Expression of protein disulfide isomerase is elevated in the endosperm of the maize floury-2 mutant

A maize protein disulfide isomerase (PDI, EC 5.3.4.1) cDNA clone was isolated and characterized. The deduced amino acid sequence contains two regions characteristic of the active sites for PDI and a carboxyl-terminal endoplasmic reticulum (ER) retention sequence, Lys-Asp-Glu-Leu. Southern blot analysis indicated the maize PDI is encoded by a single gene that maps to the short arm of chromosome 4. When isolated from the cisternal and protein body ER, the PDI protein resolves into a fast and a slow form on SDS-PAGE. During endosperm development, the PDI RNA level increases between 10 and 14 days after pollination. In floury-2 (fl2) endosperm, which contains an abnormally processed alpha-zein protein, PDI expression is significantly increased, and the level of PDI protein and RNA is positively correlated with the dosage of fl2 alleles. The increase of PDI in fl2 occurs mainly in the cisternal ER fraction, whereas the most dramatic increase of binding protein (BiP) is in the protein body ER. We propose that the induction of PDI in the fl2 mutant reflects its role as a molecular chaperone, and that PDI functions in concert with BiP at different stages of zein processing and assembly into protein bodies.

DSBC protein: a new member of the thioredoxin fold-containing family

Prediction of the DsbC protein secondary structure has been performed using a novel prediction technique which is based on consideration of both local and long-range interactions between amino acid residues. The C-terminal portion of the protein is shown to contain the thioredoxin folding motif. The N-terminal part represents a yet unknown structural domain.

Catalysis of oxidative protein folding by mutants of protein disulfide isomerase with a single active-site cysteine

Protein disulfide isomerase (PDI), a very abundant protein in the endoplasmic reticulum, facilitates the formation and rearrangement of disulfide bonds using two nonequivalent redox active-sites, located in two different thioredoxin homology domains [Lyles, M. M., & Gilbert, H. F. (1994) J. Biol. Chem. 269, 30946-30952]. Each dithiol/disulfide active-site contains the thioredoxin consensus sequence CXXC. Four mutants of protein disulfide isomerase were constructed that have only a single active-site cysteine. Kinetic analysis of these mutants show that the first (more N-terminal) cysteine in either active site is essential for catalysis of oxidation and rearrangement during the refolding of reduced bovine pancreatic ribonuclease A (RNase). Mutant active sites with the sequence SGHC show no detectable activity for disulfide formation or rearrangement, even at concentrations of 25 microM. The second (more C-terminal) cysteine is not essential for catalysis of RNase disulfide rearrangements, but it is essential for catalysis of RNase oxidation, even in the presence of a glutathione redox buffer. Mutant active sites with the sequence CGHS show 12%-50% of the kcat activity of wild-type active sites during the rearrangement phase of RNase refolding but < 5% activity during the oxidation phase. In addition, mutants with the sequence CGHS accumulate significant levels of a covalent PDI-RNase complex during steady-state turnover while the wild-type enzyme and mutants with the sequence SGHC do not. Since both active-site cysteines are essential for catalysis of disulfide formation, the dominant mechanism for RNase oxidation may involve direct oxidation by the active-site PDI disulfide. Although it is not essential for catalysis of RNase rearrangements, the more C-terminal cysteine does contribute 2-8-fold to the rearrangement activity. A mechanism for substrate rearrangement is suggested in which the second active-site cysteine provides PDI with a way to "escape" from covalent intermediates that do not rearrange in a timely fashion. The second active-site cysteine may normally serve the wild-type enzyme as an internal clock that limits the time allowed for intramolecular substrate rearrangements.

Co-expression of human protein disulphide isomerase (PDI) can increase the yield of an antibody Fab' fragment expressed in Escherichia coli

Secretion to the periplasm of Escherichia coli enables production of many eukaryotic extracellular proteins in a soluble form. The complex disulphide bond arrangement of such proteins is probably a major factor in determining the low yield of correctly folded product observed in many cases. Here we show that co-expression of human protein disulphide isomerase increased the yield of a monoclonal antibody Fab' fragment in the periplasm of E. coli.

ATP binding and hydrolysis by the multifunctional protein disulfide isomerase

We previously reported the ability of protein disulfide isomerase (PDI) to undergo an ATP-dependent autophosphorylation. Our efforts to map the modification site have been hindered by the low abundance and instability of the labeling. Results are presented in this paper on the nature of phospho-PDI, which appears as an intermediate with a half-life of 2.5-8.8 min in an ATPase reaction. ATP binds to PDI with high affinity, Kd 9.66 microM, and the kinetic parameters KmATP and kcat of the ATPase reaction were measured by using a pyruvate kinase-lactate dehydrogenase-coupled assay under various conditions. Strikingly, the ATPase reaction is stimulated in the presence of denatured polypeptides, while the disulfide oxidization activity of PDI is not affected by ATP. However, PDI is known to participate in various unrelated functions in the endoplasmic reticulum, and ATP could be involved in the regulation of one of these. The results are discussed in light of recent findings on ATP-chaperone relationships.

Roles of cysteine residues of DsbB in its activity to reoxidize DsbA, the protein disulphide bond catalyst of Escherichia coli

BACKGROUND

DsbA, a periplasmic protein, catalyses the disulphide bond formation of other cell surface proteins in E. coli. Reoxidation of DsbA for catalytic turn over is assured by DsbB, a membrane protein with four essential cysteine residues facing the periplasm. We and others previously reported that the reactive Cys30 residue of DsbA forms a mixed disulphide with DsbB in the absence of its partner Cys33 residue.

RESULTS

Under the medium condition in which the DsbA mutant lacking Cys33 forms a mixed disulphide only with DsbB, we examined cysteine mutants of epitope-tagged DsbB for their ability to form the complex. It was shown that Cys104 of DsbB is absolutely required while other three cysteines are also required for maximum interaction. Examination of the redox states of cysteines in wild-type and mutant DsbB suggested that Cys104 and Cys130 form a disulphide bond which will be transferred to DsbA. In agreement with this notion, DsbB mutants lacking one of the N-terminally located cysteines retain weak DsbB activity in vivo. The primary role of the N-terminally located thioredoxin-like motif of DsbB is probably to reoxidize Cys104 and Cys130.

CONCLUSIONS

We propose the following reaction cycle. DsbB is initially oxidized (State A in Summary Figure). Disulphide interaction between Cys30 of DsbA and Cys104 of DsbB should then trigger the recycling reaction of DsbA (State B), allowing over all electron transfer from newly secreted protein via DsbA (Cys30/Cys33) to DsbB in which intrachain electron flow from Cys104/Cys130 (State C) to Cys41/Cys44 (State D) may occur.

