Transglutaminase crosslinking of SIBLING proteins in teeth

Transglutaminase 2 ( TG2), a protein- crosslinking enzyme, participates in extracellular matrix maturation and cell adhesion in cartilage and bone. We hypothesized that TG2 has similar roles in teeth. A TG activity assay and immunoblotting of rat tooth extracts showed TG activity and the presence of high- molecular- weight forms of the SIBLING ( Small Integrin- Binding LIgand N-linked Glycoprotein) proteins: dentin matrix protein 1 ( DMP1), dentin phosphoprotein ( DPP), and bone sialoprotein ( BSP). DMP1 and BSP, each containing both glutamine and lysine residues critical for crosslink formation, readily formed polymers in vitro when incubated with TG2. The ability of glutamine- lacking DPP to form polymers in vitro and in vivo demonstrates that it could act as a lysine donor for crosslinking, potentially having protein crosslinking partner( s) in teeth. Consistent with a role in cell adhesion, the TG2 isoform was co- localized by immunohistochemistry with its substrates at cell-matrix adhesion sites, including along odontoblast tubules ( DMP1 and DPP), in the pericellular matrix of cementocytes ( DMP1), and in predentin ( BSP).