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

Functional redundancy buffers the effect of poly-extreme environmental conditions on southern African dryland soil microbial communities

Drylands' poly-extreme conditions limit edaphic microbial diversity and functionality. Furthermore, climate change exacerbates soil desiccation and salinity in most drylands. To better understand the potential effects of these changes on dryland microbial communities, we evaluated their taxonomic and functional diversities in two Southern African dryland soils with contrasting aridity and salinity. Fungal community structure was significantly influenced by aridity and salinity, while Bacteria and Archaea only by salinity. Deterministic homogeneous selection was significantly more important for bacterial and archaeal communities' assembly in hyperarid and saline soils when compared to those from arid soils. This suggests that niche partitioning drives bacterial and archaeal communities' assembly under the most extreme conditions. Conversely, stochastic dispersal limitations drove the assembly of fungal communities. Hyperarid and saline soil communities exhibited similar potential functional capacities, demonstrating a disconnect between microbial structure and function. Structure variations could be functionally compensated by different taxa with similar functions, as implied by the high levels of functional redundancy. Consequently, while environmental selective pressures shape the dryland microbial community assembly and structures, they do not influence their potential functionality. This suggests that they are functionally stable and that they could be functional even under harsher conditions, such as those expected with climate change.

Heat Shock Response and Heat Shock Proteins: Current Understanding and Future Opportunities in Human Diseases

The heat shock response is an evolutionarily conserved mechanism that protects cells or organisms from the harmful effects of various stressors such as heat, chemicals toxins, UV radiation, and oxidizing agents. The heat shock response triggers the expression of a specific set of genes and proteins known as heat shock genes/proteins or molecular chaperones, including HSP100, HSP90, HSP70, HSP60, and small HSPs. Heat shock proteins (HSPs) play a crucial role in thermotolerance and aiding in protecting cells from harmful insults of stressors. HSPs are involved in essential cellular functions such as protein folding, eliminating misfolded proteins, apoptosis, and modulating cell signaling. The stress response to various environmental insults has been extensively studied in organisms from prokaryotes to higher organisms. The responses of organisms to various environmental stressors rely on the intensity and threshold of the stress stimuli, which vary among organisms and cellular contexts. Studies on heat shock proteins have primarily focused on HSP70, HSP90, HSP60, small HSPs, and ubiquitin, along with their applications in human biology. The current review highlighted a comprehensive mechanism of heat shock response and explores the function of heat shock proteins in stress management, as well as their potential as therapeutic agents and diagnostic markers for various diseases.

Allosteric differences dictate GroEL complementation of E. coli

GroES/GroEL is the only bacterial chaperone essential under all conditions, making it a potential antibiotic target. Rationally targeting ESKAPE GroES/GroEL as an antibiotic strategy necessitates studying their structure and function. Herein, we outline the structural similarities between Escherichia coli and ESKAPE GroES/GroEL and identify significant differences in intra- and inter-ring cooperativity, required in the refolding cycle of client polypeptides. Previously, we observed that one-half of ESKAPE GroES/GroEL family members could not support cell viability when each was individually expressed in GroES/GroEL-deficient E. coli cells. Cell viability was found to be dependent on the allosteric compatibility between ESKAPE and E. coli subunits within mixed (E. coli and ESKAPE) tetradecameric GroEL complexes. Interestingly, differences in allostery did not necessarily result in differences in refolding rate for a given homotetradecameric chaperonin. Characterization of ESKAPE GroEL allostery, ATPase, and refolding rates in this study will serve to inform future studies focused on inhibitor design and mechanism of action studies.

Spatial distribution and community structure of microbiota associated with cowpea aphid (Aphis craccivora Koch)

Aphid populations were collected on cowpea, dolichos, redgram and black gram from Belagavi and Udupi locations. The samples were shotgun sequenced using the Illumina NovaSeq 6000 system to understand the spatial distribution and community structure of microbiota (especially bacteria) associated with aphids. In the present study, we identified obligatory nutritional symbiont Buchnera aphidicola and facultative symbionts Rickettsia sp. and Bacteroidetes endosymbiont of Geopemphigus sp. in all the aphid samples studied, although in varied abundance. On the other hand, Serratia symbiotica, Arsenophonus sp. and Acinetobacter sp. were only found in aphids on specific host plants, suggesting that host plants might influence the bacterial community structure. Furthermore, our study revealed that microbiota other than bacteria were highly insignificant in the aphid populations. Additionally, functional annotation of aphid metagenomes identified several pathways and enzymes involved in various physiological and ecological functions. Amino acid and vitamin biosynthesis-related pathways were predominant than carbohydrate metabolism, owing to their feeding habit and nutritional requirement. Chaperones related to stress tolerance such as GroEL and DnaK were identified. Enzymes involved in toxic chemical metabolisms such as glutathione transferase, phosphodiesterases and ABC transferases were observed. These enzymes may confer resistance to pesticides in the aphid populations. Overall, our results support the importance of host plants in structuring bacterial communities in aphids and show the functional roles of symbionts in aphid survival and development. Thus, these findings can be the basis for further detailed investigations and devising better strategies to manage the pests in field conditions.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-022-03142-1.

