Bacterial expression and characterization of the mitochondrial outer membrane channel. Effects of n-terminal modifications

In Silico Exploration of Alternative Conformational States of VDAC

VDAC (Voltage-Dependent Anion-selective Channel) is the primary metabolite pore in the mitochondrial outer membrane (OM). Atomic structures of VDAC, consistent with its physiological "open" state, are β-barrels formed by 19 transmembrane (TM) β-strands and an N-terminal segment (NTERM) that folds inside the pore lumen. However, structures are lacking for VDAC's partially "closed" states. To provide clues about possible VDAC conformers, we used the RoseTTAFold neural network to predict structures for human and fungal VDAC sequences modified to mimic removal from the pore wall or lumen of "cryptic" domains, i.e., segments buried in atomic models yet accessible to antibodies in OM-bound VDAC. Predicted in vacuo structures for full-length VDAC sequences are 19-strand β-barrels similar to atomic models, but with weaker H-bonding between TM strands and reduced interactions between NTERM and the pore wall. Excision of combinations of "cryptic" subregions yields β-barrels with smaller diameters, wide gaps between N- and C-terminal β-strands, and in some cases disruption of the β-sheet (associated with strained backbone H-bond registration). Tandem repeats of modified VDAC sequences also were explored, as was domain swapping in monomer constructs. Implications of the results for possible alternative conformational states of VDAC are discussed.

Voltage-Dependent Anion Selective Channel Isoforms in Yeast: Expression, Structure, and Functions

Mitochondrial porins, also known as voltage-dependent anion selective channels (VDACs), are pore-forming molecules of the outer mitochondrial membranes, involved in the regulation of metabolic flux between cytosol and mitochondria. Playing such an essential role, VDAC proteins are evolutionary conserved and isoforms are present in numerous species. The quest for specific function(s) related to the raise of multiple isoforms is an intriguing theme. The yeast Saccharomyces cerevisiae genome is endowed with two different VDAC genes encoding for two distinct porin isoforms, definitely less characterized in comparison to mammalian counterpart. While yVDAC1 has been extensively studied, the second isoform, yVDAC2, is much less expressed, and has a still misunderstood function. This review will recapitulate the known and poorly known information in the literature, in the light of the growing interest about the features of VDAC isoforms in the cell.

Structural and functional characterization of Solanum tuberosum VDAC36

As it forms water-filled channel in the mitochondria outer membrane and diffuses essential metabolites such as NADH and ATP, the voltage-dependent anion channel (VDAC) protein family plays a central role in all eukaryotic cells. In comparison with their mammalian homologues, little is known about the structural and functional properties of plant VDACs. In the present contribution, one of the two VDACs isoforms of Solanum tuberosum, stVDAC36, has been successfully overexpressed and refolded by an in-house method, as demonstrated by the information on its secondary and tertiary structure gathered from circular dichroism and intrinsic fluorescence. Cross-linking and molecular modeling studies have evidenced the presence of dimers and tetramers, and they suggest the formation of an intermolecular disulfide bond between two stVDAC36 monomers. The pore-forming activity was also assessed by liposome swelling assays, indicating a typical pore diameter between 2.0 and 2.7 nm. Finally, insights about the ATP binding inside the pore are given by docking studies and electrostatic calculations.

Mitochondrial VDAC1: A Key Gatekeeper as Potential Therapeutic Target

Mitochondria are the key source of ATP that fuels cellular functions, and they are also central in cellular signaling, cell division and apoptosis. Dysfunction of mitochondria has been implicated in a wide range of diseases, including neurodegenerative and cardiac diseases, and various types of cancer. One of the key proteins that regulate mitochondrial function is the voltage-dependent anion channel 1 (VDAC1), the most abundant protein on the outer membrane of mitochondria. VDAC1 is the gatekeeper for the passages of metabolites, nucleotides, and ions; it plays a crucial role in regulating apoptosis due to its interaction with apoptotic and anti-apoptotic proteins, namely members of the Bcl-2 family of proteins and hexokinase. Therefore, regulation of VDAC1 is crucial not only for metabolic functions of mitochondria, but also for cell survival. In fact, multiple lines of evidence have confirmed the involvement of VDAC1 in several diseases. Consequently, modulation or dysregulation of VDAC1 function can potentially attenuate or exacerbate pathophysiological conditions. Understanding the role of VDAC1 in health and disease could lead to selective protection of cells in different tissues and diverse diseases. The purpose of this review is to discuss the role of VDAC1 in the pathogenesis of diseases and as a potentially effective target for therapeutic management of various pathologies.

Negative Ions Enhance Survival of Membrane Protein Complexes

Membrane protein complexes are commonly introduced to the mass spectrometer solubilized in detergent micelles. The collisional activation used to remove the detergent, however, often causes protein unfolding and dissociation. As in the case for soluble proteins, electrospray in the positive ion mode is most commonly used for the study of membrane proteins. Here we show several distinct advantages of employing the negative ion mode. Negative polarity can yield lower average charge states for membrane proteins solubilized in saccharide detergents, with enhanced peak resolution and reduced adduct formation. Most importantly, we demonstrate that negative ion mode electrospray ionization (ESI) minimizes subunit dissociation in the gas phase, allowing access to biologically relevant oligomeric states. Together, these properties mean that intact membrane protein ions can be generated in a greater range of solubilizing detergents. The formation of negative ions, therefore, greatly expands the possibilities of using mass spectrometry on this intractable class of protein. Graphical Abstract ᅟ.

The N-terminus of VDAC: Structure, mutational analysis, and a potential role in regulating barrel shape

A novel feature of the voltage-dependent anion channel (VDAC, mitochondrial porin), is the barrel, comprising an odd number of β-strands and closed by parallel strands. Recent research has focused on the N-terminal segment, which in the available structures, resides in the lumen and is not part of the barrel. In this review, the structural data obtained from vertebrate VDAC are integrated with those from VDAC in artificial bilayers, emphasizing the array of native and tagged versions of VDAC used. The data are discussed with respect to a recent gating model (Zachariae et al. (2012) Structure 20:1-10), in which the N-terminus acts not as a gate on a stable barrel, but rather stabilizes the barrel, preventing its shift into a partially collapsed, low-conductance, closed state. Additionally, the role of the N-terminus in VDAC oligomerization, apoptosis through interactions with hexokinase and its interaction with ATP are discussed briefly.

