Structural properties of homogeneous protein disulphide-isomerase from bovine liver purified by a rapid high-yielding procedure

Insights on the dynamic behavior of protein disulfide isomerase in the solution environment through the SAXS technique

The dynamic behavior of Protein Disulfide Isomerase (PDI) in an aqueous solution environment under physiologically active pH has been experimentally verified in this study using Small Angle X-ray Scattering (SAXS) technique. The structural mechanism of dimerization for full-length PDI molecules and co-complex with two renowned substrates has been comprehensively discussed. The structure models obtained from the SAXS data of PDI purified from bovine liver display behavior duality between unaccompanied-enzyme and after engaged with substrates. The analysis of SAXS data revealed that PDI exists as a homo-dimer in the solution environment, and substrate induction provoked its segregation into monomer to enable the enzyme to interact systematically with incoming clients.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40203-024-00198-0.

A thermostable alkaliphilic protein-disulfide isomerase from Bacillus subtilis DR8806: cloning, expression, biochemical characterization and molecular dynamics simulation

Disulfide bonds are among the most important factors related to correct folding of the proteins. Protein disulfide isomerase (PDI) is the enzyme responsible for the correct formation and isomerization of these bonds. It is rarely studied so far and none of them showed industrial properties. In this study, the gene encoding for a putative PDI from Bacillus subtilis DR8806 was identified, cloned and expressed in Escherichia coli. It was encoded a 23.26kDa protein. The enzyme was purified by GST affinity chromatography with a specific activity of 1227u/mg. It was active and stable over a wide range of temperature (20-85°C) and pH (4.5-10) with an optimum at 65°C and pH 5.5. Its activity was enhanced by Mn²⁺ and Co²⁺ while Ag⁺ and Zn²⁺ decreased it. Some of the known PDI inhibitors such as Tocinoic acid and Bactiracin did not affect its activity. In-silico analysis shows the five amino acids changes in the protein sequence regarding to the consensus sequence of PDIs, have a positive impact toward the protein thermal stability. This was further confirmed by molecular dynamics simulations. By considering the overall results, the enzyme might be a potential candidate for applications in the respective industries.

'Something in the way she moves': The functional significance of flexibility in the multiple roles of protein disulfide isomerase (PDI)

Protein disulfide isomerase (PDI) has diverse functions in the endoplasmic reticulum as catalyst of redox transfer, disulfide isomerization and oxidative protein folding, as molecular chaperone and in multi-subunit complexes. It interacts with an extraordinarily wide range of substrate and partner proteins, but there is only limited structural information on these interactions. Extensive evidence on the flexibility of PDI in solution is not matched by any detailed picture of the scope of its motion. A new rapid method for simulating the motion of large proteins provides detailed molecular trajectories for PDI demonstrating extensive changes in the relative orientation of its four domains, great variation in the distances between key sites and internal motion within the core ligand-binding domain. The review shows that these simulations are consistent with experimental evidence and provide insight into the functional capabilities conferred by the extensive flexible motion of PDI.

Progress of Mimetic Enzymes and Their Applications in Chemical Sensors

The need to develop innovative and reformative approaches to synthesize chemical sensors has increased in recent years because of demands for selectivity, stability, and reproducibility. Mimetic enzymes provide an efficient and convenient method for chemical sensors. This review summarizes the application of mimetic enzymes in chemical sensors. Mimetic enzymes can be classified into five categories: hydrolases, oxidoreductases, transferases, isomerases, and induced enzymes. Potential and recent applications of mimetic enzymes in chemical sensors are reviewed in detail, and the outlook of profound development has been illustrated.

Polyamine effects on protein disulfide isomerase expression and implications for hypersalinity stress in the marine alga Ulva lactuca Linnaeus(1)

Full-length protein disulfide isomerase (UfPDI) cDNA was cloned from the intertidal macroalga Ulva lactuca Linnaeus. Modulation of UfPDI expression by stresses and polyamines (PA) was studied. UfPDI transcription and enzyme activity were increased by hypersalinity (90) or high light illumination (1,200 μmol photons · m(-2)  · s(-1) ), decreased by the addition of 100 μM CuSO4 . An exposure to a salinity of 90 decreased PA contents. Treating with PA biosynthetic inhibitors, D-arginine (D-Arg) or α-methyl ornithine (α-MO), led to a further decrease and also inhibited UfPDI expression and recovery of the growth rate. These results suggest that PAs are required to activate UfPDI expression with hypersalinity, even PA contents are decreased at a salinity of 90. The induction of UfPDI expression by hypersalinity of 90 and tolerance to hypersalinity could be enhanced if internal PA contents rise. Sung et al. (2011b) showed that PA contents could be increased by pretreating with putrescine (Put, 1 mM), spermidine (Spd, 1 mM), or spermine (Spm, 1 mM) at a salinity of 30. Therefore, PA pretreatment effect on UfPDI expression was examined. Pretreatment with Spd and Spm, but not with Put, enhanced UfPDI expression after transferred to a salinity of 90 and restored the growth rate. In conclusion, induction of UfPDI expression by Spd or Spm before exposure to hypersaline conditions and continuous up-regulation after hypersalinity exposure are required for the acquisition of hypersalinity tolerance in the intertidal green macroalga U. lactuca.

An additional function of the rough endoplasmic reticulum protein complex prolyl 3-hydroxylase 1·cartilage-associated protein·cyclophilin B: the CXXXC motif reveals disulfide isomerase activity in vitro

Collagen biosynthesis occurs in the rough endoplasmic reticulum, and many molecular chaperones and folding enzymes are involved in this process. The folding mechanism of type I procollagen has been well characterized, and protein disulfide isomerase (PDI) has been suggested as a key player in the formation of the correct disulfide bonds in the noncollagenous carboxyl-terminal and amino-terminal propeptides. Prolyl 3-hydroxylase 1 (P3H1) forms a hetero-trimeric complex with cartilage-associated protein and cyclophilin B (CypB). This complex is a multifunctional complex acting as a prolyl 3-hydroxylase, a peptidyl prolyl cis-trans isomerase, and a molecular chaperone. Two major domains are predicted from the primary sequence of P3H1: an amino-terminal domain and a carboxyl-terminal domain corresponding to the 2-oxoglutarate- and iron-dependent dioxygenase domains similar to the α-subunit of prolyl 4-hydroxylase and lysyl hydroxylases. The amino-terminal domain contains four CXXXC sequence repeats. The primary sequence of cartilage-associated protein is homologous to the amino-terminal domain of P3H1 and also contains four CXXXC sequence repeats. However, the function of the CXXXC sequence repeats is not known. Several publications have reported that short peptides containing a CXC or a CXXC sequence show oxido-reductase activity similar to PDI in vitro. We hypothesize that CXXXC motifs have oxido-reductase activity similar to the CXXC motif in PDI. We have tested the enzyme activities on model substrates in vitro using a GCRALCG peptide and the P3H1 complex. Our results suggest that this complex could function as a disulfide isomerase in the rough endoplasmic reticulum.

