Protein disulfide isomerase, a multifunctional protein chaperone, shows copper-binding activity

Protein disulfide isomerase (PDI) is a 55 kDa multifunctional protein of the endoplasmic reticulum (ER) involved in protein folding and isomerization. In addition to the chaperone and catalytic functions, PDI is a major calcium-binding protein of the ER. Although the active site of PDI has a similar motif CXXC to the Cu-binding motif in Wilson and Menkes proteins and in other copper chaperones, there has been no report on any metal-binding capability of PDI other than calcium binding. We present evidence that PDI is a copper-binding protein. In the absence of reducing agent freshly reduced PDI can bind a maximum of 4 mol of Cu(II) and convert to Cu(I). These bound Cu(I) are surface exposed as they can be competed readily by BCS reagent, a Cu(I) specific chelator. However, when the binding is performed using the mixture of Cu(II) and 1mM DTT, the total number of Cu(I) bound increases to 10 mol/mol, and it is slower to react with BCS, indicating a more protected environment. In both cases, the copper-bound forms of PDI exist as tetramers while apo-protein is a monomer. These findings suggest that PDI plays a role in intracellular copper disposition.     

Ribostamycin inhibits the chaperone activity of protein disulfide isomerase.

In the process of screening of proteins binding to ribostamycin in bovine liver using the affinity column chromatography, we found that ribostamycin inhibited the chaperone activity of protein disulfide isomerase (PDI), but it did not inhibit the isomerase activity. PDI was identified by SDS-PAGE, Western blotting, and N-terminal amino acid sequence analysis. A 100:1 molar ratio of ribostamycin to PDI was almost sufficient to completely inhibit the chaperone activity of PDI. The binding affinity of ribostamycin to purified bovine PDI was determined by the Biacore system, which gave a K(D) value of 3.19 x 10(-4) M. This suggests that ribostamycin binds to region distinct from the CGHC motif of PDI. This is the first report to describe the inhibitor of the chaperone activity of PDI.     

The low-density lipoprotein receptor-related protein associates with calnexin, calreticulin, and protein disulfide isomerase in receptor-associated-protein-deficient fibroblasts

The low-density lipoprotein receptor-related protein (LRP) is a large (>600 kDa) multi-ligand-binding cell surface receptor that is now known to participate in a diverse range of cellular events. To accomplish this diverse role, LRP is composed of repetitive amino acid motifs consisting of complement-type and EGF precursor-type repeats. Within these repeats are six conserved cysteine residues that form the core disulfide bond structure of each repeat. To accommodate the intricate folding that such a complex structure dictates, a specialized chaperone is present in the endoplasmic reticulum (ER) called the receptor-associated protein (RAP) that binds to LRP immediately following its biosynthesis and assists in its exocytic transport. Interestingly, RAP -/- mice show reduced LRP expression in certain cell types, but not a more global affect on LRP expression that was expected. Such a tissue-restricted effect by RAP prompted an investigation if other ER chaperones associate with LRP to assist in its complex folding requirements and compensate for the absence of RAP in RAP -/- cells. Fibroblasts obtained from RAP -/- mice demonstrate similar LRP expression levels and subcellular distribution as RAP +/+ fibroblasts. Moreover, RAP -/- cells show an identical exocytic trafficking rate for LRP as RAP +/+ cells and comparable cell surface internalization kinetics. In RAP -/- cells, three well-known ER chaperones, calnexin, calreticulin, and protein disulfide isomerase (PDI), associate with LRP and likely compensate for the absence of RAP.     

