Analysis of Free Energy Signals Arising from Nucleotide Hybridization Between rRNA and mRNA Sequences during Translation in Eubacteria

A decoding algorithm is tested that mechanistically models the progressive alignments that arise as the mRNA moves past the rRNA tail during translation elongation. Each of these alignments provides an opportunity for hybridization between the single-stranded, -terminal nucleotides of the 16S rRNA and the spatially accessible window of mRNA sequence, from which a free energy value can be calculated. Using this algorithm we show that a periodic, energetic pattern of frequency 1/3 is revealed. This periodic signal exists in the majority of coding regions of eubacterial genes, but not in the non-coding regions encoding the 16S and 23S rRNAs. Signal analysis reveals that the population of coding regions of each bacterial species has a mean phase that is correlated in a statistically significant way with species () content. These results suggest that the periodic signal could function as a synchronization signal for the maintenance of reading frame and that codon usage provides a mechanism for manipulation of signal phase.[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32]     

Proinsulin and the Genetics of Diabetes Mellitus

Insulin plays a central role in the regulation of vertebrate metabolism. The hormone, the post-translational product of a single-chain precursor, is a globular protein containing two chains, A (21 residues) and B (30 residues). Recent advances in human genetics have identified dominant mutations in the insulin gene causing permanent neonatal-onset DM² (1–4). The mutations are predicted to block folding of the precursor in the ER of pancreatic β-cells. Although expression of the wild-type allele would in other circumstances be sufficient to maintain homeostasis, studies of a corresponding mouse model (5–7) suggest that the misfolded variant perturbs wild-type biosynthesis (8, 9). Impaired β-cell secretion is associated with ER stress, distorted organelle architecture, and cell death (10). These findings have renewed interest in insulin biosynthesis (11–13) and the structural basis of disulfide pairing (14–19). Protein evolution is constrained not only by structure and function but also by susceptibility to toxic misfolding.Insulin plays a central role in the regulation of vertebrate metabolism. The hormone, the post-translational product of a single-chain precursor, is a globular protein containing two chains, A (21 residues) and B (30 residues). Recent advances in human genetics have identified dominant mutations in the insulin gene causing permanent neonatal-onset DM² (1–4). The mutations are predicted to block folding of the precursor in the ER of pancreatic β-cells. Although expression of the wild-type allele would in other circumstances be sufficient to maintain homeostasis, studies of a corresponding mouse model (5–7) suggest that the misfolded variant perturbs wild-type biosynthesis (8, 9). Impaired β-cell secretion is associated with ER stress, distorted organelle architecture, and cell death (10). These findings have renewed interest in insulin biosynthesis (11–13) and the structural basis of disulfide pairing (14–19). Protein evolution is constrained not only by structure and function but also by susceptibility to toxic misfolding.     

Algorithms for Finding Small Attractors in Boolean Networks

A Boolean network is a model used to study the interactions between different genes in genetic regulatory networks. In this paper, we present several algorithms using gene ordering and feedback vertex sets to identify singleton attractors and small attractors in Boolean networks. We analyze the average case time complexities of some of the proposed algorithms. For instance, it is shown that the outdegree-based ordering algorithm for finding singleton attractors works in time for , which is much faster than the naive time algorithm, where is the number of genes and is the maximum indegree. We performed extensive computational experiments on these algorithms, which resulted in good agreement with theoretical results. In contrast, we give a simple and complete proof for showing that finding an attractor with the shortest period is NP-hard.[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32]     

Loop 2 Structure in Glycine and GABAA Receptors Plays a Key Role in Determining Ethanol Sensitivity

