Hsp56: a novel heat shock protein associated with untransformed steroid receptor complexes

The recently-described p59 protein has been shown to be associated with untransformed steroid receptors present in rabbit uterus and rat liver cytosols (Tai, P. K., Maeda, Y., Nakao, K., Wakim, N. G., Duhring, J. L., and Faber, L. E. (1986) Biochemistry 25, 5269-5275; Renoir, J.-M., Radanyi, C., Faber, L. E., and Baulieu, E.-E. (1990) J. Biol. Chem. 265, 10740-10745), while a smaller version of this protein (p56) interacts with glucocorticoid receptors in human IM-9 cell cytosols (Sanchez, E. R., Faber, L. E., Henzel, W. J., and Pratt, W. B. (1990) Biochemistry 29, 5145-5152). In addition to interacting with glucocorticoid receptors, the p56 protein of IM-9 cell cytosol is also found as part of a large heteromeric complex that contains both the 70-kDa and 90-kDa heat shock proteins (hsp70 and hsp90, respectively). Given this association of p56 with the two major stress proteins, I have speculated that p56 may itself be a heat shock protein. In this paper, the effect of heat stress on the rate of synthesis of p56 is determined. Intact IM-9 cells were exposed to 37 or 43 degrees C for 4 h, followed by pulse-labeling with [35S]methionine. Analysis of whole cytosolic extracts by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography reveal an increased rate of radiolabeling for hsp70, hsp90, hsp100, ad hsp110, but no heat-inducible protein of smaller relative molecular mass is detected. However, immune-purification of p56 from normal and heat-stressed cytosols with the EC1 monoclonal antibody results in the presence of a 56-kDa protein that exhibits an increased rate of synthesis in response to heat stress. The results of two-dimensional gel Western blots employing the EC1 antibody demonstrate that this heat-inducible protein is indeed the EC1-reactive p56 protein and that the induction effect is not due to unequal yields of p56 during immune-purification. Heat stress has no effect on the composition of the p56.hsp.70.hsp90 complex, except that the complex derived from heat shocked-cells contains both the constitutive and heat-inducible forms of hsp70. Induction of p56 also occurs in IM-9 cells subjected to chemical stress (sodium arsenite). It is proposed that p56 is a steroid receptor-associated heat shock protein which can now be termed hsp56. Like hsp90, hsp56 likely serves in some vital cellular role apart from any specific function it provides in steroid receptor action.     

The non-DNA-binding heterooligomeric form of mammalian steroid hormone receptors contains a hsp90-bound 59-kilodalton protein

Untransformed cytosol receptors for progesterone (PR), androgen (AR), estrogen (ER), and glucocorticosteroid (GR) in rabbit tissues contain a 59-kDa protein (p59) (Tai, P.K.K., Maeda, Y., Nakao, K., Wakim, N.G., Duhring, J.L., and Faber, L.E. (1986) Biochemistry 25, 5269-5275) and a 90-kDa heat shock protein (hsp90). In the present study, receptors from calf uterus (PR, AR, ER, and GR) and from human breast cancer MCF7 cells (PR and GR) were also shown to be comprised of hsp90 and p59. These heterooligomer receptor complexes were stabilized both by transition metal oxyanions (molybdate and tungstate) and chemical cross-linking with dimethylpimelimidate. In 0.4 M KCl, tungstate-stabilized (but not molybdate-stabilized) PR, AR, ER, and GR retained hsp90, but lost p59. Dimethylpimelimidate cross-linking prevented p59 dissociation from hsp90-receptor complexes. Stabilization with tungstate and/or cross-linking permitted immunoaffinity purification of untransformed rabbit as well as calf PR and ER on EC1-Affi-Gel 10 column (an anti-p59 immunoadsorbant). Combined immunoaffinity purification and cross-linking experiments indicated that p59 is bound to hsp90 in the cytosol. We propose that in the nontransformed steroid receptor, p59 interacts with hsp90 rather than with the hormone binding subunit.     

