Distinct actions of cis and trans ATP within the double ring of the chaperonin GroEL

GroEL mediates protein folding with a two successive timer mechanism

GroEL encapsulates nonnative substrate proteins in a central cavity capped by GroES, providing a safe folding cage. Conventional models assume that a single timer lasting approximately 8 s governs the ATP hydrolysis-driven GroEL chaperonin cycle. We examine single molecule imaging of GFP folding within the cavity, binding release dynamics of GroEL-GroES, ensemble measurements of GroEL/substrate FRET, and the initial kinetics of GroEL ATPase activity. We conclude that the cycle consists of two successive timers of approximately 3 s and approximately 5 s duration. During the first timer, GroEL is bound to ATP, substrate protein, and GroES. When the first timer ends, the substrate protein is released into the central cavity and folding begins. ATP hydrolysis and phosphate release immediately follow this transition. ADP, GroES, and substrate depart GroEL after the second timer is complete. This mechanism explains how GroES binding to a GroEL-substrate complex encapsulates the substrate rather than allowing it to escape into solution.

Kinetic analysis of ATP-dependent inter-ring communication in GroEL

Different concentrations of ATP were mixed rapidly with single-ring or double-ring forms of GroEL containing the Phe44-->Trp mutation and the time-resolved changes in fluorescence emission, upon excitation at 295 nm, were followed. Two kinetic phases that were previously found for double-ring GroEL are also observed in the case of the single-ring version: (i) a fast phase with a relatively large amplitude that is associated with the T-->R allosteric transition; (ii) and a slow phase with a smaller amplitude that is associated with ATP hydrolysis. In the case of weak intra-ring positive cooperativity, the rate constant corresponding to the T-->R allosteric switch of single-ring GroEL displays a bi-sigmoidal dependence on ATP concentration that may reflect parallel pathways of the allosteric transition. The slow phase is absent when double-ring or single-ring forms of GroEL are mixed with ADP or ATP without K(+), and it has a rate constant that is independent of ATP concentration. A third fast phase that is still unassigned is observed for both single-ring and double-ring GroEL when a large amount of data is collected. Finally, a fourth phase is observed in the case of double-ring GroEL that is found to be absent in the case of single-ring GroEL. This phase is here assigned to inter-ring communication by (i) determining its dependence on ATP concentration and (ii) based on its absence from single-ring GroEL and the Arg13-->Gly, Ala126-->Val GroEL mutant, which is defective in inter-ring negative cooperativity. The value of the rate constant corresponding to this phase is found to increase with increasing intra-ring positive cooperativity, with respect to ATP. This is the first report of the rate of ATP-mediated inter-ring communication in GroEL, in the presence of ATP alone, which is crucial for the cycling of this molecular machine between different functional states.

A kinetic analysis of the nucleotide-induced allosteric transitions in a single-ring mutant of GroEL

The function of GroE requires a complex system of allosteric communication driven by protein-nucleotide interactions. These rearrangements couple the binding and hydrolysis of ATP to an overall reaction cycle in which substrate proteins are bound, encapsulated and released. Positive cooperativity with respect to ATP binding occurs within one heptameric ring of GroEL, while negative cooperativity between the two rings generates an inherent asymmetry between the two rings. A previously engineered mutant of GroEL in which the ring-ring contacts are broken gives rise to a single-ring version of the wild-type chaperonin (SR1). We have studied the kinetics of the nucleotide-induced conformational changes in a single-tryptophan variant of SR1 (Y485W-SR1) and compared the resulting data with those we reported previously for the double-ring, single-tryptophan variant of wild-type GroEL (Y485W-GroEL). Remarkably, the parting of the rings does not have a major effect on the conformational changes occurring within the heptameric ring and a kinetic model is presented to describe the sequence of structural rearrangements that occur upon ATP binding to the SR1 molecule. The observation that both the ATP-induced and ADP-induced conformational rearrangements occur more rapidly in the SR1 than they do in wild-type GroEL, indicates that intra-ring conformational changes in the double-ring structure must overcome conformational constraints provided by the presence of the second ring. Lastly, the data presented here imply a role for inter-ring allostery in controlling the dissociation-association behaviour of the GroES co-protein in the overall reaction cycle.

Crystal structures of the group II chaperonin from Thermococcus strain KS-1: steric hindrance by the substituted amino acid, and inter-subunit rearrangement between two crystal forms

The crystal structures of the group II chaperonins consisting of the alpha subunit with amino acid substitutions of G65C and/or I125T from the hyperthermophilic archaeum Thermococcus strain KS-1 were determined. These mutants have been shown to be active in ATP hydrolysis but inactive in protein folding. The structures were shown to be double-ring hexadecamers in an extremely closed form, which was consistent with the crystal structure of native alpha8beta8-chaperonin from Thermoplasma acidophilum. Comparisons of the present structures with the atomic structures of the GroEL14-GroES7-(ADP)7 complex revealed that the deficiency in protein-folding activity with the G65C amino acid substitution is caused by the steric hindrance of the local conformational change in an equatorial domain. We concluded that this mutant chaperonin with G65C substitution is deprived of the smooth conformational change in the refolding-reaction cycle. We obtained a new form of crystal with a distinct space group at a lower concentration of sulfate ion in the presence of nucleotide. The crystal structure obtained at the lower concentration of sulfate ion tilts outward, and has much looser inter-subunit contacts compared with those in the presence of a higher concentration of sulfate ion. Such subunit rotation has never been characterized in group II chaperonins. The crystal structure obtained at the lower concentration of sulfate ion tilts outward, and has much looser inter-subunit contacts compared with those in the presence of a higher concentration of sulfate ion.

