Light enhances the unfolded protein response as measured by BiP2 gene expression and the secretory GFP-2SC marker in Arabidopsis

Disruption of the protein-folding capacity in the ER induces the accumulation of unfolded proteins and ER stress, which activate the unfolded protein response (UPR). Although UPR has been extensively studied in yeast and mammals, much less is known about UPR and its relationship with light in plants. Here, we examined the effects of chemically induced UPR and light on a molecular marker of UPR (binding protein, BiP2, gene expression) and a secretory green fluorescent protein marker (GFP-2SC) that is trafficked from the ER to vacuole in Arabidopsis thaliana (L). UPR, which was induced by DTT and tunicamycin (TM), increased Bip2 mRNA levels and decreased the levels of microsomal and vacuolar forms of GFP-2SC. Treatment with protease inhibitors lessened the effects of DTT and TM on GFP-2SC, indicating the decrease in GFP levels partially involved protein degradation. Light treatments synergistically enhanced the decrease in GFP levels in both the ER and vacuole and induced the expression of UPR marker genes for BiP2 and protein disulfide isomerase (PDI, EC 5.3.4.1). DTT and TM treatments required light for maximal induction of the UPR. Light-induced UPR occurred during the daily dark to light cycle and when dark-adapted plants were exposed to light. We propose that light activates the UPR to increase the protein folding capacity in the ER to accommodate an increase in translation during dark to light transitions.     

ER stress and the unfolded protein response

Conformational diseases are caused by mutations altering the folding pathway or final conformation of a protein. Many conformational diseases are caused by mutations in secretory proteins and reach from metabolic diseases, e.g. diabetes, to developmental and neurological diseases, e.g. Alzheimer's disease. Expression of mutant proteins disrupts protein folding in the endoplasmic reticulum (ER), causes ER stress, and activates a signaling network called the unfolded protein response (UPR). The UPR increases the biosynthetic capacity of the secretory pathway through upregulation of ER chaperone and foldase expression. In addition, the UPR decreases the biosynthetic burden of the secretory pathway by downregulating expression of genes encoding secreted proteins. Here we review our current understanding of how an unfolded protein signal is generated, sensed, transmitted across the ER membrane, and how downstream events in this stress response are regulated. We propose a model in which the activity of UPR signaling pathways reflects the biosynthetic activity of the ER. We summarize data that shows that this information is integrated into control of cellular events, which were previously not considered to be under control of ER signaling pathways, e.g. execution of differentiation and starvation programs.     

Cytomegalovirus Downregulates IRE1 to Repress the Unfolded Protein Response

During viral infection, a massive demand for viral glycoproteins can overwhelm the capacity of the protein folding and quality control machinery, leading to an accumulation of unfolded proteins in the endoplasmic reticulum (ER). To restore ER homeostasis, cells initiate the unfolded protein response (UPR) by activating three ER-to-nucleus signaling pathways, of which the inositol-requiring enzyme 1 (IRE1)-dependent pathway is the most conserved. To reduce ER stress, the UPR decreases protein synthesis, increases degradation of unfolded proteins, and upregulates chaperone expression to enhance protein folding. Cytomegaloviruses, as other viral pathogens, modulate the UPR to their own advantage. However, the molecular mechanisms and the viral proteins responsible for UPR modulation remained to be identified. In this study, we investigated the modulation of IRE1 signaling by murine cytomegalovirus (MCMV) and found that IRE1-mediated mRNA splicing and expression of the X-box binding protein 1 (XBP1) is repressed in infected cells. By affinity purification, we identified the viral M50 protein as an IRE1-interacting protein. M50 expression in transfected or MCMV-infected cells induced a substantial downregulation of IRE1 protein levels. The N-terminal conserved region of M50 was found to be required for interaction with and downregulation of IRE1. Moreover, UL50, the human cytomegalovirus (HCMV) homolog of M50, affected IRE1 in the same way. Thus we concluded that IRE1 downregulation represents a previously undescribed viral strategy to curb the UPR.