Effect of lipids with different spontaneous curvature on the channel activity of colicin E1: evidence in favor of a toroidal pore

Membrane Proteins and Membrane Curvature: Mutual Interactions and a Perspective on Disease Treatments

The pathogenesis of various diseases often involves an intricate interplay between membrane proteins and membrane curvature. Understanding the underlying mechanisms of this interaction could offer novel perspectives on disease treatment. In this review, we provide an introduction to membrane curvature and its association with membrane proteins. Furthermore, we delve into the impact and potential implications of this interaction in the context of disease treatment. Lastly, we discuss the prospects and challenges associated with harnessing these interactions for effective disease management, aiming to provide fresh insights into therapeutic strategies.

Perturbation of the host cell Ca2+ homeostasis and ER-mitochondria contact sites by the SARS-CoV-2 structural proteins E and M

Coronavirus disease (COVID-19) is a contagious respiratory disease caused by the SARS-CoV-2 virus. The clinical phenotypes are variable, ranging from spontaneous recovery to serious illness and death. On March 2020, a global COVID-19 pandemic was declared by the World Health Organization (WHO). As of February 2023, almost 670 million cases and 6,8 million deaths have been confirmed worldwide. Coronaviruses, including SARS-CoV-2, contain a single-stranded RNA genome enclosed in a viral capsid consisting of four structural proteins: the nucleocapsid (N) protein, in the ribonucleoprotein core, the spike (S) protein, the envelope (E) protein, and the membrane (M) protein, embedded in the surface envelope. In particular, the E protein is a poorly characterized viroporin with high identity amongst all the β-coronaviruses (SARS-CoV-2, SARS-CoV, MERS-CoV, HCoV-OC43) and a low mutation rate. Here, we focused our attention on the study of SARS-CoV-2 E and M proteins, and we found a general perturbation of the host cell calcium (Ca2+) homeostasis and a selective rearrangement of the interorganelle contact sites. In vitro and in vivo biochemical analyses revealed that the binding of specific nanobodies to soluble regions of SARS-CoV-2 E protein reversed the observed phenotypes, suggesting that the E protein might be an important therapeutic candidate not only for vaccine development, but also for the clinical management of COVID designing drug regimens that, so far, are very limited.

Analysis of HIV-1 envelope cytoplasmic tail effects on viral replication

Trimers of the HIV-1 envelope (Env) protein perform receptor binding and virus-cell fusion functions during the virus life cycle. The cytoplasmic tail (CT) of Env forms an unusual baseplate structure, and is palmitoylated, rich in arginines, carries trafficking motifs, binds cholesterol, and interacts with host proteins. To dissect CT activities, we examined a panel of Env variants, including CT truncations, mutations, and an extension. We found that whereas all variants could replicate in permissive cells, viruses with CT truncations or baseplate mutations were defective in restrictive cells. We also identified a determinant in HIV-1 amphotericin sensitivity, and characterized variants that escape amphotericin inhibition via viral protease-mediated CT cleavage. Results additionally showed that full-length, his tagged Env can oligomerize and be co-assembled with CT truncations that delete portions of the baseplate, host protein binding sites, and trafficking signals. Our observations illuminate novel aspects of HIV-1 CT structure, interactions, and functions.

Lipid Microenvironment Modulates the Pore-Forming Ability of Polymyxin B

The ability of polymyxin B, an antibiotic used to treat infections caused by multidrug-resistant Gram-negative bacteria as a last-line therapeutic option, to form ion pores in model membranes composed of various phospholipids and lipopolysaccharides was studied. Our data demonstrate that polymyxin B predominantly interacts with negatively charged lipids. Susceptibility decreases as follows: Kdo2-Lipid A >> DOPG ≈ DOPS >> DPhPG ≈ TOCL ≈ Lipid A. The dimer and hexamer of polymyxin B are involved in the pore formation in DOPG(DOPS)- and Kdo2-Lipid A-enriched bilayers, respectively. The pore-forming ability of polymyxin B significantly depends on the shape of membrane lipids, which indicates that the antibiotic produces toroidal lipopeptide-lipid pores. Small amphiphilic molecules diminishing the membrane dipole potential and inducing positive curvature stress were shown to be agonists of pore formation by polymyxin B and might be used to develop innovative lipopeptide-based formulations.

Chromone-Containing Allylmorpholines Influence Ion Channels in Lipid Membranes via Dipole Potential and Packing Stress

Herein, we report that chromone-containing allylmorpholines can affect ion channels formed by pore-forming antibiotics in model lipid membranes, which correlates with their ability to influence membrane boundary potential and lipid-packing stress. At 100 µg/mL, allylmorpholines 1, 6, 7, and 8 decrease the boundary potential of the bilayers composed of palmitoyloleoylphosphocholine (POPC) by about 100 mV. At the same time, the compounds do not affect the zeta-potential of POPC liposomes, but reduce the membrane dipole potential by 80-120 mV. The allylmorpholine-induced drop in the dipole potential produce 10-30% enhancement in the conductance of gramicidin A channels. Chromone-containing allylmorpholines also affect the thermotropic behavior of dipalmytoylphosphocholine (DPPC), abolishing the pretransition, lowering melting cooperativity, and turning the main phase transition peak into a multicomponent profile. Compounds 4, 6, 7, and 8 are able to decrease DPPC's melting temperature by about 0.5-1.9 °C. Moreover, derivative 7 is shown to increase the temperature of transition of palmitoyloleoylphosphoethanolamine from lamellar to inverted hexagonal phase. The effects on lipid-phase transitions are attributed to the changes in the spontaneous curvature stress. Alterations in lipid packing induced by allylmorpholines are believed to potentiate the pore-forming ability of amphotericin B and gramicidin A by several times.

