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, Passalacq ua, Averna, Salamino, Melloni and Pontremoli (1999) Biochem. J, 343, 467-47 2]. In the present paper, we demonstrate that aggregated calpastatin is pre dominantly in a phosphorylated state. An increase in intracellular free [Ca 2+] induces both dephosphorylation of calpastatin, through the action of a phosphoprotein phosphatase, and its redistribution as a soluble inhibitor s pecies. cAMP, but not PMA- induced phosphorylation, reverses calpastatin di stribution favouring its aggregation. This intracellular reversible mechani sm, regulating the level of cytosolic calpastatin, could be considered a st rategy through which calpain can escape calpastatin inhibition. especially during earlier steps of its activation process.