Interactions between protein disulphide isomerase and peptides

Thiol Isomerases Orchestrate Thrombosis and Hemostasis

Significance: Since protein disulfide isomerase (PDI) was first described in 1963, researchers have shown conclusively that PDI and sibling proteins are quintessential for thrombus formation. PDI, endoplasmic reticulum protein (ERp)5, ERp57, and ERp72 are released from platelets and vascular cells and interact with integrin αIIbβ3 on the outer surface of platelets. Recent Advances: At the cell surface they influence protein folding and function, propagating thrombosis and maintaining hemostasis. TMX1, which is a transmembrane thiol isomerase, is the first family member shown to negatively regulate platelets. Targets of thiol isomerases have been identified, including integrin α2β1, Von Willebrand Factor, GpIbα, nicotinamide adenine dinucleotide phosphate oxidase (Nox)-1, Nox-2, and tissue factor, all of which are pro-thrombotic, and several of which are on the cell surface. In spite of this, PDI can paradoxically catalyze the delivery of nitric oxide to platelets, which decrease thrombus formation. Critical Issues: Although the overall effect of PDI is to positively regulate platelet activation, it is still unclear how thiol isomerases function in pro-thrombotic states, such as obesity, diabetes, and cancer. In parallel, there has been a surge in the development of novel thiol isomerase inhibitors, which display selectivity, potency and modulate thrombosis and hemostasis. The availability of selective thiol isomerase inhibitors has culminated in clinical trials, with promising outcomes for the prevention of cancer-associated thrombosis. Future Directions: Altogether, thiol isomerases are perceived as an orchestrating force that regulates thrombus development. In the current review, we will explore the history of PDI in cardiovascular biology, detail known mechanisms of action, and summarize known thiol isomerase inhibitors.

Defining the mechanism of PDI interaction with disulfide-free amyloidogenic proteins: Implications for exogenous protein expression and neurodegenerative disease

Protein disulfide isomerase (PDI) is an important molecular chaperone capable of facilitating protein folding in addition to catalyzing the formation of a disulfide bond. To better understand the distinct substrate-screening principles of Pichia pastoris PDI (Protein disulfide isomerase) and the protective role of PDI in amyloidogenic diseases, we investigated the expression abundance and intracellular retention levels of three archetypal amyloidogenic disulfide bond-free proteins (Aβ42, α-synuclein (α-Syn) and SAA1) in P. pastoris GS115 strain without and with the overexpression of PpPDI (P. pastoris PDI). Intriguingly, amyloidogenic Aβ42 and α-Syn were detected only as intracellular proteins whereas amyloidogenic SAA1 was detected both as intracellular and extracellular proteins when these proteins were expressed in the PpPDI-overexpressing GS115 strain. The binding between PpPDI and each of the three amyloidogenic proteins was investigated by molecular docking and simulations. Three different patterns of PpPDI-substrate complexes were observed, suggesting that multiple modes of binding might exist for the binding between PpPDI and its amyloidogenic protein substrates, and this could represent different specificities and affinities of PpPDI toward its substrates. Further analysis of the proteomics data and functional annotations indicated that PpPDI could eliminate the need for misfolded proteins to be partitioned in ER-associated compartments.

PDI-Regulated Disulfide Bond Formation in Protein Folding and Biomolecular Assembly

Disulfide bonds play a pivotal role in maintaining the natural structures of proteins to ensure their performance of normal biological functions. Moreover, biological molecular assembly, such as the gluten network, is also largely dependent on the intermolecular crosslinking via disulfide bonds. In eukaryotes, the formation and rearrangement of most intra- and intermolecular disulfide bonds in the endoplasmic reticulum (ER) are mediated by protein disulfide isomerases (PDIs), which consist of multiple thioredoxin-like domains. These domains assist correct folding of proteins, as well as effectively prevent the aggregation of misfolded ones. Protein misfolding often leads to the formation of pathological protein aggregations that cause many diseases. On the other hand, glutenin aggregation and subsequent crosslinking are required for the formation of a rheologically dominating gluten network. Herein, the mechanism of PDI-regulated disulfide bond formation is important for understanding not only protein folding and associated diseases, but also the formation of functional biomolecular assembly. This review systematically illustrated the process of human protein disulfide isomerase (hPDI) mediated disulfide bond formation and complemented this with the current mechanism of wheat protein disulfide isomerase (wPDI) catalyzed formation of gluten networks.

Protein Disulfide Isomerases Regulate IgE-Mediated Mast Cell Responses and Their Inhibition Confers Protective Effects During Food Allergy

The thiol isomerase, protein disulfide isomerase (PDI), plays important intracellular roles during protein folding, maintaining cellular function and viability. Recent studies suggest novel roles for extracellular cell surface PDI in enhancing cellular activation and promoting their function. Moreover, a number of food-derived substances have been shown to regulate cellular PDI activity and alter disease progression. We hypothesized that PDI may have similar roles during mast cell-mediated allergic responses and examined its effects on IgE-induced mast cell activity during cell culture and food allergy. Mast cells were activated via IgE and antigen and the effects of PDI inhibition on mast cell activation were assessed. The effects of PDI blockade in vivo were examined by treating mice with the irreversible PDI inhibitor, PACMA-31, in an ovalbumin-induced model of food allergy. The role of dietary PDI modulators was investigated using various dietary compounds including curcumin and quercetin-3-rutinoside (rutin). PDI expression was observed on resting mast cell surfaces, intracellularly, and in the intestines of allergic mice. Furthermore, enhanced secretion of extracellular PDI was observed on mast cell membranes during IgE and antigen activation. Insulin turbidimetric assays demonstrated that curcumin is a potent PDI inhibitor and pre-treatment of mast cells with curcumin or established PDI inhibitors such as bacitracin, rutin or PACMA-31, resulted in the suppression of IgE-mediated activation and the secretion of various cytokines. This was accompanied by decreased mast cell proliferation, FcεRI expression, and mast cell degranulation. Similarly, treatment of allergic BALB/c mice with PACMA-31 attenuated the development of food allergy resulting in decreased allergic diarrhea, mast cell activation, and fewer intestinal mast cells. The production of TH2-specific cytokines was also suppressed. Our observations suggest that PDI catalytic activity is essential in the regulation of mast cell activation, and that its blockade may benefit patients with allergic inflammation.

PDI Family Members as Guides for Client Folding and Assembly

Complicated and sophisticated protein homeostasis (proteostasis) networks in the endoplasmic reticulum (ER), comprising disulfide catalysts, molecular chaperones, and their regulators, help to maintain cell viability. Newly synthesized proteins inserted into the ER need to fold and assemble into unique native structures to fulfill their physiological functions, and this is assisted by protein disulfide isomerase (PDI) family. Herein, we focus on recent advances in understanding the detailed mechanisms of PDI family members as guides for client folding and assembly to ensure the efficient production of secretory proteins.

