Structural basis for the guanine nucleotide-binding activity of tissue transglutaminase and its regulation of transamidation activity

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-binding/GTPase 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+).