Spontaneous myoblast fusion is mediated by cell surface Ca2+-dependent protein kinase(s).

Mononucleated myoblasts divide in vitro until they attain confluency and fuse, forming multinucleated myotubes. Fusion is an extracellular Ca2+-dependent process. We used for our studies an established line of skeletal myoblasts (L6) as well as a non-fusing Myo- alpha-amanitin-resistant mutant of this line (Ama102). Our results show that extracellular calcium at concentrations which elicit myoblast fusion activates the phosphorylation of a protein species of 48 kD, present at the surface of mononucleated myoblasts of the fusing wild type (L6). At fusion, as the cells become independent of the extracellular calcium concentration for their further differentiation, this activation can no longer be observed. In fusion inhibition experiments, where we used lowered calcium levels, the phosphorylation of the 48 kD protein band is clearly decreased. When the myoblasts are fed with standard medium, they fuse rapidly and the phosphorylation of the 48 kD species is markedly increased. The above-described phenomenon takes place at the cell surface and is completed in a short time. The use of Myo- mutant showed that it is developmentally regulated. In view of our results, it is reasonable to postulate that Ca2+-activated phosphorylation of the cell surface could be on the basis of spontaneous myoblast fusion.