Functional differences among wheat voltage-dependent anion channel (VDAC) isoforms expressed in yeast. Indication for the presence of a novel VDAC-modulating protein?

Redox-Sensitive VDAC: A Possible Function as an Environmental Stress Sensor Revealed by Bioinformatic Analysis

Voltage-dependent anion-selective channel (VDAC) allows the exchange of small metabolites and inorganic ions across the mitochondrial outer membrane. It is involved in complex interactions that regulate mitochondrial and cellular functioning. Many organisms have several VDAC paralogs that play distinct but poorly understood roles in the life and death of cells. It is assumed that such a large diversity of VDAC-encoding genes might cause physiological plasticity to cope with abiotic and biotic stresses known to impact mitochondrial function. Moreover, cysteine residues in mammalian VDAC paralogs may contribute to the reduction-oxidation (redox) sensor function based on disulfide bond formation and elimination, resulting in redox-sensitive VDAC (rsVDAC). Therefore, we analyzed whether rsVDAC is possible when only one VDAC variant is present in mitochondria and whether all VDAC paralogs present in mitochondria could be rsVDAC, using representatives of currently available VDAC amino acid sequences. The obtained results indicate that rsVDAC can occur when only one VDAC variant is present in mitochondria; however, the possibility of all VDAC paralogs in mitochondria being rsVDAC is very low. Moreover, the presence of rsVDAC may correlate with habitat conditions as rsVDAC appears to be prevalent in parasites. Thus, the channel may mediate detection and adaptation to environmental conditions.

VDAC1 Negatively Regulates Floral Transition in Arabidopsis thaliana

Voltage-dependent anion channels (VDACs) are the most important proteins in mitochondria. They localize to the outer mitochondrial membrane and contribute to the metabolite transport between the mitochondria and cytoplasm, which aids plant growth regulation. Here, we report that Arabidopsis thaliana VDAC1 is involved in the floral transition, with the loss of AtVDAC1 function, resulting in an early-flowering phenotype. AtVDAC1 is expressed ubiquitously in Arabidopsis. To identify the flowering pathway integrators that may be responsible for AtVDAC1′s function during the floral transition, an RNA-seq analysis was performed. In total, 106 differentially expressed genes (DEGs) were identified between wild-type and atvdac1-5 mutant seedlings. However, none were involved in flowering-related pathways. In contrast, AtVDAC1 physically associated with FLOWERING LOCUS T. Thus, in the floral transition, AtVDAC1 may function partly through the FLOWERING LOCUS T protein.

Arabidopsis Mitochondrial Voltage-Dependent Anion Channels Are Involved in Maintaining Reactive Oxygen Species Homeostasis, Oxidative and Salt Stress Tolerance in Yeast

Voltage-dependent anion channels (VDACs) are conserved proteins of the mitochondria. We have functionally compared Arabidopsis VDACs using Saccharomyces cerevisiae Δpor1 and M3 yeast system. VDAC (1, 2, and 4) were able to restore Δpor1 growth in elevated temperature, in oxidative and salt stresses, whereas VDAC3 only partially rescued Δpor1 in these conditions. The ectopic expression of VDAC (1, 2, 3, and 4) in mutant yeast recapitulated the mitochondrial membrane potential thus, enabled it to maintain reactive oxygen species homeostasis. Overexpression of these VDACs (AtVDACs) in M3 strain did not display any synergistic or antagonistic activity with the native yeast VDAC1 (ScVDAC1). Collectively, our data suggest that Arabidopsis VDACs are involved in regulating respiration, reactive oxygen species homeostasis, and stress tolerance in yeast.

Arabidopsis voltage-dependent anion channel 1 (AtVDAC1) is required for female development and maintenance of mitochondrial functions related to energy-transaction

The voltage-dependent anion channels (VDACs), prominently localized in the outer mitochondrial membrane, play important roles in the metabolite exchange, energy metabolism and mitochondria-mediated apoptosis process in mammalian cells. However, relatively little is known about the functions of VDACs in plants. To further investigate the function of AtVDAC1 in Arabidopsis, we analyzed a T-DNA insertion line for the AtVDAC1 gene. The knock-out mutant atvdac1 showed reduced seed set due to a large number of undeveloped ovules in siliques. Genetic analyses indicated that the mutation of AtVDAC1 affected female fertility and belonged to a sporophytic mutation. Abnormal ovules in the process of female gametogenesis were observed using a confocal laser scanning microscope. Interestingly, both mitochondrial transmembrane potential (ΔΨ) and ATP synthesis rate were obviously reduced in the mitochondria isolated from atvdac1 plants.