Temperature Regulates Stability, Ligand Binding (Mg2+ and ATP), and Stoichiometry of GroEL-GroES Complexes

Chaperonins are nanomachines that harness ATP hydrolysis to power and catalyze protein folding, a chemical action that is directly linked to the maintenance of cell function through protein folding/refolding and assembly. GroEL and the GroEL-GroES complex are archetypal examples of such protein folding machines. Here, variable-temperature electrospray ionization (vT-ESI) native mass spectrometry is used to delineate the effects of solution temperature and ATP concentrations on the stabilities of GroEL and GroEL-GroES complexes. The results show clear evidence for destabilization of both GroEL₁₄ and GroES₇ at temperatures of 50 and 45 °C, respectively, substantially below the previously reported melting temperature (T_m ∼ 70 °C). This destabilization is accompanied by temperature-dependent reaction products that have previously unreported stoichiometries, viz. GroEL₁₄-GroES_y-ATP_n, where y = 1, 2, 8 and n = 0, 1, 2, 8, that are also dependent on Mg²⁺ and ATP concentrations. Variable-temperature native mass spectrometry reveals new insights about the stability of GroEL in response to temperature effects: (i) temperature-dependent ATP binding to GroEL; (ii) effects of temperature as well as Mg²⁺ and ATP concentrations on the stoichiometry of the GroEL-GroES complex, with Mg²⁺ showing greater effects compared to ATP; and (iii) a change in the temperature-dependent stoichiometries of the GroEL-GroES complex (GroEL₁₄-GroES₇ vs GroEL₁₄-GroES₈) between 24 and 40 °C. The similarities between results obtained by using native MS and cryo-EM [Clare et al. An expanded protein folding cage in the GroEL-gp31 complex. J. Mol. Biol. 2006, 358, 905-911; Ranson et al. Allosteric signaling of ATP hydrolysis in GroEL-GroES complexes.Nat. Struct. Mol. Biol. 2006, 13, 147-152] underscore the utility of native MS for investigations of molecular machines as well as identification of key intermediates involved in the chaperonin-assisted protein folding cycle.

Chaperonin Abundance Enhances Bacterial Fitness

The ability of chaperonins to buffer mutations that affect protein folding pathways suggests that their abundance should be evolutionarily advantageous. Here, we investigate the effect of chaperonin overproduction on cellular fitness in Escherichia coli. We demonstrate that chaperonin abundance confers 1) an ability to tolerate higher temperatures, 2) improved cellular fitness, and 3) enhanced folding of metabolic enzymes, which is expected to lead to enhanced energy harvesting potential.

Protein Nanobarrel for Integrating Chlorophyll a Molecules and Its Photochemical Performance

Taking inspiration from biology's effectiveness in nanoscale organization of chlorophylls for photosynthesis, we describe here a design for chlorophyll-protein conjugates that exploits the central hydrophobic cavity of GroEL protein nanobarrel as a binding pocket for chlorophyll. We found water-soluble conjugates of chlorophyll with GroEL could be easily generated via detergent dialysis. The number of chlorophyll units bound to GroEL is tunable by varying the equilibrium concentration of chlorophyll during dialysis. Meanwhile, it is shown that an increase in the entrapped chlorophyll amount leads to an improvement of chlorophyll-GroEL photostability. Using methyl viologen as an electron acceptor, we demonstrate that chlorophyll-GroEL has photoreduction activity, which is also switchable in on/off illumination mode. Finally, it is shown that chlorophyll-GroEL-sensitized solar cells have good photoelectric properties, yielding a high photoelectric conversion efficiency of ∼0.9%. The current strategy may be adopted for integrating other photosensitizing dyes or for other photocatalytic reactions.

Highly Surface-Active Chaperonin Nanobarrels for Oil-in-Water Pickering Emulsions and Delivery of Lipophilic Compounds

Stabilization of Pickering emulsions via particles of biological origin exhibits a great potential to be widely applied in food, cosmetic, or biomedicine formulation because of their excellent biocompatibility, biodegradability, and functional properties. This paper describes the successful development of a bioderived GroEL protein nanobarrel as a Pickering stabilizer and its protective properties on β-carotene in dispersed oil phase, as a model of labile bioactive compounds. It is shown that the GroEL nanobarrel is highly surface-active and allows the formation of Pickering emulsion by physical adsorption at the oil/water interface. The optimized formulation for generating a stable submicron oil droplet by ultrasonication includes a GroEL concentration of 0.05-0.45 wt % with an oil/water volume ratio of 0.05-0.35. The as-prepared Pickering emulsion shows pH-responsive emulsification/demulsification transition and excellent stability at temperatures less than 65 °C and ionic strength (with NaCl addition) up to 500 mM. Meanwhile, the emulsion tends to form a gel-like network structure with the oil/water ratio increasing. Finally, we demonstrate that possible factors of oxidant, reducing agent, UV radiation, and sucrose have sequentially decreasing to no effect on the stability of β-carotene encapsulated in GroEL-stabilized Pickering emulsion and that higher GroEL concentration can significantly reduce β-carotene degradation rate, thus ensuring more efficient long-term storage. We believe that the emulsion system supported by the GroEL nanobarrel could be developed to a viable tool for delivering lipophilic bioactive compounds.

Inter and intra-subunit interactions at the subunit interface of chaperonin GroEL are essential for its stability and assembly

Chaperonin GroEL helps in the folding of substrate proteins under normal and stress conditions. Although it remains stable and functional during stress conditions, the quantitative estimation of stability parameters and the specific amino-acid residues playing a role in its stability are not known in sufficient detail. The reason for poor understanding is its large size, multimeric nature, and irreversible unfolding process. The X-ray crystal structure reveals that equatorial domain forms almost all intra and inter-subunit interactions for assembly of GroEL. Considering all these facts, we adopted alternate strategies to use monomeric GroEL, native GroEL and equatorial domain mutants (GroEL_K4E/GroEL_D523K/GroEL_D473C) to study the assembly and stability of GroEL. Loss of inter-subunit interaction involving K4 residue of one subunit and E59, I60, E61, I62 residues of adjacent subunit due to K4E mutation affect the oligomerization efficiency of GroEL subunits while the equilibrium unfolding studies on wild-type monomeric GroEL, native GroEL, and the selected mutants together demonstrate that intra-subunit interactions involving K4 and D523 of the same subunit play a critical role in the thermodynamic stability of both native and monomeric GroEL without affecting the oligomerization of subunits. The stability order between the GroEL_wild-type(M) and its variants is GroEL_wild-type(M) ≥ GroEL_D473C(M)˃GroEL_D523K(M)˃GroEL_K4E.

