Intrinsic apoptosis is critical to prevent tumor formation and is engaged by many anti-cancer agents to eliminate tumor cells. BAX and BAK, the two essential mediators of apoptosis, are thought to be regulated through similar mechanisms and act redundantly to drive apoptotic cell death. From an unbiased genome-wide CRISPR/Cas9 screen, we identified VDAC2 (voltage-dependent anion channel 2) as important for BAX, but not BAK, to function. Genetic deletion of VDAC2 abrogated the association of BAX and BAK with mitochondrial complexes containing VDAC1, VDAC2, and VDAC3, but only inhibited BAX apoptotic function. Deleting VDAC2 phenocopied the loss of BAX in impairing both the killing of tumor cells by anti-cancer agents and the ability to suppress tumor formation. Together, our studies show that efficient BAX-mediated apoptosis depends on VDAC2, and reveal a striking difference in how BAX and BAK are functionally impacted by their interactions with VDAC2.

Graphical Abstract Highlights d BH3 binding to BAX groove essential for late but not early steps in BAX activation d Mutations identified in BAX helices 1 and 6 near a proposed second BH3 binding site d They reduced BH3 binding to this site and stabilized BAX in inactive conformations d They revealed allosteric changes controlling BAX mitochondrial membrane association SUMMARYTo elicit apoptosis, BAX metamorphoses from an inert cytosolic monomer into homo-oligomers that permeabilize the mitochondrial outer membrane (MOM). A long-standing puzzle is that BH3 domains apparently activate BAX by not only its canonical groove but also a proposed site involving helices a1 and a6. Our mutagenesis studies reveal that late steps like oligomerization require activation through the groove but probably not earlier steps like MOM association. Conversely, a1 or a6 obstruction and alanine mutagenesis scanning implicate these helices early in BAX activation. The a1 and a6 mutations lowered BH3 binding, altered the BAX conformation, and reduced its MOM translocation and integration; their exposure of the BAX a1-a2 loop allosterically sequestered its a9 membrane anchor in the groove. The crystal structure of an a6 mutant revealed additional allosteric effects. The results suggest that the a1 and a6 region drives MOM association and integration, whereas groove binding favors subsequent steps toward oligomerization.(E) Upon a death stimulus, unlike the active conformation assumed by WT BAX and K21E, the groove mutants translocate to the MOM but retain an inactive conformation. Bax/Bak DKO MEFs expressing the BAX variants were pre-incubated with caspase inhibitor Q-VD.oph (25 mM) for 1 h, then treated with 5 mM etoposide. After 16 h, the cells were permeabilized with 0.025% digitonin and treated with PK (20 min on ice) before fractionation, carbonate extraction, and immunoblotting for BAX. (F) Unlike WT and K21E BAX, the groove mutants do not expose the N-terminal 6A7 epitope upon a death stimulus. DKO MEFs expressing the human BAX variants, treated as in (E), were solubilized in 1% (3-((3-cholamidopropyl) dimethylammonio)-1 (CHAPS), and activated BAX was immunoprecipitated with antibody 6A7. All immunoblots are representative of at least two independent experiments.