Structure of the β₃PSI domain. (A) Two different views of the superposition of the bent (PDB 3FCS), intermediate, and extended β₃ structures on the PSI domain. Disulfide bonds, Pro33, and Leu33 are sticks. Selected cysteines are shown as Cα spheres. (B) Sequence alignment of a portion of the PSI domain containing the C26-C38 loop. Disulfide linkages are indicated. (C) Mn²⁺-stimulated binding of the ligand mimetic mAb PAC-1 to HEK293FT cells transfected with the indicated α_IIb and β₃ constructs. PAC-1 binding was performed in the presence of 1 mM of Ca²⁺/Mg²⁺ or 0.2 mM of Ca²⁺ plus 2 mM Mn²⁺. (D) Talin head (TH)–induced PAC-1 binding to HEK293FT cells transfected with the indicated α_IIb and β₃ constructs plus EGFP or EGFP-TH. PAC-1 binding was performed in the presence of 1 mM of Ca²⁺/Mg²⁺. (E) Mn²⁺-stimulated binding of fibronectin (Fn) to HEK293FT cells transfected with the indicated α_V and β₃ constructs. Fn binding was performed in the presence of 1 mM of Ca²⁺/Mg²⁺ or 0.2 mM of Ca²⁺ plus 2 mM of Mn²⁺. All the ligand binding results are presented as the mean fluorescence intensity (MFI) normalized to integrin expression. WT, wild type.