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. Also has oxidoreductase- independent functions in the STT3B-containing OST complex possibly involving substrate recognition. Could indirectly play a role in Mg(2+) transport in epithelial cells (Probable). {ECO:0000269|PubMed:25135935, ECO:0000269|PubMed:26864433, ECO:0000269|PubMed:31036665, ECO:0000269|PubMed:31831667, ECO:0000305, ECO:0000305|PubMed:15804357, ECO:0000305|PubMed:19717468}. This is the function of Ensembl gene identifier ENSG00000102158 (MAGT1, magnesium transporter 1).