Carbohydrates selectively protect a specific domain of fibronectin against proteases

To determine how the carbohydrate moiety of fibronectin influences the susceptibility of protein to proteolytic degradation, we compared the effects of various proteases on glycosylated and nonglycosylated fibronectins. Nonglycosylated fibronectin, from tunicamycin-treated chicken embryo fibroblasts, was degraded more rapidly to acid-soluble products than glycosylated fibronectin by pronase, thermolysin, trypsin, and chymotrypsin. The absence of carbohydrate did not markedly affect overall patterns of proteolytic fragments identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Except for the expected increases in electrophoretic mobilities of the nonglycosylated peptides, the only important difference was that of the nonglycosylated fragment corresponding to the carbohydrate-rich, collagen-binding domain, was completely digested by the proteases in 60 min at 30 degrees C. In contrast, the comparable fragment from glycosylated fibronectin was resistant to protease digestion. Heparin-binding domains that normally lack carbohydrate are equally susceptible to proteases in glycosylated and nonglycosylated fibronectin. We conclude that the carbohydrate component of fibronectin plays an important role in the stabilization of a specific domain of the protein against proteolytic degradation; however, the carbohydrate does not alter overall proteolytic specificity.