BAX is a pro-apoptotic protein that transforms from a cytosolic monomer into a toxic oligomer that permeabilizes the mitochondrial outer membrane. How BAX monomers assemble into a higher-order conformation, and the structural determinants essential to membrane permeabilization, remain a mechanistic mystery. A key hurdle has been the inability to generate a homogeneous BAX oligomer (BAX_O) for analysis. Here, we report the production and characterization of a full-length BAX_O that recapitulates physiologic BAX activation. Multidisciplinary studies revealed striking conformational consequences of oligomerization and insight into the macromolecular structure of oligomeric BAX. Importantly, BAX_O enabled the assignment of specific roles to particular residues and α helices that mediate individual steps of the BAX activation pathway, including unexpected functionalities of BAX α6 and α9 in driving membrane disruption. Our results provide the first glimpse of a full-length and functional BAX_O, revealing structural requirements for the elusive execution phase of mitochondrial apoptosis.

BAX是一种促凋亡蛋白，可从胞质单体转变为可透过线粒体外膜的有毒寡聚物。BAX单体如何组装成高阶构象，以及膜通透性必不可少的结构决定因素，仍然是一个机械的谜。一个关键的障碍是无法生成均质的BAX低聚物（BAX _O）进行分析。在这里，我们报告全长BAX _O的生产和表征，概括了生理BAX激活。多学科研究揭示了低聚的惊人构象后果，并深入了解了低聚BAX的大分子结构。重要的是，BAX _O能够为介导BAX活化途径各个步骤的特定残基和α螺旋分配特定的作用，包括BAXα6和α9在驱动膜破坏中的意外功能。我们的结果提供了全长和功能性BAX _O的第一印象，揭示了线粒体凋亡难以捉摸的执行阶段的结构要求。