Single-cell and bulk RNA sequencing data jointly reveals VDAC2's impacts on prognosis and immune landscape of NSCLC

Non-small cell lung cancer (NSCLC) is characterized by stronger metastatic ability and worse prognosis. In NSCLC, hypoxia is a major cause of invasion and metastasis through promoting angiogenesis. In present study, NSCLC cell clusters were extracted from single cell-sequencing dataset GSE131907, which were combined with hypoxia-related genes to group clusters. qRT-PCR and western blot were used to validate the expression of target gene. Nine NSCLC clusters were extracted, which were divided into two hypoxia-related subgroups, C1 and C2. Totally 101 differentially expressed prognostic genes were identified between subgroups. Of which, VDAC2 showed excellent prognostic value for NSCLC and was selected for further analysis. VDAC2 was upregulated in tumor samples in TCGA and was correlated with advanced stages. In vitro experiments validated this trend. Five crucial immune cells showed differential infiltration proportions between high and low VDAC2 expression groups. VDAC2 knockdown significantly inhibited the proliferation and invasion ability of NSCLC cells. Integrating single cell and bulk sequencing data as well as wet lab experiments, hypoxia-related VDAC2 exhibited important prognostic value and showed the promise of becoming immune-therapy target in NSCLC.

Cardiac-specific deletion of voltage dependent anion channel 2 leads to dilated cardiomyopathy by altering calcium homeostasis

Voltage dependent anion channel 2 (VDAC2) is an outer mitochondrial membrane porin known to play a significant role in apoptosis and calcium signaling. Abnormalities in calcium homeostasis often leads to electrical and contractile dysfunction and can cause dilated cardiomyopathy and heart failure. However, the specific role of VDAC2 in intracellular calcium dynamics and cardiac function is not well understood. To elucidate the role of VDAC2 in calcium homeostasis, we generated a cardiac ventricular myocyte-specific developmental deletion of Vdac2 in mice. Our results indicate that loss of VDAC2 in the myocardium causes severe impairment in excitation-contraction coupling by altering both intracellular and mitochondrial calcium signaling. We also observed adverse cardiac remodeling which progressed to severe cardiomyopathy and death. Reintroduction of VDAC2 in 6-week-old knock-out mice partially rescued the cardiomyopathy phenotype. Activation of VDAC2 by efsevin increased cardiac contractile force in a mouse model of pressure-overload induced heart failure. In conclusion, our findings demonstrate that VDAC2 plays a crucial role in cardiac function by influencing cellular calcium signaling. Through this unique role in cellular calcium dynamics and excitation-contraction coupling VDAC2 emerges as a plausible therapeutic target for heart failure.

SPATA33 is an autophagy mediator for cargo selectivity in germline mitophagy

Selective autophagic degradation of mitochondria (mitophagy) is important in maintaining proper cellular homeostasis. Here, we found that SPATA33 is a novel autophagy mediator for mitophagy in testis. The SPATA33 protein localizes on mitochondria via its binding of the carboxyl terminal with the outer mitochondrial membrane protein VDAC2. Upon starvation induction, SPATA33 is recruited to autophagosome by binding the autophagy machinery ATG16L1 via its N-terminal along with mitochondria. Notably, Spata33 knockout inhibited autophagy and overexpression can promote autophagosome formation for mitochondrial sequestration. Therefore, SPATA33 confers selectivity for mitochondrial degradation and promotes mitophagy in male germline cells.

