Recombinant bovine rhodanese: purification and comparison with bovine liver rhodanese

Efficient conversion of chemical energy into mechanical work by Hsp70 chaperones

Hsp70 molecular chaperones are abundant ATP-dependent nanomachines that actively reshape non-native, misfolded proteins and assist a wide variety of essential cellular processes. Here, we combine complementary theoretical approaches to elucidate the structural and thermodynamic details of the chaperone-induced expansion of a substrate protein, with a particular emphasis on the critical role played by ATP hydrolysis. We first determine the conformational free-energy cost of the substrate expansion due to the binding of multiple chaperones using coarse-grained molecular simulations. We then exploit this result to implement a non-equilibrium rate model which estimates the degree of expansion as a function of the free energy provided by ATP hydrolysis. Our results are in quantitative agreement with recent single-molecule FRET experiments and highlight the stark non-equilibrium nature of the process, showing that Hsp70s are optimized to effectively convert chemical energy into mechanical work close to physiological conditions.

Probing the dynamic process of encapsulation in Escherichia coli GroEL

Kinetic analyses of GroE-assisted folding provide a dynamic sequence of molecular events that underlie chaperonin function. We used stopped-flow analysis of various fluorescent GroEL mutants to obtain details regarding the sequence of events that transpire immediately after ATP binding to GroEL and GroEL with prebound unfolded proteins. Characterization of GroEL CP86, a circularly permuted GroEL with the polypeptide ends relocated to the vicinity of the ATP binding site, showed that GroES binding and protection of unfolded protein from solution is achieved surprisingly early in the functional cycle, and in spite of greatly reduced apical domain movement. Analysis of fluorescent GroEL SR-1 and GroEL D398A variants suggested that among other factors, the presence of two GroEL rings and a specific conformational rearrangement of Helix M in GroEL contribute significantly to the rapid release of unfolded protein from the GroEL apical domain.

Flexibility of GroES mobile loop is required for efficient chaperonin function

Chaperonin GroEL and its partner GroES assist the folding of nascent and stress-damaged proteins in an ATP-dependent manner. Free GroES has a flexible "mobile loop" and binds to GroEL through the residues at the tip of the loop, capping the central cavity of GroEL to provide the substrate polypeptide a cage for secure in-cage folding. Here, we show that restriction of the flexibility of the loop by a disulfide cross-linking between cysteines within the loop results in the inefficient formation of a stable GroEL-polypeptide-GroES ternary complex and inefficient folding. Then, we generated substrate proteins with enhanced binding affinity to GroEL by fusion of one or two SBP (strongly binding peptide for GroEL) sequences and examined the effect of disulfide cross-linking on the assisted folding. The results indicate that the higher the binding affinity of the substrate polypeptide to GroEL, the greater the contribution of the mobile loop flexibility to efficient in-cage folding. It is likely that the flexibility helps GroES capture GroEL's binding sites that are already occupied by the substrate polypeptide with various binding modes.

Analysis of peptides and proteins in their binding to GroEL

The GroEL-GroES is an essential molecular chaperon system that assists protein folding in cell. Binding of various substrate proteins to GroEL is one of the key aspects in GroEL-assisted protein folding. Small peptides may mimic segments of the substrate proteins in contact with GroEL and allow detailed structural analysis of the interactions. A model peptide SBP has been shown to bind to a region in GroEL that is important for binding of substrate proteins. Here, we investigated whether the observed GroEL-SBP interaction represented those of GroEL-substrate proteins, and whether SBP was able to mimic various aspects of substrate proteins in GroE-assisted protein folding cycle. We found that SBP competed with substrate proteins, including α-lactalbumin, rhodanese, and malate dehydrogenase, in binding to GroEL. SBP stimulated GroEL ATP hydrolysis rate in a manner similar to that of α-lactalbumin. SBP did not prevent GroES from binding to GroEL, and GroES association reduced the ATPase rates of GroEL/SBP and GroEL/α-lactalbumin to a comparable extent. Binding of both SBP and α-lactalbumin to apo GroEL was dominated by hydrophobic interaction. Interestingly, association of α-lactalbumin to GroEL/GroES was thermodynamically distinct from that to GroEL with reduced affinity and decreased contribution from hydrophobic interaction. However, SBP did not display such differential binding behaviors to apo GroEL and GroEL/GroES, likely due to the lack of a contiguous polypeptide chain that links all of the bound peptide fragments. Nevertheless, studies using peptides provide valuable information on the nature of GroEL-substrate protein interaction, which is central to understand the mechanism of GroEL-assisted protein folding.

