Subunit of the oligosaccharyl transferase (OST) complex that catalyzes the initial transfer of a defined glycan (Glc(3)Man(9)GlcNAc(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 (By similarity). Probably as part of the N-glycosylation pathway, plays a role in the regulation of tissue growth and apoptosis (PubMed:27693235). Protein modification; protein glycosylation. Component of the oligosaccharyltransferase (OST) complex. Lethal (PubMed:27693235). RNAi-mediated knockdown in the wing results in a moderate decrease in wing size, decreased N-glycosylation, increased cell-death mediated by the JNK pathway and increased ER stress mediated by the unfolded protein response pathway (PubMed:27693235). Belongs to the DAD/OST2 family.