Structure-function analysis of Bcl-2 protein. Identification of conserved domains important for homodimerization with Bcl-2 and heterodimerization with Bax

Role of apoptotic proteins in ischemic hippocampal damage

In this review, we have presented evidence that apoptotic proteins may be involved in ischemic cell death. We tried to keep in mind that focal and global ischemia almost certainly produces different forms of cell death. For example, necrotic cell death is clearly part of a focal cerebral infarct, although it is not seen in brief global insults. We also tried to make the point that not all apoptosis is the same and that various forms of non-necrotic cell death, including ischemic cell death after global ischemia, may share some of the same molecular mechanisms as classic forms of apoptosis. Finally, we wish to leave the reader with the clear impression that all the evidence is not in hand. More work needs to be done in animals to define the role of apoptotic proteins. For example, glial cells probably are important in necrosis and may play a role in ischemic cell death after transient global ischemia. Yet it is likely that glial cells die from injury in a different manner than neurons. A better understanding of these processes should lead to new therapeutic approaches that may make possible the salvaging of neurons well after an ischemic insult.

Heterodimerization-independent functions of cell death regulatory proteins Bax and Bcl-2 in yeast and mammalian cells

The pro-apoptotic protein Bax can homodimerize with itself and heterodimerize with the anti-apoptotic protein Bcl-2, but the significance of these protein-protein interactions remains unclear. Alanine substitution mutations were created in a well conserved IGDE motif found within the BH3 domain of Bax (residues 66-69) and the resulting mutant Bax proteins were tested for ability to homodimerize with themselves and to heterodimerize with Bcl-2. Correlations were made with cell death induction by these mutants of Bax both in mammalian cells where Bax may function through several mechanisms, and in yeast where Bax may exert its lethal actions through a more limited repertoire of mechanisms perhaps related to its ability to form ion channels in intracellular membranes. Two of the mutants, Bax(D68A) and Bax(E69A), retained the ability to homodimerize but failed to interact with Bcl-2 as determined by yeast two-hybrid assays and co-immunoprecipitation analysis using transfected mammalian cells. The Bax(E69A) protein exhibited a lethal phenotype in yeast, which could be specifically suppressed by co-expression of Bcl-2, despite its failure to dimerize with Bcl-2. Both the Bax(D68A) and Bax(E69A) proteins induced apoptosis when overexpressed in human 293 cells, despite an inability to bind to Bcl-2. Moreover, co-expression of Bcl-2 with Bax(D68A) and Bax(E69A) rescued mammalian cells from apoptosis. In contrast, a mutant of Bax lacking the IGDE motif, Bax(DeltaIGDE), was incapable of either homodimerizing with itself or heterodimerizing with Bcl-2 and was inactive at promoting cell death in either yeast or mammalian cells. Although failing to interact with Bcl-2, the Bax(D68A) and Bax(E69A) mutants retained the ability to bind to Bid, a putative Bax-activating member of the Bcl-2 family, and collaborated with Bid in inducing apoptosis. When taken together with previous observations, these findings indicate that (i) Bax can induce apoptosis in mammalian cells irrespective of heterodimerization with Bcl-2 and (ii) Bcl-2 can rescue both mammalian cells and yeast from the lethal effects of Bax without heterodimerizing with it. However, these results do not exclude the possibility that BH3-dependent homodimerization of Bax or interactions with Bax activators such as Bid may either assist or be required for the cell death-inducing mechanism of this protein.

Conversion of Bcl-2 to a Bax-like death effector by caspases

Caspases are a family of cysteine proteases implicated in the biochemical and morphological changes that occur during apoptosis (programmed cell death). The loop domain of Bcl-2 is cleaved at Asp34 by caspase-3 (CPP32) in vitro, in cells overexpressing caspase-3, and after induction of apoptosis by Fas ligation and interleukin-3 withdrawal. The carboxyl-terminal Bcl-2 cleavage product triggered cell death and accelerated Sindbis virus-induced apoptosis, which was dependent on the BH3 homology and transmembrane domains of Bcl-2. Inhibitor studies indicated that cleavage of Bcl-2 may further activate downstream caspases and contribute to amplification of the caspase cascade. Cleavage-resistant mutants of Bcl-2 had increased protection from interleukin-3 withdrawal and Sindbis virus-induced apoptosis. Thus, cleavage of Bcl-2 by caspases may ensure the inevitability of cell death.

Bcl-2 and Bax proteins in Lewy bodies from patients with Parkinson's disease and Diffuse Lewy body disease

Double-labelling immunohistochemistry of Bcl-2 and Bax, and ubiquitin (as a marker of Lewy bodies) was examined in the brains of patients with Parkinson's disease and Diffuse Lewy body disease to learn about possible modifications of protein expression and the presence of Lewy bodies. Bcl-2 and Bax immunoreactivities were observed in Lewy body-bearing and non-Lewy body-bearing neurons in patients with parkinsonism. These results show that Bcl-2 and Bax are probably not implicated in Lewy body formation and that the presence of Lewy bodies does not have a direct impact on the expression of Bcl-2 and Bax proteins in individual neurons.

Bcl-2 counters apoptosis by Bax heterodimerization-dependent and -independent mechanisms in the T-cell lineage

The effect of the cell death inhibitor Bcl-2 in relation to its capacity to dimerize with apoptosis promoter Bax or its homologs at their physiological expression levels was explored in the T-cell lineage. Transgenic mice expressing a BH1 mutant Bcl-2 (Bcl-2 mI-3), which fails to heterodimerize with proapoptotic members of the Bcl-2 family, such as Bax, were generated. Bcl-2 mI-3 protected immature CD4+8- thymocytes from spontaneous, glucocorticoid and anti-CD3-induced apoptosis and altered T cell maturation, resulting in increased percentages of CD3(hi) and CD4-8+ thymocytes. In contrast, apoptosis of peripheral T-cells was unaffected by transgene expression. This correlated with their high Bax expression level and insensitivity to the caspase inhibitor, zVAD-fmk, a functional hallmark of Bax-like activity. Thus, within the T-cell lineage Bcl-2 can inhibit apoptosis independent of its association with Bax or its homologs; yet, above a threshold level of their physiologic proapoptotic activity, the capacity of Bcl-2 to heterodimerize with Bax or its homologs appears essential for it to counter cell death.

Structure of detergent-resistant membrane domains: does phase separation occur in biological membranes?