The acidic C-terminal domain stabilizes the chaperone function of protein disulfide isomerase

Protein disulfide isomerase (PDI, EC 5.3.4.1) is a chaperone and catalyzes the formation and rearrangement of disulfide bonds in proteins. Domain c-(463-491), containing 18 acidic residues, is an interesting and important C-terminal extension of PDI. In this study, the PDI mutant abb'a', in which domain c is truncated, was used to investigate the relationship between the C-terminal structure and chaperone function. Reactivation and light-scattering experiments show that both wild-type PDI and abb'a' interact with lactate dehydrogenase (LDH, EC 1.1.1.27), which tends to self-aggregate during reactivation. The interaction enhances reactivation of LDH and reduces aggregation. According to these results, it seems as if domain c might be dispensable to the chaperone function of PDI. However, abb'a' is prone to self-aggregation and causes increased aggregation of LDH during thermal denaturation. In contrast, wild-type PDI remains active as a chaperone under these conditions and prevents self-aggregation of LDH. Furthermore, measurements of intrinsic fluorescence and difference absorbance during denaturation show that abb'a' is much more labile to heat or guanidine hydrochloride denaturation than wild-type PDI. This suggests that domain c is required for the stabilization and maintenance of the chaperone function of PDI under extreme conditions.     

Both chaperone and isomerase functions of protein disulfide isomerase are essential for acceleration of the oxidative refolding and reactivation of dimeric alkaline protease inhibitor

Oxidative refolding of the dimeric alkaline protease inhibitor (API) from Streptomyces sp. NCIM 5127 has been investigated. We demonstrate here that both isomerase and chaperone functions of the protein folding catalyst, protein disulfide isomerase (PDI), are essential for efficient refolding of denatured-reduced API (dr-API). Although the role of PDI as an isomerase and a chaperone has been reported for a few monomeric proteins, its role as a foldase in refolding of oligomeric proteins has not been demonstrated hitherto. Spontaneous refolding and reactivation of dr-API in redox buffer resulted in 45% to 50% reactivation. At concentrations <0.25 microM, reactivation rates and yields of dr-API are accelerated by catalytic amounts of PDI through its isomerase activity, which promotes disulfide bond formation and rearrangement. dr-API is susceptible to aggregation at concentrations >25 microM, and a large molar excess of PDI is required to enhance reactivation yields. PDI functions as a chaperone by suppressing aggregation and maintains the partially unfolded monomers in a folding-competent state, thereby assisting dimerization. Simultaneously, isomerase function of PDI brings about regeneration of native disulfides. 5-Iodoacetamidofluorescein-labeled PDI devoid of isomerase activity failed to enhance the reactivation of dr-API despite its intact chaperone activity. Our results on the requirement of a stoichiometric excess of PDI and of presence of PDI in redox buffer right from the initiation of refolding corroborate that both the functions of PDI are essential for efficient reassociation, refolding, and reactivation of dr-API.     

Mitochondrial heat shock protein 70, a molecular chaperone for proteins encoded by mitochondrial DNA

The protracted absence of muscle activation initiates complex cellular and molecular reactions aimed at restoring functional neuromuscular transmission and preventing degenerative processes. A central aspect of these reactions is the sprouting of intramuscular nerves in the vicinity of inactivated muscle fibers. Sprouts emerging from terminal nerve branches and nodes of Ranvier can reestablish functional contacts with inactive muscle fibers, and this is an essential restorative process in pathological conditions of the neuromuscular system. Due to their rapid upregulation in inactive skeletal muscle fibers and their ability to induce nerve sprouting in adult muscle, insulin-like growth factors (IGFs) are candidate signaling molecules to promote restorative reactions in the neuromuscular system. In this study we have exploited the high affinity and specificity of IGF-binding protein 4 (IGF-BP4) and IGF-BP5 for IGF1 and IGF2 to determine whether these growth factors are involved in the nerve sprouting reaction in paralyzed skeletal muscle. In tissue culture experiments with sensory- and motoneurons we demonstrate that the neurite promoting activity of IGF1 is blocked by IGF-BP4, and that a similar IGF-BP-sensitive activity is detected in muscle extracts from paralyzed, but not from control muscle. In in vivo experiments, we show that local delivery of IGF-BP4 to Botulinum toxin A-paralyzed skeletal muscle effectively prevents nerve sprouting in that muscle. Our findings indicate that muscle IGFs play an essential role in intramuscular nerve sprouting. In addition, these findings suggest that IGFs are major signaling factors from inactivated muscle to promote local restorative reactions, including interstitial cell proliferation and nerve sprouting.     

