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      Multiple roles of the ER stress sensor IRE1 demonstrated by gene targeting in rice

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          Abstract

          The endoplasmic reticulum (ER) stress sensor, IRE1, contains a kinase domain and a ribonuclease domain. Ribonuclease mediates the unconventional splicing of mRNA encoding the transcription factor AtbZIP60 in Arabidopsis, or OsbZIP50 in rice, and thereby transduces signals from stressed ER. Here, we demonstrate the additional roles of plant IRE1 using genetically modified rice plants. Using a gene targeting system based on homologous recombination, genomic IRE1 was replaced with two types of missense alleles, leading to a defect in the kinase or ribonuclease activity of IRE1. Genetic analysis of these alleles demonstrated that the kinase activity of IRE1 plays a vital role independent of ribonuclease activity. Furthermore, the existence of ribonuclease substrates other than OsbZIP50 mRNA is demonstrated for the first time. This study provides new insights into higher plant signalling using a gene targeting approach.

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          Most cited references15

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          Heat induces the splicing by IRE1 of a mRNA encoding a transcription factor involved in the unfolded protein response in Arabidopsis.

          Adverse environmental conditions produce endoplasmic reticulum (ER) stress in plants. In response to heat or ER stress agents, Arabidopsis seedlings mitigate stress damage by activating ER-associated transcription factors and a RNA splicing factor, IRE1b. IRE1b splices the mRNA-encoding bZIP60, a basic leucine-zipper domain containing transcription factor associated with the unfolded protein response in plants. bZIP60 is required for the up-regulation of BINDING PROTEIN3 (BIP3) in response to ER stress, and loss-of-function mutations in IRE1b or point mutations in the splicing site of bZIP60 mRNA are defective in BIP3 induction. These findings demonstrate that bZIP60 in plants is activated by RNA splicing and afford opportunities for monitoring and modulating stress responses in plants.
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            Fine-tuning of the unfolded protein response: Assembling the IRE1alpha interactome.

            Endoplasmic reticulum (ER) stress is a hallmark feature of secretory cells and many diseases, including cancer, neurodegeneration, and diabetes. Adaptation to protein-folding stress is mediated by the activation of an integrated signal transduction pathway known as the unfolded protein response (UPR). The UPR signals through three distinct stress sensors located at the ER membrane-IRE1alpha, ATF6, and PERK. Although PERK and IRE1alpha share functionally similar ER-luminal sensing domains and both are simultaneously activated in cellular paradigms of ER stress in vitro, they are selectively engaged in vivo by the physiological stress of unfolded proteins. The differences in terms of tissue-specific regulation of the UPR may be explained by the formation of distinct regulatory protein complexes. This concept is supported by the recent identification of adaptor and modulator proteins that directly interact with IRE1alpha. In this Review, we discuss recent evidence supporting a model where IRE1alpha signaling emerges as a highly regulated process, controlled by the formation of a dynamic scaffold onto which many regulatory components assemble.
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              Arabidopsis IRE1 catalyses unconventional splicing of bZIP60 mRNA to produce the active transcription factor

              IRE1 plays an essential role in the endoplasmic reticulum (ER) stress response in yeast and mammals. We found that a double mutant of Arabidopsis IRE1A and IRE1B (ire1a/ire1b) is more sensitive to the ER stress inducer tunicamycin than the wild-type. Transcriptome analysis revealed that genes whose induction was reduced in ire1a/ire1b largely overlapped those in the bzip60 mutant. We observed that the active form of bZIP60 protein detected in the wild-type was missing in ire1a/ire1b. We further demonstrated that bZIP60 mRNA is spliced by ER stress, removing 23 ribonucleotides and therefore causing a frameshift that replaces the C-terminal region of bZIP60 including the transmembrane domain (TMD) with a shorter region without a TMD. This splicing was detected in ire1a and ire1b single mutants, but not in the ire1a/ire1b double mutant. We conclude that IRE1A and IRE1B catalyse unconventional splicing of bZIP60 mRNA to produce the active transcription factor.
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                Author and article information

                Journal
                Sci Rep
                Sci Rep
                Scientific Reports
                Nature Publishing Group
                2045-2322
                10 December 2012
                2012
                : 2
                : 944
                Affiliations
                [1 ]Genetically Modified Organism Research Center, National Institute of Agrobiological Sciences , Kannondai 2-1-2, Tsukuba, Ibaraki 305-8602, Japan
                [2 ]These authors contributed equally to this work.
                Author notes
                Article
                srep00944
                10.1038/srep00944
                3517978
                23230509
                c67dbdd2-fc8d-4b7e-ad0d-94fd6dcd09ce
                Copyright © 2012, Macmillan Publishers Limited. All rights reserved

                This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/

                History
                : 10 October 2012
                : 12 November 2012
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