Origin of ion selectivity in Phaseolus coccineus mitochondrial VDAC

The mitochondrial voltage-dependent a nion-selective channel (VDAC) is the major permeation pathway for small ions and metabolites. Although a wealth of electrophysiological data has been obtained on different VDAC species, the physical mechanisms of their ionic selectivity are still elusive. We addressed this issue using electrophysiological experiments performed on plant VDAC. A simple macroscopic electrodiffusion model accounting for ion diffusion and for an effective fixed charge of the channel describes well its selectivity. Brownian Dynamics simulations of ion permeation performed on plant and mammalian VDACs point to the role of specific charged residues located at about the middle of the pore.

The functions of voltage-dependent anion channels in plants

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

Mitochondrial VDAC and hexokinase together modulate plant programmed cell death

The voltage-dependent anion channel (VDAC) and mitochondrially located hexokinase have been implicated both in pathways leading to cell death on the one hand, and immortalization in tumor formation on the other. While both proteins have also been implicated in death processes in plants, their interaction has not been explored. We have examined cell death following heterologous expression of a rice VDAC in the tobacco cell line BY2 and in leaves of tobacco plants and show that it is ameliorated by co-expression of hexokinase. Hexokinase also abrogates death induced by H2O2. We conclude that the ratio of expression of the two proteins and their interaction play a major role in modulating death pathways in plants.

The voltage-dependent anion channel 1 (AtVDAC1) negatively regulates plant cold responses during germination and seedling development in Arabidopsis and interacts with calcium sensor CBL1

The voltage-dependent anion channel (VDAC), a highly conserved major mitochondrial outer membrane protein, plays crucial roles in energy metabolism and metabolite transport. However, knowledge about the roles of the VDAC family in plants is limited. In this study, we investigated the expression pattern of VDAC1 in Arabidopsis and found that cold stress promoted the accumulation of VDAC1 transcripts in imbibed seeds and mature plants. Overexpression of VDAC1 reduced tolerance to cold stress in Arabidopsis. Phenotype analysis of VDAC1 T-DNA insertion mutant plants indicated that a vdac1 mutant line had faster germination kinetics under cold treatment and showed enhanced tolerance to freezing. The yeast two-hybrid system revealed that VDAC1 interacts with CBL1, a calcium sensor in plants. Like the vdac1, a cbl1 mutant also exhibited a higher seed germination rate. We conclude that both VDAC1 and CBL1 regulate cold stress responses during seed germination and plant development.

The Arabidopsis voltage-dependent anion channel 2 is required for plant growth

The voltage-dependent anion channels (VDACs) known as a major group of outer mitochondrial membrane proteins are present in all eukaryotic species. In mammalian cells, they have been established as a key player in mitochondrial metabolism and apoptosis regulation. By contrast, little is known about the function of plant VDACs. Recently, we performed functional analysis of all VDAC gene members in Arabidopsis thaliana, and revealed that each AtVDAC member has a specialized function. Especially, in spite of similar subcellular localization and expression profiling of AtVDAC2 and AtVDAC4, both the T-DNA insertion knockout mutants of them, vdac2-2 and vdac4-2, showed severe growth retardation. These results suggest that AtVDAC2 and AtVDAC4 proteins clearly have distinct functions. Here, we introduced the AtVDAC2 gene into the vdac2-2 mutant, and demonstrated that the miniature phenotype of vdac2-2 plant is abolished by AtVDAC2 expression.

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

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

Modulation of plant mitochondrial VDAC by phytosterols

We have investigated the effect of cholesterol and two abundant phytosterols (sitosterol and stigmasterol) on the voltage-dependent anion-selective channel (VDAC) purified from mitochondria of bean seeds (Phaseolus coccineus). These sterols differ by the degree of freedom of their lateral chain. We show that VDAC displays sensitivity to the lipid-sterol ratio and to the type of sterol found in the membrane. The main findings of this study are: 1), cholesterol and phytosterols modulate the selectivity but only stigmasterol alters the voltage-dependence of the plant VDAC in the range of sterol fraction found in the plant mitochondrial membrane; 2), VDAC unitary conductance is not affected by the addition of sterols; 3), the effect of sterols on the VDAC is reversible upon sterol depletion with 10 μM methyl-β-cyclodextrins; and 4), phytosterols are essential for the channel gating at salt concentration prevailing in vivo. A quantitative analysis of the voltage-dependence indicates that stigmasterol inhibits the transition of the VDAC in the lowest subconductance states.

