Transglutaminases (TGs) are Ca(2+)-dependent enzymes capable of catalyzing transamidation of glutamine residues to form intermolecular isopeptide bonds. These enzymes are involved in various biological phenomena, including blood coagulation, wound healing, cell death, tissue repair, and terminal differentiation of keratinocytes. Among the TG-family members, TG5 is one of the latest identified enzymes and therefore the less characterized at the functional level. In this work, we reported that TG5 is proteolytically processed in the baculovirus expression system and in mammal epithelial cells. Similar to other members of the TG family-TG1, TG3, and factor XIIIa -, TG5 full-length enzyme has very low enzymatic activity, while the 53-kDa proteolytically processed form is highly active.