The pro-apoptotic proteins, Bid and Bax, cause a limited permeabilization of the mitochondrial outer membrane that is enhanced by cytosol

During apoptosis, an important pathway leading to caspase activation involv es the release of cytochrome c from the intermembrane space of mitochondria , Using a cell-free system based on Xenopus egg extracts, we examined chang es in the outer mitochondrial membrane accompanying cytochrome c efflux. Th e pro-apoptotic proteins, Bid and Bar, as well as factors present in Xenopu s egg cytosol, each induced cytochrome c release when incubated with isolat ed mitochondria, These factors caused a permeabilization of the outer membr ane that allowed the corelease of multiple intermembrane space proteins: cy tochrome c, adenylate kinase and sulfite oxidase. The efflux process is thu s nonspecific. None of the cytochrome c-releasing factors caused detectable mitochondrial swelling, arguing that matrix swelling is not required for o uter membrane permeability in this system. Bid and Bar caused complete rele ase of cytochrome c but only a limited permeabilization of the outer membra ne, as measured by the accessibility of inner membrane-associated respirato ry complexes III and IV to exogenously added cytochrome c. However, outer m embrane permeability was strikingly increased by a macromolecular cytosolic factor, termed PEF (permeability enhancing factor). We hypothesize that PE F activity could help determine whether cells can recover from mitochondria l cytochrome c release.