Reconstitution of the steroid receptor.hsp90 heterocomplex assembly system of rabbit reticulocyte lysate

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.

High-molecular-weight FK506-binding proteins are components of heat-shock protein 90 heterocomplexes in wheat germ lysate

In animal cell lysates the multiprotein heat-shock protein 90 (hsp90)-based chaperone complexes consist of hsp70, hsp40, and p60. These complexes act to convert steroid hormone receptors to their steroid-binding state by assembling them into heterocomplexes with hsp90, p23, and one of several immunophilins. Wheat germ lysate also contains a hsp90-based chaperone system that can assemble the glucocorticoid receptor into a functional heterocomplex with hsp90. However, only two components of the heterocomplex-assembly system, hsp90 and hsp70, have thus far been identified. Recently, purified mammalian p23 preadsorbed with JJ3 antibody-protein A-Sepharose pellets was used to isolate a mammalian p23-wheat hsp90 heterocomplex from wheat germ lysate (J.K. Owens-Grillo, L.F. Stancato, K. Hoffmann, W.B. Pratt, and P. Krishna [1996] Biochemistry 35: 15249-15255). This heterocomplex was found to contain an immunophilin(s) of the FK506-binding class, as judged by binding of the radiolabeled immunosuppressant drug [3H]FK506 to the immune pellets in a specific manner. In the present study we identified the immunophilin components of this heterocomplex as FKBP73 and FKBP77, the two recently described high-molecular-weight FKBPs of wheat. In addition, we present evidence that the two FKBPs bind hsp90 via tetratricopeptide repeat domains. Our results demonstrate that binding of immunophilins to hsp90 via tetratricopeptide repeat domains is a conserved protein interaction in plants. Conservation of this protein-to-protein interaction in both plant and animal cells suggests that it is important for the biological action of the high-molecular-weight immunophilins.

In vivo function of Hsp90 is dependent on ATP binding and ATP hydrolysis

Heat shock protein 90 (Hsp90), an abundant molecular chaperone in the eukaryotic cytosol, is involved in the folding of a set of cell regulatory proteins and in the re-folding of stress-denatured polypeptides. The basic mechanism of action of Hsp90 is not yet understood. In particular, it has been debated whether Hsp90 function is ATP dependent. A recent crystal structure of the NH2-terminal domain of yeast Hsp90 established the presence of a conserved nucleotide binding site that is identical with the binding site of geldanamycin, a specific inhibitor of Hsp90. The functional significance of nucleotide binding by Hsp90 has remained unclear. Here we present evidence for a slow but clearly detectable ATPase activity in purified Hsp90. Based on a new crystal structure of the NH2-terminal domain of human Hsp90 with bound ADP-Mg and on the structural homology of this domain with the ATPase domain of Escherichia coli DNA gyrase, the residues of Hsp90 critical in ATP binding (D93) and ATP hydrolysis (E47) were identified. The corresponding mutations were made in the yeast Hsp90 homologue, Hsp82, and tested for their ability to functionally replace wild-type Hsp82. Our results show that both ATP binding and hydrolysis are required for Hsp82 function in vivo. The mutant Hsp90 proteins tested are defective in the binding and ATP hydrolysis-dependent cycling of the co-chaperone p23, which is thought to regulate the binding and release of substrate polypeptide from Hsp90. Remarkably, the complete Hsp90 protein is required for ATPase activity and for the interaction with p23, suggesting an intricate allosteric communication between the domains of the Hsp90 dimer. Our results establish Hsp90 as an ATP-dependent chaperone.

Association of the rat liver glucocorticoid receptor with Hsp90 and Hsp70 upon whole body hyperthermic stress

The influence of whole body hyperthermic stress (41 degrees C, 15 min) on association of the glucocorticoid receptor (GR) with heat shock proteins Hsp90 and Hsp70 was followed in rat liver cytosol during a 24 h period after the stress. Total cytosolic concentration of the GR, Hsp90 and Hsp70 and the amounts of Hsp90 and Hsp70 co-immunopurified with the GR were determined by a quantitative Western blotting using appropriate monoclonal antibodies. A significant decrease in the cytosolic GR level in response to the stress was noticed. The ratio of the amount of the GR to Hsp90 recruited by the GR was found to be unaltered by hyperthermia, in spite of the stress-induced increase in the total Hsp90 concentration in the cytosol. Hsp70 was also found in association with the GR and its 2.5-fold induction by the stress was accompanied by about 3-fold increase in its relative amount that co-immunopurified with the GR. The results suggest that heat stress influences the interaction of the GR with Hsp70 through the mechanisms controlling the untransformed rat liver GR heterocomplexes assembly process.

Heat-shock protein 90 (hsp90) binds in vitro to tubulin dimer and inhibits microtubule formation

Hsp90 interacts with steroid hormone receptors, protein kinases, and cytoskeletal proteins. The mode of action of hsp90 on microtubules and tubulin has not been investigated. Using isolated purified hsp90 and isolated tubulin, we demonstrated in vitro by difference absorption and fluorescence spectroscopy that hsp90 bound to tubulin with an apparent affinity constant of 5 x 10(5) M-1, assuming an apparent stoichiometry of 1 at 25 degrees C. Using microcalorimetry, we found a delta H of -9.8 +/- 0.8 kJ.mol-1. The binding of hsp90 to tubulin was confirmed by a sedimentation assay. Moreover, we showed that hsp90 inhibited tubulin polymerisation.

p50(cdc37) binds directly to the catalytic domain of Raf as well as to a site on hsp90 that is topologically adjacent to the tetratricopeptide repeat binding site