The ligand-binding b' domain of human protein disulphide-isomerase mediates homodimerization

Purified preparations of the recombinant b'x domain fragment of human protein-disulphide isomerase (PDI), which are homogeneous by mass spectrometry and sodium dodecyl sulfate polyacrylamide gel electrophoresis, comprise more than one species when analyzed by ion-exchange chromatography and nondenaturing polyacrylamide gel electrophoresis. These species were resolved and shown to be monomer and dimer by analytical ultracentrifugation and analytical size-exclusion chromatography. Spectroscopic properties indicate that the monomeric species corresponds to the "capped" conformation observed in the x-ray structure of the I272A mutant of b'x (Nguyen, Wallis, Howard, Haapalainen, Salo, Saaranen, Sidhu, Wierenga, Freedman, Ruddock, and Williamson, J Mol Biol 2008;383:1144-1155) in which the x region binds to a hydrophobic patch on the surface of the b' domain; conversely, the dimeric species has an "open" or "uncapped" conformation in which the x region does not bind to this surface. The larger bb'x fragment of human PDI shows very similar behavior to b'x and can be resolved into a capped monomeric species and an uncapped dimer. Preparations of recombinant b' domain of human PDI and of the bb' domain pair are found exclusively as dimers. Full-length PDI is known to comprise a mixture of monomeric and dimeric species, whereas the isolated a, b, and a' domains of PDI are found exclusively as monomers. These results show that the b' domain of human PDI tends to form homodimers--both in isolation and in other contexts--and that this tendency is moderated by the adjacent x region, which can bind to a surface patch on the b' domain.

The catalytic activity of protein-disulfide isomerase requires a conformationally flexible molecule

Protein-disulfide isomerase (PDI) catalyzes the formation of the correct pattern of disulfide bonds in secretory proteins. A low resolution crystal structure of yeast PDI described here reveals large scale conformational changes compared with the initially reported structure, indicating that PDI is a highly flexible molecule with its catalytic domains, a and a', representing two mobile arms connected to a more rigid core composed of the b and b' domains. Limited proteolysis revealed that the linker between the a domain and the core is more susceptible to degradation than that connecting the a' domain to the core. By restricting the two arms with inter-domain disulfide bonds, the molecular flexibility of PDI, especially that of its a domain, was demonstrated to be essential for the enzymatic activity in vitro and in vivo. The crystal structure also featured a PDI dimer, and a propensity to dimerize in solution and in the ER was confirmed by cross-linking experiments and the split green fluorescent protein system. Although sedimentation studies suggested that the self-association of PDI is weak, we hypothesize that PDI exists as an interconvertible mixture of monomers and dimers in the endoplasmic reticulum due to its high abundance in this compartment.

Alternative conformations of the x region of human protein disulphide-isomerase modulate exposure of the substrate binding b' domain

Protein disulphide isomerase (PDI) is a key multi-domain protein folding catalyst in the endoplasmic reticulum. The b' domain of PDI is essential for the non-covalent binding of incompletely folded protein substrates. Earlier, we defined the substrate binding site in the b' domain of human PDI by modelling and mutagenesis studies. Here, we show by fluorescence and NMR that recombinant human PDI b'x (comprising the b' domain and the subsequent x linker region) can assume at least two different conformations in solution. We have screened mutants in the b'x region to identify mutations that favour one of these conformers in recombinant b'x, and isolated and characterised examples of both types. We have crystallised one mutant of b'x (I272A mutation) in which one conformer is stabilized, and determined its crystal structure to a resolution of 2.2 A. This structure shows that the b' domain has the typical thioredoxin fold and that the x region can interact with the b' domain by "capping" a hydrophobic site on the b' domain. This site is most likely the substrate binding site and hence such capping will inhibit substrate binding. All of the mutations we previously reported to inhibit substrate binding shift the equilibrium towards the capped conformer. Hence, these mutations act by altering the natural equilibrium and decreasing the accessibility of the substrate binding site. Furthermore, we have confirmed that the corresponding structural transition occurs in the wild type full-length PDI. A cross-comparison of our data with that for other PDI-family members, Pdi1p and ERp44, suggests that the x region of PDI can adopt alternative conformations during the functional cycle of PDI action and that these are linked to the ability of PDI to interact with folding substrates.

Promotion of insulin aggregation by protein disulfide isomerase

We examined the aggregation of insulin as a result of reduction of disulfide bonds catalyzed by protein disulfide isomerase (PDI) using various techniques. We demonstrated the kinetic correlation between PDI-catalyzed insulin reduction and the aggregate formation, the relationship between aggregation and amyloid formation, and the structural information on the secondary structure of the aggregates. The initial rate of PDI-catalyzed reduction of insulin, apparent rate constants of aggregate growth for sigmoidal features, and lag times were obtained by changing the PDI concentration, temperature, and insulin concentration. In situ kinetics were studied using the dyes; thioflavin T (ThT) and Congo red (CR) in addition to turbidimetry with the insulin reduction by PDI. The ThT and CR binding assay revealed sigmoidal kinetics, and the spectrum of binding CR showed a red shift against time, suggesting an orderly formation of insulin aggregates. The secondary structure of the PDI-promoted insulin aggregates showed antiparallel beta-sheet conformation by FT-IR measurement.

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.

