Oligosaccharyltransferases catalyze the transfer of a oligosaccharyl group to an acceptor molecule, typically another carbohydrate or a lipid. The best characterized example of this activity is within the oligosaccharyltransferase complex (OST), which is a multi-span membrane protein complex that catalyzes the addition of glycans to selected Asn residues within nascent polypeptides in the lumen of the endoplasmic reticulum. This process, termed N-glycosylation, is a fundamental post-translational protein modification involved in quality control, trafficking of proteins, signal transduction, and cell-to-cell communication. Either STT3A or STT3B serves as the catalytic subunit within the OST. (Adapted from PMID:31810196.)