Catalysis of disulphide bond formation in the endoplasmic reticulum

PCRCR complex is essential for invasion of human erythrocytes by Plasmodium falciparum

The most severe form of malaria is caused by Plasmodium falciparum. These parasites invade human erythrocytes, and an essential step in this process involves the ligand PfRh5, which forms a complex with cysteine-rich protective antigen (CyRPA) and PfRh5-interacting protein (PfRipr) (RCR complex) and binds basigin on the host cell. We identified a heteromeric disulfide-linked complex consisting of P. falciparum Plasmodium thrombospondin-related apical merozoite protein (PfPTRAMP) and P. falciparum cysteine-rich small secreted protein (PfCSS) and have shown that it binds RCR to form a pentameric complex, PCRCR. Using P. falciparum lines with conditional knockouts, invasion inhibitory nanobodies to both PfPTRAMP and PfCSS, and lattice light-sheet microscopy, we show that they are essential for merozoite invasion. The PCRCR complex functions to anchor the contact between merozoite and erythrocyte membranes brought together by strong parasite deformations. We solved the structure of nanobody-PfCSS complexes to identify an inhibitory epitope. Our results define the function of the PCRCR complex and identify invasion neutralizing epitopes providing a roadmap for structure-guided development of these proteins for a blood stage malaria vaccine.

Changes in Glutathione Content in Liver Diseases: An Update

Glutathione (GSH), a tripeptide particularly concentrated in the liver, is the most important thiol reducing agent involved in the modulation of redox processes. It has also been demonstrated that GSH cannot be considered only as a mere free radical scavenger but that it takes part in the network governing the choice between survival, necrosis and apoptosis as well as in altering the function of signal transduction and transcription factor molecules. The purpose of the present review is to provide an overview on the molecular biology of the GSH system; therefore, GSH synthesis, metabolism and regulation will be reviewed. The multiple GSH functions will be described, as well as the importance of GSH compartmentalization into distinct subcellular pools and inter-organ transfer. Furthermore, we will highlight the close relationship existing between GSH content and the pathogenesis of liver disease, such as non-alcoholic fatty liver disease (NAFLD), alcoholic liver disease (ALD), chronic cholestatic injury, ischemia/reperfusion damage, hepatitis C virus (HCV), hepatitis B virus (HBV) and hepatocellular carcinoma. Finally, the potential therapeutic benefits of GSH and GSH-related medications, will be described for each liver disorder taken into account.

The role of ERp44 in glucose and lipid metabolism

Endoplasmic Reticulum Protein 44 (ERp44) is a member of the PDI family, named for a molecular weight of 44 kD. White adipose tissue has metabolic and endocrine functions that are important to metabolism. The role of ERp44 in glucose and lipid metabolism is not known yet. The current study was undertaken to investigate the implication of ERp44 in glucose and lipid metabolism. In this study, we generated and characterized ERp44^-/- mice. We used type 2 diabetes models and ERp44 knockout mice to show the implication of ERp44 in glucose and lipid metabolism. Knockout newborns had lower blood glucose compared to wild-type. Adult knockouts had abnormal intraperitoneal, glucose, insulin and pyruvic acid tolerance. Lipocytes were smaller and fewer in knockout mice compared to wild-type. Knockouts resisted to high-fat diet-induced obesity. ERp44 expression in white adipose tissue decreased significantly in type 2 diabetes models. Results suggest that ERp44 is closely associated with glucose and lipid metabolism.

Epitope Presentation of Dengue Viral Envelope Glycoprotein Domain III on Hepatitis B Core Protein Virus-Like Particles Produced in Nicotiana benthamiana

Dengue fever is currently ranked as the top emerging tropical disease, driven by increased global travel, urbanization, and poor hygiene conditions as well as global warming effects which facilitate the spread of Aedes mosquitoes beyond their current distribution. Today, more than 100 countries are affected most of which are tropical Asian and Latin American nations with limited access to medical care. Hence, the development of a dengue vaccine that is dually cost-effective and able to confer a comprehensive protection is ultimately needed. In this study, a consensus sequence of the antigenic dengue viral glycoprotein domain III (cEDIII) was used aiming to provide comprehensive coverage against all four circulating dengue viral serotypes and potential clade replacement event. Utilizing hepatitis B tandem core technology, the cEDIII sequence was inserted into the immunodominant c/e1 loop region so that it could be displayed on the spike structures of assembled particles. The tandem core particles displaying cEDIII epitopes (tHBcAg-cEDIII) were successfully produced in Nicotiana benthamiana via Agrobacterium-mediated transient expression strategy to give a protein of ∼54 kDa, detected in both soluble and insoluble fractions of plant extracts. The assembled tHBcAg-cEDIII virus-like particles (VLPs) were also visualized from transmission electron microscopy. These VLPs had diameters that range from 32 to 35 nm, presenting an apparent size increment as compared to tHBcAg control particles without cEDIII display (namely tEL). Mice immunized with tHBcAg-cEDIII VLPs showed a positive seroconversion to cEDIII antigen, thereby signifying that the assembled tHBcAg-cEDIII VLPs have successfully displayed cEDIII antigen to the immune system. If it is proven to be successful, tHBcAg-cEDIII has the potential to be developed as a cost-effective vaccine candidate that confers a simultaneous protection against all four infecting dengue viral serotypes.

