The unfolded protein response (UPR) of the Endoplasmic Reticulum (ER) is a regulatory system that protects the endoplasmic reticulum from overload of nascent or malformed proteins. The UPR is provoked during times of high secretion activity, is required for normal development, and can be activated by stress. The UPR operations include transcription and translational regulation. Core sensors that reside in the ER include IRE1, bZIP17, bZIP28, which increase the transcription of genes encoding chaperones. GCN2 is another sensor that leads to phosphorylatin of translation initiation factor eIF2 alpha and global down-regulation of protein synthesis.

IRE1 is conserved across eukaryotes but differs in copy numbers among different species. Arabidopsis has IRE1a, IRE1b, and a truncated IRE1c, while other plant species encode IRE1a and IRE1b only. IRE1 activates the RIDD, RNA degradation pathway, and splices the bZIP60 mRNA to produce a transcription factor that transits to the nucleus. bZIP17 and bZIP28 are membrane bound transcription factors, that transit to the Golgi apparatus where the tranmembrane domains are cleaved by the S1P and S2P proteases. The cytosolic domain is the TF which is released by cleave and transits to the nucleus to activate transcription of target genes. These three bZIP factors recognize similar targets, including ER resident chaperones.

The box listing “Targeted Gene Regulation” contains known genes that are upregulated by the TFs represented in the diagram.

The Plant ER UPRome includes experimentally validated genes and genes identified by interrogation of the Arabidopsis model.