How ligand binding alters integrin conformation in outside-in signaling, and how inside-out signals alter integrin affinity for ligand, have been mysterious. We address this with electron microscopy, physicochemical measurements, mutational introduction of disulfides, and ligand binding to alphaVbeta3 and alphaIIbbeta3 integrins. We show that a highly bent integrin conformation is physiological and has low affinity for biological ligands. Addition of a high affinity ligand mimetic peptide or Mn(2+) results in a switchblade-like opening to an extended structure. An outward swing of the hybrid domain at its junction with the I-like domain shows conformational change within the headpiece that is linked to ligand binding. Breakage of a C-terminal clasp between the alpha and beta subunits enhances Mn(2+)-induced unbending and ligand binding.

Integrins are important adhesion receptors in all Metazoa that transmit conformational change bidirectionally across the membrane. Integrin α and β subunits form a head and two long legs in the ectodomain and span the membrane. Here, we define with crystal structures the atomic basis for allosteric regulation of the conformation and affinity for ligand of the integrin ectodomain, and how fibrinogen-mimetic therapeutics bind to platelet integrin α IIb b β3 . Allostery in the β 3 I domain alters three metal binding sites, associated loops and a α1-and α7-helices. Piston-like displacement of the a 7-helix causes a 62° reorientation between the β 3 I and hybrid domains. Transmission through the rigidly connected plexin/semaphorin/integrin (PSI) domain in the upper β 3 leg causes a 70Å separation between the knees of the α and β legs. Allostery in the head thus disrupts interaction between the legs in a previously described low-affinity bent integrin conformation, and leg extension positions the high-affinity head far above the cell surface.Integrins are adhesion receptors that transmit signals bidirectionally across the plasma membrane 1-4 . Rearrangements in integrin extracellular, transmembrane and cytoplasmic domains underlie diverse biological processes, including cell migration, morpho-genesis, immune responses and vascular haemostasis. The platelet-specific integrin α IIb β 3 is important in both the arrest of bleeding at sites of vascular injury and pathological thrombosis leading to heart attacks and stroke. Loss of the vascular endothelium results in platelet deposition, and receptors for collagen, thrombin and other agonists then initiate