A Fox2-dependent fatty acid ß-oxidation pathway coexists both in peroxisomes and mitochondria of the ascomycete yeast Candida lusitaniae

It is generally admitted that the ascomycete yeasts of the subphylum Saccharomycotina possess a single fatty acid ß-oxidation pathway located exclusively in peroxisomes, and that they lost mitochondrial ß-oxidation early during evolution. In this work, we showed that mutants of the opportunistic pathogenic yeast Candida lusitaniae which lack the multifunctional enzyme Fox2p, a key enzyme of the ß-oxidation pathway, were still able to grow on fatty acids as the sole carbon source, suggesting that C. lusitaniae harbored an alternative pathway for fatty acid catabolism. By assaying 14Cα-palmitoyl-CoA consumption, we demonstrated that fatty acid catabolism takes place in both peroxisomal and mitochondrial subcellular fractions. We then observed that a fox2Δ null mutant was unable to catabolize fatty acids in the mitochondrial fraction, thus indicating that the mitochondrial pathway was Fox2p-dependent. This finding was confirmed by the immunodetection of Fox2p in protein extracts obtained from purified peroxisomal and mitochondrial fractions. Finally, immunoelectron microscopy provided evidence that Fox2p was localized in both peroxisomes and mitochondria. This work constitutes the first demonstration of the existence of a Fox2p-dependent mitochondrial β-oxidation pathway in an ascomycetous yeast, C. lusitaniae. It also points to the existence of an alternative fatty acid catabolism pathway, probably located in peroxisomes, and functioning in a Fox2p-independent manner.

Biting behaviour of Tabanidae on cattle in mountainous summer pastures, Pyrenees, France, and effects of weather variables

In France, during the summer, cattle in mountainous pastures can be highly exposed to tabanid bites. The persistent biting behaviour of tabanids not only causes disturbance, but is also responsible for transmitting diseases, such as bovine besnoitiosis. The purpose of this study was to better identify the level of tabanid annoyance on cattle by means of insect trapping and direct observation of cows. Tabanids were active during the entire daily observation period (10:00-16:00), except for Haematopota sp., which were less active in the morning. The tabanids collected in Nzi traps were generally representative of those that landed on cattle, except for Haematopota sp., as Nzi traps were not very effective for these species. The preferred feeding sites for most species appeared to be cow's legs or udder. Leg stamping was the defensive reaction most related to a tabanid alighting on a host. Generalized linear mixed models showed that the parameters associated with tabanid landings on hosts were related to weather and altitude, but not to landscape structure. Increased landings were mostly associated with the higher temperatures and lower wind speeds at midday, but some differences were observed between species. The results indicate that cattle-protection measures should be taken during the peak of tabanid abundance when climatic conditions favour intense biting activity. Nzi traps set close to livestock were very effective to catch tabanids and could help in reducing the annoyance caused by horse flies.

Olfactory and behavioural responses of tabanid horseflies to octenol, phenols and aged horse urine

Electrophysiological and behavioural responses of females of two tabanid species, Tabanus bromius L. and Atylotus quadrifarius (Loew) (Diptera: Tabanidae), to ammonia, octenol (1-octen-3-ol), phenols and aged horse urine were compared. Electroantennogram (EAG) responses in both species to octenol, 4-methylphenol (4MP), 3-propylphenol (3PP) and a phenol mixture (4MP and 3PP at a ratio of 16 : 1) increased in a dose-dependent fashion. The most effective stimulus was 4MP and synergism between the two phenols may exist. Aged horse urine also elicited strong EAG responses in both species. Using gas chromatography-mass spectrometry (GC-MS) analysis, we identified 29 compounds in horse urine, which included, in particular, ketones, fatty alcohols and phenols, among which 4MP was the most abundant component (~ 80%). Trapping experiments were carried out using Nzi traps baited with various odours. Octenol and the phenol mixture in combination with ammonia increased catches of tabanids by 1.8-2.8 times relative to ammonia alone. Aged horse urine increased catches of T. bromius and A. quadrifarius by 2.2 and 4.1 times, respectively. The high attractiveness of aged horse urine, especially for A. quadrifarius, is not likely to derive from 4MP alone, but from the mixture of various active compounds used in host location.

