Oligosaccharyltransferase (OST) complex - N-glycan transfer (STT3A/STT3B + accessory subunits)

The oligosaccharyltransferase (OST) complex is the endoplasmic-reticulum-membrane enzyme that performs the central, committed step of N-linked glycosylation: en-bloc transfer of the preassembled Glc3Man9GlcNAc2 glycan from dolichyl diphosphate (Dol-PP) onto the side-chain amide of asparagine residues within N-X-S/T sequons of nascent secretory and membrane proteins. Humans have two OST isoforms distinguished by their catalytic subunit. The STT3A complex (OST-A) is docked at the SEC61 translocon and glycosylates sequons co-translationally as the polypeptide enters the ER; its subunit OSTC (DC2) tethers it to the translocon. The STT3B complex (OST-B) acts post-translationally and translocon-independently, providing a proofreading second pass that glycosylates skipped, C-terminal and closely-spaced sequons; its thioredoxin-like oxidoreductase subunits MAGT1 and TUSC3 use a CXXC motif to transiently engage substrate cysteines, enabling glycosylation of cysteine-proximal sequons. Both isoforms share a set of non-catalytic scaffold subunits - ribophorin I (RPN1), ribophorin II (RPN2), OST48 (DDOST) and DAD1 - required for complex integrity and substrate presentation. Defects in individual subunits cause congenital disorders of glycosylation (STT3A-, STT3B-, DDOST-CDG), X-linked immunodeficiency with magnesium defect (MAGT1/XMEN) and autosomal-recessive intellectual disability (TUSC3).

MODULE:oligosaccharyltransferase_complexDRAFTMetabolic Pathwaymodules/oligosaccharyltransferase_complex.yaml
protein N-linked glycosylationGO:0006487 protein co-translational transfer of dolichol-linked oligosaccharideGO:0180058 protein post-translational transfer of dolichol-linked oligosaccharideGO:0180057
GO:0006487
protein N-linked glycosylation
The OST complex catalyses the en-bloc transfer of the N-glycan to asparagine (GO:0006487).
GO:0004579
dolichyl-diphosphooligosaccharide-protein glycotransferase activity
STT3A/STT3B are the catalytic subunits carrying this activity (GO:0004579).
GO:0008250
oligosaccharyltransferase complex
All nine subunits assemble into the OST complex (GO:0008250; isoforms GO:0160226/GO:0160227).
file:human/STT3A/STT3A-ai-review.yaml
STT3A gene review (human)
Co-translational OST catalytic subunit (UniProtKB:P46977, GO:0004579) matches the completed human STT3A review.
file:human/STT3B/STT3B-ai-review.yaml
STT3B gene review (human)
Post-translational OST catalytic subunit (UniProtKB:Q8TCJ2, GO:0004579) matches the completed human STT3B review.
file:human/OSTC/OSTC-ai-review.yaml
OSTC gene review (human)
STT3A-specific translocon adaptor subunit (UniProtKB:Q9NRP0, GO:0030674) matches the completed human OSTC review.
file:human/MAGT1/MAGT1-ai-review.yaml
MAGT1 gene review (human)
STT3B-specific oxidoreductase subunit (UniProtKB:Q9H0U3, GO:0015036) matches the completed human MAGT1 review.
file:human/TUSC3/TUSC3-ai-review.yaml
TUSC3 gene review (human)
STT3B-specific oxidoreductase subunit (UniProtKB:Q13454, GO:0015035) matches the completed human TUSC3 review.
file:human/RPN1/RPN1-ai-review.yaml
RPN1 gene review (human)
Shared scaffold subunit ribophorin I (UniProtKB:P04843) matches the completed human RPN1 review.
file:human/RPN2/RPN2-ai-review.yaml
RPN2 gene review (human)
Shared scaffold subunit ribophorin II (UniProtKB:P04844) matches the completed human RPN2 review.
file:human/DDOST/DDOST-ai-review.yaml
DDOST gene review (human)
Shared scaffold subunit OST48 (UniProtKB:P39656) matches the completed human DDOST review.
file:human/DAD1/DAD1-ai-review.yaml
DAD1 gene review (human)
Shared scaffold subunit DAD1 (UniProtKB:P61803) matches the completed human DAD1 review.
4Nodes
3Parts
0Variant Sets
0Variants
8Annotons
3Connections

Derived QC

Recommended-field compliance

100.0% recommended fields populated

All recommended fields populated.

