Real-time kinetic method to monitor isopeptidase activity of transglutaminase 2 on protein substrate.

Transglutaminase 2 (TG2) is a ubiquitously expressed multifunctional protein with Ca(2+)-dependent transamidase activity forming protease-resistant N(ε)-(γ-glutamyl) lysine crosslinks between proteins. It can also function as an isopeptidase cleaving the previously formed crosslinks. The biological significance of this activity has not been revealed yet, mainly because of the lack of a protein-based method for its characterization. Here we report the development of a novel kinetic method for measuring isopeptidase activity of human TG2 by monitoring decrease in the fluorescence polarization of a protein substrate previously formed by crosslinking fluorescently labeled glutamine donor FLpepT26 to S100A4 at a specific lysine residue. The developed method could be applied to test mutant enzymes and compounds that influence isopeptidase activity of TG2.