Glutathione-dependent pathways of refolding of RNase T1 by oxidation and disulfide isomerization: catalysis by protein disulfide isomerase

Protein folding, associated with oxidation and isomerization of disulfide bonds, was studied using reduced and denatured RNase T1 (rd-RNase T1) and mixed disulfide between glutathione and reduced RNase T1 (GS-RNase T1) as starting materials. Folding was initiated by addition of free glutathione (GSH + GSSG) and was monitored by electrospray mass spectrometry (ES-MS) time-course analysis. This permitted both the identification and quantitation of the population of intermediates present during the refolding process. Refolding experiments were performed in the presence of different absolute concentrations of glutathione species while keeping the redox potential fixed, in order to evaluate the effect of the glutathione concentration on the distribution of the refolding intermediates. All the analyses indicate a pathway of sequential reactions in the formation of native RNase T1 which occurs via the reiteration of two steps: (i) formation of a species containing both mixed disulfides with glutathione and free protein thiols, and (ii) formation of an intramolecular disulfide via thiol-disulfide interchange reaction between them. Refolding of rd-RNase T1 and GS-RNase T1 was also performed in the presence of protein disulfide isomerase (PDI). Addition of PDI led to a catalysis of each individual reaction of the entire process without altering the refolding pathway. Refolding reactions carried out at different absolute concentrations of glutathione proved that GSH and/or GSSG participate directly in the reaction catalyzed by PDI. On the basis of these experiments and previous results on the refolding of RNase A [Torella, C., Ruoppolo, M., Marino, G., & Pucci, P. (1994) FEBS Lett. 352, 301-306], a hypothesis of a general pathway for folding of S--S containing proteins is proposed.

Chemically and conformationally authentic active domain of human tissue inhibitor of metalloproteinases-2 refolded from bacterial inclusion bodies

The aggregation of recombinant proteins into inclusion bodies is a major problem for expression in bacterial systems. The inclusion bodies must be solubilized and the denatured protein renatured if an active molecule is to be recovered. We have developed such a procedure for the active N-terminal domain of tissue inhibitor of metalloproteinases-2 [TIMP-2-(1-127)], a small mammalian protein containing three disulfide bonds. Conditions for its renaturation were determined by studying the refolding behaviour of reduced and denatured mammalian-cell-expressed TIMP-(1-127) by intrinsic fluorescence. This strategy allows the development of a refolding protocol before generation of a bacterial expression system, and allows rapid and systematic optimization of each refolding variable by assessing its effect on the rate and extent of the refolding reaction. TIMP-(1-127) was expressed at high levels in Escherichia coli, and refolded from TIMP-2-(1-127) inclusion bodies, by means of the method developed with mammalian-cell-expressed protein, to give a refolding efficiency of 30-40% and a final yield of 11-14 mg purified protein/l culture. The chemical structure and conformation of this material was characterized by electrospray mass spectrometry and two-dimensional 1H-NMR; no significant differences were found between it and the native protein. Mass analysis of uniformly 13C-labeled and 15N-labeled protein was used to help identify a mistranslated TIMP-(1-127) contaminant in the purified refolded sample. This technique provides additional information on the nature of the modification and allows a distinction to be made between those modifications that are cell derived, and those that arise from subsequent handling of the protein.

Assembly of the B subunit pentamer of Escherichia coli heat-labile enterotoxin. Kinetics and molecular basis of rate-limiting steps in vitro

The B subunits of Escherichia coli heat-labile enterotoxin (EtxB) and cholera toxin (CtxB) assemble in vivo into exceptionally stable homopentameric complexes, which maintain their quaternary structure in a range of conditions that would normally be expected to cause protein denaturation. Recently, we showed that the simultaneous protonation of two of the COOH-terminal carboxylates in pentameric EtxB was required to cause its disassembly at pH values below 2.0 (Ruddock, L., Ruston, S. P., Kelly, S. M., Price, N. C., Freedman, R. B., and Hirst, T. R.(1995) J. Biol. Chem. 270, 29953-29958). Here, we investigate the influence of environmental parameters on the kinetics of reassembly of acid-generated EtxB monomers in vitro. Such monomers were found to undergo a further acid-mediated conformational change, with an activation energy of 76 +/- 2 J.mol-1.K-1, consistent with isomerization of the cis-proline residue at position 93, and which prevented subsequent EtxB reassembly. By using rapid neutralization of acid-generated monomers, a high proportion of the B-subunits adopted an assembly-competent conformation, which resulted in up to 75% of the protein reassembling into a stable pentameric complex, indistinguishable from native EtxB pentamers. The rate-limiting step in reassembly, over a concentration range of 50-200 microg/ml, was shown to be due to an intramolecular event, which exhibited a pH dependence with a pKa of 7.0. Modification of EtxB with amine-specific probes revealed that the protonation state of the NH2-terminal alanine residue was responsible for the pH dependence of reassembly. The implications of these findings for the biogenesis of Escherichia coli enterotoxin and related enterotoxins in vivo, are considered.

