ATP-induced structural change of the thermosome is temperature-dependent

Functional diversity in archaeal Hsp60: a molecular mosaic of Group I and Group II chaperonin

External stress disrupts the balance of protein homeostasis, necessitating the involvement of heat shock proteins (Hsps) in restoring equilibrium and ensuring cellular survival. The thermoacidophilic crenarchaeon Sulfolobus acidocaldarius, lacks the conventional Hsp100, Hsp90, and Hsp70, relying solely on a single ATP-dependent Group II chaperonin, Hsp60, comprising three distinct subunits (α, β, and γ) to refold unfolded substrates and maintain protein homeostasis. Hsp60 forms three different complexes, namely Hsp60αβγ, Hsp60αβ, and Hsp60β, at temperatures of 60 °C, 75 °C, and 90 °C, respectively. This study delves into the intricacies of Hsp60 complexes in S. acidocaldarius, uncovering their ability to form oligomeric structures in the presence of ATP. The recognition of substrates by Hsp60 involves hydrophobic interactions, and the subsequent refolding process occurs in an ATP-dependent manner through charge-driven interactions. Furthermore, the Hsp60β homo-oligomeric complex can protect the archaeal and eukaryotic membrane from stress-induced damage. Hsp60 demonstrates nested cooperativity in ATP hydrolysis activity, where MWC-type cooperativity is nested within KNF-type cooperativity. Remarkably, during ATP hydrolysis, Hsp60β, and Hsp60αβ complexes exhibit a mosaic behavior, aligning with characteristics observed in both Group I and Group II chaperonins, adding a layer of complexity to their functionality.

Recent Progress in Solution Structure Studies of Photosynthetic Proteins Using Small-Angle Scattering Methods

Utilized for gaining structural insights, small-angle neutron and X-ray scattering techniques (SANS and SAXS, respectively) enable an examination of biomolecules, including photosynthetic pigment-protein complexes, in solution at physiological temperatures. These methods can be seen as instrumental bridges between the high-resolution structural information achieved by crystallography or cryo-electron microscopy and functional explorations conducted in a solution state. The review starts with a comprehensive overview about the fundamental principles and applications of SANS and SAXS, with a particular focus on the recent advancements permitting to enhance the efficiency of these techniques in photosynthesis research. Among the recent developments discussed are: (i) the advent of novel modeling tools whereby a direct connection between SANS and SAXS data and high-resolution structures is created; (ii) the employment of selective deuteration, which is utilized to enhance spatial selectivity and contrast matching; (iii) the potential symbioses with molecular dynamics simulations; and (iv) the amalgamations with functional studies that are conducted to unearth structure-function relationships. Finally, reference is made to time-resolved SANS/SAXS experiments, which enable the monitoring of large-scale structural transformations of proteins in a real-time framework.

Heat shock response in archaea

An adequate response to a sudden temperature rise is crucial for cellular fitness and survival. While heat shock response (HSR) is well described in bacteria and eukaryotes, much less information is available for archaea, of which many characterized species are extremophiles thriving in habitats typified by large temperature gradients. Here, we describe known molecular aspects of archaeal heat shock proteins (HSPs) as key components of the protein homeostasis machinery and place this in a phylogenetic perspective with respect to bacterial and eukaryotic HSPs. Particular emphasis is placed on structure–function details of the archaeal thermosome, which is a major element of the HSR and of which subunit composition is altered in response to temperature changes. In contrast with the structural response, it is largely unclear how archaeal cells sense temperature fluctuations and which molecular mechanisms underlie the corresponding regulation. We frame this gap in knowledge by discussing emerging questions related to archaeal HSR and by proposing methodologies to address them. Additionally, as has been shown in bacteria and eukaryotes, HSR is expected to be relevant for the control of physiology and growth in various stress conditions beyond temperature stress. A better understanding of this essential cellular process in archaea will not only provide insights into the evolution of HSR and of its sensing and regulation, but also inspire the development of biotechnological applications, by enabling transfer of archaeal heat shock components to other biological systems and for the engineering of archaea as robust cell factories.