The present study tests the hypothesis that the structure of extracellular domain Loop 2 can markedly affect ethanol sensitivity in glycine receptors (GlyRs) and γ-aminobutyric acid type A receptors (GABA_ARs). To test this, we mutated Loop 2 in the α1 subunit of GlyRs and in the γ subunit of α1β2γ2GABA_ARs and measured the sensitivity of wild type and mutant receptors expressed in Xenopus oocytes to agonist, ethanol, and other agents using two-electrode voltage clamp. Replacing Loop 2 of α1GlyR subunits with Loop 2 from the δGABA_AR (δL2), but not the γGABA_AR subunit, reduced ethanol threshold and increased the degree of ethanol potentiation without altering general receptor function. Similarly, replacing Loop 2 of the γ subunit of GABA_ARs with δL2 shifted the ethanol threshold from 50 mm in WT to 1 mm in the GABA_A γ-δL2 mutant. These findings indicate that the structure of Loop 2 can profoundly affect ethanol sensitivity in GlyRs and GABA_ARs. The δL2 mutations did not affect GlyR or GABA_AR sensitivity, respectively, to Zn²⁺ or diazepam, which suggests that these δL2-induced changes in ethanol sensitivity do not extend to all allosteric modulators and may be specific for ethanol or ethanol-like agents. To explore molecular mechanisms underlying these results, we threaded the WT and δL2 GlyR sequences onto the x-ray structure of the bacterial Gloeobacter violaceus pentameric ligand-gated ion channel homologue (GLIC). In addition to being the first GlyR model threaded on GLIC, the juxtaposition of the two structures led to a possible mechanistic explanation for the effects of ethanol on GlyR-based on changes in Loop 2 structure.Alcohol abuse and dependence are significant problems in our society, with ∼14 million people in the United States being affected (1, 2). Alcohol causes over 100,000 deaths in the United States, and alcohol-related issues are estimated to cost nearly 200 billion dollars annually (2). To address this, considerable attention has focused on the development of medications to prevent and treat alcohol-related problems (3–5). The development of such medications would be aided by a clear understanding of the molecular structures on which ethanol acts and how these structures influence receptor sensitivity to ethanol.Ligand-gated ion channels (LGICs)² have received substantial attention as putative sites of ethanol action that cause its behavioral effects (6–12). Research in this area has focused on investigating the effects of ethanol on two large superfamilies of LGICs: 1) the Cys-loop superfamily of LGICs (13, 14), whose members include nicotinic acetylcholine, 5-hydroxytryptamine₃, γ-aminobutyric acid type A (GABA_A), γ-aminobutyric acid type C, and glycine receptors (GlyRs) (10, 11, 15–20) and 2) the glutamate superfamily, including N-methyl d-aspartate, α-amino-3-hydroxyisoxazolepropionic acid, and kainate receptors (21, 22). Recent studies have also begun investigating ethanol action in the ATP-gated P2X superfamily of LGICs (23–25).A series of studies that employed chimeric and mutagenic strategies combined with sulfhydryl-specific labeling identified key regions within Cys-loop receptors that appear to be initial targets for ethanol action that also can determine the sensitivity of the receptors to ethanol (7–12, 18, 19, 26–30). This work provides several lines of evidence that position 267 and possibly other sites in the transmembrane (TM) domain of GlyRs and homologous sites in GABA_ARs are targets for ethanol action and that mutations at these sites can influence ethanol sensitivity (8, 9, 26, 31).Growing evidence from GlyRs indicates that ethanol also acts on the extracellular domain. The initial findings came from studies demonstrating