Molecular characterisation of plant endoplasmic reticulum. Identification of protein disulfide-isomerase as the major reticuloplasmin

Purified endoplasmic reticulum devoid of contaminating endomembranes has been isolated from both germinating and developing castor bean endosperm by a modified two-step centrifugation procedure. These membranes have been characterised for protein and lipid composition, subfractionated into lumenal and integral membrane protein fractions, and antisera raised to these two components. A cDNA clone encoding a major lumenal protein of 55 kDa was cloned using affinity-purified antisera and shown to encode a protein with strong sequence similarity to the endoplasmic reticulum lumenal chaperone protein disulfide-isomerase. Northern and Southern blot analysis showed that the mRNA from a single-copy gene was constitutively expressed in all tissues investigated, but was preferentially expressed in developing seed where it was the most abundant lumenal protein. Expression of the recombinant protein in Escherichia coli yielded a homodimer with a molecular mass of 110 kDa with protein disulfide-isomerase catalytic activity, thus confirming identity of this protein.

Isolation from spleen of a 57-kDa protein substrate of the tyrosine kinase Lyn. Identification as a protein related to protein disulfide-isomerase and localisation of the phosphorylation sites

A 57-kDa protein (p57) has been purified to homogeneity from a microsomal fraction of rat spleen. It is specifically and efficiently phosphorylated by the Src-like tyrosine kinase Lyn purified from the same source with a Km of 0.34 microM. The tyrosine kinases c-Fgr, Fyn, C-terminal Src kinase and p72syk, as well as the Ser/Thr-specific cAMP-dependent protein kinase and protein kinases CK1 and CK2 do not phosphorylate p57. C-terminal Src kinase, which acts to down-regulate the Src-like protein-tyrosine kinases, almost completely prevents the protein phosphorylation catalysed by Lyn. Protein mass fingerprinting with tryptic fragments identified p57 as a protein related to protein disulfide-isomerase which belongs to the superfamily of Cys-Gly-His-Cys-containing sequences. Lyn phosphorylates tyrosine residues Y444, Y453 and Y466 which are located in a highly acidic region of the protein at the C-terminus. Upon phosphorylation, p57 forms a complex with Lyn which can be immunoprecipitated with anti-Lyn IgG. The association which occurs between the phosphorylated substrate and the SH2 domain of the kinase is consistent with the suggested 'processive phosphorylation' model, which implies that a primary phosphorylation site of the substrate binds to the SH2 domain of the enzyme and triggers the phosphorylation at secondary site(s).

Preferential binding of an unfolded protein to DsbA

The oxidoreductase DsbA from the periplasm of escherichia coli introduces disulfide bonds into proteins at an extremely high rate. During oxidation, a mixed disulfide is formed between DsbA and the folding protein chain, and this covalent intermediate reacts very rapidly either to form the oxidized protein or to revert back to oxidized DsbA. To investigate its properties, a stable form of the intermediate was produced by reacting the C33A variant of DsbA with a variant of RNase T1. We find that in this stable mixed disulfide the conformational stability of the substrate protein is decreased by 5 kJ/mol, whereas the conformational stability of DsbA is increased by 5 kJ/mol. This reciprocal effect suggests strongly that DsbA interacts with the unfolded substrate protein not only by the covalent disulfide bond, but also by preferential non-covalent interactions. The existence of a polypeptide binding site explains why DsbA oxidizes protein substrates much more rapidly than small thiol compounds. Such a very fast reaction is probably important for protein folding in the periplasm, because the accessibility of the thiol groups for DsbA can decrease rapidly when newly exported polypeptide chains begin to fold.

A novel Escherichia coli lipid A mutant that produces an antiinflammatory lipopolysaccharide

A unique screen was used to identify mutations in Escherichia coli lipid A biosynthesis that result in a decreased ability to stimulate E-selectin expression by human endothelial cells. A mutation was identified in the msbB gene of E. coli that resulted in lipopolysaccharide (LPS) that lacks the myristoyl fatty acid moiety of the lipid A. Unlike all previously reported lipid A mutants, the msbB mutant was not conditionally lethal for growth. Viable cells or purified LPS from an msbB mutant had a 1000-10,000-fold reduction in the ability to stimulate E-selectin production by human endothelial cells and TNF alpha production by adherent monocytes. The cloned msbB gene was able to functionally complement the msbB mutant, restoring both the LPS to its native composition and the ability of the strain to stimulate immune cells. Nonmyristoylated LPS acted as an antagonist for E-selectin expression when mixed with LPS obtained from the parental strain. These studies demonstrate a significant role for the myristate component of LPS in immune cell activation and antagonism. In addition, the msbB mutant allowed us to directly examine the crucial role that the lipid A structure plays when viable bacteria are presented to host defense cells.

Cloning, expression and genomic organization of human placental protein disulfide isomerase (previously identified as phospholipase C alpha)

Phosphoinositol-specific Phospholipase C plays an important role in transducing receptor generated signals to the rest of the cell. A cDNA encoding a phospholipase has been described (Bennett et al., 1988, Nature 334, 268-270). However it is probable that this cDNA in fact encodes a protein disulfide isomerase. Since the original work suggested that this enzyme was important in the reproductive tract we sort to clone, sequence, express and characterize the recombinant protein isolated from the placenta. We have cloned and sequenced the cDNA encoding the human homolog of this cDNA from human placenta, although the mRNA was widespread in the female reproductive tract. We have transiently expressed it in both COS cells and also 1BR fibroblasts. Cell lysates were assayed for increased phospholipase activity and protein disulfide activity. We describe the entire cDNA sequence which is highly conserved between species. We have also cloned a portion of the genomic gene and described the intron/exon boundaries. In vitro translation of this cDNA showed that it encoded a protein of 61 kD with a cleavable signal peptide. Transient expression showed the protein produced had no phospholipase activity but did show protein disulfide isomerase activity. The expression work shows that this cDNA indeed encodes a protein disulfide isomerase and not a phospholipase. The nucleotide sequence shows marked conservation of the coding and regulatory regions which may suggest that this enzyme has evolved to perform a highly specialized function.

