Properties of the mitochondrial peptide-sensitive cationic channel studied in planar bilayers and patches of giant liposomes

CALHM2 is a mitochondrial protein import channel that regulates fatty acid metabolism

For mitochondrial metabolism to occur in the matrix, multiple proteins must be imported across the two (inner and outer) mitochondrial membranes. Classically, two protein import channels, TIM/TOM, are known to perform this function, but whether other protein import channels exist is not known. Here, using super-resolution microscopy, proteomics, and electrophysiological techniques, we identify CALHM2 as the import channel for the ECHA subunit of the mitochondrial trifunctional protein (mTFP), which catalyzes β-oxidation of fatty acids in the mitochondrial matrix. We find that CALHM2 sits specifically at the inner mitochondrial and cristae membranes and is critical for membrane morphology. Depletion of CALHM2 leads to a mislocalization of ECHA outside of the mitochondria leading to severe cellular metabolic defects. These defects include cytosolic accumulation of fatty acids, depletion of tricarboxylic acid cycle enzymes and intermediates, and reduced cellular respiration. Our data identify CALHM2 as an essential protein import channel that is critical for fatty acid- and glucose-dependent aerobic metabolism.

Membrane-coated glass electrodes for stable, low-noise electrophysiology recordings in Drosophila central neurons

BACKGROUND

Electrophysiological recording with glass electrodes is one of the best techniques to measure membrane potential dynamics and ionic currents of voltage-gated channels in neurons. However, artifactual variability of the biophysical state variables that determine recording quality can be caused by insufficient affinity between the electrode and cell membrane during the recording.

NEW METHOD

We introduce a phospholipid membrane coating on glass electrodes to improve intracellular electrophysiology recording quality. Membrane-coated electrodes were prepared with a tip-dip protocol for perforated-patch, sharp-electrode current-clamp, and cell-attached patch-clamp recordings from specific circadian clock neurons in Drosophila. We perform quantitative comparisons based on the variability of functional biophysical parameters used in various electrophysiological methods, and advanced statistical comparisons based on the degree of stationariness and signal-to-noise ratio.

RESULTS

Results indicate a dramatic reduction in artifactual variabilities of functional parameters from enhanced stability. We also identify significant exclusions of a statistically estimated noise component in a time series of membrane voltage signals, improving signal-to-noise ratio.

COMPARISON WITH EXISTING METHODS

Compared to standard glass electrodes, using membrane-coated glass electrodes achieves improved recording quality in intracellular electrophysiology.

CONCLUSIONS

Electrophysiological recordings from Drosophila central neurons can be technically challenging, however, membrane-coated electrodes will possibly be beneficial for reliable data acquisition and improving the technical feasibility of axonal intracellular activities measurements and single-channel recordings. The improved electrical stability of the recordings should also contribute to increased mechanical stability, thus facilitating long-term stable measurements of neural activity. Therefore, it is possible that membrane-coated electrodes will be useful for any model system.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

Watching single proteins using engineered nanopores

Recent studies in the area of single-molecule detection of proteins with nanopores show a great promise in fundamental science, bionanotechnology and proteomics. In this mini-review, I discuss a comprehensive array of examinations of protein detection and characterization using protein and solid-state nanopores. These investigations demonstrate the power of the single-molecule nanopore measurements to reveal a broad range of functional, structural, biochemical and biophysical features of proteins, such as their backbone flexibility, enzymatic activity, binding affinity as well as their concentration, size and folding state. Engineered nanopores in organic materials and in inorganic membranes coupled with surface modification and protein engineering might provide a new generation of sensing devices for molecular biomedical diagnostics.

The therapeutic potential of mitochondrial channels in cancer, ischemia-reperfusion injury, and neurodegeneration

Mitochondria communicate with the rest of the cell through channels located in their inner and outer membranes. Most of the time, the message is encoded by the flow of anions and cations e.g., through VDAC and PTP, respectively. However, proteins are also both imported and exported across the mitochondrial membranes e.g., through TOM and MAC, respectively. Transport through mitochondrial channels is exquisitely regulated and controls a myriad of processes; from energy production to cell death. Here, we examine the role of some of the mitochondrial channels involved in neurodegeneration, ischemia-reperfusion injury and cancer in the context of their potential as therapeutic targets.

