Lateral diffusion of redox components in the mitochondrial inner membrane is unaffected by inner membrane folding and matrix density

We report the first lateral diffusion measurements of redox components in normal-sized, matrix-containing, intact mitoplasts (inner membrane-matrix particles). The diffusion measurements were obtained by submicron beam fluorescence recovery after photobleaching measurements of individual, intact, rat liver mitoplasts bathed in different osmolarity media to control the matrix density and the extent of inner membrane folding. The data reveal that neither the extent of mitochondrial matrix density nor the complexity of the inner membrane folding have a significant effect on the mobility of inner membrane redox components. Diffusion coefficients for Complex I (NADH:ubiquinone oxidoreductase), Complex III (ubiquinol: cytochrome c oxidoreductase), Complex IV (cytochrome oxidase), ubiquinone, and phospholipid were found to be effectively invariant with the matrix density and/or membrane folding and essentially the same as values we reported previously for spherical, fused, ultralarge, matrix-free, inner membranes. Diffusion of proton-transporting Complex V (ATP synthase) appeared to be 2-3-fold slower at the greatest matrix density and degree of membrane folding. Consistent with a diffusion-coupled mechanism of electron transport, comparison of electron transport frequencies (productive collisions) with the theoretical, diffusion-controlled, collision frequencies (maximum collisions possible) revealed that there were consistently more calculated than productive collisions for all redox partners. Theoretical analyses of parameters for submicron fluorescence recovery after photobleaching measurements in intact mitoplasts support the finding of highly mobile redox components diffusing at the same rates as determined in conventional fluorescence recovery after photobleaching measurements in fused, ultralarge inner membranes. These findings support the Random Collision Model of Mitochondrial Electron Transport at the level of the intact mitoplast and suggest a similar conclusion for the intact mitochondrion

Ionic strength of the intermembrane space of intact mitochondria as estimated with fluorescein-BSA delivered by low pH fusion

The electrostatic interactions of cytochrome c with its redox partners and membrane lipids, as well as other protein interactions and biochemical reactions, may be modulated by the ionic strength of the intermembrane space of the mitochondrion. FITC-BSA was used to determine the relative value of the mitochondrial intermembrane ionic strength with respect to bulk medium external to the mitochondrial outer membrane. FITC-BSA exhibited an ionic strength-dependent fluorescence change with an affinity in the mM range as opposed to its pH sensitivity in the microM range. A controlled, low pH-induced membrane fusion procedure was developed to transfer FITC-BSA encapsulated in asolectin liposomes, to the intermembrane space of intact mitochondria. The fusion procedure did not significantly affect mitochondrial ultrastructure, electron transport, or respiratory control ratios. The extent of fusion of liposomes with the mitochondrial outer membrane was monitored by fluorescence dequenching assays using a membrane fluorescent probe (octadecylrhodamine B) and the soluble FITC-BSA fluorescent probe, which report membrane and contents mixing, respectively. Assays were consistent with a rapid, low pH-induced vesicle-outer membrane fusion and delivery of FITC-BSA into the intermembrane space. Similar affinities for the ionic strength- dependent change in fluorescence were found for bulk medium, soluble (9.8 +/- 0.8 mM) and intermembrane space-entrapped FITC-BSA (10.2 +/- 0.6 mM). FITC-BSA consistently reported an ionic strength in the intermembrane space of the functionally and structurally intact mitochondria within +/- 20% of the external bulk solution. These findings reveal that the intermembrane ionic strength changes as does the external ionic strength and suggest that cytochrome c interactions, as well as other protein interactions and biochemical reactions, proceed in the intermembrane space of mitochondria in the intact cell at physiological ionic strength, i.e., 100-150 mM