The crystal structure of yeast protein disulfide isomerase suggests cooperativity between its active sites

Protein disulfide isomerase plays a key role in catalyzing the folding of secretory proteins. It features two catalytically inactive thioredoxin domains inserted between two catalytically active thioredoxin domains and an acidic C-terminal tail. The crystal structure of yeast PDI reveals that the four thioredoxin domains are arranged in the shape of a twisted "U" with the active sites facing each other across the long sides of the "U." The inside surface of the "U" is enriched in hydrophobic residues, thereby facilitating interactions with misfolded proteins. The domain arrangement, active site location, and surface features strikingly resemble the Escherichia coli DsbC and DsbG protein disulfide isomerases. Biochemical studies demonstrate that all domains of PDI, including the C-terminal tail, are required for full catalytic activity. The structure defines a framework for rationalizing the differences between the two active sites and their respective roles in catalyzing the formation and rearrangement of disulfide bonds.

The pressure effect on the structure and functions of protein disulfide isomerase

Studying on the pressure effects of the structure and functions of the multidomain protein, protein disulfide isomerase (PDI), the intrinsic Trp fluorescence spectra of PDI were measured under high pressure. PDI has 5 Trp residues and the two of all Trp residues are located at the neighborhood of the active site (WCGHC) for isomerase activity. On the basis of the red shift of center of spectral mass (CSM) of the intrinsic Trp fluorescence and the decrease in its fluorescence intensity, the changes in tertiary structure of PDI were observed above 100 MPa. These structural changes were completed at 400 MPa. The CSM of 400 MPa denatured PDI was comparable to that of 6.0 M GdnHCl denatured one. All of the Trp residues included in PDI are completely exposed to aqueous medium at 400 MPa. However, there is the significant difference between the pressure and GdnHCl-denatured PDI. The Trp fluorescence intensity was decreased with increasing pressure, but increased with the increase of the GdnHCl concentration. It is implied that the pressure-denatured state of PDI might remain compact not to be extensively unfolded. In the point of view about the reversibility of pressure-treated PDI, the tertiary structure was completely recovered after released to ambient pressure. The disulfide reduction and chaperone activity of 400 MPa-treated PDI were also recovered to be comparable to those of native one. Despite of a multidomain protein, the excellence in both structural and functional recovery of pressure-denatured PDI is quite remarkable. These unique properties of PDI against high pressure provide the insights into understanding the pressure-induced denaturation of PDI.

Protein disulphide isomerase binds ammodytoxin strongly: Possible implications for toxin trafficking

Ammodytoxin, a group IIA secreted phospholipase A(2) from the venom of the long-nosed viper (Vipera ammodytes ammodytes), is a potent presynaptically acting neurotoxin. It blocks the secretion of neurotransmitter from the nerve cell, thus hindering the communication with the neighbouring neuron or muscle cell. To express the neurotoxicity, ammodytoxin should interact with specific receptors in the axon terminal and express phospholipase activity. Our previous results indicate that, following the association with a receptor on the external side of the presynaptic membrane, the toxin penetrates into the cytosol of the nerve cell. Here, we show that the toxin associates specifically with protein disulphide isomerase, a protein in the lumen of endoplasmic reticulum, which may be crucial for the retention and concentration of the toxin in this cellular compartment and for its subsequent transport across the membrane of endoplasmic reticulum into the cytosol of the nerve cell.

Characterization of Two Protein Disulfide Isomerases from the Endocytic Pathway of Bloodstream Forms of Trypanosoma brucei

Proteins from the endocytic pathway in bloodstream forms of Trypanosome brucei are modified by the addition of linear poly-N-acetyllactosamine side chains, which permits their isolation by tomato lectin affinity chromatography. Antibodies against this tomato lectin binding fraction were employed to screen a cDNA expression library from bloodstream forms of T. brucei. Two cDNAs were prominent among those selected. These cDNAs coded for two putative protein disulfide isomerases (PDIs) that respectively contained one and two double-cysteine redox-active sites and corresponded to a single domain PDI and a class 1 PDI. Assays of the purified recombinant proteins demonstrated that both proteins possess isomerase activity, but only the single domain PDI had a reducing activity. These PDIs possess a number of unusual features that distinguish them from previously characterized PDIs. The expression of both is developmentally regulated, they both co-localize with markers of the endocytic pathway, and both are modified by N-glycosylation. The larger PDI possesses N-glycans containing poly-N-acetyllactosamine, a modification that is indicative of processing in the Golgi and suggests the presence of a novel trafficking pathway for PDIs in trypanosomes. Although generally PDIs are considered essential, neither activity appeared to be essential for the growth of trypanosomes, at least in vitro.

Identification and Characterization of Structural Domains of Human ERp57

The amino acid sequence of ERp57, which functions in the endoplasmic reticulum together with the lectins calreticulin and calnexin to achieve folding of newly synthesized glycoproteins, is highly similar to that of protein disulfide isomerase (PDI), but they have their own distinct roles in protein folding. We have characterized the domain structure of ERp57 by limited proteolysis and N-terminal sequencing and have found it to be similar but not identical to that of PDI. ERp57 had three major protease-sensitive regions, the first of which was located between residues 120 and 150, the second between 201 and 215, and the third between 313 and 341, the data thus being consistent with a four-domain structure abb'a'. Recombinant expression in Escherichia coli was used to verify the domain boundaries. Each single domain and a b'a' double domain could be produced in the form of soluble, folded polypeptides, as verified by circular dichroism spectra and urea gradient gel electrophoresis. When the ability of ERp57 and its a and a' domains to fold denatured RNase A was studied by electrospray mass analyses, ERp57 markedly enhanced the folding rate at early time points, although less effectively than PDI, but was an ineffective catalyst of the overall process. The a and a' domains produced only minor, if any, increases in the folding rate at the early stages and no increase at the late stages. Interaction of the soluble ERp57 domains with the P domain of calreticulin was studied by chemical cross-linking in vitro. None of the single ERp57 domains nor the b'a' double domain could be cross-linked to the P domain, whereas cross-linking was obtained with a hybrid ERpabb'PDIa'c polypeptide but not with ERpabPDIb'a'c, indicating that multiple domains are involved in this protein-protein interaction and that the b' domain of ERp57 cannot be replaced by that of PDI.

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.