Domain motions in GroEL upon binding of an oligopeptide

GroEL assists protein folding by preventing the interaction of partially folded molecules with other non-native proteins. It binds them, sequesters them, and then releases them so that they can fold in an ATP-driven cycle. Previous studies have also shown that protein substrates, GroES, and oligopeptides bind to partially overlapped sites on the apical domain surfaces of GroEL. In this study, we have determined the crystal structure at 3.0A resolution of a symmetric (GroEL-peptide)(14) complex. The binding of each of these small 12 amino acid residue peptides to GroEL involves interactions between three adjacent apical domains of GroEL. Each peptide interacts primarily with a single GroEL subunit. Residues R231 and R268 from adjacent subunits isolate each substrate-binding pocket, and prevent bound substrates from sliding into adjacent binding pockets. As a consequence of peptide binding, domains rotate and inter-domain interactions are greatly enhanced. The direction of rotation of the apical domain of each GroEL subunit is opposite to that of its intermediate domain. Viewed from outside, the apical domains rotate clockwise within one GroEL ring, while the ATP-induced apical domain rotation is counter-clockwise.

Role of the gamma-phosphate of ATP in triggering protein folding by GroEL-GroES: function, structure and energetics

Productive cis folding by the chaperonin GroEL is triggered by the binding of ATP but not ADP, along with cochaperonin GroES, to the same ring as non-native polypeptide, ejecting polypeptide into an encapsulated hydrophilic chamber. We examined the specific contribution of the gamma-phosphate of ATP to this activation process using complexes of ADP and aluminium or beryllium fluoride. These ATP analogues supported productive cis folding of the substrate protein, rhodanese, even when added to already-formed, folding-inactive cis ADP ternary complexes, essentially introducing the gamma-phosphate of ATP in an independent step. Aluminium fluoride was observed to stabilize the association of GroES with GroEL, with a substantial release of free energy (-46 kcal/mol). To understand the basis of such activation and stabilization, a crystal structure of GroEL-GroES-ADP.AlF3 was determined at 2.8 A. A trigonal AlF3 metal complex was observed in the gamma-phosphate position of the nucleotide pocket of the cis ring. Surprisingly, when this structure was compared with that of the previously determined GroEL-GroES-ADP complex, no other differences were observed. We discuss the likely basis of the ability of gamma-phosphate binding to convert preformed GroEL-GroES-ADP-polypeptide complexes into the folding-active state.

Conversion of the allosteric transition of GroEL from concerted to sequential by the single mutation Asp-155 -> Ala

The reaction cycle of the double-ring chaperonin GroEL is driven by ATP binding that takes place with positive cooperativity within each seven-membered ring and negative cooperativity between rings. The positive cooperativity within rings is due to ATP binding-induced conformational changes that are fully concerted. Herein, it is shown that the mutation Asp-155 --> Ala leads to an ATP-induced break in intra-ring and inter-ring symmetry. Electron microscopy analysis of single-ring GroEL particles containing the Asp-155 --> Ala mutation shows that the break in intra-ring symmetry is due to stabilization of allosteric intermediates such as one in which three subunits have switched their conformation while the other four have not. Our results show that eliminating an intra-subunit interaction between Asp-155 and Arg-395 results in conversion of the allosteric switch of GroEL from concerted to sequential, thus demonstrating that its allosteric behavior arises from coupled tertiary conformational changes.

How protein thermodynamics and folding mechanisms are altered by the chaperonin cage: molecular simulations

How the Escherichia coli GroEL/ES chaperonin assists folding of a substrate protein remains to be uncovered. Recently, it was suggested that confinement into the chaperonin cage itself can significantly accelerate folding of a substrate. Performing comprehensive molecular simulations of eight proteins confined into various sizes L of chaperonin-like cage, we explore how and to what extent protein thermodynamics and folding mechanisms are altered by the cage. We show that a substrate protein is remarkably stabilized by confinement; the estimated increase in denaturation temperature DeltaTf is as large as approximately 60 degrees C. For a protein of size R0, the stabilization DeltaTf scales as (R0/L)nu, where nu approximately 3, which is consistent with a mean field theory of polymer. We also found significant free energy cost of confining a protein, which increases with R0/L, indicating that the confinement requires external work provided by the chaperonin system. In kinetic study, we show the folding is accelerated in a modestly well confined case, which is consistent with a recent experimental result on ribulose-1,5-bisphosphate carboxylase-oxygenase folding and simulation results of a beta hairpin. Interestingly, the acceleration of folding is likely to be larger for a protein with more complex topology, as quantified by the contact order. We also show how ensemble of folding pathways are altered by the chaperonin-like cage calculating a variant of value used in the study of spontaneous folding.

Isolation and characterisation of mutants of GroEL that are fully functional as single rings

A key aspect of the reaction mechanism for the molecular chaperone GroEL is the transmission of an allosteric signal between the two rings of the GroEL complex. Thus, the single-ring mutant SR1 is unable to act as a chaperone as it cannot release bound substrate or GroES. We used a simple selection procedure to identify mutants of SR1 that restored chaperone activity in vivo. A large number of single amino acid changes, mapping at diverse positions throughout the protein, enabled SR1 to regain its ability to act as a chaperone while remaining as a single ring. In vivo assays were used to identify the proteins that had regained maximal activity. In some cases, no difference could be detected between strains expressing wild-type GroEL and those expressing the mutated proteins. Three of the most active proteins where the mutations were in distinct parts of the protein were purified to homogeneity and characterised in vitro. All were capable of acting efficiently as chaperones for two different GroES-dependent substrates. All three proteins bound nucleotide as effectively as did GroEL, but the binding of GroES in the presence of ATP or ADP was reduced significantly relative to the wild-type. These active single rings should provide a useful tool for studying the nature of the allosteric changes that occur in the GroEL reaction cycle.

