A comment on: 'The aromatic amino acid content of the bacterial chaperone protein groEL (cpn60): evidence for the presence of a single tryptophan', by N.C. Price, S.M. Kelly, S. Wood and A. auf der Mauer (1991) FEBS Lett. 292, 9-12

Back to GroEL-Assisted Protein Folding: GroES Binding-Induced Displacement of Denatured Proteins from GroEL to Bulk Solution

The main events in chaperone-assisted protein folding are the binding and ligand-induced release of substrate proteins. Here, we studied the location of denatured proteins previously bound to the GroEL chaperonin resulting from the action of the GroES co-chaperonin in the presence of Mg-ATP. Fluorescein-labeled denatured proteins (α-lactalbumin, lysozyme, serum albumin, and pepsin in the presence of thiol reagents at neutral pH, as well as an early refolding intermediate of malate dehydrogenase) were used to reveal the effect of GroES on their interaction with GroEL. Native electrophoresis has demonstrated that these proteins tend to be released from the GroEL-GroES complex. With the use of biotin- and fluorescein-labeled denatured proteins and streptavidin fused with luciferase aequorin (the so-called streptavidin trap), the presence of denatured proteins in bulk solution after GroES and Mg-ATP addition has been confirmed. The time of GroES-induced dissociation of a denatured protein from the GroEL surface was estimated using the stopped-flow technique and found to be much shorter than the proposed time of the GroEL ATPase cycle.

Dataset concerning GroEL chaperonin interaction with proteins

GroEL chaperonin is well-known to interact with a wide variety of polypeptide chains. Here we show the data related to our previous work (http://dx.doi.org/10.1016/j.pep.2015.11.020[1]), and concerning the interaction of GroEL with native (lysozyme, α-lactalbumin) and denatured (lysozyme, α-lactalbumin and pepsin) proteins in solution. The use of affinity chromatography on the base of denatured pepsin for GroEL purification from fluorescent impurities is represented as well.

Comparative cell signalling activity of ultrapure recombinant chaperonin 60 proteins from prokaryotes and eukaryotes

Heat-shock protein (hsp)60/chaperonin 60 is a potent immunogen which has recently been claimed to have cell-signalling actions upon myeloid and vascular endothelial cells. The literature is controversial with different chaperonin 60 proteins producing different patterns of cellular activation and the ever-present criticism that activity is the result of bacterial contaminants. To clarify the situation we have cloned, expressed and purified to homogeneity the chaperonin 60 proteins from Chlamydia pneumoniae, Helicobacter pylori and the human mitochondrion. These highly purified proteins were compared for their ability to stimulate human peripheral blood mononuclear cell (PBMC) cytokine synthesis and vascular endothelial cell adhesion protein expression. In spite of their significant sequence homology, the H. pylori protein was the most potent PBMC activator with the human protein the least potent. PBMC activation by C. pneumoniae and human, but not H. pylori, chaperonin 60 was blocked by antibody neutralization of Toll-like receptor-4. The C. pneumoniae chaperonin 60 was the most potent endothelial cell activator, with the human protein being significantly less active than bacterial chaperonin 60 proteins. These results have implications for the role of chaperonin 60 proteins as pathological factors in autoimmune and cardiovascular disease, and raise the possibility that each of these proteins may result in different pathological effects in such diseases.

Thermal inactivation and chaperonin-mediated renaturation of mitochondrial aspartate aminotransferase

Mitochondrial aspartate aminotransferase is inactivated irreversibly on heating. The inactivated protein aggregates, but aggregation is prevented by the presence of the chaperonin 60 from Escherichia coli (GroEL). The chaperonin increases the rate of thermal inactivation in the temperature range 55-65 degrees C but not at lower temperatures. It has previously been shown [Twomey and Doonan (1997) Biochim. Biophys. Acta 1342, 37-44] that the enzyme switches to a modified, but catalytically active, conformation at approx. 55-60 degrees C and the present results show that this conformation is recognized by and binds to GroEL. The thermally inactivated protein can be released from GroEL in an active form by the addition of chaperonin 10 from E. coli (GroES)/ATP, showing that inactivation is not the result of irreversible chemical changes. These results suggest that the irreversibility of thermal inactivation is due to the formation of an altered conformation with a high kinetic barrier to refolding rather than to any covalent changes. In the absence of chaperonin the unfolded molecules aggregate but this is a consequence, rather than the cause, of irreversible inactivation.