that α1GlyRs are more sensitive to ethanol than are α2GlyRs despite the high (∼78%) sequence homology between α1GlyRs and α2GlyRs (32). Further work found that an alanine to serine exchange at position 52 (A52S) in Loop 2 can eliminate the difference in ethanol sensitivity between α1GlyRs and α2GlyRs (18, 20, 33). These studies also demonstrated that mutations at position 52 in α1GlyRS and the homologous position 59 in α2GlyRs controlled the sensitivity of these receptors to a novel mechanistic ethanol antagonist (20). Collectively, these studies suggest that there are multiple sites of ethanol action in α1GlyRs, with one site located in the TM domain (e.g. position 267) and another in the extracellular domain (e.g. position 52).Subsequent studies revealed that the polarity of the residue at position 52 plays a key role in determining the sensitivity of GlyRs to ethanol (20). The findings with polarity in the extracellular domain contrast with the findings at position 267 in the TM domain, where molecular volume, but not polarity, significantly affected ethanol sensitivity (9). Taken together, these findings indicate that the physical-chemical parameters of residues at positions in the extracellular and TM domains that modulate ethanol effects and/or initiate ethanol action in GlyRs are not uniform. Thus, knowledge regarding the physical-chemical properties that control agonist and ethanol sensitivity is key for understanding the relationship between the structure and the actions of ethanol in LGICs (19, 31, 34–40).GlyRs and GABA_ARs, which differ significantly in their sensitivities to ethanol, offer a potential method for identifying the structures that control ethanol sensitivity. For example, α1GlyRs do not reliably respond to ethanol concentrations less than 10 mm (32, 33, 41). Similarly, γ subunit-containing GABA_ARs (e.g. α1β2γ2), the most predominantly expressed GABA_ARs in the central nervous system, are insensitive to ethanol concentrations less than 50 mm (42, 43). In contrast, δ subunit-containing GABA_ARs (e.g. α4β3δ) have been shown to be sensitive to ethanol concentrations as low as 1–3 mm (44–51). Sequence alignment of α1GlyR, γGABA_AR, and δGABA_AR revealed differences between the Loop 2 regions of these receptor subunits. Since prior studies found that mutations of Loop 2 residues can affect ethanol sensitivity (19, 20, 39), the non-conserved residues in Loop 2 of GlyR and GABA_AR subunits could provide the physical-chemical and structural bases underlying the differences in ethanol sensitivity between these receptors.The present study tested the hypothesis that the structure of Loop 2 can markedly affect the ethanol sensitivity of GlyRs and GABA_ARs. To accomplish this, we performed multiple mutations that replaced the Loop 2 region of the α1 subunit in α1GlyRs and the Loop 2 region of the γ subunit of α1β2γ2 GABA_ARs with corresponding non-conserved residues from the δ subunit of GABA_AR and tested the sensitivity of these receptors to ethanol. As predicted, replacing Loop 2 of WT α1GlyRs with the homologous residues from the δGABA_AR subunit (δL2), but not the γGABA_AR subunit (γL2), markedly increased the sensitivity of the receptor to ethanol. Similarly, replacing the non-conserved residues of the γ subunit of α1β2γ2 GABA_ARs with δL2 also markedly increased ethanol sensitivity of GABA_ARs. These findings support the hypothesis and suggest that Loop 2 may play a role in controlling ethanol sensitivity across the Cys-loop superfamily of receptors. The findings also provide the basis for suggesting structure-function relationships in a new molecular model of the GlyR based on the bacterial Gloeobacter violaceus pentameric LGIC homologue (GLIC).     

