BCL-2, BCL-X-L sequester BH3 domain-only molecules preventing BAX- and BAK-mediated mitochondrial apoptosis

Critical issues in apoptosis include the importance of caspases versus orga nelle dysfunction, dominance of anti- versus proapoptotic BCL-2 members, an d whether commitment occurs upstream or downstream of mitochondria. Here, w e show cells deficient for the downstream effectors Apaf-1, Caspase-9, or C aspase-3 display only transient protection from "BH3 domain-only" molecules and die a caspase-independent death by mitochondrial dysfunction. Cells wi th an upstream defect, lacking "multidomain" BAX, BAK demonstrate long-term resistance to all BH3 domain-only members, including BAD, BIM, and NOXA. C omparison of wild-type versus mutant BCL-2, BCL-X-L indicates these antiapo ptotics sequester BH3 domain-only molecules in stable mitochondrial complex es, preventing the activation of BAX, BAK. Thus, in mammals, BH3 domain-onl y molecules activate multidomain proapoptotic members to trigger a mitochon drial pathway, which both releases cytochrome c to activate caspases and in itiates caspase-independent mitochondrial dysfunction.