Folding of a single domain protein entering the endoplasmic reticulum precedes disulfide formation

The relationship between protein synthesis, folding, and disulfide formation within the endoplasmic reticulum (ER) is poorly understood. Previous studies have suggested that pre-existing disulfide links are absolutely required to allow protein folding and, conversely, that protein folding occurs prior to disulfide formation. To address the question of what happens first within the ER, that is, protein folding or disulfide formation, we studied folding events at the early stages of polypeptide chain translocation into the mammalian ER using stalled translation intermediates. Our results demonstrate that polypeptide folding can occur without complete domain translocation. Protein disulfide isomerase (PDI) interacts with these early intermediates, but disulfide formation does not occur unless the entire sequence of the protein domain is translocated. This is the first evidence that folding of the polypeptide chain precedes disulfide formation within a cellular context and highlights key differences between protein folding in the ER and refolding of purified proteins.

Co- and Post-Translational Protein Folding in the ER

The biophysical rules that govern folding of small, single-domain proteins in dilute solutions are now quite well understood. The mechanisms underlying co-translational folding of multidomain and membrane-spanning proteins in complex cellular environments are often less clear. The endoplasmic reticulum (ER) produces a plethora of membrane and secretory proteins, which must fold and assemble correctly before ER exit - if these processes fail, misfolded species accumulate in the ER or are degraded. The ER differs from other cellular organelles in terms of the physicochemical environment and the variety of ER-specific protein modifications. Here, we review chaperone-assisted co- and post-translational folding and assembly in the ER and underline the influence of protein modifications on these processes. We emphasize how method development has helped advance the field by allowing researchers to monitor the progression of folding as it occurs inside living cells, while at the same time probing the intricate relationship between protein modifications during folding.

Production of cell surface and secreted glycoproteins in mammalian cells

Mammalian protein expression systems are becoming increasingly popular for the production of eukaryotic secreted and cell surface proteins. Here we describe methods to produce recombinant proteins in adherent or suspension human embryonic kidney cell cultures, using transient transfection or stable cell lines. The protocols are easy to scale up and cost-efficient, making them suitable for protein crystallization projects and other applications that require high protein yields.

Stress sensing in plants by an ER stress sensor/transducer, bZIP28

Two classes of ER stress sensors are known in plants, membrane-associated basic leucine zipper (bZIP) transcription factors and RNA splicing factors. ER stress occurs under adverse environmental conditions and results from the accumulation of misfolded or unfolded proteins in the ER lumen. One of the membrane-associated transcription factors activated by heat and ER stress agents is bZIP28. In its inactive form, bZIP28 is a type II membrane protein with a single pass transmembrane domain, residing in the ER. bZIP28's N-terminus, containing a transcriptional activation domain, is oriented towards the cytoplasm and its C-terminal tail is inserted into the ER lumen. In response to stress, bZIP28 exits the ER and moves to the Golgi where it is proteolytically processed, liberating its cytosolic component which relocates to the nucleus to upregulate stress-response genes. bZIP28 is thought to sense stress through its interaction with the major ER chaperone, binding immunoglobulin protein (BIP). Under unstressed conditions, BIP binds to intrinsically disordered regions in bZIP28's lumen-facing tail and retains it in the ER. A truncated form of bZIP28, without its C-terminal tail is not retained in the ER but migrates constitutively to the nucleus. Upon stress, BIP releases bZIP28 allowing it to exit the ER. One model to account for the release of bZIP28 by BIP is that BIP is competed away from bZIP28 by the accumulation of misfolded proteins in the ER. However, other forces such as changes in energy charge levels, redox conditions or interaction with DNAJ proteins may also promote release of bZIP28 from BIP. Movement of bZIP28 from the ER to the Golgi is assisted by the interaction of elements of the COPII machinery with the cytoplasmic domain of bZIP28. Thus, the mobilization of bZIP28 in response to stress involves the dissociation of factors that retain it in the ER and the association of factors that mediate its further organelle-to-organelle movement.

