Bax mitochondrial residency is more critical than Bax oligomerization for apoptosis

Abstract

The Bax protein plays an important effector role in apoptosis by forming pores in the mitochondrial outer membrane. While doing so, Bax forms higher-order oligomers in the membrane, but it remains unclear whether this oligomer formation is essential for pore formation. Using cell-free and cellular experimental systems, we investigated two Bax C-terminus mutants, T182I and G179P. Neither mutant formed large oligomers when activated in liposomes. Nevertheless, the G179P mutant could produce membrane pores, suggesting that large oligomers are not required for permeabilization. Surprisingly, however, when G179P was transduced into Bax/Bak double knockout mouse embryonic fibroblasts, it was purely cytoplasmic and failed to mediate cell death. T182I behaved in the opposite manner. When mixed with liposomes, T182I was inefficient in both membrane insertion and permeabilization. However, transduced into cells, BaxT182I resided constitutively in mitochondria, owing to its slow retrotranslocation and mediated apoptosis as efficiently as wild-type Bax. We conclude that Baxs mitochondrial residence (regulated by targeting and retrotranslocation) is more important for apoptosis than its efficiency of membrane insertion and higher-order oligomerization.