Using monoclonal antibodies, we identified a new protein in mammalian epidermis, which we called corneodesmosin. It is located in the extracellular part of the modified desmosomes in the cornified layer of the tissue, and its proteolysis (from 52-56 to 33 kDa) is thought to be a major prerequisite of desquamation. We have now further characterized human corneodesmosin. Proteolysis of purified cornified cell envelopes produced immunoreactive fragments, confirming the covalent linkage of the protein to these structures. Sequential extraction of epidermal proteins indicated that the 52-56-kDa precursor form of the protein exists in two distinct pools, one extracted with a nondenaturing hypotonic buffer, and the other with urea. Two-dimensional gel analysis and reactivity with phosphoserine-specific antibodies showed that it is a basic phosphoprotein. Deglycosylation experiments, reactivity with lectins, and chromatography on concanavalin A-Sepharose indicated that corneodesmosin is N-glycosylated. Partial sequences, 10 and 15 amino acids long, of the purified 52-56-kDa corneodesmosin showed identity with sequences predicted from a previously cloned gene, proved to be expressed in the epidermis and designated S. This indicates that corneodesmosin is probably encoded by the S gene, the function of which was unknown until now. A model of corneodesmosin maturation during cornification is proposed.

In human epidermis and other cornified squamous epithelia, corneodesmosin is located in the desmosomes of the upper living layers, and in related structures of the cornified layers, the corneodesmosomes. During maturation of the cornified layers, the protein undergoes a series of cleavages, thought to be a prerequisite of desquamation. Partial amino acid sequencing of corneodesmosin fragments suggested that it is related to the product of the S gene, previously identified in the human major histocompatibility complex.We report the expression cloning of corneodesmosin cDNA from a human epidermis library screened with monoclonal antibodies. Sequencing demonstrated that corneodesmosin is really the product of the S gene. However, analysis of 20 alleles of the gene revealed that its product is 27 amino acids longer than initially reported. Two additional polymorphic sites were described, and the position of the unique intron was ascertained.Corneodesmosin cDNA expression in COS-7 cells led to secretion of the protein. Precise epitope mapping allowed further characterization of the molecular forms of corneodesmosin present in the most superficial cornified layers, where fragments corresponding to the central region of the protein were detected. This indicated a cleavage of the N-and C-terminal domains of corneodesmosin before desquamation. These serineand glycine-rich domains are proposed to mediate an adhesive function.The late stages of the terminal differentiation program in epidermis correspond to cornification, a particular cell death process transforming keratinocytes into anucleated, flattened corneocytes. Stacking of the corneocytes forms the most superficial epidermal layers, designated as the cornified layers or stratum corneum, and is responsible for the mechanical protection of the body. Corneocytes are continually shed from the epidermis surface in the tightly regulated process of desquamation (1, 2).Corneocyte function is achieved by peculiar structures formed during cornification. The cornified cell envelope, a rigid pericellular shell, consists of covalently linked proteins. Its external side is covered by a monomolecular lipid layer, whereas its internal surface is linked to a cytokeratin-rich fibrous matrix that occupies the entire intracellular volume (3, 4). Intercellular structures derived from desmosomes have been described in the stratum corneum. Indeed, considerable morphological modifications of the desmosomes occur when keratinocytes reach the cornified layers. The desmosomal plaque, thought to be incorporated in the cornified cell envelope, is no longer observed, and the characteristic symmetrical trilamellar structure of the extracellular core becomes transformed into a homogeneous electron dense plug (5-9). These structures, called corneodesmosomes, play a major role in stratum corneum cohesion (9 -15). Their degradation, performed at the skin surface, by the stratum corneum chymotryptic enzyme and other less characterized proteases, is required for desquamation (16 -19). Although corneod...