Function of Calpains

Abstract

Calpains are major intracellular proteases that are activated by Ca²⁺, an important modulator of cell function. Although their structures and enzymatic properties are well characterized (1,2) and new tissue-specific calpains are being found successively (3), their physiological functions are not elucidated. From many studies, it seems likely now that the major locus of calpain activation is just beneath the cytoplasmic membrane, and that cytoskeletal proteins such as membrane lining proteins are physiological substrates of calpains (4). Since alterations of membrane lining proteins increase the membrane fluidity and disrupt the asymmetry of membrane phospholipids (5, 6) that is prerequisite for membrane fusion, we examined the possibility that calpains trigger membrane fusion through degradation of cytoskeletal proteins. So far, we have shown that A2C/Ca²⁺-induced erythrocyte fusion was accompanied by activation of intracellular m-calpain and degradation of spectrin, and that inhibition of intracellular m-calpain by a cell-permeable calpain inhibitor, benzyloxycarbonyl-leucyl-leucyl-leucinal (Z-Leu-Leu-Leu-al) which had been developed in our laboratory (7), resulted in blockage of erythrocyte fusion (8). In addition to this, calpain localizes to coated vesicles, which are products of Ca²⁺-dependent intracellular membrane fusion, i.e. endocytosis, on stimulation of proerythroblastic K562 cells by phorbol ester (9). In this paper, using several calpain inhibitors, we show the possible involvement of calpain in myoblast fusion to multinucleated myotubes, a physiological cell fusion occuring in muscle differentiation.