Homogeneous Escherichia coli Chaperonin 60 Induces IL-1β and IL-6 Gene Expression in Human Monocytes by a Mechanism Independent of Protein Conformation

Escherichia coli chaperonin (cpn) 60 (groEL) is a protein-folding oligomer lacking tryptophan residues that copurifies with tryptophan-containing proteins and peptides. Cpn 60 is a major immunogen in infectious diseases, and evidence suggests that groEL and mycobacterial cpn 60s can induce cytokine synthesis, stimulate cytokine-dependent bone resorption, and up-regulate expression of vascular endothelial cell adhesion molecules. Whether such activities are due to the cpn 60 or to the copurifying/contaminating proteins/peptides has not been determined. Here we report a method for removing the protein contaminants of groEL and demonstrate that this, essentially homogeneous, groEL remains a potent inducer of human monocyte IL-1β and IL-6 production. Contaminating peptides had no cytokine-inducing activity and did not synergize with purified groEL. The LPS inhibitor polymyxin B and the CD14-neutralizing Ab MY4 had no inhibitory action on groEL demonstrating that activity is not due to LPS contamination. Heating groEL had no effect on its capacity to stimulate human monocytes to secrete IL-6. Proteolysis of groEL with trypsin, sufficient to produce low molecular mass peptides, also had no inhibitory effect. Thus, we conclude that groEL is a potent inducer of monocyte proinflammatory cytokine production, which acts through the binding of nonconformational peptide domains that are conserved after proteolysis. These data suggest that if groEL was released from bacteria it could induce prolonged tissue pathology by virtue of its cytokine-inducing activity and its resistance to proteolytic inhibition of bioactivity.

Binding of a burst-phase intermediate formed in the folding of denatured D-glyceraldehyde-3-phosphate dehydrogenase by chaperonin 60 and 8-anilino-1-naphthalenesulphonic acid

Upon dilution, D-glyceraldehyde-3-phosphate dehydrogenase (GADPH) that has been fully inactivated, but only partially unfolded, in dilute guanidine hydrochloride (GuHCl) recovers activity completely. The fully unfolded enzyme, however, is re-activated only to a limited extent after dilution, and refolds rapidly in a burst phase to a partially folded intermediate characterized by increases in both the emission intensity of intrinsic fluorescence and binding to 8-anilino-1-naphthalenesulphonic acid (ANS). This intermediate aggregates with a time lag of a few minutes, and the aggregation can be suppressed completely by chaperonin 60 (GroEL). Stoichiometric analysis of the suppression of GAPDH re-activation by GroEL suggests that the tetradecameric GroEL binds to a dimeric GAPDH folding intermediate. This intermediate can be re-activated by ATP or ATP/chaperonin 10 (GroES) to an extent considerably greater than that obtained on spontaneous re-activation of the fully denatured enzyme upon dilution. Probing with a fluorescent derivative of NAD+ shows that this folding intermediate does not have a native conformation at the active site. The similar profiles of the effects of GroEL and ANS on the re-activation of GAPDH denatured by different concentrations of GuHCl suggest that GroEL and ANS recognize and bind to the same folding intermediate, which is similar to the relatively stable, partially unfolded, state of the enzyme denatured in 0.5-1.0 MGuHCl. However, the complexes of the intermediate with GroEL or ANS appear to be different, in that GroEL, but not ANS, suppresses aggregation and assists folding in the presence of ATP.

Thermodynamic stability and folding of GroEL minichaperones

The apical domain of GroEL (residues 191 to 376) and its C-terminally truncated fragment GroEL(191-345) are expressed with high yield in Escherichia coli to give functional monomeric minichaperones. Owing to the reversible folding behaviour of the minichaperones we can analyse the folding of the polypeptide binding domain of the multidomain GroEL protein, the folding of which is known to be irreversible. The apical domain shows two reversible temperature transitions with transition midpoints at 35 degrees C and at 67 degrees C that can be attributed to the unfolding of the C-terminal helices and the domain core, respectively. The native state of the domain core is stabilized by 5.5 kcal mol-1 relative to the unfolded state. The rate constant of folding of the apical domain core is independent of the minichaperone concentration and the presence of the C-terminal alpha-helices. A folding intermediate on the folding pathway is destabilized relative to the native state by 1.6 kcal mol-1, which is also detected by equilibrium and kinetic binding of the dye bis-ANS. Reversible folding of the polypeptide domain of GroEL guarantees highly efficient chaperonin activity within the GroEL toroid.