Structural and mechanistic aspects of S-S bonds in the thioredoxin-like family of proteins

Disulfide bonds play a critical role in a variety of structural and mechanistic processes associated with proteins inside the cells and in the extracellular environment. The thioredoxin family of proteins like thioredoxin (Trx), glutaredoxin (Grx) and protein disulfide isomerase, are involved in the formation, transfer or isomerization of disulfide bonds through a characteristic thiol-disulfide exchange reaction. Here, we review the structural and mechanistic determinants behind the thiol-disulfide exchange reactions for the different enzyme types within this family, rationalizing the known experimental data in light of the results from computational studies. The analysis sheds new atomic-level insight into the structural and mechanistic variations that characterize the different enzymes in the family, helping to explain the associated functional diversity. Furthermore, we review here a pattern of stabilization/destabilization of the conserved active-site cysteine residues presented beforehand, which is fully consistent with the observed roles played by the thioredoxin family of enzymes.

Mechanistic insights on the reduction of glutathione disulfide by protein disulfide isomerase

We explore the enzymatic mechanism of the reduction of glutathione disulfide (GSSG) by the reduced a domain of human protein disulfide isomerase (hPDI) with atomistic resolution. We use classical molecular dynamics and hybrid quantum mechanics/molecular mechanics calculations at the mPW1N/6-311+G(2d,2p):FF99SB//mPW1N/6-31G(d):FF99SB level. The reaction proceeds in two stages: (i) a thiol-disulfide exchange through nucleophilic attack of the Cys53-thiolate to the GSSG-disulfide followed by the deprotonation of Cys56-thiol by Glu47-carboxylate and (ii) a second thiol-disulfide exchange between the Cys56-thiolate and the mixed disulfide intermediate formed in the first step. The Gibbs activation energy for the first stage was 18.7 kcal·mol^-1, and for the second stage, it was 7.2 kcal·mol^-1, in excellent agreement with the experimental barrier (17.6 kcal·mol^-1). Our results also suggest that the catalysis by protein disulfide isomerase (PDI) and thiol-disulfide exchange is mostly enthalpy-driven (entropy changes below 2 kcal·mol^-1 at all stages of the reaction). Hydrogen bonds formed between the backbone of His55 and Cys56 and the Cys56-thiol result in an increase in the Gibbs energy barrier of the first thiol-disulfide exchange. The solvent plays a key role in stabilizing the leaving glutathione thiolate formed. This role is not exclusively electrostatic, because an explicit inclusion of several water molecules at the density-functional theory level is a requisite to form the mixed disulfide intermediate. In the intramolecular oxidation of PDI, a transition state is only observed if hydrogen bond donors are nearby the mixed disulfide intermediate, which emphasizes that the thermochemistry of thiol-disulfide exchange in PDI is influenced by the presence of hydrogen bond donors.

Synthesis and Experimental Validation of New PDI Inhibitors with Antiproliferative Activity

Protein disulfide isomerase (PDI) is a member of the thioredoxin superfamily of redox enzymes. PDI is a multifunctional protein that catalyzes disulfide bond formation, cleavage, and rearrangement in unfolded or misfolded proteins and functions as a chaperone in the endoplasmic reticulum. Besides acting as a protein folding catalyst, several evidences have suggested that PDI can bind small molecules containing, for example, a phenolic structure, which includes the estrogenic one. Increasing studies indicate that PDI is involved in both physiology and pathophysiology of cells and tissues and is involved in the survival and proliferation of different cancers. Propionic acid carbamoyl methyl amides (PACMAs) showed anticancer activity in human ovarian cancer, both in vitro and in vivo, by inhibiting PDI. The inhibition of PDI’s activity may have a therapeutic role, in various diseases, including cancer. In the present study, we designed and synthesized a diversified small library of compounds with the aim of identifying a new class of PDI inhibitors. Most of synthesized compounds showed a good inhibitory potency against PDI and particularly 4-methyl substituted 2,6-di-tert-butylphenol derivatives (8–10) presented an antiproliferative activity in a wide panel of human cancer cell lines, including ovarian ones.

Protein Disulphide Isomerases: emerging roles of PDI and ERp57 in the nervous system and as therapeutic targets for ALS

INTRODUCTION

There is increasing evidence that endoplasmic reticulum (ER) chaperones Protein Disulphide Isomerase (PDI) and ERp57 (endoplasmic reticulum protein 57) are protective against neurodegenerative diseases related to protein misfolding, including Amyotrophic Lateral Sclerosis (ALS). PDI and ERp57 also possess disulphide interchange activity, in which protein disulphide bonds are oxidized, reduced and isomerized, to form their native conformation. Recently, missense and intronic variants of PDI and ERp57 were associated with ALS, implying that PDI proteins are relevant to ALS pathology. Areas covered: Here, we discuss possible implications of the PDI and ERp57 variants, as well as recent studies describing previously unrecognized roles for PDI and ERp57 in the nervous system. Therapeutics based on PDI may therefore be attractive candidates for ALS. However, in addition to its protective functions, aberrant, toxic roles for PDI have recently been described. These functions need to be fully characterized before effective therapeutic strategies can be designed. Expert opinion: These disease-associated variants of PDI and ERp57 provide additional evidence for an important role for PDI proteins in ALS. However, there are many questions remaining unanswered that need to be addressed before the potential of the PDI family in relation to ALS can be fully realized.

Combined ligand-observe (19)F and protein-observe (15)N,(1)H-HSQC NMR suggests phenylalanine as the key Δ-somatostatin residue recognized by human protein disulfide isomerase

Human protein disulphide isomerase (hPDI) is an endoplasmic reticulum (ER) based isomerase and folding chaperone. Molecular detail of ligand recognition and specificity of hPDI are poorly understood despite the importance of the hPDI for folding secreted proteins and its implication in diseases including cancer and lateral sclerosis. We report a detailed study of specificity, interaction and dissociation constants (K_d) of the peptide-ligand Δ-somatostatin (AGSKNFFWKTFTSS) binding to hPDI using ¹⁹F ligand-observe and ¹⁵N,¹H-HSQC protein-observe NMR methods. Phe residues in Δ-somatostatin are hypothesised as important for recognition by hPDI therefore, step-wise peptide Phe-to-Ala changes were progressively introduced and shown to raise the K_d from 103 + 47 μM until the point where binding was abolished when all Phe residues were modified to Ala. The largest step-changes in K_d involved the F11A peptide modification which implies the C-terminus of Δ-somatostatin is a prime recognition region. Furthermore, this study also validated the combined use of ¹⁹F ligand-observe and complimentary ¹⁵N,¹H-HSQC titrations to monitor interactions from the protein’s perspective. ¹⁹F ligand-observe NMR was ratified as mirroring ¹⁵N protein-observe but highlighted the advantage that ¹⁹F offers improved K_d precision due to higher spectrum resolution and greater chemical environment sensitivity.

Novel roles for protein disulphide isomerase in disease states: a double edged sword?

Protein disulphide isomerase (PDI) is a multifunctional redox chaperone of the endoplasmic reticulum (ER). Since it was first discovered 40 years ago the functions ascribed to PDI have evolved significantly and recent studies have recognized its distinct functions, with adverse as well as protective effects in disease. Furthermore, post translational modifications of PDI abrogate its normal functional roles in specific disease states. This review focusses on recent studies that have identified novel functions for PDI relevant to specific diseases.

