The pleiotropic activity of type I interferons has been attributed to the specific interaction of IFN with the cell-surface receptor components ifnar1 and ifnar2. To date, the structure of IFN has been solved, but not that of the receptor or the complex. In this study, the structure of the IFN-␣2-ifnar2 complex was generated with a docking procedure, using nuclear Overhauser 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 pairwise interactions probed, five pairs of residues were found to interact. These five interactions were incorporated as distance constraints into the flexible docking program PRODOCK by using fixed and movable energy-gradient grids attached to the receptor and ligand, respectively. Multistart minimization and Monte Carlo minimization docking of IFN-␣2 onto ifnar2 converged to a welldefined 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-␣2 and ifnar2 predicted from the model showed an almost complete superposition with the ones determined from mutagenesis studies. Based on this structure, differences in IFN-␣2 versus IFN- binding are discussed.protein-protein interaction ͉ PRODOCK ͉ Monte Carlo minimization ͉ grids T ype I interferons (IFNs) are a family of homologous cytokines that potently elicit an antiviral and antiproliferative state in cells. All human type I IFNs (IFN-␣, -, and -) bind to a cell surface receptor consisting of two transmembrane proteins, type I IFN receptor (ifnar) 1 (1) and ifnar2 (2), which associate upon binding. IFN binds with high affinity to ifnar2, probably recruiting ifnar1 subsequently. Type I IFNs belong to the class of helical cytokines and are built of five helices. The structures of human IFN-␣2 (3) and IFN- have been resolved (4). The receptor structures are unknown, but can be modeled by homology to human cytokine receptors with known structures such as tissue factor (5) and IFN-␥ receptor (6). Mutational studies have revealed the mutual binding sites on IFN-␣2 and ifnar2. On IFN-␣2, ifnar2 binds to the A helix (residues 12-15), the AB loop (residues 26-35), and the E helix (residues 144-153) (7), whereas on ifnar2, IFN-␣2 binds to three loops of the N-terminal domain of the receptor (residues 45-52, 75-82, and 102-106) with no significant binding detected toward the Cterminal domain (8-10). Determination of the receptor-ligand structure will significantly promote our understanding of IFN signaling at the molecular level.Docking of protein complexes, and calculating the conformational changes that occur at the binding interface, is a computational challenge. Rigid protein docking software algorithms are fast and can be used to dock two proteins for which the NMR or x-ray structures have been solved independently (11, 12). However, successful docking relies o...