Mitochondrial Disulfide Bond Formation Is Driven by Intersubunit Electron Transfer in Erv1 and Proofread by Glutathione

Oxidative protein folding in the intermembrane space of human mitochondria

The mitochondrial intermembrane space hosts a machinery for oxidative protein folding, the mitochondrial disulfide relay. This machinery imports a large number of soluble proteins into the compartment, where they are retained through oxidative folding. Additionally, the disulfide relay enhances the stability of many proteins by forming disulfide bonds. In this review, we describe the mitochondrial disulfide relay in human cells, its components, and their coordinated collaboration in mechanistic detail. We also discuss the human pathologies associated with defects in this machinery and its protein substrates, providing a comprehensive overview of its biological importance and implications for health.

Stimuli-Responsive Polymeric Nanocarriers Accelerate On-Demand Drug Release to Combat Glioblastoma

Glioblastoma multiforme (GBM) is a highly malignant brain tumor with a poor prognosis and limited treatment options. Drug delivery by stimuli-responsive nanocarriers holds great promise for improving the treatment modalities of GBM. At the beginning of the review, we highlighted the stimuli-active polymeric nanocarriers carrying therapies that potentially boost anti-GBM responses by employing endogenous (pH, redox, hypoxia, enzyme) or exogenous stimuli (light, ultrasonic, magnetic, temperature, radiation) as triggers for controlled drug release mainly via hydrophobic/hydrophilic transition, degradability, ionizability, etc. Modifying these nanocarriers with target ligands further enhanced their capacity to traverse the blood-brain barrier (BBB) and preferentially accumulate in glioma cells. These unique features potentially lead to more effective brain cancer treatment with minimal adverse reactions and superior therapeutic outcomes. Finally, the review summarizes the existing difficulties and future prospects in stimuli-responsive nanocarriers for treating GBM. Overall, this review offers theoretical guidelines for developing intelligent and versatile stimuli-responsive nanocarriers to facilitate precise drug delivery and treatment of GBM in clinical settings.

Privileged proteins with a second residence: dual targeting and conditional re-routing of mitochondrial proteins

Almost all mitochondrial proteins are encoded by nuclear genes and synthesized in the cytosol as precursor proteins. Signals in the amino acid sequence of these precursors ensure their targeting and translocation into mitochondria. However, in many cases, only a certain fraction of a specific protein is transported into mitochondria, while the rest either remains in the cytosol or undergoes reverse translocation to the cytosol, and can populate other cellular compartments. This phenomenon is called dual localization which can be instigated by different mechanisms. These include alternative start or stop codons, differential transcripts, and ambiguous or competing targeting sequences. In many cases, dual localization might serve as an economic strategy to reduce the number of required genes; for example, when the same groups of enzymes are required both in mitochondria and chloroplasts or both in mitochondria and the nucleus/cytoplasm. Such cases frequently employ ambiguous targeting sequences to distribute proteins between both organelles. However, alternative localizations can also be used for signaling, for example when non-imported precursors serve as mitophagy signals or when they represent transcription factors in the nucleus to induce the mitochondrial unfolded stress response. This review provides an overview regarding the mechanisms and the physiological consequences of dual targeting.

The Interaction and Effect of a Small MitoBlock Library as Inhibitor of ALR Protein-Protein Interaction Pathway

MIA40 and ALR of the MIA pathway mediate the import of protein precursors that form disulfides into the mitochondrial intermembrane space. This import pathway is suggested to be a linear pathway in which MIA40 first binds to the precursor via a disulfide linkage and oxidizes it. Subsequently, ALR re-oxidizes MIA40 and then ALR transfers electrons to terminal electron acceptors. However, the precise mechanism by which ALR and MIA40 coordinate translocation is unknown. With a collection of small molecule modulators (MB-5 to MB-9 and MB-13) that inhibit ALR activity, we characterized the import mechanism in mitochondria. NMR studies show that most of the compounds bind to a similar region in ALR. Mechanistic studies with small molecules demonstrate that treatment with compound MB-6 locks the precursor in a state bound to MIA40, blocking re-oxidation of MIA40 by ALR. Thus, small molecules that target a similar region in ALR alter the dynamics of the MIA import pathway differently, resulting in a set of probes that are useful for studying the catalysis of the redox-regulated import pathway in model systems.

Oxidation of Arabidopsis thaliana COX19 Using the Combined Action of ERV1 and Glutathione

Protein import and oxidative folding within the intermembrane space (IMS) of mitochondria relies on the MIA40-ERV1 couple. The MIA40 oxidoreductase usually performs substrate recognition and oxidation and is then regenerated by the FAD-dependent oxidase ERV1. In most eukaryotes, both proteins are essential; however, MIA40 is dispensable in Arabidopsis thaliana. Previous complementation experiments have studied yeast mia40 mutants expressing a redox inactive, but import-competent versions of yeast Mia40 using A. thaliana ERV1 (AtERV1) suggest that AtERV1 catalyzes the oxidation of MIA40 substrates. We assessed the ability of both yeast and Arabidopsis MIA40 and ERV1 recombinant proteins to oxidize the apo-cytochrome reductase CCMH and the cytochrome c oxidase assembly protein COX19, a typical MIA40 substrate, in the presence or absence of glutathione, using in vitro cysteine alkylation and cytochrome c reduction assays. The presence of glutathione used at a physiological concentration and redox potential was sufficient to support the oxidation of COX19 by AtERV1, providing a likely explanation for why MIA40 is not essential for the import and oxidative folding of IMS-located proteins in Arabidopsis. The results point to fundamental biochemical differences between Arabidopsis and yeast ERV1 in catalyzing protein oxidation.

New Insights into the Cooperativity and Dynamics of Dimeric Enzymes

A survey of protein databases indicates that the majority of enzymes exist in oligomeric forms, with about half of those found in the UniProt database being homodimeric. Understanding why many enzymes are in their dimeric form is imperative. Recent developments in experimental and computational techniques have allowed for a deeper comprehension of the cooperative interactions between the subunits of dimeric enzymes. This review aims to succinctly summarize these recent advancements by providing an overview of experimental and theoretical methods, as well as an understanding of cooperativity in substrate binding and the molecular mechanisms of cooperative catalysis within homodimeric enzymes. Focus is set upon the beneficial effects of dimerization and cooperative catalysis. These advancements not only provide essential case studies and theoretical support for comprehending dimeric enzyme catalysis but also serve as a foundation for designing highly efficient catalysts, such as dimeric organic catalysts. Moreover, these developments have significant implications for drug design, as exemplified by Paxlovid, which was designed for the homodimeric main protease of SARS-CoV-2.

Identification of disulfidptosis-related genes with immune infiltration in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a common malignant primary tumor that is usually diagnosed at an advanced stage; thus, there is an urgent need for efficient and sensitive novel diagnostic markers to determine the prognosis and halt disease progression in patients with HCC. Disulfidptosis is a recently discovered form of programmed cell death, essentially an abnormal accumulation of intracellular bisulfides. Therefore, our study aimed to investigate the role of disulfidptosis-related genes (DRGs) in the pathogenesis of HCC. Based on public databases, our work demonstrates the relationship between DRG and expression, immunity, mutation/drug sensitivity, and functional enrichment in HCC. We also revealed the significant heterogeneity of HCC in different DRGs sub-clusters and in differentially expressed genes (DEGs), respectively. Subsequently, the most relevant candidate gene, SLC7A11, was screened by machine learning to further validate the significance of SLC7A11 in the clinical features, prognosis, nomogram pattern, and immune infiltration of HCC. Our study, which elucidates the potential mechanisms of DRGs and HCC, reveals that SLC7A11 can serve as a novel prognostic biomarker and provides opportunities and challenges for individualized cancer immunotherapy strategies.

