The chaperonin GroEL binds to late-folding non-native conformations present in native Escherichia coli and murine dihydrofolate reductases

Quantifying protein unfolding kinetics with a high-throughput microfluidic platform

Even after folding, proteins transiently sample unfolded or partially unfolded intermediates, and these species are often at risk of irreversible alteration (e.g. via proteolysis, aggregation, or post-translational modification). Kinetic stability, in addition to thermodynamic stability, can directly impact protein lifetime, abundance, and the formation of alternative, sometimes disruptive states. However, we have very few measurements of protein unfolding rates or how mutations alter these rates, largely due to technical challenges associated with their measurement. To address this need, we developed SPARKfold (Simultaneous Proteolysis Assay Revealing Kinetics of Folding), a microfluidic platform to express, purify, and measure unfolding rate constants for >1000 protein variants in parallel via on-chip native proteolysis. To demonstrate the power and potential of SPARKfold, we determined unfolding rate constants for 1,104 protein samples in parallel. We built a library of 31 dihydrofolate reductase (DHFR) orthologs with up to 78 chamber replicates per variant to provide the statistical power required to evaluate the system's ability to resolve subtle effects. SPARKfold rate constants for 5 constructs agreed with those obtained using traditional techniques across a 150-fold range, validating the accuracy of the technique. Comparisons of mutant kinetic effects via SPARKfold with previously published measurements impacts on folding thermodynamics provided information about the folding transition state and pathways via φ analysis. Overall, SPARKfold enables rapid characterization of protein variants to dissect the nature of the unfolding transition state. In future work, SPARKfold can reveal mutations that drive misfolding and aggregation and enable rational design of kinetically hyperstable variants for industrial use in harsh environments.

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.

Probing the kinetic stabilities of Friedreich's ataxia clinical variants using a solid phase GroEL chaperonin capture platform

Numerous human diseases are caused by protein folding defects where the protein may become more susceptible to degradation or aggregation. Aberrant protein folding can affect the kinetic stability of the proteins even if these proteins appear to be soluble in vivo. Experimental discrimination between functional properly folded and misfolded nonfunctional conformers is not always straightforward at near physiological conditions. The differences in the kinetic behavior of two initially folded frataxin clinical variants were examined using a high affinity chaperonin kinetic trap approach at 25 °C. The kinetically stable wild type frataxin (FXN) shows no visible partitioning onto the chaperonin. In contrast, the clinical variants FXN-p.Asp122Tyr and FXN-p.Ile154Phe kinetically populate partial folded forms that tightly bind the GroEL chaperonin platform. The initially soluble FXN-p.Ile154Phe variant partitions onto GroEL more rapidly and is more kinetically liable. These differences in kinetic stability were confirmed using differential scanning fluorimetry. The kinetic and aggregation stability differences of these variants may lead to the distinct functional impairments described in Friedreich's ataxia, the neurodegenerative disease associated to frataxin functional deficiency. This chaperonin platform approach may be useful for identifying small molecule stabilizers since stabilizing ligands to frataxin variants should lead to a concomitant decrease in chaperonin binding.

Protein quality control acts on folding intermediates to shape the effects of mutations on organismal fitness

What are the molecular properties of proteins that fall on the radar of protein quality control (PQC)? Here we mutate the E. coli's gene encoding dihydrofolate reductase (DHFR) and replace it with bacterial orthologous genes to determine how components of PQC modulate fitness effects of these genetic changes. We find that chaperonins GroEL/ES and protease Lon compete for binding to molten globule intermediate of DHFR, resulting in a peculiar symmetry in their action: overexpression of GroEL/ES and deletion of Lon both restore growth of deleterious DHFR mutants and most of the slow-growing orthologous DHFR strains. Kinetic steady-state modeling predicts and experimentation verifies that mutations affect fitness by shifting the flux balance in cellular milieu between protein production, folding, and degradation orchestrated by PQC through the interaction with folding intermediates.

