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

Ca2+/Calmodulin-Dependent Protein Kinase II Disrupts the Voltage Dependency of the Voltage-Dependent Anion Channel on the Lipid Bilayer Membrane

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) is a key enzyme that plays a significant role in intracellular signaling and the modulation of mitochondrial membrane properties. It is known that the voltage-dependent anion channel (VDAC) is one of the most abundant outer mitochondrial membrane (OMM) proteins acting as a significant passageway and regulatory site for various enzymes, proteins, ions, and metabolites. Considering this, we hypothesize that VDAC could be one of the targets for CaMKII enzymatic activity. Our in vitro experiments indicate that VDAC can be phosphorylated by the CaMKII enzyme. Moreover, the bilayer electrophysiology experimental data indicate that CaMKII significantly reduces VDAC's single-channel conductivity; its open probability remains high at all the applied potentials between +60 and -60 mV, and the voltage dependency was lost, which suggests that CaMKII disrupted the VDAC's single-channel activities. Hence, we can infer that VDAC interacts with CaMKII and thus acts as a vital target for its activity. Furthermore, our findings suggest that CaMKII could play a significant role during the transport of ions and metabolites across the outer mitochondrial membrane (OMM) through VDAC and thus regulate apoptotic events.

Inhibition of mitochondrial VDAC1 oligomerization alleviates apoptosis and necroptosis of retinal neurons following OGD/R injury

Ischemia-reperfusion (I/R) injury is a common pathological mechanism in many retinal diseases, which can lead to cell death via mitochondrial dysfunction. Voltage-dependent anion channel 1 (VDAC1), which is mainly located in the outer mitochondrial membrane, is the gatekeeper of mitochondria. The permeability of mitochondrial membrane can be regulated by controlling the oligomerization of VDAC1. However, the functional mechanism of VDAC1 in retinal I/R injury was unclear. Our results demonstrate that oxygen-glucose deprivation and re-oxygenation (OGD/R) injury leads to apoptosis, necroptosis, and mitochondrial dysfunction of R28 cells. The OGD/R injury increases the levels of VDAC1 oligomerization. Inhibition of VDAC1 oligomerization by VBIT-12 rescued mitochondrial dysfunction by OGD/R and also reduced apoptosis/necroptosis of R28 cells. In vivo, the use of VBIT-12 significantly reduced aHIOP-induced neuronal death (apoptosis/necroptosis) in the rat retina. Our findings indicate that VDAC1 oligomers may open and enlarge mitochondrial membrane pores during OGD/R injury, leading to the release of death-related factors in mitochondria, resulting in apoptosis and necroptosis. This study provides a potential therapeutic strategy against ocular diseases caused by I/R injury.

Mapping the N-Terminal Hexokinase-I Binding Site onto Voltage-Dependent Anion Channel-1 To Block Peripheral Nerve Demyelination

The voltage-dependent anion channel (VDAC), the most abundant protein on the outer mitochondrial membrane, is implicated in ATP, ion and metabolite exchange with cell compartments. In particular, the VDAC participates in cytoplasmic and mitochondrial Ca²⁺ homeostasis. Notably, the Ca²⁺ efflux out of Schwann cell mitochondria is involved in peripheral nerve demyelination that underlies most peripheral neuropathies. Hexokinase (HK) isoforms I and II, the main ligands of the VDAC, possess a hydrophobic N-terminal structured in α-helix (NHKI) that is necessary for the binding to the VDAC. To gain further insight into the molecular basis of HK binding to the VDAC, we developed and optimized peptides based on the NHKI sequence. These modifications lead to an increase of the peptide hydrophobicity and helical content that enhanced their ability to prevent peripheral nerve demyelination. Our results provide new insights into the molecular basis of VDAC/HK interaction that could lead to the development of therapeutic compounds for demyelinating peripheral neuropathies.

Historical Perspective of Pore-Forming Activity Studies of Voltage-Dependent Anion Channel (Eukaryotic or Mitochondrial Porin) Since Its Discovery in the 70th of the Last Century

Eukaryotic porin, also known as Voltage-Dependent Anion Channel (VDAC), is the most frequent protein in the outer membrane of mitochondria that are responsible for cellular respiration. Mitochondria are most likely descendants of strictly aerobic Gram-negative bacteria from the α-proteobacterial lineage. In accordance with the presumed ancestor, mitochondria are surrounded by two membranes. The mitochondrial outer membrane contains besides the eukaryotic porins responsible for its major permeability properties a variety of other not fully identified channels. It encloses also the TOM apparatus together with the sorting mechanism SAM, responsible for the uptake and assembly of many mitochondrial proteins that are encoded in the nucleus and synthesized in the cytoplasm at free ribosomes. The recognition and the study of electrophysiological properties of eukaryotic porin or VDAC started in the late seventies of the last century by a study of Schein et al., who reconstituted the pore from crude extracts of Paramecium mitochondria into planar lipid bilayer membranes. Whereas the literature about structure and function of eukaryotic porins was comparatively rare during the first 10years after the first study, the number of publications started to explode with the first sequencing of human Porin 31HL and the recognition of the important function of eukaryotic porins in mitochondrial metabolism. Many genomes contain more than one gene coding for homologs of eukaryotic porins. More than 100 sequences of eukaryotic porins are known to date. Although the sequence identity between them is relatively low, the polypeptide length and in particular, the electrophysiological characteristics are highly preserved. This means that all eukaryotic porins studied to date are anion selective in the open state. They are voltage-dependent and switch into cation-selective substates at voltages in the physiological relevant range. A major breakthrough was also the elucidation of the 3D structure of the eukaryotic pore, which is formed by 19 β-strands similar to those of bacterial porin channels. The function of the presumed gate an α-helical stretch of 20 amino acids allowed further studies with respect to voltage dependence and function, but its exact role in channel gating is still not fully understood.

