A novel functional epitope formed by domains 1 and 4 of the human common β-subunit is involved in receptor activation by granulocyte macrophage colony-stimulating factor and interleukin 5

Abstract

The receptors for human interleukins 3 and 5 and granulocyte macrophage colony-stimulating factor are composed of ligand-specific α-subunits and a common β-subunit (βc), the major signaling entity. The way in which βc interacts with ligands in the respective activation complexes has remained poorly understood. The recently determined crystal structure of the extracellular domain of βc revealed a possible ligand-binding interface composed of domain 1 of one chain of the βc dimer and the adjacent domain 4 of the symmetry-related chain. We have used site-directed mutagenesis, in conjunction with ligand binding and proliferation studies, to demonstrate the critical requirement of the domain 1 residues, Tyr15 (A-B loop) and Phe79 (E-F loop), in high affinity complex formation and receptor activation. The novel ligand-receptor interface formed between domains 1 and 4 represents the first example of a class I cytokine receptor interface to be composed of two noncontiguous fibronectin III domains.