Association of calpain (Ca²⁺-dependent thiol protease) with its endogenous inhibitor calpastatin in myoblasts

Calpain isozymes (intracellular, Ca²⁺-dependent thiol proteases) are present in the cytoplasm of many cells, along with their endogenous specific inhibitor, calpastatin. Previously, we found that the levels of μ-calpain and m-calpain (activated by μM and mM Ca²⁺, respectively) remain about the same during myoblast differentiation and fusion. By contrast, the calpastatin level, which is high during the initial stages of differentiation, diminishes markedly before myoblast fusion, allowing the proteolysis that is required for myotube formation. In the present study, we used immunoprecipitation to investigate the molecular association between calpain and calpastatin in dividing myoblasts and in the initial stages of myoblast differentiation. Immunoprecipitation (IP) was performed in two ways: (1) IP of calpain, using an anti-calpain antibody that recognized both isozymes; and (2) IP of calpastatin (using anti-calpastatin). Calpastatin was co-precipitated when calpain was immunoprecipitated; calpain was co-precipitated when calpastatin was immunoprecipitated. The results indicate that calpastatin is associated with calpain in dividing myoblasts and in myoblasts during the initial stages of differentiation, thereby preventing calpain activation at this stage. Prior studies carried out in vitro have shown a Ca²⁺-dependent interaction of calpain with calpastatin. The results described here suggest that an association between calpain and calpastatin could occur within cells in the presence of physiological Ca²⁺ levels. It is proposed that the status of cellular calpain-calpastatin association is modulated by cell constituents, for which some possibilities are suggested.