Identification and immunolocalization of calreticulin in pancreatic cells: no evidence for "calciosomes".

In the present study, we have shown that calreticulin is a major Ca(2+)-sequestering protein in pancreatic microsomes. This protein is a peripheral membrane protein and could be extracted from the microsomal membrane with carbonate buffer at pH 11.4. Calreticulin was identified in the membrane fractions by immunoblotting with a specific antibody, by a 45Ca2+ overlay technique, and by NH2-terminal amino acid analysis of the purified protein. Immunocytochemical localization of calreticulin in pancreatic acinar cells and pancreatic fibroblasts showed that the protein is localized to the ER membranes in these cells. We were unable to detect calsequestrin or any calsequestrin-like proteins in the pancreas and found no evidence for the existence of large numbers of specialized, calreticulin-containing vesicles which could be an equivalent of the calsequestrin-containing calciosomes previously reported in this tissue. Purified pancreatic calreticulin binds Ca2+ with both a low and a high capacity (approximately 1 mol of Ca2+/mol of protein and approximately 20-23 mol of Ca2+/mol of protein). The concentrations of Ca2+ required for half-maximal saturation of the low and high capacity sites were approximately 4-6 microM and approximately 1.5 mM, respectively. We conclude that calreticulin, which is confined to the lumen of the ER, plays a major role in Ca2+ storage in pancreatic cells.