Regulation of c-Src nonreceptor tyrosine kinase activity by bengamide A through inhibition of methionine aminopeptidases

Methionine aminopeptidases (MetAPs) remove the N-terminal initiator methionine during protein synthesis, a prerequisite step for N-terminal myristoylation. N-myristoylation of proto-oncogene c-Src is essential for its membrane association and proper signal transduction. We used bengamides, a family of general MetAP inhibitors, to understand the downstream physiological functions of MetAPs. c-Src from bengamide A-treated cells retained its N-terminal methionine and suffered a decrease in N-terminal myristoylation, which was accompanied by a shift of its subcellular distribution from the plasma membrane to the cytosol. Furthermore, bengamide A decreased the tyrosine kinase activities of c-Src both in vitro and in vivo and eventually delayed cell-cycle progression through G(2)/M. Thus, c-Src is a physiologically relevant substrate for MetAPs whose dysfunction is likely to account for the cell-cycle effects of MetAP inhibitors including bengamide A.

Integration of short-end injection mode into electrophoretically mediated microanalysis

The possibility of integration of the short-end injection mode in the EMMA methodology is demonstrated in this work on the kinetic studies of haloalkane dehalogenase and rhodanese enzymatic reactions. The essential validations of the EMMA methods combined with the short-end and long-end injection modes were performed first to confirm their accuracy. The qualitative and quantitative parameters of both approaches such as repeatabilities of migration times and peak areas, limits of detection and correlation coefficients were in acceptable ranges. In addition, estimated Michaelis constants for the corresponding substrate(s) were comparable being in accordance with previous literature data. Moreover, the ping-pong reaction mechanism of rhodanese reaction was confirmed by means of both injection modes. This combination thus preserves the benefits of these instrumental approaches. Whereas the short-end injection procedure brought 5-6.5 times reduction of the analysis time and 2.5-4 times increase of the sensitivity, the EMMA methodology allowed full automatization of the assays while the whole kinetic studies needed only 20 microl of corresponding enzyme preparation.

Detection and analysis of protein aggregation with confocal single molecule fluorescence spectroscopy

The misfolding and aggregation of proteins is a common phenomenon both in the cell, in in vitro protein refolding, and the corresponding biotechnological applications. Most importantly, it is involved in a wide range of diseases, including some of the most prevalent neurodegenerative disorders. However, the range of methods available to analyze this highly heterogeneous process and the resulting aggregate structures has been very limited. Here we present an approach that uses confocal single molecule detection of FRET-labeled samples employing four detection channels to obtain information about diffusivity, anisotropy, fluorescence lifetimes and Förster transfer efficiencies from a single measurement. By combining these observables, this method allows the separation of subpopulations of folded and misfolded proteins in solution with high sensitivity and a differentiation of aggregates generated under different conditions. We demonstrate the versatility of the method with experiments on rhodanese, an aggregation-prone two-domain protein.

Chaperone properties of mammalian mitochondrial translation elongation factor Tu

The main function of the prokaryotic translation elongation factor Tu (EF-Tu) and its eukaryotic counterpart eEF1A is to deliver aminoacyl-tRNA to the A-site on the ribosome. In addition to this primary function, it has been reported that EF-Tu from various sources has chaperone activity. At present, little information is available about the chaperone activity of mitochondrial EF-Tu. In the present study, we have examined the chaperone function of mammalian mitochondrial EF-Tu (EF-Tumt). We demonstrate that recombinant EF-Tumt prevents thermal aggregation of proteins and enhances protein refolding in vitro and that this EF-Tumt chaperone activity proceeds in a GTP-independent manner. We also demonstrate that, under heat stress, the newly synthesized peptides from the mitochondrial ribosome specifically co-immunoprecipitate with EF-Tumt and are destabilized in EF-Tumt-overexpressing cells. We show that most of the EF-Tumt localizes on the mitochondrial inner membrane where most mitochondrial ribosomes are found. We discuss the possible role of EF-Tumt chaperone activity in protein quality control in mitochondria, with regard to the recently reported in vivo chaperone function of eEF1A.

