Acts as accessory component 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 MAGT1. 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. Magnesium transporter. Protein modification; protein glycosylation. Accessory component of the STT3B-containing form of the oligosaccharyltransferase (OST) complex. OST exists in two different complex forms which contain common core subunits RPN1, RPN2, OST48, OST4, DAD1 and TMEM258, either STT3A or STT3B as catalytic subunits, and form-specific accessory subunits. OST can form stable complexes with the Sec61 complex or with both the Sec61 and TRAP complexes. The association of TUSC3 or MAGT1 with the STT3B-containing complex seems to be mutually exclusvice. Expressed in most non-lymphoid cells and tissues examined, including prostate, lung, liver, colon, heart, kidney and pancreas. The disease is caused by variants affecting the gene represented in this entry. Belongs to the OST3/OST6 family.