Pro-apoptotic cleavage products of Bcl-x(L) form cytochrome c-conducting pores in pure lipid membranes

During apoptotic cell death, cells usually release apoptogenic proteins suc h as cytochrome c from the mitochondrial intermembrane space. If Bcl-2 fami ly proteins induce such release by increasing outer mitochondrial membrane permeability, then the pro-apoptotic, but not anti-apoptotic activity of th ese proteins should correlate with their permeabilization of membranes to c ytochrome c. Here, we tested this hypothesis using pro-survival full-length Bcl-x(L) and pro-death Bcl-x(L) cleavage products (Delta N612Bcl-x(L) and Delta N76Bcl-x(L)). Unlike Bcl-x(L), Delta N61Bcl-x(L) and Delta N76Bcl-x(L ) caused the release of cytochrome c from mitochondria in vivo and in vitro . Recombinant Delta N61Bcl-x(L) and Delta N76Bcl-x(L), as well as Bcl-xL, c leaved in situ by caspase 3-possessed intrinsic pore-forming activity as de monstrated by their ability to efficiently permeabilize pure lipid vesicles . Furthermore, only Delta N61Bcl-x(L) and Delta N76Bcl-x(L), but not Bcl-x( L), formed pores large enough to release cytochrome c and to destabilize pl anar lipid bilayer membranes through reduction of pore line tension. Becaus e Bcl-x(L) and its C-terminal cleavage products bound similarly to lipid me mbranes and formed oligomers of the same size, neither lipid affinity nor p rotein-protein interactions appear to be solely responsible for the increas ed membrane-perturbing activity elicited by Bcl-x(L) cleavage. Taken togeth er, these data are consistent with the hypothesis that Bax-like proteins ol igomerize to form lipid-containing pores in the outer mitochondrial membran e, thereby releasing intermembrane apoptogenic factors into the cytosol.