Characterization of a functional GroEL14(GroES7)2 chaperonin hetero-oligomer

Metal ions modulate the plastic nature of Mycobacterium tuberculosis chaperonin-10

Chaperonin-10s possess a highly flexible segment of approximately 10 residues that covers their dome-like structure and closes the central cavity of the chaperonin assembly. The dome loop is believed to contribute to the plasticity of their oligomeric structure. We have exploited the presence of a single tryptophan residue occurring in the dome loop of Mycobacterium tuberculosis chaperonin-10 (cpn-10), and through intrinsic fluorescence measurements show that in the absence of metal ions, the tryptophan is almost fully solvent exposed at neutral pH. The dome loop, however, assumes a closed conformation in the presence of metal ions, or at low pH. These changes are fully reversed in the presence of chelating agents such as EDTA, confirming the role of cations in modulating the metastable states of cpn-10.

Structure and mass analysis by scanning transmission electron microscopy

In the scanning transmission electron microscope (STEM) an electron beam of a few angstroms diameter is raster scanned over a thin sample and the scattered electrons are sequentially measured for each sample element irradiated. The mass, the elemental composition and the structure of a protein can be simultaneously assessed if all detector systems of the STEM are used. Aspects affecting the accuracy of the mass measurement technique and the demands placed on the instrument's dark-field detector system are outlined. In addition, the influences of some sample preparation techniques are noted and the mass-loss induced at ambient temperatures by the incidence of 80kV electrons on various biological samples is reported. Finally, the importance of the STEM for the structural analysis of proteins is documented by examples.

From minichaperone to GroEL 2: importance of avidity of the multisite ring structure

Structural studies on minichaperones and GroEL imply a continuous ring of binding sites around the neck of GroEL. To investigate the importance of this ring, we constructed an artificial heptameric assembly of minichaperones to mimic their arrangement in GroEL. The heptameric Gp31 co-chaperonin from bacteriophage T4, an analogue of GroES, was used as a scaffold to display the GroEL minichaperones. A fusion protein, MC(7), was generated by replacing a part of the highly mobile loop of Gp31 (residues 23-44) with the sequence of the minichaperone (residues 191-376 of GroEL). The purified recombinant protein assembled into a heptameric ring composed of seven 30.6 kDa subunits. Although single minichaperones (residues 193-335 to 191-376 of GroEL) have certain chaperone activities in vitro and in vivo, they cannot refold heat and dithiothreitol-denatured mitochondrial malate dehydrogenase (mtMDH), a reaction that normally requires GroEL, its co-chaperonin GroES and ATP. But, MC(7) refolded MDH in vitro. The expression of MC(7) complements in vivo two temperature-sensitive Escherichia coli alleles, groEL44 and groEL673, at 43 degrees C. Although MC(7) could not compensate for the complete absence of GroEL in vivo, it enhanced the colony-forming ability of cells containing limiting amounts of wild-type GroEL at 37 degrees C. MC(7 )also reduces aggregate formation and cell death in mammalian cell models of Huntington's disease. The assembly of seven minichaperone subunits on a heptameric ring significantly improves their activity, demonstrating the importance of avidity in GroEL function.

Staphylococcal fibronectin binding protein interacts with heat shock protein 60 and integrins: role in internalization by epithelial cells

We reported previously that internalization of Staphylococcus aureus by nonprofessional phagocytes involves an interaction between fibronectin (Fn) binding protein (FnBP) and the host cell, resulting in signal transduction, tyrosine kinase activity, and cytoskeletal rearrangement (K. Dziewanowska, J. M. Patti, C. F. Deobald, K. W. Bayles, W. R. Trumble, and G. A. Bohach, Infect. Immun. 67:4673-4678, 1999). The goal of the present study was to identify the host molecules responsible for uptake of the organism through an interaction with FnBP. First, Fn was required for internalization. Addition of small amounts of exogenous Fn stimulated the uptake of S. aureus by HEp-2 cells, which are deficient in Fn synthesis. Fn antibodies blocked internalization of the organism by MAC-T cell monolayers, a bovine epithelial cell line which expresses Fn. Second, a monoclonal antibody (MAb) specific for beta(1) integrins dramatically reduced S. aureus invasion, suggesting that the formation of a Fn bridge linking the host cell beta(1) integrin and FnBP precedes internalization. However, ligand blotting of cell membrane proteins with a functional fragment of FnBP consistently identified an additional approximately 55-kDa receptor on both human and bovine epithelial cells. This protein was purified and identified by N-terminal microsequencing as heat shock protein 60 (Hsp60). The interaction between FnBP and Hsp60 also occurred when the whole cells were used. Cell membrane localization of Hsp60 was confirmed by biotinylation with an agent nonpermeable to the cell membrane. Pretreatment of epithelial cells with a MAb specific for eukaryotic Hsp60 significantly reduced internalization of S. aureus. Combined, these results suggest that the FnBP binds directly to both Hsp60 and Fn and is linked to beta(1) integrins through a Fn bridge. The simultaneous involvement of Fn and two host cell ligands, beta(1) integrins and Hsp60, suggests that FnBP is a multifunctional adhesin that mediates internalization in a manner similar to that proposed for OpaA, the Neisseria gonorrhoeae FnBP homolog (J. P. M. van Putten, T. D. Duensing, and R. L. Cole, Mol. Microbiol. 29:369-379, 1998).

Catalysis, commitment and encapsulation during GroE-mediated folding

The Escherichia coli GroE chaperones assist protein folding under conditions where no spontaneous folding occurs. To achieve this, the cooperation of GroEL and GroES, the two protein components of the chaperone system, is an essential requirement. While in many cases GroE simply suppresses unspecific aggregation of non-native proteins by encapsulation, there are examples where folding is accelerated by GroE. Using maltose-binding protein (MBP) as a substrate for GroE, it had been possible to define basic requirements for catalysis of folding. Here, we have analyzed key steps in the interaction of GroE and the MBP mutant Y283D during catalyzed folding. In addition to high temperature, high ionic strength was shown to be a restrictive condition for MBP Y283D folding. In both cases, the complete GroE system (GroEL, GroES and ATP) compensates the deceleration of MBP Y283D folding. Combining kinetic folding experiments and electron microscopy of GroE particles, we demonstrate that at elevated temperatures, symmetrical GroE particles with GroES bound to both ends of the GroEL cylinder play an important role in the efficient catalysis of MBP Y283D refolding. In principle, MBP Y283D folding can be catalyzed during one encapsulation cycle. However, because the commitment to reach the native state is low after only one cycle of ATP hydrolysis, several interaction cycles are required for catalyzed folding.