Dual 12/15- and 5-Lipoxygenase Deficiency in Macrophages Alters Arachidonic Acid Metabolism and Attenuates Peritonitis and Atherosclerosis in ApoE Knock-out Mice

Lipoxygenase (LO) enzymes catalyze the conversion of arachidonic acid (AA) into biologically active lipid mediators. Two members, 12/15-LO and 5-LO, regulate inflammatory responses and have been studied for their roles in atherogenesis. Both 12/15-LO and 5-LO inhibitors have been suggested as potential therapy to limit the development of atherosclerotic lesions. Here we used a genetic strategy to disrupt both 12/15-LO and 5-LO on an apolipoprotein E (apoE) atherosclerosis-susceptible background to study the impact of dual LO blockade in atherosclerosis and inflammation. Resident peritoneal macrophages are the major cell type that expresses both LO enzymes, and we verified their absence in dual LO-deficient mice. Examination of AA conversion by phorbol myristate acetate-primed and A23187-challenged macrophages from dual LO-deficient mice revealed extensive accumulation of AA with virtually no diversion into the most common cyclooxygenase (COX) products measured (prostaglandin E₂ and thromboxane B₂). Instead the COX-1 by-products 11-hydroxy-eicosatetraenoic acid (HETE) and 15-HETE were elevated. The interrelationship between the two LO pathways in combination with COX-1 inhibition (SC-560) also revealed striking patterns of unique substrate utilization. 5-LO- and dual LO-deficient mice exhibited an attenuated response to zymosan-induced peritoneal inflammation, emphasizing roles for 5-LO in regulating vascular permeability. We observed gender-specific attenuation of atheroma formation at 6 months of age at both the aortic root and throughout the entire aorta in chow-fed female dual LO-deficient mice. We propose that some of the inconsistent data obtained with single LO-deficient mice could be attributable to macrophage-specific patterns of altered AA metabolism.Lipoxygenase (LO)² enzymes are an important source of lipid mediators throughout the plant and animal kingdoms (1, 2). In mammals, these mediators are predominantly formed from arachidonic acid (AA) and act in various physiological and pathological contexts (1–3). Accordingly 5-LO and 12/15-LO are two members of the LO family involved in cardiovascular and inflammatory diseases expressed to variable degrees in several cell types of the myeloid lineage, and their expression is strictly regulated and incompletely understood (2, 4, 5). Despite considerable structural homology between 5-LO and 12/15-LO, both enzymes generate distinct products. The 5-LO metabolite leukotriene (LT) A₄ is precursor to the proinflammatory LTB₄ and cysteinyl LTs, which regulate leukocyte subset-specific chemotaxis (LTB₄) and vascular permeability (cysteinyl LTs), both crucial events during acute peritonitis (1, 6, 7). 12- and 15-HETE, end products synthesized by 12/15-LO, play potential roles in cellular chemotaxis, cancer growth, and inflammation (2, 8). Transcellular interaction products derived from both 12/15-LO and 5-LO, such as lipoxins and maresins, indicate that these enzymes can possess anti-inflammatory activities in innate immunity and the resolution of inflammation (9, 10).In mice, only one cell type is known to express substantial quantities of both 5-LO and 12/15-LO, the peritoneal macrophage (PMΦ) (2, 11, 12). However, differences in subcellular localization, trafficking, and activation (8, 12–16) of these two LOs indicate that they are independently regulated and not functionally coupled. Tissue-resident MΦ (such as PMΦ) represent the first line of defense against invading pathogens and activate the immunological and inflammatory response (17). These phagocytes are capable of elaborating a wide spectrum of bioactive lipid mediators from the LO and cyclooxygenase (COX) pathways. Little is known about the regulation and putative interdependence of these pathways. Some insight was gained using mice lacking 12/15-LO where substrate shunting from the 12/15-LO into the 5-LO pathway was observed (12).The generation of knock-out mice for 12/15-LO (12) and 5-LO (18) has enabled the study of these lipid mediator pathways in models of health and disease. Because 12/15-LO and 5-LO are primarily expressed in distinct hematopoietic cells, their implication in various inflammatory disorders and models of host defense mechanisms have been investigated (2, 3). Atherosclerosis, an inflammatory disease prevalent in societies with high dietary fat intake, is initiated by low density lipoprotein (LDL) retention in the vascular wall (19) and subsequent oxidative modification. This process greatly enhances the LDL atherogenic potential, and intriguingly 12/15-LO can contribute to lipoprotein oxidation (11, 20). Initial studies using 12/15-LO- and 5-LO-deficient mice indicated proatherogenic roles for these enzymes (20, 21). Additionally mice lacking the LTB₄ receptor BLT-1 exhibit protection in early atherogenesis (22), but subsequent data from our laboratory using 5-LO-deficient mouse models have not supported an involvement of 5-LO in atherogenesis (3, 23, 24). Here we studied the consequences of simultaneous 12/15-LO and 5-LO knock-out on peritoneal inflammation and atherosclerosis in apoE-deficient mice and surmised whether some of the capricious results in atherosclerotic lesion studies could be attributable to variable eicosanoid profiles.     

The Wavelet-Based Cluster Analysis for Temporal Gene Expression Data

A variety of high-throughput methods have made it possible to generate detailed temporal expression data for a single gene or large numbers of genes. Common methods for analysis of these large data sets can be problematic. One challenge is the comparison of temporal expression data obtained from different growth conditions where the patterns of expression may be shifted in time. We propose the use of wavelet analysis to transform the data obtained under different growth conditions to permit comparison of expression patterns from experiments that have time shifts or delays. We demonstrate this approach using detailed temporal data for a single bacterial gene obtained under 72 different growth conditions. This general strategy can be applied in the analysis of data sets of thousands of genes under different conditions.[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]     

Deorphanization of GPR109B as a Receptor for the ��-Oxidation Intermediate 3-OH-octanoic Acid and Its Role in the Regulation of Lipolysis

