| Abstract: | Fibronectin binds to specific receptors on the surface of washed, thrombin-activated platelets. Evidence suggests that these receptors are closely associated with the platelet glycoprotein IIb-IIIa complex (GP IIb-IIIa). To determine whether GP IIb-IIIa itself can form a platelet receptor for fibronectin, we used a filtration assay to examine the interaction of purified fibronectin with purified GP IIb-IIIa incorporated into phospholipid vesicles. 125I-Fibronectin binding to the phospholipid vesicles required the presence of incorporated GP IIb-IIIa and was specific, time-dependent, reversible, saturable, and divalent cation-dependent (Mg2+ greater than Ca2+). The dissociation constant for 125I-fibronectin binding to the GP IIb-IIIa-containing vesicles in the presence of 2 mM MgCl2 was 87 nM. Proteins or peptides that inhibit 125I-fibronectin binding to whole platelets also inhibited 125I-fibronectin binding to the GP IIb-IIIa vesicles. Thus, specific 125I-fibronectin binding was inhibited by excess unlabeled fibrinogen or fibronectin, the anti-GP IIb-IIIa monoclonal antibody 10E5, the decapeptide from the carboxyl terminus of the fibrinogen gamma-chain, and the tetrapeptide Arg-Gly-Asp-Ser from the cell-binding domain of fibronectin. In contrast to results obtained using whole platelets, unlabeled fibronectin inhibited 125I-fibronectin binding to the GP IIb-IIIa vesicles. These results show that 125I-fibronectin binds directly to purified GP IIb-IIIa with most of the previously reported properties of 125I-fibronectin binding to washed, thrombin-stimulated platelets. Thus, GP IIb-IIIa has the potential to function as a platelet receptor for fibronectin as well as for fibrinogen. |
