A monomeric variant of GroEL binds nucleotides but is inactive as a molecular chaperone

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.

Endoplasmic reticulum chaperones and their roles in the immunogenicity of cancer vaccines

The endoplasmic reticulum (ER) is a major site of passage for proteins en route to other organelles, to the cell surface, and to the extracellular space. It is also the transport route for peptides generated in the cytosol by the proteasome into the ER for loading onto major histocompatibility complex class I (MHC I) molecules for eventual antigen presentation at the cell surface. Chaperones within the ER are critical for many of these processes; however, outside the ER certain of those chaperones may play important and direct roles in immune responses. In some cases, particular ER chaperones have been utilized as vaccines against tumors or infectious disease pathogens when purified from tumor tissue or recombinantly generated and loaded with antigen. In other cases, the cell surface location of ER chaperones has implications for immune responses as well as possible tumor resistance. We have produced heat-shock protein/chaperone protein-based cancer vaccines called “chaperone-rich cell lysate” (CRCL) that are conglomerates of chaperones enriched from solid tumors by an isoelectric focusing technique. These preparations have been effective against numerous murine tumors, as well as in a canine with an advanced lung carcinoma treated with autologous CRCL. We also published extensive proteomic analyses of CRCL prepared from human surgically resected tumor samples. Of note, these preparations contained at least 10 ER chaperones and a number of other residents, along with many other chaperones/heat-shock proteins. Gene ontology and network analyses utilizing these proteins essentially recapitulate the antigen presentation pathways and interconnections. In conjunction with our current knowledge of cell surface/extracellular ER chaperones, these data collectively suggest that a systems-level view may provide insight into the potent immune stimulatory activities of CRCL with an emphasis on the roles of ER components in those processes.

The unusual mycobacterial chaperonins: evidence for in vivo oligomerization and specialization of function

The pathogen Mycobacterium tuberculosis expresses two chaperonins, one (Cpn60.1) dispensable and one (Cpn60.2) essential. These proteins have been reported not to form oligomers despite the fact that oligomerization of chaperonins is regarded as essential for their function. We show here that the Cpn60.2 homologue from Mycobacterium smegmatis also fails to oligomerize under standard conditions. However, we also show that the Cpn60.2 proteins from both organisms can replace the essential groEL gene of Escherichia coli, and that they can function with E. coli GroES cochaperonin, as well as with their cognate cochaperonin proteins, strongly implying that they form oligomers in vivo. We show that the Cpn60.1 proteins, but not the Cpn60.2 proteins, can complement for loss of the M. smegmatis cpn60.1 gene. We investigated the oligomerization of the Cpn60.2 proteins using analytical ultracentrifugation and mass spectroscopy. Both form monomers under standard conditions, but they form higher order oligomers in the presence of kosmotropes and ADP or ATP. Under these conditions, their ATPase activity is significantly enhanced. We conclude that the essential mycobacterial chaperonins, while unstable compared to many other bacterial chaperonins, do act as oligomers in vivo, and that there has been specialization of function of the mycobacterial chaperonins following gene duplication.

The dimeric structure of the Cpn60.2 chaperonin of Mycobacterium tuberculosis at 2.8 Å reveals possible modes of function

Mycobacterium tuberculosis expresses two proteins (Cpn60.1 and Cpn60.2) that belong to the chaperonin (Cpn) family of heat shock proteins. Studies have shown that the two proteins have different functional roles in the bacterial life cycle and that Cpn60.2 is essential for cell viability and may be involved in M. tuberculosis pathogenicity. Cpn60.2 does not form a tetradecameric double ring, which is typical of other Cpns. We have determined the crystal structure of recombinant Cpn60.2 to 2.8 Å resolution by molecular replacement; the asymmetric unit (AU) contains a dimer, which is consistent with size-exclusion high-performance liquid chromatography and dynamic light-scattering measurements of the soluble recombinant protein. However, we suggest that the actual Cpn60.2 dimer may be different from that identified within the AU on the basis of surface contact stability, solvation free-energy gain, and functional aspects. Unlike the dimer found in the AU, which is formed through apical domain interactions, the dimeric form we propose here provides a free apical domain that is required for normal chaperone activity and may be involved in M. tuberculosis association with macrophages and arthrosclerosis plaque formation. Here we describe in detail the structural aspects that lead to Cpn60.2 dimer formation and prevent the formation of heptameric rings and tetradecameric double rings.