Several protein kinases (e.g. pp60(src), v-Raf) exist in heterocomplexes with hsp90 and a 50-kDa protein that is the mammalian homolog of the yeast cell cycle control protein Cdc37. In contrast, unliganded steroid receptors exist in heterocomplexes with hsp90 and a tetratricopeptide repeat (TPR) domain protein, such as an immunophilin. Although p50(cdc37) and TPR domain proteins bind directly to hsp90, p50(cdc37) is not present in native steroid receptor.hsp90 heterocomplexes. To obtain some insight as to how v-Raf selects predominantly hsp90.p50(cdc37) heterocomplexes, rather than hsp90.TPR protein heterocomplexes, we have examined the binding of p50(cdc37) to hsp90 and to Raf. We show that p50(cdc37) exists in separate hsp90 heterocomplexes from the TPR domain proteins and that intact TPR proteins compete for p50(cdc37) binding to hsp90 but a protein fragment containing a TPR domain does not. This suggests that the binding site for p50(cdc37) lies topologically adjacent to the TPR acceptor site on the surface of hsp90. Also, we show that p50(cdc37) binds directly to v-Raf, with the catalytic domain of Raf being sufficient. We propose that the combination of exclusive binding of p50(cdc37) versus a TPR domain protein to hsp90 plus direct binding of p50(cdc37) to Raf allows the protein kinase to select for the dominant hsp90.p50(cdc37) composition that is observed with a variety of protein kinase heterocomplexes immunoadsorbed from cytosols.

Studies with Purified Chaperones Advance the Understanding of the Mechanism of Glucocorticoid Receptor-hsp90 Heterocomplex Assembly

The study of the 9S, untransformed state of steroid receptors has led to the discovery of a multiprotein chaperone system that assembles heterocomplexes between hsp90 and a variety of proteins involved in signal transduction. Using the formation of glucocorticoid receptor (GR)-hsp90 heterocomplexes as a model, we have reconstituted a fully functional heterocomplex assembly system from purified components. The basic assembly system requires four proteins-hsp90, hsp70, p60/Hop and hsp40-to assemble GR-hsp90 heterocomplexes, which are then stabilized by the hsp90-interacting protein p23. The four proteins can self-assemble into an hsp90-p60/Hop-hsp70-hsp40 complex that we call a foldosome. Foldosomes isolated from reticulocyte lysate or formed from purified proteins open up a steroid-binding pocket to create a high-affinity steroid-binding state of the GR. We describe here the systematic reconstitution of the hsp90-based chaperone machinery and develop a model of the receptor-hsp90 heterocomplex assembly mechanism.

Active participation of Hsp90 in the biogenesis of the trimeric reovirus cell attachment protein sigma1

The reovirus cell attachment protein, sigma1, is a lollipop-shaped homotrimer with an N-terminal fibrous tail and a C-terminal globular head. Biogenesis of this protein involves two trimerization events: N-terminal trimerization, which occurs cotranslationally and is Hsp70/ATP-independent, and C-terminal trimerization, which occurs posttranslationally and is Hsp70/ATP-dependent. To determine if Hsp90 also plays a role in sigma1 biogenesis, we analyzed sigma1 synthesized in rabbit reticulocyte lysate. Coprecipitation experiments using anti-Hsp90 antibodies revealed that Hsp90 was associated with immature sigma1 trimers (hydra-like intermediates with assembled N termini and unassembled C termini) but not with mature trimers. The use of truncated sigma1 further demonstrated that only the C-terminal half of sigma1 associated with Hsp90. In the presence of the Hsp90 binding drug geldanamycin, N-terminal trimerization proceeded normally, but C-terminal trimerization was blocked. Geldanamycin did not inhibit the association of Hsp90 with sigma 1 but prevented the subsequent release of Hsp90 from the immature sigma1 complex. We also examined the status of p23, an Hsp90-associated cochaperone. Like Hsp90, p23 only associated with immature sigma1 trimers, and this association was mapped to the C-terminal half of sigma1. However, unlike Hsp90, p23 was released from the sigma1 complex upon the addition of geldanamycin. These results highlight an all-or-none concept of chaperone involvement in different oligomerization domains within a single protein and suggest a possible common usage of chaperones in the regulation of general protein folding and of steroid receptor activation.

Binding of hsp90 to the glucocorticoid receptor requires a specific 7-amino acid sequence at the amino terminus of the hormone-binding domain

The glucocorticoid receptor (GR) HBD must be bound to the protein chaperone hsp90 in order to acquire the high affinity steroid binding conformation. Despite this crucial role of hsp90, its binding site in GR remains poorly defined. Large portions of the GR HBD have been implicated and no similarity has been established between steroid receptor HBDs and the catalytic domains of the protein kinases (e.g. pp60(src), Raf) that also form stable heterocomplexes with hsp90. Thus, it has been thought that some general property of the proteins, such as exposure of hydrophobic residues in partially denatured regions, determines the assembly of stable hsp90 heterocomplexes. In this work, we have studied fusion proteins containing glutathione S-transferase (GST) and very short amino-terminal truncations just before and at the beginning of the rat GR HBD that are otherwise intact to the carboxyl terminus. Overexpression in COS cells of the chimeras GST537C and GST547C was found to yield receptors that were bound to hsp90 and had wild-type steroid binding affinity. However, removal of 7 more amino acids to form GST554C resulted in a fusion protein that did not bind either hsp90 or steroid. Additional mutations revealed that the role of these 7 amino acids was neither to provide a spacer between protein domains nor to expose a protein surface by introducing a bend in the conserved alpha-helix. Instead, these observations support a model in which the sequence of the 7 amino acids directly or indirectly affects hsp90 binding to the GR HBD. Thus, a region of GR that has not been thought to be relevant for hsp90 binding is now seen to be of critical importance, and these data argue strongly against the commonly accepted model of receptor-hsp90 heterocomplex assembly in which the chaperone initially interacts nonspecifically with hydrophobic regions of the partially denatured HBD and subsequently assists its folding to the steroid binding confirmation.

Fructose-1,6-diphosphate enhanced oxyradicals and nitric oxide-dependent suppressions by dexamethasone of ischemic and histamine paw edema of mice

Dexamethasone (Dex, 0.1 mg/kg, s.c.) suppressions of ischemic paw edema in mice at 1, 3, 6, 8, 18 hr were; 2, 22, 12, 11, 7%. Dex suppression in fructose-1,6-diphosphate (FDP, 100 mg/kg, i.p.)-treated mice were; 5, 49, 51, 42, 33%. Suppressions by this dose of FDP alone were less than 10% during 0-18 hr. ED30 at 6 hr of Dex +/- FDP was: 80 versus 500 mg/kg in ischemic-, and 5 versus 30 mg/kg in histamine edema. Endogenous oxyradicals or NO and protein synthesis were essential for suppressions. FDP may not change glucocorticoid receptor (GR) conformation, but increase ATP-dependent GR recycling efflux from nucleus. FDP is possible to supply this ATP. Clinical trial of FDP with low dose of Dex seems advantageous.

