Chaperonin cpn60 from Escherichia coli protects the mitochondrial enzyme rhodanese against heat inactivation and supports folding at elevated temperatures

Ectopic BH3-only protein Bim acts as a co-chaperone to positively regulate Hsp70 in yeast

The chaperone heat shock protein 70 (Hsp70) is conserved from bacteria to humans and is crucial for avoiding protein misfolding under stress. Bim functions mainly as one of the Bcl-2 family pro-apoptotic members, was identified to be a co-chaperone of Hsp70. Herein, we reported that ectopic Bim could constitute the interactions with intrinsic Hsp70 and translate its positive co-chaperone activity in vitro to the yeast growth promotion and help Hsp70 to fold its client Ras-like protein. With the help of a specific Hsp70/Bim disruptor, we illustrated that Hsp70/Bim dimers rescue yeast from heat shock. In an organism lacks apoptotic Bcl-2 factors, the pro-apoptotic Bim in mammalian cells exhibits pro-survival functions.

Chaperonin-assisted protein folding: a chronologue

This chronologue seeks to document the discovery and development of an understanding of oligomeric ring protein assemblies known as chaperonins that assist protein folding in the cell. It provides detail regarding genetic, physiologic, biochemical, and biophysical studies of these ATP-utilizing machines from both in vivo and in vitro observations. The chronologue is organized into various topics of physiology and mechanism, for each of which a chronologic order is generally followed. The text is liberally illustrated to provide firsthand inspection of the key pieces of experimental data that propelled this field. Because of the length and depth of this piece, the use of the outline as a guide for selected reading is encouraged, but it should also be of help in pursuing the text in direct order.

Mitochondrial Dynamics and Heart Failure

Mitochondrial dynamics, fission and fusion, were first identified in yeast with investigation in heart cells beginning only in the last 5 to 7 years. In the ensuing time, it has become evident that these processes are not only required for healthy mitochondria, but also, that derangement of these processes contributes to disease. The fission and fusion proteins have a number of functions beyond the mitochondrial dynamics. Many of these functions are related to their membrane activities, such as apoptosis. However, other functions involve other areas of the mitochondria, such as OPA1's role in maintaining cristae structure and preventing cytochrome c leak, and its essential (at least a 10 kDa fragment of OPA1) role in mtDNA replication. In heart disease, changes in expression of these important proteins can have detrimental effects on mitochondrial and cellular function.

Assays to characterize molecular chaperone function in vitro

Here, we describe a set of assays, using mitochondrial citrate synthase as a model substrate, which are suitable to test for chaperone function of proteins in vitro. Additionally, these assays distinguish between the ability of suppressing the aggregation of diverse substrate proteins by stable interaction (holdase function) and the ability to assist the refolding of substrate proteins (foldase function).

In silico engineering of aggregation-prone recombinant proteins for substrate recognition by the chaperonin GroEL

Background

Molecular chaperones appear to have been evolved to facilitate protein folding in the cell through entrapment of folding intermediates on the interior of a large cavity formed between GroEL and its co-chaperonin GroES. They bind newly synthesized or non-native polypeptides through hydrophobic interactions and prevent their aggregation. Some proteins do not interact with GroEL, hence even though they are aggregation prone, cannot be assisted by GroEL for their folding.

Results

In this study, we have attempted to engineer these non-substrate proteins to convert them as the substrate for GroEL, without compromising on their function. We have used a computational biology approach to generate mutants of the selected proteins by selectively mutating residues in the hydrophobic patch, similar to GroES mobile loop region that are responsible for interaction with GroEL, and compared with the wild counterparts for calculation of their instability and aggregation propensities. The energies of the newly designed mutants were computed through molecular dynamics simulations. We observed increased aggregation propensity of some of the mutants formed after replacing charged amino acid residues with hydrophobic ones in the well defined hydrophobic patch, raising the possibility of their binding ability to GroEL.

Conclusions

The newly generated mutants may provide potential substrates for Chaperonin GroEL, which can be experimentally generated and tested for their tendency of aggregation, interactions with GroEL and the possibility of chaperone-assisted folding to produce functional proteins.

Protection of GroEL by its methionine residues against oxidation by hydrogen peroxide

GroEL undergoes an important functional and structural transition when oxidized with hydrogen peroxide (H2O2) concentrations between 15 and 20mM. When GroEL was incubated for 3h with 15 mM H2O2, it retained its quaternary structure, chaperone and ATPase activities. Under these conditions, GroEL's cysteine and tyrosine residues remained intact. However, all the methionine residues of the molecular chaperone were oxidized to the corresponding methionine-sulfoxides under these conditions. The oxidation of the methionine residues was verified by the inability of cyanogen bromide to cleave at the carboxyl side of the modified methionine residues. The role for the proportionately large number (23) of methionine residues in GroEL has not been identified. Methionine residues have been reported to have an antioxidant activity in proteins against a variety of oxidants produced in biological systems including H2O2. The carboxyl-terminal domain of GroEL is rich in methionine residues and we hypothesized that these residues are involved in the protection of GroEL's functional structure by scavenging H2O2. When GroEL was further incubated for the same time, but with increasing concentrations of H2O2 (>15 mM), the oxidation of GroEL's cysteine residues and a significant decrease of the tyrosine fluorescence due to the formation of dityrosines were observed. Also, at these higher concentrations of H2O2, the inability of GroEL to hydrolyze ATP and to assist the refolding of urea-unfolded rhodanese was observed.

