Three-Dimensional Structures of Full-Length, Membrane-Embedded Human αIIbβ3 Integrin Complexes

Integrins are bidirectional, allosteric transmembrane receptors that play a central role in hemostasis and arterial thrombosis. Using cryo-electron microscopy, multireference single-particle reconstruction methods, and statistics-based computational fitting approaches, we determined three-dimensional structures of human integrin alpha(IIb)beta(3) embedded in a lipid bilayer (nanodiscs) while bound to domains of the cytosolic regulator talin and to extracellular ligands. We also determined the conformations of integrin in solution by itself to localize the membrane and the talin-binding site. To our knowledge, our data provide unprecedented three-dimensional information about the conformational states of intact, full-length integrin within membrane bilayers under near-physiological conditions and in the presence of cytosolic activators and extracellular ligands. We show that alpha(IIb)beta(3) integrins exist in a conformational equilibrium clustered around four main states. These conformations range from a compact bent nodule to two partially extended intermediate conformers and finally to a fully upright state. In the presence of nanodiscs and the two ligands, the equilibrium is significantly shifted toward the upright conformation. In this conformation, the receptor extends similar to 20 nm upward from the membrane. There are no observable contacts between the two subunits other than those in the headpiece near the ligand-binding pocket, and the alpha- and beta-subunits are well separated with their cytoplasmic tails similar to 8 nm apart. Our results indicate that extension of the ectodomain is possible without separating the legs or extending the hybrid domain, and that the ligand-binding pocket is not occluded by the membrane in any conformations of the equilibrium. Further, they suggest that integrin activation may be influenced by equilibrium shifts.