Integrin β3 regions controlling binding of murine mAb 7E3: Implications for the mechanism of integrin αIIbβ3 activation

Abciximab, a derivative of the murine mAb 7E3, protects against ischemic complications of percutaneous coronary interventions by inhibiting ligand binding to the αIIbβ3 receptor. In this study we identified regions on integrin β3 that control 7E3 binding. Murine/human amino acid substitutions were created in two regions of the βA domain that previous studies found to influence 7E3 binding: the C177-C184 loop and K125-W133. The T182W substitution and a K125Q mutation reduced 7E3 binding to human β3 in complex with αIIb. The introduction of both the human C177-C184 region and human W129 into murine β3 was necessary and sufficient to permit 7E3 binding to the human αIIb/murine β3 complex. Although we cannot exclude allosteric effects, we propose that 7E3 binds between C177-C184 and W129, which are within 15 Å of each other in the crystal structure and close to the β3 metal ion-dependent adhesion site. We previously demonstrated that 7E3 binds more rapidly to activated than unactivated platelets. Because it has been proposed that αIIbβ3 changes from a bent to an extended conformation upon activation, we hypothesized that 7E3 binds less well to the bent than the extended conformation. In support of this hypothesis we found that 7E3 bound less well to an αIIbβ3 construct locked in a bent conformation, and unlocking the conformation restored 7E3 binding. Thus, our data are consistent with αIIbβ3 existing in variably bent conformations in equilibrium with each other on unactivated platelets, and activation resulting in αIIbβ3 adopting a more extended conformation.