Electrophysiology of respiratory chain complexes and the ADP-ATP exchanger in native mitochondrial membranes

Transport of protons and solutes across mitochondrial membranes is essential for many physiological processes. However, neither the proton-pumping respiratory chain complexes nor the mitochondrial secondary active solute transport proteins have been characterized electrophysiologically in their native environment. In this study, solid-supported membrane (SSM) technology was applied for electrical measurements of respiratory chain complexes CI, CII, CIII, and CIV, the F(O)F(1)-ATPase/synthase (CV), and the adenine nucleotide translocase (ANT) in inner membranes of pig heart mitochondria. Specific substrates and inhibitors were used to validate the different assays, and the corresponding K(0.5) and IC(50) values were in good agreement with previously published results obtained with other methods. In combined measurements of CI-CV, it was possible to detect oxidative phosphorylation (OXPHOS), to measure differential effects of the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP) on the respective protein activities, and to determine the corresponding IC(50) values. Moreover, the measurements revealed a tight functional coupling of CI and CIII. Coenzyme Q (CoQ) analogues decylubiquinone (DBQ) and idebenone (Ide) stimulated the CII- and CIII-specific electrical currents but had inverse effects on CI-CIII activity. In summary, the results describe the electrophysiological and pharmacological properties of respiratory chain complexes, OXPHOS, and ANT in native mitochondrial membranes and demonstrate that SSM-based electrophysiology provides new insights into a complex molecular mechanism of the respiratory chain and the associated transport proteins. Besides, the SSM-based approach is suited for highly sensitive and specific testing of diverse respiratory chain modulators such as inhibitors, CoQ analogues, and uncoupling agents.

Dynamics of the preprotein translocation channel of the outer membrane of mitochondria

The protein translocase of the outer mitochondrial membrane (TOM) serves as the main entry site for virtually all mitochondrial proteins. Like many other protein translocases it also has an ion channel activity that can be used to study the dynamical properties of this supramolecular complex. We have purified TOM core complex and Tom40, the main pore forming subunit, from mitochondria of the filamentous fungus Neurospora crassa and incorporated them into planar lipid bilayers. We then examined their single channel properties to provide a detailed description of the conformational dynamics of this channel in the absence of its protein substrate. For isolated TOM core complex we have found at least six conductance states. Transitions between these states were voltage-dependent with a bell-shaped open probability distribution and distinct kinetics depending on the polarity of the applied voltage. The states with the largest conductance followed an Ohmic I/V characteristic consistent with a large cylindrical pore with very little interaction with the permeating ions. For the lower conductance states, however, we have observed inverted S-shaped nonlinear current-voltage curves reminiscent to those of much narrower pores where the permeating ions have to surmount an electrostatic energy barrier. At low voltages (<+/-70 mV), purified Tom40 protein did not show any transitions between its conductance states. Prolonged exposure to higher voltages induced similar gating behavior to what we observed for TOM core complex. This effect was time-dependent and reversible, indicating that Tom40 forms not only the pore but also contains the "gating machinery" of the complex. However, for proper functioning, additional proteins (Tom22, Tom7, Tom6, and Tom5) are required that act as a modulator of the pore dynamics by significantly reducing the energy barrier between different conformational states.

Electrophysiological approaches to the study of protein translocation in mitochondria

Electrophysiological techniques have been integral to our understanding of protein translocation across various membranes, and, in particular, the mitochondrial inner and outer membranes. Descriptions of various methodologies (for example, patch clamp, planar bilayers, and tip dip, and their past and potential contributions) are detailed within. The activity of protein import channels of native mitochondrial inner and outer membranes can be studied by directly patch clamping mitochondria and mitoplasts (mitochondria stripped of their outer membrane by French pressing) from various genetically manipulated strains of yeast and mammalian tissue cultured cells. The channel activities of TOM, TIM23, and TIM22 complexes are compared with those reconstituted in proteoliposomes and with those of the recombinant proteins Tom40p, Tim23p, and Tim22p, which play major roles in protein translocation. Studies of the mechanism(s) and the role of channels in protein translocation in mitochondria are prototypes, as the same principles are likely followed in all biological membranes including the endoplasmic reticulum and chloroplasts. The ability to apply electrophysiological techniques to these channels is now allowing investigations into the role of mitochondria in diverse fields such as neurotransmitter release, long-term potentiation, and apoptosis.

