N-terminal fibronectin 30-kDa fragment mediates the immobilization of soluble fibrin by factor XIIIa-coated polystyrene beads.

Polystyrene beads coated with thrombin-activated Factor XIII (plasma transglutaminase, plasma transamidase) retained soluble 125I-fibrin, if a 30-kDa fragment of fibronectin was present. The fragment was obtained by proteolytic cleavage of plasma fibronectin with trypsin, but was also available with plasmin or thrombin. It represented a fibrin-binding domain at the N-terminus of each of the two subunit chains and contained close to its amino end a transamidase-reactive site. Fibronectin was unable to mediate the binding of 125I-fibrin to Factor XIIIa-coated beads. 125I-fibrinogen was hardly recognized by the beads even in presence of the fibronectin fragment. The relatively slow binding of 125I-fibrin was inhibited by 0.15 mM putrescine or by a pretreatment of the coated beads with EDTA or N-ethylmaleinimide indicating the involvement of a transamidation in the binding reaction. Immobilization of 125I-fibrin in presence of the fibronectin fragment is assumed to require a covalent cross-linking of the two ligands at the immobilized transamidase giving rise to a product which is retained strongly. The possibility is discussed that a surface-attached transamidase might act as a fibrin receptor which requires the fibronectin fragment as a cofactor.