Tumor microenvironment responsive metal nanoparticles in cancer immunotherapy

Malignant tumors have a unique tumor microenvironment (TME), which includes mild acidity, hypoxia, overexpressed reactive oxygen species (ROS), and high glutathione (GSH) levels, among others. Recently, TME regulation approaches have attracted widespread attention in cancer immunotherapy. Nanoparticles as drug delivery systems have ability to modulate the hydrophilicity of drugs to affect drug uptake and efflux in tumor. Especially, the metal nanoparticles have been extensive applied for tumor immunotherapy due to their unique physical properties and elaborate design. However, the potential deficiencies of metal nanoparticles due to their low biodegradability, toxicity and treatment side effects restrict their clinical application. In this review, we briefly introduce the feature characteristics of the TME and the recent advances in tumor microenvironment responsive metal nanoparticles for tumor immunotherapy. In addition, nanoparticles could be combined with other treatments, such as chemotherapy, radiotherapy and photodynamic therapy also is presented. Finally, the challenges and outlook for improving the antitumor immunotherapy efficiency, side effect and potential risks of metal nanoparticles has been discussed.

Expression and copper binding studies of a Plasmodium falciparum protein with Cox19 copper binding motifs

Copper can exist in an oxidized and a reduced form, which enables the metal to play essential roles as a catalytic co-factor in redox reactions in many organisms. Copper confers redox activity to the terminal electron transport chain cytochrome c oxidase protein. Cytochrome c oxidase in yeast obtains copper for the Cu_B site in the Cox1 subunit from Cox11 in association with Cox19. When copper is chelated in growth medium, Plasmodium falciparum parasite development in infected red blood cells is inhibited and excess copper is toxic for the parasite. The gene of a 26 kDa Plasmodium falciparum PfCox19 protein with two Cx₉C Cox19 copper binding motifs, was cloned and expressed as a 66 kDa fusion protein with maltose binding protein and affinity purified (rMBP-PfCox19). rMBP-PfCox19 bound copper measured by: a bicinchoninic acid release assay; an in vivo bacterial host growth inhibition assay; ascorbate oxidation inhibition and differential scanning fluorimetry. The native protein was detected by antibodies raised against rMBP-PfCox19. PfCox19 binds copper and is predicted to associate with PfCox11 in the insertion of copper into the Cu_B site of Plasmodium cytochrome c oxidase. Characterisation of the proteins involved in Plasmodium spp. copper metabolism will help us understand the role of cytochrome c oxidase and this essential metal in Plasmodium homeostasis.

Polysaccharide-Based Stimulus-Responsive Nanomedicines for Combination Cancer Immunotherapy

Cancer immunotherapy is a promising antitumor approach, whereas nontherapeutic side effects, tumor microenvironment (TME) intricacy, and low tumor immunogenicity limit its therapeutic efficacy. In recent years, combination immunotherapy with other therapies has been proven to considerably increase antitumor efficacy. However, achieving codelivery of the drugs to the tumor site remains a major challenge. Stimulus-responsive nanodelivery systems show controlled drug delivery and precise drug release. Polysaccharides, a family of potential biomaterials, are widely used in the development of stimulus-responsive nanomedicines due to their unique physicochemical properties, biocompatibility, and modifiability. Here, the antitumor activity of polysaccharides and several combined immunotherapy strategies (e.g., immunotherapy combined with chemotherapy, photodynamic therapy, or photothermal therapy) are summarized. More importantly, the recent progress of polysaccharide-based stimulus-responsive nanomedicines for combination cancer immunotherapy is discussed, with the focus on construction of nanomedicine, targeted delivery, drug release, and enhanced antitumor effects. Finally, the limitations and application prospects of this new field are discussed.

Mitochondrial protein import machinery conveys stress signals to the cytosol and beyond

Mitochondria hold diverse and pivotal roles in fundamental processes that govern cell survival, differentiation, and death, in addition to organismal growth, maintenance, and aging. The mitochondrial protein import system is a major contributor to mitochondrial biogenesis and lies at the crossroads between mitochondrial and cellular homeostasis. Recent findings highlight the mitochondrial protein import system as a signaling hub, receiving inputs from other cellular compartments and adjusting its function accordingly. Impairment of protein import, in a physiological, or disease context, elicits adaptive responses inside and outside mitochondria. In this review, we discuss recent developments, relevant to the mechanisms of mitochondrial protein import regulation, with a particular focus on quality control, proteostatic and metabolic cellular responses, triggered upon impairment of mitochondrial protein import.

Stimuli-Responsive Gene Delivery Nanocarriers for Cancer Therapy

Gene therapy provides a promising approach in treating cancers with high efficacy and selectivity and few adverse effects. Currently, the development of functional vectors with safety and effectiveness is the intense focus for improving the delivery of nucleic acid drugs for gene therapy. For this purpose, stimuli-responsive nanocarriers displayed strong potential in improving the overall efficiencies of gene therapy and reducing adverse effects via effective protection, prolonged blood circulation, specific tumor accumulation, and controlled release profile of nucleic acid drugs. Besides, synergistic therapy could be achieved when combined with other therapeutic regimens. This review summarizes recent advances in various stimuli-responsive nanocarriers for gene delivery. Particularly, the nanocarriers responding to endogenous stimuli including pH, reactive oxygen species, glutathione, and enzyme, etc., and exogenous stimuli including light, thermo, ultrasound, magnetic field, etc., are introduced. Finally, the future challenges and prospects of stimuli-responsive gene delivery nanocarriers toward potential clinical translation are well discussed. The major objective of this review is to present the biomedical potential of stimuli-responsive gene delivery nanocarriers for cancer therapy and provide guidance for developing novel nanoplatforms that are clinically applicable.

Dual inhibition of thioredoxin reductase and proteasome is required for auranofin-induced paraptosis in breast cancer cells

Auranofin (AF), a gold (I)-containing phosphine compound, is being investigated for oncological application as a repurposed drug. We show here that 4~5 µM AF induces paraptosis, a non-apoptotic cell death mode characterized by dilation of the endoplasmic reticulum (ER) and mitochondria, in breast cancer cells. Although the covalent inhibition of thioredoxin reductase (TrxR), an enzyme that critically controls intracellular redox homeostasis, is considered the primary mechanism of AF's anticancer activity, knockdown of TrxR1 did not induce paraptosis. Instead, both TrxR1 knockdown plus the proteasome inhibitor (PI), bortezomib (Bz), and 2 μM AF plus Bz induced paraptosis, thereby mimicking the effect of 5 μM AF. These results suggest that the paraptosis induced by 5 μM AF requires the inhibition of both TrxR1 and proteasome. We found that TrxR1 knockdown/Bz or subtoxic doses of AF and Bz induced paraptosis selectively in breast cancer cells, sparing non-transformed MCF10A cells, whereas 4~5 μM AF killed both cancer and MCF10A cells. GSH depletion was found to be more critical than ROS generation for the paraptosis induced by dual TrxR1/proteasome inhibition. In this process, the ATF4/CHAC1 (glutathione-specific gamma-glutamylcyclotransferase 1) axis leads to GSH degradation, contributing to proteotoxic stress possibly due to the accumulation of misfolded thiol-containing proteins. These results suggest that the paraptosis-inducing strategy of AF plus a PI may provide an effective therapeutic strategy against pro-apoptotic therapy-resistant cancers and reduce the potential side effects associated with high-dose AF.

Maintenance of small molecule redox homeostasis in mitochondria

Compartmentalisation of eukaryotic cells enables fundamental otherwise often incompatible cellular processes. Establishment and maintenance of distinct compartments in the cell relies not only on proteins, lipids and metabolites but also on small redox molecules. In particular, small redox molecules such as glutathione, NAD(P)H and hydrogen peroxide (H₂ O₂ ) cooperate with protein partners in dedicated machineries to establish specific subcellular redox compartments with conditions that enable oxidative protein folding and redox signalling. Dysregulated redox homeostasis has been directly linked with a number of diseases including cancer, neurological disorders, cardiovascular diseases, obesity, metabolic diseases and ageing. In this review, we will summarise mechanisms regulating establishment and maintenance of redox homeostasis in the mitochondrial subcompartments of mammalian cells.