A second-site suppressor of a folding defect functions via interactions with a chaperone network to improve folding and assembly in vivo

Single amino acid substitutions in a protein can cause misfolding and aggregation to occur. Protein misfolding can be rescued by second-site amino acid substitutions called suppressor substitutions (su), commonly through stabilizing the native state of the protein or by increasing the rate of folding. Here we report evidence that su substitutions that rescue bacteriophage P22 temperature-sensitive-folding (tsf) coat protein variants function in a novel way. The ability of tsf:su coat proteins to fold and assemble under a variety of cellular conditions was determined by monitoring levels of phage production. The tsf:su coat proteins were found to more effectively utilize P22 scaffolding protein, an assembly chaperone, as compared with their tsf parents. Phage-infected cells were radioactively labelled to quantify the associations between coat protein variants and folding and assembly chaperones. Phage carrying the tsf:su coat proteins induced more GroEL and GroES, and increased formation of protein:chaperone complexes as compared with their tsf parents. We propose that the su substitutions result in coat proteins that are more assembly competent in vivo because of a chaperone-driven kinetic partitioning between aggregation-prone intermediates and the final assembled state. Through more proficient use of this chaperone network, the su substitutions exhibit a novel means of suppression of a folding defect.

Factors governing the substrate recognition by GroEL chaperone: a sequence correlation approach

The chaperonin GroEL binds to a large number of polypeptides, prevents their self-association, and mediates appropriate folding in a GroES and adenosine triphosphate-dependent manner. But how the GroEL molecule actually recognizes the polypeptide and what are the exact GroEL recognition sites in the substrates are still poorly understood. We have examined more than 50 in vivo substrates as well as well-characterized in vitro substrates, for their binding characteristics with GroEL. While addressing the issue, we have been driven by the basic concept that GroES, being the cochaperonin of GroEL, is the best-suited substrate for GroEL, as well as by the fact that polypeptide substrate and GroES occupy the same binding sites on the GroEL apical domain. GroES interacts with GroEL through selective hydrophobic residues present on its mobile loop region, and we have considered the group of residues on the GroES mobile loop as the key element in choosing a substrate for GroEL. Considering the hydrophobic region on the GroES mobile loop as the standard, we have attempted to identify the homologous region on the peptide sequences in the proteins of our interest. Polypeptides have been judged as potential GroEL substrates on the basis of the presence of the GroES mobile loop-like hydrophobic segments in their amino acid sequences. We have observed 1 or more GroES mobile loop-like hydrophobic patches in the peptide sequence of some of the proteins of our interest, and the hydropathy index of most of these patches also seems to be approximately close to that of the standard. It has been proposed that the presence of hydrophobic patches having substantial degree of hydropathy index as compared with the standard segment is a necessary condition for a peptide sequence to be recognized by GroEL molecules. We also observed that the overall hydrophobicity is also close to 30% in these substrates, although this is not the sufficient criterion for a polypeptide to be assigned as a substrate for GroEL. We found that the binding of aconitase, alpha-lactalbumin, and murine dihydrofolate reductase to GroEL falls in line with our present model and have also predicted the exact regions of their binding to GroEL. On the basis of our GroEL substrate prediction, we have presented a model for the binding of apo form of some proteins to GroEL and the eventual formation of the holo form. Our observation also reveals that in most of the cases, the GroES mobile loop-like hydrophobic patch is present in the unstructured region of the protein molecule, specifically in the loop or beta-sheeted region. The outcome of our study would be an essential feature in identifying a potential substrate for GroEL on the basis of the presence of 1 or more GroES mobile loop-like hydrophobic segments in the amino acid sequence of those polypeptides and their location in three-dimensional space.

