The interaction of the Bax C-terminal domain with negatively charged lipids modifies the secondary structure and changes its way of insertion into membranes

Curvature effect and stabilize ruptured membrane of BAX derived peptide studied by molecular dynamics simulations

BAX protein plays a key role in mitochondrial membrane permeabilization and cytochrome c release upon apoptosis. The C-terminal transmembrane domain (TMD) of BAX is supposed to act a membrane anchor when BAX is activated leading to programmed cell death. Previous studies indicate that the C-terminal transmembrane domain of BAX mediates membrane disruption and pore formation, however, the mechanism of the membrane disruption and pore-forming capability of BAX C-terminal transmembrane domain still unclear. Here, we performed all-atom (AA) molecular dynamics simulations to study the membrane effect of TMD peptide. We also conducted coarse-grained (CG) molecular dynamics simulations to study the membrane curvature and the stabilization of ruptured membrane pores effect of TMD peptides. Our results indicated that TMD peptide decreases the local POPC lipids order. The membrane binding of TMD induced a positive membrane curvature, moreover, certain numbers of TMD could stabilize ruptured membrane pore in both CG and AA simulations. These results provide insight into the structure details of membrane pore formation by TMD peptides. The diameters of the pore are qualitatively in good agreement with available experimental data.

Mitochondrial outer membrane permeabilization: a focus on the role of mitochondrial membrane structural organization

Apoptosis is important in regulating cell death turnover and is mediated by the intrinsic and death receptor-based extrinsic pathways which converge at the mitochondrial outer membrane (MOM) leading to mitochondrial outer membrane permeabilization (MOMP). MOMP results in the release of apoptotic proteins that further activate the downstream pathway of apoptosis. Thus, tight regulation of MOMP is crucial in controlling apoptosis, and a lack of control may lead to tissue and organ malformation and the development of cancers. Despite a growing number of studies focusing on the structure and activity of the proteins involved in mediating MOMP, such as the Bcl-2 family proteins, the mechanism of MOMP is not well understood. In particular, the crucial role of the various structural properties and changes in lipid components of the MOM in mediating the recruitment and activation of different Bcl-2 proteins remains poorly understood. Furthermore, the factors that control the changes in mitochondrial membrane integrity from the initiation to the final disruption of MOM have yet to be clearly defined. In this review, we provide an overview of studies that focus on the mitochondrial membrane with a biophysical analysis of the interactions of the Bcl-2 proteins with the mitochondrial membrane.

JNK3 phosphorylates Bax protein and induces ability to form pore on bilayer lipid membrane

Bax is a pro-apoptotic cytosolic protein. In this work native (unphosphorylated) and JNK3 phosphorylated Bax proteins are studied on artificial bilayer membranes for pore formation. Phosphorylated Bax formed pore on the bilayer lipid membrane whereas native one does not. In cells undergoing apoptosis the pore formed by the phosphorylated Bax could be important in cytochrome c release from the mitochondrial intermembrane space to the cytosol. The low conductance (1.5 nS) of the open state of the phosphorylated Bax pore corresponds to pore diameter of 0.9 nm which is small to release cytochrome c (∼3.4 nm). We hypothesized that JNK3 phosphorylated Bax protein can form bigger pores after forming complexes with other mitochondrial proteins like VDAC, t-Bid etc. to release cytochrome c.

The importance of evolutionarily conserved C-terminal basic residues for the stability of proapoptotic Bax protein

Bax is a protein that promotes apoptosis (a form of cell death). The atomistic details of the mechanism by which Bax is activated during apoptosis remain a subject of debate. C‐terminal basic residues in the sequence of Bax show remarkable conservation across a variety of species. The role of these charged residues in the stability of Bax was investigated by submitting substituted mutants to molecular dynamics simulations at high temperatures. Mutation of either or both K189 and K190 led to dramatic changes in helical content, radius of gyration, proximity of the C terminus to the core of the protein, exposure of the BH3 domain, and bundling of the core. These results suggest a critical role of positively charged residues close to the C terminus in the structural stability of Bax.

