Some amphiphilic cations block the mitochondrial apoptosis-induced channel, MAC

Bcl-2 family in inter-organelle modulation of calcium signaling; roles in bioenergetics and cell survival

Bcl-2 family proteins, known for their apoptosis functioning at the mitochondria, have been shown to localize to other cellular compartments to mediate calcium (Ca2+) signals. Since the proper supply of Ca2+ in cells serves as an important mechanism for cellular survival and bioenergetics, we propose an integrating role for Bcl-2 family proteins in modulating Ca2+ signaling. The endoplasmic reticulum (ER) is the main Ca2+ storage for the cell and Bcl-2 family proteins competitively regulate its Ca2+ concentration. Bcl-2 family proteins also regulate the flux of Ca2+ from the ER by physically interacting with inositol 1,4,5-trisphosphate receptors (IP3Rs) to mediate their opening. Type 1 IP3Rs reside at the bulk ER to coordinate cytosolic Ca2+ signals, while type 3 IP3Rs reside at mitochondria-associated ER membrane (MAM) to facilitate mitochondrial Ca2+ uptake. In healthy cells, mitochondrial Ca2+ drives pyruvate into the citric acid (TCA) cycle to facilitate ATP production, while a continuous accumulation of Ca2+ can trigger the release of cytochrome c, thus initiating apoptosis. Since multiple organelles and Bcl-2 family proteins are involved in Ca2+ signaling, we aim to clarify the role that Bcl-2 family proteins play in facilitating Ca2+ signaling and how mitochondrial Ca2+ is relevant in both bioenergetics and apoptosis. We also explore how these insights could be useful in controlling bioenergetics in apoptosis-resistant cell lines.

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.

VDAC1: from structure to cancer therapy

Here, we review current evidence pointing to the function of VDAC1 in cell life and death, and highlight these functions in relation to cancer. Found at the outer mitochondrial membrane, VDAC1 assumes a crucial position in the cell, controlling the metabolic cross-talk between mitochondria and the rest of the cell. Moreover, its location at the boundary between the mitochondria and the cytosol enables VDAC1 to interact with proteins that mediate and regulate the integration of mitochondrial functions with other cellular activities. As a metabolite transporter, VDAC1 contributes to the metabolic phenotype of cancer cells. This is reflected by VDAC1 over-expression in many cancer types, and by inhibition of tumor development upon silencing VDAC1 expression. Along with regulating cellular energy production and metabolism, VDAC1 is also a key protein in mitochondria-mediated apoptosis, participating in the release of apoptotic proteins and interacting with anti-apoptotic proteins. The involvement of VDAC1 in the release of apoptotic proteins located in the inter-membranal space is discussed, as is VDAC1 oligomerization as an important step in apoptosis induction. VDAC also serves as an anchor point for mitochondria-interacting proteins, some of which are also highly expressed in many cancers, such as hexokinase (HK), Bcl2, and Bcl-xL. By binding to VDAC, HK provides both metabolic benefit and apoptosis-suppressive capacity that offers the cell a proliferative advantage and increases its resistance to chemotherapy. VDAC1-based peptides that bind specifically to HK, Bcl2, or Bcl-xL abolished the cell’s abilities to bypass the apoptotic pathway. Moreover, these peptides promote cell death in a panel of genetically characterized cell lines derived from different human cancers. These and other functions point to VDAC1 as a rational target for the development of a new generation of therapeutics.

Regulation of pancreatic β-cell survival by nitric oxide: clinical relevance

The reduction of pancreatic β-cell mass is an important factor in the development of type 1 and type 2 diabetes. Understanding the mechanisms that regulate the maintenance of pancreatic β-cell mass as well as β-cell death is necessary for the establishment of therapeutic strategies. In this context, nitric oxide (NO) is a diatomic, gaseous, highly reactive molecule with biological activity that participates in the regulation of pancreatic β-cell mass. Two types of cellular responses can be distinguished depending on the level of NO production. First, pancreatic β-cells exposed to inflammatory cytokines, lipid stress or hyperglycaemia produce high concentrations of NO, mainly due to the activation of inducible NO synthase (iNOS), thus promoting cell death. Meanwhile, under homeostatic conditions, low concentrations of NO, constitutively produced by endothelial NO synthase (eNOS), promote cell survival. Here, we will discuss the current knowledge of the NO-dependent mechanisms activated during cellular responses, emphasizing those related to the regulation of cell survival.

