Specific blockers of myoblast fusion inhibit a soluble and not the membrane-associated metalloendoprotease in myoblasts.

We previously reported that the cell fusion that occurs during muscle development, when mononucleated myoblasts fuse to form multinucleated myotubes, requires endogenous metalloendoprotease activity at the time of fusion. We report here that myoblasts contain both soluble and membrane-associated metalloendoproteases, and that these proteases have different inhibitor specificities. Several inhibitors, previously shown to block myoblast fusion, inhibit only soluble and not membrane-associated metalloendoprotease activity in myoblasts. Another metalloendoprotease inhibitor, phosphoramidon, which had no effect on fusion, inhibits only the membrane-associated metalloendoprotease. These observations implicate a soluble metalloendoprotease in myoblast fusion. Two soluble metalloendoproteases can be demonstrated by column chromatofocusing, with pI values at pH 5.9 and 4.8. The soluble metalloendoprotease eluted at pH 5.9 is not inhibited by an inhibitor which blocks fusion, while the soluble metalloendoprotease eluted at pH 4.8 is inhibited. Of the three metalloendoprotease activities identified in myoblasts, the metalloendoprotease required in myoblast fusion appears to be the soluble metalloendoprotease with a pI of 4.8.