Characterization of the Ah receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin: use of chemical crosslinking and a monoclonal antibody directed against a 59-kDa protein associated with steroid receptors

The Ah receptor regulates induction of cytochrome P450IA1 (aryl hydrocarbon hydroxylase) by "3-methylcholanthrene-type" compounds and mediates the toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin and related halogenated aromatic hydrocarbons. Hepatic Ah receptor from untreated rodents is localized in the cytosol and has an apparent molecular mass of 250 to 300 kDa. This large form can be dissociated into a smaller ligand-binding subunit upon exposure to high ionic strength. The Ah receptor displays many structural similarities to the receptors for steroid hormones. Two non-ligand-binding proteins have been identified to be associated with the cytosolic forms of the steroid hormone receptors. The first is a 90-kDa heat shock protein (hsp 90); the second is a 59-kDa protein (p59) of unknown function. The cytosolic Ah receptor ligand-binding subunit previously has been shown to be associated with hsp 90. In the present study, we used a monoclonal antibody, KN 382/EC1, generated against the 59-kDa protein which is associated with rabbit steroid receptors to determine if p59 also is a component of the large cytosolic Ah receptor complex. Cytosolic forms of rabbit progesterone receptor, glucocorticoid receptor, and Ah receptor were analyzed by velocity sedimentation on sucrose gradients under low-ionic-strength conditions and in the presence of molybdate. Progesterone receptor from rabbit uterine cytosol and glucocorticoid receptor from rabbit liver each had a sedimentation coefficient of approximately 9 S. In the presence of KN 382/EC1 antibody the progesterone receptor and the glucocorticoid receptor both underwent a shift in sedimentation to a value of approximately 11 S. The increase in sedimentation velocity is an indication that the receptor-protein complexes are interacting with the antibody. Under low-ionic-strength conditions the Ah receptors from rabbit uterine cytosol and liver cytosol had a sedimentation coefficient of approximately 9 S. However, in contrast to the steroid receptors, the Ah receptor showed no change in its sedimentation properties in either tissue in the presence of KN 382/EC1, indicating that the antibody is not interacting with the Ah receptor. Multimeric Ah receptor complexes that were chemically crosslinked still did not show any interaction with KN 382/EC1. These data indicate that the 59-kDa protein either is not associated with the Ah receptor or is present in an altered form which the antibody cannot recognize.     

P59, an hsp 90-binding protein. Cloning and sequencing of its cDNA and preparation of a peptide-directed polyclonal antibody.

The primary sequence of the rabbit liver cDNA coding for protein p59 has been determined. The protein binds to the 90-kDa heat shock protein (hsp 90) and is associated with it, including when hsp 90 participates in hetero-oligomeric complexes of untransformed mammalian steroid receptors that sediment at 8-10 S. The cloned cDNA codes for an open reading frame of 458 amino acids defining a yet unknown protein. However, 55% amino acid homology to peptidyl-prolyl isomerase is found between amino acids 41 and 137, suggesting rotamase activity for p59, which speculatively may apply to bound hsp 90 and thus be implied in the intracellular trafficking of hetero-oligomeric forms of steroid hormone receptors. A polyclonal antibody derived from the COOH-terminal peptide 441-458 demonstrates a good affinity for rabbit, rat, and human "p59" protein. It interacts with at least one epitope, available in 8-10 S untransformed steroid receptor complexes and different from that recognized by the monoclonal antibody KN382/EC-1.     

Evidence that the 90-kDa phosphoprotein associated with the untransformed L-cell glucocorticoid receptor is a murine heat shock protein

Two phosphoproteins are adsorbed to protein-A-Sepharose when cytosol from 32P-labeled L-cells is incubated with a monoclonal antibody against the glucocorticoid receptor: one is a 98-100-kDa phosphoprotein that contains the steroid-binding site and the other is a 90-kDa nonsteroid-binding phosphoprotein that is associated with the untransformed, molybdate-stabilized receptor (Housley, P. R., Sanchez, E. R., Westphal, H.M., Beato, M., and Pratt, W.B. (1985) J. Biol. Chem. 260, in press). In this paper we show that the 90-kDa receptor-associated phosphoprotein is an abundant cytosolic protein that reacts with a monoclonal antibody that recognizes the 90-kDa phosphoprotein that binds steroid receptors in the chicken oviduct. The 90-kDa protein immunoadsorbed from L-cell cytosol with this antibody reacts on Western blots with rabbit antiserum prepared against the 89-kDa chicken heat shock protein. Immunoadsorption of molybdate-stabilized cytosol by antibodies against the glucocorticoid receptor results in the presence of a 90-kDa protein that interacts on Western blots with the antiserum against the chicken heat shock protein. The association between the 90-kDa protein and the receptor is only seen by this technique when molybdate is present to stabilize the complex; and when steroid-bound receptors are incubated at 25 degrees C to transform them to the DNA-binding state, the 90-kDa protein dissociates. These observations are consistent with the proposal that the untransformed glucocorticoid receptor in L-cells exists in a complex with the murine 90-kDa heat shock protein.     

