Tissue transglutaminase (TGase-2), which binds GTP and catalyzes the crosslinking of proteins (transamidation), has been implicated both in the promotion of cell death and in the protection of cells against apoptotic insults. However, a novel transcript originally identified from the brains of Alzheimer's patients, encoding a truncated form of TGase-2 (called TGase-S), shows strong apoptotic activity. TGase-S exhibits no detectable GTP-binding capability, suggesting that its ability to induce cell death might be due to its inability to bind GTP. Thus, we have examined whether eliminating the GTP-binding capability of full-length human TGase-2 would prevent it from conferring protection against apoptotic challenges and instead convert it into a protein that causes cell death. A number of point mutants of human TGase-2 defective for binding GTP, as well as a mutant that shows impaired GTP-hydrolytic activity, were generated. Similar to what we had found for TGase-S, there was a time-dependent decrease in the expression of the GTP-binding-defective TGase-2 mutants in different cell lines, whereas the expression of wild-type TGase-2 and the GTP hydrolysis-defective mutant was sustained. Moreover, the GTP-binding-defective TGase-2 mutants induced cell death. The cell death responses triggered by these mutants were not due to their transdamidation activity, because double-mutants that were both GTP-binding-and transamidationdefective also stimulated cell death. Therefore, these results point to the inability to bind GTP as being sufficient for the apoptotic activity exhibited by the TGase-S protein. They also highlight a novel example of how the loss of GTP-binding activity can convert a protein that provides protection against apoptotic stimuli into a cell death-promoting factor.
KeywordsGTP; signaling; transglutaminase; apoptosis; Alzheimer's; transamidation Tissue transglutaminase (TGase-2) belongs to a family of enzymes (transglutaminases) that catalyze the Ca 2+ -dependent, post-translational modification of proteins through the incorporation of polyamines or the formation of covalent crosslinks (1-4). The underlying chemical reaction involves the generation of an isopeptide bond between the γ-carboxamide group of a peptide-bound glutamine residue and either the ε-amino group of a lysine residue or the primary amino group of a polyamine. The expression of TGase-2 and its accompanying transamidation activity are stimulated when cells are exposed to different stresses, differentiation agents, and growth factors (5-11). A number of reports have suggested that TGase-2 is able to both promote and prevent apoptosis (4,12). For example, studies performed † This work was supported by GM61762. *To whom correspondence should be addressed. These authors contributed equally to this work. (6,7,(14)(15)(16)(17)(18)(19)(20)(21)(22). On the other hand, the regulation of TGase-2 by growth and differentiation factors has also been shown to be important in conferring a survival advantage to cells challenged with apoptotic...