Cooperation of translocase complexes in mitochondrial protein import

New insights into the structure and dynamics of the TOM complex in mitochondria

To date, there is no general physical model of the mechanism by which unfolded polypeptide chains with different properties are imported into the mitochondria. At the molecular level, it is still unclear how transit polypeptides approach, are captured by the protein translocation machinery in the outer mitochondrial membrane, and how they subsequently cross the entropic barrier of a protein translocation pore to enter the intermembrane space. This deficiency has been due to the lack of detailed structural and dynamic information about the membrane pores. In this review, we focus on the recently determined sub-nanometer cryo-EM structures and our current knowledge of the dynamics of the mitochondrial two-pore outer membrane protein translocation machinery (TOM core complex), which provide a starting point for addressing the above questions. Of particular interest are recent discoveries showing that the TOM core complex can act as a mechanosensor, where the pores close as a result of interaction with membrane-proximal structures. We highlight unusual and new correlations between the structural elements of the TOM complexes and their dynamic behavior in the membrane environment.

Cristae shaping and dynamics in mitochondrial function

Mitochondria are multifunctional organelles of key importance for cell homeostasis. The outer mitochondrial membrane (OMM) envelops the organelle, and the inner mitochondrial membrane (IMM) is folded into invaginations called cristae. As cristae composition and functions depend on the cell type and stress conditions, they recently started to be considered as a dynamic compartment. A number of proteins are known to play a role in cristae architecture, such as OPA1, MIC60, LETM1, the prohibitin (PHB) complex and the F1FO ATP synthase. Furthermore, phospholipids are involved in the maintenance of cristae ultrastructure and dynamics. The use of new technologies, including super-resolution microscopy to visualize cristae dynamics with superior spatiotemporal resolution, as well as high-content techniques and datasets have not only allowed the identification of new cristae proteins but also helped to explore cristae plasticity. However, a number of open questions remain in the field, such as whether cristae-resident proteins are capable of changing localization within mitochondria, or whether mitochondrial proteins can exit mitochondria through export. In this Review, we present the current view on cristae morphology, stability and composition, and address important outstanding issues that might pave the way to future discoveries.

NLG1, encoding a mitochondrial membrane protein, controls leaf and grain development in rice

BACKGROUND

Mitochondrion is the key respiratory organ and participate in multiple anabolism and catabolism pathways in eukaryote. However, the underlying mechanism of how mitochondrial membrane proteins regulate leaf and grain development remains to be further elucidated.

RESULTS

Here, a mitochondria-defective mutant narrow leaf and slender grain 1 (nlg1) was identified from an EMS-treated mutant population, which exhibits narrow leaves and slender grains. Moreover, nlg1 also presents abnormal mitochondria structure and was sensitive to the inhibitors of mitochondrial electron transport chain. Map-based cloning and transgenic functional confirmation revealed that NLG1 encodes a mitochondrial import inner membrane translocase containing a subunit Tim21. GUS staining assay and RT-qPCR suggested that NLG1 was mainly expressed in leaves and panicles. The expression level of respiratory function and auxin response related genes were significantly down-regulated in nlg1, which may be responsible for the declination of ATP production and auxin content.

CONCLUSIONS

These results suggested that NLG1 plays an important role in the regulation of leaf and grain size development by maintaining mitochondrial homeostasis. Our finding provides a novel insight into the effects of mitochondria development on leaf and grain morphogenesis in rice.

Adaptive Changes in Detoxification Metabolism and Transmembrane Transport of Bombyx mori Malpighian Tubules to Artificial Diet

The high adaptability of insects to food sources has contributed to their ranking among the most abundant and diverse species on Earth. However, the molecular mechanisms underlying the rapid adaptation of insects to different foods remain unclear. We explored the changes in gene expression and metabolic composition of the Malpighian tubules as an important metabolic excretion and detoxification organ in silkworms (Bombyx mori) fed mulberry leaf and artificial diets. A total of 2436 differentially expressed genes (DEGs) and 245 differential metabolites were identified between groups, with the majority of DEGs associated with metabolic detoxification, transmembrane transport, and mitochondrial function. Detoxification enzymes, such as cytochrome P450 (CYP), glutathione-S-transferase (GST), and UDP-glycosyltransferase, and ABC and SLC transporters of endogenous and exogenous solutes were more abundant in the artificial diet group. Enzyme activity assays confirmed increased CYP and GST activity in the Malpighian tubules of the artificial diet-fed group. Metabolome analysis showed increased contents of secondary metabolites, terpenoids, flavonoids, alkaloids, organic acids, lipids, and food additives in the artificial diet group. Our findings highlight the important role of the Malpighian tubules in adaptation to different foods and provide guidance for further optimization of artificial diets to improve silkworm breeding.

Mitochondrial ion channels in cardiac function

Mitochondria have been recognized as key organelles in cardiac physiology and are potential targets for clinical interventions to improve cardiac function. Mitochondrial dysfunction has been accepted as a major contributor to the development of heart failure. The main function of mitochondria is to meet the high energy demands of the heart by oxidative metabolism. Ionic homeostasis in mitochondria directly regulates oxidative metabolism, and any disruption in ionic homeostasis causes mitochondrial dysfunction and eventually contractile failure. The mitochondrial ionic homeostasis is closely coupled with inner mitochondrial membrane potential. To regulate and maintain ionic homeostasis, mitochondrial membranes are equipped with ion transporting proteins. Ion transport mechanisms involving several different ion channels and transporters are highly efficient and dynamic, thus helping to maintain the ionic homeostasis of ions as well as their salts present in the mitochondrial matrix. In recent years, several novel proteins have been identified on the mitochondrial membranes and these proteins are actively being pursued in research for roles in the organ as well as organelle physiology. In this article, the role of mitochondrial ion channels in cardiac function is reviewed. In recent times, the major focus of the mitochondrial ion channel field is to establish molecular identities as well as assigning specific functions to them. Given the diversity of mitochondrial ion channels and their unique roles in cardiac function, they present novel and viable therapeutic targets for cardiac diseases.

The Diversity of the Mitochondrial Outer Membrane Protein Import Channels: Emerging Targets for Modulation

The functioning of mitochondria and their biogenesis are largely based on the proper function of the mitochondrial outer membrane channels, which selectively recognise and import proteins but also transport a wide range of other molecules, including metabolites, inorganic ions and nucleic acids. To date, nine channels have been identified in the mitochondrial outer membrane of which at least half represent the mitochondrial protein import apparatus. When compared to the mitochondrial inner membrane, the presented channels are mostly constitutively open and consequently may participate in transport of different molecules and contribute to relevant changes in the outer membrane permeability based on the channel conductance. In this review, we focus on the channel structure, properties and transported molecules as well as aspects important to their modulation. This information could be used for future studies of the cellular processes mediated by these channels, mitochondrial functioning and therapies for mitochondria-linked diseases.

