Transglutaminase 5 cross-links loricrin, involucrin, and small proline-rich proteins in vitro

Transglutaminases (TGases) are seven enzymes, cross-linking proteins by gam ma -glutamil-epsilon -lysine bonds, four of which are expressed in the skin . A new member of the TGase family, TGase 5, has been identified recently, and in the present study we evaluated its role in keratinocyte differentiat ion in vitro. In addition to the previously described isoforms, full-length TGase 5 and Delta3 (deletion of exon 3), we identified two new splicing va riants, Delta 11 and Delta3 Delta 11 (deletion of exons 11 or 3, 11). We ex pressed full-length TGase 5, Delta3, Delta 11, and Delta3 Delta 11 isoforms in the keratinocyte and baculovirus systems. The results indicate that bot h full-length TGase 5 and All are active, whereas Delta3 and Delta3 Delta 1 1 have very low activity. Expression studies show that full-length TGase 5 is induced during the early stages of keratinocyte differentiation and is d ifferently regulated in comparison with the other epidermal TGases. Kinetic and in vitro crosslinking experiments indicate that full-length TGase 5 is very efficient in using specific epidermal substrates (loricrin, involucri n, and SPR3). In keratinocyte expression system, TGase 5 isoforms are retai ned in an intermediate filament-enriched fraction, suggesting its associati on with insoluble proteins. Indeed, TGase 5 co-localize with vimentin and i t is able to cross-link vimentin in vitro.