The role of DnaJ-like proteins in glucocorticoid receptor.hsp90 heterocomplex assembly by the reconstituted hsp90.p60.hsp70 foldosome complex

The glucocorticoid receptor (GR) is recovered from hormone-free cells in a heterocomplex with the molecular chaperone hsp90, which is required to produce the proper folding state for steroid binding. GR.hsp90 heterocomplexes are formed by a multiprotein system that appears to exist in all eukaryotic cells. Recently, we have reconstituted a receptor.hsp90 heterocomplex assembly system with purified rabbit hsp90 and hsp70 and bacterially expressed human p23 and p60. We have shown that hsp90, p60, and hsp70 form an hsp90.p60. hsp70 complex that converts the GR from a non-steroid binding to a steroid binding form (Dittmar, K. D., and Pratt, W. B. (1997) J. Biol. Chem. 272, 13047-13054). The resulting GR.hsp90 heterocomplex rapidly disassembles unless p23 is present to bind to the ATP-dependent conformation of hsp90 and stabilize its association with the receptor (Dittmar, K. D., Demady, D. R., Stancato, L. F., Krishna, P., and Pratt, W. B. (1997) J. Biol. Chem. 272, 21213-21220). In the current work, we show that the purified rabbit hsp70 utilized in prior studies is contaminated with a small amount of the rabbit DnaJ homolog hsp40. Elimination of the hsp40 from the purified GR.hsp90 assembly system reduces assembly activity, and the activity is restored by addition of the purified yeast DnaJ homolog YDJ-1. hsp40 is a component of the hsp90.p60.hsp70 foldosome complex isolated from reticulocyte lysate with antibody against p60. Under conditions that promote binding of p23 to hsp90 (elevated temperature, ATP, Nonidet P-40, molybdate), a five-membered (p23. hsp90.p60.hsp70.hsp40) complex of chaperone proteins is formed in reticulocyte lysate or from purified proteins. The hsp40-free, purified assembly system has a modest level of assembly activity that is maximally potentiated by YDJ-1 when it is present at about one-twentieth the concentration of hsp70. Although hsp40 is not in the final GR.hsp90 heterocomplex isolated from L cell cytosol, it is in the GR.hsp90 heterocomplex assembled in reticulocyte lysate. We conclude that hsp40 is a component of the multiprotein hsp90-based chaperone system where it potentiates GR.hsp90 heterocomplex assembly.

Agonist-specific modulation of glucocorticoid receptor-mediated transcription by immunosuppressants

Although the immunosuppressive drugs FK506, rapamycin and cyclosporin A have been reported to potentiate transcriptional activation mediated by a non-saturating concentration of the glucocorticoid receptor agonist dexamethasone, the precise mechanism(s) underlying these responses remains unclear. The murine L-929-derived LMCAT cell line stably transfected with the mouse mammary tumor virus promoter-chloramphenicol acetyl transferase reporter gene construct was utilized in the present study to further investigate the mechanism(s) underlying this dexamethasone potentiation as well as the possible agonist specificity of this potentiation. The present data demonstrate that pretreatment (2 h) of LMCAT cells with 10 microM FK506, rapamycin or cyclosporin A results in the potentiation of reporter gene transcription mediated not only by dexamethasone (approximately 12-fold), but also by hydrocortisone (approximately 6-fold) and triamcinolone acetonide (approximately 2.5-fold). In sharp contrast, the data show for the first time that pretreatment with any one of these immunosuppressive drugs suppresses (approximately 2-8-fold) the transcriptional responses mediated by corticosterone, deoxycorticosterone, and cortexolone. Pretreatment of intact LMCAT cells with FK506 increases the subsequent whole cell specific binding of [3H]dexamethasone, but does not increase specific cytoplasmic binding when the tritiated agonist is added directly to cytosolic extracts prepared from the pretreated cells. These data suggest that the FK506-mediated potentiation of the transcriptional responses induced by some agonists, like dexamethasone, may be related to the ability of this immunosuppressant to inhibit the membrane-associated multidrug resistance (MDR) P-glycoprotein, which actively extrudes some steroids from cells. Identical pretreatment with FK506 has no detectable effect on the subsequent whole cell specific binding of [3H]corticosterone, a steroid which is not effectively extruded by the MDR pump. Two additional MDR pump inhibitors, verapamil and quinidine, potentiate (30-fold) the dexamethasone-mediated transcriptional response as expected, but have no detectable effects on a corticosterone-mediated transcriptional response. Unlike immunosuppressive drugs, these ion channel blockers do not bind to receptor-associated immunophilins (FK506-binding proteins or cyclophilins). Collectively, these results suggest that immunosuppressants potentiate a dexamethasone-mediated transcriptional response in LMCAT cells by inhibiting efflux of this steroid. In contrast, these drugs appear to suppress a corticosterone-mediated transcriptional response by a different mechanism, perhaps one involving their binding to glucocorticoid receptor-associated immunophilins.

Sequence motifs shared between chaperone components participating in the assembly of progesterone receptor complexes

Steroid receptors typically exist in a heteromeric complex with Hsp90 and other components of the molecular chaperone machinery. Assembly of functional receptor complexes follows an ordered pathway involving at least eight chaperone components, some only participating in early assembly stages that are prerequisite for formation of mature complexes. The mechanisms directing the order of assembly steps and the nature of transitional interactions between assembly steps are largely unknown, but likely are encoded in the primary sequence and functional domains of the participating chaperones. Several common sequence motifs are shared between participants that may be key in ordering the steroid receptor assembly pathway.

