Bid acts on the permeability transition pore complex to induce apoptosis

Similar to most if not all pro-apoptotic members of the Bcl-2 family, Bid l and its truncated product t-Bid) triggers cell death via mitochondrial memb rane permeabilization (MMP). This effect can be monitored in intact cells, upon microinjection of recombinant Bid protein into the cytoplasm, as well as in purified mitochondria, upon addition of Bid protein. Here we show tha t Bid-induced MMP can be inhibited, both in cells and in the cell-free syst em, by three pharmacological inhibitors of the permeability transiton pore complex (PTPC), namely cyclosporin A, N-methyl-4-Val-cyclosporin A, and bon gkrekic acid (a ligand of the adenine nucleotide translocase, ANT, one of t he PTPC components). Bid effects on synthetic membranes were studied either in proteoliposomes or in synthetic bilayers subjected to electrophysiologi cal measurements. Full length Bid preferentially permeabilizes membranes an d induces the formation of large conductance channels at neutral pH, when a dded to liposomes or bilayers containing both purified ANT and Bax, yet has no or little effect combined with ANT or Bax alone. t-Bid acts on membrane s containing ANT alone with the same efficiency as on those containing both ANT and Bax. These results suggest that the proapoptotic effects of Bid ar e mediated, at least in part, by its functional interaction with ANT, one o f the major components of PTPC.