Real-time kinetics of high-mobility group box 1 (HMGB1) oxidation in extracellular fluids studied by in situ protein NMR spectroscopy

Some extracellular proteins are initially secreted in reduced forms via a non-canonical pathway bypassing the endoplasmic reticulum and become oxidized in the extracellular space. One such protein is HMGB1 (high-mobility group box 1). Extracellular HMGB1 has different redox states that play distinct roles in inflammation. Using a unique NMR-based approach, we have investigated the kinetics of HMGB1 oxidation and the half-lives of all-thiol and disulfide HMGB1 species in serum, saliva, and cell culture medium. In this approach, salt-free lyophilized (15)N-labeled all-thiol HMGB1 was dissolved in actual extracellular fluids, and the oxidation and clearance kinetics were monitored in situ by recording a series of heteronuclear (1)H-(15)N correlation spectra. We found that the half-life depends significantly on the extracellular environment. For example, the half-life of all-thiol HMGB1 ranged from ~17 min (in human serum and saliva) to 3 h (in prostate cancer cell culture medium). Furthermore, the binding of ligands (glycyrrhizin and heparin) to HMGB1 significantly modulated the oxidation kinetics. Thus, the balance between the roles of all-thiol and disulfide HMGB1 proteins depends significantly on the extracellular environment and can also be artificially modulated by ligands. This is important because extracellular HMGB1 has been suggested as a therapeutic target for inflammatory diseases and cancer. Our work demonstrates that the in situ protein NMR approach is powerful for investigating the behavior of proteins in actual extracellular fluids containing an enormous number of different molecules.

The endoplasmic reticulum in cardiovascular health and disease

The endoplasmic reticulum has an intricate network of pathways built to deal with the secretory and integral membrane protein synthesis demands of the cell, as well as adaptive responses set up for the endoplasmic reticulum to rely on when stressed. These pathways are both essential and complex, and because of these 2 factors, several situations can lead to a dysfunctional endoplasmic reticulum and result in a dysfunctional cell with the potential to contribute to the progression of disease. The endoplasmic reticulum has been implicated in several metabolic, neurodegenerative, inflammatory, autoimmune, and renal diseases and disorders, and in particular, cardiovascular diseases. The role of the endoplasmic reticulum in cardiovascular disease shows how the change in function of a particular microscopic organelle can lead to macroscopic changes in the form of disease.

Molecular characterization of the endoplasmic reticulum: insights from proteomic studies

The endoplasmic reticulum (ER) is a multifunctional intracellular organelle responsible for the synthesis, processing and trafficking of a wide variety of proteins essential for cell growth and survival. Therefore, comprehensive characterization of the ER proteome is of great importance to the understanding of its functions and has been actively pursued in the past decade by scientists in the proteomics field. This review summarizes major proteomic studies published in the past decade that focused on the ER proteome. We evaluate the data sets obtained from two different organs, liver and pancreas each of which contains a primary cell type (hepatocyte and acinar cell) with specialized functions. We also discuss how the nature of the proteins uncovered is related to the methods of organelle purification, organelle purity and the techniques used for protein separation prior to MS. In addition, this review also puts emphasis on the biological insights gained from these studies regarding the molecular functions of the ER including protein synthesis and translocation, protein folding and quality control, ER-associated degradation and ER stress, ER export and membrane trafficking, calcium homeostasis and detoxification and drug metabolism.

Peroxiredoxin IV protects cells from oxidative stress by removing H2O2 produced during disulphide formation

Disulphide formation within the endoplasmic reticulum (ER) requires the activity of the ER oxidase Ero1, and as a consequence, generates hydrogen peroxide. The production of hydrogen peroxide is thought to lead to oxidative stress that ultimately results in apoptosis. Here, we show that mammalian peroxiredoxin IV (PrxIV) metabolises hydrogen peroxide produced by Ero1. We demonstrate that the presence of PrxIV within the ER provides a cytoprotective effect against stresses known to enhance Ero1 activity and perturb ER redox balance. Increased Ero1 activity and production of hydrogen peroxide led to preferential hyperoxidation of PrxIV relative to peroxiredoxins in other cellular compartments. The hyperoxidation was increased by the upregulation of Ero1 and by the expression of a hyperactive Ero1. These findings provide the first evidence for an enzymatic mechanism that facilitates peroxide removal from the ER, and show that the oxidation status of PrxIV acts as a marker for ER oxidative stress.

