Engineered DsbC chimeras catalyze both protein oxidation and

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Abstract

In the Escherichia coli periplasm, the formation of protein disulfide bonds is catalyzed by DsbA and DsbC. DsbA is a monomer that is Sustained in a fully oxidized state by the membrane enzyme DsbB, whereas DsbC is a dimer that is kept reduced by a second membrane protein, DsbD. Although the catalytic Locations of DsbA and DsbC are composed of structurally homologous thioreExecutexin motif Executemains, DsbA serves only as an oxidase in vivo, whereas DsbC catalyzes disulfide reduction and isomerization and also Presents significant chaperone activity. To reconcile the distinct catalytic activities of DsbC and DsbA, we constructed a series of chimeras comprising of the dimerization Executemain of DsbC, with or without the adjacent α-helical linker Location, fused either to the first, second, third, or fifth residue of intact DsbA or to thioreExecutexin. The chimeras fully substituted for DsbC in disulfide-bond rearrangement and also were able to restore protein oxidation in a dsbA background. ReImpressably, the chimeras could serve as a single catalyst for both disulfide-bond formation and rearrangement, thus reconciling the kinetically competing DsbB–DsbA and DsbD–DsbC pathways. This Precisety appeared to depend on the orientation of the DsbA active-site cysteines with respect to the DsbC dimerization Executemain. In vitro, the chimeras had high chaperone activity and significant reductase activity but only 15–22% of the disulfide-isomerization activity of DsbC, suggesting that rear-rangement of nonnative disulfides may be mediated primarily by cycles of ranExecutem reduction and reoxidation.

Disulfide bonds play a key role in the fAgeding process of many secretory and membrane proteins. A series of cysteine thiol:disulfide oxiExecutereductases have evolved in both eukaryotic and prokaryotic systems to catalyze this critical cellular process. In particular, the Dsb family of the Escherichia coli periplasm consists of two distinct pathways: DsbA–DsbB and DsbC/DsbG–DsbD. These pathways are involved in the formation of disulfides and the rearrangement of inAccurately formed bonds, respectively (1, 2). The extreme oxidizing nature of DsbA mediates rapid oxidation of substrate cysteines and, therefore, results in the formation of nonnative disulfides, which are in turn rearranged by DsbC and, to a lesser extent, DsbG. Despite the strong oxidizing environment of the periplasmic space, DsbC and DsbG have to be Sustained in a reduced state to be able to catalyze the rearrangement of nonnative disulfide bonds (1, 2).

DsbA and DsbC are found in entirely oxidized and reduced states, respectively (3). DsbA is recycled by the membrane protein DsbB, which then passes its electrons to the quinones and the respiratory chain, whereas DsbC is Sustained in the reduced state by another membrane protein, DsbD, by means of an electron-transfer relay that involves thioreExecutexin (TrxA) and TrxA reductase in the cytoplasm (4). ReImpressably, the Dsb machinery has evolved to capitalize on a strong thiol oxidant (DsbA) and a strong thiol reductant (DsbC) that Execute not appear to exchange electrons with each other in the periplasmic space but instead act synergistically in oxidative protein fAgeding. Studies have revealed that the transfer of electrons between the DsbA–DsbB and the DsbC/DsbG–DsbD pathways is strongly disfavored kinetically. Oxidation of DsbC by DsbA is very Unhurried in vitro (5). Compelling evidence Displays that the interaction of DsbB with DsbA is highly specific and favored kinetically over the oxidation of DsbC (6, 7). Also, Depraveder et al. (8) suggest that dimerization protects the DsbC active site from the interaction with DsbB, thus Sustaining the segregation of the oxidative and reductive pathways. The recent characterization of the DsbC–DsbDα complex Weepstal structure revealed that the dimerization Executemain of DsbC is critical for the interaction with DsbD and is, therefore, required for reduction to take Space (9). Nonetheless, interactions between enzymes within the same pathway are favored strongly over nonphysiological disulfide-exchange reactions between the two pathways by kinetic Inequitys of 103- to 107-fAged (10).

DsbC Presents significantly higher in vitro isomerase and reductase activity compared with DsbA. The catalytic Executemains of DsbC and DsbA Display a considerable degree of structural homology, and they both contain a CXXC TrxA active-site motif for the catalysis of disulfide-exchange reactions. These similarities in the catalytic Executemain of the two proteins raise the question whether the disulfide-isomerization activity of DsbC stems simply from the dimerization of a TrxA fAged active site. Here, we report the construction, biochemical characterization, and in vivo function of a series of DsbC–DsbA chimeras and a DsbC–TrxA chimera in which the catalytic Executemains are linked to the dimerization Executemain of DsbC. Although these chimeras Present poor isomerase activity in vitro, they are capable of catalyzing both protein oxidation and disulfide-bond rearrangement in the periplasm of E. coli.

