ENVOPLAKIN, A NOVEL PRECURSOR OF THE CORNIFIED ENVELOPE THAT HAS HOMOLOGY TO DESMOPLAKIN

The cornified envelope is a layer of transglutaminase cross-linked pro tein that is deposited under the plasma membrane of keratinocytes in t he outermost layers of the epidermis. We present the sequence of one o f the cornified envelope precursors, a protein with an apparent molecu lar mass of 210 kD. The 210-kD protein is translated from a 6.5-kb mRN A that is transcribed from a single copy gene. The mRNA was upregulate d during suspension-induced terminal differentiation of cultured human keratinocytes. Like other envelope precursors, the 210-kD protein bec ame insoluble in SDS and beta-mercaptoethanol on activation of transgl utaminases in cultured keratinocytes. The protein was expressed in ker atinizing and nonkeratinizing stratified squamous epithelia, but not i n simple epithelia or nonepithelial cells, Immunofluorescence staining showed that in epidermal keratinocytes, both in vivo and in culture, the protein was upregulated during terminal differentiation and partia lly colocalized with desmosomal proteins. Immunogold EM confirmed the colocalization of the 210-kD protein and desmoplakin at desmosomes and on keratin filaments throughout the differentiated layers of the epid ermis, Sequence analysis showed that the 210-kD protein is homologous to the keratin-binding proteins desmoplakin, bullous pemphigoid antige n 1, and plectin. These data suggest that the 210-kD protein may link the cornified envelope to desmosomes and keratin filaments. We propose that the 210-kD protein be named ''envoplakin.''