Endoplasmic Reticulum Stress Response

Abstract

The endoplasmic reticulum (ER) is a key organelle that serves as the gateway for newly synthesized proteins into the secretory pathway. Following synthesis, secretory proteins are exported from the ER to various cellular compartments where they fulfill their inherent biological roles. The correct trafficking of proteins through the secretory pathway is accomplished by sorting-specific motifs in the secretory proteins that are recognized by transporters that translocate the proteins to their cellular destination. As a sorting and protein-processing organelle, the ER contains protein-folding and protein-processing mechanisms for the maturation of newly-synthesized secretory proteins and a quality control mechanism to prevent abnormal proteins from reaching their final destination. Under normal conditions, the ER's processing capacity is dynamically balanced with the protein synthesis rate. Disruption of the equilibrium between the cell's secretory activity and the ER's processing and folding capacities promotes a condition that is known as ER stress. In general, perturbation of ER homeostasis by ER stress leads to the accumulation of unfolded proteins in the lumen of the organelle, which triggers a cytoprotective signaling pathway designated as the unfolded protein response (UPR). To restore ER homeostasis, activation of the UPR culminates in transient and general down-regulation of protein translation, up-regulation of ER folding functions to enhance the ER's processing capacity and induction of the ER-associated protein degradation-related quality control mechanism that ensures the disposal of unfolded and abnormal proteins. However, if the stress condition persists and UPR fails to restore ER homeostasis, a cell death signal is activated as the ultimate attempt for survival. In soybean, the ER is a prime organelle partly due to the high secretory activity of developing seeds, which accumulate high levels of storage proteins in protein bodies originating in the secretory pathway. As in any other eukaryotic organism, ER stress triggers the evolutionarily conserved UPR in soybean, but it also participates in crosstalk with several other adaptive signaling responses, such as osmotic stress-induced cell death and ER stress-induced programmed cell death.