The role of cross-linking in epidermal differentiation

Cultured bovine keratinocytes contain two major classes of transglutaminase substrate proteins. These may be separated on Sephadex G-75 into a 30,000-to-50,000-dalton fraction and a fraction which is heavier than 75,000 daltons. By SDS-electrophoresis, the lighter fraction consists of two components of molecular weights 9,000 and 18,000 daltons, while the heavier fraction consists of a doublet of about 100,000 daltons. Bovine epidermis contains substrate proteins which apparently correspond to the two components of the lighter class. The 2 classes of substrate proteins may be analogous to two substrate proteins isolated from cultured human keratinocytes which are thought to be precursors of cornified envelope. The major transglutaminase substrate present in both newborn rat epidermis and cultured keratinocytes has a native molecular weight of greater than 75,000 daltons but an SDS-dissociated molecular weight of 22,000 daltons. The sera of 5 of 6 rabbits react immunologically with a protein present in cultured human keratinocytes which is a poor or minor substrate of transglutaminase. The cross-linking of keratins may play a role in epidermal differentiation since keratins from more differentiated areas of epidermis may contain a higher level of interchain isopeptide bonds. Keratin may be cross-linked itself or to stratum corneum basic protein, in vitro, to form both soluble and insoluble aggregates. No adducts between keratin and purified cornified envelope were formed when both were treated with active epidermal transglutaminase.