Palmitoylated CKAP4 regulates mitochondrial functions through an interaction with VDAC2 at ER-mitochondria contact sites

Cytoskeleton-associated protein 4 (CKAP4) is a palmitoylated type II transmembrane protein localized to the endoplasmic reticulum (ER). Here, we found that knockout (KO) of CKAP4 in HeLaS3 cells induces the alteration of mitochondrial structures and increases the number of ER-mitochondria contact sites. To understand the involvement of CKAP4 in mitochondrial functions, the binding proteins of CKAP4 were explored, enabling identification of the mitochondrial porin voltage-dependent anion-selective channel protein 2 (VDAC2), which is localized to the outer mitochondrial membrane. Palmitoylation at Cys100 of CKAP4 was required for the binding between CKAP4 and VDAC2. In CKAP4 KO cells, the binding of inositol trisphosphate receptor (IP3R) and VDAC2 was enhanced, the intramitochondrial Ca2+ concentration increased and the mitochondrial membrane potential decreased. In addition, CKAP4 KO decreased the oxidative consumption rate, in vitro cancer cell proliferation under low-glucose conditions and in vivo xenograft tumor formation. The phenotypes were not rescued by expression of a palmitoylation-deficient CKAP4 mutant. These results suggest that CKAP4 plays a role in maintaining mitochondrial functions through the binding to VDAC2 at ER-mitochondria contact sites and that palmitoylation is required for this novel function of CKAP4.This article has an associated First Person interview with the first author of the paper.

Bisindolylpyrrole triggers transient mitochondrial permeability transitions to cause apoptosis in a VDAC1/2 and cyclophilin D-dependent manner via the ANT-associated pore

Bisindolylpyrrole at 0.1 μM is cytoprotective in 2% FBS that is counteracted by cyclosporin-A (CsA), an inhibitor of cyclophilin-D (CypD). We hypothesized that the cytoprotective effect might be due to transient mitochondrial permeability transition (tPT). This study tested the hypothesis that bisindolylpyrrole can trigger tPT extensively, thereby leading to cell death under certain conditions. Indeed, CsA-sensitive tPT-mediated apoptosis could be induced by bisindolylpyrrole at > 5 μM in HeLa cells cultured in 0.1% FBS, depending on CypD and VDAC1/2, as shown by siRNA knockdown experiments. Rat liver mitochondria also underwent swelling in response to bisindolylpyrrole, which proceeded at a slower rate than Ca2+-induced swelling, and which was blocked by the VDAC inhibitor tubulin and the ANT inhibitor bongkrekate, indicating the involvement of the ANT-associated, smaller pore. We examined why 0.1% FBS is a prerequisite for apoptosis and found that apoptosis is blocked by PKC activation, which is counteracted by the overexpressed defective PKCε. In mitochondrial suspensions, bisindolylpyrrole triggered CsA-sensitive swelling, which was suppressed selectively by pretreatment with PKCε, but not in the co-presence of tubulin. These data suggest that upon PKC inactivation the cytoprotective compound bisindolylpyrrole can induce prolonged tPT causing apoptosis in a CypD-dependent manner through the VDAC1/2-regulated ANT-associated pore.

Inflammatory IFIT3 renders chemotherapy resistance by regulating post-translational modification of VDAC2 in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal cancers worldwide and effective therapy remains a challenge. IFIT3 is an interferon-stimulated gene with antiviral and pro-inflammatory functions. Our previous work has shown that high expression of IFIT3 is correlated with poor survival in PDAC patients who receive chemotherapy suggesting a link between IFIT3 and chemotherapy resistance in PDAC. However, the exact role and molecular mechanism of IFIT3 in chemotherapy resistance in PDAC has been unclear. Methods: A group of transcriptome datasets were downloaded and analyzed for the characterization of IFIT3 in PDAC. Highly metastatic PDAC cell line L3.6pl and patient-derived primary cell TBO368 were used and IFIT3 knockdown and the corresponding knockin cells were established for in vitro studies. Chemotherapy-induced apoptosis, ROS production, confocal immunofluorescence, subcellular fractionation, chromatin-immunoprecipitation, co-immunoprecipitation and mass spectrometry analysis were determined to further explore the biological role of IFIT3 in chemotherapy resistance of PDAC. Results: Based on PDAC transcriptome data, we show that IFIT3 expression is associated with the squamous molecular subtype of PDAC and an increase in inflammatory response and apoptosis pathways. We further identify a crucial role for IFIT3 in the regulation of mitochondria-associated apoptosis during chemotherapy. Knockdown of IFIT3 attenuates the chemotherapy resistance of PDAC cells to gemcitabine, paclitaxel, and FOLFIRINOX regimen treatments, independent of individual chemotherapy regimens. While IFIT3 overexpression was found to promote drug resistance. Co-immunoprecipitation identified a direct interaction between IFIT3 and the mitochondrial channel protein VDAC2, an important regulator of mitochondria-associated apoptosis. It was subsequently found that IFIT3 regulates the post-translational modification-O-GlcNAcylation of VDAC2 by stabilizing the interaction of VDAC2 with O-GlcNAc transferase. Increased O-GlcNAcylation of VDAC2 protected PDAC cells from chemotherapy induced apoptosis. Conclusions: These results effectively demonstrate a central mechanism by which IFIT3 expression can affect chemotherapy resistance in PDAC. Targeting IFIT3/VDAC2 may represent a novel strategy to sensitize aggressive forms of pancreatic cancer to conventional chemotherapy regimens.

