Structure of the α5β1 integrin headpiece in complex with SG/19 Fab. (A) Ribbon presentation of the overall structure with disulfide bonds in stick model. Individual domains are differently colored in magenta (β-propeller), red (thigh), cyan (βA), blue (hybrid), and purple (PSI), and bound metal ions are shown as spheres (yellow for Ca²⁺ and purple for Mg²⁺). SG/19 Fab is colored in gray (light chain) and wheat (heavy chain), with their CDR loop regions highlighted in green and yellow, respectively. (B) Close-up view of the SG/19-binding interface. SG/19 Fab and β1 chain are shown in surface and ribbon presentations, respectively, with the same color code used in A. CDR regions and important interface residues are labeled. (C) Superposition of three integrin headpiece structures in the ligand-unbound form. Blue, α5β1; red, αVβ3 (3IJE); green, αIIbβ3 (3FCS). On the right is a blowup of the region around the metal-binding sites in the β1 (blue) and β3 (green; 3FCS) βA domains.