The protective roles of augmenter of liver regeneration in hepatocytes in the non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) occurs in 25% of the global population and manifests as lipid deposition, hepatocyte injury, activation of Kupffer and stellate cells, and steatohepatitis. Predominantly expressed in hepatocytes, the augmenter of liver regeneration (ALR) is a key factor in liver regulation that can alleviate fatty liver disease and protect the liver from abnormal liver lipid metabolism. ALR has three isoforms (15-, 21-, and 23-kDa), amongst which 23-kDa ALR is the most extensively studied. The 23-kDa ALR isoform is a sulfhydryl oxidase that resides primarily in the mitochondrial intermembrane space (IMS), whereby it protects the liver against various types of injury. In this review, we describe the role of ALR in regulating hepatocytes in the context of NAFLD. We also discuss questions about ALR that remain to be explored in the future. In conclusion, ALR appears to be a promising therapeutic target for treating NAFLD.

Experimental Approaches for Investigating Disulfide-Based Redox Relays in Cells

Reversible oxidation of cysteine residues within proteins occurs naturally during normal cellular homeostasis and can increase during oxidative stress. Cysteine oxidation often leads to the formation of disulfide bonds, which can impact protein folding, stability, and function. Work in both prokaryotic and eukaryotic models over the past five decades has revealed several multiprotein systems that use thiol-dependent oxidoreductases to mediate disulfide bond reduction, formation, and/or rearrangement. Here, I provide an overview of how these systems operate to carry out disulfide exchange reactions in different cellular compartments, with a focus on their roles in maintaining redox homeostasis, transducing redox signals, and facilitating protein folding. Additionally, I review thiol-independent and thiol-dependent approaches for interrogating what proteins partner together in such disulfide-based redox relays. While the thiol-independent approaches rely either on predictive measures or standard procedures for monitoring protein-protein interactions, the thiol-dependent approaches include direct disulfide trapping methods as well as thiol-dependent chemical cross-linking. These strategies may prove useful in the systematic characterization of known and newly discovered disulfide relay mechanisms and redox switches involved in oxidant defense, protein folding, and cell signaling.

Mitochondrial protein translocation machinery: From TOM structural biogenesis to functional regulation

The human mitochondrial outer membrane is biophysically unique as it is the only membrane possessing transmembrane β-barrel proteins (mitochondrial outer membrane proteins, mOMPs) in the cell. The most vital of the three mOMPs is the core protein of the translocase of the outer mitochondrial membrane (TOM) complex. Identified first as MOM38 in Neurospora in 1990, the structure of Tom40, the core 19-stranded β-barrel translocation channel, was solved in 2017, after nearly three decades. Remarkably, the past four years have witnessed an exponential increase in structural and functional studies of yeast and human TOM complexes. In addition to being conserved across all eukaryotes, the TOM complex is the sole ATP-independent import machinery for nearly all of the ∼1000 to 1500 known mitochondrial proteins. Recent cryo-EM structures have provided detailed insight into both possible assembly mechanisms of the TOM core complex and organizational dynamics of the import machinery and now reveal novel regulatory interplay with other mOMPs. Functional characterization of the TOM complex using biochemical and structural approaches has also revealed mechanisms for substrate recognition and at least five defined import pathways for precursor proteins. In this review, we discuss the discovery, recently solved structures, molecular function, and regulation of the TOM complex and its constituents, along with the implications these advances have for alleviating human diseases.

Tumor-Microenvironment-Responsive Nanomedicine for Enhanced Cancer Immunotherapy

The past decades have witnessed great progress in cancer immunotherapy, which has profoundly revolutionized oncology, whereas low patient response rates and potential immune-related adverse events remain major clinical challenges. With the advantages of controlled delivery and modular flexibility, cancer nanomedicine has offered opportunities to strengthen antitumor immune responses and to sensitize tumor to immunotherapy. Furthermore, tumor-microenvironment (TME)-responsive nanomedicine has been demonstrated to achieve specific and localized amplification of the immune response in tumor tissue in a safe and effective manner, increasing patient response rates to immunotherapy and reducing the immune-related side effects simultaneously. Here, the recent progress of TME-responsive nanomedicine for cancer immunotherapy is summarized, which responds to the signals in the TME, such as weak acidity, reductive environment, high-level reactive oxygen species, hypoxia, overexpressed enzymes, and high-level adenosine triphosphate. Moreover, the potential to combine nanomedicine-based therapy and immunotherapeutic strategies to overcome each step of the cancer-immunity cycle and to enhance antitumor effects is discussed. Finally, existing challenges and further perspectives in this rising field with the hope for improved development of clinical applications are discussed.

In Vivo Structure-Function Analysis and Redox Interactomes of Leishmania tarentolae Erv

Import and oxidative folding of proteins in the mitochondrial intermembrane space differ among eukaryotic lineages. While opisthokonts such as yeast rely on the receptor and oxidoreductase Mia40 in combination with the Mia40:cytochrome c oxidoreductase Erv, kinetoplastid parasites and other Excavata/Discoba lack Mia40 but have a functional Erv homologue. Whether excavate Erv homologues rely on a Mia40 replacement or directly interact with imported protein substrates remains controversial. Here, we used the CRISPR-Cas9 system to generate a set of tagged and untagged homozygous mutants of LTERV from the kinetoplastid model parasite Leishmania tarentolae. Modifications of the shuttle cysteine motif of LtErv were lethal, whereas replacement of clamp residue Cys¹⁷ or removal of the kinetoplastida-specific second (KISS) domain had no impact on parasite viability under standard growth conditions. However, removal of the KISS domain rendered parasites sensitive to heat stress and led to the accumulation of homodimeric and mixed LtErv disulfides. We therefore determined and compared the redox interactomes of tagged wild-type LtErv and LtErv^ΔKISS using stable isotope labeling by amino acids in cell culture (SILAC) and quantitative mass spectrometry. While the Mia40-replacement candidate Mic20 and all but one typical substrate with twin Cx_3/9C-motifs were absent in both redox interactomes, we identified a small set of alternative potential interaction partners with putative redox-active cysteine residues. In summary, our study reveals parasite-specific intracellular structure-function relationships and redox interactomes of LtErv with implications for current hypotheses on mitochondrial protein import in nonopisthokonts. IMPORTANCE The discovery of the redox proteins Mia40/CHCHD4 and Erv1/ALR, as well as the elucidation of their relevance for oxidative protein folding in the mitochondrial intermembrane space of yeast and mammals, founded a new research topic in redox biology and mitochondrial protein import. The lack of Mia40/CHCHD4 in protist lineages raises fundamental and controversial questions regarding the conservation and evolution of this essential pathway. Do protist Erv homologues act alone, or do they use the candidate Mic20 or another protein as a Mia40 replacement? Furthermore, we previously showed that Erv homologues in L. tarentolae and the human pathogen L. infantum are not only essential but also differ structurally and mechanistically from yeast and human Erv1/ALR. Here, we analyzed the relevance of such structural differences in vivo and determined the first redox interactomes of a nonopisthokont Erv homologue. Our data challenge recent hypotheses on mitochondrial protein import in nonopisthokonts.