Conformational sampling and nucleotide-dependent transitions of the GroEL subunit probed by unbiased molecular dynamics simulations

GroEL is an ATP dependent molecular chaperone that promotes the folding of a large number of substrate proteins in E. coli. Large-scale conformational transitions occurring during the reaction cycle have been characterized from extensive crystallographic studies. However, the link between the observed conformations and the mechanisms involved in the allosteric response to ATP and the nucleotide-driven reaction cycle are not completely established. Here we describe extensive (in total long) unbiased molecular dynamics (MD) simulations that probe the response of GroEL subunits to ATP binding. We observe nucleotide dependent conformational transitions, and show with multiple 100 ns long simulations that the ligand-induced shift in the conformational populations are intrinsically coded in the structure-dynamics relationship of the protein subunit. Thus, these simulations reveal a stabilization of the equatorial domain upon nucleotide binding and a concomitant “opening” of the subunit, which reaches a conformation close to that observed in the crystal structure of the subunits within the ADP-bound oligomer. Moreover, we identify changes in a set of unique intrasubunit interactions potentially important for the conformational transition.

Molecular machines convert chemical energy to mechanical work in the process of carrying out their specific tasks. Often these proteins are fueled by ATP binding and hydrolysis, enabling switching between different conformations. The ATP-dependent chaperone GroEL is a molecular machine that opens and closes its barrel-like structure in order to provide a folding cage for unfolded proteins. The quest to fully understand and control GroEL and other molecular machines is enhanced by complementing experimental work with computational approaches. Here, we provide a description of the molecular basis for the conformational changes in the GroEL subunit by performing extensive molecular dynamics simulations. The simulations sample the conformational population for the different nucleotide-free and bound states in the isolated subunit. The results reveal that the conformations of the subunit when isolated resemble those of the subunit integrated in the GroEL complex. Moreover, the molecular dynamics simulations allow following detailed changes in individual interatomic interactions brought about by ATP-binding.

Cytosolic Hsp60 is involved in the NF-kappaB-dependent survival of cancer cells via IKK regulation

Cytoplasmic presence of Hsp60, which is principally a nuclear gene-encoded mitochondrial chaperonin, has frequently been stated, but its role in intracellular signaling is largely unknown. In this study, we demonstrate that the cytosolic Hsp60 promotes the TNF-alpha-mediated activation of the IKK/NF-kappaB survival pathway via direct interaction with IKKalpha/beta in the cytoplasm. Selective loss or blockade of cytosolic Hsp60 by specific antisense oligonucleotide or neutralizing antibody diminished the IKK/NF-kappaB activation and the expression of NF-kappaB target genes, such as Bfl-1/A1 and MnSOD, which thus augmented intracellular ROS production and ASK1-dependent cell death, in response to TNF-alpha. Conversely, the ectopic expression of cytosol-targeted Hsp60 enhanced IKK/NF-kappaB activation. Mechanistically, the cytosolic Hsp60 enhanced IKK activation via upregulating the activation-dependent serine phosphorylation in a chaperone-independent manner. Furthermore, transgenic mouse study showed that the cytosolic Hsp60 suppressed hepatic cell death induced by diethylnitrosamine in vivo. The cytosolic Hsp60 is likely to be a regulatory component of IKK complex and it implicates the first mitochondrial factor that regulates cell survival via NF-kappaB pathway.