On the role of symmetrical and asymmetrical chaperonin complexes in assisted protein folding

The cylindrical chaperonin GroEL of E. coli and its ring-shaped cofactor GroES cooperate in mediating the ATP-dependent folding of a wide range of polypeptides in vivo and in vitro. By binding to the ends of the GroEL cylinder, GroES displaces GroEL-bound polypeptide into an enclosed folding cage, thereby preventing protein aggregation during folding. The dynamic interaction of GroEL and GroES is regulated by the GroEL ATPase and involves the formation of asymmetrical GroEL:GroES1 and symmetrical GroEL: GroES2 complexes. The proposed role of the symmetrical complex as a catalytic intermediate of the chaperonin mechanism has been controversial. It has also been suggested that the formation of GroEL:GroES2 complexes allows the folding of two polypeptide molecules per GroEL reaction cycle, one in each ring of GroEL. By making use of a procedure to stabilize chaperonin complexes by rapid crosslinking for subsequent analysis by native PAGE, we have quantified the occurrence of GroEL:GroES1 and GroEL:GroES2 complexes in active refolding reactions under a variety of conditions using mitochondrial malate dehydrogenase (mMDH) as a substrate. Our results show that the symmetrical complexes are neither required for chaperonin function nor does their presence significantly increase the rate of mMDH refolding. In contrast, chaperonin-assisted folding is strictly dependent on the formation of asymmetrical GroEL:GroES1 complexes. These findings support the view that GroEL:GroES2 complexes have no essential role in the chaperonin mechanism.

"Half of the sites" binding of D-glyceraldehyde-3-phosphate dehydrogenase folding intermediate with GroEL

Two D-glyceraldehyde-3-phosphate dehydrogenase (GAPDH) folding intermediate subunits bind with chaperonin 60 (GroEL) to form a stable complex, which can no longer bind with additional GAPDH intermediate subunits, but does bind with one more lysozyme folding intermediate or one chaperonin 10 (GroES) molecule, suggesting that the two GAPDH subunits bind at one end of the GroEL molecule displaying a "half of the sites" binding profile. For lysozyme, GroEL binds with either one or two folding intermediates to form a stable 1:1 or 1:2 complex with one substrate on each end of the GroEL double ring for the latter. The 1:1 complex of GroEL.GroES binds with one lysozyme or one dimeric GAPDH folding intermediate to form a stable ternary complex. Both complexes of GroEL.lysozyme1 and GroEL.GAPDH2 bind with one GroES molecule only at the other end of the GroEL molecule forming a trans ternary complex. According to the stoichiometry of GroEL binding with the GAPDH folding intermediate and the formation of ternary complexes containing GroEL.GAPDH2, it is suggested that the folding intermediate of GAPDH binds, very likely in the dimeric form, with GroEL at one end only.

GroEL/GroES-dependent reconstitution of alpha2 beta2 tetramers of humanmitochondrial branched chain alpha-ketoacid decarboxylase. Obligatory interaction of chaperonins with an alpha beta dimeric intermediate

The decarboxylase component (E1) of the human mitochondrial branched chain alpha-ketoacid dehydrogenase multienzyme complex (approximately 4-5 x 10(3) kDa) is a thiamine pyrophosphate-dependent enzyme, comprising two 45.5-kDa alpha subunits and two 37.8-kDa beta subunits. In the present study, His6-tagged E1 alpha2 beta2 tetramers (171 kDa) denatured in 8 M urea were competently reconstituted in vitro at 23 degrees C with an absolute requirement for chaperonins GroEL/GroES and Mg-ATP. Unexpectedly, the kinetics for the recovery of E1 activity was very slow with a rate constant of 290 M-1 s-1. Renaturation of E1 with a similarly slow kinetics was also achieved using individual GroEL-alpha and GroEL-beta complexes as combined substrates. However, the beta subunit was markedly more prone to misfolding than the alpha in the absence of GroEL. The alpha subunit was released as soluble monomers from the GroEL-alpha complex alone in the presence of GroES and Mg-ATP. In contrast, the beta subunit discharged from the GroEL-beta complex readily rebound to GroEL when the alpha subunit was absent. Analysis of the assembly state showed that the His6-alpha and beta subunits released from corresponding GroEL-polypeptide complexes assembled into a highly structured but inactive 85.5-kDa alpha beta dimeric intermediate, which subsequently dimerized to produce the active alpha2 beta2 tetrameter. The purified alpha beta dimer isolated from Escherichia coli lysates was capable of binding to GroEL to produce a stable GroEL-alpha beta ternary complex. Incubation of this novel ternary complex with GroES and Mg-ATP resulted in recovery of E1 activity, which also followed slow kinetics with a rate constant of 138 M-1 s-1. Dimers were regenerated from the GroEL-alpha beta complex, but they needed to interact with GroEL/GroES again, thereby perpetuating the cycle until the conversion from dimers to tetramers was complete. Our study describes an obligatory role of chaperonins in priming the dimeric intermediate for subsequent tetrameric assembly, which is a slow step in the reconstitution of E1 alpha2 beta2 tetramers.