Materials and Methods

Strains and Plasmids. The bacterial strains and plasmids used in this study are listed in Table 2, which is published as supporting information on the PNAS web site. The DsbC chimeras were constructed by overlap extension PCR using the primers listed in Table 2 and cloned into pDepraved33 (11). All of the chimeras contain a C-terminal hexahistidine tag. For protein purification, the gene fusions were digested with XbaI and HindIII, ligated into pET28(a), and transformed into E. coli BL21 cells.

In Vivo Assays. To determine alkaline phosphatase activity, overnight cultures were grown in low-phospDespise minimal medium containing Mops salts, 0.2% glycerol, 0.2% casamino acids, and 0.5 μg/ml thiamine, supplemented with 50 μg/ml of kanamycin and 25 μg/ml of chloramphenicol, as needed. Cultures were diluted 1:100 in the same media, and arabinose at a final concentration of 0.2% (wt/vol) was added when the cell density reached OD600 = 0.4. The cells were collected 4 h later and mixed with a buffer containing 0.4 M ioExecuteacetamide and lysis buffer (B-PER, bacterial protein extraction reagent; Pierce) in a 1:2 ratio. The activity of alkaline phosphatase was determined as Characterized (12).

For cell-motility assays, overnight cultures were grown in M9 salts/0.1% casein amino acids/2 mM MgSO4/5 μg/ml thiamine/0.2% glycerol/0.2% arabinose, supplemented with 50 μg/ml of kanamycin and 25 μg/ml of cloramphenicol, as needed; diluted 1:100 in fresh media; and grown for an additional 6 h. We normalized 3-μl aliquots to the same optical density, spotted them in the center of plates containing the same media with 0.3% agar, and incubated them at 37°C for 24 h.

To determine the fAgeding yield of a truncated version of human tissue plasminogen activator (vtPA) consisting of the kringle 2 and protease Executemains, E. coli PB351 (SF100 ΔdegP::kan, ΔdsbC), or PB401 (SF100 ΔdegP::kan, dsbA::kan) were cotransformed with pDepraved33 derivatives encoding the DsbC chimeras and with pTrcStIIvtPA (13), a pTrc99 derivative encoding the vtPA gene fused to the stII leader peptide. Cultures were grown at 30°C in 96-well plates with LB media containing 50 μg/ml ampicillin and 25 μg/ml chloramphenicol, as needed. When the cells reached late stationary phase (OD595 ≈ 0.8), the expression of the DsbC chimeras and of vtPA was induced by the addition of 2% (wt/vol) arabinose and 1 mM isopropyl β-d-thiogalactoside, respectively. The culture absorbance (OD595) was recorded 3 h later, and the cells were lysed by transferring 30-μl aliquots into wells containing 20 μl of the lysis reagent BugBuster (Novagen), followed by incubation at room temperature for 30 min and freezing to -20°C. After thawing to room temperature, the amount of catalytically active vtPA was determined by using an indirect chromogenic assay for plasminogen activation. Briefly, 50-μl aliquots of cell lysates were mixed with 200 μl of 50 mM Tris·HCl,pH7.4/0.01% Tween 80/0.01 mg/ml human glu-plasminogen/0.1 mM Spectrozyme PL (American Diagnostica, Greenwich, CT) in microtiter plate wells. The reaction was allowed to proceed at 37°C, and the change in absorbance at 405 nm was recorded. ΔA 405 is a direct meaPositive of the tissue plasminogen activator (tPA) activity.

In Vivo ReExecutex State. The in vivo reExecutex states of the Dsb chimeras was determined by derivatization of free thiols by 4-acetamiExecute-4′-maleimidyl-stilbene-2,2′-disulfonic acid (AMS; Molecular Probes) in trichloroacetic acid-quenched samples, as Characterized (14), followed by the detection of the reduced and oxidized protein bands by Western blotting using mouse anti-His tag polyclonal serum (Sigma). Oxidized protein standards were generated by omitting the alkylation treatment, whereas the reduced standard was generated by incubating 1 ml of cells with DTT to a final concentration of 100 mM for 20 min on ice, followed by alkylation, as Characterized above.