Detergent-resistant membrane domains (DRMs) can be isolated from a variety of eukaryotic cells. DRMs are of interest because of their potential importance in processes such as intracellular membrane sorting, and signal transduction at the cell surface. One type of DRM is also present in caveolae, non clathrin-coated plasma membrane pits with proposed roles in endocytosis, lipid transport, and signal transduction. Here we review recent advances in understanding the structure of these domains, and explore the possibility that DRMs are present in a phase separate from the surrounding bilayer. DRMs are rich in sphingolipids and cholesterol. The long saturated acyl chains and high acyl chain melting temperature of sphingolipids mediate their association in detergent resistant domains. These sphingolipid and cholesterol-rich domains have the properties of the liquid-ordered phase previously described in model membranes. Several lines of investigation support the idea that DRMs are not detergent-induced artifacts, but exist as domains in cell membranes. A striking feature of the proteins in DRMs is that many of them are linked to lipids. These include both GPI anchored proteins, and acylated proteins such as Src-family kinases. The linkage of these proteins to saturated acyl chains may help in targeting them to ordered membrane domains. Caveolin, the major structural protein of caveolae, is multiply palmitoylated. The presence of a high concentration of palmitate chains in DRMs in caveolae may help stabilize ordered domains.

Signaling in unicellular eukaryotes

Aspects of intercellular and intracellular signaling systems in cell survival, proliferation, differentiation, chemosensory behavior, and programmed cell death in free-living unicellular eukaryotes have been reviewed. Comparisons have been made with both bacteria and metazoa. The central organisms were flagellates (Trypanosoma, Leishmania, and Crithidia), slime molds (Dictyostelium), yeast cells (Saccharomyces cerevisiae), and ciliates (Paramecium, Euplotes, and Tetrahymena). There are two novel aspects in this review. First, cellular responses are viewed in an evolutionary perspective, rather than from the more prevailing one, in which the unicellular eukaryotes are seen by the mammalian organisms. Second, results obtained with cell cultures in minimal, chemically defined nutrient media at low cell densities where intercellular signaling is strongly reduced are discussed. These results shed light on control mechanisms and their cooperation inside the living cell. Intracellular systems have many common features in unicellular and multicellular organisms.

The BH3 domain of Bax is sufficient for interaction of Bax with itself and with other family members and it is required for induction of apoptosis

bax is an apoptosis-inducing member of the bcl-2 multigene family. We have studied interactions of human Bax with itself, and with the apoptosis-preventing members Bcl-2 and Bcl-xL using a yeast two-hybrid system. Exhaustive Bax truncations were constructed and their interactions with full-length family members studied. Bax interacted similarly with itself as with the apoptosis-suppressing family members Bcl-2 and Bcl-xL in quantitative two-hybrid studies. A region of 41 amino acids covering the recently discovered BH3 domain of Bax was found to be necessary and sufficient for all interactions of Bax. Bax truncations containing BH3, but lacking BH1 and BH2 homology domains, interacted with the other family members markedly more strongly than full-length Bax, which may reflect conformational changes required for the interactions of full-length Bax. The minimum requirements for Bax homodimerization were found to be the BH3 domain from one Bax molecule and a region covering BH3 plus BH1 from another. We also studied the apoptosis-inducing activity of the Bax truncations upon microinjection of expression plasmids into rat fibroblasts. The BH3 region was required for the apoptosis-inducing activity of Bax, whereas BH1, BH2 and the N-terminus of Bax were dispensable.

Suppression of apoptosis by overexpression of Bcl-2 or Bcl-xL promotes survival and mutagenesis after oxidative damage

Apoptosis is the physiological process by which unwanted cells in an organism are killed. Bcl-2, a membrane-bound cytoplasmic protein, and its close relative Bcl-xL, are both effective inhibitors of apoptosis induced by a wide variety of stimuli in many different cell types. In a previous study, we reported that suppression of apoptosis by Bcl-2 or Bcl-xL, markedly elevates the levels of radiation-induced mutations at the specific locus thymidine kinase. We investigated the effect of the Bcl-2 or Bcl-xL overproduction on hydrogen peroxide-induced mutagenesis. Oxidative DNA damage has been implicated in biological processes such as mutagenesis, carcinogenesis and aging. Overexpression of either Bcl-2 or Bcl-xL enhances oxidative stress mutagenesis in cells with wild type p53 as well as with mutated p53 protein. These results support the hypothesis that apoptosis plays a crucial role in maintaining genomic integrity by selectively eliminating highly mutated cells from the population.

Integrins (alpha7beta1) in muscle function and survival. Disrupted expression in merosin-deficient congenital muscular dystrophy

Mutations in genes coding for dystrophin, for alpha, beta, gamma, and delta-sarcoglycans, or for the alpha2 chain of the basement membrane component merosin (laminin-2/4) cause various forms of muscular dystrophy. Analyses of integrins showed an abnormal expression and localization of alpha7beta1 isoforms in myofibers of merosin-deficient human patients and mice, but not in dystrophin-deficient or sarcoglycan-deficient humans and animals. It was shown previously that skeletal muscle fibers require merosin for survival and function (Vachon, P.H., F. Loechel, H. Xu, U.M. Wewer, and E. Engvall. 1996. J. Cell Biol. 134:1483-1497). Correction of merosin deficiency in vitro through cell transfection with the merosin alpha2 chain restored the normal localization of alpha7beta1D integrins as well as myotube survival. Overexpression of the apoptosis-suppressing molecule Bcl-2 also promoted the survival of merosin-deficient myotubes, but did not restore a normal expression of alpha7beta1D integrins. Blocking of beta1 integrins in normal myotubes induced apoptosis and severely reduced their survival. These findings (a) identify alpha7beta1D integrins as the de facto receptors for merosin in skeletal muscle; (b) indicate a merosin dependence for the accurate expression and membrane localization of alpha7beta1D integrins in myofibers; (c) provide a molecular basis for the critical role of merosin in myofiber survival; and (d) add new insights to the pathogenesis of neuromuscular disorders.

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.