Dimerization by domain hybridization bestows chaperone and isomerase activities

Thioredoxin, DsbA, the N-terminal active-site domain a and the non-active-site domain b of protein-disulfide isomerase are all monomeric with a thioredoxin fold, and each exhibits low or no isomerase and chaperone activity. We have linked the N terminus of the above four monomers, individually, to the C terminus of the N-terminal domain of DsbC via the flexible linker helix of the latter to produce four domain hybrids, DsbCn-Trx, DsbCn-DsbA, DsbCn-PDIa, and DsbCn-PDIb. These four hybrid proteins form homodimers, and except for DsbCn-PDIb they exhibit new or greatly elevated isomerase as well as chaperone activity. Three-dimensional structure prediction indicates that all the four domain hybrids adopt DsbC-like V-shaped structure with a broad uncharged cleft between the two arms for binding of non-native protein folding intermediates. The results provide strong evidence that dimerization creates chaperone and isomerase activity for monomeric thiol-protein oxidases or reductases, and suggesting a pathway for proteins to acquire new functions and/or higher biological efficiency during evolution.

The CXC motif: a functional mimic of protein disulfide isomerase

Protein disulfide isomerase (PDI) utilizes the active site sequence Cys-Gly-His-Cys (CGHC; E degrees ' = -180 mV) to effect thiol-disulfide interchange during oxidative protein folding. Here, the Cys-Gly-Cys-NH(2) (CGC) peptide is shown to have a disulfide reduction potential (E degrees ' = -167 mV) that is close to that of PDI. This peptide has a thiol acid dissociation constant (pK(a) = 8.7) that is lower than that of glutathione. These attributes endow the CGC peptide with substantial disulfide isomerization activity. Escherichia coli thioredoxin (Trx) utilizes the active site sequence Cys-Gly-Pro-Cys (CGPC; E degrees ' = -270 mV) to effect disulfide reduction. Removal of the proline residue from the Trx active site yields a CGC active site with a greatly destabilized disulfide bond (E degrees ' >or= -200 mV). The DeltaP34 variant retains high conformational stability and remains a substrate for thioredoxin reductase. In contrast to the reduced form of the wild-type enzyme, the reduced form of DeltaP34 Trx has disulfide isomerization activity, which is 25-fold greater than that of the CGC peptide. Thus, the rational deletion of an active site residue can bestow a new and desirable function upon an enzyme. Moreover, a CXC motif, in both a peptide and a protein, provides functional mimicry of PDI.

An atypical protein disulfide isomerase from the protozoan parasite Leishmania containing a single thioredoxin-like domain

In higher eukaryotes, secretory proteins are under the quality control of the endoplasmic reticulum for their proper folding and release into the secretory pathway. One of the proteins involved in the quality control is protein disulfide isomerase, which catalyzes the formation of protein disulfide bonds. As a first step toward understanding the endoplasmic reticulum quality control of secretory proteins in lower eukaryotes, we have isolated a protein disulfide isomerase gene from the protozoan parasite Leishmania donovani. The parasite enzyme shows high sequence homology with homologs from other organisms. However, unlike the four thioredoxin-like domains found in most protein disulfide isomerases, of which two contain an active site, the leishmanial enzyme possesses only one active site present in a single thioredoxin-like domain. When expressed in Escherichia coli, the recombinant parasite enzyme shows both oxidase and isomerase activities. Replacement of the two cysteins with alanines in its active site results in loss of both enzymatic activities. Further, overexpression of the mutated/inactive form of the parasite enzyme in L. donovani significantly reduced their release of secretory acid phosphatases, suggesting that this single thioredoxin-like domain protein disulfide isomerase could play a critical role in the Leishmania secretory pathway.

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.

Effects of macromolecular crowding on the refolding of glucose- 6-phosphate dehydrogenase and protein disulfide isomerase

The effects of polysaccharide, polyethylene glycol, and protein-crowding agents on the refolding of glucose-6-phosphate dehydrogenase (G6PDH) and protein disulfide isomerase have been examined. By increasing concentration during refolding, the reactivation yields of the two proteins decrease with the formation of soluble aggregates. In the presence of high concentrations of crowding agents the reactivation yields remain constant but with decreased refolding rates. The refolding of G6PDH changes from monophasic to biphasic first-order reactions in the presence of crowding agents, and the amplitude of the new slow phase increases with increasing concentrations of crowding agents. The molecular chaperone GroEL reverses the refolding kinetics of G6PDH from biphase back to monophase and accelerates the refolding process. Our results display the complexity and diversity of the effects of macromolecular crowding on both the thermodynamics and kinetics of protein folding.

Thermodynamics of the folding of D-glyceraldehyde-3-phosphate dehydrogenase assisted by protein disulfide isomerase studied by microcalorimetry

Thermodynamics of the refolding of denatured D-glyceraldehyde 3-phosphate dehydrogenase (GAPDH) assisted by protein disulfide isomerase (PDI), a molecular chaperone, has been studied by isothermal microcalorimetry at different molar ratios of PDI/GAPDH and temperatures using two thermodynamic models proposed for chaperone-substrate binding and chaperone-assisted substrate folding, respectively. The binding of GAPDH folding intermediates to PDI is driven by a large favorable enthalpy decrease with a large unfavorable entropy reduction, and shows strong enthalpy-entropy compensation and weak temperature dependence of Gibbs free energy change. A large negative heat-capacity change of the binding, -156 kJ.mol(-1).K(-1), at all temperatures examined indicates that hydrophobic interaction is a major force for the binding. The binding stoichiometry shows one dimeric GAPDH intermediate per PDI monomer. The refolding of GAPDH assisted by PDI is a largely exothermic reaction at 15.0-25.0 degrees C. With increasing temperature from 15.0 to 37.0 degrees C, the PDI-assisted reactivation yield of denatured GAPDH upon dilution decreases. At 37.0 degrees C, the spontaneous reactivation, PDI-assisted reactivation and intrinsic molar enthalpy change during the PDI-assisted refolding of GAPDH are not detected.

