Excess free pro-αIIb and β3 exist in stably transfected HEK293 cells. (A) Diagram localizing the epitopes of antibodies 10E5, 7E3, anti-V5, and 7H2 on the bent and the extended and ligand-bound conformations of αIIbβ3; the epitope of antibody CA3 is on αIIb, but it has not been localized to a specific region. (B) Two anti-αIIb antibodies, CA3 and anti-V5, immunoprecipitated more pro-αIIb at both 2 hours and 4 hours than the complex-specific antibody 10E5. (C) Anti-β3 antibody 7H2 bound to β3 that was in complex with either pro-αIIb or mature αIIb and also immunoprecipitated more β3 at both 2 hours and 4 hours than the 2 complex-specific antibodies 10E5 (Figure 2B) and 7E3. All gel lanes in this figure are from the same immunoblot. Equivalent amounts of protein were loaded in each lane.