Molecular packing of lipid membranes and action mechanisms of membrane-active peptides

Biomembranes are involved in diverse cellular activities. How membranes and proteins interact in the activities might hinge on the former's physical characteristics, which in turn are influenced by packing of lipid molecules. Yet, the validity of this understanding and its mechanism are unclear. By varying chain saturation of membranes, we explored correlations between lipid packing and peptide-mediated membrane disruption for the antimicrobial peptide, melittin, and amyloidogenic peptide, β-amyloid (1-42). Remarkably, reducing molecular packing flexibility enhanced the membrane disruption, possibly due to a shift from membrane perforation to micellization. A theoretical analysis suggested the energetic basis of this shift. This mechanistically shows that a peptide's mechanism might be dictated not only by its intrinsic properties but also by physical characteristics of membranes.

H1 helix of colicin U causes phospholipid membrane permeation

In light of an increasing number of antibiotic-resistant bacterial strains, it is essential to understand an action imposed by various antimicrobial agents on bacteria at the molecular level. One of the leading mechanisms of killing bacteria is related to the alteration of their plasmatic membrane. We study bio-inspired peptides originating from natural antimicrobial proteins colicins, which can disrupt membranes of bacterial cells. Namely, we focus on the α-helix H1 of colicin U, produced by bacterium Shigella boydii, and compare it with analogous peptides derived from two different colicins. To address the behavior of the peptides in biological membranes, we employ a combination of molecular simulations and experiments. We use molecular dynamics simulations to show that all three peptides are stable in model zwitterionic and negatively charged phospholipid membranes. At the molecular level, their embedment leads to the formation of membrane defects, membrane permeation for water, and, for negatively charged lipids, membrane poration. These effects are caused by the presence of polar moieties in the considered peptides. Importantly, simulations demonstrate that even monomeric H1 peptides can form toroidal pores. At the macroscopic level, we employ experimental co-sedimentation and fluorescence leakage assays. We show that the H1 peptide of colicin U incorporates into phospholipid vesicles and disrupts their membranes, causing leakage, in agreement with the molecular simulations. These insights obtained for model systems seem important for understanding the mechanisms of antimicrobial action of natural bacteriocins and for future exploration of small bio-inspired peptides able to disrupt bacterial membranes.

Characterization of the SARS-CoV-2 E Protein: Sequence, Structure, Viroporin, and Inhibitors

The COVID-19 epidemic is one of the most influential epidemics in history. Understanding the impact of coronaviruses (CoVs) on host cells is very important for disease treatment. The SARS-CoV-2 envelope (E) protein is a small structural protein involved in many aspects of the viral life cycle. The E protein promotes the packaging and reproduction of the virus, and deletion of this protein weakens or even abolishes the virulence. This review aims to establish new knowledge by combining recent advances in the study of the SARS-CoV-2 E protein and by comparing it with the SARS-CoV E protein. The E protein amino acid sequence, structure, self-assembly characteristics, viroporin mechanisms and inhibitors are summarized and analyzed herein. Although the mechanisms of the SARS-CoV-2 and SARS-CoV E proteins are similar in many respects, specific studies on the SARS-CoV-2 E protein, for both monomers and oligomers, are still lacking. A comprehensive understanding of this protein should prompt further studies on the design and characterization of effective targeted therapeutic measures.

Pore-forming proteins: From defense factors to endogenous executors of cell death

Pore-forming proteins (PFPs) and small antimicrobial peptides (AMPs) represent a large family of molecules with the common ability to punch holes in cell membranes to alter their permeability. They play a fundamental role as infectious bacteria's defensive tools against host's immune system and as executors of endogenous machineries of regulated cell death in eukaryotic cells. Despite being highly divergent in primary sequence and 3D structure, specific folds of pore-forming domains have been conserved. In fact, pore formation is considered an ancient mechanism that takes place through a general multistep process involving: membrane partitioning and insertion, oligomerization and pore formation. However, different PFPs and AMPs assemble and form pores following different mechanisms that could end up either in the formation of protein-lined or protein-lipid pores. In this review, we analyze the current findings in the mechanism of action of different PFPs and AMPs that support a wide role of membrane pore formation in nature. We also provide the newest insights into the development of state-of-art techniques that have facilitated the characterization of membrane pores. To understand the physiological role of these peptides/proteins or develop clinical applications, it is essential to uncover the molecular mechanism of how they perforate membranes.

Cation channel conductance and pH gating of the innate immunity factor APOL1 are governed by pore-lining residues within the C-terminal domain

The human innate immunity factor apolipoprotein L-I (APOL1) protects against infection by several protozoan parasites, including Trypanosoma brucei brucei Endocytosis and acidification of high-density lipoprotein-associated APOL1 in trypanosome endosomes leads to eventual lysis of the parasite due to increased plasma membrane cation permeability, followed by colloid-osmotic swelling. It was previously shown that recombinant APOL1 inserts into planar lipid bilayers at acidic pH to form pH-gated nonselective cation channels that are opened upon pH neutralization. This corresponds to the pH changes encountered during endocytic recycling, suggesting APOL1 forms a cytotoxic cation channel in the parasite plasma membrane. Currently, the mechanism and domains required for channel formation have yet to be elucidated, although a predicted helix-loop-helix (H-L-H) was suggested to form pores by virtue of its similarity to bacterial pore-forming colicins. Here, we compare recombinant human and baboon APOL1 orthologs, along with interspecies chimeras and individual amino acid substitutions, to identify regions required for channel formation and pH gating in planar lipid bilayers. We found that whereas neutralization of glutamates within the H-L-H may be important for pH-dependent channel formation, there was no evidence of H-L-H involvement in either pH gating or ion selectivity. In contrast, we found two residues in the C-terminal domain, tyrosine 351 and glutamate 355, that influence pH gating properties, as well as a single residue, aspartate 348, that determines both cation selectivity and pH gating. These data point to the predicted transmembrane region closest to the APOL1 C terminus as the pore-lining segment of this novel channel-forming protein.

