The study investigates the mechanism by which Bax, a pro-apoptotic member of the Bcl-2 protein family, induces cytochrome c (Cyt c) release from mitochondria. The researchers found that submicromolar amounts of recombinant Bax protein added to isolated mitochondria can induce Cyt c release, while a peptide representing the Bax BH3 domain was inactive. When Bax and mitochondria were placed together in purified cytosol, neither alone induced caspase activation, but their combination triggered Cyt c release and caspase activation. Recombinant Bcl-XL protein prevented Bax-induced Cyt c release and caspase activation, suggesting that Bax may directly induce Cyt c release without the need for caspases. Additionally, Bax did not induce mitochondrial swelling, indicating that its mechanism of action is different from that of Ca2+, which induces mitochondrial permeability transition. The findings suggest that Bax may directly induce Cyt c release through a pore-forming mechanism or by altering mitochondrial membrane permeability, rather than by triggering mitochondrial swelling and subsequent rupture.The study investigates the mechanism by which Bax, a pro-apoptotic member of the Bcl-2 protein family, induces cytochrome c (Cyt c) release from mitochondria. The researchers found that submicromolar amounts of recombinant Bax protein added to isolated mitochondria can induce Cyt c release, while a peptide representing the Bax BH3 domain was inactive. When Bax and mitochondria were placed together in purified cytosol, neither alone induced caspase activation, but their combination triggered Cyt c release and caspase activation. Recombinant Bcl-XL protein prevented Bax-induced Cyt c release and caspase activation, suggesting that Bax may directly induce Cyt c release without the need for caspases. Additionally, Bax did not induce mitochondrial swelling, indicating that its mechanism of action is different from that of Ca2+, which induces mitochondrial permeability transition. The findings suggest that Bax may directly induce Cyt c release through a pore-forming mechanism or by altering mitochondrial membrane permeability, rather than by triggering mitochondrial swelling and subsequent rupture.