Protein disulfide isomerase chaperone ERP-57 decreases plasma membrane expression of the human GnRH receptor

Retention of misfolded proteins by the endoplasmic reticulum (ER) is a quality control mechanism involving the participation of endogenous chaperones such as calnexin (CANX). CANX interacts with and restricts plasma membrane expression (PME) of the gonadotropin releasing hormone receptor (GnRHR), a G protein-coupled receptor. CANX also interacts with ERP-57 a thiol oxidoreductase chaperone present in the ER. CANX along with ERP-57 promotes the formation of disulfide bond bridges in nascent proteins. The human GnRH receptor (hGnRHR) is stabilized by two disulfide bond bridges (C(14)-C(200) and C(114)-C(196)), that, when broken, lead to a decrease in receptor expression at the plasma membrane. To determine if the presence of chaperones CANX and ERP-57 exerts an influence over membrane routing and second messenger activation, we assessed the effect of various mutants including those with broken disulfide bridges (Cys --> Ala) along with the hGnRHR. The effect of chaperones on mutants was insignificant, whereas the over expression of ERP-57 led to an hGnRHR retention. This effect was further enhanced by cotransfection with cDNA for CANX showing receptor retention by ERP-57 augmented by CANX, suggesting utilization of these chaperones for quality control of the GnRHR.

Physiology and pathology of proteostasis in the early secretory compartment

The endoplasmic reticulum (ER), the port of entry for proteins into the secretory pathway, is a multifunctional organelle emerging as a central integrator of numerous signalling pathways. The mechanisms that control proteostasis are integral part of this signalling network, providing cues for morphological and functional cell remodelling, proliferation, inflammation and cell death. The complexity of ER responses is exploited during physiological and pathological tissue development, cell differentiation and lifespan control. This essay outlines some of the mechanisms that link proteostasis within the early secretory compartment to signalling in development and disease.

Intracellular expression of a single domain antibody reduces cytotoxicity of 15-acetyldeoxynivalenol in yeast

15-Acetyldeoxynivalenol (15-AcDON) is a low molecular weight sesquiterpenoid trichothecene mycotoxin associated with Fusarium ear rot of maize and Fusarium head blight of small grain cereals. The accumulation of mycotoxins such as deoxynivalenol (DON) and 15-AcDON within harvested grain is subject to stringent regulation as both toxins pose dietary health risks to humans and animals. These toxins inhibit peptidyltransferase activity, which in turn limits eukaryotic protein synthesis. To assess the ability of intracellular antibodies (intrabodies) to modulate mycotoxin-specific cytotoxocity, a gene encoding a camelid single domain antibody fragment (V(H)H) with specificity and affinity for 15-AcDON was expressed in the methylotropic yeast Pichia pastoris. Cytotoxicity and V(H)H immunomodulation were assessed by continuous measurement of cellular growth. At equivalent doses, 15-AcDON was significantly more toxic to wild-type P. pastoris than was DON. In turn, DON was orders of magnitude more toxic than 3-acetyldeoxynivalenol. Intracellular expression of a mycotoxin-specific V(H)H within P. pastoris conveyed significant (p = 0.01) resistance to 15-AcDON cytotoxicity at doses ranging from 20 to 100 mug.ml(-1). We also documented a biochemical transformation of DON to 15-AcDON to account for the attenuation of DON cytotoxicity at 100 and 200 mug.ml(-1). The proof of concept established within this eukaryotic system suggests that in planta V(H)H expression may lead to enhanced tolerance to mycotoxins and thereby limit Fusarium infection of commercial agricultural crops.

Enzyme activity of phosphatase of regenerating liver is controlled by the redox environment and its C-terminal residues

Phosphatase of regenerating liver-1 (PRL-1) belongs to a unique subfamily of protein tyrosine phosphatases (PTPases) associated with oncogenic and metastatic phenotypes. While considerable evidence supports a role for PRL-1 in promoting proliferation, the biological regulators and effectors of PRL-1 activity remain unknown. PRL-1 activity is inhibited by disulfide bond formation at the active site in vitro, suggesting PRL-1 may be susceptible to redox regulation in vivo. Because PRL-1 has been observed to localize to several different subcellular locations and cellular redox conditions vary with tissue type, age, stage of cell cycle, and subcellular location, we determined the reduction potential of the active site disulfide bond that controls phosphatase activity to improve our understanding of the function of PRL-1 in various cellular environments. We used high-resolution solution NMR spectroscopy to measure the potential and found it to be -364.3 +/- 1.5 mV. Because normal cellular environments range from -170 to -320 mV, we concluded that nascent PRL-1 would be primarily oxidized inside cells. Our studies show that a significant conformational change accompanies activation, suggesting a post-translational modification may alter the reduction potential, conferring activity. We further demonstrate that alteration of the C-terminus renders the protein reduced and active in vitro, implying the C-terminus is an important regulator of PRL-1 function. These data provide a basis for understanding how subcellular localization regulates the activity of PRL-1 and, with further investigation, may help reveal how PRL-1 promotes unique outcomes in different cellular systems, including proliferation in both normal and diseased states.