A potentially versatile nucleotide hydrolysis activity of group II chaperonin monomers from Thermoplasma acidophilum

Compared to the group I chaperonins such as Escherichia coli GroEL, which facilitate protein folding, many aspects of the functional mechanism of archaeal group II chaperonins are still unclear. Here, we show that monomeric forms of archaeal group II chaperonin alpha and beta from Thermoplasma acidophilum may be purified stably and that these monomers display a strong AMPase activity in the presence of divalent ions, especially Co(2+) ion, in addition to ATPase and ADPase activities. Furthermore, other nucleoside phosphates (guanosine, cytidine, uridine, and inosine phosphates) in addition to adenine nucleotides were hydrolyzed. From analyses of the products of hydrolysis using HPLC, it was revealed that the monomeric chaperonin successively hydrolyzed the phosphoanhydride and phosphoester bonds of ATP in the order of gamma to alpha. This activity was strongly suppressed by point mutation of specific essential aspartic acid residues. Although these archaeal monomeric chaperonins did not alter the refolding of MDH, their novel versatile nucleotide hydrolysis activity might fulfill a new function. Western blot experiments demonstrated that the monomeric chaperonin subunits were also present in lysed cell extracts of T. acidophilum, and partially purified native monomer displayed Co(2+)-dependent AMPase activity.

The MitCHAP-60 disease is due to entropic destabilization of the human mitochondrial Hsp60 oligomer

The 60-kDa heat shock protein (mHsp60) is a vital cellular complex that mediates the folding of many of the mitochondrial proteins. Its function is executed in cooperation with the co-chaperonin, mHsp10, and requires ATP. Recently, the discovery of a new mHsp60-associated neurodegenerative disorder, MitCHAP-60 disease, has been reported. The disease is caused by a point mutation at position 3 (D3G) of the mature mitochondrial Hsp60 protein, which renders it unable to complement the deletion of the homologous bacterial protein in Escherichia coli (Magen, D., Georgopoulos, C., Bross, P., Ang, D., Segev, Y., Goldsher, D., Nemirovski, A., Shahar, E., Ravid, S., Luder, A., Heno, B., Gershoni-Baruch, R., Skorecki, K., and Mandel, H. (2008) Am. J. Hum. Genet. 83, 30-42). The molecular basis of the MitCHAP-60 disease is still unknown. In this study, we present an in vitro structural and functional analysis of the purified wild-type human mHsp60 and the MitCHAP-60 mutant. We show that the D3G mutation leads to destabilization of the mHsp60 oligomer and causes its disassembly at low protein concentrations. We also show that the mutant protein has impaired protein folding and ATPase activities. An additional mutant that lacks the first three amino acids (N-del), including Asp-3, is similarly impaired in refolding activity. Surprisingly, however, this mutant exhibits profound stabilization of its oligomeric structure. These results suggest that the D3G mutation leads to entropic destabilization of the mHsp60 oligomer, which severely impairs its chaperone function, thereby causing the disease.

Activation parameters for the spontaneous and pressure-induced phases of the dissociation of single-ring GroEL (SR1) chaperonin

We investigated the dissociation of single-ring heptameric GroEL (SR1) by high hydrostatic pressure in the range 0.5-3.0 kbar. The kinetics were studied as a function of temperature in the range 15-35 degrees C. The dissociation processes at each pressure and temperature showed biphasic behavior. The slower rate (k1,obs) was confirmed to be the self-dissociation of SR1 at any specific temperature at atmospheric pressure. This dissociation was pressure independent and followed concentration-dependent first-order kinetics. The self-dissociation rates followed normal Eyring plots (In k1,obs/T vs. 1/T) from which the free energy of activation (deltaG++ = 22 +/- 0.3 kcal mol(-1)), enthalpy of activation (deltaH++ = 18 +/- 0.5 kcal mol(-1)), and entropy of activation (deltaS++ = -15 +/- 1 kcal mol(-1)) were evaluated. The effect of pressure on the dissociation rates resulted in nonlinear behavior (ln k2,obs vs. pressure) at all the temperatures studied indicating that the activation volumes were pressure dependent. Activation volumes at zero pressure (V++o) and compressibility factors (beta++) for the dissociation rates at the specific temperatures were calculated. This is the first systematic study where the self-dissociation of an oligomeric chaperonin as well as its activation parameters are reported.