Artificial chaperone-assisted refolding of GuHCl-denatured alpha-amylase at low temperature: refolding versus aggregation

Refolding of GuHCl-denatured alpha-amylase was investigated using the artificial chaperone-assisted method. Three different cationic detergents (CTAB, TTAB and DTAB) and two nonionic detergents (Tween 80 and Triton X-100) were evaluated as the capturing reagents along with alpha- and beta-CD as the stripping agents. The refolding yields, at a final protein concentration of 0.15 mg/ml, were 82, 71 and 66% in the presence of beta-CD and CTAB, TTAB or DTAB, respectively. To improve the refolding yield and to suppress the extent of aggregation, the initial rate of the stripping step was slowed down by maintaining the refolding environment at 4 degrees C for about 3 min followed by raising the temperature to 25 degrees C. Under this thermal procedure, the refolding yield and the extent of aggregation were changed from 82 and 25% at 25 degrees C to 94 and 7% at 4 degrees C, respectively. These findings may assist the activity recovery of recombinant proteins at relatively high concentrations.

Recombinant expression and in vitro refolding of the yeast small heat shock protein Hsp42

Small Hsps represent a variation on the theme of protection of proteins from irreversible aggregation by reversible interaction with chaperone proteins. While different sHsps are highly heterogeneous in sequence and size, the common trait is the presence of a conserved alpha-crystallin domain. In addition sHsps assemble into large oligomeric complexes where dimers represent the basic building blocks. Hsp42, a member of the sHsp family in the cytosol of S. cerevisiae, forms ordered oligomers with a barrel-like structure. Here, we present the recombinant expression and purification of Hsp42. We demonstrate, that Hsp42 is expressed in inclusion bodies and can be resolubilized and folded to correct, active oligomers. This indicates that in contrast to thermal unfolding, the chemical disassembly and unfolding of Hsp42 is fully reversible. In comparison to the purification of mature Hsp42 from yeast, its recombinant expression leads to a substantial increase in the yield of the protein and to a reduction of contamination caused by aggregation prone proteins complexed by Hsp42. In addition, the recombinant Hsp42 is fully active as a chaperone in an energy independent manner.

Stabilization of free and immobilized enzymes using hyperthermophilic chaperonin

Chaperonins suppress the denaturation of proteins and promote protein folding in vivo. Because hyperthermophilic chaperonins are expected to be used as a stabilizer for proteins, the effects of a group II chaperonin from a hyperthermophilic archaeum, Thermococcus strain KS-1 (T. KS-1 cpn), on the stabilization of mesophilic and thermophilic free enzymes and an enzyme co-immobilized with T. KS-1 cpn were studied. T. KS-1 cpn prevented the thermal inactivation of yeast alcohol dehydrogenase (ADH), jack bean urease, and Thermus flavus malate dehydrogenase (MDH) at high temperatures. T. KS-1 cpn also improved the long-term stability of ADH at lower temperatures. Moreover, the residual ADH activity of ADH co-entrapped with T. KS-1 cpn was improved and maintained at a higher level than that of the entrapped ADH without chaperonin. T. KS-1 cpn is useful for the stabilization of free and immobilized enzymes and applicable to various fields of biotechnology.

On the chaperonin activity of GroEL at heat-shock temperature

The studies of GroEL, almost exclusively, have been concerned with the function of the chaperonin under non-stress conditions, and little is known about the role of GroEL during heat shock. Being a heat shock protein, GroEL deserves to be studied under heat shock temperature. As a model for heat shock in vitro, we have investigated the interaction of GroEL with the enzyme rhodanese undergoing thermal unfolding at 43 degrees C. GroEL interacted strongly with the unfolding enzyme forming a binary complex. Active rhodanese (82%) could be recovered by releasing the enzyme from GroEL after the addition of several components, e.g. ATP and the co-chaperonin GroES. After evaluating the stability of the GroEL-rhodanese complex, as a function of the percentage of active rhodanese that could be released from GroEL with time, we found that the complex had a half-life of only one and half-hours at 43 degrees C; while, it remained stable at 25 degrees C for more than 2 weeks. Interestingly, the GroEL-rhodanese complex remained intact and only 13% of its ATPase activity was lost during its incubation at 43 degrees C. Further, rhodanese underwent a conformational change over time while it was bound to GroEL at 43 degrees C. Overall, our results indicated that the inability to recover active enzyme at 43 degrees C from the GroEL-rhodanese complex was not due to the disruption of the complex or aggregation of rhodanese, but rather to the partial loss of its ATPase activity and/or to the inability of rhodanese to be released from GroEL due to a conformational change.

