Role of mitochondria and C-terminal membrane anchor of Bcl-2 in Bax induced growth arrest and mortality in Saccharomyces cerevisiae

Temporal dissection of Bax-induced events leading to fission of the single mitochondrion in Trypanosoma brucei

The protozoan Trypanosoma brucei has a single mitochondrion and lacks an apoptotic machinery. Here we show that expression of the proapoptotic protein Bax in T. brucei causes the release of cytochrome c, the depolarization of the mitochondrial membrane potential and mitochondrial fission. However, in contrast to mammalian cells, the three events are temporally well separated. The release of cytochrome c from the intermembrane space precedes mitochondrial fission, showing that it does not depend on mitochondrial fragmentation. Furthermore, halting Bax expression allows some cells to recover even after mitochondrial fission, the last recorded event, went to completion, indicating that all three Bax-induced events are, in principle, reversible.

Bax-induced cell death inCandida albicans

Bax is a pro-apoptotic member of the Bcl-2 family of proteins involved in the regulation of genetically programmed cell death in mammalian cells. It has been shown that heterologous expression of Bax in several yeast species, such as Saccharomyces cerevisiae, Schizosaccharomyces pombe and Pichia pastoris, also induces cell death. In this study we investigated the effects of Bax expression in the pathogenic yeast Candida albicans. Cell death inducing expression of Bax required a synthetic BAX gene that was codon-optimized for expression in Candida albicans. Expression of this BAX gene resulted in growth inhibition and cell death. By fusing Bax with the yeast enhanced green fluorescent protein of Aequoria victoria, the cell death-inducing effect of Bax was increased due to reduced proteolytic degradation of Bax. Using this fusion protein we showed that, upon expression in C. albicans, Bax co-localizes with the mitochondria. Furthermore, we showed for the first time that expression of Bax in yeast causes the mitochondria, which are normally distributed throughout the cell, to cluster in the perinuclear region.

Yeast as a tool to study Bax/mitochondrial interactions in cell death

The budding yeast Saccharomyces cerevisiae has proven to be a powerful tool in investigations of the molecular aspects of the events involved in apoptosis, particularly the steps implicating mitochondria. Yeast does not have obvious homologs of the proteins involved in the regulation of apoptosis, and provides a simplified model system in which the function of these proteins can be unraveled. This review focuses on the interactions of two of the major pro-apoptotic Bcl-2 family members, Bax and Bid, with mitochondria. It is shown that yeast has allowed questioning of several crucial aspects of the function of these two proteins, namely the molecular mechanisms driving their insertion into the mitochondrial outer membrane and those leading to the permeabilization to cytochrome c. More recently, signaling pathways leading to Bax-induced cell death, as well as other forms of cell death, have been identified in yeast. Both 'apoptosis-like' and autophagy-related forms of cell degradation are involved, and mitochondria play a central role in these two signaling pathways.

Investigation of the role of the C-terminus of Bax and of tc-Bid on Bax interaction with yeast mitochondria

Because of structural homology with the transmembrane domain of Bcl-2, the proapoptotic protein Bax has been proposed to be anchored to the outer membrane of mitochondria through its carboxy-terminal end (CT). We took advantage of the absence of Bcl-2 family members in yeast to further investigate the role of Bax CT in its mitochondrial association and function. The complete deletion or the addition of a C-terminal c-myc tag as well as the replacement of CT by a random coiled sequence enhanced membrane insertion of Bax. It has previously been suggested that conformational change in the N-terminal end of Bax would allow the C-terminal end to play its anchoring function. We found that a mutant truncated in both N- and C-termini still exhibited a strong binding activity to mitochondria. In mammals, Bax interaction with the caspase-8-generated truncated form of Bid (tc-Bid) is believed to promote a conformational change necessary for the insertion of Bax into mitochondria. We coexpressed Bax and tc-Bid in yeast and found that native Bax functions are not stimulated by tc-Bid, whereas functions of an active variant with a modified CT are. We propose that Bax CT has to undergo a conformational change to allow the insertion of Bax in mitochondria but, contrary to current views, is not a bona fide membrane anchor.

Response of yeast to the regulated expression of proteins in the Bcl-2 family

The mechanisms by which pro-apoptotic members of the Bcl-2 family of proteins promote the release of mitochondrial factors like cytochrome c, subsequently activating the apoptotic cascade, or by which anti-apoptotic family members block this release, are still not understood. When expressed in yeast, Bcl-2 family members act directly upon conserved mitochondrial components that correspond to their apoptotic substrates in mammalian cells. Here we describe a system in which the levels of representative pro- and anti-apoptotic members of the Bcl-2 family can be regulated independently in yeast. Using this system, we have focused on the action of the anti-apoptotic family member Bcl-x(L), and have defined the quantitative relationships that underlie the antagonistic action of this protein on the lethal consequences of expression of the pro-apoptotic family member Bax. This system has also allowed us to demonstrate biochemically that Bcl-x(L) has two actions at the level of the mitochondrion. Bcl-x(L) is able to inhibit the stable integration of Bax into mitochondrial membranes, as well as hinder the action of Bax that does become stably integrated into these membranes. Taken together, our results suggest that both the functional and biochemical actions of Bcl-x(L) may be based on the ability of this molecule to disrupt the interaction of Bax with a resident mitochondrial target that is required for Bax action. Finally, we confirm that VDAC (voltage-dependent anion channel) is not required for the functional responses observed following the expression of either pro- or anti-apoptotic members of the Bcl-2 family.

Model systems in drug discovery: chemical genetics meets genomics

Animal model systems are an intricate part of the discovery and development of new medicines. The sequencing of not only the human genome but also those of the various pathogenic bacteria, the nematode Caenorhabditis elegans, the fruitfly Drosophila, and the mouse has enabled the discovery of new drug targets to push forward at an unprecedented pace. The knowledge and tools in these "model" systems are allowing researchers to carry out experiments more efficiently and are uncovering previously hidden biological connections. While the history of bacteria, yeast, and mice in drug discovery are long, their roles are ever evolving. In contrast, the history of Drosophila and C. elegans at pharmaceutical companies is short. We will briefly review the historic role of each model organism in drug discovery and then update the readers as to the abilities and liabilities of each model within the context of drug development.

