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.