Conformational change of integrin α_IIbβ₃ headpiece studied by gel filtration and dynamic light scattering. (A) Gel filtration profile of α_IIbβ₃ headpiece alone or bound with antagonists. The untagged α_IIbβ₃ headpiece was mixed with saturating amounts of RUC-1, tirofiban, or eptifibatide and incubated at room temperature for 1 hour before Superdex 200 chromatography in Tris-buffered saline with 1mM Ca²⁺/Mg²⁺. The elution volumes are shown in parentheses. (B) Stokes radius measured by gel filtration or dynamic light scattering. (C-D) The structures with surface representation indicate the putative conformational rearrangement after drug binding. The structures were generated by adding the α_IIb thigh domain from the complete ectodomain structure to the headpiece crystal structures. The Ca²⁺ and Mg²⁺ ions are shown as yellow and silver spheres, respectively. RUC-1 (C) and tirofiban (D) are shown as spheres.