Interferon-stimulated gene ISG12b2 is localized to the inner mitochondrial membrane and mediates virus-induced cell death

Interferons (IFNs) are crucial for host defence against viruses. Many IFN-stimulated genes (ISGs) induced by viral infection exert antiviral effects. Microarray analysis of gene expression induced in liver tissues of mice on dengue virus (DENV) infection has led to identification of the ISG gene ISG12b2. ISG12b2 is also dramatically induced on DENV infection of Hepa 1-6 cells (mouse hepatoma cell line). Here, we performed biochemical and functional analyses of ISG12b2. We demonstrate that ISG12b2 is an inner mitochondrial membrane (IMM) protein containing a cleavable mitochondrial targeting sequence and multiple transmembrane segments. Overexpression of ISG12b2 in Hepa 1-6 induced release of cytochrome c from mitochondria, disruption of the mitochondrial membrane potential, and activation of caspase-9, caspase-3, and caspase-8. Treatment of ISG12b2-overexpressing Hepa 1-6 with inhibitors of pan-caspase, caspase-9, or caspase-3, but not caspase-8, reduced apoptotic cell death, suggesting that ISG12b2 activates the intrinsic apoptotic pathway. Of particular interest, we further demonstrated that ISG12b2 formed oligomers, and that ISG12b2 was able to mediate apoptosis through both Bax/Bak-dependent and Bax/Bak-independent pathways. Our study demonstrates that the ISG12b2 is a novel IMM protein induced by IFNs and regulates mitochondria-mediated apoptosis during viral infection.

The therapeutic potential of mitochondrial channels in cancer, ischemia-reperfusion injury, and neurodegeneration

Mitochondria communicate with the rest of the cell through channels located in their inner and outer membranes. Most of the time, the message is encoded by the flow of anions and cations e.g., through VDAC and PTP, respectively. However, proteins are also both imported and exported across the mitochondrial membranes e.g., through TOM and MAC, respectively. Transport through mitochondrial channels is exquisitely regulated and controls a myriad of processes; from energy production to cell death. Here, we examine the role of some of the mitochondrial channels involved in neurodegeneration, ischemia-reperfusion injury and cancer in the context of their potential as therapeutic targets.

Detection of the mitochondrial apoptosis-induced channel (MAC) and its regulation by Bcl-2 family proteins

Apoptosis is a phenomenon fundamental to higher eukaryotes that is integral to such diverse cellular processes as tissue homeostasis, organogenesis, and response to toxins. The release from mitochondria of apoptotic factors such as cytochrome c is a key step during apoptosis of most cells. Cytochrome c release occurs through the MAC (mitochondrial apoptosis-induced channel), a pore which forms in the mitochondrial outer membrane during early apoptosis and is exquisitely regulated by the Bcl-2 family of proteins. This unit presents basic and advanced tools for detecting MAC and defining its regulation by Bcl-2 family proteins and pharmacological agents. Protocols include the use of time-lapse video-microscopy to monitor the onset of apoptosis in living cells and patch-clamp techniques for mitochondria or proteoliposomes containing mitochondrial proteins, which allow direct detection of MAC. These approaches enable an evaluation of the role of MAC and mitochondria in apoptosis of a variety of cell types by many inducers.