VDAC Genes Expression and Regulation in Mammals

VDACs are pore-forming proteins, coating the mitochondrial outer membrane, and playing the role of main regulators for metabolites exchange between cytosol and mitochondria. In mammals, three isoforms have evolutionary originated, VDAC1, VDAC2, and VDAC3. Despite similarity in sequence and structure, evidence suggests different biological roles in normal and pathological conditions for each isoform. We compared Homo sapiens and Mus musculus VDAC genes and their regulatory elements. RNA-seq transcriptome analysis shows that VDAC isoforms are expressed in human and mouse tissues at different levels with a predominance of VDAC1 and VDAC2 over VDAC3, with the exception of reproductive system. Numerous transcript variants for each isoform suggest specific context-dependent regulatory mechanisms. Analysis of VDAC core promoters has highlighted that, both in a human and a mouse, VDAC genes show features of TATA-less ones. The level of CG methylation of the human VDAC genes revealed that VDAC1 promoter is less methylated than other two isoforms. We found that expression of VDAC genes is mainly regulated by transcription factors involved in controlling cell growth, proliferation and differentiation, apoptosis, and bioenergetic metabolism. A non-canonical initiation site termed "the TCT/TOP motif," the target for translation regulation by the mTOR pathway, was identified in human VDAC2 and VDAC3 and in every murine VDACs promoter. In addition, specific TFBSs have been identified in each VDAC promoter, supporting the hypothesis that there is a partial functional divergence. These data corroborate our experimental results and reinforce the idea that gene regulation could be the key to understanding the evolutionary specialization of VDAC isoforms.

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.

Structure, gating and interactions of the voltage-dependent anion channel

The voltage-dependent anion channel (VDAC) is one of the most highly abundant proteins found in the outer mitochondrial membrane, and was one of the earliest discovered. Here we review progress in understanding VDAC function with a focus on its structure, discussing various models proposed for voltage gating as well as potential drug targets to modulate the channel’s function. In addition, we explore the sensitivity of VDAC structure to variations in the membrane environment, comparing DMPC-only, DMPC with cholesterol, and near-native lipid compositions, and use magic-angle spinning NMR spectroscopy to locate cholesterol on the outside of the β-barrel. We find that the VDAC protein structure remains unchanged in different membrane compositions, including conditions with cholesterol.

The intrinsically disordered N-terminus of the voltage-dependent anion channel

The voltage-dependent anion channel (VDAC) is a critical β-barrel membrane protein of the mitochondrial outer membrane, which regulates the transport of ions and ATP between mitochondria and the cytoplasm. In addition, VDAC plays a central role in the control of apoptosis and is therefore of great interest in both cancer and neurodegenerative diseases. Although not fully understood, it is presumed that the gating mechanism of VDAC is governed by its N-terminal region which, in the open state of the channel, exhibits an α-helical structure positioned midway inside the pore and strongly interacting with the β-barrel wall. In the present work, we performed molecular simulations with a recently developed force field for disordered systems to shed new light on known experimental results, showing that the N-terminus of VDAC is an intrinsically disordered region (IDR). First, simulation of the N-terminal segment as a free peptide highlighted its disordered nature and the importance of using an IDR-specific force field to properly sample its conformational landscape. Secondly, accelerated dynamics simulation of a double cysteine VDAC mutant under applied voltage revealed metastable low conducting states of the channel representative of closed states observed experimentally. Related structures were characterized by partial unfolding and rearrangement of the N-terminal tail, that led to steric hindrance of the pore. Our results indicate that the disordered properties of the N-terminus are crucial to properly account for the gating mechanism of VDAC.

The voltage-dependent anion channel (VDAC) is a membrane protein playing a pivotal role in the transport of ions or ATP across the mitochondrial outer membrane as well as in the induction of apoptosis. At high enough membrane potential, VDAC is known to transition from an open state to multiple closed states, reducing the flow of ions through the channel and blocking the passage of large metabolites. While the structure of the open state was resolved more than a decade ago, a molecular description of the gating mechanism of the channel is still missing. Here we show that the N-terminus of VDAC is an intrinsically disordered region and that such a property has a profound impact on its dynamics either as a free peptide or as part of the channel. By taking disordered properties of the N-terminus into account, we managed to generate long-lived closed conformations of the channel at experimental values of the membrane potential. Our results provide new insights into the molecular mechanism driving the gating of VDAC.