Bax and Bak can localize to the endoplasmic reticulum to initiate apoptosis

Bax and Bak play a redundant but essential role in apoptosis initiated by the mitochondrial release of apoptogenic factors. In addition to their presence at the mitochondrial outer membrane, Bax and Bak can also localize to the ER. Agents that initiate ER stress responses can induce conformational changes and oligomerization of Bax on the ER as well as on mitochondria. In wild-type cells, this is associated with caspase 12 cleavage that is abolished in bax-/-bak-/- cells. In bax-/-bak-/- cells, introduction of Bak mutants selectively targeted to either mitochondria or the ER can induce apoptosis. However, ER-targeted, but not mitochondria-targeted, Bak leads to progressive depletion of ER Ca2+ and induces caspase 12 cleavage. In contrast, mitochondria-targeted Bak leads to enhanced caspase 7 and PARP cleavage in comparison with the ER-targeted Bak. These findings demonstrate that in addition to their functions at mitochondria, Bax and Bak also localize to the ER and function to initiate a parallel pathway of caspase activation and apoptosis.

Fluconazole toxicity is independent of oxidative stress and apoptotic effector mechanisms in Saccharomyces cerevisiae

Fluconazole toxicity was previously shown to require intact mitochondria in Saccharomyces cerevisiae, however, the mechanism of mitochondrial-dependent azole toxicity is unclear. Here we show that fluconazole toxicity is not attenuated by the overexpression of the mitochondrially-acting human antiapoptotic protein Bcl-2 nor is it increased by the presence of the oxidating agents in S. cerevisiae. Our data further support the notion that mitochondrial toxicity in the presence of azoles is modulated by the conversion of ergosterol precursors to toxic sterols.

Bcl-2 family members inhibit oxidative stress-induced programmed cell death in Saccharomyces cerevisiae

Selected antiapoptotic genes were expressed in baker's yeast (Saccharomyces cerevisiae) to evaluate cytoprotective effects during oxidative stress. When exposed to treatments resulting in the generation of reactive oxygen species (ROS), including H(2)O(2), menadione, or heat shock, wild-type yeast died and exhibited apoptotic-like characteristics, consistent with previous studies. Yeast strains were generated expressing nematode ced-9, human bcl-2, or chicken bcl-xl genes. These transformants tolerated a range of oxidative stresses, did not display features associated with apoptosis, and remained viable under conditions that were lethal to wild-type yeast. Yeast strains expressing a mutant antiapoptotic gene (bcl-2 deltaalpha 5-6), known to be nonfunctional in mammalian cells, were unable to tolerate any of the ROS-generating insults. These data are the first report showing CED-9 has cytoprotective effects against oxidative stress, and add CED-9 to the list of Bcl-2 protein family members that modulate ROS-mediated programmed cell death. In addition, these data indicate that Bcl-2 family members protect wild-type yeast from physiological stresses. Taken together, these data support the concept of the broad evolutionary conservation and functional similarity of the apoptotic processes in eukaryotic organisms.

Inactivation of Cdc13p triggers MEC1-dependent apoptotic signals in yeast

Inactivation of the budding yeast telomere binding protein Cdc13 results in abnormal telomeres (exposed long G-strands) and activation of the DNA damage checkpoint. In the current study, we show that inactivation of Cdc13p induces apoptotic signals in yeast, as evidenced by caspase activation, increased reactive oxygen species production, and flipping of phosphatidylserine in the cytoplasmic membrane. These apoptotic signals were suppressed in a mitochondrial (rho(o)) mutant. Moreover, mitochondrial proteins (e.g. MTCO3) were identified as multicopy suppressors of cdc13-1, suggesting the involvement of mitochondrial functions in telomere-initiated apoptotic signaling. These telomere-initiated apoptotic signals were also shown to depend on MEC1, but not TEL1, and were antagonized by MRE11. Our results are consistent with a model in which single-stranded G-tails in the cdc13-1 mutant trigger MEC1-dependent apoptotic signaling in yeast.

Programmed cell death in trypanosomatids and other unicellular organisms

In multicellular organisms, cellular growth and development can be controlled by programmed cell death (PCD), which is defined by a sequence of regulated events. However, PCD is thought to have evolved not only to regulate growth and development in multicellular organisms but also to have a functional role in the biology of unicellular organisms. In protozoan parasites and in other unicellular organisms, features of PCD similar to those in multicellular organisms have been reported, suggesting some commonality in the PCD pathway between unicellular and multicellular organisms. However, more extensive studies are needed to fully characterise the PCD pathway and to define the factors that control PCD in the unicellular organisms. The understanding of the PCD pathway in unicellular organisms could delineate the evolutionary origin of this pathway. Further characterisation of the PCD pathway in the unicellular parasites could provide information regarding their pathogenesis, which could be exploited to target new drugs to limit their growth and treat the disease they cause.

The product of the UTH1 gene, required for Bax-induced cell death in yeast, is involved in the response to rapamycin

A yeast mutant was isolated that was resistant to Bax-induced cell death. It supports a mutation leading to decreased amounts of the protein Uth1p. A strain in which the UTH1 gene is disrupted also exhibits resistance to Bax expression. The absence of Uth1p does not change the mitochondrial localization of Bax, its insertion in the mitochondrial outer membrane or its cytochrome c-releasing activity. On the other hand, the absence of Uth1p does prevent the appearance of other hallmarks related to Bax expression in yeast, such as oxidation of mitochondrial lipid, production of reactive oxygen species and maintenance of plasma membrane properties after ethanol stress. The absence of Uth1p was also found to induce resistance to rapamycin, a specific inducer of autophagy. This resistance only appears when cells are grown under respiratory conditions, but not under fermentative conditions, suggesting that Uth1p acts in an autophagic pathway involving mitochondria, in accordance with its main localization in the outer mitochondrial membrane. Taken together, these data show that Bax is able to activate a death pathway related to autophagy in yeast, which also exhibits typical hallmarks of apoptosis, revealing a possible dual function of Bax in both types of death. This hypothesis is discussed in the light of observations suggesting a co-regulation of apoptosis and autophagy in mammalian cells.

Redox Regulation of Apoptosis before and after Cytochrome C Release

Programmed cell death, or apoptosis, is one of the most studied areas of modern biology. Apoptosis is a genetically regulated process, which plays an essential role in the development and homeostasis of higher organisms. Mitochondria, known to play a central role in regulating cellular metabolism, was found to be critical for regulating apoptosis induced under both physiological and pathological conditions. Mitochondria are a major source of reactive oxygen species (ROS) but they can also serve as its target during the apoptosis process. Release of apoptogenic factors from mitochondria, the best known of which is cytochrome c, leads to assembly of a large apoptosis-inducing complex called the apoptosome. Cysteine proteases (called caspases) are recruited to this complex and, following their activation by proteolytic cleavage, activate other caspases, which in turn target for specific cleavage a large number of cellular proteins. The redox regulation of apoptosis during and after cytochrome c release is an area of intense investigation. This review summarizes what is known about the biological role of ROS and its targets in apoptosis with an emphasis on its intricate connections to mitochondria and the basic components of cell death.