Protein translocation through anthrax toxin channels formed in planar lipid bilayers

The 63-kDa fragment of the protective antigen (PA) component of anthrax toxin forms a heptameric channel, (PA63)7, in acidic endosomal membranes that leads to the translocation of edema factor (EF) and lethal factor (LF) to the cytosol. It also forms a channel in planar phospholipid bilayer membranes. What role does this channel play in the translocation of EF and LF? We report that after the 263-residue N-terminal piece of LF (LFN) binds to its receptor on the (PA63)7 channel and its N-terminal end enters the channel at small positive voltages to block it, LFN is translocated through the channel to the opposite side at large positive voltages, thereby unblocking it. Thus, all of the translocation machinery is contained in the (PA63)7 channel, and translocation does not require any cellular proteins. The kinetics of this translocation are S-shaped, voltage-dependent, and occur on a timescale of seconds. We suggest that the translocation process might be explained simply by electrophoresis of unfolded LFN through the channel, but the refolding of the N-terminal half of LFN as it emerges from the channel may also provide energy for moving the rest of the molecule through the channel.

Comparison of the TIM and TOM channel activities of the mitochondrial protein import complexes

Water-filled channels are central to the process of translocating proteins since they provide aqueous pathways through the hydrophobic environment of membranes. The Tom and Tim complexes translocate precursors across the mitochondrial outer and inner membranes, respectively, and contain channels referred to as TOM and TIM (previously called PSC and MCC). In this study, little differences were revealed from a direct comparison of the single channel properties of the TOM and TIM channels of yeast mitochondria. As they perform similar functions in translocating proteins across membranes, it is not surprising that both channels are high conductance, voltage-dependent channels that are slightly cation selective. Reconstituted TIM and TOM channel activities are not modified by deletion of the outer membrane channel VDAC, but are similarly affected by signal sequence peptides.

A novel, high conductance channel of mitochondria linked to apoptosis in mammalian cells and Bax expression in yeast

During apoptosis, proapoptotic factors are released from mitochondria by as yet undefined mechanisms. Patch-clamping of mitochondria and proteoliposomes formed from mitochondrial outer membranes of mammalian (FL5.12) cells has uncovered a novel ion channel whose activity correlates with onset of apoptosis. The pore diameter inferred from the largest conductance state of this channel is approximately 4 nm, sufficient to allow diffusion of cytochrome c and even larger proteins. The activity of the channel is affected by Bcl-2 family proteins in a manner consistent with their pro- or antiapoptotic properties. Thus, the channel activity correlates with presence of proapoptotic Bax in the mitochondrial outer membrane and is absent in mitochondria from cells overexpressing antiapoptotic Bcl-2. Also, a similar channel activity is found in mitochondrial outer membranes of yeast expressing human Bax. These findings implicate this channel, named mitochondrial apoptosis-induced channel, as a candidate for the outer-membrane pore through which cytochrome c and possibly other factors exit mitochondria during apoptosis.

Tom40, the pore-forming component of the protein-conducting TOM channel in the outer membrane of mitochondria

Tom40 is the main component of the preprotein translocase of the outer membrane of mitochondria (TOM complex). We have isolated Tom40 of Neurospora crassa by removing the receptor Tom22 and the small Tom components Tom6 and Tom7 from the purified TOM core complex. Tom40 is organized in a high molecular mass complex of approximately 350 kD. It forms a high conductance channel. Mitochondrial presequence peptides interact specifically with Tom40 reconstituted into planar lipid membranes and decrease the ion flow through the pores in a voltage-dependent manner. The secondary structure of Tom40 comprises approximately 31% beta-sheet, 22% alpha-helix, and 47% remaining structure as determined by circular dichroism measurements and Fourier transform infrared spectroscopy. Electron microscopy of purified Tom40 revealed particles primarily with one center of stain accumulation. They presumably represent an open pore with a diameter of approximately 2.5 nm, similar to the pores found in the TOM complex. Thus, Tom40 is the core element of the TOM translocase; it forms the protein-conducting channel in an oligomeric assembly.

