Toward the molecular structure of the mitochondrial channel, VDAC

Metabolically derived potential on the outer membrane of mitochondria: a computational model

The outer mitochondrial membrane (OMM) is permeable to various small substances because of the presence of a voltage-dependent anion channel (VDAC). The voltage dependence of VDAC's permeability is puzzling, because the existence of membrane potential on the OMM has never been shown. We propose that steady-state metabolically derived potential (MDP) may be generated on the OMM as the result of the difference in its permeability restriction for various charged metabolites. To demonstrate the possibility of MDP generation, two models were considered: a liposomal model and a simplified cell model with a creatine kinase energy channeling system. Quantitative computational analysis of the simplified cell model shows that a MDP of up to -5 mV, in addition to the Donnan potential, may be generated at high workloads, even if the OMM is highly permeable to small inorganic ions, including potassium. Calculations show that MDP and DeltapH, generated on the OMM, depend on the cytoplasmic pH and energy demand rate. Computational modeling suggests that MDP may be important for cell energy metabolism regulation in multiple ways, including VDAC's permeability modulation and the effect of electrodynamic compartmentation. The osmotic pressure difference between the mitochondrial intermembrane space and the cytoplasm, as related to the electrodynamic compartmentation effects, might explain the morphological changes in mitochondria under intense workloads.

Mitochondria-dependent apoptosis and cellular pH regulation

Mitochondria play a critical role in apoptosis induction in response to myriad stimuli. These organelles release proteins into the cytosol which trigger caspase activation or perform other functions relevant to apoptosis, including cytochrome c (cyt-c), caspases, AIF, and SMAC (Diablo). The mechanisms by which these proteins escape from mitochondria remain enigmatic. Moreover, it is unclear whether release of these proteins versus disturbances in core mitochondrial functions represents the cell death commitment mechanism. In this regard, suppression of apoptosis using broad-spectrum caspase inhibitory compounds has been reported in many circumstances to prevent the morphological and biochemical manifestations of apoptosis, and yet not protect cells from death and not preserve clonigenic survival. Thus, while mitochondrial damage can be coupled to caspase activation pathways, cell death commitment often occurs upstream of caspase activation when mitochondria-dependent cell death pathways are invoked. Here, we review evidence implicating dysregulation of cellular pH as a component of the cell death mechanism involving mitochondria. Cell Death and Differentiation (2000) 7, 1155 - 1165

Alternative interpretations of the oligonucleotide transport literature: insights from nature

Elucidation of the mechanism of oligonucleotide (ON) cellular internalization has met an impasse at the lipid penetration stage. ON internalization is commonly regarded to involve endocytosis, yet the method by which the ON penetrates the endosome membrane remains a mystery despite more than 10 years of research by multiple laboratories. In addition, the literature regarding this topic is fraught with discrepancies and inconsistencies. Therefore, the goal of this review is to propose and illustrate the feasibility of the notion that the literature discrepancies are perhaps an indication of a complex transport mechanism involving more than one uptake pathway. Accordingly, ON- and cell-differences in uptake may be attributed to differences in the relative importance of these pathways for different cell types and ONs. An example of one such pathway is reviewed and critiqued in this communication with respect to its hypothetical role in ON uptake. Other innovative mechanisms should similarly be considered to stimulate new ideas, discussion and research in this unique and interesting field.

MCC and PSC, the putative protein import channels of mitochondria

All but a small fraction of the hundreds of proteins in a mitochondrion are synthesized in the cytoplasm and imported into the organelle. Water-filled channels are integral to the process of translocating proteins since channels can provide an aqueous pathway through the hydrophobic environment of the membrane. The MCC (multiple conductance channel) and PSC (peptide-sensitive channel) are two high-conductance channels previously identified in electrophysiological studies of mitochondrial membranes. MCC and PSC are the putative pores of the import complexes of the inner and outer membranes, respectively. The genetic, biochemical, and biophysical evidence regarding these assignments are summarized herein. These findings support the identification of MCC and PSC as the protein import channels of mitochondria.

A 22 kDa polyanion inhibits carnitine-dependent fatty acid oxidation in rat liver mitochondria

The transport of activated fatty acids across the mitochondrial outer membrane has not been fully addressed. A polyanion (M(n)=22 kDa) inhibited the ADP-stimulated carnitine-dependent oxidation of both palmitoyl-CoA and palmitate plus CoA as well as mitochondrial hexokinase binding. In contrast, the oxidation of palmitoylcarnitine plus malate, as well as glutamate oxidation, was essentially unaffected. Mitochondrial carnitine palmitoyltransferase-1 was not inhibited by the polyanion. The data suggest an additional component in carnitine-dependent mitochondrial fatty acid oxidation, possibly porin.

Crystallization of the human, mitochondrial voltage-dependent anion-selective channel in the presence of phospholipids

Overexpressed human voltage-dependent anion-selective channel VDAC or porin from mitochondrial outer membranes has been purified to homogeneity. Electron microscopic analysis of VDAC in detergent solution revealed a uniform particle population consisting of porin monomers. After dialysis of detergent-solubilized porin in the presence of dimyristoylphosphatidylcholine at lipid-to-protein ratios between 0.2 and 0.5 (percentage by weight), mostly multilamellar crystals were obtained. Crystals adsorbed to carbon films flattened during negative staining and air-drying and exhibited different structural features due to differences in the vertical stacking of several crystalline layers, each consisting of one membrane bilayer. Adsorbed, frozen-hydrated multilamellar membrane crystals revealed uniform diffraction patterns with sharp diffraction spots extending to 8.2 A. The surface structure of VDAC was reconstructed from freeze-dried and unidirectionally metal-shadowed crystals. Major protein protrusions were observed from two VDAC monomers present in the unit cell. Differences in the surface structural features indicate alternate orientations of VDAC molecules with respect to the lipid bilayer, allowing the simultaneous imaging of both the cytosolic and intramitochondrial surfaces. Each VDAC molecule consists of a pore lumen with a diameter of 17-20 A surrounded by a protein rim of nonuniform height, suggesting an asymmetrical distribution of protein mass around the diffusion channels.