Evidences of a Direct Relationship between Cellular Fuel Supply and Ciliogenesis Regulated by Hypoxic VDAC1-ΔC

Metabolic flexibility is the ability of a cell to adapt its metabolism to changes in its surrounding environment. Such adaptability, combined with apoptosis resistance provides cancer cells with a survival advantage. Mitochondrial voltage-dependent anion channel 1 (VDAC1) has been defined as a metabolic checkpoint at the crossroad of these two processes. Here, we show that the hypoxia-induced cleaved form of VDAC1 (VDAC1-ΔC) is implicated in both the up-regulation of glycolysis and the mitochondrial respiration. We demonstrate that VDAC1-ΔC, due to the loss of the putative phosphorylation site at serine 215, concomitantly with the loss of interaction with tubulin and microtubules, reprograms the cell to utilize more metabolites, favoring cell growth in hypoxic microenvironment. We further found that VDAC1-ΔC represses ciliogenesis and thus participates in ciliopathy, a group of genetic disorders involving dysfunctional primary cilium. Cancer, although not representing a ciliopathy, is tightly linked to cilia. Moreover, we highlight, for the first time, a direct relationship between the cilium and cancer cell metabolism. Our study provides the first new comprehensive molecular-level model centered on VDAC1-ΔC integrating metabolic flexibility, ciliogenesis, and enhanced survival in a hypoxic microenvironment.

Cysteine Oxidations in Mitochondrial Membrane Proteins: The Case of VDAC Isoforms in Mammals

Cysteine residues are reactive amino acids that can undergo several modifications driven by redox reagents. Mitochondria are the source of an abundant production of radical species, and it is surprising that such a large availability of highly reactive chemicals is compatible with viable and active organelles, needed for the cell functions. In this work, we review the results highlighting the modifications of cysteines in the most abundant proteins of the outer mitochondrial membrane (OMM), that is, the voltage-dependent anion selective channel (VDAC) isoforms. This interesting protein family carries several cysteines exposed to the oxidative intermembrane space (IMS). Through mass spectrometry (MS) analysis, cysteine posttranslational modifications (PTMs) were precisely determined, and it was discovered that such cysteines can be subject to several oxidization degrees, ranging from the disulfide bridge to the most oxidized, the sulfonic acid, one. The large spectra of VDAC cysteine oxidations, which is unique for OMM proteins, indicate that they have both a regulative function and a buffering capacity able to counteract excess of mitochondrial reactive oxygen species (ROS) load. The consequence of these peculiar cysteine PTMs is discussed.

Identification of a new aggressive axis driven by ciliogenesis and absence of VDAC1-ΔC in clear cell Renal Cell Carcinoma patients

Rationale: Renal cell carcinoma (RCC) accounts for about 2% of all adult cancers, and clear cell RCC (ccRCC) is the most common RCC histologic subtype. A hallmark of ccRCC is the loss of the primary cilium, a cellular antenna that senses a wide variety of signals. Loss of this key organelle in ccRCC is associated with the loss of the von Hippel-Lindau protein (VHL). However, not all mechanisms of ciliopathy have been clearly elucidated.

Methods: By using RCC4 renal cancer cells and patient samples, we examined the regulation of ciliogenesis via the presence or absence of the hypoxic form of the voltage-dependent anion channel (VDAC1-ΔC) and its impact on tumor aggressiveness. Three independent cohorts were analyzed. Cohort A was from PREDIR and included 12 patients with hereditary pVHL mutations and 22 sporadic patients presenting tumors with wild-type pVHL or mutated pVHL; Cohort B included tissue samples from 43 patients with non-metastatic ccRCC who had undergone surgery; and Cohort C was composed of 375 non-metastatic ccRCC tumor samples from The Cancer Genome Atlas (TCGA) and was used for validation. The presence of VDAC1-ΔC and legumain was determined by immunoblot. Transcriptional regulation of IFT20/GLI1 expression was evaluated by qPCR. Ciliogenesis was detected using both mouse anti-acetylated α-tubulin and rabbit polyclonal ARL13B antibodies for immunofluorescence.

Results: Our study defines, for the first time, a group of ccRCC patients in which the hypoxia-cleaved form of VDAC1 (VDAC1-ΔC) induces resorption of the primary cilium in a Hypoxia-Inducible Factor-1 (HIF-1)-dependent manner. An additional novel group, in which the primary cilium is re-expressed or maintained, lacked VDAC1-ΔC yet maintained glycolysis, a signature of epithelial-mesenchymal transition (EMT) and more aggressive tumor progression, but was independent to VHL. Moreover, these patients were less sensitive to sunitinib, the first-line treatment for ccRCC, but were potentially suitable for immunotherapy, as indicated by the immunophenoscore and the presence of PDL1 expression.

Conclusion: This study provides a new way to classify ccRCC patients and proposes potential therapeutic targets linked to metabolism and immunotherapy.

