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. Protein modification; protein glycosylation. Component of the oligosaccharyltransferase (OST) complex, which appears to exist in two assemblies comprising OST1, OST2, OST4, OST5, STT3, SWP1, WPB1, and either OST3 or OST6 (PubMed:8175708, PubMed:16297388, PubMed:16096345, PubMed:15831493, PubMed:15886282, PubMed:9405463, PubMed:29301962). OST assembly occurs through the formation of 3 subcomplexes. Subcomplex 1 contains OST1 and OST5, subcomplex 2 contains STT3, OST3, and OST4, and subcomplex 3 contains OST2, WBP1, and SWP1 (PubMed:29301962). Interacts with SEC61, SBH1 and SSS1 (PubMed:15831493). Present with 11900 molecules/cell in log phase SD medium. Belongs to the OST1 family.