The function of ENSG00000134910 (STT3A, STT3 oligosaccharyltransferase complex catalytic subunit A) is as follows. Catalytic 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 (PubMed:19167329, PubMed:31296534, PubMed:31831667, PubMed:34653363, PubMed:38670073, PubMed:39509507). 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) (PubMed:19167329, PubMed:31296534, PubMed:31831667, PubMed:34653363, PubMed:38670073, PubMed:39509507). All subunits are required for a maximal enzyme activity (PubMed:19167329, PubMed:31831667, PubMed:34653363). This subunit contains the active site and the acceptor peptide and donor lipid-linked oligosaccharide (LLO) binding pockets (PubMed:19167329). STT3A is present in the majority of OST complexes and mediates cotranslational N-glycosylation of most sites on target proteins, while STT3B-containing complexes are required for efficient post-translational glycosylation and mediate glycosylation of sites that have been skipped by STT3A (PubMed:19167329, PubMed:38670073, PubMed:39509507). STT3A-containing OST-A complex is also required to prevent hyperglycosylation of some target proteins by preventing glycosylation of facultative sites before folding of target proteins is completed (PubMed:39509507). {ECO:0000269|PubMed:19167329, ECO:0000269|PubMed:31296534, ECO:0000269|PubMed:31831667, ECO:0000269|PubMed:34653363, ECO:0000269|PubMed:38670073, ECO:0000269|PubMed:39509507}.