Characterization of the menaquinone-dependent disulfide bond formation pathway of Escherichia coli

In the protein disulfide-introducing system of Escherichia coli, plasma membrane-integrated DsbB oxidizes periplasmic DsbA, the primary disulfide donor. Whereas the DsbA-DsbB system utilizes the oxidizing power of ubiquinone (UQ) under aerobic conditions, menaquinone (MK) is believed to function as an immediate electron acceptor under anaerobic conditions. Here, we characterized MK reactivities with DsbB. In the absence of UQ, DsbB was complexed with MK8 in the cell. In vitro studies showed that, by binding to DsbB in a manner competitive with UQ, MK specifically oxidized Cys41 and Cys44 of DsbB and activated its catalytic function to oxidize reduced DsbA. In contrast, menadione used in earlier studies proved to be a more nonspecific oxidant of DsbB. During catalysis, MK8 underwent a spectroscopic transition to develop a visible violet color (lambdamax = 550 nm), which required a reduced state of Cys44 as shown previously for UQ color development (lambdamax = 500 nm) on DsbB. In an in vitro reaction system of MK8-dependent oxidation of DsbA at 30 degrees C, two reaction components were observed, one completing within minutes and the other taking >1 h. Both of these reaction modes were accompanied by the transition state of MK, for which the slower reaction proceeded through the disulfide-linked DsbA-DsbB(MK) intermediate. The MK-dependent pathway provides opportunities to further dissect the quinone-dependent DsbA-DsbB redox reactions.     

Catalysis of disulfide bond formation and isomerization in the Escherichia coli periplasm

Disulfide bond formation is a catalyzed process in vivo. In prokaryotes, the oxidation of cysteine pairs is achieved by the transfer of disulfides from the highly oxidizing DsbA/DsbB catalytic machinery to substrate proteins. The oxidizing power utilized by this system comes from the membrane-embedded electron transport system, which utilizes molecular oxygen as a final oxidant. Proofreading of disulfide bond formation is performed by the DsbC/DsbD system, which has the ability to rearrange non-native disulfides to their native configuration. These disulfide isomerization reactions are sustained by a constant supply of reducing power provided by the cytoplasmic thioredoxin system, utilizing NADPH as the ultimate electron source.     

In vivo formation of Cu,Zn superoxide dismutase disulfide bond in Escherichia coli

We have found that the in vivo folding of periplasmic Escherichia coli Cu,Zn superoxide dismutase is assisted by DsbA, which catalyzes the efficient formation of its single disulfide bond, whose integrity is essential to ensure full catalytic activity to the enzyme. In line with these findings, we also report that the production of recombinant Xenopus laevis Cu,Zn superoxide dismutase is enhanced when the enzyme is exported in the periplasmic space or is expressed in thioredoxin reductase mutant strains. Our data show that inefficient disulfide bond oxidation in the bacterial cytoplasm inhibits Cu,Zn superoxide dismutase folding in this cellular compartment.     

Heterologous expression of lipase in Escherichia coli is limited by folding and disulfide bond formation

Functional expression of lipase B from Pseudozyma antarctica (PalB) in the cytoplasm of Escherichia coli BL21(DE3) and its mutant derivative Origami B(DE3) was explored. Coexpression of DsbA was found to be effective in enhancing PalB expression. The improvement was particularly pronounced with Origami B(DE3) as a host, suggesting that both folding and disulfide bond formation may be major factors limiting PalB expression. Fusion tag technique was also explored by constructing several PalB fusions for the evaluation of their expression performance. While the solubility was enhanced for most PalB fusions, only the DsbA tag was effective in boosting PalB activity, possibly by both enhanced solubility and correct disulfide bond formation. Our results suggest that PalB activity is closely associated with correct disulfide bond formation, and increased solubilization by PalB fusions does not necessarily result in activity enhancement.     

The interplay between the disulfide bond formation pathway and cytochrome c maturation in Escherichia coli

Heme attachment to c-type cytochromes in bacteria requires cysteine thiols in the CXXCH motif of the protein. The involvement of the periplasmic disulfide generation system in this process remains unclear. We undertake a systematic evaluation of the role of DsbA and DsbD in cytochrome c biogenesis in Escherichia coli and show unequivocally that DsbA is not essential for holocytochrome production under aerobic or anaerobic conditions. We also prove that DsbD is important but not essential for maturation of c-type cytochromes. We discuss the findings in the context of a model in which heme attachment to, and oxidation of, the apocytochrome are competing processes.     

