunit family" (SIMILARITY). Structural coverage: PDB 6S7O/6S7T (OST-A/OST-B cryo-EM), 8B6L, 8PN9, 9N9J, 9YGY.

Core biology

DDOST/OST48 is a non-catalytic subunit of the oligosaccharyltransferase (OST) complex, the ER enzyme that carries out the central step of N-linked protein glycosylation: en-bloc transfer of the preassembled Glc3Man9GlcNAc2 glycan from dolichol-pyrophosphate onto Asn within an Asn-X-Ser/Thr sequon in nascent polypeptides.
- [file:human/DDOST/DDOST-uniprot.txt "Subunit of the oligosaccharyl transferase (OST) complex that"]
- [file:human/DDOST/DDOST-uniprot.txt "catalyzes the initial transfer of a defined glycan"]
- [file:human/DDOST/DDOST-uniprot.txt "consensus motif in nascent polypeptide chains, the first step in"]
- Reactome R-HSA-446209: "This reaction is catalyzed by the oligosaccharyltransferase (OST) complex, comprising at least seven proteins; DAD1 ..., DDOST (OST48 in yeast), RPN1 ..., RPN2 ..., OST4, TUSC3 (N33), MAGT1 ... and either STT3A or STT3B ..., which contain the catalytic domain".

The catalytic subunit is STT3A or STT3B; DDOST itself has no catalytic activity. It is one of the shared core subunits (RPN1, RPN2, OST48, OST4, DAD1, TMEM258) present in both complex forms.
- [file:human/DDOST/DDOST-uniprot.txt "OST48, OST4, DAD1 and TMEM258, either STT3A or STT3B as catalytic"]

Two complex forms exist:
- OST-A (STT3A-containing, GO:0160226) — cotranslational, associates with Sec61 translocon/ribosome.
- OST-B (STT3B-containing, GO:0160227) — posttranslocational.
- PMID:31831667
- Cryo-EM confirmed DDOST as a component of both (structures OST-A and OST-B). PMID:31831667
- Gemmer et al. visualized the STT3A-OST bound to the ER translocon. PMID:36697828
- Lampson et al. determined the STT3A-OST structure and used it to define an NGI-1 druggable pocket. PMID:38670073

Functional requirement / assembly

Roboti & High (siRNA knockdown in mammalian cells) showed OST48 (and DAD1) are required for assembly of both STT3A- and STT3B-containing OST complexes; OST48 depletion destabilizes the complex and causes global hypoglycosylation.
- PMID:22467853
- PMID:22467853
- UniProt summarizes: [file:human/DDOST/DDOST-uniprot.txt "the assembly of both SST3A- and SS3B-containing OST complexes"]
This IMP is best read as DDOST contributing to complex integrity / OST catalytic activity (i.e. contributes_to the OST glycotransferase activity), NOT as an autonomous "enzyme activator activity" MF. The enzyme activator activity (GO:0008047) and regulation of protein stability (GO:0031647) IMP annotations from the same paper are over-interpretations of a structural/assembly role.

Original biochemical characterization

Kumar, Heinemann & Ozols purified human lymphocyte OST; N-terminal sequencing identified ribophorin I, ribophorin II, and "a 50-kDa homologue of Wbp1" (= OST48/DDOST). N-glycosylation activity rose ~10-fold on mitogen/IL-2 activation of T cells.
- PMID:9642163
- PMID:9642163
GOA (assigned_by UniProt) derives four IDA annotations from this paper: OST complex membership (GO:0008250), protein N-linked glycosylation (GO:0006487), plus response to cytokine (GO:0034097), T cell activation (GO:0042110), and intracellular membrane-bounded organelle (GO:0043231). The cytokine/T-cell terms reflect the experimental system (IL-2-driven T-cell activation) rather than a DDOST-intrinsic function — DDOST does not itself drive T-cell activation; it is the OST enzyme whose activity was measured to increase. These are context/over-annotations → KEEP_AS_NON_CORE / MARK_AS_OVER_ANNOTATED.