The orphan G-protein-coupled receptor GPR109B is the result of a recent gene duplication of the nicotinic acid and ketone body receptor GPR109A being found in humans but not in rodents. Like GPR109A, GPR109B is predominantly expressed in adipocytes and is supposed to mediate antilipolytic effects. Here we show that GPR109B serves as a receptor for the β-oxidation intermediate 3-OH-octanoic acid, which has antilipolytic activity on human but not on murine adipocytes. GPR109B is coupled to G_i-type G-proteins and is activated by 2- and 3-OH-octanoic acid with EC₅₀ values of about 4 and 8 μm, respectively. Interestingly, 3-OH-octanoic acid plasma concentrations reach micromolar concentrations under conditions of increased β-oxidation rates, like in diabetic ketoacidosis or under a ketogenic diet. These data suggest that the ligand receptor pair 3-OH-octanoic acid/GPR109B mediates in humans a negative feedback regulation of adipocyte lipolysis to counteract prolipolytic influences under conditions of physiological or pathological increases in β-oxidation rates.Triacylglycerols stored in the white adipose tissue serve as the major energy reserve in higher eukaryotes (1). Although they are constantly turned over by lipolysis and re-esterification, their mobilization and storage are precisely balanced by various hormones and other factors depending on the nutritional state (2). The net rate of lipolysis is increased during fasting or periods of increased energy demand. Fatty acids generated via lipolysis undergo β-oxidation in the muscle and liver to serve directly as a source of energy or as a precursor for ketone bodies (3). The major intracellular regulator of lipolysis is cyclic AMP, which stimulates cAMP-dependent kinase to activate lipolytic enzymes (2, 4–6). This lipolytic pathway is induced, for example, via β-adrenergic receptors that couple to the G-protein G_s and thereby stimulate adenylyl cyclase (7, 8). To adjust lipolysis at the appropriate rate, the effects of prolipolytic stimuli are balanced by various antilipolytic influences. Besides insulin, which promotes the degradation of cAMP via activation of phosphodiesterase 3B (2, 5, 7), several antilipolytic stimuli decrease cAMP levels by activation of G_i-coupled receptors, which mediate an inhibition of adenylyl cyclase (5, 8). One of these receptors, GPR109A, has recently been shown to mediate the anti-lipolytic effects of high concentrations of the ketone body 3-OH-butyrate thereby providing a negative feedback mechanism during fasting (9, 10). GPR109A also binds nicotinic acid (11–13) and mediates the anti-lipolytic effects of this anti-dyslipidemic drug (12).GPR109B, a close relative of GPR109A, is the result of a recent gene duplication being present in humans but not in rodents and most other mammals (14). GPR109B differs from GPR109A in an extended C-terminal tail as well as in 16 amino acids (11, 13). Despite its high homology to GPR109A, GPR109B does not bind nicotinic acid or 3-OH-butyrate with reasonable affinity (10, 11, 13). Because GPR109A and GPR109B have very similar expression patterns (11, 13, 15) and are likely to have the same basic signaling properties, agonists of GPR109B are expected to have physiological and pharmacological effects comparable with those of the GPR109A agonist 3-OH-butyrate and nicotinic acid, respectively. Recently, several synthetic compounds as well as various aromatic d-amino acids have been shown to be selective agonists at GPR109B (16–18). However, endogenous physiological anti-lipolytic ligands of GPR109B are unknown.In this study we tested endogenous carboxylic acids for their ability to activate GPR109B. We found that the fatty acid β-oxidation intermediate 3-OH-octanoic acid is a highly specific agonist of GPR109B. 3-OH-octanoic acid has anti-lipolytic activity, and its plasma concentration in humans reflects the β-oxidation flux. Our data suggest that 3-OH-octanoic acid and GPR109B mediate a negative feedback regulation of adipocyte lipolysis.     