Mycobacterium tuberculosis GroEL homologues unusually exist as lower oligomers and retain the ability to suppress aggregation of substrate proteins

Chaperonin-60s are large double ring oligomeric proteins with a central cavity where unfolded polypeptides undergo productive folding. In conjunction with their co-chaperonin, Chaperonin-60s bind non-native polypeptides and facilitate their refolding in an ATP-dependent manner. The ATPase activity of Chaperonin-60 is tightly regulated by the 10 kDa co-chaperonin. In contrast to most other bacterial species, Mycobacterium tuberculosis genome carries a duplicate set of cpn60 genes, one of which occurs on the groESL operon (cpn60.1), while the other is separately arranged on the chromosome (cpn60.2). Biophysical characterization of the mycobacterial proteins showed that these proteins exist as lower oligomers and not tetradecamers, an unexpected property much different from the other known Chaperonin-60s. Failure of the M.tuberculosis chaperonins to oligomerize can be attributed to amino acid mutations at the oligomeric interface. Rates of ATP hydrolysis of the M.tuberculosis chaperonins showed that these proteins possess a very weak ATPase activity. Both the M.tuberculosis chaperonins were partially active in refolding substrate proteins. Interestingly, their refolding activity was seen to be independent of the co-chaperonin and ATP. We hypothesize that the ATP independent chaperones might offer benefit to the pathogen by promoting its existence in the latent phase of its life cycle.

1,1'-bis(anilino)-4-,4'-bis(naphtalene)-8,8'-disulfonate acts as an inhibitor of lipoprotein lipase and competes for binding with apolipoprotein CII

Lipoprotein lipase (LPL) is dependent on apolipoprotein CII (apoCII), a component of plasma lipoproteins, for function in vivo. The hydrophobic fluorescent probe 1,1'-bis(anilino)-4,4'-bis(naphthalene)-8,8'-disulfonate (bis-ANS) was found to be a potent inhibitor of LPL. ApoCII prevented the inhibition by bis-ANS, and was also able to restore the activity of inhibited LPL in a competitive manner, but only with triacylglycerols with acyl chains longer than three carbons. Studies of fluorescence and surface plasmon resonance indicated that LPL has an exposed hydrophobic site for binding of bis-ANS. The high affinity interaction was characterized by an equilibrium constant Kd of 0.10-0.26 microm and by a relatively high on rate constant kass = 2.0 x 10(4) m(-1) s(-1) and a slow off-rate with a dissociation rate constant kdiss = 1.2 x 10(-4) s(-1). The high affinity binding of bis-ANS did not influence interaction of LPL with heparin or with lipid/water interfaces and did not dissociate the active LPL dimer into monomers. Analysis of fragments of LPL after photoincorporation of bis-ANS indicated that the high affinity binding site was located in the middle part of the N-terminal folding domain. We propose that bis-ANS binds to an exposed hydrophobic area that is located close to the active site. This area may be the binding site for individual substrate molecules and also for apoCII.

3-Mercaptopyruvate sulfurtransferase of Leishmania contains an unusual C-terminal extension and is involved in thioredoxin and antioxidant metabolism