The Chaperonin GroEL: A Versatile Tool for Applied Biotechnology Platforms

The nucleotide-free chaperonin GroEL is capable of capturing transient unfolded or partially unfolded states that flicker in and out of existence due to large-scale protein dynamic vibrational modes. In this work, three short vignettes are presented to highlight our continuing advances in the application of GroEL biosensor biolayer interferometry (BLI) technologies and includes expanded uses of GroEL as a molecular scaffold for electron microscopy determination. The first example presents an extension of the ability to detect dynamic pre-aggregate transients in therapeutic protein solutions where the assessment of the kinetic stability of any folded protein or, as shown herein, quantitative detection of mutant-type protein when mixed with wild-type native counterparts. Secondly, using a BLI denaturation pulse assay with GroEL, the comparison of kinetically controlled denaturation isotherms of various von Willebrand factor (vWF) triple A domain mutant-types is shown. These mutant-types are single point mutations that locally disorder the A1 platelet binding domain resulting in one gain of function and one loss of function phenotype. Clear, separate, and reproducible kinetic deviations in the mutant-type isotherms exist when compared with the wild-type curve. Finally, expanding on previous electron microscopy (EM) advances using GroEL as both a protein scaffold surface and a release platform, examples are presented where GroEL-protein complexes can be imaged using electron microscopy tilt series and the low-resolution structures of aggregation-prone proteins that have interacted with GroEL. The ability of GroEL to bind hydrophobic regions and transient partially folded states allows one to employ this unique molecular chaperone both as a versatile structural scaffold and as a sensor of a protein's folded states.

GroEL Ring Separation and Exchange in the Chaperonin Reaction

The bacterial chaperonin GroEL and its cofactor, GroES, form a nano-cage for a single molecule of substrate protein (SP) to fold in isolation. GroEL and GroES undergo an ATP-regulated interaction cycle to close and open the folding cage. GroEL consists of two heptameric rings stacked back to back. Here, we show that GroEL undergoes transient ring separation, resulting in ring exchange between complexes. Ring separation occurs upon ATP-binding to the trans ring of the asymmetric GroEL:7ADP:GroES complex in the presence or absence of SP and is a consequence of inter-ring negative allostery. We find that a GroEL mutant unable to perform ring separation is folding active but populates symmetric GroEL:GroES₂ complexes, where both GroEL rings function simultaneously rather than sequentially. As a consequence, SP binding and release from the folding chamber is inefficient, and E. coli growth is impaired. We suggest that transient ring separation is an integral part of the chaperonin mechanism.

The chaperone toolbox at the single-molecule level: From clamping to confining

Protein folding is well known to be supervised by a dedicated class of proteins called chaperones. However, the core mode of action of these molecular machines has remained elusive due to several reasons including the promiscuous nature of the interactions between chaperones and their many clients, as well as the dynamics and heterogeneity of chaperone conformations and the folding process itself. While troublesome for traditional bulk techniques, these properties make an excellent case for the use of single-molecule approaches. In this review, we will discuss how force spectroscopy, fluorescence microscopy, FCS, and FRET methods are starting to zoom in on this intriguing and diverse molecular toolbox that is of direct importance for protein quality control in cells, as well as numerous degenerative conditions that depend on it.

Dynamic Complexes in the Chaperonin-Mediated Protein Folding Cycle

The GroEL–GroES chaperonin system is probably one of the most studied chaperone systems at the level of the molecular mechanism. Since the first reports of a bacterial gene involved in phage morphogenesis in 1972, these proteins have stimulated intensive research for over 40 years. During this time, detailed structural and functional studies have yielded constantly evolving concepts of the chaperonin mechanism of action. Despite of almost three decades of research on this oligomeric protein, certain aspects of its function remain controversial. In this review, we highlight one central aspect of its function, namely, the active intermediates of its reaction cycle, and present how research to this day continues to change our understanding of chaperonin-mediated protein folding.

Adaptive Posttranslational Control in Cellular Stress Response Pathways and Its Relationship to Toxicity Testing and Safety Assessment

Although transcriptional induction of stress genes constitutes a major cellular defense program against a variety of stressors, posttranslational control directly regulating the activities of preexisting stress proteins provides a faster-acting alternative response. We propose that posttranslational control is a general adaptive mechanism operating in many stress pathways. Here with the aid of computational models, we first show that posttranslational control fulfills two roles: (1) handling small, transient stresses quickly and (2) stabilizing the negative feedback transcriptional network. We then review the posttranslational control pathways for major stress responses-oxidative stress, metal stress, hyperosmotic stress, DNA damage, heat shock, and hypoxia. Posttranslational regulation of stress protein activities occurs by reversible covalent modifications, allosteric or non-allosteric enzymatic regulations, and physically induced protein structural changes. Acting in feedback or feedforward networks, posttranslational control may establish a threshold level of cellular stress. Sub-threshold stresses are handled adequately by posttranslational control without invoking gene transcription. With supra-threshold stress levels, cellular homeostasis cannot be maintained and transcriptional induction of stress genes and other gene programs, eg, those regulating cell metabolism, proliferation, and apoptosis, takes place. The loss of homeostasis with consequent changes in cellular function may lead to adverse cellular outcomes. Overall, posttranslational and transcriptional control pathways constitute a stratified cellular defense system, handling stresses coherently across time and intensity. As cell-based assays become a focus for chemical testing anchored on toxicity pathways, examination of proteomic and metabolomic changes as a result of posttranslational control occurring in the absence of transcriptomic alterations deserves more attention.

Chaperonin GroEL uses asymmetric and symmetric reaction cycles in response to the concentration of non-native substrate proteins

The Escherichia coli chaperonin GroEL is an essential molecular chaperone that mediates protein folding in association with its cofactor, GroES. It is widely accepted that GroEL alternates the GroES-sealed folding-active rings during the reaction cycle. In other words, an asymmetric GroEL–GroES complex is formed during the cycle, whereas a symmetric GroEL–(GroES)₂ complex is not formed. However, this conventional view has been challenged by the recent reports indicating that such symmetric complexes can be formed in the GroEL–GroES reaction cycle. In this review, we discuss the studies of the symmetric GroEL–(GroES)₂ complex, focusing on the molecular mechanism underlying its formation. We also suggest that GroEL can be involved in two types of reaction cycles (asymmetric or symmetric) and the type of cycle used depends on the concentration of non-native substrate proteins.

