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

Molecular determinants of cysteine string protein modulation of N-type calcium channels

Cysteine string proteins (CSPs) are secretory vesicle chaperones that are important for neurotransmitter release. We have previously reported an interaction of CSP with both heterotrimeric GTP-binding proteins (G proteins) and N-type calcium channels that results in a tonic G protein inhibition of the channels. In this report we directly demonstrate that two separate regions of CSP associate with G proteins. The N-terminal binding site of CSP, which includes the J domain, binds Galpha subunits but not Galphabeta subunits whereas the C terminal binding site of CSP associates with either free Galphabeta subunits or Galphabeta in complex with Galpha. The interaction of either binding site of CSP (CSP1-82 or CSP83-198) with G proteins elicits robust tonic inhibition of N-type calcium channel activity. However, CSP1-82 inhibition and CSP83-198 inhibition of calcium channels occur through distinct mechanisms. Calcium channel inhibition by CSP83-198 (but not CSP1-82) is completely blocked by co-expression of the synaptic protein interaction site (synprint) of the N-type channel, indicating that CSP83-198 inhibition is dependent on a physical interaction with the calcium channel. These results suggest that distinct binding sites of CSP can play a role in modulating G protein function and G protein inhibition of calcium channels.

Nanostructured surfaces from size-selected clusters

The deposition of ionized beams of size-selected atomic clusters onto well-defined substrates represents a new method of preparing nanostructured surfaces, with lateral feature sizes in the range 1-10 nm. 'Pinning' of the incident clusters prevents the diffusion of the clusters on the surface, and thus preserves the gas-phase cluster size, even at room temperature and above. At the same time, advances in diblock copolymer techniques allow the preparation of ordered two-dimensional arrays of clusters. Here we discuss the creation and applications of these nanostructured surfaces, ranging from the fabrication of semiconductor nanostructures to the immobilization of protein molecules.

Mycobacterium tuberculosis chaperonin 10 is secreted in the macrophage phagosome: is secretion due to dissociation and adoption of a partially helical structure at the membrane?

To confirm that Mycobacterium tuberculosis chaperonin 10 (Cpn10) is secreted outside the live bacillus, infected macrophages were examined by electron microscopy. This revealed that the mycobacterial protein accumulates both in the wall of the bacterium and in the matrix of the phagosomes in which ingested mycobacteria survive within infected macrophages. To understand the structural implications underlying this secretion, a structural study of M. tuberculosis Cpn10 was performed under conditions that are generally believed to mimic the membrane environment. It was found that in buffer-organic solvent mixtures, the mycobacterial protein forms two main species, namely, a partially helical monomer that prevails in dilute solutions at room temperature and a dimer that folds into a beta-sheet-dominated structure and prevails in either concentrated protein solutions at room temperature or in dilute solutions at low temperature. A partially helical monomer was also found and was completely associated with negatively charged detergents in a micelle-bound state. Remarkably, zwitterionic lipids had no effect on the protein structure. By using N- and C-truncated forms of the protein, the C- and N-terminal sequences were identified as possessing an amphiphilic helical character and as selectively associating with acidic detergent micelles. When the study was extended to other chaperonins, it was found that human Cpn10 is also monomeric and partially helical in dilute organic solvent-buffer mixtures. In contrast, Escherichia coli Cpn10 is mostly dimeric and predominately beta-sheet in both dilute and concentrated solutions. Interestingly, human Cpn10 also crosses biological membranes, whereas the E. coli homologue is strictly cytosolic. These results suggest that dissociation to partially helical monomers and interaction with acidic lipids may be two important steps in the mechanism of secretion of M. tuberculosis Cpn10 to the external environment.

Folding with and without encapsulation by cis- and trans-only GroEL-GroES complexes

Although a cis mechanism of GroEL-mediated protein folding, occurring inside a hydrophilic chamber encapsulated by the co-chaperonin GroES, has been well documented, recently the GroEL-GroES-mediated folding of aconitase, a large protein (82 kDa) that could not be encapsulated, was described. This process required GroES binding to the ring opposite the polypeptide (trans) to drive release and productive folding. Here, we have evaluated this mechanism further using trans-only complexes in which GroES is closely tethered to one of the two GroEL rings, blocking polypeptide binding by that ring. In vitro, trans-only folded aconitase with kinetics identical to GroEL-GroES. Surprisingly, trans-only also folded smaller GroEL-GroES-dependent substrates, Rubisco and malate dehydrogenase, but at rates slower than the cis reaction. Remarkably, in vivo, a plasmid encoding a trans-only complex rescued a GroEL-deficient strain, but the colony size was approximately one-tenth that produced by wild-type GroEL-GroES. We conclude that a trans mechanism, involving rounds of binding to an open ring and direct release into the bulk solution, can be generally productive although, where size permits, cis encapsulation supports more efficient folding.

Protein stability induced by ligand binding correlates with changes in protein flexibility

The interaction between ligands and proteins usually induces changes in protein thermal stability with modifications in the midpoint denaturation temperature, enthalpy of unfolding, and heat capacity. These modifications are due to the coupling of unfolding with binding equilibrium. Furthermore, they can be attained by changes in protein structure and conformational flexibility induced by ligand interaction. To study these effects we have used bovine serum albumin (BSA) interacting with three different anilinonaphthalene sulfonate derivatives (ANS). These ligands have different effects on protein stability, conformation, and dynamics. Protein stability was studied by differential scanning calorimetry and fluorescence spectroscopy, whereas conformational changes were detected by circular dichroism and infrared spectroscopy including kinetics of hydrogen/deuterium exchange. The order of calorimetric midpoint of denaturation was: 1,8-ANS-BSA > 2,6-ANS-BSA > free BSA >> (nondetected) bis-ANS-BSA. Both 1,8-ANS and 2,6-ANS did not substantially modify the secondary structure of BSA, whereas bis-ANS induced a distorted alpha-helix conformation with an increase of disordered structure. Protein flexibility followed the order: 1,8-ANS-BSA < 2,6-ANS-BSA < free BSA << bis-ANS-BSA, indicating a clear correlation between stability and conformational flexibility. The structure induced by an excess of bis-ANS to BSA is compatible with a molten globule-like state. Within the context of the binding landscape model, we have distinguished five conformers (identified by subscript): BSA(1,8-ANS), BSA(2,6-ANS), BSA(free), BSA(bis-ANS), and BSA(unfolded) among the large number of possible states of the conformational dynamic ensemble. The relative population of each distinguishable conformer depends on the type and concentration of ligand and the temperature of the system.

