Peptides corresponding to helices 5 and 6 of Bax can independently form large lipid pores

Lipid unsaturation promotes BAX and BAK pore activity during apoptosis

BAX and BAK are proapoptotic members of the BCL2 family that directly mediate mitochondrial outer membrane permeabilition (MOMP), a central step in apoptosis execution. However, the molecular architecture of the mitochondrial apoptotic pore remains a key open question and especially little is known about the contribution of lipids to MOMP. By performing a comparative lipidomics analysis of the proximal membrane environment of BAK isolated in lipid nanodiscs, we find a significant enrichment of unsaturated species nearby BAK and BAX in apoptotic conditions. We then demonstrate that unsaturated lipids promote BAX pore activity in model membranes, isolated mitochondria and cellular systems, which is further supported by molecular dynamics simulations. Accordingly, the fatty acid desaturase FADS2 not only enhances apoptosis sensitivity, but also the activation of the cGAS/STING pathway downstream mtDNA release. The correlation of FADS2 levels with the sensitization to apoptosis of different lung and kidney cancer cell lines by co-treatment with unsaturated fatty acids supports the relevance of our findings. Altogether, our work provides an insight on how local lipid environment affects BAX and BAK function during apoptosis.

The regulation of the apoptotic pore-An immunological tightrope walk

Apoptotic pore formation in mitochondria is the pivotal point for cell death during mitochondrial apoptosis. It is regulated by BCL-2 family proteins in response to various cellular stress triggers and mediates mitochondrial outer membrane permeabilization (MOMP). This allows the release of mitochondrial contents into the cytosol, which triggers rapid cell death and clearance through the activation of caspases. However, under conditions of low caspase activity, the mitochondrial contents released into the cytosol through apoptotic pores serve as inflammatory signals and activate various inflammatory responses. In this chapter, we discuss how the formation of the apoptotic pore is regulated by BCL-2 proteins as well as other cellular or mitochondrial proteins and membrane lipids. Moreover, we highlight the importance of sublethal MOMP in the regulation of mitochondrial-activated inflammation and discuss its physiological consequences in the context of pathogen infection and disease and how it can potentially be exploited therapeutically, for example to improve cancer treatment.

A comparative analysis of daunorubicin and its metabolite daunorubicinol interaction with apoptotic and drug resistance proteins using in silico approach

Daunorubicin (DNR) is a chemotherapeutic drug associated with multiple side effects, including drug resistance. As the molecular mechanism related to these side effects remain unclear and mostly hypothesized, this study addresses and compares the role of DNR and its metabolite Daunorubicinol (DAUNol) to induce apoptosis and drug resistance using molecular docking, Molecular Dynamics (MD) simulation, MM-PBSA and chemical pathway analysis. The results showed that DNR's interaction was stronger with Bax protein, Mcl-1:mNoxaB and Mcl-1:Bim protein complexes than DAUNol. On the other hand, contrasting results were obtained for drug resistance proteins where stronger interaction was obtained with DAUNol compared to DNR. Further, MD simulation performed for 100 ns provided the details of protein-ligand interaction. Most notable was the interaction of Bax protein with DNR, resulting in conformational changes at α-helices 5, 6 and 9, leading to Bax activation. Finally, the chemical signalling pathway analysis also revealed the regulation of different signalling pathways by DNR and DAUNol. It was observed that DNR majorly impacted the signalling associated with apoptosis while DAUNol mainly targeted pathways related to multidrug resistance and cardiotoxicity. Overall, the results highlight that DNR biotransformation reduces its capability to induce apoptosis while enhancing its ability to induce drug resistance and off-target toxicity.Communicated by Ramaswamy H. Sarma.

Mechanisms of BCL-2 family proteins in mitochondrial apoptosis

The proteins of the BCL-2 family are key regulators of mitochondrial apoptosis, acting as either promoters or inhibitors of cell death. The functional interplay and balance between the opposing BCL-2 family members control permeabilization of the outer mitochondrial membrane, leading to the release of activators of the caspase cascade into the cytosol and ultimately resulting in cell death. Despite considerable research, our knowledge about the mechanisms of the BCL-2 family of proteins remains insufficient, which complicates cell fate predictions and does not allow us to fully exploit these proteins as targets for drug discovery. Detailed understanding of the formation and molecular architecture of the apoptotic pore in the outer mitochondrial membrane remains a holy grail in the field, but new studies allow us to begin constructing a structural model of its arrangement. Recent literature has also revealed unexpected activities for several BCL-2 family members that challenge established concepts of how they regulate mitochondrial permeabilization. In this Review, we revisit the most important advances in the field and integrate them into a new structure-function-based classification of the BCL-2 family members that intends to provide a comprehensive model for BCL-2 action in apoptosis. We close this Review by discussing the potential of drugging the BCL-2 family in diseases characterized by aberrant apoptosis.

A BAK subdomain that binds mitochondrial lipids selectively and releases cytochrome C

How BAK and BAX induce mitochondrial outer membrane (MOM) permeabilization (MOMP) during apoptosis is incompletely understood. Here we have used molecular dynamics simulations, surface plasmon resonance, and assays for membrane permeabilization in vitro and in vivo to assess the structure and function of selected BAK subdomains and their derivatives. Results of these studies demonstrate that BAK helical regions α5 and α6 bind the MOM lipid cardiolipin. While individual peptides corresponding to these helical regions lack the full biological activity of BAK, tandem peptides corresponding to α4-α5, α5-α6, or α6-α7/8 can localize exogenous proteins to mitochondria, permeabilize liposomes composed of MOM lipids, and cause MOMP in the absence of the remainder of the BAK protein. Importantly, the ability of these tandem helices to induce MOMP under cell-free conditions is diminished by mutations that disrupt the U-shaped helix-turn-helix structure of the tandem peptides or decrease their lipid binding. Likewise, BAK-induced apoptosis in intact cells is diminished by CLS1 gene interruption, which decreases mitochondrial cardiolipin content, or by BAK mutations that disrupt the U-shaped tandem peptide structure or diminish lipid binding. Collectively, these results suggest that BAK structural rearrangements during apoptosis might mobilize helices involved in specific protein-lipid interactions that are critical for MOMP.