Protein disulfide isomerase: a multifunctional protein of the endoplasmic reticulum

Protein disulfide isomerase (PDI) is a resident enzyme of the endoplasmic reticulum (ER) that was discovered over three decades ago. Contemporary biochemical and molecular biology techniques have revealed that it is present in all eukaryotic cells studied and retained in the ER via a -KDEL or -HDEL sequence at its C-terminus. However, evidence is accumulating that in certain cell types, PDI can be found in other subcellular compartments, despite possessing an intact retention sequence. A wide range of studies has established that in presence of a redox pair, PDI acts catalytically to both form and reduce disulfide bonds, therefore acting as a disulfide isomerase. Recent studies have focused on the mechanism of the isomerization process and the precise role of the two active site sequences (-CGHC-) in the process. In addition, prokaryotes have been shown to possess a set of proteins that function in a similar fashion, being able to generate disulfide bonds on polypeptides translocated into the periplasmic space. Following the recent discovery that PDI binds peptides, coupled with earlier findings that PDI is a subunit of at least two enzymatic complexes (prolyl 4-hydroxylase and microsomal triglyceride transfer protein), it seems that it may serve functions other than merely that of a disulfide isomerase. In fact, it is now clear that PDI can facilitate protein folding independently of its disulfide isomerase activity. A major challenge for the future is to define mechanistically how it accomplishes isomerization and the relationship between this process and the protein folding steps that culminate in the final, fully mature protein.

Characterization and chromosomal localization of a new protein disulfide isomerase, PDIp, highly expressed in human pancreas

Protein disulfide isomerase (PDI) catalyzes protein folding and thiol-disulfide interchange reactions. The enzyme is localized in the lumen of endoplasmic reticulum (ER) and is abundant in secretory cells of various tissues. In this study we describe the isolation and characterization from human pancreas of a new protein, PDIp, that is structurally and functionally related to PDIs. PDIp cDNA is 1,659 bp in length and predicts a protein with an open reading frame of 511 amino acids. PDIp amino acid sequence shows 46% identity and 66% similarity to that of human PDI. PDIp possesses two thioredoxin-like active sites (WCGHCQ and WCTHCK) and an endoplasmic reticulum retention signal sequence, KEEL, at the carboxyl terminus. Northern analysis of normal human tissues and various human tumor cell lines revealed PDIp mRNA (2.0 kb) expression only in the normal pancreas. Recombinant PDIp protein catalyzed reductive cleavage of insulin and renaturation of reduced RNaseA. Somatic cell genetics and fluorescence in situ hybridization localized the PDIp gene to the short arm of human chromosome 16. It is concluded that PDIp is a new member of the PDI family and is highly expressed in human pancreas.

Unproductive folding of the human G6PD-deficient variant A-

Human glucose-6-phosphate dehydrogenase (G6PD) deficiency almost invariably results from the presence of missense mutations in the X-linked gene encoding G6PD. The common African deficient variant G6PD A- differs from the normal G6PD B by two amino acid substitutions. Only one of these mutations is found on its own, resulting in the nondeficient variant G6PD A. Deficiency is always associated with decreased G6PD activity in red cells, leading to a variety of clinical manifestations. A group of deficient variants, including A-, have near-normal affinity for the substrates G6P and NADP. In these cases, deficiency is caused by a decreased number of catalytically active molecules per cell due to intracellular instability of the mutated G6PD, although the mechanism for this in vivo instability is unknown. Here we report that in vitro folding of the A- variant mainly renders partially folded polypeptides that do not undergo the dimerization required for activity. Under the same conditions, the nondeficient variants B and A undergo folding to produce active dimers with normal mobilities in native gels and normal kinetic properties. The loss of intrinsic folding determinants in the A- variant may underlie the mechanism of its in vivo instability.

Interaction of calreticulin with protein disulfide isomerase

We report here that calreticulin interacts with protein disulfide isomerase (PDI). The PDI-calreticulin complex can be dissociated by Zn(2+)-iminodiacetate-substituted Sepharose-agarose chromatography, suggesting that these interactions may be Zn2+-dependent. Direct interaction between calreticulin and PDI is also documented by calreticulin affinity chromatography. PDI was the only pancreatic microsomal protein retained on the calreticulum affinity column. Calreticulin and PDI were identified by their NH2-terminal amino acid sequence analysis, mobilities in SDS-polyacrylamide gel electrophoresis, binding of 45Ca2+, and their reactivity with specific antibodies. Using glutathione S-transferase-calreticulin fusion proteins, we show that PDI interacts strongly with the P-domain and only weakly with the N-domain of calreticulin. Expression of calreticulin domains and PDI as fusion proteins with GAL4 in the yeast two-hybrid system revealed that calreticulin interacted with PDI also under normal cellular conditions. Interaction with PDI required only the NH2-terminal region of the N-domain (amino acid residues 1-83) and the P-domain (amino acid residues 150-240) of calreticulin. Importantly, interaction between calreticulin and PDI led to the modulation of their activities. In the presence of PDI, calreticulin does not bind Ca2+ with high affinity. Calreticulin or the N-domain of calreticulin inhibited PDI ability to refold scrambled RNase A.

Protein disulfide isomerase mutant lacking its isomerase activity accelerates protein folding in the cell

We investigated the effect of protein disulfide isomerase (PDI) on in vivo protein folding of human lysozyme (h-LZM) in a specially constructed yeast coexpression system. Coexpression with PDI increased the amounts of intracellular h-LZM with the native conformation, leading to an increase in h-LZM secretion. The results indicated that PDI is a real catalyst of protein folding in the cell. The secretion of h-LZM increased even when both active sites of PDI were disrupted, suggesting that the effect of PDI resulted from a function other than the formation of disulfide bonds. This is the first finding that PDI without isomerase activity accelerates protein folding in vivo.

Characterization of the active site cysteine residues of the thioredoxin-like domains of protein disulfide isomerase

The dithiol/disulfide active sites of each of the two isolated thioredoxin-like domains of protein disulfide isomerase (PDI) expressed in Escherichia coli have been characterized in order to understand their catalytic mechanisms and their functions in PDI. In each of the folded domains, as in other proteins of the thioredoxin family, only one of the cysteine residues of the active site sequence -Cys-Gly-His-Cys- is accessible, and its thiol group is highly reactive and has a low pKa value. The kinetics and equilibria have been measured of the reactions between the active site cysteine residues and glutathione, the predominant thiol/disulfide reagent of the endoplasmic reticulum. A disulfide bond can be formed very rapidly between the pair of cysteine residues of each domain, but each disulfide bond is very unstable and reacts rapidly with reduced glutathione. The very low stabilities of these disulfide bonds, which destabilize the protein structures, account for the efficiency with which PDI and each of the isolated domains can introduce disulfide bonds into proteins. These kinetics and equilibrium data go far in helping to understand the catalytic mechanism of PDI and its individual domains.

Why is DsbA such an oxidizing disulfide catalyst?