Role of the cytosolic chaperones Hsp70 and Hsp90 in maturation of the cardiac potassium channel HERG

The human ether-a-gogo-related gene (hERG) encodes the alpha subunit of the cardiac potassium current IKr. Several mutations in hERG produce trafficking-deficient channels that may cause hereditary long-QT syndrome and sudden cardiac death. Although hERG currents have been studied extensively, little is known about the proteins involved in maturation and trafficking of hERG. Using immunoprecipitations, we show that the cytosolic chaperones heat shock protein (Hsp) 70 and Hsp90, but not Grp94, interact with hERG wild type (WT) during maturation. The specific Hsp90 inhibitor geldanamycin prevents maturation and increases proteasomal degradation of hERG WT, while reducing hERG currents in heterologous expression systems. In ventricular myocytes, inhibition of Hsp90 also decreases IKr, whereas geldanamycin had no effect on IKs or heterologously expressed Kv2.1 and Kv1.5 currents. Both Hsp90 and Hsp70 interact directly with the core-glycosylated form of hERG WT present in the endoplasmic reticulum but not the fully glycosylated, cell-surface form. For the trafficking-deficient LQT2 mutants, hERG R752W and hERG G601S, interactions with Hsp90 and Hsp70 are increased as both mutants remained tightly associated with Hsp90 and Hsp70 in the endoplasmic reticulum. Incubation at lower temperature for R752W or with the hERG blocker astemizole for G601S dissociates channel-chaperone complexes and restores trafficking. In contrast, nonfunctional but trafficking-competent hERG G628S is released from chaperone complexes during maturation comparable to WT. We conclude that Hsp90 and Hsp70 are crucial for the maturation of hERG WT as well as the retention of trafficking-deficient LQT2 mutants. The full text of this article is available online at http://www.circresaha.org.

Recombinant human heat shock protein 60 does not induce the release of tumor necrosis factor alpha from murine macrophages

Recent studies have shown that commercially available recombinant human heat shock protein 60 (rhHSP60) could induce tumor necrosis factor alpha (TNF-alpha) release from macrophages and monocytes in a manner similar to that of lipopolysaccharide (LPS), e.g. via CD14 and Toll-like receptor 4 complex-mediated signal transduction pathway. In this study, we demonstrated that a highly purified rhHSP60 preparation with low endotoxin activity (designated rhHSP60-1) was unable to induce TNF-alpha release from murine macrophages at concentrations of up to 10 microg/ml. In contrast, a less purified rhHSP60 preparation (designated rhHSP60-2) was able to induce a marked TNF-alpha release at concentrations as low as 1 microg/ml. Failure of rhHSP60-1 to induce TNF-alpha release was not due to defective physical properties because rhHSP60-1 and rhHSP60-2 contained a similar amount of HSP60 as determined by SDS gels stained with Coomassie Blue and Western blots probed with an anti-rhHSP60 antibody. Both rhHSP60 preparations also had similar enzymatic activities as judged by their ability to hydrolyze ATP. Polymyxin B added in the incubation media abolished the endotoxin activity but inhibited only about 50% of the TNF-alpha-inducing activity of rhHSP60-2. However, both the endotoxin activity and the TNF-alpha-inducing activity of rhHSP60-2 were essentially eliminated after passing through a polymyxin B-agarose column that removes LPS and LPS-associated molecules from the rhHSP60 preparation. The TNF-alpha-inducing activities of both rhHSP60-2 and LPS with equivalent endotoxin activity present in rhHSP60-2 were equally sensitive to heat inactivation. These results suggest that rhHSP60 does not induce TNF-alpha release from macrophages. Approximately 50% of the observed TNF-alpha-inducing activity in the rhHSP60-2 preparation is due to LPS contamination, whereas the rest of the activity was due to the contamination of LPS-associated molecule(s).

Immunohistochemical study of heat shock proteins 27, 60 and 70 in the normal human adrenal and in adrenal tumors with suppressed ACTH production

Heat shock proteins (HSPs) are known to protect cells against various aggressions and to assist in the correct folding of nascent proteins as well as in the recovery of denatured ones. HSP70 increases its levels in the cell in response to any stress and is induced by ACTH in the adrenal gland. HSP60 is located in the mitochondria and assists in the folding of mitochondrial peptides. HSP27 is the only small HSP that is stress-induced. HSP27 and HSP70 are known to protect cells against apoptosis while, on the contrary, HSP60 is proapoptotic, increasing caspases maturation. We studied the expression of these HSPs in human adrenal tissue both in the normal glands (12 cases) and in tumoral tissue from cortisol producing adrenal adenomas (6 cases). Besides being neoplastic, these cells live in a particular ambience of lack of ACTH due to the suppression of the hypothalamic-pituitary ACTH secretion induced by the elevated levels of cortisol. HSP27 is highly expressed in the normal adrenal and shows a marked reduction of expression in Cushing's adrenal tissue. Although with overall lower levels of expression in the normal adrenal, HSP70 exhibited a similar pattern of reduction in tumoral tissue. HSP60, on the other hand, increased significantly and consistently in adrenal Cushing tumors. Besides the possible consequences of incorrect folding of nascent peptides, the alterations observed in tumoral tissue seem to act in an apoptotic direction. The only factor that we observed that could be contributing to these changes was the lack of plasma ACTH.

Refolding of a high molecular weight protein: salt effect on collapse

Small-angle neutron scattering experiments were performed on dilute solutions of a high molecular weight protein (fibronectin, M = 580 kg/mol) in four cases: native conditions; unfolded state obtained by a denaturing agent (urea); and two badly refolded (or collapsed) states obtained by progressive elimination of the denaturing agent in salt-containing or salt-free solutions. Our main result is concerned by the conformation of the protein as the attempt for refolding is driven with or without salt. In salt-containing solution, we observe unambiguously that the protein chain collapses at large length scales but still obeys to a Gaussian statistics at short length scales. In other words, the globule embodies a large quantity of solvent compared to the compact situation. In salt-free solutions, the badly refolded protein is not globular but displays both a coil-like and an open conformation at large length scales and a local high density area. This behavior is discussed with respect to the scaling theories for polymers and polyampholytes.

alpha-synuclein aggregation: a link between mitochondrial defects and Parkinson's disease?

Protein aggregation is a shared feature of many human neurodegenerative diseases and appears to be an inevitable consequence of excessive accumulation of misfolded proteins. Recent studies suggest that accumulation of fibrillar alpha-synuclein aggregates is associated with Parkinson's disease and other Lewy body diseases. Furthermore, the missense mutations in alpha-synuclein that are responsible for some early-onset familial types of the disease promote the aggregation process of this protein. Therefore, the mechanism underlying the cellular alpha-synuclein aggregation is of great importance in understanding the pathogenic process of these diseases. This review summarizes recent advances in our understanding of the mechanisms underlying alpha-synuclein aggregation and how the mitochondrial dysfunction plays a role in this process. Protein misfolding and aggregation in vivo can be suppressed and promoted by several factors, such as molecular chaperones, protein degradation systems, and free radicals. Many of these factors are under the control of normal mitochondrial function, prompting the speculation that mitochondrial dysfunction might cause the accumulation of protein aggregates. Recent studies indeed show that mitochondrial defects can lead to the aggregation of alpha-synuclein. In addition, potentially toxic effects of alpha-synuclein have been linked to the aggregated forms rather than the monomers, both in vitro and in cultured cells. Therefore, it is postulated that aggregation of alpha-synuclein might be one of many possible links that connect mitochondrial dysfunction to neurodegeneration.