GroES in the asymmetric GroEL14-GroES7 complex exchanges via an associative mechanism

The interaction of the chaperonin GroEL14 with its cochaperonin GroES7 is dynamic, involving stable, asymmetric 1:1 complexes (GroES7.GroEL7-GroEL7) and transient, metastable symmetric 2:1 complexes [GroES7.GroEL7-GroEL7.GroES7]. The transient formation of a 2:1 complex permits exchange of free GroES7 for GroES7 bound in the stable 1:1 complex. Electrophoresis in the presence of ADP was used to resolve free GroEL14 from the GroES7-GroEL14 complex. Titration of GroEL14 with radiolabeled GroES7 to molar ratios of 32:1 demonstrated a 1:1 limiting stoichiometry in a stable complex. No stable 2:1 complex was detected. Preincubation of the asymmetric GroES7.GroEL7-GroEL7 complex with excess unlabeled GroES7 in the presence of ADP demonstrated GroES7 exchange. The rates of GroES7 exchange were proportional to the concentration of unlabeled free GroES7. This concentration dependence points to an associative mechanism in which exchange of GroES7 occurs by way of a transient 2:1 complex and excludes a dissociative mechanism in which exchange occurs by way of free GroEL14. Exchange of radiolabeled ADP from 1:1 complexes was much slower than the exchange of GroES7. In agreement with recent structural studies, this indicates that conformational changes in GroEL14 following the dissociation of GroES7 must precede ADP release. These results explain how the GroEL14 cavity can become reversibly accessible to proteins under in vivo conditions that favor 2:1 complexes.

GroEL/GroES: structure and function of a two-stroke folding machine

Recent structural and functional studies have greatly advanced our understanding of the mechanism by which chaperonins (Cpn60) mediate protein folding, the final step in the accurate expression of genetic information. Escherichia coli GroEL has a symmetric double-toroid architecture, which binds nonnative polypeptide substrates on the hydrophobic walls of its central cavity. The asymmetric binding of ATP and cochaperonin GroES to GroEL triggers a major conformational change in the cis ring, creating an enlarged chamber into which the bound nonnative polypeptide is released. The structural changes that create the cis assembly also change the lining of the cavity wall from hydrophobic to hydrophilic, conducive to folding into the native state. ATP hydrolysis in the cis ring weakens it and primes the release of products. When ATP and GroES bind to the trans ring, it forms a stronger assembly, which disassembles the cis complex through negative cooperativity between rings. The opposing function of the two rings operates as if the system had two cylinders, one expelling the products of the reaction as the other loads up the reactants. One cycle of the reaction gives the polypeptide about 15 s to fold at the cost of seven ATP molecules. For some proteins, several cycles of GroEL assistance may be needed in order to achieve their native states.

GroEL under heat-shock. Switching from a folding to a storing function

Chaperonin GroEL from Escherichia coli, together with its cochaperonin GroES, are proteins involved in assisting the folding of polypeptides. GroEL is a tetradecamer composed of two heptameric rings, which enclose a cavity where folding takes place through multiple cycles of substrate and GroES binding and release. GroEL and GroES are also heat-shock proteins, their synthesis being increased during heat-shock conditions to help the cell coping with the thermal stress. Our results suggest that, as the temperature increases, GroEL decreases its protein folding activity and starts acting as a "protein store." The molecular basis of this behavior is the loss of inter-ring signaling, which slows down GroES liberation from GroEL and therefore the release of the unfolded protein from the GroEL cavity. This behavior is reversible, and after heat-shock, GroEL reverts to its normal function. This might have a physiological meaning, since under thermal stress conditions, it may be inefficient for the cell to fold thermounstable proteins that are prone to denaturation.

Structure and function in GroEL-mediated protein folding

Recent structural and biochemical investigations have come together to allow a better understanding of the mechanism of chaperonin (GroEL, Hsp60)-mediated protein folding, the final step in the accurate expression of genetic information. Major, asymmetric conformational changes in the GroEL double toroid accompany binding of ATP and the cochaperonin GroES. When a nonnative polypeptide, bound to one of the GroEL rings, is encapsulated by GroES to form a cis ternary complex, these changes drive the polypeptide into the sequestered cavity and initiate its folding. ATP hydrolysis in the cis ring primes release of the products, and ATP binding in the trans ring then disrupts the cis complex. This process allows the polypeptide to achieve its final native state, if folding was completed, or to recycle to another chaperonin molecule, if the folding process did not result in a form committed to the native state.

Mechanisms of autoprotection and the role of stress-proteins in natural defenses, autoprotection, and salutogenesis

We hypothesize that in all physiotherapeutically oriented procedures of naturotherapy -- such as helio-, climate-, thalasso- or hydrotherapy or certain forms of physical exercise -- the transient expression of stress-proteins (heat-shock proteins, HSPs) is an important element of salutogenesis. These therapeutical procedures all cause a transitory 'disturbance' by an unspecific stressor that leads to functional responses. These functional responses can be trained and thus increase the forces and the capacity for resistance of the organism. The autoprotective mechanisms which we want to deal with in more detail are based on the functions of the heat-shock proteins (HSPs, stress-response proteins, 'chaperones') and represent archaic autoprotective responses. In addition, more complex mechanisms of autoprotection seem to have evolved that may play a role in the natural defenses against disease and which show a hierarchy of various genomically conserved strategies with different time-constants and time windows. This becomes apparent by studying autoprotective responses of the cardiovascular system of warm-blooded animals under ischemic stress. Recent extensive experimental protocols and clinical observations in elucidating the molecular basis of cardiac ischemia show that powerful autoprotective mechanisms are involved in the phenomena of 'hibernation', 'stunning', and 'ischemic preconditioning'. The system of the heat-shock proteins may therefore be regarded as a basic model for the principle of autoprotection and salutogenesis.

Structural analysis of GroE chaperonin complexes using chemical cross-linking

In this chapter, we have shown how chemical cross-linking with a bifunctional reagent, GA, can be used to investigate the structure of large oligomeric complexes such as GroEL14GroES7 and GroEL14(GroES7)2. Cross-linking, followed by denaturing electrophoresis, confirmed the number and arrangement of GroEL and GroES subunits within each individual oligomer, which was previously known from EM analysis. Furthermore, cross-linking permitted a close examination of the effect of regulatory factors, such as nucleotides and free divalent cations, on the molecular structure of GroEL14, GroEL14GroES7, and GroEL14GroES7. Finally, cross-linking analysis permitted characterization and quantitation of various chaperonin heterooligomeric complexes, GroEL14, GroEL14GroES7, and GroEL14GroES7 in solution, under conditions that also supported protein folding and ATP hydrolysis. It was shown that GA does not induce the artifactual association or the dissociation of GroES7 from the chaperonin. On the contrary, chemical cross-linking is an obligatory procedure when the subsequent analysis is carried out using methods that can displace the equilibrium.

