The crystal structure of the bacterial chaperonin GroEL at 2.8 A

Revisiting the Anfinsen cage

In the past five years, ideas about protein folding inside cells have changed as a results of experiments with the chaperonin family of molecular chaperones. The folding of at least some proteins is no longer regarded as a spontaneous energy-independent process, but as involving transient interactions with chaperonin ATPases that serve to increase the efficiency of correct folding within the highly crowded intracellular environment. This review discusses in an historical context one model for how the chaperonins function. This model suggests that proteins fold inside cells in the same way as they do in pure dilute solution, but that they do so inside macromolecular Anfinsen cages that serve as sequestration devices to prevent and reverse unproductive interactions.

Cloning and characterization of the mitochondrial HSP60-encoding gene of Schizosaccharomyces pombe

We report the isolation and characterization of a gene (designated mcp60) encoding the mitochondrial (mt) 60-kDa heat-shock protein (HSP60) in the fission yeast Schizosaccharomyces pombe. The deduced amino-acid sequence (582 aa) of this gene is highly similar to the known mt HSP60 from diverse organisms. When its sequence was related to the known functional domains of bacterial HSP60 (GroEL), the similarity was particularly high for the intermediate domains that connect the apical domain with the equatorial domain. The mRNA level of mcp60 increased several-fold upon temperature upshift (from 25 to 35 degrees C), while gradually decreased during sporulation. Gene disruption experiments revealed that mcp60 is essential for cell viability at all temperatures.

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

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

Assembly of mitochondrial ATP synthase in cultured human cells: implications for mitochondrial diseases

To study the assembly of mitochondrial F1F0 ATP synthase, cultured human cells were labeled with [35S]methionine in pulse-chase experiments. Next, two-dimensional electrophoresis and fluorography were used to analyze the assembly pattern. Two assembly intermediates could be demonstrated. First the F1 part appeared to be assembled, and next an intermediate product that contained F1 and subunit c. This product probably also contained subunits b, F6 and OSCP, but not the mitochondrially encoded subunits a and A6L. Both intermediate complexes accumulated when mitochondrial protein synthesis was inhibited, suggesting that mitochondrially encoded subunits are indispensable for the formation of a fully assembled ATP synthase complex, but not for the formation of the intermediate complexes. The results and methods described in this study offer an approach to study the effects of mutations in subunits of mitochondrial ATP synthase on the assembly of this complex. This might be of value for a better understanding of deficiencies of ATP synthase activity in mitochrondrial diseases.

Conformational cycle of the archaeosome, a TCP1-like chaperonin from Sulfolobus shibatae

The major heat shock proteins in the archaeon Sulfolobus shibatae are similar to the cytosolic eukaryotic chaperonin and form an 18-subunit bitoroidal complex. Two sequence-related subunits constitute a functional complex, named the archaeosome. The archaeosome exists in two distinct conformational states that are part of chaperonin functional cycle. The closed archaeosome complex binds ATP and forms an open complex. Upon ATP hydrolysis, the open complex dissociates into subunits. Free subunits reassemble into a two-ring structure. The equilibrium between the complexes and free subunits is affected by ATP and temperature. Denatured proteins associate with both conformational states as well as with free subunits that form an intermediate complex. These unexpected observations suggest a new mechanism of archaeosome-mediated thermotolerance and protein folding.

The chaperonin GroEL is destabilized by binding of ADP

The urea-induced dissociation and subsequent conformational transitions of the nucleotide-bound form of GroEL were studied by light scattering, 4,4'-bis(1-anilino-8- naphthalenesulfonic acid) binding, and intrinsic tyrosine fluorescence. Magnesium ion alone (10 mM) stabilizes GroEL and leads to coordination of the structural transitions monitored by the different parameters. The midpoint of the light-scattering transition that monitored dissociation of the 14-mer with bound magnesium was raised to approximately 3 M, which is considerably higher than the ligand-free form of the protein, which exhibits a transition with a midpoint at approximately 2 M urea. Binding of ADP results in destabilization of the GroEL oligomeric structure, and complete dissociation of the 14-mer in the presence of 5 mM ADP occurs at about 2 M urea with the midpoint of the transition at approximately 1 M urea. The same destabilization by ADP and stabilization by Mg2+ were seen when the conformation was followed by the intrinsic fluorescence. Complexation with the nonhydrolyzable ATP analog, 5'-adenylimidodiphosphate gave an apparent stability of the quaternary structure that was between that observed with Mg2+ and that with ADP. The ADP-bound form of the protein demonstrated increased hydrophobic exposure at lower urea concentrations than the uncomplexed GroEL. In addition, the GroEL-ADP complex is more accessible for proteolytic digestion by chymotrypsin than the uncomplexed protein, consistent with a more open, flexible form of the protein. The implication of the conformational changes to the mechanism of the GroEL function is discussed.

Functional analysis of isolated cpn10 domains and conserved amino acid residues in spinach chloroplast co-chaperonin by site-directed mutagenesis

The possibilities of independent function of the two chaperonin 10 (cpn10) domains of the cpn10 homologue from spinach chloroplasts and the role of five conserved amino acid residues in the N-terminal cpn10 unit were investigated. Recombinant single domain proteins and complete chloroplast cpn10 proteins carrying amino acid exchanges of conserved residues in their N-terminal cpn10 domain were expressed in Escherichia coli and partially purified. The function of the recombinant proteins was tested using GroEL as chaperonin 60 (cpn60) partner for in vitro refolding of denatured ribulose-1,5-bisphosphate carboxylase (Rubisco). Interaction with cpn60 was also monitored by the ability to inhibit GroEL ATPase activity. In vitro both isolated cpn10 domains were found to be incapable of co-chaperonin function. All mutants were also severely impaired in cpn10 function. The results are interpreted in terms of an essential role of the exchanged amino acid residues for the interaction between co-chaperonin and cpn60 partner and in terms of a functional coupling of both cpn10 domains. To test the function of mutant chloroplast cpn10 proteins in vivo the cpn10 deficiency of E. coli strain CG712 resulting in an inability to assemble lambda-phage was exploited in a complementation assay. Transformation with plasmids directing the expression of mutant chloroplas cpn10 proteins in two cases restored lambda-phage assembly in this bacterial strain to the same extent as did transformation with a plasmid encoding wild-type cpn10 protein. In contrast a plasmid encoded third mutant and truncated forms of chloroplast cpn10 showed significantly reduced complementation efficiencies.

