An Arabidopsis transcription factor, AtbZIP60, regulates the endoplasmic reticulum stress response in a manner unique to plants

Abstract

Analysis of transcripts of 75 genes encoding putative basic leucine zipper (bZIP) transcription factors in the Arabidopsis genome identified AtbZIP60, which was induced by tunicamycin. AtbZIP60 encodes a predicted protein of 295 aa with a putative transmembrane domain near its C terminus after a bZIP domain. A truncated form of AtbZIP60 without a transmembrane domain (AtbZIP60⌬C) fused with GFP localized to the nucleus, suggesting translocation of native protein to the nucleus by release from the membrane. AtbZIP60 was also induced by DTT and azetidine-2-carboxylate, which induce the endoplasmic reticulum (ER) stress response (also called the unfolded protein response). Expression of AtbZIP60⌬C clearly activated any of three BiP and two calnexin promoters in a dual luciferase assay using protoplasts of cultured cells. The induction was considered to be through cis-elements plant-specific unfolded protein response element and ER stress-response element. Interestingly, AtbZIP60⌬C also appeared to induce the expression of AtbZIP60 through an ER stress-response element-like sequence in the promoter of AtbZIP60. These characteristics of AtbZIP60 imply a signal transduction pathway of the ER stress response unique to plants.Arabidopsis thaliana ͉ BiP ͉ tunicamycin ͉ unfolded protein response T he endoplasmic reticulum (ER) consists of a three-dimensional structure in eukaryotic cells where proteins for the secretary pathway are synthesized. Proper folding and assembly of proteins synthesized in the ER are necessary for transport to their final destinations. When folding or assembly of proteins in the ER is disordered, unfolded proteins accumulate in the ER and expression of genes for ER-resident chaperones, such as BiP, and folding enzymes are induced. This phenomenon is conserved among eukaryotic cells and is referred to as the ER stress response or the unfolded protein response (UPR) (1-4). Recent studies conducted in yeast and mammalian cells have shown that the ER stress response plays essential roles not only under specific stresses but also under normal growth conditions (5-8). In plants, the ER stress response has been implicated in plant-specific processes, such as seed development and pathogen response (9).The mechanism of signal transduction for the ER stress response has been extensively characterized in yeast and mammalian cells. In yeast cells, IRE1, an ER membrane-located protein kinase͞ribonuclease, plays a pivotal role for the perception of ER stress (10, 11). Sensing ER stress, IRE1 dimerizes and transautophosphorylates, activating its ribonuclease activity (12, 13). Activated IRE1 catalyzes the spliceosome-independent splicing of Hac1 mRNA, encoding a basic leucine zipper (bZIP) transcription factor. Hac1 protein is efficiently synthesized from spliced Hac1 mRNA and binds to a cis-element, UPR element (UPRE; consensus sequence CAGCGTG), resulting in induction of downstream chaperone genes, such as BiP (14-16).The ER stress response pathways of mammalian cells are multiple, in contrast to that of yeas...

The endoplasmic reticulum-associated degradation is necessary for plant salt tolerance

“…There are at last three ER stress sensors, activating transcription factor 6, IRE1a, or IRE1b, and PERK that activate UPR through distinct ways [29]. The bZIP28 and bZIP60 proteins have also been shown to function as ER stress sensors in plants [21,[42][43][44]. Whether plants utilize all those sensor systems will need to be addressed in the future.…”

Manufacturing antibodies in the plant cell

“…Deleted: [44,45] Deleted: a Deleted: also Deleted: [46] Page 12 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 Once the link between UPR and SAR is established, it is interesting to look for plant regulators of UPR as potential candidates for activating the secretory pathway in plants.…”

“…Although plants lack clear orthologues of XBP1 or ATF6, the promoters of ER chaperones, such as BiP, do contain regulatory elements responsible for ER-stress response. In Arabidopsis, at least two bZIP transcription factors, bZIP60 and bZIP28, were found to regulate ER-stress [45]. Interestingly, bZIP60 is translocated to the nucleus in a truncated form, similar to the activation mechanism of mammalian ATF6 [46].…”

The physiological role of the unfolded protein response in plants

“…A new branch unique to plants was discovered by Iwata and Koizumi (2005b) in Arabidopsis thaliana that involves a transcription factor known as AtbZIP60 (Fig. 1).…”