Interplay Between Plasma Membrane Lipid Alteration, Oxidative Stress and Calcium-Based Mechanism for Extracellular Vesicle Biogenesis From Erythrocytes During Blood Storage

The shedding of extracellular vesicles (EVs) from the red blood cell (RBC) surface is observed during senescence in vivo and RBC storage in vitro. Two main models for EV shedding, respectively based on calcium rise and oxidative stress, have been proposed in the literature but the role of the plasma membrane lipid composition and properties is not understood. Using blood in K⁺/EDTA tubes stored for up to 4 weeks at 4°C as a relevant RBC vesiculation model, we showed here that the RBC plasma membrane lipid composition, organization in domains and biophysical properties were progressively modified during storage and contributed to the RBC vesiculation. First, the membrane content in cholesterol and linoleic acid decreased whereas lipid peroxidation and spectrin:membrane occupancy increased, all compatible with higher membrane rigidity. Second, phosphatidylserine surface exposure showed a first rapid rise due to membrane cholesterol decrease, followed by a second calcium-dependent increase. Third, lipid domains mainly enriched in GM1 or sphingomyelin strongly increased from the 1st week while those mainly enriched in cholesterol or ceramide decreased during the 1st and 4th week, respectively. Fourth, the plasmatic acid sphingomyelinase activity considerably increased upon storage following the sphingomyelin-enriched domain rise and potentially inducing the loss of ceramide-enriched domains. Fifth, in support of the shedding of cholesterol- and ceramide-enriched domains from the RBC surface, the number of cholesterol-enriched domains lost and the abundance of EVs released during the 1st week perfectly matched. Moreover, RBC-derived EVs were enriched in ceramide at the 4th week but depleted in sphingomyelin. Then, using K⁺/EDTA tubes supplemented with glucose to longer preserve the ATP content, we better defined the sequence of events. Altogether, we showed that EV shedding from lipid domains only represents part of the global vesiculation mechanistics, for which we propose four successive events (cholesterol domain decrease, oxidative stress, sphingomyelin/sphingomyelinase/ceramide/calcium alteration and phosphatidylserine exposure).

Inhibition of Pore-Forming Proteins

Perforation of cellular membranes by pore-forming proteins can affect cell physiology, tissue integrity, or immune response. Since many pore-forming proteins are toxins or highly potent virulence factors, they represent an attractive target for the development of molecules that neutralize their actions with high efficacy. There has been an assortment of inhibitors developed to specifically obstruct the activity of pore-forming proteins, in addition to vaccination and antibiotics that serve as a plausible treatment for the majority of diseases caused by bacterial infections. Here we review a wide range of potential inhibitors that can specifically and effectively block the activity of pore-forming proteins, from small molecules to more specific macromolecular systems, such as synthetic nanoparticles, antibodies, antibody mimetics, polyvalent inhibitors, and dominant negative mutants. We discuss their mechanism of inhibition, as well as advantages and disadvantages.

Controlling Secretion in Artificial Cells with a Membrane AND Gate

The assembly of channel proteins into vesicle membranes is a useful strategy to control activities of vesicle-based systems. Here, we developed a membrane AND gate that responds to both a fatty acid and a pore-forming channel protein to induce the release of encapsulated cargo. We explored how membrane composition affects the functional assembly of α-hemolysin into phospholipid vesicles as a function of oleic acid content and α-hemolysin concentration. We then showed that we could induce α-hemolysin assembly when we added oleic acid micelles to a specific composition of phospholipid vesicles. Finally, we demonstrated that our membrane AND gate could be coupled to a gene expression system. Our study provides a new method to control the temporal dynamics of vesicle permeability by controlling when the functional assembly of a channel protein into synthetic vesicles occurs. Furthermore, a membrane AND gate that utilizes membrane-associating biomolecules introduces a new way to implement Boolean logic that should complement genetic logic circuits and ultimately enhance the capabilities of artificial cellular systems.

Insights into the antibacterial mechanism of PEGylated nano-bacitracin A against Streptococcus pneumonia: both penicillin-sensitive and penicillin-resistant strains

Background

Multidrug-resistant (MDR) Streptococcus pneumonia constitute a major worldwide public health concern.

Materials and methods

In our preliminary study, PEGylated nano-self-assemblies of bacitracin A (PEGylated Nano-BA_12K) showed strong antibacterial potency against reference S. pneumonia strain (ATCC 49619). In this study, the possibility of applying PEGylated Nano-BA_12K against penicillin-resistant S. pneumonia was further investigated. In addition, the underlying antibacterial mechanism of PEGylated Nano-BA_12K against both sensitive and resistant S. pneumonia was also clarified systematically, since S. pneumonia was naturally resistant to its unassembled counterpart bacitracin A (BA).

Results

PEGylated Nano-BA_12K showed strong antibacterial potency against 13 clinical isolates of S. pneumonia, including five penicillin-resistant strains. Structural changes, partial collapse, and even lysis of both penicillin-sensitive and penicillin-resistant bacteria were observed after incubation with PEGylated Nano-BA_12K via transmission electron microscopy and atomic force microscopy. Thus, the cell wall or/and cell membrane might be the main target of PEGylated Nano-BA_12K against S. pneumonia. PEGylated Nano-BA_12K exhibited limited effect on the permeabilization and peptidoglycan content of cell wall. Surface pressure measurement suggested that PEGylated Nano-BA_12K was much more tensioactive than BA, which was usually translated into a good membranolytic effect, and is helpful to permeabilize the cell membrane and damage membrane integrity, as evidenced by depolarization of the membrane potential, permeabilization of membrane and leakage of calcein from liposomes.

Conclusion

Collectively, great cell membrane permeability and formidable membrane disruption may work together for the strong antibacterial activity of PEGylated Nano-BA_12K against S. pneumonia. Taken together, PEGylated Nano-BA_12K has excellent potential against both penicillin-sensitive and penicillin-resistant S. pneumonia and might be suitable for the treatment of S. pneumonia infectious diseases.