Expansion and compression of a protein folding intermediate by GroEL

The GroEL-GroES chaperonin system is required for the assisted folding of many essential proteins. The precise nature of this assistance remains unclear, however. Here we show that denatured RuBisCO from Rhodospirillum rubrum populates a stable, nonaggregating, and kinetically trapped monomeric state at low temperature. Productive folding of this nonnative intermediate is fully dependent on GroEL, GroES, and ATP. Reactivation of the trapped RuBisCO monomer proceeds through a series of GroEL-induced structural rearrangements, as judged by resonance energy transfer measurements between the amino- and carboxy-terminal domains of RuBisCO. A general mechanism used by GroEL to push large, recalcitrant proteins like RuBisCO toward their native states thus appears to involve two steps: partial unfolding or rearrangement of a nonnative protein upon capture by a GroEL ring, followed by spatial constriction within the GroEL-GroES cavity that favors or enforces compact, folding-competent intermediate states.

Active rhodanese lacking nonessential sulfhydryl groups contains an unstable C-terminal domain and can be bound, inactivated, and reactivated by GroEL

Mutation of all nonessential cysteine residues in rhodanese turns the enzyme into a form (C3S) that is fully active but less stable than wild type (WT). This less stable mutant allowed testing of two hypotheses; (a) the two domains of rhodanese are differentially stable, and (b) the chaperonin GroEL can bind better to less stable proteins. Reduced temperatures during expression and purification were required to limit inclusion bodies and obtain usable quantities of soluble C3S. C3S and WT have the same secondary structures by circular dichroism. C3S, in the absence of the substrate thiosulfate, is cleaved by trypsin to give a stable 21-kDa species. With thiosulfate, C3S is resistant to proteolysis. In contrast, wild type rhodanese is not proteolyzed significantly under any of the experimental conditions used here. Mass spectrometric analysis of bands from SDS gels of digested C3S indicated that the C-terminal domain of C3S was preferentially digested. Active C3S can exist in a state(s) recognized by GroEL, and it displays additional accessibility of tryptophans to acrylamide quenching. Unlike WT, the sulfur-loaded mutant form (C3S-ES) shows slow inactivation in the presence of GroEL. Both WT and C3S lacking transferred sulfur (WT-E and C3S-E) become inactivated. Inactivation is not due to irreversible covalent modification, since GroEL can reactivate both C3S-E and WT-E in the presence of GroES and ATP. C3S-E can be reactivated to 100%, the highest reactivation observed for any form of rhodanese. These results suggest that inactivation of C3S-E or WT-E is due to formation of an altered, labile conformation accessible from the native state. This conformation cannot as easily be achieved in the presence of the substrate, thiosulfate.

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.

A comparison of the GroE chaperonin requirements for sequentially and structurally homologous malate dehydrogenases: the importance of folding kinetics and solution environment

Escherichia coli malate dehydrogenase (EcMDH) and its eukaryotic counterpart, porcine mitochondrial malate dehydrogenase (PmMDH), are highly homologous proteins with significant sequence identity (60%) and virtually identical native structural folds. Despite this homology, EcMDH folds rapidly and efficiently in vitro and does not seem to interact with GroE chaperonins at physiological temperatures (37 degrees C), whereas PmMDH folds much slower than EcMDH and requires these chaperonins to fold to the native state at 37 degrees C. Double jump experiments indicate that the slow folding behavior of PmMDH is not limited by proline isomerization. Although the folding enhancer glycerol (<5 m) does not alter the renaturation kinetics of EcMDH, it dramatically accelerates the spontaneous renaturation of PmMDH at all temperatures tested. Kinetic analysis of PmMDH renaturation with increasing glycerol concentrations suggests that this osmolyte increases the on-pathway kinetics of the monomer folding to assembly-competent forms. Other osmolytes such as trimethylamine N-oxide, sucrose, and betaine also reactivate PmMDH at nonpermissive temperatures (37 degrees C). Glycerol jump experiments with preformed GroEL.PmMDH complexes indicate that the shift between stringent (requires ATP and GroES) and relaxed (only requires ATP) complex conformations is rapid (<3-5 s). The similarity in irreversible misfolding kinetics of PmMDH measured with glycerol or the activated chaperonin complex (GroEL.GroES.ATP) suggests that these folding aids may influence the same step in the PmMDH folding reaction. Moreover, the interactions between glycerol-induced PmMDH folding intermediates and GroEL.GroES.ATP are diminished. Our results support the notion that the protein folding kinetics of sequentially and structurally homologous proteins, rather than the structural fold, dictates the GroE chaperonin requirement.