Visualization of BOK pores independent of BAX and BAK reveals a similar mechanism with differing regulation

BOK is a poorly understood member of the BCL-2 family of proteins that has been proposed to function as a pro-apoptotic, BAX-like effector. However, the molecular mechanism and structural properties of BOK pores remain enigmatic. Here, we show that the thermal stability and pore activity of BOK depends on the presence of its C-terminus as well as on the mitochondrial lipid cardiolipin. We directly visualized BOK pores in liposomes by electron microscopy, which appeared similar to those induced by BAX, in line with comparable oligomerization properties quantified by single molecule imaging. In addition, super-resolution STED imaging revealed that BOK organized into dots and ring-shaped assemblies in apoptotic mitochondria, also reminiscent of those found for BAX and BAK. Yet, unlike BAX and BAK, the apoptotic activity of BOK was limited by partial mitochondrial localization and was independent of and unaffected by other BCL-2 proteins. These results suggest that, while BOK activity is kept in check by subcellular localization instead of interaction with BCL-2 family members, the resulting pores are structurally similar to those of BAX and BAK.

Controllable membrane remodeling by a modified fragment of the apoptotic protein Bax

Intrinsic apoptosis is orchestrated by a group of proteins that mediate the coordinated disruption of mitochondrial membranes. Bax is a multi-domain protein that, upon activation, disrupts the integrity of the mitochondrial outer membrane by forming pores. We strategically introduced glutamic acids into a short sequence of the Bax protein that constitutively creates membrane pores. The resulting BaxE5 peptide efficiently permeabilizes membranes at acidic pH, showing low permeabilization at neutral pH. Atomic force microscopy (AFM) imaging showed that at acidic pH BaxE5 established several membrane remodeling modalities that progressively disturbed the integrity of the lipid bilayer. The AFM data offers vistas on the membrane disruption process, which starts with pore formation and progresses through localized exposure of membrane monolayers leading to stable and small (height ∼ 16 Å) lipid-peptide complexes. The different types of membrane morphology observed in the presence of BaxE5 suggest that the peptide can establish different types of membrane interactions. BaxE5 adopts a rare unstructured conformation when bound to membranes, which might facilitate the dynamic transition between those different states, and then promote membrane digestion.

Folding and Insertion of Transmembrane Helices at the ER

In eukaryotic cells, the endoplasmic reticulum (ER) is the entry point for newly synthesized proteins that are subsequently distributed to organelles of the endomembrane system. Some of these proteins are completely translocated into the lumen of the ER while others integrate stretches of amino acids into the greasy 30 Å wide interior of the ER membrane bilayer. It is generally accepted that to exist in this non-aqueous environment the majority of membrane integrated amino acids are primarily non-polar/hydrophobic and adopt an α-helical conformation. These stretches are typically around 20 amino acids long and are known as transmembrane (TM) helices. In this review, we will consider how transmembrane helices achieve membrane integration. We will address questions such as: Where do the stretches of amino acids fold into a helical conformation? What is/are the route/routes that these stretches take from synthesis at the ribosome to integration through the ER translocon? How do these stretches ‘know’ to integrate and in which orientation? How do marginally hydrophobic stretches of amino acids integrate and survive as transmembrane helices?

Emerging self-assembling peptide nanomaterial for anti-cancer therapy

Recently it is mainly focused on anti-tumor comprehensive treatments like finding target tumor cells or activating immune cells to inhibit tumor recurrence and metastasis. At present, chemotherapy and molecular-targeted drugs can inhibit tumor cell growth to a certain extent. However, multi-drug resistance and immune escape often make it difficult for new drugs to achieve expected effects. Peptide hydrogel nanoparticles is a new type of biological material with functional peptide chains as the core and self-assembling peptide (SAP) as the framework. It has a variety of significant biological functions, including effective local inflammation suppression and non-drug-resistant cell killing. Besides, it can induce immune activation more persistently in an adjuvant independent manner when compared with simple peptides. Thus, SAP nanomaterial has great potential in regulating cell physiological functions, drug delivery and sensitization, vaccine design and immunotherapy. Not only that, it is also a potential way to focus on some specific proteins and cells through peptides, which has already been examined in previous research. A full understanding of the function and application of SAP nanoparticles can provide a simple and practical strategy for the development of anti-tumor drugs and vaccine design, which contributes to the historical transition of peptide nanohydrogels from bench to bedside and brings as much survival benefits as possible to cancer patients.

Membrane-dependent amyloid aggregation of human BAX α9 (173-192)

Mitochondrial outer membrane permeabilization, which is a critical step in apoptosis, is initiated upon transmembrane insertion of the C-terminal α-helix (α9) of the proapoptotic Bcl-2 family protein BAX. The isolated α9 fragment (residues 173-192) is also competent to disrupt model membranes, and the structures of its membrane-associated oligomers are of interest in understanding the potential roles of this sequence in apoptosis. Here, we used ultrafast two-dimensional infrared (2D IR) spectroscopy, thioflavin T binding, and transmission electron microscopy to show that the synthetic BAX α9 peptide (α9p) forms amyloid aggregates in aqueous environments and on the surfaces of anionic small unilamellar vesicles. Its inherent amyloidogenicity was predicted by sequence analysis, and 2D IR spectra reveal that vesicles modulate the β-sheet structures of insoluble aggregates, motivating further examination of the formation or suppression of BAX amyloids in apoptosis.

Estimation of pore dimensions in lipid membranes induced by peptides and other biomolecules: A review

The cytoplasmic membrane is one of the most frequent cell targets of antimicrobial peptides (AMPs) and other biomolecules. Understanding the mechanism of action of AMPs at the molecular level is of utmost importance for designing of new membrane-specific molecules. In particular, the formation of pores, the structure and size of these pores are of great interest and require nanoscale resolution approaches, therefore, biophysical strategies are essential to achieve an understanding of these processes at this scale. In the case of membrane active peptides, pore formation or general membrane disruption is usually the last step before cell death, and so, pore size is generally directly associated to pore structure and stability and loss of cellular homeostasis, implicated in overall peptide activity. Up to date, there has not been a critical review discussing the methods that can be used specifically for estimating the pore dimensions induced by membrane active peptides. In this review we discuss the scope, relevance and popularity of the different biophysical techniques such as liposome leakage experiments, advanced microscopy, neutron or X-ray scattering, electrophysiological techniques and molecular dynamics studies, all of them useful for determining pore structure and dimension.