The structure of the TOM core complex in the mitochondrial outer membrane

In the past three decades, significant advances have been made in providing the biochemical background of TOM (translocase of the outer mitochondrial membrane)-mediated protein translocation into mitochondria. In the light of recent cryoelectron microscopy-derived structures of TOM isolated from Neurospora crassa and Saccharomyces cerevisiae, the interpretation of biochemical and biophysical studies of TOM-mediated protein transport into mitochondria now rests on a solid basis. In this review, we compare the subnanometer structure of N. crassa TOM core complex with that of yeast. Both structures reveal remarkably well-conserved symmetrical dimers of 10 membrane protein subunits. The structural data also validate predictions of weakly stable regions in the transmembrane β-barrel domains of the protein-conducting subunit Tom40, which signal the existence of β-strands located in interfaces of protein-protein interactions.

Mitochondrial protein import as a quality control sensor

Mitochondria are organelles involved in various functions related to cellular metabolism and homoeostasis. Though mitochondria contain own genome, their nuclear counterparts encode most of the different mitochondrial proteins. These are synthesised as precursors in the cytosol and have to be delivered into the mitochondria. These organelles hence have elaborate machineries for the import of precursor proteins from cytosol. The protein import machineries present in both mitochondrial membrane and aqueous compartments show great variability in pre-protein recognition, translocation and sorting across or into it. Mitochondrial protein import machineries also interact transiently with other protein complexes of the respiratory chain or those involved in the maintenance of membrane architecture. Hence mitochondrial protein translocation is an indispensable part of the regulatory network that maintains protein biogenesis, bioenergetics, membrane dynamics and quality control of the organelle. Various stress conditions and diseases that are associated with mitochondrial import defects lead to changes in cellular transcriptomic and proteomic profiles. Dysfunction in mitochondrial protein import also causes over-accumulation of precursor proteins and their aggregation in the cytosol. Multiple pathways may be activated for buffering these harmful consequences. Here, we present a comprehensive picture of import machinery and its role in cellular quality control in response to defective mitochondrial import. We also discuss the pathological consequences of dysfunctional mitochondrial protein import in neurodegeneration and cancer.

A Dictyostelium discoideum mitochondrial fluorescent tagging vector that does not affect respiratory function

Visualizing mitochondria in living Dictyostelium discoideum cells using fluorescent dyes is often problematic due to variability in staining, metabolism of the dyes, and unknown potential effects of the dyes on mitochondrial function. We show that fluorescent labelling of mitochondria, using an N-terminal mitochondrial localization sequence derived from the D. discoideum protein GcvH1 (glycine cleavage system H1) attached to a red fluorescent protein enables clear mitochondrial imaging. We also show that this labelling has no effect upon mitochondria load or respiratory function.

The Role of Mitochondrial Stress in Muscle Wasting Following Severe Burn Trauma

Increased resting metabolic rate and skeletal muscle wasting are hallmarks of the pathophysiological stress response to severe burn trauma. However, whether these two responses occur independently in burn patients or are in fact related remains unclear. In light of recent evidence demonstrating that increased proteolysis in skeletal muscle of burned patients is accompanied by mitochondrial hypermetabolism, oxidative stress, and protein damage; in this article, we discuss the evidence for a role for the mitochondrion in skeletal muscle wasting following severe burn trauma. In particular, we focus on the role of mitochondrial superoxide production in oxidative stress and subsequent proteolysis, and discuss the role of the mitochondrion as a signaling organelle resulting in protein catabolism in other cellular compartments following severe burn trauma.

Conformational ensemble of an intrinsically flexible loop in mitochondrial import protein Tim21 studied by modeling and molecular dynamics simulations

BACKGROUND

Tim21, a subunit of a highly dynamic translocase of the inner mitochondrial membrane (TIM23) complex, translocates proteins by interacting with subunits in the translocase of the outer membrane (TOM) complex and Tim23 channel in the TIM23 complex. A loop segment in Tim21, which is in close proximity of the binding site of Tim23, has different conformations in X-ray, NMR and new crystal contact-free space (CCFS) structures. MD simulations can provide information on the structure and dynamics of the loop in solution.

METHODS

The conformational ensemble of the loop was characterized using loop modeling and molecular dynamics (MD) simulations.

RESULTS

MD simulations confirmed mobility of the loop. Multidimensional scaling and clustering were used to characterize the dynamic conformational ensemble of the loop. Free energy landscape showed that the CCFS crystal structure occupied a low energy region as compared to the conventional X-ray crystal structure. Analysis of crystal packing indicates that the CCFS provides larger conformational space for the motions of the loop.

CONCLUSIONS

Our work reported the conformational ensemble of the loop in solution, which is in agreement with the structure obtained from CCFS approach. The combination of the experimental techniques and computational methods is beneficial for studying highly flexible regions of proteins.

GENERAL SIGNIFICANCE

Computational methods, such as loop modeling and MD simulations, have proved to be useful for studying conformational flexibility of proteins. These methods in integration with experimental techniques such as CCFS has the potential to transform the studies on flexible regions of proteins.

Mitochondrial protein import regulates cytosolic protein homeostasis and neuronal integrity

Neurodegeneration is characterized by protein aggregate deposits and mitochondrial malfunction. Reduction in Tom40 (translocase of outer membrane 40) expression, a key subunit of the translocase of the outer mitochondrial membrane complex, led to accumulation of ubiquitin (Ub)-positive protein aggregates engulfed by Atg8a-positive membranes. Other macroautophagy markers were also abnormally accumulated. Autophagy was induced but the majority of autophagosomes failed to fuse with lysosomes when Tom40 was downregulated. In Tom40 RNAi tissues, autophagosome-like (AL) structures, often not sealed, were 10 times larger than starvation induced autophagosomes. Atg5 downregulation abolished Tom40 RNAi induced AL structure formation, but the Ub-positive aggregates remained, whereas knock down of Syx17, a gene required for autophagosome-lysosome fusion, led to the disappearance of giant AL structures and accumulation of small autophagosomes and phagophores near the Ub-positive aggregates. The protein aggregates contained many mitochondrial preproteins, cytosolic proteins, and proteasome subunits. Proteasome activity and ATP levels were reduced and the ROS levels was increased in Tom40 RNAi tissues. The simultaneous inhibition of proteasome activity, reduction in ATP production, and increase in ROS, but none of these conditions alone, can mimic the imbalanced proteostasis phenotypes observed in Tom40 RNAi cells. Knockdown of ref(2)P or ectopic expression of Pink1 and park greatly reduced aggregate formation in Tom40 RNAi tissues. In nerve tissues, reduction in Tom40 activity leads to aggregate formation and neurodegeneration. Rather than diminishing the neurodegenerative phenotypes, overexpression of Pink1 enhanced them. We proposed that defects in mitochondrial protein import may be the key to linking imbalanced proteostasis and mitochondrial defects.