Tiger frog virus ORF104R interacts with cellular VDAC2 to inhibit cell apoptosis

Ranaviruses belong to the family Iridoviridae, and have become a serious threat to both farmed and natural populations of fish and amphibians. Previous reports showed that ranaviruses could encode viral Bcl-2 family-like proteins (vBcl-2), which play a critical role in the regulation of cell apoptosis. However, the mechanism of ranaviruses vBcl-2 interactions with host protein in mediating apoptosis remains unknown. Tiger frog virus (TFV) belonging to the genus Ranavirus has been isolated from infected tadpoles of Rana tigrina rugulosa, and it causes a high mortality rate among tiger frog tadpoles cultured in southern China. This study elucidated the molecular mechanism underlying the interaction of TFV ORF104R with the VDAC2 protein to regulate cell apoptosis. TFV ORF104R is highly similar to other ranaviruses vBcl-2 and host Mcl-1 proteins, indicating that TFV ORF104R is a postulate vBcl-2 protein. Transcription and protein expression levels showed that TFV orf104r was a late viral gene. Western blot results suggested that TFV ORF104R was a viral structural protein. Subcellular localization analysis indicated that TFV ORF104R was predominantly colocalized with the mitochondria. Overexpressed TFV ORF104R could suppress the release of cytochrome C and the activities of caspase-9 and caspase-3. These results indicated that TFV ORF104R might play an important role in anti-apoptosis. Furthermore, the interaction between TFV ORF104R and VDAC2 was detected by co-immunoprecipitation in vitro. The above observations suggest that the molecular mechanism of TFV-regulated anti-apoptosis is through the interaction of TFV ORF104R with the VDAC2 protein. Our study provided a mechanistic basis for the ranaviruses vBcl-2-mediated inhibition of apoptosis and improved the understanding on how TFV subverts host defense mechanisms in vivo.

Microsporidia Interact with Host Cell Mitochondria via Voltage-Dependent Anion Channels Using Sporoplasm Surface Protein 1

