Binding of a fibrinogen mimetic stabilizes integrin alpha IIb beta 3's open conformation

The platelet integrin alpha IIb beta3 is representative of a class of heter odimeric receptors that upon activation bind extracellular macromolecular l igands and form signaling clusters. This study examined how occupancy of al pha IIb beta3's fibrinogen binding site affected the receptor's solution st ructure and stability. Eptifibatide, an integrin antagonist developed to tr eat cardiovascular disease, served as a high-affinity, monovalent model lig and with fibrinogen-like selectivity for alpha IIb beta3 Eptifibatide bindi ng promptly and reversibly perturbed the conformation of the alpha IIb beta 3 complex. Ligand-specific decreases in its diffusion and sedimentation coe fficient were observed at near-stoichiometric eptifibatide concentrations, in contrast to the receptor-perturbing effects of RGD ligands that we previ ously observed only at a 70-fold molar excess. Eptifibatide promoted alpha IIb beta3 dimerization 10-fold more effectively than less selective RGD lig ands, as determined by sedimentation equilibrium. Eptifibatide-bound integr in receptors displayed an ectodomain separation and enhanced assembly of di mers and larger oligomers linked through their stalk regions, as seen by tr ansmission electron microscopy. Ligation with eptifibatide protected alpha IIb beta3 from SDS-induced subunit dissociation, an effect on electrophoret ic mobility not seen with RGD ligands. Despite its distinct cleft, the open conformer resisted guanidine unfolding as effectively as the ligand-free i ntegrin. Thus, we provide the first demonstration that binding a monovalent ligand to alpha Il beta3's extracellular fibrinogen-recognition site stabi lizes the receptor's open conformation and enhances self-association throug h its distant transmembrane and/or cytoplasmic domains. By showing how epti fibatide and RGD peptides, ligands with distinct binding sites, each affect s alpha IIb beta3's conformation, our findings provide new mechanistic insi ghts into ligand-linked integrin activation, clustering and signaling.