GroEL/GroES-mediated folding of a protein too large to be encapsulated

The chaperonin GroEL binds nonnative proteins too large to fit inside the productive GroEL-GroES cis cavity, but whether and how it assists their folding has remained unanswered. We have examined yeast mitochondrial aconitase, an 82 kDa monomeric Fe(4)S(4) cluster-containing enzyme, observed to aggregate in chaperonin-deficient mitochondria. We observed that aconitase folding both in vivo and in vitro requires both GroEL and GroES, and proceeds via multiple rounds of binding and release. Unlike the folding of smaller substrates, however, this mechanism does not involve cis encapsulation but, rather, requires GroES binding to the trans ring to release nonnative substrate, which likely folds in solution. Following the phase of ATP/GroES-dependent refolding, GroEL stably bound apoaconitase, releasing active holoenzyme upon Fe(4)S(4) cofactor formation, independent of ATP and GroES.

Chaperonin-mediated protein folding

Molecular chaperones are required to assist folding of a subset of proteins in Escherichia coli. We describe a conceptual framework for understanding how the GroEL-GroES system assists misfolded proteins to reach their native states. The architecture of GroEL consists of double toroids stacked back-to-back. However, most of the fundamentals of the GroEL action can be described in terms of the single ring. A key idea in our framework is that, with coordinated ATP hydrolysis and GroES binding, GroEL participates actively by repeatedly unfolding the substrate protein (SP), provided that it is trapped in one of the misfolded states. We conjecture that the unfolding of SP becomes possible because a stretching force is transmitted to the SP when the GroEL particle undergoes allosteric transitions. Force-induced unfolding of the SP puts it on a higher free-energy point in the multidimensional energy landscape from which the SP can either reach the native conformation with some probability or be trapped in one of the competing basins of attraction (i.e., the SP undergoes kinetic partitioning). The model shows, in a natural way, that the time scales in the dynamics of the allosteric transitions are intimately coupled to folding rates of the SP. Several scenarios for chaperonin-assisted folding emerge depending on the interplay of the time scales governing the cycle. Further refinement of this framework may be necessary because single molecule experiments indicate that there is a great dispersion in the time scales governing the dynamics of the chaperonin cycle.

The mitochondrial Hsp70-dependent import system actively unfolds preproteins and shortens the lag phase of translocation

Unfolding is an essential process during translocation of preproteins into mitochondria; however, controversy exists as to whether mitochondria play an active role in unfolding. We have established an in vitro system with a kinetic saturation of the mitochondrial import machinery, yielding translocation rates comparable to in vivo import rates. Preproteins with short N-terminal segments in front of a folded domain show a characteristic delay of the onset of translocation (lag phase) although the maximal import rate is similar to that of longer preproteins. The lag phase is shortened by extending the N-terminal segment to improve the accessibility to matrix heat shock protein 70 and abolished by unfolding of the preprotein. A mutant mtHsp70 defective in binding to the inner membrane prolongs the lag phase and reduces the translocation activity. A direct comparison of the rate of spontaneous unfolding in solution with that during translocation demonstrates that unfolding by mitochondria is significantly faster, proving an active unfolding process. We conclude that access of mtHsp70 to N-terminal preprotein segments is critical for active unfolding and initiation of translocation.

Mechanisms of protein folding

The strong correlation between protein folding rates and the contact order suggests that folding rates are largely determined by the topology of the native structure. However, for a given topology, there may be several possible low free energy paths to the native state and the path that is chosen (the lowest free energy path) may depend on differences in interaction energies and local free energies of ordering in different parts of the structure. For larger proteins whose folding is assisted by chaperones, such as the Escherichia coli chaperonin GroEL, advances have been made in understanding both the aspects of an unfolded protein that GroEL recognizes and the mode of binding to the chaperonin. The possibility that GroEL can remove non-native proteins from kinetic traps by unfolding them either during polypeptide binding to the chaperonin or during the subsequent ATP-dependent formation of folding-active complexes with the co-chaperonin GroES has also been explored.