Both the isomerase and chaperone activities of protein disulfide isomerase are required for the reactivation of reduced and denatured acidic phospholipase A2.

The spontaneous reactivation yield of acidic phospholipase A2 (APLA2), a protein containing seven disulfide bonds, after reduction and denaturation in guanidine hydrochloride is very low. Protein disulfide isomerase (PDI) markedly increases the reactivation yield and prevents the aggregation of APLA2 during refolding in a redox buffer containing GSH and GSSG. S-methylated PDI (mPDI), with no isomerase but as nearly full chaperone activity as native PDI, has no effect on either the reactivation or aggregation of APLA2. However, the simultaneous presence of PDI and mPDI in molar ratios to APLA2 of 0.1 and 0.9 respectively fully reactivates the denatured enzyme, as does PDI alone at a ratio of 1. At ratios of 0.1 and 0.15 respectively, they completely suppress APLA2 aggregation, as does PDI alone at a ratio of 0.25. Moreover, delayed addition of PDI to the refolding buffer greatly diminished the reactivation yield of APLA2, but this deteriorating effect can be alleviated markedly by the presence of mPDI in the refolding buffer. Without GSSG, mPDI prevents the aggregation of APLA2 during refolding. It is proposed that the in vitro action of PDI as a foldase consists of both isomerase and chaperone activities, and the latter activity can be fully replaced by mPDI.     

Inhibition of glucose-regulated and heat shock protein induction by low temperature

The present study evaluating induction of the major stress proteins in the subphysiological temperature range (25-33 degrees C) shows that none of the agents used could effectively induce the heat shock proteins (hsp) or the glucose related protein grp95 at low temperature. However, grp82 was still induced by some amino acid analogs and by glucose deprivation while certain oxygen-regulated proteins were still induced by hypoxia at 25 degrees C. Analogs were incorporated and protein turnover was increased at low temperature even though most stress proteins were not induced. Synthesis of hsps, but not that of grps, was induced if cultures containing analog-substituted proteins were shifted to 37 degrees C. Temperature dependence of hsp induction by arsenite showed a sharp threshold between 30 degrees C and 33 degrees C. Low temperature inhibition of induction points to the existence of a temperature-dependent mechanism operating within the normal physiological temperature range and may be a useful parameter in evaluating proposed mechanisms of stress protein regulation.     

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.     

Inhibitors of protein disulfide isomerase suppress apoptosis induced by misfolded proteins

A hallmark of many neurodegenerative diseases is accumulation of misfolded proteins within neurons, leading to cellular dysfunction and cell death. Although several mechanisms have been proposed to link protein misfolding to cellular toxicity, the connection remains enigmatic. Here, we report a cell death pathway involving protein disulfide isomerase (PDI), a protein chaperone that catalyzes isomerization, reduction and oxidation of disulfides. Through a small molecule screening approach, we discovered five structurally distinct compounds that prevent apoptosis induced by mutant huntingtin protein. Using modified Huisgen cycloaddition chemistry, we then identified PDI as the molecular target of these small molecules. Expression of polyglutamine-expanded huntingtin exon 1 in PC12 cells caused PDI to accumulate at mitochondrial-associated ER membranes and trigger apoptotic cell death via mitochondrial outer-membrane permeabilization. Inhibiting PDI in rat brain cells suppressed the toxicity of mutant huntingtin exon 1 and Aβ peptides processed from the amyloid precursor protein. This pro-apoptotic function of PDI represents a new mechanism linking protein misfolding and apoptotic cell death.     

Antagonistic effects of insulin and dexamethasone on glucose-regulated and heat shock protein synthesis

The present study extends our previous observation (Kasambalides and Lanks, J. Cell. Physiol., 114:93-98, 1983), that dexamethasone inhibits the alterations in heat shock protein (HSP) and glucose-regulated protein (GRP) synthesis caused by glucose deprivation. We now show that insulin, even in the presence of high extracellular glucose concentrations, will induce 95K and 82K GRP synthesis while suppressing 85K and 69K HSP synthesis. Heat shock of insulin-treated cultures causes induction of the 82K GRP rather than the 85K and 69K HSP's. All of the insulin effects are antagonized by dexamethasone. These data suggest that the changes in GRP and HSP synthesis induced by glucose deprivation and heat shock, respectively, may reflect the operation of a normal physiological mechanism that regulates glucose metabolism.     