Active rhodanese lacking nonessential sulfhydryl groups has increased hydrophobic exposure not observed in wild-type enzyme

Mutation of all nonessential cysteine residues to serines in rhodanese turns the enzyme into a form (C3S) that is fully active but less stable than wild type (WT). bis-ANS binding studies have shown that C3S has more hydrophobic exposure than WT, although both have similar secondary structures suggesting the flexibility of its structure. Activity of C3S falls once it binds bis-ANS, and covalent binding of bis-ANS to C3S is induced by light. bis-ANS binds to C3S in its C-terminal domain as is shown by gel electophoresis and proteolysis. bis-ANS binding makes the C-terminal domain more susceptible to trypsin cleavage.

The 4,4'-dipyridyl disulfide-induced formation of GroEL monomers is cooperative and leads to increased hydrophobic exposure

The molecular chaperone, GroEL, is completely disassembled into monomers by the addition of 4,4'-dipyridyl disulfide. The dissociation leads to monomers in a kinetically controlled process. The additions of functional ligands of GroEL such as Mg(2+) or adenine nucleotides produced differences in the observed rates, but at the end of the kinetics, the dissociation was complete. In addition to the information obtained from native gels, the fluorescent probe bis-ANS was utilized to follow the monomer formation. The results demonstrate that the formation of monomers was associated with the exposure of hydrophobic surfaces. This assessment was possible without the use of added chaotropes, such as urea, to dissociate GroEL. Dissociation kinetics were also followed by light scattering. The kinetics of dissociation of the 14mer are cooperative with respect to the concentration of 4,4'-DPDS. Thermodynamic parameters for the kinetic process gave a free energy of activation (DeltaG) of 19.3 +/- 1.2 kcal mol(-1), which was decomposed to an enthalpy of activation (DeltaH) of 19.30 +/- 1.2 kcal mol(-1) and an entropy of activation (DeltaS) of -8.2 +/- 3.9 cal mol(-1) K(-1). We conclude that the dissociation of GroEL observed in this investigation is an enthalpy-controlled process.

The binding of bis-ANS to the isolated GroEL apical domain fragment induces the formation of a folding intermediate with increased hydrophobic surface not observed in tetradecameric GroEL

The extent of hydrophobic exposure upon bis-ANS binding to the functional apical domain fragment of GroEL, or minichaperone (residues 191-345), was investigated and compared with that of the GroEL tetradecamer. Although a total of seven molecules of bis-ANS bind cooperatively to this minichaperone, most of the hydrophobic sites were induced following initial binding of one to two molecules of probe. From the equilibrium and kinetics studies at low bis-ANS concentrations, it is evident that the native apical domain is converted to an intermediate conformation with increased hydrophobic surfaces. This intermediate binds additional bis-ANS molecules. Tyrosine fluorescence detected denaturation demonstrated that bis-ANS can destabilize the apical domain. The results from (i) bis-ANS titrations, (ii) urea denaturation studies in the presence and absence of bis-ANS, and (iii) intrinsic tyrosine fluorescence studies of the apical domain are consistent with a model in which bis-ANS binds tightly to the intermediate state, relatively weakly to the native state, and little to the denatured state. The results suggest that the conformational changes seen in apical domain fragments are not seen in the intact GroEL oligomer due to restrictions imposed by connections of the apical domain to the intermediate domain and suppression of movement due to quaternary structure.

Active-site sulfhydryl chemistry plays a major role in the misfolding of urea-denatured rhodanese

Unfolded bovine rhodanese, a sulfurtransferase, does not regain full activity upon refolding due to the formation of aggregates and disulfide-linked misfolded states unless a large excess of reductant such as 200 mM beta-ME and 5 mg/ml detergent are present [Tandon and Horowitz (1990), J. Biol. Chem. 265, 5967]. Even then, refolding is incomplete. We have studied the unfolding and refolding of three rhodanese forms whose crystal structures are known: ES, containing the transferred sulfur as a persulfide; E, without the transferred sulfur, and carboxymethylated rhodanese (CMR), in which the active site was blocked by chemical modification. The X-ray structures of ES, E, and CMR are virtually the same, but their tertiary structures in solution differ somewhat as revealed by near-UV CD. Among these three, CMR is the only form of rhodanese that folds reversibly, requiring 1 mM DTT. A minimum three-state folding model of CMR (N<-->I<-->U) followed by fluorescence at 363 nm, (N<-->I) by fluorescence at 318 nm, and CD (I<-->U) is consistent with the presence of a thermodynamically stable molten globule intermediate in 5-6 M urea. We conclude that the active-site sulfhydryl group in the persulfide form is very reactive; therefore, its modification leads to the successful refolding of urea-denatured rhodanese even in the absence of a large excess of reductant and detergent. The requirement for DTT for complete reversibility of CMR suggests that oxidation among the three non-active-site SH groups can represent a minor trap for refolding through species that can be easily reduced.