A novel group of glutaredoxins in the cis-Golgi critical for oxidative stress resistance

Glutaredoxins represent a ubiquitous family of proteins that catalyze the reduction of disulfide bonds in their substrate proteins by use of reduced glutathione. In an attempt to identify the full complement of glutaredoxins in baker's yeast, we found three so-far uncharacterized glutaredoxin-like proteins that we named Grx6, Grx7, and Grx8. Grx6 and Grx7 represent closely related monothiol glutaredoxins that are synthesized with N-terminal signal sequences. Both proteins are located in the cis-Golgi, thereby representing the first glutaredoxins found in a compartment of the secretory pathway. In contrast to formerly described monothiol glutaredoxins, Grx6 and Grx7, showed a high glutaredoxin activity in vitro. Grx6 and Grx7 overlap in their activity and deletion mutants lacking both proteins show growth defects and a strongly increased sensitivity toward oxidizing agents such as hydrogen peroxide or diamide. Our observations suggest that Grx6 and Grx7 do not play a general role in the oxidative folding of proteins in the early secretory pathway but rather counteract the oxidation of specific thiol groups in substrate proteins.

Assessment of endoplasmic reticulum glutathione redox status is confounded by extensive ex vivo oxidation

Glutathione (GSH) and glutathione disulfide (GSSG) form the principal thiol redox couple in the endoplasmic reticulum (ER); however, few studies have attempted to quantify GSH redox status in this organelle. To address this gap, GSH and GSSG levels and the extent of protein glutathionylation were analyzed in rat liver microsomes. Because of the likelihood of artifactual GSH oxidation during the lengthy microsomal isolation procedure, iodoacetic acid (IAA) was used to preserve the physiological thiol redox state. Non-IAA-treated microsomes exhibited a GSH:GSSG ratio between 0.7:1 to 1.2:1 compared to IAA-treated microsomes that yielded a GSH:GSSG redox ratio between 4.7:1 and 5.5:1. The majority of artifactual oxidation occurred within the first 2 h of isolation. Thus, the ER GSH redox ratio is subject to extensive ex vivo oxidation and when controlled, the microsomal GSH redox state is significantly higher than previously believed. Moreover, in vitro studies showed that PDI reductase activity was markedly increased at this higher thiol redox ratio versus previously reported GSH:GSSG ratios for the ER. Lastly, we show by both HPLC and Western blot analysis that ER proteins are highly resistant to glutathionylation. Together, these results may necessitate a re-evaluation of GSH and its role in ER function.

The unfolded protein response: a pathway that links insulin demand with beta-cell failure and diabetes

The endoplasmic reticulum (ER) is the entry site into the secretory pathway for newly synthesized proteins destined for the cell surface or released into the extracellular milieu. The study of protein folding and trafficking within the ER is an extremely active area of research that has provided novel insights into many disease processes. Cells have evolved mechanisms to modulate the capacity and quality of the ER protein-folding machinery to prevent the accumulation of unfolded or misfolded proteins. These signaling pathways are collectively termed the unfolded protein response (UPR). The UPR sensors signal a transcriptional response to expand the ER folding capacity, increase degradation of malfolded proteins, and limit the rate of mRNA translation to reduce the client protein load. Recent genetic and biochemical evidence in both humans and mice supports a requirement for the UPR to preserve ER homeostasis and prevent the beta-cell failure that may be fundamental in the etiology of diabetes. Chronic or overwhelming ER stress stimuli associated with metabolic syndrome can disrupt protein folding in the ER, reduce insulin secretion, invoke oxidative stress, and activate cell death pathways. Therapeutic interventions to prevent polypeptide-misfolding, oxidative damage, and/or UPR-induced cell death have the potential to improve beta-cell function and/or survival in the treatment of diabetes.