Bcl-wav and the mitochondrial calcium uniporter drive gastrula morphogenesis in zebrafish

Bcl-2 proteins are acknowledged as key regulators of programmed cell death. However, increasing data suggest additional roles, including regulation of the cell cycle, metabolism and cytoskeletal dynamics. Here we report the discovery and characterization of a new Bcl-2-related multidomain apoptosis accelerator, Bcl-wav, found in fish and frogs. Genetic and molecular studies in zebrafish indicate that Bcl-wav and the recently identified mitochondrial calcium uniporter (MCU) contribute to the formation of the notochord axis by controlling blastomere convergence and extension movements during gastrulation. Furthermore, we found that Bcl-wav controls intracellular Ca(2+) trafficking by acting on the mitochondrial voltage-dependent anion channel, and possibly on MCU, with direct consequences on actin microfilament dynamics and blastomere migration guidance. Thus, from an evolutionary point of view, the original function of Bcl-2 proteins might have been to contribute in controlling the global positioning system of blastomeres during gastrulation, a critical step in metazoan development.

Unsuspected pyocyanin effect in yeast under anaerobiosis

The blue–green phenazine, Pyocyanin (PYO), is a well-known virulence factor produced by Pseudomonas aeruginosa, notably during cystic fibrosis lung infections. It is toxic to both eukaryotic and bacterial cells and several mechanisms, including the induction of oxidative stress, have been postulated. However, the mechanism of PYO toxicity under the physiological conditions of oxygen limitation that are encountered by P. aeruginosa and by target organisms in vivo remains unclear. In this study, wild-type and mutant strains of the yeast Saccharomyces cerevisiae were used as an effective eukaryotic model to determine the toxicity of PYO (100–500 μmol/L) under key growth conditions. Under respiro-fermentative conditions (with glucose as substrate), WT strains and certain H₂O₂-hypersensitive strains showed a low-toxic response to PYO. Under respiratory conditions (with glycerol as substrate) all the strains tested were significantly more sensitive to PYO. Four antioxidants were tested but only N-acetylcysteine was capable of partially counteracting PYO toxicity. PYO did not appear to affect short-term respiratory O₂ uptake, but it did seem to interfere with cyanide-poisoned mitochondria through a complex III-dependent mechanism. Therefore, a combination of oxidative stress and respiration disturbance could partly explain aerobic PYO toxicity. Surprisingly, the toxic effects of PYO were more significant under anaerobic conditions. More pronounced effects were observed in several strains including a ‘petite’ strain lacking mitochondrial DNA, strains with increased or decreased levels of ABC transporters, and strains deficient in DNA damage repair. Therefore, even though PYO is toxic for actively respiring cells, O₂ may indirectly protect the cells from the higher anaerobic-linked toxicity of PYO. The increased sensitivity to PYO under anaerobic conditions is not unique to S. cerevisiae and was also observed in another yeast, Candida albicans.

Bax activation by the BH3-only protein Puma promotes cell dependence on antiapoptotic Bcl-2 family members

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.

Bax activation and mitochondrial insertion during apoptosis

The mitochondrial apoptotic pathway is a highly regulated biological mechanism which determines cell fate. It is defined as a cascade of events, going from an apoptotic stimulus to the MOM permeabilization, resulting in the activation of the so-called executive phase. This pathway is very often altered in cancer cells. The mitochondrial permeabilization is under the control of the Bcl-2 family of proteins (pBcls). These proteins share one to four homology domains (designed BH1-4) with Bcl-2, and are susceptible of homo- and/or hetero-dimerization. In spite of a poor amino-acid sequence homology, these proteins exhibit very similar tertiary structures. Strikingly, while some of these proteins are anti-apoptotic, the others are pro-apoptotic. Pro-apoptotic proteins are further divided in two sub-classes: multi-domains proteins, among which Bax and Bak, which exhibit BH1-3 domains, and BH3-only proteins (or BOPs). Schematically, BOPs and anti-apoptotic proteins antagonistically regulate the activation of the multi-domain proteins Bax and Bak and their oligomerization in the MOM, the latter process being responsible for the apoptotic mitochondrial permeabilization. Considering the critical role of Bax in cancer cells apoptosis, we focus in this review on the molecular events of Bax activation through its interaction with the other proteins from the Bcl-2 family. The mechanism by which Bax triggers the MOM permeabilization once activated will be discussed in some other reviews in this special issue.

