Abundant Accumulation of the Calcium-Binding Molecular Chaperone Calreticulin in Specific Floral Tissues of Arabidopsis thaliana

Abstract

Calreticulin (CRT) is a calcium-binding protein in the endoplasmic reticulum (ER) with an established role as a molecular chaperone. An additional function in signal transduction, specifically in calcium distribution, is suggested but not proven. We have analyzed the expression pattern of Arabidopsis thaliana CRTs for a comparison with these proposed roles. Three CRT genes were expressed, with identities of the encoded proteins ranging from 54 to 86%. Protein motifs with established functions found in CRTs of other species were conserved. CRT was found in all of the cells in low amounts, whereas three distinct floral tissues showed abundant expression: secreting nectaries, ovules early in development, and a set of subepidermal cells near the abaxial surface of the anther. Localization in the developing endosperm, which is characterized by high protein synthesis rates, can be reconciled with a specific chaperone function. Equally, nectar production and secretion, a developmental stage marked by abundant ER, may require abundant CRT to accommodate the traffic of secretory proteins through the ER. Localization of CRT in the anthers, which are degenerating at the time of maximum expression of CRT, cannot easily be reconciled with a chaperone function but may indicate a role for CRT in anther maturation or dehiscence.

Molecular cloning of a new wheat calreticulin gene TaCRT1 and expression analysis in plant defense responses and abiotic stress resistance

“…Therefore, calreticulin family members are highly conserved during its molecular evolution, even emerging before occurrence of animal and plant kingdoms. According to Arabidopsis AtCRT1, AtCRT2 and AtCRT3 structural features, Nelson et al (1997) divided plant CRT family into CRT1/2 and CRT3 subfamilies, and proposed that variation of certain amino acid residues among the CRT3 subfamily members may lead to different functions (Nelson et al, 1997). In the cluster analysis of this study, calreticulin proteins from higher plants were divided into group I, group II and group III.…”

“…In animals, the protein has demonstrated many regulatory functions, such as protein processing, inner cellular Ca 2+ balance regulation, cell adhesion and control of expression of steroid-sensitive genes and apoptosis. Calreticulin genes were initially isolated from barley (Chen et al, 1994) and spinach (Menegazzi et al, 1993), additional calreticulin cDNA sequences were subsequently cloned from tobacco (Denecke et al, 1993), corn (Kwiatkowski et al, 1995, Dresselhaus et al, 1996, Chinese cabbage (Beassica pekinensis) (Lim et al, 1996), Arabidopsis (Nelson et al, 1997), rice (Li and Komatsu, 2000), pea (Pisum sativum) and Ginkgo biloba.…”

Quantitative Proteomics of the Tonoplast Reveals a Role for Glycolytic Enzymes in Salt Tolerance

“…Digital images were captured using a Hewlett Packard flatbed scanner (Scan Jet 8250). Primary antibodies and dilutions used in this study are as follows: castor bean (Ricinus communis) anti-aldolase (1/2000) (Hodgson and Plaxton, 1998); human anti-enolase (1/1000) (purchased from Santa Cruz Biotechnology); Kalanchoe anti-VHA-B (1/1000) (Long et al, 1995); Kalanchoe anti-VHA-A (1/1000) (Long et al, 1995); anti-VHA-E (1/2000) (purchased from Agrisera; Reuveni et al, 2001); M. crystallinum anti-TIP1;2 (1/2000) (Kirch et al, 2000); Arabidopsis anti-AHA3 (1/2000) (Parets-Soler et al, 1990); M. crystallinum anti-HKT1 (1/2000) (Su et al, 2003); M. crystallinum anti-CRT1 (1/2000) (Nelson et al, 1997); anti-VDAC1 (1/2000) (purchased from Agrisera; Clausen et al, 2004); and Zea mays anti-RCA (Vargas-Suá rez et al, 2004). With the exception of anti-TIP1;2, the antibodies used did not distinguish between specific isoforms of the various proteins detected.…”

“…To confirm the origin and purity of this population of membranes for this study, FFZE fractions of M. crystallinum microsomal membranes were collected and subjected to protein blot analysis ( Figure 1B). Based on membrane protein marker analysis for different membrane compartments, including the tonoplast aquaporin TIP1;2 (Kirch et al, 2000), the plasma membrane H + -ATPase AHA3 (Parets-Soler et al, 1990), the plasma membrane Na + /K + cotransporter HKT1 (Su et al, 2003), the endoplasmic reticulum Ca 2+ binding protein calreticulin (CRT1; Nelson et al, 1997), the mitochondrial voltagedependent anion channel VDAC1 (Clausen et al, 2004), and chloroplast ribulose-1,5-bis-phosphate carboxylase/oxygenase activase (RCA; Vargas-Suá rez et al, 2004), as well as direct chlorophyll measurements ( Figure 1C), we found that the ATPdependent peak of membranes between fractions 31 to 37 corresponded to tonoplast ( Figure 1A), in agreement to our previous results (Barkla et al, 2007).…”

Effects of Calreticulin on Viral Cell-to-Cell Movement

“…Calreticulin is a Ca 21 -sequestering protein that is highly conserved between different species, including plants (Nelson et al, 1997;Borisjuk et al, 1998, and refs. therein;Michalak et al, 1998).…”