IDENTIFICATION OF A CYS MOTIF IN THE COMMON BETA-CHAIN OF THE INTERLEUKIN-3, GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR, AND INTERLEUKIN-5 RECEPTORS ESSENTIAL FOR DISULFIDE-LINKED RECEPTOR HETERODIMERIZATION AND ACTIVATION OF ALL 3 RECEPTORS

The human interleukin 3 (IL-3) and granulocyte-macrophage colony-stimu lating factor (GM-CSF) receptors undergo covalent dimerization of the respective specific alpha chains with the common beta subunit (beta(c) ) in the presence of the cognate ligand, We have now performed alanine substitutions of individual Cys residues in beta(c) to identify the C ys residues involved and their contribution to activation of the IL-3, GM-CSF, and IL-5 receptors, We found that substitution of Cys-86, Cys -91, and Cys-96 in beta(c) but not of Cys-100 or Cys-234 abrogated dis ulfide-linked IL-3 receptor dimerization, However, although Cys-86 and Cys-91 beta(c) mutants retained their ability to form non-disulfide-l inked dimers with IL-3R alpha, substitution of Cys-96 eliminated this interaction, Binding studies demonstrated that all beta(c) mutants wit h the exception of C96A supported high affinity binding of IL-3 and GM CSF, In receptor activation experiments, we found that beta(c) mutants C86A, C91A, and C96A but not C100A or C234A abolished phosphorylation of beta(c) in response to IL-3, GM-CSF, or IL-5, These data show that although Cys-96 is important for the structural integrity of beta(c), Cys-86 and Cys-91 participate in disulfide-linked receptor heterodime rization and that this linkage is essential for tyrosine phosphorylati on of beta(c). Sequence alignment of beta(c) with other cytokine recep tor signaling subunits in light of these data shows that Cys-86 and Cy s-91 represent a motif restricted to human and mouse beta chains, sugg esting a unique mechanism of activation utilized by the IL-3, GM-CSF, and IL-5 receptors.