Role of the Mitochondrial Protein Import Machinery and Protein Processing in Heart Disease

Mitochondria are essential organelles for cellular energy production, metabolic homeostasis, calcium homeostasis, cell proliferation, and apoptosis. About 99% of mammalian mitochondrial proteins are encoded by the nuclear genome, synthesized as precursors in the cytosol, and imported into mitochondria by mitochondrial protein import machinery. Mitochondrial protein import systems function not only as independent units for protein translocation, but also are deeply integrated into a functional network of mitochondrial bioenergetics, protein quality control, mitochondrial dynamics and morphology, and interaction with other organelles. Mitochondrial protein import deficiency is linked to various diseases, including cardiovascular disease. In this review, we describe an emerging class of protein or genetic variations of components of the mitochondrial import machinery involved in heart disease. The major protein import pathways, including the presequence pathway (TIM23 pathway), the carrier pathway (TIM22 pathway), and the mitochondrial intermembrane space import and assembly machinery, related translocases, proteinases, and chaperones, are discussed here. This review highlights the importance of mitochondrial import machinery in heart disease, which deserves considerable attention, and further studies are urgently needed. Ultimately, this knowledge may be critical for the development of therapeutic strategies in heart disease.

Mitochondrial Impairment by MitoBloCK-6 Inhibits Liver Cancer Cell Proliferation

Augmenter of liver regeneration (ALR) is a critical multi-isoform protein with its longer isoform, located in the mitochondrial intermembrane space, being part of the mitochondrial disulfide relay system (DRS). Upregulation of ALR was observed in multiple forms of cancer, among them hepatocellular carcinoma (HCC). To shed light into ALR function in HCC, we used MitoBloCK-6 to pharmacologically inhibit ALR, resulting in profound mitochondrial impairment and cancer cell proliferation deficits. These effects were mostly reversed by supplementation with bioavailable hemin b, linking ALR function to mitochondrial iron homeostasis. Since many tumor cells are known for their increased iron demand and since increased iron levels in cancer are associated with poor clinical outcome, these results help to further advance the intricate relation between iron and mitochondrial homeostasis in liver cancer.

Aim32 is a dual-localized 2Fe-2S mitochondrial protein that functions in redox quality control

Yeast is a facultative anaerobe and uses diverse electron acceptors to maintain redox-regulated import of cysteine-rich precursors via the mitochondrial intermembrane space assembly (MIA) pathway. With the growing diversity of substrates utilizing the MIA pathway, understanding the capacity of the intermembrane space (IMS) to handle different types of stress is crucial. We used MS to identify additional proteins that interacted with the sulfhydryl oxidase Erv1 of the MIA pathway. Altered inheritance of mitochondria 32 (Aim32), a thioredoxin-like [2Fe-2S] ferredoxin protein, was identified as an Erv1-binding protein. Detailed localization studies showed that Aim32 resided in both the mitochondrial matrix and IMS. Aim32 interacted with additional proteins including redox protein Osm1 and protein import components Tim17, Tim23, and Tim22. Deletion of Aim32 or mutation of conserved cysteine residues that coordinate the Fe-S center in Aim32 resulted in an increased accumulation of proteins with aberrant disulfide linkages. In addition, the steady-state level of assembled TIM22, TIM23, and Oxa1 protein import complexes was decreased. Aim32 also bound to several mitochondrial proteins under nonreducing conditions, suggesting a function in maintaining the redox status of proteins by potentially targeting cysteine residues that may be sensitive to oxidation. Finally, Aim32 was essential for growth in conditions of stress such as elevated temperature and hydroxyurea, and under anaerobic conditions. These studies suggest that the Fe-S protein Aim32 has a potential role in general redox homeostasis in the matrix and IMS. Thus, Aim32 may be poised as a sensor or regulator in quality control for a broad range of mitochondrial proteins.

Protein import by the mitochondrial disulfide relay in higher eukaryotes

The proteome of the mitochondrial intermembrane space (IMS) contains more than 100 proteins, all of which are synthesized on cytosolic ribosomes and consequently need to be imported by dedicated machineries. The mitochondrial disulfide relay is the major import machinery for soluble proteins in the IMS. Its major component, the oxidoreductase MIA40, interacts with incoming substrates, retains them in the IMS, and oxidatively folds them. After this reaction, MIA40 is reoxidized by the sulfhydryl oxidase augmenter of liver regeneration, which couples disulfide formation by this machinery to the activity of the respiratory chain. In this review, we will discuss the import of IMS proteins with a focus on recent findings showing the diversity of disulfide relay substrates, describing the cytosolic control of this import system and highlighting the physiological relevance of the disulfide relay machinery in higher eukaryotes.

The biogenesis of mitochondrial intermembrane space proteins

The mitochondrial intermembrane space (IMS) houses a large spectrum of proteins with distinct and critical functions. Protein import into this mitochondrial sub-compartment is underpinned by an intriguing variety of pathways, many of which are still poorly understood. The constricted volume of the IMS and the topological segregation by the inner membrane cristae into a bulk area surrounded by the boundary inner membrane and the lumen within the cristae is an important factor that adds to the complexity of the protein import, folding and assembly processes. We discuss the main import pathways into the IMS, but also how IMS proteins are degraded or even retro-translocated to the cytosol in an integrated network of interactions that is necessary to maintain a healthy balance of IMS proteins under physiological and cellular stress conditions. We conclude this review by highlighting new and exciting perspectives in this area with a view to develop a better understanding of yet unknown, likely unconventional import pathways, how presequence-less proteins can be targeted and the basis for dual localisation in the IMS and the cytosol. Such knowledge is critical to understanding the dynamic changes of the IMS proteome in response to stress, and particularly important for maintaining optimal mitochondrial fitness.

Interplay between Mitochondrial Protein Import and Respiratory Complexes Assembly in Neuronal Health and Degeneration

The fact that >99% of mitochondrial proteins are encoded by the nuclear genome and synthesised in the cytosol renders the process of mitochondrial protein import fundamental for normal organelle physiology. In addition to this, the nuclear genome comprises most of the proteins required for respiratory complex assembly and function. This means that without fully functional protein import, mitochondrial respiration will be defective, and the major cellular ATP source depleted. When mitochondrial protein import is impaired, a number of stress response pathways are activated in order to overcome the dysfunction and restore mitochondrial and cellular proteostasis. However, prolonged impaired mitochondrial protein import and subsequent defective respiratory chain function contributes to a number of diseases including primary mitochondrial diseases and neurodegeneration. This review focuses on how the processes of mitochondrial protein translocation and respiratory complex assembly and function are interlinked, how they are regulated, and their importance in health and disease.

Erv1 and Cytochrome c Mediate Rapid Electron Transfer via A Collision-Type Interaction

Being essential for oxidative protein folding in the mitochondrial intermembrane space, the mitochondrial disulfide relay relies on the electron transfer (ET) from the sulfhydryl oxidase Erv1 to cytochrome c (Cc). Using solution NMR spectroscopy, we demonstrate that while the yeast Cc-Erv1 system is functionally active, no observable binding of the protein partners takes place. The transient interaction between Erv1 and Cc can be rationalized by molecular modeling, suggesting that a large surface area of Erv1 can sustain a fast ET to Cc via a collision-type mechanism, without the need for a canonical protein complex formation. We suggest that, by preventing the direct ET to molecular oxygen (O₂), the collision-type Cc-Erv1 interaction plays a role in protecting the organism against reactive oxygen species.

The Mia40/CHCHD4 Oxidative Folding System: Redox Regulation and Signaling in the Mitochondrial Intermembrane Space

Mitochondria are critical for several cellular functions as they control metabolism, cell physiology, and cell death. The mitochondrial proteome consists of around 1500 proteins, the vast majority of which (about 99% of them) are encoded by nuclear genes, with only 13 polypeptides in human cells encoded by mitochondrial DNA. Therefore, it is critical for all the mitochondrial proteins that are nuclear-encoded to be targeted precisely and sorted specifically to their site of action inside mitochondria. These processes of targeting and sorting are catalysed by protein translocases that operate in each one of the mitochondrial sub-compartments. The main protein import pathway for the intermembrane space (IMS) recognises proteins that are cysteine-rich, and it is the only import pathway that chemically modifies the imported precursors by introducing disulphide bonds to them. In this manner, the precursors are trapped in the IMS in a folded state. The key component of this pathway is Mia40 (called CHCHD4 in human cells), which itself contains cysteine motifs and is subject to redox regulation. In this review, we detail the basic components of the MIA pathway and the disulphide relay mechanism that underpins the electron transfer reaction along the oxidative folding mechanism. Then, we discuss the key protein modulators of this pathway and how they are interlinked to the small redox-active molecules that critically affect the redox state in the IMS. We present also evidence that the mitochondrial redox processes that are linked to iron–sulfur clusters biogenesis and calcium homeostasis coalesce in the IMS at the MIA machinery. The fact that the MIA machinery and several of its interactors and substrates are linked to a variety of common human diseases connected to mitochondrial dysfunction highlight the potential of redox processes in the IMS as a promising new target for developing new treatments for some of the most complex and devastating human diseases.