Microsporidia are opportunistic intracellular pathogens that can infect a wide variety of hosts ranging from invertebrates to vertebrates. During invasion, the microsporidian polar tube pushes into the host cell, creating a protective microenvironment, the invasion synapse, into which the sporoplasm extrudes. Within the synapse, the sporoplasm then invades the host cell, forming a parasitophorous vacuole (PV). Using a proteomic approach, we identified Encephalitozoon hellem sporoplasm surface protein 1 (EhSSP1), which localized to the surface of extruded sporoplasms. EhSSP1 was also found to interact with polar tube protein 4 (PTP4). Recombinant EhSSP1 (rEhSSP1) bound to human foreskin fibroblasts, and both anti-EhSSP1 and rEhSSP1 caused decreased levels of host cell invasion, suggesting that interaction of SSP1 with the host cell was involved in invasion. Coimmunoprecipitation (Co-IP) followed by proteomic analysis identified host cell voltage-dependent anion channels (VDACs) as EhSSP1 interacting proteins. Yeast two-hybrid assays demonstrated that EhSSP1 was able to interact with VDAC1, VDAC2, and VDAC3. rEhSSP1 colocalized with the host mitochondria which were associated with microsporidian PVs in infected cells. Transmission electron microscopy revealed that the outer mitochondrial membrane interacted with meronts and the PV membrane, mitochondria clustered around meronts, and the VDACs were concentrated at the interface of mitochondria and parasite. Knockdown of VDAC1, VDAC2, and VDAC3 in host cells resulted in significant decreases in the number and size of the PVs and a decrease in mitochondrial PV association. The interaction of EhSSP1 with VDAC probably plays an important part in energy acquisition by microsporidia via its role in the association of mitochondria with the PV.IMPORTANCE Microsporidia are important opportunistic human pathogens in immune-suppressed individuals, such as those with HIV/AIDS and recipients of organ transplants. The sporoplasm is critical for establishing microsporidian infection. Despite the biological importance of this structure for transmission, there is limited information about its structure and composition that could be targeted for therapeutic intervention. Here, we identified a novel E. hellem sporoplasm surface protein, EhSSP1, and demonstrated that it can bind to host cell mitochondria via host VDAC. Our data strongly suggest that the interaction between SSP1 and VDAC is important for the association of mitochondria with the parasitophorous vacuole during microsporidian infection. In addition, binding of SSP1 to the host cell is associated with the final steps of invasion in the invasion synapse.

Ceramides bind VDAC2 to trigger mitochondrial apoptosis

Ceramides draw wide attention as tumor suppressor lipids that act directly on mitochondria to trigger apoptotic cell death. However, molecular details of the underlying mechanism are largely unknown. Using a photoactivatable ceramide probe, we here identify the voltage-dependent anion channels VDAC1 and VDAC2 as mitochondrial ceramide binding proteins. Coarse-grain molecular dynamics simulations reveal that both channels harbor a ceramide binding site on one side of the barrel wall. This site includes a membrane-buried glutamate that mediates direct contact with the ceramide head group. Substitution or chemical modification of this residue abolishes photolabeling of both channels with the ceramide probe. Unlike VDAC1 removal, loss of VDAC2 or replacing its membrane-facing glutamate with glutamine renders human colon cancer cells largely resistant to ceramide-induced apoptosis. Collectively, our data support a role of VDAC2 as direct effector of ceramide-mediated cell death, providing a molecular framework for how ceramides exert their anti-neoplastic activity.

Characterization of Oyster Voltage-Dependent Anion Channel 2 (VDAC2) Suggests Its Involvement in Apoptosis and Host Defense

Genomic and transcriptomic studies have revealed a sophisticated and powerful apoptosis regulation network in oyster, highlighting its adaptation to sessile life in a highly stressful intertidal environment. However, the functional molecular basis of apoptosis remains largely unexplored in oysters. In this study, we focused on a representative apoptotic gene encoding voltage-dependent anion channel 2 (VDAC2), a porin that abounds at the mitochondrial outer membrane. This is the first report on the identification and characterization of a VDAC gene in the Pacific oyster, Crassostrea gigas (CgVDAC2). The full length of CgVDAC2 was 1,738 bp with an open reading frame of 843 bp that encoded a protein of 281 amino acids. A four-element eukaryotic porin signature motif, a conserved ATP binding motif, and a VKAKV-like sequence were identified in the predicted CgVDAC2. Expression pattern analysis in different tissues and developmental stages as well as upon infection by ostreid herpesvirus 1 revealed the energy supply-related and immunity-related expression of CgVDAC2. CgVDAC2 was co-localized with mitochondria when it was transiently transfected into HeLa cells. Overexpression of CgVDAC2 in HEK293T cells suppressed the UV irradiation-induced apoptosis by inhibiting the pro-apoptotic function of CgBak. RNA interference induced reduction in CgVDAC2 expression showed a promoted apoptosis level upon UV light irradiation in hemocytes. The yeast two-hybrid system and co-immunoprecipitation assay indicated a direct interaction between CgVDAC2 and the pro-apoptotic protein CgBak. This study revealed the function of VDAC2 in oyster and provided new insights into its involvement in apoptosis modulation and host defense in mollusks.