A Domain in the N-terminal Part of Hsp26 is Essential for Chaperone Function and Oligomerization

Small heat-shock proteins (Hsps) are ubiquitous molecular chaperones which prevent the unspecific aggregation of non-native proteins. For Hsp26, a cytosolic sHsp from of Saccharomyces cerevisiae, it has been shown that, at elevated temperatures, the 24 subunit complex dissociates into dimers. This dissociation is required for the efficient interaction with non-native proteins. Deletion analysis of the protein showed that the N-terminal half of Hsp26 (amino acid residues 1-95) is required for the assembly of the oligomer. Limited proteolysis in combination with mass spectrometry suggested that this region can be divided in two parts, an N-terminal segment including amino acid residues 1-30 and a second part ranging from residues 31-95. To analyze the structure and function of the N-terminal part of Hsp26 we created a deletion mutant lacking amino acid residues 1-30. We show that the oligomeric state and the structure, as determined by size exclusion chromatography and electron microscopy, corresponds to that of the Hsp26 wild-type protein. Furthermore, this truncated version of Hsp26 is active as a chaperone. However, in contrast to full length Hsp26, the truncated version dissociates at lower temperatures and complexes with non-native proteins are less stable than those found with wild-type Hsp26. Our results suggest that the N-terminal segment of Hsp26 is involved in both, oligomerization and chaperone function and that the second part of the N-terminal region (amino acid residues 31-95) is essential for both functions.

MotE serves as a new chaperone specific for the periplasmic motility protein, MotC, in Sinorhizobium meliloti

The flagella of Sinorhizobium meliloti rotate solely clockwise and vary their rotary speed to provoke changes in the swimming path. This mode of motility control has its molecular corollary in two novel motility proteins, MotC and MotD, present in addition to the ubiquitous MotA/MotB energizing proton channel. MotC binds to the periplasmic portion of MotB, whereas MotD interacts with FliM at the cytoplasmic face of the rotor. We report here the assignment and analysis of a fifth motility protein, MotE. Deletion of motE resulted in aggregation and decay of the periplasmic MotC protein and, as a consequence, in paralysis of the cell. The 179-residue MotE protein bears an N-terminal signal peptide and is rapidly secreted to the periplasm, where it forms stable dimers that are linked by a disulphide bridge between the cysteine 53 residues. Both, the monomeric and the dimeric MotE bind to MotC, and dimerization is essential for MotE stability in the periplasm. We conclude that MotE is a periplasmic chaperone specific for MotC being responsible for its proper folding and stability. We also propose that the MotE dimer serves as a shuttle to target MotC to its binding site at MotB.

Hydrogen peroxide induces the dissociation of GroEL into monomers that can facilitate the reactivation of oxidatively inactivated rhodanese

Although, several studies have been reported on the effects of oxidants on the structure and function of other molecular chaperones, no reports have been made so far for the chaperonin GroEL. The ability of GroEL to function under oxidative stress was investigated in this report by monitoring the effects of hydrogen peroxide (H(2)O(2)) on the structure and refolding activity of this protein. Using fluorescence spectroscopy and light scattering, we observed that GroEL showed increases in exposed hydrophobic sites and changes in tertiary and quaternary structure. Differential sedimentation, gel electrophoresis, and circular dichroism showed that H(2)O(2) treated GroEL underwent irreversible dissociation into monomers with partial loss of secondary structure. Relative to other proteins, GroEL was found to be highly resistant to oxidative damage. Interestingly, GroEL monomers produced under these conditions can facilitate the reactivation of H(2)O(2)-inactivated rhodanese but not urea-denatured rhodanese. Recovery of approximately 84% active rhodanese was obtained with either native or oxidized GroEL in the absence of GroES or ATP. In comparison, urea-denatured GroEL, BSA and the refolding mixture in the absence of proteins resulted in the recovery of 72, 50, and 49% rhodanese activity, respectively. Previous studies have shown that GroEL monomers can reactivate rhodanese. Here, we show that oxidized monomeric GroEL can reactivate oxidized rhodanese suggesting that GroEL retains the ability to protect proteins during oxidative stress.