The physical association of multiple molecular chaperone proteins with mutant p53 is altered by geldanamycin, an hsp90-binding agent

Wild-type p53 is a short-lived protein which turns over very rapidly via selective proteolysis in the ubiquitin-proteasome pathway. Most p53 mutations, however, encode for protein products which display markedly increased intracellular levels and are associated with positive tumor-promoting activity. The mechanism by which mutation leads to impairment of ubiquitination and proteasome-mediated degradation is unknown, but it has been noted that many transforming p53 mutants are found in stable physical association with molecular chaperones of the hsp70 class. To explore a possible role for aberrant chaperone interactions in mediating the altered function of mutant p53 and its intracellular accumulation, we examined the chaperone proteins which physically associate with a temperature-sensitive murine p53 mutant. In lysate prepared from A1-5 cells grown under mutant temperature conditions, hsp70 coprecipitated with p53Val135 as previously reported by others, but in addition, other well-recognized elements of the cellular chaperone machinery, including hsp90, cyclophilin 40, and p23, were detected. Under temperature conditions favoring wild-type p53 conformation, the coprecipitation of chaperone proteins with p53 was lost in conjunction with the restoration of its transcriptional activating activity. Chaperone interactions similar to those demonstrated in A1-5 cells under mutant conditions were also detected in human breast cancer cells expressing two different hot-spot mutations. To examine the effect of directly disrupting chaperone interactions with mutant p53, we made use of geldanamycin (GA), a selective hsp90-binding agent which has been shown to alter the chaperone associations regulating the function of unliganded steroid receptors. GA treatment of cells altered heteroprotein complex formation with several different mutant p53 species. It increased p53 turnover and resulted in nuclear translocation of the protein in A1-5 cells. GA did not, however, appear to restore wild-type transcriptional activating activity to mutant p53 proteins in either A1-5 cells or human breast cancer cell lines.

Hop modulates Hsp70/Hsp90 interactions in protein folding

Hop is a 60-kDa protein characterized by its ability to bind the two chaperones, hsp70 and hsp90. We have tested the function of Hop using an assay for the refolding of denatured firefly luciferase. We show that Hop is involved in the process of refolding thermally denatured firefly luciferase in rabbit reticulocyte lysate. Hop also stimulates refolding by hsp70 and Ydj-1 in a purified refolding system. Hsp90 can also stimulate refolding, and optimal refolding is observed in the presence of both Hop and hsp90. Similar stimulation was observed when Hop was replaced by its yeast homolog Sti1. In assays of the binding of Hop to hsp70 and hsp90, Hop preferentially forms a complex with ADP-bound hsp70, and this process is unaffected by the presence of hsp90. Hop does not alter the ATPase activity or the rate of ADP dissociation of hsp70. Hop also appears to bind to the ADP-bound form of hsp90, blocking the ATP-dependent conversion of hsp90 to a form capable of interacting with p23. Conversely, once p23 is bound to hsp90, Hop binding is diminished. These results confirm that Hop provides a physical link between hsp70 and hsp90 and also indicate that Hop modulates the activities of both of these chaperone proteins.

Mutation of Hip's carboxy-terminal region inhibits a transitional stage of progesterone receptor assembly

Steroid receptor complexes are assembled through an ordered, multistep pathway involving multiple components of the cytoplasmic chaperone machinery. Two of these components are Hsp70-binding proteins, Hip and Hop, that have some limited homology in their C-terminal regions, outside the sequences mapped for Hsp70 binding. Within this region of Hip is a DPEV sequence that occurs twice; in Hop, one DPEV sequence plus a partial second sequence occurs. In an effort to better understand Hip function as it relates to assembly of progesterone receptor complexes, the DPEV region of Hip was targeted for mutations. Each DPEV sequence was mutated to an APAV sequence, singly or in combination. The combined mutation, APAV2, was further combined with a deletion of Hip's tetratricopeptide repeat region that is required for Hsp70 binding or with a deletion of Hip's GGMP repeat. An additional mutant was prepared by truncation of Hip's DPEV-containing C terminus. By comparing interactions of various Hip forms with Hsp70, it was determined that mutation of the DPEV sequences created a dominant inhibitory form of Hip. The mutant Hip-Hsp70 complex was not prevented from interacting with progesterone receptor, but the mutant caused a dose-dependent inhibition of receptor assembly with Hsp90. The behavior of the Hip mutant is consistent with a model in which Hip and Hop are required to facilitate the transition from an early receptor complex with Hsp70 into later complexes containing Hsp90.

Analysis of complex formation between Hsp80 and Hsp70, cytosolic molecular chaperones ofNeurospora crassa, by enzyme-linked immunosorbent assays (ELISA)

A physical association between Hsp70 and Hsp80, the major cytosolic stress proteins of Neurospora crassa, was demonstrated previously by interprotein crosslinking and by binding of Hsp80 to Hsp70 immobilized on ATP-agarose. In the present study, [Hsp70:Hsp80] complex formation was analyzed by enzyme-linked immunosorbent assays (ELISA), using specific antibodies. One protein was fixed onto ELISA plate wells and binding of the second mobile protein was monitored by retention of its cognate IgG. Binding of Hsp70 and Hsp80 to immobilized Hsp80 and Hsp70, respectively, was readily detectable at submicrogram levels. The effect of cations and various nucleotides on [Hsp70:Hsp80] complex was examined by inclusion of KCl, MgCl2, MnCl2, and nucleotides in the interaction mixture. K+stimulated interaction between immobilized Hsp70 and Hsp80 in solution and adenosine nucleotides exerted a stimulatory effect on complexation as well. Similarly, CTP, NAD, and NADH enhanced complex formation between plate-bound Hsp70 and Hsp80 in solution, attesting to the conformational flexibility of Hsp80. Epitope blocking revealed an overlap between protein-protein contact surfaces and antibody recognition sites. Binding to alpha -carboxymethylated lactalbumin showed that Hsp70 and Hsp80 can interact with an unfolded polypeptide, individually and in complex.Key words: molecular chaperones, Neurospora, heat shock proteins, Hsp80:Hsp70 complex.

