Structural evidence that the activation peptide is not released upon thrombin cleavage of factor XIII

The three-dimensional structure of the recombinant human factor XIII a2 dimer after cleavage by thrombin has been determined by X-ray crystallography. Factor XIII zymogen was treated with bovine alpha-thrombin in the presence of 3 mM CaCl2, and the cleaved protein was crystallized from Tris buffered at pH 6.5 using ethanol as the precipitating agent. Refinement of the molecular model of thrombin-cleaved factor XIII against diffraction data from 10.0 to 2.5 A resolution has been carried out to give a crystallographic R factor of 18.2%. The structure of thrombin-cleaved factor XIII is remarkably similar to that of the zymogen: there are no large conformational changes in the protein and the 37 residue amino terminus activation peptide remains associated with the rest of the molecule. This work shows that the activation peptide, upon thrombin cleavage, has the same conformation and occupies the same position with respect to the rest of the molecule as it does in the zymogen structure.