Structural investigation of a chaperonin in action reveals how nucleotide binding regulates the functional cycle

Chaperonins are ubiquitous protein assemblies present in bacteria, eukaryota, and archaea, facilitating the folding of proteins, preventing protein aggregation, and thus participating in maintaining protein homeostasis in the cell. During their functional cycle, they bind unfolded client proteins inside their double ring structure and promote protein folding by closing the ring chamber in an adenosine 5'-triphosphate (ATP)-dependent manner. Although the static structures of fully open and closed forms of chaperonins were solved by x-ray crystallography or electron microscopy, elucidating the mechanisms of such ATP-driven molecular events requires studying the proteins at the structural level under working conditions. We introduce an approach that combines site-specific nuclear magnetic resonance observation of very large proteins, enabled by advanced isotope labeling methods, with an in situ ATP regeneration system. Using this method, we provide functional insight into the 1-MDa large hsp60 chaperonin while processing client proteins and reveal how nucleotide binding, hydrolysis, and release control switching between closed and open states. While the open conformation stabilizes the unfolded state of client proteins, the internalization of the client protein inside the chaperonin cavity speeds up its functional cycle. This approach opens new perspectives to study structures and mechanisms of various ATP-driven biological machineries in the heat of action.

Gly-345 plays an essential role in Pyrococcus furiosus chaperonin function

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 unclear. Sequence homology between the chaperonin from Pyrococcus furiosus (PfCPN) and other group II chaperonins, together with the homo-oligomeric nature of PfCPN, suggest that PfCPN may serve as a model to clarify the role of the homologous position Gly-345 in the chaperonin-mediated protein folding. Here, we show that the purified chaperonin mutant in which the conserved residue Gly-345 is replaced by Asp (G345D) displays only about 25% ATP/ADP hydrolysis activities of the wild-type in the presence of Co(2+) and has a reduced capacity to promote folding of denatured malate dehydrogenase in vitro. This may be a reflection that Gly-345 plays an essential role in conformational change and protein refolding by archaeal group II chaperonins.

Crystal structures of a group II chaperonin reveal the open and closed states associated with the protein folding cycle

Chaperonins are large protein complexes consisting of two stacked multisubunit rings, which open and close in an ATP-dependent manner to create a protected environment for protein folding. Here, we describe the first crystal structure of a group II chaperonin in an open conformation. We have obtained structures of the archaeal chaperonin from Methanococcus maripaludis in both a peptide acceptor (open) state and a protein folding (closed) state. In contrast with group I chaperonins, in which the equatorial domains share a similar conformation between the open and closed states and the largest motions occurs at the intermediate and apical domains, the three domains of the archaeal chaperonin subunit reorient as a single rigid body. The large rotation observed from the open state to the closed state results in a 65% decrease of the folding chamber volume and creates a highly hydrophilic surface inside the cage. These results suggest a completely distinct closing mechanism in the group II chaperonins as compared with the group I chaperonins.

Maximum likelihood based classification of electron tomographic data

Classification and averaging of sub-tomograms can improve the fidelity and resolution of structures obtained by electron tomography. Here we present a three-dimensional (3D) maximum likelihood algorithm--MLTOMO--which is characterized by integrating 3D alignment and classification into a single, unified processing step. The novelty of our approach lies in the way we calculate the probability of observing an individual sub-tomogram for a given reference structure. We assume that the reference structure is affected by a 'compound wedge', resulting from the summation of many individual missing wedges in distinct orientations. The distance metric underlying our probability calculations effectively down-weights Fourier components that are observed less frequently. Simulations demonstrate that MLTOMO clearly outperforms the 'constrained correlation' approach and has advantages over existing approaches in cases where the sub-tomograms adopt preferred orientations. Application of our approach to cryo-electron tomographic data of ice-embedded thermosomes revealed distinct conformations that are in good agreement with results obtained by previous single particle studies.