Ligand-induced distortion of an active site in thymidylate synthase upon binding anticancer drug 1843U89

The anticancer drug 1843U89 inhibits thymidylate synthase (TS) at sub-nanomolar concentrations and is undergoing clinical trial. The 1.95 A crystal structure of Escherichia coli TS bound to the drug and dUMP reveals that the 1843U89 binding surface includes a hydrophobic patch that is normally buried. To reach this patch, 1843U89 inserts into the wall of the TS active site, resulting in a severe local distortion of the protein. In this new conformation, active-site groups that normally bind to the catalytic cofactor methylene-tetrahydrofolate instead bind to 1843U89 in new ways. This structure provides a rare example of a protein that can bind tightly to distinct substances using a single, flexible, binding surface. This has implications for drug design, as 1843U89 could not have been obtained from current structure-based approaches.

Conformational variability in the refined structure of the chaperonin GroEL at 2.8 A resolution

Improved refinement of the crystal structure of GroEL from Escherichia coli has resulted in a complete atomic model for the first 524 residues. A new torsion-angle dynamics method and non-crystallographic symmetry restraints were used in the refinement. The model indicates that conformational variability exists due to rigid-body movements between the apical and intermediate domains of GroEL, resulting in deviations from strict seven-fold symmetry. The regions of the protein involved in polypeptide and GroES binding show unusually high B factors; these values may indicate mobility or discrete disorder. The variability of these regions may play a role in the ability of GroEL to bind a wide variety of substrates.

Expression of an archaeal chaperonin in E. coli: formation of homo- (alpha, beta) and hetero-oligomeric (alpha+beta) thermosome complexes

Co-expression of the two genes encoding the alpha- and beta-subunit of the Thermoplasma acidophilum thermosome in Escherichia coli yielded fully assembled hetero-oligomeric complexes (alpha+beta). Surprisingly, also separate expression of both genes resulted in formation of hexadecameric complexes (alpha, beta) in the bacterial cytoplasm. On electron micrographs these complexes were indistinguishable from each other and from the native thermosome. The recombinant alpha-complex as well as the native thermosome could be reconstituted in vitro from their dissociated subunits in the presence of Mg-ATP.

Prevention of in vitro protein thermal aggregation by the Sulfolobus solfataricus chaperonin. Evidence for nonequivalent binding surfaces on the chaperonin molecule

We have studied the effects of the Sulfolobus solfataricus chaperonin on the aggregation and inactivation upon heating of four model enzymes: chicken egg white lysozyme (one 14.4-kDa chain), yeast alpha-glucosidase (one 68.5-kDa chain), chicken liver malic enzyme (four 65-kDa subunits), and yeast alcohol dehydrogenase (four 37.5-kDa subunits). When the proteins were heated in the presence of an equimolar amount of chaperonin, 1) the aggregation was prevented in all solutions; 2) the inactivation profiles of the single-chain enzymes were comparable with those detected in the absence of the chaperonin, and enzyme activities were regained in the solutions heated in the presence of the chaperonin upon ATP hydrolysis (78 and 55% activity regains for lysozyme and alpha-glucosidase, respectively); 3) the inactivation of the tetrameric enzymes was completely prevented, whereas the activities decreased in the absence of the chaperonin. We demonstrate by gel filtration chromatography that the chaperonin interacted with the structures occurring during thermal denaturation of the model proteins and that the interaction with the single-chain proteins (but not that with the tetrameric proteins) was reversed upon ATP hydrolysis. The chaperonin had nonequivalent surfaces for the binding of the model proteins upon heating: the thermal denaturation intermediates of the single-chain proteins share Surfaces I, while the thermal denaturation intermediates of the tetrameric proteins share Surfaces II. ATP binding to the chaperonin induced a conformation that lacked Surfaces I and carried Surfaces II. These data support the concept that chaperonins protect native proteins against thermal aggregation by two mechanistically distinct strategies (an ATP-dependent strategy and an ATP-independent strategy), and provide the first evidence that a chaperonin molecule bears functionally specialized surfaces for the binding of the protein substrates.

Mechanism of GroEL action: productive release of polypeptide from a sequestered position under GroES

The chaperonin GroEL is a large, double-ring structure that, together with ATP and the cochaperonin GroES, assists protein folding in vivo. GroES forms an asymmetric complex with GroEL in which a single GroES ring binds one end of the GroEL cylinder. Cross-linking studies reveal that polypeptide binding occurs exclusively to the GroEL ring not occupied by GroES (trans). During the folding reaction, however, released GroES can rebind to the GroEL ring containing polypeptide (cis). The polypeptide is held tightly in a proteolytically protected environment in cis complexes, in the presence of ADP. Single turnover experiments with ornithine transcarbamylase reveal that polypeptide is productively released from the cis but not the trans complex. These observations suggest a two-step mechanism for GroEL-mediated folding. First, GroES displaces the polypeptide from its initial binding sites, sequestering it in the GroEL central cavity. Second, ATP hydrolysis induces release of GroES and productive release of polypeptide.

A plasmid-encoded dihydrofolate reductase from trimethoprim-resistant bacteria has a novel D2-symmetric active site

Bacteria expressing R67-plasmid encoded dihydrofolate reductase (R67 DHFR) exhibit high-level resistance to the antibiotic trimethoprim. Native R67 DHFR is a 34,000 M(r) homotetramer which exists in equilibrium with an inactive dimeric form. The structure of native R67 DHFR has now been solved at 1.7 A resolution and is unrelated to that of chromosomal DHFR. Homotetrameric R67 DHFR has an unusual pore, 25 A in length, passing through the middle of the molecule. Two folate molecules bind asymmetrically within the pore indicating that the enzyme's active site consists of symmetry related binding surfaces from all four identical units.

