X-ray crystal structure of a small antagonist peptide bound to interleukin-1 receptor type 1

Interleukin (IL-1)alpha and IL-1 beta are important mediators of inflammati on. The binding of IL-l to interleukin-l receptor (IL-1R) type 1 is the ini tial step in IL-1 signal transduction and therefore is a tempting target fo r antiinflammatory therapeutics. To advance our understanding of IL-1R1 bin ding interactions, we have determined the structure of the extracellular do mains of IL-1R1 bound to a 21-amino acid IL-1 antagonist peptide at 3.0-Ang strom resolution. The antagonist peptide binds to the domain 1/2 junction o f the receptor, which is a conserved binding site for IL-1 beta and IL-1 re ceptor antagonist (IL-1ra). This co-crystal structure also reveals that con siderable flexibility is present in IL-1R1 because the carboxyl-terminal do main of the receptor is rotated almost 170 degrees relative to the first tw o domains of the receptor compared with the previously solved IL-1R1 ligand structures. The structure shows an unexpected binding mode for the peptide and may contribute to the design of smaller IL-1R antagonists.