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 all immune stimulant and vaccine adjuvant in cancer patients. GM-CSF signals via a specific alpha receptor (GM-CSFR alpha) and the shared h beta c (human common beta-subunit). The present study has investigated the role of the Ig-like domain of GM-CSFR alpha in GM-CSF binding and signalling. Deletion of the Ig-like domain abolished direct GMCSF binding and decreased growth signalling in the presence of h beta c. To locate the specific residues in the Ig-like domain of GM-CSFR alpha involved in GM-CSF binding, a structural alignment was made with a related receptor. IL-13R alpha 1 (interleukin-13 receptor alpha 1), whose structure and mode of interaction with its ligand has recently been elucidated. Mutagenesis of candidate residues in the predicted region of interaction identified Val(51) and Cys(60) as having critical roles in binding to the alpha receptor, with Ag-54 and Leu(55) also being important. High-affinity binding in the presence of h beta c was strongly affected by mutation of Cys(60) and was also reduced by mutation of Val(55), Ag-54 and Leu(55). Of the four key residues, growth signalling was most severely affected by mutation of Cys(60). The results indicated a previously unrecognized role for the Ig-like domain, and in particular Cys(60), of GM-CSFR alpha in the binding of GM-CSF and subsequent activation of cellular signalling.