Engineering of protein folding and secretion-strategies to overcome bottlenecks for efficient production of recombinant proteins

SIGNIFICANCE

Recombinant protein production has developed into a huge market with enormous positive implications for human health and for the future direction of a biobased economy. Limitations in the economic and technical feasibility of production processes are often related to bottlenecks of in vivo protein folding.

RECENT ADVANCES

Based on cell biological knowledge, some major bottlenecks have been overcome by the overexpression of molecular chaperones and other folding related proteins, or by the deletion of deleterious pathways that may lead to misfolding, mistargeting, or degradation.

CRITICAL ISSUES

While important success could be achieved by this strategy, the list of reported unsuccessful cases is disappointingly long and obviously dependent on the recombinant protein to be produced. Singular engineering of protein folding steps may not lead to desired results if the pathway suffers from several limitations. In particular, the connection between folding quality control and proteolytic degradation needs further attention.

FUTURE DIRECTIONS

Based on recent understanding that multiple steps in the folding and secretion pathways limit productivity, synergistic combinations of the cell engineering approaches mentioned earlier need to be explored. In addition, systems biology-based whole cell analysis that also takes energy and redox metabolism into consideration will broaden the knowledge base for future rational engineering strategies.

Structure of the substrate-binding b' domain of the Protein Disulfide Isomerase-Like protein of the Testis

Protein Disulfide Isomerase-Like protein of the Testis (PDILT) is a testis-specific member of the PDI family. PDILT displays similar domain architecture to PDIA1, the founding member of this protein family, but lacks catalytic cysteines needed for oxidoreduction reactions. This suggests special importance of chaperone activity of PDILT, but how it recognizes misfolded protein substrates is unknown. Here, we report the high-resolution crystal structure of the b′ domain of human PDILT. The structure reveals a conserved hydrophobic pocket, which is likely a principal substrate-binding site in PDILT. In the crystal, this pocket is occupied by side chains of tyrosine and tryptophan residues from another PDILT molecule, suggesting a preference for binding exposed aromatic residues in protein substrates. The lack of interaction of the b′ domain with the P-domains of calreticulin-3 and calmegin hints at a novel way of interaction between testis-specific lectin chaperones and PDILT. Further studies of this recently discovered PDI member would help to understand the important role that PDILT plays in the differentiation and maturation of spermatozoids.

Forming disulfides in the endoplasmic reticulum

Protein disulfide bonds are an important co- and post-translational modification for proteins entering the secretory pathway. They are covalent interactions between two cysteine residues which support structural stability and promote the assembly of multi-protein complexes. In the mammalian endoplasmic reticulum (ER), disulfide bond formation is achieved by the combined action of two types of enzyme: one capable of forming disulfides de novo and another able to introduce these disulfides into substrates. The initial process of introducing disulfides into substrate proteins is catalyzed by the protein disulfide isomerase (PDI) oxidoreductases which become reduced and, therefore, have to be re-oxidized to allow for further rounds of disulfide exchange. This review will discuss the various pathways operating in the ER that facilitate oxidation of the PDI oxidoreductases and ultimately catalyze disulfide bond formation in substrate proteins. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Peptide binding by catalytic domains of the protein disulfide isomerase-related protein ERp46

The protein disulfide isomerase (PDI) family member ERp46/endoPDI/thioredoxin domain-containing protein 5 is preferentially expressed in a limited number of tissues, where it may function as a survival factor for nitrosative stress in vivo. It is involved in insulin production as well as in adiponectin signaling and interacts specifically with the redox-regulatory endoplasmic reticulum proteins endoplasmic oxidoreductin 1α (Ero1α) and peroxiredoxin-4. Here, we show that ERp46, although lacking a PDI-like redox-inactive b'-thioredoxin domain with its hydrophobic substrate binding site, is able to bind to a large pool of peptides containing aromatic and basic residues via all three of its catalytic domains (a(0), a and a'), though the a(0) domain may contain the primary binding site. ERp46, which shows relatively higher activity as a disulfide-reductase than as an oxidase/isomerase in vitro compared to PDI and ERp57, possesses chaperone activity in vivo, a property also shared by the C-terminal a' domain. A crystal structure of the a' domain is also presented, offering a view of possible substrate binding sites within catalytic domains of PDI proteins.

The crystal structure of the protein-disulfide isomerase family member ERp27 provides insights into its substrate binding capabilities

About one-third of all cellular proteins pass through the secretory pathway and hence undergo oxidative folding in the endoplasmic reticulum (ER). Protein-disulfide isomerase (PDI) and related members of the PDI family assist in the folding of substrates by catalyzing the oxidation of two cysteines and isomerization of disulfide bonds as well as by acting as chaperones. In this study, we present the crystal structure of ERp27, a redox-inactive member of the PDI family. The structure reveals its substrate-binding cleft, which is homologous to PDI, but is able to adapt in size and hydrophobicity. Isothermal titration calorimetry experiments demonstrate that ERp27 is able to distinguish between folded and unfolded substrates, only interacting with the latter. ERp27 is up-regulated during ER stress, thus presumably allowing it to bind accumulating misfolded substrates and present them to ERp57 for catalysis.

Infection by chikungunya virus modulates the expression of several proteins in Aedes aegypti salivary glands

Background

Arthropod-borne viral infections cause several emerging and resurging infectious diseases. Among the diseases caused by arboviruses, chikungunya is responsible for a high level of severe human disease worldwide. The salivary glands of mosquitoes are the last barrier before pathogen transmission.

Methods

We undertook a proteomic approach to characterize the key virus/vector interactions and host protein modifications that occur in the salivary glands that could be responsible for viral transmission by using quantitative two-dimensional electrophoresis.

Results

We defined the protein modulations in the salivary glands of Aedes aegypti that were triggered 3 and 5 days after an oral infection (3 and 5 DPI) with chikungunya virus (CHIKV). Gel profile comparisons showed that CHIKV at 3 DPI modulated the level of 13 proteins, and at 5 DPI 20 proteins. The amount of 10 putatively secreted proteins was regulated at both time points. These proteins were implicated in blood-feeding or in immunity, but many have no known function. CHIKV also modulated the quantity of proteins involved in several metabolic pathways and in cell signalling.

Conclusion

Our study constitutes the first analysis of the protein response of Aedes aegypti salivary glands infected with CHIKV. We found that the differentially regulated proteins in response to viral infection include structural proteins and enzymes for several metabolic pathways. Some may favour virus survival, replication and transmission, suggesting a subversion of the insect cell metabolism by arboviruses. For example, proteins involved in blood-feeding such as the short D7, an adenosine deaminase and inosine-uridine preferring nucleoside hydrolase, may favour virus transmission by exerting an increased anti-inflammatory effect. This would allow the vector to bite without the bite being detected. Other proteins, like the anti-freeze protein, may support vector protection.

