Role of Dimerization in the Catalytic Properties of the Escherichia coli Disulfide Isomerase DsbC

The bacterial protein-disulfide isomerase DsbC is a homodimeric V-shaped enzyme that consists of a dimerization domain, two α-helical linkers, and two opposing thioredoxin fold catalytic domains. The functional significance of the two catalytic domains of DsbC is not well understood yet. We have engineered heterodimer-like DsbC derivatives covalently linked via (Gly₃-Ser) flexible linkers. We either inactivated one of the catalytic sites (CGYC), or entirely removed one of the catalytic domains while maintaining the putative binding area intact. Variants having a single active catalytic site display significant levels of isomerase activity. Furthermore, mDsbC[H45D]-dim[D53H], a DsbC variant lacking an entire catalytic domain but with an intact dimerization domain, also showed isomerase activity, albeit at lower levels. In addition, the absence of the catalytic domain allowed this protein to catalyze in vivo oxidation. Our results reveal that two catalytic domains in DsbC are not essential for disulfide bond isomerization and that a determining feature in isomerization is the availability of a substrate binding domain.Disulfide bonds are critical for the proper folding and structural stability of many exocytoplasmic proteins. The Dsb family of thiol:disulfide oxidoreductase enzymes catalyzes oxidative protein folding in the periplasm of Escherichia coli by means of two independent pathways (1–3). In the DsbA-DsbB oxidation pathway, DsbA, a very strong oxidant, catalyzes the formation of disulfide bonds on newly translocated proteins (4). The DsbA disulfide is rapidly recycled by DsbB, a membrane protein that transfers electrons from DsbA onto quinones (5–7). In the DsbC-DsbD isomerization pathway, non-native disulfides are reduced or rearranged by DsbC. DsbC is maintained in a reduced, catalytically active state via the transfer of electrons from the inner membrane protein DsbD that in turn accepts electrons from thioredoxin 1 and ultimately from NADPH (via thioredoxin reductase) within the cytoplasm (8, 9). Large kinetic barriers keep the oxidation and isomerization pathways isolated, preventing the establishment of a futile cycle of electron transfer. Accordingly, reactions between enzymes of the two pathways, for example the oxidation of DsbC by DsbB or the reduction of DsbA by DsbD, are 10³–10⁷-fold slower than the physiologically relevant DsbA-DsbB and DsbC-DsbD reactions (10). Nonetheless, the kinetic barrier between DsbB and DsbC can be breached by introducing mutations that result in structural changes in DsbC (11, 12).DsbC is a homodimer with each monomer comprising an N-terminal dimerization domain and a C-terminal thioredoxin-like catalytic domain fused by an α-helical linker. The crystal structure of DsbC reveals that the two monomers come together to form a V-shaped protein. The inner surface of the resulting cleft is patched with uncharged and hydrophobic residues suggesting an important role in the binding of substrate proteins. The active sites comprising the sequence Cys⁹⁸-Gly⁹⁹-Tyr¹⁰⁰-Cys¹⁰¹ in each of the monomeric subunits are located in the arms of the “V” facing each other (13). Isomerization involves an attack onto a substrate disulfide by Cys⁹⁸ resulting in the formation of a mixed disulfide, which then is resolved by either another cysteine from the substrate or by Cys¹⁰¹ from DsbC (14, 15). Besides its isomerase activity, DsbC is also known to display chaperone activity preventing protein aggregation during refolding (16). In E. coli, disulfide bond isomerization is the limiting step in the oxidative folding of many heterologous proteins that contain multiple cysteines. Overexpression of DsbC has been shown to enhance the yield of proteins such as human nerve growth factor, human tissue plasminogen activator (tPA)² and immunoglobulins (17–19).DsbC is topologically analogous to the eukaryotic protein-disulfide isomerase (PDI). The structural similarities between the two enzymes may have resulted from convergent evolution by thioredoxin-like domain replication in the case of PDI and domain recruitment in DsbC (20, 21). PDI comprises two thioredoxin-like catalytic domains (a and a′) separated by two non-catalytic domains (b and b′), in addition to a c domain (22). In PDI, the catalytic domains are different and functionally nonequivalent (23). Substrate binding is mediated primarily by the b′ domain; the two catalytic domains, a and a′, can catalyze oxidation of small model peptides indicating that they must also have low substrate binding affinity (24).The DsbC monomer is essentially devoid of RNase A isomerase activity (25). Sun and Wang (44) reported that DsbC with one catalytic site impaired by carboxymethylation is also essentially inactive but, in separate studies, Zapun et al. (26) did not detect cooperativity between the two catalytic sites indicating that they function independently of each other (26). Moreover, unlike PDI, the significance of the putative peptide binding cleft of DsbC on disulfide isomerization has not been ascertained. However, while DsbA or TrxA with a PDI active site dipeptide (CGHC) display very little isomerase activity in vitro and in vivo (27–29), we recently showed that upon fusion to a dimerization region that provides a putative substrate binding surface (the E. coli peptidyl proline isomerase FkpA) they acquire the ability to assist the folding of periplasmically expressed multidisulfide heterologous proteins (30).In the present work, we engineered heterodimer-like covalently linked DsbC derivatives in which one of the catalytic sites has been inactivated (Fig. 1A) or one of the catalytic domains has been entirely removed while maintaining the intact peptide binding cleft (which is normally formed by association of the N-terminal domains of the two monomers) (Fig. 3A). We show that DsbC forced monomers with one functional active site, or with one thioredoxin domain only, display significant isomerization activity. Interestingly, the latter variant is partially reduced in vivo indicating that the presence of both thioredoxin domains is important for the avoidance of protein oxidation by DsbB.Open in a separate windowFIGURE 1.A, protein structure of DsbC, and molecular models of mDsbC-mDsbC and the single active site covalently linked mutants. Dimerization domains are shown in gray, thioredoxin domains in black, and the active sites in white. B, gel filtration FPLC of DsbC and linked variants. Purified proteins were run on a Superdex^TM 200 column in PBS, 10% glycerol buffer.Open in a separate windowFIGURE 3.A, molecular model of mDsbC-dim. Dimerization domains are shown in gray, thioredoxin domain in black, and catalytic site in white. B, gel filtration FPLC of mDsbC-dim as compared with DsbC. Purified proteins were run on a Superdex^TM 200 column in PBS, 10% glycerol buffer. C, MALS measurement of the molar masses of the components of mDsbC-dim together with their hydrodynamic radii. The data show monomeric, dimeric, and tetrameric states. The relative concentrations were determined by the refractive index differences.     