Reaction Cycle of Chaperonin GroEL via Symmetric "Football" Intermediate

Chaperonin GroEL is an essential chaperone that assists in protein folding in the cell. Since one GroEL ring binds one GroES heptamer, the GroEL double ring permits the formation of two types of GroEL:GroES complexes: asymmetric 1:1 "bullet"-shaped and symmetric 1:2 "football"-shaped GroEL:GroES2 complexes. There have been continuing debates about the mechanism and which complex is critical to the chaperonin-assisted folding. In this review, I summarize the recent progress on the football complex.

Transcriptional profiles of Burkholderia pseudomallei reveal the direct and indirect roles of Sigma E under oxidative stress conditions

Background

Burkholderia pseudomallei, the causative agent of melioidosis, is a Gram-negative bacterium widely distributed in soil and water in endemic areas. This soil saprophyte can survive harsh environmental conditions, even in soils where herbicides (containing superoxide generators) are abundant. Sigma factor E (σ^E) is a key regulator of extra-cytoplasmic stress response in Gram-negative bacteria. In this study, we identified the B. pseudomallei σ^E regulon and characterized the indirect role that σ^E plays in the regulation of spermidine, contributing to the successful survival of B. pseudomallei in stressful environments.

Results

Changes in the global transcriptional profiles of B. pseudomallei wild type and σ^E mutant under physiological and oxidative stress (hydrogen peroxide) conditions were determined. We identified 307 up-regulated genes under oxidative stress condition. Comparison of the transcriptional profiles of B. pseudomallei wild type and σ^E mutant under control or oxidative stress conditions identified 85 oxidative-responsive genes regulated by σ^E, including genes involved in cell membrane repair, maintenance of protein folding and oxidative stress response and potential virulence factors such as a type VI secretion system (T6SS). Importantly, we identified that the speG gene, encoding spermidine-acetyltransferase, is a novel member of the B. pseudomallei σ^E regulon. The expression of speG was regulated by σ^E, implying that σ^E plays an indirect role in the regulation of physiological level of spermidine to protect the bacteria during oxidative stress.

Conclusion

This study identified B. pseudomallei genes directly regulated by σ^E in response to oxidative stress and revealed the indirect role of σ^E in the regulation of the polyamine spermidine (via regulation of speG) for bacterial cell protection during oxidative stress. This study provides new insights into the regulatory mechanisms by which σ^E contributes to the survival of B. pseudomallei under stressful conditions.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-787) contains supplementary material, which is available to authorized users.

Crystal structure of a symmetric football-shaped GroEL:GroES2-ATP14 complex determined at 3.8Å reveals rearrangement between two GroEL rings

The chaperonin GroEL is an essential chaperone that assists in protein folding with the aid of GroES and ATP. GroEL forms a double-ring structure, and both rings can bind GroES in the presence of ATP. Recent progress on the GroEL mechanism has revealed the importance of a symmetric 1:2 GroEL:GroES2 complex (the "football"-shaped complex) as a critical intermediate during the functional GroEL cycle. We determined the crystal structure of the football GroEL:GroES2-ATP14 complex from Escherichia coli at 3.8Å, using a GroEL mutant that is extremely defective in ATP hydrolysis. The overall structure of the football complex resembled the GroES-bound GroEL ring of the asymmetric 1:1 GroEL:GroES complex (the "bullet" complex). However, the two GroES-bound GroEL rings form a modified interface by an ~7° rotation about the 7-fold axis. As a result, the inter-ring contacts between the two GroEL rings in the football complex differed from those in the bullet complex. The differences provide a structural basis for the apparently impaired inter-ring negative cooperativity observed in several biochemical analyses.

Changes in the proteome of the cadmium-tolerant bacteria Cupriavidus taiwanensis KKU2500-3 in response to cadmium toxicity

Cupriavidus taiwanensis KKU2500-3 is a cadmium (Cd)-tolerant bacterial strain that was previously isolated from rice fields contaminated with high levels of Cd. In 500 μmol/L CdCl2, the KKU2500-3 strain grew slower and with a more prolonged lag-phase than when grown in the absence of Cd. A proteomic approach was used to characterize the protein expression in the Cd-tolerant bacteria C. taiwanensis KKU2500-3 during growth under Cd stress. When compared with the untreated cells, a total of 982 differentially expressed protein spots were observed in the CdCl2-treated cells, and 59 and 10 spots exhibited >2- and >4-fold changes, respectively. The level of up- and downregulation varied from 2.01- to 11.26-fold and from 2.01- to 5.34-fold, respectively. Of the 33 differentially expressed protein spots analyzed by MALDI TOF MS/MS, 19 spots were successfully identified, many of which were involved in stress responses. The most highly upregulated protein (+7.95-fold) identified was the chaperone GroEL, which indicated that this factor likely contributed to the bacterial survival and growth in response to Cd toxicity. Detection of the downregulated protein flagellin (–3.52-fold) was consistent with the less effective ATP-mediated and flagella-driven motility. The flagella-losing cells were also observed in the Cd-treated bacteria when analyzed by scanning electron microscopy. Thus, the Cd-stressed cells may downregulate pathways involving ATP utilization in favor of other mechanisms in response to Cd toxicity. When the KKU2500-3 strain was grown in the presence of Cd, H2S was not detected, suggesting a possible role of the sulfur in precipitation with Cd. Apart from a general response, no specific process could be determined using the present proteomic approach. However, the potential role of protein folding-mediated GroEL, flagella-mediated motility and CdS biotransformation in Cd toxicity response observed in this study as well as the extent of Cd-tolerant mechanisms using other methods could facilitate the future application of this strain in addressing Cd environmental contamination.

