Association of human tumor necrosis factor-related apoptosis inducing ligand with membrane upon acidification.

Tumor necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL) has been known to induce tumor-specific apoptosis and to share the structural and functional characteristics with the proteins of TNF family. Recently, the crystal structure of human TRAIL showed that TRAIL is a homotrimeric protein whose subunits contain mainly beta-sheets. We characterized the structural changes of recombinant human TRAIL induced by acidification and the biological implication of the structural characteristics at acidic pH in the interaction with the lipid bilayer. At acidic pH below pH 4.5, TRAIL resulted in substantial structural changes to a molten globule (MG)-like state. Far-UV CD spectrum of TRAIL indicated that the acidification induced alpha-helices that are absent in the native state. TRAIL at acidic pH exhibited significant change of tertiary structures as reflected in the near-UV CD spectrum. Thermal transition curve indicated that there was less cooperation at acidic pH than at neutral pH in the thermal denaturation of TRAIL. Moreover, TRAIL at the MG-like state not only enhanced the binding ability to liposomes, but also increased the release rate of a fluorescent dye, calcein, encapsulated in liposomes. The binding assay with anilinonaphthalene-8-sulfonic acid revealed that the surface hydrophobicity of TRAIL was increased while tryptophan residues became more exposed to solvent as judged by blue shift of the maximum fluorescence wavelength. Taken together, our results demonstrate that the acidification of human TRAIL induces the MG-like state in vitro and makes the membrane permeable through the favorable interaction of TRAIL with the membrane, implicating that general intrinsic properties such as TRAIL, TNF-alpha and lymphotoxin are shared by TNF family members.