Removal of cysteinylation from an unpaired sulfhydryl in the variable region of a recombinant monoclonal IgG1 antibody improves homogeneity, stability, and biological activity

The antibody MAB007 was recently shown to be cysteinylated on an unpaired cysteine residue in the CDR3 variable region. Cysteinylation at this position was not complete and resulted in heterogeneous lots of MAB007 with respect to this posttranslational modification. In this report, a mild redox step was used that effectively removed cysteinylation while keeping native inter and intra-molecular disulfide bonds intact. Biophysical methods were employed to determine what consequences cysteinylation of the variable region had by directly comparing cysteinylated and de-cysteinylated MAB007 antibodies. No differences were detected in secondary structure; however, several pieces of evidence indicated that cysteinylation may result in tertiary or quaternary structural perturbations. These included differences in the cation-exchange chromatography and fluorescence-emission spectra of the cysteinylated and de-cysteinylated antibodies as well as differences in the solvent accessibility of the unpaired cysteine residue determined by labeling experiments. Such structural changes induced by cysteinylation were shown to increase the rate of MAB007 aggregation and to decrease the melting temperature of the Fab region by as much as 6 degrees C. The bioactivity of MAB007 was also shown to be adversely affected by cysteinylation and a direct correlation was made between the percent cysteinylation and biological activity.

Protein disulfide isomerases from C. elegans are equally efficient at thiol-disulfide exchange in simple peptide-based systems but show differences in reactivity towards protein substrates

Although the formation of disulfide bonds is an essential process in every living organism, only little is known about the mechanisms in multicellular eukaryotic systems. The reason for this uncertainty is that in addition to the well-known key enzyme protein disulfide isomerase (PDI), several PDI-like proteins are present in the ER of metazoans. In total, there are now 18 PDI-family members in the human endoplasmic reticulum, with different domain architectures and active site chemistries. To understand why multicellular organisms express multiple proteins with similarity to the archetypal mammalian PDI, the properties of three PDIs from the nematode C. elegans were investigated. Here the authors demonstrate that PDI-1, PDI-2, and PDI-3 show comparable kinetic properties in catalyzing thiol:disulfide exchange reactions in two simple peptide-based assays. However, the three enzymes exhibited clear differences in their reactivity towards protein substrates. The authors therefore propose that the three PDIs can catalyze similar thiol-disulfide exchange reactions in a substrate, but due to differences in substrate binding, they can direct a folding polypeptide chain onto different folding pathways and hence fulfil distinct and different functions in the organism.

The disulfide relay system of mitochondria is connected to the respiratory chain

All proteins of the intermembrane space of mitochondria are encoded by nuclear genes and synthesized in the cytosol. Many of these proteins lack presequences but are imported into mitochondria in an oxidation-driven process that relies on the activity of Mia40 and Erv1. Both factors form a disulfide relay system in which Mia40 functions as a receptor that transiently interacts with incoming polypeptides via disulfide bonds. Erv1 is a sulfhydryl oxidase that oxidizes and activates Mia40, but it has remained unclear how Erv1 itself is oxidized. Here, we show that Erv1 passes its electrons on to molecular oxygen via interaction with cytochrome c and cytochrome c oxidase. This connection to the respiratory chain increases the efficient oxidation of the relay system in mitochondria and prevents the formation of toxic hydrogen peroxide. Thus, analogous to the system in the bacterial periplasm, the disulfide relay in the intermembrane space is connected to the electron transport chain of the inner membrane.

Inside, outside, upside down: damage-associated molecular-pattern molecules (DAMPs) and redox

Immune responses are initiated and perpetuated by molecules derived from microorganisms pathogen-associated molecular-pattern molecules or from the damage or death of host cells [damage-associated molecular-pattern (DAMP) molecules]. Many DAMPs are nuclear or cytosolic proteins with defined intracellular function that, when released outside the cell following tissue injury, move from a reducing to an oxidizing milieu resulting in their functional denaturation. Here, we discuss the consequences of DAMP oxidation on the outcome of acute inflammation. We also suggest that, outside the cell, DAMPs might adopt novel conformations or alter the redox of the extracellular environment to more closely mimic the internal one, thereby avoiding oxidation-mediated inactivation and promoting pathology. We propose that chronic inflammation associated with autoimmunity, chronic viral infection and cancer is probably mediated by persistent release and function of DAMPs, promoting and promoted by a disordered redox environment.

Curcumin induces pro-apoptotic endoplasmic reticulum stress in human leukemia HL-60 cells

Curcumin has been shown to induce apoptosis in many cancer cells. However, the molecular mechanism(s) responsible for curcumin-induced apoptosis is not well understood and most probably involves several pathways. In HL-60 cells, curcumin induced apoptosis and endoplasmic reticulum (ER) stress as evidenced by the survival molecules such as phosphorylated protein kinase-like ER-resident kinase, phosphorylated eukaryotic initiation factor-2alpha, glucose-regulated protein-78, and the apoptotic molecules such as caspase-4 and CAAT/enhancer binding protein homologous protein (CHOP). Inhibition of caspase-4 activity by z-LEVD-FMK, blockage of CHOP expression by small interfering RNA, and treatment with salubrinal, an ER inhibitor, significantly reduced curcumin-induced apoptosis. Removing two double bonds in curcumin, which was speculated to form Michael adducts with thiols in secretory proteins, resulted in a loss of the ability of curcumin to induce apoptosis as well as ER stress. Thus, the present study shows that curcumin-induced apoptosis is associated with its ability to cause ER stress.