Identifying protein stabilizing ligands using GroEL

Over the past 5 years, it has become increasingly apparent to researchers that the initial promise and excitement of using gene replacement therapies to ameliorate folding diseases are still far from being broadly or easily applicable. Because a large number of human diseases are protein folding diseases (approximately 30 to 50%), many researchers now realize that more directed approaches to target and reverse the fundamental misfolding reactions preceding disease are highly feasible and offer the potential of developing more targeted drug therapies. This is also true with a large number of so called orphan protein folding diseases. The development of a broad-based general screening array method using the chaperonin as a detection platform will enable us to screen large chemical combinatorial libraries for specific ligands against the elusive transient, primary reactions that often lead to protein misfolding. This development will provide a highly desirable tool for the pharmaceutical, academic, and medical professions.

Enterobactin synthetase-catalyzed formation of P(1),P(3)-diadenosine-5'-tetraphosphate

The EntE enzyme, involved in the synthesis of the iron siderophore enterobactin, catalyzes the adenylation of 2,3-dihydroxybenzoic acid, followed by its transfer to the phosphopantetheine arm of holo-EntB, an aryl carrier protein. In the absence of EntB, EntE catalyzes the formation of Ap(4)A, a molecule that is implicated in regulating cell division during oxidative stress. We propose that the expression of EntE during iron starvation produces Ap(4)A to slow growth until intracellular iron stores can be restored.

Posttranslational modulation on the biological activities of molecular chaperones

Molecular chaperones are a family of proteins that were first noticed to exist about 45 years ago from their increased transcription under heat shock conditions. As a result, the regulation of their encoding genes has been subject to extensive studies. Recent studies revealed that the biological activities of molecular chaperones can also be effectively modulated at the protein level. The ways of modulation so far elucidated include allosteric effect, covalent modification, protein-protein interaction, and conformational alteration induced by such macro-environmental conditions as temperature and pH. These latter aspects were reviewed here. Emphasized here is the importance of such immediate structural alterations that lead to an immediate activity increase, providing the immediate protection needed for the cells to survive the stress conditions.

Co-expression of chaperonin GroEL/GroES enhances in vivo folding of yeast mitochondrial aconitase and alters the growth characteristics of Escherichia coli

Over last two decades many researchers have demonstrated the mechanisms of how the Escherichia coli chaperonin GroEL and GroES work in the binding and folding of different aggregation prone substrate proteins both in vivo and in vitro. However, preliminary aspects, such as influence of co-expressing GroEL and GroES on the over expression of other recombinant proteins in E. coli cells and subsequent growth aspects, as well as the conditions for optimum production of recombinant proteins in presence of recombinant chaperones have not been properly investigated. In the present study we have demonstrated the temperature dependent growth characteristics of E. coli cells, which are over expressing recombinant aconitase and how the co-expression of E. coli chaperonin GroEL and GroES influence the growth rate of the cells and in vivo folding of recombinant aconitase. Presence of co-expressed GroEL reduces the aconitase over-expression drastically; however, exogenous GroEL & GroES together compensate this reduction. For the aconitase over-expressing cells the growth rate decreases by 30% at 25 degrees C when compared with the M15 E. coli cells, however, there is an increase of 20% at 37 degrees C indicating the participation of endogenous chaperonin in the folding of a fraction of over expressed aconitase. However, in presence of co-expressed GroEL and GroES the growth rate of aconitase producing cells was enhanced by 30% at 37 degrees C confirming the assistance of exogenous chaperone system for the folding of recombinant aconitase. Optimum in vivo folding of aconitase requires co-production of complete E. coli chaperonin machinery GroEL and GroES together.

Multiple Structural Transitions of the GroEL Subunit Are Sensitive to Intermolecular Interactions with Cochaperonin and Refolding Polypeptide

In this study we attempted to determine the specific roles of the numerous conformational changes that are observed in the bacterial chaperonin GroEL, by performing stopped-flow experiments on GroEL R231W in the presence of a refolding substrate protein. The apparent rate of one kinetic phase was decreased by approximately 25% in the presence of prebound unfolded malate dehydrogenase while another phase was suppressed completely under the same conditions, reflecting different effects of the unfolded protein on multiple structural transitions within GroEL. The addition of cochaperonin GroES counteracts the effect of the bound substrate protein in the former case, but had no effect on the latter, more extensive suppression. Using a chemically modified form of GroEL R231W which is incapable of releasing substrate proteins at low temperatures, we identified a conformational transition that is implicated in the release of substrate proteins. Parts of the actual process of substrate protein release were also observed through fluorescence resonance energy transfer experiments involving GroEL and labeled substrate protein. Analysis of the energy transfer data revealed an interesting relationship between substrate protein displacement and a specific structural transition in the GroEL apical domain.

Beyond transcription--new mechanisms for the regulation of molecular chaperones

Molecular chaperones are an essential part of the universal heat shock response that allows organisms to survive stress conditions that cause intracellular protein unfolding. During the past few years, two new mechanisms have been found to control the activity of several chaperones under stress conditions-the regulation of chaperone activity by the redox state and by the temperature of the environment. Hsp33, for example, is redox-regulated. Hsp33 is specifically activated by disulfide bond formation during oxidative stress, where it becomes a highly efficient chaperone holdase that binds tightly to unfolding proteins. Certain small heat shock proteins, such as Hsp26 and Hsp16.9, on the other hand, are temperature regulated. Exposure to heat shock temperatures causes these oligomeric proteins to disassemble, thereby changing them into highly efficient chaperones. The ATP-dependent chaperone folding system DnaK/DnaJ/GrpE also appears to be temperature regulated, switching from a folding to a holding mode during heat stress. Both of these novel post-translational regulatory strategies appear to have one ultimate goal: to significantly increase the substrate binding affinity of the affected chaperones under exactly those stress conditions that require their highest chaperone activity. This ensures that protein folding intermediates remain bound to the chaperones under stress conditions and are released only after the cells return to non-stress conditions.

