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Results for “Molecular chaperone”

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At least 19 recordsLinked to original sources

PubMed2005

Inducers and co-inducers of molecular chaperones.

Molecular chaperones, which are mostly heat- or stress-induced proteins (HSPs), not only regulate various cellular functions such as protein folding, refolding of partially denatured proteins, protein transport across membranes, cytoskeletal organization, degradation of disabled proteins, and apoptosis, but also act as cytoprotective factors against deleterious environmental stresses. Recent studies indicated that moderate overexpression of molecular chaperones could confer cells and tissues stress tolerance and provide beneficial effects on various pathological states associated with protein misfolding and protein aggregation. Mild heat shock, transfection of HSP genes, and some chemical compounds are the major means of overexpression of molecular chaperones. In this review, we summarize recent studies of chemical compounds that could induce or enhance the expression of molecular chaperones or HSPs.

Animals↗

PubMed1993

Prions and molecular chaperones.

Molecular chaperones are proteins involved in the folding of other proteins. Among these chaperones, some are involved in their own folding (auto-chaperones). A question arises: what is the mechanism of the chaperone folding catalysis? A model for protein folding that uses the thermodynamics of irreversible processes and statistical mechanics to describe the phenomenon is proposed; the analysis presents a clear link between these two aspects. A consequence of this model is the possible existence of misfolded proteins. This point is discussed and some experimental results arguing in this direction detailed. This thermo-kinetic model is applied to protein folding driven by a molecular chaperone. Analysis of folding shows that a misfolded chaperone can induce misfolding in protein and, in the case of autofolding (auto-chaperone), may lead to new misfolded chaperones. The consequences are explored by computer simulations. They show that such an auto-chaperone could behave as a new kind of informative molecule and replicate a misfolded structure by a process similar to infection. A quantitative model, displaying the epidemiologic characters of prion infections, is derived from this hypothesis. This hypothesis satisfactorily explains the three manifestations (infectious, genetic and sporadic) that are the characteristic features of all prion diseases. Are prions really molecular chaperones required for their own assembly? Analysis of the structure of prions revealed some features shared by true molecular chaperones. This analysis suggests the positions of the mutations likely to lead to the characteristic early onset of encephalopathy. They are in good agreement with experimental results.

Amino Acid Sequence↗

PubMed2001

Review: mechanisms of disaggregation and refolding of stable protein aggregates by molecular chaperones.

Molecular chaperones are essential for the correct folding of proteins in the cell under physiological and stress conditions. Two activities have been traditionally attributed to molecular chaperones: (1) preventing aggregation of unfolded polypeptides and (2) assisting in the correct refolding of chaperone-bound denatured polypeptides. We discuss here a novel function of molecular chaperones: catalytic solubilization and refolding of stable protein aggregates. In Escherichia coli, disaggregation is carried out by a network of ATPase chaperones consisting of a DnaK core, assisted by the cochaperones DnaJ, GrpE, ClpB, and GroEL-GroES. We suggest a sequential mechanism in which (a) ClpB exposes new DnaK-binding sites on the surface of the stable protein aggregates; (b) DnaK binds the aggregate surfaces and, by doing so, melts the incorrect hydrophobic associations between aggregated polypeptides; (c) ATP hydrolysis and DnaK release allow local intramolecular refolding of native domains, leading to a gradual weakening of improper intermolecular links; (d) DnaK and GroEL complete refolding of solubilized polypeptide chains into native proteins. Thus, active disaggregation by the chaperone network can serve as a central cellular tool for the recovery of native proteins from stress-induced aggregates and actively remove disease-causing toxic aggregates, such as polyglutamine-rich proteins, amyloid plaques, and prions.

Adenosine Triphosphatases↗

PubMed1996

Supervising the fold: functional principles of molecular chaperones.