Thermostable chaperonin from Clostridium thermocellum

Homologues of the chaperonins Cpn60 and Cpn10 have been purified from the Gram-positive cellulolytic thermophile Clostridium thermocellum. The Cpn60 protein was purified by ATP-affinity chromatography and the Cpn10 protein was purified by gel-filtration, ion-exchange and hydrophobic interaction chromatographies. The identities of the proteins were confirmed by N-terminal sequence analysis and antigenic cross-reactivity. The Cpn60 homologue is a weak, thermostable ATPase (t1/2 at 70 decrees C more than 90 min) with optimum activity (Kcat 0.07 S-1) between 60 degrees C and 70 degrees C. The ATPase activity of the authentic Cpn60 was inhibited by Escherichia coli GroES. The catalytic properties of a recombinant C. thermocellum Cpn60 purified from a GST-Cpn60 fusion protein expressed in E. coli [Ciruela (1995) Ph.D. Thesis, University of Kent] were identical with those of the authentic C. thermocellum Cpn60. Gel-filtration studies show that at room temperature the Cpn60 migrates mainly as a heptamer. Electron microscopy confirms the presence of complexes showing 7-fold rotational symmetry and also reveals a small number of particles that seem to be tetradecamers with a similar structure to E. coli GroEL complexes.

pH-dependence of the dithiol-oxidizing activity of DsbA (a periplasmic protein thiol:disulphide oxidoreductase) and protein disulphide-isomerase: studies with a novel simple peptide substrate

A decapeptide containing two cysteine residues at positions 3 and 8 has been designed for use in monitoring the disulphide bond-forming activity of thiol:disulphide oxidoreductases. The peptide contains a tryptophan residue adjacent to one of the cysteine residues and an arginine residue adjacent to the other. Oxidation of this dithiol peptide to the disulphide state is accompanied by a significant change in tryptophan fluorescence emission intensity. This fluorescence quenching was used as the basis for monitoring the disulphide bond-forming activity of the enzymes protein disulphide-isomerase (PDI) and DsbA (a periplasmic protein thiol:disulphide oxidoreductase) in the pH range 4.0-7.5, where the rates of spontaneous or chemical oxidation are low. Reaction rates were found to be directly proportional to enzyme concentration, and more detailed analysis indicated that the rate-determining step in the overall process was the reoxidation of the reduced form of the enzyme by GSSG. The pH-dependence of the enzyme-catalysed reaction reflected primarily the pKa of the reactive cysteine residue at the active site of each enzyme. The data indicate a pKapp of 5.6 for bovine PDI and of 5.1 for Vibrio cholerae DsbA.

Comparison of the expression patterns of genes coding for wheat gluten proteins and proteins involved in the secretory pathway in developing caryopses of wheat

The synthesis of gluten proteins in the developing caryopsis of wheat is highly coordinated, with mRNAs for the various groups being detected from 11 days after anthesis, and the proteins from about 14 days. In contrast, the levels of transcripts for BiP, PDI and PPI are highest at earlier stages of development. The levels of transcripts for two small GTP binding proteins involved in the secretory pathway (Rab1 and Rab5) are also highest early in development, which is consistent with the retention of most of the gluten proteins within the ER to form protein bodies.