Analysis of the regulation of the molecular chaperone Hsp26 by temperature-induced dissociation: the N-terminal domail is important for oligomer assembly and the binding of unfolding proteins

Small heat shock proteins (sHsps) are molecular chaperones that efficiently bind non-native proteins. All members of this family investigated so far are oligomeric complexes. For Hsp26, an sHsp from the cytosol of Saccharomyces cerevisiae, it has been shown that at elevated temperatures the 24-subunit complex dissociates into dimers. This dissociation seems to be required for the efficient interaction with unfolding proteins that results in the formation of large, regular complexes comprising Hsp26 and the non-native proteins. To gain insight into the molecular mechanism of this chaperone, we analyzed the dynamics and stability of the two oligomeric forms of Hsp 26 (i.e. the 24-mer and the dimer) in comparison to a construct lacking the N-terminal domain (Hsp26DeltaN). Furthermore, we determined the stabilities of complexes between Hsp26 and non-native proteins. We show that the temperature-induced dissociation of Hsp26 into dimers is a completely reversible process that involves only a small change in energy. The unfolding of the dissociated Hsp26 dimer or Hsp26DeltaN, which is a dimer, requires a much higher energy. Because Hsp26DeltaN was inactive as a chaperone, these results imply that the N-terminal domain is of critical importance for both the association of Hsp26 with non-native proteins and the formation of large oligomeric complexes. Interestingly, complexes of Hsp26 with non-native proteins are significantly stabilized against dissociation compared with Hsp26 complexes. Taken together, our findings suggest that the quaternary structure of Hsp26 is determined by two elements, (i) weak, regulatory interactions required to form the shell of 24 subunits and (ii) a strong and stable dimerization of the C-terminal domain.

Analysis of the interaction of small heat shock proteins with unfolding proteins

The ubiquitous small heat shock proteins (sHsps) are efficient molecular chaperones that interact with nonnative proteins, prevent their aggregation, and support subsequent refolding. No obvious substrate specificity has been detected so far. A striking feature of sHsps is that they form large complexes with nonnative proteins. Here, we used several well established model chaperone substrates, including citrate synthase, alpha-glucosidase, rhodanese, and insulin, and analyzed their interaction with murine Hsp25 and yeast Hsp26 upon thermal unfolding. The two sHsps differ in their modes of activation. In contrast to Hsp25, Hsp26 undergoes a temperature-dependent dissociation that is required for efficient substrate binding. Our analysis shows that Hsp25 and Hsp26 reacted in a similar manner with the nonnative proteins. For all substrates investigated, complexes of defined size and shape were formed. Interestingly, several different nonnative proteins could be incorporated into defined sHsp-substrate complexes. The first substrate protein bound seems to determine the complex morphology. Thus, despite the differences in quaternary structure and mode of activation, the formation of large uniform sHsp-substrate complexes seems to be a general feature of sHsps, and this unique chaperone mechanism is conserved from yeast to mammals.

Overexpression of DsbC and DsbG markedly improves soluble and functional expression of single-chain Fv antibodies in Escherichia coli

Single-chain Fv antibodies (scFv), a group of reconstructed molecules with several disulfide bonds, are prone to aggregate as inclusion bodies, the insoluble species of natural proteins, when expressed in Escherichia coli, especially at high level. Recovery of functionally active products from inclusion bodies is onerous and ineffective. We have increased the soluble and functional scFv yields by fusing either DsbC or DsbG, two E. coli disulfide isomerases with general chaperone function, to scFvs. Compared to the totally insoluble inclusion bodies of scFvs expressed separately, more than half of each fusion protein DsbC-scFv or DsbG-scFv was soluble, according to SDS-PAGE analysis. The more effective solubility was obtained when the fused protein DsbG-scFv was co-expressed simultaneously with DsbC under the same promoter. Under this condition, the soluble portion of DsbG-scFv increased from about 50% to 90% measured by scanning SDS-PAGE gel. Co-expression of DsbC can change fusion protein CBD-scFv from totally insoluble when expressed in E. coli separately to a considerable portion of soluble CBD-scFv. Antigen-binding activity assay showed that scFvs retained full affinity to specific antigens. We also determined that general molecular chaperones GroEL and GroES had no effects on the solubility of scFvs when co-expressed with scFv in E. coli. We propose that the correct formation of disulfide bonds in scFvs is the crucial factor responsible for solubility of scFvs.

GroEL interacts transiently with oxidatively inactivated rhodanese facilitating its reactivation

When the enzyme rhodanese was inactivated with hydrogen peroxide (H(2)O(2)), it underwent significant conformational changes, leading to an increased exposure of hydrophobic surfaces. Thus, this protein seemed to be an ideal substrate for GroEL, since GroEL uses hydrophobic interactions to bind to its substrate polypeptides. Here, we report on the facilitated reactivation (86%) of H(2)O(2)-inactivated rhodanese by GroEL alone. Reactivation by GroEL required a reductant and the enzyme substrate, but not GroES or ATP. Further, we found that GroEL interacted weakly and/or transiently with H(2)O(2)-inactivated rhodanese. A strong interaction with rhodanese was obtained when the enzyme was pre-incubated with urea, indicating that exposure of hydrophobic surfaces alone on oxidized rhodanese was not sufficient for the formation of a strong complex and that a more unfolded structure of rhodanese was required to interact strongly with GroEL. Unlike prior studies that involved denaturation of rhodanese through chemical or thermal means, we have clearly shown that GroEL can function as a molecular chaperone in the reactivation of an oxidatively inactivated protein. Additionally, the mechanism for the GroEL-facilitated reactivation of rhodanese shown here appears to be different than that for the chaperonin-assisted folding of chemically unfolded polypeptides in which a nucleotide and sometimes GroES is required.

