Brain fodrin: substrate for calpain I, an endogenous calcium-activated protease.

Abstract

The Ca-activated thiol-protease calpain I, which is present in cytosolic and membrane preparations from rat brain, was tested for its capacity to degrade the neuronal spectrin-like protein fodrin. In the presence of micromolar Ca concentrations, purified calpain I degradeed both purified fodrin and the fodrin present in hippocampal and cerebellar membranes. Fodrin was identified as a high-MW protein present in brain membranes by the following criteria: comigration on NaDodSO4 [sodium dodecylsulfate]/polyacrylamide gels with purified fodrin; reactivity with antibodies to purified fodrin; a proteolytic map following calpain activation comparable to that found after calpain-mediated degradation of purified fodrin. The fodrin breakdown was selective in that calpain I did not affect at least 15 other membrane-associated polypeptides. Fodrin degradation by the protease was rapid and was accompanied by the appearance of a lower MW breakdown product. Calpain I had a high affinity for fodrin, with a Km for degradation of about 50 nM. Purified calpain I also degraded purified spectrin and the spectrin present in erythrocyte membranes. Calpain I-mediated degradation of spectrin-like proteins could provide a mechanism by which brief increases in intracellular free Ca levels modify the structure of the submembranous cytoskeleton and the distribution of cell surface receptors and alter cell shape.