Stabilization of the R allosteric structure of Escherichia coli aspartate transcarbamoylase by disulfide bond formation

Here we report the first use of disulfide bond formation to stabilize the R allosteric structure of Escherichia coli aspartate transcarbamoylase. In the R allosteric state, residues in the 240s loop from two catalytic chains of different subunits are close together, whereas in the T allosteric state they are far apart. By substitution of Ala-241 in the 240s loop of the catalytic chain with cysteine, a disulfide bond was formed between two catalytic chains of different subunits. The cross-linked enzyme did not exhibit cooperativity for aspartate. The maximal velocity was increased, and the concentration of aspartate required to obtain one-half the maximal velocity, [Asp](0.5), was reduced substantially. Furthermore, the allosteric effectors ATP and CTP did not alter the activity of the cross-linked enzyme. When the disulfide bonds were reduced by the addition of 1,4-dithio-dl-threitol the resulting enzyme had kinetic parameters very similar to those observed for the wild-type enzyme and regained the ability to be activated by ATP and inhibited by CTP. Small-angle x-ray scattering was used to verify that the cross-linked enzyme was structurally locked in the R state and that this enzyme after reduction with 1,4-dithio-dl-threitol could undergo an allosteric transition similar to that of the wild-type enzyme. The complete abolition of homotropic and heterotropic regulation from stabilizing the 240s loop in its closed position in the R state, which forms the catalytically competent active site, demonstrates the significance that the quaternary structural change and closure of the 240s loop has in the functional mechanism of aspartate transcarbamoylase.     

Respiratory chain strongly oxidizes the CXXC motif of DsbB in the Escherichia coli disulfide bond formation pathway

Escherichia coli DsbB has four essential cysteine residues, among which Cys41 and Cys44 form a CXXC redox active site motif and the Cys104-Cys130 disulfide bond oxidizes the active site cysteines of DsbA, the disulfide bond formation factor in the periplasm. Functional respiratory chain is required for the cell to keep DsbA oxidized. In this study, we characterized the roles of essential cysteines of DsbB in the coupling with the respiratory chain. Cys104 was found to form the inactive complex with DsbA under respiration-defective conditions. While DsbB, under normal aerobic conditions, is in the oxidized state, having two intramolecular disulfide bonds, oxidation of Cys104 and Cys130 requires the presence of Cys41-Cys44. Remarkably, the Cys41-Cys44 disulfide bond is refractory to reduction by a high concentration of dithiothreitol, unless the membrane is solubilized with a detergent. This reductant resistance requires both the respiratory function and oxygen, since Cys41-Cys44 became sensitive to the reducing agent when membrane was prepared from quinone- or heme-depleted cells or when a membrane sample was deaerated. Thus, the Cys41-Val-Leu-Cys44 motif of DsbB is kept both strongly oxidized and strongly oxidizing when DsbB is integrated into the membrane with the normal set of respiratory components.     

Maturation pathway of Escherichia coli heat-stable enterotoxin I: requirement of DsbA for disulfide bond formation.

The Escherichia coli heat-stable enterotoxin STp is synthesized as a precursor consisting of pre, pro and mature regions. Mature STp is released into the culture supernatant and is composed of 18-amino-acid resides which contain three intramolecular disulfide bonds. The involvement of DsbA in the formation of the disulfide bonds of STp was examined in this study. A dsbA mutant was transformed with a plasmid harboring the STp gene, and the ST activity was significantly lower than that of the parent strain harboring the same plasmid. Furthermore, purified DsbA induced the conversion of synthetic STp peptide (inactive form) to the active form and increased the ST activity of the culture supernatant derived from the dsbA transformants. These results showed that DsbA directly catalyzes the formation of the disulfide bonds of STp. DsbA is located in periplasmic space, where STp is released as an intermediate form consisting of pro and mature regions. To examine the effect of the pro region on the action of DsbA, we replaced the cysteine residue at position 39 and tested the effect in vivo. The substitution caused a significant decrease of ST activity in the culture supernatant, the accumulation of inactive ST in periplasmic space, and an alteration in the cleavage site of the intermediate of STp. We conclude that Cys-39 is important for recognition by the processing enzymes required for the maturation of STp.     

The Escherichia coli dsbC (xprA) gene encodes a periplasmic protein involved in disulfide bond formation.