Adaptive evolution in GroEL from distantly related endosymbiotic bacteria of insects

Many symbioses between bacteria and insects resulted from ancient infections followed by strict vertical transmission within host lineages. The strong bottlenecks under which this transmission occurs promote the neutral fixation of slightly deleterious mutations by genetic drift. As predicted by Muller's ratchet, this fixation will drive endosymbiotic bacteria through an irreversible dynamics of fitness loss. The chaperonin GroEL has been proposed as a compensatory mechanism whereby endosymbiotic bacteria of aphids persist. Here, we show that endosymbiotic bacteria of insects from two phylogenetically very distant bacterial phyla have fixed amino acid substitutions by positive selection in functionally important GroEL regions involved in either GroES/peptide binding or in the en bloc movement of the GroEL apical domain. These results, together with the high levels of constitutive expression of GroEL in these endosymbionts, provide valuable insights into the evolution of a molecular mechanism responsible for the maintenance of the symbiotic lifestyle.

Functional consequences of single:double ring transitions in chaperonins: life in the cold

The cpn60 and cpn10 genes from psychrophilic bacterium, Oleispira antarctica RB8, showed a positive effect in Escherichia coli growth at low temperature, shifting its theoretical minimal growth temperature from +7.5 degrees C to -13.7 degrees C [Ferrer, M., Chernikova, T.N., Yakimov, M., Golyshin, P.N., and Timmis, K.N. (2003) Nature Biotechnol 21: 1266-1267]. To provide experimental support for this finding, Cpn60 and 10 were overproduced in E. coli and purified to apparent homogeneity. Recombinant O.Cpn60 was identical to the native protein based on tetradecameric structure, and it dissociates during native PAGE. Gel filtration and native PAGE revealed that, in vivo and in vitro, (O.Cpn60)(7) was the active oligomer at 4-10 degrees C, whereas at > 10 degrees C, this complex was converted to (O.Cpn60)(14). The dissociation reduces the ATP consumption (energy-saving mechanism) and increases the refolding capacity at low temperatures. In order for this transition to occur, we demonstrated that K468 and S471 may play a key role in conforming the more advantageous oligomeric state in O.Cpn60. We have proved this hypothesis by showing that single and double mutations in K468 and S471 for T and G, as in E.GroEL, produced a more stable double-ring oligomer. The optimum temperature for ATPase and chaperone activity for the wild-type chaperonin was 24-28 degrees C and 4-18 degrees C, whereas that for the mutants was 45-55 degrees C and 14-36 degrees C respectively. The temperature inducing unfolding (T(M)) increased from 45 degrees C to more than 65 degrees C. In contrast, a single ring mutant, O.Cpn60(SR), with three amino acid substitutions (E461A, S463A and V464A) was as stable as the wild type but possessed refolding activity below 10 degrees C. Above 10 degrees C, this complex lost refolding capacity to the detriment of the double ring, which was not an efficient chaperone at 4 degrees C as the single ring variant. We demonstrated that expression of O.Cpn60(WT) and O.Cpn60(SR) leads to a higher growth of E. coli at 4 degrees C ( micro (max), 0.22 and 0.36 h(-1) respectively), whereas at 10-15 degrees C, only E. coli cells expressing O.Cpn60 or O.Cpn60(DR) grew better than parental cells (-cpn). These results clearly indicate that the single-to-double ring transition in Oleispira chaperonin is a wild-type mechanism for its thermal acclimation. Although previous studies have also reported single-to-double ring transitions under many circumstances, this is the first clear indication that single-ring chaperonins are necessary to support growth when the temperature falls from 37 degrees C to 4 degrees C.

Identification of a major inter-ring coupling step in the GroEL reaction cycle

It has been shown previously that the double-ring structure of GroEL can be converted to a single-ring species by site-directed amino acid replacements at the ring interface and that the resultant molecule retains many of the crucial chaperonin properties; it is structurally stable, hydrolytically active, and can bind both the co-chaperonin, GroES, and unfolded substrate proteins. By comparing the behavior of the double- and single-ring structures in response to nucleotide binding and hydrolysis, we elucidate steps in the ATP-driven reaction cycle at which there is conformational coupling between the rings. Remarkably, the parting of the rings has little effect either on the thermodynamic properties of ATP binding or on the ATP-induced conformational changes prior to hydrolysis. However, there is a marked effect on the rate-limiting process in the steady-state cycle; a step that is coincident with bond cleavage in ATP. The effect of the ring-ring interaction is to increase its activation enthalpy from 42.0 to 94.2 kJ/mol. These results show that the major conformational coupling step, where structural rearrangements in one ring are propagated to the other, is the slowest process the ATPase cycle of GroEL.

Chaperonin-mediated stabilization and ATP-triggered release of semiconductor nanoparticles

Various properties of semiconductor nanoparticles, including photoluminescence and catalytic activity, make these materials attractive for a range of applications. As nanoparticles readily coagulate and so lose their size-dependent properties, shape-persistent three-dimensional stabilizers that enfold nanoparticles have been exploited. However, such wrapping approaches also make the nanoparticles insensitive to external stimuli, and so may limit their application. The chaperonin proteins GroEL (from Escherichia coli) and T.th ('T.th cpn', from Thermus thermophilus HB8) encapsulate denatured proteins inside a cylindrical cavity; after refolding, the encapsulated proteins are released by the action of ATP inducing a conformational change of the cavity. Here we report that GroEL and T.th cpn can also enfold CdS semiconductor nanoparticles, giving them high thermal and chemical stability in aqueous media. Analogous to the biological function of the chaperonins, the nanoparticles can be readily released from the protein cavities by the action of ATP. We expect that integration of such biological mechanisms into materials science will open a door to conceptually new bioresponsive devices.