The crystal structure of a sulfurtransferase from Azotobacter vinelandii highlights the evolutionary relationship between the rhodanese and phosphatase enzyme families

Rhodanese is an ubiquitous enzyme that in vitro catalyses the transfer of a sulfur atom from suitable donors to nucleophilic acceptors by way of a double displacement mechanism. During the catalytic process the enzyme cycles between a sulfur-free and a persulfide-containing form, via formation of a persulfide linkage to a catalytic Cys residue. In the nitrogen-fixing bacteria Azotobacter vinelandii the rhdA gene has been identified and the encoded protein functionally characterized as a rhodanese. The crystal structure of the A. vinelandii rhodanese has been determined and refined at 1.8 A resolution in the sulfur-free and persulfide-containing forms. Conservation of the overall three-dimensional fold of bovine rhodanese is observed, with substantial modifications of the protein structure in the proximity of the catalytic residue Cys230. Remarkably, the native enzyme is found as the Cys230-persulfide form; in the sulfur-free state the catalytic Cys residue adopts two alternate conformations, reflected by perturbation of the neighboring active-site residues, which is associated with a partly reversible loss of thiosulfate:cyanide sulfurtransferase activity. The catalytic mechanism of A. vinelandii rhodanese relies primarily on the main-chain conformation of the 230 to 235 active-site loop and on a surrounding strong positive electrostatic field. Substrate recognition is based on residues which are entirely different in the prokaryotic and eukaryotic enzymes. The active-site loop of A. vinelandii rhodanese displays striking structural similarity to the active-site loop of the similarly folded catalytic domain of dual specific phosphatase Cdc25, suggesting a common evolutionary origin of the two enzyme families.

Enzyme-mediated sulfide production for the reconstitution of [2Fe-2S] clusters into apo-biotin synthase of Escherichia coli. Sulfide transfer from cysteine to biotin

We previously showed that biotin synthase in which the (Fe-S) cluster was labelled with 34S by reconstitution donates 34S to biotin [B. Tse Sum Bui, D. Florentin, F. Fournier, O. Ploux, A. Méjean & A. Marquet (1998) FEBS Lett. 440, 226-230]. We therefore proposed that the source of sulfur was very likely the (Fe-S) centre. This depletion of sulfur from the cluster during enzymatic reaction could explain the absence of turnover of the enzyme which means that to restore a catalytic activity, the clusters have to be regenerated. In this report, we show that the NifS protein from Azotobacter vinelandii and C-DES from Synechocystis as well as rhodanese from bovine liver can mobilize the sulfur, respectively, from cysteine and thiosulfate for the formation of a [2Fe-2S] cluster in the apoprotein of Escherichia coli biotin synthase. The reconstituted enzymes were as active as the native enzyme. When [35S]cysteine was used during the reconstitution experiments in the presence of NifS, labelled (Fe35S) biotin synthase was obtained. This enzyme produced [35S]biotin, confirming the results obtained with the 34S-reconstituted enzyme. NifS was also effective in mobilizing selenium from selenocystine to produce an (Fe-Se) cluster. However, though NifS could efficiently reconstitute holobiotin synthase from the apoform, starting from cysteine, these two effectors had no significant effect on the turnover of the enzyme in the in vitro assay.

Truncations at the NH2 terminus of rhodanese destabilize the enzyme and decrease its heterologous expression

Rhodanese mutants containing sequential NH2-terminal deletions were constructed to test the distinct contributions of this region of the protein to expression, folding, and stability. The results indicate that the first 11 residues are nonessential for folding to the active conformation, but they are necessary for attaining an active, stable structure when expressed in Escherichia coli. Rhodanese species with up to 9 residues deleted were expressed and purified. Kinetic parameters for the mutants were similar to those of the full-length enzyme. Compared with shorter truncations, mutants missing 7 or 9 residues were (a) increasingly inactivated by urea denaturation, (b) more susceptible to inactivation by dithiothreitol, (c) less able to be reactivated, and (d) less rapidly inactivated by incubation at 37 degreesC. Immunoprecipitation showed that mutants lacking 10-23 NH2-terminal amino acids were expressed as inactive species of the expected size but were rapidly eliminated. Cell-free transcription/translation at 37 degreesC showed mutants deleted through residue 9 were enzymatically active, but they were inactive when deleted further, just as in vivo. However, at 30 degreesC in vitro, both Delta1-10 and Delta1-11 showed considerable activity. Truncations in the NH2 terminus affect the chemical stability of the distantly located active site. Residues Ser-11 through Gly-22, which form the NH2-proximal alpha-helix, contribute to folding to an active conformation, to resisting degradation during heterologous expression, and to chemical stability in vitro.