VDAC3 gating is activated by suppression of disulfide-bond formation between the N-terminal region and the bottom of the pore

The voltage-dependent anion channels (VDACs), VDAC1, VDAC2, and VDAC3, are pore-forming proteins that control metabolite flux between mitochondria and cytoplasm. VDAC1 and VDAC2 have voltage-dependent gating activity, whereas VDAC3 is thought to have weak activity. The aim of this study was to analyze the channel properties of all three human VDAC isoforms and to clarify the channel function of VDAC3. Bacterially expressed recombinant human VDAC proteins were reconstituted into artificial planar lipid bilayers and their gating activities were evaluated. VDAC1 and VDAC2 had typical voltage-dependent gating activity, whereas the gating of VDAC3 was weak, as reported. However, gating of VDAC3 was evoked by dithiothreitol (DTT) and S-nitrosoglutathione (GSNO), which are thought to suppress disulfide-bond formation. Several cysteine mutants of VDAC3 also exhibited typical voltage-gating. Our results indicate that channel gating was induced by reduction of a disulfide-bond linking the N-terminal region to the bottom of the pore. Thus, channel gating of VDAC3 might be controlled by redox sensing under physiological conditions.

Clustering and visualizing similarity networks of membrane proteins

We proposed a fast and unsupervised clustering method, minimum span clustering (MSC), for analyzing the sequence-structure-function relationship of biological networks, and demonstrated its validity in clustering the sequence/structure similarity networks (SSN) of 682 membrane protein (MP) chains. The MSC clustering of MPs based on their sequence information was found to be consistent with their tertiary structures and functions. For the largest seven clusters predicted by MSC, the consistency in chain function within the same cluster is found to be 100%. From analyzing the edge distribution of SSN for MPs, we found a characteristic threshold distance for the boundary between clusters, over which SSN of MPs could be properly clustered by an unsupervised sparsification of the network distance matrix. The clustering results of MPs from both MSC and the unsupervised sparsification methods are consistent with each other, and have high intracluster similarity and low intercluster similarity in sequence, structure, and function. Our study showed a strong sequence-structure-function relationship of MPs. We discussed evidence of convergent evolution of MPs and suggested applications in finding structural similarities and predicting biological functions of MP chains based on their sequence information.

The mitochondrial voltage-dependent anion channel 1 in tumor cells

VDAC1 is found at the crossroads of metabolic and survival pathways. VDAC1 controls metabolic cross-talk between mitochondria and the rest of the cell by allowing the influx and efflux of metabolites, ions, nucleotides, Ca2+ and more. The location of VDAC1 at the outer mitochondrial membrane also enables its interaction with proteins that mediate and regulate the integration of mitochondrial functions with cellular activities. As a transporter of metabolites, VDAC1 contributes to the metabolic phenotype of cancer cells. Indeed, this protein is over-expressed in many cancer types, and silencing of VDAC1 expression induces an inhibition of tumor development. At the same time, along with regulating cellular energy production and metabolism, VDAC1 is involved in the process of mitochondria-mediated apoptosis by mediating the release of apoptotic proteins and interacting with anti-apoptotic proteins. The engagement of VDAC1 in the release of apoptotic proteins located in the inter-membranal space involves VDAC1 oligomerization that mediates the release of cytochrome c and AIF to the cytosol, subsequently leading to apoptotic cell death. Apoptosis can also be regulated by VDAC1, serving as an anchor point for mitochondria-interacting proteins, such as hexokinase (HK), Bcl2 and Bcl-xL, some of which are also highly expressed in many cancers. By binding to VDAC1, HK provides both a metabolic benefit and apoptosis-suppressive capacity that offer the cell a proliferative advantage and increase its resistance to chemotherapy. Thus, these and other functions point to VDAC1 as an excellent target for impairing the re-programed metabolism of cancer cells and their ability to evade apoptosis. Here, we review current evidence pointing to the function of VDAC1 in cell life and death, and highlight these functions in relation to both cancer development and therapy. In addressing the recently solved 3D structures of VDAC1, this review will point to structure-function relationships of VDAC as critical for deciphering how this channel can perform such a variety of roles, all of which are important for cell life and death. Finally, this review will also provide insight into VDAC function in Ca2+ homeostasis, protection against oxidative stress, regulation of apoptosis and involvement in several diseases, as well as its role in the action of different drugs. We will discuss the use of VDAC1-based strategies to attack the altered metabolism and apoptosis of cancer cells. These strategies include specific siRNA able to impair energy and metabolic homeostasis, leading to arrested cancer cell growth and tumor development, as well VDAC1-based peptides that interact with anti-apoptotic proteins to induce apoptosis, thereby overcoming the resistance of cancer cell to chemotherapy. Finally, small molecules targeting VDAC1 can induce apoptosis. VDAC1 can thus be considered as standing at the crossroads between mitochondrial metabolite transport and apoptosis and hence represents an emerging cancer drug target. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Structure-guided simulations illuminate the mechanism of ATP transport through VDAC1

The voltage-dependent anion channel (VDAC) mediates the flow of metabolites and ions across the outer mitochondrial membrane of all eukaryotic cells. The open channel passes millions of ATP molecules per second, whereas the closed state exhibits no detectable ATP flux. High-resolution structures of VDAC1 revealed a 19-stranded β-barrel with an α-helix partially occupying the central pore. To understand ATP permeation through VDAC, we solved the crystal structure of mouse VDAC1 (mVDAC1) in the presence of ATP, revealing a low-affinity binding site. Guided by these coordinates, we initiated hundreds of molecular dynamics simulations to construct a Markov state model of ATP permeation. These simulations indicate that ATP flows through VDAC through multiple pathways, in agreement with our structural data and experimentally determined physiological rates.