Inactive GroEL monomers can be isolated and reassembled to functional tetradecamers that contain few bound peptides

For the first time, it has been shown that GroEL can be converted from tetradecamers (14-mers) to monomers under conditions commonly used for the preparation of this chaperonin. The essential requirements are the simultaneous presence of nucleotides such as MgATP or MgADP and a solid-phase anion-exchange medium. The monomers that are formed are metastable in that they only reassemble to a small degree in the absence of additives. These results are in keeping with previous studies on high pressure dissociation that showed the separated monomers display conformational plasticity and can undergo conformational relaxation when relieved of the constraints of the quaternary structure in the oligomer (Gorovits, B., Raman, C. S., and Horowitz, P. M. (1995) J. Biol. Chem. 270, 2061-2066). The monomers display greatly enhanced hydrophobic exposure to the probe 1,1'-bis(4-anilino)naphthalene-5,5'-disulfonic acid, although they are not active in folding functions, and they are unable to form complexes with partially folded rhodanese. The monomers can be completely reassembled to 14-mers by incubation in 1 M ammonium sulfate. There is no evidence of intermediates in the reassembly process. Compared with the original oligomers, the reassembled 14-mers have (a) very low levels of polypeptide contaminants and tryptophan-like fluorescence, two problems that previously hampered spectroscopic studies of GroEL structure and function; (b) functional properties that are very similar to the original material; (c) considerably decreased hydrophobic exposure in the native state; and (d) a similar triggered exposure of hydrophobic surfaces after treatment with urea or spermidine. This study demonstrates that the quaternary structure of GroEL is more labile than previously thought. These results are consistent with suggestions that nucleotides can loosen subunit interactions and show that changes in quaternary structure can operate under conditions where GroEL function has been demonstrated.

Isolation and biochemical characterization of highly purified Escherichia coli molecular chaperone Cpn60 (GroEL) by affinity chromatography and urea-induced monomerization

Isolated Escherichia coli molecular chaperone Cpn60 (GroEL) has been further purified from tightly bound substrate polypeptides by two different procedures: (i) group-specific affinity chromatography by using the triazine dye Procion yellow HE-3G as affinity ligand, and (ii) urea-induced monomerization and subsequent chromatography. Procion yellow binds specifically to aromatic amino-acid side chains present in the majority of proteins, but has no affinity to GroEL because of its low content of aromatic residues. Some GroEL-bound polypeptides are buried within the aqueous cavity of the GroEL oligomer, whereas others are exposed on its surface and available for affinity-ligand interactions and the complex is thereby retarded on Procion yellow columns. Pure substrate-free GroEL was obtained after ion-exchange chromatography of GroEL monomers followed by reassembly of the purified monomers into functional GroEL oligomers. The final preparation contained no substrate polypeptides bound to GroEL as judged by electrophoretic analysis and lack of tryptophan fluorescence. GroEL preparations also displayed two equally strong bands on native electrophoresis suggesting the presence of two conformers. Monomers of GroEL showed heterogeneity with respect to isoelectric point and molecular mass when analysed by MALDI-MS and electrophoresis under native and denaturing conditions respectively. By use of MALDI-MS, highly accurate molecular masses of wild-type and a truncated form of GroEL were determined and verified, by comparison with their respective gene sequences.

Refolding and reassembly of active chaperonin GroEL after denaturation

Conditions are reported that, for the first time, permit the folding and assembly of active chaperonin, GroEL, following denaturation in 8 m urea. The folding could be achieved by dilution or dialysis, and the best yields required the simultaneous presence of ammonium sulfate and the Mg2+ complexes of ATP or ADP. Ammonium sulfate was the key to this particular protocol, since there was a small recovery of oligomer in its presence, but no detectable recovery was induced by ATP or ADP without ammonium sulfate. The refolded/reassembled GroEL could arrest the spontaneous folding of rhodanese, and it could participate in the chaperonin-assisted refolding of rhodanese as effectively as GroEL that had never been unfolded. The results demonstrate that the primary sequence of GroEL contains the information required for its folding, assembly, and function. Thus, in contrast to previous studies, although chaperonins may facilitate GroEL folding, they are not necessary for the acquisition of the functional oligomeric state of this chaperone. This ability to fold denatured GroEL in vitro will facilitate studies of the influences that determine the interesting folding pattern adopted by the native protein.

Tetradecameric chaperonin 60 can be assembled in vitro from monomers in a process that is ATP independent

The present work shows that monomers of cpn60 (groEL) formed at 2.5 M urea could be assembled to tetradecamers in a process that was independent of ATP. Reassembled cpn60 was able to assist the folding of urea unfolded rhodanese. When cpn60 was incubated at urea concentrations higher than 2.75 M, assembly of tetradecameric cpn60 did not occur after dialysis, and the presence of ATP did not stimulate the assembly process. The cpn60 used here did not display the previously reported ATP-dependent self-assembly of cpn60 monomers that required a higher urea concentration (4 M) for formation (Lissen et al. (1990) Nature 348, 339-342). Assembly and disassembly of cpn60 tetradecamers were followed as a function of the urea concentration by ultracentrifugation and gel electrophoresis in the presence of urea. The electrophoresis results demonstrate that there is rapid assembly of tetradecamers following preincubation and rapid removal of urea at concentrations lower than 2.5 M. Thus, previous methods monitored irreversible dissociation of cpn60, and the present results indicate that the cpn60 assembly requirements for ATP are dependent on pretreatment conditions.