Preformed GroES oligomers are not required as functional cochaperonins

We have previously shown that the C-terminal sequence of GroES is required for oligomerization [Seale and Horowitz (1995), J. Biol. Chem. 270, 30268-30270]. In this report, we have generated a C-terminal deletion mutant of GroES with a significantly destabilized oligomer and have investigated its function in the chaperonin-assisted protein folding cycle. Removal of the two C-terminal residues of GroES results in a cochaperonin [GroESD(96-97)] that is monomeric at concentrations where GroES function is assessed. Using equilibrium ultracentrifugation, we measured the dissociation constant for the oligomer-monomer equilibrium to be 7.3 x 10(-34)M6. The GroESD(96-97) is fully active as a cochaperonin. This mutant is able to inhibit the ATPase activity of GroEL to levels comparable to wild-type GroES. It is also able to assist the refolding of urea-denatured rhodanese by GroEL. While GroESD(96-97) can function at levels comparable to wild-type GroES, higher concentrations of mutant are required to produce the same effect. These results support the idea that the performed GroES heptamer is not required for function, but they suggest that the oligomeric cochaperonin is most efficient.

Thermoalkalophilic lipase of Bacillus thermocatenulatus large-scale production, purification and properties: aggregation behaviour and its effect on activity

Escherichia coli BL321 was transformed with the expression plasmid pCYTEXP1 carrying the BTL2 gene from Bacillus thermocatenulatus under the control of the strong temperature-inducible lambda pL promoter and was cultivated in a 100 1 bioreactor. The mature lipase was produced in large quantities (54,000 U g-1 wet cells) and further purified to homogeneity by a two-step purification protocol (hydrophobic chromatography and gel filtration chromatography). The pure enzyme was characterized and its physicochemical properties compared to those of the BTL2 lipase which had previously been weakly expressed in E. coli under the control of its native promoter on pUC18, yielding 600 U g-1 wet cells. The specific activity of the overexpressed enzyme was approx. 5-fold higher than that of the weakly expressed enzyme. The two proteins showed the same pI and N-terminal sequence and had very similar thermostability, pH stability, optimum pH and temperature activity, and substrate specificity. Both enzymes were extremely stable in the presence of several organic solvents and detergents. With trioleylglycerol as a substrate, the overexpressed lipase cleaves each of the three ester bonds. The purified BTL2 lipase shows a strong tendency to aggregate. Direct evidence for changes in the aggregation state was obtained by gel filtration chromatography. The effect of aggregation on lipase activity was strongly dependent on both substrate and temperature during the assay. Under certain conditions, a direct relationship was found between the molecular mass of the lipase aggregates and the increase in activity upon the addition of 1% (w/v) sodium cholate.

Role of amino acid residues in the active site of rat liver mercaptopyruvate sulfurtransferase. CDNA cloning, overexpression, and site-directed mutagenesis

A complete amino acid structure of rat liver mercaptopyruvate sulfurtransferase (MST, EC 2.8.1.2) was determined by sequence analysis of cDNA and purified enzyme. The enzyme consists of 296 amino acid residues with a calculated molecular mass of 32,808 Da. Sequence identity in cDNA and the deduced amino acid sequence are 65 and 60% respectively, between rat MST and rhodanese. By their entire sequence similarity MST and rhodanese are confirmed to be evolutionarily related enzymes (Nagahara, N., Okazaki, T., and Nishino, T. (1995) J. Biol. Chem. 270, 16230-16235). The conversion of MST to rhodanese was attempted, and the role of amino acid residues was studied by site-directed mutagenesis with the isolated cDNA of rat liver MST. There is a strong possibility that Cys247 is a catalytic site of MST. Arg187 is suggested to be a binding site of both mercaptopyruvate and thiosulfate in MST. Arg196, which is missed in rhodanese, is important for catalysis in MST. On the other hand, the substitution of Arg for Gly248 or Lys for Ser249 facilitates catalysis of thiosulfate in MST. It is concluded that Arg187 and Arg196 of rat MST are critical residues in determining substrate specificity for mercaptopyruvate. On the other hand, Arg185, Arg247, and Lys248 of rat rhodanese are critical residues in determining substrate specificity for thiosulfate.