Import of cytochrome c into mitochondria

The import of cytochrome c into mitochondria can be resolved into a number of discrete steps. Here we report on the covalent attachment of heme to apocytochrome c by the enzyme cytochrome c heme lyase in mitochondria from Neurospora crassa. A new method was developed to measure directly the linkage of heme to apocytochrome c. This method is independent of conformational changes in the protein accompanying heme attachment. Tryptic peptides of [35S]cysteine-labelled apocytochrome c, and of enzymatically formed holocytochrome c, were resolved by reverse-phase HPLC. The cysteine-containing peptide to which heme was attached eluted later than the corresponding peptide from apocytochrome c and could be quantified by counting 35S radioactivity as a measure of holocytochrome c formation. Using this procedure, the covalent attachment of heme to apocytochrome c, which is dependent on the enzyme cytochrome c heme lyase, could be measured. Activity required heme (as hemin) and could be reversibly inhibited by the analogue deuterohemin. Holocytochrome c formation was stimulated 5–10-fold by NADH > NADPH > glutathione and was independent of a potential across the inner mitochondrial membrane. NADH was not required for the binding of apocytochrome c to mitochondria and was not involved in the reduction of the cysteine thiols prior to heme attachment. Holocytochrome c formation was also dependent on a cytosolic factor that was necessary for the heme attaching step of cytochrome c import. The factor was a heat-stable, protease-insensitive, low-molecular-mass component of unknown function. Cytochrome c heme lyase appeared to be a soluble protein located in the mitochondrial intermembrane space and was distinct from the previously identified apocytochrome c binding protein having a similar location. A model is presented in which the covalent attachment of heme by cytochrome c heme lyase also plays an essential role in the import pathway of cytochrome c

Trans-mitochondrial coordination of cristae at regulated membrane junctions

Reminiscent of bacterial quorum sensing, mammalian mitochondria participate in inter-organelle communication. However, physical structures that enhance or enable interactions between mitochondria have not been defined. Here we report that adjacent mitochondria exhibit coordination of inner mitochondrial membrane cristae at inter-mitochondrial junctions (IMJs). These electron-dense structures are conserved across species, resistant to genetic disruption of cristae organization, dynamically modulated by mitochondrial bioenergetics, independent of known inter-mitochondrial tethering proteins mitofusins and rapidly induced by the stable rapprochement of organelles via inducible synthetic linker technology. At the associated junctions, the cristae of adjacent mitochondria form parallel arrays perpendicular to the IMJ, consistent with a role in electrochemical coupling. These IMJs and associated cristae arrays may provide the structural basis to enhance the propagation of intracellular bioenergetic and apoptotic waves through mitochondrial networks within cells

The higher level of organization of the oxidative phosphorylation system: mitochondrial supercomplexes

The organization of the oxidative phosphorylation (OXPHOS) system within the inner mitochondrial membrane appears to be far more complicated than previously thought. In particular, the individual protein complexes of the OXPHOS system (complexes I to V) were found to specifically interact forming defined supramolecular structures. Blue-native polyacrylamide gel electrophoresis and single particle electron microscopy proved to be especially valuable in studying the so-called “respiratory supercomplexes”? Based on these procedures, increasing evidence was presented supporting a “solid state” organization of the OXPHOS system. Here, we summarize results on the formation, organisation and function of the various types of mitochondrial OXPHOS supercomplexes

Disruption of TBP-2 ameliorates insulin sensitivity and secretion without affecting obesity

Type 2 diabetes mellitus (T2DM) is characterized by defects in both insulin sensitivity and glucose-stimulated insulin secretion (GSIS) and is often accompanied by obesity. In this study, we show that disruption of thioredoxin binding protein-2 (TBP-2, also called Txnip) in obese mice (ob/ob) dramatically improves hyperglycaemia and glucose intolerance, without affecting obesity or adipocytokine concentrations. TBP-2-deficient ob/ob mice exhibited enhanced insulin sensitivity with activated insulin receptor substrate-1/Akt signalling in skeletal muscle and GSIS in islets compared with ob/ob mice. The elevation of uncoupling protein-2 (UCP-2) expression in ob/ob islets was downregulated by TBP-2 deficiency. TBP-2 overexpression suppressed glucose-induced adenosine triphosphate production, Ca2+ influx and GSIS. In β-cells, TBP-2 enhanced the expression level and transcriptional activity of UCP-2 by recruitment of peroxisome proliferator-activated receptor-γ co-activator-1α to the UCP-2 promoter. Thus, TBP-2 is a key regulatory molecule of both insulin sensitivity and GSIS in diabetes, raising the possibility that inhibition of TBP-2 may be a novel therapeutic approach for T2DM

