The multidomain proapoptotic molecules BAK or BAX are required to initiate the mitochondrial pathway of apoptosis. How cells maintain the potentially lethal proapoptotic effector BAK in a monomeric inactive conformation at mitochondria is unknown. In viable cells, we found BAK complexed with mitochondrial outer-membrane protein VDAC2, a VDAC isoform present in low abundance that interacts specifically with the inactive conformer of BAK. Cells deficient in VDAC2, but not cells lacking the more abundant VDAC1, exhibited enhanced BAK oligomerization and were more susceptible to apoptotic death. Conversely, overexpression of VDAC2 selectively prevented BAK activation and inhibited the mitochondrial apoptotic pathway. Death signals activate "BH3-only" molecules such as tBID, BIM, or BAD, which displace VDAC2 from BAK, enabling homo-oligomerization of BAK and apoptosis. Thus, VDAC2, an isoform restricted to mammals, regulates the activity of BAK and provides a connection between mitochondrial physiology and the core apoptotic pathway.

Voltage-dependent anion channels (VDACs) have been implicated as essential mediators of mitochondrial-dependent cell death by functioning as a channel-forming unit within the mitochondrial permeability transition (MPT) pore and the target of Bcl-2 family members. Here we report the effects of deletion of the 3 mammalian Vdac genes on mitochondrial-dependent cell death. Mitochondria from Vdac1-, Vdac3-, and Vdac1/Vdac3-null mice exhibited a Ca 2+ and oxidative stress-induced MPT that was indistinguishable from wildtype mitochondria. Similarly, Ca 2+ and oxidative-stress-induced MPT and cell death was unaltered or even exacerbated in fibroblasts lacking VDAC1, VDAC2, VDAC3, VDAC1/3, and VDAC1/2/3. Wildtype and Vdac-deficient mitochondria and cells also exhibited equivalent cytochrome c release, caspase cleavage, and cell death in response to Bax and Bid activation. These results indicate that VDACs are dispensable for both MPT and Bcl-2 family member-driven cell death.Mitochondria are intracellular organelles that mediate high-energy phosphate production, fatty acid metabolism, porphyrin synthesis, ion homeostasis and apoptotic and necrotic cell death. Apoptotic cell death is mediated by both the "extrinsic" pathway; consisting of death receptor signaling constituents, as well as the "intrinsic" pathway; consisting of pro-death Bcl-2 family members functioning at the level of the mitochondria and endoplasmic reticulum (1). Mitochondria are also critically involved in necrotic cell death following Ca 2+ overload, hypoxia, and oxidative damage, leading to swollen or ruptured mitochondria. The MPT pore, a protein complex that spans both the outer and inner mitochondrial membranes, is considered the mediator of this event and has been hypothesized to minimally consist of the VDAC in the outer membrane, the adenine nucleotide translocase (ANT) in the inner membrane, and cyclophilin-D in the matrix (2-4).The VDAC is comprised of a family of evolutionarily conserved ion channels that are the most abundant proteins in the outer mitochondrial membrane. The physiologic function of VDACs is to control the movement of adenine nucleotides, NADH, and other metabolites across the outer membrane (5,6). However, VDACs have also been proposed to possess a pathological function as mediators of mitochondrial-dependent cell death through formation of the permeability pore (7,8). In addition, VDACs have been proposed to be essential binding partners for pro-apoptotic Bcl-2 family members (9-12), combining to form protein-permeable Here we assessed whether MPT was altered in Vdac1-, Vdac3-, and Vdac1/Vdac3-null mice. Western blotting of cardiac lysates from these mice showed the complete lack of the respective VDAC protein in each line without compensatory alterations in the other VDAC isoforms (Fig. 1a). There were also no significant changes in ANT and cyclophilin D, two other putative components of the MPT pore (Fig. 1a). Cardiac mitochondria isolated from wildtype, Vdac1-, Vdac3-, and Vdac1/Vdac3-null mice were assessed for...