Coimmunoprecipitation of GMRα and β_c from primary CML cells. (A and B) CML cells were ¹²⁵I–surface-labeled, treated with (+) or without (−) GM-CSF or IL-3 (6 nmol/L) for 5 minutes at 4°C and immunoprecipitation was performed either with anti-GMRα (8G6), anti–IL-3Rα (9F5), or anti-β_c (4F3) MoAb. Immunoprecipitated proteins were separated on 7.5% SDS-PAGE and visualized by phosphorimager and are presented at exposure levels appropriate for the specific signal obtained. (C) Proteins immunoprecipitated from CML cells with different antireceptor antibodies either in the presence (+) or absence (−) of GM-CSF were subjected to Western transfer and immunoblotted using a polyclonal anti-β_c antibody.