Functional roles and efficiencies of the thioredoxin boxes of calcium-binding proteins 1 and 2 in protein folding

The rat luminal endoplasmic-recticulum calcium-binding proteins 1 and 2 (CaBP1 and CaBP2 respectively) are members of the protein disulphide-isomerase (PDI) family. They contain two and three thioredoxin boxes (Cys-Gly-His-Cys) respectively and, like PDI, may be involved in the folding of nascent proteins. We demonstrate here that CaBP1, similar to PDI and CaBP2, can complement the lethal phenotype of the disrupted Saccharomyces cerevisiae PDI gene, provided that the natural C-terminal Lys-Asp-Glu-Leu sequence is replaced by His-Asp-Glu-Leu. Both the in vitro RNase AIII-re-activation assays and in vivo pro-(carboxypeptidase Y) processing assays using CaBP1 and CaBP2 thioredoxin (trx)-box mutants revealed that, whereas the three trx boxes in CaBP2 seem to be functionally equivalent, the first trx box of CaBP1 is significantly more active than the second trx box. Furthermore, only about 65% re-activation of denatured reduced RNase AIII could be obtained with CaBP1 or CaBP2 compared with PDI, and the yield of PDI-catalysed reactions was significantly reduced in the presence of either CaBP1 or CaBP2. In contrast with PDI, neither CaBP1 nor CaBP2 could catalyse the renaturation of denatured glyceraldehyde-3-phosphate dehydrogenase (GAPDH), which is a redox-independent process, and neither protein had any effect on the PDI-catalysed refolding of GAPDH. Furthermore, although PDI can bind peptides via its b' domain, a property it shares with PDIp, the pancreas-specific PDI homologue, and although PDI can bind malfolded proteins such as 'scrambled' ribonuclease, no such interactions could be detected for CaBP2. We conclude that: (1) both CaBP2 and CaBP1 lack peptide-binding activity for GAPDH attributed to the C-terminal region of the a' domain of PDI; (2) CaBP2 lacks the general peptide-binding activity attributed to the b' domain of PDI; (3) interaction of CaBP2 with substrate (RNase AIII) is different from that of PDI and substrate; and (4) both CaBP2 and CaBP1 may promote oxidative folding by different kinetic pathways.

Thioredoxin fold as homodimerization module in the putative chaperone ERp29: NMR structures of the domains and experimental model of the 51 kDa dimer

BACKGROUND

ERp29 is a ubiquitously expressed rat endoplasmic reticulum (ER) protein conserved in mammalian species. Fold predictions suggest the presence of a thioredoxin-like domain homologous to the a domain of human protein disulfide isomerase (PDI) and a helical domain similar to the C-terminal domain of P5-like PDIs. As ERp29 lacks the double-cysteine motif essential for PDI redox activity, it is suggested to play a role in protein maturation and/or secretion related to the chaperone function of PDI. ERp29 self-associates into 51 kDa dimers and also higher oligomers.

RESULTS

3D structures of the N- and C-terminal domains determined by NMR spectroscopy confirmed the thioredoxin fold for the N-terminal domain and yielded a novel all-helical fold for the C-terminal domain. Studies of the full-length protein revealed a short, flexible linker between the two domains, homodimerization by the N-terminal domain, and the presence of interaction sites for the formation of higher molecular weight oligomers. A gadolinium-based relaxation agent is shown to present a sensitive tool for the identification of macromolecular interfaces by NMR.

CONCLUSIONS

ERp29 is the first eukaryotic PDI-related protein for which the structures of all domains have been determined. Furthermore, an experimental model of the full-length protein and its association states was established. It is the first example of a protein where the thioredoxin fold was found to act as a specific homodimerization module, without covalent linkages or supporting interactions by further domains. A homodimerization module similar as in ERp29 may also be present in homodimeric human PDI.

Aggregated proteins accelerate but do not increase the aggregation of D-glyceraldehyde-3-phosphate dehydrogenase. Specificity of protein aggregation

The effect of protein aggregates on the aggregation of D-glyceraldehyde-3-phosphate dehydrogenase (GAPDH) during unfolding and refolding has been studied. The aggregation of GAPDH follows a sigmoid course. The presence of protein aggregates increases the aggregation rate during unfolding and refolding of GAPDH but does not change the extent of aggregation and the final renaturation yield. It is suggested that protein aggregates function as seeds for aggregation via hydrophobic interaction with only GAPDH folding intermediates destined to aggregate and do not affect the distribution between pathways leading to correct folding and aggregation. Moreover, two different proteins do not interfere with each other during their simultaneous refolding together in a buffer. These findings provide insight into a mechanism by which cells prevent protein folding against the interference from aggregation of other proteins.

GroEL and protein disulfide isomerase each binds with folding intermediates of D-glyceraldehyde-3-phosphate dehydrogenase released from complexes formed with the other

Simultaneous presence of two chaperones, GroEL and protein disulfide isomerase (PDI), assists the reactivation of denatured D-glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in an additive way. Delayed addition of chaperones to the refolding solution after dilution of denatured GAPDH indicates an interaction with intermediates formed mainly in the first 5 min for PDI and formed within a longer time period for GroEL-ATP. The above indicate that the two chaperones interact with different folding intermediates of GAPDH. After delayed addition of one chaperone to the refolding mixture containing the other at 4 degrees C, GroEL binds with all GAPDH intermediates dissociated from PDI, and PDI interacts with the intermediates released from GroEL during the first 10-20 min. It is suggested that the GAPDH folding intermediates released from the chaperone-bound complex are still partially folded so as to be rebound by the other chaperone. The above results clearly support the network model of GroEL and PDI.

The CXXCXXC motif determines the folding, structure and stability of human Ero1-Lalpha

The presence of correctly formed disulfide bonds is crucial to the structure and function of proteins that are synthesized in the endoplasmic reticulum (ER). Disulfide bond formation occurs in the ER owing to the presence of several specialized catalysts and a suitable redox potential. Work in yeast has indicated that the ER resident glycoprotein Ero1p provides oxidizing equivalents to newly synthesized proteins via protein disulfide isomerase (PDI). Here we show that Ero1-Lalpha, the human homolog of Ero1p, exists as a collection of oxidized and reduced forms and covalently binds PDI. We analyzed Ero1-Lalpha cysteine mutants in the presumed active site C(391)VGCFKC(397). Our results demonstrate that this motif is important for protein folding, structural integrity, protein half-life and the stability of the Ero1-Lalpha-PDI complex.