ABBREVIATIONS

AL: autophagosome-like; Atg12: Autophagy-related 12; Atg14: Autophagy-related 14; Atg16: Autophagy-related 16; Atg5: Autophagy-related 5; Atg6: Autophagy-related 6; Atg8a: Autophagy-related 8a; Atg9: Autophagy-related 9; ATP: adenosine triphosphate; Cas9: CRISPR associated protein 9; cDNA: complementary DNA; COX4: Cytochrome c oxidase subunit 4; CRISPR: clustered regularly interspaced short palindromic repeats; Cyt-c1: Cytochrome c1; DAPI: 4,6-diamidino-2-phenylindole dihydrochloride; Dcr-2: Dicer-2; FLP: Flippase recombination enzyme; FRT: FLP recombination target; GFP: green fluorescent protein; GO: gene ontology; gRNA: guide RNA; Hsp60: Heat shock protein 60A; HDAC6: Histone deacetylase 6; htt: huntingtin; Idh: Isocitrate dehydrogenase; IFA: immunofluorescence assay; Irp-1A: Iron regulatory protein 1A; kdn: knockdown; Marf: Mitochondrial assembly regulatory factor; MitoGFP: Mitochondrial-GFP; MS: mass spectrometry; MTPAP: mitochondrial poly(A) polymerase; Nmnat: Nicotinamide mononucleotide adenylyltransferase; OE: overexpression; Pink1/PINK1: PTEN-induced putative kinase 1; polyQ: polyglutamine; PRKN: parkin RBR E3 ubiquitin protein ligase; Prosα4: proteasome α4 subunit; Prosβ1: proteasome β1 subunit; Prosβ5: proteasome β5 subunit; Prosβ7: proteasome β7 subunit; ref(2)P: refractory to sigma P; RFP: red fluorescent protein; RNAi: RNA interference; ROS: reactive oxygen species; Rpn11: Regulatory particle non-ATPase 11; Rpt2: Regulatory particle triple-A ATPase 2; scu: scully; sicily: severe impairment of CI with lengthened youth; sesB: stress-sensitive B; Syx17: Syntaxin17; TEM: transmission electron microscopy; ttm50: tiny tim 50; Tom: translocase of the outer membrane; Tom20: translocase of outer membrane 20; Tom40: translocase of outer membrane 40; Tom70: translocase of outer membrane 70; UAS: upstream active sequence; Ub: ubiquitin; VNC: ventral nerve cord; ZFYVE1: zinc finger FYVE-type containing 1.

Biomedical Informatics on the Cloud: A Treasure Hunt for Advancing Cardiovascular Medicine

In the digital age of cardiovascular medicine, the rate of biomedical discovery can be greatly accelerated by the guidance and resources required to unearth potential collections of knowledge. A unified computational platform leverages metadata to not only provide direction but also empower researchers to mine a wealth of biomedical information and forge novel mechanistic insights. This review takes the opportunity to present an overview of the cloud-based computational environment, including the functional roles of metadata, the architecture schema of indexing and search, and the practical scenarios of machine learning-supported molecular signature extraction. By introducing several established resources and state-of-the-art workflows, we share with our readers a broadly defined informatics framework to phenotype cardiovascular health and disease.

Trypanosomal mitochondrial intermediate peptidase does not behave as a classical mitochondrial processing peptidase

Upon their translocation into the mitochondrial matrix, the N-terminal pre-sequence of nuclear-encoded proteins undergoes cleavage by mitochondrial processing peptidases. Some proteins require more than a single processing step, which involves several peptidases. Down-regulation of the putative Trypanosoma brucei mitochondrial intermediate peptidase (MIP) homolog by RNAi renders the cells unable to grow after 48 hours of induction. Ablation of MIP results in the accumulation of the precursor of the trypanosomatid-specific trCOIV protein, the largest nuclear-encoded subunit of the cytochrome c oxidase complex in this flagellate. However, the trCOIV precursor of the same size accumulates also in trypanosomes in which either alpha or beta subunits of the mitochondrial processing peptidase (MPP) have been depleted. Using a chimeric protein that consists of the N-terminal sequence of a putative subunit of respiratory complex I fused to a yellow fluorescent protein, we assessed the accumulation of the precursor protein in trypanosomes, in which RNAi was induced against the alpha or beta subunits of MPP or MIP. The observed accumulation of precursors indicates MIP depletion affects the activity of the cannonical MPP, or at least one of its subunits.

Hypermetabolism and hypercatabolism of skeletal muscle accompany mitochondrial stress following severe burn trauma

Burn trauma results in prolonged hypermetabolism and skeletal muscle wasting. How hypermetabolism contributes to muscle wasting in burn patients remains unknown. We hypothesized that oxidative stress, cytosolic protein degradation, and mitochondrial stress as a result of hypermetabolism contribute to muscle cachexia postburn. Patients (n = 14) with burns covering >30% of their total body surface area were studied. Controls (n = 13) were young healthy adults. We found that burn patients were profoundly hypermetabolic at both the skeletal muscle and systemic levels, indicating increased oxygen consumption by mitochondria. In skeletal muscle of burn patients, concurrent activation of mTORC1 signaling and elevation in the fractional synthetic rate paralleled increased levels of proteasomes and elevated fractional breakdown rate. Burn patients had greater levels of oxidative stress markers as well as higher expression of mtUPR-related genes and proteins, suggesting that burns increased mitochondrial stress and protein damage. Indeed, upregulation of cytoprotective genes suggests hypermetabolism-induced oxidative stress postburn. In parallel to mtUPR activation postburn, mitochondrial-specific proteases (LONP1 and CLPP) and mitochondrial translocases (TIM23, TIM17B, and TOM40) were upregulated, suggesting increased mitochondrial protein degradation and transport of preprotein, respectively. Our data demonstrate that proteolysis occurs in both the cytosolic and mitochondrial compartments of skeletal muscle in severely burned patients. Increased mitochondrial protein turnover may be associated with increased protein damage due to hypermetabolism-induced oxidative stress and activation of mtUPR. Our results suggest a novel role for the mitochondria in burn-induced cachexia.

The Expanding Landscape of Moonlighting Proteins in Yeasts

Moonlighting proteins are multifunctional proteins that participate in unrelated biological processes and that are not the result of gene fusion. A certain number of these proteins have been characterized in yeasts, and the easy genetic manipulation of these microorganisms has been useful for a thorough analysis of some cases of moonlighting. As the awareness of the moonlighting phenomenon has increased, a growing number of these proteins are being uncovered. In this review, we present a crop of newly identified moonlighting proteins from yeasts and discuss the experimental evidence that qualifies them to be classified as such. The variety of moonlighting functions encompassed by the proteins considered extends from control of transcription to DNA repair or binding to plasminogen. We also discuss several questions pertaining to the moonlighting condition in general. The cases presented show that yeasts are important organisms to be used as tools to understand different aspects of moonlighting proteins.

Peeping at TOMs-Diverse Entry Gates to Mitochondria Provide Insights into the Evolution of Eukaryotes

Mitochondria are essential for eukaryotic life and more than 95% of their proteins are imported as precursors from the cytosol. The targeting signals for this posttranslational import are conserved in all eukaryotes. However, this conservation does not hold true for the protein translocase of the mitochondrial outer membrane that serves as entry gate for essentially all precursor proteins. Only two of its subunits, Tom40 and Tom22, are conserved and thus likely were present in the last eukaryotic common ancestor. Tom7 is found in representatives of all supergroups except the Excavates. This suggests that it was added to the core of the translocase after the Excavates segregated from all other eukaryotes. A comparative analysis of the biochemically and functionally characterized outer membrane translocases of yeast, plants, and trypanosomes, which represent three eukaryotic supergroups, shows that the receptors that recognize the conserved import signals differ strongly between the different systems. They present a remarkable example of convergent evolution at the molecular level. The structural diversity of the functionally conserved import receptors therefore provides insight into the early evolutionary history of mitochondria.

