BAG-1 modulates the chaperone activity of Hsp70/Hsc70

The 70 kDa heat shock family of molecular chaperones is essential to a variety of cellular processes, yet it is unclear how these proteins are regulated in vivo. We present evidence that the protein BAG-1 is a potential modulator of the molecular chaperones, Hsp70 and Hsc70. BAG-1 binds to the ATPase domain of Hsp70 and Hsc70, without requirement for their carboxy-terminal peptide-binding domain, and can be co-immunoprecipitated with Hsp/Hsc70 from cell lysates. Purified BAG-1 and Hsp/Hsc70 efficiently form heteromeric complexes in vitro. BAG-1 inhibits Hsp/Hsc70-mediated in vitro refolding of an unfolded protein substrate, whereas BAG-1 mutants that fail to bind Hsp/Hsc70 do not affect chaperone activity. The binding of BAG-1 to one of its known cellular targets, Bcl-2, in cell lysates was found to be dependent on ATP, consistent with the possible involvement of Hsp/Hsc70 in complex formation. Overexpression of BAG-1 also protected certain cell lines from heat shock-induced cell death. The identification of Hsp/Hsc70 as a partner protein for BAG-1 may explain the diverse interactions observed between BAG-1 and several other proteins, including Raf-1, steroid hormone receptors and certain tyrosine kinase growth factor receptors. The inhibitory effects of BAG-1 on Hsp/Hsc70 chaperone activity suggest that BAG-1 represents a novel type of chaperone regulatory proteins and thus suggest a link between cell signaling, cell death and the stress response.     

Interaction of the Hsp70 molecular chaperone, DnaK, with its cochaperone DnaJ

Chaperones of the Hsp70 family bind to unfolded or partially folded polypeptides to facilitate many cellular processes. ATP hydrolysis and substrate binding, the two key molecular activities of this chaperone, are modulated by the cochaperone DnaJ. By using both genetic and biochemical approaches, we provide evidence that DnaJ binds to at least two sites on the Escherichia coli Hsp70 family member DnaK: under the ATPase domain in a cleft between its two subdomains and at or near the pocket of substrate binding. The lower cleft of the ATPase domain is defined as a binding pocket for the J-domain because (i) a DnaK mutation located in this cleft (R167H) is an allele-specific suppressor of the binding defect of the DnaJ mutation, D35N and (ii) alanine substitution of two residues close to R167 in the crystal structure, N170A and T173A, significantly decrease DnaJ binding. A second binding determinant is likely to be in the substrate-binding domain because some DnaK mutations in the vicinity of the substrate-binding pocket are defective in either the affinity (G400D, G539D) or rate (D526N) of both peptide and DnaJ binding to DnaK. Binding of DnaJ may propagate conformational changes to the nearby ATPase catalytic center and substrate-binding sites as well as facilitate communication between these two domains to alter the molecular properties of Hsp70.     

Conformational changes of an Hsp70 molecular chaperone induced by nucleotides, polypeptides, and N-ethylmaleimide

Hsp70 molecular chaperones are highly conserved ATPases that guide the folding and assembly of proteins in many cellular pathways. They use the energy of ATP binding and hydrolysis to regulate their interactions with hydrophobic regions of unfolded proteins. The activities and the conformations of the N-terminal nucleotide- and C-terminal polypeptide-binding domains of Hsp70s are coupled. We recently reported that the sulfhydryl-modifying reagent N-ethylmaleimide (NEM) inactivates the yeast Hsp70 Ssa1p by reacting with its three cysteine residues which are located in the nucleotide-binding domain. To further characterize conformational changes associated with interdomain coupling and to determine whether NEM alters Ssa1p's conformation, the structures of Ssa1p and NEM-modified Ssa1p (NEM-Ssa1p) were compared using a variety of biophysical techniques. Size exclusion chromatography revealed that NEM-Ssa1p is more oligomeric and more resistant to nucleotide- or polypeptide-dependent depolymerization than Ssa1p. Measurement of the thermal stability indicated that NEM modification has an effect very similar to that of binding of nucleotides to the unmodified protein. Circular dichroism demonstrated small differences in the secondary structure of Ssa1p and NEM-Ssa1p, and in their complexes with nucleotides. NEM modification increased the ANS fluorescence of Ssa1p and exposed numerous trypsin-sensitive sites in its nucleotide-binding domain. The intrinsic fluorescence of Ssa1p's only tryptophan residue, which is located in a C-terminal alpha-helical region adjacent to the polypeptide-binding cleft, was quenched in the presence of ATP, but not ADP. NEM modification altered nucleotide-dependent changes in the intrinsic fluorescence of Ssa1p. Together, these results demonstrate that NEM alters the conformation of Ssa1p and disrupts, but does not eliminate, interdomain communication. Furthermore, the results provide evidence for a model in which the polypeptide-binding cleft of Hsp70s is covered by an alpha-helical lid that is open in the presence of ATP, but closed in the presence of ADP.     