Mitochondrial apoptosis-induced channel (MAC) function triggers a Bax/Bak-dependent bystander effect

Collateral spread of apoptosis to nearby cells is referred to as the bystander effect, a process that is integral to tissue homeostasis and a challenge to anticancer therapies. In many systems, apoptosis relies on permeabilization of the mitochondrial outer membrane to factors such as cytochrome c and Smac/DIABLO. This permeabilization occurs via formation of a mitochondrial apoptosis-induced channel (MAC) and was mimicked here by single-cell microinjection of cytochrome c into Xenopus laevis embryos. Waves of apoptosis were observed in vivo from the injected to the neighboring cells. This finding indicates that a death signal generated downstream of cytochrome c release diffused to neighboring cells and ultimately killed the animals. The role of MAC in bystander effects was then assessed in mouse embryonic fibroblasts that did or did not express its main components, Bax and/or Bak. Exogenous expression of green fluorescent protein-Bax triggered permeabilization of the outer membrane and apoptosis in these cells. Time-lapse videos showed that neighboring cells also underwent apoptosis, but expression of Bax and/or Bak was essential to this effect, because no bystanders were observed in cells lacking both of these MAC components. These results may guide development of novel therapeutic strategies to selectively eliminate tumors or minimize the size of tissue injury in degenerative or traumatic cell death.

Is mPTP the gatekeeper for necrosis, apoptosis, or both?

Permeabilization of the mitochondrial membranes is a crucial step in apoptosis and necrosis. This phenomenon allows the release of mitochondrial death factors, which trigger or facilitate different signaling cascades ultimately causing the execution of the cell. The mitochondrial permeability transition pore (mPTP) has long been known as one of the main regulators of mitochondria during cell death. mPTP opening can lead to matrix swelling, subsequent rupture of the outer membrane, and a nonspecific release of intermembrane space proteins into the cytosol. While mPTP was purportedly associated with early apoptosis, recent observations suggest that mitochondrial permeabilization mediated by mPTP is generally more closely linked to events of late apoptosis and necrosis. Mechanisms of mitochondrial membrane permeabilization during cell death, involving three different mitochondrial channels, have been postulated. These include the mPTP in the inner membrane, and the mitochondrial apoptosis-induced channel (MAC) and voltage-dependent anion-selective channel (VDAC) in the outer membrane. New developments on mPTP structure and function, and the involvement of mPTP, MAC, and VDAC in permeabilization of mitochondrial membranes during cell death are explored. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.

Role of mitochondrial ion channels in cell death

Ion channels located in the outer and inner mitochondrial membranes are key regulators of cellular signaling for life and death. Permeabilization of mitochondrial membranes is one of the most critical steps in the progression of several cell death pathways. The mitochondrial apoptosis-induced channel (MAC) and the mitochondrial permeability transition pore (mPTP) play major roles in these processes. Here, the most recent progress and current perspectives about the roles of MAC and mPTP in mitochondrial membrane permeabilization during cell death are presented. The crosstalk signaling of MAC and mPTP formation/activation mediated by cytosolic Ca(2+) signaling, Bcl-2 family proteins, and other mitochondrial ion channels is also discussed. Understanding the mechanisms that regulate opening and closing of MAC and mPTP has revealed new therapeutic targets that potentially could control cell death in pathologies such as cancer, ischemia/reperfusion injuries, and neurodegenerative diseases.

VDAC, a multi-functional mitochondrial protein regulating cell life and death

Research over the past decade has extended the prevailing view of the mitochondrion to include functions well beyond the generation of cellular energy. It is now recognized that mitochondria play a crucial role in cell signaling events, inter-organellar communication, aging, cell proliferation, diseases and cell death. Thus, mitochondria play a central role in the regulation of apoptosis (programmed cell death) and serve as the venue for cellular decisions leading to cell life or death. One of the mitochondrial proteins controlling cell life and death is the voltage-dependent anion channel (VDAC), also known as mitochondrial porin. VDAC, located in the mitochondrial outer membrane, functions as gatekeeper for the entry and exit of mitochondrial metabolites, thereby controlling cross-talk between mitochondria and the rest of the cell. VDAC is also a key player in mitochondria-mediated apoptosis. Thus, in addition to regulating the metabolic and energetic functions of mitochondria, VDAC appears to be a convergence point for a variety of cell survival and cell death signals mediated by its association with various ligands and proteins. In this article, we review what is known about the VDAC channel in terms of its structure, relevance to ATP rationing, Ca(2+) homeostasis, protection against oxidative stress, regulation of apoptosis, involvement in several diseases and its role in the action of different drugs. In light of our recent findings and the recently solved NMR- and crystallography-based 3D structures of VDAC1, the focus of this review will be on the central role of VDAC in cell life and death, addressing VDAC function in the regulation of mitochondria-mediated apoptosis with an emphasis on structure-function relations. Understanding structure-function relationships of VDAC is critical for deciphering how this channel can perform such a variety of functions, all important for cell life and death. This review also provides insight into the potential of VDAC1 as a rational target for new therapeutics.