Structure and function of a mitochondrial late embryogenesis abundant protein are revealed by desiccation

Few organisms are able to withstand desiccation stress; however, desiccation tolerance is widespread among plant seeds. Survival without water relies on an array of mechanisms, including the accumulation of stress proteins such as the late embryogenesis abundant (LEA) proteins. These hydrophilic proteins are prominent in plant seeds but also found in desiccation-tolerant organisms. In spite of many theories and observations, LEA protein function remains unclear. Here, we show that LEAM, a mitochondrial LEA protein expressed in seeds, is a natively unfolded protein, which reversibly folds into alpha-helices upon desiccation. Structural modeling revealed an analogy with class A amphipathic helices of apolipoproteins that coat low-density lipoprotein particles in mammals. LEAM appears spontaneously modified by deamidation and oxidation of several residues that contribute to its structural features. LEAM interacts with membranes in the dry state and protects liposomes subjected to drying. The overall results provide strong evidence that LEAM protects the inner mitochondrial membrane during desiccation. According to sequence analyses of several homologous proteins from various desiccation-tolerant organisms, a similar protection mechanism likely acts with other types of cellular membranes.

TOM22, a core component of the mitochondria outer membrane protein translocation pore, is a mitochondrial receptor for the proapoptotic protein Bax

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.

Mitochondria as the target of the pro-apoptotic protein Bax

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.

Ypr140wp, 'the yeast tafazzin', displays a mitochondrial lysophosphatidylcholine (lyso-PC) acyltransferase activity related to triacylglycerol and mitochondrial lipid synthesis

When the yeast protein Ypr140w was expressed in Escherichia coli, a lyso-PC [lysophosphatidylcholine (1-acylglycerophosphorylcholine)] acyltransferase activity was found associated with the membranes of the bacteria. To our knowledge, this is the first identification of a protein capable of catalysing the acylation of lyso-PC molecules to form PC. Fluorescence microscopy analysis of living yeasts revealed that the fusion protein Ypr140w-green fluorescent protein is targeted to the mitochondria. Moreover, in contrast with wild-type cells, in the absence of acyl-CoA, the yeast mutant deleted for the YPR140w gene has no lyso-PC acyltransferase activity associated with the mitochondrial fraction. When yeast cells were grown in the presence of lactate, the mutant synthesized 2-fold more triacylglycerols when compared with the wild-type. Moreover, its mitochondrial membranes contained a lesser amount of PC and cardiolipin, and the fatty acid composition of these latter was greatly changed. These modifications were accompanied by a 2-fold increase in the respiration rates (states 3 and 4) of the mitochondria. The relationship between the deletion of the YPR140w gene and the lipid composition of the ypr140wDelta cells is discussed.

Role of cardiolipin on tBid and tBid/Bax synergistic effects on yeast mitochondria

The apoptotic effector Bid regulates cell death at the level of mitochondria. Under its native state, Bid is a soluble cytosolic protein that undergoes proteolysis and yields a 15 kDa-activated form tBid (truncated Bid). tBid translocates to mitochondria and participates in cytochrome c efflux by a still unclear mechanism, some of them at least mediated by Bax. Using mitochondria isolated from wild-type and cardiolipin (CL)-synthase-less yeast strains, we observed that tBid perturbs mitochondrial bioenergetics by inhibiting state-3 respiration and ATP synthesis and that this effect was strictly dependent on the presence of CL. In a second set of experiments, heterologous coexpression of tBid and Bax in wild-type and CL-less yeast strains showed that (i) tBid binding and the subsequent alteration of mitochondrial bioenergetics increased Bax-induced cytochrome c release and (ii) the absence of CL favors Bax effects independently of the presence of t-Bid. These data support recent views suggesting a dual function of CL in mitochondria-dependent apoptosis.

tBid interaction with cardiolipin primarily orchestrates mitochondrial dysfunctions and subsequently activates Bax and Bak

