Isopeptide bond formation in epidermis.

Proteins which are major substrates of epidermal transglutaminases can be identified in cultured keratinocytes of human, cow, and new-born rat. Cow and human keratinocytes both contain substrate proteins which are 30 000 to 50 000 daltons in size but dissociable in SDS to 12 000 daltons or less. In both species these proteins correspond to in vivo synthesized proteins which are probable precursors of cornified envelope. Human keratinocytes synthesize a 125 000 dalton protein which is also a precursor of cornified envelope both in cells and tissue. By SDS electrophoresis two 100 000 dalton substrate proteins are seen in cow keratinocyte extracts and a 23 000 dalton substrate protein is seen in rat keratinocyte extracts. Minor substrates of transglutaminase are seen in human keratinocytes, and one has been isolated by preparative electrophoresis. Major structural proteins of epidermis which are in vitro substrates of epidermal transglutaminase include the keratins and the stratum corneum basic protein.