BH3-only proteins are tail-anchored in the outer mitochondrial membrane and can initiate the activation of Bax

17.02.2012

Wilfling F, Weber A, Potthoff S, Vögtle FN, Meisinger C, Paschen SA, Häcker G.

Cell Death Differ. 2012;19(8):1328-36

Cell Death Differ         online article

During mitochondrial apoptosis, pro-apoptotic BH3-only proteins cause the translocation of cytosolic Bax to the outer mitochondrial membrane (OMM) where it is activated to release cytochrome c from the mitochondrial intermembrane space but the mechanism is under dispute. We show that most BH3-only proteins are mitochondrial proteins that are imported into the OMM via a C-terminal tail-anchor domain in isolated yeast mitochondria, independently of binding to anti-apoptotic Bcl-2 proteins. This C-terminal domain acted as a classical mitochondrial targeting signal and was sufficient to direct GFP to mitochondria in human cells. When expressed in mouse fibroblasts, these BH3-only proteins localised to mitochondria and were inserted in the OMM. The BH3-only proteins Bim, tBid and Puma sensitized isolated mitochondria from Bax/Bak-deficient fibroblasts to cytochrome c-release by recombinant, extramitochondrial Bax. For Bim this activity is shown to require the C-terminal targeting signal and to be independent of binding capacity to and presence of anti-apoptotic Bcl-2 proteins. Bim further enhanced Bax-dependent killing in yeast. A model is proposed where OMM-tail-anchored BH3-only proteins permit passive ‘recruitment’ and catalysis-like activation of extra-mitochondrial Bax. The recognition of C-terminal membrane-insertion of BH3-only proteins will permit the development of a more detailed concept of the initiation of mitochondrial apoptosis.