The functions of voltage-dependent anion channels in plants

Voltage-dependent anion channels (VDACs), known as outer mitochondrial membrane proteins, are present in all eukaryotic cells. In mammals, they are now recognized to play crucial roles in the regulation of metabolic and energetic functions of mitochondria as well as in mitochondria-mediated apoptosis, in association with various proteins and non-protein modulators. Although there is much less information available for plant than for animal VDACs, their similar electrophysiological and topological properties suggest that some common functions are conserved among eukaryotic VDACs. Recently, it has been revealed that plant VDACs also have various important physiological functions not only in developmental and reproductive processes, but also in biotic and abiotic stress responses, including programmed cell death. In this review, we summarize recent findings about the sequence motifs, localization, and function of plant VDACs and discuss these results in the light of recent advances in research on animal VDACs.

Deletion of β-strands 9 and 10 converts VDAC1 voltage-dependence in an asymmetrical process

Voltage-dependent anion selective channel isoform1 maintains the permeability of the outer mitochondrial membrane. Its voltage-gating properties are relevant in bioenergetic metabolism and apoptosis. The N-terminal domain is suspected to be involved in voltage-gating, due to its peculiar localization. However this issue is still controversial. In this work we exchanged or deleted the β-strands that take contact with the N-terminal domain. The exchange of the whole hVDAC1 β-barrel with the homologous hVDAC3 β-barrel produces a chimeric protein that, in reconstituted systems, loses completely voltage-dependence. hVDAC3 β-barrel has most residues in common with hVDAC1, including V143 and L150 considered anchor points for the N-terminus. hVDAC1 mutants completely lacking either the β-strand 9 or both β-strands 9 and 10 were expressed, refolded and reconstituted in artificial bilayers. The mutants formed smaller pores. Molecular dynamics simulations of the mutant structure supported its ability to form smaller pores. The mutant lacking both β-strands 9 and 10 showed a new voltage-dependence feature resulting in a fully asymmetric behavior. These data indicate that a network of β-strands in the pore-walls, and not single residues, are required for voltage-gating in addition to the N-terminus.

Structure-based analysis of VDAC1: N-terminus location, translocation, channel gating and association with anti-apoptotic proteins

Structural studies place the VDAC1 (voltage-dependent anion channel 1) N-terminal region within the channel pore. Biochemical and functional studies, however, reveal that the N-terminal domain is cytoplasmically exposed. In the present study, the location and translocation of the VDAC1 N-terminal domain, and its role in voltage-gating and as a target for anti-apoptotic proteins, were addressed. Site-directed mutagenesis and cysteine residue substitution, together with a thiol-specific cross-linker, served to show that the VDAC1 N-terminal region exists in a dynamic equilibrium, located within the pore or exposed outside the β-barrel. Using a single cysteine-residue-bearing VDAC1, we demonstrate that the N-terminal region lies inside the pore. However, the same region can be exposed outside the pore, where it dimerizes with the N-terminal domain of a second VDAC1 molecule. When the N-terminal region α-helix structure was perturbed, intra-molecular cross-linking was abolished and dimerization was enhanced. This mutant also displays reduced voltage-gating and reduced binding to hexokinase, but not to the anti-apoptotic proteins Bcl-2 and Bcl-xL. Replacing glycine residues in the N-terminal domain GRS (glycine-rich sequence) yielded less intra-molecular cross-linked product but more dimerization, suggesting that GRS provides the flexibility needed for N-terminal translocation from the internal pore to the channel face. N-terminal mobility may thus contribute to channel gating and interaction with anti-apoptotic proteins.

VDAC structure, selectivity, and dynamics

VDAC channels exist in the mitochondrial outer membrane of all eukaryotic organisms. Of the different isoforms present in one organism, it seems that one of these is the canonical VDAC whose properties and 3D structure are highly conserved. The fundamental role of these channels is to control the flux of metabolites between the cytosol and mitochondrial spaces. Based on many functional studies, the fundamental structure of the pore wall consists of one α helix and 13 β strands tilted at a 46° angle. This results in a pore with an estimated internal diameter of 2.5nm. This structure has not yet been resolved. The published 3D structure consists of 19 β strands and is different from the functional structure that forms voltage-gated channels. The selectivity of the channel is exquisite, being able to select for ATP over molecules of the same size and charge. Voltage gating involves two separate gating processes. The mechanism involves the translocation of a positively charged portion of the wall of the channel to the membrane surface resulting in a reduction in pore diameter and volume and an inversion in ion selectivity. This mechanism is consistent with experiments probing changes in selectivity, voltage gating, kinetics and energetics. Other published mechanisms are in conflict with experimental results. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.

Plant VDAC: facts and speculations

The voltage-dependent anion-selective channel (VDAC) is the most abundant protein in the mitochondrial outer membrane and the major transport pathway for a large variety of compounds ranging from ions to large polymeric molecules such as DNA and tRNA. Plant VDACs feature a secondary structure content and electrophysiological properties akin to those of VDACs from other organisms. They however undergo a specific regulation. The general importance of VDAC in plant physiology has only recently emerged. Besides their role in metabolite transport, plant VDACs are also involved in the programmed cell death triggered in response to biotic and abiotic stresses. Moreover, their colocalization in non-mitochondrial membranes suggests a diversity of function. This review summarizes our current understanding of the structure and function of plant VDACs. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.

Concentration dependent ion selectivity in VDAC: a molecular dynamics simulation study

The voltage-dependent anion channel (VDAC) forms the major pore in the outer mitochondrial membrane. Its high conducting open state features a moderate anion selectivity. There is some evidence indicating that the electrophysiological properties of VDAC vary with the salt concentration. Using a theoretical approach the molecular basis for this concentration dependence was investigated. Molecular dynamics simulations and continuum electrostatic calculations performed on the mouse VDAC1 isoform clearly demonstrate that the distribution of fixed charges in the channel creates an electric field, which determines the anion preference of VDAC at low salt concentration. Increasing the salt concentration in the bulk results in a higher concentration of ions in the VDAC wide pore. This event induces a large electrostatic screening of the charged residues promoting a less anion selective channel. Residues that are responsible for the electrostatic pattern of the channel were identified using the molecular dynamics trajectories. Some of these residues are found to be conserved suggesting that ion permeation between different VDAC species occurs through a common mechanism. This inference is buttressed by electrophysiological experiments performed on bean VDAC32 protein akin to mouse VDAC.

