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The endoplasmic reticulum (ER) is the starting point for protein secretion and lipid biosynthesis in eukaryotes. ER homeostasis is precisely regulated by the unfolded protein response (UPR) to alleviate stress, involving both transcriptional and translational regulators. Autophagy is an intracellular self-eating process mediated by the double-membrane structure autophagosome for the degradation of cytosolic components and damaged organelles to regenerate nutrient supplies under nutrient-deficient or stress conditions. A recent study has revealed that besides serving as a membrane source for phagophore formation, the ER is also tightly regulated under stress conditions by a distinct type of autophagosome, namely ER-phagy. ER-phagy has been characterized with receptors clearly identified in mammals and yeast, yet relatively little is known about plant ER-phagy and its receptors. Here, we will summarize our current knowledge of ER-phagy in yeast and mammals and highlight recent progress in plant ER-phagy studies, pointing towards a possible interplay between ER-phagy and ER homeostasis under ER stress responses (ERSRs) in plants.

The endoplasmic reticulum (ER) is the starting point for protein secretion and lipid biosynthesis in eukaryotes. ER homeostasis is precisely regulated by the unfolded protein response (UPR) to alleviate stress, involving both transcriptional and translational regulators. Autophagy is an intracellular self-eating process mediated by the double-membrane structure autophagosome for the degradation of cytosolic components and damaged organelles to regenerate nutrient supplies under nutrient-deficient or stress conditions. A recent study has revealed that besides serving as a membrane source for phagophore formation, the ER is also tightly regulated under stress conditions by a distinct type of autophagosome, namely ER-phagy. ER-phagy has been characterized with receptors clearly identified in mammals and yeast, yet relatively little is known about plant ER-phagy and its receptors. Here, we will summarize our current knowledge of ER-phagy in yeast and mammals and highlight recent progress in plant ER-phagy studies, pointing towards a possible interplay between ER-phagy and ER homeostasis under ER stress responses (ERSRs) in plants. [ABSTRACT FROM AUTHOR]

Plant ER-localized HSP90 contains a conserved and charged fragment, which is required for calcium- or tunicamycin-induced ER stress resistance, and deletion of the fragment affects the protein ATPase activity.Heat-shock protein 90 (HSP90) is a highly conserved molecular chaperone that is involved in modulating a multitude of cellular processes under both physiological and stress conditions. In Arabidopsis, there are seven HSP90 isoforms (HSP90.1–HSP90.7) that are localized in the cytoplasm/nucleus, mitochondrion, chloroplast, and endoplasmic reticulum (ER) where protein folding actively takes place. In this study, we analysed the sequence of ER-localized Arabidopsis HSP90.7 and the other ER GRP94 proteins from plants and animals, and identified a short, charged region that is specifically present in the middle domain of plant-derived GRP94 proteins. To understand the role of this charged region, we analysed transgenic plants that expressed a mutant protein, HSP90.7Δ22, which had this charged region deleted. We showed that seedlings expressing HSP90.7Δ22 had significantly enhanced sensitivity to ER stress induced by tunicamycin or a high concentration of calcium, although its general chaperone activity in preventing the model protein from heat-induced aggregation was not significantly affected. We also analysed the ATP-binding and hydrolysis activity of both wild-type and mutant HSP90.7 proteins, and found that they had slightly different ATP-binding affinities. Finally, using a yeast two-hybrid screen, we identified a small set of HSP90.7 interactors and showed that the charged region is not required for the candidate client interaction, although it may affect their binding affinity, thus providing potential targets for further investigation of HSP90.7 functions. [ABSTRACT FROM AUTHOR]

Para-quinones such as 1,4-Benzoquinone (BQ) and menadione (MD) and ortho-quinones including the oxidation products of catecholamines, are derived from xenobiotics as well as endogenous molecules. The effects of quinones on major protein handling systems in cells; the 20/26S proteasome, the ER stress response, autophagy, chaperone proteins and aggresome formation, have not been investigated in a systematic manner. Both BQ and aminochrome (AC) inhibited proteasomal activity and activated the ER stress response and autophagy in rat dopaminergic N27 cells. AC also induced aggresome formation while MD had little effect on any protein handling systems in N27 cells. The effect of NQO1 on quinone induced protein handling changes and toxicity was examined using N27 cells stably transfected with NQO1 to generate an isogenic NQO1-overexpressing line. NQO1 protected against BQ–induced apoptosis but led to a potentiation of AC- and MD-induced apoptosis. Modulation of quinone-induced apoptosis in N27 and NQO1-overexpressing cells correlated only with changes in the ER stress response and not with changes in other protein handling systems. These data suggested that NQO1 modulated the ER stress response to potentiate toxicity of AC and MD, but protected against BQ toxicity. We further demonstrated that NQO1 mediated reduction to unstable hydroquinones and subsequent redox cycling was important for the activation of the ER stress response and toxicity for both AC and MD. In summary, our data demonstrate that quinone-specific changes in protein handling are evident in N27 cells and the induction of the ER stress response is associated with quinone-mediated toxicity. [ABSTRACT FROM AUTHOR]

Heat-shock protein 90 (HSP90) is a highly conserved molecular chaperone that is involved in modulating a multitude of cellular processes under both physiological and stress conditions. In Arabidopsis, there are seven HSP90 isoforms (HSP90.1–HSP90.7) that are localized in the cytoplasm/nucleus, mitochondrion, chloroplast, and endoplasmic reticulum (ER) where protein folding actively takes place. In this study, we analysed the sequence of ER-localized Arabidopsis HSP90.7 and the other ER GRP94 proteins from plants and animals, and identified a short, charged region that is specifically present in the middle domain of plant-derived GRP94 proteins. To understand the role of this charged region, we analysed transgenic plants that expressed a mutant protein, HSP90.7Δ22, which had this charged region deleted. We showed that seedlings expressing HSP90.7Δ22 had significantly enhanced sensitivity to ER stress induced by tunicamycin or a high concentration of calcium, although its general chaperone activity in preventing the model protein from heat-induced aggregation was not significantly affected. We also analysed the ATP-binding and hydrolysis activity of both wild-type and mutant HSP90.7 proteins, and found that they had slightly different ATP-binding affinities. Finally, using a yeast two-hybrid screen, we identified a small set of HSP90.7 interactors and showed that the charged region is not required for the candidate client interaction, although it may affect their binding affinity, thus providing potential targets for further investigation of HSP90.7 functions. [ABSTRACT FROM AUTHOR]