The Ig-like domain of human GM-CSF receptor α plays a critical role in cytokine binding and receptor activation

GM-CSF (granulocyte/macrophage colony-stimulating factor) is an important mediator of inducible haemopoiesis and inflammation, and has a critical role in the function of alveolar macrophages. Its clinical applications include the mobilization of haemopoietic progenitors, and a role as an immune stimulant and vaccine adjuvant in cancer patients. GM-CSF signals via a specific α receptor (GM-CSFRα) and the shared hβc (human common β-subunit). The present study has investigated the role of the Ig-like domain of GM-CSFRα in GM-CSF binding and signalling. Deletion of the Ig-like domain abolished direct GMCSF binding and decreased growth signalling in the presence of hβc. To locate the specific residues in the Ig-like domain of GMCSFRα involved in GM-CSF binding, a structural alignment was made with a related receptor, IL-13Rα1 (interleukin-13 receptor α1), whose structure and mode of interaction with its ligand has recently been elucidated. Mutagenesis of candidate residues in the predicted region of interaction identified Val⁵¹ and Cys⁶⁰ as having critical roles in binding to the α receptor, with Arg⁵⁴ and Leu⁵⁵ also being important. High-affinity binding in the presence of hβc was strongly affected by mutation of Cys⁶⁰ and was also reduced by mutation of Val⁵¹, Arg⁵⁴ and Leu⁵⁵. Of the four key residues, growth signalling was most severely affected by mutation of Cys ⁶⁰. The results indicate a previously unrecognized role for the Ig-like domain, and in particular Cys⁶⁰, of GM-CSFRα in the binding of GM-CSF and subsequent activation of cellular signalling.