Serine/threonine protein phosphatase 5 (PP5) interacts with substrate under heat stress conditions and forms protein complex in Arabidopsis

Protein phosphatase 5 plays a pivotal role in signal transduction in animal and plant cells, and it was previously shown that Arabidopsis protein phosphatase 5 (AtPP5) performs multiple enzymatic activities that are mediated by conformational changes induced by heat shock stress. In addition, transgenic overexpression of AtPP5 gene conferred enhanced heat shock resistance compared with wild-type plant. However, the molecular mechanism underlying this enhanced heat shock tolerance through functional and conformational changes upon heat stress is not clear. In this report, AtPP5 was shown to preferentially interact with its substrate, MDH, under heat stress conditions. In addition, in co-IP analysis, AtPP5 was observed to form a complex with AtHsp90 in Arabidopsis. These results suggest that AtPP5 may enhance thermotolerance via forming multi-chaperone complexes under heat shock conditions in Arabidopsis. Finally, we show that AtPP5 is primarily localized in the cytoplasm of Arabidopsis.

Both PDI and PDIp can attack the native disulfide bonds in thermally-unfolded RNase and form stable disulfide-linked complexes

Protein disulfide isomerase (PDI) and its pancreatic homolog (PDIp) are folding catalysts for the formation, reduction, and/or isomerization of disulfide bonds in substrate proteins. However, the question as to whether PDI and PDIp can directly attack the native disulfide bonds in substrate proteins is still not answered, which is the subject of the present study. We found that RNase can be thermally unfolded at 65°C under non-reductive conditions while its native disulfide bonds remain intact, and the unfolded RNase can refold and reactivate during cooling. Co-incubation of RNase with PDI or PDIp during thermal unfolding can inactivate RNase in a PDI/PDIp concentration-dependent manner. The alkylated PDI and PDIp, which are devoid of enzymatic activities, cannot inactivate RNase, suggesting that the inactivation of RNase results from the disruption of its native disulfide bonds catalyzed by the enzymatic activities of PDI/PDIp. In support of this suggestion, we show that both PDI and PDIp form stable disulfide-linked complexes only with thermally-unfolded RNase, and RNase in the complexes can be released and reactivated dependently of the redox conditions used. The N-terminal active site of PDIp is essential for the inactivation of RNase. These data indicate that PDI and PDIp can perform thiol-disulfide exchange reactions with native disulfide bonds in unfolded RNase via formation of stable disulfide-linked complexes, and from these complexes RNase is further released.

How ricin and Shiga toxin reach the cytosol of target cells: retrotranslocation from the endoplasmic reticulum

A number of protein toxins bind at the surface of mammalian cells and after endocytosis traffic to the endoplasmic reticulum, where the toxic A chains are liberated from the holotoxin. The free A chains are then dislocated, or retrotranslocated, across the ER membrane into the cytosol. Here, in contrast to ER substrates destined for proteasomal destruction, they undergo folding to a catalytic conformation and subsequently inactivate their cytosolic targets. These toxins therefore provide toxic probes for testing the molecular requirements for retrograde trafficking, the ER processes that prepare the toxic A chains for transmembrane transport, the dislocation step itself and for the post-dislocation folding that results in catalytic activity. We describe here the dislocation of ricin A chain and Shiga toxin A chain, but also consider cholera toxin which bears a superficial structural resemblance to Shiga toxin. Recent studies not only describe how these proteins breach the ER membrane, but also reveal aspects of a fundamental cell biological process, that of ER-cytosol dislocation.

Differential protein modulation in midguts of Aedes aegypti infected with chikungunya and dengue 2 viruses

Background

Arthropod borne virus infections cause several emerging and resurgent infectious diseases. Among the diseases caused by arboviruses, dengue and chikungunya are responsible for a high rate of severe human diseases worldwide. The midgut of mosquitoes is the first barrier for pathogen transmission and is a target organ where arboviruses must replicate prior to infecting other organs. A proteomic approach was undertaken to characterize the key virus/vector interactions and host protein modifications that happen in the midgut for viral transmission to eventually take place.

Methodology and Principal Findings

Using a proteomics differential approach with two-Dimensional Differential in-Gel Electrophoresis (2D-DIGE), we defined the protein modulations in the midgut of Aedes aegypti that were triggered seven days after an oral infection (7 DPI) with dengue 2 (DENV-2) and chikungunya (CHIKV) viruses. Gel profile comparisons showed that the level of 18 proteins was modulated by DENV-2 only and 12 proteins were modulated by CHIKV only. Twenty proteins were regulated by both viruses in either similar or different ways. Both viruses caused an increase of proteins involved in the generation of reactive oxygen species, energy production, and carbohydrate and lipid metabolism. Midgut infection by DENV-2 and CHIKV triggered an antioxidant response. CHIKV infection produced an increase of proteins involved in detoxification.

Conclusion/Significance

Our study constitutes the first analysis of the protein response of Aedes aegypti's midgut infected with viruses belonging to different families. It shows that the differentially regulated proteins in response to viral infection include structural, redox, regulatory proteins, and enzymes for several metabolic pathways. Some of these proteins like antioxidant are probably involved in cell protection. On the other hand, we propose that the modulation of other proteins like transferrin, hsp60 and alpha glucosidase, may favour virus survival, replication and transmission, suggesting a subversion of the insect cell metabolism by the arboviruses.

Ero1alpha requires oxidizing and normoxic conditions to localize to the mitochondria-associated membrane (MAM)

Protein secretion from the endoplasmic reticulum (ER) requires the enzymatic activity of chaperones and oxidoreductases that fold polypeptides and form disulfide bonds within newly synthesized proteins. The best-characterized ER redox relay depends on the transfer of oxidizing equivalents from molecular oxygen through ER oxidoreductin 1 (Ero1) and protein disulfide isomerase to nascent polypeptides. The formation of disulfide bonds is, however, not the sole function of ER oxidoreductases, which are also important regulators of ER calcium homeostasis. Given the role of human Ero1alpha in the regulation of the calcium release by inositol 1,4,5-trisphosphate receptors during the onset of apoptosis, we hypothesized that Ero1alpha may have a redox-sensitive localization to specific domains of the ER. Our results show that within the ER, Ero1alpha is almost exclusively found on the mitochondria-associated membrane (MAM). The localization of Ero1alpha on the MAM is dependent on oxidizing conditions within the ER. Chemical reduction of the ER environment, but not ER stress in general leads to release of Ero1alpha from the MAM. In addition, the correct localization of Ero1alpha to the MAM also requires normoxic conditions, but not ongoing oxidative phosphorylation.