Molecular chaperones are a set of conserved protein families that share the remarkable ability to recognize and selectively bind nonnative proteins under physiological and stress conditions. Thus, they prevent irreversible aggregation reactions and keep proteins on the productive folding pathway. Evidence suggests that the cell has developed several functionally distinct chaperone families to support protein folding. The importance of molecular chaperones under stress conditions is highlighted by the finding that all the major heat shock protein families (Hsp104, Hsp90, Hsp70, Hsp60/GroEL, and small Hsps) suppress irreversible unfolding reactions. Under heat shock, only the increased expression of a repertoire of different chaperones seems to convey thermotolerance and guarantee survival. The molecular mechanism by which chaperones in general influence protein folding processes is still far from clear. However, significant progress has been achieved in understanding some of the partial reactions of the chaperone folding cycles and in functionally differentiating between the different chaperone families.

Bacterial Proteins↗

PubMed1991

Chemical synthesis of 10 kDa chaperonin. Biological activity suggests chaperonins do not require other molecular chaperones.

Molecular chaperones are required for the correct folding and assembly of certain other polypeptides. It is not known whether molecular chaperones themselves require other chaperones to become functional. A 97-amino acid chaperone, the chaperonin 10 protein was chemically synthesised so that during synthesis and purification there was no contact of the chaperone with any other protein. The purified, synthetic chaperonin 10 protein formed oligomeric structures spontaneously and was biologically active as a chaperonin. This is the first description of a chemically synthesised chaperonin, and suggests that no other chaperones are required for correct folding, polymerisation and biological activity of this chaperone.

Bacterial Proteins↗

PubMedDate not supplied

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

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

Animals↗

PubMed2005

Approaches to the isolation and characterization of molecular chaperones.

Molecular chaperones are integral components of the cellular machinery involved in ensuring correct protein folding and the continued maintenance of protein structure. An understanding of these ubiquitous molecules is key to finding cures to protein misfolding diseases such as Alzheimer's and Creutzfeldt-Jacob diseases. In addition, further understanding of chaperones will enhance our comprehension of the way the body copes with the environmental stresses that humans encounter daily. Our laboratory and our collaborators specialize in the production and characterization of chaperones from a wide variety of sources in order to gain a fuller understanding of how chaperones function in the cell. In this review, we primarily use the Hsp70/Hsp40 chaperone pair as an example to discuss recent advances in technology and reductions in cost that lend themselves to chaperone purification from both native and recombinant sources. Common assays to assess purified chaperone activity are also discussed.

Adenosine Triphosphatases↗

PubMed2003

Protein refolding assisted by self-assembled nanogels as novel artificial molecular chaperone.

Molecular chaperone-like activity for protein refolding was investigated using nanogels of self-assembly of cholesterol-bearing pullulan. Nanogels effectively prevented protein aggregation (i.e. carbonic anhydrase and citrate synthase) during protein refolding from GdmCl denaturation. Enzyme activity recovered in high yields upon dissociation of the gel structure in which the proteins were trapped, by the addition of cyclodextrins. The nanogels assisted protein refolding in a manner similar to the mechanism of molecular chaperones, namely by catching and releasing proteins. The nanogels acted as a host for the trapping of refolded intermediate proteins. Cyclodextrin is an effector molecule that controls the binding ability of these host nanogels to proteins. The present nanogel system was also effective at the renaturation of inclusion body of a recombinant protein of the serine protease family.

Animals↗

PubMed1997

Functional specificity among Hsp70 molecular chaperones.

Molecular chaperones of the 70-kilodalton heat shock protein (Hsp70) class bind to partially unfolded polypeptide substrates and participate in a wide variety of cellular processes. Differences in peptide-binding specificity among Hsp70s have led to the hypothesis that peptide binding determines specific Hsp70 functions. Protein domains were identified that were required for two separate functions of a yeast Hsp70 family. The peptide-binding domain was not required for either of these specific Hsp70 functions, which suggests that peptide-binding specificity plays little or no role in determining Hsp70 functions in vivo.

Adenosine Triphosphatases↗

PubMed2001

Chemical chaperones regulate molecular chaperones in vitro and in cells under combined salt and heat stresses.