Protein structure. Why have six-fold symmetry?

Why do proteins proteins that encircle DNA have six-fold symmetry? One important factor may be the economy in protein mass with which DNA can be encircled by six globular subunits arranged in a ring.

Characterization of four new tcp-1-related cct genes from the nematode Caenorhabditis elegans

In this report we present the sequences of four new cct chaperonin genes from the nematode Caenorhabditis elegans. The four genes, cct-2, cct-4, cct-5, and cct-6 are orthologs of the mouse chaperonin genes Cctb, Cctd, Ccte, and Cctz, sharing 66%, 63%, 68%, and 67% deduced amino acid sequence identity, respectively. The C. elegans multigene family includes these four genes as well as cct-1 (tcp-1), and displays 23-35% pairwise predicted amino acid sequence identity between members, and 31-35% identity to the closely related archaebacterial chaperonin TF55. The five C. elegans cct genes are expressed in all life stages (egg, four larval stages, and adult). Members of the multigene family occur as a loosely associated group of three genes on chromosome II, and two widely separated genes on chromosome III. The predicted secondary structures of all five C. elegans CCT deduced protein sequences are nearly identical. Moreover, all chaperonins examined had comparable predicted secondary structures. Algorithmic predictions of the secondary structures of GroEL, Hsp60, and Rubisco subunit-binding protein (RuBP) are almost identical, and are very similar to the known GroEL secondary structure. The CCT/TF55 family predicted secondary structures are essentially identical to each other and are also related to GroEL, Hsp60, and RuBP. The most notable difference between the CCT/TF55 and the GroEL/Hsp60/RuBP families is in the presumed polypeptide binding domain.

On the interaction of alpha-crystallin with unfolded proteins

alpha-Crystallin, a major protein component of the lens, has chaperone-like properties whereby it prevents destabilised proteins from precipitating out of solution. It does so by forming a soluble high-molecular-weight (HMW) complex. A spectroscopic investigation of the HMW complex formed between a variety of unfolded proteins and bovine alpha-crystallin is presented in this paper. As monitored by fluorescence spectroscopy, a large amount of the hydrophobic probe, 8-anilino-1-naphthalene sulfonate (ANS) binds to the HMW complex implying that the complexed proteins (alcohol dehydrogenase (ADH), gamma-crystallin and rhodanese) are bound in an unfolded, possibly molten-globule state. The interaction between the anionic surfactant, sodium dodecyl sulfate (SDS) and ADH at high temperatures gives rise to a similar large increase in ANS fluorescence to that for the complex between alpha-crystallin and ADH. SDS, like alpha-crystallin, therefore complexes to proteins in their unfolded state leaving a large hydrophobic surface exposed to solvent. Unlike other chaperones (e.g., GroEL, DnaK and SecB), alpha-crystallin does not interact with unfolded, hydrophobic but stable proteins (e.g., reduced and carboxymethylated alpha-lactalbumin and alpha-casein). It is concluded that alpha-crystallin will only complex with proteins that are about to precipitate out of solution, i.e., ones that are severely compromised. 1H-NMR spectroscopy of the HMW complex formed between alpha-crystallin and gamma-crystallin indicates that the short C-terminal extension of alpha B-crystallin, but not that of alpha A-crystallin, has lost its flexibility in the complex implying that the former is involved in interactions with the unfolded gamma-crystallin molecule, possibly electrostatically via its two C-terminal lysine residues.

Quasi-native chaperonin-bound intermediates in facilitated protein folding

Chaperonins are known to facilitate protein folding, but their mechanism of action is not well understood. The fact that target proteins are released from and rebind to different chaperonin molecules ("cycling") during a folding reaction suggests that chaperonins function by unfolding aberrantly folded molecules, allowing them multiple opportunities to reach the native state in bulk solution. Here we show that the cycling of alpha-tubulin by cytosolic chaperonin (c-cpn) can be uncoupled from the action of cofactors required to complete the folding reaction. This results in the accumulation of folding intermediates which are chaperonin-bound, stable, and quasi-native in that they bind GTP nonexchangeably. We present evidence that these intermediates can be generated without the target protein leaving c-cpn. These data show that, in contrast to prevailing models, target proteins can maintain, and possibly acquire, significant native-like structure while chaperonin-bound.

Effects of overexpressing folding modulators on the in vivo folding of heterologous proteins in Escherichia coli

Interest continues to increase in the use of folding modulators to overcome problems with heterologous protein folding in Escherichia coli. Currently, this approach, though highly successful with a number of individual proteins, remains a somewhat hit-and-miss affair. Ongoing research directed at unraveling the precise role and specificity of these folding modulators should generate a clearer understanding of the potential and limitations of overexpressing folding catalysts in vivo. This will facilitate the development, in the not too distant future, of a more structured and rational approach to improving the folding of heterologous gene products in E. coli.

SCS1, a multicopy suppressor of hsp60-ts mutant alleles, does not encode a mitochondrially targeted protein

We identified and isolated a Saccharomyces cerevisiae gene which, when overexpressed, suppressed the temperature-sensitive phenotype of cells expressing a mutant allele of the gene encoding the mitochondrial chaperonin, Hsp60. This gene, SCS1 (suppressor of chaperonin sixty-1), encodes a 757-amino-acid protein of as yet unknown function which, nonetheless, has human, rice, and Caenorhabditis elegans homologs with high degrees (ca. 60%) of amino acid sequence identity. SCS1 is not an essential gene, but SCS1-null strains do not grow above 37 degrees C and show some growth-related defects at 30 degrees C as well. This gene is expressed at both 30 and 38 degrees C, producing little or no differences in mRNA levels at these two temperatures. Overexpression of SCS1 could not complement an HSP60-null allele, indicating that suppression was not due to the bypassing of Hsp60 activity. Of 10 other hsp60-ts alleles tested, five could also be suppressed by SCS1 overexpression. There were no common mutant phenotypes of the strains expressing these alleles that give any clue as to why they were suppressible while others were not. An epitope (influenza virus hemagglutinin)-tagged form of SCS1 in single copy complemented an SCS1-null allele. The Scs1-hemagglutinin protein was found to be at comparable levels and in similar multiply modified forms in cells growing at both 30 and 38 degrees C. Surprisingly, when localized either by cell fractionation procedures or by immunocytochemistry, these proteins were found not in mitochondria but in the cytosol. The overexpression of SCS1 had significant effects on the cellular levels of mRNAs encoding the proteins Cpn10 and Mgel, two other mitochondrial protein cochaperones, but not on mRNAs encoding a number of other mitochondrial or cytosolic proteins analyzed. The implications of these findings are discussed.