GroEL binds a late folding intermediate of phage P22 coat protein

GroEL recognizes proteins that are folding improperly or that have aggregation-prone intermediates. Here we have used as substrates for GroEL, wildtype (WT) coat protein of phage P22 and 3 coat proteins that carry single amino acid substitutions leading to a temperature-sensitive folding (tsf) phenotype. In vivo, WT coat protein does not require GroEL for proper folding, whereas GroEL is necessary for the folding of the tsf coat proteins; thus, the single amino acid substitutions cause coat protein to become a substrate for GroEL. The conformation of WT and tsf coat proteins when in a binary complex with GroEL was investigated using tryptophan fluorescence, quenching of fluorescence, and accessibility of the coat proteins to proteolysis. WT coat protein and the tsf coat protein mutants were each found to be in a different conformation when bound to GroEL. As an additional measure of the changes in the bound conformation, the affinity of binding of WT and tsf coat proteins to GroEL was determined using a fluorescence binding assay. The tsf coat proteins were bound more tightly by GroEL than WT coat protein. Therefore, even though the proteins are identical except for a single amino acid substitution, GroEL did not bind these substrate polypeptides in the same conformation within its central cavity. Therefore, GroEL is likely to bind coat protein in a conformation consistent with a late folding intermediate, with substantial secondary and tertiary structure formed.

GroEL binds artificial proteins with random sequences

Chaperonin GroEL from Escherichia coli binds to the non-native states of many unrelated proteins, and GroEL-recognizable structural features have been argued. As model substrate proteins of GroEL, we used seven artificial proteins (138 approximately 141 residues), each of which has a unique but randomly chosen amino acid sequence and no propensity to fold into a certain structure. Two of them were water-soluble, and the rest were soluble in 3 m urea. The soluble ones interacted with GroEL in a manner similar to that of a natural substrate; they stimulated the ATPase cycle of GroEL and GroEL/GroES and inhibited GroEL-assisted folding of other protein. All seven artificial proteins were able to bind to GroEL. The results suggest that the secondary structure as well as the specific sequence motif of the substrate proteins are not necessary to be recognized by GroEL.

Cyclophilin-promoted folding of mouse dihydrofolate reductase does not include the slow conversion of the late-folding intermediate to the active enzyme

Cyclophilins accelerate slow protein folding reactions in vitro by catalyzing the cis/trans isomerization of peptidyl-prolyl bonds. Cyclophilins were reported to be involved in a variety of cellular functions, including the promotion of protein folding by use of the substrate mouse dihydrofolate reductase (DHFR). The interaction of cyclophilin with DHFR has only been studied under limited conditions so far, not taking into account that native DHFR exists in equilibrium with a non-native late-folding intermediate. Here we report a systematic analysis of catalysis of DHFR folding by cyclophilins. The specific ligand methotrexate traps DHFR in its native state, permitting a specific analysis of the action of cyclophilin on both denatured DHFR with non-native prolyl bonds and denatured DHFR with all-native prolyl bonds. Cyclophilins from yeast and Neurospora crassa as well as the related prolyl isomerase b from Escherichia coli promote the folding of different forms of DHFR to the enzymatically active form, demonstrating the generality of cyclophilin-catalyzed folding of DHFR. The slow equilibrium between the late-folding intermediate and native DHFR suggests that prolyl isomerization may be required for this final phase of conversion to native DHFR. However, by reversible trapping of the intermediate, we analyze the slow interconversion between native and late-folding conformations in the backward and forward reactions and show a complete independence of cyclophilin. We conclude that cyclophilin catalyzes folding of DHFR, but surprisingly not in the last slow folding step.

