Inhibition of Oligomeric BAX by an Anti-Apoptotic Dimer of BCL-w

Abstract

Apoptosis, or programmed cell death, is governed by a complex balance of interactions between BCL-2 family proteins and culminates in mitochondrial outer membrane permeabilization. BAX is a pro-apoptotic executioner of the BCL-2 family and resides in the cytosol as a monomer until triggered by cellular stress to form an oligomer that permeabilizes mitochondria and induces apoptosis. The canonical paradigm for apoptotic blockade involves heterodimeric interaction between the -helical BH3 “killer” motif of pro-apoptotic monomers and a conserved, hydrophobic groove on anti-apoptotic monomers. A prevailing question in the field is how proteins of such homologous structure can achieve strikingly opposite functions. Here, I report that full-length anti-apoptotic BCL-w forms a distinctive, symmetric dimer (BCL-wD) with unique functions beyond those of monomeric BCL-w (BCL-wM). Specifically, BCL-wD can dissociate oligomeric BAX (BAXO), inhibit its mitochondrial translocation, promote retrotranslocation, block membrane-porating activity, and thereby influence apoptosis induction of cells. Structure-function analyses revealed discrete conformational changes upon BCL-w dimerization and reciprocal structural impacts upon BCL-wD and BAXO interaction. Beyond protein-protein interactions, SAXS analysis demonstrated that BAXO disrupts membranes by inducing negative Gaussian curvature, which is reversed by positive Gaussian curvature exerted by BCL-wD. Systematic truncation and mutagenesis dissected the core features of BCL-wD activity: dimerization relies on , BAXO engagement is mediated by non-canonical, reciprocal interactions, and membrane interaction is driven by discrete acidic residues contained within the  hairpin. These findings define a downstream layer of apoptotic control mediated by protein and membrane interactions of higher-order BCL-2 family multimers and inform novel opportunities to modulate apoptosis for therapeutic benefit.