Investigation of bax-induced release of cytochrome c from yeast mitochondria permeability of mitochondrial membranes, role of VDAC and ATP requirement

Recent studies that attempt to explore the action of pro- and anti-apoptotic proteins of the bcl2 family demonstrate the crucial role of relocalization of cytochrome c from the mitochondrial intermembrane space to the cytosol. This early event of apoptosis can be mimicked in the yeast Saccharomyces cerevisiae following expression of bax. In mammalian mitochondria, the mechanism of relocalization is thought to involve the opening of the so-called permeability transition pore. We show in this paper: (a) that bax-induced release of cytochrome c in yeast does not involve any permeability transition of the inner mitochondrial membrane but involves a general alteration of the permeability of the outer mitochondrial membrane to macromolecules. This suggests that a permeability transition of the inner mitochondrial membrane is not an event required for the relocalization of cytochrome c in yeast. (b) The outer-membrane voltage-dependent anion channel (VDAC), a putative component of the permeability transition pore, is not involved in bax-induced release of cytochrome c or in the prevention of this release by bcl-xL. (c) Bax devoid of its C-terminal putative hydrophobic alpha-helix is as efficient as full-length bax to allow the relocalization of cytochrome c, demonstrating this segment of the protein is not required for membrane-targeting. (d) We finally observe that the action of bax on the outer mitochondrial membrane requires the presence of ATP both in vitro and in vivo, and it is shown that ATP directly increases the amount of bax inserted to mitochondria.

Voltage-dependent anion channel proteins in synaptosomes of the torpedo electric organ: immunolocalization, purification, and characterization

In this study, we purified and characterized the voltage-dependent anion channel (VDAC) from the Torpedo electric organ. Using immunogold labeling, VDAC was colocalized with the voltage-gated Ca2+ channel in the synaptic plasma membrane. By immunoblot analysis, five protein bands in synaptosomes isolated from the Torpedo electric organ cross reacted with two monoclonal anti-VDAC antibody. No more than about 7 to 10% mitochondrial contains could be detected in any synaptosomal membrane preparation tested. This was estimated by comparing the specific activity in mitochondria and synaptosomes of succinate-cytochrome-c oxidoreductase and antimycin-insensitive NADH-cytochrome-c oxidoreductase activities; mitochondrial inner and outer membrane marker enzymes, respectively. [14C]DCCD (dicyclohexylcarbodiimide), which specifically label mitochondrial VDAC, labeled four 30-35 kDa protein bands that were found to interact with the anti-VDAC antibody. The distribution of the Torpedo VDAC protein bands was different among membranes isolated from various tissues. VDAC was purified from synaptosomes and a separation between two of the proteins was obtained. The two purified proteins were characterized by their single channel activity and partial amino acid sequences. Upon reconstitution into a planar lipid bilayer, the purified VDACs showed voltage-dependent channel activity with properties similar to those of purified mitochondrial VDAC. Amino acid sequence of four peptides, derived from VDAC band II, exhibited high homology to sequences present in human VDACI (98%), VDAC2 (91.8%), and VDAC3 (90%), while another peptide, derived from VDAC band III, showed lower homology to either VDAC1 (88.4%) or VDAC2 (79%). Two more peptides show high homology to the sequence present in mouse brain VDAC3 (100 and 78%). In addition, we demonstrate the translocation of ATP into synaptosomes, which is inhibited by DCCD and by the anion transport inhibitor DIDS. The possible function of VDAC in the synaptic plasma membrane is discussed.

The topology of VDAC as probed by biotin modification

The outer membrane of mitochondria contains channels called VDAC (mitochondrial porin), which are formed by a single 30-kDa protein. Cysteine residues introduced by site-directed mutagenesis at sites throughout Neurospora crassa VDAC (naturally devoid of cysteine) were specifically biotinylated prior to reconstitution into planar phospholipid membranes. From previous studies, binding of streptavidin to single biotinylated sites results in one of two effects: reduced single-channel conductance without blockage of voltage gating (type 1) or locking of the channels in a closed conformation (type 2). All sites react with streptavidin only from one side of the membrane. Here, we extend this approach to VDAC molecules containing two cysteines and determine the location of each biotinylated residue with respect to the other within the membrane. When a combination of a type 1 and a type 2 site was used, each site could be observed to react with streptavidin. Two sets of sites located on opposite surfaces of the membrane were identified, thereby establishing the transmembrane topology of VDAC. A revised folding pattern for VDAC, consisting of 1 alpha helix and 13 beta strands, is proposed by combining these results with previously obtained information on which sites are lining the aqueous pore.

Mitochondria and apoptosis

A variety of key events in apoptosis focus on mitochondria, including the release of caspase activators (such as cytochrome c), changes in electron transport, loss of mitochondrial transmembrane potential, altered cellular oxidation-reduction, and participation of pro- and antiapoptotic Bcl-2 family proteins. The different signals that converge on mitochondria to trigger or inhibit these events and their downstream effects delineate several major pathways in physiological cell death.

Bcl-2 family proteins and mitochondria

The Bcl-2 family of proteins plays a pivotal role in regulating cell life and death. Many of these proteins reside in the outer mitochondrial membrane, oriented towards the cytosol. Cytoprotective Bcl-2 family proteins such as Bcl-2 and Bcl-XL prevent mitochondrial permeability transition pore opening and release of apoptogenic proteins from mitochondria under many circumstances that would otherwise result in either apoptosis or necrosis. In contrast, some pro-apoptotic members of this family such as Bax can induce these destructive changes in mitochondria in both mammalian cells and when expressed exogenously in yeast. The mechanisms by which Bcl-2 family proteins control cell life and death remain elusive, but may include both the ability to form ion channels or pores in membranes and physical interactions with a variety of proteins implicated in apoptosis regulation.