Cytosolic 3-mercaptopyruvate sulfurtransferases (EC ) of Leishmania major and Leishmania mexicana have been cloned, expressed as active enzymes in Escherichia coli, and characterized. The leishmanial single-copy genes predict a sulfurtransferase that is structurally peculiar in possessing a C-terminal domain of some 70 amino acids. Homologous genes of Trypanosoma cruzi and Trypanosoma brucei encode enzymes with a similar C-terminal domain, suggesting that this feature, not known in any other sulfurtransferase, is a characteristic of trypanosomatid parasites. Short truncations of the C-terminal domain resulted in misfolded inactive proteins, demonstrating that the domain plays some key role in facilitating correct folding of the enzymes. The leishmanial recombinant enzymes exhibited high activity toward 3-mercaptopyruvate and catalyzed the transfer of sulfane sulfur to cyanide to form thiocyanate. They also used thiosulfate as a substrate and reduced thioredoxin as the accepting nucleophile, the latter being oxidized. The enzymes were expressed in all life cycle stages, and the expression level was increased under peroxide or hypo-sulfur stress. The results are consistent with the enzymes having an involvement in the synthesis of sulfur amino acids per se or iron-sulfur centers of proteins and the parasite's management of oxidative stress.

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.

In vivo and in vitro function of GroEL mutants with impaired allosteric properties

Escherichia coli cells that produce only plasmid-encoded wild-type or mutant GroEL were generated by bacteriophage P1 transduction. Effects of mutations that affect the allosteric properties of GroEL were characterized in vivo. Cells containing only GroEL(R197A), which has reduced intra-ring positive cooperativity and inter-ring negative cooperativity in ATP binding, grow poorly upon a temperature shift from 25 to 42 degrees C. This strain supports the growth of phages T4 and T5 but not phage lambda and produces light at 28 degrees C when transformed with a second plasmid containing the lux operon. In contrast, cells containing only GroEL(R13G, A126V) which lacks negative cooperativity between rings but has intact intra-ring positive cooperativity grow normally and support phage growth but do not produce light at 28 degrees C. In vitro refolding of luciferase in the presence of this mutant is found to be less efficient compared with wild-type GroEL or other mutants tested. Our results show that allostery in GroEL is important in vivo in a manner that depends on the physiological conditions and is protein substrate specific.

The biological action of choriogonadotropin is not dependent on the complete native quaternary interactions between the subunits

Human CG (hCG) is a member of the glycoprotein hormone family characterized by a heterodimeric structure consisting of a common alpha-subunit noncovalently bound to a hormone-specific beta-subunit. The two subunits are highly intertwined and only the heterodimer is functional, implying that the quaternary structure is critical for biological activity. To assess the dependence of the bioactivity of hCG on the heterodimeric interactions, alpha- and beta-subunits bearing mutations that prevent assembly were covalently linked to form a single chain hCG. Receptor binding and signal transduction of these analogs were tested and their structural integrity analyzed using a panel of monoclonal antibodies (mAbs). These included dimer-specific mAbs, which react with at least four different epitope sites on the hormone, and some that react only with the free beta-subunit. We showed that there was significant loss of quaternary and tertiary structure in several regions of the molecule. This was most pronounced in single chains that had one of the disulfide bonds of the cystine knot disrupted in either the alpha- or beta-subunit. Despite these structural changes, the in vitro receptor binding and signal transduction of the single chain analogs were comparable to those of the nonmutated single chain, demonstrating that not all of the quaternary configuration of the hormone is necessary for biological activity.

Chaperone-mediated protein folding

The folding of most newly synthesized proteins in the cell requires the interaction of a variety of protein cofactors known as molecular chaperones. These molecules recognize and bind to nascent polypeptide chains and partially folded intermediates of proteins, preventing their aggregation and misfolding. There are several families of chaperones; those most involved in protein folding are the 40-kDa heat shock protein (HSP40; DnaJ), 60-kDa heat shock protein (HSP60; GroEL), and 70-kDa heat shock protein (HSP70; DnaK) families. The availability of high-resolution structures has facilitated a more detailed understanding of the complex chaperone machinery and mechanisms, including the ATP-dependent reaction cycles of the GroEL and HSP70 chaperones. For both of these chaperones, the binding of ATP triggers a critical conformational change leading to release of the bound substrate protein. Whereas the main role of the HSP70/HSP40 chaperone system is to minimize aggregation of newly synthesized proteins, the HSP60 chaperones also facilitate the actual folding process by providing a secluded environment for individual folding molecules and may also promote the unfolding and refolding of misfolded intermediates.

