The current model of cornified envelope (CE) formation suggests that crosslinking between envoplakin (EVPL), periplakin (PPL), involucrin (IVL) and small proline-rich proteins (SPRs) results in the formation of a layer along the entire inner surface of the plasma membrane, including desmosomes, forming a scaffold to which other precursors are added to form the mature CE (Steinert & Marekov 1999, Kalinin et al. 2002, Candi et al. 2001).
Transglutaminases (TGs) are believed to mediate the intramolecular bonds involved in CE formation. They catalyze inter-protein bond formation by forming a thiolester acyl-enzyme intermediate and subsequently transferring the acyl residue to a primary amine (Folk & Finlayson 1977, Folk 1980). The amine acceptor is generally provided by the epsilon-amino group of a protein-bound lysine and the link formed is an N6-(gamma-glutamyl)lysine isopeptide bond.
CE assembly is thought to be initiated on the inner face of the plasma membrane between desmosomes by the cross-linking of involucrin to itself, to envoplakin and perhaps to periplakin (Steinert & Marekov 1999). The extent of homo- and heterologous cross-linking varies as the CE matures. In the immature CE, EVPL, IVL, SPR1, and SPR2 are largely cross-linked to themselves; EVPL-IVL and IVL-SPR crosslinks are common while cross-links between desmoplakin (DSP) and IVL or DSP and EVPL are not. Later there are many more cross-links between DSP and IVL, DSP and EVL, or IVL and type II keratins. Loricrin (LOR) cross-linking to other protein partners appears later.
Transglutaminase-1 (TGM1) can crosslink IVL (Simon & Green 1988, Nemes et al. 1999), LOR (Candi et al. 2001), SPR3 (Steinert et al. 1999) and is thought to be responsible for EVPL crosslinking to itself and to IVL (Steinert & Marekov 1999). TGM5 can catalyse homo-crosslinking in LOR, SPR1, SPR2, and IVL, and hetero-crosslinks between LOR-SPR3 (Candi et al. 2001).