HUMAN PROCARBOXYPEPTIDASE-U, OR THROMBIN-ACTIVABLE FIBRINOLYSIS INHIBITOR, IS A SUBSTRATE FOR TRANSGLUTAMINASES - EVIDENCE FOR TRANSGLUTAMINASE-CATALYZED CROSS-LINKING TO FIBRIN

Procarboxypeptidase U (EC 3.4.17.20) (pro-CpU), also known as plasma p rocarboxypeptidase B and thrombin-activable fibrinolysis inhibitor, is a human plasma protein that has been implicated in the regulation of fibrinolysis. In this study, we show that pro-CpU serves as a substrat e for transglutaminases. Both factor XIIIa and tissue transglutaminase catalyzed the polymerization of pro-CpU and the cross-linking to fibr in as well as the incorporation of 5-dimethylaminonaphthalene-1-sulfon yl cadaverine (dansylcadaverine), [C-14]putrescine, and dansyl-PGGQQIV . These findings show that pro-CpU contains both amine acceptor (Gln) and amine donor (Lys) residues. The amine acceptor residues were ident ified as Gln(2), Gln(5), and Gln(292), suggesting that both the activa tion peptide and the mature enzyme participate in the cross-linking re action. These observations imply that transglutaminases may mediate co valent binding of pro-CpU to other proteins and cell surfaces in vivo. In particular, factor XIIIa may cross-link pro-CpU to fibrin during t he latter part of the coagulation cascade, thereby helping protect the newly formed fibrin clot from premature plasmin degradation. Moreover , the cross-linking may facilitate the activation of pro-CpU, stabiliz e the enzymatic activity, and protect the active enzyme from further d egradation.