Role of Milk-Derived Antibacterial Peptides in Modern Food Biotechnology: Their Synthesis, Applications and Future Perspectives

Milk-derived antibacterial peptides (ABPs) are protein fragments with a positive influence on the functions and conditions of a living organism. Milk-derived ABPs have several useful properties important for human health, comprising a significant antibacterial effect against various pathogens, but contain toxic side-effects. These compounds are mainly produced from milk proteins via fermentation and protein hydrolysis. However, they can also be produced using recombinant DNA techniques or organic synthesis. This review describes the role of milk-derived ABPs in modern food biotechnology with an emphasis on their synthesis and applications. Additionally, we also discuss the mechanisms of action and the main bioproperties of ABPs. Finally, we explore future perspectives for improving ABP physicochemical properties and diminishing their toxic side-effects.

Pluronic-based nano-self-assemblies of bacitracin A with a new mechanism of action for an efficient in vivo therapeutic effect against bacterial peritonitis

Background

Although assemblies of hydrophobic-modified bacitracin A with PLGA (Nano-BA_PLGA) have demonstrated promising antibacterial activities against both Gram-positive and Gram-negative bacteria, the desirable antibacterial potency has remained challenging due to the low solubility of Nano-BA_PLGA. To address this tissue, a series of Pluronic copolymers (Pluronic^® F127, Pluronic^® P123 and Pluronic^® P85) were selected to link the N-terminus of bacitracin A to construct Pluronic-based nano-self assemblies (Nano-BA_F127, Nano-BA_P123 and Nano-BA_P85).

Results

Impressively, all the newly designed Pluronic-based Nano-BAs possessed higher solubility and stronger effectiveness against both Gram-positive and Gram-negative bacteria compared with Nano-BA_PLGA, especially the modification with Pluronic^® P85. Surface tension measurements indicated that Nano-BA_P85 was much more tensioactive than Nano-BA_PLGA, which usually translated into a good membranolytic effect. Fluorescence spectroscopy and electron microscopy analyses confirmed the speculation that the cell wall/membrane might be the main action target of Nano-BA_P85 by permeabilizing the cell membrane and damaging the membrane integrity. In vivo results further demonstrated that Nano-BA_P85 significantly suppressed bacterial growth and prolonged survival time in the bacterial peritonitis mouse model with negligible toxicity.

Conclusions

Collectively, the membrane targeting mechanism of action is entirely distinct from those of clinically used antibacterial agents. Furthermore, the new approach of construction nanoantibiotics based on the modification of commercially available antibiotics with Pluronic copolymers is demonstrated to have an efficient therapeutic effect against bacterial infection.

Electronic supplementary material

The online version of this article (10.1186/s12951-018-0397-3) contains supplementary material, which is available to authorized users.

Effect of ubiquinone-10 on the stability of biomimetic membranes of relevance for the inner mitochondrial membrane

Ubiquinone-10 (Q10) plays a pivotal role as electron-carrier in the mitochondrial respiratory chain, and is also well known for its powerful antioxidant properties. Recent findings suggest moreover that Q10 could have an important membrane stabilizing function. In line with this, we showed in a previous study that Q10 decreases the permeability to carboxyfluorescein (CF) and increases the mechanical strength of 1-palmitoyl-2-oleyl-sn-glycero-phosphocholine (POPC) membranes. In the current study we report on the effects exerted by Q10 in membranes having a more complex lipid composition designed to mimic that of the inner mitochondrial membrane (IMM). Results from DPH fluorescence anisotropy and permeability measurements, as well as investigations probing the interaction of liposomes with silica surfaces, corroborate a membrane stabilizing effect of Q10 also in the IMM-mimicking membranes. Comparative investigations examining the effect of Q10 and the polyisoprenoid alcohol solanesol on the IMM model and on membranes composed of individual IMM components suggest, moreover, that Q10 improves the membrane barrier properties via different mechanisms depending on the lipid composition of the membrane. Thus, whereas Q10's inhibitory effect on CF release from pure POPC membranes appears to be directly and solely related to Q10's lipid ordering and condensing effect, a mechanism linked to Q10's ability to amplify intrinsic curvature elastic stress dominates in case of membranes containing high proportions of palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE).

Coupling of lipid membrane elasticity and in-plane dynamics

Biomembranes exhibit liquid and solid features concomitantly with their in-plane fluidity and elasticity tightly regulated by cells. Here, we present experimental evidence supporting the existence of the dynamics-elasticity correlations for lipid membranes and propose a mechanism involving molecular packing densities to explain them. This paper thereby unifies, at the molecular level, the aspects of the continuum mechanics long used to model the two membrane features. This ultimately may elucidate the universal physical principles governing the cellular phenomena involving biomembranes.

Transmembrane oligomeric intermediates of pore forming toxin Cytolysin A determine leakage kinetics

Leakage kinetics of Cytolysin A, an α pore forming toxin, occurs through stochastic insertion of oligomeric intermediates or ‘arcs’.

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.

Extending the Hydrophobic Mismatch Concept to Amphiphilic Membranolytic Peptides

A series of nine amphiphilic, pore-forming α-helical KIA peptides (KIAGKIA repeats) with lengths between 14 and 28 residues were studied by solid-state (15)N NMR to determine their alignment in oriented lipid bilayers. In a 2:1 mixture of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) with its corresponding 1-myristoyl-2-hydroxy-sn-glycero-3-phosphocholine (lyso-MPC), which has a highly positive spontaneous curvature, the helix tilt angle was found to vary steadily with peptide length. The shortest peptide was aligned transmembrane and upright, while the longer ones successively became tilted away from the membrane normal. This behavior is in agreement with the hydrophobic matching concept, conceived so far only for hydrophobic helices. In 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine, with a negative spontaneous curvature, all KIA peptides remained flat on the bilayer surface, while the cylindrical DMPC lipids permitted a slight tilt. Peptide insertion thus depends critically on the intrinsic lipid curvature, and helix orientation is then fine-tuned by membrane thickness. A refined toroidal pore model is proposed.