Cpn60 is exclusively localized into mitochondria of rat liver and embryonic Drosophila cells

Several reports have claimed that the mitochondrial chaperonin cpn60, or a close homolog, is also present in some other subcellular compartments of the eukaryotic cell. Immunoelectron microscopy studies, using a polyclonal serum against cpn60, revealed that the protein is exclusively localized within the mitochondria of rat liver and embryonic Drosophila cells (SL2). Furthermore, no cpn60 immunoreactive material could be found within the nucleus of SL2 cells subjected to a 1 h 37 degrees C heat-shock treatment. In contrast to these findings, immunoelectron microscopy studies, using a cpn60 monoclonal antibody, revealed mitochondrial and extramitochondrial (plasma membrane, nucleus) immunoreactive material in rat liver cells. Surprisingly, the monoclonal antibody also reacted with fixed proteins of the mature red blood cell. The monoclonal antibody, as well as cpn60 polyclonal sera, only recognize mitochondrial cpn60 in Western blots of liver proteins. Furthermore, none of the cpn60 antibodies used in this study recognized blotted proteins from rat red blood cells. Therefore, we suggest that the reported extramitochondrial localization of cpn60 in metazoan cells may be due to cross-reactivity of some of cpn60 antibodies with conformational epitopes also present in distantly related cpn60 protein homologs that are preserved during fixation procedures of the cells.

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.

Interactions between the GroE chaperonins and rhodanese. Multiple intermediates and release and rebinding

Efficient renaturation of urea-denatured rhodanese using the chaperonin GroE system requires GroEL, GroES, and ATP. At high concentrations this renaturation also requires the substrate thiosulfate to have been present during GroEL-rhodanese complex formation. When thiosulfate is present the GroEL-rhodanese complex can be concentrated to greater than 1 mg/ml rhodanese with little effect on the efficiency of renaturation. However, if complex is formed in the absence of thiosulfate, renaturation of rhodanese in the presence of thiosulfate shows a critical concentration of approximately 0.4 mg/ml, above which renaturation yields drop dramatically. This critical concentration appears to be related to an aggregation event in the refolding of rhodanese. The nucleotide free or ADP-bound form of GroEL also binds to rhodanese that has been either already renatured or never denatured. The bound rhodanese has no activity but can be released from GroEL with ATP recovering 90% of control activity. The data presented herein support a release and rebinding mechanism for the GroE-assisted refolding of rhodanese. It also suggests GroEL binds several protein folding intermediates along the entire refolding pathway.

The prosegment-subtilisin BPN' complex: crystal structure of a specific 'foldase'

BACKGROUND

The folding of the bacterial protease subtilisin BPN' (SBT) is dependent on its 77-residue prosegment, which is then autocatalytically removed to give the mature enzyme. Mature subtilisin represents a class of proteins that lacks an efficient folding pathway. Refolding of mature SBT is extremely slow unless catalyzed by the independently expressed prosegment, leading to a bimolecular complex.

RESULTS

We report the crystal structure at 2.0 A resolution of the prosegment-SBT complex and consider its implications for prosubtilisin BPN' maturation and folding catalysis. The prosegment forms a compact domain that binds SBT through an extensive interface involving the enzyme's two parallel surface helices (residues 104-116 and 133-144), supplying negatively charged caps to the N termini of these helices. The prosegment C terminus binds in the enzyme active site in a product-like manner, with Tyr77 in the P1 binding pocket.

CONCLUSIONS

The structure of the complex supports a unimolecular mechanism for prosubtilisin cleavage, involving a 25 A rearrangement of the SBT N terminus in a late folding step. A mechanism of folding catalysis in which the two helices and their connecting beta strand form a prosegment-stabilized folding nucleus is proposed. While this putative nucleus is stabilized by prosegment binding, the N-terminal and C-terminal subdomains of SBT could fold by propagation.

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

The heat shock protein GroEL from Escherichia coli is a tetradecameric oligomer that facilitates the refolding of nonnative polypeptides in an ATP-hydrolysis dependent reaction. A mutant in GroEL was prepared in which lysine 3 was substituted with glutamate, which destabilizes the oligomeric structure of GroEL (Horovitz, A., Bochkareva, E.S., and Girshovich, A.S. (1993) J. Biol. Chem. 268, 9957-9959). The highly expressed and purified GroELK3E was judged to be monomeric by sedimentation equilibrium, yielding a molecular weight of 54,500, despite a weak tendency of the mutant to reversibly form higher order aggregates above 4 mg ml-1. The monomeric variant appears to be folded based on the far UV circular dichroism spectrum, which shows significant alpha-helical content, but with slight differences in conformation relative to wild-type GroEL. The increase in exposed hydrophobic surface of the monomer was probed with the dye 4,4'-bis-1-anilino-3-naphthalenesulfonate (bis-ANS). The fluorescence of bis-ANS increases approximately 150-fold in the presence of the mutant, and about 4 mol of bis-ANS bind per mol of monomer, with a binding constant of 1.6 microM. Adenosine nucleotide binding to monomeric GroELK3E resulted in considerable quenching of bis-ANS fluorescence, correlating with significant structural changes as seen in the far UV circular dichroism, and permitted the measurement of binding isotherms for ATP and ADP. Hyperbolic ATP binding isotherms yield a dissociation constant of 82 microM, about 4-fold weaker than the K0.5 for ATP seen in steady-state kinetics assays of the wild-type GroEL ATPase.A similar difference was seen for ADP binding. These results suggest that the mutation disrupts the native tetradecameric quaternary structure through conformational changes that may also weaken nucleotide binding. The monomeric mutant exhibited no chaperone activity as evidenced by a filure to inhibit or facilitate the refolding of chemically denatured enolase, an inability to refold denatured rhodanese above spontaneous levels, and a lack of binding to alpha-casein, a competitor in many chaperonin-promoted refolding reactions. Thus, the formation of assembly incompetent monomers by the lysine 3 to glutamate mutation results in a dramatic decrease in the affinity for nonnative polypeptide chains and suggests that the oligomeric nature of GroEL is crucial for its molecular chaperone function.