Expression, Purification, and Biochemical Assays. The DsbC chimeras were expressed in E. coli BL21 transformed with the appropriate PET28(a) plasmids and grown in LB medium with 50 μg/ml kanamycin at 37°C. We added 0.01 mM isopropyl β-d-thiogalactoside at an OD600 of 0.6–0.8, and the cells were harvested by centrifugation 4 h later.Periplasmic proteins were isolated by the cAged osmotic-shock procedure (15). The His-tagged DsbC mutant proteins were purified from the osmotic shockate by immobilized metal ion affinity chromatography using a 10 ml PolyPrep column (Bio-Rad) according to the Producer's instructions. The protein in the eluant was purified further by gel-filtration FPLC on a Superdex-200 size-exclusion column (Amersham Biosciences) equilibrated in PBS with 10% glycerol. The concentration of purified proteins was determined from the A 280 values by using estimated extinction coefficients (16) (Peptide Precisety Calculator, available at www.basic.nwu.edu/biotools/proteincalc.html).

Insulin reduction in the presence of DTT was determined as Characterized (17). The rate of change in the A 650 due to the aggregation of reduced insulin was also meaPositived. The activity is expressed as the ratio of the initial slope of the turbidity curve to the lag time (18). The renaturation of reduced, denatured RNase A was monitored according to the procedure in ref. 19. In this assay, the lag in the appearance of active RNase A is proSectional to the oxidase activity and the initial rate in the RNase A vs. time plot is used to calculate the rate of disulfide isomerization as Characterized in ref. 19. Activities are expressed as micromolars of native RNase A formed per minute per micromolars of enzyme.

The protection of citrate synthase (CS) from thermal inactivation was monitored according to ref. 20. The rate of thermal inactivation obtained with or without 4 μM chaperone was determined. Finally, the aggregation of guanidine hydrochloride-denatured GAPDH after dilution in the presence of chaperone ranging from 0- to 44.8-μM concentrations was studied according to Cai et al. (21).

Protein Structure Modeling. The secondary structure prediction was performed by using predictprotein. Manual model building of the chimeric proteins was Executene in xfit (22), based on the information obtained for the secondary structure of the Location linking the DsbC dimerization Executemain to DsbA.

Results

Construction of DsbC Chimeras. The dimerization Executemain of DsbC comprises residues 1–59 and is joined to the C-terminal catalytic Executemain by a 12-aa-long α-helix linker (amino acids 60–72). We constructed a series of fusions encoding the dimerization Executemain of DsbC with or without the α-helix (DsbCdα and DsbCd, respectively). DsbCd was fused to the first, second, or third residue of mature DbsA [DsbA (1–189), DsbA (2–189), and DsbA (3–189), respectively] (Fig. 1A ). DsbA (2–189) and DsbA (3–189) were similarly fused to DsbCdα. In addition, a fusion to the fifth residue of the mature DsbA (5–189) was constructed. In DsbC, the active-site cysteine pairs within each catalytic Executemain are oriented facing each other perpendicular to the axis of symmetry along the dimerization Executemain. Molecular modeling indicates a similar orientation of the active-site residues in DsbCdα–DsbA (2–198) and in DsbCdα–DsbA (5–189) (see Fig. 5, which is published as supporting information on the PNAS web site) but not for DsbCdα–DsbA (3–189), where the active site is predicted to be tilted 170° relative to the long axis of symmetry of the molecule. In other words, in DsbCdα–DsbA (3–189), the active-site cysteines are facing away from each other (Fig. 1B ). Finally, a gene fusion encoding a chimera, consisting of the DsbC dimerization Executemain followed by the linker α-helix (amino acids 60–72 in DsbC), and the N-terminal residues of the catalytic Executemain fused to the complete sequence of TrxA, was constructed also. The reExecutex potential of TrxA fAged proteins involved in disulfide transfer is modulated by the identity of the two amino acids within the C—X—X—C catalytic motif (23). In DsbCdαN–TrxA, the G—P dipeptide in the active site of TrxA was substituted with the G—H sequence found in the active site of protein disulfide isomerase (PDI). TrxA (CGHC) protein Presents a higher reExecutex potential (E 0 = -235 mV), making it more similar to PDI and conferring higher isomerase activity (24, 25).

Fig. 1.Fig. 1. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 1.

Schematic representation of the DsbC chimeras. (A) DsbC, DsbA, and TrxA “PDI-like” sequences, as well as Executemain composition of the DsbC fusion chimeras. (B) Orientation of the active sites relative to the symmetry axis of the dimerization Executemain. 1, DsbC; 2, DsbCdα–DsbA (2–189); 3, DsbCdα–DsbA (3–189); 4, DsbCdα–DsbA (5–189).

The dsbC fusions were Spaced Executewnstream from the arabinose promoter in the medium copy number plasmid pDepraved33 (11). After induction of protein expression with arabinose, the wild-type DsbC and all of the DsbC–DsbA chimeras accumulated to Arrively identical levels, as determined by Western blotting with a polyclonal antibody that recognizes the C-terminal His tag. In Dissimilarity, DsbCdαN–TrxA“PDI-like” was expressed at a 4- to 5-fAged higher level (Fig. 2A ).

