Incorporation of vitronectin into fibrin clots. Evidence for a binding interaction between vitronectin and γA/γ′ fibrinogen

Vitronectin is an abundant plasma protein that regulates coagulation, fibrinolysis, complement activation, and cell adhesion. Recently, we demonstrated that plasma vitronectin inhibits fibrinolysis by mediating the interaction of type 1 plasminogen activator inhibitor with fibrin (Podor, T. J., Peterson, C. B., Lawrence, D. A., Stefansson, S., Shaughnessy, S. G., Foulon, D. M., Butcher, M., and Weitz, J. I. (2000) J. Biol. Chem. 275, 19788-19794). The current studies were undertaken to further examine the interactions between vitronectin and fibrin(ogen). Comparison of vitronectin levels in plasma with those in serum indicates that ∼20% of plasma vitronectin is incorporated into the clot. When the time course of biotinylated-vitronectin incorporation into clots formed from ¹²⁵I-fibrinogen is monitored, vitronectin incorporation into the clot parallels that of fibrinogen in the absence or presence of activated factor XIII. Vitronectin binds specifically to fibrin matrices with an estimated K_d of ∼0.6 μM. Additional vitronectin subunits are assembled on fibrin-bound vitronectin multimers through self-association. Confocal microscopy of fibrin clots reveals the globular vitronectin aggregates anchored at intervals along the fibrin fibrils. This periodicity raised the possibility that vitronectin interacts with the γA/γ^′ variant of fibrin(ogen) that represents about 10% of total fibrinogen. In support of this concept, the vitronectin which contaminates fibrinogen preparations co-purifies with the γA/γ^′ fibrinogen fraction, and clots formed from γA/γ^′ fibrinogen preferentially bind vitronectin. These studies reveal that vitronectin associates with fibrin during coagulation, and may thereby modulate hemostasis and inflammation.