Screening Strategies for TALEN-Mediated Gene Disruption

Targeted gene disruption has rapidly become the tool of choice for the analysis of gene and protein function in routinely cultured mammalian cells. Three main technologies capable of irreversibly disrupting gene-expression exist: zinc-finger nucleases, transcription activator-like effector nucleases (TALENs), and the CRISPR/Cas9 system. The desired outcome of the use of any of these technologies is targeted insertions and/or deletions (indels) that result in either a nonsense frame shift or splicing error that disrupts protein expression. Many excellent do-it-yourself systems for TALEN construct assembly are now available at low or no cost to academic researchers. However, for new users, screening for successful gene disruption is still a hurdle. Here, we describe efficient and cost-effective strategies for the generation of gene-disrupted cell lines. Although the focus of this chapter is on the use of TALENs, these strategies can be applied to the use of all three technologies.

Grouper voltage-dependent anion selective channel protein 2 is required for nervous necrosis virus infection

Nervous necrosis virus (NNV) is a non-enveloped virus with 2 segmented positive-sense single-stranded RNAs. NNV-induced mass mortality has occurred worldwide in many species of cultured marine fish and resulted in substantial economic losses. In our previous study, we cloned the gene of voltage-dependent anion selective channel protein 2 (GVDAC2), and the NNV RNA2 expression level decreased in GVDAC2-knockdown GF-1 cells 24 h after infection. Here, we investigated the role of GVDAC2 in the NNV infection in GF-1 cells. NNV infection did not considerably affect GVDAC2 gene expression. After performing immunostaining, we detected GVDAC2 at the mitochondria and GVDAC2 was colocalized with NNV-RNA-dependent RNA polymerase. However, these 2 proteins did not interact with each other in immunoprecipitation assay. The cellular ATP level in GVDAC2-downregulated cells was lower than that in control cells, and NNV-induced apoptosis was delayed in GVDAC2-siRNA-transfected cells. Therefore, we suggest that GVDAC2 is required for NNV infection for maintaining the cellular ATP level and has positive impact on virus-induced apoptosis.

Tissue distribution of olive flounder VDAC2 and its expression in fish cell lines

Voltage-dependent anion channel (VDAC) is located in the mitochondrial outer membrane, which plays a crucial role in regulating cell life and death. In this study, the tissue distribution of olive flounder Paralichthys olivaceus VDAC2 (PoVDAC2) was detected by quantitative real-time PCR and Western blot analysis. The qRT-PCR results showed that the expression level of PoVDAC2 was abundant in heart, muscle and gill tissues. Western blot analysis revealed a protein of 32 kDa detected in all six tissues. Furthermore, a recombinant eukaryotic expression plasmid pEGFP-N3-PoVDAC2 was successfully constructed and transiently expressed the fusion protein in fish cell lines. Subcellular localization indicated that PoVDAC2-GFP was distributed in a punctate mitochondria-like pattern throughout the cytoplasm in flounder embryonic cells (FEC). The distribution of native VDAC2 in untransfected fish cells was also investigated by immunofluorescence microscopy. The punctate VDAC2 fluorescence signals of both FEC and EPC cells were identically observed in the cytoplasm but not in the nucleus. These results laid a foundation for investigating the functional relevance of VDAC response to pathogens in flounder.