Bacterial expression, purification and characterization of a rice voltage-dependent, anion-selective channel isoform, OsVDAC4

The voltage-dependent anion-selective channel (VDAC) is the most abundant protein in the mitochondrial outer membrane and forms the major conduit for metabolite transport across this membrane. VDACs from different sources show varied primary sequence but conserved functional properties. Here, we report on the characterization of a rice channel, OsVDAC4, which complements a VDAC1 deficiency in yeast. We present a consensus secondary structure prediction of an N-terminal α-helix and 19 β-strands. Bacterially expressed OsVDAC4 was purified from inclusion bodies into detergent-containing solution, where it is largely helical. Detergent-solubilized OsVDAC4 inserts spontaneously into artificial membranes of two topologies-spherical liposomes and planar bilayers. Insertion into liposomes results in an increase in β-structure. Transport of polyethylene glycols was used to estimate a pore diameter of ~2.6 nm in liposomes. Channels formed in planar bilayers exhibit large conductance (4.6 ± 0.3 nS in 1 M KCl), strong voltage dependence and weak anion selectivity. The open state of the channel is shown to be permeable to ATP. These data are consistent with a large β-barrel pore formed by OsVDAC4 on inserting into membranes. This study forms a platform to carry out studies of the interaction of OsVDAC4 with putative modulators.

VDAC isoforms in mammals

VDACs (Voltage Dependent Anion selective Channels) are a family of pore-forming proteins discovered in the mitochondrial outer membrane. In the animal kingdom, mammals show a conserved genetic organization of the VDAC genes, corresponding to a group of three active genes. Three VDAC protein isoforms thus exist. From a historically point of view most of the data collected about this protein refer to the VDAC1 isoform, the first to be identified and also the most abundant in the organisms. In this work we compare the information available about the three VDAC isoforms, with a special emphasis upon the human proteins, here considered prototypical of the group, and we try to shed some light on specific functional roles of this apparently redundant group of proteins. A new hypothesis about the VDAC(s) involvement in ROS control is proposed. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.

The 3D structures of VDAC represent a native conformation

The most abundant protein of the mitochondrial outer membrane is the voltage-dependent anion channel (VDAC), which facilitates the exchange of ions and molecules between mitochondria and cytosol and is regulated by interactions with other proteins and small molecules. VDAC has been studied extensively for more than three decades, and last year three independent investigations revealed a structure of VDAC-1 exhibiting 19 transmembrane beta-strands, constituting a unique structural class of beta-barrel membrane proteins. Here, we provide a historical perspective on VDAC research and give an overview of the experimental design used to obtain these structures. Furthermore, we validate the protein refolding approach and summarize the biochemical and biophysical evidence that links the 19-stranded structure to the native form of VDAC.

Origami in outer membrane mimetics: correlating the first detailed images of refolded VDAC with over 20 years of biochemical data

Mitochondrial porin forms an aqueous pore in the outer membrane, through which selective passage of small metabolites and ions occurs, thereby regulating both mitochondrial function and cellular respiration. Investigations of the structure and function of porin have been performed with whole mitochondria, membrane vesicles, artificial membranes, and in detergent solutions, resulting in numerous models of porin structure. The mechanisms by which this protein functions are undoubtedly linked to its structure, which remained elusive until 2008, with reports of 3 high-resolution structures of this voltage-dependent, anion-selective channel (VDAC). The barrel structure is relatively simple yet unique: it is arranged as 19 anti-parallel β-strands, with β-strands 1 and 19 aligned parallel to each other to close the barrel. The N-terminal helical component is located within the lumen of the channel, although its precise structure and location in the lumen varies. With the basic barrel structure in hand, the data obtained in attempts to model the structure and understand porin over the past 20 years can be re-evaluated. Herein, using the mammalian VDAC structures as templates, the amassed electrophysiological and biochemical information has been reassessed with respect to the functional mechanisms of VDAC activity, with a focus on voltage-dependent gating.

Crystallization of the membrane protein hVDAC1 produced in cell-free system

Structural studies of membrane proteins are in constant evolution with the development of new improvements for their expression, purification, stabilization and crystallization. However, none of these methods still provides a universal approach to solve the structure of membrane proteins. Here we describe the crystallization of the human voltage-dependent anion channel-1 produced by a bacterial cell-free expression system. While VDAC structures have been recently solved, we propose an alternative strategy for producing the recombinant protein, which can be applied to other membrane proteins reluctant to expression, purification and crystallization by classical approaches. Despite a lot of efforts to crystallize a cell-free expressed membrane protein, this study is to our knowledge one of the first reports of a successful crystallization. Focusing on expression in a soluble and functional state, in a detergent environment, is the key to get crystals. Although the diffraction of VDAC crystals is limited, the simplicity and the rapidity to set-up and optimize this technology are drastic advantages in comparison to other methods.

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.

Voltage-dependent anion channel 1-based peptides interact with Bcl-2 to prevent antiapoptotic activity

The antiapoptotic proteins of the Bcl-2 family are expressed at high levels in many types of cancer. However, the mechanism by which Bcl-2 family proteins regulate apoptosis is not fully understood. Here, we demonstrate the interaction of Bcl-2 with the outer mitochondrial membrane protein, voltage-dependent anion channel 1 (VDAC1). A direct interaction of Bcl-2 with bilayer-reconstituted purified VDAC was demonstrated, with Bcl-2 decreasing channel conductance. Expression of Bcl-2-GFP prevented apoptosis in cells expressing native but not certain VDAC1 mutants. VDAC1 sequences and amino acid residues important for interaction with Bcl-2 were defined through site-directed mutagenesis. Synthetic peptides corresponding to the VDAC1 N-terminal region and selected sequences bound specifically, in a concentration- and time-dependent manner, to immobilized Bcl-2, as revealed by the real-time surface plasmon resonance. Moreover, expression of the VDAC1-based peptides in cells over-expressing Bcl-2 prevented Bcl-2-mediated protection against staurosporine-induced apoptotic cell death. Similarly, a cell-permeable VDAC1-based synthetic peptide was also found to prevent Bcl-2-GFP-mediated protection against apoptosis. These results point to Bcl-2 as promoting tumor cell survival through binding to VDAC1, thereby inhibiting cytochrome c release and apoptotic cell death. Moreover, these findings suggest that interfering with the binding of Bcl-2 to mitochondria by VDAC1-based peptides may serve to potentiate the efficacy of conventional chemotherapeutic agents.