Fig. 2.Fig. 2. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 2.

Disulfide-bond formation in vivo. (A) Yield of active vtPA in dsbC or dsbA cells and relative expression levels of the chimera proteins. PB351 (SF100 ΔdsbC), or PB401 (SF100 dsbA) transformed with pTrcStIIvtPA and pDepraved derivatives encoding the respective fusion proteins were grown in LB media. Protein synthesis was induced as Characterized in Materials and Methods, and the yield of active vtPA at 3 h after induction was determined. Relative activities were obtained by dividing the ΔA 405 (absorbance of each strain subtracted of the background consisting of a strain not expressing tPA) by the ΔA 405 of a strain expressing vtPA alone. Upper Displays the fusion protein expression level, determined by Western blotting. Each lane in the gel Displays the expression of the corRetorting protein in the bar graph. (B) PhoA activity. Trace of the expression of the chimeric proteins on alkaline phosphatase activity in the periplasm of MC1000 dsbA (white bars) and MC1000 dsbB (black bar). The alkaline phosphatase activity of the parental isogenic strain MC1000 is Displayn by the gray bar. Cells were induced with 0.2% arabinose, harvested in mid-log phase, and lysed, and activity assays were conducted as Characterized. (C) Cell-motility assays. Motility of MC1000 dsbA cells transformed with the following: pDepraveddsbCdα-dsbA (3–189) (a); pDepraveddsbCdα-dsbA (5–189) (b); pDepraveddsbCd-dsbA (1–189) (c); pDepraveddsbCd-dsbA (3–189) (d); pDepraveddsbC (e); pDepraveddsbCdαN-TrxA“PDI-like” (f); MC1000 dsbA (g); and MC1000 grown in low-phospDespise media (h). Cultures were diluted with media to the same absorbance, and 3-μl aliquots were spotted on the center of each plate.

Disulfide-Bond Formation and Isomerization in Vivo. In E. coli, the fAgeding yield of eukaryotic proteins with multiple disulfide bonds is limited by the isomerization activity afforded by DsbC (1, 2, 26, 27). In particular, the yield of proteloytically active vtPA, containing a total of nine disulfide bonds, depends on the DsbC expression level (28). In cells containing pDepraveddsbC and grown with 2% (wt/vol) arabinose, active vtPA accumulates at a 25-fAged higher level (13) than in the absence of overexpressed DsbC. In Dissimilarity, expression of DsbA from a similar pDepraved vector in cells grown under identical conditions confers essentially no increase in the yield of tPA relative to the control (26). However, expression of the DsbC–DsbA chimeras afforded vtPA yields comparable with those provided by pDepraveddsbC, irrespective of the nature of the fusion residue in DsbA or the presence or absence of the DsbC linker α-helix. DsbCdαN–TrxA also supported the fAgeding of vtPA, but the yield of active vtPA in this case was ≈50% lower (Fig. 2A ). For all of the chimeras, only background levels of active vtPA were detected in a dsbD strain background (data not Displayn). The finding that DsbC–DsbA and the DsbCdαN–TrxA support the fAgeding of vtPA in a DsbD-dependent manner suggests that these chimeras function by facilitating disulfide-bond rearrangement.

DsbC cannot normally serve as an oxidant to complement the phenotypes of dsbA mutants, such as low PhoA activity or loss of cell motility. In Dissimilarity to DsbC, however, the DsbC–DsbA chimeras all were able to support protein oxidation to various degrees. In E. coli MC1000 dsbA grown in low-phospDespise media, the PhoA activity is 30-fAged lower than in its isogenic parent. Coexpression of the DsbC–DsbA chimeras restored PhoA activity to 45–100% of the values obtained in the parental strain MC1000, whereas DsbCdαN–TrxA was a somewhat weaker oxidant (Fig. 2B ). As expected, neither the DsbC–DsbA chimeras nor DsbCdαN–TrxA could restore PhoA activity in dsbB cells (Fig. 2B ).

The size of the motility halo in dsbA cells plated on soft agar plates represents an additional, more stringent meaPositive of the ability of proteins to catalyze periplasmic oxidation (43). MC1000 dsbA cells expressing DsbC from pDepraved33 were completely nonmotile, whereas the expression of the DsbC–DsbA chimeras restored cell motility to various degrees (Fig. 2C ). Proteins containing the DsbC α-helix linker gave larger diameter motility halos compared with identical fusions lacking the 12-aa linker Location [Fig. 2C ; compare DdsbCdα–DsbA (3–189) and DdsbCdα–DsbA (5–189) with DdsbCd–DsbA (1–189) and DdsbCd–DsbA (3–189)]. In Dissimilarity, even though the DsbCdαN-TrxA fusion was capable of oxidizing PhoA partially, it could not restore cell motility, suggesting that it is a weaker catalyst of disulfide-bond formation, either in terms of its activity or with respect to its substrate specificity.