Hepatitis B virus X-associated protein 2 is a subunit of the unliganded aryl hydrocarbon receptor core complex and exhibits transcriptional enhancer activity

Prior to ligand activation, the unactivated aryl hydrocarbon receptor (AhR) exists in a heterotetrameric 9S core complex consisting of the AhR ligand-binding subunit, a dimer of hsp90, and an unknown subunit. Here we report the purification of an approximately 38-kDa protein (p38) from COS-1 cell cytosol that is a member of this complex by coprecipitation with a FLAG-tagged AhR. Internal amino acid sequence information was obtained, and p38 was identified as the hepatitis B virus X-associated protein 2 (XAP2). The simian ortholog of XAP2 was cloned from a COS-1 cDNA library; it codes for a 330-amino-acid protein containing regions of homology to the immunophilins FKBP12 and FKBP52. A tetratricopeptide repeat (TPR) domain in the carboxy-terminal region of XAP2 was similar to the third and fourth TPR domains of human FKBP52 and the Saccharomyces cerevisiae transcriptional modulator SSN6, respectively. Polyclonal antibodies raised against XAP2 recognized p38 in the unliganded AhR complex in COS-1 and Hepa 1c1c7 cells. It was ubiquitously expressed in murine tissues at the protein and mRNA levels. It was not required for the assembly of an AhR-hsp90 complex in vitro. Additionally, XAP2 did not directly associate with hsp90 upon in vitro translation, but was present in a 9S form when cotranslated in vitro with murine AhR. XAP2 enhanced the ability of endogenous murine and human AhR complexes to activate a dioxin-responsive element-luciferase reporter twofold, following transient expression of XAP2 in Hepa 1c1c7 and HeLa cells.

Folding of the glucocorticoid receptor by the heat shock protein (hsp) 90-based chaperone machinery. The role of p23 is to stabilize receptor.hsp90 heterocomplexes formed by hsp90.p60.hsp70

In cytosols from animal and plant cells, the abundant heat shock protein hsp90 is associated with several proteins that act together to assemble steroid receptors into receptor.hsp90 heterocomplexes. We have reconstituted a minimal receptor.hsp90 assembly system containing four required components, hsp90, hsp70, p60, and p23 (Dittmar, K. D., Hutchison, K. A., Owens-Grillo, J. K., and Pratt, W. B. (1996) J. Biol. Chem. 271, 12833-12839). We have shown that hsp90, p60, and hsp70 are sufficient for carrying out the folding change that converts the glucocorticoid receptor (GR) hormone binding domain (HBD) from a non-steroid binding to a steroid binding conformation, but to form stable GR.hsp90 heterocomplexes, p23 must also be present in the incubation mix (Dittmar, K. D., and Pratt, W. B. (1997) J. Biol. Chem. 272, 13047-13054). In this work, we show that addition of p23 to native GR.hsp90 heterocomplexes immunoadsorbed from L cell cytosol or to GR.hsp90 heterocomplexes prepared with the minimal (hsp90.p60.hsp70) assembly system inhibits both receptor heterocomplex disassembly and loss of steroid binding activity. p23 stabilizes the GR.hsp90 heterocomplex in a dynamic and ATP-independent manner. In contrast to hsp90 that is bound to the GR, free hsp90 binds p23 in an ATP-dependent manner, and hsp90 in the hsp90.p60.hsp70 heterocomplex is in a conformation that does not bind p23 at all. The effect of p23 in the minimal GR heterocomplex assembly system is to stabilize GR.hsp90 heterocomplexes once they are formed and it does not appear to affect the rate of heterocomplex assembly. Molybdate has the same ability as p23 to stabilize GR heterocomplexes with mammalian hsp90, but GR heterocomplexes with plant hsp90 are stabilized by p23 and not by molybdate. We propose that incubation of the GR with hsp90.p60.hsp70 forms a GR.hsp90 heterocomplex in which hsp90 is in an ATP-dependent conformation. The ATP-dependent conformation of hsp90 is required for the hormone binding domain to have a steroid binding site, and binding of p23 to that state of hsp90 stabilizes the GR.hsp90 heterocomplex to inactivation and disassembly.

The function of steroid hormone receptors is inhibited by the hsp90-specific compound geldanamycin

The ansamycin antibiotic geldanamycin, which specifically interacts with the heat shock protein hsp90, was used to study the function of hsp90 in steroid hormone receptors. We observed inhibition of glucocorticoid-specific gene induction in several responsive cell systems. Hormone binding abilities of receptors for glucocorticoid, progestin, androgen, and estrogen were inhibited upon exposing intact cells to geldanamycin. Inhibition was only seen when geldanamycin was applied to cell cultures under growth conditions or was present during in vitro synthesis; presynthesized receptors in cell extracts were not affected. Upon withdrawal of geldanamycin, glucocorticoid binding ability was regained; this was partially independent of de novo protein synthesis. Geldanamycin caused decreased levels of immunoreactive glucocorticoid receptors in wild-type cells with enhanced degradation occurring through the ubiquitin-proteasome pathway. Analysis of receptors from treated cells revealed a heteromeric structure of normal size in which the receptor polypeptide is complexed with normal amounts of hsp90 and the immunophilin p59. These data support the view that hsp90 actively participates in steroid-induced signal transduction, and they suggest that geldanamycin affects receptor action without disrupting hsp90-containing heterocomplexes per se. Nevertheless, complexes synthesized and assembled in vitro in the presence of geldanamycin differ from receptors of cellular origin.

