Involucrin acts as a transglutaminase substrate at multiple sites.

Involucrin is a keratinocyte protein with a specialized function in terminal differentiation. Synthesized initially as a soluble protein, it later becomes a preferred substrate for a membrane-bound transglutaminase and becomes cross-linked into an insoluble envelope. When a crude keratinocyte extract containing about 2% involucrin is heated to 95 degrees, most proteins precipitate, but all of the involucrin remains in solution, where it is over 90% pure. This step has been incorporated into a simplified procedure for purification of the protein. Like intact involucrin, polypeptide fragments formed by the tryptic hydrolysis of involucrin are good substrates for the keratinocyte transglutaminase. Evidently amino acid residues participating in the enzyme-catalyzed cross-linking are distributed at numerous sites along the involucrin molecule.