Mitochondrial Hsp60, resistance to oxidative stress, and the labile iron pool are closely connected in Saccharomyces cerevisiae

In the present study, we have analyzed the role of the molecular chaperone Hsp60 in protection of Saccharomyces cerevisiae against oxidative damage. We constructed mutant strains in which the levels of Hsp60 protein, compared with wild-type cells, were four times greater, and the addition of doxycycline gradually reduces them to 20% of wild-type. Under oxidative-stress conditions, the progressive decrease in Hsp60 levels in these mutants resulted in reduced cell viability and an increase in both cell peroxide species and protein carbonyl content. Protection of Fe/S-containing enzymes from oxidative inactivation was found to be dose-dependent with respect to Hsp60 levels. As these enzymes release their iron ions under oxidative-stress conditions, the intracellular labile iron pool, monitored with calcein, was higher in cells with reduced Hsp60 levels. Consistently, the iron chelator deferoxamine protected low Hsp60-expressing cells from both oxidant-induced death and protein oxidation. These results indicate that the role of Hsp60 in oxidative-stress defense is explained by protection of several Fe/S proteins, which prevent the release of iron ions and thereby avert further damage.

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.

Microbial molecular chaperones

Protein folding in the cell, long thought to be a spontaneous process, in fact often requires the assistance of molecular chaperones. This is thought to be largely because of the danger of incorrect folding and aggregation of proteins, which is a particular problem in the crowded environment of the cell. Molecular chaperones are involved in numerous processes in bacterial cells, including assisting the folding of newly synthesized proteins, both during and after translation; assisting in protein secretion, preventing aggregation of proteins on heat shock, and repairing proteins that have been damaged or misfolded by stresses such as a heat shock. Within the cell, a balance has to be found between refolding of proteins and their proteolytic degradation, and molecular chaperones play a key role in this. In this review, the evidence for the existence and role of the major cytoplasmic molecular chaperones will be discussed, mainly from the physiological point of view but also in relationship to their known structure, function and mechanism of action. The two major chaperone systems in bacterial cells (as typified by Escherichia coli) are the GroE and DnaK chaperones, and the contrasting roles and mechanisms of these chaperones will be presented. The GroE chaperone machine acts by providing a protected environment in which protein folding of individual protein molecules can proceed, whereas the DnaK chaperones act by binding and protecting exposed regions on unfolded or partially folded protein chains. DnaK chaperones interact with trigger factor in protein translation and with ClpB in reactivating proteins which have become aggregated after heat shock. The nature of the other cytoplasmic chaperones in the cell will also be reviewed, including those for which a clear function has not yet been determined, and those where an in vivo chaperone function has still to be proven, such as the small heat shock proteins IbpA and IbpB. The regulation of expression of the genes of the heat shock response will also be discussed, particularly in the light of the signals that are needed to induce the response. The major signals for induction of the heat shock response are elevated temperature and the presence of unfolded protein within the cell, but these are sensed and transduced differently by different bacteria. The best characterized example is the sigma 32 subunit of RNA polymerase from E. coli, which is both more efficiently translated and also transiently stabilized following heat shock. The DnaK chaperones modulate this effect. However, a more widely conserved system appears to be typified by the HrcA repressor in Bacillus subtilis, the activity of which is modulated by the GroE chaperone machine. Other examples of regulation of molecular chaperones will also be discussed. Finally, the likely future research directions for molecular chaperone biology in the post-genomic era will be briefly evaluated.

Monitoring intracellular levels of XylR in Pseudomonas putida with a single-chain antibody specific for aromatic-responsive enhancer-binding proteins

We have isolated a recombinant phage antibody (Phab) that binds a distinct epitope of the subclass of the sigma(54)-dependent prokaryotic enhancer-binding proteins that respond directly to aromatic effectors, e.g., those that activate biodegradative operons of Pseudomonas spp. The DNA segments encoding the variable (V) domains of the immunoglobulins expressed by mice immunized with the C-terminal half of TouR (TouRDeltaA) of Pseudomonas stutzeri OX1 were amplified and rearranged in vitro as single-chain Fv (scFv) genes. An scFv library was thereby constructed, expressed in an M13 display system, and subjected to a panning procedure with TouR. One clone (named B7) was selected with high affinity for TouR and XylR (the regulator of the upper TOL operon of the pWW0 plasmid). The epitope recognized by this Phab was mapped to the peptide TPRAQATLLRVL, which seems to be characteristic of the group of enhancer-binding proteins to which TouR and XylR belong and which is located adjacent to the Walker B motif of the proteins. The Phab B7 was instrumental in measuring directly the intracellular levels of XylR expressed from its natural promoter in monocopy gene dosage in Pseudomonas putida under various conditions. Growth stage, the physical form of the protein produced (XylR or XylRDeltaA), and the presence or absence of aromatic inducers in the medium influenced the intracellular pool of these molecules. XylR oscillated from a minimum of approximately 30 molecules (monomers) per cell during exponential phase to approximately140 molecules per cell at stationary phase. Activation of XylR by aromatic inducers decreased the intracellular concentration of the regulator. The levels of the constitutively active variant of XylR named XylRDeltaA were higher, fluctuating between approximately 90 and approximately 570 molecules per cell, depending on the growth stage. These results are compatible with the present model of transcriptional autoregulation of XylR and suggest the existence of mechanisms controlling the stability of XylR protein in vivo.

