Apoptosis is triggered when prosurvival Bcl-2 proteins cannot restrain Bax (original) (raw)
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Mitochondria as the target of the pro-apoptotic protein Bax
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 2006
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.
FEBS Letters, 2010
Protein-protein interactions between the Bcl2 family proteins regulate apoptosis. An imbalance of this interaction network due to the upregulation of the proto-oncogene Bcl2 leads to a resistance to apoptosis associated with tumor formation. Bcl2 overexpression inhibits BAX oligomerization and mitochondrial outer membrane (MOM) permeabilization. However, Bcl2 effects on earlier steps of BAX-mediated apoptosis are not fully understood. Bcl2 overexpression inhibits BAX insertion into the MOM but spontaneously increases BAX relocalization to the mitochondria. Also, a physical interaction between BAX and Bcl2 is necessary for these two effects to occur. Taken together, these results suggest upregulated Bcl2 stabilizes BAX loose binding to mitochondrial membranes, inhibiting its insertion into the MOM and consequently cytochrome c release. Structured summary MINT-7945271: BAX (uniprotkb:Q07813) physically interacts (MI:0915) with Bcl-2 (uniprotkb:P10417) by anti bait coimmunoprecipitation (MI:0006) (L. Dejean). FEBS Letters 584 (2010) 3305-3310 j o u r n a l h o m e p a g e : w w w . F E B S L e t t e r s . o r g
Bax and Bak Coalesce into Novel Mitochondria-Associated Clusters during Apoptosis
The Journal of Cell Biology, 2001
Bax is a member of the Bcl-2 family of proteins known to regulate mitochondria-dependent programmed cell death. Early in apoptosis, Bax translocates from the cytosol to the mitochondrial membrane. We have identified by confocal and electron microscopy a novel step in the Bax proapoptotic mechanism immediately subsequent to mitochondrial translocation. Bax leaves the mitochondrial membranes and coalesces into large clusters containing thousands of Bax molecules that remain adjacent to mitochondria. Bak, a close homologue of Bax, colocalizes in these apoptotic clusters in contrast to other family members, Bid and Bad, which circumscribe the outer mitochondrial membrane throughout cell death progression. We found the formation of Bax and Bak apoptotic clusters to be caspase independent and inhibited completely and specifically by Bcl-X L , correlating cluster formation with cytotoxic activity. Our results reveal the importance of a novel structure formed by certain Bcl-2 family members during the process of cell death.
BMC cell biology, 2007
Background: Bcl-2 family proteins are key regulators of mitochondrial integrity and comprise both proand anti-apoptotic proteins. Bax a pro-apoptotic member localizes as monomers in the cytosol of healthy cells and accumulates as oligomers in mitochondria of apoptotic cells. The Bcl-2 homology-3 (BH3) domain regulates interactions within the family, but regions other than BH3 are also critical for Bax function. Thus, the N-terminus has been variously implicated in targeting to mitochondria, interactions with BH3-only proteins as well as conformational changes linked to Bax activation. The transmembrane (TM) domains (α5-α6 helices in the core and α9 helix in the C-terminus) in Bax are implicated in localization to mitochondria and triggering cytotoxicity. Here we have investigated N-terminus modulation of TM function in the context of regulation by the anti-apoptotic protein Bcl-x L .
BAX activation is initiated at a novel interaction site
Nature, 2008
BAX is a pro-apoptotic protein of the BCL-2 family stationed in the cytosol until activated by a diversity of stress stimuli to induce cell death. Anti-apoptotic proteins such as BCL-2 counteract BAX-mediated cell death. Although an interaction site that confers survival functionality has been defined for anti-apoptotic proteins, an activation site has not been identified for BAX, rendering its explicit trigger mechanism unknown. We previously developed Stabilized Alpha-Helix of BCL-2 domains (SAHBs) that directly initiate BAX-mediated mitochondrial apoptosis. Here we demonstrate by NMR analysis that BIM SAHB binds BAX at an interaction site that is distinct from the canonical binding groove characterized for anti-apoptotic proteins. The specificity of the BIM SAHB-BAX interaction is highlighted by point mutagenesis that abrogates functional activity, confirming that BAX activation is initiated at this novel structural location. Thus, we have now defined a BAX interaction site for direct activation, establishing a new target for therapeutic modulation of apoptosis.