Module deep research

✗ none found

No MODULE:oligosaccharyltransferase_complex deep-research report alongside the module YAML.

Leaf nodes lacking representative members

✓ every leaf node grounds to a representative protein.

Template conformance

✓ every declared conforms_to bundle matches its template motif.

Reaction chaining (advisory)

✓ every PRECEDES step chains, or its break is acknowledged via chaining_status.

  • cotranslational_ost → posttranslational_ost [NOT_CHECKED]

Function consistency and core coverage

5 gene-function check(s) reflected in core functions · 0 conflict(s) · 4 evidence or coverage gap(s) (9 comparison(s))

Retained noncore functions are reported separately; they do not require promotion to core functions. Support from representative genes does not establish conservation across the whole family. Missing reviews or identifiers leave a claim unverified.

AnnotonParticipantFunctionAssessment
stt3a_catalytic STT3A (human; co-translational)
UniProtKB:P46977
Family Representative 		dolichyl-diphosphooligosaccharide-protein glycotransferase activity
GO:0004579 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
ostc_adaptor OSTC (human)
UniProtKB:Q9NRP0
Family Representative 		protein-macromolecule adaptor activity
GO:0030674 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
stt3b_catalytic STT3B (human; post-translational)
UniProtKB:Q8TCJ2
Family Representative 		dolichyl-diphosphooligosaccharide-protein glycotransferase activity
GO:0004579 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
magt1_tusc3_oxidoreductase MAGT1 (human; XMEN)
UniProtKB:Q9H0U3
Family Representative 		disulfide oxidoreductase activity
GO:0015036 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
magt1_tusc3_oxidoreductase TUSC3 (human)
UniProtKB:Q13454
Family Representative 		disulfide oxidoreductase activity
GO:0015036 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
stt3a_catalytic STT3 / OST catalytic subunit family
PANTHER:PTHR13872
Family 		dolichyl-diphosphooligosaccharide-protein glycotransferase activity
GO:0004579 		Family Review Missing
No supported structured family review is available for PANTHER:PTHR13872.
ostc_adaptor OSTC / DC2 (DAD1-like) OST subunit family
PANTHER:PTHR13160
Family 		protein-macromolecule adaptor activity
GO:0030674 		Family Review Missing
No supported structured family review is available for PANTHER:PTHR13160.
stt3b_catalytic STT3 / OST catalytic subunit family
PANTHER:PTHR13872
Family 		dolichyl-diphosphooligosaccharide-protein glycotransferase activity
GO:0004579 		Family Review Missing
No supported structured family review is available for PANTHER:PTHR13872.
magt1_tusc3_oxidoreductase MAGT1/TUSC3 thioredoxin-like OST oxidoreductase family
PANTHER:PTHR12692
Family 		disulfide oxidoreductase activity
GO:0015036 		Family Review Missing
No supported structured family review is available for PANTHER:PTHR12692.

Gene-review completeness (9/9 grounded genes reviewed)

9 complete review(s) · 0 with deep research · 0 missing review · 9 reviewed but lacking deep research

Gene Review Complete Deep research
DAD1 P61803 ✓ ✓ ✗
DDOST P39656 ✓ ✓ ✗
MAGT1 Q9H0U3 ✓ ✓ ✗
OSTC Q9NRP0 ✓ ✓ ✗
RPN1 P04843 ✓ ✓ ✗
RPN2 P04844 ✓ ✓ ✗
STT3A P46977 ✓ ✓ ✗
STT3B Q8TCJ2 ✓ ✓ ✗
TUSC3 Q13454 ✓ ✓ ✗

