The C-terminal helix of BcI-xL mediates Bax retrotranslocation from the mitochondria

The proapoptotic BcI-2 protein Bax can commit a cell to apoptosis by translocation from the cytosol to the mitochondria and permeabilization of the outer mitochondrial membrane. Prosurvival BcI-2 family members, such as BcI-x(L), control Bax activity. BcI-x(L) recognizes Bax after a conformational change in the N-terminal segment of Bax on the mitochondria and retrotranslocates it back into the cytoplasm, stabilizing the inactive form of Bax. Here we show that Bax retrotranslocation depends on the C-terminal helix of BcI-X-L. Deletion or substitution of this segment reduces Bax retrotranslocation and correlates with the accumulation of GFP-tagged or endogenous Bax on the mitochondria of non-apoptotic cells. Unexpectedly, the substitution of the BcI-x(L) membrane anchor by the corresponding Bax segment reverses the Bax retrotranslocation activity of BcI-x(L), but not that of BcI-x(L) shuttling. Bax retrotranslocation depends on interaction to the BcI-x(L) membrane anchor and interaction between the Bax BH3 domain and the BcI-x(L) hydrophobic cleft. Interference with either interaction increases mitochondrial levels of endogenous Bax. In healthy cells, mitochondrial Bax does not permeabilize the outer mitochondrial membrane, but increases cell death after apoptosis induction. Cell Death and Differentiation (2013) 20, 333-342; doi:10.1038/cdd.2012.131; published online 19 October 2012