Crystal structure of the thermosome, the archaeal chaperonin and homolog of CCT

We have determined to 2.6 A resolution the crystal structure of the thermosome, the archaeal group II chaperonin from T. acidophilum. The hexadecameric homolog of the eukaryotic chaperonin CCT/TRiC shows an (alphabeta)4(alphabeta)4 subunit assembly. Domain folds are homologous to GroEL but form a novel type of inter-ring contact. The domain arrangement resembles the GroEL-GroES cis-ring. Parts of the apical domains form a lid creating a closed conformation. The lid substitutes for a GroES-like cochaperonin that is absent in the CCT/TRiC system. The central cavity has a polar surface implicated in protein folding. Binding of the transition state analog Mg-ADP-AIF3 suggests that the closed conformation corresponds to the ATP form.

Effects of the inter-ring communication in GroEL structural and functional asymmetry

The chaperonin GroEL consists of a double-ring structure that assists protein folding in the presence of GroES and ATP. Recent studies suggest that the 7-mer ring is the functional unit where protein folding takes place. Nevertheless, both GroEL rings are required to complete the reaction cycle through signals transmitted between the two rings. Electron microscopy, image processing, and biochemical analysis of GroEL, a single-ring mutant (SR1) and a inter-ring communication affected mutant (A126V), in the presence of ATP and adenylyl imidodiphosphate, have allowed the identification of a conformational change in the apical domains that is strictly dependent on the communication between the two GroEL rings. It is deduced from these results that the binding of nucleotide to both GroEL rings generates, as a consequence of the inter-ring communication, a functionally and structurally asymmetric particle. This asymmetric particle has a ring with a small conformational change in its apical domains and high affinity toward unfolded substrate and GroES, and the other ring has a larger conformational change in its apical domains and lower affinity toward substrate and GroES.

Conditions for nucleotide-dependent GroES-GroEL interactions. GroEL14(groES7)2 is favored by an asymmetric distribution of nucleotides

A still unresolved question regarding the mechanism of chaperonin-assisted protein folding involves the stoichiometry of the GroEL-GroES complex. This is important, because the activities of the Escherichia coli chaperonin GroEL are modulated by the cochaperonin GroES. In this report, the binding of GroES to highly purified GroEL in the presence of ATP, ADP, and the nonhydrolyzable ATP analogue, 5'-adenylyl beta,gamma-imidodiphosphate (AMP-PNP), was investigated by using the fluorescence anisotropy of succinimidyl-1-pyrenebutyrate-labeled GroES. In the presence of Mg2+-ATP and high [KCl] (10 mM), two GroES7 rings bind per one GroEL14. In contrast, in the presence of ADP or AMP-PNP only one molecule of oligomeric GroES can be tightly bound by GroEL. With AMP-PNP, binding of a small amount (<20%) of a second GroES can be detected. In the presence of ADP alone, a second GroES ring can bind to GroEL weakly and with negative cooperativity. Strikingly, addition of AMP-PNP to the solution containing preformed GroEL14(GroES7) complexes formed in the presence of ADP results in an increase in the fluorescence anisotropy. Analysis of this effect indicates that 2 mol of GroES oligomer can be bound in the presence of mixed nucleotides. A similar conclusion follows from studies in which ADP is added to an GroEL14 (GroES7) complex formed in the presence of AMP-PNP. This is the first demonstration of an asymmetric distribution of nucleotides bound on the 1:2 GroEL14 (GroES7)2 complex. The relation of the observed phenomena to the proposed mechanism of the GroEL function is discussed.

The N-terminal region of the luteovirus readthrough domain determines virus binding to Buchnera GroEL and is essential for virus persistence in the aphid

Luteoviruses and the luteovirus-like pea enation mosaic virus (PEMV; genus Enamovirus) are transmitted by aphids in a circulative, nonreplicative manner. Acquired virus particles persist for several weeks in the aphid hemolymph, in which a GroEL homolog, produced by the primary endosymbiont of the aphid, is abundantly present. Six subgroup II luteoviruses and PEMV displayed a specific but differential affinity for Escherichia coli GroEL and GroEL homologs isolated from the endosymbiotic bacteria of both vector and nonvector aphid species. These observations suggest that the basic virus-binding capacity resides in a conserved region of the GroEL molecule, although other GroEL domains may influence the efficiency of binding. Purified luteovirus and enamovirus particles contain a major 22-kDa coat protein (CP) and lesser amounts of an approximately 54-kDa readthrough protein, expressed by translational readthrough of the CP into the adjacent open reading frame. Beet western yellows luteovirus (BWYV) mutants devoid of the readthrough domain (RTD) did not bind to Buchnera GroEL, demonstrating that the RTD (and not the highly conserved CP) contains the determinants for GroEL binding. In vivo studies showed that virions of these BWYV mutants were significantly less persistent in the aphid hemolymph than were virions containing the readthrough protein. These data suggest that the Buchnera GroEL-RTD interaction protects the virus from rapid degradation in the aphid. Sequence comparison analysis of the RTDs of different luteoviruses and PEMV identified conserved residues potentially important in the interaction with Buchnera GroEL.

