Protein Folding in the Endoplasmic Reticulum

The endoplasmic reticulum (ER) is the principal protein-folding organelle for secretory and membrane proteins. Proteins are folded, assembled, and post-translationally modified in the ER. Chaperones and folding enzymes assist in this process. Before exiting the ER, all proteins undergo quality control such that only properly folded proteins transit to the Golgi and cell surface. The ER is also the site for sterol and lipid synthesis. As a major synthetic organelle, the ER is extremely sensitive to perturbations in homeostasis. Accumulation of misfolded or unfolded proteins within the ER induces a signaling pathway termed the unfolded protein response (UPR), which acts to restore ER homeostasis. This article discusses the basic mechanisms of protein folding in the ER, the role of the key ER chaperones, induction of the UPR, ER-associated degradation, and human diseases caused by protein misfolding and aggregation.