Sequential action of ATP-dependent subunit conformational change and interaction between helical protrusions in the closure of the built-in lid of group II chaperonins

ATP drives the conformational change of the group II chaperonin from the open lid substrate-binding conformation to the closed lid conformation to encapsulate an unfolded protein in the central cavity. The detailed mechanism of this conformational change remains unknown. To elucidate the intra-ring cooperative action of subunits for the conformational change, we constructed Thermococcus chaperonin complexes containing mutant subunits in an ordered manner and examined their folding and conformational change abilities. Chaperonin complexes containing wild-type subunits and mutant subunits with impaired ATP-dependent conformational change ability or ATP hydrolysis activity, one by one, exhibited high protein refolding ability. The effects of the mutant subunits correlate with the number and order in the ring. In contrast, the use of a mutant lacking helical protrusion severely affected the function. Interestingly, these mutant chaperonin complexes also exhibited ATP-dependent conformational changes as demonstrated by small angle x-ray scattering, protease digestion, and changes in fluorescence of the fluorophore attached to the tip of the helical protrusion. However, their conformational change is likely to be transient. They captured denatured proteins even in the presence of ATP, whereas addition of ATP impaired the ability of the wild-type chaperonin to protect citrate synthase from thermal aggregation. These results suggest that ATP binding/hydrolysis causes the independent conformational change of the subunit, and further conformational change for the complete closure of the lid is induced and stabilized by the interaction between helical protrusions.

Multiple states of a nucleotide-bound group 2 chaperonin

Chaperonin action is controlled by cycles of nucleotide binding and hydrolysis. Here, we examine the effects of nucleotide binding on an archaeal group 2 chaperonin. In contrast to the ordered apo state of the group 1 chaperonin GroEL, the unliganded form of the homo-16-mer Methanococcus maripaludis group 2 chaperonin is very open and flexible, with intersubunit contacts only in the central double belt of equatorial domains. The intermediate and apical domains are free of contacts and deviate significantly from the overall 8-fold symmetry. Nucleotide binding results in three distinct, ordered 8-fold symmetric conformations--open, partially closed, and fully closed. The partially closed ring encloses a 40% larger volume than does the GroEL-GroES folding chamber, enabling it to encapsulate proteins up to 80 kDa, in contrast to the fully closed form, whose cavities are 20% smaller than those of the GroEL-GroES chamber.

Chaperonins: The hunt for the Group II mechanism

Chaperonins are multi-subunit complexes that enhance the efficiency of protein-folding reactions by capturing protein substrates in their central cavities. They occur in all prokaryotic and eukaryotic cell types and, alone amongst molecular chaperones, chaperonin knockouts are always lethal. Chaperonins come in two forms; the Group I are found in bacteria, mitochondria and plastids [W.A. Fenton, A.L. Horwich, Q. Rev. Biophys. 36 (2003) 229-256, [1]] and the Group II in the eukaryotic cytoplasm and in archaea [N.J. Cowan, S.A. Lewis, Adv. Protein Chem. 59 (2001) 73-104, [2]]. Both use energy derived from ATP binding and hydrolysis to drive a series of structural rearrangements that enable them to capture, engulf and then release polypeptide chains that have either not yet acquired the native, biologically active state or have been denatured in the cell.

Identification of a critical chaperoning region on an archaeal recombinant thermosome

Chaperone function in water-miscible organic co-solvents is useful for biocatalytic applications requiring enzyme stability in semi-aqueous media and for understanding chaperone behavior in hydrophobic environments. Previously, we have shown that a recombinant single subunit thermosome (rTHS) from Methanocaldococcus jannaschii functions in multiple co-solvents to hydrolyze ATP, prevent protein aggregation, and refold enzymes following solvent denaturation. For the present study, a truncated analog to the thermosome in which 70 N-terminal amino acids are removed is used to identify important regions within the thermosome for its chaperoning functions in organic co-solvents. Data presented herein indicate that the N-terminal region of rTHS is essential for the chaperone to restore the native state of the enzyme citrate synthase, but it is not a critical region for either binding of unfolded proteins or ATP hydrolysis. This is the first demonstration that direct refolding by a Group II chaperonin requires the N-terminal region of the protein.