Posttranslational processing of recombinant human interferon-gamma in animal expression systems

We have characterized the heterogeneity of recombinant human interferon-gamma (IFN-gamma) produced by three expression systems: Chinese hamster ovary cells, the mammary gland of transgenic mice, and baculovirus-infected Spodopera frugiperda (Sf9) insect cells. Analyses of whole IFN-gamma proteins by electrospray ionization-mass spectrometry (ESI-MS) from each recombinant source revealed heterogeneous populations of IFN-gamma molecules resulting from variations in N-glycosylation and C-terminal polypeptide cleavages. A series of more specific analyses assisted interpretation of maximum entropy deconvoluted ESI-mass spectra of whole IFN-gamma proteins; MALDI-MS analyses of released, desialylated N-glycans and of deglycosylated IFN-gamma polypeptides were combined with analyses of 2-aminobenzamide labeled sialylated N-glycans by cation-exchange high-performance liquid chromatography. These analyses enabled identification of specific polypeptide cleavage sites and characterization of associated N-glycans. Production of recombinant IFN-gamma in the mammalian expression systems yielded polypeptides C-terminally truncated at dibasic amino acid sites. Mammalian cell derived IFN-gamma molecules displayed oligosaccharides with monosaccharide compositions equivalent to complex, sialylated, or high-mannose type N-glycans. In contrast, IFN-gamma derived from baculovirus-infected Sf9 insect cells was truncated further toward the C-terminus and was associated with neutral (nonsialylated) N-glycans. These data demonstrate the profound influence of host cell type on posttranslational processing of recombinant proteins produced in eukaryotic systems.

Kinetics of acid-mediated disassembly of the B subunit pentamer of Escherichia coli heat-labile enterotoxin. Molecular basis of pH stability

The B-subunit pentamer of Escherichia coli heat-labile enterotoxin (EtxB) is highly stable, maintaining its quaternary structure in a range of conditions that would normally be expected to cause protein denaturation. In this paper the structural stability of EtxB has been studied as a function of pH by electrophoretic, immunochemical, and spectroscopic techniques. Disassembly of the cyclic pentameric structure of human EtxB occurs only below pH 2. As determined by changes in intrinsic fluorescence this process follows first-order kinetics, with the rate constant for disassembly being proportional to the square of the H+ ion concentration, and with an activation energy of 155 kJ mol-1. A C-terminal deletion mutant, hEtxB214, similarly shows first-order kinetics for disassembly but with a higher pH threshold, resulting in disassembly being seen at pH 3.4 and below. These findings are consistent with the rate-limiting step for disassembly of human EtxB being the simultaneous disruption of two interfaces by protonation of two C-terminal carboxylates. By comparison, disassembly of the B-subunit of cholera toxin (CtxB), a protein which shows 80% sequence identity with EtxB, exhibits a much lower stability to acid conditions; with disassembly of CtxB occurring below pH 3.9, with an activation energy of 81 kJ mol-1. Reasons for the observed differences in acid stability are discussed, and the implications of these findings to the development of oral vaccines using EtxB and CtxB are considered.

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.

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.

Characterization of the subunits in an apparently homogeneous subpopulation of Clostridium thermocellum cellulosomes

Clostridium thermocellum cellulosomes isolated by cellulose affinity chromatography were fractionated by anion exchange chromatography into apparently homogeneous subpopulation that differed with respect to enzyme activity and subunit composition. One such subpopulation contained predominantly six subunits and was closely similar to the "subcellulosome" described by Kobayashi et al. (Kobayashi, T., Romaniec, M. P. M., Fauth, U., and Demain, A. L., Appl. Environ. Microbiol., 1990, 56, 3040-3046). Avicelase specific activity of this homogeneous subpopulation was slightly higher than that of unfractionated cellulosomes, but the two preparations were similarly affected by Ca2+, dithiothreitol, and cellobiose. Determination of their N-terminal sequences and enzyme activities has enabled three of the six major subunits of the subpopulation of cellulosomes to be positively identified as known components of the C. thermocellum cellulase complex; the other three subunits did not match up with previously characterized cellulosomal proteins.