DsbA, a member of the thioredoxin family of disulfide oxidoreductases, acts in catalyzing disulfide bond formation by donating its disulfide to newly translocated proteins. We have found that the two central residues within the active site Cys-30-Pro-31-His-32-Cys-33 motif are critical in determining the exceptional oxidizing power of DsbA. Mutations that change these two residues can alter the equilibrium oxidation potential of DsbA by more than 1000-fold. A quantitative explanation for the very high redox potential of DsbA was found by measuring the pKa of a single residue, Cys-30. The pKa of Cys-30 varied dramatically from mutant to mutant and could accurately predict the oxidizing power of each DsbA mutant protein.

Gap junction turnover, intracellular trafficking, and phosphorylation of connexin43 in brefeldin A-treated rat mammary tumor cells

Intercellular gap junction channels are thought to form when oligomers of connexins from one cell (connexons) register and pair with connexons from a neighboring cell en route to forming tightly packed arrays (plaques). In the current study we used the rat mammary BICR-M1Rk tumor cell line to examine the trafficking, maturation, and kinetics of connexin43 (Cx43). Cx43 was conclusively shown to reside in the Golgi apparatus in addition to sites of cell-cell apposition in these cells and in normal rat kidney cells. Brefeldin A (BFA) blocked Cx43 trafficking to the surface of the mammary cells and also prevented phosphorylation of the 42-kD form of Cx43 to 44- and 46-kD species. However, phosphorylation of Cx43 occurred in the presence of BFA while it was still a resident of the ER or Golgi apparatus yielding a 43-kD form of Cx43. Moreover, the 42- and 43-kD forms of Cx43 trapped in the ER/Golgi compartment were available for gap junction assembly upon the removal of BFA. Mammary cells treated with BFA for 6 h lost preexisting gap junction "plaques," as well as the 44- and 46-kD forms of Cx43 and functional coupling. These events were reversible 1 h after the removal of BFA and not dependent on protein synthesis. In summary, we provide strong evidence that in BICR-M1Rk tumor cells: (a) Cx43 is transiently phosphorylated in the ER/Golgi apparatus, (b) Cx43 trapped in the ER/Golgi compartment is not subject to rapid degradation and is available for the assembly of new gap junction channels upon the removal of BFA, (c) the rapid turnover of gap junction plaques is correlated with the loss of the 44- and 46-kD forms of Cx43.

Nuclear magnetic resonance characterization of the N-terminal thioredoxin-like domain of protein disulfide isomerase

A genetically engineered protein consisting of the 120 residues at the N-terminus of human protein disulfide isomerase (PDI) has been characterized by 1H, 13C, and 15N NMR methods. The sequence of this protein is 35% identical to Escherichia coli thioredoxin, and it has been found also to have similar patterns of secondary structure and beta-sheet topology. The results confirm that PDI is a modular, multidomain protein. The last 20 residues of the N-terminal domain of PDI are some of those that are similar to part of the estrogen receptor, yet they appear to be an intrinsic part of the thioredoxin fold. This observation makes it unlikely that any of the segments of PDI with similarities to the estrogen receptor comprise individual domains.

Human protein disulfide isomerase functionally complements a dsbA mutation and enhances the yield of pectate lyase C in Escherichia coli

Human PDI was expressed to the Escherichia coli periplasm, by using a plasmid encoded ompA-PDI fusion under the control of the trp promoter. Periplasmic extracts were shown to contain active PDI using the scrambled ribonuclease assay. PDI activity was also demonstrated by complementation of two phenotypes associated with a dsbA mutation. Alkaline phosphatase activity, which is reduced in dsbA cells, was restored to wild type levels by PDI. PelC, a pectate lyase from Erwinia carotovora, was shown to be DsbA dependent in E. coli. PDI was able to restore its activity to that seen in wild type cells. Increased expression of PDI was found to increase the yield of active PelC above that seen in wild type cells. PDI also enhanced the yield of PelC in DsbA- cells but only in the presence of exogenous oxidized glutathione. PDI is thus able to functionally substitute for DsbA in the folding of disulfide-bonded proteins in the bacterial periplasm and to enhance the yield of highly expressed protein when the ability of the E. coli periplasm to fold protein may be saturated. However, our results suggest that the activities of DsbA and PDI in vivo may be different.

The essential function of protein-disulfide isomerase is to unscramble non-native disulfide bonds

Protein-disulfide isomerase (PDI) is an abundant protein of the endoplasmic reticulum that catalyzes dithiol oxidation and disulfide bond reduction and isomerization using the active site CGHC. Haploid pdi1 delta Saccharomyces cerevisiae are inviable, but can be complemented with either a wild-type rat PDI gene or a mutant gene coding for CGHS PDI (shufflease). In contrast, pdi1 delta yeast cannot be complemented with a gene coding for SGHC PDI. In vitro, shufflease is an efficient catalyst for the isomerization of existing disulfide bonds but not for dithiol oxidation or disulfide bond reduction. SGHC PDI catalyzes none of these processes. These results indicate that in vivo protein folding pathways contain intermediates with non-native disulfide bonds, and that the essential role of PDI is to unscramble these intermediates.

Molecular cloning of the human glucose-regulated protein ERp57/GRP58, a thiol-dependent reductase. Identification of its secretory form and inducible expression by the oncogenic transformation

Recently it was shown that putative phospholipase C-alpha cDNA does not code for an isotype of the phospholipase C superfamily but for one of the glucose-regulated proteins (GRPs), ERp57/GRP58. We have isolated human ERp57/GRP58 cDNA from human placenta. Sequence analysis showed that ERp57/GRP58 has two Trp-Cys-Gly-His-Cys-Lys motifs completely conserved among the mammals. Bacterially expressed recombinant ERp57/GRP58 protein contained a thiol-dependent reductase activity which was completely abolished when Ser residues were substituted for Cys residues in both of the two motifs. Furthermore, we have identified a soluble form of ERp57/GRP58 by Western blotting and biosynthetic labeling. In v-onc transformants of normal rat kidney cells, the expression level of ERp57/GRP58 was elevated at the protein level. In NIH3T3 cells transformed with v-src, activated c-src (Y527F) or c-src, the expression level of ERp57/GRP58 was upregulated in proportion to their transforming abilities. These results indicate that a soluble form of ERp57/GRP58 exists and that this protein may control both extracellular and intracellular redox activities through its thiol-dependent reductase activity. Moreover, it is likely that ERp57/GRP58 is involved in the oncogenic transformation.

cDNA cloning and baculovirus expression of the human liver endoplasmic reticulum P58: characterization as a protein disulfide isomerase isoform, but not as a protease or a carnitine acyltransferase

The function of a 58-kDa liver microsomal protein (P58) is controversial. To help clarify the physiological function of this protein, particularly in humans, a full-length human liver cDNA clone was isolated, sequenced, and expressed in milligram quantities with the use of a baculovirus expression system. The deduced amino acid sequence of the mature protein contained two thioredoxin-like active site motifs (CGHC) and in its C-terminus a nuclear localization motif (KPKKKKK), and an ER-retention/retrieval motif (QEDL). The mature form of human P58 shared 95% amino acid sequence identity with the deduced amino acid sequences of a bovine liver cDNA, 93% with a murine B lymphocyte cDNA, and 91% with a rat basophilic leukemia cell cDNA. In contrast to reports on the activities of nonhuman forms of P58, the purified expressed human P58 showed no carnitine acyltransferase or protease activities. However, it did have protein disulfide isomerase activity, indicating that the physiological activity of human liver P58 may be attributed, at least in part, to this activity.

