Regulation of expression and intracellular distribution of calreticulin, a major calcium binding protein of nonmuscle cells

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Abstract
In the present study we have demonstrated the presence of calreticulin, a major Ca2+‐sequestering protein of nonmuscle cells, in a variety of cell types in tissue culture. The protein localizes to the endoplasmic reticulum in most cell types and also to the nuclear envelope or nucleoli‐like structures in some cell types. Calreticulin is enriched in the rough endoplasmic reticulum, suggesting a possible involvement in protein synthesis. Calreticulin terminates with the KDEL‐COOH sequence, which is likely responsible for its endoplasmic reticulum localization. Unlike some other KDEL proteins, calreticulin expression is neither heat‐shock nor Ca2+‐shock dependent. Using a variety of metabolic inhibitors, we have shown that the pool of calreticulin in L6 cells has a relatively slow turnover and a stable intracellular distribution. In proliferating muscle cells in culture (both L6 and human skeletal muscle) calreticulin is present in the endoplasmic reticulum, and additional intranuclear staining is observed. When fusion of the L6 cells is inhibited with either a high serum concentration or TGF‐β TPA, the nucleolar staining by anticalreticulin antibodies is diminished, although the presence of calreticulin in the endoplasmic reticulum remains unchanged. In contrast, in differentiated (i.e., fused) muscle cells neither intranuclear nor intracellular staining for calreticulin is present. We conclude, therefore, that calreticulin is abundant in the endoplasmic reticulum in proliferating myoblasts, while it is present in only small amounts in sarcoplasmic reticulum membranes in terminally differentiated myotubes. We propose a model for the domain structure of calreticulin that may explain the differential subcellular distribution of this protein. Because of its widespread distribution in nonmuscle tissues, we postulate that calreticulin is a multifunctional protein that plays an important role in Ca2+ sequestering and thus that it is the nonmuscle analog of calsequestrin.