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). Displays inhibitory activity against chymotrypsin, papain, thermolysin, subtilisin A and, to a lesser extent, elastase but not trypsin. May play an important role during desquamation by inhibiting extracellular proteases. Monomer. In the epidermis, expressed predominantly in the granular layer at the apical edge of keratinocytes (at protein level). Also detected in placenta, testis and thymus but not in epithelia of kidney, lung, small intestine or colon. Up-regulated during keratinocyte differentiation. Disease susceptibility is associated with variants affecting the gene represented in this entry. Belongs to the protease inhibitor I39 (alpha-2-macroglobulin) family.