HDL3 binds to glycosylphosphatidylinositol-anchored proteins to activate signalling pathways

Previous studies have indicated that in HepG2 cells HDL3-signalling involves glycosylphosphatidylinositol (GPI) anchored proteins. HDL3-binding to HepG2 cells was found to be enhanced by cellular preincubation with PI-PLC inhibitors and sensitive to a cellular preincubation with exogenous PI-PLC, suggesting that HDL3 binds directly on GPI-anchored proteins to initiate signaling. Moreover HDL3-binding was found to be partly inhibited by antibodies against the HDL-binding protein (AbHBP). HDL3, when binding to HepG2 cells, promoted the release in the culture medium of a 110 kDa protein that binds AbHBP, while a cellular preincubation with antibodies against the inositol-phosphoglycan (IPG) moiety of GPI-anchor (AbIPG), used to block lipolytic cleavage of the GPI-anchor, inhibits HDL3-induced release of the 110 kDa protein in the culture medium. In [3H]-PC prelabeled HepG2 cells, AbHBP were found to stimulate PC-hydrolysis and DAG generation within 5 min as did HDL3 stimulation. Cellular preincubation with AbIPG was found to inhibit only the HDL3-signal and not the AbHBP-signal, while a prior cellular pretreatment with PI-PLC from Bacillus cereus was found to inhibit the HDL3-and AbHBP-signal. Moreover cellular preincubation with AbHBP for 1 h at 37 degrees C was found to inhibit HDL3-signalling pathways. Our results suggest that in HepG2 cells a 110 kDa protein, which could be HBP, can be anchored to the membrane via GPI, and can function in HDL3-signalling pathways as binding sites.

The anti-apoptosis function of Bcl-2 can be genetically separated from its inhibitory effect on cell cycle entry

The Bcl-2 family of proteins regulate apoptosis, some antagonizing cell death and others facilitating it. It has recently been demonstrated that Bcl-2 not only inhibits apoptosis but also restrains cell cycle entry. We show here that these two functions can be genetically dissociated. Mutation of a tyrosine residue within the conserved N-terminal BH4 region had no effect on the ability of Bcl-2 or its closest homologs to enhance cell survival and did not prevent heterodimerization with death-enhancing family members Bax, Bak, Bad and Bik. Neither did this mutation override the growth-inhibitory effect of p53. However, on stimulation with cytokine or serum, starved quiescent cells expressing the mutant proteins re-entered the cell cycle much faster than those expressing comparable levels of wild-type proteins. When wild-type and Y28 mutant Bcl-2 were co-expressed, the mutant was dominant. Although R-Ras p23 has been reported to bind to Bcl-2, no interaction was detectable in transfected cells and R-Ras p23 did not interfere with the ability of Bcl-2 to inhibit apoptosis or cell cycle entry. These observations provide evidence that the anti-apoptotic function of Bcl-2 is mechanistically distinct from its inhibitory influence on cell cycle entry.

Structural and functional complementation of an inactive Bcl-2 mutant by Bax truncation

Interactions among proteins in the Bcl-2 family regulate the onset of programmed cell death. Previous work has shown that the death-inhibiting family members Bcl-2 and Bcl-xL form heterodimers with the death-promoting homologue Bax and that certain site-directed mutants of Bcl-2 and Bcl-xL lose both biological activity and the ability to bind Bax. To better understand the structural basis of heterodimer formation, we have used a yeast two-hybrid assay to screen for mutants of Bax that regain the ability to bind to these inactive Bcl-2(G145A) and Bcl-xL(G138A) mutants. This screen identified a series of C-terminally truncated Bax molecules that contain complete BH3 (Bcl-2 homology domain 3) domains but that have lost BH1 and BH2 sequences. These results indicate that while the Bcl-2 and Bcl-xL mutants fail to bind full-length Bax, they still retain a binding site for the critical BH3 domain. This suggests that conformational constraints in full-length Bax regulate its ability to bind to other Bcl-2 family members. Furthermore, we demonstrate that the normally inert Bcl-2(G145A) mutant effectively blocks apoptosis induced by a C-terminally truncated Bax molecule, but does not block apoptosis induced by wild-type Bax. This demonstrates that cell protection can be effected by directly binding pro-apoptotic members of the Bcl-2 family.

Human Bcl-2 reverses survival defects in yeast lacking superoxide dismutase and delays death of wild-type yeast

We expressed the human anti-apoptotic protein, Bcl-2, in Saccharomyces cerevisiae to investigate its effects on antioxidant protection and stationary phase survival. Yeast lacking copper-zinc superoxide dismutase (sod1Delta) show a profound defect in entry into and survival during stationary phase even under conditions optimal for survival of wild-type strains (incubation in water after stationary phase is reached). Expression of Bcl-2 in the sod1Delta strain caused a large improvement in viability at entry into stationary phase, as well as increased resistance to 100% oxygen and increased catalase activity. In addition, Bcl-2 expression reduced mutation frequency in both wild-type and sod1Delta strains. In another set of experiments, wild-type yeast incubated in expired minimal medium instead of water lost viability quickly; expression of Bcl-2 significantly delayed this stationary phase death. Our results demonstrate that Bcl-2 has activities in yeast that are similar to activities it is known to possess in mammalian cells: (a) stimulation of antioxidant protection and (b) delay of processes leading to cell death.

Modulation of cell death in yeast by the Bcl-2 family of proteins

Bcl-2 family members are regulators of cell death. The precise biochemical properties of these proteins are unclear although intrafamily protein-protein association is thought to be involved. To elucidate structure-activity relationships among Bcl-2 proteins and identify the pathways in which they act, an inducible death suppressor assay was developed in yeast. Only Bax and Bak killed yeast via a process that did not require interleukin-1beta-converting enzyme-like proteases. Bax/Bak lethality was suppressed by coexpression of Bcl-2 family members that are anti-apoptotic in vertebrates, namely Bcl-xL, Bcl-2, Mcl-1, and A1. Furthermore, Bcl-xL and Bcl-2 suppressed Bax toxicity by distinct mechanisms in yeast. Bad, Bcl-xS, and Ced-9 lacked suppressor activity. These inactive proteins bound to anti-apoptotic members of the Bcl-2 family but not to Bax or Bak. In contrast, most Bcl-2 family proteins that attenuated death bound to Bax and Bak. However, two mutants of Bcl-xL suppressed Bax-induced cell death while having no Bax binding activity. Therefore, Bcl-xL functions independently of Bax binding, perhaps by interacting with a common target or promoting a pathway that antagonizes Bax. Thus, the pathways downstream of Bax and Bcl-xL may be conserved between vertebrates and yeast. This suppressor assay could be used to isolate components of these pathways.

