IDENTIFICATION OF 3 ADJACENT AMINO-ACIDS OF INTERLEUKIN-2 RECEPTOR BETA-CHAIN WHICH CONTROL THE AFFINITY AND THE SPECIFICITY OF THE INTERACTION WITH INTERLEUKIN-2

The beta-chain of the interleukin-2 (IL-2) receptor (IL-2R-beta) and the interleukin-3 (IL-3) binding protein AIC2A are members of the family of cytokine receptors, which also includes the receptors for growth hormone (GHR) and prolactin. A four amino acid sequence of AIC2A has recently been shown to be critical for IL-3 binding. We analyze here the function of the analogous sequence of human IL-2R-beta and identify three amino acids, Ser132, His133 and Tyrl34, which play a critical role in IL-2 binding. We show that some mutant IL-2 proteins with substitutions of a critical Asp residue in the N-terminal alpha-helix bind the mutant IL-2R-beta receptor with a higher affinity than the wild-type receptor. This suggests that the critical Asp34 in the ligand and the sequence Ser-His-Tyr (positions 132-134) in the receptor interact directly. On the double barrel-beta-stranded structural model of cytokine receptors, the residues important for ligand binding in IL-2R-beta, AIC2A and GHR map to strikingly similar locations within a barrel, with the interesting difference that it is the N-terminal barrel for GHR and the C-terminal barrel for IL-2R-beta and AIC2A.