Type-2 plasminogen-activator inhibitor is a substrate for trophoblast transglutaminase and factor XIIIa. Transglutaminase-catalyzed cross-linking to cellular and extracellular structures.

Plasminogen-activator inhibitor type-2 (PAI-2), a serine-proteinase inhibitor, suppresses fibrinolysis by blocking both urokinase and tissue-type plasminogen activators. The 43-kDa PAI-2 molecule is an abundant cytosolic protein in certain cell types, but can upon appropriate stimulation be secreted as an approximately 60-70-kDa glycoprotein. However, in trophoblast membranes PAI-2 activity is associated with large covalent complexes (Jensen, P. H., Nykjaer, P., Andreasen P. A., Lund, L., Astedt, B. Lecander, I & Gliemann, J. (1989) Biochim. Biophys. Acta 986, 135-140). This study shows that PAI-2 can act as a substrate for both tissue transglutaminase and activated plasma factor XIII. In the presence of Ca2+, either of these will catalyze the incorporation of primary amines, such as putrescine, into PAI-2. Moreover, in reactions with tissue transglutaminase, PAI-2 homopolymers and, in conjunction with other biological substrates, heteropolymers were observed. As judged by the test of incorporating 125I-urokinase into SDS-resistant 125I-urokinase/PAI-2 complexes, polymerized PAI-2 retained its inhibitory activity. Furthermore, syncytiotrophoblast microvillous membranes and trophoblast detergent extracts incorporated 125I-PAI-2 into large structures in a reaction inhibited by putrescine and a synthetic inhibitor of transglutaminase. Trophoblast transglutaminase was identified as a tissue transglutaminase by non-denaturing gel electrophoresis and dansylcadaverine activity staining, fibronectin binding and Western blotting with a specific antibody. The transglutaminase-catalyzed and Ca(2+)-dependent anchoring of PAI-2 to extracellular membrane structures might have the purpose of focally regulating fibrinolysis.