Identification of key charged residues of human interleukin-5 in receptor binding and cellular activation

Interleukin-5 (IL-5) is a cytokine that plays a major role in the differentiation and activation of eosinophils. In order to identify which charged residues of human IL-5 are important in binding to its receptor and subsequent cellular activation, we have systematically replaced all of the clusters of charged amino acids with alanine residues. The mutants have been expressed in Escherichia coli, renatured, and purified. They were assayed for ability to cause proliferation of the erythroleukaemic cell line TF-1 and the up-regulation of eosinophil adhesion to ICAM-1. In addition, we studied receptor binding using either immobilized recombinant IL-5 receptor alpha-chain or the alpha/beta-receptor complex expressed on TF-1 cells. The key charged residue involved in binding to the beta-chain of the receptor is Glu-12. This residue is in an identical position to those previously identified in IL-3 and granulocyte-macrophage colony-stimulating factor (GM-CSF) involved in binding to the receptor beta-chain. The alpha-chain binding site is shown to involve the side chains Arg-90 and Glu-109, located in the second beta sheet and after the end of the fourth helix, respectively. It is unique to IL-5 and does not occur in IL-3 or GM-CSF. Understanding the topology of the interaction of IL-5 with its receptor chains will help in the search for rationally designed antagonists of IL-5 function.