Accessory component of the STT3B-containing form of the N-oligosaccharyl transferase (OST) complex which catalyzes the transfer of a high mannose oligosaccharide from a lipid-linked oligosaccharide donor to an asparagine residue within an Asn-X-Ser/Thr consensus motif in nascent polypeptide chains . Involved in N-glycosylation of STT3B-dependent substrates . Specifically required for the glycosylation of a subset of acceptor sites that are near cysteine residues; in this function seems to act redundantly with TUSC3. In its oxidized form proposed to form transient mixed disulfides with a glycoprotein substrate to facilitate access of STT3B to the unmodified acceptor site. Has also oxidoreductase-independent functions in the STT3B-containing OST complex possibly involving substrate recognition.
May be involved in Mg(2+) transport in epithelial cells.
Subcellular locations: Cell membrane, Endoplasmic reticulum, Endoplasmic reticulum membrane
Ubiquitous. Expressed at very low levels in brain, lung and kidney.