Chaperone function in organic co-solvents: experimental characterization and modeling of a hyperthermophilic chaperone subunit from Methanocaldococcus jannaschii

Molecular chaperones play a central role in maintaining protein structure within a cell. Previously, we determined that the gene encoding a molecular chaperone, a thermosome, from the hyperthermophilic archaeon Methanocaldococcus jannaschii is upregulated upon lethal heat shock. We have recombinantly expressed this thermosome (rTHS) and show here that it is both stable and fully functional in aqueous solutions containing water-miscible organic co-solvents. Based on circular dichroism the secondary structure of rTHS was not affected by one-hour exposures to a variety of co-solvents including 30% v/v acetonitrile (ACN) and 50% methanol (MeOH). By contrast, the secondary structure of a mesophilic homologue, GroEL/GroES (GroE), was substantially disrupted. rTHS reduced the aggregation of ovalbumin and citrate synthase in 30% ACN, assisted refolding of citrate synthase upon solvent-inactivation, and stabilized citrate synthase and glutamate dehydrogenase in the direct presence of co-solvents. Apparent total turnover numbers of these enzymes in denaturing solutions increased by up to 2.5-fold in the presence of rTHS. Mechanistic models are proposed to help ascertain specific conditions that could enhance or limit organic solvent-induced chaperone activity. These models suggest that thermodynamic stability and the reversibility of enzyme unfolding play key roles in the effectiveness of enzyme recovery by rTHS.

Comparative analysis of the protein folding activities of two chaperonin subunits of Thermococcus strain KS-1: the effects of beryllium fluoride

We conducted a comparative analysis of the effects of beryllium fluoride (BeFx) on protein folding mediated by the alpha- and beta-subunit homooligomers (alpha16mer or beta16mer) from the hyperthermophilic archaeum Thermococcus strain KS-1. BeFx inhibited the ATPase activities of both alpha16mer and beta16mer with equal efficiency. This indicated that BeFx replaces the gamma-phosphate of chaperonin-bound ATP, thereby forming a stable chaperonin-ADP-BeFx complex. In the presence of ATP and BeFx, both of the two chaperonin subunits mediated green fluorescent protein (GFP) folding. Gel filtration chromatography revealed that the refolded GFP was retained by both chaperonins. Protease digestion and electron microscopic analyses showed that both chaperonin-ADP-BeFx complexes of the two subunits adopted a symmetric closed conformation with the built-in lids of both rings closed and that protein folding took place in their central cavities. These data indicated that basic protein folding mechanisms of alpha16mer and beta16mer are likely similar although there were some apparent differences. While beta16mer-mediated GFP refolding in the presence of ATP-BeFx that proceeded more rapidly than in the presence of ATP alone and reached a twofold higher plateau than that achieved with AMP-PNP, the folding mediated by the alpha16mer that proceeded with much lower yields. A mutant of alpha16mer, trapalpha, which traps the unfolded and partially folded substrate protein, did not affect the ATP-BeFx-dependent GFP folding by beta16mer but it suppressed that mediated by alpha16mer to the level of spontaneous folding. These results suggested that beta16mer differed from the alpha16mer in nucleotide binding affinity or the rate of nucleotide hydrolysis.

Characterization of archaeal group II chaperonin-ADP-metal fluoride complexes: implications that group II chaperonins operate as a "two-stroke engine"

Group II chaperonins, found in Archaea and in the eukaryotic cytosol, act independently of a cofactor corresponding to GroES of group I chaperonins. Instead, the helical protrusion at the tip of the apical domain forms a built-in lid of the central cavity. Although many studies on the lid's conformation have been carried out, the conformation in each step of the ATPase cycle remains obscure. To clarify this issue, we examined the effects of ADP-aluminum fluoride (AlFx) and ADP-beryllium fluoride (BeFx) complexes on alpha-chaperonin from the hyperthermophilic archaeum, Thermococcus sp. strain KS-1. Biochemical assays, electron microscopic observations, and small angle x-ray scattering measurements demonstrate that alpha-chaperonin incubated with ADP and BeFx exists in an asymmetric conformation; one ring is open, and the other is closed. The result indicates that alpha-chaperonin also shares the inherent functional asymmetry of bacterial and eukaryotic cytosolic chaperonins. Most interestingly, addition of ADP and BeFx induced alpha-chaperonin to encapsulate unfolded proteins in the closed ring but did not trigger their folding. Moreover, alpha-chaperonin incubated with ATP and AlFx or BeFx adopted a symmetric closed conformation, and its functional turnover was inhibited. These forms are supposed to be intermediates during the reaction cycle of group II chaperonins.