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.

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 evolutionary history of mitochondrial porins

BACKGROUND

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

RESULTS

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

CONCLUSION

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

Mitochondria-associated hexokinases play a role in the control of programmed cell death in Nicotiana benthamiana

Recent findings suggest a pivotal role for mitochondria-associated hexokinase in the regulation of apoptosis in animal cells. In this study, virus-induced gene silencing (VIGS) of a hexokinase-encoding Hxk1 caused necrotic lesions on leaves, abnormal leaf morphology, and retarded plant growth in Nicotiana benthamiana. Hxk1 was associated with the mitochondria, and this association required the N-terminal membrane anchor. VIGS of Hxk1 reduced the cellular glucose-phosphorylating activity to approximately 31% of control levels without changing the fructose-phosphorylating activity and did not alter hexose phosphate content severely. The affected cells showed programmed cell death (PCD) morphological markers, including nuclear condensation and DNA fragmentation. Similar to animal cell apoptosis, cytochrome c was released into the cytosol and caspase-9- and caspase-3-like proteolytic activities were strongly induced. Furthermore, based on flow cytometry, Arabidopsis thaliana plants overexpressing Arabidopsis HXK1 and HXK2, both of which are predominantly associated with mitochondria, exhibited enhanced resistance to H(2)O(2)- and alpha-picolinic acid-induced PCD. Finally, the addition of recombinant Hxk1 to mitochondria-enriched fractions prevented H(2)O(2)/clotrimazole-induced cytochrome c release and loss of mitochondrial membrane potential. Together, these results show that hexokinase critically regulates the execution of PCD in plant cells, suggesting a link between glucose metabolism and apoptosis.

Structural and functional analysis of a salt stress inducible gene encoding voltage dependent anion channel (VDAC) from pearl millet (Pennisetum glaucum)

We have cloned and characterized a gene encoding voltage-dependent anion channel from Pennisetum glaucum (PgVDAC). PgVDAC was identified while isolating genes that were differentially up-regulated following salt stress. The genomic organization of PgVDAC clone was well conserved compared to other plant VDAC genes in terms of number of introns, their position and phasing, however, the primary amino acid sequence of voltage dependent anion channel (VDAC) proteins did not show much conservation with other plant VDACs but their secondary and tertiary structures are well conserved as predicted by in silico structural and CD spectra analyses and results show it to be a typical membrane-spanning beta-barrel leading to the formation of pore in the membrane. The heterologous expression of PgVDAC protein in yeast strain lacking the endogenous mitochondrial VDAC gene could not functionally complement it as was also previously observed for the potato VDAC. Using real-time quantitative PCR analysis it was found that transcript expression profile of PgVDAC was quantitatively and kinetically up-regulated in response to salinity, desiccation, cold and exogenous application of salicylic acid (SA); however, there was no effect of exogenous application of abscisic acid (ABA) on its expression. Constitutive over-expression of PgVDAC appears to be deleterious in transgenic rice plant; however, low level of up-regulation imparted salinity stress adaptive response. A search for a more suitable inducible transgene system is currently under way to understand PgVDAC expression levels in plant development and its role in stress adaptation.

Hunting interactomes of a membrane protein: obtaining the largest set of voltage-dependent anion channel-interacting protein epitopes

The identification of epitopes involved in protein-protein interactions is essential for understanding protein structure and function. Large scale efforts, although identifying the interactions, did not always yield these epitopes, could not confirm most of the known interactions, and seemed particularly unsuccessful for native intrinsic membrane proteins. We have developed a fluidics-based approach (non-steady-state kinetics) to obtain the broadest set of the epitopes interacting with a given target and applied it to a phage display methodology optimized for membrane proteins. Phages expressing a liver cDNA library were screened against a membrane protein (voltage-dependent anion channel) reconstituted into liposomes and captured on a chip surface. The controlled fluidics was obtained by a surface plasmon resonance (SPR) device that combined the advantages of working with minute reaction volumes and non-equilibrium conditions. We demonstrated selective enrichment of binders and could even select for different binding affinities by fractionation of the selected outputs at various elution times. With voltage-dependent anion channel as bait (a mitochondrial channel critical for cellular metabolism and apoptosis) we found at least 40% of its already reported ligands and independently confirmed 55 novel functional interactions, some of which fully blocked the channel. This highly efficient approach is generally applicable for any protein and could be automated and scaled up even without the use of a SPR device. The epitopes directly identified by this method are useful not only for unraveling interactomes but also for drug design and therapeutics.

