Structure of the outer mitochondrial membrane: analysis of X-ray diffraction from the plant membrane

Sitosterol and glucosylceramide cooperative transversal and lateral uneven distribution in plant membranes

The properties of biomembranes depend on the presence, local structure and relative distribution assumed by the thousands of components it is made of. As for animal cells, plant membranes have been demonstrated to be organized in subdomains with different persistence lengths and times. In plant cells, sitosterol has been demonstrated to confer to phospholipid membranes a more ordered structure while among lipids, glycosphingolipids are claimed to form rafts where they tightly pack with sterols. Glucosylceramides are glycosphingolipids involved in plant signalling and are essential for viability of cells and whole plant. The glucosylceramide-sitosterol structural coupling within PLPC membranes is here investigated by Langmuir films, in silico simulations and neutron reflectometry, unveiling that a strong direct interaction between the two molecules exists and governs their lateral and transversal distribution within membrane leaflets. The understanding of the driving forces governing specific molecules clustering and segregation in subdomains, such as glucosylceramide and sitosterol, have an impact on the mechanical properties of biomembranes and could reflect in the other membrane molecules partitioning and activity.

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

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

Assembly of the mitochondrial apoptosis-induced channel, MAC

Although Bcl-2 family proteins control intrinsic apoptosis, the mechanisms underlying this regulation are incompletely understood. Patch clamp studies of mitochondria isolated from cells deficient in one or both of the pro-apoptotic proteins Bax and Bak show that at least one of the proteins must be present for formation of the cytochrome c-translocating channel, mitochondrial apoptosis-induced channel (MAC), and that the single channel behaviors of MACs containing exclusively Bax or Bak are similar. Truncated Bid catalyzes MAC formation in isolated mitochondria containing Bax and/or Bak with a time course of minutes and does not require VDAC1 or VDAC3. Mathematical analysis of the stepwise changes in conductance associated with MAC formation is consistent with pore assembly by a barrel-stave model. Assuming the staves are two transmembrane alpha-helices in Bax and Bak, mature MAC pores would typically contain approximately 9 monomers and have diameters of 5.5-6 nm. The mitochondrial permeability data are inconsistent with formation of lipidic pores capable of transporting megadalton-sized macromolecules as observed with recombinant Bax in liposomes.

Regulation of the mitochondrial apoptosis-induced channel, MAC, by BCL-2 family proteins

Programmed cell death or apoptosis is central to many physiological processes and pathological conditions such as organogenesis, tissue homeostasis, cancer, and neurodegenerative diseases. Bcl-2 family proteins tightly control this cell death program by regulating the permeabilization of the mitochondrial outer membrane and, hence, the release of cytochrome c and other pro-apoptotic factors. Control of the formation of the mitochondrial apoptosis-induced channel, or MAC, is central to the regulation of apoptosis by Bcl-2 family proteins. MAC is detected early in apoptosis by patch clamping the mitochondrial outer membrane. The focus of this review is on the regulation of MAC activity by Bcl-2 family proteins. The role of MAC as the putative cytochrome c release channel during early apoptosis and insights concerning its molecular composition are also discussed.

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.

Effects of cytochromecon the mitochondrial apoptosis-induced channel MAC

Recent studies indicate that cytochrome c is released early in apoptosis without loss of integrity of the mitochondrial outer membrane in some cell types. The high-conductance mitochondrial apoptosis-induced channel (MAC) forms in the outer membrane early in apoptosis of FL5.12 cells. Physiological (micromolar) levels of cytochrome c alter MAC activity, and these effects are referred to as types 1 and 2. Type 1 effects are consistent with a partitioning of cytochrome c into the pore of MAC and include a modest decrease in conductance that is dose and voltage dependent, reversible, and has an increase in noise. Type 2 effects may correspond to “plugging” of the pore or destabilization of the open state. Type 2 effects are a dose-dependent, voltage-independent, and irreversible decrease in conductance. MAC is a heterogeneous channel with variable conductance. Cytochrome c affects MAC in a pore size-dependent manner, with maximal effects of cytochrome c on MAC with conductance of 1.9–5.4 nS. The effects of cytochrome c, RNase A, and high salt on MAC indicate that size, rather than charge, is crucial. The effects of dextran molecules of various sizes indicate that the pore diameter of MAC is slightly larger than that of 17-kDa dextran, which should be sufficient to allow the passage of 12-kDa cytochrome c. These findings are consistent with the notion that MAC is the pore through which cytochrome c is released from mitochondria during apoptosis.

