Latent fibronectin-degrading serine proteinase activity in N-terminal heparin-binding domain of human plasma fibronectin

The N-terminal 70-kDa fragment of human plasma fibronectin, purified from a cathepsin D digest, is characterized by lack of stability. It is processed proteolytically during incubation in the presence of Ca2+ into 27-kDa N-terminal heparin-binding and 45-kDa collagen-binding domains. The N-terminal residue in the 27-kDa fragment was blocked as in native fibronectin. The 45-kDa fragments began with the sequences AAVYQP, AVYQP and VYQP (residues 260, 261, 262-265 of fibronectin) that correspond to the beginning of the collagen-binding domain. In the presence of Ca2+ the purified 27-kDa fragment underwent further processing finally leading to the cleavage of the bond K85-D86 and to the simultaneous appearance of a specific proteolytic activity. Inhibition studies suggests that the newly generated enzyme is a Ca(2+)-dependent serine proteinase. Among all assayed matrix proteins, the newly generated enzyme cleaves native fibronectin and its fragments. It is proposed that this fibronectinase may originate from the N-terminal domain of fibronectin.