Molecular chaperones in the kidney

The normal milieu of the kidney includes hypoxia, large osmotic fluxes, and an enormous amount of fluid/solute reabsorption. Renal adaptation to these conditions requires a host of molecular chaperones that stabilize protein conformation, target nascent proteins to their final intracellular destination, and prevent protein aggregation. Under physiologic or pharmacologic stress, inducible molecular chaperones provide additional mechanisms for repairing or degrading non-native proteins and for inhibiting stress-induced apoptosis. In contrast to intracellular chaperones, chaperones present on the cell surface regulate the immune system and have cytokine-like effects. A diverse range of chaperones and chaperone functions provide the renal cell with an armamentarium of responses to improve the chances of survival.

Calnexin discriminates between protein conformational states and functions as a molecular chaperone in vitro

Although calnexin is thought to function as a molecular chaperone for glycoproteins, a prevalent view is that it cannot distinguish between protein conformational states, binding solely through its lectin site to monoglucosylated oligosaccharides. Using purified components in vitro, calnexin effectively prevented the aggregation not only of glycoproteins bearing monoglucosylated oligosaccharides but also proteins lacking N-glycans, an effect enhanced by ATP. It also suppressed the thermal denaturation of nonglycosylated proteins and enhanced their refolding in conjunction with other cellular components. Calnexin formed stable complexes with unfolded conformers of these proteins but not with the native molecules. Therefore, in addition to being a lectin, calnexin functions as a bona fide molecular chaperone capable of interacting with polypeptide segments of folding glycoproteins.

Analysis of GroE-assisted folding under nonpermissive conditions

The molecular chaperones GroEL and GroES facilitate protein folding in an ATP-dependent manner under conditions where no spontaneous folding occurs. It has remained unknown whether GroE achieves this by a passive sequestration of protein inside the GroE cavity or by changing the folding pathway of a protein. Here we used citrate synthase, a well studied model substrate, to discriminate between these possibilities. We demonstrate that GroE maintains unfolding intermediates in a state that allows productive folding under nonpermissive conditions. During encapsulation of non-native protein inside GroEL.GroES complexes, a folding reaction takes place, generating association-competent monomeric intermediates that are no longer recognized by GroEL. Thus, GroE shifts folding intermediates to a productive folding pathway under heat shock conditions where even the native protein unfolds in the absence of GroE.

Alkali metal ions protect mitochondrial rhodanese against thermal inactivation

Incubation of bovine liver mitochondrial rhodanese in dilute, reducing solutions at temperatures ranging between 30 and 45 degreesC conduced to a rapid loss of enzymatic activity. This inactivation was substantially reduced in the presence of millimolar concentrations of alkali metal ions, divalent cations (including Mg2+, Ca2+, and Ba2+) were ineffective. The extent of protection afforded by monovalent cations was highly dependent on their ionic radii, with K+ and Na+ ions being the most effective protective agents. The protection afforded by a number of anions, including thiosulfate, could be totally ascribed to the presence of the accompanying monovalent cation. The overall results indicate that K+ and Na+, at concentrations and temperatures within the physiological range, substantially contribute to the stabilization of the functional structure of rhodanese.

GroEL traps dimeric and monomeric unfolding intermediates of citrate synthase

The prokaryotic molecular chaperone GroE is increasingly expressed under heat shock conditions. GroE protects cells by preventing the irreversible aggregation of thermally unfolding proteins. Here, the interaction of GroE with thermally unfolding citrate synthase (CS) was dissected into several steps that occur before irreversible aggregation, and the conformational states of the unfolding protein recognized by GroEL were determined. The kinetic analysis of CS unfolding revealed the formation of inactive dimeric and monomeric intermediates. GroEL binds both intermediates without affecting the unfolding pathway. Furthermore, the dimeric intermediates are not protected against dissociation in the presence of GroEL. Monomeric CS is stably associated with GroEL, thus preventing further irreversible unfolding steps and subsequent aggregation. During refolding, monomeric CS is encapsulated inside the cavity of GroEL. GroES complexes. Taken together our results suggest that for protection of cells against heat stress both the ability of GroEL to interact with a large variety of nonnative conformations of proteins and the active, GroES-dependent refolding of highly unfolded species are important.

