During apoptosis, engagement of the mitochondrial pathway involves the permeabilization of the outer mitochondrial membrane (OMM), which leads to the release of cytochrome c and other apoptogenic proteins such as Smac/DIABLO, AIF, EndoG, Omi/HtraA2 and DDP/TIMM8a. OMM permeabilization depends on activation, translocation and oligomerization of multidomain Bcl-2 family proteins such as Bax or Bak. Factors involved in Bax conformational change and the function(s) of the distinct domains controlling the addressing and the insertion of Bax into mitochondria are described in this review. We also discuss our current knowledge on Bax oligomerization and on the molecular mechanisms underlying the different models accounting for OMM permeabilization during apoptosis.

The association of Bax with mitochondria is an essential step in the implementation of apoptosis. By using a bacterial two-hybrid assay and crosslinking strategies, we have identified TOM22, a component of the translocase of the outer mitochondrial membrane (TOM), as a mitochondrial receptor of Bax. Peptide mapping showed that the interaction of Bax with TOM22 involved the first alpha helix of Bax and possibly two central alpha helices, which are homologous to the pore forming domains of some toxins. Antibodies directed against TOM22 or an antisense knockdown of the expression of TOM22 specifically inhibited the association of Bax with mitochondria and prevented Bax-dependent apoptosis. In yeast, a haploid strain for TOM22 exhibited a decreased expression of TOM22 and mitochondrial association of ectopically expressed human Bax. Our data provide a new perspective on the mechanism of association of Bax with mitochondria as it involves a classical import pathway. Apoptosis is a cell death program, which is central in many aspects to metazoan cell physiology and pathology. 1 Proteins of the Bcl-2 family are key players in the execution phase of apoptosis and their main functions is to control the release of apoptogenic proteins, such as cytochrome c (cyt c) and apoptosis inducing factor (AIF) from the mitochondria. 2 The Bcl-2 family is composed of antiapoptotic members such as Bcl-2 and proapoptotic proteins such as Bax or Bak, which share several domains of homology called BH. 1 In addition to these proteins, a third Bcl-2-related family of proteins have been identified as major amplifiers and/inducers of apoptosis. 2 The latter family has a homology to Bcl-2 limited to the BH3 domain and thus is called the BH3 only proteins. 2 The double knockout of Bax and Bak renders cells completely resistant to cell death 2 highlighting the essential role of these proteins during apoptosis.The determination of the solution structure of Bax showed its organization around nine a-helices (Ha1 to Ha9) 3 of which Ha2 contained most of the BH3 domain and Ha5 and Ha6, a putative pore forming domain that has been shown to be involved in the integration of Bax into the membrane. 4,5 Bax resides in an inactive state in the cytosol in many resting cells and is translocated to the mitochondria at the onset of apoptosis. 6 This translocation is associated with major conformational changes in Bax as both the amino-terminal end (N-T) and carboxy terminal end (C-T) become exposed facilitating its mitochondrial addressing and membrane insertion. 5,7 Bax mitochondrial membrane insertion and oligomerization is closely associated with the release into the cytosol of several intermembrane space proteins such as cyt c, second mitochondrial apoptotic factor (Smac) and AIF. 2 The nature of the different stimuli involved in the activation or in the inhibition of Bax is still largely unknown although this step remains one of the most critical points of control of the apoptotic program during which therapeutic interventions can be envisaged. 2 The mit...

It is still unclear whether the BH3-only protein Puma (p53 up-regulated modulator of apoptosis) can prime cells to death and render antiapoptotic BH3-binding Bcl-2 homologues necessary for survival through its ability to directly interact with proapoptotic Bax and activate it. In this study, we provide further evidence, using cell-free assays, that the BH3 domain of Puma binds Bax at an activation site that comprises the first helix of Bax. We also show that, in yeast, Puma interacts with Bax and triggers its killing activity when Bcl-2 homologues are absent but not when Bcl-xL is expressed. Finally, endogenous Puma is involved in the apoptotic response of human colorectal cancer cells to the Bcl-2/Bcl-xL inhibitor ABT-737, even in conditions where the expression of Mcl-1 is down-regulated. Thus, Puma is competent to trigger Bax activity by itself, thereby promoting cellular dependence on prosurvival Bcl-2 family members.