Ionisation of cysteine residues at the termini of model alpha-helical peptides. Relevance to unusual thiol pKa values in proteins of the thioredoxin family

The physical basis of the unusually low pKa values of an active site cysteine thiol group in proteins with the thioredoxin fold is unknown. The electrostatic field associated with an alpha-helix pointing with its N terminus towards the cysteine residue has been implicated to lower the thiol pKa value by up to 5 pH units in glutaredoxin and DsbA. Here, the influence of the presence of an alpha-helical conformation on the ionisation of a cysteine thiol group located at or near the helix terminus is investigated in highly helical synthetic peptides with the generic sequence Ac-AAAAAAAAARAAAARAAAARAA-(NH2). The thiol pKa values have been determined by monitoring the pH dependence of the absorbance at 240 nm, of the alpha-helix content measured by the mean residue ellipticity at 222 nm, and of the chemical shifts of protons close to the sulphur atom of the cysteine residue. The favourable interaction between the thiolate anion at the N terminus and the alpha-helix decreases the thiol pKa value by up to 1.6 pH units when compared to a normal thiol pKa value measured in an unfolded control peptide, corresponding to a stabilisation energy of 2.1 kcal/mol. At the C terminus, the thiol pKa value is increased, but by only 0.2 pH units. The observations are consistent with an interaction of the alpha-helix dipole with the cysteine thiolate anion, involving both its charge and hydrogen-bonding. Subtle conformational effects in different model peptides appear to influence the ionisation of the thiol group significantly, with an N terminal Cys-Pro sequence having the most favourable interaction with the alpha-helix.

Defective protein folding as a basis of human disease

The ability of a polypeptide to fold into a unique, functional, three-dimensional structure in vivo is dependent upon its amino acid sequence and the function of molecular chaperone proteins and enzymes that catalyse folding. Intense study of the physical chemistry and cell biology of folding have greatly aided our understanding of the mechanisms normally employed. Evidence is accumulating that many disease-causing mutations and modifications exert their effects by altering protein folding. Here we discuss the pathobiology of these processes.

Cloning and sequencing of the cDNA encoding human P5

The cDNA encoding human P5 was cloned and sequenced. The predicted 440-amino-acid (aa) sequence of human P5 contains two thioredoxin-like domains, which are also found in members of the protein disulfide isomerase superfamily. The human and hamster P5 genes reveal 87 and 93% similarity in their nucleotide and deduced aa sequences, respectively.

Oxidation of kinetically trapped thiols by protein disulfide isomerase

The formation of a stabilized structure during oxidative protein folding can severely retard disulfide formation if the structure must be disrupted to gain access to buried cysteines. These kinetic traps can slow protein folding and disulfide bond formation to the extent that unassisted folding is too slow to be kinetically competent in the cell. Protein disulfide isomerase (PDI) facilitates the oxidation of a kinetically trapped state of RTEM-1 beta-lactamase in which two cysteines that form the single disulfide bond in the native protein are buried and approximately 500-fold less reactive than exposed cysteines. Under second-order conditions, PDI-dependent oxidation of reduced, folded beta-lactamase is 500-fold faster than GSSG-dependent oxidation. The rate difference observed between PDI and GSSG can be accounted for by the 520-fold higher kinetic reactivity of PDI as an oxidant. Noncovalent interactions between PDI (35 microM) and beta-lactamase increase the reactivity or unfolding of beta-lactamase in the steady-state by less than 3-fold. At high concentrations of PDI or alkylating agents, the reaction of beta-lactamase cysteines approaches a constant rate, limited by the spontaneous unfolding of the protein (kunfold = 0.024 +/- 0.005 min-1). PDI does not substantially increase the rate of beta-lactamase unfolding; however, once beta-lactamase spontaneously unfolds, PDI at concentrations greater than 44 +/- 4 microM, oxidizes the unfolded substrate before it can refold (kfold = 1.5 +/- 0.2 min-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that the pathway of disulfide bond formation in Escherichia coli involves interactions between the cysteines of DsbB and DsbA

Disulfide bond formation is catalyzed in the periplasm of Escherichia coli. This process involves at least two proteins: DsbA and DsbB. Recent evidence suggests that DsbA, a soluble periplasmic protein directly catalyzes disulfide bond formation in proteins, whereas DsbB, an inner membrane protein, is involved in the reoxidation of DsbA. Here we present direct evidence of an interaction between DsbA and DsbB. (Kishigami et al. [Kishigami, S., Kanaya, E., Kikuchi, M. & Ito, K. (1995) J. Biol. Chem. 270, 17072-17074] have described similar findings.) We isolated a dominant negative mutant of dsbA, dsbAd, where Cys-33 of the DsbA active site is changed to tyrosine. Both DsbAd and DsbA are able to form a mixed disulfide with DsbB, which may be an intermediate in the reoxidation of DsbA. This complex is more stable with DsbAd. The dominance can be suppressed by increasing the production of DsbB. By using mutants of DsbB in which one or two cysteines have been changed to alanine, we show that only Cys-104 is important for complex formation. Therefore, we suggest that in vivo, reduced DsbA forms a complex with DsbB in which Cys-30 of DsbA is disulfide-bonded to Cys-104 of DsbB. Cys-104 is rapidly replaced by Cys-33 of DsbA to generate the oxidized form of this protein.

The refolding of hen egg white riboflavin-binding protein: effect of protein disulphide isomerase on the reoxidation of the reduced protein

Hen egg white riboflavin-binding protein (RfBP) contains nine disulphide bonds. Provided these remain intact, the refolding of RfBP after incubation in 6 M guanidinium chloride is highly efficient with at least 95% of the binding activity regained within 3 min. Kinetic studies indicate that this regain consists of at least two phases. When the disulphide bonds of RfBP are reduced, reoxidation using a mixture of oxidized and reduced glutathione leads to less than 5% recovery of activity. However, if protein disulphide isomerase (PDI; EC 5.3.4.1) is present during the reoxidation nearly 50% activity can be regained, suggesting that PDI may play an important role in the maturation of RfBP in vivo.