Hypoxic-induced truncation of voltage-dependent anion channel 1 is mediated by both asparagine endopeptidase and calpain 1 activities

The voltage-dependent anion channel 1 (VDAC1), an outer mitochondria membrane (OMM) protein, serves as a mitochondrial gatekeeper, mediating the transport of nucleotides, Ca²⁺ and other metabolites across the OMM. VDAC1 also plays a central role in mitochondria-mediated apoptosis by facilitating the release of apoptotic proteins and by association with both pro- and anti-apoptotic proteins. Tumor cells, which are constantly exposed to hypoxic conditions, affect the cell via the transcription factor hypoxia-inducible factor (HIF) that induces transcriptional activity. In cultured cells and in lung cancer patients, hypoxia induces VDAC1 truncation at the C-terminus (VDAC1-ΔC). However, the molecular mechanisms involved in VDAC1-ΔC formation are unknown. Here, we show that hypoxia-induced VDAC1-ΔC formation is inhibited by the Ca²⁺ chelator BAPTA-AM, by calpain inhibitor-1, by inhibitor of the asparagine endopeptidase (AEP) and by si-RNA targeting HIF1-α or Ca²⁺-activated protease calpain-1 expression but not that of calpain-2. Finally, VDAC1-ΔC expressed in bacteria and reconstituted into a planar lipid bilayer exhibited decreased channel conductance relative to the full-length protein, yet retained voltage-dependent conductance. These findings suggest that hypoxia, acting via HIF-1α expression, leads to VDAC1 cleavage involving the activation of calpain 1 and AEP.

Picroside II Exerts a Neuroprotective Effect by Inhibiting mPTP Permeability and EndoG Release after Cerebral Ischemia/Reperfusion Injury in Rats

Mitochondrial membrane permeability is closely related to cerebral ischemia/reperfusion (I/R) injury. This paper explored the neuroprotective effect of picroside II (Picr), which inhibits the permeability of mitochondrial permeability transition pore (mPTP) and endonuclease G (EndoG) release from mitochondria into cytoplasm after cerebral I/R in rats. After 2 h of cerebral ischemia and 24 h of reperfusion in rats with different intervention measures, the neurobehavioral function, infarction volume, and reactive oxygen species (ROS) content in brain tissues were observed by modified neurological severity scale (mNSS), triphenyl tetrazolium chloride (TTC) staining, and enzyme-linked immunosorbent assay, respectively. The permeability of mPTP was assayed using spectrophotometry. The morphology and apoptotic cells of brain tissues were observed by hematoxylin-eosin staining and terminal deoxynucleotidyl transferase dUTP nick end labeling assay, respectively. The expressions of EndoG and voltage-dependent anion channel 1 (VDAC1) were determined by immunohistochemical assay and western blot. The Picr group exhibited clear decreases in mNSS scores, ROS content, number of apoptotic cells, mPTP permeability and expression of VDAC1, and EndoG in cytoplasm and nuclei, and the morphology of brain tissue was improved as compared with the model group (P < 0.05). Picr could attenuate cerebral I/R injury by downregulating the expression of VDAC1 and decreasing the permeability of mPTP, thereby inhibiting EndoG release from mitochondria into cytoplasm.

Two mutations in mitochondrial ATP6 gene of ATP synthase, related to human cancer, affect ROS, calcium homeostasis and mitochondrial permeability transition in yeast

The relevance of mitochondrial DNA (mtDNA) mutations in cancer process is still unknown. Since the mutagenesis of mitochondrial genome in mammals is not possible yet, we have exploited budding yeast S. cerevisiae as a model to study the effects of tumor-associated mutations in the mitochondrial MTATP6 gene, encoding subunit 6 of ATP synthase, on the energy metabolism. We previously reported that four mutations in this gene have a limited impact on the production of cellular energy. Here we show that two mutations, Atp6-P163S and Atp6-K90E (human MTATP6-P136S and MTATP6-K64E, found in prostate and thyroid cancer samples, respectively), increase sensitivity of yeast cells both to compounds inducing oxidative stress and to high concentrations of calcium ions in the medium, when Om45p, the component of porin complex in outer mitochondrial membrane (OM), was fused to GFP. In OM45-GFP background, these mutations affect the activation of yeast permeability transition pore (yPTP, also called YMUC, yeast mitochondrial unspecific channel) upon calcium induction. Moreover, we show that calcium addition to isolated mitochondria heavily induced the formation of ATP synthase dimers and oligomers, recently proposed to form the core of PTP, which was slower in the mutants. We show the genetic evidence for involvement of mitochondrial ATP synthase in calcium homeostasis and permeability transition in yeast. This paper is a first to show, although in yeast model organism, that mitochondrial ATP synthase mutations, which accumulate during carcinogenesis process, may be significant for cancer cell escape from apoptosis.