Details

Context
endoplasmic reticulum membraneGO:0005789
Oligosaccharyltransferase (OST) complexMetabolic Pathwayoligosaccharyltransferase_complex
protein N-linked glycosylationGO:0006487 protein co-translational transfer of dolichol-linked oligosaccharideGO:0180058 protein post-translational transfer of dolichol-linked oligosaccharideGO:0180057
Context
endoplasmic reticulum membraneGO:0005789

Oligosaccharyltransferase (OST) complex - the ER-membrane machine that transfers the Dol-PP-linked Glc3Man9GlcNAc2 glycan onto Asn sequons (GO:0004579 / GO:0006487) - grounded to nine completed human gene reviews. Two isoforms share four scaffold subunits: catalytic STT3A (P46977, PTHR13872) with the translocon adaptor OSTC/DC2 (Q9NRP0, PTHR13160, GO:0030674) form OST-A (GO:0160226) for co-translational glycosylation (GO:0180058); catalytic STT3B (Q8TCJ2, PTHR13872) with the thioredoxin-like oxidoreductases MAGT1 (Q9H0U3) and TUSC3 (Q13454) (both PTHR12692, GO:0015036/GO:0015035) form OST-B (GO:0160227) for post-translational proofreading (GO:0180057). Shared scaffold: RPN1 (P04843 PTHR21049), RPN2 (P04844 PTHR12640), DDOST/OST48 (P39656 PTHR10830), DAD1 (P61803 PTHR10705) - modelled as non-catalytic subunits (in_complex GO:0008250, contributes_to GO:0004579 in their gene reviews). Module node locations are the anatomical ER membrane (GO:0005789); the OST-complex CC terms live in the gene reviews' in_complex. Curation notes: MAGT1/TUSC3 historical magnesium-transporter annotations are contested/secondary to the OST oxidoreductase role (kept non-core/over-annotated in the reviews); DAD1's anti-apoptotic role is downstream of OST integrity (non-core). Diseases: STT3A/STT3B/DDOST-CDG, MAGT1/XMEN, TUSC3 AR intellectual disability. Consumes the Dol-PP-glycan from the LLO/ALG assembly (reviewed) built on dolichol from the dolichyl_phosphate_biosynthesis module. GO term ids/labels verified against the local go.db; module passes structural + term-label validation.

Connections

RPN1/RPN2/DDOST/DAD1 are shared subunits of the STT3A (OST-A) complex.
RPN1/RPN2/DDOST/DAD1 are shared subunits of the STT3B (OST-B) complex.
STT3B proofreads and glycosylates sequons that the co-translational STT3A complex skipped.
Part 1: co-translational N-glycosylation (STT3A / OST-A)
Co-translational OST (STT3A complex + translocon adaptor OSTC)Reactioncotranslational_ost

Annotons

STT3A: OST-A catalytic subunit
stt3a_catalytic
Participant: Family: STT3 / OST catalytic subunit family
Family:
STT3 / OST catalytic subunit familyPANTHER:PTHR13872
Representative Members: STT3A (human; co-translational)UniProtKB:P46977

Function

dolichyl-diphosphooligosaccharide-protein glycotransferase activityGO:0004579
Substrates: Glc3Man9GlcNAc2-PP-dolichol (Dol-PP-glycan) asparagine (N-X-S/T sequon of nascent protein)
Products: N-glycosylated protein (Glc3Man9GlcNAc2-Asn) dolichyl diphosphate (Dol-PP)

Locations

endoplasmic reticulum membraneGO:0005789

Transfer the Dol-PP glycan onto Asn sequons co-translationally.

OSTC/DC2: STT3A translocon adaptor
ostc_adaptor
Participant: Family: OSTC / DC2 (DAD1-like) OST subunit family
Family:
OSTC / DC2 (DAD1-like) OST subunit familyPANTHER:PTHR13160
Representative Members: OSTC (human)UniProtKB:Q9NRP0

Function

protein-macromolecule adaptor activityGO:0030674
Substrates: STT3A OST complex SEC61 translocon
Products: translocon-associated OST-A

Locations

endoplasmic reticulum membraneGO:0005789

Anchor the STT3A OST complex to the translocon.