Contributions of protein disulfide isomerase domains to its chaperone activity

Protein disulfide isomerase (PDI), a member of the thioredoxin (Trx) superfamily, consists of five consecutive domains, a-b-b'-a'-c. Domain combinations, AB, A'C, B'A'C and AB-C, and hybrids of PDI domains with Trx, Trx-C and Trx-B'A'C, have been constructed and expressed in Escherichia coli to examine the contributions of PDI domains to its enzyme and chaperone activities. All the combination and hybrid products are considerably less active than intact PDI in their enzyme activities. Recombinant products containing C, at low concentrations, inhibit the reactivation of lysozyme in HEPES buffer, while those without C do not. Only the intact PDI molecule and the hybrid molecule, Trx-B'A'C, but to a much lower level, show general chaperone activity in assisting the reactivation of denatured D-glyceraldehyde-3-phosphate dehydrogenase. It is suggested that all domains of PDI contribute to the binding of target protein for its chaperone activity.

The N-terminal sequence (residues 1-65) is essential for dimerization, activities, and peptide binding of Escherichia coli DsbC

Limited proteolysis of DsbC with trypsin resulted in a compact and stable C-terminal fragment (residues 66-216), fDsbC, which retains the active site sequence, -Cys(98)-Gly-Tyr-Cys(101)-, and shows only minor differences in conformation compared with that of the intact molecule. The pK(a) of active site thiol and the K(SS) with glutathione are very close to that of DsbC, respectively; however, fDsbC is inactive as an isomerase in catalyzing the formation of correct disulfide bonds in scrambled RNase A and denatured and reduced bovine pancreatic trypsin inhibitor and shows only 13% thiol-protein oxidoreductase activity (TPOR) of DsbC. In contrast to the dimeric DsbC, fDsbC exists as a monomer and has no chaperone activity in assisting the reactivation of denatured d-glyceraldehyde-3-phosphate dehydrogenase. The heterodimer of DsbC with the inactive DsbC carboxymethylated at both active site thiols shows about 50% TPOR activity of DsbC but no isomerase activity, indicating that the DsbC subunit in the heterodimer displays full TPOR activity but little, if any, isomerase activity. It is concluded that the N-terminal sequence (residues 1-65) is essential for dimer formation and chaperone activity of DsbC. The active sites in both subunits of the dimeric DsbC appear to be essential for its isomerase activity.

The thioredoxin boxes of thyroglobulin: possible implications for intermolecular disulfide bond formation in the follicle lumen

Multimerization of thyroglobulin (TG) takes place extracellularly in the thyroid follicle lumen and is regarded as a mechanism to store TG at high concentrations. Human thyroglobulin (hTG) has been shown to multimerize mainly by intermolecular disulfide cross-links. We recently noted that TG of various mammalian species contains three highly conserved thioredoxin boxes (CXXC). This sequence is known to underlie the enzymatic activity of protein disulfide isomerase (PDI). As hTG formed intermolecular disulfide bonds in the absence of other proteins depending on the redox conditions and hTG concentration, the CXXC-boxes of TG might provide the structural basis for self-assisted intermolecular cross-linking. To test this hypothesis we prepared a recombinant TG fragment containing the three thioredoxin boxes. This fragment exhibited a redox activity amounting to about 10% of the activity of PDI at redox conditions supposed to be present in the extracellular space. This activity might be supplemented by the oxidizing system of the apical cell surfaces of thyrocytes facing the follicle lumen. Indeed, incubation of hTG with peroxidase and H202 resulted in intermolecular disulfide bridge formation. Our results suggest a combined mechanism of self-assisted and peroxidase-mediated disulfide bond formation leading to the intermolecular cross-linking of lumenal hTG.

A catalytic site of protein disulfide isomerase probed with adenosine-5'-triphosphate analogs

Anthraniloyl adenosine-5'-triphosphate (Ant-ATP) and etheno-adenosine-5'-triphosphate (epsilon-ATP) complexed to Mg(2+) ions are substrates of protein disulfide isomerase (PDI). epsilon-ATP, coordinated to Tb(3+) ions, was used as a probe of the ATPase binding site. Sensitized luminescence arising from resonance energy transfer from epsilon-adenine to Tb(3+) is quenched by PDI. The luminescence results are discussed in reference to a model in which the distance of separation between epsilon-adenine (donor) and Tb(3+) (acceptor) is increased upon binding of PDI. The interaction of a small peptide of 14 amino acid residues with the b/b' domain of the protein does not influence the ATPase activity. The phosphorescence, fluorescence and fluorescence anisotropy of bound epsilon-ATP are not perturbed by the binding of the small molecular weight peptide to PDI. It is suggested that the peptide and ATP do not share a common binding site on the b/b' domain.

Isolation of ERp29, a novel endoplasmic reticulum protein, from rat enamel cells. Evidence for a unique role in secretory-protein synthesis

Recently we cloned and described ERp29, a novel 29-kDa endoplasmic reticulum (ER) protein that is widely expressed in rat tissues. Here we report our original isolation of ERp29 from dental enamel cells, and the comprehensive sequence analysis that correlated ERp29 with its cognate cDNA, both in enamel cells and liver. Fractionation of enamel cells using a new freeze-thaw procedure showed that ERp29 partitioned with known reticuloplasmins, and not with soluble proteins from mitochondria or cytosol. The absence of ERp29 in secreted enamel matrix indicated that the C-terminal tetrapeptide (KEEL motif) confers effective ER-retention in enamel cells. ERp29 behaved as a single species (approximately 40 kDa) during size-exclusion chromatography of liver reticuloplasm, suggesting that most ERp29 is not stably associated with other proteins. Immunoblot analysis showed that ERp29 was up-regulated during enamel secretion and expressed most highly in secretory tissues, indicative of a role in secretory-protein synthesis. Unlike other reticuloplasmins, ERp29 was down-regulated during enamel mineralization and thereby dissociated from a calcium-handling role. Tissue-specific variations in ERp29 molecular abundance were revealed by quantification of reticuloplasmin mole ratios.