Discrimination of ATP, ADP, and AMPPNP by chaperonin GroEL: hexokinase treatment revealed the exclusive role of ATP

The double ring chaperonin GroEL binds unfolded protein, ATP, and GroES to the same ring, generating the cis ternary complex in which folding occurs within the cavity capped by GroES (cis folding). The functional role of ATP, however, remains unclear since several reports have indicated that ADP and AMPPNP (5'-adenylyl-beta,gamma-imidodiphosphate) are also able to support the formation of the cis ternary complex and the cis folding. To minimize the effect of contaminated ATP and adenylate kinase, we have included hexokinase plus glucose in the reaction mixtures and obtained new results. In ADP and AMPPNP, GroES bound quickly to GroEL but bound very slowly to the GroEL loaded with unfolded rhodanese or malate dehydrogenase. ADP was unable to support the formation of cis ternary complex and cis folding. AMPPNP supported cis folding of malate dehydrogenase to some extent but not cis folding of rhodanese. In the absence of hexokinase, apparent cis folding of rhodanese and malate dehydrogenase was observed in ADP and AMPPNP. Thus, the exclusive role of ATP in generation of the cis ternary complex is now evident.

Folding with and without encapsulation by cis- and trans-only GroEL-GroES complexes

Although a cis mechanism of GroEL-mediated protein folding, occurring inside a hydrophilic chamber encapsulated by the co-chaperonin GroES, has been well documented, recently the GroEL-GroES-mediated folding of aconitase, a large protein (82 kDa) that could not be encapsulated, was described. This process required GroES binding to the ring opposite the polypeptide (trans) to drive release and productive folding. Here, we have evaluated this mechanism further using trans-only complexes in which GroES is closely tethered to one of the two GroEL rings, blocking polypeptide binding by that ring. In vitro, trans-only folded aconitase with kinetics identical to GroEL-GroES. Surprisingly, trans-only also folded smaller GroEL-GroES-dependent substrates, Rubisco and malate dehydrogenase, but at rates slower than the cis reaction. Remarkably, in vivo, a plasmid encoding a trans-only complex rescued a GroEL-deficient strain, but the colony size was approximately one-tenth that produced by wild-type GroEL-GroES. We conclude that a trans mechanism, involving rounds of binding to an open ring and direct release into the bulk solution, can be generally productive although, where size permits, cis encapsulation supports more efficient folding.

Closing the folding chamber of the eukaryotic chaperonin requires the transition state of ATP hydrolysis

Chaperonins use ATPase cycling to promote conformational changes leading to protein folding. The prokaryotic chaperonin GroEL requires a cofactor, GroES, which serves as a "lid" enclosing substrates in the central cavity and confers an asymmetry on GroEL required for cooperative transitions driving the reaction. The eukaryotic chaperonin TRiC/CCT does not have such a cofactor but appears to have a "built-in" lid. Whether this seemingly symmetric chaperonin also operates through an asymmetric cycle is unclear. We show that unlike GroEL, TRiC does not close its lid upon nucleotide binding, but instead responds to the trigonal-bipyramidal transition state of ATP hydrolysis. Further, nucleotide analogs inducing this transition state confer an asymmetric conformation on TRiC. Similar to GroEL, lid closure in TRiC confines the substrates in the cavity and is essential for folding. Understanding the distinct mechanisms governing eukaryotic and bacterial chaperonin function may reveal how TRiC has evolved to fold specific eukaryotic proteins.

Structural basis for GroEL-assisted protein folding from the crystal structure of (GroEL-KMgATP)14 at 2.0A resolution

Nucleotide regulates the affinity of the bacterial chaperonin GroEL for protein substrates. GroEL binds protein substrates with high affinity in the absence of ATP and with low affinity in its presence. We report the crystal structure of (GroEL-KMgATP)(14) refined to 2.0 A resolution in which the ATP triphosphate moiety is directly coordinated by both K(+) and Mg(2+). Upon the binding of KMgATP, we observe previously unnoticed domain rotations and a 102 degrees rotation of the apical domain surface helix I. Two major consequences are a large lateral displacement of, and a dramatic reduction of hydrophobicity in, the apical domain surface. These results provide a basis for the nucleotide-dependent regulation of protein substrate binding and suggest a mechanism for GroEL-assisted protein folding by forced unfolding.

Denaturation and reassembly of chaperonin GroEL studied by solution X-ray scattering

We measured the denaturation and reassembly of Escherichia coli chaperonin GroEL using small-angle solution X-ray scattering, which is a powerful technique for studying the overall structure and assembly of a protein in solution. The results of the urea-induced unfolding transition show that GroEL partially dissociates in the presence of more than 2 M urea, cooperatively unfolds at around 3 M urea, and is in a monomeric random coil-like unfolded structure at more than 3.2 M urea. Attempted refolding of the unfolded GroEL monomer by a simple dilution procedure is not successful, leading to formation of aggregates. However, the presence of ammonium sulfate and MgADP allows the fully unfolded GroEL to refold into a structure with the same hydrodynamic dimension, within experimental error, as that of the native GroEL. Moreover, the X-ray scattering profiles of the GroEL thus refolded and the native GroEL are coincident with each other, showing that the refolded GroEL has the same structure and the molecular mass as the native GroEL. These results demonstrate that the fully unfolded GroEL monomer can refold and reassemble into the native tetradecameric structure in the presence of ammonium sulfate and MgADP without ATP hydrolysis and preexisting chaperones. Therefore, GroEL can, in principle, fold and assemble into the native structure according to the intrinsic characteristic of its polypeptide chain, although preexisting GroEL would be important when the GroEL folding takes place under in vivo conditions, in order to avoid misfolding and aggregation.

Structure of glycerol dehydratase reactivase: a new type of molecular chaperone

The function of glycerol dehydratase (GDH) reactivase is to remove damaged coenzyme B(12) from GDH that has suffered mechanism-based inactivation. The structure of GDH reactivase from Klebsiella pneumoniae was determined at 2.4 A resolution by the single isomorphous replacement with anomalous signal (SIR/AS) method. Each tetramer contains two elongated 63 kDa alpha subunits and two globular 14 kDa beta subunits. The alpha subunit contains structural features resembling both GroEL and Hsp70 groups of chaperones, and it appears chaperone like in its interactions with ATP. The fold of the beta subunit resembles that of the beta subunit of glycerol dehydratase, except that it lacks some coenzyme B(12) binding elements. A hypothesis for the reactivation mechanism of reactivase is proposed based on these structural features.