Electrical dimension of the nuclear envelope

Eukaryotic chromosomes are confined to the nucleus, which is separated from the rest of the cell by two concentric membranes known as the nuclear envelope (NE). The NE is punctuated by holes known as nuclear pore complexes (NPCs), which provide the main pathway for transport of cellular material across the nuclear-cytoplasmic boundary. The single NPC is a complicated octameric structure containing more than 100 proteins called nucleoporins. NPCs function as transport machineries for inorganic ions and macromolecules. The most prominent feature of an individual NPC is a large central channel, ~7 nm in width and 50 nm in length. NPCs exhibit high morphological and functional plasticity, adjusting shape to function. Macromolecules ranging from 1 to >100 kDa travel through the central channel into (and out of) the nucleoplasm. Inorganic ions have additional pathways for communication between cytosol and nucleus. NE can turn from a simple sieve that separates two compartments by a given pore size to a smart barrier that adjusts its permeabiltiy to the metabolic demands of the cell. Early microelectrode work characterizes the NE as a membrane barrier of highly variable permeability, indicating that NPCs are under regulatory control. Electrical voltage across the NE is explained as the result of electrical charge separation due to selective barrier permeability and unequal distribution of charged macromolecules across the NE. Patch-clamp work discovers NE ion channel activity associated with NPC function. From comparison of early microelectrode work with patch-clamp data and late results obtained by the nuclear hourglass technique, it is concluded that NPCs are well-controlled supramolecular structures that mediate transport of macromolecules and small ions by separate physical pathways, the large central channel and the small peripheral channels, respectively. Electrical properties of the two pathways are still unclear but could have great impact on the understanding of signal transfer across NE and gene expression.

The isolated complex of the translocase of the outer membrane of mitochondria. Characterization of the cation-selective and voltage-gated preprotein-conducting pore

The complex of the translocase mitochondrial outer membrane (TOM), mediates recognition, unfolding, and translocation of preproteins. We have used a combination of biochemical and electrophysiological methods to study the properties of the preprotein-conducting pore of the purified TOM complex. The pore is cation-selective and voltage-gated. It shows three main conductance levels with characteristic slow and fast kinetics transitions to states of lower conductance following application of transmembrane voltages. These electrical properties distinguish it from the mitochondrial voltage-dependent anion channel (porin) and are identical to those of the previously described peptide-sensitive channel. Binding of antibodies to the C terminus of Tom40 on the intermembrane space side of the outer membrane modifies the channel properties and allows determination of the orientation of the channel within the lipid bilayer. Mitochondrial presequence peptides specifically interact with the pore and decrease the ion flow through the channel in a voltage-dependent manner. We propose that the presequence-induced closures of the pore are related to structural alterations of the TOM complex observed during the various stages of preprotein movement across the mitochondrial outer membrane.

Characterization of the yeast mitochondria unselective channel: a counterpart to the mammalian permeability transition pore?

Large and unselective permeabilities through the inner membrane of yeast mitochondria have been observed for more than 20 years, but the characterization of these permeabilities, leading to hypothesize the existence of a large-conductance unselective channel in yeast inner mitochondrial membrane, was done only recently by several groups. This channel has been tentatively identified as a yeast counterpart to the mammalian permeability transition pore, the crucial role of which is now well-documented in physiopathological phenomena, such as Ca2+ homeostasis, ischemic damages, or programmed cell death. The aim of this review is to make a point on the known characteristics of this yeast mitochondrial unselective channel (YMUC) and to analyze whether or not it can be considered as a "yeast permeability transition pore."

DNA translocation across planar bilayers containing Bacillus subtilis ion channels

The mechanisms by which genetic material crosses prokaryotic membranes are incompletely understood. We have developed a new methodology to study the translocation of genetic material via pores in a reconstituted system, using techniques from electrophysiology and molecular biology. We report here that planar bilayer membranes become permeable to double-stranded DNA (kilobase range) if Bacillus subtilis membrane vesicles containing high conductance channels have been fused into them. The translocation is an electrophoretic process, since it does not occur if a transmembrane electrical field opposing the movement of DNA, a polyanion, is applied. It is not an aspecific permeation through the phospholipid bilayer, since it does not take place if no proteins have been incorporated into the membrane. The transport is also not due simply to the presence of polypeptides in the membrane, since it does not occur if the latter contains gramicidin A or a eukaryotic, multi-protein vesicle fraction exhibiting 30-picosiemens anion-selective channel activity. The presence of DNA alters the behavior of the bacterial channels, indicating that it interacts with the pores and may travel through their lumen. These results support the idea that DNA translocation may take place through proteic pores and suggest that some of the high conductance bacterial channels observed in electrophysiological experiments may be constituents of the DNA translocating machinery in these organisms.