Relationship of the 90-kDa murine heat shock protein to the untransformed and transformed states of the L cell glucocorticoid receptor

Incubation of molybdate-stabilized L cell cytosol with a monoclonal antibody directed against the 100-kDa glucocorticoid-binding protein causes the immune-specific adsorption to protein A-Sepharose of both the 100-kDa glucocorticoid receptor and the 90-kDa murine heat shock protein (hsp90) (Sanchez, E. R., Toft, D. O., Schlesinger, M. J., and Pratt, W. B. (1985) J. Biol. Chem. 260, 12398-12401). When the glucocorticoid receptor in cytosol is transformed to the DNA-binding state, hsp90 dissociates. In this paper, we show that temperature-mediated dissociation of hsp90 from the receptor is a hormone-dependent event in the same manner as temperature-mediated transformation to the DNA-binding state. In contrast to temperature-mediated transformation, ammonium sulfate causes both dissociation of hsp90 from the receptor and conversion of the receptor to the DNA-binding form in a manner that does not require the presence of steroid. The untransformed form of the glucocorticoid receptor and the strongly negatively charged hsp90 protein behave similarly on DEAE-cellulose chromatography, suggesting that the hsp90 component may contribute significantly to the net negative charge behavior of the non-DNA-binding form of the receptor complex.     

Evidence that the 90-kilodalton heat shock protein is associated with tubulin-containing complexes in L cell cytosol and in intact PtK cells

It has been established that the 90-kilodalton murine heat shock protein, hsp90, is associated with the untransformed, non-DNA-binding form of the glucocorticoid receptor in L cell cytosol. In this work, we show that incubation of L cell cytosol with Affi-Gel-coupled monoclonal antibodies directed against either alpha-tubulin alone or both alpha- and beta-tubulin results in the immune-specific adsorption of hsp90 identified by Western blotting with the AC88 monoclonal antibody. Similarly, the AC88 antibody, which is specific for hsp90, causes the immune-specific isolation of both alpha- and beta-tubulin from hypotonic cytosol. The distribution of hsp90 in cultured Potorous tridactylis kidney cells was examined by indirect immunofluorescence using the AC88 monoclonal as primary antibody. In interphase cells, AC88-dependent fluorescence was distributed like antitubulin antibody-dependent fluorescence in a fibrillar array located in the cytoplasm and around the periphery of the nucleus. In cells undergoing mitosis, AC88 fluorescence was located in the mitotic spindle. These observations suggest that a significant portion of hsp90 is associated with a tubulin-containing complex both in a hypotonic cytosol preparation from mouse fibroblasts and in intact marsupial kidney epithelial cells. The distribution of AC88 fluorescence in interphase Potorous tridactylis kidney cells is similar to the distribution of glucocorticoid receptor demonstrated by Wikstrom, A. C., Bakke, O., Okret, S., Bronnegard, M., and Gustafsson, J. A in rat hepatoma and human uterine cells.     

Direct stoichiometric evidence that the untransformed Mr 300,000, 9S, glucocorticoid receptor is a core unit derived from a larger heteromeric complex

We have used three methods to measure the stoichiometry of the glucocorticoid receptor and the 90-kDa heat shock protein (hsp90) in L-cell glucocorticoid receptor complexes that were purified by immunoadsorption to protein A-Sepharose with an anti-receptor monoclonal antibody, followed by a minimal washing procedure that permits retention of receptor-associated protein. In two of the methods, receptor was quantitated by radioligand binding, and receptor-specific hsp90 was quantitated against a standard curve of purified hsp90, either on Coomassie blue stained SDS gels by laser densitometry or on Western blots by quantitative immunoblotting with 125I-labeled counterantibody. The stoichiometry values obtained by densitometry and immunoblotting are 7 and 6 mol of hsp90/mol of receptor, respectively. In a third method, which detects total receptor protein rather than just steroid-bound receptor, the ratio of hsp90 to receptor was determined by immunopurifying receptor complexes from [35S]methionine-labeled L cells, and the amount of 35S incorporated into receptor and hsp90 was corrected for the established methionine content of the respective proteins. In complexes from L cells which are labeled to steady state (48 h), the ratio of hsp90 to GR is 4:1. When immunoadsorbed receptor complexes are washed extensively with 0.5 M NaCl and 0.4% Triton X-100 in the presence of molybdate, the ratio of hsp90 to GR is 2:1. In addition to hsp90, preparations of [35S]methionine-labeled untransformed receptor complex also contain a 55-kDa protein that the conclusion that the untransformed L-cell glucocorticoid receptor exists in cytosol in a much larger heteromeric complex than considered to date.(ABSTRACT TRUNCATED AT 250 WORDS)     