MAC and Bcl-2 family proteins conspire in a deadly plot

Apoptosis is an elemental form of programmed cell death; it is fundamental to higher eukaryotes and essential to mechanisms controlling tissue homeostasis. Apoptosis is also involved in many pathologies including cancer, neurodegenerative diseases, aging, and infarcts. This cell death program is tightly regulated by Bcl-2 family proteins by controlling the formation of the mitochondrial apoptosis-induced channel or MAC. Assembly of MAC corresponds to permeabilization of the mitochondrial outer membrane, which is the so called commitment step of apoptosis. MAC provides the pathway through the mitochondrial outer membrane for the release of cytochrome c and other pro-apoptotic factors from the intermembrane space. While overexpression of anti-apoptotic Bcl-2 eliminates MAC activity, oligomers of the pro-apoptotic members Bax and/or Bak are essential structural component(s) of MAC. Assembly of MAC from Bax or Bak was monitored in real time by directly patch-clamping mitochondria with micropipettes containing the sentinel tBid, a direct activator of Bax and Bak. Herein, a variety of high affinity inhibitors of MAC (iMAC) that may prove to be crucial tools in mechanistic studies have recently been identified. This review focuses on characterization of MAC activity, its regulation by Bcl-2 family proteins, and a discussion of how MAC can be pharmacologically turned on or off depending on the pathology to be treated.

MAC inhibitors suppress mitochondrial apoptosis

MAC (mitochondrial apoptosis-induced channel) forms in the mitochondrial outer membrane and unleashes cytochrome c to orchestrate the execution of the cell. MAC opening is the commitment step of intrinsic apoptosis. Hence closure of MAC may prevent apoptosis. Compounds that blocked the release of fluorescein from liposomes by recombinant Bax were tested for their ability to directly close MAC and suppress apoptosis in FL5.12 cells. Low doses of these compounds (IC50 values ranged from 19 to 966 nM) irreversibly closed MAC. These compounds also blocked cytochrome c release and halted the onset of apoptotic markers normally induced by IL-3 (interleukin-3) deprivation or staurosporine. Our results reveal the tight link among MAC activity, cytochrome c release and apoptotic death, and indicate this mitochondrial channel is a promising therapeutic target.

Assembly of the mitochondrial apoptosis-induced channel, MAC

Although Bcl-2 family proteins control intrinsic apoptosis, the mechanisms underlying this regulation are incompletely understood. Patch clamp studies of mitochondria isolated from cells deficient in one or both of the pro-apoptotic proteins Bax and Bak show that at least one of the proteins must be present for formation of the cytochrome c-translocating channel, mitochondrial apoptosis-induced channel (MAC), and that the single channel behaviors of MACs containing exclusively Bax or Bak are similar. Truncated Bid catalyzes MAC formation in isolated mitochondria containing Bax and/or Bak with a time course of minutes and does not require VDAC1 or VDAC3. Mathematical analysis of the stepwise changes in conductance associated with MAC formation is consistent with pore assembly by a barrel-stave model. Assuming the staves are two transmembrane alpha-helices in Bax and Bak, mature MAC pores would typically contain approximately 9 monomers and have diameters of 5.5-6 nm. The mitochondrial permeability data are inconsistent with formation of lipidic pores capable of transporting megadalton-sized macromolecules as observed with recombinant Bax in liposomes.