Significance of chaperonin 10-mediated inhibition of ATP hydrolysis by chaperonin 60

Chaperonins are essential for the folding of proteins in bacteria, mitochondria, and chloroplasts. We have functionally characterized the yeast mitochondrial chaperonins hsp60 and hsp10. In the presence of ADP, one molecule of hsp10 binds to hsp60 with an apparent Kd of 0.9 nM and a second molecule of hsp10 binds with a Kd of 24 nM. In the presence of ATP, the purified yeast chaperonins mediate the refolding of mitochondrial malate dehydrogenase. Hsp10 inhibits the ATPase activity of hsp60 by about 40%. Hsp10(P36H) is a point mutant of hsp10 that confers temperature-sensitive growth to yeast. Consistent with the in vivo phenotype, refolding of mitochondrial malate dehydrogenase in the presence of purified hsp10(P36H) and hsp60 is reduced at 25 degrees C and abolished at 30 degrees C. The affinity of hsp10(P36H) to hsp60 as well as to Escherichia coli GroEL is reduced. However, this decrease in affinity does not correlate with the functional defect, because hsp10(P36H) fully assists the GroEL-mediated refolding of malate dehydrogenase at 30 degrees C. Refolding activity, rather, correlates with the ability of hsp10(P36H) to inhibit the ATPase of GroEL but not that of hsp60. Based on our findings, we propose that the inhibition of ATP hydrolysis is mechanistically coupled to chaperonin-mediated protein folding.

Conformational changes at the nucleotide binding of GroEL induced by binding of protein substrates. Luminescence studies

2'-Deoxy-3'-anthraniloyl adenosine-5-triphosphate (ANT-dATP) coordinated to Tb3+ was used as an environmentally sensitive probe of the nucleotide-binding site of GroEL. Tb3+.ANT-dATP recognizes the nucleotide-binding site of GroEL and inhibits ATPase activity. Sensitized luminescence, arising from resonance energy transfer from the anthraniloyl moiety to Tb3+, is substantially enhanced in the presence of GroEL. Binding of denatured mitochondrial malate dehydrogenase to the apical domain of GroEL causes a red shift in the fluorescence emitted by anthraniloyl and further enhancement in the phosphorescence emitted by Tb3+ upon excitation at 320 nm. It is suggested that binding of the protein substrate initiates domain movement, which is extended to the nucleotide-binding site. The luminescence results are discussed in reference to the structure of GroEL derived from x-ray crystallographic studies.

Importance of electrostatic interactions in the rapid binding of polypeptides to GroEL

The question of how chaperones rapidly bind non-native proteins of very different sequence and function has been examined by determining the effect of ionic strength on the refolding of barnase on GroEL, and on the thermal denaturation of barnase in the presence of GroEL and SecB. Both chaperones bind the denatured state of barnase, so lowering the T(m) value. The refolding of barnase in the presence of GroEL is multiphasic, the slowest phase corresponding to the refolding of a singly bound molecule of barnase in the complex with GroEL. The fastest phase is related to the association of barnase and GroEL. At high ratios of GroEL to barnase and low ionic strength (less than 200 mM) this fast phase corresponds to the observed rate of binding. The rate of association of barnase and GroEL was found to be highly dependent on ionic strength, and at high ionic strength (greater than 600 mM) the majority of barnase molecules escaped binding and refolded free in solution. The data are consistent with an initial, transient, ionic interaction between barnase and GroEL, before hydrophobic binding occurs, allowing diffusion-controlled association and slow dissociation of unfolded polypeptide.

GroES binding regulates GroEL chaperonin activity under heat shock

Chaperonins GroEL14 and GroES7 are heat-shock proteins implicated in the molecular response to stress. Protein fluorescence, crosslinking and kinetic analysis revealed that the bond between the two otherwise thermoresistant oligomers is regulated by temperature. As temperature increased, the affinity of GroES7 and the release of bound proteins from the chaperonin concomitantly decreased. After heat shock, GroES7 rebinding to GroEL14 and GroEL14GroES7 particles correlated with the restoration of optimal protein folding/release activity. Chaperonins thus behave as a molecular thermometer which can inhibit the release of aggregation-prone proteins during heat shock and restore protein folding and release after heat shock.

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.

Evidence for a lipochaperonin: association of active protein-folding GroESL oligomers with lipids can stabilize membranes under heat shock conditions

During heat shock, structural changes in proteins and membranes may lead to cell death. While GroE and other chaperone proteins are involved in the prevention of stress-induced protein aggregation and in the recovery of protein structures, a mechanism for short-term membrane stabilization during stress remains to be established. We found that GroEL chaperonin can associate with model lipid membranes. Binding was apparently governed by the composition and the physical state of the host bilayer. Limited proteolysis of GroEL oligomers by proteinase K, which removes selectively the conserved glycine- and methionine-rich C terminus, leaving the chaperonin oligomer intact, prevented chaperonin association with lipid membranes. GroEL increased the lipid order in the liquid crystalline state, yet remained functional as a protein-folding chaperonin. This suggests that, during stress, chaperonins can assume the functions of assisting the folding of both soluble and membrane-associated proteins while concomitantly stabilizing lipid membranes.

Chaperone-assisted protein folding

Molecular chaperones of the Hsp70 and chaperonin families are basic constituents of the cellular machinery that mediates protein folding. Recent functional and structural studies corroborate existing models for the mechanism of these components. Highlights of the past year include the X-ray crystallographic analysis of the peptide-binding domain of the Escherichia coli Hsp70 homolog, DnaK, the direct demonstration of protein folding in the central cavity of the chaperonin GroEL, and the visualization of conformational changes in GroEL during the chaperonin folding cycle.

Conformational changes in the GroEL oligomer during the functional cycle

The conformational changes that the GroEL oligomer undergoes upon nucleotide and cochaperonin GroES binding have been studied using electron microscopy and image processing techniques. Average side views of the three allosteric states (TT, TR, and RR, which correspond to none, one, or both of the two heptameric rings of the GroEL oligomer occupied by nucleotide, respectively) of GroEL and GroEL-GroES complexes for ADP, ATP, and two nonhydrolyzable analogs (AMP-PNP and ATP gamma S) have been obtained at 20-25 A resolution. Both AMP-PNP and ATP induce similar conformational shifts in the apical domains of GroEL. At the TR state, only one of the GroEL rings shows an upward and outward movement of the apical domains ("open state"). At the RR state for AMP-PNP and ATP, both GroEL rings undergo conformational changes, albeit of different magnitude, giving rise to a structurally asymmetric particle (one ring in the "open" state, while the other is in an "intermediate" state). These changes are also observed when GroEL is incubated with ADP and Pi, but not with ADP, which suggests that upon ATP binding, GroEL undergoes a conformational change that is partly maintained after ATP hydrolysis and as long as ADP and Pi are bound to the GroEL ring. The conformational changes undergone by GroEL are discussed within the framework of a proposed GroEL cycle mechanism.

