Structural and transglutaminase substrate properties of the small proline-rich 2 family of cornified cell envelope proteins

The small proline-rich (SPR) proteins are components of the cornified cell envelope of stratified squamous epithelia and become cross-linked to other proteins by transglutaminases (TGases). The SPR2 family is the most complex, as it consists of several differentially expressed members of the same size. To explore their physical and cross-linking properties, we have expressed in bacteria a human SPR2 family member, and purified it to homogeneity. By circular dichroism, it possesses no alpha or beta structure but has some organized structure associated with the central peptide repeat domain. The TGase 1, 2, and 3 enzymes expressed in epithelia use the recombinant SPR2 protein as a complete substrate in vitro, but with widely differing kinetic efficiencies, and in different ways. With TGase 1, only one glutamine on the head domain and one lysine on the tail domain were used for limited interchain cross-linking. With TGase 3, multiple head and tail domain residues were used for extensive interchain cross-linking. The total usage of glutamine and lysine residues in vitro by TGase 3 was similar to that seen in earlier in vivo studies. We conclude that SPR2 proteins are cross-linked in epithelia primarily by the TGase 3 enzyme, a minor extent by TGase 1, and probably not by TGase 2.