GroEL-substrate-GroES ternary complexes are an important transient intermediate of the chaperonin cycle

GroEL C138W is a mutant form of Escherichia coli GroEL, which forms an arrested ternary complex composed of GroEL, the co-chaperonin GroES and the refolding protein molecule rhodanese at 25 degrees C. This state of arrest could be reversed with a simple increase in temperature. In this study, we found that GroEL C138W formed both stable trans- and cis-ternary complexes with a number of refolding proteins in addition to bovine rhodanese. These complexes could be reactivated by a temperature shift to obtain active refolded protein. The simultaneous binding of GroES and substrate to the cis ring suggested that an efficient transfer of substrate protein into the GroEL central cavity was assured by the binding of GroES prior to complete substrate release from the apical domain. Stopped-flow fluorescence spectroscopy of the mutant chaperonin revealed a temperature-dependent conformational change in GroEL C138W that acts as a trigger for complete protein release. The behavior of GroEL C138W was reflected closely in its in vivo characteristics, demonstrating the importance of this conformational change to the overall activity of GroEL.

Structure of eukaryotic prefoldin and of its complexes with unfolded actin and the cytosolic chaperonin CCT

The biogenesis of the cytoskeletal proteins actin and tubulin involves interaction of nascent chains of each of the two proteins with the oligomeric protein prefoldin (PFD) and their subsequent transfer to the cytosolic chaperonin CCT (chaperonin containing TCP-1). Here we show by electron microscopy that eukaryotic PFD, which has a similar structure to its archaeal counterpart, interacts with unfolded actin along the tips of its projecting arms. In its PFD-bound state, actin seems to acquire a conformation similar to that adopted when it is bound to CCT. Three-dimensional reconstruction of the CCT:PFD complex based on cryoelectron microscopy reveals that PFD binds to each of the CCT rings in a unique conformation through two specific CCT subunits that are placed in a 1,4 arrangement. This defines the phasing of the CCT rings and suggests a handoff mechanism for PFD.

The interaction of beta(2)-glycoprotein I domain V with chaperonin GroEL: the similarity with the domain V and membrane interaction

To clarify the mechanism of interaction between chaperonin GroEL and substrate proteins, we studied the conformational changes; of the fifth domain of human beta(2)-glycoprotein I upon binding to GroEL. The fifth domain has a large flexible loop, containing several hydrophobic residues surrounded by positively charged residues, which has been proposed to be responsible for the binding of beta(2)-glycoprotein I to negatively charged phospholipid membranes. The reduction by dithiothreitol of the three intramolecular disulfide bonds of the fifth domain was accelerated in the presence of stoichiometric amounts of GroEL, indicating that the fifth domain was destabilized upon interaction with GroEL. To clarify the GroEL-induced destabilization at the atomic level, we performed H/(2)H exchange of amide protons using heteronuclear NMR spectroscopy. The presence of GroEL promoted the H/(2)H exchange of most of the protected amide protons, suggesting that, although the flexible loop of the fifth domain is likely to be responsible for the initiation of binding to GroEL, the interaction with GroEL destabilizes the overall conformation of the fifth domain.

Identification of two calcineurin B-binding proteins: tubulin and heat shock protein 60

Calcineurin (CaN) is a Ca++/calmodulin-dependent protein phosphatase with two subunits: a catalytic subunit (CaNA) and a regulatory subunit (CaNB). With four Ca(++)-binding sites and a sequence homology to calmodulin, CaNB has been defined as the regulatory subunit for CaNA. However, we have shown that mitochondrial expression of CaNB far exceeds that of CaNA. To investigate the role of this excess CaNB, we have generated glutathione-S-transferase-CaNB (GST-CaNB) fusion protein and demonstrated that the fusion protein predominantly bound to alpha-tubulin, a 57 kDa protein in bovine brain extracts, and heat shock protein 60 (Hsp60) in bovine kidney extracts. Their Ca(++)-dependent interactions with CaNB were verified by immunoprecipitation. The binding of CaNB could be demonstrated with purified alpha/beta tubulins and Hsp60, but not GroEL, a bacterial Hsp60 analog. The interaction of CaNB and Hsp60 was not disrupted by the incubation with Hsp10, ATP and Mg++, suggesting that CaNB was not associated with Hsp60 as a misfolded substrate, and may serve as a regulatory protein. Thus, CaNB may play other regulatory roles in Ca(++)-dependent events in addition to its interaction with CaNA, and may be important for Ca(++)-dependent processes in mitochondria.

Salt bridges at the inter-ring interface regulate the thermostat of GroEL

The chaperonin GroEL consists of a double-ring structure made of identical subunits and displays unusual allosteric properties caused by the interaction between its constituent subunits. Cooperative binding of ATP to a protein ring allows binding of GroES to that ring, and at the same time negative inter-ring cooperativity discharges the ligands from the opposite ring, thus driving the protein-folding cycle. Biochemical and electron microscopy analysis of wild type GroEL, a single-ring mutant (SR1), and two mutants with one inter-ring salt bridge of the chaperonin disrupted (E461K and E434K) indicate that these ion pairs form part of the interactions that allow the inter-ring allosteric signal to be transmitted. The wild type-like activities of the ion pair mutants at 25 degrees C are in contrast with their lack of inter-ring communication and folding activity at physiological temperatures. These salt bridges stabilize the inter-ring interface and maintain the inter-ring spacing so that functional communication between protein heptamers takes place. The characterization of GroEL hybrids containing different amounts of wild type and mutant subunits also indicates that as the number of inter-ring salt bridges increases the functional properties of the hybrids recover. Taken together, these results strongly suggest that inter-ring salt bridges form a stabilizing ring-shaped, ionic zipper that ensures inter-ring communication at the contact sites and therefore a functional protein-folding cycle. Furthermore, they regulate the chaperonin thermostat, allowing GroEL to distinguish physiological (37 degrees C) from stress temperatures (42 degrees C).

