Dynamics and structure of the Bax-Bak complex responsible for releasing mitochondrial proteins during apoptosis

Bax and Bak are known to play a central role in facilitating the release of mitochondrial intermembrane proteins during apoptosis. The detailed mechanism, however, is still not clear. Using live cell imaging techniques, we showed here that Bax underwent four distinct stages of dynamic redistribution during UV-induced apoptosis. At stage I, Bax was distributed diffusely in the cytosol. About an hour after UV treatment at stage II, Bax started to translocate to mitochondria and distributed uniformly at the mitochondrial outer membrane (MOM). Within a few minutes, at stage III, Bax and Bak began to form small complexes at the MOM. Later, at stage IV, these Bax and Bak complexes expanded to become large clusters. We found that the formation of Bax-Bak small complexes at stage III was responsible for permeabilizing the MOM to release cytochrome c and Smac. Using a FRET technique, we further showed that Bax binds to Bak within the complex formed at the MOM during stage III. Finally, using a quantitative fluorescence measurement, we determined that the Bax-Bak complex was about 0.25 µm wide and composed of more than 100 protein molecules. These findings suggest that the Bax-Bak structure responsible for releasing mitochondrial proteins during apoptosis is not channel-like.