Expression of β3 containing the Leu¹¹⁷ → Trp substitution in COS-1 cells. Wild-type (WT) and mutant (MK) β3 were expressed in COS-1, either alone or with αIIb. Cells were then either metabolically labeled with ³⁵S-methionine or surface labeled with biotin-labeled proteins were immunoprecipitated by the antibodies indicated below and examined by SDS-polyacrylamide gel electrophoresis. (A) Wild-type and mutant β3 were expressed alone in COS-1 cells and immunoprecipitated from cells labeled with ³⁵S-methionine using the β3-specific MoAb SSA6. (B and C) COS-1 cells were cotransfected with αIIb and either wild-type or mutant β3. After labeling the cells with ³⁵S-methionine, αIIb-β3 was immunoprecipitated using the β3-specific MoAb SSA6 (B) or the heterodimer-specific MoAb A2A9 (C). (D) The cells were treated as in (B), but were pulse-chased for the various time intervals indicated. (E) The surface of COS-1 cells cotransfected with αIIb and either wild-type or mutant β3 was biotin labeled and immunoprecipitated using SSA6. WT is the wild-type complex, MK refers to αIIb-β3^{Leu117 → Trp} complexes, and JF refers to αIIb^{Gly273 → Asp}-β3 complexes. Pro-αIIb corresponds to the single-chain αIIb precursor; αIIb-α corresponds to the αIIb heavy chain.