Cooperativity in the thermosome

The thermosome from Thermoplasma acidophilum is a type II chaperonin composed of eight alpha and eight beta subunits. The genes encoding the two types of subunit were co-expressed in Escherichia coli and the alpha8/beta8 complex purified from the cell extract. The isolated complex showed steady-state ATPase properties characteristic of the thermosome purified from the native organism and was capable of enhancing the folding yield of a thermostable enzyme at elevated temperature (55 degrees C). To compare the nucleotide response of this double-ring structure with the type I and more compositionally heterogeneous type II chaperonins, the tryptophan residue within the alpha subunit was used as a fluorescence reporter of the conformational changes within the thermosome induced by the binding of nucleotides. Stopped-flow measurements of indole fluorescence at 55 degrees C showed that there is a fast (approximately 350 s(-1)) and a slow (approximately 0.6 s(-1)) structural rearrangement when ATP binds to the thermosome. Further examination of the fast rearrangement showed that the associated rate constant followed a two-phase saturation profile, as it does for GroEL and for the type II chaperonin from the eukaryotic cytoplasm. This result, in keeping with these precedents, reveals that the thermosome is also a negatively cooperative system with respect to inter-ring communications, i.e. the first ring loads with higher affinity than the second. As in the case of GroEL, the loading of the second ring is weakened by ADP, implying that asymmetric ATP/ADP complexes are favoured over symmetric ones. Despite the difference in co-protein involvement in the type I and II chaperonins, these observations show that negative cooperativity is a common feature of all chaperonins thus far examined. This property results in a strong preference for asymmetry in nucleotide occupancy and implies at least some commonality with the reciprocating encapsulation mechanism shown for the GroE chaperonins.

Kinetics and Binding Sites for Interaction of the Prefoldin with a Group II Chaperonin

Prefoldin is a jellyfish-shaped hexameric co-chaperone of the group II chaperonins. It captures a protein folding intermediate and transfers it to a group II chaperonin for completion of folding. The manner in which prefoldin interacts with its substrates and cooperates with the chaperonin is poorly understood. In this study, we have examined the interaction between a prefoldin and a chaperonin from hyperthermophilic archaea by immunoprecipitation, single molecule observation, and surface plasmon resonance. We demonstrate that Pyrococcus prefoldin interacts most tightly with its cognate chaperonin, and vice versa, suggesting species specificity in the interaction. Using truncation mutants, we uncovered by kinetic analyses that this interaction is multivalent in nature, consistent with multiple binding sites between the two chaperones. We present evidence that both N- and C-terminal regions of the prefoldin beta sub-unit are important for molecular chaperone activity and for the interaction with a chaperonin. Our data are consistent with substrate and chaperonin binding sites on prefoldin that are different but in close proximity, which suggests a possible handover mechanism of prefoldin substrates to the chaperonin.

Crystal structures of the group II chaperonin from Thermococcus strain KS-1: steric hindrance by the substituted amino acid, and inter-subunit rearrangement between two crystal forms

The crystal structures of the group II chaperonins consisting of the alpha subunit with amino acid substitutions of G65C and/or I125T from the hyperthermophilic archaeum Thermococcus strain KS-1 were determined. These mutants have been shown to be active in ATP hydrolysis but inactive in protein folding. The structures were shown to be double-ring hexadecamers in an extremely closed form, which was consistent with the crystal structure of native alpha8beta8-chaperonin from Thermoplasma acidophilum. Comparisons of the present structures with the atomic structures of the GroEL14-GroES7-(ADP)7 complex revealed that the deficiency in protein-folding activity with the G65C amino acid substitution is caused by the steric hindrance of the local conformational change in an equatorial domain. We concluded that this mutant chaperonin with G65C substitution is deprived of the smooth conformational change in the refolding-reaction cycle. We obtained a new form of crystal with a distinct space group at a lower concentration of sulfate ion in the presence of nucleotide. The crystal structure obtained at the lower concentration of sulfate ion tilts outward, and has much looser inter-subunit contacts compared with those in the presence of a higher concentration of sulfate ion. Such subunit rotation has never been characterized in group II chaperonins. The crystal structure obtained at the lower concentration of sulfate ion tilts outward, and has much looser inter-subunit contacts compared with those in the presence of a higher concentration of sulfate ion.