Tim23, a protein import component of the mitochondrial inner membrane, is required for normal activity of the multiple conductance channel, MCC

We previously showed that the conductance of a mitochondrial inner membrane channel, called MCC, was specifically blocked by peptides corresponding to mitochondrial import signals. To determine if MCC plays a role in protein import, we examined the relationship between MCC and Tim23p, a component of the protein import complex of the mitochondrial inner membrane. We find that antibodies against Tim23p, previously shown to inhibit mitochondrial protein import, inhibit MCC activity. We also find that MCC activity is altered in mitochondria isolated from yeast carrying the tim23-1 mutation. In contrast to wild-type MCC, we find that the conductance of MCC from the tim23-1 mutant is not significantly blocked by mitochondrial presequence peptides. Tim23 antibodies and the tim23-1 mutation do not, however, alter the activity of PSC, a presequence-peptide sensitive channel in the mitochondrial outer membrane. Our results show that Tim23p is required for normal MCC activity and raise the possibility that precursors are translocated across the inner membrane through the pore of MCC.

Relationship between the peptide-sensitive channel and the mitochondrial outer membrane protein translocation machinery

The peptide-sensitive channel (PSC), a cationic channel of the mitochondrial outer membrane, is blocked by synthetic mitochondrial presequences and by nonmitochondrial basic peptides such as dynorphin B(1-13). Both types of peptides are imported into mitochondria. However, the import of dynorphin B(1-13) had to be further characterized since its properties differed from those of the general import pathway used by mitochondrial peptides. Cross-linking experiments with iodinated dynorphin B(1-13) led to the labeling of TOM 40/ISP 42, a component of the protein import machinery of the outer membrane. Accordingly, dynorphin B(1-13) could also be used as a presequence to direct the import of a cytosolic protein into the mitochondria. Pretreatment of intact mitochondria by trypsin removed components capable of discriminating between true mitochondrial presequences and other basic peptides active on the PSC. After proteolysis, both types of peptides appeared to cross the outer membrane through the same pathway. Involvement of the PSC in the translocation complex was shown by immunoprecipitation of the PSC activity by anti-ISP 42 antibodies. Taken together, the present data reinforce the hypothesis that the PSC is the pore responsible for the translocation of protein through the outer membrane.

Ionic effects of the Alzheimer's disease beta-amyloid precursor protein and its metabolic fragments

Alzheimer's disease is a progressive dementia characterized in part by deposition of proteinaceous plaques in various areas of the brain. The main plaque protein component is beta-amyloid, a metabolic product of the beta-amyloid precursor protein. Substantial evidence has implicated beta-amyloid (and other amyloidogenic fragments of the precursor protein) with the neurodegeneration observed in Alzheimer's disease. Recently, beta-amyloid precursor protein and its amyloidogenic metabolic fragments have been shown to alter cellular ionic activity, either through interaction with existing channels or by de novo channel formation. Such alteration in ionic homeostasis has also been linked with cellular toxicity and might provide a molecular mechanism underlying the neurodegeneration seen in Alzheimer's disease.

The mitochondrial protein import pathway: are precursors imported through membrane channels?

Mitochondrial biogenesis requires the import of hundreds of different proteins from the cytosol. Protein import into mitochondria is a multistep pathway that includes recognition of precursor proteins by machinery both in the cytoplasm and on the mitochondrial surface, translocation of the precursor across one or both mitochondrial membranes, and folding of the protein after its import into the organelle. Over the past several years, many components of the import machinery have been identified using both biochemical and genetic methods. Recently, significant progress has been made determining the function of some of these import proteins. One purpose of this minireview is to summarize our current understanding of the import pathway, and to introduce the topics of the minireviews that will follow. The other goal of this minireview is to discuss recent findings suggesting that proteins are translocated across both the mitochondrial inner and outer membranes through aqueous channels.

Protein import into mitochondria

Mitochondria import many hundreds of different proteins that are encoded by nuclear genes. These proteins are targeted to the mitochondria, translocated through the mitochondrial membranes, and sorted to the different mitochondrial subcompartments. Separate translocases in the mitochondrial outer membrane (TOM complex) and in the inner membrane (TIM complex) facilitate recognition of preproteins and transport across the two membranes. Factors in the cytosol assist in targeting of preproteins. Protein components in the matrix partake in energetically driving translocation in a reaction that depends on the membrane potential and matrix-ATP. Molecular chaperones in the matrix exert multiple functions in translocation, sorting, folding, and assembly of newly imported proteins.