Pore formation in regulated cell death

The discovery of alternative signaling pathways that regulate cell death has revealed multiple strategies for promoting cell death with diverse consequences at the tissue and organism level. Despite the divergence in the molecular components involved, membrane permeabilization is a common theme in the execution of regulated cell death. In apoptosis, the permeabilization of the outer mitochondrial membrane by BAX and BAK releases apoptotic factors that initiate the caspase cascade and is considered the point of no return in cell death commitment. Pyroptosis and necroptosis also require the perforation of the plasma membrane at the execution step, which involves Gasdermins in pyroptosis, and MLKL in the case of necroptosis. Although BAX/BAK, Gasdermins and MLKL share certain molecular features like oligomerization, they form pores in different cellular membranes via distinct mechanisms. Here, we compare and contrast how BAX/BAK, Gasdermins, and MLKL alter membrane permeability from a structural and biophysical perspective and discuss the general principles of membrane permeabilization in the execution of regulated cell death.

Single-molecule conformational dynamics of viroporin ion channels regulated by lipid-protein interactions

Classic swine fever is a highly contagious and often fatal viral disease that is caused by the classical swine fever virus (CSFV). Protein p7 of CFSV is a prototype of viroporin, a family of small, highly hydrophobic proteins postulated to modulate virus-host interactions during the processes of virus entry, replication and assembly. It has been shown that CSFV p7 displays substantial ion channel activity when incorporated into membrane systems, but a deep rationalization of the size and dynamics of the induced pores is yet to emerge. Here, we use high-resolution conductance measurements and current fluctuation analysis to demonstrate that CSFV p7 channels are ruled by equilibrium conformational dynamics involving protein-lipid interactions. Atomic force microscopy (AFM) confirms the existence of a variety of pore sizes and their tight regulation by solution pH. We conclude that p7 viroporin forms subnanometric channels involved in virus propagation, but also much larger pores (1-10 nm in diameter) with potentially significant roles in virus pathogenicity. Our findings provide new insights into the sources of noise in protein electrochemistry and demonstrate the existence of slow complex dynamics characteristic of crowded systems like biomembrane surfaces.

Homogeneous Oligomers of Pro-apoptotic BAX Reveal Structural Determinants of Mitochondrial Membrane Permeabilization

BAX is a pro-apoptotic protein that transforms from a cytosolic monomer into a toxic oligomer that permeabilizes the mitochondrial outer membrane. How BAX monomers assemble into a higher-order conformation, and the structural determinants essential to membrane permeabilization, remain a mechanistic mystery. A key hurdle has been the inability to generate a homogeneous BAX oligomer (BAXO) for analysis. Here, we report the production and characterization of a full-length BAXO that recapitulates physiologic BAX activation. Multidisciplinary studies revealed striking conformational consequences of oligomerization and insight into the macromolecular structure of oligomeric BAX. Importantly, BAXO enabled the assignment of specific roles to particular residues and α helices that mediate individual steps of the BAX activation pathway, including unexpected functionalities of BAX α6 and α9 in driving membrane disruption. Our results provide the first glimpse of a full-length and functional BAXO, revealing structural requirements for the elusive execution phase of mitochondrial apoptosis.

A lipid perspective on regulated cell death

Lipids are fundamental to life as structural components of cellular membranes and for signaling. They are also key regulators of different cellular processes such as cell division, proliferation, and death. Regulated cell death (RCD) requires the engagement of lipids and lipid metabolism for the initiation and execution of its killing machinery. The permeabilization of lipid membranes is a hallmark of RCD that involves, for each kind of cell death, a unique lipid profile. While the permeabilization of the mitochondrial outer membrane allows the release of apoptotic factors to the cytosol during apoptosis, permeabilization of the plasma membrane facilitates the release of intracellular content in other nonapoptotic types of RCD like necroptosis and ferroptosis. Lipids and lipid membranes are important accessory molecules required for the activation of protein executors of cell death such as BAX in apoptosis and MLKL in necroptosis. Peroxidation of membrane phospholipids and the subsequent membrane destabilization is a prerequisite to ferroptosis. Here, we discuss how lipids are essential players in apoptosis, the most common form of RCD, and also their role in necroptosis and ferroptosis. Altogether, we aim to highlight the contribution of lipids and membrane dynamics in cell death regulation.

Membrane Remodeling by the Lytic Fragment of SticholysinII: Implications for the Toroidal Pore Model

Sticholysins are pore-forming toxins of biomedical interest and represent a prototype of proteins acting through the formation of protein-lipid or toroidal pores. Peptides spanning the N-terminus of sticholysins can mimic their permeabilizing activity and, together with the full-length toxins, have been used as a tool to understand the mechanism of pore formation in membranes. However, the lytic mechanism of these peptides and the lipid shape modulating their activity are not completely clear. In this article, we combine molecular dynamics simulations and experimental biophysical tools to dissect different aspects of the pore-forming mechanism of StII_1-30, a peptide derived from the N-terminus of sticholysin II (StII). With this combined approach, membrane curvature induction and flip-flop movement of the lipids were identified as two important membrane remodeling steps mediated by StII_1-30. Pore formation by this peptide was enhanced by the presence of the negatively curved lipid phosphatidylethanolamine in membranes. This lipid emerged not only as a facilitator of membrane interactions but also as a structural element of the StII_1-30 pore that is recruited to the ring upon its assembly. Collectively, these, to our knowledge, new findings support a toroidal model for the architecture of the pore formed by StII_1-30 and provide new molecular insight into the role of phosphatidylethanolamine as a membrane component that can easily integrate into the ring of toroidal pores, thus probably aiding in their stabilization. This study contributes to a better understanding of the molecular mechanism underlying the permeabilizing activity of StII_1-30 and peptides or proteins acting via a toroidal pore mechanism and offers an informative framework for the optimization of the biomedical application of this and similar molecules.

Topology of active, membrane-embedded Bax in the context of a toroidal pore

Bax is a Bcl-2 protein critical for apoptosis induction. In healthy cells, Bax is mostly a monomeric, cytosolic protein, while upon apoptosis initiation it inserts into the outer mitochondrial membrane, oligomerizes, and forms pores that release proapoptotic factors like Cytochrome c into the cytosol. The structures of active Bax and its homolog Bak are only partially understood and the topology of the proteins with respect to the membrane bilayer is controversially described in the literature. Here, we systematically review and examine the protein-membrane, protein-water, and protein-protein contacts of the nine helices of active Bax and Bak, and add a new set of topology data obtained by fluorescence and EPR methods. We conclude based on the consistent part of the datasets that the core/dimerization domain of Bax (Bak) is water exposed with only helices 4 and 5 in membrane contact, whereas the piercing/latch domain is in peripheral membrane contact, with helix 9 being transmembrane. Among the available structural models, those considering the dimerization/core domain at the rim of a toroidal pore are the most plausible to describe the active state of the proteins, although the structural flexibility of the piercing/latch domain does not allow unambiguous discrimination between the existing models.

