Protein trafficking in the mitochondrial intermembrane space: mechanisms and links to human disease

Mitochondria fulfill a diverse range of functions in cells including oxygen metabolism, homeostasis of inorganic ions and execution of apoptosis. Biogenesis of mitochondria relies on protein import pathways that are ensured by dedicated multiprotein translocase complexes localized in all sub-compartments of these organelles. The key components and pathways involved in protein targeting and assembly have been characterized in great detail over the last three decades. This includes the oxidative folding machinery in the intermembrane space, which contributes to the redox-dependent control of proteostasis. Here, we focus on several components of this system and discuss recent evidence suggesting links to human proteopathy

Cytosolic redox components regulate protein homeostasis via additional localisation in the mitochondrial intermembrane space

Oxidative protein folding is confined to the bacterial periplasm, endoplasmic reticulum and the mitochondrial intermembrane space. Maintaining a redox balance requires the presence of reductive pathways. The major thiol-reducing pathways engage the thioredoxin and the glutaredoxin systems which are involved in removal of oxidants, protein proofreading and folding. Alterations in redox balance likely affect the flux of these redox pathways and are related to ageing and diseases such as neurodegenerative disorders and cancer. Here, we first review the well-studied oxidative and reductive processes in the bacterial periplasm and the endoplasmic reticulum, and then discuss the less understood process in the mitochondrial intermembrane space, highlighting its importance for the proper function of the cell

Oxidative protein biogenesis and redox regulation in the mitochondrial intermembrane space

Mitochondria are organelles that play a central role in cellular metabolism, as they are responsible for processes such as iron/sulfur cluster biogenesis, respiration and apoptosis. Here, we describe briefly the various protein import pathways for sorting of mitochondrial proteins into the different subcompartments, with an emphasis on the targeting to the intermembrane space. The discovery of a dedicated redox-controlled pathway in the intermembrane space that links protein import to oxidative protein folding raises important questions on the redox regulation of this process. We discuss the salient features of redox regulation in the intermembrane space and how such mechanisms may be linked to the more general redox homeostasis balance that is crucial not only for normal cell physiology but also for cellular dysfunction

The MIA pathway: a key regulator of mitochondrial oxidative protein folding and biogenesis

Mitochondria are fundamental intracellular organelles with key roles in important cellular processes like energy production, Fe/S cluster biogenesis, and homeostasis of lipids and inorganic ions. Mitochondrial dysfunction is consequently linked to many human pathologies (cancer, diabetes, neurodegeneration, stroke) and apoptosis. Mitochondrial biogenesis relies on protein import as most mitochondrial proteins (about 10-15% of the human proteome) are imported after their synthesis in the cytosol. Over the last several years many mitochondrial translocation pathways have been discovered. Among them, the import pathway that targets proteins to the intermembrane space (IMS) stands out as it is the only one that couples import to folding and oxidation and results in the covalent modification of the incoming precursor that adopt internal disulfide bonds in the process (the MIA pathway). The discovery of this pathway represented a significant paradigm shift as it challenged the prevailing dogma that the endoplasmic reticulum is the only compartment of eukaryotic cells where oxidative folding can occur. The concept of the oxidative folding pathway was first proposed on the basis of folding and import data for the small Tim proteins that have conserved cysteine motifs and must adopt intramolecular disulfides after import so that they are retained in the organelle. The introduction of disulfides in the IMS is catalyzed by Mia40 that functions as a chaperone inducing their folding. The sulfhydryl oxidase Erv1 generates the disulfide pairs de novo using either molecular oxygen or, cytochrome c and other proteins as terminal electron acceptors that eventually link this folding process to respiration. The solution NMR structure of Mia40 (and supporting biochemical experiments) showed that Mia40 is a novel type of disulfide donor whose recognition capacity for its substrates relies on a hydrophobic binding cleft found adjacent to a thiol active CPC motif. Targeting of the substrates to this pathway is guided by a novel type of IMS targeting signal called ITS or MISS. This consists of only 9 amino acids, found upstream or downstream of a unique Cys that is primed for docking to Mia40 when the substrate is accommodated in the Mia40 binding cleft. Different routes exist to complete the folding of the substrates and their final maturation in the IMS. Identification of new Mia40 substrates (some even without the requirement of their cysteines) reveals an expanded chaperone-like activity of this protein in the IMS. New evidence on the targeting of redox active proteins like thioredoxin, glutaredoxin, and peroxiredoxin into the IMS suggests the presence of redox-dependent regulatory mechanisms of the protein folding and import process in mitochondria. Maintenance of redox balance in mitochondria is crucial for normal cell physiology and depends on the cross-talk between the various redox signaling processes and the mitochondrial oxidative folding pathway

The N-terminal shuttle domain of Erv1 determines the affinity for Mia40 and mediates electron transfer to the catalytic Erv1 core in yeast mitochondria