Interaction of GroEL with a highly structured folding intermediate: iterative binding cycles do not involve unfolding

The GroE proteins are molecular chaperones involved in protein folding. The general mechanism by which they facilitate folding is still enigmatic. One of the central open questions is the conformation of the GroEL-bound nonnative protein. Several suggestions have been made concerning the folding stage at which a protein can interact with GroEL. Furthermore, the possibility exists that binding of the nonnative protein to GroEL results in its unfolding. We have addressed these issues that are basic for understanding the GroE-mediated folding cycle by using folding intermediates of an Fab antibody fragment as molecular probes to define the binding properties of GroEL. We show that, in addition to binding to an early folding intermediate, GroEL is able to recognize and interact with a late quaternary-structured folding intermediate (Dc) without measurably unfolding it. Thus, the prerequisite for binding is not a certain folding stage of a nonnative protein. In contrast, general surface properties of nonnative proteins seem to be crucial for binding. Furthermore, unfolding of a highly structured intermediate does not necessarily occur upon binding to GroEL. Folding of Dc in the presence of GroEL and ATP involves cycles of binding and release. Because in this system no off-pathway reactions or kinetic traps are involved, a quantitative analysis of the reactivation kinetics observed is possible. Our results indicate that the association reaction of Dc and GroEL in the presence of ATP is rather slow, whereas in the absence of ATP association is several orders of magnitude more efficient. Therefore, it seems that ATP functions by inhibiting reassociation rather than promoting release of the bound substrate.

Kinetic analysis of interactions between GroEL and reduced alpha-lactalbumin. Effect of GroES and nucleotides

The real-time analysis of the association and dissociation of chaperonin with respect to its substrate protein was carried out using the BIAcore system. We immobilized alpha-lactalbumin (LA) as a substrate protein on the sensor chip and the GroEL solution was passed over it. Whereas GroEL did not bind to the immobilized native LA, it associated with the immobilized Ca(2+)-depleted, disulfide bond-reduced form of LA (rLA) rapidly (kon = 1.96 x 10(5) M-1 S-1) and dissociated extremely slowly (koff = 2.08 x 10(-4) S-1), giving a low dissociation constant (KD = 1.06 nM). MgATP greatly accelerated the dissociation (koff = 0.15 +/- 0.02 S-1). The KD value remained almost unchanged when GroES and/or 10 microM ADP was included in the GroEL solution. However, when 1 mM ADP was included, the KD value of GroEL increased by 2 orders of magnitude solely due to the change in koff. When GroES and 1 mM ADP were included, no interaction with rLA was detected due to changes in both kon and koff. These results indicate that GroEL/ES has high and low affinity ADP binding sites and that occupation of the low affinity sites by ADP was responsible for the loss of ability to interact with the substrate protein. The effect of excess GroES on the preformed GroEL.rLA and GroEL/ES.rLA complexes was also examined. With increasing GroES, the dissociation of GroEL and GroEL/ES from rLA was accelerated, and thus the possibility is suggested that the substrate protein and GroES compete for the same site on GroEL.

Monomer-heptamer equilibrium of the Escherichia coli chaperonin GroES

In an effort to clarify the role of GroES in chaperonin-facilitated protein folding, a plasmid-encoding expression system for GroES incorporating a histidine-tagged, thrombin-cleavable, N-terminal sequence was constructed. This approach facilitated the rapid purification of native-like, histidine-cleaved GroES (HC-GroES). The addition of NaSCN to purification buffers to mildly promote subunit dissociation enabled the complete separation of chromosomally encoded, wild-type GroES chains from recombinant chains, allowing the production of homogeneous mutant variants of GroES. A substitution of histidine-7 to tryptophan in GroES was used to demonstrate the concentration-dependent modulation of the heptameric quaternary structure of the chaperonin. Fluorescence and light scattering studies of this mutant suggest that GroES heptamers dissociate to monomers upon dilution with half-times of 2-4 min. Sedimentation equilibrium experiments using either wild-type or HC-GroES can best be described by a monomer--heptamer equilibrium, yielding dissociation constants of 1 x 10(-38) M6 for native GroES and 2 x 10(-32) M6 for HC-GroES. These results are supported by subunit exchange experiments using mixtures of native or HC-GroES and GroES containing the complete N-terminal histidine tail. Native polyacrylamide gel electrophoresis demonstrates that these mixtures form an eight-membered hybrid set within minutes. The studies described here suggest a dynamic equilibrium for the quaternary structure of GroES, which may be an important feature for its role in GroEL-mediated protein folding reactions.

Functional significance of symmetrical versus asymmetrical GroEL-GroES chaperonin complexes

The Escherichia coli chaperonin GroEL and its regulator GroES are thought to mediate adenosine triphosphate-dependent protein folding as an asymmetrical complex, with substrate protein bound within the GroEL cylinder. In contrast, a symmetrical complex formed between one GroEL and two GroES oligomers, with substrate protein binding to the outer surface of GroEL, was recently proposed to be the functional chaperonin unit. Electron microscopic and biochemical analyses have now shown that unphysiologically high magnesium concentrations and increased pH are required to assemble symmetrical complexes, the formation of which precludes the association of unfolded polypeptide. Thus, the functional significance of GroEL:(GroES)2 particles remains to be demonstrated.