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.

Identification of two general diffusion channels in the outer membrane of pea mitochondria

Reconstitution experiments were performed on lipid bilayer membranes in the presence of detergent solubilized mitochondrial membranes of pea seedlings (Pisum sativum). The addition of the detergent-solubilized material to the membranes resulted in a strong increase of the membrane conductance. To identify the proteins responsible for membrane activity the detergent extracts were applied to a hydroxyapatite (HTP) column and the fractions were tested for channel formation. The eluate of the column contained a protein which migrated as a single band with an apparent molecular mass of 30 kDa on SDS-PAGE. This channel was identified as the porin of pea mitochondria since it formed voltage-dependent channels with single-channel conductances of 1.5 and 3.7 nS in 1 M KCl and an estimated effective diameter of about 1.7 nm. Further elution of the column with KCl containing solutions yielded fractions which resulted in the formation of transient channels in lipid bilayer membranes. These channels had a single-channel conductance of 2.2 nS in 1 M KCl and had also the characteristics of general diffusion pores with an estimated effective diameter of 1.2 nm. Zero-current membrane potential measurements suggested that pea porin was anion-selective in the open state. The selectivity of the second channel was investigated by the measurement of the reversal potential. It was also slightly anion-selective. Its possible role in the metabolism of mitochondria is discussed.

Studies on human porin. VII. The channel properties of the human B-lymphocyte membrane-derived "Porin 31HL" are similar to those of mitochondrial porins

Porin 31HL was isolated and purified from total membrane preparations of a human B-lymphocyte cell line. The protein showed a single band of apparent molecular mass 31 kDa on SDS-PAGE. Reconstitution of the protein into artificial lipid bilayer membranes defines its function as a channel-forming protein. The distribution of single-channel conductances had two maxima of 2.4 and 4.3 nS in 1M KCl. The channel formed by Porin 31HL of human B-lymphocytes was found to be voltage-dependent and switched to ion-permeable substates at membrane voltage larger than 20mV. In the open state the pore exhibited the characteristics of a general diffusion pore because the mobility sequence of the ions inside the pore was similar to that in the bulk aqueous phase. The effective diameter was estimated to be about 1.7 nm. The properties of the low conductance state of the channel were studied in detail. In this state the pore favored the passage of cations, in contrast to the open state which favored anions slightly. Monoclonal antibodies against the N-terminal end of Porin 31HL blocked its reconstitution but had otherwise no influence on the channel properties. This result suggested that the amphipathic alpha-helical structure at the N-terminal end is probably not involved in channel gating. The channel-forming properties of Porin 31HL were compared to those of porins isolated from mitochondrial outer membranes and to those of the "maxi chloride channel" observed in the cytoplasmic membrane of several eukaryotic cells.

Characterization of the mitochondrial porin from Drosophila melanogaster

Mitochondrial porin was isolated from the fruit fly Drosophila melanogaster at different developmental stages, starting from whole mitochondria. The porin from adults' mitochondria was fully characterized. The protein had a molecular mass of 31 kDa as judged from sodium dodecylsulfate electrophoretograms. It was very resistive against digestion with V8 proteinase of Staphylococcus aureus and a larger number of fragments were only obtained after digestion with papain. Drosophila porin showed little interaction with antibodies raised against mitochondrial porins from mammalia and Neurospora crassa, but a strong reactivity with antibodies raised against yeast porin. Reconstitution experiments with planar lipid bilayer membranes showed that the protein was able to form ion-permeable pores with a single-channel conductance of 0.41 nS in 0.1 M KCl. At low transmembrane voltages Drosophila porin had the properties of a general diffusion pore with an estimated effective diameter of about 1.7 nm and a small selectivity for anions over cations. Voltages larger than 20 to 30 mV resulted in a closure of the pore. The closed states of the pore were found to be cation-selective. The addition of a synthetic polyanion to the aqueous phase on one side of the membrane resulted in an asymmetric shift of the voltage dependence and the pore became already closed at very small voltages negative at the cis-side (the side of the addition of the polyanion).

