Time course of factor XIIIa-catalyzed cross-linking. Cross-linking reactions with either fibrinogen-420 (upper panel) or fibrinogen-340 (lower panel) were carried out as described in “Materials and methods.” Lane 1 contains substrate alone. Lanes 2 to 6 contain substrate with either nonactivated factor XIII (lane 2) or thrombin-activated factor XIIIa (lanes 3-6), incubated for 2 minutes (lane 3), 5 minutes (lane 4), 30 minutes (lane 5), or 60 minutes (lane 6). Proteins were separated on homogenous 12% SDS-PAGE gels under reducing conditions and stained. Positions of individual and cross-linked fibrinogen chains are indicated. dα_Erefers to degraded α_E, α_E-xlinks refers to cross-linked α_E chains, and α-xlinks refers to cross-linked α chains.