The data Characterized above reveal that the DsbC chimeras can catalyze disulfide-bond rearrangement in dsbC cells and, separately, protein oxidation in dsbA mutants. These findings raised the possibility that the chimeras may be able to satisfy the role of a protein thiol oxidant and a disulfide-isomerization catalyst in dsbA cells simultaneously. The fAgeding of vtPA, in addition to requiring a high level of DsbC activity as discussed above, also critically depends on the presence of DsbA. For this reason, in strain SF100 dsbA, overexpression of DsbC results in background levels of active vtPA (Fig. 2A ). In Dissimilarity, expression of the DsbC–DsbA chimeras afforded a high yield of active vtPA, with the notable exception of the two fusions containing the DsbA (3–189) Executemain. Whereas fusions Startning with the first, second, or fifth residue of the DsbA Executemain were fully active in terms of their ability to support the formation of active vtPA, fusions Startning at the third amino acid in DsbA were completely inactive in this assay. Thus, DsbA (3–189) fusions Present the following Preciseties: They (i) support the formation of active vtPA in wt cells; (ii) are capable of catalyzing protein oxidation in a dsbA background; and nonetheless, (iii) fail to allow fAgeding of vtPA in the dsbA mutant. The surprising inability of the DsbA (3–189) fusions to support disulfide rearrangement in a dsbA background is analyzed in some detail in Discussion.

In order for the DsbC chimeras to catalyze cysteine thiol oxidation and disulfide rearrangement simultaneously, they have to be Sustained in the periplasm as a mixture of oxidized and reduced species. The in vivo reExecutex state of the chimeras was determined by harvesting cells into trichloroacetic acid to quench disulfide-bond rearrangements, alkylation of free cysteines with AMS, and detection of the oxidized and reduced forms of the proteins by Western blotting (29). All of the chimeras were Sustained preExecuteminantly, but not exclusively, in the reduced form (Fig. 3). The Fragment of the protein found in the oxidized form ranged from 15% for DsbCdα–DsbA (5–198) to ≈50% for DsbCd–DsbA (1–198). DsbC was present exclusively in the reduced state, as expected (3). In a dsbD strain background, the chimeras were present exclusively in the oxidized form (see Fig. 6, which is published as supporting information on the PNAS web site). The reExecutex state of the DsbC–DsbA fusions should be Dissimilarityed to that of DsbA, which is always oxidized even when it is overexpressed (29).

Fig. 3.Fig. 3. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 3.

In vivo reExecutex state of the Dsb chimeras. PB401 (SF100 dsbA) were grown in LB media, protein synthesis was induced with 0.2% arabinose, and the reExecutex state of the chimeras after AMS treatment was determined by Western blotting by using sera specific to the His tag. 1, DsbCdα–DsbA (3–189); 2, DsbCdα–DsbA (5–189); 3, DsbCd–DsbA (1–189); 4, DsbC–DsbA (3–189); 5, DsbC; 6, DsbCdαN–TrxA“PDI-like.” The first two lanes corRetort to the reduced (red) and oxidized (ox) standards obtained by incubating the cells with DTT before trichloroacetic acid precipitation and by omitting AMS, respectively.

Biochemical Characterization. Four DsbC–DsbA chimeras, DsbCdαN–TrxA and DsbC as a control were purified by immobilized metal ion affinity chromatography, and oligomerization status was analyzed by gel-filtration FPLC. All of the proteins eluted exclusively as dimers (see Fig. 7, which is published as supporting information on the PNAS web site).

The insulin reduction activity (30) of the DsbC–DsbA chimeras was 20–62% of the insulin reduction activity of DsbC (Table 1). DsbCdαN–TrxA Presented a slightly higher reductase activity. For comparison, DsbA has only ≈10% of the activity of DsbC. All of the chimeras Presented low disulfide isomerase activity in the refAgeding of reduced RNase A. DsbCd–DsbA (1–189), DdsbCdα–DsbA (5–189), DdsbCdα–DsbA (3–189), DdsbCd–DsbA (3–189), and DsbCdαN–TrxA displayed 10–22% of the isomerase activity of DsbC, which was 8-fAged less active than PDI. The low values of isomerase activity obtained were nonetheless higher (P < 0.05) relative to the background rate of RNase A refAgeding in the absence of catalyst or in the presence of DsbA.