We have identified and functionally characterized a new Escherichia coli gene, dsbC, whose product is involved in disulfide bond formation in the periplasmic space. It corresponds to a previously sequenced open reading frame mapping upstream of recJ with no previously assigned function. Null mutations in dsbC were obtained using a screen for dithiothreitol (DTT)-sensitive mutants and were shown to result in the accumulation of reduced forms of a variety of disulfide bond-containing periplasmic proteins. This defect could be rescued by the addition of either oxidized DTT or cystine or by multicopy expression of dsbA, a known periplasmic disulfide oxidase. The DsbC protein is synthesized as a precursor form of 25.5 kDa which is processed to a 23.3 kDa mature species located in the periplasmic space. The DsbC protein was overexpressed, purified to homogeneity and shown to catalyse the reduction of insulin in a DTT-dependent manner at levels comparable with those of purified DsbA. The replacement of either cysteine residue of the predicted active site, F-(X4)-C-G-Y-C, completely inactivates DsbC protein function. We have further shown that in vivo overexpression of DsbC can functionally substitute for a loss of DsbA function. Taken together, all of our results demonstrate that DsbC acts in vivo as a disulfide oxidase.     

Native disulfide bond formation in proteins

Native disulfide bond formation is critical for the proper folding of many proteins. Recent studies using newly identified protein oxidants, folding catalysts, and mutant cells provide insight into the mechanism of oxidative protein folding in vivo. This insight promises new strategies for more efficient protein production.     

Statistics of disulfide bond formation in proteins

A new set of statistical expressions describing the reformation of disulfide bonds from SH groups is proposed. The results of the statistical calculations of disulfide bond reformation are discussed in terms of protein folding.     

Efficient disulfide bond formation in virus-like particles

Virus-like particles (VLPs) consist of a virus's outer shell but without the genome. Similar to the virus, VLPs are monodisperse nano-capsules which have a known morphology, maintain a high degree of symmetry, and can be engineered to encapsidate the desired cargo. VLPs are of great interest for vaccination, drug/gene delivery, imaging, sensing, and material science applications. Here we demonstrate the ability to control the disulfide bond formation in VLPs by directly controlling the redox potential during or after production and assembly of VLPs. The open cell-free protein synthesis environment, which has been reported to produce VLPs at yields comparable or greater than traditional in vivo technologies, was employed. Optimal conditions for disulfide bond formation were found to be VLP dependent, and a cooperative effect in the formation of such bonds was observed.     

Cooperative disulfide bond formation in apamin.

Apamin is being studied as a model for the folding mechanism of proteins whose structures are stabilized by disulfide bonds. Apamin consists of 18 amino acid residues and forms a stable structure consisting of a C-terminal alpha-helix and two reverse turns. This structure is stabilized by two disulfide bonds connecting Cys-1 to Cys-11 and Cys-3 to Cys-15. We used glutathione and dithiothreitol as reference thiols to measure the stabilities of the two disulfide bonds as a function of urea concentration and temperature in order to understand what contributes to the stability of the native structure. The results demonstrate modest contributions from secondary structure to the overall stability of the two disulfide bonds. The equilibrium constants for disulfide bond formation between the fully reduced peptide and the native structure with two disulfide bonds at 25 degrees C and pH 7.0 are 0.42 M2 using glutathione and 2.7 x 10(-5) using dithiothreitol. The equilibrium constant decreases by a factor of approximately 4 in 8 M urea and decreases by a factor of 3 between 0 and 60 degrees C. At least three one-disulfide intermediates are found at low concentrations in the equilibrium mixture. Using glutathione, the equilibrium constants for forming the one-disulfide intermediates with respect to the reduced peptide are approximately 0.025 M. The second disulfide bond forms with an equilibrium constant of approximately 17 M. Thus, apamin folding is very cooperative, but the native structure is only modestly stabilized by urea- or temperature-denaturable secondary structure.     

Stabilization of Escherichia coli ribonuclease H by introduction of an artificial disulfide bond.

