Multiple discontinuous ligand-mimetic antibody binding sites define a ligand binding pocket in integrin α(IIb)β₃

Integrin α(IIb)β₃, a platelet fibrinogen receptor, is critically involved in thrombosis and hemostasis. However, how ligands interact with α(IIb)β₃ has been controversial. Ligand-mimetic anti-α(IIb)β₃ antibodies (PAC-1, LJ-CP3, and OP-G2) contain the RGD-like RYD sequence in their CDR3 in the heavy chain and have structural and functional similarities to native ligands. We have located binding sites for ligand-mimetic antibodies in α(IIb) and β₃ using human-to-mouse chimeras, which we expect to maintain functional integrity of α(IIb)β₃. Here we report that these antibodies recognize several discontinuous binding sites in both the α(IIb) and β₃ subunits; these binding sites are located in residues 156-162 and 229230 of α(IIb) and residues 179-183 of β₃. In contrast, several nonligand-mimetic antibodies (e.g. 7E3) recognize single epitopes in either subunit. Thus, binding to several discontinuous sites in both subunits is unique to ligand-mimetic antibodies. Interestingly, these binding sites overlap with several (but not all) of the sequences that have been reported to be critical for fibrinogen binding (e.g. N-terminal repeats 2-3 but not repeats 4-7, of a(IIb)). These results suggest that ligand-mimetic antibodies and probably native ligands may make direct contact with these discontinuous binding sites in both subunits, which may constitute a ligand-binding pocket.