Malleable mitochondrion of Trypanosoma brucei

The importance of mitochondria for a typical aerobic eukaryotic cell is undeniable, as the list of necessary mitochondrial processes is steadily growing. Here, we summarize the current knowledge of mitochondrial biology of an early-branching parasitic protist, Trypanosoma brucei, a causative agent of serious human and cattle diseases. We present a comprehensive survey of its mitochondrial pathways including kinetoplast DNA replication and maintenance, gene expression, protein and metabolite import, major metabolic pathways, Fe-S cluster synthesis, ion homeostasis, organellar dynamics, and other processes. As we describe in this chapter, the single mitochondrion of T. brucei is everything but simple and as such rivals mitochondria of multicellular organisms.

The interplay of Hrd3 and the molecular chaperone system ensures efficient degradation of malfolded secretory proteins

A central ubiquitin ligase involved in endoplasmic reticulum (ER)–associated protein degradation is the HRD-ligase. The ER-luminal subunit Hrd3 cooperates with the cochaperone Scj1 in clearing misfolded proteins from the ER.

Misfolded proteins of the secretory pathway are extracted from the endoplasmic reticulum (ER), polyubiquitylated by a protein complex termed the Hmg-CoA reductase degradation ligase (HRD-ligase), and degraded by cytosolic 26S proteasomes. This process is termed ER-associated protein degradation (ERAD). We previously showed that the membrane protein Der1, which is a subunit of the HRD-ligase, is involved in the export of aberrant polypeptides from the ER. Unexpectedly, we also uncovered a close spatial proximity of Der1 and the substrate receptor Hrd3 in the ER lumen. We report here on a mutant Hrd3KR that is selectively defective for ERAD of soluble proteins. Hrd3KR displays subtle structural changes that affect its positioning toward Der1. Furthermore, increased quantities of the ER-resident Hsp70-type chaperone Kar2 and the Hsp40-type cochaperone Scj1 bind to Hrd3KR. Of note, deletion of SCJ1 impairs ERAD of model substrates and causes the accumulation of client proteins at Hrd3. Our data imply a function of Scj1 in the removal of malfolded proteins from the receptor Hrd3, which facilitates their delivery to downstream-acting components like Der1.

Molecular basis of the dynamic structure of the TIM23 complex in the mitochondrial intermembrane space

The presequence translocase TIM23 is a highly dynamic complex in which its subunits can adopt multiple conformations and undergo association-dissociation to facilitate import of proteins into mitochondria. Despite the importance of protein-protein interactions in TIM23, little is known about the molecular details of these processes. Using nuclear magnetic resonance spectroscopy, we characterized the dynamic interaction network of the intermembrane space domains of Tim23, Tim21, Tim50, and Tom22 at single-residue level. We show that Tim23(IMS) contains multiple sites to efficiently interact with the intermembrane space domain of Tim21 and to bind to Tim21, Tim50, and Tom22. In addition, we reveal the atomic details of the dynamic Tim23(IMS)-Tim21(IMS) complex. The combined data support a central role of the intermembrane space domain of Tim23 in the formation and regulation of the presequence translocase.

Alternative oxidase inhibitors as antiparasitic agents against scuticociliatosis

Philasterides dicentrarchi causes a severe disease in turbot, and at present there are no drugs available to treat infected fish. We have previously demonstrated that, in addition to the classical respiratory pathway, P. dicentrarchi possesses an alternative mitochondrial respiratory pathway that is cyanide-insensitive and salicylhydroxamic acid (SHAM)-sensitive. In this study, we found that during the initial phase of growth in normoxia, ciliate respiration is sensitive to the natural polyphenol resveratrol (RESV) and to Antimycin A (AMA). However, under hypoxic conditions, the parasite utilizes AMA-insensitive respiration, which is completely inhibited by RESV and by the antioxidant propyl gallate (PG), an alternative oxidase (AOX) inhibitor. PG caused significantly dose-dependent inhibition of the in vitro growth of the parasite under normoxia and hypoxia and an over-expression of heat shock proteins of the Hsp70 subfamily. RESV and PG may affect the protective role of the AOX against mitochondrial oxidative stress, leading to an impaired mitochondrial membrane potential and mitochondrial dysfunction, which the parasite attempts to neutralize by increasing the expression of Hsp70. In view of the antiparasitic effects induced by AOX inhibitors and the absence of AOX in their host, this enzyme constitutes a potential target for the development of new drugs against scuticociliatosis.

A bifunctional protein regulates mitochondrial protein synthesis

Mitochondrial gene expression is predominantly regulated at the post-transcriptional level and mitochondrial ribonucleic acid (RNA)-binding proteins play a key role in RNA metabolism and protein synthesis. The AU-binding homolog of enoyl-coenzyme A (CoA) hydratase (AUH) is a bifunctional protein with RNA-binding activity and a role in leucine catabolism. AUH has a mitochondrial targeting sequence, however, its role in mitochondrial function has not been investigated. Here, we found that AUH localizes to the inner mitochondrial membrane and matrix where it associates with mitochondrial ribosomes and regulates protein synthesis. Decrease or overexpression of the AUH protein in cells causes defects in mitochondrial translation that lead to changes in mitochondrial morphology, decreased mitochondrial RNA stability, biogenesis and respiratory function. Because of its role in leucine metabolism, we investigated the importance of the catalytic activity of AUH and found that it affects the regulation of mitochondrial translation and biogenesis in response to leucine.

Apolipoprotein e sets the stage: response to injury triggers neuropathology

Apolipoprotein (apo) E4 is the major genetic risk factor for Alzheimer's disease and is associated with poor clinical outcome following traumatic brain injury and other neuropathological disorders. Protein instability and an isoform-specific apoE property called domain interaction are responsible for these neuropathological effects. ApoE4 is the most neurotoxic isoform and can induce neuropathology through various cellular pathways. Neuronal damage or stress induces apoE synthesis as part of the repair response; however, when apoE4 is expressed in neurons, its unique conformation makes it susceptible to proteolysis, resulting in the generation of neurotoxic fragments. These fragments cause pathological mitochondrial dysfunction and cytoskeletal alterations. Here, we review data supporting the hypothesis that apoE4 (> apoE3 > apoE2) has direct neurotoxic effects and highlight studies showing that blocking domain interaction reverses these detrimental effects.

Mge1, a nucleotide exchange factor of Hsp70, acts as an oxidative sensor to regulate mitochondrial Hsp70 function

Yeast Mge1, the cochaperone of mitochondrial heat shock protein 70 (mHsp70), is essential for exchanging ATP for ADP on mHsp70 and thus for recycling of mHsp70 for mitochondrial protein import and folding. Mge1 acts as an oxidative sensor to regulate mHsp70 function.