TNFR1/Fas engagement results in the cleavage of cytosolic Bid to truncated Bid (tBid), which translocates to mitochondria. We demonstrate that recombinant tBid induces in vitro immediate destabilization of the mitochondrial bioenergetic homeostasis. These alterations result in mild uncoupling of mitochondrial state-4 respiration, associated with an inhibition the adenosine diphosphate (ADP)-stimulated respiration and phosphorylation rate. tBid disruption of mitochondrial homeostasis was inhibited in mitochondria overexpressing Bcl-2 and Bcl-XL. The inhibition of state-3 respiration is mediated by the reorganization of cardiolipin within the mitochondrial membranes, which indirectly affects the activity of the ADP/ATP translocator. Cardiolipin-deficient yeast mitochondria did not exhibit any respiratory inhibition by tBid, proving the absolute requirement for cardiolipin for tBid binding and activity. In contrast, the wild-type yeast mitochondria underwent a similar inhibition of ADP-stimulated respiration associated with reduced ATP synthesis. These events suggest that mitochondrial lipids rather than proteins are the key determinants of tBid-induced destabilization of mitochondrial bioenergetics.

Regulation of the mitochondrial apoptosis-induced channel, MAC, by BCL-2 family proteins

Programmed cell death or apoptosis is central to many physiological processes and pathological conditions such as organogenesis, tissue homeostasis, cancer, and neurodegenerative diseases. Bcl-2 family proteins tightly control this cell death program by regulating the permeabilization of the mitochondrial outer membrane and, hence, the release of cytochrome c and other pro-apoptotic factors. Control of the formation of the mitochondrial apoptosis-induced channel, or MAC, is central to the regulation of apoptosis by Bcl-2 family proteins. MAC is detected early in apoptosis by patch clamping the mitochondrial outer membrane. The focus of this review is on the regulation of MAC activity by Bcl-2 family proteins. The role of MAC as the putative cytochrome c release channel during early apoptosis and insights concerning its molecular composition are also discussed.

Distinct Domains Control the Addressing and the Insertion of Bax into Mitochondria

The translocation of Bax from the cytosol into the mitochondrial outer membrane is a central event during apoptosis. We report that beyond the addressing step, which involves its first alpha-helix (halpha1), the helices alpha5 and alpha6 (halpha5alpha6) are responsible for the insertion of Bax into mitochondrial outer membrane bilayer. The translocation of Bax to mitochondria is associated with specific changes in the conformation of the protein that are under the control of two prolines: Pro-13, which controls the unfolding of halpha1, and Pro-168, a proline located immediately before the hydrophobic carboxyl-terminal end (i.e. helix alpha9, halpha9), which controls the disclosure of halpha5alpha6. An additional step, the disruption of an electrostatic bond formed between Asp-33 (halpha1) and Lys-64 (BH3), allows the mitochondria addressing of Bax. We conclude that, although the intramolecular interactions of halpha1 with the BH3 region control the addressing of Bax to mitochondria, the Pro-168 is involved in the control of its membrane insertion through halpha5alpha6.

Organization and regulation of the cytosolic NADH metabolism in the yeast Saccharomyces cerevisiae

Keeping a cytosolic redox balance is a prerequisite for living cells in order to maintain a metabolic activity and enable growth. During growth of Saccharomyces cerevisiae, an excess of NADH is generated in the cytosol. Aerobically, it has been shown that the external NADH dehydrogenase, Nde1p and Nde2p, as well as the glycerol-3-phosphate dehydrogenase shuttle, comprising the cytoplasmic glycerol-3-phosphate dehydrogenase, Gpdlp, and the mitochondrial glycerol-3-phosphate dehydrogenase, Gut2p, are the most important mechanisms for mitochondrial oxidation of cytosolic NADH. In this review we summarize the recent results showing (i) the contribution of each of the mechanisms involved in mitochondrial oxidation of the cytosolic NADH, under different physiological situations; (ii) the kinetic and structural properties of these metabolic pathways in order to channel NADH from cytosolic dehydrogenases to the inner mitochondrial membrane and (iii) the organization in supramolecular complexes and, the peculiar ensuing kinetic regulation of some of the enzymes (i.e. Gut2p inhibition by external NADH dehydrogenase activity) leading to a highly integrated functioning of enzymes having a similar physiological function. The cell physiological consequences of such an organized and regulated network are discussed.

