A unique interhelical insertion in plasminogen activator inhibitor-2 contains three glutamines, Gln83, Gln84, Gln86, essential for transglutaminase-mediated cross-linking.

Plasminogen activator inhibitor type 2 (PAI-2) prevents fibrinolysis by blocking plasminogen activators. It is expressed principally by trophoblast cells and macrophages. PAI-2 in trophoblast membranes has been found cross-linked to large complexes apparently catalyzed by trophoblast transglutaminase (Jensen, P. H., Lorand, L., Ebbesen, P., and Gliemann, J. (1993) Eur. J. Biochem. 214, 141-146). Recombinant human PAI-2 was labeled with [14C]putrescine catalyzed by guinea pig liver transglutaminase. The [14C]putrescine-labeled PAI-2 was digested with cyanogen bromide and trypsin, and the peptides were purified by reverse-phase high performance chromatography. Amino acid sequencing and plasma desorption mass spectrometry of the labeled peptides revealed [14C]putrescine incorporation at Gln83, Gln84, and Gln86. These residues are present in a PAI-2-specific region of 33 amino acids that is inserted between helices C and D and which probably represents a unique solvent-exposed domain. A PAI-2 mutant lacking this insertion was determined not to be a substrate for transglutaminase by [14C]putrescine incorporation and could not form transglutaminase-catalyzed polymers. Thus, the unique PAI-2 insertion represents a functional domain that, by virtue of its transglutaminase acceptor sites, allows participation in binding reactions without affecting the inhibitory function of PAI-2.