Distinct Domains Control the Addressing and the Insertion of Bax into Mitochondria (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.
Regulated Targeting of BAX to Mitochondria
Journal of Cell Biology, 1998
The proapoptotic protein BAX contains a single predicted transmembrane domain at its COOH terminus. In unstimulated cells, BAX is located in the cytosol and in peripheral association with intracellular membranes including mitochondria, but inserts into mitochondrial membranes after a death signal. This failure to insert into mitochondrial membrane in the absence of a death signal correlates with repression of the transmembrane signal-anchor function of BAX by the NH 2 -terminal domain. Targeting can be instated by deleting the domain or by replacing the BAX transmembrane segment with that of BCL-2. In stimulated cells, the contribution of the NH 2 terminus of BAX correlates with further exposure of this domain after mem-brane insertion of the protein. The peptidyl caspase inhibitor zVAD-fmk partly blocks the stimulated mitochondrial membrane insertion of BAX in vivo, which is consistent with the ability of apoptotic cell extracts to support mitochondrial targeting of BAX in vitro, dependent on activation of caspase(s). Taken together, our results suggest that regulated targeting of BAX to mitochondria in response to a death signal is mediated by discrete domains within the BAX polypeptide. The contribution of one or more caspases may reflect an initiation and/or amplification of this regulated targeting.
Bax inserts into the mitochondrial outer membrane by different mechanisms
FEBS Letters, 2008
Bax insertion into the mitochondrial outer membrane is essential for the implementation of apoptosis. However, little is known about the first stage of Bax integration into the mitochondrial outer membrane. We have recently shown that TOM22, a mitochondrial outer membrane receptor, is important for insertion, although other reports have suggested that only mitochondrial lipids are involved in this process. Here, we show that monomers, but not dimers, of Bax require the presence of TOM22 and TOM40 to integrate into mitochondria. In addition we show that once inserted into the membrane, Bax can act as a receptor for cytosolic Bax.
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.
Bax mitochondrial residency is more critical than Bax oligomerization for apoptosis
2019
The Bax protein plays an important effector role in apoptosis by forming pores in the mitochondrial outer membrane. While doing so, Bax forms higher-order oligomers in the membrane, but it remains unclear whether this oligomer formation is essential for pore formation. Using cell-free and cellular experimental systems, we investigated two Bax C-terminus mutants, T182I and G179P. Neither mutant formed large oligomers when activated in liposomes. Nevertheless, the G179P mutant could produce membrane pores, suggesting that large oligomers are not required for permeabilization. Surprisingly, however, when G179P was transduced into Bax/Bak double knockout mouse embryonic fibroblasts, it was purely cytoplasmic and failed to mediate cell death. T182I behaved in the opposite manner. When mixed with liposomes, T182I was inefficient in both membrane insertion and permeabilization. However, transduced into cells, BaxT182I resided constitutively in mitochondria, owing to its slow retrotransloca...
Movement of Bax from the Cytosol to Mitochondria during Apoptosis
Journal of Cell Biology, 1997
Bax, a member of the Bcl-2 protein family, accelerates apoptosis by an unknown mechanism. Bax has been recently reported to be an integral membrane protein associated with organelles or bound to organelles by Bcl-2 or a soluble protein found in the cytosol. To explore Bcl-2 family member localization in living cells, the green fluorescent protein (GFP) was fused to the NH2 termini of Bax, Bcl-2, and Bcl-XL. Confocal microscopy performed on living Cos-7 kidney epithelial cells and L929 fibroblasts revealed that GFP–Bcl-2 and GFP–Bcl-XL had a punctate distribution and colocalized with a mitochondrial marker, whereas GFP–Bax was found diffusely throughout the cytosol. Photobleaching analysis confirmed that GFP–Bax is a soluble protein, in contrast to organelle-bound GFP–Bcl-2. The diffuse localization of GFP–Bax did not change with coexpression of high levels of Bcl-2 or Bcl-XL. However, upon induction of apoptosis, GFP–Bax moved intracellularly to a punctate distribution that partiall...
Experimental Cell Research, 2002
Bax, a proapoptotic member of the Bcl-2 family of proteins, resides in the cytosol and translocates to the mitochondrial membrane upon induction of apoptosis. It has been proposed that Bax does not translocate to mitochondria under normal physiological conditions, due to interaction between amino (ART) and carboxy (TM) terminal domains. Here, we report the physiological consequences of introducing a matrix targeting mitochondrial signal sequence (Su9) at the amino terminus of Bax and its mutants lacking ART, TM, or both segments. In vitro mitochondrial protein import assays of the fusion proteins suggests localization to the mitochondrial matrix. When expressed in Cos-1 cells, Su9 could target Bax to mitochondria in the absence of an apoptotic stimulus. However, mitochondrial localization did not result in apoptosis. When ART, TM, or both segments of Bax were deleted, expression of fusion proteins containing Su9 resulted in apoptosis via cytochrome c release. Cell death was inhibited by the pan-caspase inhibitor zVAD-fmk. We thus demonstrate that an effective mitochondrial matrix targeting signal can override the inhibition of import of Bax to the organelle, presumed to arise as a result of interaction between ART and TM segments, in the absence of apoptotic stimulus. We also demonstrate the ability of truncated variants of Bax to cause apoptosis when targeted to mitochondria by cytochrome c release from an ectopic environment. © 2002 Elsevier Science (USA)