Asymmetrical interaction of GroEL and GroES in the ATPase cycle of assisted protein folding

The chaperonins GroEL and GroES of Escherichia coli facilitate protein folding in an adenosine triphosphate (ATP)-dependent reaction cycle. The kinetic parameters for the formation and dissociation of GroEL-GroES complexes were analyzed by surface plasmon resonance. Association of GroES and subsequent ATP hydrolysis in the interacting GroEL toroid resulted in the formation of a stable GroEL:ADP:GroES complex. The complex dissociated as a result of ATP hydrolysis in the opposite GroEL toroid, without formation of a symmetrical GroEL:(GroES)2 intermediate. Dissociation was accelerated by the addition of unfolded polypeptide. Thus, the functional chaperonin unit is an asymmetrical GroEL:GroES complex, and substrate protein plays an active role in modulating the chaperonin reaction cycle.

1H NMR spectroscopy reveals that mouse Hsp25 has a flexible C-terminal extension of 18 amino acids

The small heat-shock proteins (Hsps) exist as large aggregates and function by interacting and stabilising non-native proteins in a chaperone-like manner. Two-dimensional 1H NMR spectroscopy of mouse Hsp25 reveals that the last 18 amino acids have great flexibility with motion that is essentially independent of the domain core of the protein. The lens protein, alpha-crystallin, is homologous to Hsp25 and its two subunits also have flexible C-terminal extensions. The flexible region in Hsp25 encompasses exactly that expected from sequence comparison with alpha-crystallin implying that both proteins have similar structures and that the C-terminal extensions could be of functional importance for both proteins.

Protein-mediated protein maturation in eukaryotes

Eukaryotic cells have developed particular strategies to support the critical steps in protein maturation that starts in the cytosol with the birth of a nascent polypeptide chain, and ends when the protein has reached the appropriate compartment and/or has attained its mature structure. Many of the cellular proteins that have evolved to promote maturation processes are constitutively expressed members of the highly conserved heat shock protein (hsp) family, also known as 'molecular chaperones'. Protein-mediated processes that occur in the cytosol are discussed.

Protein engineering as a tool for crystallography

The generation of large quantities of protein by overexpression technology has enabled structural studies of many important molecules that are found in only minute quantities in the cell. An increasing number of structures of proteins overexpressed in non-native systems have been solved. Crystallographers now have an extremely powerful tool, namely protein engineering, for the generation of native and derivative crystals that diffract to high resolution. The mutation of residues or generation of compact domains through truncation has resulted in crystals with enhanced diffraction properties. Heavy atom derivative crystals isomorphous to the native protein may also be engineered either by introducing cysteines or by removing cysteines whose reaction with heavy-atom compounds results in poor crystals.

Functional characterization of the higher plant chloroplast chaperonins

The higher plant chloroplast chaperonins (ch-cpn60 and ch-cpn10) have been purified and their structural/functional properties examined. In all plants surveyed, both proteins were constitutively expressed, and only modest increases in their levels were detected upon heat shock. Like GroEL and GroES of Escherichia coli, the chloroplast chaperonins can physically interact with each other. The asymmetric complexes that form in the presence of ADP are "bullet-shaped" particles that likely consist of 1 mol each of ch-cpn60 and ch-cpn10. The purified ch-cpn60 is a functional molecular chaperone. Under "nonpermissive" conditions, where spontaneous folding was not observed, it was able to assist in the refolding of two different target proteins. In both cases, successful partitioning to the native state also required ATP hydrolysis and chaperonin 10. Surprisingly, however, the "double-domain" ch-cpn10, comprised of unique 21-kDa subunits, was not an obligatory co-chaperonin. Both GroES and a mammalian mitochondrial homolog were equally compatible with the ch-cpn60. Finally, the assisted-folding reaction mediated by the chloroplast chaperonins does not require K+ ions. Thus, the K(+)-dependent ATPase activity that is observed with other known groEL homologs is not a universal property of all chaperonin 60s.

Chaperone-like activity of protein disulfide-isomerase in the refolding of rhodanese

Protein disulfide-isomerase (PDI) in near stoichiometric concentrations promotes reactivation and prevents aggregation of guanidine-hydrochloride-denatured rhodanese during refolding upon dilution. PDI also suppresses aggregation of rhodanese during thermal inactivation. The above-mentioned properties displayed by PDI completely satisfy the definition of chaperone and provide additional evidence to confirm the hypothesis proposed previously [Wang, C. C. & Tsou, C. L. (1993) FASEB J. 7, 1515-1517] that PDI is both an enzyme and a chaperone. Since rhodanese contains no disulfide bonds, the chaperone-like activity of PDI acting on rhodanese is independent of its disulfide-isomerase activity.

Binding of defined regions of a polypeptide to GroEL and its implications for chaperonin-mediated protein folding

Unfolded rhodanese in a complex with the chaperonin GroEL was subjected to limited proteolysis. Sequence analysis indentified a GroEL-bound fragment of approximately 11,000 M(r) and a well defined fragment of approximately 7,000 M(r) from the two homologous domains of rhodanese. The shorter segment contains one hydrophobic and one amphiphilic alpha-helix mapping to the domain interface while the other fragment contains the homologous regions and an additional hydrophobic helix. Our results suggest a mechanism for the GroEL-mediated folding of rhodanese in which the domain-forming regions of the polypeptide are kept apart and are then released, perhaps sequentially, resulting in correct folding.

Prediction of the structure of GroES and its interaction with GroEL

The three-dimensional structure of the GroES monomer and its interaction with GroEL has been predicted using a combination of prediction tools and experimental data obtained by biophysical [electron microscope (EM), Fourier transform infrared (FTIR), and nuclear magnetic resonance (NMR)] and biochemical techniques. The GroES monomer, according to the prediction, is composed of eight beta-strands forming a beta-barrel with loose ends. In the model, beta-strands 5-8 run along the outer surface of GroES, forming an antiparallel beta-sheet with beta 4 loosely bound to one of the edges. beta-strands 1-3 would then be parallel and placed in the interior of the molecule. Loops 1-3 would face the internal cavity of the GroEL-GroES complex, and together with conserved residues in loops 5 and 7, would form the active surface interacting with GroEL.