Despite the growing evidence of the role of oxidative stress in disease, its molecular mechanism of action remains poorly understood. The yeast Saccharomyces cerevisiae provides a valuable model system in which to elucidate the effects of oxidative stress on mitochondria in higher eukaryotes. Dimeric yeast Mge1, the cochaperone of heat shock protein 70 (Hsp70), is essential for exchanging ATP for ADP on Hsp70 and thus for recycling of Hsp70 for mitochondrial protein import and folding. Here we show an oxidative stress–dependent decrease in Mge1 dimer formation accompanied by a concomitant decrease in Mge1–Hsp70 complex formation in vitro. The Mge1-M155L substitution mutant stabilizes both Mge1 dimer and Mge1–Hsp70 complex formation. Most important, the Mge1-M155L mutant rescues the slow-growth phenomenon associated with the wild-type Mge1 strain in the presence of H₂O₂ in vivo, stimulation of the ATPase activity of Hsp70, and the protein import defect during oxidative stress in vitro. Furthermore, cross-linking studies reveal that Mge1–Hsp70 complex formation in mitochondria isolated from wild-type Mge1 cells is more susceptible to reactive oxygen species compared with mitochondria from Mge1-M155L cells. This novel oxidative sensor capability of yeast Mge1 might represent an evolutionarily conserved function, given that human recombinant dimeric Mge1 is also sensitive to H₂O₂.

SD3, an Arabidopsis thaliana homolog of TIM21, affects intracellular ATP levels and seedling development

It is poorly understood how plants control their growth by cell division, elongation, and differentiation. We have characterized a seedling-lethal mutant segregation distortion 3 (sd3) that showed a very dwarf phenotype when grown in the light and, in the dark, had short hypocotyls with reduced ploidy levels. The corresponding gene of SD3 encodes a protein with high similarity to yeast translocase on the inner mitochondrial membrane 21 (TIM21), which is a component of the TIM23 complex. Indeed, SD3 protein fused to GFP localized in the mitochondria. SD3 overexpression increased cotyledon size in the light and hypocotyl thickness in the dark. The expression of genes for several subunits of the respiratory-chain complexes III and IV was up-regulated in SD3-overexpressing plants. Furthermore, these plants showed high levels of ATP whereas those of sd3 were low. These results suggested that SD3 induced an increase in cell size by raising the expression of the respiratory-chain subunit genes and hence increased the intracellular ATP levels. We propose that intracellular ATP levels regulated by mitochondria control plant organ size.

Hydrogen sulfide (H2S) metabolism in mitochondria and its regulatory role in energy production

Although many types of ancient bacteria and archea rely on hydrogen sulfide (H(2)S) for their energy production, eukaryotes generate ATP in an oxygen-dependent fashion. We hypothesize that endogenous H(2)S remains a regulator of energy production in mammalian cells under stress conditions, which enables the body to cope with energy demand when oxygen supply is insufficient. Cystathionine γ-lyase (CSE) is a major H(2)S-producing enzyme in the cardiovascular system that uses cysteine as the main substrate. Here we show that CSE is localized only in the cytosol, not in mitochondria, of vascular smooth-muscle cells (SMCs) under resting conditions, revealed by Western blot analysis and confocal microscopy of SMCs transfected with GFP-tagged CSE plasmid. After SMCs were exposed to A23187, thapsigargin, or tunicamycin, intracellular calcium level was increased, and CSE translocated from the cytosol to mitochondria. CSE was coimmunoprecipitated with translocase of the outer membrane 20 (Tom20) in mitochondrial membrane. Tom20 siRNA significantly inhibited mitochondrial translocation of CSE and mitochondrial H(2)S production. The cysteine level inside mitochondria is approximately three times that in the cytosol. Translocation of CSE to mitochondria metabolized cysteine, produced H(2)S inside mitochondria, and increased ATP production. Inhibition of CSE activity reversed A23187-stimulated mitochondrial ATP production. H(2)S improved mitochondrial ATP production in SMCs with hypoxia, which alone decreased ATP production. These results suggest that translocation of CSE to mitochondria on specific stress stimulations is a unique mechanism to promote H(2)S production inside mitochondria, which subsequently sustains mitochondrial ATP production under hypoxic conditions.

Arabidopsis mitochondrial protein TIM50 affects hypocotyl cell elongation through intracellular ATP level

The plant hypocotyl is an excellent model for the analysis of cell elongation. We have characterized a knockout mutant of the Arabidopsis TIM50 gene that showed a reduction in the hypocotyls length of etiolated seedlings. We also found that a knockout of TIM50 caused enlargement and deformation of the mitochondrial structure and a reduction in intracellular ATP levels. TIM50 is a component of the mitochondrial TIM23 inner membrane protein complex and is involved in the import of mitochondrial proteins. The short hypocotyl phenotype was recovered by the addition of Compound C, an inhibitor of AMPK. Thus, the mitochondrial ATP level controls cell elongation in Arabidopsis hypocotyls through possible signaling via AMPK.

Inner mitochondrial translocase Tim50 interacts with 3β-hydroxysteroid dehydrogenase type 2 to regulate adrenal and gonadal steroidogenesis

In the adrenals, testes, and ovaries, 3β-hydroxysteroid dehydrogenase type 2 (3βHSD2) catalyzes the conversion of pregnenolone to progesterone and dehydroepiandrostenedione to androstenedione. Alterations in this pathway can have deleterious effects, including sexual development impairment, spontaneous abortion, and breast cancer. 3βHSD2, synthesized in the cytosol, is imported into the inner mitochondrial membrane (IMM) by translocases. Steroidogenesis requires that 3βHSD2 acts as both a dehydrogenase and isomerase. To achieve this dual functionality, 3βHSD2 must undergo a conformational change; however, what triggers that change remains unknown. We propose that 3βHSD2 associates with IMM or outer mitochondrial membrane translocases facing the intermembrane space (IMS) and that this interaction promotes the conformational change needed for full activity. Fractionation assays demonstrate that 3βHSD2 associated with the IMM but did not integrate into the membrane. Through mass spectrometry and Western blotting of mitochondrial complexes and density gradient ultracentrifugation, we show that that 3βHSD2 formed a transient association with the translocases Tim50 and Tom22 and with Tim23. This association occurred primarily through the interaction of Tim50 with the N terminus of 3βHSD2 and contributed to enzymatic activity. Tim50 knockdown inhibited catalysis of dehydroepiandrostenedione to androstenedione and pregnenolone to progesterone. Although Tim50 knockdown decreased 3βHSD2 expression, restoration of expression via proteasome and protease inhibition did not rescue activity. In addition, protein fingerprinting and CD spectroscopy reveal the flexibility of 3βHSD2, a necessary characteristic for forming multiple associations. In summary, Tim50 regulates 3βHSD2 expression and activity, representing a new role for translocases in steroidogenesis.

