Purification of calreticulin-like protein(s) from spinach leaves

In a search for the plant equivalent of calsequestrin or calreticulin, the high capacity, low affinity Ca2+ binding proteins of muscle and non-muscle cells thought to play important roles in Ca2+ storage, we purified two Ca(2+)-binding proteins from spinach leaves. The proteins had apparent molecular weights of 55 and 53 kDa. On Western blot, they did not react either with anti-rabbit skeletal muscle, anti-dog cardiac muscle calsequestrin or anti-rabbit or anti-rat liver calreticulin antibodies, indicating that they were antigenically distinct. Periodic acid Schiff staining (PAS) revealed that the larger protein was glycosylated while the 53 kDa one was PAS-negative. When the proteins were subjected to NH2-terminus amino acid sequencing, the 55 and 53 kDa proteins turned out to be identical, thus probably representing different isoforms of the same protein. Comparison with published amino acid sequences of calreticulin reveals regions of similarity indicating that the plant Ca(2+)-binding proteins probably belong to the calreticulin family.