Cloning and characterization of the groE heat-shock operon of the marine bacterium Vibrio harveyi

The DNA region of the Vibrio harveyi chromosome containing the heat-shock genes groES and groEL was cloned, and the genes were sequenced. These genes are arranged in the chromosome in the order groES-groEL. Northern hybridization experiments with RNA from V. harveyi and a DNA probe carrying both groES and groEL genes showed a single, heat-inducible transcript of approximately 2200 nt, indicating that these genes form an operon. Primer extension analysis revealed a strong, heat-inducible transcription start site 59 nt upstream of groES, preceded by a sequence typical for the Escherichia coli heat-shock promoters recognized by the sigma(32) factor, and a weak transcription start site 25 nt upstream the groES gene, preceded by a sequence typical for sigma(70) promoters. Transcription from the latter promoter occurred only at low temperatures. The V. harveyi groE operon cloned in a plasmid in E. coli cells was transcribed in a sigma(32)-dependent manner; the transcript size and the sigma(32)-dependent transcription start site were as in V. harveyi cells. Comparison of V. harveyi groE transcription regulation with the other well-characterized groE operons of the gamma subdivision of proteobacteria (those of E. coli and Pseudomonas aeruginosa) indicates a high conservation of the transcriptional regulatory elements among these bacteria, with two promoters, sigma(32) and sigma(70), involved in the regulation. The ability of the cloned groESL genes to complement E. coli groE mutants was tested: V. harveyi groES restored a thermoresistant phenotype to groES bacteria and enabled lambda phage to grow in the mutant cells. V. harveyi groEL did not abolish thermosensitivity of groEL bacteria but it complemented the groEL mutant with respect to growth of lambda phage. The results suggest that the GroEL chaperone may be more species-specific than the GroES co-chaperone.

Heat stress-dependent DNA binding of Arabidopsis heat shock transcription factor HSF1 to heat shock gene promoters in Arabidopsis suspension culture cells in vivo

Using UV laser cross-linking and immunoprecipitation we measured the in vivo binding of Arabidopsis heat shock transcription factor HSF1 to the promoters of target genes, Hsp18.2 and Hsp70. The amplification of promoter sequences, co-precipitated with HSF1-specific antibodies, indicated that HSF1 is not bound in the absence of heat stress. Binding to promoter sequences of target genes is rapidly induced by heat stress, continues throughout the heat treatment, and declines during subsequent recovery at room temperature. The molecular mechanisms underlying the differences between Hsp18.2 and Hsp70 in the kinetics of HSF1/promoter binding and corresponding mRNA expression profiles are discussed.

Getting a grip on non-native proteins

It is an underappreciated fact that non-native polypeptides are prevalent in the cellular environment. Native proteins have the folded structure, assembled state and cellular localization required for activity. By contrast, non-native proteins lack function and are particularly prone to aggregation because hydrophobic residues that are normally buried are exposed on their surfaces. These unstable entities include polypeptides that are undergoing synthesis, transport to and translocation across membranes, and those that are unfolded before degradation. Non-native proteins are normal, biologically relevant components of a healthy cell, except in cases in which their misfolding results from disease-causing mutations or adverse extrinsic factors. Here, we explore the nature and occurrence of non-native proteins, and describe the diverse families of molecular chaperones and coordinated cellular responses that have evolved to prevent their misfolding and aggregation, thereby maintaining quality control over these potentially damaging protein species.

Deletion of one of the duplicated Hsp70 genes causes hereditary myopathy of diaphragmatic muscles in Holstein-Friesian cattle

Heat-shock protein 70 (Hsp70) is a major chaperone that folds protein and prevents aggregation. The Hsp70 family contains both constitutive and stress-inducible forms. In humans, two of the inducible Hsp70 genes are located within the human major histocompatibility complex (MHC) on 6p21.3, as a duplicated locus, 12 kb apart from each other. We report that loss of one of the duplicated Hsp70 genes, the bovine homologue within the bovine MHC, is responsible for hereditary myopathy of diaphragmatic muscles (HMDM) in Holstein-Friesian cattle. Although the remaining Hsp70 gene is intact, Hsp70 protein levels are dramatically decreased in affected cattle. In normal diaphragmatic muscle, Hsp70 binds several proteins involved in energy metabolism including glycogen phosphorylase (PYGM). Immunohistochemical staining indicated that PYGM accumulated in the HMDM-specific core-like structures in affected cattle. Misfolding of energy-related proteins due to Hsp70 deficiency might lead to protein aggregation and muscle fibre degeneration.

Heat shock proteins and the pancreas

Heat shock proteins (HSPs) are cytoprotective molecules that help to maintain the metabolic and structural integrity of cells. In this review, we briefly discuss the regulation and function of HSPs. The review focuses on the current knowledge of pancreatic HSP induction, the HSP level changes during acute pancreatitis, the potential effects of the pre- and co-induction of HSPs in experimental acute pancreatitis, and the mechanisms by which HSPs might mediate cellular protection.

Periplasmic chaperones--preservers of subunit folding energy for organelle assembly

The periplasmic PapD-like chaperones have long been known to be necessary for the assembly of bacterial surface organelles. New structural work now suggests that they control assembly by arresting subunit folding. This step may be required to preserve energy for fiber formation.

Thermosensor action of GrpE. The DnaK chaperone system at heat shock temperatures

Temperature directly controls functional properties of the DnaK/DnaJ/GrpE chaperone system. The rate of the high to low affinity conversion of DnaK shows a non-Arrhenius temperature dependence and above approximately 40 degrees C even decreases. In the same temperature range, the ADP/ATP exchange factor GrpE undergoes an extensive, fully reversible thermal transition (Grimshaw, J. P. A., Jelesarov, I., Schönfeld, H. J., and Christen, P. (2001) J. Biol. Chem. 276, 6098-6104). To show that this transition underlies the thermal regulation of the chaperone system, we introduced an intersubunit disulfide bond into the paired long helices of the GrpE dimer. The transition was absent in disulfide-linked GrpE R40C but was restored by reduction. With disulfide-stabilized GrpE, the rate of ADP/ATP exchange and conversion of DnaK from its ADP-liganded high affinity R state to the ATP-liganded low affinity T state continuously increased with increasing temperature. With reduced GrpE R40C, the conversion became slower at temperatures >40 degrees C, as observed with wild-type GrpE. Thus, the long helix pair in the GrpE dimer acts as a thermosensor that, by decreasing its ADP/ATP exchange activity, induces a shift of the DnaK.substrate complexes toward the high affinity R state and in this way adapts the DnaK/DnaJ/GrpE system to heat shock conditions.

HspBP1, an Hsp70 cochaperone, has two structural domains and is capable of altering the conformation of the Hsp70 ATPase domain

We present here the first structural information for HspBP1, an Hsp70 cochaperone. Using circular dichroism, HspBP1 was determined to be 35% helical. Although HspBP1 is encoded by seven exons, limited proteolysis shows that it has only two structural domains. Domain I, amino acids 1-83, is largely unstructured. Domain II, amino acids 84-359, is predicted to be 43% helical using circular dichroism. Using limited proteolysis we have also shown that HspBP1 association changes the conformation of the ATPase domain of Hsp70. Only domain II of HspBP1 is required to bring about this conformational change. Truncation mutants of HspBP1 were tested for their ability to inhibit the renaturation of luciferase and bind to Hsp70 in reticulocyte lysate. A carboxyl terminal truncation mutant that was slightly longer than domain I was inactive in these assays, but domain II was sufficient to perform both functions. Domain II was less active than full-length HspBP1 in these assays, and addition of amino acids from domain I improved both functions. These studies show that HspBP1 domain II can bind Hsp70, change the conformation of the ATPase domain, and inhibit Hsp70-associated protein folding.