Salt and heat stresses, which are often combined in nature, induce complementing defense mechanisms. Organisms adapt to high external salinity by accumulating small organic compounds known as osmolytes, which equilibrate cellular osmotic pressure. Osmolytes can also act as "chemical chaperones" by increasing the stability of native proteins and assisting refolding of unfolded polypeptides. Adaptation to heat stress depends on the expression of heat-shock proteins, many of which are molecular chaperones, that prevent protein aggregation, disassemble protein aggregates, and assist protein refolding. We show here that Escherichia coli cells preadapted to high salinity contain increased levels of glycine betaine that prevent protein aggregation under thermal stress. After heat shock, the aggregated proteins, which escaped protection, were disaggregated in salt-adapted cells as efficiently as in low salt. Here we address the effects of four common osmolytes on chaperone activity in vitro. Systematic dose responses of glycine betaine, glycerol, proline, and trehalose revealed a regulatory effect on the folding activities of individual and combinations of chaperones GroEL, DnaK, and ClpB. With the exception of trehalose, low physiological concentrations of proline, glycerol, and especially glycine betaine activated the molecular chaperones, likely by assisting local folding in chaperone-bound polypeptides and stabilizing the native end product of the reaction. High osmolyte concentrations, especially trehalose, strongly inhibited DnaK-dependent chaperone networks, such as DnaK+GroEL and DnaK+ClpB, likely because high viscosity affects dynamic interactions between chaperones and folding substrates and stabilizes protein aggregates. Thus, during combined salt and heat stresses, cells can specifically control protein stability and chaperone-mediated disaggregation and refolding by modulating the intracellular levels of different osmolytes.

Adaptation, Biological↗

PubMed2003

Disulfide bonds convert small heat shock protein Hsp16.3 from a chaperone to a non-chaperone: implications for the evolution of cysteine in molecular chaperones.

Molecular chaperones mainly function in assisting newly synthesized polypeptide folding and protect non-native proteins from aggregation, with known structural features such as the ability of spontaneous folding/refolding and high conformational flexibility. In this report, we verified the assumption that the lack of disulfide bonds in molecular chaperones is a prerequisite for such unique structural features. Using small heat shock protein (one sub-class of chaperones) Hsp16.3 as a model system, our results show the following: (1) Cysteine-free Hsp16.3 wild type protein can efficiently exhibit chaperone activity and spontaneously refold/reassemble with high conformational flexibility. (2) Whereas Hsp16.3 G89C mutant with inter-subunit disulfide bonds formed seems to lose the nature of chaperone proteins, i.e., under stress conditions, it neither acts as molecular chaperone nor spontaneously refolds/reassembles. Structural analysis indicated that the mutant exists as an unstable molten globule-like state, which incorrectly exposes hydrophobic surfaces and irreversibly tends to form aggregates that can be suppressed by the other molecular chaperone (alpha-crystallin). By contrast, reduction of disulfide bond in the Hsp16.3 G89C mutant can significantly recover its character as a molecular chaperone. In light of these results, we propose that disulfide bonds could severely disturb the structure/function of molecular chaperones like Hsp16.3. Our results might not only provide insights into understanding the structural basis of chaperone upon binding substrates, but also explain the observation that the occurrence of cysteine in molecular chaperones is much lower than that in other protein families, subsequently being helpful to understand the evolution of protein family.

Amino Acid Sequence↗

PubMed2002

Affinity purification and characterization of the Escherichia coli molecular chaperones.

