Abstract
Maintenance of homeostasis of the endoplasmic reticulum (ER) ensures the balance between loading of nascent proteins and their secretion. Certain developmental conditions or environmental stressors affect protein folding causing ER stress. The resultant ER stress is mitigated by upregulating a set of stress-responsive genes in the nucleus modulating the mechanism of the unfolded protein response (UPR). In plants, the UPR is mediated by two major pathways; by the proteolytic processing of bZIP17/28 and by the IRE1-mediated splicing of bZIP60 mRNA. Recent studies have shown the involvement of plant-specific NAC transcription factors in UPR regulation. The molecular mechanisms activating plant-UPR transducers are only recently being unveiled. This review focuses on important structural features involved in the activation of the UPR transducers like bZIP17/28/60, IRE1, BAG7, and NAC017/062/089/103. Also, we discuss the activation of the UPR pathways, including BAG7-bZIP28 and IRE1-bZIP60, in detail, together with the NAC-TFs, which adds a new paradigm to the plant UPR.
Highlights
In eukaryotes, the endoplasmic reticulum (ER) acts as a factory site for proper folding and maturation of secretory and membrane proteins, which comprise about one-third of the total proteome (Wallin and von Heijne, 1998)
We have summarized the structural features of individual unfolded protein response (UPR)-sensors and focused on the mechanistic insights into the activation of the conserved arms of the UPR, such as the bZIP28 and inositol-requiring enzyme 1 (IRE1)-bZIP60 pathways, as well as on the plant-specific UPR transducers including NAC-transcription factors (TFs)
The nuclear-localized bZIP28 forms a transcriptional complex with the nuclear factor-Y (NF-Y) TFs, and binds to the ER stress response element (ERSE) that is located in the promoter regions of the UPR genes (Liu and Howell, 2010a)
Summary
Division of Applied Life Sciences (BK21 Plus) and Plant Molecular Biology and Biotechnology Research Center (PMBBRC), Gyeongsang National University, Jinju, South Korea. Reviewed by: Sergey Morozov, Moscow State University, Russia Sang-Soo Kwak, Korea Research Institute of Bioscience and Biotechnology (KRIBB), South Korea. The UPR is mediated by two major pathways; by the proteolytic processing of bZIP17/28 and by the IRE1-mediated splicing of bZIP60 mRNA. Recent studies have shown the involvement of plant-specific NAC transcription factors in UPR regulation. The molecular mechanisms activating plant-UPR transducers are only recently being unveiled. This review focuses on important structural features involved in the activation of the UPR transducers like bZIP17/28/60, IRE1, BAG7, and NAC017/062/089/103. We discuss the activation of the UPR pathways, including BAG7-bZIP28 and IRE1-bZIP60, in detail, together with the NAC-TFs, which adds a new paradigm to the plant UPR
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