αIIb-Specific LIBS mAbs recognize only small amounts of free pro-αIIb. (A) Epitopes of the mAb anti-V5 and the αIIb-specific LIBS mAbs are indicated by spheres or bars on the bent, unactivated, and the proposed extended structure of ligand-bound αIIbβ3. (B) αIIb-Specific LIBS mAbs B1B5 and PMI-1 reacted with subpopulations of free pro-αIIb, pro-αIIbβ3, and mature αIIbβ3. Polyclonal antibody anti-αIIb (H-160), whose epitope encompasses both the B1B5 and PMI-1 epitopes, precipitated less pro-αIIb than V5. The 1.5-hour time point is depicted from 1 of more than 5 experiments. Equivalent amounts of protein were loaded in each lane. (C) PMI-1 precipitated less pro-αIIb than CA3 from cells with an αIIbN15Q mutation that eliminates the N15 glycan.