THE CALPAIN-CALPASTATIN SYSTEM AND PROTEIN-DEGRADATION IN FUSING MYOBLASTS

Calpain (Ca2+-activated cysteine protease) induced proteolysis has bee n suggested to play a role in myoblast fusion. We previously found tha t calpastatin (the endogenous inhibitor of calpain) diminishes markedl y in myoblasts during myoblast differentiation just prior to the start of fusion, allowing Ca2+-induced calpain activation at that stage, He re, we show that a limited degradation of some proteins occurs within the myoblasts undergoing fusion, but not in proliferating myoblasts. T he protein degradation is observed at the stage when calpastatin is lo w. Protein degradation within the myoblasts and myoblast fusion are in hibited by EGTA, by the cysteine protease inhibitors calpeptin and E-6 4d and by calpastatin. The degradation appears to be selective for cer tain myoblast proteins. Integrin beta 1 subunit, talin and beta-tropom yosin are degraded in the fusing myoblasts, whereas alpha-actinin, bet a-tubulin and alpha-tropomyosin are not. A similar pattern of degradat ion is observed in lysates of proliferating myoblasts when Ca2+ and ex cess calpain are added, a degradation that is inhibited by calpastatin . The results support the notion that degradation of certain proteins is required for myoblast fusion and that calpain participates in the f usion-associated protein degradation. Participation of calpain is made possible by a change in calpain/calpastatin ratio, i.e., by a diminut ion in calpastatin level from a high level in the proliferating myobla sts to a low level in the differentiating myoblasts. Degradation of ce rtain proteins, known to be responsible for the stability of the membr ane-skeleton organization and for the interaction of the cell with the extracellular matrix, would allow destabilization of the membrane and the creation of membrane fusion-potent regions. (C) 1998 Elsevier Sci ence B.V.