TolC and DsbA are needed for the secretion of STB, a heat-stable enterotoxin of Escherichia coli

STB secretion-deficient mutants were isolated using the synthetic transposon Tn beta laM. Cultures were plated using a double-membrane system of cellulose acetate and nitrocellulose placed on Luria agar plates containing carbenicillin. The STB bound to the underlying nitrocellulose membrane was detected with anti-STB antibodies. The altered genes of two STB secretion-deficient mutants were identified by conjugation and complementation as tolC and dsbA. In cultures of well-characterized dsbA and tolC mutants, STB was absent from the culture supernatant. The role of TolC and DsbA in the secretion of peptides is discussed.

An essential role for DsbA in cytochrome c synthesis and formate-dependent nitrite reduction by Escherichia coli K-12

An Escherichia coli K-12 mutant, isolated on the basis of its inability to catalyze formate-dependent nitrite reduction, was characterized. The mutant was defective in the synthesis of all known c-type cytochromes during anaerobic growth. The mutation was localized by conjugation, transduction, and Southern blotting experiments to the dsbA gene at minute 87 on the E. coli chromosome and was complemented by the wild-type allele. Both DsbA and the recently described DipZ protein were shown to be essential for cytochrome c synthesis, suggesting that they act sequentially in a pathway for cytochrome c assembly in the E. coli periplasm.

Cytochrome P450 catalyzed covalent binding of methoxychlor to rat hepatic, microsomal iodothyronine 5'-monodeiodinase, type I: does exposure to methoxychlor disrupt thyroid hormone metabolism?

The insecticide methoxychlor is estrogenic in birds and mammals and interferes with sexual development and reproduction, but it is not known whether this toxicity is due solely to its estrogenicity. We now have found that during hepatic, microsomal metabolism of [ring-14C]- or [3H-OCH3]methoxychlor, their metabolite primarily binds to iodothyronine 5'-monodeiodinase, type I (5'-ID1). The purified, radiolabeled protein reacted with antibodies against protein disulfide isomerase, isoform Q5, which is highly homologous to 5'-ID1. Sequencing of the radiolabeled tryptic peptide indicated that methoxychlor bound to cysteine 372 or 375 or to lysine 376 of 5'-ID1. Treatment of rats with methoxychlor for 4 days decreased hepatic, microsomal 5'-ID1 activity from 2.94 to 2.20 nmol/min-mg prot (P < 0.02). Since 5'-ID1 catalyzes thyroxine conversion to the biologically active triiodothyronine, these data suggest that methoxychlor may interfere with thyroid hormone metabolism. This may be an additional factor in its environmental toxicity.

Production of rat protein disulfide isomerase in Saccharomyces cerevisiae

Protein disulfide isomerase (PDI) is an abundant protein of the endoplasmic reticulum that catalyzes the oxidation of protein sulfhydryl groups and the isomerization and reduction of protein disulfide bonds. Saccharomyces cerevisiae cells lacking PDI are inviable. PDI is a component of many different protein processing complexes, and the actual activity of PDI that is required for cell viability is unclear. A cDNA that codes for rat PDI fused to the alpha-factor pre-pro segment was expressed in a protease-deficient strain of S. cerevisiae under the control of an ADH2-GAPDH hybrid promoter. The cells processed the resulting protein and secreted it into the medium as a monomer, despite having a KDEL or HDEL sequence at its C-terminus. The typical yield of isolated protein was 2 mg per liter of culture. The catalytic activity of the PDI from S. cerevisiae was indistinguishable from that of PDI isolated from bovine liver. This expression system is unique in allowing the same plasmid to be used both to complement pdi1 delta S. cerevisiae and to produce PDI for detailed in vitro analyses. Correlations of the in vivo behavior and in vitro properties of PDI are likely to reveal structure-function relationships of biological importance.

A novel function of Escherichia coli chaperone DnaJ. Protein-disulfide isomerase

Molecular chaperones, protein-disulfide isomerases, and peptidyl prolyl cis-trans isomerases assist protein folding in both prokaryotes and eukaryotes. The DnaJ protein of Escherichia coli and the DnaJ-like proteins of eukaryotes are known as molecular chaperones and specific regulators of DnaK-like proteins and are involved in protein folding and renaturation after stress. In this study we show that DnaJ, like thioredoxin, protein-disulfide isomerase, and DsbA, possesses an active dithiol/disulfide group and catalyzes protein disulfide formation (oxidative renaturation of reduced RNase), reduction (reduction of insulin disulfides), and isomerization (refolding of randomly oxidized RNase). These results suggest that, in addition to its known function as a chaperone, DnaJ might be involved in controlling the redox state of cytoplasmic, membrane, or exported proteins.

Molecular cloning of the cDNA encoding a novel protein disulfide isomerase-related protein (PDIR)

We isolated the cDNA of a novel protein disulfide isomerase (PDI)-related protein, designated PDIR, from a human placental cDNA library. Deduced from its nucleotide sequence, PDIR has the three CXXC-like motifs (Cys-Ser-Met-Cys, Cys-Gly-His-Cys and Cys-Pro-His-Cys), which are found in proteins within the PDI superfamily and are responsible for oxidoreductase activity. PDIR has a hydrophobic stretch at its amino terminus, which may serve as a signal sequence, and the putative endoplasmic reticulum (ER) retention signal 'Lys-Glu-Glu-Leu' at its carboxy terminus, indicating that PDIR is an ER resident protein. Northern blots showed that PDIR is preferentially expressed in cells actively secreting proteins and that the expression of PDIR is stress-inducible. These results suggested that PDIR has oxidoreductase activity of disulfide bonds against polypeptides and that it acts as a catalyst of protein folding in the lumen of the ER.

Functional properties of the individual thioredoxin-like domains of protein disulfide isomerase

The two thioredoxin-like domains of human protein disulfide isomerase (PDI) have been produced in bacteria as individual soluble, folded protein molecules, and their functional properties have been compared to those of intact PDI. The two individual domains were very similar in their functional properties, and there were no indications of synergy between them, so it is unlikely that they have intrinsically different functions in PDI. Both domains efficiently introduced disulfide bonds into unfolded model proteins and peptides but were less efficient than PDI with folded substrate protein molecules. Relative to PDI, neither domain had substantial activity in catalyzing disulfide bond isomerization. This pattern of activities is very similar to that of the bacterial catalyst DsbA and probably reflects similarities in the catalytic mechanisms of these proteins. The differences in activity between PDI and its thioredoxin-like domains suggest that other features of the PDI molecule are also required for its complete range of thiol-disulfide exchange activities.