From genetics and genomics to drug discovery: yeast rises to the challenge

Yeast expands its role from eukaryotic genetics and genomics to drug discovery.

Evidence for crucial electrostatic interactions between Bcl-2 homology domains BH3 and BH4 in the anti-apoptotic Nr-13 protein

Nr-13 is an anti-apoptotic member of the Bcl-2 family previously shown to interact with Bax. The biological significance of this interaction was explored both in yeast and vertebrate cells and revealed that Nr-13 is able to counteract the pro-apoptotic activity of Bax. The Bax-interacting domain has been identified and corresponds to alpha-helices 5 and 6 in Nr-13. Site-directed mutagenesis has revealed that the N-terminal region of Nr-13 is essential for activity and corresponds to a genuine Bcl-2 homology domain (BH4). The modelling of Nr-13, based on its similarity with other Bcl-2 family proteins and energy minimization, suggests the possibility of electrostatic interactions between the two N-terminal-conserved domains BH4 and BH3. Disruption of these interactions severely affects Nr-13 anti-apoptotic activity. Together our results suggest that electrostatic interactions between BH4 and BH3 domains play a role in the control of activity of Nr-13 and a subset of Bcl-2 family members.

Bcl-x Complements Saccharomyces cerevisiae Genes That Facilitate the Switch from Glycolytic to Oxidative Metabolism

All eukaryotic organisms have mechanisms to adapt to changing metabolic conditions. The mammalian cell survival gene Bcl-x(L) enables cells to adapt to changes in cellular metabolism. To identify genes whose function can be substituted by Bcl-x(L) in a unicellular eukaryote, a genetic screen was performed using the yeast Saccharomyces cerevisiae. S. cerevisiae grows by anaerobic glycolysis when glucose is available, switching to oxidative phosphorylation when carbohydrate in the media becomes limiting (diauxic shift). Given that Bcl-x(L) appears to facilitate the switch from glycolytic to oxidative metabolism in mammalian cells, a library of yeast mutants was tested for the ability to efficiently undergo diauxic shift in the presence and absence of Bcl-x(L). Several mutants were identified that have a defect in growth when switched from a fermentable to a nonfermentable carbon source that is corrected by the expression of Bcl-x(L). These genes include the mitochondrial chaperonin TCM62, as well as previously uncharacterized genes. One of these uncharacterized genes, SVF1, promotes cell survival in mammalian cells in response to multiple apoptotic stimuli. The finding that TCM62 and the analogous human prohibitin gene also inhibit mammalian cell death following growth factor withdrawal implicates mitochondrial chaperones as regulators of apoptosis. Further characterization of the genes identified in this screen may enhance our understanding of Bcl-x(L) function in mammalian cells, and of cell survival pathways in general.

Bax-induced cell death in yeast depends on mitochondrial lipid oxidation

The oxidant function of pro-apoptotic protein Bax was investigated through heterologous expression in yeast. Direct measurements of fatty acid content show that Bax-expression induces oxidation of mitochondrial lipids. This effect is prevented by the coexpression of Bcl-xL. The oxidation actually could be followed on isolated mitochondria as respiration-induced peroxidation of polyunsaturated cis-parinaric acid and on whole cells as the increase in the amount of thiobarbituric acid-reactive products. Treatments that increase the unsaturation ratio of lipids, making them more sensitive to oxidation, increase kinetics of Bax-induced death. Conversely, inhibitors of lipid oxidation and treatments that decrease the unsaturation ratio of fatty acids decrease kinetics of Bax-induced death. Taken together, these results show that Bax-induced mitochondrial lipid oxidation is relevant to Bax-induced cell death. Conversely, lipid oxidation is poorly related to the massive Bax-induced superoxide and hydrogen peroxide accumulation, which occurs at the same time, as chemical or enzymatic scavenging of ROS does not prevent lipid oxidation nor has any effects on kinetics of Bax-induced cell death. Whatever the origin of mitochondrial lipid oxidation, these data show that it represents a major step in the cascade of events leading to Bax-induced cell death. These results are discussed in the light of the role of lipid oxidation both in mammalian apoptosis and in other forms of cell death in other organisms.

Cytochrome c release and mitochondria involvement in programmed cell death induced by acetic acid in Saccharomyces cerevisiae

Evidence is presented that mitochondria are implicated in the previously described programmed cell death (PCD) process induced by acetic acid in Saccharomyces cerevisiae. In yeast cells undergoing a PCD process induced by acetic acid, translocation of cytochrome c (CytC) to the cytosol and reactive oxygen species production, two events known to be proapoptotic in mammals, were observed. Associated with these events, reduction in oxygen consumption and in mitochondrial membrane potential was found. Enzymatic assays showed that the activity of complex bc(1) was normal, whereas that of cytochrome c oxidase (COX) was strongly decreased. This decrease is in accordance with the observed reduction in the amounts of COX II subunit and of cytochromes a+a(3). The acetic acid-induced PCD process was found to be independent of oxidative phosphorylation because it was not inhibited by oligomycin treatment. The inability of S. cerevisiae mutant strains (lacking mitochondrial DNA, heme lyase, or ATPase) to undergo acetic acid-induced PCD and in the ATPase mutant (knockout in ATP10) the absence of CytC release provides further evidence that the process is mediated by a mitochondria-dependent apoptotic pathway. The understanding of the involvement of a mitochondria-dependent apoptotic pathway in S. cerevisiae PCD process will be most useful in the further elucidation of an ancestral pathway common to PCD in metazoans.

A metacaspase of Trypanosoma brucei causes loss of respiration competence and clonal death in the yeast Saccharomyces cerevisiae

Metacaspases constitute a new group of cysteine proteases homologous to caspases. Heterologous expression of Trypanosoma brucei metacaspase TbMCA4 in the budding yeast Saccharomyces cerevisiae resulted in growth inhibition, mitochondrial dysfunction and clonal death. The metacaspase orthologue of yeast, ScMCA1 (YOR197w), exhibited genetic interaction with WWM1 (YFL010c), which encodes a small WW domain protein. WWM1 overexpression resulted in growth arrest and clonal death, which was suppressed by concomitant overexpression of ScMCA1. GFP-fusion reporters of WWM1, ScMCA1 and TbMCA4 localized to the nucleus. Taken together, we suggest that metacaspases may play a role in nuclear function controlling cellular proliferation coupled to mitochondrial biogenesis.

