Recognition of RGD peptide by α5β1. (A and B) Close-up views of the region surrounding MIDAS before (A) and after (B) ligand peptide soaking. Fo-Fc omit maps, in which RGD, metals, and β1 residues Tyr133–Met135 were removed from the models, are shown in magenta wireframes at a contour level of 3 σ. For reduction of model bias from the omitted atoms, the partial models were subjected to refinement before phase calculation. Note that the electron density corresponding to ADMIDAS Ca²⁺ disappeared in the RGD-bound structure. (C) Specific recognition of RGD peptide by three different classes of integrins: α5β1 (top), αVβ3 (1L5G; middle), and αIIbβ3 (2VDR; bottom). Hydrogen and coordination bonds are shown in dashed lines. Metal ions in the βA domain are shown in spheres. (D) Sequence alignment around the Arg acceptor residues of RGD-dependent α subunits. Head-on and side-on binding residues are highlighted in pink and yellow, respectively.