Isolation and characterization of AtXBP1, a functional homolog of XBP1/Hac1, in Arabidopsis

A number of environmental and physiological stimuli, such as perturbation in calcium homeostasis, shift of endoplasmic reticulum (ER) redox potential, altered glycosylation and elevated protein synthesis lead to accumulation of unfolded or misfolded proteins in the ER lumen, and subsequently impose stress to ER. Such condition activates a set of signaling pathway termed the unfolded protein response (UPR). To date, a large number of components for UPR signaling pathway has been isolated and characterized in yeast or mammals. Plants also show a quite similar response compared to the UPR in yeast and mammals when they are exposure to the stimuli. However, the plant UPR and its signaling pathway are rarely studied. In this report, we present our research regarding to the characterization of a UPR signaling component AtXBP1 in Arabidopsis. AtXBP1 is alternatively spliced generating two isoforms; long isoform (AtXBP1L) and short isoform (AtXBP1S). Overexpression of AtXBP1S has shown to up regulate the expression of ER stress responsive genes. Futhermore, expression of the luciferase gene, driven by BiP3 promoter, under the ER stress condition has been significantly decreased in the atxbp1 mutant. Electrophoretic mobility shift assay revealed that only AtXBP1S is able to interact with putative XBP1 binding cis-elements in AtBiP2 and AtBiP3 promoters. These results support our hypothesis that the AtXBP1 is an important regulatory component in the UPR signaling in Arabidopsis. [Supported by BK21 program]