The evolution of the heat-shock protein GroEL from Buchnera, the primary endosymbiont of aphids, is governed by positive selection

The heat-shock protein GroEL is a double-ring-structured chaperonin that assists the folding of many newly synthesized proteins in Escherichia coli and the refolding in vitro, with the cochaperonin GroES, of conformationally damaged proteins. This protein is constitutively overexpressed in the primary symbiotic bacteria of many insects, constituting approximately 10% of the total protein in Buchnera, the primary endosymbiont of aphids. In the present study, we perform a maximum likelihood (ML) analysis to unveil the selective constraints in GroEL. In addition, we apply a new statistical approach to determine the patterns of evolution in this highly interesting protein. The main conclusion derived from our analysis is that GroEL has suffered an accelerated rate of amino acid substitution upon the symbiotic integration of Buchnera into the aphids. It is most interesting that the ML analysis of codon substitutions in the different branches of the phylogenetic tree strongly supports the action of positive selection in the different lineages of BUCHNERA: Additionally, the new sliding window analysis of the complete groEL sequence reveals different regions of the molecule under the action of positive selection, mainly located in the apical domain, that are important for both peptide and GroES binding.

Protein folding in the post-genomic era

Protein folding is a topic of fundamental interest since it concerns the mechanisms by which the genetic message is translated into the three-dimensional and functional structure of proteins. In these post-genomic times, the knowledge of the fundamental principles are required in the exploitation of the information contained in the increasing number of sequenced genomes. Protein folding also has practical applications in the understanding of different pathologies and the development of novel therapeutics to prevent diseases associated with protein misfolding and aggregation. Significant advances have been made ranging from the Anfinsen postulate to the "new view" which describes the folding process in terms of an energy landscape. These new insights arise from both theoretical and experimental studies. The problem of folding in the cellular environment is briefly discussed. The modern view of misfolding and aggregation processes that are involved in several pathologies such as prion and Alzheimer diseases. Several approaches of structure prediction, which is a very active field of research, are described.

Sequential SNARE disassembly and GATE-16-GOS-28 complex assembly mediated by distinct NSF activities drives Golgi membrane fusion

Characterization of mammalian NSF (G274E) and Drosophila NSF (comatose) mutants revealed an evolutionarily conserved NSF activity distinct from ATPase-dependent SNARE disassembly that was essential for Golgi membrane fusion. Analysis of mammalian NSF function during cell-free assembly of Golgi cisternae from mitotic Golgi fragments revealed that NSF disassembles Golgi SNAREs during mitotic Golgi fragmentation. A subsequent ATPase-independent NSF activity restricted to the reassembly phase is essential for membrane fusion. NSF/alpha-SNAP catalyze the binding of GATE-16 to GOS-28, a Golgi v-SNARE, in a manner that requires ATP but not ATP hydrolysis. GATE-16 is essential for NSF-driven Golgi reassembly and precludes GOS-28 from binding to its cognate t-SNARE, syntaxin-5. We suggest that this occurs at the inception of Golgi reassembly to protect the v-SNARE and regulate SNARE function.

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.

New aspects on the mechanism of GroEL-assisted protein folding

The mechanism of assisted protein folding by the chaperonin GroEL alone or in complex with the co-chaperonin GroES and in the presence or absence of nucleotides has been subject to extensive investigations during the last years. In this paper we present data where we have inactivated GroEL by stepwise blocking the nucleotide binding sites using the non-hydrolyzable ATP analogue, (Cr(H2O)4)3+ATP. We correlated the amount of accessible nucleotide binding sites with the residual ATP hydrolysis activity of GroEL as well as the residual refolding activity for two different model substrates. Under the conditions used, folding of the substrate proteins and ATP hydrolysis were directly proportional to the residual, accessible nucleotide binding sites. In the presence of GroES, 50% of the nucleotide binding sites were protected from inactivation by CrATP and the resulting protein retains 50% of both ATPase and refolding activity. The results strongly suggest that under the conditions used in our experiments, the nucleotide binding sites are additive in character and that by blocking of a certain number of binding sites a proportional amount of ATP hydrolysis and refolding activities are inactivated. The experiments including GroES suggest that full catalytic activity of GroEL requires both rings of the chaperonin. Blocking of the nucleotide binding sites of one ring still allows function of the second ring.

Molecular chaperones--cellular machines for protein folding

Proteins are linear polymers synthesized by ribosomes from activated amino acids. The product of this biosynthetic process is a polypeptide chain, which has to adopt the unique three-dimensional structure required for its function in the cell. In 1972, Christian Anfinsen was awarded the Nobel Prize for Chemistry for showing that this folding process is autonomous in that it does not require any additional factors or input of energy. Based on in vitro experiments with purified proteins, it was suggested that the correct three-dimensional structure can form spontaneously in vivo once the newly synthesized protein leaves the ribosome. Furthermore, proteins were assumed to maintain their native conformation until they were degraded by specific enzymes. In the last decade this view of cellular protein folding has changed considerably. It has become clear that a complicated and sophisticated machinery of proteins exists which assists protein folding and allows the functional state of proteins to be maintained under conditions in which they would normally unfold and aggregate. These proteins are collectively called molecular chaperones, because, like their human counterparts, they prevent unwanted interactions between their immature clients. In this review, we discuss the principal features of this peculiar class of proteins, their structure-function relationships, and the underlying molecular mechanisms.