Single channel activity of OmpF-like porin from Yersinia pseudotuberculosis

To gain a mechanistic insight in the functioning of the OmpF-like porin from Yersinia pseudotuberculosis (YOmpF), we compared the effect of pH variation on the ion channel activity of the protein in planar lipid bilayers and its binding to lipid membranes. The behavior of YOmpF channels upon acidification was similar to that previously described for Escherichia coli OmpF. In particular, a decrease in pH of the bathing solution resulted in a substantial reduction of YOmpF single channel conductance, accompanied by the emergence of subconductance states. Similar subconductance substates were elicited by the addition of lysophosphatidylcholine. This observation, made with porin channels for the first time, pointed to the relevance of lipid-protein interactions, in particular, the lipid curvature stress, to the appearance of subconductance states at acidic pH. Binding of YOmpF to membranes displayed rather modest dependence on pH, whereas the channel-forming potency of the protein tremendously decreased upon acidification.

Fullerenol C60(OH)24 increases ion permeability of lipid membranes in a pH-dependent manner

Fullerenols are water-soluble analogs of fullerene exhibiting both antioxidant and prooxidant activities in vitro and in vivo. Here we report, for the first time, that fullerenol C60(OH)24 can induce ion permeability of a planar lipid bilayer membrane via the formation of ion pores or conductive defects with a preference for cations over anions. The fullerenol-mediated electrical current displayed non-linear concentration dependence and was reversibly enhanced by alkalinization. Calcium and magnesium ions decreased the fullerenol-induced potassium ion permeability. Voltage dependence of the current was sensitive to membrane composition, with the conductance being well pronounced in fully saturated diphytanoylphosphatidylcholine. Fullerenol did not induce carboxyfluorescein leakage from liposomes, suggesting a small size of fullerenol-induced pores. In contrast to ion permeability, the binding of C60(OH)24 to liposomes increased at acidic pH, as measured by fluorescence quenching of pyrene-labeled lipid. In line with this, the photodynamic action of fullerenol on the peptide gramicidin A also increased at low pH. It is hypothesized that aggregates of fullerenol may stabilize transient conductive lipid defects or pores formed under a variety of stress conditions.

How Lipid Membranes Affect Pore Forming Toxin Activity

Pore forming toxins (PFTs) evolved to permeate the plasma membrane of target cells. This is achieved in a multistep mechanism that usually involves binding of soluble protein monomer to the lipid membrane, oligomerization at the plane of the membrane, and insertion of part of the polypeptide chain across the lipid membrane to form a conductive channel. Introduced pores allow uncontrolled transport of solutes across the membrane, inflicting damage to the target cell. PFTs are usually studied from the perspective of structure-function relationships, often neglecting the important role of the bulk membrane properties on the PFT mechanism of action. In this Account, we discuss how membrane lateral heterogeneity, thickness, and fluidity influence the pore forming process of PFTs. In general, lipid molecules are more accessible for binding in fluid membranes due to steric reasons. When PFT specifically binds ordered domains, it usually recognizes a specific lipid distribution pattern, like sphingomyelin (SM) clusters or SM/cholesterol complexes, and not individual lipid species. Lipid domains were also suggested to act as an additional concentration platform facilitating PFT oligomerization, but this is yet to be shown. The last stage in PFT action is the insertion of the transmembrane segment across the membranes to build the transmembrane pore walls. Conformational changes are a spontaneous process, and sufficient free energy has to be available for efficient membrane penetration. Therefore, fluid bilayers are permeabilized more readily in comparison to highly ordered and thicker liquid ordered lipid phase (Lo). Energetically more costly insertion into the Lo phase can be driven by the hydrophobic mismatch between the thinner liquid disordered phase (Ld) and large protein complexes, which are unable to tilt like single transmembrane segments. In the case of proteolipid pores, membrane properties can directly modulate pore size, stability, and even selectivity. Finally, events associated with pore formation can modulate properties of the lipid membrane and affect its organization. Model membranes do not necessarily reproduce the physicochemical properties of the native cellular membrane, and caution is needed when transferring results from model to native lipid membranes. In this context, the utilization of novel approaches that enable studying PFTs on living cells at a single molecule level should reveal complex protein-lipid membrane interactions in greater detail.

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.

Assembling the puzzle: Oligomerization of α-pore forming proteins in membranes

Pore forming proteins (PFPs) share the ability of creating pores that allow the passage of ions, proteins or other constituents through a wide variety of target membranes, ranging from bacteria to humans. They often cause cell death, as pore formation disrupts the membrane permeability barrier required for maintaining cell homeostasis. The organization into supramolecular complexes or oligomers that pierce the membrane is a common feature of PFPs. However, the molecular pathway of self-assembly and pore opening remains unclear. Here, we review the most recent discoveries in the mechanism of membrane oligomerization and pore formation of a subset of PFPs, the α-PFPs, whose pore-forming domains are formed by helical segments. Only now we are starting to grasp the molecular details of their function, mainly thanks to the introduction of single molecule microscopy and nanoscopy techniques. This article is part of a Special Issue entitled: Pore-forming toxins edited by Mauro Dalla Serra and Franco Gambale.

