MAPPING RECEPTOR-BINDING SITES IN INTERLEUKIN (IL)-1 RECEPTOR ANTAGONIST AND IL-1-BETA BY SITE-DIRECTED MUTAGENESIS - IDENTIFICATION OF A SINGLE-SITE IN IL-1RA AND 2 SITES IN IL-1-BETA

Interleukin-l receptor antagonist (IL-1ra), an IL-1 family member, bin ds with high affinity to the type I IL-1 receptor (IL-1RI), blocking I L-1 binding but not inducing an IL-l-like response. Extensive site-dir ected mutagenesis has been used to identify residues in IL-1ra and IL- 1 beta involved in binding to IL-1RI. These analyses have revealed the presence of two discrete receptor binding sites on IL-1 beta. Only on e of these sites is present on IL-1ra, consisting of residues Trp-16, Gln-20, Tyr-34, Gln-36, and Tyr-147. Interestingly, the absent second site is at the location of the major structural difference between IL- 1 beta and IL-1 beta, which are otherwise structurally similar. The tw o receptor binding sites on IL-1 beta are also present on IL-1 alpha. Thus, it appears that the two IL-1 agonist molecules have two sites fo r IL-1RI binding, and the homologous antagonist molecule, IL-1ra, has only one. Based on these observations, a hypothesis is presented to ac count for the difference in activity between the agonist and antagonis t proteins. It is proposed that the presence of the two receptor bindi ng sites may be necessary for agonist activity.