Erv1 and Mia40 constitute the two important components of the disulfide relay system that mediates oxidative protein folding in the mitochondrial intermembrane space. Mia40 is the import receptor that recognizes the substrates introducing disulfide bonds while it is reduced. A key function of Erv1 is to recycle Mia40 to its active oxidative state. Our aims here were to dissect the domain of Erv1 that mediates the protein–protein interaction with Mia40 and to investigate the interactions between the shuttle domain of Erv1 and its catalytic core and their relevance for the interaction with Mia40. We purified these domains separately as well as cysteine mutants in the shuttle and the active core domains. The noncovalent interaction of Mia40 with Erv1 was measured by isothermal titration calorimetry, whereas their covalent mixed disulfide intermediate was analyzed in reconstitution experiments in vitro and in organello. We established that the N-terminal shuttle domain of Erv1 is necessary and sufficient for interaction to occur. Furthermore, we provide direct evidence for the intramolecular electron transfer from the shuttle cysteine pair of Erv1 to the core domain. Finally, we reconstituted the system by adding in trans the N- and C- terminal domains of Erv1 together with its substrate Mia40

Financial Liberalization: The African Experience

Almost a decade after their initiation, financial reforms appear to have had little effect on the economies of Sub-Sahara Africa. Whether the blame is to fall on their initial design itself, or on the partial nature of their implementation, liberalization policies have not mobilized savings, deepened intermediation or raised investment. Yet, Africa needs properly functioning financial markets for a more efficient allocation of resources for growth and risk diversification. How can African governments “correct’ their approach towards financial policy reform? A first step towards refining future policy choices requires an assessment of the short African experience with financial reform. How has progress in institutional and policy reform affected the financial depth of these economies? How have the gains in financial depth, if any, affected saving, consumption and investment? How does the African reform experience compare with that of other developing countries? How do the countries that during the period of reforms experienced substantial increases in aggregate saving compare to those that experienced substantial declines? These are some of the key issues we address in this paper.librlization capital flows credit interest rates

Dollar Shortages and Crises

Emerging markets do not handle adverse shocks well. In this paper, we lay out an argument about why emerging markets are so fragile, and why they may adopt contractual mechanisms—such as a dollarized banking system—that increase their fragility. We draw on this analysis to explain why dollarized economies may be prone to dollar shortages and twin crises. The model of crises described here differs in some important aspects from what are now termed the first-, second-, and third-generation models of crises. We then examine how domestic policies, especially monetary policy, can mitigate the adverse effects of these crises. Finally, we consider the role, potentially constructive, that international financial institutions may undertake both in helping to prevent the crises and in helping to resolve them.

Iron–sulfur clusters: from metals through mitochondria biogenesis to disease

Iron–sulfur clusters are ubiquitous inorganic co-factors that contribute to a wide range of cell pathways including the maintenance of DNA integrity, regulation of gene expression and protein translation, energy production, and antiviral response. Specifically, the iron–sulfur cluster biogenesis pathways include several proteins dedicated to the maturation of apoproteins in different cell compartments. Given the complexity of the biogenesis process itself, the iron–sulfur research area constitutes a very challenging and interesting field with still many unaddressed questions. Mutations or malfunctions affecting the iron–sulfur biogenesis machinery have been linked with an increasing amount of disorders such as Friedreich’s ataxia and various cardiomyopathies. This review aims to recap the recent discoveries both in the yeast and human iron–sulfur cluster arena, covering recent discoveries from chemistry to disease

Oxidative protein folding in the mitochondrial intermembrane space

Disulfide bond formation is a crucial step for oxidative folding and necessary for the acquisition of a protein's native conformation. Introduction of disulfide bonds is catalyzed in specialized subcellular compartments and requires the coordinated action of specific enzymes. The intermembrane space of mitochondria has recently been found to harbor a dedicated machinery that promotes the oxidative folding of substrate proteins by shuttling disulfide bonds. The newly identified oxidative pathway consists of the redox-regulated receptor Mia40 and the sulfhydryl oxidase Erv1. Proteins destined to the intermembrane space are trapped by a disulfide relay mechanism that involves an electron cascade from the incoming substrate to Mia40, then on to Erv1, and finally to molecular oxygen via cytochrome c. This thiol–disulfide exchange mechanism is essential for the import and for maintaining the structural stability of the incoming precursors. In this review we describe the mechanistic parameters that define the interaction and oxidation of the substrate proteins in light of the recent publications in the mitochondrial oxidative folding field

Design of a Seismically Isolated Railway Viaduct over Axios River in Northern Greece

The paper presents the salient features of the design of the Railway viaduct over Axios River on the new high-speed double railway line, which is the longest (800m) railway bridge in Greece constructed using the travelling gantry method. Located in a high seismic hazard area, the viaduct is provided with an isolation system aiming to reduce the structural response to seismic loading, a solution that presents several challenges in the case of railway bridges and cannot be implemented solely on the basis of existing codes. Lead-rubber bearings are provided at each pier to deck connection and system damping is further increased through fluid viscous dampers at each abutment. To verify the performance of the isolation system, analysis for seismic actions is conducted in three discrete stages of increasing complexity. Seismic forces and displacements are found to be within acceptable limits and serviceability requirements are also met. Conclusions are drawn regarding the feasibility of using passive systems in railway bridges in seismic areas