Relevance of Viroporin Ion Channel Activity on Viral Replication and Pathogenesis

Modification of host-cell ionic content is a significant issue for viruses, as several viral proteins displaying ion channel activity, named viroporins, have been identified. Viroporins interact with different cellular membranes and self-assemble forming ion conductive pores. In general, these channels display mild ion selectivity, and, eventually, membrane lipids play key structural and functional roles in the pore. Viroporins stimulate virus production through different mechanisms, and ion channel conductivity has been proved particularly relevant in several cases. Key stages of the viral cycle such as virus uncoating, transport and maturation are ion-influenced processes in many viral species. Besides boosting virus propagation, viroporins have also been associated with pathogenesis. Linking pathogenesis either to the ion conductivity or to other functions of viroporins has been elusive for a long time. This article summarizes novel pathways leading to disease stimulated by viroporin ion conduction, such as inflammasome driven immunopathology.

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.

Modifiers of membrane dipole potentials as tools for investigating ion channel formation and functioning

Electrostatic fields generated on and within biological membranes play a fundamental role in key processes in cell functions. The role of the membrane dipole potential is of particular interest because of its powerful impact on membrane permeability and lipid-protein interactions, including protein insertion, oligomerization, and function. The membrane dipole potential is defined by the orientation of electric dipoles of lipid headgroups, fatty acid carbonyl groups, and membrane-adsorbed water. As a result, the membrane interior is several hundred millivolts more positive than the external aqueous phase. This potential decrease depends on the lipid, and especially sterol, composition of the membrane. The adsorption of certain electroneutral molecules known as dipole modifiers may also lead to significant changes in the magnitude of the potential decrease. These agents are widely used to study the effects of the dipole potential on membrane transport. This review presents a critical analysis of a variety of data from studies dedicated to ion channel formation and functioning in membranes with different dipole potentials. The types of ion channels found in cellular membranes and pores formed by antimicrobial agents and toxins in artificial lipid membranes are summarized. The mechanisms underlying the influence of the membrane dipole potential on ion channel activity, including dipole-dipole and charge-dipole interactions in the pores and in membranes, are discussed. A hypothesis, in which lipid rafts in both model and cellular membranes also modulate ion channel activity by virtue of an increased or decreased dipole potential, is also considered.

Perforin oligomers form arcs in cellular membranes: a locus for intracellular delivery of granzymes

Perforin-mediated cytotoxicity is an essential host defense, in which defects contribute to tumor development and pathogenic disorders including autoimmunity and autoinflammation. How perforin (PFN) facilitates intracellular delivery of pro-apoptotic and inflammatory granzymes across the bilayer of targets remains unresolved. Here we show that cellular susceptibility to granzyme B (GzmB) correlates with rapid PFN-induced phosphatidylserine externalization, suggesting that pores are formed at a protein-lipid interface by incomplete membrane oligomers (or arcs). Supporting a role for these oligomers in protease delivery, an anti-PFN antibody (pf-80) suppresses necrosis but increases phosphatidylserine flip-flop and GzmB-induced apoptosis. As shown by atomic force microscopy on planar bilayers and deep-etch electron microscopy on mammalian cells, pf-80 increases the proportion of arcs which correlates with the presence of smaller electrical conductances, while large cylindrical pores decline. PFN appears to form arc structures on target membranes that serve as minimally disrupting conduits for GzmB translocation. The role of these arcs in PFN-mediated pathology warrants evaluation where they may serve as novel therapeutic targets.

The effects of lipids and surfactants on TLR5-proteoliposome functionality for flagellin detection using surface plasmon resonance biosensing

The use of proteoliposomes as affinity elements in conjunction with a surface plasmon resonance sensor is a high-sensitivity alternative for the detection of multiple analytes. However, one of the most important aspects of these conformations is maintaining the functionality of the immobilized protein, which is determined by the choice of lipids and surfactants employed in the reconstitutions. Previously, we demonstrated the functionality of TLR5-proteoliposomes as screening affinity elements of bacterial flagellin. In this new study we change the conditions of immobilization of TLR5 and evaluate how the fluidity of the membrane and the final size of the liposomes affect the functionality of the construct and thus increase their utility as an affinity element for design of new biosensors. In particular, we used reconstructions into preformed liposomes composed of the lipids POPC, POPC-DMPC and POPC-POPE mediated by the use of surfactants OG, Triton X100, and DDM, respectively. The affinity results were evaluated by SPR technology proteoliposomes and were correlated with the anisotropic change in the membrane status; the final sizes of the proteoliposomes were estimated. Our results clearly show the dependence of fluidity and final size of the proteoliposomes with surface plasmon resonance affinity measurements.

Incomplete pneumolysin oligomers form membrane pores

Pneumolysin is a member of the cholesterol-dependent cytolysin (CDC) family of pore-forming proteins that are produced as water-soluble monomers or dimers, bind to target membranes and oligomerize into large ring-shaped assemblies comprising approximately 40 subunits and approximately 30 nm across. This pre-pore assembly then refolds to punch a large hole in the lipid bilayer. However, in addition to forming large pores, pneumolysin and other CDCs form smaller lesions characterized by low electrical conductance. Owing to the observation of arc-like (rather than full-ring) oligomers by electron microscopy, it has been hypothesized that smaller oligomers explain smaller functional pores. To investigate whether this is the case, we performed cryo-electron tomography of pneumolysin oligomers on model lipid membranes. We then used sub-tomogram classification and averaging to determine representative membrane-bound low-resolution structures and identified pre-pores versus pores by the presence of membrane within the oligomeric curve. We found pre-pore and pore forms of both complete (ring) and incomplete (arc) oligomers and conclude that arc-shaped oligomeric assemblies of pneumolysin can form pores. As the CDCs are evolutionarily related to the membrane attack complex/perforin family of proteins, which also form variably sized pores, our findings are of relevance to that class of proteins as well.

