Developmental regulation of a secreted gelatin-binding protein during myogenesis in vitro

Collagen has a stimulatory effect on the differentiation of skeletal muscle cells in culture. Putative collagen-binding proteins were isolated from detergent-solubilized cultures of the L6 rat muscle cell line and primary clonal cultures of human skeletal muscle satellite cells, using gelatin-Sepharose affinity chromatography. In addition to fibronectin, which has been reported by others to be synthesized by cultured muscle cells, we found that muscle cultures synthesized gelatin-binding proteins of lower apparent molecular weight. Only one of these proteins was secreted into the growth medium and bound to type I collagen. Binding of this protein to gelatin and collagen-Sepharose was resistant to repeated washing with 1 M NaCl and nonionic detergent. The secreted gelatin-binding protein had an apparent molecular weight of 63,000-72,000, depending upon the conditions of electrophoresis. The lack of reactivity of the secreted protein with polyclonal antisera against fibronectin, the lack of effect of protease inhibitors on its appearance in the medium, and the rapid de novo production of the protein during pulse labeling with radioactive methionine indicated that it was not a fibronectin fragment. The rate of synthesis of the secreted gelatin-binding protein increased markedly during the myogenesis of rat and human cultures.