Reversible oligomerization and denaturation of the chaperonin GroES

The chaperonin GroEL can assist protein folding and normally acts with the co-chaperonin GroES. These Escherichia coli proteins are encoded on the same operon, with GroES positioned first. In this report, we have investigated the reversible folding of GroES. Using fluorescence anisotropy of dansyl-labeled GroES, intrinsic fluorescence, bis-ANS binding, sedimentation velocity, and limited proteolysis, we show that GroES unfolds in a single, two-state transition. Importantly, intrinsic fluorescence and sedimentation velocity analyses show that GroES is capable of refolding and reassembling from a urea denatured state. The refolded GroES is fully active as shown by its ability to assist GroEL in the refolding of rhodanese. These results indicate that chaperonins may not require other chaperonins for successful folding/assembly. We also show that GroES is capable of assisting in the refolding/reassembly of fully denatured GroEL. The reversible folding of GroES coupled with the ability of GroES to assist the refolding/reassembly of GroEL suggest that the groE operon may be organized in a manner that provides a structural role in GroES/GroEL assembly as well as a functional role.

Ligand-induced conformational changes in the apical domain of the chaperonin GroEL

Although the role of nucleotides in the catalytic cycle of the GroESL chaperonin system has been extensively studied, the molecular effects of nucleotides in modulating exposure of sites on GroEL has not been thoroughly investigated. We report here that nucleotides (ATP, ADP, or adenosine 5'-(beta, gamma-imino)triphosphate) in the presence of Mg2+ make the oligomer selectively sensitive to trypsin proteolysis in a fashion suggesting conformational changes in the monomers of one heptameric ring. The site of proteolysis in the monomer that is exposed upon nucleotide binding by the oligomer is in the apical domain (Arg-268). Further, complexes of GroEL with GroES or rhodanese display the same sensitivity to proteolysis, unlike the GroEL-GroES-rhodanese complex, which is protected from proteolysis. The influence of various cations on trypsin proteolysis is investigated to elucidate the differential effects that monovalent and divalent cations have on the oligomeric structure of the chaperonin. These results are discussed in relation to the molecular basis for the chaperonin activity.

Inactive GroEL monomers can be isolated and reassembled to functional tetradecamers that contain few bound peptides

For the first time, it has been shown that GroEL can be converted from tetradecamers (14-mers) to monomers under conditions commonly used for the preparation of this chaperonin. The essential requirements are the simultaneous presence of nucleotides such as MgATP or MgADP and a solid-phase anion-exchange medium. The monomers that are formed are metastable in that they only reassemble to a small degree in the absence of additives. These results are in keeping with previous studies on high pressure dissociation that showed the separated monomers display conformational plasticity and can undergo conformational relaxation when relieved of the constraints of the quaternary structure in the oligomer (Gorovits, B., Raman, C. S., and Horowitz, P. M. (1995) J. Biol. Chem. 270, 2061-2066). The monomers display greatly enhanced hydrophobic exposure to the probe 1,1'-bis(4-anilino)naphthalene-5,5'-disulfonic acid, although they are not active in folding functions, and they are unable to form complexes with partially folded rhodanese. The monomers can be completely reassembled to 14-mers by incubation in 1 M ammonium sulfate. There is no evidence of intermediates in the reassembly process. Compared with the original oligomers, the reassembled 14-mers have (a) very low levels of polypeptide contaminants and tryptophan-like fluorescence, two problems that previously hampered spectroscopic studies of GroEL structure and function; (b) functional properties that are very similar to the original material; (c) considerably decreased hydrophobic exposure in the native state; and (d) a similar triggered exposure of hydrophobic surfaces after treatment with urea or spermidine. This study demonstrates that the quaternary structure of GroEL is more labile than previously thought. These results are consistent with suggestions that nucleotides can loosen subunit interactions and show that changes in quaternary structure can operate under conditions where GroEL function has been demonstrated.