In vitro interactions between the two mitochondrial membrane proteins VDAC and cytochrome c oxidase

VDAC, a mitochondrial outer membrane channel, is involved in the control of aerobic metabolism and in apoptotic processes via numerous protein-protein interactions. To unveil those interactions, we screened a human liver cDNA library with the phage display methodology optimized to target VDAC reconstituted into a membrane environment. One positively selected clone yielded a sequence matching a part of the subunit I of human cytochrome c oxidase (COX), a mitochondrial inner membrane enzyme. Such putative interaction was never reported before. This interaction proved to be functional as evidenced by the effect of the human and yeast isoforms of VDAC on the oxidation of cytochrome c by the pure holoenzyme and by the effect of the COX epitope on VDAC permeability. Our results providing four independently obtained evidences of VDAC-COX interaction in vitro, would support a novel and potentially important level of mitochondrial regulation given the respective locations and functions of both proteins.

Processes underlying the upregulation of Tom proteins in S. cerevisiae mitochondria depleted of the VDAC channel

It has been shown recently that in Saccharomyces cerevisiae mitochondria depleted of the VDAC channel (delta por1 mitochondria), the TOM complex channel substitutes for the VDAC channel. The additional function of the TOM complex channel is probably facilitated by the upregulation of nuclear-encoded components of the TOM complex as has been shown for Tom40 (a major component of the channel) and Tom70 (one of the surface receptors). Here we report that in S. cerevisiae cells the VDAC channel seems to be an important signal in the expression of the TOM complex components. S. cerevisiae cells depleted of the VDAC channel (delta por1 cells) contain distinctly increased levels of Tom40mRNA, and Tom70mRNA, but their synthesis and translation are affected differentially by the applied inhibitors of transcription and translation. Consequently, it may be concluded that depletion of the VDAC channel might influence differentially the expression of TOM40 and TOM70 genes.

VDAC: the channel at the interface between mitochondria and the cytosol

The mitochondrial outer membrane is not just a barrier but a site of regulation of mitochondrial function. The VDAC family of proteins are the major pathways for metabolite flux through the outer membrane. These can be regulated in a variety of ways and the integration of these regulatory inputs allows mitochondrial metabolism to be adjusted to changing cellular conditions. This includes total blockage of the flux of anionic metabolites leading to permeabilization of the outer membrane to small proteins followed by apoptotic cell death.

Origami in the outer membrane: the transmembrane arrangement of mitochondrial porins

Voltage-dependent anion-selective channels (VDAC), also known as mitochondrial porins, are key regulators of metabolite flow across the mitochondrial outer membrane. Porins from a wide variety of organisms share remarkably similar electrophysiological properties, in spite of considerable sequence dissimilarity, indicating that they share a common structure. Based on primary sequence considerations, analogy with bacterial porins, and circular dichroism analysis, it is agreed that VDAC spans the outer membrane as a beta-barrel. However, the residues that form the antiparallel beta-strands comprising this barrel remain unknown. Various predictive methods, largely based on the known structures of bacterial beta-barrels, have been applied to the primary sequences of VDAC. Refinement and confirmation of these predictions have developed through numerous investigations of wild-type and variant porins, both in mitochondria and in artificial membranes. These experiments have involved VDAC from several sources, precluding the generation of a unified model. Herein, using the Neurospora VDAC sequence as a template, the published structural information and predictions have been reassessed to delineate a model that satisfies most of the available data.

Sequence analysis, transcriptional and posttranscriptional regulation of the rice vdac family

The voltage-dependent anion-selective channel (VDAC) is a mitochondrial outer membrane ion channel. Different isoforms exist in plants but information about their specific role remains to be established. Our purpose is to find out the structural features common to three rice VDAC isoforms and to investigate their (post)transcriptional regulation in response to an osmotic stress. Two new cDNAs encoding mitochondrial VDAC from rice (Oryza sativa) were isolated, sequenced and characterized: a phylogenetic reconstruction permitted identification of orthologues in Poaceae and computer-based analyses predicted 18 transmembrane beta-strands, one amphipathic alpha-helix and two different phosphorylation motifs. The expression of three rice vdac genes was investigated. Northern blot analyses indicated that they were expressed in all plant tissues. There was a differential expression of osvdac1 and osvdac3, whereas osvdac2 was homogeneously expressed in all tissues. No change in vdac expression was observed under an osmotic stress. However, a fast-enhanced expression of vdac was observed in roots during the recovery period after stress release. This enhanced expression is not correlated to the amount of VDAC protein detected in roots suggesting a posttranscriptional regulation.

