Anti-(Arg-Gly-Asp-Ser) antibody and its interaction with fibronectin, fibrinogen and platelets

An antibody population recognizing the sequence Arg-Gly-Asp-Ser (RGDS) in fibronectin, anti-(RGDS)N, was isolated by immunoadsorption. Between 2.5% and 4.9% of antibodies were obtained from two different anti-fibronectin sera indicating that this region represents an antigenic epitope in native fibronectin. Complete inhibition of binding of 125I-fibronectin to anti-(RGDS)N was produced only by nonreduced and reduced fibronectin. Fibrinogen and synthetic RGDS tetrapeptide, each at concentration of 10 microM, showed only a slight inhibition of 22% and 17%, respectively. Measurements of the conformational constant, the equilibrium constant for the interconversion of the non-native and native conformations of this epitope, showed that less than 0.0001% of the RGDS molecules adopt the native conformation in aqueous solutions. It indicates that long-range interactions in fibronectin and fibrinogen result in different conformations of the RGDS sequence in both proteins. Anti-(RGDS)N antibodies purified from anti-fibronectin serum had a strong inhibitory effect on thrombin-stimulated platelet aggregation. They also inhibited binding of fibronectin and fibrinogen to thrombin-stimulated platelets, supporting the primary role of the RGDS sequence in the direct interaction of these proteins with platelet membrane receptors.