Fig29.14 ((a) Crystal structure of the complex of IL-2 (purple) with its heterotrimeric receptor (turquoise). The small-molecule ligand 29.31 was discovered by screening at Roche and binds to IL-2 with micromolar affinity. To achieve this binding, the cytokine undergoes a conformational change in the surface region that accommodates the bioarylalkyne moiety (b). Without this conformational change, the ligand would experience steric repulsion on the surface of IL-2 (c, purple surface and section blow-up).