TUSC3
Protein-coding gene in the species Homo sapiens
From Wikipedia, the free encyclopedia
Tumor suppressor candidate 3 (TUSC3), formerly known as N33 or OST3A, is a protein that in humans is encoded by the TUSC3 gene on chromosome 8p22.[5]
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| Aliases | TUSC3, D8S1992, M33, MRT22, MRT7, N33, OST3A, tumor suppressor candidate 3, MagT2, SLC58A2 | |||||||||||||||||||||||||||||||||||||||||||||||||||||
| External IDs | OMIM: 601385; MGI: 1933134; GeneCards: TUSC3 | |||||||||||||||||||||||||||||||||||||||||||||||||||||
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Structurally, TUSC3 is an integral membrane protein localized to the endoplasmic reticulum (ER). It contains a large N-terminal thioredoxin-like domain in the ER lumen followed by a C-terminal four-pass transmembrane domain that anchors it in the membrane.[6][7][8]
Functionally, TUSC3 exhibits a dual biochemical role. It acts as an accessory unit of the oligosaccharyltransferase (OST) complex, facilitating N-linked glycosylation of nascent proteins.[6] In addition, TUSC3 forms a complex with the ER magnesium transporter ERMA(TMEM94) and is required for efficient magnesium uptake into the ER lumen. Disruption of this TUSC3--ERMA complex leads to magnesium depletion, activation of the PERK--eIF2α stress pathway, impaired synaptic protein translation and neurodevelopmental deficits, linking TUSC3 loss to autosomal recessive intellectual disability (ARID).[9]
TUSC3 has also been studied as a candidate tumor suppressor gene, with loss or silencing associated with progression in several human cancers.[8][10][11]
Structure
TUSC3 is an ER membrane subunit of the STT3B-containing OST complex. It contains a large luminal N-terminal thioredoxin domain, containing the redox reactive Cys-X-X-Cys motif, followed by a compact C-terminal four-pass transmembrane bundle that anchors the protein in the ER membrane. TUSC3 functions as one of two alternative oxidoreductase accessory subunits (TUSC3 or MAGT1) that are incorporated into the hetero-oligomeric STT3B OST complex.[12]
Function
Role in N-linked glycosylation
N-linked glycosylation is the process in which a preassembled oligosaccharide (sugar chain) is attached to the nitrogen atom of specific asparagine residues within Asn-X-Ser/Thr sequons of nascent polypeptides. This modification occurs in the ER and is catalyzed by the OST complex. Disruptions to this pathway cause congenital disorders of glycosylation (CDG).[13][14]
TUSC3 encodes an essential accessory subunit of the OST complex. In the ER it helps the complex transfer the oligosaccharide onto the target asparagine residues of newly synthesized proteins.[15]
Molecular mechanism
Mammalian cells express two distinct catalytic forms of the OST complex: STT3A, which acts co-translationally at the translocon channel, and STT3B, which acts post-translationally to glycosate sites missed by STT3A.[15] TUSC3; along with its paralog MAGT1, specifically associates with the STT3B complex.[15]
Structurally, TUSC3 features an N-terminal luminal domain containing a thioredoxin-like fold with a canonical CxxC active-site motif.[8] Through this oxidoreductase domain, TUSC3 forms transient mixed-disulfide bonds with cysteine residues on unfolded polypeptide substrates.[13] This interaction temporarily delays local protein folding, giving the neighboring STT3B catalytic subunit time to access and glycosylate sterically hindered or disulfide-adjacent sequons.[13]
Substrate specificity and redundancy
TUSC3 and MAGT1 share high sequence identity and act redundantly within the STT3B OST complex.[16] Simultaneous loss of both TUSC3 and MAGT1 severely compromises STT3B dependent glycosylation, leading to profound luminal protein misfolding, activation of the unfolded protein response (UPR), and ER stress-induced apoptosis.[15][8]
Knockout (KO) of TUSC3 alone does not cause systemic N-glycan loss or alter serum glycoprotein patterns in most tissues, because MAGT1 compensates for it’s absence.[8] However, while TUSC3 and MAGT1 demonstrate functional redundancy in vitro, their endogenous expression profiles are distinct: MAGT1 is predominantly expressed in immune cells, whereas TUSC3 is highly enriched in the central nervous system.[16] Consequently, loss of TUSC3 cannot be adequately compensated for in brain tissue, leading to neurological pathology.[16]
Discovery and initial classification as a congenital disorder of glycosylation
(in progress)
Clinical significance
Intellectual disability
(content in progress)
Cancer
A candidate tumor suppressor gene. TUSC3 is located within a deleted region of a metastatic prostate cancer. The gene is expressed in most nonlymphoid human tissues including prostate, lung, liver, and colon. Expression was also detected in many epithelial tumor cell lines. Two transcript variants encoding distinct isoforms have been identified for this gene.[17]