Refolding and reassociation of glycerol dehydrogenase from Bacillus stearothermophilus in the absence and presence of GroEL

The refolding of the tetrameric, metalloenzyme glycerol dehydrogenase (GDH) from Bacillus stearothermophilus has been investigated using stopped-flow fluorescence and circular dichroism spectroscopy. The effects of metal ions on the refolding of the native enzyme and the refolding of a monomeric mutant ([A208]GDH) have also been studied. The refolding process of the wild-type enzyme is at least biphasic; 70% of the respective signal changes occur in the first 2 ms followed by a slower process with a half-life of 3 s. The presence of the metal ion does not affect the slowest biphasic refolding rate, which is virtually the same for all three versions of the enzyme. The presence of GroEL slows down the first phase of refolding. The reassociation of subunits was examined by measuring the regain in catalytic activity and the enhancement in the fluorescence emission from NADH on binding to the oligomeric form of the enzyme. The rate and extent of reassociation is dependent on enzyme concentration and the extent of reactivation is dependent on the presence of the metal ion. The reassociation process was more efficient in the presence of NADH particularly for the metal-depleted enzyme (apo-GDH). The presence of GroEL or GroEL plus ATP leads to a higher yield of reassociation and therefore catalytically active enzyme. The additional presence of Mg-ATP does not affect the extent of reassociation, but has a small positive effect on the rate of reassociation. These data suggest that GDH is bound weakly to GroEL and that GroES is not required for release of the protein.

Legionella pneumophila heat-shock protein-induced increase of interleukin-1 beta mRNA involves protein kinase C signalling in macrophages

Heat-shock proteins (hsp) are chaperon molecules important in protein folding and assembly. Furthermore, they may have functions in immunoregulatory processes, like T-cell stimulation and antigen presentation, which are not yet fully understood. It has been shown that several hsp of various species and family derivations modulate functions in macrophage immunity by directly increasing cytokine production. In the present study we showed that the 60,000 MW hsp of Legionella pneumophila (Lp-hsp 60) increased cellular steady-state levels of interleukin-1 beta (IL-1 beta) mRNA measured by quantitative reverse transcription-polymerase chain reaction and Northern blotting as well as IL-1 secretion, when added to cultures of thioglycollate-elicited mouse peritoneal macrophages in vitro. The level of mRNA increased in a dose-dependent manner with a minimum effective concentration of 0.5 microgram/ml and peaked 3 hr after stimulation. Lp-hsp 60-coated latex beads also increased IL-1 beta mRNA levels in the presence of cytochalasin D, which inhibits bead uptake but permits binding, indicating that binding to the macrophage surface was sufficient for induction. Accumulation of IL-1 beta mRNA was completely blocked by pretreatment with the protein kinase C (PKC) inhibitor, H7, but not decreased by prior treatment with cycloheximide. The cell lysates of macrophages stimulated with Lp-hsp 60 showed an increased PKC activity measured by phosphorylation of PKC pseudosubstrate. The IL-1 bioactivity in culture supernatants after 24 hr of stimulation with Lp-hsp 60 was increased in a dose-dependent manner but at hsp concentrations in excess of those needed to increase mRNA. Thus, the present study demonstrates that Lp-hsp 60 rapidly increases the steady-state level of IL-1 beta mRNA, possibly through a cell surface receptor system involving a PKC-dependent signalling pathway.

Chaperonins GroEL and GroES: views from atomic force microscopy

The Escherichia coli chaperonins, GroEL and GroES, as well as their complexes in the presence of a nonhydrolyzable nucleotide AMP-PNP, have been imaged with the atomic force microscope (AFM). We demonstrate that both GroEL and GroES that have been adsorbed to a mica surface can be resolved directly by the AFM in aqueous solution at room temperature. However, with glutaraldehyde fixation of already adsorbed molecules, the resolution of both GroEL and GroES was further improved, as all seven subunits were well resolved without any image processing. We also found that chemical fixation was necessary for the contact mode AFM to image GroEL/ES complexes, and in the AFM images. GroEL with GroES bound can be clearly distinguished from those without. The GroEL/ES complex was about 5 nm higher than GroEL alone, indicating a 2 nm upward movement of the apical domains of GroEL. Using a slightly larger probe force, unfixed GroEL could be dissected: the upper heptamer was removed to expose the contact surface of the two heptamers. These results clearly demonstrate the usefulness of cross-linking agents for the determination of molecular structures with the AFM. They also pave the way for using the AFM to study the structural basis for the function of GroE system and other molecular chaperones.

Molecular chaperones and protein folding in plants

Protein folding in vivo is mediated by an array of proteins that act either as 'foldases' or 'molecular chaperones'. Foldases include protein disulfide isomerase and peptidyl prolyl isomerase, which catalyze the rearrangement of disulfide bonds or isomerization of peptide bonds around Pro residues, respectively. Molecular chaperones are a diverse group of proteins, but they share the property that they bind substrate proteins that are in unstable, non-native structural states. The best understood chaperone systems are HSP70/DnaK and HSP60/GroE, but considerable data support a chaperone role for other proteins, including HSP100, HSP90, small HSPs and calnexin. Recent research indicates that many, if not all, cellular proteins interact with chaperones and/or foldases during their lifetime in the cell. Different chaperone and foldase systems are required for synthesis, targeting, maturation and degradation of proteins in all cellular compartments. Thus, these diverse proteins affect an exceptionally broad array of cellular processes required for both normal cell function and survival of stress conditions. This review summarizes our current understanding of how these proteins function in plants, with a major focus on those systems where the most detailed mechanistic data are available, or where features of the chaperone/foldase system or substrate proteins are unique to plants.