Salt causes ion disequilibrium-induced programmed cell death in yeast and plants

Programmed cell death (PCD) is a fundamental cellular process conserved in metazoans, plants and yeast. Evidence is presented that salt induces PCD in yeast and plants because of an ionic, rather than osmotic, etiology. In yeast, NaCl inhibited growth and caused a time-dependent reduction in viability that was preceded by DNA fragmentation. NaCl also induced the cytological hallmarks of lysigenous-type PCD, including nuclear fragmentation, vacuolation and lysis. The human anti-apoptotic protein Bcl-2 increased salt tolerance of wild-type yeast strain and calcineurin-deficient yeast mutant (cnb1Delta) that is defective for ion homeostasis, but had no effect on the NaCl or sorbitol sensitivity of the osmotic hypersensitive hog1Delta mutant -- results that further link PCD in the response to the ion disequilibrium under salt stress. Bcl-2 suppression of cnb1Delta salt sensitivity was ENA1 (P-type ATPase gene)-dependent, due in part to transcriptional activation. Salt-induced PCD (TUNEL staining and DNA laddering) in primary roots of both Arabidopsis thaliana wild type (Col-1 gl1) and sos1 (salt overly sensitive) mutant seedlings correlated positively with treatment lethality. Wild-type plants survived salt stress levels that were lethal to sos1 plants because secondary roots were produced from the shoot/root transition zone. PCD-mediated elimination of the primary root in response to salt shock appears to be an adaptive mechanism that facilitates the production of roots more able to cope with a saline environment. Both salt-sensitive mutants of yeast (cnb1Delta) and Arabidopsis (sos1) exhibit substantially more profound PCD symptoms, indicating that salt-induced PCD is mediated by ion disequilibrium.

Mitochondrial reactive oxygen species in cell death signaling

During apoptosis, mitochondrial membrane permeability (MMP) increases and the release into the cytosol of pro-apoptotic factors (procaspases, caspase activators and caspase-independent factors such as apoptosis-inducing factor (AIF)) leads to the apoptotic phenotype. Apart from this pivotal role of mitochondria during the execution phase of apoptosis (documented in other reviews of this issue), it appears that reactive oxygen species (ROS) produced by the mitochondria can be involved in cell death. These toxic compounds are normally detoxified by the cells, failing which oxidative stress occurs. However, ROS are not only dangerous molecules for the cell, but they also display a physiological role, as mediators in signal transduction pathways. ROS participate in early and late steps of the regulation of apoptosis, according to different possible molecular mechanisms. In agreement with this role of ROS in apoptosis signaling, inhibition of apoptosis by anti-apoptotic Bcl-2 and Bcl-x(L) is associated with a protection against ROS and/or a shift of the cellular redox potential to a more reduced state. Furthermore, the fact that active forms of cell death in yeast and plants also involve ROS suggests the existence of an ancestral redox-sensitive death signaling pathway that has been independent of caspases and Bcl-2.

Signalling apoptosis: a radical approach

Reactive oxygen species (ROS) are frequently associated with cytotoxicity, often being described as damaging, harmful or toxic. It is generally assumed that, under pathological circumstances, ROS elicit wide-spread and random acts of oxidation. This passive attack of cellular components by ROS, in conditions where oxidative stress is the initiating stimulus for apoptosis, is assumed to simply trigger cell death as a result of cumulative oxidative damage. However, accumulating evidence now suggests that ROS may act as signalling molecules for the initiation and execution of the apoptotic death programme in many, if not all, current models of apoptotic cell death. Signalling by ROS would not appear to be random, as previously assumed, but targeted at specific metabolic and signal transduction cellular components. There is also evidence that the enzymatic generation of ROS may not simply be an unwanted by-product of the primary reaction catalysed, but that ROS may be used as signalling molecules to regulate cellular processes including apoptosis. This view of ROS as signalling molecules (as opposed to toxic metabolites) has been further bolstered by the findings that cellular antioxidants such as glutathione and thioredoxin not only serve to regulate ROS levels but also act as reversible redox modifiers of enzyme function. This review will attempt to delineate the involvement of ROS in apoptosis in light of these recent discoveries and provide evidence for a crucial role for ROS in the initiation and execution of the death process.

Soybean ascorbate peroxidase suppresses Bax-induced apoptosis in yeast by inhibiting oxygen radical generation

Bax, a mammalian proapoptotic member of the Bcl-2 family, can induce cell death when expressed in yeast or plant cells. To identify plant Bax inhibitors, we cotransformed a soybean cDNA library and the Bax gene into yeast cells and screened for expressed genes that prevented Bax-induced apoptosis. From the Bax-inhibiting genes isolated, ascorbate peroxidase (sAPX) was selected for characterization. The transcription of sAPX in plants was specifically induced by oxidative stress. Moreover, overexpression of sAPX partially suppressed the H(2)O(2)-sensitive phenotype of yeast cytosolic catalase T (Deltactt)- and thermosensitive phenotype of cytochrome c peroxidase (Deltaccp)-deleted mutant cells. Examination of reactive oxygen species (ROS) production using the fluorescence method of dihydrorhodamine 123 oxidation revealed that expression of Bax in yeast cells generated ROS, which was greatly reduced by coexpression with sAPX. Our results collectively suggest that sAPX inhibits the generation of ROS by Bax, which in turn suppresses Bax-induced cell death in yeast.

Programmed cell death in the unicellular protozoan parasite Leishmania

In the present study we have demonstrated some features characterizing programmed cell death (PCD) in the unicellular protozoan parasite Leishmania donovani, the causative agent of visceral Leishmaniasis. We report that PCD is initiated in stationary phase cultures of promastigotes and both in actively growing cultures of axenic amastigotes and promastigotes upon treatment with anti Leishmanial drugs (Pentostam and amphotericin B). However, the two cell types respond to antileishmanial drugs differently. The features of PCD in L. donovani promastigotes are nuclear condensation, nicked DNA in the nucleus, DNA ladder formation, increase in plasma membrane permeability, decrease in the mitochondrial membrane potential (DeltaPsi m) and induction of a PhiPhiLux (PPL)-cleavage activity. PCD in both stationary phase culture and upon induction by amphotericin B resulted first in the decrease of mitochondrial membrane potential followed by simultaneous change in plasma membrane permeability and induction of PPL-cleavage activity. Of the total PPL-cleavage activity, several caspase inhibitors inhibited a significant amount (21-34%). Inhibitors of cathepsin or calpain did not inhibit PPL-cleavage activity. Taken together this study demonstrates that the characteristic features of PCD exist in unicellular protozoan Leishmania donovani. The implication of PCD on the Leishmania pathogenesis is discussed.

