Tissue transglutaminase (TG) is a Ca 2؉ -dependent acyltransferase with roles in cellular differentiation, apoptosis, and other biological functions. In addition to being a transamidase, TG undergoes a GTP-binding͞GTPase cycle even though it lacks any obvious sequence similarity with canonical GTP-binding (G) proteins. Guanine nucleotide binding and Ca 2؉ concentration reciprocally regulate TG's transamidation activity, with nucleotide binding being the negative regulator. Here we report the x-ray structure determined to 2.8-Å resolution of human TG complexed with GDP. Although the transamidation active site is similar to those of other known transglutaminases, the guanine nucleotide-binding site of TG differs markedly from other G proteins. The structure suggests a structural basis for the negative regulation of transamidation activity by bound nucleotide, and the positive regulation of transamidation by Ca 2؉ .T issue transglutaminase (TG, also called type II transglutaminase) catalyzes the Ca 2ϩ -dependent formation of a new amide bond between the ␥-carboxamide of glutamine and the -amino group of lysine or another primary amine (1) (see scheme below).TG activity, which is found in the cytosol, plasma membrane, and nucleus of cells, has been implicated in a variety of physiological activities and pathological processes, including neuronal growth and regeneration (2-4), bone development (5-6), angiogenesis (7), wound healing (7), cellular differentiation, and apoptosis (8-10). During apoptosis, for example, TG-catalyzed crosslinking of proteins results in the irreversible formation of scaffolds that could prevent the leakage of harmful intracellular components (11). Retinoic acid (RA)-stimulated increases in TG expression and activation accompany RA-induced cellular differentiation (8,12). This increased TG expression, coupled with the finding that two of the primary targets for TG, the eukaryotic initiation factor eIF-5A and the retinoblastoma gene product (13,14), are essential for cell viability has led to the suggestion that TG activity is necessary for ensuring cell survival under conditions of differentiation or cellular stress. It has also been proposed that the dysregulation of TG activity may be associated with neurodegenerative conditions such as Alzheimer's disease and Huntington's disease (15-17).TG's ability to bind and hydrolyze GTP with affinity and rates like those of traditional G proteins distinguishes it from other transglutaminases and suggests that TG, like other G proteins, participates in signaling pathways (18)(19)(20)(21). Among the studies implicating TG as a signal transducer in biological response pathways, the best documented is its role in ␣1-adrenergic receptor-mediated stimulation of phospholipase C-␦ activity (21-23). It was originally reported that an Ϸ70-to 80-kDa GTP-binding protein (named Gh) was responsible for coupling ␣1-adrenergic agonists to the stimulation of phosphoinositide lipid metabolism (24), and it was subsequently demonstrated that Gh was identical to TG (21)....

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