Membrane interactions and cellular effects of MACPF/CDC proteins

The cell membrane is crucial for protection of the cell from its environment. MACPF/CDC proteins are a large superfamily known to be essential for bacterial pathogenesis and proper functioning of the immune system. The three most studied groups of MACPF/CDC proteins are cholesterol-dependent cytolysins from bacteria, the membrane attack complex of complement and human perforin. Their primary function is to form transmembrane pores in target cell membranes. The common mechanism of action comprises water-soluble monomeric proteins binding to the host cell membrane, oligomerization, and formation of a functional pore. This causes a disturbance in gradients of ions and other molecules across the membrane and can lead to cell death. Cells react to this form of attack in a complex manner. Responses can be general, like removing the perforated part of the membrane, or more specific, in many cases depending on binding of proteins to specific receptors to trigger various signalling cascades.

Lipid charge regulation of non-specific biological ion channels

Lipid charge regulation effects in different protein–lipid conformations highlight the role of electrostatic interactions in conductance and selectivity of non-specific biological ion channels.

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.

Three-dimensional structure of Bax-mediated pores in membrane bilayers

B-cell lymphoma 2 (Bcl-2)-associated X protein (Bax) is a member of the Bcl-2 protein family having a pivotal role in triggering cell commitment to apoptosis. Bax is latent and monomeric in the cytosol but transforms into its lethal, mitochondria-embedded oligomeric form in response to cell stress, leading to the release of apoptogenic factors such as cytochrome C. Here, we dissected the structural correlates of Bax membrane insertion while oligomerization is halted. This strategy was enabled through the use of nanometer-scale phospholipid bilayer islands (nanodiscs) the size of which restricts the reconstituted system to single Bax-molecule activity. Using this minimal reconstituted system, we captured structural correlates that precede Bax homo-oligomerization elucidating previously inaccessible steps of the core molecular mechanism by which Bcl-2 family proteins regulate membrane permeabilization. We observe that, in the presence of BH3 interacting domain death agonist (Bid) BH3 peptide, Bax monomers induce the formation of ~3.5-nm diameter pores and significantly distort the phospholipid bilayer. These pores are compatible with promoting release of ions as well as proteinaceous components, suggesting that membrane-integrated Bax monomers in the presence of Bid BH3 peptides are key functional units for the activation of the cell demolition machinery.

Effects of MACPF/CDC proteins on lipid membranes

Recent work on the MACPF/CDC superfamily of pore-forming proteins has focused on the structural analysis of monomers and pore-forming oligomeric complexes. We set the family of proteins in context and highlight aspects of their function which the direct and exclusive equation of oligomers with pores fails to explain. Starting with a description of the distribution of MACPF/CDC proteins across the domains of life, we proceed to show how their evolutionary relationships can be understood on the basis of their structural homology and re-evaluate models for pore formation by perforin, in particular. We furthermore highlight data showing the role of incomplete oligomeric rings (arcs) in pore formation and how this can explain small pores generated by oligomers of proteins belonging to the family. We set this in the context of cell biological and biophysical data on the proteins' function and discuss how this helps in the development of an understanding of how they act in processes such as apicomplexan parasites gliding through cells and exiting from cells.

Analysis of SARS-CoV E protein ion channel activity by tuning the protein and lipid charge

A partial characterization of the ion channels formed by the SARS coronavirus (CoV) envelope (E) protein was previously reported (C. Verdiá-Báguena et al., 2012 [12]). Here, we provide new significant insights on the involvement of lipids in the structure and function of the CoV E protein channel on the basis of three series of experiments. First, reversal potential measurements over a wide range of pH allow the dissection of the contributions to channel selectivity coming from ionizable residues of the protein transmembrane domain and also from the negatively charged groups of diphytanoyl phosphatidylserine (DPhPS) lipid. The corresponding effective pKas are consistent with the model pKas of the acidic residue candidates for titration. Second, the change of channel conductance with salt concentration reveals two distinct regimes (Donnan-controlled electrodiffusion and bulk-like electrodiffusion) fully compatible with the outcomes of selectivity experiments. Third, by measuring channel conductance in mixtures of neutral diphytanoyl phosphatidylcholine (DPhPC) lipids and negatively charged DPhPS lipids in low and high salt concentrations we conclude that the protein–lipid conformation in the channel is likely the same in charged and neutral lipids. Overall, the whole set of experiments supports the proteolipidic structure of SARS-CoV E channels and explains the large difference in channel conductance observed between neutral and charged membranes.

•

SARS-CoV E protein channel structure is analyzed by tuning lipid charge.

•

Proteolipidic channel conformation is similar in charged and neutral lipids.

•

Lipid charge modulation of conductance is biphasic.

Hydrophobic pulmonary surfactant proteins SP-B and SP-C induce pore formation in planar lipid membranes: evidence for proteolipid pores

Pulmonary surfactant is a complex mixture of lipids and specific surfactant proteins, including the hydrophobic proteins SP-B and SP-C, in charge of stabilizing the respiratory surface of mammalian lungs. The combined action of both proteins is responsible for the proper structure and dynamics of membrane arrays in the pulmonary surfactant network that covers the respiratory surface. In this study, we explore the possibility that proteins SP-B and SP-C induce the permeabilization of phospholipid membranes via pore formation. To this end, electrophysiological measurements have been carried out in planar lipid membranes prepared with different lipid/protein mixtures. Our main result is that channel-like structures are detected in the presence of SP-B, SP-C, or the native mixture of both proteins. Current traces show a high variety of conductance states (from pS to nS) that are dependent both on the lipid composition and the applied potential. We also show that the type of host lipid crucially determines the ionic selectivity of the observed pores: the anionic selectivity observed in zwitterionic membranes is inverted to cationic selectivity in the presence of negatively charged lipids. All those results suggest that SP-B and SP-C proteins promote the formation of proteolipid channels in which lipid molecules are functionally involved. We propose that proteolipidic membrane-permeabilizing structures may have an important role to tune ionic and lipidic flows through the pulmonary surfactant membrane network at the alveolar surfaces.