Grx2 Regulates Skeletal Muscle Mitochondrial Structure and Autophagy

Glutathione is an important antioxidant that regulates cellular redox status and is disordered in many disease states. Glutaredoxin 2 (Grx2) is a glutathione-dependent oxidoreductase that plays a pivotal role in redox control by catalyzing reversible protein deglutathionylation. As oxidized glutathione (GSSG) can stimulate mitochondrial fusion, we hypothesized that Grx2 may contribute to the maintenance of mitochondrial dynamics and ultrastructure. Here, we demonstrate that Grx2 deletion results in decreased GSH:GSSG, with a marked increase of GSSG in primary muscle cells isolated from C57BL/6 Grx2^-/- mice. The altered glutathione redox was accompanied by increased mitochondrial length, consistent with a more fused mitochondrial reticulum. Electron microscopy of Grx2^-/- skeletal muscle fibers revealed decreased mitochondrial surface area, profoundly disordered ultrastructure, and the appearance of multi-lamellar structures. Immunoblot analysis revealed that autophagic flux was augmented in Grx2^-/- muscle as demonstrated by an increase in the ratio of LC3II/I expression. These molecular changes resulted in impaired complex I respiration and complex IV activity, a smaller diameter of tibialis anterior muscle, and decreased body weight in Grx2 deficient mice. Together, these are the first results to show that Grx2 regulates skeletal muscle mitochondrial structure, and autophagy.

The mitochondrial intermembrane space: the most constricted mitochondrial sub-compartment with the largest variety of protein import pathways

The mitochondrial intermembrane space (IMS) is the most constricted sub-mitochondrial compartment, housing only about 5% of the mitochondrial proteome, and yet is endowed with the largest variability of protein import mechanisms. In this review, we summarize our current knowledge of the major IMS import pathway based on the oxidative protein folding pathway and discuss the stunning variability of other IMS protein import pathways. As IMS-localized proteins only have to cross the outer mitochondrial membrane, they do not require energy sources like ATP hydrolysis in the mitochondrial matrix or the inner membrane electrochemical potential which are critical for import into the matrix or insertion into the inner membrane. We also explore several atypical IMS import pathways that are still not very well understood and are guided by poorly defined or completely unknown targeting peptides. Importantly, many of the IMS proteins are linked to several human diseases, and it is therefore crucial to understand how they reach their normal site of function in the IMS. In the final part of this review, we discuss current understanding of how such IMS protein underpin a large spectrum of human disorders.

CHCHD4 (MIA40) and the mitochondrial disulfide relay system

Mitochondria are pivotal for normal cellular physiology, as they perform a crucial role in diverse cellular functions and processes, including respiration and the regulation of bioenergetic and biosynthetic pathways, as well as regulating cellular signalling and transcriptional networks. In this way, mitochondria are central to the cell's homeostatic machinery, and as such mitochondrial dysfunction underlies the pathology of a diverse range of diseases including mitochondrial disease and cancer. Mitochondrial import pathways and targeting mechanisms provide the means to transport into mitochondria the hundreds of nuclear-encoded mitochondrial proteins that are critical for the organelle's many functions. One such import pathway is the highly evolutionarily conserved disulfide relay system (DRS) within the mitochondrial intermembrane space (IMS), whereby proteins undergo a form of oxidation-dependent protein import. A central component of the DRS is the oxidoreductase coiled-coil-helix-coiled-coil-helix (CHCH) domain-containing protein 4 (CHCHD4, also known as MIA40), the human homologue of yeast Mia40. Here, we summarise the recent advances made to our understanding of the role of CHCHD4 and the DRS in physiology and disease, with a specific focus on the emerging importance of CHCHD4 in regulating the cellular response to low oxygen (hypoxia) and metabolism in cancer.

Glutathionylated and Fe-S cluster containing hMIA40 (CHCHD4) regulates ROS and mitochondrial complex III and IV activities of the electron transport chain

Human MIA40, an intermembrane space (IMS) import receptor of mitochondria harbors twin CX9C motifs for stability while its CPC motif is known to facilitate the import of IMS bound proteins. Site-directed mutagenesis complemented by MALDI on in vivo hMIA40 protein shows that a portion of MIA40 undergoes reversible S-glutathionylation at three cysteines in the twin CX9C motifs and the lone cysteine 4 residue. We find that HEK293T cells expressing hMIA40 mutant defective for glutathionylation are compromised in the activities of complexes III and IV of the Electron Transport Chain (ETC) and enhance Reactive Oxygen Species (ROS) levels. Immunocapture studies show MIA40 interacting with complex III. Interestingly, glutathionylated MIA40 can transfer electrons to cytochrome C directly. However, Fe–S clusters associated with the CPC motif are essential to facilitate the two-electron to one-electron transfer for reducing cytochrome C. These results suggest that hMIA40 undergoes glutathionylation to maintain ROS levels and for optimum function of complexes III and IV of ETC. Our studies shed light on a novel post-translational modification of hMIA40 and its ability to act as a redox switch to regulate the ETC and cellular redox homeostasis.

The C-terminal region of the oxidoreductase MIA40 stabilizes its cytosolic precursor during mitochondrial import

BACKGROUND

The mitochondrial intermembrane space (IMS) is home to proteins fulfilling numerous essential cellular processes, particularly in metabolism and mitochondrial function. All IMS proteins are nuclear encoded and synthesized in the cytosol and must therefore be correctly targeted and transported to the IMS, either through mitochondrial targeting sequences or conserved cysteines and the mitochondrial disulfide relay system. The mitochondrial oxidoreductase MIA40, which catalyzes disulfide formation in the IMS, is imported by the combined action of the protein AIFM1 and MIA40 itself. Here, we characterized the function of the conserved highly negatively charged C-terminal region of human MIA40.

RESULTS

We demonstrate that the C-terminal region is critical during posttranslational mitochondrial import of MIA40, but is dispensable for MIA40 redox function in vitro and in intact cells. The C-terminal negatively charged region of MIA40 slowed import into mitochondria, which occurred with a half-time as slow as 90 min. During this time, the MIA40 precursor persisted in the cytosol in an unfolded state, and the C-terminal negatively charged region served in protecting MIA40 from proteasomal degradation. This stabilizing property of the MIA40 C-terminal region could also be conferred to a different mitochondrial precursor protein, COX19.

CONCLUSIONS

Our data suggest that the MIA40 precursor contains the stabilizing information to allow for postranslational import of sufficient amounts of MIA40 for full functionality of the essential disulfide relay. We thereby provide for the first time mechanistic insights into the determinants controlling cytosolic surveillance of IMS precursor proteins.