Role of the helical protrusion in the conformational change and molecular chaperone activity of the archaeal group II chaperonin

To elucidate the exact role of the helical protrusion of a group II chaperonin in its molecular chaperone function, three deletion mutants of the chaperonin from a hyperthermophilic archaeum (Thermococcus sp. strain KS-1) lacking one-third, two-thirds, and the whole of the helical protrusion were constructed. The helical protrusion is thought to be substituted for the co-chaperonin GroES of a group I chaperonin and to be important for binding to unfolded proteins. Protease sensitivity assays and small angle x-ray scattering experiments were performed to demonstrate the conformation change of the wild type protein and the deletion mutants by adenine nucleotides. Whereas the binding of ATP to the wild type protein induced a structural transition corresponding to the closure of the built-in lid, it did not cause significant structural changes in deletion mutants. Although the mutants effectively protected proteins from thermal aggregation, ATP-dependent protein folding ability was remarkably diminished. We conclude that the helical protrusion is not necessarily important for binding to unfolded proteins, but its ATP-dependent conformational change mediates folding of captured unfolded proteins.

Roles of molecular chaperones in protein misfolding diseases

Human misfolding diseases result from the failure of proteins to reach their active state or from the accumulation of aberrantly folded proteins. The mechanisms by which molecular chaperones influence the development of these diseases is beginning to be understood. Mutations that compromise the activity of chaperones lead to several rare syndromes. In contrast, the more frequent amyloid-related neurodegenerative diseases are caused by a gain of toxic function of misfolded proteins. Toxicity in these disorders may result from an imbalance between normal chaperone capacity and production of dangerous protein species. Increased chaperone expression can suppress the neurotoxicity of these molecules, suggesting possible therapeutic strategies.

ATP binding is critical for the conformational change from an open to closed state in archaeal group II chaperonin

Group II chaperonins, found in archaea and in eukaryotic cytosol, do not have a co-chaperonin corresponding to GroES. Instead, it is suggested that the helical protrusion extending from the apical domain acts as a built-in lid for the central cavity and that the opening and closing of the lid is regulated by ATP binding and hydrolysis. However, details of this conformational change remain unclear. To investigate the conformational change associated with the ATP-driven cycle, we conducted protease sensitivity analyses and tryptophan fluorescence spectroscopy of alpha-chaperonin from a hyperthermophilic archaeum, Thermococcus strain KS-1. In the nucleotide-free or ADP-bound state, the chaperonin, especially in the helical protrusion region, was highly sensitive to proteases. Addition of ATP and ammonium sulfate induced the transition to the relatively protease-resistant form. The fluorescence intensity of the tryptophan residue introduced at the tip of the helical protrusion was enhanced by the presence of ATP or ammonium sulfate. We conclude that ATP binding induces the conformational change from the lid-open to lid-closed form in archaeal group II chaperonin.

Nested cooperativity and salt dependence of the ATPase activity of the archaeal chaperonin Mm-cpn

The properties of the ATPase activity of the type II chaperonin from Methanococcus maripaludis (Mm-cpn) were examined. Mm-cpn can hydrolyze not only ATP, but also CTP, UTP, and GTP, albeit with different effectiveness. The ATPase activity is dependent on magnesium and potassium ions, and is effectively inhibited by sodium ions. Maximal rates of ATP hydrolysis are achieved at 600 mM potassium. Initial rates of ATP hydrolysis by Mm-cpn were determined at various ATP concentrations, revealing for the first time the presence of both positive intra-ring and negative inter-ring cooperativity in the archaeal chaperonin.

