THE ROLE OF CALPASTATIN (THE SPECIFIC CALPAIN INHIBITOR) IN MYOBLAST DIFFERENTIATION AND FUSION

Using red cells as an experimental model, we previously showed that a limited degradation of certain membrane proteins by calpain (Ca2+-acti vated thiol protease) was a necessary prerequisite for cell fusion and that fusibility depended on the ratio of calpain to its endogenous in hibitor calpastatin. Here we show that fusion of rat L8 line myoblasts is accompanied by a dramatic change in the calpain/calpastatin ratio. The protein levels of mu-calpain and m-calpain increased only slightl y during myoblast differentiation. In contrast, calpastatin diminished by a factor of 10 at the stages of myoblast alignment and start of fu sion, allowing calpain activity to become apparent. Calpastatin reappe ared at a late stage of myoblast fusion (myotube formation). The resul ts indicate that calpastatin is regulated during myoblast differentiat ion, and that its diminution is important in determining the activity of the calpain required for myoblast fusion. (C) 1996 Academic Press, Inc.