Recent evidence suggests that Bax induces apoptosis by moving from the cytosol to the mitochondrial membrane, independent of Bcl-2 or Bcl-X. Subcellular fractionation revealed that Bax is in the cytosol of non-apoptotic cells and in the membrane fraction of apoptotic cells.^{1, 2} In live cells, a green fluorescent protein-Bax chimera (GFP-Bax)³ was in the cytosol of healthy cells but on the mitochondrial membrane after induction of apoptosis. Relocation occurred early (within 2 hours after treatment with staurosporin or gamma-irradiation) and was followed by cell shrinkage (15-30 m after relocation) and nuclear fragmentation (~10 hours after relocation). Relocation required the mitochondrial-targeting C-terminus of Bax; this sequence appears to be masked in non-apoptotic cells and revealed during apoptosis.

Figure 1. Bax moves from the cytosol to the mitochondria before cell shrinkage and nuclear fragmentation. a. Induction of apoptosis with staurosporin or gamma-irradiation; b. movement of bax from the cytosol to the membranes; c. cell shrinkage; d. nuclear fragmentation.

Some confusion about the oligomerization and subcellular distribution of Bax appears to result from the effects of detergents on conformational changes.^{1, 4} Non-ionic detergents expose Bax determinants that mediate dimerization with Bcl-2 and Bcl-X. The zwitterionic detergent, CHAPS, did not expose these determinants. Thus, the association of Bax with other Bcl-2 family members and its subcellular distribution can be altered by detergents used to solubilize or permeabilize cells.

Supporting the interpretation that Bax may not bind to Bcl-2 or Bcl-X in live cells was the finding that the amount of Bax relocating to the mitochondria was independent of the amount of Bcl-X and/or Bcl-2 present in the mitochondrial membrane.³ In addition, Bax can induce apoptosis when its BH3 domain, the domain required for dimerization with Bcl-X, is deleted.⁵ Taken together these results suggest that Bax acts alone by inserting into the mitochondrial membrane during apoptosis.

The results raise questions about the oligomerization state of other Bcl-2 family members and about the possibility that other pro-apoptotic Bcl-2 family members change their intracellular location during the induction of apoptosis.

References

1. Hsu, Y-T. and R.J. Youle (1998) J. Biol. Chem. 273:10777.
2. Hsu, Y-T. et al. (1997) PNAS USA 94:3668.
3. Wolter, K.G. et al. (1997) J. Cell Biol. 139:1281.
4. Hsu, Y-T. and R.J. Youle (1997) J. Biol. Chem. 272:13829.
5. Simonian, P.L. et al. (1996) J. Biol. Chem. 271:22764.