The electrostatics of VDAC: implications for selectivity and gating

The voltage-dependent anion channel (VDAC) is the major pathway mediating the transfer of metabolites and ions across the mitochondrial outer membrane. Two hallmarks of the channel in the open state are high metabolite flux and anion selectivity, while the partially closed state blocks metabolites and is cation selective. Here we report the results from electrostatics calculations carried out on the recently determined high-resolution structure of murine VDAC1 (mVDAC1). Poisson-Boltzmann calculations show that the ion transfer free energy through the channel is favorable for anions, suggesting that mVDAC1 represents the open state. This claim is buttressed by Poisson-Nernst-Planck calculations that predict a high single-channel conductance indicative of the open state and an anion selectivity of 1.75--nearly a twofold selectivity for anions over cations. These calculations were repeated on mutant channels and gave selectivity changes in accord with experimental observations. We were then able to engineer an in silico mutant channel with three point mutations that converted mVDAC1 into a channel with a preference for cations. Finally, we investigated two proposals for how the channel gates between the open and the closed state. Both models involve the movement of the N-terminal helix, but neither motion produced the observed voltage sensitivity, nor did either model result in a cation-selective channel, which is observed experimentally. Thus, we were able to rule out certain models for channel gating, but the true motion has yet to be determined.

Effects of ergosterol on the structure and activity of Neurospora mitochondrial porin in liposomes

Mitochondrial porins (also known as voltage-dependent anion-selective channels (VDACs)) regulate and contribute to cellular metabolism. These proteins copurify with sterols, and some purified forms of the protein require sterol for insertion into planar artificial membranes. Recently, interactions between detergent-solubilized mitochondrial porins and sterols have been detected by NMR and spectroscopic methods, but the effects of sterols on pore function remained to be assessed. Therefore, in this work, a freeze–thaw technique was used to introduce recombinant Neurospora porin into liposomes containing, or lacking, the native fungal sterol ergosterol. In both types of liposomes, insertion of the protein converts it to a protease-resistant state and low levels of dimeric and trimeric forms are observed. There are only minor differences between the secondary structural components of the protein in the presence or absence of sterol. Ergosterol in proteoliposomes alters their osmotic responses to sucrose, possibly due to increased membrane rigidity or interactions with the protein that were not revealed by the methods used in this study. The presence of ergosterol is associated with an increased change in conformation and loss of function of liposome-embedded porin at high temperature. Taken with other evidence for direct interactions of sterols with porins, these results support a link between these two molecules in mitochondrial membrane activity.

Structure of the voltage dependent anion channel: state of the art

The eukaryotic porin or Voltage Dependent Anion-selective Channels (VDAC) is the protein forming the aqueous pore channel in the mitochondrial outer membrane. It can modulate the energy-dependent metabolism of the cell forming a diffusion barrier to ions, adenine-nucleotides and other metabolites and it is probably involved in the regulation of apoptotic-relevant events. For these reasons, VDAC co-responsibility in unphysiological events leading to important pathologies such as onset or sustainment of cancer has been envisaged very early. The knowledge of the VDAC atomic structure is thus a relevant step in the design of modern drugs acting upon the mitochondrial function and its related apoptotic balance. This goal, despite many efforts, has not been gained until now. Several predictive or descriptive techniques have been employed to obtain models or representations of the pore-structure. The results obtained are reported in this review. The emerging picture arising from these many results is coherent and sufficiently informative. From these efforts it appears that VDAC is functionally monomeric but can cluster in tight but regular groups; it is asymmetric with larger exposed domains on the cytosolic side of the outer mitochondrial membrane; the diameter of the pore is between 2.5-3.0 nm and it is apparently free from obstructions (in the open state); the channel wall is mainly formed by typical amphipathic beta-strands; mobile components (the N-terminal ?) can have functional relevance to the pore regulation.

Approaching the structure of human VDAC1, a key molecule in mitochondrial cross-talk

The voltage dependent anion-channel, VDAC, is the major constitutive protein of the outer membrane of mitochondria. Functionally, VDAC is involved in the exchange of small metabolites over the mitochondrial outer membrane and supports enzymes of the cytoplasm with energy precursors i.e. ATP. Moreover, the channel alone or in complex with proteins of the inner mitochondrial membrane or the intermembrane space provides a basis for docking of cytosolic proteins which can regulate outer membrane permeability in several ways. Structurally, this channel has a bacterial origin by evolution and partly resembles bacterial porin functions. However, the structure seems more complex as a variety of interactions on both channel sides can occur. Therefore, our work described is aiming to determine the structure of VDAC at atomic resolution and together with functional data to understand better how this channel can carry out such a variety of differing functions.

Reflections on VDAC as a voltage-gated channel and a mitochondrial regulator

There is excellent agreement between the electrophysiological properties and the structure of the mitochondrial outer membrane protein, VDAC, ex vivo. However, the inference that the well-defined canonical "open" state of the VDAC pore is the normal physiological state of the channel in vivo is being challenged by several lines of evidence. Knowing the atomic structure of the detergent solubilized protein, a long sought after goal, will not be sufficient to understand the functioning of this channel protein. In addition, detailed information about VDAC's topology in the outer membrane of intact mitochondria, and the structural changes that it undergoes in response to different stimuli in the cell will be needed to define its physiological functions and regulation.

The role of solution NMR in the structure determinations of VDAC-1 and other membrane proteins

The voltage-dependent anion channel (VDAC) is an essential protein in the eukaryotic outer mitochondrial membrane, providing the pore for substrate diffusion. Three high-resolution structures of the isoform 1 of VDAC in detergent micelles and bicelles have recently been published, using solution NMR and X-ray crystallography. They resolve longstanding discussions about the membrane topology of VDAC and provide the first eukaryotic beta-barrel membrane protein structure. The structure contains a surprising feature that had not been observed in an integral membrane protein before: A parallel beta-strand pairing and thus an odd number of strands. The studies also give a structural and functional basis for the voltage gating mechanism of VDAC and its modulation by NADH; however, they do not fully explain these functions yet. With the de novo structure of VDAC-1, as well as those of half a dozen other proteins, the number of integral membrane protein structures solved by solution NMR has doubled in the past two years. Numerous further structural and functional studies on many different membrane proteins show that solution NMR has become an important tool for membrane protein molecular biology.

