Curator sequence-similarity transfer from experimentally characterized orthologs supplies the ISS annotations of TUSC3 to protein N-linked glycosylation (GO:0006487) and to mitochondrion (GO:0005739); the first agrees with direct human evidence, while the mitochondrial location is not corroborated by any experimental study of TUSC3 and conflicts with its established ER-membrane residence.

PAINT phylogenetic inference places TUSC3 in the Ost3p/Ost6p thioredoxin-domain OST subunit clade, supporting the IBA annotations to oligosaccharyltransferase complex (GO:0008250) and protein N-linked glycosylation (GO:0006487); the same GO_REF also supplies an IBA to magnesium ion transmembrane transport (GO:1903830), which rests on the contested Mg2+-transporter interpretation of the family rather than on its thioredoxin/OST function.

UniProt subcellular-location keyword mapping supplies the IEA annotation of TUSC3 to endoplasmic reticulum membrane (GO:0005789), which matches the multi-pass ER topology and lumenal thioredoxin domain described in the UniProt entry.

ARBA machine-learning rules supply IEA annotations of TUSC3 to protein N-linked glycosylation (GO:0006487), oligosaccharyltransferase complex (GO:0008250) and magnesium ion transmembrane transporter activity (GO:0015095); the first two are well supported, whereas the transporter activity inherits the disputed Mg2+ assignment.

Proteomic dissection of ribosome-associated mammalian OST resolves multiple subcomplexes of 500-700 kDa with different subunit compositions, the analysis that places TUSC3 among OST-associated subunits.

The paper identifies DC2 as an OST-copurifying protein weakly homologous to the C-terminal half of yeast Ost3p and Ost6p, the yeast proteins to which TUSC3 is the thioredoxin-domain counterpart.

A homozygous 1 bp insertion in TUSC3 (c.787_788insC, p.N263fsX300) causing mRNA decay segregates with autosomal-recessive non-syndromic intellectual disability, the IMP basis for the cognition (GO:0050890) annotation.

TUSC3 encodes a subunit of the oligosaccharyltransferase complex that carries out the key step of N-glycosylation - the NAS basis for the protein N-linked glycosylation annotation from this paper.

Patient fibroblasts showed normal bulk N-glycan synthesis and transfer, which the authors attribute to functional compensation; this is why loss of TUSC3 does not present as a classical CDG and is consistent with MAGT1 redundancy.

An independent consanguineous family with seven affected individuals carries a homozygous deletion partly removing TUSC3, with complete absence of a functional transcript, replicating TUSC3 loss as a cause of non-syndromic AR intellectual disability.

The authors note that, unlike other glycosylation defects, TUSC3 inactivation causes isolated intellectual disability rather than a multisystem CDG phenotype.

Knockdown of TUSC3 lowers total and free intracellular Mg2+ in mammalian cell lines, the IMP basis for the magnesium ion transport (GO:0015693) annotation.

The authors interpret their yeast-complementation and knockdown data as showing TUSC3 is a plasma-membrane Mg2+ transporter; this interpretation is the source of the plasma membrane (GO:0005886) and transporter-activity annotations and is contested by later work placing TUSC3 in the ER as an OST oxidoreductase subunit.

Morpholino knockdown of TUSC3 in zebrafish arrests early development and is rescued by excess Mg2+ or by mammalian mRNA, indicating a physiological requirement but not by itself establishing direct transport by TUSC3.

TUSC3 has a membrane-anchored N-terminal thioredoxin domain in the ER lumen that can form transient mixed disulfide complexes with OST substrates - the structural basis of its protein-disulfide oxidoreductase molecular function.

X-ray structures of TUSC3 bound to two model peptides reveal a defined peptide-binding groove adjacent to the active site that accepts peptides in either orientation, giving TUSC3 genuine substrate selectivity rather than indiscriminate thiol chemistry.

The authors' model is that TUSC3 raises glycosylation efficiency for a subset of human glycoproteins by slowing their folding, which is the mechanistic rationale for modelling TUSC3 as contributing to OST activity rather than catalysing glycan transfer.

The lumenal CXXC active site of the MagT1/TUSC3 oxidoreductase is required for glycosylation of STT3B-dependent acceptor sites, including sequons bracketed by disulfides or containing cysteine as the internal residue.

The oxidoreductase is predominantly oxidized in vivo, consistent with a mechanism in which it forms a transient mixed disulfide with the substrate to give STT3B access to unmodified acceptor sites.

TUSC3 fully complements MagT1-depleted cells, establishing functional redundancy between the two thioredoxin-domain OST-B subunits and explaining the normal bulk glycosylation seen in TUSC3 patient fibroblasts.

This paper directly addresses the competing Mg2+-transporter interpretation, noting that MagT1 overexpression does not raise cellular Mg2+ and that MagT1-deficient lymphocytes have normal cellular Mg2+ levels.

CRISPR/Cas9 was used to make viable HEK293 lines null for the two STT3B-specific accessory subunits MagT1 and TUSC3, providing clean genetic tools free of the residual expression that confounds siRNA knockdown.

TUSC3 is described as an accessory subunit assembled with the catalytic STT3B and a shared subunit set, consistent with modelling its molecular function as a contribution to complex-level activity.

Cryo-EM structures show that MAGT1 - and by extension its paralogue TUSC3 - occupies the OST-B-specific subunit position contacting the catalytic STT3B, whereas OST-A uses DC2 in the equivalent position.

An acceptor peptide and dolichylphosphate were resolved bound to STT3B but only dolichylphosphate in STT3A, suggesting the two complexes differ in substrate affinity - relevant to why the OST-B-specific TUSC3 matters for particular acceptor sites.

Mammals express two homologs of yeast Ost3p - TUSC3 and MAGT1 - which are assembled with the shared OST subunits into complexes of significantly different enzymatic activity.

Tissue- and cell-type-specific expression of the STT3 homologs is proposed to tune OST enzymatic properties, the context in which TUSC3's restricted tissue distribution (relative to the ubiquitous MAGT1) is interpreted.

Reactome lists TUSC3 (N33) among the subunits of the ER-membrane OST complex that transfers the preassembled 14-sugar glycan onto nascent protein, with STT3A or STT3B supplying the catalytic domain.

This is a Reactome holding pathway for transport and binding events that cannot yet be placed in an existing pathway; it is the source of the broad transmembrane transport (GO:0055085) TAS annotation for TUSC3 and carries no TUSC3-specific evidence.

Reactome models TUSC3 as an essential protein for cellular Mg2+ uptake, citing Zhou & Clapham 2009; this record is the sole source of the magnesium ion transmembrane transporter activity (GO:0015095) and plasma membrane (GO:0005886) TAS annotations and inherits the disputed interpretation of that paper.

TUSC3 is listed as an OST-complex participant in the ER N-glycosylation of the SARS-CoV-2 spike protein; a substrate-specific reaction record supporting the ER-membrane localization rather than a distinct TUSC3 function.

TUSC3 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 TUSC3 function.

TUSC3 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.

TUSC3 appears as an OST-complex participant in the co-folding N-glycosylation of dengue virus NS1; notably this substrate is glycosylated while folding, the situation in which the TUSC3 oxidoreductase activity is expected to matter.

TUSC3 appears as an OST-complex participant in the ER N-glycosylation of PD-L1 (CD274); the record attributes catalysis to STT3A/STT3B, consistent with TUSC3 being a non-catalytic accessory subunit.