Complete integrin headpiece opening in eight steps

Carefully soaking crystals with Arg-Gly-Asp (RGD) peptides, we captured eight distinct RGD-bound conformations of the alpha(IIb)beta(3) integrin headpiece. Starting from the closed beta I domain conformation, we saw six intermediate beta I conformations and finally the fully open beta I with the hybrid domain swung out in the crystal lattice. The beta 1-alpha 1 backbone that hydrogen bonds to the Asp side chain of RGD was the first element to move followed by adjacent to metal ion-dependent adhesion site Ca2+, alpha 1 helix, alpha 1' helix, beta 6-alpha 7 loop, alpha 7 helix, and hybrid domain. We define in atomic detail how conformational change was transmitted over long distances in integrins, 40 angstrom from the ligand binding site to the opposite end of the beta I domain and 80 angstrom to the far end of the hybrid domain. During these movements, RGD slid in its binding groove toward alpha(IIb), and its Arg side chain became ordered. RGD concentration requirements in soaking suggested a >200-fold higher affinity after opening. The thermodynamic cycle shows how higher affinity pays the energetic cost of opening.