Conformational changes in the mitochondrial channel protein, VDAC, and their functional implications

The voltage-dependent, anion-selective channel (VDAC) is generally considered the main pathway for metabolite diffusion across the mitochondrial outer membrane. It also interacts with several mitochondrial and cytosolic proteins, including kinases and cytochrome c. Sequence analysis and circular dichroism suggest that the channel is a bacterial porin-like beta-barrel. However, unlike bacterial porins, VDAC does not form tight trimeric complexes and is easily gated (reversibly closed) by membrane potential and low pH. Circular dichroism indicates that the protein undergoes a major conformational change at pH < 5, involving decreased beta-sheet and increased alpha-helical content. Electron microscopy of two-dimensional crystals of fungal VDAC provides direct information about the size and shape of its lumen and suggests that the N-terminal domain forms a mobile alpha-helix. It is proposed that the N-terminal domain normally resides in a groove in the lumen wall and that gating stimuli favor its displacement, destabilizing the putative beta-barrel. Partial closure would result from subsequent larger-scale structural rearrangements in the protein, possibly corresponding to the conformational change observed at pH < 5.

Binding of rat brain hexokinase to recombinant yeast mitochondria: effect of environmental factors and the source of porin

Heterologous binding of rat brain hexokinase to wild type, porinless, and recombinant yeast mitochondria expressing human porin was assessed, partially characterized, and compared to that in the homologous system (rat liver mitochondria). With porin-containing yeast mitochondria it is shown that (i) a significant, saturable association occurs; (ii) its extent and apparent affinity, correlated with the origin of porin, are enhanced in the presence of dextran; (iii) the binding requires Mg ions and apparently follows a complex cooperative mechanism. This heterologous association does not seem to differ fundamentally from that in the homologous system and represents a good basis for molecular studies in yeast. With porinless yeast mitochondria, binding occurs at much lower affinity, but to many more sites per mitochondrion. The results indicating a major but not exclusive role for porin in the binding are discussed in terms of (i) the mode and mechanism of binding, and (ii) the suitability of the rat hexokinase-yeast mitochondria couple for the study of heterogeneous catalysis in reconstituted cellular model systems.

Double-stranded DNA can be translocated across a planar membrane containing purified mitochondrial porin

The transport of genetic material across biomembranes is a process of great relevance for several fields of study. However, much remains to be learned about the mechanisms underlying transport, one of which implies the involvement of proteic DNA-conducting pores. Entry of genetic material into mitochondria has been observed under both physiological and pathological conditions. We report here that double-stranded DNA can move through a planar bilayer membrane containing isolated mitochondrial porin (voltage-dependent anion channel). The transport is driven by the applied electrical field, and the presence of DNA is associated with a decrease of current conduction by the pores. The passage of DNA does not take place if the bilayer has not been doped with any protein or in the presence of both reconstituted porin and anti-porin antibody. Translocation does not occur if the bilayer contains Shigella sonnei maltoporin, gramicidin A channels, or a 30 pS anion-selective channel plus other proteins. These results show that mitochondrial porin is capable of mediating the transport of genetic material, revealing a new property of this molecule and further confirming the idea that DNA can move through proteic pores.

The intracellular potassium and chloride channels: properties, pharmacology and function (review)

Channels selective for potassium or chloride ions are present in membranes of intracellular organelles such as sarcoplasmic (endoplasmic) reticulum, mitochondria, nucleus, synaptic vesicles, and chromaffin, and zymogen granules. They probably play an important role in cellular events such as compensation of electrical charges during transport of Ca2+, delta pH formation in mitochondria or V-ATPase containing membrane granules, and regulation of volume changes, due to potassium and chloride transport into intracellular organelles. Intracellular potassium and chloride channels could also be the target for pharmacologically active compounds. This mini-review describes the basic properties, pharmacology, and current hypotheses concerning the functional role of intracellular potassium and chloride channels.

Minireview: on the structure and gating mechanism of the mitochondrial channel, VDAC

There is considerable evidence that the voltage-gated mitochondrial channel VDAC forms a beta-barrel pore. Inferences about the number and tilt of beta-strands can be drawn from comparisons with bacterial beta-barrel pores whose structures have been determined by x-ray crystallography. A structural model for VDAC is proposed (based on sequence analysis and electron crystallography) in which the open state is like that of bacterial porins with several important differences. Because VDAC does not occur as close-packed trimers, there are probably fewer interpore contacts than in the bacterial porins. VDAC also appears to lack a large, fixed intraluminal segment and may not have as extensive a region of uniformly 35 degrees -tilted beta-strands as do the bacterial porins. These structural differences would be expected to render VDAC's beta-barrel less stable than its bacterial counterparts, making major conformational changes like those associated with gating more energetically feasible. A possible gating mechanism is suggested in which movement of the N-terminal alpha-helix out of the lumen wall triggers larger-scale structural changes.