Mutation in the interdomain tether influences the stability and refolding of the enzyme rhodanese

Rhodanese is a single polypeptide chain of 293 amino acids that is folded into two globular domains of nearly equal size that are connected by a 16 amino acid tether. Two amino acids, Val-Asp (VD), were inserted into the interdomain tether through site-directed mutagenesis to produce the new interdomain sequence, E145PSRPEPAIFKAVDTLNR. The purified mutant protein, when unperturbed, was virtually indistiguishable in all properties tested and gave a specific activity that was at least 90% of the WT. However, the tether mutant was considerably less stable to perturbation compared with the WT enzyme. The interdomain hydrophobic surfaces in the mutant were more easily exposed, and the formation of intermediate folding states was facilitated. The rate of unassisted refolding was slightly less for the mutant, and the yield of active enzyme was somewhat reduced. The mutation introduced a new V8 proteinase cleavage site, but this site was not accessible in the native mutant which was as resistant to proteolysis as the WT enzyme. However, perturbation with low concentrations of urea that could form folding intermediate(s), allowed facile cleavage of the mutant to give fragments that appeared to represent the individual domains. In addition, the perturbed mutant could be proteolyzed close to one end of the polypeptide, a position that is far from the site of mutation, and which was not readily cleaved in the WT enzyme or the native form of the mutant. These results indicate that mutation in the interdomain tether can have dramatic effects on the stability and conformational transitions of rhodanese.

Refolding and reassembly of active chaperonin GroEL after denaturation

Conditions are reported that, for the first time, permit the folding and assembly of active chaperonin, GroEL, following denaturation in 8 m urea. The folding could be achieved by dilution or dialysis, and the best yields required the simultaneous presence of ammonium sulfate and the Mg2+ complexes of ATP or ADP. Ammonium sulfate was the key to this particular protocol, since there was a small recovery of oligomer in its presence, but no detectable recovery was induced by ATP or ADP without ammonium sulfate. The refolded/reassembled GroEL could arrest the spontaneous folding of rhodanese, and it could participate in the chaperonin-assisted refolding of rhodanese as effectively as GroEL that had never been unfolded. The results demonstrate that the primary sequence of GroEL contains the information required for its folding, assembly, and function. Thus, in contrast to previous studies, although chaperonins may facilitate GroEL folding, they are not necessary for the acquisition of the functional oligomeric state of this chaperone. This ability to fold denatured GroEL in vitro will facilitate studies of the influences that determine the interesting folding pattern adopted by the native protein.

Cytosolic mercaptopyruvate sulfurtransferase is evolutionarily related to mitochondrial rhodanese. Striking similarity in active site amino acid sequence and the increase in the mercaptopyruvate sulfurtransferase activity of rhodanese by site-directed mutagenesis

Rat liver mercaptopyruvate sulfurtransferase (MST) was purified to homogeneity. MST is very similar to rhodanese in physicochemical properties. Further, rhodanese cross-reacts with anti-MST antibody. Both purified authentic MST and expressed rhodanese possess MST and rhodanese activities, although the ratio of rhodanese to MST activity is low in MST and high in rhodanese. In order to compare the active site regions of MST and rhodanese, the primary structure of a possible active site region of MST was determined. The sequence showed 66% homology with that of rat liver rhodanese. An active site cysteine residue (Cys246; site of formation of persulfide in catalysis) and an arginine residue (Arg185; substrate binding site) in rhodanese were also conserved in MST. On the other hand, two other active site residues (Arg247 and Lys248) were replaced by Gly and Ser, respectively. Conversion of rhodanese to MST was tried by site-directed mutagenesis. After cloning of rat liver rhodanese, recombinant wild type and three mutants (Arg247-->Gly and/or Lys248-->Ser) were constructed. The enzymes were expressed in Escherichia coli strain BL21 (DE3) with a T7 promoter system. The mutation of these residues decreases rhodanese activity and increases MST activity.

The molecular chaperonin cpn60 displays local flexibility that is reduced after binding with an unfolded protein

