Myonexin: an 80-kDa glycoprotein that binds fibronectin and is located at embryonic myotendinous junctions.

The distribution and function of an 80-kDa glycoprotein located at the surface of skeletal muscle cells and enriched in gelatin-binding fractions of skeletal muscle extracts are examined in the present study. The glycoprotein was purified by concanavalin A affinity chromatography followed by gel filtration and anion exchange chromatography. The purified protein did not display gelatin-binding although the protein bound to fibronectin in several assays. First, the glycoprotein bound to fibronectin-Sepharose and did not elute in high salt buffers although subsequent basic elutions displaced the 80-kDa protein from the column. Second, gel filtration of the 80-kDa glycoprotein in the presence of fibronectin showed separate peaks corresponding to the mass of the 80-kDa glycoprotein and fibronectin as well as a third, higher mass peak shown in immunoblots to contain both fibronectin and the 80-kDa glycoprotein. Third, immunoprecipitation with affinity-purified anti-80-kDa glycoprotein in the presence of the glycoprotein and radioiodinated fibronectin precipitated labeled fibronectin. The quantity of labeled fibronectin precipitated was reduced by the addition of nonradiolabeled fibronectin. Immunofluorescent microscopy using affinity-purified, anti-80-kDa showed this protein located at the myotendinous junctions of frog tadpoles and embryonic chicks. In chicks, it was discernible by immunofluorescence only during the morphogenetic stages that myotendinous junctions were being assembled. Amino acid analysis shows that the 80-kDa glycoprotein has a high concentration of acidic residues. There is only one cysteine per molecule in the 80-kDa glycoprotein and comparisons of reducing and nonreducing gels show that no disulfides are present, indicating that this is not an integrin protein. Amino terminal sequencing reveals that the protein contains marked similarity to the amino terminal of calsequestrin although the protein is distinct from calsequestrin in lacking Ca2(+)-dependent phenyl sepharose affinity and in its molecular weight and distribution. The observations indicate that the 80-kDa glycoprotein is a fibronectin receptor present at chick myotendinous junctions during junction morphogenesis. This apparently novel protein is named "myonexin" to reflect its location and likely function in attaching fibronectin to the surface of muscle cells.