The contributions of protein disulfide isomerase and its homologues to oxidative protein folding in the yeast endoplasmic reticulum

In vitro, protein disulfide isomerase (Pdi1p) introduces disulfides into proteins (oxidase activity) and provides quality control by catalyzing the rearrangement of incorrect disulfides (isomerase activity). Protein disulfide isomerase (PDI) is an essential protein in Saccharomyces cerevisiae, but the contributions of the catalytic activities of PDI to oxidative protein folding in the endoplasmic reticulum (ER) are unclear. Using variants of Pdi1p with impaired oxidase or isomerase activity, we show that isomerase-deficient mutants of PDI support wild-type growth even in a strain in which all of the PDI homologues of the yeast ER have been deleted. Although the oxidase activity of PDI is sufficient for wild-type growth, pulse-chase experiments monitoring the maturation of carboxypeptidase Y reveal that oxidative folding is greatly compromised in mutants that are defective in isomerase activity. Pdi1p and one or more of its ER homologues (Mpd1p, Mpd2p, Eug1p, Eps1p) are required for efficient carboxypeptidase Y maturation. Consistent with its function as a disulfide isomerase in vivo, the active sites of Pdi1p are partially reduced (32 +/- 8%) in vivo. These results suggest that PDI and its ER homologues contribute both oxidase and isomerase activities to the yeast ER. The isomerase activity of PDI can be compromised without affecting growth and viability, implying that yeast proteins that are essential under laboratory conditions may not require efficient disulfide isomerization.     

