Immunoprecipitation analysis of biotin-labeled surface-expressed proteins from transfected CHO cells expressing Pen^a (Arg₁₄₃) or Pen^b(Gln₁₄₃) alloantigens. Stably transfected CHO cells were surface-labeled with biotin. Cell extracts were immunoprecipitated with the indicated antibodies. (A) Surface-expressed GPIIb-IIIa was recognized by different monoclonal antibodies. MoAb AP3 (specific for GPIIIa, lane 1) and Tab (specific for GPIIb, lane 2) immunoprecipitated the respective subunit and its associated subunit. AP2 recognized the intact mature complex (lane 3). Normal mouse IgG was used as a negative control (lane 4). (B) Surface-expressed GPIIb-IIIa was recognized by allele-specific alloantibodies. Whereas Pl^A1 (lane 3) reacted with GPIIb-IIIa derived from both stable cell lines, anti-Pen^a alloantibodies (lane 1) recognized the Arg₁₄₃ form of GPIIb-IIIa but not the Gln₁₄₃form. Anti-Pen^b alloantibodies (lane 2) only reacted with the Gln₁₄₃ form, as expected. Normal human IgG (lane 4) was used as a control.