Apoptotic foci at mitochondria: in and around Bax pores

The permeabilization of the mitochondrial outer membrane by Bax and Bak during apoptosis is considered a key step and a point of no return in the signalling pathway. It is always closely related to the reorganization of mitochondrial cristae that frees cytochrome c to the intermembrane space and to massive mitochondrial fragmentation mediated by the dynamin-like protein Drp1. Despite multiple evidence in favour of a functional link between these processes, the molecular mechanisms that connect them and their relevance for efficient apoptosis signalling remain obscure. In this review, we discuss recent progress on our understanding of how Bax forms pores in the context of Drp1-stabilized signalling platforms at apoptotic foci in mitochondria.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

Pore formation by dimeric Bak and Bax: an unusual pore?

Apoptotic cell death via the mitochondrial pathway occurs in all vertebrate cells and requires the formation of pores in the mitochondrial outer membrane. Two Bcl-2 protein family members, Bak and Bax, form these pores during apoptosis, and how they do so has been investigated for the last two decades. Many of the conformation changes that occur during their transition to pore-forming proteins have now been delineated. Notably, biochemical, biophysical and structural studies indicate that symmetric homodimers are the basic unit of pore formation. Each dimer contains an extended hydrophobic surface that lies on the outer membrane, and is anchored at either end by a transmembrane domain. Membrane-remodelling events such as positive membrane curvature have been reported to accompany apoptotic pore formation, suggesting Bak and Bax form lipidic pores rather than proteinaceous pores. However, it remains unclear how symmetric dimers assemble to porate the membrane. Here, we review how clusters of dimers and their lipid-mediated interactions provide a molecular explanation for the heterogeneous assemblies of Bak and Bax observed during apoptosis.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

Systemic Delivery of Tumor-Targeted Bax-Derived Membrane-Active Peptides for the Treatment of Melanoma Tumors in a Humanized SCID Mouse Model

Melanoma is a highly metastatic and deadly form of cancer. Invasive melanoma cells overexpress integrin α_vβ₃, which is a well-known target for Arg-Gly-Asp-based (RGD) peptides. We developed a sophisticated method to synthetize milligram amounts of a targeted vector that allows the RGD-mediated targeting, internalization, and release of a mitochondria-disruptive peptide derived from the pro-apoptotic Bax protein. We found that 2.5 μM Bax[109-127] was sufficient to destabilize the mitochondria in ten different tumor cell lines, even in the presence of the anti-apoptotic Bcl2 protein, which is often involved in tumor resistance. This pore-forming peptide displayed antitumor activity when it was covalently linked by a disulfide bridge to the tetrameric RAFT-c[RGD]₄-platform and after intravenous injection in a human melanoma tumor model established in humanized immuno-competent mice. In addition to its direct toxic effect, treatment with this combination induced the release of the immuno-stimulating factor monocyte chimoattractant protein 1 (MCP1) in the blood and a decrease in the level of the pro-angiogenic factor FGF2. Our novel multifunctional, apoptosis-inducing agent could be further customized and assayed for potential use in tumor-targeted therapy.

Connecting mitochondrial dynamics and life-or-death events via Bcl-2 family proteins

The morphology of a population of mitochondria is the result of several interacting dynamical phenomena, including fission, fusion, movement, elimination and biogenesis. Each of these phenomena is controlled by underlying molecular machinery, and when defective can cause disease. New understanding of the relationships between form and function of mitochondria in health and disease is beginning to be unraveled on several fronts. Studies in mammals and model organisms have revealed that mitochondrial morphology, dynamics and function appear to be subject to regulation by the same proteins that regulate apoptotic cell death. One protein family that influences mitochondrial dynamics in both healthy and dying cells is the Bcl-2 protein family. Connecting mitochondrial dynamics with life-death pathway forks may arise from the intersection of Bcl-2 family proteins with the proteins and lipids that determine mitochondrial shape and function. Bcl-2 family proteins also have multifaceted influences on cells and mitochondria, including calcium handling, autophagy and energetics, as well as the subcellular localization of mitochondrial organelles to neuronal synapses. The remarkable range of physical or functional interactions by Bcl-2 family proteins is challenging to assimilate into a cohesive understanding. Most of their effects may be distinct from their direct roles in apoptotic cell death and are particularly apparent in the nervous system. Dual roles in mitochondrial dynamics and cell death extend beyond BCL-2 family proteins. In this review, we discuss many processes that govern mitochondrial structure and function in health and disease, and how Bcl-2 family proteins integrate into some of these processes.

The membrane activity of BOK involves formation of large, stable toroidal pores and is promoted by cBID

The BCL-2 family members are key regulators of the intrinsic apoptotic pathway, which is defined by permeabilization of the mitochondrial outer membrane by members of the BAX-like subfamily. BOK is classified as a BAX-like protein; however, its (patho-)physiological role remains largely unclear. We therefore assessed the membrane permeabilization potential of C-terminally truncated recombinant BOK, BOK^∆C . We show that BOK^∆C can permeabilize liposomes mimicking the composition of mitochondrial outer membrane, but not of endoplasmic reticulum, forming large and stable pores over time. Importantly, pore formation was enhanced by the presence of cBID and refractory to the addition of antiapoptotic BCL-X_L . However, isolated mitochondria from Bax^-/- Bak^-/- cells were resistant to BOK-induced cytochrome c release, even in the presence of cBID. Taken together, we show that BOK^∆C can permeabilize liposomes, and cooperate with cBID, but its role in directly mediating mitochondrial permeabilization is unclear and may underlie a yet to be determined negative regulation.