Functional Roles of the N- and C-Terminal Regions of the Human Mitochondrial Single-Stranded DNA-Binding Protein

Biochemical studies of the mitochondrial DNA (mtDNA) replisome demonstrate that the mtDNA polymerase and the mtDNA helicase are stimulated by the mitochondrial single-stranded DNA-binding protein (mtSSB). Unlike Escherichia coli SSB, bacteriophage T7 gp2.5 and bacteriophage T4 gp32, mtSSBs lack a long, negatively charged C-terminal tail. Furthermore, additional residues at the N-terminus (notwithstanding the mitochondrial presequence) are present in the sequence of species across the animal kingdom. We sought to analyze the functional importance of the N- and C-terminal regions of the human mtSSB in the context of mtDNA replication. We produced the mature wild-type human mtSSB and three terminal deletion variants, and examined their physical and biochemical properties. We demonstrate that the recombinant proteins adopt a tetrameric form, and bind single-stranded DNA with similar affinities. They also stimulate similarly the DNA unwinding activity of the human mtDNA helicase (up to 8-fold). Notably, we find that unlike the high level of stimulation that we observed previously in the Drosophila system, stimulation of DNA synthesis catalyzed by human mtDNA polymerase is only moderate, and occurs over a narrow range of salt concentrations. Interestingly, each of the deletion variants of human mtSSB stimulates DNA synthesis at a higher level than the wild-type protein, indicating that the termini modulate negatively functional interactions with the mitochondrial replicase. We discuss our findings in the context of species-specific components of the mtDNA replisome, and in comparison with various prokaryotic DNA replication machineries

Measurement of the Absolute Magnitude and Time Courses of Mitochondrial Membrane Potential in Primary and Clonal Pancreatic Beta-Cells

The aim of this study was to simplify, improve and validate quantitative measurement of the mitochondrial membrane potential (ΔψM) in pancreatic β-cells. This built on our previously introduced calculation of the absolute magnitude of ΔψM in intact cells, using time-lapse imaging of the non-quench mode fluorescence of tetramethylrhodamine methyl ester and a bis-oxonol plasma membrane potential (ΔψP) indicator. ΔψM is a central mediator of glucose-stimulated insulin secretion in pancreatic β-cells. ΔψM is at the crossroads of cellular energy production and demand, therefore precise assay of its magnitude is a valuable tool to study how these processes interplay in insulin secretion. Dispersed islet cell cultures allowed cell type-specific, single-cell observations of cell-to-cell heterogeneity of ΔψM and ΔψP. Glucose addition caused hyperpolarization of ΔψM and depolarization of ΔψP. The hyperpolarization was a monophasic step increase, even in cells where the ΔψP depolarization was biphasic. The biphasic response of ΔψP was associated with a larger hyperpolarization of ΔψM than the monophasic response. Analysis of the relationships between ΔψP and ΔψM revealed that primary dispersed β-cells responded to glucose heterogeneously, driven by variable activation of energy metabolism. Sensitivity analysis of the calibration was consistent with β-cells having substantial cell-to-cell variations in amounts of mitochondria, and this was predicted not to impair the accuracy of determinations of relative changes in ΔψM and ΔψP. Finally, we demonstrate a significant problem with using an alternative ΔψM probe, rhodamine 123. In glucose-stimulated and oligomycin-inhibited β-cells the principles of the rhodamine 123 assay were breached, resulting in misleading conclusion