Properties of purified recombinant human polyamine oxidase, PAOh1/SMO

The discovery of an inducible oxidase whose apparent substrate preference is spermine indicates that polyamine catabolism is more complex than that originally proposed. To facilitate the study of this enzyme, the purification and characterization of the recombinant human PAOh1/SMO polyamine oxidase are reported. Purified PAOh1/SMO oxidizes both spermine (K(m)=1.6 microM) and N(1)-acetylspermine (K(m)=51 microM), but does not oxidize spermidine. The purified human enzyme also does not oxidize eight representative antitumor polyamine analogues; however, specific oligamine analogues were found to be potent inhibitors of the oxidation of spermine by PAOh1/SMO. The results of these studies are consistent with the hypothesis that PAOh1/SMO represents a new addition to the polyamine metabolic pathway that may represent a new target for antineoplastic drug development.

Polyglutamine expansion induces a protein-damaging stress connecting heat shock protein 70 to the JNK pathway

Polyglutamine diseases, including Huntington's disease, designate a group of nine neurodegenerative disorders characterized by the presence of a toxic polyglutamine expansion in specific target proteins. Using cell and mouse models, we have shown that expanded polyglutamine led to activation of the stress kinase JNK and the transcription factor AP-1, which are implicated in neuronal death. Polyglutamine expansion-induced stress shared common features with protein-damaging stress such as heat shock, because activation of JNK involved inhibition of JNK phosphatase activities. Indeed, expanded polyglutamine impaired the solubility of the dual-specificity JNK phosphatase M3/6. Aggregation of M3/6 by polyglutamine expansion appeared to be indirect, because M3/6 was not recruited into polyglutamine inclusions. The heat shock protein HSP70, which is known to inhibit JNK during the heat shock response, suppressed polyglutamine-mediated aggregation of M3/6 and activation of JNK. Interestingly, levels of HSP70 were down-regulated by polyglutamine expansion. We suggest that reduction of HSP70 by expanded polyglutamine is implicated in aggregation and inhibition of M3/6 and in activation of JNK and AP-1.

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.

The Hsp70 and TRiC/CCT chaperone systems cooperate in vivo to assemble the von Hippel-Lindau tumor suppressor complex

The degree of cooperation and redundancy between different chaperones is an important problem in understanding how proteins fold in the cell. Here we use the yeast Saccharomyces cerevisiae as a model system to examine in vivo the chaperone requirements for assembly of the von Hippel-Lindau protein (VHL)-elongin BC (VBC) tumor suppressor complex. VHL and elongin BC expressed in yeast assembled into a correctly folded VBC complex that resembles the complex from mammalian cells. Unassembled VHL did not fold and remained associated with the cytosolic chaperones Hsp70 and TRiC/CCT, in agreement with results from mammalian cells. Analysis of the folding reaction in yeast strains carrying conditional chaperone mutants indicates that incorporation of VHL into VBC requires both functional TRiC and Hsp70. VBC assembly was defective in cells carrying either a temperature-sensitive ssa1 gene as their sole source of cytosolic Hsp70/SSA function or a temperature-sensitive mutation in CCT4, a subunit of the TRiC/CCT complex. Analysis of the VHL-chaperone interactions in these strains revealed that the cct4ts mutation decreased binding to TRiC but did not affect the interaction with Hsp70. In contrast, loss of Hsp70 function disrupted the interaction of VHL with both Hsp70 and TRiC. We conclude that, in vivo, folding of some polypeptides requires the cooperation of Hsp70 and TRiC and that Hsp70 acts to promote substrate binding to TRiC.

Chaperone properties of Escherichia coli thioredoxin and thioredoxin reductase

Thioredoxin, thioredoxin reductase and NADPH form the thioredoxin system and are the major cellular protein disulphide reductase. We report here that Escherichia coli thioredoxin and thioredoxin reductase interact with unfolded and denatured proteins, in a manner similar to that of molecular chaperones that are involved in protein folding and protein renaturation after stress. Thioredoxin and/or thioredoxin reductase promote the functional folding of citrate synthase and alpha-glucosidase after urea denaturation. They also promote the functional folding of the bacterial galactose receptor, a protein without any cysteines. Furthermore, redox cycling of thioredoxin/thioredoxin reductase in the presence of NADPH and cystine stimulates the renaturation of the galactose receptor, suggesting that the thioredoxin system functions like a redox-powered chaperone machine. Thioredoxin reductase prevents the aggregation of citrate synthase under heat-shock conditions. It forms complexes that are more stable than those formed by thioredoxin with several unfolded proteins such as reduced carboxymethyl alpha-lactalbumin and unfolded bovine pancreatic trypsin inhibitor. These results suggest that the thioredoxin system, in addition to its protein disulphide isomerase activity possesses chaperone-like properties, and that its thioredoxin reductase component plays a major role in this function.

Structure-activity relations of successful pharmacologic chaperones for rescue of naturally occurring and manufactured mutants of the gonadotropin-releasing hormone receptor

We expressed a test system of wild-type (WT) rat (r) and human (h) gonadotropin-releasing hormone (GnRH) receptors (GnRHRs), including naturally occurring (13) and manufactured (five) "loss-of-function" mutants of the GnRHR. These were used to assess the ability of different GnRH peptidomimetics to rescue defective GnRHR mutants and determine their effect on the level of membrane expression of the WT receptors. Among the manufactured mutants were the shortest rGnRHR C-terminal truncation mutant that resulted in receptor loss-of-function (des(325-327)-rGnRHR), two nonfunctional deletion mutants (des(237-241)-rGnRHR and des(260-265)-rGnRHR), two nonfunctional Cys mutants (C(229)A-rGnRHR and C(278)A-rGnRHR); the naturally occurring mutants included all 13 full-length GnRHR point mutations reported to date that result in full or partial human hypogonadotropic hypogonadism. The 10 peptidomimetics assessed as potential rescue molecules ("pharmacoperones") are from three differing chemical pedigrees (indoles, quinolones, and erythromycin-derived macrolides) and were originally developed as GnRH peptidomimetic antagonists. These structures were selected for this study because of their predicted ability to permeate the cell membrane and interact with a defined affinity with the GnRH receptor. All peptidomimetics studied with an IC(50) value (for hGnRHR) Collapse

Key Words Collapse

MESH Headings

• Animals
• Antineoplastic Agents, Hormonal/pharmacology
• Binding Sites/drug effects
• Buserelin/pharmacology
• COS Cells
• Chlorocebus aethiops
• Genetic Vectors/genetics
• Humans
• Indoles/pharmacology
• Ligands
• Macrolides/pharmacology
• Molecular Chaperones/chemistry
• Molecular Chaperones/pharmacology
• Mutation/genetics
• Mutation/physiology
• Quinolones/pharmacology
• Rats
• Receptors, LHRH/drug effects
• Receptors, LHRH/genetics
• Structure-Activity Relationship
• Transfection

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Grants

• HD-18185 NICHD NIH HHS
• HD-19899 NICHD NIH HHS
• RR-00163 NCRR NIH HHS
• TW/HD-00668 FIC NIH HHS

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Affiliation(s)

Jo Ann Janovick Oregon Health and Science University/Oregon National Primate Research Center, 505 N.W. 185th Avenue, Beaverton, Oregon 97006, USA
Mark Goulet
Eugene Bush
Jonathan Greer
David G Wettlaufer
P Michael Conn

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Collaborators Collapse

Drug discovery and p53

In the past two decades, the identification of commonly mutated oncogenes and tumour suppressor genes has driven an unprecedented growth in our understanding of the genetic basis of human cancer. Although oncogenes can clearly serve as classically defined drug targets whose inactivation by small molecules could place a brake on cancer cell proliferation, the restoration of mutated tumour suppressor gene activity by small molecules might appear on the surface to be unrealistic. However, there is a growing realization that many eukaryotic regulatory proteins are partially unfolded and such intrinsically disordered proteins acquire a folded structure after binding to their biological target. Molecular characterization of the p53 protein has shown that its conformational flexibility and intrinsic thermodynamic instability provide a foundation from which its conformation can be quickly post-translationally modified.

