Cell surface magnesium transporter. 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. May be involved in substrate-specific N-glycosylation. Protein modification; protein glycosylation. Accessory component of the STT3B-containing form of the oligosaccharyltransferase (OST) complex. Early developmental arrest. Belongs to the OST3/OST6 family.