Folding of newly translated proteins in vivo: the role of molecular chaperones

Recent years have witnessed dramatic advances in our understanding of how newly translated proteins fold in the cell and the contribution of molecular chaperones to this process. Folding in the cell must be achieved in a highly crowded macromolecular environment, in which release of nonnative polypeptides into the cytosolic solution might lead to formation of potentially toxic aggregates. Here I review the cellular mechanisms that ensure efficient folding of newly translated proteins in vivo. De novo protein folding appears to occur in a protected environment created by a highly processive chaperone machinery that is directly coupled to translation. Genetic and biochemical analysis shows that several distinct chaperone systems, including Hsp70 and the cylindrical chaperonins, assist the folding of proteins upon translation in the cytosol of both prokaryotic and eukaryotic cells. The cellular chaperone machinery is specifically recruited to bind to ribosomes and protects nascent chains and folding intermediates from nonproductive interactions. In addition, initiation of folding during translation appears to be important for efficient folding of multidomain proteins.

ATP-bound states of GroEL captured by cryo-electron microscopy

The chaperonin GroEL drives its protein-folding cycle by cooperatively binding ATP to one of its two rings, priming that ring to become folding-active upon GroES binding, while simultaneously discharging the previous folding chamber from the opposite ring. The GroEL-ATP structure, determined by cryo-EM and atomic structure fitting, shows that the intermediate domains rotate downward, switching their intersubunit salt bridge contacts from substrate binding to ATP binding domains. These observations, together with the effects of ATP binding to a GroEL-GroES-ADP complex, suggest structural models for the ATP-induced reduction in affinity for polypeptide and for cooperativity. The model for cooperativity, based on switching of intersubunit salt bridge interactions around the GroEL ring, may provide general insight into cooperativity in other ring complexes and molecular machines.

GroEL channels the folding of thioredoxin along one kinetic route

Many proteins display complex folding kinetics, which represent multiple parallel folding pathways emanating from multiple unfolded forms and converging to the unique native form. The small protein thioredoxin from Escherichia coli is one such protein. The effect of the chaperonin GroEL on modulating the complex energy landscape that separates the unfolded ensemble from the native state of thioredoxin has been studied. It is shown that while the fluorescence change accompanying folding occurs in five kinetic phases in the absence of GroEL, only the two slowest kinetic phases are discernible in the presence of saturating concentrations of GroEL. This result is shown to be consistent with only one out of several available folding routes being operational in the presence of GroEL. It is shown that native protein, which forms via fast as well as slow routes in the absence of GroEL, forms only via a slow route in its presence. The effect of GroEL on the folding of thioredoxin is shown to be the consequence of it binding differentially to the many folding-competent forms. While some of these forms can continue folding when bound to GroEL, others cannot. All molecules are then drawn into the operational folding route by the law of mass action. This observation indicates a new role for GroEL, which is to bias the energy landscape of a folding polypeptide towards fewer available pathways. It is suggested that such channeling might be a mechanism to avoid possible aggregation-prone routes available to a refolding polypeptide in vivo.

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.

Dual function of protein confinement in chaperonin-assisted protein folding

The GroEL/GroES chaperonin system mediates the folding of a range of newly synthesized polypeptides in the bacterial cytosol. Using a rapid biotin-streptavidin-based inhibition of chaperonin function, we show that the cage formed by GroEL and its cofactor GroES can have a dual role in promoting folding. First, enclosure of nonnative protein in the GroEL:GroES complex is essential for folding to proceed unimpaired by aggregation. Second, folding inside the cage can be significantly faster than folding in free solution, independently of ATP-driven cycles of GroES binding and release. This suggests that confinement of unfolded protein in the narrow hydrophilic space of the chaperonin cage smoothes the energy landscape for the folding of some proteins, increasing the flux of folding intermediates toward the native state.

Chaperonins--keeping a lid on folding proteins

Two classes of chaperonins are known in all groups of organisms to participate in the folding of newly synthesized proteins. Whereas bacterial type I chaperonins use a reversibly binding cofactor to temporarily sequester folding substrate proteins within the cylindrical chaperonin cavity, type II chaperonins in archaea and the eukaryotic cytosol appear to have evolved a built-in lid for this purpose. Not entirely surprisingly, this has consequences for the folding modes of the two types of chaperonins.

Nucleotide-induced transition of GroEL from the high-affinity to the low-affinity state for a target protein: effects of ATP and ADP on the GroEL-affected refolding of alpha-lactalbumin

We studied the refolding kinetics of alpha-lactalbumin in the presence of wild-type GroEL and its ATPase-deficient mutant D398A at various concentrations of nucleotides (ATP and ADP). We evaluated the apparent binding constant between GroEL and the alpha-lactalbumin refolding intermediate quantitatively by numerical simulation analysis of the alpha-lactalbumin refolding curves in the presence and absence of GroEL. The binding constant showed a co-operative decrease with an increase in ATP concentration, whereas the binding constant decreased in a non-co-operative manner with respect to ADP concentration. For the D398A mutant, the ATP-induced decrease in affinity occurred much faster than the steady-state ATP hydrolysis by this mutant, suggesting that ATP binding to GroEL rather than ATP hydrolysis, was responsible for the co-operative decrease in the affinity for the target protein. We thus analyzed the nucleotide-concentration dependence of affinity of GroEL for the target protein using an allosteric Monod-Wyman-Changeux model in which GroEL underwent an ATP-induced co-operative conformational transition between the high-affinity and low-affinity states of the target protein. The transition midpoint of the ATP-induced transition of GroEL has been found to be around 30 microM, in good agreement with the midpoint evaluated in other structural studies of GroEL. The results show that the observed difference between ATP and ADP-induced transitions of GroEL are brought about by a small difference in an allosteric parameter (the ratio of the nucleotide affinities of GroEL in the high-affinity and the low-affinity states), i.e. 4.1 for ATP and 2.6 for ADP.

