Chick myotendinous antigen. II. A novel extracellular glycoprotein complex consisting of large disulfide-linked subunits.

Abstract

This report describes the biochemical characterization of a novel extracellular matrix component, myotendinous antigen, which appears early in chick limb morphogenesis at sites connecting developing muscle fibers, tendons and bone. This extracellular matrix antigen is a major component of the secretory proteins released into the medium by fibroblast and muscle cultures; the soluble form is characterized here. This form of myotendinous antigen is a large glycoprotein complex consisting of several disulfide linked subunits (MW .apprx. 150,000-240,000). The differently sized antigen subunits are related, since they yielded very similar proteolytic cleavage patterns. M1 antibody can bind to the denatured subunits. The antigen subunits, as well as a MW .apprx. 80,000 pepsin-resistant antigenic domain derived from them, are resistant to bacterial collagenase. Despite possessing subunits similar in size to fibronectin, myotendinous antigen appears to be both structurally and antigenically unrelated to fibronectin or to other known extracelluar matrix components. About 7 times more M1 antigen per cell nucleus was released into the medium in fibroblast as compare to muscle cultures. In muscle conditioned medium, myotendinous antigen is noncovalently complexed to very high MW material that could be heavily labeled by[3H]glucosamine and [35S]sulfate. This material is sensitive to chondroitinase ABC and hence appears to contain sulfated glycosaminoglycans. Myotendinous antigen might interact with proteoglycans on the surface of muscle fibers, thereby acting as a link to tendons.