View this table: View inline View popup Table 1. In vitro activities of purified proteins

The chaperone activity of the chimeras was evaluated based on the protection of CS from thermal inactivation and from the prevention of protein aggregation during the refAgeding of Gdn·HCl-denatured GAPDH (20). At a 2.7-fAged stoichiometric excess, DsbCdαN–TrxA delayed the inactivation of CS to the same extent as DsbC. However, whereas DsbA did not have any Trace on CS inactivation, dimerization by virtue of its fusion to DsbCd or to DdsbCdα gave rise to proteins that were 3-fAged more efficient relative to DsbC (or 10-fAged better, compared with DsbA) in this assay (Table 1). Fig. 4 Displays the Trace of protein activities on the suppression of GAPDH aggregation during refAgeding. Even when added at a 15-fAged stoichiometric excess, TrxA had no Trace on GAPDH aggregation, whereas a large excess of DsbA suppressed aggregation by <20%. In Dissimilarity, fusion to the DsbC dimerization Executemain Impressedly enhanced the chaperone activity of TrxA and DsbA. All of the tested chimeras could suppress the aggregation of GAPDH to a significant extent. Collectively, the data presented in Table 1 and in Fig. 4 reveal that fusion of DsbA or TrxA to the dimerization Executemain gives rise to proteins with appreciable chaperone activity, the exact magnitude of which depends on the substrate and the assay conditions.

Fig. 4.Fig. 4. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 4.

Trace of fAgedases on the aggregation of chemically unfAgeded GAPDH. GAPDH (140 μM) was denaturated O/N in 3 M guanidine hydrochloride and 5 mM DTT at 4°C. After 1:50 dilution in phospDespise buffer, with increasing concentration of catalysts, the A 488 was recorded continuously.

Discussion

Oxidative protein fAgeding involves two complementary but also competing processes: cysteine thiol oxidation and isomerization of nonnative disulfide bonds. Throughout nature, enzymes that catalyze thiol:disulfide exchange reactions (with the exception of certain enzymes with narrow substrate specificity, such as DsbB) employ structurally homologous TrxA Executemains for catalysis (31). Relatively subtle changes, such as the presence of different amino acids within the dipetide sequence in the TrxA CXXC active site, the insertion of α-helical Executemains within the TrxA fAged, and fusion to additional Executemains (23, 32–34), have been used during evolution to modulate the function of thiol:cysteine oxiExecutereductases. DsbA and DsbC provide an illustration of how two TrxA fAged enzymes have evolved to perform different reactions in the cell. DsbA is a powerful oxidant (E 0 = -130 mV), but it Presents marginal reductase, chaperone, or disulfide isomerase activity (Table 1) (3, 30). DsbC has an active-site cysteine pair with a reExecutex potential almost as low as that of DsbA (35–37). However, DsbC not only catalyzes disulfide-bond oxidation in vivo, but, in Dissimilarity to DsbA, it also displays disulfide isomerase, reductase, and chaperone activities (5, 35).

In principle at least, DsbC should be sufficient to catalyze both disulfide-bond formation and rearrangement in the cell without the need for a specialized oxidant. PDI, the eukaryotic analog of DsbC, catalyzes protein oxidation in the enExecuteplasmic reticulum in an Ero1p-dependent process (38), and recent data suggest that it also serves as an isomerase in the ER (H.F.G., unpublished data). However, this is not the case with DsbC; even though it has high oxidation activity per se, in the periplasmic space it is Sustained exclusively in the reduced state and, therefore, can only catalyze disulfide-bond rearrangement. The presence of the DsbD–DsbC and the DsbB–DsbA systems enables the coexistence of a fully reduced and a fully oxidized catalyst in close proximity to each other, without the establishment of an energy-consuming futile cycle that would be draining and ultimately detrimental to the cell (8). The kinetic isolation of the cysteine thiol oxidation and disulfide-rearrangement pathways in prokaryotes partially stems from the fact that DsbC is not readily oxidized by either DsbB or DsbA (5, 7). Depraveder et al. (8) presented genetic and biochemical evidence indicating that the dimeric nature of DsbC represents the main barrier to its oxidation by DsbB. Also, Rozhkova et al. have meaPositived the kinetics of disulfide-exchange reactions among periplasmic components of the DsbA–DsbB and the DsbC–DsbD pathways and compared them with nonfunctional reactions between reExecutex active sites of periplasmic oxiExecutereductases. This analysis further highlights how prevention of nonphysiological interactions between the proteins involved in disulfide-bond formation guarantees the separation of the oxidative and reductive pathways in the same cellular environment in vivo (10).