Selective photoaffinity labelling of one mitochondrial protein in living cells of Saccharomyces cerevisiae with the fluorescent probe APMC. Identification of the target protein as subunit I of cytochrome c oxidase

The lipophilic, cationic fluorochrome azopentylmethylindocarbocyanine (APMC) specifically stains the mitochondria in living yeast cells (Saccharomyces cerevisiae WT X 2180). It contains a photosensitive diazirine ring and is suitable for photoaffinity labelling. By combining photoaffinity labelling, micro-gel electrophoresis (SDS-PAGE), and detection of the APMC fluorescence with a microfluorimeter, we established a highly sensitive procedure for determining the apparent molecular weight of the APMC-labelled proteins in yeast cells. On vital staining at 0.1 microM APMC for 30 min, only one mitochondrial protein with an apparent molecular weight of 40 kDa is labelled with high intensity. At increased dye concentrations proteins of 47 and 49 kDa are labelled too, however not until all binding sites of the 40 kDa protein are occupied. Obviously, the APMC cations have a pronounced affinity for this protein. It was shown by fractional centrifugation that the labelled 40 kDa protein is a constituent of the inner mitochondrial membrane. One driving force for the accumulation of the APMC cations is the trans-membrane potential (TMP) across the inner mitochondrial membrane. Consequently, uncouplers like dinitrophenol (DNP) and carbonylcyanidechlorophenyl-hydrazone (CCCP), ionophores (valinomycin, gramicidin), and inhibitors of the respiratory chain (myxothiazol, KCN), which decrease the TMP, also diminish the APMC accumulation and labelling. And conversely, drugs, which hyperpolarize the inner membrane (nigericin, atractyloside), favour APMC labelling. Another driving force of APMC accumulation is the dye's lipophilicity, which facilitates dye accumulation by hydrophobic interaction with the very lipophilic proteins of the inner mitochondrial membranes. This was shown by competitive double staining experiments. Thiamine strongly inhibits APMC labelling. Obviously, the transport of the APMC cations is facilitated by the thiamine carrier, and thiamine competes for the same binding sites, which are occupied by the dye cations. Chloramphenicol is an inhibitor of the mitochondrial protein synthesis without affecting the TMP. On preincubation, chloramphenicol completely quenches the signal of the 40 kDa protein. Therefore, this protein must be encoded on the mtDNA. The only 40 kDa protein with adequate properties is the subunit I of cytochrome c oxidase. Obviously, it is the preferred target of the APMC cations on photoaffinity labelling. This assignment agrees with the strong hydrophobicity of the labelled 40 kDa protein, which was tested with various detergents. It also agrees with the solvatochromism of the protein-bound APMC label, and finally with the paralellism of the labelled protein with cytochrome c oxidase on fractional ammonium sulfate precipitation.

Is the mammalian porin channel, VDAC, a perfect cylinder in the high conductance state?

The mammalian porin channel (VDAC, porin-31BM) was reconstituted in planar lipid bilayers under voltage clamp conditions. The radii of both entrances of the channel were examined using a method that consisted in filling the channel with different non-electrolytes through its cis or trans entrances while recording single channel conductances. As a result it was found that the geometry of channels formed by porin-31BM could not be approximated by a perfectly cylindrical pore. In fact there is an asymmetry in the geometry of the channel: the diameters of the cis and trans entrances were estimated to be approximately 2 nm and approximately 4 nm respectively.

Isolation and characterization of an amino acid-selective channel protein present in the chloroplastic outer envelope membrane

The reconstituted pea chloroplastic outer envelope protein of 16 kDa (OEP16) forms a slightly cation-selective, high-conductance channel with a conductance of Lambda = 1,2 nS (in 1 M KCl). The open probability of OEP16 channel is highest at 0 mV (Popen = 0.8), decreasing exponentially with higher potentials. Transport studies using reconstituted recombinant OEP16 protein show that the OEP16 channel is selective for amino acids but excludes triosephosphates or uncharged sugars. Crosslinking indicates that OEP16 forms a homodimer in the membrane. According to its primary sequence and predicted secondary structure, OEP16 shows neither sequence nor structural homologies to classical porins. The results indicate that the intermembrane space between the two envelope membranes might not be as freely accessible as previously thought.

Electron tomography of neuronal mitochondria: three-dimensional structure and organization of cristae and membrane contacts

The structure of neuronal mitochondria from chick and rat was examined using electron microscope tomography of chemically fixed tissue embedded in plastic and sliced in approximately 500 nm-thick sections. Three-dimensional reconstructions of representative mitochondria were made from single-axis tilt series acquired with an intermediate voltage electron microscope (400 kV). The tilt increment was either 1 degree or 2 degrees ranging from -60 degrees to +60 degrees. The mitochondrial ultrastructure was similar across species and neuronal regions. The outer and inner membranes were each approximately 7 nm thick. The inner boundary membrane was found to lie close to the outer membrane, with a total thickness across both membranes of approximately 22 nm. We discovered that the inner membrane invaginates to form cristae only through narrow, tubular openings, which we call crista junctions. Sometimes the cristae remain tubular throughout their length, but often multiple tubular cristae merge to form lamellar compartments. Punctate regions, approximately 14 nm in diameter, were observed in which the inner and outer membranes appeared in contact (total thickness of both membranes approximately 14 nm). These contact sites are known to a play a key role in the transport of proteins into the mitochondrion. It has been hypothesized that contact sites may be proximal to crista junctions to facilitate transport of proteins destined for the cristae. However, our statistical analyses indicated that contact sites are randomly located with respect to these junctions. In addition, a close association was observed between endoplasmic reticulum membranes and the outer mitochondrial membrane, consistent with the reported mechanism of transport of certain lipids into the mitochondrion.

The murine voltage-dependent anion channel gene family. Conserved structure and function

Voltage-dependent anion channels (VDACs) are pore-forming proteins found in the outer mitochondrial membrane of all eucaryotes. VDACs are the binding sites for several cytosolic enzymes, including the isoforms of hexokinase and glycerol kinase. VDACs have recently been shown to conduct ATP when in the open state, allowing bound kinases preferential access to mitochondrial ATP and providing a possible mechanism for the regulation of adenine nucleotide flux. Two human VDAC cDNAs have been described previously, and we recently reported the isolation of mouse VDAC1 and VDAC2 cDNAs, as well as a third novel VDAC cDNA, designated VDAC3. In this report we describe the structural organization of each mouse VDAC gene and demonstrate that, based on conserved exon/intron boundaries, the three VDAC isoforms belong to a single gene family. The 5'-flanking region of each VDAC gene was shown to have transcription promoter activity by transient expression in cultured cells. The promoter region of each VDAC isoform lacks a canonical TATA box, but all are G+C-rich, a characteristic of housekeeping gene promoters. To examine the conservation of VDAC function, each mouse VDAC was expressed in yeast lacking the endogenous VDAC gene. Both VDAC1 and VDAC2 are able to complement the phenotypic defect associated with the mutant yeast strain. VDAC3, however, is only able to partially complement the mutant phenotype, suggesting an alternative physiologic function for the VDAC3 protein.