The proteomic 2D-DIGE approach reveals the protein voltage-dependent anion channel 2 as a potential therapeutic target in epithelial thyroid tumours

We investigated the role of VDAC2 in human epithelial thyroid tumours using proteomic 2D-DIGE analysis and qRT-PCR. We found a significant up-regulation of VDAC2 in thyroid tumours and in thyroid tumour cell lines (TPC-1 and CAL-62). We did not detect overexpression of VDAC2 in a normal thyroid cell line (Nthy-ori 3-1). Silico analysis revealed that two proteins, BAK1 and BAX, had a strong relationship with VDAC2. BAK1 gene expression showed down-regulation in thyroid tumours (follicular and papillary tumours) and in TPC-1 and CAL-62 cell lines. Transient knockdown of VDAC2 in TPC-1 and CAL-62 promoted upregulation of the BAK1 gene and protein expression, and increased susceptibility to sorafenib treatment. Overexpression of the BAK1 gene in CAL-62 showed lower sorafenib sensitivity than VDAC2 knockdown cells. We propose the VDAC2 gene as a novel therapeutic target in these tumours.

Influence of protein-micelle ratios and cysteine residues on the kinetic stability and unfolding rates of human mitochondrial VDAC-2

Delineating the kinetic and thermodynamic factors which contribute to the stability of transmembrane β-barrels is critical to gain an in-depth understanding of membrane protein behavior. Human mitochondrial voltage-dependent anion channel isoform 2 (hVDAC-2), one of the key anti-apoptotic eukaryotic β-barrel proteins, is of paramount importance, owing to its indispensable role in cell survival. We demonstrate here that the stability of hVDAC-2 bears a strong kinetic contribution that is dependent on the absolute micellar concentration used for barrel folding. The refolding efficiency and ensuing stability is sensitive to the lipid-to-protein (LPR) ratio, and displays a non-linear relationship, with both low and high micellar amounts being detrimental to hVDAC-2 structure. Unfolding and aggregation process are sequential events and show strong temperature dependence. We demonstrate that an optimal lipid-to-protein ratio of 2600∶1 – 13000∶1 offers the highest protection against thermal denaturation. Activation energies derived only for lower LPRs are ∼17 kcal mol⁻¹ for full-length hVDAC-2 and ∼23 kcal mol⁻¹ for the Cys-less mutant, suggesting that the nine cysteine residues of hVDAC-2 impart additional malleability to the barrel scaffold. Our studies reveal that cysteine residues play a key role in the kinetic stability of the protein, determine barrel rigidity and thereby give rise to strong micellar association of hVDAC-2. Non-linearity of the Arrhenius plot at high LPRs coupled with observation of protein aggregation upon thermal denaturation indicates that contributions from both kinetic and thermodynamic components stabilize the 19-stranded β-barrel. Lipid-protein interaction and the linked kinetic contribution to free energy of the folded protein are together expected to play a key role in hVDAC-2 recycling and the functional switch at the onset of apoptosis.

A chemical glycoproteomics platform reveals O-GlcNAcylation of mitochondrial voltage-dependent anion channel 2

Protein modification by O-linked β-N-acetylglucosamine (O-GlcNAc) is a critical cell signaling modality, but identifying signal-specific O-GlcNAcylation events remains a significant experimental challenge. Here, we describe a method for visualizing and analyzing organelle- and stimulus-specific O-GlcNAcylated proteins and use it to identify the mitochondrial voltage-dependent anion channel 2 (VDAC2) as an O-GlcNAc substrate. VDAC2(-/-) cells resist the mitochondrial dysfunction and apoptosis caused by global O-GlcNAc perturbation, demonstrating a functional connection between O-GlcNAc signaling and mitochondrial physiology through VDAC2. More broadly, our method will enable the discovery of signal-specific O-GlcNAcylation events in a wide array of experimental contexts.

[Expression of the voltage-dependent anion channel gene in human ejaculated spermatozoa]

OBJECTIVE

To identify the genes involved in sperm motility.

