Functional Replacement of Cytokine Receptor Extracellular Domains by Leucine Zippers

Granulocyte-macrophage colony-stimulating factor receptor signals by a complex which includes the ligand and two different receptor subunits: a low affinity α receptor binding chain (granulocyte-macrophage colony-stimulating factor receptor α subunit (GM-Rα)) and a signal-transducing β chain (GM-Rβ). To investigate two unresolved issues in the initiation of signaling, the role of receptor extracellular domains and receptor stoichiometry, we replaced the mouse GM-Rα and GM-Rβ extracellular domains with the leucine zipper domain of either the Fos or Jun molecule. We co-transfected combinations of chimeric receptors into Ba/F3 cells and found that both simple heterodimers of the GM-Rα and GM-Rβ intracellular domains and homodimers of the GM-Rβ intracellular domain signaled for proliferation. Surprisingly, homodimers of the GM-Rα intracellular domain also signaled for prevention of apoptosis in transfected cells. We similarly engineered dimers of the intracellular domain of the human interferon γ receptor β subunit and found that homodimers of the intracellular domain signaled for proliferation. When Fos peptide was added to Ba/F3 cells expressing the human interferon γ receptor β subunit construct, thereby preventing homodimer formation, the cells no longer proliferated in the absence of mouse interleukin 3.