Liberation of an N-terminal proline-rich peptide from the cryptic proteinase of fibronectin by auto-proteolysis

Fibronectin (Fn) is a modular glycoprotein present in both the extra-cellular matrix and blood plasma. It has a cryptic zinc-metalloproteinase activity (Fn-proteinase) in the gelatin-binding domain (GBD). The nature of the enzyme’s substrates and the specificity of the peptide bonds cleaved are not yet precisely known. We used mass spectrometry to demonstrate the auto-proteolytic cleavage of Fn-proteinase. A 14-mer N-terminal peptide is the most important product released. This peptide has a very peculiar sequence, AAVYQPQPHPQPPP, demonstrating that Fn-proteinase cleaves after three consecutive proline residues.