Refolding by disulfide isomerization: the mixed disulfide between ribonuclease T1 and glutathione as a model refolding substrate

Protein folding, associated with isomerization of disulfide bonds, was studied using the mixed disulfide between glutathione and reduced ribonuclease T1 (GS-RNase T1) as a stable soluble and homogeneous starting material; conditions were selected to model those within the lumen of the endoplasmic reticulum where native disulfide bonds are formed in protein biosynthesis. Folding was initiated by addition of free glutathione (GSH +/- GSSG) to promote thiol-disulfide interchange and was monitored by intrinsic protein fluorescence, appearance of native ribonuclease activity, HPLC, and nonreducing SDS-PAGE. All the analyses indicated that native RNase T1 was recovered in high yield in a variety of redox conditions. Appearance of native activity followed first-order kinetics; kinetic analysis of the intrinsic fluorescence changes indicated an additional rapid process in some conditions, interpreted as the formation of a nonnative intermediate state. Analysis by HPLC and SDS-PAGE also indicated the formation of transient intermediates. In 1.5 M NaCl, GS-RNase T1 adopts a compact native-like conformation; refolding by thiol-disulfide interchange in these conditions was accelerated approximately 2-fold. Refolding of GS-RNase T1 was catalyzed by protein disulfide isomerase (PDI); substoichiometric quantities of PDI accelerated refolding several-fold. GS-RNase T1 refolding was inhibited by BiP; refolding was completely blocked in presence of a 5-fold molar excess of BiP, and the yield of refolding was substantially reduced by equimolar concentrations of BiP; the refolding was then restored by the addition of ATP. GS-RNase T1 is a convenient model substrate for studying protein folding linked to native disulfide formation in conditions comparable to those within the lumen of the endoplasmic reticulum.

A molecular model for the redox potential difference between thioredoxin and DsbA, based on electrostatics calculations

The disulphide active sites of thioredoxin and DsbA are known to possess a high degree of structural homology. However, DsbA is a much stronger oxidant than thioredoxin. The redox potential difference between DsbA and thioredoxin has been measured to be 160 mV, equivalent to a shift of 15.4 kJ/mol in the reduced/oxidised equilibrium. Electrostatics calculations have been used to study the relative stabilities of the reduced forms of the two proteins. Model calculations suggest that much of the redox potential difference between DsbA and thioredoxin arises form altered stabilisation of the exposed and ionised thiolates of the reduced forms, supporting suggestions previously made on the basis of experimental studies. The calculations have been used to construct a molecular model for the difference in thiolate stabilisation. Although specific interactions, such as thiolate-NH 35 (thioredoxin)/33 (DsbA), provide substantial stabilisation in each reduced protein, the difference between thioredoxin and DsbA is predicted to reside in several side-chain and main-chain groups acting in concert. Residues H32 and Q97 in DsbA are predicted to contribute, along with substantial regions of the polypeptide backbone in the protein domain which is common to DsbA and thioredoxin. Increased thiolate stabilisation by the peptide dipoles is suggested to arise from altered main-chain disposition, and the effect of the additional protein domain of DsbA on the electric field. Peptide dipoles in a region of about 20 residues close to the active site disulphide are predicted to contribute significantly to the redox potential difference.

N-glycosylation of recombinant human interferon-gamma produced in different animal expression systems

Recombinant human interferon-gamma (IFN-gamma) was expressed in Chinese hamster ovary cells, baculovirus-infected Sf9 insect cells and the mammary gland of transgenic mice. The N-linked carbohydrate populations associated with both Asn25 and Asn97 glycosylation sites were characterized by matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) in combination with exoglycosidase array sequencing. A site-specific analysis of dual (2N) and single (1N) site-occupancy variants of IFN-gamma derived from Chinese hamster ovary cells showed that N-glycans were predominantly of the complex bi- and triantennary type. Although Asn25-linked glycans were substituted with a core fucose residue, Asn97 N-glycans were predominantly non-fucosylated, and truncated complex and high-mannose oligosaccharide chains were also evident. Transgenic mouse derived IFN-gamma exhibited considerable site-specific variation in N-glycan structures. Asn25-linked carbohydrates were of the complex, core fucosylated type, Asn97-linked carbohydrates were mainly of the oligomannose type, with smaller proportions of hybrid and complex N-glycans. Carbohydrates associated with both glycosylation sites of IFN-gamma from Sf9 insect cells were mainly tri-mannosyl core structures, with fucosylation confined to the Asn25 site. These data demonstrate the profound influence of host cell type and protein structure on the N-glycosylation of recombinant proteins.

The formation of protein disulphide bonds

The past year has provided more detail on the formation of native disulphide bonds during protein folding at biosynthesis and has identified important cellular factors in the oxidative folding compartments, namely the eukaryotic endoplasmic reticulum and the bacterial periplasm. This information has enabled traditional in vitro refolding studies to be re-evaluated and their relevance as models for folding in the cell to be established.