Subunit of the oligosaccharyl transferase (OST complex that catalyzes the initial transfer of a defined glycan (Glc(3Man(9GlcNAc(2 in eukaryotes from the lipid carrier dolichol-pyrophosphate to an asparagine residue within an Asn-X-Ser/Thr consensus motif in nascent polypeptide chains, the first step in protein N-glycosylation. N-glycosylation occurs cotranslationally and the complex associates with the Sec61 complex at the channel-forming translocon complex that mediates protein translocation across the endoplasmic reticulum (ER. All subunits are required for a maximal enzyme activity. May be involved in N-glycosylation of APP (amyloid-beta precursor protein. Can modulate gamma-secretase cleavage of APP by enhancing endoprotelysis of PSEN1.