Cysteine Oxidations in Mitochondrial Membrane Proteins: The Case of VDAC Isoforms in Mammals

Cysteine residues are reactive amino acids that can undergo several modifications driven by redox reagents. Mitochondria are the source of an abundant production of radical species, and it is surprising that such a large availability of highly reactive chemicals is compatible with viable and active organelles, needed for the cell functions. In this work, we review the results highlighting the modifications of cysteines in the most abundant proteins of the outer mitochondrial membrane (OMM), that is, the voltage-dependent anion selective channel (VDAC) isoforms. This interesting protein family carries several cysteines exposed to the oxidative intermembrane space (IMS). Through mass spectrometry (MS) analysis, cysteine posttranslational modifications (PTMs) were precisely determined, and it was discovered that such cysteines can be subject to several oxidization degrees, ranging from the disulfide bridge to the most oxidized, the sulfonic acid, one. The large spectra of VDAC cysteine oxidations, which is unique for OMM proteins, indicate that they have both a regulative function and a buffering capacity able to counteract excess of mitochondrial reactive oxygen species (ROS) load. The consequence of these peculiar cysteine PTMs is discussed.

Vectorial Import via a Metastable Disulfide-Linked Complex Allows for a Quality Control Step and Import by the Mitochondrial Disulfide Relay

Disulfide formation in the mitochondrial intermembrane space (IMS) is an essential process. It is catalyzed by the disulfide relay machinery, which couples substrate import and oxidation. The machinery relies on the oxidoreductase and chaperone CHCHD4-Mia40. Here, we report on the driving force for IMS import and on a redox quality control mechanism. We demonstrate that unfolded reduced proteins, upon translocation into the IMS, initiate formation of a metastable disulfide-linked complex with CHCHD4. If this interaction does not result in productive oxidation, then substrates are released to the cytosol and degraded by the proteasome. Based on these data, we propose a redox quality control step at the level of the disulfide-linked intermediate that relies on the vectorial nature of IMS import. Our findings also provide the mechanistic framework to explain failures in import of numerous human disease mutants in CHCHD4 substrates.

Redox characterisation of Erv1, a key component for protein import and folding in yeast mitochondria

The mitochondrial import and assembly (MIA) pathway plays a vitally important role in import and oxidative folding of mitochondrial proteins. Erv1, a member of the FAD-dependent Erv1/ALR disulphide bond generating enzyme family, is a key player of the MIA pathway. Although considerable progress has been made, the molecular mechanism of electron transfer within Erv1 is still not fully understood. The reduction potentials of the three redox centres were previously determined to be -320 mV for the shuttle disulphide, -150 mV for the active-site disulphide and -215 mV for FAD cofactor. However, it is unknown why FAD of Erv1 has such a low potential compared with other sulfhydryl oxidases, and why the shuttle disulphide has a potential as low as many of the stable structural disulphides of the substrates of MIA pathway. In this study, the three reduction potentials of Erv1 were reassessed using the wild-type and inactive mutants of Erv1 under anaerobic conditions. Our results show that the standard potentials for the shuttle and active-site disulphides are approximately -250 mV and -215 ~ -260 mV, respectively, and the potential for FAD cofactor is -148 mV. Our results support a model that both disulphide bonds are redox-active, and electron flow in Erv1 is thermodynamically favourable. Furthermore, the redox behaviour of Erv1 was confirmed, for the first time using Mia40, the physiological electron donor of Erv1. Together with previous studies on proteins of MIA pathway, we conclude that electron flow in the MIA pathway is a thermodynamically favourable, smoothly downhill process for all steps. DATABASE: Erv1: EC 1.8.3.2.

Kinetic characterisation of Erv1, a key component for protein import and folding in yeast mitochondria

Yeast (Saccharomyces cerevisiae) essential for respiration and viability 1 (Erv1; EC number http://www.chem.qmul.ac.uk/iubmb/enzyme/1/8/3/2.html), a member of the flavin adenine dinucleotide‐dependent Erv1/ALR disulphide bond generating enzyme family, works together with Mia40 to catalyse protein import and oxidative folding in the mitochondrial intermembrane space. Erv1/ALR functions either as an oxidase or cytochrome c reductase by passing electrons from a thiol substrate to molecular oxygen (O₂) or cytochrome c, respectively. However, the substrate specificity for oxygen and cytochrome c is not fully understood. In this study, the oxidase and cytochrome c reductase kinetics of yeast Erv1 were investigated in detail, under aerobic and anaerobic conditions, using stopped‐flow absorption spectroscopy and oxygen consumption analysis. Using DTT as an electron donor, our results show that cytochrome c is ~ 7‐ to 15‐fold more efficient than O₂ as electron acceptors for yeast Erv1, and that O₂ is a competitive inhibitor of Erv1 cytochrome c reductase activity. In addition, Mia40, the physiological thiol substrate of Erv1, was used as an electron donor for Erv1 in a detailed enzyme kinetic study. Different enzyme kinetic k_cat and K_m values were obtained with Mia40 compared to DTT, suggesting that Mia40 modulates Erv1 enzyme kinetics. Taken together, this study shows that Erv1 is a moderately active enzyme with the ability to use both O₂ and cytochrome c as the electron acceptors, indicating that Erv1 contributes to mitochondrial hydrogen peroxide production. Our results also suggest that Mia40‐Erv1 system may involve in regulation of the redox state of glutathione in the mitochondrial intermembrane space.

Erv1

EC number http://www.chem.qmul.ac.uk/iubmb/enzyme/1/8/3/2.html.

Structure of the mitochondrial import gate reveals distinct preprotein paths

The translocase of the outer mitochondrial membrane (TOM) is the main entry gate for proteins^1-4. Here we use cryo-electron microscopy to report the structure of the yeast TOM core complex^5-9 at 3.8-Å resolution. The structure reveals the high-resolution architecture of the translocator consisting of two Tom40 β-barrel channels and α-helical transmembrane subunits, providing insight into critical features that are conserved in all eukaryotes^1-3. Each Tom40 β-barrel is surrounded by small TOM subunits, and tethered by two Tom22 subunits and one phospholipid. The N-terminal extension of Tom40 forms a helix inside the channel; mutational analysis reveals its dual role in early and late steps in the biogenesis of intermembrane-space proteins in cooperation with Tom5. Each Tom40 channel possesses two precursor exit sites. Tom22, Tom40 and Tom7 guide presequence-containing preproteins to the exit in the middle of the dimer, whereas Tom5 and the Tom40 N extension guide preproteins lacking a presequence to the exit at the periphery of the dimer.

S-Glutathionylation of mouse selenoprotein W prevents oxidative stress-induced cell death by blocking the formation of an intramolecular disulfide bond

Mouse selenoprotein W (SELENOW) is a small protein containing a selenocysteine (Sec, U) and four cysteine (Cys, C) residues. The Sec residue in SELENOW is located within the conserved CXXU motif corresponding to the CXXC redox motif of thioredoxin (Trx). It is known that glutathione (GSH) binds to SELENOW and that this binding is involved in protecting cells from oxidative stress. However, the regulatory mechanisms controlling the glutathionylation of SELENOW in oxidative stress are unclear. In this study, using purified recombinant SELENOW in which Sec13 was changed to Cys, we found that SELENOW was glutathionylated at Cys33 and that this S-glutathionylation was enhanced by oxidative stress. We also found that the S-glutathionylation of SELENOW at Cys33 in HEK293 cells was due to glutathione S-transferase Pi (GSTpi) and that this modification was reversed by glutaredoxin1 (Grx1). In addition to the disulfide bond between the Cys10 and Cys13 of SELENOW, a second disulfide bond was formed between Cys33 and Cys87 under oxidative stress conditions. The second disulfide bond was reduced by Trx1, but the disulfide bond between Cys10 and Cys13 was not. The second disulfide bond was also reduced by glutathione, but the disulfide bond in the CXXC motif was not. The second disulfide bond of the mutant SELENOW, in which Cys37 was replaced with Ser, was formed at a much lower concentration of hydrogen peroxide than the wild type. We also observed that Cys37 was required for S-glutathionylation, and that S-glutathionylated SELENOW containing Cys37 protected the cells from oxidative stress. Furthermore, the SELENOW (C33, 87S) mutant, which could not form the second disulfide bond, also showed antioxidant activity. Taken together, these results indicate that GSTpi-mediated S-glutathionylation of mouse SELENOW at Cys33 is required for the protection of cells in conditions of oxidative stress, through inhibition of the formation of the second disulfide bond.