To examine the effect of the introduction of a disulfide bond on the stability of Escherichia coli ribonuclease H, a disulfide bond was engineered between Cys13, which is present in the wild-type enzyme, and Cys44, which is substituted for Asn44 by site-directed mutagenesis. The disulfide bond was only formed between these residues upon oxidation in vitro with redox buffer. The conformational and thermal stabilities were estimated from the guanidine hydrochloride and thermal denaturation curves, respectively. The oxidized (cross-linked) mutant enzyme showed a Tm of 62.3 degrees C, which was 11.8 degrees C higher than that observed for the wild-type enzyme. The free energy change of unfolding in the absence of denaturant, delta G[H2O], and the mid-point of the denaturation curve, [D]1/2, of the oxidized mutant enzyme were also increased by 2.1-2.8 kcal/mol and 0.36-0.48 M, respectively. Introduction of a disulfide bond thus greatly enhanced both the thermal and conformational stabilities of the enzyme. In addition, kinetic analyses for the enzymatic activities of mutant enzymes suggest that Thr43 and Asn44 are involved in the substrate-binding site of the enzyme.     

Crystal structure of the protein disulfide bond isomerase, DsbC, from Escherichia coli

DsbC is one of five Escherichia coli proteins required for disulfide bond formation and is thought to function as a disulfide bond isomerase during oxidative protein folding in the periplasm. DsbC is a 2 x 23 kDa homodimer and has both protein disulfide isomerase and chaperone activity. We report the 1.9 A resolution crystal structure of oxidized DsbC where both Cys-X-X-Cys active sites form disulfide bonds. The molecule consists of separate thioredoxin-like domains joined via hinged linker helices to an N-terminal dimerization domain. The hinges allow relative movement of the active sites, and a broad uncharged cleft between them may be involved in peptide binding and DsbC foldase activities.     

Mode of disulfide bond formation of a heat-stable enterotoxin (STh) produced by a human strain of enterotoxigenic Escherichia coli

To determine the modes of three disulfide linkages in the heat-stable enterotoxin (STh) produced by a human strain of enterotoxigenic Escherichia coli, we synthesized STh(6-18), which consists of 13 amino acid residues and has the same intramolecular disulfide linkages as native STh [(1985) FEBS Lett. 181, 138-142], by stepwise and selective formation of disulfide bonds using different types of removable protecting groups for the Cys residues. Synthesis of the peptide with different modes of disulfide bond formation provided three peptides consistent with standard STh(6-18) in their physicochemical and biological properties, thereby indicating that the disulfide bonds in STh(6-18) are Cys-Cys-Glu-Leu-Cys-Cys-Asn-Pro-Ala-Cys-Thr-Gly-Cys.     

Pathways for protein disulphide bond formation

The folding of many secretory proteins depends upon the formation of disulphide bonds. Recent advances in genetics and cell biology have outlined a core pathway for disulphide bond formation in the endoplasmic reticulum (ER) of eukaryotic cells. In this pathway, oxidizing equivalents flow from the recently identified ER membrane protein Ero1p to secretory proteins via protein disulphide isomerase (PDI). Contrary to prior expectations, oxidation of glutathione in the ER competes with oxidation of protein thiols. Contributions of PDI homologues to the catalysis of oxidative folding will be discussed, as will similarities between eukaryotic and prokaryotic disulphide-bond-forming systems.     

Role of the intramolecular disulfide bond in FlgI, the flagellar P-ring component of Escherichia coli

The P ring of the bacterial flagellar motor consists of multiple copies of FlgI, a periplasmic protein. The intramolecular disulfide bond in FlgI has previously been reported to be essential for P-ring assembly in Escherichia coli, because the P ring was not assembled in a dsbB strain that was defective for disulfide bond formation in periplasmic proteins. We, however, found that the two Cys residues of FlgI are not conserved in other bacterial species. We then assessed the role of this intramolecular disulfide bond in FlgI. A Cys-eliminated FlgI derivative formed a P ring that complemented the flagellation defect of our DeltaflgI strain when it was overproduced, suggesting that disulfide bond formation in FlgI is not absolutely required for P-ring assembly. The levels of the mature forms of the FlgI derivatives were significantly lower than that of wild-type FlgI, although the precursor protein levels were unchanged. Moreover, the FlgI derivatives were more susceptible to degradation than wild-type FlgI. Overproduction of FlgI suppressed the motility defect of DeltadsbB cells. Additionally, the low level of FlgI observed in the DeltadsbB strain increased in the presence of l-cystine, an oxidative agent. We propose that intramolecular disulfide bond formation facilitates the rapid folding of the FlgI monomer to protect against degradation in the periplasmic space, thereby allowing its efficient self-assembly into the P ring.