The molecular chaperones are a group of proteins that are effective in vitro and in vivo folding aids and show a well-documented affinity for proteins lacking tertiary structure. The molecular chaperones were induced from lon(-) Escherichia coli mutants, affinity purified with an immobilized beta-casein column, and assayed for refolding activity with thermally and chemically denatured carbonic anhydrase B (CAB). Chaperones were induced with three treatments: heat shock at 39 degrees C, heat shock 42 degrees C, and alcohol shock with 3% ethanol (v/v). Lysates were applied to an immobilized beta-casein (30 mg/g beads) column. After removing nonspecifically bound proteins with 1 M NaCl, the molecular chaperones were eluted with cold water or 1 mM Mg-ATP. The cold water and Mg-ATP eluates were analyzed by SDS-PAGE. Western analysis identified five E. coli molecular chaperones including DnaK, DnaJ, GrpE, GroEL, and GroES. The purity of eluted chaperones was 58% with cold water and 100% with Mg-ATP. Refolding denatured CAB in the presence of Mg-ATP resulted in a 97% recovery of heat-denatured CAB and a 68% recovery of chemically denatured CAB. The use of affinity matrices for the chaperone purification which are effective as in vitro folding aids will be presented.

Bacterial Proteins↗

PubMed1989

The molecular chaperone concept.

Molecular chaperones are a ubiquitous family of proteins whose proposed role is to mediate the folding and assembly of other proteins into oligomeric structures. The essential function of molecular chaperones is to prevent the formation of incorrect structures which may result from the transient exposure of charged or hydrophobic surfaces normally involved in interactions between or within polypeptide chains. Such transient exposure may occur during the synthesis of polypeptides, the unfolding and refolding that occurs during their transport across membranes, the association of polypeptides made in one subcellular compartment with those made in another, changes in protein-protein interactions during the normal functioning of a complex, and recovery from stresses such as heat shock. Three classes of molecular chaperone are discussed: the nucleoplasmins, the BiP group, and the chaperonins.

Bacterial Proteins↗

PubMed2001

Liberation of the intramolecular interaction as the mechanism of heat-induced activation of HSP90 molecular chaperone.

The molecular chaperone function of HSP90 is activated under heat-stress conditions. In the present study, we investigated the role of the interactions in the heat-induced activation of HSP90 molecular chaperone. The preceding paper demonstrated two domain-domain interactions of HtpG, an Escherichia coli homologue of mammalian HSP90, i.e. an intra-molecular interaction between the N-terminal and middle domains and an intermolecular one between the middle and C-terminal domains. A bacterial two-hybrid system revealed that the two interactions also existed in human HSP90alpha. Partners of the interaction between the N-terminal and middle domains of human HSP90alpha could, but those between the middle and C-terminal domains could not, be replaced by the domains of HtpG. Thus, the interface between the N-terminal and middle domains is essentially unvaried from bacterial to human members of the HSP90-family proteins. The citrate synthase-binding activity of HtpG at an elevated temperature was solely localized in the N-terminal domain, but HSP90alpha possessed two sites in the N-terminal and other domains. The citrate-synthase-binding activity of the N-terminal domain was suppressed by the association of the middle domain. The complex between the N-terminal and middle domains is labile at elevated temperatures, but the other is stable even at 70 degrees C. Taken together, we propose the liberation of the N-terminal client-binding domain from the middle suppressor domain is involved in the temperature-dependent activation mechanism of HSP90 molecular chaperone.

Animals↗

PubMed2002

Chaperoning signaling pathways: molecular chaperones as stress-sensing 'heat shock' proteins.

Heat shock proteins interact with multiple key components of signaling pathways that regulate growth and development. The molecular relationships between heat shock proteins, various signaling proteins and partner proteins appear to be critical for the normal function of signal transduction pathways. The relative levels of these proteins may be important, as too little or too much Hsp70 or Hsp90 can result in aberrant growth control, developmental malformations and cell death. Although the functions of heat shock proteins as molecular chaperones have been well characterized, their complementary role as a 'stress-induced' proteins to monitor changes and alter the biochemical environment of the cell remains elusive. Genetic and molecular interactions between heat shock proteins, their co-chaperones and components of signaling pathways suggest that crosstalk between these proteins can regulate proliferation and development by preventing or enhancing cell growth and cell death as the levels of heat shock proteins vary in response to environmental stress or disease.

Cell Differentiation↗

PubMed2006

The molecular chaperone Hsp104--a molecular machine for protein disaggregation.