Closing the folding chamber of the eukaryotic chaperonin requires the transition state of ATP hydrolysis

Chaperonins use ATPase cycling to promote conformational changes leading to protein folding. The prokaryotic chaperonin GroEL requires a cofactor, GroES, which serves as a "lid" enclosing substrates in the central cavity and confers an asymmetry on GroEL required for cooperative transitions driving the reaction. The eukaryotic chaperonin TRiC/CCT does not have such a cofactor but appears to have a "built-in" lid. Whether this seemingly symmetric chaperonin also operates through an asymmetric cycle is unclear. We show that unlike GroEL, TRiC does not close its lid upon nucleotide binding, but instead responds to the trigonal-bipyramidal transition state of ATP hydrolysis. Further, nucleotide analogs inducing this transition state confer an asymmetric conformation on TRiC. Similar to GroEL, lid closure in TRiC confines the substrates in the cavity and is essential for folding. Understanding the distinct mechanisms governing eukaryotic and bacterial chaperonin function may reveal how TRiC has evolved to fold specific eukaryotic proteins.

Nucleotide-dependent protein folding in the type II chaperonin from the mesophilic archaeon Methanococcus maripaludis

We report the characterization of the first chaperonin (Mm-cpn) from a mesophilic archaeon, Methanococcus maripaludis. The single gene was cloned from genomic DNA and expressed in Escherichia coli to produce a recombinant protein of 543 amino acids. In contrast with other known archaeal chaperonins, Mm-cpn is fully functional in all respects under physiological conditions of 37 degrees C. The complex has Mg(2+)-dependent ATPase activity and can prevent the aggregation of citrate synthase. It promotes a high-yield refolding of guanidinium-chloride-denatured rhodanese in a nucleotide-dependent manner. ATP binding is sufficient to effect folding, but ATP hydrolysis is not essential.

Molecular chaperones in the cytosol: from nascent chain to folded protein

Efficient folding of many newly synthesized proteins depends on assistance from molecular chaperones, which serve to prevent protein misfolding and aggregation in the crowded environment of the cell. Nascent chain--binding chaperones, including trigger factor, Hsp70, and prefoldin, stabilize elongating chains on ribosomes in a nonaggregated state. Folding in the cytosol is achieved either on controlled chain release from these factors or after transfer of newly synthesized proteins to downstream chaperones, such as the chaperonins. These are large, cylindrical complexes that provide a central compartment for a single protein chain to fold unimpaired by aggregation. Understanding how the thousands of different proteins synthesized in a cell use this chaperone machinery has profound implications for biotechnology and medicine.

Glycine at the 65th position plays an essential role in ATP-dependent protein folding by Archael group II chaperonin

In the previous study, we have found that G65C and I125T double mutant of alpha chaperonin homo-oligomer from a hyperthermophilic archaeum, Thermococcus sp. strain KS-1, lacks ATP-dependent protein refolding activity despite showing ATPase activity and the ability to bind the denatured proteins. In this study, we have characterized several mutant Thermococcus chaperonin homo-oligomers with the amino acid substitutions of Gly-65 or Ile-125. The results showed that amino acid residue at 65th position should be a small amino acid such as glycine or alanine for the ATP-dependent refolding activity. The alpha chaperonin homo-oligomers with amino acid substitution of Gly-65 by amino acids whose side chains are larger than the methyl group did not have ATP-dependent protein refolding activity, but exhibited an increase of the binding affinity for unfolded proteins in the presence of ATP or AMP-PNP. (c)2001 Elsevier Science.

The Chaperones of the archaeon Thermoplasma acidophilum

Chaperonesare an essential component of a cell's ability to respond to environmental challenges. Chaperones have been studied primarily in bacteria, but in recent years it has become apparent that some classes of chaperones either are very divergent in bacteria relative to archaea and eukaryotes or are missing entirely. In contrast, a high degree of similarity was found between the chaperonins of archaea and those of the eukaryotic cytosol, which has led to the establishment of archaeal model systems. The archaeon most extensively used for such studies is Thermoplasma acidophilum, which thrives at 59 degrees C and pH 2. Here we review information on its chaperone complement in light of the recently determined genome sequence.