Studies in Vitamin E: Biochemistry and Molecular Biology of Tocopherol Quinones

Tocopherols and tocotrienols, parent congeners in the vitamin E family, function as phenolic antioxidants. However, there has been little interest in their quinone electrophiles formed as a consequence of oxidation reactions, even though unique biological properties were suggested by early studies conducted immediately after the discovery of vitamin E. Oxidation of tocopherols and tocotrienols produces para- and ortho-quinones, and quinone methides, while oxidation of their carboxyethyl hydroxychroman derivatives produces quinone lactones. These quinone electrophiles are grouped in two subclasses, the nonarylating fully methylated alpha-family and the arylating desmethyl beta-, gamma-, and delta-family. Arylating quinone electrophiles form Michael adducts with thiol nucleophiles, provided by cysteinyl proteins or peptides, which can be identified and quantified by tetramethylammonium hydroxide thermochemolysis. They have striking biological properties which differ significantly from their nonarylating congeners. They are highly cytotoxic, inducing characteristic apoptotic changes in cultured cells. Cytotoxicity is intimately associated with the induction of endoplasmic reticulum stress and a consequent unfolded protein response involving the pancreatic ER kinase (PERK) signaling pathway that commits overstressed cells to apoptosis. The step-function difference between arylating and nonarylating tocopherol quinones is conceivably the basis for distinct biological properties of parent tocopherols, including the epigenetic modification of a histone thiol, the ceramide pathway, natriuresis, and the activity of COX-2, NF-kappaB, PPARgamma, and cyclin. The role of alpha-tocopherol in the origin and evolution of the western hominin diet, the so-called "Mediterranean" diet, and the prominence of alpha-tocopherol in colostrum, mother's milk, and infant nutrition are considered. Finally, the discordance introduced into the diet by arylating tocopherol quinone precursors through the wide use of vegetable oils in deep-frying is recognized.

Calcium-dependent Dimerization of Human Soluble Calcium Activated Nucleotidase

Mammals express a protein homologous to soluble nucleotidases used by blood-sucking insects to inhibit host blood clotting. These vertebrate nucleotidases may play a role in protein glycosylation. The activity of this enzyme family is strictly dependent on calcium, which induces a conformational change in the secreted, soluble human nucleotidase. The crystal structure of this human enzyme was recently solved; however, the mechanism of calcium activation and the basis for the calcium-induced changes remain unclear. In this study, using analytical ultracentrifugation and chemical cross-linking, we show that calcium or strontium induce noncovalent dimerization of the soluble human enzyme. The location and nature of the dimer interface was elucidated using a combination of site-directed mutagenesis and chemical cross-linking, coupled with crystallographic analyses. Replacement of Ile(170), Ser(172), and Ser(226) with cysteine residues resulted in calcium-dependent, sulfhydryl-specific intermolecular cross-linking, which was not observed after cysteine introduction at other surface locations. Analysis of a super-active mutant, E130Y, revealed that this mutant dimerized more readily than the wild-type enzyme. The crystal structure of the E130Y mutant revealed that the mutated residue is found in the dimer interface. In addition, expression of the full-length nucleotidase revealed that this membrane-bound form can also dimerize and that these dimers are stabilized by spontaneous oxidative cross-linking of Cys(30), located between the single transmembrane helix and the start of the soluble sequence. Thus, calcium-mediated dimerization may also represent a mechanism for regulation of the activity of this nucleotidase in the physiological setting of the endoplasmic reticulum or Golgi.

Evaluation of the membrane-spanning domain of ClC-2

The ClC family of chloride channels and transporters includes several members in which mutations have been associated with human disease. An understanding of the structure-function relationships of these proteins is essential for defining the molecular mechanisms underlying pathogenesis. To date, the X-ray crystal structures of prokaryotic ClC transporter proteins have been used to model the membrane domains of eukaryotic ClC channel-forming proteins. Clearly, the fidelity of these models must be evaluated empirically. In the present study, biochemical tools were used to define the membrane domain boundaries of the eukaryotic protein, ClC-2, a chloride channel mutated in cases of idiopathic epilepsy. The membrane domain boundaries of purified ClC-2 and accessible cysteine residues were determined after its functional reconstitution into proteoliposomes, labelling using a thiol reagent and proteolytic digestion. Subsequently, the lipid-embedded and soluble fragments generated by trypsin-mediated proteolysis were studied by MS and coverage of approx. 71% of the full-length protein was determined. Analysis of these results revealed that the membrane-delimited boundaries of the N- and C-termini of ClC-2 and the position of several extramembrane loops determined by these methods are largely similar to those predicted on the basis of the prokaryotic protein [ecClC (Escherichia coli ClC)] structures. These studies provide direct biochemical evidence supporting the relevance of the prokaryotic ClC protein structures towards understanding the structure of mammalian ClC channel-forming proteins.