Mitochondrial ADP/ATP carrier can be reversibly converted into a large channel by Ca2+

Single-channel current measurements of excised patches with reconstituted purified mitochondrial ADP/ATP carrier (AAC) indicates the presence of a large low cation selective (PK+/PCl- = 4.3 +/- 0.6) channel. The channel conductance has multiple sublevels and varies from 300 to 600 pS. It has low probability of current fluctuations at Vhold up to 80-100 mV of both signs and is reversibly gated at Vhold > 150 mV. The opening of the channel is Ca(2+)-dependent (1 mM Ca2+) and can be reversibly closed on removal of Ca2+. It is strongly pH dependent and closes completely at pHex 5.2. The AAC-specific inhibitor bongkrekate inhibits the channel partially and completely in combination with ADP, whereas carboxyatractylate did not affect the conductance. The effects of these AAC-specific ligands prove that the channel activity belongs to AAC. The AAC-linked conductance can clearly be differentiated from the porin channel, rarely detected in our preparations. The properties of the AAC-linked channel coincide with the mitochondrial permeability transition pore (MTP), which is also affected by the AAC ligands [Hunter, D. R., & Haworth, R. A. (1979) Arch. Biochem. Biophys. 195, 453-459] and resembles the mitochondrial "multiconductance channel" [Kinnally, K. W., Campo, M. L., & Tedeschi, H. T. (1989) J. Bioenerg. Biomembr. 21, 497-506] or "megachannel" [Petronilli, V., Szabo, I., & Zoratti, M. (1989) FEBS Lett. 259, 137-143]. Therefore we conclude that the AAC, when converted into a large unselective channel, is a key component in the MTP and thus is involved in the ischemia-reperfusion damage and cytosolic Ca2+ oscillations. The channel opening in AAC is proposed to be caused by binding of Ca2+ to the cardiolipin, tightly bound to AAC, thus releasing positive charges within the AAC which open the gate.

The high-conductance channels of yeast mitochondrial outer membranes: a planar bilayer study

The high-conductance channels present in the outer membranes of wild-type and porin-less yeast mitochondria have been characterized electrophysiologically after incorporation in planar bilayer membranes. The most prominent activity was ascribed to a voltage-dependent, substate-rich, cationic channel which generally inactivated at potentials positive in the cis compartment, in agreement with the observations from patch-clamp experiments on porin-less mitoplasts. This channel has been identified as the so-called "peptide-sensitive channel" (PSC). We also observed similar channels displaying either no inactivation, or inactivation at both positive and negative potentials. These latter properties match those already described for mammalian and yeast PSC, respectively. These different behaviors are tentatively explained as arising from the presence, or lack of, peptides bound to the PSC. Very high conductances, apparently due to cooperative gating, were frequently displayed. In wild-type membranes, activity ascribable to the porin was also observed.

Characterization and function of the mitochondrial outer membrane peptide-sensitive channel

The PSC (peptide-sensitive Channel), a cationic channel of large conductance, has been characterized in yeast and mammalian mitochondria by three different methods, "tip-dip," patch clamp of giant liposomes, and planar bilayers. The yeast and mammalian PSC share the common property to be blocked by basic peptides such as pCyt OX IV (1-12)Y which contains the first 12 residues of the presequence of cytochrome C oxidase subunit IV. The electrophysiological data are consistent with a translocation of the peptide through the pore. Analysis of the frequency of observation of the PSC in different fractions indicates that the channel is located in the outer mitochondrial membrane. Uptake measurements of iodinated peptides by intact mitochondria from a porin-less mutant show that the peptides are translocated through the outer membrane, presumably at the level of PSC. Among the peptides active on PSC, several, such as pCyt OX IV (1-22) and the reduced form of the mast cell degranulating peptide, induce an alteration of the voltage dependence or of the inactivation rate subsisting after washing and which is eliminated only by proteolysis of the interacting peptide. These irreversible effects may account for the variability of the properties of the PSC which would interact with cytosolic or intermembrane cations, peptides, or proteins, thus modulating the channel permeability. Finally, several lines of evidence suggest the participation of the PSC in protein translocation and some interaction with the general insertion pore of the outer membrane translocation machinery.

