Endoplasmic reticulum (ER) stress response and its physiological roles in plants

Abstract

The endoplasmic reticulum (ER) stress response is a highly conserved mechanism that results from the accumulation of unfolded or misfolded proteins in the ER. The response plays an important role in allowing plants to sense and respond to adverse environmental conditions, such as heat stress, salt stress and pathogen infection. Since the ER is a well-controlled microenvironment for proper protein synthesis and folding, it is highly susceptible to stress conditions. Accumulation of unfolded or misfolded proteins activates a signaling pathway, called the unfolded protein response (UPR), which acts to relieve ER stress and, if unsuccessful, leads to cell death. Plants have two arms of the UPR signaling pathway, an arm involving the proteolytic processing of membrane-associated basic leucine zipper domain (bZIP) transcription factors and an arm involving RNA splicing factor, IRE1, and its mRNA target. These signaling pathways play an important role in determining the cell's fate in response to stress conditions.

Figure 1

Structure of the N-linked oligosaccharide Glc3Man9GlcNAc2. N-linked oligosaccharides on glycoproteins consist of three glucoses (orange circle), nine mannoses (blue hexagon) and two N-acetylglucosamines (pink square) and is a branched structure with three branches, A, B and C. The numbers inside the residues indicate the order in which they are added during synthesis, and the numbers inside the dark blue circles indicate the order of their modification. The types of residue linkages and the glucosidases and mannosidase involved in modification processes are also shown.

Figure 2

Protein folding and misfolding leading to export or endoplasmic reticulum associated degradation (ERAD). Preassembled oligosaccharides are transferred from dolichyl pyrophosphate (Dol-PP) to newly synthesized polypeptides, catalyzed by oligosaccharide transferase (OST). The two outermost glucose (Glc) residues are rapidly removed by GI and GII, allowing the nascent proteins to be picked up by the CRX or CRT folding apparatus and to fold, aided by members of the family of protein-disulfide isomerases (PDI) proteins. Removal of the last Glc residue allows properly folded proteins to be exported to Golgi, whereas misfolded proteins are reglucosylated by glycoprotein glucosyltransferase (UGGT) and re-enter the CNX/CRT folding cycle. Misfolded proteins that fail to achieve their native conformation are extracted from further CNX/CRT folding cycles by sequential removal of the outer α1,2 mannoses (Man) and recognized and ubiquitinated by Hrd1 (for ERAD-M and ERAD-L substrates) or Doa10 (for ERAD-C substrates) E3 complexes. The misfolded proteins are dislocated from the ER by the Cdc48 complex and sent to 26S proteasome for degradation. The numbers inside the blue circles indicate the order of the modification, which is consistent with the ones in Figure 1. The genes representing the EBSs in Arabidopsis are also shown.

Figure 3

The unfolded protein response in Arabidopsis. Arabidopsis plants have two arms of the ER stress signaling pathway, an arm involving the proteolytic processing of membrane-associated basic leucine zipper domain (bZIP) transcription factors (TFs) and an arm involving RNA splicing factor, IRE1. The membrane-associated bZIP TFs, having a cytosol-facing TF domain, a single transmembrane domain and a canonical S1P site on their lumen-facing domain, are translocated to Golgi (by some means likely involving the COPII vesicle machinery; red question mark) and proteolytically processed by site-1 protease (S1P) and site-2 protease (S2P) in response to ER stress. The cytoplasmic components of the released TFs (e.g., bZIP28(t)) enter the nucleus and activate unfolded protein response (UPR) target genes together with CCAAT-box binding proteins composed of NF-Y subunits. Upon ER stress, IRE1 splices bZIP60 mRNA, causing a frameshift leading to the synthesis of a TF without a transmembrane domain, but having acquired a nuclear targeting signal. The spliced form of bZIP60 (bZIP60(s)) is imported into the nucleus to activate UPR target genes. On the other hand, recent studies suggested that IRE1-dependent decay (RIDD) of specific mRNAs may also occur in Arabidopsis (red question mark). It is important to note that it is still unknown whether the activation of IRE1 and bZIP17/bZIP28 involve the disassociation of BiPs and whether bZIP60 functions as a TF together with NF-Y complexes (red question marks). K, kinase domain; R, RNase domain.

Figure 4

Hypothetical time course of the unfolded protein response in mammals. It has been proposed that the translation repression by PERK and IRE1-dependent decay (RIDD) occur early in response to ER stress, followed by ATF6-induced upregulation of ERQC and ERAD related genes. The upregulation of UPR target genes by XBP1 and PERK-ATF4 occurs next, since the activation of XBP1 and ATF4 themselves also require de novo protein synthesis. The responses in all three arms of the UPR undergo attenuation after the onset of stress, even under continuous stress conditions. While the IRE1 response attenuates quickly within eight hours through phosphatase-regulated IRE1 dephosphorylation, the ATF6 activation response lasts for about 20 h and, then, is suppressed by its own induced NUCB1 and WFS1. On the other hand, the PERK-mediated response persists even after 30 h and finally attenuates through eIF2α dephosphorylation regulated by GADD34/P58IPK, which are also induced by the PERK pathway itself.