Hierarchical regulation of mitochondrion-dependent apoptosis by BCL-2 subfamilies
Nature Cell Biology, 2006
Apoptosis, or programmed cell death, results from the activation of an evolutionarily conserved genetic pathway 1 . Apoptosis manifests in irreversible damage to cellular constituents through caspase activation and mitochondrial dysfunction 2,3 . Mammals evolved a more complex regulation of caspase activation with the involvement of mitochondria, namely the requirement for cytochrome c 4,5 . The BCL-2 family members are major regulators of mitochondrial integrity and mitochondrion-initiated caspase activation 2,6 .
Conformation of the Bax C-terminus regulates subcellular location and cell death
The EMBO Journal, 1999
Bax, a pro-apoptotic member of the Bcl-2 family, translocates from the cytosol to the mitochondria during programmed cell death. We report here that both gain-of-function and loss-of-function mutations can be achieved by altering a single amino acid in the Bax hydrophobic C-terminus. The properly mutated Cterminus of Bax can target a non-relevant protein to the mitochondria, showing that specific conformations of this domain alone allow mitochondrial docking. These data along with N-terminus epitope exposure experiments suggest that the C-and the N-termini interact and that upon triggering of apoptosis, Bax changes conformation, exposing these two domains to insert into the mitochondria and regulate the cell death machinery.
The N-terminal End of Bax Contains a Mitochondrial-targeting Signal
Journal of Biological Chemistry, 2003
The translocation of Bax ␣, a pro-apoptotic member of the BCL-2 family from the cytosol to mitochondria, is a central event of the apoptotic program. We report here that the N-terminal (NT) end of Bax ␣, which contains its first ␣ helix (⌯␣1), is a functional mitochondrial-addressing signal both in mammals and in yeast. Similar results were obtained with a newly described variant of Bax called Bax , which lacks the first 20 amino acids of Bax ␣ and is constitutively associated with mitochondria. Deletion of ⌯␣1 impairs the binding of Bax to mitochondria, whereas a fusion of the N terminus of Bax ␣, which contains ⌯␣1 with a cytosolic protein, results in the binding of the chimeric proteins to mitochondria both in a cell-free assay and in vitro. More importantly, the mitochondria-bound chimeric proteins inhibit the interaction of Bax with mitochondria as well as Baxapoptogenic properties. The mutations of the ⌯␣1, which inhibit Bax ␣ and Bax translocation to mitochondria, also block the subsequent activation of the execution phase of apoptosis. Conversely, a deletion of the C terminus does not appear to influence Bax ␣ and Bax mitochondrial addressing. Taken together, our results suggest that Bax is targeted to mitochondria by its NT and thus through a pathway that is unique for a member of the BCL-2 family.
Bcl-2 on the Endoplasmic Reticulum Regulates Bax Activity by Binding to BH3-only Proteins
Journal of Biological Chemistry, 2002
Bcl-2 family members have been shown to be key mediators of apoptosis as either pro-or anti-apoptotic factors. It is thought that both classes of Bcl-2 family members act at the level of the mitochondria to regulate apoptosis, although the founding anti-apoptotic family member, Bcl-2 is localized to the endoplasmic reticulum (ER), mitochondrial, and nuclear membranes. In order to better understand the effect of Bcl-2 localization on its activity, we have utilized a Bcl-2 mutant that localizes only to the ER membrane, designated Bcl-2Cb5. Bcl-2Cb5 was expressed in MDA-MB-468 cells, which protected against apoptosis induced by the kinase inhibitor, staurosporine. Data presented here show that Bcl-2Cb5 inhibits this process by blocking Bax activation and cytochrome c release. Furthermore, we show that Bcl-2Cb5 can inhibit the activation of a constitutively mitochondrial mutant of Bax, indicating that an intermediate between Bcl-2 on the ER and Bax on the mitochondria must exist. We demonstrate that this intermediate is likely a BH3-only subfamily member. Data presented here show that Bcl-2Cb5 can sequester a constitutively active form of Bad (Bad3A) from the mitochondria and prevent it from activating Bax. These data suggest that Bcl-2 indirectly protects mitochondrial membranes from Bax, via BH3-only proteins.