IN CONCLUSION

(a) ERp29 is a novel reticuloplasmin of general functional importance; (b) a unique role in protein processing is implicit from the distinctive expression patterns and molecular structure; (c) ERp29 is primarily involved in normal protein secretory events, not the ER stress response; (d) a major role is likely in tissues where ERp29 was equimolar with established molecular chaperones and foldases. This study implicates ERp29 as a new member of the ER protein-processing machinery, and identifies tissues where the physiological role of ERp29 is most likely to be clearly manifested.

Physical proximity and functional association of glycoprotein 1balpha and protein-disulfide isomerase on the platelet plasma membrane

Platelet function is influenced by the platelet thiol-disulfide balance. Platelet activation resulted in 440% increase in surface protein thiol groups. Two proteins that presented free thiol(s) on the activated platelet surface were protein-disulfide isomerase (PDI) and glycoprotein 1balpha (GP1balpha). PDI contains two active site dithiols/disulfides. The active sites of 26% of the PDI on resting platelets was in the dithiol form, compared with 81% in the dithiol form on activated platelets. Similarly, GP1balpha presented one or more free thiols on the activated platelet surface but not on resting platelets. Anti-PDI antibodies increased the dissociation constant for binding of vWF to platelets by approximately 50% and PDI and GP1balpha were sufficiently close on the platelet surface to allow fluorescence resonance energy transfer between chromophores attached to PDI and GP1balpha. Incubation of resting platelets with anti-PDI antibodies followed by activation with thrombin enhanced labeling and binding of monoclonal antibodies to the N-terminal region of GP1balpha on the activated platelet surface. These observations indicated that platelet activation triggered reduction of the active site disulfides of PDI and a conformational change in GP1balpha that resulted in exposure of a free thiol(s).

Isolation, purification, and characterization of a rat liver mitochondrial protein disulfide isomerase

The isolation and purification to electrophoretical homogeneity and characterization of a protein disulfide isomerase from rat liver mitochondria is reported. The purified enzyme exhibits a single band on sodium dodecylsulfatepolyacrylamide gel electrophoresis with an apparent molecular weight of approximately 54 kDa. Comparatively, the microsomal form shows an apparent molecular weight of 57 kDa indicating that the two forms are slightly different. The antibody raised against the microsomal isoform does not recognize the mitochondrial enzyme. To characterize the enzyme, different classical methodologies utilized for protein disulfide isomerase estimation have been adopted. The isolated enzyme is active with all of them, indicating that it comprises all the features of a typical protein disulfide isomerase. At the mitochondrial level the enzyme appears mostly localized at the membrane level. Its potential involvement in mitochondrial membrane permeability control is also discussed.

Metal regulation of metallothionein participation in redox reactions

Like glutathione or dithiothreitol, metallothionein effects the formation of pancreatic ribonuclease A from its S-sulfonated derivative catalyzed by protein disulfide isomerase. EDTA increases the yield of ribonuclease A activity recovery with metallothionein but does not affect the reaction with glutathione or dithiothreitol. EDTA also increases the reactivity of thiol groups in metallothionein with 5,5'-dithiobis-(2-nitrobenzoic acid) by chelation of zinc ions. It is suggested that the thiol groups in metallothionein form a part of the pool of cellular thiols in the regulation of cellular redox reactions and their availability is modulated by zinc chelation.

Recognition of protein substrates by protein-disulfide isomerase. A sequence of the b' domain responds to substrate binding

Refolding of partially folded mitochondrial malate dehydrogenase (mMDH) is assisted by protein-disulfide isomerase (PDI). The addition of a 20-fold molar excess of PDI over denatured protein (0. 1 microM) accelerates the recovery of catalytic activity. PDI fluorescence measurements show that 1 mol of PDI binds 1 mol of denatured mMDH when their concentrations approach 1 microM. The binding of PDI, derivatized with the fluorescence probe iodoacetamide fluorescein, to partially folded mMDH is characterized by a dissociation constant of 0.2 microM. It is shown that the fluorescence probe is covalently attached to a SH residue located in the b' domain. Based on the fluorescence measurements of native and derivatized PDI, it is suggested that recognition of the unfolded substrate involves conformational changes propagated to several domains of PDI.

Chaperone activity of DsbC

DsbC, a periplasmic disulfide isomerase of Gram-negative bacteria, displays about 30% of the activities of eukaryotic protein disulfide isomerase (PDI) as isomerase and as thiol-protein oxidoreductase. However, DsbC shows more pronounced chaperone activity than does PDI in promoting the in vitro reactivation and suppressing aggregation of denatured D-glyceraldehyde-3-phosphate dehydrogenase (GAPDH) during refolding. Carboxymethylation of DsbC at Cys98 decreases its intrinsic fluorescence, deprives of its enzyme activities, but lowers only partly its chaperone activity in assisting GAPDH reactivation. Simultaneous presence of DsbC and PDI in the refolding buffer shows an additive effect on the reactivation of GAPDH. The assisted reactivation of GAPDH and the protein disulfide oxidoreductase activity of DsbC can both be inhibited by scrambled and S-carboxymethylated RNases, but not by shorter peptides, including synthetic 10- and 14-mer peptides and S-carboxymethylated insulin A chain. In contrast, all the three peptides and the two nonnative RNases inhibit PDI-assisted GAPDH reactivation and the reductase activity of PDI. DsbC assists refolding of denatured and reduced lysozyme to a higher level than does PDI in phosphate buffer and does not show anti-chaperone activity in HEPES buffer. Like PDI, DsbC is also a disulfide isomerase with chaperone activity but may recognize different folding intermediates as does PDI.

Identifying and characterizing a second structural domain of protein disulfide isomerase

Recent protein engineering studies have confirmed the multidomain nature of protein disulfide isomerase previously suggested on the basis of analysis of its amino acid sequence. The boundaries of three domains, denoted a, a' and b, have been determined, and each domain has been expressed as an individual soluble folded protein. In this report, the boundaries of the final structural domain, b', are defined by a combination of restricted proteolysis and protein engineering approaches to complete our understanding of the domain organization of PDI. Using these data an optimized polypeptide construct has been prepared and characterized with a view to further structural and functional studies.

