Kinetic resolution of two mechanisms for high-affinity granulocyte-macrophage colony-stimulating factor binding to its receptor

Granulocyte-macrophage colony-stimulating factor (GMCSF) is an import-ant h ematopoietic cytokine that exerts its effects by interaction with the GM-CS F receptor (GMR) on the surface of responsive cells. The GM-CSF receptor co nsists of two subunits: GMR alpha, which binds GM-CSF with low affinity, an d GMR beta, which lacks intrinsic ligand-binding capability but complexes w ith GMR alpha to form a high-affinity receptor (GMR alpha/beta). We conduct ed dynamic kinetic analyses of GM-CSF receptors to define the role of GMR b eta in the interaction of ligand and receptor. Our data show that GMR alpha /beta exhibits a higher k(on) than GMR alpha, indicating that GMR beta faci litates ligand acquisition to the binding pocket. Heterogeneity with regard to GM-CSF dissociation from GMR alpha/beta points to the presence of loose and tight. ligand-receptor complexes in high-affinity binding. Although th e loose complex has a k(off) similar to GMR alpha, the lower k(off) indicat es that GMR beta inhibits GM-CSF release from the tight receptor complex. T he two rates of ligand dissociation may provide for discrete mechanisms of interaction between GM-CSF and its high-affinity receptor. These results sh ow that the beta subunit functions to stabilize ligand binding as well as t o facilitate ligand acquisition. (C) 1999 by The American Society of Hemato logy.