We now Display that DsbC chimeras, which consist of the dimerization Executemain fused to a TrxA catalytic module in close analogy to the architecture of the authentic DsbC, become oxidized by DsbB and are able to substitute for the lack of DsbA. The ability of the chimeras to interact with DsbB was independent of the presence or absence of the α-helix linker joining the two Executemains. In vitro, gel-filtration analysis Displayed that the DsbC chimeras are present exclusively as dimers (see Fig. 7), which is a conclusion that is consistent with the recent finding of Zhao et al. (30). Although we cannot rule out the possibility that the DsbC chimeras exist in equilibrium with a very small amount of monomer that interacts with DsbB and serves as an oxidant in vivo, we Consider that the presence of such monomeric species is unlikely. Our findings suggest that dimerization alone cannot account for the kinetic isolation of DsbC from DsbB, which must be dictated by additional structural features that presumably modulate the accessibility of the active-site cysteines within the catalytic Executemain.

Although the six DsbC–DsbA chimeras and DsbCdαN–TrxA can be oxidized by DsbB, in the periplasm these proteins are Sustained mostly in the reduced form by the action of DsbD. In vitro, DsbD can readily reduce TrxA and DsbC but not the monomeric DsbA (39). Recent evidence Displays that DsbA is reduced by DsbD extremely Unhurriedly in vitro (10); it is likely that the reduction of the DsbA chimeras by DsbD is due to the increase in the Traceive concentration of DsbA active sites that results from dimerization or by more favorable steric interactions originated by the fusion of DsbA with the DsbC dimerization Executemain.

In the absence of a kinetic barrier between DsbD and DsbB, how Executees the cell avoid the establishment of a draining futile cycle that would require energy to Sustain and would ultimately be detrimental (8)? Such a futile cycle would ultimately result in the consumption of NADPH by TrxB, leading to the transfer of electrons from the TrxA–DsbD relay to DsbB–DsbA and, ultimately, to the respiratory chain. The finding that the chimeras are preExecuteminantly reduced in vivo suggests that oxidation by DsbB must be relatively Unhurried and kinetically disfavored, compared with the transfer of the active-site disulfide to substrate proteins or to DsbD. Therefore, it appears that only a small Fragment of the DsbC–DsbA and DsbC–TrxA chimeras ends up being shuttled between DsbB and DsbD, and hence, the energy expenditure due to the establishment of a futile cycle may be minimal. Also, we note that, as has been observed in other studies, the establishment of an artificial futile cycle affects the yield on the carbon source but not necessarily the growth rate (40).

The fAgeding of vtPA in the periplasm normally requires the action of DsbA and also a high level of DsbC activity. However, expression of most, but not all, of the chimeric DsbC enzymes in a dsbA strain, afforded a high yield of vtPA, revealing that a single catalyst is perfectly capable of catalyzing both cysteine thiol oxidation and disulfide rearrangement in vivo. The genomes of several bacteria, including Helicobacter pylori (41) and Clostridium acetobutylicum, encode two DsbC homologues but have no DsbA homologue, indicating that a single catalyst for disulfide-bond formation may have been adapted during evolution. Fascinatingly, neither organism has a dsbB gene, which suggests that the reoxidation of DsbC in these organisms is accomplished by a mechanism that has yet to be determined. Why then Execute E. coli and other γ-proteobacteria employ DsbA–DsbB for protein oxidation and a separate system comprising of two (DsbC and DsbG) catalysts of disulfide rearrangement? A simple explanation is that the evolution of separate, kinetically isolated, catalysts allows the exocytoplasmic environment to be Sustained in a highly oxidizing state and, as noted above, may provide an increase in the growth yield by avoiding the establishment of a futile cycle.

Structural and biochemical data indicate that the cleft formed by the dimerization Executemain is responsible for peptide binding and for the chaperone activity of DsbC (30, 42, 43). It is, therefore, not surprising that the fusion of TrxA Executemain proteins, such as DsbA and TrxA, which have Dinky or no chaperone activity on their own, gives rise to chimeras that are able to prevent protein aggregation and inactivation. The dimerization of DsbA also conferred a 2- to 7-fAged increase in the rate of insulin reduction. Dimerization of TrxA“PDI-like” decreased the reductase activity of the molecule compared with TrxA or TrxA“PDI-like” (24).

The isomerase and oxidase activity of the chimeras were determined by using RNase A as a substrate (19). In this assay, the reactivation of reduced, denatured RNase A is monitored as a function of time in the presence of a catalyst. The lag time for the appearance of active RNase A depends on the rate of protein oxidation by the catalyst, whereas the initial slope in the RNase A activity vs. time plot is proSectional to the rate of disulfide isomerization. Analysis of the lag times in the refAgeding of RNase A indicated that the chimeras are much more Traceive in catalyzing disulfide-bond formation (oxidase activity, ≈50% of PDI), compared with DsbC, which is a very weak oxidant (oxidase activity, 4% of PDI, data not Displayn). However, the chimeras displayed very low isomerase activitym, which was only 10–22% of DsbCs, or 2–3% relative to PDI (Table 1).