A mutant at position 87 of the GroEL chaperonin is affected in protein binding and ATP hydrolysis

The highly conserved aspartic acid residue at position 87 of the Escherichia coli chaperonin GroEL was mutated to glutamic acid. When expressed in an E. coli groEL mutant strain deficient for phage morphogenesis, plasmid-encoded GroEL mutant D87E restored T4 phage morphogenesis. It did not, however, reactivate the transcription of a recombinant luciferase operon from Vibrio fischeri. In vitro, GroEL mutant D87E was found to be impaired in the ability to bind nonnative proteins and to hydrolyze ATP, resulting in less efficient refolding of urea-denatured ribulose-1,5-bisphosphate carboxylase/oxygenase. Mutant oligomer D87E GroEL14 was able to bind GroES7 as efficiently as wild-type GroEL14. The conserved aspartic acid residue at position 87 located in the equatorial domain of GroEL (Braig, K., Otwinowski, Z., Hegde, R., Boisvert, D.C., Joachimiak, A., Horwich, A.L., and Sigler, P.B. (1994) Nature 371, 578-586) is thus inferred to have a dual effect on the binding of nonnative proteins to the GroEL14 core chaperonin and on ATP hydrolysis.

On the distribution of ligands within the asymmetric chaperonin complex, GroEL14.ADP7.GroES7

In the presence of MgATP or MgADP the E. coli chaperonin proteins, GroEL and GroES, form a stable asymmetric complex with a stoichiometry of two GroEL7:one GroES7: seven MgADP. The distribution of the ligands between the two heptameric GroEL rings is crucial to our understanding of the mechanism of chaperonin-assisted folding, being either cis (i.e. [GroEL7.MgADP7.GroES7]-[GroEL7]) or trans (i.e. [GroEL7.MgADP7]-[GroEL7.GroES7]. On the basis of cross-linking experiments with 8-azido-ATP and the heterobifunctional reagent, N-succinimidyl 3-(2-pyridyldithio) propionate (SPDP), it was suggested that GroES and MgADP are bound to the same GroEL ring which resists proteinase K digestion [Nature 366 (1993) 228-233]. However, we find that the SPDP-promoted cross linking of GroES and GroEL occurs in the absence of Mg2+, ADP or ATP, which are required for the formation of the asymmetric complex. Cross-linking is shown to occur only when the SPDP-modified GroES is co-precipitated with GroEL by trichloracetic acid. Furthermore, there are structural grounds for questioning whether SPDP can crosslink, in a physiologically relevant manner, an amino group of GroES with any of the cysteinyl groups of GroEL.

The molecular chaperonin cpn60 displays local flexibility that is reduced after binding with an unfolded protein

Steady-state fluorescence polarization was used to examine the chaperonin cpn60 that was covalently labeled with pyrene. Two compounds, 1-pyrenesulfonyl chloride or N-(1-pyrene)maleimide, were used to incorporate up to 8 mol of pyrene per mol of cpn60 14-mer. The fluorescence lifetime of the cpn60-pyrenesulfonyl chloride conjugate exhibited a double exponential decay: 5.36 ns, with a fractional contribution to the intensity of 7%, and 48.77 ns, with a fractional contribution to the intensity of 93%. These yield a second-order average lifetime of 45.58 ns at 20 degrees C. Analysis of the fluorescence polarization data for the pyrene probe by the Perrin-Weber treatment revealed the existence of two components that account for the depolarization. The fast component accounted for 24% of the depolarization at 20 degrees C. The rotational relaxation time for the cpn60 14-mer derived from the low viscosity part of the Perrin-Weber plot which accentuates the slow motion gave rho h = 1113 +/- 55 ns. When this value of rho h is compared with the rho h calculated based on the Stokes radius of cpn60 from ultracentrifugation, rho Stokes, it leads to rho h/rho Stokes = 0.4 which is considerably smaller than the value expected (rho h/rho Stokes = 1) or actually found in the cpn60-rhodanese complex (rho h/rho Stokes = 0.93). These considerations and the observed presence of the fast motion suggest that cpn60 is not a rigid protein. Analysis of the polarization data as a function of temperature, which is weighted more toward the fast motion, showed that the rotational relaxation time assessed by temperature variation is greatly increased (from 552.5 to 2591 ns) for the complex of cpn60 with partially folded rhodanese (34-kDa monomeric protein). No change in rho h was observed upon formation of the cpn60.ATP complex (rho h = 556.9 ns). These data indicate that there is local motion in the cpn60 14-mer molecule that can be frozen by formation of a binary complex with partially folded proteins. This conclusion is in keeping with results showing that the structure of cpn60 is generally stabilized when it forms complexes with passenger proteins (Mendoza, J. A., and Horowitz, P. M. (1994) J. Biol. Chem. 269, 25963-25965).

Mammalian 60-kDa stress protein (chaperonin homolog). Identification, biochemical properties, and localization

Mammalian chaperonin homolog (HSP60) was purified from porcine livers cytosol using a tandem ATP-Sepharose column and Mono Q column chromatography. A partial amino acid sequence (96 amino acid residues) of this protein was determined and coincided with those of human HSP60 with 96.9% homology, which was deduced from the nucleotide sequence of the cDNA. The sequence of the NH2 termini of the purified protein (5 amino acid residues) coincided with the signal sequence of HSP60. These facts led to the identification of the 60-kDa liver protein with the chaperonin homolog. Dihydrofolate reductase was able to form a stable complex with the liver chaperonin homolog. The liver chaperonin homolog was detected by at least five spots around pI = 5.6 on two-dimensional gel electrophoresis. Immunoblotting studies using an antibody against chaperonin homolog showed that the chaperonin homolog was localized in the cytosol, mitochondrial, and nuclear fractions of porcine liver. The chaperonin homolog was localized both in the mitochondria and cytoplasm of rat kidneys at the electron microscopic level. The chaperonin homolog in the cytosol, but not in the other subcellular fractions, was cross-reacted with an antibody against the synthetic peptide corresponding to the signal peptide of HSP60 as well as the purified chaperonin homolog on immunoblotting. These results suggested that the functional chaperonin homolog in the cytosol may be transported into the mitochondria and the protein may be processed to mitochondrial HSP60 in the organella.

