SECONDARY STRUCTURE AND TOPOLOGY OF INTERLEUKIN-1 RECEPTOR ANTAGONIST PROTEIN DETERMINED BY HETERONUCLEAR 3-DIMENSIONAL NMR-SPECTROSCOPY

Interleukin-1 (IL-1) proteins, such as IL-1-beta play a key role in immune and inflammatory responses. Interaction of these cytokines with the IL-1 receptor induces a variety of biological changes in neurologic, metabolic, hematologic, and endocrinologic systems. Interleukin- 1 receptor antagonist protein (IRAP) is a naturally occurring inhibitor of the interleukin-1 receptor. The 153-residue protein binds to the receptor with an affinity similar to that of IL-1-beta but does not elicit any physiological responses. As a first step toward understanding IRAP's mode of action, we have used multidimensional, heteronuclear NMR spectroscopy to determine the antagonist's solution secondary structure and global fold. Using a combination of 3D H-1-N-15 NOESY-HMQC and TOCSY-HMQC and 3D H-1-N-15-C-13 HNCA and HN(CO)CA experiments on uniformly N-15- or doubly C-13/N-15-enriched IRAP, we have made resonance assignments for more than 90% of the main-chain atoms. Analysis of short- and long-range NOE's indicates that IRAP is predominantly beta-sheet, with the same overall topology as IL-1-beta but with different regions of the primary sequence comprising the beta-strands. Two short helical segments also were identified. The 14% sequence identity between IL-1-beta and IRAP increases to 25% when differences in the locations of secondary structure elements in the primary sequences are taken into account. Still, numerous differences in side chains, which ultimately play a major role in receptor interaction, exist. Comparison of the structure of IRAP with that of IL-1-beta provides a basis for understanding the structure/function relationships at the molecular level that account for the drastically different physiological effects of these two proteins.