Mechanism of arylating quinone toxicity involving Michael adduct formation and induction of endoplasmic reticulum stress

Quinones permeate our biotic environment, contributing to both homeostasis and cytotoxicity. All quinones generate reactive oxygen species through redox cycling, while partially substituted quinones also undergo arylation (Michael adduct formation) yielding covalent bonds with nucleophiles such as cysteinyl thiols. In contrast to reactive oxygen species, the role of arylation in quinone cytotoxicity is not well understood. We found that the arylating quinones, including unsubstituted 1,4-benzoquinone (1,4-BzQ) and partially substituted vitamin E congener gamma-tocopherol quinone (gamma-TQ), were cytotoxic, with gamma-TQ > 1,4-BzQ, whereas the fully substituted nonarylating vitamin E congener alpha-tocopherol quinone was not. In vitro, both arylating quinones formed Michael adducts with the thiol nucleophile N-acetylcysteine (NAC) at rates where 1,4-BzQ > gamma-TQ. In cultured cells, concurrent addition of NAC eliminated 1,4-BzQ caused toxicity, but preincubation was required for the same NAC detoxification effect on gamma-TQ. These data clearly established the role of arylation in quinone toxicity and revealed that arylating quinone structure affects cytotoxicity by governing detoxification through the rate of adduct formation. Furthermore, arylating quinones induced endoplasmic reticulum (ER) stress by activating the pancreatic ER kinase (PERK) signaling pathway including elF2alpha, ATF4, and C/EBP homologous protein (CHOP). Detoxification by NAC greatly attenuates CHOP induction in arylating quinone-treated cells, suggesting that ER stress is a cellular mechanism for arylating quinone cytotoxicity.

An introduction to thiol redox proteins in the endoplasmic reticulum and a review of current electrochemical methods of detection of thiols

This aim of this paper is to expound the complexity of thiol redox systems in the endoplasmic reticulum of eukaryotic cells to the electroanalytical community. A summary of the state of the art in electrochemical methods for detection of thiols gives an insight into the challenges that need to be addressed to bridge the disparity between current analytical techniques and applications in a 'real' biological scenario.

Phylogenetic analyses identify 10 classes of the protein disulfide isomerase family in plants, including single-domain protein disulfide isomerase-related proteins

Protein disulfide isomerases (PDIs) are molecular chaperones that contain thioredoxin (TRX) domains and aid in the formation of proper disulfide bonds during protein folding. To identify plant PDI-like (PDIL) proteins, a genome-wide search of Arabidopsis (Arabidopsis thaliana) was carried out to produce a comprehensive list of 104 genes encoding proteins with TRX domains. Phylogenetic analysis was conducted for these sequences using Bayesian and maximum-likelihood methods. The resulting phylogenetic tree showed that evolutionary relationships of TRX domains alone were correlated with conserved enzymatic activities. From this tree, we identified a set of 22 PDIL proteins that constitute a well-supported clade containing orthologs of known PDIs. Using the Arabidopsis PDIL sequences in iterative BLAST searches of public and proprietary sequence databases, we further identified orthologous sets of 19 PDIL sequences in rice (Oryza sativa) and 22 PDIL sequences in maize (Zea mays), and resolved the PDIL phylogeny into 10 groups. Five groups (I-V) had two TRX domains and showed structural similarities to the PDIL proteins in other higher eukaryotes. The remaining five groups had a single TRX domain. Two of these (quiescin-sulfhydryl oxidase-like and adenosine 5'-phosphosulfate reductase-like) had putative nonisomerase enzymatic activities encoded by an additional domain. Two others (VI and VIII) resembled small single-domain PDIs from Giardia lamblia, a basal eukaryote, and from yeast. Mining of maize expressed sequence tag and RNA-profiling databases indicated that members of all of the single-domain PDIL groups were expressed throughout the plant. The group VI maize PDIL ZmPDIL5-1 accumulated during endoplasmic reticulum stress but was not found within the intracellular membrane fractions and may represent a new member of the molecular chaperone complement in the cell.