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 involving acceptor sites that are near cysteine residues. 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. Belongs to the OST3/OST6 family.