Bax monomers form dimer units in the membrane that further self-assemble into multiple oligomeric species

Abstract

Bax is a key regulator of apoptosis that mediates the release of cytochrome c to the cytosol via oligomerization in the outer mitochondrial membrane before pore formation. However, the molecular mechanism of Bax assembly and regulation by other Bcl-2 members remains obscure. Here, by analysing the stoichiometry of Bax oligomers at the single-molecule level, we find that Bax binds to the membrane in a monomeric state and then self-assembles in <1 min. Strikingly, active Bax does not exist in a unique oligomeric state, but as several different species based on dimer units. Moreover, we show that cBid activates Bax without affecting its assembly, while Bcl-xL induces the dissociation of Bax oligomers. On the basis of our experimental data and theoretical modelling, we propose a new mechanism for the molecular pathway of Bax assembly to form the apoptotic pore.

Abstract

The proapoptotic protein Bax triggers cell death by forming pores in the outer mitochondrial membrane. Using single-particle TIRF imaging, the authors show that Bax binds the membrane in a monomeric state before forming dimers and multimers of dimers, which are disassembled by the survival protein Bcl-xL.