Crystallisation of RNA-protein complexes. II. The application of protein engineering for crystallisation of the U1A protein-RNA complex

The hairpin is one of the most commonly found structural motifs of RNA and is often a binding site for proteins. Crystallisation of U1A spliceosomal protein bound to a RNA hairpin, its natural binding site on U1snRNA, is described. RNA oligonucleotides were synthesised either chemically or by in vitro transcription using T7 RNA polymerase and purified to homogeneity by gel electrophoresis. Crystallisation trials with the wild-type protein sequence and RNA hairpins containing various stem sequences and overhanging nucleotides only resulted in a cubic crystal form which diffracted to 7-8 A resolution. A new crystal form was grown by using a protein variant containing mutations of two surface residues. The N-terminal sequence of the protein was also varied to reduce heterogeneity which was detected by protein mass spectrometry. A further crystallisation search using the double mutant protein and varying the RNA hairpins resulted in crystals diffracting to beyond 1.7 A. The methods and strategy described in this paper may be applicable to crystallisation of other RNA-protein complexes.

Identification of GroEL as a constituent of an mRNA-protection complex in Escherichia coli

An RNA-binding activity has been identified in Escherichia coli that provides physical protection of RNA against ribonucleases in an ATP- and Mg(2+)-dependent manner. This binding activity is stimulated under growth conditions known to cause a decrease in the rate of mRNA decay. RNA protection is mediated by a protein complex that contains a modified form of the chaperonin GroEL as an indispensable constituent. These results suggest a new role for GroEL as an RNA chaperone.

Specificity in chaperonin-mediated protein folding

Chaperonins are ubiquitous multisubunit toroidal complexes that aid protein folding in an ATP-dependent manner. Current models of folding by the bacterial chaperonin GroEL depict its role as unfolding and releasing molecules that have misfolded, so that they can return to a potentially productive folding pathway in solution. Accordingly, a given target polypeptide might require several cycles of binding and ATP-driven release from different chaperonin complexes before reaching the native state. Surprisingly, cycling of a target protein does not guarantee its folding, and we report here that unfolded beta-actin or alpha-tubulin both form tight complexes when presented to either GroEL or its mitochondrial homologue, and both undergo cycles of release and rebinding upon incubation with ATP, but no native protein is produced. We conclude that different chaperonins produce distinctive spectra of folding intermediates.

The chaperonin containing t-complex polypeptide 1 (TCP-1). Multisubunit machinery assisting in protein folding and assembly in the eukaryotic cytosol

Many proteins in the cell require assistance from molecular chaperones at stages in their life cycles in order to attain correctly folded states and functional conformations during protein synthesis or during recovery from denatured states. A recently discovered molecular chaperone, which is abundant in the eukaryotic cytosol and is called the chaperonin containing TCP-1 (CCT), has been shown to assist the folding of some proteins in cytosol. This chaperone is a member of the chaperonin family which includes GroEL, 60-kDa heat shock protein (Hsp60), Rubisco subunit binding protein (RBP) and thermophilic factor 55 (TF55), but is distinct from the other members in several respects. Presently the most intriguing feature is the hetero-oligomeric nature of the CCT; at least eight subunit species which are encoded by independent and highly diverged genes are known. These genes are calculated to have diverged around the starting point of the eukaryotic lineage and they are maintained in all eukaryotes investigated, suggesting a specific function for each subunit species. The amino acid sequences of these subunits share approximately 30% identity and have some highly conserved motifs probably responsible for ATPase function, suggesting this function is common to all subunits. Thus, each subunit is thought to have both specific and common functions. These observations, in conjunction with biochemical and genetic analysis, suggest that CCT functions as a very complex machinery for protein folding in the eukaryotic cell and that its chaperone activity may be essential for the folding and assembly of various newly synthesized polypeptides. This complex behaviour of CCT may have evolved to cope with the folding and assembly of certain highly evolved proteins in eukaryotic cells.

Proteasome: a complex protease with a new fold and a distinct mechanism

The structure of the proteasome from Thermoplasma acidophilum introduces threonine proteases as a fifth class of proteolytic enzymes, and offers insights into the catalytic activity of this complicated piece of molecular machinery with its 14 active sites.

Spinach chloroplast cpn21 co-chaperonin possesses two functional domains fused together in a toroidal structure and exhibits nucleotide-dependent binding to plastid chaperonin 60

Chloroplasts contain a 21-kDa co-chaperonin polypeptide (cpn21) formed by two GroES-like domains fused together in tandem. Expression of a double-domain spinach cpn21 in Escherichia coli groES mutant strains supports growth of bacteriophages lambda and T5, and will also suppress a temperature-sensitive growth phenotype of a groES619 strain. Each domain of cpn21 expressed separately can function independently to support bacteriophage lambda growth, and the N-terminal domain will additionally suppress the temperature-sensitive growth phenotype. These results indicate that chloroplast cpn21 has two functional domains, either of which can interact with GroEL in vivo to facilitate bacteriophage morphogenesis. Purified spinach cpn21 has a ring-like toroidal structure and forms a stable complex with E. coli GroEL in the presence of ADP and is functionally interchangeable with bacterial GroES in the chaperonin-facilitated refolding of denatured ribulose-1,5-bisphosphate carboxylase. Cpn21 also inhibits the ATPase activity of GroEL. Cpn21 binds with similar efficiency to both the alpha and beta subunits of spinach cpn60 in the presence of adenine nucleotides, with ATP being more effective than ADP. The tandemly fused domains of cpn21 evolved early and are present in a wide range of photosynthetic eukaryotes examined, indicating a high degree of conservation of this structure in chloroplasts.