Misrouted cell surface receptors as a novel disease aetiology and potential therapeutic target: the case of hypogonadotropic hypogonadism due to gonadotropin-releasing hormone resistance

Molecules that are incorrectly folded or defectively assembled are recognised by cellular quality control mechanisms. This leads such conformationally abnormal molecules to intracellular retention and eventual degradation. A number of diseases caused by mutations that interfere with proper processing and intracellular trafficking of key cell surface proteins have been described. These include a particular variant of hypogonadotropic hypogonadism, which results from mislocalisation of the gonadotropin-releasing hormone (GnRH) receptor. It has been shown recently that membrane expression and function of misfolded GnRH receptor mutants can be rescued by a peptidomimetic antagonist of GnRH (IN3) that permeates into the cell and reaches the abnormally manufactured nascent receptor, stabilising a conformation compatible with cell-surface transport and reversing intracellular retention. This approach seems applicable for the development of defined therapeutic strategies for an array of diseases caused by incorrectly routed cell surface or secreted proteins.

Phosphorylation of serine 303 is a prerequisite for the stress-inducible SUMO modification of heat shock factor 1

The heat shock response, which is accompanied by a rapid and robust upregulation of heat shock proteins (Hsps), is a highly conserved protection mechanism against protein-damaging stress. Hsp induction is mainly regulated at transcriptional level by stress-inducible heat shock factor 1 (HSF1). Upon activation, HSF1 trimerizes, binds to DNA, concentrates in the nuclear stress granules, and undergoes a marked multisite phosphorylation, which correlates with its transcriptional activity. In this study, we show that HSF1 is modified by SUMO-1 and SUMO-2 in a stress-inducible manner. Sumoylation is rapidly and transiently enhanced on lysine 298, located in the regulatory domain of HSF1, adjacent to several critical phosphorylation sites. Sumoylation analyses of HSF1 phosphorylation site mutants reveal that specifically the phosphorylation-deficient S303 mutant remains devoid of SUMO modification in vivo and the mutant mimicking phosphorylation of S303 promotes HSF1 sumoylation in vitro, indicating that S303 phosphorylation is required for K298 sumoylation. This finding is further supported by phosphopeptide mapping and analysis with S303/7 phosphospecific antibodies, which demonstrate that serine 303 is a target for strong heat-inducible phosphorylation, corresponding to the inducible HSF1 sumoylation. A transient phosphorylation-dependent colocalization of HSF1 and SUMO-1 in nuclear stress granules provides evidence for a strictly regulated subnuclear interplay between HSF1 and SUMO.

Identification and characterization of a Hsp70 (DnaK) chaperone system from Meiothermus ruber

We have cloned the genes encoding the chaperones of Meiothermus ruber, Hsp70 (Mru.Hsp70), Hsp40 (Mru.Hsp40) and Hsp22 (Mru.Hsp22). The genes hsp70, hsp22 and hsp40 of M. ruber are organized into an operon. The amino acid sequences of the three M. ruber chaperones show strong similarity with the heat shock proteins of Thermus thermophilus. Both Mru.Hsp40 and its homolog from T. thermophilus lack a cysteine-rich region. However, recombinant Mru.Hsp70 and Mru.Hsp40 associate in an ATP-dependent manner, and assemble into a complex in the absence of other proteins, unlike their counterparts from T. thermophilus, which require DafA for assembly. The analysis revealed that Mru.Hsp70 and Mru.Hsp40 assemble as monomers into the complex, although their homologs from T. thermophilus enter the complex as trimers. The Mru.Hsp70 and Mru.Hsp40 complex increases the spontaneous rate of refolding of denatured mitochondrial malate dehydrogenase by tenfold.

Between genotype and phenotype: protein chaperones and evolvability

Protein chaperones direct the folding of polypeptides into functional proteins, facilitate developmental signalling and, as heat-shock proteins (HSPs), can be indispensable for survival in unpredictable environments. Recent work shows that the main HSP chaperone families also buffer phenotypic variation. Chaperones can do this either directly through masking the phenotypic effects of mutant polypeptides by allowing their correct folding, or indirectly through buffering the expression of morphogenic variation in threshold traits by regulating signal transduction. Environmentally sensitive chaperone functions in protein folding and signal transduction have different potential consequences for the evolution of populations and lineages under selection in changing environments.

Ribosome-tethered molecular chaperones: the first line of defense against protein misfolding?

Folding of many cellular proteins is facilitated by molecular chaperones. Analysis of both prokaryotic and lower eukaryotic model systems has revealed the presence of ribosome-associated molecular chaperones, thought to be the first line of defense against protein aggregation as translating polypeptides emerge from the ribosome. However, structurally unrelated chaperones have evolved to carry out these functions in different microbes. In the yeast Saccharomyces cerevisiae, an unusual complex of Hsp70 and J-type chaperones associates with ribosome-bound nascent chains, whereas in Escherichia coli the ribosome-associated peptidyl-prolyl-cis-trans isomerase, trigger factor, plays a predominant role.

The composition, structure and stability of a group II chaperonin are temperature regulated in a hyperthermophilic archaeon

The hyperthermoacidophilic archaeon Sulfolobus shibatae contains group II chaperonins, known as rosettasomes, which are two nine-membered rings composed of three different 60 kDa subunits (TF55 alpha, beta and gamma). We sequenced the gene for the gamma subunit and studied the temperature-dependent changes in alpha, beta and gamma expression, their association into rosettasomes and their phylogenetic relationships. Alpha and beta gene expression was increased by heat shock (30 min, 86 degrees C) and decreased by cold shock (30 min, 60 degrees C). Gamma expression was undetectable at heat shock temperatures and low at normal temperatures (75-79 degrees C), but induced by cold shock. Polyacrylamide gel electrophoresis indicated that in vitro alpha and beta subunits form homo-oligomeric rosettasomes, and mixtures of alpha, beta and gamma form hetero-oligomeric rosettasomes. Transmission electron microscopy revealed that beta homo-oligomeric rosettasomes and all hetero-oligomeric rosettasomes associate into filaments. In vivo rosettasomes were hetero-oligomeric with an average subunit ratio of 1alpha:1beta:0.1gamma in cultures grown at 75 degrees C, a ratio of 1alpha:3beta:1gamma in cultures grown at 60 degrees C and a ratio of 2alpha:3beta:0gamma after 86 degrees C heat shock. Using differential scanning calorimetry, we determined denaturation temperatures (Tm) for alpha, beta and gamma subunits of 95.7 degrees C, 96.7 degrees C and 80.5 degrees C, respectively, and observed that rosettasomes containing gamma were relatively less stable than those with alpha and/or beta only. We propose that, in vivo, the rosettasome structure is determined by the relative abundance of subunits and not by a fixed geometry. Furthermore, phylogenetic analyses indicate that archaeal chaperonin subunits underwent multiple duplication events within species (paralogy). The independent evolution of these paralogues raises the possibility that chaperonins have functionally diversified between species.

