Structure of the interferon-receptor complex determined by distance constraints from double-mutant cycles and flexible docking

The pleiotropic activity of type I interferons has been attributed to the s pecific interaction of IFN with the cell-surface receptor components ifnar1 and ifnar2. To date, the structure of IFIN has been solved, but not that o f the receptor or the complex. In this study, the structure of the IFN-alph a2-ifnar2 complex was generated with a docking procedure, using nuclear Ove rhauser effect-like distance constraints obtained from double-mutant cycle experiments. The interaction free energy between 13 residues of the ligand and 11 of the receptor was measured by double-mutant cycles. Of the 100 pai rwise interactions probed, five pairs of residues were found to interact. T hese five interactions were incorporated as distance constraints into the f lexible docking program PRODOCK by using fixed and movable energy-gradient grids attached to the receptor and ligand, respectively. Multistart minimiz ation and Monte Carlo minimization docking of IFN-alpha2 onto ifnar2 conver ged to a well-defined average structure, with the five distance constraints being satisfied. Furthermore, no structural artifacts or intraloop energy strain were observed. The mutual binding sites on IFN-alpha2 and ifnar2 pre dicted from the model showed an almost complete superposition with the ones determined from mutagenesis studies. Based on this structure, differences in IFN-alpha2 versus IFN-beta binding are discussed.