How potassium affects the activity of the molecular chaperone Hsc70. I. Potassium is required for optimal ATPase activity

Several functions of the 70-kilodalton heat shock cognate protein (Hsc70), such as peptide binding/release and clathrin uncoating, have been shown to require potassium ions. We have examined the effect of monovalent ions on the ATPase activity of Hsc70. The steady-state ATPase activities of Hsc70 and its amino-terminal 44-kDa ATPase fragment are minimal in the absence of K+ and reach a maximum at approximately 0.1 M [K+]. Activation of the ATPase turnover correlates with the ionic radii of monovalent ions; those that are at least 0.3 A smaller (Na+ and Li+) or larger (Cs+) than K+ show negligible activation, whereas ions with radii differing only approximately 0.1 A from that of K+ (NH4+ and Rb+) activate to approximately half the turnover rate observed with K+. Single turnover experiments with Hsc70 demonstrate that ATP hydrolysis is 5-fold slower with Na+ than with K+. The equilibrium binding of ADP or ATP to Hsc70 is unperturbed when K+ is replaced with Na+. These results are consistent with a role for monovalent ions as specific cofactors in the enzymatic hydrolysis of ATP.     

Constitutive expression of heat shock proteins Hsp90, Hsc70, Hsp70 and Hsp60 in neural and non-neural tissues of the rat during postnatal development

Heat shock proteins (Hsps) are a group of highly conserved proteins, that are constitutively expressed in most cells under normal physiological conditions. Previous work from our laboratory has shown that neurons in the adult brain exhibit high levels of Hsp90 and Hsc70 mRNA and protein, as well as basal levels of Hsp70 mRNA. We have now investigated the expression of Hsp90, Hsc70, Hsp60 and Hsp70 in neural and non-neural tissues of the rat during postnatal development, a time of extensive cell differentiation. Western blot analysis revealed constitutive expression of these Hsps early in postnatal development. Developmental profiles of these Hsps suggest that they are differentially regulated during postnatal development of the rat. For example, while levels of Hsp90 decrease somewhat in certain developing brain regions, levels of Hsp60 show a developmental increase, and Hsc70 protein is abundant throughout postnatal neural development. Low basal levels of Hsp70 are also observed in the developing and adult brain. A pronounced decrease in Hsp90 and Hsc70 was observed during postnatal development of the kidney while levels of Hsp60 increased. In addition, tissue-specific differences in the relative levels of these Hsps between brain and non-brain regions were found. Immunocytochemical studies demonstrated a neuronal localization of Hsp90, Hsc70 and Hsp60 at all stages of postnatal development examined as well as in the adult, suggesting a role for Hsps in both the developing and fully differentiated neuron. The developmental expression of subunit IV of cytochrome oxidase was similar to that of Hsp60, a protein localized predominantly to mitochondria.     

The peptide-binding domain of the chaperone protein Hsc70 has an unusual secondary structure topology

Modern NMR methods were used to determine the secondary structure topology of the 18 kDa peptide binding domain of the chaperone protein Hsc70 in solution. This report constitutes the first experimental conformational information on this important domain of the class of Hsp70 proteins. The domain consists of two four-stranded antiparallel beta-sheets and a single alpha-helix. The topology does not resemble at all the topology observed in the human leukocyte antigen (HLA) proteins of the major histocompatibility complex. This is significant because such resemblance was predicted on the basis of limited amino acid homology, secondary structure prediction, and related function. Moreover, the exact meander-type beta-sheet topology identified in Hsc70 has to our best knowledge not been observed in any other known protein structure.     