Denaturation and reassembly of chaperonin GroEL studied by solution X-ray scattering

We measured the denaturation and reassembly of Escherichia coli chaperonin GroEL using small-angle solution X-ray scattering, which is a powerful technique for studying the overall structure and assembly of a protein in solution. The results of the urea-induced unfolding transition show that GroEL partially dissociates in the presence of more than 2 M urea, cooperatively unfolds at around 3 M urea, and is in a monomeric random coil-like unfolded structure at more than 3.2 M urea. Attempted refolding of the unfolded GroEL monomer by a simple dilution procedure is not successful, leading to formation of aggregates. However, the presence of ammonium sulfate and MgADP allows the fully unfolded GroEL to refold into a structure with the same hydrodynamic dimension, within experimental error, as that of the native GroEL. Moreover, the X-ray scattering profiles of the GroEL thus refolded and the native GroEL are coincident with each other, showing that the refolded GroEL has the same structure and the molecular mass as the native GroEL. These results demonstrate that the fully unfolded GroEL monomer can refold and reassemble into the native tetradecameric structure in the presence of ammonium sulfate and MgADP without ATP hydrolysis and preexisting chaperones. Therefore, GroEL can, in principle, fold and assemble into the native structure according to the intrinsic characteristic of its polypeptide chain, although preexisting GroEL would be important when the GroEL folding takes place under in vivo conditions, in order to avoid misfolding and aggregation.

Maintenance of glucocorticoid receptor function following severe heat-shock of heat-conditioned cells

The competence of the glucocorticoid receptor to regulate gene expression is thought to depend on Hsp70-driven continuous reactivation following spontaneous inactivation of its hormone-binding state. We show here that the glucocorticoid-binding capacity of HeLa cells fell with increasing temperature in the range 43-45 degrees C in a manner that closely paralleled the loss of soluble receptor protein. Receptor activity was maintained during moderate (43 degrees C) but not severe (45 degrees C) heat shock. Hsp70 was rapidly rendered insoluble and was replenished by soluble chaperone at 43 but not 45 degrees C. In heat-conditioned cells expressing different levels of Hsp70, we observed a positive correlation between the concentration of active receptor and the amount of Hsp70 rendered insoluble by heat shock. Much higher amounts of Hsp70 were rendered insoluble and receptor competence to regulate gene expression was preserved after severe heat shock of appropriately heat-conditioned cells. An excess of Hsp90 was found associated with resolubilized heat-inactivated receptor from severely heat-shocked cells. The data indicate that GR activity is maintained, provided that denaturation and/or aggregation of the receptor is prevented by Hsp70; and that the concentration of the chaperone is the limiting determinant of receptor activity in heat-shocked HeLa cells.

Detection of a concerted conformational change in the ATPase domain of DnaK triggered by peptide binding

The molecular chaperone DnaK is composed of two functional domains, the ATPase domain and the substrate-binding domain. In this report, we show that peptide binding to DnaK can be sensed in real time through a labeled nucleotide bound in the ATPase domain. Specifically, when N8-(4-N'-methylanthraniloylaminobutyl)-8-aminoadenosine 5'-triphosphate (MABA)-ATP.DnaK complexes are rapidly mixed with excess peptide, MABA fluorescence rapidly increases and the rate of increase is proportional to peptide concentration. Analysis of the formation traces yield on and off rate constants that are exactly equal to the rate constants obtained from experiments that directly probe peptide binding to DnaK. These results are the first to show that peptide binding to ATP.DnaK triggers a concerted conformational change in the ATPase domain.

The 1.6-A crystal structure of the class of chaperones represented by Escherichia coli Hsp31 reveals a putative catalytic triad

Heat shock proteins (Hsps) play essential protective roles under stress conditions by preventing the formation of protein aggregates and degrading misfolded proteins. EcHsp31, the yedU (hchA) gene product, is a representative member of a family of chaperones that alleviates protein misfolding by interacting with early unfolding intermediates. The 1.6-A crystal structure of the EcHsp31 dimer reveals a system of hydrophobic patches, canyons, and grooves, which may stabilize partially unfolded substrate. The presence of a well conserved, yet buried, triad in each two-domain subunit suggests a still unproven hydrolytic function of the protein. A flexible extended linker between the A and P domains may play a role in conformational flexibility and substrate binding. The alpha-beta sandwich of the EcHsp31 monomer shows structural similarity to PhPI, a protease belonging to the DJ-1 superfamily. The structure-guided sequence alignment indicates that Hsp31 homologs can be divided in three classes based on variations in the P domain that dramatically affect both oligomerization and catalytic triad formation.

Folding and insertion of the outer membrane protein OmpA is assisted by the chaperone Skp and by lipopolysaccharide

We have studied the folding pathway of a beta-barrel membrane protein using outer membrane protein A (OmpA) of Escherichia coli as an example. The deletion of the gene of periplasmic Skp impairs the assembly of outer membrane proteins of bacteria. We investigated how Skp facilitates the insertion and folding of completely unfolded OmpA into phospholipid membranes and which are the biochemical and biophysical requirements of a possible Skp-assisted folding pathway. In refolding experiments, Skp alone was not sufficient to facilitate membrane insertion and folding of OmpA. In addition, lipopolysaccharide (LPS) was required. OmpA remained unfolded when bound to Skp and LPS in solution. From this complex, OmpA folded spontaneously into lipid bilayers as determined by electrophoretic mobility measurements, fluorescence spectroscopy, and circular dichroism spectroscopy. The folding of OmpA into lipid bilayers was inhibited when one of the periplasmic components, either Skp or LPS, was absent. Membrane insertion and folding of OmpA was most efficient at specific molar ratios of OmpA, Skp, and LPS. Unfolded OmpA in complex with Skp and LPS folded faster into phospholipid bilayers than urea-unfolded OmpA. Together, these results describe a first assisted folding pathway of an integral membrane protein on the example of OmpA.

Atomic-level observation of macromolecular crowding effects: escape of a protein from the GroEL cage

Experimental work has demonstrated that the efficient operation of the GroEL-GroES chaperonin machinery is sensitive to the presence of macromolecular crowding agents. Here, I describe atomically detailed computer simulations that provide a microscopic view of how crowding effects are exerted. Simulations were performed to compute the free energy required to extract the protein rhodanese from the central cavity of GroEL into solutions containing a range of crowder concentrations. The computed energetics allow the total yield of folded protein to be predicted; the calculated yields show a nonlinear dependence on the concentration of crowding agent identical to that observed experimentally. The close correspondence between simulation and experiment prompts the use of the former in a truly predictive setting: simulations are used to suggest that more effective crowding agents might be designed by exploiting an "agoraphobic effect."

MecA, an adaptor protein necessary for ClpC chaperone activity

ClpC of Bacillus subtilis is an ATP-dependent HSP100Clp protein involved in general stress survival. A complex of ClpC with the protease ClpP and the adaptor protein MecA also controls competence development by regulated proteolysis of the transcription factor ComK. We investigated the in vitro chaperone activity of ClpC and found that the presence of MecA was crucial for the major chaperone activities of ClpC. In particular, MecA enabled ClpC to solubilize and refold aggregated proteins. Finally, in the presence of ClpP, MecA allowed the ClpC-dependent degradation of unfolded or heat-aggregated proteins. This study demonstrates that adaptor proteins like MecA through interaction with their cognate ClpC proteins can have a dual role in the protein quality-control network by rescuing, or together with ClpP, by degrading, aggregated proteins. MecA can thereby coordinate substrate targeting with ClpC activation, adding another layer to the regulation of HSP100/Clp protein activity.