Partitioning of rhodanese onto GroEL. Chaperonin binds a reversibly oxidized form derived from the native protein

The mammalian mitochondrial enzyme, rhodanese, can form stable complexes with the Escherichia coli chaperonin GroEL if it is either refolded from 8 M urea in the presence of chaperonin or is simply added to the chaperonin as the folded conformer at 37 degreesC. In the presence of GroEL, the kinetic profile of the inactivation of native rhodanese followed a single exponential decay. Initially, the inactivation rates showed a dependence on the chaperonin concentration but reached a constant maximum value as the GroEL concentration increased. Over the same time period, in the absence of GroEL, native rhodanese showed only a small decline in activity. The addition of a non-denaturing concentration of urea accelerated the inactivation and partitioning of rhodanese onto GroEL. These results suggest that the GroEL chaperonin may facilitate protein unfolding indirectly by interacting with intermediates that exist in equilibrium with native rhodanese. The activity of GroEL-bound rhodanese can be completely recovered upon addition of GroES and ATP. The reactivation kinetics and commitment rates for GroEL-rhodanese complexes prepared from either unfolded or native rhodanese were identical. However, when rhodanese was allowed to inactivate spontaneously in the absence of GroEL, no recovery of activity was observed upon addition of GroEL, GroES, and ATP. Interestingly, the partitioning of rhodanese and its subsequent inactivation did not occur when native rhodanese and GroEL were incubated under anaerobic conditions. Thus, our results strongly suggest that the inactive intermediate that partitions onto GroEL is the reversibly oxidized form of rhodanese.

Isolation of a periplasmic molecular chaperone-like protein of Rhodobacter sphaeroides f. sp. denitrificans that is homologous to the dipeptide transport protein DppA of Escherichia coli

A periplasmic protein has been found to prevent aggregation of the acid-unfolded dimethyl sulfoxide reductase (DMSOR), the periplasmic terminal reductase of dimethyl sulfoxide respiration in the phototroph Rhodobacter sphaeroides f. sp. denitrificans, in a manner similar to that of the Escherichia coli chaperonin GroEL (Matsuzaki et al., Plant Cell Physiol. 37:333-339, 1996). The protein was isolated from the periplasm of the phototroph. It had a molecular mass of 58 kDa and had no subunits. The sequence of 14 amino-terminal residues of the protein was completely identical to that of the periplasmic dipeptide transport protein (DppA) of E. coli. The 58-kDa protein prevented aggregation to a degree comparable to that of GroEL on the basis of monomer protein. The 58-kDa protein also decreased aggregation of guanidine hydrochloride-denatured rhodanese, a mitochondrial matrix protein, during its refolding upon dilution. The 58-kDa protein is a kind of molecular chaperone and could be involved in maintaining unfolded DMSOR, after secretion of the latter into the periplasm, in a competent form for its correct folding.

Structural and mechanistic consequences of polypeptide binding by GroEL

The remarkable ability of the chaperonin GroEL to recognise a diverse range of non-native states of proteins constitutes one of the most fascinating molecular recognition events in protein chemistry. Recent structural studies have revealed a possible model for substrate binding by GroEL and a high-resolution image of the GroEL-GroES folding machinery has provided important new insights into our understanding of the mechanism of action of this chaperonin. Studies with a variety of model substrates reveal that the binding of substrate proteins to GroEL is not just a passive event, but can result in significant changes in the structure and stability of the bound polypeptide. The potential impact of this on the mechanism of chaperonin-assisted folding is not fully understood, but provides exciting scope for further experiment.

GroE modulates kinetic partitioning of folding intermediates between alternative states to maximize the yield of biologically active protein

The central issue of chaperone function is the mechanism whereby partitioning of folding polypeptides along the productive pathway may be maximized, while non-productive folding pathways are minimized. We have found that the GroE chaperone is capable of accelerating the rate of the productive pathway of bacterial luciferase alphabeta heterodimer formation. At intermediate temperatures at which the productive pathway and non-productive pathways leading to dimerization-incompetent monomeric forms of the subunits coexist, GroE enhances the yield of native enzyme while minimizing the yield of misfolded protein. These results suggest that GroE releases the subunits in forms capable of achieving the native structure faster than the forms initially bound by the chaperone. At higher temperatures, at which the native enzyme is stable but the dimerization reaction is diminished, GroE is unable to force the productive folding reaction to occur. However, the chaperone decreases the rate of formation of the heterodimerization-incompetent species, thereby enhancing the final yield of active enzyme when the temperature is reduced to the permissive range. Our results suggest a mechanism by which the chaperone functions to maximize the yield of the biologically active form of the protein while maintaining or even accelerating the essential rapid kinetics of folding reactions.

GroEL-mediated protein folding

I. Architecture of GroEL and GroES and the reaction pathway A. Architecture of the chaperonins B. Reaction pathway of GroEL-GroES-mediated folding II. Polypeptide binding A. A parallel network of chaperones binding polypeptides in vivo B. Polypeptide binding in vitro 1. Role of hydrophobicity in recognition 2. Homologous proteins with differing recognition-differences in primary structure versus effects on folding pathway 3. Conformations recognized by GroEL a. Refolding studies b. Binding of metastable intermediates c. Conformations while stably bound at GroEL 4. Binding constants and rates of association 5. Conformational changes in the substrate protein associated with binding by GroEL a. Observations b. Kinetic versus thermodynamic action of GroEL in mediating unfolding c. Crossing the energy landscape in the presence of GroEL III. ATP binding and hydrolysis-driving the reaction cycle IV. GroEL-GroES-polypeptide ternary complexes-the folding-active cis complex A. Cis and trans ternary complexes B. Symmetric complexes C. The folding-active intermediate of a chaperonin reaction-cis ternary complex D. The role of the cis space in the folding reaction E. Folding governed by a "timer" mechanism F. Release of nonnative polypeptides during the GroEL-GroES reaction G. Release of both native and nonnative forms under physiologic conditions H. A role for ATP binding, as well as hydrolysis, in the folding cycle V. Concluding remarks.