Protein disulfide isomerase: the multifunctional redox chaperone of the endoplasmic reticulum

Protein disulfide isomerase (PDI) is a protein-thiol oxidoreductase that catalyzes the oxidation, reduction and isomerization of protein disulfides. In the endoplasmic reticulum PDI catalyzes both the oxidation and isomerization of disulfides on nascent polypeptides. Under the reducing condition of the cytoplasm, endosomes and cell surface. PDI catalyzes the reduction of protein disulfides. At those locations, PDI has been demonstrated to participate in the regulation of reception function, cell-cell interaction, gene expression, and actin filament polymerization. These activities of PDI will be discussed, as well as its activity as a chaperone and subunit of prolyl 4-hydroxylase and microsomal triglyceride transfer protein.     

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.     

Display of disulfide-rich proteins by complementary DNA display and disulfide shuffling assisted by protein disulfide isomerase

We report an efficient system to produce and display properly folded disulfide-rich proteins facilitated by coupled complementary DNA (cDNA) display and protein disulfide isomerase-assisted folding. The results show that a neurotoxin protein containing four disulfide linkages can be displayed in the folded state. Furthermore, it can be refolded on a solid support that binds efficiently to its natural acetylcholine receptor. Probing the efficiency of the display proteins prepared by these methods provided up to 8-fold higher enrichment by the selective enrichment method compared with cDNA display alone, more than 10-fold higher binding to its receptor by the binding assays, and more than 10-fold higher affinities by affinity measurements. Cotranslational folding was found to have better efficiency than posttranslational refolding between the two investigated methods. We discuss the utilities of efficient display of such proteins in the preparation of superior quality proteins and protein libraries for directed evolution leading to ligand discovery.     

The thioredoxin-like fold: hidden domains in protein disulfide isomerases and other chaperone proteins

Although protein disulphide isomerase (PDI) has been known for nearly 40 years, several new PDIs have recently been described that reveal a remarkable diversity in both structure and function. This article reviews our current knowledge of the PDI family members and identifies four novel PDIs in the human genome. These include human transmembrane proteins that have C. elegans or Drosophila orthologues for which a developmental role has been proven. Their role in development, together with other functional roles for PDIs such as conferring resistance to apoptosis under hypoxia and a potential role in the oxygen-sensing apparatus are discussed.     

Small heat shock proteins, protein degradation and protein aggregation diseases

Small heat shock proteins have been characterized in vitro as ATP-independent molecular chaperones that can prevent aggregation of un- or mis-folded proteins and assist in their refolding with the help of ATP-dependent chaperone machines (e.g., the Hsp70 proteins). Comparison of the functionality of the 10 human members of the small HSPB family in cell models now reveals that some members function entirely differently and independently from Hsp70 machines. One member, HSPB7, has strong activities to prevent toxicity of polyglutamine-containing proteins in cells and Drosophila, and seems to act by assisting the loading of misfolded proteins or small protein aggregates into autophagosomes.     

Besnoitia besnoiti protein disulfide isomerase (BbPDI): molecular characterization, expression and in silico modelling

