Preservation of mitochondrial structure and function after Bid- or Bax-mediated cytochrome c release

Proapoptotic members of the Bcl-2 protein family including Bid and Bax, can activate apoptosis by directly interacting with mitochondria to cause cyto chrome c translocation from the intermembrane space into the cytoplasm, the reby triggering Apaf-1-mediated caspase activation. Under some circumstance s, when caspase activation is blocked, cells can recover from cytochrome c translocation; this suggests that apoptotic mitochondria may not always suf fer catastrophic damage arising from the process of cytochrome c release. W e now show that recombinant Bid and Bax cause complete cytochrome c loss fr om isolated mitochondria in vitro, but preserve the ultrastructure and prot ein import function of mitochondria, which depend on inner membrane polariz ation. We also demonstrate that, if caspases are inhibited, mitochondrial p rotein import function is retained in UV-irradiated or staurosporine-treate d cells, despite the complete translocation of cytochrome c, Thus, Bid and Bax act only on the outer membrane, and lesions in the inner membrane occur ring during apoptosis are shown to be secondary caspase-dependent events.