Changes in intracellular calpastatin localization are mediated by reversible phosphorylation

We have previously reported that, in neuroblastoma LAN-5 cells, calpastatin is in an aggregated state, close to the cell nucleus [De Tullio, Passalacqua, Averna, Salamino, Melloni and Pontremoli (1999) Biochem. J. 343, 467–472]. In the present paper, we demonstrate that aggregated calpastatin is predominantly in a phosphorylated state. An increase in intracellular free [Ca²⁺] induces both dephosphorylation of calpastatin, through the action of a phosphoprotein phosphatase, and its redistribution as a soluble inhibitor species. cAMP, but not PMA-induced phosphorylation, reverses calpastatin distribution favouring its aggregation. This intracellular reversible mechanism, regulating the level of cytosolic calpastatin, could be considered a strategy through which calpain can escape calpastatin inhibition, especially during earlier steps of its activation process.