BAX activation is initiated at a novel interaction site

BAX is a pro- apoptotic protein of the BCL- 2 family that is stationed in the cytosol until activated by a diversity of stress stimuli to induce cell death. Anti- apoptotic proteins such as BCL- 2 counteract BAX- mediated cell death. Although an interaction site that confers survival functionality has been defined for anti- apoptotic proteins, an activation site has not been identified for BAX, rendering its explicit trigger mechanism unknown. We previously developed stabilized alpha- helix of BCL- 2 domains ( SAHBs) that directly initiate BAX- mediated mitochondrial apoptosis. Here we demonstrate by NMR analysis that BIM SAHB binds BAX at an interaction site that is distinct from the canonical binding groove characterized for anti- apoptotic proteins. The specificity of the human BIM- SAHB - BAX interaction is highlighted by point mutagenesis that disrupts functional activity, confirming that BAX activation is initiated at this novel structural location. Thus, we have now defined a BAX interaction site for direct activation, establishing a new target for therapeutic modulation of apoptosis.