In silico survey of the mitochondrial protein uptake and maturation systems in the brown alga Ectocarpus siliculosus

The acquisition of mitochondria was a key event in eukaryote evolution. The aim of this study was to identify homologues of the components of the mitochondrial protein import machinery in the brown alga Ectocarpus and to use this information to investigate the evolutionary history of this fundamental cellular process. Detailed searches were carried out both for components of the protein import system and for related peptidases. Comparative and phylogenetic analyses were used to investigate the evolution of mitochondrial proteins during eukaryote diversification. Key observations include phylogenetic evidence for very ancient origins for many protein import components (Tim21, Tim50, for example) and indications of differences between the outer membrane receptors that recognize the mitochondrial targeting signals, suggesting replacement, rearrangement and/or emergence of new components across the major eukaryotic lineages. Overall, the mitochondrial protein import components analysed in this study confirmed a high level of conservation during evolution, indicating that most are derived from very ancient, ancestral proteins. Several of the protein import components identified in Ectocarpus, such as Tim21, Tim50 and metaxin, have also been found in other stramenopiles and this study suggests an early origin during the evolution of the eukaryotes.

Mitochondrial targeting of human NADH dehydrogenase (ubiquinone) flavoprotein 2 (NDUFV2) and its association with early-onset hypertrophic cardiomyopathy and encephalopathy

Background

NADH dehydrogenase (ubiquinone) flavoprotein 2 (NDUFV2), containing one iron sulfur cluster ([2Fe-2S] binuclear cluster N1a), is one of the core nuclear-encoded subunits existing in human mitochondrial complex I. Defects in this subunit have been associated with Parkinson's disease, Alzheimer's disease, Bipolar disorder, and Schizophrenia. The aim of this study is to examine the mitochondrial targeting of NDUFV2 and dissect the pathogenetic mechanism of one human deletion mutation present in patients with early-onset hypertrophic cardiomyopathy and encephalopathy.

Methods

A series of deletion and point-mutated constructs with the c-myc epitope tag were generated to identify the location and sequence features of mitochondrial targeting sequence for NDUFV2 in human cells using the confocal microscopy. In addition, various lengths of the NDUFV2 N-terminal and C-terminal fragments were fused with enhanced green fluorescent protein to investigate the minimal region required for correct mitochondrial import. Finally, a deletion construct that mimicked the IVS2+5_+8delGTAA mutation in NDUFV2 gene and would eventually produce a shortened NDUFV2 lacking 19-40 residues was generated to explore the connection between human gene mutation and disease.

Results

We identified that the cleavage site of NDUFV2 was located around amino acid 32 of the precursor protein, and the first 22 residues of NDUFV2 were enough to function as an efficient mitochondrial targeting sequence to carry the passenger protein into mitochondria. A site-directed mutagenesis study showed that none of the single-point mutations derived from basic, hydroxylated and hydrophobic residues in the NDUFV2 presequence had a significant effect on mitochondrial targeting, while increasing number of mutations in basic and hydrophobic residues gradually decreased the mitochondrial import efficacy of the protein. The deletion mutant mimicking the human early-onset hypertrophic cardiomyopathy and encephalopathy lacked 19-40 residues in NDUFV2 and exhibited a significant reduction in its mitochondrial targeting ability.

Conclusions

The mitochondrial targeting sequence of NDUFV2 is located at the N-terminus of the precursor protein. Maintaining a net positive charge and an amphiphilic structure with the overall balance and distribution of basic and hydrophobic amino acids in the N-terminus of NDUFV2 is important for mitochondrial targeting. The results of human disease cell model established that the impairment of mitochondrial localization of NDUFV2 as a mechanistic basis for early-onset hypertrophic cardiomyopathy and encephalopathy.

Possible existence of lysosome-like organella within mitochondria and its role in mitochondrial quality control

The accumulation of unhealthy mitochondria results in mitochondrial dysfunction, which has been implicated in aging, cancer, and a variety of degenerative diseases. However, the mechanism by which mitochondrial quality is regulated remains unclear. Here, we show that Mieap, a novel p53-inducible protein, induces intramitochondrial lysosome-like organella that plays a critical role in mitochondrial quality control. Mieap expression is directly regulated by p53 and is frequently lost in human cancer as result of DNA methylation. Mieap dramatically induces the accumulation of lysosomal proteins within mitochondria and mitochondrial acidic condition without destroying the mitochondrial structure (designated MALM, for Mieap-induced accumulation of lysosome-like organelles within mitochondria) in response to mitochondrial damage. MALM was not related to canonical autophagy. MALM is involved in the degradation of oxidized mitochondrial proteins, leading to increased ATP synthesis and decreased reactive oxygen species generation. These results suggest that Mieap induces intramitochondrial lysosome-like organella that plays a critical role in mitochondrial quality control by eliminating oxidized mitochondrial proteins. Cancer cells might accumulate unhealthy mitochondria due to p53 mutations and/or Mieap methylation, representing a potential cause of the Warburg effect.

Role of low native state kinetic stability and interaction of partially unfolded states with molecular chaperones in the mitochondrial protein mistargeting associated with primary hyperoxaluria

The G170R variant of the alanine:glyoxylate aminotransferase (AGT) is the most common pathogenic allele associated to primary hyperoxaluria type I (PH1), leading to mitochondrial mistargeting when combined with the P11L and I340M polymorphisms (minor allele; AGT(LM)). In this work, we have performed a comparative analysis on the conformation, unfolding energetics and interaction with molecular chaperones between AGT(wt), AGT(LM) and AGT(LRM) (G170R in the minor allele) proteins. Our results show that these three variants share similar conformational and functional properties as folded dimers. However, kinetic stability analyses showed a ≈1,000-fold increased unfolding rate for apo-AGT(LRM) compared to apo-AGT(wt), as well as a reduced folding efficiency upon expression in Escherichia coli. Pyridoxal 5'-phosphate (PLP)-binding provided a 4-5 orders of magnitude enhancement of the kinetic stability for all variants, suggesting a role for kinetic stabilization in pyridoxine-responsive PH1. Conformational studies at mild acidic pH and moderate guanidium concentrations showed the formation of a molten-globule-like unfolding intermediate in all three variants, which do not reactivate to the native state and strongly interact with Hsc70 and Hsp90 chaperones. Additional expression analyses in a mammalian cell-free system at neutral pH showed enhanced interaction of AGT(LRM) with Hsc70 and Hsp90 proteins compared to AGT(wt), suggesting kinetic trapping of the mutant by chaperones along the folding process. Overall, our results suggest that mitochondrial mistargeting of AGT(LRM) may involve the presentation of AGT partially folded states to the mitochondrial import machinery by molecular chaperones, which would be facilitated by the low native state kinetic stability (partially corrected by PLP binding) and kinetic trapping during folding of the AGT(LRM) variant with molecular chaperones.