ATP hydrolysis induces an intermediate conformational state in GroEL

The conformational properties of the molecular chaperone GroEL in the presence of ATP, its non-hydrolyzable analog 5'-adenylimidodiphosphate (AMP-PNP), and ADP have been analyzed by differential scanning calorimetry (DSC), Fourier-transform infra-red (FT-IR) and fluorescence spectroscopy. Nucleotide binding to one ring promotes a decrease in the Tm value of the GroEL thermal transition that is reversed when both rings are filled with nucleotide, indicating that the sequential occupation of the two protein rings by these nucleotides has different effects on the GroEL thermal denaturation process. In addition, ATP induces a conformational change in GroEL characterized by (a) the appearance of a reversible low temperature endotherm in the DSC profiles of the protein, and (b) an enhanced binding of the hydrophobic probe 8-anilino-naphthalene-1-sulfonate (ANS), which strictly depends on ATP hydrolysis. The similar sensitivity to K+ of the temperature range where activation of the GroEL ATPase activity, the low temperature endotherm, and the increase of the ANS fluorescence are abserved strongly indicates the existence of a conformational state of GroEL during ATP hydrolysis, different from that generated on ADP or AMP-PNP binding. To achieve this intermediate conformation, GroEL mainly modifies its tertiary and quaternary structures, leading to an increased exposure of hydrophobic surfaces, with minor rearrangements of its secondary structure.

Stable expression and rapid purification of Escherichia coli GroEL and GroES chaperonins

An Escherichia coli expression vector pRE (P. Reddy, A. Peterkofsky, and K. McKenney, 1989, Nucleic Acids Res. 17, 10473-10488), originally developed for the cloning and expression of lethal genes, was used for cloning and hyperexpression of GroEL and GroES genes. Regulated gene expression is achieved in the pRE vector under the tight control of the lambda PL promoter. Upon induction of the promoter, stable expression of GroEL to about 60% of the total cell protein was observed. Similarly, stable expression of GroES to about 40% of the total cell protein was achieved. GroES was found to be a heat-stable protein while GroEL was not. Both GroE chaperonins were purified in a single chromatographic step with a yield of about 100 mg GroEL and 25 mg GroES per liter of E. coli culture. GroE chaperonins purified by the protocols described here were active in the renaturation of urea-denatured rhodanese.

Refolding chromatography with immobilized mini-chaperones

Mini-chaperones (e.g., a peptide consisting of residues 191-345 of GroEL) that are immobilized on agarose have very efficient chaperoning activity with several proteins that are otherwise recalcitrant to renaturation by conventional methods. We have used immobilized mini-chaperones both in column chromatography and batchwise to renature an insoluble protein from an inclusion body, to refold apparently irreversibly denatured proteins, and to recondition enzymes that have lost activity on storage. Refolding chromatography offers an efficient and simple means to renature proteins in high yield and with biological activity.

Chaperone activity and structure of monomeric polypeptide binding domains of GroEL

The chaperonin GroEL is a large complex composed of 14 identical 57-kDa subunits that requires ATP and GroES for some of its activities. We find that a monomeric polypeptide corresponding to residues 191 to 345 has the activity of the tetradecamer both in facilitating the refolding of rhodanese and cyclophilin A in the absence of ATP and in catalyzing the unfolding of native barnase. Its crystal structure, solved at 2.5 A resolution, shows a well-ordered domain with the same fold as in intact GroEL. We have thus isolated the active site of the complex allosteric molecular chaperone, which functions as a "minichaperone." This has mechanistic implications: the presence of a central cavity in the GroEL complex is not essential for those representative activities in vitro, and neither are the allosteric properties. The function of the allosteric behavior on the binding of GroES and ATP must be to regulate the affinity of the protein for its various substrates in vivo, where the cavity may also be required for special functions.