Mitochondrial AAA-type protease Yme1p is involved in Bax effects on cytochrome c oxidase

Expression of the pro-apoptotic protein Bax in yeast Saccharomyces cerevisiae induces a release of cytochrome c accompanied by a decrease of the amount of cytochrome c oxidase. Here we show that the decrease of cytochrome c oxidase is due to the activation of mitochondrial protease Yme1p, of which cytochrome c oxidase subunit 2 (Cox2p) is a substrate. The absence of Yme1p slightly delays Bax-induced cell death, suggesting a role of this protease in yeast cell death and thus of its mammalian homologue in apoptosis.

Apoptosis in yeast: a new model for aging research

Apoptosis is a form of programmed cell death with a central role in development and homeostasis of metazoan organisms. Recent research indicates the presence of an apoptotic cell death program in unicellular eukaryotes. Yeast can be killed by expression of mammalian proapoptotic genes or in response to oxygen stress, which is an inducer of mammalian apoptosis. The dying yeast cells show morphological alterations typical for apoptosis. Yeast provides a simple model for cellular aging. The observation that old yeast cells produce oxygen radicals and die apoptotically may provide clues to a similar sequence of events in mammalian aging.

The Arabidopsis thaliana ethylene-responsive element binding protein (AtEBP) can function as a dominant suppressor of Bax-induced cell death of yeast

We identified genes based on screening of an Arabidopsis cDNA library for functional suppressors of mouse Bax-induced cell death of yeast cells. Interestingly, the cDNA encoding AtEBP, known as Arabidopsis thaliana ethylene-responsive element binding protein, was isolated numerous times in the functional screen (82% of all suppressors). Full-length AtEBP and its localization to the nucleus were essential for the suppression of Bax-induced cell death. Morphological abnormality of intracellular network that is a hallmark of Bax-induced cell death was attenuated by expression of AtEBP.

A streamlined process to phenotypically profile heterologous cDNAs in parallel using yeast cell-based assays

To meet the demands of developing lead drugs for the profusion of human genes being sequenced as part of the human genome project, we developed a high-throughput assay construction method in yeast. A set of optimized techniques allows us to rapidly transfer large numbers of heterologous cDNAs from nonyeast plasmids into yeast expression vectors. These high- or low-copy yeast expression plasmids are then converted quickly into integration-competent vectors for phenotypic profiling of the heterologous gene products. The process was validated first by testing proteins of diverse function, such as p38, poly(ADP-ribose) polymerase-1, and PI 3-kinase, by making active-site mutations and using existing small molecule inhibitors of these proteins. For less well-characterized genes, a novel random mutagenesis scheme was developed that allows a combination selection/screen for mutations that retain full-length expression and yet reverse a growth phenotype in yeast. A broad range of proteins in different functional classes has been profiled, with an average yield for growth interference phenotypes of approximately 30%. The ease of manipulation of the yeast genome affords us the opportunity to approach drug discovery and exploratory biology on a genomic scale and shortens assay development time significantly.

Mammalian Bax-induced plant cell death can be down-regulated by overexpression of Arabidopsis Bax Inhibitor-1 (AtBI-1)

We recently isolated the AtBI-1 (Arabidopsis Bax Inhibitor-1) gene, the expression of which suppressed Bax-induced cell death in yeast. To determine whether the same is true in the plant system, transgenic Arabidopsis plants overexpressing Bax protein under a dexamethasone (DEX)-inducible promoter were generated. On DEX treatment, such transgenic plants exhibited marked cell death at the whole-plant level, cell shrinkage, membranous destruction, and other apoptotic phenotypes. Transgenic Bax plants were retransformed with a vector containing the AtBI-1 gene (tagged with green fluorescent protein) under the control of the cauliflower mosaic virus 35S promoter. Plants expressing both Bax and AtBI-1 were able to maintain growth on DEX-treatment by sustaining intracellular integrity. Thus, we present here direct genetic evidence that the plant antiapoptotic protein AtBI-1 is biologically active in suppressing the mammalian Bax action in planta.

Toxicity of human adenovirus E4orf4 protein in Saccharomyces cerevisiae results from interactions with the Cdc55 regulatory B subunit of PP2A

The E4orf4 protein of human adenovirus induces p53-independent apoptosis, a process that may promote cell death and viral spread. When expressed alone, E4orf4 kills transformed cells but not normal human cells. The only clear target of E4orf4 in mammalian cells is the Balpha (B55) subunit of protein phosphatase 2A (PP2A), a member of one of three classes of regulatory B subunits. Here we report the effects of E4orf4 in Saccharomyces cerevisiae, which encodes two PP2A regulatory B subunits, CDC55 and RTS1, that share homology with mammalian B and B' subunits, respectively. E4orf4 expression was found to be toxic in yeast, resulting in the accumulation of cells in G2/M phase that failed to grow upon removal of E4orf4. E4orf4-expressing yeast also displayed an elongated cell morphology similar to cdc55 deletion strains. E4orf4 required CDC55 to elicit its effect, whereas RTS1 was dispensable. The recruitment of the PP2A holoenzyme by E4orf4 was entirely dependent on Cdc55. These studies indicate that E4orf4-induced apoptosis in mammalian cells and cell death in yeast require functional interactions with B-type subunits of PP2A. However, some inhibition of growth by E4orf4 was observed in the cdc55 strain and with an E4orf4 mutant that fails to interact with Cdc55, indicating that E4orf4 may possess a second Cdc55-independent function affecting cell growth.

Adenovirus E4orf4 protein induces PP2A-dependent growth arrest in Saccharomyces cerevisiae and interacts with the anaphase-promoting complex/cyclosome

Adenovirus early region 4 open reading frame 4 (E4orf4) protein has been reported to induce p53-independent, protein phosphatase 2A (PP2A)-dependent apoptosis in transformed mammalian cells. In this report, we show that E4orf4 induces an irreversible growth arrest in Saccharomyces cerevisiae at the G2/M phase of the cell cycle. Growth inhibition requires the presence of yeast PP2A-Cdc55, and is accompanied by accumulation of reactive oxygen species. E4orf4 expression is synthetically lethal with mutants defective in mitosis, including Cdc28/Cdk1 and anaphase-promoting complex/cyclosome (APC/C) mutants. Although APC/C activity is inhibited in the presence of E4orf4, Cdc28/Cdk1 is activated and partially counteracts the E4orf4-induced cell cycle arrest. The E4orf4-PP2A complex physically interacts with the APC/C, suggesting that E4orf4 functions by directly targeting PP2A to the APC/C, thereby leading to its inactivation. Finally, we show that E4orf4 can induce G2/M arrest in mammalian cells before apoptosis, indicating that E4orf4-induced events in yeast and mammalian cells are highly conserved.