Redox-dependent domain rearrangement of protein disulfide isomerase coupled with exposure of its substrate-binding hydrophobic surface

Protein disulfide isomerase (PDI) is a major protein in the endoplasmic reticulum, operating as an essential folding catalyst and molecular chaperone for disulfide-containing proteins by catalyzing the formation, rearrangement, and breakage of their disulfide bridges. This enzyme has a modular structure with four thioredoxin-like domains, a, b, b', and a', along with a C-terminal extension. The homologous a and a' domains contain one cysteine pair in their active site directly involved in thiol-disulfide exchange reactions, while the b' domain putatively provides a primary binding site for unstructured regions of the substrate polypeptides. Here, we report a redox-dependent intramolecular rearrangement of the b' and a' domains of PDI from Humicola insolens, a thermophilic fungus, elucidated by combined use of nuclear magnetic resonance (NMR) and small-angle X-ray scattering (SAXS) methods. Our NMR data showed that the substrates bound to a hydrophobic surface spanning these two domains, which became more exposed to the solvent upon oxidation of the active site of the a' domain. The hydrogen-deuterium exchange and relaxation data indicated that the redox state of the a' domain influences the dynamic properties of the b' domain. Moreover, the SAXS profiles revealed that oxidation of the a' active site causes segregation of the two domains. On the basis of these data, we propose a mechanistic model of PDI action; the a' domain transfers its own disulfide bond into the unfolded protein accommodated on the hydrophobic surface of the substrate-binding region, which consequently changes into a "closed" form releasing the oxidized substrate.

Protein disulfide isomerase: a critical evaluation of its function in disulfide bond formation

Disulfide bond formation is probably involved in the biogenesis of approximately one third of human proteins. A central player in this essential process is protein disulfide isomerase or PDI. PDI was the first protein-folding catalyst reported. However, despite more than four decades of study, we still do not understand much about its physiological mechanisms of action. This review examines the published literature with a critical eye. This review aims to (a) provide background on the chemistry of disulfide bond formation and rearrangement, including the concept of reduction potential, before examining the structure of PDI; (b) detail the thiol-disulfide exchange reactions that are catalyzed by PDI in vitro, including a critical examination of the assays used to determine them; (c) examine oxidation and reduction of PDI in vivo, including not only the role of ERo1 but also an extensive assessment of the role of glutathione, as well as other systems, such as peroxide, dehydroascorbate, and a discussion of vitamin K-based systems; (d) consider the in vivo reactions of PDI and the determination and implications of the redox state of PDI in vivo; and (e) discuss other human and yeast PDI-family members.

A protein-based set of reference markers for liver tissues and hepatocellular carcinoma

Background

During the last decade, investigations have focused on revealing genes or proteins that are involved in HCC carcinogenesis using either genetic or proteomic techniques. However, these studies are overshadowed by a lack of good internal reference standards. The need to identify "housekeeping" markers, whose expression is stable in various experimental and clinical conditions, is therefore of the utmost clinical relevance in quantitative studies. This is the first study employed 2-DE analysis to screen for potential reference markers and aims to correlate the abundance of these proteins with their level of transcript expression.

Methods

A Chinese cohort of 224 liver tissues samples (105 cancerous, 103 non-tumourous cirrhotic, and 16 normal) was profiled using 2-DE analysis. Expression of the potential reference markers was confirmed by western blot, immunohistochemistry and real-time quantitative PCR. geNorm algorithm was employed for gene stability measure of the identified reference markers.

Results

The expression levels of three protein markers beta-actin (ACTB), heat shock protein 60 (HSP60), and protein disulphide isomerase (PDI) were found to be stable using p-values (p > 0.99) as a ranking tool in all 224 human liver tissues examined by 2-DE analysis. Of high importance, ACTB and HSP 60 were successfully validated at both protein and mRNA levels in human hepatic tissues by western blot, immunohistochemistry and real-time quantitative PCR. In addition, no significant correlation of these markers with any clinicopathological features of HCC and cirrhosis was found. Gene stability measure of these two markers with other conventionally applied housekeeping genes was assessed by the geNorm algorithm, which ranked ACTB and HSP60 as the most stable genes among this cohort of clinical samples.

Conclusion

Our findings identified 2 reference markers that exhibited stable expression across human liver tissues with different conditions thus should be regarded as reliable reference moieties for normalisation of gene and protein expression in clinical research employing human hepatic tissues.

Redox-regulated export of the major histocompatibility complex class I-peptide complexes from the endoplasmic reticulum

In contrast to the fairly well-characterized mechanism of assembly of MHC class I-peptide complexes, the disassembly mechanism by which peptide-loaded MHC class I molecules are released from the peptide-loading complex and exit the endoplasmic reticulum (ER) is poorly understood. Optimal peptide binding by MHC class I molecules is assumed to be sufficient for triggering exit of peptide-filled MHC class I molecules from the ER. We now show that protein disulfide isomerase (PDI) controls MHC class I disassembly by regulating dissociation of the tapasin-ERp57 disulfide conjugate. PDI acts as a peptide-dependent molecular switch; in the peptide-bound state, it binds to tapasin and ERp57 and induces dissociation of the tapasin-ERp57 conjugate. In the peptide-free state, PDI is incompetent to bind to tapasin or ERp57 and fails to dissociate the tapasin-ERp57 conjugates, resulting in ER retention of MHC class I molecules. Thus, our results indicate that even after optimal peptide loading, MHC class I disassembly does not occur by default but, rather, is a regulated process involving PDI-mediated interactions within the peptide-loading complex.

Novel role of protein disulfide isomerase in the regulation of NADPH oxidase activity: pathophysiological implications in vascular diseases

Vascular cell NADPH oxidase complexes are key sources of signaling reactive oxygen species (ROS) and contribute to disease pathophysiology. However, mechanisms that fine-tune oxidase-mediated ROS generation are incompletely understood. Besides known regulatory subunits, upstream mediators and scaffold platforms reportedly control and localize ROS generation. Some evidence suggest that thiol redox processes may coordinate oxidase regulation. We hypothesized that thiol oxidoreductases are involved in this process. We focused on protein disulfide isomerase (PDI), a ubiquitous dithiol disulfide oxidoreductase chaperone from the endoplasmic reticulum, given PDI's unique versatile role as oxidase/isomerase. PDI is also involved in protein traffic and can translocate to the cell surface, where it participates in cell adhesion and nitric oxide internalization. We recently provided evidence that PDI exerts functionally relevant regulation of NADPH oxidase activity in vascular smooth muscle and endothelial cells, in a thiol redox-dependent manner. Loss-of-function experiments indicate that PDI supports angiotensin II-mediated ROS generation and Akt phosphorylation. In addition, PDI displays confocal co-localization and co-immunoprecipitates with oxidase subunits, indicating close association. The mechanisms of such interaction are yet obscure, but may involve subunit assembling stabilization, assistance with traffic, and subunit disposal. These data may clarify an integrative view of oxidase activation in disease conditions, including stress responses.