Single-molecule observation of protein-protein interactions in the chaperonin system

We have analyzed the dynamics of the chaperonin (GroEL)-cochaperonin (GroES) interaction at the single-molecule level. In the presence of ATP and non-native protein, binding of GroES to the immobilized GroEL occurred at a rate that is consistent with bulk kinetics measurements. However, the release of GroES from GroEL occurred after a lag period ( approximately 3 s) that was not recognized in earlier bulk-phase studies. This observation suggests a new kinetic intermediate in the GroEL-GroES reaction pathway.

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.

Review: allostery in chaperonins

Chaperonins mediate protein folding in an ATP-dependent manner. ATP binding and hydrolysis by chaperonins are subject to both homotropic and heterotropic allosteric regulation. In the case of GroEL and CCT, homotropic regulation by ATP is manifested in nested cooperativity, which involves positive intra-ring cooperativity and negative inter-ring cooperativity in ATP binding. Both types of cooperativity are modulated by various heterotropic allosteric effectors, which include nonfolded proteins, ADP, Mg2+, monovalent ions such as K+, and cochaperonins in the case of type I chaperonins such as GroEL. Here, the allosteric properties of chaperonins are reviewed and new results of ours are presented with regard to allosteric effects of ADP. The role of allostery in the reaction cycle and folding function of chaperonins is discussed.

Structures of unliganded and ATP-bound states of the Escherichia coli chaperonin GroEL by cryoelectron microscopy

We have developed an angular refinement procedure incorporating correction for the microscope contrast transfer function, to determine cryoelectron microscopy (cryo-EM) structures of the Escherichia coli chaperonin GroEL in its apo and ATP-bound forms. This image reconstruction procedure is verified to 13-A resolution by comparison of the cryo-EM structure of unliganded GroEL with the crystal structure. Binding, encapsulation, and release of nonnative proteins by GroEL and its cochaperone GroES are controlled by the binding and hydrolysis of ATP. Seven ATP molecules bind cooperatively to one heptameric ring of GroEL. This binding causes long-range conformational changes that determine the orientations of remote substrate-binding sites, and it also determines the conformation of subunits in the opposite ring of GroEL, in a negatively cooperative mechanism. The conformation of GroEL-ATP was determined at approximately 15-A resolution. In one ring of GroEL-ATP, the apical (substrate-binding) domains are extremely disordered, consistent with the high mobility needed for them to achieve the 60 degrees elevation and 90 degrees twist of the GroES-bound state. Unexpectedly, ATP binding also increases the separation between the two rings, although the interring contacts are present in the density map.

Review: nucleotide binding to the thermoplasma thermosome: implications for the functional cycle of group II chaperonins

Structural information on group II chaperonins became available during recent years from electron microscopy and X-ray crystallography. Three conformational states have been identified for both archaeal and eukaryotic group II chaperonins: an open state, a spherical closed conformation, and an intermediate asymmetric bullet-shaped form. However, the functional cycle of group II chaperonins appears less well understood, although major principles are conserved when compared to group I chaperonins: binding of the substrate polypeptide to the apical domains of the open state and MgATP-driven conformational changes that result in encapsulation of the substrate where folding can proceed presumably in the closed ring of the bullet-shaped form. Binding of the transition state analogue MgADP-AlF3-H2O in the crystal structure of the Thermoplasma acidophilum thermosome suggests that the closed geometry is the enzymatically active conformation that performs ATP hydrolysis. Domain movements observed by electron microscopy suggest a coupling of ATP hydrolysis and domain movement similar to that in the GroE system. The hydrophilic interior of the closed thermosome corresponds to the cis-ring of the asymmetric GroEL-GroES complex implicated in protein folding.

Review: a structural view of the GroE chaperone cycle

The GroE chaperone system consists of two ring-shaped oligomeric components whose association creates different functional states. The most remarkable property of the GroE system is the ability to fold proteins under conditions where spontaneous folding cannot occur. To achieve this, a fully functional system consisting of GroEL, the cochaperone GroES, and ATP is necessary. Driven by ATP binding and hydrolysis, this system cycles through different conformational stages, which allow binding, folding, and release of substrate proteins. Some aspects of the ATP-driven reaction cycle are still under debate. One of these open questions is the importance of so-called "football" complexes consisting of GroEL and two bound GroES rings. Here, we summarize the evidence for the functional relevance of these complexes and their involvement in the efficient folding of substrate proteins.

Application of fluorescence resonance energy transfer to the GroEL-GroES chaperonin reaction

Fluorescence resonance energy transfer (FRET) is a sensitive and flexible method for studying protein-protein interactions. Here it is applied to the GroEL-GroES chaperonin system to examine the ATP-driven dynamics that underlie protein folding by this chaperone. Relying on the known structures of GroEL and GroES, sites for attachment of fluorescent probes are designed into the sequence of both proteins. Because these sites are brought close in space when GroEL and GroES form a complex, excitation energy can pass from a donor to an acceptor chromophore by FRET. While in ideal circumstances FRET can be used to measure distances, significant population heterogeneity in the donor-to-acceptor distances in the GroEL-GroES complex makes distance determination difficult. This is due to incomplete labeling of these large, oligomeric proteins and to their rotational symmetry. It is shown, however, that FRET can still be used to follow protein-protein interaction dynamics even in a case such as this, where distance measurements are either not practical or not meaningful. In this way, the FRET signal is used as a simple proximity sensor to score the interaction between GroEL and GroES. Similarly, FRET can also be used to follow interactions between GroEL and a fluorescently labeled substrate polypeptide. Thus, while knowledge of molecular structure aids enormously in the design of FRET experiments, structural information is not necessarily required if the aim is to measure the thermodynamics or kinetics of a protein interaction event by following changes in the binding proximity of two components.