The VDAC1 N-terminus is essential both for apoptosis and the protective effect of anti-apoptotic proteins

The release of mitochondrial-intermembrane-space pro-apoptotic proteins, such as cytochrome c, is a key step in initiating apoptosis. Our study addresses two major questions in apoptosis: how are mitochondrial pro-apoptotic proteins released and how is this process regulated? Accumulating evidence indicates that the voltage-dependent anion channel (VDAC) plays a central role in mitochondria-mediated apoptosis. Here, we demonstrate that the N-terminal domain of VDAC1 controls the release of cytochrome c, apoptosis and the regulation of apoptosis by anti-apoptotic proteins such as hexokinase and Bcl2. Cells expressing N-terminal truncated VDAC1 do not release cytochrome c and are resistant to apoptosis, induced by various stimuli. Employing a variety of experimental approaches, we show that hexokinase and Bcl2 confer protection against apoptosis through interaction with the VDAC1 N-terminal region. We also demonstrate that apoptosis induction is associated with VDAC oligomerization. These results show VDAC1 to be a component of the apoptosis machinery and offer new insight into the mechanism of cytochrome c release and how anti-apoptotic proteins regulate apoptosis and promote tumor cell survival.

Functional characterization of the conserved "GLK" motif in mitochondrial porin from Neurospora crassa

Mitochondrial porin facilitates the diffusion of small hydrophilic molecules across the mitochondrial outer membrane. Despite low sequence similarity among porins from different species, a "glycine-leucine-lysine" (GLK) motif is conserved in mitochondrial and Neisseria porins. To investigate the possible roles of these conserved residues, including their hypothesized participation in ATP binding by the protein, we replaced the lysine residue of the GLK motif of Neurospora crassa porin with glutamic acid through site-directed mutagenesis of the corresponding gene. Although the pores formed by this protein have size and gating characteristics similar to those of the wild-type protein, the channels formed by GLEporin are less anion selective than the wild-type pores. The GLEporin retains the ability to be cross linked to [alpha-(32)P]ATP, indicating that the GLK sequence is not essential for ATP binding. Furthermore, the pores formed by both GLEporin and the wild-type protein become more cation selective in the presence of ATP. Taken together, these results support structural models that place the GLK motif in a part of the ion-selective beta-barrel that is not directly involved in ATP binding.

The influence of sterols on the conformation of recombinant mitochondrial porin in detergent

Mitochondrial porins (voltage-dependent anion-selective channels, VDAC) are key contributors to cellular metabolism. When isolated from mitochondria porins copurify with sterols, and some isolated forms of the protein require sterol for insertion into artificial membranes. Nonetheless, the contributions of sterols to the folded state of mitochondrial porin are not understood. Recently, with the goal of high-resolution structural studies, several laboratories have developed methods for folding recombinant porins at high concentration in detergent. In the present study, recombinant Neurospora crassa porin solubilized in detergent–sterol mixtures was examined. Sterols do not significantly alter the secondary structure of porin in lauryl dimethylamine oxide, nor in a mixture of sodium dodecylsulfate and dodecylmaltopyranoside. However, as detected by near-UV circular dichroism spectropolarimetry and fluorescence spectroscopy, the environments surrounding the aromatic amino acids in the detergent–sterol solubilized protein are measurably different from those in detergent alone. Furthermore, the effects are different in the presence of ergosterol, the native sterol in fungal mitochondria, and cholesterol. While these influences on the tertiary arrangement of detergent-solubilized porin are subtle, they may contribute to the generation of a form of the protein competent for insertion into the artificial bilayers used for electrophysiological analyses, and should be considered in future structural studies of porin.

The crystal structure of mouse VDAC1 at 2.3 A resolution reveals mechanistic insights into metabolite gating

The voltage-dependent anion channel (VDAC) constitutes the major pathway for the entry and exit of metabolites across the outer membrane of the mitochondria and can serve as a scaffold for molecules that modulate the organelle. We report the crystal structure of a beta-barrel eukaryotic membrane protein, the murine VDAC1 (mVDAC1) at 2.3 A resolution, revealing a high-resolution image of its architecture formed by 19 beta-strands. Unlike the recent NMR structure of human VDAC1, the position of the voltage-sensing N-terminal segment is clearly resolved. The alpha-helix of the N-terminal segment is oriented against the interior wall, causing a partial narrowing at the center of the pore. This segment is ideally positioned to regulate the conductance of ions and metabolites passing through the VDAC pore.

Solution structure of the integral human membrane protein VDAC-1 in detergent micelles

The voltage-dependent anion channel (VDAC) mediates trafficking of small molecules and ions across the eukaryotic outer mitochondrial membrane. VDAC also interacts with antiapoptotic proteins from the Bcl-2 family, and this interaction inhibits release of apoptogenic proteins from the mitochondrion. We present the nuclear magnetic resonance (NMR) solution structure of recombinant human VDAC-1 reconstituted in detergent micelles. It forms a 19-stranded beta barrel with the first and last strand parallel. The hydrophobic outside perimeter of the barrel is covered by detergent molecules in a beltlike fashion. In the presence of cholesterol, recombinant VDAC-1 can form voltage-gated channels in phospholipid bilayers similar to those of the native protein. NMR measurements revealed the binding sites of VDAC-1 for the Bcl-2 protein Bcl-x(L), for reduced beta-nicotinamide adenine dinucleotide, and for cholesterol. Bcl-x(L) interacts with the VDAC barrel laterally at strands 17 and 18.