The periplasmic peptidyl prolyl cis-trans isomerases PpiD and SurA have partially overlapping substrate specificities

One of the rate-limiting steps in protein folding has been shown to be the cis-trans isomerization of proline residues, catalysed by a range of peptidyl prolyl cis-trans isomerases (PPIases). In the periplasmic space of Escherichia coli and other Gram-negative bacteria, two PPIases, SurA and PpiD, have been identified, which show high sequence similarity to the catalytic domain of the small PPIase parvulin. This observation raises a question regarding the biological significance of two apparently similar enzymes present in the same cellular compartment: do they interact with different substrates or do they catalyse different reactions? The substrate-binding motif of PpiD has not been characterized so far, and no biochemical data were available on how this folding catalyst recognizes and interacts with substrates. To characterize the interaction between model peptides and the periplasmic PPIase PpiD from E. coli, we employed a chemical crosslinking strategy that has been used previously to elucidate the interaction of substrates with SurA. We found that PpiD interacted with a range of model peptides independently of whether they contained proline residues or not. We further demonstrate here that PpiD and SurA interact with similar model peptides, and therefore must have partially overlapping substrate specificities. However, the binding motif of PpiD appears to be less specific than that of SurA, indicating that the two PPIases might interact with different substrates. We therefore propose that, although PpiD and SurA have partially overlapping substrate specificities, they fulfil different functions in the cell.

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.

Production of active eukaryotic proteins through bacterial expression systems: a review of the existing biotechnology strategies

Among the various expression systems employed for the over-production of proteins, bacteria still remains the favorite choice of a Protein Biochemist. However, even today, due to the lack of post-translational modification machinery in bacteria, recombinant eukaryotic protein production poses an immense challenge, which invariably leads to the production of biologically in-active protein in this host. A number of techniques are cited in the literature, which describe the conversion of inactive protein, expressed as an insoluble fraction, into a soluble and active form. Overall, we have divided these methods into three major groups: Group-I, where the factors influencing the formation of insoluble fraction are modified through a stringent control of the cellular milieu, thereby leading to the expression of recombinant protein as soluble moiety; Group-II, where protein is refolded from the inclusion bodies and thereby target protein modification is avoided; Group-III, where the target protein is engineered to achieve soluble expression through fusion protein technology. Even within the same family of proteins (e.g., tyrosine kinases), optimization of standard operating protocol (SOP) may still be required for each protein's over-production at a pilot-scale in Escherichia coli. However, once standardized, this procedure can be made amenable to the industrial production for that particular protein with minimum alterations.

Production of active eukaryotic proteins through bacterial expression systems: a review of the existing biotechnology strategies

Among the various expression systems employed for the over-production of proteins, bacteria still remains the favorite choice of a Protein Biochemist. However, even today, due to the lack of post-translational modification machinery in bacteria, recombinant eukaryotic protein production poses an immense challenge, which invariably leads to the production of biologically in-active protein in this host. A number of techniques are cited in the literature, which describe the conversion of inactive protein, expressed as an insoluble fraction, into a soluble and active form. Overall, we have divided these methods into three major groups: Group-I, where the factors influencing the formation of insoluble fraction are modified through a stringent control of the cellular milieu, thereby leading to the expression of recombinant protein as soluble moiety; Group-II, where protein is refolded from the inclusion bodies and thereby target protein modification is avoided; Group-III, where the target protein is engineered to achieve soluble expression through fusion protein technology. Even within the same family of proteins (e.g., tyrosine kinases), optimization of standard operating protocol (SOP) may still be required for each protein's over-production at a pilot-scale in Escherichia coli. However, once standardized, this procedure can be made amenable to the industrial production for that particular protein with minimum alterations.

The endoplasmic reticulum: folding, calcium homeostasis, signaling, and redox control

The endoplasmic reticulum (ER) plays a major role in regulating synthesis, folding, and orderly transport of proteins. It is also essentially involved in various cellular signaling processes, primarily by its function as a dynamic Ca(2+) store. Compared to the cytosol, oxidizing conditions are found in the ER that allow oxidation of cysteine residues in nascent polypeptide chains to form intramolecular disulfide bonds. However, compounds and enzymes such as PDI that catalyze disulfide bonds become reduced and have to be reoxidized for further catalytic cycles. A number of enzymes, among them products of the ERO1 gene, appear to provide oxidizing equivalents, and oxygen appears to be the final oxidant in aerobic living organisms. Thus, protein oxidation in the ER is connected with generation of reactive oxygen species (ROS). Changes in the redox state and the presence of ROS also affect the Ca(2+) homeostasis by modulating the functionality of ER-based channels and buffering chaperones. In addition, a close relationship exists between oxidative stress and ER stress, which both may activate signaling events leading to a rebalance of folding capacity and folding demand or to cell death. Thus, redox homeostasis appears to be a prerequisite for proper functioning of the ER.

Protein disulfides and protein disulfide oxidoreductases in hyperthermophiles

Disulfide bonds are required for the stability and function of a large number of proteins. Recently, the results from genome analysis have suggested an important role for disulfide bonds concerning the structural stabilization of intracellular proteins from hyperthermophilic Archaea and Bacteria, contrary to the conventional view that structural disulfide bonds are rare in proteins from Archaea. A specific protein, known as protein disulfide oxidoreductase (PDO) is recognized as a potential key player in intracellular disulfide-shuffling in hyperthermophiles. The structure of this protein shows a combination of two thioredoxin-related units with low sequence identity which together, in tandem-like manner, form a closed protein domain. Each of these units contains a distinct CXXC active site motif. Due to their estimated conformational energies, both sites are likely to have different redox properties. The observed structural and functional characteristics suggest a relation to eukaryotic protein disulfide isomerase. Functional studies have revealed that both the archaeal and bacterial forms of this protein show oxidative and reductive activity and are able to isomerize protein disulfides. The physiological substrates and reduction systems, however, are to date unknown. The variety of active site disulfides found in PDOs from hyperthermophiles is puzzling. Nevertheless, the catalytic function of any PDO is expected to be correlated with the redox properties of its active site disulfides CXXC and with the distinct nature of its redox environment. The residues around the two active sites form two grooves on the protein surface. In analogy to a similar groove in thioredoxin, both grooves are suggested to constitute the substrate binding sites of PDO. The direct neighbourhood of the grooves and the different redox properties of both sites may favour sequential reactions in protein disulfide shuffling, like reduction followed by oxidation. A model for peptide binding by PDO is proposed to be derived from the analysis of crystal packing contacts mimicking substrate binding interactions. It is assumed, that PDO enzymes in hyperthermophilic Archaea and Bacteria may be part of a complex system involved in the maintenance of protein disulfide bonds. The regulation of disulfide bond formation may be dependent on a distinct interplay of thermodynamic and kinetic effects, including functional asymmetry and substrate-mediated protection of the active sites, in analogy to the situation in protein disulfide isomerase. Numerous questions related to the function of PDO enzymes in hyperthermophiles remain unanswered to date, but can probably successfully be studied by a number of approaches, such as first-line genetic and in vivo studies.