Functional characterization of an N-terminally-truncated mitochondrial porin expressed in Neurospora crassa

Mitochondrial porin, which forms voltage-dependent anion-selective channels (VDAC) in the outer membrane, can be folded into a 19-β-stranded barrel. The N terminus of the protein is external to the barrel and contains α-helical structure. Targeted modifications of the N-terminal region have been assessed in artificial membranes, leading to different models for gating in vitro. However, the in vivo requirements for gating and the N-terminal segment of porin are less well-understood. Using Neurospora crassa porin as a model, the effects of a partial deletion of the N-terminal segment were investigated. The protein, ΔN2-12porin, is assembled into the outer membrane, albeit at lower levels than the wild-type protein. The resulting strain displays electron transport chain deficiencies, concomitant expression of alternative oxidase, and decreased growth rates. Nonetheless, its mitochondrial genome does not contain any significant mutations. Most of the genes that are expressed in high levels in porin-less N. crassa are expressed at levels similar to that of wild type or are slightly increased in ΔN2-12porin strains. Thus, although the N-terminal segment of VDAC is required for complete function in vivo, low levels of a protein lacking part of the N terminus are able to rescue some of the defects associated with the absence of porin.

VDAC in cancer

The voltage-dependent anion channel (VDAC) is a pore located at the outer membrane of the mitochondrion. It allows the entry and exit of numerous ions and metabolites between the cytosol and the mitochondrion. Flux through the pore occurs in an active way: first, it depends on the open or closed state and second, on the negative or positive charges of the different ion species passing through the pore. The flux of essential metabolites, such as ATP, determines the functioning of the mitochondria to a noxious stimulus. Moreover, VDAC acts as a platform for many proteins and in so doing supports glycolysis and prevents apoptosis by interacting with hexokinase, or members of the Bcl-2 family, respectively. VDAC is thus involved in the choice the cells make to survive or die, which is particularly relevant to cancer cells. For these reasons, VDAC has become a potential therapeutic target to fight cancer but also other diseases in which mitochondrial metabolism is modified. This article is part of a Special Issue entitled Mitochondria in Cancer, edited by Giuseppe Gasparre, Rodrigue Rossignol and Pierre Sonveaux.

VDAC3 As a Potential Marker of Mitochondrial Status Is Involved in Cancer and Pathology

VDAC3 is the least known isoform of the mammalian voltage-dependent anion selective channels of the outer mitochondrial membrane. It has been recently shown that cysteine residues of VDAC3 are found over-oxidized. The VDAC3 cysteine over-oxidation was associated with the oxidizing environment and the abundance of reactive oxygen species (ROS) in the intermembrane space. In this work, we have examined the role of VDAC3 in general pathogenic mechanisms at the basis of mitochondrial dysfunction and involving the mitochondrial quality control. Many of the diseases reported here, including cancer and viral infections, are often associated with significant changes in the intracellular redox state. In this sense, VDAC3 bearing oxidative modifications could become marker of the oxidative load in the mitochondria and part of the ROS signaling pathway.

News about VDAC1 in Hypoxia

The voltage-dependent anion channel (VDAC) is the main interface between the cytosol and mitochondria of cells. It plays a crucial role in both mitochondrial metabolism and cell death. The main basic function of this channel is to mediate and gate the flux of small ions, metabolites, and adenosine triphosphate. Changes in its structure, and thus conformation, are expected to affect its activity and modulate the ability of cancer cells to expand. In this review, we describe a novel mechanism by which mitochondria of cells in hypoxia, a low level of oxygen, protects from apoptosis. In hypoxia, some mitochondria become enlarged due to hyperfusion. These mitochondria possess a truncated form of VDAC1 (VDAC1-ΔC), which is linked to the higher metabolic capacity and the greater resistance to cell death of hypoxic cells. However, not all of the VDAC1 protein is truncated, but the amount of the full-length form is diminished compared to the amount in normoxic cells. First, we describe how such a decrease effects cell proliferation, respiration, glycolysis, and other processes. Second, we report on a novel mitochondrial-endolysosomal crosstalk that leads to VDAC1 truncation. By pharmacological targeting of VDAC1-ΔC, the production of energy could be turned off and the sensitivity to cell death restored. This could counteract the favorable microenvironment that gives cancer cells a growth advantage and thereby disrupts the balance between life and death, which is controlled by VDAC1.

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.

N-helix and Cysteines Inter-regulate Human Mitochondrial VDAC-2 Function and Biochemistry

Human voltage-dependent anion channel-2 (hVDAC-2) functions primarily as the crucial anti-apoptotic protein in the outer mitochondrial membrane, and additionally as a gated bidirectional metabolite transporter. The N-terminal helix (NTH), involved in voltage sensing, bears an additional 11-residue extension (NTE) only in hVDAC-2. In this study, we assign a unique role for the NTE as influencing the chaperone-independent refolding kinetics and overall thermodynamic stability of hVDAC-2. Our electrophysiology data shows that the N-helix is crucial for channel activity, whereas NTE sensitizes this isoform to voltage gating. Additionally, hVDAC-2 possesses the highest cysteine content, possibly for regulating reactive oxygen species content. We identify interdependent contributions of the N-helix and cysteines to channel function, and the measured stability in micellar environments with differing physicochemical properties. The evolutionary demand for the NTE in the presence of cysteines clearly emerges from our biochemical and functional studies, providing insight into factors that functionally demarcate hVDAC-2 from the other VDACs.