Purification and characterization of the pore forming protein of yeast mitochondrial outer membrane

One of the major outer membrane proteins of yeast mitochondria was isolated and purified. It migrated as a single band with an apparent molecular weight of 30 kDa on a SDS-electrophoretogram. When reconstituted in lipid bilayer membranes the protein formed pores with a single channel conductance of 0.45 nS in 0.1 M KCl. The pores had the characteristics of general diffusion pores with an estimated diameter of 1.7 nm. The pore of mitochondrial outer membranes of yeast shared some similarities with the pores formed by mitochondrial and bacterial porins. The pores switched to substates at voltages higher than 20 mV. The possible role of this voltage-dependence in the metabolism of mitochondria is discussed.

Electron microscopy and image analysis of the mitochondrial outer membrane channel, VDAC

The channel protein in the outer membrane of Neurospora crassa mitochondria, VDAC, forms extended planar crystals on the membrane. The arrays, which are induced by phospholipase A2, are polymorphic, varying from parallelogram (P) to near-rectangular (R) geometry with increased phospholipase treatment. Computer-based analysis of projection images of negatively stained VDAC arrays indicates that the protein forms a transmembrane channel in the P array. Comparison of average images of arrays embedded in different negative stains suggests that the bore of the channel is 2-2.5 nm. The locations of functionally important lysine clusters on VDAC are inferred from the effects of succinylation on projection images of arrays negatively stained with phosphotungstate. Projection images of unstained frozen-hydrated arrays indicate the general shape of the channel and suggest each channel is formed by one 31-kDa VDAC polypeptide.

The 35 kDa DCCD-binding protein from pig heart mitochondria is the mitochondrial porin

The protein which can be labelled by low concentrations of dicyclohexylcarbodiimide in the Mr region of 30 000-35 000 has been purified from pig heart mitochondria with a high yield and as a single band of apparent Mr 35 000 in dodecyl sulphate-containing gels. The protein is not identical with the phosphate carrier as suggested before, since the two proteins behave differently during isolation. Incorporation of the isolated 35 kDa dicyclohexylcarbodiimide-binding protein into lipid bilayer membranes causes an increase of the membrane conductance in definite steps, due to the formation of pores. The specific pore-forming activity increases during the purification procedure. The single pore conductance is about 4.0 nS, suggesting a diameter of 1.7 nm of the open pore. The pore conductance is dependent on the voltage across the membrane. Anion permeability of the pore is higher than cation permeability. These properties are similar to those described for isolated mitochondrial and bacterial porins. It is concluded that the 35 kDa dicyclohexylcarbodiimide-binding protein from pig heart mitochondria is identical with porin from outer mitochondrial membrane.

Porin from bacterial and mitochondrial outer membranes

The outer membrane of gram-negative bacteria acts as a molecular filter with defined exclusion limit for hydrophilic substances. The exclusion limit is dependent on the type of bacteria and has for enteric bacteria like Escherichia coli and Salmonella typhimurium a value between 600 and 800 Daltons, whereas molecules with molecular weights up to 6000 can penetrate the outer membrane of Pseudomonas aeruginosa. The molecular sieving properties result from the presence of a class of major proteins called porins which form trimers of identical subunits in the outer membrane. The porin trimers most likely contain only one large but well-defined pore with a diameter between 1.2 and 2 nm. Mitochondria are presumably descendents of gram-negative bacteria. The outer membrane of mitochondria contains in agreement with this hypothesis large pores which are permeable for hydrophilic substances with molecular weights up to 6000. The mitochondrial porins are processed by the cell and have molecular weights around 30,000 Daltons. There exists some evidence that the pore is controlled by electric fields and metabolic processes.