Regulation of NAD(P)H oxidase by associated protein disulfide isomerase in vascular smooth muscle cells

NAD(P)H oxidase, the main source of reactive oxygen species in vascular cells, is known to be regulated by redox processes and thiols. However, the nature of thiol-dependent regulation has not been established. Protein disulfide isomerase (PDI) is a dithiol/disulfide oxidoreductase chaperone of the thioredoxin superfamily involved in protein processing and translocation. We postulated that PDI regulates NAD(P)H oxidase activity of rabbit aortic smooth muscle cells (VSMCs). Western blotting confirmed robust PDI expression and shift to membrane fraction after incubation with angiotensin II (AII, 100 nm, 6 h). In VSMC membrane fraction, PDI antagonism with bacitracin, scrambled RNase, or neutralizing antibody led to 26-83% inhibition (p < 0.05) of oxidase activity. AII incubation led to significant increase in oxidase activity, accompanied by a 6-fold increase in PDI refolding isomerase activity. AII-induced NAD(P)H oxidase activation was inhibited by 57-71% with antisense oligonucleotide against PDI (PDIasODN). Dihydroethidium fluorescence showed decreased superoxide generation due to PDIasODN. Confocal microscopy showed co-localization between PDI and the oxidase subunits p22(phox), Nox1, and Nox4. Co-immunoprecipitation assays supported spatial association between PDI and oxidase subunits p22(phox), Nox1, and Nox4 in VSMCs. Moreover, in HEK293 cells transfected with green fluorescent protein constructs for Nox1, Nox2, and Nox4, each of these subunits co-immunoprecipitated with PDI. Akt phosphorylation, a known downstream pathway of AII-driven oxidase activation, was significantly reduced by PDIasODN. These results suggest that PDI closely associates with NAD(P)H oxidase and acts as a novel redox-sensitive regulatory protein of such enzyme complex, potentially affecting subunit traffic/assembling.

Oxidative folding of conotoxins sharing an identical disulfide bridging framework

Conotoxins are short, disulfide-rich peptide neurotoxins produced in the venom of predatory marine cone snails. It is generally accepted that an estimated 100,000 unique conotoxins fall into only a handful of structural groups, based on their disulfide bridging frameworks. This unique molecular diversity poses a protein folding problem of relationships between hypervariability of amino acid sequences and mechanism(s) of oxidative folding. In this study, we present a comparative analysis of the folding properties of four conotoxins sharing an identical pattern of cysteine residues forming three disulfide bridges, but otherwise differing significantly in their primary amino acid sequence. Oxidative folding properties of M-superfamily conotoxins GIIIA, PIIIA, SmIIIA and RIIIK varied with respect to kinetics and thermodynamics. Based on rates for establishing the steady-state distribution of the folding species, two distinct folding mechanisms could be distinguished: first, rapid-collapse folding characterized by very fast, but low-yield accumulation of the correctly folded form; and second, slow-rearrangement folding resulting in higher accumulation of the properly folded form via the reshuffling of disulfide bonds within folding intermediates. Effects of changing the folding conditions indicated that the rapid-collapse and the slow-rearrangement mechanisms were mainly determined by either repulsive electrostatic or productive noncovalent interactions, respectively. The differences in folding kinetics for these two mechanisms were minimized in the presence of protein disulfide isomerase. Taken together, folding properties of conotoxins from the M-superfamily presented in this work and from the O-superfamily published previously suggest that conotoxin sequence diversity is also reflected in their folding properties, and that sequence information rather than a cysteine pattern determines the in vitro folding mechanisms of conotoxins.

Catalysis of Creatine Kinase Refolding by Protein Disulfide Isomerase Involves Disulfide Cross-link and Dimer to Tetramer Switch

Protein disulfide isomerase (PDI) functions as an isomerase to catalyze thiol:disulfide exchange, as a chaperone to assist protein folding, and as a subunit of prolyl-4-hydroxylase and microsomal triglyceride transfer protein. At a lower concentration of 0.2 microm, PDI facilitated the aggregation of unfolded rabbit muscle creatine kinase (CK) and exhibited anti-chaperone activity, which was shown to be mainly due to the hydrophobic interactions between PDI and CK and was independent of the cross-linking of disulfide bonds. At concentrations above 1 microm, PDI acted as a protector against aggregation but an inhibitor of reactivation during CK refolding. The inhibition effect of PDI on CK reactivation was further characterized as due to the formation of PDI-CK complexes through intermolecular disulfide bonds, a process involving Cys-36 and Cys-295 of PDI. Two disulfide-linked complexes containing both PDI and CK were obtained, and the large, soluble aggregates around 400 kDa were composed of 1 molecule of tetrameric PDI and 2 molecules of inactive intermediate dimeric CK, whereas the smaller one, around 200 kDa, was formed by 1 dimeric PDI and 1 dimeric CK. To our knowledge this is the first study revealing that PDI could switch its conformation from dimer to tetramer in its functions as a foldase. According to the observations in this research and our previous study of the folding pathways of CK, a working model was proposed for the molecular mechanism of CK refolding catalyzed by PDI.

Increased catalytic activity of protein disulfide isomerase using aromatic thiol based redox buffers

PDI is an enzyme that acts as a chaperone, shufflase, and oxidase during the folding of disulfide-containing proteins. The ability of aromatic thiols to increase the activity of PDI-catalyzed protein folding over that of the standard thiol glutathione (GSH) was measured. 4-Mercaptobenzoic acid (ArSH) increased the activity of PDI by a factor of three.

Glutathione directly reduces an oxidoreductase in the endoplasmic reticulum of mammalian cells

The formation of disulfide bonds is an essential step in the folding of many glycoproteins and secretory proteins. Non-native disulfide bonds are often formed between incorrect cysteine residues, and thus the cell has dedicated a family of oxidoreductases that are thought to isomerize non-native bonds. For an oxidoreductase to be capable of performing isomerization or reduction reactions, it must be maintained in a reduced state. Here we show that most of the oxidoreductases are predominantly reduced in vivo. Following oxidative stress the oxidoreductases are quickly reduced, demonstrating that a robust reductive pathway is in place in mammalian cells. Using ERp57 as a model we show that the reductive pathway is cytosol-dependent and that the component responsible for the reduction of the oxidoreductases is the low molecular mass thiol glutathione. In addition, ERp57 is not reduced following oxidative stress when inhibitors of glutathione synthesis or glutathione reduction are added to cells. Glutathione directly reduces ERp57 at physiological concentrations in vitro, and biotinylated glutathione forms a mixed disulfide with ERp57 in microsomes. Our results demonstrate that glutathione plays a direct role in the isomerization of disulfide bonds by maintaining the mammalian oxidoreductases in a reduced state.

Functional differences between human and yeast protein disulfide isomerase family proteins

Previously, it has been reported that a mammalian protein disulfide isomerase (PDI), when expressed on a single copy number plasmid, can rescue growth of a PDI1-disrupted yeast. However, here, for the first time we demonstrated by tetrad analysis that human PDI (hPDI) is unable to replace yeast PDI (yPDI) when hPDI cDNA is integrated into the yeast chromosome. This observation indicates that hPDI is not functionally equivalent to yPDI. Estimation of the actual copy number of the plasmid, as well as comparison of isomerase and chaperone activities between human and yeast PDI homologues, indicates that one copy of hPDI cDNA is not sufficient to rescue the PDI1-disrupted strain. Notably, the isomerase activities of yPDI family proteins, Mpd1p, Mpd2p, and Eug1p, were extremely low, although yPDI itself exhibited twice as much isomerase activity as hPDI in vitro. Moreover, with the exception of Mpd1p, all hPDI and yPDI family proteins had chaperone activity, this being particularly strong in the case of yPDI and Mpd2p. These observations indicate that the growth of Saccharomyces cerevisiae is completely dependent on the isomerase activity of yPDI.

