Activation of the Transducers of Unfolded Protein Response in Plants

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.

Multiple ER-to-nucleus stress signaling pathways become active duringPlantago asiatica mosaic virusandTurnip mosaic virusinfection inArabidopsis thaliana

“…This study also examined the roles of two NAC factors that are known to relay information from the bZIP factors for UPR gene activation (Nawkar et al , 2018). The results indicate that NAC089 and NAC103 respond to TGB3 and 6K2 transient expression.…”

“…Environmental challenges such as heat, drought, and pathogen attack can drastically alter the protein maturation process and cause a bottleneck of malformed proteins to accumulate in the ER. The unfolded protein response (UPR) describes alternative ER-to-nucleus signaling pathways leading to increased nuclear gene expression (Bao et al , 2018, Nawkar et al , 2018).…”

“…Upon activation, the BAG7/bZIP28/BiP complex dissociates, and the bZIP28 and BAG7 are available to move between the ER and the nucleus(Williams et al , 2010, Li et al , 2017). The SITE-1 Protease (S1P) and S2P remove the transmembrane domains releasing the bZIP17 and bZIP28 transcription factors to enter the nucleus (Liu et al , 2007b, Liu et al , 2007a, Iwata et al , 2017) and activate expression of BiPs and other chaperones, similar to bZIP60 (Nawkar et al , 2018). The transmembrane domain of BAG7 is also proteolytically removed, and the truncated BAG7 translocates to the nucleus where it principally interacts with WRKY28, independent of bZIP28.…”

“…We know very little about which stretches of sequences containing G-boxes are differential targets for homo-and heterodimers of these bZIP factors and cannot yet precisely predict the transcription pattern of genes by examining promoter sequences (Ezer et al , 2017). Nevertheless, experiments have demonstrated that the bZIP60 and bZIP28 recognize the promoters of at least two NAC (NAM/ATAF/CUC) transcription factors (Nawkar et al , 2018). NAC103 is activated by bZIP60 through the UPRE-III cis -elements and upregulates the expression of genes that are categorically described as “pro-survival” factors (Sun et al , 2013).…”

Endoplasmic reticulum stress‐induced accumulation of VAMP721/722 requires CALRETICULIN 1 and CALRETICULIN 2 in Arabidopsis

“…Endoplasmic reticulum (ER) stress, which is caused by accumulation of misfolded proteins in the ER, can also be induced by environmental stresses, leading to a higher burden on secretory proteins in the ER in plants (Bao and Howell ; Nawkar et al ). This ER stress often results in growth retardation in Arabidopsis (Christensen et al ; Christensen et al ).…”