A reversible component of mitochondrial respiratory dysfunction in apoptosis can be rescued by exogenous cytochrome c

Multiple apoptotic pathways release cytochrome c from the mitochondrial intermembrane space, resulting in the activation of downstream caspases. In vivo activation of Fas (CD95) resulted in increased permeability of the mitochondrial outer membrane and depletion of cytochrome c stores. Serial measurements of oxygen consumption, NADH redox state and membrane potential revealed a loss of respiratory state transitions. This tBID-induced respiratory failure did not require any caspase activity. At early time points, re-addition of exogenous cytochrome c markedly restored respiratory functions. Over time, however, mitochondria showed increasing irreversible respiratory dysfunction as well as diminished calcium buffering. Electron microscopy and tomographic reconstruction revealed asymmetric mitochondria with blebs of herniated matrix, distended inner membrane and partial loss of cristae structure. Thus, apoptogenic redistribution of cytochrome c is responsible for a distinct program of mitochondrial respiratory dysfunction, in addition to the activation of downstream caspases.

Mitochondria-dependent apoptosis and cellular pH regulation

Mitochondria play a critical role in apoptosis induction in response to myriad stimuli. These organelles release proteins into the cytosol which trigger caspase activation or perform other functions relevant to apoptosis, including cytochrome c (cyt-c), caspases, AIF, and SMAC (Diablo). The mechanisms by which these proteins escape from mitochondria remain enigmatic. Moreover, it is unclear whether release of these proteins versus disturbances in core mitochondrial functions represents the cell death commitment mechanism. In this regard, suppression of apoptosis using broad-spectrum caspase inhibitory compounds has been reported in many circumstances to prevent the morphological and biochemical manifestations of apoptosis, and yet not protect cells from death and not preserve clonigenic survival. Thus, while mitochondrial damage can be coupled to caspase activation pathways, cell death commitment often occurs upstream of caspase activation when mitochondria-dependent cell death pathways are invoked. Here, we review evidence implicating dysregulation of cellular pH as a component of the cell death mechanism involving mitochondria. Cell Death and Differentiation (2000) 7, 1155 - 1165

A novel plant glutathione S-transferase/peroxidase suppresses Bax lethality in yeast

The mammalian inducer of apoptosis Bax is lethal when expressed in yeast and plant cells. To identify potential inhibitors of Bax in plants we transformed yeast cells expressing Bax with a tomato cDNA library and we selected for cells surviving after the induction of Bax. This genetic screen allows for the identification of plant genes, which inhibit either directly or indirectly the lethal phenotype of Bax. Using this method a number of cDNA clones were isolated, the more potent of which encodes a protein homologous to the class theta glutathione S-transferases. This Bax-inhibiting (BI) protein was expressed in Escherichia coli and found to possess glutathione S-transferase (GST) and weak glutathione peroxidase (GPX) activity. Expression of Bax in yeast decreases the intracellular levels of total glutathione, causes a substantial reduction of total cellular phospholipids, diminishes the mitochondrial membrane potential, and alters the intracellular redox potential. Co-expression of the BI-GST/GPX protein brought the total glutathione levels back to normal and re-established the mitochondrial membrane potential but had no effect on the phospholipid alterations. Moreover, expression of BI-GST/GPX in yeast was found to significantly enhance resistance to H(2)O(2)-induced stress. These results underline the relationship between oxidative stress and Bax-induced death in yeast cells and demonstrate that the yeast-based genetic strategy described here is a powerful tool for the isolation of novel antioxidant and antiapoptotic genes.

Bax and Bcl-xL independently regulate apoptotic changes of yeast mitochondria that require VDAC but not adenine nucleotide translocator

Mitochondria play an essential role in apoptosis by releasing apoptogenic molecules such as cytochrome c and AIF, and some caspases, which are all regulated by Bcl-2 family proteins. Pro-apoptotic Bax and Bak have been shown to induce cytochrome c release and loss of membrane potential (Deltapsi) leading to AIF release in the isolated mitochondria. We have previously shown that Bax and Bak open the voltage-dependent anion channel (VDAC) allowing cytochrome c to pass through the channel, and Bcl-xL closes the channel. However, it has been reported that it is adenine nucleotide translocator (ANT) with which Bax/Bcl-xL interacts that modulate the channel activity. Here, we investigated the role of ANT and VDAC in the changes of isolated mitochondria triggered by Bax and by chemicals that induce permeability transition (PT). In rat and yeast mitochondria, Bax did not affect the ADP/ATP exchange activity of ANT. VDAC-deficient but not ANT-deficient yeast mitochondria showed resistance to cytochrome c release, Deltapsi loss, and swelling caused by Bax and PT inducers. Bcl-xL showed similar inhibition of all these changes in ANT-deficient and wild type yeast mitochondria. Furthermore, Bax induces cytochrome c release in wild type yeast cells but not VDAC1-deficient yeast cells. These data indicate that VDAC, but not ANT, is essential for apoptotic mitochondrial changes. The data also indicate that Bcl-xL and Bax possess an ability to regulate mitochondrial membrane permeability independently of other Bcl-2 family members.