Protein phosphatase 5 is a major component of glucocorticoid receptor.hsp90 complexes with properties of an FK506-binding immunophilin

Steroid receptors are recovered from hormone-free cells in multiprotein complexes containing hsp90, p23, an immunophilin, and often some hsp70. The immunophilin, which can be of the FK506- or cyclosporin A-binding class, binds to hsp90 via its tetratricopeptide repeat (TPR) domain, and different receptor heterocomplexes exist depending upon which immunophilin occupies the TPR-binding region of hsp90. We have recently reported that a protein serine/threonine phosphatase that is designated PP5 and contains four TPRs binds to hsp90 and is co-purified with the glucocorticoid receptor (GR) (Chen, M.-S., Silverstein, A. M., Pratt, W. B., and Chinkers, M. (1996) J. Biol. Chem. 271, 32315-32320). In this work, we show that PP5 is recovered with both GR that is nuclear and GR that is cytoplasmic in hormone-free cells. Approximately one-half of the GR.hsp90 heterocomplexes in L cell cytosol contains an immunophilin with high affinity FK506 binding activity, such as FKBP51 or FKBP52, and approximately 35% contains PP5. Only a small (but undetermined) fraction of the native GR.hsp90 heterocomplexes contain the cyclosporin A-binding immunophilin CyP-40. PP5, FKBP52, and CyP-40 exist in separate heterocomplexes with hsp90, and competition binding experiments with the PP5 TPR domain suggest that the three proteins occupy a common binding site on hsp90. A 55-residue connecting region between the N-terminal TPR domain of human PP5 and its C-terminal phosphatase domain has 50% amino acid homology and 22% identity with the central portion of the peptidylprolyl isomerase domain of human FKBP52. Of the 9 residues in this portion of FKBP52 involved in high affinity interactions with FK506, 3 residues are retained and 4 have homologous substitutions in PP5. Although immunoadsorbed PP5 did not bind [3H]FK506, we found that both rabbit PP5 in reticulocyte lysate and purified rat PP5 were specifically retained by an FK506 affinity matrix. Thus, we propose that PP5 possesses properties of an immunophilin with low affinity FK506 binding activity and that it determines a major portion of the native GR heterocomplexes in L cell cytosol.

Steroid receptor interactions with heat shock protein and immunophilin chaperones

We have provided a historical perspective on a body of steroid receptor research dealing with the structure and physiological significance of the untransformed 9S receptor that has often confused both novice and expert investigators. The frequent controversies and equivocations of earlier studies were due to the fact that the native, hormone-free state of these receptors is a large multiprotein complex that resisted description for many years because of its unstable and dynamic nature. The untransformed 9S state of the steroid and dioxin receptors has provided a unique system for studying the function of the ubiquitous, abundant, and conserved heat shock protein, hsp90. The hormonal control of receptor association with hsp90 provided a method of manipulating the receptor heterocomplex in a manner that was physiologically meaningful. For several steroid receptors, binding to hsp90 was required for the receptor to be in a native hormone-binding state, and for all of the receptors, hormone binding promoted dissociation of the receptor from hsp90 and conversion of the receptor to the DNA-binding state. Although the complexes between tyrosine kinases and hsp90 were discovered earlier, the hormonal regulation or steroid receptor association with hsp90 permitted much more rapid and facile study of hsp90 function. The observations that hsp90 binds to the receptors through their HBDs and that these domains can be fused to structurally different proteins bringing their function under hormonal control provided a powerful linkage between the hormonal regulation of receptor binding to hsp90 and the initial step in steroid hormone action. Because the 9S receptor hsp90 heterocomplexes could be physically stabilized by molybdate, their protein composition could be readily studied, and it became clear that these complexes are multiprotein structures containing a number of unique proteins, such as FKBP51, FKBP52, CyP-40, and p23, that were discovered because of their presence in these structures. Further analysis showed that hsp90 itself exists in a variety of native multiprotein heterocomplexes independent of steroid receptors and other 'substrate' proteins. Cell-free systems can now be used to study the formation of receptor heterocomplexes. As we outlined in the scheme of Fig. 1, the multicomponent receptor-hsp90 heterocomplex assembly system is being reconstituted, and the importance of individual proteins, such as hsp70, p60, and p23, in the assembly process is becoming recognized. It should be noted that our understanding of the mechanism and purpose of steroid receptor heterocomplex assembly is still at an early stage. We can now speculate on the roles of receptor-associated proteins in receptor action, both as individuals and as a group, but their actual functions are still vague or unknown. We can make realistic models about the chaperoning and trafficking of steroid receptors, but we don't yet know how these processes occur, we don't know where chaperoning occurs in the cell (e.g. Is it limited to the cytoplasm? Is it a diffuse process or does chaperoning occur in association with structural elements?), and, with the exception of the requirement for hormone binding, we don't know the extent to which the hsp90-based chaperone system impacts on steroid hormone action. It is not yet clear how far the discovery of this hsp90 heterocomplex assembly system will be extended to the development of a general understanding of protein processing in the cell. Because this assembly system is apparently present in all eukaryotic cells, it probably performs an essential function for many proteins. The bacterial homolog of hsp90 is not an essential protein, but hsp90 is essential in eukaryotes, and recent studies indicate that the development of the cell nucleus from prokaryotic progenitors was accompanied by the duplication of genes for hsp90 and hsp70 (698). (ABSTRACT TRUNCATED)

Folding of the glucocorticoid receptor by the reconstituted Hsp90-based chaperone machinery. The initial hsp90.p60.hsp70-dependent step is sufficient for creating the steroid binding conformation

Rabbit reticulocyte lysate contains a multiprotein chaperone system that assembles steroid receptors into a complex with hsp90. The glucocorticoid receptor (GR) is bound to hsp90 via its hormone binding domain (HBD), which must be associated with hsp90 to have a steroid binding conformation. Recently, we have reconstituted a receptor.hsp90 heterocomplex assembly system with purified rabbit hsp90 and hsp70 and bacterially expressed human p23 and p60 (Dittmar, K. D., Hutchison, K. A., Owens-Grillo, J. K., and Pratt, W. B. (1996) J. Biol. Chem. 271, 12833-12839). In this work we show that when the GR is incubated with hsp90, hsp70, and p60, steroid binding sites are generated despite the absence of p23. In this minimal reconstituted system, the GR is incubated with the chaperones in the presence of [3H]triamcinolone acetonide ([3H]TA), which binds to the receptor as GR.hsp90 complexes are formed. When molybdate or p23 is also present during the incubation with chaperones at 30 degrees C, the formation of steroid binding sites can be assayed by incubating the washed GR with [3H]TA after heterocomplex assembly at 30 degrees C. However, in the absence of p23 or molybdate, rapid disassembly of GR.hsp90 complexes apparently occurs simultaneously with assembly, such that [3H]TA must be present during the assembly process to trap evidence of conversion of the GR HBD from a non-steroid binding to a steroid binding conformation. Mixture of purified rabbit hsp90 and hsp70 with bacterial lysate containing human p60 results in spontaneous formation of an hsp90.p60.hsp70 complex that can be adsorbed with anti-p60 antibody, and the resulting immune complex converts the GR HBD to a steroid binding state in an ATP-dependent and K+-dependent manner. When the GR is incubated with hsp90, hsp70, and p60 in the presence of the hsp90-binding antibiotic geldanamycin, GR.hsp90.p60. hsp70 complexes are formed, but they have no steroid binding activity. Our data suggest that hsp90, hsp70, and p60 work together as a chaperone complex that possesses all of the folding/unfolding activity necessary to generate the high affinity steroid binding conformation of the receptor.

