The structural basis for the regulation of tissue transglutaminase by calcium ions

The role of calcium ions in the regulation of tissue transglutaminase is in vestigated by experimental approaches and computer modeling. A three-dimens ional model of the transglutaminase is computed by homology building on cry stallized human factor XIII and is used to interpret structural and functio nal results. The molecule is a prolate ellipsoid (6.2 x 4.2 x 11 nm) and co mprises four domains, assembled pairwise into N-terminal and C-terminal reg ions. The active site is hidden in a cleft between these regions and is ina ccessible to macromolecular substrates in the calcium-for form, Protein dyn amics simulation indicates that these regions move apart upon addition of c alcium ions, revealing the active site for catalysis. The protein dimensions are consistent with results obtained with small-angl e neutron and X-rap scattering. The gyration radius of the protein (3 nm) i ncreases in the presence of calcium ions (3.9 nm), but it is virtually unaf fected in the presence of GTP, suggesting that only calcium ions can promot e major structural changes in the native protein. Proteolysis of an exposed loop connecting the N-terminal and C-terminal regions is linearly correlat ed with enzyme inactivation and prevents the calcium-induced conformational changes.