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 (PubMed:31831667). Involved in N-glycosylation of STT3B-dependent substrates (PubMed:31831667). 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. Protein modification; protein glycosylation. Accessory component of the STT3B-containing form of the oligosaccharyltransferase (OST) complex (PubMed:31831667). 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 (PubMed:31831667). 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. Ubiquitous. Expressed at very low levels in brain, lung and kidney. Up-regulated by low extracellular Mg(2+). The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Belongs to the OST3/OST6 family. Although MAGT1 has been reported to be involved in intellectual disability (PubMed:18455129), its pathological role is questionable (PubMed:23871722).