Signal transduction by IRE1‐mediated splicing of bZIP50 and other stress sensors in the endoplasmic reticulum stress response of rice

Abstract

SUMMARYThe endoplasmic reticulum (ER) stress sensor IRE1 transduces signals by inducing the unconventional splicing of mRNAs encoding key transcription factors: HAC1 in yeast and XBP1 in animals. However, no HAC1 or XBP1 homologues have been found in plants, and until recently the substrate for plant IRE1 has remained unknown. This study demonstrates that the Oryza sativa (rice) OsbZIP50 transcription factor, an orthologue of Arabidopsis AtbZIP60, is regulated by IRE1-mediated splicing of its RNA. Despite the presence of a transcriptional activation domain, OsbZIP50 protein is not translocated into the nucleus efficiently in the absence of OsbZIP50 mRNA splicing. Unconventional splicing of OsbZIP50 mRNA causes a frame shift, which results in the appearance of a nuclear localization signal in the newly translated OsbZIP50. OsbZIP50 mRNA is spliced in a similar manner to HAC1 and XBP1 mRNAs; however, this splicing has very different effects on the translation products, a finding that shows the diversity of IRE1-related transcription factors in eukaryotes. In addition, the expression of OsbZIP50 is affected by ER stress sensor proteins OsIRE1, OsbZIP39 and OsbZIP60. ER stress-related genes differ with respect to their dependency on OsbZIP50 for their expression. The findings of this study improve our understanding of the molecular mechanisms underlying the plant ER stress response.

Effect of overexpression of kinase- or RNase-deficient OsIRE1 on the endoplasmic reticulum stress response in transgenic rice plants

“…[5][6][7] IRE1-OE rice plants display a similar expression pattern of ER stress response-related genes to that of rice under ER stress conditions, even under normal non-stress condition and its seed phenotype is chalky. 5,7 By contrast, IRE1-KD exhibits a severe dysfunction in vegetative growth, suggesting that the appropriate expression of OsIRE1 is required for normal plant growth. 6 In this study, to investigate the induction mechanism of the ER stress response through OsIRE1 signaling, we generated new types of transgenic rice plants overexpressing disrupted OsIRE1s deficient in kinase activity (K519A-OE) or RNase activity (K833A-OE).…”

“…1B) transgenic rice plants were produced in previous studies. 5,6 In this study, we generated transgenic rice plants overexpressing OsIRE1 deficient in kinase activity (K519A-OE) (Fig. 1C) or RNase activity (K833A-OE) (Fig.…”

“…By contrast, under ER stress conditions, unconventional cytoplasmic splicing of the OsbZIP50 mRNA, which is mediated by the active form of OsIRE1, causes a frameshift in the C-terminal region of OsbZIP50, resulting in the appearance of a nuclear localization signal in the newly-translated OsbZIP50. 5 Thus, activated OsbZIP50 can be smoothly translocated into the nucleus, where it functions as a transcriptional factor to induce many ER stress-responsive genes such as chaperones.…”

“…[1][2][3][4] In rice, we previously reported that the OsbZIP50 transcription factor is activated by OsIRE1-mediated cytoplasmic splicing of its mRNA. 5 In normal cells, inactive OsbZIP50 translated from its unspliced mRNA is IRE1 is an endoplasmic reticulum (ER) stress sensor protein in eukaryotes. In this study, we generated transgenic rice plants overexpressing three types of OsIRE1, including wild-type OsIRE1 (IRE1-OE) and two disrupted-IRE1s deficient in either kinase activity (K519A-OE) or RNase activity (K833A-OE), under the control of a constitutive promoter.…”

“…[1][2][3][4][5] IRE1 is an ER transmembrane protein that consists of an ER stress sensing domain in the ER lumen, a kinase domain and an RNase domain facing the cytosol. BiP-bound IRE1 is the inactive form of this protein and is localized as a monomer in the ER membrane in normal cells.…”