Endoplasmic reticulum protein quality control and its relationship to environmental stress responses in plants

Abstract

The endoplasmic reticulum (ER) has a sophisticated quality control (QC) system to eliminate improperly folded proteins from the secretory pathway. Given that protein folding is such a fastidious process and subject to adverse environmental conditions, the ER QC system appears to have been usurped to serve as an environmental sensor and responder in plants. Under stressful conditions, the ER protein folding machinery reaches a limit as the demands for protein folding exceed the capacity of the system. Under these conditions, misfolded or unfolded proteins accumulate in the ER, triggering an unfolded protein response (UPR). UPR mitigates ER stress by upregulating the expression of genes encoding components of the protein folding machinery or the ER-associated degradation system. In Arabidopsis thaliana, ER stress is sensed and stress signals are transduced by membrane-bound transcription factors, which are activated and mobilized under environmental stress conditions. Under acute or chronic stress conditions, UPR can also lead to apoptosis or programmed cell death. Despite recent progress in our understanding of plant protein QC, discovering how different environmental conditions are perceived is one of the major challenges in understanding this system. Since the ER QC system is one among many stress response systems in plants, another major challenge is determining the extent to which the ER QC system contributes to various stress responses in plants.

Figure 1.

Protein Folding and Modification in the ER. The signal peptide sequence directs the translocation of polypeptides through the Sec61 translocon into the ER lumen. The ADP-bound state of the chaperone BiP binds to nascent polypeptides to prevent protein aggregation. If an N-glycosylation site is detected, a preassembled oligosaccharide core (Glc3Man9GlcNAc2) is transferred to the nascent protein catalyzed by the OST. Subsequently, rapid deglucosylation of the two outermost glucose residues (G) on the oligosaccharide core structures permits the entry of the nascent protein into the CNX/CRT protein folding machinery. PDI family proteins accelerate the folding by catalyzing formation of intra- and intermolecular disulfide bonds on the nascent proteins. The oxidizing environment in the ER lumen is maintained by Ero1. Removal of the innermost glucose residues by glucosidase II releases correctly folded proteins for export from the ER. Incorrectly folded proteins are reglucosylated by UGGT and are cycled again through the CNX/CRT apparatus. Terminal improperly folded proteins are recognized by EDEM and targeted to the retrotranslocon for ERAD. The functions of the illustrated proteins in Arabidopsis have not, in large part, been demonstrated experimentally but have been inferred from their homology to proteins in yeast and mammalian cells.

Figure 2.

The Structure of Lipid-Linked Oligosaccharide Unit Glc3Man9GlcNAc2. Two N-acetylglucosamine (N1 and N2, gray squares) and five mannose residues (M3 to M7, gray, pink, and blue circles) are added subsequently to the polyisoprenoid lipid dolichol (Dol) on the cytoplasmic side of the ER. The above lipid-linked oligosaccharide (LLO) unit Man5GlcNAc2 is then flipped over to face the ER lumen, where four additional mannose (M8 to M11, pink circles) and three glucose residues (G12 to G14, blue and gray triangles) are added to form the LLO unit Glc3Man9GlcNAc2. The moieties containing 14 monosaccharide units are then transferred from the lipid Dol to proteins and then subjected to further glucose and/or mannose trimming at bond sites indicated by arrows. The enzymes that catalyze the respective hydrolysis reactions are indicated. The folding status of the protein is recognized either by the CNX/CRT through residues indicated in blue or EDEM through residues indicated in pink. The properly folded proteins are secreted out or the unfolded/misfolded proteins are retrieved for degradation through ERAD.

Figure 3.

ER Stress Response Pathways in Plants. Three bZIP membrane-bound transcription factors (MTFs) are localized on the ER membrane under normal conditions. Under environmental stress conditions as indicated, the MTFs are transported to the Golgi apparatus and proteolytically processed by Golgi-localized proteases, site-1 protease (S1P; cleavage site indicated by red x) and site-2 protease (S2P; cleavage site indicated by black x). The processed forms of MTFs are imported into the nucleus to activate stress response genes. The activation mechanism for At-bZIP60 is not known.

Figure 4.

ER Stress–Induced Cell Death (Apoptosis) in Plants. ER stress activates the expression of NRPs, which induces the caspase-3–like activity to trigger cell death. ER stress promotes the formation of Gαβ heterodimer to induce PCD, presumably by activating PLC or IP3 receptor pathways. Negative regulators, such as BAGs and BI-1, repress PCD in plants. Note that the counterparts of mammalian BAX or BCL-2 have not been identified in plants so far.