The role of thiols and disulfides in platelet function

Disulfide bonds formed in newly synthesized proteins in the endoplasmic reticulum of cells are important for protein structure and stability. Recent research, however, emphasizes a role for thiol-disulfide reactions with disulfide bond rearrangement as a dynamic process in cell and protein function, and in platelet function in particular. Protein disulfide isomerase was found on the platelet surface where it appears to play an important role in the platelet responses of aggregation and secretion, as well as activation of the platelet fibrinogen receptor, the alphaIIbbeta3 integrin. Additionally, sulfhydryl groups in alphaIIbbeta3 have been implicated in the activation of this integrin. Physiologic concentrations of reduced glutathione generate sulfhydryls in alphaIIbbeta3 and potentiate sulfhydryl-dependent reactions in alphaIIbbeta3. Sulfhydryl labeling in alphaIIbbeta3 is inhibited by phenylarsine oxide, a reagent that binds to vicinal thiols. As vicinal thiols are in equilibrium with disulfide bonds, they provide redox-sensitive sites in alphaIIbbeta3 able to respond to external or cytoplasmic reducing equivalents. Furthermore, protein disulfide isomerase and sulfhydryls are now implicated in platelet adhesion by a second platelet integrin, the alpha2beta1 collagen receptor. Most recently, extracellular sulfhydryls in the P2Y12 ADP receptor were found to be required for platelet activation by this receptor. We here provide an overview of this field with a focus on recent developments, and conclude with a working model.

Replacement of domain b of human protein disulfide isomerase-related protein with domain b' of human protein disulfide isomerase dramatically increases its chaperone activity

We have reported that human protein disulfide isomerase-related protein (hPDIR) has isomerase and chaperone activities that are lower than those of the human protein disulfide isomerase (hPDI), and that the b domain of hPDIR is critical for its chaperone activity [J. Biol. Chem. 279 (2004) 4604]. To investigate the basis of the differences between hPDI and hPDIR, and to determine the functions of each hPDIR domain in detail, we constructed several hPDIR domain mutants. Interestingly, when the b domain of hPDIR was replaced with the b' domain of hPDI, a dramatic increase in chaperone activity that was close to that of hPDI itself was observed. However, this mutant showed decreased oxidative refolding of alpha1-antitrypsin. The replacement of the b domain of hPDIR with the c domain of hPDI also increased its chaperone activity. These observations suggest that putative peptide-binding sites of hPDI determine both its chaperone activity and its substrate specificity.

Molecular Characterization of the Principal Substrate Binding Site of the Ubiquitous Folding Catalyst Protein Disulfide Isomerase

Disulfide bond formation in the endoplasmic reticulum of eukaryotes is catalyzed by the ubiquitously expressed enzyme protein disulfide isomerase (PDI). The effectiveness of PDI as a catalyst of native disulfide bond formation in folding polypeptides depends on the ability to catalyze disulfide-dithiol exchange, to bind non-native proteins, and to trigger conformational changes in the bound substrate, allowing access to buried cysteine residues. It is known that the b' domain of PDI provides the principal peptide binding site of PDI and that this domain is critical for catalysis of isomerization but not oxidation reactions in protein substrates. Here we use homology modeling to define more precisely the boundaries of the b' domain and show the existence of an intradomain linker between the b' and a' domains. We have expressed the recombinant b' domain thus defined; the stability and conformational properties of the recombinant product confirm the validity of the domain boundaries. We have modeled the tertiary structure of the b' domain and identified the primary substrate binding site within it. Mutations within this site, expressed both in the isolated domain and in full-length PDI, greatly reduce the binding affinity for small peptide substrates, with the greatest effect being I272W, a mutation that appears to have no structural effect.

Different Contributions of the Three CXXC Motifs of Human Protein-disulfide Isomerase-related Protein to Isomerase Activity and Oxidative Refolding

Human protein-disulfide isomerase (hPDI)-related protein (hPDIR), which we previously cloned from a human placental cDNA library (Hayano, T., and Kikuchi, M. (1995) FEBS Lett. 372, 210-214), and its mutants were expressed in the Escherichia coli pET system and purified by sequential nickel affinity resin chromatography. Three thioredoxin motifs (CXXC) of purified hPDIR were found to contribute to its isomerase activity with a rank order of CGHC > CPHC > CSMC, although both the isomerase and chaperone activities of this protein were lower than those of hPDI. Screening for hPDIR-binding proteins using a T7 phage display system revealed that alpha1-antitrypsin binds to hPDIR. Surface plasmon resonance experiments demonstrated that alpha1-antitrypsin interacts with hPDIR, but not with hPDI or human P5 (hP5). Interestingly, the rate of oxidative refolding of alpha1-antitrypsin with hPDIR was much higher than with hPDI or hP5. Thus, the substrate specificity of hPDIR differed from that associated with isomerase activity, and the contribution of the CSMC motif to the oxidative refolding of alpha1-antitrypsin was the most definite of the three (CSMC, CGHC, CPHC). Substitution of SM and PH in the CXXC motifs with GH increased isomerase activity and decreased oxidative refolding. In contrast, substitution of GH and PH with SM decreased isomerase activity and increased oxidative refolding. Because CXXC motif mutants lacking isomerase activity retain chaperone activity for the substrate rhodanese, it is clear that, similar to PDI and hP5, the isomerase and chaperone activities of hPDIR are independent. These results suggest that the central dipeptide of the CXXC motif is critical for both redox activity and substrate specificity.

Mutations in domain a' of protein disulfide isomerase affect the folding pathway of bovine pancreatic ribonuclease A

Protein disulfide isomerase (PDI, EC 5.3.4.1), an enzyme and chaperone, catalyses disulfide bond formation and rearrangements in protein folding. It is also a subunit in two proteins, the enzyme collagen prolyl 4-hydroxylase and the microsomal triglyceride transfer protein. It consists of two catalytically active domains, a and a', and two inactive ones, b and b', all four domains having the thioredoxin fold. Domain b' contains the primary peptide binding site, but a' is also critical for several of the major PDI functions. Mass spectrometry was used here to follow the folding pathway of bovine pancreatic ribonuclease A (RNase A) in the presence of three PDI mutants, F449R, Delta455-457, and abb', and the individual domains a and a'. The first two mutants contained alterations in the last alpha helix of domain a', while the third lacked the entire domain a'. All mutants produced genuine, correctly folded RNase A, but the appearance rate of 50% of the product, as compared to wild-type PDI, was reduced 2.5-fold in the case of PDI Delta455-457, 7.5-fold to eightfold in the cases of PDI F449R and PDI abb', and over 15-fold in the cases of the individual domains a and a'. In addition, PDI F449R and PDI abb' affected the distribution of folding intermediates. Domains a and a' catalyzed the early steps in the folding but no disulfide rearrangements, and therefore the rate observed in the presence of these individual domains was similar to that of the spontaneous process.