Programmed death in bacteria

Programmed cell death (PCD) in bacteria plays an important role in developmental processes, such as lysis of the mother cell during sporulation of Bacillus subtilis and lysis of vegetative cells in fruiting body formation of Myxococcus xanthus. The signal transduction pathway leading to autolysis of the mother cell includes the terminal sporulation sigma factor Esigma(K), which induces the synthesis of autolysins CwlC and CwlH. An activator of autolysin in this and other PCD processes is yet to be identified. Autolysis plays a role in genetic exchange in Streptococcus pneumoniae, and the gene for the major autolysin, lytA, is located in the same operon with recA. DNA from lysed cells is picked up by their neighbors and recombined into the chromosome by RecA. LytA requires an unknown activator controlled by a sensory kinase, VncS. Deletion of vncS inhibits autolysis and also decreases killing by unrelated antibiotics. This observation suggests that PCD in bacteria serves to eliminate damaged cells, similar to apoptosis of defective cells in metazoa. The presence of genes affecting survival without changing growth sensitivity to antibiotics (vncS, lytA, hipAB, sulA, and mar) indicates that bacteria are able to control their fate. Elimination of defective cells could limit the spread of a viral infection and donate nutrients to healthy kin cells. An altruistic suicide would be challenged by the appearance of asocial mutants without PCD and by the possibility of maladaptive total suicide in response to a uniformly present lethal factor or nutrient depletion. It is proposed that a low rate of mutation serves to decrease the probability that asocial mutants without PCD will take over the population. It is suggested that PCD is disabled in persistors, rare cells that are resistant to killing, to ensure population survival. It is suggested that lack of nutrients leads to the stringent response that suppresses PCD, producing a state of tolerance to antibiotics, allowing cells to discriminate between nutrient deprivation and unrepairable damage. High levels of persistors are apparently responsible for the extraordinary survival properties of bacterial biofilms, and genes affecting persistence appear to be promising targets for development of drugs aimed at eradicating recalcitrant infections. PCD in unicellular eukaryotes is also considered, including aging in Saccharomyces cerevisiae. Apoptosis-like elimination of defective cells in S. cerevisiae and protozoa suggests that all unicellular life forms evolved altruistic programmed death that serves a variety of useful functions.

Bax, Bid and the permeabilization of the mitochondrial outer membrane in apoptosis

Mitochondria provide a key amplification step in the apoptotic pathway of many cells by releasing apoptogenic proteins into the cytosol. Recent studies have provided insights into how Bax and Bid may operate synergistically to recruit mitochondria into the pathway and how GD3 ganglioside, a metabolite of the sphingomyelin pathway, may also be used. In ischaemic disease, activation of the mitochondrial permeability transition pore may bypass the requirement for these factors.

Molecular steps of cell suicide: an insight into immune senescence

The cellular and molecular basis of immune senescence is unclear. A number of mechanisms have been proposed. In this issue of the Journal of Clinical Immunology, some of the mechanisms for various immunologic abnormalities in aging are presented. In this article, various molecular steps of both death receptor and mitochondrial pathways of apoptosis in general are reviewed. In particular, the role of apoptosis in T-cell immune senescence is discussed.

Detection of damage to the mitochondrial genome in the oncocytic cells of Warthin's tumour

Warthin's tumour of the salivary glands is composed of oncocytic cells containing excessive numbers of mitochondria which show frequent structural abnormalities and reduced metabolic function. Recent evidence of a strong association between cigarette smoking and the occurrence of Warthin's tumour prompted this study, to look for evidence of damage to mitochondrial DNA (mtDNA) that could be the result of an increase in oxidative stress; two-colour fluorescence in situ hybridization (FISH) was developed to show the distribution of mitochondria with deleted mtDNA in paraffin wax-embedded material. Approximately 10% of mtDNA bears the 'common' 4977 bp deletion. Using the polymerase chain reaction (PCR), the 4977 bp deletion was further quantified, in Warthin's tumour and age-matched normal parotid control tissue. Whilst the deletion was present in all parotid tissue, its presence was significantly higher in oncocytic tumour cells. In a small number of controls, there was a trend towards higher concentrations of the deletion in smokers.

Bcl-x and Bax regulate mouse primordial germ cell survival and apoptosis during embryogenesis

Restricted germ cell loss through apoptosis is initiated in the fetal gonad around embryonic day 13.5 (E13.5) as part of normal germ cell development. The mechanism of this germ cell attrition is unknown. We show that Bcl-x plays a crucial role in maintaining the survival of mouse germ cells during gonadogenesis. A bcl-x hypomorphic mouse was generated through the introduction of a neomycin (neo) gene into the promoter of the bcl-x gene by homologous recombination. Mice that contained two copies of the hypomorphic allele had severe reproductive defects attributed to compromised germ cell development. Males with two mutant alleles lacked spermatogonia and were sterile; females showed a severely reduced population of primordial and primary follicles and exhibited greatly impaired fertility. Primordial germ cells (PGCs) in bcl-x hypomorph mice migrated to the genital ridge by E12.5 but were depleted by E15.5, a time when Bcl-x and Bax were present. Two additional bcl-x transcripts were identified in fetal germ cells more than 300 bp upstream of previously reported start sites. Insertion of a neo cassette led to a down-regulation of the bcl-x gene at E12.5 in the hypomorph. Bax was detected by immunohistochemistry in germ cells from bcl-x hypomorph and control testes at E12.5 and E13.5. Bcl-x function was restored, and animals of both genders were fertile after removal of the neo selection cassette using Cre-mediated recombination. Alternatively, the loss of Bcl-x function in the hypomorph was corrected by the deletion of both copies of the bax gene, resulting in a restoration of germ cell survival. These findings demonstrate that the balance of Bcl-x and Bax control PGC survival and apoptosis.

Determinants of cytochrome c pro-apoptotic activity. The role of lysine 72 trimethylation

Cytochrome c released from vertebrate mitochondria engages apoptosis by triggering caspase activation. We previously reported that, whereas cytochromes c from higher eukaryotes can activate caspases in Xenopus egg and mammalian cytosols, iso-1 and iso-2 cytochromes c from the yeast Saccharomyces cerevisiae cannot. Here we examine whether the inactivity of the yeast isoforms is related to a post-translational modification of lysine 72, N-epsilon-trimethylation. This modification was found to abrogate pro-apoptotic activity of metazoan cytochrome c expressed in yeast. However, iso-1 cytochrome c lacking the trimethylation modification also was devoid of pro-apoptotic activity. Thus, both lysine 72 trimethylation and other features of the iso-1 sequence preclude pro-apoptotic activity. Competition studies suggest that the lack of pro-apoptotic activity was associated with a low affinity for Apaf-1. As cytochromes c that lack apoptotic function still support respiration, different mechanisms appear to be involved in the two activities.

Apoptosis in yeast--a monocellular organism exhibits altruistic behaviour

Apoptosis is a highly regulated form of programmed cell death crucial for life and health in metazoan animals. Apoptosis is defined by a set of cytological alterations. The recent discovery of these markers in yeast indicates the presence of the basic mechanisms of apoptosis already in unicellular eukaryotes. Oxygen radicals regulate both mammalian and yeast apoptosis. We suggest that apoptosis originated in unicellular organisms as an altruistic response to severe oxidative damage. Later, cells developed mechanisms to purposely produce reactive oxygen species as a regulator of apoptosis. Yeast may become an important model to investigate the conserved steps of apoptosis.

