β3-Specific LIBS antibodies variably recognize free β3 and the pro-αIIββ3 complex but not the mature αIIbβ3 complex. (A) Epitopes of mAb 7E3, 7H2, and the β3-specific LIBS antibodies are indicated by spheres on the bent and extended structures of ligand-bound αIIbβ3. The epitopes of LIBS1 and PMI-2 are unknown. (B) Precipitation of αIIb and β3 by the mAbs at various times after initiating the chase. The mAbs LIBS2 and AP5 precipitated virtually the entire pool of free β3 (defined by the binding of the non-LIBS mAb 7H2), a portion of the pro-αIIb in complex with β3 but no mature αIIbβ3. The mAbs PMI-2, LIBS1, and LIBS6 precipitated subpopulations of uncomplexed β3 but no complexed β3. Data shown are from one of more than 5 experiments. The top row of blots is from a single experiment and is overexposed to show the fainter pro-αIIb bands. The bottom row is from a separate experiment and earlier time point to show the absence of mature αIIb in the AP5 precipitate. Equivalent amounts of protein were loaded in each lane.