A unique zinc-binding site revealed by a high-resolution X-ray structure of homotrimeric Apo2L/TRAIL

Apoptosis-inducing ligand 2 (Apo2L, also called TRAIL), a member of the tum or necrosis factor.(TNF) family, induces apoptosis in a variety of human tu mor cell lines but not in normal cells [Wiley, S. R., Schooley, K., Smolak, P. J., Din, W. S., Huang, C.-P., Nicholl, J. K., Sutherland, G. R., Smith, T. D., Rauch, C., Smith, C. A., and Goodwin, R. G. (1995) Immunity 3, 673- 682; Pitti, R. M., Marsters, S. A., Ruppert, S., Donahue, C. J., Moore, A., and Ashkenazi, A. (1996) J. Biol. Chem. 271, 12687-12690]. Here we describ e the structure of Apo2L at 1.3 Angstrom resolution and use alanine-scannin g mutagenesis to map the receptor contact regions. The structure reveals a homotrimeric protein that resembles TNF with receptor-binding epitopes at t he interface between monomers. A zinc ion is buried at the trimer interface , coordinated by the single cysteine residue of each monomer. The zinc ion is required for maintaining the native structure and stability and, hence, the biological activity of Apo2L. This is the first example of metal-depend ent oligomerization and function of a cytokine.