Distinct lipid effects on tBid and Bim activation of membrane permeabilization by pro-apoptotic Bax

After exposure to stressful stimuli, apoptotic signals can be relayed to mitochondria by pro-apoptotic activator proteins, tBid (truncated Bid/p15) and Bim (Bcl-2 interacting mediator), which activate Bax (Bcl-2 associated X protein) and or Bak (Bcl-2 antagonist/killer) to induce mitochondrial outer membrane (MOM) permeabilization (MOMP). These protein–protein and protein–membrane interactions are critical for apoptosis regulation, since MOMP irreversibly leads to cell death. Whereas the distinct roles of tBid and Bim as sensors of different types of stress are well recognized, it is not known whether the molecular mechanisms whereby they initiate MOMP are the same. In the present study, we compare membrane permeabilization by Bax activated by either cBid [cleaved Bid (p7 and p15)] or Bim and we examine the role of membrane lipids in the recruitment and activation of these three Bcl-2 (B-cell lymphoma 2) pro-apoptotic proteins. We employ fluorescently-labelled proteins and liposomes to quantify the effects of specific lipids on each of the well-characterized steps in Bax-mediated membrane permeabilization. We show that high levels of cholesterol in the membrane inhibit permeabilization by categorically identifying the recruitment of Bax by the activators and Bax insertion in the membrane as the steps being hindered by cholesterol. Furthermore, we show that binding of both cBid and Bim to membranes is facilitated by electrostatic interactions with anionic phospholipids. However, whereas Bim does not require any particular anionic lipids, the conformational change in tBid depends on cardiolipin (CL). This suggests that CL can activate tBid in a similar manner to Mtch2 (mitochondrial carrier homologue 2). Thus, lipids modify multiple aspects of Bax-mediated membrane permeabilization.

Bak apoptotic pores involve a flexible C-terminal region and juxtaposition of the C-terminal transmembrane domains

Bak and Bax mediate apoptotic cell death by oligomerizing and forming a pore in the mitochondrial outer membrane. Both proteins anchor to the outer membrane via a C-terminal transmembrane domain, although its topology within the apoptotic pore is not known. Cysteine-scanning mutagenesis and hydrophilic labeling confirmed that in healthy mitochondria the Bak α9 segment traverses the outer membrane, with 11 central residues shielded from labeling. After pore formation those residues remained shielded, indicating that α9 does not line a pore. Bak (and Bax) activation allowed linkage of α9 to neighboring α9 segments, identifying an α9:α9 interface in Bak (and Bax) oligomers. Although the linkage pattern along α9 indicated a preferred packing surface, there was no evidence of a dimerization motif. Rather, the interface was invoked in part by Bak conformation change and in part by BH3:groove dimerization. The α9:α9 interaction may constitute a secondary interface in Bak oligomers, as it could link BH3:groove dimers to high-order oligomers. Moreover, as high-order oligomers were generated when α9:α9 linkage in the membrane was combined with α6:α6 linkage on the membrane surface, the α6-α9 region in oligomerized Bak is flexible. These findings provide the first view of Bak carboxy terminus (C terminus) membrane topology within the apoptotic pore.

Functions of the C-terminal domains of apoptosis-related proteins of the Bcl-2 family

Bcl-2 family proteins are involved in cell homeostasis, where they regulate cell death. Some of these proteins are pro-apoptotic and others pro-survival. Moreover, many of them share a similar domain composition with several of the so-called BH domains, although some only have a BH3 domain. A C-terminal domain is present in all the multi-BH domain proteins and in some of the BH3-only ones. This C-terminal domain is hydrophobic or amphipathic, for which reason it was thought when they were discovered that they were membrane anchors. Although this is indeed one of their functions, it has since been observed that they may also serve as regulators of the function of some members of this family, such as Bax. They may also serve to recognize the target membrane of some of these proteins, which only after an apoptotic signal, are incorporated into a membrane. It has been shown that peptides that imitate the sequence of C-terminal domains can form pores and may serve as a model to design cytotoxic molecules.