Protein disulphide isomerase and a lumenal cyclophilin-type peptidyl prolyl cis-trans isomerase are in transient contact with secretory proteins during late stages of translocation

The transport of a presecretory protein into the mammalian endoplasmic reticulum can be divided into early translocation events which include specific targeting of the presecretory protein to and insertion into the endoplasmic reticulum membrane and late translocation events, comprising signal sequence cleavage, completion of translocation and folding of the secretory protein into a functional conformation. The microsomal membrane proteins Sec61 alpha p and translocating-chain-associating membrane protein were previously identified as being in close contact with a nascent presecretory protein at an early step of translocation. Here, we investigated whether additional microsomal proteins are in contact with translocating chains during or immediately after transit. This was addressed by crosslinking after release of the nascent chain from Sec61 alpha p. We observed two additional membrane proteins interacting with the nascent precursor in the early stages of translocation and three lumenal proteins interacting with the processed polypeptide chain in the late stages of translocation. One of the lumenal proteins was identified as protein disulphide isomerase by immunoprecipitation. Another of the lumenal proteins was suggested to be a lumenal cyclophilin-type peptidyl prolyl cis-trans isomerase by the effect of cyclosporin A. We propose that molecular chaperones, such as protein disulphide isomerase and cyclophilin may represent two of the lumenal proteins which are involved in completion of translocation.

Localization of protein disulfide isomerase to the external surface of the platelet plasma membrane

Protein disulfide isomerase (PDI) is an enzyme that catalyzes the formation as well as the isomerization of disulfide bonds. In this study, antibodies against PDI were used to show PDI antigen on the platelet surface by indirect immunofluorescence microscopy and by flow cytometry. The platelets were not activated, as evidenced by the absence of staining by an antibody against P-selectin. Permeabilized platelets showed little cytosolic PDI by indirect immunofluorescence microscopy, suggesting that the majority of platelet PDI is localized to the platelet surface. PDI activity against "scrambled" RNase was shown with intact platelets. The activity was inhibited by inhibitors of PDI and by an antibody against PDI. Other blood cells showed little PDI. Platelet surface PDI may play a role in the various physiological and pathophysiologic processes in which platelets are involved.

Components of the protein synthesis and folding machinery are induced in vascular smooth muscle cells by hypertrophic and hyperplastic agents. Identification by comparative protein phenotyping and microsequencing

Vascular smooth muscle cells (VSMC) are the principal cellular component of the blood vessel wall. Atherosclerosis, hypertension, and angiogenesis are associated with abnormal VSMC growth. Angiotensin II is hypertrophic for cultured adult rat aortic VSMC, whereas platelet-derived growth factor and serum are hyperplastic. To identify changes in specific proteins associated with either hyperplastic or hypertrophic growth, high resolution two-dimensional gel electrophoresis was performed on extracts from quiescent rat aortic VSMC and from VSMC exposed for 24 h to growth factors (10% fetal calf serum, platelet-derived growth factor, or angiotensin II). 12 proteins were up-regulated and 5 down-regulated by treatment with growth factors. Eight of the up-regulated and one of the down-regulated proteins were identified by internal protein microsequencing from electroblotted two-dimensional gels or by co-electrophoresis of purified proteins in two-dimensional gels. Four of the proteins up-regulated by growth factors were identified as mediators of protein folding. These were heat shock proteins, HSP-60 and HSP-70, protein disulfide isomerase, and protein disulfide isomerase isozyme Q-2. Additional proteins were identified as elongation factor EF-1 beta, a component of the protein synthesis apparatus, and calreticulin, another putative molecular chaperone. Vimentin and actin were also up-regulated, whereas an isoform of myosin heavy chain was down-regulated. Hyperplastic and hypertrophic growth were accompanied by similar changes in protein expression, suggesting that both types of growth require up-regulation of the protein synthesis and folding machinery.

Alteration of hybridoma viability and antibody secretion in transfectomas with inducible overexpression of protein disulfide isomerase

Monoclonal antibody (mAb)-secreting transfectomas with dexamethasone inducible expression of the mammalian endoplasmic reticulum foldase and chaperone protein disulfide isomerase (PDI, ERp59) were generated from the murine 9.2.27 hybridoma in order to obtain in vivo evidence of whether alteration of the level of PDI, believed to be involved in immunoglobulin (Ig) assembly, results in alteration of mAb secretion kinetics. Using an RNase refolding assay, the specific activity of endogenous PDI in the 9.2.27 hybridoma was found to be constant during batch growth. An expression vector for glucocorticoid-inducible overexpression of PDI, pMMTVPDI, was constructed from pMAMneo using a rat PDI cDNA. Cell lysates of stable transfectomas contained 2-4-fold higher levels of PDI mRNA and increased levels of PDI protein, detected by immunoblotting, following induction with 0.1 microM dexamethasone. Monoclonal antibody secretion kinetics were evaluated in 12.5 mL shake flasks, a 100 mL spinner, and a 1 L aerated batch reactor. A transfectoma was found with altered mAb secretion kinetics during cell growth following dexamethasone induction of PDI overexpression. Specific mAb secretion rate was not significantly increased following dexamethasone induction; however, hybridoma viability was sustained longer during the stationary phase of cell growth and hence total antibody yield was increased in comparison to the parent 9.2.27 hybridoma.

Purification and characterization of a trypanothione-glutathione thioltransferase from Trypanosoma cruzi

Although trypanothione [T(S)2] is the major thiol component in trypanosomatidae, significant amounts of glutathione are present in Trypanosoma cruzi. This could be explained by the existence of enzymes using glutathione or both glutathione and T(S)2 as cofactors. To assess these hypotheses, a cytosolic fraction of T. cruzi epimastigotes was subjected to affinity chromatography columns using as ligands either S-hexylglutathione or a non-reducible analogue of trypanothione disulphide. A similar protein of 52 kDa was eluted in both cases. Its partial amino acid sequence indicated that it was identical with the protein encoded by the TcAc2 cDNA previously described [Schoneck, Plumas-Marty, Taibi et al. (1994) Biol. Cell 80, 1-10]. This protein showed no significant glutathione transferase activity but surprisingly catalysed the thiol-disulphide exchange between dihydrotrypanothione and glutathione disulphide. The kinetic parameters were in the same range as those determined for trypanothione reductase toward its natural substrate. This trypanothione-glutathione thioltransferase provides a new target for a specific chemotherapy against Chagas' disease and may constitute a link between the glutathione-based metabolism of the host and the trypanothione-based metabolism of the parasite.