Solid-state NMR, electrophysiology and molecular dynamics characterization of human VDAC2

The voltage-dependent anion channel (VDAC) is the most abundant protein of the outer mitochondrial membrane and constitutes the major pathway for the transport of ADP, ATP, and other metabolites. In this multidisciplinary study we combined solid-state NMR, electrophysiology, and molecular dynamics simulations, to study the structure of the human VDAC isoform 2 in a lipid bilayer environment. We find that the structure of hVDAC2 is similar to the structure of hVDAC1, in line with recent investigations on zfVDAC2. However, hVDAC2 appears to exhibit an increased conformational heterogeneity compared to hVDAC1 which is reflected in broader solid-state NMR spectra and less defined electrophysiological profiles.

Glucose-6-phosphate tips the balance in modulating apoptosis in cerebellar granule cells

A metabolic shift from oxidative phosphorylation to glycolysis (i.e. the Warburg effect) occurs in Alzheimer's disease accompanied by an increase of both activity and level of HK-I. The findings reported here demonstrate that in the early phase of apoptosis VDAC1 activity, but not its protein level, progressively decreases, in concomitance with the physical interaction of HK-I with VDAC1. In the late phase of apoptosis, glucose-6-phosphate accumulation in the cell causes the dissociation of the two proteins, the re-opening of the channel and the recovery of VDAC1 function, resulting in a reawakening of the mitochondrial function, thus inevitably leading to cell death.

Long-term oral resveratrol intake provides nutritional preconditioning against myocardial ischemia/reperfusion injury: involvement of VDAC1 downregulation

SCOPE

This study elucidates the effects of long-term nutritional preconditioning by resveratrol on ischemia/reperfusion (I/R) injury and its underlying mechanisms.

METHODS AND RESULTS

Mice were treated with resveratrol at 2.0 mg/kg/day by gastric gavages for 6 wk. Then hearts were isolated and subjected to I/R injury in a Langendorff apparatus. Resveratrol significantly improved left ventricular pressure, ±dp/dtmax, and coronary flow; decreased the lactate dehydrogenase and creatine phosphokinase activities; and reduced the infarction size. Additionally, long-term oral resveratrol intake prevented mitochondrial permeability transition pore opening and subsequently inhibited mitochondria-mediated apoptosis, as demonstrated by decrease of cytochrome c release, inactivation of caspase-3, and reduction of terminal deoxynucleotidyl transferase mediated nick end labeling positive cells. Furthermore, resveratrol inhibited the upregulation of voltage-dependent anion channel 1 (VDAC1) expression induced by I/R injury. Local left-ventricle overexpression of VDAC1 by adenovirus diminished the protective effect of resveratrol against I/R injury, indicating that VDAC1 plays an important role in resveratrol-mediated cardioprotection.

CONCLUSION

Our data revealed that long-term oral intake of resveratrol sets nutritional preconditioning to cope with myocardial I/R injury. Strikingly, we found that resveratrol downregulates VDAC1, leading to prevention of mitochondrial permeability transition pore opening and cardiomyocyte apoptosis.

Magic angle spinning nuclear magnetic resonance characterization of voltage-dependent anion channel gating in two-dimensional lipid crystalline bilayers

The N-terminus of the voltage-dependent anion channel (VDAC) has been proposed to contain the mechanistically important gating helices that modulate channel opening and closing. In this study, we utilize magic angle spinning nuclear magnetic resonance (MAS NMR) to determine the location and structure of the N-terminus for functional channels in lipid bilayers by measuring long-range ¹³C–¹³C distances between residues in the N-terminus and other domains of VDAC reconstituted into DMPC lipid bilayers. Our structural studies show that the distance between A14 C_β in the N-terminal helix and S193 C_β is ∼4–6 Å. Furthermore, VDAC phosphorylation by a mitochondrial kinase at residue S193 has been claimed to delay mitochondrial cell death by causing a conformational change that closes the channel, and a VDAC-Ser193Glu mutant has been reported to show properties very similar to those of phosphorylated VDAC in a cellular context. We expressed VDAC-S193E and reconstituted it into DMPC lipid bilayers. Two-dimensional ¹³C–¹³C correlation experiments showed chemical shift perturbations for residues located in the N-terminus, indicating possible structural perturbations to that region. However, electrophysiological data recorded on VDAC-S193E showed that channel characteristics were identical to those of wild type samples, indicating that phosphorylation of S193 does not directly affect channel gating. The combination of NMR and electrophysiological results allows us to discuss the validity of proposed gating models.

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.

