Endoplasmic Reticulum (ER) Stress Response and Its Physiological Roles in Plants

Proc. Natl. Acad. Sci. U.S.A.

et al. 2015

Cellular homeostasis in response to internal and external stimuli requires a tightly coordinated interorgannellar communication network. We recently identified methylerythritol cyclodiphosphate (MEcPP) as a novel stress-specific retrograde signaling metabolite that accumulates in response to environmental perturbations to relay information from plastids to the nucleus. We now demonstrate, using a combination of transcriptome and proteome profiling approaches, that mutant plants (ceh1) with high endogenous levels of MEcPP display increased transcript and protein levels for a subset of the core unfolded protein response (UPR) genes. The UPR is an adaptive cellular response conserved throughout eukaryotes to stress conditions that perturb the endoplasmic reticulum (ER) homeostasis. Our results suggest that MEcPP directly triggers the UPR. Exogenous treatment with MEcPP induces the rapid and transient induction of both the unspliced and spliced forms of the UPR gene bZIP60. Moreover, compared with the parent background (P), ceh1 mutants are less sensitive to the ER-stress-inducing agent tunicamycin (Tm). P and ceh1 plants treated with Tm display similar UPR transcript profiles, suggesting that although MEcPP accumulation causes partial induction of selected UPR genes, full induction is triggered by accumulation of misfolded proteins. This finding refines our perspective of interorgannellar communication by providing a link between a plastidial retrograde signaling molecule and its targeted ensemble of UPR components in ER.T he endoplasmic reticulum (ER) function is crucial to adjustment and maintenance of a balance between protein loads and folding capacity in response to frequently changing intracellular and environmental conditions. To maintain balance (homeostasis) under stressful conditions, the ER activates conserved intracellular signal transduction pathways collectively termed the unfolded protein response (UPR) (1). The UPR monitors ER protein-folding capacity and communicates the ER status to gene expression programs that up-regulate genes encoding components of the protein folding machinery or the ER-associated degradation system (1, 2). In plants, two distinct and parallel branches of the UPR signaling pathway have been identified. One pathway involves two integral membrane-bound transcription factors (bZIP17 and bZIP28). The other pathway involves an ER membrane-localized dual-functioning (kinase/ribonuclease) protein, inositol-requiring protein-1 (IRE1). IRE1 catalyzes unconventional cytoplasmic splicing of mRNA encoding basic leucine zipper 60 (bZIP60), the transcription factor responsible for the induction of ER quality control genes (3, 4). Stress causes activation and nuclear relocation of bZIP17 and bZIP28, and activation of IRE1 responsible for splicing of bZIP60 mRNA that encodes transcriptionally active nuclear localized bZIP60. These activated transcription factors induce transcription of target genes, including genes that mediate the UPR.Protein folding is coupled to many biological processes,...

Section: Discussionmentioning

confidence: 87%

Plastid-produced interorgannellar stress signal MEcPP potentiates induction of the unfolded protein response in endoplasmic reticulum

Walley

Xiao

Proc. Natl. Acad. Sci. U.S.A.

et al. 2015

“…The second category comprised four genes encoding ER-resident protein chaperones, including BIP1 (Gillece et al, 1999), precursor of BIP3, a Heat shock protein (Hsp)20-like chaperone superfamily member, and a member of the Hsp20/a crystallin protein family. The third category possessed one gene (UDP-N-acetylglucosamine transporter) encoding an enzyme involved in protein folding (Deng et al, 2013). In addition, there were five genes encoding antiapoptosis proteins as the inhibitors of ER stress-responsive PCD, which were three Bax (pro-apoptotic protein) inhibitor1 (BI1) protein family members (Ishikawa et al, 2011) and two apoptosis regulator Bcl2 (anti-apoptotic protein)-associated athanogenes (BAGs; Briknarová et al, 2001; Table I).…”

Section: Fl4 Endosperm Exhibits Altered Expression Of Er Stress-assocmentioning

confidence: 99%

“…The ER lumen-resident ERO1 and PDI family proteins possibly identify ERAD substrates based on the time spent in the folding cycle (Tsai et al, 2001;Tsai and Rapoport, 2002;Martínez and Chrispeels, 2003;Deng et al, 2013). Four Derlin-like genes encoding the homologs of yeast (Saccharomyces cerevisiae) Der1p protein were identified in maize, which are involved in ERAD by escorting proteins to a retrotranslocon (Kirst et al, 2005).…”