Functional analysis of the yeast 40 kDa cyclophilin Cyp40 and its role for viability and steroid receptor regulation

We have identified and characterized a homolog of the 40 kDa cyclophilins in the budding yeast Saccharomyces cerevisiae. At the amino acid level, this novel yeast cyclophilin, termed Cyp40, is 47% identical to human cyclophilin-40. Recombinant Cyp40 produced in bacteria has a peptidyl-prolyl cis-trans isomerase activity with a catalytic efficiency (k[cat]/K[m]) of 0.5 x 10(6)M(-1)s(-1), which can be inhibited by cyclosporin A with an IC50 value of 60nM. Using a polyclonal antibody against Cyp40 we have found that Cyp40 is predominantly cytoplasmic, and that its expression is induced 3-4-fold by heat shock. Moreover, Cyp40 can be coprecipitated from yeast extracts with the cytosolic molecular chaperone Hsp90. Surprisingly, a Cyp40-deficient yeast strain is fully viable at normal and elevated temperatures. Cyp40 is also dispensable for normal regulation of vertebrate steroid receptors in yeast. While other immunophilins could conceivably compensate a Cyp40 defect, our results are compatible with the notion that immunophilins may be fortuitous partners in the biochemically established steroid receptor-Hsp90 complex.

Ligand-dependent interaction of the aryl hydrocarbon receptor with a novel immunophilin homolog in vivo

In an effort to identify regulators of aryl hydrocarbon receptor (AHR) signaling, we have employed the yeast two-hybrid system to screen for human proteins that interact in a ligand-dependent manner with the AHR. After screening 1.4 x 10(6) clones from a human B cell library, two distinct clones were identified that associated specifically with the liganded receptor. No clones were identified that interacted preferentially with the unliganded AHR. One of the ligand-dependent clones, ARA9, encodes a novel 330-amino acid protein with regions of amino acid sequence similarity to the 52-kDa FK506-binding protein known to be associated with the glucocorticoid receptor. Yeast two-hybrid experiments with ARA9 demonstrated a strong interaction with the AHR that is enhanced 11-fold in the presence of the ligand beta-naphthoflavone. In vitro experiments using proteins generated in reticulocyte lysates confirmed this interaction and indicated that ARA9 can be co-immunoprecipitated with the AHR using antisera raised specifically for either the AHR or the 90-kDa heat shock protein. The observation that ARA9 has a high affinity for both the 90-kDa heat shock protein-associated and ligand-activated forms of the AHR suggests that ARA9 is a component of the AHR-signaling pathway in vivo.

Crystal structure of an Hsp90-geldanamycin complex: targeting of a protein chaperone by an antitumor agent

The Hsp90 chaperone is required for the activation of several families of eukaryotic protein kinases and nuclear hormone receptors, many of which are protooncogenic and play a prominent role in cancer. The geldanamycin antibiotic has antiproliferative and antitumor effects, as it binds to Hsp90, inhibits the Hsp90-mediated conformational maturation/refolding reaction, and results in the degradation of Hsp90 substrates. The structure of the geldanamycin-binding domain of Hsp90 (residues 9-232) reveals a pronounced pocket, 15 A deep, that is highly conserved across species. Geldanamycin binds inside this pocket, adopting a compact structure similar to that of a polypeptide chain in a turn conformation. This, and the pocket's similarity to substrate-binding sites, suggest that the pocket binds a portion of the polypeptide substrate and participates in the conformational maturation/refolding reaction.

The hsp90-binding antibiotic geldanamycin decreases Raf levels and epidermal growth factor signaling without disrupting formation of signaling complexes or reducing the specific enzymatic activity of Raf kinase

We have expressed the mitogenic signaling proteins Src, Ras, Raf-1, Mek (MAP kinase kinase), and Erk (MAP kinase) in baculovirus-infected Sf9 insect cells in order to study a potential role for the chaperone hsp90 in formation of multiprotein complexes. One such complex obtained by immunoadsorption with anti-Ras antibody of cytosol prepared from cells simultaneously expressing Ras, Raf, Mek, and Erk contained Ras, Raf, and Erk. To detect directly the protein-protein interactions involved in forming multiprotein complexes, we combined cytosols from single infections in vitro in all possible combinations of protein pairs. We detected complexes between Ras.Raf, Ras.Src, Raf.Mek, and Raf.Src, but no complex containing Erk was obtained by mixing cytosols. Thus, cellular factors appear to be required for assembly of the Erk-containing multiprotein complex. One cellular factor thought to be involved in signaling protein complex formation is the chaperone hsp90, and we show that Src, Raf, and Mek are each complexed with insect hsp90. Treatment of Sf9 cells with geldanamycin, a benzoquinone ansamycin that binds to hsp90 and disrupts its function, did not decrease coadsorption of either Raf or Erk with Ras, although it did decrease the level of cytosolic Raf. To study geldanamycin action, we treated rat 3Y1 fibroblasts expressing v-Raf and showed that the antibiotic blocked assembly of Raf.hsp90 complexes at an intermediate stage of assembly where Raf is still bound to the p60 and hsp70 components of the assembly mechanism. As in Sf9 cells, Raf levels decline with geldanamycin treatment of 3Y1 cells. To determine if geldanamycin affects mitogenic response, we treated HeLa cells with epidermal growth factor (EGF) and showed that geldanamycin treatment decreased EGF signaling and decreased the level of Raf protein without affecting the EGF-mediated increase in Raf kinase activity. We conclude that hsp90 is not required for forming complexes between the mitogenic signaling proteins or for Raf kinase activity and that EGF signaling is decreased indirectly by geldanamycin because the antibiotic increases degradation of Raf and perhaps other components of the signaling pathway.