The N-terminal end of Bax contains a mitochondrial-targeting signal

The translocation of Bax alpha, 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 alpha, which contains its first alpha helix (Eta alpha 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 psi, which lacks the first 20 amino acids of Bax alpha and is constitutively associated with mitochondria. Deletion of Eta alpha 1 impairs the binding of Bax psi to mitochondria, whereas a fusion of the N terminus of Bax alpha, which contains Eta alpha 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 psi with mitochondria as well as Bax-apoptogenic properties. The mutations of the Eta alpha 1, which inhibit Bax alpha and Bax psi 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 alpha and Bax psi 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.

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.

A novel, high conductance channel of mitochondria linked to apoptosis in mammalian cells and Bax expression in yeast

During apoptosis, proapoptotic factors are released from mitochondria by as yet undefined mechanisms. Patch-clamping of mitochondria and proteoliposomes formed from mitochondrial outer membranes of mammalian (FL5.12) cells has uncovered a novel ion channel whose activity correlates with onset of apoptosis. The pore diameter inferred from the largest conductance state of this channel is approximately 4 nm, sufficient to allow diffusion of cytochrome c and even larger proteins. The activity of the channel is affected by Bcl-2 family proteins in a manner consistent with their pro- or antiapoptotic properties. Thus, the channel activity correlates with presence of proapoptotic Bax in the mitochondrial outer membrane and is absent in mitochondria from cells overexpressing antiapoptotic Bcl-2. Also, a similar channel activity is found in mitochondrial outer membranes of yeast expressing human Bax. These findings implicate this channel, named mitochondrial apoptosis-induced channel, as a candidate for the outer-membrane pore through which cytochrome c and possibly other factors exit mitochondria during apoptosis.

Yeast mitochondrial dehydrogenases are associated in a supramolecular complex

Separation of yeast mitochondrial complexes by colorless native polyacrylamide gel electrophoresis led to the identification of a supramolecular structure exhibiting NADH-dehydrogenase activity. Components of this complex were identified by N-terminal Edman degradation and matrix-assisted laser desorption ionization mass spectrometry. The complex was found to contain the five known intermembrane space-facing dehydrogenases, namely two external NADH-dehydrogenases Nde1p and Nde2p, glycerol-3-phosphate dehydrogenase Gut2p, D- and L-lactate-dehydrogenases Dld1p and Cyb2p, the matrix-facing NADH-dehydrogenase Ndi1p, two probable flavoproteins YOR356Wp and YPR004Cp, four tricarboxylic acids cycle enzymes (malate dehydrogenase Mdh1p, citrate synthase Cit1p, succinate dehydrogenase Sdh1p, and fumarate hydratase Fum1p), and the acetaldehyde dehydrogenase Ald4p. The association of these proteins is discussed in terms of NADH-channeling.

Analysis of protein sequences and protein complexes by matrix-assisted laser desorption/ionization mass spectrometry

In the context of proteome analysis, matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) can fulfil the two tasks of primary structure verification and protein identification. As an illustration of the first of these tasks, the sequence of Eschericha coli isoleucyl-tRNA synthetase, a protein with 15 reported sequence conflicts, has been established by means of MALDI mass mapping. The identification of mitochondrial proteins participating in a yeast supramolecular complex exhibiting NADH dehydrogenase activity highlights the performances of MALDI-MS for the second task. The spectral suppression phenomenon occurring for complex peptide mixtures analysed by MALDI is discussed, as well as the role of post-source decay analysis for confident protein identification.

The substitution of the C-terminus of bax by that of bcl-xL does not affect its subcellular localization but abrogates its pro-apoptotic properties

The interaction of the anti-apoptotic members of the Bcl-2 family with mitochondria, through their hydrophobic C-terminus, has been proposed to play a crucial role in the execution phase of apoptosis. We report here that a substitution of the C-terminal end of pro-apoptotic bax by that of anti-apoptotic bcl-xL (baxCxL) does not modify its association with mitochondria in human and rat cells or in Saccharomyces cerevisiae. In addition, while bax sensitizes these cells to apoptotic stimuli, the construct baxCxL does not affect the apoptotic response in transfected cells. These results suggest that the C-terminus of bax plays an important role in apoptosis independently of its membrane addressing/targeting mechanism.