Crystallization and preliminary X-ray crystallographic studies of human voltage-dependent anion channel isoform I (HVDAC1)

The major channel by which metabolites can pass through the outer mitochondrial membrane is formed by the voltage-dependent anion-channel (VDAC) family. Functionally, VDAC is involved in the limited exchange of ATP, ADP and small hydrophilic molecules across the outer membrane. Moreover, there is compelling evidence that VDAC isoforms in mammals may act in the cross-talk between mitochondria and the cytoplasm by direct interaction with enzymes involved in energy metabolism and proteins involved in mitochondrial-induced apoptosis. To obtain a high-resolution structure of this channel, human VDAC protein isoform I was overproduced in Escherichia coli. After refolding and testing the correct fold using circular dichroism, a subsequent broad-range screening in different detergents resulted in a variety of crystals which diffracted to 3.5 A resolution. The crystal lattice belongs to the trigonal space group P321, with unit-cell parameters a = 78.9, c = 165.7 A and one monomer in the asymmetric unit.

Two-step folding of recombinant mitochondrial porin in detergent

Precise information regarding the transmembrane topology of mitochondrial porin is essential for understanding the mechanisms by which this protein functions. Porin acts as a channel in the outer membrane and interacts with small solutes and proteins to regulate mitochondrial function. The acquisition of high-resolution structural data requires a method of maintaining high concentrations of unaggregated, properly folded porin. In the current studies, several mixed detergent systems were analyzed for their ability to fold Neurospora mitochondrial porin expressed in and isolated from Escherichia coli. A mixture of sodium dodecyl sulfate and dodecyl-beta-D-maltopyranoside in a 1:6 molar ratio supports a beta-strand-rich conformation. In this state, the two tryptophan residues in the protein reside in hydrophobic environments, and about half of the nine tyrosines are solvent exposed. Most importantly, heat-labile tertiary contacts, as detected by near-UV circular dichroism spectropolarimetry, in the sodium dodecyl sulfate/dodecyl-beta-D-maltopyranoside-solubilized porin are very similar to those of the protein following functional reconstitution into liposomes. Similarly, both forms are protease resistant. Thus, a method has been identified with the potential to solubilize high concentrations of mitochondrial porin in a state virtually indistinguishable from the membrane-embedded form.

High-level expression, refolding and probing the natural fold of the human voltage-dependent anion channel isoforms I and II

The voltage-dependent anion channel (VDAC) is the major protein found in the outer membrane of mitochondria. The channel is responsible for the exchange of ATP/ADP and the translocation of ions and other small metabolites over the membrane. In order to obtain large amounts of pure and suitably folded human VDAC for functional and structural studies, the genes of the human isoforms I and II (HVDAC1 and HVDAC2) were cloned in Escherichia coli. High-level expression led to inclusion body formation. Both proteins could be refolded in vitro by adding denatured protein to a solution of zwitterionic or nonionic detergents. A highly efficient and fast protocol for refolding was developed that yielded more than 50 mg of pure human VDACs per liter of cell culture. The native and functional state of the refolded porins was probed by Fourier transform infrared spectroscopy to determine the secondary structure composition and by electrophysiological measurements, demonstrating the pore-forming activity of HVDAC1. Furthermore, binding of HVDAC1 to immobilized ATP was demonstrated. Limited proteolysis of HVDAC1 protein embedded in detergent micelles in combination with matrix-assisted laser desorption ionization mass spectrometric analysis was applied to identify micelle-exposed regions of the protein and to develop an improved topology model. Our analysis strongly suggests a 16-stranded, antiparallel beta-barrel with one large and seven short loops and turns. Initial crystallization trials of the protein yielded crystals diffracting to 8 Angstrom resolution.

RAS-RAF-MEK-dependent oxidative cell death involving voltage-dependent anion channels

Therapeutics that discriminate between the genetic makeup of normal cells and tumour cells are valuable for treating and understanding cancer. Small molecules with oncogene-selective lethality may reveal novel functions of oncoproteins and enable the creation of more selective drugs. Here we describe the mechanism of action of the selective anti-tumour agent erastin, involving the RAS-RAF-MEK signalling pathway functioning in cell proliferation, differentiation and survival. Erastin exhibits greater lethality in human tumour cells harbouring mutations in the oncogenes HRAS, KRAS or BRAF. Using affinity purification and mass spectrometry, we discovered that erastin acts through mitochondrial voltage-dependent anion channels (VDACs)--a novel target for anti-cancer drugs. We show that erastin treatment of cells harbouring oncogenic RAS causes the appearance of oxidative species and subsequent death through an oxidative, non-apoptotic mechanism. RNA-interference-mediated knockdown of VDAC2 or VDAC3 caused resistance to erastin, implicating these two VDAC isoforms in the mechanism of action of erastin. Moreover, using purified mitochondria expressing a single VDAC isoform, we found that erastin alters the permeability of the outer mitochondrial membrane. Finally, using a radiolabelled analogue and a filter-binding assay, we show that erastin binds directly to VDAC2. These results demonstrate that ligands to VDAC proteins can induce non-apoptotic cell death selectively in some tumour cells harbouring activating mutations in the RAS-RAF-MEK pathway.

Determination of the conformation of the human VDAC1 N-terminal peptide, a protein moiety essential for the functional properties of the pore

Mitochondrial porin or VDAC (voltage-dependent anion-selective channel) is the most abundant protein in the mitochondrial outer membrane. The structure of VDAC has been predicted to be a transmembrane beta-barrel with an alpha-helix at the N terminus. It is a matter of debate as to whether this putative alpha-helix plays a structural role as a component of the pore walls or a function in the pore activity. We have synthesised the human VDAC1 (HVDAC1) N-terminal peptide Ac-AVPPTYADLGKSARDVFTK-NH2 (Prn2-20) and determined its structure by CD and NMR spectroscopy. CD studies show that the Prn2-20 peptide exists in aqueous solvent as an unstructured peptide with no stable secondary structure. In membrane-mimetic SDS micelles or water/trifluoroethanol, however, it assumes an amphipathic alpha-helix conformation between Tyr5 and Val16, as deduced from NMR. No ordered structure was observed in dodecyl beta-maltoside. Differential scanning calorimetric measurements were carried out in order to examine the membrane affinity of the peptide. Upon interaction with the negatively charged 1,2 dipalmitoyl-sn-glycero-3-phosphoserine membrane, Prn2-20 exhibited distinctive behaviour, suggesting that electrostatics play an important role. Interaction between the peptide and artificial bilayers indicates that the peptide lies on the membrane surface. Recombinant HVDAC1 deletion mutants, devoid of seven or 19 N-terminal amino acids, were used for transfection of eukaryotic cells. Over-expression of HVDAC1 increases the number of Cos cells with depolarised mitochondria, and this effect is progressively reduced in cells transfected with HVDAC1 lacking those seven or 19 amino acids. The mitochondrial targeting of the deletion mutants is unaffected. The overall picture emerging from our experiments is that the VDAC N-terminal peptide plays a role in the proper function of this protein during apoptotic events.