The essentials of protein import in the degenerate mitochondrion of Entamoeba histolytica

Several essential biochemical processes are situated in mitochondria. The metabolic transformation of mitochondria in distinct lineages of eukaryotes created proteomes ranging from thousands of proteins to what appear to be a much simpler scenario. In the case of Entamoeba histolytica, tiny mitochondria known as mitosomes have undergone extreme reduction. Only recently a single complete metabolic pathway of sulfate activation has been identified in these organelles. The E. histolytica mitosomes do not produce ATP needed for the sulfate activation pathway and for three molecular chaperones, Cpn60, Cpn10 and mtHsp70. The already characterized ADP/ATP carrier would thus be essential to provide cytosolic ATP for these processes, but how the equilibrium of inorganic phosphate could be maintained was unknown. Finally, how the mitosomal proteins are translocated to the mitosomes had remained unclear. We used a hidden Markov model (HMM) based search of the E. histolytica genome sequence to discover candidate (i) mitosomal phosphate carrier complementing the activity of the ADP/ATP carrier and (ii) membrane-located components of the protein import machinery that includes the outer membrane translocation channel Tom40 and membrane assembly protein Sam50. Using in vitro and in vivo systems we show that E. histolytica contains a minimalist set up of the core import components in order to accommodate a handful of mitosomal proteins. The anaerobic and parasitic lifestyle of E. histolytica has produced one of the simplest known mitochondrial compartments of all eukaryotes. Comparisons with mitochondria of another amoeba, Dictystelium discoideum, emphasize just how dramatic the reduction of the protein import apparatus was after the loss of archetypal mitochondrial functions in the mitosomes of E. histolytica.

All eukaryotic organisms have mitochondria, organelles cordoned by a double membrane, which are descendants of an ancestral bacterial endosymbiont. Nowadays, mitochondria are fully integrated into the context of diverse cellular processes and serve in providing energy, iron-containing prosthetic groups and some of the cellular building blocks like lipids and amino acids. In multi-cellular organisms, mitochondria play an additional vital role in cell signaling pathways and programmed cell death. In some unicellular eukaryotes which inhabit oxygen poor environments, intriguing mitochondrial adaptations have taken place resulting in the creation of specialized compartments known as mitosomes and hydrogenosomes. Several important human pathogens like Entamoeba histolytica, Giardia intestinalis, Trichomonas vaginalis and microsporidia contain these organelles and in many cases the function and biogenesis of these organelles remain unknown. In this paper, we investigated the protein import pathways into the mitosomes of E. histolytica, which represent one of the simplest mitochondria-related compartment discovered yet. In accordance with the limited organellar proteome, we show that only core components of mitochondria-related protein import machines are present in E. histolytica to serve for the import of a small set of substrate proteins.

Evolution of macromolecular import pathways in mitochondria, hydrogenosomes and mitosomes

All eukaryotes require mitochondria for survival and growth. The origin of mitochondria can be traced down to a single endosymbiotic event between two probably prokaryotic organisms. Subsequent evolution has left mitochondria a collection of heterogeneous organelle variants. Most of these variants have retained their own genome and translation system. In hydrogenosomes and mitosomes, however, the entire genome was lost. All types of mitochondria import most of their proteome from the cytosol, irrespective of whether they have a genome or not. Moreover, in most eukaryotes, a variable number of tRNAs that are required for mitochondrial translation are also imported. Thus, import of macromolecules, both proteins and tRNA, is essential for mitochondrial biogenesis. Here, we review what is known about the evolutionary history of the two processes using a recently revised eukaryotic phylogeny as a framework. We discuss how the processes of protein import and tRNA import relate to each other in an evolutionary context.

Electrophysiology clarifies the megariddles of the mitochondrial permeability transition pore

After a brief review of the early history of mitochondrial electrophysiology, the contribution of this approach to the study of the mitochondrial permeability transition (MPT) is recapitulated. It has for example provided evidence for a dimeric nature of the MPT pore, allowed the distinction between two levels of control of its activity, and underscored the relevance of redox events for the phenomenon. Single-channel recording provides a means to finally solve the riddle of the biochemical entity underlying it by comparing the characteristics of the pore with those of channels formed by candidate molecules or complexes. The possibility that this entity may be the protein import machinery of the inner mitochondrial membrane is emphasized.

Mitochondrial protein quality control systems in aging and disease

Preserving the integrity of proteins, biomolecules prone to molecular damage, is a fundamental function of all biological systems. Impairments in protein quality control (PQC) may lead to degenerative processes, such as aging and various disorders and diseases. Fortunately, cells contain a hierarchical system of pathways coping protein damage. Specific molecular pathways detect misfolded proteins and act either to unfold or degrade them. Degradation of proteins generates peptides and amino acids that can be used for remodelling of impaired pathways and cellular functions. At increased levels of cellular damage whole organelles can be removed via autophagy, a process that depends on the activity oflysosomes. In addition, cells may undergo apoptosis, a form of programmed cell death, which in single-cellular and lower multicellular organisms can lead to death of the individual. Molecular damage of cellular compartments is mainly caused by reactive oxygen species (ROS). ROS is generated via different cellular pathways and frequently arises in the mitochondrial electron transport chain as a by-product of oxygenic energy transduction. Consequently, mitochondrial proteins are under high risk to become damaged. Perhaps for this reason mitochondria contain a very efficient PQC system that keeps mitochondrial proteins functional as long as damage does not reach a certain threshold and the components of this system themselves are not excessively damaged. The mitochondrial PQC system consists of chaperones that counteract protein aggregation through binding and refolding misfolded polypeptides and of membrane-bound and soluble ATP-dependent proteases that are involved in degradation of damaged proteins. During aging and in neurodegenerative diseases components of this PQC system, including Lon protease present in the mitochondrial matrix, become functionally impaired. In this chapter we summarise the current knowledge of cellular quality control systems with special emphasis on the role of the mitochondrial PQC system and its impact on biological aging and disease.

Subcellular localization of Nox4 and regulation in diabetes

Oxidative stress is implicated in human diseases. Some of the oxidative pathways are harbored in the mitochondria. NAD(P)H oxidases have been identified not only in phagocytic but also in somatic cells. Nox4 is the most ubiquitous of these oxidases and is a major source of reactive oxygen species (ROS) in many cell types and in kidney tissue of diabetic animals. We generated specific Nox4 antibodies, and found that Nox4 localizes to mitochondria. (i) Immunoblot analysis in cultured mesangial cells and kidney cortex revealed that Nox4 is present in crude mitochondria, in mitochondria-enriched heavy fractions, and in purified mitochondria; (ii) immunofluorescence confocal microscopy also revealed that Nox4 localizes with the mitochondrial marker Mitotracker; and (iii) the mitochondrial localization prediction program MitoProt indicated that the probability score for Nox4 is identical to mitochondrial protein cytochrome c oxidase subunit IV. We also show that in purified mitochondria, siRNA-mediated knockdown of Nox4 significantly reduces NADPH oxidase activity in pure mitochondria and blocks glucose-induced mitochondrial superoxide generation. In a rat model of diabetes, mitochondrial Nox4 expression is increased in kidney cortex. Our data provide evidence that a functional Nox4 is present and regulated in mitochondria, indicating the existence of a previously undescribed source of ROS in this organelle.