Comparison of the effects of bax-expression in yeast under fermentative and respiratory conditions: investigation of the role of adenine nucleotides carrier and cytochrome c

A new system for bax-expression in yeast has been devised to investigate bax's effect under fermentative and respiro-fermentative conditions. This has allowed us to show unambiguously that the ability of bax to kill yeast is higher under respiratory conditions than under purely fermentative conditions. The extent of killing under respiro-fermentative conditions (non-repressive sugars) is intermediate. It has been proposed that the two proteins adenine nucleotides carrier (ANC) and cytochrome c play a crucial role in bax-induced cell death. We have investigated the effects of deletion of the genes encoding the two proteins on the toxicity induced by bax, using this new system. The absence of ANC did not modify bax-induced lethality in any way. Moreover, the absence of cytochrome c also did not prevent bax-induced death. Only the kinetics of lethality were altered. All these effects are prevented by co-expression of bcl-xL.

Investigation of bax-induced release of cytochrome c from yeast mitochondria permeability of mitochondrial membranes, role of VDAC and ATP requirement

Recent studies that attempt to explore the action of pro- and anti-apoptotic proteins of the bcl2 family demonstrate the crucial role of relocalization of cytochrome c from the mitochondrial intermembrane space to the cytosol. This early event of apoptosis can be mimicked in the yeast Saccharomyces cerevisiae following expression of bax. In mammalian mitochondria, the mechanism of relocalization is thought to involve the opening of the so-called permeability transition pore. We show in this paper: (a) that bax-induced release of cytochrome c in yeast does not involve any permeability transition of the inner mitochondrial membrane but involves a general alteration of the permeability of the outer mitochondrial membrane to macromolecules. This suggests that a permeability transition of the inner mitochondrial membrane is not an event required for the relocalization of cytochrome c in yeast. (b) The outer-membrane voltage-dependent anion channel (VDAC), a putative component of the permeability transition pore, is not involved in bax-induced release of cytochrome c or in the prevention of this release by bcl-xL. (c) Bax devoid of its C-terminal putative hydrophobic alpha-helix is as efficient as full-length bax to allow the relocalization of cytochrome c, demonstrating this segment of the protein is not required for membrane-targeting. (d) We finally observe that the action of bax on the outer mitochondrial membrane requires the presence of ATP both in vitro and in vivo, and it is shown that ATP directly increases the amount of bax inserted to mitochondria.

Dependence of yeast mitochondrial unselective channel activity on the respiratory chain

The dependence of yeast mitochondrial unselective channel activity on the respiratory chain was investigated. Modulation of the respiratory chain with different substrates and inhibitors showed that channel activity was dependent on the electron flow rate through the chain and that external NADH only could provide a sufficient rate to activate the channel. These results support the hypothesis that the yeast mitochondrial unselective channel may be involved in the oxidation of cytosolic NADH without coupling to ATP synthesis.

Role of the C-terminal domain of Bax and Bcl-XL in their localization and function in yeast cells

It has been suggested that the C-terminal domain of Bcl-2 family members may contain a signal anchor sequence that targets these proteins to the mitochondrial outer membrane. We have investigated the consequence of deleting this domain upon cytochrome c release in yeast strains that coexpress truncated forms of Bax (i.e. BaxA) and Bcl-X(L) (i.e. Bcl-X(L)delta). We find that (i) Bax(delta) is as efficient as full-length Bax in promoting cytochrome c release, but Bcl-x(L)delta has remarkably reduced rescuing ability compared to full-length Bcl-x(L); (ii) full-length Bcl-X(L) protein acts by relocalizing Bax from the mitochondrial fraction to the soluble cytosolic fraction; (iii) Bax undergoes N-terminal cleavage when expressed in yeast, which is prevented by coexpression of Bcl-X(L), suggesting that Bcl-x(L) may mask the cleavage site of Bax through a direct physical interaction of the two proteins.

Characterization of the yeast mitochondria unselective channel: a counterpart to the mammalian permeability transition pore?