Ca2+ regulation of interactions between endoplasmic reticulum chaperones

Casade Blue (CB), a fluorescent dye, was used to investigate the dynamics of interactions between endoplasmic reticulum (ER) lumenal chaperones including calreticulin, protein disulfide isomerase (PDI), and ERp57. PDI and ERp57 were labeled with CB, and subsequently, we show that the fluorescence intensity of the CB-conjugated proteins changes upon exposure to microenvironments of a different polarity. CD analysis of the purified proteins revealed that changes in the fluorescence intensity of CB-ERp57 and CB-PDI correspond to conformational changes in the proteins. Using this technique we demonstrate that PDI interacts with calreticulin at low Ca2+ concentration (below 100 microM), whereas the protein complex dissociates at >400 microM Ca2+. These are the Ca2+ concentrations reminiscent of Ca2+ levels found in empty or full ER Ca2+ stores. The N-domain of calreticulin interacts with PDI, but Ca2+ binding to the C-domain of the protein is responsible for Ca2+ sensitivity of the interaction. ERp57 also interacts with calreticulin through the N-domain of the protein. Initial interaction between these proteins is Ca2+-independent, but it is modulated by Ca2+ binding to the C-domain of calreticulin. We conclude that changes in ER lumenal Ca2+ concentration may be responsible for the regulation of protein-protein interactions. Calreticulin may play a role of Ca2+ "sensor" for ER chaperones via regulation of Ca2+-dependent formation and maintenance of structural and functional complexes between different proteins involved in a variety of steps during protein synthesis, folding, and post-translational modification.

Autodegradation of protein disulfide isomerase

Protein disulfide isomerase (PDI) and its degradation products were found in HepG2, COS-1, and CHO-K1 cells. Whether or not the products were formed through autodegradation of PDI was examined, since PDI contains the CGHC motif, which is the active center of proteolytic activity in ER-60 protease. Commercial bovine PDI was autodegraded to produce a trimmed PDI. In addition, human recombinant PDI also had autodegradation activity. Mutant recombinant PDIs with CGHC motifs of which cysteine residues were replaced with serine or alanine residues were prepared. However, they were not autodegraded, suggesting the cysteine residues of motifs are necessary for autodegradation.

Refolding of thioredoxin reductase assisted by groEL and PDI

Thioredoxin reductase was unfolded in 2 M guanidine hydrochloride as revealed by fluorescence and CD spectroscopy. Spontaneous refolding of denatured species resulted in low recovery of 10% catalytic activity after 4 h incubation at 25 degrees C. Addition of groEL or protein disulfide isomerase to the renaturation buffer accelerated the rate of recovery of catalytic activity to a level of 35 and 15%, respectively. Fluorescence spectroscopy has been used to investigate the interaction of groEL and protein disulfide isomerase with denatured thioredoxin reductase tagged with a fluorescent probe. The fluorescence emitted by the denatured protein was quenched upon binding to either groEL or protein disulfide isomerase. It is suggested that encapsulation of the protein substrate by the chaperone plays an important role in the process of folding by facilitating the formation of correctly folded species.

The role of calcium on the activity of ERcalcistorin/protein-disulfide isomerase and the significance of the C-terminal and its calcium binding. A comparison with mammalian protein-disulfide isomerase

ERcalcistorin/protein-disulfide isomerase (ECaSt/PDI) shows a 55% identity with mammalian protein-disulfide isomerase (PDI) (Lucero, H. A., Lebeche, D., and Kaminer, B. (1994) J. Biol. Chem. 269, 23112-23119) is a high capacity low affinity Ca2+-binding protein and behaves as a Ca2+ storage protein in the ER of a living cell (Lucero, H. A., Lebeche, D., and Kaminer, B. (1998) J. Biol. Chem. 273, 9857-9863). Here we show that recombinant ECaSt/PDI bound 26 mol of Ca2+/mol and a C-terminal truncated mutant bound 14 mol of Ca2+/mol, both with a Kd of 2.8 mM in 50 mM KCl and 5.2 mM in 150 mM KCl. The percentage reduction in Ca2+ binding in the mutant corresponded with the percentage reduction of deleted pairs of acidic residues, postulated low affinity Ca2+-binding sites. 5 mM Ca2+ moderately increased the PDI activity of both ECaSt/PDI and the C-terminal truncated mutant on reduced RNase and insulin. Surprisingly, ECaSt/PDI in the absence of Ca2+ prevented the spontaneous reactivation of reduced bovine pancreatic trypsin inhibitor. In the presence of 1-5 mM Ca2+ (or 10 microM polylysine) ECaSt/PDI augmented the bovine pancreatic trypsin inhibitor reactivation rate. In contrast, the C-terminal truncated ECaSt/PDI augmented rBPTI reactivation in the absence of Ca2+ and 1-5 mM Ca2+ further accelerated the reactivation rate, responses similar to those obtained with mammalian PDI.

Redox control of exofacial protein thiols/disulfides by protein disulfide isomerase

Protein disulfide isomerase (PDI) facilitates proper folding and disulfide bonding of nascent proteins in the endoplasmic reticulum and is secreted by cells and associates with the cell surface. We examined the consequence of over- or underexpression of PDI in HT1080 fibrosarcoma cells for the redox state of cell-surface protein thiols/disulfides. Overexpression of PDI resulted in 3.6-4. 2-fold enhanced secretion of PDI and 1.5-1.7-fold increase in surface-bound PDI. Antisense-mediated underexpression of PDI caused 38-53% decreased secretion and 10-33% decrease in surface-bound PDI. Using 5,5'-dithio-bis(2-nitrobenzoic acid) to measure surface protein thiols, a 41-50% increase in surface thiols was observed in PDI-overexpressing cells, whereas a 29-33% decrease was observed in underexpressing cells. Surface thiol content was strongly correlated with cellular (r = 0.998) and secreted (r = 0.969) PDI levels. The pattern of exofacial protein thiols was examined by labeling with the membrane-impermeable thiol reactive compound, 3-(N-maleimidylpropionyl)biocytin. Fourteen identifiable proteins on HT1080 cells were labeled with 3-(N-maleimidylpropionyl)biocytin. The intensity of labeling of 11 proteins was increased with overexpression of PDI, whereas the intensity of labeling of 3 of the 11 proteins was clearly decreased with underexpression of PDI. These findings indicated that secreted PDI was controlling the redox state of existing exofacial protein thiols or reactive disulfide bonds.