Molecular chaperones and protein folding in plants

Protein folding in vivo is mediated by an array of proteins that act either as 'foldases' or 'molecular chaperones'. Foldases include protein disulfide isomerase and peptidyl prolyl isomerase, which catalyze the rearrangement of disulfide bonds or isomerization of peptide bonds around Pro residues, respectively. Molecular chaperones are a diverse group of proteins, but they share the property that they bind substrate proteins that are in unstable, non-native structural states. The best understood chaperone systems are HSP70/DnaK and HSP60/GroE, but considerable data support a chaperone role for other proteins, including HSP100, HSP90, small HSPs and calnexin. Recent research indicates that many, if not all, cellular proteins interact with chaperones and/or foldases during their lifetime in the cell. Different chaperone and foldase systems are required for synthesis, targeting, maturation and degradation of proteins in all cellular compartments. Thus, these diverse proteins affect an exceptionally broad array of cellular processes required for both normal cell function and survival of stress conditions. This review summarizes our current understanding of how these proteins function in plants, with a major focus on those systems where the most detailed mechanistic data are available, or where features of the chaperone/foldase system or substrate proteins are unique to plants.

Fluorescence detection of symmetric GroEL14(GroES7)2 heterooligomers involved in protein release during the chaperonin cycle

The GroEL14 chaperonin from Escherichia coli was labeled with 5-((((2-iodoacetyl)amino)ethyl)amino)naphthalene-1-sulfonic acid (I-AEDANS), a hydrophobic probe whose fluorescent emission is sensitive to structural changes within the protein. Increasing concentrations of ATP or adenylyl imidodiphosphate but not ADP caused two successive GroES7-dependent changes in the fluorescence intensity of AEDANS-GroEL14, corresponding to the sequential binding of two GroES7 heptamers and the formation of two types of chaperonin heterooligomers, GroEL14GroES7 and GroEL14(GroES7)2. The binding of thermally denatured malate dehydrogenase (MDH) caused a specific increase in fluorescence intensity of AEDANS-GroEL14 that allowed the direct measurement in solution at equilibrium of ATP- and GroES7-dependent protein release from the chaperonin. Structure/function analysis during the generation of ATP from ADP indicated the following sequence of events: 1) ADP-stabilized MDH-GroEL14GroES7 particles bind newly formed ATP. 2) MDH-GroEL14GroES7 particles bind a second GroES7. 3) MDH-GroEL14(GroES7)2 particles productively release MDH. 4) Released MDH completes folding. Therefore, the symmetrical GroEL14(GroES7)2 heterooligomer is an intermediate after the formation of which the protein substrate is productively released during the chaperonin-mediated protein folding cycle.

Molecular chaperones in cellular protein folding

The folding of many newly synthesized proteins in the cell depends on a set of conserved proteins known as molecular chaperones. These prevent the formation of misfolded protein structures, both under normal conditions and when cells are exposed to stresses such as high temperature. Significant progress has been made in the understanding of the ATP-dependent mechanisms used by the Hsp70 and chaperonin families of molecular chaperones, which can cooperate to assist in folding new polypeptide chains.

Nucleotide binding-promoted conformational changes release a nonnative polypeptide from the Escherichia coli chaperonin GroEL

The Escherichia coli chaperonins GroEL and GroES facilitate the refolding of polypeptide chains in an ATP hydrolysis-dependent reaction. The elementary steps in the binding and release of polypeptide substrates to GroEL were investigated in surface plasmon resonance studies to measure the rates of binding and dissociation of a normative variant of subtilisin. The rate constants determined for GroEL association with and dissociation from this variant yielded a micromolar dissociation constant, in agreement with independent calorimetric estimates. The rate of GroEL dissociation from the nonnative chain was increased significantly in the presence of 5'-adenylylimidodiphosphate (AMP-PNP), ADP, and ATP, yielding maximal values between 0.04 and 0.22 s(-1). The sigmoidal dependence of the dissociation rate on the concentration of AMP-PNP and ADP indicated that polypeptide dissociation is limited by a concerted conformational change that occurs after nucleotide binding. The dependence of the rate of release on ATP exhibited two sigmoidal transitions attributable to nucleotide binding to the distal and proximal toroid of a GroEL-polypeptide chain complex. The addition of GroES resulted in a marked increase in the rate of nonnative polypeptide release from GroEL, indicating that the cochaperonin binds more rapidly than the dissociation of polypeptides. These data demonstrate the importance of nucleotide binding-promoted concerted conformational changes for the release of chains from GroEL, which correlate with the sigmoidal hydrolysis of ATP by the chaperonin. The implications of these findings are discussed in terms of a working hypothesis for a single cycle of chaperonin action.

High resolution surface structure of E. coli GroES oligomer by atomic force microscopy

Using atomic force microscopy (AFM) in aqueous solution, we show that the surface structure of the oligomeric GroES can be obtained up to 10 angstroms resolution. The seven subunits of the heptamer were well resolved without image averaging. The overall dimension of the GroES heptamer was 8.4 +/- 0.4 nm in diameter and 3.0 +/- 0.3 nm high. However, the AFM images further suggest that there is a central protrusion of 0.8 +/- 0.2 nm high and 4.5 +/- 0.4 nm in diameter on one side of GroES which displays a profound seven-fold symmetry. It was found that GroEL could not bind to the adsorbed GroES in the presence of AMP-PNP and Mg2+, suggesting that the side of GroES with the central protrusion faces away from the GroEL lumen, because only one side of GroES was observed under these conditions. Based on the results from both electron and atomic force microscopy, a surface model for the GroES is proposed.

Molecular chaperones are present in the thylakoid lumen of pea chloroplasts

A soluble protein fraction was obtained from pea chloroplast thylakoids, which represents highly enriched lumenal components. Using antisera against chaperonin 60 (cpn60), chaperonin 10 (cpn10) and the heat shock cognate protein of 70 kDa (hsc70) we are able to demonstrate, that the thylakoid lumen contains a separate set of molecular chaperones, which is distinct from the stromal one. In contrast to the alpha and beta subunits of cpn60 present in the stroma the lumen contains only one cpn60 isoform of distinct isoelectric point. Furthermore the lumenal cpn10 is of 'normal' size and not like its stromal counterpart of a double-domain tandem architecture. The immunoreactive hsc70 isoforms in the lumen seem also to be different from the stromal ones. Thus, chloroplasts seem to contain the largest number of molecular chaperone isoforms present in one organelle.

