The Point of No Return; 3D Structure of Bax-Mediated Pores in Membrane Bilayers

Abstract

Bax is a protein that is responsible for the decisive point-of-no-return event in cell death. Bax is normally monomeric, but it transforms into its lethal oligomeric form in response to cell stress. To map this transformation on the molecular level, we used nanometer-scale phospholipid bilayer islands (nanodiscs), each able to accommodate only a single BAX molecule. Using this minimal reconstituted system, we captured structural correlates that precede Bax homo-oligomerization elucidating previously inaccessible steps of the core molecular mechanism by which Bcl-2 family proteins regulate membrane permeabilization. The three-dimensional reconstructions of these membrane assemblies demonstrate that in the presence of a BID BH3 peptide, an individual BAX molecule can insert into the nanodisc membrane, distort the lipid bilayer, and form a pore. This implies that monomeric membrane-inserted BAX is the key functional unit responsible for initiating mitochondrial outer membrane pore formation and destabilization, leading to apoptosis and cell death. This study provides a new set of tools and previously not accessible data to gain new understanding of the molecular basis of mitochrondrial permeabilization.