A 50-kDa cytosolic protein complexed with the 90-kDa heat shock protein (hsp90) is the same protein complexed with pp60v-src hsp90 in cells transformed by the Rous sarcoma virus.

We have previously shown that a 50-kDa protein is one component of a heteromeric complex immunoprecipitated by the 90-kDa heat shock protein (hsp90) monoclonal antibodies 8D3 and 3G3 (Perdew, G. H., and Whitelaw, M. L. (1991) J. Biol. Chem. 266, 6708-6713). In this report, we compare the 50-kDa protein with that found in pp60v-src-hsp90-p50 complexes immunoprecipitated from Rous sarcoma virus-transformed cells with antibodies to pp60v-src. 35S- and 32P-labeled p50 proteins from each system were identical in their mobilities by sodium dodecyl sulfate-polyacryl-amide gel electrophoresis. The profile of N-chlorosuccinimide cleavage products derived from each 32P-labeled p50 protein were also identical when resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. We have developed a mouse monoclonal antibody, 3M/1B5p50, capable of detecting p50 on Western blots. This antibody detected the 50-kDa protein which co-purified with the pa104 pp60v-src mutant of the avian sarcoma virus oncoprotein in 44A rat fibroblasts. We did not detect p50 in association with native glucocorticoid receptor in L cells or with the overexpressed glucocorticoid receptor in Chinese hamster ovary cells. Two experiments utilizing immunochemical staining implied that essentially all cytosolic p50 is associated with hsp90. Firstly, immunoprecipitating hsp90 from Hepa 1 cytosol with monoclonal antibody 3G3 left the cytosol depleted of p50. Secondly, cytosol fractionated by sucrose gradient revealed that p50 cosedimented with hsp90, confirming the existence of p50 only in association with hsp90.     

The molybdate-stabilized glucocorticoid binding complex of L-cells contains a 98-100 kdalton steroid binding phosphoprotein and a 90 kdalton nonsteroid-binding phosphoprotein that is part of the murine heat-shock complex

This paper summarizes our work performed with glucocorticoid-binding complexes in molybdate-stabilized cytosol prepared from 32P-labeled L-cells. In our early work, we showed that cytosol prepared from 32P-labeled L-cells contains two phosphoproteins (a 90 and a 98-100 kdalton protein) that elute from an affinity resin of deoxycorticosterone agarose in a manner consistent with the predicted behavior of the glucocorticoid receptor. Both phosphoproteins are immunoadsorbed onto protein-A-Sepharose from molybdate-stabilized cytosol incubated with a monoclonal antibody against the receptor. The 98-100 kdalton phosphoprotein binds steroid and the 90 kdalton phosphoprotein is a structurally different, nonsteroid-binding protein that is bound to the untransformed, molybdate-stabilized glucocorticoid receptor. The 90 kdalton protein reacts on Western blots with a monoclonal antibody raised against a 90 kdalton protein from the water mold Achlya ambisexualis. This antibody recognizes an epitope that is conserved in 90 kdalton phosphoproteins from rodent and human cells, and it reacts with the 90 kdalton phosphoprotein that copurifies with the molybdate-stabilized, untransformed chick oviduct progesterone receptor. The 90 kdalton nonsteroid-binding phosphoprotein is an abundant cytosolic protein that dissociates from the glucocorticoid receptor when it is transformed, and unlike the steroid-binding protein, it does not bind to DNA. The 90 kdalton phosphoprotein determines the acidic behavior of the untransformed glucocorticoid receptor on DEAE-cellulose. This abundant cytosolic 90 kdalton phosphoprotein reacts with rabbit antiserum raised against the gel purified 89 kdalton chicken heat-shock protein (hsp89). This antiserum recognizes 90 kdalton heat-shock proteins in human, rodent, frog and Drosophila cells. Immunoadsorption of molybdate-stabilized cytosol with antibody directed against the 98-100 kdalton steroid receptor results in the immune-specific adsorption of a 90 kdalton phosphoprotein that reacts with anti-hsp89 antibody on Western blots. These observations suggest that, like the transforming proteins from several avian sarcoma viruses, the untransformed glucocorticoid receptor exists in a complex with the 90 kdalton heat-shock protein.     

