Factor XIIIa does not cross-link r_EC. Factor XIII-catalyzed cross-linking reactions were performed as described in Materials and Methods with either the r_EC domain (upper panel) or fragment D₁ (lower panel) as substrate. Lanes Ø contain substrate alone. Samples in the adjacent lane in each panel were further supplemented with nonactivated factor XIII (FXIII), whereas those in the remaining lanes were treated with thrombin-activated factor XIII (FXIIIa) and incubated for the lengths of time indicated. Samples were run on 4% to 20% gradient SDS-PAGE under reducing conditions. Positions of factor XIII, factor XIIIa, the fragment D₁ subunits (D, βD, and γD) as well as the γD-dimer are indicated in the margins.