Hsp70 and Hsc70 are preferentially expressed in differentiated epithelial cells in normal human endometrium and ectocervix

This paper addresses parametric system identification of linear and nonlinear dynamic systems by analysis of the input and output signals. Specifically, we investigate the relationship between estimation of the system using a feedforward neural network model and estimation of the system by use of linear and nonlinear autoregressive moving-average (ARMA) models. By utilizing a neural network model incorporating a polynomial activation function, we show the equivalence of the artificial neural network to the linear and nonlinear ARMA models. We compare the parameterization of the estimated system using the neural network and ARMA approaches by utilizing data generated by means of computer simulations. Specifically, we show that the parameters of a simulated ARMA system can be obtained from the neural network analysis of the simulated data or by conventional least squares ARMA analysis. The feasibility of applying neural networks with polynomial activation functions to the analysis of experimental data is explored by application to measurements of heart rate (HR) and instantaneous lung volume (ILV) fluctuations.     

The carboxy-terminal domain of Hsc70 provides binding sites for a distinct set of chaperone cofactors

The modulation of the chaperone activity of the heat shock cognate Hsc70 protein in mammalian cells involves cooperation with chaperone cofactors, such as Hsp40; BAG-1; the Hsc70-interacting protein, Hip; and the Hsc70-Hsp90-organizing protein, Hop. By employing the yeast two-hybrid system and in vitro interaction assays, we have provided insight into the structural basis that underlies Hsc70's cooperation with different cofactors. The carboxy-terminal domain of Hsc70, previously shown to form a lid over the peptide binding pocket of the chaperone protein, mediates the interaction of Hsc70 with Hsp40 and Hop. Remarkably, the two cofactors bind to the carboxy terminus of Hsc70 in a noncompetitive manner, revealing the existence of distinct binding sites for Hsp40 and Hop within this domain. In contrast, Hip interacts exclusively with the amino-terminal ATPase domain of Hsc70. Hence, Hsc70 possesses separate nonoverlapping binding sites for Hsp40, Hip, and Hop. This appears to enable the chaperone protein to cooperate simultaneously with multiple cofactors. On the other hand, BAG-1 and Hip have recently been shown to compete in binding to the ATPase domain. Our data thus establish the existence of a network of cooperating and competing cofactors regulating the chaperone activity of Hsc70 in the mammalian cell.     

Characterization of interactions between the anti-apoptotic protein BAG-1 and Hsc70 molecular chaperones

The anti-cell death protein BAG-1 binds to 70-kDa heat shock proteins (Hsp70/Hsc70) and modulates their chaperone activity. Among other facilitory roles, BAG-1 may serve as a nucleotide exchange factor for Hsp70/Hsc70 family proteins and thus represents the first example of a eukaryotic homologue of the bacterial co-chaperone GrpE. In this study, the interactions between BAG-1 and Hsc70 are characterized and compared with the analogous GrpE-DnaK bacterial system. In contrast to GrpE, which binds DnaK as a dimer, BAG-1 binds to Hsc70 as a monomer with a 1:1 stoichiometry. Dynamic light scattering, sedimentation equilibrium, and circular dichroism measurements provided evidence that BAG-1 exists as an elongated, highly helical monomer in solution. Isothermal titration microcalorimetry was used to determine the complex stoichiometry and an equilibrium dissociation constant, KD, of 100 nM. Kinetic analysis using surface plasmon resonance yielded a KD consistent with the calorimetrically determined value. Molecular modeling permitted a comparison of structural features between the functionally homologous BAG-1 and GrpE proteins. These data were used to propose a mechanism for BAG-1 in the regulation of Hsp70/Hsc70 chaperone activity.     

ATPase activity and molecular chaperone function of the stress70 proteins

The codons for the amino acid residues making up the proposed ATP-binding sites of the maize (Zea mays L.) endoplasmic reticulum and tomato (Lycopersicon esculentum) cytoplasmic Stress70 proteins were deleted from their respective cDNAs. The deletions had little effect on the predicted secondary structure characteristics of the encoded proteins. Both wild-type and mutant proteins were expressed in Escherichia coli and purified to electrophoretic homogeneity. The mutant recombinant proteins did not bind to immobilized ATP columns, had no detectable ATPase activity, and were unable to function in vitro as molecular chaperones. Additionally, the inability to bind ATP was associated with changes in the oligomerization state of the Stress70 proteins.     