Dichroic ratios in polarized Fourier transform infrared for nonaxial symmetry of beta-sheet structures

The transition moments for the amide bands from beta-sheet peptide structures generally do not exhibit axial symmetry about the director in linearly polarized Fourier transform infrared (FTIR) measurements on oriented systems. The angular dependences of the dichroic ratios of the amide bands are derived for beta-sheet structures in attenuated total reflection (ATR) and polarized transmission experiments on samples that are oriented with respect to the normal to the substrate and are randomly distributed with respect to the azimuthal angle in the plane of the orienting substrate. The orientational distributions of both the beta-strands and the beta-sheets are considered, and explicit expressions are given for the dichroic ratios of the amide I and amide II bands. The dichroic ratio of the amide II band, which is parallel polarized, can yield the orientation of the beta-strands directly, but to specify the orientations of the beta-sheets completely requires measurement of the dichroic ratios of both the amide I and amide II bands, or generally two bands with parallel and perpendicular polarizations. A random distribution in tilt of the planes of the beta-sheets does not give rise to equal dichroic ratios for bands with perpendicular and parallel polarizations, such as the amide I and amide II bands. The results are applied to previous ATR and polarized transmission FTIR measurements on a potassium channel-associated peptide, the Escherichia coli outer membrane protein OmpA, and the E. coli OmpF porin protein in oriented membranes.

Autodirected insertion: preinserted VDAC channels greatly shorten the delay to the insertion of new channels

VDAC, a mitochondrial outer membrane channel, has the ability to catalyze and direct the insertion of other VDAC channels into planar phospholipid membranes. The spontaneous rate of insertion of detergent-solubilized VDAC channels into phospholipid membranes is estimated to be 1.5 x 10(-5) channels min-1 micron-2. VDAC channels already in the membrane can increase this rate by a factor of 10(9). The presence of 5 M urea on the opposite side of the membrane increases this 10-fold to 4.5 x 10(5) channels min-1 microns-2. Similar but weaker effects are observed with Triton X100 addition (10(-3)% (v/v)). These agents are not acting on uninserted channels because they do not affect the delay from sample addition to first insertion. Under the chosen conditions, this delay is long (240 s) without preinserted channels. However, the presence of a few VDAC channels in the membrane reduces this delay to 14 s, close to the diffusion limit. Therefore, urea and Triton, added to the side of the membrane opposite that to which the VDAC sample was added, likely increase the flexibility of the VDAC channels in the membrane, allowing them to be more efficient catalysts for VDAC insertion. There are obvious implications for membrane protein insertion and targeting.

Peripheral benzodiazepine receptor in cholesterol transport and steroidogenesis

Steroidogenesis begins with the metabolism of cholesterol to pregnenolone by the inner mitochondrial membrane cytochrome P450 side-chain cleavage (P450scc) enzyme. The rate of steroid formation, however, depends on the rate of cholesterol transport from intracellular stores to the inner mitochondrial membrane and loading of P450scc with cholesterol. In previous in vitro studies, we demonstrated that a key element in the regulation of cholesterol transport is the mitochondrial peripheral-type benzodiazepine receptor (PBR). We also showed that the polypeptide diazepam binding inhibitor (DBI), an endogenous PBR ligand, stimulates cholesterol transport and promotes loading of cholesterol to P450scc in vitro, and that its presence is vital for hCG-induced steroidogenesis by Leydig cells. Based on these data and the observations that i) the mitochondrial PBR binding and topography are regulated by hormones; ii) the 18-kDa PBR protein is functionally coupled to the mitochondrial contact site voltage-dependent anion channel protein; iii) the 18-kDa PBR protein is a channel for cholesterol, as shown by molecular modeling and in vitro reconstitution studies; iv) targeted disruption of the PBR gene in steroidogenic cells dramatically reduces the ability of the cells to transport cholesterol in the mitochondria and produce steroids; v) endocrine disruptors, with known anisteroidogenic effect, inhibit PBR ligand binding; and vi) in vivo reduction of adrenal PBR expression results in reduced circulating glucocorticoid levels, we conclude that PBR is an indispensable element of the steroidogenic machinery.

Pore formation in lipid bilayer membranes made of phosphatidylinositol and oxidized cholesterol followed by means of alternating current

The kinetics of porin incorporation into black lipid membranes (BLM) made of phosphatidylinositol (PI) or oxidized cholesterol (Ox Ch) were studied by means of alternating current; the set-up was able to acquire resistance and capacitance simultaneously by means of a mixed double-frequency approach at 1 Hz and 1 KHz, respectively. Conductance was dependent on the interaction between protein-forming pores and lipids. For PI membranes below a porin concentration of 12.54 ng/ml, there was no membrane conductivity, whereas at 200 ng/ml a steady-state value was reached. Different behavior was displayed by Ox Ch membranes, in which a concentration of 12.54 ng/ml was sufficient to reach a steady state. The incorporation kinetics when porin was added after membrane formation were sigmoidal. When porin was present in the medium before membrane formation, the kinetics were sigmoidal for PI membranes but became exponential for Ox Ch membranes. Furthermore, for BLM made of PI, the conductance-versus-porin concentration relationship is sigmoidal, with a Hill coefficient of 5.6 +/- 0.07, which is functional evidence corroborating the six-channel repeating units seen previously. For BLM made of Ox Ch, this relationship followed a binding isotherm curve with a Hill coefficient of 0.934 +/- 0.129.