Section: Zeins With Defective Signal Peptides Stimulate Er Stressmentioning

confidence: 99%

“…Signal peptide removal is an early step in protein folding, the failure of which can trigger ER stress responses (Braakman and Hebert, 2013). Cells are equipped with an exquisite ER quality control system (Deng et al, 2013) and a group of cell-sparing mechanisms under conditions of ER stress, including the ERassociated degradation (ERAD; Gillece et al, 1999), the unfolded protein response (UPR), and increased phosphorylation of eukaryotic translational initiation factor2 a-subunit (eIF2a), which slows protein synthesis (Harding et al, 1999). Programmed cell death (PCD) may occur if these cell-sparing mechanisms are exhausted (Liu and Howell, 2010).…”

mentioning

confidence: 99%

See 1 more Smart Citation

Identification and Characterization of Maizefloury4as a Novel Semidominant Opaque Mutant That Disrupts Protein Body Assembly

et al. 2014

Plant Physiology

Zeins are the major seed storage proteins in maize (Zea mays). They are synthesized on the endoplasmic reticulum (ER) and deposited into protein bodies. Failure of signal peptide cleavage from zeins can cause an opaque endosperm in the mature kernel; however, the cellular and molecular mechanisms responsible for this phenotype are not fully understood. In this study, we report the cloning and characterization of a novel, semidominant opaque mutant, floury4 ( fl4). fl4 is caused by a mutated z1A 19-kD a-zein with defective signal peptide cleavage. Zein protein bodies in fl4 endosperm are misshapen and aggregated. Immunolabeling analysis indicated that fl4 participates in the assembly of zeins into protein bodies, disrupting their proper spatial distribution. ER stress is stimulated in fl4 endosperm, as illustrated by dilated rough ER and markedly up-regulated binding protein content. Further analysis confirmed that several ER stress pathways are induced in fl4 endosperm, including ERassociated degradation, the unfolded protein response, and translational suppression by the phosphorylation of eukaryotic translational initiation factor2 a-subunit. Programmed cell death is also elevated, corroborating the intensity of ER stress in fl4. These results provide new insights into cellular responses caused by storage proteins with defective signal peptides.

“…After entering the ER lumen, endogenous and secreted recombinant proteins attain their correct folding and native conformation with the assistance of ER chaperons. However, the protein-folding process could be hampered by stress on the ER evoked by abiotic and biotic factors such as drought, salinity, heat stress, and pathogen infection (Deng et al 2013). To maintain cellular homeostasis, the ER possesses a quality control system (ER QC) that responds to stress.…”

Section: Introductionmentioning

confidence: 99%

Expression of bioactive anti-CD20 antibody fragments and induction of ER stress response in Arabidopsis seeds

Appl Microbiol Biotechnol

Sun

et al. 2015

Seed-based expression system is an attractive platform for the production of recombinant proteins in molecular farming. Despite the many advantages of molecular farming, little is known about the effect of the different subcellular accumulation of recombinant proteins on the endoplasmic reticulum (ER) quality control system in host plants. In this study, we analyzed the expression of anti-CD20 antibody fragments in seeds of Arabidopsis thaliana (ecotype Columbia) and corresponding glycosylation mutants, and evaluated the influence of three different signal sequences on the expression levels of scFv-Fc of C2B8. The highest protein accumulation level, with a maximum of 6.12 % total soluble proteins, was observed upon fusing proteins to the signal peptide of Arabidopsis seed storage albumin 2. The ER stress responses in developing seeds at 13 days post-anthesis were also compared across different transgenic lines under normal and heat shock conditions. Based on the gene expression profiles of ER stress transducers, our results suggest that accumulation of antibody fragments in the ER exerts more stress on ER homeostasis. In addition, quantitative PCR results also implicate enhanced activation of ER-associated degradation in transgenic lines. Last but not the least, we also demonstrate the anti-tumor potency of plant-derived proteins by showing the anti-tumor activity of purified scFv-Fc proteins against Daudi cells. Together, our data implies that better understanding of the interaction between exogenous protein production and the cellular quality control system of the host plant is necessary for the development of an optimal expression strategy that will be especially beneficial to commercial protein manufacturing.