Bax transmembrane domain interacts with prosurvival Bcl-2 proteins in biological membranes

The Bcl-2 (B-cell lymphoma 2) protein Bax (Bcl-2 associated X, apoptosis regulator) can commit cells to apoptosis via outer mitochondrial membrane permeabilization. Bax activity is controlled in healthy cells by prosurvival Bcl-2 proteins. C-terminal Bax transmembrane domain interactions were implicated recently in Bax pore formation. Here, we show that the isolated transmembrane domains of Bax, Bcl-x_L (B-cell lymphoma-extra large), and Bcl-2 can mediate interactions between Bax and prosurvival proteins inside the membrane in the absence of apoptotic stimuli. Bcl-2 protein transmembrane domains specifically homooligomerize and heterooligomerize in bacterial and mitochondrial membranes. Their interactions participate in the regulation of Bcl-2 proteins, thus modulating apoptotic activity. Our results suggest that interactions between the transmembrane domains of Bax and antiapoptotic Bcl-2 proteins represent a previously unappreciated level of apoptosis regulation.

Bax and Bak Pores: Are We Closing the Circle?

Bax and its homolog Bak are key regulators of the mitochondrial pathway of apoptosis. On cell stress Bax and Bak accumulate at distinct foci on the mitochondrial surface where they undergo a conformational change, oligomerize, and mediate cytochrome c release, leading to cell death. The molecular mechanisms of Bax and Bak assembly and mitochondrial permeabilization have remained a longstanding question in the field. Recent structural and biophysical studies at several length scales have shed light on key aspects of Bax and Bak function that have shifted how we think this process occurs. These discoveries reveal an unexpected molecular mechanism in which Bax (and likely Bak) dimers assemble into oligomers with an even number of molecules that fully or partially delineate pores of different sizes to permeabilize the mitochondrial outer membrane (MOM) during apoptosis.

Pro-apoptotic Bax molecules densely populate the edges of membrane pores

How the pro-apoptotic Bax protein permeabilizes the mitochondrial outer membrane is not fully understood. Previously, using cryo-electron microscopy (cryo-EM), we showed that activated Bax forms large, growing pores. Whether formed in liposomes or in mitochondrial outer membranes, Bax-induced pores exhibit the same morphology, with negative curvature flanking the edges and with no visible protein structure protruding from the membranes. Here we used cryo-EM to show that gold-labeled Bax molecules, after activation by Bid, became localized strictly at pore edges. This argues that Bax acts at short range to deform the membrane. Also, Bax molecules populated the walls of both small and large pores at the same density, implying that Bax is continuously recruited to the pores as they widen. Moreover, because all Bax molecules became oligomerized after membrane insertion, we infer that Bax oligomers are present at pore edges. We suggest that oligomerization may promote pore enlargement.

The Toxoplasma Dense Granule Proteins GRA17 and GRA23 Mediate the Movement of Small Molecules between the Host and the Parasitophorous Vacuole

Toxoplasma gondii is a protozoan pathogen in the phylum Apicomplexa that resides within an intracellular parasitophorous vacuole (PV) that is selectively permeable to small molecules through unidentified mechanisms. We have identified GRA17 as a Toxoplasma-secreted protein that localizes to the parasitophorous vacuole membrane (PVM) and mediates passive transport of small molecules across the PVM. GRA17 is related to the putative Plasmodium translocon protein EXP2 and conserved across PV-residing Apicomplexa. The PVs of GRA17-deficient parasites have aberrant morphology, reduced permeability to small molecules, and structural instability. GRA17-deficient parasites proliferate slowly and are avirulent in mice. These GRA17-deficient phenotypes are rescued by complementation with Plasmodium EXP2. GRA17 functions synergistically with a related protein, GRA23. Exogenous expression of GRA17 or GRA23 alters the membrane conductance properties of Xenopus oocytes in a manner consistent with a large non-selective pore. Thus, GRA17 and GRA23 provide a molecular basis for PVM permeability and nutrient access.

A Pore Idea: the ion conduction pathway of TMEM16/ANO proteins is composed partly of lipid

Since their first descriptions, ion channels have been conceived as proteinaceous conduits that facilitate the passage of ionic cargo between segregated environments. This concept is reinforced by crystallographic structures of cation channels depicting ion conductance pathways completely lined by protein. Although lipids are sometimes present in fenestrations near the pore or may be involved in channel gating, there is little or no evidence that lipids inhabit the ion conduction pathway. Indeed, the presence of lipid acyl chains in the conductance pathway would curse the design of the channel's aqueous pore. Here, we make a speculative proposal that anion channels in the TMEM16/ANO superfamily have ion conductance pathways composed partly of lipids. Our reasoning is based on the idea that TMEM16 ion channels evolved from a kind of lipid transporter that scrambles lipids between leaflets of the membrane bilayer and the modeled structural similarity between TMEM16 lipid scramblases and TMEM16 anion channels. This novel view of the TMEM16 pore offers explanation for the biophysical and pharmacological oddness of TMEM16A. We build upon the recent X-ray structure of nhTMEM16 and develop models of both TMEM16 ion channels and lipid scramblases to bolster our proposal. It is our hope that this model of the TMEM16 pore will foster innovative investigation into TMEM16 function.

Protein-lipid interactions and non-lamellar lipidic structures in membrane pore formation and membrane fusion

Pore-forming proteins and peptides act on their targeted lipid bilayer membranes to increase permeability. This approach to the modulation of biological function is relevant to a great number of living processes, including; infection, parasitism, immunity, apoptosis, development and neurodegeneration. While some pore-forming proteins/peptides assemble into rings of subunits to generate discrete, well-defined pore-forming structures, an increasing number is recognised to form pores via mechanisms which co-opt membrane lipids themselves. Among these, membrane attack complex-perforin/cholesterol-dependent cytolysin (MACPF/CDC) family proteins, Bax/colicin family proteins and actinoporins are especially prominent and among the mechanisms believed to apply are the formation of non-lamellar (semi-toroidal or toroidal) lipidic structures. In this review I focus on the ways in which lipids contribute to pore formation and contrast this with the ways in which lipids are co-opted also in membrane fusion and fission events. A variety of mechanisms for pore formation that involve lipids exists, but they consistently result in stable hybrid proteolipidic structures. These structures are stabilised by mechanisms in which pore-forming proteins modify the innate capacity of lipid membranes to respond to their environment, changing shape and/or phase and binding individual lipid molecules directly. In contrast, and despite the diversity in fusion protein types, mechanisms for membrane fusion are rather similar to each other, mapping out a pathway from pairs of separated compartments to fully confluent fused membranes. Fusion proteins generate metastable structures along the way which, like long-lived proteolipidic pore-forming complexes, rely on the basic physical properties of lipid bilayers. Membrane fission involves similar intermediates, in the reverse order. I conclude by considering the possibility that at least some pore-forming and fusion proteins are evolutionarily related homologues. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Membrane Disintegration Caused by the Steroid Saponin Digitonin Is Related to the Presence of Cholesterol