Crystal structure of chaperonin-60 from Paracoccus denitrificans

The crystal structure of chaperonin-60 from Paracoccus denitrificans (P.cpn60) has been determined at 3.2 A resolution by the molecular replacement method. Two heptameric rings of identical subunits of P.cpn60 in adjacent asymmetric units are stacked in a back-to-back manner and form a cylinder, as found in GroEL, cpn60 from Escherichia coli. With respect to the unliganded GroEL structure, each subunit of P.cpn60 tilts 2 degrees outwards and the apical domain twists 4 degrees counter-clockwise in the top view in a hinge-like manner, rendering the central hole 5 A wider. Despite the subunit tilts, both rings in P.cpn60 contact at two sites of the equatorial domain in the same way as in GroEL. Interactions between residues 434 and 434, and 463 and 463 observed in GroEL were not found in P.cpn60, and the interaction between 452 and 461 was weaker in P.cpn60 than in GroEL. The unique hydrogen bond between 468 and 471 was observed at the right site in P.cpn60, which could account for why the subunits tilt outwards. The contact surface area was reduced at the left site, which is similar to the observed changes in the GroEL structures induced by ATP binding. In general, inter-ring interactions in P.cpn60 were weakened, which is consistent with findings that P.cpn60 is observed in single-ring forms as well as in double-ring forms.

Complex effects of molecular chaperones on the aggregation and refolding of fibroblast growth factor-1

Fibroblast growth factor one (FGF-1) exists in a molten globule (MG)-like state under physiological conditions (neutral pH, 37 degrees C). It has been proposed that this form of the protein may be involved in its atypical membrane transport properties. Macromolecular chaperones have been shown to bind to MG states of proteins as well as to be involved in protein membrane transport. We have therefore examined the effect of such proteins on the aggregation and refolding of FGF-1 to evaluate whether they might play a role in FGF-1 transport. The proposed chaperone alpha-crystallin was found to strongly inhibit the aggregation of the MG state of FGF-1. Curiously, two other proteins of similar size and charge (thyroglobulin and a monoclonal IgM immunoglobulin) with no previously reported chaperone properties were also found to have a related effect. In contrast, the chaperone GroEL/ES induced further aggregation of MG-like FGF-1 but had no effect on the native conformation. Both chaperones stimulated refolding to the native state (25 degrees C) but had no detectable effect when FGF-1 was refolded to the MG state (37 degrees C). This suggests that disordered intermediates are present in the folding pathways of the native and MG-like FGF conformations which differ from the MG-like state induced under physiological conditions. FGF-1 does, therefore, interact with molecular chaperones, although this may involve both the MG and the native states of the protein.

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.

Structural changes in GroEL effected by binding a denatured protein substrate

In the absence of nucleotides or cofactors, the Escherichia coli chaperonin GroEL binds select proteins in non-native conformations, such as denatured glutamine synthetase (GS) monomers, preventing their aggregation and spontaneous renaturation. The nature of the GroEL-GS complexes thus formed, specifically the effect on the conformation of the GroEL tetradecamer, has been examined by electron microscopy. We find that specimens of GroEL-GS are visibly heterogeneous, due to incomplete loading of GroEL with GS. Images contain particles indistinguishable from GroEL alone, and also those with consistent identifiable differences. Side-views of the modified particles reveal additional protein density at one end of the GroEL-GS complex, and end-views display chirality in the heptameric projection not seen in the unliganded GroEL. The coordinate appearance of these two projection differences suggests that binding of GS, as representative of a class of protein substrates, induces or stabilizes a conformation of GroEL that differs from the unliganded chaperonin. Three-dimensional reconstruction of the GroEL-GS complex reveals the location of the bound protein substrate, as well as complex conformational changes in GroEL itself, both cis and trans with respect to the bound GS. The most apparent structural alterations are inward movements of the apical domains of both GroEL heptamers, protrusion of the substrate protein from the cavity of the cis ring, and a narrowing of the unoccupied opening of the trans ring.

Excluded volume effects on the refolding and assembly of an oligomeric protein. GroEL, a case study

We have studied the effect of macromolecular crowding reagents, such as polysaccharides and bovine serum albumin, on the refolding of tetradecameric GroEL from urea-denatured protein monomers. The results show that productive refolding and assembly strongly depends on the presence of nucleotides (ATP or ADP) and background macromolecules. Nucleotides are required to generate an assembly-competent monomeric conformation, suggesting that proper folding of the equatorial domain of the protein subunits into a native-like structure is essential for productive assembly. Crowding modulates GroEL oligomerization in two different ways. First, it increases the tendency of refolded, monomeric GroEL to undergo self-association at equilibrium. Second, crowding can modify the relative rates of the two competing self-association reactions, namely, productive assembly into a native tetradecameric structure and unproductive aggregation. This kinetic effect is most likely exerted by modifications of the diffusion coefficient of the refolded monomers, which in turn determine the conformational properties of the interacting subunits. If they are allowed to become assembly-competent before self-association, productive oligomerization occurs; otherwise, unproductive aggregation takes place. Our data demonstrate that the spontaneous refolding and assembly of homo-oligomeric proteins, such as GroEL, can occur efficiently (70%) under crowding conditions similar to those expected in vivo.

Limits of protein folding inside GroE complexes

The GroE chaperones of Escherichia coli promote the folding of other proteins under conditions where no spontaneous folding occurs. One requirement for this reaction is the trapping of the nonnative protein inside the chaperone complex. Encapsulation may be important to prevent unfavorable intermolecular interactions during folding. We show here that, especially for oligomeric proteins, the timing of encapsulation and release is of critical importance. If this cycle is decelerated, misfolding is observed inside functional chaperone complexes.

The affinity of the GroEL/GroES complex for peptides under conditions of protein folding

The affinity of four short peptides for the Escherichia coli molecular chaperone GroEL was studied in the presence of the co-chaperone GroES and nucleotides. Our data show that binding of GroES to one ring enhances the interaction of the peptides with the opposite GroEL ring, a finding that was related to the structural readjustments in GroEL following GroES binding. We further report that the GroEL/GroES complex has a high affinity for peptides during ATP hydrolysis when protein substrates would undergo repeated cycles of assisted folding. Although we could not determine at which step(s) during the cycle our peptides interacted with GroEL, we propose that successive state changes in GroEL during ATP hydrolysis may create high affinity complexes and ensure maximum efficiency of the chaperone machinery under conditions of protein folding.

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.