Amide hydrogen exchange shows that malate dehydrogenase is a folded monomer at pH 5

Although there is general agreement that native mitochondrial malate dehydrogenase (MDH) exists as a dimer at pH 7, its aggregation state at pH 5 is less certain. The present amide hydrogen exchange study was performed to determine whether MDH remains a dimer at pH 5. To detect pH-induced changes in solvent accessibility, MDH was exposed to D(2)O at pH 5 or 7, then fragmented with pepsin into peptides that were analyzed by mass spectrometry. Even after adjustments for the effect of pH on the intrinsic rate of hydrogen exchange, large increases in deuterium levels were found at pH 5 only in peptic fragments derived from the subunit binding surface of MDH. In parallel experiments, elevated deuterium levels were also found in the same regions of MDH monomer trapped inside a mutant form of the chaperonin GROEL: This selective increase in hydrogen exchange rates, which was attributed to increased solvent accessibility of these regions, provides new evidence that MDH is a monomer at pH 5.

Synchronized domain-opening motion of GroEL is essential for communication between the two rings

Escherichia coli chaperonin GroEL consists of two stacked rings of seven identical subunits each. Accompanying binding of ATP and GroES to one ring of GroEL, that ring undergoes a large en bloc domain movement, in which the apical domain twists upward and outward. A mutant GroEL(AEX) (C138S,C458S,C519S,D83C,K327C) in the oxidized form is locked in a closed conformation by an interdomain disulfide cross-link and cannot hydrolyze ATP (Murai, N., Makino, Y., and Yoshida, M. (1996) J. Biol. Chem. 271, 28229-28234). By reconstitution of GroEL complex from subunits of both wild-type GroEL and oxidized GroEL(AEX), hybrid GroEL complexes containing various numbers of oxidized GroEL(AEX) subunits were prepared. ATPase activity of the hybrid GroEL containing one or two oxidized GroEL(AEX) subunits per ring was about 70% higher than that of wild-type GroEL. Based on the detailed analysis of the ATPase activity, we concluded that inter-ring negative cooperativity was lost in the hybrid GroEL, indicating that synchronized opening of the subunits in one ring is necessary for the negative cooperativity. Indeed, hybrid GroEL complex reconstituted from subunits of wild-type and GroEL mutant (D398A), which is ATPase-deficient but can undergo domain opening motion, retained the negative cooperativity of ATPase. In contrast, the ability of GroEL to assist protein folding was impaired by the presence of a single oxidized GroEL(AEX) subunit in a ring. Taken together, cooperative conformational transitions in GroEL rings ensure the functional communication between the two rings of GroEL.

ClpA mediates directional translocation of substrate proteins into the ClpP protease

The intracellular degradation of many proteins is mediated in an ATP-dependent manner by large assemblies comprising a chaperone ring complex associated coaxially with a proteolytic cylinder, e.g., ClpAP, ClpXP, and HslUV in prokaryotes, and the 26S proteasome in eukaryotes. Recent studies of the chaperone ClpA indicate that it mediates ATP-dependent unfolding of substrate proteins and directs their ATP-dependent translocation into the ClpP protease. Because the axial passageway into the proteolytic chamber is narrow, it seems likely that unfolded substrate proteins are threaded from the chaperone into the protease, suggesting that translocation could be directional. We have investigated directionality in the ClpA/ClpP-mediated reaction by using two substrate proteins bearing the COOH-terminal ssrA recognition element, each labeled near the NH(2) or COOH terminus with fluorescent probes. Time-dependent changes in both fluorescence anisotropy and fluorescence resonance energy transfer between donor fluorophores in the ClpP cavity and the substrate probes as acceptors were measured to monitor translocation of the substrates from ClpA into ClpP. We observed for both substrates that energy transfer occurs 2--4 s sooner with the COOH-terminally labeled molecules than with the NH(2)-terminally labeled ones, indicating that translocation is indeed directional, with the COOH terminus of the substrate protein entering ClpP first.

Genetic analysis of bacteriophage-encoded cochaperonins

Early genetic studies identified the Escherichia coli groES and groEL genes because mutations in them blocked the growth of bacteriophages lambda and T4. Subsequent genetic and biochemical analyses have shown that GroES and GroEL constitute a chaperonin machine, absolutely essential for E. coli growth, because it is needed for the correct folding of many of its proteins. In spite of very little sequence identity to GroES, the bacteriophage T4-encoded Gp31 protein and the bacteriophage RB49-encoded CocO protein are bona fide GroEL cochaperonins, even capable of substituting for GroES in E. coli growth. A major functional distinction is that only Gp31 and CocO can assist GroEL in the correct folding of Gp23, the major bacteriophage capsid protein. Conserved structural features between CocO and Gp31, which are absent from GroES, highlight their potential importance in specific cochaperonin function.

The importance of a mobile loop in regulating chaperonin/ co-chaperonin interaction: humans versus Escherichia coli

Chaperonins are universally conserved proteins that nonspecifically facilitate the folding of a wide spectrum of proteins. While bacterial GroEL is functionally promiscuous with various co-chaperonin partners, its human homologue, Hsp60 functions specifically with its co-chaperonin partner, Hsp10, and not with other co-chaperonins, such as the bacterial GroES or bacteriophage T4-encoded Gp31. Co-chaperonin interaction with chaperonin is mediated by the co-chaperonin mobile loop that folds into a beta-hairpin conformation upon binding to the chaperonin. A delicate balance of flexibility and conformational preferences of the mobile loop determines co-chaperonin affinity for chaperonin. Here, we show that the ability of Hsp10, but not GroES, to interact specifically with Hsp60 lies within the mobile loop sequence. Using mutational analysis, we show that three substitutions in the GroES mobile loop are necessary and sufficient to acquire Hsp10-like specificity. Two of these substitutions are predicted to preorganize the beta-hairpin turn and one to increase the hydrophobicity of the GroEL-binding site. Together, they result in a GroES that binds chaperonins with higher affinity. It seems likely that the single ring mitochondrial Hsp60 exhibits intrinsically lower affinity for the co-chaperonin that can be compensated for by a higher affinity mobile loop.

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.