The guanidine-induced conformational changes of the chaperonin GroEL from Escherichia coli. Evidence for the existence of an unfolding intermediate state

Equilibrium unfolding experiments of the E. coli chaperonin GroEL were performed in guanidine hydrochloride. A reversible unfolding intermediate was observed in very low concentrations of denaturant (< 0.5 M guanidine hydrochloride). This intermediate was characterized by a decreased light scattering intensity and an increased binding of the fluorescent probe 1-anilino-8-naphthalene sulfonate. No significant changes in circular dichroism spectra were observed for this unfolding intermediate. A second decrease in fluorescence intensity and light scattering was observed in higher concentrations of guanidine hydrochloride, with a transitional midpoint of 1.15 M. This transition was accompanied by the complete loss of secondary structure, as monitored by circular dichroism spectroscopy. This second transition agreed well with the results previously reported in this journal (Price et al. (1993) Biochim. Biophys. Acta 1161, 52-58).

Refolding and recognition of mitochondrial malate dehydrogenase by Escherichia coli chaperonins cpn 60 (groEL) and cpn10 (groES)

In vitro refolding of pig mitochondrial malate dehydrogenase is investigated in the presence of Escherichia coli chaperonins cpn60 (groEL) and cpn10 (groES). When the enzyme is initially denatured with 3 M guanidinium chloride, chaperonin-assisted refolding is 100% efficient. C.d. spectroscopy reveals that malate dehydrogenase is almost unfolded in 3 M guanidinium chloride, suggesting that a state with little or no residual secondary structure is the optimal 'substrate' for chaperonin-assisted refolding. Malate dehydrogenase denatured to more highly structured states proves to refold less efficiently with chaperonin assistance. The enzyme is shown not to aggregate under the refolding conditions, so that losses in refolding efficiency result from irreversible misfolding. Evidence is advanced to suggest that the chaperonins are unable to rescue irreversibly misfolded malate dehydrogenase. A novel use is made of 100 K Centricon concentrators to study the binding of [14C]acetyl-labelled malate dehydrogenase to groEL by an ultrafiltration binding assay. Analysis of the data by Scatchard plot shows that acetyl-malate dehydrogenase, which has previously been extensively unfolded with guanidinium chloride, binds to groEL at a specific binding site(s). At saturation, one acetyl-malate dehydrogenase homodimer (two polypeptides) is shown to bind to each groEL homooligomer with a binding constant of approx. 10 nM.

Purification and characterization of chaperonin 60 and chaperonin 10 from the anaerobic thermophile Thermoanaerobacter brockii

Chaperonin 60 and chaperonin 10 (GroEL and GroES homologues, respectively) have been isolated from extracts of the anaerobic thermophile Thermoanaerobacter brockii. A simple and rapid purification for chaperonin 60 made use of hydrophobic and anion-exchange chromatographies, and could be readily scaled up; approximately 2 mg pure chaperonin 60 was obtained/g cells. In contrast with all other prokaryotic chaperonin 60 proteins that have been studied, which are tetradecamers, including those from Thermus sp., the T. brockii protein is a heptamer, and as isolated was not in association with chaperonin 10. The preparation is readily crystallized using 2-propanol or poly(ethylene glycol) with MgCl2. The N-terminal amino acid sequence of this preparation is similar to other thermophilic chaperonin 60 proteins. Chaperonin 10 was purified from the flow-through of the first hydrophobic column (which bound chaperonin 60) using a more hydrophobic adsorbent to remove contaminating proteins, followed by anion-exchange chromatography. Chaperonin 10 was obtained with a yield of approximately 10% that of chaperonin 60. The subunit molecular mass of chaperonin 10 determined by electrospray mass spectrometry is 10254 +/- 0.4 Da, which is very similar to the molecular mass of Escherichia coli GroES. Similarly, the subunit size of chaperonin 60 determined by mass spectrometry is very similar to that of GroEL, at 57949 +/- 10 Da. T. brockii chaperonin 60 has an ATPase activity that is suppressed by chaperonin 10, and the two proteins together are active in protein-folding assays. Mitochondrial malate dehydrogenase was successfully refolded at 37 degrees C after denaturation in guanidine hydrochloride, using T. brockii chaperonin 60 and chaperonin 10, or chaperonin 60 and E. coli GroES. The denatured enzyme was protected from aggregation by association with chaperonin 60. Guanidine-hydrochloride-denatured preparations of isocitrate dehydrogenase and secondary alcohol dehydrogenase isolated from T. brockii were also refolded at 60-65 degrees C. In each case, refolding required chaperonin 60, chaperonin 10 and ATP, giving up to 80% regeneration of control activity.