Glucocorticoid receptor phosphorylation in mouse L-cells

This paper summarizes our observations on the phosphorylation state of untransformed and transformed glucocorticoid receptors isolated from 32P-labeled L-cells. The 300-350-kDa 9S untransformed murine glucocorticoid receptor complex is composed of a 100-kDa steroid-binding phosphoprotein and one or possibly two units of the 90-kDa heat shock protein (hsp90), which is also a phosphoprotein. Transformation of this complex to the 4S DNA-binding state is accompanied by dissociation of hsp90. When receptors in cytosol are transformed by heating at 25 degrees C, there is no gross change in the degree of phosphorylation of the steroid-binding protein. Both receptors that are bound to DNA after transformation under cell-free conditions and receptors that are located in the nucleus of cells incubated at 37 degrees C in the presence of glucocorticoid are labeled with 32P. The results of experiments in which the 32P-labeled receptor was submitted to limited proteolysis suggest that the 16-kDa DNA-binding domain is phosphorylated and that the 28-kDa steroid-binding domain is not.     

Identification of a 60-kilodalton stress-related protein, p60, which interacts with hsp90 and hsp70.

Immunoaffinity purification of hsp90 from chick oviduct cytosol reveals two major proteins, hsp70 and a 60-kDa protein (p60), copurifying with hsp90. A similar result is obtained when hsp90 is immunoaffinity purified from chick liver and brain cytosols, avian fibroblasts, and rabbit reticulocyte lysate. This p60 is the same protein previously identified in certain assembly complexes of chick progesterone receptor generated in a cell-free reconstitution system. Tryptic and cyanogen bromide peptide fragments were generated from gel-purified p60, and partial N-terminal sequences were determined from eight peptides. The sequences show a striking similarity to the sequence of a 63-kDa human protein (IEF SSP 3521) whose abundance is increased in MRC-5 fibroblasts following simian virus 40 transformation. A monoclonal antibody was prepared against avian p60; Western immunoblot analysis showed that p60 was present in each of eight chick tissues examined and in each of the human, rat, rabbit, and Xenopus tissues tested. Immunoaffinity purifications from both chick oviduct cytosol and rabbit reticulocyte lysate using anti-p60 and anti-hsp70 monoclonal antibodies confirm that there is a relatively abundant complex in these extracts containing hsp90, hsp70, and p60. This complex appears to comprise an important functional unit in the assembly of progesterone receptor complexes. However, judging from the abundance and widespread occurrence of this multiprotein complex, hsp90, hsp70, and p60 probably function interactively in other systems as well.     

A heat shock protein complex isolated from rabbit reticulocyte lysate can reconstitute a functional glucocorticoid receptor-Hsp90 complex.

When unliganded glucocorticoid receptor that has been stripped free of associated proteins is incubated with rabbit reticulocyte lysate, the receptor becomes associated with the 70- and 90-kDa heat shock proteins (hsp70 and hsp90), and the untransformed state of the receptor is functionally reconstituted [Scherrer, L. C., Dalman, F. C., Massa, E., Meshinchi, S., & Pratt, W. B. (1990) J. Biol. Chem. 265, 21397-21400]. Recently, an hsp70-containing protein complex (200-250 kDa) purified from rabbit reticulocyte lysate was shown to maintain a fusion protein bearing the mitochondrial matrix-targeting signal in a state that is competent for mitochondrial import [Sheffield, W. P., Shore, G. C., & Randall, S. K. (1990) J. Biol. Chem. 265, 11069-11076]. In this work, we show that this partially purified mitochondrial import-competent fraction contains both hsp90 and hsp70. When the purified fraction is immunoadsorbed with a monoclonal antibody specific for hsp90, a significant portion of the hsp70 is co-immunoadsorbed, suggesting that hsp90 and hsp70 are present together as a complex. The partially purified fraction maintains a hybrid precursor protein containing the mitochondrial matrix-targeting signal of rat pre-ornithine carbamyl transferase in an import-competent state. Incubation of immunopurified glucocorticoid receptor with this fraction of reticulocyte lysate results in ATP-dependent association of the receptor with both hsp70 and hsp90, and the resulting complexes are functional as assessed by return of the receptor to the high-affinity steroid binding conformation. The glucocorticoid receptor hetero-complex reconstituting activity of the lysate fraction is low relative to its mitochondrial import activity. Importantly, however, this is the first demonstration of the functional and structural reconstitution of the untransformed state of any steroid receptor utilizing a partially purified system.     