Molecular basis for membrane pore formation by Bax protein carboxyl terminus

Bax protein plays a key role in mitochondrial membrane permeabilization and cytochrome c release upon apoptosis. Our recent data have indicated that the 20-residue C-terminal peptide of Bax (BaxC-KK; VTIFVAGVLTASLTIWKKMG), when expressed intracellularly, translocates to the mitochondria and exerts lethal effect on cancer cells. Moreover, the BaxC-KK peptide, as well as two mutants where the two lysines are replaced with glutamate (BaxC-EE) or leucine (BaxC-LL), have been shown to form relatively large pores in lipid membranes, composed of up to eight peptide molecules per pore. Here the pore structure is analyzed by polarized Fourier transform infrared, circular dichroism, and fluorescence experiments on the peptides reconstituted in phospholipid membranes. The peptides assume an α/β-type secondary structure within membranes. Both β-strands and α-helices are significantly (by 30-60 deg) tilted relative to the membrane normal. The tryptophan residue embeds into zwitterionic membranes at 8-9 Å from the membrane center. The membrane anionic charge causes a deeper insertion of tryptophan for BaxC-KK and BaxC-LL but not for BaxC-EE. Combined with the pore stoichiometry determined earlier, these structural constraints allow construction of a model of the pore where eight peptide molecules form an "α/β-ring" structure within the membrane. These results identify a strong membranotropic activity of Bax C-terminus and propose a new mechanism by which peptides can efficiently perforate cell membranes. Knowledge on the pore forming mechanism of the peptide may facilitate development of peptide-based therapies to kill cancer or other detrimental cells such as bacteria or fungi.

Translocation of a Bak C-terminus mutant from cytosol to mitochondria to mediate cytochrome C release: implications for Bak and Bax apoptotic function

BACKGROUND

One of two proapoptotic Bcl-2 proteins, Bak or Bax, is required to permeabilize the mitochondrial outer membrane during apoptosis. While Bax is mostly cytosolic and translocates to mitochondria following an apoptotic stimulus, Bak is constitutively integrated within the outer membrane. Membrane anchorage occurs via a C-terminal transmembrane domain that has been studied in Bax but not in Bak, therefore what governs their distinct subcellular distribution is uncertain. In addition, whether the distinct subcellular distributions of Bak and Bax contributes to their differential regulation during apoptosis remains unclear.

METHODOLOGY/PRINCIPAL FINDINGS

To gain insight into Bak and Bax targeting to mitochondria, elements of the Bak C-terminus were mutated, or swapped with those of Bax. Truncation of the C-terminal six residues (C-segment) or substitution of three basic residues within the C-segment destabilized Bak. Replacing the Bak C-segment with that from Bax rescued stability and function, but unexpectedly resulted in a semi-cytosolic protein, termed Bak/BaxCS. When in the cytosol, both Bax and Bak/BaxCS sequestered their hydrophobic transmembrane domains in their hydrophobic surface groove. Upon apoptotic signalling, Bak/BaxCS translocated to the mitochondrial outer membrane, inserted its transmembrane domain, oligomerized, and released cytochrome c. Despite this Bax-like subcellular distribution, Bak/BaxCS retained Bak-like regulation following targeting of Mcl-1.

CONCLUSIONS/SIGNIFICANCE

Residues in the C-segment of Bak and of Bax contribute to their distinct subcellular localizations. That a semi-cytosolic form of Bak, Bak/BaxCS, could translocate to mitochondria and release cytochrome c indicates that Bak and Bax share a conserved mode of activation. In addition, the differential regulation of Bak and Bax by Mcl-1 is predominantly independent of the initial subcellular localizations of Bak and Bax.