Large and unselective permeabilities through the inner membrane of yeast mitochondria have been observed for more than 20 years, but the characterization of these permeabilities, leading to hypothesize the existence of a large-conductance unselective channel in yeast inner mitochondrial membrane, was done only recently by several groups. This channel has been tentatively identified as a yeast counterpart to the mammalian permeability transition pore, the crucial role of which is now well-documented in physiopathological phenomena, such as Ca2+ homeostasis, ischemic damages, or programmed cell death. The aim of this review is to make a point on the known characteristics of this yeast mitochondrial unselective channel (YMUC) and to analyze whether or not it can be considered as a "yeast permeability transition pore."

Investigation of the yeast mitochondrial unselective channel in intact and permeabilized spheroplasts

The existence of an activity corresponding to the nucleotide-induced Yeast Mitochondria Unselective Channel (YMUC2) of isolated mitochondria was investigated in permeabilized and intact spheroplasts of the baker's yeast Yeast Foam. In nystatin-permeabilized spheroplasts, ATP and GDP-beta-S induced a decavanadate-sensitive stimulation of the respiration only under conditions equivalent to those previously reported for isolated mitochondria (low phosphate concentration, presence of a salt). On intact spheroplasts parallel measurements of respiration rate, [ATP]/[ADP] ratio and mitochondrial transmembrane potential allowed to show that the addition of the glucose analog 2-deoxyglucose decreased the permeability of the inner mitochondrial membrane owing to cellular ATP depletion. This strongly supports the hypothesis that Yeast Mitochondria Unspecific Channel is active in situ and inhibited by cellular [ATP] depletion.

Release of cytochrome c and decrease of cytochrome c oxidase in Bax-expressing yeast cells, and prevention of these effects by coexpression of Bcl-xL

The characteristics of mitochondria of yeast cells expressing the pro-apoptotic gene Bax or coexpressing Bax and the anti-apoptotic gene Bcl-xL have been investigated in whole cells, isolated mitochondria and permeabilized spheroplasts. It is found that Bax-induced growth arrest of yeast cells is related to two defects in the respiratory chain: (i) a decrease in the amount of cytochrome c oxidase, the terminal enzyme of the respiratory chain, and (ii) a dramatic increase in the release of cytochrome c to the cytosol. Other components of the inner mitochondrial membrane (bc1 complex and F0F1-ATPase) are unaffected. Coexpression of Bcl-xL almost fully prevented the effect of Bax. Surprisingly, these results obtained in yeast parallel similar observations reported in mammalian cells.

Modulation of the electrophoretic ATP-induced K(+)-transport in yeast mitochondria by delta pH

The modulation of the ATP-induced K(+)-transport pathway of the yeast inner mitochondrial membrane by delta pH was investigated in two ways. First, the inhibitory effect of phosphate was compared to the effect of other permeant acids, demonstrating that a part of the effect of phosphate was linked to its electroneutral transport down delta pH. However, an additional effect specific for phosphate also occurred inside the matrix. Second, the stimulation of the respiration by ATP in the presence of K+ was compared to the effects of the protonophore ClCCP2 and of the K(+)-ionophore valinomycin. Quite unexpectedly, the effect of ATP looked more like the effect of ClCCP than the effect of valinomycin. This and previous results (Manon et al., Biochimica et Biophysica Acta 1231, 282-288 (1995)) show that the rate of electrophoretic K(+)-entry is limited by the rate of electroneutral K(+)-exit via the K+/H+ exchange and is therefore delta pH-dependent.

Conditions allowing different states of ATP- and GDP-induced permeability in mitochondria from different strains of Saccharomyces cerevisiae

The effect of ATP and other nucleotides on the respiration of Saccharomyces cerevisiae mitochondria was investigated. It was observed that ATP induced a stimulation of the respiration rate only in the presence of a salt in mitochondria from the baker's yeast Yeast Foam, whereas an ATP-induced stimulation occurred even in the absence of salt in mitochondria from three different laboratory strains. In both cases, the stimulation was related to a collapse of the transmembrane potential, suggesting the opening of ion- and/or proton-conducting pathways. Not only ATP, but also GTP and CTP, induced these pathways. Moreover, a similar stimulation was obtained with GDP and its analog GDP-beta-S. The fact that, as opposed to NTPs, GDP did not induce any non-specific anion channel, allowed us to use it to demonstrate unambiguously that a proton-conducting pathway was opened through the inner mitochondrial membrane of laboratory strains but not of Yeast Foam. Three additional aspects of this nucleotide-induced permeability were investigated. (i) The proton-conducting pathway was insensitive to Mg2+, whereas the anion-conducting pathway was fully inhibited by 4 mM Mg2-. (ii) The proton-conducting pathway of mitochondria isolated from laboratory strains was opened by the action of nucleotides outside the mitochondrion, since it was fully insensitive to (carboxy)atractyloside, and fully active in mitochondria isolated from op1 and delta anc strains. On the other hand, the cation-conducting pathway of Yeast Foam mitochondria was partly sensitive to (carboxy)atractyloside and insensitive to bongkrekic acid, suggesting a role of the conformational state of ANC in this activity. (iii) Both the proton and cation-conducting pathways were inhibited by very low concentrations of vanadate, under conditions where this oxyanion was polymerized to decavanadate: a competitor to nucleotide-binding sites on some enzymes.