Prevention of in vitro protein thermal aggregation by the Sulfolobus solfataricus chaperonin. Evidence for nonequivalent binding surfaces on the chaperonin molecule

We have studied the effects of the Sulfolobus solfataricus chaperonin on the aggregation and inactivation upon heating of four model enzymes: chicken egg white lysozyme (one 14.4-kDa chain), yeast alpha-glucosidase (one 68.5-kDa chain), chicken liver malic enzyme (four 65-kDa subunits), and yeast alcohol dehydrogenase (four 37.5-kDa subunits). When the proteins were heated in the presence of an equimolar amount of chaperonin, 1) the aggregation was prevented in all solutions; 2) the inactivation profiles of the single-chain enzymes were comparable with those detected in the absence of the chaperonin, and enzyme activities were regained in the solutions heated in the presence of the chaperonin upon ATP hydrolysis (78 and 55% activity regains for lysozyme and alpha-glucosidase, respectively); 3) the inactivation of the tetrameric enzymes was completely prevented, whereas the activities decreased in the absence of the chaperonin. We demonstrate by gel filtration chromatography that the chaperonin interacted with the structures occurring during thermal denaturation of the model proteins and that the interaction with the single-chain proteins (but not that with the tetrameric proteins) was reversed upon ATP hydrolysis. The chaperonin had nonequivalent surfaces for the binding of the model proteins upon heating: the thermal denaturation intermediates of the single-chain proteins share Surfaces I, while the thermal denaturation intermediates of the tetrameric proteins share Surfaces II. ATP binding to the chaperonin induced a conformation that lacked Surfaces I and carried Surfaces II. These data support the concept that chaperonins protect native proteins against thermal aggregation by two mechanistically distinct strategies (an ATP-dependent strategy and an ATP-independent strategy), and provide the first evidence that a chaperonin molecule bears functionally specialized surfaces for the binding of the protein substrates.

Chaperone-like activity of protein disulfide-isomerase in the refolding of rhodanese

Protein disulfide-isomerase (PDI) in near stoichiometric concentrations promotes reactivation and prevents aggregation of guanidine-hydrochloride-denatured rhodanese during refolding upon dilution. PDI also suppresses aggregation of rhodanese during thermal inactivation. The above-mentioned properties displayed by PDI completely satisfy the definition of chaperone and provide additional evidence to confirm the hypothesis proposed previously [Wang, C. C. & Tsou, C. L. (1993) FASEB J. 7, 1515-1517] that PDI is both an enzyme and a chaperone. Since rhodanese contains no disulfide bonds, the chaperone-like activity of PDI acting on rhodanese is independent of its disulfide-isomerase activity.

Transient interaction of Hsp90 with early unfolding intermediates of citrate synthase. Implications for heat shock in vivo

At normal temperatures, Hsp90 is one of the most abundant proteins in the cytosol of various eucaryotic cells. Upon heat shock, the level of Hsp90 is increased even more, suggesting that it is important for helping cells to survive under these conditions. However, studies so far have been almost exclusively concerned with the function of Hsp90 under non-stress conditions, and therefore only little is known about the role of Hsp90 during heat shock. As a model for heat shock in vitro, we have monitored the inactivation and subsequent aggregation of dimeric citrate synthase (CS) at elevated temperatures. Hsp90 effectively "stabilized" CS under conditions where the enzyme is normally inactivated and finally aggregates very rapidly. A kinetic dissection of the unfolding pathway of CS succeeded in revealing two intermediates which form and subsequently undergo irreversible aggregation reactions. Hsp90 apparently interacts transiently with these highly structured early unfolding intermediates. Binding and subsequent release of the intermediates favorably influences the kinetic partitioning between two competing processes, the further unfolding of CS and the productive refolding to the native state. As a consequence, CS is effectively stabilized in the presence of Hsp90. The significance of this interaction is especially evident in the suppression of aggregation, the major end result of thermal unfolding events in vivo and in vitro. These effects, which are ATP-independent, seem to be a general function of members of the Hsp90 family, since yeast and bovine Hsp90 as well as the Hsp90 homologue from Escherichia coli gave similar results. It seems likely that this function also reflects the role of Hsp90 under heat shock conditions in vivo. We therefore propose that members of the Hsp90 family convey thermotolerance by transiently binding to highly structured early unfolding intermediates, thereby preventing their irreversible aggregation and stabilizing the active species.