FTIR microspectroscopy discriminates anticancer action on human leukemic cells by extracts of Pinus kesiya; Cratoxylum formosum ssp. pruniflorum and melphalan

Apoptosis is the principal molecular goal of chemotherapeutics for effective anticancer action. We studied the effect of 50% ethanolic-water extracts of Pinus kesiya, Cratoxylum formosum ssp. pruniflorum and melphalan on cytotoxicity and apoptosis induction for human leukemic U937 cells, and explored the mode of action using FTIR microspectroscopy. The number of viable U937 cells in vitro was decreased in a concentration-dependent manner by all tested compounds, although potency differed between the U937 and Vero cells. Melphalan and the extract of C. formosum exhibited relatively lower IC(50) values (15.0 ± 1.0 and 82.7 ± 3.2 μg/mL respectively) and higher selectivity (selective index>3) than the extract of P. kesiya (299.0 ± 5.2 μg/mL; selective index<3) on the U937 cells. All three compounds significantly induced apoptosis through the late stage - seen by the indicative DNA ladder - with the most effective being melphalan, then the P. kesiya and C. formosum extracts. FTIR microspectroscopy revealed that all three compounds raised the intensity of the β-pleated sheet - higher than that of the untreated U937 cells - corresponding to a shift in the α-helix band associated with an alteration in the secondary structure of the protein band, confirming induction of apoptosis via pro-apoptotic proteins. The differences in intensity of the FTIR bands associated with lipids, proteins and nucleic acids were responsible for discrimination of the anticancer mode of action of each of the three compounds. The FTIR data suggest that the two plant extracts possessed anticancer activity with a different mode of action than melphalan.

Bax forms two types of channels, one of which is voltage-gated

When activated, the proapoptotic protein Bax permeabilizes the mitochondrial outer membrane, allowing the release of proteins into the cytosol and thus initiating the execution phase of apoptosis. When activated Bax was reconstituted into phospholipid membranes, we discovered a new, to our knowledge, property of Bax channels: voltage gating. We also found that the same Bax sample under the same experimental conditions could give rise to two radically different channels: Type A, which is small, well behaved, homogeneous, and voltage-gated, and Type B, which is large, noisy, and voltage-independent. One Type B channel can be converted irreversibly into a population of Type A channels by the addition of La(3+). This conversion process appears to involve a two-dimensional budding mechanism. The existence of these two types of Bax channels suggests a process for controlling the degree of mitochondrial outer membrane permeabilization.

Membrane docking of the C2 domain from protein kinase Cα as seen by polarized ATR-IR. The role of PIP₂

We have used attenuated total internal reflection infrared spectroscopy (ATR-IR) spectroscopy to study the association of the C2 domain from protein kinase Cα (PKCα) with different phospholipid membranes, so as to characterise the mode of membrane docking and its modulation by the second-messenger lipid PIP₂. In parallel, we have also examined the membrane interaction of the C2 domain from cytosolic phospholipase A₂. PIP₂ did not induce significant changes in secondary structure of the membrane-bound PKCα-C2 domain, nor did binding of the PKCα-C2 domain change the dichroic ratios of the lipid chains, whereas the C2 domain from phospholipase A₂ did perturb the lipid chain orientation. Measurements of the dichroic ratios for the amide I and amide II protein bands were combined so as to distinguish the tilt of the β-sheets from that of the β-strands within the sheet. When associated with POPC/POPS membranes, the β-sandwich of the PKCα-C2 domain is inclined at an angle α=35° to the membrane normal, i.e., is oriented more nearly perpendicular than parallel to the membrane. In the process of membrane docking, the tilt angle increases to α=44° in the presence of PIP₂, indicating that the β-sandwich comes closer to the membrane surface, so confirming the importance of this lipid in determining docking of the C2 domain and consequent activation of PKCα.