Developmental regulation of neuraminidase-sensitive lectin-binding glycoproteins during myogenesis of rat L₆ myoblasts

Intact monolayers of L6 myoblasts were treated with neuraminidase, with the aim of selectively removing sialic acid residues of cell-surface glycoproteins. Neuraminidase treatment unmasked binding sites for Ricinus communis agglutinin I and peanut agglutinin, thus allowing the identification of the major binding proteins for these lectins. For Ricinus communis agglutinin I these neuraminidase-sensitive glycoproteins had apparent Mr values of 136000, 115000, 87000, 83000 and 49000. For peanut agglutinin the major neuraminidase-sensitive glycoproteins had apparent Mr values of 200000, 136000, 87000 and 83000. We found highly reproducible, developmentally regulated, changes in the lectin-binding capacity of certain of these glycoproteins as L6 myoblasts differentiated into myotubes. Coincident with myoblast fusion there was a co-ordinate decrease in Ricinus communis agglutinin I binding by glycoproteins of apparent Mr of 136000 and 49000. There was also a co-ordinate shift in mobility of the broad band of glycoprotein, centred at an apparent Mr of 115000 in myoblasts, to a new average apparent Mr of 107000 in mid-fusion cultures and myotube cultures. Peanut agglutinin binding by the major protein of apparent Mr 136000 also decreased at the mid-fusion stage of myogenesis, and was barely detectable in 7-day-old fused cultures. These developmentally regulated changes in neuraminidase-sensitive glycoproteins were all inhibited by growth of myoblasts in 6.4 microM-5-bromo-2′-deoxyuridine, indicating that they are associated with myoblast differentiation. In contrast, an increase in fibronectin was seen in mid-fusion cultures, which was not inhibited by growth of myoblasts in 5-bromo-2′-deoxyuridine. This initial increase in fibronectin is, therefore, unlikely to be directly related to myoblast fusion or differentiation.