Nonionic detergents induce dimerization among members of the Bcl-2 family

Members of the Bcl-2 family (including Bcl-2, Bcl-XL, and Bax) play key roles in the regulation of apoptosis. These proteins are believed to be membrane-associated and have been proposed to regulate apoptosis through both homodimerization and heterodimerization. We have found that whereas Bcl-2 is predominantly membrane-associated as previously reported, significant amounts of Bcl-XL and most of the Bax proteins are not membrane-associated and thus appear in the cytosolic fraction of thymocyte and splenocyte extracts. This finding allows the study of the dimerization properties and conformation of these proteins in the absence of detergent perturbation. For this analysis, we have produced monoclonal antibodies that are specific for known epitopes of Bax, Bcl-2, and Bcl-XL. An antibody to an N-terminal epitope (alpha uBax 6A7) between amino acids 12 and 24 fails to bind the soluble cytosolic form of Bax, indicating that this epitope is normally buried. Nonionic detergents alter the Bax conformation to expose this epitope. In the presence of nonionic detergent, the 6A7 antibody avidly binds the monomeric form of Bax, but not Bax complexed with either Bcl-XL or Bcl-2. In contrast, a monoclonal antibody to an adjacent epitope of Bax (alpha mBax 5B7) within amino acids 3-16 binds the soluble and detergent-altered forms of Bax and also binds the Bax.Bcl-XL or the Bax.Bcl-2 complex. Surprisingly, in the absence of detergent Bax fails to form homodimers or heterodimers with Bcl-XL. These results demonstrate a novel conformational state of members of the Bcl-2 family under a physiological condition that is distinct from the detergent-altered state that forms dimers and is currently believed to regulate apoptosis.

Developmental regulation, expression, and apoptotic potential of galectin-9, a beta-galactoside binding lectin

Galectin-9, a beta-galactoside binding lectin, has recently been isolated from murine embryonic kidney. In this study, its biological functions and expression in embryonic, newborn, and adult mice tissues were investigated. By Northern blot analyses, it was found widely distributed and its expression was developmentally regulated. In situ hybridization studies revealed an accentuated expression of galectin-9 in liver and thymus of embryonic mice. In postnatal mice, antigalectin-9 immunoreactivity was observed in various tissues, including thymic epithelial cells. The high expression of galectin-9 in the thymus led us to investigate its role in the clonal deletion of thymocytes. Fusion proteins were generated, which retained lactose-binding activity like the endogenous galectin-9. Galectin-9, at 2.5 microM concentration, induced apoptosis in approximately 30% of the thymocytes, as assessed by terminal deoxytransferase-mediated dUTP nick end labeling method. The apoptotic effect was dose dependent and lactose inhibitable. At higher concentrations, it induced homotypic aggregation of the thymocytes. Electron microscopy revealed approximately 60% of the thymocytes undergoing apoptosis in the presence of galectin-9. By immunofluorescence microscopy, some of the thymocytes undergoing apoptosis had plasmalemmal bound galectin-9. Galectin-9 failed to induce apoptosis in hepatocytes. Taken together, these findings indicate that galectin-9, a developmentally regulated lectin, plays a role in thymocyte-epithelial interactions relevant to the biology of the thymus.

Bcl-2 does not require Raf kinase activity for its death-protective function

It has been widely accepted that the oncogene product bcl-2 protects mammalian cells from programmed cell death (apoptosis). The molecules and signalling pathways upon which bcl-2 acts are, however, still ill-defined. Recently, bcl-2 was shown to interact with c-raf-1 in vitro. Furthermore, an active form of c-raf-1 delayed apoptosis induced by trophic factor deprivation and enhanced the death-suppressive function of bcl-2 when co-expressed. This has led to the hypothesis that bcl-2 communicates cell-death protection via a raf-dependent signal transduction pathway. Here we show, by various immunological and biochemical methods, that bcl-2 does not stably associate with c-raf-1 in cellular extracts prepared from fibroblasts before or after treatment with agents that induce apoptosis. Unexpectedly, bcl-2 function is entirely maintained, if not improved, when raf-dependent signalling is experimentally abrogated. In fact, bcl-2 allows the stable overexpression of a kinase-defective dominant-negative raf mutant that usually interferes with cell viability and/or proliferation. Our results indicate that bcl-2 does not require c-raf-1 kinase activity and an associated mitogen-activated protein kinase signalling pathway for its survival function. This property may be exploited to dissect cellular events that are dependent or independent of c-raf-1 kinase activity.

Channel formation by antiapoptotic protein Bcl-2

Bcl-2 is the prototypical member of a large family of apoptosis-regulating proteins, consisting of blockers and promoters of cell death. The three-dimensional structure of a Bcl-2 homologue, Bcl-XL, suggests striking similarity to the pore-forming domains of diphtheria toxin and the bacterial colicins, prompting exploration of whether Bcl-2 is capable of forming pores in lipid membranes. Using chloride efflux from KCl-loaded unilamellar lipid vesicles as an assay, purified recombinant Bcl-2 protein exhibited pore-forming activity with properties similar to those of the bacterial toxins, diphtheria toxin, and colicins, i.e., dependence on low pH and acidic lipid membranes. In contrast, a mutant of Bcl-2 lacking the two core hydrophobic alpha-helices (helices 5 and 6), predicted to be required for membrane insertion and channel formation, produced only nonspecific effects. In planar lipid bilayers, where detection of single channels is possible, Bcl-2 formed discrete ion-conducting, cation-selective channels, whereas the Bcl-2 (Deltah5, 6) mutant did not. The most frequent conductance observed (18 +/- 2 pS in 0.5 M KCl at pH 7.4) is consistent with a four-helix bundle structure arising from Bcl-2 dimers. However, larger channel conductances (41 +/- 2 pS and 90 +/- 10 pS) also were detected with progressively lower occurrence, implying the step-wise formation of larger oligomers of Bcl-2 in membranes. These findings thus provide biophysical evidence that Bcl-2 forms channels in lipid membranes, suggesting a novel function for this antiapoptotic protein.