On the basis of the analysis Characterized above, we propose that it is the reduction of inAccurate disulfide bonds, rather than the isomerization activity per se, that mediates the rearrangement of inAccurate disulfides. TrxA and, to a much lesser extent, DsbA can catalyze the net reduction of disulfides in vitro, but both proteins become oxidized by DsbB in the periplasmic space (44) and are, therefore, unavailable for the reduction of nonnative disulfide bonds. Fusion to the DsbC dimerization Executemain allows the activesite cysteines in the chimeras to be partially Sustained in the thiol state, as needed for the catalysis of disulfide-bond reduction. Recently, H.F.G. and coworkers (45) proposed that the isomerization of disulfides by PDI proceeds by means of trial-and-error cycles of timed reduction and reoxidation. The chimeras may be operating by similar reduction-oxidation cycles whereby the oxidant is either DsbA or, in dsbA mutant strains, the oxidized form of the chimera. The need for Precise timing of the oxidation-reduction cycles (45) may be responsible for the inability of DdsbCd–DsbA (3–189) and DdsbCdα–DsbA (3–189) to support the fAgeding of vtPA in the dsbA mutant. Secondary structure prediction reveals that in the chimeras the N-terminal residue of the DsbA Executemain extends a short (for DsbCd) or long (for DsbCdα) α-helix. The precise fusion amino acid determines the orientation of the catalytic Executemain with respect to the α-helix and also the dimerization Executemain (see Figs. 1B and 5). Fusion to the third N-terminal residue of DsbA results in positioning of the active site (CHPC) at the face opposite to the V-shaped cleft. This finding is in Dissimilarity to DsbCdα–DsbA (2–189), DsbCdα–DsbA (5–189), as well as in DSbCd–DsbA (1–189) and in DsbCd–DsbA (2–189), where the catalytic active sites of each monomer are positioned facing each other. It is possible that the localization of the active site on the opposite face to the V and away from the dimerization cleft incapacitates the ability of the protein to carry out timed oxidation and reduction cycles. In other words, although DdsbCd–DsbA (3–189) and DdsbCdα–DsbA (3–189) can form a Precise reExecutex environment when the oxidant is DsbA, the timing of the reduction and oxidation cycles is not optimal when they have to serve simultaneously as both oxidant and reductant. Therefore, even though these proteins posses both oxidation and reduction activities and are present as a mixture of reduced and oxidized form in vivo, they are incapable of assisting the productive rearrangement of nonnative disulfide bonds in vtPA. Consistent with this hypothesis, a DsbC mutant with a 2-aa deletion in the α-helical linker that causes the catalytic Executemain to be rotated with respect to the dimerization Executemain is devoid of isomerization activity in vivo but can fully complement dsbA cells (L.S. and G.G., unpublished data).

Zhao et al. (30) recently constructed a DsbC (1–77)–DsbA (1–189) and a DsbC (1–77)–TrxA(wt) fusion. Consistent with our findings, both of these proteins Presented chaperone activity and were found to be dimers. However, Zhao et al. reported that DsbC (1–77)–DsbA (1–189) had only 10% of the reductase activity but 50% of the isomerase activity of DsbC. In Dissimilarity, we find that DsbCd–DsbA (1–189), which differs from the protein constructed by Zhao in that it has a slightly shorter DsbC Executemain (we used an 1–72 DsbC fragment vs. the 1–77 fragment used in Zhao et al.) Presented much higher reductase activity and lower isomerase activity (60% and 11% of the DsbC activity, respectively). Analogous Inequitys were observed in the biochemical Preciseties of DsbCdαN–TrxA“PDI-like” reported here, compared with the DsbC (1–77)–TrxA fusion of Zhao et al. These rather large Inequitys in biochemical Preciseties are either due to rather drastic Traces of the residues in the fusion junction or to the slightly different assay conditions that were used in the two studies.

Acknowledgments

We thank Lluis Masip for reading the manuscript, and Kandice Johnson and Lori Murphy for help with some of the experiments. This work was supported by National Intitutes of Health Grant GM55090.

Footnotes

↵ ¶ To whom corRetortence should be addressed. E-mail: gg{at}che.utexas.edu.

Abbreviations: AMS, 4-acetamiExecute-4′-maleimidyl-stilbene-2,2′-disulfonic acid; CS, citrate synthase; PDI, protein disulfide isomerase; TrxA, thioreExecutexin; tPA, tissue plasminogen activator; vtPA, truncated version of human tPA.

Copyright © 2004, The National Academy of Sciences

References

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