Organization of the Mitochondrial Apoptotic BAK Pore OLIGOMERIZATION OF THE BAK HOMODIMERS

Background: BAK and BAX permeabilize the mitochondrial membrane during apoptosis. Results: Helices 2-5 of BAK form the BH3-in-groove homodimer in the membrane, which oligomerizes by juxtaposing the carboxyl termini of 3 and 5, respectively. Conclusion: A novel 3:3, 5:5 oligomerization interface exists in the BAK oligomeric pore. Significance: These results support a model for BAX/BAK pore formation, which constitutes a key regulatory step in mitochondrial apoptosis. The multidomain pro-apoptotic Bcl-2 family proteins BAK and BAX are believed to form large oligomeric pores in the mitochondrial outer membrane during apoptosis. Formation of these pores results in the release of apoptotic factors including cytochrome c from the intermembrane space into the cytoplasm, where they initiate the cascade of events that lead to cell death. Using the site-directed spin labeling method of electron paramagnetic resonance (EPR) spectroscopy, we have determined the conformational changes that occur in BAK when the protein targets to the membrane and forms pores. The data showed that helices 1 and 6 disengage from the rest of the domain, leaving helices 2-5 as a folded unit. Helices 2-5 were shown to form a dimeric structure, which is structurally homologous to the recently reported BAX BH3-in-groove homodimer. Furthermore, the EPR data and a chemical cross-linking study demonstrated the existence of a hitherto unknown interface between BAK BH3-in-groove homodimers in the oligomeric BAK. This novel interface involves the C termini of 3 and 5 helices. The results provide further insights into the organization of the BAK oligomeric pores by the BAK homodimers during mitochondrial apoptosis, enabling the proposal of a BAK-induced lipidic pore with the topography of a worm hole.