The role of the hsp90-based chaperone system in signal transduction by nuclear receptors and receptors signaling via MAP kinase

The multicomponent heat-shock protein (hsp) 90-based chaperone system is an ubiquitous protein-folding system in the cytoplasm of eukaryotes. Several signal transduction systems utilize an interaction with hsp90 as an essential component of the signaling pathway. The steroid and dioxin receptors are bound to hsp90 through their hormone-binding domains, and several of them must be bound to hsp90 in order to have a ligand-binding site. The binding of ligands to these receptors promotes their dissociation from hsp90, an event that is the first step in their signaling pathways. Several protein kinases, including the Src and Raf components of the MAP kinase system, are also bound to hsp90. Genetic studies in yeast have demonstrated that hsp90 is required for normal signaling via steroid and dioxin receptors and for the activity of Src in vivo. The hsp90-based chaperone system has been reconstituted from purified components, permitting detailed analysis of the molecular basis of the chaperone's role in signal transduction.

In vivo analysis of the Hsp90 cochaperone Sti1 (p60)

Hsp90 interacts with Sti1 (p60) in lysates of yeast and vertebrate cells. Here we provide the first analysis of their interaction in vivo. Saccharomyces cerevisiae mutations that eliminate Sti1 or reduce intracellular concentrations of Hsp90 individually have little or no effect on growth at normal temperatures. However, when combined, the mutations greatly reduce or eliminate growth. Furthermore, overexpression of Sti1 has allele-specific effects on cells carrying various hsp90ts point mutations. These genetic interactions provide strong evidence that Hsp90 and Sti1 interact in vivo and that their functions are closely allied. Indeed, deletion of STI1 reduces the in vivo activity of the Hsp90 target protein, glucocorticoid receptor (GR). Mutations in GR that eliminate interaction with Hsp90 also eliminate the effects of the sti1 deletion. Examination of GR protein complexes in the sti1 deletion mutant reveals a selective increase in the concentration of GR-Ydj1 complexes, supporting previous hypotheses that Ydj1 functions at an early step in the maturation of GR and that Sti1 acts at an intermediate step. Deletion of STI1 also reduces the in vivo activity of another, unrelated Hsp90 target protein, v-Src. Our data indicate that Sti1 is a general factor in the maturation of Hsp90 target proteins and support earlier suggestions that Hsp90 matures even very different target proteins by a similar mechanism.

The tetratricopeptide repeat domain of protein phosphatase 5 mediates binding to glucocorticoid receptor heterocomplexes and acts as a dominant negative mutant

We previously identified a protein-serine phosphatase designated PP5, based on the binding of its tetratricopeptide repeat (TPR) domain to the atrial natriuretic peptide receptor (Chinkers, M. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 11075-11079). We have now identified another protein complex to which PP5 is targeted through its TPR domain. A 90-kDa protein, identified as heat shock protein 90 (hsp90) by immunoblotting, specifically co-immunoprecipitated from COS-7 cell lysates with the FLAG-tagged TPR domain of PP5. hsp90 also co-immunoprecipitated with full-length FLAG-tagged PP5 overexpressed in COS-7 cells and with endogenous PP5 from untransfected COS-7 cells or rat brain. During gel filtration, PP5 and hsp90 comigrated in a high molecular weight complex. Since glucocorticoid receptors (GR) exist as large heterocomplexes containing hsp90 bound to TPR proteins, we hypothesized that PP5 might be associated with these complexes. Consistent with this hypothesis, PP5 specifically co-immunoprecipitated with GR from mouse L cell lysates. To test the functional importance of this TPR-mediated association in living cells, we used a dominant negative PP5 mutant consisting only of its TPR domain. The mutant inhibited GR-mediated transactivation by approximately 70% in transfected CV-1 cells. This is the first evidence that the TPR proteins in steroid receptor heterocomplexes may be required for signaling in vivo.

Binding of immunophilins to the 90 kDa heat shock protein (hsp90) via a tetratricopeptide repeat domain is a conserved protein interaction in plants

In animal cell lysates, multiprotein complexes containing hsp90, hsp70, p60, p23, and several immunophilins can assemble steroid receptors and oncogenic protein kinases, such as v-Src and v-Raf, into heterocomplexes that contain hsp90 and either immunophilins or, in the case of protein kinases, p50. The complexes with hsp90 are required for the proper functioning of these signal transduction systems. Wheat germ lysate contains a similar protein folding activity that forms functional steroid receptor complexes with hsp90, but not all the components of this system have been identified. The plant chaperone system has conserved interactions with animal chaperones in that wheat hsp70 functions in the rabbit reticulocyte lysate heterocomplex assembly system and human p23 functions in the wheat germ lysate. Here, we ask if wheat germ lysate also contains immunophilins of the FK506-binding class (FKBPs) that bind to the hsp90 component of the chaperone complex via tetratricopeptide repeat (TPR) domains. To demonstrate the plant heterocomplex, we add purified mammalian p23, preadsorbed with the JJ3 antibody to protein A-Sepharose, to wheat germ lysate and allow ATP-dependent formation of an animal p23. plant hsp90 complex. The complex is then washed and incubated with the radiolabeled immunosuppressant drug [3H]FK506, which binds in a specific manner to a coimmunoadsorbed plant FKBP. Binding of the plant FKBP to plant hsp90 is prevented by adding to wheat germ lysate a purified fragment containing the TPR domains of human cyclophilin-40. Geldanamycin, a benzoquinone ansamycin that binds to animal hsp90s and prevents their chaperone activity, binds in a temperature-dependent manner to wheat hsp90 to block formation of the p23.hsp90.FKBP heterocomplex. These data show that immunophilin binding to hsp90 via TPR domains is conserved in the plant kingdom as well as in the animal kingdom and that geldanamycin will be an important tool for the study of hsp90-mediated protein chaperoning in plant cells.