Immunoprecipitation of biotin-labeled surface or total (surface and intracellular) proteins from CHO cells cotransfected with cDNAs encoding GPIIb and either normal or [T¹⁸⁴⁶]GPIIIa. CHO cells were transiently cotransfected with cDNAs encoding GPIIb and either normal or [T¹⁸⁴⁶]GPIIIa in the expression plasmid pcDNA3. (A) Protein labeling was performed by exposure of intact cells to biotin-NHS, and the GPIIb-IIIa complexes were immunoprecipitated with either anti-GPIIIa (P37) or anti-GPIIb (M3) MoAbs as described in Materials and Methods. (B) The experimental design was similar to that described in (A), except that total cell lysates rather intact cells were exposed to biotin-NHS and the immunoprecipitation was also performed with a GPIIb-IIIa complex-specific antibody (CII). Mock cells were transfected with void pcDNA3 plasmid.