Résumé:
Apoptosis is a tightly regulated form of programmed cell death essential for development and tissue homeostasis. A key event in this process is the permeabilization of the mitochondrial outer membrane, driven by the pro-apoptotic BCL-2 family of proteins; Bax and Bak. VDAC, the major channel of the mitochondrial outer membrane, has long been proposed to play a central role in apoptosis through interactions with BCL-2 family proteins. However, despite extensive cellular studies, the molecular basis of these interactions remains poorly understood, and direct structural and biochemical characterization of VDAC–Bax complexes is lacking. Here, we address these questions by a structure-function in vitro approach. We produced and purified the VDAC2–Bax complex in a minimal membrane system. Using complementary biochemical, biophysical, and structural approaches, we demonstrate that Bax directly forms a stable complex with VDAC2. We identify the C-terminal helix α9 as a key determinant of the interaction and show that VDAC2-associated Bax adopts a membrane-embedded, ‘primed’ conformation in which the N-terminal region and BH3 domain remain accessible. Finally, we establish the specificity of this interaction for the isoform VDAC2 over VDAC1 and propose a mechanistic framework by which VDAC2 defines a distinct intermediate in the Bax activation pathway.