The role of thiols in antioxidant systems

The sulfur biochemistry of the thiol group endows cysteines with a number of highly specialized and unique features that enable them to serve a variety of different functions in the cell. Typically highly conserved in proteins, cysteines are predominantly found in functionally or structurally crucial regions, where they act as stabilizing, catalytic, metal-binding and/or redox-regulatory entities. As highly abundant low molecular weight thiols, cysteine thiols and their oxidized disulfide counterparts are carefully balanced to maintain redox homeostasis in various cellular compartments, protect organisms from oxidative and xenobiotic stressors and partake actively in redox-regulatory and signaling processes. In this review, we will discuss the role of protein thiols as scavengers of hydrogen peroxide in antioxidant enzymes, use thiol peroxidases to exemplify how protein thiols contribute to redox signaling, provide an overview over the diverse set of low molecular weight thiol-based redox systems found in biology, and illustrate how thiol-based redox systems have evolved not only to protect against but to take full advantage of a world full of molecular oxygen.

Development of the Mitochondrial Intermembrane Space Disulfide Relay Represents a Critical Step in Eukaryotic Evolution

The mitochondrial intermembrane space evolved from the bacterial periplasm. Presumably as a consequence of their common origin, most proteins of these compartments are stabilized by structural disulfide bonds. The molecular machineries that mediate oxidative protein folding in bacteria and mitochondria, however, appear to share no common ancestry. Here we tested whether the enzymes Erv1 and Mia40 of the yeast mitochondrial disulfide relay could be functionally replaced by corresponding components of other compartments. We found that the sulfhydryl oxidase Erv1 could be replaced by the Ero1 oxidase or the protein disulfide isomerase from the endoplasmic reticulum, however at the cost of respiration deficiency. In contrast to Erv1, the mitochondrial oxidoreductase Mia40 proved to be indispensable and could not be replaced by thioredoxin-like enzymes, including the cytoplasmic reductase thioredoxin, the periplasmic dithiol oxidase DsbA, and Pdi1. From our studies we conclude that the profound inertness against glutathione, its slow oxidation kinetics and its high affinity to substrates renders Mia40 a unique and essential component of mitochondrial biogenesis. Evidently, the development of a specific mitochondrial disulfide relay system represented a crucial step in the evolution of the eukaryotic cell.

Oxidative protein folding: state-of-the-art and current avenues of research in plants

Contents Summary 1230 I. Introduction 1230 II. Formation and isomerization of disulfides in the ER and the Golgi apparatus 1231 III. The disulfide relay in the mitochondrial intermembrane space: why are plants different? 1236 IV. Disulfide bond formation on luminal proteins in thylakoids 1240 V. Conclusion 1242 Acknowledgements 1242 References 1242 SUMMARY: Disulfide bonds are post-translational modifications crucial for the structure and function of thousands of proteins. Their formation and isomerization, referred to as oxidative folding, require specific protein machineries found in oxidizing subcellular compartments, namely the endoplasmic reticulum and the associated endomembrane system, the intermembrane space of mitochondria and the thylakoid lumen of chloroplasts. At least one protein component is required for transferring electrons from substrate proteins to an acceptor that is usually molecular oxygen. For oxidation reactions, incoming reduced substrates are oxidized by thiol-oxidoreductase proteins (or domains in case of chimeric proteins), which are usually themselves oxidized by a single thiol oxidase, the enzyme generating disulfide bonds de novo. By contrast, the description of the molecular actors and pathways involved in proofreading and isomerization of misfolded proteins, which require a tightly controlled redox balance, lags behind. Herein we provide a general overview of the knowledge acquired on the systems responsible for oxidative protein folding in photosynthetic organisms, highlighting their particularities compared to other eukaryotes. Current research challenges are discussed including the importance and specificity of these oxidation systems in the context of the existence of reducing systems in the same compartments.

VHL-Mediated Regulation of CHCHD4 and Mitochondrial Function

Dysregulated mitochondrial function is associated with the pathology of a wide range of diseases including renal disease and cancer. Thus, investigating regulators of mitochondrial function is of particular interest. Previous work has shown that the von Hippel-Lindau tumor suppressor protein (pVHL) regulates mitochondrial biogenesis and respiratory chain function. pVHL is best known as an E3-ubiquitin ligase for the α-subunit of the hypoxia inducible factor (HIF) family of dimeric transcription factors. In normoxia, pVHL recognizes and binds hydroxylated HIF-α (HIF-1α and HIF-2α), targeting it for ubiquitination and proteasomal degradation. In this way, HIF transcriptional activity is tightly controlled at the level of HIF-α protein stability. At least 80% of clear cell renal carcinomas exhibit inactivation of the VHL gene, which leads to HIF-α protein stabilization and constitutive HIF activation. Constitutive HIF activation in renal carcinoma drives tumor progression and metastasis. Reconstitution of wild-type VHL protein (pVHL) in pVHL-defective renal carcinoma cells not only suppresses HIF activation and tumor growth, but also enhances mitochondrial respiratory chain function via mechanisms that are not fully elucidated. Here, we show that pVHL regulates mitochondrial function when re-expressed in pVHL-defective 786O and RCC10 renal carcinoma cells distinct from its regulation of HIF-α. Expression of CHCHD4, a key component of the disulphide relay system (DRS) involved in mitochondrial protein import within the intermembrane space (IMS) was elevated by pVHL re-expression alongside enhanced expression of respiratory chain subunits of complex I (NDUFB10) and complex IV (mtCO-2 and COX IV). These changes correlated with increased oxygen consumption rate (OCR) and dynamic changes in glucose and glutamine metabolism. Knockdown of HIF-2α also led to increased OCR, and elevated expression of CHCHD4, NDUFB10, and COXIV in 786O cells. Expression of pVHL mutant proteins (R200W, N78S, D126N, and S183L) that constitutively stabilize HIF-α but differentially promote glycolytic metabolism, were also found to differentially promote the pVHL-mediated mitochondrial phenotype. Parallel changes in mitochondrial morphology and the mitochondrial network were observed. Our study reveals a new role for pVHL in regulating CHCHD4 and mitochondrial function in renal carcinoma cells.

Cysteine residues in mitochondrial intermembrane space proteins: more than just import

The intermembrane space (IMS) is a very small mitochondrial sub-compartment with critical relevance for many cellular processes. IMS proteins fulfil important functions in transport of proteins, lipids, metabolites and metal ions, in signalling, in metabolism and in defining the mitochondrial ultrastructure. Our understanding of the IMS proteome has become increasingly refined although we still lack information on the identity and function of many of its proteins. One characteristic of many IMS proteins are conserved cysteines. Different post-translational modifications of these cysteine residues can have critical roles in protein function, localization and/or stability. The close localization to different ROS-producing enzyme systems, a dedicated machinery for oxidative protein folding, and a unique equipment with antioxidative systems, render the careful balancing of the redox and modification states of the cysteine residues, a major challenge in the IMS. In this review, we discuss different functions of human IMS proteins, the involvement of cysteine residues in these functions, the consequences of cysteine modifications and the consequences of cysteine mutations or defects in the machinery for disulfide bond formation in terms of human health. LINKED ARTICLES: This article is part of a themed section on Chemical Biology of Reactive Sulfur Species. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.4/issuetoc.

