Comparison of cartilage structural glycoproteins with matrix proteins and fibronectin

After extraction of proteoglycans and soluble matrix proteins from canine puppy rib cartilage, with 4 M guanidine . HCl, the insoluble collagen-containing residue has been shown to contain two collagenase-resistant structural glycoproteins, A and G. The characteristic subunits of these insoluble structural glycoproteins have been identified by solubilizing them with 50 mM dithiothreitol (DTT) in 1% sodium dodecyl sulfate (SDS) and 8 M urea and comparing the SDS disc gel patterns with those of more readily soluble matrix proteins. Three subunit bands which did not occur in gels from the soluble matrix proteins were found in the solubilized material from both the collagen-containing residue and the structural glycoproteins. One band, of about 87 000 daltons, was found equally in both A and G glycoproteins. The other two bands formed a closely spaced doublet, of about 30 000 and 27 500 daltons, of which the lower band is present in higher concentration in G. Although none of these SDS gel bands corresponds with the 220 000 dalton band produced by pure fibronectin under the same conditions, a fraction of the solubilized glycoprotein material resembles fibronectin in showing an affinity for collagen, fibrinogen, heparin, and an antibody to plasma fibronectin. Crude fibronectin from human plasma contains minor components (including one of about 87 000 daltons) which show partial identify in immunodiffusion reactions with components of the solubilized cartilage structural glycoproteins. Solubilized A gave a stronger reaction with anti-plasma fibronectin than did G and the soluble matrix proteins have no reaction. It is possible either that the intercellular structural glycoproteins are formed by selection of some of the partial cleavage products of fibronectin which occur in connective tissues as well as in plasma, or that cleavage products of tissue structural glycoproteins occur in plasma which cross-react with anti-plasma fibronectin.