Circular dichroism studies of the mitochondrial channel, VDAC, from Neurospora crassa

The protein that forms the voltage-gated channel VDAC (or mitochondrial porin) has been purified from Neurospora crassa. At room temperature and pH 7, the circular dichoism (CD) spectrum of VDAC suspended in octyl beta-glucoside is similar to those of bacterial porins, consistent with a high beta-sheet content. When VDAC is reconstituted into phospholipid liposomes at pH 7, a similar CD spectrum is obtained and the liposomes are rendered permeable to sucrose. Heating VDAC in octyl beta-glucoside or in liposomes results in thermal denaturation. The CD spectrum irreversibly changes to one consistent with total loss of beta-sheet content, and VDAC-containing liposomes irreversibly lose sucrose permeability. When VDAC is suspended at room temperature in octyl beta-glucoside at pH < 5 or in sodium dodecyl sulfate at pH 7, its CD spectrum is consistent with partial loss of beta-sheet content. The sucrose permeability of VDAC-containing liposomes is decreased at low pH and restored at pH 7. Similarly, the pH-dependent changes in the CD spectrum of VDAC suspended in octyl beta-glucoside also are reversible. These results suggest that VDAC undergoes a reversible conformational change at low pH involving reduced beta-sheet content and loss of pore-forming activity.

Alpha-helical, but not beta-sheet, propensity of proline is determined by peptide environment

Proline is established as a potent breaker of both alpha-helical and beta-sheet structures in soluble (globular) proteins. Thus, the frequent occurrence of the Pro residue in the putative transmembrane helices of integral membrane proteins, particularly transport proteins, presents a structural dilemma. We propose that this phenomenon results from the fact that the structural propensity of a given amino acid may be altered to conform to changes imposed by molecular environment. To test this hypothesis on proline, we synthesized model peptides of generic sequence H2N-(Ser-LyS)2-Ala- Leu-Z-Ala-Leu-Z-Trp-Ala-Leu-Z-(Lys-Ser)3-OH (Z = Ala and/or Pro). Peptide conformations were analyzed by circular dichroism spectroscopy in aqueous buffer, SDS, lysophosphatidylglycerol micelles, and organic solvents (methanol, trifluoroethanol, and 2-propanol). The helical propensity of Pro was found to be greatly enhanced in the membrane-mimetic environments of both lipid micelles and organic solvents. Proline was found to stabilize the alpha-helical conformation relative to Ala at elevated temperatures in 2-propanol, an observation that argues against the doctrine that Pro is the most potent alpha-helix breaker as established in aqueous media. Parallel studies in deoxycholate micelles of the temperature-induced conformational transitions of the single-spanning membrane bacteriophage IKe major coat protein, in which the Pro-containing wild type was compared with Pro30 --> Ala mutant, Pro was found to protect the helix, but disrupt the beta-sheet structure as effectively as it does to model peptides in water. The intrinsic capacity of Pro to disrupt beta-sheets was further reflected in a survey of porins where Pro was found to be selectively excluded from the core of membrane-spanning beta-sheet barrels. The overall data provide a rationale for predicting and understanding the structural consequences when Pro occurs in the context of a membrane.

Detection of likely transmembrane beta strand regions in sequences of mitochondrial pore proteins using the Gibbs sampler

The mitochondrial channel VDAC is presumed to fold as a beta-barrel although the number and identity of transmembrane beta-strands in the protein are controversial. Previously, a novel multiple alignment algorithm called the Gibbs sampler was used to detect a residue-frequency motif in sequences of bacterial outer-membrane proteins that corresponds to transmembrane beta-strands in bacterial porins of known structure (Neuwald et al., 1995, Protein Science, 4, 1618. In the present study, this bacterial motif has been used to screen sets of mitochondrial membrane protein sequences, with matches occurring in only two classes of proteins: VDACs and the outer-membrane protein import pore (1SP42, M0M38). These results suggest a structural (and perhaps evolutionary) relatedness between the bacterial and mitochondrial pore proteins, with the mitochondrial subsequences that match the bacterial motif corresponding to transmembrane beta-strands as in the porins.

Indications of a common folding pattern for VDAC channels from all sources

Previous research on the mitochondrial channel VDAC from the yeast S. cerevisiae had identified protein strands forming the wall of VDAC's aqueous pore. Here we report the results of analyzing the primary sequences of VDAC from various sources to see if the transmembrane folding pattern identified from this yeast is conserved for VDAC of different species. We analyzed the primary sequences of VDAC from higher plants, fungi, invertebrates, and vertebrates and found that all have a very similar "beta-pattern" profile with 12-15 peaks indicating potential sided beta strands that are candidates for protein strands forming the wall of the aqueous pore. All these VDAC sequences can be put into the 13 transmembrane strand folding pattern previously identified for yeast VDAC. These folding patterns agree with available experimental data: both electrophysiological and protease digestion data. Although the primary sequences of VDAC from very diverse organisms show low homology, sequence similarity in the proposed corresponding 13 transmembrane strands is substantial. Competing proposals utilizing 16 transmembrane beta strands are in conflict with electrophysiological experimental observations and violate the constraints on such strands, such as no charged amino acids facing the phospholipid membrane and sufficient number of residues to span the membrane.

