Establishment and maintenance of the proper protein folding environment of the endoplasmic reticulum

Proteins enter the secretory pathway of eukaryotic cells through the endoplasmic reticulum (ER), which must then ensure that such proteins reach their native state before allowing them to progress to their final destinations. Like other compartments in which folding occurs, the ER contains a number of factors that directly assist folding.; The ER is unique, however, in that protein folding in this compartment is coupled to covalent modifications, including disulfide bond formation. The machinery responsible for maintaining the oxidizing environment favorable to disulfide bond formation is beginning to be elucidated with the identification of a gene, ERO1, whose activity is responsible for determining the ER oxidation state. Mutations in ero1 cause an increased sensitivity to reducing agents, while overexpression of ERO1 leads to increased resistance. Moreover, mutation of ERO1 results in a specific defect in the folding of those proteins that are dependent on disulfide bond formation.; Biochemical studies have identified the activity carried out by the Ero1p protein: the FAD-dependent oxidation of protein disulfide isomerase. This system is analogous to that found in the bacterial periplasm for the oxidation of proteins. However, Ero1p is not coupled to the respiratory chain, as depletion of respiratory chain components does not affect the ability of yeast cells to oxidize secretory proteins. Contrary to previous expectations, glutathione is not necessary for Ero1p-dependent oxidation.; In addition to assisting the folding of secretory proteins, the ER also contains a quality control system to recognize misfolded species and then remove them in a process known as ER-associated degradation. In the event that misfolded proteins accumulate, the ER initiates a response known as the unfolded protein response (UPR). Our analysis of transcriptional targets of the UPR has revealed regulation of nearly all aspects of secretory pathway function, suggesting that maintaining an efficient protein folding environment requires more than directly assisting the folding of proteins. This idea was tested for one specific class of UPR targets, those involved in the process of ERAD. This analysis revealed that the UPR and ERAD together perform an essential task for the cell: the removal of misfolded proteins from the secretory pathway.