In the present investigation we studied the molecular mechanisms of the monodesmosidic saponin digitonin on natural and artificial membranes. We measured the hemolytic activity of digitonin on red blood cells (RBCs). Also different lipid membrane models (large unilamellar vesicles, LUVs, and giant unilamellar vesicles, GUVs) in the presence and absence of cholesterol were employed. The stability and permeability of the different vesicle systems were studied by using calcein release assay, GUVs membrane permeability assay using confocal microscopy (CM) and fluorescence correlation spectroscopy (FCS) and vesicle size measurement by dynamic light scattering (DLS). The results support the essential role of cholesterol in explaining how digitonin can disintegrate biological and artificial membranes. Digitonin induces membrane permeability or causes membrane rupturing only in the presence of cholesterol in an all-or-none mechanism. This effect depends on the concentrations of both digitonin and cholesterol. At low concentrations, digitonin induces membrane permeability while keeping the membrane intact. When digitonin is combined with other drugs, a synergistic potentiation can be observed because it facilitates their uptake.

Direct Activation of Bax Protein for Cancer Therapy

Bax, a central cell death regulator, is an indispensable gateway to mitochondrial dysfunction and a major proapoptotic member of the B-cell lymphoma 2 (Bcl-2) family proteins that control apoptosis in normal and cancer cells. Dysfunction of apoptosis renders the cancer cell resistant to treatment as well as promotes tumorigenesis. Bax activation induces mitochondrial membrane permeabilization, thereby leading to the release of apoptotic factor cytochrome c and consequently cancer cell death. A number of drugs in clinical use are known to indirectly activate Bax. Intriguingly, recent efforts demonstrate that Bax can serve as a promising direct target for small-molecule drug discovery. Several direct Bax activators have been identified to hold promise for cancer therapy with the advantages of specificity and the potential of overcoming chemo- and radioresistance. Further investigation of this new class of drug candidates will be needed to advance them into the clinic as a novel means to treat cancer.

Interaction of fluorescent phospholipids with cyclodextrins

Fluorescent analogs of phospholipids are often employed to investigate the structure and dynamics of lipids in membranes. Some of those studies have used cyclodextrins e.g., to modulate the lipid phase. However, the role of the fluorescence moiety of analogs for the interaction between cyclodextrins and fluorescent lipids has not been investigated so far in detail. Therefore, in the present study the interaction of various fluorescent phospholipid analogs with methylated α-, β- and γ- cyclodextrins was investigated. The analogs differed in their structure, in the length of the fatty acyl chain, in the position of the fluorescence group, and in the attached fluorescence moiety (7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) or dipyrrometheneboron difluoride (BODIPY)). In aqueous buffer, cyclodextrins bind fluorescent lipids disturbing the organization of the analogs. When incorporated into lipid vesicles, analogs are selectively extracted from the membrane upon addition of cyclodextrins. The results show that the interaction of cyclodextrins with fluorescent phospholipids depends on the cyclodextrin species, the fluorescence moiety and the phospholipid structure. The presented data should be of interest for studies using fluorescent phospholipids and cyclodextrins, since the interaction between the fluorescence group and the cyclodextrin may interfere with the process(es) under study.

More Than a Pore: The Interplay of Pore-Forming Proteins and Lipid Membranes

Pore-forming proteins (PFPs) punch holes in their target cell membrane to alter their permeability. Permeabilization of lipid membranes by PFPs has received special attention to study the basic molecular mechanisms of protein insertion into membranes and the development of biotechnological tools. PFPs act through a general multi-step mechanism that involves (i) membrane partitioning, (ii) insertion into the hydrophobic core of the bilayer, (iii) oligomerization, and (iv) pore formation. Interestingly, PFPs and membranes show a dynamic interplay. As PFPs are usually produced as soluble proteins, they require a large conformational change for membrane insertion. Moreover, membrane structure is modified upon PFPs insertion. In this context, the toroidal pore model has been proposed to describe a pore architecture in which not only protein molecules but also lipids are directly involved in the structure. Here, we discuss how PFPs and lipids cooperate and remodel each other to achieve pore formation, and explore new evidences of protein-lipid pore structures.

Emerging understanding of Bcl-2 biology: Implications for neoplastic progression and treatment

Bcl-2, the founding member of a family of apoptotic regulators, was initially identified as the protein product of a gene that is translocated and overexpressed in greater than 85% of follicular lymphomas (FLs). Thirty years later we now understand that anti-apoptotic Bcl-2 family members modulate the intrinsic apoptotic pathway by binding and neutralizing the mitochondrial permeabilizers Bax and Bak as well as a variety of pro-apoptotic proteins, including the cellular stress sensors Bim, Bid, Puma, Bad, Bmf and Noxa. Despite extensive investigation of all of these proteins, important questions remain. For example, how Bax and Bak breach the outer mitochondrial membrane remains poorly understood. Likewise, how the functions of anti-apoptotic Bcl-2 family members such as eponymous Bcl-2 are affected by phosphorylation or cancer-associated mutations has been incompletely defined. Finally, whether Bcl-2 family members can be successfully targeted for therapeutic advantage is only now being investigated in the clinic. Here we review recent advances in understanding Bcl-2 family biology and biochemistry that begin to address these questions.

Lipid-dependent bimodal MCL1 membrane activity

Increasing evidence indicates that the mitochondrial lipid membrane environment directly modulates the BCL2 family protein function, but the underlying mechanisms are still poorly understood. Here, we used minimalistic reconstituted systems to examine the influence of mitochondrial lipids on MCL1 activity and conformation. Site-directed mutagenesis and fluorescence spectroscopic analyses revealed that the BCL2 homology region of MCL1 (MCL1ΔNΔC) inhibits permeabilization of MOM-like membranes exclusively via canonical BH3-into-groove interactions with both cBID-like activators and BAX-like effectors. Contrary to currently popular models, MCL1ΔNΔC did not require becoming embedded into the membrane to inhibit membrane permeabilization, and interaction with cBID was more productive for MCL1ΔNΔC inhibitory activity than interaction with BAX. We also report that membranes rich in cardiolipin (CL), but not phosphatidylinositol (PI), trigger a profound conformational change in MCL1ΔNΔC leading to membrane integration and unleashment of an intrinsic lipidic pore-forming activity of the molecule. Cholesterol (CHOL) reduces both the conformational change and the lipidic pore-forming activity of MCL1ΔNΔC in CL-rich membranes, but it does not affect the interaction of MCL1ΔNΔC with proapoptotic partners in MOM-like liposomes. In addition, we identified MCL1α5 as the minimal domain of the protein responsible for its membrane-permeabilizing function both in model membranes and at the mitochondrial level. Our results provide novel mechanistic insight into MCL1 function in the context of a membrane milieu and add significantly to a growing body of evidence supporting an active role of mitochondrial membrane lipids in BCL2 protein function.