Dynamin-2-dependent targeting of mannheimia haemolytica leukotoxin to mitochondrial cyclophilin D in bovine lymphoblastoid cells

Exotoxins which belong to the family containing the RTX toxins (repeats in toxin) contribute to a variety of important human and animal diseases. One example of such a toxin is the potent leukotoxin (LKT) produced by the bovine respiratory pathogen Mannheimia haemolytica. LKT binds to CD18, resulting in the death of bovine leukocytes. In this study, we showed that internalized LKT binds to the outer mitochondrial membrane, which results in the release of cytochrome c and collapse of the mitochondrial membrane potential (psi(m)). Incubation of bovine lymphoblastoid cells (BL-3 cells) with the mitochondrial membrane-stabilizing agent cyclosporine (CSA) reduced LKT-mediated cytotoxicity, cytochrome c release, and collapse of the psi(m). Coimmunoprecipitation and intracellular binding studies suggested that LKT binds to the mitochondrial matrix protein cyclophilin D. We also demonstrated that LKT mobilizes the vesicle scission protein dynamin-2 from mitochondria to the cell membrane. Incubation with CSA depleted mitochondrial dynamin-2 in BL-3 cells, making it unavailable for vesicle scission and LKT internalization. The results of this study show that LKT trafficking and LKT-mediated cell death involve dynamin-2 and cyclophilin D, in a process that can be prevented by the mitochondrial membrane-protecting function of CSA.

A tale of two mitochondrial channels, MAC and PTP, in apoptosis

The crucial step in the intrinsic, or mitochondrial, apoptotic pathway is permeabilization of the mitochondrial outer membrane. Permeabilization triggers release of apoptogenic factors, such as cytochrome c, from the mitochondrial intermembrane space into the cytosol where these factors ensure propagation of the apoptotic cascade and execution of cell death. However, the mechanism(s) underlying permeabilization of the outer membrane remain controversial. Two mechanisms, involving opening of two different mitochondrial channels, have been proposed to be responsible for the permeabilization; the permeability transition pore (PTP) in the inner membrane and the mitochondrial apoptosis-induced channel (MAC) in the outer membrane. Opening of PTP would lead to matrix swelling, subsequent rupture of the outer membrane, and an unspecific release of intermembrane proteins into the cytosol. However, many believe PTP opening is a consequence of apoptosis and this channel is thought to principally play a role in necrosis, not apoptosis. Activation of MAC is exquisitely regulated by Bcl-2 family proteins, which are the sentinels of apoptosis. MAC provides specific pores in the outer membrane for the passage of intermembrane proteins, in particular cytochrome c, to the cytosol. The electrophysiological characteristics of MAC are very similar to Bax channels and depletion of Bax significantly diminishes MAC activity, suggesting that Bax is an essential constituent of MAC in some systems. The characteristics of various mitochondrial channels and Bax are compared. The involvement of MAC and PTP activities in apoptosis of disease and their pharmacology are discussed.

The Bax pore in liposomes, Biophysics

The protein BAX of the Bcl-2-family is felt to be one of the two Bcl-2-family proteins that directly participate in the mitochondrial cytochrome c-translocating pore. We have studied the kinetics, stoichiometry and size of the pore formed by BAX in planar lipid bilayers and synthetic liposomes. Our data indicate that a cytochrome c-competent pore can be formed by in-membrane association of BAX monomers.

Is MAC the knife that cuts cytochrome c from mitochondria during apoptosis?

Apoptosis is a phenomenon fundamental to higher eukaryotes and essential to mechanisms controlling tissue homeostasis. Bcl-2 family proteins tightly control this cell death program by regulating the permeabilization of the mitochondrial outer membrane and, hence, the release of cytochrome c and other proapoptotic factors. Mitochondrial apoptosis-induced channel (MAC) is the mitochondrial apoptosis-induced channel and is responsible for cytochrome c release early in apoptosis. MAC activity is detected by patch clamping mitochondria at the time of cytochrome c release. The Bcl-2 family proteins regulate apoptosis by controlling the formation of MAC. Depending on cell type and apoptotic inducer, Bax and/or Bak are structural component(s) of MAC. Overexpression of the antiapoptotic protein Bcl-2 eliminates MAC activity. The focus of this review is a biophysical characterization of MAC activity and its regulation by Bcl-2 family proteins, and ends with some discussion of therapeutic targets.

