Regulation of human tissue transglutaminase function by magnesium-nucleotide complexes - Identification of distinct binding sites for Mg-GTP and Mg-ATP

Tissue transglutaminase (tTG) catalyzes a Ca2+- dependent transglutaminase (TGase) activity that stabilizes tissues and a GTP hydrolysis activity that regulates cell receptor signaling, The purpose of this study was to examine the true substrates for nucleotide hydrolysis and the effects of these substrates on modulating the dual enzymatic activities of tTG. We found that Mg-GTP and Mg-ATP are the true substrates of the hydrolysis reaction, tTG hydrolyzed Mg-GTP and Mg-ATP at similar rates and interacted with Mg-ATP (K-m = 39 +/- 10 mu M) at a 3-fold greater steady-state affinity than with Mg-GTP (K-m = 130 +/- 35 mu M). In addition, Mg-ATP inhibited GTP hydrolysis (IC50 = 24 mu M), whereas 1 mm Mg-GTP reduced ATP hydrolysis by only 20%, Furthermore, the TGase activity of tTG was inhibited by Mg-GTP, Mg-GDP, and Mg-GMP, with IC50 values of 9, 9, and 400 mu M, respectively, whereas the Mg-adenine nucleotides were ineffective, Kinetic analysis of the hydrolysis reaction demonstrates the presence of separate binding sites far Mg-GTP and Mg-ATP. Finally, we found that Mg-GTP protected tTG from proteolytic degradation by trypsin, whereas Mg-ATP was ineffective, In conclusion, we report that Mg-GTP and Mg-ATP can bind to distinct sites and serve as substrates for nucleotide hydrolysis. Fur- thermore, binding of Mg-GTP causes a conformational change and the inhibition of TGase activity, whereas Mg-ATP is ineffective, The implication of these findings in regulating the intracellular and extracellular function of tTG is discussed.