Context-dependent Bcl-2/Bak Interactions Regulate Lymphoid Cell Apoptosis

The release of cytochrome c from mitochondria, which leads to activation of the intrinsic apoptotic pathway, is regulated by interactions of Bax and Bak with antiapoptotic Bcl-2 family members. The factors that regulate these interactions are, at the present time, incompletely understood. Recent studies showing preferences in binding between synthetic Bcl-2 homology domain 3 and antiapoptotic Bcl-2 family members in vitro have suggested that the antiapoptotic proteins Mcl-1 and Bcl-x_L, but not Bcl-2, restrain proapoptotic Bak from inducing mitochondrial membrane permeabilization and apoptosis. Here we show that Bak protein has a much higher affinity than the 26-amino acid Bak Bcl-2 homology domain 3 for Bcl-2, that some naturally occurring Bcl-2 allelic variants have an affinity for full-length Bak that is only 3-fold lower than that of Mcl-1, and that endogenous levels of these Bcl-2 variants (which are as much as 40-fold more abundant than Mcl-1) restrain part of the Bak in intact lymphoid cells. In addition, we demonstrate that Bcl-2 variants can, depending on their affinity for Bak, substitute for Mcl-1 in protecting cells. Thus, the ability of Bcl-2 to protect cells from activated Bak depends on two important contextual variables, the identity of the Bcl-2 present and the amount expressed.The release of cytochrome c from mitochondria, which leads to activation of the intrinsic apoptotic pathway, is regulated by Bcl-2 family members (1–5). This group of proteins consists of three subgroups: Bax and Bak, which oligomerize upon death stimulation to form a putative pore in the outer mitochondrial membrane, thereby allowing efflux of cytochrome c and other mitochondrial intermembrane space components; Bcl-2, Bcl-x_L, Mcl-1, and other antiapoptotic homologs, which antagonize the effects of Bax and Bak; and BH3-only proteins² such as Bim, Bid, and Puma, which are proapoptotic Bcl-2 family members that share only limited homology with the other two groups in a single 15-amino acid domain (the BH3 domain, see Ref. 6). Although it is clear that BH3-only proteins serve as molecular sensors of various stresses and, when activated, trigger apoptosis (3, 6–11), the mechanism by which they do so remains incompletely understood. One current model suggests that BH3-only proteins trigger apoptosis solely by binding and neutralizing antiapoptotic Bcl-2 family members, thereby causing them to release the activated Bax and Bak that are bound (reviewed in Refs. 9 and 10; see also Refs. 12 and 13), whereas another current model suggests that certain BH3-only proteins also directly bind to and activate Bax (reviewed in Ref. 3; see also Refs. 14–17). Whichever model turns out to be correct, both models agree that certain antiapoptotic Bcl-2 family members can inhibit apoptosis, at least in part, by binding and neutralizing activated Bax and Bak before they permeabilize the outer mitochondrial membrane (13, 18, 19).Much of the information about the interactions between pro- and antiapoptotic Bcl-2 family members has been derived from the study of synthetic peptides corresponding to BH3 domains. In particular, these synthetic peptides have been utilized as surrogates for the full-length proapoptotic proteins during structure determinations (20–22) as well as in functional studies exploring the effect of purified BH3 domains on isolated mitochondria (14, 23) and on Bax-mediated permeabilization of lipid vesicles (15).Recent studies using these same peptides have suggested that interactions of the BH3 domains of Bax, Bak, and the BH3-only proteins with the “BH3 receptors” of the antiapoptotic Bcl-2 family members are not all equivalent. Surface plasmon resonance, a technique that is widely used to examine the interactions of biomolecules under cell-free conditions (24–26), has demonstrated that synthetic BH3 peptides of some BH3-only family members show striking preferences, with the Bad BH3 peptide binding to Bcl-2 and Bcl-x_L but not Mcl-1, and the Noxa BH3 peptide binding to Mcl-1 but not Bcl-2 or Bcl-x_L (27). Likewise, the Bak BH3 peptide exhibits selectivity, with high affinity for Bcl-x_L and Mcl-1 but not Bcl-2 (12). The latter results have led to a model in which Bcl-x_L and Mcl-1 restrain Bak and inhibit Bak-dependent apoptosis, whereas Bcl-2 does not (10).Because the Bak protein contains multiple recognizable domains in addition to its BH3 motif (28, 29), we compared the binding of Bak BH3 peptide and Bak protein to Bcl-2. Surface plasmon resonance demonstrated that Bcl-2 binds Bak protein with much higher affinity than the Bak 26-mer BH3 peptide. Further experiments demonstrated that the K_D for Bak differs among naturally occurring Bcl-2 sequence variants but is only 3-fold higher than that of Mcl-1 in some cases. In light of previous reports that Bcl-2 overexpression contributes to neoplastic transformation (30–33) and drug resistance (34–36) in lymphoid cells, we also examined Bcl-2 expression and Bak binding in a panel of neoplastic lymphoid cell lines. Results of these experiments demonstrated that Bcl-2 expression varies among different lymphoid cell lines but is up to 40-fold more abundant than Mcl-1. In lymphoid cell lines with abundant Bcl-2, Bak is detected in Bcl-2 as well as Mcl-1 immunoprecipitates; and Bak-dependent apoptosis induced by Mcl-1 down-regulation can be prevented by Bcl-2 overexpression. Collectively, these observations shed new light on the role of Bcl-2 in binding and neutralizing Bak.