Refined characterization of corneodesmosin proteolysis during terminal differentiation of human epidermis and its relationship to desquamation

Corneodesmosin is a putative adhesion glycoprotein located in the extracell ular part of the desmosomes in the upper layers of the epidermis. Synthesiz ed by granular keratinocytes as a 52-56-kDa protein, corneodesmosin is prog ressively proteolysed during corneocyte maturation. This processing is a pr erequisite for desquamation. Two glycine- and serine-rich domains of the pr otein might take on the conformation of adhesive secondary structures simil ar to glycine loops. Corneodesmosin proteolysis was further characterized. Deglycosylation exper iments and reactivity with lectins demonstrated that the corneodesmosin car bohydrate moiety does not prevent the proteolysis. Immunoblotting, immunohi stochemistry, and immunoelectron microscopy experiments using affinity-puri fied antipeptide antibodies raised to four of the five structural domains o f corneodesmosin and a monoclonal antibody against its fifth central domain showed that the first step in corneodesmosin processing is the cleavage of its extremities and probably occurs before its incorporation into desmosom es. Then the glycine loop-related domains are cleaved, first the N-terminal and then part of the C-terminal domain. At the epidermis surface, the mult istep proteolytic cleavage leaves intact only the central domain, which was detected on exfoliated corneocytes and probably lacks adhesive properties. Importantly, corneodesmosin was demonstrated to be a preferred substrate o f two serine proteases involved in desquamation, the stratum corneum trypti c and chymotryptic enzymes.