Structure of the outer mitochondrial membrane: ordered arrays of porelike subunits in outer-membrane fractions from Neurospora crassa mitochondria

Light-membrane fractions obtained by hypoosmotic lysis of neurospora crassa mitochondria exhibit buoyant densities and marker-enzyme activities characteristic of outer mitochondrial membranes. SDS PAGE of these membrane fractions indicates that a polypeptide of M(r) 31,000 is the main protein component. Under negative-stain electron microscope examination many of the membranes in these fractions appear as large (0.5-1- mum diameter), collapsed vesicles. The surfaces of flattened, open (i.e., ripped) vesicles often exhibit extended two-dimensional arrays of subunits are arranged into hexagons within each parallelogram unit cell, 12.6x11.1 nm (lattice angle = 109 degrees).

K+ transport in mitoplasts

K+ transport into mitoplasts, prepared by digitonin disruption and removal of the outer membranes from rat liver mitochondria, has been studied. Unidirectional K+ influx has been measured by means of 42K, in the presence of the respiratory substrate succinate. K+ influx is inhibited by CN-, antimycin A and dicyclohexylcarbodiimide, but is insensitive to oligomycin. A linear dependence of the reciprocal of the K+ -influx rate on the reciprocal of the external K+ concentration is observed. Under the conditions studied, the apparent Km for K+ of the transport mechanism is approx. 6 mM, while the Vmax of K+ influx is approx. 5 mu mol K+/g protein per min. The rate of K+ influx increases with increasing external pH over the range from 6.8 to 8.0. The observed kinetics, pH dependence and inhibitor sensitivity are essentially similar to previously reported characteristics of K+ transport into intact rat liver mitochondria. It is concluded that the outer mitochondrial membrane does not not have a role in controlling K+ flux into rat liver mitochondria.

Evidence for identity between the hexokinase-binding protein and the mitochondrial porin in the outer membrane of rat liver mitochondria

Hexokinase-binding protein and mitochondrial porin were isolated from rat liver mitochondria by different procedures. It was found that the hexokinase-binding protein made lipid vesicles permeable to ADP and formed asymmetric pores in lipid bilayer membranes identical to those obtained from the mitochondrial porin. On the other hand, the mitochondrial porin confers the ability to bind hexokinase. In addition, evidence is presented that both hexokinase-binding protein and mitochondrial porin bind glycerol kinase.

Identification and characterization of the pore-forming protein in the outer membrane of rat liver mitochondria

The proteins of the outer membrane from rat liver mitochondria have been subfractionated by means of density gradient centrifugation. The different polypeptides of the membrane were incorporated into asolectin vesicles and black lipid membranes. It was observed that a polypeptide of Mr 32 000 renders asolectin vesicles permeable to ADP and forms pores in bilayer membrane. These pores showed the same properties as the channels which are formed in the lipid membrane after addition of Triton X-100 solubilized complete outer membrane. The properties of the pore are as follows: (1) The formation of pores depends on the type of phospholipid used for the preparation of the black membranes. (2) The pore is inserted asymmetrically into the membrane. (3) The pore is voltage gated but does not switch off completely at higher voltages. The pore seems to show different conductance states decreasing conductance being observed at increasing voltage. The implications of these findings for the regulation of transport processes across the outer membrane are discussed.

Purification and characterisation of a pore protein of the outer mitochondrial membrane from Neurospora crassa

The major protein of the outer mitochondrial membrane of Neurospora was purified. On dodecylsulfate-containing gels it displayed a single band with an apparent molecular weight of 31 000. Reconstitution experiments with artificial lipid bilayers showed that this protein forms pores. Pore conductance was dependent on the voltage across the membrane. The protein inserted into the membrane in an oriented fashion, the membrane current being dependent on the sign of the voltage. Single pore conductance was 5nS, suggesting a diameter of 2 nm of the open pore. This mitochondrial protein shows a number of similarities to the outer membrane porins of gram-negative bacteria.