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We showed previously that replacement of Lys-145 in the IL-1 receptor antagonist (IL-1ra) with Asp resulted in an analog (IL-1ra K145D) with partial agonist activity. To identify additional amino acids that affect IL-1 bioactivity, we created second site mutations in IL-1ra K145D. Substitutions of single amino acids surrounding position 145 were made; none of these substitutions increased the bioactivity of IL-1ra K145D. However, the insertion of the ␤-bulge (QGEESN) of IL-1␤ at the corresponding region of IL-1ra K145D resulted in a 3-4-fold augmentation of bioactivity. An additional increase in agonist activity was observed when the ␤-bulge was coexpressed with a second substitution (His-54 3 Pro) in IL-1ra K145D. We also show that the bioactivity of both IL-1ra K145D and the triple mutant IL-1ra K145D/H54P/ QGEESN is dependent on interaction with the newly cloned IL-1 receptor accessory protein.

Using oligonucleotide-directed mutagenesis, the binding site on human interleukin-1 alpha (IL-1 alpha) for the human type I IL-1 receptor (IL-1R) has been analyzed. Substitution of seven amino acids (Arg12, Ile14, Asp60, Asp61, Ile64, Lys96 and Trp109) resulted in a significant loss of binding to the receptor. Based on crystallographic information, the side chains of these residues are clustered in one region of IL-1 alpha and exposed on the surface of the protein. Five of the residues in the IL-1 alpha binding site align with the binding residues previously determined in human IL-1 beta, demonstrating that the type I IL-1R recognizes homologous regions in both ligands. Unexpectedly, only three of the aligned residues are identical between IL-1 alpha and IL-1 beta. These observations suggest that the composition of contact residues in the binding site is unique for each ligand-receptor complex in the IL-1 system.

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