Role of Bag-1 in the survival and proliferation of the cytokine-dependent lymphocyte lines, Ba/F3 and Nb2

The expression and function of the newly identified Bcl-2- and Raf-1- binding protein, Bag-1, during the cytokine-regulated growth of B and T cell lines was examined. Immunoblot analysis of lysates from the interleukin-3 (IL-3)-dependent B cell line Ba/F3, and the PRL-dependent T cell line Nb2, revealed that variations in Bag-1 levels paralleled alterations in cellular proliferation, viability, and apoptosis induced by the presence or absence of growth factor. To test whether up-regulation of Bag-1 levels altered cellular survival and proliferation, Ba/F3 cells were transfected with a Bag-1 expression construct. The overexpression of Bag-1 in transfected Ba/F3 cells induced an IL-3-independent state. Such transfectants demonstrated sustained viability and proliferation, with minimal apoptosis, in the complete absence of exogenous IL-3. Bag-1 expression was also compared in glucocorticoid-sensitive Nb2 cells and a PRL-independent, glucocorticoid-resistant subline, SFJCD1, during culture of these lines in dexamethasone (Dex). Bag-1 levels were profoundly decreased by the addition of Dex to Nb2 cells, precedent to the onset of apoptotic cell death. In contrast, Dex treatment or PRL withdrawal had no effect on levels of Bag-1 within the SFJCD1 line. These findings establish that the overexpression of Bag-1 in the appropriate cellular context promotes cellular survival and growth, events that may result from the juxtaposition of this protein with mitogenic and antiapoptotic signaling pathways.

Human Bak induces cell death in Schizosaccharomyces pombe with morphological changes similar to those with apoptosis in mammalian cells

Apoptosis as a form of programmed cell death (PCD) in multicellular organisms is a well-established genetically controlled process that leads to elimination of unnecessary or damaged cells. Recently, PCD has also been described for unicellular organisms as a process for the socially advantageous regulation of cell survival. The human Bcl-2 family member Bak induces apoptosis in mammalian cells which is counteracted by the Bcl-x(L) protein. We show that Bak also kills the unicellular fission yeast Schizosaccharomyces pombe and that this is inhibited by coexpression of human Bcl-x(L). Moreover, the same critical BH3 domain of Bak that is required for induction of apoptosis in mammalian cells is also required for inducing death in yeast. This suggests that Bak kills mammalian and yeast cells by similar mechanisms. The phenotype of the Bak-induced death in yeast involves condensation and fragmentation of the chromatin as well as dissolution of the nuclear envelope, all of which are features of mammalian apoptosis. These data suggest that the evolutionarily conserved metazoan PCD pathway is also present in unicellular yeast.

Bcl-2 relieves the trans-repressive function of the glucocorticoid receptor and inhibits the activation of CPP32-like cysteine proteases

Glucocorticoid induces apoptosis in immature lymphocytes which is inhibitable by Bcl-2. Although glucocorticoid-mediated signal transduction is well understood, the mechanism of the induction of apoptosis by the activated glucocorticoid receptor as well as the inhibition of apoptosis by Bcl-2 remains enigmatic. Here we report that overexpressed Bcl-2 relieves the glucocorticoid receptor-mediated repressive function on the AP-1 activity and completely inhibits the activation of CPP32-like cysteine proteases. In contrast, glucocorticoid receptor-mediated transactivation was not affected by Bcl-2. This suggests that glucocorticoid may induce apoptosis by repressing transactivation by AP-1 which is relieved by Bcl-2. Furthermore, we report evidence that, in contrast with CPP32-like proteases, ICE-like proteases are not involved in this apoptotic pathway.

Two N-terminal self-association domains are required for the dominant negative transcriptional activity of WT1 Denys-Drash mutant proteins

Patients with Denys-Drash syndrome (DDS) have been shown to be constitutionally heterozygous for mutations of the WT1 gene. Almost all DDS mutations inactivate or remove the DNA-binding zinc finger region of WT1 and the resulting mutant proteins appear to act in a dominant negative manner. This may occur via WT1 self-association, which has been shown to involve the first 180 amino acids. By creating a series of N-terminal deletions, we have further investigated WT1 self-association using a yeast di-hybrid system and an in vitro protein binding assay. Our results suggest that there are two distinct domains within the N-terminal region facilitating self-association, residing from amino acids 1-45 and 157-253. Co-transfection of WT1 with progressively shorter N-terminal constructs demonstrates that both of these sites are required for a dominant negative activity as assessed by activation of a reporter construct.

A common binding site mediates heterodimerization and homodimerization of Bcl-2 family members

Bcl-2 inhibits apoptosis induced by a wide variety of stimuli. In contrast, the Bcl-2 homologue, Bax, antagonizes Bcl-2's death protecting function. Bcl-2 forms protein-protein homodimers with itself and heterodimers with Bax, and previous experiments have shown that point mutations in Bcl-2 can abrogate Bax binding while leaving homodimerization intact. These mutagenesis results can be interpreted to suggest that Bcl-2 has separate binding sites that are responsible for homodimer and heterodimer formation. Results from yeast two-hybrid studies have also suggested that homodimerization and heterodimerization reflect distinct modes of interaction. However, using quantitative plate binding assays, we now show that Bax as well as peptides derived from the BH3 domains of Bax and Bak block both Bcl-2/Bax binding and Bcl-2/Bcl-2 binding. Similar assays demonstrate that Bcl-xL can form both homodimers and heterodimers and that these interactions are also inhibited by Bax and the BH3-derived peptides. These results demonstrate that the same binding motifs are responsible for both homodimerization and heterodimerization of Bcl-2 family members.

Apoptosis in the failing human heart

BACKGROUND

Loss of myocytes is an important mechanism in the development of cardiac failure of either ischemic or nonischemic origin. However, whether programmed cell death (apoptosis) is implicated in the terminal stages of heart failure is not known. We therefore studied the magnitude of myocyte apoptosis in patients with intractable congestive heart failure.

METHODS

Myocardial samples were obtained from the hearts of 36 patients who underwent cardiac transplantation and from the hearts of 3 patients who died soon after myocardial infarction. Samples from 11 normal hearts were used as controls. Apoptosis was evaluated histochemically, biochemically, and by a combination of histochemical analysis and confocal microscopy. The expression of two proto-oncogenes that influence apoptosis, BCL2 and BAX, was also determined.