Targeting peptides transiently block a mitochondrial channel

The effects of synthetic targeting peptides on the activity of the multiple conductance channel (MCC) of mouse and yeast mitochondria were investigated using patch-clamp techniques. Amino-terminal targeting peptides of two inner membrane proteins reversibly decreased the open probability and mean open time of MCC. One of these targeting peptides had no effect on two other voltage-dependent mitochondrial channels. Furthermore, the effects induced by the two targeting peptides on MCC were not elicited by two peptides of an outer membrane protein. The specific interactions of targeting peptides with MCC suggest that this channel may be involved in protein import across the inner mitochondrial membrane.

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.

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.

Reversible and irreversible effects of basic peptides on the mitochondrial cationic channel

We have previously shown that a 13-residue basic peptide, derived from the presequence of a mitochondrial precursor, blocked the cationic channel of the outer mitochondrial membrane. The properties of the blockade suggested that the peptide could go through the pore in the presence of a sufficient driving force. In an attempt to evaluate more precisely the relevance of such an interpretation, we have examined the effect on the same channel of basic peptides from 16 to 34 residues, most of which are parts of or derive from mitochondrial presequences. Two peptides were found to induce a reversible voltage-dependent blockade, the properties of which were the same as those of the blockade induced by the 13-residue peptide. The others had a similar effect, but triggered in addition a modification of the voltage gating that persisted after washing the peptide out. The modification was in turn abolished by trypsin added to the side of the channel previously exposed to the peptide. The protease acted on the bound peptide and not on the channel itself. The irreversible modification of the voltage gating, the mechanism of which remains obscure, was not specific for mitochondrial-addressing sequences.

Interaction of phospholipids with proteins and peptides. New advances IV

1. The review deals with the newest achievements in the field of the various interactions between phospholipids and proteins and peptides. 2. Interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecular assemblies is also discussed.

The mitochondrial permeability transition pore may comprise VDAC molecules. II. The electrophysiological properties of VDAC are compatible with those of the mitochondrial megachannel

The electrophysiological properties of isolated mitochondrial porin (VDAC), reconstituted in planar bilayers or proteoliposomes, resemble those of the mitochondrial megachannel believed to be the permeability transition pore. In particular, a correspondence was found with regard to the voltage dependence: VDAC was driven to closed states by potentials of either sign, but the effect was not symmetrical; voltages negative in the compartment to which VDAC was added were more effective. The results are consistent with the hypothesis that the PTP may consist of two cooperating VDAC channels, plus presumably an adenine nucleotide carrier dimer and a third component known to be part of the mitochondrial benzodiazepine receptor.

Translocation of proteins across the endoplasmic reticulum

The past year has seen significant advances in the field of protein translocation: the roles of the signal recognition particle and its receptor have been understood in greater detail; many membrane components responsible for translocation have been identified; and insight has been gained into how proteins cross membranes.

Channels in mitochondrial membranes: knowns, unknowns, and prospects for the future

Rapid diffusion of hydrophilic molecules across the outer membrane of mitochondria has been related to the presence of a protein of 29 to 37 kDa, called voltage-dependent anion channel (VDAC), able to generate large aqueous pores when integrated in planar lipid bilayers. Functional properties of VDAC from different origins appear highly conserved in artificial membranes: at low transmembrane potentials, the channel is in a highly conducting state, but a raise of the potential (both positive and negative) reduces drastically the current and changes the ionic selectivity from slightly anionic to cationic. It has thus been suggested that VDAC is not a mere molecular sieve but that it may control mitochondrial physiology by restricting the access of metabolites of different valence in response to voltage and/or by interacting with a soluble protein of the intermembrane space. The latest application of the patch clamp and tip-dip techniques, however, has indicated both a different electric behavior of the outer membrane and that other proteins may play a role in the permeation of molecules. Biochemical studies, use of site-directed mutants, and electron microscopy of two-dimensional crystal arrays of VDAC have contributed to propose a monomeric beta barrel as the structural model of the channel. An important insight into the physiology of the inner membrane of mammalian mitochondria has come from the direct observation of the membrane with the patch clamp. A slightly anionic, voltage-dependent conductance of 107 pS and one of 9.7 pS, K(+)-selective and ATP-sensitive, are the best characterized at the single channel level. Under certain conditions, however, the inner membrane can also show unselective nS peak transitions, possibly arising from a cooperative assembly of multiple substrates.