METHODS

We hybridized asthenospermia and normal motile sperm cDNA samples with the human whole genome Affymetrix chip to screen differentially expressed genes. Then we detected the mRNA expressions of the voltage-dependent anion channel genes (VDACs) in human organs and spermatozoa by RT-PCR and compared their expressions in the poor and normal motility spermatozoa.

RESULTS

Differentially expressed genes VDACs were identified by analysis of the hybridization signals, including the 3 subtypes VDAC1, VDAC2 and VDAC3. The expression of VDAC2 mRNA was significantly decreased in the poor motility sperm (0.568 +/- 0.036), as compared with the healthy men (0.803 +/- 0.043, P < 0.01).

CONCLUSION

The decreased expression of VDAC2 in the ejaculated spermatozoa is possibly associated with the reduction of sperm motility.

Voltage-dependent anion channel 2 of Arabidopsis thaliana (AtVDAC2) is involved in ABA-mediated early seedling development

The voltage-dependent anion channel (VDAC) is the major transport protein in the outer membrane of mitochondria and plays crucial roles in energy metabolism, apoptosis, and metabolites transport. In plants, the expression of VDACs can be affected by different stresses, including drought, salinity and pathogen defense. In this study, we investigated the expression pattern of AtVDAC2 in A. thaliana and found ABA suppressed the accumulation of AtVDAC2 transcripts. Further, phenotype analysis of this VDAC deregulated-expression transgenic Arabidopsis plants indicated that AtVDAC2 anti-sense line showed an ABA-insensitivity phenotype during the early seedling development under ABA treatment. The results suggested that AtVDAC2 might be involved in ABA signaling in A. thaliana.

The voltage dependent anion selective channel family in Drosophila melanogaster

VDACs (voltage-dependent anion-selective channels) or mitochondrial PORINS are transmembrane proteins forming pores in the outer membrane. In eukaryotic genomes multiple genes coding for VDAC homologues have been discovered, but the functional meaning of this gene redundancy is unknown. In Drosophila melanogasterthree additional genes homologous to the gene porin (CG6647) have been found. As in other occurences, the presence of a gene revealed by genome analysis raises the questions: are these genes really expressed? What are the molecular features of the putative proteins, if they are expressed? Where and when in the organism are they expressed? Consequently have they any specific activity justifying the presence of more isoforms in the organism? To answer to these questions we have produced antibodies against the recombinant proteins corresponding to the whole (VDAC1 and 2) or to substantial portions (VDAC3 and 4) of the known or predicted proteins. Immunohistological and transcriptional analysis has been performed, showing that VDAC2 and 3 are expressed, while VDAC4 was not detected. Structural predictions of VDAC3 are consistent with the presence of additional alpha-helices at the N-terminus of the protein.

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.

Genetic demonstration that the plasma membrane maxianion channel and voltage-dependent anion channels are unrelated proteins

The maxianion channel is widely expressed in many cell types, where it fulfills a general physiological function as an ATP-conductive gate for cell-to-cell purinergic signaling. Establishing the molecular identity of this channel is crucial to understanding the mechanisms of regulated ATP release. A mitochondrial porin (voltage-dependent anion channel (VDAC)) located in the plasma membrane has long been considered as the molecule underlying the maxianion channel activity, based upon similarities in the biophysical properties of these two channels and the purported presence of VDAC protein in the plasma membrane. We have deleted each of the three genes encoding the VDAC isoforms individually and collectively and demonstrate that maxianion channel (approximately 400 picosiemens) activity in VDAC-deficient mouse fibroblasts is unaltered. The channel activity is similar in VDAC1/VDAC3-double-deficient cells and in double-deficient cells with the VDAC2 protein depleted by RNA interference. VDAC deletion slightly down-regulated, but never abolished, the swelling-induced ATP release. The lack of correlation between VDAC protein expression and maxianion channel activity strongly argues against the long held hypothesis of plasmalemmal VDAC being the maxianion channel.