Localization

• ER membrane is the authoritative location (SUBCELLULAR LOCATION). [file:human/DDOST/DDOST-uniprot.txt "Endoplasmic reticulum membrane"]
• HPA IDA to endoplasmic reticulum (GO:0005783) — consistent.
• Reactome TAS "endoplasmic reticulum membrane" (multiple pathway-specific reactions) — consistent, redundant.
• membrane (GO:0016020) IEA/HDA — correct but non-informative parent.
• Azurophil granule membrane (GO:0035577) and plasma membrane (GO:0005886) (Reactome "Neutrophil degranulation" / azurophil-granule exocytosis, R-HSA-6798739) are large-scale granule-proteome inclusions, not the site of DDOST's molecular function. Over-annotations (neutrophil granule proteomics sweep OST subunits in); keep as non-core / over-annotated. Not experimental for DDOST specifically here (TAS from Reactome).

Interactions (IPI "protein binding")

• STING1 (Q86WV6) — DDOST appears in the STING1 innate-immunity interactome (HI5). UniProt records P39656; Q86WV6: STING1 in the INTERACTION block; GOA IPI GO:0005515 WITH UniProtKB:Q86WV6 from PMID:21903422 (Li et al., innate-immunity interactome; STING/UNC93B1 pull down many ER membrane/translocation proteins).
• [file:human/DDOST/DDOST-uniprot.txt "P39656; Q86WV6: STING1; NbExp=2; IntAct=EBI-358866, EBI-2800345"]
• PMID:21903422
• SMIM22 / CASIMO1 (K7EJ46) — GOA IPI GO:0005515 WITH UniProtKB:K7EJ46 from PMID:29765154; UniProt SUBUNIT "Interacts with SMIM22". The cached paper is about CASIMO1/SQLE and does not narrate a DDOST-functional consequence, so this binding is not a core function.
• [file:human/DDOST/DDOST-uniprot.txt "SMIM22 (PubMed:29765154)"]
  Both are bare "protein binding" IPIs → MARK_AS_OVER_ANNOTATED (per policy, never REMOVE a protein-binding IPI). They inform interactome context, not DDOST's molecular function.

Disease

DDOST deficiency causes Congenital disorder of glycosylation type 1R (CDG1R / DDOST-CDG), MIM 614507; variant Gly217Asp identified by whole-exome sequencing (Jones et al. 2012, PMID:22305527).
- [file:human/DDOST/DDOST-uniprot.txt "Congenital disorder of glycosylation 1R (CDG1R) [MIM:614507]"]

Curation summary (actions)

• Core (ACCEPT / represent core): OST complex membership (GO:0008250 IDA/IBA/IPI/ISS), OST-A (GO:0160226) & OST-B (GO:0160227) IDA, protein N-linked glycosylation (GO:0006487 IDA/IMP/IBA), ER membrane location (GO:0005789), ER (GO:0005783 IDA).
• Redundant-but-correct IEA/TAS for own function → ACCEPT (e.g. GO:0005789 IEA/ISS/TAS, GO:0006487 IEA).
• Broad parents → KEEP_AS_NON_CORE (GO:0016020 membrane; GO:0043231 intracellular membrane-bounded organelle; GO:0009101 glycoprotein biosynthetic process ISS).
• Over-annotations (not DDOST's molecular role) → MARK_AS_OVER_ANNOTATED: GO:0008047 enzyme activator activity, GO:0031647 regulation of protein stability, GO:0034097 response to cytokine, GO:0042110 T cell activation, GO:0035577 azurophil granule membrane, GO:0005886 plasma membrane, GO:0005515 protein binding (STING1, SMIM22).

Core function model (non-catalytic subunit)

• in_complex: GO:0008250 oligosaccharyltransferase complex (CC protein complex; def: "A protein complex that is found in the endoplasmic reticulum membrane of eukaryotes...").
• contributes_to_molecular_function: GO:0004579 dolichyl-diphosphooligosaccharide-protein glycotransferase activity (MF; the OST catalytic activity, carried by STT3A/B, to which DDOST contributes structurally).
• directly_involved_in / BP: GO:0006487 protein N-linked glycosylation.
• location: GO:0005789 endoplasmic reticulum membrane.
• NOTE: GO:0018279 ("protein N-linked glycosylation via asparagine") is OBSOLETE (merged into GO:0006487) — use GO:0006487.