Hop as an adaptor in the heat shock protein 70 (Hsp70) and hsp90 chaperone machinery

Hop, an abundant and conserved protein of unresolved function, binds concomitantly with heat shock protein 70 (Hsp70) and Hsp90, participates with heat shock proteins at an intermediate stage of progesterone receptor assembly, and is required for efficient assembly of mature receptor complexes in vitro. A largely untested hypothesis is that Hop functions as an adaptor that targets Hsp90- to Hsp70-substrate complexes; if true, then loss of either Hsp70 binding or Hsp90 binding by Hop should equally disrupt its ability to promote assembly of mature receptor complexes. To generate Hop mutants that selectively disrupt heat shock protein interactions, highly conserved amino acids in the previously mapped Hsp70 and Hsp90 binding domains of Hop and in a conserved C-terminal domain were targeted for small substitutions and deletions. In co-precipitation assays, these mutants displayed selective loss of association with heat shock proteins. In assays using Hop-depleted rabbit reticulocyte lysate for the cell-free assembly of receptor complexes, none of the Hop mutants inhibited Hsp70 binding to receptor, but all mutants were defective in supporting Hsp90-receptor interactions. Thus, Hop has a novel role in the chaperone machinery as an adaptor that can integrate Hsp70 and Hsp90 interactions.     

The molecular chaperone Hsp90 can negatively regulate the activity of a glucocorticosteroid-dependent promoter

Hsp90, a molecular chaperone required for the functioning of glucocorticosteroid receptor (GR), ensures, by direct interaction, the conformational competence of the steroid-binding pocket. In addition to having this positive function, Hsp90 maintains steroid receptors in an inactive form in the absence of hormone. However, neither the participation of Hsp90 once the pathway has been activated by the ligand nor the importance of increased Hsp90 levels in determining the amplitude of the response has ever been assessed directly. Here, by increasing the Hsp90/GR ratio in the nuclear compartment, we found an attenuation of the response to glucocorticosteroids that was not due to a nonspecific or toxic effect of the Hsp90 modified by nuclear targeting. Since this negative effect was more pronounced at high levels of hormone, when receptor and Hsp90 are maximally dissociated, the possibility of an interaction between Hsp90 and GR, already activated to a DNA-binding form, was directly investigated. Indeed GR, after in vivo activation by ligand, was still able to reassociate with Hsp90, suggesting that this interaction plays a role in vivo, possibly in receptor recycling. Moreover, the GR binding to its DNA response element was inhibited by an excess of Hsp90, pointing to a function of Hsp90 in the nuclear compartment. It is thus proposed that an increased Hsp90/GR ratio influences the responsiveness to ligand at a step that is after receptor activation. This increased ratio may be of pathophysiological relevance in the different circumstances that lead to an elevated level of nuclear Hsp90.     

Oligomerization of the 17-kDa peptide-binding domain of the molecular chaperone HSC70

Crystallographic and biochemical studies have indicated that the peptide-binding site of the molecular chaperone HSC70 is located in a small subdomain comprising a beta-sheet motif followed by a helical region, and there is some evidence of the involvement of this site in oligomerization of the protein. To determine the structure of this subdomain in solution and examine its involvement in oligomerization of HSC70, a 17-kDa protein (residues 385-540 of HSC70) consisting mainly of the peptide-binding site was constructed and analyzed for oligomerization properties. This small domain was found to bind peptides and to form oligomers in solution, probably tetramers, which dissociated into monomers on peptide binding in a manner comparable with that observed for the whole protein. Furthermore, in the 60-kDa fragment of HSC70, which is composed of the 17-kDa domain and the 44-kDa ATPase domain, not only were the oligomerization properties conserved, but dissociation of multimeric species into monomers on ATP binding also occurred and peptide stimulation of ATPase activity was restored. These results indicate that the isolated 17-kDa peptide-binding domain is necessary and sufficient for oligomerization of the whole protein, suggesting that the peptide-binding site may be involved in the oligomerization process.     

BAG-1, a negative regulator of Hsp70 chaperone activity, uncouples nucleotide hydrolysis from substrate release

Molecular chaperones influence the process of protein folding and, under conditions of stress, recognize non-native proteins to ensure that misfolded proteins neither appear nor accumulate. BAG-1, identified as an Hsp70 associated protein, was shown to have the unique properties of a negative regulator of Hsp70. Here, we demonstrate that BAG-1 inhibits the in vitro protein refolding activity of Hsp70 by forming stable ternary complexes with non-native substrates that do not release even in the presence of nucleotide and the co-chaperone, Hdj-1. However, the substrate in the BAG-1-containing ternary complex does not aggregate and remains in a soluble intermediate folded state, indistinguishable from the refolding-competent substrate-Hsp70 complex. BAG-1 neither inhibits the Hsp70 ATPase, nor has the properties of a nucleotide exchange factor; instead, it stimulates ATPase activity, similar to that observed for Hdj-1, but with opposite consequences. In the presence of BAG-1, the conformation of Hsp70 is altered such that the substrate binding domain becomes less accessible to protease digestion, even in the presence of nucleotide and Hdj-1. These results suggest a mechanistic basis for BAG-1 as a negative regulator of the Hsp70-Hdj-1 chaperone cycle.     