Activation of Ha-ras p21 by substitution, deletion, and insertion mutations.

The transforming activity of naturally arising ras oncogenes results from point mutations that affect residue 12 or 61 of the encoded 21-kilodalton protein (p21). By use of site-directed mutagenesis, we showed that deletions and insertions of amino acid residues in the region of residue 12 are also effective in conferring oncogenic activity on p21. Common to these various alterations is the disruption that they create in this domain of the protein, which we propose results in the inactivation of a normal function of the protein.     

Phenotypical characterization and molecular identification of clinical isolates of Candida tropicalis

Background

Candida tropicalis is an increasingly important human pathogen which usually affects neutropenic oncology patients with common hematogenous seeding to peripheral organs and high mortality rates. Candida pathogenicity is facilitated by several virulence attributes, including secretion of hydrolytic enzymes; however, little is known regarding the C. tropicalis ability to secrete them and their role in the disease.

Hyperthermic isolated limb perfusion increases circulating levels of inflammatory cytokines

Hyperthermic isolated limb perfusion is an established method of treatment for regionally advanced melanoma. Recent studies suggest that exogenously administered cytokines potentiate tumor response in patients with in-transit melanoma. We hypothesized that isolated limb perfusion induces an immunogenic response characterized by increased circulating levels of cytokines in the pump circuit, potentially contributing to the antitumor effect. We obtained blood samples from the perfusion circuit and systemic circulation at various intervals from patients undergoing isolated chemotherapeutic perfusion for melanoma. Samples were analyzed for serum cytokine profiles by enzyme-linked immunosorbent assay. When compared with baseline values, significant increases in serum levels of interleukin-6 (IL-6), interleukin-8 (IL-8), and tumor necrosis factor (TNF) occurred within the perfusion circuit during isolated limb perfusion (P<0.05). In addition, there was a corresponding increase in IL-8 within the systemic circulation at the 60-min interval (P<0.05), suggesting some degree of leakage from the isolated circuit due to the extremely high levels of IL-8 in the perfusion circuit. A transient but insignificant decrease in circulating levels of neutrophils was also observed during the perfusion process, which may be attributed to margination. Increased levels of cytokines IL-6, IL-8, and TNF occurred within the isolated circuit during hyperthermic limb perfusion and may contribute to tumor response seen in patients treated with isolated limb perfusion.Presented at the Annual Meeting of the Society of Surgical Oncology, 17–20 March 1994 Houston, Texas     

Temperature effects in the stimulus-secretion process from isolated chromaffin cells

Temperature effects on the stimulus-secretion coupling process was studied by inducing release of catecholamines (CA) from isolated chromaffin cells of the bovine adrenal medulla. Use was made of three different secretagogues: acetylcholine (ACH), high potassium concentration, and the calcium ionophore A23187, at various incubation temperatures. The latter two agents induced a monotonic increase in secretion with rise in temperature, suggesting different regions of the dependence of total release on temperature. The ACH-induced secretion was, however, markedly different and exhibited a maximal release at 30 degrees C. Kinetic experiments using ACH stimulus revealed that this maximum is produced by different temperature dependence in the stages of activation and desensitization. A proposed model for the total release process yields temperature-dependent parameters that can be divided into three regions of initial rates of secretory activity corresponding to the above independent findings using high K+ concentration and the calcium ionophore. The transitions between the various regions indicate possible transitions in the physical properties of the plasma and secretory granule membranes. Elucidation of the interaction between the membranes is of primary importance in the determining mechanism of CA secretion from the isolated adrenal medulla cell.