Coupling between protein folding and allostery in the GroE chaperonin system

GroEL is an allosteric protein that facilitates protein folding in an ATP-dependent manner. Herein, the relationship between cooperative ATP binding by GroEL and the kinetics of GroE-assisted folding of two substrates with different GroES dependence, mouse dihydrofolate reductase (mDHFR) and mitochondrial malate dehydrogenase, is examined by using cooperativity mutants of GroEL. Strong intra-ring positive cooperativity in ATP binding by GroEL decreases the rate of GroEL-assisted mDHFR folding owing to a slow rate of the ATP-induced transition from the protein-acceptor state to the protein-release state. Inter-ring negative cooperativity in ATP binding by GroEL is found to affect the kinetic partitioning of mDHFR, but not of mitochondrial malate dehydrogenase, between folding in solution and folding in the cavity underneath GroES. Our results show that protein folding by this "two-stroke motor" is coupled to cooperative ATP binding.

Interaction with GroEL destabilises non-amphiphilic secondary structure in a peptide

The Escherichia coli molecular chaperone GroEL can functionally interact with non-native forms of many proteins. An inherent property of non-native proteins is the exposure of hydrophobic residues and the presence of secondary structure elements. Whether GroEL unfolds or stabilises these structural elements in protein substrates as a result of binding has been the subject of extended debate in the literature. Based on our studies of model peptides of pre-formed helical structure, we conclude that the final state of a GroEL-bound substrate is dependent on the conformational flexibility of the substrate protein and the distribution of hydrophobic residues, with optimal association when these are able to present a cluster of hydrophobic residues in the binding interface.

Cooperative effects of potassium, magnesium, and magnesium-ADP on the release of Escherichia coli dihydrofolate reductase from the chaperonin GroEL

Previous investigation has shown that at 22 degrees C and in the presence of the chaperonin GroEL, the slowest step in the refolding of Escherichia coli dihydrofolate reductase (EcDHFR) reflects release of a late folding intermediate from the cavity of GroEL (Clark AC, Frieden C, 1997, J Mol Biol 268:512-525). In this paper, we investigate the effects of potassium, magnesium, and MgADP on the release of the EcDHFR late folding intermediate from GroEL. The data demonstrate that GroEL consists of at least two conformational states, with apparent rate constants for EcDHFR release that differ by four- to fivefold. In the absence of potassium, magnesium, and ADP, approximately 80-90% of GroEL resides in the form with the faster rate of release. Magnesium and potassium both shift the distribution of GroEL forms toward the form with the slower release rate, though cooperativity for the magnesium-induced transition is observed only in the presence of potassium. MgADP at low concentrations (0-50 microM) shifts the distribution of GroEL forms toward the form with the faster release rate, and this effect is also potassium dependent. Nearly identical results were obtained with a GroEL mutant that forms only a single ring, demonstrating that these effects occur within a single toroid of GroEL. In the presence of saturating magnesium, potassium, and MgADP, the apparent rate constant for the release of EcDHFR from wild-type GroEL at 22 degrees C reaches a limiting value of 0.014 s(-1). For the single ring mutant of GroEL, the rate of EcDHFR release under the same conditions reaches a limiting value of 0.024 s(-1), suggesting that inter-ring negative cooperativity exists for MgADP-induced substrate release. The data suggest that MgADP preferentially binds to one conformation of GroEL, that with the faster apparent rate constant for EcDHFR release, and induces a conformational change leading to more rapid release of substrate protein.

Chaperonin function: folding by forced unfolding

The ability of the GroEL chaperonin to unfold a protein trapped in a misfolded condition was detected and studied by hydrogen exchange. The GroEL-induced unfolding of its substrate protein is only partial, requires the complete chaperonin system, and is accomplished within the 13 seconds required for a single system turnover. The binding of nucleoside triphosphate provides the energy for a single unfolding event; multiple turnovers require adenosine triphosphate hydrolysis. The substrate protein is released on each turnover even if it has not yet refolded to the native state. These results suggest that GroEL helps partly folded but blocked proteins to fold by causing them first to partially unfold. The structure of GroEL seems well suited to generate the nonspecific mechanical stretching force required for forceful protein unfolding.