Mitochondrial and plastid evolution in eukaryotes: an outsiders' perspective

The eukaryotic organelles mitochondrion and plastid originated from eubacterial endosymbionts. Here we propose that, in both cases, prokaryote-to-organelle conversion was driven by the internalization of host-encoded factors progressing from the outer membrane of the endosymbionts towards the intermembrane space, inner membrane and finally the organelle interior. This was made possible by an outside-to-inside establishment in the endosymbionts of host-controlled protein-sorting components, which enabled the gradual integration of organelle functions into the nuclear genome. Such a convergent trajectory for mitochondrion and plastid establishment suggests a novel paradigm for organelle evolution that affects theories of eukaryogenesis.

Ups1p and Ups2p antagonistically regulate cardiolipin metabolism in mitochondria

Cardiolipin, a unique phospholipid composed of four fatty acid chains, is located mainly in the mitochondrial inner membrane (IM). Cardiolipin is required for the integrity of several protein complexes in the IM, including the TIM23 translocase, a dynamic complex which mediates protein import into the mitochondria through interactions with the import motor presequence translocase–associated motor (PAM). In this study, we report that two homologous intermembrane space proteins, Ups1p and Ups2p, control cardiolipin metabolism and affect the assembly state of TIM23 and its association with PAM in an opposing manner. In ups1Δ mitochondria, cardiolipin levels were decreased, and the TIM23 translocase showed altered conformation and decreased association with PAM, leading to defects in mitochondrial protein import. Strikingly, loss of Ups2p restored normal cardiolipin levels and rescued TIM23 defects in ups1Δ mitochondria. Furthermore, we observed synthetic growth defects in ups mutants in combination with loss of Pam17p, which controls the integrity of PAM. Our findings provide a novel molecular mechanism for the regulation of cardiolipin metabolism.

Energy-dependent immunity protein release during tol-dependent nuclease colicin translocation

Nuclease colicins bind their target receptor in the outer membrane of sensitive cells in the form of a high affinity complex with their cognate immunity proteins. Upon cell entry the immunity protein is lost from the complex by means that are poorly understood. We have developed a sensitive fluorescence assay that has enabled us to study the molecular requirements for immunity protein release. Nuclease colicins use members of the tol operon for their translocation across the outer membrane. We have demonstrated that the amino-terminal 80 residues of the colicin E9 molecule, which is the region that interacts with TolB, are essential for immunity protein release. Using tol deletion strains we analyzed the cellular components necessary for immunity protein release and found that in addition to a requirement for tolB, the tolA deletion strain was most affected. Complementation studies showed that the mutation H22A, within the transmembrane segment of TolA, abolishes immunity protein release. Investigation of the energy requirements demonstrated that the proton motive force of the cytoplasmic membrane is critical. Taken together these results demonstrate for the first time a clear energy requirement for the uptake of a nuclease colicin complex and suggest that energy transduced from the cytoplasmic membrane to the outer membrane by TolA could be the driving force for immunity protein release and concomitant translocation of the nuclease domain.

Tim23-Tim50 pair coordinates functions of translocators and motor proteins in mitochondrial protein import

Mitochondrial protein traffic requires coordinated operation of protein translocator complexes in the mitochondrial membrane. The TIM23 complex translocates and inserts proteins into the mitochondrial inner membrane. Here we analyze the intermembrane space (IMS) domains of Tim23 and Tim50, which are essential subunits of the TIM23 complex, in these functions. We find that interactions of Tim23 and Tim50 in the IMS facilitate transfer of precursor proteins from the TOM40 complex, a general protein translocator in the outer membrane, to the TIM23 complex. Tim23-Tim50 interactions also facilitate a late step of protein translocation across the inner membrane by promoting motor functions of mitochondrial Hsp70 in the matrix. Therefore, the Tim23-Tim50 pair coordinates the actions of the TOM40 and TIM23 complexes together with motor proteins for mitochondrial protein import.

The translocator maintenance protein Tam41 is required for mitochondrial cardiolipin biosynthesis

The mitochondrial inner membrane contains different translocator systems for the import of presequence-carrying proteins and carrier proteins. The translocator assembly and maintenance protein 41 (Tam41/mitochondrial matrix protein 37) was identified as a new member of the mitochondrial protein translocator systems by its role in maintaining the integrity and activity of the presequence translocase of the inner membrane (TIM23 complex). Here we demonstrate that the assembly of proteins imported by the carrier translocase, TIM22 complex, is even more strongly affected by the lack of Tam41. Moreover, respiratory chain supercomplexes and the inner membrane potential are impaired by lack of Tam41. The phenotype of Tam41-deficient mitochondria thus resembles that of mitochondria lacking cardiolipin. Indeed, we found that Tam41 is required for the biosynthesis of the dimeric phospholipid cardiolipin. The pleiotropic effects of the translocator maintenance protein on preprotein import and respiratory chain can be attributed to its role in biosynthesis of mitochondrial cardiolipin.

Recombinant mitochondrial transcription factor A with N-terminal mitochondrial transduction domain increases respiration and mitochondrial gene expression

We developed a scalable procedure to produce human mitochondrial transcription factor A (TFAM) modified with an N-terminal protein transduction domain (PTD) and mitochondrial localization signal (MLS) that allow it to cross membranes and enter mitochondria through its "mitochondrial transduction domain" (MTD=PTD+MLS). Alexa488-labeled MTD-TFAM rapidly entered the mitochondrial compartment of cybrid cells carrying the G11778A LHON mutation. MTD-TFAM reversibly increased respiration and levels of respiratory proteins. In vivo treatment of mice with MTD-TFAM increased motor endurance and complex I-driven respiration in mitochondria from brain and skeletal muscle. MTD-TFAM increases mitochondrial bioenergetics and holds promise for treatment of mitochondrial diseases involving deficiencies of energy production.

The basal flux of Akt in the mitochondria is mediated by heat shock protein 90

Akt is a known client protein of heat shock protein 90 (HSP90). We have found that HSP90 is responsible for Akt accumulation in the mitochondria in unstimulated cells. Treatment of SH-SY5Y neuroblastoma cells and human embryonic kidney cells with the HSP90 inhibitors novobiocin and geldanamycin caused substantial decreases in the level of Akt in the mitochondria without affecting the level of Akt in the cytosol. Moreover, intracerebroventricular injection of novobiocin into mice brains decreased Akt levels in cortical mitochondria. Knockdown of HSP90 expression with short interfering RNA also caused a significant decrease in Akt levels in the mitochondria without affecting total Akt levels. Using a mitochondrial import assay it was found that Akt is transported into the mitochondria. Furthermore, it was found that the mitochondrial import of Akt was independent of Akt activation as both an unmodified Akt and constitutively active mutant Akt; both readily accumulated in the mitochondria in an HSP90-dependent manner. Interestingly, incubation of isolated mitochondria with constitutively active Akt caused visible alterations in mitochondrial morphology, including pronounced remodeling of the mitochondrial matrix. This effect was blocked when Akt was mostly excluded from the mitochondria with novobiocin treatment. These results indicate that the level of Akt in the mitochondria is dependent on HSP90 chaperoning activity and that Akt import can cause dynamic changes in mitochondrial configuration.