The key role of the energized state of Saccharomyces cerevisiae mitochondria in modulations of the outer membrane channels by the intermembrane space proteins

Mitochondria of the yeast Saccharomyces cerevisiae constitute a perfect model to study the outer membrane channel modulation as besides the TOM complex channel they contain only a single isoform of the VDAC channel and it is possible to obtain viable mutants devoid of the channel. Here, we report that the fraction of the intermembrane space isolated from wild type and the VDAC channel-depleted yeast mitochondria, except of the well-known VDAC channel modulator activity, displays also the TOM complex channel modulating activity as measured in the reconstituted system and with intact mitochondria. The important factor influencing the action of both modulating activities is the energized state of mitochondria. Moreover, the presence of the VDAC channel itself seems to be crucial to properties of the intermembrane space protein(s) able to modulate the outer membrane channels because in the case of intact mitochondria quantitative differences are observed between modulating capabilities of the fractions isolated from wild type and mutant mitochondria.

An interplay between the TOM complex and porin isoforms in the yeast Saccharomyces cerevisiae mitochondria

The outer mitochondrial membrane of Saccharomyces cerevisiae contains two isoforms of mitochondrial porin, known also as the voltage-dependent anion channel. The isoform termed here porin1 displays channel-forming activity enabling metabolite transport whereas the second one, termed here porin2, does not form a channel and its function is still not clear. We have shown recently that in the absence of porin1, the channel within the protein import machinery (the TOM complex) is essential for metabolite transport across the outer membrane [Kmita and Budzińska, Biochim. Biophys. Acta 1509 (2000) 6044-6050]. Here, we report that the TOM complex channel may also serve as a supplementary pathway for metabolites in the presence of porin1 when the permeability of the latter is limited and the role of the TOM complex seems to increase when porin2 is depleted.

Under conditions of insufficient permeability of VDAC1, external NADH may use the TOM complex channel to cross the outer membrane of Saccharomyces cerevisiae mitochondria

Thus far, only three channel-forming activities have been identified in the outer membrane of the yeast Saccharomyces cerevisiae mitochondria. Two of them, namely the TOM complex channel (translocase of the outer membrane) and the PSC (peptide-sensitive channel) participate in protein translocation and are probably identical, whereas a channel-forming protein called VDAC (voltage-dependent anion channel) serves as the major pathway for metabolites. The VDAC is present in two isoforms (VDAC1 and VDAC2) of which only VDAC1 has been shown to display channel-forming activity. Moreover, the permeability of VDAC1 has been reported to be limited in uncoupled mitochondria of S. cerevisiae. The presented data indicate that in S. cerevisiae-uncoupled mitochondria, external NADH, applied at higher concentrations (above 50 nmoles per 0.1 mg of mitochondrial protein), may use the TOM complex channel, besides VDAC1, to cross the outer membrane. Thus, the permeability of VDAC1 could be a limiting step in transport of external NADH across the outer membrane and might be supplemented by the TOM complex channel.

Structure and orientation of two voltage-dependent anion-selective channel isoforms. An attenuated total reflection fourier-transform infrared spectroscopy study

Two VDAC (voltage-dependent anion-selective channel) isoforms were purified from seed cotyledons of Phaseolus vulgaris by chromatofocusing chromatography. Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy was used to study the structural properties of the two isoforms reconstituted in a mixture of asolectin and 5% stigmasterol. The IR spectra of the two VDAC isoforms were highly similar indicating 50 to 53% anti-parallel beta-sheet. The orientation of the beta-strands relative to the barrel axis was calculated from the experimentally obtained dichroic ratios of the amide I beta-sheet component and the amide II band. Comparing the IR spectra of the reconstituted VDAC isoforms with the IR spectra of the bacterial porin OmpF, for which a high resolution structure is available, provided evidence for a general structural organization of the VDAC isoforms similar to that of bacterial porins. Hydrogen-deuterium exchange measurements indicated that the exchange of the amide protons occurs to a higher extent in the two VDAC isoforms than in the OmpF porin.