Secretion, surface localization, turnover, and steady state expression of protein disulfide isomerase in rat hepatocytes

Protein disulfide isomerase in isolated rat hepatocytes was present at a concentration of 7 micrograms/mg cell protein, representing a approximately 2-fold enrichment compared to isolated hepatic non-parenchymal cells. Though localized mainly in microsomal fractions of hepatocytes, direct immunofluorescence and cell surface radioiodination followed by immunoprecipitation revealed the presence of M(r) 57,000 disulfide isomerase at the cell surface. Electrostatic interaction of the protein with the cell surface was suggested by susceptibility to carbonate washing. Metabolic radiolabeling and immunoprecipitation studies also indicated that some of the newly synthesized M(r) 57,000 disulfide isomerase was secreted. Treatment of cells with colchicine markedly reduced the recovery of disulfide isomerase from the media, indicating microtubular-directed secretion of the protein. Partial staphlococcal V8 proteolytic digestion of the secreted protein revealed a peptide pattern similar to that of the cellular protein. Immunoprecipitation with antibody specific to the -KDEL peptide retention sequence confirmed the presence of this sequence in the secreted protein. Studies of the turnover of disulfide isomerase revealed a half-life of approximately 96 h. Treatment of cells with tunicamycin or heat shock resulted in an increased recovery of newly synthesized disulfide isomerase from cell lysates but diminished recovery from the media. The secretion and cell surface distribution of disulfide isomerase in hepatocytes may be important for the pathogenesis of immune mediated liver injury.

Vibrio spp. secrete proaerolysin as a folded dimer without the need for disulphide bond formation

Proaerolysin is an extracellular dimeric protein that is secreted across the inner and outer membranes of Aeromonas spp. in separate steps. To investigate the role of protein folding in the second step, one or more cysteine residues were introduced and the mutant proaerolysins were expressed in Aeromonas hydrophila and Aeromonas salmonicida, as well as Vibrio cholerae. Replacing Met-41 with Cys resulted in expression of a protein that could form a dimer in which the monomers were linked together by a disulphide bridge. A double mutant was also made, in which Gly-202 and Ile-445 were replaced with cysteine in order to allow the formation of an intrachain disulphide bridge when the molecule was correctly folded. The M41C covalent dimer and G202C/I445C proaerolysin with the new intrachain bridge were both easily detected inside the bacteria, and they later appeared in the culture supernatants. Small amounts of incorrectly folded proaerolysin were also observed in the cells, but they were not secreted. We observed in the cells, but they were not secreted. We conclude that proaerolysin folds and dimerizes before being released from the cell, and that correct folding is a requirement for secretion to occur. The proton ionophore CCCP reduced release of the folded proteins. Unoxidized protein was secreted by cells grown in beta-mercaptoethanol and by a dsbA mutant of V. cholerae, indicating that disulphide bond formation may not be essential for release.

Active site peptides with CXXC motif on map-resin can mimic protein disulfide isomerase activity

Two conserved Trp-Cys-Gly-His-Cys (WCGHC) sequences are assigned to act as catalytic sites for protein disulfide isomerase. Peptides containing the active site sequence, Ala-Pro-Trp-Cys-Gly-His-Cys-Lys(APWCGHCK), were synthesized both in a mono-molecular form and on multiple antigen peptide (MAP) resin or Wang resin by the 9-fluoroenylmethoxycarbonyl (Fmoc)-based solid-phase method. With scrambled RNase as a substrate, the (APWCGHCK)8-MAP was first shown to mimic the PDI activity, which was one thousandth of that of bovine PDI and comparable to that of thioredoxin. APWCGPCK and APWCGHCK, however, did not display a disulfide isomerase activity even at a concentration 8 times higher than that of (APWCGHCK)8-MAP. It was assumed that a sterically proper proximity of at least two active site peptides with CXXC motif was required for the expression of PDI activity.

Proton sharing between cysteine thiols in Escherichia coli thioredoxin: implications for the mechanism of protein disulfide reduction

Proton sharing between acidic groups has been observed in the active sites of several enzymes, including bacteriorhodopsin, aspartic proteases, and ribonuclease HI. We here report NMR observations suggestive of proton sharing between cysteine thiols in the active site of the oxidation-reduction enzyme thioredoxin. The pKas of the two cysteine thiols in the Escherichia coli protein are removed from the expected value of 8.4 by approximately 1 pH unit in either direction, upward and downward. Further, the C beta resonances of both residues show clearly the effects of both of these pKas, indicating that the titrations of the two thiol groups are intimately linked. This behavior strongly suggests that the low pKa ascribed to the deprotonation of the Cys 32 thiol most likely arises through the interaction and close approach of the thiol of Cys 35, with the thiolate anion of Cys 32 stabilized through the sharing of the remaining thiol proton, nominally attached to Cys 35. These observations provide a rationale for the mediation of active site pH control, an important aspect of the mechanism of thioredoxin and other proteins with catalytic thioredoxin domains, such as protein disulfide isomerases.

Isolation and characterization of a yeast gene, MPD1, the overexpression of which suppresses inviability caused by protein disulfide isomerase depletion

MPD1, a yeast gene the overexpression of which suppresses the inviability caused by the loss of protein disulfide isomerase (PDI) was isolated and characterized. The MPD1 gene product retained a single disulfide isomerase active site sequence (APWCGHCK), an N-terminal putative signal sequence, and a C-terminal endoplasmic reticulum (ER) retention signal, and was a novel member of the PDI family. The gene product, identified in yeast extract, contained core size carbohydrates. MPD1 was not essential for growth, but overexpression of the gene suppressed the maturation defect of carboxypeptidase Y caused by PDI1 deletion, indicative of the related function to PDI in the yeast ER.

Repression of transcriptional activity by heterologous KRAB domains present in zinc finger proteins

We report the characterization of three novel members of the KRAB-domain containing C2-H2 zinc finger family (ZNF133, 136 and 140). KRAB (Krüppel-associated box) is an evolutionarily conserved protein domain found N-terminally with respect to the zinc finger repeats that encodes the DNA binding domain. ZNF133 and ZNF140 have both the KRAB A- and KRAB B-boxes present at their N-terminus, whereas ZNF136 contains only the KRAB A-box. We have previously demonstrated that the KRAB domains derived from ZNF133 and ZNF140 are potent transcriptional repression domains [Margolin et al. (1994) Proc. Natl. Acad. Sci. USA 91, 4509-4513]. The KRAB domain from ZNF136, containing only subdomain A, is a considerable weaker suppression domain; however, when fused to the heterologous KRAB B subdomain of ZNF10 (KOX1) the two subdomains from a KRAB domain which induces repression as potently as previously reported KRAB domains.