Structure of the heat shock protein chaperonin-10 of Mycobacterium leprae

Members of the chaperonin-10 (cpn10) protein family, also called heat shock protein 10 and in Escherichia coli GroES, play an important role in ensuring the proper folding of many proteins. The crystal structure of the Mycobacterium leprae cpn10 (Ml-cpn10) oligomer has been elucidated at a resolution of 3.5 angstroms. The architecture of the Ml-cpn10 heptamer resembles a dome with an oculus in its roof. The inner surface of the dome is hydrophilic and highly charged. A flexible region, known to interact with cpn60, extends from the lower rim of the dome. With the structure of a cpn10 heptamer now revealed and the structure of the E. coli GroEL previously known, models of cpn10:cpn60 and GroEL:GroES complexes are proposed.

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

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

Biochemical characterization of symmetric GroEL-GroES complexes. Evidence for a role in protein folding

When chaperonins GroEL and GroES are incubated under functional conditions in the presence of ATP (5 mM) and K+ (150 mM), GroEL-GroES complexes appear in the incubation mixture, that are either asymmetric (1:1 GroEL:GroES oligomer ratio) or symmetric (1:2 GroEL:GroES oligomer ratio). The percentage of symmetric complexes present is directly related to the [ATP]/[ADP] ratio and to the K+ concentration. Kinetic analysis shows that there is a cycle of formation and disappearance of symmetric complexes. A correlation between the presence of symmetric complexes in the incubation mixture and its rhodanese folding activity suggests some active role of these complexes in the protein folding process. Accordingly, under functional conditions, symmetric complexes are found to contain denatured rhodanese. These data suggest that binding of substrate inside the GroEL cavity takes place before the symmetric complex is formed.

Kinetic significance of GroEL14.(GroES7)2 complexes in molecular chaperone activity

BACKGROUND

Symmetrical GroEL14.(GroES7)2 complexes, nicknamed 'footballs', have been observed by electron microscopy to form in the presence of excess ATP. But the significance of these footballs in the molecular chaperone cycle is controversial. We have analyzed the folding of barnase in the presence of GroEL, GroES and various nucleotides to probe the importance of footballs.

RESULTS

A stoichiometric concentration of GroES7 binds to the GroEL14.nucleotide.denatured barnase complex to produce a slow-folding state. Higher concentrations of GroES in the presence of ATP or AMP-PNP, but not ADP, produce a proportion of a fast-folding state, rising to 50% at a GroES7:GroEL14 stoichiometry of > or = 2:1.

CONCLUSIONS

These results imply that there is a transiently formed GroEL14.(GroES7)2.denatured protein complex that dissociates into a 50:50 mixture of slow-folding cis and fast-folding trans GroEL14.GroES7.denatured protein complexes. The transient formation of a symmetrical football could provide a means of opening the cage that encapsulates folded cis-bound proteins.

The C-terminal sequence of the chaperonin GroES is required for oligomerization

The Escherichia coli protein GroES is a co-chaperonin that is able to assist GroEL in the refolding of proteins. GroES is a heptamer of seven identical subunits. Recent work has focused on the structural aspects of GroES. We have investigated the role of the C-terminal portion of GroES on its oligomerization. Limited proteolysis of GroES by carboxypeptidase Y gives a product in which the C-terminal 7 amino acid residues have been removed. Sedimentation velocity analysis reveals that the truncated form of GroES is unable to reassemble. The results presented here implicate the C-terminal sequence in intermonomer actions within the GroES oligomer. In addition, this work provides experimental verification of predictions implied in the recent x-ray determination of the GroES structure (Hunt, J. F., Weaver, A. J., Landry, S. J., Gierasch, L. M., and Deisenhofer, J. Nature, in press).

The protein-folding activity of chaperonins correlates with the symmetric GroEL14(GroES7)2 heterooligomer

Chaperonins GroEL and GroES form, in the presence of ATP, two types of heterooligomers in solution: an asymmetric GroEL14GroES7 "bullet"-shaped particle and a symmetric GroEL14(GroES7)2 "football"-shaped particle. Under limiting concentrations of ATP or GroES, excess ADP, or in the presence of 5'-adenylyl imidodiphosphate, a correlation is seen between protein folding and the amount of symmetric GroEL14(GroES7)2 particles in a chaperonin solution, as detected by electron microscopy or by chemical crosslinking. Kinetic analysis suggests that protein folding is more efficient when carried out by a chaperonin solution populated with a majority of symmetric GroEL14(GroES7)2 particles than by a majority of asymmetric GroEL14GroES7 particles. The symmetric heterooligomer behaves as a highly efficient intermediate of the chaperonin protein folding cycle in vitro.

Increased efficiency of GroE-assisted protein folding by manganese ions

This study addresses the role of ATP-bound and free Mg2+ and Mn2+ ions in the activation and modulation of chaperonin-assisted refolding of urea-denatured malate dehydrogenase. As compared with Mg2+, Mn2+ ions caused a significant increase in the rate of GroE-assisted malate dehydrogenase refolding and, concomitantly, a decrease in the rate of ATP hydrolysis. Moreover, Mn2+ increases the affinity of GroES for GroEL, even in the presence of saturating amounts of Mg2+. Chemical cross-linking showed that lower concentrations of Mn-ATP as compared with Mg-ATP are needed to form both asymmetric GroEL14GroES7 and symmetric GroEL14(GroES7)2 particles. The manganese-dependent increase in the rate of protein folding concurred with a specific increase in the amount of symmetric GroEL14-(GroES7)2 particles detected in a chaperonin solution. Thus, Mn2+ is a cofactor that can markedly increase the efficiency of the chaperonin reaction in vitro. Mn2+ ions can serve as an important tool for analyzing the molecular mechanism and the structure of chaperonins.