Detailed analysis of the IL‐5‐IL‐5R alpha interaction: characterization of crucial residues on the ligand and the receptor

The receptor for interleukin‐5 (IL‐5) is composed of two different subunits. The IL‐5 receptor alpha (IL‐5R alpha) is required for ligand‐specific binding while association with the beta‐chain results in increased binding affinity. Murine IL‐5 (mIL‐5) has similar activity on human and murine cells, whereas human IL‐5 (hIL‐5) has marginal activity on murine cells. We found that the combined substitution of K84 and N108 on hIL‐5 by their respective murine counterpart yields a molecule which is as potent as mIL‐5 for growth stimulation of a murine cell line. Since the unidirectional species specificity is due only to the interaction with the IL‐5R alpha subunit, we have used chimeric IL‐5R alpha molecules to define regions of hIL‐5R alpha involved in species‐specific hIL‐5 ligand binding. We found that this property is largely determined by the NH2‐terminal module of hIL‐5R alpha, and detailed analysis defined D56 and to a lesser extent E58 as important for binding. Moreover, two additional residues, D55 and Y57, were identified by alanine scanning mutagenesis within the same region. Based on the observed homology between the NH2‐terminal module and the membrane proximal (WSXWS‐containing) module of hIL‐5R alpha we located this stretch of four amino acid residues (D55, D56, Y57 and E58) in the loop region that connects the C and D beta‐strands on the proposed tertiary structure of the NH2‐terminal module.