The evolutionary history of mitochondrial porins

BACKGROUND

Mitochondrial porins, or voltage-dependent anion-selective channels (VDAC) allow the passage of small molecules across the mitochondrial outer membrane, and are involved in complex interactions regulating organellar and cellular metabolism. Numerous organisms possess multiple porin isoforms, and initial studies indicated an intriguing evolutionary history for these proteins and the genes that encode them.

RESULTS

In this work, the wealth of recent sequence information was used to perform a comprehensive analysis of the evolutionary history of mitochondrial porins. Fungal porin sequences were well represented, and newly-released sequences from stramenopiles, alveolates, and seed and flowering plants were analyzed. A combination of Neighbour-Joining and Bayesian methods was used to determine phylogenetic relationships among the proteins. The aligned sequences were also used to reassess the validity of previously described eukaryotic porin motifs and to search for signature sequences characteristic of VDACs from plants, animals and fungi. Secondary structure predictions were performed on the aligned VDAC primary sequences and were used to evaluate the sites of intron insertion in a representative set of the corresponding VDAC genes.

CONCLUSION

Our phylogenetic analysis clearly shows that paralogs have appeared several times during the evolution of VDACs from the plants, metazoans, and even the fungi, suggesting that there are no "ancient" paralogs within the gene family. Sequence motifs characteristic of the members of the crown groups of organisms were identified. Secondary structure predictions suggest a common 16 beta-strand framework for the transmembrane arrangement of all porin isoforms. The GLK (and homologous or analogous motifs) and the eukaryotic porin motifs in the four representative Chordates tend to be in exons that appear to have changed little during the evolution of these metazoans. In fact there is phase correlation among the introns in these genes. Finally, our preliminary data support the notion that introns usually do not interrupt structural protein motifs, namely the predicted beta-strands. These observations concur with the concept of exon shuffling, wherein exons encode structural modules of proteins and the loss and gain of introns and the shuffling of exons via recombination events contribute to the complexity of modern day proteomes.

NMR structural investigation of the mitochondrial outer membrane protein VDAC and its interaction with antiapoptotic Bcl-xL

Bcl-2 family proteins are essential regulators of cell death and exert their primary pro- or antiapoptotic roles at the mitochondrial outer membrane. Previously, pro- and antiapoptotic Bcl-2 proteins have been shown to interact with the voltage-dependent anion channel (VDAC) of the outer mitochondrial membrane. VDAC is a 283-residue integral membrane protein that forms an aqueous pore in the outer mitochondrial membrane, through which metabolites and other small molecules pass between the cytosol and intermembrane space. The essential life-sustaining function of VDAC in metabolite trafficking is believed to be regulated by proteins of the Bcl-2 family. The protective role of antiapoptotic Bcl-xL may be through its interaction with VDAC. Here, VDAC has been expressed, purified, and refolded into a functional form amenable to NMR studies. Various biophysical experiments indicate that micelle-bound VDAC is in intermediate exchange between monomer and trimer. Using NMR spectroscopy, gel filtration, and chemical cross-linking, we obtained direct evidence for binding of Bcl-xL to VDAC in a detergent micelle system. The VDAC-interacting region of Bcl-xL was characterized by NMR with chemical shift perturbation and transferred cross-saturation. The interaction region was mapped to a putative helical hairpin motif of Bcl-xL that was found to insert into detergent micelles. Our results suggest that Bcl-xL can bind to one or two VDAC molecules forming heterodimers and heterotrimers. Our characterization of the VDAC/Bcl-xL complex offers initial structural insight into the role of antiapoptotic Bcl-xL in regulating apoptotic events in the mitochondrial outer membrane.

Correct folding of the beta-barrel of the human membrane protein VDAC requires a lipid bilayer

Spontaneous membrane insertion and folding of beta-barrel membrane proteins from an unfolded state into lipid bilayers has been shown previously only for few outer membrane proteins of Gram-negative bacteria. Here we investigated membrane insertion and folding of a human membrane protein, the isoform 1 of the voltage-dependent anion-selective channel (hVDAC1) of mitochondrial outer membranes. Two classes of transmembrane proteins with either alpha-helical or beta-barrel membrane domains are known from the solved high-resolution structures. VDAC forms a transmembrane beta-barrel with an additional N-terminal alpha-helix. We demonstrate that similar to bacterial OmpA, urea-unfolded hVDAC1 spontaneously inserts and folds into lipid bilayers upon denaturant dilution in the absence of folding assistants or energy sources like ATP. Recordings of the voltage-dependence of the single channel conductance confirmed folding of hVDAC1 to its active form. hVDAC1 developed first beta-sheet secondary structure in aqueous solution, while the alpha-helical structure was formed in the presence of lipid or detergent. In stark contrast to bacterial beta-barrel membrane proteins, hVDAC1 formed different structures in detergent micelles and phospholipid bilayers, with higher content of beta-sheet and lower content of alpha-helix when inserted and folded into lipid bilayers. Experiments with mixtures of lipid and detergent indicated that the content of beta-sheet secondary structure in hVDAC1 decreased at increased detergent content. Unlike bacterial beta-barrel membrane proteins, hVDAC1 was not stable even in mild detergents such as LDAO or dodecylmaltoside. Spontaneous folding of outer membrane proteins into lipid bilayers indicates that in cells, the main purpose of membrane-inserted or associated assembly factors may be to select and target beta-barrel membrane proteins towards the outer membrane instead of actively assembling them under consumption of energy as described for the translocons of cytoplasmic membranes.