Calreticulin is a endoplasmic reticulum-located calcium binding protein important in calcium homeostasis as well as in protein folding. This protein binds calcium but also associates with other proteins to conduct quality control of newly formed proteins in the endoplasmic reticulum. In the model plant Arabidopsis thaliana, three calreticulin isoforms have been distinguished, crt1a and crt1b belonging to one subgroup and crt3 to a subgroup that diverged before the split into monocots and dicots. These calreticulins have overlapping as well as specific localisations, both seen with GUS-staining of tissues and using immunotechniques for cellular and subcellular studies. Using mutation studies we have concluded that overlapping as well as specific functions also could be assigned to the respective calreticulin isoforms. Crt1a and crt1b are mainly related to general protein folding events, whereas crt3 is involved in pathogen responses.
Coexpression analyses indicated several components with similar expression patterns as the respective calreticulins, suggesting that they might be part of the same functional units in the cell. This is now further investigated at the protein level, using endoplasmic reticulum-enriched fractions from A. thaliana cells.
A. thaliana seedlings. Wildtype (left group of seven) and crt1a/crt1b knockout mutants (right group of seven). Note the difference in size.
Wild-type (above) and crt1a/crt1b double knock-out A. thaliana root (below). Note the surface changes.