Coronavirus E protein forms ion channels with functionally and structurally-involved membrane lipids

Coronavirus (CoV) envelope (E) protein ion channel activity was determined in channels formed in planar lipid bilayers by peptides representing either the transmembrane domain of severe acute respiratory syndrome CoV (SARS-CoV) E protein, or the full-length E protein. Both of them formed a voltage independent ion conductive pore with symmetric ion transport properties. Mutations N15A and V25F located in the transmembrane domain prevented the ion conductivity. E protein derived channels showed no cation preference in non-charged lipid membranes, whereas they behaved as pores with mild cation selectivity in negatively-charged lipid membranes. The ion conductance was also controlled by the lipid composition of the membrane. Lipid charge also regulated the selectivity of a HCoV-229E E protein derived peptide. These results suggested that the lipids are functionally involved in E protein ion channel activity, forming a protein-lipid pore, a novel concept for CoV E protein ion channel entity.

Membrane lipid composition regulates tubulin interaction with mitochondrial voltage-dependent anion channel

Elucidating molecular mechanisms by which lipids regulate protein function within biological membranes is critical for understanding the many cellular processes. Recently, we have found that dimeric αβ-tubulin, a subunit of microtubules, regulates mitochondrial respiration by blocking the voltage-dependent anion channel (VDAC) of mitochondrial outer membrane. Here, we show that the mechanism of VDAC blockage by tubulin involves tubulin interaction with the membrane as a critical step. The on-rate of the blockage varies up to 100-fold depending on the particular lipid composition used for bilayer formation in reconstitution experiments and increases with the increasing content of dioleoylphosphatidylethanolamine (DOPE) in dioleoylphosphatidylcholine (DOPC) bilayers. At physiologically low salt concentrations, the on-rate is decreased by the charged lipid. The off-rate of VDAC blockage by tubulin does not depend on the lipid composition. Using confocal fluorescence microscopy, we compared tubulin binding to the membranes of giant unilamellar vesicles (GUVs) made from DOPC and DOPC/DOPE mixtures. We found that detectable binding of the fluorescently labeled dimeric tubulin to GUV membranes requires the presence of DOPE. We propose that prior to the characteristic blockage of VDAC, tubulin first binds to the membrane in a lipid-dependent manner. We thus reveal a new potent regulatory role of the mitochondrial lipids in control of the mitochondrial outer membrane permeability and hence mitochondrial respiration through tuning VDAC sensitivity to blockage by tubulin. More generally, our findings give an example of the lipid-controlled protein-protein interaction where the choice of lipid species is able to change the equilibrium binding constant by orders of magnitude.

Mechanical properties of lipid bilayers and regulation of mechanosensitive function: from biological to biomimetic channels

Material properties of lipid bilayers, including thickness, intrinsic curvature and compressibility regulate the function of mechanosensitive (MS) channels. This regulation is dependent on phospholipid composition, lateral packing and organization within the membrane. Therefore, a more complete framework to understand the functioning of MS channels requires insights into bilayer structure, thermodynamics and phospholipid structure, as well as lipid-protein interactions. Phospholipids and MS channels interact with each other mainly through electrostatic forces and hydrophobic matching, which are also crucial for antimicrobial peptides. They are excellent models for studying the formation and stabilization of membrane pores. Importantly, they perform equivalent responses as MS channels: (1) tilting in response to tension and (2) dissipation of osmotic gradients. Lessons learned from pore forming peptides could enrich our knowledge of mechanisms of action and evolution of these channels. Here, the current state of the art is presented and general principles of membrane regulation of mechanosensitive function are discussed.

Perforin rapidly induces plasma membrane phospholipid flip-flop

The cytotoxic cell granule secretory pathway is essential for host defense. This pathway is fundamentally a form of intracellular protein delivery where granule proteases (granzymes) from cytotoxic lymphocytes are thought to diffuse through barrel stave pores generated in the plasma membrane of the target cell by the pore forming protein perforin (PFN) and mediate apoptotic as well as additional biological effects. While recent electron microscopy and structural analyses indicate that recombinant PFN oligomerizes to form pores containing 20 monomers (20 nm) when applied to liposomal membranes, these pores are not observed by propidium iodide uptake in target cells. Instead, concentrations of human PFN that encourage granzyme-mediated apoptosis are associated with pore structures that unexpectedly favor phosphatidylserine flip-flop measured by Annexin-V and Lactadherin. Efforts that reduce PFN mediated Ca influx in targets did not reduce Annexin-V reactivity. Antigen specific mouse CD8 cells initiate a similar rapid flip-flop in target cells. A lipid that augments plasma membrane curvature as well as cholesterol depletion in target cells enhance flip-flop. Annexin-V staining highly correlated with apoptosis after Granzyme B (GzmB) treatment. We propose the structures that PFN oligomers form in the membrane bilayer may include arcs previously observed by electron microscopy and that these unusual structures represent an incomplete mixture of plasma membrane lipid and PFN oligomers that may act as a flexible gateway for GzmB to translocate across the bilayer to the cytosolic leaflet of target cells.

Structural insights into the oligomerization and architecture of eukaryotic membrane pore-forming toxins

Pore-forming toxins (PFTs) are proteins that are secreted as soluble molecules and are inserted into membranes to form oligomeric transmembrane pores. In this paper, we report the crystal structure of Fragaceatoxin C (FraC), a PFT isolated from the sea anemone Actinia fragacea, at 1.8 Å resolution. It consists of a crown-shaped nonamer with an external diameter of about 11.0 nm and an internal diameter of approximately 5.0 nm. Cryoelectron microscopy studies of FraC in lipid bilayers reveal the pore structure that traverses the membrane. The shape and dimensions of the crystallographic oligomer are fully consistent with the membrane pore. The FraC structure provides insight into the interactions governing the assembly process and suggests the structural changes that allow for membrane insertion. We propose a nonameric pore model that spans the membrane by forming a lipid-free α-helical bundle pore.