HISTIDINE-367 OF THE HUMAN COMMON BETA-CHAIN OF THE RECEPTOR IS CRITICAL FOR HIGH-AFFINITY BINDING OF HUMAN GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR

High-affinity receptors for granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin 3, and interleukin 5 consist of ligand-specific alpha chains (low-affinity subunits) and a common beta chain (beta(c)) that converts each complex to a high-affinity form. Although beta(c) alone has no detectable cytokine-binding activity, amino acid substitutions for Glu-21 of human GM-CSF significantly reduce high-affinity but not low-affinity binding, implying that beta(c) interacts directly with GM-CSF during formation of the high-affinity receptor but only in the presence of the alpha chain. A potential GM-CSF-binding determinant was identified in the second hemopoietin domain of beta(c), and the role of individual residues within this region was investigated by determining the ability of mutated beta(c) chains to confer high-affinity binding when coexpressed with the a subunit of the GM-CSF receptor in COS cells. Substitutions involving Met-363, Arg-364, Tyr-365, and Glu-366 did not affect high-affinity binding. However, substitution of His-367 by lysine or glutamine abolished high-affinity binding, suggesting that this residue may form an important part of the high-affinity GM-CSF-binding determinant. Consistent with the loss of high-affinity binding, higher concentrations of human GM-CSF were required to stimulate proliferation of CTLL-2 cell lines transfected with cDNAs for GM-CSF receptor alpha chain and His-367 beta(c) mutant than those expressing GM-CSF receptor alpha subunit and beta(c), wild type.