A study of structure of the mitochondrial membrane system

Demonstration and characterization of human cardiac porin: a voltage-dependent channel involved in adenine nucleotide movement across the outer mitochondrial membrane

The porins are a class of voltage-dependent, anion-selective, channel-forming proteins located in the outer mitochondrial membrane (OMM). The porins are responsible for passage of adenine nucleotides across the OMM, as well as for specific binding of hexokinase and glycerol kinase. This porin-kinase complex has direct access to ATP generated by mitochondrial oxidative phosphorylation and may be important in the regulation of glycolysis. Porin had not been described previously in humans but, due to its importance in bioenergetics, would be expected to be present, especially in organs requiring a large and constant supply of energy. We therefore postulated that porin would occur in human myocardium where it would be important in cardiac function. Polyclonal antibodies to bovine myocardial and rat liver porins were utilized in transblotting experiments after polyacrylamide gel electrophoresis of human heart preparations from atria, ventricles, papillary muscles, and interventricular septum. These immunoblots demonstrated selective staining of a 34-kDa band. This was identical to the results obtained with purified porin and the antibodies. Also notable was the finding that the vast majority of this staining was found in the homogenate pellet after high speed centrifugation (20,000g), as would be expected for a mitochondrial protein. The demonstration of human cardiac porin by immunoblotting with rat liver and bovine myocardial porin antibodies is the first demonstration of cross-species identification of the porins. The success of this approach undoubtedly occurred because of strong homology between porins from a variety of species.

Porin of Paramecium mitochondria isolation, characterization and ion selectivity of the closed state

Porin was isolated and purified from mitochondria of Paramecium tetraurelia. The protein showed a single band of apparent Mr 37,000 on sodium dodecyl sulfate polyacrylamide electrophoretograms. The reconstitution of the protein into artificial lipid bilayer membranes revealed it to be a porin giving pores with an average single-channel conductance of 0.26 nS in 0.1 M KCl. This conductance is about half of that of other eukaryotic porins studied to date. The pore formed by the mitochondrial porin of Paramecium was found to be voltage-dependent and switched to a defined substrate at membrane voltages larger than 20 mV. 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 1.3 nm. The properties of the low conductance state of the pore were studied in detail. In this state the pore favored the passage of cations, in contrast to the open state which favored anions slightly. The possible role of the low-conductance state in the regulation of transport processes across the outer mitochondrial membrane and in mitochondrial metabolism is discussed.

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.

Pore formation by the mitochondrial porin of rat brain in lipid bilayer membranes

The porin of the outer membrane of rat-brain mitochondria was isolated and purified. The protein showed a single band of apparent Mr 35,500 on dodecyl sulfate-containing polyacrylamide gels. The incorporation of rat-brain porin into artificial lipid bilayer membranes showed that it is able to form pores with an average single-channel conductance of 400 pS in 0.1 M KCI. The pores were found to be voltage-dependent and switched to substrates at higher transmembrane potentials. The voltage-dependence of the rat brain pore was considerably smaller than that of the other known eukaryotic porins. The possible role of the rat-brain porin in the regulation of transport process across the outer mitochondrial membrane is discussed.

Evidence for the existence of an inner membrane anion channel in mitochondria

Mitochondria normally exhibit very low electrophoretic permeabilities to physiologically important anions such as chloride, bicarbonate, phosphate, succinate, citrate, etc. Nevertheless, considerable evidence has accumulated which suggests that heart and liver mitochondria contain a specific anion-conducting channel. In this review, a postulated inner membrane anion channel is discussed in the context of other known pathways for anion transport in mitochondria. This anion channel exhibits the following properties. It is anion-selective and inhibited physiologically by protons and magnesium ions. It is inhibited reversibly by quinine and irreversibly by dicyclohexylcarbodiimide. We propose that the inner membrane anion channel is formed by inner membrane proteins and that this pathway is normally latent due to regulation by matrix Mg2+. The physiological role of the anion channel is unknown; however, this pathway is well designed to enable mitochondria to restore their normal volume following pathological swelling. In addition, the inner membrane anion channel provides a potential futile cycle for regulated non-shivering thermogenesis and may be important in controlled energy dissipation.

Are there knobs on energy transducing membranes in situ?

It is argued here that the projections which are frequently seen on the matrix side of the mitochondrial inner membranes and which are characterized as globules or spherical projections or as knob-on-stalks are an artifact of the preparation and/or of the staining of the submitochondrial particles or mitochondria. In sectioned or freeze-fractured preparations of intact cells or mitochondria, the externalized spheres are rarely seen on the membranes. They are, however, almost always seen on fragmented preparations, especially if they have been negatively stained with phosphotungstate.

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.

Molecular alterations in the plasma membrane of sporangiospores of Phycomyces related to germination

Replicas of concave fractures of freeze-fractured plasma membrane of dormant sporangio­-spores ofPhycomycesshow particles of three types, namely, (i) small (5-8 nm†); (ii) large homogeneous (30 to 35 nm); (iii) compound (30 to 35 nm). The compound particles appear to be composed of four oblong subparticles (23 nm x 12 nm). The convex fracture shows only small particles (5 to 8 nm), and also depressions of two types that are complementary to the homogeneous and compound particles on the opposite (concave) face. Heat shocking initiates germination of spores. The compound particles are no longer seen by 6 h after heat-shocking. Aggregates of homogeneous particles appear upon heat-shocking, and these aggregates grow to large arrays by about 12 h. These aggregates and the homo­geneous particles disappear by the time the spore germinates, that is, about 20 h after heat-shocking. It therefore appears that both homogeneous and compound particles are related to controlled dormancy and germination of spores. The number of small particles increases on both the fractured faces as the spore germinates. Partial digestion of homogeneous particles by incubation with pronase suggests that these macromolecules may contain protein and are amphipathic in structure. The small particles are apparently unaffected by treatment with pronase. Evidence indicative of pinocytotic activity is seen in germinating spores.

Preparation of isolated rat liver mitochondria for electron microscopy

A comparative study of the fixation of isolated rat liver mitochondria was undertaken. If the criterion is adopted that after processing, the mitochondria should resemble as closely as possible rat liver mitochondria in situ, the procedure found to produce such preservation was that of fixation in suspension in veronal-buffered 2% potassium permanganate. Fixation in osmium tetroxide produced variable results, while mitochondria fixed in glutaraldehyde were contracted. We suggest that in cases where fixation procedures modify the morphological appearance of mitochondria, the significance of such changes must be treated with caution.

Ruthenium Tetroxide for Fixing and Staining Cytoplasmic Membranes

Rat liver and kidney were fixed for electron microscopy with ruthenium tetroxide. When compared with tissues fixed with osmiun tetroxide, the membranes showed well without additional staining. The trilaminar structuire of the cytoplasmic membranes was clearly seen. In width the nuclear, mitochondrial, and cytoplasmic membranes investigated resembled the cell memnbrane.