GTP-dependent conformational changes associated with the functional switch between Galpha and cross-linking activities in brain-derived tissue transglutaminase.

GTP and Ca2+, two well-known modulators of intracellular signaling pathways, control a structural/functional switch between two vital and mutually exclusive activities, cross-linking and Galpha activity, in the same enzyme. The enzyme, a brain-derived tissue-type transglutaminase (TGase), was recently cloned by us in two forms, one of which (s-TGN) lacks a C-terminal region that is present in the other (l-TGN). Immunoreaction with antibodies directed against a peptide present in the C-terminus of l-TGN but missing in s-TGN suggested that this site, which is located in the C-terminal fourth domain, undergoes conformational changes as a result of interaction between l-TGN and GTP. Site-directed mutagenesis suggested that the third domain is involved in mediating the inhibition of the cross-linking activity. These results were supported by molecular modeling, which further suggested that domains III and IV both participate in conformational changes leading to the functional switch between the Ca2+-dependent cross-linking activity and the Galpha activity.