RESULTS

Heart failure was characterized morphologically by a 232-fold increase in myocyte apoptosis and biochemically by DNA laddering (an indicator of apoptosis). The histochemical demonstration of DNA-strand breaks in myocyte nuclei was coupled with the documentation of chromatin condensation and fragmentation by confocal microscopy. All these findings reflect apoptosis of myocytes. The percentage of myocytes labeled with BCL2 (which protects cells against apoptosis) was 1.8 times as high in the hearts of patients with cardiac failure as in the normal hearts, whereas labeling with BAX (which promotes apoptosis) remained constant. The near doubling of the expression of BCL2 in the cardiac tissue of patients with heart failure was confirmed by Western blotting.

CONCLUSIONS

Programmed death of myocytes occurs in the decompensated human heart in spite of the enhanced expression of BCL2; this phenomenon may contribute to the progression of cardiac dysfunction.

Structural and biochemical similarities reveal a family of proteins related to the MAL proteolipid, a component of detergent-insoluble membrane microdomains

The MAL gene encodes a proteolipid protein displaying a cell type-specific pattern of expression, including T lymphocytes, myelin-forming cells, and epithelial renal MDCK cells, which has been recently identified as a component of detergent-insoluble membranes known to be enriched in glycolipids and cholesterol. Sequence alignment revealed a high degree of conservation of the MAL protein across species and evidenced the existence of a significant level of overall identity between MAL and two other proteins, BENE and the plasmolipin proteolipid. Moreover, using subcellular fractionation of transiently transfected COS-7 cells, both MAL and BENE were identified in detergent-resistant membranes. These results suggest the existence of a novel family of MAL-related proteins (including MAL, BENE, and plasmolipin) with primary structure homologies and with the distinctive features of having unusual biochemical properties such as lipid-like behaviour and/or the ability to reside in glycolipid-enriched membrane microdomains.

Kaposi's sarcoma-associated herpesvirus encodes a functional bcl-2 homologue

Kaposi's sarcoma-associated herpesvirus (KSHV) is a newly discovered herpesvirus etiologically associated with Kaposi's sarcoma (KS) and two lymphoproliferative disorders. We describe a KSHV vbcl-2 gene with homology to the proto-oncogene bcl-2. It is expressed in KS lesions and in cell lines derived from primary effusion lymphomas. Using yeast and human cells we demonstrate the ability of KSHV vBcl-2 protein to suppress Bax toxicity. We show that KSHV vBcl-2 heterodimerizes with human Bcl-2 in a yeast two-hybrid system. These results suggest that KSHV vBcl-2 plays an anti-apoptotic role in virus infected cells.

Structure of Bcl-xL-Bak peptide complex: recognition between regulators of apoptosis

Heterodimerization between members of the Bcl-2 family of proteins is a key event in the regulation of programmed cell death. The molecular basis for heterodimer formation was investigated by determination of the solution structure of a complex between the survival protein Bcl-xL and the death-promoting region of the Bcl-2-related protein Bak. The structure and binding affinities of mutant Bak peptides indicate that the Bak peptide adopts an amphipathic alpha helix that interacts with Bcl-xL through hydrophobic and electrostatic interactions. Mutations in full-length Bak that disrupt either type of interaction inhibit the ability of Bak to heterodimerize with Bcl-xL.

Bax- and Bak-induced cell death in the fission yeast Schizosaccharomyces pombe

The effects of the expression of the human Bcl-2 family proteins Bax, Bak, Bcl-2, and Bcl-XL were examined in the fission yeast Schizosaccharomyces pombe and compared with Bax-induced cell death in mammalian cells. Expression of the proapoptotic proteins Bax and Bak conferred a lethal phenotype in this yeast, which was strongly suppressed by coexpression of the anti-apoptotic protein Bcl-XL. Bcl-2 also partially abrogated Bax-mediated cytotoxicity in S. pombe, whereas a mutant of Bcl-2 (Gly145Ala) that fails to heterodimerize with Bax or block apoptosis in mammalian cells was inactive. However, other features distinguished Bax- and Bak-induced death in S. pombe from animal cell apoptosis. Electron microscopic analysis of S. pombe cells dying in response to Bax or Bak expression demonstrated massive cytosolic vacuolization and multifocal nuclear chromatin condensation, thus distinguishing this form of cell death from the classical morphological features of apoptosis seen in animal cells. Unlike Bax-induced apoptosis in 293 cells that led to the induction of interleukin-1 beta-converting enzyme (ICE)/CED-3-like protease activity, Bax- and Bak-induced cell death in S. pombe was accompanied neither by internucleosomal DNA fragmentation nor by activation of proteases with specificities similar to the ICE/CED-3 family. In addition, the baculovirus protease inhibitor p35, which is a potent inhibitor of ICE/CED-3 family proteases and a blocker of apoptosis in animal cells, failed to prevent cell death induction by Bax or Bak in fission yeast, whereas p35 inhibited Bax-induced cell death in mammalian cells. Taken together, these findings suggest that Bcl-2 family proteins may retain an evolutionarily conserved ability to regulate cell survival and death but also indicate differences in the downstream events that are activated by overexpression of Bax or Bak in divergent cell types.

Bcl-2 gene family in the nervous system

A growing family of genes that share homology with the bcl-2 proto-oncogene is involved in the regulation of cell death. Many of these proteins show widespread expression and are expressed in the nervous system in developing and adult organisms. A physiologic role for Bcl-2 and Bcl-x in neuron survival has been shown. In addition, these proteins have been shown to protect neurons from a wide array of toxic insults. In this review, we discuss the Bcl-2 family of proteins with regard to their structure and interactions. We then discuss the role of apoptotic cell death in the development of the nervous system and as a response to neuronal injury. Lastly, we discuss the evidence for a role for these cell death regulators in neuronal death decisions.

A role for Jun-N-terminal kinase in anoikis; suppression by bcl-2 and crmA

The disruption of interactions between extracellular matrix and specific cognate integrins triggers apoptosis in epithelial cells, in a process termed "anoikis." To understand anoikis, the connections between epithelial cell integrin signaling and the apoptosis-regulatory proteins are being explored. We report herein that early after detachment from matrix, epithelial cells activate Jun-N-Terminal Kinases (JNKs; alternatively known as Stress-activated Protein Kinases), which are also activated by other apoptotic stimuli. The activity of this pathway was required for anoikis. Another early response to cell suspension was the activation of the ICE-related cysteine protease, ICE/LAP3; this activation and anoikis were suppressed by the ICE-protease inhibitor, crmA. The overexpression of bcl-2 suppressed ICE/LAP3 activation as well. Surprisingly, bcl-2 and crmA attenuated the activation of JNKs following cell suspension, suggesting that the JNK pathway is regulated directly or indirectly by proteolysis. In addition, the blockage of the JNK pathway attenuated the activation of ICE/LAP3, suggesting a positive feedback loop between the ICE and JNK systems. These results indicate the following sequence of information flow in anoikis: integrins-->bcl-2/bax-->(ICE-proteases<-->JNK)-->apopt osis. Cell-cell interactions, which were previously shown to sensitize cells to anoikis, caused bcl-2 mRNA to be downregulated, a permissive event for downstream apoptotic signaling.