Biochemical and genetic analysis of the mitochondrial response of yeast to BAX and BCL-X(L)

The BCL-2 family includes both proapoptotic (e.g., BAX and BAK) and antiapoptotic (e.g., BCL-2 and BCL-X(L)) molecules. The cell death-regulating activity of BCL-2 members appears to depend on their ability to modulate mitochondrial function, which may include regulation of the mitochondrial permeability transition pore (PTP). We examined the function of BAX and BCL-X(L) using genetic and biochemical approaches in budding yeast because studies with yeast suggest that BCL-2 family members act upon highly conserved mitochondrial components. In this study we found that in wild-type yeast, BAX induced hyperpolarization of mitochondria, production of reactive oxygen species, growth arrest, and cell death; however, cytochrome c was not released detectably despite the induction of mitochondrial dysfunction. Coexpression of BCL-X(L) prevented all BAX-mediated responses. We also assessed the function of BCL-X(L) and BAX in the same strain of Saccharomyces cerevisiae with deletions of selected mitochondrial proteins that have been implicated in the function of BCL-2 family members. BAX-induced growth arrest was independent of the tested mitochondrial components, including voltage-dependent anion channel (VDAC), the catalytic beta subunit or the delta subunit of the F(0)F(1)-ATP synthase, mitochondrial cyclophilin, cytochrome c, and proteins encoded by the mitochondrial genome as revealed by [rho(0)] cells. In contrast, actual cell killing was dependent upon select mitochondrial components including the beta subunit of ATP synthase and mitochondrial genome-encoded proteins but not VDAC. The BCL-X(L) protection from either BAX-induced growth arrest or cell killing proved to be independent of mitochondrial components. Thus, BAX induces two cellular processes in yeast which can each be abrogated by BCL-X(L): cell arrest, which does not require aspects of mitochondrial biochemistry, and cell killing, which does.

Role of oxidative phosphorylation in Bax toxicity

The Bcl-2-related protein Bax is toxic when expressed either in yeast or in mammalian cells. Although the mechanism of this toxicity is unknown, it appears to be similar in both cell types and dependent on the localization of Bax to the outer mitochondrial membrane. To investigate the role of mitochondrial respiration in Bax-mediated toxicity, a series of yeast mutant strains was created, each carrying a disruption in either a component of the mitochondrial electron transport chain, a component of the mitochondrial ATP synthesis machinery, or a protein involved in mitochondrial adenine nucleotide exchange. Bax toxicity was reduced in strains lacking the ability to perform oxidative phosphorylation. In contrast, a respiratory-competent strain that lacked the outer mitochondrial membrane Por1 protein showed increased sensitivity to Bax expression. Deficiencies in other mitochondrial proteins did not affect Bax toxicity as long as the ability to perform oxidative phosphorylation was maintained. Characterization of Bax-induced toxicity in wild-type yeast demonstrated a growth inhibition that preceded cell death. This growth inhibition was associated with a decreased ability to carry out oxidative phosphorylation following Bax induction. Furthermore, cells recovered following Bax-induced growth arrest were enriched for a petite phenotype and were no longer able to grow on a nonfermentable carbon source. These results suggest that Bax expression leads to an impairment of mitochondrial respiration, inducing toxicity in cells dependent on oxidative phosphorylation for survival. Furthermore, Bax toxicity is enhanced in yeast deficient in the ability to exchange metabolites across the outer mitochondrial membrane.

A cytochrome c-GFP fusion is not released from mitochondria into the cytoplasm upon expression of Bax in yeast cells

To study Bax-induced release of cytochrome c in vivo, we have expressed a cytochrome c-GFP (green fluorescent protein) fusion in Saccharomyces cerevisiae cells null for the expression of the endogenous cytochrome. We show here that cytochrome c-GFP is efficiently localised to mitochondria and able to function as an electron carrier between complexes III and IV of the respiratory chain. Strikingly, while natural cytochrome c is released into the cytoplasm upon expression of Bax, the cytochrome c-GFP fusion is not. Nevertheless, cells co-expressing Bax and the cytochrome c-GFP fusion die, indicating that mitochondrial release of cytochrome c is not essential for cell death to occur in yeast. The failure to release cytochrome c-GFP is presumed to arise from increased bulk due to the GFP moiety. We propose that in intact yeast cells, Bax-induced release of cytochrome c into the cytoplasm occurs through a selective pore and not as a consequence of the non-specific breakage of the mitochondrial outer membrane.

Structural basis of BFL-1 for its interaction with BAX and its anti-apoptotic action in mammalian and yeast cells

BFL-1 is the smallest member of the BCL-2 family and has been shown to retard apoptosis in various cell lines. However, the structural basis for its function remains unclear. Molecular modeling showed that BFL-1 could have a similar core structure as BCL-xL, consisting of seven alpha helices, although both proteins share only the conserved BCL-2 homology domains (BH1 and BH2 domains), but otherwise have very limited sequence homology, particularly in the N-terminal region. We demonstrated in the yeast two-hybrid system that BFL-1 interacts strongly with human BAX but is not able to form homodimers nor to interact with human BCL-2 or BCL-xL. Overexpression experiments in REF52 rat fibroblasts showed that BFL-1 conferred increased resistance to apoptosis induced by serum deprivation. BFL-1 had also the ability to neutralize BAX lethality in yeast. BAX requires the BH3 domain for interaction with BFL-1. However, the minimal region of BFL-1 for the interaction with BAX in coimmunoprecipitation experiments was not sufficient to protect cells from apoptosis. Further examination of BFL-1 and several other anti-apoptotic proteins suggests a more general type of structure based on structural motifs, i.e. a hydrophobic pocket for the binding of proapoptotic proteins, rather than extended sequence homologies.

The cytotoxic action of Bax on yeast cells does not require mitochondrial ADP/ATP carrier but may be related to its import to the mitochondria

The effect of the expression of murine Bax protein on growth and vitality was examined in Saccharomyces cerevisiae and compared with the effect of Bax in mutant cells lacking functional mitochondria. The cytotoxic effect of Bax on yeast does not require functional oxidative phosphorylation, respiration, or mitochondrial proteins (ADP/ATP carriers) implicated in the formation of the permeability transition pore in mammalian mitochondria. In the wild type S. cerevisiae the expression of Bax does not result in a severe effect on mitochondrial membrane potential and respiration. On the basis of Bax induced differences in the fluorescence of green fluorescent protein fused to mitochondrial proteins, it is proposed that Bax may interfere with one essential cellular process in yeast: the mitochondrial protein import pathway that is specific for the proteins of the mitochondrial carrier family.