Biophysical characterization of involucrin reveals a molecule ideally suited to function as an intermolecular cross-bridge of the keratinocyte cornified envelope.

Involucrin is a 68-kDa precursor of the keratinocyte cornified envelope. During keratinocyte terminal differentiation glutamine residues of involucrin become covalently cross-linked to other envelope precursors via covalent epsilon-(gamma-glutamyl)lysine bonds. In the present study we examine the secondary and tertiary structure of human involucrin using computer algorithms, circular dichroism, and electron microscopy. Our results indicate that involucrin is an extended, flexible, rod-shaped molecule that has a length of 460 A, an axial ratio of 30:1 and possesses between 50 and 75% alpha-helical content. Glutamine residues are circumferentially distributed along the length of the alpha-helical segments of the molecule, a distribution that is conserved in all species. We hypothesize that this distribution of glutamine residues together with the elongated shape of the molecule permits optimal interaction of involucrin glutamyl side chains with the lysine residues of other para-membranous proteins during transglutaminase-mediated cross-linking. Moreover, its long length allows involucrin to cross-link molecules that are separated by substantial distances in the cornified envelope. These properties allow a single involucrin molecule to form multiple cross-links, in multiple spatial planes, with other envelope precursors. Thus, the structure of involucrin is that of an ideal intermolecular cross-bridge.