The crystal structure of α_IIbβ₃ headpiece in closed conformation. (A) Cartoon diagram of the α_IIbβ₃ headpiece complex with 10E5 Fab. Ca²⁺ and Mg²⁺ ions are yellow and silver spheres, respectively. (B) Superposition of the closed headpiece structure found in this study (2 molecules in one asymmetric unit, red) with the indicated previously reported structures (number of molecules per asymmetric unit). The structures were aligned on the α_IIbβ-propeller and the β₃ βI domains by the super command of Pymol. (C) Superposition of β₃ βI domains of the closed α_IIbβ₃ headpiece structure in this study (red) and the closed headpiece structure of α_IIbβ₃ entire ectodomain (blue). Metal ions are shown as spheres. (D) 2Fo-Fc maps of metals and coordinating waters in β₃ βI domain in the presence of 5mM Mg²⁺ and 1mM Ca²⁺. The maps are contoured at 1.5 σ. Ca²⁺ (gold) and Mg²⁺ (green) ions are large spheres. Waters (red) are small spheres. Nitrogen atoms are blue and oxygen atoms are red. Metal coordination and hydrogen bonds are dashed. (E) The β₃ βI/Hybrid interface in the complete α_IIbβ₃ ectodomain structure. (F) The β₃ βI/Hybrid interface in the closed headpiece structure. The 2 structures were aligned on the β₃ βI domain.