The ATP-induced K(+)-transport pathway of yeast mitochondria may function as an uncoupling pathway

The effect of the presence of K+ during oxidative phosphorylation measured on isolated yeast mitochondria was dependent on phosphate concentration. At 0.5 mM phosphate, K+ did promote an uncoupling of oxidative phosphorylation, which was prevented by decavanadate, a potent inhibitor of the ATP-induced K(+)-transport pathway. AT 5 mM phosphate, no uncoupling effect of K+ could be evidenced. These data suggest that the ATP-induced K(+)-transport pathway may, under certain conditions, function as an uncoupling pathway of oxidative phosphorylation.

Stimulation of oxidative phosphorylation by electrophoretic K+ entry associated to electroneutral K+/H+ exchange in yeast mitochondria

The effect of the addition of KCl, at constant osmolarity, was investigated on oxidative phosphorylation in isolated yeast mitochondria. KCl stimulated both respiration and ATP synthesis rates without changing the ATP/O ratio. KCl did not change the relationships between respiration rates and the protonmotive force. Since the K+/H+ exchange activity was active under these conditions, the stimulatory effect of respiration could be explained by the net proton entry caused by the electrophoretic K+ entry/electroneutral K+/H+ exchange cycle. On the other hand, K+ entry stimulated phosphate accumulation and transport under non-phosphorylating conditions and decreased the kinetic control by phosphate transport under phosphorylating conditions. Additionally, the stimulation of ATP synthesis strongly depended on the activity of phosphate transport. Taken together, these data showed that electrophoretic K(+)-entry and electroneutral K+/H+ exchange occurred in phosphorylating yeast mitochondria but did not promote any uncoupling between respiration and ATP synthesis.

Decoupling of the bc1 complex in S. cerevisiae; point mutations affecting the cytochrome b gene bring new information about the structural aspect of the proton translocation

Four mutations in the mitochondrial cytochrome b of S. cerevisiae have been characterized with respect to growth capacities, catalytic properties, ATP/2e- ratio, and transmembrane potential. The respiratory-deficient mutant G137E and the three pseudo-wild type revertants E137 + I147F, E137 + C133S, and E137 + N256K were described previously (Tron and Lemesle-Meunier, 1990; Di Rago et al., 1990a). The mutant G137E is unable to grow on respiratory substrates but its electron transfer activity is partly conserved and totally inhibited by antimycin A. The secondary mutations restore the respiratory growth at variable degree, with a phosphorylation efficiency of 12-42% as regards the parental wild type strain, and result in a slight increase in the various electron transfer activities at the level of the whole respiratory chain. The catalytic efficiency for ubiquinol was slightly (G137E) or not affected (E137 + I147F, E137 + C133S, and E137 + N256K) in these mutants. Mutation G137E induces a decrease in the ATP/2e- ratio (50% of the W.T. value) and transmembrane potential (60% of the W.T. value) at the bc1 level, whereas the energetic capacity of the cytochrome oxidase is conserved. Secondary mutations I147F, C133S, and N256K partly restore the ATP/2e- ratio and the transmembrane potential at the bc1 complex level. The results suggest that a partial decoupling of the bc1 complex is induced by the cytochrome b point mutation G137E. In the framework of the protonmotive Q cycle, this decoupling can be explained by the existence of a proton wire connecting centers P and N in the wild type bc1 complex which may be amplified or uncovered by the G137E mutation when the bc1 complex is functioning.