The role of the mitochondrial apoptosis induced channel MAC in cytochrome c release

Permeabilization of the mitochondrial outer membrane is a crucial event during apoptosis. It allows the release of proapoptotic factors, like cytochrome c, from the intermembrane space, and represents the commitment step in apoptosis. The mitochondrial apoptosis-induced channel, MAC, is a high-conductance channel that forms during early apoptosis and is the putative cytochrome c release channel. Unlike activation of the permeability transition pore, MAC formation occurs without loss of outer membrane integrity and depolarization. The single channel behavior and pharmacology of reconstituted MAC has been characterized with patch-clamp techniques. Furthermore, MAC's activity is compared to that detected in mitochondria inside the cells at the time cytochrome c is released. Finally, the regulation of MAC by the Bcl-2 family proteins and insights concerning its molecular composition are also discussed.

The mitochondrial channel VDAC has a cation-selective open state

The mitochondrial channel VDAC is known to have two major classes of functional states, a large conductance "open" state that is anion selective, and lower conductance substates that are cation selective. The channel can reversibly switch between open and half-open states, with the latter predominant at increasing membrane voltages of either polarity. We report the presence of a new functional state of VDAC, a cation-selective state with conductance approximately equal to that of the canonical open state. This newly described state of VDAC can be reached from either the half-open cation-selective state or from the open anion-selective state. The latter transition implies that a mechanism exists for selectivity gating in VDAC that is separate from partial closure, which may be relevant to the physiological regulation of this channel and mitochondrial outer membrane permeability.

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

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

Oligomeric Bax is a component of the putative cytochrome c release channel MAC, mitochondrial apoptosis-induced channel

Bcl-2 family proteins regulate apoptosis, in part, by controlling formation of the mitochondrial apoptosis-induced channel (MAC), which is a putative cytochrome c release channel induced early in the intrinsic apoptotic pathway. This channel activity was never observed in Bcl-2-overexpressing cells. Furthermore, MAC appears when Bax translocates to mitochondria and cytochrome c is released in cells dying by intrinsic apoptosis. Bax is a component of MAC of staurosporine-treated HeLa cells because MAC activity is immunodepleted by Bax antibodies. MAC is preferentially associated with oligomeric, not monomeric, Bax. The single channel behavior of recombinant oligomeric Bax and MAC is similar. Both channel activities are modified by cytochrome c, consistent with entrance of this protein into the pore. The mean conductance of patches of mitochondria isolated after green fluorescent protein-Bax translocation is significantly higher than those from untreated cells, consistent with onset of MAC activity. In contrast, the mean conductance of patches of mitochondria indicates MAC activity is present in apoptotic cells deficient in Bax but absent in apoptotic cells deficient in both Bax and Bak. These findings indicate Bax is a component of MAC in staurosporine-treated HeLa cells and suggest Bax and Bak are functionally redundant as components of MAC.

The mitochondrial apoptosis-induced channel (MAC) corresponds to a late apoptotic event

We have investigated the mechanism responsible for mitochondria permeabilization occurring during cell apoptosis. We have developed an in vivo model of apoptotic rat liver. Mitochondria appeared as an homogenous population in control liver. On the contrary, mitochondria varied in size, morphology, and the matrical density in apoptotic liver. Mitochondria were purified from control and apoptotic livers. In control conditions, a single mitochondrial population was identified; whereas three populations of mitochondria were purified from apoptotic liver. Our data show that these apoptotic populations correspond to early, intermediate, and late apoptotic mitochondria, which are characterized by an increasing extent of permeabilization of their outer membrane and a gradual enrichment in oligomerized Bax protein. Remarkably, a new ionic channel was observed in apoptotic but not in control mitochondria. The biophysical and pharmacological properties of this channel are in good agreement with those reported for a previously described mitochondrial apoptosis-induced channel (MAC) (Pavlov, E. V., Priault, M., Pietkiewicz, D., Cheng, E. H., Antonsson, B., Manon, S., Korsmeyer, S. J., Mannella, C. A., and Kinnally, K. W. (2001) J. Cell Biol. 155, 725-731). However, MAC activity was only observed in the late apoptotic mitochondrial population. Thus, our study establishes that MAC activity is related to the overall apoptotic process but corresponds to a late event.