Overexpression in Escherichia coli, purification and characterization of the molecular chaperone HSC70

Two different tetrapeptides, AlaTrpTrpPro and AlaIleIlePro, were inserted near the C-terminus of the protein ZZT0. The Trp-rich peptide unit strongly increased both the partitioning of ZZT0 into the polyethylene glycol (PEG)-rich phase in a PEG-potassium phosphate aqueous two-phase system and its retention on PEG and propyl hydrophobic interaction chromatographic columns with potassium phosphate as eluent. Both the partitioning and the retention increased with increasing number of Trp-rich peptide units inserted into ZZT0. Insertion of Ile-rich tetrapeptide units affected the partitioning and retention to a much lesser extent. Partition data also indicated a folding of inserted Trp tetrapeptides units, probably to minimize their water contact.     

A chaperone in the HSP70 family controls production of extracellular fibrils in Myxococcus xanthus

Three independent Tn5-lac insertions in the S1 locus of Myxococcus xanthus inactivate the sglK gene, which is nonessential for growth but required for social motility and multicellular development. The sequence of sglK reveals that it encodes a homologue of the chaperone HSP70 (DnaK). The sglK gene is cotranscribed with the upstream grpS gene, which encodes a GrpE homologue. Unlike sglK, grpS is not required for social motility or development. Wild-type M. xanthus is encased in extracellular polysaccharide filaments associated with the multimeric fibrillin protein. Mutations in sglK inhibit cell cohesion, the binding of Congo red, and the synthesis or secretion of fibrillin, indicating that sglK mutants do not make fibrils. The fibR gene, located immediately upstream of the grpS-sglK operon, encodes a product which is predicted to have a sequence similar to those of the repressors of alginate biosynthesis in Pseudomonas aeruginosa and Pseudomonas putida. Inactivation of fibR leads to the overproduction of fibrillin, suggesting that M. xanthus fibril production and Pseudomonas alginate production are regulated in analogous ways. M. xanthus and Pseudomonas exopolysaccharides may play similar roles in a mechanism of social motility conserved in these gram-negative bacteria.     

ADP-ribosylation of the molecular chaperone GRP78/BiP

Starvation of mouse hepatoma cells for essential amino acids or glucose results in the ADP-ribosylation of the molecular chaperone BiP/GRP78. Addition of the missing nutrient to the medium reverses the reaction. The signal mediating the response to environmental nutrients involves the translational efficiency. An inhibitor of proteins synthesis, cycloheximide, or reduced temperature, both of which reduce translational efficiency, stimulate the ADP-ribosylation of BiP/GRP78. Inhibition of N-linked glycosylation of proteins results in the overproduction of BiP/GRP78. The over produced protein is not ADP-ribosylated suggesting that this is the functional form of BiP/GRP78. The over produced BiP/GRP78 can, however, be ADP-ribosylated if the cells are starved for an essential amino acid. BiP/GRP78 resides in the lumen of the endoplasmic reticulum where it participates in the assembly of secretory and integral membrane proteins. ADP-ribosylation of BiP/GRP78 during starvation is probably part of a nutritional stress response which conserves limited nutrients by slowing flow through the secretory pathway.     

Members of the Hsp70 family of proteins in the cell wall of Saccharomyces cerevisiae

Western blot (immunoblot) analysis of cell wall and cytosolic extracts obtained from parental and ssa1 and ssa2 single- and double-mutant strains of Saccharomyces cerevisiae showed that the heat shock protein 70 (Hsp70) products of these genes, previously thought to be restricted to the cell interior, are also present in the cell wall. A cell wall location was further confirmed by indirect immunofluorescence with intact cells and biotinylation of extracellular Hsp70. Hsp70s have been implicated in translocation across the membrane and as molecular chaperones, and changes in the profile of cell wall proteins suggested that these proteins may have a similar role in the cell wall.