Steady-state fluorescence polarization was used to examine the chaperonin cpn60 that was covalently labeled with pyrene. Two compounds, 1-pyrenesulfonyl chloride or N-(1-pyrene)maleimide, were used to incorporate up to 8 mol of pyrene per mol of cpn60 14-mer. The fluorescence lifetime of the cpn60-pyrenesulfonyl chloride conjugate exhibited a double exponential decay: 5.36 ns, with a fractional contribution to the intensity of 7%, and 48.77 ns, with a fractional contribution to the intensity of 93%. These yield a second-order average lifetime of 45.58 ns at 20 degrees C. Analysis of the fluorescence polarization data for the pyrene probe by the Perrin-Weber treatment revealed the existence of two components that account for the depolarization. The fast component accounted for 24% of the depolarization at 20 degrees C. The rotational relaxation time for the cpn60 14-mer derived from the low viscosity part of the Perrin-Weber plot which accentuates the slow motion gave rho h = 1113 +/- 55 ns. When this value of rho h is compared with the rho h calculated based on the Stokes radius of cpn60 from ultracentrifugation, rho Stokes, it leads to rho h/rho Stokes = 0.4 which is considerably smaller than the value expected (rho h/rho Stokes = 1) or actually found in the cpn60-rhodanese complex (rho h/rho Stokes = 0.93). These considerations and the observed presence of the fast motion suggest that cpn60 is not a rigid protein. Analysis of the polarization data as a function of temperature, which is weighted more toward the fast motion, showed that the rotational relaxation time assessed by temperature variation is greatly increased (from 552.5 to 2591 ns) for the complex of cpn60 with partially folded rhodanese (34-kDa monomeric protein). No change in rho h was observed upon formation of the cpn60.ATP complex (rho h = 556.9 ns). These data indicate that there is local motion in the cpn60 14-mer molecule that can be frozen by formation of a binary complex with partially folded proteins. This conclusion is in keeping with results showing that the structure of cpn60 is generally stabilized when it forms complexes with passenger proteins (Mendoza, J. A., and Horowitz, P. M. (1994) J. Biol. Chem. 269, 25963-25965).

Exposure of hydrophobic surfaces on the chaperonin GroEL oligomer by protonation or modification of His-401

Hydrophobic exposure on the chaperonin GroEL is increased 6-10-fold after the protein is treated with the His-reactive reagent diethyl pyrocarbonate (DEP), or the solution pH is lowered to 5.5. The induced hydrophobic surfaces have the same 1,1'-bis(4-anilino)naphthalene-5,5'-disulfonic acid (bis-ANS) binding characteristics as unperturbed GroEL: a Kd approximately equal to 3.5 microM, a maximum intensity at approximately 500 nm, and an average fluorescence lifetime of approximately 8.0 ns. The pKa for the pH-induced transition is 6.6, most likely attributable to the only histidine in GroEL, His-401, located in the intermediate domain. The modification of one histidine residue per monomer upon DEP treatment is supported by the correlation between the change in the absorbance at 242 nm for the N-carbethoxyhistidyl derivative and the increase in bis-ANS fluorescence. GroEL at pH 5.5 is tetradecameric and can capture urea-denatured rhodanese and release it as active enzyme. The GroEL-rhodanese and release it as active enzyme. The GroEL-rhodanese complex is more stable to dissociation by 2.25 M urea than the complex formed at pH 7.8. We propose that His-401 is in a conformationally sensitive region such that protonation or modification can lead to increased exposure of hydrophobic surfaces capable of binding folding intermediates.

Thermally perturbed rhodanese can be protected from inactivation by self-association

A fluorescence-detected structural transition occurs in the enzyme rhodanese between 30-40 degrees C that leads to inactivation and aggregation, which anomalously decrease with increasing protein concentration. Rhodanese at 8 micrograms/ml is inactivated at 40 degrees C after 50 min of incubation, but it is protected as its concentration is raised, such that above 200 micrograms/ml, there is only slight inactivation for at least 70 min. Inactivation is increased by lauryl maltoside, or by low concentrations of 2-mercaptoethanol. The enzyme is protected by high concentrations of 2-mercaptoethanol or by the substrate, thiosulfate. The fluorescence of 1,8-anilinonaphthalene sulfonate reports the appearance of hydrophobic sites between 30-40 degrees C. Light scattering kinetics at 40 degrees C shows three phases: an initial lag, a relatively rapid increase, and then a more gradual increase. The light scattering decreases under several conditions; at increased protein concentration; at high concentrations of 2-mercaptoethanol; with lauryl maltoside; or with thiosulfate. Aggregated enzyme is inactive, although enzyme can inactivate without significant aggregation. Glutaraldehyde cross-linking shows that rhodanese can form dimers, and that higher molecular weight species are formed at 40 degrees C but not at 23 degrees C. Precipitates formed at 40 degrees C contain monomers with disulfide bonds, dimers, and multimers. We propose that thermally perturbed rhodanese has increased hydrophobic exposure, and it can either: (a) aggregate after a rate-limiting inactivation; or (b) reversibly dimerize and protect itself from inactivation and the formation of large aggregates.