Nuclear localization of two steroid receptor-associated proteins, hsp90 and p59

It has been proposed that the unliganded nontransformed form of steroid hormone receptor is a heterooligomer comprising, in addition to the hormone-binding subunit, two associated proteins: a heat shock protein of MW 90,000 (hsp90) and another protein of MW 59,000 (p59). Using monoclonal antibodies, we demonstrate immunocytochemically the presence of both hsp90 and p59 in cell nuclei of progesterone target cells of the rabbit uterus. While steroid receptors (e.g., progesterone receptors) appear to be exclusively nuclear, we find p59 predominantly in the cell nuclei and hsp90 in both the nucleus and the cytoplasm. In addition, Western blotting of high-salt extracts of nuclear proteins detects the presence of hsp90 and p59 in the nuclei of rabbit uterus. These observations are consistent with the presence of the untransformed heterooligomeric form of steroid hormone receptors in the nuclei of target cells.     

Transformation of glucocorticoid and progesterone receptors to the DNA-binding state

This brief review explores some recent observations relating to the structure of untransformed glucocorticoid and progesterone receptors and the mechanism by which the receptors are transformed to the DNA-binding state. In their molybdatestabilized, untransformed state, progesterone and glucocorticoid receptors exist as a heteromeric 8-9S complex containing one unit of steroid binding phosphoprotein and one or two units of the 90 kD heat shock protein hsp90. When the receptors are transformed, the steroid-binding protein dissociates from hsp90. In cytosol preparations, temperature-mediated dissociation proceeds much more rapidly in the presence of hormone. The dissociated receptor binds to DNA with high affinity, regardless of whether it is in the hormone-bound or the hormone-free state. These observations raise the possibility that the primary, and perhaps the only, role for the hormone is to promote dissociation of the receptor-hsp90 complex. Molybdate, vanadate, and tungstate inhibit receptor transformation to the DNA-binding form, an effect that appears to reflect the ability of these transition metal oxyanions to stabilize the complex between the steroid receptor and hsp90. By promoting the formation of disulfide bonds, hydrogen peroxide also stabilizes the glucocorticoid receptor-hsp90 complex and prevents receptor transformation. A small, heat-stable factor present in all cytosol preparations inhibits receptor transformation, and, when the factor is removed, glucocorticoid receptors are rapidly transformed. This ubiquitous factor has the physical properties of a metal anion, and it is proposed that molybdate and vanadate affect steroid receptor complexes by interacting with a metal anion-binding site that is normally occupied by this endogenous receptor-stabilizing factor.     

Evidence that the 90-kDa heat shock protein (HSP90) exists in cytosol in heteromeric complexes containing HSP70 and three other proteins with Mr of 63,000, 56,000, and 50,000

Monoclonal antibody (mAb) 8D3 and 3G3 are unique antibodies capable of precipitating both free 90-kDa heat shock protein (HSP90) and HSP90-protein complexes. Immunoprecipitation of [35S]methionine-labeled Hepa 1c1c7 cytosolic extracts were performed using mAb 8D3 or 3G3. The resulting immunoprecipitates can be dissociated from the mAb with a 500 mM NaCl wash. These washes were subjected to both sodium dodecyl sulfate-polyacrylamide gel electrophoresis and two-dimensional gel electrophoresis. Five major protein spots were detected in addition to HSP90 with the following relative molecular weights: 68,000, 63,000, 56,000, 50,000, and 188,000. On Western blots mAb 3G3 was capable of specifically binding to HSP90. Each of these proteins was localized on two-dimensional gels. Using one-dimensional gel electrophoresis and immunochemical localization on Western blots, the p68 spot was identified as HSP70, and the p56 spot was found to cross-react with polyclonal antibody JP-1 raised against a 59-kDa protein. This 59-kDa protein has been found previously to be associated with several steroid hormone receptors in rabbit uterine cytosol. Immunoprecipitation of [32P]orthophosphate-labeled Hepa 1c1c7 cytosol with mAb 8D3 or 3G3 revealed two major phosphorylated proteins with relative molecular weights of 90,000 and 50,000. The identities of p63 and p188 are currently unknown. This is the first report examining the major proteins that are complexed with HSP90 in mammalian cells.     