Pore formation by the cytotoxic islet amyloid peptide amylin

Amylin is a 37-amino acid cytotoxic constituent of amyloid deposits found in the islets of Langerhans of patients with type II diabetes. Extracellular accumulation of this peptide results in damage to insulin-producing beta cell membranes and cell death. We report here that at cytotoxic concentrations, amylin forms voltage-dependent, relatively nonselective, ion-permeable channels in planar phospholipid bilayer membranes. Channel formation is dependent upon lipid membrane composition, ionic strength, and membrane potential. At 1-10 microM, cytotoxic human amylin dramatically increases the conductance of lipid bilayer membranes, while non-cytotoxic rat amylin does not. We suggest that channel formation may be the mechanism of cytotoxicity of human amylin.

The CHIP28 water channel visualized in ice by electron crystallography

Electron crystallography of frozen-hydrated two-dimensional crystals of deglycosylated human erythrocyte CHIP28 reveals an aqueous vestibule in each monomer leading to the water-selective channel that is enclosed by multiple transmembrane alpha-helices.

Peptide-specific antibodies as probes of the topography of the voltage-gated channel in the mitochondrial outer membrane of Neurospora crassa

The voltage-dependent anion-selective channel (VDAC) in mitochondrial outer membranes is formed by a polypeptide (M(r) 31,000) coded by a nuclear gene whose cDNA sequence is known for several organisms. Antibodies have been raised against synthetic peptides corresponding to four different regions in the predicted sequence of the VDAC polypeptide of the fungus Neurospora crassa (residues 1-20, amino terminus; 195-210, 251-268, and 272-283, carboxyl terminus). Specificity of the antibodies has been characterized in terms of binding to peptides or fungal mitochondria on microtiter plates and binding to mitochondrial proteins of several species in Western blots. Reactivity of three of the four antibodies with fungal mitochondria in suspension increases with lysis of outer membranes, indicating that the respective epitopes (including those near the amino and carboxyl termini) are exposed on the surface of the outer membrane that faces inside the mitochondrion. Preincubation of mitochondria with a polyanion that modulates VDAC voltage dependence strongly inhibits binding of the antibody against residues 251-268, whose epitopes are on the outer mitochondrial surface.

Role of the peripheral-type benzodiazepine receptor and the polypeptide diazepam binding inhibitor in steroidogenesis

Steroidogenesis begins with the metabolism of cholesterol to pregnenolone by the inner mitochondrial membrane cytochrome P450 side-chain cleavage (P450scc) enzyme. The rate of steroid formation, however, depends on the rate of (i) cholesterol transport from intracellular stores to the inner mitochondrial membrane and (ii) loading of P450scc with cholesterol. We demonstrated that a key element in the regulation of cholesterol transport is the mitochondrial peripheral-type benzodiazepine receptor (PBR) and that the presence of the polypeptide diazepam binding inhibitor (DBI) was vital for steroidogenesis. We also showed that DBI, as the endogenous PBR ligand, stimulates cholesterol transport. In addition, DBI directly promotes loading of cholesterol to P450scc. We review herein our studies on the structure, function, topography and hormonal regulation of PBR and DBI in steroidogenic cells. Based on these data we propose a model where the interaction of DBI with PBR, at the outer/inner membrane contact sites, is the signal transducer of hormone-stimulated and constitutive steroidogenesis at the mitochondrial level. Hormone-induced changes in PBR microenvironment/structure regulate the affinity of the receptor. PBR ligand binding to a higher affinity receptor results in increased cholesterol transport. In addition, hormone-induced release (processing?) of a 30,000 Mw DBI-immunoreactive protein from the inner mitochondrial membrane may result to the intramitochondrial production of DBI which directly stimulates loading of P450scc with cholesterol. Thus, in vivo, hormonal activation of these two mechanisms results in efficient cholesterol delivery and utilization and thus high levels of steroid synthesis.

The high-conductance channel of porin-less yeast mitochondria

Patch-clamp and planar bilayer experiments on porin-less yeast mitochondria have allowed the characterization of a cationic channel activated at matrix-side positive (unphysiological) potentials. In voltage-pulse experiments, inactivation was a faster process than activation and the time constant for inactivation was more steeply dependent on voltage than the one for activation. The channel exhibited various conductance states whose occupancy depended on the applied transmembrane potential. In bilayer experiments, the presence of the pCOx-IV leader peptide induced fast gating in a voltage-dependent manner. A comparison with previously described activities suggests that the pore may coincide with the peptide-sensitive channel (PSC) (Thieffry et al. (1988) EMBO J. 7, 1449-1454) as well as with two other activities (Dihanich et al. (1989) Eur. J. Biochem. 181, 703-708; Tedeschi et al. (1987) J. Membr. Biol. 97, 21-29) assigned to the mitochondrial outer membrane. The possible relationship of this channel to the mitochondrial megachannel is discussed.

Role of sterols in the functional reconstitution of water-soluble mitochondrial porins from different organisms

Experiments were performed on lipid bilayer membranes with water-soluble mitochondrial porins from different eukaryotic organisms, such as Dictyostelium discoideum, Paramecium, and rat liver, to study the requirements of functional reconstitution of the porins. The water-soluble porins lost their associated lipids and sterols and are unable to form channels in lipid bilayer membranes. We demonstrate that the water-soluble porins regain their channel-forming ability after preincubation of the polypeptides with sterols in the presence of detergents. Mitochondrial porin from Dictyostelium discoideum maintained after this procedure its original properties, in particular the voltage dependence. Water-soluble mitochondrial porins from Paramecium tetraurelia and from rat liver were also activated upon preincubation with different sterols in detergent but showed voltage-dependences that were different from those of detergent-purified porins. Furthermore, the voltage dependence depended on the sterol used for preincubation. Interestingly, the preincubation with sterols can likewise be used to activate detergent-purified mitochondrial porins that may have lost associated sterol during isolation and purification procedures.