Recombinant mouse Bcl-2(1-203). Two domains connected by a long protease-sensitive linker

Bcl-2 is a cytoplasmic integral membrane protein with potent anti-apoptotic activity but whose mechanism of action is poorly understood. The purpose of this paper was to obtain large amounts of soluble Bcl-2 protein for structural and functional studies. Mouse Bcl-2(1-203) (missing the COOH-terminal hydrophobic tail) was produced in bacterial inclusion bodies, solubilized in guanidine, and refolded by dialysis. The resulting protein was monomeric in nondenaturing solution and was active in protecting mouse T hybridoma cells from glucocorticoid-induced apoptosis. Refolded Bcl-2(1-203) showed no tendency to homodimerize by gel filtration or analytical ultracentrifugation. Limited proteolysis experiments identified a region between the BH3 and BH4 homology domains of Bcl-2(1-203) which was extremely susceptible to digestion by several common proteases, but not by a cell extract known to contain CPP-32-like (interleukin-1beta-converting enzyme family) protease activity. The protease-sensitive sites were located within a 50-residue stretch that contained most of the nonconserved and proline residues of Bcl-2(1-203). Trypsin-cleaved Bcl-2(1-203) eluted in the same position as the undigested protein on gel filtration in nondenaturing solution, indicating that the two portions of the molecule connected by the protease-sensitive region associate stably and noncovalently. The solution properties of Bcl-2(1-203) suggest that it consists of two noncovalently associated domains connected by a long protease-sensitive linker and that its structure is similar to that of Bcl-xL, which has been determined by x-ray and NMR analysis.

Bcl-2 targets the protein kinase Raf-1 to mitochondria

A green fluorescent protein (GFP)-Raf-1 fusion protein was used to show that Bcl-2 can target this kinase to mitochondria. Active Raf-1 fused with targeting sequences from an outer mitochondrial membrane protein protected cells from apoptosis and resulted in phosphorylation of BAD, a proapoptotic Bcl-2 homolog. Plasma membrane-targeted Raf-1 did not protect from apoptosis and resulted in phosphorylation of ERK-1 and ERK-2. Untargeted active Raf-1 improved Bcl-2-mediated resistance to apoptosis, whereas a kinase-inactive Raf-1 mutant abrogated apoptosis suppression by Bcl-2. Bcl-2 can therefore target Raf-1 to mitochondrial membranes, allowing this kinase to phosphorylate BAD or possibly other protein substrates involved in apoptosis regulation.

Expression pattern of candidate cell death effector proteins Bax, Bcl-2, Bcl-X, and c-Jun in sensory and motor neurons following sciatic nerve transection in the rat

Numerous studies have demonstrated a prolonged expression of c-Jun transcription factor in neurons following axotomy, and it has been hypothesized that c-Jun may be causally involved in neuroregeneration in vivo. By contrast, there is growing evidence from in vitro studies that induction of c-Jun may be necessary for neuronal cell death induced by growth factor starvation. It has been demonstrated that protein levels of cell death repressor Bcl-2 and cell death promotor Bax determine the threshold for neuronal cell death and that their expression is dynamically modulated at the onset of neurodegeneration. In the present study, we investigated by double-immunolabeling methods activation of c-Jun transcription factor and expression of members of the Bcl-2 family of cell death effector proteins in axotomized neurons. Six days after transection of the sciatic nerve in young rats, when axotomized neurons start to degenerate, strong nuclear Jun immunostaining in spinal cord motoneurons was associated with intense cytoplasmic Bax labeling and signs of neuronal atrophy. Bcl-2 and Bcl-X proteins were present only at moderate to low levels. In situ end-labeling by terminal transferase revealed nuclear DNA fragmentation in scattered motoneurons of the ipsilateral ventral horn (1 or 2 labeled nuclei per section). In the L5 dorsal root ganglia (DRG) levels of Bax, Bcl-2, and Bcl-X proteins were highly variable. High levels of Bax immunoreactivity together with intense Jun immunofluorescence were frequently observed in small-diameter sensory neurons. RT-PCR analysis revealed expression of exclusively the anti-apoptotic bcl-xL mRNA isoform in rat DRG which decreased significantly following sciatic nerve transection. These findings indicate that the high susceptibility of central neurons and small-sized DRG neurons to axotomy-induced cell death might be related to their low ratio of cell death repressor Bcl-2 and Bcl-XL to cell death promotor Bax expression. It should be noted, however, that numerous strongly Jun-positive DRG neurons contained low levels of Bax or high levels of Bcl-2 and Bcl-X immunoreactivity. Thus, high levels of c-Jun protein in axotomized neurons do not necessarily suggest a destination to die, and other factors may determine the outcome of axotomy.

Structure-function comparisons of the proapoptotic protein Bax in yeast and mammalian cells

Expression of the proapoptotic protein Bax under the control of a GAL10 promoter in Saccharomyces cerevisiae resulted in galactose-inducible cell death. Immunofluorescence studies suggested that Bax is principally associated with mitochondria in yeast cells. Removal of the carboxyl-terminal transmembrane (TM) domain from Bax [creating Bax (deltaTM)] prevented targeting to mitochondrial and completely abolished cytotoxic function in yeast cells, suggesting that membrane targeting is crucial for Bax-mediated lethality. Fusing a TM domain from Mas70p, a yeast mitochondrial outer membrane protein, to Bax (deltaTM) restored targeting to mitochondria and cytotoxic function in yeast cells. Deletion of four well-conserved amino acids (IGDE) from the BH3 domain of Bax ablated its ability to homodimerize and completely abrogated lethality in yeast cells. In contrast, several Bax mutants which retained ability to homodimerize (deltaBH1, deltaBH2, and delta1-58) also retained at least partial lethal function in yeast cells. In coimmunoprecipitation experiments, expression of the wild-type Bax protein in Rat-1 fibroblasts and 293 epithelial cells induced apoptosis, whereas the Bax (deltaIGDE) mutant failed to induce apoptosis and did not associate with endogenous wild-type Bax protein. In contrast to yeast cells, Bax (deltaTM) protein retained cytotoxic function in Rat-1 and 293 cells, was targeted largely to mitochondria, and dimerized with endogenous Bax in mammalian cells. Thus, the dimerization-mediating BH3 domain and targeting to mitochondrial membranes appear to be essential for the cytotoxic function of Bax in both yeast and mammalian cells.