Besnoitia besnoiti is an apicomplexan parasite responsible for bovine besnoitiosis, a disease with a high prevalence in tropical and subtropical regions and re-emerging in Europe. Despite the great economical losses associated with besnoitiosis, this disease has been underestimated and poorly studied, and neither an effective therapy nor an efficacious vaccine is available. Protein disulfide isomerase (PDI) is an essential enzyme for the acquisition of the correct three-dimensional structure of proteins. Current evidence suggests that in Neosporacaninum and Toxoplasmagondii, which are closely related to B. besnoiti, PDI play an important role in host cell invasion, is a relevant target for the host immune response, and represents a promising drug target and/or vaccine candidate. In this work, we present the nucleotide sequence of the B. besnoiti PDI gene. BbPDI belongs to the thioredoxin-like superfamily (cluster 00388) and is included in the PDI_a family (cluster defined cd02961) and the PDI_a_PDI_a′_c subfamily (cd02995). A 3D theoretical model was built by comparative homology using Swiss-Model server, using as a template the crystallographic deduced model of Tapasin-ERp57 (PDB code 3F8U chain C). Analysis of the phylogenetic tree for PDI within the phylum apicomplexa reinforces the close relationship among B. besnoiti, N. caninum and T. gondii. When subjected to a PDI-assay based on the polymerisation of reduced insulin, recombinant BbPDI expressed in E. coli exhibited enzymatic activity, which was inhibited by bacitracin. Antiserum directed against recombinant BbPDI reacted with PDI in Western blots and by immunofluorescence with B. besnoiti tachyzoites and bradyzoites.     

In vivo chaperone activity of heat shock protein 70 and thermotolerance

Heat shock protein 70 (Hsp70) is thought to play a critical role in the thermotolerance of mammalian cells, presumably due to its chaperone activity. We examined the chaperone activity and cellular heat resistance of a clonal cell line in which overexpression of Hsp70 was transiently induced by means of the tetracycline-regulated gene expression system. This single-cell-line approach circumvents problems associated with clonal variation and indirect effects resulting from constitutive overexpression of Hsp70. The in vivo chaperone function of Hsp70 was quantitatively investigated by using firefly luciferase as a reporter protein. Chaperone activity was found to strictly correlate to the level of Hsp70 expression. In addition, we observed an Hsp70 concentration dependent increase in the cellular heat resistance. In order to study the contribution of the Hsp70 chaperone activity, heat resistance of cells that expressed tetracycline-regulated Hsp70 was compared to thermotolerant cells expressing the same level of Hsp70 plus all of the other heat shock proteins. Overexpression of Hsp70 alone was sufficient to induce a similar recovery of cytoplasmic luciferase activity, as does expression of all Hsps in thermotolerant cells. However, when the luciferase reporter protein was directed to the nucleus, expression of Hsp70 alone was not sufficient to yield the level of recovery observed in thermotolerant cells. In addition, cells expressing the same level of Hsp70 found in heat-induced thermotolerant cells containing additional Hsps showed increased resistance to thermal killing but were more sensitive than thermotolerant cells. These results suggest that the inducible form of Hsp70 contributes to the stress-tolerant state by increasing the chaperone activity in the cytoplasm. However, its expression alone is apparently insufficient for protection of other subcellular compartments to yield clonal heat resistance to the level observed in thermotolerant cells.     

Molecular characterization of a Bombyx mori protein disulfide isomerase (bPDI)

We have isolated a complementary deoxyribonucleic acid clone that encodes the protein disulfide isomerase of Bombyx mori (bPDI). This protein has a putative open reading frame of 494 amino acids and a predicted size of 55.6 kDa. In addition, 2 thioredoxin active sites, each with a CGHC sequence, and an endoplasmic reticulum (ER) retention signal site with a KDEL motif were found at the C-terminal. Both sites are typically found in members of the PDI family of proteins. The expression of bPDI messenger ribonucleic acid (mRNA) was markedly increased during ER stress induced by stimulation with calcium ionophore A23187, tunicamycin, and dithiothreitol, all of which are known to cause an accumulation of unfolded proteins in the ER. We also examined the tissue distribution of bPDI mRNA and found pronounced expression in the fat body of insects. Hormonal regulation studies showed that juvenile hormone, insulin, and a combination of juvenile hormone and transferrin (although not transferrin alone) affected bPDI mRNA expression. A challenge with exogenous bacteria also affected expression, and the effect peaked 16 hours after infection. These results suggest that bPDI is a member of the ER-stress protein group, that it may play an important role in exogenous bacterial infection of the fat body, and that its expression is hormone regulated.