Structural model of active Bax at the membrane

Bax plays a central role in the mitochondrial pathway of apoptosis. Upon activation, cytosolic Bax monomers oligomerize on the surface of mitochondria and change conformation concertedly to punch holes into the outer membrane. The subsequent release of cytochrome c initiates cell death. However, the structure of membrane-inserted Bax and its mechanism of action remain largely unknown. Here, we propose a 3D model of active Bax at the membrane based on double electron-electron resonance (DEER) spectroscopy in liposomes and isolated mitochondria. We show that active Bax is organized at the membrane as assemblies of dimers. In addition to a stable dimerization domain, each monomer contains a more flexible piercing domain involved in interdimer interactions and pore formation. The most important structural change during Bax activation is the opening of the hairpin formed by helices 5 and 6, which adopts a clamp-like conformation central to the mechanism of mitochondrial permeabilization.

Functional characterization of sticholysin I and W111C mutant reveals the sequence of the actinoporin's pore assembly

The use of pore-forming toxins in the construction of immunotoxins against tumour cells is an alternative for cancer therapy. In this protein family one of the most potent toxins are the actinoporins, cytolysins from sea anemones. We work on the construction of tumour proteinase-activated immunotoxins using sticholysin I (StI), an actinoporin isolated from the sea anemone Stichodactyla helianthus. To accomplish this objective, recombinant StI (StIr) with a mutation in the membrane binding region has been employed. In this work, it was evaluated the impact of mutating tryptophan 111 to cysteine on the toxin pore forming capability. StI W111C is still able to permeabilize erythrocytes and liposomes, but at ten-fold higher concentration than StI. This is due to its lower affinity for the membrane, which corroborates the importance of residue 111 for the binding of actinoporins to the lipid bilayer. In agreement, other functional characteristics not directly associated to the binding, are essentially the same for both variants, that is, pores have oligomeric structures with similar radii, conductance, cation-selectivity, and instantaneous current-voltage behavior. In addition, this work provides experimental evidence sustaining the toroidal protein-lipid actinoporins lytic structures, since the toxins provoke the trans-bilayer movement (flip-flop) of a pyrene-labeled analogue of phosphatidylcholine in liposomes, indicating the existence of continuity between the outer and the inner membrane leaflet. Finally, our planar lipid membranes results have also contributed to a better understanding of the actinoporin's pore assembly mechanism. After the toxin binding and the N-terminal insertion in the lipid membrane, the pore assembly occurs by passing through different transient sub-conductance states. These states, usually 3 or 4, are due to the successive incorporation of N-terminal α-helices and lipid heads to the growing pores until a stable toroidal oligomeric structure is formed, which is mainly tetrameric.

Conformational rearrangements in the pro-apoptotic protein, Bax, as it inserts into mitochondria: a cellular death switch

The B-cell lymphoma 2 (Bcl-2) family of proteins regulates the activation of apoptosis through the mitochondria pathway. Pro- and anti-apoptotic members of this family keep each other in check until the correct time to commit to apoptosis. The point of no return for this commitment is the permeabilization of the outer mitochondrial membrane. Translocation of the pro-apoptotic member, Bax, from the cytosol to the mitochondria is the molecular signature of this event. We employed a novel method to reliably detect Förster resonance energy transfer (FRET) between pairs of fluorophores to identify intra-molecular conformational changes and inter-molecular contacts in Bax as this translocation occurs in live cells. In the cytosol, our FRET measurement indicated that the C-terminal helix is exposed instead of tucked away in the core of the protein. In addition fluorescence correlation spectroscopy (FCS) showed that cytosolic Bax diffuses much slower than expected, suggesting possible complex formation or transient membrane interaction. Cross-linking the C-terminal helix (α9) to helix α4 reduced the potential of those interactions to occur. After translocation, our FRET measurements showed that Bax molecules form homo-oligomers in the mitochondria through two distinct interfaces involving the BH3 domain (helix α2) and the C-terminal helix. These findings have implications for possible contacts with other Bcl-2 proteins necessary for the regulation of apoptosis.

Apoptotic pore formation is associated with in-plane insertion of Bak or Bax central helices into the mitochondrial outer membrane

The pivotal step on the mitochondrial pathway to apoptosis is permeabilization of the mitochondrial outer membrane (MOM) by oligomers of the B-cell lymphoma-2 (Bcl-2) family members Bak or Bax. However, how they disrupt MOM integrity is unknown. A longstanding model is that activated Bak and Bax insert two α-helices, α5 and α6, as a hairpin across the MOM, but recent insights on the oligomer structures question this model. We have clarified how these helices contribute to MOM perforation by determining that, in the oligomers, Bak α5 (like Bax α5) remains part of the protein core and that a membrane-impermeable cysteine reagent can label cysteines placed at many positions in α5 and α6 of both Bak and Bax. The results are inconsistent with the hairpin insertion model but support an in-plane model in which α5 and α6 collapse onto the membrane and insert shallowly to drive formation of proteolipidic pores.

Regulating cell death at, on, and in membranes

Bcl-2 family proteins are central regulators of apoptosis. Various family members are located in the cytoplasm, endoplasmic reticulum, and mitochondrial outer membrane in healthy cells. However during apoptosis most of the interactions between family members that determine the fate of the cell occur at the membranes of intracellular organelles. It has become evident that interactions with membranes play an active role in the regulation of Bcl-2 family protein interactions. Here we provide an overview of various models proposed to explain how the Bcl-2 family regulates apoptosis and discuss how membrane binding affects the structure and function of each of the three categories of Bcl-2 proteins (pro-apoptotic, pore-forming, and anti-apoptotic). We also examine how the Bcl-2 family regulates other aspects of mitochondrial and ER physiology relevant to cell death.

Unconventional secretion of fibroblast growth factor 2--a novel type of protein translocation across membranes?