Hsc70 regulates accumulation of cyclin D1 and cyclin D1-dependent protein kinase

The cyclin D-dependent kinase is a critical mediator of mitogen-dependent G1 phase progression in mammalian cells. Given the high incidence of cyclin D1 overexpression in human neoplasias, the nature and complexity of cyclin D complexes in vivo have been subjects of intense interest. Besides its catalytic partner, the nature and complexity of cyclin D complexes in vivo remain ambiguous. To address this issue, we purified native cyclin D1 complexes from proliferating mouse fibroblasts by affinity chromatography and began to identify and functionally characterize the associated proteins. In this report, we describe the identification of Hsc70 and its functional importance for cyclin D1 and cyclin D1-dependent kinase maturation. We demonstrate that Hsc70 associates with newly synthesized cyclin D1 and is a component of a mature, catalytically active cyclin D1/CDK4 holoenzyme complex. Our data suggest that Hsc70 promotes stabilization of newly synthesized cyclin D1, thereby increasing its availability for assembly with CDK4. In addition, our data demonstrate that Hsc70 remains bound to cyclin D1 following its assembly with CDK4 and Cip/Kip proteins, where it ensures the formation of a catalytically active complex.

Trigger Factor and DnaK possess overlapping substrate pools and binding specificities

Ribosome-associated Trigger Factor (TF) and the DnaK chaperone system assist the folding of newly synthesized proteins in Escherichia coli. Here, we show that DnaK and TF share a common substrate pool in vivo. In TF-deficient cells, deltatig, depleted for DnaK and DnaJ the amount of aggregated proteins increases with increasing temperature, amounting to 10% of total soluble protein (approximately 340 protein species) at 37 degrees C. A similar population of proteins aggregated in DnaK depleted tig+ cells, albeit to a much lower extent. Ninety-four aggregated proteins isolated from DnaK- and DnaJ-depleted deltatig cells were identified by mass spectrometry and found to include essential cytosolic proteins. Four potential in vivo substrates were screened for chaperone binding sites using peptide libraries. Although TF and DnaK recognize different binding motifs, 77% of TF binding peptides also associated with DnaK. In the case of the nascent polypeptides TF and DnaK competed for binding, however, with competitive advantage for TF. In vivo, the loss of TF is compensated by the induction of the heat shock response and thus enhanced levels of DnaK. In summary, our results demonstrate that the co-operation of the two mechanistically distinct chaperones in protein folding is based on their overlap in substrate specificities.

Revisiting monomeric HIV-1 protease. Characterization and redesign for improved properties

Interactions between the C-terminal interface residues (96-99) of the mature HIV-1 protease were shown to be essential for dimerization, whereas the N-terminal residues () and Arg(87) contribute to dimer stability (Ishima, R., Ghirlando, R., Tozser, J., Gronenborn, A. M., Torchia, D. A., and Louis, J. M. (2001) J. Biol. Chem. 276, 49110-49116). Here we show that the intramonomer interaction between the side chains of Asp(29) and Arg(87) influences dimerization significantly more than the intermonomer interaction between Asp(29) and Arg(8'). Several mutants, including T26A, destablize the dimer, exhibit a monomer fold, and are prone to aggregation. To alleviate this undesirable property, we designed proteins in which the N- and C-terminal regions can be linked intramolecularly by disulfide bonds. In particular, cysteine residues were introduced at positions 2 and 97 or 98. A procedure for the efficient preparation of intrachain-linked polypeptides is presented, and it is demonstrated that the Q2C/L97C variant exhibits a native-like single subunit fold. It is anticipated that monomeric proteases of this kind will aid in the discovery of novel inhibitors aimed at binding to the monomer at the dimerization interface. This extends the target area of current inhibitors, all of which bind across the active site formed by both subunits in the active dimer.

Human and bacterial oxidative demethylases repair alkylation damage in both RNA and DNA

Repair of DNA damage is essential for maintaining genome integrity, and repair deficiencies in mammals are associated with cancer, neurological disease and developmental defects. Alkylation damage in DNA is repaired by at least three different mechanisms, including damage reversal by oxidative demethylation of 1-methyladenine and 3-methylcytosine by Escherichia coli AlkB. By contrast, little is known about consequences and cellular handling of alkylation damage to RNA. Here we show that two human AlkB homologues, hABH2 and hABH3, also are oxidative DNA demethylases and that AlkB and hABH3, but not hABH2, also repair RNA. Whereas AlkB and hABH3 prefer single-stranded nucleic acids, hABH2 acts more efficiently on double-stranded DNA. In addition, AlkB and hABH3 expressed in E. coli reactivate methylated RNA bacteriophage MS2 in vivo, illustrating the biological relevance of this repair activity and establishing RNA repair as a potentially important defence mechanism in living cells. The different catalytic properties and the different subnuclear localization patterns shown by the human homologues indicate that hABH2 and hABH3 have distinct roles in the cellular response to alkylation damage.

Characterization of the Escherichia coli YedU protein as a molecular chaperone

We have cloned, purified to homogeneity, and characterized as a molecular chaperone the Escherichia coli YedU protein. The purified protein shows a single band at 31 kDa on SDS-polyacrylamide gels and forms dimers in solution. Like other chaperones, YedU interacts with unfolded and denatured proteins. It promotes the functional folding of citrate synthase and alpha-glucosidase after urea denaturation and prevents the aggregation of citrate synthase under heat shock conditions. YedU forms complexes with the permanently unfolded protein, reduced carboxymethyl alpha-lactalbumin. In contrast to DnaK/Hsp70, ATP does not stimulate YedU-dependent citrate synthase renaturation and does not affect the interaction between YedU and unfolded proteins, and YedU does not display any peptide-stimulated ATPase activity. We conclude that YedU is a novel chaperone which functions independently of an ATP/ADP cycle.

Two Hsp70 family members expressed in atherosclerotic lesions

Gene expression profiling was carried out comparing Con A elicited peritoneal macrophages from C57BL6 and FVBN wild-type and apolipoprotein (apo)E knockout mice. An EST, was expressed at higher levels in C57BL6 compared with FVBN mice. mapped to an atherosclerosis susceptibility locus on chromosome 19 revealed in an intercross between atherosclerosis-susceptible C57BL6 and atherosclerosis-resistant FVBN apoE knockout mice. A combination of database search and Northern analysis confirmed that corresponded to 3'-UTR of a hitherto predicted gene, named HspA12A. Blasting the National Center for Biotechnology Information database revealed a closely related homologue, HspA12B. HspA12A and -B have very close human homologues. TaqMan analysis confirmed the increased HspA12A expression (2.6-fold) in elicited peritoneal macrophages from C57BL6 compared with FVBN mice. TaqMan analysis also revealed increased HspA12A and HspA12B expression (87- and 6-fold, respectively) in lesional versus nonlesional portions of the thoracic aorta from C57BL6 apoE knockout mice on a chow diet. In situ hybridization confirmed that both genes were expressed within lesions but not within nonlesional aortic tissue. Blasting of HspA12A and HspA12B against the National Center for Biotechnology Information database (NR) revealed a hit with the Conserved Domain database for Hsp70 (pfam00012.5, Hsp70). Both genes appear to contain an atypical Hsp70 ATPase domain. The BLAST search also revealed that both genes were more similar to primitive eukaryote and prokaryote than mammalian Hsp70s, making these two genes distant members of the mammalian Hsp70 family. In summary, we describe two genes that code for a subfamily of Hsp70 proteins that may be involved in atherosclerosis susceptibility.

