Long-range Coupling Between Separate Docking Sites in Interleukin-1β

The human cytokine interleukin-1β (IL-1β) interacts with the interleukin type I receptor using two large docking surfaces designated A and B. Crystallographic studies reveal that a single histidine residue (His30) in IL-1β makes critical electrostatic interactions at the receptor/ligand interface. To study the function of this residue at site A, four mutant forms of IL-1β (H30A, H30D, H30F and H30R) were investigated. The mutation that introduces charge repulsion at His30 destabilizes the protein, but paradoxically causes the least effect on receptor binding (H30D). Mutations that enhance hydrophobic or electrostatic interactions have little effect on protein stability yet markedly impair receptor binding (H30F, H30R). All mutations can transmit effects from site A to site B, as evidenced by changes in the binding of a single-chain antibody highly specific for site B. Dihedral scalar coupling constants for the wild-type IL-1β and the four His mutant proteins showed changes in backbone angles in residues located around site B, some ∼30 A away from His30 in site A. A comparison of native solvent exchange in wild-type and mutated IL-1β shows transmission of local destabilization along the hydrogen bond network of the β-sheet. Taken together, the data indicate that a single residue in site A of IL-1β can impact stability and function through perturbations in both local and long-range contacts.