Charged residues distribution modulates selectivity of the open state of human isoforms of the voltage dependent anion-selective channel

Voltage Dependent Anion-selective Channels (VDACs) are pore-forming proteins located in the outer mitochondrial membrane. They are responsible for the access of ions and energetic metabolites into the inner membrane transport systems. Three VDAC isoforms exist in mammalian, but their specific role is unknown. In this work we have performed extensive (overall ∼5 µs) Molecular Dynamics (MD) simulations of the human VDAC isoforms to detect structural and conformational variations among them, possibly related to specific functional roles of these proteins. Secondary structure analysis of the N-terminal domain shows a high similarity among the three human isoforms of VDAC but with a different plasticity. In particular, the N-terminal domain of the hVDAC1 is characterized by a higher plasticity, with a ∼20% occurrence for the 'unstructured' conformation throughout the folded segment, while hVDAC2, containing a peculiar extension of 11 amino acids at the N-terminal end, presents an additional 310-helical folded portion comprising residues 10' to 3, adhering to the barrel wall. The N-terminal sequences of hVDAC isoforms are predicted to have a low flexibility, with possible consequences in the dynamics of the human VDACs. Clear differences were found between hVDAC1 and hVDAC3 against hVDAC2: a significantly modified dynamics with possible important consequence on the voltage-gating mechanism. Charge distribution inside and at the mouth of the pore is responsible for a different preferential localization of ions with opposite charge and provide a valuable rationale for hVDAC1 and hVDAC3 having a Cl-/K+ selectivity ratio of 1.8, whereas hVDAC2 of 1.4. Our conclusion is that hVDAC isoforms, despite sharing a similar scaffold, have modified working features and a biological work is now requested to give evidence to the described dissimilarities.

Characterization of functional reprogramming during osteoclast development using quantitative proteomics and mRNA profiling

In addition to forming macrophages and dendritic cells, monocytes in adult peripheral blood retain the ability to develop into osteoclasts, mature bone-resorbing cells. The extensive morphological and functional transformations that occur during osteoclast differentiation require substantial reprogramming of gene and protein expression. Here we employ -omic-scale technologies to examine in detail the molecular changes at discrete developmental stages in this process (precursor cells, intermediate osteoclasts, and multinuclear osteoclasts), quantitatively comparing their transcriptomes and proteomes. The data have been deposited to the ProteomeXchange with identifier PXD000471.

Our analysis identified mitochondrial changes, along with several alterations in signaling pathways, as central to the development of mature osteoclasts, while also confirming changes in pathways previously implicated in osteoclast biology. In particular, changes in the expression of proteins involved in metabolism and redirection of energy flow from basic cellular function toward bone resorption appeared to play a key role in the switch from monocytic immune system function to specialized bone-turnover function. These findings provide new insight into the differentiation program involved in the generation of functional osteoclasts.

Amyloid beta-induced glycogen synthase kinase 3β phosphorylated VDAC1 in Alzheimer's disease: implications for synaptic dysfunction and neuronal damage

Glycogen synthase kinase 3 (GSK3) is a serine/threonine protein kinase that is involved in the multiple signaling processes of a cell. Increasing evidence suggests that GSK3β plays a key role in multiple cellular processes in the progression of diabetes, obesity, Alzheimer's disease (AD), Parkinson's disease (PD), inflammatory diseases, schizophrenia, bipolar and several mood disorders, and mitochondrial diseases. Recent research has found that increased GSK3β activity is linked to the pathogenesis of AD through amyloid beta (Aβ), phosphorylated tau and mitochondrial dysfunction. Recent research has also revealed that GSK3β is elevated in AD-affected tissues and is critically involved in dissociating the voltage-dependent anion channel 1 (VDAC1) protein from hexokinases, and causing disrupted glucose metabolism, mitochondrial dysfunction and activating apoptotic cell death. The purpose of this article is to review recent research that is elucidating the role of GSK3β in AD pathogenesis. We discuss the involvement of GSK3β in the phosphorylation of VDAC1 and dissociation of VADC1 with hexokinases in AD neurons.

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.

VDAC1: from structure to cancer therapy

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

β-Barrel mobility underlies closure of the voltage-dependent anion channel

The voltage-dependent anion channel (VDAC) is the major protein in the outer mitochondrial membrane, where it mediates transport of ATP and ADP. Changes in its permeability, induced by voltage or apoptosis-related proteins, have been implicated in apoptotic pathways. The three-dimensional structure of VDAC has recently been determined as a 19-stranded β-barrel with an in-lying N-terminal helix. However, its gating mechanism is still unclear. Using solid-state NMR spectroscopy, molecular dynamics simulations, and electrophysiology, we show that deletion of the rigid N-terminal helix sharply increases overall motion in VDAC's β-barrel, resulting in elliptic, semicollapsed barrel shapes. These states quantitatively reproduce conductance and selectivity of the closed VDAC conformation. Mutation of the N-terminal helix leads to a phenotype intermediate to the open and closed states. These data suggest that the N-terminal helix controls entry into elliptic β-barrel states which underlie VDAC closure. Our results also indicate that β-barrel channels are intrinsically flexible.

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.