Purification and characterization of two voltage-dependent anion channel isoforms from plant seeds

Mitochondria were isolated from imbibed seeds of lentil (Lens culinaris) and Phaseolus vulgaris. We copurified two voltage-dependent anion channel from detergent solubilized mitochondria in a single purification step using hydroxyapatite. The two isoforms from P. vulgaris were separated by chromatofocusing chromatography in 4 M urea without any loss of channel activity. Channel activity of each isoform was characterized upon reconstitution into diphytanoyl phosphatidylcholine planar lipid bilayers. Both isoforms form large conductance channels that are slightly anion selective and display cation selective substates.

Characterization of a cDNA encoding a rice mitochondrial voltage-dependent anion channel and its gene expression studied upon plant development and osmotic stress

The voltage-dependent anion channel (VDAC) of mitochondria forms a large pore in the outer envelope membrane. Here, the full Oryza sativa OSVDAC1 cDNA was sequenced and is shown to belong to a small multigene family in the rice genome. This cDNA is 1093 bp long and codes for a protein of 274 amino acids. Expression studies of the osvdac1 gene show a regulation of its level in function of the plantlets maturation and organs. In contrast with several bacterial porins, osmotic stress does not have any effect on the plant osvdac1 gene expression.

Binding of rat brain hexokinase to recombinant yeast mitochondria: identification of necessary molecular determinants

The association in vitro of rat brain hexokinase to mitochondria from rat liver or yeast (wild type, porinless, or expressing recombinant human porin) was studied in an effort to identify minimal requirements for each component. A short hydrophobic N-terminal peptide of hexokinase, readily cleavable by proteases, is absolutely required for its binding to all mitochondria. Mammalian porins are significantly cleaved at two positions in putative cytoplasmic loops around residues 110 and 200, as determined by proteolytic-fragment identification using antibodies. Recombinant human porin in yeast mitochondria is more sensitive to proteolysis than wild-type porin in rat liver mitochondria. Recombinant yeast mitochondria, harboring several natural or engineered porins from various sources, bind hexokinase to variable extent with marked preference for the mammalian porin1 isoform. Genetic alteration of this isoform at the C-, but not the N-terminal, results in a significant reduction of hexokinase binding ability. Macromolecular crowding (dextran) promotes a stronger association of the enzyme to all recombinant mitochondria, as well as to proteolytically digested organelles. Consequently, brain hexokinase association with heterologous mitochondria (yeast) in these conditions occurs to an extent comparable to that with homologous (rat) mitochondria. The study, also pertinent to the topology and organization of porin in the membrane, represents a necessary first step in the functional investigation of the physiological role of mammalian hexokinase binding to mitochondria in reconstituted heterologous recombinant systems, as models to cellular metabolism.

Studies on human porin. XVI: Polyamines reduce the voltage dependence of human VDAC in planar lipid bilayers--spermine and spermidine inducing asymmetric voltage gating on the channel

We present the first data on the effects of polyamines on human VDAC. Purified VDAC in lipid bilayers shows lowered voltage dependence whenever putrescine, cadaverine, spermine, spermidine, or the histamine releaser Compound 48/80, respectively, are applied. Only spermine and spermidine induce an asymmetric reaction on the channel. However, we state a groupwise different reaction of polyamines, which are organic polycations, on human porin.

Novel aspects of the electrophysiology of mitochondrial porin

The recent findings that mitochondrial porin, VDAC, participates in supramolecular complexes and is present in the plasmamembrane need to be reconciled with its biophysical properties. We report here that VDAC often displays previously unobserved or unappreciated behaviors. Reconstituted VDAC can: a) exhibit fast gating when in any of many conductance substates; b) close completely, although briefly, on its own; c) close for a long periods, in the presence of König's polyanion; d) take several milliseconds to re-open when an applied transmembrane potential is switched off; e) be desensitized by prolonged exposure to high voltages, so that it will not re-open to the full conductance state upon subsequent return to zero voltage; f) display polarity-dependent voltage-induced closure. These behaviors are especially noticeable when the observations are conducted on a single reincorporated channel, suggesting that interactions between copies of VDAC may play a role in determining its electrophysiological properties. Any model of VDAC's structure, gating and function should take these observations into account.