Is able to inhibit all four classes of proteinases by a unique 'trapping' mechanism. This protein has a peptide stretch, called the 'bait region' which contains specific cleavage sites for different proteinases. When a proteinase cleaves the bait region, a conformational change is induced in the protein which traps the proteinase. The entrapped enzyme remains active against low molecular weight substrates (activity against high molecular weight substrates is greatly reduced). Following cleavage in the bait region a thioester bond is hydrolyzed and mediates the covalent binding of the protein to the proteinase (By similarity). Homotetramer; disulfide-linked. Expressed in uterus, mesometrial lymphoid aggregate and mammary tissue during pregnancy. Expressed in ovary, testis and kidney. Low level expression in heart. Not expressed in liver. Expressed in uterus of pregnant females during decidualization from 6 dpc with highest level around 10 dpc declining throughout the rest of the pregnancy. Belongs to the protease inhibitor I39 (alpha-2-macroglobulin) family.