Functional Characterization of Arabidopsis Calreticulin1a: A Key Alleviator of Endoplasmic Reticulum Stress

Abstract

The chaperone calreticulin plays important roles in a variety of processes in the endoplasmic reticulum (ER) of animal cells, such as Ca2+ signaling and protein folding. Although the functions of calreticulin are well characterized in animals, only indirect evidence is available for plants. To increase our understanding of plant calreticulins we introduced one of the Arabidopsis isoforms, AtCRT1a, into calreticulin-deficient (crt-/-) mouse embryonic fibroblasts. As a result of calreticulin deficiency, the mouse crt-/- fibroblasts have decreased levels of Ca2+ in the ER and impaired protein folding abilities. Expression of the AtCRT1a in mouse crt-/- fibroblasts rescued these phenotypes, i.e. AtCRT1a restored the Ca2+-holding capacity and chaperone functions in the ER of the mouse crt-/- fibroblasts, demonstrating that the animal sorting machinery was also functional for a plant protein, and that basic calreticulin functions are conserved across the Kingdoms. Expression analyses using a beta-glucuronidase (GUS)-AtCRT1a promoter construct revealed high expression of CRT1a in root tips, floral tissues and in association with vascular bundles. To assess the impact of AtCRT1a in planta, we generated Atcrt1a mutant plants. The Atcrt1a mutants exhibited increased sensitivity to the drug tunicamycin, an inducer of the unfolded protein response. We therefore conclude that AtCRT1a is an alleviator of the tunicamycin-induced unfolded protein response, and propose that the use of the mouse crt-/- fibroblasts as a calreticulin expression system may prove useful to assess functionalities of calreticulins from different species.

Arabidopsis Stromal-derived Factor2 (SDF2) Is a Crucial Target of the Unfolded Protein Response in the Endoplasmic Reticulum

“…1, D-F, supplemental Fig. S1) correlates with reported expression pattern of the ER-chaperones BIP2 (32) and CRT1a (33) and the UPR sensors IRE1-1/2 (34). Taken together, expression profiles, promoter elements, and the very similar transcriptional induction pattern of Arabidopsis SDF2, BIP1-3, CRT, CNX, and PDI upon treatment with various agents that are known to induce protein unfolding but feature different modes of action qualify SDF2 as a general component of the UPR in plants.…”

Clinicopathological significance of calreticulin in breast invasive ductal carcinoma

“…41 Cancer cells are known to acquire enhanced survival mechanisms, which include regulating calcium homeostasis in the endoplasmic reticulum 30 to counter hypoxia and endoplasmic reticulum stress. 42 Calreticulin, being the main alleviator of endoplasmic stress, 43 could therefore have a key role in the survival of cancer cells. Calreticulin has also been reported to mediate cell adhesion and motility.…”

Arabidopsis bZIP60 Is a Proteolysis-Activated Transcription Factor Involved in the Endoplasmic Reticulum Stress Response

“…The relative changes in gene expression for these 129 genes in wild-type and bzip60 seedling are shown in Supplemental Table 1 online. The numerically dominant component of the tunicamycin response comprises 25 genes encoding ER chaperones and folding enzymes, including BiP, GRP94 (Klein et al, 2006), J protein (Yamamoto et al, 2008), calreticulin (Christensen et al, 2008), calnexin, protein disulfide isomerase (Houston et al, 2005), ERO1, and ROC7, and enzymes involved in modification, such as glycosylation. An additional 21 of the tunicamycin-induced genes code for proteins involved in protein transport into or through the secretory pathway.…”