Adseverin: a novel cisplatin-resistant marker in the human bladder cancer cell line HT1376 identified by quantitative proteomic analysis

Cisplatin is currently the most effective antitumor agent available against bladder cancer. However, a majority of patients eventually relapse with cisplatin-resistant disease. Chemoresistance thus remains a major obstacle in bladder cancer therapy. To clarify the molecular mechanisms underlying cisplatin resistance in bladder cancer, we established a cisplatin-resistant subline from the human bladder cancer cell line HT1376 (HT1376-CisR), and conducted large-scale analyses of the expressed proteins using two-dimensional (2D) gel electrophoresis coupled with mass spectrometry (MS). Comparative proteomic analysis of HT1376 and HT1376-CisR cells revealed 36 differentially expressed proteins, wherein 21 proteins were upregulated and 15 were downregulated in HT1376-CisR cells. Among the differentially regulated proteins, adseverin (SCIN), a calcium-dependent actin-binding protein, was overexpressed (4-fold upregulation) in HT1376-CisR, with the increase being more prominent in the mitochondrial fraction than in the cytosol fraction. SCIN mRNA knockdown significantly reduced cell proliferation with mitochondria-mediated apoptosis in HT1376-CisR cells. Immunoprecipitation analysis revealed voltage-dependent anion channels (VDACs) to be bound to SCIN in the mitochondrial fraction. Our results suggest that the VDAC-SCIN interaction may inhibit mitochondria-mediated apoptosis in cisplatin-resistant cells. Targeting the VDAC-SCIN interaction may offer a new therapeutic strategy for cisplatin-resistant bladder cancer.

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.

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.

Hydrophobicity drives the cellular uptake of short cationic peptide ligands

Short cationic linear peptide analogs (LPAs, prepared as Arg-C( n )-Arg-C( n )-Lys, where C( n ) represents an alkyl linkage with n = 4, 7 or 11) were synthesized and tested in human breast carcinoma BT-20 and CCRF-CEM leukemia cells for their application as targeting ligands. With constant LPA charge (+4), increasing the alkyl linkage increases the hydrophobic/hydrophilic balance and provides a systematic means of examining combined electrostatic and hydrophobic peptide-membrane interactions. Fluorescently conjugated LPA-C(11) (F-LPA-C(11)) demonstrated significant uptake, whereas there was negligible uptake of the shorter LPAs. By varying temperature (4°C and 37°C) and cell type, the results suggest that LPA-C(11) internalization is nonendocytic and nonspecific. The effect of LPA binding on the phase behavior, structure, and permeability of model membranes composed of dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylserine (DPPC/DPPS, 85/15) was studied using differential scanning calorimetry (DSC), cryogenic transmission electron microscopy (cryo-TEM), and fluorescence leakage studies to gain insight into the LPA uptake mechanism. While all LPAs led to phase separation, LPA-C(11), possessing the longest alkyl linkage, was able to penetrate into the bilayer and caused holes to form, which led to membrane disintegration. This was confirmed by rapid and complete dye release by LPA-C(11). We propose that LPA-C(11) achieves uptake by anchoring to the membrane via hydrophobicity and forming transient membrane voids. LPAs may be advantageous as drug transporter ligands because they are small, water soluble, and easy to prepare.

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.

Characterization of human VDAC isoforms: a peculiar function for VDAC3?

VDACs are a family of pore-forming proteins mainly located in the mitochondrial outer membrane. In mammals three isoforms exist. In this work we review the information available about them with the addition of new results. We have compared the human VDACs transformed in a yeast strain lacking the endogenous porin. VDAC1 and 2 are able to complement the lack of porin in mitochondrial respiration and modulation of ROS. VDAC3 has a limited ability to support the mitochondrial respiration and has no influence in the control of ROS production. The over-expression of VDAC isoforms in wild type yeast strain led to a dramatic sensitivity to oxidative stress, especially for VDAC3, and a shorter lifespan in respiratory conditions. Real-time PCR comparison of the isoforms indicated that in HeLa cells VDAC1 is 10 times more abundant than VDAC2 and 100 times than VDAC3. The over-expression of any single isoform caused a 10 times increase of the transcripts of VDAC2 and VDAC3, while VDAC1 is not changed by the over-expression of the other isoforms. Models of VDAC2 and VDAC3 isoform structure showed that they could be made of a 19-strand beta-barrel and an N-terminal sequence with variable features. In this work we show for the first time a functional characterization of VDAC3 in a cellular context.

A tag at the carboxy terminus prevents membrane integration of VDAC1 in mammalian mitochondria

beta-Barrel proteins are found in the outer membranes of bacteria, chloroplasts and mitochondria. The evolutionary conserved sorting and assembly machinery (SAM complex) assembles mitochondrial beta-barrel proteins, such as voltage-dependent anion-selective channel 1 (VDAC1), into complexes in the outer membrane by recognizing a sorting beta-signal in the carboxy-terminal part of the protein. Here we show that in mammalian mitochondria, masking of the C-terminus of beta-barrel proteins by a tag leads to accumulation of soluble misassembled protein in the intermembrane space, which causes mitochondrial fragmentation and loss of membrane potential. A similar phenotype is observed if the beta-signal is shortened, removed or when the conserved hydrophobic residues in the beta-signal are mutated. The length of the tag at the C-terminus is critical for the assembly of VDAC1, as well as the amino acid residues at positions 130, 222, 225 and 251 of the protein. We propose that if the recognition of the beta-signal or the folding of the beta-barrel proteins is inhibited, the nonassembled protein will accumulate in the intermembrane space, aggregate and damage mitochondria. This effect offers easy tools for studying the requirements for the membrane assembly of beta-barrel proteins, but also advises caution when interpreting the outcome of the beta-barrel protein overexpression experiments.

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.

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.