Characterization of human loricrin. Structure and function of a new class of epidermal cell envelope proteins.

We have isolated and characterized a full-length cDNA clone encoding human loricrin. Curiously, this protein displays major differences from the recently described mouse loricrin (Mehrel, T., Hohl, D., Nakazawa, H., Rothnagel, J.A., Longley, M.A., Bundman, D., Cheng, C.K., Lichti, U., Bisher, M.E., Steven, A. C., Steinert, P.M., Yuspa, S.H., and Roop, D.R. (1990) Cell 61, 1103-1112). Although both proteins are glycine-serine-cysteine-rich, the sequences have not been conserved. However, analysis of the sequences reveals a common motif of quasi-peptide repeats of an aliphatic or aromatic amino acid residue followed by several glycine and/or serine and cysteine residues. These sequences are interspersed and flanked by short glutamine- or glutamine/lysine-rich peptides. Thus loricrins consist of a family of cell envelope proteins of highly variable sequences that nevertheless retain common structural elements. We show that unlike all other putative protein components of the cell envelope, loricrins are highly insoluble, due at least in part to cross-linking by disulfide bonds. Furthermore, we have isolated four peptides from purified human cell envelopes that contain recognizable loricrin sequences and which are cross-linked by the N epsilon-(gamma-glutamyl)lysine isodipeptide bond. The presence of such bonds thus affords an explanation for the extraordinary insolubility of loricrin by cross-linking to the cell envelope and can also explain the low steady-state levels of monomeric loricrin in cytoskeletal extracts of epidermis. This study represents the first report of this isodipeptide cross-link in a protein component of the cornified cell envelope. We propose a model for the structure of loricrin in which (i) the unusual glycine-serine-rich sequences adopt a flexible loop conformation, indexed on the recurrent aliphatic residues; (ii) inter- or intramolecular isodipeptide and disulfide cross-links induce or stabilize folding of loricrin so as to form a more compact rosette-like structure; and (iii) the presence of the flexible glycine-rich loops necessarily will impact a flexible character to the cell envelope and entire epithelium

Identification of a major keratinocyte cell envelope protein, loricrin

During epidermal cell cornification, the deposition of a layer of covalently cross-linked protein on the cytoplasmic face of the plasma membrane forms the cell envelope. We have isolated and characterized cDNA clones encoding a major differentiation product of mouse epidermal cells, which has an amino acid composition similar to that of purified cell envelopes. Transcripts of this gene are restricted to the granular layer and are as abundant as the differentiation-specific keratins, K1 and K10. An antiserum against a C-terminal peptide localizes this protein in discrete granules in the stratum granulosum and subsequently at the periphery of stratum corneum cells. Immunofluorescence and immunoelectron microscopy detect this epitope only on the inner surface of purified cell envelopes. Taken together, these results suggest that it is a major component of cell envelopes. On the basis of its presumed function, this protein is named loricrin