N-terminal signal peptides are a hallmark of the vast majority of soluble secretory proteins that are transported along the endoplasmic reticulum/Golgi-dependent pathway. They are recognized by signal recognition particle, a process that initiates membrane translocation into the lumen of the endoplasmic reticulum followed by vesicular transport to the cell surface and release into the extracellular space. Beyond this well-established mechanism of protein secretion from eukaryotic cells, a number of extracellular proteins with critical physiological functions in immune surveillance and tissue organization are known to be secreted in a manner independent of signal recognition particle. Such processes have collectively been termed "unconventional protein secretion" and, while known for more than two decades, their underlying mechanisms are only beginning to emerge. Different types of unconventional secretory mechanisms have been described with the best-characterized example being based on direct translocation of cytoplasmic proteins across plasma membranes. The aim of this review is to critically assess our current knowledge of this type of unconventional secretion focusing on fibroblast growth factor 2 (FGF2) as the most established example.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

Mitochondrial alterations in apoptosis

Besides their conventional role as energy suppliers for the cell, mitochondria in vertebrates are active regulators of apoptosis. They release apoptotic factors from the intermembrane space into the cytosol through a mechanism that involves the Bcl-2 protein family, mediating permeabilization of the outer mitochondrial membrane. Associated with this event, a number of additional changes affect mitochondria during apoptosis. They include loss of important mitochondrial functions, such as the ability to maintain calcium homeostasis and to generate ATP, as well as mitochondrial fragmentation and cristae remodeling. Moreover, the lipidic component of mitochondrial membranes undergoes important alterations in composition and distribution, which have turned out to be relevant regulatory events for the proteins involved in apoptotic mitochondrial damage.

Death upon a kiss: mitochondrial outer membrane composition and organelle communication govern sensitivity to BAK/BAX-dependent apoptosis

For stressed cells to induce the mitochondrial pathway of apoptosis, a cohort of pro-apoptotic BCL-2 proteins must collaborate with the outer mitochondrial membrane to permeabilize it. BAK and BAX are the two pro-apoptotic BCL-2 family members that are required for mitochondrial outer membrane permeabilization. While biochemical and structural insights of BAK/BAX function have expanded in recent years, very little is known about the role of the outer mitochondrial membrane in regulating BAK/BAX activity. In this review, we will highlight the impact of mitochondrial composition (both protein and lipid) and mitochondrial interactions with cellular organelles on BAK/BAX function and cellular commitment to apoptosis. A better understanding of how BAK/BAX and mitochondrial biology are mechanistically linked will likely reveal novel insights into homeostatic and pathological mechanisms associated with apoptosis.

Building blocks of the apoptotic pore: how Bax and Bak are activated and oligomerize during apoptosis

The central role of the Bcl-2 family in regulating apoptotic cell death was first identified in the 1980s. Since then, significant in-roads have been made in identifying the multiple members of this family, characterizing their form and function and understanding how their interactions determine whether a cell lives or dies. In this review we focus on the recent progress made in characterizing the proapoptotic Bcl-2 family members, Bax and Bak. This progress has resolved longstanding controversies, but has also challenged established theories in the apoptosis field. We will discuss different models of how these two proteins become activated and different 'modes' by which they are inhibited by other Bcl-2 family members. We will also discuss novel conformation changes leading to Bak and Bax oligomerization and speculate how these oligomers might permeabilize the mitochondrial outer membrane.

The rheostat in the membrane: BCL-2 family proteins and apoptosis

Apoptosis, a mechanism for programmed cell death, has key roles in human health and disease. Many signals for cellular life and death are regulated by the BCL-2 family proteins and converge at mitochondria, where cell fate is ultimately decided. The BCL-2 family includes both pro-life (e.g. BCL-XL) and pro-death (e.g. BAX, BAK) proteins. Previously, it was thought that a balance between these opposing proteins, like a simple 'rheostat', could control the sensitivity of cells to apoptotic stresses. Later, this rheostat concept had to be extended, when it became clear that BCL-2 family proteins regulate each other through a complex network of bimolecular interactions, some transient and some relatively stable. Now, studies have shown that the apoptotic circuitry is even more sophisticated, in that BCL-2 family interactions are spatially dynamic, even in nonapoptotic cells. For example, BAX and BCL-XL can shuttle between the cytoplasm and the mitochondrial outer membrane (MOM). Upstream signaling pathways can regulate the cytoplasmic-MOM equilibrium of BAX and thereby adjust the sensitivity of cells to apoptotic stimuli. Thus, we can view the MOM as the central locale of a dynamic life-death rheostat. BAX invariably forms extensive homo-oligomers after activation in membranes. However, recent studies, showing that activated BAX monomers determine the kinetics of MOM permeabilization (MOMP), perturb the lipid bilayer and form nanometer size pores, pose questions about the role of the oligomerization. Other lingering questions concern the molecular mechanisms of BAX redistribution between MOM and cytoplasm and the details of BAX/BAK-membrane assemblies. Future studies need to delineate how BCL-2 family proteins regulate MOMP, in concert with auxiliary MOM proteins, in a dynamic membrane environment. Technologies aimed at elucidating the structure and function of the full-length proteins in membranes are needed to illuminate some of these critical issues.

Proapoptotic Bax and Bak proteins form stable protein-permeable pores of tunable size

The Bcl-2 proapoptotic proteins Bax and Bak mediate the permeabilization of the mitochondrial outer membrane during apoptosis. Current models consider that Bax and Bak form pores at the mitochondrial outer membrane that are responsible for the release of cytochrome c and other larger mitochondrial apoptotic factors (i.e. Smac/DIABLO, AIF, and endoglycosidase G). However, the properties and nature of Bax/Bak apoptotic pores remain enigmatic. Here, we performed a detailed analysis of the membrane permeabilizing activity of Bax and Bak at the single vesicle level. We directly visualized that cBid-activated Bax and BakΔC21 can form membrane pores large enough to release not only cytochrome c, but also allophycocyanine, a protein of 104 kDa. Interestingly, the size of Bax and BakΔC21 pores is not constant, as typically observed in purely proteinaceous channels, but evolves with time and depends on protein concentration. We found that Bax and BakΔC21 formed long-lived pores, whose areas changed with the amount of Bax/BakΔC21 but not with cardiolipin concentration. Altogether, our results demonstrate that Bax and BakΔC21 follow similar mechanisms of membrane permeabilization characterized by the formation of protein-permeable pores of dynamic size, in agreement with the proteolipidic nature of these apoptotic pores.