High glucose and insulin promote O-GlcNAc modification of proteins, including alpha-tubulin

Increased flux through the hexosamine biosynthesis pathway has been implicated in the development of glucose-induced insulin resistance and may promote the modification of certain proteins with O-linked N-acetylglucosamine (O-GlcNAc). L6 myotubes (a model of skeletal muscle) were incubated for 18 h in 5 or 25 mM glucose with or without 10 nM insulin. As assessed by immunoblotting with an O-GlcNAc-specific antibody, high glucose and/or insulin enhanced O-GlcNAcylation of numerous proteins, including the transcription factor Sp1, a known substrate for this modification. To identify novel proteins that may be O-GlcNAc modified in a glucose concentration/insulin-responsive manner, total cell membranes were separated by one- or two-dimensional gel electrophoresis. Selected O-GlcNAcylated proteins were identified by mass spectrometry (MS) analysis. MS sequencing of tryptic peptides identified member(s) of the heat shock protein 70 (HSP70) family and rat alpha-tubulin. Immunoprecipitation/immunoblot studies demonstrated several HSP70 isoforms and/or posttranslational modifications, some with selectively enhanced O-GlcNAcylation following exposure to high glucose plus insulin. In conclusion, in L6 myotubes, Sp1, membrane-associated HSP70, and alpha-tubulin are O-GlcNAcylated; the modification is markedly enhanced by sustained increased glucose flux.

Proteomics in brain research: potentials and limitations

The advent of proteomics techniques has been enthusiastically accepted in most areas of biology and medicine. In neuroscience, a host of applications was proposed ranging from neurotoxicology, neurometabolism, determination of the proteome of the individual brain areas in health and disease, to name a few. Only recently, the limitations of the method have been shown, hampering the rapid spreading of the technology, which in principle consists of two-dimensional gel electrophoresis with in-gel protein digestion of protein spots and identification by mass-spectrometrical approaches or microsequencing. The identification, including quantification using specific software, of brain protein classes, like enzymes, cytoskeleton proteins, heat shock proteins/chaperones, proteins of the transcription and translation machinery, synaptosomal proteins, antioxidant proteins, is a clear domain of proteomics. Furthermore, the concomitant detection of several hundred proteins on a gel allows the demonstration of an expressional pattern, rather generated by a reliable, protein-chemical method than by immunoreactivity, proposed by protein-arrays. An additional advantage is that hitherto unknown proteins, so far only proposed from their nucleic acid structure, designated as hypothetical proteins, can be identified as brain proteins. As to shortcomings and disadvantages of the method we would point to the major problem, the failure to separate hydrophobic proteins. There is so far no way to analyse the vast majority of these proteins in gels. Several other analytical problems need to be overcome, but once the latter problem can be solved, there is nothing to stop the method for a large scale analysis of membrane proteins in neuroscience.

Prion: toxic or infectious agent?

Prions are proteins that cause a number of invariably fatal neuro-degenerative diseases, which can be classified into two groups: genetic or sporadic diseases (GSD) and transmissible spongiform encephalopathies (TSE). Both types of disease require the development of both normal prion (PrP) and abnormal prion (PrP(sc)) which differs from PrP in having a tertiary structure rich in beta-sheets. In fact, PrP(sc) is a totally dehydrated protein with an anhydrous environment, probably a thin carbon dioxide gas gap, that is why it appears highly resistant to proteases, to chemical disinfectants in water phase except in certain conditions to sodium hydroxide and sodium hypochlorite, to heat and to radiation. GSD and TSE diseases differ in incubation time, primary symptoms, and nature of CNS lesions. This paper argues that diseases of the GSD type as inherited or hereditary metabolic disorders and diseases of the TSE type could be regarded as chemical poisonings. TSE is caused by a deficiency in the chemo-defense system (CDS), which is unable to destroy or eliminate PrP(sc). As a result, the immune defense system (IDS) accommodates PrP(sc) as an inert particle if not a virus lure and routes it through to the nervous central system and the brain via the body's lymphoreticular system. In TSE PrP(sc) acts inside the cells as a toxic disruptor of post-translational phase of PrP biosynthesis. Unfortunately, CDS and IDS appear unable to neutralize PrP(sc).

Molecular chaperones Hsp90 and Hsp70 deliver preproteins to the mitochondrial import receptor Tom70

The role of cytosolic factors in protein targeting to mitochondria is poorly understood. Here, we show that in mammals, the cytosolic chaperones Hsp90 and Hsp70 dock onto a specialized TPR domain in the import receptor Tom70 at the outer mitochondrial membrane. This interaction serves to deliver a set of preproteins to the receptor for subsequent membrane translocation dependent on the Hsp90 ATPase. Disruption of the chaperone/Tom70 recognition inhibits the import of these preproteins into mitochondria. In yeast, Hsp70 rather than Hsp90 is used in import, and Hsp70 docking is required for the formation of a productive preprotein/Tom70 complex. We outline a novel mechanism in which chaperones are recruited for a specific targeting event by a membrane-bound receptor.

Endotoxin contamination in recombinant human heat shock protein 70 (Hsp70) preparation is responsible for the induction of tumor necrosis factor alpha release by murine macrophages

Using commercially available recombinant human heat shock protein 70 (rhHsp70), recent studies have shown that rhHsp70 could induce the production of tumor necrosis factor alpha (TNFalpha) by macrophages and monocytes in a manner similar to lipopolysaccharide (LPS) e.g. via CD14 and Toll-like receptor 4-mediated signal transduction pathway. In the current study, we demonstrated that a highly purified rhHsp70 preparation (designated as rhHsp70-1) with a LPS content of 1.4 pg/microg was unable to induce TNFalpha release by RAW264.7 murine macrophages at concentrations up to 5 microg/ml. In contrast, a less purified rhHsp70 preparation (designated as rhHsp70-2) at 1 microg/ml with a LPS content of 0.2 ng/microg was able to induce TNFalpha release to the same extent as that induced by 0.2 ng/ml LPS. Failure of rhHsp70-1 to induce TNFalpha release was not because of defective physical properties since rhHsp70-1 and rhHsp70-2 contained identical hsp70 content as determined by SDS gels stained with Coomassie Blue and Western blots probed with an anti-rhHsp70 antibody. Both rhHsp70 preparations also had similar enzymatic activities as judged by their ability to remove clathrin from clathrin-coated vesicles. Removal of LPS from rhHsp70-2 by polymyxin B-agarose column or direct addition of polymyxin B to the incubation medium essentially eliminated the TNFalpha-inducing activity of rhHsp70-2. The addition of LPS at the concentration found in rhHsp70-2 to rhHsp70-1 resulted in the same TNFalpha-inducing activity as observed with rhHsp70-2. The TNFalpha-inducing activities of rhHsp-2, LPS alone, and LPS plus rhHsp70-1 were all equally sensitive to heat inactivation. These results suggest that rhHsp-70 does not induce TNFalpha release from murine macrophages and that the observed TNFalpha-inducing activity in the rhHsp70-2 preparation is entirely due to the contaminating LPS.