Identification of the GPIIb/IIIa complex in cells expressing normal or mutant GPIIb. GPIIIa was precipitated from lysates of normal or mutant-containing cells by polyclonal antibody against GPIIIa. Precipitates were subjected to SDS–polyacrylamide gels with (A) or without (B) β-mercaptoethanol, and Western blot analysis was carried out using monoclonal antibodies against GPIIb (SZ22) and GPIIIa (AP3) for detection of coprecipitated GPIIb with GPIIIa. (C) GPIIb/IIIa complex precipitated from lysates of normal or mutant-containing cells by monoclonal antibody P2 recognizing GPIIb only when complexed with GPIIIa, and subjected to SDS–polyacrylamide gel electrophoresis with β-mercaptoethanol. Western blots carried out with SZ22 show the same pattern as in panel A. (D) Coprecipitation of GPIIb with AP3 anti-GPIIIa monoclonal antibody analyzed in reduced gel by SZ22 monoclonal antibody. Overexposure of the gel detected a weak band corresponding to mature form of GPIIb in cells containing GPIIb-alt.25 and a faint band in cells containing GPIIb-del.20, which correspond to mature aberrant GPIIb.