Immunoelectron microscopic study of the distribution of porin on outer membranes of rat heart mitochondria

The distribution of porin on the outer membranes of rat heart mitochondria has been studied by means of immunogold labelling with antibodies to the N-terminal part of the human protein. It was found that only a minority of isolated, unfixed mitochondria are labelled by these antibodies, with the gold particles frequently organized in threads or bands. Extensive immunogold labelling is frequently observed on regions of outer membranes stripped away from mitochondria and on regions separating two mitochondrial compartments whose cristae display different configurations (possibly representing two mitoplasts covered by a common outer membrane). Also, pairs of connected mitochondria are sometimes heavily labelled in the "neck" regions, which may represent the junctions involved in electrical communication between mitochondria in cardiac tissue.

Electrophysiology of the inner mitochondrial membrane

The application of electrophysiological techniques to mitochondrial membranes has allowed the observation and partial characterization of several ion channels, including an ATP-sensitive K(+)-selective one, a high-conductance "megachannel", a 107 pS anionic channel and three others studied at alkaline pH's. A reliable correlation with the results of non-electrophysiological studies has been obtained so far only for the first two cases. Activities presumed to be associated with the Ca2+ uniporter and with the adenine nucleotide translocator, as well as the presence of various other conductances have also been reported. The review summarizes the main properties of these pores and their possible relationship to permeation pathways identified in biochemical studies.

Topography of the Leydig cell mitochondrial peripheral-type benzodiazepine receptor

Native MA-10 mouse Leydig tumor cell mitochondrial preparations were examined by transmission electron (TEM) and atomic force (AFM) microscopic procedures in order to investigate the topography and organization of the peripheral-type benzodiazepine receptor (PBR). Mitochondria were immunolabeled with an anti-PBR antiserum coupled to gold-labeled secondary antibodies. Results obtained indicate that the 18,000 MW PBR protein is organized in clusters of 4-6 molecules. Moreover, on many occasions, the interrelationship among the PBR molecules was found to favor the formation of a single pore. Taking into account recent observations that the 18,000 MW PBR protein is functionally associated with the pore forming 34,000 MW voltage-dependent anion channel (VDAC) these results suggest that (i) the mitochondrial PBR complex could function as a pore, thus allowing the translocation of cholesterol and other molecules to the inner mitochondrial membrane, and (ii) the native receptor is a multimeric complex of an approximate 140,000 MW composed on an average of five 18,000 PBR subunits, one 34,000 VDAC subunit, and associated lipids.

Characterization and partial purification of the VDAC-channel-modulating protein from calf liver mitochondria

The mitochondrial channel, VDAC, mediates metabolic flux across the mitochondrial outer membrane. When reconstituted into planar phospholipid membranes, VDAC is voltage-dependent, existing in multiple conformational states with different selectivities and permeabilities. At low membrane potentials, these channels are in the open state and are anion-selective. VDAC channels switch to lower-conductive closed states at high membrane potentials. The VDAC modulator, a soluble mitochondrial protein, has been demonstrated to dramatically increase the voltage dependence of VDAC channels and induce the channels to enter closed states even at low membrane potentials. We have isolated and partially purified this modulating protein and the activity is associated with a 54 kDa protein on SDS-PAGE. Under native reduced conditions the activity eluted around 100 kDa from a gel filtration column. As little as 200 ng/ml of the partially purified protein was sufficient to modulate reconstituted VDAC channels. This protein had a pI of 5.1. A second activity with a pI of 4.8 was far more potent, making VDAC-channel-containing membranes virtually non-conductive in some experiments. The effects of both modulator activities could be completely reversed by the addition of pronase. Simple perfusion of the chamber did not reverse the effect of the modulator on VDAC. By controlling the gating of VDAC channels, the VDAC modulator could play an important role in regulating cellular metabolism.

The outer mitochondrial membrane channel, VDAC, is modulated by a protein localized in the intermembrane space

The mitochondrial outer membrane channel, VDAC, provides a pathway for the flux of metabolites between the cytoplasm and mitochondrion. VDAC is voltage-dependent and occupies states of differing conductivity and ion selectivity that are dependent on transmembrane potential. A protein, derived from preparations of mitochondria, has been shown to increase the voltage dependence of VDAC and is called the VDAC modulator. Both VDAC and the VDAC modulator have been extensively characterized by reconstitution into planar lipid bilayers. In order for the VDAC modulator to have physiological significance it must have physical access to VDAC in the cell. This constraint dictates that the modulator be an extrinsic outer mitochondrial membrane protein, occupy the mitochondrial intermembrane space, or be a cytoplasmic constituent. To address the question of subcellular localization, purified mitochondria were selectively lysed with digitonin or treated with trypsin while resuspended in hypo-osmotic or iso-osmotic medium. Marker enzymes and modulator activity were monitored during the various treatments. Results indicate that the integrity of the outer membrane was necessary to prevent modulator release or protection from trypsin digestion. Outer membrane lysis, under conditions where the inner membrane remained intact, resulted in modulator release or inactivation by trypsin. These results suggest an intermembrane space location for the VDAC modulator in the mitochondrion.

The mitochondrial permeability transition pore may comprise VDAC molecules. I. Binary structure and voltage dependence of the pore

Electrophysiological records suggest that the pore responsible for the mitochondrial Ca(2+)-dependent permeability transition (PTP), identified as the mitochondrial megachannel (MMC) observed in patch-clamp experiments, may comprise two cooperating porin (VDAC) molecules. We have re-investigated the voltage dependence of the megachannel, which favors the closed state(s) at negative (physiological) transmembrane potentials. This behavior confirms that MMC corresponds to the permeabilization pore. As detailed in the accompanying paper [(1993) FEBS Lett. 330, 206-210] this voltage dependence resembles that of VDAC. Alpidem, a ligand of the mitochondrial benzodiazepine receptor, which reportedly comprises VDAC, the adenine nucleotide carrier and a third component, elicited currents from silent mitoplast patches, suggesting that the benzodiazepine receptor may be identical to the PTP/MMC.