Part 2: post-translational N-glycosylation (STT3B / OST-B)
Post-translational OST (STT3B complex + oxidoreductases MAGT1/TUSC3)Reactionposttranslational_ost

Annotons

STT3B: OST-B catalytic subunit
stt3b_catalytic
Participant: Family: STT3 / OST catalytic subunit family
Family:
STT3 / OST catalytic subunit familyPANTHER:PTHR13872
Representative Members: STT3B (human; post-translational)UniProtKB:Q8TCJ2

Function

dolichyl-diphosphooligosaccharide-protein glycotransferase activityGO:0004579
Substrates: Glc3Man9GlcNAc2-PP-dolichol (Dol-PP-glycan) asparagine (skipped/C-terminal N-X-S/T sequon)
Products: N-glycosylated protein dolichyl diphosphate (Dol-PP)

Locations

endoplasmic reticulum membraneGO:0005789

Glycosylate skipped/C-terminal Asn sequons post-translationally.

MAGT1/TUSC3: STT3B-specific oxidoreductase subunits
magt1_tusc3_oxidoreductase
Participant: Family: MAGT1/TUSC3 thioredoxin-like OST oxidoreductase family
Family:
MAGT1/TUSC3 thioredoxin-like OST oxidoreductase familyPANTHER:PTHR12692
Representative Members: MAGT1 (human; XMEN)UniProtKB:Q9H0U3 TUSC3 (human)UniProtKB:Q13454

Function

disulfide oxidoreductase activityGO:0015036
Substrates: substrate glycoprotein cysteine thiol/disulfide
Products: transient mixed-disulfide intermediate (engaged sequon)

Locations

endoplasmic reticulum membraneGO:0005789

Engage substrate cysteines to expose Cys-proximal sequons for STT3B.

Part 3: shared non-catalytic scaffold subunits
Shared OST scaffold subunits (RPN1, RPN2, DDOST/OST48, DAD1)Reactionshared_scaffold

Annotons

RPN1: ribophorin I (substrate presentation)
rpn1_scaffold
Participant: Family: RPN1 / ribophorin I family
Family:
RPN1 / ribophorin I familyPANTHER:PTHR21049
Representative Members: RPN1 (human)UniProtKB:P04843

Processes

protein N-linked glycosylationGO:0006487

Locations

endoplasmic reticulum membraneGO:0005789

Present nascent chains to the catalytic STT3 subunit (both isoforms).

RPN2: ribophorin II
rpn2_scaffold
Participant: Family: RPN2 / ribophorin II family
Family:
RPN2 / ribophorin II familyPANTHER:PTHR12640
Representative Members: RPN2 (human)UniProtKB:P04844

Processes

protein N-linked glycosylationGO:0006487

Locations

endoplasmic reticulum membraneGO:0005789

Structural scaffold subunit of both OST isoforms.

DDOST/OST48: 48 kDa scaffold subunit
ddost_scaffold
Participant: Family: DDOST / OST48 family
Family:
DDOST / OST48 familyPANTHER:PTHR10830
Representative Members: DDOST (human)UniProtKB:P39656

Processes

protein N-linked glycosylationGO:0006487

Locations

endoplasmic reticulum membraneGO:0005789

Core scaffold subunit required for complex integrity; DDOST-CDG.

DAD1: complex-stabilising subunit
dad1_scaffold
Participant: Family: DAD1 / OST small-subunit family
Family:
DAD1 / OST small-subunit familyPANTHER:PTHR10705
Representative Members: DAD1 (human)UniProtKB:P61803

Processes

protein N-linked glycosylationGO:0006487

Locations

endoplasmic reticulum membraneGO:0005789

Stabilise the OST complex; its loss abolishes glycosylation and triggers apoptosis.