A model of glucocorticoid receptor unfolding and stabilization by a heat shock protein complex

It has recently been reported that incubation of avian progesterone receptors, mouse glucocorticoid receptors, or the viral tyrosine kinase pp60^src with rabbit reticulocyte lysate reconstitutes their association with the 90 kDa heat shock protein, hsp90. The reassociation is thought to require unfolding of the steroid receptor or pp60^src before hsp90 can bind. The unfoldase activity may be provided by hsp70, which is also present in the reconstituted receptor heterocomplex. In this paper we review evidence that hsp70 and hsp90 are associated in cytosolic heterocomplexes that contain a limited number of other proteins. From an analysis of known receptor-hsp interactions and a predicted direct interaction between hsp90 and hsp70 we have developed an admittedly very speculative model of glucocorticoid receptor unfolding and stabilization. One important feature of the model is that the receptor becomes attached to a heat shock protein heterocomplex rather than undergoing independent unfolding and stabilization events. The model requires that hsp70 and hsp90 bind directly to the receptor at independent sites. Importantly, the model accomodates the stoichiometry of 2 hsp90 per 1 molecule of receptor that has been assayed in the untransformed GR heterocomplex in cytosols prepared from hormone-free cells.     

Differential roles of heat shock protein 70 in the in vitro nuclear import of glucocorticoid receptor and simian virus 40 large tumor antigen.

Nuclear import of glucocorticoid receptors (GRs) was analyzed in vitro with digitonin-permeabilized cells (S. A. Adam, R. Sterne-Marr, and L. Gerace, J. Cell Biol. 111:807-816, 1990). Indirect immunofluorescence methods were used to monitor the transport of GRs from rat hepatoma and fibroblast cell cytosol into HeLa nuclei. In vitro nuclear import of GRs was shown to be hormone dependent and to require ATP and incubation at ambient temperatures (i.e., 30 degrees C). Hormone-dependent dissociation of GR-bound proteins, such as the 90-kDa heat shock protein, hsp90, is part of an activation process that is obligatory for the expression of the receptor's DNA-binding activity. Inhibition of in vitro GR activation by Na2MoO4 blocked hormone-dependent nuclear import, demonstrating that receptor activation is required for nuclear import. The addition to GR-containing cytosol of antiserum directed against the cytosolic 70-kDa heat shock protein, hsp70, while effective in blocking the nuclear import of simian virus 40 large tumor antigen (SV40 TAg), did not affect hormone-dependent nuclear import of endogenous, full-length GRs or an exogenously added truncated GR protein (i.e., XGR556) that lacks a hormone-binding domain but possesses a constitutively active nuclear localization signal sequence (NLS). Depletion of hsp70 from HeLa cell cytosol did not affect the nuclear import of exogenously added XGR556 but led to inhibition of SV40 TAg nuclear import. Thus, two closely related NLSs, one contained within GRs and the other contained within SV40 TAg, are distinguished by their differential requirements for hsp70 in vitro.     

Reconstitution of the multiprotein complex of pp60src, hsp90, and p50 in a cell-free system.

A rabbit reticulocyte lysate system that has been used to reconstitute functional complexes between steroid receptors and the 90-kDa heat shock protein (hsp90) has been used here to form complexes between the pp60src tyrosine kinase and hsp90. Reticulocyte lysate forms complexes between hsp90 and a temperature-sensitive mutant of Rous sarcoma virus pp60v-src, which is normally present in cytosol virtually entirely in the multiprotein complex form. In addition, hsp90 in the lysate complexes with wild-type pp60v-src, of which only a small portion is normally recovered in cytosol in the native multiprotein complex, and with the cellular homolog, pp60c-src, which has never been recovered in cytosol in the form of a native multiprotein complex with hsp90. Moreover, the reticulocyte lysate-reconstituted complex also contains the 50-kDa phosphoprotein component of the native pp60v-src multiprotein complex. The native and reconstituted pp60src-hsp90 complexes have similar thermal stability and, like steroid receptor heterocomplexes, they are stabilized by molybdate. As previously shown with reticulocyte lysate-reconstituted steroid receptor heteroprotein complexes, the reconstituted pp60src multiprotein complex contains hsp70, which is a major candidate for providing the protein unfoldase activity required for hsp90 association.