Protein Folding and The Endoplasmic Reticulum (ER)
Proteins that are destined for the plasma membrane or export to the extracellular environment in eukaryotic cells are translated on ribosomes that sit on the rough endoplasmic reticulum (RER). Most proteins are co-translationally transported into the ER (i.e., while the ribosome is translating the mRNA code into a polypeptide code, the polypeptide is simultaneously inserted via the "transloconpore" into the ER). The ER provides an environment that helps these nascent polypeptides fold into and become functional or partially functional proteins. The ER provides an oxidizing environment (for formation of disulfide bonds) and the necessary chaperones (folding assisting agents that are not part of the final protein) that help in polypeptide folding. Numerous exported proteins form disulfide bonds as these covalent bonds stabilize the protein structure in harsh extracellular environments (a classical example are antibodies which are secreted by B-cell of our immune system, the heavy and light chain polypeptide of antibodies are held together via disulfide bonds). Another key event that takes place in the ER is called N-linked glycosylation. In this process, polypeptides that have a unique stretch of 3 amino acids (Asparagine - X - Serine/threonine, where X represents any amino acid except proline), the amide group of asparagine is modified with a complex sugar moiety. Other types of glycosylations found include S-linked (via cysteine residues), C-linked (via tryptophan) and O-linked (via serine or threonine). By far, N-linked glycosylation is the most abundant post-translational modification found in eukaryotic cells.
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