ER chaperone functions during normal and stress conditions.

Nearly all resident proteins of the organelles along the secretory pathway, as well as proteins that are expressed at the cell surface or secreted from the cell, are first co-translationally translocated into the lumen of the endoplasmic reticulum (ER) as unfolded polypeptide chains. Immediately after entering the ER, they are often modified with N-linked glycans, are folded into the appropriate secondary and tertiary structures, which are stabilized by disulfide bonds, and finally in many cases are assembled into multimeric complexes. These processes are aided and monitored by ER chaperones and folding enzymes. When cells experience conditions that alter the ER environment, protein folding can be dramatically affected and can lead to the accumulation of unfolded proteins in this organelle. This in turn activates a signaling response, which is shared among all eukaryotic organisms, termed the unfolded protein response (UPR). The hallmark of this response is the coordinate transcriptional up-regulation of ER chaperones and folding enzymes. A major role for the increased levels of chaperones and folding enzymes during conditions of ER stress is to provide the same functions they carry out during normal physiological conditions. This includes preventing unfolded and incompletely folded proteins from aggregating and promoting the proper folding and assembly of proteins in the ER. During conditions of ER stress, many proteins are unable to fold properly and the requirements for chaperones are therefore increased. However, more recently it has become clear that some ER chaperones are also involved in signaling the ER stress response, targeting misfolded proteins for degradation and perhaps even shutting down the UPR when the stress subsides. In addition, during some normal physiological conditions, like plasma cell differentiation where there is an increased demand in the secretory capacity of B cells, the levels of various ER chaperones are also up-regulated via at least part of the UPR pathway. In order to discuss these various functions of ER chaperones, we will begin with the roles of ER chaperones and folding enzymes during normal physiological conditions and then discuss their roles during ER stress.