Identification of the Discontinuous Binding Site in Human Interleukin 1β for the Type I Interleukin 1 Receptor

Human interleukin 1β (IL-1β) exerts its diverse biological effects by binding to specific receptors on target cells. Two types of IL-1 receptor (IL-1R) have been identified: the type I IL-1R (p80) and the type II IL-1R (p68). Using site-specific mutagenesis, we have identified the binding site on IL-1β for the murine type I IL-1R. Analogs of the IL-1β protein containing defined amino acid substitutions were produced and tested for competitive binding to the two IL-1Rs. Substitutions of the amino acids at seven positions resulted in analogs that had ≥100-fold reductions in competitive binding to the type I IL-1R, while maintaining substantial binding to the type II IL-1R. These seven amino acids (Arg-4, Leu-6, Phe-46, Ile-56, Lys-93, Lys-103, and Glu-105) are clustered in the IL-1β molecule, forming a discontinuous binding site. The side chains of all seven residues are exposed on the surface of IL-1β. The cumulative binding energies contributed by each of the residues predict a binding affinity that is consistent with the observed Kdof the wild-type protein for the type I IL-1R.