At the Cold Spring Harbor Meeting on 'Molecular Chaperones and the Heat Shock Response' in May 1996, Susan Lindquist presented evidence that a chaperone of yeast termed Hsp104, which her group had been investigating for several years, is able to dissolve protein aggregates (Glover, J.R., Lindquist, S., 1998. Hsp104, Hsp70, and Hsp40: a novel chaperone system that rescues previously aggregated proteins. Cell 94, 73-82). Among many of the participants this news stimulated reactions reaching from decided skepticism to utter disbelief because protein aggregation was widely considered to be an irreversible process. Several years and publications later, it is undeniable that Susan had been right. Hsp104 is an ATP dependent molecular machine that-in cooperation with Hsp70 and Hsp40-extracts polypeptide chains from protein aggregates and facilitates their refolding, although the molecular details of this process are still poorly understood. Meanwhile, close homologues of Hsp104 have been identified in bacteria (ClpB), in mitochondria (Hsp78), and in the cytosol of plants (Hsp101), but intriguingly not in the cytosol of animal cells (Mosser, D.D., Ho, S., Glover, J.R., 2004. Saccharomyces cerevisiae Hsp104 enhances the chaperone capacity of human cells and inhibits heat stress-induced proapoptotic signaling. Biochemistry 43, 8107-8115). Observations that Hsp104 plays an essential role in the maintenance of yeast prions (see review by James Shorter in this issue) have attracted even more attention to the molecular mechanism of this ATP dependent chaperone (Chernoff, Y.O., Lindquist, S.L., Ono, B., Inge-Vechtomov, S.G., Liebman, S.W., 1995. Role of the chaperone protein Hsp104 in propagation of the yeast prion-like factor [PSI+]. Science 268, 880-884).

Amino Acid Motifs↗

PubMed1994

Kinetics of molecular chaperone action.

Molecular chaperones of the Hsp70 type transiently sequester unfolded segments of proteins and promote their correct folding. Target peptides were labeled with an environmentally sensitive fluorophore so that their binding to the molecular chaperone DnaK of Escherichia coli could be followed in real time. The two-step process was characterized by relaxation times of 27 seconds and 200 seconds with 2 microM DnaK and 0.1 microM ligand at 25 degrees C. In the presence of adenosine triphosphate, the formation of the complex was greatly accelerated and appeared to be a single-exponential process with a relaxation time of 0.4 second. The binding-release cycle of DnaK thus occurs in the time range of polypeptide chain elongation and folding and is too fast to be stoichiometrically coupled to the adenosine triphosphatase activity of the chaperone (turnover number, 0.13 per minute at 30 degrees C).

2-Naphthylamine↗

PubMed2002

Function and regulation of cytosolic molecular chaperone CCT.

Molecular chaperones are a group of proteins that assists in the folding of newly synthesized proteins or in the refolding of denatured proteins. The cytosolic chaperonin-containing t-complex polypeptide 1 (CCT) is a molecular chaperone that plays an important role in the folding of proteins in the eukaryotic cytosol. Actin, tubulin, and several other proteins are known to be folded by CCT, and an estimated 15% of newly translated proteins in mammalian cells are folded with the assistance of CCT. CCT differs from other chaperonin family proteins in its subunit composition, which consists of eight subunit species comprising the CCT 16-mer double-ring-like complex. CCT preferentially recognizes quasinative (or partially folded) intermediates, whereas its Escherichia coli homologue GroEL recognizes more unfolded intermediates, especially those displaying hydrophobic surfaces. Molecular evolutionary analyses have suggested that each subunit species has a specific function in addition to contributing to a common ATPase activity. Consistent with this view, it has been suggested that each subunit recognizes specific substrate proteins (or their parts) and that they collectively modulate the ATPase activity of the complex. The overall expression of CCT in mammalian cells is primarily dependent on cell growth, but each subunit exhibits an individual patterns of expression. Recent progress in CCT research is reviewed, focusing particularly on CCT function and expression. From these observations, the possible roles of the distinct subunits in CCT-assisted folding in the eukaryotic cytosol are discussed.

Animals↗