Chaperonin GroE and ADP facilitate the folding of various proteins and protect against heat inactivation

In the presence of ADP, the molecular chaperones GroEL and GroES from Escherichia coli not only facilitated the refolding of various proteins, but also prevented their irreversible heat inactivation in vitro. Without nucleotides the refolding reactions were arrested by GroEL. Addition of GroES and ADP to the reaction mixture initiated the refolding reactions and the enzyme activities were regained efficiently. The presence of GroE (GroEL and GroES) and ADP also protected against heat inactivation of native enzymes at various temperatures. These findings suggest that in the presence of GroES, nucleotide binding is an important event in the mechanism of GroEL-facilitated protein folding.

Heat-shock proteins as molecular chaperones

Functional proteins within cells are normally present in their native, completely folded form. However, vital processes of protein biogenesis such as protein synthesis and translocation of proteins into intracellular compartments require the protein to exist temporarily in an unfolded or partially folded conformation. As a consequence, regions buried when a polypeptide is in its native conformation become exposed and interact with other proteins causing protein aggregation which is deleterious to the cell. To prevent aggregation as proteins become unfolded, heat-shock proteins protect these interactive surfaces by binding to them and facilitating the folding of unfolded or nascent polypeptides. In other instances the binding of heat-shock proteins to interactive surfaces of completely folded proteins is a crucial part of their regulation. As heat shock and other stress conditions cause cellular proteins to become partially unfolded, the ability of heat-shock proteins to protect cells against the adverse effects of stress becomes a logical extension of their normal function as molecular chaperones.

Correlation of high-temperature stability of alpha-chymotrypsin with 'salting-in' properties of solution

A correlation between the stability of alpha-chymotrypsin against irreversible thermal inactivation at high temperatures (long-term stability) and the coefficient of Setchenov equation as a measure of salting-in/out efficiency of solutes in the Hofmeister series has been found. An increase in the concentration of salting-in solutes (KSCN, urea, guanidinium chloride, formamide) leads to a many-fold decrease of the inactivation rate of the enzyme. In contrast, addition of salting-out solutes has a small effect on the long-term stability of alpha-chymotrypsin at high temperatures. The effects of solutes are additive with respect to their salting-in/out capacities; the stabilizing action of the solutes is determined by the calculated Setchenov coefficient of solution. The correlation is explained by a solute-driven shift of the conformational equilibrium between the 'low-temperature' native and the 'high-temperature' denatured forms of the enzyme within the range of the kinetic scheme put forward in the preceding paper in this journal: irreversible inactivation of the high-temperature form proceeds much more slowly compared with the low-temperature form.

Genetically engineering mammalian cell lines for increased viability and productivity

The generation of new host cell lines for the production of foreign proteins can be achieved by cell engineering. This approach can be used to enhance the cell's ability to produce proteins that are properly processed and secreted at elevated levels and consequently can increase the overall productivity of an expression system. One potential target for cell engineering is the modification of the cell's protein folding capacity. The appropriate folding, assembly, localization and secretion of newly synthesized proteins is dependent upon the action of a group of proteins known as molecular chaperones. Improving the host cell's chaperoning capacity might increase the yield of properly folded recombinant proteins by preventing the formation of insoluble aggregates. Another potentially beneficial cell engineering goal is the inhibition of physiological cell death. The productivity of genetically engineered cells is dependent upon the maintenance of high levels of cell viability throughout the bioprocess period. Fluctuations in a cell's environment can trigger a deliberate form of cell death known as apoptosis. The proteins that mediate this self-destruction are currently being characterized. Regulating the expression of these death genes by cellular engineering could limit the loss of productivity that results from the physiological death of the recombinant cell line.

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.

GroE-mediated folding of bacterial luciferases in vivo

In this study we present evidence indicating that GroE chaperonins mediate de novo protein folding of heterodimeric and monomeric luciferases under heat shock or sub-heat shock conditions in vivo. The effects of additional groESL and groEL genes on the bioluminescence of Escherichia coli cells expressing different bacterial luciferase genes at various temperatures were directly studied in cells growing in liquid culture. Data indicate that at 42 degrees C GroESL chaperonins are required for the folding of the beta subunit polypeptide of the heterodimeric alpha beta luciferase from the mesophilic bacterium Vibrio harveyi MAV (B392). In contrast, the small number of amino acid substitutions present in the luciferase beta subunit polypeptide from the thermotolerant V. harveyi CTP5 suppresses this requirement for GroE chaperonins, and greatly reduces interaction between the beta subunit polypeptide and GroEL chaperonin. In addition, GroESL are required for the de novo folding at 37 degrees C of a MAV alpha beta luciferase fusion polypeptide that is functional as a monomer. No such requirement for luciferase activity is observed at that temperature with a fusion of the CTP5 alpha and beta subunit polypeptides, although GroE chaperonins can still mediate folding of the CTP5 fusion luciferase. Bacterial luciferases provide a unique system for direct observation of the effects of GroE chaperonins on protein folding and enzyme assembly in living cells. Furthermore, they offer a sensitive and simple assay system for the identification of polypeptide domains required for GroEL protein binding.