Potential Proteolytic Activity of Human Plasma Fibronectin: Fibronectin Gelatinase

Human plasma fibronectin contains a latent proteinase that after activation cleaves gelatin and fibronectin. The autoactivation propensity of the two purified cathepsin D-produced fragments of fibronectin (190 and 120 kDa) was compared. Both polypeptides were spontaneously activated in the presence of Ca2+. This activation was inhibited by EDTA. The active gelatinase was isolated from the autodigest of the 190-kDa fragment. Among various protein substrates, including laminin and native type I and IV collagens, the purified enzyme degraded only gelatin and fibronectin. We have named this proteinase FN-gelatinase. FN-gelatinase is inhibited by phenylmethanesulfonyl fluoride and also by pepstatin A like retroviral aspartic proteinases. The amino-acid composition of the purified enzyme (35 kDa) was compared with the entire fibronectin sequence using the computer programme FIT. The optimal fit indicated that the 35-kDa fragment corresponds to the stretch # 1043-1404. This sequence contains a 93-residue segment (# 1140-1233) analogous to retroviral aspartic proteinases, comprising the sequence DTG of their putative active site.