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 SIPR2 family is the most complex, as it consists of several differentially expressed m embers of the same size. To explore their physical and cross-linking p roperties, we have expressed in bacteria a human SPR2 family member, a nd purified it to homogeneity. By circular dichroism, it possesses no alpha or beta structure but has some organized structure associated wi th the central peptide repeat domain. The TGase 1, 2, and 3 enzymes ex pressed in epithelia use the recombinant SPR2 protein as a complete su bstrate 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 cr oss-linking. With TGase 3, multiple head and tail domain residues were used for extensive interchain cross-linking. The total usage of gluta mine 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-l inked in epithelia primarily by the TGase 3 enzyme, a minor extent by TGase 1, and probably not by TGase 2.