A Novel Protease Inhibitor of the α2-Macroglobulin Family Expressed in the Human Epidermis

Abstract In the course of a large scale analysis of late-expressed genes in the human epidermis, we identified a new member of the α2-macroglobulin (α2M) protease inhibitor family, A2ML1 (for α2-macroglobulin-like 1). Like A2M and PZP, A2ML1 is located on chromosome 12p13.31. A2ML1 encodes a protein of 1454 amino acids, which fits the characteristics of α2Ms: 1) strong conservation in amino acid sequence including most of cysteine positions with α2M; 2) a putative central bait domain; 3) a typical thiol ester sequence. Northern blot and reverse transcriptase-PCR studies revealed a single 5-kb A2ML1 mRNA, mainly in the epidermis granular keratinocytes. A2ML1 is also transcribed in placenta, thymus, and testis. By Western blot analysis, α2ML1 is detected as a monomeric, ∼180-kDa protein in human epidermis. In vitro keratinocyte differentiation is associated with increased expression levels. By immunohistochemistry, α2ML1 was detected within keratinosomes in the granular layer of the epidermis, and as a secreted product in the extracellular space between the uppermost granular layer and the cornified layer. Recombinant α2ML1 displayed inhibitory activity toward chymotrypsin, papain, thermolysin, subtilisin A, and to a lesser extent, elastase but not trypsin. Incubation with chymotrypsin and the chymotrypsin-like kallikrein 7 protease indicated that α2ML1 binds covalently to these proteases, a feature shared with other members of the family. Therefore, α2ML1 is the first α2M family member detected in the epidermis. It may play an important role during desquamation by inhibiting extracellular proteases.