Importance of the Bcl-2 family in cell death regulation

Bcl-2 was first identified as a novel transcript associated with the t(14;18) chromosomal breakpoint which occurs in most follicular lymphomas. The deregulated expression of bcl-2 was found to contribute to multistep neoplasia through the suppression of cell death, or apoptosis, in transgenic mouse models. Bcl-2 was subsequently shown to be normally expressed in a variety of tissues and to significantly inhibit the induction of apoptosis in many experimental systems. Bcl-2 is now known to be structurally similar to other proteins, in particular within the domains referred to as BH1 and BH2. This multigene family of cell death regulators includes members which enhance rates of apoptosis, including bcl-xs and bax, and those which inhibit apoptosis, including MCL-1 and bcl-xL. Members of the bcl-2 family physically interact with other proteins, including other family members and these interactions appear to modulate their function. The mechanism(s) by which bcl-2 family members regulate cell death remain in large part unknown, although recent evidence suggests that bcl-2 may interfere with cellular signalling events involved in apoptosis induction.

Expression of bcl-2 protein, an inhibitor of apoptosis, and Bax, an accelerator of apoptosis, in ventricular myocytes of human hearts with myocardial infarction

BACKGROUND

In general, myocyte death in myocardial infarctions (MIs) is attributed to necrosis, but recently the involvement of apoptosis has been suggested. The ratio of bcl-2 protein, an inhibitor of apoptosis, to Bax protein, an inducer of apoptosis, determines survival or death after an apoptotic stimulus. We speculated that bcl-2 or Bax expression is induced by ischemia and that it may be related to myocyte death in human hearts.

METHODS AND RESULTS

We studied immunohistochemically 37 autopsied human hearts (acute MI, n = 15; old MI, n = 12; normal hearts as a control, n = 10) with the use of bcl-2 and Bax antibodies. There were no myocytes with positive bcl-2 immunoreactivity in the controls or hearts with old MI. However, myocytes with positive bcl-2 immunoreactivity were seen in 9 of 15 hearts (60%) with acute MI, in that it was localized only in salvaged areas surrounding the infarcted tissues. Myocytes with slightly positive Bax immunoreactivity were observed in the control hearts. In the salvaged myocytes surrounding the infarcted tissues, Bax was overexpressed in 2 of 15 hearts (13%) with acute MI but in 10 of 12 hearts (83%) with old MI.

CONCLUSIONS

bcl-2 protein is induced in salvaged myocytes at the acute stage of infarction, but Bax protein is overexpressed at the old stage. The expression of bcl-2 and the overexpression of Bax may play an important pathophysiological role in the protection or acceleration of the apoptosis of human myocytes after ischemia and/or reperfusion.

Bcl-2 inhibits the mitochondrial release of an apoptogenic protease

Bcl-2 belongs to a family of apoptosis-regulatory proteins which incorporate into the outer mitochondrial as well as nuclear membranes. The mechanism by which the proto-oncogene product Bcl-2 inhibits apoptosis is thus far elusive. We and others have shown previously that the first biochemical alteration detectable in cells undergoing apoptosis, well before nuclear changes become manifest, is a collapse of the mitochondrial inner membrane potential (delta psi m), suggesting the involvement of mitochondrial products in the apoptotic cascade. Here we show that mitochondria contain a pre-formed approximately 50-kD protein which is released upon delta psi m disruption and which, in a cell-free in vitro system, causes isolated nuclei to undergo apoptotic changes such as chromatin condensation and internucleosomal DNA fragmentation. This apoptosis-inducing factor (AIF) is blocked by N-benzyloxycarbonyl-Val-Ala-Asp.fluoromethylketone (Z-VAD.fmk), an antagonist of interleukin-1 beta-converting enzyme (ICE)-like proteases that is also an efficient inhibitor of apoptosis in cells. We have tested the effect of Bcl-2 on the formation, release, and action of AIF. When preventing mitochondrial permeability transition (which accounts for the pre-apoptotic delta psi m disruption in cells), Bcl-2 hyperexpressed in the outer mitochondrial membrane also impedes the release of AIF from isolated mitochondria in vitro. In contrast, Bcl-2 does not affect the formation of AIF, which is contained in comparable quantities in control mitochondria and in mitochondria from Bcl-2-hyperexpressing cells. Furthermore, the presence of Bcl-2 in the nuclear membrane does not interfere with the action of AIF on the nucleus, nor does Bcl-2 hyperexpression protect cells against AIF. It thus appears that Bcl-2 prevents apoptosis by favoring the retention of an apoptogenic protease in mitochondria.

Evidence for alpha-helical conformation of an essential N-terminal region in the human Bcl2 protein

A region occupying approximately 24 amino acids near the N terminus of human Bcl2 is essential for this cytoplasmic membrane protein's ability to inhibit apoptosis. Systematic mutagenesis of this N-terminal region indicates that only five hydrophobic and aromatic residues within it are specifically required for function. Computerized secondary structure prediction, together with circular dichroism spectroscopy of synthetic peptides, indicates that the region encompassing these five residues has the propensity to take on an alpha-helical conformation in the presence of SDS micelles, which presumably mimic the hydrophobic surfaces of cellular membranes or polypeptides. The five critical residues are predicted to be clustered on one face of this putative helix, where they might serve to mediate protein-protein contacts involved in the multimerization of Bcl2 or in the interaction of Bcl2 with other, as yet unidentified components of the apoptotic pathway. Apparent structural homologues of this helical motif are also present in at least some other anti-apoptotic proteins from the Bcl2 family but not in those family members that tend to potentiate, rather than inhibit, apoptosis.

Stopping death cold

The three-dimensional structure of Bcl-xL, an inhibitor of apoptosis, suggests how different combinations of proteins in the same family might be utilized to control apoptosis.