The mitochondrial oxidoreductase CHCHD4 is present in a semi-oxidized state in vivo

Disulfide formation in the mitochondrial intermembrane space is an essential process catalyzed by a disulfide relay machinery. In mammalian cells, the key enzyme in this machinery is the oxidoreductase CHCHD4/Mia40. Here, we determined the in vivo CHCHD4 redox state, which is the major determinant of its cellular activity. We found that under basal conditions, endogenous CHCHD4 redox state in cultured cells and mouse tissues was predominantly oxidized, however, degrees of oxidation in different tissues varied from 70% to 90% oxidized. To test whether differences in the ratio between CHCHD4 and ALR might explain tissue-specific differences in the CHCHD4 redox state, we determined the molar ratio of both proteins in different mouse tissues. Surprisingly, ALR is superstoichiometric over CHCHD4 in most tissues. However, the levels of CHCHD4 and the ratio of ALR over CHCHD4 appear to correlate only weakly with the redox state, and although ALR is present in superstoichiometric amounts, it does not lead to fully oxidized CHCHD4.

Augmenter of liver regeneration: A key protein in liver regeneration and pathophysiology

Liver is constantly exposed to pathogens, viruses, chemicals, and toxins, and several of them cause injury, leading to the loss of liver mass and sometimes resulting in cirrhosis and cancer. Under physiological conditions, liver can regenerate if the loss of cells is less than the proliferation of hepatocytes. If the loss is more than the proliferation, the radical treatment available is liver transplantation. Due to this reason, the search for an alternative therapeutic agent has been the focus of liver research. Liver regeneration is regulated by several growth factors; one of the key factors is augmenter of liver regeneration (ALR). Involvement of ALR has been reported in crucial processes such as oxidative phosphorylation, maintenance of mitochondria and mitochondrial biogenesis, and regulation of autophagy and cell proliferation. Augmenter of liver regeneration has been observed to be involved in liver regeneration by not only overcoming cell cycle inhibition but by maintaining the stem cell pool as well. These observations have created curiosity regarding the possible role of ALR in maintenance of liver health. Thus, this review brings a concise presentation of the work done in areas exploring the role of ALR in normal liver physiology and in liver health maintenance by fighting liver diseases, such as liver failure, non-alcoholic fatty liver disease/non-alcoholic steatohepatitis, viral infections, cirrhosis, and hepatocellular carcinoma.

Clinical and genetic aspects of defects in the mitochondrial iron-sulfur cluster synthesis pathway

Iron-sulfur clusters are evolutionarily conserved biological structures which play an important role as cofactor for multiple enzymes in eukaryotic cells. The biosynthesis pathways of the iron-sulfur clusters are located in the mitochondria and in the cytosol. The mitochondrial iron-sulfur cluster biosynthesis pathway (ISC) can be divided into at least twenty enzymatic steps. Since the description of frataxin deficiency as the cause of Friedreich's ataxia, multiple other deficiencies in ISC biosynthesis pathway have been reported. In this paper, an overview is given of the clinical, biochemical and genetic aspects reported in humans affected by a defect in iron-sulfur cluster biosynthesis.

A glutaredoxin in the mitochondrial intermembrane space has stage-specific functions in the thermo-tolerance and proliferation of African trypanosomes

Trypanosoma brucei glutaredoxin 2 (Grx2) is a dithiol glutaredoxin that is specifically located in the mitochondrial intermembrane space. Bloodstream form parasites lacking Grx2 or both, Grx2 and the cytosolic Grx1, are viable in vitro and infectious to mice suggesting that neither oxidoreductase is needed for survival or infectivity to mammals. A 37 °C to 39 °C shift changes the cellular redox milieu of bloodstream cells to more oxidizing conditions and induces a significantly stronger growth arrest in wildtype parasites compared to the mutant cells. Grx2-deficient cells ectopically expressing the wildtype form of Grx2 with its C31QFC34 active site, but not the C34S mutant, regain the sensitivity of the parental strain, indicating that the physiological role of Grx2 requires both active site cysteines. In the procyclic insect stage of the parasite, Grx2 is essential. Both alleles can be replaced if procyclic cells ectopically express authentic or C34S, but not C31S/C34S Grx2, pointing to a redox role that relies on a monothiol mechanism. RNA-interference against Grx2 causes a virtually irreversible proliferation defect. The cells adopt an elongated morphology but do not show any significant alteration in the cell cycle. The growth retardation is attenuated by high glucose concentrations. Under these conditions, procyclic cells obtain ATP by substrate level phosphorylation suggesting that Grx2 might regulate a respiratory chain component.

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Bloodstream T. brucei lacking glutaredoxin 2 are fully viable in vitro and in vivo.

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A temperature rise shifts the cellular redox state to more oxidizing conditions.

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Glutaredoxin 2-deficiency confers bloodstream cells with thermo-tolerance.

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The insect stage requires redox-active glutaredoxin 2 for viability and morphology.

A single-cysteine mutant and chimeras of essential Leishmania Erv can complement the loss of Erv1 but not of Mia40 in yeast

Mia40/CHCHD4 and Erv1/ALR are essential for oxidative protein folding in the mitochondrial intermembrane space of yeast and mammals. In contrast, many protists, including important apicomplexan and kinetoplastid parasites, lack Mia40. Furthermore, the Erv homolog of the model parasite Leishmania tarentolae (LtErv) was shown to be incompatible with Saccharomyces cerevisiae Mia40 (ScMia40). Here we addressed structure-function relationships of ScErv1 and LtErv as well as their compatibility with the oxidative protein folding system in yeast using chimeric, truncated, and mutant Erv constructs. Chimeras between the N-terminal arm of ScErv1 and a variety of truncated LtErv constructs were able to rescue yeast cells that lack ScErv1. Yeast cells were also viable when only a single cysteine residue was replaced in LtErv^C17S. Thus, the presence and position of the C-terminal arm and the kinetoplastida-specific second (KISS) domain of LtErv did not interfere with its functionality in the yeast system, whereas a relatively conserved cysteine residue before the flavodomain rendered LtErv incompatible with ScMia40. The question whether parasite Erv homologs might also exert the function of Mia40 was addressed in another set of complementation assays. However, neither the KISS domain nor other truncated or mutant LtErv constructs were able to rescue yeast cells that lack ScMia40. The general relevance of Erv and its candidate substrate small Tim1 was analyzed for the related parasite L. infantum. Repeated unsuccessful knockout attempts suggest that both genes are essential in this human pathogen and underline the potential of mitochondrial protein import pathways for future intervention strategies.

Protein Translocation into the Intermembrane Space and Matrix of Mitochondria: Mechanisms and Driving Forces

Mitochondria contain two aqueous subcompartments, the matrix and the intermembrane space (IMS). The matrix is enclosed by both the inner and outer mitochondrial membranes, whilst the IMS is sandwiched between the two. Proteins of the matrix are synthesized in the cytosol as preproteins, which contain amino-terminal matrix targeting sequences that mediate their translocation through translocases embedded in the outer and inner membrane. For these proteins, the translocation reaction is driven by the import motor which is part of the inner membrane translocase. The import motor employs matrix Hsp70 molecules and ATP hydrolysis to ratchet proteins into the mitochondrial matrix. Most IMS proteins lack presequences and instead utilize the IMS receptor Mia40, which facilitates their translocation across the outer membrane in a reaction that is coupled to the formation of disulfide bonds within the protein. This process requires neither ATP nor the mitochondrial membrane potential. Mia40 fulfills two roles: First, it acts as a holdase, which is crucial in the import of IMS proteins and second, it functions as a foldase, introducing disulfide bonds into newly imported proteins, which induces and stabilizes their natively folded state. For several Mia40 substrates, oxidative folding is an essential prerequisite for their assembly into oligomeric complexes. Interestingly, recent studies have shown that the two functions of Mia40 can be experimentally separated from each other by the use of specific mutants, hence providing a powerful new way to dissect the different physiological roles of Mia40. In this review we summarize the current knowledge relating to the mitochondrial matrix-targeting and the IMS-targeting/Mia40 pathway. Moreover, we discuss the mechanistic properties by which the mitochondrial import motor on the one hand and Mia40 on the other, drive the translocation of their substrates into the organelle. We propose that the lateral diffusion of Mia40 in the inner membrane and the oxidation-mediated folding of incoming polypeptides supports IMS import.