Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), existing as homotrimer in solution, contains a unique zinc-binding site coordinated by three Cys230 residues at the tip of trimeric interface. TRAIL mutant with replacements of Cys230 with Ala (TRAIL(C230A)) negligibly formed trimeric structure and showed no apoptotic activity. Here, to elucidate the relationship between the trimeric stability and the apoptotic activity of TRAILC230A, we rationally designed mutations to induce homotrimerization of TRAILC230A by substituting for the three residues involved in hydrogen bonding (Tyr(183) and Tyr(243)) and putative repulsive electrostatic (Arg227) interactions at the buried trimeric interface into hydrophobic residues, like Y183F, Y243F, and R227I. The TRAIL(C23OA)-derived mutants exhibited enhanced hornotrimerization, but only the mutants containing R227I exhibited significant apoptosis-inducing activity in cancer cells. These results, together with the induction of rigid local structure around the zinc-binding region by R227I in TRAILC230A, suggest that ordered, rigid structure around the zinc-binding region is critical for the hornotrimerization and apoptotic activity of TRAIL. (c) 2007 Elsevier Inc. All rights reserved.