Final answer:
Oligosaccharyl Transferase (OST) is an enzyme that mediates the glycosylation of nascent polypeptides during their synthesis in the ER, which is critical for protein folding, stability, and function.
Step-by-step explanation:
Oligosaccharyl Transferase (OST) is an enzyme that plays a critical role in the post-translational modification of proteins. Its main function is mediating protein glycosylation in the endoplasmic reticulum (ER). During the process of protein synthesis, as a nascent polypeptide is being synthesized by the ribosome, OST co-translationally adds a glycan, specifically a preassembled oligosaccharide, to certain asparagine residues within the polypeptide chain. This modification, known as N-linked glycosylation, is essential for proper folding, stability, and function of glycoproteins.
Understanding the role of OST is crucial for grasping how proteins acquire their final structure and function. This glycosylation process occurs as the growing polypeptide chain enters the lumen of the RER. The addition of sugars to the polypeptide chain is a significant step in determining the eventual destination and role of the protein within the cell, be it within the cell membrane or secreted outside the cell.