Transglutaminase 2 (TGase 2) is a bifunctional enzyme that catalyzes calcium-dependent transamidation and GTP binding/hydrolysis. The transamidation activity is proposed to be associated with several neurodegenerative disorders such as Alzheimer's and Hungtinton's disease. However, the regulation mechanism by which TGase 2 causes neurodegeneration is unknown. In this study, we show that two activities of TGase 2 have a differential stability; transamidation activity is less stable than GTP hydrolytic activity, and that GTP was required to stabilize and to display transamidation activity. Moreover, GTP binding-defective mutant of TGase 2 did not show any transamidation activity in transfection experiments. These results indicate that GTP binding is crucial for transamidation activity of TGase 2, suggesting that protein cross-linking by TGase 2 might be associated with G-protein coupled receptor signaling system. Thus, our data could contribute to understand the regulation of TGase 2 activity and TGase 2-associated pathogenesis. (C) 2002 Elsevier Science (USA). All rights reserved.